Rapid Prototyping, Rapid Tooling and Reverse Engineering: From Biological Models to 3D Bioprinters 9783110664904, 9783110663242

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Kaushik Kumar, Divya Zindani, J. Paulo Davim Rapid Prototyping, Rapid Tooling and Reverse Engineering

De Gruyter Series in Advanced Mechanical Engineering

Series Editor J. Paulo Davim

Volume 5

Kaushik Kumar, Divya Zindani, J. Paulo Davim

Rapid Prototyping, Rapid Tooling and Reverse Engineering From Biological Models to 3D Bioprinters

Authors Dr. Kaushik Kumar Department of Mechanical Engineering Birla Institute of Technology Mesra, Ranchi 835215, Jharkhand India [email protected]

Prof. Dr. J. Paulo Davim Dept. of Mechanical Engineering University of Aveiro Campus Santiago 3810-193 Aveiro Portugal [email protected]

Divya Zindani Department of Mechanical Engineering National Institute of Technology, SILCHAR Cachar, Silchar 788010, Assam India [email protected]

ISBN 978-3-11-066324-2 e-ISBN (PDF) 978-3-11-066490-4 e-ISBN (EPUB) 978-3-11-066345-7 ISSN 2367-3796 Library of Congress Control Number: 2020936761 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2020 Walter de Gruyter GmbH, Berlin/Boston Cover image: Sky_blue/iStock/thinkstock Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Preface The authors are pleased to present the book Rapid Prototyping, Rapid Tooling and Reverse Engineering. From Biological Models to 3D Bioprinters under the book series Advanced Mechanical Engineering. Book title was chosen as it converges upcoming technologies in mechanical engineering and provides an interdisciplinary concept in the coming future. This book provides an insight on the new technology called rapid prototyping. At present, “rapid prototyping” is the buzzword for all disciplines working toward product development and many scholars are working in these areas. This book provides an insight for all researchers, academicians, postgraduate or senior undergraduate students working in the area. With globalization of market and advances in science and technology, the life span of products has shortened considerably. For early realization of products and short development period, engineers and researchers are constantly working together for more and more efficient and effective solutions. The most effective solution identified has been usage of computers in both designing and manufacturing. This gave birth to the nomenclatures CAD (computer-aided design) and CAM (computer-aided manufacturing). This was the initiation that ensured short product development and realization period. Researchers coined the concept as rapid prototyping. In contrast to prototyping, rapid prototyping is a group of techniques used to quickly fabricate a scale model of a physical part or assembly using three-dimensional CAD data. Construction of the part or assembly is usually done using 3D printing or “additive or subtractive layer manufacturing” technology. The first methods for rapid prototyping became available in the late 1980s and were used to produce models and prototype parts. Today, they are used for a wide range of applications and are used to manufacture production quality parts in relatively small numbers if desired without the typical unfavorable short-run economics. This economy has encouraged online service bureaus for early product realization or physical products for actual testing. This book contains seven chapters. Chapter 1 explains product life cycle and the product development phase in the same. It also introduces role of rapid prototyping techniques in product development phase. Chapter 2 deals with the concept, origin and working cycle of rapid prototyping processes. Chapter 3 concentrates on the applications of rapid prototyping technology apart from elaboration of engineering and nonengineering applications. In engineering applications, prototype for concept evaluation and prototype for functional evaluation have been discussed. In case of nonengineering aspects, applications in biomedical models for surgical planning, development of prosthesis and implants, molecular models, architectural models, sculptured models, food article models,

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Preface

models for NEMS/MEMS applications, models for toys, psychoanalysis models and others have been discussed Chapter 4 introduces the various rapid prototyping systems available worldwide. The various processes usually come under three umbrellas: liquid-based processes, powder-based processes and solid-based processes. The chapter also introduces the technique of generating human organs from live cells/tissues of the same human being named 3D BIOPRINTERS, hence ensuring low rejection rate by the human body. As the rapid prototyping techniques are for tailor-made products and not for mass manufacturing, Chapter 5 elaborates on the mass manufacturing of rapid prototyped products. This includes casting and rapid tooling. Chapter 6 deals with reverse engineering and the role played by rapid prototyping techniques toward the same. The concluding chapter of the book, that is, Chapter 7 explains 3D bioprinting in detail. At the end, Glossary has been added, which is a collection of various technical terms defined clearly to make the readers at par with this new budding concept. First and foremost, the authors would like to thank God for providing the opportunity, believe in passion, hard work and pursue dreams. In the process of putting this book together, it was realized how true this gift of writing is for anyone. This could never have been done without power provided by you and of course, the faith in You, the Almighty. The authors would also like to thank all their colleagues, the editorial board members, project development editor and the complete team of De Gruyter for their availability on this editorial project. The efforts will come to a level of satisfaction if the professionals concerned with all the fields working in product development, CAD/CAM and rapid prototyping technology get benefited. Kaushik Kumar Divya Zindani J. Paulo Davim

About the authors Kaushik Kumar, B.Tech (Mechanical Engineering, REC (Now NIT), Warangal), MBA (Marketing, IGNOU) and Ph.D (Engineering, Jadavpur University), is presently an associate professor in the Department of Mechanical Engineering, Birla Institute of Technology, Mesra, Ranchi, India. He has 17 years of teaching and research and over 11 years of industrial experience in a manufacturing unit of global repute. His areas of teaching and research interest are conventional and nonconventional quality management systems, optimization, nonconventional machining, CAD/ CAM, rapid prototyping and composites. He has nine patents, 28 books, 15 edited book volume, 43 book chapters, 136 international journals, 21 international and eight national conference publications to his credit. He is on the editorial board and review panel of seven international and one national journals of repute. He has been felicitated with many awards and honors. Divya Zindani (BE, Mechanical Engineering, Rajasthan Technical University, Kota), M.E. (Design of Mechanical Equipment, BIT Mesra), is presently pursuing PhD (National Institute of Technology, Silchar). He has over 2 years of industrial experience. His areas of interests are optimization, product and process design, CAD/CAM/CAE, rapid prototyping and material selection. He has one patent, four books, six edited books, 18 book chapters, two SCI journals, seven Scopus Indexed international journals and four international conference publications to his credit. J. Paulo Davim received his Ph.D. degree in Mechanical Engineering in 1997, M.Sc. degree in Mechanical Engineering (materials and manufacturing processes) in 1991, Mechanical Engineering degree (5 years) in 1986, from the University of Porto (FEUP), the Aggregate title (Full Habilitation) from the University of Coimbra in 2005 and the D.Sc. (Higher Doctorate) from London Metropolitan University in 2013. He is Senior Chartered Engineer by the Portuguese Institution of Engineers with an MBA and Specialist titles in Engineering and Industrial Management as well as in Metrology. He is also Eur Ing by FEANI-Brussels and Fellow (FIET) of IET-London. Currently, he is Professor at the Department of Mechanical Engineering of the University of Aveiro, Portugal. He is also distinguished as honorary professor in several universities/colleges. He has more than 30 years of teaching and research experience in Manufacturing, Materials, Mechanical and Industrial Engineering, with special emphasis in Machining & Tribology. He has also interest in Management, Engineering Education and Higher Education for Sustainability. He has guided large numbers of postdoc, Ph.D. and master’s students as well as has coordinated and participated in several financed research projects. He has received several scientific awards and honours. He has worked as evaluator of projects for ERC-European Research Council and other international research agencies as well as examiner of Ph.D. thesis for many universities in different countries. He is the Editor in Chief of several international journals, Guest Editor of journals, books Editor, book Series Editor and Scientific Advisory for many international journals and conferences. Presently, he is an Editorial Board member of 30 international journals and acts as reviewer for more than 100 prestigious Web of Science journals. In addition, he has also published as editor (and co-editor) more than 150 books and as author (and co-author) more than 15 books, 100 book chapters and 500 articles in journals and conferences (more than 250 articles in journals indexed in Web of Science core collection/h-index 55+/9500+ citations, SCOPUS/h-index 60+/12000+ citations, Google Scholar/h-index 77+/19500+).

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Contents Preface

V

About the authors Chapter 1 1.1 1.2 1.2.1 1.2.1.1 1.2.2 1.2.2.1 1.2.3 1.2.3.1 1.2.4 1.2.4.1

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1.4

Product life cycle 1 Introduction 1 Product development phase in product life cycle 3 Phase 1: conceive 5 Meaning: idea generation, specify, plan and innovate 5 Phase 2: design 6 Delineate, define, develop, test, analyze and validate 6 Phase 3: realize 6 Fabricate, make, develop, procure, sell and deliver 6 Phase 4: service 7 Utilize, operate, maintenance, support, sustain, phase-out, obsolete, recycle and disposal 7 Rapid prototyping technology in product development and realization 7 Conclusion 9

Chapter 2 2.1 2.2 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.4

Rapid prototyping processes 11 Introduction 11 Origins of rapid prototyping 12 Design process 13 The concept 13 Preliminary designs 14 Preliminary prototype fabrication 14 Short-run production 14 Final production 15 Rapid prototyping cycle 15

Chapter 3 3.1 3.2 3.2.1 3.3 3.3.1 3.3.1.1 3.3.1.2 3.3.1.3

Applications of rapid prototyping processes 17 Introduction 17 Engineering application 17 Prototype for concept evaluation 17 Nonengineering applications 19 Biomedical models for surgical planning 19 Steps in production of rapid prototyping models 19 Design evaluation and surgical planning 19 Additive manufacturing and production of the model 20

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3.3.1.4 3.3.2 3.3.3 3.3.3.1 3.3.3.2 3.3.3.3 3.3.3.4 3.3.3.5 3.3.3.6 3.3.3.7 3.3.4 3.3.4.1 3.3.4.2 3.3.4.3 3.3.4.4 3.3.4.5 3.3.4.6 3.3.5 3.3.6 3.3.6.1 3.3.6.2 3.3.6.3 3.3.7 3.3.7.1 3.3.7.2 3.3.7.3 3.3.7.4 3.3.7.5 3.3.7.6 3.3.8 3.3.8.1 3.3.9 3.3.9.1 3.3.9.2 3.3.9.3 3.4

Surgical simulation and virtual planning 20 Development of prosthesis and implants 21 Molecular models 21 Design of physical models 22 Fabrication of models 23 Augmented reality interface 24 Examples 25 HIV protease 25 Superoxide dismutase 26 Ribosome 26 Architectural models 26 Architectural needs 26 Models and architectural designs 26 Limitations of models 27 Representation of models 28 Digital models for representations 29 Rapid prototyping 29 Sculptured models 30 Food article models 34 Rapid prototyping food and beverage packaging 3D printing and food 35 Applications of 3D printed food 35 Models for NEMS/MEMS applications 36 Evolution and rise of MEMS 36 Impact in medicine 37 MEMS evolving manufacturing alternatives 37 Tools for modeling and simulation 38 Prototyping and MEMS 38 Virtual reality prototyping and MEMS 41 Models for toys 42 The challenge of toy design 42 Psychoanalysis models 43 Rapid prototyping 43 Nuclear magnetic resonance imaging 44 Procedure 44 Conclusion 45

Chapter 4 4.1 4.2 4.2.1 4.2.1.1

Rapid prototyping/manufacturing processes Introduction 47 Liquid-based processes 48 Stereolithography 49 The stereolithography apparatus 49

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4.2.1.2 4.2.1.3 4.2.1.4 4.2.2 4.2.3 4.2.4 4.2.5 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.7.1 4.3.7.2 4.3.8 4.3.9 4.3.10 4.4 4.4.1 4.4.1.1 4.4.2 4.4.2.1 4.4.2.2 4.4.2.3 4.4.2.4 4.4.2.5 4.4.2.6 4.4.2.7 4.4.2.8 4.4.2.9 4.5 4.5.1 4.5.1.1 4.5.1.2 4.5.1.3 4.6

Stereolithography apparatus operation 50 Relation to other rapid prototyping technologies 51 Advantages and disadvantages 51 Jetting systems 52 Direct light processing technologies 52 High-viscosity jetting 53 The Maple process 53 Powder-based processes 54 Selective laser sintering (polymers) 54 Selective laser sintering (ceramics and metals) 55 Direct metal laser sintering 55 Three-dimensional printing 56 Fused metal deposition systems 56 Electron beam melting 56 Selective laser melting 57 Process 57 Advantages of SLM 58 Selective mask sintering 58 Electrophotographic layered manufacturing 58 High-speed sintering 59 Solid-based processes 60 Fused deposition modeling 60 FDM process parameters 61 Sheet stacking technologies 62 System hardware 62 Process 64 Software 64 Part orientation 64 Crosshatching 65 System parameters 65 Technique 66 Finishing 68 Advantages and disadvantages 68 Bioprinter 69 Three-dimensional bioprinting techniques 70 Ink-jet-based printing 70 Extrusion-based printing 73 Laser direct write 74 Conclusion 76

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Chapter 5 5.1 5.2 5.2.1 5.2.1.1 5.2.1.2 5.2.1.3 5.2.2 5.2.2.1 5.2.2.2 5.2.2.3 5.2.2.4 5.2.2.5 5.2.2.6 5.2.2.7 5.2.2.8 5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.3.4 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.2 5.3.2.1 5.3.2.2

Contents

5.3.3 5.3.4 5.3.4.1 5.3.5 5.3.6 5.4

Mass manufacturing from rapid prototyped products 77 Introduction 77 Casting processes 77 Investment casting 77 The process 77 Investment casting design parameters 78 Advantages and disadvantages of investment casting 84 Sand casting 85 Pattern 86 Types of pattern 89 Parting line 90 Core and core box 90 Binders 91 Molding material 91 Molding sands 92 Refractory sands 93 Permanent mold casting processes 94 Pressure die casting process 94 Squeeze casting 95 Centrifugal casting 95 Continuous casting 95 Rapid tooling 96 Direct rapid prototyped tooling 96 3D printing (Z402 system) 97 Laser engineered net shaping 100 Silicone rubber tooling 101 Process 102 Dimensional accuracy of products manufactured using silicone rubber tooling 102 Investment-cast tooling 103 Powder metallurgy tooling 103 3D Keltool 103 Spray metal tooling 104 Desktop machining 105 Conclusion 105

Chapter 6 6.1 6.2 6.2.1 6.2.1.1 6.2.1.2

Reverse engineering using rapid prototyping Introduction 107 Process 108 Phase 1 – Scanning 108 Contact scanners 108 Noncontact scanners 109

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6.2.2 6.2.3 6.2.4 6.2.4.1 6.2.4.2 6.2.4.3 6.2.4.4 6.2.4.5 6.2.4.6 6.3 6.3.1 6.3.1.1 6.3.2 6.3.2.1 6.3.3 6.3.3.1 6.4

Phase 2 – point processing 109 Phase 3 – Application geometric model development 110 Integration of RE and RP for layer-based model generation 110 The adaptive slicing approach for cloud data modeling 111 Planar polygon curve construction for a layer 112 Initial point determination 113 Constructing the first line segment 114 Constructing the remaining line segments (Si) 116 Determination of adaptive layer thickness 117 Other reverse engineering applications 117 Reverse engineering and the automotive industry 117 Reverse engineering – workflow for automotive body design 118 Reverse engineering in the aerospace industry 119 Reducing costs of hard tooling 119 Reverse engineering and the medical device industry 121 Reverse engineering – A better knee replacement 121 Conclusion 122

Chapter 7 7.1 7.2 7.3 7.4 7.5 7.6

3D bioprinting 124 Introduction 124 Extrusion-based bioprinting 125 Laser-assisted bioprinting 128 Stereolithography-based bioprinting 129 Challenges, applications and future perspective Conclusion 132

Glossary

135

Bibliography Index

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Chapter 1 Product life cycle 1.1 Introduction Product is the physical representation of any concept or any idea in the form of three-dimensional objects that can be used to satisfy the requirements of the potential customer base. From marketing point of view, a product is a representation of concrete ideas that can be recommended to a market so that it can satisfy requirements of its potential clients. In the retail field the products are known as merchandise. From the perspective of manufacturing sector, anything that is brought as a raw material and then converted and sold in the market as finished good is a product. The term commodity encompasses a wide spectrum of material ranging from any raw material to anything that is widely available in the market. For instance, the metals or the agricultural product or any other product can be termed as commodity. Products can be defined in the domain of project management as the elements and deliverables that contribute to the entire project and as such delivers the objectives and goals of the project. In insurance discipline, the products are the policies that are offered for sale by the insurance company. A broader spectrum of goods encompasses the definition of products in the field of economics and commerce. Political economist Adam Smith was the first to use the economic meaning of a product. The two main categorization of a product are tangible and intangible. A tangible product is a physical representation of concrete ideas that one can feel by sense of touch. Some of the examples of tangible products are building, gadget, clothing or vehicle. On the other hand, the physical representation of ideas that one can feel indirectly is called intangible product, an insurance policy for instance. One can further classify intangible products into virtual digital goods (VDG) and real digital goods (RDG). VDG, such as JPEG and MP3 files, are the ones that are located virtually on an operating system. These are accessible to users as conventional file types. Such type of goods may be subjected to license and/or rights of digital transfer. On the other hand, the elements that exist within the environment of data program and are independent of the conventional file type are termed as RDG. RDG are viewed commonly as three-dimensional objects or as a presentational item that can be controlled easily by the user. Open-source codes can be used to convert the basic VDG into process-oriented RDG. Android and GNU/Linux are few among the available open-source codes. Application process or service in the manufactured domain may be seen on the operating system such as the personal data assistant. These may be also visualized on the other available hand-held tangible devices. Before a product is developed, it is necessary to note that products can be of three levels:

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Chapter 1 Product life cycle

a) Core product b) Actual product c) Augmented product New product development is basically a mammoth part of any manufacturing process. Most enterprises comprehend that all products have a limited life cycle, so new products need to be manufactured to replace the old products so that the company always remains in business. Developing a new product entails a number of phases that typically centered on the following key areas: The idea: Every product has to incept with an idea or a concept. In some of the situations, the ideas may exist already in the open market and in other cases, it may be pretty complex meaning that the idea generation part of the process is much more involved leading to the generation of something revolutionary and unique. In fact, there are many industrial units or organizations that have a dedicated department that focuses solely on generation of new ideas and then converting those ideas into a new product that can be launched in the open market. Research: The next step is to start researching the market once an organization has selected the best of ideas for a new product from the large number of available alternatives. The thorough research enables the organization to see if there’s likely to be a demand for the new type of product, and also what specifications need to be developed in order to best meet the needs of the potential customer base. Development: The next phase is the development of the new product. Prototypes may be altered through various iterations of design and manufacturing stages in order to come up with a finished product that meets the need of the potential customer base. Testing: Most companies will test their new product with a small group of actual consumers from the potential customer base, before launching its new product in the market because the manufacturer spends a huge amount of money on production and promotion. This helps to make sure of the viability of product in the market that will be profitable. Also it makes sure that there are no changes that need to be made before a new product is launched. Analysis: Analysis of the feedback from consumer and testing enables the manufacturer to make any vital changes to the product. It also helps in deciding as to how the product will be launched in the market. With valuable information from consumers, the manufacturer will be able to build a number of strategic decisions such as what price to sell at and how the product will be marketed which will be crucial to the product’s success. Introduction: Finally, introduction of the product to the market is done when the new product has made it all the way through the new product development stage.

1.2 Product development phase in product life cycle

3

In order to ensure that the manufacturer makes the most of all their effort and investment, a good product life cycle management (PLM) is necessary. Thousands of new products are put on sale every year. It is about managing the product once it has been launched and then throughout its lifetime, which is a key to creating a profitable product. Making sure the product life cycle curve is as long and profitable as possible, the PLM process entails a range of different marketing and production strategies.

1.2 Product development phase in product life cycle The product life cycle describes the stages a new product goes through in the marketplace: development, introduction, growth, maturity and decline. In industry, PLM may be defined as the management process that aids in the management of entire life cycle of product. The management process takes care of the different stages of product life cycle beginning from the inception and then passing through the engineering design and fabrication phase and then terminating at the service and the discard phase of the fabricated products. As such, PLM provides a backbone for the organizations and their extended enterprise in the form of product information by integrating people, data, processes and business systems. The inspiration for PLM came from American Motors Corporation (AMC). There was an atmosphere of competition and every automaker company wanted to have a competitive edge. According to François Castaing, Vice President for Product Engineering and Development, AMC was also looking forward to have a competitive edge in this atmosphere of competition. AMC exploited the benefits of PLM, which led to the acceleration of its product development process. Saving production time by increasing the production efficiency of its workforce was one of the motives of AMC toward faster product development. This was achieved with the aid of computer-aided design (CAD) software systems. Another was to have a system in place that could lead to alteration in the product with the involvement of least cost. This could have also lead to improvement in the product at each and every step of design process. An effective product data management was employed to do away with the conflicts and thereby helped in reducing cost in making any alterations. As a result, Chrysler was able to gain a competitive edge over its competitors and therefore became the auto industry’s leader after early adoption of PLM technology. Product development (NPD) is the process in business and engineering, of bringing a new product to market. The process of developing a new product aims to convert the available market opportunity into a ready for sale product. The product developed may be tangible or intangible. The important factors leading to the success of the new product in the market are good understanding of customer needs and requirements, the competitive environment and the nature of the market. Based on the

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three variables of cost, time and quality, which drives the customer needs, the best strategies and practices are established by the organizations by taking the requirements of customers into consideration. With the new approaches, the organizations tend to enhance their market share through continuous product development process aiding them in coming up with the new products. Companies face many uncertainties and challenges throughout the process of product development. The main concerns for the management of NPD process are the employment of best practices and the elimination of barriers to communication. The product development process consists of different activities that are employed by firms in the intricate process of supplying new products to the market. Each and every new product passes through a number of stages starting from idea and concept generation and then going through design and manufacturing finally leading to market introduction. The product development process basically has four main stages which have been discussed next: a) Fuzzy front-end. There are group of activities that are required to ensure the achievement of complete product specifications and requirements. As such, these activities are required to be defined accurately so that product can be developed with due consideration to customer requirements. Requirements should be able to meet the market or business needs through the product. b) Product design. It is the development of effective and detailed-level design of the product, which will answer to the question that what requirements should a product have that will meet the market and business needs specifically. From the point of view of the marketing and planning, this step ends at precommercialization analysis stage. c) Product implementation. In this phase, the detailed engineering drawing of electrical or mechanical hardware is being carried out. Likewise the software engineering of software or embedded software, or detailed design of soft goods or other product forms is being effectively carried out in this phase. Also, any test that may be required, in order to validate the prototype of the thought out objects that will actually meet the design specifications and hence fulfill the market and business requirements, may also be designed priori. d) Fuzzy back-end. This is also known as the commercialization phase. This phase consists of the action steps leading to the production and thereby the market launch. Engineering design is the iterative procedure carried out to obtain a technical solution to solve a problem at hand. The engineering design stage is a very vital stage because at this stage much of the costs relating to the product life cycle are engaged. According to the researchers, it is in the product design stage that 70% of the cost relating to the entire product life cycle and 70–80% of the quality of the final product are evaluated, therefore it is the design-manufacturing interface that provides for the greatest opportunity for cost reduction. The time frame for which design projects are

1.2 Product development phase in product life cycle

5

carried out ranges from a few weeks to 3 years, averaging nearly a year or so. When the high-level design is terminated, prototypes of the same will be manufactured by the manufacturing plant. Implementation of practices such as quality function deployment (QFD) and design for manufacturing (DFM)/design for assembly (DFA) is done for developing a concurrent engineering approach. Product and process specifications are the output of the design phase which is mostly in the form of drawings, and the sale ready product is the output of manufacturing. Generally, the design team will develop technical drawings comprising specifications representing the product that will meet the business and market demands, and will send the technical drawings to the manufacturing plant for further execution in order to obtain the sale ready product. Deciphering product/process problems is of utmost priority in case of projects relating to information communication because if any changes are made after the release of the product then 90% of the development effort must be discarded. The views about new product development differ from industry to industry. Most industry leaders perceive new product development as an ardent process where resources are allocated for identifying changes in the market and capture the opportunities in the potential market before they arrive. This is in contrast to a reactive strategy in which no action is taken until problems occur or any other competitor introduces a new product or any innovation in the market. New product development is seen as an ongoing process for many industry leaders (usually referred to as continuous development) in which the entire enterprise is always looking for opportunities. There are several models for new product development. Industries have the option of choosing the best among these to suit their requirement. Most of these models are rather similar. Phases of one of the most popular model describing hardwareoriented products can be discussed as follows. There is a wide range of product base for the industries including nontechnical and software-based products. Similar models would delineate any form of product.

1.2.1 Phase 1: conceive 1.2.1.1 Meaning: idea generation, specify, plan and innovate Determination of the requirements that a product should have is the first stage of the new product development model. Viewpoints of customer, market, company and regulatory bodies can lead to systematically obtaining the requirements that a product is required to have. From these requirements, the technical specifications to the product can be derived. And these technical specifications can in turn lead to technical parameters for the product. Further, analysis is carried out to define the aesthetics of the product. This is done by carrying out the initial high-level design work, which also leads to defining of the functionality of the product. 3D computeraided software packages are employed to accomplish the above-mentioned processes. Even clay models can be handy.

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Necessary investments are to be done on research and evaluation to have a number of alternatives to a product. This can be included in the conception phase, for example, investment in bringing the technology to the desired level of maturity. It is also possible that something may not work out at any phase. In some cases this may be all the way back to conception phase. Therefore, life cycle engineering process is an iterative process, where evaluation of different processes is carried out at each and every step of the new product development.

1.2.2 Phase 2: design 1.2.2.1 Delineate, define, develop, test, analyze and validate After defining the different designing parameters in phase 1, a further detailing relating to the design and development is carried out in phase 2. This phase also involves the development of prototypes that can be used for testing and validation. Even the pilot release to have customer feedback is carried out within this phase. Redesigning and ramping for improvement to existing products as well as obsolete products can also be involved as an exercise in this phase. CAD is the main tool used for design and development of the product. There are different 2D drafting CAD software for simple 2D drawing/drafting and also many 3D software for 3D parametric featurebased solid/surface modeling. Such software consists of technology like reverse engineering, hybrid modeling and assembly construction. This phase covers diversified engineering disciplines such as mechanical, electrical, software (embedded), electronics, architectural, automotive and aerospace. Analysis of the components and product assemblies can also be carried out along with the actual creation of geometry. Computer-aided engineering (CAE) software is employed to carry out simulation, validation and optimization tasks. Tasks such as stress analysis, kinematics, computational fluid dynamics, finite element analysis and mechanical event simulation can be carried out using the CAE software packages. Dimensional tolerance (engineering) analysis can be carried out using the computer-aided quality. Even the sourcing of bought out components can be performed at this stage with the aid of procurement systems.

1.2.3 Phase 3: realize 1.2.3.1 Fabricate, make, develop, procure, sell and deliver The method of manufacturing comes into picture in this phase of new product development. The final design for this purpose is obtained from the designing phase. CAM, that is, computer-aided manufacturing software, is used to aid in manufacturing of the product. CAD software packages are used for designing of tool. Computernumerically controlled (CNC) machining instructions are developed. This further

1.3 Rapid prototyping technology in product developmentand realization

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entails the analysis tools required for carrying out process simulation for different manufacturing processes such as casting, molding and die press forming. CPM then comes into play after the identification of manufacturing method has been completed. This involves for carrying out plant, factory and facility layout and simulation of production with the aid of computer-aided production engineering. There are computer-aided software packages that are used for checking the geometrical form and size of the manufactured components. Sales product configuration and marketing documentation work take place alongside to the engineering tasks. This includes moving engineering data such as geometry and part list data to a web-based sales configurator and other desktop publishing systems.

1.2.4 Phase 4: service 1.2.4.1 Utilize, operate, maintenance, support, sustain, phase-out, obsolete, recycle and disposal Managing of in-service information is the last and final phases of the life cycle. Support information for maintenance and repair as well as waste management/recycling information is provided to the customers and service engineers. Maintenance, repair and operations management (MRO) software can be employed as a tool. Information pertaining to the fact that the product is to be disposed or destructed at the end of life is also recommended. Making a client or customer to have a real feel of the product that will meet their needs and requirements is a major challenge faced by the designers. This is the case even after the advancements in 3D CAD technologies. Explaining the aspects of design through digital models always ends up with “can you prove-it?” Layer-by-layer manufacturing, additive manufacturing or the rapid prototyping technology is the recent advancement in the field of new product development that offers a fast and accurate way to make a customer or the client to have a real feel of the potential of the product and is therefore gaining importance among the engineering designers and manufacturers. Employing rapid prototyping brings higher constancy from the conceptualized design compared to the traditional paper prototyping.

1.3 Rapid prototyping technology in product development and realization One of the fastest growing processes of manufacturing is that of rapid prototyping that manufactures a physical object from its scale model. The assembly or the object is obtained through a 3D CAD software package. Manufacturing of the model or assembly is

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usually done using 3D printing or “additive layer manufacturing” technology. Some of the considerable advantages of this manufacturing technology include the following: 1) Design concepts realization Through different CAD packages, it is only possible for a designer to visualize their discrete concepts on the screen. It is really important to have a real feel of the product. Additive manufacturing or rapid prototyping techniques are employed by designers to enhance their scope of visualization. Hence, prior to finalization, the designers can carry forward their thought-out ideas and implement them in their design. It also provides a proof of concept for the end customers, who can have a feel of the realistic product design rather than just a mere visualization of the design on screen. 2) Incorporating the changes instantly Through pilot launch of the physical model, the important feedback from the customer can help in making further improvements in the product. However, the change needs to be incorporated instantly with the involvement of least cost. Several iterations are carried prior to finalizing the design. The design improves further with each iterative process. The improved product as a result of iterations, leads to strengthening of bond and confidence between a designer and the potential customer base of the market. This also makes possible to develop competitive products with better acceptance rate after identifying the actual requirement of the market. 3) Cost and time saving With the use of additive manufacturing or rapid prototyping one could save a considerable time to develop molds, patterns and special tools. A wide variety of product with difference in their geometries can be produced from similar CAD software and the printing equipment. This leads to saving in cost of production. Further, rapid prototyping only makes use of the required amount of construction material for building an object. Therefore, the amount of waste produced is least unlike traditional prototyping methods such as CNC machining. 4) Customizing designs Rapid prototyping can produce products to suit the needs and wants of the customer. The products can be customized to suit the end client’s requirements. Elimination of special tools or process to implement design changes in the product further adds to its advantages. One can make a small change in the CAD model. No change is required in the rest of the process of product development. This leads to competitive edge for any manufacturers. 5) Minimizing design flaws The flaws in the design prior to mass production can be identified by employing the layer-by-layer manufacturing. Performance of physical tests becomes easy as the

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materials available for rapid prototyping closely resemble the properties and strength of the actual product. The risks of flaws and usability related issues can be identified earlier to eliminate problems that might occur during later phase of manufacturing process. The employability of rapid prototyping in product design and development is indeed a profitable decision and is always motivated in the manufacturing organization. In today’s fiercely competitive environment, the rapid prototyping tool helps in building innovative products cost-effectively and early.

1.4 Conclusion Traditionally, the design of any product was made on a piece of paper and then drawn to have a three-dimensional view of the same. It is always valuable to have feedback of the customer and in initial days the feedback was based on the models made from wax, wood and sheet metal. The model could also be used to test functionality of the product and its ability to be assembled. It could take weeks to produce prototypes and in particular when the required prototyping tools were developed from press tools, injection molding and casting. CAD software packages were developed with the advancements in the field of computers. These CAD packages gave graphical representations of the object on the screen. During the early times, although representations were possible they were simple and static three-dimensional representations. These representations then gave way to images that could be manipulated through the views from different angles. The drawings could be converted into some form of programming code through the aid of postprocessing software. Then this programming code could be used by the CNC machines directly for further processing. Although this was a great leap in the realm of rapid manufacturing, the tools were still required for complex parts and products. 3D CAD models can be created and seen on a screen due to further developments in computing power. Any product can be analyzed for strength, thermal performance and thermal properties with the aid of software tools such as finite element analysis. Other similar CAD data can be extracted using similar software packages. Extraction of digital data from drawings was impossible to be utilized to produce solid three-dimensional objects. But now numerous technologies have been developed that leads to creation of solid objects from digital data. “Rapid prototyping” is a term that can be coined for the production of solid models from technical drawings and as the name for the process suggests, it was designed for the production of prototypes for the new product at a relatively rapid pace. The early systems that were invented made the creation of solid models quicker, thereby leading to product visualization and limited functionality testing. Once fabrication of solid models became easier, engineers demanded much more functionality from the objects. The demanded functionality is dependent to a great

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extent on the materials that are employed for model building. Gradually the materials have been developed to the extent that any level of functionality can be demanded from the solid models. This has led to the production of solid models in plastics or metals with advanced modeling techniques playing their crucial role in the process. From the aforementioned discussion, it is obvious that the rapid prototyping process is well suited to develop the product and as such the product development process. Once the incepted concept and idea is digested, the rapid manufacturing process can develop a physical object or assembly with lesser developmental time. Certain circumstances may require additional inputs for some of the rapid prototyping processes. Although rapid prototyping has been an expensive process it is still a preferred manufacturing process that can produce physical prototype of models. It ceases to exist in some cases of article to let it be mass produced or even letting it to be produced on a small scale in commercial quantities with the required materials.

Chapter 2 Rapid prototyping processes 2.1 Introduction Rapid prototyping (RP) is a layer-by-layer fabrication of three-dimensional models from a computer-aided design (CAD). RP is often referred to as solid free-form fabrication or layered manufacturing. The term “rapid” has nothing to do with the speed; rather it aims at the automated step from CAD data to machine. Production times can be as long as a few days depending on the dimensions and the complexity of the object. The production time is still much faster in comparison to the time required by traditional production process. The relatively fast production time allows for analyzing the parts in a very early stage of designing, thereby decreasing the resulting design cost. The engineers and designers can print out their ideas in three dimensions using this additive manufacturing process. This manufacturing process allows designers to produce physical prototypes of their thought-out designs quickly, which then can be used to test various aspects of their design, such as dimensional checks and wind tunnel tests, besides serving as visual aids for communicating ideas with coworkers or customers. In addition to the production of physical prototypes, RP techniques can also be used for production of molds or mold inserts and other rapid tooling and even fully functional end-use parts (rapid manufacturing). This layerby-layer manufacturing process is often the best manufacturing processes available for small and complex parts. The layer-by-layer fabrication of three-dimensional physical models allows designers to develop complex parts that would be seemingly impossible to machine. RP can hassle freely develop the complex structures, inner structures of parts and very thin-walled models. RP construct models by manufacturing very thin cross sections of the model, stacking one section on top of the other section, the process being continued until the solid part is finished. The stacking of twodimensional slices together greatly simplifies the complex three-dimensional model construction processes. For example, in order to manufacture a sphere, layers of different-sized “circles” are built successively in the RP machine and stacked together, instead of carrying out a detailed machining process to construct the sphere. RP also decreases the operational time required to build the three-dimensional complex models. The RP process is intermittent as the RP machines, once started it runs unattended until the part is complete. It is only at the beginning of the process that the operator has to spend a small amount of time in setting up the control program for operating the RP process and in postprocessing, where time needs to be given to clean-up operation which is also essential. Therefore, the user intervention time is very much lesser than that required for https://doi.org/10.1515/9783110664904-002

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traditional machining processes. Models can generally be built within hours and thereby the time and cost savings involved with such a process can be imagined. Also the build materials are usually cheap. Some RP machines are small and eco friendly just like a common photocopy machine and therefore occupy very less official space. Further, RP is an asset to have for any establishment in the business of producing three-dimensional components because of its capability to manufacture prototypes that can verify concepts in the beginning phases of a project. Some of the advantages of RP process can be listed as below: – Freedom of design: The production of complex parts is reduced to the stacking of layers one over another. – Well automated: No human intervention is needed during the entire building process barring at the initial and postprocessing stages. – Relatively easy: No skilled operator required. The operator is required only for initial little preparation and postprocessing. – Inexpensive: The RP process avoids the high cost of prototype tooling, thereby allowing for (more) design iterations. – Error free: Three-dimensional models are easy to check for errors. Some of the disadvantages of RP process can be listed as follows: – Accuracy is usually greater than 0.1 mm. – Fragility: Products can be very delicate and fragile, and therefore some of them even need postprocessing. – Staircasing effect: Staircasing will occur because a surface at an angle is constructed using large number of layers.

2.2 Origins of rapid prototyping It is the CAD industry that gave birth to the RP technology. There are numerous CAD software packages in the market to suit different needs of the manufacturer. There are CAD packages that aid in solid modeling of the product to suit the requirements of the market. RP stems from this solid modeling side of CAD. Any entity that has volume and occupies space is a solid model. The different data related to the solid model, be it geometrical data or data related to the mechanical properties, is converted to some standard format. This then can be used as input to different RP machines. Solid modeling was introduced in the 1980s, but before that the objects were represented using wireframes. In the wireframe representations, the models were represented similar to the ones drawn with pencil or chalkboard. Sometimes the terminology of two-and-a-half-dimensional models was used for wireframe. To enhance the visual representation, the wireframes were equipped with properties of surfaces. The surface would enclose the wireframes, that is, for example six squares joined at

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the edges to represent a cube. However, the slow transformation of wireframes to solid model led to the development of RP processes. The first RP process was developed in 1986 and 3D Systems is credited for its development. It was Charles Hull who helped 3D Systems in developing different RP techniques. Stereolithography is considered as one of the oldest RP techniques. The process builds the 3D model from the slices, that is, consecutive slices of construction material are stacked together to produce a three-dimensional solid model. The different layers are cured using UV light. It became possible for the designers to make the customers to have a real feel of the object. It was possible for the manufacturers to have a valuable feedback from customers, to perform tests and validate the model. Different iterations could be carried out to improve the product. Table 2.1 briefs about the development of RP processes. Table 2.1: Brief about the development of RP processes. Year of origin

Technology



Processes began to mechanize.



First computer was invented.



First numerical control (NC) machine tool came into existence.



Robots came into existence.



Computer-aided design software packages began to roll out in the market.



First RP process came into existence.

2.3 Design process In the proceeding section, the different phases involved in a typical RP process have been discussed. However, the approach may vary from manufacturer to manufacturer, but the stages discussed can be generalized to different RP processes.

2.3.1 The concept Before modeling a new product there must be some concept or idea to be converted to real life object. The concepts or ideas may arise to fulfill the needs and wants of the potential customer base. Different ideas from different sets of people can then be clubbed to a single solid concept that when converted to product will fulfill the needs and wants of the customer. Therefore, idea generation is an integral part of all designing to manufacturing process.

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2.3.2 Preliminary designs There are different forms to a preliminary design. These include a two-dimensional drawing on a piece of paper. It may also be a three-dimensional model developed in some CAD software packages. Besides developing these models on CAD software packages, iterative procedures are adopted to validate the design. Improvements are made after incorporating ideas and feedbacks from colleagues, coworkers. Even the presentations to management can elicit important feedbacks that can improve the product in the initial design phase. Different analyses such as stress analysis, checks on fits and tolerances are also carried out in the preliminary phase of design. RP techniques can be employed in this phase to help build a 3D model of a product that can be used to obtain valuable feedback from clients. These prototypes can be used to validate the model as different tests can be performed on them. Further, this can aid in gaining a proper understanding of the functionality of the product. This use of RP is referred to in this text as concept verification.

2.3.3 Preliminary prototype fabrication After an intensive process of preliminary design phase, the design is given a go ahead for implementation to have a real object. In the process of obtaining the same, prototypes are developed to determine if the design is of correct size, shape etc., before RP came into existence; these prototypes were constructed using hands. This was a time-consuming process then. Also, to develop a prototype using hands is a costly affair. With the advent of RP, it was possible to use wide range of materials such as durable plastics to build prototypes from the initial design at a quicker pace. Fit-check analysis then can be performed to validate the correctness of the design. Several iterations are necessary at this stage of RP process. With RP techniques the iterative procedure can be carried out with least effort.

2.3.4 Short-run production After a proper analysis and validation of the model, the parts are manufactured in small quantities. This is necessary to final proof the model before it goes on for the production at large scale. Parts from short run production can be distributed for testing, eliciting customer feedback etc., to manufacture few hundreds of parts, RP can be used. Even rapid tooling can come in handy to manufacture small number of parts. Performing shorter runs may be advantageous to eradicate any design flaws before going for the final production. The parts that are to be produced in only small quantities, this phase would be the last phase. This eliminates the need for expensive traditional tools to manufacture the parts.

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2.3.5 Final production For parts that are to be manufactured in large quantities, further machining processes are required. The parts can be injection molded or casting can be employed to obtain large number of parts. In order to decrease the cost of manufacturing of cast parts, the patterns are made from reusable materials such as aluminum or steel. With the development of direct metal and ceramic processes, RP may yet reach this phase in the near future.

2.4 Rapid prototyping cycle Different steps are performed in the RP techniques to convert a 3D model into the final product. These steps are discussed next. 1. First, a 3D model of the part to be manufacture is produced using different CAD packages. This can be obtained either by scanning the already existing object or creating a new design. 2. The CAD model is then converted to some standard format for further processing. The standard triangulation language (STL) is one such standard format. This has been used widely across industries. The STL format visualizes a product to be composed of number of triangles. Increasing the number of triangles, curved surfaces can be approached. Thus, the STL format is the concrete visualization of the product’s geometry. A greater precision can be achieved by increasing the number of triangles. But at the same time the file size will increase. Therefore, a compromise needs to be struck between number of triangles and the part accuracy. 3. Once the conversion to STL format has been achieved, the next step is that of slicing the STL file. The STL file is sliced into number of layers which when built will represent the cross-sectional shape of the part to be built. The layers are sliced with a determined thickness. 4. There may be areas in the part to be built that could float away during its manufacturing. These are the overhanging features of a part. In order to take care of these features, support structures are generated. The support can also prevent the part to get distorted. This support structure can however be easily removed in the post-processing stage. 5. The sliced layers are stacked together to build the product. The process is fully automated. 6. The last stage of any RP process is the post-processing stage. In this stage the parts are cured or infiltrated to obtain desired mechanical properties. Any support or base structure is also removed. The RP process belongs to the class of additive (or generative) production processes unlike forming processes or subtractive processes, which include lathing, grinding,

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milling or coining, wherein an object is fabricated by plastic deformation or material removal. The object is fabricated in all commercial RP processes by stacking of layers in a (x–y) plane two dimensionally. The stacking up of single layers on top of each other results in the third dimension (z) but not as an intermittent z-coordinate. Therefore, the prototypes are very precise on the x–y plane but will have stairstepping effect in the z-direction. However, the stair-casing effect can be minimized and the model looks like an original one if model deposition is done with very fine layers. RP can be divided into two elementary process steps, namely generation of physical layer model and generation of mathematical layer information. The RP process incepts with a three-dimensional CAD model of the part and then export of the STL file takes place by tessellating the geometric three-dimensional models. Tessellation is the process where different surfaces of a three-dimensional CAD model are approximated piecewise by a series of triangular elements. The number and size of triangles are decided by facet deviation or chordal error. The STL files are then checked for any defects like missing facets, flip triangles, dangling edges or faces, overlapping facets etc. The defects are repaired and then the defect-free STL files are used as an input to various slicing software. At this phase, the orientation of part deposition is the most critical factor as it in turn influences surface quality, amount of support structures, cost building time and so on. Then the tessellated three-dimensional geometric models are sliced and the generated data, which is in the standard data formats like common layer interface (CLI) or stereolithography contour (SLC) is stored. This information is used to go to step 2, wherein the laser scanning paths (in processes like stereolithography and selective laser sintering) or material deposition paths (in processes like fused deposition modeling) are employed by the software running the RP systems for the development of physical threedimensional models. This phase differs for different RP processes as it depends on the basic deposition principle employed in RP machine. The stored information computed earlier is used in this phase to deposit the part layer by layer on RP system platform. The last and the final step in the process is the postprocessing task. During this stage, generally some manual operations like that of cleaning the excess materials that gets adhered with the part or removal of support structures are carried out. This necessitates the requirement of skilled operator. Postprocessing operations are also necessary to provide an increased level of accuracy to the final product. Some of the postprocessing operations are polishing, removal of support structure and so on. This also provides enhanced aesthetic appearance. Finally, the testing and validation of prototypes is carried out and the changes are included to yield the final product.

Chapter 3 Applications of rapid prototyping processes 3.1 Introduction Additive manufacturing or rapid prototyping (RP) technology is flexible and has the capability to customize the manufacturing process. The widely studied RP techniques include stereolithography apparatus (SLA), three-dimensional (3D) printing, selective laser sintering (SLS), 3D plotting, fused deposition modeling (FDM), multiphase jet solidification, laminated object manufacturing (LOM) and solid ground curing. Different techniques are associated with different materials and/or processing principles and thus are devoted to specific applications. Professor Herbert Voelcker has been credited to the foundation of rapid manufacturing processes. In particular, the professor was responsible for devising basic tools of mathematics and also the theories for fabrication and solid modeling. However, the true impetus came in the year 1987 when the researchers at Texas University, USA, were able to develop layered manufacturing and employed lasers to fuse metal powders in solid prototypes and hence print 3D model. With the advancements, RP technologies have been extended to address the problems of industrial manufacturing. Presently, RP technologies have contributed to almost every domain of engineering as well as nonengineering realm such as mechanical and biomedical. Today, the RP technology has contributed to almost all the spheres of engineering and nonengineering areas. These include mechanical, materials, industrial, aerospace, electrical and most recently biomedical engineering.

3.2 Engineering application 3.2.1 Prototype for concept evaluation It has been believed by the researchers worldwide that RP will be the technology of the twenty-first century that will offer a new degree of freedom to learn and create 3D printed models at ease. The 2D computers are now being replaced with computers having 3D displays and voice input that allows the user to sculpt the desired geometry. Building of prototypes aid the small design and engineering firms to test their ideas and hence experiment on their design to choose the right project. Large companies can model and share the concepts within the departments and learn from their ideas to hone the initially incepted idea. 3D printed prototypes are a valuable communication tool that can aid in effective communication of ideas between https://doi.org/10.1515/9783110664904-003

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clients and designers. RP helps the designers to visualize their ideas and properly understand the look and feel of their design ideas. The designers can input their ideas into their designs and hence refine the final product before final implementation. The 3D printed model also provides a means of visualization for the end consumers who can realistically feel design products instead of just visualizing them. The changes can be incorporated merely by having feedback of customers on the physical model. A number of iterations are taken to improve the product. Design improves with each passing iteration and builds confidence for both the customers as well as the designers. The actual market can be realized and thereby allowing for development of better products and better rate of their acceptance. The cost and build-up time of the product decreases with the iterative process. Internet can be considered as a noticeable partner to rapid manufacturing technologies. There are companies worldwide that do not have equipment for rapid manufacturing and transfer CAD files through Internet to the service stations. The service stations can work immediately to process and convert the received files into final prototypes. With continual advancements, it would be possible to deliver the products by means of downloading on the Internet and thereby making it possible to print the objects directly in customers’ office and homes. There will arise a possibility when the designers, creative people and artisans become independent from the conventional production and distribution circuits. With the introduction of such evolutionary techniques, a new relationship between the producers and consumers is formed that has shortened the distance of concept to the final product by minimizing the intermediate stages. Presently, there has been widespread commercialization of the equipment associated with RP as well as the technology of 3D printers. They represent different processes and accordingly different materials. So the mechanical characteristics are usually poor as other properties like fusibility (FDM) and photosensitivity (SLA) should be more dominating for the product realization. The ideas generated at the early stages of design are thereby tested through the developed product. The designer will be able to visualize the iterative process of creation without indulging into the prospects that attract economic limitations. The designer is only required to send the CAD file to any of the available RP equipment placed anywhere worldwide. With such advancement, it becomes easier to testify their ideas with greater precision. As the product produced is for concept evaluation, the products made are usually weak as far as strength is concerned. The designers use sparse models to reduce the build time as well as cost.

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3.3 Nonengineering applications 3.3.1 Biomedical models for surgical planning Medical science is a challenging field. The advancements are recurrent in this sphere of ever-changing field. Professor Herbert Voelcker laid the foundation of one such technology in the late 1960s, which is additive manufacturing or RP, that has revolutionized the medical field. It is the process of manufacturing in which prototypes can be built in a relatively lesser time and with relatively lesser cost. Further, with the advent of this technology, the design can pass through number of iterations of testing and validation before it has been finalized for production. The term RP has got number of connotations such as digital fabrication, 3D printing, solid free form fabrication, laser prototyping and solid imaging. Impressive strides have been made by RP in the sphere of medical field. Some of the major application of RP in the medical field includes maxilla-facial, orthopedic surgery and preparation of scaffold for tissue engineering to name a few. RP can be used as an educational tool in fields as diverse as obstetrics and gynecology and forensic medicine to plastic surgery and has now gained wide acceptance and is likely to have far-reaching impact on how complicated cases are treated and various conditions are taught in medical schools. 3.3.1.1 Steps in production of rapid prototyping models The different steps that go into producing a RP model are discussed briefly: 1. Computed tomography scan or magnetic resonance imaging (MRI) scan is used for imaging 2. Acquisition of DIACOM files 3. DIACOM files converted to STL files 4. Design evaluation 5. Surgical planning and superimposition if desired 6. Creation of model using additive manufacturing 7. Validation and testing of the model 3.3.1.2 Design evaluation and surgical planning It is necessary to screen the collected data. This is done with the combined effort of bioengineer and surgeon. The process of screen decreases the time required for creating the model. Also the cost of production decreases. If the model is intended for teaching purpose then the data can be sent directly to machine for production. The real use, however, is in surgical planning in which it is critical that the surgeon and designer brain storm to create the final prototype. There may be a need to incorporate other objects such as fixation devices, prosthesis and implants. The step may involve a surgical simulation carried out

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by the surgeon and creation of templates or jigs. This may require in addition to the existing converting software, computer-aided designing (CAD) software like Pro-Engineer, Auto CAD or Turbo CAD. 3.3.1.3 Additive manufacturing and production of the model Stereolithography, FDM, ink-jet printing, solid ground curing, LOM and SLS are some of the technologies that are available to create RP model. The choice of the technology depends on the need for accuracy, finish and surface appearance, number of desired colors, strength and property of the materials. In order to plan for minimum machine running time, one needs to plan to orient the part. The model can also be made on different scales to original size like 1:0.5, this ensures a faster turnaround time for production and sometimes especially for teaching purpose this may be convenient and sufficient. 3.3.1.3.1 Validation and testing of the model The model needs to be tested and validated in order to ensure that the model serves the intended purpose and meets the requirements. 3.3.1.4 Surgical simulation and virtual planning Preoperative templating is a well-known term among the surgeons. This provides an opportunity to plan a complex surgery before it is done. Computer-aided surgical simulation digital templating are some of the advanced technologies that are gaining ground. Once the process of generation of model is completed, the surgeon can analyze and study the configuration of the fracture or the deformity that is to be worked upon. Different surgical options and modalities can be explored and even be simulated upon the model. After carefully planning and studying, the next stage is to contour the desired implant according to bony anatomy. The implant in three cases, that is, actetabulum, calcaneum and other periarticular area, is necessary. For accurate, easy preplanning of the osteotomies and screw trajectories computer-generated interpositioning templates or jigs can be used. The intraoperative time is also saved as the surgeons can measure the screw sizes that are to be used for the planned surgery. The model could also be referred to intraoperatively should a help is required in understanding the orientation during the surgery. 1. Better understanding of the fracture configuration or disease pathology 2. Helped to achieve near anatomical reduction 3. Reduced the surgical time 4. Decreased intraoperative blood loss 5. Decreased the requirement of anesthetic dosage

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3.3.2 Development of prosthesis and implants As an anatomy of the part can be scanned into a computer system slice by slice, similarly an object can be reproduced slice by slice using the 3D computer data integrated with RP machine. As the model is created layer by layer, it contains nearly all the details of its internal contour geometry. By using this technology, life size 3D, implants and solid skull models were made and used to select optimal bone and tissue graft donor sites or used as a guide to fabricate implants. The development of the biomaterials such as those can be fabricated using RP technology with controllable strength and properties to simulate real tissues.

3.3.3 Molecular models With the advent and prevalence of computer technology, it has become very easy to explore and collaborate, in order to gain access to the knowledge related to the molecular biology. Computer has been assisting the humans in exploring the data related to scientific research in the field of molecular biology and testing of the related scientific hypothesis. Computer has also been assisting humans in collaborating, that is, sharing of scientific research of two or more scientists related to same work. However, this comes with the disadvantage too. By keeping on exploring and collaborating, it is quite obvious that the database will increase. This may also include more and more complex structures and software packages becoming diverse. In such a scenario, it becomes a tedious task to access and manipulate the digital information. The computer graphics have been used widely in exploration of research. 3D representations in molecular biology are utilized for the exploratory research. The sharing of the visual representation aids in collaboration, both local and remote. However, most of the biologist still doesn’t use the advancements of human–computer interface. All the work related to genomics and biomolecular structure is still done at the workstations, mouse and keyboard being used as input devices. The 3D form needs to be visualized and understood in order to perform various physical manipulations. Our sense of touch that relays information from the receptors of our skin as well as the senses that make us able to detect weight, movement of joints, muscles and other body parts, helps us in gaining an ability to understand the 3D form clearly. Physical models in 3D form have been used by different scientists to produce scientific results of importance. The space filling models invented by Pauling was used to prognosticate the basic folding unit of proteins. Similarly the popular brass-wire model by Watson and Crick was used to arrive at the possible structure of DNA. With the developments in research in the field of molecular biology, the available molecular models are not adequate enough to focus on larger assemblies. Also

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it’s not possible to visualize the increasing complexity of interaction with the aid of available models. However, it is possible to create an environment of merging the physical virtual objects. This environment is the augmented reality environment. Such an environment can lead to exploitation of complex information represented by tangible models and hence new modes of interaction. It is now possible to make physical models for complex assemblies by making use of the 3D printing technology. In the proceeding discussion we have presented an application to gain an understanding as to how the scientific environment of exploration and collaboration be enhanced with the aid of tangible models produced using autofabrication tools in the augmented reality environment. The 3D model is generated by the Python Molecular Viewer in the augmented reality environment. This is then overlaid on autofabricated model. ARToolKit package developed at the University of Washington could be used for the precise representation of virtual objects with the real world. The researchers can use the package to access different information such as that of the molecular properties of the molecules. Creation of tangible models from known atomic structure has been discussed. Further, the focus has been made on ARToolKit and its integration with Python framework. 3.3.3.1 Design of physical models Python Molecular Viewer can be used to produce virtual objects. Further, the package has the capability of designing tangible models. The package integrates the models with the virtual environment very easily. Wide range of representations and design can be made using this software package. Representations of molecular surface, backbone ribbons, extruded volumes and others can be made using Python Model Viewer. Design of models can be done to suit the needs at different levels of abstractions. For example, for large systems, representations can be made to focus on molecular shape and when it is required to look at the function at atomic level, then detailing can be done at the atomic level. Python Model Viewer as the name suggests is built using Python language. Python language has been in use since decades because of its characteristics such as it is open source and object oriented, extensible and efficient, platform independent. It is because of these characteristics it has the capability to interconnect different components of a software at higher levels, thus acting like a glue. A generic 3D visualization component known as DejaVu is an integral part of the Python Model Viewer. The component is made to provide interface to the OpenGL library and as well as to other applications of geometry viewing. Apart from DejaVu, there are several other components that provide for molecular modeling at different levels of abstractions. In addition, these also provide for visualization functionality.

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Some of the components are MSMS (which helps in obtaining calculations for solvent-excluded surfaces), GLE (which is used for the extrusion along an arbitrary path for 2D shapes), Babel (which has its application for handling files and coordinates at the molecular level) and last but not the least RAPID (which finds application for identification of intersections between polygonal models). The components from the software package can be exported for use in RP industry. For this, conversion into STL and VRML format is done. Software packages have been developed to generate surface/feature-based representations from the atomic coordinates. This can be used for mold making, after transformation into STL representation. The designing of the molecular model is similar to designing of mechanical components. Similar to the mechanical design context, the atoms are represented using small-sized spheres. The molecular model is obtained by using Boolean operations on the atoms or the small-sized spheres. The software package identifies the hydrogen bond. The bonds are simulated using magnets. Further, the package simulates the backbone with the internal smooth tubes. It therefore creates an internal structure very similar to that of mechanical design. For mold making, the software can fix the atoms to a particular plane, part the model into two halves and create inner and outer features for injection molding. Thus, the software package can be used for mass production. 3.3.3.2 Fabrication of models The process of fabrication is revolutionizing with the recent advancements. Automated layer-by-layer manufacturing is one such development that has revolutionized the process of fabrication. The process is known as RP. Models are built up by gluing thin layers of construction material. This creates a solid, hollow object of any desired shape. Nearly any shape can be obtained using RP fabrication process. Only the structural integrity of the constructed model is a cause of concern. The Z-corp process is one such RP process that aids in the fabrication of models. The process employs a 3D printer, that is, Z-corp 406 color 3D printer. The inkjet print heads are used to apply a pigment-binder mixture to powdered gypsum. Strengthening agents are used to provide a finish to the fabricated model. If rigid models are desired, then the infiltrating agents used may be wax or cynoacrylate glue. However, to fabricate flexible models, elastomers could be used as infiltrating agent. The process is quick and hence saves considerable amount of fabrication time. It employs materials that are relatively inexpensive. Further, it is fully automated. The major source of concern is the fragility and hence the structural integrity of the model produced. Stratasys is another technique for the production of models. This employs a fused deposition method. In this process, the thin layers are produced by extruding a molten ABS plastic filament. The process is relatively slower and thus timeconsuming. The cost of material employed is almost twice that used for the Z-corp.

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Further, the models produced are monochrome as opposed to full color models of the Z-corp. However, the models fabricated are durable and therefore leads to finer representations. 3.3.3.3 Augmented reality interface The properties of a structure cannot be delineated in a physical molecular model. However, the physical models are more informative in comparison to 2D drawings. Further they are far more than textual descriptions. In order to enhance the capability to represent dynamic properties, computer-based spatial tracking are being utilized. The rendering methods further aids in enhancing the same. The augmented reality environment gives a feel of real interaction with the physical model. This is done by exploring the structural and dynamic properties of physical object. The augmented reality environment combines the presence of real world and virtual object. The computational models and data are used by the augmented reality environment that combines the physical models and the real-world user. Model is displayed on the screen of the computer, with the aid of tracking device such as a camera, and the manipulation can be performed by the user in the augmented reality environment. The superimposition of the virtual representation of say another 3D rendering of the same model takes place over a video display. This is done in parallel to the exploration of the structure made by the structure. ARToolKit is an open-source software that is widely used for developing visionbased augmented reality environment. It is used for calculation of the position of video camera and orientation of the model relative to markers. This allows for superimposition of virtual objects onto physical markers. The features of this opensource library software help in tracking of the relative position of the camera with respect to the position of the markers. It also generates the marker tracking code which is in the form of black squares. Further, it has the capability to use arbitrary marker patterns. PyARTK is the integration of ARToolKit and the Python Molecular Viewer. Thus PyARTK can be considered as a standalone software package based on Python. The software package provides for a framework to manage markers. Even it manages the visual representation of the composite images from any of the video input devices. Besides all this, it accesses the functionality of the ARToolKit library. The standalone software has also been integrated with the Python Molecular Viewer. Specific augmented reality environment markers are assigned with the geometric properties and animations, with the aid of geometric manager. Modifications are inevitable and therefore can be made as the modeling proceeds. Besides all the aforementioned features the PyARTK is also equipped with variety of camera operations, controls pertaining to lighting and clipping. The standalone package combines the video display created by it after tracking the embedded markers

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and that produced by the Python Molecular Viewer. A run-time paging can also be performed interactively which consists of the molecular representations as the exploration of the molecular model is proceeded. Often, the virtual objects interpenetrate the real world physical model. Therefore in order to have a better feel of the real-world environment, masking is done. This occludes the virtual object. The geometry of the tangible object is directly used to create mask. The square shaped markers are used for tracking and therefore recognizing the tangible models. These are placed on the surface of a model. The markers are used to produce superimpositions of the virtual object with the manipulated real-world object. Often, few plates are added to the surface of model, when designing using Python Molecular Viewer. The markers are placed onto these plates, once the model is built. The transformation between the markers and the model is recorded at different levels of the designing process. This is used in the latter stage of the process where superimposition of the virtual object and the tangible model is done. The accuracy of superimposition is of great importance during the entire process. This is ensured by employing increasing number of markers. 3.3.3.4 Examples In the proceeding sections, we will demonstrate the application with the aid of some examples. This will also make us aware of the teaching feature that the application has. Also, one will get to know that how the manipulation of the augmented physical models can be used with ease to enhance the perception of interactions of biological molecules and complex shape. 3.3.3.5 HIV protease The physical backbone of protein for the HIV protease is integrated with various inhibitor molecules that are effective in the treatment of AIDS. The integration is performed with the graphical display of the inhibitor molecules. The backbone for the HIV protease is fabricated using the Z-corp printer, which is a 3D printer as mentioned earlier. The extruded tube is used to represent the geometry. This is colored resembling the amino acid. Totally three markers are used to track the model. This is done so that one of the markers always remains in the field of view of the camera. The bound conformation of the inhibitors in the active protease site is accomplished using augmented reality overlay. The visualization of the inhibitors and their representation can be perceived to be space filling. Using the built in animation player of the PyARTK, one can page through the different inhibitors.

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3.3.3.6 Superoxide dismutase The tangible model of the superoxide dismutase (SOD) can be produced using Stratasys printer. SOD is an enzyme of detoxification. This has got a very strong electrostatic funneling effect. The color enhancement of the tangible model is done using the augmented reality environment. This also aids in showing the dynamic properties. The spherical harmonic surface represents the SOD. The representation shows the overall shape of the protein. The atomic detail has been smoothed out. The positive and negative potential is depicted with volume-rendered clouds, in blue and red respectively. One can manipulate the volume rendered electrostatic potential using a transfer function widget integrated within the molecular viewer. Further, the interface can also be used to manipulate interactions of two SOD proteins. 3.3.3.7 Ribosome Ribosome is composed by aligning tRNA molecules along an mRNA strand. Therefore, ribosome is a complex biomolecular structure. Z-corp printer can be employed to produce a small sub unit, using a smooth spherical harmonics representation. The small subunit is then augmented with the larger unit. This shows how the two subunits assemble to generate the complex shape.

3.3.4 Architectural models 3.3.4.1 Architectural needs In order to create architectural designs, the role of drawing is well understood. Drawing has been playing a key role in building the architectural designs. However, there has not been much known about the role the models play as a design tool. Further, the relationship of architectural designs and RP has also not been explored to a vast extent. In the proceeding sections, we will discuss the roles that models play in architectural designs. We will then graduate toward identifying the areas in which the developments need to take place. 3.3.4.2 Models and architectural designs Models play a crucial role in architectural designs. Models find varied applications in architectural designs. In order to study the different aspects of design idea, models are created in the early stages of design. Using models, it becomes easier to visualize different aspects. The assembly of the created models is done rapidly in order to seek the feedback at the earliest. The feedback then can be used to make the required modifications.

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After seeking feedback, modifications are done. The models are then carefully assembled in the latter stages of design. The carefully assembled models are then employed to seek the ideas from different decision making bodies. The models are often used for presentations and therefore are highly articulated so that they resemble very truly to the end product. Models are used as a communication tool to communicate the design intent. It is very difficult for laymen to understand the drawing images. Since the models represent different aspects of the idea in a lucid manner, drawings are being supplemented with the models. Therefore the models instead of drawings can be used for communicating the aspects lucidly both to the laymen as well as to the trained professionals. Models are being used at different scales ranging from the entire town planning to a particular component of the building. The sequences in which the end product will be spaced in space as well as the complex mass-void relationships can be lucidly delineated with the aid of models. Models can also be used to analyze the various processes that will go in making the end product, besides delineating the various aspects. The construction of the building can be validated using models. Also the number of complex processes that goes into making the plan successful can be studied well in advance. The different building components can be studied in a much detailed manner. One can have a perception about the space a particular building component will occupy in the entire planning. The hidden aspects can also be seen in a very lucid manner. Thus creating models not only aids in communication but also enhances the visualization. Models help the designers to gain an insight into the spatial arrangement of the building components, the 2D drawing of which has already been created. The complexities in the geometries can be delineated in the models. The intricate ideas can be clearly visualized by assembling and disassembling the models. The behavioral aspects such as lighting, ventilation, acoustic and structure, to name a few, can be analyzed in a lucid manner. One can test the models several times. The iterative procedure for testing can be continued until the entire design is made free from failures. This can also lead to alternate design and modeling procedures. Thus models provide a safer and cost-effective approach toward implementation of the failure proof design to obtain the end product. The experimental results can be used to validate the theoretical understanding of the subject. One can even have a futuristic design of a building which cannot be realized in practical and elicit the appreciation from the world around. Even after the building is completed, the designs can be used to enhance the understanding. 3.3.4.3 Limitations of models Models are often conceived to be a connotation of the building to be constructed. Models therefore cannot be considered to be a true replication of the building. The models often overlook some of the behavioral aspects such as finishing. This is also

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the case when the model has been built to its full scale. The skill of the professional puts a limit on the level of detailing and hence the accuracy of building the model. The models are used at various stages of design by the designers, but since the models are only interpretive, it becomes the responsibility of the designer to judge the compromises made during the articulation of the idea. The design team needs to decide on to whether the same model could be used in the subsequent decision making processes. It is very vital to consider the relevant properties of the model, since the models are built for experimentation to allow for various design alternatives. This should ensure the similitude to the properties of the true product. If this is not done, transferability of the experimental result to the true object cannot be processed. The validation of the theoretical understanding also becomes a problem. It is a relatively cumbersome task to consider the similitude of these properties because the model is not an actual representation of the true object as mentioned earlier. 3.3.4.4 Representation of models Since models are connotation to the true objects, it becomes very crucial to choose the material and process for model construction very wisely. However, it is problematic to achieve the true abstraction using the chosen material and process. Designers try to use the materials similar to those that will be used for the construction of true object. By doing so, designers try to get away with the difference in the reel and real object. This makes the clients to have a feel of real object. However there are problems faced in translating the material used for the model to the real object to be constructed. There are number of factors that play their role in material selection, for instance, workability, and safety in working and other properties. Also, longevity is another criterion. One needs to make different use of materials for a model that is used for international exhibition and some other to explore a quicker feedback from the customer or clients. Typically one can categorize models into three different classes: preliminary, experimental and final. The function of the preliminary model is to gain an insight into the shape and volume of the end product. As a result these are made much quicker. Therefore, the preliminary models can be made from any material in hand. These classes of models can be refined to preliminary model. This leads to greater control over the dimensions. The experimental models are constructed to similitude the properties that are to be present in the final design. The property selected for approximation depends on the particular aspect of the design that needs to be tested and validated. The designers can find a variety of tools and processes in numerous texts for model making. There are tools to orchestrate the materials to help in representing concrete design ideas. Instructions on materials for sheet work can be made easily available through these texts. Even the materials that pertain to certain applications

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such as materials for cutting, milling, etc., for sheet metal work can also be found. Elements with high level of detailing and accuracy can be made using materials such as rubber and plaster. Choice of different processes to accomplish the task of detailing can be well studied in advanced. The finishing of the model, which is critical from the visualization perspective, takes about one-third of the time that is required for assembling the model. Model making is a cumbersome and time-consuming process. It also requires a high level of skilled professionals to carve out a model representing the true model accurately. It becomes therefore impossible for the designers to utilize the models for representations. It is only the preliminary models that can be used in early design stages. However, the models are must to save cost in the subsequent design phases. The importance of RP in design can be realized. 3.3.4.5 Digital models for representations The analogue models that are time consuming can be thought to be supplemented with the digital models. This has become a reality with the advent of CAD software packages. A digital model of a building can be transformed into physical model. This is done by using the same data set that has been used for producing visualization on the screen. The physical models then can be employed for presentations. Physical models are used to lucidly delineate the concrete design concept. One can clearly visualize the appearance of the product to be built. Hence, physical models are used as tool to communicate visual representation. In the sense one can know about how the end product will look like. The digital models give a complete visualization of the components, assembly and hence the product as a whole. The data set for digital models can be directly used to produce the model. Digital models offer the advantage of manipulation over the physical models. This has been made possible with the advent of RP. For example a designer of an auditorium will find it easier to execute Boolean operator on two spaces, which would have been otherwise tedious and cumbersome in the case of physical models. The linking of RP to the digital models has led to generation of geometries considering the algorithmic designs. 3.3.4.6 Rapid prototyping RP is a layer-by-layer manufacturing technique. It consists of a vast range of techniques that have the capability to take as input the CAD models produced by different computer-aided software. The CAD models are then converted to the physical form. This physical form of the model is the output of any RP technique. The process is fully automated and even the complex geometries can be fabricated with ease, without the need of special tools and fixtures. The 3D model is produced using layers of construction materials. The resemblance to the end product depends on

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the thickness of the layers. A very fine thickness ensures that the end product is close enough to meet the requirements and functions for which it is needed. The RP techniques make use of software packages to design and fabricate complex and intricate models. If a conventional technique is utilized to fabricate these complex geometries, then it would become difficult to analyze, test and manufacture. This would also inculcate huge amount of time and requirement of skilled labor. It is here that RP techniques come in handy to reduce the time and cost. It is also useful to provide the design data required at different stages of product development.

3.3.5 Sculptured models Directly or in abstract sense, a link has been set up between the artistic work and the technological developments. It is the recent developments and advancements that have controlled on how we see things around us. It debates the very essence of our perception. Further, it debates the way we acquire knowledge. It has controlled the very essence of how we fathom the world. The recent advancements in technology have had a greater influence on the artistic representation, as far as the domain of art is concerned. One can clearly understand the difference that has been created by the technological advancements in the way one perceives the object and therefore creating its representations by carefully examining the attitudes of the artists of the Italian Renaissance and that of the seventeenth century artists from Dutch. On one hand, the belief on perfect poises and beauty formed the basis of Italian representations. Artists were chosen from the once with heightened beauty and mathematical harmony. That means the artists who would perceive the things from the beauty point of view and represent them were not the once to make it to the selection list. It was the section of artist that would represent the things on the basis of informed choices and judgments that were given more consideration. On the other hand the technology was employed by the artists of the seventeenth century Dutch to build representations of the object and change the way people would perceive the representations. Several research and advancements were carried out to help increase the accuracy of the technology to assist in perfect visualization of the things. The advancements lead to the innovation of new technologies such as the camera and the different lenses to help the artists to perfect their observations of nature. The new technologies met the popular needs of the artist to have a microscopic close-up of the distant views. This was seen as the new way of gaining knowledge. This also enhanced the understanding of visualization and hence their representations. It is interesting to note that researchers have revealed that the Italian artists used a mechanical device that consisted of convex lens or aperture in a darkened box which was used for projecting the outside object onto a screen, also known as

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camera obscura. The artists did not use the direct effects of this mechanical device into their landscape paintings but used it only to achieve accuracy in placement. Each technological tool has its own sets of characteristic. These mechanical aids have their own strengths and weaknesses. The peculiar nature of each device influences the nature and structure of artistic conceptualization and production. In the subsequent sections, we would discuss the use of modern-day computers and eye trackers to build 3D sculptures. Creation of artwork from computers is not new. It all started as early as 1960s. Gaussian Quadratic was created by some of the researchers in the year 1963. Mathematical functions were used for transformation of visuals, generated using computers. This work by a group of scientist remains influential even today. With the recent developments, Eye tracker is a software that is now used to create artwork. But the software has only been used to produce 2D images. Eye writer project was one of the major outcomes of the eye tracker software. This project has helped the people suffering from a disease known as amyotrophic lateral sclerosis (ALS), which leads to the death of neurons controlling the voluntary muscles. One such exemplary example is that of Los Angeles-based activist and graffiti writer, Tony Quan, who was left paralyzed after being diagnosed with ALS in 2003. The project helped the people to create graffiti and to project the same onto the walls and buildings. Number of variable is involved in producing visual representations of a 2D portrait on a screen. Eye position, the path through which the eye follows while looking at the object, and the time spent in looking at the objects are some of the variables to name a few. All these variables aid in the collection of data in order to produce a 2D portrait of an object. The data can also be used to identify, where the users are looking to help in some of the experiments. However, despite the use of eye tracking software over a decade, some of the interesting variables for the collection of data have been overlooked during analysis. Tracksys ETG is one of the applications of eye tracking concept. The Tracksys ETG works with the aid of infrared cameras, which are built in the glasses. An initial calibration is necessary while looking at an object. Once the calibration is accomplished, the X and Y coordinates of the user’s pupil is gathered using specialized software. The gathered data produced from the fast movement of the eye generates a scan path. Then with the help of analysis software, the gathered data is transformed into visual results that can be projected onto a screen. One can closely examine the eye movement for the user who is either alert or for not being even aware of the things around, using the eye tracking devices and methods. The capacity to gain an understanding of the cognitive process such as memory, decision-making and language compression is greatly increased with the aid of eye tracking devices. The eye movements of the humans are linked to the perceptual systems. The closer relation of these eye movements to different attention

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mechanisms and rapid movements of the eye provides an insight to the aforementioned process. The researchers have found that the movement of the eye increases during mental imagery. Further research has provided insight that eye movements result from a cumulative of perceptual experience and different cognitive process. The eye movements are also a result of sequentially correlating different fixation points that the pupils perceive. The beautiful correlation can then be used to gain an insight into how one thinks. This correlation has also led to the foundation of different aesthetic related research and many other related works. The eye tracking has been used to gain an insight of visual exploratory behavior of paintings. Wherever the pupil of an observer focuses attention for a longer period of time and thereby increasing the density of eye fixation, it is often perceived to be observer’s keen interest in gaining the informative element of the image. The neural primitives often drive the studies of aesthetic judgment that mainly focuses on the deep root analysis of image composition. The aforementioned process has been explored for the generation of 3D sculptured models. The task to produce the 3D sculptured models is done systematically and sequentially following different steps. Firstly, a mental plan is generated by visualizing the entire field and then processing the same parallel. This generated mental plan is weighted according to the task. Next a bottom-up driven process is performed. In this process the eye movements are performed in order. The eye first tracks the most interesting or the strongest feature. The movement then gradually moved toward the weaker or the least significant feature. The other factor where the fixation is for a longer time includes curves, lines, edges, corners, color and contrast of luminance. The research has further provided evidence of increase in the aesthetic appeal of visual stimuli with the modest degree of complexity. An object on which the form could be based was required to create 3D sculptured models using eye tracking process. Human faces were realized to be the main source of inspiration. In human perception, the human face is perceived to be a special object. Even the infants learn quicker to recognize a human face than any other object. One can look it as if we were born to have the inherent visual systems to learn to recognize the human faces in comparison to recognizing other objects. One can also observe that the top-down approach that was mentioned earlier could be adopted. This is accomplished with two distinct types of attention to the feature. That is, one being an intentional and second being focused. The intentional direction as well as the focused attention helps in drawing out the simulation of emotions and sensations. The face is considered to be constructed of discrete 3D objects. The mind perceives this discrete set of 3D objects in a holistic way, that is, the objects are construed to be interconnected. This also forms the reasoning as to why the face is generally the first part of the body that is scanned in portraits. Thus, instead of common analysis of individual features, a well-organized visual encoding

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can be performed. Studies have indicated that the gaze can be prioritized, while observing a face. The gaze strategy should be able to extract as much information as possible. This can result from the similarity between the face and its visual representation. Generally, the gaze is directed toward the internal region of the face, while observing the face for the first time. In particular, it is the eyes and the bridge of the nose on which the gaze is directed initially. The gaze strategy will allow the observer to maximize the perceptual span. This leads to the extended covering of the stimulus. Therefore the gaze strategy optimizes the region to be included for high-resolution acuity. This results in gathering optimized data from eye tracking which can further aid in constructing an exact copy of the face in the form of a 3D sculpture. The approach in creating a 3D object is to collect the data set of points generated by eye tracking. The points combined with the relative positions of the 3D object and the observer is then placed in a virtual space. The combined data is then projected to a virtual 3D feature in this virtual space. This resulted in creation of voxels, produced as a result of intersection of the projection with the virtual 3D object. These then can be used as a basis for the creation of 3D sculptures. However, a number of problems arise while transforming the captured data into a virtual space. First, it is a natural tendency that an observer will not be able to keep one’s own head straight, since to get a better view of the object the head is to be tilted. Hence, a proper recording mechanism needs to be adopted that can record precisely the relative motion of the head. However, one can do away with this problem by a number of ways. Researchers have made the use of Polhemus Patriot which is an electromagnetic positional tracker. The device senses the electromagnetic field generated from an electromagnetic coil containing sensor. Since the sensor is connected to its electronic unit by employing a cable, the use of the aforementioned device is restricted. The use of optical motion capture is the second option. Typically, there are two versions of it: passive and active. Both the versions make use of special markers. The positions of these markers are tracked with the aid of special cameras. The passive version consists of infrared LEDs around the camera lens and the infrared filter over the contact lens. The light emitted from the camera by the LEDs is reflected by the markers. The reflected light is then sensed and captured by the camera. No external source is required to power the LEDs. However, the system has the disadvantage of requirement of extra time to clean up the data that could have been affected because of unwanted noise. On the other hand, the active version employs the pulsed LED. These measure LEDs instead of measuring the reflected light from the markers. However, the LEDs are needed to be externally powered, which adds on to the disadvantage for these devices. In our discussion, we have made use of passive systems. In order to increase the accuracy of the passive systems, more and more markers need to be employed. However, care must be taken while placing the markers. They should be placed

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where the head movement is the least. This can be ensured by allowing a proper rest support for the chin using a chin mount. The object is made to have movements in such a manner that the head movement is least. The next stage is to import all the generated data to a 3D CAD system. One of the software packages developed by scientists is the PO3TS, that is, Positional Observational 3D Tracking Software. The data collected as a result of eye tracking is resolved into components. The individual fixations produced as a result are selected along with their time stamp and increment number. The selected fixations are placed onto a plane which is at a distance equal to that of the scanned model’s head. The points are then made to intersect with the virtual head surface. This is accomplished by projecting a vector from the eye of the observer and passing through the points placed onto a plane. As the number of intersection points created increases in number, the area becomes dense with the data points. This leads to a basic 3D heat map.

3.3.6 Food article models A number of challenges are associated with the production of food. Global warming is one of the major challenges that the food industry is facing. Even the production of greenhouse gases is a cause of concern for the food industry. Other challenges are that of growing demand of food with the population. Also the fair treatment of livestock is a major issue gripping the food industry. There is about 18% of greenhouse gas production from livestock, according to the recent FAO (Food and Agriculture Organization of the United Nations) report. The situation is getting worse with the increasing population. Also, the growing need for food has led to the deterioration of the livestock treatment. However, the growing population demands for the better treatment of animals and the transparency of the process concerned with converting livestock into food. It is here the need of RP could be realized. Some of the companies are using it to produce artificial protein and even meat. RP has found its way into food industry as well. Packaging of food and beverages as well as the production and prototyping of food are the two main fields where RP has been playing a very important role. These have been discussed in the subsequent sections. 3.3.6.1 Rapid prototyping food and beverage packaging The use of RP for packaging of food and beverage was not very prominent. This was not adapted to the scale it was adopted in other industries such as aerospace and automotive. However, with the recent advancement, RP is being used widely in the food industry. The industry was able to save time using RP techniques for

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packaging of food and beverages. Industry found the application of 3D printers handy as well. It was possible to produce prototypes, which resembled very closely to the real packaging. It is the packaging of a product that plays a very important role in its marketing. Industries are able to make necessary modifications. The time involved is also very less. This leads to the realization that how important it is to employ various RP techniques to gain a competitive advantage over the competitors in the market. 3D printing is fast becoming popular in the food industry. The technique of 3D printing is still not very mature in comparison to its other conventional RP techniques such as those of CNC prototyping. However, the economic advantages associated with 3D printing are to the extent that the industries have started to do away with the traditional way of packaging of food and beverages. 3.3.6.2 3D printing and food 3D printed food helps in reducing global warming. It also caters to the need of customers, taking utmost care of the scarce natural resources. 3D printed foods are becoming fast popular. However, already there are machines in the machine that can produce the food we want, but because of the added advantages associated with 3D printed foods, 3D printing has become very popular. 3D printers are trying to fill the gap of pleasing the eye and the taste of the customers. A balance is needed to be stricken for achieving the objective. Foodini is one such machine that has been developed by Natural Machine in the year 2014. It is a 3D food printer. One needs to place the required ingredients into this machine and can get the prepared food within a quick span of time. The machine is capable of producing 3D printed food of all types ranging from savory to sweet. 3D printers are now available with WiFi and Internet. Bocusini is one such 3D food. This also supports a wide range of recipes ranging from fruits and vegetables to products of bakery. 3.3.6.3 Applications of 3D printed food Some of the interesting applications of 3D printers are discussed next. – Modern food designs: with the advent of recent advancements in 3D printing, it has now become possible to produce any design with the food, which otherwise would have been difficult. The appeal to the eye of the designed food is such that one often neglects the footfalls in the taste. – Food for the elderly: the play field with the advent of companies like Biozoon has transformed completely when it comes to the food for elderly. Earlier, the elderly had no option other than to take puree with different flavors. The technology of 3D printing has done away with the problem of chewing and swallowing. Food can be chewed and swallowed in the form of puree.

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– Food printing in space: space agencies like NASA are looking forward to ways for producing 3D printed foods. The first project in this direction for the space agency is a 3D food printer for pizza.

3.3.7 Models for NEMS/MEMS applications Highly specialized skills are required for designing and innovating MEMS. Also expertise from different interdisciplinary background is required for the same. Physics, chemistry, advanced mathematical modeling (MM), materials engineering and manufacturing technology are some of disciplines from which the expertise is sought. Modeling and simulation software with the capability to reduce the time required for prototyping and hence optimization is required for industrial development. Therefore there is a restriction to the industrial development on a wider scale because of the required skills and expertise. The time required for prototyping and optimization was reduced with the advent of CAD packages. The advent of different CAD packages lead to the widespread development of VLSI devices, which made the goal of time-saving much easier. However, the CAD packages have not matured enough for the development of MEMS devices despite high demand of such devices. The different CAD packages that are available support only MM and lack realistic and useful applications. A strong interdisciplinary background is still required. There are widespread applications of MEMS devices. Some of the applications for MEMS include airbag triggers, optical, ink-jet print heads and medical to name a few. With the recent advancements in MEMS, the MEMS industry is on the growth particularly in the medical and optical applications. MEMS have their domain in the microscopic world. There is a difference of dominant forces in the microscopic and the macroscopic domain. As far as the microscopic domain is concerned the dominating forces are friction and adhesion where as for the macroscopic domain it is gravity and inertia that are the dominant ones. Hence one cannot think of downscaling the macroscopic machines to microscopic dimensions. The same applies to MEMS. For example, if a fluid pump is downscaled to microscopic dimensions then it would not function effectively with this version. 3.3.7.1 Evolution and rise of MEMS MEMS were first applied for reducing the dimensions of transistor to submicron scale in the late eighties nineties. Also with the aid of MEMS, the process of microlithography patterning progressed toward increased accuracy. Initially the MEMS were several millimeters big, but with the advancements and intentions of the researchers they were downscaled to micron scale. The basic idea was to keep these

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devices as small as possible since the days of inception. The smaller the size of MEMS, the lesser the material that goes into its manufacturing and hence lesser the cost. This also adds onto the flexibility. Silicon is not only the material used for the fabrication of MEMS but with recent advancements, materials like polymers or metals are also used. MEMS were employed in microelectronic manufacturing industries. These industries had the advantage of fabricating MEMS without the involvement of extra cost since the steps and the technology that were used in these industries could also be used for fabrication of MEMS. The scope of application of MEMS further increased, when polymeric materials could be used for their fabrication. MEMS are often perceived as complex devices. MEMS could be structurally complex or functionally complex. However, there is difference between the structural and functional complexities. MEMS can be fabricated from a group of simple parts that will perform complex functions, that is, MEMS being functionally complex in this case. A movable mirror in an optical switch is an example for functionally complex MEMS. On the other hand MEMS could perform simple functions if they are built with a group of several components in a complex arrangement. A microfluidic pump is an example for structurally complex MEMS. MEMS can replace large, heavy equipment because of their submicron size level. Space and time can be saved by employing MEMS. 3.3.7.2 Impact in medicine MEMS have revolutionized the medical industry. Robots are being employed during the surgery. MEMS have been utilized in variety of medical devices ranging from microgrippers to the devices used for endoscopy. Some other applications of MEMS include ultrasonic surgery and microsurgery for eyes, to name a few. The revolutionized effort of MEMS to the medical field has transformed some of the medical industries into multibillion dollar organizations in less than 20 years. MEMS also find their application in the field of biophysics. Ion-sensitive fieldeffect transistor arrays have been employed by some of the researchers to design MEMS for the effective measurement of metabolic activity and the electrical activity in a network of neurons. 3.3.7.3 MEMS evolving manufacturing alternatives As discussed previously, MEMS can be composed of a single sophisticated component or MEMS can have multiple components. Whatever be their composition, some circuitry is required for interaction or to control them. There could be huge costsaving in fabrication for the silicon manufacturing, if it was possible to fabricate the MEMS and the related circuitry onto the same wafer. However, this is not possible because the heat involved in the processing will damage the previously built structures. The structures of MEMS are produced either from the removal process

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on the solid materials or built up in a deposition process. This depends on the material used. The fabrication of MEMS is a very arduous task. It requires semiconductor foundry that are capable of handling around 200 processing steps. Further the fabrication process requires expensive machinery thereby making it a costly affair. In order to combat the high cost of the process and the equipment, the industries are forced to use the in-house facilities for the fabrication. The other way round could be the collaboration between manufacturers and research institutes. Besides using silicon as the material for manufacturing of MEMS, other materials such as glass, polymers are also being used. A number of technologies are now available to manufacture the same. These include reactive-ion etching (RIE) and excimer laser (UV laser), to name a few. Some researchers have utilized photopatternable UV-sensitive adhesives. This results in low cost since no baking is required. Even the requirement of a clean room is not a necessity with such type of technologies. 3D printing is another technological advancement for the purpose of manufacturing MEMS. 3.3.7.4 Tools for modeling and simulation Design of MEMS requires different range of modeling techniques. A different set of modeling techniques are required for designing the circuitry for functioning of MEMS, while for fabrication some other set of modeling techniques are required. Some other techniques would be required to find out the desired properties MEMS must have, depending on the field of application. CAD tools are the recent advancements for carrying out different sets of activities at different stages of MEMS design. The essential for any design process is the MM. For MEMS CAD software, the MM forms the underlying simulation tool. This is a very essential part of MEMS design that there cannot be any serious outcome without it. Also it is necessary to evaluate the dynamic behavior of the other components that make up the MEMS. The mathematical and physical modeling accomplishes this task of evaluation. Solvers such as MATLABTM and finite element analysis are used for solving the different differential equations. COMSOL MultiphysicsTM is another solver used for the same. 3.3.7.5 Prototyping and MEMS Prototyping has led to substantial development in the industries. It is the technique of manufacturing prototypes of a physical 3D model. This is mainly done to study the functionality of an existing object with the sole aim of replicating the same. Also, the production feasibility can be studied using these prototypes. If the aim is to study the functioning, then neither materials nor the production process need to be the same as the intended ones for regular fabrication. Neither the production

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methods nor the materials that are used for the production of the existing production are needed to be replicated if the sole purpose is to study its functionality. However, the production techniques are needed to be replicated if the aim of producing prototype is to study the feasibility of a production process. Whatever the case may be, prototyping aims at reduction in production time. Studying the feasibility of a production process before going for the mass production and detecting any flaws in the design of a product have led to the widespread utilization of prototyping in the recent years. Even RP requires to some extent a worked-out design. The prototyping has also found its application for the production of MEMS devices. Since prototyping aims at faster and cheaper production, it has led to the fabrication of MEMS devices not only from silicon but also from some other materials such as glass and polymers. A number of available alternatives for technologies have made it easier to fabricate medical MEMS products inexpensively. One of the problems faced by the manufacturing giants was the availability of rapid tooling for the production of submicron dimensioned MEMS. However, with the advent of stereolithography, the problem could be solved only partially. But when the stereolithography was replaced by one of the other rapid tools, that is, microstereolithography, the problem of the manufacturing giants seemed to be solved. In microstereolithography, rapid tooling was also not fully developed at that time as it was possible to manufacture layers of thickness ranging between 0.05 and 0.2 mm but the aspect ratio was on the higher side. With the recent advancements, researchers have utilized photoresist to fabricate the microfluidic channels. A microfluidic channel as deep as only few micrometers was developed by the researchers. One of the ways to fabricate the microfluidic channel was to employ a two-step baking process in which the photoresist could be adhesively bonded to the glass. The etching process was subsequently followed. In order to ensure smoother etching results, an iterative procedure of wet dipping and ultrasonic agitation was done. The sealing of microfluidic systems with glass chips was done at around 580 ºC. The process led to the RP of microfluidic system faster and cheaper. Some of the researchers have also developed free moving structures by utilizing photoresists. A spring and a piston was developed using this technique. First, insulation was developed for the silicon wafer by employing 20 μm layered photoresists (SU-8). Then subsequently a 50 μm layer of crystal silicon was applied on the top of the insulating spacer using wafer bonding. RIE was then followed to produce the desired structures. The layer-by-layer building, that is, additive manufacturing, and the subtractive process, that is, removal of material are done to obtain the desired MEMS structures. This process allows for the alternative materials that can be used for the fabrication of MEMS devices. Further, these do not require any clean room for fabrication process. Some of the researchers are using these processes to fabricate the MEMS devices. They have developed an ultrasonic based powder feeding

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mechanism for microsized particles. This has been adopted for deposition of microsized dry powder onto the substrate. Patterning is subsequently followed. This is done by sintering the powder patterns with a microsized laser beam to clad them onto a substrate. Shape deposition modeling is one of the other developed techniques. Researchers have developed some of the techniques for the ultraprototyping of some of the MEMS devices. One such technique employed is liquid phase photopolymerization. This finds application in building of microfluidic channels by using micromolding. However, microfluidic devices without micromolding can also be fabricated using the same technique. The microfluidic channels fabricated using this process finds application in life sciences, where fluids are used, mixed, etc., and discarded, in very small quantities. A multichanneled cartridge was used as a master. This channel was filled with photoresist. The unwanted parts were then discarded. Subsequently, to harden the channel geometry, UV exposure was followed. After some time of exposure to UV, the rinsing of structure was done to result in desired channels. Plasma processing with low-pressure chemical vapor deposition has been developed by researchers for the fabrication of trenches with high aspect ratio, that is, deep narrow trenches with straight walls. However, the technique used depends on the construction material used and also on the intended application and life span. Direct write laser ablation is a technique that has been developed by the scientists for the fabrication of capacitive microaccelerometer based on polymeric material. The technique has advantage of producing MEMS in smaller quantities. This has overcome the hurdle faced with the traditional lithography in which producing small quantities of MEMS is an expensive affair. Apart from expensive techniques, a group of scientists have developed cheap technology to produce actuators. These actuators allow microfluid droplets to move, employing electrodes separated by a micrometer range distance of 50–60 μm. Recycled circuit boards were used in place of gold and metals. Even compact discs were used. Razor blade was used for developing ink and ink masking for the purpose of electrode patterning in lieu of expensive photolithography with UV exposure. Cling wrap was utilized for dielectric coating. Teflon was substituted by car windshield, for carrying out protective hydrophobic treatment. The process was successfully carried out thereby validating their approach toward a cheaper technique. With the advancements in research, scientists have developed a manufacturing technique using a cutter plotter (a “printer” that removes material). Utilizing this technique, cutting is done through the polymeric substrate. This is followed by patterning the structures layer by layer with holes and trenches. The stacking of the patterned polymeric sheet is done and then the sheets are bonded. The process resulted in fabrication of containers for the fluids. This fabricating technique is fast

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enough to produce 20 m wide and 30 m deep channels in less than 30 min. The technique also do not require clean room environment for its successful implementation. The technique has done away with conventional manufacturing which otherwise would have been expensive and even slow to fabricate large quantities of MEMS for medical field. Another interesting development is the use of special ink to print MEMS onto paper. Scientists have printed resistive sensors onto paper substrates employing special method and the ink-jet equipment. The development finds application in design of complex models of sensors to be utilized in low-cost application. The low-cost application may include consumer goods packaging and medical disposal to name a few. Recently 3D printing has been developed for speeding up the production time. It is a faster approach toward building up of devices using layer-by-layer manufacturing technique. 3D printing finds application for electronic packaging of MEMS devices. This is useful at the wafer stage. The process is suitable for parts larger than 1 μm. However, 3D printing cannot be employed for structural elements such as cantilevers and springs. 3.3.7.6 Virtual reality prototyping and MEMS Virtual prototyping is another manufacturing technique that is gaining importance in recent years. It involves faster algorithms and hence relatively faster in comparison to the traditional prototyping approach. It has evolved over a decade, but its application for the manufacturing of MEMS is still limited. This is because of the complexity of the manufacturing. Scientists have carried out comprehensive research for the application of virtual prototyping to production of MEMS. Some scientists have proposed for the development of a service driven MEMS CAD design tool. Researchers have developed a partial software prototype for production of bond graphs. Behavioral modeling was proposed by some, which was also compared with the two most popular modeling methods: finite element analysis and boundary element method. Voxels instead of pixels is another interesting development for the display and animation. Virtual realitybased environment for microassembly has been invented by a group of scientists. The software can be linked with the physical manufacturing. This aids in visual representation of the tools and their movements on a screen. This is helpful from the operator point of view. Genetic algorithms are used by an automated assembly sequence generator which is aimed to optimize assembly sequences. The virtual environment aids in producing a well schedule for the fabrication of MEMS. This also generates the assembly instructions for the physical part (tools and autonomous robots) to be assembled by the available work cell resources.

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3.3.8 Models for toys A toy is any object that can be used for playing. For example, a water bottle can be used as a toy, an umbrella can be used as a toy and even a wheel of a bicycle. But none of them is a toy product. A toy product is a product that is designed and manufactured for the sole purpose of playing. For example, a football and tennis racket. Therefore, the important distinction between a toy and a toy product can be realized. However, there are number of challenges that the toy making company is faced with. Technology is required to manufacture toy products in large quantities. The cost and the benefit to the customer need to be balanced. Some of the challenges are discussed in the proceeding section. 3.3.8.1 The challenge of toy design There are several reasons for difficulty that is faced by the toy making industry. Designing toys is not as simple as one might perceive. This is mainly due to the playful nature that comes with the toy products. Other reasons are discussed next. – Unlike any other product, the customer base of any toy product comprises mainly two types of customers. One is the purchaser who is also a caregiver, while the other is the end user who is a child. These two types of customers have got differences in needs and preferences and hence it is a quite challenging task to satisfy both simultaneously. This challenge is in addition to the form, manufacturability and function, which are concerns that are inherent in all product design. – Any toy product must be tested for safety to the child. There are a number of international standard tests that a toy product needs to pass. These include tests related to chemicals, projectiles, moving parts, choking hazard, strangulation, drowning hazard and magnets. – The majority of demand of any toy product is mainly during the vacations. Also with time, the trends, character keeps on changing. Therefore, there remains a very less time from concept generation to the end product. It is also very difficult to plan inventory and get realistic feedback, which is very important to improve the product. – It becomes very difficult for a toy manufacturer to operate independently to get their product onto the shelves. The number of toy retailer has gone down since the 1970s. this is because of the decreasing profit margins and time constraints. The emerging technology of RP has eased the burden of toy manufacturing companies. RP has broken away the shackles of challenges which had gripped the toy manufacturing companies. The additive manufacturing is found to be cost-effective and time-saving. Now the manufacturers can produce toy products rapidly. The wastage of material is least. Several alterations can be made before the product is

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finalized for design. Manufacturers can have the product on their shelves as and when required. Several tests can be performed with ease. This all increases the profit margin of the companies involved in the toy making industry.

3.3.9 Psychoanalysis models A highly great set of skill and experience is required for carrying out brain surgery. It is a high-risk procedure where careful planning is a must. Any aberration from the planning can lead to trouble for both the surgeon and the patient. This could even lead to fatality of patient if the surgery is not carried out as planned. Therefore, for the success of the brain surgery careful planning is essential to deter the risks. Being a risky procedure, nervousness in patient undergoing the surgery is inevitable. The patients even doubt the benefits the surgery can have and can the procedure outweigh the associated risk. In order to avert the associated risk, a tangible model of the brain is required. The model should depict the main functional areas of the brain as well as the white matter tract. The model can also help neurosurgeons to communicate and make their patients understand the functioning of the brain properly. This could also help in explaining as to how a particular function is affecting the functioning of the brain. The planning for performing the surgery can also be explained to the patient with the aid of tangible models. Such a communication can help the neurosurgeons in building a sense of trust with their patients by building a healthy working relationship. Further, the models can also come in handy for educational purpose. 3.3.9.1 Rapid prototyping For building of tangible models, the additive manufacturing process can be employed. The process is known as RP. A 3D model can be produced by employing layer-by-layer process. Even the prototypes of human organs can be produced using the technology. This also includes building of tangible models for human brain. CAD software packages are often used to completely build the models. 3D printers such as Z Corporation Spectrum Z510 3D color printer could be used for layer-by-layer deposition of the material to produce the model. Successive cross sections of 3D objects are produced from plaster powder. The Z Corp printer creates successive cross section of 3D objects from a plaster powder. The powder is fed into the machine bed by the roller and the subsequent layers are produced in the machine bed. Spraying of the binder is done by the ink-jet, as the model build-up process proceeds. The machine bed is lowered to accommodate the formation of the new layer. On completion of the model, the excess powder from the model is blown off. In order to strengthen the part, the completed part is then infiltrated with glue. The process is very quick depositing about two layers per minute and at the same

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time cost-effective. However, the surface finish and accuracy is relatively less compared to other techniques of RP. 3.3.9.2 Nuclear magnetic resonance imaging One of the forms of medical imaging is the nuclear MRI (NMRI). In this form of medical imaging, the magnetic properties of the protons are exploited. NMRI further contains various forms and since proton is an integral part of almost all elements, it is required that NMRI must focus on particular elements. Isotropic abundance, physiological concentration and a strong magnetic moment are some of the characteristics of a good element for MRI) applications. MRI makes use of high frequency electromagnetic waves to produce high resolution image of the tissue. Further, hydrogen has a very large physiologic concentration and magnetic moment, making it a perfect element for MRI. It is a common knowledge that water makes up 70% of human composition. Further a water molecule is composed of two hydrogen atoms. The MRI activates the hydrogen atom and as a result the proton jumps up to a new energy level after absorbing the high frequency electromagnetic waves. However, to be in the equilibrium state the proton will try to get back to its ground state by releasing the RF wave’s frequency. A signal is produced which is detected by the MRI machine. However, this signal is dependent on the environment surrounding the water molecule. As a result a high resolution image of the tissue is produced. 3.3.9.3 Procedure The MRI scans once obtained are converted and saved in a PNG file format. However, conversion of images to TIFF file format is essential for mimic software to produce a 3D model of the human brain. For this purpose scripted image processor is used. The images from the scripted image processor are then processed using a new project wizard in the mimic software. Grayscale threshold values ranging between 107 and 254 were used for creating a mask in order to isolate the region of study. The tissues that are not connected to brain are removed by the region growing feature. Using edit feature, one can manually remove the portions of the skull and peripheral tissues with similar grey scale values. The complete model then can be based on this brain model. It may be possible that the MRI images of each anatomical area of the brain may not match truly with the base brain model. By creating a new mask for each part on the original brain scan, the obstacle can be overcome. Each of the brain’s primary area of function is needed to be defined to the new mask. This can be accomplished by using the edit feature. The contour lines of the anatomical images could be used as reference. The new masks are superimposed over the original scan images of the brain. Along with the MRI data, DTI data are also compiled using Mimics software. The masks from the MRI data and the DTI data overlap when loaded into the same

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project file. Boolean operations are then used to do away with the overlapping of the masks. This will also avert the problems that the Z Corp printer could face. The Boolean operations will remove the area that overlaps with the base brink mask. Any roughness can be removed using smoothing operation. The masks are converted to STL file format. These are then processed in a software package known as Magics. Three slices are made to show the interior of the brain. One slice is made where the right hemisphere meets the left. The other two are made in the middle of the right hemisphere. The slices are held together using cylindrical magnets. Boolean operations are used to remove the cylindrical sections. Z Corp printer is input with the masks for production. As a result, a layer thickness of 0.1 mm of the model of the brain is produced. Each part of the brain could be differentiated from the other using color coding.

3.4 Conclusion RP and other recent advancements in 3D CAD technologies are helping the designers to make their customers and clients have a real feel of the product that will meet their needs and perform the intended function. The questions like “can you prove-it” can be tackled with the aid of such technologies. Further, the layer-by-layer manufacturing process or RP saves a considerable amount of production time. The accuracy is important in designing and fabricating a product. The RP has the capability of producing the products with high level of precision and accuracy. The discrete concepts can be made reality using such technologies. Thus the RP manufacturing process is gaining a lot of importance in the industries. Some of the considerable benefits of RP are discussed next: 1. Realizing the design concepts The RP enhances the visualization of concepts. Through 3D CAD models, one can have a real feel of the discrete ideas, rather than just assuming. The scope of virtual visualization is enhanced to the extent that the designers can carry forward their ideas and implement them practically without much effort and time. The feedback from the clients can make the designer to achieve the more realistic product design that latter on can be translated into a useful product. The 3D CAD models can act as a proof of the design which otherwise would have entertained the questioning of “can you prove it?” 2. Modifications in the nick of time Normally a product design passes through several iterations before it is made final. Using RP as a tool, designers can make modifications in nick of time after having customer feedback. This helps to improve the product toward the finalization of the

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product. This leads to the bridge of confidence between the designer and the end customer. This also provides a competitive edge in the market as it becomes possible to manufacture a product with better acceptance rate. 3. Cost and time-saving The use of additive manufacturing saves a considerable amount of time. The requirement to produce special tools and other aids of manufacturing is totally eliminated. There is also considerable saving in cost as the same CAD software package can be used to produce different geometries. Further the amount of waste produced is also relatively less, since RP only prints the material that is required to build the model. 4. Customized designs The recent advancements of RP can help the manufacturers to produce the customized products as per the need and requirements of the customer. No special tooling is required to produce the customized products. Further, only a small change in the CAD model is needed. The entire process of manufacturing remains the same. This also provides a connected experience platform that can be shared by the customer and the manufacturer, after the sale of the product. 5. Design flaws are minimized Several iterations are possible before the design is finalized. The prototypes can be tested for faults. This averts the risk of failure when the product is out in the market to perform its intended function. The designs can be made free from any faults before they are processed for manufacturing. All the above-mentioned advantages suggest that it is indeed profitable to employ additive manufacturing or RP in the manufacturing process. These technologies provide for a competitive edge to an organization by aiding them to develop innovative products with ease and cost-effectively.

Chapter 4 Rapid prototyping/manufacturing processes 4.1 Introduction Processes such as stereolithography and selective laser sintering (SLS) are some of the much known and well-established techniques as far as the rapid prototyping (RP) or rapid tooling industry is concerned. However, for the rapid manufacturing (RM) industry, the processes are still new. One of the reasons behind this is the recent advent of RM. The fact that RM technologies are still in their nascent stage can be established by making a comparison between SLS process for polymers and injection molding. As far as injection molding is concerned, one can infer that the time from filing of the first patent by Hyatt brothers (1872) to the time when injection molding came to be accepted worldwide is very wide. The lack of suitable material is one of the major reasons for this. Initially, injection molding was based on cellulose materials that are in fact highly flammable. However, with time new materials came into existence, which escalated the use of injection molding technique worldwide. Materials such as polystyrene made this possible. There was also escalated demand of products during World War II, which led to the further upsurge of applications for injection molding. By 1979, plastic production met the demand of the rising market. Steel has also found ground with the recent developments. On the other hand, as can be fathomed from the timeline, the first patent for the SLS came in 1979 by Ross Householder. However, in contrast to the injection molding, within a span of 23 years of the patent, an upsurge was seen in applications of SLS. There are more than 20 RP technologies at present but not all can be considered suitable for RM. This is because of the certain material properties that lead to the fabrication of parts with limited functionality or to have a limited scope of visualization. Parts may have limited functionality when fabricated with some of the processes employing particular material of construction, for example, parts from Z-Corp and infiltrated with epoxy. On the other hand, the parts from thermojet process will have only visualization properties with limited mechanical properties. In this chapter, we will discuss different processes of RM which were originally intended for RP/rapid tooling. Also discussions of the technologies that are recently developing with RM in mind have been made. According to the raw material used, the different technologies for RM have been categorized and discussed comprehensively. These technologies are categorized as follows: – Liquid-based systems – Powder-based systems – Solid-based systems

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Further, different technologies under each of the category that are used widely in industries have been discussed. The list of technologies consists of those that have found a widespread use in different RM industries. Also some of the emerging technologies in different categories of RM have been elaborated. Further, the nature of processing of technologies that have not yet been commercialized has been discussed. RM systems have gradually advanced toward 2D methods for processing materials from 0D. This has greatly improved the production capability of different RP systems. The processes have become cost-effective for both the manufacturer and the end users. This has led to mass production of different parts through RP systems. A single spot laser was employed to cure resin. Although the spot has a dimensional area of πr2, it is considered to be 0D. In order to cover the entire build area, the spot needs to be scanned in X and Y directions. A 1D array of ink-jets is employed to process the material in multijet modeling. The print heads traverse only in one direction, since the array is 1D. This figure shows how the light is reflected to cure the resin surface. This is done using a 2D array of selected mirror. This also does away with the need of traversing or scanning as is the case with 1D and 2D methods, respectively.

4.2 Liquid-based processes Curing of the selective regions of the photosensitive polymers to produce a solid part is the basic underlying principle of the liquid-based layer additive manufacturing (AM) techniques. Rapid freeze prototyping produces ice sculptures, which is an exception to the liquid layer-by-layer manufacturing. The technology builds ice parts from water, which are beyond the scope of visualization. Stereolithography is one of the oldest liquid-based processes. The process associates with itself a number of advantages such as that of accuracy. The parts produced through curing of resin have a much similar appearance and precision when compared to that produced using injection molding. This has led to the commercialization of different liquid-based processes in recent years. The industries have adopted these RM processes over injection molding. However, there are certain disadvantages as far as the construction material is concerned. The continuous exposure to sunlight can lead to further curing of the resin material. As such the appearance and the mechanical properties get affected adversely. Thus, with aging the quality of product can degrade. Other factor is that of humidity, since photocured parts are sensitive to humidity. However, the recent advancements to develop photocured materials that stabilize with time and with different working environment had provided a new set of potential to various liquid-based AM processes. Some of the liquid-based processes are discussed in the proceeding sections.

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4.2.1 Stereolithography Stereolithography is considered to be the first and oldest processes of the RP field. Chuck Hull is credited with for its invention in 1986. 3D Systems was the first company to come up with the commercial machine for stereolithography in 1987. The process makes use of laser to cure the selected area of the surface with resin. Computer-aided design (CAD) software is used to drive the laser. The platform on which the resin has been cured and solidified is lowered to about 100 mm. A new layer of liquid resin is deposited onto the previous layer. The bonding between two adjacent layers is achieved by the scanning operation performed by laser. Supports for the overhang features are created automatically by the machine. The supports are removed to reveal the final product. The part once built is removed from the platform. Further, postprocessing is done in the oven curing any uncured resin thermally or with the use of ultraviolet (UV). The stereolithography hardware, software, build materials and operation of the machine are discussed in the proceeding sections. 4.2.1.1 The stereolithography apparatus 4.2.1.1.1 Software The software for operating stereolithography apparatus (SLA) has gone through a number of transformations. Different packages were required for different operations to be performed. SLA view UNIX-based software was used for viewing and positioning. For support structures, Bridge-WorksTM has been used. This is also UNIX-based software. SLA SliceTM was used for performing slicing operations. The three software packages formed the total software package for SLA machines. With the advancement, the viewing/positioning and the support building softwares were clubbed to a giant UNIX-based software known as Maestro™. Still the DOS (Disk Operating System) platform was used for the system operation. At present, there are single software based to perform operations using Microsoft® Windows NT® platform. 3D Lightyear™ is used for viewing and positioning, support generation and slicing operations. Buildstation™ is used for operating the machine. However, the newer software systems are capable of writing codes for DOS operating machines also. 4.2.1.1.2 Build materials Acrylate was used as a build material for the SLA process. Later on, epoxy-based materials were used. This is known as the ACES build style, that is, Acrylic Clear Epoxy System. The better mechanical properties of the epoxy-based material provide the leading edge over the acrylate materials. Also, epoxy-based materials are less hazardous than the acrylate ones. Although longer time is required to expose

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proper curing of the material, the advantages make the epoxy-based material a preferred choice over the acrylate materials. There are now a wide variety of resins available not only from the vendor but also from third-party vendors. The competitive market continues to open up with higher performance build materials at slightly lower costs. Resins can be purchased to improve resolution, temperature capacity or even speed of the build. 4.2.1.1.3 The SLA hardware A removable vat is the integral part of the build chamber of the SLA. The vat contains resin. The chamber also consists of automated resin-level checking apparatus. The z-axis elevator frame has a detachable and perforated platen. The machine allows for very small amount of motion for the vat. This helps in keeping track of the exact height per layer. A track is provided at the top of the vat. The track works as a guide for the recoater blade. In order to prevent the rounding of edges, the liquid is smoothed across the part surface by the recoater blades. Now the machines come equipped with the Zephyr recoater blades. These blades have siphon mechanism that siphons up the resin. This is then applied evenly across the part surface. The resin is cured using laser. The setup for the laser and optics is located above and behind the build chamber. The setup is enclosed in a chamber. The laser unit is around 4 feet long and rectangular. The unit remains stationary. The laser is focused on the bottom of the part surface by a series of optics. Postprocessing components consist of UV oven, an alcohol bath that is large enough to accommodate the entire built platens with parts attached. The parts are washed in the bath to remove the extra resin that clings to the surface of the part. To finish curing, the parts are required to be placed in the UV oven. 4.2.1.2 Stereolithography apparatus operation The SLA process starts with a 3D model made in any of the CAD software packages. The CAD format model must be transformed to the standard file format such as . STL. However, before doing so, it must be ensured that the model must consist of enclosed volumes. The model in the standard file format is oriented in the positive octant of the Cartesian coordinate system. To allow for building of supports, the model must be translated up the –z-axis. The translation is usually 0.25″. The supports serve the dual purpose of safe removal of the part and that of allowing for platen to be built. The orientation should ensure the optimum build of the part. The complex curvatures should be placed in the XY plane. Rotations if any should be in the Z-axis for the least translation along the axis. A compromise must be struck by the operator to obtain the optimized product. The STL file format should be verified of any missing vertices and triangles. An egg-crate support with about 0.2″ crosshatch is created. The support is provided to

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fix the part to the base. Also, for any overhanging structures it provides a fixture. In the absence of the support, the structure would fall or float away. Once the STL file has been verified, the part model is sliced into horizontal cross sections. This is then saved as slice files. Four separate files ending with file extensions L, R, V and PRM are created once the slice files are merged. The file ending with the extension V is the vector file. This is the largest of all the remaining files. The file has the line data that is used by the laser to cure the shape of the part. Another important file is the file ending with extension R. This is known as orange file, which contains the data for open or solid fills, as well as the recoater blade parameters. The different files with different versions are downloaded to the SLA machine. The machine starts to build the supports. Curing of the first few layers takes place in the platen, and therefore, provides a solid support for the rest of the parts. SLA employs the laser that scans across the cross section and fills across the resin. The resin is then cured and then takes the space into the crosssectional space when it gets hardened. As the slices get build up, the platen is lowered and then more resin is deposited onto the upper surface of the part that is under construction. The build platen is brought back out of the resin vat, once the part has been completed. This results in draining of excess resin. Once the excess resin has been drained off, the postprocessing is carried out. Final product is removed from the base and then the support structures are removed. All this is done manually. 4.2.1.3 Relation to other rapid prototyping technologies SLA system provides currently the most accurate functional prototyping of the products. However, postprocessing is quiet laborious and requires to smooth finish the final product to high tolerances. Presently, there are more than 1,000 of SLA systems worldwide and the number will continue to grow. 4.2.1.4 Advantages and disadvantages Stereolithography provides umpteen benefits to industrial houses involved in the RP process. SLA provides best quality of surface of all the present RP processes. The SLA process is also highly competitive in terms of dimensional tolerances. Since its inception, the number of advancements has resulted in enhanced speed of product build up and fulfills the goal of RP. SLA systems provide the potential ability to build thin vertical walls, tall vertical columns and sharp corners. The list of available printable resin materials is growing by each passing day and therefore pushes the envelope on strength as well as temperature characteristics. One of the major disadvantages of SLA process is the requirements on postprocessing. Procedures are required to handle well the raw materials for SLA. Relatively higher cost of photocurable resins is another limitation to the SLA systems.

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4.2.2 Jetting systems Photocurable resins can be produced using jetting systems. Two widely used jet systems in industries are PolyJetTM and the InVision. Objet is the companies from Israel to commercialize PolyJetTM, while InVision was commercialized by 3D Systems. The PolyJetTM process was developed in the late 1990s, which was then commercialized in the year 2000. Since then the process has been in use worldwide for RM. The machine deposits acrylate-based photopolymer. The machine employs an array of printing heads to deposit the same. The photopolymer is then cured by using a UV lamp. The support structure for overhanging parts is built simultaneously from a series of second jet. For the easy removal of the supporting structure, these are cured to gel-like state with the UV lamp. The technology was initially used widely for the RP and rapid tooling. However, because of its popularity, the same technology has now been adopted by different RM industries as well. On the other hand, the InVision machine was developed and later on commercialized in the year 2003 by 3D Systems. The process had the capability of producing the parts at a speed comparable to that produced by ThermoJet process. Further, the construction material had the properties similar to that used for the process of stereolithography. The capabilities were exploited by the RP and rapid tooling industries. The industries found the process profitable that the InVision technology got widely accepted as a RM process too. The InVision also employs the series of jets to build the part. This is one of the features of InVision process similar to that of Objet. However, there is a difference in the construction material for build removed by some means. Also, one can have an improved aesthetics, as the machine employs colored resins. This could improve the visual appeal of the final product. The capability of the Objet is limited by the material properties that can be handled by it.

4.2.3 Direct light processing technologies PerfactoryTM is one of the machines that make use of the digital mirror devices (DMDs) to build parts. The DMD technology was developed by National Instruments. This is used for switching on and off the mirrors that reflect the UV light. The PerfactoryTM was first developed and then commercialized in the year 2003. EnvisionTec of Germany is credited with its commercialization. The technology has been used widely in the field of RM since its inception. The connotation PerfactoryTM is the abbreviation for personal factory. As the name of the machine goes, the technology is intended to produce customized products. In addition to its customization capability, the technology has other distinguishing

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aspects as well. The parts are built downward instead of fabricating it in the upward manner. This is one of the distinguishing aspects. The machine makes use of acrylate-based photopolymer similar to that used for PolyJetTM and InVision. The photopolymer is, however, cured using a 2D matrix of mirrors unlike using the 1D array of ink-jets. The mirrors operate on the DMD technology. The selective curing is therefore performed by switching on and off mirrors that reflect the UV light from a source. The buildup time is also less. Approximately 10–15 s are required to finish a layer. However, the technology is limited to building small parts. The capability is limited by the use of single DMD with the finite matrix of pixels. As such industries for small parts manufacturing have found this technology very useful. The hearing aid industry is an example of such industries.

4.2.4 High-viscosity jetting The process involves the change in the pattern of the layer continuously and is in accordance with the object to be manufactured. The mechanism involves the accurate displacement of the small drop of printable material to a desired location on the substrate. A single jet is the fundamental unit and is controlled by the pressure of the air jet, the length of the jetting pulse and the distance from the substrate. Desired shapes and sizes of the product can be achieved with different experimental runs on the single jets. This single jet is then extended to encompass multijets controlled in parallel and depositing the layer of printable material in accordance with the desired product. The high-viscosity jetting systems provide solution to the number of limitations with the conventional printing techniques. Furthermore, the process has more flexibility in the degree of accuracy which depends on the hole size being used. Speed of production is similar to many existing high-volume methods of fabrication and is also possible to load paste with any composition of powder.

4.2.5 The Maple process Naval Research Laboratory, Washington, is credited with the development of MAPLE DW. The abbreviation stands for Matrix Assisted Pulsed Laser Evaporation: Direct Write. The ribbon has a few thickness layer of construction material underneath it. Laser with high repetition rate is employed in this process. As the laser is directed toward the ribbon, the construction material is transferred to the substrate. This is the basic underlying principle for the Maple process.

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4.3 Powder-based processes The powder-based processes produce parts that are more stable and have improved mechanical properties than the parts produced from the liquid-based processes. This is also an acceptable trend in the industries as powder-based SLS process is more prominent than its liquid-based counterpart stereolithography. Also parts can be produced from vast range of construction material that includes metals, ceramics and polymers. Further, the advantages of the process are to the extent that even functionally graded material can be used to produce parts in the ever-growing RM industry. In the proceeding sections, SLS process employing polymers as the construction material is discussed briefly. Next follows the description about a number of metal-based powder processes. These were developed for the RP and rapid tooling industries but have found their way in the growing RM industry as well. Finally, processes suitable for medium- to high-volume manufacturing have been described. These processes employ 1D and 2D processing.

4.3.1 Selective laser sintering (polymers) SLS is one of the oldest powder-based processes. Ross Householder invented the process in the year 1979. Efforts were made by Carl Deckard to commercialize the process in the late 1980s. DTM Corporation was the first company to commercialize the machine for production of parts from polymers and other ceramics and metals. The company came up with the machine in the year 1992. EOSINT machine is another product by EOS GmbH developed in the year 1994. The machine enjoys a significant market share. The SLS process employs a laser to sinter or melt the powdered raw material. The laser selectively does so to produce the required 2D shapes. The process shares much similarity to the commonly used liquid-based process of stereolithography. The process of selectively scanning the powder is done on a powder bed. Once a layer is completed, a fresh layer of raw material is added onto the previously built layer. The layer is typically 100 mm thick. The same 2D shapes are then traced by the layer on this fresh layer. Layers are deposited one over another until the 3D part with required thickness has been built. The unfused powder acts as a supporting material, thereby eliminating the need for support removal. Before laser is used for selective scanning, the powdered raw material is heated to the temperature just below the sintering temperature. Infrared heaters are utilized for heating the raw powder on the powder bed. Increasing the temperature of the powdered material just below the sintered temperature reduces the energy that the laser would require to sinter the powder.

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Different materials are sintered by the lasers to different temperature below the sintering temperature. Highly crystalline polymers – notably nylons – are raised to their melt temperature, whereas for amorphous materials such as polycarbonate the temperature is just the same as that of glass transition temperature. Better mechanical properties and good contact between the crystalline powders are obtained by raising the temperature to their melting point. On the other hand, a weaker part is obtained by elevating the temperature of the amorphous polycarbonate powders. But these have been widely accepted by the casting industry, since strength is not the priority criteria over there.

4.3.2 Selective laser sintering (ceramics and metals) Efforts have been made to produce complex parts such as molds and cores for sand casting application, using sand particles coated with a polymer binder. Companies like EOS and DTM have carried out quantum of work for producing parts with ceramics and metal. DTM has produced steel parts in the green state by using coated powders to the metallic parts. This was achieved using the existing machinery for SLS. The parts were than heated in a furnace to burn away the polymer binder. The steel particles were sintered further in the furnace. Finally, bronze was used to infiltrate the porous parts. However, commercialization of the various works carried out to use ceramics and metals as the construction material could not be achieved. However, with the existing machinery setup and with a little compromise, the process offers potential for RM of parts using ceramics and metals.

4.3.3 Direct metal laser sintering With the continuing research and advancements, companies have tried to build parts with SLS but without employing the binder. They have also made efforts to do away with the postprocessing phase. Metals with some additional properties were required for the process to be carried out. Mixture of components having different melting points formed the composition for these metals. These were produced by EOS. The metals were sintered or melted employing the SLS machine. Ytterbium fiber laser is used to produce the required shape. The machine has the build envelope of 250 mm × 250 mm × 325 mm and thereby has the ability to grow multiple parts together. The process is therefore cost-effective. However, the parts produced would have grainy surface finish. Also machining is required during the postprocessing phase, which can be time-consuming.

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4.3.4 Three-dimensional printing Jeremy Rifkin is credited to the invention of 3D printing. In its nascent stage of development, 3D printing was referred to as the process employing ink-jet print heads. The process is most suitable for the parts that do not require a much needed surface finish. 3D printing comes in handy for applications where polymers will not suffice. The process bears resemblance to SLS. However, machining is required to obtain the desired surface finish. Therefore, postprocessing can be time-consuming. 3D printing has been used to create ceramic filters. The technology has a higher efficiency to produce parts in green state.

4.3.5 Fused metal deposition systems S. Scott Crump is credited to the invention of fused deposition modeling (FDM). The process was, however, commercialized in the year 1990 by Stratasys. Some of the companies that have commercialized the process include Optomec, Aeromet and POM. The companies have rolled out different versions of the technology. The process has the capability to employ functionally graded materials as the construction material. Further, the technology is useful for repairing broken parts by adding material where required. However, there are certain disadvantages. The process has a relatively low deposition rate. Further, the surface finish is also relatively poor. Because of its capability to repair broken parts, the FDM has become a niche for the RM area of product maintenance and repair. A plastic filament is utilized as an extrusion material that is unwound from a coil and is supplied to the extrusion nozzle. The material is heated as it passes through the extrusion nozzle and because of the heat, the material is melted and is deposited layer by layer on coming out of the extrusion nozzle. The flow of material is regulated by the nozzle that moves both vertically and horizontally. The nozzle is guided by the aid of a CAD software. The material solidifies immediately and as such the model is built layer by layer. Various thermoplastic materials can be used to produce the FDM models in a variety of colors.

4.3.6 Electron beam melting Arcam, based in Sweden, was the first company to commercialize the process of electron beam machining. The company did so in the year 1997. The laser used in SLS has been replaced with an electron beam. Apart from this difference there is no difference in the approach when compared to SLS. The use of electron beam has increased the range of materials that can be handled. This is because the electron beam possesses a very high energy that can melt materials such as titanium alloy.

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The list of materials is, however, restricted to conducting materials. The scanning rate is relatively higher. Further, the need for the mirrors and lenses to direct the beam at a particular spot is eliminated. This can be achieved through the changing magnetic field. The process requires extensive finishing operation, which adds onto its disadvantage. The process of electron beam machining makes use of 0D scanning technology. However, the elimination of other equipments and further furnace operation makes it a leading contender for RM. The higher temperature and medical field have found this technology useful in producing parts with intricate geometries.

4.3.7 Selective laser melting Generative manufacturing methods have been established since the late 1980s and have been employed as manufacturing systems to develop products across the industries. Processing capabilities of such technologies have been enhanced through the omnipresent plastics and newly developed materials as such metal matrix composites. SLS is one such process that processes metallic materials without the need of binders or any other additives. Selective laser melting (SLM) is one of the AM processes that can generate complex structures directly through the utilization of CAD files. The parts are produced through metallic powder. The SLM process is used in particular for the production of tools required for plastic injection molding process as well as for die casting. Filigree structures can also be produced through SLM process and can be used for dental and human implant applications. There are diverse applications of this RP process. However, the applications are limited by the number of viable printing material as such high-quality steels, and titanium-, aluminum- and nickel-based alloys with powder grain sizes between 10 and 60 μm. The producible layer thickness is between 20 and 50 μm. It is possible to achieve a component accuracy of ±50 μm. The speed of processing ranges to 5–20 cm3/h and depends on the utilization of space. The parts generated through the SLM process are homogeneous and have uniform density. The laser used is power intensive. 4.3.7.1 Process The SLM process can be divided into three phases and these occur repeatedly. Lowering of the substrate plate marks the beginning phase of the SLM process. In the second phase, a new layer is applied onto the substrate plate. This is done with the application of coating. The powder is then scanned by the laser in the third phase. As such the powder is fused at the scanned area. The three phases are repeated until the entire component is completed.

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4.3.7.2 Advantages of SLM SLM process has the potential ability to build up components with undercuts and hollow components. The developer is leveraged greater degree of freedom with the part geometry and are able to produce components with lesser restriction in comparison to the conventional machining processes. Furthermore, integration of multiple functions in the component is also possible with the SLM process. Several procedural steps have also been omitted from the SLM process as such the laser sintering process. Thermal and cost-intensive posttreatment processes have also been removed with the introduction of SLM processes. As such the manufacturing time of the product reduces. The saving in time provides a big competitive advantage for the SLM process.

4.3.8 Selective mask sintering Selective mask sintering (SMS) is an innovative AM process, based on the layerwise application and selective consolidation of a thermoplastic starting powder. The required thermal energy for melting the powder is supplied by planar infrared radiation. In addition to significantly improved speed this also results in a more homogeneous energy input, especially in comparison to laser-based AM. At the Institute of Mineral Engineering, material systems are being developed that allow the realization of 3D ceramic components using the SMS method. Initially, the powdered raw material is heated to a temperature slightly below its melting point and transported to the building platform by means of the so-called powder shuttle. The sword (bottom of the shuttle) will be opened there and subsequently closed again, whereby a flat and smooth powder layer is deposited. The cross-sectional area of the individual component layers is completely exposed in one process step to infrared radiation. By placing a mask between powder bed and infrared radiator, the irradiated area is defined for each layer. So far, the SMS method was used only for the production of plastic prototypes and components, in which materials were used with a maximum melting temperature of 250 °C. As ceramic materials have no significant thermoplasticity in this temperature range, a combination with polymers is required. Moreover, the powder-based SMS method requires a raw material, which can be applied to homogeneous layers. For this reason, a spray-drying process was chosen to produce ceramic–polymer composite granulates based on aqueous suspensions

4.3.9 Electrophotographic layered manufacturing The technique is a layer-by-layer manufacturing process that makes use of electrophotography. The powdered raw material is deposited using a charged photoconducting

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surface. Ashok Kumar is credited to the innovation of the process. The process is discussed next. The manufacturing employs a photoconductor drum. The surface of the drum is coated with the photoreceptive material. A charging roller is used to charge the drum. The photoreceptive material, acting as an insulator, makes the drum capable to hold the charge. A laser beam is projected onto the surface of the drum to discharge the selective regions of the drum. As a result, a latent image is formed on the surface of the drum. The powdered raw material is contained in the image developer. The powder is brought closer to the surface of the drum. Depending on the polarity of the powder, the powdered particles either jump onto the charged region or onto the discharged regions of the photoconductor drum. The image that is formed onto the surface of the drum is transferred to the top layer of the deposited powder on the build platform. To accelerate the pace of the transfer, a voltage can be applied to the build platform. Care must be taken to ensure zero relative velocity of the platform with respect to the drum. A heated roller then passes onto the deposited top layer. This heats the top layer to ensure sufficient green strength. The process is repeated until the desired thickness of the part has been achieved.

4.3.10 High-speed sintering Layered manufacturing has been in development phase since few years. The development has been credited to Loughborough University. The process has resemblance to 3DP and selective laser sintering (SLS) processes. However, the novelty of the developed process suggests that it can compete with the injection molding process. A radiation absorbing material first prints each layer instead of the expensive laser that dictates the sintered cross-sectional area. The basic working of the process is that the radiation absorbing material absorbs the radiation at a rate higher than that absorbed by the nylon powder. The process as such is also known as high-speed sintering (HSS). A number of advantages is offered by HSS over SLS. The machining cost is not only reduced with the elimination of laser within HSS but the speed of build is also reduced drastically. Radiation system equipped with infrared increases the product build speed. Elimination of laser results in removal of time-consuming scan and the process of sintering occurs in a time that is independent of the size, amount and shape of the 2D profiles in each layer. The advantages associated with HSS have resulted into its expansion in the field of RM.

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4.4 Solid-based processes The solid-based process makes use of nonpowdered form of raw material to build parts. The processes were developed in the early 1990s and since then have been an integral part of the RP industry. In the proceeding sections, we have discussed two most dominant techniques, that is, FDM and laminated object manufacturing. Both have been commercialized but improvements are still in place from both the suppliers and the academic institutions worldwide.

4.4.1 Fused deposition modeling As mentioned earlier, S. Crump is credited with the invention of FDM and Stratasys was the first company to commercialize the process in the year 1992. The process became so popular worldwide that Stratasys was able to surpass its record of number of units sold annually in the year 2003. Due to its cost-effectiveness, Stratasys was able to install more than 3,000 units of the machine in the cheaper end of the market globally. FDM process makes use of the 3D model produced in the CAD packages. The model is converted into a standard file format such as STL. The model is converted into number of slices using surface triangular algorithm. The STL format is taken in by the software. The software generates the tool path in the XY plane. The raw material is extruded from the nozzle. Separate set of nozzles are provided in the X- and Y-directions, thereby forming a layer in the XY plane. The process is very easy and can be easily operated. However, the diameter of the nozzle puts a limit on the accuracy of the part built. The resolution is also limited. The build speed is limited by the movement of nozzle to traverse the built area manually. Support structures are built by the nozzles, which can be latter on removed manually. In some cases, the support is made from water-soluble materials that then can be dissolved to be removed. The most commonly used standard materials for FDM are acrylonitrate butadiene styrene, polylactic acid polyamides, polystyrene and polyethylene. The capability of the process to handle the wide range of materials and its simplicity has made it an integral part of the RM industry. Aerospace, automotive and medical are some of the industries that have used FDM to their advantage. Few examples are that of gun mounts and pill dispensing machine. Contour crafting is an interesting development of FDM. This is used to create smooth and accurate planar surface. The process exploits the surface-forming capability of troweling. Mainly ceramics are used as the construction material that is extruded from the nozzle. A side trowel attaches to the extrusion nozzle. A smoother outer and top surface was created when the trowel traverses and the material is extruded. Nonorthogonal surfaces are created as the deflection of the side trowel takes place. The outside edges of each layer is only constructed by the process of

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extrusion. Filler material as such concrete may be required to be added after the completion of extrusion of each closed section of a given layer. 4.4.1.1 FDM process parameters Superior quality of parts has become one of the important requirements for any manufacturing process. As such the FDM process is one of the reliable AM processes that can deliver parts with superior quality, higher rates of productivity, shorter lead times and low cost of manufacturing. The process conditions for AM are required to be established for each application so the needs and requirements of customer can be met. Proper selection of the process parameters is the key to the success of the AM process. Hence, the process parameters must be able to yield optimum manufacturing results and therefore forms an integral part of the job of production engineers. Determination of optimal set of parameters is, however, a tedious task owing to the complex nature of the FDM process and as such the presence of large number of parameters that are conflicting in nature that influences the quality of the end product. The different process variables that are required to be studied and hence optimized in the FDM process have been depicted in Figure 4.1. Numerous optimization techniques have been employed to carry out the successful optimization of process conditions associated with the FDM AM process. The meaning of few FDM process parameters has been depicted in Figure 4.2. Description of few of the process parameters has been provided as follows: (i) Build orientation delineates the way in which the part under construction is oriented inside the build platform. The orientation is with respect to the X-, Y-, Z-axes, as depicted in Figure 4.2a. (ii) The thickness of the layer deposited by the nozzle tip is referred to as the layer thickness and has been delineated in Figure 4.2b. Size of the nozzle tip and the properties of the material determine the value of layer thickness. (iii) The gap between the adjacent raster tool paths is known as the air gap, as shown in Figure 4.2c. (iv) The angle of the raster pattern can be depicted through the raster angle. The angle is measured with respect to the X-axis at the bottom layer. For parts encompassing small curves, specification of raster angle is very important and the values typically ranges from 0° to 90°. (v) The width of the material bead is referred to as the raster width. A part will be built with stronger interiors when there is a larger value of raster width. While it is another topic of discussion that lesser time of production and material is required with smaller value of raster width. The size of the nozzle tip determines the value of raster width.

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Build material spool

Support material spool Drive wheels

Liquefier head (moves in X and Y directions)

Heating element

Extrusion nozzles Build material Foam base

Support material

Build platform (moves in Z direction)

Figure 4.1: Principle of FDM process (source: http://link.springer.com/journal/40436).

4.4.2 Sheet stacking technologies The innovation of laminated object manufacturing (LOM) is credited to Helisys, Corp led by Michael Feygin. LOM is a subtractive process as well as an additive process. Therefore, one can term it as a hybrid RP–RM process. The process is additive in the sense that layers are used to build the object and the layers are then cut to provide for the cross-sectional shape of the object; hence, the LOM process is subtractive as well. A layer is cut by using a laser. LOM is one of the fastest RP processes, which use paper, composites or plastics to build a part. The process is mainly employed for large size objects. 4.4.2.1 System hardware LOM 1015 and LOM 2030 are two of the LOM systems that are used for building up the parts. The two machines differ in the limit of dimensions of the part that can be

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(b)

ness thick Layer

(c) Contour width

Ai

rg ap

Number of contours

Contour-to-contour air gap

h dt wi r e st Ra

Raster angle

Perimeter-to-raster air gap

Figure 4.2: (a) Build orientations, (b) layer thickness and (c) FDM tool path parameters (source: http://link.springer.com/journal/40436).

handled. The LOM 1015 can build parts up to 10″ × 15″ × 14″, whereas the LOM2030 is larger and can build parts up to 20″ × 30″ × 24″. Both the machines employ the same technique to build the part. The construction material employed is paper. The final product has a wood-like texture and appearance. The thickness of the construction material used ranges from 0.0038″ to 0.005″, which is about the same as the thickness of two or three sheets of paper of notebook. There is heat-sensitive and pressure-adhesive backing for the build material. The LOMSlice™ is a software system that forms the basis of operation of the machines. This software provides a bridge for the communication between the machine and the operator. The STL file slices continuously, once the parameters are loaded into the LOMSlice™, and thus the machine does not requires the preslice of the STL file. The process of continuous slicing is called slice on the fly. The LOM is equipped with two spindles that feed the material and store the excess material. The spindle that feeds the material is the feed spindle, whereas the spindle that stores the excess material after the layer is cut is known as the take-up spindle. Use of adhesive is done to bond the different layers together. This is done by a heated roller that traverses across the face of the part. In doing so, the roller applies the required adhesive onto the layers and thus bonds the layers together. For cutting the excess material, an invisible 25 W carbon dioxide laser is employed.

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A focusing lens is placed on the carriage. The laser is focused onto the focusing lens with the aid of three mirrors. The three mirrors reflect the laser and focus it on the lens placed on the carriage. The movement of the carriage is in the Xdirection, while that of the lens placed on it moves in the Y-direction. This provides for the local cutting point, allowing the laser to behave as a plotter pen to cut the material to the desired shape. A metal part holds the part stationary, until the buildup process is completed. The plate itself is fixed by means of bolts to the platen. The plate moves in the Z-direction. The motion in Z-direction is provided by a threaded shaft. The entire machine can run on the household current. However, since the laser is employed for the cutting operation, smoke is inevitable during the process. Therefore, ventilation must be provided to the outside air. This can be accomplished by employing a large filtering device that is capable of venting the smoke at the discharge rate of around 500 cubic feet per minute. In case of LOM2030, a lifting truck is required for the loading of larger material rolls. Further, an overhead crane access for the removal of material is also required. This demands for a lot of operating space for LOM2030. 4.4.2.2 Process A brief description to build parts using LOM is discussed next. Paper is used as the building material for building the part. However, the process is similar for other construction materials. 4.4.2.3 Software Files must be converted to a certain standard format for being processed. Usually, STL file format is loaded into LOMSliceTM. Once the STL file is loaded, the software LOMSliceTM performs the different initialization functions which then are used for controlling the LOM machine. One can view the model on the screen. Several parameters need to be taken into account for carrying out the building process. The STL is loaded into which graphically represents the model on screen. 4.4.2.4 Part orientation The orientation of part while building the part is an important factor that needs to be taken into account. This is necessary to provide better finish to the part. In order to determine for the part orientation, the shape of the part needs to be evaluated and analyzed. Number of considerations should be evaluated. To have an accurate curvature is one of the first considerations that need to be factored in. With accurate curvatures come the added advantage of better laminar strength across the length of the part. The accurate curvatures can be had if the cutting operation is performed in the XY plane. This is because the laser can dimensionally hold better tolerances

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while cutting in the XY plane. A part should be held at an angle to the axes, if the part contains curvature in more than one plane. The time required to build the part is another important consideration that needs to be factored in while building a part. Most of the time taken during the buildup process is consumed while providing the desired thickness, that is, in stacking and bonding of the sheets to provide the desired thickness to the part. This is mainly due to the fact that more layers need to be placed at the top of finished layer by the LOM. The adhesive needs to be applied to bond the old and the new layers effectively. Therefore, in order to save time, the thumb rule is to place the lengthiest portion in the XY plane. An operator can scale, rotate or translate a part to orient the part to have better accuracy and hence better surface finish. There are some software packages available, which can orient the part to have better surface finish with least time of production. 4.4.2.5 Crosshatching Crosshatching is done to get rid of the excess paper. This is done for each layer individually. A range of layers to have a certain crosshatch pattern is set in LOMSliceTM by the operator. The crosshatch varies across the part. Easy part removal and the buildup time are the two main factors that are to be considered while providing the crosshatch. Operator cannot have a very small hatch size for large parts, as this will increase the buildup time considerably. Also, a larger crosshatch size for parts with complexities will make it a tedious process to remove the excess of material. This is the main reason as to why the operator needs to have a varying crosshatch size. From the discussion it is clear that one can set a larger crosshatch for the part that does not have intricate features or cavities. By doing so, a part can be build faster. However, a finer crosshatch is required for thin-walled sections. The same applies for parts with hollowed-out areas. The input for the crosshatch size is in terms of X- and Y-values. Hence, the hatch pattern can vary from squares to long, thin rectangles. An operator can visualize the part on the screen and determine where and how the part should be crosshatched. Alternatively, the simulations can be run to determine the crosshatch sizes for the layers. The crosshatch size values can be changed as the buildup process progresses. This can be done by stopping the operation and then typing the new crosshatch values. 4.4.2.6 System parameters There are some system parameters such as heater speed, laser power, support-wall thickness, heater compression and material advance margin. These are certain system parameters that can be set to the specific values for different ranges of the part to be built. These parameters are discussed in brief next.

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The percentage of the total laser output wattage is known as laser power. The laser power will differ from material to material and from machine to machine. Usually 2.25 W is usually required for operating the LOM 1015. However, the maximum wattage is 25 W. Thus, the laser power is about 9%. The rate at which the hot roller passes across the top of the part is termed as heater speed. The heater speed is measured in inches per second. The bond strength between sheets is dependent on the heater speed. Therefore, it must be carefully set. Also to build the part faster, different speeds for the initial and returning passes should be selected. About 6″ per second is usually selected as the speed for initial pass, whereas a speed of 3″ per second is selected for the returning pass. The distance the paper is advanced in addition to the length of the part is known as material advance margin. This is measured in inches. A compromise needs to be stuck to ensure that the scorched paper does not get included and the paper does not get wasted too much. Usually, a value of 1″ is selected; however, this can be changed to a lower value. The support-wall thickness is given to accommodate the part outside the build envelope. This controls walls of the outer support box. This is measured in inches. The value can be altered by the operator but it is recommended to not do so. A value of 0.25 is the typical value of the support-wall thickness specified in the Xand Y-directions. The operator can change this value, however, to suit the requirement. For example, a support-wall thickness of 0.15″ would be required to build along the axis if the part is 0.1″ long to be accommodated within the build envelope. The heater roller exerts certain pressure to strengthen the bond between the layers. The pressure is determined by the value of compression. The pressure is calibrated in terms of distance by which the roller is lifted from its initial trek by the top surface of the part. The value typically ranges between 0.015″ and 0.045″. This differs for varying materials and machines. 4.4.2.7 Technique Once the different parameters have been calculated, the buildup process for the part can begin after plugging in the calculated parameters. Utmost care must be taken for cleaning of the lens and mirrors before a buildup process is initialized. The feed spindle and the take-up spindle are located at the bottom of the machine. The feed spindle takes up the paper, which gets rolled over it. The paper then passes onto the platform and then the take-up spindle takes down the paper. The feed of the paper is monitored with the aid of an electrical device attached to the spring-loaded wheel. The assembly to monitor the feed is placed against one of the rollers. The monitoring system avoids the wastage of the construction material by allowing only the necessary amount of paper that is required to cover the crosssectional area of the part to be built. A plate mounted on the platform is raised to

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the position known as the home position. Laser is employed to cut out the paper. The paper is cut to represent the largest cross-sectional area of the part to be built. The blank is then removed. Next, a layer of double-sided foam tape is applied across the build plate. The foam tape is then cut with the aid of laser to match the previous blank size. The support base of the tape serves the dual purpose. It makes the removal of the part easier and second provides a support to the part to be built. The first purpose of the tape is made clear in the fact that if the part was to build directly on the plate, then there would have been chances of the part adhering very tightly to the plate. Further, it protects the part from being damaged when it is chiseled for removal. For strengthening the support further, few layers of paper are added onto the support base of the tape. Typically 5–10 layers are added. Apart from providing for a rigid support, it also provides for checking the various parameters of the machine. The operator can assure that the machine is functioning as it is intended to be. Operator can check other parameters related to the bonding of layers, functioning of heated roller and so on. After the initial setup is completed, the LOM machine starts building the part. The part is built from its bottom. The building proceeds according to the directives from the software. Gradually, the paper is fed from the feed spindle in the required amount. The spindle turns to provide for the new paper across the build plate. Next, the heated roller senses the top of the paper. It then moves across to apply the adhesive material. The compression of the roller applies the adhesive. The roller then makes a return pass to its home position. This completes the application of adhesive material. The laser is directed to a focal point using lens and the mirror setup. The laser behaving as the plotter pen cuts through the outline of the current cross section of the part. To avoid the wastage of material, it must be ensured that the layers cut out from the sheet of paper must have uniform cross section. This is also necessary to prevent the excess material from adhering to the part. This could be easily managed for the parts with uniform cross section. However, if the part to be built is of nonuniform cross section, then there will be extra material that could be used to fill the remaining rectangular area. The wastage can be avoided by employing nesting. Nesting is the strategic placement of the other parts in the STL file to fill into such voids. If this is not possible then in order to prevent the excess material to adhere to the part, the excess material can be cut into small crosshatches. Similar ones can be used for internal cavities. A support wall, as mentioned earlier, is formed using two rectangular perimeters. The rectangular perimeter encompassing the outer edge is cut, thus forming one of two rectangular perimeters. Another rectangular perimeter is made at an offset distance to the outer rectangular perimeter, thereby forming a support wall with the outer one. Within the support wall lies the crosshatched cubes and the part or the parts.

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Once the final cut is completed, the platform is lowered. The excessive material is then taken up by the take-up spindle. As the spindle rolls, the fresh layer of paper comes onto the surface through the feed spindle. In order to ensure continuous feeding of the paper, the sheet between the two spindles is not served full. There is always a wide paper margin outside the build area. Thus, the excess material consists of the remaining margins of the cutout layers. This is what is taken up by the take-up spindle. This excessive material will be discarded. The remaining process continues to build the part by supplying fresh sheet of paper onto the previously built layer. The process is terminated when the part with the desired thickness has been built. The completed part is in the form of a rectangular block. The support wall with the crosshatched cubes needs to be removed to have the finished product. The part along with the plate is detached from the LOM machine. The part is removed from the plate thereafter. The crosshatched cubes are removed with the help of hands. A citrus solvent is sprayed to remove any material sticking to the foam support layer. The plate is cleaned before it is again attached to the machine. The new part then can be built on this plate. The finishing process has been discussed briefly in the proceeding section. 4.4.2.8 Finishing The process of decubing is followed next to reveal the actual part. The process removes the support base, crosshatched cubes and support walls from the rectangular block of the part generated by the LOM machine. However, the technique varies from person to person but a procedure can be used to generalize the process. This is discussed next. Angular block is turned upside down on a table. The support tape and layers are then removed with the aid of tools such as chisel. The outer crosshatches can be removed with the help of fingers. In some cases, they fall apart automatically. However, care must be taken while removing the crosshatched cubes from the internal cavities and the other sensitive areas. Otherwise, damage can be caused to the final product. Sealing is necessary to prevent moisture from getting into the finished part. This is because excessive moisture can distort or delaminate the part. Therefore, after decubing, the part can be sealed and painted accordingly. Standard wood sanding sealer is used for paper parts. The sanding sealer serves the purpose of providing a better surface finish by removing any remaining effects of the crosshatches. After sealing is complete, parts can be painted. 4.4.2.9 Advantages and disadvantages As can be seen throughout the process, LOM has the ability to produce large sized parts with the use of inexpensive construction material such as paper. Further, the

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parts produced have got better surface finish. The production time involved is relatively less. The materials used are eco-friendly and therefore are not injurious to health. However, everything comes with a certain disadvantages too and LOM is not an exception to this. The applying of adhesive through heated roller produces fumes that can be a nuisance for the people who are not accustomed to it. Also the process of decubing is time-consuming and labor-intensive.

4.5 Bioprinter One of the layer-by-layer manufacturing or RP processes that have been in use particularly for cell and tissue engineering applications is the 3D bioprinters. 3D bioprinters are capable of constructing very intricate structures. Various types of cells and scaffold structures can be obtained in the desired 3D pattern. Its potential has made it an integral part of the medical engineering applications. 3D bioprinters have the potential to deposit living cells and biochemical molecules such as proteins simultaneously at the desired location. Further, the process has the capability to create specially designed microenvironment. The 3D pattern created greatly resembles the native tissue architecture. 3D bioprinters can control the way in which the stem cells are deposited. Thus, it is a versatile tool, which allows for the microenvironment to adjust itself according to the deposition of stem cell. Different stem cell environment have been generated using 3D bioenvironments, for example, the patterning of stem cell in a single cell level in an efficient and reproductive manner. The culture environment then goes onto controlling embryonic formation. This has become possible because of the flexibility and power of the 3D bioprinters. It is also important to develop visualization techniques in order to properly observe stem cell fate, interactions among different cells and structural features of an engineered tissue. This is because the 3D printed structures are made of scaffolds that are opaque, cells that are numerous and aggregate themselves in the form of dense cloud. Destructive techniques such as western blots and quantitative polymerase chain reactions have been in use to gain a biological understanding of the stem cell differentiation and hence its functions. But with the currently available techniques of visualization, it becomes very difficult to monitor the stem cell interaction with its tissue environment. This is because the monitoring needs to take place nondestructively. This also necessitates the need for unperturbed environment. Hence, this calls for the need of sophisticated visualization techniques. In the proceeding sections, a review of different bioprinting technologies used for tissue engineering has been made. Next, the potential of each technique in the field of tissue engineering has been done. Biomedical imaging modalities have also been discussed briefly. The vast array of imaging techniques used has been elucidated.

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Advantages and disadvantages of the different techniques of the stem cell engineering have been summarized. The ways in which the different bioprinting techniques can be integrated to different imaging techniques conclude the discussion on bioprinters.

4.5.1 Three-dimensional bioprinting techniques 4.5.1.1 Ink-jet-based printing This is one of the first types of bioprinter. The biological ink replaced the printing ink of the 2D ink-jet printer, which converted the commercial printer into a bioprinter. There is now a wide variety of bioprinters available to handle the wide range of biomaterials. The printers operate at a very high resolution and speed. Thus, the accuracy and the time saving add to the advantages of these printers. The different versions of ink-jet printers include thermal, piezoelectric and microvalve. These differ in the way the drops of printing material are generated. In all these versions, a very small volume of aqueous biological materials are dispensed in the form of drops, and the other form is that of extrusion-based printing system. 4.5.1.1.1 Printing mechanism Increasing the temperature of the heating filament to generate pulses of pressure is the basic principle underlying the functioning of thermal ink-jet printers. The pressure pulse leads to the production of droplets of bio-ink. The temperature of the heating element is raised to about 200–300 °C. However, even because of such high range of temperature, the postprinting of mammalian cells does not get influenced. On the other hand, the piezoelectric ink-jet printers employ piezoelectric crystals. Vibrations are generated inside the printer head when electrical signal is applied to the piezoelectric crystals. The vibrations lead to the breaking of bio-ink to droplets, which is then forced out of the nozzle. The size of the nozzle in both the thermal and piezoelectric printers determines the size of the bio-ink droplets. Further, in addition to nozzle size, the amplitude of vibrations in piezoelectric printers also plays an important role in the droplet dimension. Viscosity of the bio-ink is another factor that needs to be factored in. The force required to eject droplets depends on the viscosity. Higher the viscosity higher is the force required to eject the ink droplets. The microvalve-based ink-jet printers include a valve ball, valve coils and closing elements in the printer head. There is a wide range of microvalve printers to suit different applications. However, the mentioned components are common to all the ranges of microvalve printers. Additional pressure derived using pneumatic arrangements is applied on the biomaterial that is loaded inside the printer head. The electrical signal triggers the valve coils. This further leads to the lifting of valve ball to dispense a droplet. The factors that affect the droplet size are the duration for

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which the valve opens, the frequency of actuation, magnitude of pressure applied and last but not the least is the viscosity of the material. 4.5.1.1.2 Resolution and patterning capability The typical resolution of an ink-jet bioprinter ranges between 50 μm and 1 mm. The resolution of the printer depends on the minimum size of the droplet. However, the deposition rate ranges between 1 and 10,000 droplets per second. Therefore, there are chances that two consecutive droplets may get merged. Hence, the resolution is generally less than the minimum size of the droplet. There are number of factors on which the droplet size depends. These have been discussed in the previous section. The size of the droplet and the deposition rate can be controlled electronically to obtain the desired level of resolution. The droplet size ranges from picoliter to nanoliter. Adjustments to density of cell suspension and droplet size can lead to the printing of single cell. 4.5.1.1.3 Available materials Biocompatible and cytocompatible materials are used in bioprinting. Further, they should have properties to provide for structural integrity. Also the intended functions should also be fulfilled. However, one is always skeptical of the viability of the end products produced using bioprinters. The doubt about the intended function has limited the range of available mechanical, operating temperature, rheological properties and the chemistries. Commonly used scaffold materials include fibrin, collagen, alginate, chitosan and other naturally derived hydrogel. Also some of the synthetic polymers such as polyethylene glycol are available as a construction material for the scaffold. The hydrogel naturally derived polymers are printed in aqueous precursor form. Postprinting processes such as cross-linking or gelatin are used to solidify the aqueous form. Some of the other postprinting process includes enzymatic cross-linking and photo-cross-linking. Naturally derived hydrogels have the sole advantage of only supporting biological functions. But synthetic polymers in addition also provide for tailoring of scaffold. They are also cost-effective and have the reproducibility capability. Since hydrogel polymers are meant for supporting, they must possess high mechanical strength. The higher mechanical strength ensures structural integrity during and postprinting. The mechanical strength depends to a great extent on the viscosity of the polymer. Higher the viscosity higher the mechanical strength it associates with itself. Also concentration is another factor on which the mechanical strength depends. The higher the concentration of the polymer, the more sturdy and rigid the structure will be. In some cases, it can lead to high resolution. There may be cases such as that of embedded cells, which need to degrade the matrix in order to migrate and proliferate. Therefore, these conditions may not be beneficial. Therefore, the mechanical properties must be such that a balance

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between structure integrity and preferred cell culture conditions is maintained. This will also help in optimizing the results. The ink-jet printers can handle most of the hydrogel polymers. There are a number of advantages of ink-jet-based printing such as that of use of vibrations or pneumatic pressure to produce a gentle ejecting force. This further minimizes cell death, functionality loss or phenotype alteration. However, the ink-jet printers find it difficult to handle material with high viscosity. This is because high viscosity materials can clog the printers printing head. This can further lead to irregular printing patterns. Proper ejecting force is required to eject highly viscous materials. This can be achieved in the case of microvalve-based printers. 4.5.1.1.4 2D and 3D cell printing applications The printing mechanism is capable of producing gradient patterns of cells. Further, soluble growth factors can be generated too. This can be achieved by either altering the droplet size, spacing between droplets and frequency of printing. Thus, for dispensing cell suspensions and soluble growth factors, the ink-jet printers are more beneficial to use. Also, ink-jet bioprinters find application in generating vascular tissue models and 3D in vitro coculture models. This is the direct cell dispensing application of the ink-jet printers. 4.5.1.1.5 Advantages and disadvantages There are numerous advantages that ink-jet printers have. These include costeffectiveness, higher production speed and last but not least the ability to handle a wide range of biocompatible and cytocompatible materials. The ink-jet bioprinters are generally suited for materials with low viscosity. The postprinting viability is really high as the printers make use of gentle force to dispense materials. They have shown great potential in handling delicate biostructures, for example, progenitor cells. However, exposure to thermal conditions is a bit of concern for such printers. The thermal stress can lead to damage of cells and loss of functionality of the biostructure. The ability to handle less viscous material limits its capability. For building of bioprinting platform, generally aqueous form of hydrogel is used. Postprinting processes such as cross-linking and gelatin process are required. All these processes produce UV radiation and temperature changes. The required physiological cell density is difficult to achieve which puts further limitation on the potential of ink-jet bioprinters. The high concentration of material cannot be used for generating biostructures. These can lead to clogging of nozzle, irregular dispensing trajectory and others.

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4.5.1.2 Extrusion-based printing For nonbiological 3D printing, the most commonly used technique is that of microextrusion. The printing is performed in AM manner. The material comes out in the form of filament from the heated printer head. The technique has become widely accepted in the field of bioengineering. The technique finds application in hard tissue replacement/regeneration. Further, it is also used for designing of scaffold. 4.5.1.2.1 Printing mechanism Three-axis robotic stages and mechanical or pneumatic dispensing system are the major components of extrusion-based bioprinters. Materials in the extrusion-based bioprinters are forced to come out in the form of continuous filament with the application of continuous pressure. Mechanical and pneumatic systems have got their benefits and limitations. Since mechanical dispensing system can avoid the delay in gas compression; hence, it has good control over the extrusion of material from the nozzle. Further, mechanical dispensing systems can handle highly viscous materials as opposed to the pneumatic systems. On the other hand, pneumatic systems can handle a wide range of materials as opposed to that of mechanical dispensing systems. Pneumatic systems also provide for a large range of dispensing pressure. Whereas there is a limit to the range of dispensing pressure in case of mechanical systems, since it contains complex components. 4.5.1.2.2 Resolution and patterning capability The typical resolution of extrusion-based bioprinter ranges between 5 μm and 1 mm. The diameter of the extruded filament decides the resolution of extrusionbased bioprinter. There are number of factors that decide on the dimension of the extruded element. These include the size of the nozzle orifice, deposition speed, the magnitude of pressure applied and on the mechanical properties of the bioink. The typical value of printing speed varies between 10 and 50 μm/s. The moving capability of the robotic motors determines the speed of deposition of the filament. The printing speed is also critical to determining the filament dimension. Since it is very challenging to maintain cell viability during printing; hence, it becomes difficult to print biostructures of millimeter or centimeter scale. This is one of the major obstacles for extrusion-based printing. 4.5.1.2.3 Available materials Materials with wide range of viscoelastic properties are supported by the extrusion based bioprinters. Also hydrogels and other biocompatible materials are easily handled by the extrusion-based bioprinters. However, since the magnitude of applied pressure to obtain the filament of material is relatively large, the handling of material with low viscosity is the area of concern. Cells need to be encapsulated in order

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to have direct cell printing. For bioprinting platforms, the materials that have the shear thinning properties can be employed. 4.5.1.2.4 2D and 3D cell printing applications Some of the applications of extrusion-based bioprinters are fabrication of cardiovascular tissue structure, 3D cancer coculture models, and in vitro multilayer tissue models. These are few applications but due to wide array of material handling capability, the extrusion-based process has shown a great potential in tissue engineering. The required level of physiological density and cell matrix can be achieved using the technology. This is because of the capability of the extrusion-based process to handle highly viscous materials. The extrusion-based processes have the potential to build interconnected channels for sufficient nutrients and oxygen supply, porous scaffolds and other perfusion biostructures. Extrusion-based bioprinters can deposit cell spheroids at desired locations of the desired 3D structure. 4.5.1.2.5 Advantages and disadvantages Extrusion-based bioprinters have the potential to handle biocompatible materials with high viscosity and high density. With these bioprinters, the desired level of cell density and matrix can be achieved, which is generally beyond the capabilities of many bioprinting techniques. Extrusion base printers are capable of generating cell–hydrogel mixture with highly dense population of cells they allow for direct printing of cells. Further, spheroids can be deposited at the desired location in the cell matrix. This allows for self-assembling of the spheroids and their fusing obtaining the desired 3D biostructures. The cell viability, postprinting is an important factor that limits the capability of the extrusion-based bioprinters. The higher dispensing pressure and increased shear stress leads to more cell deaths when compared with other bioprinting techniques. Generally, the cell viability is lower in comparison to other techniques. Other types of cell damages such as phenotype alterations of stem cells and progenitor cell death are the most critical limitation of current extrusion bioprinting. Loss of functionality further adds onto the limitations of extrusion-based bioprinting process. 4.5.1.3 Laser direct write 4.5.1.3.1 Printing mechanism Laser energy is utilized in laser direct write (LDW) to force the hydrogel droplet to a growth surface. Thus, it is a noncontacting technique of the depositing material. The laser transparent print ribbon and a receiving substrate are the two main components of the LDW technology. The transfer layer and the sacrificial layer are both present on the transparent print ribbon. The laser with the configurable energy and

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frequency is directed toward the print ribbon. The laser energy is absorbed by the sacrificial layer. A vapor pocket is formed at the ribbon–material interface as the sacrificial layer volatizes. The expanding of the vapor pocket takes place rapidly to eject a droplet of the transfer layer to a receiving substrate. The modifications to the laser properties can control the transfer of material to the receiving substrate. Only a small amount of laser energy is transmitted to the deposited material, as the amount of material transfer always exceeds the transfer of heat. The laser interacts with the sacrificial layer only. The sacrificial layer must adhere to the transfer layer properly. The technique uses a cellular suspension using cultured mammalian cells resuspended in a medium or a noncytotoxic hydrogel. In order to dampen the energy of the falling droplet, a hydrogel layer is uniformly applied onto the surface of the receiving substrate. The prepared cellular suspension is evenly distributed onto the surface of the sacrificial layer. These preparations promote the deposition of 2D cell pattern with a high resolution. One can also accomplish the custom placement of cells and other biopayloads in a 3D isolated microenvironment. This includes alginate deposition. The receiving substrate can also be coated by calcium chloride. This coating helps in cross-linking of the hydrogel cellular suspension into a 3D microbead. The typical hydrogelcontrolled microenvironment enables release or sequestration of the biopayload. Direct-written micromedia, growth factors and waste products can be allowed to diffuse in and out of this microenvironment through microbead fabrication. 4.5.1.3.2 Resolution and patterning capability With the aid of CAD and computer-aided manufacturing technology (CAM), a very high level of resolution and precision can be achieved for spatial patterning. The LDW has the capability to automatically monitor the movements related to the transfer layer and the receiving substrate. There is no limitation to the size of transfer material as is the case with other techniques. The size can be easily controlled by varying the laser parameters. Further, the LDW system has a camera lens that is coincident with the path of the laser. This enhances the visualization capability. One can easily monitor the placement of the transfer material onto the receiving substrate. The assistance of CAD–CAM has enhanced the resolution and patterning capability to within the microscopic levels. 4.5.1.3.3 Available materials for LDW A wide range of materials can be used for printing of different biostructures. The materials include polymer-based biomaterials and nucleic acids. Hydrogels such as matrigel and gelatin can be used to form the sacrificial layer. Even some of the metals can also be used for the same. Each has been successful for multiple cell types. For transfer layer, the available materials include alginate, gelatin and glycerol. For the receiving substrate, a large number of materials are available but for cushioning

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effect to damp the kinetic energy of the transfer layer, a hydrogel layer is often applied to its surface. For substrate coating, typically matrigel is used. 4.5.1.3.4 2D cell printing applications The LDW technique can be used to deposit mammalian cells in prescribed patterns. Other applications include deposition of epithelial cells, fibroblasts, endothelial cells and neuroblasts. The technique has also been used for studying of stem cells. Specifically, mesenchymal stromal cells (MSCs), human embryonic stem cells (hESCs) and mouse embryonic stem cells (mESCs) have also been printed successfully.

4.6 Conclusion RP has changed the perception of companies in the way they design and build their products. Numerous advancements have been made in the domain of RM and have revolutionized the manufacturing industry. One such advancement relates to the increased speed. The machines in the realm of RM are slow in terms of speed. The build time has reduced drastically with the faster and complex systems and enhanced materials. Continual reductions in the product buildup time will ultimately make RM economical for a wide variety of products. Another area requiring exhaustive research is the improvements in surface finish and accuracy. As of today, commercial machines have accuracy as close to 0.08 mm in the x–y plane. However, the accuracy in the z-direction is much lesser. The surface accuracy can be improved in all directions with the enhanced laser optics and laser control. Also, the companies into RM are developing polymers that are less prone to curing and temperature-induced warpage. The introduction of other nonpolymeric materials, composites, ceramics and metals has opened another frontier for open research in the domain of RM. With such introduction, RM users can produce varied functional parts. In the present-day scenario, the plastic prototypes work well as far as visualization is concerned but does not fair well as far as testing is concerned. Therefore, the introduction of rugged materials can aid their prototypes to be tested well. The RM capabilities can be expanded with the introduction of metal matrix composites and other metallic. Another critical advancement is that associated with the increased size capacity. Currently, the size of RM machines is limited to objects of dimensions of 0.125 m3 or less. Larger parts are required to be joined manually. The problem has been remedied with the large prototype techniques. The aforementioned improvements have aided the continual expansion of RM industry.

Chapter 5 Mass manufacturing from rapid prototyped products 5.1 Introduction Rapid manufacturing (RM) is often considered to be the evolution of recently developed prototyped techniques that aid in directing the manufacturing process toward a more mature process. However, the research in this direction still remains elusive. With the continual advancements, the applications of rapid prototyping (RP) have been expanded to industrial applications. The potential of expanding RP business can be accessed through the identification of possible advantages and disadvantages. Identification and removal of weak points should then be able to enhance the usability of RP process. This could be achieved through efficient dissemination, marketing and education.

5.2 Casting processes 5.2.1 Investment casting In case of investment casting process, the wax pattern is coated with a refractory material as such ceramic material. The internal geometry takes the shape of casting when the hardening of the ceramic material takes place. Wax is then melted out and then the molten metal is poured into the cavity. When the molten metal solidifies within the ceramic mold, the casting is broken out. The process of manufacturing is known as a lost wax pattern. Development of investment casting dates back to both India and Egypt. Some of the prominent industrial parts manufactured through investment casting process are turbine blades, ratchets, gears and dental fixtures. 5.2.1.1 The process Fabrication of wax pattern is the beginning step for the investment casting process. The pattern for this purpose may be made from plastics; however, mostly the pattern is constructed from wax so that the molten material may be melted out easily. The pattern is destroyed after the process and as such one will be needed for production of each casting. The size of the die must be calculated judiciously. Calculations should take into consideration the shrinkage of wax, ceramic material and the metal casting. It should make some trial and error in order to obtain the right size. As such the expensiveness of the mold cannot be ignored. https://doi.org/10.1515/9783110664904-005

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Since the mold does not need to be opened, castings of very complex geometry can be manufactured. Several wax patterns may be combined for a single casting. Or, as often the case, many wax patterns may be connected and poured together producing many castings in a single process. This is done by attaching the wax patterns to a wax bar; the bar serves as a central sprue. A ceramic pouring cup is attached to the end of the bar. This arrangement is called a tree, denoting the similarity of casting patterns on the central runner beam to branches on a tree. The metal casting pattern is then dipped in refractory slurry whose composition includes extremely fine-grained silica, water and binders. A ceramic layer is obtained over the surface of the pattern. The pattern is then repeatedly dipped into the slurry to increase the thickness of the ceramic coat. In some cases, the pattern may be placed in a flask and the ceramic slurry poured over it. The coat of refractory material casted over the pattern is thick and is allowed to dry in air so that it gets hardened. The next step in this manufacturing process is the key to investment casting. The hardened ceramic mold is turned upside down and heated to a temperature of around 200–375 °F. This causes the wax to flow out of the mold, leaving the cavity for metal casting. Heating of the ceramic mold takes place at temperatures ranging from 1,000 to 2,000 °F. This results in strengthening the mold and eliminates any leftover wax or foreign material. Higher heating temperature range also allows the water to escape from the mold. The molten metal is poured while the mold is in the heated state. Pouring the casting with molten metal when the mold is hot allows the liquid metal to flow through the mold cavity easily and fills the thin sections very easily. Better dimensional accuracy is achieved when the molten metal is poured in the heated state. The molten metal once poured into the mold is allowed to solidify and as such the solidification process starts. As a final step to the manufacturing process, the ceramic mold is broken and the parts are cut out from the tree. 5.2.1.2 Investment casting design parameters In the following section, the dimensional and structural topics in relation to investment casting are discussed. The dimensional and structural properties are proportional to the cost of the final casting. The rule is valid for fabricated parts, machined parts and that made from investment casting. Investment casting produced parts with close dimensional tolerances with minimal cost. There are number of factors that affect the parts to have close dimensional tolerances. The factors play a critical role in determining the tolerance band for the investment casting products. The factors can be controlled to a great extent by foundry, however, are inevitable. Other than foundry, machining can also achieve closer tolerances. In order to achieve

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higher yield of production and thus the reduced cost per casting, a review of the design to permit the expansion of tolerance band, undercuts and so on can be undertaken. Further, the machining required for the parts produced from investment casting is substantially less in comparison to the parts produced from other conventional machining techniques. Another advantage of investment casting is its ability to add radii and other details without the involvement of much cost. Thus, the process is less expensive in comparison to other traditional techniques as far as adding details is concerned. 5.2.1.2.1 Variables There are a number of factors that affect investment casting tolerances. The tolerance is determined by the casting size and shape. The factors affecting tolerance band are wax temperature, wax die tie temperature, cooling rate of metal, firing temperature of the ceramic shell, the composition of the ceramic mold refractory, the magnitude of pressure that needs to apply to inject wax into a die and many more. 5.2.1.2.2 Standard linear tolerances For investment casting, a normal linear tolerance is taken to be ±0.005 of an inch per inch or ±0.125 of an mm per 25 mm (Table 5.1).

Table 5.1: Partial listing of dimensions and tolerances. Dimension

Inches

Dimension

mm

″ up to ″

±.

 up to .

±.

″ + up to ″

±.

. + up to 

±.

″ + up to ″

±.

. + up to 

±.

″ + up to ″

±.

. + up to 

±.

″+ up to ″

±.

. + up to 

±.

″ + up to ″

±.

. + up to 

±.

″ + up to ″

±.

. + up to 

±.

″ + up to ″

±.

. + up to 

±.

″ + up to ″

±.

. + up to 

±.

Each inch after

±.

Each  mm after

±.

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5.2.1.2.3 Flatness The word flatness is often confused with the term straightness. The tolerance zone defined by two parallel planes is the flatness tolerance (Table 5.2). It is within these parallel planes the surface must lay. The distance between two parallel planes must be minimal. The volumetric shrinkage of the wax pattern and the liquid shrinkage determine the degree of flatness in an investment casting. It is the center of the casting where shrinkage takes place. The term “sink” is used to denote such type of shrinkage. Table 5.2: A rough guide for the flatness tolerance. Section thickness

Volume of section

Possible sink per face of casting



Inches

In

. .  

.   .

Millimeters

mm

   

   

Not significant . . .

Not significant . . .

There are a number of methods of measuring flatness. These include surface plate and feeler gauge. The purchaser should specify the method of measuring flatness. 5.2.1.2.4 Straightness The zone within which an axis or the part under consideration must lie is known as the straightness tolerance. It is a usual practice to measure the tolerance zone within which the axis or the axial plane lies in order to correctly measure the axial straightness of a shaft, bar or plate. There are a number of problems with the straightness for some types of castings. For example, a casting that is relatively thin and short parts will bend while the castings that are heavy and consist of long parts may not bend at all. Thus, it is a tedious task to know the extent to which the casting will bend. One may, however, be sure that a casting will bend or not. This comes out of design experience.

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5.2.1.2.5 Parallelism For a given datum plane, if all the points on a surface are equidistant from the datum plane, then one can say that the condition of parallelism is fulfilled. The same may apply for an axis, in which case the parallelism is verified using a datum axis. It is a tedious task to maintain parallelism in the as-cast parts. This is because the parallelism being the function of complexity of part. 5.2.1.2.6 Roundness Any casted circular element must lie within two concentric circles that specify the roundness tolerance zone. The tolerance value is obtained by taking half the distance between the maximum and the minimum condition or is the total indicator reading (TIR) when the part is rotated to 360º. It is up to the purchaser to specify the inspection method. However, the TIR method is preferred over the former method of inspection. 5.2.1.2.7 Concentricity If two or more features such as cylinders, cones, spheres and hexagon have the same axis, then it is known to be the condition of concentricity. Any difference between the distance of axes of two parts results in eccentricity. It should be kept in mind that concentricity is not affected by the roundness in diameter; it is the axis that is considered for concentricity of parts. The concentricity is greatly affected by the straightness. This is particularly the case if the casting has a shaft or a tube feature. 5.2.1.2.8 Angularity Angularity is the measure of an angle made by the surface, center plane or axis with respect to a datum plane or a datum axis. In certain situations, it should be known that some straightness or certain reworking of die may be required to maintain the angularity of the component. The angle-forming configurations decide on the angular tolerance that must be given. 5.2.1.2.9 Positioning The parent casting configuration determines the position tolerance. There is much deviation in position tolerance for a part that is unsymmetrical while for the parts that have symmetric configurations yield the best positioning tolerances. Some of the restricting features that affect in proper shrinkage of the casting affect the position features around them. These restricting features are holes, slots or pockets. Because of the restricting features, the linear dimension may be affected approximately to about 5%. The volumetric shrinkage of heavier sections can affect the other features near to them. The features nearer to the heavy sections will get distorted in the direction of

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heavy features. However, it is a tedious task to determine the amount of distortion that these features will face. 5.2.1.2.10 Hole tolerance It is a tedious task to maintain the roundness of hole in a casting. If the surrounding metal has uneven distribution of mass, then the hole may be pulled out of its roundness. However, standard tolerances of as close as ±0.005″ (0.013 cm) can be maintained for a diameter of 0.5″ (1.3 cm) if the mass of the surrounding metal is uniformly distributed. The effect of pull will be more pronounced if a hole is longer and there is more mass around it. This is the case where the dimensions of the top and bottom of the hole are true, whereas the center will have a relatively large diameter. The hole shrinkage concavity is unavoidable. For larger thru holes that require clearances, this can be ignored, if the diameter at the center is not taken into consideration. This situation of hole shrinkage concavity can be avoided if the sidewalls of the hole are used as bearing surfaces in which case the simple ramming would be sufficient. Sketch “B,” on the other hand, shows the effect of heavier sections on the hole diameter. The diameter of the hole is affected due to shrinkage of the adjacent heavier sections. 5.2.1.2.11 Curved holes The normal tolerances tend to double in case of curved holes. This is because these are formed using soluble wax or preformed ceramic cores. The tolerances associated with all the dimensions are multiplied by 2. An additional dimensional tolerance of ±0.005″ per inch is added, since such holes cannot be sized. 5.2.1.2.12 Internal radii, fillets Widest tolerances are required for internal radii and fillets. This is because the presence of internal radii and fillets reduces the shrinkage and cracking. Further, these improve the integrity of the casting. However, it is difficult to control the sizes of internal radii and fillets and require frequent checks. 5.2.1.2.13 Contours, radii and cams External radii and contours are affected due to volumetric shrinkage that occurs during cooling. This fact should be considered when dimensioning a casting radius. It is the center of radius where most of the volumetric shrinkage occurs. A profile tolerance must be used for the contours, radii and fillets. The volumetric shrinkage in case of a concave radius will occur in the center and the outer extremities. The result is the decrement in radius. In this case, the profile tolerance must also be used with the basic dimensions for the radius.

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The use of profile tolerance also applies to the convex radius. The radius in the case of convex radius is increased due to volumetric shrinkage at the center. 5.2.1.2.14 Surface texture As defined in ASNI-ASME B 46.1-1985 “Surface texture includes roughness, waviness, lay, and flaws.” The unit for measurement of surface roughness is roughness average denoted by Ra. Profilometer measures the surface roughness. For ferrous alloy, the typical value of surface roughness ranges between 125 and 150 Ra, and it is around 180–200 Ra for aluminum castings. Postfinishing processes can improve the surface texture. 5.2.1.2.15 Perpendicularity A line that is at 90° from the datum plane or datum axis is said to be perpendicular. Generally, the longest plane for reference is used for specifying perpendicularity. The longest reference plane establishes three tooling points. The shortest surface will be perpendicular to the longest within 0.005″ per inch of length of the shortest surface.

Example: Length of B = 3″ 0.005″ × 3″ = 0.015″ Therefore: Within 0.015 TIR, surface B should be perpendicular to surface A. Mechanical straightening can be used for some improvement on the tolerance.

5.2.1.2.16 Casting integrity The casting integrity will be sacrificed in pursuit of producing casting with the greatest number of details. This will, however, reduce the overall cost. The casting integrity is, however, customer dependent and should be thoroughly discussed with the foundry. There are certain guidelines related to casting integrity that are discussed next. These will also aid in producing a sound casting. – Directional solidification There will be solidification of the molten metal after it is poured into the mold cavity. Thus, shrinkage will occur. To obtain a sound casting, it is required to feed the molten metal where there is a decrement in the volume. Directional solidification is used for feeding the molten metal to the solidifying areas. This also helps in keeping a check on the casting integrity.

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– Wall thickness The material and the distance that the molten metal can flow decide on the achievable minimum wall thickness. The molten metal initially has a very high temperature before it enters the mold cavity. As it flows down the cavity, the molten metal loses heat continuously. The molten metal will not be able to fill out all the details of the mold, if it loses out enough heat. The ratio of the mass of a metal to the mass of a shell is less for a thinner section; therefore, the molten metal is cooled relatively faster in comparison to when it enters the thicker section. This may result in improper filling of the thin section. Higher temperature is maintained when the molten metal fills a thicker section. This is because the thick castings have more mass. This will result in proper filling of the casting and hence a sound casting. For a length of 1″, wall thicknesses of 1/8″ are possible. In case of aluminum, the wall thickness can be as thin as 3/32″ over a distance of 2″ in length. – Radii Part strength and casting ability are improved to a great extent by providing radii. Further, radii help in improving the solidification of casting also. For internal or external corners, radius of 1/64″ to 1/32″ is generally preferred. A recessed corner can be provided where a casting mates with other part that has sharp outside corner. By providing recessed corners, the strength of the part improves greatly. – Engraving Engraving of letters and other engraved features should be done on a recessed pad. This will ensure that the engraving is below the surface of the part. Further, the engraved features will be produced without defects. However, recessed features affect the function of the part. – Hole length The integrity of a casting is greatly affected by the hole features. Thus, the ratio of hole diameter and its length is a very important consideration that should be determined precisely (Table 5.3). A proper diameter-to-length ratio will ensure the best possible casting result. 5.2.1.3 Advantages and disadvantages of investment casting Advantages: – Excellent surface finish – Tight dimensional tolerances – Complex and intricate shapes may be produced – Capability to cast thin walls – Wide variety of metals and alloys (ferrous and nonferrous) may be cast

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Table 5.3: A rough idea of the diameter to length ratio for different range sizes of the hole. Type of hole

Ranges of size

Diameter-to-length ratio

Blind thru

/ and up / to / / to / / to / / to   and up

:–/ : :–/ : : :

– Draft is not required in the molds design – Low material waste Disadvantages: – Individual pattern is required for each casting – Limited casting dimensions – Relatively high cost (tooling cost, labor cost)

5.2.2 Sand casting Sand molded casting is another term that can be used for sand casting. The mold for preparing the castings is made using sand as the construction material. Some binders, fiber reinforcements and other ingredients are mixed with the sand to give it the desired mechanical properties. Water is added to the mixture to obtain the required strength and plasticity for molding. The sand prepared mold can also be used for steel castings. Special factories called foundries are used to produce casting using the process of sand casting. The basic steps involved in making sand castings are discussed next. 1. Pattern making. For making the sand molds, patterns or the prototypes are required. Different types of patterns are used to prepare the final mold for receiving the molten metal to produce the final casting. Some of these types are loose piece pattern, split pattern, skeleton pattern and gated pattern. A particular pattern can be selected according to the requirement to form the cavity in the mold. Also, allowances are added for the easy removal of the pattern from the mold. 2. Molding. It is the operation necessary to prepare a mold for receiving the metal. Generally, a mold consists of two parts: cope and drag or swing and ram. The terminology for the mold halves depends on the orientation of the mold. If the mold is horizontal, then the two halves are called cope and drag. The top half is known as cope, while the lower half is known as drag. While with the vertical molding the terminology for mold halves is swing and ram. The leading half is

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swing while the back half is the ram. Molding sand is packed around the pattern to obtain the mold for the casting to be fabricated. By making a mold into two parts, removal of the pattern becomes easier. The imprints of the pattern provide the cavity for the production of the final casting. The pattern is withdrawn after bringing the two halves of the mold together. Spruce and riser pins, the gating system are provided for the flow of molten metal into the cavity produced after the withdrawal of the pattern to produce the final casting. For making hollow portions of the casting, cores and core boxes are used. These are supported using chaplets. The core and core boxes are placed in the mold and is then packed around with molding sand. Thus, the void formed between the mold and the core forms the casting. 3. Melting and pouring. The material of which the final casting is to be made is melted at a suitable temperature to form the pool of molten metal. The molten metal is then poured into the prepared mold with the use of transfer ladles. 4. Cleaning. After the solidification of the molten metal, a postprocessing operation is followed to remove the gates and risers, the sand adhering to the casting, pins, scales and other foreign material sticking to the final casting. 5. Inspection and testing. Casting is inspected for any defect post cleaning. Check on the size and shape is performed by comparing with the dimensions specified on the drawing. After rectifying the defects in the castings, further treatments such as the heat treatment and surface treatments are undertaken to provide the final finishing to the casting.

5.2.2.1 Pattern A pattern is the replica or the prototype of the casting to be produced. The pattern consists of additional attachments with it to obtain the final casting. These may include core prints and cores. Some allowances are also added to the pattern to take into account various factors such as that of expansion and contraction during solidification of the molten metal. Different materials are used for producing a pattern. The quality of the final casting depends on the material used for producing a pattern, the other factor being design and construction of the pattern. The final cost of the casting depends on the cost incurred on the pattern and the related equipment. The cost can be justified in producing substantial quantities of castings. The casting defects can be reduced if the pattern used has a proper finish and smoothness. The pattern forms the cavity for the molten metal to flow into it. 5.2.2.1.1 Pattern material Different materials are available to produce a pattern. The material to be used depends on the field of application. Material used includes wood, metals and alloys,

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plaster of paris, wax, resins and plastics. The material used for pattern should be light in weight, strong, hard and durable. Further, the material should provide for better machining characteristics so that it could be worked, shaped and joined with ease. Material should be durable and stable even at high temperatures and must be corrosion resistant. The availability and cost of the material should be considered while making a judicious choice for the material. Every material has its own limitations and advantages. For example, wood is the most commonly used material as it costs low and is readily available. The shaping is much easier to be performed on wood. However, wood has the tendency to absorb moisture and retain it. This can cause distortions and hence the dimensional changes. Other commonly used materials include metals and plastics. The newer materials such as araldite are also now used for pattern making. Araldite refers to a wide range of epoxy, acrylic and polyurethane adhesives. With the recent advancements, the additive layer manufacturing processes or the RP are now used for making patterns. The RP patterns can be manufactured with less time and less cost. Even intricate shaped patterns can be produced with ease. 5.2.2.1.2 Pattern allowances Allowances are added to a pattern to obtain the final casting with the desired specifications. Allowances also help in minimizing the machining costs and the rejections. Thus, allowances are a vital feature for the patterns. The different allowances given are shrinkage or contraction allowance, machining or finish allowance, draft or taper allowance, rapping allowance and distortion or camber allowance. These are discussed next. (1) Shrinkage or contraction allowance On cooling, there is shrinkage in the cast metal. Therefore, allowance must be provided to the pattern so that the final casting is of accurate dimension. Typically, the metal shrinkage is of two types: liquid shrinkage and solid shrinkage. i. Liquid shrinkage: the liquid shrinkage occurs when the liquid molten metal solidifies to the solid state. To account for this shrinkage, risers are provided in the mold. Risers also serve the purpose of feeding the mold with the molten metal. ii. Solid shrinkage: the solid shrinkage occurs during the cooling that takes place in the solid state. A shrink rule is used for considering the shrinkage allowance. The shrink rule varies from material to material. For example, it is 1/8″ per foot longer than a standard rule. The rate of contraction is material dependent. For example, aluminum contracts less when compared with steel. Therefore, care must be taken while providing for shrinkage or contraction allowance.

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(2) Draft or taper allowance For easy removal of the pattern from the mold, a draft or taper must be provided on all the vertical edges of the pattern. This is required to prevent the tearing of the sides of the sand mold. If no draft is provided to the vertical edges of the pattern, then the sides of the mold will break when removing the pattern from the mold. This is because the sides will try to remain in contact of the sand particles at the edges and therefore the pattern will take away the sand particles at the edges of the sand mold. The edges are well away from the sides of the mold. Thus, the pattern can be removed from the sand without tearing its edges. Typically, a higher draft is required on the inner surfaces in comparison to the outer surfaces. The draft allowance depends on a number of factors such as the method of molding, method adopted for removal of the pattern, the pattern material and, last but not the least, length of the vertical sides of the pattern. (3) Machining or finish allowance A proper surface finish is required in the final casting. But the casting produced does not have the desired accuracy in surface finish. Some of the materials will be removed during machining to obtain a better quality product. In order to account for machining, machining or finish allowance is provided. The allowance depends on the method of molding adopted. Further, size and shape of the casting, the metal used and the casting orientation are some of the other factors that affect the machining allowance (Table 5.4).

Table 5.4: The recommended value of machining allowance. Metal

Dimension

Allowance

Cast iron

Up to   to   to 

. . .

Cast steel

Up to   to   to 

. . .

Nonferrous

Up to   to   to 

. . .

(4) Distortion or camber allowance There are situations when the distortion takes place in the castings as the process of solidification progresses toward the finish stage. This may arise particularly

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when the castings are of peculiar and complex shapes. As for instance, the casting will contract at the closed end when it is in the form of letters such as L, T, V or U. The contraction process results in sight inclination of the vertical legs. Such distortions can be avoided by making the legs of different lettered shape castings slightly inward, so that after the solidification process the legs of the casting become vertical. Internal stresses are also the primary reason behind such distortion in castings. The internal stresses arise due to unequal cooling of the different sections of the casting. Some of the important measures that can be taken to prevent the distortion in castings are listed as follows: i. Modifications in the design of casting ii. Sufficient machining allowance needs to be provided so as to cover the effect of distortion iii. Suitable allowances on the pattern must be provided in the form of distortion allowance, which is also referred to as inverse reflection (5) Rapping allowance The pattern is rapped all around the vertical faces so that the cavity is enlarged and the casting is withdrawn from the sand mold easily. It is therefore required to suitably dimension the pattern and as such reduced dimensioning to that required should be provided so as to take into account the increased size of the cavity. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. 5.2.2.2 Types of pattern There are different types of patterns. Different requirements related to casting can be met with different types of pattern. A broad classification along with their brief description is given in the proceeding section. – Solid pattern – a pattern made for a model in a single piece is a solid pattern. One can easily manufacture the solid patterns. However, making a mold for solid patterns is a tedious task. Solid patterns are advantageous where parts are to be manufactured in small quantities. – Split pattern – as the name indicates, the patterns that are made into two parts are split patterns. These two parts are called cope and drag. The upper part is known as cope, while the lower part is the drag. The cope and drag separate at the parting line. The mold cavities in the split pattern can be made separately in cope and drag. Typically, split patterns are used for parts that have complex shape and that are required to be manufactured in moderate quantities. – Match-plate pattern – a pattern for the model created in two halves, with each half being on opposite side of a plate. The plate is known as the match plate, which can be made either in wood or metal. Thus, it is similar to a split pattern

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with the exception of a match plate. The runners and gates are made on the match plate. For the parts that are to be made in large quantities, match-plate pattern is one of the best alternatives. – Cope and drag pattern – in case of large castings, it becomes difficult to make two halves of the pattern and align them precisely. Therefore, in such cases the two halves of the pattern are made separately. Such patterns are known as cope and drag pattern. The two halves are aligned on match plates separately. The runner and gates are incorporated on the match plate. The cope and drag patterns are employed where large castings are to be produced in large quantities. – Gated patterns A gated pattern has a gate and runner system common to a number of loose patterns. Since gates and runners are common, removal of individual castings after solidification is a problem. However, there is a consistency regarding the flow of molten metal to each of the castings attached to the system. The gated patterns are used for producing small number of parts. – Shell patterns The patterns are used for making cylindrical castings. Mostly, it is used for producing piping and drainage systems. 5.2.2.3 Parting line The term parting line is a misnomer as it is the plane where two halves of the molds, that is, cope and drag, meet. It is the plane where draft angle changes the direction. To facilitate the removal of pattern, the orientation of the features should be perpendicular to the parting line. The part designer takes care of the designing of the parting line and parting surface. 5.2.2.4 Core and core box For producing internal cavities, cores are used in the casting and molding processes. A core is a disposable item that gets destroyed after the casting is completed. Cores are costly and creation of draft is also a tedious process. There are certain requirements that a core must fulfil: (1) it should have sufficient green strength to handle the molten metal; (2) the typical range of compressive strength should be between 100 and 300 psi to handle the forces of the casting; (3) it should have high value of permeability to vent the gases produced; (4) cores must break down easily after the casting gets completed and must lend themselves for their easy removal; (5) cores must have a good refractoriness; and last but not the least, (6) cores should possess a good surface finish. There are different types of cores available to suit different needs. Some of these are green sand cores, dry sand cores and lost cores. Green sand cores leave much material to be removed. The lost cores are used for producing complex cavities. According to the shape and position of the core, the cores can be classified as follows:

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– Horizontal core: this is the most common and the simplest mold types. – Vertical core: the core is arranged vertically. It is the usual practice to have greater part of the mold in the drag. – Hanging core: if the core overhangs in the core and does not have any support, then it is known as the hanging core. – Balanced core: this is used for making a blind hole. It is balanced and supported from its one end only. 5.2.2.5 Binders Binders are used to impart strength to the core. Vegetable oil was the oldest binder. Synthetic oils are now used in conjunction with some cereals or clay. After the application of binder, the core is baked in an oven. This helps in polymerization. There are some binders such as Portland cement and sodium silicate, which does not require the baking operation. Sodium silicate can be hardened by blowing carbon dioxide through the mixture. The following reaction produced silica gel that binds the sand grains together: Na2 Si2 O5 .H2 OðlÞ + CO2ðgÞ ! SiO2ðgÞ + Na2 CO3 .H2 OðglassÞ Different processes vis-à-vis hot box process, cold box process and air set sands are used for the binder application. In the hot box process, thermosets are used as a binder. The thermoset mixed with the sand is packed into the core. The core is heated to a temperature of 250 °C. The binder comes in contact with the hot surface of the core and the application process gets completed. In the cold box process, special gases are used to harden the binder. Toxic gases like amine and SO2 are used for the process and therefore special handling systems are required. The air set process is carried out at room temperature. The process employs organic binders. Other than binders, internal wires and rods can also be used. Straws can be used to enhance the collapsibility of the core. In certain situations where only one core print is used, chaplets are required to provide support to the core. The material of the chaplets used must be similar to that used for the casting. This is because they become the part of the casting after the process is completed. Moreover, the chaplets should be designed optimally. 5.2.2.6 Molding material Since a mold is always in contact with high-temperature molten material, the molding material should possess high refractoriness. Metallic materials such as iron, alloy steels and mild steels can be used as a molding material. On the other hand, some of the nonmetallic materials such as plaster of paris, silicon carbide and ceramics can also be used as the construction material for producing the mold.

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5.2.2.7 Molding sands River beds, sea, lakes and desert are the main sources of molding sands. Molding sand must possess certain properties to make it functionally fit for the molding process. These include the following: – Porosity: the property allows for the passage of gases produced during the molding process. The grain size and shape, moisture and clay affect the porosity of the molding sand. – Plasticity: the property that allows for the proper filling of the mold. Sand with good plasticity will produce a good mold. – Adhesiveness: the property of sand that provides for sticking to the surfaces of the mold box. – Cohesiveness: defines the strength of the molding sand. – Refractoriness: determines the resistance of the molding sand to high temperature of the molten metal. Different types of sands are used for molding process. These include (i) natural sand, (ii) synthetic sand and (iii) loam sand. These are discussed next in brief. – Natural sand: Natural sands contain 7–20% of clay and are used directly for the molding process. Some amount of water in proper proportion is required to provide it the intended functionality. The low refractoriness is the major drawback of the natural sand. In order to improve the molding properties of the natural sand, in general, binders such as bentonite are added. Because of the added binders, sometimes the natural sand is referred to as semisynthetic sand. – Synthetic sands: Silica is the main constituent of the synthetic sand. It may contain clay. The synthetic sand is formed by adding different ingredients that give it the desired properties. Properties like permeability and refractoriness can be obtained as per the requirements. Synthetic sand is typically employed for heavier castings. Further, these are suitable for casting of metallic and nonmetallic components. – Loam sand: Loam sands are used for lining of big molds made by a brick framework. This is because loam sand becomes hard when mixed with water. The consistency resembles closely to that of mortar. In many cases, loam sand consists of clay in a percentage of 50% or more. It also consists of other ingredients such as graphite, fiber reinforcements and binders. Generally, binders are added to different types of molding sand. The binders impart the necessary strength to the molding sand. It increases the cohesion between sand grains of different shapes and sizes. However, binders tend to reduce

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permeability and refractoriness, and hence must be added in the optimal quantity. Further, beyond certain quantity there will be no effect on the properties of the sand. Typical binders are fireclay, illite, bentonite, limonite and kaolinite. Fireclay particles are nearly 400 times in size as compared to bentonite and hence this is one of the main reasons for their poor bonding strength. Coal mines are the major source of fireclay. Generally, they are in the form of hard black lumps when taken out from the mines. These are further processed to be used as a binder for molding sand. On the other hand, illite is formed as a result of decomposition of micaceous materials. Natural sands are the major source of illite. The bonding strength is lower than that of bentonite. The commonly used binder is the bentonite. This is used in two forms: sodium montmorillonite or calcium montmorillonite. Because of low binding strength, limonite and kaolinite are not commonly used as binders. 5.2.2.8 Refractory sands Besides molding sand, different types of refractory sands are also used. These include silica sand, zircon, magnesite, silimanite, graphite/carbon and olivine. Silica sand is generally used in foundries; this is because it is a good refractory material. The sand does not fuse even at higher temperatures of 1,650 °C. Further, silica sand possesses excellent permeability and porosity. They can be easily molded to intricate shapes. Silica sand is easily available and is cheap. These can be used repeatedly. However, silica sand has a high coefficient of thermal expansion. Steps involved in mold making are discussed next. 1. First, a molding box of suitable size needs to be selected. While selecting the box, the space occupied by the riser, mold cavity and the gating system must also be taken into account. 2. Now, the drag portion of the molding flask is placed on the ramp-up board. 3. Next, the facing sand is sprinkled all around the pattern. The facing sand prevents the molding. The facing sand prevents the molding sand to get stick to the pattern during the pattern removal. 4. Now, the drag portion of the molding flask is packed with the molding sand. The molding sand is rammed uniformly. The process of filling and ramming is performed until the drag portion of the flask gets packed completely with the molding sand. A strike off bar is used to remove the excess of sand, that is, sand that is more than the required height of the drag. 5. Parting sand is then sprinkled over the top of the drag. 6. Next, the cope part of the molding flask is aligned properly over the drag portion. The alignment is done using dowel pins. 7. Parting sand is now sprinkled again around the cope pattern. 8. To form cavities for pouring of molten metal, sprue and riser pins are placed vertically at suitable locations.

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9. Next, the cope is filled with the molding sand. The sand is rammed uniformly as done for drag portion of the molding flask. 10. Vent holes are created using vent wires after the removal of sprue and riser pins. 11. The mold is repaired after the removal of cope and drag. 12. Next, the gating system and the runners are cut in the mold. The gating system connects the lower basin of the sprue with the runner and then the mold cavity. 13. In case of dry sand mold, the baking of the mold in ovens is done. 14. If hollow cavities are to be casted, then cores are placed in the mold. Chaplets can be placed for providing proper support to the cores in the molding. 15. The mold is closed by inverting cope over the drag. Pins are used for proper alignment of the two halves of the mold.

5.2.3 Permanent mold casting processes Permanent mold casting processes involve the use of metallic dies that are permanent in nature and can be used repeatedly. The metal molds are also called dies and provide superior surface finish and close tolerance than the typical sand molds. The permanent mold casting processes broadly include pressure die casting, squeeze casting, centrifugal casting and continuous casting. 5.2.3.1 Pressure die casting process For aluminum, zinc and magnesium castings, pressure die casting is the most common of all the casting techniques. These metals have got low melting points. The molten metal is injected into the die with a high pressure. The solidification takes place to maintain the high pressure. Postprocessing operations are performed after the casting is taken out from the die or mold. Generally, pressure die casting is used for mass production. There are typically two categories of pressure die casting, that is, high-pressure die casting and low-pressure die casting. Castings with close dimensional control can be produced at a mass rate. However, the dies cannot withstand very high temperature and the process of pressure die casting is not suitable for materials with high melting temperatures. Porosity is another major area of concern for castings produced using the process of pressure die casting. Furthermore, the dies in the pressure die casting process are usually very costly. In the hot-chamber die casting process, the furnace to melt material is part of the die itself; hence, this process is suitable primarily for low-melting point temperature materials such as aluminum and magnesium.

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5.2.3.2 Squeeze casting Squeeze casting employs two dies. One of the die is stationary while the other squeezes the molten metal with pressure. The molten metal is forced into the desired cavity. Squeeze casting has been used to produce automobile pistons. Fiber reinforced casting with SiC or Al2O3 fibers mixed in metal matrix has been successfully squeezed cast. However, squeeze casting is limited only to shallow part or part with smaller dimensions. 5.2.3.3 Centrifugal casting The molten metal is poured at the center of a rotating mold or dies in case of centrifugal casting. The centrifugal force causes the impurities to move toward the center of the rotating die or mold. The axis of rotation is placed at the center of the casting to produce hollow parts. The centripetal force of the order of 60–80 g is produced as a result of the high rotational speed of the die or mold. As a result of the centrifugal force, the nonmetallic inclusions are segregated near the center, which then can be removed. The process of centrifugal casting does not require the use of cores to produce hollow parts. Solid parts can also be casted with great precision. Further, the need of complex feeding systems is eliminated because the molten metal is fed by the centrifugal action. Production of axisymmetric parts is very popular with centrifugal casting. Both horizontal and vertical centrifugal castings are widely used in the industry. 5.2.3.4 Continuous casting Steel industry widely makes use of the continuous casting process. There is no enclosed mold in case of continuous casting process, which differentiates it from the other casting processes. Steel is melted down in a furnace from where it is transferred to the ladle. The accumulated molten steel undergoes treatments such as alloying and degassing. This is done to raise the temperature of the molten steel to a required temperature. The ladle with the molten steel is transported to the top of the setup. A refractory shroud (pipe) is used to transport the molten steel from ladle to tundish, which is a holding bath. The tundish allows a reservoir of metal to feed the casting machine. Metal is then allowed to pass through a open base copper mold. The mold is water-cooled to solidify the hot metal directly in contact with it and removed from the other side of the mold. The continuous casting process is used for casting metal directly into billets or other similar shapes that can be used for rolling. The process involves continuously pouring molten metal into a externally chilled copper mold or die walls, and hence, can be easily automated for large size production. Since the molten metal solidifies from the die wall and in a soft state as it comes out of the die wall such that the same can be directly guided into the rolling mill or can be sheared into a selected size of billets.

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5.3 Rapid tooling Rapid tooling produces molds for producing casting using RP. Sometimes tools are also produced by combining the nontraditional methods of casting with the RP process. In this case, the prototype of the tool is produced using RP and then the casting methods are employed to produce the final tooling. Thus, the tooling time is shorter in comparison to the conventional machining process. The expensive tooling cost can be offset with the mass production. The main disadvantage is that of less service life. The rapid tooling process can be broadly classified into soft tooling, reaction injection molding and bridge tooling. All the three processes are used for low volume production; on the other hand, metal-filled epoxy tooling and powdered metal tooling are for intermediate levels of production. The choice depends on the purpose, volume of casting to be produced, the functional and accuracy requirements and the applied RP processes.

5.3.1 Direct rapid prototyped tooling Rapid tooling produces parts capable of withstanding high pressure and temperature. The materials used impart such mechanical properties that low volume injection molding can be processed with no complexity. The RP metal tooling is fabricated using different processes and techniques. These include laser engineered net shaping (LENS), selective laser sintering (SLS) and three-dimensional printing (3DP). These techniques are capable to fabricate metal tooling that can produce thousands of parts before tool failure. The components fabricated using LENS have adequate strength to be used directly. On the other hand, sintering process is required for the components fabricated using SLS and 3D printing. The fabricated components are infiltrated with a lower temperature metal so that adequate strength can be imparted for their use. The major drawback of the 3DP technology is the lack of surface finish and smoothness. For fabricating tools for short run, stereolithography (SL) can be used. The SL machine is used to fabricate an epoxy shell of the tool. A thermally conductive material is then backfilled to provide the adequate strength and the required mechanical properties. For the molds, cooling channels can be inbuilt that help in cooling down the mold before it is ready for the next injection molding process. SL tools can be used for about 100 parts, but typically it is employed for less than 50 parts.

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5.3.1.1 3D printing (Z402 system) MIT has licensed the 3DP process. The 3DP process produces a component by spraying liquid binder onto the metal powder media. The jets are used for spraying of the binder incorporated into the printing head of the printer. Thus, the process prints a physical part using the data from the different CAD software packages. Z402 system of the Z Corporation (Z Corp) incorporates the 3DP process. The system is relatively inexpensive. The system is intended to produce components for the purpose of verification of the concepts. The components can be used for the validation and testing. This is because the components have less accuracy and surface accuracy as required by the higher end systems. Initially starch-based powders were used with water-based binders. With the advancements in research gypsumbased material is being used by the system. The building takes place from the bottom to top direction. The starch-based layers are added progressively from bottom to top to produce the end component. The resulting component is porous and is infiltrated with wax or some other hardener to provide for the adequate strength. The Z402 systems are much faster than the other RP techniques and processes. The dimension of the model that can be produced using Z402 systems is around 8″ × 11″ × 8″. Although the infiltrated parts have adequate strength, the components can also be used as pattern for investment casting. Cyanoacrylates are also used as binder, which imparts enough strength to the part to make it durable to survive significant handling. Advancements have been made in the 3DP systems of the Z Corp since its inception. The corporation released an advanced cartridge system that can be used for longer runs, producing the components with the similar required strength. The system of cartridge is termed as the type 3 system. The company then went on to produce parts with little or with no postprocessing and infiltration requirements. The systems were ZP100 Microstone. Finally, the corporation came up with an automated waxing system to control the amount of wax to be applied during the infiltration process. The Z402 modeler consists of the following hardware components: 1. Build and feed pistons. As the name indicates, the build piston provides for the area to construct a part while the feed piston provides for the construction material. The build piston lowers down as the material layers deposit one over the other. The feed piston, on the other hand, rises to feed the construction material for the formation of a new layer. 2. Printer gantry. The printer gantry houses important parts such as print head, the wiper/roller and the printer cleaning station. The motion on the XY plane for the part building is provided by the printer gantry. 3. Powder overflow system. To scrap off the excess powdered material, an opening just opposite the feed piston known as the powder overflow system is provided. This system collects the excess of powdered material. A vacuum system helps in collecting the excess powdered material in a disposable bag.

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4. Binder feed/take-up system. Separate containers are provided to feed the binder material and to collect the excess of it. The binder is fed to the build area using siphon technique. Sensors are provided near the containers to take care of the feed rate of the binder. A COM port is used to operate the Z402 printing system. Computers are provided on board to regularly diagnose the process. The printer is compatible with different operating systems. The Z402 requires very few commands to build a part. Thus, the system has a user-friendly interface. Also the system does not require any support structure to support overhanging surfaces, which is the case with other RP systems. 5.3.1.1.1 Software The part from the CAD file is converted to STL format. The Z Corp software slices the STL part file into number of layers. This is then saved as a BUILD file. The part imported in STL format is oriented automatically in the shortest Z-direction. This increases the build speed of the machine. The other RP systems orient the part model in the X- and Y-directions. However, for the better visualization, the part can be oriented manually too. A number of parts can be produced simultaneously by importing multiple STL files. A part can be rotated or moved by entering coordinates or by a simple drag-anddrop method. Parts can be copied to produce a new part simultaneously. This operation is performed by simply highlighting the part using a copy command. This is only possible if enough build envelope is present to occupy a new part. If this is not the case, then the new part will be built over the existing part. Simple menu commands can be used to justify a part to either side of the build envelope. 3D nesting can be employed to accommodate parts to be built in floating space. This increases the process capability of the machine. “3D Print” command is given to the machine to print the STL file. On issuing the command, the part file is sent to the machine. One can monitor the percentage of progress of the process as the machine builds the part. This can be seen from the progress bar. The information of the start time of the process and the expected completion time is displayed by the machine. A dialog box with the information of time taken to complete the process will be displayed once the process is completed. Volume of material used and also the average size of the droplet are also displayed. 5.3.1.1.2 Machine preparation for a build The machine is checked before it is ready to build parts. Checks should be performed on the level of binder in the container. The sensors should be checked carefully too. The feed piston and the take-up pistons should be checked for any mechanical faults. A wiper blade levels the metal powder on the table. This also helps in defining the

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build area. The vacuum bag that collects the excess of powder should be replaced every week. The jet cleaning station is squirted with a very small quantity of distilled water. This helps in optimizing the performance of the machinery. 5.3.1.1.3 Build technique Following steps are performed in sequential order to obtain the final component. The machine builds the part using slices, which is similar to that of other RP systems. The steps are discussed next: – For marking of the starting point, that is, the starting layer on which the other layers are deposited a manual operation is performed to spread the blank layer of powders. This is termed as landscaping. The remaining steps are performed automatically. – As a result of manual operation, the bottom cross section of the part is printed. – Supply of powder for the next layer is made from the feed piston. – The layer of powder is spread by the printer gantry. – The operation of depositing layers one over the other is performed automatically by the machine. – After a desired thickness has been obtained, the final part is obtained. This is then ready to be postprocessed.

5.3.1.1.4 Postprocessing The parts produced from starch are fragile and needs to be handled with care. A wide range of infiltrates are used to provide adequate strength and the desired mechanical properties to the starch parts. Generally, parts are infiltrated using paraffin wax. A wide range of other materials available are plastics to cyanoacrylate. The postprocessing steps used are discussed next. The whole process takes about 15–20 min. 1. Powder removal. A small air brush system integrated with the machine removes the excess powder from the part. This is performed immediately after removing the part from the machine. A small glove box is also provided with the air brush system. The excess powder is removed gently by the airbrush system. A vacuum cleaner is used to blow of the excess powder from the glove box. The entire process is performed for about 5 min. 2. Heating. In order to provide for the wicking characteristic, the part is heated to about 200 °F. The operation is performed in small ovens. The entire process may take around 10 min. 3. Infiltration. The heated part is then dipped in the vat of molten wax for a few seconds. The part is then removed from the molten vat and is then placed on a sheet for drying. The process of infiltration and drying takes about 5 min.

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The actual processing time is minimal when compared to other RP systems. However, the actual time depends on the skill of the user, complexity of the part and last but not the least on the skill of the operator. 5.3.1.1.5 Advantages and disadvantages of the Z402 The Z402 system has a machining time of around one vertical inch per hour. With this build speed, the system can build a part with several inches height in a normal working day. The speed of the machine is advantageous to companies where the time is a factor in sales and production. The rough surface is one of the major disadvantages of the Z402 systems. However, this is the major issue with the systems employing powdered material for building the part. The issue of surface roughness can be rectified by employing sanding. One of the other disadvantages of Z402 systems is the frequent replacement of ink-jet cartridge system. The ink cartridge needs to be replaced after 100 h of operation. 5.3.1.2 Laser engineered net shaping The LENS is developed by Sandia National Laboratories and various industry members on a Cooperative Research and Development Agreement. The LENS is referred to as the “true” direct metal RP system. This is because the strength of the part obtained from the machine is similar to the raw material used for the production of the part. 5.3.1.2.1 Build materials A wide range of building materials is used for building the part. Different build materials include tooling steel (HI3), stainless steel 316 (SS316) and titanium with 4% vanadium and 6% aluminum. Apart from these materials, other ceramics and metallic materials have also been researched Current build materials with an extensive operational database on the system include SS316 and HI3. 5.3.1.2.2 Build process The LENS system works similar to that of other RP techniques. The LENS system too produces parts using layer-by-layer manufacturing process. The STL file is sliced and downloaded to the machine for further processing to produce the final part. The system adopts a bottom-up approach. 5.3.1.2.3 Deposition head A 700 W Nd:Yag laser is employed by the LENS system. The metal powder is focused to the laser. The powder is supplied from the feeder tubes. The powder is placed atop the previous layer and is welded properly. Argon is used to create inert

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atmosphere to prevent the powders from being oxidized. The powder is deposited to form the boundary of the cross section. This is then used to produce solid areas using raster fill. The deposition head provides for the transverse motion along the X- and Ydirections. The Z-axis motion is achieved by a moving platform. LENS system has the capability to produce the parts ranging from simple to semicomplex parts. But due to need of rigid supports, fabrication of free hanging surfaces is not easy. The system has a wall angle capability of 18°. Research is going on for providing rotational and tilts capability to the deposition head. The software is being developed to enhance the capability of the deposition head. Once achieved, the LENS systems would be able to produce complex geometries also. 5.3.1.2.4 Build substrate The part is built upon the substrate, which is of the same material as that used for building the part model. The substrate gets welded with the part itself. This also helps in preventing any movement and deformation to the part being built. The build substrate is removed from the part after the completion of the process. 5.3.1.2.5 Postprocessing Since the parts produced have rough surface, machining is required to make it smooth. The substrate must be cut from the final part. However, since the part has full strength, sintering and curing are not required. Thus, the overall manufacturing time is reduced. 5.3.1.2.6 Advantages and disadvantages The LENS systems do not require postprocessing operations such as sintering and curing. The LENS systems can produce strong parts directly from the CAD data. The parts have excellent mechanical properties. However, the system has the main disadvantage of rough surface and that of low-dimensional accuracy. This requires some cleaning and polishing operations.

5.3.2 Silicone rubber tooling One of the most popular applications of RP is that of rapid tooling. Production of vulcanizing silicon rubber tooling is one such application. The purpose of RTV tools lies in the creation of epoxy or urethane prototypes under the influence of vacuum. A rubber mold making process encompasses making of master pattern on RP machine, finishing the created pattern to the desired accuracy. Then the RTV silicon

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rubber is poured onto the pattern in order to form the mold. The mold is injected with two-part thermoset materials and as such molded plastic parts are produced. The advantages of silicon rubber tooling lie in the production of inexpensive molds, parts with excellent cosmetics and the option of employing multiple materials. The process is much more suitable for medium- and small-sized parts. Creation of negative draft is another benefit of silicon rubber tooling. The primary weakness of process is that the urethane material has distinct properties in comparison to the thermoplastics. Tool life also plays a major restriction to the production and as such the quantity of produced parts is restricted to small quantity. The cost of parts produced is therefore high. 5.3.2.1 Process A special pattern is used for the vacuum mold casting process. It is either a match plate or a cope and drag pattern with tiny holes to enable a vacuum suction. A thin plastic sheet is placed over the casting pattern and vacuum pressure is turned on, causing the sheet to adhere to the surface of the pattern. A special flask is used for this manufacturing process. The flask has holes to utilize vacuum pressure. This flask is placed over the casting pattern and filled with sand. A pouring cup and sprue are cut into the mold for the pouring of the metal casting. In the next stage, the vacuum on the special casting pattern is turned off and the pattern is removed. The vacuum pressure from the flask is still on. This causes the plastic film on the top to adhere to the bottom. The drag portion of the mold is manufactured in the same fashion. The two halves are then assembled for the pouring of the casting. 5.3.2.2 Dimensional accuracy of products manufactured using silicone rubber tooling The accuracy of the products made using vacuum casting and tooling methods are of the order of 99%. The same can be established by the following: L = original pattern length T = temperature difference of SR mold curing and plastic casting Silicone rubber linear shrinkage = 0.1% Silicone rubber mold length = L – 0.001L = 0.999L α = coefficient of expansion of silicone rubber = 35 × 10–6 mm/mm/°C Increase in the length of silicone rubber while preheating during casting = (0.999L) × 35 × 10–6 × t mm = 3.4965 × 10–5 × t × L mm New length of SR mold = L(1 + 3.4965 × 10–5 × t) mm

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Linear shrinkage of resin (SG 95) = 0.2% Decrease in length of plastic prototype = 0.002 × (1 + 3.4965 × 10–5 × t) × L Actual length of plastic prototype = L × (1 + 3.4965 × 10–5 × t) × (1 – 0.002) mm = L × (1 + 3.4965 × 10–5 × t) × 0.998 mm For L = 100 mm and t = (70−40)°C = 30°C Actual length of the plastic prototype = 99.905 mm

5.3.3 Investment-cast tooling Another alternative to the machined tool is the tools produced using investment casting process. However, the process is not viable because of the metal shrinkage problems and the like involved in the casting process. Also one of the other reasons being the demand of higher accuracy for injection mold tools. The process of fabrication is similar to that of normal investment casting of the component. The RP produces the prototype of the tool, which then can be used to form the cavity for the molten metal to flow in. The pattern can be used repeatedly. Due to the number of steps involved in the process, the final tooling suffers from the dimensional error. Further, the turnaround time is also more. However, in certain conditions, the process is more suitable than the other traditional approaches of producing the tool.

5.3.4 Powder metallurgy tooling With the recent advancements, powder metallurgy is used for the fabrication of tools and inserts. The fabricated tools and inserts have more life of service in comparison to the machined tools. The machined tools, however, are made from RP but the powder metallurgy tools and inserts are far more superior to the machine tools. A number of ways are there to fabricate the tools and inserts using powder metallurgy process. 3D Keltool is one such process. The process was developed by 3D Systems Corp. a silicon rubber positive is created using a negative master. The negative master fabricated using RP. The silicon rubber positive is then injected with the proprietary metal mixture. Sintering process is followed to shape it. The fabricated tools have properties similar to that of tool steels. The production time is also relatively faster. 5.3.4.1 3D Keltool 3D Keltool has been employed to develop injection-molded inserts. This powder metal process is also used for the development of other durable tooling from the

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master patterns. 3M is credited to the development of Keltool in the year 1976 and then the technology was sold and later on developed by Keltool Inc. Keltool Inc. then transferred the Keltool technology to 3D Systems and as such it was renamed as 3D Keltool. 3D Systems continued their developmental task to further enhance the capabilities of 3D Keltool. Being the proprietary, it is difficult to decipher the secrets of 3D Keltool publically. DTM was later on acquired by 3D Systems in the year 2001 and then the SLS tooling was favored over the 3D Keltool. As such the developments associated with 3D Keltool was very quiet between 2001 and 2003 and thereafter nonexistent. The processes under the realm of 3D Systems have been licensed to several companies. 3D Keltool technology incepts with a CAD design of the cavity insets and the core. Then SL process is adopted to create core and cavity patterns. Silicon rubber is cast against the created core and cavity patterns and molds are created into which is poured the metal powder and the binder. The metallic mixture is composed of finely powdered A6 tool steel and finer tungsten carbide particles. During this stage, the cavity inserts and the core exist in green state. The binder is burnt away by firing the green inserts within the hydrogen reduction furnace. The metal particles are sintered and inserts are infiltrated with copper. The produced inserts are machined and drilled for ejector pins.

5.3.5 Spray metal tooling There are numerous industries that have been working in collaboration with government agencies to develop spray-metal tooling methodology. Low-temperature substrates are coated through the employability of thermal metal deposition technologies as such vacuum plasma deposition and wire-arc spray techniques. The resulting number of products and their quality aids in low-cost tooling and the tooling capabilities aid in providing various degrees of durability with different injection pressures. The logic is to connect shell material possessing high-temperature capabilities as well as high hardness to RP positive and then fill the remaining tool shell with the material possessing low strength, temperatures and even cooling channels. Such an arrangement aids in providing hard and durable face that can withstand the high forces and temperatures in the injection molding process. Furthermore, the objective lies in providing a backing that can aid in working for optimal thermal conductivity and heat transfer from the mold. Although success has been achieved with such an arrangement, there lies challenge to deposit high temperature and harder material onto the RP pattern directly without affecting the integrity of the intricate geometry. An alternative solution can be to produce silicon-rubber mold that in turn can be employed to create ceramic spray substrate. As such, higher

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temperatures can be endured by the ceramic substrates. However, the process is cost and time expensive. The material to be deposited in case of wire-arc spray method comes in the form of filament. There are two filaments, that is, one is positively charged and the other is negatively charged. Both these filaments are fed into the device until they meet and create electrical arc. The arc created melts the filament and there is flow of high velocity gas through the arc zone that accelerates the atomized particles onto the RP pattern. While, on the other hand, high melting temperature metals are processed more easily through the vacuum plasma spray technologies. The material to be deposited is in the form of powder that is melted, accelerated and deposited by the generated plasma. While there is limitation to process lower temperature RP patterns with vacuum plasma spray technique, some success has been achieved in this direction through the employability of sturdier RP patterns such as SLS.

5.3.6 Desktop machining Small and portable machines are now used as an alternative to RP for producing tools and inserts. This is referred to as desktop machining, which uses a graphical interface to import a STL file to fabricate the tools and inserts. The machining does not require a high level of knowledge skills to produce the tools and inserts. The elimination of numerical-code programming does away with the need of skilled manpower. Even a novice can operate the machinery. This reduces the overall cost of fabrication. The aluminum-based material can be milled out quickly without the human intervention. Cooling and tool changing capabilities are also present in some of the machining systems.

5.4 Conclusion The manufacturing operations are impacted positively through the RM techniques. With the employability of RM techniques it is possible to reduce cost of production through improvements in the logistics, inventory and manpower handling abilities. It is also obvious that with the RM techniques the ability to deal with unstable demand patterns also enhances. Cost reductions become apparent when the volume of manufacturing is high, the manufactured goods are used for the benefit of country’s population and there is no pressure from the cost of labor. With RM, it is possible to realize truly the advantages associated with the flexible just in time (JIT) supply chain proposition wherein the customer demands can be met as and when required.

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Furthermore, the quality of products manufactured is enhanced even with the customizable product. However, a number of questions arise that pertain to as to how the developments in the domain of RM can be implemented successfully. Successful implementation of such advanced technologies can aid in understanding the profound impact that the RM techniques have on the manufacturing environment. With the successful implementation, it will become easy to manufacture products that were not produced easily may be because of small market size or the complexity of the part involved.

Chapter 6 Reverse engineering using rapid prototyping 6.1 Introduction Engineering is a cumulative of different processes performed simultaneously to obtain the desired product. The processes are that of designing, modelling, manufacturing, assembling and maintenance of a product after its sale. The engineering processes can be broadly classified into reverse and forward engineering. Forward engineering translates the concept or idea to the 3D model which is then converted to obtain the end product. On the other hand, reverse engineering is the process of acquiring the set of points from the 3D model to fabricate a product that doesn’t have the technical detail. That is, it is the process of replicating an already existing part, assembly. The technical details can be details with regard to drawings, bill-of-materials, etc. Legacy parts can be recreated using reverse engineering. This aids in their restoration. Reverse engineering can also be utilized to recreate commercial parts of very high value. The profits earned are of appreciable amount. The requirements of the market should be met while fabricating parts using reverse engineering. The functions and the performance of the original par, that is being replicated, are needed to be understood for accomplishing this task. Also the engineer needs an understanding of the skills required to replicate the original product. Reverse engineering encompasses a very broad range of disciplines. These include mechanical engineering, software engineering, microchips, animation/entertainment industry chemicals, pharmaceutical products, electronics, etc. As far as the mechanical engineering field is concerned, reverse engineering is a technique of designing and data documentation from existing parts. An existing part and its assemblies are recreated by acquiring surface and geometrical features data. This is done by using contact or noncontact digitizing or measuring devices. Many mechanical parts have been manufactured using reverse engineering tool. Seals, bolts and nuts, O-rings, gaskets and engine parts are some of the examples of the parts produced using reverse engineering. The recent advancement in reverse engineering has made this technology one of the top and primary methodologies employed in many industries. Some of the major industries where it has found its root are aerospace, automotive, medical device, sports equipment and consumer electronics. It is also employed in forensic science and accident investigations. Reverse engineering shortens the product development cycle and therefore enormous gains can be imagined from such a technique. Reverse engineering has been made easy with the aid of new technologies. Three-dimensional (3D) laser scanning and high-resolution microscopy are some of the new technologies. https://doi.org/10.1515/9783110664904-006

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The process of manufacturing products to meet the requirements of customer base has become more accurate. This is because the reverse engineering has become integral with different manufacturing processes. A cloud of data points is generated by employing contact and noncontact scanners. The generated data set in the various CAD packages is then converted to some standard file format. NURB and STL are few of the standard file formats. The STL or NURB converted model can be used as an input to the manufacturing machines to manufacture the end product. In recent years the fabrication is done using layer-by-layer manufacturing known as rapid prototyping (RP). The RP can manufacture models with complex geometries. Recently, much research has been carried out to adaptively determine the thickness of each layer and the number of layers required to fabricate a product using RP. Different algorithms have been employed to fabricate a product. For example the algorithm for modeling in the reverse engineering and the algorithm used for RP is that of adaptive slicing algorithm. The modeling and the slicing algorithms are used consecutively to produce a part from the cloud of point data.

6.2 Process The reverse engineering process can be subdivided into three main phases. The three phases are the following: – The object is scanned for obtaining the cloud of points. – Segregation and further processing to construct layer of desired thickness. – Generation of the required geometric model. 6.2.1 Phase 1 – Scanning The first and important phase is to obtain a correct set of data points by carefully scanning the object. Therefore selection of proper scanning technique is critical. The scanning should be carried out carefully, which would help in producing the part with increased level of accuracy. The scanning obtains information such as that relating to slots, holes and pockets together with all the other geometrical data. The scanners are either integrated with the CNC machines or can be provided as add-ons. Generally the scanners can be categorized into contacting and noncontacting scanners. 6.2.1.1 Contact scanners The contact scanners obtain the point data by physical contact of the surface. The contact to the surface is made possible by employing probes. These probes follow the contours of the physical surface and records the set of points as it proceeds. The

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tolerance range of probes employing CMM technology ranges between +0.01 and 0.02 mm. Since each point is generated in a sequential manner, the process of scanning may incur sufficient amount of time. A degree of pressure is required to be maintained while scanning the part. This makes scanning of soft materials such as rubber relatively difficult. 6.2.1.2 Noncontact scanners The part surface can be scanned without any physical contact with the device. Cloud of point data is generated by employing optics, lasers and charge-coupled device (CCD) sensors. The noncontact scanners have the advantage of capturing a large amount of data points in a very less amount of time. However there are certain disadvantages associated with noncontact scanners. Some of these have been discussed next. – The noncontact scanners have a typical tolerance value ranging ±0.025 to 0.2 mm and hence the accuracy gets reduced. – It becomes a difficult task to generate points from surfaces parallel to the axis of the laser. – Since light is employed to capture data points, a problem is created when scanning shiny surfaces. In order to combat the problem some prior preparations are required such as application of temporary coating of fine powder before scanning process is started. Therefore the use of noncontacting devices gets restricted to the engineering areas where time is much more important than the accuracy of the information. However, the accuracy of the commercially available noncontact scanning device has begun to improve with the advancements and research in optical technology. Thus after scanning a part surface, point cloud data sets are generated in the most convenient format.

6.2.2 Phase 2 – point processing Different filter algorithms are used to remove the unwanted points obtained after scanning of the objects. In order to ensure accuracy in the end-product, one should have a proper understanding of the different filter algorithms. Further, in this phase the noise in the data is also reduced. Sometimes it requires merging the multiple set of data points. This is because multiple scans are sometimes taken to ensure that all the features have been scanned. The multiple scanning processes should be properly planned as this has a direct impact on the point processing stage. Scanning involves rotating of parts and hence to ensure reduction in noise in the collected data points, proper scanning is very important. Further, this will reduce the effort in this phase and also avoid introduction of errors from merging

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multiple scan data. A large number of array of softwares are available for performing the point processing phase. With different algorithms in place it is possible to obtain a clean, merged, point cloud data set in the most convenient format. The phase 2, that is, the point processing phase is very important as the points are used for further processing in the subsequent stages to fabricate accurate and precise products.

6.2.3 Phase 3 – Application geometric model development The clean, merged cloud data set obtained from the phase 2 is now used for generating 3D CAD models employing different reverse engineering software packages. The current reverse engineering technologies are helping to reduce the time to create 3D CAD models in the electronic format from existing point cloud data sets. It becomes very important for any RE process to generate a true 3D model of any object. However, it becomes a tedious task to process large number of data points to produce an accurate model. Different fitting algorithms are required to generate a true replication of the existing object. The existing software packages lacks in processing a large database of points and hence the need of new software packages for RE is the need of the hour. It’s still very subjective to generate surfaces from the scanned data points. Feature-based algorithms are beginning to emerge. This will enable to develop 3D models by interacting with the point cloud data. Since a wide range of software modules are available, it becomes critical to choose the one that will suit the business as well as the customer needs. Therefore the output of this phase will depend on the business objective. For example, for producing a replacement tool for injection moulding, we would be interested in obtaining the ISO G code data apart from the geometrical parameters. This will help the manufacturer to manufacture the same in the shortest possible time. Reverse engineering can be employed to analyze “as designed” to “as manufactured.” This involves comparing the two sets of data points generated for designed and manufactured part. This is done by superimposing the scanned point cloud data set of the manufactured part on the imported as designed CAD model. Geometric 3D mode is obtained in one of the proprietary formats such as IGES, VDA, STL, DXF, OBJ, VRML, ISO G Code, etc., as an output of this phase.

6.2.4 Integration of RE and RP for layer-based model generation The adaptive slicing algorithms and the cloud data modeling algorithms of RP and reverse engineering respectively generates end product with greater precision. This is made possible by the integrated shape-control mechanism. The point cloud data is converted to construct a layer-based model using RP. The task is

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accomplished by sequentially employing the modeling algorithm of the reverse engineering and the adaptive slicing algorithm of RP. However this is not as simple as it appears to be. The shape-error control is a very tedious task. The shape error of the final Rapid Prototyped model is generated from 3 stages: (1) shape error generated by the error between the point data set and the generated surface model, (2) then there is error from the surface model and the converted STL format and (3) finally the error from the STL model and that from layer-based RP model. An STL model is generated on application of triangulation algorithm which itself is arduous. A surface model (e.g., NURBS) will be produced in the reverse engineering process, if a segment-and-fit algorithm is applied. Further the produced surface model is then converted to an STL file format or any other suitable acceptable format and lastly a layer-by-layer model (RP model) is generated by adaptively slicing the STL file format model. The shape errors from different stages are not directly related. As a result of which it becomes very difficult to control the shape error as a whole since all the 3 stages are carried out one after the other. The remodeling computations are expensive in RP and reverse engineering. The data processing also results in large file transmissions. However, if the data processing is integrated in the RE and RP then the problem can be resolved to a greater extent. One of the approaches to integrate the data processing is slicing the point cloud into layers of points. This must then be followed by constructing a 2D contours for the points on the same level of thickness. The shape error can be identified for each layer. This error then can be controlled effectively by varying the layer thickness. Producing a layer-based model directly from point cloud data, which being an automated segmentation, approach has been developed. Three major steps are used to accomplish the task. First, an error known as subdivision error is calculated to segregate the points into certain set of regions. The error is calculated by obtaining the distance between the cloud of data points and their respective projection plane. The data in each region are then compressed by using a digital image reduction method. The feature points (FPs) are kept within the user-defined shape and tolerance. Second, an intermediate point-based curve model is generated on the basis of feature points of each region. The layers to be used for building the part is then extracted directly from the models, in particular their contours. Finally, the contours of the extracted Rapid Prototyped layer are closed to obtain the final product. 6.2.4.1 The adaptive slicing approach for cloud data modeling Adaptive slicing approach comprises of 2 basic steps for cloud data modeling: (1) the generated cloud of points is sliced into number of layer of points. The cloud data are segmented into building direction, that is, the direction defined by the user. The data points in the space are projected onto a particular plane in the building direction. A layer represented by a 2D contour and a specific thickness is constructed. This is

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done by projecting data points which is used to reconstruct a 2D polygon approximating the contour. Finally, the constructed 2D polygon is extruded in the building direction. In this processing step, the number of points for further processing gets reduced. This is because some of the points form the vertices of 2D polygon. In this step, a 2D polygon constructed using projected data points. The polygon is then extruded in the RP building direction to a maximum possible layer thickness. Some points that form the vertices of the polygon will be retained while the remaining points will be eliminated. Thus data thinning is carried out in an effective manner. Calculation of shape error is a very important calculation which is a difficult process, as mentioned earlier. The maximum deviation is calculated first to obtain the shape error between the original set of data points and the 2D polygon on the projection plane. The shape error between the RP fabricated part and the original set of data points can be obtained by obtaining the maximum shape error amongst different layers. There is a tolerance value to keep a check on the shape error for each layer. The maximum thickness of each layer is obtained by keeping the shape error value very close to the prescribed tolerance value. To obtain the maximum layer thickness, an adaptive slicing method is employed in which maximizing the layer thickness is achieved through an iterative process. The 2 extreme points are identified in the slicing direction. Then from one end the layer construction is started. Initially the layer thickness is kept as minimum as possible. A plane is considered in the middle of the layer. The points are projected on to this plane. A 2D polygon is constructed with points as vertex on the projected plane. The distance between actual set of data points and the constructed polygon gives the actual shape error. If the shape error is below the prescribed tolerance value then the layer thickness is increased adaptively. This is done until the shape error value becomes closer to the prescribed tolerance value. After obtaining the final thickness value, the layer is extruded in the RP building direction. Thus the first layer is constructed. Similarly other layers are also constructed. 6.2.4.2 Planar polygon curve construction for a layer The collected set of data points once projected onto a projection plane construct multiple polygonal curves. Since these polygonal curves are closed, construction of each polygon should be done separately. In case of multiple loops different curves split naturally. In the proceeding section, the construction of single loop has been discussed. The collected set of data points is in fact unorganized and the process of curve reconstruction approximates these sets by a curve. The curve used in our case is straight line segments. This is because the projected points have local linearity. These segments can be used to construct a polygon. To ensure the accuracy in obtaining the original shape, the feature points must be retained by the polygon. An

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algorithm is in place to construct a 2D polygon from the unorganized set of data points. The algorithm uses an oriented vector from a point to segregate the data of points. A fixed radius of neighborhood points is used for determining the oriented vector of a point. From this oriented vector the other feature points are obtained repeatedly. The data set is compressed while retaining the feature based points. It is the radius of the oriented vector that determines the accuracy of the process. If the radius is too large, then a large number of points could be segregated including feature points. On the other hand if the radius is too small, then the efficiency of the process gets affected. A regression curve can be obtained using the concept of correlation. Further, the concept can be extended to determining the radius of neighborhood adaptively. Correlation Coefficient is used to measure the strength of the linear relationship between 2 variables on the 2 axes in a 2D plot. The correlation coefficient of 2 variables A and B can be calculated using eq. (6.1) ρðA, BÞ =

CovðA, BÞ SðAÞSðBÞ

(6:1)

where Cov (A, B) = E [(A–E(A))(B–E(B)] = E(AB)–E(A) E(B) and E(φ) represents an expectation of a random variable φ. S(φ) denotes a standard deviation of a random variable φ. Let (A, B) stand for a set of N data points {Pi = (Ai, Bi), {i = 1, . . ., N}, then eq. (6.1) can be rewritten as follows:   N    P   Ai − A Bi − B  i=1 (6:2) ρðA, BÞ = sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N  N   P 2 P 2  Bi − B Ai − B  i=1

i=1

Where, A and B are the average values of {Ai} and {Bi}, respectively. The Value of ρ (A, B) lies between 0 and 1. Calculation of correlation coefficient is done to verify the linearity of the points within a neighborhood. The correlation coefficients for R1 and R2 are 0.928 and 0.602, respectively. Points within R1 have better linearity. Our aim in the problem of 2D polygon construction is to fit the points with a line segment accurately. We need to find the maximum value of the radius of the neighborhood. Using the correlation coefficient, we can determine the radius of the neighborhood adaptively. 6.2.4.3 Initial point determination The start point of the first line segment of the 2D polygon is defined by the reference point known as the initial point or IP. The 2D polygon is to be constructed using set of data points. The selection of IP is very difficult but it is very crucial. A starting

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point is selected at random using the random search approach. The points in the vicinity of this starting point are selected. Center of these selected points is calculated. The point closest to the center point will be the initial point. The center point O of the neighborhood will be at a distance from the original shape. This would be the case if point Q is selected as the starting point. Also, the closest point to center point O will also be the worst point to be the initial point. The problem can be resolved if it is ensured that the points within the first neighborhood have good linearity. The correlation coefficient (ρ) of this neighborhood is then calculated. If correlation coefficient is greater than a preset limit then this neighborhood is utilized to obtain the initial point, that is, the point that is closest to the center of this neighborhood. Otherwise an iterative process for finding the initial point is continued by reselecting a point. For the case in hand point P can be utilized as the origin point to decipher the initial point. It is to be noted that point Q will be left out due to its poor linearity. The initial point P can then be utilized as a reference point for the construction of the first segment. 6.2.4.4 Constructing the first line segment Once the initial point is found then for the first line segment, a neighborhood circle (N-circle) S1 is obtained in order to satisfy the user requirement for the correlation coefficient. For polygon to have the least number of line segments, it becomes necessary to make the radius (R) of the neighborhood circle as big as possible. Hence, the radius of the neighborhood circle needs to be evaluated adaptively. So, based on the correlation coefficient we start with a smaller value of radius and then hunt for that value of radius that will be approximate to the radius of optimal N-circle. A smaller value of correlation coefficient means poor linearity, and thus we need to decrease the radius of the N-circle. On the other hand, a larger value of correlation coefficient means good linearity, and we can increase the radius of the N-circle. This is an iterative process and can be summarized as follows: Algorithm for finding neighborhood circle S1. Given: A planar data set C, The initial point (IP), R = initial radius of N-circle, ΔR = the increment of radius, ρlow = the predefined lower limit of correlation coefficient and, ρhigh = upper limit of correlation coefficient. (a) From the data set C, select all the points Pi, such that ‖Pi – IP‖ ≤ R, to form a new set of data points, C1 (b) Determine the value of correlation coefficient ρ using eq. (6.2) for the new data set C1.

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(c) If ρ is greater than ρhigh, then Set the value of R = R + 2ΔR and go to step (a). However, if ρ is less than ρlow, then Set R = R – ΔR and go to step (a). (d) Once the iterative process is carried out then we will have the final value of radius R of the Ncircle and also the final set of data points.

Now we can construct a straight line segment that will best fit these set of points locally since the points within the N-circle will have good linearity. For this purpose, we utilize the least-squares method to obtain a regression line, which will pass through the initial points (XIP and YIP) and will best fit these points within the N-circle. Let C1 = {Pi (Xi, Yi)| i = 1 . . . N} be the set of data points within the N-circle. Then, a straight line, L1: Y = m(X–XIP) + YIP, can be computed minimizing eq. (6.3): ε=

N X

ðmðXi − XIP Þ + YIP − Yi Þ2

(6:3)

i=1

The N-circle which is centered at the initial point and has radius R, intersects with line 1* 1* 1* L1 at points, P1* start and Pend . Pstart and Pend can be assumed to be the starting and ending points of the line segment L1. But there are all possibilities that these two points may not be among the points that lies within the N-circle. In order to combat this problem we can have two points inside the circle and in the vicinity to the points P1* start and , respectively, as the starting and ending points. Let the point in the vicinity to P1* end 1 1* 1 be P , and the point in the vicinity to P be P . A new N-circle is obtained P1* start start end end with P1start P1end as the diameter. The unit oriented vector of this neighborhood can be defined using points P1start P1end as (ŝ1 = P1end – P1start )/ ‖P1end – P1start ‖). All the other points that lie in this circle are also deleted. The data set C with the remaining points will also get updated. The role of initial radius is very critical in drawing the line segment. If the value of initial radius R is very small, for example, R1, then only a small number of points are included for the first round, giving a bad correlation coefficient. This leads to the reduction of R in the subsequent iteration and then the iteration terminated with an even smaller value of R which may be even 2 to 3 points inside. On the other hand, if the initial R is too large, for example, R2, we may have a satisfactory correlation coefficient but loss of the fine corner feature is inevitable. Therefore in our algorithm, we select the initial value of R so that there are at least 30–50 data points in the region selected producing satisfactory results. However, this number also depends on the scanning resolution and so a finer resolution must be opted for so that there are sufficient data points.

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6.2.4.5 Constructing the remaining line segments (Si) There is a slight difference in the methodology for constructing the remaining segments. We start with P1end as the start point for the second segment, and let it be represented by P2start . N-circle for S2 is then determined adaptively. The remaining algorithm for constructing the remaining line segments is same as that used for constructing the first segment. The algorithm is discussed next: Algorithm for finding neighborhood Si Given: a planar data set C, Pistart The initial start point R = initial radius of N-circle, ΔR = increment of radius, ρlow = the predefined lower limit of correlation coefficient and, ρhigh = upper limit of correlation coefficient. (a) N-circle with radius R is constructed with center at Pistart . From C, select all points Pk, such that ‖Pk – Pistart ‖ ≤ R. the points selected will form a data set Ck (k = 1, 2, . . ., n). Calculate the correlation coefficient ρ of data set Ck. If ρ is less than ρlow, then Select R = R – ΔR and go to step (b). However, if value of ρ is greater than ρhigh, then Set the value of R = R + 2ΔR and go to step (b). To compute a regression line that passes through Pistart , the least-squares method is employed. The N-circle intersects with the regression line at O1 and O2. n X Now consider Pave = Pk =n k=1

If ‖ Pave – O1 ‖ is greater than ‖ Pave –O2 ‖ Then *

Ŝi = ðO2 − O1 Þ= k O2 − O1 k ; Otherwise; *

Ŝi = ðO1 − O2 Þ= k O2 − O1 k . (b) Now N-circle having radius R and with center at Pc, where Pc = Pistart + RŜi* is constructed. Select all the points Pk from C, such that ‖ Pk – Pc ‖ ≤ R, to form a data set Ci. Calculate the value of the correlation coefficient (ρ) of data set Ci (c) If the value of the correlation coefficient ρ is greater than ρhigh, then Select R = R + 2ΔR and go to step (d). However if the value of ρ is less than ρlow, then Select R = R − ΔR and go to step (d). (d) Use the least-squares method to compute a regression line that passes through Pistart . This line * i* i i* has two intersection points, Pistart and Pi* end , with the N-circle. Set Ŝi = (Pend – Pstart )/ ‖ Pend – i Pstart ‖. Go to step (b).

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Because we do not have any prior knowledge about the neighborhood of Si, that is., the unit oriented vector Ŝi*, we need to find a reasonable estimate to start the iterative process. This is achieved in step (a) of algorithm to find neighborhood Si. We start by choosing a small R to create an N-circle (centered at Pistart ), so that the points within this circle have good linearity. Then, we compute a regression line that passes through Pistart , which helps determine a good estimate of Ŝi*. From step (b), we start with an N-circle (centred at Pc = Pistart + RŜi*) and adaptively find the maximal allowable neighborhood radius. Pistart and Pi*end , and all the set of points from data set Ci are the outputs from the above algorithm. The above algorithm is applied iteratively to construct Si+1, until the remaining planar data set C is a null set. The computation time is very large because in each round, a regression procedure is implemented. In order to decrease the computation time, one of the solution is to utilize the Ŝi* obtained from step (a) throughout the remaining iterative process. Doing this the computation can be made more efficient. 6.2.4.6 Determination of adaptive layer thickness The next step after construction of 2D polygon is to determine the thickness for each layer. The layer is then extruded to the RP direction. The thickness is determined adoptively by comparing the shape error with the prescribed tolerance value. The shape error is calculated by calculating the distance between the original set of data points in the space and the projected points on the projection plane. Initially, small thickness is chosen for the layer. The layer thickness is increased adaptively, that is, the thickness is increased until the shape error reaches the prescribed shape error tolerance. This is an iterative procedure to obtain the final layer thickness. This is how the construction of first layer is constructed. The construction of subsequent layers takes place in the similar manner. After stacking the different layers, RP model is generated.

6.3 Other reverse engineering applications 6.3.1 Reverse engineering and the automotive industry There are number of applications of reverse engineering in the modern automotive industry. Some of the applications are discussed next. 1. The reverse engineering is used to create free-form shapes. Creating free-form shapes is difficult in the different available CAD modules. 2. The data integrity and the data exchange can be performed hassle free using RE.

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3.

There are number of complex geometries that can be produced very easily and at much quicker pace in comparison to other CAD modules. This gives a competitive advantage to the automotive industry. 4. Due to the difference in the CAD model and the actual tooling, discrepancies are inevitable. These can be resolved using RE. 5. The processes of combustion design, ergonomics design and aerodynamics is much quicker relative to the other software modules. The CAD module adds unnecessary steps during such designs. 6. RE allows for computer-aided inspection and engineering analysis. This aids in improving the quality of the product. In this section, we will look at an example of reverse engineering in the automotive industry. 6.3.1.1 Reverse engineering – workflow for automotive body design After conducting proper survey, the information from the survey is extracted using sophisticated methods. Consumers and the focus groups are involved in the survey process. The design models of the car are analyzed with mathematical simulations and production constraints. Out of 6 full-scale car design models 2 models will be redesigned on the basis of customer feedback and manufacturing constraints. At the end of the redesigning process one of the car design model goes through production and is finally sent in the market. Using conventional computer aided design manufacturing, the time a typical automotive process takes right from its design to manufacturing till its launch is usually around 3 months. However, this is not acceptable from the point of view of competition aspect. Therefore employing reverse engineering process, the time of 3 months can be reduced to 3 days. This is illustrated in the steps below. A one-quarter-scale model of the car body is made by the designers in the beginning of the 3-day process. The 3D optical noncontact scanner as discussed earlier in the previous section is employed for the measurement of the small-scale model. The output is in the form of multiple pieces of dense 3D point cloud data. The merging of the multiple pieces of 3D point cloud data is done to obtain a highly dense single point cloud, truly representing the shape of the small car model. Surface in a polygonal mesh is constructed with the aid of reverse engineering software packages. This is then used as output to RP machine and other engineering analysis software. Also a non-uniform rational B-spline (NURBS) surface is constructed which is utilized as an output to computer-aided design and manufacturing software. The result is a digital model which is sent for refinement to surface design group. The digital model produced is also sent for initial calculation of manufacturing constraints to the manufacturing group.

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When the digital model of the small car model goes to the manufacturing group, the model is scaled up to full size and manufactured using clay or any another material. Measurement of the full sized model is then done using optical noncontact scanner. A comparison regarding the tolerance of the designer created small car model and the full sized physical model of the car is done using dense point cloud generated from the scanner. Once the model passes the tolerance test, the full-size CAD surface is reconstructed by the designers using reverse engineering software. The traditional CAD software is used to construct the functional design. However if model fails in the tolerance test, the clay model is modified, and the scan, remodel and compare steps are carried out iteratively until the physical model passes the tolerance test. The output of the engineering group is the final CAD model. While the output from the body design group is a final surface model. The two outputs are merged as the “golden” model and the remaining production is carried out using this merged model.

6.3.2 Reverse engineering in the aerospace industry In an organization as mammoth as an aerospace, instituting changes with so many suppliers is a bit difficult task. The changes may include reducing the physical inventories of the legacy parts by digitizing them. The change may include creating digital data for parts that are no longer manufactured in the country of origin. But changes are an inevitable part of an organization and therefore most of the aerospace companies have been adopting reverse engineering as a tool to help the organization go for instituting changes. The improvised scanners, surfacing software and computers have together made the process much faster and better. The maintenance of parts and the saving in the storage cost justifies for the investment in reverse engineering by the aerospace industry. The case study is discussed next for cost reductions of hard tooling. 6.3.2.1 Reducing costs of hard tooling Tooling is needed when drawings and digital models do not exist. It’s also needed when the drawings or digital models available are not accurate enough. Hard tooling is very vital for making repairs of the aeroplane spare parts. One of the major goals for any manufacturing industry would be to take care of the costs incurred in tooling. Similar strategy was adopted by the airline manufacturing company Boeing. Apart from Boeing there were different industries that took care of the rapid tooling cost to make their end product cost effective. For the aerospace industries it is very important to keeping aging aircraft flying. The importance can be compared to the importance of working computer to information technology.

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Reduction in the cost incurred for hard tooling optimized the profit margins for the airline manufacturer. Boeing decided to have reverse engineering process to save time for digitizing the physical models. Boeing gave the responsibility of reducing the hard tooling cost and finally its eradication to its manufacturing research & development (MR&D) department. The department was asked to develop reverse engineering techniques and a schematic process to accomplish their goal. Boeing utilized the 747 wing to examine the reverse engineering technique. The entire process of examination was split into different phases and all to be carried out in a single day. The different phase were (1) replicating of the tip of the wing using reverse engineering software, (2) using the scanned cloud of points to generate a CAD model and (3) using the RP processes to generate the part. The goal was to manufacture a part with a tolerance value as close to 0.003 of an inch. The Boeing’s custom made systems were employed for scanning the wing tip. Engineers in Boeing used the linear-laser scan. Further, mechanical motion controller was also used by the team. CAD model was created using different software packages of the reverse engineering. The part consistent with Boeing’s new product design was manufactured on site. The goal of the company was to obtain a CAD model that could be universally used for the manufacturing the spare parts of any old plane and production of a new airplane. Also if the process could be made easier and cost-effective, then there would be a huge cost saving in maintaining hard tooling. It became possible for the company to store a database of 3D CAD files for hard tools. This task was made possible by employing the reverse engineering process. It was possible for the company to produce parts for repairs using an on-demand manufacturing process. The technology at that time was not mature enough as the entire process of duplicating a wing took more than a day, although the wing was produced within permissible tolerances. Pre-release betas of first-generation reverse engineering software were employed by the Boeing. Today, the technology is mature enough to reproduce a part like the wing tip within a day using different software packages of reverse engineering. However, since the entire process has not been tested in the real environment, the software packages of reverse engineering have not been adopted widely to reduce the costs of hard-tooling maintenance. Although companies spend large amount in purchasing new components but they are still reluctant to spend money on producing duplicate spare parts, despite the huge cost-saving potential. However the long term benefits of creating a digital inventory of hard tooling has been realized, especially in an economic environment where profit matters.

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6.3.3 Reverse engineering and the medical device industry Reverse engineering is becoming the integral part of manufacturing medical devices. Fabrication of medical devices to suit the needs of the people affected in one or the other way has led to the improvement in quality of life. With the aid of reverse engineering modules, medical devices can be manufactured in large quantities in a lesser period of time. The process of reverse engineering starts by scanning the part of the body that is to be replicated. The process then goes on to manufacturing the true 3D shape while ensuring a perfect fit. However in the recent years, the medical industry has been able to widely benefit by the same technology advancements that crept into automotive and aerospace industries, as has been discussed previously. Now the 3D scanners and industrial desktop CT can cost-effectively measure firm surfaces, such as ear and dental impressions. Even the nontechnical operators can easily operate the custom software. The medical-grade materials for automated fabrication systems have made these systems production-friendly and efficient. Production of any medical device typically consists of 4 major systems. This has been discussed next: 1. Measurement system: Hardware that measures and captures human anatomy. The result is a data set of spatial points. 2. Software design system: For reading into the generated spatial points different softwares have been developed. The design process is automated. The result is the digital data that can be used for manufacturing. 3. Fabrication system: The fabrication system should be such that from one-of-akind digital data, a large number of medical devices can be manufactured. To accomplish this, different hardware system has been developed. 4. New materials: The materials used for the fabrication of medical device are classified under new materials that are better than materials used in traditional manual processes. A significant impact has been made by the turnkey systems. These systems have combined the 4 technological advances. Some of the medical fields being benefitted from these systems are orthodontics, general dental and hearing instruments to name a few. The employability of reverse engineering in medical field has been explained using a knee replacement example, which is discussed next. 6.3.3.1 Reverse engineering – A better knee replacement One of the major problems that have grappled the knee replacement procedure is the wear and tear. This problem has limited the life span of knee implants to 15 or 20 years. The limited life span of knee replacement is becoming a concern, because more and more young patients are being diagnosed with knee joint problems.

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In recent years the evaluation and testing for artificial knee designs are carried out by researchers. This is done by simulating a real-world environment which is specific to individual patients. Reflective markers are placed on the patient’s skin and clothing. These markers capture the patient’s motion by employing LCD devices. The obtained data is stored in a computer. Dynamic X-ray procedure is then augmented with the motion capture process. This dynamic Xray procedure is known as fluoroscopy that accurately measures knee joint motion for natural or artificial knees. CAD models of the implant components were obtained by the researchers from the manufacturer. The obtained 3D models were matched with each of the 2D images produced from fluoroscopy using custom software. Quantifying of patient’s motion of the knee was done by the image-matched components . On the other hand it is arduous task to study natural knees. This is because of the unavailability of CAD models of the bones. CT scans are used for producing models of natural knees. Static 2D image slices of a patient’s leg are produced through the CT scans. The data from the CT scans are imported into image-processing software. The 2D slices are stacked to create a 3D point cloud model. The generated cloud of points is converted to 3D model using the reverse engineering software. In order to compare the built model with the already existing part, polygonal models are constructed first. The polygonal models are then transformed to mathematical surface models. These models can be used for carrying out analysis, tests and so on. Analysis such as contact stress analysis is carried out to judge the function ability of the fabricated part. After incorporating the different test results and analysis, the final wear model is created. This wear model aids in pinpointing the places of failures of an artificial knee.

6.4 Conclusion From the discussions in the preceding sections, the role of reverse engineering to replicate the existing product to fabricate the object by using cloud data points has been clearly delineated. This can be useful in particular where there are no sufficient technical details available to manufacture a product directly using the different RP processes. There are 3 major steps that RE processes employ: first is the proper scanning of the object by employing different 3D scanners. This is done to obtain the cloud of data points. Second, the cloud of data points is projected to construct 2D polygons. In the last step, the thickness of each layer is determined adaptively. The layers are then stacked together to obtain the desired product. The algorithm has been used widely. The two main aspects of the algorithm are presented as follows:

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(1) The construction of polygon starts automatically after a feasible starting point is detected. It takes care of different intersecting polynomials and segments with the multiple loops. (2) The process of determining the thickness is also adaptive. The final thickness is arrived after following the algorithm iteratively. This adaptive control leads to the building of the part at a much quicker pace.

Chapter 7 3D bioprinting 7.1 Introduction Tissue regeneration is one of the research areas that is being studied extensively as it is the main process involved in the cell growth. The tissue engineering mechanism also takes care of the reconstruction of the organs. Transplantation, repair and replacement of organs are viable options for patients with damaged organs. There is an extensive waiting list for organ transplantation around the world. As such scientists around the globe are working hard to discover alternative ways that can aid in doing away with the shortage of organs around the globe. Tissue engineering as such has been considered to be the effective method that can help not only save lives but also aid in improving the overall quality of life. Tissue engineering since its inception in 1993 has been employed to aid in development of practical solutions that result in replacements for damaged tissues through the application of biological and to the principles associated with engineering. Scaffolds have been employed extensively in tissue engineering as templates for interaction among cells and also aid in providing support to the freshly developed tissues. Scaffolds also act as delivery vehicles to control and enhance the growth of tissues. Printing precursors in the form of combinatorial cells and biomaterials are employed for bioprinting of scaffolds. Controlled fabrication of scaffolds and cellular distribution can be thought of with advanced techniques of printing such as bioprinting. Printing resolution of 3D printing techniques ranges from 10 to 10,000 µm, thereby demonstrating its flexibility over other assembly methods. 3D printing technology is based on biomaterial deposition at microlevel and therefore develops subtle structures bearing resemblance to the tissues. The movement of extruders printing in bioink is controlled by platform controlled using the three-axis mechanism. The designer creates the coordinates and saves the created coordinates in a file format such as g-code. This g-code format can be easily followed by the bioprinter. 3D printing has been used extensively for wide range of applications, owing to its numerous advantages such as cost-effectiveness, precise deposition and controllability for cell distribution. Wider acceptability has called for new bioinks that can provide for the requisite properties resulting in successful printing. This chapter therefore presents some of the outstanding works in the major bioprinting methods. The challenges in development of bioink development and applications have been presented toward the end of the chapter.

https://doi.org/10.1515/9783110664904-007

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7.2 Extrusion-based bioprinting Scaffold fabrication has been made possible through the introduction of alternative method in the form of extrusion bioprinting which has been studied widely and hence employed on a larger scale. Bioprinting relies on clear-cut processing methodology that ultimately results in simplicity, diversity and predictability for this technique. Extrusion printing has been supported by the bioinks with the viscosity ranging 30–6 × 107 mPa s. Higher cellular densities are offered by extrusion-based bioprinting in comparison to the ink-jet bioprinting. However, the extrusion-based bioprinting offers lower speed and resolution in comparison to the ink-jet bioprinting. Some of the other advantages are the availability of printable biomaterials at wider scale and the presence of economical and user-friendly equipment. Commercial bioprinters are used extensively by the researchers to carry out research activities and the scientific community focusses on to enhance the quality and suitability of bioprinters to print wide range of biomaterials. The three major factors that need consideration toward printability via extrusion bioprinting involve adjustability of viscosity, biofabrication window depending on the material characteristics and the bioink phase before the extrusion process. Viscosity is dependent on temperature and shear thinning and is required to be adjusted for different methods of printing. Clogging of nozzles needs to be avoided by maintaining liquid phase of the bioink. Numerous studies have been carried out in the domain of extrusion bioprinting. Rees and his group in 2015 considered two types of 3D printed structures in the realm of nanocellulose and employed it for wound dressing. (2,2,6,6Tetramethylpiperidin-1-yl) oxidanyl was used in the preparation of the first type and combinatorial periodate oxidation with the process of carboxymethylation led to the development of the second type. 3D porous structures were printed using the developed nanocellulose ink and were studied as a support structure for bacterial growth. The study revealed that the developed 3D porous structure had the capability to carry as well as release antimicrobial components, while these structures revealed to not support the bacterial growth process. Tissue strands as a bioink were printed from the coaxial nozzle system by Yu and Ozbolat. The bioink was revealed to print the organs successfully. A mouse TC3 cell viability as close to 90% was revealed through the developed alginate-based bioink. The applicability of the developed bioink was further demonstrated for pancreatic tissue by incorporating human umbilical vein smooth muscle cells into the developed bioink. In another study, 3D printed tissue constructs laden with cells were printed using combinatorial collagen, hydrogenbased gelatin and alginate. The degradation rate of hydrogel and its control was one of the main factors for their study and this was achieved by considering changes in the molar ratio of sodium alginate and citrate present, respectively, in medium and hydrogel. The alginate bioink was improved and the improvement was indicated through high cell proliferation rate.

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Although there has been extensive employability of bioprinting, the current technologies in the bioprinting domain does not have the potential application to print functional solid organs. Researchers have been working on this major limitation of bioprinting and they have been successful in fabrication and development of templates that could be employed in vivo and aid in providing support as well as the development of vascularized solid organs. Gamma-irradiated alginatebased bioinks were encapsulated with stem cells and poly(ε-caprolactone) (PCL) fibers further reinforced the bioink. The osteogenesis was enhanced by the incorporation of Arginylglycylaspartic acid (RGD) peptides. A construct of cartilaginous structure exhibiting similar appearance to the vertebral body conceptualized as a result of the fabrication done and it was demonstrated to successfully support the vascularized bone development process. Combination of different biomaterials has been used by the researchers to achieve the targeted properties and hence fulfill the requirements associated with the desired applications. Alginate and gellan were employed in one of the studies for the preparation of new-fashioned bioink. This was done along with BioCartilage and was used for printing cartilage grafts. The cartilage grafts were able to support proliferation of chondrocytes. A polymeric material loaded with cations was also employed in order to provide support and hence bring stabilization to the overhanging structures. Alginate has been a commonly used biomaterial as bioink, and most of the studies have been based on native alginate bearing limited degradation characteristics. The alginate hydrogels with its oxidized variants were studied and were revealed to exhibit various degrees of oxidation and during the study the due consideration was given to control the degradation rate (Jia et al., 2014). Alginate solutions were also studied for the effect of viscosity and density extensively by researchers and these studies were meant to address their printability aspect. human adipose derived stem cells (hASCs)-loaded alginate solutions with different biodegradability levels have been studied for their potential ability to control the distribution of cells. Low cell-activating properties have been exhibited by alginate-based bioinks. This is one of the weaknesses exhibited by bioink and was overcome by Lee et al. (2015). They have bioprinted porous 3D structures using novel bioink consisting of alginate and collagen. The developed bioink demonstrated decent cell viability and relatively higher activity associated with osteogenicity. One of the factors that played a crucial role in the development of bioink was the chosen biomaterial. Owing to this reason, the biomaterials compatible with cells in commercial or noncommercial devices are normally chosen. The variables are therefore minimized and it becomes viable to predict the outcome. For instance, RGD-phase solution was employed by few researchers who displayed versatility with the cell printing ability and in another study the proliferation of MC3T3 cells was revealed to improve proportionally with the concentration of phases. Pati and his research team in 2015 devised a new methodology to aid in cell printing using DAT bioink.

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Gelatin was employed in another study to fabricate 2D and 3D cell encapsulating structures. The printing precision was enhanced using polyethylene oxide (PEO). The hydrogels printed showed support for cell proliferation as well as cell spreading. A blend of gelatin and hyaluronic acid was considered in another study and was used to prepare liver-specific bioink. The developed bioink was also expanded for its applicability toward other tissue types. Tandem gelatin mechanism was adopted by Kesti and others to cross-link a blend of poly(N-isopropylacrylamide) grafted hyaluronan. A good fidelity was exhibited by the developed bioink and also a suitable mechanical stability was demonstrated. Furthermore, no direct toxicity associated with the cell cultured on the surface was observable. There are materials that may or may not be compatible to be 3D printed. For instance, Pluronic is a thermosensitive polymer that has been used extensively for drug delivery systems and dressings of wound. Good printing properties are exhibited by Block Pluronic but it has poor properties of cell culture. The biocompatibility was, however, improved through the method proposed by Muller and his group in 2015 and was achieved by blending acrylate with unmodified variant of Pluronic. The blending was followed with cross-linking through the employability of ultraviolet (UV). With recent advancements, new extrusion-based methods have evolved. A core–shell nozzle is being utilized for printing the cross-linking agent and it was revealed that the printing is accomplished at the same time as the bioink from the core of the nozzle. Core–shell printing method has been employed for rapid printing of alginate 3D constructs. Cell viabilities of 93% and 92% were observed with the printed mesh structures for hASCs and preosteoblasts. Cell-laden bioink was printed by Yeo et al. (2016) by employing this methodology. The core barrel was filled with the cell encapsulating collagen bioink while the shell was filled using the alginate and the cell viability was revealed to improve. A multilayered meshed structure was obtained using the aerosol cross-linking method. A noticeable higher cell viability was demonstrated by this methodology and was far better in comparison to the alginate-based bioink. Another viable material that is used widely for the 3D printing of structures as that of tissues is the silk protein. Its potential alone has been explored widely and has also been explored in tandem with the other known biomaterials. It has been revealed that silk possesses higher mechanical properties and is biocompatible. Furthermore, silk possesses diverse side chain characteristics and therefore allows for decoration with growth and adhesion factors. A 3D spider silk structure was fabricated in one of the studies by Schacht (2015). The constructs were printed through robotic dispensing mechanism with no incorporation of cross-linking additives. Hydrogels were cultivated through the incorporation of different cellular lines. Once the cultivation was achieved, studies associated with cell proliferation and adhesion were undertaken. One of the major advantages was that no post print crosslinking was required as is required with the other bioinks. For a week of notice, the

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construct was revealed to have good cell viability as well as cell proliferation. 3D tissues were also built by the utilization of blend of silk fibroin and gelatin. Cross-linking mechanism encompassed mushroom tyrosinase and physical crosslinking. The cross-linking was achieved through the sonication process. Bioink has been studied for its potential application in surrounding the nasal cavity interior, that is, on the mesenchymal progenitor cells derived from turbinate tissue. The 3D structures developed were revealed to exhibit differentiation of encapsulated stem cells. The printing resolution was enhanced through the blended silk and glycerol. As a result, it was possible to meet the patient-specific requirements and needs associated with the soft tissue regeneration. The studies that were conducted on the developed material demonstrated the stability and biocompatibility of the developed material and at the same time supports integration of tissues.

7.3 Laser-assisted bioprinting In 1986, Bohandy presented laser-induced forward transfer (LIFT) technique that allows for deposition of material in liquid and solid phases at high resolution. There are several existing versions of the same. One such variation is that of pulsed-laser evaporation direct wire and has been employed for cell printing. NIH3T3 mouse cells were incorporated onto sodium alginate and was employed as the bioink in tandem to calcium chloride. Calcium chloride here was the cross-linking agent. Studies have been made on the effects of alginate gelation and concentration as well as at the time of gelation. The effects were studied on the cell viability. The longer gelation time resulted in decreasing cell viability after a duration of 24 h incubation. The reason could be attributed to reduced oxygen and nitrogen transfer through the thick gel wall. The LIFT technique demonstrated a successful cell transfer, and the survival rate was, however, below 85%. The main cause of cell death was revealed to be the nanosecond laser irradiation. Femtosecond lasers were therefore employed to diminish the probability of cell death. An improved variant of LIFT referred to as absorbing film-assisted LIFT methodology has been studied by Hopp (2012) and was reported to transfer the living cells in a controlled manner onto the surfaces of the acceptor. The experimental result revealed that the proposed femtosecond LIFT methodology was able to achieve the higher rates of fatality in cells in comparison to the LIFT method within the nanosecond range. The reason could be attached to the strong photochemical influence of laser beam. Bioprinting with laser was also investigated for the printing of cell-laden 3D structures through the LIFT method. 3D skin tissue like structures were printed employing the collagen encapsulating fibroblasts and keratinocytes. Resistance to damage was exhibited by the cellular lines during the process of bioprinting through laser.

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Printing of cells can be achieved either by printing onto the layer of extracellular matrix (ECM) or can be printed as encapsulated particles in a biomaterial with similar characteristics to that of ECM printable material. It is also critical to understand the viable effects of parameters associated with printing on the cellular feasibility. In one of the conducted studies, the effects on the cell viability of different parameters such as ECM thickness, energy of the laser pulse and the viscosity of the developed bioink were investigated. The different print settings were compared by measuring the cell viability after 24 h of printing. It was reported that a higher laser energy results in a higher probability of cell fatality while on the other hand the cellular feasibility increased with increasing thickness of the bioink film as well as its viscosity. Guillotin (2012), on the other hand, investigated the effects of bioink viscosity, printing speed and laser energy on the resolution of printing. The resolution at microscale was reported to be achievable along with the frequency of printing at 5 kHz.

7.4 Stereolithography-based bioprinting Polymerization of light-sensitive polymers forms the major working ground for stereolithography printing process. The process is accomplished through the aid of controlled light glinting from digital micromirrors. Stereolithography has higher quality of printing, better cell viability and quality of speed in comparison to other methods of printing. However, there are certain drawbacks associated with the method such as the harmful nature of UV light source that possesses danger to the DNA cells. The harmful UV rays can even lead to skin cancer. The disadvantages have been addressed by the development of visible light stereolithography bioprinting systems. Wang (2015) employed such systems of bioprinting that encompasses blends of polyethylene(glycol)diacrylate and eosin Y-based photoinitiators bioink and beam projector. Melchels (2010) studied the versatility, precision and controllability of stereolithography process. They fabricated porous scaffolds using poly(D,L-lactide-co-єcaprolactone)-based resin. The mechanical properties were revealed to be controlled through the variation of macromeres and the porous architecture. Scaffolds were also prepared using photo-cross-linkable PCL-based resin encompassing networked gel. Stereolithography was used to prepare the porous scaffolds by employing the resin prepared using the photoinitiator such as Irgacure 369, dye and the inhibitor. Scaffolds that were fabricated were revealed to match the design requirements and the suitability of the developed resin to construct scaffolds in tissue engineering was also proved and demonstrated. In one of the studies, the 3D scaffolds were designed using a projection stereolithography platform. This was done through the employability of computer-aided design (Gauvin et al., 2012). The mechanical properties were controlled using

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various structural forms of GeIMA and its concentration. Complex porous constructs were seeded using human umbilical vein endothelial cells and then the in vitro studies were conducted. The cell growth was demonstrated to be supported with the fabricated scaffolds encompassing precisely connected pores. This in turn resulted in enhanced cell densities. Stereolithography was also employed for the fabrication of porous constructs from the developed resin formed from combinatorial 2-armed poly(D,L-lactide), inhibitor, ethyl lactate, dye and photoinitiator. Better characteristics such as preosteoblast adherence and comparable proliferation were reported. 3D printers with blue light digital printing capabilities commercialized for stereolithography were used for the preparation of polyurethane with hyaluronic acid. The development was employed in particular for the purpose of tissue repair. Photosensitivity was exhibited by the developed material and it was reported that they were nontoxic, cytocompatible and supported higher resolution. Furthermore, they also revealed better promotion of cell adhesion, differentiation and proliferation. In another study, development of poly(D,L-lactide)/N-vinyl-2-pyrrolidone resin was studied and it was functionalized using fumaric acid monoethyl ester. The developed resin was employed for the fabrication of scaffolds with the aid of stereolithography. Mouse preosteoblasts were revealed to adhere onto the surface of the material. Further, the uniform spreading was also revealed. Yet in another study, the resin composed of diethyl fumarate, poly(propylene fumarate) and bisacrylphosphrine oxide was developed and was used for the fabrication of scaffolds by stereolithography. Fabrication of constructs was carried out in a study in a controlled approach by optimization of resin and parameters associated with laser. Utilization and hence commercialization of projection-based stereolithography process employing particles of sugar have been demonstrated for their feasibility and optimized microstructures with higher porosity have been achieved. The porosity of the scaffolds was revealed to enhance by twofold in comparison with the already existing methods of stereolithography. Utilization of PEG hydrogels with the stereolithography process has been delineated in a number of literatures. The presence of photoreactive groups was the source for cross-linking of PEG into hydrogels. 3D layered structure of 3D PEG hydrogel builds has been developed by employing two different molecular weights of PEG and through the aid of stereolithography process. Investigations were carried out on the effects of different factors such as concentration of photopolymer, photoinitiator and dose of energy on the properties of the two PEG hydrogels. The effects of the parameters associated with the stereolithography process were also investigated through the in vitro cell studies. Employability of poly(trimethylene)-based carbonate resin was investigated for stereolithography by Schüller‐Ravoo (2013). The results were revealed to enhance the attachment of bovine chondrocytes. Even the diffusion as well as the differentiation of the bovine chondrocytes was improved. As

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such evidence of the applicability of the developed resin for the domain of cartilage tissue engineering was demonstrated and justified. Scaffolds made using stereolithography have been vastly employed for tissue engineering associated with the heart valve. The resin was formed using combinatorial thermoplastic elastomer, a polyhydroxyoctanoate and poly-4-hydroxybutyrate. Synchronous opening and closing of the valves was revealed using the direct pressure measurement approach. Development of sacrificial molds has also been approached through the application of stereolithography and has been employed for the preparation of scaffolds. The architecture of the gel-cast glass ceramic tissue scaffold was controlled in a study conducted by Chopra (2012). Similarly, ceramic stereolithography was employed by Bian in the same year to fabricate beta-tricalcium phosphate/collagen scaffolds. The scaffolds produced were revealed to have high resolution and were suitable for bone tissue engineering. In another study, development of epoxy/hydroxyapatite was investigated for its employability with stereolithography scaffolds by Scalera (2014).

7.5 Challenges, applications and future perspective A number of challenges exist in the domain of tissue engineering. The associated challenges can be categorized into two diversified groups: first, being that of biomanufacturing that involves the fabrication of biomaterials and cells and the second group of challenges can be categorized into in vivo integration that involves postimplantation integration and functionality. Clogging of nozzle is one of the challenges associated with biomanufacturing in case of nozzle-based manufacturing. Fabrication time can take several hours of manufacturing. As such the printing precursor requires to be homogeneous. Furthermore, shear thinning properties as well as proper viscosity are other recommended required aspects. Stability of 3D construct is another challenge so that successful transplantation can be ensured. For instance, higher elastic modulus is required in case of hard tissue repair so that design structure could be maintained and also the porosity. Any newly formed tissue will fail owing to the scaffold deformation if the scaffold fails to maintain the required design and structure during the prolonged functioning. Bioprinted builds are required to support and endure the very phenomenon of vascularization so that the cellular population provided the sufficient nutrition, oxygen and other factors necessary for the growth. The distance of the capillaries is within the distance range of few micrometers from the cellular walls and as such the cells can survive owing to the sufficient diffusion. However, additional means of diffusion may be required in case of distances more than 100 µm such as in the case of printing thick tissues. In order to overcome this challenge, an artificial vascular system was suggested by Hutmacher and others who aided to enhance and

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improvise the transportation of nutrient matter, oxygen and also in removal of generated waste products. There has been swift expansion of 3D bioprinting industry toward the potential and diverse applications. It is predicted that the 3D bioprinting size is expected to reach $10.8 billion in 2021. Currently, there are numerous companies that have been working on 3D bioprinting of structures and constructs for tissue engineering applications such as liver tissue, bone, breast and cartilage. One such company is Tissue Regeneration Systems that has been working on bioprinting of tissues and has been successful in production of such systems. The company has been developing bioprinted PCL-based solutions that are customizable as per the patient requirements. The solution provided has been approved by the Food and Drug Administration for skeletal reconstruction. Another company by the name Organove has developed and introduced the bioprinted human tissues such as the liver tissue that was designed and investigated to evaluate and analyze the toxicity associated with the drug. A commercial liver tissue has, however, been not developed till date even though the product offers in vitro drug screening. Generally, there are two groups into which the bioprinting applications can be categorized: tissue regeneration and bioengineering and biomedical applications. The first category is all about applications of bioprinted constructs such as skin, bone, neuron and vascular grafts, and the second category relates to drug delivery and biopreservation. Latest technologies in the form of bioprinting have been evolving continuously, and research and development activities have grown exponentially over the past decade. This exponential growth seems to continue for the upcoming decades. The quality of printing and its resolution will increase. As such bioprinting experience is being enhanced to encompass complex 3D structures and builds. The complexity associated with natural organs is very high and encompasses different types of tissues. Biofabrication of complex structures as such becomes real experience that accurately mimics the natural organs. An improvement in the structural complexity of bioprinted products and builds has also been improved through increased precision in fabrication of multimaterial 3D builds and constructs. The vascularization challenge can be overcome with the future developments in 3D bioprinting. Employment of microfluidic systems aids in realization of biofabrication of microstructures. It is also predicted that the advancement in biofabrication can benefit fields such as diagnostic applications.

7.6 Conclusion This chapter provides an overview of research and development in bioinks and 3D bioprinting methods. Extensive research has been carried out on past decades and is therefore a sign of its wide range of engineering as well as nonengineering applications. However, more research needs to be carried out to understand and wane

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the challenges associated with 3D bioprinting techniques. Research and development is still required at exhaustive levels so that more accurate bioprinting methods can be developed. Furthermore, development of multimaterial hydrogels and combinatorial approach combining different printing methods is some of the future research directions. Few of the bioprinted products are already available in the market. With the continual advancement, it is believed that the bioprinted builds and products will become expansively available in the open market and therefore aid patients to solve out their suffering from different diseases. Advanced techniques in the form of 3D bioprinting will become one of the strong fabrication tools aiding tissue regeneration and it is engineering.

Glossary 2D Abbreviation for two-dimensional. Often applied to the description of CAD systems (e.g., 2D CAD) indicating that the resulting file is a flat representation that has dimensions in only x- and y-axes. 3D Abbreviation for three-dimensional. Often applied to the description of CAD systems (e.g., 3D CAD) indicating that the resulting file is a volumetric representation that has dimensions in the x-, y- and z-axes. 3DP

See 3D printing.

3D Keltool® An indirect rapid tooling process where powdered metals are formed against a pattern and sintered. This technology is owned and licensed by 3D Systems. 3D printing (1) Rapid prototyping processes that use systems that are low cost, small in size, fast and easy to use. Often suitable for an office environment. Original process and terminology developed at MIT (Massachusetts Institute of Technology); now commonly used as a generic term. (2) Collective term for all rapid prototyping activities. 3-axis

Devices that have simultaneous motion in the x-, y- and z-axes.

5-axis Devices that have simultaneous motion in the x-, y- and z-axes and two rotational axes. ACES (accurate clear epoxy solid) Stereolithography build style that offered increased accuracy and improved surface finish when compared to earlier build styles. Additive manufacturing See rapid prototyping or rapid manufacturing. Alpha test In-house testing of preproduction products to find and eliminate the most obvious design deficiencies. See also beta test. ARP

(additive rapid prototyping) See rapid prototyping.

ASCII Coding system for representing characters in a numeric form. ASCII (pronounced “asskee”) files are text files that can be displayed on a screen or printed without special formatting or specific software program requirements. Aspect ratio Relative relationship between height and width. Expressed in integer form (not percentage) as the ratio of height to width, where each is divided by the width to yield a ratio of X:1. Associative geometry Placing and controlling graphic elements based on a relationship of previously defined graphic elements. Elements placed associatively maintain the relationship an element is manipulated. Associativity Operating under a single, integrated database structure. Allows changes in any application (i.e., design, drawing, assembly, mold, etc.) that are then reflected instantly throughout all associated applications as well as in every deliverables (e.g., drawings, bill of materials and NC tool paths) Axis (CAD) Imaginary line segment upon which all measurements are made when creating or documenting a CAD model in 3D space. The complete Cartesian coordinate system is comprised of x-, y- and z-axes.

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Glossary

BASSTM (break-away support structure) A style of support structure for the fused deposition modeling process that is manually removed after prototype creation. Benching For shop floor or model making operations, the process of finishing a part or prototype, typically with manual operations and hand tools. Examples are sanding, filing, joining and bonding. B-Rep (boundary representation) CAD software methodology that defines the model as a set of vertices, edges and faces (points, lines, curves and surfaces). B-spline (bicubic spline) Sequence of parametric polynomial curves (typically quadratic or cubic polynomials) forming a smooth fit between a sequence of points in 3D space. Beta test External operation of preproduction products in field situations to find those faults that go undetected in controlled in-house tests but may occur when in actual use. See also alpha test. Bezier curves Quadratic (or greater) polynomial for describing complex curves and surfaces. Binary system

Numbering system in base-2, using ones and zeros.

Bit Single digit number in base-2, or binary notation (either a one or zero). The smallest piece of information understood by a computer. Bitmap Matrix of pixels representing an image. Blow molding Manufacturing process in which plastic material, in a molten state, is forced under high pressure into a mold, causing the plastic to conform to the shape of the tool with a consistent wall thickness. Often used to produce hollow items such as bottles. BOM (bill of materials) Listing of all subassemblies, intermediate parts and raw materials that go into a parent assembly, showing the required quantity of each. BPM (ballistic particle manufacturing) Rapid prototyping process where wax materials are deposited with a multiaxis, ink jet print head. Process is no longer available. Bridge tooling Relating to molds or dies intended to fill demand between early prototype or soft tooling, and production tooling. Build time Length of time for the physical construction of a rapid prototype, excluding preparation and postprocessing time. Also known as run time. CAD (computer-aided design or computer-aided drafting) Software program for the design and documentation of products in either two- or three-dimensional space. CAE (computer-aided engineering) Software method using the design data of CAD for the analysis of mechanical and thermal attributes and behavior. This is accomplished through the use of finite element analysis (FEA) software for determining mechanical strength and thermal analysis. CAM (computer-aided manufacturing) Software program that uses the design data of CAD to build tool paths, and similar manufacturing data, for the purposes of machining prototypes, parts, fixtures or tooling. Children (1) Components of a design instance in a product structure tree. Also referred to as parts. (2) Nodes in a database tree structure that have a parent. (3) Also refers to features in

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137

parametric modeling. These features are dependent on others for establishing location in space. If the parent features are changed drastically, the children can become “orphans,” or unassociated. Chord Line segment that connects two distinct points on an arc. Chord height Distance from the chord to the surface that the chord approximates. One of several terms that relate to the control and tolerance of the STL file. CMM (coordinate measuring machine) A device that determines 3D spatial coordinates from a physical part. The output is typically used for inspection and can be used for reverse engineering. CNC (computer numerical control) Numerical control (NC) system in which the data handling, control sequences and response to input is determined by an on-board computer system at the machine tool. Coincidence Geometry that occupies the same spatial location. For example, coincident lines can have differing lengths while one occupies the same location as the other. Compression Process of compacting digital data to reduce file size for electronic transmission of data archival. Computer model Set of computer data representing a product or process and capable of being used to simulate the physical product or process behavior. Concept model Physical model intended primarily for design review is not meant to be sufficiently accurate or durable for full functional or physical testing. Examples are foam models, 3D printed parts and rapid prototype parts. Concept optimization/concept study Research approach that evaluates how specific product benefits or features contribute to a concept’s overall appeal to consumers. Product development tasks that help determine unknowns about the market, technology or production processes. Concurrent engineering Organization of product design, development, production planning and procurement that occurs in parallel rather than in series. The use of a project-oriented team structure to include input from all concerned parties. Conformal cooling Water lines in tooling that follow the geometry of the part to be produced, which creates higher cooling rates and lower cycle times. Unattainable prior to rapid prototyping techniques, significant research and development efforts are being made to understand and device optimal cooling strategies. Cavity Core

Mold component that forms the exterior or external surface of the closure. Mold component that forms the internal surface of the closure.

CSG (constructive solids geometry) CAD modeling technique that uses a hierarchical representation of instances of solids and combination operations (union, intersection, difference). CT (computed tomography) (1) Scanning system based on x-ray technology used to reverse engineer or dimensionally verify physical parts. (2) X-ray-based volumetric scanning used for solid objects (e.g., bone in humans, but also industrial components) with internal features.

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Cycle time Period between the start of an operation and the start of the next occurrence of the same operation. Dp (penetration depth) Variable for photocurable materials that specifies the depth of solidification at a known level of power input. Combined with Ec, these variables identify the photo speed of a resin. Design for manufacturability Process to ensure that a product or its components can be manufactured. The objective is to maximize the process rate and minimize the cost to produce. DFA (design for assembly) Application of a design philosophy to insure that parts and part designs are optimized for use in the assembly process. This step is important when automated assembly equipment is used to insure parts can be handled, oriented and positioned accurately. DFM

See design for manufacturability.

Die casting Manufacturing process that produces metal components through the pressurized injection of molten alloys into a metal tool (die). Typically used for high volume production. Digital modeling The concept of holding the master product design definition in purely digital form; the total information set required to specify and document the product. Related terms include virtual prototyping, virtual product development, soft prototyping and electronic product development. Direct When applied to rapid tooling and rapid manufacturing applications, the production of a tool or part from a rapid prototyping device without secondary manufacturing operations. Direct AIM Injection-mold tooling produced directly from a stereolithography process, where AIM stands for ACES injection molding. See ACES. Direct digital manufacturing Application of additive technologies (rapid prototyping) to the production of finished goods without the use of tooling or secondary processes. Direct digital tooling Application of additive technologies (rapid prototyping) to the creation of molds or dies without the use of secondary or intermediary steps. Direct metal deposition (DMDTM) Proprietary rapid tooling process from precision optical manufacturing (POM). Laser-based technology produces fully dense metal tools. Often applied to tool restoration. Direct metal laser sintering (DMLS) Rapid prototyping and tooling process from EOS GmbH that sinters metal powders. Direct shell production casting (DSPC) Rapid prototyping and tooling process from Soligen based on MIT’s 3DP technology. Ink-jet deposition of liquid binder onto ceramic powder to form shell molds for investment casting. DirectTool® Trademarked rapid tooling process from EOS GmbH for the production of metal tools using the company’s DMLS technology. DMLS

See direct metal laser sintering

Glossary

139

Drop-on-demand (DOD) Ink-jet methodology now incorporated in rapid prototyping systems, where the material is deposited in a noncontinuous stream. Drops are produced and deposited only as required. DOE (design of experiments) Methodology for running a statistically significant battery of tests (or computer simulations) on a design to determine its sensitivity to, or robustness for, design or manufacturing variations. DPI (dots per inch) Measure or resolution common to computer monitors and also applied to some raster-based rapid prototyping technologies where dots are equated to pixels or a single droplet of the material. DSPC

See direct shell production casting.

DXF (drawing exchange file) File format that allows for transfer of CAD data among dissimilar systems. Originally devised by Autodesk for the AutoCAD software program. Ec (critical energy) A variable for photocurable materials that specifies the energy required to solidify a given thickness of material. Combined with Dp, these variables identify the photo speed of a resin. EDM (electrical discharge machining) Electric current passed through a graphite or copper alloy electrode that machines metal with spark erosion. The electrodes have the same geometry as the intended part or profile to be produced (machined). Early adopters Customers who, relying on their intuition or vision, buy into new product concepts or new manufacturing processes very early in the product life cycle. Economies of scale Achieving low per-unit costs by producing in volume and permitting fixed costs to be distributed over a large number of products. Economies of scope Achieving low per-unit costs by computerizing production; allows goods to be manufactured economically in small lot sizes. Electron beam melting Proprietary rapid prototyping and tooling process from Arcam AB that solidifies metal powder with an electron beam. Element The basic building block used in geometric modeling. Elements include points, lines, curves, surfaces and solids. Enterprise Prototyping Center Rapid prototyping devices characterized by higher throughput, larger physical size, increased operator control, improved accuracy and enhanced surface finish. Often operated by a dedicated staff in a lab-like setting. Epoxy tooling Indirect rapid tooling process where the mold is created by casting an epoxy resin, usually mixed with aluminum powder, against a pattern. Suitable for injection molding in low quantities. Ergonomics Interaction of technological and work situations with the human being. Also called human factors. Extrusion Process where material, often in a molten or semimolten state, is forced through an orifice that gives the material shape.

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Family mold

Tool that has cavities for two or more different parts.

Facet Polygonal element that represents the smallest unit of a 3D mesh. These elements can be either three or four sided. The mesh represents an approximation of the actual geometry. Threesided (triangular) facets are used in STL files. Both three- and four-sided elements are used in finite element modeling. Facet deviation Maximum distance between the triangular element of an STL file and the surface that it approximates. See also chord height. FDM FEA

See fused deposition modeling. See finite element analysis.

Feature Discrete attributes of a model or prototype that include holes, slots, ribs, bosses, snap fits and other basic elements of a product design. Feature-based modeling CAD modeling method defined by a series of rules that are used to describe how features interact with each other to construct a specific solid. For example, the through-hole feature understands the rule that it must pass completely through the part and will do so no matter how the part changes. Finite element analysis Method used in CAD/CAE for determining the structural integrity of a part by mathematical simulation of the part and its loading conditions. Also used to predict the behavior of parts under a thermal load. First-to-market Fixture

Initial product that creates a new product category.

Used to hold and position the workpiece for a manufacturing operation.

Form and fit Shape and size of a component and its relationship to mating components. Often used in the context of design analysis of the adequacy of a part in terms of its size, shape and conformance to constraints imposed by mating or nested components. Free-form fabrication Alternative description of rapid prototyping. Intended to describe a broader base of application where components are generated directly from digital data. See rapid prototyping. Free-form surface Contours that cannot be defined with simple linear or quadratic mathematical equations. Many natural shapes, such as the human face, are examples. FTP (file transfer protocol) Communication standard for transferring data over the Internet or internal networks. Functional testing Evaluation of a prototype, in conditions similar to those that the product will experience, to determine its ability to operate as specified. Fused deposition modeling Rapid prototyping process by Stratasys Inc. The process extrudes a thermoplastic material and deposits it on a layer-by-layer basis to form a part. GARPA (Global Alliance of Rapid Prototyping Associations) Alliance of rapid prototyping associations, such as RPA/SME, from around the world that fosters the transfer of information related to rapid prototyping.

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141

Gradient material Graduated displacement of one material with another that yields a gradual transition between two materials. Gross profit Financial measure that equals sales revenue less variable expense. IGES (Initial Graphic Exchange Specification) Standard format for the exchange of 2D and 3D CAD data between dissimilar CAD software systems. Indirect When applied to rapid tooling and rapid manufacturing applications, the production of a tool or part from a rapid prototyping devices where secondary manufacturing operations are required between the rapid prototyping operation and the production of the desired item. Injection molding Manufacturing process where molten plastic is introduced into a tool or die with the use of pressure. Commonly applied to both prototype and production requirements. Interference checking CAD capability that automatically examines the intersection of objects within a 3D model. Investment casting Manufacturing process, which utilizes an expendable pattern (the investment), to produce metal parts. A ceramic mold is made by repeatedly dipping the pattern in a ceramic slurry solution followed by fine grain silica sand. The pattern is then burned out in an autoclave or furnace, which simultaneously sinters and strengthens the ceramic shell. Molten metal is then poured into the shell. After cooling and solidification, the shell is destroyed to reveal the final metal part. Keltool®

See 3D Keltool

Kirksite Low melting point metal used in the casting of large mold and form tools to produce low quantities of parts. This material is generally used to make large parts. Laminated object manufacturing Patented rapid prototyping system, originally from Helisys Inc. and now offered by Cubic Technologies, uses a laser to cut a cross section from the sheet material. These cross sections are stacked and bonded together to create an object. Laser curing Derived from the concept of fusing. Rapid prototyping and tooling process from Concept Laser GmbH that produces fully dense metal parts from powders that are fused with a high energy laser. Laser sintering Rapid prototyping processes that use heat, often from a laser, to fuse powdered materials, including plastics and metals. Layer (CAD) A logical separation of data to be viewed individually or in combination. Similar in concept to transparent acetate overlays. Layer (RP) A thin horizontal slice of the STL file used to fabricate a rapid prototype. Typically between 0.001 and 0.010 in. (0.025 and 0.25 mm) in thickness. Also see slice. Layer thickness Vertical dimension of a single slice of an STL file. Smaller dimensions often lead to smoother surfaces but may increase build time. Layer-based manufacturing

See rapid prototyping or rapid manufacturing.

142

Glossary

LENS (laser engineered net shaping) Rapid prototyping and tooling process that injects metal powder into a pool of molten metal created by a focused laser beam. Originally developed at Sandia and later commercialized by Optomec, Inc. LOM

See laminated object manufacturing.

LS See laser sintering. Machining General term for all manufacturing processes that produce parts or tools through the removal of material. Manufacturability Extent to which a product can be easily and effectively manufactured at minimum cost and with maximum reliability. Mass customization Method of production that stresses the manufacturing of small lots of customized goods rather than large volumes of standardized products. Mass production Large-scale, high-volume manufacturing of standardized parts. Relies on “economies of scale” to achieve low per-unit costs. Mass properties Characteristics of a solid that includes volume, weight, center of gravity and moments of inertia. MJM

See multijet modeling.

Mold inserts (1) Components of a mold core or cavity used to change geometry features in the mold. Provides alternatives to making multiple molds. Or, it is used in the repair of hardened molds to prevent degradation of the surrounding metal if welding was used for the repair. (2) Used in insert molds to insert a complete core and cavity complete with ejector mechanism and cooling into a frame, which is then installed into a molding machine. MRI (magnetic resonance imaging) (1) used to generate cross-sectional images of a solid part. Typically used for reverse engineering parts when 2D or 3D documentation is not available. (2) Used medically to can patients as a nonevasive method to check internal structure. (3) Process uses magnets to “align electrons” before creating a computer image. This image can be used to generate a 3D file then used to generate a rapid prototype. (4) Technique similar to CT scanning to examine internal geometry or structures. Multijet modeling Rapid prototyping processes from 3D Systems that use ink-jet technology to deposit materials. NC (numerical control) Method of controlling the cutter motion of a machine tool through the use of numeric data and standardized codes. In contrast to CNC devices, NC tools offer automation with limited programming ability and logic beyond direct input. Neutral file Format for electronic data that can be both imported and exported by dissimilar software programs. Examples include DXF, IGES, STEP and STL. NURBS (nonuniform rational B-spline) Mathematical description of a surface created by two or more B-splines. OEM (original equipment manufacturer) Company that uses product components from one or more companies to build a product that it sells under its own company name and brand.

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143

Outsource To subcontract services, such as prototyping, design or manufacturing, to an organization that is independent of the buying (requesting) organization. Paper lamination technology (PLT) Rapid prototyping process from Kira Corporation that laminates paper and then cuts the layer profile with a computerized knife. Parametric CAD Type of CAD methodology that relates the geometry of different elements of a part such that the change of one element changes related features. The association is based on a predetermined correlation. Pattern Physical representation of a design that is used to produce molds, dies or tools. Also called master pattern. PDM (product data management) Technology for managing and controlling all engineering and manufacturing data. PHAST Proprietary rapid tooling process developed by Procter & Gamble that was granted to the Milwaukee School of Engineering for further process development and refinement. Photopolymer Liquid resin material that utilizes light (visible, ultraviolet) as a catalyst to initiate polymerization, in which the material cross-links and solidifies. This technique is used by various rapid prototyping techniques. Pipeline management A process that integrates product strategy, project management and functional management to continually optimize the cross-project management of all developmentrelated activities. Pixel Individual dot placed on a cathode-ray tube that, when combined with neighboring dots, creates an image (e.g., television or computer monitor). Plaster mold casting Process for creating small quantities of metal parts in aluminum, zinc or magnesium. Often used as a prototype method for the simulation of die castings. The mold is created from a pattern with several intermediate steps. Metal is cast into the mold, as with investment casting, the mold is destroyed to yield the metal casting. PolyJetTM Rapid prototyping process from Objet Geometries that deposits photocurable materials through an ink-jet process. Postprocessing Common practice required with rapid prototype systems that refers to clean-up and finishing procedures on RP models after they are removed from the RP machine. May include mechanical or chemical removal of support structures, powder removal and surface finishing. Preproduction unit Product that looks and works like the intended final product, but is made either by hand of in-pilot facilities rather than by the final production process. Primitives Lowest state of a solid model. A solid surface that is not derived from other elements, such as a cube, cone, cylinder or sphere. Product data All engineering data necessary to define the geometry, function and behavior of a product over its entire life span, including logistic elements for quality, reliability, maintainability, topology, relationship, tolerances, attributes and features necessary to define the item completely for the purpose of design, analysis, manufacture, test and inspection.

144

Glossary

Production tooling (1) Hardened tooling intended to create large volumes (quantities) of parts. The molds should last the life of the products produced. Typically machined from steel, it is used for the mass production manufacturing of wax, polymer or metal components. ProMetal® Rapid prototyping and tooling process commercialized by Extrude Hone, Inc. that is based on the MIT 3DP technology. The process generates a “green” part by solidifying metal powder with a binder. The green part is placed in a furnace to burn off the binder, sinter the powder and infiltrate with an alloy. Prototype Physical model of a part or product during the product development process. Depending upon the purpose, prototypes may be nonworking, functionally working or both functionally and aesthetically complete. Derived from Latin term for “first form.” Prototype tooling Short-life molds and dies used in the fabrication of molded, stamping and dies and other parts. This approach has a low life expectancy compared to hardened production tooling. May yield from one to as many as 50,000 parts depending on methods and materials utilized. QuickCastTM A trademarked process of 3D Systems for a stereolithography build style that reduces the mass of the pattern to accommodate the investment casting process. Rapid manufacturing Production of end use parts – directly or indirectly – from a rapid prototyping technology. Rapid prototyping Collection of technologies that are driven by CAD data to produce physical models and parts through an additive process. Rapid tooling technology.

Production of tools, molds or dies – directly or indirectly – from a rapid prototyping

Raster (1) A two-dimensional array of pixels which, when displayed, form an image or representation of an original document. (2) A scan pattern (as of the electron beam in a cathode-ray tube) in which an area is scanned from side to side in lines from top to bottom. Antonym – vector. Reaction injecting molding Manufacturing process where thermoset resins are injected into rigid tools. Red lining (1) Facility for annotating on-screen documents by transmitting overlaid comments and sketches. (2) Process of marking documentation for requested changes to part, tooling or specification documentation. Rendering Process of adding shading, colors, reflectivity, textures and other visual elements to a solid model to make it appear realistic. Resin General classification of nonmetallic materials and compounds. For rapid prototyping, the term is most often associated with the liquid state of stereolithography photopolymers. For molding operations, the term is a reference to any thermoplastic or thermoset material. Reverse engineering Process for the capture of the geometric definition of a physical part through scanning technologies. Resulting data, often a set of discrete points that are spatially oriented, is imported into a CAD system and used for further product refinement, prototype creation, tooling creation or manufacturing.

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145

Return on investment Financial calculation that illustrates the value of an investment in a specific period of time. ((financial gain – cost)/cost) × 100%. RFP (request for proposal) Bid package, submitted to potential vendors, that solicits price and delivery information for a program or project. RFQ (request for quotation) Similar to an RFP, but generally used when requesting individual parts. RIM

See reaction injection molding.

Road Term applied to the fused deposition modeling process that describes the extrusion of material in a single pass. ROI

See return on investment.

RP See rapid prototyping. RPA/SME (Rapid Prototyping Association of the Society of Manufacturing Engineers) Association dedicated to the collection and sharing of information on rapid prototyping, tooling and manufacturing. RP-ML (rapid prototyping mailing list) Internet forum for the online discussion of topics related to rapid prototyping. RTV molding

See silicone rubber molding.

Rubber molding

See silicone rubber molding.

Rubber plaster molding See plaster mold casting. Sand casting Manufacturing process for the production of metal, including gray iron, castings. Sand is packed against a form (tool) to create each half of the tool. After combining the tool halves, metal is cast into the cavity and allowed to cool. To remove the metal casting, the sand tool is destroyed. Selective laser melting Rapid prototyping and tooling process from F&S GmbH that produces 100% dense metal parts by melting a power with an infrared laser. Selective laser sintering Rapid prototyping process, originally developed by DTM Corp. and now owned by 3D Systems, which uses CO2 laser to fuse powdered materials, including plastics and metals. Service bureau (1) Company or group of companies providing engineering, prototyping or manufacturing support to other companies who do not have the capability. (2) For rapid prototyping, a commercial entity that specializes in providing rapid prototyping and peripheral services to a customer base. SGC

See solid ground curing.

Short run tooling Molds created for low volume (e.g., less than 100 samples) production. Silicone rubber tooling Soft tooling technique that utilizes room-temperature vulcanized (RTV) rubber material to form molds that are cast from machined or rapid prototype patterns. Commonly used to produce small lots (25–100 pieces) in urethane materials.

146

Glossary

Sinter Heating a material to a temperature below its melting point to cause it to fuse to create a solid mass. SL See stereolithography. SLA (stereolithography apparatus) A trademarked name by 3D Systems for the machines that use the stereolithography process. Also used interchangeably with SL. Slice

Single layer of an STL file that becomes the working surface for the additive process.

SLM See selective laser melting. SLS®

See selective laser sintering.

Solid ground curing Rapid prototyping process that solidifies photocurable materials through a photomask. The use of the mask allows curing of a complete layer with one flash of UV light. Process is no longer available. Solid imaging An alternative term for rapid prototyping. Solid freeform fabrication An alternative term for rapid prototyping. Solid modeling 3D CAD technique that represents all physical characteristics of an object; including volume, mass and weight. Solid object ultraviolet-laser printer Stereolithography process offered by CMET. SOUP

See solid object ultraviolet-laser printer.

Spin casting Process that uses rubber molds to create metal castings in low melting temperature alloys. The mold is rotated and material is poured into its center. Centrifugal force fills the mold with molten material. Sprayform A trade name and technology owned by the Ford Motor Company. This process uses wire arc spray of metal alloy onto a ceramic mold pattern to generate tooling. Spray metal tooling Process for creating prototype or bridge tooling through metal deposition onto a pattern using wire arc spray, vacuum plasma deposition or similar techniques. After creation of the metal tool face, epoxy or other materials are used to backfill the tool to add strength. Often used for injection molds. SRPTM (Subtractive Rapid Prototyping) Trademarked name of Roland Corporation used to identify rapid prototyping devices that remove material for prototype creation. Antonym –additive rapid prototyping (ARP). Stair stepping Result of additive processes where surfaces that are neither vertical nor horizontal are not smooth, since they are approximated by individual layers. STEP (standard for the exchange of product model data). File format standard for the transfer of data between dissimilar CAD systems. Adopted by the International Organization for Standardization (ISO) in December 1994. Stereolithography Process that builds an object, a layer at a time, by curing photosensitive resin with a laser-generated beam of ultraviolet radiation. Originally applied to 3D Systems’ technology,

Glossary

147

the use of the term has broadened to include all technologies that process prototypes in this manner. STL Neutral file format is exported data from CAD systems for use as input to rapid prototyping equipment. The file contains point data for the vertices of the triangular facets that combine to approximate the shape of an object. The acronym is derived from the word STereoLithography. Surface

Boundary defining an exterior or interior face of a 3D CAD model.

Surface normal Vector that is perpendicular to a surface or facet in an STL file. For the facets of the STL file, the direction of the vector indicates the outward-facing side of the facet. Surfaced wireframe Method of 3D CAD modeling that represents part geometry with bounding edges and skins that stretch between the boundaries. The CAD model is defined by its innermost and outermost boundaries and does not contain any mass between these boundaries. Support structure Common to many rapid prototyping processes. Scaffold of sacrificial material upon which overhanging geometry is built. Also used to rigidly attach the prototype to the platform upon which it is built. After prototype construction, these are removed in a postprocessing operation. Surface modeling See surface wireframe. TALC (technology adoption life cycle) Business model that describes the adoption of technology through an analysis of purchasing traits. Thermoplastic Plastic compound that is processed (molded) in a liquid state that is achieved with elevated temperatures. This class of plastic can be repeatedly cycled through a liquid and solid state. Common applications: injection molding, blow molding and vacuum forming. Thermoset Plastic compound that is processed in a liquid state where two or more liquid components are blended just prior to molding. Upon blending, an exothermic, chemical reaction causes the liquid to change to a solid state. Unlike thermoplastics, once solidified these materials cannot be returned to a liquid state. Common applications: rubber molding and reaction injection molding. Time to market Period to conceive, develop, manufacture and deliver a new product. Tooling Generic term used to describe molds or dies used in the production of parts and assemblies. Examples include injection molds, blow molds, die cast dies and stamping dies. Ultrasonic consolidation Proprietary rapid prototyping and tooling process from Solidica, Inc. that ultrasonically welds sheet metal to deliver homogeneous material properties. After welding of the sheet material, the profile is CNC machined. Urethane Thermoset material commonly used in rubber molding and RIM molding processes. Any of various polymers that contain NHCOO linkages and are used especially in flexible and rigid foams, elastomers and resins. UV (ultraviolet) Light energy situated beyond the visible spectrum at its violet end – having a wavelength shorter than wavelengths of visible light and longer than those of x-rays Often used in the curing of photopolymer resins.

148

Glossary

Vacuum forming Process for producing plastic parts by heating plastic sheet and drawing it against a form when air is pulled through the form. Virtual prototyping Computer-based generation of 3D geometry for analyzing product design features. Often associated with immersive environments where the digital data is presented with realism and offers an ability to interact with the digital design as if it were real. More commonly applied to computer-based testing and analysis methods such as finite element analysis. Vector Quantity that has magnitude and direction and that is commonly represented by a directed line segment whose length represents the magnitude and whose orientation in space represents the direction. Voxel

Volume cell. The 3D equivalent of the pixel.

WaterWorksTM Trademarked and patented process of Stratasys, used with the FDM rapid prototyping process, that allows models (or assemblies) to be made with movable parts already assembled. The support material is dissolved in a water-based solution. Wireframe CAD modeling method defines a part by its innermost and outermost boundaries. The model does not contain any mass between the boundaries nor any bounding surfaces.

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Index 2D 6, 17, 23, 24, 31, 48, 53, 59, 70, 72, 74, 75, 76, 111, 112, 113, 117, 122, 127 3D 5, 6, 7, 9, 17, 18, 19, 21, 22, 23, 24, 25, 29, 31, 32, 33, 34, 35, 36, 38, 41, 43, 45, 49, 50, 52, 56, 60, 69, 72, 73, 74, 75, 96, 97, 98, 103, 104, 107, 110, 118, 121, 122, 124, 125, 126, 127, 128, 129, 130, 131, 132 3D computer aided design (CAD) 7 3D printing 8, 19, 22, 35, 38, 41, 56, 73, 96, 124, 127 ABS 23, 60 actetabulum 20 additive layer manufacturing 8, 87 additive manufacturing 7, 8, 19, 39, 42, 43, 46, 48, 57, 58, 61, 73 Allowance 87, 88, 89 Architectural 26 architectural designs 26 Artists 30 ARToolKit 22 Binders 91 bio printers 69, 70, 72, 73, 74 bioink 124, 125, 126, 127, 128, 129 Bio-medical 19 Boolean 23, 29, 45 build material 49, 63, 100 CAD 3, 6, 7, 8, 9, 18, 20, 29, 34, 36, 38, 41, 43, 45, 46, 50, 56, 57, 60, 75, 97, 98, 101, 104, 108, 110, 117, 118, 119, 120, 122 calcaneum 20 centrifugal 94, 95 ceramic mould 78 CNC 6, 8, 9, 35, 108 Computer 3, 20, 21, 49 computer graphics 21 Comsol 38 concept evaluation 17 Contact Scanners 108 continuous casting 94, 95 CT scan 19 designers 7, 8, 18, 27, 28, 29, 45, 118, 119 Desktop Machining 105 DNA 21, 129 https://doi.org/10.1515/9783110664904-010

Electron Beam Melting 56 Engineering design 4 Extrusion-based bioprinting 125 Eye tracker 31 eye tracking 31, 32, 33, 34 Fabrication of models 23 FDM 17, 18, 56, 60, 61, 62, 63 Finite element analysis 38 Food article 34 Fused Deposition Modeling 20, 56, 60 Geometric Model Development 110 HIV protease 25 implants 21 inkjet bio-printer 71 intangible product 1 investment casting 77, 78, 79, 80, 84, 97, 103 Investment-cast Tooling 103 Keltool technology 104 Laminated Object Manufacturing 60, 62 Laser-assisted bioprinting 128 layer by-layer fabrication 11 layers 23, 29, 39, 43, 49, 51, 58, 62, 63, 65, 66, 67, 68, 97, 98, 99, 108, 111, 112, 117, 122 LDW 74, 75, 76 LENS 96, 100 Liquid-based 47, 48 Liquid-based systems 47 LOM 17, 20, 62, 63, 64, 65, 66, 67, 68, 69 lost wax pattern 77 MEMS 36, 37, 38, 39, 40, 41 Mimics software 44 Modeling 36, 38, 41, 56, 111 models 5, 7, 9, 10, 17, 18, 19, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 32, 34, 36, 41, 42, 43, 45, 56, 72, 74, 108, 110, 111, 118, 119, 120, 122 Molecular models 21 MRI 19, 44 MRI scan 19

160

Index

NEMS 36 New product development 2, 5 Noncontact Scanners 109 orientation 20, 24, 50, 61, 64, 85, 88, 90 packaging 34, 35, 41 Pattern 86, 87, 89, 102 periarticular 20 Permanent Mold Casting Processes 94 PLA 60 Postprocessing 99, 101 Powder Metallurgy Tooling 103 Powder-based 47 Powder-based systems 47 Preoperative Templating 20 Pressure die casting process 94 Product 1, 3, 4, 5, 7 product development 2, 3, 4, 5, 6, 7, 8, 10, 30, 107 Product Life Cycle 3, 4 prosthesis 21 Prototyping 34, 35, 38, 41, 44, 111 PyARTK 24, 25 Python Molecular Viewer 22, 24, 25 rapid manufacturing 9, 10, 17, 18, 47, 48, 52, 54, 55, 56, 57, 59, 60, 76, 77 rapid prototyping 7, 8, 9, 10, 17, 18, 19, 23, 26, 29, 34, 35, 39, 42, 43, 45, 46, 47, 49, 51, 52, 54, 57, 60, 69, 77, 87, 96, 97, 101, 103, 105, 108, 110, 111, 117, 118, 122 Rapid Prototyping Technology 7 Rapid tooling 96 RE 110, 111, 117, 118, 122 Reverse Engineering 6, 107, 108, 110, 111, 117, 118, 119, 120, 121 Ribosome 26 RP 17, 20, 21, 29, 30, 48, 51, 98, 99, 100, 101, 104, 105, 110, 111, 112, 117, 120

Sand 85, 92 Scaffold fabrication 125 Sculptured 30 selective laser sintering 17, 47, 54, 55, 56, 96 Silicone Rubber Tooling 101, 102 simulation 38 SLA 17, 18, 49, 50, 51 SLM 57, 58 SMS 58 SOD 26 Solid modeling 12 Solid-based 47 Solid-based systems 47 Spray Metal Tooling 104 Squeeze casting 95 stereolithography 17, 20, 39, 47, 48, 49, 50, 51, 52, 54, 96, 129, 130, 131 STL 19, 23, 45, 50, 51, 60, 63, 64, 67, 98, 100, 105, 108, 110, 111 STL file 45, 50, 51, 63, 64, 67, 98, 100, 105, 111 Stratasys 23, 26, 56, 60 Superoxide Dismutase 26 tangible product 1 three dimensional 1, 9, 43, 54, 69, 110 three-dimensional printing 17 toy 42 UV 38, 40, 49, 52, 53, 72, 127, 129 virtual 20, 22, 24, 25, 33, 34, 41, 45 VRML 23, 110 wax pattern 77, 80 wireframe 12 Z402 97, 98, 100 Z-corp 23, 25

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