RAPID PROTOTYPING RAPID PROTOTYPING Laser-Based and Other Technologies

RAPID PROTOTYPING RAPID PROTOTYPING Laser-based and Other Technologies

Patri K. V enuvinod and Weiyin Ma Department of Manufacturing Engineering and Engineering Management City University of Hong Kong

.... Springer'' Science+Business Media, LLC Library of Congress Cataloging-in-Publication

Title: Rapid Prototyping Laser-based and Other Technologies Author (s): Patri K. Venuvinod and Weiyin Ma ISBN 978-1-4419-5388-9 ISBN 978-1-4757-6361-4 (eBook) DOI 10.1007/978-1-4757-6361-4

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photo-copying, microfilming, recording, or otherwise, without the prior written permission of the publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Permissions for books published in the USA: permissions@wkap. com Permissions for books published in Europe: [email protected] Printed on acid-free paper. Table of Contents

PREFACE xi

ACKNOWLEDGMENTS xvii

1. INTRODUCTION 1 1.1 THE IMPORTANCE OF BEING RAPID 1 1.2 THE NATURE OF RP/T 6 1.3 HISTORY OF RP 13 1.4 THE STATE OF RP/T INDUSTRY 21

2. MATERIALS BASICS 25 2.1 ATOMIC STRUCTURE AND BONDING 25 2.2 CERAMICS 31 2.3 POLYMERS 33 2.3.1 Nature ofPolymers 33 2.3.2 Free Radical Polymerization 36 2.3.3 Cationic Polymerization 38 2.3.4 Thermoplastic and Thermosetting Polymers 39 2.3.5 Polymer Structures 41 2.3 .6 Properties of Polymers 42 2.3.7 Degradation ofPolymers 47 2.4 POWDERED MATERIALS 48 2.4.1 Types ofPowders 48 2.4.2 Compaction and Sintering of Powders 49 2.5 COMPOSITES 52 vi Rapid Prototyping

3. LASERS FOR RP 57 3.1 THE PRINCIPLE OF LASER 57 3 .1.1 The Nature of Light 57 3 .1.2 Emission Radiation 59 3 .1.3 Light Amplification by Stimulated Emission Radiation 60 3.2 LASER SYSTEM 63 3.3 LASER BEAM CHARACTERISTICS 65 3.4 LASER BEAM CONTROL 69 3.5 TYPES OF LASERS USED IN RP 71

4. REVERSE ENGINEERING AND CAD MODELING 75 4.1 BASIC CONCEPT OF REVERSE ENGINEERING 75 4.2 DIGITIZING TECHNIQUES FOR REVERSE ENGINEERING 78 4.2.1 Mechanical Contact Digitizing 79 4.2.2 Optical Non-Contact Measurement 81 4.2.3 CT Scanning Method 91 4.2.4 Data Pre-processing for Surface Reconstruction 96 4.3 MODEL REPRESENTATION 98 4.3.1 Basic Geometric Features 98 4.3 .2 General Algebraic Surfaces 98 4.3.3 Parametric Surfaces 99 4.3.4 Subdivision Surfaces 101 4.3 .5 Other Approaches and Recommendations 102 4.4 B-SPLINE BASED MODEL RECONSTRUCTION 103 4.4.1 Parametrization of Measured Points 103 4.4.2 Knots Allocation 105 4.4.3 Least Squares Fitting 107 4.5 NURBS BASED MODEL RECONSTRUCTION 110 4.5.1 A Two-Step Linear Approach 112 4.5.2 Numerical Algorithms for Weights Identification 115 4.6 OTHER APPROACHES FOR MODEL RECONSTRUCTION 119 4.6.1 Basic Geometric Features 119 4.6.2 General Algebraic Surfaces 119 4.6.3 Subdivision Surface Fitting 120 4.7 SURFACE LOCAL UPDATING 121 4.7.1 Related Work and General Strategies 122 4.7.2 Pre-Processing Steps for Surface Local Updating 123 4.7.3 Computing Updated Control Points 124 4.8 EXAMPLES ON MODEL RECONSTRUCTION 125 4.8.1 Parametrization for Surface Reconstruction 126 4.8.2 B-Spline Surfaces 127 4.8.3 NURBS Surfaces 129 Table of Contents vii

4.8.4 Subdivision Surfaces 130 4.8.5 Surface Local Updating 132

5. DATA PROCESSING FOR RAPID PROTOTYPING 135 5.1 INTRODUCTION 135 5.2 CAD MODEL PREPARATION 140 5.3 DATA INTERFACING FOR RAPID PROTOTYPING 144 5.3.1 STL Interface Specification 144 5.3.2 STL Data Generation 147 5.3.3 STL Data Manipulation 149 5.3.4 Alternative RP interfaces 151 5.4 PART ORIENTATION AND SUPPORT GENERATION 152 5 .4.1 Factors Affecting Part Orientation 152 5.4.2 Various Models for Part Orientation Determination 153 5.4.3 The Functions ofPart Supports 158 5.4.4 Support Structure Design 159 5.4.5 Automatic Support Structure Generation 162 5.5 MODEL SLICING AND CONTOUR DATA ORGANIZATION165 5.5.1 Model Slicing and Skin Contour Determination 165 5.5.2 Identification of Internal and External Contours 169 5.5.3 Contour Data Organization 171 5.6 DIRECT AND ADAPTNE SLICING 173 5.6.1 Identification of Peak Features 174 5 .6.2 Adaptive Layer Thickness Determination 178 5.6.3 Skin Contours Computation 180 5.7 ASELECTNEHATCHINGSTRATEGYFORRP 185 5.8 TOOL PATH GENERATION 188

6. STEREOLITHOGRAPHY (SL) 195 6.1 THE STEREOLITHOGRAPHY (SL) PROCESS 195 6.1.1 Part Building Using SL 195 6.1.2 Post-build Processes 197 6.1.3 Pre-build Processes 198 6.2 PHOTO-POLYMERIZATION OF SL RESINS 199 6.2.1 SL Polymers 199 6.2.2 Radical Photo-polymerization 200 6.2.3 Cationic Polymerization 204 6.2.4 Vinylethers and Epoxies 204 6.2.5 Developments in SL Resins 205 6.3 ABSORPTION OF LASER RADIATION BY THE RESIN 207 6.3.1 Beam Size and Positioning over the Resin Surface 207 6.3 .2 Laser Scanning Patterns 208 V111 Rapid Prototyping

6.3.3 Total Exposure from a Single Laser Scan 208 6.3 .4 Total Exposure of Interior Resin Layers 210 6.3.5 Shape of a Cured Strand 211 6.3.6 Cure Depth and Width 212 6.3.7 Multi-layer Part Building, Overcure, and Undercure 214 6.4 RECOATING ISSUES 216 6.4.1 Recoating Cycle 216 6.4.2 Resin Level Control 220 6.4.3 Gap Control 221 6.5 CURING AND ITS IMPLICATIONS 222 6.5.1 Degree of Curing and 'Green Strength' 222 6.5.2 Effects During Post-curing 225 6.6 PART QUALITY AND POCESS PLANNING 227 6.6.1 Shrinkage, Swelling, Curl and Distortion 227 6.6.2 Surface Deviation and Accuracy 231 6.6.3 Build Styles and Decisions 235 6.6.4 Build-time and Build-cost 238 6.6.5 Functional Prototyping using SL 240 6. 7 OTHER LASER LITHOGRAPHY SYSTEMS 242

7. SELECTIVE LASER SINTERING (SLS) 245 7.1 THE PRINCIPLE OF SLS 245 7.2 INDIRECT AND DIRECT SLS 249 7 .2.1 Powder Structures 249 7.2.2 Indirect SLS using Coated Powders 250 7.2.3 Direct SLS using Mixed Powders and LPS 254 7.3 MODELING OF SLS 258 7.3.1 Modeling ofMaterial Properties 258 7.3.2 Energy Input Sub-model 263 7.3.3 Heat Transfer Sub-model 266 7.3.4 Sintering Sub-model and Solution 268 7.4 POST-PROCESSING 272 7.5 PROCESS ACCURACY 275

8. OTHER RP SYSTEMS 279 8.1 SELECTIVE LASER CLADDING (SLC) 279 8.2 LAMINATED OBJECT MANUFACTURING (LOM) 281 8.3 FUSED DEPOSITION MODELING (FDM) 288 8.4 3D PRINTING AND DESKTOP PROCESSES 294 8.5 SHAPE DEPOSITION MANUFACTURING (SDM) 300 8.6 VACUUM CASTING 303 8.7 ELECTROFORMING 304 Table of Contents lX

8.8 FREEZE CASTING 305 8.9 CONTOUR CRAFTING 306 8.10 3D WELDING 307 8.11 C NC MACHINING AND HYBRID SYSTEMS 308

9. RAPID TOOLING 311 9.1 CLASSIFICATION OF RT ROUTES 312 9.2 RP OF PATTERNS 313 9.3 INDIRECT RT 316 9.3.1 Indirect Methods for Soft and Bridge Tooling 316 9.3.2 Indirect Methods for Production Tooling 322 9.3.3 Direct RT Methods for Soft and Bridge Tooling 324 9.3.4 Direct RT Methods for Production Tooling 325 9.4 OTHERRT APPROACHES 327 lO.APPLICATIONS OF RP 329 10.1 HETEROGENEOUS OBJECTS 330 10.2 ASSEMBLIES 332 10.3 MEMS AND OTHER SMALL OBJECTS 333 10.4 MEDICINE 337 10.5 MISCELLANEOUS AREAS INVOLVING ART 340

REFERENCES 345

INDEX 377 Preface

Since the dawn of civilization, mankind has been engaged in the conception and manufacture of discrete products to serve the functional needs of local customers and the tools (technology) needed by other craftsmen. In fact, much of the progress in civilization can be attributed to progress in discrete product manufacture. The functionality of a discrete object depends on two entities: form, and material composition. For instance, the aesthetic appearance of a sculpture depends upon its form whereas its durability depends upon the material composition. An ideal manufacturing process is one that is able to automatically generate any form (freeform) in any material. However, unfortunately, most traditional manufacturing processes are severely constrained on all these counts. There are three basic ways of creating form: conservative, subtractive, and additive. In the first approach, we take a material and apply the needed forces to deform it to the required shape, without either adding or removing material, i.e., we conserve material. Many industrial processes such as forging, casting, sheet metal forming and extrusion emulate this approach. A problem with many of these approaches is that they focus on form generation without explicitly providing any means for controlling material composition. In fact, even form is not created directly. They merely duplicate the external form embedded in external tooling such as dies and molds and the internal form embedded in cores, etc. Till recently, we have had to resort to the 'subtractive' approach to create the form of the tooling. The production of such tooling can be quite expensive and time consuming, thus making unit costs highly sensitive to production volume. Xll Rapid Prototyping

The subtractive approach involves taking a block of material and chipping away unwanted segments. This is the way Michelangelo had created his brilliant sculptures. In modem industry, CNC machines work on the subtractive principle. An advantage of CNC is that it can utilize information embedded in a CAD model of the part. Further, form generation depends on the relative motion between the subtractive tool (e.g., an end mill) and the blank. In other words, it is not necessary to have tooling embedded with the required form, so small-volume production becomes possible. As a result, much of the tooling industry today depends upon CNC machining. However, when applied to the direct manufacture of products, CNC machining is not economical for high production volumes. Further, only those form features accessible by the subtractive tools can be created. This means that reentrant comers and internal form cannot be created. Another problem is that, although the machine is computer controlled, the physical side of machining requires manual attention. Lastly, like the conservative approach, the subtractive approach merely focuses on the generation of form without providing a means of controlling material composition. The 'additive' approach starts with nothing and builds an object incrementally by adding material. The material added each time can be the same or different. Thus, one is able to address the problems of form generation and material composition at once through the same process. The smaller the volume of each material increment, the greater are the form accuracy achievable and the degree of control on material composition. In the ultimate, the principle is capable of even achieving the dream of "from bits to atoms". The immediate advantage, however, is that, in principle, any solid 3D freeform can be generated without the aid of external tooling with embedded form, so most of the problems associated with the conservative and subtractive methods are totally sidestepped. Unfortunately, till recently, the additive principle could not be implemented in industry owing to the lack of suitable materials and supporting technologies. However, by the 1980s, progress in photopolymers, laser technologies, CAD modeling, etc. had matured sufficiently to enable layer-wise additive creation of 3D physical objects through selective polymerization of a photosensitive resin. The first commercially available equipment based on this principle was StereoLithography Apparatus-1 (SLA-1) released by 3D Systems, Inc. in 1987. This started a new revolution in manufacturing. The revolution is still in progress as evident from the growing number of commercially available SFF technologies: selective laser sintering (SLS), fused deposition modeling (FDM), layered object manufacturing (LOM), 3D Printing (3DP), etc. Preface Xlll

From a different viewpoint, the arrival of the additive SFF technologies was timely. By the 1990s, the world of manufacturing was experiencing an unprecedented transformation owing to rapidly changing customer attitudes and globalization of manufacture. Increasingly affluent consumers had started demanding greater product variety. This meant that production runs had to become smaller and the pace of product innovation had to pick up by an order of magnitude. In addition to productivity and quality, time-to• market became an important competitive weapon. Companies had to conceive and deliver products as rapidly as possible. But, product development is an iterative process requiring evaluation of intermediate designs through virtual (computerized) and physical prototypes. Fortunately, progress in CAD modeling and virtual reality (VR) enabled overcome several bottlenecks in the creation of virtual prototypes, whereas the new SFF technologies were found to be helpful in overcome bottlenecks in preparing physical prototypes. Naturally, these technologies started being referred to as rapid prototyping (RP) technologies. While a variety of RP technologies have been developed in the last fifteen or so years, they share many common features. Almost all of them are capable of creating an object in a layered manner directly from information c ontained in a CAD model and without the need for e xtemal tooling. Many of them use lasers as the energy source to solidify selected voxels within a layer. Together, these technologies are capable of producing objects from a variety of materials including polymers and, metallic/ceramic powders. The objects can be of a single material, multiple-material, or, even functionally graded. Initially, RP was used to facilitate visualization of design concepts via prototypes produced from wax and other polymers. However, through the development of new materials and processes, RP is now being applied to produce functional prototypes and tooling components (rapid tooling-RT). RP is also being applied to direct manufacture of components in small to medium sized batches (rapid manufacture--RM). Thus, RP has now become a billion dollar industry. The authors of this book believe that RP has now matured to a level that a course on RP can be included within an undergraduate industrial/manufacturing/mechanical engineering curriculum. In particular, a course on RP can act as an integrative or 'capstone' course. The study ofRP requires basic concepts drawn from several fields such as computational geometry, polymer chemistry, control, heat transfer, metallurgy, ceramics, optics, fluid mechanics, and manufacturing technology to be integrated. RP machines are truly mechatronic in nature, i.e., they synergistically combine mechanical, electrical/electronic and embedded computer technologies. RP can be viewed as high technology ('hi-tee') since it uses the latest developments in lasers, CNC control, computers, etc. The study of RP also XlV Rapid Prototyping provides insights into the emerging fields of MEMS and nano-technology. By studying RP, a student becomes aware of the latest trends in several engineering fields. The authors have already introduced such a course at the Department of Manufacturing Engineering and Engineering Management at the City University of Hong Kong. It appears that similar courses are being introduced in several other universities around the world. A look at the books published so far on RP (surprisingly, there are already over 20 of them) reveals that few of these are immediately suitable for use by undergraduate engineering students. Firstly, owing to the rapid and continuing progress in the rapid prototyping and tooling (RP/T) technologies, several of these books are already partly outdated. Further some books are biased towards the particular RP techniques that the author(s) had helped develop. This book intends to fill these gaps by providing a balanced and updated understanding of the wide range of RP/T technologies available today. At the same time, we hope that the book would be useful to R&D personnel and professional engineers working directly or indirectly in areas related to RP. The book consists of ten chapters. Chapter 1 provides an introduction to the general field of RP/T by clarifying why it is important to adopt 'rapid' technologies in contemporary industry. Next, a broad overview of the various RP/T technologies is given while emphasizing the common threads that runs through all of them. The historical development of RP/T technologies is also outlined so as to strengthen the view of RP/T as an emerging field. Much of the success of RP process technologies is derived from successes in the development of new materials. Hence, a deep understanding of RP processes and their capabilities in terms of product accuracy and integrity requires an equally deep understanding of the physical, chemical, and mechanical characteristics of the materials involved. The materials involved can be broadly grouped into polymers, metallic and ceramic powders, and composites. This, in tum, requires the student to have a basic understanding of atomic structure and bonding. Almost all the available books on RPIT assume that the reader is already equipped with knowledge of such materials basics. However, often, this is not true. Chapter 2 is therefore designed to fill this gap. Lasers represent yet another technology underpinning the development ofRP/T. Processes such as SL, SLS, and LOM would not have been possible if there were no lasers. Further, the capabilities oflaser-based SFF processes are intimately tied to our ability to control the laser beam characteristics and manipulate the laser beam. Again, many RP books assume that the reader possesses sufficient understanding of lasers to fully appreciate these issues. This book deviates from this assumption by devoting a full chapter (Chapter Preface XV

3) to laser basics. The chapter utilizes the principles of atomic structure outlined in Chapter 2 to explain the principle of lasers. The discussion then moves on to certain characteristics of lasers that are of particularly useful in understanding most laser-based RP processes. Finally, the general characteristics of some important lasers used in RP are described. The starting point of an RP exercise is a CAD model of the object. Owing to its importance to engineering in general, courses on CAD are usually included in most engineering curricula. Hence, a separate chapter on CAD is not included in this book. However, many times, we do not have a CAD model available. In the automobile industry, e.g., we often design the external aesthetic shape of a car by manipulating clay or wooden models, the so-called mockups. We then have to tum the physical mockup in digital form in order to survive with today's NC technologies, including rapid prototyping. At other times, we might have a piece of patient's bone from which we need to quickly produce a prosthetic. Such problems are solved in modem industry through a technology called reverse engineering (RE). This technology is of particular importance to industries engaged in original equipment manufacturing (OEM), as in China and other developing economies. Interestingly, few books on RP include a detailed discussion of RE. This book deviates from this tradition by including a full chapter (Chapter 4) on reverse engineering techniques and preparing a CAD model on the basis of RE data. While materials, lasers and precision mechatronics provide the essential substance and infrastructure for rapid prototyping, a CAD model, however, has to be processed in order to make the RP equipment function. Chapter 5 is devoted to discussions of various topics in data processing for rapid prototyping. One of the important topics addressed is on CAD/RP interfacing. With today's RP industry, the de-facto standard is the STL interface that has been adopted by almost all RP equipment manufacturers as a standard input. With the STL interface, an object is defined by tessellating an original CAD model as an approximate triangular faceted surface model. Due to the simplicity of the geometric algorithms in processing a tessellated model, many data processing steps can be made automatic. Next, the determination of an optimal build orientation and support structure design are addressed. Model slicing is then presented followed by discussions on direct and adaptive slicing. An algorithm for selective hatching/solidification among different layers is also introduced in this chapter to further improve the building speed. The chapter is closed with discussions on tool path generation. Chapters 6 to 8 discuss the various processing technologies available for RP in as much detail as possible within the confines of the available space. Since SL (stereolithography) presently holds the major share in the RP XVI Rapid Prototyping industry, a full chapter (chapter 6) is devoted to it. The discussion on SL also acts as an introduction to subsequent discussions on other RP processes by clarifying issues related to laser utilization, implications of layered fabrication with regard to product quality, etc. Chapter 7 provides a fairly detailed description of selective laser sintering (SLS) processes and issues concerning their modeling. A full chapter is devoted to this process in view of potential of SLS in the fabrication of functional parts and hard tooling. Chapter 8 discusses a host of other processes including laser cladding, FDM andLOM. Chapter 9 is entirely devoted to the important area of rapid tooling (RT). Both direct and indirect tool fabrication techniques are discussed. Finally, Chapter 10 discusses the major areas of application of RP techniques: Among the areas discussed are the fabrication of heterogeneous and functionally graded objects, assemblies and MEMS. Further areas discussed include architectural and medical applications. Acknowledgments

The authors wish to gratefully acknowledge the Department of Manufacturing Engineering and Engineering Management (MEEM) of City University of Hong Kong (CityU) for providing various resources to the authors' RP-related teaching and research activities. The authors also acknowledge the financial support provided by the Research Grants Council of Hong Kong Special Administrative Region (HKSAR) through CERG research grants CityU1041/96E and CityU1131/03E, and the City University of Hong Kong through Strategic Research Grants 7000589, 7000861, 7001074, 7001241 and 7001296. The authors also wish to acknowledge the funding support received from Industry Department of the Government of HKSAR for setting up the Rapid Prototyping Technology Center (RPTC) jointly set up by the MEEM department of City University of Hong Kong and the Hong Kong Productivity Council (HKPC). In connection with RPTC, the authors wish to thank Mr. H.Y. Wong, Dr. Edmund H.M. Cheung, Dr. I.K. Hui, Dr. K.S. Chin, Dr. Ralph W.L. Ip, Dr. Meng Hua, Dr. H.W. Law and Dr. Ricky W.H. Yeung from CityU. Thanks are also extended to Dr. T.L. Ng, Dr. S.W. Lui, Mr. L.M. Li, Dr. L.K. Yeung, Mr. Lawrence Cheung, Mr. Thomas Chow, and Mr. Norman Chan from HKPC, and Ms. Annie Choi and Mr. K.S. Chiang from the Industry Department for collaborating on various R&D projects. The authors also wish to gratefully acknowledge input from their coauthors of related publications. Among others, particular mention is made of Prof. Jean-Pierre Kruth ofKatholieke Universiteit Leuven, and Dr. S. Liu, Dr. P. He, Dr. N. Zhao, Mr. W.C. But and Mr. K.Y. Chu from City University of Hong Kong. Special thanks also go to Dr. K.M. Yu (Hong xviii

Kong Polytechnic University) for providing valuable information on some related topics, and Mr. C.C. Leung and Mr. Y.S. Tai of the Integrated Design and Prototyping Laboratory (MEEM, CityU) and the CAD and Software Applications Laboratory (MEEM, CityU), respectively, for their daily support and maintenance of related facilities. The authors also wish to thank Mr. Deepak Patri for the book cover design, Mr. Michael Hackett (Kluwer Academic Publishers) for his patient support throughout the writing of the book and Dr. D.R. Vij (Kurukshetra University, India) for communications relating to the publication. We would also like to express our special gratitude to our families for their support and understanding in the preparation of this publication.

Patri K. V enuvinod and Weiyin Ma