1996 International Solid Freeform Fabrication Symposium

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Proceedings for the 1996 International Solid Freeform Fabrication Symposium. For more information about the symposium, please see the Solid Freeform Fabrication website.

The Seventh Solid Freeform Fabrication (SFF) Symposium, held at The University of Texas in Austin on August 12-14, 1996, was attended by over 200 national and international researchers. Papers addressed SFF issues in computer software, machine design, materials synthesis and processing, and integrated manufacturing. The continued growth in the research, application and development of SFF approaches was readily apparent from the increased participation over previous years and the diverse domestic and foreign attendees from industrial users, SFF machine manufacturers, universities, and government. The excitement generated at the Symposium reflects the participants' total involvement in SFF and the future technical health of this growing technology. The Symposium organizers look forward to its being a continuing forum for technical exchange among the expanding body of researchers involved in SFF.

The Symposium was again organized in a manner to allow the multi-disciplinary nature of the SFF research to be presented coherently, with various sessions emphasizing computer issues, machine topics, and the variety of materials aspects of SFF. The demand for a forum at the SFF Symposium posed a conflict with the desire to maintain the meeting at three days. The compromise was to move to afternoon parallel sessions for the first time and to extend the meeting to three full days. The poster session was also expanded. We believe that documenting the constantly changing state of SFF art as represented by these Proceedings will serve both the people presently involved in this fruitful technical area as well as the large flux of new researchers and users entering the field.

The Japanese Technology Evaluation Center and its companion World Technology Evaluation Center at Loyola College, under a cooperative agreement with the National Science Foundation, assessed the state of SFF in Europe and Japan. Dr. Fritz Prinz of Stanford University chaired the study and a review of this assessment at the Symposium. The panel participants were Clint L. Atwood (Sandia National Labs), Richard Aubin (United Technologies Research Center), Joe Beaman (University of Texas), Robert L. Brown (The Gillette Company), Paul Fussell (Alcoa Technical Center), Allan Lightman (University of Dayton Research Institute), Emanuel Sachs (Massachusetts Institute of Technology), and Lee Weiss (Carnegie Mellon University). Slides of the original presentation and JTEC report ordering infotn1ation is available on the worldwide web at http://itri.loyola.edu/rp/top.htm.

The editors would like to extend a warm "Thank You" to Glorya Gutchess for her detailed handling of the logistics of the meeting and the Proceedings, as well as her excellent perfotn1ance as registrar and problem solver during the meeting. We also acknowledge the support efforts of Cindy Pflughoft throughout. We would like to thank the organizing committee, the session chaitn1en, the attendees for their enthusiastic contributions, and the speakers both for their significant contribution to the meeting and for the relatively prompt delivery of the manuscripts comprising this volume. We look forward to the continued close cooperation of the SFF community in organizing the Symposium. We also want to thank ONR through Grant No. N00014-96-1-0441, ARPA, and The Minerals, Metals and Materials Society and the University of Connecticut at Storrs for co-sponsoring the Symposium with the Mechanical Engineering Department and the Center for Materials Science and Engineering at the University of Texas at Austin.


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    1996 International Solid Freeform Fabrication Symposium Table of Contents
    (1996) Laboratory for Freeform Fabrication and University of Texas at Austin
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    The Influence of Natural Convection and Radiation Heat Transfer on Sintering of Polycarbonate Powders
    (1996) Norrell, J. L.; Kandis, .M; Bergman, T. L.
    The influence ofsurface radiation and natural convection on sintering of polycarbonate powders processed under non-isothermal conditions is investigated. These modes of heat transfer affect local powder temperatures and thus local sintering rates which in turn influence part growth (uncontrolled sintering). This paper presents a 2-dimensional sintering simulation of powder whose free surface exchanges energy with the surrounding enclosure surfaces. Modeling is accomplished using a commercial finite element code (FIDAP) in conjunction with a model for viscous sintering.
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    Techniques for Improved Speed and Accuracy in Layered Manufacturing
    (1996) Novae, Andrei; Kaza, Srinivas; Wang, Zetian; Lee, CheoI; Thomas, Charles
    The ability to improve the construction accuracy and/or the build speed for layered manufacturing techniques is demonstrated using a series of new techniques: (1) Parts can be decomposed into sections which are constructed in parallel and then assembled. This reduced the build time and material waste for a sheet foam process. (2) A more accurate interface based on direct slicing ofthe CAD model can be used to eliminate the need for the intermediary tessellation file. (3) The layer thickness can be adapted based on the part's geometric complexity to increase the surface quality, build speed, and z-axis accuracy.
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    Parametric Representation of Part Contours in SLS Process
    (1996) Chen, Kenwei; Crawford, Richard H.; Beaman, Joseph J.
    Current layer-based SFF technologies process faceted geometric input data to produce polygonal contours of the part's boundary in each layer. However, for improved part quality, other more accurate representations of part contours are desirable. Likewise, implementation of Wu's minimum time optimal laser tracking control method for selective laser sintering (SLS) requires contour curves that exhibit higher order continuity. In this paper, we first analyze the requirements of optimal laser tracking to develop evaluation criteria for choosing a contour representation. Several possible representation methods are reviewed. We show that the NonUniform Rational B-Spline (NURBS) curve meets the criteria. A demonstration program illustrates the advantages of NURBS curves for representing contours with uniform point distributions. The results can be used in other control areas where uniform point distribution or constant velocity is required.
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    Feature Extraction From Tessellated And Sliced Data in Layered Manufacturing
    (1996) Tata, Kamesh; Fadel, Georges
    When parts are built in layers, the cross sectional area of each layer has to be defined and filled with a pattern of vectors. This filling process is called hatching and the vectors defme the hatch pattern. To accurately reproduce a three dimensional object, key features need to be identified. In particular, top and bottom surfaces, edges. holes and protrusions must be recognized to ensure the slice plane does carry-the critical information required for the build. This paper describes a technique to extract relevant features from a tessellated model to generate a correct sliced representation.
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    Extraction of Fault Patterns on SLS Part Surfaces Using the Karhunen-Loeve Transform
    (1996) Turner, Irem Y.; Wood, Kristin L.; Busch-Vishniac, Ilene J.
    To gain a thorough understanding of the fault mechanisms in SLS machines, we decompose SLS profile signals into independent features using a novel tool called Karhunen-Loeve (KL) transform. These individual features can then be studied separately to monitor the occurrence of fault patterns on manufactured parts and determine their nature. Analytical signals with known fault patterns, simulating profile measurement signals from SLS parts, are used to determine the suitability of the proposed method. Multi-component patterns are assumed to manifest on SLS part surfaces, resulting from faults in the machine, for example, the roller mechanism. The results of this work determine the suitability of the KL transform for condition monitoring and extraction of fault-indicating patterns.
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    Modelling Simple Feature Creation in Selective Laser Sintering
    (1996) Ryder, G. J.; Berzins, M.; Childs, T. H. C
    A two dimensional finite difference thermal sintering model has been created to describe the Selective Laser Sintering process(SLS). It includes thermal property variation with position and temperature, and especially adaptive meshing to refine information in regions of high temperature gradients. It has been used to predict density and temperature in both single and multi layer sintering operations, corresponding to experimental results. This paper will present comparisons of theory and experiment for the SLS of simple geometries such as blocks, steps, and cylinders.
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    Finite Element Analysis of Curl Development in the Selective Laser Sintering Process
    (1996) Dalgarno, K.W.; Childs, T.R.C.; Rowntree, I.; Rothwell, L.
    The work reported within this paper is concerned with the development of analytical procedures which will allow the accuracy of parts generated by selective laser sintering to be predicted. One source of inaccuracy is curl, which results in curved part edges of flat plates manufactured lying horizontally in the part bed. This paper reports on the use of finite element techniques to model the development of curl. The models have been validated through comparison of f.e. results with the results of experimental builds, and extended to allow the influence of "bases" on the development of curl to be examined.
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    Selective Laser Sintering Part Strength as a Function of Andrew Number, Scan Rate and Spot Size
    (1996) Williams, John; Miller, David; Deckard, Carl
    Selective Laser Sintering has been modeled analytically and numerically, and studied experimentally. Further investigation is necessary to couple the results of modeling with experimental data. At Clemson University, numerical modeling of heat transfer phenomena is used to predict temperatures within the powder layer as a function of process parameters. Efforts are focused on delivering process speed up through improved process understanding. Initial modeling results and current understanding of the effects of process parameters on the strength properties offreeform parts produced by the SLS process are presented.
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    Part Quality Prediction Tools for Fused Deposition Processing
    (1996) Yardimci, M. Atif; Guceri, Selcuk I.; Agarwala, Mukesh; Danforth, Stephen C.
    Fused Deposition process fabricates requested part geometries by sequentially depositing discrete curvilinear beads ofmaterial next to and on top of each other. The part integrity depends strongly on the bonding quality at the bead interfaces. Since diffusion bonding of thermoplastic components in the material system is thermally driven, temperature history ofinterfaces determine the bonding quality. Detailed thermal analysis of deposition region and layer building simulation for a model geometry have been performed to investigate local and global material behavior during processing. A simple transport property prediction model has also been developed for the determination of thermal transport properties of the particle loaded systems used in Fused Deposition. Based on the information obtained from thermal models, a computationally efficient part building model has been developed to predict bonding quality in the whole part. The model is driven by the same command file, sml file, that drives the Fused Deposition hardware; and hence is capable of replicating the building process. The model has been tested for a model geometry, spur gear, and three dimensional bonding quality distribution has been predicted for the part.
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    Finite Element Analysis and Strain Gauging of the Stereolithography/Investment Casting System
    (1996) Hague, Richard; Dickens, Phill
    Many metal parts have been produced from stereolithography (SL) models via the investment casting route. However, it is still not possible for every foundry to directly use SL models as thermally expendable patterns and gain the same success as achieved with wax patterns. Significant drawbacks still exist with the QuickCastTM structure that restricts its use to specialist investment casting foundries who are willing to alter their standard techniques. As part of a continuing work programme at the University ofNottingham, the stresses that are created in the SL/ceramic construction have been determined using simple stress analysis and finite element analysis techniques. Further work has involved connecting strain gauges and thermocouples to SL parts in order to confirm the results obtained with the theoretical stress analysis. Inspection of the results obtained is aiding the generation of new build structures to enable the successful autoclaving of SL models. Details ofthe work to date are outlined in this paper, along with the results obtained.
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    Process Insight About LOM Systems
    (1996) Chi, Chen
    A Laminated Object Manufacturing (LOM) machine offers much freedom in terms ofsystem parameters: laser cutting speed, laser power setting, roller speed, roller temperature, and so on. Because ofthis freedom, users can choose any number, within certain limitations, to create wellconstructed objects. Obviously, each user has a different definition for the quality of an object. Therefore this freedom has induced some confusion. Most commonly, each customer has his own preferred parameter data sets. These sets may not be the same but they are good sets. We need to devise a method as a guideline for system parameters to ensure a consistency in the construction of objects. We have been studying actual laser power at different cutting speeds, actual cutting curves and bonding curves for different materials, actual temperature distribution, etc. These results have helped us find a proper way to set system parameters so that any user can run LOM machines without difficulty and confusion. The research methodology and results are elaborated in this paper.
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    Thermomechanical Modeling of Successive Material Deposition in Layered Manufacturing
    (1996) Chin, R.K.; Beuth, J.L.; Amon, C.H.
    Residual stress build-up due to successive deposition of superheated molten metal onto metal substrates is modeled for application to layered manufacturing methods. This work is specifically applied to microcasting, which is a deposition process used within shape deposition manufacturing. One-dimensional thennal and mechanical models are used to predict temperature and stress evolution related to two physical phenomena. First, the effect of thennal cycling by newly deposited material on stress states in previously deposited and cooled layers is investigated. Here, deposited molten metal solidifies and cools to room temperature before new molten metal is deposited. For this case, predicted stress distributions as a function of depth are relatively uncomplicated and can be related to residual stress-induced part tolerance loss. In the second case, the effect of localized preheating by previously deposited material is investigated. In this model, molten metal is successively deposited at a rate comparable to that used to deposit individual droplets in the microcasting process. Results indicate that although preheating by previously deposited material strongly affects transient stresses, final stress states are not substantially altered.
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    IVECS, Interactively Correcting .STL Files in a Virtual Environment
    (1996) Morvan, Stephane M.; Fadel, Georges M.
    Free Form Fabrication (FFF) machines transform objects merely existing as Os or 1s in a computer into a tangible object. FFF machines shift the paradigm of standard 2 Dimensional printers/paper printouts to 3 Dimensional printers/volumetric printouts (or 3D hardcopies). Currently, this technology is weakened by the link between computers and FFF machines: the .STL file, which contains a series oftriangles representing the skin ofthe object to be prototyped. A prototype, reflecting precisely the evolution of a concept within a design cycle and allowing a systematic inspection/verification, is essential. A system responding to this need was designed at Clemson University for the inspection and the correction ofsuch a file. IVECS, the Interactive Virtual Environment for the Correction of.STL files, is a tool that allows minute surgery to be performed on faulty tessellated models. IVECS allows STL files to be imported, tessellation errors to be detected and automatically or manually fixed. This paper expands on the use of IVECS for the inspection and the correction of .STL files. It extends the usefulness of the STL format by allowing designers to virtually prototype before actually building a physical model, thus contributing to a shorter design cycle.
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    Interfacing Reverse Engineering Data to Rapid Prototyping
    (1996) Vail, N.K.; Wilke, W.; Bieder, H.; Jiinemann, G.
    Rapid prototyping has become an increasing part of product development process chains resulting in reduced time to market and reduced development costs. As manufacturers strive to further reduce development cycles to maintain market competitiveness, the use ofreverse engineering technologies have started to play key roles in the product development cycles. Integration of these technologies into existing development cycles provides tools to maintain design integrity during development stages as well as between successive product lines. One aspect ofreverse engineering is the intert'acing of data obtained from these technologies to manufacturing processes such as rapid prototyping. This paper discusses work at Daimler-Benz to develop a set ofinterlacing tools as part of a larger reverse engineering process loop. These tools include facilities to generate contiguous surt'ace meshes from a collection ofmeasured views as well as automatic feature detection and hole closure.
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    Low Shrinkage, High T Liquid Crystal Resins For Sterelithography
    (1996) Ullett, J. S.; Chartoff, R. P.; Schultz, J. W.; Bhatt, J. C.; Dotrong, M.; Pogue, R. T.
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    Forced Convection in a Polymeric Powders
    (1996) Norrell, Jeffery L.; Wood, Kristin L.; Crawford, Richard H.; Bergman, Theodore L.
    In a Selective Laser Sintering (SLS) powder bed, thermal transfer occurs through multiple modes. Forced convection through the powder, or downdraft, has recently been implemented in SLS machines in an effort to enhance thermal transfer within the powder bed. In this paper, forced convection is analytically shown to be a significant thermal transfer mode for low porosities, such as seen in SLS powder beds. A polymeric powder bed subjected to downdraft is investigated with the goal of quantitatively determining thermal behavior. A numerical model describing heat transfer within a powder is presented. The design and construction of an experimental apparatus to measure the temperature profiles within a powder subjected to forced convection is described. Using the information gained in these experiments, it may be possible to better control the thermal environment of SLS powder beds, reducing growth and internal stress build-up.
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    Filament Feed Materials for Fused Deposition Processing of Ceramics and Metals
    (1996) Agarwala, M.K.; Weeren, R. van; Bandyopadhyay, A.; Safari, A.; Danforth, S.C.; Priedeman, W.R.
    Fused Deposition of Ceramics (FDC) and Metals (FDMet) are SFF techniques, based on commercial FDMTM technology, for fabrication of ceramic and metal components. The FD processes use feed material in the form of filaments which require certain phy~ical and mechanical properties. FDC and FDMet processes employ fila.n::ents formed from ?erannc ~r metal powders mixed with thermoplastic polymers. The thermoplastic polymers act as bmder dunng the FDC and FDMet processing in forming a green part. Development of green ceramic or metal fIlaments for FDC or FDMet processing involves three critical steps : selection of an appropriate binder chemistry, appropriate mixing procedures and fIlament fabrication techniques. This study describes the properties required for filaments for successful FD processing and the approach taken in the development of a series of binder which meets these requisite properties for a wide range of ceramics and metals. Appropriate mixing and filament forming techniques are also discussed.
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    Recent Developments in Extrusion Freeform Fabrication (EFF) Utilizing Non-Aqueous Gel Casting Formulations
    (1996) Hilmas, Greg E.; Lombardi, John L.; Hoffman, Robert A.; Stuffle, Kevin
    Extrusion Freeform Fabrication (EFF) was shown to be an extremely versatile method for fabricating Functionally Graded Materials (FGMs) The approach is inexpensive and potentially feasible for grading between any thermodynamically compatible ceramic-metal, ceramic-ceramic, or metal-metal material combination. Several material systems were investigated in this study including alumina-304 stainless steel, zirconia-304 stainless steel, alumina-Inconel 625, zirconiaInconel625, alumina-nickel aluminide, zirconia-nickel aluminide, titanium carbide-InconeI625, titanium diboride-nickel aluminide, and tungsten carbide-nickel aluminide. A controlled gradient was demonstrated between the end members for all of the above compositions. The FGMs were hot pressed to achieve near theoretical densities, providing flexural strengths as high as 1000 MPa for the zirconia-304 stainless steel FGM. The FGM systems developed in this program have a wide variety of potential commercial and government applications including cutting tools and other components requiring wear resistant surfaces, aircraft engine and automotive engine components, light and heavy armor systems, and electrical insulators and heat-sinks for the electronics industry, to name a few