1994 International Solid Freeform Fabrication Symposium

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Proceedings for the 1994 International Solid Freeform Fabrication Symposium. For more information about the symposium, please see the Solid Freeform Fabrication website.

This Proceedings of the Fifth Solid Freeform Fabrication Symposium, held at The University of Texas in Austin on August 8-10, 1994, was the best attended and continued the dynamic nature of the first four. Intense interest was shown by researchers for the latest in the basic aspects of Solid Freeform Fabrication (SFF) that highlighted the papers presented at this Symposium. The speakers addressed problems in computer software, machine design, materials synthesis and processing, and SFF in integrated manufacturing. The continued growth in the research, application and development of SFF approaches was readily apparent from the additional papers presented and the attendees from industrial users, SFF machine manufacturers, universities, and government. There was a very large international involvement in the meeting, both as attendees and as contributors. Research presented in the Symposium showed the continued movement forward toward the goal of structurally sound parts using a wide range of SFF techniques. This continued advancement in the state-of-the-art of SFF and the drive for continually improving and reaching out for standardization of the technology will continue to drive its exponential growth and cooperative efforts. The excitement generated at the Symposium reflects the participants' total involvement in SFF and the future technical health of SFF. The Symposium organizers look forward to its being a continued 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 aspects, machine topics, and the variety of materials aspects of SFF. Application-related efforts were scattered throughout the Symposium. To avoid parallel sessions, a poster session was organized, and a panel session on SFF was held. The dynamic, loosely organized panel discussion on "Where does SFF go in the Next Five Years?" was led by Joel Barlow, Michael Cima, Thomas Pang, Fritz Prinz, Sean O'Reilly, and Michael Wozny. The written versions of the presented papers are incorporated into these Proceedings. The editors would like to thank the speakers for their timely delivery of the manuscripts. We believe that documenting the constantly changing state of the SFF art as represented by these Proceedings will serve both the people presently involved in this fruitful area as well as the large flux of new researchers and users entering the field of SFF. The evenings were highlighted with Texas-style vittles and entertainment featuring the Geezinslaws.

The editors again would like to extend a warm "Thank You" to Renee Loyless-May for her extensive efforts in the detailed handling of the logistics of the meeting and the Proceedings and the support efforts of Vicki Lehmeier and Cindy Pflughoft throughout. We would also like to thank the organizing committee, the speakers, the session chairmen, panel members, and the attendees for their enthusiastic contributions. 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. N00014-94-1-0829, ARPA, .and The Minerals, Metals and Materials Society for cosponsoring the Symposium with the Mechanical Engineering Department and the Center for Materials Science and Engineering at the University of Texas at Austin.

Organizing Committee: Dick Aubin, United Technologies; Joel W. Barlow, The University of Texas at Austin; Joseph J. Beaman/The University of Texas at Austin; David L. Bourell, The University of Texas at Austin; Robert L. Brown, The Gillette Company; William Coblenz, ARPA; Richard Crawford, The University of Texas at Austin; Samuel Drake, University of Utah; Steven Fishman, Office of Naval Research; Harris L. Marcus, The University of Texas at Austin; Fritz Prinz, Carnegie Mellon University; Emanuel Sachs, Massachusetts Institute of Technology; Susan Smyth, General Motors Corporation; Sean O'Reilly, Ford Motor Company; Ralph Wachter, Office of Naval Research; Michael Wozny, Rennselaer Polytechnic Institute

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1994 International Solid Freeform Fabrication Symposium Table of Contents
(1994) Laboratory for Freeform Fabrication and University of Texas at Austin
Advances in Selective Area Laser Deposition of Silicon Carbide
(1994) Tompkins, J.V.; Laabi, R.; Birmingham, B.R.; Marcus, H.L.
Selective Area Laser Deposition (SALD) is a Solid Freeform Fabrication (SFF) technique which uses a scanning laser beam to produce solid material by locally decomposing a gas precursor. In this work, a focused C02 laser beam strikes a substrate in the presence oftetramethylsilane (TMS) or diethylsilane (DES), producing silicon carbide objects with high density and no binder phase. Recent investigation has yielded growth rates up to 2.7mlnJmin in the beam area, and has eliminated previously noted contamination ofthe optics by a byproduct which mass spectroscopy identifies as silicon dioxide. This paper reviews a cause of non-uniform growth and delTIOnstrates the addition of hydrogen and reduced scan speeds to lTIake lTIultilayer parts. In addition, it presents a lTIethod for in-situ measurement of height of deposited material.
Adaptive Slicing of Solid Freeform Fabrication Processes
(1994) Suh, Young Seok; Wozny, Michael J.
The Solid Freeform Fabrication (SFF) process significantly reduces part specific setup manufacturing lead time. This process has been primarily used in fabricating prototypes for design visualization and verification. However, the major impact of this process on the future of manufacturing technology would be the possibility offabricating functional parts for end use. One ofthe obstacles to this goal is the insufficient accuracy ofthe final physical part produced by the process. From the software point of view, the major sources of the inaccuracy come from the inappropriate data transfer format and the 3D aliasing' or Stair-stepping' problem. The '3D aliasing' problem can be reduced by adapting the layer thickness to the geometry of the part. In this paper, the procedure of adaptive slicing from the exact representation ofthe part model is described. This will improve part accuracy and minimize building time especially for the parts with highly curved surfaces. The procedures are implemented and a comparison to the conventional uniform layer thickness method will be discussed.
Ultraviolet Curing of Highly Loaded Ceramic Suspensions for Stereolithography of Ceramics
(1994) Halloran, John W.; Griffith, Michelle L.
Ceramic green bodies can be created using stereolithography methods where a ceramic slip consisting of 45-55 vlo ceramic powder is dispersed within an ultraviolet-curable aqueous acrylamide solution. Two ceramic materials were investigated: silica [Si02] for investment casting purposes, and alumina [AI203] for structural parts. After mixing the powders in the curable solution, the ceramic slip is tape cast onto a substrate for cure under a high intensity ultraviolet lamp (220-450 nm) at different exposure times. The materials systems were evaluated at different solids loadings (10-50 v/o) for cure thickness and viscosity control. Silica had a cure depth of 330 f.lm at a solids loading of 55 vlo, and at 50 vlo, alumina had a cure depth of 300 f.lm. Preliminary work utilizing scattering theory revealed the cure depth is controlled by the particle size and the refractive index difference between the ceramic and ultraviolet solution. The refractive index difference is the dominating factor. Two particle size distributions of alumina were used to more accurately determine the effect of particle size.
Modeling of Polymer Degradation in SLS
(1994) Vail, N.K.; Barlow, J.W.
A simple computer model has been developed to predict the thermal degradation of polymer binders used in the fabrication of composite green shapes from high temperature ceramic materials. Decomposition rate kinetics of the polymer materials were determined and incorporated into the model. The polymer degradation occurring in three separate powder systems was determined as a function of applied laser energy. Agreement between model results and experimental data is quite good. (Key Words: Polymer, Degradation, Selective Laser Sintering, Composites).
Fundamentals of Liquid Phase Sintering Related to Selective Laser Sintering
(1994) Bunnell, D.E.; Bourell, D.L.; Marcus, H.L.
Liquid phase sintering is one of the underlying principles that must be modeled and understood when the Selective Laser Sintering (SLS) process is used. This paper describes the initial studies being conducted to measure surface tension of metal alloys used for SLS. A low melting point solder was used to verify the wetting balance and pendant drop techniques and equipment for determining surface tension. The liquid-solid, liquid-vapor, and solid vapor surface tension of 80 Sn - 20 Pb solder on mild steel was determined to be 245, 417, and 662 dynes/cm.
Manufacture Of Injection Molds Using SLS
(1994) Badrinarayan, B.; Barlow, J.W.
This paper describes the use of SLS technology for the fabrication of injection mold cavities. Green shapes were made from metal - copolymer powder mixtures by SLS. The copolymer was gradually burnt out and the metal was oxidized in an air furnace. The porous oxidized metal part was subsequently infiltrated with an epoxy resin and cured. Effect of process variables in SLS, effect of oxidation cycle, dimensional changes on oxidation and epoxy infiltration of the oxidized metal part are discussed.
Selective Laser Pyrolysis for Solid Free-FonTI Fabrication of Silicon Carbide
(1994) Jakubenas, K.; Marcus, H.L.
Selective laser pYrolysis rapidly decomposes a polymeric precursor to form a cohesive ceramic shape. The considerable shrinkage and porosity during pYrolysis of pure precursor can be modified by the addition of either inert or reactive fillers. With polycarbosilane as the polymeric precursor, the process forms shapes of ~-SiC and, by using fillers, composites of ~-SiC/Al4C3/Al, ~-SiC/TiC/Ti, and ~-SiC/ZrC/Zr. The technique offers some potential for ceramic shapes with custom designed composition and microstructure including nanometer grain size.
Selective Laser Sintering of Polycarbonate at Varying Powers, Scan Speeds and Scan Spacings
(1994) Childs, T.H.C.; Cardie, S.; Brown, J.M.
A benchmark study (1) has shown selective laser sintering to be the equal of or to have accuracy advantages over other processes for creating parts of size over 10 mm. Experience is needed to achieve best accuracies, as with other processes. This paper is (for us) a first step in understanding the relation between sintering parameters, part size and acuracy. Work at the University of Texas at Austin (2-4) has established that the sintering of polycarbonate can be understood in terms of a rate model driven by viscous and surface tension effects. Material properties are such that a sharp boundary exists between sintered and unsintered material. When full density is not achieved in a part, density within a single layer varies from fully sintered to totally unsintered; measured part density is thus a mean of widely varying values. Published work (3-4) uses a onedimensional non-steady state heat flow model to calculate the temperature profile and densification beneath the surface and concentrates on the effects on this of material properties varying with temperature and during sintering. In this paper, these variations are ignored but a three dimensional non-steady heat flow is used to enable edge effects to be estimated. Density gradients at edges are assumed to be responsible for variations of accuracy with sintering parameters, part size, part shape and orientation.
Silicon Carbide Shapes By Selected Area Laser Deposition Vapor Infiltration
(1994) Birmingham, B.R.; Tompkins, J.V.; Marcus, H.L.
Selected Area Laser Deposition Vapor Infiltration (SALDVI) is a unique combination ofselected area laser deposition, chemical vapor infiltration and layered powder handling techniques that can be used to fabricate silicon carbide (SiC)/SiC composite shapes. This paper discusses a SALDVI process under investigation which selectively infiltrates SiC powder with SiC generated by decomposition of a gas precursor under a scanned laser beam. A general description of the process, including some of its inherent advantages is presented. Experimental results which explore beam interaction, powder size and infiltration time effects are also presented.
Selective Laser Sintering of Alumina-Boron Oxide Composites
(1994) Lee, Insup; Manthiram, A.; Marcus, H.L
The selection of an optimum composite system for selective laser sintering (SLS) is based on materials properties such as the melting point and the wettability between the components in the composite powder. The alumina-boron oxide composite system is attractive for SLS because the presence of the low melting component B203 (melting point 4500 C) can enhance sintering. A better wetting of solid alumina powder by molten boron oxide can also aid densification process. The alumina-boron oxide conlposite system has been investigated by SLS and selective laser reactive sintering (SLRS). The role of boron oxide content as a binder, laser power density, and secondary heat treatment on the microstructure and mechanical properties is discussed.
Anisotropy in Alumina Processed by SLS.
(1994) Kamatchi Subramanian, P.; Vail, N.K.; Barlow, J.W.; Marcus, H.L.
Alumina powders of 15~m size and 2~m size were processed by SLS using PMMA and a copolymer. The 2~m powders were agglomerated and mixed with the polymer powder before being processed by SLS. SLS bend strength specimens were made with parts built along different orientations. The variation of the strength with incident energy density and with orientation was studied.
Measurement and Prediction of the Thermal Conductivity of Powders at High Temperatures
(1994) Sumin Sih, Samuel; Barlow, Joe W.
An equation for the prediction of the thermal conductivity of powder beds up to high temperatures is suggested by the authors. The predicted values by the equation are compared with the values of the thermal conductivity of alumina powder, magnesia powder and zirconia powder reported in the literature, and are found to be consistent. The predicted values by the equation are also compared with the measured values of the thermal conductivity of calcium hydroxyapatite powder, at various temPeratures, up to 500°C by the laser-heated method.
Applications of 3D Laser Digitizing and Surfacing Technologies
(1994) Winkelman, Wayne
New Developments in Processing and Control of Selected Area Laser Deposition of Carbon
(1994) Thissell, W. Richards; Marcus, H.L.
Selected area laser deposition (SALD) has been used to deposit carbon from methane, hydrogen, oxygen, and argon mixtures using a third generation deposition system. The effect of two laser scanning hardware/software designs on the development of morphological instability in the resulting deposit is compared. One method uses programmed I/O using the main process control CPU to calculate and download beam position and desired laser power. Another method is presented which uses dedicated direct memory access (DMA) controllers and a dedicated counter/timer to download the required information. Its improvements to the process include better coordination between laser power and beam speed resulting in an improved beam power delivery uniformity and an improved ability to utilize one CPU for control of more of the SALD process variables.
Modeling of Selected SFF Process Limits
(1994) Mendez, Patricio; Brown, Stuart
An analytical model of the thermal field for one scan line during SLS is developed. Quantitative relationships between net heat input and beam velocity are stated for sintering at a given distance from the center of the beam and for the case of maximum surface temperature. For the maximum surface temperature, two extreme cases have been analyzed: pure conduction heat transport, and highly convective molten consolidation. It is suggested that a highly convective process allows significantly higher net heat input than pure conduction. It is found that for certain conditions, the relationship between net heat input and beam velocity is independent of the thermal conductivity of the material. Key Words: model, melting, selective laser sintering, thermal, process window.
Structural Design for Freeform Fabrication using Composite Materials
(1994) Johanson, Roy; Kikuchi, Noboru; Papalambros, Panos
Advances in the development of methods to Perform topology optimization offer the ability to design novel structures composed of dense composite materials. These structures, which possess superior mechanical proPerties, can only be produced through the use of layered manufacturing techniques. In this paper, we demonstrate a technique for the design of layered structures composed of composite materials. In addition, this procedure allows the design of the composite materials used for fabrication of such components on a microstructural level.
Shape and Topology Structural Redesign by Large Perturbations
(1994) Bernitsas, Michael M.; Suryatama, Danet; Byungsik, Kang; Dale, G. Karr
The ultimate goal in concurrent engineering of structures is to achieve simultaneously in the design stage the following objectives: (1) A shape that performs itsfunction, conforms with the boundary conditions,and can support the external loads. (2) A product with structural integrity, i.e. with stress levels remaining below acceptable limits. (3) A product with acceptable performance, e.g. modal dynamics, i.e. with natural frequencies and mode shapes that do not amplify external dynamic loads; and static, i.e. acceptable deflection. (4) A composite microstructure that can optimally satisfy the above topology/ shape, load, and performance constraints. (5) A microstructurefabrication process that efficiently produces the above optimal structure. The purpose of our ONR funded project is to address the complete problem in concurrent structural design by further developing the LargE Admissible Perturbations (LEAP) theory which is being developed at the University of Michigan since 1983, and combiningit with micromechanicsconstitutive equations. At the fabrication end, the Selective Laser Sintering (SLS) process will be simulated so that the SLS variables are defined as the final product of the concurrent structuraLdesign optimization process. LEAP theory -- as implemented in Code RESTRUCT (REdesign of STRUCTures) -- produces the final design without trial and error or repeated Finite Element Analyses (FEAs), thus, shortening the redesign process and contributing to rapid prototyping.
Temperature-Controlled Selective Laser Sintering
(1994) Benda, J.A.
A control scheme for laser sintering has been developed which maintains sintering powder at constant temperature by actively controlling laser power. It uses a sensor to monitor the temperature of powder at the focus of a moving laser beam. The control scheme corrects for variations ofthermal conductivity and powder reflectivity due to the proximity of previously sintered material, as well as for statistical fluctuations. The sensor also serves as a useful diagnostic, and is used to confirm model predictions ofthe variation of powder temperature with process parameters. A second temperature-controlled laser beam, concentric with the first, but of larger spot size, can be used to locally heat the powder around the sintering powder. This is shown to reduce curling as well as the balling or agglomeration of molten material
Product Model Driven Direct Manufacturing
(1994) Carleberg, Per
The input to the freeform fabrication process is essentially geometric data, raw material, material data and process parameters. Optimal process parameters depend upon current material and the part geometry. This paper describes an research approach in which all necessary input including process parameters are obtained or derived from the product model. The part geometry with its process parameters is transferred as a STEP model to the SFF system. In the SFF system this model is converted to the internal format, coupled to the process parameters. The approach is exemplified with the SLS machine from DTM as SFF system.

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