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 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
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
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.
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.
(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.
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.
(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.
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,
(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.
(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.
(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.
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.
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
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.
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.
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.
(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.
(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.
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.
(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