Computer Aspects of Solid Freeform Fabrication: Geometry, Process Control, and Design

Solid Freefonn Fabrication (SFF) is a class of manufacturing technologies aimed at the production of mechanical components without part-specific tooling or process planning. Originally used for creating modelsfor visualization, many industrial users of SFF technologies are realizing the greater potentialofSFF as legitimate manufacturing processes for producing patterns and, in some cases, functional.parts. Thus, SFF is becoming an important aspect of the product realization process in these industries. Solid Freefonn Fabrication arose from the dream of "push-button" prototyping, in which solid reproductions of three-dimensional geometric models are created automatically under computer control. Perhaps more than any other class of manufacturing technologies, computer software development has been an integral part of the emergence of SFF. As SFF technologies evolve toward the ability to create functional parts, computer issues gain more importance. This paper discusses three aspects of software design for SFF: processing of geometric data, global and local control of SFF processes, and computer-based analysis and design for SFF manufacturing. The discussion of geometric processing issues focuses on accuracy and completeness of input models, and the algorithms required to process such models. The interplay between the physics of SFF processing and the desired output geometry is discussed in terms of the development of model-based control algorithms for SFF. These two areas, geometric processing and control, are necessary for the practical implementation of any SFF technology. However, for SFF to realize its potential as an alternative for manufacturing functional parts, engineers must be provided with analysis and design tools for predicting mechanical properties, ensuring dimensional accuracy, choosing appropriate materials, selecting process parameter values, etc. For each of these three different but related areas of software design, the state-of-theart is assessed, contemporary research is summarized, and future needs are outlined.Mechanical Engineerin

A Method to Generate Exact Contour Files for Solid Freeform Fabrication

Existing methods to create contour files generate a polygonal approximation of the contours instead of an exact representation. This paper presents a method to generate exact contours from Constructive Solid Geometry (CSG) representations. The method preserves the accuracy of the contour files provided the primitives used to generate the CSG tree are polygonal or quadric objects. Due to the inclusion of quadric objects into the primitive set an additional effort to solve for the intersection points between two quadric curves is required. The paper also presents a method to convert piecewise quadratic contours to toggle point files for raster scanning solid freeform fabrication processes.Mechanical Engineerin

Software Testbed for Selective Laser Sintering

Computer software plays an important role in the implementation of Solid Freeform Fabrication (SFF) technologies. This paper describes a software testbed for processing part geometry for a particular SFF technology, selective laser sintering (SLS), that is built around the separation of the slicing and rasterization operations to accommodate geometric information from a variety of sources. The paper also discusses the process control software being developed for a new high-temperat rkstation for SLS of metal powders. This program features a high-resolution data rmat, the ability to interpolate to achieve a desired resolution, and a menu-driven user interface with graphical feedback and process simulation capabilities.Mechanical Engineerin

Agile Product Testing with Constrained Prototypes

The means to acquire reliable functional information is a critical factor that differentiates product development time and cost. Thanks to advances in solid freeform fabrication techniques, industries can produce geometrically complex parts within dramatically reduced time and cost. Even though industries can save significant efforts by performing functional tests rapid prototypes, they still prefer full-scale product tests, especially in later design phases, due to inherent limitations in traditional similarity methods (TSM). This paper describes a new method to perform reliable functional tests with rapid prototypes that cannot be properly handled by the TSM.Mechanical Engineerin

Selective Laser Sintering of Passive Dynamic Ankle-Foot Orthoses

Passive dynamic ankle-foot orthoses (AFO’s) are used to improve gait performance in those with various neuromuscular disorders. An important design characteristic of passive dynamic AFOs is the storage and release of elastic energy within its structure to help satisfy the energetic demands of walking. Thus, minimizing energy dissipation through internal friction is a fundamental criterion for selecting the appropriate AFO material. This study compared the mechanical damping of a carbon-fiber AFO to three geometrically identical AFO’s fabricated using selective laser sintering with different materials. Mechanical damping characteristics ranked the materials as Nylon 11 (best), followed by DuraformTM PA and DuraformTM GF (worst).Mechanical Engineerin

Implementation of a Functionally Gradient Material Modeling and Design System

New advancements in Solid Freeform Fabrication (SFF) processes promise the capability to produce Functionally Gradient Material (FGM) parts, in which the material compositions vary spatially. To realize this potential there is a need for CAD methods and design software to model, design, represent and exchange material information and instructions to the manufacturing process. However, currently available commercial CAD systems are limited to representing and storing only geometric information, which is not adequate for material design purposes. This work presents an extension of a theoretical approach based on Volumetric MultiTexturing (VMT) and hypertexturing schemes to make the material design process intuitive and user controllable. Inverse distance weighted interpolation is used in conjunction with procedural material functions to accomplish axial or linear material gradient directions from surface to surface across a solid. This offers the capability of specifying fixed material composition values to the faces in the solid and blending them across the interior of the solid. The extension of the proposed approach to the modeling of discrete material domains is also discussed. These material regions can be combined using special sets of operators depending on the form of the material functions. Finally, a design environment has been developed, which allows users to systematically apply material information to geometry and captures design intent.Mechanical Engineerin

Optimization of SLS Process Parameters Using D-Optimality

Solid Freeform Fabrication (SFF) refers to a group of processes that manufacture parts of arbitrarily complex geometry without tooling. Currently, the operation of most SFF machines requires skilled operators with expertise in choosing process parameters in ordered to achieve the desired part quality. Thus, the “push-button 3D hardcopy” promise of SFF has yet to be realized. This paper presents a framework for selecting optimal process parameter values automatically for the selective laser sintering (SLS) process. The research described considered five process parameters that are important for the SLS process. To achieve quality measures from the five process parameters, optimization is inevitable. The method optimizes these process parameters of SLS with respect to a set of desired quality measures, based on user input of the relative importance of each of the quality measures. The basis for the framework is the so-called Doptimality criterion applied to a series of factorial experiments that capture empirically the relationships between the process parameters and part quality measures. The framework is implemented in MINITAB™ and a macro is used to perform the optimization.Mechanical Engineerin

Parametric Representation of Part Contours in SLS Process

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.Mechanical Engineerin

Impact of gender and generational differences in work values and attitudes in an Arab culture

In this article, the work values and attitudes of 241 nationals from the United Arab Emirates are examined. Specifically, we assessed the impact of generational membership and gender on the Islamic work ethic, individualism, attitudes toward women at work, and perceptions of the utility of wasta. Results suggest that values and attitudes held by people in the Middle East are changing on the one hand (e.g., individualism) but are also deeply held on the other (e.g., the Islamic work ethic). Results also suggest that the perceptions about the utility of wasta differ across generations. The implications of these findings and suggestions for future research are discussed. © 2006 Wiley Periodicals, Inc

Functionally Gradient Material Design and Modeling Using Hypertexture for Solid Freeform Fabrication

SFF technologies have the potential to become manufacturing process that are capable of producing parts that have not been feasible by other techniques. The fabrication of Functionally Gradient Material (FGM) is one of the possible candidate. It is critical.to provide three dimensional material gradient data in the solid model to fabricate FGM. The approach is to model spatially varying material density distribution on implicit solid geometry using a hypertexturing scheme and a procedural volumetric modeling. It is essentially an extension of procedural solid texture synthesis, which has been effectively used to model cloud, gas, and flow stream. It will save the amount of information storage especially when the gradient pattern is repeating. Geometric operation becomes convenient since the material data are linked to the geometry only by the reference surfaces.Mechanical Engineerin