Lee E. Weiss
The goal of rapid mechanical prototyping (RP) is to be able to quickly fabricate complex-shaped, three-dimensional parts directly from CAD models. One approach for accomplishing this is to use solid freeform fabrication (SFF) processes. SFF methodologies have the following attributes:
Current SFF systems are based upon a layered manufacturing paradigm (Fig. 2.1). In this method, a solid 3D CAD model of the object is first decomposed into cross-sectional layer representations in the process planner. The planner then generates trajectories for guiding material additive processes to physically build up these layers in an automated fabrication machine to form the object. Sacrificial supporting layers are also simultaneously built up to fixture the object. For example, shapes are first decomposed into 2½-dimensional layers, i.e., layers that can be represented by a planar cross-section with an associated uniform thickness.
Fig. 2.1. Solid freeform fabrication using a layered manufacturing paradigm.
Each physical layer, which consists of the cross-section and a complementary shaped sacrificial layer, is then deposited and fused to the previous layer (Fig. 2.2a) using one of several available deposition and fusion technologies. The sacrificial material has two primary roles: first, it holds the part, analogous to a "fixture" in traditional fabrication techniques; second, it serves as a substrate upon which "unconnected regions" and overhanging features can be deposited. The unconnected regions require this support since they are not joined with the main body until subsequent layers are deposited. Another use of sacrificial material is to form blind cavities in the part.
Other building approaches use support structures only where required, i.e., for the unconnected regions and steep overhanging features (Fig. 2.2b). These explicit support structures are deposited with the same material as the object being formed, but are drawn out in a semisolid fashion so that they are easy to remove once the part is completed. For example, they may be deposited as thin wall structures.
SFF can rapidly and automatically be planned and executed, independent of part shape, for several reasons: (1) the shape decomposition operation maps complex 3D geometry into simple 2½D representations, (2) custom fixturing is not required, and (3) the machinery to implement these systems is relatively easy to operate.
Building up structures in layers is not a new idea; in fact, it goes back to the days of the pyramids -- although this was hardly automated construction (Fig. 2.3). Practical implementations of layered manufacturing for modern manufacturing needs have been made possible by several enabling technologies, including CAD-based solids modeling, lasers, ink-jet printing, and high-performance motion controllers, integrated with more traditional manufacturing processes, such as powdered metallurgy, extrusion, welding, CNC (computer numerical control) machining, and lithography, into novel arrangements (Fig. 2.4).
Fig. 2.4. SFF enabling technologies.
Machining also plays an important role in rapid prototyping. CNC machining, however, is not generally considered to be an SFF methodology, not only because it requires skillful human intervention to help plan the operations and to operate the equipment, but also because machining often requires custom fixturing and has inherent geometric limitations. Still, machining can be effective in many rapid prototyping applications. In this JTEC/WTEC mission, the panel visited mostly with researchers and manufacturers of layered manufacturing processes. These processes are described in the next section. The panel also visited with several groups working on machining, and this work is described below in the section on the role of machining in RP.