MATERIALS

There is a clear need to improve the mechanical properties of the stereolithography resins and plastics currently being used. Cubital is searching for an epoxy-type material that will match the properties of 3D Systems' epoxy resin and for a wax-like material that can be used for investment casting of metal parts. The Institute for Polymer Testing and Polymer Science at the University of Stuttgart has helped EOS develop a polystyrene for selective laser sintering. As laser-sintered, this material has relatively low strength and is generally used for investment casting patterns or visualization models. Although it can be infiltrated to produce high-strength rigid parts, polyamide (nylon) is generally used for parts requiring higher strength and/or toughness. Users such as Daimler Benz desire models that have the mechanical properties of ABS; that is, with ductility to test snap fit, flexible hinges, and other functional properties of a design.

Full-density resins, which may be produced by stereolithography, selective laser sintering, or infiltration, will not replicate the mechanical properties of a part produced by plastic injection molding, due to rheological considerations in the molded plastic parts that can align the anisotropic polymers to produce different mechanical properties in different directions within the part. In a similar manner, sintered metal parts do not replicate the mechanical properties of fully dense wrought or cast products. Properties in plastic parts that approach what might be expected from production parts can only be achieved using rapid tooling to produce injection molded parts. Equipment and resin makers, research institutes, and others, see the need to develop rapid tooling methods and materials to (1) produce parts that approach the mechanical properties of production parts, and (2) be able to make a few hundred parts for functional testing.

Stereolithography Resins -- Properties and Cost

A prime consideration in both Europe and Japan is the development of resins that have improved mechanical properties, better long-term dimensional stability, higher temperature resistance, and lower cost. Better mechanical properties give designers greater latitude in using RP models to test functional characteristics (Sites, 1996, Olympus report, 95-98). Several companies expressed the desire to achieve properties equivalent to those of ABS.

Greater dimensional stability over time (i.e., 30 days) is also needed. When Japan's Ministry of International Trade and Industry (MITI) made funds available to develop better resins, one attribute that it specified as an objective was time-dependent dimensional stability (Sites, 1996, D-MEC report, 47-53). Resin makers are also developing epoxy-based UV curable resins to reduce warping.

Higher thermal stability is desirable in rapid prototyping resins when used for (a) producing plastic injection molds that are used for making a few hundred prototype parts in a common plastic, and (b) making functional models that have to withstand elevated temperatures, such as for functional testing of the intake manifold of an internal combustion engine.

Stereolithography resins are also expensive. Service bureaus such as Schneider in Germany would like to see price breakthroughs in resins to make them more competitive (Sites, 1996, Cubital report, 7). A single large vat of stereolithography resin can cost $100,000 in Japan. For companies that make large parts, such as intake manifolds for engines, this is a significant issue (Sites, 1996, Hino Motors report, 58-9).

Metals

In Europe there is considerable activity in devising processes that will directly yield metal components. In processes using powder metals in selective laser sintering-type methods, the resulting RP products are porous. They can be furnace-sintered to consolidate the objects, but this results in extensive shrinkage and loss of dimensional accuracy. The alternative is to infiltrate with a lower-melting-point alloy or a high-temperature epoxy resin that does not require furnace sintering. Infiltrated products represent a compromise: they do not have good high-temperature properties and often have too low a hardness and wear resistance compared to fully dense steel objects; however, they are often adequate for pilot tooling for making a few thousand injection-molded parts. The Fraunhofer Institute for Production Technology (IPT) is developing a process to directly make fully dense metal parts by a laser fusion process called Laser-Generated RP, which incorporates a milling cutter to trim the walls and surfaces of each layer to improve accuracy and surface finish. The process was first shown publicly December 1995 at Euromold 95 in Frankfurt, Germany. Similar techniques are being pursued in the United States, but without integrated machining.

There are two forms of metal tooling: pilot tooling and production tooling. The long-term objective of European research institutes and equipment makers is to develop RP hardware and processes to produce production tooling. However, accuracy is a major issue. In producing pilot metal tooling, achievement of a high degree of accuracy and of mechanical properties approaching those of wrought metal are serious considerations. Achieving both of these properties at the same time is not currently possible.

It is clear that in Europe there is a need for metal parts produced by rapid prototyping for tooling and structural components. At the RP service bureau Schneider Prototyping near Frankfurt, 15-20% of orders are for metal parts and tooling. Panelists were told that German companies often send their rapid prototyping models or CAD files to the United States to have them cast into metal parts because price and quality are better than in Germany. German casting foundries have been slow to learn casting methods that use rapid prototyping masters for investment casting patterns.

The JTEC/WTEC panel found no company in Japan developing RP processes to directly build metal components, but did find considerable interest in making metal components by investment casting, metal spray, and so forth.

Ceramics and Paper

Compared to the United States, there does not appear to be much interest in Europe and Japan in building ceramic parts by RP. At the time of the panel's visit, several companies and institutes in Europe were working on developing ceramic processes, but the emphasis appeared to be on polymer resins and metal products.

In Japan, Kira Corporation, which builds a small laminated paper (or object) manufacturing (LOM) machine, would like to see its process capable of greater accuracy. It is now limited to ±0.1 mm/25 mm in the horizontal plane (x-y direction) and ±0.3 mm/25 mm in the vertical build direction (z direction). Swelling of the completed part in the z direction due to humidity can also be a problem, which Kira researchers hope to correct with an improved paper (Sites, 1996, 75-83).

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Published: March 1997; WTEC Hyper-Librarian