EOS GmbH of Germany markets the EOSINT S rapid prototyping machine, which fabricates casting patterns directly in sand. The process is a modification of EOS' standard sintering machine, in which a coated refractory sand is used as the powder. The mold information is written using a CO2 laser that causes the sand particles to adhere by heating and binding their coating. EOS has termed the process "Direct Croning®," referring to the particular refractory sand employed. Molds for complex parts can be built quickly, and castings can be made directly into the sand mold (Fig. 7.1).
Researchers at the Fraunhofer Institute for Applied Materials Research (IFAM) in Bremen (http://www.ifam.fhg.de) have already produced ceramic part (SiC) fabrication using their Multiphase Jet Solidification (MJS) system (Fig. 7.2). Current materials development for MJS is focused on metals, but the system developers have stated that they are considering ceramic materials for future development.
Fig. 7.2. Ceramic parts produced by MJS: (left) green parts (tip and SLA test part); (right) sintered parts.
Researchers at both Teijin Seiki and D-MEC have openly discussed their efforts to incorporate ceramic (glass) spheres within their photopolymers for use in their standard RP stereolithography (SL) machines. The objective is to produce a prototype that can be used as a mold face for a limited number of injection molding operations. The ceramic provides added strength to withstand the molding pressures, and it provides thermal conduction to keep the mold faces from deteriorating rapidly due to the injection of molten plastic. Typically, the mold halves are backed by metal frames with cooling lines. The Teijin Seiki material has been demonstrated to withstand ABS temperature and pressure requirements for at least 22 shots (Fig. 7.3). A D-MEC mold shown informally in 1995 at the Sixth International Conference on Rapid Prototyping (Dayton, OH) evidenced use at elevated temperature; no information was available on the longevity of the mold. Introduction of ceramic considerably raises resin viscosity, creating problems for many of the leveling systems used by vendors (viscosities as high as 49,000 cps have been mentioned by Teijin Seiki engineers). This high viscosity restricts the loading concentration used and the range of dimensions of the particles, since both higher concentrations and smaller-diameter particles (loaded to a constant percent) result in increased viscosities.
Professor Nakagawa of the University of Tokyo mentioned to JTEC/WTEC panelists that ceramic parts are typically fabricated in plaster molds, usually using slip casting. He recently developed a nonaqueous carrier for the ceramic, which does not require a porous mold. Nakagawa successfully cast ceramic into a rubber mold, which opens the possibility of having reusable molds that can be created rapidly using an RP master.
In the United States, universities, industries, and government laboratories have been actively working with ceramic materials. Several licensees are commercializing aspects of MIT's "Three-Dimensional Printing" program (http://web.mit.edu/afs/athena/org/t/tdp/www/home.html). These include Soligen, which offers the "Direct Shell Production Casting" machine. The machine "writes" patterns for molds directly into ceramic powder using a binder dispersed via an ink-jet printer head. The resulting pattern is then cleaned of loose powder and sintered to provide a shell into which metal can be cast. A host of other processes are under development, most of which are tied to modifications of existing commercial systems. Some of these efforts are mentioned below.
Selective laser sintering of ceramic powders and fusing of coated ceramics are being investigated by DTM and the University of Texas (http://lff.me.utexas.edu). Both Lone Peak Engineering and the University of Dayton are investigating production of ceramic tapes and use of these tapes in the laminated object manufacturing (LOM) environment. In addition, the University of Dayton (http://www.udri.udayton.edu/mat_eng/rpdl.htm) is extending this process to ceramic composites using both chopped and continuous fiber reinforcement in its tape systems. Ceramic loading of photopolymers for use in stereolithography systems is being developed at the University of Michigan. Argonne National Laboratories (http://www.anl.gov/ITD/rapid.html) and Rutgers University (http://www.caip.rutgers.edu/~jumalata/sff-others.html) are developing ceramic-loaded filaments that will be compatible with fused deposition molding systems, similar to the multiphase jet solidification (MJS) system being developed in Europe. Case Western Reserve University is developing the CAM-LEM system, which utilizes ceramic material delivered in sheet format. Each material layer is cut by a 5-axis laser cutter that shapes the edge to match the slope of the part at every location. The layers are then robotically stacked and sintered to form the part. Other efforts include the program at Stanford Research Institute to develop a filled photopolymer.
The U.S. effort encompasses the development of ceramic molds for casting and the fabrication of both monolithic and composite ceramic parts. The particular ceramics under study include lower-temperature oxides and the higher-temperature materials such as SiC and AlN. RP fabrication of ceramic components could potentially open a variety of application areas that heretofore have been cost-prohibitive.