Three-dimensional printing, as shown in Fig. 6.1, is a rapid prototyping process being developed at MIT. In 3D Printing (3DP), an ink jet printing head can be used to selectively inject a binder into a metal powder bed. The selectively bound part, when later removed from the bed, is a relatively low-density (about 50% dense) "green" part. The green part is then subsequently fired and infiltrated to make a dense metal part. Fig. 6.2 shows an early example of a mold made in the process. Even at this stage of development, there are some interesting features in this mold. This mold has internal cooling channels built right into it -- something very difficult to achieve with standard machining processes. Fig. 6.3 shows a more recent mold, illustrating the advances being made. This tool is made of stainless steel-bronze, and the injection-molded plastic parts are glass-filled nylon.
Fig. 6.2. 3DP mold exhibiting internal cooling channels.
Fig. 6.3. 3DP mold and resulting injection-molded connectors.
Selective laser sintering (SLS) is a rapid prototyping process first developed at the University of Texas. The indirect process for fabricating metal parts and molds is commercially available from DTM Corporation. In this process, metal powders are coated with a thermoplastic binder. These coated powders are then selectively fused together with a laser in the SLS process. This bonds the metal powders together to form mold components represented by a CAD file, thus producing a green part. The green part is then post-processed in a furnace, where the binder is burned out, and the metal powders are bonded together through traditional sintering mechanics. This part is now referred to as a "brown" part; it exhibits geometry but is also porous in nature. The brown part is then infiltrated with a second metal to form a fully dense mold. Fig. 6.4 is a schematic of the entire process. Fig. 6.5 shows a commercial core and cavity set that was created with this process. The properties of this mold are similar to those of 7075 aluminum.
Fig. 6.5 Core and cavity sets produced by RapidTool.