A new technology that uses MIM to rapidly produce mold inserts is said to cut tooling production time by a factor of five to 10 and tool price by half compared to traditional methods for making production molds. Jean-Marc BoÃ©chat, director of MIM Systems Ltd., reported on his company's new approach to rapid tooling at Euro PM2000 in Germany last October. The method is called Mold in Mold, a fitting name for a process that uses MIM to injection mold inserts that can then be used to injection mold plastic, metal, or ceramic parts.
Figure 1. The Mold in Mold rapid tooling process allowed for a trial-and-error approach to mold development for this medical application. The original part was metal, far left, which was first replaced by Catamold 17-4 PH, second from left, and subsequently by ceramic. Trial and error led to the final mold design, and acceptable part, far right.
One major advantage of the process is that the rapid tools are made out of steel, giving them similar characteristics to production tools, reported BoÃ©chat. The technology is well suited for small or limited production runs and for bridge tooling.
The starting point of the Mold in Mold process is the CAD file of the part to be molded. From the file, prototypes are built of either the part itself or the mold elements. The prototype is then inserted into a standard cavity and MIM material is injected. The finished product is a steel core or cavity that can be used to injection mold a few hundred or a few thousand polymer, MIM, or CIM parts. The length of the run depends on feedstock, BoÃ©chat said.
MIM allows for rapid and continuous mold modifications, he added. Using the process, several identical green inserts can be produced. After testing the first sintered insert, modifications then can be easily made to the next green part. This modified green part is then sintered and becomes the next iteration of the mold insert to be tested. For multicavity molding, several green parts can be sintered at once to create multiple identical cavities.
MIM Systems is still exploring the limitations of the process. Currently, it is comparable to other rapid prototyping methods in terms of practical accuracy and resolution limit, which is on the order of .1 mm (although this resolution limit is reduced to .06 mm after debinding and sintering). The biggest concern at present is the shrinkage factor during the entire process, which needs to be mastered to reach a reasonable quality level, said BoÃ©chat.
"The more the homogenity of the produced parts is controlled, the more the shrinkage can be precisely controlled, hence the better the precision of the sintered molds," he said.
Trial and Error
MIM Systems used the Mold in Mold process in the development of tooling for a current medical part. The flexibility of the rapid prototyping method allowed the company to take a trial-and-error approach when specifying a new material to eliminate extensive machining, said BoÃ©chat (Figure 1).
The part was originally metallic, made of martensitic stainless steel phase hardened to more than 50 Rockwell C. First, a Catamold 17-4 PH was tried, but after these parts failed to produce acceptable mechanical properties, ceramic was chosen. Catamold TPZ-A, a zirconium oxide based material from BASF, was selected.
"The first ceramic parts were produced with the same design as the ones out of 17-4 PH," BoÃ©chat reported. However, the shrinkage factor for the two materials is very different, he added, which resulted in a visible parting line in the thickest portion of the part. A new prototype mold was built within five working days, and further testing was done. Three more design changes resulted in three new inserts, the last of which was built in three days. The price for each iteration reportedly represented 25 percent of the cost of the production tool.
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