Despite all the advancements in polymer chemistry, some times metal is the only material that will do the job. And, sometimes machining individual metal parts one-by-one isn’t practical. Metal Injection Molding (MIM or sometimes called PIM: Powder Injection Molding) is just what it sounds like: injection molding metal… The material is shot in standard plastic injection molding machines and molds. To make the metal moldable, metal power is combined with a polymer (plastic) binder and shot at normal plastics injection molding temperatures.
But, due to the presence of the polymer binder, mold cavities are designed approximately 20% larger than the final part size. After molding, green parts are sintered at temperatures up to 2,600°F. At those temperatures, the polymer binder breaks down and dissipates while the metal particles retain all of the molded features. The metal particles fuse together during sintering and the part shrinks approximately 20% to form a solid metal part.
Kinetics has a really nice design guide to give you an overview of how to design parts for MIM. You can download it as a PDF (accessible from the home page), or you can view it in an interactive flash presentation.