Process for metal injection molding
In the monograph P.O. Gribovsky, published in 1956, describes in detail the technology of hot casting (hot molding) ceramic products under pressure (now, Low Pressure Powder Injection Molding) and, in particular, notes that "hot casting technology provides the ability to manufacture products from any solid materials, ranging from natural minerals, pure oxides, carbides, metals, etc., and ending with multicomponent composite synthetic materials and their combinations". This indication of the possibility of MIM-casting, which was implemented by Dr. Raymond E. Wiech Jr. in the 1970s, who refined MIM technology as co-founder of a California company named Parmatech, the name being condensed from the phrase "particle materials technology". Wiech later patented his process, and it was widely adopted for manufacturing use in the 1980s.
MIM gained recognition throughout the 1990s as improvements to subsequent conditioning processes resulted in an end product that performs similarly to or better than those made through competing processes. MIM technology improved cost efficiency through high volume production to "net-shape", negating costly, additional operations such as machining although MIM is weak in terms of tight dimensional specifications.
The process steps involve combining metal powders with polymers such as wax and polypropylene binders to produce the "feedstock" mix that is injected as a liquid into a mold using plastic injection molding machines. The molded or "green part" is cooled and ejected from the mold. Next, a portion of the binder material is removed using solvent, thermal furnaces, catalytic process, or a combination of methods. The resulting, fragile and porous (40 volume percent "air") part, is in a condition called the "brown" stage. To improve handling often the debinding and sintering are combined into a single process. Sintering heats the powder to temperatures near the melting point in a protective atmosphere furnace to densify the particles using capillary forces in a process called sintering. MIM parts are often sintered at temperatures nearly high enough to induce partial melting in a process termed liquid phase sintering. For example, a stainless steel might be heated to 1350 to 1400 degrees Celsius). Diffusion rates are high leading to high shrinkage and densification. If performed in vacuum, it is common to reach 96–99% solid density. The end-product metal has comparable mechanical and physical properties with annealed parts made using classic metalworking methods. Post sintering heat treatments for MIM are the same as with other fabrication routes, and with high density the MIM component is compatible with the metal conditioning treatments such as plating, passivating, annealing, carburizing, nitriding, and precipitation hardening.
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