Metal Injection Molding (MIM for short) is a typical cross-disciplinary product. It is a new type of near-net-shape processing technology for metal parts formed by introducing modern plastic injection molding technology into the field of powder metallurgy. The flexibility of plastic injection molding and forged metals' strength and integrity are combined to provide cost-effective solutions for highly complex part geometries. It is a high-tech that has developed rapidly in the powder metallurgy discipline and industrial field in recent years.The MIM process is generally explained as 4 distinct processing steps (mixing, molding, degreasing, and sintering) to generate the final part.
Mixing
The MIM begins with the mixing or raw material preparation. Micron-sized custom-made metal powder is mixed with the high molecular polymer in a precise amount. The mixture is placed in special mixing equipment and heated to a temperature that melts the binder until the binder is uniformly coated with the metal powder particles, which finally is made into pellets of about 3-4mm, commonly known as feeds, so that these granular feeds can fall freely in the hopper of the molding machine when used.
Injection molding
Injection molding of MIM is the same as plastic injection molding in equipment and technology. The granular raw material is fed into an injection molding machine, where it is heated and injected into the mold cavity under high pressure. The molded part (now called "green body") is allowed to cool and then removed from the mold so that the process can be repeated. Since only the binder melts (to carry the metal powder), the whole process takes place at about 200°C. The mold can be with multiple cavities for high productivity. The dimensions of the mold cavity are approximately 20% larger to compensate for the shrinkage that occurs during sintering. The shrinkage of each material is precisely known.Degreasing (the process of removing the binder)
After injection molding, the molded part goes to the next step, called the degreasing or binder removal process. Binder removal is the process of removing binder material from molded MIM parts. This process is usually done in two steps, a portion of which is removed before the sintering step, and the residual skeletal binder maintains the shape of the product so that the degreased part can be moved into the sintering furnace for removal. Binder removal can be accomplished by a variety of methods, the most popular being catalysis and solvent extraction. After removing the binder, the metal powder of the brown billet is porous, and finally sintered and densified to obtain the final product.
Sintering - Sintered Parts
The degreased parts are placed on a ceramic holder, which is loaded into a vacuum or atmosphere sintering furnace. The degreased parts are slowly heated in a protective atmosphere to decompose the remaining skeleton binder, and the metal powder is densified into a complete metal body at a temperature close to the melting point, after cooling, the sintered part with a nearly finished shape is obtained, the relative density usually can reach more than 97%.