Metal injection molding (MIM), sometimes referred to as Powder Injection Molding, merges two established technologies, plastic injection molding and powdered metal. It combines the strength and integrity of metal with the design flexibility of the injection molding process to offer simple solutions for highly complex part geometries. The Powder Injection Molding process is typically explained as four unique processing steps (Compounding, Molding, Debinding and Sintering) to produce a final part that may or may not need final finishing operations.
Fine metal powder less than 20µ particle size is compounded
with binders in precise amounts. The ratio of metal powder to binder is approximately
60% metal, 40% binder. Both pre-alloyed metal powders and combinations of
elemental metal powders are suitable for MIM processing, and the varieties of
metal, available for MIM are constantly expanding.
Powder injection molding is identical in equipment and technique to how plastic injection molding takes place. The feedstock is injected into a machine where it is heated and into a mold cavity under high pressure. Once molded, the component is referred to as a “green” part.The tooling can be of multiple cavities for high production rates. The mold cavity is sized approximately 20% larger to compensate for shrinkage that takes place during sintering. The shrinkage change is precisely known for each material.
De-binding is a process where the binder materials are removed from the molded component. The process uses heat to remove the binders and prepare the part for the final step sintering. Once this phase is complete, the component is referred to as “brown.”
The brown parts are placed on sintering furnaces. At the beginning of this phase, the “brown” part is held together by just a small amount of the binder, and so is fragile. Sintering eliminates the remaining binder, then densifies and gives the part its final geometry. The part is subjected to temperatures near the melting point of the material, and the entire sintering process takes 15-20 hours.
Depending on final requirements, certain finishing operations may be performed to Powder Injection Molding parts. To improve physical properties, heat treatment can be done as on any metal. Any type of machining operations can be performed to achieve tolerances that are more precise than what the process can capably supply. Plating and coatings can be readily applied due to the high material density. Numerous mechanical assembly techniques such as welding or cold working can also be successfully used.