Metal Injection Molding (MIM)
This process is a multi-stage process that converts fine metal powders into near full density, high strength metal components. MIM competes with cast, wrought, and machined metal components on the basis of both manufacturing cost effectiveness and material properties. The MIM process can yield very complex shapes with high degree of repeatability. The process consists of mixing fine metal powders with thermoplastic binders to form a feedstock which is injection molded into a closed mold. After ejection from the mold, the thermoplastic “binders” are chemically or thermally removed from the part so that the part can be sintered to high density. During the sintering process, the individual particles metallurgically bond together as material diffusion occurs to remove most of the porosity left by the removal of the binder. The sintering process shrinks the part, providing a net shape that can be used as-is or further worked to add additional features or improve tolerances.
The sintering process tempers carbon steel and heat treatable alloys such that they are soft. These alloys are typically cheaper that the precipitation or air hardenable alloys, therefore there is strong incentive for their use for MIM parts. However, subsequent heat treating processes use to harden the parts lead to distortion which requires hard machining.Low distortion laser heat treating of localized areas such as sliding pin cavities, bearing, tool, and gear surfaces enables heat treating as the last step after soft machining.