Tech Talk
What is MIM?
Metal injection molding of parts has been around for 35 years. Generally, MIM parts weigh less than 250g and are, at most, the size of a folding cell phone. However, larger components are becoming more common. If 20,000 to several million parts are needed, and the parts have complex geometries – 20 to 30 measurable dimensions that would call for three or more machining setups – MIM is a viable option. Some of the parts it has been used for include computer hard drives and other peripherals, hand tools, firearms, fluid injectors and sprayers, medical products and cutting tools. MIM requires three steps: 1. Injection under heat and pressure of a mix of powdered metal and thermoplastics or other binder into a mold cavity, analogous to thermoplastics injection molding; 2. Removal of the binder by heat or solvent, leaving just metal powder; and 3. Sintering of the metal powder at a temperature below the melting point of the metal, but high enough to cause the almost-microscopic powder to coalesce virtually into solid metal (the resulting density is greater than 96 per-cent that of solid metal). MIM can be done with carbide, other metals and metal allows, including precious metals. The metal feedstock particle size is far smaller than metal particles used in other powder metallurgy and might better be described as metal “flour”. It is as fine as metallic paint pigment (less than 20µm). Attributes of MIM parts include: 1. Net shape with good dimensional tolerancing; 2. Great freedom of shape, including molded-in blind- or cross-holes, small radii, thin sections and other complex geometry; and 3. Mechanical properties nearly equivalent to metal processed with conventional cutting, welding, forging and stamping. Depending on the application, sintered parts might be hardened (including case-hardened), polished and finished. Machining might be required to add features, remove molded-in supports for sintering, touch up the part or achieve a final dimension. Ongoing and future developments for MIM include environmentally friendly binders, continued refinement of metal powders, process simplification and cost reduction, and design tools (including mold analysis) that accommodate the shrink factors in the process. D. Gehman
