Metal injection molding takes fine metal powder, mixes it with a polymer binder, and molds it exactly as a plastic part is molded. The green part is then debound and sintered, during which the binder leaves and the part shrinks substantially to near full density. It is the way to make small, complex, high volume metal parts that would otherwise need a great deal of machining.
The four stages
Feedstock mixing, injection molding, debinding and sintering. The molding stage is conventional and fast. Debinding removes the polymer chemically or thermally without collapsing the part. Sintering fuses the powder at high temperature and is where the shrinkage happens, typically a substantial and predictable percentage that the tool is scaled up to compensate for.
What it is good at
Metal injection molding services earn their tooling on small parts, complex geometry, high volume, and materials that are hard to machine. Surgical instruments, firearm components, watch cases, connectors, orthodontic brackets and a great many automotive small parts are made this way. The economics resemble injection molding: an expensive tool amortised over a large number of near net shape parts that need very little finishing.
Metal injection molding vs die casting
Die casting handles larger parts, thicker sections and much shorter cycles, and works in a narrow set of low melting alloys, mostly aluminum, zinc and magnesium. Metal injection molding handles small intricate parts in a much wider alloy range including stainless and tool steels, at higher cost per unit mass and better density and mechanical properties. Part size and alloy usually decide it before cost does.
Design constraints that come from sintering
Uniform wall thickness matters even more than in plastic molding, because uneven sections shrink unevenly and distort. Very thick sections are difficult to debind. Large flat spans sag in the furnace and need setters. And the part must be supported through sintering without marking, which is a real limit on geometry.
Questions people ask about metal injection molding
What is metal injection moulding?
A process that molds a mixture of metal powder and polymer binder like a plastic part, then removes the binder and sinters the part to near full density.
How much do MIM parts shrink?
A substantial and material dependent percentage during sintering, compensated by scaling the tool. The important consequence is that uneven parts shrink unevenly.
Is MIM cheaper than machining?
In volume, usually much cheaper for small complex parts, because it is near net shape. The tool has to be amortised, so low volumes favor machining.
MIM or die casting?
Part size and alloy usually decide. Die casting suits larger parts in aluminum, zinc and magnesium. MIM suits small intricate parts in steels and other high melting alloys.