Metal injection molding is a manufacturing option for producing complicated parts in medium to high volumes (10,000 to over 1,000,000 parts annually). The technique uses fine (<20-m m) metal powders that are intimately mixed with various thermoplastic binders in what's called a feedstock. The feedstock injects into molds similar to those used in plastic injection molding forming a composite "green" part.

A multistage powder-metallurgy process first removes most of the thermoplastic binder from the part using thermal or solvent processing, or a combination of both. The so-called debinding method employed is dependent on the polymer formulation as well as part cross section. Following binder removal, parts are sintered with a temperature and atmospheric profile specific to the alloy being processed. This removes the remaining traces of binder and as the temperature rises, the powder-metal particles fuse together, pore volume shrinks, and grain boundaries form at particle contacts.

Sintering temperatures for MIM range from 1,200 to 1,400°C. The fine particle size in combination with the high sintering temperatures produces greater sinter densities than those possible with conventional powder-metallurgy processes. MIM densities between 96 and 99% theoretical are common and interconnected porosity is less than 0.2%.

MIM is well suited for parts weighing up to 250 gm. Cross sections are typically less than 0.25 in. (6.35 mm). However, parts are not restricted to this combination of mass and cross section. Tolerances are on the order of ±0.3 to 0.5%, although specific dimensions can be held as close as ±0.1%.

Injection molding accurately duplicates mold surfaces. This results in parts having assintered surface finishes of <32 RMS (microinches). Common secondary operations, such as plating or heat treating, are also used to enhance the look of the part or boost performance. Because interconnected porosity is so low, parts don't need to be resin impregnated prior to plating and close control over case depth is possible in carburizing.

The Metal Powder Industries Federation (MPIF), Princeton N.J., offers Standard 35, Materials Standards for Metal Injection Molded Parts. The standard covers the most commonly used MIM materials with details on chemical and mechanical properties of each.