Design Considerations for Metal Injection Molding

Metal injection molding (MIM) produces relatively small, complex, net or near-net parts which can include features such as holes, radii, logos, and text. Moreover, MIM can make such parts for half the cost of CNC machining or investment casting. Better yet, MIM allows the consolidation of multiple components into a single design, eliminating the need for downstream assembly operations. Finished parts have excellent strength, surface finish, and corrosion resistance.

MIM material comprises fine (less than 20 micron) metal powder hot-mixed with polymeric binders, forming a homogenous mixture. Injection-molded metals include titanium, tool steels, stainless steels, and high-temperature alloys, among others. The hot mixture is cooled and granulated to form the feedstock for specially equipped injection-molding machines.

The machines mold the feedstock into so-called “green” parts which get palletized onto flat ceramic fixtures or “setters”. A catalyst removes 90% of the binder from the green part, creating what is called a “brown part”. Finally, the brown parts are sintered using a temperature and atmosphere appropriate for the alloy being processed. The heat removes the residual polymer and sinters or bonds neighboring particles together, giving parts with densities from 96 to 99% of theoretical. Shrinkage during sintering is a predictable 14 to 22%, linear. Oversizing the mold cavity compensates, with typical as-sintered tolerances being ±0.003 to ±0.005 in./in.

Design features possible with MIM are similar to those of plastic injection molding or die casting. Typically, draft angles range from 0.25 to 0.50° and walls should be a uniform thickness. Wall thicknesses should target 0.050 to 0.250 in., but parts can be produced with walls as thin as 0.005 in. Rib thickness should not exceed that of the adjoining wall. The addition of fillets and radii eliminates sharp corners and facilitates the flow of feedstock into the mold cavity. MIM can also make external and internal threads.

In general, follow these guidelines when designing for Metal Injection Molding (MIM):
• Orient features perpendicular to the parting line (the plane where the mold halves meet) to facilitate removing the part from the mold.
• Locate gates so they direct the flow onto a core pin or cavity wall. Where wall thickness varies, position gates so the material flows from the thicker to the thinner sections.
• A MIM component is likely to have witnesses (marks from ejector pins, parting lines, and gates). When designing critical features, carefully consider the location of witnesses.
• MIM parts are typically placed on flat setters for sintering so parts with long cantilevers and spans generally require additional support from ribs or custom fixtures. If possible, part designs should include a flat surface to eliminate the need for custom fixtures.

Many parts sinter to their net shape, but it might be necessary to use secondary operations, such as machining, for tighter tolerances. Tapping produces internal threads with tighter tolerances than possible from MIM. Tumbling and polishing produce aesthetic surfaces. Other options include plating, heat treating, PVD, and black-oxide coating.