Metal injection molding (MIM) has gotten more versatile in the last few years. For starters, it combines the flexibility of injection molding with the material properties of machined components and produces net or near net-shape parts. Advances in the technology let it handle complex components such as blades, needles, finger loops, and fiber-optic connectors that measure less than 75-mm long and weigh less than 140 gm. Part tolerances out of the mold are within 0.3% of any dimension. MIM also requires fewer set-ups and generates less wasted materials, as compared to metal injection or machining processes.

MIM works like this: Operators first mix a recipe of metal powders, binders, and carrying agents specific to the finished product's performance and density requirements. Typically, recipes use metal powders with particle sizes of 20μ, or less, and binders selected primarily for their melting characteristics. This creates a homogenous slurry, similar to those found in high-density-plastic molding. The mixture is fed into an injection-molding machine to form the parts.

Molded pieces right out of the injection machine are called “green parts.” They are slightly oversized and relatively soft. Secondary operations such as drilling cross-holes and tapping internal threads can be performed while the parts are green. Doing so eliminates complicated mold designs, reduces the cost of adding features, and maintains precision tolerances.

Next, binders are removed. In the past, MIM companies used solvents and high temperatures. More recently, several companies have developed water-based binders that are just as effective while being environmentally friendly.

Next, an industrial oven heats the parts, using special gases to remove the small amount of residual high-molecular-weight material. This stage results in porous structures called a “brown parts,” which contain only metal powder.

These are placed in a vacuum sintering furnace with a controlled atmosphere. When parts reach a temperature close to their melting point, the controls add a small amount of hydrogen. This fuses the powder particles together to form a dense, uniform structure. In fact, MIM produces parts with densities of 98.8% and mechanical properties comparable to bar stock. When needed, parts can be heat-treated, plated, or bead blasted. And using video-measurement equipment confirms that dimensions and densities are within tolerances.