Engineers think of injection molding as a plastic forming technology, using polyethylene, polystyrene, glass-filled nylon, and similar materials. Metals and ceramics have several desirable properties when compared with plastics. They are stronger, tougher, stiffer, have better chemical resistance, and offer significant latitude in electrical and thermal conductivity, magnetic response, wear behavior, and operate to high temperatures. MIM is a synergistic combination of the performance properties associated with inorganic materials and injection molding associated with plastics. Most applications for powder injection molding are in ferrous metals, especially stainless steels.
In MIM, small powders are molded in the desired shape using a thermoplastic binder. Binders can be as simple as paraffin wax (candle wax), but usually are a mixture of several engineering polymers. The binder softens and melts on heating and then freezes in the mold to hold the particles into the desired shape. After molding, the binder is removed and the shaped powder is sintered to attain the target properties. A schematic of the SMP process is illustrated here. The polymeric binder bonds the particles during the shaping process and is extracted as the particles are heated. Once the polymer is extracted, the porous powder structure looks like the molded shape, but it is weak. However, when the powder is heated to a high temperature, the particles bond and densify due to capillary attraction, in what is called sintering. There is a tremendous strength gain in sintering, to the point that the sintered solid is essentially indistinguishable in strength from wrought metal. So in MIM the plastic or polymer is only a transient phase, added to help the metal particles flow and fill out the mold.
The MIM process has many variants, reflecting different combinations of powders, binders, molding techniques, de-binding routes, and sintering furnaces. However, all variants share three technology areas:
- Feedstock; powders, binders, mixing
- Molding; rheology, tool design, machine/press operation
- Thermal processing; de-binding, sintering, heat treatment