Metal injection molding (MIM) is a near-net-shape process suitable for small, three-dimensional complex shape products with special performance requirements. This technology is a new near-net-shape technology in powder metallurgy, which is formed by introducing modern plastic injection molding technology into the field of powder metallurgy.
MIM process flow is as follows: all kinds of fine metal powder (generally less than 20 μ m) and the preset binder, mixed with a certain proportion, are made to the feedstock with rheological properties. By injecting the feedstock into the mold cavity through an injection machine, the semifinished products are formed. After removing the binder and sintering at a high temperature, various metal parts can be obtained.
The particle size, tap density and particle shape of MIM powder are the key performance indicators to determine whether the powder can be successfully used in the MIM process. MIM process requires very fine powder (~10μm) to ensure uniform dispersion, good rheological properties and a high sintering rate.
The ideal MIM powder has the features as follows: the powder size is 2-8μm; the bulk density is 40 %~50 %; The tap density is more than 50%; the powder particles are nearly spherical with a large specific surface area.
At present, MIM metal powder raw materials include iron, nickel, titanium, stainless steel, precious metals, superalloys and other materials. At the same time, it is developing towards diversification, such as structural materials, functional materials, magnetic materials and so on.
The main production methods of MIM powder are carbonyl method, ultra-high pressure water atomization method, high pressure gas atomization method, plasma atomization method and laminar flow atomization method. Different powder preparation technologies have different effects on the grain size of powders, particle shape, microstructure, chemical properties, manufacturing cost and so on.
Binder is the core technology of metal injection molding. Compared with the conventional powder metallurgy method, MIM has a significant difference of high binder content. The main function of the binder is to act as a carrier for the flow of bonded metal powder particles and to maintain the shape of the workpiece after molding.
The binder for MIM shall meet the following requirements:
Small contact angle with powder, strong adhesion and no reaction with powder;
The viscosity changes little in the range of injection temperature, but the viscosity changes fast in cooling, so it is difficult to stick the mold;
A relatively small amount of binder can make the mixture have better rheological properties.