Metal Injection Molding Process

The key steps for the MIM process are as follows:

Step 1: Feedstock Mixing and Granulating

Very fine metal powders are combined with thermoplastic and wax binders in a precise recipe. A proprietary compounding process creates a homogenous pelletized feedstock that can be injection molded just like plastic. This achieves ultra-high density and close tolerances over high-production runs. After granulation, the metal mixture – the feedstock – is viscous and can now be processed in the injection molding process.

Step 2: Molding

The next step is the molding of the part in a conventional injection molding machine. The feedstock pellets are gravity fed from a hopper into the machine’s barrel where heaters melt the binder, bringing the feedstock to the consistency of toothpaste. A reciprocating screw forces the material into a two-part mold through openings called gates. Once cooled, the part is ejected from the mold with its highly complex geometry fully formed. Once the component is ejected it is known as a "green part."If necessary, additional design features not feasible during the molding process (undercuts or cross holes, for example) can be easily added at this stage by machining or another secondary operation.

Step 3: Debinding

the "green part" is then put through a controlled process called debinding that removes the binder and prepares the part for the final step. This step is to remove most of the binder, leaving behind only enough to serve as a backbone holding the size and geometry of the part completely intact. This process, commonly referred to as "debinding," may be performed chemically (catalytic debinding) or thermally, which in some cases may involve a solvent bath as the initial step. The choice of debinding method depends on the material being processed, required physical and metallurgical properties, and chemical composition. After debinding, the part is referred to as a "brown part".

Step 4: Sintering

In order to make a precision molded part with the desired geometric and mechanical properties out of the "brown body", it has to be sintered at high temperatures. The "brown part" is held together by a small amount of binder and is still fragile. During sintering temperatures reach near the melting point of the material. The remaining binder is removed in the early part of this cycle, followed by the elimination of pores and the fusing of the metal particles as the part shrinks isotropically to its design dimensions and transforms into a dense solid. The sintered density is approximately 98% of theoretical. The end result is a net-shape or near-net-shape metal component, with properties similar to those of one machined from bar stock. Of course, if necessary, post-sintering operations such as coining, machining, heat treating, coating, and others, may be performed on the part to achieve tighter tolerances or enhanced properties.