MIM process (Metal Injection Molding)

Metal injection molding

An increasingly widespread process for complex parts

The Metal Injection Molding (M.I.M.) process was developed in the early nineties in the United States, essentially for applications in the military sector. In the following years the method became increasingly widespread, generally for the production of small-medium sized pieces.

Numerous aerospace, military, medical, precision mechanics, watchmaking, weapons and automotive companies currently use this technology.

The advantages of this technology are many: high precision in detail, notable mechanical properties of the obtained parts, ability to produce complex parts with tough and hard steels.
Complex parts

Purpose and features

The main purposes of the MIM process is to create small-medium pieces, with a complex shape, in medium-large batches, with high-resistance materials, with automated processing cycles.

The producion starts with the usual injection molding systems, with high production speed and minimal defects. Thus formed parts undergo the debinding step (resin weight loss) and sintering phase (to reach the final size) without subsequent manipulation. The final processing may ends with tumbling or polishing.
Steps of the MIM process
Wave

Process phases in detail

1
Injection
Injection
Injection
The feedstock is injected by a special screw machine (similar to plastic injection).
The result is a solid structure component (powder metal/resin mix) called Green Body, over dimensioned according to a shrinkage factor dependent to each kind of alloy.
2
Debinding
Debinding
Debinding
The injected components (Green Body) are introduced into an oven to be subjected to catalytic debinding: removal of the binding resin causes reduction of the weight of the part. The week solid structure composed by metal powder aggregates it’s called Brown Body. During this phase no dimensional changing occurs: the only variation is a weight reduction (because of the loss of the resin component)
3
Sintering
Sintering
Sintering
Once removed the resin, parts are introduced in a special sintering oven. Specific thermal cycles in vacuum or monitored atmosphere effect the coalescence of the metal powder particles. The part assumes the configuration of the minimum volume until compacting. Acquisition of mechanical and dimensional characteristics are measurable by final density and dimensional shrinkage (about 20%): the obtained part is in complete, similar to a casting.

Grinding and polishing give the workpiece gloss and low roughness values. Any heat treatment may be required.