Metal
Injection Molding (MIM) Process Overview
Powder Injection Molding (PIM) includes Metal Injection Molding (MIM), and is a growth technology for manufacturing
complex-shaped, high-volume metal or alloy parts.
Metal Injection Molding (MIM) parts can be produced from ferrous and non-ferrous
metals and alloys.
The Metal Injection Molding (MIM) process is outlined in the flowchart below.
The process begins with the atomization of molten metal to
form metal powders.
Then, the metal powder is sieved followed by gas
classification to alter the particle size distribution.
The metal powder is mixed with thermoplastic binders to
produce a homogeneous feedstock; with approximately 60 volume %
metal powder and 40 volume % binders.
Then, the feedstock is placed into an injection molder and
molded to form a net shape
green
part.
Injection molding occurs at relatively low temperatures and
pressures in conventional plastic injection molding machines.
The molds are similar to those used for plastic injection
molding including slides and multi-cavity configurations.
After injection molding, two thermal processes occur.
First, the binder is removed from the green part via an
evaporative process called “debinding.”
Second, after debinding, the part is sintered to form a
high-density metal part.
Sintering occurs at high temperatures, up to 2300oF
(1260oC), near the melting point
of the metal; under a dry H2 atmosphere or inert gas
atmosphere.
During sintering, the part will shrink isotropically to
form a dense shape.
Since, the complex shape of the molded part is retained
through the process, close tolerances in the as-sintered part can
be achieved.
Scrap is eliminated or significantly reduced since
machining of the part after sintering is usually not necessary.
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Like other volume production
methods, Metal Injection Molding (MIM) conserves material and time. Additionally, it has the
following advantages:
Density
- Typically
95-98% of wrought material.
Mechanical
Properties -
Comparable to wrought alloys of similar composition.
Complexity
- Geometries
comparable to plastic injection molding.
Surface
Finish - As
processed, 32 rms or better.
Corrosion
Resistance -
Comparable to wrought alloys of similar composition.
Alloy
Selection -
Many ferrous, nickel-based and copper-based alloys available.
Comparing
MIM to other fabrication methods:
| Specifications
or Characteristics |
MIM |
MACHINING |
INVESTMENT
CASTING |
POWDERED
METAL |
| Density |
96% |
100% |
98% |
88% |
| Elongation |
High |
High |
High |
Low |
| Tensile
Strength |
High |
High |
High |
Low |
| Hardness |
High |
High |
High |
Low |
| Complexity |
High |
High |
Medium |
Low |
| Surface
Finish |
High |
High |
Medium |
Medium |
| Cost |
Medium |
High |
Medium |
Low |
| Production
Volumes |
High |
Low |
Medium |
Low |
|
