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Metal Injection Molding Process Overview

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

 

 

Case Study # 3
Case Study #3: Curved Scissor Blades
Component: Curved Scissor Blades
Challenge:  To develop a coining process that would produce consistent dimensions throughout the length of the curved blade thus assuring blade interference for cutting efficacy.
Solution: Use of controls, including SPC, to step up and maintain coining and monitor of the coining process.
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