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D.  Alternative PM Processing Methods

 1.  Metal Injection Molding

Metal injection molding (MIM) is used to manufacture small parts of complex shape.  The process (Figure 9) uses fine powders (typically <20 µm dia.) that are heated and mixed with polymer binders in the form of thermoplastics, waxes, or other organics.  After cooling, the mix is granulated and fed into an injection molding machine.  The granulated mixture is heated to the consistency of a paste and injected into a closed mold to form the “green” compact.  In comparison with the conventional PM process where the organic content is ~1w/o, the binder content of the green MIM compact may be as high as 40 w/o.  The green part is cooled and removed from the mold.  Chemical and/or thermal methods are used to remove the majority of the binder.  The remainder of the binder is removed from the porous compact by heating at an intermediate temperature, then elevating the temperature for sintering.

Figure 9: Metal Injection Molding Process: Source "Powder Metallurgy Design Manual" Third Edition, MPIF.

MIM can be used to make parts with much greater complexity than by press-and-sinter PM but smaller in size.  A typical MIM part weighs 100 g or less and has a cross-sectional diameter <0.25" (6.35mm).  The sintering temperatures used are usually higher than in conventional PM and as a consequence, the sintered parts contain only 2-5 v/o porosity.  The high density results in exceptionally high ductility and elongation.  Secondary surface treatments are performed easily in the absence of interconnected porosity.