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The Powder Injection Molding Process
PIM technology is the combination of thermoplastic injection molding and conventional powder metallurgy. Polymer binders mixed with fine metal or ceramic powders are injected into molds similar to those used for plastic injection molding. Undercuts, coring for lighter weights, blind holes, angled holes, crossholes, irregular shapes, and splines are some of the more complex features that the molding process is able to produce.
Injection Molding gives many design freedoms not easily matched by other traditional metal-forming processes. PIM provides near-theoretical densities resulting in mechanical properties superior to most other forms of powder metallurgy and approaching those of wrought material. Low alloy and stainless steels, soft magnetic tool steels and ceramic are some of Britt Manufacturing's standard materials. See Standard Material list for details.
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Molding
MIM technology is a union of thermoplastic injection molding and conventional powder metallurgy. Polymer binders mixed with fine metal powders are injected into molds similar to those used for plastic injection molding. Slides, cores, and other common tools used in conventional injection molding, are able to produce more complex features than conventional machining allows or that can be made using castings.
Injection molding accurately duplicates mold surfaces. This results in parts having as-sintered surface finishes of <32 RMS (microinches). In addition, high-resolution features such as raised or subsurface lettering and logos are easily molded. Internal or external threads, and knurled features, costly machined features, are essentially free with injection molding.
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MIM uses many of the same design principles applied in molding plastic parts. The molds for injection molding have different characteristics than conventional investment cast molds or dies. Injection molding uses two mold halves to form the parts. Material is forced into the mold cavity through openings called gates. Once the parts cool and harden, the mold opens and the parts are ejected. Parts in the molded stage are referred to as "Green Parts," and are approximately 20% larger than finished dimensions.
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Post Molding Secondary
Molded products are referred to as "Green Parts". Parts may be machined or other secondaries may be performed adding
features in the green state. Machining is easier in the green state, such as undercuts or cross holes that may not be feasible in the mold.
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Debind
Debinding is the removal of binders mixed with the metal or ceramic powders, which takes place after molding. Britt capabilities include chemical (catalytic), solvent, and water debind system. The debind method is determined by the material to be processed, required physical properties, metallurgical requirements and chemical composition.
Debound parts are referred to as "Brown Parts".
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Sinter
Parts are sintered with a temperature and atmosphere profile specific to the alloy being processed. Debound or brown parts are sintered in vacuum type furnaces. At the lower temperatures of the sintering cycle (300 to 400 degrees C) the residual binder is removed.
As the temperature increases, particles fuse, pore volume shrinks, and grain boundaries form at particle contacts. The grain size and part density depend on sintering time and
temperature.
The sintering temperatures in MIM range from 1,200 to 1,800 degrees C. Parts shrink 17-22 % to nearly full density. Secondary machining heat treatment and surface treatment are also available.
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Post Sintering Secondary
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Coining: parts designed without a flat surface needed for sintering may need straightening or "coining" to correct minor
shape and dimensional distortions. |
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Machining: Grinding, milling or other machining operations are available, if necessary |
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Heat Treat: Standard hardening methods are applicable |
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Coatings: Any coating your application requires is available, (chrome plate, black oxide etc.). |
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Finishing: Post sinter average finish 32 RMS. A better surface finish may be obtained with our in-house mass
finishing department. |
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