Powder injection molding (abbreviated PIM) is a variant of plastic injection molding used to produce parts made of metal (MIM = metal injection molding) or made of ceramics (CIM = ceramic injection molding)
with complex geometries.
Powder injection molding is the general term for the process of injection molding of polymer melt–based mixes containing a high loading of ceramic or metal powders. This process is similar to injection molding of thermoplastics except that metal or ceramic particles are compounded with polymer binder into pellets before the process. The injection unit can be based on a reciprocating screw type or a piston. The molding process follows the same principles as thermoplastic injection molding. Parts are solidified by cooling in the mold, and then heated to remove binder and sinter.
Ceramic or metal powder incorporation into the polymer melt can be accomplished by high shear mixing in a twin screw extruder, for example. The powders used in this process have particles in the micron size range for ceramics and less than 20 µm or so for metals. Lubricants are also added to lessen wear of machine from contact with hard particles. At least two polymer binders are used. The major binder is a higher molecular weight polymer that is responsible for the solidification and strength of the molded part. The minor binder is a lower molecular weight polymer that enhances flow behavior and assists in the binder burnout process. The minor binder decomposes before the major binder, which clears out some pore channels and allows easier removal of the major binder. Plasticizer or solvent is also added to assist in flow. The mixes can have up to 50–70 vol% particles.
Post-processing is the rate limiting step in the powder injection molding. Decomposition and removal of large amounts of polymer from the parts requires time and ovens with atmosphere control. Of concern is retention of the shape of the parts considering that the polymers are thermoplastic. The time needed to remove the polymer increases with part size and hence powder injection molding applications are restricted to small parts. Given the high content of polymer there is considerable shrinkage during the polymer removal and then final sintering steps. Nonetheless, the advantage of creating complex shapes drives the use of this process.
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