Although the density of the resulting component approaches that of a machined part, dimensional accuracies are only marginally better than with investment casting, and many items still require secondary finishing work. In the mid to late 1990s, efforts to improve the MIM process focused on finding a way to mass-produce intricate metal components using permanent molds similar to those employed for plastic injection molding. The PMIM process derived from this desire to overcome some of the shortcomings of MIM and fabricate complex, tight-tolerance components without the need for secondary operations. The finest powders are submicron in size, with a significant portion of the entire mix made up of particles less than 4 mm in diameter. The finest powders in the mix are composed of a eutectic iron, the coarsest are prealloyed stainless steel, while those of intermediate size are composed of a master alloy with a higher alloying content.
Several elements are present in the plastic portion of the mix.
One, which appears in Figure 1 as a coating on the powder surface, is a plasticized, cross-linkable thermoset polymer that behaves virtually like a glue. A second, low-viscosity polymer that is easily removed after injection molding fills out the rest of the space. However, when put into hot liquid, PEG will easily dissolve. A number of other polymer components and additives occur in the mix, among them antioxidants, coupling agents, internal lubricants, compatibilizers, surfactants, and flex agents. The metal-plastic mixture is processed in an advanced plastic injection molding machine.