Capillary Rheometer Tests on Feedstock for Metal Injection Molding (MIM)

Not only standard plastics or elastomers can be tested with a capillary rheometer. If a material can be injection molded without special processing aids, and it doesn't undergo curing (setting) during the process, it can be tested. One interesting example is given by feedstock for Metal Injection Molding (MIM). The MIM technique is used to manufacture small solid metal parts with complex shape for a variety of applications, but the starting point is a metal powder mixed with waxes or other polymeric materials. This compound has a very high density being made of metal for the biggest percentage in mass, but behaves like plastics when heated and forced to flow. In fact, the polymeric part melts and flows carrying the metal particles as a filler. The compound is injection molded and this gives a precursor (aka green) of the final object. Treatments at high temperatures burn away the polymer and let the metal particles glue together by means of a sintering process. This gives life to accurate and complex metal parts without doing any metal casting or precision machining, hence great saves in terms of time, energy, complexity of manufacturing process. Final products are used in a broad range of industrial, commercial, medical, dental, firearms, aerospace, and automotive applications.

We were asked to test several samples of feedstock for MIM on a CEAST SR20 capillary rheometer, in order to confirm that this equipment was suitable for process control and optimization once in the lab supporting the MIM manufacturing. The tests were carried out without noticeable problems, since the samples behave almost like a commodity plastics from the point of view of flow and stability. The metal particles are actually controlled in shape (rounded) and size (in the range from micrometers to less than a tenth of millimeter), so they are not creating agglomerates clogging the capillary die, neither they scratch the surface of the barrel. Dies made of tungsten carbide with very high surface hardness offer a better abrasion resistance compared to steel dies, and guarantee a longer life with the correct dimensional tolerance for this application. The temperatures and pressures required to generate the processing flow conditions are easily covered by the rheometer, so we found that the best configuration was a 20-kN unit with standard barrel and 1-mm diameter capillary dies.