Non-monotonic shrinkage behavior of a highly filled polymer-metal feedstock during injection molding and its relation to binder phase transitions

Abstract

Highly filled polymer–metal feedstocks exhibit complex thermophysical behavior because of their high solid loading and multicomponent binder systems. In this study, the green-part shrinkage of a 17-4PH MIM feedstock containing approximately 92 wt% metal powder was investigated as a function of mold temperature, packing pressure and melt temperature. The feedstock exhibited a statistically significant non-monotonic response to mold temperature, with a maximum shrinkage of approximately 0.5% at 60 °C, whereas the polypropylene reference showed a monotonic trend. DSC analysis identified a lower-temperature transition with an onset at 51.4 °C, a peak at 62.9 °C and an enthalpy of 4.4 J/g. The Tait model described the overall PVT behavior with R² = 0.98 but did not reproduce the local thermophysical change observed between approximately 55 and 65 °C. The correspondence between the DSC transition and the shrinkage maximum suggests that binder phase behavior may contribute to the non-monotonic response; however, direct causality was not established. Changes in gate-freeze time, effective packing duration, viscosity, pressure transmission and particle rearrangement may also influence the measured shrinkage. The results identify the 55–60 °C mold-temperature interval as a critical processing range for dimensional control of the investigated feedstock.