Cooling rate influence in bonded magnet injection moulding

Recently the use of injection moulded bonded magnets has become very widespread. This is mainly due to the interesting price of injection moulded magnets. However the process is actually limited by some critical aspects that are related to rheological and thermal properties of the melt. Compared to a non-filled polymer melt, the high filler content (about 90% in weight), leads to a much higher viscosity, heavily affected by temperature. Furthermore, injection moulding of bonded magnets is characterized by high cooling rate, due to the high diffusivity of the metal filler. This can often lead to a premature gate freezing, negatively affecting the quality of the moulded part. Therefore, a reliable no-flow temperature is a prerequisite for a robust numerical simulation of the process. This paper presents a full characterization of the no-flow temperature of a commercial polyamide-based compound. To this aim, a method based on differential scanning calorimetry (DSC) has been compared with another approach that relies on rheological data. Great attention was paid to evaluate the influence of cooling rate on no-flow temperature and a regression model was proposed for describing the relation between these two variables. Furthermore, a comparison between numerical and experimental results was carried out in order to validate the numerical simulation of the process.