Engineering near-surface enrichment of glass-ceramic particles in polypropylene composites via magnetic-field-assisted injection molding

Particle-based polymer micro-composites offer tunable surface functionalities, which are maximized when particles are concentrated at the component’s surface. However, mass-producing these functional components via injection molding (IM) remains challenging, as fillers naturally migrate to the core region during processing, compromising surface-specific performance. Conventionally, achieving surface functionality requires either costly secondary coating processes or uneconomical high bulk filler concentrations. To overcome these limitations, this study integrates a magnetic circuit with permanent magnets directly into an injection mold to actively drive ferrimagnetic micro-particles toward the mold surface during the IM cycle. To maximize the applicability of this strategy, a polymer composite comprising a commodity plastic matrix (polypropylene) and ferrimagnetic glass-ceramic micro-particles was utilized. A simplified force-balance model, combining magnetostatic and numerical simulations, was used to preliminarily analyze the competition between magnetic attraction and viscous drag during the IM cycle. Subsequent experimental validation confirmed the effectiveness of magnetic-field-assisted IM in promoting particle migration toward the components’ surface. By experimentally investigating the influence of processing parameters, high mold temperature and low polymer viscosity were proven to serve as critical parameters to maximize particle redistribution under a magnetic field. In optimized conditions, a particle area fraction of approximately 27% was achieved in the skin layer, marking a 7.7% increase over non-optimized reference samples. Ultimately, this work presents a scalable engineering route for tailoring particle distribution in commodity polymers. It provides a transferable framework for manufacturing functionalized composites with surface-concentrated properties, such as sensing, anti-icing, or antibacterial behaviors, eliminating the need for complex post-processing.