Process-induced fiber matrix separation in long fiber-reinforced thermoplastics

This work explores the fiber migration during injection molding of long glass fiber-reinforced polypropylene. It was found that the simplified assumption of uniform fiber concentration distribution is inaccurate, and the process causes substantial variations in the fiber concentration along the flow path and through the thickness of injection molded parts. This was tested for a simple plate geometry molded at varying nominal fiber concentrations. The fiber concentration was measured by pyrolysis to obtain a global concentration, and using micro computed-tomography for a through-thickness analysis. Additionally, the fiber concentration at the melt front of partially filled moldings was investigated. A new measurement protocol using micro computed-tomography and digital image processing is proposed to calculate the through-thickness fiber concentration. The results of this study show substantial heterogeneity of the fiber concentration throughout the molded plates. Fibers agglomerated in the core layer with volume fractions up to 1.5 times the nominal fiber concentration.