A study of “worms” melt fracture in polyethylene extrusion blow molding process using capillary rheometry

Abstract

During the polyethylene (PE) blow molding process of large size drums, string-like defects, which are referred to here as worm melt fracture, can sometimes be observed on the extrudate surface. Such string-like defects, in various shapes and sizes, are also observed in the capillary extrusion at high shear rates after the slip-stick transition. The PE resin with broader molecular weight distribution (MWD) exhibits a greater degree of worm melt fracture while narrow MWD PE resin, which has higher slip velocity and a uniform slip layer, shows a lesser degree of worm melt fracture. It is hypothesized that the worm melt fracture is related to the die build-up. Based on the mechanism of the fast die build-up, it is proposed that the cohesive slip layer, which is a failure within the polymer melts at an internal surface, could emerge out from the die as these string-like materials attached on the extrudates. The broader MWD resin, which has more small polymer chains and a lower plateau modulus, is postulated to have a weaker polymer melt, which then makes it easier to have such an internal failure and consequently have more string-like defects at high shear rates. POLYM. ENG. SCI., 56:650–656, 2016. © 2016 Society of Plastics Engineers