INLET INSTABILITIES IN THE CAPILLARY-FLOW OF POLYETHYLENE MELTS

Inlet instabilities in the capillary flow of polyethylene melts were s tudied in this work. Extrudate distortions in branched polyethylenes, produced by unstable upstream flow, were found to be accompanied by pr essure oscillations that do not have their origin in the slip phenomen on, but on polymer compressibility. The absence of slip was clearly ev idenced in the experiments, and the differences between pressure oscil lations occurring in linear and branched polymers are shown. Pressure oscillations in the capillary flow of branched polyethylenes were foun d to be made up of at least two components of different frequency and amplitude. These two components were identified with different bulk de fects appearing in the extrudates. Information about the dynamics of v ortices upstream of the contraction and extrudate distortions is obtai ned from the analysis of pressure oscillations. The influence of capil lary entrance angle on flow curves was also investigated. From the res ults, it is concluded that the extensional component of the flow in th e contraction is the main factor responsible for the slope change usua lly found in the log-log flow curves of both Linear and branched polye thylenes.