INSTABILITIES IN STARTUP FLOWS OF POLYOLEFIN MELTS IN AXISYMMETRIC CONTRACTION GEOMETRIES

摘要

A flow visualisation cell with an abrupt entry pseudo-axisymmetric geometry has been used in the study of a low density polyethylene melt flow. The flow cell was mounted on a Davis Standard Betol BK38 single screw extruder. Birefringence and imaging velocimetry techniques have been used to observe changes in the stress and velocity fields in the melt at various processing conditions. Flow development of the melt was observed using pigment tracers. Start-up and steady state flow regimes are investigated. A transient instability occurred on start-up flows. The time to stabilisation of the flow increased with dwell time between extrusion trials. The instability was observed as oscillatory motion of the stress birefringence pattern and fluctuations in measured pressure.