Two low density polyethylene melts are studied using a coextrusion flow visualisation cell. The cell design enabled two melt streams to be observed at the confluent region and into the die land to the die exit. The melt streams converge at an angle of 30° and flow into a common die land of 1.0 mm height. The relative stream velocities, hence layer thickness ratio, of the two streams was varied using a restriction plate in one of the streams. Stress and velocity fields are quantified using stress birefringence and particle image velocimetry techniques. Numerical simulation is conducted using flow history dependent viscoelastic stress calculation based on a modified Leonov model and Flow 2000? software. Simulated stress fields are compared to those experimentally. The simulated stress magnitudes were in good agreement with those determined experimentally. The simulation predicted a high local extensional stress in the minor flow stream just upstream of the entry to the die land. This local stress was also observed in the stress birefringence pattern. Extrudate instability was noted even when the stress field (birefringent pattern) in the convergent region and the beginning of the die land was stable. However, disturbances were noted in the birefringence and velocity fields near the exit of the die land when the major stream - minor stream layer thickness ratio was greater than 1.5:1. Surface distortion appeared on the surface of the extrudate on the side of the minor flow stream when process conditions promoted instability. The recently proposed TNSD sign criterion has been used to predict the onset of the wave interfacial instabilities in the coextrusion geometry and good agreement with the experimental observation has been found.
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