Visualisation and analysis of interfacial instability in coextrusion of LDPE melt

Details presented cover the study of interfacial instability of a low density polyethylene melt flow in a coextrusion flow visualisation cell. The cell design splits a single melt feed into two meltstreams using a flow divider. The meltstreams are made to converge at an angle of 30° and flow into a common die land of 1.0 mm height. The flows in the confluent region and die land to the die exit were observed through the side windows of the cell. The relative stream velocities, hence layer thickness ratio, of the two streams were varied using an adjustable restriction plate in the bottom channel. Stress and velocity fields were quantified using stress birefringence and particle image velocimetry techniques. Wave type interfacial instability was observed in the extrudate when the major–minor stream layer thickness ratio exceeded 2: 1. Flow at immediate entry to the die land appeared stable when the extrudate exhibited instability. However, a disturbance was observed in the flow towards the exit of the die land. The frequency of the disturbance was quantified and found to be the same as that of the wave disturbance in the extrudate. A modified Leonov model and Flow 2000 software were used to simulate the melt flow through the geometry. A total normal stress difference (TNSD) sign criterion has been used to predict the presence of the wave interfacial instabilities in the coextrusion geometry.