Experimental observations and analysis of LDPE melt flow in coextrusion geometries

Abstract

The study of two low density polyethylene melt flows in slit coextrusion flow cells with 30° and 90° confluent geometries is presented. The stream flows and polymer-polymer interface in the confluent region and die land are observed through side windows of the cell. Birefringence and image processing techniques are used to quantify stress and velocity fields in the upstream and down stream melt flows. Interfacial instabilities were observed in the LDPE melt flows in both geometries. Instabilities occurred at certain stream mass flow ratios. Digital image processing shows the wave type extrudate instabilities have the same periodicity as perturbation in the flow field in the die land. A recently proposed "TNSD sign criterion" reasonably predicted the onset of the interfacial instabilities in these die geometries.