The effect of channel geometry and wall boundary conditions on the formation of extrusion surface instabilities for LLDPE

It is believed that surface instabilities can occur during the extrusion of linear low density polyethylene due to high extensional stresses at the ex it of the die. Local crack development can occur at a critical stress level when melt rupture is reached. This high extensional stress results from th e rearrangement of the flow at the boundary transition between the wall exi t and the free surface. The stress is highest at the extrudate surface and decreases into the bulk of the material. The location of the region where t he critical level is reached can determine the amplitude of the extrudate s urface distortion, This paper studies the effect of wall slip on the numeri cally simulated extensional stress level at the die exit and correlates thi s to the experimentally determined amplitude of the surface instability. Th e effect of die exit radius and die wall roughness on extrusion surface ins tabilities is also correlated to the exit stress level in the same way. Whe reas full slip may completely suppress the surface instability, a reduction in the exit stress level and instability amplitude is also shown for a rou nded die exit and a slight increase in instability is shown to result from a rough die wall. A surface instability map demonstrates how the shear rate for onset of extrusion surface instabilities can be predicted on the basis of melt strength measurements and simulated stress peaks at the exit of th e die. (C) 2001 Elsevier Science B.V. All rights reserved.