The gas-assisted extrusion of molten polyethylene

This paper reports for the first time our preliminary findings of a new gas-assisted extrusion process. We have discovered that if gas is injected at a metal die/molten polymer interface at a low flow rate, it is possible to establish a stable gas layer at the interface, which can give rise to an essentially full slip wall boundary condition. We report experimental optical observations, flow birefringence data, pressure difference, and die swell data for both a slit and rod geometry extrusion. We also match some of the experimental results with a viscoelastic numerical simulation. The introduction of wall slip induced by the presence of the gas layer has a profound effect on the magnitude of the die swell observed for polyethylene processed using gas-assisted extrusion. The experiments demonstrate, without ambiguity, that wall boundary conditions can play a crucial role in the overall extrusion flow of high viscosity viscoelastic fluids, such as polyethylene. (C) 2001 The Society of Rheology.