The Occurrence of Periodic Distortions in the Extrusion of Polymeric Melts. Part 1: Modelling and Numerical Results

In the extrusion of polymeric melts at high ow rates, often ow instabilities observed as surface distortions of the extrudate occur. The ow instability 'spurt' in piston-driven ows is accompanied by persistent oscillations in the pressure. In this paper spurt is explained in terms of constitutive instabilities (mechanical failure of the polymeric uid itself), while the no-slip boundary condition at the wall of the die is maintained. The innuence of compression on the onset of spurt is investigated for an extrusion process that is modelled by the ow of a JSO-uid through a contraction from a wide barrel into a narrow cylindrical capillary. Numerical computations disclose that persistent oscillations in the pressure as well as in the volumetric ow rate occur for a bounded range of prescribed plunger speeds. The onset of the persistent oscillations is found to depend on both the material parameters of the polymer and the dimensions of the barrel and the capillary. Numerical computations on loading and unloading processes, in which the plunger speed is gradually raised or lowered, clarify the innuence of the deformation history.