The occurrence of periodic distortions in the extrusion of polymeric melts

In the extrusion of polymeric melts at high flow rates, often flow instabilities observed as surface distortions of the extrudate occur. The flow instability ‘spurt’ in piston-driven flows is accompanied by persistent oscillations in the pressure. In this paper spurt is explained in terms of constitutive instabilities (mechanical failure of the polymeric fluid itself), while the no-slip boundary condition at the wall of the die is maintained. The influence of compression on the onset of spurt is investigated. The polymeric melt is modelled as a JSO-fluid, but two other constitutive models are also considered. Numerical computations disclose that persistent oscillations in the pressure as well as in the volumetric flow rate occur for a bounded range of prescribed plunger speeds. The occurrence of the persistent oscillations is also explained by a linearized stability analysis. The frequency of the persistent oscillations is determined both from the linearized stability theory as well as by means of a Fourier spectral analysis. In conclusion, the theory is validated by a qualitative comparison with experimental results.