The influence of die design and extensional rheology on the onset of 'wave' interfacial instabilities in coextrusion flows

Abstract

The influence of die design and rheology on the onset of 'wave' interfacial instabilities in coextrusion flows is evaluated through flow history dependent viscoelastic stress calculation using a modified Leonov model and the velocity field determination through FEM with the help of a newly proposed criterion based on the difference of normal stress differences across the layer interface in the merging area. The investigation shows how this criterion can be used to investigate the role of the die design and rheology of coextruded materials from the interfacial instability point of view. It is shown that both the die geometry and the extensional rheology have a crucial effect on the interfacial wave instability.