Theoretical and experimental investigation of interfacial instability phenomena occurring during viscoelastic coextrusion

The fully viscoelastic finite element method (FEM) together with the flow visualisation technique were employed to quantify the effect of die design on wave type interfacial instabilities in coextrusion. It has been shown experimentally that minor channel geometry has a strong impact on wave type interfacial instabilities and the results can be correlated through novel criteria called Total Stress Difference (TSN) which takes into account the bulk change in total stress in the normal as well as the tangential direction with respect to the interface. It has also been shown that a specific type of high stress area occurs around the interface at the end of the converging section. This total stress has been found to be non-monotonic along the interface and is related to the onset of zigzag type interfacial instabilities when the total stress achieved 200 kPa.