BOUNDARY-CONDITIONS FOR CONTACT LINES IN COEXTRUSION FLOWS

A bicomponent coextrusion process is modelled using a 3-D finite eleme nt formulation. The layer uniformity problem in coextrusion is address ed by examining the effects of the polymer melt/polymer melt/die wall contact line boundary condition. It has been observed that the less vi scous polymer layer will tend to displace the more viscous polymer lay er near the die wall. The behaviour of the contact line is considered to be either a ''stick'' or ''slip'' boundary condition. In the ''stic k'' boundary condition, the contact line does not move from its origin al position after the two polymer layers meet. A slip boundary conditi on allows the contact line to move along the die wall. The calculated interfaces which result from different contact line assumptions are de termined. Results show that if a ''stick'' boundary condition is appro priate for a given fluid/fluid/solid contact line, then a very thin en trained layer of the more viscous polymer melt will be trapped between the less viscous polymer melt and the die wall. Slip boundary conditi ons would allow complete displacement of the contact line along the di e wall. Both slip and stick boundary conditions produce similar interf ace profiles far away from the die wall for small viscosity ratios. In certain cases, the displacement of the more viscous material by the l ess viscous material will cease and a static interface structure is pr oduced regardless of die length. Experimental work with polycarbonate melts is compared with the numerical simulations.