Three dimensional simulation of viscoelastic polymer melts flow in a cast film process

Three-dimensional simulation of the cast film process for viscoelastic polymer materials was carried out using the finite element method. The flow between the extrusion die and chill roll was assumed to be steady-state and isothermal and the rheological property of material was characterized by a single-mode PTT model. Gravity and inertial flow were considered in the simulation work, but neglecting die swell at the die exit, surface tension, crystallization, transient disturbance and film sag. Simulation results of the cast film production line were compared with the experimental data on the velocity profiles and neck-in values. Neck-in and edge bead was well predicted and the influence of strain-hardening nature, elasticity of materials and operation conditions on the final film shape was also investigated. It was found that the effect of strain-hardening nature was masked for the material with a very low relaxation time when the single-mode PTT model was adopted. Greater elasticity helped to produce a film with smaller neck-in and less waste edge. When the air gap length was increased, it was predicted that neck-in phenomenon would be promoted as well. Moreover, it was found that ratio of neck-in values under different air gap lengths was approximately equal to that of air gap lengths, which was consistent with experimental evidence.