In the first part of this paper a numerical strategy is developed for the numerical simulation of the coextrusion process. Coextrusion consists of extruding many polymers in the same die in order to combine their respective properties. The die is generally flat and quite large and consequently a two-dimensional approximation is sufficient. The main difficulty is to accurately predict the interfaces between the different layers of polymers. A finite element method based on a pseudoconcentration function is developed to calculate these fluid interfaces. Numerical results are presented for the coextrusion of up to five fluids.
In the second part of the paper the above strategy is slightly modified to simulate the film-casting process. In this case a polymer is stretched (with a draw velocity UL) at the exit of the die in order to produce a very thin layer of polymer that is cooled in contact with a chill roll. Only one polymer-air interface has to be computed. The draw ratio is defined as Dr = UL/U, where U is the mean velocity of the polymer at the exit of the die. As the draw ratio is increased, instabilities appear and numerical results put in evidence the draw resonance phenomenon.