4. Conclusion

The models presented can be used to predict the velocity field in the end section of multi-channel coextrusion dies used in sheath blowing, tube or multilayer film manufacture. A velocity profile marked by strong shear rate discontinuities at the interfaces will be the source of interfacial instabilities described in [AM 3 661] . These shear rate discontinuities cannot be predicted by a simple Newtonian model. Indeed, coextrusion flow will generate highly heterogeneous shear rates between layers and within each layer, which can only be accounted for by using viscous behavior laws....