5. Conclusion

Interface instabilities in the coextrusion process are convective in nature, and their conditions of occurrence can be determined in simple cases using linear stability mathematical tools (e.g. Declic software, see [Doc. AM 3661]). Studies show that instabilities are more severe the higher the level (jumps) of shear rate and stress in the die. However, this first step requires a good knowledge of the viscoelastic rheological properties, in the non-linear domain, of the products used, which is still an area of research.

In the case of complex geometries and flow conditions, the use of more powerful direct simulation codes (e.g. Ximex, Rem 3D, Polyflow software, see [Doc. AM 3661]) will, in the near future, provide optimization tools, both from the point of view of the process and the choice of products used.