4. Conclusion

Although less widespread than corotative bivis extrusion, contrarotative bivis extrusion remains an important polymer processing method. It is characterized by a variety of geometric configurations (interpenetrating or non-interpenetrating screws, with wide or narrow threads, fitted or offset, cylindrical or conical) and therefore operating conditions, which can be adapted to different applications. While interpenetrating C-chamber screws are widely used in rigid PVC extrusion (pipe and profile manufacture), other geometries can be found in other fields, mainly in reactive extrusion. Although studies devoted to this process have been more limited than those devoted to corotating extruders, there is now a good understanding of flow conditions in the various configurations. Local and global models have been developed, but to the best of our knowledge, they have not yet spread outside the laboratories...