Counter-Rotating Twin-Screw Extrusion

In terms of the quantity of material involved, extrusion remains the leading process for transforming and shaping polymers. In fact, this very general term covers a wide range of applications and processes. The two main applications are the continuous manufacture of finished or semi-finished products of constant cross-section (tubes, films, sheets, profiles, etc.) and the production of complex materials by mixing and compounding (polymer blends, composites and nanocomposites, etc.). The two main extrusion processes differ essentially in the number of screws used: a single screw for single-screw extrusion, and two screws for twin-screw extrusion. Single-screw extrusion is mainly used for the first type of application. In the case of twin-screw extrusion, there are two further categories, depending on the direction of rotation of the screws. Corotative twin-screw extruders are the most widespread, and are the preferred tool for compounding. Counter-rotating bivices are less common, but offer a greater variety of geometric configurations, which means they can be found in pipe and profile production (mainly rigid PVC), as well as in compounding tools and reactive extrusion. In this article, we first present a detailed description of the different configurations encountered. We will then explain their operating modes (total or partial filling, melting mechanism, conveying mode, etc.), before introducing a few aspects of flow modeling in these different systems.