Every day, new polymer materials are created to meet increasingly specific needs, such as lighter, more resistant, flame-retardant, antistatic materials and materials that comply with new environmental standards. These complex materials require efficient mixing tools, allowing controlled and competitive manufacturing. Among continuous mixing processes, twin-screw extruders are widely used, but co-kneaders also play an important role. A co-kneader is a modular single-screw extruder, driven by a double movement of rotation and oscillation back and forth. The threads of the screw are interrupted from place to place and mixing fingers are fixed to the sheath. The kinematics involved allow these fingers to sweep the screw channels by passing through the thread interruptions, which causes very effective mixing, while having relatively low average shear rates, inducing little heating. Co-kneaders are therefore generally used in the case of thermosensitive polymers, such as PVC, or when high loading rates are required.
In this article, we will first present in detail the configuration of a co-kneader, explaining its mode of operation, which is very different from that of a conventional single-screw extruder. We will focus on the kinematics of the screw, the role of the mixing fingers, the flow modes, etc. The influence of operating parameters will then be discussed, before introducing some aspects concerning the modeling of flows in these particular extruders.