Single screw extrusion of long discontinuous fiber-reinforced polymers: Pellet motion and heat transfer

Single screw extrusion is widely used in material pre-compounding, extrusion additive manufacturing, and injection molding. A single screw extruder has three stages-the solid conveying zone, the transition zone, and the melt-conveying zone. In starve-fed extrusion, the screw channels are partially filled, and melting cannot be analyzed by traditional plug-flow melting models. However, understanding individual pellet motion and melting is an essential precursor to modeling pellet deformation and fiber length attrition in the transition and melt-conveying zones. Therefore, a sequentially coupled framework has been developed to capture individual pellet trajectories and their melting in a single screw extruder. The discrete element method has been used to study motion of long-discontinuous fiber-reinforced pellets which are represented as multi-spheres. Individual pellet melting is captured in a static heat-transfer analysis with dynamic boundary conditions based on the contact information obtained from pellet interactions with its surroundings. Results indicate a translational-conveying motion followed by a rotational-conveying motion of individual pellets as they move along the screw. The rotational-conveying motion coincides with an increase in screw pitch and corresponds to the highest rate of pellet melting. Increasing screw rotation speed reduced the residence time causing insufficient pellet melting. Decreasing the feeding rate did not significantly affect the residence time but resulted in improved melting through increased thermal contacts. Melting is initiated locally on the pellet surface due to thermal contacts and proceeds radially inward. In this process, pellets first experience a molten surface shell before the core reaches melt temperature.Highlights Multi-sphere representation of LDF reinforced pellets. A discrete element model to capture pellet flow in a single screw extruder. Individual pellet trajectories extracted from single screw extrusion. A sequentially coupled finite-element model to study LDF pellet heat transfer. Study of screw speed and feeding rate on average pellet melting behavior.