A phenomenological study on twin screw extruders

Abstract

Although more and more twin screw extruders are being used in the polymer industry, the theoretical background is relatively undeveloped. The literature abounds in contradictions and often informs the reader that all extrusion problems can be solved if a certain new design is considered. The development of successful machines has mainly been possible through the application of commercially valuable empirical knowledge. In this research some understanding has been gained of the relevant phenomena that control the working of a counter-rotating intermeshing twin screw extruder. In these machines the two screws form more or less C shaped chambers with leakage gaps between them. The basis of the model presented in this thesis is that the extruder can be regarded as two series of chambers moved from hopper to die by the rotation of screws, while the leakage flows provide interactions between these chambers. The throughput of a twin screw extruder can be calculated with this model by subtracting the total amount of leakage from the total volume of the C shaped chambers that come free per unit time. This has been checked with model extruders fully filled with Newtonian liquids giving completely satisfactory results. For these conditions the throughput to pressure relationship, presented in terms of two new dimensionless groups, Q/2NmV and AP/Nn, can be represented by a straight line independent of viscosity and rotational speed. In order to obtain insight into the importance of the geometrical variables for the throughput to pressure relationship a sensitivity analysis has been made using computer programs based on the calculations of the leakage gaps. When using the two dimensionless numbers the characteristics are retained when the extruder is scaled up geometrically. Many quantitative articles report that twin screw extruders provide a better homogenisation on micro scale and that there is a narrower spread in residence time distributions than in single screw extruders. The experiments described in this thesis show that the dimensionless exit age distribution of a twin screw extruder can be worse than that of single screw extruders. Video recordings of tracer injections in a perspex model extruder have shown that the mixing within the chambers is also generally overestimated. The last two chapters deal with the extrusion of real polymers. The model as proposed in the first part of the thesis is extrapolated to practical situations. An explanation of the relative insensitivity of the throughput to the die pressure in a twin screw extruder when working with polymer granules is given. Experiments in which a real twin screw polymer extruder was stopped and dismantled confirm the predictions of the model that the length over which the extruder is fully filled with melt changes with die pressure. Finally it is shown quantitatively that the melting process in an intermeshing counter rotating twin screw extruder is markedly different from that in a single screw extruder.Aerodynamics & Wind EnergyApplied Science