Melting of thermoplastics in single screw extruders

Some experiments on the melting of thermoplastic polymeric materials in single screw extruders are described. Although these were of the now familiar screw extraction type, special care was taken to distinguish between material melted by screw rotation and that melted during the subsequent cooling operation. A barrel which could be split longitudinally was also used, thus avoiding some of the disadvantages of axial extraction. A theoretical model is proposed which, unlike previous models, allows the solid bed of material in the screw channel to accelerate naturally, and also allows for the presence of a film of molten material between the bed and the screw. This model gives satisfactory predictions of melting performance. Comparison with experimental results shows that break-up of the solid bed occurs when the model predicts rapid acceleration of the bed. Bed break-up and the resulting surging may be reduced or prevented by the use of screw cooling which has the effect of inhibiting the formation of a melt film at the screw surface.