Instabilities of the single-screw extrusion process

Instabilities were measured in a single-screw (at 10 and 40 rpm) extruder, including those in the screw rotation speed, die and barrel (last zone) heating powers (P-d and P-b), pressure (p) of molten polymer in the die entrance, takeoff speed (v), and temperature of molten polymer (T); extrudate (rod) diameter (D) and dimensional stability were followed with a laser device. Statoil's HDPE H870 (MFI 2 g/10 min) was used for the measurements. The instabilities observed were classified as low or very low frequent or even random (Figs. 1, 2) and believed to originate from an unstable borderline between the feeding and the plasticating zone, where shifting of the melting start-point position could produce preessure and temperature fluctuations in the diehead.