A study of the stick-slip phenomenon in single-screw extrusion of linear polyethylene

Abstract : We present results of a study on the stick-slip phenomenon observed for two linear polyethylenes (HDPE and LLDPE). Experiments were carried out in a single-screw extruder using dies of different diameters and LID ratios. The pressure and the mass flow rate have been measured simultaneously and the instant mass flow rate was determined using laser Doppler velocimetry. Our results clearly confirm the hypothesis that the flow jumps from one stable branch to the other during oscillations as suggested in the literature. The role of the melt compressibility in the reservoir is also established. The effect of the average mass flow rate imposed by the extruder Q(IN) on the respective compression and decompression phases is in good agreement with the effect predicted by the mass balance in the reservoir. We also used different reservoir volumes to confirm that the period of oscillations increases with the compressed melt volume upstream of the die.