New aspects of melt fracture

The movement of coloured markers (polydimethyl siloxane and low density polythene) placed near the die entry before extrusion or injected through the side of the die during extrusion has been measured both above and below the onset of instability. During stable flow the observed velocity distributions are similar to those predicted by assuming that polymer at the die wall is stationary, but when melt fracture occurs there is movement at the wall. The possibility of a stick‐slip mechanism being responsible for unstable flow is examined and evidence in support of this hypothesis is presented. Available evidence suggests that for a given molecular weight, the critical shear stress increases with the polarity of the polymer, and the material of which the die is made has been found to affect the instability.