Melt flow of polyethylene at high rates

The flow of high‐density polyethylene at high shear rates in round dies in laboratory instruments and in extruders depends in an unusual way on capillary diameter. With small diameters this dependence can be explained in terms of slippage of the melt at the wall of the capillary; with large diameters, in terms of temperature increases developed through viscous flow. A rapid increase in slip velocity appears to be responsible for a discontinuity in the flow curve determined in laboratory instruments. Recurring transitions between the two branches of the flow curve are responsible for the oscillating flow observed in equipment operating at constant or nearly constant throughput rate. The energy stroage capability of the over‐all system, apparatus plus melt, controls the dynamic aspects of flow in the vicinity of the discontinuity.