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The objective of the present study is to confirm the hypothesis presented by one of the authors that the initiating mechanism of melt flow fracture is affected by the behaviors of distribution and vibration of pressure in shear flow field of high density polyethylene melts. It was recognized that the pressure at reservoir wall and in the reservoir, which was measured in rectangular direction with the flow direction, varied with the positions of measurement points i.e., the pressure distribution curves were similar to the profile of shear rate distribution curves under normal flow region or low shear rate region. The mean pressure in a pressure distribution curve increases with the increase of shear rate. The pressure vibrations occurred at particular measurement points, which were presented in the previous paper as fracture region at the critical condition of normal flow to spiralling flow under moderate shear rate. At these positions of measurement points the vibration phenomena generally initiated near the capillary inlet under low shear rate, and moved to the reservoir wall from the reservoir axis with the increase of shear rate and ultimately, disappeared with further increase of shear rate. The larger melt index of high density polyethylene, the larger pressure vibration becomes. Both flow rate excess induced by the melt flow fracture and the effects of L/D of capillaries upon flow rate excess can be explained by the hypothesis utilizing the particular fracture surface in shear flow field.

A Consideration on the Behavior of Capillary Flow Fracture on the Basis of Distribution and Vibration of Pressure in Reservoir of Polymer Melt