The oscillating shear phenomenon in high density polyethylenes

Studies of capillary extrusion measurements using an Instron rheometer show that certain types of high density polyethylene exhibit a discontinuity in the shear stress/shear rate curve. This behavior which is separate and distinct from the phenomenon of melt fracture is caused by a change in the flow regime. It is manifested by oscillation of the recorded load while the rheometer piston is operated at a constant rate. Therefore, this behavior has been termed “oscillating shear phenomenon”. The location and magnitude of the fluctuations are functions of the molecular parameters of the polymer as well as the melt temperature, capillary geometry, and other rheometer variables. Evidence is presented which indicates that the change in flow regime could result from the orientation of the melt as it enters the capillary.