Dependence of the secondary flow of a low-density polyethylene on processing parameters as investigated by laser-Doppler velocimetry

The flow of a low-density polyethylene (LDPE) in the entry regions of slit dies with various contraction ratios was studied experimentally for difference temperatures and throughputs. Laser-Doppler velocimetry was used to investigate the flow kinematics upstream of the contraction plane. For the LDPE large secondary flow regions were found in the corners of the die which significantly depend on the processing parameters temperature, throughput and die geometry. In the literature a heuristic assumption was made that the vortex size is directly related to the strain hardening of a polymer melt. To verify this hypothesis the influence of processing parameters on the vortex size was investigated systematically. The experimental results could be related to the strain hardening expressed by the viscosity ratio μ+/3η0. Master curves of the viscosity ratio could be obtained by applying a time-temperature shift. Using the same shifting procedure master curves of the vortex size could be created, too, which show trends very similar to the viscosity ratio. All the experimental findings of this study support the assumption that there exists a correlation between the vortex size and strain hardening at least in a qualitative way.