Investigation of LDPE converging flows using fieldwise measurements techniques

Abstract : This study is devoted to the analysis of the secondary flow of a low-density polyethylene melt occurring in the entrance region of extrusion dies. From a processing point of view, the appearance of secondary flows can lead to flow instabilities (as observed for helical defect in circular channels) and polymer degradation (due to long residence times). The flow is investigated in the entrance region of a flow channel by means of both Laser Doppler Velocimetry (LDV) and Flow Induced Birefringence (FIB) using a transparent slit die with a planar contraction of 14:1. We focus on the influence of temperature T and flow rate Q on the vortex size of the secondary flow, which strongly depends on balance between shear and extensional rheological properties of the material, as previously observed by means of LDV. The same dependence with T and Q is also observed on FIB patterns, in which one can identify a boundary line which exhibits a strong contrast in the birefringence pattern. This contrast could be related to the fact that molecular conformations (and consequently the birefringence pattern) change locally due to a different thermomechanical history, which indeed would occur close to the vortex boundary.