THE EXPERIMENTAL-OBSERVATION AND NUMERICAL PREDICTION OF PLANAR ENTRY FLOW AND DIE SWELL FOR MOLTEN POLYETHYLENES

We report experimental observations and matching numerical simulations for the planar entry flow and die swell of two high-density polyethylenes (HDPEs) and one low-density polyethylene (LDPE). Experimental data for stress fields, centreline velocities and die swell are reported for each polymer. These results are compared with numerical simulation. The materials are characterized in simple shear using a Wagner integral constitutive equation with a discrete spectrum of relaxation times and a single parameter damping function. The numerical simulation has been carried out using a finite element software package, Polyflow. Self consistency in the stress and die swell data are found for one HDPE, but the other HDPE and the LDPE show an extensional strain hardening response which is not predicted using the simple shear rheology data. In the latter cases, the numerical predictions consistent with entry flow experimental observations can be achieved if extensional flow damping parameters, rather than simple shear damping parameters, are chosen. For the LDPE, an increase in the strain hardening parameter results in the numerical prediction of upstream recirculation vortices in the entry region, which qualitatively agrees with the experimental observations. Apparent inconsistencies in the absolute values of measured and simulated velocity profiles are explained in terms of the 2D nature of the simulation and a 3D component to the experimental flow.