Simulation of planar welding flows: Part 2. Strain history, stress calculation, and experimental comparison

Abstract

A numerical method is described for calculating the stress a viscoelastic melt exhibits in a flow, based on approximate kinematics. The method assumes that the kinematics are reasonably close to those of a shear-thinning fluid such as the Carreau model. The strain history of a given flow and the resulting stress are calculated via a tracking method from finite element kinematics. Fullfield flow birefringence experiments were done for lowdensity polyethylene and polystyrene flowing past a thin plate divider in a 1.254-mm planar slit die. By digitally analyzing birefringence photographs of the flow field, the birefringence was measured over two dimensions. These birefringence results are in good agreement with birefringence fields calculated from the numerical simulations and the stress-optical law. The flow fields were most highly oriented in a region surrounding the weld interface just downstream of the plate divider. This orientation relaxed farther downstream, with polystyrene relaxing faster than low-density polyethylene.