Analysis of the capillary extrusion of low-density polyethylene by using velocimetry

The knowledge of the flow kinematics of polymer melts is relevant for their processing as well as for the design of molds and dies. However, the analysis of the flow behavior of polymer melts during extrusion has been typically performed by using rheometric measurements and numerical simulation. In this work, a description of the kinematics of a low-density polyethylene flowing under continuous extrusion through a transparent capillary die was performed by using the particle image velocimetry (PIV) technique coupled with rheometric measurements. The velocity maps for the polymer melt exhibited fully developed flow, meanwhile the flow rate data obtained from the velocity profiles agreed very well with the rheometric ones. The maximum difference in the volumetric flow rates by using the two methods was 6.5%, which shows the reliability of the PIV technique to describe the flow behavior of the polymer melt. The true flow and viscosity curves for the polymer melt were obtained from the measured wall shear stress and velocity profiles, in a wider shear rate range than the accessible by the capillary measurements, including the transition between the Newtonian and power-law regions. This allowed the viscosity data to be very well fitted by a Carreau constitutive equation.