Birefringence studies in die flows of an HDPE melt

Abstract

Flow studies of a high-density polyethylene (HDPE) melt have been undertaken through planar slit dies with the purpose of establishing the birefringence pattern. Experimental results are compared with numerical solutions obtained by using an integral constitutive equation of the K-BKZ type with a spectrum of relaxation times. The material parameters have been obtained by fitting viscosity and normal stress data as measured in shear, and elongational viscosity data available in the literature. The numerical simulations have been undertaken for a long (L/2H=2) and a short (L/2H=0.5) die at 180 °C. They show that as the flow rate increases, viscoelastic effects become important and manifest themselves in delayed relaxation of stresses along the slit and in stress overshoots at the die exit. This behavior is in close agreement with experimental birefringence patterns and in sharp contrast with purely viscous simulations which cannot predict such strong viscoelastic phenomena. Elastic recovery is also captured in an enhanced extrudate swell which is always higher for the short die.