VISCOELASTIC SIMULATIONS OF EXTRUDATE SWELL FOR AN HDPE MELT THROUGH SLIT AND CAPILLARY DIES

Extrudate swell studies of a high-density polyethylene (HDPE) melt have been undertaken in flows through slit and capillary dies with the purpose of finding out the effect of die-length/diameter (gap) (L/D or L/2H) ratio on the viscoelastic behavior. Numerical solutions have been obtained by using the finite element method (FEM) and an integral constitutive equation of the K-BKZ type with a spectrum of relaxation times. The material parameters have been obtained by fitting experimental viscosity and normal stress data for the melt as measured in shear, and elongational viscosity data available in the literature. Different L/D (L/2H) ratios have been considered ranging from very short to infinitely long dies. The numerical simulations reveal that as the flow rate increases, viscoelastic effects exhibited by the HDPE melt become important and manifest themselves in an enhanced swelling behavior after the die exit, while small, Newtonian-like vortices exist in the contraction before entry to the die. Elastic recovery is also captured in an enhanced extrudate swell, which is always higher at the same apparent shear rate for the capillary than the slit dies and decreases drastically as the L/D (L/2H) ratio increases, reaching asymptotic values for very long dies. Such behavior is in agreement with experimental findings from flows through slit and capillary dies and in sharp contrast with purely viscous simulations which cannot predict such strong viscoelastic phenomena associated with the memory of the polymer melt.