A STUDY OF STRESS-DISTRIBUTION IN CONTRACTION FLOWS OF AN LLDPE MELT

Numerical simulations are carried out for a linear low-density polyethylene (LLDPE) flowing through an 8: 1 planar contraction equipped with slit dies of different length (L/2H = 2 and 8) at two different temperatures (145 and 205-degrees-C). The emphasis is on determining the stress distribution and comparing it with birefringence experimental results that have previously appeared in the literature. The working constitutive equation is a realistic integral model of a K-BKZ type with a-spectrum of relaxation times. The material parameters have been obtained by fitting experimental viscosity and normal stress data as measured in shear, and by using elongational viscosity data available in the literature. The numerical simulations are performed for a wide range of apparent shear rates (10 s-1 < gamma(a) < 140 s-1) with good and fast convergence of the iterative scheme. The results show that as the apparent shear rate increases, viscoelastic effects dominate, exhibiting a delayed relaxation of stresses along the slit and stress over-shoots at the die exit. This behaviour is in close agreement with the experimental birefringence patterns. Elastic recovery is also captured in an enhanced extrudate swell which is always higher for the short die at the same temperature.