The significance of slip in matching polyethylene processing data with numerical simulation

Abstract

Precise measurements of pressure difference (Δ P) in capillary and slit flow of a commercial grade linear low-density polyethylene (LLDPE) were carried out using a multi-pass rheometer (MPR) fitted with stainless-steel die inserts. In addition, the experimental stress field for slit flow was determined from isochromatic retardation bands of the flow-induced birefringence (FIB) patterns by assuming the linear stress optical rule to be valid. The MPR results were compared with steady-state numerical predictions of the viscoelastic integral Wagner model, simulated using a finite element code, POLYFLOW. The effect of slip boundary conditions on the numerical predictions of Δ P and slit principal stress difference (PSD) was investigated; the comparison was made for the capillary Δ P data obtained at two temperatures (170 and 190°C) and the slit results at 170°C. For the lower stress data at 190°C, a good match between experiment and simulation was obtained. At the lower temperature and higher stress it was necessary to introduce slip in order to match the results. Non-isothermal power law simulations suggest that shear heating effects cannot fully explain the low temperature extreme L/D data. Therefore, we tentatively conclude the necessity to introduce a wall high shear stress slip effect. For the slit geometry, the centreline principal stress difference birefringence tracking shows good agreement with the simulations. It was found that the simulated centreline |PSD| was not very sensitive to the addition of wall slip. However, in order to obtain a good match with the overall pressure difference for the slit geometry, it was again necessary to introduce a slip component at the wall.