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

Precise measurements of pressure difference (Delta P) in capillary and slit flow of a commercial grade linear low-density polyethylene (LLDPE) were ca rried out using a multi-pass rheometer (MPR) fitted with stainless-steel di e inserts. In addition, the experimental stress field for slit flow was det ermined from isochromatic retardation bands of the flow-induced birefringen ce (FIB) patterns by assuming the linear stress optical rule to be valid. T he 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 predicti ons of Delta P and slit principal stress difference (PSD) was investigated; the comparison was made for the capillary Delta P data obtained at two tem peratures (170 and 190 degrees C) and the slit results at 170 degrees C. Fo r the lower stress data at 190 degrees C, a good match between experiment a nd simulation was obtained. At the lower temperature and higher stress it w as necessary to introduce slip in order to match the results. Non-isotherma l power law simulations suggest that shear heating effects cannot fully exp lain the low temperature extreme L/D data. Therefore, we tentatively conclu de the necessity to introduce a wall high shear stress slip effect. For the slit geometry, the centreline principal stress difference birefring ence tracking shows good agreement with the simulations. It was found that the simulated centreline \PSD\ was not very sensitive to the addition of wa ll 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 sl ip component at the wall. (C) 2000 Elsevier Science B.V. All rights reserve d.