Slip effects in HDPE flows

The capillary flow of a high-density polyethylene (HDPE) melt was studied both experimentally and numerically. The excess pressure drop due to entry (entrance pressure drop), the compressibility, the effect of pressure and temperature on viscosity, and the slip effects on the capillary data analysis have been examined. Using a series of capillary dies having different diameters, D, and length-to-diameter LID ratios, a full theological characterization has been carried out, and the experimental data have been fitted both with a viscous model (Cross) and a viscoelastic one (the Kaye-Bernstein, Kearsley, Zapas/Papanastasiou, Scriven, Macosko or K-BKZ/PSM model). Particular emphasis has been placed on the effects of wall slip (significant for HDPE). For the viscous model, the viscosity is a function of both temperature and pressure. For the viscoelastic K-BKZ model, the time-temperature shifting concept has been used for the non-isothermal calculations, while the time-pressure shifting concept has been used to shift the relaxation moduli for the pressure-dependence effect. It was found that only the viscoelastic simulations were capable of reproducing the experimental data well. On the other hand, viscous modeling underestimates the pressures drops, especially at the higher apparent shear rates and L/D ratios. It is concluded that wall slip effects are significant for HDPE flow, whereas viscous heating is not. (C) 2011 Elsevier B.V. All rights reserved.