Thermoplastic melts and rubber compounds are viscoelastic fluids.
They show a complex flow behavior, which is influenced by various factors
such as polymer type, molar mass distribution, recipe, filler-filler network and
in some cases wall slippage. Most of the state-of-the-art simulation software
packages use viscous material models for the calculation of the flow field as
well as pressure and temperature distribution, neglecting the viscoelastic
nature of polymers. This simplification may lead to an underestimated pressure
demand in injection molding simulation.
This contribution presents how to correctly measure viscosity data (shear
and extensional viscosity) for thermoplastics and rubber compounds taking into
account the pressure dependency of the viscosity and the influence of viscous
dissipation in capillary rheometry at higher shear rates. Moreover, a guideline
on how to best fit rheological data with the viscoelastic K-BKZ/Wagner model
is outlined. Comparing CFD simulation results to experimental data, only the
K-BKZ/Wagner model is able to correctly predict pressure losses of contraction
flow dominated geometries. Examples will be given for NBR and PP-PNC.