THE IMPORTANCE OF VISCOELASTICITY AND PRESSURE DEPENDENCE OF VISCOSITY FOR SIMULATION OF MELT FLOW IN INJECTION MOLDING AND CAPILLARY RHEOMETRY

To establish closed-loop control of the injection molding process with regard to a self-optimizing process,
quality models derived from injection molding simulations acc. to DoE are of crucial importance. Quality
functions derived from simulation linking the process parameters with desired part quality (part weight,
dimensions) allows running the production within the allowed process window realizing a robust injection
molding process. It is well known that viscoelasticity of polymer melts [1,2] and pressure dependence of
viscosity [3] play a major role in processing. Nevertheless these effects are commonly neglected in simulation
of polymer melts resulting in remarkable deviations between the real pressure demand and the prediction via
simulation. The effects increase strongly with raising pressure level for processing.
The aim of this study was to compare measured data for the pressure demand in capillary rheometry as well
as in injection molding with calculated values based on simulations. Particular emphasis had been given on
viscoelasticity and the pressure-dependence of viscosity. The results from viscous calculations for different
conical dies having different diameters, D, and length-to-diameter L/D ratios, were compared with those from
viscoelastic modeling and viscoelastic modeling taking into account the pressure effect. Four injection molding
nozzle geometries had been also used to reach apparent shear rates up to 800,000 s-1. Using oscillatory and
capillary rheometry a full rheological characterization had been carried out for both a polypropylene-filled
nanocomposite and a rubber compound (SBR).
It was found that only the viscoelastic simulations were capable of reproducing the measured pressure drop
very well [4], while any viscous modeling always strongly underestimates the pressures, especially at the higher
apparent shear rates and L/D ratios. 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 KBKZ/
PSM model).The data for pressure dependence of viscosity were measured on an injection molding
machine rheometer with the help of a special rheological mold. Experimental data are evaluated taking into
account the melt temperature rise due to dissipative heating.
In addition the importance of pressure-dependence of viscosity for injection molding simulations of practical
parts is shown for different thermoplastics (ABS, PS, isotactic PS) and a rubber compound (NBR) comparing
measured pressures in the screw antechamber or the cavity with simulated pressure values. For pressures higher
than 100 MPa the pressure-dependence of viscosity should be taken into account.