Isothermal flow of neat polypropylene through a slit die and its die swell : bridging experiments and 3D numerical simulations

The relevance of 3D numerical simulations for the isothermal flow of neat polypropylene through a slit die and the subsequent swelling upon die exit is highlighted based on the finite element method using the ANSYS Polyflow software package. The input parameters of (i) the generalized Newtonian constitutive model with the Cross law viscosity function and (ii) the multimode viscoelastic Phan-Thien-Tanner (PTT) constitutive model are estimated from rheological data obtained with rotational and capillary rheometers. Model validation is performed based on die pressure and real time experimental HD imaging for 3D die swell characterization. The evolution of the swell ratios in both width and height directions is investigated downstream the die exit. It is demonstrated that the PTT based swell ratio in the width direction increases in a much lower rate than the swell ratio in the height direction until equilibrium is achieved in both directions. Therefore, the distance from the die exit in which the final extrudate shape is obtained strongly depends on the die width, which is ignored in conventional 2D numerical simulations. The findings are underpinned through a detailed analysis of the simulated 3D stress and velocity field distributions