The impact of upstream contraction flow on three-dimensional polymer extrudate swell from slit dies

The relevance of upstream contraction for extrudate swell behavior of polymer melts is numerically investigated for slit dies with an aspect ratio of 10 and a length-to-height ratio ranging from 2.5 to 40. Three-dimensional (3D) flow simulations are performed with ANSYS Polyflow software at different flow rates and a constant temperature of 200 degrees C, selecting a differential multimode Phan-Thien-Tanner (PTT) constitutive model to describe the viscoelastic behavior of polypropylene. It follows that decreasing the die length results in an opposite trend of the swell ratio in the width and height directions. The width swell ratio decreases up to 28%, whereas the middle height swell ratio increases up to 55% and the edge height contraction increases with 37%, all highlighting the 3D anisotropy of the extrudate swell deformation. However, especially for low flow rates, the area swell ratio remains almost unchanged and cannot be used to capture the anisotropic swelling behavior. This is predominantly attributed to the similar relative redistribution degree of the axial velocity (v(x)). An analysis of the transversal (v(y)) and lateral velocity (v(z)) combined with the stress field variation is performed as well to fully relate the anisotropic extrudate swell behavior to the various die lengths. The current study therefore contributes to completing the understanding of the relation of slit die design parameters and 3D extrudate flow under isothermal conditions, covering both developing and fully developed flow.