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°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.

对于长径比为10且长高比为2.5至40的狭缝模具，数值研究了上游收缩与聚合物熔体挤出物膨胀行为的相关性。使用ANSYS进行三维（3D）流动模拟Polyflow软件在不同的流量和200°C的恒定温度下，选择差分多模Phan-Thien-Tanner（PTT）本构模型来描述聚丙烯的粘弹性行为。随之而来的是，减小模头长度导致在宽度和高度方向上的溶胀比的相反趋势。宽度膨胀率最多可降低28％，而中间高度膨胀率最多可提高55％，边缘高度收缩可增加37％，所有这些都突出了挤出物膨胀变形的3D各向异性。然而，特别是对于低流速，面积膨胀率几乎保持不变，不能用于捕捉各向异性溶胀行为。这主要归因于轴向速度的相对相对重新分布程度（v _x）。结合横向应力（v _y）和横向速度（v _z）以及应力场变化进行分析，以使各向异性挤出物的溶胀行为与各种模具长度完全相关。因此，本研究有助于在等温条件下全面理解狭缝模头设计参数与3D挤出物流动之间的关系，涵盖了已发展和已充分发展的流动。