Free surface instabilities produced by slip at die exit in polymer melt flow and its suppression

在聚合物挤出液流动中，在高挤出速度下，挤出液经常遭受表面断裂，其特征是规则的细粗糙度或波浪形变形。规则的粗糙表面被称为鲨鱼皮断裂，并推测在模具出口扰动是其原因。在这项研究中，我们开发了一种紧凑的二维挤出流动装置和微观可视化系统，通过该系统可以观察狭缝出口内外的流动不稳定性。发现周期性流动是由粘接失效引起的。接下来，我们使用毛细管模具进行了挤出实验，观察到与狭缝模具挤出相似的不稳定性。此外，在模唇处观察到局部黏附失效沿模唇圆周移动，这种运动导致螺旋断裂。锥形出口模具表明，在这里应用的模具中，熔体断裂起始的临界剪切应力较低。这一结果表明，模具的几何变形可能抑制了熔体断裂。

In a polymer extrudate flow, extrudate often suffers surface fracture characterized by regular fine roughness or wavy distortion at a high extrudate velocity. The regular rough surface is called sharkskin fracture, and disturbance at a die exit is speculated to be its cause. In this study, we developed a compact device for two-dimensional extrudate flow and a microscopic visualization system, by which the flow instability was observed at the inside and the outside of the slit exit. It was found that the periodical flow was produced by adhesion failure. Next, we carried out the extrudate experiments using capillary dies, where the similar instability as the slit die extrudate was observed. In addition, the local adhesion failure at the die lip was observed to move circumferentially along the die lip, and this movement caused the spiral fracture. A tapered-exit die indicated lower critical shear stress for the initiation of melt fracture among the dies applied here. This result suggested the possibility that the geometrical modification of dies suppresses the melt fracture.