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

在聚合物挤出物流动中，在高挤出速度下，挤出物经常会发生表面断裂，其特征是规则的细小粗糙度或波状变形。这种规则的粗糙表面被称为鲨鱼皮断裂，模具出口处的扰动被认为是其原因。在这项研究中，我们开发了一种紧凑的二维挤出物流动装置和一套微观可视化系统，通过该系统可以观察到狭缝出口内外的流动不稳定性。结果表明，周期性流动是由粘接失效引起的。接下来，我们用毛细管口模进行了挤出实验，观察到了与缝隙口模挤出物类似的不稳定性。此外，还观察到模具唇处的局部粘结失效沿模具唇沿模具唇周移动，这一运动导致了螺旋断裂。锥形出口机头具有较低的熔体断裂临界剪应力。这一结果表明，模具的几何修改可能抑制了熔体的破裂。

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.