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

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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.