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Extensional Deformation, Cohesive Failure, and Boundary Conditions During Sharkskin Melt Fracture



Kalman D. Migler, Y Son, F Qiao, Kathleen M. Flynn


We measure the flow kinetics of a polyethylene extruded through the exit of a capillary tube in order to understand the nature of sharkskin, a surface roughness in the extruded material. In the absence of a polymer process additive (PPA), optical velocimetry shows that the polymer sticks (or weakly slips) at the sapphire wall just upstream of the tube exit and the flow is smooth. Downstream of the exit, high-speed video-microscopy reveals that the material splits cohesively into a slow moving surface layer and a fast moving core region. When we modify the surface by using a PPA, we observe strong slip at the wall but find that sharkskin does occur at significantly higher flow rates. We find that the extensional strain rate at the onset of sharkskin is significantly higher in the case of the PPA than without it. We then define the deformation rate, which is related to the product of the extensional strain rate with the material deformation. We find that the deformation rate is comparable at the onset of sharkskin for the two boundary conditions. We use a simplified argument and literature data to show that the deformation rate may also describe the onset of sharkskin as a function of radius.
Journal of Rheology
No. 2


extrusion, instabilities, polyethylene, rheology, sharkskin, velocimetry


Migler, K. , Son, Y. , Qiao, F. and Flynn, K. (2002), Extensional Deformation, Cohesive Failure, and Boundary Conditions During Sharkskin Melt Fracture, Journal of Rheology, [online], (Accessed May 23, 2024)


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Created March 31, 2002, Updated October 12, 2021