Influence of wall boundary conditions on sharkskin

Extrudate polymer flow is unstable at high throughputs, and some polymers cause sharkskin roughness on the extrudate surface. Two rotating rollers were installed in a planar die exit to give a moving die wall, and optimal control of the rollers eliminated sharkskin. Extrusion instability for polydimethylsiloxane was investigated as a function of the die wall velocity and extrudate velocities. The minimum die wall velocity VwL required to eliminate sharkskin increased with an increase in the extrudate velocity Vf, and this relationship between VwL and Vf was investigated over the range of Vf = 3.2 to 28.0 mm/s. The apparent reconfiguration rate was introduced for the flow on a moving wall of a viscous power law fluid, and was found to be useful as the sharkskin onset criterion over the Vf range investigated. A new formula was derived for VwL and reasonably agreed with the measured results. The roller die and the introduced methodology allowed extrudate instability to be investigated under extended exit conditions.