Disappearance of extrusion instabilities in brass capillary dies

We have extruded a linear low-density polyethylene through capillary dies fabricated from stainless steel and brass. We confirm a result first reported by Ramamurthy [Ramamurthy, A. V., “Wall Slip in Viscous Fluids and Influence of Materials of Construction,” J. Rheol. 30, 337–357 (1986)]: sharkskin can be eliminated by the use of a brass die. We also find a substantially enhanced throughput from the brass die relative to the stainless-steel die at stresses in the range where sharkskin is observed with the latter. Finally, the large pressure oscillations and periodic extrudate distortions observed in the “slip–stick” region with stainless steel are absent with brass, where the transition to the “upper branch” of the flow curve is more gradual.