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. (C) 1999 The Society of Rheology. [S0148-6055(99)00302-8].