Flow instabilities of linear polyethylenes have been studied in capillary dies. The effects of shear stress, temperature, molecular weight and material of construction of the dies on both the critical shear stress for the onset of slip and slip velocities have been evaluated. Flow curves were obtained using capillary dies with constant L/D and different diameters. Slip velocities were calculated using the Mooney technique  as a function of shear stress. The onset of slip and slip velocity are affected by the material of construction of the capillary die. Flow instabilities were eliminated when brass dies were used under specific conditions. Instabilities persisted when naval brass and copper capillary dies were used.