A stability diagram for the onset of HDPE, MDPE and LLDPE sharkskin extrudates

Abstract

Sharkskin defect and flow instability of FIDPE, MDPE, and LLDPE polymer melts were studies by capillary and parallel plates rheometers. The onset conditions of HDPE. MDPE, and LLDPE sharkskin were investigated in terms of molecular weight, melt temperature, and die geometry. The recoverable shear (SR), obtained by normalizing the critical wall shear stress with the liquidlike entanglement storage modulus, was found to depend only on material type. Stability diagram of sharkskin defects can be constructed by using the normalized length scale ratio of the sharkskin wavelength (γS) and amplitude (εS) and the recoverable shear (SR). There are two sharkskin boundaries depending on the material type: one unique line for HDPE, and one unique line for LLDPE and MDPE.