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This study examines the effects of average surface roughness and composition of the die wall surface on the critical shear stress for the onset of instabilities in die extrusion. Increasing the average surface roughness of the die wall from 0.1 to 15m produced an increase of the critical shear stress for the onset of flow instabilities by as much as 20% during the extrusion of HDPE. The surface of the extrudates obtained with dies having an average surface roughness greater than 5lm showed lines and grooves along the direction of extrusion. Chrome-plated and nickel-plated dies showed an average 28% increase in the critical shear stress as compared to conventional steel dies. The chemical composition of uncoated steel dies wall surfaces, including the presence of iron oxide, did not affect the critical shear stress for the onset of sharkskin significantly regardless of the type of resin used.

Effect of the Chemical and Morphological Conditions of the Die Wall on the Extrusion of Linear Polyolefins