Effect of the Radial Flow and Average Molecular Weight on the Surface Defect in PDMS Extrusion

This study examines the role of both a radial flow created at the entrance of the die and the average molecular weight of polymer, on the appearance and development of the sharkskin defect. To do so, three linear polydimethylsiloxanes (PDMS) of different viscosities and molecular weights were considered. A convergent radial flow was created at the entrance zone by imposing a rod upstream the die. It was found, that when the radial flow is well established in the entrance zone (i.e., for very low gaps between the bottom of the rod and the plane of the die) the onset of sharkskin defect can be translated to higher flow rate. As the influence of molecular weight of polymer, it was shown that the extrusion of lower molecular weight PDMS can delay the occurrence of sharkskin instability. Besides, it was remarked that the morphology of the extrudate associated to the trigger of surface defect is not affected with the presence of radial flow in the entrance region or with the molecular weight of the extruded polymer.