Effect of rheological and physical properties on mitigation of melt fracture instability during extrusion of polymer melts through a radial flow die

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 helical defect. To do so, three linear polydimethylsiloxanes (PDMSs) of different viscosities and molecular weights were considered. A convergent radial flow is created at the entrance zone when a radial geometry is generated upstream the extrusion die. As for the influence of the considered radial flow, extrudate photographs show that helical instability can be mitigated since its frequency is higher and its amplitude is smaller than what they are when radial flow geometry is not installed. However, it is possible to relate the observed effects to the establishment of the radial flow at the die entrance where the shear deformations are enhanced with respect to the elongational ones. In fact, the development of the helical defect is shaped with amplitude, which is sensitive to the gap width of the radial flow and a frequency imposed by both the entrance geometry and the molecular weight of the polymer. It has also been shown that the ratio of frequencies of the onset of helical instability and the ratio of the Newtonian viscosities, for two given polymers, are inversely equal. Moreover, results obtained confirm that the frequency of the instabilities at their onset is inversely proportional to a mean characteristic time tau (M) as to the plateau viscosity of the considered PDMS.