Effect of die geometry on helical defect during extrusion of PDMS across a radial flow upstream the contraction

The present work aims at analyzing the influence of both die geometry and entry flow on the appearance and the evolution of the helical defect. The experiments have been performed with linear polydimethylsiloxane (PDMS) via a capillary rheometer in which a convergent radial flow was created upstream the contraction. For a small gap width of radial flow, it was demonstrated that the decrease of the die length-to-die diameter ratio allows the reduction of the helical defect. Moreover, with a short die, when the gap width-to-die diameter ratio is reduced, the helical defect is lessened. In fact, compared to the average diameter of extrudate, the helix pitch and the distortion amplitude are reduced when the die is of a small length and a large diameter as well as the radial flow is of a small gap width.

► Die dimensions effect on helical defect in presence of radial flow.
► The decrease of the die length-to-diameter ratio allows reducing the helical defect.
► For same die length-to-diameter ratios the higher diameter reduces the helical defect.