Extrudate swell and melt fracture with different exit geometry of dies

Abstract

The extrudate swell, die swell, or Barus effect is the ratio of extruded product diameter to the die diameter through which the material was extruded. The extrudate diameter or area is obtained after the material is allowed to relax completely and care should be taken to make sure that there is no draw down followed by extrusion. Post-extrusion swelling and extrudate distortion phenomenon reveal the viscoelastic nature of elastomeric materials and are particulary critical in polymer processing. With the short dies used in the polymer industry, it is clear that extrudate swell depends not only on the rehological properties of the compounds, but also on the praticular geometry of the extrusion system. There are several main factors affecting post-extrusion swelling and the critical conditions for the onset of extrudate distortion. In our experiments, the effects of die exit geometry on die swell and melt fracture were investigated. The geometry of die exit was changed to see whether it could reduce the die swell ratios. An experimental study of extrudate swell of several melts (low-density polyethylene, styrene-butadiene rubber(SBR), SBR/HAF compound) has been carried out in the absence of applied take-up force. Extrudate swell for a melt emerging from dies with differing diameters correlates with capillary-wall shear rate. Extrudate swell increased with shear rate. Swell passed through a minium value at about 45$^\circ$ die exit angle. The melt fracture region decrease with increase of the die exit angle.