Factors affecting extrudate swell and melt flow rate

The effects of temperature, load and L/D ratio of a capillary die on the melt flow rate and the swelling characteristics of different polyethylenes have been studied. The selection of the polyethylenes was made carefully to reflect the molecular structure effect on the two chosen polymer properties. It has been shown that whilst the melt flow rate increased with increasing temperature and load, it decreased with increasing L/D ratio. The extrudate swell measured at the die exit was found to have a concave-downwards characteristics with temperature. Sagging of the extrudate was believed to be responsible for the maximum swelling ratio found. The experimental data also indicated that this maximum swelling ratio appeared to shift as the L/D ratio varied. Since the melt flow rate and the extrudate swell ratio are often required in product design and process control, a mathematical expression of practical significance has been attempted to reveal the relationship between these two polymer characteristics. It was found that melt flow rate was proportional to e((extrudate swell) (ratio)). (C) 1998 Elsevier Science S.A. All rights reserved.