Die swell behaviour of polypropylene — An experimental investigation

The die swell behaviour of polypropylene samples was studied over a range of shear rates by employing dies of several length/diameter (L/D)-ratios. The swelling ratio increases with shear rate but decreases as the (L/D)-ratio of the die increases. The samples annealed above the polymer melting temperature at 230°C were found to have the swelling ratio considerably higher than that of unannealed samples. At a constant shear rate the swelling ratio was found to decrease as the temperature was raised from 220–240°C. For the dies employed, the swelling ratios were found to be inversely proportional to the average transit or residence time of the polymer melt through the die. The die swell data were then used to predict the first normal-stress difference values by means of various die swell theories and the results were discussed in the light of polymer viscoelasticity.