The melt die-swell behaviour during capillary extrusion of LDPE/PP blends

Two blow-film extrusion grade thermoplastics, polypropylene (PP) and low density polyethylene (LDPE), have been melt-blended in a screw extruder. The die-swell behaviour of the blend melts and their affecting factors were investigated by using a capillary rheometer. The die-swell ratio (B) of the blends decreased approximately linearly with increasing temperature and the die length-diameter ratio (UD); with increasing wall shear stress the end pressure losses and B increase non-linearly; with addition of the weight percentage content (phi(pp)) of PP, the values of B decreased, but the maximum (a peak of the curve) appeared at phi(pp) = 20%. This phenomenon can be attributed to the elastic stored energy amount in the melt and its transition due to the viscoelasticity difference between the two phases. (C) 1998 Elsevier Science Ltd. All rights reserved.