The flow curves

extrudate distortions

extrusion pressure variety and the characters of viscoelasticity of linear and branched polyolefine melts in high speed extrusion was investigated by a double capillary rheometer of constant speed type. The unstable flow behaviors and the trouble headstream of different melts were discussed. The results indicate that there are three kinds of trouble headstreams in high speed extrusion flow field

such as the destabilization on die entrance

on die wall

and on die exit. Branched polymer is easily disturbed by the destabilization on die entrance

which will cause a irregular extrudate fracture ;while the linear polymer is easily disturbed by the destabilization on die wall

which will cause an extrusion pressure oscillation and a regular extrudate distortion. The sharkskin-like distortion is caused by an oscillatory destabilization on die exit.