Laser-Doppler velocimetry - a powerful tool to investigate the flow behavior of polymer melts

The flow behavior of a low density polyethylene (LDPE) and a high density polyethylene (HDPE) melt in the entrance region, the interior of slit, and within a slit die was investigated by means of laser-Doppler velocimetry (LDV). Large vortices were found upstream of the die with for the LDPE. The HDPE did not exhibit any vortices. The velocities within the vortices are about 100 times smaller than those in the center of the flow. Far-reaching consequences for processing arise as the residence time of the melt in the vortices is long. The velocity distribution along the channel axis reveals large elongational deformations near the contraction. The elongational velocities are much higher than those attainable by elongational rheometers. For the HDPE it could be shown that the material slips at the wall.