A visualization study of poly(ethylene terephthalate) flow using potential chain‐ordering die geometries

Streak‐photographic and stress birefringence techniques were used to analyze the flow of poly(ethylene terephthalate) through potential chain‐ordering die geometries. The streak photographs were used to determine velocity distributions and streamlines in various convergent dies. The different contours were seen to have a significant effect on the polymer streamlines and velocity distributions. The measured velocities were used to develop empirical equations, specific for each geometry, which relate velocity to position within the die and to throughput rate. Flow birefringence was used to determine the extent of molecular ordering. The optimum chain‐ordering die geometry was determined to be one which included a rapid initial decrease in cross‐sectional area. Birefringence was also used to monitor polymer flow instability. An unusual mechanism for instability was observed at intermediate throughput rates.