Flow visualization studies were made during extrusion by using a hyperbolic die, streak photography, and flow birefringence techniques. Velocity was found to be a linear function of position along the die axis. This is equivalent to a constant elongational strain rate. Considering that the hyperbolic die maintains the stretching rate achieved in the upstream over an extended time period, this profile seems to be the optimum die design for maximum chain‐ordering potential. A unique analysis of the birefringence and velocity distribution data allowed determination of characteristic relaxation time for the PET melt of 5.5 × 10−3 at 285°C.
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Ihm et al. (1987) studied this question.