Mechanics Analysis of the Die Drawing Process for Producing Oriented Polyethylene Terephthalate (PET) Tube

Abstract
An analysis is presented of the die drawing process used to produce transparent polyethylene terephthalate (PET) tube having a high degree of molecular orientation and consequently superior mechanical properties to tube in the isotropic state. The stress in an element of material being drawn is determined from equilibrium considerations and by using the Tresca yield criterion. Flow stress values used in the analysis allow for strain rate effects during drawing and were obtained from uniaxial stress-strain data and applied to the problem using equivalent stress-strain concepts. Results are presented of die drawing tests carried out on a large-scale die drawing facility on PET tube at 100°C. Predicted drawing loads for an assumed value of friction coefficient of 0.2 compared well with experimental values for billet velocities greater than 7 mm/min. Draw ratios of up to 3.5 were achieved in tests when there was evidence of crystallization during the free drawing of the material downstream of the die exit.