HIGH-SPEED TEMPERATURE CONTROL FOR POLYMERIC FIBERS

A description is given of a method devised for high-speed open-loop control of temperature in polymeric fibres, suitable for either running threadlines in processing machinery or static fibre specimens in testing equipment. It is based on impingement on the fibre by a variable-temperature transverse air jet, formed by the mixing of varying proportions of hot and cold air. Mixing is carried out in a stepper-motor-driven rotary valve controlled by microcomputer. Under these conditions, typical polymeric fibres of diameters less than c. 100-mu-m behave as thermally 'thin', and temperature gradients within them can be ignored. Nominal heating/cooling rates of up to c. 500 K s-1 were achieved while maintaining a unique (but somewhat non-linear) relationship between the air-jet temperature and valve-shaft position. Nominal rates of up to c. 5000 K s-1 were usable, but, beyond 1000 K s-1, there was noticeable lag in the temperature response (50 ms), as a result of the finite time required for the two air streams to mix. The maximum practical heating rate is in the region of 3600 K s-1. An analytical model of the valve was constructed for predicting the air-jet temperature for a given step-sequence of the motor, including the transient effect, by using only three adjustable parameters. A reasonable fit to measured temperatures was obtained for experiments covering a range of two decades of nominal heating rate.