Cooling and calibrating the extrudate
When the melt leaves the die, it has low dimensional stability. It should therefore be cooled as close as possible to the die without influencing the temperature control at the head of the die. Water baths, water spray jets, air showers or a combination of these systems can be used for cooling. To prevent any deformation of the extrudate while still in a plastic state, cooling should be carried out gently. The length of the cooling section will depend on the type of article, wall thickness and take-off speed.
The final temperature should be 40 °C.
Calibration systems such as sizing/draw plates, floating plugs and vacuum calibration can only be used with TPU if a lubricating film builds up between the extrudate and the calibrating surface.
Fig. 1 shows a vacuum low-friction calibrator.
Fig. 1: Low-friction calibration