The primary tube is then pulled to the top of the orientation tower, using a Pull Unit. At the top of the tower, the primary tube is drawn in a downward direction and, at the same time, it is heated to a temperature just below the melt point, supported by infra-red resistances. At the start-up, when this temperature is reached, air pressure is introduced expanding the tube to the gauge required. The biaxial orientation takes place as the tube is expanded and the downward speed is accelerated.