 Film thickness and blow-up ratio can be varied by altering the take-up rate of the rollers, the internal pressure in the blown tube, and the melt extrusion rate.As the film is drawn upwards, it is cooled by a ring of air blowers so that the melt first becomes an amorphous viscoelastic solid, and then a semicrystalline solid, at what is known as the frost line.After solidification, the blown film tube continues to be cooled as it is pulled up by several sets of rollers, deflating the film to form lay-flat tubing.