Blown films are created by plastic granules being fed into an extruder where they are melted and homogenised before they are pumped through a circular blown film die. The melted plastic forms a continuous tube which is drawn from the die. It is inflated and simultaneously cooled by rapidly moving air so that the plastic solidifies quickly. The tube is them collapsed between rollers and wound onto a reel.
Cast films also start life as plastic granules fed into an extruder where they too are melted. The difference is that they are fed through a flat die onto a chilled roller so that they cool extremely quickly. Once the plastic has solidified, it is wound onto a reel.