This technique is widely used in the manufacture of polyethylene and other plastic films. Through this method, the molten polymer from the extruder head enters the die, where it flows around a mandrel and emerges through a ring-shaped opening in the form of a tube.
The tube expands into a bubble in the required diameter by the pressure of internal air admitted through the center of the mandrel. The air contained in the bubble cannot escape as it’s sealed by the die at one end and by the nip (or pinch) rolls at the other end. Hence, it acts as a permanent shaping mandrel once it has been injected.
An even pressure of air is maintained to ensure uniform thickness of the film bubble. The film bubble is cooled below the softening point of the polymer by subjecting it to blowing air from a cooling ring placed around the die.

Resins used

• Low Density polyethylene (LDPE)
• Linear Low Density Polyethylene (LLDPE)
• High Density Polyethylene (HDPE)
• Polypropylene (PP)
• Ethylene Vinyl Acetate (EVA)
• Ethylene Butyl Acrylate (EBA)
• Ethylene-vinyl alcohol (EVOH)