Cast vs blown stretch film: how are they made and how do they differ?

How stretch film is made

1. Polyethylene pellets (mainly LLDPE) and additives are fed into an extruder, melted and pushed through a die.
2. The molten film is cooled: on a chilled roll (cast) or as an air-cooled bubble (blown).
3. The finished film is wound onto large jumbo rolls.
4. The jumbo rolls go to rewinding machines, which turn them into rolls ready for sale.

The cooling step is where cast and blown film part ways, and it shapes almost everything about how the film behaves.

Cast film

In the cast process the melt leaves a flat die as a thin curtain and lands on a polished, water-cooled chill roll. It cools very quickly, which lets a cast line produce up to about four times faster than a blown line. Stretch film is often co-extruded in several thin layers at once, so each layer can be designed for a job: cling on the outside, strength in the middle.

Blown film

In the blown process the melt leaves a round die as a tube, which is inflated with air into a tall bubble and cooled slowly on its way up. It is then flattened and wound. The bubble stretches the film in two directions, and the slow cooling makes it less clear than cast film.

Side-by-side comparison

Neither film is better in every case; each has its advantages. If the load does not need blown film's puncture resistance and toughness, cast film is usually the more economical choice. For even thickness, the quality of the line and the know-how of its operators matter more than the process itself.

Does it matter for rewinding?

Both cast and blown jumbo rolls can be rewound. What changes from one jumbo roll to the next, even from the same producer, is the cling and the way the film reacts to tension. Machines that control unwinding and rewinding with separate servo motors, such as the SM-1300 2S and SM-1300 2S PRO, let the operator adjust film tension precisely from the touchscreen, which makes them well suited to jumbo rolls with varying cling.