Stretch film is commonly produced by using one of two extrusion processes, cast extrusion, and blown extrusion. At U.S. Packaging & Wrapping we only supply cast extruded film. Cast stretch film is more affordable, but does have some disadvantages over blown stretch film. For a detailed post about the advantages and disadvantages of each type of stretch film view our post about stretch film differences. This post we are going to explain the process of each extrusion technique. See below for a description of each type of plastic extrusion process.
Blown Extrusion– Blown extrusion uses a melted plastic resin extracted into a closed circular bubble with air blown through to cool the resin. The inflated bubble is pulled upward by air and continuously adding more resin. Airflow is also used on the outside of the bubble to cool the film as it is being pulled upward. A continuous seamless tube of thin film is created. After the film has cooled it is slit, then flattened and wound into rolls.
Because blown film uses a slower air cooling method, the film can crystallize and have a slight haze. Blown film is often stronger than cast film and can offer the ability to use a lower gauge compared to cast stretch film. We recommend blown film for companies wrapping very heavy pallet loads with sharp edges and corners.
See the basic image below to see how blown extrusion works.
Cast Extrusion– Initial machinery start-up costs are often more for cast extrusion than blown, but production is much cheaper than blown extrusion. Cast film extrusion offers the ability to make a very thin uniform film. Hybrid stretch films and equivalent stretch films are made with the cast extrusion process.
Cast extrusion uses rollers to cool plastic resin. Once the resin is extracted from the extruder, it is immediately cooled and solidified on cooling rolls. When the film reaches the end of the cooling rolls it is trimmed and can be rolled directly onto a material roll for resale.
There are a variety of cast extrusion processes used the image below is a basic explanation to the cast extrusion process.