Blown films, sometimes referred to as tubular films, are manufactured by extruding molten resin vertically through a circular die. Introducing air through the center of the die creates a bubble. The air drives the bubble upward, and slowly cools the material. Nip rollers flatten the material into a tube that can be reeled for creating bags and pouches, or slit and then reeled as a flat sheet of film. Blown films are pre-stretched by virtue of their manufacturing process.
Cast films are made by extruding melted resin horizontally through a flat die to create a sheet of material that is pinned to a highly polished chilled roller by means of an air curtain or vacuum box. Additional chill rollers quickly cool the film prior to trimming and rolling. Cast films are minimally pre-stretched allowing for improved depth of drawing for thermoforming operations. Typically, cast films have an even distribution of polyolefins due to increased gauge consistency due to the manufacturing process.
Differences in cooling lead to differences in the final films’ characteristics.
| BLOWN FILM | CAST FILM |
Cooling | Slow | Rapid |
Resin Molecule Alignment | Random
(in all directions) | Linear
(in one direction) |
Clarity | Potentially Hazy | Very clear |
Puncture Resistance | Very good | Good |
Holding Power(resistance to stretch) | Higher | Lower |
Unwind noise | High | Low |
Choosing between blown and cast film begins with examination of the unique needs of your product and process. General properties can also vary with processing systems and product storage conditions. The best first step to creating and optimizing the perfect packaging film for your product and process is talking with the experts at Flair.
Contact Flair today!