Stretch film, also known as stretch wrap, is a highly elastic plastic film primarily used for wrapping and stabilizing palletized loads during transportation, handling, and storage operations. One of the main industrial processes used to manufacture this flexible packaging material is cast extrusion, from which so-called cast film is produced.
The material most commonly used for stretch film is linear low-density polyethylene (LLDPE), selected for its excellent elasticity, puncture resistance, and surface adhesion properties (cling). Additives may be incorporated into the LLDPE resin to enhance specific characteristics such as UV resistance, coloration, and tack level.
The production of cast stretch film takes place through a continuous and carefully controlled process:
- Resin preparation and extrusion
The process begins with feeding plastic resin pellets into an extruder. Inside the machine, the material is gradually heated until it reaches a molten state. The molten polymer is then forced through a flat die, from which it exits as a thin, continuous sheet of film.
- Casting and cooling
The still-hot plastic film is then deposited onto a series of temperature-controlled cooling rolls. The rapid cooling and solidification of the material give cast film its typical characteristics of high transparency and surface gloss. The speed of the rolls and the operating conditions of the line contribute to determining the final film thickness.
- Winding
After cooling, the solidified film is wound into large-diameter rolls (jumbo rolls). These rolls can subsequently undergo longitudinal slitting operations to obtain rolls of specific widths, which are then rewound onto smaller cores to facilitate their use.
Cast film is distinguished by its high optical clarity, which facilitates barcode reading and visual inspection of packaged goods. In addition, it unwinds more quietly from the roll and generally features more uniform thickness.
Why flexography is the dominant technology for printing on cast stretch film?
Flexography is currently the most widely used technology for printing on cast stretch film. This predominance derives from the combination of high production speed, versatility, and cost efficiency, characteristics that are particularly well suited to the needs of the flexible packaging industry.
- Substrate versatility
Flexography is designed to print on a wide range of materials, particularly non-porous substrates. Cast stretch film, typically made of LLDPE, is a flexible and non-porous material that is well suited to this process. Photopolymer or elastomer printing plates easily conform to the film surface, ensuring consistent ink transfer.
- High productivity
The packaging industry requires large-scale production, and stretch film is a high-volume product. Flexographic printing presses are designed to operate at high speeds. Modern lines in advanced configurations can reach speeds of up to approximately 790–800 m/min.
- Cost efficiency for medium-to-long runs
Flexography is particularly cost-effective for medium-to-high volume production. Flexographic plates are less expensive and faster to produce than the engraved cylinders used in rotogravure printing, making the process more economical, especially for simple graphics such as logos and text.
- Use of fast-drying inks
Printing on plastic surfaces requires inks with very fast drying times to prevent smearing during film rewinding. Flexography uses water-based, solvent-based, or UV inks, which allow high production speeds while maintaining good print quality. Furthermore, the development of more sustainable inks, such as water-based and UV formulations, has made flexography an increasingly environmentally compatible choice.
- Precise control of ink transfer
The flexographic process ensures accurate ink metering through the anilox roll, an engraved cylinder with calibrated microcells that transfer a controlled amount of ink to the printing plate. The use of a doctor blade also removes excess ink, ensuring uniform application.
- Integration with inline operations
Modern flexographic lines can integrate several finishing operations directly inline, such as longitudinal slitting or other treatments. This integration simplifies the production process and reduces overall processing time.
Flexography, thanks to its ability to deliver good print quality at high speeds on complex flexible substrates while maintaining competitive production costs, represents today the dominant and most suitable technology for inline printing on cast stretch film.
Written by Fabrizio V. | Team Giugni®
