Stretch film produced by blown extrusion (blown stretch film) consists of a polymeric film characterized by significant elasticity and elongation capacity, used in the packaging sector for wrapping and protecting food products, particularly fresh foods arranged on trays and intended for distribution through large-scale retail.
Thanks to its mechanical strength and ability to conform to the profile of the packaged product, it helps preserve the integrity of the packaging and protect the food during handling, transport, and storage along the supply chain. The film is generally wrapped around the food product placed on a rigid or semi-rigid support, creating a tight closure that protects the content and improves its presentation at the point of sale.
Among the technologies for producing stretch film, the blown extrusion process allows for materials with particularly high structural performance. In the production process, the molten polymer is extruded through a circular die and subsequently expanded using air blowing, forming a vertical bubble. This configuration induces a bidirectional molecular orientation that provides the film with high resistance to puncture, tearing, and tear propagation, features essential for ensuring packaging integrity even during handling and distribution of packaged food products.
The most commonly used material is linear low-density polyethylene (LLDPE), selected for its combination of flexibility, strength, and ability to develop an adequate cling effect, i.e., controlled adhesion between overlapping film layers during wrapping of the product or tray. Functional additives can also be incorporated into the polymer matrix to enhance specific properties, such as UV protection, anti-slip characteristics, pigmentation, thermal stability, or surface adhesion levels.
1. Resin preparation and extrusion
The process begins with feeding resin pellets into the extruder. Inside the heated barrel, the material is progressively melted and plasticized until it reaches a fully fluid state. The molten polymer is then conveyed to a circular die, from which it exits as a continuous tube.
2. Bubble formation and cooling
At the die exit, the polymer tube is expanded using air blowing, forming a vertical bubble. During this stage, the film undergoes controlled stretching in both the machine and transverse directions, establishing the orientation of the polymer chains.
Bubble cooling, generally achieved through air flows or ring systems, is a critical process parameter: the cooling rate directly influences the material’s microstructure and therefore the final properties of the film, such as toughness, elasticity, and load retention capacity.
3. Bubble collapse and winding
Once dimensionally stabilized, the bubble is guided through collapsing frames, which flatten it into a double-layered flat web. The resulting film is then wound onto large-diameter rolls (jumbo rolls).
Subsequently, the rolls undergo longitudinal cutting (slitting) and rewinding to obtain rolls of specific width and length intended for manual or automatic packaging lines.
Flexography and printing on blown stretch film
Even in the case of blown stretch film, flexographic printing is one of the most widely used technologies for applying logos, identifying marks, or technical information directly onto the surface of the film. This process combines good graphic quality, high productivity, and low operational costs, making it particularly suitable for high-volume industrial production.
Adaptability to plastic substrates
Flexography is particularly suitable for thin, non-porous plastic substrates. Photopolymer flexo plates, thanks to their elasticity, allow uniform contact with the substrate and controlled ink transfer without compromising film integrity.
Production efficiency
Modern flexographic presses can operate at very high speeds, up to approximately 800 m/min, allowing efficient handling of large production volumes.
Economic convenience
Flexo plates require shorter production times and have lower costs compared to other industrial printing systems, making them especially advantageous for simple graphics such as logos, identifying symbols, or text.
Fast-drying inks
Printing on blown stretch film uses fast-drying inks, generally water-based, solvent-based, or UV. Rapid drying is essential to prevent smudging or transfer during rewinding and to maintain high production speeds.
Controlled ink dosing
The flexographic system, based on anilox rollers and doctor blades, ensures precise control of the ink amount transferred to the substrate, guaranteeing print uniformity and process stability.
Integration with the converting process
Modern flexographic lines can be integrated with converting processes, combining printing with slitting, rewinding, or surface treatments such as corona treatment, necessary to improve film wettability and ink adhesion.
Flexographic printing represents a technologically efficient solution for customizing blown stretch film, allowing a combination of graphic quality, substrate reliability, and industrial productivity in food packaging applications.
Written by Michele R. | Team Giugni®
