Innovations in plastic continue to enhance the way we live — it has also had a profound influence on the packaging industry.
Shrink-wrapping use is overgrowing worldwide, especially in India, for light/small articles and heavy/huge pallets. The blown extrusion process produces low-cost polyethylene shrink film on conventional equipment. No additional machinery is needed. According to the film’s application, processing conditions and resin characteristics must be selected appropriately. The film shrinks because it introduces a high degree of molecular orientation or internal stress during its manufacture. These stresses are ‘frozen’ in the film by the air cooling.
When the LDPE film is reheated to a softening point (such as in a shrink-tunnel or oven), the film’s molecules tend to regress to their entangled unstrained state. Thus, the internal stresses are released, forcing the film to shrink.
Two shrink film classes are mainly used. At present, bi-oriented shrink film is more widespread than mono-oriented film. In the future, the mono-oriented film is anticipated to make considerable gains in the total pallet-wrapping and sleeve-wrapping business. The bi-oriented film will remain for full overwrap of small and medium-sized packages.
The most widely used film thicknesses are:
- 25-50 μm (thin shrink film) and
- 57-150 μm (heavy-duty shrink film).
Shrink Film Properties
The properties that fully describe a shrink film are the following:
(1) Thickness and thickness uniformity,
(2) Percent shrinkage (MD, TD),
(3) Shrink strength (MD, TD),
(4) Tear resistance (Elmendorf),
(5) Impact strength (dart drop),
(6) Puncture resistance,
(7) Clarity (haze, gloss),
(8) Slip and anti-block, and
(9) UV resistance.
The LDPE shrink-wrapping process consists of bundling and heat-sealing the products loosely in the film. The flexible film circumference should exceed the product circumference by no more than 7-10%. The container is then transferred through a shrink tunnel or into an oven. Numerous heating systems are used, the best using hot circulating air because it provides more uniform heating.
The last two factors depend on film thickness and the product specifications to be packed. Both these characteristics affect the amount of heat required to reach the softening point of the film. Ultimately, it is worth indicating that the maximum shrink strength is performed when the film cools further the shrink tunnel and not inside. In fact, inside the oven, the film shrinks with a small shrink force because it is hot and soft. When it cools rapidly to room temperature outside the oven, the film shrinks tightly around the article, with a much higher shrink force.
Low-density polyethylene (LDPE film) is one of today’s multiple used plastics for shrink bundling. The polymerization causes of ethylene in high-pressure reactors. After that, the resin can be blown, slit, and wound to make film rolls, which plants increasingly use for diverse packaging purposes. LDPE’s solid and flexible properties in shrink bundling produce are vital to protecting, utilizing, and allocating packaged goods.