Shrink film is manufactured so that polymer chains are stretched and oriented during the extrusion process, then quickly cooled to lock that stretched, oriented state into the film before it can relax back to its original dimensions. The film stores that orientation as latent stress. When the wrapped package later passes through a heat tunnel, typically in the range of 200 to 300 degrees Fahrenheit depending on the resin, the heat allows the polymer chains enough mobility to relax back toward their unstretched state, and the film physically shrinks and tightens around whatever it's wrapped around.
Shrink film is widely used to bundle multipacks of bottles or cans, unitize cases on pallets, and wrap products for retail display, since it conforms tightly to irregular shapes in a way that a rigid package or a non-shrink film simply cannot. Polyolefin shrink film, often based on LDPE or LLDPE blends, is common for general bundling and multipack applications, prized for its toughness, clarity, and seal strength, while PVC and polypropylene-based shrink films are used in other specific applications, particularly some label and retail display uses.
Shrink force and shrink percentage, meaning how much tightening force the film generates and how much it physically shrinks in each direction when heated, are key specifications, and they're tested under controlled heating conditions since both depend heavily on the degree of orientation built into the film during manufacturing.
Glossary
Shrink Film
Film that has been stretched and oriented during production so it shrinks tightly around a product when heat is applied, commonly used to bundle multipacks.
Example
A six-pack of beverage cans held together by a single piece of film that's been heat-shrunk tightly around them, conforming to the shape of the cans, is a textbook polyolefin shrink film application.
Nuance
Shrink film and stretch film are sometimes confused because both involve stretching a film and both are used to bundle or wrap products, but they work in opposite ways. Shrink film is stretched and locked during manufacturing, then shrinks with applied heat at the point of use, while stretch film is stretched at the point of use itself, with no heat involved, and relies on elastic recovery and cling to hold a load together.