Shrink film packaging was first applied in France in 1936 and became widely used in the 1960s. The most common production method is to use circular die extrusion inflation method or narrow die extrusion casting method. For PVC, rolling method and solvent casting method can also be used for production. The film thickness uniformity is good, but due to the large investment in production equipment and high production cost, they are rarely used. Shrink film is made by stretching polymer sheets or pipes at a temperature above the softening point to orient polymer chains, and fixing this orientation with cooling method. When in use, as long as this film is used to package the item and then heated to the temperature required for stretching, the film will shrink, releasing the stress and energy stored during stretching. The shrinkage of the film is determined by the degree of stretching during the orientation process, as well as the composition of the polymer and the process conditions during production. The total shrinkage of shrink wrap can reach 15% to 80%, while general use only requires 15% to 30%. Products with unique shapes require a shrinkage rate of over 50%, and shrink wrap with balanced longitudinal and transverse shrinkage rates is generally needed. However, for packaging of sleeve shaped items and shrink label films, only unidirectional shrinkage is required
The shrinkage temperature is generally between 65.5 and 176-6 ℃ (150T to 350 ℃), depending on the type of film. For example, the shrinkage temperature of PVC shrink film is 655 to 148.8 ℃ (150F to 300 ℃), while the shrinkage temperature of P is 1655C (330F). Some heat shrink films have good temperature tolerance and can be heated multiple times to obtain packaging with different shrinkage rates.
