Shrinkage film packaging was first applied in France in 1936 and began to be widely used in the 1960s. The most common production methods for PVC are circular die extrusion and blowing method or narrow and elongated die extrusion and 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, it is rarely used. Shrinkage film is made by stretching polymer sheets or pipes at temperatures above the softening point to orient polymer chains, and fixing this orientation with cooling methods. When in use, as long as the film is packaged and heated to the temperature during 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, and is also related to the composition of the polymer and the production process conditions. The total shrinkage of the shrink film can reach 15% to 80%, while general use only requires 15% to 30%. Strange shaped products require a shrinkage rate of more than 50%. Generally, a shrink film with a balanced shrinkage rate in both vertical and horizontal directions is required. However, for packaging and shrink label films of tubular items, only unidirectional upward shrinkage is required
The shrinkage temperature is generally between 65.5~176-6 ℃ (150T~350 ℃), which varies with the type of film. For example, the shrinkage temperature of PVC shrinkage film is 655~148.8 ℃ (150F~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. The principle is to use the minimum amount of heat and time to achieve sufficient contraction.
