Shrink ratio of heat shrinkable film

①Shrinkage rate and shrinkage ratio The shrinkage rate includes longitudinal and transverse directions. The test method is to measure the film length L1 first, then immerse the film in glycerin at 120°C for 1 to 2 seconds, take it out and cool it with cold water, and then measure the length L2. Ll is the length of the film before shrinking; L2 is the length of the film after shrinking. At present, the shrink film used for packaging generally requires the same shrinkage in both longitudinal and transverse directions, about 50%; but in special cases, there are also unidirectional shrinkage, with a shrinkage rate of about 25% to 50%. There are also partially stretched films with unequal shrinkage rates in the longitudinal and transverse directions. The ratio of the shrinkage ratio in the vertical and horizontal directions is called the shrinkage ratio. ②Shrink tension Shrink tension refers to the tension applied to the package after the film shrinks. The shrinkage tension generated at the shrinkage temperature is closely related to the protection of the product. Rigid products such as packaging metal cans allow greater shrinkage tension, while some fragile or wrinkled products will be deformed or even damaged if they shrink too much. Therefore, the shrinkage tension of the shrink film must be selected appropriately. ③Shrinking temperature The shrinking film reaches a certain temperature and starts to shrink after heating, and the temperature rises to a certain height to stop shrinking. The temperature in this range is called the shrinkage temperature. For packaging operations, the package is heated in the heat shrinkage channel, and the temperature reached when the film shrinks to produce a predetermined tension is called the shrinkage temperature under the tension. The shrinkage temperature has a certain relationship with the shrinkage rate, and various films are different. In shrink packaging, the lower the shrinking temperature, the less adverse effects on the packaged products, especially fresh vegetables, fruits and textiles. ④ In heat-sealable shrink packaging operations, two or three sides heat sealing must be performed before heat shrinking, and the sealing seam is required to have high strength. The commonly used sealing methods for packaging include heat sealing, pulse fusion sealing, radiation sealing, and ultrasonic sealing. The heat sealing method is the most common method. During sealing, a flat plate (or disc) heated to a certain temperature is pressed on the superimposed film to seal. In order to prevent thermal shrinkage of the film during sealing, it should be sealed quickly, and it is best to take timely cooling measures immediately after sealing.