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At present, the shrinkage film used for packaging mostly requires a shrinkage of about 50% in both longitudinal and transverse directions; a special film for sealing at the bottle cap also requires a one-way shrinkage rate of 25% to 50%, both horizontally and vertically. The shrinkage ratio is not equal, and the ratio is called the shrinkage ratio. The lower the shrinkage temperature, the better, and the adverse effect on the articles (especially fresh foods, fiber products, etc.) is less. 
Because the packaged articles need to be sealed on both sides or sides before entering the heat-shrink tunnel for shrinkage, the heat-sealability of the film should also be good, and sufficient heat-seal strength is required. 
After repeated trials and comparisons, we chose polyethylene (PE) and polyvinyl chloride (PVC) transparent shrink plastic films as shrink packaging materials.
Third, method selection
Shrink packaging operations are generally performed in two steps. The first is pre-packaging, packaging the product with a shrink film, heat-sealing the necessary mouth and seams, and then heat-shrinking to heat the pre-packaged product in a heat-shrinking device. 
1. Pre-packaged work. When pre-packaged, the film size should be 10% to 20% larger than the size of the device. If the size is too small, the filling is inconvenient, and the shrinkage tension is too large, and the film may be punctured; if the size is too large, the shrinkage tension is not enough and the package is not tight or smooth.
The maximum shrink film used can be determined based on the size and weight of the device and the required contraction tension. There are three types of shrink packaging methods:
1 Open at both ends: If you use a tubular film, you can use the slide slot to push the device into the tube; if you use a flat film, you need to roll up and down two flat films. After the previous package has been cut off and sealed, the two films are sealed and the product is then manually pushed into the upright film by machine or hand. After being in place, the closure mechanism falls, and the other side of the device is heat sealed and simultaneously sealed. shear. After heat shrinking, both ends of the package shrink to form an oval opening. 
2 Sealed on all sides: Wrap around the product (when flat film is used) or both ends (when using tube film). For equipment packaging that requires good sealing.
The L-type sealing method can be used for the folded film. In this method, the film is folded on the roll, the film is pulled out to a certain length in a horizontal position, the open end is stretched by means of a mechanism or by hand, and the equipment is pushed to the crease. Before this, the last heat-sealed cut is left. The next transverse joint, plus a total of two seams, does not need to be sealed. Then use a "L" type heat-sealing shear to press down from the back of the equipment product and the film joint and heat-seal the shear seam to complete one longitudinal joint at a time. Easy to operate, consignment or semi-automatic can be; with a single roll flat film can be pillow-shaped bag. (To be continued)
Shrink packaging and transport packaging (center)
Shrinkage includes longitudinal and transverse aspects: The test method is to first measure the original length of the film L1, then immersed in 120 °C glycerin 1 ~ 2s, and then cooled with water; measured length L2, then: Shrinkage (% ) = (L1-L2)/L1 x 100% where: L1 - the length of the film before shrinking: L2 - the length of the film after shrinking.