The shrinkage rate of the material refers to the percentage of the difference between the size of the plastic bag at the molding temperature and the size after it is taken out of the mold and cooled to room temperature. It reflects the extent to which the plastic part is reduced in size after it is removed from the mold and cooled. The factors that affect the plastic shrinkage rate are: plastic variety, molding conditions, mold structure, etc. Different polymer materials have different shrinkage rates, and the shrinkage rate of plastics is also related to the shape of the plastic parts, the complexity of the internal structure, and whether there are inserts. 1. The influence of the molding process on the shrinkage rate of plastic (structure: synthetic resin, plasticizer, stabilizer, pigment) bag: food bag, food packaging bag (1) The molding temperature remains unchanged, the injection pressure increases, and the shrinkage rate decreases (2) The shrinkage rate decreases when the holding pressure increases; (3) The shrinkage rate decreases when the melt temperature increases; (4) The shrinkage rate increases when the mold temperature is high; (5) The shrinkage rate increases when the holding time is long; The shrinkage rate decreases, but the shrinkage rate is not affected after the gate is closed; (6) The cooling time in the mold is long, and the shrinkage rate decreases; (7) The injection speed is high, the shrinkage rate tends to increase slightly, and the effect is small; (8) The molding shrinkage is large, and the post-shrinkage is small. Food plastic bags commonly used in plastic bags are mostly made of polyethylene film, which is non-toxic and can be used to contain food. There is also a film made of polyvinyl chloride, which itself is non-toxic, but the additives added according to the use of the film are often harmful to the human body and have certain toxicity. Therefore, such films and plastic bags made of the films are not suitable for containing food. To identify polyvinyl chloride plastic bags and polyethylene plastic bags, the following simple method can be used for identification. The post-contraction was two days old at first and stabilized for about a week. 2. The influence of plastic structure on the shrinkage rate of plastic bags: (1) The shrinkage rate of thick-walled plastic parts is larger than that of thin-walled plastic parts (but most plastic 1mm thin-walled parts have a larger shrinkage rate than 2mm, because the melt ( (2) The shrinkage rate of plastic parts with inserts is smaller than that without inserts; (3) The shrinkage rate of plastic parts with complex shapes is smaller than that of simple shapes; ( 4) The shrinkage rate of the plastic parts in the height direction is generally smaller than that in the horizontal direction; (5) The shrinkage rate of the slender plastic parts in the length direction is smaller; (6) The shrinkage rate of the length direction of the plastic parts is smaller than that of the thickness direction; ( 7) The inner hole shrinkage rate is large, and the shape shrinkage rate is small. Plastic bags are an indispensable item in people’s daily life and are often used to hold other items. It is widely used because of its advantages of low cost, extremely light weight, large capacity, and easy storage. However, due to the shortcomings of extremely long degradation cycle and difficult disposal of plastic bags, it has been banned by some countries for production and use. 3. The influence of mold structure on plastic (structure: synthetic resin, plasticizer, stabilizer, pigment) bag shrinkage: (1) The gate size is large, the shrinkage rate is reduced; (2) The vertical gate direction shrinks The shrinkage rate decreases, and the shrinkage rate increases in the direction parallel to the gate; (3) The shrinkage rate farther from the gate is smaller than that near the gate; (4) The shrinkage rate of the plastic part with mold limitation is small, and the unrestricted plastic part has a small shrinkage rate. The shrinkage rate is large. Vacuum bag packaging materials are usually three-sealed packaging bags made of double-layer composite films or three-layer aluminum thin composite films. Two types of vacuum packaging machines are commonly used: cavity type and external pumping type. China’s vacuum packaging technology was developed in the early 1980s, and vacuum inflatable packaging technology began to be used in the early 1990s. Fourth, the influence of plastic properties on the shrinkage rate of plastic bags: (1) The shrinkage rate of crystalline plastics (structure: synthetic resin, plasticizer, stabilizer, pigment) is greater than that of amorphous plastics; (2) Plastics with good fluidity, The molding shrinkage rate is small; (3) When fillers are added to the plastic, the molding shrinkage rate decreases significantly; (4) The molding shrinkage rate of different batches of the same plastic is also different. The outer shell of the food packaging bag can separate the food from oxygen, water vapor, stains, etc., and anti-leakage is also a necessary factor in packaging design. Some packages include desiccants or deoxidizers to extend shelf life. Vacuum packaging or de-airing of packaging bags is also the main method of food packaging. Keeping food clean, fresh and safe within the shelf life is the primary function of the bag. [vacuum bag] [aluminum foil bag]