Plastic bags are made of plastic as the main raw material. They are indispensable items in people's daily life and are often used to hold other items. Because of its low cost, light weight, large capacity and easy to collect, it is widely used, so why does the plastic bag shrink?
Shrinkage is the percentage of the difference between the size of the bag at the molding temperature and the size taken out of the mold and cooled to room temperature. It reflects the reduction of the size of the plastic part after it is taken out of the mold and cooled. The factors that affect the shrinkage of plastics are: the variety of plastics, forming conditions, mold structure, etc. The shrinkage rate of different polymer materials is different. The shrinkage rate of plastic is different from the shape of plastic, the complexity of internal structure and nesting.
1、 Effect of molding process on shrinkage of plastics (structure: synthetic resin, plasticizer, stabilizer, color material):
(1) The molding temperature remains unchanged, the injection pressure (increases, the shrinkage decreases;
(2) Improve maintenance pressure and reduce shrinkage;
(3) The shrinkage decreases with the increase of melt temperature;
(4) The mold temperature is high, the shrinkage increases;
(5) The shrinkage decreases with a long holding time, but it is not affected after the gate is closed;
(6) The mould has a long cooling time and a low shrinkage;
(7) The injection speed was fast, the contraction tendency increased slightly, and the shadow was small;
(8) After forming, the shrinkage is large. Plastic bags commonly used in food plastic bags are made of polyethylene film, the film is non-toxic, so it can be used to contain food. Another kind of film is made of PVC. PVC itself is non-toxic, but additives added according to the use of the film are often harmful to human body and have certain toxicity. So the plastic bags made of film and film are not suitable for food. If you want to distinguish PVC plastic bags and polyethylene plastic bags, you can use the following simple method for identification. It was larger in the first two days after contraction and stable in about a week.
II. Effect of plastic structure on shrinkage of plastic bags:
(1) The shrinkage rate of thick wall plastic parts is higher than that of thin wall plastic parts (but the shrinkage rate of most 1mm thick wall plastic parts is higher than 2mm because of the increase of melt resistance in mold cavity);
(2) The shrinkage of plastic parts with inserts is smaller than that without inserts;
(3) The shrinkage of complex plastic parts is smaller than that of simple plastic parts;
(4) The shrinkage in the height direction of the plastic part is generally smaller than that in the horizontal direction;
(5) The shrinkage of slender plastic parts is smaller in the length direction;
(6) The shrinkage of length dimension is smaller than that of thickness dimension;
(7) The shrinkage rate of inner hole is large and that of outer hole is small. Plastic bags are an indispensable part of people's daily life, often used to hold other things. It has the advantages of low cost, light weight, large capacity and convenient storage, and has been widely used. However, some countries have banned the production and use of plastic bags due to its long degradation cycle and difficult to handle.