Blow molding pressure is a method of using air pressure to inflate the hot melt parison in a closed mold to form a hollow product.

Abstract

Blow molding pressure, air pressure, refers to the air pressure blown into the parison during hollow blow molding. Here it mainly refers to hollow blow molding (also known as blow molding), which is a method of using gas pressure to inflate the hot melt parison closed in the mold to form a hollow product. It is the third most commonly used plastic processing method and also a plastic molding method with rapid development. The mold used for blow molding only has a female mold (concave mold). Compared with injection molding, the equipment cost is lower, the adaptability is stronger, the molding performance is good (such as low stress), and the products with complex undulating curves (shapes) can be molded.

Technology History

Blow molding originated in the 1830s. It was not until after 1979 that blow molding entered the stage of widespread application. During this period, blow molding plastics include: polyolefins, engineering plastics and elastomers; the application of blow molding products involves automobiles, office equipment, household appliances, medical treatment and other aspects; 60,000 bottles can be produced per hour, and large blow molding parts (weight up to 180kg) can be manufactured. Multilayer blow molding technology has been greatly developed; blow molding equipment has adopted closed-loop control systems of microcomputers and solid-state electronics, and computer CAE/CAM technology has become increasingly mature; and blow molding machinery is more specialized and more distinctive.

Blow molding method

Different blow molding methods have different advantages in processing different products due to differences in raw materials, processing requirements, output and cost. For detailed blow molding process, please refer to the literature. Here we introduce the characteristics of blow molding from a macro perspective. Blow molding of hollow products includes three main methods: extrusion blow molding: mainly used for unsupported parison processing; injection blow molding: mainly used for parison processing supported by metal cores; stretch blow molding: including extrusion-stretch-blow molding and injection-stretch-blow molding. It can process biaxially oriented products, greatly reducing production costs and improving product performance. In addition, there are multi-layer blow molding, compression blow molding, dip blow molding, foam blow molding, three-dimensional blow molding, etc. However, 75% of blow molded products are formed by extrusion blow molding, 24% by injection blow molding, and 1% by other blow molding; among all blow molded products, 75% are biaxially stretched products. The advantages of extrusion blow molding are high production efficiency, low equipment cost, and a wide range of mold and machinery selection. The disadvantages are high scrap rate, poor recycling and utilization of waste materials, limited thickness control of products, and limited dispersion of raw materials, and trimming operations must be performed after molding. The advantages of injection blow molding are that no waste is generated during the processing process, the wall thickness of the product and the dispersion of the material can be well controlled, the molding precision of the thin-necked product is high, the product surface is smooth, and small batch production can be carried out economically. The disadvantage is that the molding equipment cost is high, and to a certain extent, it is only suitable for small blow molded products. The process conditions of hollow blow molding require that the compressed air of the parison in the blowing mold must be clean. The air pressure for injection blow molding is 0.55 ~ 1MPa; the pressure for extrusion blow molding is 0.2l ~ 0.62MPa, and the pressure for stretch blow molding often needs to be as high as 4MPa. During the solidification of plastic, low pressure makes the internal stress of the product low, the stress dispersion is more uniform, and the low stress can improve the tensile, impact, bending and other properties of the product. 1.2 Product Types Blow molding products are divided into two categories: containers and industrial parts. Among them, containers include: packaging containers, large-volume storage barrels/tanks, and foldable containers. However, with the maturity of blow molding technology, there are more and more blow molding products for industrial parts, and the scope of application is becoming increasingly wide. At present, containers account for about 80% of the market share, with an annual growth of about 4%; while industrial and structural products account for 20% of the total, with an annual growth rate of 12%. The growth in container consumption lies in the continuous expansion of the application range of twistable plastic containers, and the growth in the consumption of industrial products is mainly due to the improvement of new processing technologies, such as multi-layer parison extrusion, biaxial extrusion, non-axisymmetric blow molding, etc. Table 2 lists the application and performance requirements of some blow molded products. Pressure is an aspect that is carried out simultaneously with our selection of a certain material and processing method, that is, our motivation, which here refers to pressure. The size of the pressure is closely related to the material and process we choose.

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