Blow molding is also called hollow blow molding, a rapi […]
Blow molding is also called hollow blow molding, a rapidly developing plastic processing method. The tubular plastic parison obtained by extrusion or injection molding of the thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state). After the mold is closed, compressed air is injected into the parison to blow the plastic parison It expands and clings to the inner wall of the mold, and after cooling and demolding, various hollow products are obtained.
The manufacturing process of blown film is very similar in principle to blow molding of hollow products, but it does not use molds. From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion. The blow molding process was used to produce low-density polyethylene vials during the Second World War. In the late 1950s, with the birth of high-density polyethylene and the development of blow molding machines, blow molding technology was widely used. The volume of the hollow container can reach thousands of liters, and some production has adopted computer control. Plastics suitable for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc. The resulting hollow containers are widely used as industrial packaging containers.
According to the manufacturing method of the parison, blow molding can be divided into extrusion blow molding and injection blow molding, and the newly developed multi-layer blow molding and stretch blow molding.
Blow molding machines can be divided into three categories: extrusion blow molding machines, injection blow molding machines and special structure blow molding machines. Stretch blow molding machines can belong to each of the above categories. Extrusion blow molding machine is a combination of extruder, blow molding machine and mold clamping mechanism. It is composed of extruder and parison die, inflation device, mold clamping mechanism, parison thickness control system and transmission mechanism. The parison die is one of the important components that determine the quality of blow molded products, and there are usually side feed die and central feed die. When large-scale products are blow-molded, the storage cylinder type billet die is often used. The storage tank has a minimum volume of 1kg and a maximum volume of 240kg. The parison thickness control device is used to control the wall thickness of the parison. The control points can reach up to 128 points, generally 20-30 points. The extrusion blow molding machine can produce hollow products with a volume ranging from 2.5ml to 104l.
Injection blow molding machine is a combination of injection molding machine and blow molding mechanism, including plasticizing mechanism, hydraulic system, control electrical appliances and other mechanical parts. Common types are three-station injection blow molding machine and four-station injection blow molding machine. The three-station machine has three stations: prefabricated parison, inflation and demoulding, and each station is separated by 120°. The four-station machine has one more preforming station, and each station is 90° apart. In addition, there is a double-station injection blow molding machine with a 180° separation between stations. The plastic container produced by the injection blow molding machine has precise dimensions and does not require secondary processing, but the cost of the mold is relatively high.