Blow molding, also known as hollow blow molding, is a rapidly developing plastic processing method. During the Second World War, the blow molding process began to be used to produce low density polyethylene vials. 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 hollow containers can reach thousands of liters, and some production has adopted computer control. The 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 parison production method, blow molding can be divided into extrusion blow molding and injection blow molding. The newly developed ones are multi-layer blow molding and stretch blow molding.
Injection stretch blow molding
At present, injection stretch blow molding technology is more widely used than injection blow molding. This blow molding method is also injection blow molding, but it only increases the axial tension, making blow molding easier and reducing energy consumption. The volume of the products that can be processed by injection drawing and blowing is larger than that by injection blowing. The volume of the container that can be blown is 0.2-20L, and its working process is as follows:
1. The principle of injection molding is the same as that of ordinary injection molding.
2. Then turn the parison to the heating and temperature regulation process to make the parison soft.
3. Turn to the pull-blowing station and close the mold. The push rod in the core stretches the parison along the axial direction, while blowing air to make the parison close to the mold wall and cool.
4. Transfer to demoulding station to take parts
Note - pulling - blowing process:
Injection molding parison → heating parison → closing, drawing and blowing → cooling and taking parts
Schematic diagram of mechanical structure of injection, drawing and blowing
Extrusion blow molding
Extrusion blow molding is one of the most widely used blow molding methods. Its processing range is very wide, from small products to large containers and auto parts, aerospace chemical products, etc. The processing process is as follows:
1. First, melt and mix the rubber, and the melt enters the machine head to become the tubular parison.
2. After the parison reaches the predetermined length, the blow molding mold is closed and the parison is clamped between the two halves of the mold.
3. Blow air, blow the air into the parison, blow the parison to make it close to the mold cavity for molding.
4. Cooling products
5. Open the mold and take away the hardened products.
Extrusion blow molding process:
Melting → extruding parison → mold closing and blow molding → mold opening and part taking
Schematic diagram of extrusion blow molding principle
(1 - extruder head; 2 - blow mold; 3 - parison; 4 - compressed air blow pipe; 5 - plastic parts)
Injection blow molding
Injection blow molding is a molding method that combines the characteristics of injection molding and blow molding. At present, it is mainly applied to drink bottles, medicine bottles and some small structural parts with high blowing accuracy.
1. In the injection molding station, the mold embryo is injected first, and the processing method is the same as that of ordinary injection molding.
2. After the injection mold is opened, the mandrel and parison move to the blow molding station.
3. The mandrel puts the parison between the blow molding molds and closes the mold. Then, the compressed air is blown into the parison through the middle of the mandrel, and then it is blown to make it close to the mold wall and cooled.
4. When the mold is opened, the mandrel is transferred to the demoulding station. After the blow molding part is taken out, the mandrel is transferred to the injection station for circulation.
Working process of injection blower:
Blow molding parison → injection mold opening to film blowing station → mold closing, blow molding and cooling → rotating to demoulding station to take parts → parison
Schematic diagram of injection blow molding principle
Advantages and disadvantages of injection blow molding:
advantage
The product has relatively high strength and high precision. There is no joint on the container and no need to repair. The transparency and surface finish of the blow molded parts are good. It is mainly used for hard plastic containers and wide mouth containers.
shortcoming
The equipment cost of the machine is very high, and the energy consumption is large. Generally, only small containers (less than 500ml) can be formed. It is difficult to form containers with complex shapes and elliptical products.
Whether it is injection blow molding, injection pull blow molding, extrusion pull blow molding, it is divided into one-time molding and twice molding process. The one-time molding process has high automation, high precision of parison clamping and indexing system, and high equipment cost. Generally, most manufacturers use the twice molding method, that is, molding the parison first through injection molding or extrusion, and then putting the parison into another machine (injection blow machine or injection pull blow machine) to blow out the finished product, with high production efficiency.
Post time: Mar-22-2023