1.Blow molding
Blow molding also known as hollow blow molding, is a rapidly evolving plastic processing method. The tubular plastic parison obtained by extrusion or injection molding of thermoplastic resin is heated (or heated to a softened state), placed in a split mold, and compressed air is introduced into the parison immediately after closing the mold to blow the plastic parison It is inflated and adhered to the inner wall of the mold, and is cooled and released to obtain various hollow products. The manufacturing process of the blown film is very similar in principle to the blow molding of hollow products, but it does not use a mold. From the perspective of classification of plastic processing technology, the molding process of blown film is usually included in the extrusion. The blow molding process began 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.
Hollow containers can be up to several thousand liters in volume, and some have been computer controlled. Suitable plastics for blow molding include polyethylene, polyvinyl chloride, polypropylene, polyester, etc., and the resulting hollow containers are widely used as industrial packaging containers.
According to the parison making method, blow molding can be divided into extrusion blow molding and injection blow molding, and newly developed ones have multi-layer blow molding and stretch blow molding.
2.Injection molding
Injection molding of thermoplastics by melting the plastic material and then injecting it into the membrane cavity. Once the molten plastic enters the mold, it is shaped into a shape by the cold cavity.
The resulting shape is often the final product and no further processing is required prior to installation or use as a final product. Many details, such as bosses, ribs, threads, can be formed in one injection molding operation.
Injection molding machines have two basic components: an injection device and a clamping device for melting and feeding plastic into the mold. The function of the mold device is as follows: 1. The mold is closed while receiving the injection pressure; 2. The product is taken out of the injection device to melt the plastic before it is injected into the mold, and then the pressure and speed are controlled to inject the melt into the mold. There are two types of injection devices currently in use: a screw pre-plasticizer or a two-stage device, and a reciprocating screw. The screw pre-plasticizer uses a pre-plasticized screw (first stage) to inject molten plastic into the shot bar (second stage).
The advantages of the screw pre-plasticizer are constant melt mass, high pressure and high speed, and precise injection volume control (using mechanical thrust stops at both ends of the piston stroke). These strengths are required for transparent, thin-walled products and high production rates. Disadvantages include uneven residence time (resulting in material degradation), higher equipment costs, and maintenance costs.
The most common reciprocating screw injection device does not require the plunger to melt and inject the plastic.
