Injection Molding of Thermoplastics

　　Thermoplastic injection molding is a process in which plastic material is melted and injected into a mold cavity.

　　The melted plastic enters the mold, and it is cooled to form a certain shape according to the mold cavity. The resulting shape is often the final product and requires no further processing before installation or use as a final product. Many details, such as bosses, ribs, and threads, can be formed in a single injection molding operation.

　　An injection molding machine has two basic components: the injection unit for melting and feeding the plastic into the mold, and the clamping unit. The function of the mold clamping device is 1. Make the mold close under the injection pressure; 2. Take the product out of the injection device to melt the plastic before injecting it into the mold, and then control the pressure and speed to inject the melt into the mold.

　　Two designs of injection units are currently used: screw pre plasticizers or two-stage units, and reciprocating screws. Screw pre-plasticizers use a pre-plasticizing screw (first stage) to inject molten plastic into the injection rod (second stage).

　　The advantages of screw pre-plasticizers are the constant quality of molten mass, high pressure and high speed, and precise injection volume control (using mechanical thrust devices at both ends of the piston stroke). These advantages are required for transparent, thin-walled products and high production rates. Disadvantages include non-uniform residence time (leading to material degradation), higher equipment costs, and maintenance costs. The most commonly used reciprocating screw injection device does not need a plug to melt and inject the plastic.