Injection system structure and parameters

Structural composition of the injection system

The main components of the injection system include plasticizing injection device, screw drive device and metering device, etc..

(1) Plasticizing injection device

It mainly includes screw, barrel (barrel) and heating elements. A non-return ring or various shearing and mixing heads can also be set at the head of the screw to prevent the melt from flowing back.

① Screw:

The injection screw has the function of plasticizing the plastic material and injecting the melt into the mold cavity. When plasticizing, the heating of the barrel and the rotation of the screw make the rubber material change from a solid state to a viscous melt during the forward conveying process; when injecting, the screw moves forward to remove the melt at the front end of the barrel Inject into the mold cavity. The structure and working characteristics of the screw have a great influence on the production efficiency of the injection molding machine and the quality of the injection molded products.

The working characteristics of the screw: the screw moves axially during the plasticizing and injection processes; the work of the screw is intermittent, and the plasticizing process of the screw is an unsteady melting process, and the plasticizing process is only a process of preparing molten material for injection; After the screw conveys and plasticizes, it still stays in the front section of the barrel for a period of time, and can still continue to receive the heating of the barrel for further plasticization.

Screw structure: In order to meet the processing requirements of plastics with different properties, the screw is designed in different structural forms. The currently used screw structure forms are: gradual change type, sudden change type and universal type.

Screw material: divided into full hard screw (SKD61, SKH-91 or HPT full hardened powder alloy steel), stainless steel screw (9Cr18Mov), double alloy screw (base material 38CrMALA or SACM645), nitride screw (base material 38CrMALA), Tungsten carbide screw (surface covered with cobalt and tungsten carbide composite alloy powder coating), chrome-plated screw, etc. Ordinary screws from our suppliers are generally stainless steel screws or nitrided screws; for fiber-reinforced materials or materials above V0, double alloy screws or tungsten carbide screws are generally used.

The screw classification is as follows:

1.Gradient screw is a screw with a longer compression section (melting section), which is characterized by a relatively gentle conversion of energy during plasticization. As shown in Table 7(a), the compression section L2 is longer, and the depth of the screw groove in this section gradually becomes shallower, and the energy conversion is relatively gentle during plasticization. It is suitable for processing plastics with a wide range of melt temperatures, because such plastics require sufficient plasticizing process to completely melt. Gradient screw is suitable for processing amorphous plastics with wide softening temperature range and high viscosity, such as PS and PVC.

2.The mutation screw is a screw with a relatively small compression section, which is characterized by a relatively severe energy conversion during plasticization. As shown in Table 7(b), the compression section L2 is short, and the groove depth of the screw in this section changes sharply from deep to shallow, and the energy conversion is relatively severe during plasticization. It is suitable for processing plastics with a narrow melt temperature range, because such plastics can be softened and melted in a short time as long as they are preheated enough to reach the melting point. The mutant screw is suitable for processing polyamide and polyolefin crystalline plastics.

3.The length of the compression section of the universal screw is between the gradual and abrupt screw. In order to avoid replacement of screws for different types of plastic materials, general-purpose screws are usually used. At this time, as long as the process parameters (such as barrel temperature, screw speed and back pressure, etc.) are properly adjusted, the molding requirements of different plastic materials can also be met. As shown in Chart 7(C), the length of the compression section L2 is generally 4 to 5 pitches. At the same time, it is suitable for the processing of crystalline and non-crystalline plastics. It not only considers that non-crystalline materials cannot withstand the excessive shear plasticization of sudden-type screws in the compression section, but also considers that crystalline materials need sufficient preheating to soften and melt. process characteristics.

In order to avoid degradation of the rubber compound due to local overheating, the screw needs to be cooled and controlled during production. At the beginning of the manufacture of injection molding machines, most of them used mineral oil to flow into the center hole of the screw for cooling; but now heat pipes are generally used to cool the screw, which is characterized by a capillary liquid suction core or a special oil tank placed on the inner wall of the screw to penetrate the working fluid. When the heat pipe is working, the evaporating section of the heat pipe located in the metering section of the screw is heated, and the medium enclosed in it is sublimated into vapor and continuously flows back from the evaporating section to the condensing section of the heat pipe located inside the plasticizing section of the screw to be cooled to liquid, and the liquid is re- Through the action of capillary, it continuously flows to the evaporation section to form a cycle. The medium in the heat pipe includes water, oil and some organic solvents.

② Screw head

During screw injection, for low-viscosity rubber, it is necessary to prevent problems such as low injection efficiency, large pressure loss, and difficulty in maintaining pressure caused by melt backflow; for high-viscosity rubber, it is necessary to solve the problem of incomplete discharge during injection. Therefore, the structural form of the screw head has a non-reverse screw head, a screw head to prevent the decomposition of stagnant material, and a special-purpose screw head.

③ Material barrel: 

The material barrel is generally an integral structure, there is a certain gap between the inner hole of the material barrel and the screw, and the heating and cooling device is installed outside. Generally, 38CrMOALA surface nitriding is used, and tungsten carbide or HPT fully hardened powder alloy steel is used for fire-proof V0 grade or fiber-reinforced engineering plastics.