Engineering plastics injection molding process injection process - plasticization control

The injection molding process of engineering plastics mainly includes three stages: preparation before molding, injection process, and post-treatment of plastic parts. The injection process generally includes several processes, such as material addition, plasticization, mold filling, pressure holding, backflow, cooling, and demolding. However, from the viewpoint of the change of plastic state in the injection molding process, there are only two processes: plasticization and melt molding. The following is an introduction to the plasticization control of engineering plastics injection process. Plasticization refers to the process in which engineering plastic raw materials are heated in the barrel to a flowing state and have good plasticity. Therefore, it can be said that plasticization is the preparation process of injection molding. As far as the production process is concerned, the general requirements for this process are: engineering plastics raw materials should reach the specified molding temperature before entering the mold cavity, and can provide a sufficient amount of molten plastic within a specified period of time, the temperature of the melt at all points should be uniform, no or very little thermal decomposition to ensure the quality of engineering plastics products.

Injection process of engineering plastics injection molding process - plasticization control

Since injection molding is an intermittent process, it is important to keep the quantity of material added to keep the operation stable and the plasticization uniformity in order to obtain qualified products. Too much material and too long heating time will easily cause thermal degradation of the melt and increase the power loss of the injection molding machine; too little material, the lack of pressure transfer medium in the barrel, the pressure of the plastic melt in the mold cavity will be reduced, making it difficult to make up the plastic, which will easily cause shrinkage, depression, hollow and other defects.

Plasticization quality and preplasticization quantity of injection molding machine are mainly determined by process parameters, such as barrel temperature, nozzle temperature, screw speed, preplasticization back pressure, metering device metering, etc.

(1) Barrel temperature. This is one of the key process parameters to ensure the quality of plasticization, and its selection is related to the characteristics of the processed plastic. For amorphous plastics, the barrel temperature should be controlled above its viscous flow temperature; for crystalline plastics, it is controlled above the melting point temperature, but the maximum barrel temperature of both must be controlled below their decomposition temperature. In actual production, the control of barrel temperature is not only related to the type and performance of the plastic, but also related to the structure of the product, the equipment used, the structure of the mold, the operation time and other factors. The distribution of the barrel temperature is lower near the hopper, and the temperature rises gradually in the direction of the nozzle, so that the material is gradually plasticized in the barrel, but when the moisture content in the raw material is high, the temperature at the back end can be increased appropriately to facilitate the early discharge of moisture. For shear-sensitive plastics, if the screw injection molding machine, due to the frictional heat of shear screw helps plasticize the plastic, the temperature of the front section of the material can be appropriately lower than the middle section to prevent the melt from overheating degradation.

(2) nozzle temperature. Injection molding machine nozzle temperature is generally lower than the front of the barrel temperature, if the temperature is too high, prone to salivation, if the temperature is too low, it is easy to generate condensate blocking the nozzle, or become cold material into the mold cavity, affecting the quality of the product. Nozzle temperature selection according to plastic varieties, mold structure and other process parameters to consider.

(3) back pressure. This refers to the injection molding screw in the back of a certain resistance, so that the end of the screw melt to build up a certain pressure, this pressure is known as back pressure. This pressure can be achieved by adjusting the back pressure valve in the hydraulic system. In the plasticizing process, the size of the back pressure depends on the screw structure, the quality of the product, the variety and characteristics of the raw material, etc. At a certain screw speed, increasing the back pressure will increase the temperature of the melt and its uniformity, which is conducive to the discharge of gas from the melt and the uniformity of color, but will reduce the plasticizing rate.

For high viscosity plastics, such as PC, PPO, PSF, etc., the screw speed and back pressure should not be too high, so as not to increase the load on the screw; for thermally effective plastics, such as HPvC, POM, etc., too high back pressure will promote the superheated degradation of the melt, so the screw speed and back pressure should not be too high. In general, the back pressure should not be greater than 2MPa.