Environmental requirements for injection molding

To complete the intensive cultivation of injection molding processing, it is very important to master the on-site environment of plastic parts molding, such as the trend of temperature, pressure, time and molecular structure, the trend of fiber filler and its molecular crystallization during the molding process, all of which will affect the specification accuracy of injection molded parts. The shrinkage and shaking of injection molded parts caused by the molding environment is the root cause of the specification deviation of injection molded parts.

First, the influence of mold shell temperature. The temperature in injection molding has a great influence on the accuracy of product specifications, especially the mold shell temperature. Mold temperature is based on the orientation and crystallinity of the molten polymer, which directly affects the shrinkage of the product, causing specification deviations. For amorphous resins, the mold temperature is high, the solution is cooled slowly, the molecules have enough time to relax, the crystallization tends to be more complete, the degree of crystallinity is high, and the shrinkage rate increases. For crystalline resins, high mold temperatures can reduce aftershrinkage and stabilize product specifications throughout the injection molding process. If the mold temperature is low, the cooling is fast, and the molecular structure of the solution is frozen and cleaned, it is not easy to crystallize, the volume change is small, and the product shrinkage is small. However, the low mold temperature increases the post-shrinkage of the product and prolongs the shrinkage time, so it is not easy to stabilize the specifications of the injection molded parts. It is a sensitive factor in the whole crystallization process, and the crystallization rate can be several times different with a temperature difference of 1°C. Uneven or unstable mold temperature will immediately affect crystallinity, causing shrinkage of injection molded products and causing specification deviation.

The second is the influence of injection pressure. Variations in injection pressure lead to convex product dimensions. When other process parameters remain unchanged, increasing the injection pressure can increase the charging activity of the melt, increase the active length, accelerate the charging speed, make the product dense and reduce shrinkage, but it is easy to generate directional stress. When the pressure is low, the product cannot be fully compacted and the shrinkage rate increases.

Third, the impact of pressure. Maintaining pressure plays an important role in stabilizing product dimensions. In the injection molding process, increasing the pressure protection can supplement and compact the shrinkage of the product in the mold and reduce the shrinkage rate. However, if the pressure protection pressure is too high, the residual stress will increase when the product is demolded, resulting in deformation or cracking, which will affect the size of the product.

Fourth, the impact of injection time. The shorter the injection time, the greater the shrinkage rate and the greater the change in shrinkage rate. When the injection time reaches or exceeds the injection solidification time, the quality and shrinkage of the injection molded product will not change even if the injection time is prolonged. Injection time control is closely related to the designed gate thickness, which largely controls the closing time of the gate.

Fifth, the impact of holding time. The longer the holding time, the better the melt is for filling and compacting the plastic. The higher the injection molding density, the smaller the shrinkage. When the gate is sealed, the compression protection is not effective in reducing the shrinkage of the injection molded product, and the compression protection time is too long to prolong the molding cycle.