Injection molding process to improve dimensional accuracy of plastic parts
Dimensional accuracy of plastic parts is a key indicator of injection molded product quality, directly impacting their assembly performance and reliability. Improving dimensional accuracy in the injection molding process involves multiple steps, including raw material selection, mold design, and process parameter optimization. These steps require coordinated effort to achieve precise control of part dimensions. In today’s industrial production, as product precision requirements continue to rise, optimizing the injection molding process to improve dimensional accuracy has become a critical issue for the industry.
Raw material selection and pretreatment have a significant impact on the dimensional accuracy of plastic parts. Different types of plastics have different molding shrinkage rates, and shrinkage stability is a key factor in determining the dimensional accuracy of plastic parts. Therefore, plastic raw materials with low shrinkage and a narrow shrinkage fluctuation range, such as polystyrene and polymethyl methacrylate, should be selected. These plastics have good dimensional stability and are conducive to ensuring the dimensional accuracy of plastic parts. Crystalline plastics such as polyethylene and polypropylene experience significant volume changes and shrinkage fluctuations during crystallization, necessitating strict control of their crystallinity to minimize dimensional deviation. Raw material pretreatment requires thorough drying to remove moisture and volatiles. Moisture evaporates into gas at high temperatures, causing bubbles in the melt, affecting melt flow and filling efficiency, and ultimately, the dimensional accuracy of the plastic part. Raw material purity must also be ensured to avoid contamination with impurities or different plastic types, which can prevent uneven shrinkage caused by raw material inhomogeneity.
Mold design is fundamental to improving the dimensional accuracy of plastic parts. A reasonable mold structure can effectively reduce dimensional deviation in plastic parts. The mold cavity dimensions must be precisely calculated based on the plastic’s molding shrinkage to ensure that the part reaches the designed dimensions after cooling. The mold’s guide mechanism should be highly precise to ensure accurate alignment of the movable and fixed molds during mold closing, preventing dimensional deviation from mold misalignment. The location and size of the gate are also critical. The gate should be located in a thick-walled area or symmetrically at the center of the part to ensure uniform melt flow and minimize dimensional deviation caused by uneven filling. A gate size should be appropriate. Too small will result in excessive melt flow resistance and insufficient filling, while too large will prolong the gate solidification time and impair the pressure holding effect. Furthermore, the mold’s cooling system should be rationally designed to ensure uniform cooling across all parts of the part, minimizing shrinkage differences caused by uneven cooling and ultimately improving the part’s dimensional accuracy.
Optimizing process parameters is key to improving the dimensional accuracy of plastic parts. This involves properly setting parameters such as injection pressure, injection speed, holding pressure, holding time, and cooling time. Injection pressure directly affects the fill density and compaction of the melt. Properly increasing injection pressure allows the melt to more fully fill the mold cavity, reducing shrinkage and improving part dimensional accuracy. However, excessively high injection pressure can cause internal stress in the part, negatively impacting dimensional stability. Therefore, the appropriate injection pressure must be determined based on the part’s structure and raw material properties. The injection speed should be adjusted according to the part’s shape and wall thickness. For thin-walled parts, a faster injection speed should be used to prevent premature cooling of the melt during the filling process. For thick-walled parts, a slower injection speed can ensure even filling of the melt and reduce the formation of bubbles and shrinkage cavities.
The holding stage is particularly important for part dimensional accuracy. Properly setting the holding pressure and holding time effectively compensates for melt shrinkage during cooling. The holding pressure is generally set between 50% and 80% of the injection pressure, and the holding time should be long enough to ensure continuous melt replenishment into the cavity before the gate solidifies, compensating for shrinkage. Insufficient cooling time can cause the part to continue shrinking after demolding, affecting dimensional accuracy. Excessive cooling time can reduce production efficiency and increase costs. Therefore, the optimal cooling time should be determined based on the part’s wall thickness and thermal properties, ensuring that the part is sufficiently cooled and dimensionally stable upon demolding. Furthermore, controlling barrel and mold temperatures is crucial. Excessively high barrel temperatures reduce melt viscosity and increase fluidity, but also prolong cooling time and increase shrinkage. Excessively low mold temperatures can cause the melt to cool too quickly, making filling difficult and resulting in dimensional deviation. Barrel and mold temperatures should be appropriately set based on the type of plastic and part requirements to ensure a balance between melt fluidity and cooling rate.
Stability control during the production process is also crucial for improving the dimensional accuracy of plastic parts. The performance stability of injection molding machines directly impacts the consistency of process parameters. Therefore, regular maintenance and calibration are necessary to ensure the precise control of parameters such as injection pressure, temperature, and speed. The temperature and humidity of the production environment can also affect the dimensional accuracy of plastic parts. Excessive temperature fluctuations can cause changes in the plastic’s shrinkage rate, while high humidity causes hygroscopic plastics to absorb moisture, affecting their molding properties. Therefore, the production environment should maintain a constant temperature and humidity, generally between 20-25°C and 50%-60% relative humidity. Regular inspection of plastic part dimensions is also necessary during the production process to promptly identify dimensional deviations and correct them through adjustment of process parameters. The use of automated testing equipment can improve inspection efficiency and accuracy, facilitate timely feedback and adjustments, and thus stabilize the dimensional accuracy of plastic parts. Through these comprehensive measures, the dimensional accuracy of plastic parts can be significantly improved to meet the quality requirements of high-end products.
