The relationship between plastic part quality and injection time
There is a close and complex relationship between plastic part quality and injection molding time. As a key parameter in the injection molding process, injection molding time directly affects the filling, cooling, and solidification of the melt within the mold cavity, which in turn has a significant impact on multiple aspects of the plastic part, such as dimensional accuracy, mechanical properties, and surface quality. During the injection molding process, injection molding time generally consists of three main components: injection time, holding time, and cooling time. The rational allocation and precise control of these three components are important prerequisites for ensuring stable plastic part quality. For example, an injection time that is too short may result in insufficient melt filling, causing defects such as missing material and obvious weld marks in the plastic part; while insufficient cooling time will result in insufficient cooling of the plastic part, leading to deformation after demolding and affecting dimensional accuracy. Therefore, a deep understanding and mastery of the relationship between plastic part quality and the various components of injection molding time is of great significance for optimizing the injection molding process and improving product quality.
Injection time refers to the time it takes for the melt to enter the mold cavity from the nozzle until the preset fill volume is reached. Its length directly affects the melt filling efficiency and the surface quality of the molded part. Within a certain range, appropriately extending the injection time can allow the melt to more fully fill every corner of the mold cavity, reducing defects such as material shortages and bubbles. It also facilitates pressure transfer within the mold cavity, improving the density and compactness of the molded part. However, excessively long injection time increases the residence time of the melt in the barrel, potentially causing plastic degradation and resulting in quality issues such as black spots and silver streaks on the part surface. It also reduces production efficiency and increases production costs. For different types of plastic parts, the injection time setting needs to be adjusted according to factors such as structural complexity, wall thickness, and plastic fluidity. For example, thin-walled parts require a shorter injection time to prevent premature cooling of the melt during the filling process, while thick-walled parts require a longer injection time to ensure sufficient melt filling.
The holding time refers to the duration of pressure applied after the melt fills the mold cavity to compensate for shrinkage due to cooling. It has a crucial influence on the dimensional accuracy and density of the plastic part. During the holding phase, appropriately extending the holding time allows more melt to enter the mold cavity, compensating for volume shrinkage caused by cooling, thereby reducing defects such as sink marks and dents in the part and improving dimensional stability. However, excessive holding times can increase internal stress in the part, making it more susceptible to cracking and deformation. It also increases the clamping force and shortens the mold’s lifespan. The holding time setting is closely related to the wall thickness of the part. Thick-walled parts cool more slowly, requiring a longer holding time to ensure full solidification in the center. Thin-walled parts, on the other hand, cool faster, allowing for a shorter holding time. Furthermore, the shrinkage rate of the plastic plays a crucial role in determining the holding time. Plastics with higher shrinkage rates require longer holding times to compensate for this shrinkage.
Cooling time refers to the time required for a plastic part to cool within the mold cavity, fully solidify, and reach demolding conditions. It has a direct impact on the part’s shape stability and production efficiency. Sufficient cooling time allows the part to fully cool and solidify, ensuring it will not deform after demolding. It also reduces internal stress and improves the part’s mechanical properties. If the cooling time is insufficient, the part’s temperature will be too high, and deformations such as warping and twisting may occur due to external forces during demolding, seriously affecting product quality. However, excessive cooling time significantly reduces production efficiency, increases energy consumption, and raises production costs. The cooling time setting depends primarily on the part’s wall thickness, the plastic’s thermal properties, and the efficiency of the mold’s cooling system. Thick-walled parts typically require longer cooling times, while molds with efficient cooling systems (such as conformal cooling channels) can shorten cooling times, improving production efficiency while ensuring part quality.
In actual production, repeated trials and optimization are required to determine the optimal combination of injection time, holding time, and cooling time to achieve a balance between part quality and production efficiency. With the development of injection molding technology, modern injection molding machines are equipped with advanced time control and monitoring systems that can precisely control the time parameters of each stage and dynamically adjust them through feedback from real-time monitoring of part quality. For example, in-mold sensors are used to monitor the cooling temperature of plastic parts. When the temperature reaches a preset value, the cooling time is automatically terminated to ensure consistent cooling effects. An adaptive control system automatically adjusts the holding time of the next mold based on the part quality data of the previous mold, enabling continuous optimization of process parameters. In the future, with the in-depth application of intelligent and digital technologies, injection time control will be even more precise, and the correlation model between part quality and injection time will be even more complete, providing strong support for achieving high-quality and efficient injection molding production.
