Changes in cavity pressure during injection molding
The change in cavity pressure during the injection molding process is a key parameter reflecting the melt filling, holding, and cooling processes within the cavity, directly impacting the density, dimensional accuracy, mechanical properties, and appearance quality of the plastic part. Cavity pressure refers to the pressure exerted on the cavity walls by the plastic melt under the action of injection pressure. Its variation is closely related to factors such as injection process parameters, mold structure, and plastic material properties. By monitoring and analyzing the cavity pressure curve, we can gain a deeper understanding of the melt’s flow state and solidification process within the cavity, providing a scientific basis for optimizing process parameters and improving mold structure, thereby improving the stability of plastic part quality.
The filling phase is the process in which the mold cavity pressure gradually increases from zero. This pressure change is primarily influenced by factors such as injection speed, melt temperature, mold temperature, and cavity structure. At the beginning of filling, the melt enters the cavity under the pressure of the injection. Because the cavity is initially at atmospheric pressure, the cavity pressure rises rapidly, but at a relatively slow rate. During this phase, the melt primarily fills the majority of the cavity at a relatively high rate. As the cavity gradually fills with melt, the resistance to melt flow increases, and the cavity pressure begins to rise rapidly. When the melt reaches the end of the cavity and fills the entire cavity, the cavity pressure reaches its peak during the filling phase, known as the peak filling pressure. The magnitude of the peak filling pressure depends on the volume and complexity of the cavity. For large and complex cavities, the peak filling pressure is typically higher, potentially reaching 50-150 MPa; for small and simple cavities, the peak filling pressure is relatively lower. Insufficient pressure during the filling phase can result in incomplete cavity filling, resulting in underfill defects. Excessive pressure can cause mold deformation and even flash.
The cavity pressure during the holding phase primarily manifests itself as pressure is maintained and then slowly decreased. This ensures continued melt replenishment into the cavity as the melt cools and shrinks, ensuring part density and dimensional accuracy. At the beginning of the holding phase, the cavity pressure typically remains near the peak fill pressure and then gradually decreases based on the set hold pressure and hold time. The hold pressure is typically between 50% and 80% of the fill pressure. Excessive hold pressure increases internal stress in the part, leading to warping and cracking after demolding. Excessive hold pressure fails to fully replenish the melt, making the part susceptible to defects such as shrinkage cavities and sink marks. The rate of change in cavity pressure during the holding phase is related to the melt cooling rate. In areas with faster cooling rates, the melt solidifies early and the pressure drops rapidly. In areas with slower cooling rates, the pressure drops more slowly. By monitoring changes in cavity pressure during the holding phase, it is possible to assess melt replenishment and part shrinkage, providing a basis for optimizing holding parameters.
During the cooling phase, the cavity pressure gradually drops to zero. At this point, the melt has begun to solidify and no more melt needs to be added to the cavity. The cavity pressure decreases primarily as the melt cools and shrinks. In the early stages of the cooling phase, the cavity pressure drops relatively slowly because the melt still has a certain degree of fluidity. As the melt temperature decreases, the viscosity increases, the fluidity deteriorates, and the cavity pressure drops faster. When the melt is completely solidified and a small gap appears between the plastic part and the cavity wall, the cavity pressure drops rapidly to zero. Changes in cavity pressure during the cooling phase reflect the solidification process and shrinkage of the melt. If the cooling rate is too fast, the cavity pressure drops rapidly, potentially leading to vacuum bubbles inside the plastic part. If the cooling rate is too slow, the production cycle will be extended, reducing production efficiency. Therefore, reasonable control of cavity pressure changes during the cooling phase requires appropriate cooling system design and cooling time settings to ensure that the plastic part solidifies smoothly as the pressure gradually releases.
The uniformity of cavity pressure distribution has a significant impact on the consistency of plastic part quality. In multi-cavity molds or large, complex cavities, cavity pressure can vary significantly due to varying melt flow distances and resistance at different locations. This can lead to inconsistent quality across cavities or across different parts of the same part. For example, in a multi-cavity mold, cavities near the gate tend to fill first, resulting in higher cavity pressure, while cavities farther from the gate may experience insufficient filling pressure, resulting in smaller parts. To improve cavity pressure uniformity, gating system design can be optimized, such as using balanced runners to ensure simultaneous melt flow to all cavities; injection process parameters can be adjusted, such as using segmented injection speeds to distribute pressure throughout the melt flow; and an appropriate venting system can be implemented to reduce pressure fluctuations caused by trapped air. Furthermore, pressure sensors can be installed at key locations in the mold to monitor cavity pressure in real time. Through feedback control, process parameters can be adjusted to achieve uniform cavity pressure distribution.
Cavity pressure monitoring and control technologies are widely used in modern injection molding production, providing strong support for improving part quality and production stability. Cavity pressure monitoring typically involves installing a pressure sensor inside the mold cavity or near the gate. The pressure signal is converted into an electrical signal, and the pressure curve is recorded in real time via a data acquisition system. By analyzing characteristic parameters of the pressure curve, such as peak fill pressure, hold pressure duration, and pressure drop rate, it is possible to determine whether the molding process is stable, detect anomalies, and make adjustments promptly. For example, a sudden increase in peak fill pressure may be due to a drop in melt temperature or mold cavity blockage, requiring prompt inspection of the heating system and cleaning of the cavity. A rapid drop in hold pressure may indicate insufficient hold time or an under-set hold pressure; in this case, the hold time should be increased or the hold pressure should be appropriately increased. With the development of intelligent injection molding technology, cavity pressure control systems now achieve closed-loop control with the injection molding machine. These systems automatically adjust parameters such as injection and hold pressure based on real-time pressure data, ensuring that cavity pressure remains within the optimal range and enabling precise control of part quality.
