Pay attention to the relationship between injection and pressure holding
Injection and holding pressure are two key stages in the injection molding process that are closely connected but have very different functions. Correctly distinguishing the relationship between the two is crucial to ensuring product quality. The injection stage is the process by which the melt fills the mold cavity through the runner and gate under the action of injection pressure. Its core function is “filling”. Sufficient pressure and speed are required to push the melt to overcome flow resistance and ensure that the mold cavity is completely filled. The holding pressure stage, on the other hand, continues to apply pressure after the mold cavity is full, so that the melt fills the space created by cooling and shrinkage in the mold cavity. Its core function is “shrinkage compensation”. The pressure is usually lower than the injection pressure, but it needs to be maintained for a certain period of time. The connection between the two needs to be smoothly transitioned. If the holding pressure is entered before the injection is completed, insufficient filling will result. If the holding pressure is started too late, the melt will lose fluidity due to cooling and solidification, and it will not be able to effectively compensate for shrinkage, resulting in sink marks.
The matching of pressure and time between injection and holding pressure varies significantly, requiring appropriate parameters based on the product structure. The injection pressure depends on factors such as melt viscosity, runner length, and part wall thickness. Thin-walled or complex parts require higher injection pressures (80-150 MPa) and speeds (50-100 mm/s) to quickly fill the cavity. Thick-walled parts can use lower pressures (50-80 MPa) and speeds (20-50 mm/s) to avoid turbulent air entrainment. Holding pressure is typically 50%-80% of the injection pressure. For example, if the injection pressure is 100 MPa, the holding pressure can be set to 50-80 MPa. Too high a pressure can easily lead to flash, while too low a pressure can lead to insufficient shrinkage compensation. The holding time is determined based on the part’s wall thickness. Thick-walled parts require longer holding times (10-30 seconds), while thin-walled parts require shorter holding times (2-5 seconds). The holding time ends when the melt solidifies at the gate. Terminating holding pressure too early can result in sink marks, while terminating it too late can increase the molding cycle.
Injection and holding pressure have different focuses on the impact of product performance, and targeted adjustments are required to optimize quality. The pressure and speed in the injection stage mainly affect the appearance of the product and the strength of the weld mark. High-speed injection can quickly fill the melt and reduce the occurrence of weld marks. However, if the speed is too fast, shear overheating will cause silver streaks or scorch marks on the surface of the product. The holding pressure stage mainly affects the density and internal stress of the product. Properly extending the holding time can increase the density of the product and reduce the shrinkage rate. However, excessive holding pressure will increase the internal stress of the product and make it prone to warping or cracking. For example, for structural parts requiring high strength, the holding pressure needs to be increased to increase the density; for transparent products, the holding pressure needs to be reduced to reduce the optical distortion caused by internal stress.
Determining the boundary between injection and holding pressure is crucial for parameter setting, with cavity fill typically used as the switching point. When the melt fills 95%-98% of the cavity, the injection phase should switch to the holding phase. The remaining space can then be filled with pressure packing. Fill can be determined by the injection molding machine’s shot volume feedback or pressure curve. A clear inflection point (a sudden increase in pressure) indicates the cavity is nearly full and holding pressure can be initiated. For equipment without metering feedback, a trial mold can be used to observe changes in part weight. When increasing the shot volume has little effect on part weight, the switching point has been reached. Properly setting the switching point can prevent overfilling caused by overinjection while ensuring effective pressure packing.
The coordinated optimization of injection and holding pressure requires the combination of raw material characteristics and mold structure to form a systematic parameter system. Crystalline plastics (such as PP and PA) have a large shrinkage rate during the cooling process, so the holding time needs to be extended and the holding pressure needs to be increased accordingly to compensate for the large volume shrinkage; non-crystalline plastics (such as PC and ABS) have a small shrinkage rate, so the holding time can be shortened and the pressure can be appropriately reduced. The gate size of the mold has a significant impact on the holding effect. Small gates (diameter 0.5-1mm) cool quickly and the holding time needs to be short, otherwise the holding pressure will be ineffective after the gate solidifies; large gates (diameter 2-5mm) cool slowly, which can extend the holding time and enhance the shrinkage compensation effect. By repeatedly adjusting the injection and holding parameters through mold trials, the optimal balance between product appearance, dimensional accuracy and mechanical properties can be achieved, reducing quality defects caused by the imbalance between the two.
