Pressure changes and pressure peaks at each stage of the die casting process
The die-casting process is a complex physical process involving high pressure and high speed. Pressure fluctuations directly affect the filling and solidification of the molten metal, as well as the final quality of the die-cast part. The entire die-casting process can be divided into four main stages: slow shot, fast shot, pressure holding, and pressure relief. Pressure fluctuations in each stage have their own unique patterns and effects, and the occurrence of pressure peaks is a key characteristic of the filling stage. A thorough understanding of these pressure fluctuations is crucial for optimizing die-casting process parameters and improving the quality of die-cast parts.
The pressure changes relatively smoothly during the slow-speed injection phase. Its primary purpose is to smoothly push the molten metal to the front of the shot chamber, avoiding the entrainment of air and slag. During this phase, the injection pressure is low, typically 5-15 MPa, and rises slowly over time, primarily to overcome the flow resistance of the molten metal within the shot chamber. During this phase, the molten metal moves forward at a relatively low speed (0.1-0.5 m/s), with a stable liquid surface and no severe turbulence. The pressure must be adjusted based on the diameter of the shot chamber and the viscosity of the molten metal. Excessive pressure may cause the molten metal to backflow at the shot chamber inlet, forming vortices. Excessive pressure prevents the molten metal from being smoothly pushed to the desired position, impacting the subsequent rapid-speed injection phase.
The rapid injection stage is the stage with the most dramatic pressure changes and is also the critical period for the emergence of pressure peaks. When the front end of the molten metal approaches the gate, the injection speed rapidly increases to 3-10m/s, and the injection pressure also rises sharply, reaching its maximum value in a very short time, forming a pressure peak. The magnitude of this pressure peak is typically 50-150MPa. Its function is to overcome the flow resistance of the molten metal in the mold cavity and ensure that the molten metal can fill the entire cavity before solidification. The occurrence time and peak value of the pressure peak have a significant impact on the quality of the die casting: a peak that is too high may cause the mold to bear excessive impact loads, accelerate mold wear, and even produce flash; a peak that is too low may lead to insufficient filling, resulting in defects such as cold shut and insufficient pouring. The formation of the pressure peak is closely related to factors such as injection speed, mold runner design, and molten metal temperature. The characteristics of the pressure peak require precise control of these parameters.
Pressure fluctuations during the holding phase are characterized by a stable, typically 60%-80% of the peak pressure. After the molten metal fills the mold cavity, the holding pressure continues to act on the partially solidified metal, compensating for its solidification shrinkage and preventing defects such as shrinkage cavities and porosity. The holding time during the holding phase depends on the wall thickness of the die-cast part, with longer holding times (3-5 seconds) for thick-walled parts and shorter holding times (1-2 seconds) for thin-walled parts. If the holding pressure is insufficient or the holding time is too short, the molten metal will not be adequately compensated during solidification, resulting in pores forming within the casting. If the holding pressure is too high or the holding time is too long, internal stresses in the casting will increase, increasing the risk of subsequent deformation. Pressure stability is crucial during the holding phase; even slight pressure fluctuations can affect the compensation effect. Therefore, modern die-casting machines typically utilize closed-loop control technology to ensure stable holding pressure.
The pressure change during the decompression phase manifests as a rapid drop. Its purpose is to reduce the pressure in the injection chamber after the casting has essentially solidified, preparing for mold opening and ejection. The decompression process must proceed smoothly, and the pressure drop should not be too rapid, otherwise it may cause cracks or bubbles within the casting. Typically, the decompression time is controlled within 0.5-1 second, and the pressure drops from the holding pressure to near atmospheric pressure. Although the pressure change during the decompression phase is brief, it still affects the final quality of the casting: if the pressure is released too early, the casting may deform during ejection due to incomplete internal pressure release; if the pressure is released too late, the production cycle will be extended and production efficiency will be reduced. By properly setting the decompression parameters, the production rhythm can be improved while ensuring the quality of the casting.
Monitoring and controlling pressure fluctuations is a crucial tool for optimizing the die-casting process. Using a pressure sensor installed on the injection cylinder, the pressure curve can be collected in real time throughout the die-casting process. By analyzing the pressure curve’s morphology (such as the height of the pressure peak, the stability of the holding pressure, and the pressure relief rate), the stability of the die-casting process can be determined and abnormal process parameters can be detected promptly. For example, an abnormally high pressure peak may be caused by a runner blockage or excessive injection speed; excessive fluctuations in the holding pressure may be due to a leak in the hydraulic system. Based on this monitoring data, parameters such as the injection speed, shot pressure ratio, and holding time can be adjusted to optimize the pressure profile, thereby stabilizing the production process and improving the yield rate of die-cast parts. With the development of intelligent die-casting technology, adaptive control algorithms have been applied to pressure regulation, automatically adjusting process parameters based on real-time pressure fluctuations to achieve precise pressure control.
