Metal die casting principle and die casting process
Metal die casting is a precision casting method in which molten metal is rapidly forced into a mold cavity under high pressure, where it solidifies under pressure. Its principle is based on the fluidity and plasticity of metal. Through the constraints of the mold and the application of pressure, castings with complex shapes, high precision, and excellent surface quality are achieved. The core principles of metal die casting include high-pressure filling and pressure solidification. High-pressure filling enables the molten metal to overcome flow resistance and quickly fill the complex mold cavity, ensuring a clear outline of the casting. Pressure solidification reduces metal shrinkage during solidification, improving the density and mechanical properties of the casting. Different metal materials have different die-casting characteristics. For example, zinc alloy has good fluidity and low die-casting temperatures, making it suitable for thin-walled, complex parts. Aluminum alloy has high strength and low density, making it widely used in the automotive and aviation industries. Copper alloy has good thermal conductivity and strong corrosion resistance, and is often used in electrical appliances and bathroom products. Understanding the principles of metal die casting is important for optimizing the die-casting process and improving casting quality.
The metal die-casting process primarily involves pre-casting preparation, mold closing, injection molding, holding pressure, pressure relief, mold opening, and ejection, each of which significantly impacts casting quality. Pre-casting preparation is crucial and includes cleaning and preheating the mold, melting and degassing the metal, and adjusting die-casting machine parameters. Mold cleaning involves removing scale and residue from the cavity surface to prevent it from affecting the casting’s surface quality. Preheating the mold to a certain temperature (typically 150-300°C) reduces the cooling rate of the molten metal, improves fluidity, and avoids defects such as cold shuts and under-casting. The melting of metal requires strict temperature and composition control. For example, the melting temperature of aluminum alloys is generally 650-700°C. Refining agents are added during the melting process to remove gases and impurities, thereby improving the purity of the molten metal. Adjustment of die-casting machine parameters, including injection speed, injection pressure, holding pressure, and holding time, must be appropriately set based on the structure and material properties of the casting.
Mold closing and injection molding are key steps in the die-casting process. During mold closing, the dynamic and fixed molds of the die-casting machine are tightly fitted together under the action of the clamping force, forming a closed cavity. This clamping force must be sufficient to prevent the molten metal from overflowing from the parting surface during injection. The injection process is divided into two stages: slow injection and fast injection. During the slow injection stage, the molten metal enters the injection chamber smoothly under low pressure to avoid eddy currents and air entrainment. When the molten metal approaches the gate, the injection molding process switches to fast injection, allowing the molten metal to fill the cavity at a higher speed (typically 5-15 m/s) to ensure that the entire cavity is filled before solidification. The injection pressure is determined by the complexity and wall thickness of the casting. Thin-walled complex parts require higher injection pressures (up to 100-200 MPa) to ensure effective filling. Thick-walled parts can use lower injection pressures to reduce mold wear. The control accuracy of injection speed and pressure directly affects the filling quality of the casting. Modern die-casting machines often use computer-controlled servo systems to precisely adjust injection parameters.
The holding and unloading stages have a significant impact on the density and dimensional accuracy of the casting. When the molten metal fills the mold cavity, a certain holding pressure must be maintained so that the unsolidified molten metal can continue to replenish the mold cavity, compensate for the shrinkage of the metal, and reduce defects such as shrinkage cavities and shrinkage. The holding pressure is usually 50%-80% of the injection pressure. The holding time is determined according to the wall thickness of the casting. Thick-walled parts have a longer holding time to ensure that the center part can be fully solidified. After the holding pressure is completed, the unloading stage begins, and the pressure is gradually reduced to avoid cracks in the casting due to a sudden drop in pressure. The unloading process needs to be smooth, and the pressure drop rate should not be too fast. It is usually controlled within a certain range to ensure the structural integrity of the casting.
Mold opening and ejection are the final steps in the die-casting process, directly impacting the quality of the casting’s demolding and production efficiency. Once the casting has fully solidified within the mold cavity and cooled to a certain temperature, the die-casting machine drives the movable mold to open the mold. The mold opening speed must be moderate; too fast a speed may cause adhesion or damage to the casting and the mold surface, while too slow a speed will extend the production cycle. The ejection mechanism operates during or after mold opening to eject the casting from the mold cavity. The ejection force must be evenly distributed, and the ejection point should be located in a rigid area of the casting to avoid deformation or damage during ejection. For castings with complex structures that are prone to mold sticking, multiple ejections or delayed ejection can be used to ensure smooth demolding. After ejection, the casting needs to be cleaned to remove excess parts such as gates and flash. Heat treatment and surface treatment are required to improve the mechanical properties and appearance of the casting.
The metal die-casting process is a complex physical and chemical process, influenced by a variety of factors, such as molten metal temperature, injection parameters, mold temperature, and mold structure. With technological advancements, intelligent die-casting systems are increasingly being used. These systems use sensors to monitor key parameters in the die-casting process in real time, combined with artificial intelligence algorithms for dynamic adjustment. This allows for precise control of the die-casting process, improving the quality stability and production efficiency of castings. Furthermore, the application of numerical simulation technology allows for virtual simulation of the die-casting process before actual production, predicting potential defects and optimizing process parameters and mold structure, providing strong support for efficient, high-quality production of metal die-casting. A deep understanding of the principles and processes of metal die-casting is crucial for promoting the advancement and application of die-casting technology.
