Die casting process
The die-casting process is a systematic process that integrates metal melting, high-pressure filling, rapid solidification, and mold coordination. Precise control of each link directly affects the quality and production efficiency of die-cast parts. The entire process can be divided into three main stages: preliminary preparation, die-casting cycle, and post-processing. Each stage is closely linked to form an efficient production process.
Preliminary preparation is essential for a smooth die-casting process, primarily involving smelting the molten metal and preheating the mold. The composition of the molten metal must be strictly controlled based on the material requirements of the die-cast part. For example, aluminum alloy die-casting requires a silicon content of 6%-12% to enhance fluidity. Gases and impurities are also removed from the molten metal. This can be achieved through treatment with refining agents (such as hexachloroethane) and static precipitation, ensuring a molten metal purity exceeding 99.5%. Mold preheating requires the use of specialized heating equipment (such as an electric heating plate or heat gun) to gradually raise the mold temperature to the operating range. Zinc alloy molds should be preheated to 150-200°C, aluminum alloy molds to 200-250°C, and copper alloy molds to 300-400°C to avoid thermal shock from contact between the cold mold and the hot molten metal. Furthermore, the die-casting machine’s injection, clamping, and hydraulic systems must be checked for proper function, ensuring that parameters such as injection force and clamping force meet process requirements.
The die-casting cycle is the core process, consisting of seven steps: mold closing, pouring, injection, holding pressure, cooling, mold opening, and ejection. The entire cycle typically takes 10 to 60 seconds. During the clamping phase, the movable and fixed molds are tightly closed under the clamping force. This force is calculated based on the projected area of the die-casting part, typically 4 to 8 MPa x projected area, to prevent molten metal leakage. During the pouring phase, molten metal, reaching the required temperature (650 to 700°C for aluminum alloys), is poured into the injection chamber. The amount poured must be precisely controlled. Too much will cause flash, while too little will result in underfill. The injection phase is divided into two stages: slow and fast. The slow stage (0.1 to 1 m/s) steadily pushes the molten metal into the sprue. The fast stage (5 to 50 m/s) allows the molten metal to fill the mold cavity at high speed, ensuring that the complex structure is fully filled before solidification. During the holding phase, 50% to 80% of the injection pressure is continuously applied to push the unsolidified molten metal to fill the shrinkage space. The holding time is determined by the wall thickness of the die-cast part: 0.5 to 2 seconds for thin-walled parts and 3 to 10 seconds for thick-walled parts. During the cooling phase, heat is removed through the mold water channels, causing the molten metal to solidify within 1 to 10 seconds. During the mold opening phase, the movable and fixed molds separate, and the core pulling mechanism operates synchronously to extract the core. During the ejection phase, the ejector pushes the die-cast part out of the cavity, completing the die-casting cycle.
The mold plays a critical role in the die-casting process, with its structure and condition directly impacting the resulting die-casting. The mold cavity must perfectly match the die-casting’s shape, with a surface roughness of Ra 0.8-1.6μm to ensure surface quality. The pouring system guides the molten metal to fill the mold in an orderly manner. The position and shape of the ingrowth are optimized through simulation to prevent deformation caused by molten metal impacting the core. The exhaust system (vent groove depth 0.05-0.1mm) promptly removes air and volatiles from the cavity to prevent porosity in the die-casting. Furthermore, the mold’s guide mechanism (with a clearance of 0.02-0.05mm between the guide pin and guide sleeve) ensures precise clamping accuracy, while the core-pulling mechanism handles complex structures such as side holes and undercuts. All components must work together and precisely synchronize with the die-casting machine’s movements.
Dynamic adjustment of injection parameters is key to ensuring die-casting quality. The injection speed directly affects the filling state of the molten metal. Too low a speed can easily cause a cold shut, while too high a speed can cause gas entrainment. Therefore, the injection speed must be adjusted based on the die-cast part structure. For example, thin-walled parts like mobile phone midframes require a high speed of 15-25 m/s, while thick-walled parts like automotive cylinder blocks require a medium speed of 5-10 m/s. The injection pressure must overcome both the flow resistance of the molten metal and the forces of solidification shrinkage. Aluminum alloy die-casting typically requires 50-100 MPa, while copper alloys require 80-150 MPa. Mold temperature uniformity is equally important. A zoned temperature control system is used to maintain temperature differences within ±10°C across the mold to prevent deformation of the die-cast part due to uneven cooling. In actual production, parameters are continuously optimized through trial molds. For example, a die-casting project for automotive wheels reduced the filling time by 0.2 seconds and increased the die-casting qualification rate by 8%.
Post-processing steps ensure final quality improvement for die-cast parts, primarily including cleaning, trimming, inspection, and surface treatment. The cleaning process removes flash and burrs from the die-cast surface, either manually or through automated robotic cleaning, ensuring an edge roughness of Ra ≤ 3.2μm. The trimming process corrects deformation in die-cast parts by applying pressure at room temperature using a specialized fixture to control deformation to within 0.1mm/m. Inspection uses a coordinate measuring machine to check dimensional accuracy (tolerances are controlled within IT7-IT10), and X-rays are used to detect internal porosity and shrinkage, ensuring that the die-cast parts meet design requirements. Surface treatment is performed as needed. For example, aluminum alloy die-castings can undergo anodizing (to increase wear resistance), electrophoretic coating (to enhance corrosion resistance), or electroplating (to enhance appearance). Zinc alloy die-castings are often passivated (to prevent oxidation). Through rigorous post-processing, the performance and appearance of die-castings are further enhanced to meet the needs of diverse applications.
