Refurbishment and processing of die castings
The finishing and treatment of die-castings is a crucial step after die-casting production. Its purpose is to remove surface defects, improve dimensional accuracy, and ensure that the castings meet assembly and use requirements. The finishing and treatment process requires a targeted plan based on the casting’s material, structure, and defect type. Common treatments include removing burrs, flash, gates, risers, and other excess parts, correcting casting deformation, and repairing surface defects. For aluminum alloy die-castings, due to their soft texture and easily oxidized surface, excessive force that could cause deformation must be avoided during the finishing process. Dedicated finishing tools are typically used for fine processing. For zinc alloy die-castings, due to their lower hardness, greater attention must be paid to controlling the force during finishing to prevent scratches or dents on the surface.
Removing burrs and flash is a fundamental process in die-casting finishing. During the die-casting process, burrs and flash often form on the casting surface due to gaps between the molds or overflowing molten metal. These unwanted parts not only affect the casting’s appearance but can also interfere with assembly and even injure operators. Deburring methods include manual filing, mechanical grinding, and vibration finishing. Manual filing is suitable for small castings or complex areas. It removes burrs manually using tools like files and sandpaper. While highly flexible, it is inefficient and its quality is significantly affected by operator skill. Mechanical grinding, using equipment such as grinding wheels and grinders, polishes the casting surface. It is suitable for large-scale production and can quickly remove larger burrs and flash. However, it is important to control the grinding force to avoid excessive grinding and dimensional deviations. Vibration finishing involves placing the casting, abrasive, and abrasive fluid in a vibrating tank. Vibration causes the abrasive to rub against the casting surface, removing burrs and smoothing the surface. It is suitable for complex shapes and small castings, can process multiple parts simultaneously, and is highly efficient without damaging the casting surface.
Correcting casting deformation is a critical step in ensuring dimensional accuracy. Die castings are prone to deformations such as bending and twisting during solidification due to uneven temperature distribution and shrinkage. This is particularly problematic for thin-walled and slender castings. The main methods for correcting deformation include mechanical correction and heat treatment. Mechanical correction uses specialized correction tools or dies to apply external force to the deformed area to restore it to its correct shape. For example, a bent aluminum alloy bracket can be corrected slowly to the required dimensions by applying reverse pressure using a press. Mechanical correction requires careful control of the force and duration to avoid cracking the casting due to overcorrection. Heat treatment correction is suitable for temperature-sensitive alloys, such as magnesium alloy die castings. By heating the casting to a specific temperature (usually below the alloy’s recrystallization temperature), holding it for a period of time, and then slowly cooling it, the metal’s plastic recovery at high temperatures is utilized to eliminate internal stresses and achieve deformation correction. Heat treatment correction can simultaneously improve the casting’s internal structure and mechanical properties, but the process is lengthy and relatively expensive.
Removing gates and risers is a crucial part of die-casting refinishing. The gate is the passage for molten metal to enter the mold cavity, while the riser compensates for shrinkage during the solidification process. After die-casting, these sections need to be removed from the casting. The removal method depends on the size and material of the casting. Small castings can be removed by manual sawing or shearing, while large castings require specialized cutting machines, punching machines, and other equipment. For castings with more demanding requirements, the remaining sections after gate and riser removal require grinding and polishing to ensure a surface roughness consistent with the rest of the casting to avoid affecting assembly and appearance. For example, after removing the gate from an automobile engine block, the interface needs to be polished with a grinding wheel to create a smooth surface and ensure sealing with other components.
Surface treatment of die-castings is also a crucial step in the finishing process, aiming to enhance the casting’s corrosion resistance, wear resistance, and decorative qualities. Common surface treatment processes include electroplating, anodizing, painting, and plastic spraying. Aluminum alloy die-castings are typically anodized, forming an oxide film on the surface through electrolysis. This not only improves corrosion resistance but also allows for dyeing to impart different colors for decorative purposes. Zinc alloy die-castings are often treated with electroplating processes, such as zinc and chrome plating, to enhance surface hardness and wear resistance. For die-castings used in harsh environments, painting or plastic spraying can be used to form a dense protective film to isolate them from corrosive media. Before surface treatment, the casting surface must be thoroughly cleaned to remove impurities such as oil, scale, and other impurities to ensure secure adhesion of the treated coating. For example, die-castings intended for use in marine environments require phosphating before being coated with an anti-corrosion coating to achieve long-term salt spray corrosion resistance.
