Post-processing of die-castings is a critical step in ensuring their stable performance and meeting usage requirements. It covers a series of processing and adjustment processes from demolding to final product. The content of post-processing varies depending on the purpose of the casting. For structural parts, aging treatment may be required to eliminate internal stress and prevent deformation during subsequent use. For example, aluminum alloy die-castings will generate internal stress due to uneven cooling rates during the die-casting process. By heating them to 120-180°C and holding them at this temperature for 2-4 hours, the internal structure can be stabilized and the internal stress can be significantly reduced. For load-bearing components, post-processing also includes mechanical property testing, such as tensile testing and hardness testing, to ensure that their strength, toughness and other indicators meet design standards. In addition, for die-castings that require assembly, post-processing requires dimensional accuracy re-inspection, and critical dimensions are inspected using precision equipment such as three-dimensional coordinate measuring machines to avoid dimensional deviations that affect assembly accuracy.
Surface treatment of die-castings is an important means of improving their appearance quality and performance. Its main purpose is to enhance corrosion resistance and wear resistance while beautifying the appearance. Common surface treatment processes are selected according to the casting material and the use environment. The most widely used treatment for aluminum alloy die-castings is anodizing, which forms an oxide film on the surface through electrolysis. This oxide film is not only dense and wear-resistant, but can also present different colors through dyeing to meet decorative needs. For example, aluminum alloy die-castings used in electronic equipment casings, after anodizing and dyeing, have good scratch resistance and a beautiful metallic texture. For zinc alloy die-castings, galvanizing is a common method. The zinc coating can effectively isolate air and moisture, preventing corrosion of the substrate. At the same time, the corrosion resistance of the coating can be further improved through passivation treatment.
In addition to traditional electroplating and oxidation processes, modern surface treatment technologies also include spraying, electrophoresis, and vapor deposition. The spraying process is suitable for die-castings with high weather resistance requirements, such as outdoor lighting housings. By spraying polyester powder coating, a uniform protective film is formed, which can resist erosion from ultraviolet rays, rain, and other factors, and has a service life of more than 10 years. Electrophoretic coating is suitable for castings with complex shapes. The coating adheres evenly to the surface under the action of an electric field, with a consistent coating thickness and good environmental performance. It is widely used in the surface treatment of automotive parts. Vapor deposition technologies, such as physical vapor deposition (PVD), can form a carbide coating on the surface of die-castings, significantly improving wear resistance. It is often used for die-castings with high strength requirements, such as mold accessories.
Pretreatment prior to surface treatment significantly impacts the final result. It is crucial to ensure that the die-casting surface is clean, free of oil, scale, and impurities. Pretreatment typically includes degreasing, pickling, neutralization, and phosphating. Degreasing can be performed with an alkaline solution or organic solvent to remove surface oil. Pickling uses a dilute acid solution to remove scale and rust. Neutralization eliminates residual acidic substances after pickling to prevent corrosion of the casting. Phosphating forms a thin phosphate film on the surface, enhancing the adhesion of subsequent coatings. For example, automotive wheel die-castings must undergo rigorous pretreatment before painting; otherwise, the coating is prone to peeling and blistering, affecting both appearance and service life.
The post-processing and surface treatment of die-cast parts also need to consider environmental and cost factors. Traditional electroplating processes produce wastewater containing heavy metals, which pollutes the environment. They are currently being replaced by more environmentally friendly processes such as chromium-free passivation and water-based paint spraying. At the same time, the choice of treatment process must take into account cost-effectiveness. For ordinary castings produced in large quantities, highly automated continuous processing lines can be used to improve efficiency and reduce unit costs. For high-precision, small-batch castings, refined processing processes are required to ensure that quality standards are met. For example, an automotive parts company introduced an automated electrophoretic coating line, which not only increased surface treatment efficiency by 40% but also reduced paint consumption by 30%. This reduced costs and pollutant emissions, achieving a win-win situation for environmental protection and efficiency.
