Commonly used die casting coatings and their uses
Commonly used die-casting coatings can be divided into several types based on their main ingredients and uses. Each type has its own unique properties and applicable scenarios. The correct selection and use of these coatings is key to ensuring smooth die-casting production. Graphite-based coatings are one of the most widely used types. They are made with natural or artificial graphite as the main functional ingredient, supplemented by binders and solvents. They have excellent lubricity and high-temperature resistance and are suitable for high-temperature die-casting processes such as aluminum alloys and copper alloys. Graphite itself has good thermal conductivity and chemical stability, and can form a stable solid lubricating film on the mold surface, effectively reducing adhesion and wear between the molten metal and the mold. For example, in the die-casting of aluminum alloy wheels, graphite-based coatings can withstand temperatures exceeding 700°C, allowing for smooth demolding of the casting while reducing thermal fatigue damage to the mold and extending the life of the mold.
Water-based coatings use water as a dispersion medium and are made with various functional additives (such as lubricants, film-forming agents, and preservatives). They are environmentally friendly, safe, and non-flammable, and are widely used in medium- and low-temperature die-casting scenarios such as zinc alloys and magnesium alloys. Compared with traditional solvent-based coatings, water-based coatings have low emissions of volatile organic compounds (VOCs), pose less harm to operators and the environment, and are relatively low in cost. Water-based coatings dry quickly, drying in 10-15 minutes at room temperature. If the ambient humidity is high, drying can be accelerated by heating. For example, the water-based emulsified wax coating used in the die-casting of zinc alloy toy accessories not only has an excellent demolding effect, but also forms a thin wax film on the surface of the casting, reducing subsequent cleaning steps and improving production efficiency.
Solvent-based coatings, using organic solvents (such as kerosene and alcohol) as carriers and containing mineral oils, resins, and other ingredients, are known for their strong adhesion and long-lasting lubrication. They were once widely used in the die-casting industry. However, their high VOC content and environmental impact have led to a gradual decline in their use, with limited application in specialized applications requiring extremely high coating performance, such as die-casting of large, thick-walled castings. The advantages of solvent-based coatings include fast drying speeds, drying in 3-5 minutes under well-ventilated conditions, and excellent coating toughness and erosion resistance. For example, in copper alloy die-casting, due to the high pouring temperatures (900-1100°C), high heat and erosion resistance are required of the coating. Therefore, solvent-based ceramic coatings are still used in limited quantities to ensure effective protection of the mold surface.
Ceramic-based coatings, with ceramic powders (such as alumina and zirconia) as their primary functional ingredients, offer excellent heat and wear resistance, withstanding the intense erosion and high-temperature corrosion of molten metal. They are suitable for die-casting large, thick-walled castings or high-melting-point alloys. The coatings formed by ceramic-based coatings have high hardness and low thermal conductivity, which reduces the heat load on the mold while also resisting the mechanical impact of molten metal, significantly extending mold life. For example, in the production of large stainless steel die-castings, ceramic-based coatings can remain stable at temperatures exceeding 1000°C, extending mold life by 2-3 times. However, ceramic-based coatings are relatively costly and brittle, making them unsuitable for mass production where mold life is critical.
The application of common die-casting coatings requires adherence to standardized procedures to ensure effectiveness. First, the coating is mixed. Different types of coatings require a diluent (water or a specialized solvent) added in appropriate proportions and stirred until uniform and free of sediment. For example, graphite-based coatings require an appropriate amount of water and stirring for 10-15 minutes to ensure even dispersion of the graphite particles. Water-based coatings require the dilution ratio adjusted according to the ambient temperature, with less dilution required at lower temperatures. Application methods include spraying, brushing, and dipping. Spraying is suitable for large areas and complex cavities and can be applied manually or with automated equipment. Controlled spray distance (15-25cm) and pressure (0.2-0.4MPa) are crucial to ensure uniform coating. Brushing is ideal for corners and deep cavities, ensuring adequate coverage. Dipping is suitable for small, simple molds and is easy to use, but consumes more paint. After application, ensure the coating is thoroughly dried to avoid residual moisture or solvent outgassing at high temperatures. Furthermore, after each die-casting cycle, inspect the coating for wear and apply recoat promptly to ensure the mold surface remains fully protected. For example, an aluminum alloy die-casting company reduced the sticking rate of castings from 8% to 1% and extended the mold life by 15% by standardizing the use process of graphite-based coatings.
