Die casting coatings
Die-casting coatings are specialized substances applied to surfaces such as the mold cavity and core during die-casting production. Their primary function is to reduce friction and adhesion between the molten metal and the mold surface, protecting the mold surface from direct erosion and corrosion by the hot molten metal. They also improve the release properties and surface quality of die-cast parts. Die-casting coatings typically consist of a base material, solvent, suspending agent, lubricant, and release agent. The coating’s formulation and properties vary depending on the type of die-casting alloy (such as zinc alloy, aluminum alloy, and magnesium alloy) and the mold’s operating conditions. High-quality die-casting coatings should exhibit excellent high-temperature stability, lubricity, mold release, and adhesion. They should also leave no residue on the mold surface after evaporation and should not adversely affect the performance of the die-cast part.
The primary functions of die-casting coatings are reflected in several aspects. First, lubrication and mold release. Once applied to the mold surface, the coating forms a uniform film. This film reduces the friction coefficient between the molten metal and the mold surface, reducing the adhesion between the die-casting and the mold, enabling smooth demolding and avoiding defects such as strain and deformation. For example, in aluminum alloy die-casting, without coating or with poor coating performance, the adhesion between the die-casting and the mold cavity can reach 5-10 MPa, making surface strain very likely during demolding. However, with high-quality coatings, the adhesion can be reduced to 1-2 MPa, significantly improving demolding effectiveness. Second, thermal insulation and protection are provided by the coating. The coating forms an insulating layer on the mold surface, slowing the heat transfer from the molten metal to the mold, reducing temperature fluctuations on the mold cavity surface, minimizing thermal fatigue damage, and extending the mold’s service life. Furthermore, the coating prevents high-temperature molten metal from directly eroding the mold surface, thus protecting it from erosion and wear.
There are many types of coatings for die casting. Based on their form, they can be divided into water-based and oil-based coatings. Based on their application, they can be categorized into cavity coatings, core coatings, and punch coatings. Water-based coatings use water as a solvent and offer advantages such as environmental friendliness, non-flammability, and low cost. However, their high-temperature stability is relatively poor, making them suitable for medium- and low-temperature die casting (such as zinc alloy die casting). Oil-based coatings use mineral oils, vegetable oils, and other solvents, offering excellent high-temperature stability and lubricity, making them suitable for high-temperature die casting (such as aluminum and magnesium alloy die casting). However, their cost is higher, and they emit high amounts of volatile organic compounds, which have a certain impact on the environment. Cavity coatings require excellent mold release and high-temperature stability, typically containing a high proportion of lubricants and high-temperature-resistant ingredients. Core coatings require excellent adhesion and lubricity to ensure close contact between the core and the inner surface of the die casting. Punch coatings are primarily used to lubricate the injection punch and the pressure chamber, reducing friction and wear between them, and typically contain a high proportion of solid lubricants (such as graphite and molybdenum disulfide).
The coating process for die-casting coatings significantly impacts their effectiveness. The main coating methods include spraying, brushing, and dipping. Spraying is the most commonly used method, suitable for coating large areas and complex cavities, ensuring uniform coating distribution and controllable coating thickness. Brushing is more flexible and suitable for localized areas or small molds, but offers less uniform coating thickness. Dip coating is more efficient for small cores or simple molds, but consumes more coating material. Coating thickness must be strictly controlled. Excessively thick coatings can cause surface defects such as pitting and bubbles in die-cast parts, and volatilization gases can enter the die-casting, forming pores. Excessively thin coatings ineffectively lubricate and protect, leading to demolding difficulties and mold wear. For example, in aluminum alloy die-casting, cavity coating thickness is typically controlled at 5-15μm, and punch coating thickness at 10-20μm. This is achieved by adjusting the spray pressure (typically 0.2-0.5MPa) and nozzle distance (150-300mm).
The selection and application of die-casting coatings must be tailored to the specific die-casting conditions. Different die-casting alloys require specialized coatings. For example, aluminum alloy die-casting requires high-temperature-resistant and anti-oxidation coatings, while zinc alloy die-casting can choose coatings with excellent lubricity and low cost. During production, the type of coating and coating process must be adjusted promptly based on the quality of the die-casting. If die-castings exhibit scratches or mold sticking, this may indicate insufficient lubricity or a thin coating. In this case, a coating with better lubricity should be replaced or the coating thickness should be increased. If defects such as pores and pitting appear on the die-casting surface, this may indicate excessive volatiles or an overly thick coating. In this case, a coating with a faster evaporation rate should be selected or the coating thickness should be reduced. Furthermore, proper storage and management of the coating are crucial. Avoid direct sunlight and high temperatures to prevent deterioration, and stir thoroughly before use to ensure uniform composition. Proper selection and application of die-casting coatings can significantly improve die-casting quality, extend mold life, and reduce production costs, making them an essential component of die-casting production.
