The requirements for die-casting coatings are fundamental to ensuring their effectiveness during the die-casting process. These requirements encompass multiple aspects, including performance, applicability, and environmental friendliness, and are directly related to casting quality, mold life, and production efficiency. First and foremost, die-casting coatings must exhibit excellent high-temperature stability, resisting decomposition and carbonization at the high temperatures of the molten metal (typically 600-800°C), maintaining stable physical and chemical properties. This is because if the coating is not sufficiently stable when in direct contact with the hot molten metal, it will decompose and produce gases. These gases can be drawn into the casting, forming pores. Furthermore, decomposition residues may adhere to the casting surface, causing inclusion defects. For example, coatings used for aluminum alloy die-casting must withstand temperatures of around 700°C. Their base material should be a heat-resistant silicone resin or ceramic powder to ensure a continuous protective film even at high temperatures.
Secondly, die-casting coatings must have excellent lubricity and mold release properties, effectively reducing the coefficient of friction between the molten metal and the mold cavity to ensure smooth demolding of the casting. Insufficient lubricity can cause the casting to adhere to the mold, resulting in strain, deformation, and even breakage during demolding, while also increasing mold wear. Therefore, the coating must contain an appropriate amount of lubricating ingredients such as graphite, molybdenum disulfide, and polytetrafluoroethylene. These substances form a lubricating film on the mold surface that significantly reduces frictional resistance. Mold release requires that the coating be able to quickly separate from the casting after solidification without causing adhesion. This requires the coating to have appropriate surface tension, ensuring that it adheres firmly to the mold without exerting excessive adhesion to the casting. For example, water-based graphite coatings can reduce demolding force by 30%-50% by adjusting the graphite particle size and content, significantly improving the demolding effect.
Die-casting coatings must not negatively impact the surface quality of the casting. This requires a thin, even coating, and easy-to-clean residue. Excessively thick coatings can cause pitting and sink marks on the casting surface, affecting appearance and dimensional accuracy. Difficult-to-clean residues increase the difficulty and cost of subsequent processing. Therefore, the coating must exhibit excellent film-forming properties, forming a uniform, thin layer of 5-20μm on the mold surface and resisting detachment after drying. Furthermore, the coating’s components must be easily volatile or decomposed. After the solvent and organic components evaporate at high temperatures, any remaining solid residues must be removable with simple cleaning (such as hot water rinsing or sandblasting). For example, coatings used for decorative die-castings often utilize low-residue formulations. After demolding, surface marks can be removed with a simple hot water rinse, eliminating the need for complex cleaning.
Die-casting coatings must also exhibit good compatibility with the mold material and die-casting alloy, resisting chemical reactions. Molds are often constructed of hot-work die steel (such as H13). Any reaction between the coating and the mold can corrode the mold surface, damaging its hardness and finish, and shortening its lifespan. Chemical reactions between the coating and the molten metal can also produce harmful compounds, affecting the chemical composition and mechanical properties of the casting. For example, acidic coatings used in magnesium alloy die-casting can react with magnesium to produce hydrogen, leading to porosity in the casting. Neutral coatings, however, can avoid this problem. Therefore, coating selection requires compatibility testing to ensure no adverse effects on the mold or casting.
Finally, die-casting coatings also have strict requirements in terms of use and environmental protection. In terms of use, the coating must be easy to store, mix and apply, have appropriate viscosity to suit spraying or brushing, and dry quickly (usually 5-15 minutes) without affecting the production rhythm. For automated production lines, the coating must also have good stability and no stratification or precipitation after long-term use. In terms of environmental protection, the coating should reduce the content of volatile organic compounds (VOCs) and toxic substances (such as heavy metals) and meet national environmental protection standards. At present, water-based coatings and solvent-free coatings have become mainstream due to their low VOCs and low toxicity. For example, the VOCs emissions of a certain brand of water-based coatings are only 10% of traditional solvent-based coatings, and do not contain heavy metals such as lead and chromium, which not only protects the health of operators but also reduces environmental pollution.
