Requirements for die casting coatings
Die-casting coatings must first possess excellent high-temperature stability, maintaining their performance under the extreme temperatures of the die-casting process without decomposition, carbonization, or failure. During the die-casting process, the surface temperature of the mold cavity rises dramatically as the molten metal fills. For example, during aluminum alloy die-casting, mold temperatures can reach 200°C to 300°C, while the molten metal reaches temperatures as high as 650°C to 720°C. The coating, in direct contact with the hot molten metal, must be able to withstand such thermal shocks. If the coating’s high-temperature stability is insufficient, it will decompose at high temperatures, producing volatile gases. These gases can be drawn into the molten metal, forming pores and affecting the quality of the casting. Furthermore, decomposition residues may adhere to the casting surface, forming inclusions or stains. Therefore, high-quality die-casting coatings require high-temperature-resistant base materials, such as silicone resins and ceramic powders. These materials are resistant to high-temperature decomposition and maintain the coating’s integrity and functionality. For example, coatings containing ceramic microbeads can withstand temperatures exceeding 800°C, fully meeting the temperature requirements of aluminum and copper alloy die-casting.
Die-casting coatings must possess excellent lubricity and mold release properties to ensure easy release of the casting from the mold and minimize release forces. Insufficient lubricity can increase friction between the casting and the mold, potentially causing damage, deformation, or even breakage during demolding. It can also increase mold wear. Therefore, the coating must contain an appropriate amount of lubricating ingredients such as graphite, molybdenum disulfide, and polytetrafluoroethylene. These materials have a low coefficient of friction and form a lubricating film on the mold surface, reducing friction between the molten metal and the mold. Demolding properties require the coating to quickly separate from the casting surface after solidification without causing adhesion. This requires the coating to possess appropriate surface tension and adhesion, ensuring a firm bond to the mold surface without excessively strong adhesion. For example, water-based graphite coatings, by adjusting the graphite particle size and content to form a uniform lubricating layer on the mold surface, can reduce release forces by 30% to 50%, significantly improving demolding results.
Die-casting coatings must not negatively impact the surface quality of the casting. They must be applied thinly and evenly, with residues that are easily cleaned. Overly thick or uneven coatings can lead to surface defects such as pitting and sink marks, affecting the casting’s appearance and dimensional accuracy. Difficult-to-remove coating residues can complicate subsequent cleaning processes like shot blasting and pickling, increasing production costs. Therefore, the coating must exhibit excellent film-forming properties, forming a continuous, uniform, thin layer on the mold surface. A typical coating thickness is between 5μm and 20μm. Furthermore, the coating’s components should be easily volatile or decomposable. The high-temperature molten metal vaporizes the solvent and some organic components, leaving any remaining solid residues that are easily cleaned. For example, coatings used for decorative die-castings often use low-residue formulations. After demolding, surface residues can be removed with a simple hot water rinse, eliminating the need for complex cleaning procedures.
Die-casting coatings must be highly compatible with the mold material and die-casting alloy, resisting chemical reactions. Molds are often made of hot-work die steel. Any chemical reaction between the coating and the mold material can corrode the mold surface, damaging its hardness and finish, and shortening its lifespan. Furthermore, chemical reactions between the coating and the molten metal can produce harmful compounds, affecting the chemical composition and mechanical properties of the casting. For example, in magnesium alloy die-casting, acidic coatings can react with the magnesium alloy to produce hydrogen, leading to porosity in the casting. Neutral or weakly alkaline coatings, on the other hand, can avoid this chemical reaction. Therefore, when selecting die-casting coatings, it is crucial to conduct targeted screening based on the mold material and die-casting alloy, and conduct compatibility testing to ensure that the coating will not adversely affect the mold and casting.
Die-casting coatings also have strict requirements regarding ease of use and environmental friendliness. They must be user-friendly and environmentally friendly. Ease of use requires the coating to be easy to store, mix, and apply. For example, the coating should have an appropriate viscosity for easy spray or brush application, and dry quickly after spraying to form a coating that doesn’t disrupt production. For automated production lines, the coating must also exhibit excellent stability, preventing delamination and sedimentation during long-term cycling. Environmentally friendly, the coating should minimize volatile organic compounds (VOCs) and toxic and hazardous substances, complying with national environmental standards to protect operator health and minimize environmental pollution. Currently, water-based and solvent-free coatings are becoming mainstream due to their low VOC and toxicity profiles. For example, one brand of water-based die-casting coating emits only 10% of the VOCs of traditional solvent-based coatings and contains no heavy metals like lead and chromium, fully meeting environmental requirements. Furthermore, coating waste disposal should be simple and easy, avoiding the generation of persistent pollutants, thereby achieving green and environmentally friendly die-casting production.
