Metal coating process
The metal coating process is a systematic undertaking, encompassing a complete process from substrate pretreatment to final coating curing. Its core goal is to impart superior protective properties and decorative effects to metal surfaces through scientific process design. The typical process includes five key steps: surface pretreatment, primer application, midcoat and topcoat application, drying and curing, and quality inspection. Quality control at each step directly impacts the adhesion, corrosion resistance, and service life of the final coating. In the automotive industry, a comprehensive coating process can ensure a rust-free coating for over five years in salt spray environments, while parts treated with inferior processes may experience coating flaking within months.
Surface pretreatment is the foundation of the metal coating process, and its quality directly determines the bonding strength between the coating and the substrate. Traditional pretreatment methods mainly include degreasing, rust removal, and phosphating. Alkaline degreasing agents remove stamping oil and fingerprint oil stains from the metal surface through emulsification, pickling and rust removal use hydrochloric acid or sulfuric acid solution to dissolve the oxide scale, and phosphating forms a porous phosphate film on the metal surface through chemical reaction, providing an anchoring effect for subsequent coatings. In recent years, environmentally friendly pretreatment technologies have gradually replaced traditional processes. For example, the electrodeposited silica film technology developed by Zhejiang University uses electrochemical methods to form a nanoporous structure on the metal surface. Its bonding strength with the epoxy coating is more than 30% higher than that of the traditional phosphating layer, and it is completely free of phosphorus and heavy metals.
Primer coating, as the first line of defense in a protective system, plays a crucial role in corrosion prevention and connecting the upper and lower parts. Common types of primers include epoxy primers, polyurethane primers, and zinc-rich primers. Epoxy primers are widely used in construction machinery due to their excellent adhesion and chemical resistance. Zinc-rich primers provide cathodic protection for steel structures through the sacrificial anode action of zinc powder and are indispensable in marine engineering. Primer application often uses dipping or electrophoretic coating processes. Electrophoretic coating uses an electric field to deposit paint particles in a targeted manner, evenly covering the inner cavities and corners of complex workpieces, with film thickness deviations controlled within ±5μm. Practical data from a certain automobile factory shows that car bodies using cathodic electrophoretic primers can withstand salt spray for over 1,000 hours, far exceeding the 500-hour standard of traditional spray coating.
The midcoat and topcoat processes determine the decorative and functional properties of the coating. Midcoat is primarily used to fill minor imperfections and improve smoothness, typically employing a polyester resin system with a dry film thickness of 30-40μm. Topcoats are categorized as natural, metallic, and clearcoats. Metallic paints, which create a angularly chromatic effect through the directional arrangement of aluminum powder or mica flakes, are widely used in high-end automobiles and home appliances. Topcoat application is typically performed using electrostatic spraying or robotic automated spraying. Electrostatic spraying utilizes a high-voltage electric field to charge paint particles, achieving an adsorption efficiency exceeding 85%, reducing paint loss by 30% compared to air spraying. During the drying and curing process, different coatings require specific temperature profiles: amino paints require baking at 120°C for 30 minutes, while two-component polyurethane paints can be cured at a low temperature of 60°C, meeting the coating requirements of heat-sensitive substrates.
Quality inspection and process optimization are the final steps in the metal coating process, ensuring that coating performance meets standards through a series of quantitative indicators. Routine testing includes: cross-cut testing to assess adhesion (requires a grade of 0 with no peeling), pencil hardness testing to assess surface abrasion resistance (minimum 2 hours), and gloss measurement at a 60° angle (with a deviation of less than 5 units). Advanced companies have implemented online inspection systems, such as infrared thermal imaging cameras to monitor curing oven temperature distribution and laser thickness gauges to provide real-time feedback on film thickness, enabling closed-loop control of process parameters. By analyzing 120 process parameters during the coating process using big data, one appliance company reduced its coating defect rate from 3% to 0.5%, saving over 2 million yuan in annual rework costs. With increasingly stringent environmental regulations, the use of low-VOC coatings and powder coatings continues to increase, driving the evolution of metal coating processes towards greener and more precise processes.
