How to control the quality of product surface painting by controlling spraying parameters in a high-temperature and high-humidity coating environment?
Painting in a high-temperature and high-humidity environment is a severe challenge. Excessive humidity can cause the paint film to “whiten” (due to water vapor condensation), while excessive temperature can easily cause “dry spraying,” “orange peel,” or “sagging.” To stably produce products with both acceptable performance and appearance, it is necessary to systematically adjust the spray gun parameters and film thickness control, supplemented by other key measures. The following is a detailed introduction on how to make systematic adjustments and related measures:
I. Spray Gun Parameter Adjustment Strategy
(1) Appropriately increase the atomizing air pressure (e.g., increase by 5-10%)
Principle: Enhances the atomization effect, making the paint particles finer, which helps the solvent evaporate faster in humid air, reducing the chance of water vapor mixing. It also speeds up the rate at which the paint mist reaches the workpiece.
(2) Appropriately reduce the paint output (paint valve)
Principle: Directly reducing the amount of paint deposited per unit area is the most crucial means of preventing “sagging” and “pinholes.” This must be combined with a faster gun travel speed.
(3) Maintain or slightly reduce the fan-shaped air pressure.
Principle: At high temperatures, the fluidity of the paint itself increases, and an excessively wide fan shape can easily lead to uneven film thickness. Slightly narrowing the fan shape can achieve a more concentrated and controllable spray pattern.
(4) Slightly increase the spraying distance (e.g., adjust to 18-22cm).
Principle: Increasing the distance provides more space for solvent evaporation in the paint mist and reduces the “thermal shock” of the workpiece surface (which may be even hotter at high temperatures) to the paint, which is beneficial for leveling. However, it should not be too far to avoid dry spraying.
(5) Increase the gun moving speed.
This is the most important operational change. Increasing the travel speed is a necessary requirement to accommodate the reduced paint output, ensuring the formation of a thinner wet film, thus allowing the solvent to evaporate effectively and orderly even in adverse environments.
II. Film Thickness Control
(1) Single-coat film thickness: Must be strictly controlled, and it is recommended to be 70-80% of the single-coat film thickness under normal conditions. For example: The normal dry film thickness of a single coat is 20μm, which should be controlled at 14-16μm under high temperature and high humidity.
(2) Total film thickness: The final total film thickness still needs to meet the process specifications, but this must be achieved by increasing the number of coats (e.g., from 2 to 3). It is forbidden to slow down the spray gun or increase the paint output to achieve the desired thickness in one go.
(3) Intercoat time: This must be extended. Surface drying is extremely fast at high temperatures, but actual drying may be delayed due to high humidity. Depending on the site conditions, ensure that the previous coating has fully evaporated (not sticky to the touch) and shows no whitening before applying the next coat.
III. Supporting Key Measures
(1) Adjustment of paint and thinner: Use slow-drying or summer-specific thinners to delay surface drying time, creating a window for leveling and solvent evaporation. Appropriately increase the dilution ratio (e.g., increase by 5-10%) to reduce viscosity and improve atomization and leveling, but this must be done within the paint technical specifications. If possible, a coating system specifically designed for high-temperature and high-humidity environments can be selected.
(2) Workpiece and Ambient Temperature and Humidity Control
- Workpiece Temperature: Ensure the surface temperature of the workpiece does not exceed 40℃, preferably below 35℃. Nighttime spraying or cooling treatment of the workpiece (e.g., moving it to an air-conditioned room for cooling) can be arranged.
- Air Dehumidification: A dehumidifier must be used in the spray booth to control the relative humidity below 70% (65% is preferred).
Compressed Air: Ensure the oil-water separator operates efficiently, providing dry and clean compressed air.
(3) Spraying Techniques and Inspection
- Uniform Coverage: Use a stable, uniform spray gun with a 50% overlap rate.
- Flash-off Inspection: After each coat of paint is applied, carefully check for defects such as whitening and pinholes, and ensure these are completely resolved before continuing.
- Edge and Corner Priority: For edges, corners, and other areas prone to dry spraying or dripping, quickly apply a thin coat first, then spray the entire area.
