Various difficulties often exist in zinc-aluminum coating process, and how to find the true cause of these difficulties has become a difficult point in the coating industry.
Apart from the product workpiece itself, the most important raw material for zinc-aluminum coating is the zinc-aluminum micro-coating solution. Poor control of zinc-aluminum coating solution can lead to many undesirable phenomena, such as solution accumulation, overall black appearance, watermark sagging, poor adhesion, and salt spray failure, etc.
The accumulation of solution is mostly due to too high viscosity and temperature of the coating solution and the centrifugal failure to effectively shake out the excess coating solution.
Overall black appearance is mainly because the coating solution is not stirred evenly and the solid content of the upper layer of coating solution is low, so even if the coating has been adsorbed on the workpiece, the coating will be lost (effective solid ingredients are lost for part of the location) through the flow of the coating solution itself after entering the drying channel.
Watermark sagging is primarily caused by uneven mixing and inconsistent color of the coating solution.
Poor adhesion is mainly due to too many invalid substances in the coating solution (such as steel shot, oxidized resin, and iron powder dust).
There are many reasons for salt spray failure, and any subtle changes in the zinc-aluminum coating solution will have an impact on it. However, salt spray is the most important performance we need to achieve the goal.
Therefore, it’s important to make sure that the maintenance and use of the coating solution are controlled.
Maintenance and use notes of zinc-aluminum coating solution in the coating process
1. Working solution indicator measurement of coating solution
Measure viscosity every 2h, measure temperature and humidity every 2h, and measure solid content once per shift.
2. Mixing of paint working solution
A large mixer should be used to fully mix the working coating solution in the dipping tank for 15min before entering the coating line, and the oil-based coating solution on the coating line must be pulled off the line after 12 hours of continuous work and re-mixed for 10min in the dispensing room before online for use.
According to the production scheduling plan, water-based environmental protection coating solution should be pulled back to the dispensing room sealed at a constant temperature to prevent the aging of coating solution if no production plan is available for at least three days.
3. Filtration
Filter the oil-based coating solution once in 3 working days, the oil-top coating solution once in 7 working days, and the water-based coating solution once in 10 working days. While filtering, remove steel shot and iron powder from the coating solution. The frequency of filtration should be increased in hot weather or in case of quality problems.
4. Renewal
During the normal consumption of the coating solution in the dipping tank, the coating solution and thinner that are mixed in the dispensing room are added and renewed.
Data inspection should be completed for the coating solution that has not been used for at least one week in the dipping tank before it is put on the coating line again, and it cannot be put on line unless the inspection is qualified. In case of any slight deviation, scoop out 1/4 of the coating solution in the dipping tank, add 1/4 of the new solution for renewal, and scoop out part of the original solution to be added in the form of 1:1 when mixing the new solution for subsequent production.
5. Storage management
The storage temperature and humidity (especially in summer) should be controlled and recorded in strict accordance with the instructions, and reported in time once the standard is exceeded.
The storage temperature of the coating solution tank in the dispensing room should be as close as possible to the outdoor temperature to avoid water droplets due to dew point to affect the solution performance. The storage temperature of the new coating solution tank is 20±2℃ before opening. When the difference between the new coating solution and the outdoor temperature is large, the solution tank must be sealed outside for 4 hours before adding to ensure that the temperature inside and outside the tank is the same.
6. Precautions for use
(1) Any coating solution tank entering or leaving the dispensing room must be sealed with a wrap-around film and covered with the tank lid.
(2) Take protective measures when it’s rainy and highly humid.
(3) During temporary shutdown caused by various equipment problems, the dipping tank must not be exposed in a non-working state for more than 4 hours.
(4) To ensure the stability of the coating solution, no hot objects (especially workpieces that have not been cooled to room temperature) should be in contact with the coating solution on all lines.
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