Spraying profiles are colorful and popular among customers, but the complexity of the cross-sectional shape of aluminum profiles brings certain difficulties to spray production and coating thickness inspection. Regardless of the shape of the aluminum profile, the minimum local film thickness of the coating on its decorative surface is ≥40um. This is a mandatory implementation clause stipulated in GB5237.4-2008 and must be strictly observed and implemented; others such as: inner corners, transverse grooves, and grooves allow insufficient coating film thickness, but the bottom cannot be exposed.
In actual production, due to the influence of electrostatic spraying equipment and the cross-sectional shape of aluminum profiles, accurate control of coating thickness is a certain matter. The characteristics of the spraying production line can better control the coating thickness from the following aspects.
1. Hanging method
Choose appropriate hangers according to the shape of aluminum profiles. Choosing the correct hanging method is the key to controlling the coating thickness. It can not only improve the quality of aluminum profile coating, but also reduce the workload of powder room operators to manually add powder with guns. In particular, the spacing of the hangers can adjust the thickness of the powder coating. The spacing of the hangers is generally required to be: the width of the aluminum profile = the spacing of the aluminum profile.
2. Contact between workpiece and hanger
Due to poor contact between the workpiece and the hanger (i.e. poor conductivity), it will seriously affect the coating efficiency and coating quality, resulting in low powder application rate on the workpiece, resulting in low coating film thickness or even bottom exposure, and causing great pressure on the recycling system, so that the excess powder cannot be recycled in time and overflows outside the powder room, which not only reduces the utilization rate of the powder but also pollutes the environment.
When the workpiece is in poor contact, the coating thickness of the product measured at different points on the same material may differ by 30um-70um. In some places, due to the excessive thickness of the coating, there will be defects such as orange peel, pinholes, flow marks, etc., and the color and gloss will also be affected. In addition, the sand grain powder profile has obvious zebra-shaped defects under natural light. Therefore, it is necessary to ensure good conductivity between the workpiece and the hanger.
3. Conveyor chain speed and production process
During production, the speed of the conveyor chain and the production process play a major role in the thickness of the powder spray coating, such as: the travel of the elevator, the gun distance, the powder pressure, the electrostatic voltage, etc. The height of the elevator travel is generally required to be: travel height = workpiece height + 200mm. The more complex the shape of the workpiece, the closer the distance between the spray gun and the workpiece, the smaller the powder output and the lower the voltage; conversely, the simpler the shape of the workpiece, the farther the distance between the spray gun and the workpiece, the greater the powder output and the higher the voltage.
4. How to use a hand spray gun to add powder ?
Due to the complexity of the cross-sectional shape of aluminum profiles, during the production process, the inner corners, horizontal grooves, and slots of the decorative surface are not easy to be sprayed by automatic spray guns, and must be sprayed by hand. When the nozzle of the hand spray gun is facing the place to be sprayed on the workpiece, the transmission pressure is adjusted too large, and the distance between the hand spray gun and the workpiece is too close, it is easy to cause orange peel, flow marks, pinholes, gun dots, fat edges, etc. on the surface of the workpiece. Under normal circumstances, when the nozzle of the hand spray gun is facing the place to be sprayed on the workpiece, the transmission pressure is: 1.2~1.8bar, and the distance between the hand spray gun and the workpiece should be controlled at 80~150mm.
5. Powder pump factors
The common uneven powder discharge in the production process is mainly caused by the uncleanness of the Venturi powder pump. Since the powder pump presses the powder suction pipe tightly, it is easy to accumulate powder and stick to the powder, causing blockage, which will affect the receiving force in the powder pipe and thus affect the coating thickness of the workpiece. In general, the powder room operator should check the cleanliness of each part of the Venturi powder pump in time.
