Metal powder coating methods and precautions

Spray-coated profiles come in a variety of colors and are popular with customers. However, the complexity of aluminum profile cross-sectional shapes presents challenges for spray-coating production and coating thickness inspection. Regardless of the profile shape, the minimum local coating thickness on the decorative surface must be ≥40µm. This is a mandatory requirement of GB5237.4-2008 and must be strictly adhered to. Other areas, such as inner corners, horizontal grooves, and slots, may have slightly thinner coatings, but the substrate must not be exposed.

In actual production, accurate coating thickness control requires careful consideration due to the influence of electrostatic spraying equipment and the cross-sectional shape of the aluminum profile. The characteristics of the spray-coating production line can be used to better control coating thickness in the following ways:

1. Hanging Method

Selecting appropriate hangers based on the shape of the aluminum profile is crucial for controlling coating thickness. This not only improves the quality of the aluminum profile coating but also reduces the workload of manual powder application by powder booth operators. In particular, the spacing of the hangers can adjust the powder coating thickness. The general requirement for the hanger spacing is: aluminum profile width = aluminum profile spacing.

2. Workpiece-Hanger Contact

Poor contact between the workpiece and the hanger (i.e., poor conductivity) severely impacts coating efficiency and quality, leading to low powder application and resulting in thin coatings or even exposed substrate. It also puts significant pressure on the recovery system, causing excess powder to overflow into the powder booth, reducing powder utilization and polluting the environment.

Products with poor workpiece contact may exhibit coating thickness variations of 30µm-70µm at different points on the same substrate. In some areas, excessive coating thickness can result in defects such as orange peel, pinholes, and flow marks. Color and gloss will also be affected, and under natural light, textured powder coatings will show noticeable zebra-stripe defects. Therefore, ensuring good conductivity between the workpiece and the hanger is crucial.

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 coating. Factors such as the lift's stroke, gun distance, powder pressure, and electrostatic voltage are important. The lift's stroke height is generally required to be: Stroke height = Workpiece height + 200 mm. The more complex the workpiece shape, the closer the spray gun is to the workpiece, requiring a smaller powder output and lower voltage; conversely, the simpler the workpiece shape, the farther the spray gun is from the workpiece, requiring a larger powder output and higher voltage.

4. Hand-held Powder Replenishment Method

Due to the complexity of aluminum profile cross-sectional shapes, areas such as inner corners, grooves, and slots (which are decorative surfaces) are difficult for automatic spray guns to reach during production. Hand-held spray gun replenishment is necessary. If the hand-held spray gun nozzle is aimed at the area to be replenished, the transmission pressure is set too high, or the distance between the hand-held spray gun and the workpiece is too close, it can easily cause orange peel, flow marks, pinholes, gun dots, and thick edges on the workpiece surface. Generally, when the nozzle of the hand spray gun is directly aimed at the area to be sprayed on the workpiece, the transmission pressure should be 1.2–1.8 bar, and the distance between the hand spray gun and the workpiece should be controlled between 80–150 mm.

5. Powder Pump Factors

Uneven powder output is commonly caused by uncleanliness of the Venturi powder pump during production. Because the powder pump presses tightly against the suction pipe, powder accumulation and adhesion can easily occur, causing blockages. This affects the receiving force within the powder pipe, thus impacting the coating thickness on the workpiece. Generally, powder booth operators should regularly check the cleanliness of all parts of the Venturi powder pump.

Source: Coating and Painting Experts Discuss