Analysis and Prevention of Surface Defects in Aluminum Powder Coating

[Mini Encyclopedia] Powder coating uses powder coating equipment (electrostatic powder coating machine) to spray powder coating onto the surface of a workpiece. Under electrostatic action, the powder is evenly adsorbed onto the workpiece surface, forming a powdery coating. This powdery coating is then baked at high temperature to level and cure, resulting in a final coating with varying effects (depending on the type of powder coating). Powder coating offers superior mechanical strength, adhesion, corrosion resistance, and aging resistance compared to spray painting, while also being less expensive than spray painting for the same effect.

Electrostatic powder coating technology for architectural aluminum profiles is a rapidly developing surface treatment technology in recent years. Its advantages include being solvent-free, environmentally friendly, having recyclable powder, easy automation, ease of use, and a variety of coating colors to meet different weather resistance requirements. However, some defects frequently occur in electrostatic powder coating of aluminum profiles, affecting the product's appearance quality and performance.

The following is a brief introduction to the common causes and control measures of coating defects:

I. Defect Name: Particles

Cause Analysis: The substrate surface has burrs, aluminum shavings, and graphite marks, indicating poor pretreatment; residue from the pretreatment bath adheres to the profile surface, and water rinsing is inadequate; the chromate has been left for an extended period before coating, resulting in dust adsorbed on the surface; improper use of new curing racks leads to dust settling on the chromate surface; the powder or recycled powder may contain impurities; the spraying area and curing oven are unclean, containing suspended matter.

Control Measures: Grind or sandblast the surface of the blank before loading to ensure proper pretreatment; regularly empty the spray tank, clean residue, and extend the pure water rinsing time; carefully blow away dust after loading the chromate; apply a thin coating to new curing racks before use; recycled powder should be sieved before use; implement 6S management in the workshop, maintain a clean spraying area, and regularly clean the curing oven.

II. Defect Name: Exposed Base Coating

Cause Analysis: Poor coverage of powder coating; complex profile cross-sectional structure easily causing electrostatic shielding effect; improper coating application method; lack of responsibility and poor skills among spray booth operators, resulting in inaccurate judgment of powder film thickness; excessive powder accumulation at the spray gun nozzle, causing blockage; excessively fast spray gun movement speed or chain speed; unclear understanding of the profile's decorative surface.

Control Measures: Strictly control powder coating quality; manually replenish powder in profile grooves and inner corners; adjust the profile coating angle; improve the skills of spray booth operators, ensuring they measure the cured profile film thickness as soon as possible; regularly clean the spray gun nozzle to maintain unobstructed flow; adjust the spray gun movement speed and appropriately reduce the chain speed; understand customer spraying requirements and clearly identify the profile's decorative surface.

III. Defect Name: Uneven Coating

Cause Analysis: Improper suspension and arrangement of chromate; poor grounding of profiles (poor conductivity of individual hangers); uneven conveyor chain speed, or mismatch between chain speed and spray gun lifting speed, resulting in inconsistent powder coverage on the profile decorative surface; improper powder output and atomization from the spray gun; unstable compressed air pressure; unstable voltage of the high-voltage static generator, causing fluctuations in high-voltage static electricity.

Control Measures: Strengthen communication between the spray booth operator and the spray booth worker, and ensure proper selection of hangers corresponding to the profiles; check the conductivity of the hangers; improve the skills of the spray booth worker, ensuring proper coordination of hanger density, chain speed, and lifting speed during spraying; control the ratio of powder extraction air to powder delivery air; strengthen equipment maintenance to maintain stable equipment operation.

IV. Defect Name: Miscellaneous Colors

Cause Analysis: Insufficient cleaning during color change and spray booth cleaning by the spray booth worker, resulting in powder seepage and miscellaneous colors; failure to promptly bake the powder (recycled powder) to correct the color; unclean environment around the spray booth.

Control Measures: When cleaning the spray booth, operators must meticulously clean every detail, especially the powder nozzles of the spray guns. The large cyclone separator must be thoroughly cleaned. After cleaning, a baking test must be performed before painting to confirm the absence of discoloration. Recycled powder must be baked to confirm its quality before use. The area around the spray booth must be kept clean at all times.

V. Defect Name: Pinholes

Cause Analysis: Incomplete degreasing during pretreatment; substandard compressed air quality, with oil in the powder supply and atomizing gases; potential oil or water contamination during chromate material handling; deformation of the chain's oil tray, causing oil dripping from the chain; resin incompatibility, resulting in pinholes due to mixed powders.

Control Measures: Adjust the degreasing process to ensure thorough oil removal from the profiles; carefully inspect the surface quality of the chromate material, preventing oil and water contamination during storage and handling; check the quality of compressed air and the operation of the degreasing and oil removal equipment, promptly draining condensate and oil from the compressed air; check the chain lubricant usage to avoid contamination; use a fine cleaning mode for color changes, thoroughly cleaning the powder nozzles and powder spraying system.

VI. Defect Name: Coarse Sand

Cause Analysis: Coarse sand refers to a rough surface on the powder coating. The main causes include: high voltage due to discharge at the edges or tips of the profile during electrostatic spraying; insufficient distance between the spray gun and the profile, resulting in excessive electric field strength; poor conductivity of the mounting bracket leading to poor powder application or severe reverse ionization; and poor powder quality causing a rough coating after curing.

Control Measures: Strictly control electrostatic voltage and the distance between the spray gun and the profile, and adhere to powder coating process requirements; before applying the powder, thoroughly grind the joints between the mounting bracket, chain, and profile to ensure good conductivity; strictly control the quality of the powder coating.

VII. Defect Name: Color Difference

Cause Analysis: Color difference is caused by uneven powder pigments; different batches of powder coating; different curing temperatures and times; and uneven coating thickness.

Control Measures: Strengthen powder quality monitoring; conduct baking inspection on new powder coatings upon arrival at the warehouse; maintain consistent curing temperature and time for the same batch of sprayed profiles; special powders must be applied to aluminum profiles on a fixed production line; strengthen powder coating process control to avoid excessive differences in coating thickness. VIII. Defect Name: Coating Scratches

Cause Analysis: Before finished coatings are discharged, transferred, or laminated, scratches occur on the decorative surface due to friction between materials of different lengths or improper manual operation.

Control Measures: Distribute finished coatings according to their length; workers must cooperate and handle materials gently.

IX. Defect Name: Powder Spillage

Cause Analysis: Powder agglomerates due to moisture, resulting in poor fluidization and uneven powder concentration in the powder delivery pipe; the fluidizing plate is blocked by powder, leading to poor fluidization of the powder supply device; powder buildup or severe wear on accessories such as powder pipes, powder pumps, venturi tubes, electrostatic needles, and spray gun nozzles causes uneven powder output and powder spillage.

Control Measures: Improve the powder storage environment; seal powder boxes promptly after use to prevent moisture absorption; perform regular maintenance on the powder supply equipment; clean powder buildup from spray booth equipment accessories promptly and replace worn parts.

In actual powder coating production, meticulous management of each process is essential. Addressing coating defects directly and taking targeted measures to prevent recurrence of similar defects is crucial for improving powder coating quality while reducing production costs.

Source: Aluminum Friends Community