Common defects in electrostatic powder coating, their causes, and corresponding preventive measures

1. Shrinkage

Defect characteristics: surface defects resembling pores, pinholes, and volcanic craters.

Causes and preventive measures for powder coatings: 1. Insufficient amount of leveling agents and defoaming agents. 2. Uneven mixing of leveling agents and defoaming agents. 3. Contamination among incompatible powder coatings during the manufacturing process. 4. Compressed air containing oil and moisture during the manufacturing process.

Reasons for coating process issues and corresponding preventive measures: 1. Inadequate degreasing. 2. Uneven surface of the workpiece. 3. Insufficient drying after surface treatment. 4. Incomplete cleaning when changing powder varieties. 5. Contamination of the compressed air used for spraying with oil and water, or oil leakage from chains. 6. Powder is damp or the workshop air is humid.

2. Granules

Defect characteristic: protruding particles on the coating surface.

Causes and preventive measures for powder coatings: 1. Poor quality of raw materials, with gel particles contained in the resin. 2. Mechanical impurities contained in pigments, fillers, or additives. 3. The self-cleaning action of the extruder is poor, bringing in unmelted gel particles. 4. Mechanical impurities such as dust in the environment are brought into the powder coating.

Causes of coating process issues and corresponding preventive measures: 1. The surface of the workpiece has burrs or is rough, with fine pits, etc. 2. Chrome residues and other impurities in the surface treatment solution still adhere to the workpiece surface. 3. The environment around the spraying chamber is not clean, with dust and particles in the air being brought into the powder coating or spraying chamber, or charged impurities being attracted to the workpiece due to static electricity. 4. The coating is too thin to conceal small, non-fusing particles on the coating film. 5. Recycled coating has not been sifted, or the mesh of the sieve is too coarse. 6. Particles of cured powder coating fall into the coating film from the conveying chain fixture.

3. Film thickness exceeds standards

Defect characteristics: The membrane thickness of the same workpiece exceeds the range specified in the contract.

Reasons for the spraying process and corresponding preventive measures: 1. The arrangement method of the number of spray guns in the spraying area. 2. The output of the spray gun and air pressure. 3. The operation speed of the conveying chain. 4. The hanging method and arrangement of the workpiece. 5. Improper use ratio or uneven mixing of reclaimed powder.

4.  do not add powder

Defect characteristic: The ratio of the powder amount on the workpiece surface to the powder spraying amount is less than 60%.

Reasons and preventive measures for powder coatings: 1. For the corona discharge system, it is advisable to select fillers with good electrostatic properties or to add special additives such as charging agents to enhance the electrostatic properties of the coating; 2. For the friction charging system, friction charging additives should be added to the formulation; 3. A high mass fraction of pigments and fillers leads to a larger density and coarser particles, resulting in gravitational force greater than electrostatic adhesion; 4. Alternatively, if the particle size is too small, there will be less charge and a smaller electrostatic adhesion force.

Reasons for the spraying process and corresponding preventive measures: 1. The voltage of the corona spray gun is too low or too high; the resistance of the workpiece is too large (poor grounding or the fixture is not cleaned); 2. The amount of powder sprayed is too large; 3. The air pressure is too high.

5. Loss of brightness and yellowing

Defect characteristic: Gloss limit value exceeds at 60°.

Reasons and prevention measures for powder coatings: 1. Interference between powder coatings of different resin types. 2. Interference is also likely between powder coatings formulated with two types of resins with different reactivities.

Reasons for the spraying process and corresponding preventive measures: 1. Incomplete cleaning of the powder spraying system. 2. Curing temperature is too high, and the time is too long, exceeding the heat resistance temperature of the coating.

6. Color difference

Defect characteristic: The color difference between the coating and the standard color sample exceeds the specified limit.

Reasons for powder coatings and preventive measures: The heat resistance of raw materials selected in the formulation design, such as pigments and matting agents, is not ideal, and there is a certain gap between the baking and curing conditions of the coating factory and those during the coloring process at the powder production factory.

Reasons for the spraying process and corresponding preventive measures: 1. Uneven film thickness. 2. Uneven furnace temperature. 3. When workpieces with significantly different thicknesses are cured together in the same curing furnace, the thicker workpiece takes longer to heat than the thinner one, resulting in color difference. 4. Incomplete cleaning of the powder spraying system.

7. Physical performance is not up to standard.

Reasons and preventive measures for powder coatings: The mismatch between resin and curing agent in the formulation design, such as inappropriate selection of resin reactivity or curing agent type and dosage, or an excessive mass fraction or volumetric fraction of fillers.

Reasons for spray coating issues and corresponding preventive measures: 1. The curing temperature does not meet the requirements of the coating. 2. The curing time does not meet the requirements of the coating. 3. Poor surface treatment of the workpiece. 4. Excessive film thickness. 5. Testing temperature is too low. 6. Curing temperature is too high, and baking time is too long.

8. Unqualified chemical resistance performance

Causes and preventive measures for powder coating: In the formulation design, the matching of the resin and curing agent system, the chemical stability of pigments and fillers, and the inappropriate mass fraction of these components in the formulation.

Reasons for the spraying process and corresponding preventive measures: 1. Poor surface treatment quality. 2. Incomplete curing. 3. Inconsistent thickness of the film.

9. Poor coverage

Reasons for powder coating and preventive measures: The coating formula does not use enough pigments with covering power, resulting in insufficient coverage of the coating film.

Reasons for the spraying process and corresponding prevention measures: The coating thickness is too thin.