1. Material Factors

Excess Volatile Content in Powder Coating:

The quality of powder coating is directly influenced by its composition, especially the ratio of resin and additives. If there is moisture or low-molecular monomers in the powder that have not been removed, these components will vaporize during curing, forming bubbles. While these bubbles may appear small on the surface, they are the root cause of pinholes. If these bubbles are not released in time during curing, they will form tiny holes on the surface or inside the coating, which will become visible as the coating cools.

Mechanism:

During the curing process of powder coatings, volatile components such as water and low-molecular monomers start to vaporize at temperatures between 150–200°C. If these volatile components are not released in time before the coating cures, they will form gas pockets inside the coating, leading to pinholes. Since gas molecules are small and vaporize quickly, if the coating does not fully cure or the temperature is unstable, the gas gets “trapped” inside the coating, ultimately forming pinholes on the surface.To learn more about how volatile content affects powder coatings, check out Powder Coating Selection, Application Techniques, and Material Overview.

Verification Method:

To verify the volatile content in powder coatings, testing can be done according to the GB/T 21782.13 standard. After heating the sample at 150°C for 1 hour, the weight change can be measured. The moisture content can be detected using the Karl Fischer method, with the acceptable moisture content being ≤ 0.1%. If the volatile content or moisture content exceeds the standard, pinhole problems may arise after spraying.

2. Equipment Factors

Improper Spray Gun Parameters:

The settings of the spray gun are critical to the powder coating result. If the spray gun voltage is set too high (over 80kV) or the spraying distance is too close (less than 150mm), the kinetic energy of the powder particles will be too high, causing dents and the accumulation of air bubbles upon impact with the substrate, resulting in pinholes. Additionally, when the coating is too thick (over 100μm), the gases inside the coating are unable to escape, leaving pinholes inside.”For more detailed information on how to optimize spray gun parameters for better coating results, check out Powder Coating Challenges: Overcoming Industry Barriers.“

Misuse of Recycled Powder:

Using recycled powder is a common cost-saving practice during the spraying process. However, after multiple baking cycles, the volatile content gradually accumulates. These volatiles will be released during spraying, creating gas pockets that lead to pinholes. The accumulation of volatiles in recycled powder can exacerbate this issue.

Verification Method:

To ensure the coating thickness is up to standard, a thickness gauge can be used to measure the coating, with the target range being 60-80μm. Comparing the volatile content between new powder and recycled powder can help identify the accumulation of volatiles. The volatile content in recycled powder should not exceed 1.5 times that of new powder.

3. Environmental Factors

High Workshop Humidity (>70%):

High humidity in the workshop is a key factor leading to pinholes in coatings. When the humidity exceeds 70%, moisture can be absorbed by the substrate. During spraying, the moisture evaporates rapidly with the increased baking temperature, forming bubbles and resulting in pinholes on the coating surface. High humidity can also cause uneven curing during the curing process, which affects the coating’s quality and appearance.”For more information on how high humidity impacts coating quality, refer to EPA – Powder Coating and Environmental Protection.“

Moisture or Oil in Compressed Air:

Poor quality compressed air is another common cause of powder coating problems. If the compressed air contains moisture or oil, it can be introduced into the coating, where these impurities release gas during curing, leading to pinholes on the coating surface. Therefore, ensuring clean, dry compressed air is essential.

Verification Method:

To ensure the workshop humidity is within the suitable range, a hygrometer can be used to monitor the humidity, which should be controlled between 40-60%. Additionally, the dew point of the compressed air should be tested, with the dew point temperature lower than -20°C to ensure the air is free of moisture and oil.

4. Process Factors

Incomplete Curing:

Curing is a critical step in the powder coating process. If the curing temperature is insufficient (less than 180°C) or the time is too short (less than 10 minutes), the internal stress in the coating will not be released, and the gases will remain trapped inside, forming pinholes. This is especially true when the temperature is low or the curing time is inadequate, which prevents the volatiles from escaping fully, leading to pinholes on the surface.”For detailed guidance on the powder coating curing process, refer to What Is the Powder Coating Application Process?.”

Verification Method:

The temperature uniformity in the curing oven can be checked using a multi-point thermometer to ensure the temperature difference does not exceed ±5°C, ensuring an even and adequate curing process. The temperature and time should match the manufacturer’s recommendations to achieve the best curing results.

5. Substrate Factors

Incomplete Pretreatment:

If the substrate surface is not thoroughly pretreated, such as incomplete cleaning or uneven phosphating, the adhesion between the coating and the substrate will be affected, causing gas to accumulate in these areas and leading to pinholes on the coating surface. Therefore, ensuring thorough pretreatment of the substrate is crucial.

Verification Method:

After pretreatment, the substrate surface can be wiped with a solvent to check its cleanliness. The thickness of the phosphating layer should be measured, with the ideal thickness being between 1-3μm and evenly distributed.