Powder coatings and powder coating processes have been used in industry for decades, especially since the 1990s when the world placed increasingly higher demands on environmental protection. Powder coatings contain no solvents, produce almost no volatile organic compounds (VOCs), and the small amount of powder pollution is easily eliminated; they offer high coating efficiency, oversprayed powder can be recycled with a loss rate of less than 5%, and a thick coating can be achieved in a single coat, with a preference for thicker coatings over thinner ones.
1. Powder Coating Spraying Application Process
The powder spraying application process utilizes the principle of a high-voltage electrostatic corona field. A high-voltage negative electrode is connected to the metal guide at the spray gun head, and the workpiece being sprayed is grounded to form a positive electrode, creating a strong electrostatic field between the spray gun and the workpiece. When compressed air, acting as the carrier gas, delivers powder coating from the powder supply hopper through the powder pipe to the guide rod of the spray gun, corona discharge generated by the high-voltage negative electrode connected to the guide rod creates a dense negative charge in its vicinity. This causes the powder to acquire a negative charge and enter a high-intensity electrostatic field. Under the combined action of electrostatic force and the carrier gas, the powder is evenly sprayed onto the grounded workpiece surface, forming a uniform powder layer. This layer is then heated and cured to form a durable coating film.
Powder Coating Application Process: Pretreatment - Drying to Remove Moisture - Spraying - Inspection - Baking - Inspection - Finished Product
2. Powder Coating Spraying Requirements
2.1 General Powder Coating Spraying Requirements：
2.1.1 To fully utilize the properties of powder coating and extend the coating's service life, the surface of the workpiece must undergo rigorous pretreatment.
2.1.2 During spraying, the workpiece must be completely grounded to increase the spraying efficiency of the powder coating.
2.1.3 For objects with significant surface defects, conductive putty should be applied to ensure a smooth and even coating.
2.1.4 After spraying, the object needs to be heat-cured. Curing conditions should adhere to the powder coating's technical specifications, but the curing temperature and time must be adequately guaranteed to avoid insufficient curing and quality issues.
2.1.5 Inspect immediately after powder coating. If defects are found, address them promptly. If defects are found after curing and are small and localized, they can be repaired using a powder of the same color diluted with acetone. If the defects are large and affect surface quality, sand the surface and recoat, or remove the coating with paint remover and reapply powder.
2.1.6 Recycled powder must be screened to remove impurities before being mixed with new powder in a specific ratio.
2.1.7 The powder supply tank, spraying chamber, and recycling system should be kept free from contamination by powders of different colors. Therefore, they must be thoroughly cleaned before each color change.
2.2 Requirements for Artistic Powder Coating:
Artistic powder coatings are characterized by their aesthetic appeal, strong three-dimensional effect, and excellent decorative properties. However, their application process requires strict adherence to the following:
2.2.1 The input air pressure during powder coating should not be too high, generally controlled between 0.5 and 1.5 kg/cm². Excessive air pressure will result in poor pattern clarity or the formation of pits. The electrostatic voltage should also not be too high, generally controlled around 60–70 kV. Excessive voltage will cause the powder adhering to the workpiece surface to rebound, resulting in pitting and poor leveling.
2.2.2 It is crucial to ensure the thickness of the coating film during powder coating. Generally, a thickness between 70 and 100 μm is necessary to facilitate the formation of clear and large patterns. A thinner coating will result in less distinct and smaller patterns, and may also lead to pitting and exposed substrate.
2.2.3 Curing must be performed at the specified temperature and time. If the temperature is too low or the time is too short, the powder will fail to form a pattern, and the mechanical properties will also be greatly reduced due to incomplete curing.
Furthermore, due to the special nature of the production process of artistic patterned powder, the pattern will become smaller or less noticeable after re-coating with recycled powder. Therefore, it is generally recommended not to use recycled powder for artistic patterns. If it must be used, it must be tested. As can be seen from the above, although the application requirements are relatively strict, it is believed that as long as the above factors are well controlled during the coating process, ideal and satisfactory results will be achieved.
3. Characteristics of Electrostatic Powder Coating
Electrostatic powder coating is the most widely used method, and its emergence has greatly advanced powder coating technology. Its biggest features are:
3.1. The workpiece can be coated at room temperature without preheating;
3.2. Powder utilization rate is as high as 95% or more;
3.3. The coating film can be relatively thin, uniform, and without sagging;
3.4. A continuous, smooth coating film can be formed on the sharp edges and rough surfaces of the workpiece;
3.5. It is easy to automate production. There are two methods for charging powder: one is through the collision of charges generated by corona discharge and air ionization in an electrostatic field; the other is through friction between powder particles or with the walls of a container. The charged powder particles fly towards the workpiece with the airflow in the electrostatic field between the electrode and the grounded workpiece. During flight, they repel and disperse with particles carrying the same charge, and finally adhere to the workpiece by Coulomb forces.
