Electrophoretic painting, a widely used surface coating technique, is favored for its efficiency, corrosion resistance, and excellent finish quality, especially in the automotive, home appliance, and hardware industries. However, a common issue that puzzles many manufacturers is the occurrence of bubbles in the electrophoretic paint during the baking process. These bubbles not only mar the aesthetic appeal of the product but also compromise the coating’s protective properties, potentially leading to premature product failure. So, what exactly causes these bubbles to form during the baking of electrophoretic paint?

　　One of the primary reasons for bubble formation is inadequate surface preparation of the workpiece. Before electrophoretic coating, the workpiece must undergo thorough degreasing, derusting, and phosphating. If this pre-treatment is incomplete, residual oil, rust, or impurities on the surface can react with the paint during electrophoresis, weakening the adhesion of the coating and causing bubbles5. These contaminants act as barriers, preventing the paint from forming a uniform and tight bond with the substrate, which leads to air pockets trapped beneath the coating.

　　Another significant factor is the instability of the electrophoretic bath parameters. The pH value, conductivity, and solid content of the bath play a crucial role in coating quality. An improper pH level, unstable conductivity, or uneven solid content can disrupt the coating’s formation, leading to the generation of bubbles during curing5. For instance, if the pH is too high or too low, it can interfere with the electrochemical reactions involved in paint deposition, causing inconsistencies that manifest as bubbles.

　　Electrophoretic voltage is also a critical parameter. Excessive voltage can result in an overly thick coating, which increases internal stress and makes the paint prone to bubble formation. Conversely, insufficient voltage may lead to uneven coating thickness, with some areas having poor adhesion, again resulting in bubbles5. Proper voltage adjustment, tailored to the workpiece’s shape and size, is essential to ensure a uniform and bubble-free finish.

　　The baking and curing process itself can contribute to bubble formation if not executed correctly. If the baking temperature is too high or the heating rate is too rapid, the solvents or moisture trapped within the coating will evaporate too quickly, forming bubbles5. Similarly, insufficient baking time can leave the coating partially cured, making it more susceptible to bubble formation. To avoid this, it’s crucial to follow the paint manufacturer’s recommendations for temperature and time, ensuring a gradual and complete curing process.

　　Environmental conditions during the coating process also play a role. High humidity can cause moisture to adhere to the workpiece surface, which then evaporates during baking, leading to bubbles5. Maintaining a controlled environment with appropriate humidity levels is therefore essential for preventing this issue.

　　The physical characteristics of the workpiece surface can also contribute to bubble formation. Uneven surface roughness can disrupt the uniformity of the coating, creating areas where bubbles are more likely to form5. Ensuring a smooth and consistent surface finish through proper machining or post-processing steps can mitigate this risk.

　　To address these issues, several measures can be taken. First, optimizing the pre-treatment process is vital. This includes ensuring thorough degreasing, derusting, and phosphating, as well as using appropriate cleaning agents and regularly monitoring the concentration and pH of the pre-treatment solutions5. For complex-shaped workpieces, methods like spray washing or ultrasonic cleaning can enhance the effectiveness of surface preparation.

　　Second, strict control of electrophoretic bath parameters is necessary. Regularly testing the pH, conductivity, and solid content ensures they remain within the required range. Circulating and filtering the bath to remove impurities and particles maintains its cleanliness, while timely replenishment or replacement of the bath prevents degradation5.

　　Adjusting the electrophoretic voltage according to the workpiece’s specifications can prevent coating thickness issues. For complex shapes, implementing segmented voltage control can ensure even coverage5.

　　Optimizing the baking and curing process is also key. Setting appropriate baking temperatures and heating curves, as well as ensuring sufficient curing time, helps avoid rapid solvent evaporation and incomplete curing5. For large workpieces, segmented baking can promote uniform curing.

　　Improving the workpiece surface quality by controlling roughness and using methods like sanding or polishing can enhance surface uniformity5. Additionally, controlling environmental humidity through dehumidification equipment in the coating area is beneficial.

　　In conclusion, the formation of bubbles in electrophoretic paint during baking is a multifaceted issue stemming from factors like inadequate surface preparation, unstable bath parameters, improper voltage, flawed baking processes, environmental conditions, and surface roughness. By systematically addressing these factors through optimized processes and careful monitoring, manufacturers can significantly reduce or eliminate bubble formation, ensuring high-quality, bubble-free coatings that enhance both the appearance and durability of their products.