How to Address Abnormalities in Electrocoat Films

In coating operations, common paint film defects such as particles and shrinkage (cratering) are frequently encountered. These defects are closely related to pretreatment processes and the on-site environment. When these issues arise, they often lead to production halts, workpiece rework, and significant losses for the company.

1. Particles

Particles refer to hard, rough (or visually noticeable) grains on the surface of the dried electrocoat film.

Causes:

1. Presence of sediment, aggregates, or other foreign matter in the electrophoretic bath liquid, coupled with poor filtration.

2. Dirty rinse water after electrophoresis or excessive paint concentration in the rinse water.

3. Contamination in the drying oven, with particulate debris falling onto the surface.

4. Unclean coated objects entering the electrophoretic bath or inadequate post-rinsing.

5. Dirty coating environment.

6. Insufficient wettability of the electrocoat.

Solutions:

1. Reduce dust contamination and enhance bath liquid filtration. Ensure all circulating bath liquid passes through filtration equipment, ideally using 25 μm precision filter bags. Eliminate dead zones and exposed metal areas in the bath, strictly control pH and alkaline substances to prevent sedimentation, resin precipitation, or aggregation.

2. Improve the cleanliness of post-electrophoresis rinse water, keeping solids content as low as possible. Maintain overflow from downstream to upstream tanks, and filter the bath liquid to reduce foam.

3. Clean the drying chamber, air filters, and check the balance system and air leaks.

4. Enhance pre-electrophoresis workpiece rinsing to remove surface residues. Check whether the deionized water rinse tank filter is clogged to prevent secondary contamination of the coated object’s surface.

5. Maintain a clean coating environment, particularly the area between electrophoresis and the drying chamber, and eliminate sources of airborne dust.

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2. Shrinkage (Cratering)

Shrinkage is one of the most common paint film issues in electrocoating and a critical concern in pretreatment. Once shrinkage occurs, it often leads to batch quality issues, affecting the paint film’s appearance and corrosion resistance, significantly impacting product quality.

Types and Causes of Shrinkage:

1. Particle/Fiber Shrinkage: Caused by low-surface-tension or solid foreign matter (e.g., fibers or particles) landing on the wet electrocoat film. These defects are characterized by visible foreign matter at the center of the crater.

2. Oil-Induced Shrinkage: Caused by low-surface-tension substances like oil or silicones on the paint film. These craters typically lack raised particles at the center, do not expose the substrate, and range in size from 0.5–3 mm. This type usually occurs before the paint film cures; post-curing, it may result in oil stains or shallow depressions.

3. Bath Liquid Shrinkage: Results from contamination of the bath liquid by equipment, environmental factors, or workpieces, accumulating over time. This type is always accompanied by oil-induced shrinkage, with similar appearance. Its impact is initially minor but can be monitored through monthly bath liquid inspections.

4. Bubble-Induced Shrinkage: Caused by solvent bubbles or small foam accumulating on the wet paint film surface, not fully rinsed or defoamed before entering the drying oven, leading to large craters that often expose the substrate after curing.

Solutions:

1. Adjust the pigment-to-binder ratio of the bath liquid and add pigment paste to increase pigment content.

2. Strengthen management of the degreasing process for coated workpieces to prevent secondary contamination of the phosphating film.

3. Install oil-removal filtration in the bath liquid circulation system and identify and eliminate sources of oil contamination.

4. Enhance monitoring of post-rinse water quality, regularly clean or replace filter bags to ensure filtration quality.

3. Conclusion

Maintain a clean coating environment and eliminate substances harmful to coating, especially those containing silicones (e.g., cables, drawing oils, rust inhibitors, welding slag adhesives, sealants). Raw materials and auxiliary materials used in coating workshops and related equipment or processes must be free of ester ketones. Regularly clean the drying chamber, maintain cleanliness of the drying chamber and circulating hot air, and strengthen on-site management.