When appearance issues arise with workpieces at the coating site, a comprehensive investigation should first be conducted to identify the root cause.
1. Shrinkage (Cratering)
Shrinkage defects are not visible in wet paint films but appear after drying as pinholes (0.5–3.0 mm in diameter) that may expose the substrate (shrinkage) or form crater-like depressions without exposing the substrate (craters/pits). These defects often originate from dust, oil, or incompatible particles in or on the wet film, which act as the center of the crater, leading to coating imperfections. In many cases, these defects are also related to the material of the coated substrate, such as microcracks or micropores in metal substrates.
Cause 1: Contamination of the electrocoat film by external oil, which adheres to the workpiece surface and affects film formation. This is a common cause of shrinkage.
Solution: Inspect the conveyor system and racks to prevent oil droplet contamination. Ensure that no contaminants (e.g., oil, silicone oil, wax, greasy hydrocarbons, glue) affect the workpieces, equipment, or bath liquid, starting from the manufacturing and installation of electrophoretic equipment. New parts should be inspected for contamination before being introduced into the coating process.
Cause 2: Incomplete degreasing during pretreatment, leading to poor wettability and shrinkage in the dried paint film.
Solution: Enhance pretreatment cleaning processes.
Cause 3: Oil or foreign matter in the bath liquid, affecting the appearance of the electrocoat film.
Solution: Use oil-absorbing cotton or oil-removal filter bags to remove oil, eliminate foreign matter from the bath liquid, and maintain bath cleanliness to prevent contamination.
Cause 4: Uneven mixing of electrocoat during addition, leading to incomplete maturation of the bath liquid and poor film quality.
Solution: Ensure thorough mixing of added electrocoat and enhance bath circulation to achieve complete maturation.
Cause 5: Oil in rinse water or an unclean drying oven (e.g., oil in circulating air), causing oil to adhere to the paint film and form shrinkage after drying.
Solution: Regularly replace rinse water and clean the drying oven. Use high-temperature-resistant oil for oven chain tracks that does not volatilize at high temperatures.
2. Particles
Particles refer to the presence of foreign matter oneffect the electrocoat film surface, resulting in a rough texture with hard particles or fine visible grains after drying.
Cause 1: Large impurities in the electrophoretic bath liquid.
Solution: Inspect the bath filtration system and enhance filtration of the bath liquid.
Cause 2: Dust or other airborne particles in the workshop settling on workpieces in the drip-drying area.
Solution: Maintain workshop cleanliness to prevent dust from floating.
Cause 3: Dust or debris in the drying oven.
Solution: Regularly clean the drying oven.
Cause 4: Incomplete cleaning during pretreatment.
Solution: Strengthen pretreatment rinsing and degreasing.
Cause 5: Particles on the metal workpiece surface, such as burrs or blisters on plated surfaces.
Solution: Inspect the workpiece surface for issues before hanging.
3. Thin Coating Film
A thin coating film refers to an excessively thin paint film on the workpiece surface after electrophoresis, resulting in unsatisfactory color gloss, hiding power, and reduced quality.