Abnormal Conditions and Solutions for Electrocoating (23 Issues) - III

I. Overview

Many abnormal issues in the electrocoating system can be addressed by identifying the causes through daily and periodic monitoring and analysis of the bath solution, thereby resolving coating film abnormalities. Since the actual conditions of each electrocoating line vary, the elimination method is usually adopted to troubleshoot problems. The following analysis and treatment suggestions serve as general operational guidelines.

II. Methods for Handling Abnormal Conditions

(I) Clarify the Problem

1. Distinguish whether the problem occurs in the system or on related surfaces.
2. Determine the type of issue and whether it affects all workpieces.
3. Identify if the problem affects the entire workpiece or consistently occurs at the same location.
4. Ascertain the timing of the problem.
5. Understand the persistence of the problem—whether it occurs frequently, occasionally, or is related to changes in certain factors.

(II) Identify the Cause of the Fault

1. Analyze common causes of the fault and check for any changes in the production line.
2. Judge if there is a correlation between the bath solution parameters and the problem symptoms.
3. Inspect mechanical aspects, and test whether equipment, instruments, and electrophoretic production line devices are operating normally.
4. Verify process compliance—check for any operations that violate processes or procedures.
5. Confirm if there are changes in the quality of pretreatment.

(III) Determine Adjustment Measures

1. Clarify which variables can be quickly measured on the line.
2. Identify which variables can be measured with minimal interruption to production.
3. Know which variables need to be measured in the laboratory.
4. Judge if technical support from suppliers is required for parameter measurement.

(IV) Implement Adjustment Measures

Each variable needs to be tested to eliminate or confirm potential solutions. If multiple solutions exist, they should be verified one by one. If no solution is found, re-clarify the problem before further handling.

(V) Follow-Up

Determine constant variables to prevent the problem from recurring. Maintaining detailed records enables rapid response to future similar issues.

(VI) Preventive Measures

1. Ensure the normal operation of electrocoating equipment, and conduct regular maintenance or cleaning annually.
2. Maintain stable pretreatment processes and control pretreatment quality.
3. Keep all parameters of the electrophoretic bath solution within the process-specified range.

III. Specific Abnormal Conditions and Solutions

13. Craters and Dimples

• Poor degreasing of the workpiece during pretreatment or re-contamination with oil/dust after cleaning: Strengthen the workpiece degreasing process and ensure the phosphating film is not secondarily contaminated.
• Oil contamination in the bath solution, floating on the liquid surface or emulsified in the bath solution: Install oil-removing filter bags in the bath solution circulation system and identify the pollution source.
• Oil contamination in the post-electrophoresis washing solution: Improve the quality of post-washing water, enhance filtration, and replace with clean water if necessary.
• Contaminated drying chamber with oil in the circulating air: Keep the drying chamber and circulating hot air clean.
• Replenished paint with cratering issues, or poor resin dissolution/neutralization: Strengthen the management of replenished paint; do not add substandard paint to the tank, and ensure the replenished paint is well-dissolved, neutralized, and filtered.
• Oil in the compressed air at the water-blowing station Vent the buffer tank of the compressed air.
• Oil dripping from the overhead chain: Install oil collection devices.
• Low pigment-to-binder ratio in the bath solution, resulting in poor coating leveling and increased tendency for cratering: Adjust the pigment-to-binder ratio of the bath solution and add color paste appropriately to increase pigment content.
• Low solvent content in the bath solution, resulting in poor coating leveling: Replenish solvent to improve coating leveling.

Note: Invisible on the wet film, but crater-like pits (0.5-3.0 mm in diameter) appear on the dried coating surface. Pits that do not expose the substrate are called "dimples"; those that expose the substrate are called "craters"; and those with particles in the center are called "fish eyes". These defects mainly occur because dust, oil, or substances incompatible with the electrophoretic paint exist on or in the wet electrophoretic film, acting as centers for dimple formation and causing uneven leveling during drying.

14. Pinholes

• Too low or too high pigment-to-binder ratio in the bath solution: Replenish paint to adjust the pigment-to-binder ratio to within the process range.
• Low pH value of the bath solution: Discharge the anode solution to increase the bath solution pH value.
• Excessively high content of impurity ions in the bath solution, causing intense electrolytic reactions and excessive gas generation on the workpiece surface: Discharge the UF solution and add pure water to reduce the content of impurity ions.
• In electrophoretic tanks with segmented voltage, small spacing between workpieces leads to potential differences between the rear and front workpieces, with the rear workpiece acting as an anode and causing pinholes through reaction: Replenish solvent to improve coating leveling.
• High anode solution conductivity, intensifying electrolytic reactions and increasing bubble generation on the workpiece surface: Discharge the anode solution and replenish with pure water; unclog the return pipeline of the anode tube promptly if blocked.
• Bath solution temperature too low or too high: Adjust the bath solution temperature to within the process range.
• Delayed post-washing of the workpiece after electrophoresis, causing redissolution of the wet coating: Wash the workpiece with UF solution or pure water promptly after it leaves the bath (within 1 minute).
• High porosity and roughness of the phosphating film: Adjust the phosphating solution parameters to ensure a uniform and dense phosphating film.
• Poor circulation leading to inadequate defoaming of the wet film: Increase the bath solution agitation intensity to remove bubbles from the workpiece surface.
• Low solvent content in the bath solution, resulting in poor coating leveling: Replenish solvent to improve the leveling property of the coating film.
• Low flow rate of the bath solution at the workpiece entry end, causing foam accumulation: Maintain a liquid surface flow rate of over 0.2 m/s to eliminate foam accumulation at the entry end.
• Excessively large liquid level difference between the main tank and auxiliary tank, forming bubbles that enter the main tank after circulation: Replenish pure water or paint to control the liquid level difference between the main and auxiliary tanks to approximately 10 cm.

Note: Tiny pinhole-like pits or holes formed on the dried electrocoating film are called "pinholes". They differ from craters (pits) in that they have smaller diameters, no foreign matter in the center, and no coating accumulation around them. Pinholes caused by redissolution of the wet film are called "redissolution pinholes"; those caused by gas generation during electrophoresis or inadequate defoaming of the wet film are called "stepwise pinholes from charged tank entry", which generally occur on the lower part of the workpiece.

15. Water Stains

• High solvent content in the bath solution: Discharge ultrafiltrate and replenish pure water to reduce the solvent content.
• Water droplets or beads on the surface of the wet electrocoating film not volatilized or blown off before drying Blow off water droplets/beads using compressed air or fans to remove moisture.
• Water droplets dripping from hangers and overhead chains: Implement measures to prevent water droplets from falling onto the workpiece.
• Water accumulation on the workpiece: Take steps to drain accumulated water from the workpiece.
• Poor water resistance and electroosmosis of the wet film: Improve the water droplet resistance of the used electrocoat.
• Excessively rapid temperature rise after entering the drying chamber: Avoid abrupt temperature increases or add a preheating stage (install a low-temperature zone).
• Insufficient pure water washing: Enhance the intensity of pure water washing.

Note: After drying the electrophoretic film, uneven water droplet-like spots appear on local areas of the coating surface, affecting the flatness of the film. This defect is called "water stains". It occurs when water droplets or accumulated water remain on the wet film, causing boiling during drying and forming uneven coating surfaces at the droplet locations.

16. Secondary Flow Marks

• Complex workpiece structure with numerous gaps: Improve the structure if possible and open reasonable process holes.
• Improper selection of pre- and post-electrophoresis cleaning processes: For complex workpieces (e.g., pipes), adopt a combined immersion-spray cleaning process, or use water flushing and compressed air to blow out bath solution from gaps.
• Abrupt temperature rise when entering the drying chamber: Strengthen water draining and air-drying functions, and add a preheating stage before drying.

Note: After post-washing, the surface of the wet electrocoating film appears normal, but flow marks form at the gap structures of the workpiece after drying. This coating defect is called "secondary flow marks".

17. Orange Peel

• Excessively thin or thick coating film: Adjust the coating film thickness to the process-specified range.
• Low pigment-to-binder ratio in the bath solution: Replenish color paste to increase the pigment-to-binder ratio.
• Excessively low or high solvent content in the bath solution: Control the solvent content through the ultrafiltration system and paint feeding.
• Abrupt temperature rise in the drying tunnel, causing the upper coating layer to dry too quickly: Adjust the temperature curve in the drying tunnel and set up a low-temperature baking zone.

Note: Wrinkles appearing on the surface of the electrocoating film are mainly caused by imbalanced bath solution parameters or improper application conditions. This defect is referred to as "orange peel" due to its resemblance to the texture of an orange rind.

18. Coating Surface Stains

• Poor degreasing effect, with local oil residues remaining: Enhance the degreasing effect.
• Poor water washing effect after degreasing: Maintain water quality and ensure no oil contamination on the surface of the water washing tank.
• Mottled, blue-tinted phosphating film or flow marks on the phosphating film: Strengthen water washing, adjust phosphating solution parameters to ensure a uniform and fine phosphating film.
• Poor water quality for post-phosphating water washing: Strengthen water quality management for post-phosphating washing; the conductivity of dripping water after pure water washing should not exceed 30 μs/cm.
• Recontamination of the phosphated coated surface: Prevent secondary contamination of the treated surface, maintain a clean environment, and prevent water dripping from hangers.

Note: Due to contamination on the substrate surface or phosphating film, streak or map-like stains remain visible on the surface of the dried coating film after electrocoating. This defect is called "coating surface stains". Unlike water stains and paint marks, the coating surface remains flat, but the water resistance and corrosion resistance of the film are affected.