Small batch sheet metal electrophoresis is prone to the following typical defects due to the characteristics of small order quantities, large fluctuations in pre-processing, and low frequency of tank maintenance. Targeted solutions should be taken based on process principles:

1、 Poor adhesion of coating (powder shedding, peeling)

Defect manifestation: The coating separates from the substrate, causing powder to fall off with light scraping and large-scale peeling with heavy scraping. It is commonly found at the edges, welds, and areas with incomplete pre-treatment.

  • Core causes:
  1. Inadequate pre-treatment: residual oil stains after incomplete oil removal, thin or uneven phosphating film, and missing passivation film, resulting in the inability of the coating to form chemical/mechanical bonding with the substrate.
  2. Abnormal electrophoretic parameters: low voltage (insufficient coating deposition), high current density (localized burning), high bath temperature (rapid coating cross-linking).
  3. Tank contamination: Impurities (such as pre-treatment residues and dust) are mixed into the tank, affecting the uniform deposition of coating particles.
  • Solution:
  1. Strengthen pre-treatment: Oil removal adopts the four step method of “alkali washing+hot water washing+acid washing+hot water washing” to ensure no oil stains or rust stains; Phosphating film thickness ≥ 5 μ m, uniform without flower spots; The passivation film is intact without spots.
  2. Calibration of electrophoresis parameters: The cathodic electrophoresis voltage is controlled at 180220V, the bath temperature is 2832 ℃, the current density is 0.81.2A/dm ², and the solid content (1215%) and pH value (7.5-8.5) are regularly detected.
  3. Filter tank solution: Use a 100200 mesh filter element for circulating filtration, filter twice a day, and replace a portion of the tank solution (10% and 20%) weekly to avoid the accumulation of impurities.

2、 Uneven coating thickness (large difference in thickness)

Defect manifestation: The difference in coating thickness on the surface of the workpiece is greater than 10 μ m, with thin grooves and corners, and thick surfaces and protrusions, which affects the anti-corrosion performance and appearance.

  • Core causes:
  1. Improper hanging method of workpieces: The distance between hanging fixtures is too large, and the distance between workpieces is too close, resulting in uneven distribution of electric field, insufficient or excessive local deposition.
  2. Insufficient agitation of the tank solution: The flow rate of the circulating pump is insufficient, and the distribution of the tank solution components (such as resin and pigment) is uneven, which affects the migration of coating particles.
  3. Differences in pre-treatment film layers: The thickness of the phosphating film is inconsistent, resulting in different adsorption amounts of the coating during electrophoresis.
  • Solution:
  1. Optimize hanger design: hanger spacing ≤ 10cm, workpiece spacing ≥ 20cm, ensuring uniform electric field; Complex workpieces adopt “multi-point suspension” to avoid local obstruction.
  2. Strengthen the circulation of the tank solution: The flow rate of the circulation pump should be ≥ 10m ³/h, stirred twice per hour, and the solid content and pH value of the tank solution should be regularly checked to ensure uniformity.
  3. Unified pre-treatment standard: The thickness of the phosphating film should be controlled within 5-8 μ m, uniform without spots, and the passivation film should be intact without spots to avoid differences in pre-treatment affecting the electrophoresis effect.

3、 Coating pinholes and bubbles

Defect manifestation: Needle like small holes (pinholes) or circular bubbles (bubbles) appear on the surface of the coating, affecting the anti-corrosion performance and appearance.

  • Core causes:
  1. Residual moisture in pre-treatment: The workpiece has not been dried, and the moisture evaporates during electrophoresis to form bubbles.
  2. High gas content in the tank solution: Air leakage in the circulation system and intense stirring of the tank solution can cause air to mix into the tank solution, leading to the deposition of paint particles carrying bubbles.
  3. Impurities introduced into the workpiece: corrosion of fixtures, dust on the surface of the workpiece, and the formation of local “air gaps” during electrophoresis, leading to coating defects.
  • Solution:
  1. Thoroughly dry the workpiece: After pre-treatment, dry at a temperature of 6080 ℃ for 1015 minutes to ensure no residual moisture.
  2. Eliminate the gas in the tank: check the sealing of the circulation system to avoid air leakage; The stirring speed should be controlled at 50-80rpm to avoid introducing air during vigorous stirring; Regularly use defoamers to treat the tank solution.
  3. Cleaning fixtures and workpieces: The fixtures are made of stainless steel and regularly polished to remove rust; Thoroughly remove dust and oil stains during workpiece pre-treatment to avoid impurities from entering the tank solution.