Unveiling the "L Effect" in Electrophoretic Coating: Why Does the Coating "Slip" Away?

The "L Effect" in electrophoretic coating refers to a phenomenon where the coating appearance differs between the flat (horizontal) and vertical surfaces of a workpiece during the coating process. Specifically, the flat surface typically exhibits lower gloss, a rougher texture, uneven film thickness, particle impurities, and poor leveling compared to vertical surfaces. This effect not only impacts the aesthetic quality of the workpiece but may also compromise its protective performance. Below, we detail the causes of the L Effect and corresponding solutions.

01 Causes of the L Effect

① Frontend: Pretreatment

1. Poor degreasing or insufficient wetting, leading to particles, dust, or bubbles, which can cause pitting or, in severe cases, substrate exposure.
2. Rough phosphating film, resulting in poor coating leveling and reduced gloss.
3. Excessive phosphating sludge, leading to particle formation.
4. Contaminated pretreatment liquid or incomplete rinsing before entering the bath, increasing impurities and forming particles.

② Mid-End: Bath Liquid

1. Imbalanced pigment-to-binder ratio (high ratio causes pigment settling, increasing particles).
2. pH imbalance: High pH leads to poor wetting, while low pH causes redissolution, thinning the film on flat surfaces and creating thickness differences between vertical and flat surfaces.
3. Low solvent content, resulting in poor film formation and insufficient wetting, causing pitting or substrate exposure.
4. Bacterial growth, increasing particle formation.
5. Insufficient circulation flow, leading to bath liquid sedimentation and increased particles.
6. Poor anode system performance (e.g., blocked anode membranes, poor conductivity, or corroded anode tubes/plates), causing uneven electric fields.
7. High conductivity, leading to inconsistent film formation rates, bath liquid aging, resin failure, pigment settling, or breakdown bubbling.

③ Backend

1. UF (Ultrafiltration) Tank:
   • Incomplete rinsing of floating paint, causing streaks, flow marks, or particles.
   • Low pH causing redissolution, thinning the film and creating thickness differences between vertical and flat surfaces.
2. Draining Area + Oven: Dust or impurities settling on the surface cause particle formation, while uneven airflow leads to temperature variations in the oven, resulting in inconsistent gloss across the workpiece.

02 Solutions to the L Effect

① Frontend: Pretreatment

1. Increase the concentration and temperature of the pretreatment liquid as needed.
2. Adjust parameters of the phosphating solution.
3. Remove phosphating sludge.
4. Frequently replace water or increase overflow rinsing.

② Mid-End: Bath Liquid

1. Add emulsion to balance the pigment-to-binder ratio.
2. Add acid, drain ultrafiltration, or drain anolyte.
3. Increase solvent content.
4. Sterilize, precipitate, or transfer the bath liquid.
5. Increase circulation flow and inspect nozzles.
6. Check for membrane blockages (sterilize if needed) and inspect anode tubes/plates for corrosion.
7. Drain ultrafiltration, reduce conductivity, and add fresh paint.

③ Backend

1. UF Tank:
   • Check for clogged nozzles, frequently replace water, or increase overflow (in immersion tanks).
   • Drain ultrafiltration or anolyte.
2. Maintain a clean environment, ensure even airflow, measure the oven temperature curve, and clean the oven regularly.

Summary

The L Effect is a common coating defect in electrophoretic coating. By strengthening pretreatment and bath liquid management, along with implementing auxiliary measures, the L Effect can be effectively addressed. These measures not only enhance production efficiency and quality stability but also improve the protective performance and aesthetic appeal of the coated workpieces.