Causes and Solutions for "Powder Failure" in Powder Coating

1. Introduction

"Powder failure" is a common and complex defect in electrostatic powder coating. It affects film thickness and appearance, and leads to powder waste and rework. This article analyzes its root causes from five perspectives: powder formulation performance, equipment condition, process parameters, workpiece pretreatment, and environmental conditions, and proposes targeted solutions to improve transfer efficiency and coating stability.

2. Powder Formulation and Performance

The inherent properties of a powder determine its ability to effectively charge and deposit. An unbalanced resin/hardener ratio can result in poor flowability, weak charging ability, and even premature curing. Excessive resin can hinder the formation of a dense coating film; excessive hardener can trigger a "premature reaction," causing the powder to lose flowability immediately after leaving the spray gun.

Particle size is crucial: coarse particles (>100 μm) tend to scatter and have poor adhesion; fine particles (<10 μm) cannot retain their charge and will be blown away, leaving bare areas. Prolonged storage in a humid environment can cause moisture absorption and agglomeration, clogging powder feed lines or causing discharge agglomerates. Recommendations: Use a narrow, medium D50 (35–45 μm); control additive dosage and avoid high levels of low-Tg materials; store below 25°C and ≤60% RH; pass through an 80-mesh screen before use to ensure flowability and uniformity.

3. Equipment and Electrostatic System

The spray gun and feed material properties determine the degree of charging. If the spray gun voltage is low (<60 kV), charging is insufficient; if the current is too high, corona discharge can cause powder dispersion. Worn nozzles/electrodes distort the magnetic field, resulting in uneven charging. Contaminated compressed air (water/oil/particulate matter) can reduce quality; clogged feed/recovery devices or insufficient airflow can lead to unstable and poor delivery.

Recommendations: Regularly calibrate the high voltage (60–100 kV/10–30 μA); inspect the nozzle/needle; use three-stage filtration to remove water and oil; maintain air pressure at 0.4-0.5 MPa; and regularly clean the powder hose and venturi.

4. Process Parameters

Even with high-quality powder and equipment, improper settings can lead to poor pickup. Gun distances greater than 300 mm can cause charge decay; less than 150 mm can cause reverse ionization, edge buildup, or pinholes. Excessive movement speed can result in weak or exposed areas; tilt angles can concentrate the electric field on one side. Complex components (e.g., grooves/corners) can create "blind spots."

Recommendations: Set gun distance to 200–250 mm; traverse speed 0.8–1.2 m/min; for complex components, use multi-angle channels or auxiliary guns; verify the gun path at each shift change to maintain a symmetrical field.

5. Workpiece Pretreatment

Workpiece surface cleanliness is a core prerequisite for subsequent adsorption and adhesion quality, directly affecting process stability. Incomplete degreasing, leaving residual oil, dust, rust, metal oxides, or mold release agents, will form a dense insulating layer on the workpiece surface, directly hindering the normal functioning of electrostatic attraction and preventing effective adhesion in subsequent processes. Common defects in the pretreatment stage also require close attention: insufficient phosphating film thickness weakens the substrate's protective ability; aging of the plating solution leads to an imbalance in component ratios; and excessively low processing temperature slows down the chemical reaction rate. These problems all result in a loose and porous film structure, significantly reducing adhesion to the workpiece surface. Surface roughness control is also crucial: when the surface roughness (Ra) is below 3μm, the surface is too smooth, making it difficult to form effective anchor points and affecting adhesion stability; while when Ra exceeds 10μm, the surface unevenness is too great, easily forming local accumulation in depressions, leading to uneven coating thickness. Recommended operating points: Strictly follow the standardized process of "degreasing → rinsing → phosphating → DI rinsing → drying" to ensure each step is fully effective; precisely control the surface roughness within the optimal range of 3–8 micrometers; after pretreatment, thoroughly blow away residual dust and moisture from the surface using clean compressed air; before spraying, professional testing is required to confirm that the workpiece surface is clean, dry, and has good conductivity to avoid affecting the final product quality due to improper pretreatment.

6. Environmental and Application Conditions

High humidity (>70% RH) promotes moisture absorption, reducing powder flowability and charge; condensation on parts hinders deposition. Extremely low temperatures (<10°C) reduce powder flowability; extremely high temperatures (>35°C) risk premature reaction or agglomeration. Poor ventilation and high dust concentration in the spray booth can induce reverse ionization, neutralizing the powder charge.

Recommendations: Maintain shop temperature at 18–28°C and relative humidity at 40–60%; use dehumidifiers/HVAC systems; regularly clean overspray and maintain good air circulation. In hot seasons, shorten storage periods and use insulated transport.

7. Conclusion

This problem is rarely singular; it stems from imbalances in formulation, equipment, process, pretreatment, and environment. Recommended troubleshooting sequence:

7.1 Check powder (size, flow rate, humidity);

7.2 Verify spray gun voltage/current and air pressure;

7.3 Adjust spray gun distance/angle;

7.4 Confirm cleanliness and phosphate quality;

7.5 Stabilize temperature/humidity and ventilation. Standardized parameter tables and regular equipment checks can improve transfer stability and film consistency, and reduce waste and rework.