In powder electrostatic spraying, surface defects are a frequent headache for industry professionals – orange peel, exposed substrate, particles, pinholes, and other issues not only lower the product's appearance but can also affect performance, leading to a surge in rework rates. These defects are not entirely undetectable; most are related to material selection, process parameters, equipment status, or environmental control. Today, we'll break down the causes of eight common defects and share practical troubleshooting techniques to help you avoid pitfalls and improve your spraying success rate.

1. Orange Peel

Orange peel is the most obvious visual defect. The surface displays irregular bumps and ridges resembling orange peel, and the texture is clearly visible under magnification. The main causes are concentrated in three aspects:

 Poor leveling properties of the powder itself: Either the curing speed is too fast, solidifying before it can be spread evenly after melting; or the curing temperature is too low, resulting in poor powder flowability; other possibilities include poor powder dispersion (uneven mixing of pigments, resins, and additives), insufficient fineness (containing large pigment particles), or low-grade raw materials and a chaotic formula, leading to inconsistent surface tension during melt flow.

Inconsistent spraying operation: Intermittent powder output from the spray gun, inconsistent distance from the workpiece, or electrostatic shielding on the workpiece can all cause uneven powder layer thickness—too thin will form muscle-like wrinkles, while too thick will result in mottled orange peel.

Incorrect powder mixing: Different manufacturers use different formulations and raw materials for the same color powder, resulting in differences in melt flow speed and surface tension. Mixing these can easily lead to orange peel or even pinholes.

Solution: Prioritize high-quality powders with good leveling properties and uniform fineness, avoiding mixing powders of the same color from different brands; adjust spray gun parameters to ensure uniform powder output and stable gun distance (200-300mm recommended); strictly control the curing temperature and time according to powder requirements to allow the powder to fully melt and level.

2. Exposed Base Layer

Exposed base layer essentially indicates insufficient powder coverage on the workpiece surface, either due to low single-pass powder application or poor powder hiding power. Specific causes include 7 points:

Improper process parameters: Conveyor chain speed too fast, insufficient powder spraying time; low powder output (low conveyor air pressure) or blocked powder supply lines, resulting in poor powder delivery.

Poor electrostatic effect: Electrostatic generator voltage too low (normally should be maintained around 80KV), poor powder charging properties; hangers not cleaned for a long time, resulting in an excessively thick powder layer on the surface leading to poor conductivity (resistance > 1MΩ); poor workpiece grounding.

Operational and Equipment Issues: Workpiece dimensions are too large; improper spray gun position or powder distribution adjustment; internal spray gun component malfunction (e.g., broken discharge needle, abnormal resistance), affecting powder output.

Powder Quality Issues: Poor powder hiding power; even with adequate coating thickness, it cannot completely cover the substrate. This is related to pigment grade, fineness (especially titanium dioxide), and the selection of matting agents or fillers.

Solutions: Optimize process parameters, adjust chain speed and powder output, and regularly clean the powder supply pipeline; check the voltage of the electrostatic generator and the conductivity of the hangers, and replace faulty spray gun parts promptly; select powders with adequate hiding power, and request test reports from suppliers if necessary.

3. Particles

Particle defects are one of the main causes of rework. Fine particles adhering to the surface can severely affect coating smoothness. Common causes include: Environmental and Substrate Contamination: Dust and impurities remain on the workpiece surface after pretreatment; an unclean powder coating workshop environment, with dust settling on the workpiece after powder coating or during curing; prolonged uncleaning of the drying tunnel, causing internal dust to fall onto the coating surface.

Powder and Equipment Issues: Particles are mixed into the recycled powder, and the powder box vibrating screen is damaged or poorly sealed; the powder itself contains dust and large-diameter pigment particles; the powder supply pipe or Venturi throat has been used for a long time, resulting in a powder skin that has not been cleaned; residual impurities remain after the powder removal process by burning the hangers; the powder becomes damp and clumps, resulting in uneven atomization.

Solutions: Strengthen the cleanliness management of the workshop and drying tunnel, and clean the inside of the equipment regularly; ensure dust protection for the workpiece after pretreatment; inspect and maintain equipment such as the vibrating screen and powder supply pipeline to prevent impurities from entering; store the powder properly to prevent it from becoming damp and clumping, and ensure it is fully fluidized before use.

4. Pinholes

Pinholes are small, round depressions that appear on the coating surface, mainly caused by oil contamination or powder compatibility issues: Oil contamination: Incomplete degreasing of the workpiece during pretreatment, or contamination by oil from gloves or transport boxes during transport; oil in the compressed air; the conveyor chain was recently lubricated with grease, which evaporates during high-temperature baking, or oil condensed on the inner wall of the drying tunnel evaporates and adheres to the workpiece surface.

Powder Issues: Mixing powders of the same color from different manufacturers leads to poor compatibility due to formulation differences; improper powder raw material ratios or insufficient grade; oil contamination of the powder hopper fluidizing plate (the root cause is oil in the compressed air).

Solutions: Strengthen the pretreatment degreasing process to avoid secondary contamination during workpiece transfer; regularly check compressed air purity and install oil removal devices; regularly clean the drying tunnel and conveyor chains to prevent oil buildup; do not mix powders from different brands, and choose products with stable formulations.

5. Poor Mechanical Properties

Poor mechanical properties include adhesion, flexibility, impact strength, and hardness not meeting standards, directly affecting product lifespan. Main causes include: Improper curing parameters: Curing temperature too low or time insufficient, resulting in incomplete curing; or curing temperature too high or time too long, leading to thermal aging of the coating and decreased flexibility. Different powders have different curing conditions; for example, epoxy powders can be cured at 140℃/25min or 220℃/5min, and product requirements must be strictly followed.

Powder Quality Issues: Defects in the powder itself, such as substandard components like curing agents, resins, and fillers; excessively long storage time exceeding the shelf life, leading to performance degradation.

Improper Pre-treatment: Oil and dust on the workpiece surface are not properly cleaned, affecting powder adhesion; the phosphating film is not fine or dense enough, failing to provide a good foundation for coating adhesion.

Solutions: Use an oven temperature tracker to precisely control curing temperature and time to avoid under- or over-curing; select reliable powders and use them within their shelf life; strictly control the pre-treatment process to ensure clean workpiece surfaces and generate a high-quality phosphating film.

6. Loss of Gloss/Discoloration

Loss of gloss and discoloration often occur together, mainly affecting the consistency of product appearance. The causes are concentrated in three points:

Exceeding Curing Parameter Standards: Excessively high curing temperature or time leads to insufficient heat resistance of pigments (especially red, yellow, and blue pigments), resulting in yellowing; resin aging causes the coating to become brittle and lose gloss.

Powder Quality Issues: Inconsistent gloss levels between different batches of powder, or mixing powders with different gloss levels; improper selection of pigments, matting agents, gloss enhancers, and other additives in the powder.

Powder Mixing: Significant differences in powder formulations between manufacturers, with variations in resins, pigments, and additives, can easily lead to discoloration and loss of gloss after mixing.

Solutions: Strictly control curing parameters according to powder requirements, avoiding prolonged high-temperature baking; use powders from the same brand and batch for the same product, avoiding indiscriminate mixing; select high-quality powders with stable gloss levels.

7. Pinholes

Pinholes are tiny penetrating holes on the coating surface, mainly caused by two factors:

Improper Electrostatic Parameters: Excessively high spray gun voltage, or excessive distance from the workpiece (less than 200mm), causing the powder layer to be punctured by high voltage, forming pinholes.

Pretreatment Issues: Rough phosphating film with scum; even if the powder coating thickness meets standards, it cannot completely cover underlying defects, ultimately resulting in pinholes.

Solutions: Adjust the spray gun voltage and distance to a reasonable range (recommended distance 200-300mm); optimize the phosphating process to ensure a fine, slag-free phosphating film and improve surface smoothness.

8. Common Problems in the Production Process

Besides defects in the coating itself, problems such as powder escape and intermittent powder output from the spray gun may occur during production, indirectly affecting coating quality:

8.1 Powder Escape

This not only pollutes the environment and harms worker health but can also lead to cross-contamination between different colored powders. Common causes include: the spray gun being too close to the powder spraying chamber inlet and outlet; excessive air pressure and powder flow rate; poor sealing of the powder spraying chamber recovery component, insufficient suction of the worm gear fan, or blockage of the recovery filter (caused by damp powder, low backflushing air pressure, etc.).

Solutions: Adjust the spray gun position and optimize air pressure parameters; check and repair the seal of the recovery component, regularly clean or replace the recovery filter to ensure sufficient backflushing air pressure; adjust the powder output according to the fan suction to avoid exposing the workpiece.

8.2 Intermittent Powder Discharge from the Spray Gun

This will cause uneven powder layer thickness, leading to defects such as orange peel and exposed substrate. The main causes are: unstable compressed air; blocked, tortuous, or excessively long powder supply pipeline (recommended not to exceed 8m); poor powder fluidization in the powder supply hopper (caused by unstable compressed air, blocked fluidizing plate, or damp powder).

Solutions: Ensure stable compressed air supply; regularly clean the powder supply pipeline and avoid excessively long bends; check the condition of the fluidizing plate to ensure good air permeability; avoid moisture absorption during powder storage; and ensure thorough fluidization before use.

9. Summary

The core of electrostatic powder coating is the synergistic coordination of "materials, processes, equipment, and environment". To reduce defects, firstly, select reliable powders with stable formulations, avoiding mixing products from different brands or batches. Secondly, precisely control process parameters, especially curing temperature, time, electrostatic voltage, and gun distance, using tools such as oven temperature trackers and voltage detectors to ensure parameters meet standards. Thirdly, perform routine equipment maintenance, regularly cleaning powder supply pipelines, drying tunnels, and recycling components to ensure stable equipment operation. Finally, optimize the production environment, strengthening workshop cleanliness and dust control to prevent substrate and coating contamination.

In fact, most spraying defects can be resolved through "prevention + adjustment during process + post-process investigation." By being familiar with powder characteristics, mastering key process points, and understanding equipment status, problems can be specifically avoided, improving coating quality and production efficiency, ensuring powder coating is both aesthetically pleasing and performs exceptionally well.