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Common Issues and Solutions in UV Vacuum Plating (Part 5)

Release time:

2026-06-08 17:02

Common Issues and Solutions in UV Vacuum Plating (Part 5)

In the practical production of UV vacuum plating, pitting is one of the common defects that compromise surface quality. Pitting manifests as tiny depressions on the coated surface, undermining its smoothness and mirror‑like finish. Unlike particles, which appear as raised features, pitting represents a concave defect; under illumination, it casts shadows along its edges and produces muted internal reflections. Beyond detracting from appearance, pitting also compromises the coating’s density and integrity. To address this defect, a systematic approach is required—covering aspects such as control of coating‑related air bubbles, the use of defoaming agents, primer sealing, and spray‑process parameters—in order to effectively minimize pitting and enhance the surface quality of plated products.

I. Countermeasures for Controlling Coating Bubbles

1. Mixing and standing to remove air bubbles

Before use, the coating should be thoroughly mixed until uniform; however, avoid excessive stirring speed and prolonged mixing time to minimize air entrainment. During mixing, tilt the container to reduce air incorporation. After mixing, allow the coating to stand for a period of time to enable any entrained air bubbles to rise and escape naturally. The standing time should be adjusted according to the coating’s viscosity and bubble content—higher‑viscosity formulations require longer settling periods. For coatings with a high bubble content, vacuum degassing can be employed to rapidly remove internal bubbles under reduced pressure.

2. Sealing and Venting of the Conveying System

During paint pumping, filtration, and delivery, pipeline connections must remain sealed to prevent air from being drawn in at the joints. The piping design should avoid sharp bends and abrupt changes in cross-section to minimize bubble formation caused by turbulence. Paint tanks and delivery lines should be equipped with venting devices to promptly remove accumulated air from the system. When switching between paint batches, purge the air from the piping before initiating circulation.

3. Control of Compressed Air Quality

Compressed air used for spraying must be dried and filtered to remove moisture and oil. Moisture and oil entering the coating can cause bubbling and may also lead to defects such as craters. The compressed-air system should be equipped with an oil‑water separator and a precision filter, and accumulated condensate in the air receiver tank should be drained regularly. Filter elements should be replaced on schedule to ensure effective filtration.

II. Measures for the Use of Defoamers

1. Optimization of defoamer dosage

When the defoamer dosage in a coating is insufficient, the amount added should be appropriately increased. The optimal defoamer level should be determined through testing, using the lowest concentration that effectively eliminates bubbles as the benchmark. If the dosage is too low, the defoaming effect will be inadequate; if it is too high, it may lead to cratering or other surface defects. Defoamer requirements can vary slightly between batches of coating, so adjustments should be made on an as‑needed basis.

2. Selection of Defoamer Types

The compatibility of a defoamer with the coating system is a critical factor influencing its defoaming performance. If an improperly selected defoamer results in suboptimal efficacy, it should be replaced with a formulation that exhibits better compatibility. Silicone‑based defoamers offer high defoaming efficiency but have relatively poor compatibility; excessive use can easily lead to cratering. Non‑silicone defoamers generally exhibit superior compatibility, though their defoaming effectiveness is comparatively lower. The appropriate type of defoamer should be chosen based on the coating formulation and application requirements.

3. Control of the timing of defoamer addition

The timing of defoamer addition and its dispersion state can significantly affect defoaming performance. The defoamer should be added during the initial stage of paint mixing to ensure thorough dispersion. When adding, introduce it slowly while stirring to prevent localized overconcentration. If the defoamer is unevenly dispersed, areas with excessive concentration may develop craters, while regions with insufficient concentration will exhibit poor defoaming.

III. Measures to Improve the Sealing Performance of the Primer

1. Control of primer coating thickness

The primer coat thickness should be sufficient to seal the substrate’s micropores and minor surface defects. A primer that is too thin will fail to adequately fill these defects, potentially leading to pinholes after coating. The coating thickness should be determined based on the substrate’s surface condition; for substrates with high surface roughness or numerous micropores, the primer thickness should be increased accordingly. Edges and recessed areas are typically weak points in the coating application and require particular attention to ensure adequate coverage.

2. Improvement of primer leveling properties

The primer should exhibit excellent leveling properties, enabling it to fill microscopic surface imperfections on the substrate before curing. When the primer’s viscosity is too high, its leveling performance deteriorates; this can be mitigated by adjusting the formulation or preheating to reduce viscosity. Sufficient leveling time is essential to ensure the coating spreads evenly prior to cure. Adding an appropriate amount of a leveling agent can lower surface tension and enhance leveling.

3. Optimization of the primer’s curing state

When the primer is not fully cured, residual unreacted monomers in the coating may volatilize under vacuum conditions, leading to bubble formation and pinholing. It is essential to ensure complete curing of the primer, with curing energy and duration meeting the specifications of the primer formulation. Over‑curing can render the primer brittle, potentially generating microcracks in a vacuum environment, which likewise results in pinhole defects. The primer’s cure state should be verified through adhesion and surface hardness tests to strike an optimal balance between under‑curing and over‑curing.

4. Control of Volatiles in the Substrate

For substrates with high hygroscopicity, drying should be performed prior to coating to remove moisture adsorbed within the substrate. The drying temperature and duration should be determined based on the substrate’s thermal stability to prevent deformation at elevated temperatures. Residual low-molecular-weight additives and unreacted monomers in the substrate also contribute to volatile emissions; selecting substrates with low volatile content can help mitigate pinholing issues.

IV. Optimization Strategies for Spraying Parameters

1. Adjustment of Spray Pressure

Spray pressure should be maintained within an appropriate range to avoid being too high or too low. When the pressure is excessive, the high‑velocity coating droplets striking the workpiece surface may rebound, creating microscopic spatter craters; when the pressure is insufficient, atomization is poor, resulting in coarse coating particles and interparticle gaps that can lead to pitting. Determine the optimal spray pressure through experimentation and maintain it consistently during production.

2. Control of Spray Distance

The spray distance should be maintained within an appropriate range to avoid being too close or too far. If the distance is too short, the impact force of the coating material may be excessive, potentially damaging the previously applied layer; if the distance is too great, excessive solvent evaporation can increase viscosity, reduce leveling, make it harder for air bubbles to escape, and promote the formation of pinholes. Typically, the distance is kept between 15 and 25 centimeters, with the specific setting adjusted according to the type of spray gun and the characteristics of the coating.

3. Matching the spray gun’s travel speed with the paint output rate

The spray gun’s travel speed and paint output should be properly matched to ensure uniform coating thickness and prevent overspray. If the travel speed is too fast and the paint output is insufficient, the coating will be too thin and discontinuous; if the travel speed is too slow and the paint output is excessive, the coating will be overly thick and prone to spatter-induced craters. The spray gun’s path should remain parallel with appropriate overlap to avoid localized buildup and missed areas.

V. Countermeasures for Controlling Pinholes in the Coating Process

1. Cleaning and maintenance of the coating chamber

Regularly clean the interior of the coating chamber to remove metal deposits adhering to the chamber walls and baffles. Tiny particles generated by flaking deposits can settle on the workpiece surface; after coating, these particles may detach, leaving pitting defects. The cleaning frequency should be determined based on the number of coating batches and the thickness of the deposited layer. Components inside the coating chamber should be inspected periodically: loose parts must be tightened, and worn components should be replaced.

2. Quality Control of Coating Materials

High-purity coating materials are selected; impurities can rupture during evaporation, generating fine particulate splatter that, upon deposition, forms protrusions. When these protrusions detach, they give rise to pitting defects. Prior to use, the materials may undergo a pre‑melting treatment to remove low‑melting-point impurities. During the coating process, the heating rate should be carefully controlled to prevent rapid heating that could cause the material to boil and spatter.

VI. Comprehensive Management Measures

1. Standardization of Process Parameters

Establish standardized process parameter files, including mixing time, settling time, spray pressure, spray distance, and leveling time. Operators must adhere strictly to these standards to minimize variability caused by human factors. Process parameters for each production batch should be recorded and analyzed in conjunction with the corresponding product test results.

2. Quality Inspection and Feedback

Each batch of products undergoes a surface‑quality inspection, with the number and size of pinholes assessed under standardized lighting. If the number of pinholes exceeds the specified limit, the root cause must be promptly investigated and corrective actions implemented. Microscopic examination of the pinhole morphology enables an initial assessment of their origin, facilitating targeted process adjustments.

VII. Conclusion

Addressing the issue of pinholes requires a multi‑faceted approach, encompassing control of coating bubbles, proper use of defoamers, effective primer sealing, and optimization of spray parameters. Bubbles in the coating are the primary cause of pinholes; they can be minimized through thorough mixing, adequate settling to allow degassing, and stringent control of compressed air quality. Insufficient or improperly selected defoamers can leave residual bubbles, necessitating careful adjustment of both dosage and formulation. Poor primer sealing allows substrate micropores and volatile substances to escape, leading to pinholes; therefore, it is essential to regulate primer thickness, leveling characteristics, and curing conditions. Improper spray settings—such as incorrect pressure or distance—can also give rise to pinholes, calling for fine‑tuning of these parameters. Additionally, particle shedding during the coating process can leave behind pinholes; regular cleaning of the coating chamber and rigorous quality control of coating materials are critical. Through systematic process optimization, pinhole defects can be effectively mitigated, resulting in a significant improvement in the surface smoothness of coated products.

Disclaimer: The above content has been compiled from publicly available sources and is provided for reference only. If any infringement occurs, please contact us, and we will address it promptly.

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B-113	Bisphenol A epoxy acrylate	High hardness, high gloss, high fullness, containing 20% TPGDA.
B-151	Modified epoxy acrylate	Low halogen, yellowing-resistant, excellent plating performance, and strong adhesion.
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B-163	Modified epoxy acrylate	Good flexibility, excellent pigment wetting, and strong adhesion.
B-165	Modified epoxy acrylate	Good flexibility and strong adhesion
B-212A	Aromatic polyurethane acrylate	High cost-performance, excellent plating performance, good toughness, and resistant to boiling water.
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B-268M	Aliphatic polyurethane acrylate	Good flexibility, excellent adhesion, superior plating performance, and strong hiding power.
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Intermediate coat
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B-374	Aliphatic polyurethane acrylate	Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing.
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BM6263 (DPHA-90)	Dipentaerythritol hexaacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.

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Common Issues and Solutions in UV Vacuum Plating (Part 6)

Common Issues and Solutions in UV Vacuum Plating (Part 4)