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Common Issues in UV Vacuum Plating (Part 5)

Release time:

2026-06-01 17:11

Common Issues in UV Vacuum Plating (Part 5)

In the production process 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 protrude from the surface, pitting is an indentation that casts shadows along its edges under illumination and produces muted internal reflections. Pitting not only detracts from the appearance but also compromises the coating’s density and integrity. Understanding the characteristics and root causes of pitting helps identify associated risks during manufacturing.

I. Manifestations of Pitting

Pits are minute depressions on the coated surface, the opposite of the raised morphology of particles. Their diameters range from tiny pinholes to clearly visible craters. Under illumination, the edges of pits cast shadows while their interiors reflect faintly, making them readily discernible to the naked eye. Pits may occur as isolated features or in dense clusters; in severe cases, the surface can take on a spongy, porous appearance. Beyond detracting from the aesthetic appearance, pits compromise the coating’s density and protective performance, allowing moisture and contaminants to penetrate into the coating layer or the underlying primer.

II. Pinholes Caused by Air Bubbles in Coatings

Bubbles in coatings are the primary cause of pinholing. During mixing, the high-speed rotation of the agitator blades entrains air into the coating, forming tiny bubbles. The faster and longer the mixing, the more bubbles are incorporated. Splashing during stirring can also introduce air bubbles. Furthermore, during pumping, filtration, and transfer, leaks at pipe joints or inadequate pump sealing can draw air into the coating.

Bubbles can also be introduced during the spraying process. If the compressed air contains water or oil, it will mix with the coating and form bubbles. During atomization, the compressed air mixes with the coating, and improper spray pressure can likewise entrain air into the coating. Additionally, if the spray distance is too close, the rebound of the high‑velocity coating as it impacts the workpiece surface can also introduce bubbles into the coating.

Bubbles in the coating adhere to the workpiece surface after spraying or remain trapped within the coating. During the leveling process, some bubbles rise to the coating surface and rupture. If the leveling time is insufficient or the bubbles are highly stable, they fail to break before the coating cures and solidifies, leaving craters on the surface.

III. Bubble Residue Caused by Defoamer Issues

Defoamers in coatings are used to reduce surface tension and promote the rupture and removal of bubbles. When the defoamer dosage is insufficient, bubbles exhibit greater stability and are less likely to break. During the leveling process, tiny air bubbles in the coating system cannot rise and escape promptly, remaining trapped near the coating’s surface. Upon curing, as the coating rapidly sets, these bubbles become entrapped within the film or rupture, resulting in pinholes.

Improper selection of a defoamer can also compromise its performance. Different types of defoamers are suited to specific coating systems and application processes. If a defoamer exhibits poor compatibility with the coating system, it may fail to effectively reduce the surface tension at the bubble interface or may aggregate into particles, thereby becoming a new source of defects. Moreover, the timing of defoamer addition and its dispersion state can also influence its effectiveness.

Excessive addition of defoamer can also cause problems. An overabundance of defoamer may form incompatible droplets in the coating, which can lead to craters or pinholes in the finished film.

IV. Pinholes Caused by Poor Primer Sealing

Poor primer sealing is a major cause of pinholing. When the substrate’s micropores, fine cracks, or loose areas are not adequately filled by the primer, microscopic depressions form at these defects. During vacuum coating, the metal deposition behavior in these depressed regions differs from that in smooth areas, resulting in a coating layer that may be thinner or exhibit a different microstructure, which manifests as pinholes on the finished surface.

Volatile substances in the substrate, when released under vacuum conditions, can also form bubbles within the primer or coating layer; upon bursting, these bubbles leave behind pitting defects. Residual moisture, unreacted monomers, and low-molecular-weight additives remaining in the plastic substrate may volatilize in a vacuum environment, permeate the primer layer, or directly nucleate at the interface between the primer and the coating layer, thereby forming bubbles.

When the primer is incompletely cured, residual unreacted monomers in the coating may volatilize under vacuum conditions, also leading to bubble and pinhole defects. Conversely, if the primer is over‑cured, the coating becomes brittle and may develop microcracks in a vacuum environment; these microcracks can subsequently manifest as pinhole‑like defects after coating deposition.

5. Pinholing Caused by Insufficient Flow of the Primer

Insufficient leveling of the primer can also indirectly give rise to pinhole defects. If the primer surface exhibits minute craters or traces of burst bubbles prior to curing, these imperfections fail to be fully eliminated during the leveling process and become permanently fixed upon curing. Subsequent coating and topcoat layers not only fail to fill these depressions but instead inherit the surface morphology, thereby propagating the pinhole defects onto the finished product’s surface.

Excessive primer viscosity, insufficient leveling time, and low ambient temperatures can all compromise the primer’s leveling performance, preventing minor surface depressions from fully filling. Additionally, when air bubbles in the primer rupture, if the surrounding coating lacks adequate flowability to promptly flow into the voids and level them, pitting defects may remain.

The leveling of the primer is also influenced by the surface condition of the substrate. Non-uniform surface tension on the substrate can lead to localized shrinkage during leveling, resulting in minute depressions.

VI. Pinholes Caused by Improper Spraying Parameters

Improper spray parameters can also lead to pinhole defects. When the spray pressure is too high, the coating is atomized into excessively fine droplets; however, as these high‑velocity droplets impact the workpiece surface, they may rebound, leaving microscopic spatter pits on the coating. Conversely, if the spray pressure is too low, atomization is poor and the coating particles are coarse; as these large particles accumulate, the voids between them can form minute depressions.

If the spray distance is too close, the high‑velocity paint stream exerts excessive impact on the workpiece surface, potentially damaging the existing coating and creating impact‑induced craters. Conversely, if the spray distance is too far, some of the solvent evaporates before the paint reaches the surface, increasing its viscosity, reducing its leveling properties, and making it harder for air bubbles to escape, thereby promoting the formation of pinholes.

VII. Causes of Pinholes in the Coating Process

During vacuum coating, minute particles or droplets present in the coating chamber may deposit on the workpiece surface, obstructing the deposition of metal vapor. After coating, these particles or droplets can detach, leaving uncoated areas in the deposited layer and resulting in pitting defects. Material adhering to the chamber walls or baffles may also flake off, serving as another source of such defects.

Non-uniform coating rates or sputtering during the deposition process can cause large metal particles to deposit directly onto the workpiece surface. These particles may detach during subsequent topcoat application, leaving pinholes. Impurities in the coating material that rupture during evaporation can also generate fine particles, which deposit on the workpiece and form protrusions; these protrusions may subsequently fall off, resulting in pinholes.

VIII. Pinholing Issues in the Topcoat

The topcoat layer can also develop pinholing defects. Air bubbles in the topcoat, issues with defoaming agents, and inadequate leveling can all leave pinholes on the surface. If air bubbles present on the surface before the topcoat cures rupture, and the coating’s flow is insufficient to fill in the resulting depressions prior to curing, the pinholes will become permanently fixed.

Interfacial contamination between the topcoat and the coating layer can also result in insufficient local adhesion; during curing, the topcoat contracts from the contaminated areas, leading to the formation of microscopic depressions. If the topcoat cures too rapidly, concentrated shrinkage stresses may develop, potentially leaving a pitted‑like defect on the surface of the coating layer.

IX. Conclusion

Pinholes are a common defect in UV vacuum plating that compromises surface smoothness, with their origins stemming from multiple factors, including coating bubbles, inadequate primer sealing, poor leveling, and the plating process itself. Coating bubbles are the primary cause of pinholes; air entrained during mixing, transfer, and spraying forms bubbles, and insufficient or improperly selected defoamers prevent their timely removal, leaving depressions after curing. Poor primer sealing—whether due to unfilled substrate micropores or the escape of volatile substances—can also give rise to pinholes. Insufficient primer leveling fails to close minute pits. Improper spray parameters may likewise induce pinholes. Particle shedding or sputtering during plating can likewise leave behind pinholes. The topcoat layer, too, is susceptible to pinhole defects. Understanding the appearance and root causes of pinholes is the essential first step in identifying this defect.

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.

Bosheng Related Product Recommendations – Vacuum Plating
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Product Model/English Abbreviation	Product Name/Product Type	Product Features
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.
B-160D	Modified epoxy acrylate	Good flexibility, yellowing resistance, and excellent adhesion.
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.
B-221	Aliphatic polyurethane acrylate	Fast curing, resistant to boiling water
B-268M	Aliphatic polyurethane acrylate	Good flexibility, excellent adhesion, superior plating performance, and strong hiding power.
B-574C	Polyester acrylate	Low viscosity, low odor, excellent wettability, suitable for LED UV.
B-619W	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance.
Intermediate coat
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-374	Aliphatic polyurethane acrylate	Good flexibility, excellent leveling, resistant to abrasion and chemicals, and yellowing‑resistant.
B-601	Aromatic polyurethane acrylate	High hardness, scratch resistance, chemical resistance, and excellent cost-effectiveness.
B-6020	Special functional group acrylate	Resistant to boiling water, excellent color development, and strong interlayer adhesion.
Topcoat
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B-221	Aliphatic polyurethane acrylate	Fast curing, resistant to boiling water
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B-368	Aliphatic polyurethane acrylate	Good toughness, excellent leveling, excellent bend resistance, and excellent heat resistance.
B-374	Aliphatic polyurethane acrylate	Good flexibility, excellent leveling, resistant to abrasion and chemicals, and yellowing‑resistant.
B-574C	Polyester acrylate	Low viscosity, low odor, excellent wettability, suitable for LED UV.
B-601	Aromatic polyurethane acrylate	High hardness, scratch resistance, chemical resistance, and excellent cost-effectiveness.
B-6016C	Special functional group acrylate	Easy to apply, resistant to yellowing and boiling water, and improves the appearance of the paint film.
B-6019	Special functional group acrylate	Good leveling, excellent wetting, resistant to boiling water, and superior color dispersion.
B-609	Aliphatic polyurethane acrylate	Fast curing, high hardness, scratch resistance, and chemical resistance.
B-615A	Aliphatic polyurethane acrylate	Fast curing, excellent toughness, wear resistance, and chemical resistance.
B-619W	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance.
B-6210	Aliphatic polyurethane acrylate	Low viscosity, chemical resistance, environmental resistance, and dual photocatalytic–thermal curing.
B-6211	Aliphatic polyurethane acrylate	Fast curing, high hardness, scratch-resistant, and free of organotin.
B-919B	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, and superior chemical and wear resistance.
Monomer Recommendation
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BM2223 (TPGDA)	Di(propylene glycol) diacrylate	Good flexibility and low volatility
BM3231 (TMPTA)	Trimethylolpropane triacrylate	High crosslink density, high hardness, high gloss, and excellent wear resistance.
BM3235 (PET3A)	Pentaerythritol triacrylate	Fast curing, high crosslink density, high hardness, and excellent chemical resistance.
BM3380 (3EO-TMPTA)	Pentaerythritol triacrylate	More flexible and less irritating than TMPTA.
BM6261 (DPHA-80)	Dipentaerythritol hexaacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.
BM6263 (DPHA-90)	Dipentaerythritol hexaacrylate	High crosslink density, high hardness, chemical and wear resistance, and water resistance.

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Common Issues in UV Vacuum Plating (Part 6)

Common Issues in UV Vacuum Plating (Part 4)