Language
- Chinese

language

Tel

Tel

+8618142863185
Follow us

Official Accounts

Official Accounts
Top

News

Common Issues in UV Vacuum Plating (Part 11)

Release time:

2026-06-04 17:14

Common Issues in UV Vacuum Plating (Part 11)

In the production process of UV vacuum plating, coating pinholes are one of the common defects that compromise the density and surface quality of the deposited film. These pinholes appear as tiny circular voids on the coated surface, visible to the naked eye or clearly discernible under high magnification. Such minute pores penetrate the entire coating layer, exposing the underlying primer or substrate directly. Beyond disrupting the continuity of metallic luster, pinholes also serve as pathways for moisture and contaminants to infiltrate, thereby degrading the protective performance of the coating. Understanding the characteristics and root causes of coating pinholes is essential for identifying associated risks during manufacturing.

I. Manifestations of Coating Pinholes

Coating pinholes appear as tiny circular or elliptical voids of varying sizes, ranging from a few micrometers to several hundred micrometers in diameter, distributed across the coated surface. The edges of these pinholes are typically well-defined and circular, and their interiors may reveal the color of the primer or the bare substrate. Under magnification, a raised, ring‑like rim can often be observed around the pinhole, resulting from the accumulation of metal deposits at the periphery of the defect.

The distribution of pinholes may be either isolated, discrete points or clustered groups. Isolated pinholes typically correspond to individual defects on the substrate surface, whereas clustered pinholes are often associated with large‑area poor sealing in the primer or with regionally localized substrate defects. Under illumination, pinholes do not reflect light and appear as dark spots, contrasting with the surrounding bright metallic surface. Microscopic examination of cross‑sections reveals that pinholes have a funnel‑shaped morphology—wide at the top and narrow at the bottom—and extend through the coating layer down to the primer or the substrate.

II. Inadequate Sealing of the Primer

Poor sealing by the primer is the primary cause of pinholes in the coating. The substrate surface is rarely perfectly flat and smooth; plastic injection‑molded parts may exhibit microscopic defects such as gas pores, sink marks, flow lines, and orange‑peel texture. During primer application, if the primer lacks sufficient filling capability or is applied too thinly, these defects remain unfilled. In the vacuum‑coating process, metal particles deposit on the primer surface, but they cannot form a continuous film over the defect sites. The recessed geometry of these defects results in discontinuous metal films, leaving behind pinhole‑like voids.

The leveling property of the primer directly affects the sealing performance. A primer with poor leveling fails to spread adequately after application, resulting in insufficient coating coverage at surface defects. When the primer’s viscosity is too high, the coating struggles to penetrate fine pores; during the coating process, trapped air within these pores expands and escapes, also giving rise to pinholes. If the primer layer is applied too thinly, microscopic protrusions on the substrate surface may pierce through it, leaving the tips of these protrusions as pinhole‑like defects that remain uncovered after coating.

III. Emission of Volatile Substances from the Substrate

The release of volatile substances from the substrate is another major cause of pinholes in the coating. During injection molding, plastic substrates may retain residual moisture, which exists either in molecular form within the material or adsorbed on the surface of fillers. In the vacuum environment of vacuum coating, where pressures are extremely low, the water molecules inside the substrate gain the energy needed to escape. These water molecules diffuse from the interior toward the surface, passing through the primer layer and exiting. At the point of escape, the water vapor disrupts the continuity of the coating, resulting in pinhole‑like voids.

Low-molecular-weight substances in the plastic substrate also serve as sources of volatile compounds. Unreacted monomers, low‑molecular‑weight additives, plasticizers, antioxidants, and other such components may volatilize at elevated temperatures. These low‑molecular‑weight species exhibit even greater volatility under vacuum conditions; upon escape, they can likewise leave pinholes in the coating layer. Additionally, gas bubbles trapped within the substrate during injection molding expand and rupture under vacuum, and the released gases can likewise give rise to pinholes.

The volatile‑substance content varies among different substrates. Recycled materials contain higher levels of degradation products and impurities, resulting in a greater total volatile‑substance content. Substrates stored in humid environments absorb moisture; if used without prior drying, the risk of pinholing increases markedly. In glass‑fiber‑reinforced plastics, moisture tends to adsorb at the glass‑fiber–resin interface, which also serves as a source of volatiles.

IV. Insufficient Curing of the Primer

Insufficient curing of the primer can exacerbate the issue of volatile‑compound release. A poorly cured primer exhibits low crosslink density and a loose coating structure, failing to effectively prevent the penetration of volatiles from the substrate. Low‑molecular‑weight substances can more readily diffuse through the molecular gaps in the primer layer to the surface, leading to pinholes in the coating.

When the primer is not fully cured, unreacted monomers and oligomers remain within the coating. These substances can volatilize even under vacuum conditions, thereby increasing the total amount of volatile emissions. A surface with insufficient primer cure may feel tacky; such a tacky surface is more prone to adsorbing volatile compounds, further complicating the issue of pinholes.

The degree of cure in the primer is influenced by multiple factors, including curing energy, primer formulation, and coating thickness. When curing energy is insufficient, reactions deep within the primer do not proceed adequately. In thick coatings, the underlying layers may exhibit a lower degree of cure than the surface, resulting in a structure where the surface is dense while the bulk remains porous. Volatile substances that accumulate in the substrate can then escape collectively, potentially leading to pinholes of considerable size.

V. Contamination of the Substrate Surface

Contaminants on the substrate surface can also cause pinholes in the coating. Residual release agents are among the most common contaminants; they form a low‑surface‑energy film on the substrate. During primer application, areas contaminated by release agents exhibit poor wetting, preventing the primer from achieving uniform coverage and resulting in thin spots or missed areas. After plating, these regions manifest as pinhole defects.

Oil and fingerprints can similarly compromise the coverage of the primer. Oil forms a barrier on the surface, reducing the primer’s adhesion; once cured, it may crack or peel off. During coating, the primer layer over oily areas can delaminate entirely, leaving large voids. The salts and fatty acids in fingerprints can also corrode the primer surface, adversely affecting the uniformity of the coating.

VI. Internal Bubbles in the Substrate

Another source of pinholes is the expansion and rupture of internal bubbles within the substrate under vacuum conditions. Plastic injection‑molded parts may contain microscopic bubbles that remain compressed at atmospheric pressure. When the part is placed in a vacuum coating chamber, the external pressure drops to near zero, causing the internal pressure of the bubbles to far exceed the ambient pressure and leading to rapid expansion. If these bubbles are located close to the substrate surface, their expansion can cause the underlying primer layer to crack, and once the gas inside escapes, it leaves voids or holes in the coating layer.

Bubble pinholes are characterized by relatively large openings with irregular edges, and sometimes exhibit radial cracks in the primer layer. These defects typically occur in clusters on specific batches of parts and are often linked to fluctuations in injection‑molding process parameters. Excessively high mold temperatures, overly rapid injection speeds, or insufficient drying of the raw material can all contribute to an increase in internal voids within the substrate.

VII. Gas Release During the Coating Process

During the coating process, residual gases or desorbed gases remaining in the coating chamber can also give rise to pinholes. Insufficient pumping speed of the vacuum system, or inadequate degassing of the workpiece prior to coating, may result in gases adsorbed on the workpiece surface being released during deposition. These gases form tiny bubbles on the workpiece surface, impeding the deposition of metal vapor; upon bursting, they leave behind pinholes.

Outgassing from the metal substrate itself cannot be overlooked during the coating process. As the metal is heated and evaporated, gases adsorbed within the material are released, increasing the partial pressure of gas in the deposition chamber. These gases coexist with the coating process and, during metal deposition, give rise to gas inclusions that can result in pinholes in the deposited film.

VIII. The Relationship Between Coating Rate and Pinholes

The deposition rate influences the formation of pinholes. When the deposition rate is too slow, the substrate surface remains exposed to the vacuum environment for a longer period, increasing the likelihood of adsorbed gases and volatile species escaping. Moreover, the extended deposition time creates favorable conditions for the continuous release of volatiles, which may give rise to new pinholes in the already deposited film layer.

When the deposition rate is too high, metal particles deposit vigorously, potentially trapping adsorbed gases beneath the film layer. Once these gases are trapped, they may rupture the overlying film during subsequent deposition or upon pressure changes after the chamber is opened, resulting in pinholes. Excessively rapid deposition can also lead to a loose film microstructure, with inherent microscopic porosity.

IX. Conclusion

Pinholes in the coating are a common defect in UV vacuum plating that compromises the density of the deposited film. Their formation stems from multiple factors, including the sealing condition of the primer, volatile substances present in the substrate, the degree of primer curing, surface contamination, subsurface bubbles in the substrate, and the plating process itself. If the primer does not seal adequately, microscopic defects on the substrate surface remain unfilled, preventing the formation of a continuous coating in those areas during deposition. Moisture and low‑molecular‑weight compounds within the substrate can volatilize under vacuum conditions, leaving pinholes in the coating layer. Insufficient primer curing fails to effectively block the penetration of volatile species and may itself release volatiles. Release agents and oil contaminants on the substrate surface can interfere with primer adhesion. Internal bubbles in the substrate may expand and rupture in the vacuum environment. Additionally, gas evolution during plating and an inappropriate plating rate can also give rise to pinholes. Understanding the characteristics and root causes of coating pinholes is essential for 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
Primer
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-113	Bisphenol A epoxy acrylate	High hardness, high gloss, high fullness, contains 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 adhesion, 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	Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing.
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
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-221	Aliphatic polyurethane acrylate	Fast curing, resistant to boiling water
B-301	Aromatic polyurethane acrylate	Fast curing, excellent toughness, and good sandability.
B-302	Aromatic polyurethane acrylate	Fast curing, high strength, excellent toughness, and good grindability.
B-368	Aliphatic polyurethane acrylate	Good toughness, excellent leveling, excellent bend resistance, and excellent heat resistance.
B-374	Aliphatic polyurethane acrylate	Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing.
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 photothermal 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 outstanding chemical and wear resistance.
Monomer Recommendation
Product Model/English Abbreviation	Product Name/Product Type	Product Features
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 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.

Share to:

Common Issues in UV Vacuum Plating (12)

Common Issues in UV Vacuum Plating (10)