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Common Issues and Solutions in UV Vacuum Plating (Part 11)

Release time:

2026-06-11 16:43

Common Issues and Solutions in UV Vacuum Plating (Part 11)

In the practical production 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, penetrating the entire coating layer and exposing the underlying primer or substrate. Not only do they disrupt the continuity of the metallic luster, but they also serve as pathways for the ingress of moisture and contaminants. To address this defect, a systematic approach is required, encompassing multiple aspects such as primer sealing, substrate preparation, curing control, and plating process optimization.

I. Measures to Improve the Sealing Performance of the Primer

1. Enhanced primer fill capability

Poor sealing by the primer is the primary cause of pinholes in the coating. A primer with excellent filling properties should be selected to ensure effective coverage of micro‑pores and minor surface defects on the substrate. If the primer’s viscosity is too high, an appropriate amount of reactive diluent can be added to reduce it, thereby enhancing the coating’s ability to penetrate fine pores. For substrates with high surface roughness, a two‑coat priming process may be employed: after the first coat has cured, lightly sand the surface before applying the second coat to improve the sealing performance.

2. Improvement of primer leveling properties

Poor leveling of the primer can result in inadequate coating coverage at surface defects. A primer with superior leveling properties should be selected to ensure thorough spreading after application. Adding an appropriate amount of a leveling agent can reduce surface tension and enhance leveling performance. Sufficient leveling time is essential to allow the primer to fully fill microscopic imperfections on the substrate surface before curing.

3. Control of Primer Coating Thickness

When the primer coat is applied too thinly, microscopic protrusions on the substrate surface may penetrate the primer layer. The primer thickness should be appropriately increased to ensure that even the smallest defects on the substrate are fully covered. Edges and recessed areas are particularly prone to insufficient coating; special attention must be paid to ensure adequate film thickness in these regions. Use a film‑thickness gauge to periodically measure the primer thickness and verify that it meets process specifications.

II. Countermeasures for Controlling Volatiles in the Substrate

1. Substrate Drying Treatment

Residual moisture in the substrate is the primary source of volatile compounds. For substrates with high hygroscopicity, pre‑drying is essential—use an oven or a desiccant dryer to remove moisture adsorbed within the material. Drying temperature and duration should be set according to the substrate’s thermal stability to prevent deformation at elevated temperatures. After drying, store the substrate in a dry environment and remove it only immediately before use.

2. Control of Substrate Quality

Recycled materials contain a higher proportion of degradation products and impurities, with elevated levels of volatile substances. The proportion of recycled material used should be carefully controlled, and for products with stringent pinhole‑free requirements, virgin resin should be employed. Residual low‑molecular‑weight additives in the substrate also contribute to volatiles; therefore, substrate grades with low volatility should be selected. Substrates from different batches must undergo small‑scale testing to confirm that pinhole issues are manageable before being put into full‑scale production.

3. Optimization of the Injection Molding Process

Excessive injection molding temperature and excessively high injection speed can lead to an increase in internal bubbles within the substrate. Injection molding process parameters should be optimized by reducing both the molding temperature and the injection speed to minimize bubble formation. Raw materials must be thoroughly dried before molding to prevent moisture from being introduced. For batches with severe pinhole defects, annealing of the substrate can be performed to relieve internal stresses and eliminate microscopic bubbles.

III. Measures to Address the Curing Status of the Primer

1. Optimization of Curing Parameters

Insufficient curing of the primer can result in low crosslink density, failing to effectively prevent the penetration of volatile substances. Ensure that the primer is fully cured, with curing energy and duration meeting the specifications of the primer formulation. Regularly monitor the output energy of UV lamps and replace aged lamps promptly. For thick‑coated primers, consider extending the curing time or increasing the lamp power as needed.

2. Verification of the degree of curing

Regularly verify the degree of primer cure using either the cotton‑ball wipe test or an adhesion test. A poorly cured primer will feel tacky, and the cotton ball will pick up color upon wiping. Over‑cured primer, on the other hand, develops an excessively dense surface, which may reduce adhesion. Assess the primer’s cure state through adhesion and surface hardness tests to strike a balance between under‑cure and over‑cure.

IV. Measures for Cleaning the Substrate Surface

1. Removal of the release agent

Residual release agents on the substrate surface can form a low‑surface‑energy film, compromising primer adhesion. Remove these release agents using ultrasonic water rinsing or a dedicated cleaning agent, followed by a final rinse with deionized water to prevent any residual cleaner. For workpieces with complex geometries, employ spray‑type cleaning to ensure the cleaning solution reaches all areas. After cleaning, the workpieces should be thoroughly dried.

2. Control of oil stains and fingerprints

During operation, wear clean gloves to avoid direct contact between bare hands and the workpiece surface. The workpiece storage area must be kept clean to prevent oil‑based contamination. For contaminated workpieces, wipe them with a dedicated cleaning agent prior to coating; allow the cleaner to fully evaporate before applying the primer.

V. Countermeasures for Coating Processes

1. Degassing treatment prior to coating

Prior to entering the coating chamber, workpieces should undergo thorough degassing to remove gases adsorbed on their surfaces. This can be achieved by pre‑evacuating the workpieces in a vacuum chamber for an appropriate duration; coating may commence only after the vacuum level has stabilized. For substrates with high hygroscopicity, the pre‑evacuation time may be extended as needed. The vacuum level within the coating chamber must meet the process specifications before coating is initiated.

2. Control of the coating rate

When the deposition rate is too slow, the substrate surface remains exposed to the vacuum environment for a longer period, increasing the likelihood of volatile species escaping. The deposition rate should be appropriately increased to shorten the coating time. Conversely, an excessively fast deposition rate may result in a loose film microstructure; therefore, a balance must be struck between deposition speed and film quality. For products with severe pinhole issues, a stepwise deposition strategy—fast at the outset followed by a slower rate—can be employed: rapid deposition in the initial stage forms a protective layer, while slower deposition completes the remaining thickness in the later stages.

3. Cleaning and Maintenance of the Coating Chamber

Regularly clean the interior of the coating chamber to remove metal deposits and contaminants adhering to the chamber walls and baffles. Such deposits may release gases during the coating process, increasing the partial pressure of gases within the chamber. Components inside the coating chamber should be inspected periodically to ensure they are securely fastened and exhibit no abnormal outgassing. The vacuum pump should undergo routine maintenance to maintain its pumping efficiency.

VI. Comprehensive Process Management Measures

1. Standardization of Process Parameters

Establish standardized process‑parameter documentation, including primer coating thickness, curing energy, substrate drying conditions, pre‑evacuation time, and coating rate. Operators must adhere strictly to these standards to minimize batch‑to‑batch variability caused by human error. For each production batch, record the process parameters and correlate them with the corresponding product test results for analysis.

2. Quality Inspection and Feedback

Each production batch undergoes pinhole inspection, with the surface examined under a standard light source for any minute voids. For severe pinholes, magnification tools such as a loupe or microscope may be used. When pinholes are detected, their morphology and distribution should be evaluated to determine whether they stem from inadequate primer sealing or volatile‑related issues, allowing for targeted process adjustments. Quality data should be compiled regularly to analyze the incidence of pinhole defects and enable ongoing process optimization.

VII. Conclusion

Addressing pinhole defects in coatings requires a multi‑faceted approach, encompassing primer sealing, substrate preparation, curing control, and coating process optimization. Inadequate primer sealing is the primary cause of pinholes; this can be mitigated by enhancing filler capacity, improving leveling, and carefully controlling coating thickness. Moisture and low‑molecular‑weight volatiles migrating from the substrate are significant contributors to pinholing; these can be reduced through thorough drying, careful substrate quality control, and optimized injection‑molding processes to minimize volatile emissions. Insufficient primer cure fails to prevent volatiles from penetrating, necessitating fine‑tuning of curing parameters and rigorous verification of cure degree. Release agents and surface contaminants on the substrate can compromise primer adhesion, requiring enhanced cleaning procedures. During the coating process, degassing and precise control of the coating rate are essential. Through systematic process optimization, pinhole defects can be effectively managed, leading to marked improvements in the density and aesthetic quality 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.

Bosheng’s Recommended Products – 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, 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 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 superior 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.

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Common Issues and Countermeasures in UV Vacuum Plating (Part 12)

Common Issues and Solutions in UV Vacuum Plating (Part 10)