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Common Issues in UV Vacuum Plating (Part 7)

Release time:

2026-06-03 07:15

Common Issues in UV Vacuum Plating (Part 7)

During UV vacuum plating production, color shifting of the coating is one of the common defects that affect product appearance quality. Color shifting refers to the appearance of iridescent spots on the surface of the metallic coating, causing reflected light to exhibit colors rather than a pure silvery white. These colored patterns form uneven patches on what should otherwise be a mirror‑like, highly reflective coating, compromising the uniformity and purity of the metal’s aesthetic. Understanding the manifestations and underlying causes of color shifting helps identify associated risks during the manufacturing process.

I. Manifestations of Coloration in Coatings

Coloring in the coating primarily manifests as red, green, blue, yellow, and other colored patterns on the surface of the metallic film, resembling the interference colors seen in an oil slick on water. These colored patterns may be distributed uniformly over large areas or appear as irregular patches localized to specific regions. Under natural light or fluorescent illumination, the colored zones stand out clearly against the normal silvery‑white areas, with reflected light exhibiting hues rather than a pure silver tone. As the viewing angle changes, the coloration may shift, a hallmark of thin‑film interference.

The severity of color iridescence varies: mild iridescence is visible only under specific lighting angles, whereas severe iridescence is clearly discernible from any viewing angle. Iridescence zones may exhibit sharp boundaries with the surrounding normal areas or transition gradually. Coating‑induced iridescence is particularly pronounced on coated products with light‑colored or clear topcoats, as the transparency of the topcoat does not obscure the underlying coating’s color.

II. Non-uniform coating thickness

Non-uniformity in coating thickness is the primary cause of color iridescence in coatings. The thickness of metallic thin films typically falls at the nanometer scale; when the film thickness lies within a specific range, the reflected light from the top and bottom interfaces of the film interferes with the incident light. The outcome of this interference depends on the relationship between the film thickness and the wavelength of the light: different film thicknesses enhance or suppress particular wavelengths, thereby producing distinct colors.

When the coating thickness is uniform, the entire surface exhibits a consistent interference color; if the thickness is maintained within an appropriate range, a pure silvery-white hue can be achieved. However, when the coating thickness varies, differences in film thickness across different areas give rise to variations in interference colors, resulting in colorful mottling on the surface. Areas with thinner coatings may appear pale yellow or pale blue, moderately thick regions display a silvery-white tone, while thicker areas may take on reddish or purplish hues.

Non-uniform electric field distribution within the coating chamber is a common cause of non-uniform film thickness. In thermal evaporation coating, the design of the heating system, the shape and positioning of the evaporation source, as well as the distance and angle between the substrate holder and the evaporation source, all influence the spatial distribution of metal vapor. Regions near the evaporation source exhibit higher deposition rates, whereas areas farther away show slower deposition. In magnetron sputtering coating, factors such as the etching condition of the target, the magnetic field distribution, and the rotation mode of the substrate holder likewise affect the uniformity of film thickness.

Uneven rotation of the workpiece holder or an inappropriate rotational speed can also lead to non-uniform film thickness. During coating, the workpiece holder must rotate uniformly so that each component receives a consistent deposition of metal vapor. If the rotating mechanism seizes, the rotational speed fluctuates, or the rotation path is eccentric, different parts of the workpiece will experience varying deposition times and angles, resulting in thickness variations.

III. Coating Rate Fluctuations

Fluctuations in the deposition rate also lead to non-uniform film thickness. When the deposition rate is too high, metal particles deposit in a disordered manner, resulting in a loose film structure; moreover, rate fluctuations more readily give rise to thickness variations. Conversely, when the deposition rate is too low, the coating process is prolonged, and even minor rate changes during this extended period will be reflected in the final film thickness.

In evaporation coating, instability in the heating power leads to fluctuations in the evaporation rate. Variations in the voltage and current of the power supply, localized overheating or ablation of the heating filament, and changes in the surface condition of the metal melt pool all affect the evaporation rate. During the consumption of the metallic material, variations in the depth and surface area of the melt pool also alter the evaporation rate. In sputter coating, changes in the target surface condition, fluctuations in gas pressure, and power supply instability likewise cause variations in the deposition rate.

IV. Residuals on the Primer Surface

Residues on the primer surface can also cause localized coloration. Contaminants such as oil, fingerprints, and detergent residues on the primer surface can alter the local surface energy and surface chemical state. During vacuum coating, the deposition behavior of metal particles in contaminated areas differs from that on a clean primer surface; consequently, the resulting film thickness, density, or crystal structure may vary, leading to abnormal localized interference colors.

Oil and fingerprints are common sources of contamination. When operators handle the primer surface with bare hands, the oils on their skin leave fingerprint marks. These fingerprint‑marked areas exhibit altered surface energy, leading to abnormal wetting and film‑forming behavior of metal particles; as a result, the coating develops colored, fingerprint‑like blemishes. Similarly, if the cleaning agent does not fully evaporate before coating, residual solvents or surfactants can also compromise coating quality.

The tiny particles on the primer surface can also affect the local coating thickness. Around these particles, shadowed regions form, leading to reduced metal deposition and a thinner film; under interference effects, this results in distinct color variations. Moreover, once the particles are coated with metal, they become raised, and the film thickness at their tops differs from that of the flat areas, further causing localized color differences.

V. Influence of Primer Surface Roughness

The surface roughness of the primer affects the interference colors of the coating. When the surface roughness is high, metal particles deposit on surfaces with varying orientations, resulting in an uneven microstructure of the film; local variations in film thickness lead to changes in the interference colors. By contrast, a smooth and even primer surface promotes the formation of a uniform coating, yielding more consistent and uniform interference colors.

Defects such as scratches, orange‑peel texture, and bubbles on the primer surface can also compromise the uniformity of the coating. In these defective areas, the deposition behavior of metal particles is altered, leading to deviations in film thickness and localized color differences. The color variations induced by these defects are typically correlated with their morphology, making them readily distinguishable from those arising from non‑uniform film thickness.

VI. Influence of Coating Material Purity

The purity of the coating material affects the optical performance of the deposited layer. During evaporation or sputtering, impurities in the metallic substrate can also be deposited onto the surface. The presence of these impurity elements alters the optical constants of the film, thereby influencing its reflectance and interference effects. Moreover, certain impurity elements may form compounds that modify the film’s color and transparency.

If the purity of coating materials varies between batches, the optical performance of the deposited film will also exhibit batch-to-batch fluctuations. Some batches may display abnormal interference colors due to elevated impurity levels, manifesting as color‑shift defects. Furthermore, surface oxidation or contamination of the coating material during storage can adversely affect its evaporation behavior and the quality of the resulting film layer.

VII. Influence of Substrate Temperature

The substrate temperature during the coating process significantly influences film growth. It affects the surface mobility of metal particles, thereby impacting the film’s density, crystalline structure, and thickness uniformity. When the substrate temperature is non-uniform, the growth behavior of the film varies across different regions, potentially leading to uneven film thickness or structural disparities and resulting in abnormal interference colors.

The position of the workpiece within the coating chamber varies, resulting in uneven thermal radiation and an non-uniform substrate temperature distribution. Differences in thermal capacity between thick‑walled and thin‑walled components further influence the temperature profile. During the coating process, heat accumulates; as coating time progresses, the workpiece temperature rises, and the film‑growth conditions evolve, potentially leading to spatially non‑uniform film thickness and microstructure.

VIII. Effects of Post-Coating Treatment

The application and curing of the topcoat after coating can also influence the color‑shift effect. Because the refractive index of the topcoat differs from that of the coating layer, the original interference conditions are altered once the topcoat is applied. When the topcoat thickness is uneven, the interference effects vary across different areas, potentially enhancing or masking the color‑shift inherent to the coating layer itself.

Shrinkage stresses during the curing of the topcoat may induce minute deformations or cracking in the coating layer, and these mechanical changes can also affect the optical performance of the film. If solvents or monomers from the topcoat penetrate into the coating layer, they may alter the refractive index or thickness of the film, leading to shifts in interference colors.

IX. Conclusion

Color iridescence in UV vacuum plating is a common defect that compromises the purity of metallic luster, with its origins rooted in multiple factors, including coating thickness distribution, the condition of the primer surface, and process parameters. Non-uniform coating thickness is the primary cause of iridescence; uneven electric field distribution, unstable rotation of the workpiece holder, and fluctuations in the deposition rate can all lead to variations in film thickness. Residual contaminants such as oils and fingerprints on the primer surface alter local surface energy, influencing metal deposition and resulting in localized color differences. Furthermore, primer surface roughness and defects can also undermine coating uniformity. The purity of the plating material and the substrate temperature likewise affect interference colors, while post‑plating topcoat treatments can further impact the manifestation of iridescence. Understanding the characteristics and underlying causes of coating iridescence 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’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	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
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	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 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)	Dipropylene 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 in UV Vacuum Plating (Part 8)

Common Issues in UV Vacuum Plating (Part 6)