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Common Issues in UV Vacuum Plating (Part 8)

Release time:

2026-06-03 17:18

Common Issues in UV Vacuum Plating (Part 8)

During UV vacuum plating, edge whitening is one of the common defects that compromise product appearance quality. It manifests as a lighter shade in the coating along the edges and sharp corners of the workpiece, appearing whitish or dull, in stark contrast to the metallic luster of the central area. This unevenness between the edges and the center undermines the overall visual consistency of the product and is particularly critical in applications with stringent aesthetic requirements, such as cosmetic packaging and consumer electronics. Understanding the characteristics and root causes of edge whitening helps identify potential risks during the manufacturing process.

I. Manifestations of Whitening at the Edges

Edge whitening primarily manifests as a lighter coloration of the coating at the edges and sharp corners of the workpiece, resulting in a grayish-white or dull, hazy appearance. Under illumination, the metallic luster at the edges is noticeably weaker than in the central area, with lower reflectance and an overall pale, muted visual effect. The whitened zones typically follow the edges, corners, and raised features of the part, appearing as linear bands or annular patterns. In severe cases, the coating at the edges virtually ceases to reflect light, adopting a matte finish.

The degree of whitening gradually diminishes from the edge toward the center; near the edge, the whiteness is pronounced, while further inward, it transitions smoothly back to the normal metallic luster. On curved‑surface components, this gradual whitening manifests as an arcuate band of whitening along the edge. In cross‑hatch adhesion testing, the coating in the whitened areas typically exhibits lower adhesion than in the unaffected regions, making it prone to delamination and lifting starting at the edges.

II. Shadowing Effect in Vacuum Coating

The shadowing effect during vacuum coating is the primary cause of edge whitening. In a vacuum coating environment, metal vapor emitted from the evaporation source or target surface propagates in straight lines. Due to the varying spatial relationships between different regions on the substrate and the evaporation source, the amount of metal vapor received and the angle of incidence differ across the substrate surface.

The edges and sharp corners of the workpiece are geometrically disadvantaged. For raised edges, the side surfaces often deviate from the direct line of sight to the evaporation source, resulting in a shallower incidence angle for the metal vapor and a lower deposition rate. In recessed areas or on interior surfaces, the metal vapor may be blocked by other parts of the workpiece, creating shadowed regions. These areas receive less metal deposition than the planar regions directly facing the evaporation source, leading to thinner coating layers, insufficient metallic luster, and a whitish appearance.

III. Influence of Workpiece Rack Configuration and Rotation Method

The layout and rotational configuration of the substrate holder directly affect the uniformity of the coating layer. The positioning of the substrates within the deposition chamber determines their angular orientation and distance relative to the evaporation source. Substrates located directly above the evaporation source experience more favorable deposition conditions, whereas those positioned at the periphery receive less favorable conditions. If certain locations on the substrate holder consistently fall within regions of poor coating uniformity, edge‑whitening defects on the substrates in those areas will become even more pronounced.

The rotation method and speed of the substrate holder also influence the edge‑coating performance. With a single‑axis rotating holder, the substrate’s own spin can enhance coating uniformity across all directions; however, shadows may still persist on certain concave surfaces and edges. Dual‑axis or planetary rotation can further improve coating uniformity and minimize shadowing effects. If the rotation mechanism is poorly designed or operates at too low a speed, some areas of the substrate may never face the evaporation source directly, resulting in insufficient deposition at the edges and causing a whitish appearance.

IV. Influence of Workpiece Geometry

The geometry of the workpiece significantly influences the occurrence of edge‑whitening defects. Workpieces with complex, uneven surfaces are more prone to edge‑whitening. For components featuring deep holes, internal cavities, recesses, grooves, and similar features, the incidence of metal vapor at the edges and sharp corners is restricted, resulting in reduced deposition.

Sharp edges and acute corners are particularly prone to whitening. Sharp edges have a small radius of curvature and limited surface area, reducing the likelihood of metal vapor deposition from the side. Acute edges and corners are even more likely to cast shadows, resulting in an extremely thin coating layer and making the whitening effect especially pronounced. In contrast, passivated edges and rounded corners, with their smoother transition surfaces, allow for a wider range of incident angles of the metal vapor, leading to better coating uniformity and a significantly reduced risk of whitening.

V. Spatial Distribution within the Coating Chamber

The spatial distribution within the coating chamber significantly affects coating uniformity. With the evaporation source or target positioned at a fixed location, the chamber exhibits regions of high and low deposition rates. The workpiece’s position inside the chamber determines the amount of metal vapor that each part receives.

The spatial distribution of metal vapor in the coating chamber is influenced by factors such as the electric field, magnetic field, and gas flow. Non-uniform electric field strength leads to an uneven spatial distribution of evaporation rates, while the magnetic field affects the trajectories of sputtered particles. Together, these factors determine the deposition rate distribution within the chamber; workpieces located in regions with lower deposition rates are more prone to whitening at their edges.

VI. Influence of Coating Time and Rate

Coating time and coating rate both influence the degree of edge whitening. When the coating time is short, although the difference in coating thickness between the edge and the center is small, the relative disparity is significant, making the whitening phenomenon more readily noticeable. As the coating time is extended, the deposition at the edge increases; while the difference with the center persists, the metallic luster at the edge may improve, thereby reducing the extent of whitening.

When the coating rate is too low, deposition efficiency declines, the coating at the edges becomes thin, and whitening becomes pronounced. Conversely, when the coating rate is too high, metal particles deposit in a disordered manner, resulting in a loose film structure, reduced adhesion at the edges, and insufficient gloss—conditions that can likewise manifest as whitening.

VII. The Influence of Primer Condition on Edge Whitening

The application condition of the primer at the edges of the workpiece also affects the edge appearance after coating. When the primer is applied too thinly or unevenly at the edges, its inherent smoothness and gloss are inadequate, which in turn compromises the deposition of the coating layer on the primer. In edge areas where the primer thickness is insufficient, the surface energy may be lower, leading to poorer wetting and film‑forming performance of the metal particles; consequently, the coating layer tends to be thin and lack sufficient gloss.

When the primer fails to cure properly, the edges may be more prone to incomplete curing than other areas. An incompletely cured primer surface becomes tacky, causing metal particles to deposit abnormally on this sticky layer, resulting in a coating with low gloss and a whitish appearance.

VIII. Conclusion

Edge whitening is a common defect in UV vacuum plating that compromises the consistency of product appearance. Its causes are multifaceted, involving shadowing effects during vacuum coating, workpiece holder configuration, workpiece geometry, and coating process parameters. The linear propagation of metal vapor in vacuum deposition results in lower deposition at edges and sharp corners compared to flat surfaces, which is the fundamental reason for edge whitening. Improper workpiece holder design and rotation can leave certain areas perpetually in shadow; sharp edges and complex geometries exacerbate these shadowing effects. The spatial positioning of the workpiece within the coating chamber influences the distribution of deposition rates. The settings for coating time and deposition rate further modulate the severity of edge whitening, while the application quality of the primer at the edges also affects the post‑coating edge appearance. Understanding the manifestations and root causes of edge whitening 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, 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
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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
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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 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 9)

Common Issues in UV Vacuum Plating (Part 7)