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Common Issues in UV Vacuum Plating (Part 9)

Release time:

2026-06-03 23:18

Common Issues in UV Vacuum Plating (Part 9)

In the production process of UV vacuum plating, substrate exposure is one of the most serious defects, compromising both the product’s appearance and its protective performance. Substrate exposure manifests as a lack of coating at the edges and sharp corners of the workpiece, revealing the underlying primer or the bare substrate and creating a stark contrast with the coated areas. Unlike edge whitening, substrate exposure involves the complete absence of the coating layer, leaving the substrate or primer directly exposed. This defect not only detracts from the aesthetic appeal but can also result in a loss of protective functionality at the affected area. Understanding the characteristics and root causes of substrate exposure helps identify associated risks during manufacturing.

I. Manifestations of Exposed Underlying Layers

Bare‑base defects manifest as the absence of the coating layer at edges, sharp corners, and raised areas of the workpiece, exposing the underlying primer or, in some cases, the plastic substrate itself. Under illumination, these bare‑base regions stand out sharply against the surrounding coated areas: the coated zones exhibit a metallic sheen, while the bare‑base zones reveal the natural color of the plastic or the primer. Such defects typically occur along the outer edges or at elevated points of the part, appearing as linear or punctate marks; in certain instances, they may extend continuously around the entire perimeter.

The distinction between substrate exposure and edge whitening lies in whether the coating layer remains intact. Edge whitening occurs when the coating is present but excessively thin, resulting in insufficient gloss, whereas substrate exposure means the coating has been completely removed. In exposed areas, the primer or substrate is directly visible, with no metallic layer to provide coverage. Even after the topcoat is applied, these exposed regions remain devoid of the metallic luster, and their visual contrast with the surrounding areas remains pronounced.

II. Masking in Vacuum Coating

During vacuum coating, certain areas of the workpiece may be shielded by the workpiece itself or by other workpieces, preventing metal vapor from reaching them and resulting in a bare‑metal defect. When two workpieces are positioned too close to each other, their opposing surfaces can obstruct one another. Since metal vapor travels in straight lines, the shaded regions receive no deposition, leaving the coating thickness at zero and creating a bare‑metal area.

The workpiece’s own geometry can also cause shadowing. For components featuring deep holes, narrow slots, or internal cavities, the interiors of these features may fall within shadowed regions. As metal vapor enters through the opening, the amount of deposition decreases with increasing depth, and deposition may be absent in the deepest areas. In holes with a large depth-to-width ratio, the bottom often remains unplated, resulting in exposed substrate.

III. Unreasonable Workpiece Rack Design

The design of the workpiece rack significantly influences the occurrence of under‑coating defects. When the fixture is improperly designed, certain areas of the workpiece may be obscured by components of the fixture itself. If the fixture’s hooks, support rods, or clamping devices are positioned in front of critical regions of the workpiece, they can impede the flow of metal vapor, creating shadows on the workpiece surface and resulting in under‑coating in those areas.

The orientation and angle of the workpiece on the rack determine the deposition conditions at different locations. If the workpiece is oriented such that certain edges remain consistently facing away from the evaporation source, those edges will fail to receive metal deposition, resulting in bare‑metal areas. Overly dense placement of workpieces can cause adjacent parts to shade one another, each developing exposed‑base regions. When the gaps between workpieces are too narrow, metal vapor struggles to penetrate deep into the spaces, leaving the edges on both sides of the gap prone to bare‑metal exposure.

IV. Motion Issues During the Coating Process

The motion state of the workpiece during the coating process affects under‑coating defects. When the chuck rotation speed is too slow, certain areas of the workpiece remain facing away from the evaporation source for an extended period; if this exposure lasts long enough to prevent metal deposition, under‑coating will occur. Non‑uniform rotational speed—alternating between fast and slow—can also result in insufficient deposition in specific directions.

When the workpiece rotation mechanism fails, the workpiece cannot rotate uniformly during the coating process, resulting in excessive deposition on the side facing the evaporation source and insufficient or even no deposition on the side opposite it. For complex workpieces requiring multi‑angle coating, inadequate rotational motion leads to poor coating quality in shadowed areas, often exposing the substrate.

V. Insufficient edge coating during spraying

Insufficient application of primer or topcoat at the edges of the workpiece is a key precursor to substrate exposure after coating. The primer serves as the foundation for adhesion of the coating layer; if it is inadequately applied or absent at the edges, the coating cannot achieve effective adhesion in those areas. Even if metal particles deposit at the edges, the lack of a primer interface may cause the coating to delaminate during subsequent processing steps, resulting in visible substrate exposure.

Improper adjustment of spray parameters can result in insufficient edge coverage. If the gun travel speed is too high, the gun spends too little time at the edges, leading to inadequate coating application. A low paint output from the gun may also fail to deliver enough material per unit time to fully coat all surfaces. Additionally, if the spray pattern does not match the workpiece geometry, certain edge areas will consistently remain at the periphery of the spray zone, resulting in insufficient coating.

An improper spray angle can also result in insufficient edge coverage. When the spray gun is held perpendicular to the workpiece surface, the coating effectively covers flat areas, but its adhesion to edges that are perpendicular to the plane is poor. The spray gun should be adjusted to an appropriate angle to ensure that the spray pattern reaches the side edges; otherwise, the edges may remain uncoated with primer.

VI. Electrostatic Shielding Effect

The electrostatic shielding effect is a peculiar phenomenon in vacuum coating that leads to substrate exposure. Plastic substrates are insulators; during vacuum deposition, the metal particles deposited on the surface become charged, and these charges cannot be dissipated quickly. Once a certain thickness of metallic film has been deposited in a particular area, that region acquires electrical conductivity. Consequently, the charged metal particles experience electrostatic repulsion from the already deposited layer, causing subsequent particles to preferentially deposit on other areas.

At the edges and sharp features of a workpiece, the charge density is higher, and the electrostatic repulsion effect is more pronounced. If the initial coating layer at the edge is relatively thin, electrostatic repulsion will further reduce the deposition of subsequent particles, potentially even preventing deposition altogether and exposing the substrate. This phenomenon is especially pronounced on workpieces with sharply defined edges.

7. The primer has insufficient surface energy.

If the surface energy of the primer at the edges is too low, it can also prevent the formation of the coating layer. After the primer has cured, an abnormally low surface energy at the edges makes it difficult for metal particles to wet and nucleate on the surface. Metal particles that reach the surface cannot remain, or the initial crystal nuclei that do form exhibit extremely weak adhesion; during subsequent deposition, they may detach, resulting in a lack of coating in that area.

The low surface energy of the primer may be attributable to factors such as the intrinsic properties of the primer formulation, excessive curing that renders the surface overly dense, or differing curing conditions at the edges compared with other areas. Residual release agents or other contaminants on the primer’s edges can likewise reduce surface energy, thereby inhibiting the formation of the coating layer.

VIII. Conclusion

Bare substrate is a severe defect in UV vacuum plating, resulting from the loss of the coating layer. Its causes are multifaceted, involving factors such as shadowing effects during vacuum deposition, fixture design, coating processes, and surface conditions. In vacuum plating, both the workpiece’s own shadowing and mutual shadowing among workpieces can prevent certain areas from receiving metal deposition; inadequate fixture design and overly dense workpiece placement exacerbate these shadowing issues; insufficient rotational motion during deposition leaves some orientations perpetually facing away from the evaporation source; edge‑coating deficiencies during spraying lead to bare substrate, leaving no viable sites for the coating to adhere; electrostatic shielding at edges and sharp features repels subsequent deposition; and excessively low surface energy of the primer inhibits the formation of the coating layer. Understanding the manifestations and root causes of bare substrate is the fundamental prerequisite 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 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	Good flexibility, excellent 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 in UV Vacuum Plating (10)

Common Issues in UV Vacuum Plating (Part 8)