In the practical production of UV vacuum plating, edge whitening is one of the common defects that compromise the consistency of product appearance. It manifests as a lighter shade in the coating layer along the edges and at sharp corners of the workpiece, resulting in a whitish or dull appearance that contrasts sharply with the metallic luster of the central area. To address this defect, a systematic approach is required, addressing factors such as shadowing effects in vacuum deposition, fixture design, optimization of workpiece geometry, and the condition of the primer, in order to effectively enhance the coating quality at the edges.

I. Measures to Mitigate the Shadow Effect

1. Optimization of the evaporation source layout

In vacuum coating, the rectilinear propagation of metal vapor is the fundamental cause of shadowing effects. To mitigate shadowing at the edges, the layout of the evaporation sources should be optimized. For thermal evaporation coating, multiple-point evaporation sources can replace a single-point source to broaden the coverage of the metal vapor. The evaporation sources should be positioned as close as possible beneath the center of the substrate holder, ensuring that each substrate experiences a relatively uniform vapor field. Additionally, the height of the evaporation sources should be adjusted appropriately so that the vapor’s diffusion angle adequately encompasses the edge regions of the substrates.

2. Adjustment of the Distance Between the Workpiece and the Evaporation Source

When the workpiece is positioned too close to the evaporation source, the deposition non-uniformity between the edges and the center becomes more pronounced. The distance between the workpiece and the evaporation source should be appropriately increased to allow the metal vapor to diffuse over a larger spatial area, thereby improving coating uniformity. Adjusting this distance requires balancing coating efficiency with uniformity, and the optimal value should be determined through experimentation. For workpieces with complex geometries, further increasing the distance between the workpiece and the evaporation source can help mitigate the effects of shadowing.

3. Setting the tilt angle of the workpiece rack

For workpieces with severe edge whitening, the angle of the holder can be adjusted to expose the edge regions to a greater flux of metal vapor. By tilting the holder so that the edges face the evaporation source, the deposition rate at the edges can be increased. The tilt angle should be determined based on the workpiece geometry and the location of the whitening, and optimized through experimentation.

II. Optimization Strategies for Workpiece Rack Layout and Rotation

1. Adjustment of the workpiece rack layout

The positioning of workpieces within the coating chamber significantly affects coating uniformity. Workpieces should be avoided in the lower‑rate deposition zones near the chamber edges. Regularly measure the deposition rate distribution inside the chamber, generate a coating uniformity map, and adjust the workpiece rack configuration accordingly. For workpieces with pronounced edge‑whitening issues, prioritize placing them in the higher‑rate central region.

2. Optimization of Rotational Speed and Mode

The rotation speed of the workpiece holder should be appropriately set; if it is too slow, certain areas of the workpiece may remain facing away from the evaporation source for extended periods, leading to shadowing effects. To mitigate this, the rotation speed should be increased so that all parts of the workpiece frequently alternate their orientation relative to the evaporation source, thereby reducing the duration of shadowing. For workpieces with complex geometries, a dual-axis or planetary rotation configuration can be employed, enabling continuous changes in the workpiece’s orientation during coating and yielding a more uniform coating layer. The rotary mechanism should undergo regular inspection and maintenance to ensure smooth operation.

3. Improvements to Fixture Design

The design of the rack has a direct impact on the edge‑plating quality. The rack’s hooks and support rods should be positioned so as not to obscure the critical areas of the workpiece. For parts with severe edge whitening, a dedicated rack can be designed to suspend the workpiece at a more favorable angle. Contact points between the rack and the workpiece should be minimized to prevent coating defects in the vicinity of these contact areas. The rack material should be selected to minimize debris generation, thereby avoiding contamination of the plating chamber.

III. Optimization Strategies for Workpiece Geometry

1. Edge blunting treatment for sharp edges

Sharp edges and sharp corners are particularly prone to edge whitening. During the product design phase, sharp edges can be deburred to increase the radius of curvature. Deburred edges feature smoother transition surfaces, allowing for a wider range of incident angles of metal vapor and improving coating uniformity. For finished parts, edge deburring can be achieved through grinding; however, care must be taken to avoid compromising dimensional accuracy.

2. Coating Strategies for Complex Workpieces

For workpieces with complex geometries—such as deep holes, internal cavities, and recesses—segmented coating or multi‑angle coating strategies can be employed. Segmented coating involves applying the coating in two passes, with intermediate reorientation of the workpiece to ensure that all surfaces are properly aligned with the evaporation source. Multi‑angle coating entails adjusting either the angle of the evaporation source or the orientation of the workpiece during deposition, thereby increasing the likelihood that metal vapor reaches even the most challenging areas. For hard-to‑coat features like internal cavities, sputter coating may be considered, as its particle scattering characteristics are superior to those of evaporation coating.

IV. Optimization Strategies for Coating Process Parameters

1. Extension of the coating time

When the coating time is short, the thickness difference between the edge and the center is relatively large, and the whitening effect becomes more pronounced. Appropriately extending the coating time to increase deposition at the edges can mitigate the degree of whitening. The extent of the extension should be determined based on the severity of the whitening, with the goal of achieving an acceptable level of gloss at the edges. At the same time, care must be taken to ensure that the overall film thickness does not exceed the specified limits.

2. Adjustment of the coating rate

When the deposition rate is too low, edge deposition becomes insufficient; in this case, the deposition rate should be appropriately increased to boost the amount deposited per unit time. Conversely, an excessively high deposition rate may result in a loose film structure and reduced gloss, necessitating a balance between deposition speed and film quality. To address edge whitening, a higher deposition rate can be used during the initial stage of coating, followed by a reduction in rate later on, thereby optimizing both uniformity and film quality.

3. Optimization of Airflow Distribution in the Coating Chamber

The airflow distribution within the coating chamber can influence the diffusion path of metal vapor. The position of the vacuum system’s exhaust ports and the settings of baffles should be carefully checked to prevent airflow from causing uneven metal vapor deposition. For sputter coating, the location of the process gas inlet and the flow‑rate distribution can affect coating uniformity; therefore, the design of the gas distribution ring should be optimized. Regularly clean the chamber walls to prevent deposits from disrupting airflow distribution.

V. Measures for Controlling the Condition of the Primer Layer

1. Reinforcement of edge primer application

One of the factors causing edge whitening is the application of a relatively thin primer coat at the edges of the workpiece. Spray parameters should be adjusted to enhance primer coverage at the edges. During spraying, consider increasing edge touch‑ups or adjusting the spray angle to ensure adequate coating on the edge faces. For workpieces with complex geometries, manual touch‑up can be used to build up primer thickness along the edges. After priming, inspect edge coverage to confirm there are no areas with missed or insufficient coating.

2. Improvement of primer leveling properties

Poor leveling of the primer at edge areas can compromise surface smoothness, thereby affecting coating gloss. A primer with excellent leveling properties should be selected to ensure thorough leveling after application. If the primer’s viscosity is too high, it can be reduced by preheating or adding a diluent. Sufficient leveling time should be allowed to enable the primer to form a smooth, even surface at the edges.

3. Optimization of the primer’s curing state

When the primer fails to cure properly, edge areas are particularly prone to incomplete curing. Ensure that the primer is fully cured by meeting the formulation’s specified curing energy and curing time. For edge regions, consider increasing the curing energy or extending the curing time to guarantee thorough curing of the edge primer. After curing, inspect the hardness at the edges to confirm that the curing condition is satisfactory.

VI. Comprehensive Process Management Measures

1. Regular inspection of coating uniformity

Regularly monitor the thickness distribution of the coating layer by measuring film thickness at various locations on the workpiece to identify differences between the edges and the center. Use glass or metal test substrates, positioning them at the edge and in the center of the workpiece holder, and measure the thickness after coating to assess the variation. Based on these results, adjust the process parameters to reduce the thickness disparity between the edge and the center.

2. Standardization of Process Parameters

Establish standardized process parameter files for coating, including evaporation source power, coating time, substrate holder rotation speed, and substrate placement position. Operators must adhere strictly to these standards to minimize batch-to-batch variability caused by human factors. Process parameters should be recorded for each production batch and analyzed in conjunction with the corresponding product test results.

3. Quality Inspection and Feedback

Each batch of products undergoes an edge‑appearance inspection, with gloss and color at the edge assessed under standardized lighting. If edge whitening is detected, measure the thickness difference between the edge and the center to determine whether it is caused by a shadowing effect or a primer‑related issue, and make process adjustments accordingly. Regularly compile quality data to analyze the incidence and severity of edge whitening, and continuously optimize the coating process.

VII. Conclusion

Addressing edge‑whitening requires a multifaceted approach, considering factors such as shadowing effects, workpiece rack configuration, workpiece geometry, and the condition of the primer. In vacuum coating, the straight‑line propagation of metal vapor is the root cause of shadowing; this can be mitigated by optimizing the arrangement of evaporation sources, adjusting the distance between the workpiece and the source, and setting an appropriate tilt angle for the rack. Workpiece rack layout and rotation strategy influence coating uniformity; thus, it is essential to refine placement, optimize rotational speed, and improve fixture design. Sharp edges and complex geometries exacerbate whitening, which can be alleviated through passivation treatments and segmented coating processes. Coating time and deposition rate also affect the degree of whitening; extending the coating duration and fine-tuning the deposition rate are advisable. Furthermore, the application quality of the primer at the edges impacts coating performance, necessitating enhanced edge‑coating techniques and optimized curing conditions. Through systematic process optimization, edge‑whitening can be effectively controlled, significantly improving the consistency of gloss between the edges and the center of coated products.

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