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Typical Defects of UV 3C Coatings (Part 11)

Release time:

2026-06-29 16:34

Typical Defects of UV 3C Coatings (Part 11)

During the application of UV 3C coatings, sagging is one of the most common defects that compromise the uniformity of coating thickness. It manifests as downward flow of the coating on vertical surfaces, resulting in an uneven film—thick in some areas and thin in others. Such defects frequently occur on the sidewalls and edges of workpieces and are particularly pronounced in 3C products with vertical surfaces, such as mobile phone housings and laptop panels. Sagging not only compromises the aesthetic consistency of the coating but can also lead to localized film‑thickness exceedances, potentially triggering additional issues like incomplete curing. Understanding the characteristics and root causes of sagging helps identify this defect during production.

I. Appearance of Flow Marks

Sagging manifests as tear‑drop‑shaped or wavy flow marks on the coating surface, resulting from the paint flowing downward under gravity and accumulating in thicker deposits at the lower end of the flow path. The direction of these sagging marks typically aligns with the gravitational force, extending from the upper to the lower portion of the workpiece.

The coating thickness in sagging areas is significantly greater than in the surrounding regions, resulting in strip‑like ridges along the flow path. At the ends of sags, thick droplet‑like accumulations often form, with paint volumes far exceeding those in normally coated areas. Under illumination, these sag marks cast distinct shadows, contrasting sharply with the smooth surrounding surface. When touched, the surface feels uneven and textured.

Sagging typically occurs on the side walls, edges, and corners of workpieces. At these locations, the coating exhibits a stronger tendency to flow under gravity, and its spreading behavior differs from that observed on flat surfaces.

II. Excessive Spray Film Thickness

Excessive spray film thickness is the primary cause of sagging. When the spray gun moves too slowly, the amount of coating applied per unit time accumulates on the workpiece surface, forming an excessively thick wet film. Under the influence of gravity, such a thick coating cannot remain stable and flows downward.

Excessive single‑coat application volume can also result in an overly thick coating. Setting the spray gun’s flow rate too high, or positioning the gun too close to the workpiece during spraying, will both cause the coating thickness per unit area to exceed the normal range. A wet film that is too thick remains in a liquid state for an extended period before curing, allowing gravity to draw it downward.

Excessive coating thickness not only directly causes sagging but also gives rise to other issues. In areas with thick coatings, the degree of cure may differ from that in other regions; while the surface layer cures, the interior may remain incompletely cured.

III. Coating Viscosity Too Low

Another major cause of sagging is excessively low paint viscosity. Paint viscosity determines its resistance to flow; paints with lower viscosity tend to flow more readily under gravity, resulting in inadequate sag resistance.

Paint viscosity that is too low may stem from the formulation itself or from the addition of thinners during application. At higher ambient temperatures, paint viscosity naturally decreases, which in turn increases the risk of sagging. Variations in viscosity between batches can also necessitate adjustments to application conditions.

IV. Excessive Addition of Thinner

Excessive diluent addition and slow evaporation are also factors that can cause sagging. The purpose of the diluent is to reduce the coating’s viscosity to meet application requirements; however, when too much is added, the viscosity drops excessively, resulting in overly high flowability.

The evaporation rate of the thinner also affects sagging. Thinner with a slower evaporation rate keeps the coating in a low-viscosity state for an extended period, allowing ample time for the paint to flow under gravity. Conversely, a thinner that evaporates too quickly may cause the paint to partially dry by the time it reaches the substrate, compromising leveling but reducing the risk of sagging. The type and amount of thinner must be carefully balanced to suit the specific application conditions.

V. The Impact of Flow Marks on Coating Quality

The impact of sagging on coating quality is multifaceted. From an aesthetic standpoint, sagging defects compromise the coating’s smoothness and uniform gloss, preventing the product from meeting high‑quality appearance standards. In the context of 3C electronic products, which demand stringent visual quality, sagging is a major factor contributing to product nonconformity.

In terms of performance, the coating thickness in sagging areas tends to be excessive, which may lead to incomplete curing due to insufficient UV penetration. The disparity in film thickness between sagging and non-sagging regions results in inconsistent coating performance; the thicker‑coated areas exhibit higher internal stresses, potentially compromising long-term durability.

Sagging also complicates subsequent processing steps. Sagging marks must be sanded down before further coating can be applied, thereby increasing both labor and time costs.

VI. Conclusion

Sagging is a common defect in UV 3C coatings that compromises the uniformity of coating thickness, with its causes stemming from both the spraying process and the coating’s condition. Excessively thick spray films cannot maintain stability under gravity, while insufficient coating viscosity results in inadequate sag resistance; moreover, over‑dilution with a diluent that evaporates too slowly can keep the coating at low viscosity for an extended period. These factors cause the coating to flow downward on vertical surfaces, leading to an uneven film. Sagging not only undermines aesthetic consistency but may also give rise to curing issues caused by localized thickness variations. Understanding the manifestations and root causes of sagging is essential for identifying and analyzing 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.

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Typical Defects of UV 3C Coatings (12)

Typical Defects of UV 3C Coatings (10)