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Flexographic UV coating has gained widespread adoption among printing enterprises due to its rapid curing speed, energy efficiency, and environmentally friendly characteristics. The resulting coated products exhibit excellent water resistance, abrasion resistance, and biodegradability, making them highly favored by consumers. However, in actual production, various factors can lead to poor gloss and insufficient brightness on the surface of printed materials after UV coating. This article analyzes the primary causes of this issue.
Insufficient UV Varnish Coating
When the UV varnish is inadequately applied, the cured coating becomes too thin, failing to achieve the desired surface brightness. Several key reasons contribute to this problem.
One cause is low viscosity of the UV varnish. UV varnish typically consists of photosensitive resin, reactive diluent, photoinitiator, polymerization inhibitor, and other additives. Among these components, photosensitive resin plays a critical role in forming a glass-like film after UV curing. It directly influences the varnish's viscosity, curing speed, and post-curing smoothness. When the viscosity is too low, the adhesion between the varnish and the anilox roller decreases, leading to insufficient transfer and a thinner-than-desired coating.
Solution: Select a UV varnish with higher viscosity based on product specifications or add an appropriate amount of UV modifier to increase viscosity.
Another factor is an excessively fine anilox roller. Flexographic printing relies on the engraved cells of the anilox roller to store and transfer ink or varnish. A higher line count results in smaller cell volumes, which reduces the amount of varnish transferred. Consequently, the resulting UV film layer is too thin, resulting in diminished gloss and brightness.
Solution: Choose an anilox roller with a lower screen count according to product requirements to ensure sufficient film thickness. Typically, an anilox roller with 450 lines per inch is recommended for UV varnishing applications.
Excessive Dilution with Non-reactive Solvents
In practical operations, UV varnish may be diluted using non-reactive solvents such as industrial ethanol to improve flowability and transfer performance. While this method temporarily lowers viscosity without altering the chemical composition of the varnish, excessive use of ethanol can introduce moisture that fails to evaporate completely during curing. This leads to fog formation during the heating process, compromising the transparency of the cured film. In severe cases, incomplete curing and surface tackiness may occur.
Solution: Minimize the use of non-reactive solvents and opt for high-purity ethanol when necessary.
Uneven Application of UV Varnish
Inconsistent application of UV varnish significantly affects the final gloss and brightness of the printed surface. Several interrelated factors influence the uniformity of the coating:
(1) Variations in the height of raised areas on the printing plate can hinder varnish transfer, affecting coating quality.
(2) Uneven printing pressure may result in inconsistent varnish thickness on the substrate.
(3) Clogged anilox roller cells reduce ink storage capacity and impair uniform varnish transfer.
(4) Incompatibility between the printing ink and UV varnish, along with excessive adhesive substances on the ink surface, may locally obstruct varnish transfer.
Solution: Use high-quality flexographic plates with consistent relief heights. Ensure proper developer effectiveness and rinsing quality. Adjust printing pressure to maintain consistency and stability. Clean the anilox roller thoroughly to remove any residue from the cells. Preferably select inks and UV varnishes from the same manufacturer and avoid incorporating highly viscous additives into the ink formulation.