Common Faults and Solutions of Flexographic UV Coating (Part 2)

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Surface Smoothness of Paper Is Suboptimal

The roughness of the paper surface directly impacts the leveling ability of the UV varnish. An overly rough surface slows down varnish flow, allowing most of the varnish to be absorbed into the paper fibers before it can form a continuous film. This diminishes the gloss and brightness of the finished product. Additionally, uneven surfaces may interfere with UV light reflection, diffraction, and interference, reducing curing efficiency.

However, excessively smooth paper surfaces also pose challenges. Poor absorbency can prevent ink penetration, causing ink to crystallize on the surface and create a non-absorbent layer. When UV varnish is applied over such a surface, the resin remains on top, potentially leading to incomplete curing.

Solution: Select paper with optimal printability. Alternatively, pre-print a diluted white ink layer to adjust surface roughness and slow absorption. Applying a water-based varnish as a base coat can also enhance surface printability and improve the mechanical durability of the printed material.

Incomplete Curing of UV Varnish

UV curing systems typically employ high-pressure mercury lamps emitting wavelengths between 200 and 400 nm, with a recommended irradiance intensity exceeding 80 W/cm. Under normal conditions, the lamp lifespan ranges from 800 to 1,200 hours. As the lamp ages, UV output diminishes, preventing full activation of the photoinitiators. This results in slower cross-linking reactions between the photosensitive resin and reactive diluent, leading to incomplete curing, surface tackiness, and reduced gloss.

Moreover, UV varnish curing may be compromised on black, white, and luminous blue ink surfaces. These pigments and the photoinitiators in the varnish often absorb UV light at similar wavelengths, competing for limited energy and reducing the available light for effective curing.

Solution: Operators should establish optimal lamp replacement schedules based on product requirements and monitor the quality of the varnish film throughout production. When applying UV varnish over black, white, or luminous blue ink layers, use a higher-power light source and regularly clean the reflector to maintain efficient UV light utilization.