• Yellowing of the Coating

The primary cause of yellowing after UV curing is twofold. First, if the UV varnish is left for too long, especially lower-quality varnishes tend to discolor more easily. Second, excessive UV exposure can lead to over-curing, which also causes yellowing.

Solution: If the issue stems from poor varnish quality, replace it with a higher-grade product. If overexposure is the problem, consider reducing the UV lamp power or increasing the glazing speed to prevent excessive curing.

• Poor Adhesion on Non-Absorbent Substrates

When working with non-absorbent or non-porous materials such as PVC or PET film, the UV varnish may not adhere well to the surface, leading to peeling or flaking.

Solution: To test adhesion, use a tape test by making a cross-cut on the coating and applying pressure with adhesive tape. If the coating remains intact, the adhesion is good. If large areas come off, the varnish may not be suitable for non-porous substrates, and a different formulation should be considered.

• Fingerprint Marks on the Surface

Fingerprints often appear on printed products, particularly when black ink is coated. This happens because some additives in the varnish—like defoamers or leveling agents—do not fully cure and may migrate to the surface, forming an uncured layer that leaves marks when touched.

Solution: Ensure complete drying of the coating and minimize the migration of these substances to the surface. Proper formulation and application techniques can help reduce this issue.

• Unstable Glazing Effect

In flexographic printing, amine substances are often added to water-based inks to maintain viscosity. While this helps control ink consistency, certain amines can interfere with UV varnish curing, causing inconsistent results.

Solution: If the varnish and ink are incompatible, try replacing the varnish or adjusting its formulation. Compatibility testing is essential before full-scale production.

• Unstable Matte Oil Concentration

Long print runs often experience issues with matte varnishes. To achieve a matte finish, silica is added, which increases viscosity and affects the overall performance. In some cases, this leads to streaks or uneven coatings.

Solution: Choose a matte varnish designed for long-run applications. It should maintain stability and avoid unwanted visual defects.

• Adhesive Compatibility and Printability Issues

A high-quality UV varnish forms a solid, smooth film on the ink layer, similar to a plastic coating. While this is ideal for aesthetics, it can make post-processing like gluing difficult, as adhesives may not stick effectively.

Solution: Always test the varnish before use. Work closely with your supplier to ensure the varnish has good compatibility and printability for your specific application.

• Poor Scratch Resistance

UV varnish may lack scratch resistance due to being too brittle, which can happen from over-curing. Alternatively, if the ink layer isn't fully dry, the varnish coating may not adhere properly, resulting in poor durability.

Solution: Check and adjust the ink drying system to ensure complete drying. Improve air circulation to speed up the process and enhance coating performance.

• Poor Water Resistance

Standard UV varnishes often have weak water resistance due to hydrophilic components in their formulation, which can compromise the coating’s durability when exposed to moisture.

Solution: Opt for water-resistant UV varnishes, although they may cure more slowly. Adjust the printing speed accordingly to accommodate this.

• Inadequate Curing on Black, White, and Blue Inks

Applying UV varnish on black, white, or polished blue inks can be problematic. Both the pigment and varnish absorb similar UV wavelengths, leading to competition for light energy and potential under-curing of the varnish.

Solution: Increase UV lamp power, clean reflectors, replace old lamps, or slow down the printing speed. In the long term, developing a universal varnish that cures effectively on all ink colors would be ideal.

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