UV glazing plays an important role in flexo printing and is generally used online. The UV curing unit consists of two UV tubes and a reflecting device. Water-based ink printed after printing UV varnish, can play a very good protection of water-based inks. Some faults often occur during UV glazing.
• Yellow coating
The main reason for the yellowing of UV varnish after curing is twofold: UV varnish is left for too long, especially the yellowing of inferior varnish is more serious; UV irradiation is excessive, and UV varnish is excessively cured.
Solution: If it is the quality problem of UV varnish itself, you must replace the UV varnish; if it is caused by overexposure, reduce the power of the UV source or increase the glazing speed.
• poor adhesion of the coating to the surface of the non-absorbent substrate
The substrate is a non-absorbent or non-porous substrate (such as PVC or PET film), which will result in poor adhesion of the varnish coating to the substrate surface.
Solution: Prepare the scotch tape and the mesh tool. Make a cross on the coating and then tape it. The size of the coating tape torn off by the tape represents the degree of cure and adhesion of the coating. If the coating is not torn off, the adhesion is good. If the coating is torn off over a large area, this varnish is not suitable for use on non-porous substrates and should be replaced.
• Fingerprint problem
Fingerprints were found on the printed products after glazing, especially when the black ink was glazed. This is because varnish often adds some additives to improve the varnish properties, such as defoamers, leveling agents, etc., they do not participate in the curing cross-linking reaction, and will eventually be absorbed by the substrate, or may be removed after curing. To the surface of the coating. If the varnish is not stabilized in the coating, it will migrate to the surface of the coating, forming an uncured layer that will leave a fingerprint when it is touched.
Solution: Ensure that the coating is completely dry and reduces the migration of residues to the surface of the coating.
• Glazing effect is unstable
In the flexographic printing process, in order to stabilize the viscosity of the water-based ink, it is necessary to add a certain amine substance to the ink. Although the viscosity of the ink can be effectively maintained, some amine substances may have a negative effect on the curing effect of the UV varnish.
Solution: If the varnish and ink do not match, replace the varnish or change the varnish formulation.
• Matte oil concentration is unstable
The problem of printing a long version of the work is more prominent. In order to reduce the gloss of UV varnish or water varnish to achieve the matting effect, a large amount of silica needs to be added, which causes the viscosity of the varnish to increase, directly affecting the performance of the varnish. If a debonding agent is added, the curing speed is affected, causing streaks and stains on the coating.
Solution: Choose a matt varnish suitable for long-run printing.
• Adhesive suitability and printability
A good varnish coating forms a 100% solid film on the surface of the ink layer. The coating is like a plastic film with a smooth surface. This is very disadvantageous for post-processing such as gluing, and the adhesive and ink cannot adhere at all.
Solution: The varnish coating must have good applicability and printability. It is best to test it before use and contact the varnish supplier to find a solution.
• poor scratch resistance
The UV varnish coating has poor scratch resistance. On the one hand, the coating is too brittle and easily peeled off, which is caused by excessive curing; on the other hand, the ink layer is not completely dried, resulting in poor scratch resistance of the glazing coating. There are three reasons why the ink layer is not completely dried: the residual substance in the ink diffuses to the surface of the ink layer; the ink is slow-drying; the drying device of the printing machine is not suitable.
Solution: Check and adjust the drying device of the ink to ensure that the ink is completely dry; keep the air flowing and speed up the drying of the ink.
• poor water resistance
The general-purpose UV varnish is poor in water resistance because the important components in the UV varnish are hydrophilic, which affects the water resistance of the coating.
Solution: There is water-resistant UV varnish on the market, but the curing speed is slow, and the printing speed is appropriately reduced when used.
　 • The varnish is poorly cured on the black ink, white ink, and the blue ink layer.
Printing varnish on black, white and polished blue inks presents many headaches. The photoinitiator of the pigment and varnish in the ink absorbs the ultraviolet light of the same wavelength band, and the two compete for the limited ultraviolet light energy, which has a greater influence on the varnish, which may cause the varnish to be incompletely cured.
Solution: Increase the power of the UV lamp, clean the reflector, replace the UV lamp, reduce the printing speed of the varnish. In the long run, develop a versatile varnish that can be completely cured on any color ink layer.
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