UV glazing occupies an important position in flexo printing, and generally adopts online mode. The UV curing device consists of two ultraviolet lamps and a reflecting device. After the water-based ink is printed, the UV varnish is printed, which can play a good role in protecting the water-based ink. During the UV coating process, some failures often occur.
Yellow coating
There are two main reasons for UV varnish yellowing after curing: UV varnish is left for too long, especially the yellowing phenomenon of inferior varnish is more serious; excessive UV irradiation, UV varnish over-curing.
Solution: If it is the quality problem of the UV varnish itself, the UV varnish must be replaced; if it is caused by overexposure, reduce the power of the ultraviolet light source or increase the glazing speed.
Poor adhesion of coating on non-absorbent substrate surface
The printing material is a non-absorbent or non-porous substrate (such as PVC or PET film), and there will be poor adhesion of the varnish coating on the surface of the substrate.
Solution: Prepare transparent tape and meshing tools, draw cross lines on the coating, and then use adhesive tape to adhere. The area of ​​the coating torn off by the tape represents the curing degree 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 are found on the printed product after glazing, especially when the black ink full version is glazed. This is because some additives are often added to the varnish to improve the performance of the varnish, such as defoamers, leveling agents, etc., they do not participate in the curing cross-linking reaction, and will eventually be absorbed by the substrate, and may also move after curing To the coated surface. If the varnish is not stabilized in the coating, it will migrate to the surface of the coating and form an uncured thin layer, which will leave fingerprints when touched.
Solution: Ensure that the coating is completely dry to reduce the migration of residues to the surface of the coating.
Unstable glazing effect
In the flexographic printing process, in order to stabilize the viscosity of the water-based ink, it is necessary to add certain amines to the ink. Although it can effectively maintain the viscosity of the ink, some amines will negatively affect the curing effect of UV varnish.
Solution: If the varnish and ink do not match, replace the varnish or change the varnish formula.
Matte oil concentration is unstable
The issue of printing long work is more prominent. In order to reduce the gloss of UV varnish or water-based varnish to achieve the matting effect, a large amount of silica needs to be added, which causes the viscosity of the varnish to increase, which directly affects the performance of the varnish. If you add an adhesive remover, it will affect the curing speed, causing streaks and stains on the coating.
Solution: Choose matt varnish suitable for long live printing.
Glue 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 unfavorable for post-processing such as glue application. The adhesive and ink cannot be attached at all.
Solution: The varnish coating must have good glue suitability and printing suitability. It is best to test before use and contact the varnish supplier to find a solution together.
Poor scratch resistance
The UV varnish coating has poor scratch resistance. On the one hand, the coating is too brittle and easily peels 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 glazed coating. There are three reasons why the ink layer is not completely dried: the residual substances in the ink diffuse to the surface of the ink layer; the ink is a slow-drying type; the drying device of the printing machine is not suitable.
Solution: Check and adjust the ink drying device to ensure that the ink is completely dried; maintain air circulation to speed up the drying of the ink.
Poor water resistance
Universal UV varnish has poor water resistance, because the important components in UV varnish are hydrophilic, which affects the water resistance of the coating.
Solution: There are water-resistant UV varnishes on the market, but the curing speed is slow, and the printing speed is appropriately reduced when used.
Poor curing of black ink, white ink, and blue ink layer
There are many headaches when printing varnish on black, white ink and light blue ink. The pigment in the ink and the photoinitiator of the varnish absorb ultraviolet light in the same wavelength band, and the two compete for limited ultraviolet light energy, which has a greater impact on the varnish and will cause the varnish to cure incompletely.
Solution: Increase the power of the UV lamp, clean the reflector, replace the UV lamp tube, and reduce the printing speed of the varnish. In the long-term perspective, a versatile varnish that can be completely cured on any color ink layer should be developed.
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