Solutions to 9 major problems of Chocolate Boxes flexo UV coating
Feb 21, 2022
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Solutions to 9 major problems of Chocolate Boxes flexo UV coating
UV varnishing plays an important role in Chocolate Boxes flexo printing, and generally adopts online method. The UV curing device consists of two UV lamps and a reflector. The UV varnish is printed after the water-based ink is printed, which can protect the water-based ink well. There are often some glitches in the UV coating process.
The coating turns yellow
There are two main reasons for the yellowing of UV varnish after curing: UV varnish is left for too long, especially the yellowing phenomenon of inferior varnish is more serious; excessive ultraviolet irradiation and excessive curing of UV varnish.
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 UV light source or increase the glazing speed.
Poor adhesion of the coating to the surface of non-absorbent substrates
The printing material is a non-absorbent or non-porous substrate (such as PVC or PET film), and the adhesion of the varnish coating on the surface of the substrate will be poor.
Solution: Prepare scotch tape and a grid tool, draw a cross line on the coating, and then stick it with tape. The size of the coating area 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 largely torn off, the varnish is not suitable for use on non-porous substrates and should be replaced.
fingerprint problem
Fingerprints are found on the Chocolate Boxes after glazing, especially when the black ink is full of glazing. This is because some additives are often added to the varnish to improve the properties of the varnish, such as defoamer, leveling agent, etc. They do not participate in the curing cross-linking reaction, and will eventually be absorbed by the substrate, or may 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, forming a thin uncured layer that can leave fingerprints when touched by fingers.
Solution: Make sure the coating is completely dry to reduce the migration of residues to the coating surface.
The glazing effect is unstable
In the flexographic printing process, in order to stabilize the viscosity of water-based ink, it is necessary to add certain amine substances to the ink. Although the ink viscosity can be effectively maintained, some amine substances will have a negative impact on the curing effect of UV varnish.
Solution: If the varnish and ink do not match, replace the varnish, or change the varnish formula.
The concentration of matt oil is unstable
The problem of printing long runs is even more prominent. In order to reduce the gloss of UV varnish or water-based varnish to achieve a matting effect, a large amount of silica needs to be added, which will increase the viscosity of the varnish and directly affect the performance of the varnish. If a detackifier is added, it will affect the cure speed and cause streaks and stains in the coating.
Solution: Choose a matte varnish suitable for long-run live printing.
Glueability and printability
A good varnish coating forms a 100% solid film on the surface of the ink layer, and the coating is like a plastic film with a smooth surface, which is very unfavorable for post-processing such as gluing, and the adhesive and ink cannot be attached at all.
Solution: The varnish coating must have good gluing suitability and printing suitability. It is best to test it before use, and contact the varnish supplier to find a solution together.
Poor scratch resistance
The scratch resistance of UV varnish coating is not good. On the one hand, the coating is too brittle and easy to peel 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: residual substances in the ink diffuse 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 circulating to speed up the drying of the ink.
Poor water resistance
The water resistance of general-purpose UV varnish is poor, 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 should be appropriately reduced when using.
The varnish is poorly cured on the black ink, white ink and light-emitting blue ink layer
Printing varnishes on black, white and glossy blue inks presents many headaches. Because the pigment in the ink and the photoinitiator of the varnish absorb the ultraviolet light in the same band, they 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. From a long-term perspective, an all-round varnish that can be completely cured on any color ink layer should be developed.