UV coating in flexo printing common problems and solutions

UV coating occupies an important position in flexo printing and is generally used online. The UV curing device consists of two UV lamps and a reflector. Water-based ink printing after printing UV Varnish can play a very good role in the protection of water-based ink. Some failures often occur during UV polishing.

Yellow coating

There are two main reasons for the yellowing of the UV varnish after curing: the varnish is left for a long time, especially the yellowing phenomenon of the inferior varnish is more serious; the ultraviolet varnish is excessive and the UV varnish is excessively cured.

Solution: If it is the quality of UV varnish itself, you must replace the UV varnish; if it is caused by overexposure, reduce the power of the UV light source or increase the speed of the varnish.

The poor adhesion of the coating on the non-absorbent substrate surface

Substrate materials are non-absorbent or non-porous substrates (such as PVC or PET films), which can cause poor adhesion of the varnish coating on the substrate surface.

Solution: Prepare the transparent tape and the tool that draws the grid, draw the cross line on the coating, and then stick it with tape. The area of ​​the coating removed by the tape represents the degree of curing 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 nonporous substrates and should be replaced.

Fingerprint problem

Fingerprints were found on the glazing products, especially when the black ink was full. This is due to the fact that some additives are often added to the varnish to improve 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 shifted 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 thin layer that will leave fingerprints when touched.

Solution: Make sure the coating is completely dry, reducing the migration of residue to the coating surface.

The glazing effect is not stable

In the flexographic printing process, in order to stabilize the viscosity of the water-based ink, it is necessary to add a certain amount of amines to the ink. Although the viscosity of the ink can be effectively maintained, some amines may negatively affect the curing effect of the UV light.
Solution: If the varnish and ink do not match, replace the varnish or change the varnish formulation.

Matte oil instability

The problem of printing long-lived books is even more prominent. In order to reduce the gloss of UV varnish or water varnish to achieve matting effect, it is necessary to add a large amount of silica, which causes the varnish viscosity to increase and directly affect the varnish use performance. If an ungreaser is added, the curing speed will be affected and the coating will produce streaks and stains.
Solution: Choose a matt varnish that is suitable for long live printing.

Applicability and printability

The good varnish coating forms a 100% solid film on the surface of the ink layer. The coating is like a smooth plastic film. This 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 applicability and printability. It is best to test it before use and contact the varnish supplier to find a solution.

Poor scratch resistance

The scratch resistance of the UV varnish coating is not good. On the one hand, the coating is too brittle and easily peel off. This is caused by excessive curing; on the other hand, the ink layer is not completely dried, resulting in poor scratch resistance of the varnish coating. There are three reasons why the ink layer is not completely dried: residual substances in the ink diffuse into the surface of the ink layer; the ink is a slow-drying type; the drying device of the printing press is not suitable.

Solution: Check and adjust the ink drying device to ensure that the ink is completely dry; maintain air circulation and accelerate ink drying.

Poor water resistance

Universal UV Varnish has poor water resistance because the important components in UV Varnish are hydrophilic and affect the water resistance of the coating.

Solution: Water-resistant UV varnish is available on the market, but the curing speed is slow, and the printing speed is appropriately reduced during use.

Varnish cured poorly on black ink, white ink, and cast blue ink layers

There are many headaches in printing black oil, white ink, and blue light. The pigments in the ink and the photoinitiator of the varnish absorb the ultraviolet light in the same wavelength band, and the two compete for limited ultraviolet light energy, which has a greater influence on the varnish and causes the varnish to be not completely cured.

Solution: Increase the power of the UV lamp, clean the reflector, replace the UV lamp, reduce the speed of the varnish, and in the long term, develop an all-around varnish that can be fully cured on any color ink layer.

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