Responding to several major problems in flexo UV glazing
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UV coatings occupy an important position in flexo printing and are usually selected online. The UV curing device consists of two UV tubes and a reflective device. Water-based inks can be printed with UV varnish after printing, which can provide excellent maintenance for water-based inks. There are often some problems in UV glazing.
Yellow coating
The main reason for the yellowing of UV varnish after curing is twofold: the UV varnish is placed for too long, especially the yellowing of the residual varnish is more serious; the ultraviolet ray is excessively over, and the UV varnish is excessively cured.
Solution: If it is the quality of UV varnish itself, it is necessary to 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
Imprinted data for fault-absorbent or non-porous substrates (such as PVC or PET film) will show poor adhesion of the varnish coating to the substrate.
Solution: Prepare the clear tape and the meshed object, cross the thread on the coating, and then stick it with tape. The size of the coating tape 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, clarifying that the varnish is not suitable for use on a non-porous substrate, it should be replaced.
Fingerprint question
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 function, 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 appearance of the coating. If the varnish is not stabilized in the coating, it will move to the outer surface of the coating, forming an uncured thin layer that will leave fingerprints when touched.
Solution: Ensure that the coating is completely boring and reduce the relocation of residues to the exterior of the coating.
Glazing is not stable
In the flexographic printing process, in order to make the water-based ink stable, it is necessary to participate in the ink to participate in certain amines. Although the viscosity of the ink can be effectively adhered to, some amines 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 modify the varnish formula.
Matte oil concentration is not stable
The question of printing a long version of the book is more prominent. In order to reduce the gloss of UV varnish or water varnish to achieve matting effect, it is necessary to participate in a lot of silica, which constitutes an increase in the viscosity of varnish, which directly affects the function of varnish. If the adhesive is removed, it will affect the curing speed and form streaks and stains on the coating.
Solution: Pick a suitable long-lived print matt varnish.
Adhesive suitability and printability
A good varnish coating forms a 100% solid film on the outer surface of the ink layer. The coating is like a plastic film that is lubricated on the outside. This is very disadvantageous for post-processing such as gluing, and the adhesive and ink cannot adhere at all.
Solution: It is necessary for the varnish coating to have good applicability and printability. Before the application, it is best to test 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 simply falls off, which is an over-curing composition; on the other hand, the ink layer is not completely boring, and the glazing coating is poor in scratch resistance. There are three reasons why the ink layer is not completely boring: the residual substance in the ink is dispersed to the outside of the ink layer; the ink is slow-drying; the boring equipment of the printer is not suitable.
Solution: Check and adjust the boring equipment of the ink to ensure that the ink is completely boring; adhere to air circulation and accelerate the boring 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 lowered during the operation.
Varnish is poorly cured on black ink, white ink, and blue ink layer
Printing varnish on black, white, and 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, and constitutes incomplete curing of the varnish.
Solution: Increase the power of the UV lamp, clean the reflector, replace the UV lamp, reduce the printing speed of the varnish, and think about it for a long time. Develop a versatile varnish that can be completely cured on any color ink layer.