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Since the invention of UV curing technology in the 1960s, it has been widely used by the end of the 20th century. In China, UV printing and UV glazing have been widely used in aluminum foil paper and aluminum foil cardboard printing, magnetic card printing, commercial printing, metal printing, food, medicine, cigarette packs, stickers and other packaging printing fields. UV paper product glazing is growing at an annual rate of more than ten percent, so the application prospect of UV varnish is very broad.

For flexo printing, post-print glazing is very common, because glazing can improve the water resistance and wear resistance of printed products to a certain extent, and it also plays a role in beautifying products and improving product grades. At the same price level, many post-press processing methods are not comparable to glazing effects. Therefore, in the past 10 years, the development of varnish technology is quite rapid, and chemical raw materials are becoming cheaper. The application range is getting wider and wider, and the performance is more and more stable.

At present, UV coating plays an important role in flexographic coating. And generally use online mode. The UV curing device consists of one or two ultraviolet lamps and a reflecting device. Printing UV varnish after water-based ink printing can play a good role in protecting water-based ink. Of course, there are some special occasions that require water-based varnish. For example, when the final product needs to be code-printed, it is necessary to use a special water-based varnish for printing. Other manufacturers also choose water-based varnish for matt glazing Because UV matt oil is easy to thicken when printing long version.

The final unit of the flexographic printing machine and the special coating machine are used for coating, and the processing speed is 100 ～ 200m / min.

light source. The UV irradiation light source generally uses a high-pressure mercury lamp or a metal halide lamp (preferably an electrodeless lamp), and the power of the light source also needs to be stable. The output power of the high-pressure mercury lamp is generally 80 ~ 120W / cm, in order to ensure the curing speed of UV coating <0.5s;

Paper UV coating is not suitable for processing paper that is easy to penetrate. Because the low-molecular materials in UV coatings easily penetrate into the paper, causing the paper to darken or even penetrate. In this case, only to increase the printing speed and increase the number of lamps. In order to prevent the penetration phenomenon, the bottom layer can be coated with infrared primer to avoid penetration when applying UV coating;

Ink. Need to pay attention to the combination of UV paint and ink, if necessary, apply a layer of primer first;

coating. After the glazed coating is irradiated with ultraviolet light, the photoinitiator is initiated to generate free radicals or ions. These free radicals or ions cross-link with the double bond in the prepolymer or unsaturated monomer to form a monomer group. These monomer groups are used to form polymer solid polymers through a chain reaction to complete the drying process of the glazing paint. The coating layer is too thick, and the drying time is relatively long under the same power light source. On the one hand, it affects the drying of UV varnish, on the other hand, the water content of the paper is relatively reduced, which makes the sheet of paper brittle and easy to break, affecting the UV. Wear resistance of oil layer;

Thinner. UV glazing oil generally needs to use a special thinner, the dilution amount is controlled at about 1: 0.5 to 1: 1, too much will affect the glazing effect (depending on the amount of coating film);

In the process of UV glazing, some problems often occur, and the author makes some simple discussions below.

1. Yellow coating

The main reasons for UV varnish yellowing after curing are generally three aspects: one is that the UV varnish has been left for too long, and it has turned yellow, especially the yellowing phenomenon of inferior varnish is more serious; the second is excessive ultraviolet radiation, UV varnish is over-cured; third, the glaze coating is too thick.

Solution: If it is a quality problem of UV varnish itself, it must be replaced by UV varnish: if it is yellowing caused by overexposure, reduce the power of the ultraviolet light source or increase the glazing speed; if the varnish is too thick, it is appropriate Consider thinning the varnish thickness (the varnish concentration can be appropriately reduced).

2. The gloss layer is whitish and the crosslink density is low

It is very likely that the thinner is used improperly

Solution: Due to the fast evaporation rate of alcohol solvents in summer, the thinner should be replaced or ester thinner or special thinner should be used.

3. Poor adhesion of coating on non-absorbent substrate surface

If the printing material is a non-absorbent or non-porous substrate (such as PVC or PET film), there may be a poor adhesion fastness of the varnish coating on the surface of the substrate.

It is generally believed that the harder the coating, the better the effect, but for non-absorbent substrates, the toughness of the coating is the first priority, otherwise there will be problems with poor adhesion. Using some special raw materials, choose photo-curable resin and active monomer with good toughness and good adhesion.

The crystallization of the ink layer of the printed matter or the improper use of ink additives and anti-slip agents should improve the printing process.

Solution: To test the adhesion fastness of the coating, the cross-cut test method is generally used. First prepare the transparent tape and the tool for drawing the grid, and cross the grid line on the coating. 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 at all. This shows that the powder adhesion of the coating is quite good. If the coating is torn off in a large area, it means that this varnish is not suitable for use on non-porous substrates. You should consider replacing other types of varnish. No or less ink additives are used; the viscosity of the varnish is too low, the varnish should be adjusted and the viscosity of the varnish should be increased.

4. Brightness difference

Poor brightness is generally most likely due to the following two reasons. First, the quality of the paper is poor or the surface is rough, and the oil absorption is large; the coating is uneven or the varnish is diluted too thin and the viscosity is too low.

If it is the first problem, there is no better way to replace the paper; the second problem may be to adjust the glazing machine and increase the viscosity of the varnish.

5. Light film is too soft

Fingerprints are often found on the printed products after glazing, especially when the black ink is full of glazing. Why would such phenomenon happen? This is because some additives are often added to the varnish to improve the performance of the varnish, such as defoamers. Leveling agent, etc., these components are not involved in curing crosslinking reaction. They may eventually be absorbed by the substrate, or they may migrate to the surface of the coating after curing. If a component of varnish is not stabilized in the coating, it will directly migrate to the surface of the coating and form an uncured thin layer, which will leave a clear fingerprint when it is accidentally touched by the finger.

Solution: When encountering such problems. The key is to ensure that the coating is completely dry and reduce the migration of residues to the surface of the coating.

6. Unstable glazing effect

Unstable varnishing is a big headache for many flexo printing companies. In the flexographic printing process, in order to maintain the viscosity of the water-based ink, it is necessary to add a certain amount of amines to the ink. Although it can effectively adjust and maintain the viscosity of the ink, some amines will affect the UV varnish The curing effect has a negative effect. Sometimes, the unstable coating is caused by the amine added to the ink.

Solution: Try to use other new methods to control the viscosity of the ink. If the varnish does not match the ink used, you must replace the varnish. Or change the formula of varnish.

7. Matte oil concentration is unstable

UV matt oil has the problem of concentration instability in printing and coating, especially when printing long plates. Why is there such a problem? In order to reduce the gloss of UV varnish or water-based varnish and make it reach the matting effect, it is generally necessary to add a large amount of silica. This will inevitably cause the viscosity of the varnish to increase, and the viscosity increase will directly affect the performance of the varnish. If adding a debonding agent will affect the curing speed and bring some uncertain factors, causing streaks and stains on the coating. Many printing companies avoid UV matt oil as much as possible because they cannot find a suitable solution to the problem.

Solution: Contact the varnish supplier to choose the matt varnish suitable for long live printing.

8. Adhesion (not dry)

There are three possibilities, one is that the coating is too thick; the second is that the power of the UV lamp is insufficient or aging (the life of the UV lamp is generally about 800 hours, and the curing efficiency will decrease after this time); the third is that the varnish has expired.

Solution: If the coating is too thick, the viscosity should be reduced; if the UV lamp power is insufficient or aging, the UV lamp should be replaced to reach the rated power; if the varnish has expired, it should be stopped.

9. Glue suitability and printability

We often hear that some manufacturers praise their printed products for their good glazing effect, which looks like a mirror. A good varnish coating forms a 100% solid film on the surface of the ink layer. Strictly speaking, the performance of the coating is more like a dense and smooth plastic film on the surface. But from another point of view, this is very unfavorable for the subsequent processing such as coating, because on the surface of the coating that is too smooth, the adhesive and ink cannot be attached at all.

Solution: If the printed product needs to be pasted or printed after glazing, the coating surface should not be too smooth to ensure that the adhesive or ink can adhere to it. That is to say, the varnish coating must have good glue suitability and printing suitability, but it is worth noting that even if the varnish coating suitability and printing suitability are relatively good, it may not be suitable for all adhesives. It is compatible with ink, so it is best to do a test before using it. If the test result is not ideal, it is best to get in touch with the varnish supplier to find a solution together.

10. Poor scratch resistance

The UV varnish coating has poor scratch resistance, and the root cause of the problem is still the adhesion fastness. On the one hand, the coating is too brittle and easily peels off, which may be caused by excessive curing. On the other hand, the ink layer is not completely dried, which will also cause poor scratch resistance of the glazed coating. There are three main reasons why the ink layer is not completely dried: first, the residual substances in the ink diffuse to the surface of the ink layer; second, the ink is slow-drying ink; third, the drying device of the printing machine is not suitable.

Solution: Check and adjust the ink drying device. Ensure that the ink can be completely dried; keep air circulation as much as possible to speed up the drying of the ink. If it still doesn't work, you can only change the varnish.

11. Poor water resistance

The water resistance of general-purpose UV varnish is generally poor, because some important components contained 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 of this varnish is slow. Therefore, when using this water-resistant varnish, the printing speed should be appropriately reduced.

12. Poor curing of varnish on some ink surfaces

When printing varnish on black ink, white ink, and light blue ink layers, various unexpected problems often occur, which is also one of the most headaches for printing companies.

We can understand and treat this problem in this way: the pigment in the ink and the photoinitiator in the varnish absorb ultraviolet light in the same band, and the two compete for limited ultraviolet light energy. The curing of varnish requires energy, so this has a greater impact on varnish, which may cause incomplete curing of varnish.

Solution: increase the power of the UV lamp, clean the reflector, replace the UV lamp, and reduce the printing speed of the varnish, but this can only be used as a stopgap measure. From a long-term perspective, a color that can be used in any color should be developed and developed. An all-around varnish that is fully cured on the ink layer.

Listed above are some of the problems that I often encounter in my work. I would like to share with you here, hoping to help the actual operation of glazing.

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