Flexo UV glazing FAQ
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Since the invention of UV UV curing technology in the 1960s, it has been widely used by the end of the 20th century. UV printing and UV glazing in China have been widely used in the field of aluminum foil paper and aluminum foil card printing, magnetic card printing, commercial printing, metal printing, food, medicine, cigarette packets, stickers and other packaging and printing. UV paper products are growing at a rate of more than 10% per year, so the application of UV coatings is very promising.
For flexo printing, post-press glazing is an unusual thing, because glazing can improve the water resistance and wear resistance of printed matter to a certain extent, and at the same time it can beautify products and enhance product quality. At the same price level, many post-press processing methods are not comparable to the glazing effect. Therefore, in the past 10 years, the development of varnish technology has been quite rapid, and chemical raw materials have become cheaper and cheaper. The application range is wider and wider, and the performance is more and more stable.
At present, UV glazing plays an important role in flexographic printing. And generally use the online method. The UV curing device consists of one or two UV lamps and a reflecting device. After printing the water-based ink, the UV varnish can be printed to provide a good protection for the water-based ink. Of course, there are also some special occasions requiring water-based varnish. For example, when the final product needs to be printed, it must be printed with special water-based varnish. Other manufacturers choose water-based varnish for matt glazing. Because UV matt oil is easy to thicken when printing long plates.
The final unit of the flexographic printing machine and the special coating machine are coated, and the processing speed is 100-200 m/min.
Light source. The UV illumination source generally uses a high pressure mercury lamp or a metal halide lamp (preferably an electrodeless lamp), and the power of the light source is also required 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 coatings are not suitable for processing easily permeable paper. Because the low molecular materials in the UV coating easily penetrate into the paper, causing the paper to darken or even soak. In this case, only speed up the printing and increase the number of lamps. In order to prevent the phenomenon of penetration, the bottom layer may be first coated with an infrared primer to avoid penetration when the UV coating is applied;
Ink. Need to pay attention to the combination of UV coating and ink, if necessary, first apply a primer;
Coating. After the glazing coating is irradiated by ultraviolet light, the photoinitiator is initiated to generate radicals or ions which crosslink with the double bonds in the prepolymer or unsaturated monomer to form a monomer group. These monomers are opened to form a polymer solid polymer by a chain reaction to complete the drying process of the glazing coating. The coating layer is too thick, and the drying time is relatively long under the irradiation of the same power source. On the one hand, it affects the drying of the UV varnish, on the other hand, the water content of the paper is relatively reduced, which makes the sheet paper brittle and easy to break, affecting UV. Wear resistance of the oil layer;
Thinner. UV varnish generally requires the use of special thinner, the dilution is controlled at about 1:0.5 to 1:1, too much will affect the glazing effect (depending on the amount of film);
There are often some problems in the UV glazing process. Here I will do some simple discussion.
1. Yellow coating
The main reason for the yellowing of UV varnish after curing is generally three aspects: First, the UV varnish is placed for too long, and it has turned yellow, especially the yellowing of inferior varnish is more serious; The UV varnish is over-cured; the third is that the glazing coating is too thick.
Solution: If it is the quality problem of UV varnish itself, you must replace the UV varnish: if it is caused by overexposure, it will reduce the power of the UV source or increase the glazing speed; if the varnish is too thick, it should be appropriate. Consider reducing the thickness of the varnish (the varnish concentration can be appropriately reduced).
2. The glazing layer is whitish and the crosslink density is low.
It is very likely that the diluent is not used properly.
Solution: Due to the rapid evaporation of alcohol solvents in summer, replace the diluent or use ester diluent or special diluent.
3. Poor adhesion of the coating to the surface of the non-absorbent substrate
If the substrate is a non-absorbent or non-porous substrate (such as PVC or PET film), the adhesion of the varnish coating to the surface of the substrate may be poor.
It is generally believed that the harder the coating is, the better the effect is. However, for non-absorbent substrates, the toughness of the coating is the first priority, otherwise the problem of poor adhesion will occur. In order to obtain a soft coating, it is required in the varnish formulation. Using some special raw materials, choose the photo-solid resin and active monomer with good toughness and good adhesion.
The printing ink layer crystallization or ink additives and anti-slip agents are not used properly, and the printing process should be improved.
Solution: Test the adhesion fastness of the coating. Generally, use the cross-cut test method. First, prepare the transparent tape and the meshing tool, and cross the grid line on the coating. Be careful not to damage the substrate and then stick it with tape. The size of the coating area torn off by the tape represents the degree of cure and adhesion of the coating. If the coating is not torn off at all. It shows that the adhesion of the coating is quite good. If the coating is torn off over a large area, it means that the varnish is not suitable for use on non-porous substrates and should be considered for replacement of other types of varnish. No or little use of ink additives; the viscosity of the varnish is too low, the glazing machine should be adjusted and the viscosity of the varnish should be increased.
4. Brightness difference
The brightness is not good. Generally, there are two reasons. First, the paper quality is poor or the surface is rough, and the oil absorption is large; the coating is uneven or the glazing oil is too thin and the viscosity is too low.
For the first problem, there is no better way to replace the paper; the second problem can be considered to adjust the glazing machine and improve the viscosity of the varnish.
5. Light film is too soft
Fingerprints are often found on prints after glazing, especially when the black ink is full. Why would such phenomenon happen? This is because auxins are often added to improve the varnish properties, such as defoamers. Leveling agent, etc., these components are not involved in the curing cross-linking reaction. They may eventually be absorbed by the substrate and may migrate to the surface of the coating after curing. If a component of the varnish is not stabilized in the coating, it will migrate directly to the surface of the coating and form an uncured layer that will leave a distinct fingerprint when the finger accidentally touches it.
Workaround: When encountering such a problem. The key is to ensure that the coating is completely dry and reduces the migration of residues to the surface of the coating.
6. Glazing effect is unstable
The glazing effect is unstable, which is a big headache for many flexo companies. In the flexographic printing process, in order to keep the viscosity of the water-based ink stable, it is necessary to add a certain amount of amines to the ink. Although the viscosity of the ink can be effectively adjusted and maintained, some amines will be used for UV varnish. The curing effect has a negative impact. Sometimes, glazing instability is caused by the amine added to the ink.
Solution: Try a new method of controlling the viscosity of the ink. If the varnish does not match the ink used, the varnish must be replaced. Or change the formulation of varnish.
7. Matte oil concentration is unstable
UV matt oil has a problem of concentration instability in printing and coating, especially when printing long versions. Why is this problem occurring? In order to reduce the gloss of the UV varnish or the aqueous varnish and thereby achieve the matting effect, it is generally necessary to add a large amount of silica thereto. This will inevitably lead to an increase in the viscosity of the varnish, and the increase in viscosity will directly affect the performance of the varnish. If a debonding agent is added, it will affect the curing speed and bring some uncertain factors, causing streaks and stains on the coating. Many printing companies avoid using UV matt oil as much as possible because they cannot find a suitable solution.
Solution: Contact the varnish supplier to select a matt varnish suitable for long-run printing.
8. Adhesion (not dry)
There are three possibilities, one is that the coating is too thick; the other is that the UV lamp has insufficient power 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 glazing oil has expired.
Solution: If the coating is too thick, reduce the viscosity; if the UV lamp is insufficient or aging, replace the UV lamp to reach the rated power; if the varnish has expired, stop using it.
9. Adhesive suitability and printability
We often hear that some manufacturers praise the glazing effect of their prints and look like mirrors. A good varnish coating forms a 100% solid film on the surface of the ink layer. Strictly speaking, the coating is more like a dense and smooth plastic film. However, from another point of view, this is very disadvantageous for post-processing such as gluing, because the adhesive and ink cannot adhere at all on the surface of the coating that is too smooth.
Solution: If the printed product is subjected to a process such as gluing or printing after glazing, the surface of the coating 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 applicability and printability. However, it is worth noting that even if the varnish coating has good applicability and printability, it is not necessarily all adhesives. Both ink and ink are suitable. Therefore, it is best to do some tests before use. If the test results are not satisfactory, it is best to contact the varnish supplier to find a solution.
10. Poor scratch resistance
The scratch resistance of the UV varnish coating is not good, 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, and the glazing coating is also poor in scratch resistance. There are three main reasons why the ink layer is not completely dried: one is that the residual substance in the ink diffuses to the surface of the ink layer, the second is that the ink is a slow-drying ink; and the third is that the drying device of the printing machine is not suitable.
Solution: Check and adjust the drying device of the ink. Ensure that the ink is completely dry; keep the air flowing as much as possible and speed up the drying of the ink. If it doesn't work, you can only change the varnish.
11. Bad water resistance
The general-purpose UV varnish is generally poor in water resistance because some of the important components contained in the UV varnish are hydrophilic, which affects the water resistance of the coating.
Solution There is a water-resistant UV varnish on the market, but this varnish has a slower curing speed. Therefore, the printing speed should be appropriately reduced when using this water-resistant varnish.
12. Varnish does not cure well on some ink surfaces
When printing varnish on black ink, white ink and luminescent blue ink layer, various unexpected problems often occur, which is one of the most headaches for printing companies.
We can understand and see this problem in this way: the pigment in the ink and the photoinitiator in the varnish absorb the same band of ultraviolet light, which together compete for limited UV energy. The varnish curing requires energy, so the effect on the varnish is greater, which may cause the varnish to not cure completely.
Solution: increase the power of the UV lamp, clean the reflector, replace the UV lamp, reduce the printing speed of the varnish, but this can only be used as a stopgap measure. In the long run, it should be developed and developed in any color. Fully cured varnish on the ink layer.