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At present, UV coating plays an important role in flexo glazing, and it is generally used online. The UV curing device consists of a UV lamp and a reflector. Water-based inks are printed with UV Varnish after printing, which can protect water-based inks. Of course, there are also special requirements for the use of water-based varnishes. For example, when the final product needs to be printed with a special code, special water-based varnishes are used for printing. In addition, some manufacturers choose water-based varnishes for matte glazing. Because the UV matte oil tends to thicken when it is printed.
In the process of UV coating, there are often some inexplicable failures. This article lists nine common problems in flexographic UV glazing and their solutions. We would like to share this with you.
1. The coating is yellow
The main reason for the yellowing of UV varnish after curing is generally twofold. First, UV varnish has been left for a long time and has turned yellow. In particular, the yellowing phenomenon of inferior varnish is even more serious; the second is excessive UV irradiation. UV Varnish cures excessively.
Solution: If it is the quality of UV varnish itself, you must replace the UV varnish; if it is caused by excessive yellowing, it is necessary to reduce the power of the UV light source or improve the speed of light.
2. Poor adhesion of the coating on the non-absorbent substrate surface
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 substrate surface may be poor.
It is generally believed that the harder the coating is, the better the effect is, but for non-absorbent substrates, the toughness of the coating is the most important, otherwise the problem of poor adhesion will occur. In order to obtain a "soft" coating, some special raw materials need to be used in the varnish formulation, and photocurable resins and reactive monomers with good toughness and good adhesion are selected.
Solution: The test coating adhesion is generally used cross-cut test method, first to prepare the transparent tape and the tool to draw a grid, cross the grid lines on the coating, taking care not to damage the substrate, and then sticking with tape, The area of ​​the coating removed by the tape represents the degree of cure and adhesion of the coating. If the coating is not torn off at all, the adhesion of the coating is quite good. If the coating is torn off over a large area, this shows that the varnish is not suitable for use on a non-porous substrate, and other types of varnish should be considered.
3. Fingerprint problem
Fingerprints are often found on glazing, especially when full-size glazing is applied to black ink. Why would such phenomenon happen? 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. These components do not participate in the curing cross-linking reaction. They may eventually be absorbed by the substrate. Will migrate to the coating surface after curing. If a component of the varnish is not stabilized within the coating, it will migrate directly to the coating surface and form an uncured thin layer that will leave a distinct fingerprint when the finger accidentally touches it.
Solution: The key to this type of problem is to ensure that the coating is completely dry and to reduce the migration of residue to the coating surface.
4. The glazing effect is not stable
The glazing effect is unstable, which is a big headache for many flexo 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 the viscosity of the ink can be effectively adjusted and maintained, some amines may affect the UV varnishes. The curing effect has a negative effect. Sometimes, the coating is unstable because of the amine added to the ink.
Solution: Try new methods to control the viscosity of the ink. If the varnish does not match the ink used, you must change the varnish or change the varnish formulation.
5. The instability of the matt oil concentration
There is a problem of unstable concentration of UV matte oil in printing and coating, and this problem is particularly prominent in the long-lived printing. Why does this happen? In order to reduce the gloss of UV-varnish or water-based gloss oils and thereby achieve a matt effect, it is generally necessary to add a large amount of silica to them, which will inevitably cause the viscosity of the varnish to rise, and the increase in viscosity will directly affect Varnish use performance. If you add an ungreaser, it will affect the cure speed and cause some uncertainties, causing the coating to produce streaks and stains. Many printing companies avoided the use of UV matte as much as they could not find a suitable solution to the problem.
Solution: Contact the varnish supplier to select a matt varnish suitable for long live printing.
6. Applicability and printability
We often hear that some manufacturers boast that their products have a good glazing effect and look like a mirror. The good varnish coating forms a 100% solid film on the surface of the ink layer. Strictly speaking, the coating performance is more like a dense and smooth plastic film on the surface. But from another perspective, this is very disadvantageous for subsequent processing such as gluing, because on the surface of a coating that is too smooth, the adhesive and ink cannot be attached at all.
Solution: If the printed product is pasted with glue or printed, then the surface of the coating must not be too smooth to ensure that the adhesive or ink can adhere to it. In other words, the varnish coating must have good applicability and printability, but it is worth noting that even if the varnish coating has good applicability and printability, it may not necessarily The adhesives and inks are adaptable. 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.
7. Scratch resistance is poor
The scratch resistance of the UV varnish coating is not good, and the root cause of the problem is the attachment fastness. On the one hand, the coating is too brittle and can easily peel off, which may be caused by excessive curing; on the other hand, the ink layer is not completely dried and can also result in poor scratch resistance of the coating coating. There are three main reasons why the ink layer is not completely dried: First, the residual material in the ink diffuses to the surface of the ink layer; second, the ink is a slow-drying ink; and third, the drying device of the printing press is not suitable.
Solution: Check and adjust the ink drying device to ensure that the ink can be completely dry; keep the air circulation as much as possible to speed up the drying of the ink. If it does not work, only the varnishes are replaced.
8. poor water resistance
Universal UV Varnish is generally poor in water resistance because UV Varnish contains some important components that are hydrophilic, affecting the water resistance of the coating.
Solution: Waterborne Varnish is available on the market, but this Varnish cures at a slower rate. Therefore, when using this water-resistant Varnish, the printing speed should be appropriately reduced.
9. Varnish is poorly cured on black ink, white ink, and light blue ink layers
In the black ink, white ink and light blue ink layer printing Varnish often appear a variety of unexpected problems, which is the biggest headache for the majority of printing companies.
We can understand and look at the problem in this way: the pigment in the ink and the photoinitiator in the varnish absorb the same wavelength band of ultraviolet light, both of which compete for the limited ultraviolet light energy, while the varnish curing requires energy, so this The effect on varnish must be greater, which may result in incomplete cure of the varnish.
Solution: Increase the power of the UV lamp, clean the reflector, replace the UV lamp, and reduce the printing speed of the varnish. However, this can only be used as an expedient measure. In the long run, it should be developed and developed in any color. Completely cured all-around varnish on the ink layer.
Post-printing oil selection
For flexo printing, the post-printing coating is an unusual thing, because the coating can improve the water resistance and wear resistance of printed products to a certain extent. At the same time, it also plays a role in beautifying the products and improving the product quality. At the same price level, many post-press processing methods cannot match the glazing effect. Therefore, in the past 10 years, the development of optical oil technology has been very rapid, chemical materials have become cheaper and cheaper, the application range has become wider and wider, and the performance has become more and more stable.