Flexible uv coating common problems and precautions

The uv UV curing technology has been widely used since the invention in the 1960s until the end of the 20th century. In our country uv printing and uv glazing has been widely used in aluminum foil and aluminum foil jam printing, magnetic card printing, commercial printing, metal printing, food, medicine, cigarette packets, stickers and other packaging printing. The uv paper product coating is increasing at a rate of more than 10% per year, so the application prospect of uv coating oil is very broad.

For flexo printing, post-printing is an unusual thing, because it can improve the water resistance and abrasion resistance of printed products to a certain extent, and at the same time, it can also beautify products and improve the quality of products. 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 quite rapid, and chemical materials have become cheaper and cheaper. The application area is more and more extensive, the performance is more and more stable.

At present, uv coating plays a very important role in flexo glazing. And generally use online. The uv curing device consists of one or two UV lamps and reflectors. After the printing of water-based ink, the uv gloss oil is printed, which can protect the water-based ink. 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.

The final unit of the flexographic printing press and the special coating machine are coated with a processing speed of 100-200 m/min.

light source. UV light sources generally use high-pressure mercury lamps or metal halide lamps (ideal lamps are preferred), and the power of the light source is also required to be stable. High-pressure mercury lamp output power is generally 80 ~ 120w/cm, in order to ensure that the UV coating curing speed <0.5s;

Paper uv coatings are not suitable for the processing of easily permeable paper. Because the low-molecular materials in the uv coating can easily penetrate into the paper, causing the paper to become dark or even saturated. In this case, only speed up printing and increase the number of tubes. In order to prevent the infiltration phenomenon, the underlayer can be coated with an infrared primer to avoid infiltration when the uv coating is applied.

Ink. Need to pay attention to the combination of uv coating and ink, if necessary, first coat a primer;

coating. After the glazing coating is irradiated with ultraviolet light, the photoinitiator is initiated to generate free radicals or ions, and these radicals or ions react with the double bonds in the prepolymer or unsaturated monomer to form monomeric groups. These monomer radicals form a polymer solid polymer through a chain reaction and complete the drying process of the gloss coating. The overcoating layer is too thick to dry for a long time under the irradiation of the same power source. On the one hand, it affects the drying of the uv light oil, and on the other hand, the water content of the paper is relatively reduced, which makes the paper brittle and breaks easily and affects the uv. The wear resistance of the oil layer;

Thinner. UV glazing oil generally requires the use of special thinner, dilution control in the 1:0.5 to 1:1 or so, too much will affect the glazing effect (specific depending on the amount of film);

In the uv glazing process, there are often some problems, the following author made some simple discussions.

1. Yellow coating

The main reason for the yellowing of uv varnish after curing is generally threefold. First, uv varnish is 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 ultraviolet radiation. Uv light oil curing excessive; Third, the coating is too thick.

Solution: If it is the quality of uv varnish itself, you must replace uv varnish: if the yellowing caused by overexposure is to reduce the power of the ultraviolet light source or increase the speed of the varnish; if it is too thick varnish, to be appropriate Consider reducing the thickness of the varnish (the varnish concentration may be reduced appropriately).

2. The glazing layer is white and the cross-linking density is low

It is very likely that the thinner is used improperly

Solution: Due to the high volatilization rate of alcohol solvents in summer, thinners should be changed or ester diluents or special thinners should be used instead.

3. The 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 first, otherwise there will be poor adhesion problems. In order to get a soft coating, the light oil formula needs Use some special raw materials to choose photohardenable resins and reactive monomers with good toughness and good adhesion.

Inappropriate use of print ink layer crystallization or ink additives and anti-slip agents should improve the printing process.

Solution: The test coating adhesion is generally used cross-cut test method, the first preparation of transparent tape and mesh tools, the cross-line grid lines on the coating, taking care not to damage the substrate and then tape, was 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 It shows that the powder coating 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 non-porous substrates. Consideration should be given to replacing other types of varnish. Do not use or less use ink additives; coating oil viscosity is too low, you should adjust the polishing machine and increase the viscosity of the coating oil.

4. Light brightness difference

The brightness is not good generally the following two most likely reasons, First, the poor quality of paper or rough surface, large oil absorption; uneven coating or diluted coating is too thin, the viscosity is too low.

If question 1 is the case, there is no better way to replace the paper; question 2 can be considered adjusting the glosser and increasing the viscosity of the coating.

5. Light film is too soft

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 curing cross-linking reaction. They may eventually be absorbed by the substrate and may also 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.

Workaround: When you encounter this type of problem. The key is to ensure that the coating is completely dry, reducing the migration of residue to the coating surface.

6. 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 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 may affect the uv light oil. 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 and the ink used do not match, you must replace the varnish. Or change the formula of varnish.

7. Matte oil instability

The uv matte oil has a problem of unstable concentration 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 the uv gloss oil or the waterborne gloss oil and thereby achieve a matting effect, it is generally preferable to add a large amount of silica thereto. This will inevitably cause the viscosity of the varnish to rise, and the increase in viscosity will directly affect the performance of varnish. The addition of a release agent can affect the cure speed and cause some uncertainties, resulting in streaks and stains on the coating. Many printing companies avoided the use of uv matte oil as much as possible because 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.

8. Adhesion (not dried)

There are three possibilities. First, the coating is too thick. Second, the uv lamp has insufficient power or aging (usually the life of the uv lamp is approximately 800 hours. The curing efficiency will be reduced beyond this time). Third, the coating oil has expired.

Solution: If the coating is too thick, reduce the viscosity; if the uv lamp power is insufficient or aging, you should replace the uv lamp to achieve the rated power; if the coating oil has expired, should stop using.

9. 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 unfavorable 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 after the coating, the coating surface should 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 be suitable for all adhesives. Both ink and ink are adaptable. Therefore, it is best to do a test before use. If the test result is not satisfactory, it is best to contact with the varnish supplier to find a solution together.

10. Poor scratch resistance

The scratch resistance of the uv light oil 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 easily peel off. This may be caused by excessive curing. On the other hand, the ink layer is not completely dried, and the scratch resistance of the coating coating is also poor. The reason why the ink layer is not completely dried mainly includes three aspects: one is that the residual substances in the ink diffuse into the surface of the ink layer; secondly, the ink is a slow-drying type ink; and third, the drying device of the printing press is not suitable.

Solution: Check and adjust the ink drying device. Make sure the ink can be completely dry; keep the air circulation as fast as possible to speed up the drying of the ink. If it does not work, only the varnishes are replaced.

11. Poor water resistance

Universal uv gloss oils generally have poor water resistance because some of the important components contained in the uv gloss oil are hydrophilic, affecting the water resistance of the coating.

Solution The market has a water-resistant UV varnish, but this varnish cures slowly. Therefore, when using this varnish, the printing speed should be appropriately reduced.

12. Varnish is poorly cured on some ink surfaces

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 this problem in this way: The pigment in the ink and the photoinitiator in the varnish absorb the ultraviolet light in the same wavelength band, and they both compete for the limited ultraviolet light energy. Varnish curing requires energy, so this has a greater effect on varnish, which may result in incomplete curing of varnish.

Solution: Increase the uv lamp power, clean the reflector, replace the uv tube, and reduce the 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.

The above lists some of the problems that the writer can often encounter in his work. He communicated with everyone here and hoped to help the actual operation of the glazing.