Causes and countermeasures of insufficient surface gloss and brightness of UV coating

Flexographic UV coating has won the favor of printing companies for its fast curing, energy-saving and pollution-free environmental protection concept. Printed products have good water resistance, wear resistance, and degradability, and can be comparable to the film coating effect and are deeply loved by consumers. However, due to various reasons during the actual printing process, the phenomenon of poor surface gloss and insufficient brightness after the UV coating on the surface of the printed product occurs. The main causes are:

1. UV Varnish viscosity is too low and the coating is too thin

UV varnish usually consists of a photoactive oligomer resin, a reactive diluent, a photoinitiator, a polymerization inhibitor, and other additives. Among them, the photosensitive oligomer resin is the main film-forming substance of the UV varnish, and its properties directly affect the viscosity of the varnish, the curing speed, and the degree of smoothness after curing. When the viscosity of the UV varnish is too low, the content of the photoactive oligomer inherent in the varnish decreases, and the thickness of the film after light curing becomes thin, and the phenomenon that the gloss of the surface of the printed article is not good. In addition, too low a viscosity will reduce the adhesion of the varnish to the surface of the anilox roller, resulting in too thin a varnish coating on the surface of the printing product and insufficient surface brightness after curing.

Solution: Select a varnish with a relatively high viscosity of UV varnish. Add a suitable amount of UV thickener to UV varnish to increase its viscosity.

2. Excessive dilution of non-reactive solvents such as ethanol

For various reasons, we have to dilute the UV Varnish used for various reasons. The most economical and convenient method is to directly add appropriate amounts of non-reactive solvents such as industrial ethanol in UV Varnish to change the instantaneous viscosity of UV Varnish. After curing, the surface of the printed product is smooth, and the inherent components of the UV varnish are not changed at the same time. However, if ethanol is added in excess, the water contained in ethanol cannot be completely volatilized, and water mist is easily generated in the photocuring and heating, and remains in the glazing film, thereby affecting the transparency of the photocured oil after film formation, and even leading to the varnish. The curing is not complete and the printed surface is tacky.

Solution: Ethanol with slightly higher purity may be used, and the amount of addition should be controlled as much as possible during use.

3. UV Varnish uneven coating

The surface gloss of the printed product is not good, the brightness is not enough, and it is related to uneven coating of UV varnish. The reasons for this are the unsatisfactory quality of the stencil printing coating, the uneven printing pressure, the blockage of the anilox roller net, the rejection or interference of the printing ink and the UV varnish, and the excessive viscosity of the ink surface.

Solution: Select excellent flexographic plate for plate base to ensure that the plate surface is flat; printing pressure is even and moderate; thoroughly clean the anilox roller to ensure that there is no foreign matter in the cell site; use the ink and varnish produced by the same manufacturer; try to avoid the ink Add viscous additives.

4. The paper surface is rough, absorptive too strong or not strong

The rough surface of the paper directly affects the leveling performance of the UV varnish on the surface of the paper. Absorption is too strong and the varnish is almost completely absorbed by the rough fibers, resulting in thinning of the cured oil film and loss of gloss and brightness. In addition, the rough surface of the paper also affects the reflection interference of the ultraviolet light wave, which results in attenuation of the light wave and affects the curing effect. On the contrary, the absorption of the paper is too weak, so that the penetration, solidification, and conjunctival action of the varnish in the leveling can be significantly reduced, and a smooth film cannot be formed on the surface of the print.

Solution: Select paper with good printability. You can also preprint and dilute the washed white ink on the surface of the paper to change the surface roughness of the paper and slow the absorption speed. Or use water-based primer to improve the surface of the paper, and at the same time improve the flexural properties of the print.

5. The anilox roller is too thin and the oil supply is too small

Flexographic ink transfer is based on the surface of the anilox roller surface to complete the ink storage and transmission, the higher the number of network lines, the finer the anilox roller, the smaller the amount of ink storage point, so the oil passed to the printing plate The amount is also small, the oil film on the surface of the photocured printing product is relatively thin, the gloss of the surface of the printing product is not good, and the brightness is not enough.

Solution: According to the requirements of the product, select anilox rollers with a relatively small number of screen lines to ensure the thickness of the oil film. In general, 450 line anilox rollers should be selected.

UV coating usually uses light from 200 to 400 nm in the ultraviolet light as the light source, and requires that the radiation intensity should be greater than 80 W/cm, and the life time should be 800 to 1200 hours (in the normal state). After the failure of the ultraviolet light wave in the light source, the photoinitiator in the varnish cannot be fully activated, the crosslinking reaction between the photoactive oligomer resin and the reactive diluent is slowed down, resulting in incomplete film formation of the varnish, and the surface of the printed product is sticky. The brightness is not enough.

In addition, the phenomenon of poor light curing may also occur on the surface of black ink, white ink, and shot blue ink. The reason is that the pigments in the inks and the photoinitiators in the varnish absorb the same wavelengths of ultraviolet light, and both of them compete for limited ultraviolet light energy, so that the energy required for the varnish curing is insufficient, resulting in incomplete curing of the varnish. .

Solution: The operator should always check whether the oil film on the surface of the printed product is sticky. Summarize the replacement period of the pressure tribute lamp in the long version to ensure the timely replacement of the light source. When UV ink is applied on the surface of black ink, white ink and blue light, it is recommended to use a relatively high-power light source. At the same time, the reflector should be cleaned frequently to ensure that the reflective surface is smooth and the unnecessary loss of ultraviolet light waves is minimized. (Text/Su Pingan)