UV glazing method on ordinary ink (2)

5. It is very important to choose the right ink

The ink has two adverse effects on the glazing coating:

Due to the repulsion of the ink, the moisture of the dispersion coating on the printed area of ​​the printed sheet cannot be fully absorbed by the substrate. Therefore, the degree of mixing of the ink and the paint depends on the viscosity of both.

The ink that has just been printed starts its drying process, and even if the second coating, an ultraviolet (UV) coating, has hardened, its drying continues to proceed below. This can cause the surface of the coating to distort and may be affected by the product formed during oxidative drying. These reactions reduce the gloss and destroy the bond.

Due to the interrelationships between the two, the gloss of the UV coating depends largely on the ink type chosen. In order to ensure a certain degree of gloss, fast absorbing inks should be used. However, ink spots may occur, limiting the absorption. When the first printing unit uses an ultra-absorbent ink, the subsequent printing unit, the ink splits and transfers to the blanket, resulting in ink spots. Therefore, different situations must be treated separately. The fountain solution should be used as little as possible during printing, which can reduce the accumulation of ink on the blanket and thus reduce the risk of ink spots. The selected ink also has a certain relationship to the situation that the delivery pile has back blemishes.

6. Priming material

Ultraviolet (UV) coating on common inks, usually with a dispersion coating. Normal offset inks and ultraviolet (UV) coatings are incompatible chemicals, and this "intermediate coating" needs to be applied wet on wet sheets in a very short time. The water-based dispersion synthetic solution contains 40% to 45% solids, and the temperature-dependent viscosity can be adjusted by adding water.

Once the dispersion coating is applied on the sheet, a considerable amount of moisture must be absorbed into the substrate, and some will disappear. Only at this time does the polymer begin its combination and form a dense coating film. Therefore, in the online production process, the primer material must be able to dry as soon as possible. The achievable drying speed, flexibility, infiltration capacity, superimposition force, viscosity, gloss, and degree of adhesion, etc., depend on the base material and the additives used.

From the use of different types of primer materials and the increase in the number of use, the important function of the primer material can be clearly seen. Especially on surfaces with high ink coverage, the impact of the primer material is even more pronounced.

Replace the 13m3/m3 anilox roller with 18cm3/m3 anilox roller, the UV coating is more smooth and the gloss is higher. This is particularly evident on high-coverage surfaces.

We can easily identify the effects of different types of rendering materials. Severe deformation on the 200% solid surface can only be seen when using soft primers. The hard primed material appeared to have a smooth surface before it was UV coated. However, the scratch resistance is reduced. The smaller the amount of use, the greater the effect of the type of primer material. These tests clearly show that drying in the coating units is extremely important. If the drying time and the degree of drying are not sufficient before UV light is applied, the best gloss is not achieved even with the best primer material and optimum amount. Therefore, the use of two coating units for coating, it is recommended to use an extended drying part.

7. Ultraviolet (UV) coating

Nowadays, UV-curing coating is mostly used in offset printing. These UV coatings contain binders, reactive diluents, and photoinitiators. Under the irradiation of ultraviolet (UV) light, a networked reaction occurred to cure the coating. Ultraviolet (UV) light causes the photoinitiator to release chemical moieties that cause the acrylate to polymerize and form long-chain network linkages. Oxygen hinders this reaction.

For cationic hardened coatings, ultraviolet (UV) light initiates ion formation and chain reaction forms hardening. Due to the continuity of the curing, it can be completely cured although the curing speed is slow. Cation hardening Ultraviolet (UV) coating methods are used in food packaging because they are odorless and harden thoroughly. Our tests focus on chemically hardened UV coatings.

Increasing the amount of UV coating can improve gloss, but the effect is not as good as increasing the amount of primer. During the test, the amount of UV coating was increased from 20 cm3/m3 to 30 ccm3/m3, and the gloss increased by at least 10 points regardless of the ink coverage. When determining the amount of UV coating, do not forget that, due to the viscosity, if the amount is too large, it is difficult to smoothly apply. Heating the UV coating to 40°C enhances the flow and also increases the gloss.

In addition, the geometry of the anilox roller also affects the flow of the paint. In addition to the carrying capacity, the number of screen lines also works. Only a non-foamed coating can produce a good gloss effect. When the mesh of the anilox roller is filled with paint, fine bubbles form micro-foam in the paint. This situation can be improved with defoamers, combined with a reasonable combination of paint circulation system, to ensure that the paint will not stir too much.

8. Drying is the key

In a few seconds, the color and glossy materials are applied to the substrate. Although suppliers try their best to make these layers quickly from liquid to solid, high speed printing has limitations. Therefore, the coating process must be supported by the drying method chosen.

1 Drying is determined by the production speed

High-speed production is of course what we want, but it must be remembered that the drying method described only contributes to the drying device. With the increase of production speed, it is difficult to completely dry and harden. After printing, this situation directly results in the reaction of the upper and lower sides of the delivery stack sheet, and if it can be seen, it is called a back-grinding. The quality characteristics of UV varnish prints can be stabilized after days. Depending on the ink and coating used, gloss and bond quality can still change after 24 hours. However, changes are not always a disadvantage.

To keep the surface of the high-speed hardening coating stable, after drying, it must not be allowed to change underneath. Sheets pile up one by one on the take-up pile and are under considerable pressure. Therefore, the surface of the sheet is required to be sufficiently hardened to ensure that it is not damaged by contact with the back of the previous sheet. This hardened surface is due to the proper beam energy generated by the UV drying device.

Drying raises the temperature of the delivery pile, causing smearing on the back and even paper sticking. Therefore, the most important thing is to apply proper dry energy at the right time. Therefore, after printing and glazing units, IR lamps are really needed, plus cold and hot air and pumping gas. It is recommended to use two transfer forms of extended drying unit between the two coating units. If you think that only the drying time determines the gloss effect, then add a transfer part between the two coating units to ensure that the production speed can be increased by 40% without affecting the gloss effect. The output energy of the drying device must match the production speed. The output energy of UV must follow the speed of the printing press, but the nonlinear relationship. The output energy of IR is linear mode.

As the speed of the press increases, the gloss decreases as ink coverage changes. As described above, the time interval for applying each coating determines the formation of a stable "base layer." The paper moved between the printing units at a rate of 10,000 sheets per hour and the ink penetrated into the paper in less than a second. These tests show that, at normal production speeds, the gloss of the printing ink on the first and last printing units is compared, resulting in poor gloss of the final printing ink. These conditions can be used as a reference, which is helpful for us to compose the layout and determine the printing sequence. However, when printing with ordinary inks, the gloss decreases as the printing speed increases. The inherent problem with this ink is an irreversible fact. It has been said that the selective combination of inks, coatings and drying devices can minimize the loss of quality.