1. Closed blade provides uniform ink film thickness Manroland, in cooperation with its customers and suppliers, developed a closed blade method in 1993. This method ensures that the thickness of the glazing coating is extremely uniform. Anilox rolls with different load capacities can ensure accurate glazing and can be repeated. The amount of paint carried by the rollers differs depending on the work. The normal specification is between 6 and 30 cm3 per square meter. In fact, only 25% to 35% of the above-mentioned nominal load is transferred to the surface of the substrate. The different screen lines on the surface of the anilox roller enable the machine to transfer different amounts of paint. Good circulation system is an important factor. The stable quality of the coating is related to the feed volume, storage capacity, and pumping principle of the coating, plus the stability of the temperature. These factors must be matched with the glazing unit used.
2. Use the reflected light to measure gloss. A certain reflection angle can be used to measure gloss. The "mirror surface" reflects 100% of the light. Since only a part of the light is reflected on the surface of the coating, considering the degree of gloss, the angle of measurement must be selected to provide reliable data on the change in gloss. The angle of measurement is very important because the pigment penetrates into the depths and is dispersed. The human eye reacts more strongly to gloss on a black background. 3. Scratch resistance and adhesion test The scratch resistance and adhesion of the coating is an important factor in the post-press processing process, and it is also an important factor for the durability of the printed matter. Users sometimes judge these characteristics by unscientific methods, such as scratching with nails or sticking with tape. These are subjective judgment methods, which mainly rely on the judgment of each customer. There is no measurable data, but it can be used as a reference. Without objective test conditions, after production, if the adhesion of the coating is not stable after many days, it can only be maintained for a few hours. During this period, the ink oxidizes to dry, causing gas to build up between the ink layer and the paint layer, thereby breaking the bond. Therefore, it is reasonable to check the degree of adhesion only a few days after printing. Acetone is very suitable for testing the curing degree of ultraviolet (UV) coatings. The method is to wipe the coating with a cloth saturated with acetone. If the curing is not good, the coating will be dissolved. The German Printing Association (FOGRA) recommends some testing devices to check the ability to resist peeling and blocking. The resistance of the printed matter can be tested according to the methods provided by the "German Industrial Standards" DIN16524 and DIN16525.
4. The substrate is the basic element. The important factors that affect the glazing effect are the basic materials, the method of machine coating, and the quality and uniformity of these factors. In our tests, eight substrates of different qualities were used. Including recycling of recycled materials (GT and GD) and new fiber materials (GC). These materials are not only different in grade, but also the quality of machine coating. Usually two or three machine coatings are used. The coating tools include doctor blade roller, air brush, air knife and mucosa machine. (Note: In order to avoid confusion, we must distinguish between different coatings. The coatings (powders) added to paper or cardboard in the paper mill and the coatings that enhance the effect of the printed matter, that is, glazing coatings, are useful for a series of applications under the same conditions. Test, the thickness of each material used is the same. On different grades of paper, the number of powder coatings has a significant impact on quality. The effect of recycled materials plus two coatings on the machine is much worse than the three coatings on the machine The cardboard made of new fiber is only coated twice and then glued. Its surface is smooth and has a gloss like coating three times. The hardness of the surface coated on the machine also affects the gloss-a hard coating tool can produce a smoother The surface has increased gloss. It is not useful to predict the gloss that may be achieved based on water absorption and oil absorption. In the laboratory, after using glazing coating treatment, the gloss characteristics of different grades of cardboard are better than online ultraviolet (UV) glazing. More practical. Depending on the amount of ink and paint used, the characteristics of the substrate itself have a very important effect on the degree of gloss. The smoothness of the powder coating surface and Yield determines the effect of the coating. High-grade paperboard has a uniform smooth surface, although slight changes are inevitable, especially paperboard made from natural materials, but also affects gloss. The effect of paperboard grade on the results Up to 30%. It should be noted that even if the amount of paint is increased, the difference in gloss can be reduced, but this test has nothing to do with the grade of the paint (because the paint of different manufacturers is used).
On the other hand, the same amount of paint is used to coat the cardboard on the three times to obtain a better smooth surface. The difference between the three coating materials on the machine can be said to be caused by the formulation of the coating powder on the machine, the coating tool and the processing order.
5. Choosing the right ink is very important. The ink has two adverse effects on the glaze coating: due to the ink repulsion, the moisture of the dispersing paint used as a primer on the printing area of ​​the printed sheet cannot be fully absorbed by the substrate. Therefore, the degree of mixing of ink and paint depends on the viscosity of the two. The ink that has just been printed starts its drying process. Even if the second layer of ultraviolet (UV) paint has hardened, its drying continues below. This will cause the surface of the coating to be deformed and may be affected by the product during oxidative drying. These reactions reduce the gloss and break the bond. Due to the interrelationship between the two, the gloss of ultraviolet (UV) varnish depends mainly on the type of ink used. In order to ensure a certain gloss, fast-absorbing inks should be used. However, ink spots may occur, limiting the amount of absorption. When the first printing unit uses ultra-fast absorption ink, in the subsequent printing unit, the ink splits and transfers to the blanket, creating ink spots. Therefore, different situations should be treated separately. When printing, you should use as little fountain solution as possible to reduce the accumulation of ink on the blanket, thereby reducing the risk of ink spots. The selected ink also has a certain relationship with the back staining of the pile.
6. Priming materials are subjected to on-line ultraviolet (UV) glazing on ordinary inks, usually with dispersing paints. Ordinary offset printing inks and ultraviolet (UV) coatings are incompatible chemicals, and this "intermediate coating" needs to be applied to the printed sheet in a wet stacked wet form in a short time. The water-based dispersion synthesis solution contains 40% to 45% of solids, and the viscosity that changes with temperature can be adjusted by adding water. Once the dispersive paint is applied to the printed sheet, a considerable amount of moisture must be drawn into the substrate, and some will evaporate and disappear. It is only at this time that the polymer starts its bonding and forms 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, penetration ability, overprinting force, viscosity, gloss and adhesion 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, it is clear that the important functions of primer materials. Especially on the surface with high ink coverage, the effect of primer material is more significant. Replace the 13m3 / m3 anilox roller with an 18cm3 / m3 anilox roller. The ultraviolet (UV) coating is smoother and has a higher gloss. This situation is particularly evident on surfaces with high coverage. We can easily identify the influence of different types of primer materials. The severe deformation on the surface of 200% of the ground can only be seen when using a soft primer material. The hard primer material appears to have a smooth surface before ultraviolet (UV) glazing. However, scratch resistance is reduced. The smaller the amount used, the greater the influence of the type of primer. These tests clearly show that drying between coating units is extremely important. If the drying time and degree of drying are insufficient before ultraviolet (UV) glazing, even if the best primer material and the best dosage are used, the best gloss cannot be achieved. Therefore, two glazing units are used for glazing, and it is recommended to use one extended drying part.
7. Ultraviolet (UV) glazing coatings Currently, chemical-based hardening ultraviolet (UV) glazing coatings are often used in offset printing. These ultraviolet (UV) coatings contain binders, reactive diluents and photoinitiators. Under ultraviolet (UV) light irradiation, a networked reaction takes place, curing the coating. Ultraviolet (UV) light causes the photoinitiator to release a chemical group, which causes the acrylate to polymerize and form a long-chain network connection. Oxygen hinders this reaction. For cationic hardening coatings, ultraviolet (UV) light initiates ion formation, and the chain reaction forms hardening. Due to the continuity of curing, although the curing speed is slow, it can be completely cured. Cationic hardening ultraviolet (UV) glazing coating methods are mostly used in food packaging because they have no odor and harden thoroughly. Our testing focuses on chemically hardened ultraviolet (UV) glazing coatings. Increasing the amount of UV coating can improve the gloss, but the effect is not as good as increasing the amount of primer.
During the test, increase the amount of UV coating from 20cm3 / m3 to 30ccm3 / m3, no matter how large the ink coverage is, the gloss will increase by at least 10 points
When deciding the amount of UV coating, do not forget that due to viscosity, if the amount is too large, it is difficult to apply smoothly. Heating the UV coating to 40 ° C can enhance fluidity and also increase gloss. In addition, the geometry of the anilox roller also affects the flow of paint. In addition to the carrying capacity, the number of network cables also plays a role. Only a foam-free coating can produce a good gloss effect. When the anilox roller cells are filled with paint, fine bubbles form micro-foam in the paint. This situation can be improved by adding a defoamer and a reasonably combined paint circulation system to ensure that the paint is not stirred too much.
8. Drying is the key. Within a few seconds, the material that gives color and gloss is applied to the substrate. Although suppliers try their best to make these layers quickly change from liquid to solid, high-speed printing has limitations. Therefore, the glazing process must be supported by the selected drying method. â‘ Drying is determined by the production speed. Of course, high-speed production is what we want, but it must be remembered that the drying method described only benefits from the drying device. As the production speed increases, it is difficult to completely dry and harden. After printing, this situation directly causes the upper and lower sides of the stack of printed sheets to react. If it causes a visible situation, it is called back rubbing. The quality of print quality of UV coating can be stable after several days. Depending on the ink and coating used, the gloss and bond quality can still change after 24 hours. However, change is not always a disadvantage. To maintain the stability of the surface of the high-speed hardening coating, after drying, it should not be allowed to change underneath. The printed sheets are piled up one by one in the receiving pile and are subjected to considerable pressure. Therefore, the surface of the printed sheets needs to be sufficiently hardened to ensure that they are not damaged by contact with the back of the previous sheet. This hardened surface is caused by the UV drying device generating appropriate beam energy. Drying raises the temperature of the stack, causing dirt on the back and even paper sticking. Therefore, the most important thing is to apply the proper drying energy at the right time. Therefore, after the printing and glazing unit, infrared (IR) lamps are indeed required, plus cold and hot air and gas extraction assistance. It is recommended to use two extended drying parts in the form of transfer between the two glazing units. If it is felt that only the drying time determines the gloss effect, then, between the two glazing units, a transfer part is added 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 it has a non-linear relationship. The output energy of IR is in linear mode. As the speed of the printing press increases, the gloss decreases due to changes in ink coverage.
As mentioned above, the interval between the application of each coating determines the stable "base layer" formed. The paper moves between printing units at 10,000 sheets per hour, and the ink penetrates into the paper in less than a second. These tests show that at normal production speeds, comparing the gloss of the printing ink on the first and last printing units, the result is that the gloss of the printing ink on the last unit is poor. These conditions can be used as a reference, which is helpful for us to edit the layout and determine the printing color sequence. However, when printing with ordinary inks, as the printing speed increases, the gloss decreases. The inherent problem of this ink is a fact that cannot be concealed. It has been said that the selective matching of ink, paint and drying devices can reduce the loss of quality to the slightest extent.
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