1. Closed-type scrapers provide uniform ink film thickness Manroland has worked with its customers and supply partners to develop a closed-type scraper method in 1993. This method ensures a very uniform coating thickness. Anilox rollers with different carrying capacities ensure an accurate amount of glazing and can be carried out repeatedly. The amount of paint carried by the rollers varies from job to job. The normal rule is between 6 and 30 cm3 per square meter. In fact, only 25% to 35% of the wet paint of the above nominal load is transferred to the surface of the substrate. Different screen conditions on the surface of the anilox roller allow the machine to transfer different required amounts of coating. A good circulatory system is an important factor. The quality of the coating's stability is related to the paint's feed, storage, and pumping principles, plus temperature stability. These factors must be matched to the glazing unit used.
2. Measuring Luster with Reflected Light You can choose a reflective angle to measure gloss. The "mirror surface" reflects 100% of light. Since the coating surface reflects only a portion of the light, the degree of gloss must be chosen to provide a reliable measure of the gloss change. The angle of measurement is very important, because the pigment penetrates deep and has a phenomenon of dispersion. The human eye responds more strongly to gloss on a black background.
3. Scratch Resistance and Adhesion The scratch resistance and adhesion of the test coating are important factors in the post-printing process and an important factor in the durability of the printed product. Users sometimes use unscientific methods to determine these properties, such as scratching with nails or sticking with tape. These are subjective methods of judgment, relying primarily on the judgment of each client, and have no measurable data, but they can be used as a reference. Without objective test conditions, the adhesion of the coating, if not stabilized after a few days after production, can only be maintained for several hours. During this time, the ink is oxidized and dried, resulting in the generation of gas that builds 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 degree of curing of UV coatings. The method is to use a cloth impregnated with acetone to wipe over the coating. If the curing is poor, the coating will be dissolved. The German Printing Association (FOGRA) recommends several test devices for checking the resistance to peeling and blocking. The resistance of printed matter can be tested according to the methods provided by "German Industrial Standard" DIN 16524 and DIN 16525.
4. Substrates are the basic elements 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 test, eight different quality substrates were used. Including recycled materials (GT and GD) and new fiber materials (GC). Not only do these materials have different grades, but the quality of machine coating is also different. Usually two or three machine coats are used. The tools used include a doctor roll, air brush, air knife, and mucous membrane machine. (Note: In order to avoid confusion, we must distinguish between different coatings. Coatings (powder) on paper or paperboard in paper mills, and coatings that enhance the effects of prints are glazing coatings, in order to perform a series of useful under the same conditions. In the test, the thickness of each material used is the same, and on different grades of paper, the coating frequency of the powder has a significant impact on the quality.The effect of the recoating material plus two on-machine coating is much less than the effect of the three coating on the machine. The cardboard made of new fiber is coated only twice and then glued, and its surface is smooth, with a gloss like three times of coating.The surface hardness of the machine coated also affects the gloss - the hard coating tool can produce a smoother The surface is shiny, and it is not useful to predict the gloss that can be achieved based on water absorption and oil absorption.In the laboratory, after the use of the coating treatment, the gloss properties of different grades of paperboard are lighter than those of ultraviolet (UV). More practical According to the amount of ink and coating used, the properties of the substrate itself have a very important influence on the gloss level. Absorption determines the effect of the coating.Higher grade paperboards have a consistent surface smoothness, although slight variations are unavoidable, especially for paperboard made from natural materials, but it also affects gloss. Up to 30%. It should be noted that even if increasing the amount of coating can reduce the difference in gloss, this test is not related to the grade of the coating (due to the use of different manufacturers' coatings). On the other hand, three times each time the coated board is used The same amount of paint can achieve a better smooth surface.The difference between the three on-machine coating materials can be said to be due to the on-machine coating powder formulation, the coating tool, and the processing sequence.
5. Choosing the Right Ink It is very important that the ink has two undesirable effects on the glazing coating: Due to the repellence 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 is used for on-line ultraviolet (UV) glazing on common inks, which are usually primed 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) Glazing Coatings Nowadays, chemical-based UV curing coatings are 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 gloss imparting material is 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 speed of production High-speed production is of course what we want, but remember 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.
Microfiber Fabric Mini Matt Co., Ltd. , http://www.printedseries.com