UV glazing on plain ink
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Large format presses naturally provide this special reinforcement when the medium format press demonstrates two coaters plus multiple dryers for initial drying, inter-unit drying and tail drying. Large format printer users are generally interested in the method of making ultraviolet (UV) glazing after printing with normal ink in the production of folding cartons.
1 Closed scraper provides uniform ink film thickness
A closed knife method that ensures a very uniform thickness of the glazing coating. Anilox rolls with different load capacities ensure accurate glazing and can be repeated. The amount of paint carried by the rolls 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 capacity is transferred to the surface of the substrate. The different wire mesh conditions on the anilox roll surface allow the machine to transfer different amounts of paint required. A good circulatory system is an important factor. The quality of the coating is related to the amount of coating, the amount of storage, and the pumping principle, plus the stability of the temperature. These factors must match the coating unit used.
2 Measuring gloss with reflected light
A certain angle of reflection can be used to measure the gloss. The "mirror surface" reflects 100% of the light. Since the surface of the coating reflects only a portion of the light, considering the degree of gloss, the angle of measurement must be chosen to provide reliable data on the change in gloss. The angle of measurement is very important because the pigment penetrates deep and is dispersed. The human eye responds strongly to the luster on a black background.
3 Test for scratch resistance and adhesion
The scratch resistance and adhesion of the coating are important factors in the post-press process and an important factor in the durability of the print. Users sometimes judge these characteristics in an unscientific way, such as scratching with a nail or sticking with a tape. These are subjective judgment methods, relying mainly on the judgment of each customer, and there is no measurable data, but they can be used as a reference. There are no objective test conditions. After the production, the adhesion of the coating can only be maintained for several hours if it is not stable after many days. During this time, the ink is oxidatively dried, resulting in the generation of gas, which builds up between the ink layer and the coating layer, thereby breaking the bond. Therefore, it is only reasonable to check the degree of adhesion a few days after the completion of printing. Acetone is well suited for testing the degree of cure of ultraviolet (UV) coatings. The method is to wipe the coating with a cloth soaked in acetone, and if the curing is poor, the coating will be dissolved. The German Printing Association (FOGRA) recommends some test equipment to check the resistance to peeling and anti-bonding into blocks. The resistance of the printed matter can be tested according to the methods provided by the German Industrial Standards DIN 16524 and DIN 16525.
4 Substrate is the basic element
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 different quality substrates were used. Includes recycled materials (GT and GD) and new fiber materials (GC). These materials are not only of different grades, but also the quality of machine coating. Machine coating is usually done twice or three times, and the coated tools include a doctor roll, an air brush, an air knife, and a mucosal machine. (Note: In order to avoid confusion, we have to distinguish between different coatings. Coatings (powders) added to paper or paperboard in paper mills, and glazing coatings that enhance the effect of printed matter, in order to perform a series of useful under the same conditions. The thickness of each material used was the same. On different grades of paper, the number of times the powder was applied had a significant effect on the quality.
The effect of the two-coating of the recycled material plus the two coats is much worse than that of the three coats on the machine. The new fiber-made paperboard is coated twice and then glued. The surface is smooth and has a gloss like coating three times. The surface hardness of the coating on the machine also has an effect on the gloss – hard coating tools produce a smoother surface and increase gloss.
It is not useful to predict the gloss that can be achieved based on water absorption and oil absorption. In the laboratory, the gloss properties of different grades of paperboard are more practical than on-line ultraviolet (UV) glazing after glazing application.
Depending on the amount of ink and coating used, the properties of the substrate itself have a significant influence on the degree of gloss. The smoothness and absorbency of the powder coated surface determine the effect of the coating. High grade cardboard with consistent smoothness on the surface, although slight variations are inevitable, especially for cardboard made from natural materials, but also affect gloss.
The grade of cardboard has an effect on the results of up to 30%. It should be noted that even if the amount of coating is increased, the difference in gloss can be reduced, but this test is independent of the grade of the coating (due to the use of coatings from different manufacturers).
On the other hand, the coated paperboard on the machine can obtain a better smooth surface each time using the same amount of paint. The difference between the coating materials on the three machines can be said to be caused by the formulation of the on-machine coating powder, the coating tool, and the processing sequence.
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-type coating on the printed area of the printed sheet cannot be sufficiently absorbed by the substrate. Therefore, the degree to which the ink is mixed with the coating depends on the viscosity of both.
The ink that has just been printed begins its drying process, and even if the second layer of coating, the ultraviolet (UV) coating, has hardened, its drying continues to proceed below. This can cause deformation of the surface of the coating and may be affected by the product during oxidative drying. These reactions reduce gloss and break the bond.
Due to the interrelationship between the two, the gloss of the ultraviolet (UV) glaze depends primarily on the type of ink selected. In order to ensure a certain degree of gloss, fast absorption inks should be used. However, ink spots may be generated to limit the amount of absorption. When the first printing unit uses an ultra-fast absorption ink, in the subsequent printing unit, the ink is split and transferred to the blanket to generate ink spots. Therefore, different situations should be treated separately. The dampening solution should be used as little as possible during printing, which reduces the build-up of ink on the blanket and reduces the risk of ink spots. The ink used is also related to the situation in which the back of the delivery pile is dirty.
6 primer material
On-line ultraviolet (UV) glazing is applied to conventional inks, usually with a dispersion coating. Ordinary offset inks and ultraviolet (UV) coatings are incompatible chemicals, and this "intermediate coating" needs to be applied to the sheet in a wet, wet form in a short period of time. The water-based dispersion synthesis solution contains 40% to 45% solids, and the viscosity as a function of temperature can be adjusted by adding water.
Once the dispersion coating is applied to the sheet, a significant amount of moisture must be drawn into the substrate and some will disappear. Only at this time, the polymer begins its binding to form a dense coating film. Therefore, in the online production process, the primer material must be able to dry as quickly as possible. The achievable drying speed, flexibility, penetration, overprinting force, viscosity, gloss and adhesion depend on the base material and the additives used.
The important function of the primer material can be clearly seen from the increase in the use and use of different types of primer materials. Especially on the surface with high ink coverage, the influence of the primer material is more significant.
Replace the 13m3/m3 anilox roll with an 18cm3/m3 anilox roll. The UV (UV) coating is smoother and has a higher gloss. This is especially true on high coverage surfaces.
We can easily identify the effects of different types of primer materials. Severe deformation on the surface of 200% of the ground can only be seen when using soft primers. Hard primer materials appear to have a smooth surface before UV (UV) glazing. However, the scratch resistance is reduced. The smaller the amount used, the greater the influence of the type of primer material. These tests clearly show that drying between the coating units is extremely important. If the drying time and the degree of drying are insufficient before the ultraviolet (UV) glazing, that is, using the best primer and the optimum amount, the optimum gloss cannot be achieved. Therefore, using two coaters for glazing, it is recommended to use an extended drying unit.
7 ultraviolet (UV) glazing coating
Nowadays, chemically hardened ultraviolet (UV) glazing coatings are often used in offset printing. These ultraviolet (UV) coatings contain a binder, a reactive diluent, and a photoinitiator. Under ultraviolet (UV) light, a network reaction occurs to cure the coating. Ultraviolet (UV) light causes the photoinitiator to release a chemical group, causing the acrylate to polymerize to form a long chain network linkage. Oxygen blocks this reaction.
For cationic hardening coatings, ultraviolet (UV) light initiates ion formation and the chain reaction forms hardening. Due to the continuity of the curing, although the curing speed is slow, it can be completely cured. The methods of cationic hardening ultraviolet (UV) glazing coatings are mostly used in food packaging because they have no odor and are hardened thoroughly. Our tests focus on chemically based hardening (UV) glazing coatings.
Increasing the amount of UV coating can increase the 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 cm 3 /m 3 to 30 ccm 3 /m 3 , and the gloss increased by at least 10 points regardless of the ink coverage. When deciding on the amount of UV coating, don't forget that it is difficult to apply smoothly if the amount is too large due to the viscosity. Heating the UV coating to 40 ° C enhances fluidity and increases shine.
In addition, the geometry of the anilox roll also affects the flow of the coating. In addition to the amount of capacity, the number of network lines also works. Only a non-foamed coating produces a good gloss. When the anilox roll cell is filled with paint, fine bubbles form micro-foam in the paint. This situation can be combined with defoamer improvement, combined with a reasonable combination of paint circulation system to ensure that the paint will not be over-mixed.
8 drying is the key
In a matter of seconds, the material giving the color and gloss is applied to the substrate. While suppliers try to make these layers change quickly from liquid to solid, high speed printing has limitations. Therefore, the glazing process must be supported by the drying method chosen.
1 drying is determined by the production speed
High speed production is of course our desired, but it is important to remember that the drying method described is only useful for drying equipment. 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 delivery stack to react. If it is visible, it is called back smearing. The quality characteristics of UV glazing prints can be stabilized after several days. Gloss and bond quality can vary after 24 hours, depending on the ink and coating used. However, change is not always a disadvantage.
To maintain the stability of the surface of the high-speed hardening coating, after drying, it is not allowed to change underneath. The sheets are piled up one by one in the delivery stack and are subjected to considerable pressure. Therefore, the surface of the 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 beam drying device producing the appropriate beam energy.
Drying causes the temperature of the delivery stack to rise, causing the back to become dirty and even the paper to stick. Therefore, it is of utmost importance to apply appropriate drying energy when appropriate. Therefore, after printing and glazing units, infrared (IR) lamps are indeed required, plus cold and hot air and pumping. It is recommended to use two transfer forms of extended drying parts between the two coaters. If it is only the drying time that determines the gloss effect, then a transfer part is added 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 unit must match the production speed. The output energy of the UV must follow the speed of the press, but the nonlinear relationship. The output energy of IR is linear.
As the speed of the press increases, the gloss decreases as the ink coverage changes. As described above, the interval between coating each coating determines the stable "base layer" formed. The paper moves between the printing units at a speed of 10,000 sheets per hour, and the ink penetrates into the paper in less than one second. These tests show that the gloss of the printed ink on the first and last printing units is compared at normal production speeds, resulting in a poor gloss of the last unit printing ink. These situations can be used as a reference, which is helpful for us to format the layout and determine the print color sequence. However, when printing with ordinary inks, the gloss is degraded as the printing speed increases, and the inherent problem of this ink is an unavoidable fact. It has been said that the selective matching of inks, coatings and drying devices can minimize the loss of quality.
The effect that ultraviolet (UV) glazing can achieve depends mainly on the substrate. The effect of coating a high gloss dispersion coating on a substrate, the desired effect of UV (UV) glazing on such a substrate, provides a reference to high coverage inks that can cause gloss It is reduced, but it is possible to improve the composition of the ink and its adhesion to the primer by using a high concentration of pigment and an absorbent ink, and the adhesion is really stable a few days after printing. Correct selection of the type and amount of primer used, which is decisive for the achievable gloss. Ultraviolet (UV) glazing coating as a surface layer helps to prevent scratching; at the same time, if the coating is uniform, it can form a bright The surface is the prerequisite for the best results of the press by purchasing the appropriate machine components, especially the dry components, and paying attention to the proper matching of the materials.