If a particularly high gloss is required, ultraviolet (UV) coatings are the most desirable. This type of paint is resistant to scratches, not only provides a good protective layer, its bright surface makes it visually attractive. Packaging printers are particularly interested in these advantages. Therefore, when the medium format models prove the effectiveness of these reforms (two coating units plus multiple dryers for preliminary drying, inter-unit and tail drying), large format models naturally follow this special enhancement combination. General large format printer users, who produce folding boxes, are particularly interested in using ordinary inks and then applying ultraviolet (UV) coatings.
Process technology research gets better results
To get the best results, there are some prerequisites for printing presses, coating and drying methods. With the highest ideals, the elements of municipal presses and the special requirements of individual customers are the only methods. Many tests help to understand the impact of more different process parameters. Whether the test can be carried out closely with partners in the supply industry is extremely important.
Chamber knife coating provides uniform film thickness
The squeegee coating developed in 1993 ensures that the thickness of the coating layer is extremely uniform. Reticulated reels with different loading capacities allow the exact amount of paint to be applied and can be repeated. The amount of paint contained in the roller reel is different for each job. The normal requirement is between six and thirty cubic centimeters per square meter.
This is the nominal capacity. In actual transmission, only 25-35% of the wet paint reaches the surface of the substrate. Reticulated reels with different wire surfaces allow the amount of paint transfer to meet the needs. If it is often suitable to take care of the reticulated reel, it can ensure the stability of production quality.
Good paint circulation is an element
To be able to handle a variety of different paints, the paint circulation system must be the most appropriate combination. The quality of the coating layer is uniform, depending on the delivery volume, storage volume, and pumping principle, plus factors such as temperature stability, they must match the coating machine used.
As various applications continue to increase, the coating unit provides solutions for related applications. A coating unit unit has several completely separate coating circuits, which can be quickly and economically transferred from one coating method to another. It not only saves time for calibration, but also saves paint and cleaning agent. Diffusion coatings and ultraviolet (UV) coatings are not compatible. At the same time, changing from pigmented paint to clear paint requires thorough cleaning. When changing paints, hygiene is very important to ensure stable quality.
Figure 1: The process of using ordinary ink and ultraviolet (UV) paint to increase the effect, Roland R906LTTLV printing machine and the influencing factors.
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How to assess coating quality
On the large-format flowing coating surface, the change of different brightness is particularly significant. Therefore, the most important thing is to apply the paint evenly so that the entire surface maintains a uniform brightness.
There is a so-called "fading effect" that causes difficulties. Due to the difference in printing time and the occurrence of different physical processes, the ink layer and the paint layer are dried at different speeds. When different material layers are mixed to change the volume and emit gas, the surface will be deformed. The structure produced in this way leads to a decrease in gloss, depending on their combination.
Measure gloss with reflected light
Can choose a certain light reflection angle to measure gloss. A surface like a "mirror" reflects 100% of the light, or only part of it, depending on the quality of the coated surface. Consider the degree of gloss, choose the angle of measurement, be sure to change the gloss, provide reliable data. The angle of measurement is very important, because the pigment penetrates into the depth and will be dispersed. The human eye reacts more strongly to gloss against an ink background.
Scratch resistance and adhesion test
The scratch resistance and adhesion characteristics of the coating are important factors required to complete the processing, and also show the durability of the printed matter. Customers sometimes use unscientific methods to test these characteristics, such as a nail scraping test or adhesive tape test. These are subjective methods, but mainly rely on the judgment of the customer. There is no measurement or copyable data, but it can be used for reference and guidance. 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 time, the ink dries and oxidizes, generating gas that builds up between the ink layer and the paint layer, breaking the bond. Therefore, only a few days after the completion of printing, and then check the degree of adhesion, it is reasonable. Acetone is very suitable for testing the curing process of ultraviolet (UV) coatings: wipe the coating with a cloth soaked in acetone. If the curing is not good, the coating will be dissolved.
The German Printing Research Association (FORGA) recommends some test devices to check the resistance of peeling and sticking into pieces. The resistance of the printed matter can be tested according to the methods provided by the "German Industrial Standards" DIN 16524 and DIN 16525.
Influential parameters
For the relevant elements, we bring together a number of basic conclusions. Pay special attention to the effect structure of the entire system and use multiple test methods to determine: how does the combination of different substrates, inks, primer materials, paints and dryers affect the results? We use a group of reliable quality judgment factors to ensure the test results.
Printing materials
We use ordinary quality folding box cardboard for testing. Including recycled materials (GT and GD) and new fiber materials (GC). These basic materials not only have different grades, but also have different coating quality. Traditionally, two or three powder coatings are applied on the machine. The coating methods include: roller reel blade, air brush, air knife, and laminating machine.
Note: To avoid confusion, we need different paints: paint (powder) added to paper or cardboard in the paper mill and paint (glazing paint) added to paper or cardboard to increase the effect of printed matter.
In this way, the important factors of coating are: basic materials, machine coating methods and the quality and uniformity of these elements.
When selecting printing materials, the suitability and economy of the technology should be reasonably flat, and the ink and glazing paint used should be matched to achieve the desired gloss. [next]
Basic elements of printing materials
We use eight kinds of printing materials with different quality to test. Each material has the same thickness, and a series of useful tests are conducted under the same conditions. On different grades of paper, the number of times the powder is applied has significantly different effects.
The effect of reconstituted materials plus two coatings on the machine is much worse than the three coatings on the machine. The new fiberboard is coated only twice but with glue added, and its surface is smooth and has the gloss of coating three times. The hardness of the surface layer coated on the machine also has an effect on the gloss-the pressure tool during coating causes a smooth surface and increases gloss.
Water absorption and oil absorption properties do not provide any useful information for predicting the gloss that may be achieved. In the laboratory, different grades of cardboard are coated with glazing paint, and the gloss characteristics obtained are useful for evaluating the effect of linked ultraviolet (UV) coatings.
Depending on the amount of printing ink and paint, the characteristics of the substrate itself have a very to very serious impact on the gloss process. The smoothness and inhalation characteristics of the powder coating surface determine the effect of the coating. The high-grade cardboard has a uniform smoothness on the surface. Although slight changes are unavoidable, especially the cardboard made of natural materials, it will also affect the gloss.
Figure 2: Compare substrates. Apply five different combinations and different amounts of paint on 100% black: row 1 with a large amount of UV; row 2 with a small amount of UV; rows 3 to 5 with a moderate amount of UV
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The grade of cardboard can affect the results by up to 30% (see Figure 2). It should be noted that even if the amount of paint is increased, the difference in gloss level can be reduced. This test does not depend on the grade of the paint (due to the use of paint from different manufacturers).
Photos 1 and 2 clearly show that after coating, the structure and fibers of the onboard coated cardboard can still be clearly seen. On the other hand, the smooth surface of the coated paper on the tertiary machine makes the same amount of paint better. The difference between the three coating materials on the machine can be said to be due to the formulation of the coating powder on the machine, the coating tools, and the processing order.
Ink
Offset inks mainly contain pigments, vehicles, thinners and additives. The ink is applied to the substrate in the form of water, ink, and film. As the pigment content increases, the same amount of color can be achieved with less ink. After printing, the "inhaled and condensed" ink dries in seconds, but the oxidative drying takes hours or even days to complete. [next]
Choosing the right ink is very important
The ink has two undesirable effects on the glaze coating layer:
(1) Due to the relationship of ink cover, the moisture on the printed part of the paper and the diffusing paint used for priming cannot be fully absorbed, which affects the degree of mixing of the paint and the ink, depending on the flow resistance (viscosity) of the two .
(2) The ink just applied starts its drying process. Even if the second layer of coating (ultraviolet (UV) coating) added last is solid, it will continue to proceed below. In this case, the primer layer is deformed and may be affected by objects released during oxidation and drying. These interactions reduce gloss and destroy bonding.
As mentioned above, due to the relationship between the two, the degree of gloss of ultraviolet (UV) coatings is very dependent on the type of ink used. To maintain gloss to a certain level, quick-sucking inks are required. However, the quality of the cardboard and the quality required for the printed parts are different, and ink stains may occur, limiting the suction capacity. When the first printing unit uses ultra-fast suction ink, the subsequent printing unit will produce ink spots when the ink is split and transferred in the tape. Therefore, each situation must be individually governed by a municipality. When printing, use minimal wetting. This can reduce the accumulation of ink on the adhesive tape, thereby reducing the risk of ink spots. The selected ink also has a certain effect on the reverse printing of the pile.
Photos 1 and 2: Two-sided coated GD3 cardboard (top) and three-sided coated GT2 cardboard (bottom), the effect of first coating and then ultraviolet (UV) coating.
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Shijiazhuang Asa Technology Co., Ltd. , https://www.asaantiwrinkle.com