Apply UV coating to common inks (1)

More than two decades ago, Manroland was the first press manufacturer to recommend paint to the industry. Since then, paints can be used to enhance the effects of packaging or advanced brochures such as glossy and protective layers without additional processes.

In recent years, Manroland has continued to expand the range and application of enhancements: gold, silver, pearlescent effects, special gloss or matt, partitions, embossments, fragrances, etc. All of these coatings can be applied in conjunction with a given thickness. Behind these extensions, the driving force behind the scenes comes from the presence of two coating unit presses, which use a chamber-type scraper or web-texture coating technology.

Ultra-violet (UV) coatings are among the most desirable if a particularly high gloss is required. This type of paint can resist rubbing, not only provide a good protective layer, its bright surface makes it visually, very attractive. Packaging printers are particularly interested in these advantages. Therefore, when the medium-format models demonstrated the effectiveness of these reforms (two coating units plus multiple dryers for preliminary drying, inter-unit and tail drying, respectively), large format models naturally follow this particular enhancement. combination. Users of large-format printers in general, who produce folding cartridges, are particularly interested in methods that use common inks and then coat ultraviolet (UV) coatings.

Process technology research gains better results

For best results, some prerequisites must be met for the printing press, coating, and drying methods. With the highest ideals, coordinating the elements of the press and the special requirements of individual customers is the only way. Many tests help to understand the effects of many different process parameters. It is extremely important that testing can be conducted closely with the partners in the supply industry.

Chamber scraper coating provides uniform film thickness

The chamber scraper coating developed in 1993 ensures that the thickness of the coating layer is extremely uniform. Reticulate crucibles with different loading capacity allow the exact amount of coating to be applied and can be repeated. The amount of paint contained in the roll, which varies from job to job, is normally between six and thirty cubic centimeters per square meter.

This is the nominal load. In fact, only 25-35% of the wet paint is transferred to the surface of the substrate. Reticulated enamel mesh surface, allowing the transfer of paint components meet the needs. If you regularly take care of webbing, you can ensure the stability of production quality.

The good circulation of paint is a factor

To be able to handle a variety of different coatings, the coating's circulatory system must be the most appropriate combination. The quality of the coating layer is uniform, depending on the conveying component, the storage component and the pumping principle, plus the temperature must be stable and other factors, they must match the coating machine used.

Figure 1: Production process using ordinary ink and ultraviolet (UV) coatings to increase the effect, Roland R906 LTTLV presses and influencing factors

As various applications continue to increase, coating units provide solutions for the application. A coating unit unit has several completely separate coating circuits that can be quickly and economically changed from one coating method to another. It not only saves school time, but also saves paint and cleaning agents. Diffusion coatings and UV coatings are not compatible. At the same time, changing from a pigmented paint to a clear paint requires thorough cleaning. When converting paints, hygiene is very important for ensuring stable quality.

How to assess coating quality

On large-format, flow-through coated surfaces, the changes in light intensity are particularly significant. Therefore, the most important thing is to apply the coating evenly so that the entire surface maintains uniform brightness.

There is a so-called "recession effect" that causes difficulties. Due to the difference in printing time and different physical processes, the ink layer and the paint layer are dried at different rates. When different material layers are mixed to change volume and emit gas, the surface will be deformed. The resulting structure leads to a decrease in gloss, depending on their combination.

Measure gloss with reflection rays

One light reflection angle can be used to measure gloss. Like the surface of a "mirror", reflecting 100% of the light, or reflecting only part of the light, depends on the quality of the coating surface. Consider the degree of gloss, select the angle of the measurement, and be sure to provide information on the change in gloss and provide reliable data. The angle of measurement is very important because the pigment penetrates deep and there is dispersion. The human eye reacts more strongly to gloss when it is under an ink background.

Scratch resistance and adhesion test

The scratch resistant and adhesive properties of the coating are an important factor needed to complete the process and also indicate the durability of the printed product. Customers sometimes test these features in an unscientific way, such as scratching with nails or sticking with adhesive tape. These are subjective methods, but mainly rely on customer judgment. There are no measured or reproducible data, but it can be used as a guideline. After production, if the coating adhesion remains stable after a few days, it can only be maintained for several hours. During this time, the ink is oxidized and dried to generate gas that builds up between the ink layer and the paint layer, breaking the bond. Therefore, it is reasonable to only check the adhesiveness for a few days after printing. Acetone is well-suited for testing the degree of cure of UV coatings: Wipe the coating with an acetone-soaked cloth. If the cure is poor, the coating will dissolve.

The German Printing Research Association (FORGA) recommends several test devices to check the resistance of peeling and sticking into blocks. The resistance of printed matter can be tested according to the methods provided by "German Industrial Standard" DIN16524 and DIN16525.

Influential parameters

For the relevant elements, we gather a number of basic conclusions. Pay special attention to the effect structure of the entire system, and use a number of test methods to determine: How do different substrates, inks, primer materials, paint, and dryer combinations affect the results? We use a certain set of reliable quality judgment factors to ensure the test results.

Substrate

We use ordinary quality folding paperboard test. Includes recycled materials (GT and GD) and new fiber materials (GC). These basic materials not only have different grades, but also have different machine coating qualities. Conventionally, the powder coating is performed twice or three times on the machine. The coating methods include: roll squeegee, air brush, air knife and film applicator.

Figure 2: Comparison of substrates. Apply five different combinations of different amounts of paint on 100% black:
The first row, a large amount of UV; the second row, a small amount of UV; the third to the fifth row, a moderate amount of UV.

Note: To avoid confusion, we need to have separate coatings: paint (powder) added to paper or paperboard in paper mills, and paint (glazing paint) added to paper or paperboard for effect of printed matter.

In this way, the important factors for coating are: basic materials, the way the machine is coated, and the quality and evenness of these elements.

With regard to the choice of substrates, the appropriateness and economy of the technology must be reasonably balanced to match the ink and coating used to achieve the desired gloss.

Basic quantity of printing materials

We use eight different quality substrates for testing. Each material has the same thickness and a series of useful tests are performed under the same conditions. On different grades of paper, the coating times of powders have distinctly different effects.

The effect of re-creating the material plus two on-machine coating is much worse than the three coating on the machine. The cardboard made of the new fiber was coated only twice but with glue. The surface was smooth and had three coats of gloss. The hardness of the surface layer coated on the machine also has an effect on the gloss - the pressing tool during application creates 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 lab, different grades of paperboard, coated gloss coating, and the resulting gloss properties are useful for evaluating the effect of UV coating on linkage.

Depending on the weight of the applied ink and coating, the properties of the substrate itself have a very, very serious effect on the degree of gloss. The smoothness and inhalation characteristics of the powder coating surface determine the effect of the coating. High-grade paperboards have uniform smoothness on the surface, and slight variations are inevitable. In particular, paperboard made from natural materials will also affect gloss.

Cardboard grades can affect results by up to 30% (see Figure 2). It should be noted that even if you increase the amount of paint, you can reduce the difference in gloss levels. This test does not depend on the grade of the paint (due to the use of paints from different manufacturers).

Photos 1 and 2 clearly show that after the paint was applied, the structure and fibers of the two coated cardboards were clearly visible. On the other hand, the smooth surface coated with cardboard on a third machine gives better results for the same amount of paint. The difference between the coating materials on the third machine can be said to be due to the formulation of the coating powder on the machine, the coating tools, and the processing sequence.

Ink

Offset inks mainly contain pigments, vehicles, diluents, and additives. The ink is applied to the substrate in the form of water, ink, or film. When the pigment component increases, the same chromaticity can be achieved with less ink. After printing, the "inhalation and coagulation" ink components dry in a matter of seconds, but oxidation drying takes hours or even days to complete.

Choosing the right ink is very important

The ink has two undesirable effects on the coating layer:

(1) Because of the ink coverage, the printed part of the printed sheet, the moisture of the diffusion coating used for the primer cannot be fully absorbed, and the degree of mixing of the coating and the ink is affected. The condition depends on the flow resistance (viscosity) of the both. .

(2) The newly applied ink starts its drying process and continues to be performed even if the second coating (UV coating) finally added is firm. In this case, the primer layer is deformed, and the flaw may be affected by the object released during the oxidation drying. These interactions reduce the gloss and destroy the bond.

As mentioned above, due to the mutual relationship between the two, the degree of gloss of an ultraviolet (UV) coating depends very much on the type of ink selected. To maintain gloss to a certain level, fast inhalation inks are used. However, the quality of the paperboard and the quality of the prints may not be the same, resulting in the possibility of ink spots and limiting the inhalation capacity. When the first printing unit uses an ultra-fast suction ink, the subsequent printing unit, the ink will be generated when the tape is split and transferred. Therefore, each situation should be coordinated individually. When printing, use minimal wetting. In this way, the accumulation of ink on the blanket can be reduced, thereby reducing the risk of ink spots. The selected inks also have a certain relationship with the occurrence of anti-printing on the delivery piles.

(Photos 1 and 2: Two-sided coated GD3 board (top) and three-side coating on board