UV coating technology basic knowledge

First, the application of UV coating technology Paper coating products after UV treatment, can improve the appearance of the effect, so that the printing quality is thick and full, glossy and full of moisture, improve the decorative effect of paper products, and can significantly improve the folding durability and Chemical resistance, improve its performance, reduce the loss of the product during transportation and storage. For products with high moisture-proof requirements (such as electrical appliances, etc.), their paper-based packaging products are treated with UV coating to increase the moisture resistance of the package.

UV glazing has developed rapidly in recent years, and its proportion in the packaging and printing industry is gradually increasing, but at the same time there are still many problems to be solved. The first is security issues. At present, reactive diluents used in UV varnish have certain volatility or irritation and are still polluted to the environment. The water-based UV varnish currently being developed is an ideal environment-friendly product because it replaces the active diluent with a thickener, which solves the problems of volatility and irritation.

Followed by the odor problem. Now it is gradually developing towards low odor. At present, UV-glazed packaging products have not been fully recognized by the international food and drug companies and cannot be directly contacted with food. Therefore, it is necessary to improve the working conditions and environment, improve the drying effect, and obtain a complete cure, thereby reducing the odor.

Again, there is a problem of adhesion on oxidative polymerization inks. There is not only the problem of odor, low gloss, but also difficulty in adhesive attachment. Double glossing is often used to improve the quality of UV coatings, but it also increases costs. Therefore, the adaptability of UV light oil and ink needs to be improved.

The last is to reduce the cost, although the current cost has dropped a lot, compared with the cost of BOPP composite film composite cheaper. However, in China, there are many types of UV coatings and their properties are different. So the price difference is greater. In general, the cost is still too high, and it is necessary to further reduce the cost of lacquer, coating costs, and equipment operating costs.

Second, the advantages of UV coating

1. No environmental pollution, no solvent evaporation, safe and environmentally friendly.

2. Drying speed, high production efficiency. Because the photo-curing reaction can be completed in an instant, the UV varnish can be directly stacked or entered into the next process after the printing is cured, which accelerates the production process and improves the production efficiency.

3. Not limited by the type of substrate, not only on the surface of paper, aluminum, plastic materials, etc., but also can be used for surface coating of polymer panels, furniture, metal materials, wireless materials.

4. Save space and energy, due to the UV varnish curing process, do not need auxiliary drying equipment, such as drying drying tunnel of ordinary varnish, and the UV curing machine is small in size, and the effective utilization rate is high, thereby greatly saving space and energy. , reduce early stage equipment investment.

5.Good quality, UV print surface smooth after printing, no warping, wrinkling, blistering and other phenomena, and high surface hardness, abrasion resistance, scratches, chemical corrosion resistance, and the past Compared with lamination, the cost is only about 1/4-1/3 of the lamination.

Three, double coating system introduced

Previously, for those printers who wish to continue to use standard inks and are unwilling to use UV gloss for certain printing jobs, double-glazing is undoubtedly their most suitable choice. Two coating units are usually located at the end of the press. To eliminate or reduce the possibility of loss of light (glazing oils or substrates), an aqueous coating must be applied between the wet offset ink and the final coating to isolate the offset ink and use infrared light Or heat radiation drying, can be used as the basis for UV coating adhesion. At the same time, every coating machine must have an integrated drying device, which increases the printing machine's external dimensions and equipment costs. However, the range of gloss that such devices can achieve is greater than that of a pure UV printer. In addition to the use of waterborne varnish plus UV varnish, these devices can also be used to apply the same waterborne varnish twice (eg, to increase the final gloss, apply two coats of varnish to the substrate), Or apply two different coatings (eg, applying the standard coating to the first coating unit and applying special effects to the coating through the second coating unit).

There is another model of the double coater: a flexographic printing unit and a drying device are installed before the first offset printing unit to apply opaque white or a special effect primer.

Fourth, pure UV coating system introduced

Printing lines that use UV inks and are last UV-cured can achieve extremely high gloss for prints. However, once such a pure UV system is used, it is almost impossible to switch to other types of coating operations.

Fifth, composite glazing printing equipment introduction

Unlike pure UV printers, composite printers generally refer to printers that can apply compound inks and perform final UV coating, as well as apply conventional offset inks and perform aqueous glossing. This type of printer can be easily and flexibly converted between these two modes of operation without the need to replace the rollers, blankets, and dryers. In order to adapt to different modes of operation, the composite press is equipped with IR (infrared), hot air and UV curing devices.

The use of composite presses is much broader than pure UV printers and does not require the operator to have experience with UV printing production. The gloss obtained by UV-coating on compound inks is much higher than the gloss obtained by applying two coats of polish on standard inks and can approach the level of a pure UV system. Moreover, the composite printing press performed particularly well in the combination of no-gloss and gloss effects, and the obtained gloss contrast effect was much higher than the operation of applying a layer of water-based coating oil before the UV coating. And composite glazing offers additional options such as a blend of particle and gloss effects. In addition, if a suitable configuration is used, the composite printing press can print with a small amount of alcohol or even no alcohol, which is a guarantee of environmental protection.

Composite presses are the largest collection of UV printing and double-glazing printing equipment, so composite printing presses are another ideal tool for the UV printing and glossing markets. However, the compound type printing press can use a small amount of printing even if it does not use alcohol. Therefore, the environmental protection certificate it obtains is highly praised.

Six, UV coating on the impact of print color

The use of UV coating on the print, sometimes yellowing phenomenon, there are two main reasons: First, UV coating itself yellow; Second, in the coating process, the coating oil is only cured on the surface of the print. The interior is not completely cured.

The yellowing of the varnish is related to the use of the oligomers and reactive diluents with a certain yellow color. Therefore, colorless oligomers and reactive diluents should be used to formulate the varnish. In addition, the influence of photoinitiators is also great, and some photoinitiators will make the system yellow after curing, such as benzoin dimethyl ether, so the use of such photoinitiators should be avoided in the preparation of varnishes.

In the coating process, when the interior of the coating oil is not completely cured, the internal coating oil thinner will gradually be absorbed by the ink and the paper, and at the same time, it will have an effect on the binder in the ink and cause the fat to be lost. The pigment is not sufficiently protected and the pigment particles are free. At this time, the smoothness of the surface of the paper coating and ink layer will be reduced. The rough surface will produce diffuse reflections of light, adding some of the plant's white components to the colors, which will increase the brightness and decrease the saturation, giving the impression of overall yellowing and lightening. Therefore, the curing speed of the UV coating should be increased or an efficient UV curing light source should be selected. In addition, the use of water-based UV varnish can avoid the chemical effect of the thinner Liu ink layer and paper coating, and can avoid yellowing of the printed matter.

Seven, UV coating method

There are many UV glazing methods, and there are many classification methods. There are mainly the following classification methods:

1. According to the relationship between the machine and printing press classification can be divided into: offline coating and online coating two ways.

(1) Off-line glazing

Off-line glazing uses a special glazing machine to polish the prints, ie, printing and glazing are performed on their own special equipment. This kind of glazing method is more flexible and convenient, and the glazing equipment investment is small, which is more suitable for professional post-press processing manufacturers. However, this glazing method increases the transportation and transfer between the printing and glazing processes, and the production efficiency is low.

(2) Online coating

Online coating directly connects the glazing unit to the printing unit, ie, printing and glazing are performed on the same machine, which has high speed, high production efficiency, low processing cost, and reduces the handling of the printed matter and overcomes the problem caused by dusting. The various types of quality failures are the future direction of development. However, the on-line coating has high requirements for coating technology, coating oil, drying equipment, and coating equipment.

2. According to the glazing method, the classification can be divided into two types: roller coating and printing glazing.

(1) Roller coating

Roller coating is the most common method of glazing. The coating roll is used to spread the coating on the surface of the printed product.

(2) Printing glazing

Printing glazing coats lacquer on the prints through a glazing plate so that it can be applied locally. Currently used are gravure coating, flexo coating, offset printing, and screen polishing.

3. According to the classification of glazing product types, it can be divided into: full-width surface, partial glazing, extinction glazing, and artistic glazing.

(1) Full-width light

The main role of full-frame light is to protect prints and improve the surface gloss of prints. Full-frame light is generally applied by roller coating.

(2) Partially glazing

Partial glossing is generally used to polish parts of graphics that need to be emphasized in printed matter. The use of a high-gloss picture of the glossed part is compared with a low-gloss picture that has no glossiness, resulting in wonderful artistic effects. The partial coating is performed by the method of printing glazing, in which the film obtained by screen printing is thicker, the effect is more obvious than other methods, and the cost is low. Currently, it is used more domestically, but its production efficiency is general. Lower.

(3) Extinction coating

Extinction coating uses UV matte oil, which has the opposite effect of ordinary coating. It reduces the glossiness of the print surface and produces a special effect. Because the gloss is too high to stimulate the human eye to a certain extent, extinction coating is currently a popular glazing method.

(4) Art glazing

The role of artistic glazing is to obtain special artistic effects on the surface of glazing products. If using UV pearlescent coating on the surface of the print, it will produce a pearl effect on the printed surface, making the printed material look grand, elegant and elegant.

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