Discussion on the precautions of UV glazing process
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China's printing industry's "Twelfth Five-Year" development plan proposes to vigorously develop green printing and green packaging, which is the essential requirement for the development of circular economy, and is a powerful measure to establish a resource-saving society and promote the harmonious development of people and nature. Both the laminating process and the U V glazing process belong to the finishing process of the printed surface, and all have the functions of increasing the surface smoothness and gloss of the printed matter, protecting the printed graphic and improving the product grade. In contrast, the U V glazing process is more environmentally friendly and energy-saving than the laminating process. It is currently a widely used method of decorative art surface decoration. In this paper, the precautions for U V glazing process are summarized as follows, in order to achieve the purpose of ensuring the quality of U V glazing and the normal production.
1. U V varnish selection and quality inspection before glazing
There are many varieties of UV varnish. The varnish should first be selected according to the glazing method. Different glazing methods need to be matched with different performance varnishes, that is, glazing oil with different applications for offline glazing and online glazing. Roller coating and printing glazing should use different varnishes. When printing glazing, the varnish used in different printing methods such as offset printing and flexo printing is different.
The adhesion fastness of U V varnish varies depending on the type of substrate. It is required that the surface tension of the varnish should be less than the surface tension of the substrate or ink, so that the varnish can be well wetted, adhered, and soaked on the surface of the printed matter. Otherwise, the coated varnish layer will have a certain shrinkage, unevenness, and even some problems such as blisters or no adsorption in some places. Therefore, if you want to replace the new printing materials, you must carry out the glazing test before the official production.
U V varnish has a certain storage period and has certain requirements for the environment. UV varnish usually has a shelf life of 1 year. It should be stored in a cool or ventilated place. If it is stored at a higher temperature, it will easily deteriorate or harden. UV varnish cannot be mixed with ordinary varnish. Can use UV varnish special adjuvant, different brands should not be mixed;
2 . Selection and use of UV curing light source
Since the curing of U V varnish relies on the photoinitiator to absorb the radiant energy of ultraviolet light to form a radical, thereby initiating polymerization and cross-linking reaction of the monomer and the prepolymer. Therefore, the choice of UV curing light source has an important influence on the drying speed of U V varnish. The ultraviolet light source is generally a high-pressure mercury lamp or a metal halide lamp, and the output power is generally 80 to 120 w / c m to ensure that the curing time of the U V varnish is less than 0.5 seconds. The intensity of the UV curing light source should be appropriate, the light is too strong, the free radicals are generated too fast, the concentration is too high, the reaction speed is increased, and the crosslinking reaction is adversely affected; the intensity is too low, the free radicals are generated too slowly, and the oxygen inhibition is effected. Very strong, it will make the UV varnish dry too slowly.
Generally, the tubular U V lamp can be used to distribute the light evenly on the printed matter, and it is easy to install, cheap, safe and reliable. Also, a good lampshade is used for concentrating, and the same lampshade is good or bad, which can make the efficiency difference by a dozen times.
The U V lamp has a certain service life, the UV lamp will gradually age, and the light intensity of the expired UV lamp will be much lower, making the U V varnish dry too slowly. The effective power should be regularly checked during production, and should be replaced in time when the usage time is close to 1000 hours.
After the mercury lamp is turned on, it needs a warm-up time so that the mercury can evaporate and the ultraviolet light should not be obtained during the preheating time. After the mercury lamp is turned off, it cannot be restarted immediately. It needs to be cooled for a while, which is not convenient to use.
3 . U V varnish viscosity, coating amount and glazing speed control
The viscosity of U V varnish has an important influence on the leveling property on the print and the coating suitability of the ink, which will eventually affect the gloss of the glazing product. The control of the viscosity of the varnish should take into account the absorbency of the print and the thickness of the coating. The smaller the viscosity of the varnish, the more it is absorbed by the print. Therefore, if the absorbency of the printed matter is too strong, the viscosity of the varnish can be appropriately increased to shorten the leveling time, otherwise the varnish is absorbed in a large amount during the long-time leveling process, and it is difficult to form on the surface of the printed matter. The flat film layer is easy to form a film layer similar to the surface of the printed matter.
The coating amount of the varnish should be appropriate and the coating should be uniform. The leveling property of UV varnish is not as good as that of solvent-based varnish. If the coating amount is too small, the varnish cannot spread evenly, and the surface smoothness of the coating is poor. If the coating amount is too much, it is beneficial to the varnish. The leveling also enhances the compensation effect on the surface defects of the printed matter, but the drying of the glazing oil is not thorough, only the surface layer is dried, causing adhesion after rewinding, and the cost is high.
The determination of the amount of glazing coating is related to the type of coating, the surface condition of the printed matter, and the coating conditions. When determining, it is necessary to take care of each other and make overall arrangements. It is necessary to meet the glazing quality requirements and to be suitable for various processes. In coating, the change in coating amount can be achieved by adjusting the glazing coater control mechanism or changing the speed of the coater. When the amount of change is small, it can also be achieved by changing the viscosity of the varnish.
The glazing speed should be determined according to the curing speed and coating amount of the varnish. When the curing speed of the varnish is fast, the speed of the coater can also be increased. At this time, the coating has a short leveling time and the coating layer is relatively thick, and vice versa. In addition, the speed of the coater is also related to the conditions of the curing machine of the coater, the condition of the printed matter, and the like.
When glazing operation, it should be noted that UV varnish viscosity, glazing coating speed, coating amount and coating roller pressure are mutually restricted. It is necessary to wet and absorb the varnish according to different paper surfaces. Adjust the relationship between them.
4 . Pay attention to the safety protection of the operator
In UV glazing, UV light may cause damage to the skin and eyes. It should be shielded from UV light to the human body. The ozone generated by the UV light is harmful to the human body. Pay attention to the ventilation of the production environment. Smooth; Pay attention to the skin protection of the operator, otherwise the skin will be stimulated by UV varnish, causing itching, redness, pimples, and even peeling. Splash the skin in time, rinse with soapy water and change the overalls in time. U V varnish should be cleaned with a special cleaning agent or ethyl acetate. It is difficult to wash off with gasoline and kerosene.
5 . U V glazing common faults and solutions
1 U V glazing products are sticky on the surface and have a high residual odor.
Main reasons: The coating layer is too thick, the UV light intensity is insufficient or the machine speed is too fast; U V varnish storage time is too long; the diluent that does not participate in the reaction is added too much.
Solution: The coating layer is too thick, the coating thickness should be reduced. For some special wrinkles, crystals and other artistic effects, the coating layer is too thick, so the irradiation time is relatively long under the same power source, which affects the UV light on the one hand. The solidification of the oil, on the other hand, makes the water content of the paper relatively reduced, which makes the sheet brittle and fragile, affecting the abrasion resistance of the UV varnish layer. More importantly, the excessive temperature will increase the temperature and cause a fire. Such as accidents; when the UV lamp power is insufficient or aging, the curing efficiency will be reduced, and the UV lamp can be replaced in time to reach the rated power; the UV varnish with fast curing speed and low odor can be selected. Add a certain amount of U V varnish curing accelerator or replace varnish. Pay attention to the proper use of thinner.
2 U V After glazing, the product is prone to cracking.
The main reasons are: The paper itself is brittle and easy to crack, especially thick paper or cardboard; the varnish coating layer is too thick; the UV varnish is too hard and the flexibility is insufficient; the light source is too strong, the UV curing is overexposed; 5 Post-press processes such as cut indentations do not match.
For the above reasons, the following methods can be used to eliminate: Select paper with good toughness and not easy to break; Reduce the coating amount of UV varnish and reduce the coating thickness; Select UV varnish with better flexibility, also in UV light Add a small amount of plasticizer and flexible additive to the oil; on the basis of ensuring the curing of the varnish, try to avoid overexposure or baking of UV light; adjust the postpress process to adapt it to the processing of thick paper.
3 U V The glazing layer is yellowish.
The main reasons are: U V varnish itself is yellow; U V light is excessively irradiated, U V varnish is excessively cured.
Solution: U V varnish is too long, it has turned yellow, you must replace the appropriate U V varnish; to moderately reduce the power of the U V light source or open the U V light group to speed up the glazing speed.