Glazing is a common process for post press processing of printed materials at present, and is also one of the main methods for post press processing of self-adhesive labels. The purpose is to increase the glossiness of the surface of printed materials, so as to achieve the functions of antifouling, moisture prevention, and image and text protection. Here, the author sorted out some experience and shared it with the industry insiders based on the relevant situation of the selection of UV glazing process and reverse glazing process. Selection of UV glazing process and effect description

UV glazing process is closely related to the equipment used by processing enterprises. At present, the commonly used glazing processes include printing unit plate glazing, unit end coating unit glazing (belonging to anilox roll coating) and single machine coating and glazing. Among them, the printing plate glazing of the printing unit can realize local glazing and full plate glazing, while the glazing of the coating unit at the end of the unit and the stand-alone coating glazing can only achieve full plate glazing. For offset printing units and embossing printing units, only the ink et type UV varnish products with high viscosity can be used, otherwise, the thinner varnish is easy to leak out from the side of the ink roller and pollute the equipment and printing materials. Moreover, thinner varnish will cause printing failures such as "flying ink".

The commonly used ink et type UV varnish is divided into bright varnish and matt varnish, which can be directly printed on the surface of the ink layer, on the one hand to protect the ink layer, on the other hand to create a bright or matt effect with decorative effect. Among them, when using UV varnish in offset printing process, it is recommended that the front end color ink should not be cured by UV light irradiation, and the printing method of "wet pressing wet" should be used directly to improve the glossiness, otherwise, the overall glossiness of UV printed products is likely to decline. However, for UV matt oil, it is required that the front end color ink should be printed with matt oil after irradiation and curing, that is, "wet press dry" printing, to achieve a better matt effect. For satellite or intermittent embossing machines, each color deck is equipped with a UV lamp curing device behind it, which cannot achieve the "wet press wet" glazing process. Therefore, the effect of using ink et type UV bright oil is not good, and more UV matt oil is used. It should be further noted that due to the high viscosity and poor leveling of the ink et type UV varnish, and the extremely thin printing ink layer, the thickness is usually only 1~5 μ m. Therefore, the overall brightness is far from the display effect of coated UV varnish. When many enterprises use anilox roll or intaglio coating equipment, in order to reduce costs and improve production efficiency, they choose coated UV bright oil or matt oil to replace the bright film and sub film of the film covering process. In this way, the coated product not only has the required gloss effect, but also has good texture, good solvent resistance and scratch resistance. The disadvantage is that it can not achieve local glazing, and can only be full plate glazing.

The biggest advantage of screen printing is the ink layer thickness (the ink layer can be 10~50 μ M), controllable hiding power, high saturation, can achieve some special effects, such as snowflakes, wrinkles, frosting, etc. The disadvantage is that the printing speed is slow, the cost is high, and it is not suitable for rapid printing labels in large quantities. Analysis of reverse glazing process

At present, UV glazing process is not limited to a single device or a specific varnish product, generally a combination of multiple types. With the emergence of new types of varnish products and the continuous development and updating of glazing equipment, more and more one-time molding special effects are available. In the past, if processing enterprises wanted to achieve the effect of "local bright, local matt" on the surface of the same label, they generally needed to use offset printing, screen printing or flexo printing and other printing methods, which were inefficient and difficult to register. In addition, in order to achieve accurate local glazing effect, the scrap rate may also be high. But now this situation has improved, and a "reverse glazing process" is popular in the market, that is, the process of typical bright and matt effects forming on a label at the same time. The working principle of the reverse glazing process is to use the "mutual exclusion" principle of the coated UV varnish and the base printing UV varnish (ink et type UV varnish) to achieve the effect of bump texture and different gloss at one time, that is, to apply the varnish on the place without the base oil to achieve the bright effect, and to apply the varnish on the place with the base oil to achieve the matt effect. This process can control the amount of UV primer or the dot size to achieve the change of light and shade (or wrinkle shrinkage strength) after UV glazing.

The recommended printing sequence of reverse glazing process mainly includes the following three types: (1) UV color ink printing → UV irradiation curing → UV primer printing → UV irradiation curing → UV varnish coating → UV irradiation curing; (2) UV color ink printing → UV irradiation curing → UV primer printing → UV varnish coating → UV irradiation curing (no irradiation in the middle); (3) UV color ink printing → UV irradiation curing+(offline secondary printing)+UV primer printing → UV varnish coating → UV irradiation curing. For the reverse glazing process, the author also summarized some factors affecting the glazing effect and adjustment methods for reference. From the influence of printing speed, slow speed will make varnish have longer reaction time and strong concavity and convexity; From the perspective of the influence of UV curing degree between the units, the higher the curing degree, the more likely to produce "mutual exclusion", and the less concave convex feeling; From the perspective of the amount of ink on the UV primer, a large amount of ink is more likely to produce "mutual exclusion", and the bump feeling is weak; From the perspective of the coating amount of UV varnish, less coating amount is more likely to produce "mutual exclusion", and the bump feeling is weak; From the viscosity of coated UV varnish, low viscosity is more likely to produce "mutual exclusion", smaller particles; From the perspective of the coating pressure of coated UV varnish, small pressure is more likely to produce "mutual exclusion" and smaller particles. In the reverse glazing process, there are some matters needing attention. For example, when the UV primer is over cured, the subsequent coated varnish may fall off when scraping. Please be sure to confirm the adhesion of the varnish when printing. In order to achieve the required "mutual exclusion" effect, it is sometimes necessary to make fine adjustments. In addition, the use of different conditions, the mutual exclusion between varnish is also different. Different printing process, UV lamp curing power, UV lamp position, UV primer and coating varnish will cause different glazing effects. Therefore, it is necessary to strictly control the printing conditions to ensure the reproducibility of labels.