Varnishing is the process of applying a layer of colorless and transparent paint on the surface of the printed matter, and after leveling, drying, calendering, and forming a thin and uniform transparent bright layer on the surface of the printed matter. Glazing can not only enhance the surface brightness, protect printed graphics, but also does not affect the recycling of paper. Therefore, it is widely used in the surface processing of printed products such as packaging cartons, books, picture books, posters and so on. The glazing processing technology of paper printed matter includes coating glazing, UV glazing and pearlescent pigment glazing.
1 Various glazing process characteristics and requirements
1) Coating glazing
The process of coating glazing is actually the process of applying paint? Commonly known as glazing oil? To the surface of paper prints to dry. Varnishes include solvent varnishes, water-based varnishes, UV varnishes and pearlescent pigment varnishes.
Since the solvent varnish is not conducive to environmental protection, the scope of application is increasingly restricted.
Water-based glazing oil uses water as a solvent, non-toxic and odorless, eliminating the harm to the human body and the pollution to the environment. It has fast drying speed, good film transparency, stable performance, good wear resistance and smoothness of the glazed surface. The characteristics of wide post-processing adaptability, good heat sealing performance, safe and reliable use, convenient storage and transportation, etc., are increasingly valued by food, medicine, tobacco carton packaging and printing companies.
UV glazed prints have high surface gloss, heat resistance, wear resistance and water resistance. There is no solvent evaporation during curing, which will not cause pollution to the environment. It is suitable for glazing of high-end packaging.
The pearlescent pigment glazing can obtain good spot color gloss effect and has good development prospects.
Generally, after glazing, after the varnish is dried (the time is short, you can work online), you can perform calendering, the purpose is to form a high-gloss surface on the glazed surface.
Performance requirements for glazing coatings
â‘ The film has high transparency and does not change color.
â‘¡ Has a certain degree of flexibility. The bright film formed by any varnish on the surface of the printed matter must maintain good elasticity in order to be compatible with the flexibility of the paper or cardboard, so as not to be damaged, cracked or peeled off.
â‘¢The film layer has certain wear resistance. Some glazed prints require certain abrasion resistance and scratch resistance after glazing. Such as the production process of high-speed box making machine, cardboard box packaging machine device, book cover and other production lines, the surface of the printed matter is subject to friction, so it must be wear-resistant.
â‘£It has a certain adhesion to the surface of the printed matter. Due to the influence of the graphic ink layer on the surface of the printed matter, the surface adhesion is significantly reduced. In order to prevent the ink layer from drying out and cracking during use, the film layer is required to have strong adhesion, and it has a certain degree of ink and various auxiliary materials for ink adjustment Of adhesion.
⑤Good leveling and smooth film surface. There are many kinds of printing materials, and the influence of printed graphics and texts, the surface absorption, smoothness, wettability and other differences are very different. In order to make smooth coatings on different product surfaces can form a smooth film, varnish is required Good leveling, smooth film surface after film formation.
â‘¥The membrane layer has better performance in adapting to the environment. After glazing, some printed products are used to make various types of packaging cartons. In order to play a good role in protecting the packaged products, the glazing film layer must adapt to the environment. For example, the packaging of food, cigarettes, cosmetics, clothing and other commodities must have moisture and mildew resistance. In addition, the chemical properties of the dried film should be stable, and the performance cannot be changed by contact with chemical substances such as weak acids or alkalis in the environment.
⑦ Wide adaptability after printing. After the printed product is polished, it generally needs to be processed after the process, such as: good heat resistance, after hot stamping of anodized aluminum, no sticking phenomenon can be produced; good solvent resistance, the dried ink layer cannot be subjected to post-processing Under the influence of the adhesive, blistering, wrinkling and stickiness appear.
The main factors affecting the quality of glazing
The factors affecting the glazing quality are mainly the glazing suitability, temperature, ink quality and crystallization of the printed matter.
â‘ Gloss adaptability of printed matter. The glazing suitability of the printed matter mainly refers to the influence of the printing paper and printing ink on the glazing coating. a. What is the quality and characteristics of the paper, which will directly affect the quality of the glazing products? For example, the coated paper is smooth, and after coating, the coating is easy to level, and the coating layer is on the paper surface after drying. The formation of a film surface with a higher degree of smoothness significantly improves the gloss effect; while the surface of the coated paper of low quality is rough, and the coating is not easy to level after glazing, and the glazing effect is relatively poor. To this end, you can take the varnish twice to ensure the quality of glazing. b. If the heat resistance and alcohol resistance of the ink are poor, the quality of the printed image and text will be discolored or wrinkled. Therefore, glazing products should use alcohol-resistant, ester-based solvents and acid and alkali resistant printing inks. In addition, the ink is also required to change color for a long time and have good gloss, and strong adhesion to the paper.
②The influence of temperature on the glazing quality of the product. Under normal circumstances, the temperature of the glazing of the printed matter can be controlled at about 20 ℃ to obtain a more ideal effect. However, if it is glazed in winter, due to the low temperature, the glazing oil is relatively easy to solidify, which is not conducive to the normal flow of the glazing oil, so that the oil film on the surface of the product is not uniform, so its brightness is necessarily poor. In order to overcome the disadvantages caused by the defects of environmental conditions, varnish should be placed in a relatively warm place. If necessary, dilute with solvent as appropriate.
â‘¢Influence of crystallization of printed matter on glazing quality. If the printing product is placed for too long or excessive dry oil is added to the background ink, the ink layer will form a crystallization phenomenon on the paper surface, so that the varnish will not attach to the ink layer when glazing, and the varnish layer will appear to bloom, not Both phenomena. In this case, as long as 5% lactic acid is added to the glazing oil, it can be glazed after stirring evenly. Applying this modified varnish to the printed product can damage the crystallized film on the surface of the printed product, thereby achieving the requirement of evenly absorbing the varnish.
2) UV coating
UV glazing is a method of curing UV glazing coatings by UV irradiation? UV glazing relies on the irradiation of UV light to cause a chemical reaction inside the UV coating to complete the curing process. Due to the action of ultraviolet rays, UV drying instantly when glazing, the curing speed is fast, the curing speed can reach 60 ~ 120m / min, the glazing oil has strong affinity for the ink, and the adhesion is firm. There is no volatilization of solvent during curing, which will not cause pollution to the environment. Prints using UV varnish have high surface gloss, heat resistance, abrasion resistance, water resistance, light resistance, and UV glazing products are non-blocking. They can be stacked and stacked after offline, which is beneficial to post-processing processing operations such as binding. However, due to the high price of UV glazing oil and the high performance requirements of the machine, it is currently only used for glazing high-end carton. There is also a special type of glazing, which is to apply local glazing to the parts of the package print that need to be highlighted. Using the contrast between the bright part and the non-gloss part of the local glazing product can produce a unique artistic effect.
The domestically produced UV varnishes have also developed rapidly in recent years, and the quality has reached a high level. For example: the flexible UV varnish produced by Shaanxi New Century Printing Materials Co., Ltd. and the UV varnish for textbook covers are already in China. Promotion and use, praised by users.
3) Pearlescent pigment coating
Pearlescent pigment glazing is to uniformly apply a pigment with a lustrous and semi-transparent flake crystal structure with partial hiding power to the surface of the printed product. Pearlescent pigments can be printed separately after being blended with colorless and transparent binders, and can be mixed with other inks and used afterwards, and can also be overlapped with other ink layers. In the modern printing industry, the pearlescent effect is an irreplaceable spot color gloss effect. It has good application prospects in the fields of high-end packaging such as cigarettes, medicines, food, and cosmetic packaging. [next]
2 Common faults and troubleshooting methods
1) Small particles of impurities appear on the surface of the glazed printed matter. Cause: Impurities are mixed in the glazing paint.
Solution: The glazing paint should be filtered before use. Before coating, the coating bucket and rubber roller of the coating machine should be cleaned to avoid mixing of dirt.
2) Poor gloss of the glazed film. Causes of failure: a. Low temperature during coating drying and calendering; b. Poor coating quality, low gloss after film formation; c. Insufficient coating amount due to low coating concentration, coating Too thin.
Solution: a. Adjust the drying temperature of the glazing coating to increase the calendering temperature; b. Change the coating, or increase the coating concentration and increase the coating amount; c. Secondary coating can be used.
3) Causes of streaks or wrinkles on the film surface: a. High viscosity of glazing paint; b. Too large coating amount of glazing paint; c. Glazing paint has poor wettability to the ink layer on the surface of printed matter; d. The pressure between the rubber roller of the varnishing machine and the impression cylinder is uneven.
Solution: a. Add a small amount of thinner to reduce the viscosity value of the coating; b. Reduce the coating amount of the glazing coating; c. Adjust the pressure between the rubber roller and the impression cylinder of the glazing machine to eliminate streaking.
4) After the printed matter is calendered, the surface is easy to break (thick paper is more obvious)
Causes of failure: a. The high temperature during calendering reduces the water content of the printed matter and makes the paper fibers brittle; b. The high pressure during calendering makes the extensibility and flexibility of the printed matter poor; c. The post-processing process conditions are not suitable for selection ; D. Poor processing suitability after coating.
Solution: a. Reduce the calendering drying temperature and take effective measures to change the moisture content of the printed matter; b. Reduce the calendering pressure; c. Adjust the post-processing conditions to make it compatible with the printed matter (after calendering) match.
5) The blank part of the printed product is light-colored after calendering, and the light-colored part is discolored. The cause of the failure: a. Poor ink drying and poor solvent resistance of the ink layer; b. The coating solvent has a certain dissolving effect on the ink layer; c. Drying of the coating layer Incomplete, high residual solvent.
Solution: a. The printed matter is dried before glazing; b. Reduce the amount of solvent in the glazing paint (solvent that has a dissolving effect on the ink), the conditions allow to change the solvent or change the paint; c. Increase the drying temperature and reduce the coating Internal solvent residual amount.
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