In packaging and printing, with the continuous improvement of printing technology and equipment, and the increasingly fierce competition for goods, people also pay more and more attention to the appearance requirements of packaging and printing products, and constantly introduce special and new packaging methods and packaging printing surface treatment technology Among them, the glazing technology has been widely used, making the product packaging leap to a new height. Glazing not only protects the printed matter, but also changes the appearance and visual effects of the printed matter and promotes the sale of goods. In the context of increasing consumer demand for printed matter or packaging, glazing of printed matter has become an important means of value-added printed matter and the promotion of modern goods.
The characteristics of glazing and the main factors affecting the quality of packaging glazing
Varnishing is to apply a colorless and transparent coating on the surface of the printed matter that has been printed to obtain a bright film layer. When glazing, the leveling of the coating on the surface of the printed product enhances the smoothness of the printed surface, not only improves the surface gloss of the printed product, but also improves the surface performance of the printed product, enhances the light resistance of the ink, increases the heat resistance of the ink layer and Moisture resistance has played a role in protecting printed graphics and beautifying products. It has been widely used in book cover processing and packaging and decoration, picture books, large decoration, posters and other printed surface processing.
There are many methods of coating, according to the coating can be divided into oxidative polymer coating coating, solvent volatile coating coating, thermal curing coating coating, photocurable coating coating and so on. According to the coating method, it can be divided into spray coating, printing coating and special coating machine coating. According to the glazing equipment, it can be divided into glazing machine and printing glazing linkage. In addition, according to the glazed products, it can be divided into whole surface glazing, partial glazing, extinction, and artistic glazing.
No matter what technical methods and materials are used for glazing, they require excellent glazing quality to achieve the intended purpose. From the material point of view, the main factors affecting the glazing quality are printing paper, printing ink and glazing paint with good glazing adaptability.
(1) Printing paper. Printing paper has the greatest influence on the glazing coating is the smoothness and surface absorption of the paper, they affect the leveling of the coating, and affect the coating to form a high smoothness bright film on the surface of the printed product. The smoothness of the paper is high, the surface of the paper is in good contact with the coating layer, and the coating is easy to level, thereby forming a high smoothness film surface. The surface of the paper is highly absorbent, and when the paint is leveled, the paper fibers absorb the paint too quickly, which makes the paint flow poor and difficult to level. On the contrary, the absorption of paper surface is weak, the paper fiber absorbs paint too slowly, the coating is not easy to congeal and solidify during leveling, and it is difficult to form a bright film with high smoothness. Therefore, different papers should choose the matching paint, adjust the process conditions such as glaze coating leveling time and drying temperature.
(2) Printing ink. The particle size of the ink affects the wetting effect of the ink layer and thus the leveling and brightness of the coating. If the ink particles are large, the surface of the printed product must be rough, not easy to wet, and not easy to level.
(3) Glazing paint. Glazing paint is also the main factor affecting the quality of glazing. The glazing paint should not only have the properties of colorless, odorless, non-toxic, strong gloss, chemical resistance, fast drying, etc., but also have good transparency, not easy to change color, and have a comparative Good flexibility, heat resistance, leveling and adhesion. The most important effect of glazing coatings on glazing quality is its viscosity, followed by surface tension and solvent volatility. The solvent in the coating will be less and less due to volatilization, penetration, diffusion and other effects, and the viscosity will gradually increase. The driving force for leveling is the surface tension of the coating. The surface tension flattens the traced surface into a smooth surface in the process of making the coating shrink to the smallest shape of the surface. Therefore, the surface tension of the paint has a greater effect on leveling.
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Problems to be noted in the use of the two main varnishes
The glazing paint can be divided into oxidative polymerization type (mainly depends on the polymerization reaction with oxygen to dry to form a film), solvent volatile type (mainly depends on the solvent in the paint to evaporate and dry to form a film, its leveling is good, suitable for various High-quality large-scale printed products are glazed), thermosetting type (mainly relying on the reaction of resin and catalyst when heated to dry to form a film), photocuring type (through light irradiation to the coating molecules to polymerize to dry the film, the coating has good gloss and the film layer is resistant Grinding, folding resistance, heat resistance) and so on.
At present, UV glazing oil and water-based glazing oil are more ideal and have better development prospects.
1.Using technology of UV varnish and troubleshooting of some UV varnish mainly consists of photoinitiator, prepolymer (oligomer), monomer (active diluent), auxiliary agent (leveling agent, plasticizer) Agent, antioxidant) and other components. Under ultraviolet irradiation of appropriate wavelength and light intensity, the photoinitiator is excited to generate free radicals, which initiates the polymerization reaction between the unsaturated groups (usually acryloyl groups) on the prepolymer and the monomer, and eventually changes rapidly from liquid to liquid. Solid, generally only 0.1 to 0.3 seconds.
The UV glazing of printed products mainly has two processes: coating UV oil and curing. In fact, printing itself is the process of applying ink to the substrate, and UV varnish can also be applied to the substrate by printing, but the surface of the substrate is required to be dense and not opaque because of the viscosity of the varnish Much lower than ink. Coated paper, whiteboard paper, gold cardboard, plastic film, etc. can be coated with UV oil.
UV varnish is applied to the substrate. After a period of time, the varnish flows into the ultraviolet radiation area and solidifies. The curing process is a chemical cross-linking process, that is, the unsaturated carbon-carbon double bond (liquid) in the prepolymer and monomer receives the energy transferred by the photoinitiator to cross-link and polymerize into molecules (solids) in a network structure. But the curing changes with the thickness, time and UV radiation intensity of the UV oil. The relationship between radiant energy transfer and the thickness of the UV oil is exponentially decreasing. About 90% of the UV energy is absorbed within 1 micron of the uppermost layer, and only 9% can be obtained within 2 microns of the lowermost layer. If you want to get the same energy within 2 microns of the lower layer, the initial UV irradiation energy should be increased by 10 times. Generally, the thickness of the coating layer is only 3 to 5 microns, and the upper and lower varnishes are cured almost simultaneously. However, when the coating is thick (such as glazing with a wire mesh machine or roll coater), the upper layer will be cured, and the lower layer will not be cured.
The curing changes with time. The initiation time of a large amount of photoinitiator is generally 0.1 to 0.3 seconds, but a part of the remaining (incompletely reacted) photoinitiator will continue to react for 6 to 24 hours, so-called post-polymerization. If the light is too much at the beginning, the UV oil will become more and more brittle due to the post-polymerization reaction, so the exposure should not be too large when UV glazing.
When using UV curing equipment, generally use a lamp and adjust the distance between the tube and the object to be illuminated (generally 5 to 15cm). If a lamp cannot be cured, a curing lamp needs to be added to enhance the exposure. The curing changes with the UV radiation intensity. When the UV intensity per unit area increases, the curing rate will accelerate, but it is not a linear relationship but increases several times. UV glazing oil is very suitable for glazing high-end printed products.
When using a UV lamp, the high temperature above 800 ℃ around the lamp tube is conducive to the vaporization and conduction of mercury molecules, but the object needs to be cooled. Therefore, the design of lamps and cooling devices should be fully considered. When using a focus lampshade, do not place the subject in focus to avoid overexposure.
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At present, there are two main ways of using UV coating
â‘ Three-roller type (ie ink roller, coating roller, pressure roller). The advantage of this method is that it is fast and adjustable; the disadvantage is that it is not suitable for coating on thin paper. Although some glazing machines have air knives, the glazing effect on thin paper is not good, and the oil layer is thick. Therefore, this method is more suitable for cardboard.
â‘¡ Gripper type (that is, the impression cylinder has gripper teeth). The advantage of this method is that both thin and thick paper are suitable, fuel saving, and the thickness of the oil layer is easy to adjust, because the diameter of the impression cylinder is large and the pressure is large; the disadvantage is that the speed is not easy to adjust. When using UV varnish in packaging and printing, due to the influence of technology, materials, equipment, environment and other factors, there will often be some failures. In summary, there are mainly the following problems:
(1) UV varnish is not compatible with ink, and has stripes and orange peel. The specific performance is that the UV oil is coated on a certain ink base and beads, which is like orange peel after curing, or although it can be coated evenly without foaming, but if it is scraped with a sharp tool, the UV oil layer will separate from the ink layer. The main reason is that the viscosity of the UV varnish is too high; the anilox roller screen is too thick (the coating amount is too large), the surface is not smooth; the coating pressure is uneven; the leveling of the UV varnish is poor.
The solution is to first apply a layer of primer, and then apply UV oil, or apply UV oil after treatment with electric spark, and mix in an appropriate amount of alcoholic infrared varnish.
(2) The adhesion of UV varnish is not good, the oil can not be coated or flowered. The main reason is that the viscosity of UV varnish is too small, the coating is too thin; the content of ink or dry oil in the ink is too high; the surface of the printing ink is crystallized; the additives in the printing ink are not suitable; the anti-sticking material on the ink surface (silicone oil) , Dusting) too much; the anilox roller wire is too thin; light curing conditions are not suitable. The solution is to reduce the viscosity of the UV varnish and reduce the amount of coating; adjust the pressure evenly; the coating roller should be ground and polished; add a bright leveling agent. Corresponding measures should be taken according to the glazing conditions when printing; UV varnish can be applied thicker; if necessary, apply primer or special varnish; apply primer to enhance adhesion on the printed products.
(3) The gloss and brightness after UV glazing are not ideal. The main reason is insufficient oil supply; the surface of the paper is not bright enough or the surface is rough and not dense, and the absorption is too strong; the viscosity of the UV varnish is too small, the coating is too thin; the non-reactive solvent such as ethanol is too diluted; the UV oil is unevenly coated . The solution is to prime the oil first. In addition, UV oil expired or UV oil quality problems will also make it not bright after glazing. According to the different paper, increase the viscosity and coating amount of UV varnish appropriately. The paper with strong penetration and absorption can be coated with a layer of primer or replaced with a smooth paper.
(4) The UV oil layer is brittle, and the creases are brittle when folded. Polyurethane oil can be used instead, indentation should be made when folding, and thin paper should be folded by hand instead of bamboo. This fault will also occur when the UV oil layer is too thick, and the amount of oil can be adjusted.
(5) Alcoholic UV oil is easily dissolved by ethanol after curing. If the wine box is filled with UV oil, if the wine bottle breaks during transportation, the wine will dissolve the UV oil layer. The solution is to use UV oil that is insoluble in alcohol.
(6) Poor drying, incomplete curing, and sticky surface. The main reason is that the UV intensity is not enough; the UV lamp is aging and weakened; the UV varnish storage time is too long; the diluent that does not participate in the reaction is added too much; the machine speed is too fast. Solution: In the case of curing speed requirements less than 0.5s, generally should ensure that the power of high-pressure mercury lamp is not less than 120W / cm; the lamp tube should be updated in time: if necessary, add a certain amount of UV varnish curing accelerator to accelerate drying.
(7) Too thick UV oil coating is neither economical nor brittle. The solution is to heat the UV oil to 50-55 ° C or add a thinner to the UV oil.
(8) The UV oil bursts automatically after curing. The reason is that the UV light is excessive and the surface temperature of the illuminated object is too high. The solution is to increase the distance between the lamp tube and the surface of the illuminated object so that the illuminated object is far away from the focus of the lampshade. After solidification, it cools quickly. It can be cooled with cold air or cold roller or with air-cooled or water-cooled light source.
(9) Oily spots are produced by glazing. The main reason is that the thickness of the oil consumption is not enough, the calendering temperature is not enough, the calendering plate is not suitable, and the quality of the paper is inconsistent. The appropriate materials should be adjusted or replaced according to the specific situation.
(10) The glaze coating has white spots and pinholes. The main reason is that the coating is too thin; the anilox roller is too thin; the non-reactive thinner (such as ethanol) is added excessively; the surface of the printed matter is dusty. The production environment and the surface of the printed matter should be kept clean; the thickness of the coating should be increased; a small amount of smoothing aid should be added: the thinner is preferably an active thinner participating in the reaction.
(11) The residual odor is large. The main reason is that the UV varnish is not cured completely; the ultraviolet ray is insufficient or the lamp tube is aging; the UV varnish has poor anti-oxidation ability; the non-reactive diluent is added too much in the UV varnish. The UV varnish should be thoroughly cured, strengthen the ventilation, and replace the UV varnish if necessary.
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2. Issues to be noted in the application of water-based varnish
Water-based glazing oil is also called dispersing glazing agent, which is colorless, odorless, transparent and non-toxic, free of organic volatiles (VOC), low cost, wide source of materials, good gloss, folding resistance, Features such as abrasion resistance and chemical resistance.
Because the water-based glazing oil uses water as a diluent and uses a small amount of volatile ethanol as an auxiliary solution, it can eliminate the harm to the human body and the pollution to the environment during the operation, which is conducive to safe production, improves the working conditions of employees, and meets the requirements of environmental protection , Especially suitable for food, cigarettes, medicines and other packaging.
Aqueous varnish is mainly composed of three main categories: main agent, solvent and auxiliary agent. The main agent of the water-based glazing oil is the film-forming resin, which is the film-forming substance of the glazing agent, usually a synthetic resin, which affects and controls various physical properties of the deep layer and the glazing quality of the film layer, such as gloss, adhesion Sex, dryness, etc. There are many varieties of water-based film-forming resins, but the most commonly used at home and abroad are acrylic resin copolymer resins. The performance of the waterborne acrylic copolymer resin depends on the composition of the monomer, the synthesis of the monomer and the synthesis process. Water-based acrylic copolymer resin has a series of excellent properties such as good film forming, good gloss, high transparency, rapid drying, abrasion resistance, water resistance, etc., and is widely used in water-based glazing agents and water-based glazing coatings.
Adjuvants are used to improve the physical and chemical properties and processing characteristics of water-based glazing agents. The auxiliary agents mainly include the following
①Curing agent: improve the film-forming property of the water-based main agent and increase the cohesive strength of the film layer. ② Surfactant: reduce the surface tension of the aqueous solvent and improve the leveling. ③ Antifoaming agent: It can control the foaming of polishing agent for a long time and eliminate the quality defects such as fish eyes and pinholes. ④Drying agent: increase the drying speed of water-based polishing agent and improve the suitability of paper printing. ⑤ Adhesion promoter: Improve the adhesion of film-forming materials and substrates. ⑥ Wetting and dispersing agent: improve the dispersion of the main agent, prevent sticking and improve wear resistance. ⑦Other additives: plasticizers that can improve folding resistance.
There are many types of adjuvants, and there are different usages depending on the type of polish. Such as infrared water-based glazing oil, matt glazing oil, etc. are different types of glazing oil, so the composition of the auxiliary agent is also different. However, it should be noted that the amount of various additives should not exceed 5% of the total amount, otherwise it will affect the processing suitability of the polish.
The main role of the solvent is to disperse or dissolve synthetic resin and various additives. The solvent of water-based varnish is mainly water. It is colorless, odorless, non-toxic, wide source and low price. The volatility of water is almost zero, and the leveling performance is very good. However, the solvent of the water-based polishing agent also has shortcomings. For example, the drying speed is slow, which may cause process failures such as unstable product size. Therefore, ethanol is often added in use to improve the drying performance of the aqueous solvent and improve the processing suitability of the aqueous varnish. In addition, because UV varnish is ideal for drying, etc., if it is used in water-based varnish, it can complement each other. According to this, glazing oil using a new drying method has been developed. Ultraviolet glazing paint can form a bright film in a few seconds, and there is a glazing paint that is dried by mixing ultraviolet and infrared rays.
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Compared with solvent-based varnish, the advantages of water-based varnish are
(1) Strong transparency and good preservation. It can be stored for a long time on the printed matter coated on it. Under long-term strong sunlight, the water-based varnish is not easy to yellow and does not change color.
(2) Good wear resistance. Packaging prints require strong abrasion resistance, especially cigarette packs, which bear a large mechanical load during high-speed operation at speeds of up to 300 to 500 packs / minute. Therefore, the printed products are glazed to improve wear resistance, and water-based glazing oil can meet this requirement.
(3) The conjunctiva is fast. The drying process of water-based varnish is 30% by evaporation, 70% is absorbed by material permeation and absorption, drying quickly, which can be completed within a few seconds. When the varnish still contains 20% to 30% moisture, it can conjunctiva. Therefore, no drying method is required. If using infrared or hot air drying method, the drying speed is faster.
(4) Good heat sealing performance. At present, PP film is widely used as the outer packaging of the cigarette pack. The water-based varnish has good heat sealing performance. Even if celluloid is used, good heat sealing effect can be obtained.
(5) The polished finished product has good flatness and strong anti-rolling force.
A headache in gravure printing is the continuous drying of the substrate after several color groups, the paper has a very low moisture content, and is particularly prone to curling. Apply water-based varnish to glaze the soft package. The product is cut into large pieces, and then it is gilded and slitted to maintain good flatness and roll resistance. This provides a new way for future offset printing to change to gravure printing, especially for cigarette labels that require bronzing.
In the application of water-based glazing, factors such as glazing method, paper type, and product quality requirements should be considered comprehensively.
(1) Viscosity control: During the coating process, the viscosity and solid content of the varnish must be reasonably controlled. Therefore, dilution can only be carried out within a certain solid content range. The thinner of water-based varnish is generally a 1: 1 mixture of ethanol and water, which has obvious viscosity reduction effect.
(2) Select the paper type. Thick paper has good dimensional stability and thin paper has poor dimensional stability. Dimensional stability is one of the key indicators of printing quality, and it is closely related to the amount of water used in printing, the time and method of moisture adjustment and drying of paper, and the control of viscosity. If the printed product is paper under 90g / m2, be careful with water-based varnish.
(3) Control of coating amount. The coating amount of water-based varnish is not easy to control. Because ethanol is easily volatilized during coating, and water is colorless and transparent, it is difficult to distinguish the amount of coating with the naked eye when applying an aqueous liquid on white paper, so the control of viscosity is crucial. The content of ethanol should also be increased in time to ensure the stability of the product.
(4) Use of anti-sticking agent. When using water-based varnish, you can completely eliminate or reduce the use of anti-sticking agent, because the water-based varnish is completely cured and rarely sticks. After glazing with a printing press (wet wet printing), the paper can be punched and creasing after 2 hours.
(5) Control of drying time. Solvent-based glazing oils are almost all volatile and drying, while the permeation drying of water-based glazing oils has great use value. However, in multi-color printing, if the last color is coated with varnish, the drying time will be slower because the paper has absorbed more ink and dampening fluid inside. Infrared water-based glazing oil can be used for drying with the aid of infrared drying device. When drying, the glazing oil is heated, the bottom layer quickly penetrates to dry, and at the same time, the moisture in the glazing agent is heated and evaporated by high temperature, and is discharged through the exhaust pipe.
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