Guarantee of glazing quality in packaging printing
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In packaging printing, with the continuous improvement of printing technology and equipment, as well as the increasingly fierce competition of products, people pay more and more attention to the appearance requirements of packaging printed matter, and constantly introduce new and unique packaging methods and processing techniques for the surface of packaging printed matter. Among them, glazing technology has been widely used, making commodity packaging a new height. Polishing not only protects the printed matter, but also changes the appearance and visual effect of the printed matter, and promotes the sale of goods. In the case of consumers' increasingly demanding prints or packaging, printed glazing has become an important means of value-added printing and modern merchandise promotion.
The characteristics of glazing and the main factors affecting the quality of light on the package
Glazing is to apply a layer of colorless and transparent paint on the printed surface of the printed matter to obtain a bright film layer. When glazing, due to the leveling of the paint on the surface of the printed product, the smoothness of the surface of the printed product is enhanced, which not only improves the surface gloss of the printed matter, but also improves the surface properties of the printed matter, enhances the light resistance of the ink, and increases the heat resistance of the ink layer. The moisture-proof ability plays a role in protecting printed graphics and beautifying products. It has been widely used in the surface processing of book cover processing and packaging decoration, albums, large decorations, posters and other printed materials.
There are many methods for glazing. According to the glazing coating, it can be divided into oxidative polymerization coating glazing, solvent evaporation coating glazing, thermosetting coating glazing, and photocuring coating glazing. According to the glazing coating method, it can be divided into spray coating glazing, printing coating glazing and special glazing coating glazing. If you press the glazing device, you can divide it into a glazing machine and a printing splicer. In addition, the products after glazing can be divided into whole surface light, partial glazing, extinction, and artistic glazing.
Regardless of the technical method and material glazing, excellent glazing quality is required to achieve the intended purpose. From the material point of view, the factors affecting the quality of glazing mainly include good glazing adaptability between printing paper, printing ink and glazing coating.
(1) Printing paper. The most influential effect of printing paper on glazing coating is the smoothness and surface absorbency of the paper, which affect the leveling of the coating and affect the coating to form a highly smooth bright film on the surface of the printing. 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 highly smooth film surface. The surface of the paper is highly absorbent. When the paint is leveled, the paper fibers absorb the paint too quickly, making the paint flowability worse and difficult to level. On the contrary, the surface of the paper is weakly absorbed, the paper fiber absorbs the paint too slowly, the coating in the leveling is not easy to form a film, and it is difficult to form a high-smooth bright film. Therefore, different papers should be selected to match the coating, and the process conditions such as varnish coating leveling time and drying temperature should be adjusted.
(2) Printing ink. The particle size of the ink affects the wetting of the ink layer and thus affects 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 difficult to level.
(3) glazing coating. Glazing coating is also the main factor affecting the quality of glazing. Glazing coatings must not only have colorless, odorless, non-toxic, strong gloss, chemical resistance, fast drying, etc., but also good transparency, not easy to change color, Good flexibility, heat resistance, leveling, and adhesion. The most important effect of varnish on the quality of glazing 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, etc., and the viscosity will gradually increase. The power generated by the leveling is the surface tension of the coating, and the surface tension flattens the surface with traces into a smooth surface as much as possible to shrink the coating into the smallest shape of the surface. Therefore, the surface tension of the coating has a large influence on the leveling.
Two main glazing oils to pay attention to
The glazing coating can be mainly divided into oxidative polymerization type (mainly relying on polymerization with oxygen to form a film), solvent evaporation type (mainly relying on solvent evaporation in the coating to form a film, its leveling property is good, suitable for various Grade high-volume printing glazing), heat curing type (mainly relying on resin and catalyst to react with heat to form a film), photocuring type (drying conjunctiva by polymerization of light-emitting coating molecules, good coating gloss, film resistance Four kinds of grinding, folding, heat resistance, etc.
At present, UV varnish and water-based varnish are better than ideal and development prospects.
1. UV varnish technology and some troubleshooting UV varnish is mainly composed of photoinitiator, prepolymer (oligomer), monomer (reactive diluent), auxiliaries (leveling agent, plasticizing) Composition, antioxidants, etc. At the appropriate wavelength and intensity of ultraviolet light, the photoinitiator is excited to generate free radicals, which initiate polymerization of the unsaturated groups (usually acryloyl groups) on the prepolymer and monomer, and eventually change from liquid to liquid. Solid, generally only 0.1 to 0.3 seconds.
Print UV glazing mainly has two processes of coating UV oil and curing. In fact, printing itself is the process of coating the ink on 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, not permeable, because of the viscosity of the varnish. It is much lower than the ink. Coated paper, whiteboard paper, gold card paper, plastic film, etc. can be coated with UV oil.
UV varnish is applied to the substrate, and the varnish is leveled into the ultraviolet radiation zone for a period of time to solidify. The curing process is a chemical crosslinking process, that is, the prepolymer, the unsaturated carbon-carbon double bond (liquid) in the monomer, and the molecules (solids) which are cross-linked and polymerized into a network structure by the energy transmitted by the photoinitiator. However, the curing varies with the thickness of the UV oil, the time, and the intensity of the UV radiation. The relationship between radiant energy transfer and UV oil thickness is exponentially decreasing. About 90% of the UV energy is absorbed in the uppermost layer of 1 micron, and only 9% is obtained in the lower layer of 2 microns. If the same energy is also obtained within 2 microns of the lower layer, the initial UV irradiation energy is increased by a factor of 10. Generally, the thickness of the coating layer is only 3 to 5 micrometers, and the upper and lower varnishes are hardened at the same time. However, when the coating is thick (such as with a wire machine or a roller coater), the upper layer will be cured and the lower layer will be uncured.
Curing varies with time, and a large amount of photoinitiator initiation time is typically from 0.1 to 0.3 seconds, but a portion of the remaining (under-reacted) photoinitiator will continue to react for 6 to 24 hours, the so-called post-polymerization. If the light is excessive at the beginning, the UV oil will become more and more brittle due to the post-polymerization reaction, so the exposure amount during UV glazing should not be too large.
When using UV curing equipment, generally use a lamp to light, and adjust the distance between the lamp and the object (usually 5 ~ 15cm), if a lamp can not be cured, you need to add a curing lamp to enhance the exposure. Curing varies with the intensity of UV radiation. When the UV intensity per unit area increases, the curing rate will increase, but it is not a linear relationship but is increased several times. UV varnish is ideal for glazing of high-end prints.
When using a UV lamp, it is advantageous to keep the mercury molecules vaporized and conductive because the temperature around 800 ° C is maintained around the lamp tube, but the object to be heated needs to be cooled. Therefore, the design of the luminaire and the cooling device should be considered in full. When using a focusing type lampshade, do not place the object in focus to avoid overexposure.
Currently, there are two main ways to use UV glazing.
1 three-roll type (ie, ink roll, coating roll, pressure roll). The advantage of this method is that it is fast and adjustable; the disadvantage is that it is not suitable for glazing on thin paper. Although some glazing machines have air knives, the glazing effect on thin paper is poor and the oil layer is thick. Therefore, this method is more suitable for board paper.
2 type (ie impression cylinder with fangs). The advantage of this method is that thin and thick paper are suitable, fuel-saving, and the thickness of the oil layer is easy to adjust, because the impression cylinder has a large diameter and a large pressure; the disadvantage is that the speed is not easy to adjust. When using UV coating oil in packaging printing, due to the influence of technology, materials, equipment, environment and other factors, there are often some failures. In summary, there are mainly the following problems:
(1) UV varnish is incompatible with ink, with streaks and orange peel. The specific performance is that the UV oil is coated on a certain ink bottom to be beaded, and after curing, it is like orange peel, or although it can be uniformly coated without foaming, if it is scraped with a sharp object, the UV oil layer will be separated from the ink layer. The main reason is that the viscosity of the UV varnish is too high; the mesh of the rubberized anilox roll is too thick (the coating amount is too large), the surface is not smooth; the coating pressure is not uniform; the leveling property of the UV varnish is poor.
The solution is to first apply a layer of base oil, then apply UV oil, or use an electric spark to treat the UV oil, and mix the alcoholic infrared varnish.
(2) The adhesion of UV varnish is not good, and the oil is not coated or scented. The main reason is that the viscosity of UV varnish is too small, the coating is too thin; the content of varnish or dry oil in the ink is too high; the surface of the printed ink is crystallized; the auxiliaries in the printing ink are not suitable; the surface of the ink is anti-adhesive material (silicone oil) Too much dusting; the rubberized anilox roller mesh is too thin; the photocuring conditions are not suitable. The solution is to reduce the viscosity of the UV varnish, reduce the coating amount; 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 during printing; UV varnish can be appropriately thickened; if necessary, the base oil or special varnish should be used; and the adhered oil should be coated on the printed product.
(3) The gloss and brightness after UV glazing are not ideal. The main reason is that the oil supply is insufficient; the surface brightness of the paper is not enough or the surface is not dense, 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 excessively diluted; the coating of the UV oil is uneven. . The solution is also to make the base oil first. In addition, if the UV oil expires or the UV oil quality is defective, it will not light up after glazing. Appropriately increase the viscosity and coating amount of UV varnish according to the difference of paper. The paper with strong osmosis can be coated with a primer or a smooth paper.
(4) The UV oil layer is brittle, and the folds are brittle when folding. Polyurethane oil can be used instead, and the folds should be indented first. The thin paper should not be folded by hand. This fault can also occur when the UV oil layer is too thick, and the oil can be adjusted.
(5) The alcoholic UV oil is easily dissolved by ethanol after curing. If the UV oil is on the wine box, if the 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) The drying is not good, the curing is not thorough, and the surface is sticky. The main reason is that the UV intensity is not enough; the UV lamp is aging and the strength is weak; 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: When the curing speed requirement is less than 0.5s, the power of the high pressure mercury lamp should generally be guaranteed to be no less than 120W/cm; the lamp should be updated in time: if necessary, add a certain amount of UV varnish curing accelerator to accelerate drying.
(7) It is neither economical nor too economical to cause brittle cracking. The solution is to heat the UV oil to 50-55 ° C or add a thinner to the UV oil.
(8) The UV oil will automatically burst after curing. The reason is that the UV light is excessive and the surface temperature of the object is too high. The solution is to increase the distance between the lamp and the surface of the object to be illuminated, to keep the object away from the focus of the lampshade, and to cool it quickly after solidification. The cold or cold roller can be used or the air-cooled or water-cooled light source can be used.
(9) Polishing produces oil spots. The main reason is that the thickness of the oil is not enough, the calendering temperature is not enough, the calender is not suitable, and the paper quality is inconsistent. The appropriate material should be adjusted or replaced for specific situations.
(10) The glazing coating has white spots and pinholes. The main reason is that the coating is too thin; the rubberized anilox roll is too fine; the non-reactive diluent (such as ethanol) is added in excess; the surface of the print is more dusty. The production environment and the surface of the printed matter should be kept clean; increase the thickness of the coating; add a small amount of smoothing aid: the diluent is preferably a reactive diluent that participates in the reaction.
(11) The residual odor is large. The main reason is that the UV varnish is not completely cured; the UV is insufficient or the lamp is aging; the UV varnish has poor oxidation resistance; the non-reactive diluent in the UV varnish is added too much. The UV varnish should be completely cured, ventilated, and replaced with UV varnish if necessary.
Problems to be paid attention to in the application of water-based varnish
Water-based varnish, also known as dispersing glazing agent, has colorless, odorless, strong transparency, non-toxic, no organic volatiles (VOC), low cost, wide material source, good gloss, folding resistance, Wear resistance and chemical resistance.
Since water-based varnish uses water as a diluent to use a small amount of volatile ethanol as an auxiliary solution, it can eliminate harm to the human body and pollution to the environment during operation, is conducive to safe production, improves labor conditions of employees, and meets environmental protection requirements. It is especially suitable for packaging of food, cigarettes, medicines, etc.
The aqueous glazing oil is mainly composed of three main categories: main agent, solvent and auxiliary agent. The main agent of water-based varnish is a film-forming resin, which is a film-forming substance of a glazing agent, usually a synthetic resin, which affects and governs various physical properties of the deep layer and the glazing quality of the film layer, such as gloss and adhesion. Sex, dryness, etc. There are many varieties of water-based film-forming resins, but copolymers of acrylic systems are commonly used at home and abroad. The properties of the aqueous acrylic copolymer resin depend on the composition of the monomer, the synthesis of the monomer, and the synthesis process. The water-based acrylic copolymer resin has a series of excellent properties such as good film forming property, good gloss, high transparency, rapid drying, abrasion resistance and water resistance, and is widely used in water-based glazing agents and water-based glazing coatings.
The auxiliary agent is used to improve the physical and chemical properties and processing characteristics of the aqueous polishing agent, and the auxiliary agent mainly includes the following
1 curing agent: improving the film forming property of the aqueous main agent and increasing the cohesive strength of the film layer.
2 Surfactant: reduce the surface tension of the aqueous solvent and improve leveling.
3 antifoaming agent: It can control the foaming of the polishing agent for a long time and eliminate the quality defects such as fish eyes and pinholes.
4 desiccant: increase the drying speed of the waterborne coating agent, improve the printability of the paper.
5 adhesion promoter: improve the adhesion of film-forming substances to substrates.
6 Wetting and dispersing agent: Improve the dispersibility of the main agent, prevent sticking and improve wear resistance.
7 Other additives: plasticizers that improve the folding resistance.
There are many types of adjuvants, and there are different uses depending on the type of the polish. For example, infrared water glazing oil, matt glazing oil, etc. are different kinds of glazing oil, so the composition of the auxiliary agent is also different. However, it should be noted that the use of various additives should not exceed 5% of the total amount, otherwise it will affect the processing suitability of the polish.
The main function of the solvent is to disperse or dissolve the synthetic resin and various auxiliary agents. The solvent for aqueous glazing oil is mainly water. 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 aqueous glazing agent also has disadvantages, such as a slow drying speed, and it is easy to cause process failures such as unstable product dimensions. Therefore, ethanol is often added during use to improve the drying performance of the aqueous solvent and to improve the processing suitability of the aqueous varnish. In addition, since UV varnish is ideal for drying and the like, it can be complemented by reference to water-based varnish. Accordingly, glazing oils using new drying methods have been developed. Ultraviolet glazing coatings form a bright film in seconds, as well as a glazing coating that is dried with UV and IR.
Compared with solvent-based varnishes, the advantages of water-based varnishes are:
(1) Strong transparency and good preservation. It can be stored for a long time on the printed matter it is coated on. Under the long-term strong sunlight, the aqueous varnish is not easy to yellow and does not change color.
(2) Good wear resistance. Packaging-type prints require a high degree of wear resistance, especially for cigarette packs, which are subject to large mechanical loads during high-speed operation at speeds of up to 300-500 packs/min. Therefore, the printed matter is glazed to improve the abrasion resistance, and the aqueous varnish can satisfy this requirement.
(3) The conjunctival speed is fast. The drying process of water-based glazing oil is 30% by evaporation, 70% by material absorption, and drying quickly, in about a few seconds. Conjunctiva is formed when the glazing oil still contains 20% to 30% moisture. Therefore, no drying method is required. If you use infrared or hot air drying, the drying speed is faster.
(4) The heat sealing performance is good. At present, the PP film is widely used as an outer package, and the water-based varnish has good heat-sealing performance, and even if the celluloid is used, a good heat-sealing effect can be obtained.
(5) The glazing finished product has good flatness and strong anti-rolling force.
One headache in gravure printing is the continuous drying of the substrate through several color groups, which have extremely low moisture content and are particularly susceptible to curling. The water-based varnish is applied to the soft bag, and the product is cut into large sheets, and then the gold is cut and cut, and the flatness and the curl resistance are maintained. This will provide a new way for the future offset printing to change the gravure, especially the need for hot stamping.
The application of water-based glazing oil should take into account factors such as glazing mode, paper type, product quality requirements, etc., and pay attention to the following aspects when using water-based varnish.
(1) Viscosity control: During the coating process, the viscosity and solid content of the varnish must be properly controlled. Therefore, the dilution can only be carried out within a certain solid content range. The thinner of the aqueous glazing oil generally adopts a 1:1 mixture of ethanol and water, and the viscosity reducing effect is obvious.
(2) Select the paper type. Thick paper has good dimensional stability and poor dimensional stability of tissue. Dimensional stability is one of the key indicators of print quality, which is closely related to the amount of water used in printing, the humidity control of the paper, the time and manner of drying, and the control of viscosity. If the printed product is 90g/m2 or less, be careful with water-based varnish.
(3) Control of the amount of coating. The coating amount of the aqueous glazing oil is not easily controlled. Since ethanol is easily volatilized during coating, and water is colorless and transparent, it is difficult to discriminate the amount of coating by coating the 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. The use of anti-sticking agents can be completely eliminated or reduced when using water-based coatings because the aqueous coatings cure completely and are less sticky. After the glazing of the printing machine (wet wet printing), the paper can be punched and indented after 2 hours.
(5) Control of drying time. Solvent-based glazing oils are almost all volatilized and dried, and the osmotic drying of water-based glazing oil has great value. However, in multi-color printing, if the last color is coated with varnish, the drying time will be slower because the inside of the paper has absorbed more ink and dampening fluid. Infrared aqueous varnish can be used and dried by means of an infrared drying device. When drying, the glazing oil is heated, and the bottom layer is quickly permeated and dried, and the moisture in the glazing agent is heated and evaporated by the high temperature, and is discharged through the exhaust pipe.