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The glazing process is a technique and method of coating a layer of transparent coating on the surface of the printed product, after leveling, drying, and calendering, to form a thin and uniform transparent bright layer on the surface of the printed product.
Varnishing process and classification
The glazing process of printed matter generally includes two operations of coating and calendering of glazing paint. The coating of glazing paint is the process of applying a layer of glazing paint evenly on the surface of the printed matter in a certain way. There are three common coating methods: spray coating, printing coating and glaze coating machine coating.
Spray coating is a manual operation. It has a slow working speed and poor coating quality, but it is easy to operate and has strong flexibility. It is suitable for the coating of small batches and rough surfaces.
Printing and coating is usually used as a coating for glazing paint after being modified by printing equipment. This method has better quality and higher production efficiency than manual spray coating. It can use existing printing equipment, one machine and two uses, it is simple and easy to use, suitable for glazing coating processing in general medium and small printing plants. However, solvent-based varnishes should generally be used. During operation, care should be taken to ensure that the viscosity of the paint is relatively stable, and a certain amount of coating should be controlled.
Special glaze coating machine coating can achieve accurate control of coating amount. The coating quality is stable and reliable, and it is suitable for the glazing coating processing of various grades of prints. It is currently the most commonly used coating method.
The quality requirements for glaze coating are: uniform coating on the coated surface, appropriate coating amount, good leveling, and a certain adhesion to the surface of the printed matter. After the glazing coating is applied on the surface of the printed matter, it usually needs to be calendered by a calender. The calendering of the calender can change the surface state of the dried glaze coating to form an ideal mirror surface.
The glazing process can be comprehensively classified in different ways: according to the glazing mode, it can be divided into offline glazing process and online glazing process; according to the glazing coating, it can be divided into oxidative polymerization coating glazing, solvent volatile coating glazing, and glazing Curing coating glazing and thermal curing coating glazing; according to the product can be divided into full cap glazing, partial glazing, extinction and artistic glazing; according to the printed material input method can be divided into manual paper feeding and automatic paper feeding.
Varnish
Various types of glazing coatings currently developed have their own process advantages; oxidative polymerization glazing coatings mainly rely on oxygen in the air to polymerize and dry to form a film. The requirements for drying sources are not high, and the equipment investment is small; the solvent Volatile glazing paint relies on the solvent in the paint to evaporate and dry to form a film. It has good leveling in the process of coating, drying and film formation. Its processing performance and wide application range are suitable for various grades and large quantities of printed products. Glazing processing; heat-curing glazing coatings rely on the active functional groups contained in the polymer structure of the film-forming resin and the catalysts in the coating. Crosslinking reaction occurs in the presence of heat to dry the film, fast curing, high production efficiency, suitable for automation Glazing processing; photocurable glazing coating is to absorb the radiation light energy, the internal structure of the coating molecule undergoes polymerization reaction and dried to form a film, the gloss of the glazing coating is high, the wear resistance and folding resistance of the film layer , Heat resistance is relatively good, suitable for glazing processing of high-end prints.
The basic composition of various types of glazing paints is roughly the same, that is, they are composed of a main agent (film-forming tree finger), an auxiliary agent and a solvent. The main agent is the film-forming substance of the glazing paint. After the printed product is polished, the quality and physical and chemical properties of the film layer, such as gloss, folding resistance, and post-processing suitability, are all related to the selected main agent. The main agent is a natural resin glazing paint, which has poor film-forming transparency, is easily yellowed, and is prone to sticking back; the glazing paint with synthetic resin as the main agent has good film-forming properties, high gloss and transparency, and resistance. Grinding, water resistance, aging resistance, and strong applicability.
Additives are some auxiliary substances that need to be added in order to improve the physical and chemical properties and process characteristics of the glazing coating. For example, the curing agent added to improve the film-forming property of the main agent resin and increase the cohesive strength of the film layer; the surface active agent added to improve the leveling of the glazing coating and reduce its surface tension; Defoamer added during synthesis and coating operations; plasticizer added to improve film elasticity, enhance water resistance and folding resistance, etc.
The role of the solvent is to disperse, dissolve, dilute the main agent and auxiliary agents. Commonly used solvents include aromatics, esters, alcohols, etc. The toxic, odor, drying, leveling and other physical and chemical properties of the glazing coating are directly related to the choice of solvent. Aromatic solvents have low evaporation heat, fast volatilization speed, and high solubility, but these solvents are more toxic; ester solvents have good solubility, fast volatilization, and low cost, but have a large odor; alcohol solvents have The volatilization rate is not as good as the above two types, but it is non-toxic, odorless and pollution-free. If water can be used as a solvent for glazing paint, the cost is the lowest, the source is the widest, no harm to human body, and no pollution to the environment. Therefore, the development of water-based glazing coatings in recent years is attracting great attention at home and abroad. [next]
Process factors affecting glazing quality
1. Process factors affecting the quality of glazing coating. The essence of the glazing coating process is the process of glazing paint leveling and drying on the surface of the printed matter. The main influencing factors are the adaptability of the glazing of the printed matter, the types and properties of the glazing coating, the processing conditions of the coating process, etc.
(1) The glazing suitability of the printed matter refers to the effect of the paper and the graphic performance of the printed matter on the glazing coating. In the glazing coating, the glazing coating is easy to level on the paper surface with high smoothness. In the drying process, with the curing of the glazing paint, a smooth surface can be formed. Therefore, the higher the smoothness of the paper surface, the better the coating effect, and vice versa. The absorber on the surface of the paper is too strong, the absorption rate of the coating fiber by the paper fiber is high, and the solvent penetration is fast, which causes the viscosity value of the coating to increase, and the shear stress of the coating layer flowing on the surface of the printed product increases, which affects the leveling of the coating. It is difficult to form a smoother film layer; on the contrary, the absorption is too weak, which significantly reduces the penetration, solidification and film formation of the glazing paint in the leveling, and also cannot form a high-quality film layer on the surface of the printed matter.
The quality of the printed ink also directly affects the coating quality and leveling of the glazing paint. The particles of the ink are fine, and the dispersion is high. The graphic ink layer is easily wetted by the glazing paint. Under the action of the coating pressure, the leveling is good, and the formed film layer has high smoothness. On the contrary, the ink particles are coarse, the printing ink layer spreads poorly, and it is not easy to form a high-quality film layer during coating.
(2) Different types of glazing coatings have different performances. Even under the same process conditions, the conditions of the coatings obtained after coating and calendering are different. For example, the viscosity of glazing paint has an important influence on the leveling and wettability of the paint. The absorption rate of the glazing paint with paper of the same absorption intensity is inversely proportional to the viscosity value of the paint, that is, the smaller the viscosity value of the paint, the greater the absorption rate, which will cause the leveling to end prematurely, causing some large coatings on the surface of the printed matter to affect The smoothness and brightness of the film after drying and calendering.
Varnishes with different surface tension values ​​have different effects on wetting, adhesion and penetration of the same printed matter, and the film forming effect after coating and calendering varies greatly. A glazing paint with a low surface tension value can wet, attach, and soak the solid surface and graphic ink layer of various types of printed matter, "leveling into a smooth and uniform film surface; a glazing paint with a large surface tension value, right The wetting of the ink layer on the surface of the printed matter is restricted, and even the glazed coating will produce a certain amount of shrinkage and affect the film formation quality. The volatility of the solvent also affects. The volatilization rate is too fast, which will make the coating layer too flat to form a uniform film. In turn, it will cause insufficient drying of the glazing paint, the hardened conjunctiva is blocked, and the anti-stickiness is poor.
(3) The choice of coating process conditions also has a great influence on the coating quality. If the coating layout is too small, the coating cannot spread the entire surface to be coated evenly, and the smoothness after drying and calendering is poor; if the coating amount is too thick, it will affect the drying and increase the cost. In order to dry the thicker film layer, the temperature during coating and calendering should be relatively increased. The drying time should be lengthened, which will lead to a decrease in the moisture content of the printed product, the paper fiber becomes brittle, and the surface of the printed product is easily cracked.
Process conditions such as coating machine speed, drying time, and drying temperature also affect each other. When the machine speed is fast, the coating leveling time is short, and the coating is thick. To obtain the same drying effect, the drying time should be recognized and the temperature should be high; the machine speed is slow, the coating leveling time is long, and the coating is thin. The drying time can be shortened, and the drying temperature can be reduced appropriately. The above process factors sometimes have an influence on the quality of L-light coating with cross-linking properties. In the work, in order to obtain good L-light coating quality, these factors need to be comprehensively considered in order to obtain an appropriate match between the factors.
2. Technological factors affecting calendering quality. In calendering, the main factors affecting the calendering quality of the printed matter are the calendering temperature, pressure and machine speed.
During the calendering process, there should be a suitable temperature in each stage of hot pressing, glazing and cooling peeling to facilitate the secondary wetting, adhesion and penetration of the main agent molecules in the coating on the surface of the printed matter and enhance the contact effect between the two. Under certain temperature conditions, the plasticity of the coating film layer can be improved, and the smoothness of the calendered surface of the printed matter can be greatly improved under the action of pressure. If the temperature is too high, the adhesion strength of the coating layer will decrease, the deformation value will increase, and the moisture content of the printed matter will decrease sharply. This is detrimental to the light and peeling process; on the contrary, if the hot pressing temperature is too low, the coating layer cannot be completely plasticized. The second of the printed matter: insufficient subwetting, adhesion and penetration ability, poor adhesion of the coating layer, so the calendering effect is poor, and it is not easy to form an ideal film layer with high smoothness after calendering.
In the film layer after coating and drying, the arrangement of paint molecules is relatively loose, and there are many tiny holes in between. During calendering, under a certain temperature and pressure, the movement between molecules intensifies, which is manifested by the volume change of the film layer. The percentage change of this volume is called compression rate, which is the ratio of the volume reduced when the paint layer is pressed to the original volume before pressure. The high compression rate is beneficial to the formation of a smooth film on the surface of the coating; on the contrary, the amount of compression of the paint layer is small, and the surface is not easy to form a smooth film. However, when the pressure is too high, the extensibility and plasticity of the printed matter will be reduced, resulting in fracture.
The surface smoothness and adhesion strength of the glazed film layer generally increase with the increase of curing time, but the rate of increase is getting smaller and smaller, and no longer increases after reaching a certain value. The curing time depends on the calendering speed. When the glazing paint is in contact with the glazing belt, its molecular activity gradually weakens as the coating temperature decreases. If the curing time is short and the attenuation rate is fast, the coating molecules and the ink layer on the surface of the printed matter cannot fully function, the adhesion strength of the coating layer to the ink layer is poor, and the surface smoothness of the film layer after drying and cooling is low.
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