part 1 Glazing process and analysis of quality factors
The glazing process is a technique and method for applying a layer of uncoated transparent coating on the surface of the printed product, after leveling, drying, calendering, and forming a thin and uniform transparent bright layer on the surface of the printed matter.
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Glazing process and classification
The glazing process of printed matter generally involves the application of coating and calendering of the glazing coating. The coating of the glazing coating, that is, the process of uniformly applying a glazing coating on the surface of the printed matter in a certain manner. Common coating methods include spray coating, printing coating and glazing coater coating.
The spray coating is a manual operation, and the working speed is slow and the coating quality is poor, but the operation is convenient and the flexibility is strong, and the coating is suitable for the printing of small batches and rough surface.
Printing coatings are typically applied by coating equipment, modified to be used as a glazing coating. Compared with the manual spray coating, the quality is high and the production efficiency is high. The existing printing equipment can be utilized, and the machine is easy to use, and is suitable for general coating processing in small and medium-sized printing plants. However, solvent-based glazing coatings should generally be used. During operation, care should be taken to ensure that the viscosity of the coating is relatively stable and that a certain amount of coating is required.
Special glazing coater coating can achieve accurate control of coating amount. The coating quality is stable and reliable, and it is suitable for glazing coating processing of various grades of prints. It is the most common coating method currently used.
The quality requirements for glazing coating are: uniform coating on the coated surface, suitable coating amount, good leveling property, and a certain adhesion to the surface of the printed matter. After the glazing coating is applied to the surface of the print, it is usually necessary to pass the calendering of the calender. The calender calendering can change the surface state of the dried glazing coating to form the desired mirror surface.
The glazing process can be classified into different ways according to different ways: according to the glazing method, it can be divided into offline glazing process and on-line glazing process; according to glazing coating, it can be divided into oxidative polymerization coating glazing, solvent volatilization coating glazing and light. Curing coating glazing and heat curing coating glazing; according to the product can be divided into full cap glazing, partial glazing, extinction and artistic glazing; according to the print input method can be divided into manual paper feeding and automatic paper feeding.
Varnish coating
All kinds of glazing coatings developed at present have their own technological advantages; oxidative polymerization glazing coatings mainly rely on the polymerization of oxygen in the air to form a film, which has low requirements on drying sources and less equipment investment; solvent Volatile glazing coating relies on solvent evaporation in the coating to form a film, which has good leveling property in coating, drying and film forming process. Its processing property and wide application range are suitable for various grades and large quantities of printed matter. Glazing processing; thermosetting glazing coating relies on the reactive functional groups contained in the polymer structure of the film-forming resin and the catalyst in the coating, and is formed into a film by cross-linking reaction in the presence of heat, which is fast in curing and high in production efficiency, and is suitable for automation. The glazing processing; the photocurable glazing coating is obtained by absorbing the radiant light energy, and the internal structure of the coating molecule is polymerized to form a film, and the gloss of the glazing coating is high, and the abrasion resistance and folding resistance of the coating layer are high. It has good heat resistance and is suitable for glazing processing of high-grade printed matter.
The basic composition of all kinds of glazing coatings is basically the same, that is, it consists of three parts: main agent (film forming tree), auxiliary agent and solvent. The main agent is a film-forming substance of the glazing coating. After the printed matter is glazed, the quality and physical and chemical properties of the film, such as gloss, folding resistance and post-processing suitability, are related to the selected main agent. The main agent is a glazing coating of natural resin, the transparency of the film formation is poor, easy to yellow, and the back tack phenomenon is easy to occur; the glazing paint with synthetic resin as the main agent has good film forming property, high gloss and transparency, and resistance. Grinding, water and aging resistance, and strong applicability.
Additives are some auxiliary substances that need to be added to improve the physical and chemical properties and process characteristics of glazing coatings. a curing agent added to improve the film forming property of the main resin and increase the cohesive strength of the film; a surfactant added to improve the leveling property of the glazing coating and reduce the surface tension thereof; An antifoaming agent added by synthesizing and coating operation; a plasticizer added to enhance the elasticity of the film layer and to enhance water resistance and folding resistance.
The role of the solvent is to disperse, dissolve, dilute the main agent and auxiliary. Commonly used solvents are aromatics, esters, alcohols, and the like. The physicochemical properties of glazing coatings such as toxicity, odor, drying and leveling are directly related to the choice of solvent. The aromatic solvent has low evaporation heat, high volatilization speed and high solubility, but the solvent is more toxic; the ester solvent has good solubility, high volatilization speed and low cost, but the odor is relatively large; the alcohol solvent is in solubility, The volatilization rate is not as good as the above two types, but it is non-toxic, tasteless and has no pollution. If water can be used as a solvent for glazing coatings, the cost is the lowest, the source is the widest, and it is harmless to the human body and does not pollute the environment. Therefore, the development of water-based glazing coatings in recent years is attracting great attention at home and abroad.
Process factors affecting glazing quality
1. Process factors that affect the quality of glazing coating. The essence of the glazing coating process is the process in which the glazing coating is leveled and dried on the surface of the print. The main influencing factors are the glazing suitability of the printed matter, the type and performance of the glazing coating, and the coating processing conditions.
(1) The glazing suitability of printed matter refers to the influence of the performance of printed paper and printed graphic on glazing coating. In glazing coatings, glazing coatings tend to level on highly smooth paper surfaces. During the drying process, as the glazing coating is cured, a smoother film surface can be formed. Therefore, the higher the surface smoothness of the paper, the better the glazing coating effect, and vice versa. The absorbent member on the surface of the paper is too strong, the absorption rate of the paper fiber to the glazing paint is high, the solvent penetration is fast, the viscosity value of the coating becomes large, and the shear stress of the coating layer flowing on the surface of the printed matter increases, which affects the leveling of the glazing paint. It is difficult to form a smoother film layer; on the contrary, the absorption is too weak, so that the penetration, solidification and conjunctiva of the glazing coating in the leveling are significantly reduced, and a high-quality film layer cannot be formed on the surface of the printed matter.
The quality of the printed ink also directly affects the coating quality and leveling properties of the glazing coating. The particles of the ink are fine and the dispersion degree is high, and the ink layer of the ink is easily wetted by the glazing paint. Under the application of the coating pressure, the leveling property is good, and the smoothness of the formed film layer is high. On the contrary, the ink particles are coarse, the printing ink layer spreads poorly, and it is difficult to form a high-quality film layer during coating.