Polishing process and classification

The glazing process of printed products generally includes coating and calendering of the glazing coating. The application of glazing coatings, in a certain way, applies a layer of lacquer coating to the surface of printed products. Commonly used coating methods include spray coating, printing coating, and glazing coating machine coating.

Spray coating is manual operation. Its working speed is slow and the coating quality is poor. However, it is easy to operate and flexible. It is suitable for the coating of printed products with small batches and rough surface.

Printing coating is generally a coating device that has been modified for use as a gloss coating. Compared with manual spray painting, this method has better quality and higher production efficiency. It can use existing printing equipment and can be used in one machine for two purposes. It is easy to use and is suitable for general medium and small-sized printing factories. However, a solvent-based glazing coating should be generally used. During the operation, care should be taken to ensure that the viscosity of the coating is relatively stable and a certain amount of coating must be controlled.

Special coating coater coating can realize accurate control of coating amount. The coating quality is stable and reliable, and it is suitable for coating coating of various grades of printed matter. It is currently the most common coating method.

The coating quality requirements for coating are: coating on the coating surface should be uniform, suitable coating amount, good leveling, and a certain degree of adhesion with the surface of the print. After applying the gloss coating on the surface of the print, it is usually required to be calendered by the calender. Calender calendering can change the surface state of a dried glazing coating to form an ideal mirror surface.

The glazing process can be comprehensively classified in different ways: according to the glazing method, it can be divided into an offline glazing process and an on-line glazing process; the glazing coating can be divided into an oxidative polymerization type coating glazing, a solvent evaporation type coating glazing, and light Curing paint coating and heat-curing coating coating; according to the product can be divided into full-cap glazing, local glazing, extinction and artistic glazing; according to the print input method can be divided into manual feeding and automatic feeding.

Glazing coating

At present, all types of coating coatings developed have their own technological advantages; oxidative polymerization coatings mainly rely on the oxygen in the air for polymerization and drying to form films. The requirements for drying sources are not high and the investment in equipment is low. Volatile coating coatings rely on the evaporation of solvent in the coating to form a film. They have good leveling during coating, drying, and film formation. Their processing performance and application range are wide. They are applicable to various grades and large-volume prints. Glazing processing; thermosetting glazing coating relies on the active functional groups contained in the polymer structure of the film-forming resin and the catalyst in the coating. When heat is generated, cross-linking reaction takes place and the film is formed. The curing is rapid and the production efficiency is high. It is suitable for automation. Polishing; Light-curing coating coating is absorbed by the radiation, the coating structure of the internal molecular polymerization and dry film, the gloss of the coating coating is high, the wear resistance of the film, folding resistance , Heat resistance is relatively good, suitable for high-grade printed products of the glazing processing.

The basic composition of all kinds of coating coatings is roughly the same, that is, they are composed of the main agent (membrane tree), additives and solvents. The main agent is the film-forming substance of the varnish coating. After the prints are glazing, the quality and physiochemical properties of the film, such as glossiness, folding resistance, post processing suitability, etc., are all related to the selected main agent. The main agent is a natural resin glazing coating. The transparency of the film is poor, yellowing is easy, and sticking is also easy to occur. The glazing coating with synthetic resin as the main agent has good film forming property, high gloss and transparency, and high resistance. Grinding, water resistance, aging resistance, and strong applicability.

Additives are auxiliary substances that need to be added in order to improve the physicochemical properties and process characteristics of glazing coatings. For example, a curing agent added to improve the film forming property of the main agent resin and increasing the cohesive strength of the film; a surface activating agent added for improving the leveling property of the coating agent and lowering the surface tension thereof; Antifoaming agents added by synthesis and coating operations; plasticizers added to improve the elasticity of the film and enhance water resistance and flex resistance.

The role of the solvent is to disperse, dissolve, and dilute the main agent and auxiliary agent. Commonly used solvents are aromatics, esters, alcohols and so on. The physicochemical properties such as toxicity, odor, drying and leveling of the coating coating are directly related to the selection of the solvent. Aromatic solvents have low evaporation heat, high volatilization rate, and high solubility, but these solvents are more toxic. Ester solvents have good solubility, high volatilization rate, and low cost, but they have a relatively large odor. Alcohol solvents have good solubility, The volatilization rate is neither higher than the above two categories, but it is non-toxic, odorless, and free from pollution. If water can be used as a coating solvent, the cost is the lowest and the source is the widest. It does no harm 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 Polish Quality

1. Process factors that affect the coating quality of coating. The essence of the coating process is the process of smoothing and drying the coating on the surface of the printed matter. 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 effect of the properties of the printed paper and printing graphics on coating. In gloss coating, the gloss coating tends to level out on the surface of high smoothness paper. During the drying process, a smoother film surface can be formed as the coating coating cures. Therefore, the higher the surface smoothness of the paper is, the better the coating effect is, and vice versa. Absorption of the surface of the paper is too strong, paper fiber absorption rate of the coating coating is high, the solvent penetration is fast, resulting in the coating viscosity value becomes larger, the shear stress of the coating layer flowing on the surface of the print increases, affecting the leveling of the coating coating. However, it is difficult to form a relatively smooth film layer; on the contrary, absorption is too weak, so that the penetration, solidification, and conjunctival function of the glazing coating in the leveling are significantly reduced, and a high-quality film layer cannot be formed on the print surface as well.

The quality of printed inks also directly affects the coating quality and leveling of glazing coatings. The ink has a fine particle size, a high degree of dispersion, and the ink layer is easily wetted by the glazing coating. Under the effect of the coating pressure, the leveling property is good, and the resulting film has a high degree of smoothness. On the contrary, the ink particles are coarse, the printing ink layer is spread poorly, and a high-quality film layer is not easily formed during coating.

(2) The type of glazing coating is different, and its performance is also different. Even under the same process conditions, the condition of the film obtained after coating and calendering is not the same. The viscosity of the above light coating has an important influence on the leveling and wettability of the coating. Absorption rate of glazing paint on the same absorption strength paper is inversely proportional to coating viscosity value, that is, the smaller the coating viscosity value is, the greater the absorption rate will be, the premature end of leveling will be caused, resulting in some localized large paint on the surface of printed matter and affecting Film drying and calendering smoothness and brightness.

The glazing coatings with different surface tension values ​​have different effects on the wetting, adhesion, and impregnation of the same printed matter, and the filming effects after coating and calendering vary greatly. The glazing coating with a small surface tension value can wet, adhere to and saturate the surface and ink layers of various types of printed products. “The surface of the coating is smooth and uniform, and the surface coating has a large surface tension value. The wetting of the ink on the surface of the printed matter is limited, and even after coating the coating will produce a certain amount of shrinkage and affect the quality of the film.The volatility of the solvent also has an effect.The too high volatilization rate will make the paint layer not ready to flow into a uniform film On the contrary, it will cause the paint coating to dry, the hardened conjunctiva is blocked, and the anti-sticking property is bad.

(3) The selection of coating process conditions also has a great influence on coating quality. The coating arrangement is too little, the paint cannot spread evenly over the entire surface to be coated, and the smoothness after drying and calendering is poor; a too thick coating amount will affect the drying and increase the cost. In order to dry the thick film layer, the temperature at the time of coating and calendering should be relatively increased. The drying time needs to be longer, which will result in the reduction of the moisture content of the printed matter, the paper fibers becoming brittle, and the surface of the prints easily broken.

The coating speed, drying time, drying temperature and other process conditions also influence each other. When the machine speed is fast, the coating leveling time is short and the coating is thick. In order to obtain the same drying effect, the drying time must be recognized and the temperature must 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 appropriately reduced. The above-mentioned process factors sometimes have a cross-linking effect on the quality of L-light coating. At 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 various factors.

2. Process factors that affect calendering quality. In calendering, the main factors influencing the calender quality of the printed matter are calendering temperature, pressure and machine speed.

During the calendering process, the appropriate temperatures for the hot press, glazing, and cooling stripping stages should be used to facilitate the secondary wetting, adhesion, and penetration of the main agent molecules in the coating on the surface of the print, and to 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 pressure. When the temperature is too high, the adhesion strength of the coating layer decreases, the deformation value increases, and the moisture content of the printed matter decreases rapidly. This is disadvantageous to the light and peeling process. On the contrary, the hot pressing temperature is too low, and the paint layer is not completely plasticized. Second, the printed product has poor secondary wetting, adhesion, and penetration capabilities, and the coating layer has poor adhesion and poor calendering performance. After calendering, it is not easy to form an ideal film with high smoothness.

In the coated and dried film layer, the arrangement of the coating molecules is relatively loose, with many tiny holes in between. When calendering, under certain temperature and pressure, the intermolecular movement is intensified, showing the volume change of the film. The percentage of change in volume is called the compression ratio, which is the ratio of the reduced volume of the paint layer to the original volume before pressurization. The large compression rate is favorable for the formation of a smooth film layer on the surface of the coating; conversely, the paint layer has a small amount of compression and the surface is not likely to form a smooth film layer. However, if the pressure is too high, the printability and plasticity of the printed matter will be reduced, resulting in breakage.

The surface smoothness and adhesion strength of the coating film generally increase with the increase of the curing time, but the rate of increase is getting smaller and smaller, and it does not increase after reaching a certain value. The length of the curing time depends on the calendering speed. When the glazing coating is in contact with the glazing tape, its molecular mobility gradually decreases as the coating temperature decreases. For example, if the curing time is short and the weakening rate is fast, the coating molecules can not fully function with the ink layer on the surface of the printed matter, and the adhesion of the paint layer to the ink layer is poor, and the smoothness of the film surface after drying and cooling is low.