Polishing process and quality control of packaging printing products II

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 applicability of glazing products, the types and properties of glazing coatings, 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 the coating of glazing. In gloss coating, the gloss coating is easily leveled on the surface of high-smooth 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, the absorption rate of the paper fiber to the coating material is high, the solvent penetration is fast, the viscosity value of the coating becomes large, the shear stress of the coating layer flowing on the surface of the printing product increases, and the leveling of the coating coating is affected. It is difficult to form a relatively smooth film; on the contrary, absorption is too weak, so that the penetration, solidification, and conjunctival action 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 it is difficult to form a high-quality film layer 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, and the leveling will end prematurely, causing some local large paint on the surface of printed matter to affect 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 print is limited, and even the coating after coating will produce a certain amount of shrinkage and affect the quality of the film.The volatility of the solvent will also have an effect.The too high volatilization rate will make the paint layer not evenly leveled 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 is prolonged, which leads to the reduction of the moisture content of the printed matter, the paper fibers become brittle, and the surface of the printed matter is 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 a 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 the quality of calendering. 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 should be provided for each stage of hot pressing, glazing, and cooling in order to facilitate the secondary wetting, adhesion and penetration of the main agent molecules in the coating on the surface of the printed matter, and to enhance the contact effect between the two. Under certain temperature conditions, the plasticity of the coating film layer can be improved, and under the effect of pressure, the smoothness of the calendered surface of the printed matter is greatly improved. 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 for 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 layer 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 percent change in this volume is called the compression ratio, which is the ratio of the reduced volume of the paint layer upon pressurization to the original volume prior to pressurization. The large compression rate is favorable for the formation of a smooth film layer on the coating surface; conversely, the coating layer is compressed to a small amount, and a smooth film layer is not easily formed on the surface. 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 adhesive 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 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 print. The adhesion of the paint layer to the ink layer is poor, and the surface smoothness of the film after drying and cooling is low.

Source: Postpress Principles and Processes