Film pasting technology
1. The essence of film binding technology is the leveling and drying process of binding film on the surface of printed matter. The main influencing factors are the gloss of printed matter, the types and properties of adhesive film, coating processing conditions and so on.
(1) The glossiness of printed matter refers to the influence of the performance of paper and printed graphics printed on printed matter on glazing paint. In this adhesive film, it is easy to flatten on the surface of high smoothness paper. In the drying process, with the curing of the adhesive film, a film surface with high smoothness can be formed. Therefore, the higher the smoothness of the paper surface, the better the effect of glazing coating, and vice versa. The absorption site on the surface of paper is too strong, the absorption rate of paper fiber to glazing paint is high, and the solvent permeates quickly, which leads to the increase of paint viscosity and the increase of shear stress of coating flowing on the surface of printed matter, which affects the leveling of glazing paint and makes it difficult to form a smoother film. On the contrary, the absorption is too weak, which obviously reduces the penetration, curing and conjunctival effect of glazing paint in leveling, and can not form a high-quality film on the surface of printed matter.
The quality of ink also directly affects the coating quality and leveling of glazing coatings. The ink particles are fine and its dispersibility is high, so the graphic ink layer is easily wetted by glazing paint. Under the action of coating pressure, the leveling property is good and the flatness of the formed film is high. On the contrary, the ink particles are coarse, and the printing ink layer has poor spreadability, so it is not easy to form a high-quality film layer when coating.
(2) Different types of glazing coatings have different properties. Even under the same process conditions, the coating conditions obtained after coating and calendering are different. For example, the viscosity of glazing coatings has an important influence on the leveling and wettability of coatings. The absorption rate of paper with the same absorption strength is inversely proportional to the viscosity of the coating, that is, the smaller the viscosity of the coating, the greater the absorption rate, which will lead to the premature end of leveling, resulting in some local large-area coatings on the surface of printed matter, which will affect the smoothness and brightness of the film after drying and calendering.
Glazing coatings with different surface tension values have different wetting, adhesion and penetration effects on the same printed matter, and the film-forming effects after coating and calendering are very different. Polishing paint with small surface tension can wet, adhere and soak the solid surface and graphic ink layer of various printed matter, and "level off into a smooth and uniform film surface;" The polishing coating with high surface tension limits the wetting of the ink layer on the surface of printed matter, and even the polished coating will shrink to a certain extent, which will affect the film quality. The volatility of solvents also has an effect. The evaporation speed is too fast, which will make the coating too late to level into a uniform film surface; On the contrary, it will cause insufficient drying of glazing paint, blocking the hardened conjunctiva and poor adhesion resistance.
(3) The selection of coating process conditions also has a great influence on coating quality. The coating arrangement is too few, which makes the coating not spread evenly on the whole surface to be coated, and the smoothness after drying and calendering is poor; Too thick coating will affect drying and increase the cost. In order to dry the thick film layer, the temperature during coating and calendering should be relatively increased, and the drying time should be prolonged, which will lead to the decrease of the water content of the printed matter, the brittleness of paper fibers and the easy cracking of the printed matter surface.
Coating speed, drying time and drying temperature also affect each other. When the machine speed is high, the coating leveling time is short and the coating is thick. In order to obtain the same drying effect, the drying time should be recognized and the temperature should be high. If the machine speed is slow and the coating leveling time is long, the coating will be thin, which can shorten the drying time and reduce the drying temperature appropriately. The above process factors sometimes have cross-linking effect on the quality of L-ray coating. In work, in order to obtain good quality of L-ray coating, it is necessary to comprehensively consider these factors in order to obtain a proper match between them.
2. Calendering quality. In calendering, the main factors affecting the quality of printed matter are calendering temperature, pressure and machine speed.
In the calendering process, all stages of hot pressing, glazing and cooling stripping should have appropriate temperatures, which is beneficial to the secondary wetting, adhesion and penetration of the main agent molecules in the coating to the printed surface and enhances the contact effect between them. Under certain temperature conditions, the plasticity of the coating film can be improved, and the smoothness of the calendered surface of printed matter can be greatly improved under pressure. When the temperature is too high, the adhesion strength of the coating decreases, the deformation value increases, and the water content of the printed matter drops sharply, which is not conducive to the inch light and peeling process; On the other hand, if the hot pressing temperature is too low, the coating layer cannot be completely plasticized, the secondary wettability, adhesiveness and permeability of printed matter are insufficient, and the adhesion of the coating layer is poor, so the calendering effect is poor, and it is not easy to form an ideal film layer with high smoothness after calendering.
In the dried film, the coating molecules are loosely arranged and there are many tiny holes in it. During calendering, under the action of certain temperature and pressure, the motion between molecules is intensified, which shows the volume change of the film. This volume change percentage is called compression ratio, that is, the ratio of the reduced volume of the coating under pressure to the original volume before pressure. The compression ratio is large, which is beneficial to form a smooth film on the surface of the coating; On the contrary, the coating layer is compressed less, and it is not easy to form a smooth film on the surface. But when the pressure is too high, the ductility and plasticity of printed matter will be reduced, which will lead to fracture.
The surface smoothness and adhesion strength of glazing film generally increase with the increase of curing time, but the increase range is smaller and smaller, and will not increase after reaching a certain value. The curing time depends on the calendering speed. When the varnish is in contact with the varnish belt, its molecular activity decreases with the decrease of coating temperature. If the curing time is short and the attenuation speed is fast, the coating molecules cannot fully interact with the ink layer on the surface of the printed matter, the adhesion strength of the coating to the ink layer is poor, and the surface smoothness of the film layer is low after drying and cooling.