The wetting of the surface of the offset printing plate refers to the process in which the air on the graphic part of the printing plate is replaced by ink, and the process in which the air on the blank part of the printing plate is replaced by the wetting fluid.
The main components of ink are pigments and binders. The binder constitutes the liquid part of the ink, and the binder is an organic liquid except natural resin and synthetic resin. In organic chemistry, organic liquids are collectively called "oil". Therefore, the wetting of the printing plate by the ink is regarded as the wetting of the printing plate by the oil.
The main component of the wetting liquid is water, which is the wetting of the printing process, that is, the wetting of the blank part of the printing plate by water. Different water delivery devices are used for the transmission of the wetting fluid. Regardless of the type of printing plate with water offset, its surface structure is composed of graphic and non-graphic parts, forming two areas of hydrophilic and oleophilic, each selectively adsorbing the wetting fluid and ink, This selective adsorption depends firstly on the chemical structure of the blank and graphic parts of the printing plate. But in the actual printing process, whether this selective adsorption can be achieved depends on whether certain conditions are met.
Selective adsorption of printing plate to water and ink
In the state of equal humidity and pressure, the work required to increase one square unit of the surface of a substance is called surface energy or surface free energy.
The surface energy depends on the structure of the surface of the substance and the inward pulling force of the surface molecules. For the surface energy of liquid, it is reflected in the contraction force of the surface per unit length of the liquid surface, that is, surface tension. It can be seen that surface energy and surface tension are physical quantities that describe the amount of energy on the liquid surface or solid surface. They have the same dimension and the same value, but the two have different physical meanings (in special cases, they can be converted into the same unit).
According to the test, the surface of metal objects and the surface formed by inorganic compounds have high surface energy and are called high-energy surfaces, while the surfaces of organic and polymer materials have low surface energy and are called low-energy surfaces. Obviously, the sun-printed PS plate is divided into two areas: a high-energy surface and a low-energy surface. In addition, the surface energy formed by the wetting fluid and the ink is very low, and the surface energy of the ink is lower than that of the wetting fluid. Under the condition of constant pressure, when the system's high energy changes to low energy, this change process is a theory that can proceed spontaneously. The adsorption of water and ink on the printing plate, the hydrophilic blank part of the printing plate is not selective, and the graphic part of the treated plate is oleophilic (selective to adsorb ink and repel wetting fluid).
According to the above analysis, the printing plate must first be filled with water (wetting fluid), which is absorbed by the hydrophilic blank part, and the oleophilic graphic part is basically not adsorbed, and then the printing plate is inked. Because the oil and water are not miscible, the ink cannot be attached due to the wetting fluid in the blank part, so it only wets the graphic part of the printing plate. Selective, so as to ensure the normal progress of printing. This is the reason why the dampening device of the offset press is in front and the inking device is behind.
Since the hydrophilic surface of the printing plate is a high-energy surface, it is easy to adsorb impurities and destroy the hydrophilicity of the surface. For this reason, appropriate protection measures must be taken on the surface of the printing plate.
The gum arabic solution (peach gum) is coated on the printing plate, and the oleophilic graphic part is not glued, because it is a low-energy surface and no protection is needed. The hydrophilic blank part is glued, and the high-energy surface of the metal is replaced by the low-energy surface of the polymer compound, that is, the blank part is covered with gum arabic. In this way, the adsorption of impurities on the blank part of the printing plate is greatly reduced, and the printing plate is protected. This is why the printing plate should be glued or the printing plate should be wiped out during a short stop.
[Expert Tips]
Applying glue to the printing plate is almost all the work that offset press operators have to do. It is aimed at "uniformity and thinness" rather than "thickness and more". In addition, the glue solution is required to be free of impurities, so that the printing plate can be protected, and the protection time is longer.
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