Printing plate selective adsorption of water, ink

In a state of equal humidity and isobaric pressure, the work required to increase the surface area by one square unit is called surface energy or surface free energy.

The surface energy depends on the structural condition of the surface of the material and the inward pull of the surface molecules. The surface energy of the liquid is also reflected in the shrinkage force per unit length of the surface of the liquid surface, that is, the surface tension. Therefore, the surface energy and the surface tension are physical quantities that describe the energy level of the liquid surface or the solid surface. They have the same dimensions, the same values, but they have different physical meanings (in special cases they can be converted into the same units).

According to the test, the surface of a metal object, the surface formed by an inorganic compound, has a high surface energy, which is called a high energy surface, while the surface of an organic matter and a high molecular material has a low surface energy and is called a low energy surface. Obviously, the slick version is divided into two areas: high-energy surface and low-energy surface. In addition, the surface energy of the wetting fluid and the ink is very low, and the ink surface energy is lower than the wetting fluid, according to the constant temperature, Under constant pressure conditions, when the system's high energy changes to low energy, this change process is a theory that can proceed spontaneously. The printing plate's adsorption of water and ink, and the blank part of the printing plate hydrophilic, are not selective, and the graphic part of the treated printing plate lipophilic is selective (adsorption ink, repellent wetting liquid).

According to the above analysis, the printing plate water (wetting liquid) must be firstly adsorbed by the hydrophilic blank portion during printing, and the lipophilic graphic portion is not adsorbed, and then the printing plate is inked. Since the oil and water are insoluble, the ink cannot adhere due to the wetting liquid in the blank part, and only the graphic part of the printing plate is wetted. Thus, the blank part and the graphic part of the printing plate have both wetting fluid and ink. Selective, thus ensuring the normal printing. Why is the offset printing machine moistening device before the inking device is the reason.

Since the hydrophilic surface of the printing plate is a high-energy surface, it is easy to adsorb impurities and degrade the hydrophilicity of the surface. Therefore, appropriate protection measures must be taken for the printing plate surface.

The gum arabic (powder gum) is applied to the printing plate. The lipophilic graphic part is not glued because it is a low-energy surface and does not require protection. The hydrophilic blank 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 portion 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 it is necessary to tamper with the printing plate, or to ditch the plate during a brief shutdown.

[Expert Tips]

Giving gum to the printing plate is almost all the work that the operator of the offset printing press has to do. The silicone is trying to be “even and thin” and is not “thick and numerous”. In addition, the glue is required to be free of impurities so that the printing plate can be protected and the protection time is longer.