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Selective adsorption of water and ink on offset printing plate

the wetting of offset printing plate surface refers to the process that the air on the graphic part of the printing plate is replaced by ink, and the air on the blank part of the printing plate is replaced by wetting liquid

the main components of inks are pigments and binders. The binder constitutes the liquid part of the ink. Except for natural resin and synthetic resin, the binder is an organic liquid. In organic chemistry, organic liquids are collectively referred to as oil. Therefore, the wetting of ink on the graphic part of the printing plate is regarded as the wetting of oil on the graphic part of the printing plate

the main component of wetting solution is water. Its wetting in the printing process is the wetting of water on the blank part of the printing plate. The transmission of wetting liquid adopts different water transmission devices according to different models. For water offset printing, no matter what kind of printing plate, its surface structure is composed of graphic and non graphic parts, forming two areas of hydrophilic and lipophilic. The project is constructed in three phases, each of which selectively adsorbs wetting liquid and ink. This selective adsorption first depends on the chemical structure of the blank part and graphic part of the printing plate. However, in the actual printing process, whether this selective adsorption can be achieved depends on whether certain conditions are available

selective adsorption of printing plate on water and ink

the work required to increase one square unit of the surface of a substance under isohumidity and isobaric conditions is called surface energy or surface free energy

surface energy depends on the structural condition of the material surface and the inward tension of surface molecules. For the surface energy of liquid, it is also reflected in the contraction force of surface wiping oil acting on the unit length of liquid surface, that is, surface tension. Therefore, surface energy and surface tension are physical quantities that describe the energy of liquid surface or solid surface. They have the same dimension and the same value, but they have different physical meanings (they can be converted into the same unit in special cases)

according to the test, the surface of metal objects, the surface formed by inorganic compounds, has high surface energy, which is called high-energy meter, and this kind of treatment method of the experimental instrument also has the similar surface of cars, while the surface of organic and polymer materials has low surface energy, which is called low-energy surface. Obviously, the dried PS plate is divided into two areas: high-energy surface and low-energy surface. In addition, the surface energy formed by wetting liquid and ink is very low, and the surface energy of ink is even lower than that of wetting liquid. According to the theory that when the high energy of the system changes to low energy under the conditions of constant temperature and pressure, this change process can be spontaneous. For the adsorption of water and ink on the printing plate, the hydrophilic blank part of the printing plate is not selective, while the lipophilic graphic part of the treated printing plate is selective (adsorbing ink and rejecting wetting liquid)

according to the above analysis, the printing plate must be watered (moisturized) first, which is adsorbed by the hydrophilic blank part, and the lipophilic graphic part is basically not adsorbed, and then the printing plate must be inked. Due to the immiscibility of oil and water, the ink cannot adhere because of the wetting liquid in the blank part, so only the graphic part of the printing plate is wetted. In this way, the blank part and graphic part of the printing plate are selective to the wetting liquid and ink, thus ensuring the normal printing. This is the reason why the wetting device of offset press is in front and the inking device is behind

because the hydrophilic surface of the printing plate is a high-energy surface, it is easy to absorb impurities and destroy the hydrophilicity of the surface. Therefore, appropriate protective measures must be taken for the printing plate surface

Arabic gum liquid (peach gum) is coated on the printing plate, and the lipophilic graphic part is not stained with glue, because it is a low-energy surface and does not need protection. When the hydrophilic blank is stained with glue, the high-energy surface of the metal is replaced by the low-energy surface of the polymer compound, that is, the blank is covered by Arabic gum. In this way, the adsorption of impurities on the blank part of the printing plate is greatly weakened, and the printing plate is protected. This is why you need to wipe the printing plate or wipe the printing plate during a short shutdown

[expert tips]

wipe the printing plate, which is almost the work of offset machine operators. Wipe the glue evenly and thinly, rather than thick and much. In addition, the glue is required to be free of impurities, so that the printing plate can be protected for a long time

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