Commonly used screens are stainless steel mesh, nylon mesh, polyester mesh, etc. The specific surface energy of stainless steel is the highest, the specific surface energy of polyester is the lowest (the critical surface tension is approximately 4.3×10-6N/m), and the specific surface energy of nylon (the critical surface tension is approximately 4.6×10-2N/m). Between stainless steel and polyester. Therefore, the best combination of photosensitivity and ink penetration is stainless steel mesh, followed by nylon mesh, and the worst is polyester mesh. Porogram plates made of stainless steel wire mesh are used to print fine products with high quality requirements. In order to improve the screen printing performance and reduce the cost, screen printing made of composite materials is often used. E.g. The nickel-plated polyester mesh is coated with a nickel metal with a thickness of about 2-5 μm on the polyester mesh. The braided node of this mesh is fixed with nickel plating to avoid loosening of the metal mesh due to metal fatigue. The disadvantages of low binding force and poor ink-permeability of Chi and polyester webs and sensitized offset printing films have become webs with high printability and high printability with high tension and low elongation. The stencil made of nickel-plated polyester mesh has small mesh deformation during printing, smooth ink flow, and even thickness of the ink layer obtained on the substrate.

3. Lithographic surface structure and wettability. Plates used for lithography include PS plates, gravure plates, protein plates, multilayer metal plates, and the like. The characteristics of the surface structure of these printing plates are that the graphic part of the printing plate and the non-graphical blank part are almost on the same plane. When printing, we must first wetting the non-graphic part of the printing plate with a wetting fluid to form a uniform water film of a certain thickness, and then wetting the graphic part of the printing plate with ink to form a uniform ink with a certain thickness. Membrane, using oil, water immiscible principle, non-graphic parts and graphic parts rely on the water film and ink film to resist mutual infiltration, as shown in Figure 1-17. The surface of the lithographic printing plate must form hydrophilic, oleophobic and lipophilic hydrophobic surface areas. This can be achieved in two ways. One is to use the same metal to form the plate, but the physical and chemical treatment is used to change the wettability of the plate to water and ink to form two types of surface areas with different properties, such as the PS plate. The plate, plate, plate, etc. are made of aluminum plate or zinc plate as plate materials; the other is that two types of surface areas such as multi-layered metal plate are used to form two kinds of metal with different wettability to water and ink.

Figure 1-17 Schematic of lithographic and lithographic printing

Among all kinds of metals, copper has the best lipophilicity, but its hydrophilicity is the worst; chromium has the best hydrophilicity, but its lipophilicity is the worst, and both the hydrophilicity and the lipophilicity of zinc and aluminum are in the middle.

Metal oxides and inorganic salts are mostly substances that are hydrophilic, oleophobic, and ion crystal structures, and have a strong affinity for water. On the aluminum plate or zinc plate that has certain hydrophilicity and certain oleophilicity, the water-insoluble metal oxide or inorganic salt layer is formed, which greatly improves the hydrophilicity of this part.

Fatty acids, fatty acid salts, and diazo photosensitive resins are all substances with a weakly polar non-ionic structure. When these substances are coated on a zinc plate or an aluminum plate, they adhere to the plate with the polar part between the molecules, and the non-polar part with the lipophilicity extends into the space, which makes this part of the printing plate The lipophilicity has greatly improved (Figure 1-18).

Figure 1-18 Change in lipophilicity of printing plate

The PS version is an ideal plate in the lithography. The positive PS plate is made of aluminum plate. The hydrophilic non-graphic part is aluminum oxide (Al2O3). The oleophilic graphic part is a hardened diazo photosensitive resin.

Flat intaglio plate and protein plate are both made of zinc plates. The hydrophilic non-graphic part is inorganic zinc phosphate (Zn3(PO4)2). The lipophilic graphic parts are non-polar polar and hardened respectively. The protein photo film.

The multi-layer metal plate is made of two kinds of metal plates, and the non-graphic part is made of metal chromium with good hydrophilicity, and the graphic part is made of metal copper with good lipophilicity. In this way, two areas of hydrophilic and oleophilic are formed on the same plate, each of which selectively adsorbs the wetting fluid and the ink.

In summary, the non-graphic part and the graphic part of the lithographic printing plate must selectively absorb the wetting liquid and the ink. This selective adsorption first depends on the non-graphical part of the printing plate and the graphic part of the layout. The chemical structure. However, in the actual printing process, this selective adsorption can be achieved, but also depends on whether there are certain conditions.

The non-graphic portion of the lithographic plate is composed of a metal, a metal oxide, an inorganic salt, etc. having a higher specific surface energy and belongs to a high-energy surface. For example, the specific surface energy of zinc phosphate is as high as 9.0×10 −1 J/m 2 , and the specific surface energy of alumina is as high as 7.0×10 −1 J/m 2 , which is more than 10 times the surface tension of the wetting liquid. Therefore, from the chemical structure similarity and surface energy data of the wetting fluid and the non-graphical part, the wetting fluid can wet the non-graphic part of the printing plate. This is because the wetting fluid's wetting causes a decrease in the free energy of the system's non-graphical blank, and makes γW-γI ≤ 0 (γW is the surface tension of the wetting fluid, and γII is a surface excess of the non-graphical portion. Free energy) is therefore a spontaneous process. On the other hand, the surface tension of the ink is very low, which is only about 1/20 of the excess free energy of the non-graphical surface. Therefore, although the chemical structure of the ink and the non-graphical part of the material is not similar, the ink can still Spread over the non-graphic part. This is because the spread of the ink also results in a decrease in the free energy of the system, so that γ0-γI≤0 (γ0 is the surface tension of the ink), and thus is also a spontaneous process. In short, the non-graphic part of the printing plate can be wetted by the wetting fluid and wetted by the ink. It can be seen that the non-graphical part of the printing plate is not selective with respect to the wetting fluid and the ink.

The graphic part of the lithography is a non-polar organic compound with a low surface energy and belongs to the low-energy surface. Its specific surface energy is generally similar to the surface tension value of the ink. For example, the deuterium and the hardened diazo photosensitive resin have specific surface energy. All were between 3.0×10 -2 and 4.0×10 -2 J/m 2 , and the normal wetting fluid was slightly higher than 7.2×10 -2 N/m due to the surface tension of some electrolytes. Therefore, the ink can be spread on the graphic portion of the printing plate, and the wetting liquid cannot be spread. This is because the exhibition of inks leads to a decline in the free energy of the system, so that γ0-γ′I≤0 (γ′I is the surface free energy of the graphic part), which is a spontaneous process; and if the wetting fluid spreads, To increase the free energy of the system, γW-γ′I ≥ 0, which is a non-spontaneous process. In short, the graphic part of the printing plate can only be wetted by the ink, but it cannot be wetted by the wetting liquid. It can be seen that the graphic part of the printing plate is selective for wetting fluid and ink in wetting.

In order to make the non-graphic part of the lithographic plate only wetted by the wetting liquid, we must first supply the wetting fluid to the printing plate, and then supply the ink to the printing plate after the non-graphic part is covered with the wetting fluid. Miscible, the ink can not adhere due to the non-graphic part of the wetting liquid, and only wet the graphic part of the printing plate. In this way, the non-graphic part and the graphic part of the lithographic printing plate are selective to the wetting liquid and the ink.