Magnetic Printing Technology and Mechanical Discussion--lithographic Printing (III)

(3) Basic types of lithographic plates

The printing plates used in lithographic printing are different in terms of plate material and plate making operation. Protein plates, flat gravure plates, multi-layered metal plates, and pre-coated photosensitive plates (PS plates) are commonly used.

1. Protein version

Protein plates are also called negative plates or flat plates, and substrates are aluminum plates or zinc plates. The lipophilic hydrophobic film in the graphic part is a sclerostin film with a high publication plane of about 3-5 [mu]m, and the blank part of the hydrophilic oleophobic film layer is an inorganic salt layer.

Protein plate making process is simple, the cost of raw materials used is low, but the printing rate is low, generally 5000 to 10000 India. Since the photosensitive resin base is not strong, the operation during the printing process must be very careful, and the concentration of the dampening aqueous solution should not be too high. The blank part of the plate is easy to get dirty. Therefore, it is rarely used at present and it is mainly used for copying prints with a small number of prints.

2. Flat intaglio

The flat intaglio plate is also called the positive plate version, and the plate base is the zinc plate (as shown in Figure 2-26).

The plain intaglio plate is printed on the positive plate of the positive image, and the photo-hardened film is dissolved and removed by the developing solution, exposing the metal of the graphic part, and the metal is slightly corroded by the etching solution so that the metal of the graphic part is lower than the plate surface by 5 to 8 μm. Then it was painted with a oleophilic base paint, and the hydrophobic and hydrophobic graphic part was formed.

After the flat intaglio graphic part was formed, the hardened sensation film was removed and treated with a phosphoric acid solution, followed by the hydrophilic gum arabic, and a hydrophilic oleophobic blank was also formed.

The oleophobic part of the plain intaglio is a base paint formulated with a variety of organic compounds, commonly known as "Lak." Base paint formulations include: phenolic resin, nitro lacquer, shellac, ethanol, isopropanol, various lipids and aldehydes compounds. The base paint is a non-polar hydrophobic material.

The hydrophilic part of the intaglio plate is zinc phosphate. Zinc phosphate is insoluble in water and has a high stability in weakly acidic media. Water spreads easily on the surface of zinc phosphate salts.

The gravure printing process is more complicated than the protein version, and the cost of raw materials is also slightly higher, but the printing rate is high; up to 20,000 to 40,000 prints is a widely used printing plate in offset printing. Because the fountain solution concentration is slightly higher or the printing plate is slightly more moisture, the damage to the printing plate is not as obvious as the protein plate, so the ink layer of the printed matter is thick, the dots are bright and clean, and the quality is high.

3. Multi-layered metal plate

Multi-layered metal plates use two kinds of metal with opposite hydrophilicity and oleophilicity as the printing plate. There are two kinds of metal plates and three metal plates. According to the shape of the concave and convex figures, it is divided into a flat gravure version and a peaceful type. The most commonly used are two-layer gravure and three-layer gravure. Copper chrome plated to make a two-layer gravure; metal plated copper and then chrome or nickel will be made of three-layer gravure. The printing process of multilayer metal plates is shown in Figure 2-27.
The oleophilic part of the multi-layered metal plate is the copper metal with the best lipophilicity and is treated with potassium ethylxanthogenate to directly adsorb the ink.

The hydrophilic part of the multi-layered metal plate is the most hydrophilic nickel and chromium. There is no inorganic salt layer and oxide layer. The water is directly spread on the surface of the metal chromium or nickel. Chromium or nickel has stable chemical properties and good abrasion resistance, so the blank part of the printing plate is not easy to wear, and the printing plate has a high printing rate.

The multi-layer metal plate directly forms a stable graphic portion and a blank portion using the oleophilicity of copper and the hydrophilicity of chromium or nickel. During the printing process, the water consumption on the printing plate can be reduced, thereby reducing the printing paper's flexibility, ensuring the accuracy of overprinting, reducing the emulsification value of the ink, ensuring the vividness of the ink and reproduction of the color tone, and improving the printed product. the quality of.

The disadvantage of the multi-layered metal plate is that the plate making process is complicated, the speed is slow, the cost is high, the electroplating waste liquid is polluted to the environment, and the dot reducibility is also poor. Therefore, there are not many factories that use this plate.

4. Pre-coated version (PS version)

PS version is the abbreviation of pre-coated version, is a new type of offset printing plate. In foreign countries, the PS version has been widely used. At home, the PS version has also become the main printing plate in offset printing.

PS version is divided into "photopolymerization type" and "photolytic type" two. The photopolymerization type is used to print the original negative plate, and the photoreactive film of the diazo film in the graphic part is photohardened and remains on the plate. The non-graphical part of the diazo film does not see light, does not harden, and is dissolved and removed by the developer. The photodecomposition type is positively printed on the positive plate, and the diazo compound in the non-graphic part is decomposed by light, and is dissolved and removed by the developer. The diazo compound remaining on the plate is still not visible, and the structure of the cross section of the PS plate is as follows: See Figure 2-28.

The oleophilic part of the PS plate is a diazo photosensitive resin with a high publishing base plane of about 3 μm. It is a good hydrophobic film, and the ink is easily spread on it, but water is difficult to spread on it. Diazo photosensitive resin also has good abrasion resistance and acid resistance. If the film is baked at a temperature of 230 to 240° C. for 5 to 8 minutes and the photosensitive film is deuterated, the hardness of the printing plate can be improved, and the printing plate can withstand printing rates of up to 200,000 to 300,000 sheets.

The hydrophilic part of the PS plate is an Al2O3 thin film. The high-printing substrate plane is about 0.2 to 1 μm. The hydrophilicity, abrasion resistance, and chemical stability are all relatively good. Therefore, the printing plate has a higher printing rate.

PS version of the sand fine, high resolution, forming a smooth network of complete, it is good color reproduction, image clarity; PS version of the blank part has a higher moisture content, printing plate printing water consumption Water and ink balance are easy to control. As the PS version has many of the above advantages, it has become the ideal printing plate in the current offset printing.

The base plate of the PS plate is a 0.5 mm, 0.3 mm, or 0.15 mm thick aluminum plate. The aluminum plate has undergone electrolytic roughening, anodization, and pore sealing, and forms an oxide film on the plate surface. The diazo photosensitive solution is then coated to make a pre-coated plate. When printing, the photosensitive liquid is no longer applied, and the original plate is directly used for printing.

For now, the PS version is the ideal plate. The disadvantage is that the cost is higher and the printing plate can only be used once. Repeated use of the recycled PS version is for further study.