Section 2 Lithography

First, the principle and characteristics of lithographic printing 1. Lithographic printing principle The lithographic printing of the graphic and blank parts are almost on the same plane, but the two parts have completely opposite properties for water and oil intimacy. The graphic part is lipophilic and hydrophobic, while the blank part is hydrophilic and oleophobic. Therefore, in the printing process, the graphic part absorbs the ink and repels the fountain solution; while the blank part of the printing plate absorbs the fountain solution and repels the ink. During the printing process, the blank portion is first wetted by the fountain solution, and then the graphic portion is coated with the ink, and then the graphic portion of the ink is transferred onto the paper. Therefore, lithography is based on the principle of oil, water immiscibility, the transfer of graphics on the plate above the substrate.
Modern lithographic printing generally uses indirect printing, which means that the ink on the graphic part of the printing plate is transferred to the surface of the substrate by means of intermediate carrier transfer. Therefore, lithographic printing is also called lithographic offset printing, that is, according to indirect Printing principle, the printing plate transfers the graphic through the blanket transfer roller to the substrate for printing.
The basic principle of the general lithography printing department is shown in Figure 1-21.
The printing plate on the plate cylinder 2 first applies water on the plate surface with water roller 9 (ink is blank and resists ink) before the ink is applied on the plate cylinder 2, and then the ink is applied to the plate by the ink roller 1 (image 3 is inked). Reject the principle of printing. Therefore, in addition to an ink inking device, a water supply device is also provided. The ink on the upper part of the printing plate is not directly transferred to the surface of the printing material, but is first transferred onto the blanket and then transferred onto the paper by the blanket. Therefore, the blanket cylinder 4 is specially provided. When the paper passes between the blanket cylinder and the impression cylinder, printing is performed under the effect of the printing pressure to complete the printing.
Plates for typographic plates are generally chosen to have moderate levels of metal aluminum and zinc, both hydrophilic and lipophilic. In order to improve the hydrophilicity of the printing plate made of this plate material, on the surface of the aluminum printing plate or the zinc printing plate, a hydrophilic metallic flower film of a stable metal inorganic salt or metal oxide is formed by chemical treatment or Chromizing the printing plate; In order to make the surface of the printing plate absorb enough water, the surface of the printing plate has a certain degree of roughness by grinding to increase the surface area of ​​the plate and increase the water storage capacity; in order to improve the oleophilicity of the printing plate, The plate is coated with various non-polar layers in the graphic part or copper plated in the graphic part. Through the mechanical or chemical treatment of the printing plate surface, the oleophilic graphic part of the printing plate is selective for the adsorption of water and ink, that is, it only adsorbs ink, not only water. Through grinding, the roughness of the printing plate is increased, so that the blank portion of the printing plate is only adsorbed to the ink. The hydrophilic blank portion of the printing plate has no selectivity for the adsorption of water and ink, that is, it adsorbs both water and ink. In short, the adsorption of water and ink, the oleophilic graphic part of the printing plate is selective, and the hydrophilic blank part of the printing plate does not have such selectivity. In the printing process, if the ink is applied first, the oleophilic graphic portion on the printing plate and the hydrophilic blank portion are adsorbed simultaneously, and the printing process cannot be performed.
In actual printing, water must be supplied to the printing plate. The water is adsorbed by the hydrophilic blank and cannot be absorbed by the oleophilic graphic part. Then the plate is inked, and the ink is oleophilic. When adsorbed, the blank portion wetted by the fountain solution is not substantially adsorbed. This will ensure the normal printing process.
2. Lithographic Features Lithographic printing process has unique features compared with the processes of wafer printing and gravure printing. It is summarized as follows:
In a lithographic printing plate, the blank part and the graphic part are almost on the same plane of the printing plate. The lithographic printing plate forms, the blank part and the graphic part are almost on the same plane of the plate without the aid of the instrument identification. Using the laws of oil and water incompatibility, the hydrophobic and hydrophobic sections and the hydrophilic and oleophobic sections are formed on the plate of the same plane.
2 The lithographic printing method is an indirect printing, and the pattern on the printing plate is "positive". The ink on the lithographic printing plate is not directly transmitted to the print sheet, but is selected through the middle roller of the elastomer. That is, during the printing process, the ink on the printing plate graphic is first transferred to the blanket cylinder, and the ink on the blanket cylinder is transferred to the printing substrate. Therefore, the graphic is "informal" and is indirect printing.
The rubber blanket wrapped on the surface of the blanket has two characteristics: First, it has better hydrophobic, lipophilic characteristics, can fully transfer ink, and restrict the transfer of water; Second, the blanket is flexible, can be larger The printing pressure and compression deformation of the case are printed with a solid and small rate of expansion.
3 dot fineness. The color of flat-printed products is ever-changing, and the formation of color is formed by nests and color inks, and dot color is the main one. It is guided by the theory of light color, using overlapping dots and juxtapositions, and using the theory of three primary colors plus black (four colors), through the photo separation or electronic separation, the color of the picture is decomposed into yellow with different reticulation angles. Magenta, cyan, black and other four color editions, and then use four-color printing plates, overlapping reproduction of numerous levels and colors, to obtain a very rich color reproduction of the art.
4 multi-color printing, a wide range of printing format. Lithography prints monochrome text, but mostly multi-color overlays. The printing format can be large or small (4 sheets or less to double sheets) and it can be printed with multi-colored products of dots and fields at the same time. Finished products are flat and colorful, which is not covered by letterpress printing.
3. Lithographic printing plates can be divided into lithographic plates, protein plates, indigo plates, gravure plates, multi-layered metal plates, photosensitized plates and pre-coated photosensitive plates in accordance with the different processes of plates and plates. This section mainly introduces the following types.
(1) Protein version Protein version is also called plano-plate. Zinc or aluminum is used as a base plate, and the photosensitive liquid is coated after sanding. The photosensitive solution consisted of protein, ammonium dichromate and water, and was exposed with a negative image. The process of plate making was shown in Figure 1-22.
Yin pictures
Plate pre-processing → Coating photosensitive solution → Exposure → Coating ink → Water development → Trimming → Post-corrosion → Coating protective film → Printing plate Figure 1-22 The graphic of the protein plate in the process of protein plate making is the lipophilic and hydrophobic sclerostin Membrane, high publication base surface 3 ~ 5μm, blank is hydrophilic and oleophobic inorganic salt layer.
Protein plate making process is simple in operation, and the cost of raw materials used is low, but the printing plate is not resistant to printing force, and is generally 5000 to 10,000 India, and is mainly used for short editions.
(2) The gravure plate uses the zinc plate as the base plate, and the positive image is used to print. After the development and corrosion, the graphic and graphic department is slightly lower than the blank plate of the blank. The process of its plate making process is shown in Figure 1-23.
Yin pictures
Plate base pretreatment → coating photosensitive solution → printing plate → developing → etching → coating lacquer → squeegee → de-squeezing → printing plate Figure 1-23 Plano-gravure plate making process After the entire plate making process is completed, the intaglio plate layout is See Figure 1-24.
The gravure plate engraving process is more complicated than the protein plate, and the raw material cost is higher. However, the printing plate's resistance to printing force can reach 20,000 to 40,000 prints. It is a kind of flat printing plate that is widely used in lithographic offset printing.
(3) Multilayer metal plate with steel or aluminum skin as the base plate, copper as the ink-receptive layer, and chromium or nickel as the lithographic plate of the hydrophilic layer.
1 types and main features. The multi-layered metal plate is electroplated on the plate base. If the plate is composed of two layers of metal, such as a copper-chromium structure, it is called a two-layer metal plate; if the plate is composed of three layers of metal, if an iron-copper-chromium structure is used, a three-layer metal plate is called. In addition, according to the layout of the printing plate layout, there are also the points of the plain intaglio and the peaceful typography. At present, most of them use three-layer gravure.
2 three-layer gravure plate making process. The iron-copper-chrome three-layer gravure plate is used as an example to illustrate the process of platemaking, as shown in Figure 1-25.
Yang pictures
Plate base preparation→electroplating→coating photosensitive solution→printing→developing→hardening and double exposure→etching→removal of film→sense grease→retouch→printing plate Figure 1-25 Three-layer metal plate making process After the plate making process is completed The structure of the plate is shown in Figure 1-26.
The oleophilic part of the multi-layer metal plate is a strong lipophilic metal copper, and is treated with a sense of fat to further enhance the sense of fat. The hydrophilic part of the layout is hydrophilic metal chromium or nickel, without inorganic salt layer and oxide layer, the wetting liquid can be directly spread on the blank part, and the amount of water can be reduced in the printing process, which can reduce the scale of the substrate. Deformation helps to ensure the accuracy of registration, while reducing the emulsification value of the ink, in order to help improve the quality of printed products. In addition, the chemical stability of metallic chromium is good, there is a good abrasion resistance, and the printing plate has a higher resistance to printing. The multi-layer metal plate making process is more complicated, the plate making cycle is longer, the printing plate cost is higher, and the dot gain is increased. The reproducibility of color tone is not as good as that of the PS Plate. Generally, it is only applicable to the long plate version.
(4) Pre-coated photosensitive lithographic plate A pre-coated photosensitive layer that can be printed at any time, referred to as the "PS" version. The PS version is produced by a professional manufacturer and directly provides users with printing plates.
1PS plate production.
a. Edition base. PS plate base is generally made of aluminum. The thickness of Aluminum Plate is usually 0.15mm, 0.3mm and 0.5mm.
b. Plate production process. PS plate production process shown in Figure 1-27.
Plate base preparation → Electrolytic roughening → Anodizing → Sealing → Coating photosensitive solution → PS plate Figure 1-27 PS plate production process 2PS plate making process. At present, there are mainly two PS versions that are widely used, namely the positive PS version and the negative PS version.
a. Positive PS version. It is also known as Photo-Solution PS version. The process of plate making is shown in Figure 1-28.
Yang pictures
Plate→Stencil→Development→Correction→Roasting→Drying→Printing Plate Figure 1-28 Positive Pattern PS Platemaking Process Figure 1-29 shows the plate structure of the positive PS version.
In addition, in order to facilitate the inspection and protection of the layout, it is also possible to apply ink and paint gum arabic.
b. Negative PS version. Also called photopolymerization type PS plate. Using the negative image exposure, the photoimageable portion of the light was photopolymerized and hardened, and the blank portion where light was not seen was not hardened without being hardened and was dissolved during development.
The production process of the negative PS type printing plate and the structure of the printing plate are basically the same as those of the positive PS type. Both are flat and convex versions.
PS version has a high resolution, easy to use, and can be saved, has a good printability and high resistance to printing force, is currently more ideal flat plate, has been popularized abroad, the domestic is offset printing The main media.

Second, lithographic offset press Lithographic offset press, in addition to round flat proofer, all round press circular presses. There are many forms of modern offset printing presses. There are manual feeding and manual feeding according to the way of feeding paper. There are also single-side printing and double-sided printing according to the number of printing surfaces; There are also many colors; there are paper types according to the types of substrates, there are also iron sheets, etc.; there are single sheets in the form of substrates, and there are reels;