1000 Offset Printing (257-273)

257. What is offset printing plate?

A: The offset printing plate is a type of printing plate in which the graphic part and the blank part (non-graphic part) lie flat on the same plane. Its use of oil-water incompatibility principle, graphic part of the oil-and water-repellent and adsorptive ink, blank part of the hydrophilic and oil-repellent and adsorption of water (fluid solution), when the offset printing plate is printed on the first water after the ink, through selectively Adsorption finishes printing. At present, the most commonly used offset printing plate is the PS plate.

258. What is the basic structure of offset printing plates?

A: The offset printing plate consists of a base plate and a printing section. Plate base is the support of the printing part, generally consists of materials with certain mechanical strength and chemical stability; the printing part is the working surface in the printing process, there are two parts of the graphic part and the blank part, it is through the coating of the photosensitive liquid. , After exposure, development, fixing; After washing and other processing in the system.

259. What are the classifications of offset printing plates?

Answer: 1 According to the different printing plate structure, the offset printing plate is divided into a plano convex type and a concave type;

2 According to the material of the base plate, the offset printing plate has a lithographic plate and a metal plate. Glass plate (Cara version) and paper base plate;

3 According to the structural pattern of the base material, the offset printing plate is divided into a single flat printing plate and a roller printing plate;

4 According to the roughening state of the surface of the substrate, the offset printing plate is divided into ordinary sand and sandless printing plates;

5 According to the different graphic basis, the offset printing plate has a protein plate. Photosensitive resin plate, multi-layer metal plate, etc.

6 According to blank basis, offset printing plate is divided into ordinary type and anhydrous type, etc.

7 According to different uses, offset printing plates have proofing plates, printing plates, etc.

260. What is the PS version?

A: The PS version (Presensitized Offset P,ates) is an abbreviation of pre-coated photosensitive resin plate. It is an offset printing plate formed by pre-coating the photosensitive liquid on the surface of the plate base. Currently widely used in China is aluminum-based positive PS version.

261. What is the basic composition and function of the PS version?

A: PS version from the base. Oxide layer. Hydrophilic layer and photosensitive layer.

1 Edition base: The carrier of the printing plate picture and text, plays the role of supporting the photosensitive layer, the plate base generally uses the aluminum plate;

2 Oxide layer: After anodizing the plate base, it will form a hard oxide film on the surface of the plate base. This layer of oxide film has strong adsorption, abrasion resistance and corrosion resistance, and can firmly connect the photosensitive material. Adhesive and aluminum base:

3 Hydrophilic layer: The hydrophilic film formed after anodization and sealing;

4 photosensitive layer: from the photosensitive agent, film-forming resin. A photosensitive solution composed of an organic solvent, a dye, and the like is coated on the surface of the plate base and dried. The photosensitive layer of the positive-type PS plate has good oleophilic repellency, abrasion resistance, and acid resistance.

262. How to choose the base material?

Answer: According to the principle of offset printing, the base material should have good lipophilicity. Hydrophilicity, excellent bending strength, high abrasion resistance, and adaptability to chemical processing.

In fact, the plate base material that completely meets the above conditions does not exist. However, in combination with the oleophilic and hydrophilic requirements of offset printing plates, among copper, iron, zinc, aluminum, nickel, and chromium metals, copper has the best lipophilicity and the worst hydrophilicity; chromium has the worst lipophilicity. Hydrophilicity is the best, and zinc and aluminum are in between. Therefore, offset printing plates based on zinc and aluminum are widely used.

263. What is the surface treatment of the base?

A: The aluminum plate used in the PS plate produced by the aluminum plate plant has a very smooth surface, a low hardness, and poor adsorptivity, hydrophilicity, abrasion resistance, and corrosion resistance. Therefore, it cannot be directly used to produce a PS plate. In order to meet the need for offset printing, aluminum plates must first be surface-treated.

264. What are the methods of surface treatment?

A: The surface treatment of the substrate is chemically treated and mechanically sanded. Liquid grinding. Electrolytic grits and other methods. Mechanical grinding is divided into ball grinding method. Brush grinding method and sandblasting method, electrolyzed sand have static hanging contact method, non-contact spraying method and non-contact power supply method. Among them, electrolytic sanding and ball milling are most commonly used.

265. What is the process flow of electrolytic sand?

A: The process flow of electrolytic sand: degreasing → washing → pickling → washing → electrolytic roughing → washing → anodizing → washing → sealing → washing → drying.

266. What is sand?

A: The grain size refers to the micropores formed after the surface of the aluminum plate is roughened. Sand spot is the basis for the formation of layout and blank sections, and the sand appearance on the plate base can best represent the surface treatment level of the base.

267. What is the role of surface treatment of the plate?

Answer: The surface roughening treatment of the plate base is to form sand spots. Its role is:

1 layout pores are conducive to the printing plate to receive and store the fountain solution. The specific surface area of ​​the printing plate increases, which plays a role of better adsorption of liquid and wetting of the plate surface;

2 There is coating left on the photosensitive resin;

(3) to generate surplus surface energy to activate the layout, to create a condition for firmly and stably adsorbing substances such as oil and water with lower surface free energy;

Numerous micropores in 4 layouts have good oleophilicity for the layout. Hydrophilic properties create the conditions.

5 negative effects: too fine sand, layout is easy to dirty; too coarse sand. Affect the reproduction of the tone level of the printed product; the wear of the sand will reduce the service life of the printing plate, and make the printing plate resist the printing force under the action of the acidic dampening liquid.

268. What are the performance requirements of photosensitizers?

A: The sensitizer is the photosensitive resin, which is the main component of the photosensitive layer of the PS plate. Performance requirements of sensitizers: Strong adhesion to the plate base, good affinity to the ink, excellent imaging sensitivity and high resolution, low pollution to the environment, high chemical stability, and convenience Save, low cost, etc.

269. Photosensitizers can be divided into several categories?

Answer: 1 According to the photochemical reaction mechanism of photosensitizers, photosensitizers are classified into photolysis, photocrosslinking, and photopolymerization;

2 According to the photosensitizer after the photosensitivity. Adhesion and hydrophilic hydrophobic properties change, divided into photosensitive soluble, photosensitive insoluble, photosensitive release type. Hydrophilic hydrophobic base variants and so on.

270. What are the most commonly used photosensitizers for PS?

A: The commonly used sensitizers for PS are o-naphthoquinones, diazonium salts, azides, and polyvinyl cinnamates. The former is used in the positive PS version, and the latter three are used in the negative PS version. Among them, o-naphthoquinone naphthalocyanine sensitizers are most widely used in PS plates, and can be used in combination with other resins, and can also be chemically bonded to molecular chains of some polymers to form various unique photosensitive resins. .

271. What is the photosensitivity mechanism of PS version?

A: There are two ways to say about the sensitization mechanism of the diazonaphthalene sensitizers:

1 Under irradiation of ultraviolet rays, the diazonium group is decomposed, nitrogen gas is precipitated, and internal rearrangement of molecules is performed to obtain fluorenone, which is hydrolyzed to produce indole carboxylic acid. The carboxylic acid is developed in a dilute alkaline developer to produce a carboxylate salt which is removed after it is dissolved in water.

2 Under the irradiation of ultraviolet rays, the surface molecules of the diazonaphthoquinones decompose to form fluorenone, which immediately reacts with the adjacent diazonaphthoquinone molecule that has no light in the lower layer to form lactones. The lactone is developed in a dilute alkaline developer to produce a carboxylate salt that is dissolved in water and removed.

272. PS version how to classify?

Answer: 1 According to the material of the base plate, the PS plate is divided into paper base and film base. Aluminum plate type;

2 According to the photochemical reaction type of photosensitive resin, the PS version is divided into photopolymerization type. Light decomposition type. Photosensitive peeling type and light cross-linked type;

3 According to the original image drawn in the printing process, the PS version is divided into two types: positive and negative.

273. What are the advantages of the PS version?

A: The surface of the 1 base is finely ground. Uniform, good moisture-retaining, hard and wear-resistant plate oxide film, high resolving power of the photosensitive layer, good dot reducibility, excellent ink-receiving and ink-transfer performance, and accurate reproduction of the tone and color of the original;

2 The plate making process is simple, the plate making speed is fast, and it is easy to use and can be prepared in advance;

3 layout with small water consumption, stable performance, strong printability, high printing plate resistance, suitable for high-speed printing;

4 The printing plate is rich in levels, and the dot reproduction is good. It can accurately reproduce the tone and color of the manuscript. Especially suitable for printing fine products;

5 The cost is low and the storage time is long.