Offset blanket structure and printing characteristics

Offset blankets are products of high molecular compounds. Understanding and mastering the structure and printing characteristics of offset blankets is of great significance for rational use. This article gives a brief discussion on this aspect of knowledge, and initiates a discussion to discuss the research.

First, the type of printing blanket

There are many kinds of printing blankets, and they can be divided into blankets for impression rolls and transfer blankets according to their functions. The gravure printing blanket and the printing blanket are all belong to the printing roller blanket (the blanket printing process is not in contact with the ink), the sheet-fed offset printing machine blanket, the web offset printing blanket and the printing iron Blankets, etc. are transfer blankets. Offset blankets from raw materials to products, not only from the special synthetic rubber and excellent composition, but also have dedicated rubber processing equipment and advanced processing technology. With the continuous development of the printing industry, the variety and quality of printing blankets have also been greatly improved. At present, in addition to ordinary blankets, air-cushion blankets have also been used in large quantities.

Second, the structure of offset blanket

The structure of the offset blanket is designed based on the total distance between the cylinder spacing of the printing press and the thickness of the outer liner of the blanket cylinder, taking into account the printing technology requirements (such as ink, solvent and printing pressure). Offset blanket structure shown in Figure l.

l. Total thickness: The thickness of domestic blankets is the same as that of similar foreign products, with a total thickness of 1.80-1.90 mm. There are also a small number of blankets with a total thickness of 160-1.70 mm, which consists of three base fabrics.

2, the thickness of the surface glue: the thickness of the surface glue should be appropriate, generally 0.6 ~ 0.7 mm, if the thickness is too thin, the elasticity is low, the hardness is too high, will wear the printing plate, reduce the printing plate resistance to printing force, will also make the network point not Full, the screen from the ink (that is, the screen from the article), different shades of black; surface glue is too thick will make the print network dot deformation (such as dot gain, elongation, etc.) lead to imprint movement, overprinting is not allowed, the picture is blurred.

3, the thickness of the cloth glue; cloth glue mainly from the bonding bottom cloth, increase the role of flexibility. Its thickness is limited by the thickness of the surface glue and the total thickness. It is generally controlled at 1.2 to 1.3 mm (including four base fabrics). The thickness of the adhesive layer between the four backings is also different, as shown in Fig. 2. The adhesive layer thickness and performance design between 1 and 2 backings are mainly designed to increase the compressibility and softness of the adhesive layer close to the printing surface. Sex. The thickness and performance of the adhesive layer between the 3 to 4 layers of the backing cloth only serve to bond the backing cloth.

4. The number and thickness of the bottom cloth: The radial force of the blanket during printing is very large, which is nearly 100O kg. Therefore, the blanket must be selected as the skeleton material of high-strength cotton.

5, the structure of air-cushion blanket: Air-cushion blanket appearance and ordinary blanket no obvious difference, its thickness is roughly divided between several 1.65 ~ 1.9 mm specifications, air-cushion blanket and ordinary blanket The main difference is that it not only has different fabric layers and rubber layers, but also has a gas-filled layer sandwiched between them to form an elastic layer. The gas-filled layer is mainly divided into microbubbles and gas channels.

Third, offset rubber in the characteristics.

Offset blankets are different from general rubber products in that they are responsible for transferring the ink on the plate to the paper. In order to make the printed ink uniform, clear dots, rich layers, offset blanket must have moderate hardness, compression deformation, good ink transfer performance and small elongation characteristics, it must have the ability to absorb ink and adsorption fountain solution in chemical properties The role of the chemical must be such that it does not react chemically with oils and medicines and is eroded by oils and medicines. Because of the lithography, the flatness of the blanket is demanding. The technical requirements for offset blankets are as follows:

l. Hardness: refers to the ability of rubber to resist the pressing of other substances onto its surface. From the printing requirements, the hardness is not too high too low, we must consider three aspects: the quality of printed matter, the life of the printing plate, the precision of the printing press and the blanket itself.

2. Elasticity: refers to the ability of the blanket to restore its original shape immediately after removing its deformed external force. During the printing process, when the rubber skin roller comes into contact with the impression cylinder, the blanket is deformed by a certain pressure. When the surface of the impression roller is turned away from the surface of the rubber roller, the blanket is required to be restored to its original shape and then the printing plate is accessed. Point part of the ink, so the blanket must have a very high elasticity.

3, compression deformation: refers to the rubber after repeated compression of the rubber deformation strength. When the blanket is printed, it is compressed by thousands of times per hour. Innumerable compression recovery processes cause the blanket to be compressed and permanently deformed. At this time, the blanket thickness will be reduced and the elasticity will be reduced. Reduced, so that the blanket can not continue to use. Therefore, the smaller the compression deformation of the blanket, the better.

4, tearing force: refers to the force used when the blanket is torn off. The blanket is subjected to nearly 1,000 kilograms of pulling force during printing. Therefore, when considering the skeleton material, the warp yarn of the base fabric must have a relatively high strength, because the blanket is mainly supported by the base fabric to withstand these forces when the blanket is subjected to a tensile force; in addition, the surface rubber layer Must also have a certain strength, so that the surface glue will not be crushed by the sand or folded paper in the paper.

5. Ink transferability: refers to the ability of the blanket to transfer ink. The blanket must not only have a strong ability to accept ink (ie sucking capacity), but also have the ability to properly transfer ink.

6. Oil and solvent resistance of surface glue: It refers to the ability of the surface layer rubber of blanket to resist the penetration of oil or certain solvents. If this resistance is lacking, the portion of the blanket that contacts the ink and solvent will expand, affecting printing.

7. Flatness: When the blanket is processed, the uniformity of the thickness of each point is required, that is, the processing thickness must have a certain degree of accuracy. Offset blankets are used for lithographic offset printing. The compression of the blanket and impression cylinders during printing is only 0.1 to 0.2 mm. Therefore, the flatness of the blanket must be determined properly, and the flatness error must not exceed 0.04 mm. If it exceeds 0.04 mm, the ink of the print will be uneven and the dot shape will change. It is sometimes necessary to fill the back of the blanket before applying it. Therefore, flatness is especially important for offset blankets.

8, elongation: refers to the blanket under a certain amount of tension beyond the amount of the original length, the size of the blanket extension is generally used to express the elongation, the smaller the blanket elongation is better, the elongation is large, the rubber is easy When stretched, the adhesive layer will be thinned and the elasticity will be reduced. The size of the blanket's elongation mainly depends on the strength of the four-layer base fabric.

9. Appearance quality: The surface of the blanket should be surface-treated like a printing plate so that numerous fine blisters are uniformly distributed on the surface of the blanket, and the surface is finely smooth and smooth, with no fine foreign matter. If it is not surface-treated, the surface of the blanket Too smooth, the ink absorption is poor. On the other hand, tests have shown that blankets with too high a smoothness (also known as smoothness) absorb much more of the paper's hair than do rough-surfaced blankets.