Current situation and development of the hottest o

2022-08-17
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The current situation and development of offset blanket

in 1982, the former National Economic Commission established a coordination group for printing technology and equipment. Under the leadership of Comrade Fan Muhan, it formulated a 16 character development policy of "Laser Phototypesetting, electronic color separation, offset printing and binding linkage", which played a major guiding role in the development of printing technology and equipment in China, making offset printing flourish. Offset printing blanket (hereinafter referred to as blanket) is a very important and essential printing equipment in offset printing. With the continuous development of printing technology, new requirements are constantly put forward for its variety and quality. The blanket plays an intermediate role in the offset printing process. Based on the principle of "hydrophilic and oleophobic" in the blank part and "oleophobic and hydrophobic" in the graphic part, the ink part on the printing plate is first transferred to the blanket, and then the blanket transfers the ink to the substrate as faithfully as possible. In the early 1960s, the blanket used in China's printing press was all imported. It has been more than 40 years since the first piece of blanket (ordinary field blanket) was trial produced by Shanghai printing equipment factory in 1960. At present, the field blanket has basically replaced the import and is used on the printing machine with a printing speed of about 4000 pieces/hour

ordinary blanket generally has a split (915cm × It can really make graphene grow 915cm × 1.85mm) and full sheet (1200cm × 1200cm × 1.85mm) in two specifications, its structure is that after four layers of cotton cloth are bonded (natural glue is applied between each layer), the surface is coated with synthetic rubber glue resistant to ink solvent

rubber blanket is a kind of fine rubber product. It should faithfully transfer the ink adsorbed on the graphic part of the printing plate to the substrate (generally various types of paper), and the blank part of the printing plate is coated with wetting solution by the dampening rubber roller in advance to ensure that it is free of oil and ink. Therefore, the surface rubber must adopt synthetic rubber that is ink resistant, solvent resistant (wipe the rubber blanket when changing the printing plate) and acid and alkali resistant, The four layers of rubber blanket should have good adhesion between them, and give the rubber blanket a certain elasticity. Generally, natural rubber is used as the main glue, and a reasonable formula is designed. In general offset printing, the transfer of ink to the paper is completed by the reverse operation of the blanket and an embossing cylinder under a certain pressure. In addition, the general color printing products are at least yellow, magenta, cyan and black, and the advanced printing products are 6 or even 8 colors. To ensure the accuracy of overprint, the blanket must ensure sufficient tension (the blanket is tightened on the cylinder) and good constant load elongation, To ensure that the blanket will not stretch longer and longer in the printing process, resulting in point deformation, false handwriting and other ills. In addition, the blanket must also have good overall flatness, and the surface glue must not have impurities of any size, so as to ensure that the ink is transmitted evenly, the picture level is clear, and the text is sharp and clear. It can be seen that the overall design of blanket involves many fields such as chemistry, polymer chemistry and textile, especially the base cloth as the framework material of blanket. In the weaving process, cotton yarn is specially spun, cotton cloth is specially woven, and the finishing is specially processed. The warp direction (that is, the direction of the blanket wrapped in the drum) should maintain a certain elongation and deformation. It is a special product with high technical requirements in the textile industry

with the rapid development of printing technology, especially the continuous development of printing towards multi-color and high-speed (web paper, double-sided printing), the development of its matching blanket must adapt to it. The original ordinary blanket is far from meeting the needs of printing, and air cushion compressible blanket (hereinafter referred to as air cushion blanket) came into being. The so-called air cushion blanket is to add a very fine closed cell rubber sponge layer between the two layers of bottom cloth close to the surface adhesive layer, so that the blanket as a whole has extremely excellent compression deformation resistance and instantaneous elastic recovery, So as to ensure that the printing can be carried out smoothly under the condition of high-speed operation (the printing machine has developed into two-color, four-color, six color, eight color machine, web printing machine, and its speed has exceeded 10000 sheets/hour, and even reached 60000 sheets/hour)

during the printing process, the rubber blanket on the ground contacts the embossing cylinder (there is no compression layer in its internal structure) to produce shear stress, which causes the rubber on the surface of the rubber blanket to expand (or convex hull) on both sides of the contact cylinder, thus affecting the printing quality. When the air cushion blanket is in contact with the drum, the compression layer of its inner layer shrinks vertically, so that the shear stress is absorbed and the expansion phenomenon will not occur, which not only ensures the smooth progress of high-speed printing, but also ensures the high quality of printing

the completion of the printing process depends on a certain printing pressure, and the printing pressure given by the offset printing machine is achieved by adjusting the gap between the blanket cylinder and the embossing cylinder. Due to the special requirements of traditional gravure printing and embossing printing, the printing pressure is generally about 40 threads, while the printing pressure of offset printing with the printing speed of 5000 sheets/hour is generally about 25 threads, but for the printing speed of more than 10000 sheets/hour, especially the web printing machine, the printing speed can be as high as more than 60000 sheets/hour, Such a high printing speed must be well maintained, and the printing pressure should be about 10 threads to make the printing smooth. Therefore, the technical performance requirements of air cushion blanket are higher. At present, only the United States, Japan, Germany, Britain and other industrialized countries have air cushion blanket production plants in the world. The overall flatness error of air cushion blanket is generally required to be about 1 wire, while the surface adhesive layer can only be within 40 wires (the flatness error of ordinary blanket can be about 4 wires, and the surface adhesive layer is about 60 wires), otherwise it will cause defects such as point deformation and unclear graphic hierarchy. Therefore, in the design of air cushion rubber blanket, the four layers of bottom cloth used as the framework layer of the bottom cloth must be designed with different thicknesses. The thin cloth with a thickness of about 15 wires must be in contact with the surface adhesive layer to ensure that the rubber blanket is tight on the roller without exposing the texture structure of the bottom cloth, which will affect the printing quality. The thickness of the middle two layers of bottom cloth is about 25 wires, while the thickness of the last layer of bottom cloth is 35 wires, At the same time, to give the air cushion rubber blanket excellent compression deformation resistance and instantaneous elastic recovery, the key is the production of (closed cell) foam rubber sponge layer. Throughout the development process of air cushion blanket at home and abroad, foreign countries began trial production in the early 1970s, domestic trial production began in the mid-1980s, and small batch production began in succession after the mid-1990s. There are generally three methods for making air cushion closed cell foaming layer: (1) salting out method (2) chemical foaming agent method (3) microsphere method

salting out method uses a very fine MgSO4. The crystals are mixed into the rubber formula, and then the salt crystals are dialyzed with water after vulcanization to maintain a very fine closed cell layer. Considering its long production cycle and low efficiency, it has generally been less used

chemical foaming agent method is to decompose and release CO2, N2 and other gases under certain temperature and pressure conditions, so as to match the vulcanization rate of rubber. The lattice of rubber cross-linking layer contains this gas, so as to produce a closed cell rubber sponge layer. Now, most air cushion blanket manufacturers at home and abroad use this method

microsphere method uses a specially processed microsphere, which is mixed into the rubber formula, and vaporizes and expands under certain temperature and pressure to form a compression layer with a highly uniform closed cell structure. Due to patent rights and other issues, only a few foreign manufacturers adopt this method

in the whole production process of air cushion blanket, in addition to the design of base cloth and formula, vulcanization process is also an important factor to determine the quality of air cushion blanket. Due to the limitations of various conditions, domestic manufacturers that can produce air cushion blanket currently use drum vulcanizing machine to produce air cushion blanket, which has some negative effects on the uniformity and fineness of closed cell foaming layer and the overall elasticity of products when the breaking point ratio of air cushion blanket is adjusted to 80. In addition, in order to ensure the excellent flatness of air cushion blanket, the product needs to be ground by continuous grinding machine after vulcanization. In addition to the above purposes, another important purpose of this process is to give the rubber on the surface of the blanket a certain smoothness. People generally have a one-sided view that the smoother the surface of the blanket, the better. In fact, it is not true. Although the smoother the surface, the more complete the transfer of printing plate points, the worse the transfer of printing plate ink concentration (less ink), and because the printing speed is faster and faster, especially in web printing, the smoother the surface of the blanket, the worse the paper layout, and even affect the normal progress of printing

it can be seen that there are two purposes of blanket surface grinding, one is to give the blanket a good flatness as a whole, and the other is to give it a certain surface smoothness (but not the smoother the better), so as to meet the needs of high-quality printing

as a new printing equipment with strong vitality, air cushion blanket is attracting the attention of domestic and foreign manufacturers, especially in China. According to the author's incomplete statistics, the demand ratio of domestic air cushion blanket to ordinary blanket is 1:3, and ordinary blanket still accounts for a large share, but economically developed southern regions, such as Shenzhen, Guangdong and Fujian, The ratio of air cushion blanket to ordinary blanket has changed fundamentally, basically 4:1, which is equivalent to that of foreign advanced countries. It can be seen that with the rapid development of economic construction, the printing technology continues to develop, and the demand for air cushion blanket will be increasing. With the continuous emergence of printing refresh technologies such as UV (UV curing) printing and commercial form printing, new requirements are also put forward for the variety and specification of air cushion blanket. In addition, in order to maximize economic benefits, make it faster and more convenient to replace the blanket, and also to make the tension more uniform when the blanket is loaded on the drum. Some blanket manufacturers install aluminum splints on both ends of each blanket in advance before the blanket leaves the factory, so as to replace the iron splints with more than a dozen screws when disassembling the blanket

in 1999, the China printing and Equipment Industry Association put forward a 28 word policy for the development of China's printing industry before 2010: "digital networking before printing, multi-color and efficient printing, diversified automation after printing, and high-quality serialization of equipment". Among them, the requirements for equipment are the current goal of equipment production in China. The quality of rubber blanket in China is still in the middle and low grades, but it has not reached high quality, and the variety is relatively single, so it cannot form a series, which is also the direction of printing equipment production

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