First, 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.
Second, the characteristics of offset blanket
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.
When printing off-site or large-area graphic products with poor quality offset paper, the phenomenon of paste blanketing is likely to occur, which is a headache for operators. Paste blankets not only affect the production speed, increase the labor intensity, and waste raw materials, but also pay little attention to quality accidents. Below, the author analyzes the phenomenon of paste blanket caused by various reasons encountered in the work.
1. Printing pressure is too small
The printing process is a process in which the printing plate selectively adsorbs the ink and transfers the ink to the printing material through the blanket. However, due to the addition of paper and paper powder, the transfer of ink is hindered, so that the ink cannot be smoothly transferred to the substrate but remains on the blanket. By appropriately increasing the pressure between the three rollers, forcing the ink to be transferred to the substrate unconditionally, the problem of the paste blanket can be solved. However, while increasing the pressure between the rollers, it is also necessary to consider the printing plate's resistance to printing. When printing large quantities of products, baking can be done to increase the printing plate's resistance to printing.
2. Eraser layout uneven
If the blankets and pads are used for too long, irregularities will appear. It is easy to pile up paper and paper powder in the sunken place, causing a partial paste blanket. In the sunken place, wipe the reducing agent or use 1 or 2 layers of transparent plastic tape to stick to the back of the blanket to restore it to a flat surface. If you can't solve the problem of the local paste blanket, you can only replace the blanket and pad.
3. Poor flow of ink
If the fluidity of the ink is not good or too viscous, the peeling force on the surface of the paper will increase, and the paper surface and the paper powder on the surface of the paper can be easily peeled off and transferred to the printing plate, ink roller or ink through the blanket. Slots create a vicious circle.
When printing large quantities of products, timely cleaning should be performed according to the amount of paper and paper powder in the ink roller, ink tank, and so on, which can also guarantee the printing plate's resistance to printing forces.
Adding an appropriate amount of auxiliary material (No. 6 oil or thinner) in the ink can increase the fluidity of the ink, and then add an appropriate amount of the diluting agent, which in turn increases the ink's activity, so that the ink can be transferred to the printing well. On the object, reduce the phenomenon of paste blanket.
Encountered above, some operators will mistakenly believe that adding an appropriate amount of sizing agent can increase the fluidity of the ink, but at the same time, adding the sizing agent also reduces the motility of the ink and makes it stick on the plate and eraser. The ink on the cloth is difficult to clean, and the phenomenon of sticking blankets will not be reduced.
4. The arrangement of printing color sequence is unreasonable
If it is a four-color machine, you can use a small amount of text and a small amount of ink on the front to keep paper and paper powder on the previous print.
The reasons for sticking blankets are various. The above points are some of the experiences that the author has summarized in practice and are for reference only by peers.