2.3 Rubber roller
The rubber roller mainly has the following functions:
1. Transfer ink. The rubber cylinder transfers the ink from the printing plate to the surface of the paper to complete the printing. Unlike other printing methods, the printing plate transfers the ink directly to the surface of the paper.
2. Make up for the comprehensive error of the machine. Due to the errors in the processing and installation of the machine, the places where the rollers are in contact with each other cannot be completely uniform. However, when printing, it is impossible to allow some places to be printed, and some places cannot be printed, which means that the ink on the printing plate needs to be completely transferred. This can be done with a rubber cloth, because the covering on the surface of the rubber roller (general term for rubber cloth and pad) has a certain compressibility. In this way, where the gap between the rollers is small, the deformation of the liner is larger; where the gap between the rollers is large, the deformation of the liner is smaller, but the liner is always completely close to the surface of the roller it contacts, so that the ink is completely Transfer from the plate cylinder to the paper. This is one of the important functions of the rubber roller.
3. Compared with other contact printing, printing pressure is greatly reduced. Basically similar to the above analysis, if the surfaces of the two rigid cylinders are completely in close contact, the cylinder surface must have a considerable degree of deformation to overcome the machining and installation errors of the machine, so a large printing pressure is required. A large printing pressure will bring a lot of disadvantages, which will be analyzed in detail later. The use of compressible elastic lining can greatly reduce the printing pressure, because the compressive deformation force of the lining is much smaller than that of the cylinder surface. A lower printing pressure will bring a lot of benefits to the working state and printing quality of the machine. This will be analyzed in detail in the chapter on printing pressure. This is one of the main reasons for the continuous development of offset presses.
4. Buffer vibration absorption. During normal operation, the surface of the drum is subjected to many forces, and the magnitude of the force is constantly changing, which means that the drum is always in a complex vibration state. That is, the pressure of the lining can not be consistent at every contact place on the surface of the drum, and the pressure of the lining in different places is also inconsistent due to the unevenness of the surface of the drum. When the pressure is high, the liner deforms more; when the pressure is small, the liner deforms less, but there is no large deformation on the drum surface. This changes the pressure changes caused by machine vibration and machine processing and installation errors into the elastic energy of the blanket. When contacted, elastic energy is produced; when contacted, elastic energy is released. When it comes in contact again, new elastic energy is produced, and when it comes out of contact, it is released again. This process is repeated continuously to minimize the damage caused by the impact and vibration of the machine.
5. Safety effect. When paper or other foreign matter (which greatly exceeds the elastic compression of the liner) enters between the rubber cylinder and the impression cylinder, the liner will be damaged, but the accuracy of the surface of the cylinder is not affected or is minimally affected. To protect the machine.
2.3.1 Lining
The function of the lining is exactly the same as that of the rubber roller above. In fact, the above-mentioned function of the rubber roller is completely expressed through the lining. The performance of the lining determines the role of the rubber cylinder, and also determines the level of print quality. The ideal printing process should be that the graphics on the printing plate are transferred to the paper (or substrate), and the size and shape of the imprint remain unchanged, that is, completely true reproduction. For offset printing, the rubber cylinder is used as an intermediate medium and should not have any impact on the transfer of imprints. But this can only be achieved if the diameter of the plate cylinder and the diameter of the impression cylinder are exactly the same, and the surface contact conditions are exactly the same, as shown in Figure 2.3.
(Figure 2.3)
It can be seen from the figure that the printing length of the printing plate is L = Rp · Z, the printing length of the rubber cylinder opposite to the printing plate cylinder is Lbp = Rb · Z, and the printing length of the rubber cylinder opposite to the impression cylinder is Lbi = Rb · Z, the printing length on the impression cylinder is Li = Ri · Z, then the difference between the impression on the impression cylinder and the plate cylinder is: L = (Li-Lbi)-(Lp-Lbp) = Li-Lp-LB = Li-Lp = (Ri-Rp) · Z, where LB is the difference between the marks on both sides of the rubber Lbi = Lbp, so LB = 0.
When Rp> Ri, the print is shortened; when Rp = Ri, the print is the original length; when Rp <Ri, the print is enlarged.
Therefore, when the pad of the printing plate (the diameter of the plate cylinder) or the thickness of the paper (the diameter of the impression cylinder) changes, the length of the print will be affected, and the change rule is the same as above.
The LB mentioned in the above analysis is zero only assuming that the contact conditions on both sides are exactly the same. But in fact, the contact conditions on both sides cannot be exactly the same, so how does the value of LB change? Suppose the ink is placed on a smooth surface (convex surface) and a rough surface (concave surface), as shown in Figure 2.4. When subjected to the same pressure, its deformation is shown by the dotted line. The rough surface prevents the ink from deforming more than the smooth surface. To make the ink deformation in the two states equal, the pressure on the rough surface should be greater than the pressure on the smooth surface. [next]
Because the current general offset printing version is a flat convex PS version, that is, the text part is a little higher than the blank part. When the ink is transferred from the rubber cylinder to the surface of the paper, the roughness of the surface of the paper is larger than that of the PS version (of course, different papers are different, the offset paper is rougher than the coated paper), and it is in plane contact, so The deformation of the ink is small when pressed.
(Figure 2.4)
When printing, in theory, L = (Li-Lib)-(Lbp-Lp) = 0 is required. For this formula to be established, Li-Lib = Lbp-Lp is required, so that the pressure between the plate cylinder and the rubber cylinder must be less than the pressure between the impression cylinder and the rubber cylinder. It is relatively easier to control the deformation of rough plane ink than smooth plane. In all possible cases, try to find ways to increase the roughness of the contact surface. If you change the flat letterpress to flat intaglio, it might be better from this point.
2.3.2 Selection of lining
The choice of lining needs to be considered from the two points of machine accuracy and print quality. In terms of the accuracy of the machine, the greater the compressibility of the liner, the better the ability to compensate for machining and installation errors; from the quality of the printed matter, the smaller the compression of the liner, the better, because In this way, the smaller the contact surface, the closer to pure rolling, that is, the smaller the deformation of the imprint. From the point of view of machine accuracy, under the premise of satisfying the quality of the print, try to choose the most compressed liner; from the point of view of the quality of the print, on the premise of meeting the accuracy of the machine, try to choose the least compressed liner, but The accuracy of the machine should be the main factor, which must be kept in mind.
The bag liner is usually composed of two parts, the pad and the blanket. The role of the bag liner is actually the combined effect of the pad and the blanket.
Gaskets are generally divided into three categories: soft gaskets, neutral gaskets and hard gaskets. The soft pad is mainly made of woolen cloth, which has the strongest ability to make up for the accuracy error of the machine, but the deformation of the imprint is the largest. The accuracy of randomizers continues to improve, and such pads have tended to be eliminated. Neutral liners are mainly tape, which is less compressible than soft liners, but the deformation of the imprint is small, so there are still many used at present. The hard liner is mainly made of special backing paper. This type of liner has the worst ability to compensate for machine accuracy errors, but the imprint is clear. Therefore, as the accuracy of the machine is further improved, the amount of hard liner is also increasing.
Rubber blankets are generally divided into two categories: elastic blankets and air cushion blankets. The elastic blanket is easily deformed after being stressed, but it cannot be compressed and cannot compensate for machine accuracy errors. The air cushion blanket is compressible and easy to deform, to a certain extent, it can compensate for machine accuracy errors. When the pressure increases, the contact area increases less than the elastic blanket, so a larger printing pressure can be used to make the print clearer and stronger. The function of the lining is reflected by the deformation of the lining. Without deformation, its function will not exist, so the key is how to create the conditions for its deformation, and at the same time, it can help the deformation to compensate for machine errors and ensure printed products The purpose of quality. Therefore, the blanket and pad or both have compressibility and deformation; or one of them has; or one has deformation and one has compressibility. In a word: the two can compensate each other, creating conditions for deformation and compressibility. Starting from this principle, the matching of blanket and pad can be as follows:
â‘ Elastic blanket + soft liner;
â‘¡Elastic blanket + neutral liner;
â‘¢ Air cushion blanket + neutral liner;
â‘£ Air cushion blanket + hard liner.
The other two are not permitted in use, the reasons are:
â‘ The liner formed by elastic rubber cloth + rigid liner only has the ability of deformation, but not compressibility, and cannot compensate the accuracy error of the machine.
â‘¡The deformation and compressibility of the lining formed by the air cushion blanket + soft liner are too large to reflect the complementary effects of both sides, and the air cushion blanket is relatively expensive, so it makes no sense to do so. [next]
2.3.3 Selection of gasket
Exactly what kind of liner is used depends on the specific situation, but in the same state, there is an optimal state in its selection method. Let's analyze this best state.
After the cushion is installed, it cannot be allowed to slide forward and backward under the blanket. If it slides, it is likely to deform the print. How can it not slide relatively? As shown in Figure 2.5, the outer surface of the pad is in contact with the blanket, and the inner side is in contact with the roller, so these two contact surfaces will produce friction that hinders the movement of the pad. When this friction is insufficient, the pad may slide relatively. Therefore, in order to prevent it from sliding relatively, a sufficiently large friction must be created. As mentioned earlier, there are two ways to increase friction:
(Figure 2.5)
â‘ Put the blanket on the surface of the drum that tightly and evenly surrounds it, that is, generate enough friction.
â‘¡ Make each contact surface have the largest friction coefficient:
a. The surface of the blanket should have the largest possible roughness (under the premise of ensuring accuracy);
b. The surface of the pad should have the largest possible roughness;
c. The surface of the rubber cylinder body should have the maximum roughness (under the premise of ensuring accuracy).
The way to achieve the first one is to increase the tension of the blanket, so that it generates greater pressure in the contact surface; the key to the second way is the choice of materials. From the actual situation, the materials of the blanket and the roller are determined, especially the roller can not be replaced under normal circumstances. The surface characteristics of the blanket will vary depending on the manufacturer. What the user can choose is the roughness of the pad surface. From the perspective of increasing the coefficient of friction, the rougher the surface of the pad, the better. There are two reasons for this requirement:
â‘ From the perspective of the contact surface, the coefficient of friction between the pad and the blanket is much larger than the coefficient of friction with the roller, that is, the possibility of the pad sliding relative to the surface of the roller is large. The possibility can only be to increase the friction coefficient.
â‘¡ From the inside of the pad, because the pad is generally stacked together, it is not allowed to slide on the contact surface. The friction force generated between them that hinders relative sliding must be greater than the friction force generated by the two external contact surfaces (and the blanket, and the roller body). Therefore, the rougher the surface of the liner, the less the possibility of relative slippage. The fewer the layers of the liner, the less the possibility of slippage.
In summary, the principles for selecting pads are:
â‘ The friction coefficient is as large as possible, that is, the surface is as rough as possible;
â‘¡ As few layers as possible;
â‘¢ The surface of the pad should be as flat as possible.
The last point is required by the printing process, otherwise the transfer of the print will be affected. Starting from this principle, the following three methods of selecting gaskets are analyzed:
Soft padding: Soft padding is usually made of woolen cloth, so the friction coefficient of its surface is absolutely guaranteed. When the pressure is insufficient, it can be adjusted by adding very little paper. In general, the number of layers is not large.
Neutral liner: Neutral liner generally uses rubber and paper to form the friction coefficient of the tape surface is still relatively large. The key issue is the choice of paper. Paper commonly used by printing manufacturers include: coated paper, newsprint, offset paper, cardboard and kraft paper. Paper jams are inconvenient to use because of their low flexibility. The surface of coated paper is relatively smooth, so it should not be selected. The choices are offset paper, newsprint and kraft paper. These three types of paper can produce a large friction coefficient, but from the perspective of the least number of layers, the use of kraft paper is slightly better. No matter which kind of paper is used, Article 3 of the above principles must be absolutely guaranteed.
The installation method of the gasket is generally to put it on the surface of the drum so that it is even and evenly contacted. Some equipment is equipped with a device for clamping the gasket, which provides convenience for the installation of the gasket. If the blanket is not tight enough, it will cause the paper to crease forward or backward.
2.3.4 Installation of blanket
After the blanket is installed, it is required to enclose it evenly and tightly on the outer surface of the liner, and the tension across the surface is consistent. This is an important condition for ensuring the uniform quality of the surface of the printed product. The blanket is held by two special clips, each with six to eight clamping screws. There are several points to note when installing the blanket:
â‘ The blanket is cut into the required rectangle, which is a very important part to ensure that the blanket is evenly stressed. If one side is long and the other side is short, the tightness after installation may be inconsistent.
â‘¡Let the edge of the rubber cloth tightly against the stop in the clip, so as to prevent more clips and less clips, resulting in uneven stress.
â‘¢ When tightening the blanket, first the middle and then the two sides, so as to prevent the clamped sides from becoming wavy. When tightening the screws, be sure to approach them step by step, otherwise sometimes it is thought that they are clamped, but in fact not clamped, causing the blanket to slide inside.
â‘£ When installing the blanket on the roller, be sure to put the clip of the blanket in the clip above the roller, otherwise the clip of the blanket will be thrown out when the machine is running.
⑤ Both sides of the blanket are locked with a worm gear mechanism. The worm gear mechanism is self-locking, and its anti-loosening can be achieved by the elasticity of the rubber cloth itself.
â‘¥ In general, the blanket cannot guarantee an absolute rectangle. If it is a slight difference (compared to its length), it will not have a great impact on the tension, but if the difference is large, one way is to remove it and cut it again; another way is to pad paper on the shaft of the worm gear. This can be partially tightened.
2.3.5 Matters needing attention during the use of the blanket
1. Check the flatness of its surface. The way to check is to play full version, but the printing pressure should be lower than normal pressure. Only in this way can the unevenness of the surface be exposed. If the pressure is too great and the ground is thick, it is difficult to see the difference.
2. If the unevenness of the surface is unacceptable (specific indicators can be judged by experience), you can check the uniformity of the surface of the blanket and liner and whether there are foreign objects on the surface of the roller. After removing the foreign objects, if the unevenness still exists, you can use the method of drawing a "map". First draw each low (or false) place, and then place a sticker on the back of the blanket (the thickness of the paper is selected according to the situation).
3. If the surface of the blanket is damaged during use, the solution is the same as 2. However, if the blanket is scratched, the image formed by it is emptied or blank in the middle, and it continuously transitions to the surroundings to a place that is not crushed. This is one of the most obvious signs of judging the blanket being sagged.
4. The surface of the blanket should be acid and oil resistant to prevent its surface layer from being destroyed. The frequent contact of the blanket with oil can cause its aging, and finally loosen its internal organization. The frequent contact between the blanket and the acidic substance will cause the surface to corrode. It should be cleaned with fast volatile detergents, such as gasoline, instead of volatile substances such as kerosene.
5. Creep and stress relaxation of the blanket. After the blanket is stressed for a long time, there are two kinds of performance: a if the length is constant, the stress gradually decreases, which is called the stress relaxation; b if the stress is constant, the length will gradually increase, which is called the creep.
After the blanket is installed on the roller, the length will not change, which will cause stress relaxation. Therefore, after a period of use, the blanket needs to be re-tightened. If you don't use it for a long time, you should loosen the blanket to avoid stress relaxation.
6. The surface of the blanket should be kept clean. The effective parts of printing need to be kept clean frequently. If it is not clean, it will affect the printing pressure. When padding the blanket, the paper usually used is around 0.05mm, and there is a clear difference in the reproduction of the pad and non-pad pads. This shows the effect of this 0.05mm thick paper. In fact, the paper with a thickness of 0.05mm increases the printing pressure by 0.05mm (generally, the printing pressure is between 0.05-0.25mm). If ink or other impurities are fixed on the surface of the blanket, the printing pressure will change, thus Print quality has an impact. Non-printed parts should also be kept clean. This is because when the blanket at the printed part is damaged, sometimes the second half of the rubber (usually the printed surface will not exceed the entire blanket) is borrowed. If this part is not kept clean, as the sludge and other impurities continue to condense, and the printing pressure continues to increase, the surface of the blanket will be damaged and cannot be used normally. The increase in printing pressure will also bring other problems, which will be described later. Keeping the surface of the blanket clean should also pay attention to the junction of the effective parts of the printing, which has the most condensed ink and other impurities. Due to the increased pressure, the fatigue of the blanket is accelerated. In practice, it has been found that the blanket in this part is most prone to cracks. Therefore, impurities must be prevented from accumulating here.
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