Offset rubber blanket analysis (1)

2.3 Rubber Roller

Rubber roller mainly has the following functions:

1. Transfer ink. The blanket cylinder transfers ink from the 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. Since there are errors in the processing and installation of the machine, it is impossible for the rollers to be in complete contact with each other. However, when printing, some places cannot be allowed to print on, and some places cannot be printed, which means that the ink on the plate needs to be completely transferred. The use of blankets can do this because the blanket lining on the surface of the blanket (collectively referred to as blankets and gaskets) is somewhat compressible. Such a small gap between the drum, the liner lining deformation is greater; where the gap between the drum is large, the liner lining deformation is smaller, but the lining is always close to the surface of the drum in contact with it, so that the ink can be guaranteed completely Transfer from plate cylinder to paper. This is one of the important roles of the blanket cylinder.

3. Compared with other contact printing, the printing pressure is greatly reduced. Basically similar to the above analysis, if the surfaces of two rigid rollers are brought into close contact with each other, the roller surface must have a considerable degree of deformation to overcome the machining and installation errors of the machine, thus requiring a large printing pressure. There is a lot of pressure in printing, and it will bring a lot of disadvantages. It will be analyzed in detail later. The use of compressible elastic liners can greatly reduce the printing pressure because the force that compresses the liner is much less than the force that compresses the cylinder surface. Smaller printing pressure will bring many benefits to the working condition of the machine and the quality of printed products. The chapter on printing pressure will analyze this in detail. This is one of the main reasons why offset presses can continue to move forward.

4. Shock absorption. During normal operation, the surface of the drum is subjected to a lot of force, and the magnitude of the force is constantly changing, which means that the drum is always in a complex vibration state. That is, at each contact surface of the drum surface, the pressure of the lining cannot be consistent, and because of the uneven surface of the drum, the pressure of the lining at different places is also inconsistent. When the pressure is high, the lining is more deformed; when the pressure is less, the lining is less deformed, but the roller surface is not deformed greatly. This changes the pressure caused by the vibration of the machine and the processing and installation errors of the machine into the elastic energy of the blanket. When in contact, elastic energy is generated; when disengaged, elastic energy is released. Once in contact again, new elastic energy is generated, and once released, it is released again. This process is carried out repeatedly to minimize the damage caused by the impact and vibration of the machine.

5. Security role. When there is paper or other foreign matter (largely over the elastic compression of the liner) into the gap between the blanket cylinder and the impression cylinder, its lining will be damaged, but the accuracy of the surface of the cylinder is not affected or affected minimally, from To protect the role of the machine.

2.3.1 Lining

The function of the lining is exactly the same as that of the squeegee above. In fact, the above function of the squeegee is completely manifested by the lining. The performance of the lining determines the role of the blanket cylinder and also determines the quality of the printed product. The ideal printing process should be that the graphic on the plate is transferred to the paper (or substrate), the size and shape of the print remain unchanged, that is completely true reproduction. For offset printing, the blanket cylinder acts as an intermediary and should not have any effect on the transfer of the imprint. However, this can only be achieved if the plate cylinder diameter is exactly the same as the diameter of the impression cylinder 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 imprint length of the printing plate is L=Rp·Z, the length of the imprint on the blanket cylinder opposite to the plate cylinder is Lbp=Rb·Z, and the length of the imprint on the blanket cylinder opposite to the impression cylinder is Lbi=Rb· Z, the length of the imprint on the impression cylinder is Li = Ri · Z, then the difference between the imprinting cylinder and the printing plate roller is: L = (Li-Lbi) - (Lp-Lbp) = Li-Lp-LB =Li-Lp=(Ri-Rp)·Z, where LB is the difference between the two sides of the eraser Lbi=Lbp, so LB=0.

The blot was shortened when Rp > Ri; the original length was blotted when Rp = Ri; the blot was amplified when Rp < Ri.

Therefore, when the pad of the 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 analysis above is zero when it is assumed 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, then what is the value of LB? Assume that the ink is placed on a smooth plane (convex surface) and a rough plane (concave surface), as shown in Figure 2.4. When subjected to the same pressure, the deformation is shown by a dotted line. The force to deform the ink due to the rough surface is greater than the smooth plane. To equalize the ink deformation in both conditions, the pressure on the rough surface should be greater than the pressure on the smooth surface.