Plate deformation and its control (1)

The plane printing plate, which is mounted on the plate cylinder, first undergoes bending deformation, and when it is tightened, it undergoes tensile deformation. In order to improve the overprint accuracy of printed matter, it is necessary to control these two types of deformation of the printing plate.

First, the bending deformation of the printing plate

Plates such as lithographic plates, rubber plates, and photosensitive plates, etc., are mounted on the plate cylinder of a printing press and then change from flat to cylindrical, as shown in Figure 8-5. Assuming that the axial direction (Z-direction) of the plate cylinder is not affected by external forces, the deformation of the plate in the XOY plane necessarily includes bending deformation.

The bending deformation of the plate in the XOY plane can be regarded approximately as the pure bending deformation of a straight beam with a rectangular cross section, as shown in the left figure of Figure 8-6. According to the theory of material mechanics, in a purely curved process of a straight beam, any one of the cross-sections maintains its planar shape and rotates at an angle. If one assumes that the beam is composed of longitudinal fibers, as the cross-section rotates, the concave fiber of the beam will be shortened, the flange fiber of the beam will be elongated, and there will be a constant length of fiber in the middle, the so-called “neutral layer”. The position of the neutral layer of the short section beam is consistent with the position of the middle layer of the beam. For a printing plate, the length of the neutral layer is equal to the width of the plate before deformation, and the surface used for printing on the printing plate, which is equivalent to the outer surface of the flange of the beam, is the most severely deformed.

Let the length of the printing plate along the circumference of the roller be L (the width before printing plate deformation), the thickness of the printing plate be b, and the radius of the printing plate roller is R, as shown in the right figure of Figure 8-5.

K=L/2Ï€(R+b/2)

K is the plate utilization factor. Based on this, the absolute elongation ΔL1 and the relative elongation ΔL1/L of the printing plate due to bending can be determined, that is,

ΔL1=[2π(R+b)-2π(R+b/2)]·K=b/2R+bL (8-4)

ΔL1/L=b/2R+b (8-5)

From (8-4) and (8-5), we can see that when the plate thickness is constant, the bending deformation of the plate decreases with the increase of the radius of the plate cylinder; when the plate cylinder radius is constant, the plate bending deformation Increase with plate thickness.

The surface plating of multi-layer metal plate should not be too thick, otherwise it will be excessively deformed due to bending of the plate, resulting in partial cracking of the coating layer, forming a dense pattern of moiré, causing the blank part of the printing plate to be sticky. The flexographic plate and rubber flexo plate installed on the rotary printing press have a thick plate base and a lot of bending deformation. Therefore, in the image layout design, the amount of deformation should be taken into account.

The printing speed of the high-speed rotary printing press is high, the radius of the printing plate cylinder is small, and the utilization factor is large, so that the amount of bending deformation produced by the printing plate is large. In order to control the bending deformation of the printing plate, it is necessary to use a thin printing plate. For example, the PS plate used in high-speed web offset presses is typically 0.3 mm or 0.15 in thickness. m.

In addition, multi-color printing presses should use printing plates of the same thickness so that the bending deformation produced by each printing plate is close to ensure the accuracy of overprinting.