Plate deformation and its control (2)

Second, the tensile deformation of the printing plate

The printing plate is installed on the plate cylinder of the offset printing machine and must be fixed by means of a plate clamp. Due to the tension, the plate must be stretched and deformed.

From Fig. 8-7(a), it can be seen that the plate fixed to the roller is subjected to the following forces: the tension T of the plate is tightened, the frictional force F' of the pad to the plate, and the positive pressure N. The bending moments formed by these forces together cause the printing plate to undergo a bending deformation, and the tensile force formed along the circumferential direction of the printing plate cylinder causes the printing plate to undergo tensile deformation. Next, the approximate estimation of the printing plate stretching is discussed.

Assuming that the radius of the plate cylinder is quite large, a section of the plate is considered to be approximately a plane, and regardless of the force F′ and N of the plate to the plate, the stretching of the plate can be regarded as equal section. The straight shaft tension, as shown in Figure 8-7(a), the relative strain ΔL2/L can be calculated as follows:

DL2/L=T/EA (8-6)

In the formula, E is the elastic modulus of the plate, the common unit is N/m2, and A is the cross-sectional area of ​​the printing plate. From (8-6), we can see that when the E of the plate and the A of the printing plate are constant, the relative deformation ΔL2/L of the plate produced by the stretching is proportional to the tension T of the printing plate; when the printing plate When T and A are constant, ΔL2/L is inversely proportional to E; when T and E are constant, ΔL2/L is inversely proportional to A. It should be pointed out that these inferences are both approximate and qualitative. According to the above inference, when using plate clamps to fix lithographic plates such as PS plates and gravure plates, the tension should not be too large, and the degree of tension should not be fixed. A plate with a small elastic modulus must not be too thin. Otherwise, the plate will be excessively stretched due to a large decrease in cross-sectional area.

Further consider the unevenness of the tensile deformation of the printing plate. Referring to Figure 8-7(b), due to the tension force T, on the one hand, the plate is required to withstand the pressure N given by the pad, and on the other hand, due to the relative slippage between the plate and the pad, The direction of the frictional force F', F' against the pad is along the circumferential direction of the plate cylinder. If a section of the printing plate is taken out, the frictional force production and the positive pressure N are taken into consideration, and the force diagram is as shown in Fig. 8 -7(c) shows. P1 and P2 in the figure indicate the tensile forces on the two end faces of the printing plate. Due to the presence of friction, there must be P1>P2 during the balance. From this point, from point A to point C (from point B to point C), the axial stress on each section of the plate (that is, the tensile force per unit area) decreases, so the tensile deformation of the plate is also Decreasing, A and B point of the maximum tensile deformation, C point is not deformed, this is the uneven deformation of the printing plate. Inhomogeneities in the deformation of the printing plate can result in local over-displacement of the printing plate (at locations near A and B) and can also affect the relative positions of the printing plate sections.