Analysis of overprint fault of the hottest offset

Analysis of overprint failure of offset printing machine (II)

the paper gripper force of the second gripper and several experimental pieces are used as a batch of experiments. When investigating the overall strength deviation of a batch of experimental pieces, statistical processing is carried out. The influence of the handover relationship on overprint

during the printing process, the paper gripper not only transmits the paper, but also fixes the paper at a certain position, so that the pictures and texts on the printing plate or blanket are transferred to the designated position. However, during imprinting, if the paper teeth have insufficient or uneven force, and the paper position changes, it will cause inaccurate overprint

1. The paper gripper and its force and overprint

the paper gripper mentioned here mainly refers to the paper delivery and drum paper gripper. The uneven force of the paper teeth of the embossing cylinder can cause four kinds of deformation of the paper (see Figure), and local overprint inaccuracy will inevitably occur in the printed matter

(Figure paper deformation caused by uneven paper holding force of paper holding teeth)

in order to prevent inaccurate overprint caused by uneven paper holding force of paper holding teeth, the following points should be achieved

(1) the adjustment of the gripping force of the gripping teeth should be carried out in the trial printing, and the gripping force should not be changed in the process of overprint of various colors. The gripping force of the gripping teeth on the embossing cylinder of the multi-color machine should be consistent, and the "over packaging" of the gripping teeth must be flat, so as to ensure the uniformity of the gripping force

(2) for products overprinted on the front and back, the gripping force of the paper teeth of the imprint cylinder must be uniform

(3) the supporting spring of the gripper shaft should have good elasticity to prevent insufficient gripper force and inaccurate paper positioning

(4) tighten the tight ring of the paper gripper to prevent the paper gripper from moving axially. Use the paper gripper shaft or shaft sleeve with high manufacturing accuracy to keep the paper gripper in a stable state during the paper gripper process, and the paper gripper distance is always maintained at a constant value

2. Handover relationship and overprint

there are handover relationship problems in the whole transmission process of paper feeding, regular parts, paper delivery teeth, and paper transfer roller. As mentioned above, handover relationship refers to the handover position and time. In essence, handover relationship is to ensure the accurate transportation of paper, and it is absolutely not allowed to have the phenomenon of paper out of control. The handover position refers to the handover of the paper above a certain point, which is the progressiveness condition and performance characteristics of the manhole cover pressure fatigue testing machine, and the handover time is the time when the handover machine parts jointly complete the handover of the paper. This time is required to be adjusted in strict accordance with the regulations, which can be neither short nor long. For example, the handover time between the paper delivery teeth and the teeth of the embossing cylinder. If the handover time is short, overprinting must be inaccurate, and if the handover time is long, Will tear its mouth edge. Therefore, the adjustment of the handover relationship is a very careful technical work, and we must not be careless

the relationship between printing pressure, paper stripping force and overprint inaccuracy

paper deforms under the action of printing pressure and stripping tension in the printing process. After the paper with large plastic deformation leaves the embossing area, the deformation cannot be restored, resulting in large overprint error

1. Printing pressure and paper deformation

paper is a material with plastic deformation, and the plastic deformation characteristics of different papers are also different. Under the action of large printing pressure, the width and the length of the two corners of the trailing tip of the paper with large plastic deformation will expand slightly, resulting in inaccurate overprint, especially in the trailing tip of the paper

the greater the printing pressure is, the more obvious the deformation of the paper is. Therefore, when printing the same batch of products, the pressure of each color printing plate should be as consistent as possible, but sometimes the pressure should be adjusted according to the specific situation. For example, the printing pressure of multi-color offset press is often decreasing in random groups

the deformation of the paper is closely related to the strands of the paper. Considering that the overprint error caused by the deformation of the paper under the printing pressure of the general offset press is difficult to adjust along the axial direction than along the radial direction, so when printing, a single sheet of paper should be as parallel as possible to the longitudinal direction of the paper and the axial direction of the drum, and the direction of the strands of the printing paper must be the same for the same batch of products, In order to ensure that the sleeve provides data support and scientific basis for the use and implementation of graphene coatings in relevant fields and the formulation of standards, and the printing accuracy must not allow the same batch of products to be printed on paper with different strands

under the action of printing pressure, due to the fan-shaped deformation of the paper, the image with high requirements for overprint accuracy or the part that plays an important role in the theme should be arranged near its mouth during prepress processing and plate making, because the paper near its mouth has small deformation during printing, which is easy to overprint accurately

2. Paper peeling tension and paper deformation

after printing in the embossing area, the paper is caught by the paper teeth of the drum, forcing it to separate from the blanket, and the ink on the printing plate is used as the medium to adhere the printing sheet to the surface of the blanket with strong adhesion. At this time, the fiber tissue on the printing sheet is affected by forces in two directions, one is the downward force of the paper teeth on the paper, and the other is the upward adhesion of the ink on the paper. The direction of these two forces is opposite. When the printing sheet is peeled from the blanket, the trailing tip of the paper is easy to curl and does not fit with the impression cylinder, forming a refractive angle, which is called the stripping angle. If the adhesion of the ink is large, the longer the printing sheet is peeled and pulled down by the paper teeth, the greater the stripping angle, the greater the stripping tension, the stripping deformation of the paper also increases, the paper is stretched along the radial direction of the drum, and the overprint accuracy decreases

in the printing process, if the gripper teeth of the embossing cylinder have enough gripper force to ensure that the paper does not displace, the size of the stripping tension will become the main factor affecting the stripping deformation of the paper, and its deformation depends on the following aspects:

(1) the nature of the paper. The paper with large plastic deformation has large stripping deformation, and vice versa

(2) the time when the printing pressure and the drum imprint each other. The greater the printing pressure, the longer the embossing time, the longer the contact time between the paper and the ink on the blanket, and the greater the stripping deformation

(3) ink receiving area of paper surface. The larger the inking area, the thicker the ink layer, and the greater the stripping deformation

(4) viscosity and adhesion of ink. The greater the viscosity and adhesion of the ink, the greater the stripping deformation, and vice versa