Analysis and Exclusion of Common Faults Affecting Print Quality (Part 2)

Adjustment method: Check each part of the water supply and ink supply device, replace the aging parts, and adjust the pressure between the rollers to make them consistent.
8. Print Overprinting is not allowed for fault analysis and resolution

(1) The paper is stretched and deformed. When the moisture content of the paper and the humidity of the printing environment cannot reach the balance, the paper will absorb water or dehydrate and cause telescopic deformation, which will affect the accuracy of overprinting. In addition, the paper will cause telescopic deformation under pressure and cause overprinting. Adjustment method: humidity treatment for printing workshop, the method described above.

(2) The plate is not allowed to register due to the tensile deformation of the plate. Adjustment method: When the upper plate is used, tighten the tightening screw to be even and moderate, and don't cut the part of the screw tightly to the limit, so as to avoid serious deformation of the plate.

(3) Blanket deformation caused by blanket deformation is not allowed. Adjustment method: use a blanket with small elongation; during the overprinting process, keep the tightness of the blanket unchanged.

9. Analysis and resolution of the ghosting problem of printed products

According to different ghosting directions, ghosts can be divided into three types: horizontal ghosting, longitudinal ghosting, and AB ghosting. The horizontal ghosting network is located on the left and right sides of the original outlet, and the position is basically fixed. Longitudinal ghosting is also called upper and lower ghosting. The ghosting of the dot is at the upper and lower ends of the original dot, and the position is fixed in the direction perpendicular to the axial direction of the roller. AB ghosting, some of the virtual point of the network point in the upper and lower side of the original network, some in the original site left and right sides, the position is not fixed. AB ghosting is sometimes found on every sheet, but it sometimes appears every Other sheet or sheets.

(1) Causes of horizontal ghosting and adjustment methods

1Axial movement of the gripper shaft causes misfeeds, misalignment, and ghosting. Adjustment method: Adjust the gap between the two ends of the roller gripper shaft and the end face of the sleeve to overcome the ghost caused by the axial movement of the gripper.

2 Roller thrust bearing screws Loosening, wearing, or loosening the nuts of the thrust bearing can cause relative axial displacement during high-speed operation, causing ghost images. Adjustment method: if it is found that the thrust bearing wear should be replaced immediately; find a drum with a large amount of axial rotation (generally require the machine's axial string 0.03 ~ 0.05mm), tighten the thrust bearing nut, you can eliminate this Ghost image.

3 The printing paper is not stable, the paper is unbalanced in the local force during the printing process, causing the paper to deform, causing horizontal ghosting. Adjustment method: According to the requirements of the machine manual, adjust the parameters of each part of the paper feeding system; check the inside suction and blowing environment of the machine, and solve any problems in time.

(2) Causes of longitudinal ghosting and adjustment methods

1 roller journal and bushing, eccentric sleeve and wall plate hole accuracy is poor, with the gap is too large, especially the plate cylinder with a large gap, the drum in the process of vibration, the drum rotation angular velocity is uneven, there ghosting. Most of these ghosts occur at the mouth of printed products. Adjustment method: After long-term use, when the clearance between the shaft diameter of the drum and the shaft sleeve is too large, it must be replaced in time. In addition, it is necessary to adjust the clearance of the roller sleeve, and the clearance of the roller sleeve is preferably 0.01 to 0.02 mm.

2 printing pressure is too large. This is usually related to improper roller lining. Excessive pressure makes it difficult for the blanket to recover to its original state in a timely manner at a high speed, resulting in longitudinal ghosting. At the same time, large pressure peeling force is large, and the force of rubbing the teeth is less than the peeling force of the paper from the blanket, causing longitudinal displacement of the paper. Longitudinal ghosting occurs. Adjustment method: According to the requirements of the specification, choose a reasonable lining to avoid longitudinal gravitation caused by improper lining of the roller; at the same time, reasonably adjust the printing pressure. In general, the pressure between the printing plate and the blanket is 0.10-0.15mm. The printing pressure between the blanket and the impression cylinder is 0.15 to 0.20 mm.

3 Roller gears have low machining accuracy, serious wear, large backlash of gears, and large radial runout, which may cause the drums to fail to operate properly. Instantaneous relative displacement between drums results in longitudinal ghosting. Adjustment method: choose high-precision gears, replace worn gears; adjust the printing pressure by adjusting the thickness of the lining; use pad paper to pad the mouth of the blanket into a trapezoid to reduce the impact Hit the resulting vibration and so on.

4 The multi-color offset printing presses are inaccurately connected to each other, resulting in displacement of the ink of the first printing color, and the blotting of each color transfer cannot be overlapped to cause ghosting. Adjustment method: Check and adjust the degree of wear, installation accuracy, and position of the paper positioning transfer parts to ensure accurate paper transfer.

5 When the vertical fine-adjustment mechanism of the multi-color machine is loosened and the bearing wear is severe, under the effect of printing pressure, the plate cylinder has momentum from the idle to the crested mouth, which causes the plate cylinder to move in the circumferential direction and cause longitudinal ghosting. Adjustment method: Adjust the gap of the fine adjustment mechanism so that it does not affect the rotation of the plate cylinder, and it can exert its fixing effect on the plate cylinder.

(3) Causes and adjustment methods of AB ghosting

AB ghosting generally occurs on printing machines with double or multiple diameter rollers. The possible causes and adjustment methods of the faults are as follows.

(1) The precision of the manufacture and installation of the dental caries is not enough or the wear is serious, which reduces the work accuracy and causes ghosting. Adjustment method: Check the degree of wear of the carious device and replace it in time. Check the force of the carious teeth. According to the installation and adjustment instructions on the manual, correct the pad and the force of the caries; check the working conditions of the caries and adjust them to normal. , to achieve a flexible grippers, to ensure the accuracy of the transfer.

2 ink viscosity is too large, so that the paper peel tension increased. For papers with high tensile strength and high retractility, AB stretching may also occur due to the difference in elongation at peeling. Adjustment method: According to the printability requirements, the ink viscosity is properly adjusted so that it can meet the printing requirements.

The above briefly introduces the possible causes and adjustment methods of common faults in offset printing. There are many reasons for the failure, both the reasons for the machine, but also for the error of the operation, can not be generalized, there is no fixed routines to follow, which requires the printing operator to strive to master the printing process and mechanical properties of various machines The relevant knowledge, etc., carefully sum up production experience, better prevent and eliminate all kinds of printing failures, thus reducing the production of defects and improving the quality of printed products.