Detailed explanation of offset printing - ghosting failure (1)

Ghosting

Double shadow is also called double shadow. Refers to the appearance of light lines next to the printed lines or the silhouette that appears next to the printed dot.

The emergence of ghosting makes the thin lines thick, and the original halftones become one half or two (see Figure 8-2). The entire printed matter is blurry and its sharpness is significantly reduced.

Figure 8-2 Type of ghost

The production and type of a ghost

With offset printing, each time the drum is rolled, tiny displacements occur between the rollers, and the corresponding dots do not overlap. The remaining ink layer on the blanket can not be completely overlapped with the transferred ink layer. In addition to the ink layer transferred this time, it also shows the light-colored footprint of the remaining ink layer. As shown in Figure 8-2, the overlapping dark and light inks are displayed on the substrate and a ghosting fault is formed.

Ghosting failures often occur at the beginning of printing, when the drum pressure is combined with the drum pressure separation; when the printing drum is in contact with and separated from the printing surface of the blanket cylinder, the printing surface of the blanket roller and the impression cylinder are in contact with and separated from each other. When the plate cylinder and the transfer cylinder transfer the paper; when the ink roller starts to contact the plate or the plate and when the printing speed changes.

The type of ghosting is classified into vertical ghosting, horizontal ghosting, and A and B ghosting, depending on the direction of ghosting.

Longitudinal ghosting, also known as upper and lower ghosting. The phantom of the dot is at the upper and lower ends of the original dot, and is perpendicular to the axial direction of the roller. Its position is fixed, as shown in the left figure on Figure 8-2.

Lateral ghosting is also called ghosting. The ghosts on the left and right side of the original dot are parallel or at a certain angle to the axis of the drum. The ghosts are light and heavy and their positions are fixed, as shown in the right figure on Figure 8-2.

A, B ghosting, the virtual point of the network, generally in the original site of the upper and lower side, and some in the original site of the left and right, its position is not fixed.

Second, the cause of ghosting and how to eliminate it

There are many reasons for ghosting. You must analyze the cause according to the type of ghosting and find out the exclusion method.

(A) Longitudinal ghosting

1, roller journal and bushing, eccentric sleeve and wall plate hole with a large gap, especially the plate cylinder with a large gap, so that the drum vibration during operation, the drum rotation angular velocity is not uniform, plate cylinder and rubber The cloth cylinder can not be contacted at the same position each time, and the ink on the printing plate is successively transferred to a different position of the blanket cylinder in a second time and ghosting occurs. Most of this type of ghosting occurs at the mouth of prints. It is generally believed that the fitting clearance of the roller bushing is preferably 0.01 to 0.02 mm, and when it exceeds 0.07 to 0.08 mm, maintenance is required to ensure the printing accuracy of the printing press.

2. The backlash and radial run-out of the roller gear is too large, the service life of the offset press is too long, the gear wears, the tooth thickness decreases, and the backlash of the meshing teeth increases, resulting in that each roller cannot run at a constant speed, resulting in instantaneous relative motion between the rollers. Displacement, ghosting. Conditional factories may exchange gears or bearings with severe wear; reduce the center distance of the rollers to keep the backlash within a certain range; use padding paper to pad the mouth of the blanket into a trapezoid to reduce The shock caused by the impact of the small jaw can overcome the ghost caused by the excessive backlash of the roller gear and excessive radial runout in different degrees.

3, roller lining improperly, the pressure is too large, the surface line speed is inconsistent, so that there is excessive squeeze force between the rollers, changing the value of the blanket extrusion. Due to the large displacement of the blanket, it cannot be fully restored to its pre-compression state, resulting in ghosting during ink transfer.

In the rolling process of the three rollers of the offset printing press, it must be a reasonable rolling, so that the three rollers can obtain pure rolling rolling without slip at the same angular velocity. In fact, a slight amount of slip is always present, but it is required that the amount of slip does not exceed a certain amount under the condition of synchronous rolling. Therefore, according to the requirements of the machine instructions, reasonable lining should be adopted to avoid ghosting caused by improper lining.

4. The multi-color offset printing presses are inconsistent with each other. The ink of the multi-color offset press is superimposed under wet and wet conditions. Therefore, the printed ink marks can be easily transferred to the back-colored blanket surface. When the positioning between the units is inaccurate, the first ink will be displaced so that each color transfer can not overlap and cause ghosting. During printing, from the start of paper feeding to the delivery table, many parts are required. The more times the paper is handed over, the greater the cumulative overprinting error and the greater the chance of displacement, ie, the greater the possibility of ghosting. Therefore, it is required that the positioning and transfer components between the rollers be positioned accurately and cannot be loosened or worn to ensure that the rollers transfer the paper at the tangent point.

In addition, failure of the roller pressure, excessive pressure of the roller, poor quality of the blanket, excessive peeling force of the paper, looseness of the plate, or cracking of the plate at the platen may cause longitudinal ghosting failure. .

[Case 8-1]
After a certain factory has a four-color, four-opening imported offset printing machine that prints a heavy paper with a basis weight of 300 g/m2, followed by a single-sided coated paper cigarette marking with a printing basis weight of 80 g/m2, a large area of ​​vertical gravitation appears in the second color. (From the mouth to the regular midline position), the distance between light and deep money tracks is 1.5mm.

[case analysis]
The main reason for the above-mentioned failures is that during the printing of 300 g/m2 thick paper, the impression cylinder grippers work under a high load condition for a long time, resulting in the partial fatigue of the small compression springs of the grippers and the decrease in strength. The paper is unstable, causing ghosting.

[Approach]
(1) Perform a comprehensive inspection of the impression cylinder grippers, replace some small fatigue springs that are ineffective, ensure that the angles of the grippers are in place, and that the lengths of the grippers are the same and the force is consistent.
(2) Clean and lubricate the original gripper.
(3) The machine is idle for about half an hour, then try again.
[Expert Tips]
There are many machines that print thick paper first, and then print thin paper. The beginning is a double image, but after 1,000 to 2,000 prints, the ghost image disappears. This shows that some machines pick up the spring force can be restored.

(b) Horizontal ghosting

1. The axial movement of the gripper shaft, the gripper is mounted on the gripper shaft, and the gripper shaft is tangled, resulting in inaccurate transfer of paper and displacement of the paper, resulting in ghost images. Generally adjusting the gap between the tight end of the roller gripper shaft and the end face of the sleeve can overcome the ghost caused by the axial movement of the gripper teeth.

2. Loosen the thrust bearing of the drum or loosen the thrust bearing or tighten the nut of the thrust bearing. Loosen these parts to make the axial displacement of the plate cylinder, the blanket cylinder and the impression cylinder during high-speed operation, resulting in ink. Transfers do not coincide, resulting in ghosting. General requirements for the axial direction of the machine does not exceed 0.03mm, so find out the roller, tighten the nut of the thrust bearing, you can eliminate this ghosting.

In addition, the string of water, string ink roller axial movement; paper "foliar"; improper front and side baffle adjustment, so that the paper edge pressure is too large to produce curl; paper is not smooth, etc. will cause horizontal ghost.