Identification and Analysis of Offset Ghosting
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Ghosting refers to the appearance of shadows or ghosts around printed graphics, which can be divided into axial ghosting and circumferential ghosting according to different directions. The axial ghost is at a small angle to the axis of the roller and the circumferential ghost is perpendicular to the axis of the roller. The reasons for several ghosting faults are described below.
1. The blanket is not stretched tightly, resulting in full or partial circumferential ghosting, which is divided into two cases.
(1) The overall tension of the blanket is not tight, so that the blanket cannot be reset in time after being pressed, resulting in an overall ghosting.
(2) The blanket is cut or installed so that the local tension is not tight and local ghosting is caused. At this point, tighten the blanket or reinstall the blanket to solve the problem.
2. The roller pressure is too large, resulting in an overall circumferential ghosting. The drum pressure should be adjusted according to the thickness of the printed paper.
3. The paper is not dry, causing more than a dozen sheets of paper on the upper layer. The direction of this ghost is not fixed and will become lighter and lighter.
4. The paper is uneven, the water content is uneven, causing local ghosting, and the position is not fixed, sometimes it is not, when it is light and heavy.
5. The printing plate is not tightened or the two ends of the mouth are cracked, resulting in a full-page or partial circumferential ghosting.
6. Two-color machine embossing roller The crepe pad is lower than the roller surface, and the embossing roller licks the paper, and the paper cannot be close to the roller after the nip of the embossing roller. After the first color blanket is imprinted, most of the arches disappear. When the second color blanket is embossed, there is still a small amount of arching. Due to the slight difference in the curvature, sometimes it cannot be combined with the first color blanket. The dot blots overlap completely, causing ghosting.
7. The impression cylinder is too small and uneven. The gripper and the pad are worn out too much, too smooth, or the amount of ink is large, and the viscosity is large, so that the paper moves in the gripper. Due to the different amount of movement, the dots of the two-color blanket cannot be overlapped, and a ghost image appears.
8. The drum gear and bearing wear and the gap becomes large. When the machine is suddenly loaded, the drum produces a ghost due to vibration. In addition, if the roller bearing is loose, the thrust bearing is not locked, etc., which may cause ghosting.
There are many reasons for ghosting. When you encounter a ghosting problem, you should first analyze the type of ghosting, find the cause according to its type, and troubleshoot.