Author: Shandong Lukang Pharmaceutical Group Wu Xiangyang Industrial Company printing plant
In a sheet-fed offset press, the impression roller gripper is one of the most important parts. Among them, the gripping power of the gripper is very critical. If the gripper is not properly maintained or maintained, or wears out after long-term use, it may cause the gripper's gripping force to decrease or the gripping force of the entire row of grippers. Inconsistent, and cause printing failures such as inaccurate overprinting and ghosting.
There are 15 grippers on the gripper shaft of the impression cylinder. The gripping force of grippers is mainly adjusted by the pressure of the springs at both ends of the gripper. The greater the pressure of the spring, the greater the gripping force of grippers. The factors that cause the change of gripping force of paper teeth are mainly in the following aspects.
1. Long-term use leads to a reduction in the spring force of the spring, which reduces the gripping force.
The gripping force of the gripper teeth of the impression cylinder is greater than the peeling force of the rubber cylinder, to ensure that the gripper teeth of the impression cylinder should be able to peel off the paper from the rubber cylinder after the gripper and the rubber cylinder are pressed. After a long period of use, the elastic force of the springs at both ends of the tooth bar decreases, so that the gripping force of the gripper teeth of the impression cylinder becomes smaller, and the paper will come out of the gripper teeth, which cannot be printed on the rubber roller. Even if the gripping force of the gripper of the impression cylinder is slightly greater than the peeling force, and the paper does not come out of the gripper, displacement may occur in the gripper, which may cause overprinting, ghosting and other failures of the printed matter.
One ghosting problem I once encountered was caused by the gripping force of the platen of the impression cylinder being too small. When printing a white cardboard with a basis weight of 250g /? Using Beiren J2205 offset press, the first color is printed separately (that is, when the second color is not pressed), ghosting does not occur. When performing two-color printing, the first color The ghosting of the graphic. At first it was suspected that it was caused by the second-color blanket. After replacing the new blanket and printing again, the ghosting problem still exists. After careful inspection, it was found that the root cause of the fault was the improper gripping force of the impression cylinder grippers. The image area of ​​the first color is small, and the peeling force of the rubber cylinder is less than the gripping force of the gripper. Therefore, no ghosting occurs when the first color is printed separately; while the image area of ​​the second color is larger, the peeling force of the rubber cylinder is corresponding It is slightly larger. When printing the first color and the second color at the same time, the paper is displaced in the second color group, causing the image and text of the first color to produce a ghost image on the second color blanket.
2. After long-term use, the dental pad wears, resulting in reduced gripping force or inconsistent gripping force of grippers.
During the printing process, the long-term mechanical external force causes the dental pad to wear, and the degree of wear will be different, and the dental pad will become uneven or the pattern of the teeth will be smoothed, which will inevitably affect the gripping force of the gripper. In addition, if the offset offset press is often used to print small-format paper, it will also cause different wear procedures for the dental pads, that is, the central dental pad wears more heavily, and the dental pads on both sides wear lighter. When printing large-format paper again, the gripping force of the entire row of gripper teeth will be reduced, the gripper force of the gripper teeth in the middle is small, the gripper force of grippers on both sides is large, and the gripper of the paper is wavy when gripping the paper. During printing, problems such as pleating, flicking, and cracking may occur.
3. Improper adjustment of grippers and inconsistent opening and closing times will also result in different degrees of wear of the dental pads, resulting in inconsistent gripping power of grippers.
If the gripper is improperly adjusted, and the opening and closing time may be early or late, the closed gripper will first come in contact with the pressure immediately, not only the dental pad is worn seriously, but also the small compression spring in contact with the dental pad is easy to break, causing Dead teeth, causing tearing during printing.
4. Foreign materials such as paper powder, powder spray, and ink are often stuck to the grippers during printing. If they are not cleaned in time, the wear of the grippers will be accelerated, resulting in a reduction in gripping force of the grippers and various failures.
In short, the operator must be familiar with the structure of the gripper of the impression cylinder, make correct adjustments and maintenance, and should pay attention to the following matters.
1. The height of the dental pad should be adjusted uniformly.
2. The length of the dental tablets should be adjusted uniformly.
3. The opening and closing time of the gripper should be adjusted uniformly.
4. Regularly check the wear of grippers and replace worn grippers in time.
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