Sheetfed offset printing machine is not allowed to be faulty--longitudinal overprinting is not allowed (two)
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2. Overprinting caused by paper passing over the unilateral position
Inaccurate overprinting caused by the unilateral passing of the paper means that the longitudinal cross-line operation surface or the side of the transmission surface is not overprinted. Moreover, the crosshairs that are not inaccurate are in the direction of the tip of the standard line. The common reasons are as follows.
(1) One of the front gauge tongues on this side is too high. When the height of the paper stop is more than 3 times the thickness of the printed paper, there is a possibility of overprinting misalignment. Solution: Loosen the adjusting screw, put 3 sheets of printed paper under the paper stopper, and then tighten the screw to retest. 3 sheets of paper can be pulled out gently, and 4 sheets of paper can be tightly packed.
(2) A linear groove on the front gauge plate
Although the front gauge plate is rigid, it will produce a linear groove after thousands of times of paper rubbing. Paper may enter the linear groove or may not enter the linear groove. When the paper enters the linear groove, especially when printing fine products, the quality of the overprint will be affected. Solution: Remove the front gauge tongue and level with a planer, grinder or file. However, manual operation is inevitably uneven, and after re-testing, it must be re-tested to make the paper edge contact with the front gauge line to prevent point contact.
(3) The paper feeding positioning distance is too short
After the front gauge tongue is positioned, the front edge of the paper should be kept at 6-8mm from the front gauge. If it is less than 6mm, it may cause the overprint of one side to pass the position to be faulty. Because it is impossible to maintain absolute parallelism when conveying paper, there are always some papers that cause a small amount of skew due to various reasons. When these papers are skewed into the front gauge, the front gauge has not yet settled due to the short paper feeding time. Walking through the position, resulting in inaccurate overprinting. And leave traces of crushing on the edge of the paper.
Solution: Adjust the paper feeding positioning distance to 6-8mm.
(4) The brush wheel is too heavy
The brush wheel on the cardboard plays an important role in the accuracy of overprinting. However, when printing paper of 80 g/m2 or less, the pressure on the paper surface of the brush wheel is strictly controlled. Because it is installed on the belt, the front of the paper is generated: the medium force is large, and the paper of 80g/m2 or less has small supporting force, so that the front edge of the paper is curled, causing the overprinting to be inaccurate, and even causing paper in serious cases. Wrinkles make the product obsolete.
Solution: When printing paper of 80g/m2 or less, the pressure of the brush wheel on the paper should be lightened. When adjusting, you can first retract the adjusting screw so that it does not turn, and then tighten it clockwise to make the brush wheel rotate gently on the paper.
(5) Pressing the ball against the edge of the paper
Sometimes the size of the two batches of prints is different or the paper size of the same batch is inconsistent. The paper of slightly larger size is pressed by the paper ball at the tip of the paper to press the paper edge, and the paper generates a front force, causing the position to pass, and the overprint is inaccurate. .
Solution: Move the platen back backwards against the edge of the paper and not press it into the paper.