During the printing process, the paper fan-shaped expansion is mostly caused by quality problems such as loose paper structure, uneven surface, unstable or uneven moisture content, especially when the moisture on the plate is too large and the printing pressure is too large, the paper is embossed and unfolded. It is wider than the original, and spread out longer than the original. This is because the gripping part of the paper is fixed by grippers during the printing process. When the roller rolls from the gripper of the paper toward the trailing direction, the extension and squeezed deformation of the paper cause the trailing tip to be wider than the gripper , That is, it is fan-shaped and enlarged, and there is a problem that the overprint graphic overprint is accurate and the trailing overprint is not allowed.
Another main reason for the fan-shaped expansion of the paper: due to improper adjustment or other reasons, the leading edge of the paper gripper edge is deformed during positioning, transmission or handover, causing the edge of the gripper edge to be uneven when the paper enters the roller for imprint Wavy. E.g:
(1) When the front regulations are not straight, if the front regulations are not straight, the edge of the paper will be bent when positioning; when there are only two front regulations, due to the improper adjustment of the brush wheel and paper pressure wheel on the paperboard, When the paper reaches the front rule, the middle part is arched, and when the paper teeth are held, the mouth is wavy.
(2) The gap between the two front gauges and the paper board is appropriate, and the gap between the delivery tooth pad and the paper board is too large, and the delivery tooth pad is higher than the two front rules.
(3) The gap between the delivery tooth pad and the surface of the impression cylinder is too large. When the paper is transferred to the impression cylinder to grip the teeth, the edge of the grip mouth will also be wavy.
(4) The tooth pads of the impression roller gripper, the transfer roller gripper, and the paper transfer gripper are severely worn due to long-term use, or due to improper adjustment of unevenness, the edge of the paper will also show uneven waves when gripping .
In this way, the length of the paper is shortened. After entering the impression cylinder, after several rollings, the paper is gradually released from the gripper back, resulting in inaccurate lateral overprinting, and the trailing part is more serious than the gripper part. In production, sometimes the paper is severely stretched horizontally, and the front and back color images are difficult to register. Operators often think that it is due to the poor quality of the paper. In fact, it is due to the above reasons. If the surface of the paper itself is uneven, the problem will be more serious.
The elongation and deformation of the paper and the uneven edge of the gripper are caused by the inaccurate failure of the fan-shaped expansion of the paper caused by the imprint. The phenomenon is very similar, which makes it difficult for the operator to distinguish. It must be carefully observed and analyzed to distinguish it. First of all, the edge of the mouth is not straight and enters the imprint. If it is a multi-color printing machine, the wave shape of the edge of the paper will change during the handover of the teeth for several consecutive times. Therefore, the edge of the mouth will be small. Wrinkles; if the tooth marks on the edge of the mouth are obvious, the pads are badly worn and need to be replaced or smoothed. In addition, after the four-color printing on the first side of the paper, it has been adapted to the roller imprint and can be balanced with the moisture in the printing. Especially for the single-color machine, it needs several printings on one side, and the paper absorbs moisture and releases moisture after many times. The water content is already quite stable. When printing on the back side, it generally will not be stretched and deformed again. If the fan-shaped expansion phenomenon still occurs, it may be due to improper mechanical adjustment or other problems.
To avoid the fan-shaped expansion of the paper, it is mainly to prevent the machine adjustment. When reprinting paper of different thicknesses in production, the gaps between the front gauge and the paper conveying table, the delivery tooth pad and the delivery table, the delivery tooth pad and the impression cylinder should be adjusted accordingly, and the general adjustment amount is the paper thickness plus 0.2mm. In addition, usually pay more attention to the wear of dental pads. It is worth noting that in the printing process of single-color and double-color machines, if it is necessary to level the dental pad, replace the worn dental pad, or reduce the gap between the delivery dental pad and the impression cylinder, the semi-finished product must be marked separately. Because the pads are flush and the gap is reduced, the corresponding paper is longer, and the image size on the paper is changed. It may be difficult to correctly overprint due to the different image sizes before and after, so they cannot be mixed together. In terms of paper quality, if the quality of the paper is poor, it is best to do a moisture balance before printing. If the situation permits, you can also print water once to adapt the paper to the printing requirements. At the same time, the printing process should also pay attention to control the moisture of the layout, to ensure that the printing is under an ideal pressure, and to minimize the water absorption and extrusion deformation of the paper.
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