Talking about paper sector expansion and overprint failure

In 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 layout is over-sized and the printing pressure is too high. The paper is imprinted and unfolded. It's wider than it was and it's spread longer than it was originally. This is because the mouth portion of the paper is fixed by the tusks in the printing process. When the tumbling rolls from the mouth of the paper to the direction of the hauling, the paper is stretched and deformed by the squeezing, resulting in a wider tip than the chin. , that is, a fan-shaped expansion, there is an accurate overprinting of cornices, and the failure of overprinting is not allowed.

Another main reason for the fan-shaped paper expansion is: due to improper adjustment or other reasons, the leading edge of the paper squeezing is deformed during positioning, transmission, or transfer. As a result, when the paper is imprinted between the rollers, the edge of the scallop is not straight. Wavy. E.g:

(1) In the case of multiple pre-registrations, if the front rule is not straight, the edges of the paper are not flat when positioning; when there are only two front specified positions, the brush roller and the pressure roller on the paper supply board are improperly adjusted. When the paper arrives at the front, the middle part of the paper is arched, and after the paper teeth are held, the mouth is wavy.
(2) The clearance between the front and the back of the paper conveying board is proper, and the gap between the paper feeding pad and the paper feeding board is too large, and the paper feeding tooth pad is higher than the two front rules. When the paper is handed, the edge of paper is wavy.
(3) The gap between the transfer paper pad and the surface of the impression cylinder is too large. When the paper is transferred to the impression roller, the chin will be wavy.
(4) Impression roller dents, transfer roller dents, and dentition of the transfer tooth due to long-term use, or due to improper adjustment uneven, edging paper edge will be uneven when the wavy .
In this way, the length of the paper is shortened. After entering the impression cylinder, after several times of rolling, the paper is gradually released from the mouth of the mouth, resulting in misalignment in the transverse direction, and the position of the dragging point is more serious than that of the mouthwash. In the production process, sometimes the paper is stretched horizontally, and the front and back images are difficult to register. The operator often thinks that the paper quality is poor. In fact, it is caused by the above reasons. If the surface of the paper itself is not even, the problem will be more serious.

Paper elongation and edge unevenness, imprinting caused by the expansion of the fan paper overprint is not allowed to fault, in the phenomenon is very similar, so that the operator is difficult to distinguish, must be carefully observed and analyzed before being able to distinguish. First of all, if the mouth is uneven, it will enter the imprinting. If it is a multi-color printing press, the shape of the wavy edge of the paper will change during several successive tooth transfer processes. Therefore, there will be small gingival edges. The creases; if the enamel gingival crevice is obvious, it should be serious tooth pad wear, need to be replaced or flattened. In addition, after the four-color printing on the first side of the paper, it has been adapted to roller embossing and can be balanced with the moisture in the printing process. In particular, the monochrome machine requires printing on several occasions. The paper absorbs moisture and releases moisture after many times. The water content is quite stable. When the back side is printed, it will not normally be stretched. If there is a fan-shaped expansion phenomenon, it may be due to improper mechanical adjustment or other problems.

To avoid paper fanning, the main aspect of machine adjustment is prevention. When changing the thickness of the paper in production, the gap between the front gauge and the feeding table, the transfer pad and the transfer table, the transfer pad and the impression cylinder should be adjusted according to the standard. Generally, the adjustment amount is the thickness of the paper. 0.2mm. In addition, usually pay more attention to the wear of the pad. It is worth noting that in the printing process of single- and dual-color machines, if you must flatten the pad, replace the worn pad, or adjust the gap between the transfer pad and the impression roller, be sure to mark the semi-finished product separately. Since the tooth pads are flush and the gap is reduced, the corresponding paper is long, and the image size on the paper is also changed. It may be difficult to correctly overprint due to different front and rear image sizes, 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 humidity balance before printing. If circumstances permit, you can also print clean water first to allow the paper to adapt to the printing requirements. At the same time, the printing process should also pay attention to control the layout of water, to ensure that the printing under the ideal pressure, minimize paper absorption and extrusion deformation.

Author: Mr Hu Jin

Reprinted from: Printing Technology