Analysis of the Upset and Shrinkage of Printing Paper

In the printing process of thin paper, we often encounter this phenomenon: Sometimes the deviation appears in the horizontal dimension of the printing plate, the error is generally less than 1mm, but sometimes it reaches 3 ~ 4mm. And the farther away from the unit, the greater the error in the horizontal overprint. In addition, sometimes the problem of overprinting is also reflected in the diagonal layout of the entire layout. One overprint is intact, but the other end is not small. Through years of practical operation, it has been found that these problems are mainly caused by the combination of the two opposite processes of paper moisture absorption and heat drying.

Gravure printing speed is very fast, drying temperature is quite high, the temperature of the paper is basically consistent, how can it absorb moisture? There are two aspects of the paper's moisture absorption, one is in the unwinding part, after the roll paper is unrolled, In the part of the paper, the application of the front side of the paper will hinder some of the moisture from entering the paper, while the back side will be exposed to more moisture. Under normal circumstances, the paper will increase the moisture by 2% to 3%. Second, green printing is now advocated, and water-based inks are gradually being used in gravure printing, including waterborne varnish. If a large area of ​​printing water-based ink, the surface of the paper will be temporarily deformed due to moisture, after drying at high temperatures, the damp part will shrink and make the paper lateral size becomes smaller. If the heating temperature of the oven is higher, the more moisture in the paper will be volatilized, the greater the deformation and shrinkage of the paper, and the overprinting will occur when the next color is printed. In addition, whether the moisture content of the paper in the web is uniform or not, and if the water content in the front and back of the paper roll and in the middle of the paper roll are not uniform, such a problem also occurs.

Solve the thin paper printing and shrinking method is to properly increase the speed of printing, to reduce the heating temperature as much as possible, reduce unnecessary hot air drying, printing ink does not anti-stick, do not fade the premise. In addition, excessive paper tension will also cause the paper width to change during the transfer process, resulting in inaccurate horizontal overprint, but it also needs to be analyzed when encountering specific problems.