Multi-color overprinting and moisture absorption deformation of paper

In multi-color printing, there are many factors that affect the accuracy of overprinting. The paper positioning device and the paper feeding system are not functioning properly, the plates are excessively worn, the pads are too thick or too thin, the printing pressure is too large, the blanket is aged and deformed, the wetting liquid is too much, and the accuracy of the printing press is lost. Accuracy has different degrees of influence. The influence of these factors is complex, and some may complement each other, while others may overlap each other.

In addition to the above factors, the most prominent and most common factor affecting the accuracy of overprinting is the hygroscopic deformation of the paper due to changes in water content during the printing process.

When the water content of the paper changes, not only the length and width of the paper change due to the expansion or contraction of the paper, but also the local size change of the paper due to the increase or decrease of the water content, resulting in various deformations of the paper, such as curling , creases, ripples, etc. Some of these distortions are reversible, while others are irreversible. Some deformations bring difficulties to the printing operation, and some deformations seriously affect the quality of the printed matter. For example, when a map is printed, if the paper is clearly stretched after it has absorbed the fountain solution, the boundary of one country may be printed on the border of another country when the second color is printed, so that the printed matter cannot be printed. Not obsolete. For another example, when the relative humidity of the printing shop is relatively high and the original moisture content of the paper is low, after the paper is piled in the workshop for a period of time, the water-absorbing elongation of the edge portion will generate a “foliage” phenomenon. On the contrary, the relative humidity of the air in the printing shop is relatively low, and the original moisture content of the paper is relatively high. After the storage, quicker edge dehumidification will result in a "tight edge." When printing on such a paper, overprinting is not allowed due to the fact that the edges of the paper are not flat and the front gauge and the side set position are easily inaccurate.

Therefore, it is necessary to study the influence laws of moisture absorption performance and dimensional stability of water content changes, and formulate the printing process and humidity regulation in accordance with this rule. It is also necessary to discuss measures to overcome overprint due to paper deformation, and thus reduce the Print quality accidents and improve print quality.

Source: Paper Easy