Research on Printing Suitability of Embossed Paper (2)

3. The surface condition of embossed paper affects the quality of printed matter

Due to the pattern on the embossed paper surface, the uniformity of the printed image will be reduced, especially in the solid part; due to the effect of wrinkles, the gloss of the printed surface will be reduced.

For the particularity of the surface of embossed cover paper, in order to ensure the quality of printed products, it is necessary to properly adjust some characteristics of the ink or change the printing process parameters to adapt to the characteristics of the paper: when printing, the printing pressure may be appropriately increased, or the fluidity may be used more appropriately. Large, wet ink, in order to compensate to a certain extent, due to the uneven paper and imprints appear false.

4. Embossed book paper moisture absorption deformation, affect the print overprint accuracy

In multi-color offset printing, there are many factors that affect the accuracy of overprinting, but the most prominent and common factor is the hygroscopic deformation of the paper due to the change in moisture content during the printing process.

1) The characteristics of embossed book paper moisture absorption and deformation The embossed book paper has large moisture deformation and the overprint accuracy will be affected. Therefore, it is necessary to grasp the characteristics of moisture absorption and deformation. Embossed book paper deformation characteristics are mainly the following two points: First, directionality, the general transverse expansion rate is greater than the longitudinal, overprinting inaccurate often occurs in the transverse direction of the large rate of expansion. Therefore, as long as the horizontal control of the expansion rate is within the allowable range; the second is the two sides, the opposite side of the extension is larger than the front, the paper curled to the front. Similarly, the degree of contraction on the front and back surfaces is also different when the moisture is shrunken. The contraction on the opposite side is larger than that on the front side, and it is expressed as a curl on the reverse side. If the paper is directly infiltrated, there are also inconsistent characteristics of these two deformations. No matter which side is curled, it is always oriented along the longitudinal direction of the paper.

2) Printing failure caused by moisture deformation of embossed cover paper The printing failure caused by moisture deformation of embossed cover paper is mainly due to overprinting, followed by printing wrinkling and curling. Imprinting is not allowed to show that the misprints of various colors are not allowed, including inaccurate on-site printing and overprinting of outlets (or subtle parts). If slight overprinting is not allowed, it is not obvious in the solid part of the slightly larger area, but changes can be seen in the dot or thin line part, as the thin line becomes thicker or the dot becomes larger.

The overprint caused by the undulating and tight edges of the paper is not allowed, and is generally caused by improper stacking of the paper. When the paper is wavy and bent to a large extent due to moisture absorption, wrinkles may occur during printing.

3) Prevention and remediation of embossed cover paper moisture absorption distortion treatment of embossed cover paper:
Before printing, the embossed cover paper is hung in a paper room or a humidity chamber. After a period of time, the moisture content of the paper reaches or approaches the ambient humidity, and the water content of the paper is kept uniform throughout the entire paper. The degree of elongation of the paper due to fountain solution is reduced.

Embossed wavy bending and tight edge correction:
The humidity control method may be adopted, and the method of infrared heating may also be adopted. That is, the side of the paper cassette is directly irradiated with infrared rays on the paper transporting table to correct the wave bending of the paper, and also prevent the paper from being printed when printing in a high temperature environment. Leveling failure. For the occurrence of tight-edge paper, steam spray can be used to correct the edge of the paper so that moisture will stretch and flatten in a short time.

The embossed cover paper also undergoes elongation during the printing process due to the relatively high pressure, and it also deforms in varying degrees due to the peeling tension when passing through the embossing area. Therefore, in order to overcome overprinting due to paper deformation, can also be achieved through the adjustment of printing equipment.

5. The ink absorbency of embossed cover paper affects the quality of prints

1) The purpose of printing and absorbing the ink on the embossed cover paper is to transfer the ink image on the printing plate to the paper by embossing. Therefore, the acceptance of the ink on the embossed cover paper determines the image reproduction effect. That is, the sharpness of the print and the ink saturation. Because of the pattern on the surface of the embossed cover paper, the acceptability of the ink is poor, and relatively speaking, the transfer rate and the transfer amount are relatively small.

In an embossed book print, the dot shape changes. Embossed book paper is poor in acceptability of the ink or printing pressure is too small will cause the dot to become smaller; embossed book paper sizing degree is too low, the ink is too fluid or too much pressure will cause the dot to increase; embossing The subtle unevenness of the surface of the cover paper will cause the ink dots to be uneven. Therefore, the embossed cover must have good ink absorption to promote the fixation of the ink.

2) Failures and Solutions for Ink Transfer Drying and Formation of Images

Poor fixation and back stains:
The embossed cover paper that is accumulated on the printing press receiving table is under the pressure of the upper print sheet, and the back surface is dirty. The reason is that embossed book paper absorbs ink slowly. Solution: Reduce the printing speed, reduce the supply of ink, change to fast-drying ink, etc. Of course, these measures should be kept within proper limits, otherwise new problems will arise.

Poor drying and sticky ink:
In the late drying of ink fixation, stickiness may occur due to poor drying. The reason for this is that the absorption of the embossed cover paper is not strong or the pH of the embossed cover paper is too low. The solution is to increase the amount of desiccant and to enhance the ventilation of the printing process.

Chen Yongchang Source: Shaanxi University of Science and Technology