Paper and ink and print quality

The effect of paper on print quality is due to the ink absorbency of the paper. Paper absorbency refers to the paper's ability to absorb ink, and it relates to the absorption of low-viscosity components in the ink by the capillary pores and the penetration of certain components of the ink into the paper. Therefore, it is one of the important factors affecting the quality of printed products. The following is a brief discussion:

First, the paper absorption process of ink

The ink absorption of paper is not only related to the looseness of the paper and the state of the capillary, but also related to the surface properties of the paper fiber, the content of the filler, the pigment, the rubber material, the composition and characteristics of the ink, the printing method, and the printing pressure.

To properly evaluate the ink absorption of a paper, it is necessary to first understand the law of ink absorption in the printing process. In actual printing, the absorption of ink by paper can be divided into two stages.

The first stage is the imprinting moment of the printing press. The ink transferred to the surface of the paper is pressed into the larger pores of the paper by the effect of the printing pressure. That is, the entire ink (including the pigment in the ink) enters into the pores of the paper. This process is generally called adding. Pressure infiltration phase. The absorption of ink by paper at this stage depends only on the size of the printing pressure, the structure of the paper, and the viscosity of the ink. Into the printing pressure or paper loose structure, paper absorption ability of the ink will be stronger. In general, loosely-printed papers such as newsprint, letterhead and other non-coated papers should be printed at a lower pressure and the ink viscosity should be lower. On the other hand, printing papers with tight structure, such as coated paper, can be printed with appropriate pressure. The ink viscosity can also be slightly higher.

The second stage is to leave the embossed area from the paper until the ink is completely dry. This phase mainly relies on the growth of capillary action to absorb the ink, known as the free-penetration stage. At this stage, the binder is separated from the ink as a whole, and passes through the rough surfaces of the small pores and the paper fibers, entering the interior of the paper at a relatively slow speed. Therefore, this process is actually a process in which the binder migrates from the ink to the pores of the paper. Moreover, this process is more important because the separation of the binder from the entire ink will change the nature of the conjunctiva that remains on the ink film on the paper, and the fixation and drying of the ink must also be completed in this process. At this stage, the rate of ink absorption by the paper determines the gloss of the print, and whether or not printing and chalking occur. When the ink is transferred to the paper, as the time goes by, the low molecular weight binder (solvent) will begin to penetrate into the paper layer, so that the solvent content of the ink on the paper surface will decrease, and the ink viscosity will increase. The layer produces condensation.

The absorbency of paper is generally determined by the number of paper capillaries and the diameter of the capillary. Paper is a kind of porous material. There are many gaps between the fiber and fiber inside the paper, between the fiber and the filler, and between the pigment particles. These gaps are equivalent to a lot of capillaries. Under the action of these capillaries, the binder in the ink can be absorbed, and the thicker the diameter of the capillary, the faster the ink can be absorbed. Therefore, the pore structure of the paper determines its ink absorption.

In general, the looser the paper, the greater the porosity, the stronger the ink absorption. For example, newsprint is loose and has strong ink absorption. Therefore, the transfer of ink on newsprint is fast, which is conducive to the improvement of printing speed. Coated paper is not as loose as offset paper. The porosity is not as large as that of offset paper. However, the fixation of ink during printing is not necessarily slower than that of offset paper. There are two main reasons. One is that when the coated paper is printed, the ink supply is smaller; the other is that there are many fine capillaries between the pigment particles attached to the surface of the coated paper, and the absorption capacity of the more small capillary is greater than that of a few coarse capillaries. many. In addition, the absorption selectivity of the coated paper to the binder is very strong, and only the low molecular components with strong flowability in the binder are allowed to penetrate deep into the paper, and the polymer components in the binder still remain in the ink layer, so the ink layer can be improved. The firmness and glossiness.

The amount of ink sucked can be considered from the perspective of the pore size. After the ink is filled in the pores, the capillary in the paper absorbs the binder in the ink. If the pores between the fibers of the paper are small, the capillary action of the fibers is damaged and the ink-absorbing ability is poor; if the pores between the paper fibers are too large The ink absorption is too strong. When the viscosity of the printing ink is low, the capillary not only absorbs the linking material but also absorbs the pigment, which will cause the ink to penetrate the back of the paper, cause the phenomenon of overprinting, and affect the thickness of the ink layer after drying, so that the ink Can not appear in the paper color saturation, affect the quality of print.

Both the permeation stage and the free permeation stage have a great relationship with the ink-absorbing properties of the paper, especially the free-penetration stage. Therefore, the stronger the ink absorbency of the paper, the faster the paper absorbs the binder in the ink, and the faster the ink dries.

Second, the paper's ink absorption of print quality

The paper's ink absorption has a lot to do with the quality of prints. Many print failures are caused by the mismatch between the paper's ink absorption and the printing conditions used.

1, the paper is too strong ink absorption

Paper's ink absorption is too strong and often leads to the following printing failures.

(1) blots are no light, chalking

In the printing, the paper mainly absorbs the binder in the ink. If the absorption energy of the ink to the paper is too strong, the binder is absorbed into the paper layer or the coating excessively, resulting in the lack of the binder in the ink layer on the paper surface. The pigment particles are suspended on the surface of the paper. After the conjunctiva is dried, the imprint of the print is old or discolored. Even if some of the imprints are rubbed, the pigment will be exfoliated in a granular form, causing the phenomenon of chalking. When the pulverization is serious, in the process of overprinting, the ink of the previous color will be peeled off from the surface of the paper after printing, which will seriously affect the smooth progress of printing.

The solution is as follows:

a, replace the appropriate paper.

b. Use a tackifier to increase the viscosity of the ink, or add a high content of varnish to the ink.

c. Condition the paper.

(2) through the printing

When the ink viscosity is small, when the printing pressure is too high, the paper absorbs the ink too strongly, because the binder penetrates into the internal pores of the paper, replacing some of the air in the paper pores, resulting in air-fiber, air-filler, air-coating The reduction of the interfacial scattering interface reduces the light scattering ability of the paper, and enhances the transparency, which may lead to the occurrence of a print-through phenomenon.

The solution is as follows:

a, replace the appropriate paper.

b. Perform the grey balance data test again on the medium paper.

2, the paper is too weak ink absorption

The ink absorption of the paper is too weak, and the speed of absorbing the ink is too slow. In the printing process, though a colorful ink layer can be obtained, the penetration and fixation of the ink and the oxidative polymerization are affected, so that the fixing speed of the ink is reduced, and the ink is transferred to After the paper is not quickly absorbed by the paper, the ink that has not dried will be transferred to the back of the other paper covered above, resulting in stickiness and, in severe cases, blocking. In addition, the poor ink absorption of the paper is not conducive to the improvement of the printing speed, thereby affecting the production efficiency.

The solution is as follows:

a, replace the appropriate paper.

b. Increase printing pressure appropriately.

c, lower ink viscosity.

d, reduce the printing speed.

Source: China Ink Technology Network