Ink penetration during printing (5)

When printing on non-coated paper, if the texture of the paper used is loose, the viscosity of the used ink is small, and the printing pressure used is large, then the ink will overflow from the edge of the printed graphic, causing the “moisture phenomenon” and the edge of the graphic. Obscure, as shown in Figure 7-13 (a). In addition, the penetration depth of the link material in the ink may exceed the thickness of the paper, so that it penetrates the paper and penetrates into the back of the print, causing the “printing phenomenon” of the ink. This also makes the print chaotic, as shown in Figure 7-13 ( b) shows.

The degree of strikethrough is related to the reflectivity on the back of the print. If RA is used to indicate the reflectivity of white paper and RB is the reflectivity on the back of the print, the degree of printout can be evaluated using the Carlson test method. When printing on coated paper, the ink The pigment particles in excess remain on the surface of the printed material and will fall off when rubbed. This phenomenon, which is somewhat like the character written on the blackboard, is called "powdering of ink."

Offset and chalking of inks are common failures that occur in non-coated paper and coated paper respectively. The occurrence of these two types of failures is mainly determined by the distribution of the diameter of the paper capillary pores.

When discussing the ink penetration of non-coatings and coated papers, the paper is regarded as a porous sheet material, and the paper capillary is simulated with a thin straight pipe of equal radius, and it is assumed that the straight straight tube is arranged in parallel to the paper surface. Such a paper model greatly simplifies the discussion of the penetration of ink in paper and has yielded many useful results. However, such models are, after all, rough and in-depth discussions first involve the issue of the probability distribution of capillaries of different radii.

A large number of experimental results show that the probability density distribution of paper capillary radii is shown in Figure 7-14, which is approximately consistent with the lognormal distribution. In the figure, the abscissa is the radius r of the capillary of the paper, the unit is μm, and the ordinate is (r), which is the probability density of r.

Coated paper is a coated paper composed of a coating layer and a paper substrate. Fig. 7-15 is a probability distribution curve of capillaries obtained after separating a coating layer and a paper base layer of a coated paper. The capillary radius of the coating layer is between 0.01 and 0.1 μm, and the capillary radius of the paper substrate is between 0.1 μm and 2 μm, bounded by 0.1 μm. The left half of the curve in Figure 7-15 indicates the capillary radius of the coated paper layer. The probability distribution, the right half shows the probability distribution of capillary radius of coated paper base layer.

Taking into account the probability distribution of paper capillary radii, the discussion of the penetration of ink in paper will be more complicated.

Coating paper is smoother than non-coating papers such as offset paper and newsprint. This can be confirmed from the graph in Figure 7-14. The peak value of the probability distribution curve of the capillary radius of the coated paper is high and advanced. Qualitatively, it is a small aperture. There are many capillary tubes, so the surface is smooth. Baker smoothness of coated paper can generally exceed 600s, and some are as high as 1000s or even higher. For example, 150 g/m2 of coated paper produced in the former West Germany has a Baker smoothness of 1960s. Therefore, printing with coated paper does not require a great deal of printing pressure. The penetration of ink in coated paper depends mainly on the capillary action of the paper, relying mainly on free penetration.

It has been pointed out earlier that the penetration of the ink in the paper is mainly the penetration of the binder in the ink in the paper. However, as the binder penetrates, the pigment particles in the ink also participate in the process of infiltrating the paper to varying degrees. As the pigments play a role of coloration on the printed matter, the penetration of the pigments seriously affects the quality of the printed matter.

Only if the size of the pigment particle is smaller than the pore size of a capillary in the paper, it is possible for the pigment particle to “bleed” into the capillary along with the binder in the ink; if the size of the pigment particle is larger than the pore size of a capillary in the paper, the pigment particle Stuck "at the top of this capillary. Because the pigment particles have different sizes, different shapes, different paper capillary radii, and different shapes, the penetration and retention of pigment particles in the ink penetration process are very different. If there are too many pigment particles penetrating into the capillary of the paper, that is, the depth of penetration of the pigment particles into the capillary of the paper is too large, the print will appear dull due to the low pigment concentration on the surface of the paper; if too little pigment particles penetrate into the capillary of the paper That is, a large number of pigment particles remain on the surface of the paper, and the excessively accumulated pigment particles will fall off as soon as they are rubbed. The quality of the prints is also not high, and only the amount of pigment particles that penetrate into the paper and remain on the paper surface is moderate, so that printing can be guaranteed. the quality of.

Experiments have shown that the faster the paper's oil absorption speed, the deeper the pigment particles penetrate into the paper, and the more likely the ink powdering occurs. For example, the experimentally measured certain type of coated paper, its surface oil absorption is 70s, the penetration depth of pigment particles is 2.5μm, the ink after printing is powdered; the other kind of coated paper, its surface oil absorption is 80s, the penetration of pigment particles The depth is 3.8μm and the ink does not chalk after printing.

The distribution of paper capillary pore size and the distribution of ink pigment particle size are similar to the normal distribution, but the shape of the curve is different. The former is low and flat, and the latter is high and steep. The relationship between them affects the penetration of ink in the paper. If the paper used is certain, the probability distribution curve of the capillary aperture is shown in Figure 7-16. Changing the ink used, such as the ink particle size, the probability distribution curve remains unchanged but in a different position, as shown in Figure 7-16(a)(b). (a) shows that more pigment material than the paper capillary pore size may cause ink pulverization; (b) shows that there are fewer pigment particles larger than the capillary pore size of the paper, so that ink pulverization does not occur. Such as the amount of pigment particles infiltrated into the paper is moderate, which is the ideal distribution.

The phenomenon of ink pulverization occurs mainly when using coated paper. This is because the paper is thick and smooth, and the capillary diameter of the paper is very small. The pigment particles contained in the ink are mostly coarse, so they remain on the surface of the paper during the printing process. The pigment may be excessive, resulting in ink pulverization. The print-through phenomenon mainly occurs when printing on non-coated paper. This is because the non-coated paper is thin and loose, and the capillary diameter of the paper is large. The pigment particles used in the ink are mostly fine, so the printing process penetrates. The web's binder and the pigment particles it carries create a print-through phenomenon. In order to prevent the occurrence of these two kinds of printing failures, the main measure is to make the distribution of paper capillary pore size and ink pigment particle size reasonable; secondly, to increase the viscosity of the ink, accelerate the drying speed of the ink, and appropriately reduce the printing pressure, to slow down the ink The powdering and strikethrough also have a certain effect.



Source: Bison