On the relationship between paper opacity and transparent printing

In the offset printing process, the paper is prone to see-through phenomenon. In order to fundamentally eliminate the problem of see-through, many people think that the opacity of the paper is directly related to the occurrence of the pass-through phenomenon. The possibility that the printing house stipulates some kind of opacity is aimed at preventing annoying through-printing effects. It is very important to clarify the relationship between the opacity of the paper and the through-printing. Since the two are affected by the ink and the paper, in the printing process, the composition of the ink generally used is fixed, so master The effect of paper on opacity and print-through is very necessary.

1. The meaning of opacity and print-through

In general, opacity is a basic property of printed paper and one of very important indicators, referring to the ability of materials to prevent light transmission. Although opacity is determined by the total amount of transmitted light, contrast ratio is usually used to describe the opacity of paper. Contrast ratio is the ratio of diffuse reflectance when a single sheet of paper is backed by a black body and when it is backed by a white body.

Common methods of opacity are TAPPI opacity, printing opacity and IGT printing opacity. In the field of printed paper, it mainly refers to the ability of the paper to hide the phenomenon existing under or on the reverse side when the upper surface of the paper is irradiated with diffuse light, and when the eyes are viewed under normal daylight. There are also many factors that affect the opacity of paper. In theory, any factor that can increase the scattering ability and absorption ability will affect the opacity of the paper, including the reflection ability, the wavelength of the measuring light, the dye, the quantitative, the beating and the Wet pressing, apparent density, filler, wax coating, slurry, etc. For pass-through printing, it mainly refers to the content printed on one side of the paper, and the general term for the phenomenon that can be partially seen on the other side. There are many cases of through-printing, including revealing and penetrating. The causes of printing through printing are various. The printing pressure, the structural condition and opacity of the paper, the penetration of ink into the paper, and the printing operation process are the main factors that affect the printing through printing. If the print-through value is expressed as PT, then:

PT = PTST + PTPP + PTVS

Among them, PTST represents the translucent component caused by the paper through back, PTPP represents the translucent component caused by the penetration of the pigment in the ink, and PTVS represents the translucent caused by the separation of the binder in the ink from the ink as a whole to penetrate the paper pores instead of air ingredient. As can be seen from the above formula, the degree of through-printing depends on the penetration of ink into the paper and the opacity of the paper itself.

Regarding the definition of printing through-printing value, the quality standard of the domestic paper industry is based on the opacity of paper to evaluate the through-printing after printing. Obviously, through-printing is affected by opacity. The lower the opacity, the higher the through-printing. However, many papers with the same opacity have completely different levels of through-print. It is not very accurate to predict the issue of print-through only by relying on opacity without fully understanding the interaction between ink and paper.

Generally speaking, through-printing is determined by the penetrability of the ink to the paper and the opacity of the paper itself. The total through-printing can be expressed as:

G = GST + GIP

Among them, GST is a component produced by direct transillumination, and GIP is a component produced when ink penetrates into paper. Analyzing two different components makes it easier to easily determine the influencing factors of through-printing. Both the penetrability of the paper surface and the transparency of the paper itself are related to the composition of the paper itself and the method of papermaking. It is more conducive to analyze the relationship between the transparency of the paper and the print through to define the characteristics of the paper.

2. Other factors that affect the through printing

Through analysis, it is clear that opacity has a certain effect on the through-printing. However, this effect is not all the factors that affect the through-printing. The reasons for the through-printing are mainly determined by the paper and the ink. In paper, not only Only opacity has an influence on the through-printing, and also includes the papermaking methods and components of the paper.

1. The impact of papermaking methods

Paper is formed by consolidation of pulp-like wood fibers. Among them, mechanical pulping and chemical pulping represent two different methods. A large amount of lignin is retained in mechanical wood pulp, and there will be many tiny fiber fragments and fine Fibers: The lignin in chemical pulp is removed. Although most of the fibers are retained, they are not excessively chopped. Although there is not much difference in material content, it directly leads to paper made from mechanical pulp having higher opacity than paper made from chemical pulp.

In terms of printing paper, in order to improve opacity and surface properties, it is often necessary to add inorganic filler materials to the ingredients. The opacity of paper is not only restricted by the type of pulp, but also by the content and type of filler.

2. The effect of paper filler additives

For paper fillers, the difference between opacity and through-printing can be obtained through research. In terms of a certain amount of ink, the amount of through-printing on paper with filler and wood pulp without filler is higher than that without filler Throughput on mechanical pulp is much higher. Therefore, the addition of fillers can appropriately increase the opacity, but it cannot produce the effect of reducing offset as produced by mechanical pulp. In terms of a certain printing density, as the filler content increases, the requirements for inks will increase, especially when the amount of ink used is large, pay attention to the properties of the paper when selecting paper, and special requirements can be used. Additives will greatly reduce through-printing. Of course, the nature of the ink can not be completely ignored, the ink has a very important position in the through-printing process. Therefore, when considering the issues of opacity and through-printing, it is also necessary to pay attention to the selection of ink and behavior in the case of clear paper performance.

For papers containing mechanical pulp and mineral fillers of different properties, although their opacity may be the same, under standard printing conditions, the use of a certain ink may result in very different levels of print-through of. In the paper with the same opacity, the paper based on mechanical pulp is better than the paper based on high filler content and based on chemical pulp in terms of offset value.

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