Analysis of the relationship between paper opacity and offset
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In the offset printing process, paper is prone to print-through phenomenon. In order to fundamentally eliminate the problem of offset printing, many people think that the opacity of paper is directly related to the phenomenon of paper-printing. It is often judged that the paper is printed according to the opacity of the paper. The possibility that the printing house stipulates some kind of opacity in order to prevent an unpleasant offset effect. It is very important to clarify the relationship between opacity and offset of paper. Because of the influence of ink and paper between the two, the composition of the ink used in the printing production process is fixed, so master The effect of paper on opacity and offset is necessary.
First, the meaning of opacity and print
In general, opacity is a fundamental property of printed paper and is one of the most important indicators, referring to the ability of materials to block light transmission. Although the opacity is determined by the total amount of transmitted light, the contrast is often used to describe the opacity of the paper. The contrast ratio is the ratio of the diffuse reflectance of a single sheet of paper backed to a black body and the diffuse reflectance of the backing of the white body, as shown in Figure 1.
Common opacity representations are TAPPI opacity, print opacity, and IGT print opacity. In the field of printing paper, it mainly refers to the ability of the paper to hide the phenomenon existing underneath or on the reverse side when the upper surface of the paper is irradiated with diffused light, and when the normal eye is viewed under simulated sunlight. There are also many factors that affect paper opacity. In theory, any factor that increases the ability to absorb and absorb light will affect the opacity of the paper, including the ability to reflect light, the wavelength of the measured light, dye, quantification, beating and Wet pressure, apparent density, filler, wax coating, slurry, etc. For the offset printing, it is mainly a general term for the phenomenon that the content printed on one side of the paper is partially visible on the other side. There are many cases of throughprinting, including revealing and penetrating. The causes of printing and printing are various. The printing pressure, the structural condition and opacity of the paper, the penetration of the ink into the paper, and the printing operation process are all major factors affecting the printing offset. If the print-through value is expressed as PT, then:
PT=PTST+PTPP+PTVS
Among them, PTST indicates the offset component due to the paper through the back, PTPP indicates the offset component due to the penetration of the pigment in the ink, and PTVS indicates that the binder in the ink is separated from the ink as a whole and penetrates into the pore of the paper to replace the air. ingredient. As can be seen from the above formula, the degree of offset depends on the degree of penetration of the ink into the paper and the opacity of the paper itself.
For the definition of printing offset value, the quality standard of the domestic paper industry is based on paper opacity to evaluate the offset of paper after printing. Obviously, the offset is affected by the opacity, and the lower the opacity, the higher the offset. However, many papers with the same opacity have completely different degrees of offset. It is not accurate to fully understand the interaction between ink and paper, and rely solely on opacity to predict the problem of offset.
In general, offset is determined by the ability of the ink to penetrate the paper and the opacity of the paper itself. The total offset can be expressed by the formula:
G=GST+GIP
Among them, GST is a component produced by direct penetration printing, and GIP is a component generated when ink penetrates into paper. Analysis of two different components makes it easier to determine the factors that influence the print. Whether it is the penetration of the paper surface or the transparency of the paper itself, it is related to the composition of the paper itself and the way of making the paper. It is better to analyze the characteristics of the paper to analyze the relationship between paper transparency and offset.
Second, other factors affecting the penetration
It can be clear from the analysis that opacity has a certain effect on the offset printing. However, this effect is not all the factors that affect the offset printing. The reason for the influence of the offset printing is mainly determined by the paper and the ink. In the paper, not only Only opacity has an impact on the print-through, but also the papermaking methods and components of the paper.
1. The influence of papermaking methods
The paper is made of pulp-like wood fiber. Among them, mechanical pulping and chemical pulping represent two different methods. A large amount of lignin is retained in the mechanical wood pulp, and there are many tiny fiber fragments and small pieces. Fiber; lignin is removed from chemical pulp, and most of the fiber is retained, but not excessively fibrillated. Although there is little difference in the content of the material, it directly causes the paper made of mechanical pulp to have a higher opacity than the paper made of chemical pulp.
In terms of printing paper, in order to improve opacity and surface properties, it is often necessary to add an inorganic filler material to the ingredients. The opacity of the paper is not only limited by the type of pulp, but also by the content and type of filler.
2. The influence of paper filler additives
For paper fillers, the difference between opacity and offset can be obtained through research. For a certain amount of ink, the amount of offset on the wood-free paper with filler is better than that without filler. The amount of print on mechanical pulp is much higher. Therefore, the addition of the filler can appropriately increase the opacity, but it does not produce a reduction in the offset effect as produced by mechanical pulp. As far as the printing density is concerned, as the filler content increases, the requirements on the ink are increased. Especially when the amount of ink used is large, the paper should be paid attention to when selecting the paper. When there are special requirements, special use can be used. Additives will greatly reduce the offset. Of course, the properties of the ink cannot be completely ignored, and the ink has a very important position in the process of printing. Therefore, in considering the problem of opacity and offset, in the case of clarifying the performance of the paper, it is also necessary to pay attention to the choice of ink and behavior.
For papers with mechanical pulp and mineral fillers of different nature, although their opacity may be the same, under standard printing conditions, the use of a certain ink may result in very different print levels. of. In the same opacity paper, the paper based on mechanical pulp is better than the paper based on the high filler content and chemical pulp.