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The influence of the characteristics of paper and ink on dot enlargement

release date: Source: China Packaging

for dot image printing, although dot enlargement is inevitable, how to effectively control dot enlargement within the normal specified range is the guarantee to improve the printing quality. Generally speaking, the 50% point increment value specified in the printing requires that the fine products should be 12% and the general products should be 18%. Once it exceeds this range, there will be quality problems such as printing tone loss, density increase, color change and so on. In the prepress production and printing process, certain compensation will be made for the normal point increase, but the abnormal point increase is often irregular, so in the prepress production process, reasonable compensation cannot be made, but can only be controlled in the printing process. There are many factors to expand the starting point, but paper and ink are two important materials in printing. Their characteristics and use play an important role in the quality of points. Today, China paper industry takes you into the characteristics of paper ink and introduces the relevant contents in detail

I. Influence of paper characteristics on point increase

1 The influence of paper absorbency and smoothness on point increase

point increase has a great influence on the change of replication hue. The amount of ink printed on the paper will affect the dot increase, and the dot increase will affect the printing contrast. When printing black self or color eye tone images, point increase will change the contrast of the image and cause the loss of image detail and clarity. In multicolor printing, dot increase will lead to loss of contrast, dark image, dot paste and sharp color changes. Before printing, you must choose the appropriate paper for the printed matter, especially considering the factor of dot increase, and you should be more careful about the choice of paper. The printability of paper depends on its surface characteristics. For example, the faster the transfer ink is accepted, the greater the dot increase of the print. Absorbency is a characteristic that determines the speed and amount of ink penetrating into paper. The better the absorption performance of the paper, the higher the percentage of dot increase. When the ink is fixed on the absorbent paper, it will penetrate and diffuse. The absorption rate of paper to ink plays a key role in the degree of dot increase. For example, do not use highly absorbent paper for printing 300 lines. The flatness of paper is the deviation between the actual surface of paper and the ideal plane. The relationship with point increase is that the higher the paper flatness is, the smaller the point increase is, and the lower the flatness is, the worse the point increase is. The smoothness of the paper is high, and the layout graphics and texts on the print can be reproduced truthfully, especially the small dots can be accurately restored. On the other hand, the paper with low smoothness is easy to make dot printing unclear due to the concave shape on the paper surface, and the print layout level is not clear. The smoothness and thickness uniformity of paper have a direct impact on whether the printing pressure and ink color can be kept uniform. Therefore, using pressure to realize the production process of printing requires that the thickness of the paper be as uniform as possible. Smoothness is also the guarantee of improving printing quality. However, the method to improve the smoothness of the paper is mainly to add fillers and calendering. In this way, the tightness of the paper is improved, and its elasticity and ink absorption are reduced, so the back of the printed matter is prone to rubbing. In addition, when the printing pressure is too high, it is also easy to make the ink layer spread, imprint and dot increase deformation. Based on these conditions, the paper with uneven smoothness and obvious differences in rough light should be classified and separated, so that when printing the paper with poor smoothness, the clear impression can be obtained by appropriately increasing the printing pressure and ink volume, so as to improve the printing quality. The smooth surface of the ink produces the best field density on high-quality paper. When the paper quality is reduced, the field ink density decreases and a certain point increases, affecting the quality of the printed matter. Obviously, the absorbed ink cannot produce a smooth printing surface. The higher the number of lines, the smaller the dot diameter, and the small pits on the paper can also distort or omit small printing plate dots

therefore, the higher the number of lines, the higher the smoothness of the paper is required, which is a principle of selecting paper. However, in practice, some properties of ink can also be adjusted appropriately or process parameters can be changed to adapt to the characteristics of paper. There are many kinds of coated paper on the market, such as glossy coated paper and matte coated paper. Coated paper helps to improve the fidelity and color clarity of dot printing because its absorption is lower than that of non coated paper. As we all know, low absorption paper, point increase is also small. Non coated paper has high absorption performance. High absorption non coated paper prints high-line images, resulting in a high percentage of point increase. In theory, the point increase of recycled paper is the same as that of base paper, but in fact it is larger than that of base paper. When the ink is transferred to the phenolic insulation board on the surface of the paper under pressure, the porous phenolic foam board made of the condensation polymer of phenol and formaldehyde mixed with other additives, due to the absorption of the paper, the ink diffuses around when penetrating, and the production point increases, making the whole image darker. Obviously, different point enlargement effects will be produced for different quality paper. The stronger the paper absorption, the greater the point increase value. Specifically, the paper is the largest and the coated paper is the smallest. The dot increase formed by the paper performance generally presents the expansion law of exponential mode. In the process of prepress processing, the influence of dot increase must be compensated. For points of different sizes, the value of point increase is different. Generally, the increase value of high-key and dark key points is small, and the increase value of middle key points is large. It is generally recommended to use the default setting. For example, when printing on coated paper, the default value is about 17%, offset paper is about 22%, and paper is about 30%

2. The influence of paper moisture content and pH value on point quality

the moisture content in paper is an important work based on the weight of water in paper and the weight of paper. It is also expressed by the percentage of anti bribery. Generally speaking, the moisture content of printing paper should be controlled between 7% ± 1%, and the error of moisture content between the center of paper and the periphery of paper should not be greater than 1%. When the water content of the paper is too large, the binding force between the fiber, filler and rubber in the paper is easy to be reduced, which will affect the surface strength of the paper, resulting in poor nitrile conditions of powder and paper scraps after printing, thus reducing the hardness and stiffness of the paper edge, so it is also easy to produce positioning instability during paper feeding, resulting in misregistering. If the water content of the paper is too low, the elasticity of the paper is poor, and the imprint is easy to expand when printing points, that is, the point increase rate is large. In addition, static electricity and moisture absorption deformation are also easy to occur in the printing process, making the product overprint inaccurate. The pH value of the paper is also the pH value. Generally, the pH value of the printing paper is neutral. If the pH value of the paper is not appropriate, it can change the color of some acid and alkali resistant ink and affect the dryness of the imprint. The pH value of domestic offset paper, letterpress paper, white card, glass card and whiteboard is generally below 7.0, which is slightly acidic or weakly acidic, and the drying speed of imprinting is relatively slow. Therefore, when printing large-area layouts, attention should be paid to adding drying oil in the ink to dry them, so as to avoid poor overprinting or over bottom quality problems. The pH value of coated paper, imported glass card and other high-grade paper and paperboard is mostly above 7.0, which is weakly alkaline or alkaline. The imprinting drying speed is relatively fast, which is conducive to overprint. It is worth mentioning that no matter what kind of paper, its pH value will decrease with the extension of storage time

second, the influence of ink on dot enlargement

many factors in ink, such as pigment, viscosity, viscosity and resin, are related to dot enlargement. The strength and coloring power of ink play an important role in point increase. The ink is squeezed by the printing plate and the blanket, and then between the blanket and the paper. The ink layer formed after two squeezing is usually very thin. The color intensity of ink depends partly on the pigment in the ink. Pigments are often the most expensive part of inks. However, reducing pigments means reducing the coloring power of inks. When printing, the ink layer is thick, and the dot will increase. The ink has high coloring power, and the ink layer of printing is thin. In this way, the desired ideal color can be obtained. In the increase of control point, the adhesion of ink is also very important. The higher the viscosity of the ink, the smaller the dot increase. F. when the viscosity of dannvkou ink is too high, and its degree exceeds the strength of the paper, the paper will still be found hairy, and even tear the paper seriously. Viscosity, also known as the fluidity of ink, mainly depends on versicolor and pigment. Of course, temperature and water also affect viscosity. When the temperature is low, the viscosity of the ink is high, and the degree of dot increase is also low; When the temperature is high, the viscosity of the ink is low, and the degree of dot increase is high. As we all know, in the printing process, the viscosity of ink will change with the change of temperature. In lithography, the feed rate of water will affect the viscosity and dot increase of ink. After the ink absorbs water in the printing process, the viscosity will decrease and the viscosity will decrease, thus affecting the dot increase. If there is too much water, the viscosity of the ink will decrease, and the degree of dot increase will increase, so the problem of overprint will also occur. On the contrary, if there is water shortage, the viscosity of the ink will increase, and the fuzzing problem of the paper will occur. According to another study, the increase of influence points of soybean based inks is smaller than that of petroleum based inks. As the degree of dot increase decreases, it also improves the contrast of prints

III. Influence of water and ink volume control on point increase

on the offset press, the inking device can not only apply ink on the printed image, but also perform other functions, such as homogenizing ink to improve its activity performance, receiving wetting liquid to emulsify it with ink. The above factors will increase the influence point. The thicker the ink layer provided by the ink roller system, the more non directional points increase. The wetting system provides water-based wetting solution to the printing plate, which is used to quickly and completely distinguish the image and non image parts on the printing plate. The operator must maintain a balance between water supply and ink supply. Too much water supply may cause the ink viscosity to decrease, thereby increasing the degree of increase

although electronic prepress technology, laser image and text output technology and optical density measurement technology provide some means for the increase of control points, the complexity of lithography process and the rapid technological changes will still bring trouble to the printer. Therefore, understanding how, where and how to control the increase of points will certainly help the printer to obtain high-quality prints

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