In color newspaper printing, newsprint is the substrate, and the graphic information is finally presented to the reader through the paper. Therefore, the suitability of newsprint printing directly affects the final color newspaper printing quality. The smoothness, ink absorption, whiteness, mechanical properties and dimensional stability of the paper all affect the print quality of the color newsletter.
1. Smoothness of newsprint
In printing, the first condition for obtaining prints faithful to the original is the smoothness of newsprint. The smoothness of newsprint reflects the degree of unevenness of the microscopic geometry of the surface of newsprint. Only with a certain degree of smoothness can the reproduction effect of graphics and text be guaranteed. There are many capillary holes and pores on the surface of newsprint. In order to obtain the ideal printing effect, that is, the imprint is clear, the ink color is uniform, and the layer is rich, you must ensure that the paper and the blanket are in good contact. If you want to get rich layers and vivid colors, you must display the four-color dots on the paper as much as possible. The smoothness of the paper plays a decisive role. If the smoothness is small, the ink cannot fully contact with the paper, and the fine dots will inevitably be lost, without layers. Therefore, the smoothness of the paper is an indispensable condition for determining the number of screen lines and obtaining high quality dots. Paper with high flatness can obtain enough ink, and the ink layer is uniform, the thickness is consistent, the transfer rate of ink is high, the solid density of the printed matter is high, the dot expansion value is small, the color is rich, and the reproducibility is strong. Paper with poor smoothness has incomplete dots, infrequent colors, and poor color reproducibility. Appropriately increasing the fluidity, ink supply and printing pressure of the ink can make up for the void and unrealistic imprint caused by the poor smoothness. But the consequence of this is that the dots are expanded, the dark tone is blurred, and the color is biased. You can also print a lighter color first to achieve the purpose of primer. Therefore, the newsletter must use newsprint with high smoothness.
Second, the whiteness of newsprint
Paper is the carrier of graphic information, and its whiteness directly affects the color of printed matter. After the light is irradiated on the paper, it will be partially absorbed, and the amount of absorption depends on its whiteness. The higher the whiteness, the more light reflected. After the ink is transferred to the paper, the color is formed by the color material subtractive color method and the color light additive color method. If the whiteness of the paper is low, it absorbs too much light, and the color of the ink cannot be fully exerted, which affects the hue, brightness, and saturation of the image color.
3. The absorbency of newsprint
The ink absorption of paper includes the transfer and penetration of ink into the paper, which depends not only on the absorption capacity of newsprint to the ink binder, but also depends on the absorption of ink by newsprint under imprinting conditions, so the ink absorption should be paper (Newsprint) The comprehensive reflection of the performance of the ink under the printing conditions, that is, the quality of the ink absorption is not entirely dependent on the performance of the newsprint, but also depends on the performance of the ink and printing conditions. When newsprint is fixed, changing the performance of ink or printing conditions will cause changes in ink absorption. The ink absorption directly affects the drying speed of the ink on the paper surface, and also affects the color effect of the ink and the fastness of the conjunctiva. The essence of ink absorption is a comprehensive reflection of the paper's absorption and adhesion of ink during the printing process. It is not only related to the surface properties, porosity, compressibility, adsorption, etc. of the paper, but also related to the composition, rheology and Dryness is also related to printing method, printing pressure and imprint time.
The smoothness of newsprint is low, and under the condition of less ink supply, it is impossible to obtain a larger transfer amount. When the amount of ink supply is large enough, a large amount of ink transfer can be obtained when the ink layer contacts the surface of the paper at the moment of imprinting. Therefore, the amount of ink transfer varies depending on the surface properties of the paper. In order to achieve the desired printing effect, under the same conditions, the amount of ink supply must be determined according to the surface properties of the paper to meet the amount of cured ink required for different papers and the amount of free ink required to cover the surface split on the paper Of ink.
During the transfer of the ink to the paper, the binder of the ink undergoes sorption and pressure penetration, and the surface part undergoes physical and chemical changes to form a solid dry ink film. If the paper's ink absorption is too large, the ink absorption will be large. Too much binder and some pigment particles will be sucked into the paper, so that the pigment particles will not be effectively protected, resulting in weak marks, powdering, dryness, and dark ink. Light, reduce the brightness of the color, and the saturation and hue are also affected to a certain extent. If the ink absorption is small, the ink film is thick and the solid density is high, which is conducive to color reproduction. If the amount of ink supply is too large, it is easy to cause the back to be dirty. Web offset presses mostly use newsprint and the printing speed is fast, so the paper has strong ink absorption capacity. [next]
4. The surface efficiency of newsprint
The color effect produced by the ink transferred to the paper depends mainly on the nature of the ink, but it is also inseparable from the nature of the paper. Printed on different papers with the same ink, not only the color intensity and contrast are different, but also the color deviation and gray scale are very different. Even if printed on both sides of the same paper, the color effect is different. This is because the absorbency and gloss of the paper surface are different.
The surface efficiency of paper mainly considers absorption and gloss. The absorption of paper is mainly the strength of the liquid absorption capacity, and the absorption of ink is mainly to absorb the binder in the ink. The color effect of the ink is formed after a certain amount of ink is consolidated on the surface of the paper, but the paper absorbs the ink or the ink layer in the ink layer to a different degree, causing the gloss, color density and hue of the ink layer on the paper to change. The change is strong when the sex is strong; the change is small when the absorption is small. Glossiness is the optical performance of the paper surface-the degree of specular reflection. The level of gloss mainly depends on the roughness of the surface of the paper and the nature of the substances that make up the paper. The ink is dry on the surface of the paper, and its continuity and uniformity after drying the conjunctiva must be affected by the surface state of the paper, resulting in different color effects. The surface efficiency is high. Under the same conditions, the color intensity of the ink is the largest and the color deviation and grayscale are the smallest. Otherwise, the color intensity of the ink layer is low, and the color deviation and grayscale are relatively high, which makes the gray balance of color overprint difficult.
Paying attention to the surface efficiency of newsprint is of great help in improving the quality of color newspaper printing, because the surface efficiency of paper can provide powerful data for some quality problems, which is helpful to find the cause of the problem. In the process of plate making and printing, according to the surface efficiency of the paper, the color deviation and gray scale of the ink color are effectively corrected, so as to achieve the gray balance of the color superposition and reduce the factors that affect the color quality.
5. The mechanical strength of newsprint
For web offset presses, due to newsprint slurry reasons. Its mechanical strength is an important factor affecting print quality. This is mainly manifested in two aspects: tensile strength and surface strength. The paper path of the web offset press is generally longer. During the high-speed printing process, the tension of each part of the paper path is different and has a large difference. Once the tension exceeds its tensile strength, the paper will break and affect the normal printing. The surface strength of newsprint is another important technical indicator. In the printing process, when the ink splits between the paper and the blanket, it is peeled off due to the pulling force of the ink adhesion and the adhesion between the blanket and the paper. tension. When the peeling tension is greater than the surface strength (the internal binding force of the vertical paper surface), the paper tape will peel off from the middle and wrap around the rubber roller, causing the paper to break and crush the rubber cloth. The surface of the paper is subject to peeling tension, which causes powder loss and hair loss, and accumulates on the blanket. After a certain amount of printing, the excessive accumulation of paper powder and paper hair will make the imprint unrealistic. The broken text will seriously affect the printing quality. It will also cause the grinding of the plate due to the increased pressure and reduce the service life of the blanket. . For color newspapers, the ink layer transferred to the surface of the paper is reduced. The color is dim and lacks layers, and the outlets are seriously expanding. During the printing of color newsletters, the ink transfers from "wet to wet", so there is also "wet fluff". Therefore, in addition to the use of paper with high surface strength, the corresponding technological measures must be taken when printing the color newsletter. The viscosity is small Of ink, appropriately reduce printing pressure and printing speed.
6. Dimensional stability of newsprint
Newsprint is made of interwoven plant fibers. The porous surface has hydrophilicity and hygroscopicity. Its water content has a great influence on its dimensional stability. During the printing process of color newspapers, it needs to be exposed to water many times, causing dimensional stability. Variety. Dimensional stability not only directly affects whether the printing production can be carried out smoothly, but also affects the quality of the printing quality. The change of dimensional stability includes the expansion and contraction of the plane size and the change of shape. Newsprint is highly hygroscopic, and therefore has poor dimensional stability. Under different external conditions, the dimensional stability is different. For newsprint, due to its poor mechanical properties, dimensional stability is important for the smooth progress of the printing process and obtaining high-quality prints, otherwise it will cause problems such as interruption of the printing process, printing wrinkles, and overprinting. Newsprint must control the temperature and humidity during transportation and storage to avoid large changes, otherwise it will have a greater impact on dimensional stability.
In short, newsprint as an indispensable printing material in the printing of newspapers and periodicals, must pay enough attention to its performance, in order to ensure the normal production of printing and obtain high-quality printed materials to provide due material protection.
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