Paper properties and their impact on printing
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In actual production, the performance of the paper directly affects the quality of the printed product, so the paper for printing should be selected according to the quality requirements, use, content and customer requirements of the printing before printing. The following describes the performance of several types of paper and its impact on printing, for your reference.
First, the thickness of the paper
The printing pressure for different paper selections is different. Before printing, the printing pressure between the blanket cylinder and the impression cylinder is adjusted according to the thickness of the paper, by adjusting the center distance of the two rollers or the thickness of the liner. The paper of the same batch of printed materials requires uniform thickness. If the thickness is not uniform, the ink transfer rate will be different due to different printing pressures, resulting in a change in color depth. In severe cases, the image and text information may not be completely reproduced due to insufficient printing pressure.
When printing on thin paper, the offset is likely to occur due to its good ink absorbing property. In order to avoid the problem of offset, when printing thin paper, use thicker ink and as little printing pressure as possible. When printing with cardboard, because of its small tightness and loose structure, the amount of deformation of the paper in the vertical direction during printing is large, and it is necessary to increase the maximum compression of the blanket cylinder and the impression cylinder to obtain sufficient printing pressure.
Second, the direction of the silk
The fiber arrangement of the paper is directional. According to the arrangement of the paper fibers, it is divided into "longitudinal silk paper" and "cross thread paper".
The dimensional elongation caused by the water absorption of the paper is different in the direction of the vertical twist and the direction of the silk, and the elongation in the direction of the vertical twist is about 3 to 8 times that of the yarn in the direction of the silk twist; Under the action of printing pressure, the horn-shaped elongation is easy to occur, and it is particularly easy to cause a large overprinting error at the two corners of the trailing edge; the tensile strength of the paper in the direction of the wire is better than the direction perpendicular to the wire. Generally, it is preferred to use a sheet-fed printing machine to use paper in the direction of the longitudinal threads. Because the long side of the paper is always parallel to the cylinder axis of the printing machine when printing, the elongation of the long side is small when the longitudinal paper is used, and the elongation of the short side is large, so that the absolute elongation of the long side and the short side of the paper can be made. On the other hand, the short edge with a large elongation is inaccurate due to the change of the paper size, and can be remedied by adjusting the pad of the plate cylinder, and the overprinting of the roller axis is not allowed. Unable to make up.
Longitudinal silk paper and horizontal silk paper can not be printed together. Because under the same conditions, the length of the long side and the wide side of the two papers with different fiber directions are different. If the printing is inaccurate due to paper deformation in the next color printing, even if some measures are taken to remedy it, It can't be done because there is no uniform deformation.
Third, the surface gloss of the paper
Gloss is the ability of a paper surface to reflect concentrated light at an angle to the incident light, which indicates the glossiness of the paper. In general, the higher the smoothness of the paper, the higher the gloss, and the effect of gloss on printing is as follows.
1 The gloss of the print will increase as the gloss of the paper itself increases, and the gloss of the print will make the color bright and full. In actual printing, high-gloss prints are generally used, and high-gloss paper is generally used.
2 High-gloss paper can achieve higher printing density when the same ink film thickness is printed on the paper surface than paper with low gloss.
3 Due to the influence of glare, too high gloss often leads to a decline in reading quality and makes people feel tired. Therefore, text-based publications often use low gloss.
Fourth, smoothness
Smoothness is a technical indicator for evaluating the degree of unevenness on paper and board surfaces. Generally, the smoothness of the paper under the printing pressure is referred to as the printing smoothness, and the smoothness in the free state is referred to as the apparent smoothness.
Paper smoothness has a great influence on printing, mainly in the effect of dot reduction and printing gloss.
1 The effect of smoothness on blotting reduction. The smoothness of printing determines the degree of embossing moment, the degree of contact between the surface of the paper and the surface of the blanket, which is an important factor that affects whether the ink transfer is comprehensive and the image is clear.
2 The effect of paper smoothness on the gloss of the print. The ink has a certain fluidity on the paper before it is transferred from the blanket to the paper until it is officially dried. When the ink and printing conditions are constant, the smoothness of the paper, the better the level of ink leveling, and the stronger the gloss of printing. In order to obtain uniform and complete ink transfer and reasonable solid density, papers with poor smoothness are generally used to increase printing pressure and increase the amount of ink and the fluidity of the ink.
Fifth, the ink absorption of paper
In the printing process, the ink absorption of the paper is related to the tightness of the paper, the surface properties of the papermaking fiber, the content of the filler and the rubber compound, and is related to the characteristics of the printing ink, the printing method, the printing speed, the printing pressure and the like. . The ink absorbing property of the paper affects the fixing and drying speed of the print, and also affects the quality of the print.
When the ink absorbability of the paper is too high, the binder in the ink is excessively sucked into the inside of the paper, and the binder in the ink layer is correspondingly reduced excessively, and the pigment particles in the ink are floated on the paper surface, and the ink layer is dried. After lack of luster. When printing on paper with too low ink absorption, the drying speed of the ink will be slow, and the back surface will be easily smeared. The paper with too small ink absorption capacity should control the thickness of the ink layer, and use fast drying ink and spray. Powder and other measures to prevent back sputum.
Sixth, the water content of the paper
The ratio of the mass of water contained in the paper to the mass of the paper is called the moisture content of the paper. The moisture content of the paper has an important influence on the printing process and product quality. Too high or too little water content will cause many printing failures.
1 If the water content of the paper is too high, the surface strength and tensile strength of the paper will be significantly reduced, which will intensify the pulling and elongation deformation during the printing process; and the elasticity and stiffness of the paper will decrease.
2 If the water content of the paper is too small, the paper will become brittle, hard, and easily damaged, and the insulation of the paper will be improved to generate static electricity. When printing, it is prone to double and empty sheets.
3 The change in paper size caused by the change in paper moisture content is the most important cause of the failure of offset printing. In addition, the "leaf edge", "tight edge", "curl", etc. caused by the uneven change of the water content of the paper pulp may cause wrinkles, inaccurate printing, paper feeding and paper delivery difficulties. Therefore, in the usual production of production, it is necessary to strictly control the change of paper moisture content, including controlling the change of paper moisture content caused by the surrounding environment of the paper.