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The influence of paper grain on printing and post press processing

paper is the most important substrate in printing production. The maximum experimental load of the breaking amount of the sample should be between 20% and (9) 0% of the range of the experimental machine. The performance of the paper directly determines the printing quality and post press processing. The properties of paper include surface properties, structural properties, optical properties, chemical properties, etc. any of these properties will affect the quality of printed matter. The following is an analysis of the impact of paper structure performance on printing and post press processing. Paper is mainly a thin thing intertwined with plant fibers. The length, strength, arrangement direction and tightness of the fibers will affect the quality of the paper, and the arrangement direction of the fibers is the problem of the grain of the paper

1 formation of grain

in pulp, the arrangement of plant fibers after beating is disorderly, but when making paper on a high-speed paper machine, due to the effect of center of gravity and inertia, most fibers will be aligned along the direction of copper movement of the paper machine, just like wood in water is aligned along the direction of water flow. Although a small amount of fibers are still disordered due to the vibration of the paper machine, which weakens the directionality of the fiber arrangement, the fibers in the paper still show obvious longitudinal and transverse differences, which is the grain of the paper

2 the influence of paper grain on printing

the influence of paper grain on printing has not been paid much attention by people all the time. It is thought that the influence of paper grain on printing is small, and it has no greater influence on printing quality than the surface strength, smoothness, gloss and whiteness of paper. So does it really have a small impact? Of course not, its impact on printing can be analyzed from the following aspects

1) the influence of paper deformation on overprint

we know that the main components of fiber are cellulose, hemicellulose and lignin, while cellulose is a polymer compound composed of multiple glucose monomers (-c6h10o5-), while hemicellulose is a polysaccharide molecule composed of pentose and hexose. The molecular structure contains multiple hydroxyl groups. The polarity of hydroxyl groups is very strong, which is easy to absorb water, which is also one of the reasons why paper is easy to absorb water. Both the diameter direction and the length direction of the fiber will absorb water and expand, but the deformation is different. The amplitude of expansion and deformation in the diameter direction is almost 2~8 times that in the length direction; Dehydration will also produce shrinkage, and the amplitude of shrinkage deformation is much larger in the diameter direction than in the length direction, which will cause different deformation in the length and width directions of the paper. This deformation will affect the image quality transferred to the paper, especially the overprint inaccuracy

in addition, the deformation of paper under pressure and tension is also related to the arrangement direction of fibers, with small longitudinal deformation and large transverse deformation. Therefore, in order to solve this problem, we should recognize the direction of the paper when printing paper, otherwise it will cause great quality problems. So how to judge the direction of the paper? Generally, there will be obvious marks on the outer package of paper, which can also be judged in this way. Generally, when the long side of paper is consistent with the fiber length direction, it is called longitudinal silk paper, and when the long side of paper is perpendicular to the fiber length direction, it is called horizontal silk paper. At the same time, you can also tear the paper. After tearing the paper, the straight direction of the crack is longitudinal, and the jagged or oblique direction of the crack is transverse. Pay attention to the use of longitudinal wire paper in printing, because when the diameter direction of the fiber is consistent with the printing direction, the deformation of the paper can be compensated for the possible overprint inaccuracy of the image by increasing or decreasing the roller liner, while the axial deformation is difficult to compensate

2) other effects

in addition to causing a relatively large impact on the deformation of the grain of paper, which will affect the printing quality, it will also affect the tensile strength, folding resistance, tearing degree, etc

① tensile strength. It is the ability of the paper to resist stretching and produce elongation. Its size depends on the strength and adhesion of the fibers in the paper. It can be seen from the grain properties of the paper that the longitudinal direction of the paper has better tensile strength, because the longitudinal direction is the direction of fiber length, and its strength is better; However, the tensile strength is poor because of the binding force between fiber molecules in the transverse direction. Therefore, for web paper, its length direction should be the longitudinal direction of the paper, otherwise it is easy to cause paper breakage and affect the printing efficiency and quality

② folding resistance. It is the ability of the paper to resist repeated folding under a certain tension. The folding resistance is mainly affected by the strength and length of fibers and the adhesion between fibers. Because the longitudinal direction of the paper is the length direction of the fiber, it has a better ability to resist folding, while the transverse direction depends only on the molecular binding force of the fiber, and the ability to resist folding is poor. Therefore, for some printed materials that need to be folded repeatedly, such as banknote paper, various securities paper, map paper, books and periodicals, and the cover of exercise books, we should choose paper with good folding resistance, especially in the direction with more folding times, we should pay attention to the grain of paper. Otherwise, it is easy to break after a certain period of use

in addition, there are certain requirements for the longitudinal grain of the paper in the process of folding and binding. When folding several times in a certain direction, the folding resistance in this direction should be better, so as to avoid the breakage of the paper caused by the folding knife during folding. However, when binding books and periodicals, in order to make the book better when opening, the book can be well fixed. At this time, the direction of the paper should be the same as that of the spine

③ tear degree. The tear resistance of paper. Similarly, the tearing degree of paper depends on the fiber strength and adhesion. The transverse direction of the longitudinal paper, that is, the long side direction of the paper, has good tear resistance and is not easy to tear due to the joint action of the bonding force between the fibers and the fiber strength, and the tear surface will appear jagged; The longitudinal direction of the longitudinal paper, that is, the short side direction, has only fiber bonding force, which is easy to tear, and the tearing surface is neat. Therefore, the paper should pay attention to the tearing degree when printing paper feeding and folding. At the same time, when binding, we should also pay attention to which direction is easy to cause damage, so the tear resistance of this direction should be better

in a word, the grain problem of paper is still a problem that cannot be ignored in the process of printing, post press processing and packaging. In the case that the current papermaking technology can not effectively avoid the grain problem, we must take various methods to pay attention to the grain direction of the paper in the process of printing, post press processing and packaging, so as to ensure the printing quality and service life

source: PRINT world author: Chen Wenge

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