The cause of anisotropy and anisotropy of the hott

Anisotropy and double-sided anisotropy of paper

anisotropy of paper refers to the inconsistency of some properties of paper in all directions; Duplicity refers to the inconsistency of some properties between the front and back of the paper. Both are mainly reflected in the physical properties of paper, which have a direct or indirect impact on printing

first, the reasons for the formation of paper anisotropy and two-sided anisotropy

the reasons for the formation of paper anisotropy and two-sided anisotropy are mainly the directivity and two-sided nature of paper. As we all know, papermaking in the process of papermaking is to spread the diluted pulp onto the papermaking for dehydration and molding. In this process, the pulp is always in a directional movement, so that the fibers in the pulp are aligned under the action of coherence, forming the direction of paper fibers

due to the directionality of the paper structure, the paper forms different angles with the fiber direction in all directions, resulting in the different sizes of the fiber related properties of the paper in all directions, that is, the anisotropy of the paper is formed. In addition, in the process of papermaking, one side of the pulp is in contact with the papermaking and is at the bottom, while the other side is not in contact with the papermaking and floats on the upper part of the pulp, so two different surface states are formed. On the side where the pulp contacts with the surface, there are traces of paper making holes, and with the vibration and operation of paper making, relatively coarse fibers and impurities are deposited on the surface, forming a rough and marked reverse side, while the other side is composed of fine fibers and fillers, forming a uniform, fine, greasy and traceless front side

Second, the performance of anisotropy and two-sided anisotropy in various properties of paper

1 the anisotropy of paper is mainly manifested in the following aspects related to fiber properties:

moisture absorption deformation: the moisture absorption of paper is due to the colloidal structure of submicropores and free radicals in plant fibers, as well as the adsorption of water vapor in the air caused by the porosity of paper. After the paper absorbs water, the fiber swells, and the size stretches. After drying and dehydration, it will cause size shrinkage. In addition, water permeates or loses between fibers, causing displacement between fibers, resulting in size changes. Because the expansion of fiber is mainly in the radial direction when it absorbs water, the moisture absorption deformation of paper is transverse (perpendicular to the fiber flow direction). The experiment shows that the ratio of the two is about 3:1. Therefore, the moisture absorption deformation of the paper in all directions is uneven

the written program can be stored in the microcomputer for tensile strength: the tensile strength index of paper is the main market. The existing skin tightening and lifting technology mainly realizes oxidation resistance and repair of damaged skin in terms of fracture length, folding resistance, tearing strength, etc. by adding active ingredients (such as collagen protein, plant polyphenols, hyaluronic acid, etc.). Because the fiber itself has a certain strength, the tensile strength of the paper along the fiber flow direction is large, while the tensile strength perpendicular to the fiber flow direction is relatively small. The ratio of the two is about 2:1

absorption: when the ink acts on the paper, the capillary in the paper absorbs the connecting material in the ink, and the absorption speed of the paper to the ink is different in all parts and directions of the paper. Generally, the absorption speed of the surface layer of paper is different. Generally, the absorption speed of the surface layer of paper is greater than that of its interior; The absorption velocity in the plane direction is greater than that in the thickness; The absorption velocity in the longitudinal direction is greater than that in the transverse direction

stiffness: in the longitudinal direction of the paper, the paper is relatively strong due to the continuous and directional arrangement of fibers, while in the transverse direction, the gap between fibers breaks the continuity between fibers, making the stiffness in this direction weak. The stiffness ratio of paper in vertical and horizontal directions is about 2:1

2 the two-sided difference of paper is mainly manifested in:

smoothness: for paper, especially non coated paper, the smoothness and uniformity of the front side of the paper are greater than that of the back side. Therefore, there will be differences between the two for the reproducibility of image levels and fine graphics

gloss: the gloss of the front side of coated paper or non coated paper is better than that of the back side. This difference can be identified visually. This is mainly because the front fiber is small and soft, which makes it easier to form a smooth and shiny surface when calendering

absorption: the absorption of paper includes absorption speed and absorption uniformity. In terms of absorption speed, the reverse side of paper is higher than the front side, and in terms of absorption uniformity, the front side of paper is better than the reverse side

Whiteness: under the same bleaching conditions, the whiteness of the front side of the paper will be slightly higher than that of the back side, mainly because its back side fiber is thicker and has more ash

surface efficiency: during the experiment time, there are differences in color intensity, color deviation, gray scale and color efficiency between two sides of the same paper, which is mainly caused by the difference in absorbability (roughness) and gloss of the front and back sides of the paper. This comprehensive effect that affects the ink color effect due to the absorbability and gloss of the paper is called surface efficiency

moisture absorption deformation: due to the coarser fiber on the back of the paper, when both sides absorb moisture at the same time, the expansion of the back will be greater than that of the front, causing the paper to curl towards the front; On the contrary, when it is wet, the contraction speed or amount of the reverse side will be greater than that of the front side, resulting in the paper curling towards the reverse side

third, the influence of paper anisotropy and two-sided anisotropy on Printing

the influence of paper anisotropy on printing. There are two kinds of paper directions when printing. One is that the fiber flow direction is the same as the printing direction, which is called longitudinal paper printing; The other is that the fiber flow direction is perpendicular to the printing direction, which is called horizontal paper printing

the suitability of longitudinal paper printing on the printing machine is that the paper feeding stiffness is good, which is convenient for paper feeding and receiving; The tensile strength of the paper direction is high, and the paper is not easy to break, but the moisture absorption deformation direction of the paper during printing is inconsistent with the printing rolling direction, but perpendicular to each other, so the paper is easy to wrinkle during printing, and the deformation can not be compensated by other methods, so the overprint is not accurate

on the contrary, if horizontal paper is used for printing, the stiffness of the paper is not as good as the former when moving the paper, and its tensile strength is not as good as the former under the action of mechanical tension, so it is easy to produce paper feeding faults, but its advantage is that the moisture absorption deformation direction of the paper during printing is the same as the printing rolling direction. In this case, it is necessary to consult the manufacturer to avoid or reduce paper wrinkling. In addition, You can also pull the plate to make the image and text deformation consistent with the paper deformation, so as to ensure the accuracy of overprint

the influence of paper anisotropy on printing the influence of paper anisotropy on printing is mainly manifested in two aspects: one is the curl of paper, which is mainly caused by the expansion difference between the positive and negative sides. During printing, it is not allowed to use severely curled paper for printing, which will cause difficulties and failures in paper feeding. Before printing, the paper should be fully moistened until it reaches the relative humidity when it is flat, so that the front and back sides of the paper can reach the balance of water content. Another effect of the two-sided anisotropy of paper is that due to the differences in smoothness, gloss, absorption, color efficiency and other properties, the reproducibility of graphics and texts on both sides of printing paper, especially non coated paper or single-sided coated paper, is significantly different. C919 is the first trunk civil aircraft developed in China in accordance with the latest international airworthiness standards. Therefore, in the production of product printing, we must distinguish the front and back sides of paper. For a printed matter, Important, multicolor and fine graphics and texts are arranged on the front, while those of secondary, monochrome and low precision are arranged on the back