The dimensional stability of paper is an important indicator of paper, which has an important impact on the stability of printing quality.
The dimensional stability of paper is also called shape stability, which refers to the change of the paper size (shape) when the paper moisture changes. It is expressed as the percentage of the size change before and after the paper moisture change to the original size of the paper. Generally speaking, all kinds of paper basically have the conditions of elongation when absorbing moisture and contraction when reducing moisture. The rate and degree of change vary with the type of fiber raw material in the paper. The greater the rate and degree of change, the poorer the paper's form stability; otherwise, the form stability is better.
Causes of paper deformation and its effect on printing quality
In the process of manufacturing and use, there are many factors that affect the deformation of the paper. The physical and chemical characteristics of the pulp, the beating situation, the choice of chemicals and fillers, the papermaking performance, the equipment status of the paper machine, and the water gain and loss during use will all affect The size of the paper changes. Here we only discuss the dimensional changes caused by the loss of moisture during the transportation and printing of finished paper.
Paper is mainly composed of cellulose, and cellulose is a very hydrophilic substance, which has strong water absorption and dehydration. During the storage, transportation and printing of paper, changes in the water content of the paper due to changes in temperature and humidity cause expansion and contraction. The main reasons for paper size changes caused by paper moisture changes are: When the paper moisture changes, the single fibers that make up the paper can absorb water expansion or lose water shrinkage, causing the entire fiber shape to change, which causes the paper size to change; there is hydrogen inside the paper Bonding, when the paper absorbs or loses moisture, the role of hydrogen bonding is strengthened or weakened, causing the fibers to tighten or push apart.
The changes in the shape of the fibers inside the paper due to the loss of water will eventually lead to changes in the size of the paper, which will adversely affect the printing. When the moisture content of the paper is too large, the binding force between the fiber, filler and rubber in the paper is easy to decrease, which will affect the surface strength of the paper, resulting in the occurrence of undesirable phenomena such as dusting and lint after printing. The occurrence of powder loss and hair loss reduces the hardness and stiffness of the edge of the paper, and it is easy to cause the problem of unstable positioning during paper feeding, resulting in misregistration. If the moisture content of the paper is too low, the paper will have poor elasticity, and the dot print will easily expand during printing, resulting in a large dot expansion rate.
Therefore, the paper should not be printed under high humidity conditions that are not suitable for its water content, but should be printed under medium humidity, which is conducive to the guarantee of overprint accuracy. In order to keep the moisture content of the paper uniform on the entire paper surface, and to adapt to the temperature and humidity of the printing workshop, at the same time, in order to reduce the sensitivity of the paper to the environmental humidity and improve the stability of the paper size, it is generally necessary to take Certain measures.
Treatment method of paper deformation
In order to improve the dimensional stability of the paper, the paper is usually processed by the following methods:
1. Coating. After the paper is coated, the deformation of the paper can be improved to a certain extent. However, after the paper is coated on one side and dried, the balance system of fiber and water molecules is destroyed, which will cause uneven distribution of water molecules inside the paper and inconsistent internal stress, causing deformation and shrinkage of various parts. . If it is not modified, it will affect the running suitability of the paper.
Conditioning is a process that can eliminate the undesirable stress that has been generated in the paper during the processing and exists in the paper, so that the paper can obtain the appropriate moisture evenly distributed on the whole, and ensure that the paper is exposed to the normal room Maintain flat and stable dimensions under temperature and humidity. Therefore, in the process of coating and processing of paper, humidity control is an important process that is often applied. There are roughly two types of traditional process methods: the older method is to hang the paper sheet in a certain humidity environment or in a large sealed space, and fill it with a large amount of steam to form a high-temperature and stable humidity space. In this state, according to the principle of fiber water absorption balance, the paper sheet slowly absorbs moisture until the absolute moisture content of the paper web is in a state of equilibrium with the surrounding air moisture content. With this process, the effect of humidifying and conditioning the paper sheet is better , But it takes up a lot of space, takes a long time, and is not suitable for web, so this method has been gradually eliminated; the second more common method is to use liquid water to humidify, in a small sealed device, spray Disperse or change the shape of small droplets to form an atomized space. The principle of electrostatic adsorption is used to make the small droplets contact the paper surface to achieve humidification, but this method can only humidify the surface of the paper. Atomized small droplets will not penetrate into the internal fibers of the paper sheet, and it is easy to cause excessive water on the surface of the paper sheet to cause swelling or stains and other paper diseases, thereby reducing the quality of the paper.
According to many years of production experience and practice, after the paper is humidified and conditioned, it can eliminate the deformation of the paper sheet, the flatness and dimensional stability of the paper sheet, calendering and finishing, printing, slitting, Packaging is of great significance. When the paper is in the "free state", if the moisture in both the width and the longitudinal direction can be uniformly adjusted to the required value, then it is possible to produce "non-deformed" flatter paper, the risk of static interference It can be reduced, and the running performance of the paper on the printing machine is improved, and the adaptability of the paper page printing is improved.
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