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 humidification and conditioning of 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, spray in a small sealed device 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.
2. Wrinkle. Paper deformation will affect other properties and printing quality of the paper, but in order to improve some properties of the paper, we can artificially make some wrinkles on the paper, this is the wrinkling technology. Paper wrinkling can greatly improve the deformation properties of the paper, increase the elongation and dynamic strength of the paper, obtain softness, air permeability and elasticity, and improve the absorption characteristics, air permeability and other special properties of the paper.
The method of wrinkling the paper is the "printing roller" method. In addition, the most popular and widely used is the scraper method of wrinkling the dryer with a scraper and peeling the paper from the surface of the cylinder. According to the moisture of the paper web in contact with the scraper, it can be divided into three types: wet wrinkling, wrinkling at 60% moisture; semi-dry wrinkling, wrinkling at 40% moisture; dry wrinkling, Wrinkles at 8% moisture content. Whether it is dry wrinkle or wet wrinkle, the basic principles are similar. Wet wrinkling and semi-dry wrinkling are easy to grasp, and the wrinkles after wrinkling are more uniform, but the paper is dry and stiff, and the board is hard. It is only suitable for the production of low-grade toilet paper.
3. Humidity adjustment. Humidity adjustment (hanging) process, the moisture content of the paper is expressed as the percentage of the weight of water contained in the paper and the weight of the paper. When the moisture content of the paper changes by 0.1%, it will affect the overprint, so controlling the moisture content of the paper is the prerequisite to ensure the accuracy of the overprint. In general, the moisture content of printing paper should be controlled at 7% ± 1%, and the moisture content error between the center of the paper and the periphery of the paper is not more than 1%. Usually the printing company's method of drying paper is to transport the paper to the printing workshop 24 hours in advance or put it in a workshop close to the printing workshop's temperature and humidity for use. Due to the limitation of the site, many companies only need to print paper only a few hours before printing and turn it into the printing workshop, which is very detrimental to the stability of the paper size. The best way is to humidify the drying room that is 6% to 8% higher than the relative humidity of the printing workshop or put it in a humid place to humidify first, and then go to the printing workshop to balance the moisture to make the moisture content of the paper even. stable.
4. Active humidification. For offset printing, the principle of water and ink separation is adopted. During printing, the paper is deformed due to moisture absorption. According to the hysteresis effect of paper moisture change, the paper can be printed once before the official printing and the water content of the paper can be actively increased (commonly known as "run water") , Let the paper have a pre-hygroscopic process, so that the amount of paper deformation during formal printing will be much smaller. After offset printing, especially UV printing, due to the influence of drying temperature, the paper is in the process of dehumidification, and the size usually becomes smaller. Due to the influence of high-intensity heat after laminating or off-line polishing, the size of the paper is greatly deformed. Although it has been left for more than ten hours or more, the effect on the moisture of the paper is not very obvious, and the severe deformation of the paper size is difficult to recover. In this case, the moisture content of the paper can be adjusted by active humidification to make the paper rebound.
The above methods are to improve the stability of the paper by adjusting the state of the paper. In addition, you should also start from the pre-press and do appropriate processing on the printing settings to compensate for the deformation of the paper to ensure the printing quality.
Feature:
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