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.
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 40-60% moisture; semi-dry wrinkling, wrinkling at 20-40% moisture; dry method Wrinkle, the paper web is wrinkled at 5-8%. 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) treatment, the moisture content of the paper is expressed as a 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. The paper is deformed due to moisture absorption during printing. 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.
1. Selection of printing paper format. In the layout, the deformation of the paper must be considered, and the amount of deformation of different papers is different. Some papers are loose in texture and are easily affected by changes in moisture. For these papers, the layout should not be too large. Many printing companies' offset printing equipment is folio format, so it is hoped that paper can be produced in folio format, so as to improve production efficiency. However, some products have more requirements for the next processing procedure and higher accuracy of overprinting. If the machine is printed on a folio format, the relative deformation of the paper is also relatively large, resulting in great difficulty in post-processing overprinting. This requires that it is best to use full-open paper for printing, but sometimes it will cause waste of paper. Therefore, it is necessary to formulate specific and detailed process plans according to the specific requirements of the products.
2. The printing direction of the paper. Since the deformation of the paper fiber in the length direction is greater than the radial direction, it is advantageous to improve the accuracy of overprinting by using paper that is parallel to the longitudinal axis of the printing cylinder. It is necessary for us to study the influence law of paper hygroscopicity and dimensional stability of water change, and to formulate printing process and humidity regulation according to this law. Improve the quality level of printing.
3. Set the position of the overprint pattern. Some printed products, especially packaging boxes, require bronzing or bump printing. During imposition, pay attention to arranging bronzing patterns as close as possible to the side of the body. This is because the rules on the side of the body are better than those on the outside, especially some larger sizes. It is more difficult to register on the outside than on the body side. According to the requirements of the packaging structure design, when placing the long side of the packaging box, the paper fiber direction should be perpendicular to the long side of the packaging box during imposition, so that the maximum carton strength can be obtained, but this type of layout is often not conducive to the next step. The registration and design of the overprint pattern should be paid special attention.
BBQ grills are cooking appliances that are used for grilling or barbecuing food. They are typically used outdoors and are designed to cook food over an open flame or hot coals. BBQ grills come in various types, including gas grills, charcoal grills, electric grills, and pellet grills. Gas grills use propane or natural gas as fuel, while charcoal grills use charcoal briquettes or lump charcoal. Electric grills are powered by electricity, and pellet grills use wood pellets as fuel. BBQ grills are popular for cooking various types of food, including burgers, steaks, hot dogs, vegetables, and seafood. They are commonly used for outdoor gatherings, parties, and picnics.
Metal Bbq Tool,Metal Bbq Grill Mesh Basket,Bbq Metal Skewers,Stainless Steel Ring Brush,barbeque tools
vchomy , https://www.vichenivchomy.com