The moisture content of the paper has a great influence on the printing. The moisture is too low, the paper is brittle, and static electricity is easily generated during printing; if the content is too high, the ink will be difficult to dry. Moisture changes also have a great influence on the various characteristics of paper. With the change of moisture, its basis weight, tensile strength, flexibility, folding resistance, etc. will change, and the paper size will also expand and contract, even curling, warping Edge, wrinkle, lotus leaf and other phenomena.
All kinds of paper have an optimal moisture content value. At this value, the various properties of the paper are in a good state. The optimal moisture content of cast coated paper and cast coated white cardboard is 7% ± 2%. White cardboard 4% -7%, single-sided coated white paperboard is 8% ± 2%. This is the moisture content of the paper when it leaves the factory.
During storage in the printing house, paper acts as a hygroscopic material, it can absorb moisture from the air, the rate of absorption depends on the relative humidity and temperature of the air; it can also pass moisture into the air and lose moisture, the rate depends on the moisture content of the paper And ambient temperature and humidity. When the rate of moisture absorption and the rate of water loss are the same, the paper and the air are in equilibrium, and the moisture content in the paper is unchanged. In this case, the moisture content of the paper is called equilibrium moisture.
The relative humidity refers to the ratio of the moisture content in the air at a certain temperature to the moisture content that the air can saturate at that temperature. The same kind of paper has different equilibrium moisture under different relative humidity.
How to balance moisture
The changes in the equilibrium moisture have the following characteristics:
â‘ Affected by the nature of the paper. At the same relative humidity, the more hydrophilic the paper, the higher its equilibrium moisture; otherwise, the lower it is. The paper with no auxiliary materials applied has a higher equilibrium moisture, while the paper with filler, sizing and coating has a lower equilibrium moisture. Thick products of the same variety have a higher equilibrium moisture than thin products because of the high ratio of water-absorbent base paper in thick products.
② Influenced by temperature. At the same relative humidity, when the temperature changes by about 15 ° C, the maximum change in paper equilibrium moisture is about 0.5%. However, the change of paper moisture content in overprinting during printing needs to be controlled within ± 0.1%, otherwise it will affect the accuracy of overprinting. Therefore, while controlling the relative humidity, the color printing workshop should control the temperature change to ± 3 ℃.
â‘¢ It is affected by the journey to equilibrium-moisture absorption or dehumidification. The moisture content of the paper at a certain relative humidity when absorbing moisture from low moisture to reach equilibrium is lower than that at the same relative humidity when it is dehumidified by high moisture to achieve equilibrium. This is the hysteresis effect of paper moisture absorption. In order to make the paper moisture content consistent with the original, we must adopt the "overcorrection" method. That is, a paper that has reached equilibrium at a certain relative humidity. If it needs to be dehumidified to reach the original equilibrium moisture after absorbing moisture in a high humidity environment, it must be placed in an environment with a lower relative humidity, and vice versa.
In addition, the speed of moisture absorption and dehumidification is also different, the dehumidification speed is much slower. But whether it is absorbing or dehumidifying, the starting speed is relatively fast, and the closer it is to equilibrium, the slower it is. This process is also related to paper quality and air mobility. Under standard humidity and temperature, a single sheet of cigarette paper can be treated for 35 minutes to achieve a 5.8% equilibrium moisture; various printing papers require 2-4 hours to reach a 5% -8% equilibrium moisture; packaging cardboard takes longer . This lag between the change in relative humidity and the change in paper moisture content will bring about the lag effect between the deformation of the paper and the change in relative humidity.
â‘£ Affected by the directionality of the paper. The so-called directivity refers to the paper's balanced moisture is not consistent in the longitudinal and transverse directions, the lateral expansion and contraction rate is much greater than the longitudinal expansion and contraction rate, therefore, the paper's lateral expansion and contraction deformation is greater than the longitudinal direction. Tests have shown that the expansion and contraction of a single fiber in the transverse direction is equivalent to about 20 times the longitudinal direction. However, the direction of the fibers in the paper is varied, but it is usually the number of fibers arranged in the longitudinal direction.
Therefore, the gap between the stretch ratio of the paper in the vertical and horizontal directions is not as large as that of a single fiber. Taking the relative humidity from 50% to 60% as an example, the ratio of vertical and horizontal expansion is about 3: 7, which is about 2.3 times. This ratio varies according to the degree of fiber alignment. The stronger the directivity, the greater the ratio. Making paper with random and irregular fiber arrangement directions is one of the key problems to reduce the lateral expansion and contraction rate and avoid misregistration.
⑤ Influenced by both sides of paper. A very flat sheet of paper will absorb or dehumidify in the air when its environment changes. If it is hygroscopic, the elongation of the reverse side is larger than the front side, and the paper will curl to the front side; if it is dehumidification, the shortening of the reverse side is larger than the front side, the paper will curl to the reverse side. Regardless of the direction of curling, the longitudinal direction of the paper is always taken as the axis, which is caused by the different degrees of orientation of the front and back sides of the paper. Especially cardboard, the performance is even greater in this regard.
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, changing the shape of the entire fiber, 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.
Discussion on the overprinting caused by paper stretching
Paper is a material containing variable plant fibrous tissue. It has the function of absorbing moisture and releasing moisture. When the moisture in the air is constantly changing, it will adjust the balance with the air humidity. When the humidity of the air is higher than the moisture content of the paper, it will absorb the moisture in the air and expand and stretch; otherwise it will release the moisture and shrink to shorten to achieve a relative balance with the humidity of the environment.
Lithographic offset printing is based on the principle that oil and water are immiscible. Therefore, after printing, the paper generally stretches, and shrinkage becomes shorter. In order to eliminate such failures, the related printing process is now discussed:
1. Use horizontal thread locks when printing. The paper is divided into horizontal threads and longitudinal threads. In the papermaking part of the papermaking process, because the pulp (vegetable fiber) flows vertically to the moving wire mesh, the fibers are suddenly accelerated, and most of them will be connected along the direction of the wire mesh, forming what we call Dreadlocks. The plant fibers in the paper are hollow and translucent tubes. We can vividly say that the length direction of the fiber itself is what we call the longitudinal thread lock, and the direction of the gap between the fiber and the fiber is the horizontal thread lock.
The experimental results show that the cohesive force of the fiber itself is greater than the bonding force between the fibers (mainly hydrogen bonding force). After the paper is damp, the expansion and contraction in the direction of the longitudinal threads is smaller than that in the direction of the horizontal threads. The two are roughly 1: 2. Therefore, products with more accurate overprinting must consider the deformation relationship of the horizontal and vertical threads of the paper. Therefore, when printing, make the longitudinal thread direction of the paper parallel to the cylinder axis, and the thread direction of the same batch of printing paper should be consistent.
2. Overprint clear water (commonly known as pressurized water). In the case that the printing paper is not easy to master, in order to prevent the paper from stretching during printing, you can use the method of overprinting once to make the paper adapt to the printing requirements, and then start printing the color. This method is not accurate for solving overprinting. Has a certain role.
3. Control the moisture of the printing plate. The moisture content of the printing plate is the key factor to control the expansion and contraction of the paper. The moisture of the plate is transferred to the paper through the blanket. The larger the moisture of the plate, the greater the expansion and contraction. Therefore, the smaller the moisture control of the plate, the better, especially when printing 80 grams of double-coated paper.
4. The printing plate is elongated by a plate-drawing machine. Generally, the single-color machine is in the third and four-color printing process, and the two-color machine is in the second printing process, and the image on the paper may be elongated, resulting in inaccurate overprinting. This is because the PS plate material has better ductility, and can be elongated by a plate-drawing machine to make the plate image and the image on the paper stretch evenly, so that the overprinting is accurate.
At present, most domestic small and medium-sized enterprises generally do not have air-conditioning equipment. Most of the practice is to spray some water around the machine to increase the air humidity. For products with low requirements for non-dot printing, you can use a spray cylinder to sprinkle water on the edge of the paper and then print on the machine; for high air humidity in the workshop, the cut paper is best covered with a plastic cover to prevent moisture from becoming longer ; If it is a secondary printing, you can use the method of borrowing lines to make up for the shrinkage of the image.
In general, there are many process treatment methods to solve the inaccurate overprint caused by the expansion and contraction of paper. As long as the discussion and management of the printing process are strengthened, it can generally be solved.
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