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How does paper balance moisture affect printing?

The moisture content of paper has a great influence on printing, the moisture content is too low, the paper is brittle, and static electricity is easily generated in printing. If the content is too high, the ink will be difficult to dry. Water changes also have a great influence on various characteristics of paper. With the change of water, its quantity, tensile strength, flexibility, folding endurance, etc. will all change, and the paper size will also expand and contract, and even curl, warp, wrinkle, lotus leaf, etc. All kinds of paper have an optimum moisture content value, under which all kinds of properties of paper are in good condition, the optimum moisture content of cast coated paper and cast coated white cardboard is 7 % 2 %, white cardboard is 4 % - 7 %, and single coated white cardboard is 8 % 2 %. This is the moisture content of paper when it is produced. During storage in printing shops, paper as a hygroscopic material can absorb moisture from the air, and the absorption rate depends on the relative humidity and temperature of the air. It can also transfer moisture into the air and lose moisture, depending on the moisture content of the paper and the ambient humidity temperature. When the moisture absorption rate and the water loss rate are the same, the paper and air are in equilibrium, and the water content in the paper remains unchanged. In this case, the water content in the paper is called equilibrium water. The so-called 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 hold when saturated at that temperature. The balance moisture of the same paper is different under different relative humidity.
In addition, the change of equilibrium moisture has the following characteristics: ① under the same relative humidity affected by the properties of paper, the more hydrophilic the paper is, the higher the equilibrium moisture is; On the contrary, the lower. Paper with no auxiliary materials has higher equilibrium moisture, while paper with fillers, sizing and coating has lower equilibrium moisture. The same variety and its thick products have a higher ratio of base paper rich in water absorption than the thin products.
( 2 ) Under the same relative humidity affected by temperature, when the temperature changes about 15 ℃, the maximum change of paper balance moisture is 0. 5%。 However, the change of water content in paper caused by overprinting in printing should be controlled within 0. 1 %, otherwise it will affect the accuracy of overprinting. Therefore, the color printing workshop should control the change of temperature at 3 ℃ while controlling the relative humidity.
( 3 ) under the influence of the journey to balance - moisture absorption or dehumidification, the moisture content of the paper when it reaches balance from low moisture absorption under a certain relative humidity is lower than that when it reaches balance from high moisture desorption under the same relative humidity, which is the moisture absorption hysteresis effect of the paper. If you want to make the paper water content consistent with the original. We must adopt a " overcorrection" approach. That is, if the paper reaches equilibrium under a certain relative humidity, it must be placed in an environment lower than the original relative humidity if it is to be dehumidified to reach the original equilibrium moisture after absorbing moisture in a high humidity environment, and vice versa. In addition, the speed of moisture absorption and dehumidification is also different, and the speed of dehumidification is much slower. However, no matter moisture absorption or dehumidification, the initial speed is relatively fast, and the closer the balance is, the slower it will be. This process also has something to do with paper and air mobility. At standard humidity and temperature, 35 mm of single cigarette paper can reach 5. 8 % equilibrium moisture; It takes 2 - 4h for all kinds of printing paper to reach 5 - 8 % equilibrium moisture. Packaging cardboard takes longer. This lag phenomenon between the change of relative humidity and the change of paper moisture content will lead to the lag effect of paper deformation and the change of relative humidity.
( 4 ) Affected by the directionality of paper, the so-called directionality means that the balance moisture of paper is inconsistent in the longitudinal and transverse directions, and the transverse expansion ratio is much larger than the longitudinal expansion ratio, so the transverse expansion change of paper is larger than the longitudinal expansion ratio. Tests show that the expansion and contraction of the single heel fiber in the transverse direction is about 20 times that in the longitudinal direction. However, the arrangement direction of the fibers inside the paper is various, but the number of fibers arranged along the longitudinal direction is usually more. Therefore, the difference in the ratio of paper stretching in the vertical and horizontal direction is not as large as that of a single fiber stretching in the vertical and horizontal direction. Taking the relative humidity from 50 % to 60 % as an example, the ratio of vertical to horizontal contraction is about 3: 7, or 2. Three times or so. This ratio varies according to the degree of orientation of the fiber arrangement. The stronger the orientation, the greater the ratio. Making paper with scattered and regular fiber arrangement direction is one of the key problems to reduce the transverse expansion rate and avoid misregister. The paper balance moisture characteristics, paper balance moisture content, paper moisture content, paper moisture meter, paper moisture meter.
⑤ Due to the dual nature of the paper, a very flat paper will absorb moisture or dehydrate in the air when its environment changes. If it is hygroscopic, the elongation of the reverse side is larger than that of the front side, and the paper will curl like the front side. If it is dehumidified, the shortening of the reverse side is larger than that of the front side, and the paper will curl like the reverse side. No matter which direction the paper is curled, it always takes the longitudinal direction of the paper as its axis, which is caused by the different degree of orientation of the front and back sides of the paper. In particular, cardboard has performed even more in this respect.
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