Paper moisture content
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.
Balance moisture
In addition, the changes in 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.
②Affected 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‘¢Affected by the journey to balance-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 than the original, 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.
⑤Affected by both sides of the 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.
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