How much do you know about the impact of changes in paper moisture on the printing of Wine Boxes?

How much do you know about the impact of changes in paper moisture on the printing of Wine Boxes?

The moisture content of the paper has a great influence on the printing of Wine Boxes. If the moisture content is too low, the paper will be brittle, and static electricity will easily be generated during printing. If the content is too high, it will be difficult for the ink to dry. The change of moisture also has a great influence on the various properties of paper. With the change of moisture, its basis weight, tensile strength, flexibility, folding resistance, etc. will change, and the size of the paper will also expand and contract, and even curl and warp will occur. Edges, wrinkles, lotus leaf edges and other phenomena.

All kinds of paper have an optimal moisture content value. Under this value, the various properties of the paper are in good condition. The optimal moisture content of cast-coated paper and cast-coated white cardboard is 7%±2%, and white cardboard 4%-7%, single-sided coated white cardboard is 8%±2%. This is the moisture content of the paper when it leaves the factory.

During storage in a printing plant, paper, as a moisture-absorbing material, can absorb moisture from the air at a rate that depends on the relative humidity and temperature of the air; it can also transfer moisture to the air and lose moisture at a rate that depends on the moisture content of the paper. And environmental temperature and humidity. When the moisture absorption rate and the water loss rate are the same, the paper and the air are in equilibrium, and the water content in the paper remains unchanged. In this case, the water content of the paper is called the equilibrium moisture.

　　 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 same kind of paper has different equilibrium moisture under different relative humidity.

　　How to balance water

　　The changes in equilibrium moisture have the following characteristics:

　　① Affected by the nature of the paper. Under the same relative humidity, the more hydrophilic the paper, the higher its equilibrium moisture content; on the contrary, the lower it is. The paper without auxiliary materials has higher equilibrium moisture, while the paper with filler, sizing, and coating has lower equilibrium moisture. The balance of moisture in thick products of the same variety is higher than that of thin products. This is because the ratio of water-absorbent base paper in thick products is high.

　　② Affected by temperature. Under the same relative humidity, the temperature change is about 15℃, and the maximum change of paper equilibrium moisture is about 0.5%. However, the change of paper moisture content during printing should be controlled within ±0.1%, otherwise it will affect the accuracy of overprinting. Therefore, the color printing workshop should control the temperature change at ±3℃ while controlling the relative humidity.

　　③ Affected by the stroke to reach equilibrium-moisture absorption or dehumidification. Under a certain relative humidity, the moisture content of paper when it reaches equilibrium by low moisture absorption is lower than the moisture content when it reaches equilibrium when high moisture is dehumidified under the same relative humidity. This is the hysteresis effect of paper. In order to make the paper moisture content the same as the original, we must adopt an "overcorrection" approach. That is to say, paper that has reached equilibrium under a certain relative humidity, if it is 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, and the dehumidification speed is much slower. But regardless of moisture absorption or dehumidification, the starting speed is relatively fast, and the closer it is to the equilibrium, the slower it is. This process is also related to paper quality and air mobility. Under standard humidity and temperature, a single cigarette paper can reach 5.8% equilibrium moisture after 35 minutes; all kinds of printing paper need 2-4 hours to reach 5%-8% equilibrium moisture; packaging cardboard takes longer . This hysteresis between the change in relative humidity and the change in paper moisture content will bring about a hysteresis effect between the deformation of the paper and the change in relative humidity.

　　④ Affected by the directionality of the paper. The so-called directionality means that the balance of moisture in the paper is inconsistent in the vertical and horizontal directions, and the horizontal expansion rate is much larger than the vertical expansion rate. Therefore, the horizontal expansion and contraction deformation of the paper is larger than the vertical expansion rate. Tests show that the expansion and contraction of a single fiber in the transverse direction is equivalent to about 20 times that in the longitudinal direction. However, the arrangement direction of the fibers inside the paper is varied, but usually the number is more arranged along the longitudinal direction.

　　 Therefore, the gap between the vertical and horizontal stretch ratio of the paper is not as large as the vertical and horizontal stretch 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 arrangement and orientation. The stronger the directionality, the greater the ratio. Making paper with scattered and irregular fiber arrangement directions is one of the key problems to reduce the transverse expansion and contraction rate and avoid the occurrence of inaccurate registration.

　　⑤ Affected by the two sides of paper. A very flat sheet of paper will absorb or dehumidify moisture in the air when its environment changes. If it is moisture absorption, the elongation of the back side is greater than that of the front side, and the paper will curl toward the front; if it is moisture removal, the shrinkage of the back side is greater than that of the front side, and the paper will curl toward the reverse side. No matter which direction it is curled, it always takes the longitudinal direction of the paper as the axis. This happens because the front and back sides of the paper are oriented differently. Especially cardboard, the performance in this aspect is even greater.

　　 The causes of paper deformation and its influence 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 properties of the pulp, the beating condition, the choice of chemicals and fillers, the papermaking performance, the equipment condition of the paper machine, and the moisture gain or loss during use will all affect The size of the paper changes. Here we only discuss the dimensional changes caused by moisture gains and losses 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 paper storage, transportation, and printing, changes in temperature and humidity cause changes in the moisture content of the paper, leading to expansion and contraction. The main reasons for the change of paper size caused by the change of paper moisture are: when the paper moisture changes, the individual fibers that make up the paper can absorb water and expand or lose water and shrink, which changes the shape of the entire fiber, which causes the size of the paper to change; there is hydrogen in the paper. Bond bonding, when the paper absorbs or loses moisture, the role of hydrogen bonding is strengthened or weakened, causing the fibers to tighten or push away from each other.

　　 The shape change of the inner fiber of the paper due to the moisture gain and loss will eventually cause the size of the paper to change, which will adversely affect the printing. When the water content of the paper is too high, the bonding force between the fibers, fillers and sizing material in the paper will be easily reduced, which will affect the surface strength of the paper, resulting in the occurrence of undesirable phenomena such as powder and lint after printing. The occurrence of powder and lint reduces the hardness and stiffness of the paper edge, and it is easy to cause positioning instability during paper feeding, resulting in misregistration. If the water content of the paper is too low, the elasticity of the paper is poor, and the dot print is easy to expand during printing, resulting in a large dot expansion rate.

　　 Therefore, paper is not suitable for printing under high humidity conditions that are not compatible with its moisture content, but should be printed under medium humidity conditions, 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 environmental humidity and improve the stability of the paper size, it is generally necessary to take measures before printing. Certain treatment measures.

　　 Paper stretch-induced overprinting is not allowed

　　Paper is a material containing variable plant fiber tissue. It has the function of absorbing moisture and releasing moisture. When the moisture in the air is constantly changing, it will adjust and balance with the air humidity. When the air humidity is higher than the moisture content of the paper, it will absorb the moisture in the air and expand and expand; on the contrary, it will release the moisture and shrink and shorten to achieve a relative balance with the environmental humidity.

　　 Offset offset printing is based on the principle that oil and water are immiscible with each other for printing. Therefore, the paper generally stretches after printing, but it is rare that the shrinkage becomes shorter. In order to eliminate this type of failure, the relevant printing process is now discussed:

　　 1. Try to use horizontal strands when printing. Paper is divided into horizontal and vertical strands. In the papermaking part of the papermaking process, since the pulp (plant fiber) flows vertically to the moving screen, the fibers are suddenly accelerated, and most of them are connected along the moving direction of the screen, forming what we call Vertical and horizontal strands of silk. The plant fibers in the paper are hollow and translucent tubes. We can say it vividly: the length of the fiber itself is what we call the longitudinal strands, and the direction between the fibers and the fiber is the horizontal strands.

　　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 longitudinal direction is smaller than the expansion and contraction in the transverse direction, and the two are roughly 1:2. Therefore, products with more accurate overprinting must consider the deformation relationship of the vertical and horizontal strands of the paper. Therefore, when printing, make the paper's longitudinal yarn direction parallel to the cylinder axis, and keep the same batch of printing paper in the same direction.

　　2, overprint clear water (commonly known as pressurized water). When the printing paper is not well mastered, in order to prevent the paper from elongating during printing, the method of overprinting with clean water can be used to adapt the paper to the printing requirements, and then start to print the color. This method can solve the problem of inaccurate overprinting. Has a certain effect.

　　 3. Control the moisture content of the printing plate. The moisture content of the printing plate is the key factor to control the expansion of the paper. The moisture of the printing plate is transferred to the paper through the blanket. The greater the moisture of the printing plate, the greater the expansion. Therefore, the lower the moisture content of the layout, the better, especially when printing 80 grams of offset paper.

　　4. Use a drawing machine to lengthen the printing plate. Generally, the single-color machine is in the third and fourth-color printing process, and the two-color machine is in the second printing process. The image on the paper is stretched and enlarged, resulting in inaccurate overprinting. This is because the PS plate has good ductility and can be stretched by a drawing machine to make the printing plate image and the image on the paper evenly elongated to make the overprint accurate.

　　 At present, most small and medium-sized enterprises in China generally do not have air-conditioning equipment. Most of the methods are to sprinkle some water around the machines to increase the air humidity. For products with low requirements for non-dot printing, you can use a spray cylinder to spray water on the edge of the paper and print on the machine; when the air humidity in the workshop is high, the cut paper is best covered with a plastic cover to prevent it from becoming longer due to moisture ; If it is a second edition printing, you can use the method of borrowing the plate line to make up for the profitability of the image.

　　 Generally speaking, there are many process methods to solve the problem of inaccurate overprinting caused by the expansion of the paper. As long as the discussion and management of the printing process are strengthened, they can generally be solved.

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