Influence of moisture change of the hottest paper

Influence of paper moisture change on printing

release date: Source: packaging belt

during the printing process, it is necessary to pay attention to the change of paper moisture content. Only when the moisture is moderate can we print more exquisite prints. Too much or too little moisture will bring a series of problems. Today, China's paper industry will take you into the water content of paper and give you ways to balance the water content

the moisture content of paper has a great impact on printing. Too low moisture makes the paper brittle and easy to generate static electricity during printing; If the content is too high, the ink will be difficult to dry. Moisture change also has a great impact on various characteristics of paper. With the change of moisture, its quantitative, tensile strength, flexibility, folding resistance, etc. will change, and the paper size will also stretch, and even curl, curl, wrinkle, ruffle and other phenomena will occur

all kinds of paper have an optimal moisture content value. Under this value, all kinds of paper properties are in good condition. The optimal moisture content of cast coated paper and cast coated white paperboard is 7% ± 2%, white paperboard is 4%-7%, and 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 plant, 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 transmit moisture to the air and lose moisture. The rate depends on the moisture content of the paper and the ambient temperature and humidity. When the moisture absorption rate and the water loss rate are the same, the paper and air are in balance, and the water content in the paper remains unchanged. In this case, the water content in the paper is called balanced 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 it reaches saturation at that temperature. The equilibrium moisture of the same paper is different under different relative humidity

how to balance moisture

the change of balance moisture has the following characteristics:

① it is affected by the properties of paper. At the same relative humidity, the more hydrophilic the paper is, the higher its equilibrium moisture is; Otherwise, the lower. The paper without auxiliary materials has higher equilibrium moisture, while the paper with filler, sizing and coating has lower equilibrium moisture. For the same variety, the balanced moisture of thick products is higher than that of thin products, because the proportion of base paper rich in water absorption in thick products is high

② affected by temperature. At the same relative humidity, when the temperature changes by about 15 ℃, the maximum change of paper equilibrium moisture is about 0.5%. However, the change of water content of the paper during overprint is required to be controlled at ± 0.1%, otherwise the accuracy of overprint will be affected. Therefore, the color printing workshop has become a social problem of general concern. While controlling the relative humidity, the change of temperature should be controlled at ± 3 ℃

③ it is affected by the stroke to reach equilibrium -- moisture absorption or dehumidification. Under a certain relative humidity, the moisture content of paper when reaching equilibrium by low moisture absorption is lower than that when reaching equilibrium by high moisture dehumidification under the same relative humidity, which is the moisture absorption hysteresis effect of paper. To make the moisture content of the paper consistent with the original We must adopt the way of "overcorrection". That is, if the paper reaches equilibrium at a certain relative humidity, it must be placed in an environment with lower relative humidity, and vice versa, if it wants to dehumidify to reach the original equilibrium moisture after moisture absorption in a high humidity environment

in addition, the speed of moisture absorption and dehumidification is also different, and the dehumidification speed is much slower. However, whether moisture absorption or dehumidification, the starting speed is relatively fast, and the closer to equilibrium, the slower. This process is also related to paper and air mobility. Under standard humidity and temperature, a single cigarette paper can reach 5.8% equilibrium moisture after being treated for 35min; All kinds of printing paper need h to reach the equilibrium moisture of 5%-8%; The packaging cardboard takes longer. This hysteresis between the change of relative humidity and the change of paper moisture content will bring the hysteresis effect between paper deformation and the change of relative humidity

④ affected by the directivity of the paper. The so-called directionality refers to that the balance moisture of paper is inconsistent in the longitudinal and transverse directions, and the transverse expansion rate is much larger than the longitudinal expansion rate. Therefore, the transverse expansion deformation of paper is larger than that of longitudinal direction. The test shows that the horizontal expansion and contraction of a single fiber is about 20 times that of the longitudinal. However, the arrangement direction of the fibers inside the paper is diverse, but usually the number of fibers arranged longitudinally is more

therefore, the difference in the vertical and horizontal expansion ratio of paper is not as large as that of a single fiber. Taking the change of relative humidity from 50% to 60% as an example, the ratio of vertical and horizontal expansion and contraction of foam granulator is about 3:7, that is, about 2.3 times. This ratio varies according to the degree of orientation of the fiber arrangement. The stronger the directivity, the greater the ratio. Making paper with disordered and irregular fiber arrangement direction is one of the key problems to reduce the horizontal expansion rate and avoid inaccurate overprint

⑤ affected by the two sides of the paper. When the environment of a very flat paper changes, the paper will absorb moisture or dehumidify in the air. If it is moisture absorption, the extension of the reverse side can further reduce the electrode linewidth and increase the illumination area, which is larger than that of the front side, and the paper will curl towards the front side; In case of dehumidification, the shortening of the reverse side is larger than that of the front side, and the paper will curl towards the reverse side. No matter which direction to curl, it always takes the longitudinal direction of the paper as the axis, which is caused by the different degree of directional arrangement of the front and back sides of the paper. Paperboard, in particular, performs even better in this regard

reasons for paper deformation and its impact on printing quality

in the process of manufacturing and use, there are many factors affecting paper deformation. The physical and chemical properties of pulp, beating conditions, selection of chemicals and fillers, papermaking performance, equipment status of paper machine and moisture gain and loss in the process of use will all affect the size change of paper. Here we only discuss the dimensional changes caused by the gain and loss of moisture in the transportation and printing process of the finished paper

paper is mainly composed of cellulose, which is a highly hydrophilic material with strong water absorption and dehydration. During the storage, transportation and printing of paper, the change of temperature and humidity causes the change of water content of paper, resulting in expansion and contraction. The main reasons for the change of paper size caused by the change of paper moisture are as follows: when the paper moisture changes, the single fiber that makes up the paper can absorb moisture and expand or lose moisture and contract, so that the shape of the whole fiber changes, resulting in the change of paper size; There are hydrogen bonds inside the paper. When the paper absorbs or loses water, the role of hydrogen bonds is strengthened or weakened, making the fibers tighten or push away from each other

the shape change of the internal fiber of the paper due to the gain and loss of moisture will eventually lead to the size change of the paper, which will have an adverse impact on printing. When the water content of the paper is too large, the binding force between the fiber, filler and rubber in the paper is easy to be reduced, which will affect the surface strength of the paper, resulting in the occurrence of adverse phenomena such as powder and wool loss after printing. The occurrence of powder and wool dropping reduces the hardness and stiffness of the paper edge, and it is easy to produce the problem of unstable positioning 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, the paper should not be printed under high humidity conditions that are incompatible with its water 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 whole paper surface, and 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, certain treatment measures should be taken before printing

paper stretch induced overprint inaccuracy discussion

paper is a kind of material containing changeable plant fiber tissue. It has the function of moisture absorption and moisture release. When the moisture in the air is constantly changing, it will be adjusted and balanced with the air humidity. When the air humidity is higher than the moisture contained in the paper, it will absorb the moisture in the air and expand and elongate; On the contrary, it will release water and shorten the contraction, so as to achieve a relative balance with the environmental humidity

offset lithography is based on the principle that oil and water are immiscible. Therefore, the paper will generally lengthen after printing, and the shrinkage will become shorter, which is rare. In order to eliminate this kind of fault, this paper discusses the relevant printing technology:

1. Try to use horizontal grain during printing. Paper can be divided into horizontal grain and vertical grain. In the papermaking process, because the pulp (plant fiber) flows vertically to the moving silk, the fibers are suddenly accelerated, and most of them will be connected along the moving direction of the silk, forming what we call vertical and horizontal grain. 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 grain, and the direction of the gap between the fiber and the fiber is the transverse grain

the experimental results show that the cohesion of the fiber itself is greater than the bonding force between the fibers (mainly hydrogen bonding force). After the paper is damped, the expansion and contraction in the longitudinal grain direction is smaller than that in the horizontal grain direction, and the two are roughly 1:2. Therefore, the deformation relationship between the vertical and horizontal grain of the paper must be considered for products with more accurate overprint. Therefore, when printing, make the longitudinal grain direction of the paper parallel to the roller axis, and keep the grain direction of the same batch of printing paper consistent

2. Overprint clear water (commonly known as water pressing). When the printing paper is not easy to master, in order to prevent the paper from stretching during printing, the method of overprinting once with clear water can be adopted to make the paper adapt to the printing requirements, and then start printing color. This method has a certain effect on solving the inaccurate overprinting

3. Control the moisture content of the printing plate. The moisture content of the printing page is the key factor to control the paper expansion and contraction. The moisture content of the page is transferred to the paper through the blanket. The greater the moisture content of the page, the greater the expansion and contraction. Therefore, the smaller the moisture of the layout, the better, especially when printing 80g double offset paper

4. Use a plate drawing machine to stretch the printing plate. Generally, the monochrome 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 overprint. This is because PS plates have good ductility, which can be elongated by a plate drawing machine, so that the printing plate image is the same as the image on the paper. 1. The common universal material testing machine can make the overprinting accurate by the extension of the type balance

at present, most small and medium-sized enterprises in China generally do not have air conditioning equipment. Most of the practice is to sprinkle some water around the machine to increase the air humidity. For products with low requirements for non dot printing, the spray can be used to spray water on the edge of the paper and then printed on the machine; When the air humidity in the workshop is high, it is best to cover the cut paper with a plastic cover to prevent it from becoming longer due to moisture; If it is secondary printing, the method of borrowing plate line can be used to make up for the profit and shrinkage of the image

generally speaking, there are many processing methods to solve the inaccurate overprint caused by paper expansion. As long as we strengthen the discussion and management of printing process, it can generally be solved

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