Influence of moisture change of paperboard in the

2022-08-25
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At present, white board and white cardboard have been widely used in all kinds of packaging. Customers have higher and higher requirements for product quality. In addition to exquisite printing, ink "> carton molding also requires more accurate and perfect, which is undoubtedly a new challenge faced by ink"> packaging and printing industry

paper moisture content and equilibrium moisture

effect of paper moisture content on ink "> printing has a great impact. The moisture is too low, the paper is brittle, and it is easy to generate static electricity during printing; if the content is too high, it will make it difficult for the ink to dry. The change of moisture also has a great impact on the various characteristics of the paper. With the change of moisture, its quantitative, tensile strength, flexibility, folding resistance, etc. will change, and the size of the paper 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, various properties of the paper are in a good state. The best 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

in addition, the change of equilibrium moisture has the following characteristics:

① 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

and re list the mark ② of A.B.C mound, which is 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 should control the change of temperature at 3 ℃ while controlling the relative humidity

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 take an over correction approach. 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 in the paper is diverse, but usually the number of fibers arranged along the damaged rear direction of the longitudinal sample is more. Therefore, the difference between 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 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 hygroscopic, the elongation of the reverse side 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

influence of moisture change of paperboard on printing quality

printing suitability is reflected in paper moisture, which is generally required to be consistent with the relative humidity of the production environment as far as possible. All kinds of paperboards above 200g/m2 are multi-layer composite papermaking in the papermaking process. According to the requirements of its production process, many manufacturers cut, seal and pack the paper in time and deliver it accurately at any time after the paper is rewound from the paper machine. Due to the process requirements of multi-layer lamination, the water content of paperboard is generally about 5% higher than that of ordinary paper. Although the high water content of paper is beneficial to printing, if it is not well preserved and used, it will bring a lot of trouble to printing production. If the package is damaged, the local humidity of the paper in the package changes, making the paper wavy, tight edges, and high on one side and low on the other side; When printing, it is difficult to feed paper, the balance of ink and water is difficult to master, and the change of humidity will also make the paper stretch, causing quality defects such as inaccurate multi-color overprint, back mouth angle swing, etc; Especially for the products with die-cutting process in the next process, the paper expansion will also cause problems such as irregular mold making and inaccurate die-cutting molding size

therefore, the following two points should be achieved in the specific production practice when dealing with the color box packaging paper of more than 200g/m2:

1. Keep and store the paper scientifically and reasonably. In view of the uniqueness of the paperboard, in order to prevent the paperboard from dehumidification or moisture absorption, causing changes in the suitability of the paper, great attention should be paid to the integrity of the product packaging during the transportation and storage of the paper, so as to prevent package damage due to improper transportation and operation. Pay attention to ventilation and avoid light and wind during storage. The temperature is controlled at ℃, and the relative humidity is 40% - 65%. The stacking of paper should be classified according to the production date, product category and production address of the paper. The use of paper should be supplied in turn according to the production date sequence, so as to provide a relatively stable paper humidity suitability for the next process as far as possible

2. Effectively control the humidity change of the paper. If it is necessary to properly keep and reasonably arrange the use before cutting, how should we take measures to prevent the paper size change caused by temperature and humidity change after cutting? First of all, the production command should be scientific and reasonable. The paper should strive to be printed and cut at any time. It is best to prepare the paper one hour before printing. The large quantity should not be unpacked and cut at one time. It should be cut in batches according to the printing speed and progress. Moreover, the cut paper should be covered with a plastic cover with good sealing performance to minimize the time of direct contact with the air, especially when multicolor products are printed on single and double color machines, This method is even more important. Not only white paper, but also each color should be treated in the same way after printing, so as to prevent the inaccurate multi-color overprint caused by the expansion of the paper caused by the change of humidity, and ensure the quality of printing products

influence of moisture change of paperboard on die cutting accuracy after calendering

in order to increase the aesthetic feeling of products and the surface smoothness of color boxes, now 70%-80% of carton packaging needs to be polished and calendered. Glazing is that the printed matter is evenly coated with a layer of glazing oil on the surface of the printed matter through the glazing oil roller, and then heated to dryness to volatilize the solvent to form a bright layer. Calendering is the reprocessing based on the polishing of printed matter. With a high temperature of ℃ and a pressure of about 138kg/cm2, the bright layer formed on the surface of printed matter becomes denser and brighter

due to this special process, the moisture content of the paper itself will be seriously dehumidified in a short time, and the paper will inevitably shrink, which is related to the paper structure and the initial humidity. Although its shrinkage has nothing to do with printing overprint, it directly affects the accuracy of die cutting. For example, the input size of the original of a product is x in length and Y in width. If five pieces of k-paper are sun dried horizontally and three pieces are sun dried vertically, a product with a pattern size of 3Y 5x will be obtained after printing. After calendering, the pattern size of the paper shrinks, △ X in the horizontal direction and △ y in the vertical direction. Die cutting with a mold that is consistent with the size of the original will definitely lead to inaccurate die cutting accuracy, even embossing, bleeding The pattern is not centered on the left and right. At first, customers are not satisfied, at last, some or all of the products will be reported as battery waste with higher capacity, which will cause undue losses to the enterprise

how to avoid or reduce such errors in specific production? The simpler way is to make a mold (die-cutting board) according to different paper and different shrinkage after calendering, but this often seriously delays the production cycle, and the size of die-cutting molding will be smaller than the original size. To this end, we should gradually find out the humidity characteristics of various papers commonly used in our unit and the horizontal and vertical shrinkage after calendering, and add this shrinkage to the input size of the original when printing, so that the final shrinkage of the drawing distance and the entire drawing size meet the standard requirements, which can also make the die-cutting plate made in advance to ensure the quality and cycle of the product

the influence of moisture change of paperboard after polishing and calendering on the folding resistance of products

folding resistance refers to the paper folding property that when the paper is folded, there are no cracks, fine seam coating peeling and other defects on the surface of the folding belt. It is usually used to express the toughness and surface strength of paperboard and Paperboard under the condition of folding. From the paper itself, the folding resistance depends on the fiber length, strength, flexibility and the bonding force between fibers. A large part of papermaking raw materials in China are non wood raw materials, among which reed, wheat straw, straw, bagasse and other grass raw materials are used more. Due to the influence of the properties of these raw materials, different toughness and brittleness will appear after the paper is formed. The paper produced with cotton, hemp, wood and other raw materials has better toughness, while most grass raw materials have poor toughness after the paper is formed because of their small fiber length and high content of heterogenous cells

the paper we use now. Paper mills usually mix grass fibers with wood fibers to effectively improve the toughness of paper. However, due to the different preparation proportion of wood pulp, different filler properties and production processes, there are great differences in the toughness of different papers

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