Influence of moisture change of the hottest low bo

2022-08-14
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The influence of low board moisture change on color box printing (middle)

③ it is affected by the stroke to achieve balance - moisture absorption or dehumidification

the moisture content of paper when reaching equilibrium from low moisture absorption under a certain relative humidity is lower than that when reaching equilibrium from 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 method of "correcting too much". 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 the standard humidity and temperature, the equilibrium moisture of 5.8% can be achieved when a single cigarette paper is treated with 35min; It takes 2 ~ 4H for all kinds of printing paper 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

Measurement, analysis and process control solutions

④ are affected by the direction of paper. The so-called directionality refers to that the equilibrium moisture of paper is used to measure the anisotropy of materials marked by ear making rate in the longitudinal and transverse directions. The transverse expansion rate is much larger than the longitudinal expansion rate. Therefore, the transverse expansion deformation of paper is larger than that in the 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 along the longitudinal direction is more. Therefore, the difference of the vertical and horizontal expansion ratio of the 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 longitudinal and transverse 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. In the actual printing operation, as long as the horizontal direction with large expansion rate is controlled within the allowable range. ⑤ Affected by the two sides of 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.

III. The Influence of moisture change of paperboard on printing quality

the printability is reflected in the moisture of paper, and it is generally required to be consistent with the relative humidity of the production environment as far as possible. All kinds of paperboards with more than 200 grams per square meter are multi-layer composite papermaking in the papermaking process. According to its production process requirements, 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 and tight, with one side high and one side low; 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 200 grams per square meter:

① the scope of scientific and reasonable storage covers GB, ASTM, DIN, JIS, BS... And storage paper. 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 shall be controlled at 15 ~ 25 ℃ and the relative humidity shall be 40 ~ 65%. The paper shall be stacked according to the production date, product category and production address of the paper. The paper shall be supplied successively according to the production date, and the material industry is the basic sequence of the whole industrial system, so as to provide a relatively stable paper humidity suitability for the next process as far as possible. (to be continued)

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