The influence and control of paper moisture content on printing effect
Paper is the most used printing material in printing. The characteristics of paper greatly affect the quality of printed matter, and water content is one of its important properties. The size of water content is directly related to the printed product. Therefore, it is particularly important to correctly analyze the moisture content of paper and its impact on printing.
1 Factors affecting the moisture content of paper
There are three main factors that affect the moisture content of paper:
1.1 Hydrophilic properties of paper components The main components of paper are plant fibers and other hydrophilic materials mainly used as fillers, such as clay, kaolin, gypsum powder, etc., their physical and chemical properties make the paper have water absorption properties. The paper will not only absorb water from the fountain solution, put water in the drying box (dehydration), but also absorb moisture from the humid air or distribute moisture to the dry air.
1.2 Relative humidity The paper will maintain a balance with the ambient humidity when it is in contact with the surrounding air. Therefore, the same paper has different water contents under different humidity, and different papers have different water contents under the same humidity. The moisture content of different papers under different air relative humidity is shown in the table below. It can be seen that the moisture content of any paper increases as the relative humidity of the air increases and decreases as the humidity decreases. When the relative humidity of the air changes by 10%, the moisture content of the paper changes by 1%.
1.3 The temperature experiment proves that under the condition of a certain air relative humidity, the paper moisture content and temperature change are inversely proportional. As the temperature increases, the paper moisture content decreases, and as the temperature decreases, the paper moisture content increases. On the premise that the relative humidity is 45%, for every change of temperature ± 5%, the change of paper moisture content is ± 0.17% on average. In the process of multi-color overprinting, the moisture content of the paper cannot change more than ± 0.1%, otherwise it will affect the accuracy of overprinting, so the temperature change of the workshop should be controlled within ± 3 ℃.
2 The effect of paper moisture content on printing effect
The paper is made of fibers and fillers according to the papermaking direction of the paper machine. The expansion of different fibers is different. Generally, when the fiber expands, its diameter increases by 30%, and its length increases by only 1 to 2%. About 2 ～ 1/3, has little effect on the accuracy of overprinting. The amount of lateral expansion and contraction is large, and the maximum can reach more than 0.3mm, which greatly affects the printing quality.
At the same time, the imbalance of paper moisture content will make the overprint inaccurate. During the storage of a large amount of paper, only four sides can be in contact with the air, and the middle part is less. Therefore, as the temperature and humidity of the surrounding environment change, the water content of the paper is unbalanced, and the phenomenon of "lotus edge" or "tight edge" appears. Then after bearing the printing pressure, it will have different deformation under pressure. If the paper has "lotus leaf edge" or "tight edge" before printing, the graphic imprint obtained on the paper surface after imprinting cannot be consistent with the printing plate, resulting in inaccurate overprinting. Therefore, in the multi-color printing, when the semi-finished products suddenly produce "lotus edge" or "tight edge" due to the change of the surrounding humidity, the graphics and text printed on the paper will inevitably change, causing overprint errors. As shown below:
The moisture content of the paper will not only cause the deformation of the paper and cause inaccurate overprint, but also change the other properties of the paper due to the change of moisture, which directly affects the printability. If the moisture content of the paper is too large, the tensile strength and surface strength of the paper will be reduced, the plasticity will be enhanced, and the drying speed of the blot will be affected, which will easily make the back of the paper dirty; if the moisture content of the paper is too small, the paper will be brittle and hard. In the process, a lot of static electricity will be generated due to friction, resulting in uneven paper collection, thereby wasting paper.
3 Control of paper moisture
3.1 The paper should be humidity-adjusted before printing to make the paper moisture content balanced, and to adapt to the temperature and humidity of the printing workshop, to improve the stability of the paper size, so that its hysteresis effect is fully displayed, thereby improving the accuracy of overprinting. The longer the humidity adjustment time, the smaller the paper's elasticity and the higher its stability. At the same time, we must pay attention to control the temperature and humidity of the printing workshop, and try to make it have the ideal conditions of constant temperature and constant humidity.
3.2 Control the water and ink balance in the printing process. When printing, the amount of dampening water should be uniform and should not be large or small. Especially for papers with strong water absorption, the expansion and contraction rate is also relatively large, so pay more attention to this problem.
3.3 Do not remove the paper outer packaging prematurely, otherwise it will cause a certain degree of humidity difference between the two sides and the inside of the paper, and the amount of water absorption and expansion will be different, so that the overprint is not accurate. In short, the moisture content of paper is an important factor that cannot be ignored in the printing process.As long as we analyze it carefully and combine it with actual production, we will be able to understand it, master it, and avoid its negative impact on printing, thereby improving the printed matter. quality.
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