Influence of paper moisture content on printing effect and its control
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Paper is the most used printing material in printing. The characteristics of paper greatly affect the quality of printed matter. Water content is one of its important properties. The water content is directly related to printed products. Therefore, it is especially important to correctly analyze the water content of paper and its impact on printing.
1 Factors affecting the moisture content of the paper
There are three main factors that affect the moisture content of paper:
1.1 Hydrophilic properties of paper constituents The main components of paper are plant fibers and other hydrophilic substances mainly used as fillers, such as clay, kaolin, gypsum powder, etc., and their physicochemical properties make the paper water absorbing. The paper not only absorbs water from the dampening solution, but also releases water (dehydrated) in the drying oven, and absorbs moisture from the moist air or emits moisture into the dry air.
1.2 Relative humidity When the paper is in contact with the surrounding air, it will be in balance with the ambient humidity. Therefore, the same paper has different water contents under different humidity conditions, and different papers have different water contents under the same humidity condition. The water content of different papers under different air relative humidity is shown in the table below. It can be seen that the water content of any paper increases with the increase of the relative humidity of the air and decreases with the decrease of the humidity. The moisture content of the paper changes by 1% for every 10% change in air relative humidity.
1.3 Temperature experiments have shown that the paper moisture content is inversely proportional to the change in temperature under conditions of constant air relative humidity. As the temperature increases, the water content of the paper decreases, and the temperature decreases the water content of the paper increases. Under the premise of a relative humidity of 45%, the change in water content per paper is ±0.17% for every ±5% change in temperature. In the multi-color overprinting process, the moisture content of the paper should not change by more than ±0.1%, otherwise the overprinting will be affected accurately, so the temperature change of the workshop should be controlled within ±3 °C.
2 Influence of paper moisture content on printing effect
The paper is made of fibers and fillers in the direction of the paper machine. Different fibers have different expansion conditions. Generally, when the fiber expands, the diameter increases by 30%, and the length direction increases by only 1-2%. Therefore, the longitudinal expansion and contraction amount of the wire is small during the printing process, which is 1/ of the transverse direction. 2 to 1/3 or so, has little effect on the accuracy of overprinting. The lateral expansion amount is large, and the maximum is up to 0.3 mm, which greatly affects the printing quality.
At the same time, the imbalance of paper moisture content will make the overprinting inaccurate. During the storage of a large amount of paper, only four sides can be in contact with the air, and there is less contact in the middle. Therefore, as the temperature and humidity of the surrounding environment change, the water content of the paper is unbalanced, and the phenomenon of "raw edge" or "tight edge" occurs. Then, after being subjected to printing pressure, there will be different compression deformations. If the paper has "ruffled edge" or "tight edge" before printing, the graphic print obtained on the surface of the paper after imprinting cannot be consistent with the printing plate, resulting in inaccurate overprinting. Therefore, in multi-color printing, when the semi-finished product suddenly produces "flange edge" or "tight edge" due to the change of the surrounding humidity, the printed image on the paper will inevitably change, resulting in overprinting error. As shown below:
The water content of the paper not only causes deformation of the paper, but also causes inaccurate overprinting. It also causes other changes in the paper properties due to changes in moisture, which directly affects the printability. If the water content of the paper is too large, the tensile strength and surface strength of the paper will be reduced, the plasticity will be enhanced, the drying speed of the blot will be affected, and the back of the paper will be easily smeared. If the water content of the paper is too small, the paper will be brittle and hard. During the process, a large amount of static electricity is generated due to friction, which causes the paper to be misaligned, thereby wasting paper.
3 Control of paper moisture content
3.1 The paper should be conditioned before printing to balance the water content of the paper and adapt to the temperature and humidity of the printing workshop to improve the stability of the paper size and fully display the hysteresis effect, thus improving the overprinting accuracy. The longer the humidity control time, the smaller the paper's stretchability and the higher its stability. At the same time, we must also pay attention to controlling the temperature and humidity of the printing workshop, and try to make it ideal for constant temperature and humidity.
3.2 Control the ink and water balance during the printing process. The amount of dampening water should be uniform and consistent during printing. It should not be too big or small. Especially for papers with strong water absorption, the expansion ratio is relatively large, so it is necessary to pay attention to this problem.
3.3 Do not remove the paper packaging too early, otherwise there will be a certain degree of humidity difference between the two sides of the paper, and the amount of water absorption and expansion will be different, which will make the overprinting inaccurate. In short, the water content of paper is an important factor that can not be ignored in the printing process. As long as we carefully analyze it and combine it with actual production, we will be able to understand it, master it, avoid its negative impact on printing, and thus improve the printing. quality.