Influence of Paper Moisture Content on Printing Effect and Its Control

Paper is the most used printing material in printing. The characteristics of paper affect the quality of printed matter to a great extent. Water content is one of its important properties. The amount of water content has a direct relationship with the printed product. Therefore, it is very important to correctly analyze the moisture content of paper and its impact on printing.

1 Factors affecting the moisture content of paper

The main factors affecting the moisture content of paper are the following three:

1.1 Hydrophilic properties of paper components The main components of paper are plant fibers and other hydrophilic substances that are mainly used as fillers, such as clay, kaolin, and gypsum powder. Their physical and chemical properties make the paper water-absorbent. Paper not only absorbs water from fountain solution, releases water (dehydration) in the drying oven, but also draws moisture from humid air or releases moisture into the dry air.

1.2 Relative Humidity When the paper is in contact with the surrounding air, it will maintain a balance with the ambient humidity, so the same paper will have different moisture content under different humidity, and the moisture content of different paper under the same humidity conditions will also be different. The moisture content of different papers under different air relative humidity is shown in the following table. It can be seen that the moisture content of any paper increases with the relative humidity of the air and decreases with the decrease of the humidity. For every 10% change in relative air humidity, the moisture content of the paper changes by 1%.

1.3 The temperature experiment proves that under certain conditions of relative air humidity, the moisture content of the paper changes inversely with the temperature. As the temperature increases, the moisture content of the paper decreases, and the temperature decreases the moisture content of the paper. At a relative humidity of 45%, the change in paper moisture content averages ±0.17% for every ±5% change in temperature. In the process of multicolor printing, the moisture content of the paper cannot vary by more than ±0.1%. Otherwise, the overprinting accuracy will be affected. Therefore, the temperature change of the shop should be controlled within ±3°C.

2 paper moisture content on the printing effect

Paper is made from fibers and fillers in the papermaking machine's papermaking direction. Different fibers have different expansion conditions. Generally, when the fiber expands, its diameter increases by 30% and its length increases by only 1 to 2%. Therefore, the longitudinal shrinkage of the silk yarn in the printing process is small, which is 1/1 in the transverse direction. 2 to 1/3 or so, little effect on overlay accuracy. The horizontal stretching amount is large, and the maximum can reach 0.3mm or more, which greatly affects the printing quality.

At the same time, the uneven moisture content of the paper will make overprinting inaccurate. In the process of storing large amounts of paper, only four sides can be in contact with the air, and there is less contact in the middle. Therefore, with changes in the ambient temperature and humidity, the water content of the paper is unbalanced, and "foliage" or "tight edge" phenomenon occurs. Then after being subjected to printing pressure, it will have different compression deformation. If the paper has "foil" or "tight edge" before printing, the imprint of the imprinted paper surface cannot be identical with the printing plate, resulting in inaccurate overprinting. Therefore, in the multi-color printing, when the semi-finished products suddenly generate “flounce” or “tight edges” due to changes in the surrounding humidity, the printed images on the paper will inevitably change, resulting in overprinting errors. As shown below:

The moisture content of the paper will not only cause the paper to be deformed, which will result in inaccurate overprinting, but also cause other changes in the properties of the paper due to changes in the moisture, directly affecting its 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 increased, the drying speed of the print will be affected, and the back surface of the paper will be stained easily. If the water content of the paper is too small, the paper will be brittle and hard to print. In the process, a large amount of static electricity is generated due to friction, resulting in uneven delivery and waste of paper.

3 paper moisture content control

3.1 The paper undergoes humidity conditioning before printing to balance the water content of the paper and adapt it to the temperature and humidity of the printing shop to improve the stability of the paper size and allow the hysteresis effect to be fully displayed, thereby improving the overlay accuracy. The longer the humidity control time, the smaller the stretchability of the paper and the higher its stability. At the same time, we must also pay attention to controlling the temperature and humidity of the printing shop and try to make it ideal for constant temperature and humidity.

3.2 Control the ink balance during the printing process. The amount of dampening water should be uniform when printing, must not be big or small, especially for paper with strong water absorption, the expansion rate is relatively large, so we must pay attention to this issue.

3.3 Paper packaging should not be removed prematurely, otherwise it will cause a certain degree of humidity difference between the two sides of the paper and the inside, and its water absorption and expansion will be different, so that overprinting is not allowed. In short, the moisture content of paper is an important factor that can not be ignored in the printing process. As long as we analyze carefully and combine with actual production, we will be able to understand it, master it, and avoid its negative impact on printing, so as to improve printing products. quality.