Paper is a kind of material containing variability of plant fibrous tissue. It has the function of moisture absorption and moisture release. When the moisture in the air is constantly changing, it will adjust to the humidity of the air. When the air humidity is higher than the water content of the paper, it will absorb the moisture in the air and expand and expand; otherwise it will release moisture and contraction shortened to achieve a relative balance with the ambient humidity.
Offset offset printing is based on the principle of immiscibility between oil and water. Therefore, the printed paper will generally stretch, and shrinkage is shortened. In order to eliminate such failures, the relevant printing process is discussed:
1, try to use cross-filament when printing. The paper has horizontal and vertical threads. During the papermaking process, as the pulp (vegetable fiber) flows vertically to the moving wire mesh, the fibers are suddenly accelerated and most of them are linked together along the direction of movement of the wire mesh, forming what we call Vertical and horizontal wire. The plant fibers in the paper are all hollow translucent tubes. We can say vividly that the length direction of the fibers themselves is what we call longitudinal warp, and the direction of the fiber and fiber gaps is transverse warp. The experimental results show that the cohesive force of the fiber itself is greater than the binding force (mainly hydrogen bonding force) between fibers. After the paper is wet, the telescopic extension in the longitudinal direction is smaller than that in the horizontal direction, which is approximately 1:2. Therefore, overprinting more accurate products must take into account the paper's vertical and horizontal deformation. Therefore, when printing, make the longitudinal twist direction of the paper parallel to the axis of the drum, and the same batch of printing paper should be consistent in the direction of the strand.
2, overprinting water (commonly known as water pressure). In the case that printing paper is not well controlled, in order to prevent the paper from elongating in printing, a method of overprinting the water once may be used to adapt the paper to the printing requirements and then start printing the color. This method is not suitable for overprinting. There is a certain role.
3, control the printing plate surface moisture. Moisture content of the printing plate is the key factor to control the expansion and contraction of the paper. The moisture of the plate is transmitted to the paper through the blanket. The greater the moisture content of the plate, the greater the expansion and contraction. Therefore, the smaller the water content of the layout is, the better, especially when printing 80 g of offset paper.
4, using a pull machine to lengthen the printing plate. Generally monochrome machine in the third, four-color printing process, two-color machine in the second printing process, the image of the paper has a longer stretch, resulting in overprinting is not allowed. This is because the PS plate material has good ductility and can be stretched by the plate-pulling machine so that the image of the plate plate and the image on the paper can be elongated in a balanced manner and the overprinting is accurate.
At present, most domestic small and medium-sized enterprises generally do not have air-conditioning equipment. Most of the measures are to sprinkle some water around the machine to increase air humidity. For non-network printing products that do not require high printing quality, you can use the spray cylinder to spray water on the edge of the paper and then print on the machine. When the air humidity in the workshop is high, the cut paper is best covered with a plastic cover to prevent the moisture from getting too long. If it is the printing of secondary printing, you can use the method of borrowing lines to compensate for the image's contractility.
In general, there are many processing methods that can solve the problem of over-printing and lead to overprinting. As long as the discussion and management of the printing process are strengthened, it can generally be solved.