Paper is a material containing variable plant fiber tissue. It has the function of absorbing moisture and releasing moisture. When the moisture in the air is constantly changing, it will adjust and balance with the air humidity. When the air humidity is higher than the moisture content of the paper, it will absorb the moisture in the air and expand and expand; otherwise, it will release the moisture and shrink and shorten to achieve a relative balance with the environmental humidity.
Lithographic offset printing uses the principle that oil and water are incompatible with each other for printing. Therefore, the paper generally stretches after printing, but it is rare that the shrinkage becomes shorter. In order to eliminate this type of failure, the relevant printing process is discussed:
1. Try to use horizontal strands when printing
Paper is divided into horizontal strands and vertical strands. In the papermaking part of the papermaking process, since the pulp (plant fiber) flows vertically to the moving screen, the fibers are suddenly accelerated, and most of them are connected along the moving direction of the screen, forming what we call Vertical and horizontal strands of silk. The plant fibers in the paper are all hollow and translucent tubulars. We can vividly say that the length of the fiber itself is what we call the longitudinal strands, and the direction between the fibers and the fiber is the horizontal strands. The experimental results show that the cohesive force of the fiber itself is greater than the bonding force between the fibers (mainly hydrogen bonding force). After the paper is damp, the expansion and contraction in the longitudinal direction is smaller than the expansion and contraction in the transverse direction, and the two are roughly 1:2. Therefore, products with overprint comparison* must consider the deformation relationship of the vertical and horizontal strands of the paper. Therefore, when printing, make the paper's longitudinal yarn direction parallel to the cylinder axis, and keep the same batch of printing paper in the same direction.
2. Overprinting clear water (commonly known as pressurized water)
When the printing paper is not well mastered, in order to prevent the paper from elongating during printing, the method of overprinting with clean water can be used to adapt the paper to the printing requirements, and then start printing the color. This method can solve the problem of overprinting Has a certain effect.
3. Control the moisture of the printing page
The moisture content of the printing page is a key factor in controlling the expansion and contraction of the paper. The moisture in the layout is transferred to the paper through the blanket. The greater the moisture in the layout, the greater the expansion. Therefore, the lower the moisture content of the layout, the better, especially when printing 80 grams of offset paper.
4. Use a drawing machine to lengthen the printing plate
Generally, the single-color machine is in the third and fourth-color printing process, and the two-color machine is in the second printing process, and the image on the paper is stretched and enlarged, resulting in inaccurate registration. This is because the PS plate has good ductility and can be stretched by a drawing machine to make the printing plate image and the image on the paper evenly elongated to make the overprint accurate.