The Basic Principle and Process of Offset Printing
1. The basic principle of offset printing
Oil-water immiscibility principle;
The principle of selective adsorption of printing plates;
Dot fineness principle;
Indirect printing principle.
2. Offset printing process
Choice of Overprint or Trap
If the trapping is not overprinted correctly, it is easy to show white and affect the appearance, and overprinting is easy to cause the back to be dirty because the ink layer is too thick. trapping;
Wet pressure overprinting must take into account the properties of the ink. The viscosity and adhesiveness of the first printed ink must be greater than that of the later printed ink, otherwise the later printed ink will appear reverse overprinting. The post-printed ink must also have high transparency;
When printing gold ink and silver ink on coated paper, the color obtained by printing gold ink and silver ink on white paper by trapping is thicker and purer than printing on other background colors;
When printing black, if there is only a large solid image on the black plate, the ink volume of the black plate is increased by means of trapping, and the ink color obtained by overprinting on the white paper is thicker than that obtained by overprinting on other solid background colors. However, fine lines are generally overprinted.
In order to ensure that the trapping plate in the front row is covered by the ink layer behind, its trapping volume is usually increased by 0.03~0.05mm on the basis of the registration accuracy of its equipment. If the registration accuracy is 0.15mm, then the trapping part is generally 0.18~0.20mm larger.
