How to use trapping and overprinting in offset printing
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The term "overprint" as used herein refers to the adhesion of the latter color ink to the previous color ink film layer during the printing process.
Trapping refers to the printing of one color of ink in an image that has been hollowed out at the corresponding position of another different color ink.
In terms of color, it is generally necessary to overprint several colors to get the color that the customer needs. For example, if the red color is copied by the four-color printing process, it must be overprinted by magenta ink and yellow ink. However, the results obtained by overprinting and trapping seem to be similar, such as black, gold, silver and other colors overprinting or trapping, the results are not very different; dark and hue similar to the light overprint The results obtained from trapping are similar.
Choosing overprinting or trapping in actual production requires comprehensive consideration of relevant issues and selection of appropriate processes.
1 from the perspective of registration and drying
Overprinting does not cause whitening problems even if the overprinting is not allowed, but it is easy to cause the backside to be dirty due to the too thick ink layer. Trap is not easy to appear dirty on the back, but if the overprint is not allowed, it is easy to white and affect the appearance.
For a monochrome machine, after the previous color is dried, it is printed after the color, which is good for the drying of the ink, but it is easy to use the overprint due to the inaccurate overprinting caused by the expansion and elongation of the paper.
For multi-color machines, multi-color printing is completed in an instant, and there is basically no problem that the paper absorbs water and stretches, resulting in inaccurate overprinting. The registration is improved, but the ink layer is deposited too thick in a wet overprint. It is prone to dryness, so it tends to use trapping.
2 from the printing color sequence
Wet overprint must take into account the nature of the ink, the viscosity of the first printed ink. The adhesion must be greater than the ink printed on the back, otherwise the reverse printed ink will appear reverse overprinting. The post-printing ink must also have a high degree of transparency. The trapping is not limited by the nature of the ink in the color sequence arrangement, and it is more advantageous to select the most suitable printing color sequence according to other factors.
3 from the color effect
When printing gold ink or silver ink on coated paper, the color obtained by printing full ink and silver ink on white paper in a trapping manner is thicker and purer than that printed on other ground colors. On the black plate, there is only a large-area solid image, and the ink amount of the black plate is increased by the trapping method, and the ink color printed on the white paper is thicker than the ink color obtained by overprinting on other solid ground colors. However, when printing only words and lines, because the human eye is not strong in distinguishing the density of fine lines, it is not necessary to print too small characters and lines, and the thin lines will be trapped. It is difficult to overprint accurately, so the printing of small lines is generally done by overprinting.