How to choose trapping and overprinting in offset printing

Overprinting here 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 a color ink in an image that is hollowed out at a corresponding position in another different color ink.

From the color point of view, it is generally necessary to superimpose several colors to obtain the color desired by the customer. For example, if a large red is reproduced by a four-color printing process, it must be overprinted with magenta ink and yellow ink. However, some colors do not look too similar to those obtained by overprinting and trapping. Overprinting or trapping like black, gold, silver, and other colors results in a difference that is not very great; dark colors are similar to light colors The results obtained with trapping are similar.

The choice of overprinting or trapping in actual production requires comprehensive consideration of the relevant issues and the selection of suitable processes.
1 From the point of view of registering and drying, it is considered that overprinting will not cause the problem of missing white even if overprinting is inaccurate, but it is easy to cause back rubbing due to the thick ink layer. Trap trapping is not easy to appear on the back, but if overprinting is not allowed, it will easily dew white and affect the appearance.

For a monochrome machine, the previous color is dried and printed one after the other, which is beneficial to the drying of the ink, but it is prone to overprinting due to the overprinting of the paper due to water expansion and elongation.

For multi-color machines, multi-color printing is completed in an instant. Basically, there is no problem of overprinting due to water absorption elongation. The registration is improved, but the ink layer that is too thick is accumulated in a wet manner. It is prone to dry defects and tends to use trapping.

2 Considering wet-overprinting from the printing sequence must take into account the nature of the ink and the viscosity of the ink printed first. Adhesion must be greater than the printed ink, otherwise the ink will appear after the reverse printing phenomenon. Post-printing inks must also have a high degree of transparency. Trapping on the arrangement of the color sequence is not limited by the nature of the ink, and it is more favorable for selecting the most suitable printing sequence according to other factors.

3 Considering the effects of color When printing gold and silver inks on coated paper, the colors obtained by printing the ink and silver ink on white paper in a trapping manner are more thick and pure than those obtained on other background colors. When printing black, if there is only a larger solid image on the black plate, use the trapping method to increase the ink volume of the black plate to print thicker on the white paper than on the other solid ground. But when printing only words and lines, because the human eye does not have a strong ability to discriminate the fine line density, printing small texts and lines does not need to be printed too heavily, and thin lines will be trapped if trapping is used. It is difficult to overprint accurately, therefore, the printing of small lines generally uses overprinting.

Source: Shenzhen Printing Network