Trapping in Printing

If you hear a printer use the term trapping, they are probably not talking about trapping beavers.

The term trapping to printers means printing ink over ink. If you look at a printed piece that, let's say has a black solid panel and inside that panel there is red text. You may or may not see a small white line between the black and red. If you see this white line, it is probably because the printer did not trap the job properly.

In this scenario, it is necessary for the printer to increase the width of the stroke of the red text so that it actually overlaps the black background slightly. Because the black background is a darker color, your eye does not see the red text as being bolder. The amount of overlap is very slight, approx .006" and is absolutely necessary for colors that appear to touch each other. There are several reasons why it is necessary.

As precise as printing presses are, they are not perfect but an even larger reason is the movement of the sheet of paper as it travels through the press. When paper travels through a press a tremendous amount of pressure is applied to the sheet. In addition to pressure, moisture is also introduced to the sheet. Paper will stretch with pressure and actually swell with moisture. The .006" of trap is needed to cover for these variables.

Written by Jay Atkinson
The Odee Company