[EN] Trapping key is neither „true“ nor „false“

What does this message mean?

This message refers to „trapping“ (Aussteuerungen von Überfüllung, Unterfüllung und Überdrucken Einstellungen), a step in production.*

*Not to be confused with it’s second meaning: the transfer of color when printing also being called trapping.

This step in production can take place at different times. You can take this step within your layout program. But is also possible to let the printer do the trapping.

Both possibilities are allowed under the pdf x-norms. Is is, however, absolutely necessary to state whether the trapping has been done (Trapping Key is set to "true”) or not (Trapping Key is set to„false“). It is not allowed to not make a selection at all (Überfüllungsschlüssel steht auf „not defined“).

What do I need to do?

Let the printing house do the trapping and simply use the DUON joboptions.

All DUON portal joboptions are designed to show that no trapping has been made and the printer has to do the trapping. The jobtoptions are setting the Überfüllungsschlüssel on ”false” (/PDFXTrapped/False).

In case you want to do the trapping yourself, you can do that – depending on the software you use and the selections – with not much effort and with more or less good results.

In certain cases it can be necessary fort he printing house to change the trapping.

CAUTION! Don’t make life difficult for yourself! In order to use proper trapping settings you need a lot of experience as well as good software. Wrong settings can lead to unexpected results.

Background

When printing four colors (4C printing) all colors are being created from the translucent basic colors cyan, magenta, yellow and black. The autotypical color mixing (Raster imaging) is being used for this.

In order to display the colors properly it is necessary to print the colors exactly above each other. This is technically possible only to a certain degree.

Reasons are, amongst others, the character of the paper and the run of the paper within the printing machine. The fibers of the wood based paper are stretching out due to humidity. While printing in offset process printing technology the paper naturally becomes wet. But also when the paper is being transfered within the printing machine can it become humid.

When both forms fit perfectly on top of each other you get a problem when you don’t print precisely on top of each other. You can see the paper on the side.

 

This schematic illustration shows this: the magenta area would fit perfectly on top of the cut out area but has not been printed ther properly. From a distance the magenta on top of the black does barely register but the white areas are immediately noticable.

Since it is not always possible to print both colors 100% on top of each other a solution has to be found how to deal with it. The solution is an under- or overfill.

Overfill

When overfilling usually the brighter of both colors is being made bigger. The strength of the overfill depends on the rastering grid (frequency) and quality of paper.

As shown in the graphic the magenta area has not been printed properly in the middle. But since the area is bigger than the area printed on no white areas arise.

Underfill

Underfilling is when the color in the „background“ is brighter than the color in the foreground.

A black area which is being printed solely with black color is often not black enough. Therefore a certain amount of cyan ink is being printed as well. The black in combination with the cyan ink results in a deeper, darker black.

A problem arises when a white text is to be printed in this black area. The white text is being achieved by not printing the area where the text is to be seen.

Due to the inevitable inconsistency in printing all the colors perfectly on top of each other it could happen that the emtpy spaces contain cyan. In order to avoid this cyan is being printed with bigger spacing than black. Even when cyan is being printed a little of center, it will not be visible.

Overprinting

If in the example above black and magenta would have been exchanged you would not overfill black. You would also not spare magenta, but print black above – overprinting. It really doesn’t matter if black is being printed after magenta or not. The wording is the same, the results as well.

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