Page 154 - Graphic Design and Print Production Fundamentals

142 Chapter 5. Pre-press
trap values of their inks. This refers to the ability of one printing ink to stick to another. Inks vary in
viscosity depending on the density and types of pigments they are carrying. The trap characteristics and
transparency of a printing ink are part of what determines the printing order in which they are applied
to the substrate. For example, a process primary yellow ink is very transparent and will not stick (trap)
well if printed on top of a heavy silver metallic ink. The metallic silver is thick and very opaque, so it
will hide everything that it overprints. A graphics technician must generate trap lines for a graphic that
has metallic silver abutting to a process yellow shape. The technician will increase (spread) the shape
of the yellow graphic to go under the abutment to the silver. The silver shape will not be altered, and
when it overprints, the yellow ink will stick to and hide the yellow trap line shape. The best analogy, we
have heard is from a press person — the peanut butter sandwich analogy. We know the jelly sticks to the
peanut butter and the peanut butter will not stick to the bread if the jelly is spread first. If a press person
does not know the trap values of the inks, he or she can make as big a mess of the press sheet as an
upside-down peanut butter and jelly sandwich makes on the front of your shirt! For this reason, trapping
should be left to the specialists and is usually applied to a final PDF file before it is sent to a RIP. Ninety
percent of trap lines for lithographic and flexographic imaging reproduction are generated automatically
by specialized trapping software. Operators are trained to recognize shapes and colour combinations that
will cause problems on the press. They will custom trap those documents with the Acrobat plug-ins we
talked about earlier.

Special Consideration for Black

There is one trapping combination that should be considered and applied to all four imaging
technologies. It is the way that black ink is handled in the document and applied on the imaging device.
Most type is set in black ink, and much of it overprints coloured backgrounds. In all four imaging
technologies, black is the strongest ink and hides most of what it overprints. It is still a transparent ink
and most process black ink is more dark brown than the rich dark black we love to see in our documents.
If the size of the black type or graphic is large enough, we will be able to see the black colour shift as it
overprints stronger or weaker colours under it. Graphic designers should pay attention to setting whether
black type or graphics overprint the background, or knock out the background to print a consistent black
colour. A useful rule of thumb is that type above 18 points should be knocked out and boosted. Raise
this threshold for very fine faces such as a script where larger point sizes can overprint, and reduce it for
excessively heavy fonts like a slab serif. If the graphic is large enough, it should also be ‘boosted’ with
other process colours.

The way we handle black ink or toner deserves special consideration in all four imaging technologies.
Black is a supplemental colour to the three primary process colours. It is intended to print only when the
other three colours are all present in some kind of balance. In all imaging technologies, we must watch
that our total ink coverage does not approach 400%, or 100% of each ink overprinting the other inks in
the same place. This is usually too much ink or toner for the substrate to absorb. As a result, it will not
dry properly and will offset on the back of the next sheet, or bubble and flake off the media in the fuser.
Therefore, we must pay attention to how our photographs are colour separated, and how we build black
in our vector graphics.