Adobe Illustrator CC 2015 - Manual (Page 500)

495
Printing
Last updated 6/5/2015
Overprinting
By default, when you print opaque, overlapping colors, the top color knocks out the area underneath. Overprinting
prevents knockouts and makes the topmost overlapping printing ink appear transparent in relation to the underlying
ink.
Where colors printed from separate plates overlap or adjoin one another, press misregistration can cause gaps between
colors on the final output. To compensate for potential gaps between colors in artwork, print shops use a technique
called trapping to create a small area of overlap (called a trap) between two adjoining colors. You can use a separate,
dedicated trapping program to create traps automatically, or you can use Illustrator to create traps manually.
About overprinting
By default, when you print opaque, overlapping colors, the top color knocks out the area underneath. You can use
overprinting to prevent knockout and make the topmost overlapping printing ink appear transparent in relation to the
underlying ink. The degree of transparency in printing depends on the ink, paper, and printing method used. Consult
your print shop to determine how these variables will affect your final artwork.
You may want to overprint in the following situations:
• Overprint black ink to aid in registration. Because black ink is opaque (and usually the last to be printed), it doesn’t
look much different when printed over a color as opposed to a white background. Overprinting black can prevent
gaps from appearing between black and colored areas of your artwork.
• Overprint when the artwork does not share common ink colors and you want to create a trap or overlaid ink effects.
When overprinting process color mixes or custom colors that do not share common ink colors, the overprint color
is added to the background color. For example, if you print a fill of 100% magenta over a fill of 100% cyan, the
overlapping fills appear violet, not magenta.
After you set overprinting options, you should use the Overprint Preview mode (View > Overprint Preview) to see
an approximation of how the overprinting colors will print. You should also carefully check overprinted colors on
separated artwork using integral proofs (where each separation is shown in register on a single piece of paper) or
overlay proofs (where the separations are shown in register on separate plastic sheets stacked on top of each other).
Set up overprinting
1 Select the object or objects that you want to overprint.
2 In the Attributes panel, select Overprint Fill, Overprint Stroke, or both.
If you use the Overprint option on a 100% black stroke or fill, the black ink may not be opaque enough to prevent
the underlying ink colors from showing through. To eliminate the show-through problem, use a four-color (rich)
black instead of a 100% black. Consult with your print shop about the exact percentages of color to add to the black.