Prepress 101: Trapping and Overprinting

As a sheet runs through a printing press, being passed from unit to unit, it shifts. It doesn’t shift around much — in fact, the amount is barely even measurable, often just a fraction of a millimeter — but it does move, especially when flying through the press at high speed. This is simply a physical hurdle that must be compensated for when the art is being prepared. If this is not dealt with, the natural shifting in the press creates haloes and gaps where the unprinted stock peeks through.

Intended appearance: magenta star on cyan background, no overlap.
Printed result with no trapping. White peeks through when registration shifts.

Click the images to see larger.

Compensating for these shifts involves extending images and colors by a mere three thousandths of an inch (8 hundredths of a millimeter), an amount invisible to the naked eye. This microscopic overlap of color compensates for the shifting of the sheet, completing an illusion of two colors butting up to each other and appearing to touch without overlapping. That 0.003-inch overlap is called a trap.

Printed result with trapping. Full ink coverage regardless of shift. Overlap becomes invisible to the naked eye.

As small as 0.003 inches may sound, however, it makes up the majority of the size of thin type — for example, legal fine print. Suppose that a paragraph of cyan fine print is placed on a yellow background. This would be a problem for a few reasons.

Intended appearance: cyan type on yellow background.
Intended appearance: cyan type on yellow background.

First, the trap (0.003 inches all around both sides of each letter) would make up the majority of the type, creating significant overlap of the cyan and yellow, thus making the type appear green. Whoops.

Appearance after attempting to trap. Yellow trapping into cyan makes all type appear green.
Appearance after attempting to trap. Yellow trapping into cyan makes all type appear green.

But a more common example is black type on a background.


Whether that background is a photo, a gradient, or a solid color, the problem ends up being the same. Fine print — in fact, most type — is ultimately too thin to trap. But now what? If the type can’t be trapped, the shifting of the sheet will produce haloes of white around the letters unless the black type falls exactly into the space left for it in all three of the other plates — which simply will not happen because the sheet will shift between print units. Here are the four images in such an example:


And a close-up of the (simulated) resulting sheet after running through the press:

White peeks through and magenta registration can be seen when the black type does not overprint.
White peeks through and magenta registration can be seen when the black type does not overprint.

Obviously, the “Launch Day” type is very large and thick lettering, and could easily be trapped if we wanted to. But for all black (and small) type, the solution is overprinting. Setting the black type to overprint means that the cyan, magenta, and yellow will not leave a space for the black. They will print their images normally, and the black type goes on top of it. The result?


Crisp, smooth type over a pleasant background. No peeking white, no registration issues.

These are just some of the nuts-and-bolts details that our prepress team obsesses over, day in and day out. We hope the next time you need quality, professional printing, you’ll allow us to take care of these details for you.

Diecutting: All About Precision

The diecutter is another of the more fascinating aspects of a printing job because these amazing machines do so many things simultaneously. In a single pass through the diecutter, a sheet is scored, embossed, cut, and scrapped, all at virtually the same time. What enters is a printed sheet, but what comes out is a nearly-finished carton, needing only to be glued and folded.


Here, all of the job’s planning and preparation is put to use. The press sheet layout, the blueprint by which the sheet was printed, is sent to a die maker, who constructs the cutting die specifically for the job being run.

Like a massive industrial cookie-cutter, a cutting die is pressed down onto the press sheet. Sharp cutting rule is bordered by soft rubber, preserving the carton’s finish as it is cut. Rounded scoring rule presses down to form the scores, while a counter plate below — with a groove in the shape of the score — presses up, holding the rest of the carton steady around the scores. Wooden stripping boards push out hanger holes and windows, and the sheet’s scrap is separated from the cartons by the blanking module…all in a heartbeat. The diecutter then drops the flat cartons in its delivery area, forming neat stacks ready to be loaded into the gluer.


Prepress 101: CMYK vs. RGB


When the prepress department receives art for a job, one of the most common oversights we see in any given file is the use of RGB (Red/Green/Blue) color space, when print technology is based on CMYK (Cyan/Magenta/Yellow/BlacK). So what’s the difference? Obviously in the example image here, the RGB version is more vivid, colorful and all-around better looking. So what’s the problem? Can’t that press use RGB?

Well, it’s not really that simple. RGB and CMYK are opposites of each other — additive and subtractive color spaces. In plain English, that simply means that in RGB, if you add all colors in the spectrum together, you get white. This is how the human eye perceives color, and is how computer monitors and TV screens present color. RGB, essentially, is based on light.

CMYK, on the other hand, is necessarily based on ink. As all of the colors are added together, the result is black. If you’ve ever mixed colors in a home painting kit, you may have been disappointed after accidentally producing black paint when you tried to mix too many tones. This is the same concept. While RGB adds more light to create brighter colors, CMYK absorbs (subtracts) light, creating darker tones. The difference between the two color spaces is most apparent in blue and green tones, as shown in the traffic light image.

cmyk-rgbThe issue comes into play because Adobe Photoshop, the most widely used program for graphic design, selects the RGB color space by default when creating a new file. It’s an understandable choice — most design work is created for media that’s viewed on a screen (light-based color) and RGB has a wider range of colors. Most designers don’t give it a second thought, but these are major matters to a printer who then has to attempt to match the vivid colors with a more limited color spectrum. Depending on the image, a great deal of manual adjustment may be needed to return the color to something even resembling the original RGB image… and even that may not be possible. The problem is that even if the printer is able to color-correct the image back into a pleasing range, it can take an inordinate amount of time — which means a greater cost for both customer and printer.

The safest solution is to simply design in CMYK from the beginning. If this is not possible, convert the source images to CMYK before sending to the printer so you can see how much of a difference the conversion will make.

But even this can lead to false results at times. To truly see how a file will really look when printed, it must be previewed with overprinting, an option in Adobe Illustrator that’s available only in CMYK mode.


To preview overprinting, select the “Window” pulldown in Illustrator, and then “Separations Preview” (highlighted). In the window that appears, check the “Overprint Preview” box. This allows colors to be toggled on and off, previewing how each individual print plate will appear. This gives a more accurate impression of how colors will appear when mixed together (especially useful if spot colors are present).

Soon we’ll look at other uses for the Overprint Preview and how it can help detect a range of print issues before they happen.