The Basics of Trapping

The quality of a printer’s work depends in large part on getting different inks to print exactly in register. To minimize the effects of misregistration, commercial printers developed a technique called trapping.

What is Trapping?


Trapping is a method of adjusting areas where two distinct, adjacent colors meet so that press misregistration won't cause white spaces. A trap is a small overlap of two colors.

There is another use of "trap" in printing; the stickiness of the inks, describing the ability of an ink to stick to the paper and to previously printed, wet, inks. This is normally a concern for the press operator, not the designer.

If you knock out graphics or type you may have to create a trap to ensure that you don't have white spaces due to misregistration. The default for computer graphics applications is to knock out the colors under the top object. Objects can be given the overprint attribute for fills or strokes.

trapping

There are two types of trap:

1. A spread
, in which a lighter object overlaps a darker background and seems to expand into the background; and

2. A choke, in which a lighter background overlaps a darker object that falls within the background and seems to squeeze or reduce the object.

spread-choke

Spread (object overlaps background) compared to choke (background overlaps object)

 

Registration, or lack of...

When an offset printed document uses more than one ink on the same page, each ink must be printed in register (perfectly aligned) with any other inks that it abuts, so that there is no gap where the different inks meet.

It’s impossible to ensure exact registration for every object on every sheet of paper running through a printing press, so misregistration of inks will occur. Misregistration causes an unintended gap between inks.

How do you fix this?

You can compensate for misregistration by slightly expanding one object so that it overlaps an object of a different color—a process known as trapping.

By default, placing one ink over another knocks out, or removes, any inks underneath to prevent unwanted color mixing; but trapping requires that inks overprint, or print on top of each other, so that at least a partial overlap is achieved.

trap

Most traps employ spreading—expanding a light object into a dark object.

Because the darker of two adjacent colors defines the visible edge of the object or text, expanding the lighter color slightly into the darker color maintains the visual edge.

How to prevent registration problems

By being aware of how you are designing to minimize the need for trapping.

1. Avoid Touching or Almost Touching Colors

trapping

 

2. Use Common Process Colors to Close Gaps

trapping

trapping

trapping

 

When using process colors you can sometimes avoid the need to trap by making sure that adjacent colors share a certain percentage of color.

For example, if each object shares at least 20% of magenta, any shift in registration will result in the gap being the lesser percentage of magenta rather than the white paper. In some circumstances this is acceptable and not very noticeable.

The closer the percentage of common colors, the less trapping that may be required. (The shift between 20% and 30% cyan is less apparent than the difference between 20% and 80%, for example.)


3. Overprint Black to Prevent Gaps

trapping

You can avoid misalignment or the need to trap by printing black on top of your other colors. Black outlines around objects and black text over color areas are examples.



4. Use Chokes and Spreads to Fill the Gaps

spread-choke

Spread (object overlaps background) compared to choke (background overlaps object)


5. Overprinting / Surprinting Avoids Need to Trap

trapping

If you overprint (also called surprinting) you are actually printing one color of ink over the top of another layer of ink. Depending on the colors you will get varying degrees of noticeable changes in color where the two objects overprint.

6. Manual and Automatic Trapping

If trapping is necessary you have the option of trapping it yourself or leaving that job to your printer. Talk with your service provider about their preferences and recommendations for trapping your files.

There are three basics programs available for trapping.

Graphics Software
In vector-based drawing programs such as Adobe Illustrator you can create simple manual traps by applying outlines (overprinting strokes) to objects or there may be a trapping function, such as the Trap in Illustrator's Pathfinder.

Trapping in the printer's RIP
Most print shops will have automatic trapping software in their Raster Image Processors. The printer will do the trapping and you will not need to do it in your files.

Artwork for printing on t-shirts, mugs, and other specialty items may still need a manually built trap. Check with your supplier to determine what you may need to do.

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