Trapping color

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.

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.


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


About ink trapping

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 can 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.


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.


So how do you prevent having your work come off the printing press out of register?

By being aware of how you are designing and avoiding the need for trapping.

1. Avoid Touching or Almost Touching Colors



2. Use Common Process Colors to Close Gaps





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


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

5. Overprinting / Surprinting Avoids Need to Trap


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 service bureau or printer. Talk with your service provider about their preferences and recommendations for trapping your files.

There are three basics programs available for trapping.

Illustration Software
In vector-based drawing programs such as Adobe Illustrator you can create simple manual traps by applying outlines (overprinting strokes) to objects or use plug-ins that apply the appropriate trap.

Page Layout Software
High-end tools such as QuarkXPress, Adobe PageMaker, and Adobe InDesign have built-in trapping.

Dedicated Trapping Software
Usually used by service bureaus and printers these programs are designed to do nothing but trapping. TrapWise and Trapper are two such programs.


Exercise (Teams of 2) :

After reviewing the information provided you will be able to create a digital document that demonstrates...

1. Use process colors that don't need trapping

2. Use two PMS colors that overprint to create a third color

3. Overprint PMS black over another PMS color

4. Manually create a spread using two pms colors

Other useful links:


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