What is Trapping?
Trapping when related to screen printing allows you to more easily maintain registration or fit of a graphic when it is broken down into spot colors. It is essentially an overlap of colors to whatever degree is tolerable. An ideal trap splits an outline at its thinnest point by 50% to allow the most room for variation with the the spot color that lies beneath the outline or overlapping color. Trapping can also be used to slightly spread a lighter color under a darker one where it would normally butt against another color so no white space shows with minimal variations in registration. Trapping is used to compensate for registration issues of successive colors or images. It ensures that there are no noticeable gaps or overlaps in the printed result. Registration is the aligning of each separate color/screen with one another to produce the desired printed result. A few circles with crosshairs are placed next to a printed area to make registration quicker and more accurate. Misregistration can cause the final image to look blurred or “off register” and can occur because of:
Mechanical inaccuracies in an imagesetter or CTP system
Printing plates or film stretching
Inaccuracy in copying film to either film or plates
Instability of the printing press
Dimensional instability of the paper or medium that is printed on
There are several reasons trapping is needed; in screenprinting a piece of paper is pulled through the press and ink is applied using one or more screens. As more screens (inks) are used the paper is stretched more and more. This stretching results in gaps where designs have one ink abutting another, which attracts the readers eye to the light colored paper showing through. By slightly overlapping these inks (trapping) the eye is less drawn to the slightly darker border. One artifact that is less objectionable than the other. When you have two colors that meet - like in this image below; you can overlap them so that - if any misregistration occurs - the background color of the substrate or fabric will not show through and the image will not look “off register.”
The example relates to translucent inks used at InHouse, but the general concept should become apparent when you study the information shown there. InHouse typically uses translucent inks, but if the PMS color chosen has more than 40% white in its formula, the ink will have more opacity. Opaque inks often eliminate the concerns listed about changing colors.
How to trap
Years ago trapping was done photographically. Now that all content is digital, software takes care of trapping. Adding an overprint stroke to compound paths in Illustrator or expanding selections in Photoshop are two common ways to trap. There are different techniques used to trap layout elements.
As a general rule, lighter colors should be spread into and under darker colors. This minimizes the visual effect of trapping.
A spread (more common) means that an element is slightly enlarged or ‘spread’ so that it overlaps the background.
A choke means that the size of an element is slightly reduced so that the its background slightly overlaps the object.
How much trapping is needed?
As a general rule, the width of traps should be between .75 points (.01”) and 1.5 points (.02”). Graphics with a lot of detail or very thin lines may require smaller trapping. Basic graphics with minimal detail and a lot of big spaces would be fine with bigger trapping.
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