- 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 material 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 Pro Ad Co, but the general concept should become apparent when you study the information shown there. Pro Ad Co 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.