Trapping, and the white line around everything

The problem: paper moves

A four-colour sheet passes four printing units. Between them the paper stretches slightly, absorbs a little moisture, and is gripped and released. The four impressions therefore do not land exactly on top of each other. A drift of 0.1 to 0.2 mm is normal on offset litho; flexo on film is worse; screen printing on stretchy fabric is worse still.

Where two different colours meet edge to edge, that drift shows as a sliver of unprinted paper between them. A thin white line around every letter and every shape.

Knockout, which causes it

By default, an object knocks out what is beneath: the press does not lay the background ink where the object sits, so the inks do not mix and the colour is what you specified. Knockout means the two inks meet along an exact shared edge, and an exact shared edge is precisely what misregistration exposes.

Trapping, which fixes it

Trapping deliberately overlaps the two colours by a small amount, so that when registration drifts, the drift reveals an overlap rather than a gap. An overlap is a slightly darker line, which the eye forgives. A white gap, it does not.

Two directions, and the choice is not arbitrary:

• Spread: the lighter colour is expanded outwards into the darker one.
• Choke: the darker background is contracted inwards, which amounts to the same thing from the other side.

The lighter colour always moves. The reason is that the eye reads the edge of a shape from its darker side; enlarging the light colour slightly leaves the perceived boundary where it was, while enlarging the dark one visibly fattens the shape.

Typical trap width: 0.08 to 0.15 mm on sheetfed offset, more on web, considerably more on screen print and flexo. Ask.

Who does it, and why it is usually not you

On almost all commercial offset work, the printer’s prepress or the RIP applies trapping automatically, using in-RIP trapping settings tuned to their press. If you also trap by hand, you get double traps and visibly fat edges.

So the default answer is: do not trap, and say so. Put “no manual trapping applied” on the docket and let them do their job.

You trap by hand when:

• Screen printing, especially on garments, where registration is coarse and the shop expects you to supply trapped separations.
• Flexo packaging on film, where the trap values are large and job-specific.
• A two-colour job at a small shop with no in-RIP trapping.
• Vinyl and signage where two cut colours butt together.

Doing it by hand

The manual method is an offset plus overprint:

1. Duplicate the lighter object.
2. Offset the duplicate outwards by the trap width (Object ▸ Path ▸ Offset Path, or a stroke of twice the trap width set to the lighter colour).
3. Set that duplicate to overprint fill and overprint stroke, so it prints on top of the darker colour rather than knocking it out.
4. Place it beneath the original.

Illustrator has Window ▸ Separations Preview plus Effect ▸ Pathfinder ▸ Trap for simple cases. Scribus has per-object overprint control in the properties palette. CorelDRAW has Overprint fill and Overprint outline in Object Properties and an in-RIP trapping section in its PDF export dialog.

The two cases you must know without being told

Black text overprints. Small black type set to knock out will show coloured fringes at the slightest drift. Set 100% K text to overprint and no trap is needed at all, because the black prints over whatever is there. Most workflows do this automatically for 100% K; check rather than assume.

White never overprints. White is the absence of ink. An object at 0/0/0/0 set to overprint prints nothing, and whatever is behind it shows through. Your white logo vanishes from the middle of the photograph, and the file looked perfect on screen the whole time. Turn on overprint preview before every export and look.

Where trapping is somebody’s whole job

On a garment screen-printing floor, trapping is the difference between a saleable shirt and a reject, because the fabric moves under the squeegee and registration on a manual carousel is measured in whole millimetres rather than tenths. Separations there routinely carry traps of 0.5 mm or more, plus a choked underbase, and the shop will tell you the values they want if you ask for them by name.

That is worth remembering as a general principle. Trap width is a property of the process, not of the artwork, so the only correct way to choose one is to ask the person running the machine. A number you found on the internet belongs to somebody else’s press.