A small, deliberate overlap between two adjacent colors. Just enough to absorb the mechanical drift that happens when two plates don't land in exactly the same spot. Without it, a shifted plate leaves a thin white seam between colors.
Printing presses are mechanical. Every color runs through a different station, and between stations the substrate stretches, slips, and drifts by a fraction of a millimeter. If you have a blue shape sitting inside a yellow background and the blue plate lands 0.2mm to the right, the right side of the shape shows a thin blue line over yellow (fine) and the left side shows a thin strip of white paper where blue used to be (not fine).
Trapping solves this by extending one color slightly into the other. If blue expands 0.3pt outward into the yellow, now when the blue plate drifts 0.2mm, the overlap just becomes a 0.1mm overlap. No white gap.
| Trap type | What it does | When to use |
|---|---|---|
| Spread | Lighter color expands outward into darker | Light shape on dark background (e.g. yellow text on blue) |
| Choke | Darker color's inner edge shrinks inward, lighter color fills in | Dark shape with a light hole (e.g. black text with white dropped inside) |
| Centerline | Both colors overlap by equal amounts at the boundary | Two colors of similar luminance meeting |
The universal rule: the lighter color spreads into the darker. The eye follows the darker edge, so extending the lighter side is invisible; extending the darker side would look like a fat outline.
Spot-to-spot joins on flexo. Two solid spot colors meeting edge-to-edge on a flexo press, particularly on flexible packaging, can still benefit from explicit manual trapping because flexo plates distort more than offset or digital. Your prepress provider will usually handle this.
Specialty inks (white underbase, varnish, foil). A white underbase needs to be slightly smaller than the color artwork on top (a choke) so the white doesn't peek out past the color. Varnish plates often get a small choke too.
Metallic inks meeting process colors. Metallics have different ink density behavior and often need explicit traps set by a prepress operator.
Vintage or specialty presses. Some older flexo presses and specialty equipment don't have modern auto-trap software. For those, traps are baked into the file.
Most digital and offset work goes through a RIP (Raster Image Processor) that applies trapping automatically based on ink densities, screen frequencies, and substrate profile. If you then add manual traps on top of that in Illustrator, you end up with double-traps. The RIP extends your already-extended shape, producing fat outlines at every color join.
Before adding manual traps, ask your printer: "Is your RIP handling trapping, or should I trap in the file?" Most will say the RIP is handling it.
| Press type | Typical trap width |
|---|---|
| Digital (HP Indigo, Xeikon) | 0.15–0.25pt (0.05–0.09mm) |
| Offset lithography | 0.25–0.4pt (0.09–0.14mm) |
| Flexo (labels) | 0.3–0.5pt (0.1–0.18mm) |
| Flexo (flexible packaging) | 0.4–0.75pt (0.14–0.26mm) |
| Screen printing | 0.5–1pt (0.18–0.35mm) |
Most digital and offset jobs trap at the RIP. Send your file to your printer and ask. Preflight checks for the artwork conditions that tend to cause white-line registration gaps.
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