Foil stamping needs a separate art layer that defines where the foil will be applied. That art is used to make the die. The print file must have a knockout (no ink) where foil will go, and a trap (slight overlap or gap) so foil and ink do not misregister.
Foil layer
Provide a vector layer or spot color that defines the foil area. The printer uses this to make the die. The layer should be closed shapes only; no open paths or raster. Name it as the printer specifies (e.g. "Foil," "Stamp").
Knockout and trap
Where foil will print, ink is knocked out so foil sits on the substrate (or on a base coat if required). Trap is a small overlap or gap between ink and foil so slight misregistration does not show a white line or overlap. The printer will specify the trap value.
Checklist
• One foil layer with closed vector shapes only.
• Knockout in the print where foil will go.
• Trap applied per printer spec (typically 0.25–0.5 pt).
Common mistake
Sending a file with no knockout so ink prints under the foil area. Foil does not adhere well over wet ink and can look muddy. Knock out the foil area in the print.