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Gripper margin and tail margin
The gripper margin is the unprintable strip along the leading edge of the sheet, where the press holds it. On the opposite edge, the tail, there is usually a narrower strip the press cannot print or where printing is not safe; that is the tail margin.
Together they define the area of the sheet you can actually use. The layout should be made on what is left after both margins are taken off, not on the full sheet size.
How many millimetres?
The gripper margin varies from press to press; on most sheetfed offset presses it is between 8 and 12 mm, and the tail margin is usually smaller. The press catalogue gives a figure, but the safest value is the one you measure on a sheet printed on your own machine.
The catalogue and the measured value can differ by a few millimetres. On a job that only just fits, that difference can cost a whole row of pieces or cut off the marks on the gripper side.
How it changes the number of pieces
The gripper margin shortens the sheet in one direction only, so laying the same piece across or along the sheet can give different results:
- On a 70 × 100 cm sheet, a 10 mm gripper and a 5 mm tail margin leave 985 mm usable in the gripper direction.
- If the long side of the piece runs in the gripper direction, losing 15 mm can cost a full row; with the short side there, often nothing is lost.
- When the gripper is on the long edge and the job is borderline, try turning the sheet or the piece.
- Trim marks and the colour bar must fit in the usable area too; a mark placed on the gripper strip will not print.
Turning the sheet and the gripper edge
Which edge becomes the gripper when the back is printed depends on how the sheet is turned. In work-and-turn the sheet is flipped sideways and the same leading edge stays the gripper. In work-and-tumble it is flipped head over tail and the opposite edge becomes the gripper.
With work-and-tumble both edges act as the gripper in turn, so a gripper margin has to be left on both. That shrinks the usable area and can change which turning method suits a borderline job.
Digital printing
Digital presses have no grippers like offset, but they cannot print to the very edge of the sheet either. Every machine leaves an unprintable strip of a few millimetres on all four sides.
Because that strip is the same on all four edges, it does not shorten the layout in one direction, and turning the piece makes less difference than in offset. On a job that only just fits, it still has to be counted.