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The press sheet is not the page
A press prints on sheets far larger than the finished piece. A business card is 85 × 55 mm; a sheet-fed offset press takes 50 × 70 or 70 × 100 cm, a digital press SRA3 or a long sheet of around 33 × 48 cm. Printing one card per sheet would be absurd, so the job is repeated across the sheet, or — for a book — several pages are printed on each side of one sheet and the sheet is folded into a signature.
Imposition is deciding that arrangement before anything is printed: which piece goes where, which way round, with what spacing, and on which sheet size. It is done in prepress, and its result is the file the plate or the digital press receives.
Three kinds of imposition
Almost every job a print shop takes falls into one of three patterns:
- Step and repeat — the same design many times on the sheet: labels, business cards, postcards, flyers. The question is how many fit and how they are cut apart.
- Signature imposition — the pages of a book, magazine or brochure placed so that the folded sheet reads in order. Half of the pages sit upside down on the flat sheet, and that is correct.
- Nesting — irregular shapes such as folding cartons, interlocked on the sheet and cut out by a die rather than a guillotine.
What the press takes away from the sheet
Not all of the sheet can carry print. On a sheet-fed press the grippers hold one edge of the sheet as it travels through; nothing can print on that strip, commonly around 10 mm. The opposite edge, the tail, needs its own margin too. A digital press has no grippers, but it cannot print to the very edge either, so an equal strip is lost on all four sides.
Inside what is left, the layout needs bleed — the artwork extended past the trim, usually 3 mm (⅛ inch in the US), so a slightly off cut does not leave a white edge — and room outside the block for crop marks, registration marks and a colour control bar.
How the sheet turns
A job printed on both sides has to turn over between the two passes, and how it turns changes the imposition:
- Sheetwise — the front and the back each have their own plates. The simplest case, and the one that needs the most plates.
- Work-and-turn — front and back share one set of plates; the sheet is turned side to side and keeps its gripper edge. Each sheet yields two copies, and half as many plates are needed.
- Work-and-tumble — the sheet is turned head to tail, so the edge that was the tail becomes the gripper for the second pass, and both edges need a gripper margin.
- Perfecting — the press turns the sheet itself and prints both sides in one pass.
Choosing the sheet
The sheet that fits the most pieces is often not the cheapest one. Paper is bought by weight, so what matters is how much paper each finished piece costs, after the gripper, the margins and the offcuts are counted. A slightly smaller sheet that wastes less can beat a larger one that fits a few more pieces.
Grain direction matters as well: a sheet that folds across the grain cracks at the fold and lies badly in a book, so the sheet is chosen with the fold in mind, not only the fit.
The mistakes imposition prevents
- Pages bound in the wrong order, or a signature folded from the wrong side and mirrored.
- Page numbers that print upside down because they were set to the sheet instead of to their page.
- A white hairline at the edge because the file arrived without bleed.
- Text cut off at the fore-edge of a thick saddle-stitched booklet, where creep pushed the inner pages out.
- Marks or a colour bar that land in the gripper and never print.