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Why does creep happen?
In saddle stitching the signatures are nested inside each other and stitched together through the spine. Each outer leaf has to wrap around the thickness of the ones inside, so when folded its fore-edge sits slightly back from them. The further in you go, the more the leaves push out.
When the excess is trimmed, the inner pages end up narrower than the outer ones. The thicker the paper and the more pages, the bigger the difference.
Which bindings need it?
Creep is a problem of bindings where the signatures are nested:
- Saddle stitch (magazines, brochures, catalogues): creep allowance is needed, more so as page count and paper weight grow.
- Perfect binding and section sewing: signatures are gathered one after another, each a consecutive block of pages; there is no creep between them.
- Spiral and Wire-O: the leaves are not folded but stacked; no creep.
- On thin 8–16 page jobs creep is usually small enough to ignore.
How to work out creep
The rough calculation: the creep of the innermost leaf is about the number of folded sheets outside it times the paper thickness. Example: a 48-page saddle-stitched catalogue, each folded sheet carrying 4 pages, so 12 sheets nested. The paper is 115 gsm coated, about 0.1 mm thick. There are 11 sheets outside the innermost one: 11 × 0.1 = about 1.1 mm.
That value goes to the innermost signature and is shared out decreasingly towards the outside; the outermost gets none. In imposition the pages are shifted towards the spine by that amount, so after trimming every page has the same margin.
Rather than guessing thickness from weight, ask your supplier for the caliper in microns; papers of the same weight can differ in thickness.
When the cover is separate
If the cover is printed on a heavier stock and added outside (a 300 gsm cover on a 115 gsm text, say), its thickness adds to the creep of everything inside. Count the cover by its own thickness, not as one more text sheet.
Designing with creep in mind
Creep is corrected in imposition, but the design has to allow for it:
- Keep important elements away from the edge; leave a safe area of at least 5 mm at the fore-edge.
- Don't push folios right up against the fore-edge; on a creeping page they are the first thing cut.
- Avoid frame lines running parallel to the page edge; creep narrows the frame on the middle pages and it shows.
- On images spreading across two pages (other than the centre spread), creep shifts the image slightly; don't put a critical line exactly on the spine.
Check it with a dummy
However careful the calculation, have a blank dummy made from the job's own paper, fold it, stitch it and measure the fore-edge. The difference between the middle leaf and the outer one is the real creep. On a long run, those few minutes of measuring can save the whole job.