Bleed and gutters

Three settings, and one relationship between them that decides whether a ganged sheet can be cut at all.

The relationship is this: where two pages share a cut, the gutter between them has to be at least twice the bleed — one bleed for each page — or the two allowances overlap and each page prints over its neighbour. The alternative is a gutter of zero with a bleed of zero, which is a butt cut. Anything in between is the problem case.

One number, all four edges

This is the first thing to internalise if you have come from a layout application, where bleed is four separate fields and it is entirely normal to set 3 mm at the head and foot and 5 mm at the fore-edge. FastImpose's file bleed is a single scalar. Whatever you type is applied symmetrically around the placed page.

That has two consequences. If your artwork was built with asymmetric bleed, the imposition will reserve the largest of them on every side, or the smallest on every side, depending which number you enter — there is no way to describe the asymmetry. And a page with 3 mm of bleed occupies 6 mm more than its trim size in each axis, not 3 mm: three at the top and three at the bottom, three at the left and three at the right.

The symmetric model is the right one for imposition, because imposition is about how pieces sit next to each other on a sheet and every gutter has two sides. Per-edge bleed is a property of a document; a symmetric allowance is a property of a piece about to be ganged.

The gutter arithmetic

Two adjacent pages on a sheet meet in one of two ways, and the correct gutter for each is different.

A shared cut

The pieces bleed, so between them there is a band of artwork belonging to the left-hand piece and a band belonging to the right-hand piece, with a single knife position dividing them. That band has to be wide enough for both bleeds:

1. 3 mm bleed → 6 mm gutter.
2. 2 mm bleed → 4 mm gutter.
3. 5 mm bleed → 10 mm gutter.

In other words, gutter ≥ 2 × bleed, or equivalently bleed ≤ half the gutter. With the default 1 mm Gap X, the largest bleed you can honestly carry across a shared cut is 0.5 mm — which is to say, none. Change the gap the moment you set a bleed.

A butt cut

The pieces do not bleed. Artwork stops short of the trim on every side, and the two pieces sit edge to edge with a single cut between them and nothing to overlap. Set bleed to 0 and both gaps to 0. This wastes no sheet at all and it is why a non-bleeding job yields more up than a bleeding one — often a whole column or row more, as the worked example on Step and Repeat shows.

What happens when bleed exceeds half the gutter

Nothing stops you. Set a 3 mm bleed and leave Gap X at its 1 mm default and the file exports. What you have made is a sheet where each page's bleed extends 3 mm into a 1 mm gap, so 2.5 mm of the right-hand page's bleed is drawn on top of the left-hand page's bleed and vice versa.

On press this shows up in a predictable order:

• A visible seam of mixed artwork between every pair of pieces, most obvious where two different images meet.
• Crop marks drawn in the gutter sitting on top of that mixed artwork, so the cutter cannot see them.
• A trimmed piece with a strip of the neighbouring job along one edge — the classic symptom, and it is usually noticed after the whole run is cut.

Gaps and the sheet

Gap X and Gap Y have different defaults — 1 mm and 2 mm — and are set independently. That asymmetry is often what you want: cutting a sheet down in horizontal strips first, then dividing each strip, means the vertical gutters take more knife passes and a little more room is cheap insurance.

Every millimetre of gutter is a millimetre off the block, and the block still has to leave 8 mm of clear sheet at each outside edge for the crop marks. Work outward: sheet, minus 16 mm for marks, minus the gutters, divided by the pieces. If the answer is a fraction, you lose the row.

Common mistakes

Setting the bleed and leaving the gaps alone

The single most common fault on this page. The bleed field is the one people think about; the gaps sit at their defaults and quietly guarantee an overlap. Treat bleed and gutter as one decision made in one sitting — you cannot set one correctly without the other.

Bleed already in the source, and set again here

If the artwork was exported from the layout application with 3 mm of bleed already outside its trim box, that bleed is part of the placed page. Adding another 3 mm here reserves space for a second allowance that does not exist, wasting sheet and, worse, pushing the crop marks 3 mm away from the actual trim. Know which side of the workflow owns the bleed, and set it once.

Expecting per-edge bleed

Entering the head bleed and hoping the fore-edge sorts itself out. It will not — the number you type goes on all four sides. If your artwork genuinely needs different allowances per edge, resolve that in the layout application before export and give FastImpose a single figure it can apply evenly.

Butt-cutting artwork that bleeds

Zero gutter with a non-zero bleed is the maximum-overlap configuration: every pair of neighbours prints fully into each other. If you want the yield of a butt cut, the artwork has to be designed not to bleed. You cannot have both.

Spending the whole sheet on gutters

Wide gutters look safe and then there is nowhere for the marks to go. If tightening the block to make room means dropping to a smaller bleed, that is often the better trade — 2 mm of bleed that the cutter can see beats 5 mm they cannot.