Imposition: How a Printer Arranges Pages on a Sheet

A sheet of paper does not come off the press with the pages in reading order. A book is printed on large sheets that carry many pages at once, and those pages are positioned so that when the sheet is folded and the folds are trimmed, the numbers run 1, 2, 3 down the block. The arrangement is called imposition, and it is one of the first things prepress settles before a plate is made.

Why the order changes

Print one page per sheet and the job is simple and slow. Fold the sheet into a signature and you can print 16, 32 or 64 pages on each side in a single pass. The pages are laid out on the flat in the positions they will occupy after folding, not in the order a reader sees. A 16-page signature, for example, stands for a sheet folded four times. Page 1 sits beside page 16, page 2 beside page 15, and so on, because each pair ends up facing each other once the sheet is folded and the folds are cut.

Perfect Binding: How a Flat Spine Holds a Book Together

A paperback with a square spine has no stitches and no wire. The pages are held by a strip of glue along one edge, and the cover is a single printed sheet wrapped around the whole block. The method is called perfect binding, and almost every trade paperback, annual report and thick catalogue is made this way.

From folded sections to loose leaves

The text starts as folded sections, the same signatures that a stitched book would use, gathered in order. In a stitched book the folds at the spine stay intact and the thread or staples pass through them. In perfect binding the fold on the spine edge is cut away, so the sections open into single leaves. The binder clamps the block, mills off the fold, and leaves a flat, slightly rough edge for the adhesive.

Bleed: Why Artwork Runs Past the Trim Line

Print a solid panel of colour to the edge of a sheet and two limits appear at once. A press cannot put ink on the very edge of the sheet, because the gripper needs bare paper to hold it, and the guillotine that trims the job cuts to a tolerance rather than to a hairline. Bleed is the allowance that keeps the printed edge clean despite both.

What bleed is

Bleed is the part of the artwork that sits outside the finished size. When the job is trimmed, that extra image is cut away, and any small movement in printing or cutting lands in the waste instead of opening a white sliver at the edge. The usual allowance is 3 mm on every edge that reaches the trim. Some work, such as a heavy cover board or a job with a wide cutting tolerance, is given 5 mm.

Trapping: The Small Overlap That Keeps a White Line Out

Two colours meet on a printed sheet. One is a solid blue panel, the next is a red band, and where they touch the edge should be a clean line. On press the paper moves and stretches, and the four inks do not always land in exactly the same place. If the artwork simply butts the two colours against each other, a thin sliver of bare paper can show between them. Trapping is the fix.

What trapping does

Trapping overlaps the two colours slightly along their shared edge, so normal movement on press cannot open a gap. The overlap is invisible because the two colours mix in the sliver, but it is wide enough to hide registration drift. The usual width is small, roughly a quarter of a point to half a point, and it is measured in the file.

Varnish, Aqueous and UV: Three Ways to Protect a Printed Sheet

A printed sheet leaves the press with wet ink on it. Ink that rubs off on a hand, a bag or the next sheet in the stack is a problem, so most commercial work gets a clear coating before it is folded or cut. Three coatings do that job, and they are not interchangeable.

Varnish

Varnish is a clear ink with no pigment in it. It runs through a normal printing unit using a plate, so it can be laid down in a precise spot, around small type or a single logo, while the rest of the sheet stays bare. It dries by absorption and oxidation, slower than the other two, so sheets may need to sit before folding. Oil based varnish can yellow over time on white paper. Of the three it gives the least protection against scuffing and moisture, and it is the cheapest.

Registration: How Four Colours Land in the Same Place

A sheet on an offset press passes through one unit for each colour. Cyan goes on first, then magenta, then yellow and black. Between units the sheet travels a short distance, and each plate has to put its colour exactly where the previous one did. That alignment is registration.

What goes wrong without it

Set four impressions slightly apart and the edge of every letter picks up a halo of the colours that missed. Fine type closes up. A solid headline takes on a coloured fringe. On a four colour image a shift of a tenth of a millimetre is enough to soften the whole picture.

Letterpress: Relief Printing and the Impression Left in the Paper

A letterpress plate works like a rubber stamp scaled up to a printing press. The image stands raised above the surface, ink touches only that raised face, and the paper is pressed against it. Almost everything else about the process follows from that one geometry.

The plate

Older shops set metal type by hand and locked it into a chase, with wood type and carved blocks handling the larger display work. A modern shop builds the same raised surface from a photopolymer plate. A light-sensitive plastic on a metal base is exposed through a film positive, the exposed areas harden, and the unexposed background washes away in water. What is left is a plate where the artwork sits higher than everything around it.