Web offset imposition: cut-offs, ribbons and what the folder delivers

Sheetfed imposition starts with a sheet you choose. Web imposition starts with a press that has decided most of it already: the plate cylinder fixes the length of every form, the roll its width, and the folder which signatures come out. The job is to fit the book to the machine.

What's different about a web press

• Cut-off, not sheet length. A form is the length of web printed in one turn of the plate cylinder, fixed for the press. A common heatset cut-off for letter-size work is 578 mm (22.75 in); A4 presses are usually around 620 to 630 mm.
• Web width, not sheet width. You choose the roll, within what the press takes. The narrowest roll that holds the pages wastes least.
• Both sides in one pass, each with its own plates. No work-and-turn or work-and-tumble.
• No gripper. The plate gap at the cut, where the plate is locked up, has to fall in paper that is trimmed off.
• Grain always runs along the web. On the usual 16-page magazine signature the spine fold runs along the web too, so grain is parallel to the spine.

A typical 16-page heatset press carries four pages across the web and two round the cylinder, on both sides: 4 × 2 × 2 = 16 pages per turn, delivered as one 16-page signature. A 32-page press doubles the web width, or the cylinder's circumference with four pages round it.

Ribbons, the former and the folder

Before the folder (on heatset, after the dryer and chill rolls) the web can be slit lengthwise into ribbons. Angle bars, often called turn bars, move ribbons sideways and lay them on top of each other, so two ribbons, or two webs, go over the former together and come out as one thicker product, the inner ribbon's pages nested inside the outer's.

The former fold is always the first fold and always runs along the web. The knife then cuts the web off at the cut-off, and the folder makes its cross folds:

Folder	Folds	Delivers
Former only	The former fold	4-page broadsheet sections
Jaw	Former and one jaw (parallel) fold	8-page tabloid
Chopper	Former, jaw, then a quarter fold along the web	16-page magazine
Double parallel	Former and two jaw folds	16-page digest
Combination	Former, jaw, second parallel and chopper	Up to 32 pages when four pages fit round the cylinder

Coldset newspaper presses mostly deliver broadsheet and tabloid sections from jaw folders; heatset presses mostly deliver 16s. Two more folder details matter. A pin folder leaves pin holes at the cut, so the trim there must take them off; a pinless folder doesn't. And two rows of pages meet at the jaw fold either foot to foot, heads at the cut where the knife and pins are (the usual web choice), or head to head, as most sheetfed 16s are laid out.

A worked example

A 64-page saddle-stitched catalogue, 8.125 × 10.875 in (206.4 × 276.2 mm), on a 16-page heatset press with a 577.85 mm (22.75 in) cut-off and a chopper folder. Trims are 3 mm, with an 8 mm lip.

• Across the web: four pages, four face trims and the lip on two of them make 853.5 mm. With 8 mm unprinted edges that needs an 869.5 mm roll. A 35 in (889 mm) roll works, but trims another 19.5 mm off every cut-off.
• Round the cylinder: two rows of pages and their head and foot trims take 564.45 mm, so 13.4 mm of each cut-off is spare. The knife and jaw fold are fixed on the cylinder, so the rows share it evenly. Foot to foot, the spare goes to the heads: each head trim becomes 9.7 mm.
• At the cut: that leaves 6.7 mm of clear paper outside the bleed on each side of the knife, for the plate gap and any pin holes.
• The run: four 16s, four forms, eight plate sets, four cut-offs per copy.

Change the page count and the sums change. 56 pages are three 16s and an 8 that runs two to a cut-off: 3.5 cut-offs per copy. 60 pages are four 16s with 4 blanks, or 16 + 16 + 16 + 8 + 4 with none and one more signature to gather. On a double-web press, 64 pages print as two 32-page products, a 16 from the second web inside a 16 from the first: still eight plate sets, but 2 cut-offs per copy instead of 4.

Where it goes wrong by hand

• Ribbons in the wrong order. In a stacked product the outside ribbon carries the first and last pages. Swap the ribbons and every plate is right but the product is out of sequence.
• Heads at the wrong end, so the spare cut-off, pin holes and plate gap land at the foot, where the trims weren't planned for them.
• Page counts that don't divide. The remainder needs a signature the folder can make, blanks, a second web, a separate cover or an insert, and each changes plates, cut-offs and bindery work.
• A press setup that lives in someone's head, so prepress works from an old template: the plates fit the cylinder, the pages don't fit the folder.

All of these show up after the plates are made and the web is threaded: a plate set, another makeready, and paper wasted by the metre while the press comes up to colour.

Creep, the lip and fan-out

Nested 16s creep like any saddle-stitched book. Each 16 is four sheets, so 64 pages nest 16 sheets deep; on 0.076 mm paper the innermost pages move about 1.1 mm toward the spine to keep their face margins. The stitcher still needs its lip on the high or low folio.

Fan-out is different: the web takes up water and grows across its width, so later colours print slightly wider. It's corrected on press or by scaling plates for particular units, not in the imposition.

How AutomatePrint plans a web job

Define the press once. In Equipment, an offset press can be web-fed: give it its cut-off, web folder (chopper, jaw, double parallel, combination or former only), how many webs it combines, its pin trim (0 for pinless), its roll range, the plate gap at the cut, the bands at the web's edges and a short code for file names.

Pick it for the booklet, in the wizard or the editor, and Booklets takes those settings with it. The largest signature the folder makes that fits the cut-off becomes the main one, and the folder's smaller ones take the remainder. The rows of pages share the whole cut-off and the spare goes into the trim at the cut, never smaller than the pin trim: the 9.7 mm head trim above, worked out rather than typed in.

Ribbons and webs. Blocks side by side are ribbons, with a Between ribbons allowance trimmed out at each slit. Stacked ribbons and Webs build products from parts that nest, the first web's ribbons outside; each web gets its own form, and forms that print together count as one press run. A panel shows the ribbons, copies round the cylinder, press runs, cut-offs per copy and the roll the form needs, with a button to use it.

The wizard compares. On a web press it tries each signature the folder makes, and stacks of 2 to 4 ribbons, and ranks the plans by fewest plate sets, then fewest cut-offs per copy, then fewest blanks.

Check before plating. The Sheet view draws each form at roll width by cut-off, with every slit; the Fold view folds the real pages; the Book view shows nesting and creep for every leaf. The checks flag a scheme the folder doesn't make, a plate gap that would cut into bleed, a roll outside the press's range or wider than needed, ribbons that don't fit across, and pages short for the cut-off.

Marks and output. The colour bar runs across the web in the trim at one cut, registration targets and the slug in the trim at the other, with ticks at each slit; collating marks go on each product's outside part. Download PDF forms or a JDF package for your RIP, named by your convention: booklet.cutoff gives web width by cut-off (35.00x22.75) and booklet.press_code the press. A separate cover gets its own form on its own press, so a sheetfed cover round a web text stays one booklet.

What Booklets doesn't do: plan the angle-bar path (it lists which part each ribbon carries; set the press to match), compensate fan-out, or model web waste such as splices. Estimates treat a web press as an offset press with a cut-off counted as a sheet.

Checklist: setting up a web booklet

1. Set up the web press in Equipment: cut-off, folder, webs, pin trim, roll range, plate gap and short code.
2. Ask the press room which way rows face round the cylinder, and the bindery which folio takes the lip.
3. Get the paper's real thickness: it drives creep.
4. Run the wizard with every press the job could go on, web and sheetfed, and compare plate sets and cut-offs per copy.
5. Decide the remainder: a smaller signature, blanks, a second web or a separate cover.
6. Set the roll to what the form needs.
7. Clear the checks, fold the first signature on screen, then fold a dummy from a proof.

See AutomatePrint booklet imposition, or read web offset presses in Booklets and setting up a web press.