Wash-Up: What It Takes to Clean a Press Between Colours

A press does not stop when the last sheet of a job leaves the delivery. The ink has to come off the rollers, the blankets and the plates before the next job can run, and that cleaning step is called wash-up.

Offset ink is a paste of pigment, resin and oil. Water will not move it. The wash is a solvent or a vegetable-oil based cleaner that dissolves the resin and carries the pigment away. On a modern press the wash-up unit does the work: a blade or a trough presses solvent against the ink rollers while they turn, the dirty liquid drains into a waste container, and the cycle repeats with clean solvent until the rollers stop giving up colour.

Kiss Cutting: Why a Sticker Stays on the Sheet Until Someone Peels It

A sheet of stickers looks like it was punched out of one piece of paper. It was not. The labels are still attached to the backing, and that is the whole point of a kiss cut.

A kiss cut is a die cut that goes through the face material and the adhesive but stops at the release liner. The label stays on the liner, the liner stays whole, and the sheet can run through a printer, a laminator or a mail slot without falling apart. A through cut goes through every layer, so the label and the liner separate. Many label jobs use both cuts on the same sheet: the label outline is a kiss cut, the outer edge of the sheet is a through cut.

Wire-O and Spiral: How a Book Is Built to Lie Flat

A perfect-bound book is a block of pages glued into a cover. It opens to about 180 degrees, and past that the spine starts to fight back. A wire-o or spiral book opens through 360 degrees, folds back on itself and sits flat on a table. The trade is that the binding stays visible: a row of punched holes and a metal or plastic element running through them. For a workbook, a manual, a swatch set or a cookbook that has to stay open while both hands are busy, that trade is usually worth taking.

Two machines, one preparation step

Wire-o, also called twin loop, uses a continuous length of formed wire. The wire is fed through the punched holes from the back of the block and the loops are closed at a closing station. The finished spine is rigid; the pages stay in line with each other and the book holds its shape.

Ink Tack: The Sticky Number That Decides Whether a Sheet Backs Up Cleanly

Ink is not a liquid that flows onto paper. It is a paste that gets split. Between two rollers, and again between the blanket and the sheet, the ink film is pulled apart thousands of times a second. Tack is the force needed to pull that film apart, and it is measured, quoted and argued over in every pressroom.

How it is measured

An inkometer lays a known film of ink on a roller, runs that roller at a set speed against a second roller, and measures the torque the film resists. The reading is in gram-metres. Standard conditions are 32 C, 1200 rpm and one minute of running. A tack number means nothing without those conditions, which is why suppliers print them next to the figure.

Fan-Out: The Register Error That Grows Across a Web

A sheetfed press holds one sheet at a time, so every unit prints on paper that is the same width. A web press does not. The paper runs through the machine as a continuous ribbon, and that ribbon gets wider as it travels. The error this creates is called fan-out, and it is the reason a web job can be in register at the centre of the sheet and out at the edges.

What happens to the web

Paper is pulled through the press under tension, from the unwind stand to the chill rolls and the folder. Tension alone stretches the web lengthwise, and a stretched web narrows slightly, the way a rubber band does. Moisture does the opposite. Each printing unit lays fountain solution on the web, and the paper fibres take it up. Fibres swell most in the cross direction, which is the direction of the web width, so the web widens a little at every unit.

Buckle and Knife: Two Ways a Folder Makes a Fold

A folded brochure and a folded book signature look alike on the table. The creases may have been made by two different machines, and the choice between them is settled by paper weight, fold direction and the number of folds needed in one pass.

Buckle folding

The common folding machine works with buckle plates. The sheet runs between two rollers, hits a stop plate, and the paper behind it keeps moving. The sheet has nowhere to go, so it buckles upward into the gap between a second pair of rollers. Those rollers catch the buckle and pull it through, pressing a fold into the paper at the point where it bent.

Creep: Why the Pages of a Saddle-Stitched Booklet Step Outward

Nest sixteen folded sheets inside each other and the spine gets thick while the face stays thin. The inner sheets sit closer to the fold, so their fore-edge stops short of the outer sheet. That offset is creep, and on a thick booklet it is visible before anyone opens the book.

What causes it

A saddle-stitched booklet is not a stack of separate leaves. It is a set of nested sheets folded together and stitched through the fold. Each sheet adds its own thickness at the spine. The second sheet from the outside has to wrap around the first, so it starts a little further out. By the time you reach the centre sheet, the accumulated board has pushed it several millimetres toward the fore-edge.