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.

Gripper Margin: The Strip of Sheet a Press Cannot Print

On a sheet-fed press the paper is not placed on the cylinder by hand. A row of small metal fingers closes on the leading edge and drags the sheet through each unit. Those fingers need somewhere to hold. The strip they occupy, usually 8 to 12 millimetres along the leading edge, can never carry an image, and every layout has to allow for it.

The margin is set in the machine, not in the file. A large offset press running a 70 by 100 centimetre sheet may need 10 millimetres of gripper, while a half-format machine might take 8. The exact figure depends on the press, so a file sent without allowance will either be scaled down or run with a white strip where the design expected colour.

Paper Moisture: Why a Sheet Curls After It Leaves the Press

Paper is mostly air and fibre, and the fibre keeps trading water with the room. A sheet that sat in a cold warehouse overnight and then meets a warm pressroom will move before the first impression. Most of the curl and wave problems blamed on the press start in that exchange.

Moisture content and relative humidity

Wood fibre holds water in the cell walls. At 50 percent relative humidity and 23 C, most printing papers settle around 6 to 8 percent moisture content. Change the room and the sheet follows, slowly. A pallet in a wrapper holds its own climate until the wrapper comes off.

The Back Gauge: How a Guillotine Makes the Same Cut Twice

A guillotine makes one straight cut at a time, and every copy in the run has to end up the same size. That repeatability comes from the back gauge, the plate that pushes the pile forward and holds it against the knife. Move it a fraction of a millimetre and the whole run drifts.

What the back gauge does

The back gauge sits behind the pile on a threaded lead screw. The operator dials a measurement, the screw turns, and the plate advances or retreats. On older machines the position is read from a mechanical counter. On newer ones a servo motor drives the screw and the counter is digital, often with a memory that stores a cut sequence.