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.

Jogging and Aeration: How a Pile Is Made Ready to Feed

A guillotine cut pile and a press-ready pile look the same from a metre away. They are not. The press-ready pile has been jogged until the edges are square and aerated until air sits between the sheets. Skip those two steps and the feeder will pull doubles, miss sheets, or stop every few hundred impressions.

Why a pile needs air

Paper arrives from the mill under pressure. A ream of coated stock that has sat in a wrapper for a month is closer to a block than a stack. Sheets pressed together hold each other by friction, by static, and by the thin film of moisture at the surface.

Anilox Rolls: The Engraving That Meters Flexo Ink

A flexo press does not meter ink with a duct and a train of rollers the way an offset press does. It uses one roll with hundreds of thousands of tiny cells cut into its surface, and the size of those cells decides how much ink reaches the plate. That roll is the anilox.

What the roll is

An anilox starts as a steel or aluminium cylinder. The surface is coated with ceramic, usually chromium oxide, and then polished. The cells are burned into the ceramic by a laser, one pass per cell pattern, and the finished roll is polished again to remove the ridge left around each cell.

Screen Printing: How Mesh Count Sets the Ink Film

Screen printing pushes ink through a woven mesh with a rubber blade. Everything that decides how much ink lands on the sheet is set before the job starts: the thread count of the mesh, the thickness of the emulsion on it, and the hardness of the squeegee. Change the mesh and the same artwork prints as a thin tint or as a raised patch of colour.

The stencil

A screen starts as a frame stretched with woven polyester or steel mesh. The mesh is coated with a light-sensitive emulsion, dried, and exposed through a film positive. The exposed emulsion hardens; the unexposed area washes out and leaves an open window in the shape of the image. Ink can only pass where the mesh is open.

Three-Knife Trimming: How a Book Block Gets Its Final Size

A bound book is not the size of the sheets that went into it. After folding, gathering and binding, the block is still oversized on three sides, and the edges are rough: loops of folded paper at the fore edge, signatures that have jogged a little at the head, and a tail that has crept along the spine. The machine that makes every copy the same size is a three-knife trimmer.

Why the cut comes after binding

A trim before folding would not survive the process. Each fold takes up a fraction of a millimetre. Gathering compresses the signatures. Glue adds thickness at the spine. In a saddle-stitched book the inner pages push outward, and in a perfect-bound book the block is clamped and milled before the cover goes on. Every one of these steps moves the paper relative to the trim marks that were printed on the flat sheet.

Punch Register: How a Punched Plate Holds Its Position

Every plate that goes on a press has to sit in a known position. On a four colour job, four plates have to sit in the same position relative to each other, or the image will not line up. The tool that makes this repeatable is a punch and a set of pins.

The idea

A punch makes holes in the plate along one edge. The plate cylinder carries pins at the same spacing. The operator hangs the plate on the pins, wraps it around the cylinder, and clamps the tail. The holes decide where the plate sits along the cylinder, and the pins decide where it sits across.