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.

Blade Coating: How Paper Gets Its Smooth Surface

Open a ream of coated paper and the surface looks like a solid white sheet. Under a microscope it is a layer of pigment sitting on top of a web of fibres, and that layer was applied as a wet slurry and then scraped to thickness by a steel blade.

Why coat paper at all

An uncoated sheet has fibres, pits and gaps. Ink sinks into those gaps, and a halftone dot spreads unevenly. A pigment coating fills the low points and covers the fibres, so the surface holds ink closer to where it was placed. The result is higher gloss, better opacity, finer dot reproduction and a more even solid.

Aluminium Plate Graining: Why the Surface Is Roughened

Every offset plate starts as a sheet of aluminium, and the surface of that sheet is not smooth. It is roughened before anything is coated on it, and that roughness is what lets the plate hold water where no ink should go.

Two jobs on one surface

A litho plate has to do two opposite things at once. The image area has to take ink and shed water. The non image area has to hold a film of water and refuse ink. Aluminium is naturally hydrophilic, so it prefers water, but a bare polished sheet holds that water in beads rather than an even film. A grain gives the water somewhere to sit.

Plate Scumming: When the Non-Image Area Takes Ink

On an offset plate, ink belongs on the image and water keeps the rest clean. Scumming is the loss of that division: a thin tint of ink creeps onto the area that should print nothing at all.

Two surfaces on one plate

A litho plate carries two kinds of surface side by side. The image area is oleophilic, so it takes ink and turns water away. The non-image area is the opposite. It holds a film of fountain solution and refuses ink. The press keeps the two apart by feeding water and ink at the same time and by the small chemical difference between them.