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.

UCR and GCR: When Black Replaces Three Inks

A four colour press lays down cyan, magenta, yellow and black, and a neutral grey would need all four. Under colour removal changes that. Part of the grey the three colours were carrying is taken out and handed to black, so the same area prints with less ink and more control.

What the three colours were doing

In a grey built from cyan, magenta and yellow, the three inks overlap almost completely. Each adds density the eye reads as darkness, yet the sum of three wet films is hard to hold steady on press. A small shift in any one of them turns the grey warm or cool. Black is a single ink, and one ink is easier to keep in line than three.

RIP: How a Page Becomes Dots on a Plate

A page arrives as text, shapes and images, and it leaves the prepress room as a set of instructions a platesetter can expose. The step in between is the RIP, short for raster image processor. It is the software that turns the page into a bitmap the output device can handle.

What the RIP does

An output device cannot draw a curve or scale a font. A RIP reads the page description, whether it comes as PostScript or PDF, and converts every element into a grid of tiny cells. Text is rendered at the machine resolution, images are resampled to the right size and colour space, and vectors are drawn as filled shapes. The result is a raster, one large bitmap of the page.