Rounding and Backing: How a Book Block Gets Its Spine Curve

A hardcover starts as a stack of folded and sewn sections. Before the case goes on, that block is shaped at the spine, first rounded and then backed. Rounding and backing are two steps that turn a flat block into one that opens properly and holds its form for years.

Why the spine is rounded

A sewn block trimmed square has a flat spine. In a thick book the swell from the sewing thread and the folded sections makes the spine thicker than the fore edge, and a flat spine fights the boards every time the book is opened. Rounding presses the block between rollers or under a hammering machine and curves the spine into a shallow convex arc. The arc gives the block a shape that rolls open instead of snapping, and it spreads the swell so the boards can lie flat.

Cylinder Packing: Why a Thin Underlay Changes Print Length

The gap between the blanket cylinder and the impression cylinder is set by the packing under each one. Packing is the sheet of paper, film or board laid under the plate and under the blanket, and its thickness decides how hard the two cylinders press together. Change the packing by a few hundredths of a millimetre and the printed image itself changes length.

What the packing does

On an offset press the plate cylinder carries the image, the blanket cylinder transfers it, and the impression cylinder presses the sheet against the blanket. Each cylinder has an underlay, or packing, that brings its surface to the right height. The plate packing sets the contact between plate and blanket. The blanket packing sets the contact between blanket and paper, and this is the one that rules how much the image is squeezed onto the sheet.

Embossing and Debossing: Raising and Sinking a Printed Surface

An embossed area stands above the sheet. A debossed area sits below it. Both are made without ink, by pressing the paper between a matched pair of dies, and both give a surface the reader can feel as well as see. They show up on covers, cartons, certificates and cards.

How the pair works

An emboss uses a male die that carries the raised shape and a female die, often called the counter, that matches it. The two are mounted in a press and the sheet is squeezed between them, hot or cold. The fibre of the paper stretches and sets into the shape. The result is a permanent relief rather than a coating, so it cannot rub off.

Stochastic Screening: When a Halftone Leaves the Grid

Conventional halftone screening places every dot on a fixed grid, and varies the size of the dot to build tone. Stochastic screening keeps the dot size fixed and small, then varies the number of dots by scattering them at random. The change sounds small. It alters how a printed image is built and how the press has to run.

How the two screens differ

An amplitude modulated screen, the classic halftone, uses a grid of cells at a set angle for each colour. Light tones get small dots inside their cells, dark tones get large ones that touch and merge. The eye blends the pattern. A frequency modulated screen, the stochastic method, drops the grid. The dot is a fixed micro-dot, and a light tone simply has fewer of them. The screen angle problem disappears, because there are no rows left to interfere.

Letterpress: A Raised Surface That Presses Ink Into Paper

Letterpress is the oldest of the mechanical print methods still in daily use. The image sits above the surface of the plate, and the plate is pressed directly into the sheet. Ink transfers from the raised lines, and the paper is left slightly indented along every stroke. That indentation is not a fault. It is the signature of the process.

A raised image, not a flat one

On an offset plate the printing area sits level with the non-printing area, and the two are separated by water and chemistry. On a letterpress plate the printing area stands proud of the base. Only the tops of those raised forms touch the paper, so the ink lands where the form meets the sheet and nowhere else.

Flexography: How a Rubber Plate Prints a Web

Flexography is the method behind much of the packaging a person handles in a week. Corrugated boxes, labels, flexible film, cartons and paper bags are all likely to be flexo work. It prints from a raised image on a soft plate, and it runs on a roll of material rather than a stack of sheets. That pairing is what makes it quick and forgiving on rough surfaces.

A raised image on a flexible plate

The image sits above the plate surface, the way a rubber stamp sits above its base. Ink is metered onto the raised areas by a roller, and the plate is pressed against the moving web. The soft plate deforms slightly under pressure, which lets it reach into the texture of a board that a hard metal plate would miss.

Overs and Shorts: Why a Print Run Arrives as a Range

A print order is usually written as a single number. Five thousand copies, say. When the pallets arrive, the count is rarely exactly that. It is a little over or a little under, and that is normal in the trade.

Why the count moves

A press does not stop the moment the last good sheet comes off. It runs until the pile is complete, and then a few more sheets are pulled for the finishing line. Some of those sheets are lost to trimming, to a jam, to a spot of dirt on the blanket, or to a fault found during inspection. The printer plans for that loss and sets the press to run a little long.