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.

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.