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.

Deinking: How Used Paper Comes Back as Clean Fibre

A used sheet of newspaper or a printed office page can be turned back into pulp. The process that removes ink from that pulp is called deinking, and it is the reason recycled fibre can be used for anything that has to look clean.

Why ink has to come off

Paper fibre can be reused several times, but the ink sitting on it does not disappear when the sheet is repulped. Left in the pulp, the ink breaks into fine particles that tint the new sheet grey or speckle it with dark flecks. For newsprint a little grey is tolerable. For a white carton or a printed book it is not, so the ink has to be separated from the fibre before the pulp is made into new paper.

Dot Gain: Why a Printed Dot Grows

A halftone image is built from thousands of tiny dots. The file decides how big each dot should be, and the plate carries that size to the press. On the sheet the dot is almost never the size that was drawn. It grows, and the growth is called dot gain.

Where the gain comes from

Ink is a paste. When it is pressed onto paper it spreads a little under pressure, and it soaks into the fibres of an uncoated stock. A 50 percent tint, which should cover half the area, can measure 55 or 65 percent on the sheet. The dots also pick up a small halo from light scattering inside the paper, which makes the edge of each dot look larger to a densitometer and to the eye.

Paper Conditioning: Why Stock Rests Before It Prints

Paper arrives from the mill in a sealed wrapper, and the first thing a careful print shop does is let it rest. The reason is moisture. Paper takes water from the air and gives it back until it reaches the same moisture content as the room around it. Depending on the stock, that is usually between 4 and 6 percent by weight. A sheet that has not settled will move after it is printed, and the movement shows up as misregistration, wavy edges or a sheet that will not feed flat.

Why the mill wraps the pallet

The wrapper is not only packaging. A mill trims and packs paper at a controlled moisture content, so the pallet reaches the printer in a known state. Cut the wrap in a cold warehouse and let the air in, and the edges of the pile start to swell or dry before the centre does. The result is a pile whose edges are a different size from its middle, which is exactly the condition that makes a sheet cockle.

Kiss Cutting: When a Cut Goes Through the Label but Not the Liner

A label needs to be cut and still stay on its backing sheet until someone peels it off. Kiss cutting does exactly that. The die cuts through the printed face material and the adhesive, then stops at the release liner underneath. What comes off the press is a shaped label sitting flat on a continuous roll, ready for a dispenser or an applicator.

The name comes from the way the blade only touches the liner instead of slicing through it. In practice the die depth is set a few hundredths of a millimetre above the liner surface. Set it too shallow and the face does not separate cleanly. Set it too deep and the blade scores the liner, which then tears when the web is pulled through a printer or an applicator.