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.

Acid, Lignin and the Life of a Printed Page

Two copies of the same book, printed a century apart, can look decades different on the shelf. One keeps its paper white and its pages flexible, the other has gone yellow and brittle at the edges. The difference is chemistry, and it starts with the pulp.

Lignin is the problem

Wood is held together by lignin, the woody binder around the cellulose fibres. Chemical pulping, the kraft process used for most book and office paper, dissolves most of that lignin away and leaves long, strong cellulose fibres. Mechanical pulping, used for newsprint and cheap groundwood, keeps much more of the lignin because it grinds the wood rather than cooking it. Lignin is not stable. Over years it oxidises, and the paper yellows and loses strength. Newsprint is the familiar example: it turns brown within days in sunlight and embrittles far sooner than a book page.

GSM: What Paper Weight Actually Tells You

Grams per square metre sounds like a simple number. It is the mass of one square metre of a paper, and it is often the first figure on a spec sheet. What it does not tell you is how thick the sheet is, how stiff it feels, or how it will take ink. Those come from other measurements.

What GSM measures

GSM is grammage, the weight of a sheet normalised to one square metre. A 100 gsm paper weighs 100 grams per square metre, and a 300 gsm board weighs three times that for the same area. Because it is weight per area, GSM lets you compare different sheet sizes and different formats on the same scale. It is the number a merchant uses to price stock and the number a press operator uses to set a sheet through the feeder.

Coated and Uncoated: What the Surface Does to a Printed Sheet

Two sheets can weigh the same and print completely differently. The difference sits in the top layer. An uncoated sheet is paper through and through, while a coated sheet carries a thin mineral layer that changes how ink behaves the moment it touches down.

What the coating is

A coated paper is a base sheet with a layer of mineral pigment, usually clay or calcium carbonate, bound with a latex or starch adhesive. The coating is applied on the paper machine or in a separate pass, then calendered between rolls to control how smooth and glossy it ends up. One coating gives a matte or silk finish, and two or three coatings give the high gloss used on magazine covers and packaging. The mineral layer is thin, measured in microns per side, but it decides almost everything about how the finished job looks.