How does a printer work?

Every printer does the same job: it turns a page on your screen into millions of tiny dots on paper. The two common kinds do it in very different ways.

An inkjet (left) pulls one sheet from the tray with a rubber pick roller. Feed rollers step the sheet forward a strip at a time over a flat platen. Above it, a carriage holding the ink cartridges zooms left and right on a steel rail, pulled by a belt and a small motor. Under the carriage is the printhead, a chip with hundreds of nozzles thinner than a hair.

A laser printer (right) has no nozzles. A green drum is given a coating of static charge, a laser bouncing off a spinning mirror draws the page on it, and fine plastic powder called toner sticks where the laser hit. The powder is rolled onto the paper and a hot fuser melts it in. That's why pages come out warm.

Under the carriage is the printhead, a silicon chip with a row of tiny nozzles. An ink-tank printer's head can have 1,792 of them. Each nozzle sits over its own little chamber of ink.

In a thermal inkjet, the chamber has a tiny heater in it. A pulse of current lasting about 3 microseconds heats a thin skin of ink to around 300 °C. It boils instantly into a vapour bubble that shoves a drop out of the nozzle at about 10 m/s. The bubble collapses, and fresh ink is sucked back in from behind. A nozzle can do this more than 10,000 times a second.

A piezo head, used by Epson, has no heater. A crystal that bends when you put a voltage across it squeezes the chamber instead, like pressing a dropper.

The drops are measured in picolitres, trillionths of a litre. A 5 pL drop is about 21 µm across, smaller than a hair is thick. On paper it spreads into a dot about twice as wide.

A printer can't print "a bit of cyan". Each spot on the page either gets a dot of ink or stays white. So how does it print a pale sky or a soft shadow?

It cheats your eye. Where the picture is light, it prints few, small or spread-out dots. Where it is dark, it prints many. From reading distance your eye blends them into a smooth shade. This trick is called halftoning. Newspapers use neat rows of dots that grow and shrink (an AM halftone). Most inkjets scatter dots in a clever random-looking way called dithering.

Colour works by taking light away. White paper reflects red, green and blue. Cyan ink soaks up red, magenta soaks up green, yellow soaks up blue. Overlap them and less light gets back, so colours get darker. This is subtractive mixing. A TV screen does the opposite: it adds red, green and blue light (see TVClear).

In theory C + M + Y makes black. Real inks give a muddy brown and waste three times the ink, and text looks fuzzy. So printers add a fourth ink, K for "key", black. That's CMYK.

The heart of a laser printer is the drum, a tube with a special coating called a photoconductor. In the dark it holds static charge like a balloon rubbed on your hair. Where light shines on it, the charge leaks away.

1. Charge. A roller coats the drum with negative charge, about −700 volts. 2. Expose. A laser draws the page, one thin line at a time. A spinning mirror with six faces sweeps the beam across the drum, thousands of lines a second. Every spot the beam hits loses its charge. Now the page exists as an invisible picture of charge.

3. Develop. Toner, a plastic powder a few micrometres across, is also given a negative charge. It is pushed away from the charged drum but lands on the spots the laser drew. 4. Transfer. A roller under the paper pulls the toner off the drum onto the page. 5. Fuse. Hot rollers at about 150 to 200 °C melt the toner into the paper. 6. Clean. A blade and a lamp wipe the drum for its next turn.

The same trick, xerography, was invented in 1938 for photocopiers. A photocopier uses a lamp and a photo of your page instead of a laser.

Dot matrix. A print head with a column of 9 or 24 steel pins, each fired by a tiny electromagnet, slams through an inked ribbon onto the paper. Letters are built from a grid of dots, 5 across and 7 down here. Because it strikes hard, it can print through carbon copies and onto thick bank passbooks. That's why Indian banks still use passbook printers, and railway counters have long printed tickets on them.

Thermal. Shop receipts use no ink at all. The paper is coated with a colourless dye and an acid developer. A row of tiny heaters, 8 per millimetre, warms dots of the coating until they melt together and turn black. It's cheap and quiet, but receipts fade in sunlight and heat.

3D printing. A common home 3D printer (FDM) pushes a plastic thread through a hot nozzle at about 200 °C. It draws one thin layer, about 0.2 mm thick, then moves up and draws the next on top, like building a cup from coils of clay.

Printer ink can cost more per millilitre than perfume. An 8.5 ml black cartridge listed at about ₹1,025 works out at around ₹1.2 lakh a litre. Why? Many companies sell cartridge printers cheaply and make their money on the ink, like selling a cheap razor with expensive blades. The cartridge also holds chips and sometimes the printhead itself.

Ink tank printers flip this: the printer costs more, but you refill it from bottles, and a black page costs around 10 paise instead of about ₹2. These printers took off in India and other Asian countries in the 2010s.

A laser printer's toner doesn't dry out, so it suits people who print lots of black text now and then. Refilled and compatible toner is common in India and much cheaper, though quality varies.

Caring for a printer. Print a page every week or so, or ink can dry in the nozzles. Fan paper before loading and don't overfill the tray, which prevents most paper jams. Pull a jammed sheet out slowly, in the direction it travels. Give empty toner and ink cartridges to the maker's recycling scheme or a refiller rather than the bin.