How Does a Computer Printer Work?

Your screen makes a picture out of light. Paper makes no light of its own, so a printer has to do something completely different: it has to put a physical substance onto the page, in exactly the right places, without smudging it.

There are two common ways to do that, and they have almost nothing in common.

The Parts

• Cartridges — hold the ink (a liquid) or the toner (a powder).
• Print head — the part that travels across the paper putting ink down.
• Paper tray — where blank paper waits.
• Feeder and rollers — pull one sheet in and move it through.
• A cable or wireless link — how the printer hears from your computer.

Inkjet: Spraying Tiny Drops

An inkjet printer squirts liquid ink through nozzles far too small to see.

That is why a printer sounds like it is going back and forth — it is. A page is built up in narrow strips, and the paper creeping forward between strips is the quiet clicking you can hear between the louder sweeps.

Laser: Melting Powder On

A laser printer holds no liquid at all. It uses toner, a very fine plastic powder, and it prints a whole page at once instead of in strips.

A laser draws the page onto a spinning metal drum as a pattern of electric charge. Toner powder sticks to the charged parts and nowhere else. The drum rolls the powder onto the paper — where it is still only sitting loose, like dust. Then the paper passes through the fuser, which is hot, and the powder melts and sets into the fibers.

Have a Look for Yourself

Find a printed page and look at it very closely — a magnifying glass helps, and so does a phone camera zoomed all the way in. Solid-looking color is not solid at all. It is thousands of separate dots, and where two colors overlap you can often see them sitting side by side rather than mixed.

You cannot take a printer apart, but you can usually spot the paper tray, the rollers and the cartridges just by opening the lid. Ask first.