How does a laser printer work?


Jake Port explains what's going on as the office printer whirrs into life.


dem10 / Getty Images

You can find a laser printer in almost every office. From the outside, it whirs away, churning out warm, freshly printed pages.

Inside, though, is a lightshow combining laser beams and electrical currents working perfectly together to turn million of bytes worth of information into an image of anything from a diagram of a cell dividing to a fluffy cat. But how does this all happen?

You can thank static electricity.

It all starts when the user presses the print button on their computer, sending a couple million pieces (bytes) of information to a small chip in the printer.

This mini brain converts the data into a two-dimensional image ready to be printed.

Two components are then simultaneously activated that begin the process of turning a digital image to a hard copy.

A large drum, positively charged by a wire known as a corona wire, begins to spin.

This drum, known as a photoreceptor drum, will be the main interface between the ink and the paper. It spins at a speed in time with the movement of the paper.

At the same time, a laser begins to fire at a mirror that reflects the light across the drum millions of times a second.

When the laser beam makes contact with the drum, that precise spot has its positive charge removed, making it negatively charged. This attracts positively charged toner, a fine black or coloured powder.

By passing the laser over the drum again and again, a negatively charged image begins to form.

A roller then applies toner to the drum to a thickness of around 15 microns or about a thousandth of a centimetre.

Toner only “sticks” to the drum where the laser has converted the charge to negative.

Think of it as a rolling pin that has an image traced in glue on it. If the pin rolls over something like flour, it will only stick to where glue has been applied.

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Now comes the time to put that toner onto paper.

As a feeder draws paper from the paper tray, it is given a strong negative charge by another corona wire inside a small drum. This makes the paper a lot more attractive to the toner than the photoreceptor drum, causing it to jump across to the paper.

The paper moves along, attracting the toner, with the full image eventually being transferred.

At this point, you would think the process is complete, but there is still one more important step.

Toner does not dry like ink and therefore is currently only stuck to the paper by the electrostatic charge. So the paper is then passed between two rollers known as the fuser unit, rapidly heating the surface to 200 °C and melting the toner onto the page. This is why paper is hot when it’s fresh out of the printer.

When the print is done, the photoreceptor drum is reset with a new positive charge, the leftover toner is cleaned off and the whole process begins again.

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