Laser printers: printing with static electricity

Rub a balloon on your jumper and it sticks to the wall. A laser printer is that trick, industrialised.

Toner is a fine dry powder (plastic plus pigment), not ink. The whole printer is a game of opposite charges attract and like charges repel, finished off with heat.

"Describe how a laser printer prints a page." Usually 4 to 6 marks.

There's one mark per correct stage, so the marks come from naming each stage in the right order with the right key word: drum, charge, laser, toner, paper, fuser, discharge.

1. The data for the whole page is sent to the printer and stored in its memory (buffer).
2. The printing drum is given a positive static charge as it turns.
3. A laser beam, steered by a rotating mirror, scans across the drum and removes the charge where the text and images will go. This leaves an electrostatic image of the page.
4. The drum is coated with positively charged toner. It is repelled by the charged areas and sticks only to the discharged areas.
5. The paper is given a negative charge (opposite to the toner) and rolls over the drum, so the toner is pulled off the drum onto the paper.
6. The paper's charge is removed so it doesn't stick to the drum, then it passes through the fuser: heated rollers melt the toner and press it permanently onto the paper.
7. A discharge lamp removes the remaining charge from the drum, ready for the next page.

Step 5 is the one people skip. Without the charged paper there's no reason for the toner to leave the drum.