How do laser printers work?

Marcel Beaudoin
23 January 2012

It all starts with static electricity, which is the build-up of positive or negative electrical charge on a surface. If you have ever rubbed a balloon in your hair and stuck it on the wall; shuffled along a carpet in socks and "shocked" someone with your outstretched finger; or been struck by lightning, you have experienced static electricity. The attraction between these opposing electric charges is what sticks the balloon to the wall or causes “static shock”. Static electricity combined with lasers is what makes laser printers work.

Did You Know?
Static shock and lightning both represent the transfer of built up negative charges, which actually involves an electrical current. Thankfully, the current involved in static shock is many time smaller than that of lightning!

Once the printer gets a printing job, a roller in the center of the printer starts turning. While it is turning, the outside of the roller is given an overall positive electric charge. Rollers in laser printers are made with photoconductive materials, which are materials that are electrically excited or charged when they are exposed to light. The printer then uses a laser, which is just a very focused beam of light, to draw letters or shapes on the roller. As the laser draws on the surface of the roller, its photoconductive nature causes the drawings or letters to be made up of negative charge. Simply put, this would be similar to writing with a pen or marker that uses negative charges as ink.

Did You Know?
Photoconductive rollers are also used in Xerox or photocopying machines.

The surface of the roller with its positively charged background and negatively charged letters/drawings then passes in front of a toner cartridge. This cartridge contains very fine, positively charged powder that is attracted to negatively charged areas, like those just created by the laser. The end result is the roller having letters on it. The roller continues rolling and then meets the paper in the printer. The paper is given a stronger negative charge than the roller, so the powder leaves the roller and is transferred to the paper.

Two things then happen. First, the paper passes through heated rollers. The paper gets hot enough that the powder melts onto the paper, but passes through the rollers fast enough that it doesn't catch on fire. While this is happening, the roller is exposed to a bright light which "erases" the positive and negative electrical charges from the surface of the photoconductive roller. The process then starts over again.

Article first published on January 25, 2010.

Marcel Beaudoin

I am in my mid-30s, and currently work for the Government of Canada. I was, and still am, a geek! From the time I was very young, I was interested in math, science and the like. How the world worked. It fascinated me that there were these rules and laws that the universe obeyed.   In university, I got my PhD in chemistry, and spent a bunch of time visiting various schools doing chemistry demonstrations. All in all, a great time. I was lucky to have had the opportunity to meet a bunch of teachers that really encouraged my interest in science and my interest in the questions "Why does...?" "What happens when...?" They didn't always know the answers, but they would point me in the right direction.

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