How do Laser Printers Work?


Laser printers have become ubiquitous in the office and copying environment. The first laser printer was developed in 1971 by Xerox, and in 1977 it was brought to the commercial market. The rapid, crisp printing soon made laser printers a popular choice for many consumers, although the internal workings of the printer remained mysterious to many. Because of the name, some consumers think that laser printers use a laser actively on the paper in some way. In fact, laser printers actually harness static electricity to print, although lasers do play a role in the printing process.

Laser printers begin a print job by receiving data from a computer and routing it through a central controller, a small computer inside the printer which manages the printer. Many laser printers have a controller which is capable of handling several jobs at once, enqueuing them, and then printing them. This ability to handle multiple sets of data makes laser printers quite popular. After the controller has determined what is going to be printed, the process begins.

Inside a laser printer there is a drum which holds an electric charge. Next to the drum is a transfer corona roller, which can negatively or positively charge the drum as needed, as well as a toner unit. In most laser printers, the drum starts out positively charged, although this process can also work in reverse. The controller manipulates a small laser to “write” on the drum with a negative charge, creating an electrostatic image.

Then, the drum is rolled through the toner, which is positively charged so that it will cling to the areas of negative charge on the printer drum. The printer feeds a piece of paper, which is given an even stronger negative charge by the transfer corona wire before being rolled past the drum. The electrostatic image on the drum will transfer to the paper, which is then discharged to prevent it from clinging to the drum. Then it is fed through a fuser which heats the toner and causes it to bind with the fibres in the paper.

Meanwhile, the drum passes a discharge lamp, which will expose the entire surface of the drum and erase the electrostatic image. The transfer corona wire applies another positive charge, and the printer is ready for the next page or job.

Colour laser printers work by performing multiple passes. Most printers have blue, red, and yellow ink, in addition to black, which can be combined to form any colour. Some printers progressively lay the ink onto the drum so that the image will print with one pass of the paper, while others recirculate the paper multiple times to apply progressive layers of colour. Large colour printers sometimes have separate drum and toner assemblies for each colour, with the paper passing each drum separately.