As in photocopiers, static electricity is the primary principle that is used in the operation of a laser printer. This is the same static electricity that causes lightning or other oppositely charged particles to attract each other. This attraction is used to temporally hold small dry ink particles called toner to a statically charged image on an electrophotographic drum. A laser beam is used to draw this image.

The central part of the laser printer is its electrophotographic drum. The drum is a metal cylinder that is coated with a light sensitive insulating material. The drum is precharged by the corona wire. When a beam of laser light strikes the drum , it becomes a conductor and discharges at the point where the light hits it. As the drum rotates, the laser beam draws an electrostatic image upon the drum called the image. The latent image, that is the undeveloped image, is passed by a supply of dry ink or toner that is attracted to it. The drum turns and brings this image in contact with the paper, which attracts the toner from the drum. The paper is passed through a fuser that is made up of hot rollers which melts the toner into the paper.

Caution: The photosensitive drum should never be exposed to light for long periods of time.

This laser printing process is broken into six steps:

1. Cleaning – When an image has been deposited on the paper and the drum has separated from the paper, any remaining toner must be removed from the drum. One method uses a blade to scrape all excess toner from the drum. Other printers use an AC voltage on a wire that removes the charge from the drum surface and allows the excess toner to fall away from the drum. The excess toner is stored in a used toner container that may be emptied or discarded.
2. Conditioning – This step involves removing the old latent image from the drum and clearing or conditioning the drum for a new latent image. Placing a special wire, grid, or roller that is charged to about -6000 volts DC uniformly across the surface of the drum accomplishes this. This charged wire or grid is referred to as the primary corona. Some printers provide this charge by using a conditioning roller. The charge impressed upon the surface of the drum is -600 to -1000 Volts DC.

   Caution: The voltage device that is used to erase the drum, called the primary corona wire or grid, or the conditioning roller, is dangerous. This voltage runs as high as -6000 volts. Take proper care when servicing this unit and make sure that it is properly discharged.

3. Writing – This process involves scanning the photosensitive drum with the laser beam. Every portion of the drum that is exposed to the light has the surface charge reduced to about -100 Volts DC. This electrical charge has a lower negative charge than the remainder of the drum. As the drum turns, an invisible latent image is created on the drum. Figure illustrates the various voltage transitions involved with creating a printed page in the laser printing process.
4. Developing –This is the step where the toner is applied to the latent image. Inside the toner unit is developer particles made up of magnetic materials. These magnetic particles are coated with a plastic-like material. A turboelectric charge on the developer particles causes it to attract toner. The toner as, a result, is charged to around -200 volts DC, which causes it to be attracted to the more positive (-100 volts DC) areas of the photosensitive drum, but repelled by the more negative (-600 volts DC) areas. Typically a cylinder within the toner unit releases toner so it can fall against a control blade or developer blade, as the developer rollers travel. Also known as the restricting blade, the control blade keeps toner from pouring onto the drum. Instead, it holds the toner at a microscopic distance from the drum, and toner leaps from it to the drum where it is attracted by the more positively charged latent image.
5. Transferring – In this step, the toner attached to the latent image is transferred to the paper. The transfer, or secondary corona places a positive charge on the paper (remember that drum was charged negatively) so that it attracts the negative toner image from the drum to the paper. The image is now on the paper and is held in place by the positive charge.
6. Fusing – The toner particles on the paper are only there because of the charge that is present. They are kept in place permanently by the "fusing process". In this process, the printing paper is rolled between a heated roller and a pressure roller. As the paper rolls, the top fuser roller is heated to about 350 degrees. This melts the plastic, which is mixed with the carbon black to make toner, into the paper. This operation is called fixing by some manufacturers. After the fusing operation is complete, the paper is moved to the output tray, as a printed page.

Caution: When working around the fuser, remember that it is hot enough to burn the skin. Always allow it to cool before removing it or servicing it.