A laser printer is a high-quality, fast printer that uses a laser beam to create an image. The central part of the laser printer is its imaging drum. The drum is a metal cylinder that is coated with a light-sensitive insulating material. When a beam of laser light strikes the drum, it becomes a conductor at the point where the light hits it.
As the drum rotates, the laser beam draws an electrostatic image upon the drum. The undeveloped or latent image is passed by a supply of dry ink or toner that is attracted to it. While the image is being exposed on the drum, an individual sheet of paper has been pressed between a separation pad and pickup roller and is fed toward the drum.
The drum turns and brings the exposed image in contact with the paper, which attracts the ink from the drum. The paper is then passed through a fuser assembly that is made up of hot rollers, which melts the toner into the paper.
The laser printer process involves seven steps to print information onto a single sheet of paper.
1. Processing - The data from the source must be converted into a printable form. The printer converts data from common languages, such as Adobe PostScript (PS) or HP Printer Command Language (PCL), to a bitmap image stored in the printer’s memory. Some laser printers have built in Graphical Device Interface (GDI) support. GDI is used by Windows applications to display printed images on a monitor so there is no need to convert the output to another format such as PostScript or PCL.
2. Charging - The previous latent image on the drum is removed and the drum is conditioned for the new latent image. A wire, grid, or roller receives a charge of approximately -600 volts DC uniformly across the surface of the drum. The charged wire or grid is called the primary corona. The roller is called a conditioning roller.
3. Exposing - To write the image, the photosensitive drum is exposed with the laser beam. Every portion of the drum that is scanned with 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.
4. Developing - The toner is applied to the latent image on the drum. The toner is a negatively charged combination of plastic and metal particles. A control blade holds the toner at a microscopic distance from the drum. The toner then moves from the control blade to the more positively charged latent image on the drum.
5. Transferring - The toner attached to the latent image is transferred to the paper. A corona wire places a positive charge on the paper. Because the drum was charged negatively, the toner on the drum is attracted to the paper. The image is now on the paper and is held in place by the positive charge. Because color printers have three cartridges of ink, a colored image must go through multiple transfers to be complete. To ensure precise images, some color printers write multiple times onto a transfer belt that transfers the complete image to paper.
6. Fusing - The toner is permanently fused to the paper. The printing paper is rolled between a heated roller and a pressure roller. As the paper moves through the rollers, the loose toner is melted and fused with the fibers in the paper. The paper is then moved to the output tray as a printed page. Laser printers with duplex assemblies can print on both sides of a sheet of paper.
7. Cleaning - When an image has been deposited on the paper and the drum has separated from the paper, the remaining toner must be removed from the drum. A printer might have a blade that scrapes the excess toner. Some 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 is either emptied or discarded.
These are some advantages of a laser printer:
- Low cost per page
- High ppm
- High capacity
- Prints are dry
These are some disadvantages of a laser printer:
- High cost of start up
- Toner cartridges are expensive
- Require a high level of maintenance