Steps of Laser Printing

A laser printer processes and prints an entire page at one time. The page comes to the printer encoded in a printer language and the firmware inside the printer processes the incoming data to produce a bitmap of the final page, which is stored in the printer's memory. One bitmap image is produced for monochrome images. For color images, one bitmap is produced for each of four colors. (The colors are blue, red, yellow and black, better known as cyan, magenta, yellow and black, and sometimes written as CMYK)

The drum is conditioned by a roller that places a high uniform electrical charge of -600 V on the surface of the drum. The roller is called the primary charging roller or primary corona, which is charged by a high-voltage power supply assembly. For some printers, a corona wire is used instead of the charging roller to charge the drum.

A laser beam controlled by motors and a mirror scans across the drum until it completes the correct number or passes. The laser beam is turned on and off continually as it makes a single pass down the length of the drum, once for each raster line, so that dots are exposed only where toner should go to print the image. For example, for a 1200 dots per inch (dpi) printer, the beam makes 1200 passes for every one inch of the drum circumference. For a 1200-dpi printer, 1200 dots are exposed or not exposed along the drum for every inch of linear pass.

The developing cylinder applies to toner to the surface of the drum. The toner is charged and sticks to the developing cylinder because of a magnet inside the cylinder. A control blade prevents too much toner from sticking to the cylinder surface. As the cylinder rotates very close to the drum, the toner is attracted to the part of the surface of the drum that has a -100 V charge and repelled from the -600 V part of the drum surface. The result is that toner sticks to the drum where the laser beam has hit and is repelled from the area where the laser beam has not hit.

In the transferring step a strong electrical charge draws the toner off the drum onto the paper. This is the first step that takes place outside the cartridge and the first step that involves the paper. The soft, black transfer roller puts a positive charge on the paper to pull the toner from the drum onto the paper. Then the static charge eliminator weakens the charges on both the paper and the drum so that the paper does not stick to the drum. The stiffness of the paper and the small radius of the drum also help the paper move away from the drum and toward the fusing assembly. Very thin paper can wrap around the drum, which is why printer manually usually instruct you to use only paper designated for laser printers.

The fuser assembly uses heat and pressure to fuse the toner to the paper. Up to this point, the toner is merely sitting on the paper. The fusing rollers apply heat to the paper, which causes the toner to melt, and the rollers apply pressure to bond the melted toner into the paper. The temperature of the rollers is monitored by the printer. If the temperature exceeds an allowed maximum value (410 degrees F) the printer shuts down.

A sweeper strip cleans the drums of any residual toner, which is swept away by a sweeping blade. The charge left on the drum is then neutralized. Some printers use erase lamps in the top cover of the printer for this purpose. The lamps use red light so as not to damage the photosensitive drum.