Printers, Laser Printers, How A Laser Printer Works,Developing

1. Data from the PC is received by the formatter.

2. Data is then passed on to the DC controller that controls the laser unit.

3. The laser beam is initiated and directed toward the octagonal mirror called the

scanning mirror. The scanning mirror is turned clockwise by the scanning motor.

There are eight mirrors on the eight sides of the scanning mirror. As the mirror turns, the laser beam is directed in a sweeping motion that can cover the entire length of the drum.

4. The laser beam is reflected off the scanning mirror, focused by the focusing lens , and sent on to the mirror.

5. The mirror deflects the laser beam to a slit in the removable cartridge and on to the drum.

The speed of the drum motor and the speed of the scanning motor are synchronized so that the laser beam completes one pass, or scan line, across the drum and returns to the beginning of the drum (right side of the drum in the above figure) to begin a new pass.

For a 300 dots per inch (dpi) printer, the beam makes 300 passes for every one inch of the drum circumference. The laser beam is turned on and off continually as it makes a single pass down the length of the drum so that dots are written along the drum on every pass.

For a 300 dpi printer, 300 dots will be written along the drum for every inch of linear pass. The 300 dots per inch along each pass and the 300 passes per inch of drum circumference result in a resolution of 300 x 300 dots per square inch.

Where the laser beam strikes the surface of the drum, the drum discharges from its conditioned charge of -600 V down to -100 V. Toner will not stick to the highly charged areas of the drum.

To synchronize the output of data to the scan line, the beam detect mirror detects the initial presence of the laser beam by reflecting the beam to an optical fiber. The light travels along the optical fiber to the DC Controller, where it is converted to an electrical signal that is used to synchronize the data output. The signal is also used to diagnose problems with the laser or scanning motor.

When the writing process is complete, the area on the drum surface that will become an image during the printing process contains a -100V charge that will be used in the developing stage to transmit toner to the drum surface.

Step 4: Developing

The following details shows the developing step, in which toner is applied to the discharged areas of the drum by the developing cylinder.

1. Toner transfers from the cylinder to the drum as the two rotate very close together.

2. The cylinder is coated with a layer of toner, made of black resin bonded to iron. The toner is held on the cylinder surface by its attraction to a magnet inside the cylinder.

(A toner cavity keeps the cylinder supplied with toner.) A control blade prevents too much toner from sticking to the cylinder surface.

3. The toner on the cylinder surface takes on a negative charge (between –200 V and – 500 V) because the surface is connected to a DC power supply called the DC bias.

4. The negatively charged toner is less negative than the -100V on the drum surface but more negative than the -600V surface. Because of this fact, the toner is attracted to the less negative drum surface just as though the surface were positive.

5. The toner is repelled from the drum surface that is more negative than it is. 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 did not hit.

6. When you adjust the print density control of the laser printer, the DC bias charge is reduced or increased. When more charge is applied, more toner is attracted to the cylinder, which results in a denser print. Less charge results in a less dense print.