Xerox 3450B Service Manual - Page 35

Overview of the Phaser 3450 Laser Printer Summary of the Printing Process The Phaser 3450 Laser Printer is a desktop monochrome laser printer, applying the principals of an electrophotographic system. The system, comprising a drum and developing unit, places the toner image onto print media producing a monochrome image. The following procedures are summarized. The printing process is composed of the following steps. See the illustration on the following page as a reference. 1. Charging: The charge roller is negatively charged at approximately -1400 VDC by the high voltage power supply (HVPS). The charge roller is kept in contact with the drum surface to provide a uniform negative charge of approximately 800 VDC on the drum surface as it rotates at a constant speed. 2. Exposure: The laser unit emits laser beams in response to image data from the Main board. The laser beams are directed onto the drum surface through a system of mirrors and lenses. A rotating polygonal mirror causes the laser beams to scan the drum surface from end to end (axially) as it rotates. The beams are turned on to print a pixel and off when no printing is required. The negative charge on the drum surface is reduced to approximately -250 VDC at each point where the energized laser beam strikes, to form an invisible electrostatic latent image on the drum surface. 3. Development: Negatively charged toner particles from the toner hopper are applied to the supply roller and are then applied to the developer roller in a even layer controlled by the metering blade. The developer roller turns against the drum and the toner particles are attracted to the relatively positive latent image. The toner forms a visible image on the drum surface. 4. Pre-Clean: The pre-transfer lamp exposes the developed suface of the drum lowering the surface potential and thereby providing enhanced transfer efficiency. 5. Transfer: The finished toner image on the drum is transferred onto the print media using the voltage supplied by the transfer roller. The conductive transfer roller receives a high positive voltage (approximately +1000 VDC) from the HVPS that puts it at a higher potential than the drum. Since the transfer roller is located behind the print media, the toner image is attracted to the high potential and deposits on the surface of the print media. 6. Fixing: The finished toner image is impermanent and easily smeared. To fix the image, the print media goes through the Fuser where it passes between a pressure roller and the heat roller. The toner is fused onto the print media by the combination of heat and pressure. The heat roller is heated by a Halogen lamp. The roller surface temperature is detected by a Thermistor. The information is fed back to the lamp control to maintain a surface temperature of 185º C during printing and 145º C during standby. If the thermistor detects a Fuser overheat condition, it disconnects AC power to the Fuser. 7. Cleaning: After the image is transferred to the print media, a cleaning blade inside the cartridge removes any remaining toner particles from the drum. 2-2 Phaser 3450 Laser Printer Service Manual