HP LaserJet 4250 Service Manual - Page 169

Electrostatic latent-image formation Forming the electrostatic latent image on the photosensitive drum requires applying a uniform negative charge (bias) to the surface of the drum and then the exposing it to the laser beam. The laser beam neutralizes the section of the drum that will it attract toner during the developing operation. Primary charging The conditioning process consists of the primary charging roller applying a uniform negative charge to the surface of the drum. The primary charging roller is coated with conductive rubber. An ac bias is applied to erase any residual charges and maintain a constant drum-surface charge. The printdensity setting modifies the amount of dc voltage. Figure 5-37 Primary charging of the photosensitive drum Writing the image The laser/scanner contains two diodes in the laser unit. During the writing process, the modulated laser diodes project two beams onto the rotating six-sided scanning mirror. As the mirror rotates, the beams reflect off the mirror, through a set of focusing lenses, through a slot in the top of the toner cartridge, and onto the photosensitive drum. The beams sweep the drum from left to right, discharging the negative potential wherever the beams strike the surface. This creates a latent electrostatic image, which later is developed into a visible image. Laser beam Unexposed area Exposed area Figure 5-38 Writing the image to the photosensitive drum Because the beams are sweeping the entire length of the drum and the drum is rotating, the entire surface area of the drum can be covered. The speed of the scanner motor (which turns the scanning mirror) and the speed of the main motor (which turns the drum) are synchronized, and each successive sweep of a beam is offset by 1/1200th of an inch. The beams can be turned on and off to place a dot of light every 1/1200th of an inch. This is how the printer achieves its true 1200-by-1200dpi resolution. After the writing process, the drum surface holds an invisible (latent) electrostatic image. Image-formation system 155