HP LaserJet 4 Service Manual - Page 134

Developing Stage The developing station develops the latent electrostatic image into a visible image on the drum. The developing unit consists of a metallic cylinder that rotates around a fixed magnetic core inside the toner cavity. Toner is a powdery substance made of black plastic resin bound to iron particles, which is attracted to the magnetic core of the cylinder. A rubber blade "brushes" the toner on the developing cylinder to a uniform thickness. The toner particles obtain a negative surface charge by rubbing against the developing cylinder which is connected to a negative DC supply. The negatively charged toner is attracted to the discharged (grounded) areas of the drum exposed to laser light, and repelled from the negatively charged (non-exposed) areas. An AC potential is applied to the developing cylinder to decrease the attraction between the toner and the magnetic core of the cylinder, and to increase the repelling action of toner against the areas of the drum not exposed to laser light. This AC potential improves density and contrast. Figure 5-12 Image Development 5-22 Functional Overview