HP 2100 Service Manual - Page 94

Developing Stage Figure 5-12 Developing the Latent Electrostatic Image into a Visible Image NOTE: At the developing stage, the latent electrostatic image is developed into a visible image on the photosensitive drum. As shown in figure 5-12, the developing cylinder 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 the magnetic core of the developing cylinder attracts. A rubber blade brushes the toner on the developing cylinder to a uniform thickness. Charges in the photosensitive drum's exposed areas are shown as positive in figure 5-12 even though they are actually negative. This means that the photosensitive drum potential is higher than that of the developing cylinder. To obtain a negative surface charge, the toner particles rub against each other and the developing cylinder which is connected to a negative DC supply. The discharged (grounded) areas of the drum exposed to laser light attract the negatively charged toner. The negatively charged (non-exposed) areas repel the toner. An AC potential is applied to the developing cylinder to decrease the attraction between the toner and the cylinder's magnetic core 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. The print density setting (through the software) adjusts the developing cylinder's DC bias to change the force of attraction between the toner and drum. 84 5 Functional Overview C4170-90959