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122 Theory of operation Q2431-90912
Developing the image
The developing process makes the latent electrostatic image 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 uniformly attracted to the magnetic core of the cylinder.
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 (exposed, grounded) areas on the drum. 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 the toner against the areas of the drum that have not been
exposed to the laser beam. This ac potential improves density and contrast.
Figure 53. Developing the image
The print-density control in the print-quality menu adjusts the dc bias of the developing cylinder
by changing the force of attraction between the toner and drum. A change in the dc bias causes
either more or less toner to be attracted to the drum, which in turn either increases or decreases
the print density. Both the primary and developer dc bias voltages are changed in response to
the density setting.
Blade
Stirring plate
Stirring unit
Developing cylinder
Cylinder magnet
ac bias
dc bias
Photosensitive
drum