Brother 1660E Service Manual page 24

3.2.2 Developing stage
Development places particles of toner onto the areas of the drum that have been cleared of
charge by the laser beam. This makes a visible image. This printer uses the toner projection
development method with a single-component toner.
Step 3 Development
Note: The charges on the light areas on the photosensitive drum are shown as positive in this
figure. Actually they are negative, but they are more positive than the developing
cylinder and explanation is simplified by regarding them as positive.
As shown in Figure 2.11, the developing unit consists of a developing cylinder and rubber blade.
The developing cylinder rotates around a fixed internal magnet. The single-component toner
consists of magnetite and a resin binder, and is held to the cylinder by magnetic attraction. The
toner is an insulator, and acquires a negative charge by friction due to the rotation of the cylinder.
The areas on the drum that were exposed to the laser beam have a higher potential (are less
negative) than the negatively charged toner particles on the developing cylinder. When these
areas approach the cylinder, the potential difference projects the toner particles onto them. This
is called toner projection, and the latent image on the drum becomes visible.
An AC bias is applied to the developing cylinder to help project the toner particles to the drum
surface and improve the contrast of the printed image. The center voltage of the AC bias (1600
Vp-p) varies with the DC bias voltage.
The IMAGE DENSITY ADJUSTMENT signal (sent from the Engine CPU to the high-voltage
power supply) changes the DC bias, and thus the potential difference between the cylinder and
drum. This changes the density of the print.
This printer has a stirring mechanism to supply toner in the cartridge smoothly to the cylinder.
Blade
Photo-
sensitive
drum
AC bias
DC bias
Figure 2.11
Developing cylinder
surface potential
+V
0
-V
Time t
Figure 2.12
II - 8
Stirrer
Developing cylinder
Cylinder
Magnet
Drum surface
potential
(exposed area)
DC bias
Drum surface
potential
(unexposed area)