HP 3150 EN - Manual (Page 77)

EN Image formation system 75
CAUTION The printer's toner cartridge does not include a light-blocking shutter.
Be careful to avoid exposing the drum to light, which can permanently
damage the drum. Protect the cartridge whenever removing it from the
printer.
Step 1: Primary charging
This process consists of applying a uniform negative charge to the
surface of the drum with the primary charging roller, located in the
toner cartridge. The primary charging roller is coated with conductive
rubber. An AC bias is applied to the roller to erase any residual
charges from any previous image. In addition, a negative DC bias is
applied by the charging roller to create a uniform negative potential on
the drum surface. The amount of DC voltage is modified by the print
density setting.
Step 2: Scanning exposure
During this process, a modulated laser diode projects the beam onto
a rotating scanning mirror. As the mirror rotates, the beam reflects off
the mirror, first through a set of focusing lenses, then off a mirror, and
finally through a slot in the top of the toner cartridge, and onto the
photosensitive drum. The beam sweeps the drum from left to right,
discharging the negative potential wherever the beam strikes the
surface. This creates a latent electrostatic image, which later is
developed into a visible image.
Because the beam is sweeping the entire length of the drum and the
drum is rotating, the entire surface area of the drum can be covered.
At the end of each sweep, the beam strikes the beam detect lens,
generating the beam detect signal (BD signal). The BD signal is sent
to the ECU, where it is converted to an electrical signal used to
synchronize the output of the next scan line of data.
Step 3: Developing
At this stage of the process, the latent electrostatic image is present
on the drum. 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 of the drum, and repelled from
the negatively charged (unexposed) areas.