CANON LBP1120 - LASER PRINTER - WORKING PRINCIPLE
For preparation of latent image formation, a uniform
negative potential is applied to the photosensitive drum surface. Primary charging of this printer is
performed by the method that directly charges the photosensitive drum. The primary charging roller consists of semi
conductive rubber. In addition to DC bias, AC bias is applied to the primary
charging roller to maintain the potential on the drum surface uniform. The DC
bias changes with the developing DC bias that changes according to the IMAGE
DENSITY DATA signal sent from the interface controller. This direct charging method has advantages such as the lower
applied voltage and almost no ozone generation over the conventional corona
charging method.
SCANNING EXPOSURE
When the laser beam scans onto the photosensitive drum
surface, charges on the light area are neutralized to form an electrostatic
latent image.
DEVELOPING STAGE
Toner is applied to the electrostatic latent image on
photosensitive drum surface to form a visible image. This printer performs the
toner projection development with a single-component toner.
## Charges in the exposed areas of the photosensitive drum
are shown as positive in this figure, even though they are actually negative.
It means that the photosensitive drum potential is higher as compared with that
of cylinder.
The developing unit consists of a developing cylinder and
rubber blade. The developing cylinder consists of a fixed magnet and a cylinder
which rotates around the magnet. The
single-component toner mainly consists of magnetite and resin binder and is held
to the cylinder by magnetic
attraction. The toner functions as an insulator, and
acquires a negative charge by friction due to the rotating 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 toner layer of the cylinder (with negative charges), the potential
difference (higher on the drum) projects the toner particles to them. This is called toner projection development,
which develops the latent image on the drum into a visible image.
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 centre voltage of the AC bias (1460 Vp-p) varies with the
developing DC bias. This printer
changes the developing DC bias value according to the IMAGE DENSITY DATA signal
sent from the interface controller. The changes of the developing DC bias value
makes the potential difference between the cylinder and drum so that the
density of the print can be adjusted.
In the transfer stage, the toner image is transferred from
the drum surface onto the paper.
Positive charges are applied to the back of the paper to
attract the negatively charged toner particles to the paper.
The elasticity of the paper causes it to separate from the
drum (curvature separation). To
stabilize the paper feed system and to prevent droplets on the printed image at
low temperature and low humidity, the charge on the back of the paper is
reduced by the static charge eliminator.
FIXING STAGE
As the toner image transferred onto the paper in the
transfer stage is held to the paper only by the static electricity, even a
light touch will smear the image. The
toner particles are fused to the paper to make a permanent image by applying
pressure and heat to the paper and toner.
As the toner particles transferred onto the paper are held
to the paper by the positive charge applied to the back of the paper, they may
be dispersed at fixing. This printer
applies the negative DC voltage to the fixing film via the pressure roller
shaft. The negative charge is applied to the paper surface by the fixing film
before the paper reaches the fixing heater. This improves the adhesion force of
the toner particles to the paper and prevents dispersion of the toner
particles. This printer utilizes an
on-demand fixing method which uses the fixing film that requires small heat
capacity. This method can heat the roller quickly and does not require the
power supply to the fixing heater at the standby mode. Therefore, the wait time is reduced to eight seconds or
less, and the energy can be saved. The
fixing film is a cylindrical polyamide film. The surface is coated with
fluorine to prevent offset.
At the photosensitive drum cleaning stage, the
photosensitive drum surface is cleaned in preparation for the next print operation.
Prior to the next printing, the residual toner on the drum
surface is scraped away by the cleaning blade to clean the drum surface. The
removed toner is collected into the cleaner container.
