Mita-Kyocera FS-6900 Service Manual PART4
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Electrophotography is the technology used in laser printing which transfers data representing
texts or graphics objects into a visibl e imag e which is developed on the photosensitive drum,
finally fusing on paper, using light beam generated by a laser diode.
The key features for the electrophotography system used in the printer are:
600 dpi resolution
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Newly developed amorphous silicon drum with no heating device
•
Diode laser scanning
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Mono component toner
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The electrophotography system of the printer performs a cyclic action made of seven steps as
diagrammed below.
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The printer uses the long lasting amorphous silicon drum. The drum surface is a composite of
five substances coated in five layers as shown in page 4-3, Figure 4.2.
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The amorphous silicon layer is photoconductive, reducing its electrical conductivity when
exposed to laser light.
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Figure below is a simplified diagram of the electrophotographics components. Charging the
drum is done by the main charger wire (in the main charger unit) marked “A” in the diagram.
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As the drum (“B” above) rotates in a “clean (neutral)” state, its photoconductive layer is given a
uniform, positive (+) electrical charge dispersed by the main charger wire (“A”).
Due to high-voltage scorotron charging, the charging wire can get contaminated by oxidization
and therefore must be cleaned periodically from time to time using the method explained on
page 3-5,
such as black streaks caused by the oxide accumulated around the charging wire.
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. Cleaning the charging wire prevents print quality problems
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The toner is fed from the toner pack TK-20/H. The toner is comprised of the following substances.
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The charged surface of the drum ("B") is then scanned by the laser beam from the scanner unit
("A").
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The laser beam is switched on for a black dot and off for a white (blank) dot according to the
print data. Whenever it is illuminated by the laser beam, the electrical resis-tance of the photoconductor is reduced, the potential on the photoconductor is also lowered to 20 V, effectively
driving the charge through the a-Si layer down to the aluminum base.
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The 600 dpi scanner unit includes the diode laser that produces the 670 nm wave-length laser
beam. This wavelength is specifically designed to match the photocon-ductive response of
amorphous silicon.
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Laser diode Emits diffused, visible laser.
Collimeter le ns Aligns the laser be am to the cylindrical lens.
Cylindrical lens Compensates the slant angle at which the laser beam hits a polygon mirror
segment.
Polygon mirror (motor) Has six mirror segments around its oc ta gonal circumference; each mirror
corresponding to one scanned line width on the drum when laser beam
scans on it.
Primary f-theta lens See page 4-7, Figure 4.4 and below.
Secondary f-theta lens See page 4-7, Figure 4.4. The primary (above) and secondary ftheta lenses
equalize focusing distortion on the drum edges. The effective length of
line ("A," "B" below) the lase r bea m draws on the drum becomes longer
as the laser beam hits closer to the drum edges. In the figure below, dis-
tances represented by "A" and "B" are not the same (A>B) until the ftheta
lenses are provided be tween the polygon m irror and the drum (A=B) .
Diversion mirror diverts the laser beam vertica ll y onto the drum. Note the diffused la ser
beam finally pin-point s on the drum.
Protective glass Pre ve nts dust, debris, etc., from entering the scanner asse m bly.
Sensor mirror Bends the very first shot of a laser scan towards the beam detection sensor
(See next.).
Beam detector sensor When shone by the sensor mirror above, this photosensor generates a trig-
ger signal for the engine controller to start a ctivating the paper feeding
system.
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The laser beam hits one of 5 sides of t he polygo nal mi rror. As the mi rror rev olves (at the revo lution of 26600 rpm), the laser beam reflects off of it and reaches the charged drum surface in a
lengthwise manner.
A pair of (plastic) lenses provides focusing the horizontally sweeping laser beam onto the drum.
As the drum rotates, the laser beam sweeps the entire length of the drum so th at the drumís
entire circumference is exposed to the laser beam. The revolution of the polygon mirror motor
and the drum itself is timing-controlled so that each suc-cessive sweeping of the laser beam produces a 1/600 inch offset. The printer’s controller system continuously turns the laser beam on
and off to put a dot at every 1/600 inch distance horizontally. The diameter of a dot is typically
70 to 80 µm. When KIR is on, the intensity of the beam is switched in fou r deg rees compensating for the smoothed image.
Synchronizing the output data with one scanning line is achieved by the photo sensor provided
next to the first mirror. At the beginning of each laser sweeping, the beam hits the photo sensor
which in turn sends a command to the logic controller for syn-chronization.
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The latent image constituted on the drum is developed into a visible image. The de-veloping
roller contains a 3-pole (S-N-S) magnet core and an aluminum cylinder ro-tating around the
magnet core. Toner attracts to the developing roller since it is pow-dery ink made of black resin
bound to iron particles. A magnetized blade positioned approximately 0.3 to 0.4 mm above the
developing roller constitutes a smooth layer of toner in accordance with the roller revolution.
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The developing roller is connected to a AC-weighted, positive DC power source. Toner on the
developing roller is given a positive charge. The positively charged toner is then attracted to the
areas of the drum which was exposed to the laser l ight. The non- exposed areas of th e drum repel
the positively-charged toner as these areas maintain the positive charge.
The developing roller is also biased with an ac potential to apply com pensation to the toner’s
attraction and repelling actions for more contrast in the development.
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The following diagram depicts the change in the drum surface potential during development.
The vertical distance rerepresents the depth of bias potential.
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The toner replenishment sensor is provided within the developer. As the toner supply from the
toner container dwindles and the toner level low e rs in the reservoir, the sensor translates it
through its diaphragm, urging the toner motor to feed more toner.
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The image developed by toner on the drum (“A” below) is transferred onto the paper using the
electric charge attraction given by the toner itself and the transfer roller (“B” below). The transfer roller is negatively biased so that the positively charged toner is attracted onto the paper
while it is pinched by the drum and the transfer roller.
The paper is automatically peeled off the drum because of the small diameter of the drum. To
prevent thin paper wrapping around the drum, the static discharger brush is provided to reduce
the attraction of the negatively charged paper to the positiv ely charged drum.
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The nominal transfer bias is set to approximately -1.80 kV (limit) with the current of 65±2 mA.
Since thicker paper (91 to 200 g/m
more bias potential for the satisfactory transferring process, the transfer bias is user-switchable
to -2.45 kV (limit) by using the printerís operator panel. Double-sided printing using a DU-25
duplexer automatically increases the transfer bias to the above value.
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The toner on the paper is permanently fused onto the paper as it passes between the florin-finished heat roller (“A” below) and the pressure roller (“B” below) in the fuser unit. The toner is
molten and pressed into the paper. The heat roller has a halogen lamp, t urning frequ ently on and
off to maintain a preheat temperature at approxi-mately 175°C.
The heat roller temperature is constantly monitored by the engine control circuit us-ing a thermistor. For safety against overheating, the fuser system is protected by a triac which automatically opens power to the halogen lamp. If the temperature ex-ceeds 350°C, it activates the
thermo-cut device to interrupt open power to the halo-gen lamp.
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The drum needs to be physically cleaned of toner remaining on its surface in the pre-vious rotation. The cleaning blade (A below) is constantly pressed against the drum and scrapes the residual toner on the drum off to the refresher roller (B below). The refresher roller drives the toner to
the spiral (fins) roller (C below) at one end of which the waste toner bottle is connected to collect the waste toner.
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After the drum is physically cleaned, it then must be cleaned to an electrically neu-tral state.
This is necessary to erase any residual positive charges, ready to accept th e next uniform charg e.
The residual charge is canceled by exposing the drum to the light emitted from the eraser LED
4-11
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