HP 4 Plus Manual

							Conditioning Stage  
After the drum is physically cleaned, it must be conditioned. This
process consists of applying a uniform negative charge on the
surface of the drum with the primary charging roller, located in
the toner cartridge. The primary charging roller is coated with
conductive rubber with an AC bias applied to erase any residual
charges and maintain a constant drum surface potential. In
addition, a negative DC bias is applied by the charging roller to
create a uniform negative potential on the drum’s surface. The
amount of DC voltage is modified by the print density setting
from the Control Panel.
Figure 5-9 Primary Charging Roller
Functional Overview  5-19 
						

							Writing Stage  
At the writing station, the laser beam is used to discharge the
drum’s negative potential by focusing laser light onto areas of the
photosensitive drum. This creates a latent electrostatic image,
which later is developed into a visible image.
Laser light is produced by a small modulated laser diode. The
beam created by the diode shines onto a rotating polygonal
mirror. As the mirror rotates, the beam reflects off the mirror and
sweeps the page from left to right (see Figure 5-10).
A set of lenses and mirrors focuses the horizontal sweeping beam
through the laser beam access slot in the rear of the toner
cartridge, and onto the photosensitive drum. Because the beam is
sweeping the entire length of the drum and the drum is rotating,
the entire circumference of the drum can be covered. The speed of
the scanner motor (which turns the scanning mirror) and the
speed of the main motor (which turns the drum) are
synchronized, and each successive sweep of the beam is offset
1/600th of an inch. The beam can be turned on and off to place a
dot of light every 1/600th of an inch in the horizontal direction.
This is how the printer achieves its 600 x 600 DPI resolution.
Figure 5-10 Image Writing
5-20 Functional Overview 
						

							At the beginning of each sweep (before the beam reaches the
drum), the beam is reflected off the beam detect mirror. The
momentary pulse of light is sent to the DC Controller, where it is
converted to an electrical signal used to synchronize the output of
data (
VDO) for one sweep (scan line). This pulse, referred to as
the Beam Detect signal (
BD), is used to diagnose problems with
the laser diode or scanner motor also.
After the writing station, the drum surface has an invisible
(latent) electrostatic image. Portions of the drum not exposed to
the laser are still at the uniform negative potential (placed there
by the primary charging roller), but those portions exposed to
light have been discharged to ground, to form the latent
electrostatic image.
Figure 5-11 Drum Signals
Functional Overview  5-21 
						

							Developing Stage  
The developing station develops the latent electrostatic image
into 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 attracted to
the magnetic core of the cylinder. A rubber blade “brushes ” the
toner on the developing cylinder to a uniform thickness.
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 (grounded) areas of the drum exposed to laser light,
and repelled from the negatively charged (non-exposed) areas.
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 toner against the
areas of the drum not exposed to laser light. This AC potential
improves density and contrast. 
Figure 5-12 Image Development
5-22 Functional Overview 
						

							The DC bias of the developing cylinder is adjusted to change the
force of attraction between the toner and drum by changing the
print density setting from the control panel. A change in DC bias
causes either more or less toner to be attracted to the drum,
which in turn either increases or decreases the print density. The
print density variation is approximately ±10% (see Figure 5-13).
Both the primary and developer DC bias voltages are changed in
response to the density setting.
Figure 5-13 Developing Potentials
Functional Overview  5-23 
						

							Transferring Stage   
At the transferring station, the toner image on the drum’s surface
is transferred to the paper. A positive charge applied to the back
of the paper by the transfer roller causes the negatively charged
toner on the drum ’s surface to be attracted to the page.  
The small radius of the drum and the stiffness of the paper peel
the paper away from the drum. The static charge eliminator also
helps separate the paper from the drum. The static charge
eliminator weakens the attractive forces between the negatively
charged drum surface and the positively charged paper. Without
this help, thin paper can wrap around the drum. After separation,
the paper moves to the fusing station and the drum rotates to the
cleaning and conditioning stations.
Figure 5-14 Transfer of the Toner Image and Paper Separation
5-24 Functional Overview 
						

							Fusing Stage    
At the fusing station, the toner is fused into the paper by heat
and pressure to produce a permanent image. The paper passes
between a heated fusing roller and a soft pressure roller. This
melts the toner and presses it into the paper. To keep the toner or
paper from sticking, the upper fusing roller is covered with a
non-stick, “Teflon-type”  sleeve. The fusing roller contains an
halogen quartz lamp that provides heat for the fusing process.
The fusing roller temperature is monitored by the DC Controller
PCA, using a thermistor (TH1). The DC Controller maintains a
temperature of about 172 ° C during standby mode and 183 ° C
during print mode. If the fusing system overheats (exceeds 230 °
C), a thermoswitch opens the power circuit to the fusing heat
lamp, and a  50 SERVICE  message is displayed.  
Figure 5-15 Fusing the Toner to the Paper
Functional Overview  5-25 
						

							Paper Feed System  
NoteThe LJ 5 printer designators differ as follows.
LJ 4 / 4 Plus
Multipurpose (MP) Tray Paper Cassette (PC) Tray Lower Cassette (LC) Tray
LJ 5 Tray 1Tray 2Tray 3
The following paper paths are described in this section of the
manual:
•PC Tray (Tray 2) to Output Trays
•MP Tray (Tray 1) to Output Trays
The paper feed system is responsible for picking paper from
either the PC tray (Tray 2) or MP tray (Tray 1) and delivering it
to the image formation system, feeding it into the fusing station,
and delivering the paper to the output tray (see Figure 5-16).
The Paper Feed Assembly is a modular, replaceable paper-feeder
mechanism which allows media to be input from either of four
sources. The four paper input sources of the printer are:  
•The MP Tray (Tray 1)
•The PC Tray (Tray 2)
•The Lower Cassette (Tray 3)
•The Envelope Feeder
Each input has features which makes it the best choice for
various applications. For instance, short media and heavy stock
can be fed from the MP tray (Tray 1) because it senses media
length and has a relatively straight paper path. Large print jobs
can be fed reliably from the optional Lower Cassette (Tray 3).  
The Paper Feed Assembly contains a state machine which
monitors input sensors and generates commands for the solenoids
and motors on the Paper Feed Assembly. In addition, the Paper
Feed Assembly communicates with the DC Controller through
serial data lines. An illustration of this is M2, the Paper Feed
Motor. M2 is controlled solely by the Paper Feed Assembly state
machine. M2 begins rotation when the State Machine receives
the correct set of conditions from its inputs and the serial data
lines. M2 stops rotation when the Input Registration sensor (PS1)
reports the presence of paper to the State Machine. The State
Machine outputs the presence of paper to the DC Controller
through the serial data line.  
5-26 Functional Overview 
						

							The Paper Feed Assembly performs horizontal registration of the
print media through the Oblique Rollers. As the print media
passes through the Oblique Rollers, the media is moved
uniformly to the left side of the print path. The movement
ensures that each print image is oriented properly on the
horizontal axis.
Figure 5-16 Input Paper Feed System Diagram
Functional Overview  5-27 
						

							Figure 5-17 Paper Feed Assembly (Sectional View)
PartDescription
PS1Leading Edge Registration Sensor
PS2PC Tray (Tray 2) Paper Out Sensor
PS3Exit Sensor
PS4MP Tray (Tray 1) Paper Out Sensor
PS5MP Tray (Tray 1) Paper End Sensor
SL1Cassette (Tray 2) Pickup Roller Clutch Solenoid
SL2MP (Tray 1) Tray Pickup Roller Clutch Solenoid
Table 5-6
5-28 Functional Overview