Charging Systems and Dependent
Processes in Xerography
Corona Systems and Dependent Processes in Xerography
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OUTLINE
OVERVIEW
- Charging system applications
- Product examples
CORONA DEVICES
- Geometry
- Performance criteria
- Capacitive charging model
- Device dependent electrical behavior
BIAS CHARGING & TRANSFER ROLLS
Corona Systems and Dependent Processes in Xerography
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Corona Systems and Dependent Processes in Xerography
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Charging System Applications
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Tandem Architecture – Single Pass Color
K
ROS
Intermediate Belt (IBT)
Paper
Corona Systems and Dependent Processes in Xerography
ROS
ROS
C
M
Y
ROS
Bias transfer roll (BTR)
BTR
Fuser
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Image on Image – Single Pass Color
Hybrid
Scavenge less
Development
ROS
DC&AC
Charge
scorotrons
ROS
Photoreceptor
ROS
Cleaner
Preclean
dicorotron
ROS
Acoustic
Transfer
Assist
Pre-transfer
discorotron
Transfer
assist blade
Corona Systems and Dependent Processes in Xerography
Transfer
dicorotrons
Fuser
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Corona Devices and Characteristics
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Dry Air at Atmospheric Pressure.
Positive Needle-Plane Corona
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BareWire Corona Emission
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Corona Devices
Corotron: Uses small diameter wire or pin array electrode and
is the simplest of all corona devices. Used in many products.
(DC and or AC)
Wire Scorotron: Small diameter wire electrode behind a screen.
Wire and screen voltages are independently set. Typically DC..
Pin Scorotron: Similar to a wire scorotron, except that an
array of pins is used for the coronode. Pins eliminate wire
vibration, enable width, improve reliability and generate less
ozone . Negative DC device.
Discorotron: The coronode is a glass-coated wire. The
dielectric coating is Xerox-unique technology that enables
exceptional uniformity (+/- 5%) and reliability.
Dicorotron: a discorotron without the grid (screen). First
technology used by Xerox with negative charging belt
photoreceptors.
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Device Characteristics
Shape Factor
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Device Characteristics
Uniformity
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I-V Behavior
I total
Vc
I shield
Iplate/L(length)
Increasing V wire (Itotal)
Slope= ΔI plate/L
Δ V plate
I plate
Bareplate voltage (Vplate)
V plate
V intercept
• The slope and intercept voltage are important attributes of corona devices.
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Corona Devices and Characteristics
Capacitive Charging Model
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DEFINITIONS
Power Supply Operating modes:
•Constant Current – Implies constant current delivered to the process independent of device
characteristics.
•Constant Total Current – Total device corona current is maintained constant. Current
delivered to the process may vary.
•Constant Coronode Voltage (wire or pins) – Applied voltage is held constant.
•Constant Shield Voltage – Unique to dicorotrons. The shield bias is maintained constant.
•Constant Grid Voltage – Unique to scorotrons. The grid bias is maintained constant.
Process Operating modes:
•Constant Voltage – Charge receiving surface(s) are charged to a constant voltage
independent of receiver electrical and mechanical characteristics. Requires high slope,
voltage sensitive I-V behavior.
•Constant Charge Density – Charge receiving surface(s) are charged to a constant charge
density independent of receiver electrical and mechanical characteristics. Requires low
slope, voltage insensitive I-V behavior.
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Ideal Capacitive Model
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Capacitive Charging Model
Iplate/L(length)
I total
Vc
I plate
Slope (S)= ΔI plate/L
Δ V plate
I=-S (Vintercept-Vplate)
V plate
Bareplate voltage (Vplate)
V intercept
I total
J(x) current density
Vc
Beam Profile (Gx)
∫G(x) = 1
Vinitial
Vfinal
dielectric
Velocity (v)
Corona Systems and Dependent Processes in Xerography
J(x) = I G(x)
J(x) = -S(Vintercept-Vplate) G(x)
Q(x) = CV(x)
C = capacitance of charge
receiving surface
dQ(x)/dt = CdV(x)/dt = J(x)
CdV(x)/dt = -S[Vintercept-V(x)] G(x)
dt = dx/v
CdV(x) v/dx = -S[Vintercept-V(x)] G(x)
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Final Voltage
dV(x) / (Vintercept – V(x)) = - S / CvG(x)d(x)
Vfinal
Vinitial
∞
∫ dV(x) / (Vintercept – V(x)) = - S / Cv ∫ G(x) dx
0
∞
where ∫ G(x) dx
0
Vfinal = V intercept [1- exp-(S/Cv)] + [exp-(S/Cv)]Vinitial
Dynamic Charging Current
Q(x) = CV(x)
J(x) = (dV(x)/dt)C = C(dV(x)/dx)(dx/dt)
J(x) = CvdV(x)/dx
∞
∫ J(x) dx = J = CvdV(x)
0
(where dV(X) = Vfinal - Vinitial )
J = CvdV(x) = Cv (Vfinal - Vinitial ) (substitute for Vfinal)
J = Cv(Vintercept – Vinitial) (1-exp-SCv)
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Photoreceptor Charging
and
Sample Calculations
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Constant
Voltage
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P/R Charging Current Solution
J = Idyn./L=Cv(Vintercept-Vinitial)1-exp-S/Cv
Idyn./L = P/R dynamic charging current per unit length (amps/meter)
Substitute values:
Idyn./L = (.95)(.254)[-2000-(-25)][1-exp-(0.2/(0.95)(0.254)]
Idyn./L = -268.5X10- 6 amps/meter
Idyn. = Idyn./L x L = -268.5 X 10 - 6 amps/meter x 0.3 m. = 80 x 10
Corona Systems and Dependent Processes in Xerography
– 6 amps
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P/R Charging Problem Statement
2. An AC scorotron will be utilized to charge the same ideal photoreceptor to
within 98% of its –800 volt grid potential at the same P/R surface velocity
(10 ips.=0.254 m./sec.). The static I-V characteristics show that the intercept
voltage is approximately equal to the grid bias as expected. The initial residual
photoreceptor potential entering the charge device is 0. What must the voltage
sensitivity (slope) of the device be to achieve this function?
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P/R Charging Problem (cont’d)
Vfinal = V intercept [1- exp-(s/cv)] + [exp-(s/cv)]Vinitial
solving for slope (s):
s=-cvln[(Vfinal - Vintercept)/( Vinitial - Vintercept)]
Vinitial = 0
Vintercept = Vgrid = -800 volts
Vfinal = .98Vgrid = .98(-800)
Vfinal = -784 volts
C = .95 x 10-6 farads/m2
v = 0.254 m./sec.
s=-cvln[(Vfinal - Vintercept)/( Vinitial - Vintercept)]
s= - (.95 x 10-6 farads/m2)(.254m./sec.) ln[(-784+800)/(0+800)]
s= 0.94x10-6 amps/m-volt
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Electrical Analogy of
Photoreceptor Charging
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Charging Process
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Current Voltage Sensitivity
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Equivalent Circuits
and
Corotron Current-Voltage Behavior
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DC Corotrons
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Ozone Generation
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Corona Materials
Wire Corotrons/Scorotrons
tungsten/oxidized tungsten (3.5 mil + corona; 2mil neg. corona)
Platinum (field replacement for + corona in legacy products)
Gold coated tungsten (some) neg. corona
Pin Corotrons/Scorotrons
Beryllium copper, phosphor bronze
Dicorotron
3-4 mil diameter triple polished tungsten core with glass overcoat, 9 mil overall
diameter (core+glass)
Grid Materials
304 stainless steel with Electro dag overcoat to inhibit “Parking Deletions”
Corona Compatible Plastics
Talc filled Polypropylene is preferred.
Dielectric grade Noryl (Polyphenylene oxide with minimum 10% mineral or talc filler)
Corona Systems and Dependent Processes in Xerography
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Bias Charging and Transfer Rolls
(BCR / BTR)
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Photoreceptor Charging
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Effect of AC
fac=1000Hz
• A steady state DC voltage approximately equal to the DC bias
is achieved when the applied AC is high enough to generate
both positive and negative corona.
Corona Systems and Dependent Processes in Xerography
Palghat Ramesh
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BIAS CHARGING ROLL
ADVANTAGES
- Size
- Low Ozone
- Applied voltages are lower than corona devices
- “Doubles” as P/R charge neutralizer (some low end products)
DISADVANTAGES
- “Robust” uniform charging appears to require AC
- AC adds to power supply UMC
- AC capacitive currents can be high
- AC (positive corona ½ cycle) degrades P/R transport layer
LIMITATIONS
-Extensibility to higher process speeds?
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REFERENCES (cont’d.)
EXTERNAL ARTICLES
• “Pin” Models – (K. Pietrowski, Walsh)
• Corona Charging – (K. Pietrowski, et al)
• Corona Physics – (C. Gallo, W. Lama)
EXTERNAL REFERENCES
• Williams, E.M. (1984), Physics and Technology of
Xerographic Processes, John Wiley and Sons, New York.
• Schaffert, R.M. (1975), Electrophotography, 5th ed., Focal
Press, London.
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