Corona Systems and Dependent Processes in Xerography Charging Systems and Dependent Processes in...

Corona Systems and Dependent Processes in Xerography

Charging Systems and DependentProcesses in Xerography

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OVERVIEW- Charging system applications- Product examples

CORONA DEVICES- Geometry- Performance criteria- Capacitive charging model- Device dependent electrical behavior

BIAS CHARGING & TRANSFER ROLLS

OUTLINE

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Corona Systems and Dependent Processes in Xerography 3


Charging System Applications

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ROSROSROS ROS

Intermediate Belt (IBT) Bias transfer roll (BTR)

BTRPaper

Fuser

K C M Y

Tandem Architecture – Single Pass Color

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Image on Image – Single Pass Color

Hybrid Scavenge lessDevelopment

ROS

ROS

ROS

ROS

Cleaner

FuserTransferdicorotrons

Photoreceptor

Transfer assist blade

AcousticTransferAssist

Precleandicorotron

DC&AC Charge scorotrons

Pre-transferdiscorotron

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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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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.

Corona DevicesCorona 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)

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Device Characteristics

Shape Factor

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Device Characteristics Uniformity

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I total

I shield

I plate

V plate

Vc

Ipla

te/L

(leng

th)

Bareplate voltage (Vplate)

Increasing V wire (Itotal)

Slope= ΔI plate/L Δ V plate

V intercept

• The slope and intercept voltage are important attributes of corona devices.

I-V Behavior

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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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I total

I plate

V plate

Vc

Capacitive Charging Model

I total

Vc

Vfinal Vinitial

dielectric Velocity (v)

J(x) current density

Beam Profile (Gx)

∫G(x) = 1

Ipla

te/L

(leng

th)

Bareplate voltage (Vplate)

Slope (S)= ΔI plate/L Δ V plate

V intercept

I=-S (Vintercept-Vplate)

J(x) = I G(x)J(x) = -S(Vintercept-Vplate) G(x)Q(x) = CV(x)C = capacitance of charge receiving surfacedQ(x)/dt = CdV(x)/dt = J(x)CdV(x)/dt = -S[Vintercept-V(x)] G(x)dt = dx/vCdV(x) v/dx = -S[Vintercept-V(x)] G(x)

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dV(x) / (Vintercept – V(x)) = - S / CvG(x)d(x)

∫ dV(x) / (Vintercept – V(x)) = - S / Cv ∫ G(x) dx

Vfinal

Vinitial

∞

0

where ∫ G(x) dx0

∞

Vfinal = V intercept [1- exp-(S/Cv)] + [exp-(S/Cv)]Vinitial

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) (where dV(X) = Vfinal - Vinitial )

J = CvdV(x) = Cv (Vfinal - Vinitial ) (substitute for Vfinal)

J = Cv(Vintercept – Vinitial) (1-exp-SCv)

0

∞

Dynamic Charging Current

Final Voltage

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Photoreceptor Charging

and

Sample Calculations

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Constant Voltage

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P/R Charging Current SolutionP/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 – 6 amps

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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?

P/R Charging Problem Statement

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Vfinal = V intercept [1- exp-(s/cv)] + [exp-(s/cv)]Vinitialsolving for slope (s):s=-cvln[(Vfinal - Vintercept)/( Vinitial - Vintercept)]Vinitial = 0Vintercept = Vgrid = -800 voltsVfinal = .98Vgrid = .98(-800)Vfinal = -784 voltsC = .95 x 10-6 farads/m2v = 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

P/R Charging Problem (cont’d)

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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/ScorotronsBeryllium copper, phosphor bronze

Dicorotron3-4 mil diameter triple polished tungsten core with glass overcoat, 9 mil overall diameter (core+glass)

Grid Materials304 stainless steel with Electro dag overcoat to inhibit “Parking Deletions”

Corona Compatible PlasticsTalc filled Polypropylene is preferred.Dielectric grade Noryl (Polyphenylene oxide with minimum 10% mineral or talc filler)

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Bias Charging and Transfer Rolls

(BCR / BTR)

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Photoreceptor Charging

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Effect of ACEffect 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.

Palghat Ramesh112


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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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.

REFERENCES (cont’d.)

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