Xerography/Electrophotography: The Technology of Photocopiers and Laser Printers

Xerography/Electrophotography:
The Technology of Photocopiers
and Laser Printers
MAE Seminar
April 14, 2003

Fa-Gung Fan
Wilson Center for Research & Technology
Xerox Corporation
Webster, NY

Fa-Gung Fan, Xerox Corporation MAE Dept. Seminar, Clarkson University, 4/14/2003

Outline of the Talk

Part I. General Overview of Xerography
Xerographic Process
Brief Description of Subsystems
Transport, Deposition and Removal of Fine
Particles (Toner) in Xerography

Part II. Modeling of Electrostatics
Charging & Transfer Subsystems Using Biased
Rolls
Modeling the Electrostatics of a Biased Rolls
Modeling a Transfer Subsystem Using A Biased
Transfer Roll


Part I.
General Overview of Xerography


Xerographic Process
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3
C. Duke, J. Noolandi, T. Thieret, “The surface
science of xerography,” Surface Science, 500, p.
1005, (2002)


Xerox Phaser 7700 Color Desktop Laser
Printer
22 prints/min, full-color

Xerography is a versatile technology that scales from desktop, to
office, to production machines; black-and-white, hightlight color,
and full color.


DocuColor 40 Pro Color Office
Multifunction Machine


DocuTech 6180 Production Publisher

180 prints/min
Black-and-White


DocuColor iGen3 Digital Production Color
Press


iGen3
Xerographic Unit

Finisher/Binder


Photoreceptor
A semiconductor whose
conductivity is a strong 0
0 5

Voltage on Surface (V)
function of light exposure.

_ _ _ _ _ _ _ _ _ _ _
_+_+
-1000
++ + + + + + ++ ++ + + +
Exposure (ergs/cm2)
Electron/hole pairs
• Requirements
– Insulator in the dark.
– Conductor when exposed to light
– Builds up enough voltage.
– Uniform properties

Charging Subsystem

HV Power HV Power HV Power
Supply (-) Supply (-) Supply (-)

Free ions are attracted Rapidly moving electrons Electrons continue to
to wire; Free electrons are and ions collide with air follow Electric Field lines
repelled. Counter-charges molecules, ionizing them to Photoreceptor until
build up on grounded surfaces. and creating a corona. uniform charge builds up

Positive Ions
Electrons


Imaging/Exposure

Traditional Analog Copier Laser Printer


Development

Photoreceptor Photoreceptor
Toner
Apply E
Field E

Development roll Development roll
Development
Photoreceptor

Roll
Charge particles
triboelectrically
Electric field moves
Mixing
Charging particles from developer
roll to photoreceptor
Triboelectrification of
toner particles and carrier
beads

Toner

• Charging
• Adhesion/cohesion
• Powder flow
• Rheology
5-10 microns
• Color - hue and density
• Pigment dispersion


Toned Carrier Bead

q ≅ 2 x 104 e
m ≅ 2 x 10-10 gm

q/m ≅ 16 µC/gm

20 µm


Transfer

Paper

Apply E Paper
Paper Field &
Separate E
Photoreceptor Photoreceptor
Photoreceptor

Electric field moves particles from
photoreceptor to paper or transparency
Detachment field must overcome toner
adhesion to photoreceptor


Fusing Subsystem

Permanently affix the image to the final substrate
paper of various roughnesses and surface treatment
transparency (plastic)
Apply heat and/or pressure

Hot Roll Fuser: Pressure Roll

Unfused toner Fused toner Paper

Elastomer Heat Roll


Cleaning and Erase

Removes unwanted residual toner and charge
from photoreceptor before next imaging cycle
Physical agitation removes toner (blade or brush)
Light neutralizes charge by making entire photoreceptor
conductive

Photoreceptor
_
_
_
_
_
Charge

Residual toner

Particle Transport, Deposition and
Removal in Xerography

Toner must flow smoothly
down dispenser

Toner must develop
onto roll uniformly

Toner must transfer from
roll to paper

paper


Toner Adhesion Forces

Fa Particle adhesion
depends on:
++ + ++
+ Size, shape, &
roughness
+ Materials
+
+
+ + Flow agents
++ ++
Charge
Surface charge
Fad distribution on
particle
Detachment when Fa > Fad


Electrostatic Image Force Model
Q
Fa σ=
E R 4 π R2
Fi

Image Force Applied Force
Q2
Fi = − α Fa = β QE − γ π ε o R 2 E 2
16 π ε o R2
αQ
Ed ≅ ≈ 1 V / µm
β 16 π ε o R 2

Charge Patch Adhesion Model
Q = σ At
Fa
Ac
Ac
Fad f =
At

2
σ
F ad = − A c − W A c
2 εo
 σ W
= −Qf 
 2ε + 
 o σ 


Electric Field Detachment of Fine Particles

Measure Many Particle Adhesion

transparent
conductive
electrodes

Donor Receiver
V

E. Eklund, W. Wayman, L. Brillson, D. Hays, 1994 IS&T Proc.,
10th Int. Cong. on Non-Impact Printing, 142-146


Toner Transferred
When FE > FAD
FE

-
-
Donor Surface
- -
FAD

E. Eklund, W. Wayman, L. Brillson, D. Hays, 1994 IS&T Proc.,
10th Int. Cong. on Non-Impact Printing, 142-146


Adhesion Control Additives

Changing type of additive modifies adhesion
Atomic Force Microscopy results


Part II.
Modeling of Electrostatics


Xerographic Machine Using Drum
Photoreceptor and Biased Rolls

Biased Charging Roll

1
6
1) Charge
2) Expose
2 3) Develop
3
4) Transfer
5 5) Clean
6) Erase

4
Paper or
Intermediate Belt

Biased Transfer Roll

J. Swift and S. Badesha, Surface
Science, 500, p.1024, (2002)


Biased Charging/Transfer Rolls
(BCR/BTR)

pre-nip nip post-nip Applications of Biased Rolls:
VA Charging
Transfer

Roller Drum
Shaft Insulator
Elastomer Ground plane
Field probe
C. DiRubio, G. Fletcher, Proceedings of IS&T NIP 12:
International Conference on Digital Printing Technologies,
p.334, 1996.


Basic Equations

2 ρ
Electrical Potential ∇ φ =− (1)
ε

Interface Condition  ∂φ   ∂φ  σ
− k  = − k  + (2)
 ∂n matl 2  ∂n  matl1 ε 0

Electric Conduction j = γ (| E |) E where E = −∇φ (3)

Conservation of ∂σ ∂σ ∂ 2φ
+v = γ s 2 + j matl1 ⋅ n − j matl 2 ⋅ n (4)
Surface Charge ∂t ∂s ∂s

Ref. P. Ramesh, private communication


KEY Requirements for the Integrated
Model:
1. Allows Implementation volume and surface
charge density
2. Solves Poisson equation for composite
domain, and obtain electric field
3. Calculates current density
4. Solves surface charge transport
5. Handles air breakdown

In our integrated model, 1 and 2 are done in
ABAQUS/Standard (version 5.7), and 3 to 5 are
implemented as an in-house C program.


Flowchart of the Integrated Model
Start

Input/Pre
child process ABAQUS

check to see if ABAQUS job is completed
UNIX fork() parent process
UNIX shell
script
Check if
air breakdown
occurs Handling air breakdown σ brk = ε 0 (Eairgap − EPaschen )
(5)
Update & solve
surface charge

Y
t < tmax

N

Stop/Post


Modeling of Biased Charging Roll (BCR) &
Comparison with Field Probe Measurement
C. DiRubio, G. Fletcher, “Field profile Parameters used in the field probe
measurements in biased charging experiment:
systems,” Proceedings of Image
Science &Technology NIP 12, p.334, Elastomer:
1996 outer diameter = 37.83 mm
inner diameter = 25.24 mm
pre-nip nip post-nip dielectric constant = 4.4
resistivity = 5.0E+9 ohm-cm
VA

Insulator (kapton tape):
thickness = 33 microns
dielectric constant = 2.7

Roller Drum Field Probe (magnet wire):
Shaft Insulator diameter = 1.72 mm
Elastomer Ground plane
Field probe
Nip Width (contact) = 2.7 mm
Schematic of the nip region
of the experimental Charge Relaxation Time = 0.22 sec
apparatus.

σ probe
Eair = (6)
ε0 30

Τ=32
25 Τ=1
where σ probe = Q / Aprobe
20

Eair (V/µ m)
15
Τ=0.35
dwell time based on effective nip width
10
T= Τ=0.13
charge relaxation time
5

0
T roll surface -3 -2 -1 0
x (mm)
1 2 3

speed, v
(mm/sec)
32 0.656 The field profiles for different values of T.
1 23.65 (DiRubio and Fletcher)
0.35 65.8
0.13 251.6
(data from C. DiRubio)

The computational mesh used. (yellow mesh --- elastomer; white mesh --- air gap;
blue mesh --- insulator on the drum).


Potential map for T=32. The color scale is from 0V (dark blue) to 400V (red).


Potential map for T=1. The color scale in this figure is from 0V (dark blue) to
697V (red).


Nip

The blow-up of the nip region shown in previous slide. The contours represent equal
electrical potentials (for T=1). The field lines are orthogonal to these equi-potential
lines.

Potential map for T=0.13. The color scale in this figure is from 0V (dark blue) to
822V (red).


Pre-Nip Nip Post-Nip

The model predictions of field profile for various values of T as evaluated from Eq.(6).


30

Τ=32
25 Τ=1

20

Eair (V/µ m)
15
Τ=0.35

10
Τ=0.13
5

0
-3 -2 -1 0 1 2 3
x (mm)

The field profiles for different values
of T as measured with 1.72mm probe.
The field profiles seen by a 1.72mm probe as (DiRubio and Fletcher)
predicted by the model. (The high-resolution field profiles
from the model were first averaged with a circular area of 1.72mm
diameter, and then shifted so that the position of the peak field for
each curve is at x=0.) The profiles on this figure should
be compared with the measured ones (at the right).

Transfer Subsystem Using A Biased
Transfer Roll

Photoreceptor
(PR)
Photoreceptor:

ITB:

BTR Elastomer:
Nip
Post-Nip
Pre-Nip

Intermediate Transfer Belt (ITB)
v = Belt speed

Bias Transfer
Roll (BTR)


Air Breakdown & Paschen Curve

50
45
Paschen Curve
40
Eair (V= -200)
35 Eair (V= -400)
Eair (V/µ m)

30 Eair (V= -600)
Eair (V= -800)
25
20
15
10
5
0
0 50 100 150 200
dair (µm)
µ


The region that is included in the model and the computational mesh used. (yellow
mesh --- BTR elastomer; magenta mesh --- ITB; cyan mesh --- PR material coated on
the drum; white mesh --- air gap)

The steady-state potential map of the modeled domain. The contours displayed in this
figure are the equi-potential contours.

Post-Nip Nip Pre-Nip

Field profile in the ITB-PR air gap. The Paschen breakdown limit for
the air gap is also plotted.

Post-Nip Nip Pre-Nip

The field profile in the BTR-ITB air gap.


Nip

The variations of charge densities on different surfaces. The relevant surfaces shown
here are the photoreceptor surface, the upper surface of ITB, the lower surface of
ITB, and the BTR surface.

Transfer Subsystem I-V Curves
increase in
resistivity

I-V curves for different values of BTR resistivities.


Transfer Subsystem I-V Curves

I-V curves for four different field-dependent BTR
resistivities.

Summary:

We discussed:

Part I. General Overview of Xerography
Xerographic Process
Brief Description of Subsystems
Transport, Deposition and Removal of Fine Particles (Toner)
in Xerography

Part II. Modeling of Electrostatics
Charging & Transfer Subsystems Using Biased Rolls
Modeling the Electrostatics of Biased Rolls
Modeling a Transfer Subsystem Using A Biased Transfer
Roll


Xerography/Electrophotography: The Technology of Photocopiers and Laser Printers

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