The Science and Technology of Laser Printers and Photocopiers

The Science and Technology
of Laser Printers and
Photocopiers
August 27, 2007

Fa-Gung Fan
Xerox Research Center Webster
Xerox Corporation
Webster, NY
http://www.xerox.com/innovation/wcrt.shtml

Fa-Gung Fan, Xerox Corporation Academia Sinica, Taipei, Taiwan, 8/27/2007

Outline of the Talk
Part I. General Overview of Xerography
• Xerographic Process
• Description of Subsystems
Part II. Transport, Adhesion/Cohesion, and Removal of Fine
Particles (toner) in Xerography
• Measuring Toner Charge --- Cage Blowoff, Charge
Spectrograph
• Electrostatic Adhesion/Detachment of Toner Particles
• Measuring Toner Adhesion --- Atomic Force Microscopy
(AFM), Centrifuge Detachment, Electric Field Detachment
• Measuring Cohesion --- Fluidized Bed
Part III. Modeling & Simulation of Xerographic Subsystems
• Electro-hydrodynamic Flow (Corona Wind) in Corotron
Devices
• Charging & Transfer Subsystems Using Biased Rolls
• Modeling/Analysis of Electrostatics of Biased Rolls


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)


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

4


Xerox C118 Copier & M118 Desktop
Multifunction
18 prints/minute, black-and-white
(low end)

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


Office Multifunction Machines

DocuColor 40 Pro DocuColor 242/260
40 prints/minute 40 to 60 prints/minute
full-color full-color


DocuTech 180 HighLight Production
Publisher
180 prints/minute
highlight color


Xerox iGen3 Digital Production Color Press

110 prints/minute
full-color


iGen3
Xerographic Unit

Finisher/Binder


Subsystems
and
Components


Photoreceptor
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


Photoreceptor
A semiconductor whose
conductivity is a strong 0
0 5

Voltage on Surface (V)
function of light exposure.
light

_ _ _ _ _ _ _ _ _ _ _
_+_+
-700 ~ -800V
-1000
++ + + + + + ++ ++ + + +
Exposure (ergs/cm2)
Electron/hole pairs
• Requirements
– Insulator in the dark.
– Conductor when exposed to light
– Builds up enough voltage.
– Uniform properties

1. Charging Step
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


Charging Subsystem (a corotron)

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


2. Imaging/Exposure Step
data input
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


Imaging/Exposure

original
document

Traditional Analog Copier Laser Printer


3. Development Step
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


Development
development
housing Photoreceptor Photoreceptor
Toner
Apply E
Field E

Development roll Development roll
Development Development step:
Photoreceptor

Roll
Charge particles
triboelectrically
Electric field moves
Mixing
Charging particles from developer
roll to photoreceptor

Toner Particles + Carrier Beads
--- triboelectrification

Polymer particles loaded with
Toner pigment and some other additives

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

Traditional toner ---
produced by mechanical
means from breaking down
of large polymer chunks.


Toned Carrier Bead

q ≅ 2 x 104 e
m ≅ 2 x 10-10 gm

q/m ≅ 16 µC/gm

20 µm


Chemical Toners
Xerox Emulsion polymerization and Aggregation (EA) Process

Bottom-up approach in contrast to the top-down mechanical process that
produces traditional toner particles.

4. Transfer Step
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


Transfer Subsystem (a corotron device)

transfer corotron
Paper

Apply E Paper
Paper Field &
Separate E
Photoreceptor Photoreceptor
Photoreceptor

In the Transfer step:
Electric field moves particles from
photoreceptor to paper or transparency
Detachment field must overcome toner
adhesion to photoreceptor


5. Fusing Step
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


Fusing Subsystem

Fusing step:
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

Fused toner Paper

Elastomer Heat Roll


6. Cleaning & Erase Step
5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


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

Part II.
Transport,
Adhesion/Cohesion, and
Removal of Fine Particles
(toner) in Xerography


Transport, Adhesion/Cohesion and Removal
of Fine Particles (Toner) in Xerography

photoreceptor development housing
drum Toner must flow smoothly
down dispenser

Toner must develop
DC40 Pro onto roll uniformly

Toner must transfer from
roll to paper

carrier bead
paper

Transport, Adhesion/Cohesion and Removal
of Fine Particles (Toner) in Xerography

photoreceptor development housing
drum Toner must flow smoothly
down dispenser

Toner must develop
onto roll uniformly
Mag.
roll
Toner must transfer from
roll to paper
P/R
drum
carrier bead
paper

Toner Charge Measurements

Blowoff Tribo

Air
50

40
Q/M (µC/gm)
30
Blow toner
µ

from toned 20
beads in cage
10
Measure
charge & mass 0
difference 0 1 2 3 4
Calculate Toner Concentration (%)
average Q/M


Toner Charge Measurements

Charge Spectrograph

1.5
99%

95%

q/D (fC/µm)
1 90%
Air Flow
µ 80%
E Field 50%
0.5 Lycopodium

0
Inject toner
0 5 10 15 20 25 30 35
Displacement
∝ q/D -0.5 Diameter (µm)
µ
Measure D --- diameter of toner particle
position &
size of
particles

Toner Adhesion Forces

Fa Particle adhesion
depends on:
++ + ++
+ Size, shape, &
roughness
E + 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 Coulomb polarization
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 


Additives Control Adhesion

Changing type of additive modifies adhesion
Atomic Force Microscopy results


Atomic Force Microscopy (AFM)

Bring toner Push toner Retract toner until
near surface against surface probe releases

200 µm
20 µm


AFM

Measure Single Particle Adhesion

Laser
Photodetector

Particle
Cantilever
Piezoelectric:
moves up
and down


Centrifuge Detachment

Measure Many Particle Adhesion

Putt, putt, putt,... Vroom, vroom,... Whoosh,...

Observe Donor Plate after Each Spin

H. Mizes, “Adhesion of Small Particle”, Electro. Soc. Amer.
Univ. of Rochester, 6/23/95


Electric Field Detachment

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


Electrical Field Detachment of
Charged Toner

Detachment Cell
Adhesion of Triboelectrically Charged Toner
1
Transferred Fraction
0.8

0.6

0.4

0.2

0
0 2 4 6 8 10 12 14 16 18

Detachment Field (V/µm)
µ


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


Fluidized Bed

Measure Powder Cohesion

Stresses on the toner bed

τ σ
α α
h
σ
∆P

Tension Compression
σ = ρ (1−ε) gh cos α − ∆ P
(1−
Shear
(1−
τ = ρ (1−ε) gh sin α

P.K. Watson, “Yield Locus of Cohesive Granular Materials”, Workshop on
Dynamics of Granular Materials: Understanding & Control, Univ. of Chicago,
5/11/95

70
Te nsi on
60 C ompression Less Additives
τ , She ar Stre s s (N/m2 )

50
More Additives
40

30

20

10

0
-30 -20 -10 0 10 20 30 40 50 60 70
σ , Compre s s ive Stre s s (N/m2 )


Summary

I. General Overview of Xerography
• Xerographic Process
• Subsystems & Components
II. Transport, Adhesion/Cohesion, and Removal of Fine
Particles (toner) in Xerography
• Measuring Toner Charge
• Electrostatic Adhesion/Detachment of Toner Particles
• Measuring Toner Adhesion
• Measuring Cohesion
Part III. Modeling & Simulation of Xerographic Subsystems
• Electro-hydrodynamic Flow (Corona Wind) in Corotrons
• Charging & Transfer Subsystems Using Biased Rolls
• Modeling/Analysis of Electrostatics of Biased Rolls


Part III.
Modeling & Simulation of
Xerographic Subsystems


Corona Wind in Corotrons

5
2 Fusing
Exposure
1 6
Cleaning
Charging

4
Transfer
Photoreceptor
SN
SN

Substrate
N

S N

Development
Paper
3


Corona Wind
Corona wind phenomenon is a coupling of electrical and fluid-dynamic
problems. The study of this subject belongs to a branch of fluid
mechanics called electrohydrodynamics (or EHD).

Corotron Device Charging of a Photoreceptor

Governing Equations (2-D, steady-
state)
 ∂ 2V ∂ 2V 
Gauss’ Law ε0 2 + 2
 ∂x  = −q

 ∂y 
Conservation of Charge
∂q ∂q  ∂ 2 q ∂ 2 q   ∂  ∂V  ∂  ∂V 
u +v = D 2 + 2  + b   q
 ∂x  + q
 

∂x ∂y  ∂y   ∂x  ∂x
  ∂y  ∂y 
charge convection

Conservation of Mass
∂u ∂v
+ =0
∂x ∂y
EHD
Conservation of Momentum pumping
 ∂u ∂u  ∂p  ∂ 2u ∂ 2u 
ρ  u + v  = − + µ  2 + 2  + qE x
 ∂x
 ∂y 
 ∂x  ∂x
 ∂y  
 ∂v ∂v  ∂p  ∂ 2v ∂ 2v 
ρ  u + v  = − + µ  2 + 2  + qE y

 ∂x ∂y 
 ∂y 
 ∂x ∂y  

Kaptsov’s Condition at Coronating
Wire Surface
1
E onset = ( A + B ) Peek’s law
Rw

Condition at Moving Substrate
∂σ ∂V V
u paper = bq σ = ε0
∂x ∂y td


Electrical Potential Charge Density Air Flow Patterns

Zamankhan et al., J. Imaging Sci. Technol.,
50, pp. 375–385, 2006

Control of Air Flow in Corotrons

Fan et al., US Patent 7085512


Machines that Use Biased Rolls
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

DocuColor 40 Pro Office Biased Transfer Roll
Color Multifunction
Machine


Electrostatics of Biased Charging/Transfer
Rolls

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 =−
ε

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

Electric Conduction j =γ E where E = −∇φ


Modeling of Biased Charging Roll &
Comparison with Field Probe Measurement
Parameters used in the field probe
C. DiRubio, G. Fletcher, “Field profile experiment:
measurements in biased charging systems,”
Proceedings of Image Science Elastomer:
&Technology NIP 12, p.334, 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 =
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 -- elastomer; white -- air gap;
blue -- insulator on the drum)


Potential map for T=32.


Potential map for T=1.


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.


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

Post-Nip Pre-Nip

Intermediate Belt

V
Biased
Transfer
Roll


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 field profile in the ITB-PR air gap.


The field profile in the BTR-ITB air gap.


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.

increase in
resistivity

The I-V curves for different values of BTR resistivities.


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