U.S. patent number 3,781,105 [Application Number 05/309,562] was granted by the patent office on 1973-12-25 for constant current biasing transfer system.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Thomas Meagher.
United States Patent |
3,781,105 |
Meagher |
December 25, 1973 |
CONSTANT CURRENT BIASING TRANSFER SYSTEM
Abstract
Electrostatic transfer of charged particles to a transfer member
is accomplished using a roller electrode having electrically
relaxable and self-leveling layers. The roller, along with the
original support for the particles, defines a nip through which the
transfer member is passed. Asymmetrical fields associated with the
roller permit desirable post-nip air ionization while suppressing
undesired pre-nip ionization. Constant current regulation of the
roller current automatically corrects for electrical parameter
variations affecting the desired field levels, such as variations
in the thickness of the transfer member and variations in the
resistance of the roller with relative humidity.
Inventors: |
Meagher; Thomas (Webster,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23198720 |
Appl.
No.: |
05/309,562 |
Filed: |
November 24, 1972 |
Current U.S.
Class: |
399/314;
430/125.5; 15/1.51; 250/324; 250/325; 250/326; 399/66;
15/256.52 |
Current CPC
Class: |
G03G
15/1675 (20130101) |
Current International
Class: |
G03G
15/16 (20060101); G03g 015/16 () |
Field of
Search: |
;355/3,16,17 ;96/1.4
;117/17.5 ;250/49.5ZC ;118/637 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Greiner; Robert P.
Claims
What is claimed is:
1. Transfer apparatus for accurately transferring an image of
electrically charged particles between an original support and a
transfer support comprising:
an original support for carrying the particles, said support being
electrically insulating at least in the absence of certain
wavelengths of light;
an electrically conductive backing electrode underlying said
original support;
a transfer electrode positioned adjacent said original support and
said backing electrode to form pre-nip, nip, and post-nip regions
and air gaps providing passage therethrough of said transfer
support;
and variable electrical bias means connected internally to said
transfer electrode for providing current and variable voltage
thereto for generating electrical fields between said backing and
transfer electrodes in said pre-nip, nip and post-nip regions for
transfer of said particles from said original support to said
transfer support,
said transfer electrode including internal electrically relaxable
means to provide asymmetrical external electrical field levels from
said variable electrical bias means which are below substantial air
ionization levels in said pre-nip region and substantially above
air ionization levels at said post-nip regions,
said variable electrical bias means comprising variable voltage
applying means for regulating automatically said pre-nip, nip and
post-nip field levels in response to changing electrical parameters
affecting said fields by sensing and controlling the current
applied to said transfer electrode from said electrical bias
means.
2. The apparatus of claim 1 wherein said variable electrical bias
means is a constant current energy source.
3. The apparatus of claim 1 wherein said electrically charged
particles to be transferred are xerographic toner particles, and
wherein said original support includes a photoconductive layer.
4. The apparatus of claim 3 wherein said photoconductive layer is
carried by a cylindrical surface.
5. The apparatus of claim 3 wherein said photoconductive layer is
carried by a belt member.
6. The apparatus of claim 1 further including a paper sheet
comprising said transfer member.
7. The apparatus of claim 6 wherein said paper sheet is a cut
sheet.
8. The apparatus of claim 1 wherein said transfer electrode
comprises a roller having an electrically conductive core connected
to said bias means, and wherein said relaxable means is a thick
layer of selectively resistive material around said core.
9. The apparatus of claim 8 wherein said transfer electrode further
includes an electrically self-leveling layer comprising a thinner
layer of substantially higher resistivity material than said
relaxable layer overlying said relaxable layer.
10. The apparatus of claim 1 wherein in said post-nip region the
air gap between said transfer member and said transfer electrode is
larger than the air gap between said transfer member and said
original support.
11. In electrophotographic apparatus, charging apparatus for
controlled air ionization charging of a moving surface to be
charged, comprising:
a rotatable electrode roller positioned adjacent to said surface to
be charged and defining at least one air gap therebetween;
and variable electrical bias means internally connected to said
roller electrode for applying current and variable voltage thereto
for generating controlled air ionizing electrical fields in said
air gap between said roller electrode and said surface to be
charged;
said roller electrode having a single conductive core to which said
bias means is connected and a thick body of resistive material
surrounding said core,
said electrical bias means comprising variable voltage means for
regulating automatically said electrical fields between said roller
electrode and said surface to be charged in response to changing
electrical parameters affecting said fields, said electrical bias
means including bias current sensing and controlling means for
sensing and controlling the current applied to said roller
electrode from said electrical bias means so that the amount of
said air ionization in said gap is controlled.
Description
BACKGROUND OF THE INVENTION
This invention relates to the transfer of electrically charged
particles between supports. Specifically, this invention relates to
methods and apparatus for the improved electrostatic transfer of
xerographic toner particles with electric fields established by
roller or endless belt electrodes.
Roller electrode transfer systems employ D.C. electric fields to
move charged particles such as xerographic toner from first to
second supporting surfaces. (By D.C. it is meant that the direction
of the field vectors are not reversed 180.degree. on a unit time
basis.) The purpose is to exert an electrical force on the charged
particles that moves them from the first to the second support.
Among the difficulties is the fact that the direction and intensity
of the electrical fields acting on a particle will vary at
different locations relative to the roller electrode because the
electric fields are goemetrically dependent on the electrode
configurations. Accordingly, the direction and intensity of the
forces exerted on a particle by these fields will also vary with
position and time because there is relative movement between the
particles and the electrodes during transfer.
The electrodes establishing the transfer field generally include a
roller electrode and a backing electrode. The backing electrode is
one adjacent the support initially carrying the particles. The
backing electrode is often in the configuration of either a flat
plate or a cylinder, and is supported for movement relative to the
roller electrode. The region of closest proximity between the two
electrodes defines a nip region between which passes the second, or
final, particle support or transfer member. Conventionally, the
geometry of the various apparatus of roller transfer systems is
symmetrical.
The initial particle support may be a conductive member and thereby
comprise the backing electrode. Alternately, the initial support
may be, for example, an insulator, a photosensitive semi-conductor,
or have an insulative substrate, at least in a link or web
positioned between the two electrodes, and may carry charges
affecting the transfer field.
Historically, the transfer of toner images between supporting
surfaces is accomplished with the electrostatic transfer of either
a corotron or a roller electrode biased to constant potential
(constant voltage) levels. Various trade-offs are made in choosing
between the corotron or roller transfer systems. The corotron
system is particularly noted for its relative simplicity, but the
charges deposited by the corotron electrostatically tack the
transfer support (e.g., paper) to the original toner support (e.g.,
a photoconductor) in addition to creating the desired electric
field effecting transfer of the toner to the paper. This strong
(tacking) attraction between the paper and the original toner
support makes it mechanically difficult to separate or detack the
two supports.
The detack problem is less severe when the transfer member is a web
or other mechanically gripped member, and for that reason web
transfer members have been generally felt to be better suited for
higher speed copying machines, and for biased roller transfer
systems. No commercially successful copying machine is believed to
have employed a roller transfer system with cut sheet transfer
members.
As indicated, practical commercial corotron transfer systems have
used constant voltage regulation for creating, directly or
indirectly, the electric fields effecting transfer of the toner
image. This is appropriate because a corotron is a voltage
sensitive device that provides a current proportional to the
potential difference between it and the surface from which it is
spaced, by the generation of a corona with ion flow toward the
surface. Current control is usually not a problem. To express it
another way, corotron current control, when needed, may be
accomplished by operating well above some minimum current level
with a total current (largely to the shield) which is high and
relatively constant. Furthermore, variable current sources are
believed in the art to be preferred in applications where the
objective is the charging of a non-uniformly charged surface to a
more uniform potential. A photoconductor bearing an electrical
image into a transfer area is such a non-uniformly charged
surface.
Thus, in contrast, constant current, especially in connection with
bias roller transfer systems, has heretofore been of little or no
interest. The roller transfer systems rely upon some minimum
voltage difference between the bias roller electrode and the
photoconductor (or other initial toner support) to transfer the
charged toner particles to the paper. However, some, but excessive,
charging of the sheet of paper is highly desirable within the
post-nip region to keep the transferred toner particles tacked to
the paper after the paper has left the region of high field around
the nip formed between the roller and paper. This corona charging
can be provided in the present invention by the selective
ionization of air at the nip exit region.
Significantly, it has been found by the present inventors that
constant current control of the bias voltage supply in a transfer
roller system as disclosed herein, rather than constant voltage
control, provides a highly desirable and novel result. It can
provide a controlled amount of desired post-nip corona with
suppressed pre-nip corona over a wide range of physical variations
in operating conditions, including substantial relative humidity
changes.
In regard to the construction and materials of the bias roller
described herein, incorporation by reference is made herein of a
prior U. S. Pat. No. 3,702,482 by C. Dolcimascolo et al., issued
Nov. 7, 1972. Further, some prior art in corotron charging voltage
and current controls is shown, for example, in U. S. Pats. Nos,
2,576,047, 3,062,956 and 3,335,273, 4 and 5. Gundlach Pat. No,
2,912,586 discloses xerographic charging rollers and materials.
Fitch U. S. Pat. No. 2,807,233 illustrates roller electrical toner
transfer.
It is an object of this invention to employ constant current
biasing in a toner transfer system.
Another object of the present invention is to enhance xerographic
imaging systems by maintaining constant corona currents to members
being charged in a biased member transfer system.
Yet another object of the invention is to reliably control the
pre-nip and post-nip corona currents generated in a roller
electrode system over a wide range of humidity and equipment
variations.
These and other objects of the instant invention are accomplished
by a novel system employing a special roller electrode biased with
a constant current energy source. The present transfer system is
capable of handling a wide range of transfer members wherein
pre-nip and post-nip corona currents are effectively regulated,
including cut sheets.
DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the instant
invention will be apparent from the present description and from
the drawings wherein:
FIG. 1 is a schematic elevational side view of an exemplary biased
roller electrode transfer system in accordance with the present
invention.
FIG. 2 is a schematic of one example of an electrical circuit
capable of serving as the constant current energy source for the
present transfer system.
FIG. 3 is a graph of electrical field as a function of time (where
the center of the transfer roller nip is time) in three regions: at
the nip entrance (pre-nip); in the nip; and after the nip
(post-nip).
FIG. 4 is a graph comparing the applied bias to an exemplary
transfer roll versus roll resistivity and relative humidity.
FIG. 5 is a graph comparing nip fields versus roll resistivities
for an exemplary roll.
FIG. 6 is a schematic cross-sectional side view of another biased
roller, transfer sheet, and photoconductor (or other insulating
surface) illustrating the post-nip ionization of air around the
roller during the transfer process and the relaxation and
self-leveling abilities of the transfer roller.
DETAILED DESCRIPTION
Xerographic toner 10 typically comprises microscopic size particles
(0.1 - 20 microns) that are opaque or include opaque materials.
Electrically, the toner is highly insulating and carries a net
electrical charge. The desired polarity of the toner charge depends
on the polarity scheme of the imaging system. In the presently
described embodiment the toner is assumed to have a net negative
charge, which thereby suggests the other polarities illustrated,
discussed and intimated. Quite clearly, alternative system based on
positively charged toner particles are also benefited by the
present invention, although a detailed description thereof is
omitted here, as it would be redundant.
Referring particularly to FIG. 6, the photoconductor
(photoreceptor) 11 is, for the purposes of the present discussion,
a moving electrical insulator web. It is supported by a conductive
core (roller) 12 which is electrically coupled here to a ground
potential 13 as a convenient and safe potential level. The backing
electrode may also be a continuous conductive backing layer of the
photoreceptor belt, grounded by a contacting grounded wiper, in
which case the backing roller 12 can be non-conductive. The
transfer roller should not be allowed to contact any grounded
surface during operation. The plus signs 14 on the photoconductor
11 represents positive charges associated with an electrical latent
image on it. In one xerographic system the latent image is a
pattern of charge 14 created by steps including uniformly charging
the photoreceptor and then exposing it to a light image.
Alternately, the latent electrical image may be created even on a
non-photosensitive insulator by selectively depositing charge on
the insulator through a stencil shaped in the form of an image, or
other imaging means. In most systems the latent electrical image is
developed by steps including bringing toner particles 10 adjacent
the latent images. The fields associated with charge 14 then
electrostatically tack the charged toner particles to the insulator
11.
The transfer of the toner to a transfer member can be accomplished
electrostatically by a roller electrode, as illustrated in Fitch U.
S. Pat. No. 2,807,233, or by a corotron as described in Shaffert U.
S. Pat. No. 2,576,047. In these references the bias supplied to the
transfer device (roller or corotron) is indicated as a constant
voltage energy source.
The present transfer system uses a novel transfer roller biasing
scheme to greatly enhance the electrostatic transfer operation.
Still referring to FIG. 6, a transfer roller 15 is appropriately
journaled for rotation at an angular velocity such that the
peripheral speed of the roller is substantially equal to the speed
of the insulator 11. A cut sheet transfer member 16, e.g., 81/2
.times. 11 inch, 20 pound bound paper, is fed by appropriate means
into the nip 17 formed between the roller 15 and insulator 11. The
arrows shown indicate the relative direction of movement for the
roller 15, insulator 11 and paper 16. The terms "pre-nip" and
"post-nip" used herein refer to the direction of travel of the
transfer sheet 16 through the nip, and in FIG. 6 correspond to the
right and left hand regions respectively adjacent the nip 17.
The exemplary roller 15 here includes an electrically "self-
leveling" outer layer 20, an electrically "relaxable" next (inner)
layer 21 and a central conductive core or axle 22. The constant
current electrical bias or energy source 23 is electrically
connected to the conductive core 22.
The heart of the roller electrode 15 is the thick relaxable layer
21, which has a bulk resistivity falling in a well defined
operating range selected in relation to roll diameter and surface
velocity. For the specific system described herein, the bulk
resistivity of the relaxable layer can vary over the range from
about 10.sup.7 to about 10.sup.11 ohms per centimeter. A variation
in this resistivity of about two orders of magnitude, primarily as
a result of static and dynamic changes in relative humidity or RH
(extending generally from 5-to-10% RH to 85-to-100% RH), is
observed for practical available commercial materials in this
resistivity range. The preferred resistivity ranges may vary for
transfer systems designed to operate at different throughput speeds
of the transfer sheet 16. These numbers are for a roller 15
diameter of about 3 inches, a roller 12 diameter of about 5 inches,
and a paper speed of from about 10 to about 20 inches per second. A
properly selected resistivity range is critical for the relaxable
layer 21 operation, even for present day copying speeds of 10 to 20
inches per second. (Doubling the speed is generally equivalent to
halving the resistivity).
The relatively soft, thick, electrically relaxable body 21 may be
mounted directly on the axle 22 of the bias roll. The relatively
low durometer of this material allows good mechanical contact in
the transfer zone at moderate pressures and eliminates "hollow
character" transfer under normal operating conditions. Since the
relaxation time of the core material is long compared to the ion
transfer time of gaseous discharges, during air breakdown the roll
acts like an insulator, protects against arcing and helps control
the amount of charge transferred at any point on the surface.
The relaxable layer 21 comprises a material that functionally takes
a selected time period to transmit a charge from the conductive
core 22 to the interface 47 between the relaxable layer 21 and the
self-leveling layer 20 sufficient to restore said interface 47 to
about the bias potential applied to the core 22. This selected time
period is that corresponding to the roller surface speed and nip
region width, i.e., roughly greater than the time any point on the
transfer roller is in the nip region, and is chosen to be
approximately one quarter of the roller revolution time.
Functionally, this means that the magnitude of the external
electric field increases significantly from the pre-nip entrance
toward the post-nip exit, while the field within the relaxable
layer diminishes. Thus, a relaxable layer is one that has an
external voltage profile which is non-symmetrical about the
transfer nip. As will be discussed shortly, the ideal conditions
are to have a field strength below that for substantial air
ionization in the air gap at the entrance to the nip, and a field
strength above that required for air ionization in the air gap just
beyond the exit of the nip. [Some pre-nip ionization may be
allowable]. The present invention realizes these goals.
The (outer) self-leveling layer 20 is a leaky insulator. The layer
20 is selected for substantially higher resistive values, which in
the present embodiments means in the order of about 10.sup.11 to
10.sup.14 ohms per centimeter. In addition, the self-leveling layer
includes materials, (or is so related to the relaxable layer), such
that charges applied to the outer surface 24 of the self-leveling
layer 20 will be generally dissipated within one revolution of the
roller 15. This dissipation of charge is desirable to prevent
suppression of the transfer field in the nip.
It is desired that the self-leveling layer 20 thickness divided by
its dielectric constant should be substantially greater than any
other material in the nip in order for its capacitance to be much
less than such other materials, as desired. The self-leveling layer
20 also acts as a thin insulating layer coated on the surface of
the relaxable core material to help protect the roll during air
breakdown, to act as a moisture barrier, to limit current flow
through the roll, and to make the roll surface easy to clean.
However, if the relaxable material is durable and cleanable the
self-leveling layer 20 is not essential.
A constant current supply can compensate for non-leveling as long
as the voltage buildup across this layer 20 does not cause the
power supply to exceed its rated maximum output voltage, and as
long as the charge on this layer is reasonably uniform. Some
non-leveling is tolerable.
The paper or other transfer sheet 16 is here a cut sheet (versus a
web) that is fed into and extracted from the transfer system by
conventional or appropriate means. One example is illustrated in
FIG. 1 and discussed in more detail later. The comparison of the
sheet to the web is intended to differentiate sheet 16 from those
transfer members that are guided through the transfer system by
being mechanically coupled to the transfer roller, as illustrated
in the above-cited Dolcimascolo et al. patent, or wound between
spools or the like as illustrated in the Fitch patent, supra.
The transfer sheet typically will be conventional 20 pound bond
paper with or without a plastic coating. It should be understood,
however, that an advantage of the present system is that it can
operate with paper weights ranging from nine pound "vellum" to 100
pound or greater card stock. Alternately, the transfer sheet here
may include various transparent materials, such as polyester resin
sheet sold commercially under the trade name "Mylar."
Electrically, paper is generally a fair insulator at low RH and a
fair conductor at high RH. Consequently, the charge illustrated by
the plus signs 30 on the non-image side of the transfer sheet 16
may actually leak onto the image side of the sheet if the sheet is
reasonably conductive. The plastics are, of course, generally
always highly insulating.
A key factor in the improved operation of the present transfer
system is the constant current energy (bias) source 23. Its
automatic current control controls pre-nip ionization to tolerable
levels while allowing a desired amount of post-nip ionization even
when RH variations, roller material aging, paper thickness changes,
and other factors change the electrical parameters of the transfer
system, and yet while maintaining high transfer fields. Before the
bias circuit 23 is described here, however, it is helpful to first
discuss the electric fields holding the toner to its support and
the roles the relaxable and self-leveling layers of roller 15 play
in the transfer process.
While the toner 10 is carried by the insulator 11 toward the nip
region 17, the toner is tacked to the insulator by the fields
associated with the latent image charge 14 and by other adhesive
forces such as VanderWaal forces. In some imaging systems, there
may also be charge in the non-image areas, i.e., areas adjacent
charge 14, but this non-image charge is at a different potential or
is otherwise separated from the image charge by some discontinuity
that causes the toner particles 10 to preferentially adhere to the
image areas. Furthermore, the charge 14 may be substantially
altered before reaching the nip, as in systems where a
photoconductor is involved, by exposing the photoconductor to light
discharge. Even so, the original support is still able to retain
the toner particles in place with such reduced field strengths. The
reduced tacking fields are advantageous for the simple reason that
transfer can occur with a lower nip field. Another reason is that
if charge 14 is not reduced, excessive fields prior to entering the
nip might cause air ionization, or cause "fuzzy" images (or loss in
sharpness) due to premature transfer of the toner while the gap is
too large. It is to be noted, however, that the present transfer
system is effective whether or not the latent image, e.g., charge
14, is altered prior to transfer. One reason for this is that it is
considered that the constant current bias 23 may offset the bad
effects that excessive image charge 14 may otherwise have on the
transfer operation.
With constant current bias, the pre-transfer illumination of the
photoconductor is desirable because it insures that the transfer
current density in image regions is nearly equal to the current
density in background regions. Without it the current density in
background regions would be much larger than in image regions,
particularly with low resistivity bias rolls, thus reducing
transfer efficiency.
The operation of roller 15 may be explained in connection with the
generalized curves in FIG. 3. The time scale along the horizontal
axis represents the movement of the transfer sheet 16 through the
nip region. The pre-nip period is to the left of the nip period 43,
and the post-nip period is to its right. Since velocities are
assumed constant here the horizontal axis also corresponds to path
distances from the nip area 17.
The volts-per-micron scale along the vertical axis of FIG. 3
represents relative transfer field intensity along the path of the
transfer sheet. The field observed is that between the outer
surface 24 of the roller 15 and the free surface of the toner
support 11. It is that field which effects the transfer of the
toner 10 between supports 11 and 16.
Curve 40 in FIG. 3 is the Paschen curve which represents the field
intensities at or above which ionization of air will normally occur
(on both sides of the nip). Curve 41 is the field curve generated
by the roller transfer system of FIGS. 1 and 6. Curve 42 is an
exemplary curve for prior art rollers not having a relaxable layer
21, e.g., conductive rollers and conductive rollers overcoated with
high resistance and/or high dielectric materials. This curve 42 is
included to comparatively dramatize the desirable asymmetrical
nature of the subject curve 41, which permits post-nip but prevents
pre-nip ionization of air.
Discontinuities (not shown) may occur in curves 41 and 42 at the
exit 44 to the nip due to the charge associated with the transfer
of toner particles 10. When the toner 16 is not present, the
portions 41A and B and 42A and B of curves 41 and 42, respectively,
are continuous, as shown. (In contrast to curve 41B, the charge
density of a non-coated conductive roller, or the field between an
insulator and a conductive core, would begin to fall off
immediately at the exit to the nip).
Curve 42 represents the prior approach to roller transfer in that
it is symmetrical about the nip contact region (represented by the
time period 43) in the absence of toner and air ionization effects.
Curve 41 is asymmetrical because of the effects of the relaxable
and self-leveling layers during and just after exit from the nip
region 43. The object is to have portion 41B of the curve 41
continue upward in post-nip until the Paschen curve is reached,
thereby initiating the desired post-nip corona ionization. Yet
pre-nip portion 41A is selected to remain below the Paschen curve
40 to realize the preferred condition of no pre-nip ionization. In
contrast, it is apparent from an inspection of symmetrical curve 42
that the prior art rollers must be biased either above or below the
Paschen curve in both pre-nip and post-nip. If they are biased
above, damaging pre-nip ionization accompanies the desired post-nip
ionization. If they are biased below, the pre-nip ionization is
suppressed, but so is the post-nip, and other means of keeping
toner 10 tacked to sheet 16 must therefore be employed in lieu of
post-nip ionization.
The transfer conditions depicted by FIG. 3 is schematically
illustrated by the plus signs 48 in FIG. 6. The plus signs 48
represent charge at the roller internal interface 47. Prior to
entering the nip, the relaxable layer 21 is not subjected to high
internal fields; that is, its outer surface is at substantially the
same potential as the core 22. Just prior to and in the nip area
the roller surface becomes closely spaced from the grounded backing
electrode (support) 12. This tends to draw charge toward the roller
15 surface, but charge movement is resisted by the roller
resistivity. Thus, the charge density at interface 47 increases as
the relaxable layer proceeds through the nip in proportion to the
resistivity of the relaxable layer. Initially after exiting the
nip, the charge density will generally continue to increase due to
the internal field in the relaxable layer 21, or the induced charge
may have nearly reached equalibrium; in either case the rapid
increase in the air gap soon after separation occurs causes the
ionization level to be reached for the field strength corresponding
to the residual charge density. (The Paschen curve level at which
ionization occurs in a function of spacing as well as field
strength, and in the present case it is mainly reached by the
increase in the air gap rather than by an increase in the
field).
Ions from this air breakdown are drawn to the opposing surfaces 24
and 30. Then, as the gap becomes substantially wider, the air gap
field falls below the Paschen curve, and, as discussed above,
charge relaxation occurs in the relaxable layer 21; so ionization
stops.
The plus signs 30 and negative signs 49 represent positive and
negative ions deposited on the transfer sheet 16 and the outer
surface 24 of the roller 15, respectively, as a result of the
post-nip ionization of the air in the gap. (Note that a plus sign
48 is positioned at interface 47 opposite each negative sign 49 to
represent an induced counter-charge within the roller which was
brought to interface 47 during relaxation of material 21 in the
nip). The positive charge 30 holds (and continues to maintain) the
transferred toner 10 to sheet 16. The negative charge 49, on the
other hand, is dissipated by current flow through the self-leveling
layer 20 during the subsequent one to five revolutions of the
roller.
The field intensity required to break the bond of the toner 10 to
the initial support 11, and to tack the toner to the sheet 16, is
reached at some time after the entrance to the nip but before
post-nip ionization occurs. (In FIG. 3, the transfer is depicted
for convenience as having occurred at the exit of the nip).
However, a continued "holding" or tacking field (from charge 30)
must also be present during the subsequent stripping of the paper
16 from the support 11 for high efficiency and stable toner
transfer.
With the above information, the significance of the constant
current energy source is better understood. Stated simply the
significance is that suggested earlier; namely, the constant
current source provides automatic correction of post-nip fields to
compensate for changes in the electrical parameters of the roller
and its environment. The parameters that normally experience the
greatest and most frequent fluxuations are roller resistivity,
which is very sensitive to RH, and transfer sheet thickness. In
terms of FIG. 3, constant current biasing is the method and means
for keeping curve portion 41A below the Paschen curve 40 to prevent
pre-nip ionization and for insuring that curve portion 41B
intersects the Paschen curve in the post-nip region. This control
of the extent of post-nip ionization controls the amount of
deposited charge 30, and therefore the toner "holding" field on the
paper 16 is more constant, and maintainable at a moderate level
providing good toner holding, but also easier paper stripping.
Thus, high transfer efficiency is achieved with a relatively lower
applied current and charge density on the transfer member.
Curves 54, 55, and 56 in FIG. 4 depict the beneficial operation of
constant current biasing. Curves 54 and 56 represent the field
levels for pre-nip and post-nip ionization, respectively, versus
changes in the resistivity of roller 15 for the previously defined
resistivity range. The roller resistivity variations are exemplary
changes in the resistivity of the relaxable layer with changes in
RH. Curve 55, which lies in the ideal region between curves 54 and
56, is obtained by keeping the roller current constant.
Curves 60 and 61 in FIG. 5 illustrate a family of curves for
constant current that permit field intensities below the pre-nip
ionization level. Curve 62 represents the pre-nip ionization level
for the resistance range shown.
The current referred to as being held constant throughout this
description is the current to the roller 15 core 23, I.sub.R . This
roller current I.sub.R is, by reason of conservation of charge,
basically equal to the post-nip ionization current I.sub.N .
(Substantially zero pre-nip current is, of course, one of the
desired operating conditions here.) The constant current bias
source 23 may be described as a device for automatically widely
varying the potential level coupled to roller 15 to automatically
compensate for I.sub.R changes, due to the connected load
(resistance) changes, which are due to changes in ambient RH and
temperature and aging of materials plus other factors tending to
effect the pre-nip, nip and post-nip field levels such as paper
thickness, charge build-up on the self-leveling layer, etc. In the
specific system described herein, the constant current source
output I.sub.R is equal to about 1.5 microamps per inch, where the
inch refers to the length of the roller along its axis
(perpendicular to the plane of FIG. 6). The wide internal roller
resistivity swing previously discussed requires the bias potential
on core 22 here to vary from about 800 to about 4,000 volts to
maintain this constant current of 1.5 microamps per inch (Note FIG.
4). Thus, the bias source 23 output voltage must vary automatically
over this voltage range.
It is important to point out at this time that effective constant
current biasing is closely interrelated here to the
earlier-discussed self-leveling ability of the outer layer 20. If
the negative charge 49 on the roller surface 24 is not dissipated,
it may suppress transfer performance during subsequent revolutions
of the roller by exceeding the voltage compensation capability of
the bias source 23. Alternatively providing external charge
leveling means, such as a conductive roller in contact with the
outer surface 24, or a biased neutralizing corotron, would not be
satisfactory for the present constant roller current operation. The
current drawn by any such external leveling devices, because of
conservation of charge, would be substantially equal to the
post-nip current I.sub.N. This means that I.sub.R would be zero,
leaving nothing to regulate by the bias source 23 in response to
changes in the roller electrical parameters. The currents in an
external leveling device could be regulated by a constant current
biasing circuit, but such a circuit would be insensitive to the
roller electrical parameters. Consequently, it may be seen that the
desired results of the present invention would be quite difficult
to achieve with a constant current biasing of external, rather than
internal, charge leveling means.
The air gaps W, X, Y and Z (FIG. 6) around roller 15, sheet 16 and
support 11 are important. The paper transfer sheets with which this
invention is particularly concerned will necessarily involve such
various air gaps although possibly in some altered manner from that
illustrated. As long as pre-nip ionization is suppressed, gaps W
and X cause no particular electrical problem to the transfer
operation and the relation of gaps W and X to each other is not
particularly critical. Pre-nip ionization in gap W results in
charging of the sheet 16 which in turn may cuase premature toner
transfer resulting in poor image resolution. It also induces
ionization in gap X. Ionization in gap X causes the charge
associated with toner 10 to be altered. Pre-nip ionization effects
will be less if X is smaller than W, i.e., if the paper is closest
to the photoconductor. High fields in gap X can also cause the
toner to jump prematurely across the air gap W.
Air gaps Y and Z are critical because the desired post-nip
ionization must occur in gap Y. The ionization in gap Y enables the
charge 30 of appropriate sign to tack the transferred toner to
sheet 16. (However, as noted, it also tacks the sheet 16 to the
photoconductor 11.) Ionization, if any, in gap Z usually follows
that in gap Y and reduces the net charge in the paper and adds to
the negative charge associated with the transferred toner 10,
bonding it more strongly to the paper.
Ionization in gap Z before gap Y would lead to a negative charge on
the paper and untacked toner. Ionization in gap Y in preference to
gap Z, and efficient transfer prior to stripping, is insured by
making gap Y open at a faster rate than, and prior to, gap Z. This
is accomplished respectively in the present embodiment by selecting
the radius of roller surface 24 to be less (e.g, by a factor of 2 -
4 times) than the radius of support 11, and by the partial wrap in
post-nip of the sheet 16 and support 11 together on the backing
roller 12. I.e., stripping occurs substantially after the transfer
nip. The radius of support 11 is established for the apparatus of
FIG. 6 by the radius of the backing roller 12.
It should also be noted that the shape of the Paschen curve for an
air gap can be affected by the paper position. When the paper
splits the inter-roller air gap into two gaps, as X and W, air
ionization will require a higher field level than for a single gap,
as Y (which is desired here.)
The exemplary electrical circuit shown in FIG. 2 is capable of
providing the constant current biasing voltage source 23 for roller
15. Transistors Q301 and Q302 level out input power E.sub.in to a
transformer T1 primary. Initially assume the circuit is working in
a steady state condition, supplying current I.sub.R within the
above-discussed specifications to the load (core 22). If the load
resistance suddenly increases (e.g., when paper is fed between the
bias roller and photoreceptor), the load current tends to decrease
from its steady state value. A load current sensing return path is
provided up through zener CR308, current setting pot R8 and
resistance R312 and then to the transformer T1 secondary at tap P7.
(The low side of the rectified output). Thus, as the load current
decreases the voltage drops across R8 and R312 are reduced, and
hence, the voltage across capacitor C305 to ground tends to go up.
This decreases the connected base voltage input to transistor Q305,
thereby decreasing its emitter current. The base of adjacent
transistor Q304 is clamped to a fixed voltage (determined by
voltage divider R308 and R317), so the voltage at its emitter is
effectively constant. Current to transistors Q304 and Q305 is
shared through a common emitted supply resistor R310. Thus, when
the current through Q305 decreases, the current through Q304
proportionally increases, thereby increasing the voltage drop
across its output resistor R311. Resistor R311 connects to the base
of a transistor Q303, and an amplified rectified a.c. output
appears across its output resistor R306 which is connected to
primary tap P5 as a control voltage E.sub.C. The increase in
E.sub.C increases the total peak-to-peak voltage in the transformer
T1 primary and, therefore, the output voltage developed across the
load at the transformer secondary. The output voltage will thereby
increase until the load current reaches its original value
determined by setting of pot R8.
It will be appreciated that many other suitable applicable constant
current supply designs are available in the electronics art, and
that the above-described circuit is merely exemplary.
The mechanical features of the particular transfer system depicted
in FIG. 1 are also described in copending applications. Briefly,
transfer sheets 70 are fed through chute 71 in registration with a
toner image on the photoconductive belt 72. Prior to reaching the
nip region 73, the charge on the surface of the belt 72 may be
altered by an appropriately biased corotron 74 and/or a
pre-transfer lamp. The transfer roller 75 and constant current bias
source 76 are of the same design as that described in connection
with FIG. 6. As discussed in the previously cited Dolcimascolo
application, a specific example of an appropriate self-leveling
layer 77 is a polyurethane material commercially available from the
DuPont Company under the tradename "Adiprine" (Type L315) while
examples of the relaxable layer 78 include materials such as the
polyester urethanes. An outer overcoating of low moisture
permiability polymer material such as polyvinylidene chloride may
be additionally applied. The speed of the sheet through the nip 73
is about 10-20 inches per second. The cleaning brush 80 with its
associated vacuum housing 81 is positioned to clean stray toner and
dirt from the outer surface of roller 75.
The corotron 82, in the post-nip area in FIG. 1, is commonly
referred to as a detack corotron. It is designed to neutralize or
lower the potential of the charge deposited onto the sheet 70 by
transfer post-nip ionization. That is, corotron 82 is designed to
neutralize most of the charge represented by the plus signs 30 in
FIG. 6. Lowering the charge on the non-image areas of the transfer
sheet makes the sheet easier to strip from the belt 72. In this
regard, the output of the present constant current bias source 76
or 23 can be adjusted so that the tacking force of the charge on
the non-image side of sheet 70 does not tack the sheet 70 to the
belt 72 too strongly. In other words, the bias current previously
described as I.sub.N can be adjusted so that a detack corotron 82
is not necessary, although transfer efficiency may be lowered.
The extraction of sheet 70 from the transfer system and belt 72 is
accomplished by a vacuum belt transport 83. Transport 83 includes
the continuous belt 84 moving around roller 85 in the direction
indicated. Acting through the bottom section of the belt 84 is a
vacuum chamber 86 that pulls the sheet 70 to the bottom of the belt
84.
Obviously, the foregoing embodiments may be altered without
departing from the spirit of the present invention. Clearly, a
xerographic system using a photoconductor arranged on the surface
of a cylinder is equally profited by the present system. Likewise,
other variations in environments or in details of the
above-described system are possible without departing from the
instant invention.
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