Heated bias transfer roll

A bias transfer member including an internal heating element is disclosed. The transfer member includes a thermocouple or voltage activated switch for allowing current flow through the heating element in response to predetermined conditions to raise the temperature of the transfer member and thereby decrease and maintain a substantially constant resistivity in the transfer member so as to extend the electrical life of the transfer member.

The present invention relates generally to a system for transfer of charged 
toner particles in an electrostatographic printing apparatus, and more 
particularly concerns an electrically biased contact transfer member 
including a heating element for applying heat to the transfer member. 
Generally, the process of electrostatographic copying is executed by 
exposing a light image of an original document onto a substantially 
uniformly charged photoreceptive member. Exposing the charged 
photoreceptive member to a light image discharges a photoconductive 
surface thereon in areas corresponding to non-image areas in the original 
document while maintaining the charge in image areas, thereby creating an 
electrostatic latent image of the original document on the photoreceptive 
member. This latent image is subsequently developed into a visible image 
by depositing charged developing material onto the photoreceptive member 
such that the developing material is attracted to the charged image areas 
on the photoconductive surface thereof. Thereafter, the developing 
material is transferred from the photoreceptive member to a copy sheet or 
to some other image support substrate to create an image which may be 
permanently affixed to the image support substrate, thereby providing an 
electrophotographic reproduction of the original document. In a final step 
in the process, the photoconductive surface of the photoreceptive member 
is cleaned to remove any residual developing material thereon in 
preparation for successive imaging cycles. 
The described electrostatographic copying process is well known and is 
commonly used for light lens copying of an original document. Analogous 
processes also exist in other electrostatographic printing applications 
such as, for example, digital laser printing or ionographic printing and 
reproduction where charge is deposited on a charge retentive surface in 
response to electronically generated or stored images. 
The operation of transferring developing material from the photoreceptive 
member to the image support substrate is realized at a transfer station. 
In a conventional transfer station, transfer is achieved by applying 
electrostatic force fields in a transfer nip sufficient to overcome forces 
holding the toner particles to an original support surface on the 
photoreceptive member. These electrostatic force fields operate to attract 
and transfer the toner particles over onto the copy sheet or other support 
surface. 
Typically, transfer of toner images between support surfaces in 
electrostatographic applications is accomplished via electrostatic 
induction using a corotron or other corona generating device. In such 
corona induced transfer systems, the final support sheet or other image 
support substrate is placed in direct contact with the toner image while 
the image is supported on the photoconductive surface. Transfer is induced 
by spraying the back of the support sheet with a corona discharge having a 
polarity opposite that of the toner particles, thereby electrostatically 
transferring the toner particles to the sheet. An exemplary corotron ion 
emission transfer system is disclosed in U.S. Pat. No. 2,836,725. 
Biased roll transfer systems have also been used successfully to accomplish 
toner transfer. This type of transfer was first disclosed by Fitch in U.S. 
Pat. No. 2,807,233 which disclosed the use of a metal roll coated with a 
resilient coating having an approximate resistivity of at least 10.sup.6 
ohm-cm, providing a means for controlling the magnetic and non-magnetic 
forces acting on the toner during transfer. One shortcoming in such bias 
roll transfer systems arises because the resistivity of the resilient 
coating introduces a limit to the amount of bias that can be applied to 
the roll due to the fact that, at higher ranges, the air in and about the 
transfer zone begins to break down, or "ionizes", causing the image to 
degrade during transfer. Nonetheless, bias roll transfer has become the 
transfer method of choice in many state-of-the-art xerographic copying 
systems and apparatus. Notable examples of biased roll transfer systems 
are described in U.S. Pat. No. 3,702,482 by C. Dolcimascolo et al., and 
U.S. Pat. No. 3,781,105, issued to T. Meagher. Other general examples of 
biased roll transfer systems can be found in U.S. Pat. Nos. 3,043,684; 
3,267,840; 3,328,193; 3,598,580; 3,625,146; 3,630,591; 3,684,364; 
3,691,993; 3,832,055; and 3,847,478. 
As described, the process of transferring development materials in an 
electrostatographic system involves the physical detachment and 
transfer-over of charged particulate toner materials from one surface into 
attachment with a second surface via electrostatic force fields. The 
critical aspect of the transfer process focuses on maintaining the same 
pattern and intensity of electrostatic fields as on the original latent 
electrostatic image being reproduced to induce transfer without scattering 
or smearing of the developer material. This difficult requirement is met 
by careful control of the electrostatic fields which, by necessity, must 
be high enough to effect toner transfer while being low enough so as not 
to cause arcing or excessive ionization at undesired locations. Such 
electrical disturbances can create copy or print defects by inhibiting 
toner transfer or by inducing uncontrolled transfer of the development 
materials. 
The problems associated with successful image transfer are well known. In 
the pre-transfer air gap region or the so called pre-nip region, 
immediately in advance of copy sheet contact with the image, excessively 
high transfer fields can result in premature transfer across the air gap, 
leading to decreased resolution or blurred images. High transfer fields in 
the pre-nip air gap can also cause ionization which may lead to loss of 
transfer efficiency, strobing or other image defects, and a lower latitude 
of system operating parameters. Conversely, in the post-transfer air gap 
region or the so called post-nip region, at the photoconductor/copy sheet 
separation area, insufficient transfer fields can cause image dropout and 
may generate hollow characters. Improper ionization in the post-nip region 
may also create image stability defects or cause copy sheet detacking 
problems. The overriding consideration in providing an effective transfer 
system centers on the fact that the transfer field should be as large as 
possible in the region directly adjacent the transfer nip where the copy 
paper contacts the image so that high transfer efficiency and stable 
transfer can be achieved. 
Variations in ambient environment conditions, copy paper resistivity, 
contaminants, and field strength, can all effect necessary transfer 
parameters. Material resistivity can change greatly with humidity and 
other environmental parameters. Further, in bias transfer roll systems, 
conduction of the bias charge from the bias transfer roll is greatly 
affected by the magnitude of transfer current through, as well as the 
resistivity of the material of the bias roll. Moreover, the functional 
life of the bias transfer roll is directly related to maintaining a 
constant controlled resistivity of the coating through which the transfer 
current flows. 
It has been shown that charge control additives such as organic salts and 
specifically tetrahepthlammonium bromide (THAB) can be used in bias 
transfer system components to attain specific resistivity levels (U.S. 
Pat. Nos. 4,062,812; 4,116,894). However, as transfer current flows 
through the biased transfer member, the charge control additives in the 
base material thereof tend to migrate toward the biasing source, thereby 
depleting ions in the base material and increasing the resistivity of the 
material. Resistivity also increases as a function of relative humidity 
and temperature This causes the bias voltage across the roll to increase 
in response to constant transfer current applied thereto. As a result, the 
pre-nip fields increase correspondingly, generating severe copy quality 
problems. Resultant increased voltages also complicate hardware design and 
add to the expense of the system. 
Thus, a problem associated with bias transfer roll systems is that the 
electrical life of the bias roll material is inversely proportional to the 
transfer current therethrough. Various approaches and solutions to the 
problems inherent to the use of bias transfer rolls and specifically 
directed toward extending the electrical life thereof have been proposed. 
The following disclosures may be relevant to various aspects of the 
present invention: 
U.S. Pat. No. 4,062,812. 
Patentee: Safford et al. 
Issued: Dec. 13, 1977. 
U.S. Pat. No. 4,116,894. 
Patentee: Lentz et al. 
Issued: Sep. 26, 1978. 
JP-A-2-39182. 
Patentee: Koichi Okuda et al. 
Issued: Feb. 8, 1990. 
U.S. patent application Ser. No. 07/789,506. 
Inventor: Gross et al. 
Filed: Nov. 8, 1991. 
U.S. patent application Ser. No. 07/801,568. 
Inventor: Gross 
Filed: Dec. 2, 1991. 
The relevant portions of the foregoing disclosures may be briefly 
summarized as follows: 
U.S. Pat. No. 4,062,812 discloses a method for extending the electrical 
life of copolymers used in bias transfer rolls. That patent recognizes 
that control of, and minimization of the variations in the resistivity 
under applied voltages with respect to time is important. Thus, certain 
salts having a particular geometric make-up which are useful for extending 
the functional electrical life and electrical stability of materials are 
incorporated into the materials used in xerographic devices. 
U.S. Pat. No. 4,116,894 also discloses compositions and a method for 
enhancing the electrical life of copolymers used in xerographic devices. 
That patent discloses a specific method for enhancing the electrical life 
of butadiene copolymers having solublized conductivity control agents 
incorporated therein by varying specified quantities of terminally 
unsaturated hydrocarboned nitriles in the butadiene. 
JP-A-2-39182 discloses an image forming device utilizing a bias transfer 
roller for impressing a transfer bias when a paper is fed into a transfer 
nip. A transfer bias lower than the bias applied during transfer but 
having a polarity opposite the polarity of the toner is impressed when 
paper is not fed into the transfer nip so as to prevent paper trace or 
back trace to a transfer member. 
U.S. patent application Ser. No. 07/789,506 is directed toward a method and 
apparatus for extending material life in a biased transfer roll by 
enabling reverse current flow therethrough. The apparatus of that 
invention includes a biasing member including a bias roll or other 
charging device for reversing current flow through the bias transfer roll. 
U.S. patent application Ser. No. 07/801,568 is directed toward a system for 
extending the electrical life of a bias transfer member including a 
switching device for selectively coupling the bias transfer member to an 
electrical biasing source for either permitting current flow through the 
bias transfer member to induce toner transfer or inhibiting or reversing 
current flow through the bias transfer member to replenish ions depleted 
therefrom. 
In accordance with the present invention, a transfer apparatus for 
electrostatically transferring electrically charged particles from an 
image support surface to a copy support substrate is disclosed, comprising 
a transfer member positioned adjacent the image support surface, biasing 
means coupled to the transfer member for applying an electrical bias to 
the transfer member to generate electric fields, thereby attracting toner 
particles from the image support surface to the copy substrate, and means 
for maintaining the transfer member at a substantially constant 
resistivity. 
In another aspect of the invention, an electrostatographic printing 
apparatus is disclosed, including a transfer assembly for transferring 
toner particles from a photoconductive image support surface to a copy 
support substrate, wherein the transfer assembly includes a transfer 
member positioned adjacent the image support surface, biasing means 
coupled to the transfer member for applying an electrical bias thereto, 
generating electric fields to attract the toner particles from the image 
surface to the copy substrate, and means for maintaining the transfer 
member at a substantially constant resistivity.

While the present invention will be described with reference to preferred 
embodiments thereof, it will understood that the invention is not to be 
limited to these preferred embodiments. On the contrary, it is intended 
that the present invention cover all alternatives, modifications, and 
equivalents as may be included within the spirit and scope of the 
invention as defined by the appended claims. Other aspects and features of 
the present invention will become apparent as the description proceeds, 
wherein like reference numerals have been used throughout to designate 
identical elements. 
For a general understanding of an electrostatographic printing machine in 
which the features of the present invention may be incorporated, reference 
is initially made to FIG. 3, before providing a description of the 
specific features of the present invention, wherein a schematic depiction 
of the various components of an exemplary electrophotographic reproducing 
apparatus incorporating the transfer assembly of the present invention is 
provided. Although the apparatus of the present invention is particularly 
well adapted for use in an automatic electrophotographic reproducing 
machine as shown, it will become apparent from the following discussion 
that the present transfer assembly is equally well suited for use in a 
wide variety of electrostatographic processing machines as well as in any 
other system utilizing a bias transfer device. Further, the invention is 
not necessarily limited in its application to the particular embodiment or 
embodiments shown herein. 
The exemplary electrophotographic reproducing apparatus of FIG. 5 employs a 
belt 10 including a photoconductive surface 12 deposited on an 
electrically grounded conductive substrate 14. Drive roller 22, coupled to 
motor 24 by any suitable means, as for example a drive belt, engages with 
belt 10 to move belt 10 about a curvilinear path defined by drive roller 
22, and rotatably mounted tension rollers 20, 23. This system of rollers 
is used for advancing successive portions of photoconductive surface 12 in 
the direction of arrow 16, through various processing stations disposed 
about the path of movement thereof, as will be described. 
Initially, a segment of belt 10 passes through charging station A. At 
charging station A, a corona generating device or other charging 
apparatus, indicated generally by reference numeral 26, charges 
photoconductive surface 12 to a relatively high, substantially uniform 
potential. 
Once charged, the photoconductive surface 12 is advanced to imaging station 
B where an original document 28, positioned face down upon a transparent 
platen 30, is exposed to a light source, i.e., lamps 32. Light rays from 
this light source are reflected from the original document 28 to form a 
light image thereof for transmission through a lens 34 which focuses the 
light image onto the charged portion of photoconductive surface 12, This 
imaging process selectively dissipates the charge on the photoconductive 
surface 12 and records an electrostatic latent image corresponding to the 
original document 28 onto photoconductive surface 12. Although an optical 
system has been shown and described for forming the light image of the 
information used to selectively discharge the charged photoconductive 
surface 12, one skilled in the art will appreciate that a properly 
modulated scanning beam of energy (e.g., a laser beam) may be used to 
irradiate the charged portion of the photoconductive surface 12 for 
recording the latent image thereon. 
After the electrostatic latent image is recorded on photoconductive surface 
12, belt 10 advances to development station C where a magnetic brush 
development system, indicated generally by reference numeral 36, deposits 
developing material onto the electrostatic latent image. Preferably, 
magnetic brush development system 36 includes a single develop roller 38 
disposed in developer housing 40, wherein toner particles are mixed with 
carrier beads, generating an electrostatic charge therebetween which 
causes the toner particles to cling to the carrier beads to form 
developing material. The developer roller 38 rotates and attracts this 
developing material to form a magnetic brush having carrier beads and 
toner particles magnetically attached thereto. Thus, as the developer 
roller 38 rotates, developing material is brought into contact with the 
photoconductive surface 12 such that the latent image thereon attracts the 
toner particles of the developing material and the latent image on 
photoconductive surface 12 is developed into a visible image. A toner 
particle dispenser, indicated generally by the reference numeral 42, 
furnishes a supply of additional toner particles to housing 40 to sustain 
the developing process. 
After the toner particles have been deposited onto the electrostatic latent 
image for development thereof, belt 10 advances the developed image to 
transfer station D, where a sheet of support material 46 is moved into 
contact with the developed toner image via sheet feeding apparatus 48 and 
chute 54. Preferably, sheet feeding apparatus 48 includes a feed roller 50 
for rotation while in contact with the uppermost sheet of stack 52 to 
advance the uppermost sheet into chute 54. Chute 54 directs the advancing 
sheet of support material 46 into contact with photoconductive surface 12 
of belt 10 in a timed sequence so that the developed image thereon 
contacts the advancing sheet of support material 46 and is transferred 
thereon at transfer station D. A bias transfer roll 56 is provided for 
establishing a directional force field capable of attracting toner 
particles from the photoconductive surface 12 to support material 46. The 
support material 46 is subsequently transported in the direction of arrow 
58 for placement onto a conveyor (not shown) which advances the sheet to a 
fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 60, for permanently affixing the transferred image to 
sheet 46. Fuser assembly 60 preferably comprises a heated fuser roller 62 
and a support roller 64 spaced relative to one another for receiving a 
sheet of support material 46 therebetween. The toner image is thereby 
forced into contact with the support material 46 between fuser rollers 62 
and 64 to permanently affix the toner image to support material 46. After 
fusing, chute 66 directs the advancing sheet of support material 46 to 
receiving tray 68 for subsequent removal of the finished copy by an 
operator. 
Invariably, after the support material 46 is separated from the 
photoconductive surface 12 of belt 10, some residual developing material 
remains adhered to belt 10. Thus, a final processing station, namely 
cleaning station F, is provided for removing residual toner particles from 
photoconductive surface 12, subsequent to separation of the support 
material 46 from belt 10. Cleaning station F can include a rotatably 
mounted fibrous brush 70 for physical engagement with photoconductive 
surface 12 to remove toner particles therefrom by rotation thereacross. 
Removed toner particles are stored in a cleaning housing chamber (not 
shown). Cleaning station F can also include a discharge lamp (not shown) 
for flooding photoconductive surface 12 with light in order to dissipate 
any residual electrostatic charge remaining thereon in preparation for a 
subsequent imaging cycle. 
The foregoing description should be sufficient for purposes of the present 
application for patent to illustrate the general operation of an 
electrophotographic reproducing apparatus incorporating the features of 
the present invention. As described, the electrophotographic reproducing 
apparatus may take the form of any of several well known devices or 
systems. Variations of specific electrostatographic processing subsystems 
or processes may be expected without affecting the operation of the 
present invention. 
Referring now in particular to FIG. 1, a particular embodiment of the bias 
transfer assembly in accordance with the present invention will be 
described. The use of the term "bias transfer roll" or "bias transfer 
assembly" refers to a transfer assembly having an electrically biased 
contact member for cooperating with an image support surface to attract 
electrically charged particles from the image support surface onto a 
second support surface, such as a copy sheet or the like. Specifically, a 
bias transfer assembly including a bias transfer roll is shown in FIG. 1, 
wherein the bias transfer roll 56 is shown in a configuration adapted to 
form a transfer nip for receiving the copy substrate 46, which allows the 
copy substrate 46 to cooperate, in conjunction with the bias transfer roll 
56, with a toner image on the photoconductive surface 12 of belt 10 when 
brought into contact therewith. The bias transfer roll 56 electrically 
attracts charged toner particles from the photoconductive surface 12 in 
the direction of the bias transfer roll 56 so as to transfer the developed 
images on the photoconductive surface from the belt 10 to the copy 
substrate 46 positioned therebetween. 
For the purposes of the present discussion, a configuration is shown 
wherein the bias transfer roll 56 is urged physically against belt 10, 
forming a nip therebetween and having no opposing support member 
thereagainst, such that the bias transfer roll 56 causes the path of belt 
10 to be slightly bowed, thereby increasing the contact dwell time between 
the belt 10 and the bias transfer roll 56. It will be understood, however, 
that a backup roll (not shown) may be provided opposite the transfer roll 
56 for urging the belt 10 into contact with the transfer roll 56 with 
minimal or no distortion in the path of belt 10. For example, the transfer 
station D can be positioned adjacent drive roll 23 (FIG. 3), or an 
independent roll may be added to the system configuration along the path 
of belt 10, opposite the transfer roll 56. Alternatively, the bias 
transfer roll system can be integrated into a machine having a drum type 
photoreceptor. The bias transfer roll 56 is appropriately journaled for 
rotation at an angular velocity so that the peripheral speed of the roll 
56 is substantially equal to the speed of the belt 10. The arrows shown in 
FIG. 1 indicate the relative direction of movement for the copy substrate 
46, the transfer roll 56 and belt 10, as the copy support substrate 46 is 
fed by appropriate means into the nip formed between transfer roll 56 and 
belt 10. As such, the terms "pre-nip" and "post-nip" used herein, refer to 
the direction of travel of the copy substrate 46 through the transfer nip. 
The exemplary transfer roll 56 of the present invention includes an 
electrically "self-leveling" outer layer 70, and an electrically 
"relaxable" inner layer 72 on an electrically conductive central core or 
axle 74. Thicknesses of the various layers shown are provided for 
illustrative purposes only and are not necessarily drawn to scale. An 
electrical biasing device in the form of a constant current source 76 is 
electrically coupled to the conductive core 74 for providing an electrical 
bias thereto. An internal heating element 57 is also provided for 
maintaining the temperature of the bias transfer roll 56, and, more 
specifically, the resistivity of the electrically conductive inner and 
outer layers 70, 72 at a predetermined level. 
In a preferred embodiment, the relaxable layer 72 of the transfer roll 56 
is comprised of a relatively thick blanket of a resilient elastomeric 
polyurethane material, which may include a butadiene based copolymer 
having a hardness of between about 40 Shore 00 and about 90 Shore A. This 
elastomeric polyurethane blanket may be about 0.030 to about 0.625 inches 
in thickness (preferably 0.25 inches in thickness), and may have 
sufficient resiliency to allow the bias transfer roll 56 to deform when 
brought into moving contact with an opposingly supported portion of belt 
10. The relative deformable characteristics of the relaxable layer 72 as 
well as the belt 10 allow for good mechanical contact in the transfer zone 
at moderate pressures to eliminate "hollow character" transfer under 
normal operating conditions. This deformable feature also provides an 
extended contact region for increasing the dwell time in which toner 
particles of the developer material can be transferred between support 
surfaces. It will be understood that the deformable feature provided by 
relaxable layer 72 is not a necessary feature of the present invention, as 
for example in a configuration, as shown, wherein transfer is conducted 
against an unsupported portion of belt 10. 
The material making up the relaxable layer 72 is further selected so that 
it functionally takes a selected time period to transmit a charge from the 
conductive inner core 74 to the interface between the relaxable layer 72 
and the self-leveling layer 70. The relaxable layer 72 has a bulk 
resistivity failing in a well-defined operating range selected relative to 
the diameter of the transfer roll 56 and the surface velocity thereof. The 
preferred resistivity ranges may vary for transfer systems designed to 
operate at different transfer sheet throughput speeds. This selected 
resistivity corresponds to the roller surface speed and nip region 
dimension such that the time necessary to transmit a charge from the 
conductive core 74 to the self-leveling layer 70 is roughly greater than 
the dwell time for any point on the transfer roll 56 in the transfer nip 
region. Ideally, the external voltage profile of the bias transfer roll 56 
provides a field strength below that is necessary for substantial air 
ionization in the air gap at the entrance of the nip, and above that 
required for air ionization in the air gap just beyond the exit of the 
nip. As a general rule, 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 72 diminishes. It has been 
found that a resistivity of between about 10.sup.7 and 5.0.times.10.sup.11 
ohm-cm, and preferably a resistivity of about 10.sup.8 to about 10.sup.10 
ohm-cm is sufficient for this requirement. 
The transfer roll 56 is covered with a relatively thin external coating, so 
called self-leveling layer 70, which may comprise an elastomeric material 
such as polyurethane having a resistivity of between 10.sup.10 and 
10.sup.15 ohm-cm, preferably having a thickness of approximately 0.0025 
inches and a hardness of about 65 to 75 Durometer. The material of the 
self-leveling layer is generally selected for its higher resistive values, 
providing a so-called leaky insulator. In addition, the self-leveling 
layer includes material (or is so related to the relaxable layer) so that 
charges applied to the outer surface of the self-leveling layer 70 will be 
generally dissipated within one revolution of the transfer roll 56 in 
order to prevent suppression of the transfer fields in the transfer nip. 
The self-leveling layer 70 also acts as a thin insulating layer to protect 
the bias transfer roll 56 during air breakdown and to limit current flow 
through the roll 56. In addition, an external coating having increased 
hardness provides a moisture barrier for the inner components of the roll 
56 and makes the roll structure easy to clean. It will be noted, however, 
that other materials which are resilient, durable and cleanable have been 
used to form the relaxable layer 72, such that the self-leveling layer 70 
described herein is not essential to the functionality of the bias 
transfer roll. 
Electrical biasing source 76 is provided for generating current flow 
through the bias transfer roll 56. The biasing source 76 provides a 
constant current source for creating high transfer fields while 
maintaining pre-nip ionization at tolerable levels and allowing a desired 
amount of post-nip ionization. A discussion of the electric fields 
developed by the bias transfer roll 56 and the roles of the relaxable and 
self-leveling layers, as well as a detailed description of a preferable 
circuit for the electrical biasing source 76 are provided in U.S. Pat. No. 
3,781,105, issued to Meagher, the contents of which are hereby 
incorporated by reference. 
The functional life of a bias transfer contact member, such as a bias 
transfer roll, is directly related to the maintenance of a constant 
controlled resistivity region. However, most ionic additives utilized for 
reducing the resistivity in polymer materials used in bias transfer roll 
members migrate toward higher potential energy, causing an increase in 
ionic mobility which therefore results in a more rapid variation in 
resistivity over the life of the material. It is known that the electrical 
life of materials used in bias transfer devices and subsystems as 
described above can be improved by controlling and maintaining constant 
resistivity with time under an applied electrical field. It is also known 
that resistivity of a material is directly related to the temperature 
thereof. Thus, it has been found by the present invention, that electrical 
life of a bias transfer member can be improved by selectively applying 
heat to the bias transfer member for maintaining the temperature thereof 
at a predetermined elevated temperature. Variation of the temperature of 
the bias transfer roll allows for control of the resistivity thereof. For 
this reason, the present invention provides a resistive heating element 57 
internal to the conductive core 74 of the bias transfer roll 56 for 
controlling the temperature thereof. 
The operation of roller 57 may be explained in connection with the 
generalized curves in FIG. 2. The time scale along the horizontal axis 
represents the movement of the copy sheet 46 through the nip region. The 
pre-nip period is to the left of the nip transfer period 62, and the 
post-nip period is to the right of transfer nip period 43. Since 
velocities are assumed constant here the horizontal axis also corresponds 
to path distances relative to the transfer nip area. 
The volts-per-micron scale along the vertical axis of FIG. 2 represents 
relative transfer field intensity along the path of the transfer sheet. 
The field observed is that between the outer surface of the roller 57 and 
the photoreceptive surface 12 of belt 10. It is that field which effects 
the transfer of the toner particles between the photoreceptor belt 10 and 
the copy sheet 46. 
Curve 60 in FIG. 2 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 61 is the transfer field curve generated by 
the roller transfer system of FIG. 1. Curve 62 is an exemplary curve for a 
typical bias transfer roll, e.g., conductive rollers and conductive 
rollers overcoated with high resistance and/or high dielectric materials. 
Curve 62 is shown in order to comparatively dramatize the desirable 
asymmetrical nature of the subject curve 61, which permits post-nip, but 
minimizes pre-nip, ionization of air. 
Curve 62 represents a typical approach to bias roll transfer in that the 
curve 62 is symmetrical about the nip contact region (represented by the 
time period 63) in the absence of toner and air ionization effects. 
Conversely, curve 61 is asymmetrical because of the effects of the 
resistivity control provided by the heating element of the present 
invention. The object is to maximize the transfer field without having 
detrimental pre-nip ionization. This is achieved by forcing portion 61B of 
the curve 61 continue upward in the post-nip region until the Paschen 
curve is reached while pre-nip portion 61A is selected to remain below the 
Paschen curve 60 to realize the preferred condition of no pre-nip 
ionization. In contrast, it is apparent from an inspection of symmetrical 
curve 62 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 
the Paschen curve, 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 ionization such that other means of 
keeping toner tacked to the copy sheet 46 must be employed in lieu of 
post-nip ionization. 
The transfer conditions depicted by FIG. 2 can be discussed with respect to 
FIG. 1 wherein it can be seen that prior to entering the nip, the bias 
transfer roll 56 is not subjected to high internal fields; that is, the 
outer surface of layer 72 is at substantially the same potential as the 
core 74 . Just prior to and in the nip area the roller surface becomes 
closely spaced to the photoconductive belt 10 which tends to draw charge 
toward the roller 56 surface. However, charge movement is resisted by the 
roller resistivity such that the charge density at the interface between 
the layers 70 and 72 increases in proportion to the resistivity of the 
relaxable layer as the relaxable layer proceeds through the nip. After 
exiting the nip, the charge density will generally continue to increase 
initially due to the internal field in the relaxable layer 72, or the 
induced charge may have nearly reached equilibrium; 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 is 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 of the 
roller 56 and the copy sheet 46. 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 72, thereby 
inhibiting ionization. 
The field intensity required to break the bond of toner to the 
photoreceptor surface 12, and to tack the toner to the copy sheet 46, is 
reached at some time after the entrance to the nip but before post-nip 
ionization occurs. However, a continued "holding" or tacking field must 
also be present during the subsequent stripping of the paper 46 from the 
photoreceptor belt 10 for high efficiency and stable toner transfer. 
With the above information, the significance of the heating element in 
combination with the constant current energy source is better understood. 
Stated simply, the significance is that, as previously suggested, the 
heating element provides the capability to control the resistivity of the 
bias roll 56 to compensate for changes in the electrical parameters of the 
roller and its environment. The parameter that normally experiences the 
greatest and most frequent fluxuations are roller resistivity, which is 
very sensitive to RH, and temperature. In terms of FIG. 2, temperature 
control is the method and means for keeping curve portion 61A below the 
Paschen curve 60 to prevent pre-nip ionization and for insuring that curve 
portion 61B intersects the Paschen curve in the post-nip region. 
Controlling the extent of post-nip ionization also controls the amount of 
deposited charge such that the toner "holding" field on the copy sheet 46 
is more constant and maintainable at a moderate level, providing good 
toner holding as well as easier paper stripping. Thus, high transfer 
efficiency is achieved with a relatively lower applied voltage and charge 
density on the transfer member. Moreover, since bias roll electrical life 
is a function of the applied field and therefore the voltage across the 
bias transfer roll, maintenance of a constant, lower resistivity extends 
the electrical life of the roll. 
The relationship between resistivity and temperature, as well as relative 
humidity (RH), is shown in graphic form in FIG. 3. It can be seen from 
this Figure that, for a nominal 10% RH environment, the resistivity of a 
roll can be decreased from approximately 4.0.times.10.sup.9 ohm-cm to 
approximately 1.0.times.10.sup.9 ohm-cm by changing the operating 
temperature of the roll from 60.degree. to 120.degree. F. Resistivity is 
less and the relative decease in resistivity is even greater at higher RH 
as can be seen from the graphic representations for 50% RH and 85% RH. The 
particular test data shown in FIG. 3 provides a representation of mean 
data within an envelope of minimum to maximum resistivity variation 
measured over a given sample of bias transfer rolls tested. FIG. 3 also 
shows the resistivity breakdown thresholds for nominal nip dimensions of 4 
mm and 10 mm at 75 microamps. 
The current referred to as being held constant throughout this description 
is the current to the roll core 74. This roll current is, by reason of 
conservation of charge, basically equal to the post-nip ionization 
current. (Substantially zero pre-nip current is, of course, one of the 
desired operating conditions here.) The constant current bias source 76 
may be described as a device for automatically widely varying the 
potential level coupled to roll 56 to automatically compensate for 
variation in current to the core 74, due to the connected load 
(resistance) changes, which are due to changes in ambient RH and 
temperature and aging of materials as well as various other factors 
tending to effect the pre-nip, nip and post-nip field levels (e.g., paper 
thickness, charge build-up on the self-leveling layer, etc.). In the 
specific system described herein, the constant current source output is 
equal to about 1.5 microamps per inch, where the inch refers to the length 
of the roller along its axis. Thus an internal roll resistivity on the 
order discussed with respect to FIG. 3 requires the bias potential at core 
22 to vary from about 4 to about 9 KVolts in order to maintain a constant 
current of 1.5 microamps per inch. Thus, the bias source 76 output voltage 
varies automatically over this voltage range while providing a constant 
current signal. 
With further reference to FIG. 1 and in accordance with the present 
invention, the heating element 57 is coupled to a voltage source 78 via a 
detecting system such as a thermocouple switch 79 for selectively 
activating the heating element in response to a detected temperature at 
the bias transfer roll to maintain a constant predetermined temperature 
within the bias transfer roll 56. In addition, or alternatively, the 
detection system can include a voltage measurement device so that the 
heating element 37 can be selectively activated in response to a 
predetermined resistivity measurement at the bias transfer member. For 
example, a voltmeter 80 is provided for monitoring the voltage across the 
constant current source 76 for maintaining a predetermined constant 
current through the bias transfer member 56; when the measured voltage 
exceeds a predetermined voltage level corresponding to a defined 
resistivity level, the heating element 57 is activated. 
Tests have shown that a robust system can be maintained by heating the bias 
transfer member to approximately 80.degree. F. Such temperature 
requirements can be satisfied via a relatively low rated, inexpensive 
heating element. At the stated temperature, the electrical life of the 
bias transfer member has been shown to be extended by a factor of two. 
FIG. 4 shows a graphical representation of the bias transfer roll life 
extension achieved by the present invention when the bias transfer roll 56 
is heated to a temperature of approximately 80.degree. F. FIG. 4 shows 
that as temperature is increased, the voltage across the roll 57 
(generated by constant current source 76) decreases, resulting in extended 
electrical life. 
In recapitulation, the electrophotographic printing apparatus of the 
present invention includes a toner transfer system having a bias transfer 
roll including an internal resistive heating element for heating the bias 
transfer roll to a predetermined temperature to reduce and maintain the 
resistivity of the bias transfer roll. Heating the bias transfer roll 
results in a decrease in resistivity thereof which results in extended 
electrical life of the bias transfer roll in an electrophotographic 
printing apparatus. 
It is therefore evident that there has been provided, in accordance with 
the present invention, an electrophotographic printing apparatus that 
fully satisfies the aims and advantages of the invention as hereinabove 
set forth. While this invention has been described in conjunction with 
preferred embodiments thereof, it is evident that many alternatives, 
modifications, and variations will be apparent to those skilled in the 
art. Accordingly, the present application for patent is intended to 
embrace all such alternatives, modifications and variations as are within 
the broad scope and spirit of the appended claims.