US3924943A

US3924943A - Segmented biased transfer member

Info

Publication number: US3924943A
Application number: US478184A
Authority: US
Inventors: Gerald M Fletcher
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1974-06-11
Filing date: 1974-06-11
Publication date: 1975-12-09
Anticipated expiration: 1992-12-09
Also published as: FR2274964A1; FR2274964B1; GB1487116A; JPS519459A; DE2523722A1; NL7506577A

Abstract

In an electrostatographic copying system wherein an image is transferable from an image support surface to a copy utilizing an electrically biased transfer roller positionable thereagainst, the transfer roller conductive core is divided along its axis into electrically discrete cylindrical segments, and commonly covered with an electrically relaxable elastomer layer. An electrical bias circuit provides discrete electrical bias supplies for these core segments, whereby a central segment can be connected to a constant current (variable voltage) source, and end segments slaved to the voltage level on the central segment to prevent roll and leakage in no copy areas from adversely affecting the constant current source.

Description

United States Patent i191 Fletcher Dec. 9, 1975 SEGMENTED BIASED TRANSFER MEMBER Primary ExaminerRichard L. Moses [75] Inventor: Gerald M. Fletcher, Pittsford, N.Y. 1:35: 2 Agent or Flrm Bemal-d chlama; Earl e [73] Assignee: Xerox Corporation, Stamford,

Conn. S 57 AB TRACT [22] Filed: June 11, 1974 1 In an electrostatographic copying system wherein an [21] Appl 478l84 image is transferable from an image support surface to a copy utilizing an electrically biased transfer roller [52] US. Cl 355/3 R; 96/1.4; 355/16 positionable thereagainst, the transfer roller conduc- [51] Int. Cl. G03G 15/00 tive core s divided al ng ts a is int t al y d s- [58] Field of Search 355/3 R, 3 DD, 16, 11, crete cylindrical segments, and commonly Covered 3355/ 12, 3 TR, 3 TE; 96/ 1,4; 118/62] with an electrically relaxable elastomer layer. An electrical bias circuit provides discrete electrical bias [56] References Cited supplies for these core segments, whereby a central UNITED STATES PATENTS segment can be connected to a constant current (variable voltage) source, and end segments slaved to the 3,640,249 2/1972 Young 355/3 DD voltage level on the central Segment to prevent r0 3,644,034 2/1972 Nelson 355/3 R 3,827,800 8/1974 Tanaka et al. 355/3 R and leakage COPY areas from adversely affectmg 3,832,053 8/1974 Goel et al. 355/3 R the constant Current Source 3,837,741 9 1974 S 6 1.4 X

/ pence 9 10 Claims, 3 Drawing Figures 49TH, I 23 2/ 22 l I R CONSTANT CURRENT SOURCE k if 48 Y !Y- 7 5 I: /7

U.S. Patent Dec. 9, 1975 Sheet 1 of3 3,924,943

CONSTANT CURRENT SOURCE U.S. Patent Dec. 9, 1975 Sheet 2 of3 3,924,943

E ends FIG. 2

'zgcnsoa U.S. Patent Dec. 9, 1975 Sheet 3 of3 3,924,943

wk an & mm R 1mm E 6 mm em v E am SEGMENTED BIASED TRANSFER MEMBER The present invention relates to an electrostatographic copying system with an electrically biased transfer member providing electrically discrete areas transverse the direction of copy motion.

In a conventional transfer station in xerography, a developed image of toner particles (from the image developer material) is transferred from a photoreceptor (the imaging surface) to a cut or roll fed copy sheet (the final image support surface), either directly or after an intermediate image transfer to an intermediate surface. Such image transfers are also required in other electrostatographic processing systems, such as electrophoretic development. In TESI systems the intermediately transferred image may be an undeveloped latent electrostatic image on a non-photoconductive insulator.

Transfer is most commonly achieved by applying electrostatic force fields in a transfer nip sufficient to overcome the forces holding the toner to its original support surface and to attract most of the toner to transfer over onto the contacting second surface. These transfer fields are generally provided in one of two ways, by ion emission from a transfer corona generator onto the back of the copy sheet, as in U.S. Pat. No. 2,807,233, or by a DC. biased transfer roller or belt rolling along the back of the copy sheet. Examples of bias roller transfer systems are described in U.S. Pat. No. 3,781,105, issued Dec. 25, 1973, to Thomas Meagher, U.S. Class 355/3, Int. Class G03g /16, and in U.S. Pat. Nos. 2,807,233; 3,043,684; 3,267,840; 3,328,193; 3,598,580; 3,625,146; 3,630,591; 3,684,364; 3,691,993; and 3,702,482, by C. Dolcimascolo et al., issued Nov. 7, 1972.

The difficulties of successful image transfer are well known. In the pre-transfer (pre-nip) region, before the copy paper contacts the image, if the transfer fields are high the image is susceptible to premature transfer across the air gap, leading to decreased resolution or fuzzy images. Further, if there is ionization in the prenip air gap from high fields, it may lead to strobing or other image defects, loss of transfer efficiency, and a lower latitude of system operating parameters. Yet, in the directly adjacent nip region itself the transfer field should be large as possible (greater than approximately volts per micron) to achieve high transfer efficiency and stable transfer. In the next adjacent post-nip region, at the photoconductor/copy sheet separation (stripping) area, if the transfer fields are too low hollow characters may be generated. On the other hand, improper ionization in the post-nip region may cause image instability or copy sheet detacking problems. Variations in ambient conditions, copy paper, contaminants, etc., can all affect the necessary transfer parameters. The bias roll material resistivity and paper resistivity can change greatly with humidity, etc. Further, conduction of the bias charge from the bias roller is also greatly affected by the presence or absence of the copy paper between it and the imaging surface. To achieve these different transfer field parameters consistently, and with appropriate transitions, is difficult The above-cited Meagher U.S. Pat. No. 3,781,105 teaches a constant current electrical bias system for improvement in an unsegmented transfer bias roll operation under these variable conditions, and further technical background in this area. The present invention 2 represents an improvement over this disclosed system. Accordingly, this patent is specifically made a part of this specification.

Various bias transfer systems are known in which the bias roll or belt has discrete peripheral conductive linear strips extending radially along the roller surface, i.e., segmented circumferentially. One example is U.S. Pat. No. 3,640,249, issued Feb. 8, 1972, to E. F. Young. ,Another example is U.S. Pat. No. 3,684,364, issued Aug. 15, 1972, to Fred W. Schmidlin. This patent teaches a xerographic roller electrode transfer system in which appropriate transfer potential can be provided to the roller from a transfer bias source through a plurality of fixed contacts. These contacts slidably engage moving segmented conductors mounted inside the roller, spaced around the circumference of the roller.

U.S. Pat. No. 3,574,301, issued Apr. 13, 1971, to J. S. Bernhard discloses a segmented bias roll to enable different biases and different functions to occur at different areas of the roll circumference. However, this bias roll is in a developer station rather than a transfer station.

U.S. Pat. No. 3,647,292, issued Mar. 7, 1972, to D. J. Weikel, J r., discloses a transfer belt system for carrying a copy sheet through the transfer station, vacuum means for holding the sheet on the belt, and transfer field generating means, which in one embodiment includes multiple stationary transfer electrodes extending transverse the system movement path in a stationary segmented plate with different (increasing) applied potentials acting at the back of the transfer belt along its path.

U.S. Pat. No. 3,644,034, issued Feb. 22, 1972, to R. L. Nelson, discloses a segmented wide conductive strip transfer belt to which two different bias potentials are applied by two support rollers to those segments passing over the rollers. The conductive segments are separated by l/16 inch insulative segments.

Unsegmented electrically monolithic bias transfer members are well known in the art. A bias transfer roll is disclosed by Fitch in the above-cited U.S. Pat. No. 2,807,233, where a metal roll coated with a resilient coating having a resistivity of about 10 to 10 ohm cm is used as a bias transfer member. Shelffo in U.S. Pat.

No. 3,520,604 suggests a transfer roll made of a conductive rubber having a resistivity in the range of from about 10 to about 1.0 ohm cm. A biased roll transfer member is described by Dolcimascolo et al. in the above-cited U.S. Pat. No. 3,702,482, issued Nov. 7, 1972, incorporated herein by reference. It comprises a conductive substrate (core) for supporting a uniform bias potential thereon; an intermediate resilient blanket placed around said substrate having an electrical resistivity of 10 to 10 ohm cm such that the blanket is capable of. transmitting said bias potential on said substrate to the outer periphery of said blanket; and an outercoating placed over said blanket having an electrical resistivity of 10 to 10 ohm cm to minimize ionization of the surrounding air when the transfer member is placed in electrical cooperation with said support surface. A polyurethane material manufactured by the DuPont Company under the tradename Adiprene is suggested therein as the outer coating of the roll, capable of providing a relatively smooth surface and exhibiting relatively good mechanical release properties in respect to the toner materials employed. This outer coating may be approximately 0.0025 inch in thickness. The relatively thick resilient intermediate blanket may be of elastomeric material having'a hardness of between 15-25 durometers, preferably a polyurethane rubber approximately 0.25 inch in thickness.

In a copending U-.S. patent application with the same assignee, filed April, 1974, by Clifford O. Eddy, James 'A. Lentz and StepheriStrella, entitled Compositions and Method for Making Biasable Members (D/74167) there is more recently disclosed appropriate transfer members. A transfer member there is in the form of a roll formed upon a rigid hollow conductive metal cylinder core, such as aluminum, copper or the like, capable of readily responding to the full biasing potential placed thereon. Over the core'is placed a coating or layer which is a hydrophobic elastomeric polyurethane. This coating is formed of a resilient elastomeric material preferably about 0.25 inch in thickness,'having a hardness between 40 Shore and about 40 Shore A and preferably about -25 durometers. Where this first coating above sufficiently minimizes ionization of the atmosphere in and about the contact region of the bias transfer member with the photoconductor, and where it has suitable mechanicalstability, and where it is easy to clean, then it maybe the outermost coating on the bias transfer member- It is' preferred that the resilient hydrophobic elastomeric polyurethane have a resistivity of between about 10 and 5.0 X 10" ohm cm. This coating is known, as the relaxable layer. By coating the biasable transfer member (roll) with this particular class of polyurethanes, resistivity of the biasable transfer roll is controlled in relationship to changes in relative humidity, and more specifically, resistivity remains substantially unchanged when changes in relative humidity occur.

In another embodiment of the Eddy et a1 application the resilient relaxable blanket of hydrophobic elastomeric polyurethane material is similar, but is an intermediate. It may be about 0.125 inch to .about 0.625 inch in thickness and is preferably 0.25 inch in thickness. This intermediate blanket should be capable of responding rapidly to the, biasing potential to impart electrically the charge potential'on the core to the outer extremities of the roll surface. The blanket therefore should have a resistivity of between about l0 and 5.0 X10 ohm cm, and preferably about 10 to about 10 ohm cm. Over the intermediate-hydrophobic blanket is placed,*in this second embodiment, a relatively thin'outer coating,which according to the prior art bias transfer rolls, may be an elastomeric material such as a polyurethane having a resistivity of between 10 and 10 ohm cm and which preferably has a thickness of about 0.0025 inch and a hardness of about 65-75 D durometer. Ionization of the atmosphere in and about the contact region isminimized, as discussed above, in relation to the resistivity of the outer coating. The outer coating, known also as a self-leveling layer,'is a leaky insulator, and is generally selected for a higher resistive value than those of the intermediate 'relaxable blanket underlying it. In addition, the outer layer preferably includes materials, or is so related to the relaxable layer, such that charges applied to the outer surface of the outer coating will be generally dissipated within one revolution of the roll. The outer layer also acts as a thin insulating layer to help protect the resilient intermediate blanket during air breakdown, to limitcurrent flow through the roll, and to make the roll surface easy to clean. When the relaxable material itself is durable and cleanable the outer self-leveling is not required. Since in accordance with this Eddy et a1 transfer member, relative humidity problems are substantially reduced or eliminated, the outer layer need not act as a moisture barrier to prevent resistivity changes in the resilient intermediate blanket due to changes in relative humidity. The ratio of the relaxable material resistivity at a relative humidity of 10 percent to the resistivity at a relative humidity at 80 percent, should be about 1 to about 12 to provide a suitable hydrophobic biasable transfer member in accordance with the Eddy et a1 transfer member.

In the event the hydrophobic elastomeric polyurethane has a resistivity higher than the desired resistivity, the resistivity may be adjusted by the addition of a suitable ionicadditive for reducing the resistivity of the particular polyurethane. For example, a particular hydrophobic --elastomeric polyurethane may have a low RH sensitivity, but it may have a resistivity of 10. By the use of a suitable additive, such as a quarternary ammonium compound, that resistivity may be reduced from 10 to within the range of 10 and 5.0 X l0 -ohm cm without any adverse effect upon the RH sensitivity.

A particular problem with present bias transfer rolls, particularly where a constant current bias is utilized, is known at the end leakage problem. The transfer roller frequently extends axially beyond the width of the copy sheet. This means that the roller ends extend into no paper areas in which they directly overly the imaging surface. The electrical characteristics inthese roller end areas, including the current drawn there from the roller surface, are therefor quite different from those in the'central area where the copy sheet is interposedbetween the roller surface and the imaging surface. These differences are due to the different (and variable) electrical characteristics of the papers, and also spacing or contact pressure differences, and direct exposure to charges on the imaging surface. Excessive current leakage can occur in these non-paper areas, especially for conventional relatively low resistivity rolls. The response of the constant current bias circuit is adversely affected, andits output does not .keep the current density to the paper constant. Excessive impedance changes can cause the effective transfer voltage range of the constant current source to be exceeded as that circuit attempts to keep the current constant. With shorter paper being copied the no paper areaof the roller will, of course, increase.

The transfer system of the invention may be utilized in any desired path, orientation or configuration. It may be utilized for transfer with an image bearing surface of any desired configuration or material, including either a cylinder or a belt. Photoconductive belt imaging surfaces in electrographic copying systems are exemplified by US. Pat. Nos. 3,093,039; 3,697,285; 3,707,138; 3,713,821, and 3,719,165.

The above-cited and other references and their foreign counterparts teach details of various suitable exemplary structures, materials, and functions to those skilled in the art. Further examples are disclosed in the books Electrophotography by R. M. Schaffert, and Xerography and Relatd Processes by John H. Dessauer and Harold E. Clark; both first published in 1965 by Focal Press Ltd., London, England. All references cited herein are incorporated in this specification, where appropriate.

Further objects,-'features and advantages of the "present invention pertain to the particular apparatus and details wherebythe above-mentioned aspects of the inyention are attained. Accordingly, the invention will be better understood by reference to the following description of one example thereof and to the drawings forming a part of that description wherein:

FIG. 1 is a central axial cross-sectional view of an exemplary xerographic bias roll transfer system embodiment in accordance with the present invention;

FIG. 2 is a schematic of an electrical bias circuit, including the constant current source, for this exemplary transfer system; and

FIG. 3 is a cross-sectional view of the transfer member of FIG. 1 taken along the axis thereof and showing exemplary end bearing and electrical connections.

Referring now to the drawings, FIGS. 1-3, and particularly to FIG. 1, this structure is described in further detail in the above-cited Meagher US. Pat. No. 3,781,105. This patent also discusses the theory of bias transfer and constant current biasing. A photoconductor (photoreceptor) 11 is shown here as 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. [The polarities disclosed herein may be reversed with other suitable systems]. 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 adjacent the latent images. The fields associated with charge 14 then electrostatically tack the charged toner particles to the insulator 11.

The transfer roller 15 is appropriately journaled for rotation (as shown in FIG. 3) so that the peripherial speed of the roller is substantially equal to the speed of the insulator 11. A cut sheet transfer member 16, e.g.,

conventional copy 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 l1 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. 1 corresponds to the right and left hand regions, respectively, adjacent the nip 17.

The exemplary roller 15 here includes a thin electrically self-leveling outer layer or overcoating 20, an electrically relaxable next (inner) layer 21 and a central cylindrical conductive core or axle 22. The core 22 here is a hollow relatively thin walled conductive metal tube, e.g., aluminum. 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 or blanket 21, which has a bulk resistivity falling in a well defined operating range selected in relation to roll diameter and surface velocity. Appropriate bulk resistivities of this relaxable layer can vary over the ranges previously described. A variation in this resistivity of about two orders of magnitude, primarily as a result of static and dynamic changes in relative humidity or RI-I (extending generally from 5-to-10% RH to to-% 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. I

The relatively soft, thick, electrically relaxable body 21 may be mounted directly on the conductive core 22 of the bias roll. The relatively low durometer of this elastomer relaxable 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 postnip 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. 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-nipionization may be allowable].

The (outer) self-leveling layer 20 is a leaky insulator. The layer 20 is selected for substantially higher resistive values, e.g., in the order of about 10 to l0 ohm centimeters. 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 selfleveling 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.

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.

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 9 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 7 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 plasticsare, of course, generally always highly insulating.

. The constant current energy (bias) source 23 automatically 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.

While the toner 10 is carried by the insulator 11 toward the nip region 17, the toner is tacked to the insulatorby the fields associated with the latent image charge 14 and by other adhesive forces such as VanderWaal forces.

The transfer 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.

The transfer conditions are schematically illustrated by the plus signs 48 in FIG. 1. 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 air gap (Y 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 with time by current flow through the self-level- 8 ing layer 20 during the subsequent revolution of the roller. 1

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. 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.

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. Constant current biasing keeps the field intensity below the Paschen curve to prevent pre-nip ionization and controls the extent of post-nip ionization that controls the amount of post-nip 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.

The current referred to as being held constant throughout this description is the current to the roller 15 core 22, I 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 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 exemplary system described herein, the constant current source output I may be 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. 1). The wide internal roller resistivity swing previously discussed requires the bias potential on core 22 here to vary from about 800 to about 4000 volts to maintain this constant current of 1.5 microamps per inch. Thus, the bias source 23 output voltage must vary automatically over this voltage range.

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.

The exemplary electrical circuit shown in FIG. 2 is capable of providing the constant current biasing voltage source 23 for roller 15. Transistors 0301 and Q302 chop D.C. input power E, to a transformer T1 primary. Initially assume the circuit is working in a steady state condition, supplying current I within the above-discussed specifications to the load (core 22). If the load resistance increases (e. g., when paper is fed between 'the' bias roller and photoreceptor), the load current zener CR308, current setting pot R8 and resistance R312 and then to the transformer T1 secondary at top 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 0305, 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 0305 is shared through a common emitter supply resistor R310. Thus, when the current through 0305 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 output appears across its output resistor R306 which is connected to primary tap P as a control voltage E The increase in E increases the total peakto-peak voltage in the transformer T1 primary and, therefore, the output voltage developed across the load at both transformer secondaries at their taps P8 and P9. The output voltages will thereby increase until the load current reaches its original value determined by setting of pot R8.

Considering now the aspects of the circuit 23 different from that disclosed in the cited Meagher U.S. Pat. No. 3,781,105, it may be seen that there are two almost identical T1 secondary windings and associated conventional output rectifier and filter circuits here, rather than one as in that patent. The second windings preferably has the same number of tums, so that the variable output voltage developed at its tap P9 will be the same as that developed at tap P8 for the constant current supply. This second output circuit 50 is electrically discrete and isolated from the constant current I output. It is not associated with the above-described feedback circuit. Thus, the circuit 50 output E is a variable voltage which is controlled directly by (slaved to) the output voltage of the constant current circuit. Therefore, the circuit 50 output is effectively independent of its own load impedance fluctuations and is not a constant current output. The output voltage E is preferably maintained to within approximately less than 50 volts of the output voltage (at I of the constant current supply. This prevents any significant voltage differences between any of the roll segments. Such voltage differences between segments could cause visible transfer differences by causing direct resistive current flow between the segments, which would correspondingly change the charge flow to the paper from the outer segment, since its total current supply is held constant.

Referring now particularly to FIG. 3, this is a crosssectional view taken along the axis of the roller illustrating an exemplary mounting arrangement for the roller 15 as well as exemplary electrical connections thereto. Numerous other mounting arrangements and electrical connection arrangements may be utilized. The rotational end mounting of only one end of the roller 15 is illustrated, and the main central portion of the roller (the segment 22) is greatly foreshortened in order to allow an enlarged view here'for drawing clarity. A similar mounting and bearing arrangement may be utilized for the opposite end of the roller 1'5. However, with the arrangement shown both electrical connections are made from the end illustrated. Therefore, the opposite end bearing not illustrated here may be a simple insulated arbor end bearing without any electrical connections to the roller.

'

cores

22A and 22B extending axially from the opposite ends of the central core 22 and mounted coaxially therewith. These

end cores

22A and 22B and the central core may be seen to have the same external radial dimensions. However, the central core section 22 has a much greater axial length. For example, the central core section may have an axial length of approximately 7.5 inches, as opposed to an axial length of approximately 3.75 inches for each of the two

end cores

22A and 22B.

Both of the

conductive end cores

22A and 22B are electrically insulated from the central core by insulator rings 71 and 72. It may be seen that these insulator rings have a central band extending out to the common surface of the conductive cores which fully electrically separates the core segments by approximately 1 centimeter of insulation here. They may be phenolic or other suitable material, preferably providing the mechanical connection of the core segments. However, all three of the

core segments

22, 22A, and 22B are fully commonly overlaid by the single uniform layer 21 of resistive relaxable material. This layer 21 in the example here may have depth of approximately 1 centimeter.

The overlying resistive material 21 does, of course, provide a small zone of lateral electrical conductivity between the conductive core segments along the roll axis since it electrically by-passes or bridges the insulator rings 71 and 72. However, since the layer 21 is relatively thin in comparison to the segment 22 length, and quite resistive, and since the voltages applied to all three segments are maintained at substantially the same voltage level at all times by the circuit 23, these conductive zones of the layer 21 overlying the insulator rings 71 and 72 are not conductive enough to exceed the capabilities of the constant current circuit, and do not present any visible transfer effects on the copy sheet. That is, this axial resistance through the zones of axial conductivity between the roller segments is much greater than the radial resistance of the layer 21, in view of the vastly greater circumferential area of the core 22 overlaid by the relatively thin layer 21. (The external diameter of the conductive cores here can be approximately 2.5 centimeters).

For a given desired resistivity and layer thickness of the resistive material 21, the axial insulative gap provided by the insulator rings 71 and 72 can be varied to control the resistance between conductive segments caused by the bridging resistive material over the insulator rings. A substantial (e.g., 2 megohm) resistance between segments is desirable. This provides a sufficiently low current loss from the center core section 22 as long as the voltage difference between core segments is kept sufficiently small as previously described. Both this high roller inter-segment resistance and the low inter-segment voltage difference together provide a current flow between roller segments which is much lower than the current I Since the pre and post-nip ionization control operation of the relaxable material 21 utilizes the radial bulk resistivity time constant of this material, a mini- 11 mum preferable design constraint on the roll splitting insulation thickness, (i.e., the axial length of the insulation gap provided by the insulator rings 71 and 72) is that the conduction time for charge flow axially along the bias roller in this insulation gap region is faster,.so that uneven charge patterns will not occur from the electrical relaxation as this insulation gap region is con- I tinually rotated into the nip. lt has'be'en calculated that this axial flow time constant across the insulation gap will be faster than the radial bulk flow time constant of the relaxable material if the axial length (width) of the insulation gap is less than the thickness ofthe relaxable material 21. This relationship is satisfied,- for example,

for a A inch thick bias roll layer 21, by a l 16 inch axial insulation gap. It will be appreciated that while two

end core segments

22A and 22B are illustrated herein, that with an edge registration system (in which the copy sheet would alwaysextend to one end of the roller 15) rather than a center registration system as here, that one roller end section 22A or. 228 could be eliminated, leaving two rather than three roller segments. In any case, however, the central section'22 will have an axial length and position such that during transfer it will always overlay only an areain which the copy sheet 16 is present. That is,-- even for shorter paper, the core segment 22 should 'not extend over a no paper area in which theresistive material 21 thereon would directly overlie the original imaging surface (photoreceptor) 1.1. This is important to prevent the constant current power supply connection to this central section 22 from being af fected byany-significant end leakage. Accordingly, the axial length of the segment 22 is less than the width of the copy sheet. at all times. a

That portion of the copy sheet which extends under the-'end-

core segments

22A and 22B is, however, subjecthere to equal transfer fields of the same transfer effectiveness applied with the same interposed relaxable material 21, so as to provide simultaneous and visibly identical transfer of anytransfer material at. any position along the roller at any time, and irrespective of the rotational position of the roller; 2? I Considering now the exemplary electrical connections of the power supply 23 of FIG. 2 to the roller 15 as illustrated in FIG. 3, it may be seen that the power supply terminals are here connected to,- two conventional stationary

electrical brushes

66 and 68, respectively. These two brushes in turn respectively electrically .slidably connect'with separate conductive slip rings 62 and 64 on a rotating cone arbor unit 60. The arbor unit 60 provides the concentric end bearing for the roller 15. The brush 66 and slip ring 62 provide a direct electrical connection of the constant current circuit output l to a conductve strip81 which extends in a corresponding groove along an insulative core 80. This conductive strip 81 extends inside along the roll along core 80 to a contact pad 82 located inside the conductive core segment 22. There it makes electrical contact through a spring contact 84 with (only) the core segment 22.

a core 80. The strip 74 has a curled arbor end 76 which makes a spring loaded connection with the inside of the slip ring 64. The other end of the conductor strip 74 connects'with a contact pad 76, which is contacted by a spring contact 78 on the core segment 22B.

:Thus, from the exemplary electrical connections described herein itmay be seen that the constant current power supply output I connects only with the central conductive core segment 22, .whereas the slaved voltage E from the circuit 50 is separately applied {simultaneously to both the

end cores

22A and 22B.

,ing the entire roller as a single monolithic cylindrical blanket. The onlysharp transitions in conductive/nonconductive or differently biased conductors occur internally of the roll, undeer the resistive material layer The layer of resistive material 21, could, of course,

itself also be segmented by insulative layers or a more core segment 22.

resistive material at the roll segments. This, however, is not desirable as it could cause a sharp volt age transition at the roll surface rather than the smooth gradual change between segments provided by the present structure whenever the voltage on the

end cores

22A and 22B is not exactly. the same as the voltage on the It will be appreciated that numerous other structures can be provided to achieve the same above-described functions. Various wiring or printed circuit arrangements may be utilized in place of those described here. Even the conductive core itself may be formed, for example, by plating conductive segments onto an insulative core. The electrical connections with the roll can be provided by wipers which are axially mounted coil springs, or the slip rings can be internal oron the ends of the rollers, etc. g

Although the circuit 23 has been described herein as providing a constant current supply I it will be appreciated that lower and/or uppeer voltage lirnits may be placed on its constant current operation, so that under extreme operating conditions the applied voltage will not go below or exceed a desired range of transfer voltage. This can be provided here by various conventional circuit techniques, such as modifying the feedback circuit illustrated in'FlG. 2. The feedback circuit input to the base of transistor 0305, for example, can be clamped by zener diodes or current limiters, or the feedback control voltage E output can be limited. This would cause the voltage at the constant current I output as well as the voltage E to automatically limit its excursions to within a selected range. However, it-will be appreciated that if, say, a 1500 volt low voltage limit is placed on the constant current power supply at a current level I for example, providing current densities to the copy sheet near 1.5 microamps per roller inch, then as the resistivity of the paper or roller decreases below this limit the bias current I will then increase. This will cause an increase in the charge density applied to the copy sheets, which will make sheet detacking and stripping more difficult,'and may cause per-nip ionization if the overcoat 20 resistivity is low.

In conclusion, it may be seen that there has been dis- 1 closed herein a novel and improved bias transfer system. The exemplary embodiment described herein is presently considered to be preferred; however, it is 13 contemplated that further variations and modifications within the purview of those skilled in the art can be made herein. The following claims are intended to cover all such variations and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An improved transfer member for transferring an image from an image support surface to a copy moved between the transfer member and support surface, the improvement comprising:

at least two segments which are electrically discrete in a direction perpendicular to the direction of movement of the copy and electrical bias means for supplying discrete electrical biases to at least two of the segments.

2. In an electrostatographic copying system wherein an image is transferable from an image support surface to an adjacent copy utilizing an electrically biased transfer member positioned so as to contact and keep said copy in contact with said support surface while said copy is moved between said support surface and said transfer member, the improvement in said electrically biased transfer member wherein:

said transfer member comprises at least two endless loop core segments which are electrically discrete in a direction transverse to the direction of movement of said copy, and a resistive material layer commonly overlying said segments; and

electrical bias means for supplying discrete electrical biases to at least two of said segments.

3. The copying system of claim 2, wherein said electrical bias means provides a substantially constant current and variable voltage bias to a first one of said segments, and a non-constant current variable voltage bias to another of said segments corresponding to the variable voltage applied to said first one of said segments.

4. In an electrostatographic copying system wherein an image is transferable from an image support surface to an adjacent copy utilizing an electrically biased generally cylindrical transfer roller mounted for rotation about its longitudinal axis and positioned so as to contact and keep said copy in contact with said support surface while said copy is moved between said support surface and said transfer member, the improvement in said electrically biased transfer roller wherein:

14 said transfer roller is divided along its axis into a plurality of electrically discrete generally cylindrical segments, and electrical bias means are connected to said transfer roller for providing electrical bias supplies which are discrete for at least two of said segments.

5. The copying system of claim 4, wherein said electrical bias means provides a substantially constant current and variable voltage bias to a first one of said segments, and a non-constant current variable voltage bias to another of said segments corresponding to the variable voltage applied to said first one of said segments.

6. The copying system of claim 4 wherein said transfer roller comprises conductive core segments insulated from one another and a commonly overlying resistive material layer.

7. In an electrostatographic copying system wherein an image is transferable from an image support surface to an adjacent copy utilizing an electrically biased generally cylindrical transfer roller mounted for rotation about its longitudinal axis and positioned so as to contact and keep said copy in contact with said support surface while said copy is moved between said support surface and said transfer member, the improvement in said electrically biased transfer roller wherein said transfer roller comprises:

a conductive generally cylindrical central core;

conductive end cores axially extending from opposite ends of said central core and mounted co-axially therewith,

said end cores being electrically insulated from said central core;

and a layer of resistive material overlying both said central core and said end cores.

8. The copying system of claim 7, wherein said layer of resistive material is a single monolithic generally cylindrical blanket providing a resistive electrical connection between said central core and said end cores.

9. The copying system of claim 7 wherein said resistive material is an elastomeric electrically relaxable material.

10. The copying system of claim 7 further including electrical bias means for biasing said central core with a substantially constant current and variable voltage bias, and for discretely biasing said end cores with a variable voltage maintained substantially equal to the variable voltage applied to said central core.

Claims

1. An improved transfer member for transferring an image from an image support surface to a copy moved between the transfer member and support surface, the improvement comprising: at least two segments which are electrically discrete in a direction perpendicular to the direction of movement of the copy and electrical bias means for supplying discrete electrical biases to at least two of the segments.

2. In an electrostatographic copying system wherein an image is transferable from an image support surface to an adjacent copy utilizing an electrically biased transfer member positioned so as to contact and keep said copy in contact with said support surface while said copy is moved between said support surface and said transfer member, the improvement in said electrically biased transfer member wherein: said transfer member comprises at least two endless loop core segments which are electrically discrete in a direction transverse to the direction of movement of said copy, and a resistive material layer commonly overlying said segments; and electrical bias means for supplying discrete electrical biases to at least two of said segments.

4. In an electrostatographic copying system wherein an image is transferable from an image support surface to an adjacent copy utilizing an electrically biased generally cylindrical transfer roller mounted for rotation about its longitudinal axis and positioned so as to contact and keep said copy in contact with said support surface while said copy is moved between said support surface and said transfer member, the improvement in said electrically biased transfer roller wherein: said transfer roller is divided along its axis into a plurality of electrically discrete generally cylindrical segments, and electrical bias means are connected to Said transfer roller for providing electrical bias supplies which are discrete for at least two of said segments.

7. In an electrostatographic copying system wherein an image is transferable from an image support surface to an adjacent copy utilizing an electrically biased generally cylindrical transfer roller mounted for rotation about its longitudinal axis and positioned so as to contact and keep said copy in contact with said support surface while said copy is moved between said support surface and said transfer member, the improvement in said electrically biased transfer roller wherein said transfer roller comprises: a conductive generally cylindrical central core; conductive end cores axially extending from opposite ends of said central core and mounted co-axially therewith, said end cores being electrically insulated from said central core; and a layer of resistive material overlying both said central core and said end cores.