Patent Publication Number: US-6668146-B2

Title: Hybrid scavengeless development using direct current voltage shift to remove wire history

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a Hybrid Scavengeless Development (HSD) apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a method to prevent toner or other particulate contamination of wires in such an HSD developer unit. 
     2. Brief Description of Related Developments 
     Generally, the process of electrophotographic printing includes charging a photoreceptor member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoreceptor surface is exposed to a light image from either a scanning laser beam, an LED source, or an original document being reproduced. This records an electrostatic latent image on the photoreceptor surface. After the electrostatic latent image is recorded on the photoreceptor surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single-component developer material typically comprises toner particles. Toner particles are attracted to the latent image, forming a toner powder image on the photoreceptor surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration. 
     Hybrid scavengeless development technology develops toner via a conventional magnetic brush onto the surface of a donor roll. A plurality of electrode wires are closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the electrode wires to generate a toner cloud in the development zone. This donor roll generally consists of a conductive core covered with a thin (50-200 microns) partially conductive layer. The magnetic brush roll is held at an electrical potential difference relative to the donor roll to produce the field necessary for toner to adhere to the donor roll. The toner layer on the donor roll is then disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles. Typical ac voltages of the wires relative to the donor are 700-900 Vpp at frequencies of 5-15 kHz. These ac signals are often square waves, rather than pure sinusoidal waves. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image. 
     A problem with developer systems using electrode wires is “Wire History.” Wire history involves highly charged (though sometimes low charged) and generally small toner or other particles being attracted to the wire and sticking to the wire as a result of either adhesive or electrostatic attractive forces. The result is that contaminants build up on the electrodes, as a response to the image area coverage history, causing visible streaks on prints. U.S. Pat. No. 6,049,686 discloses the use of direct current (DC) offset applied to the electrode wires to reduce wire history. It is not practical to routinely work at high direct current (DC) electrode bias offsets because at the same time the offsets improve wire history they reduce the overall level of developability. The electrode DC offset being defined as the DC potential of the electrodes with respect to the magnetic roll DC level. The present invention overcomes the problems of the prior art as will be described in greater detail below. 
     SUMMARY OF THE INVENTION 
     An image transfer apparatus and a method for removing wire history from the electrodes in a Hybrid Scavengeless Development system. 
     One embodiment of the invention comprises an image transfer apparatus with a development unit having a development zone containing marking material; an electrode for transporting developing material positioned in the development zone; a donor member that moves in the development zone; a movable imaging member with imaging regions and inter-imaging regions between the imaging regions, the movable imaging member moving both the imaging regions and inter-imaging regions into and out of the development zone; and a voltage supply to electrically bias the electrode, the voltage supply generating a shift relative to nominal in the direct current component of the electrode bias relative to an electrical bias of the donor member during the movement of at least one of the inter-imaging regions through the development zone, wherein the electrode is cleaned. 
     A second embodiment of the invention comprises an image transfer apparatus, with a development unit having a development zone; a donor member for transporting marking particles to the development zone adjacent an imaging member, the imaging member, having image receiving regions and inter-image areas between the image receiving regions, the imaging member advancing the image receiving regions and the inter-image areas into and out of the development zone; and a voltage supply to electrically bias the donor member relative to the imaging member, the voltage supply generating an electrical bias shift in the donor member from a first electrical bias to a second electrical bias, the electrical bias shift being generated, during the advancement of the inter-image area through the development zone, wherein an electrode in the development zone is cleaned. 
     A third embodiment of the invention comprises a method of cleaning an image transfer apparatus with the steps of: providing a voltage supply; and supplying voltage from the voltage supply for electrically biasing an electrode with respect to a donor roll; and with the voltage supply, generating a shift in a direct current component of the electrical bias relative to another electrical bias of the donor roll during advancement of an inter-image area. 
     A fourth embodiment of the invention is a method of transferring an image, with the steps of: generating image regions on an image receiving member, the image regions being separated by inter-image areas; transporting marking particles with a development member to a development zone having an electrode positioned between the image receiving member and the development member; supplying voltage for electrically biasing the development member relative to the image receiving member; and varying at least a direct current component of the electrical bias of the development member to shift at least the direct current component from an initial voltage to another voltage during passage of the inter-image areas through the development zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
     FIG. 1 is a schematic elevational view of an illustrative electrophotographic printing or imaging machine or apparatus incorporating a development apparatus having the features of the present invention therein; 
     FIG. 2 shows a typical voltage profile of an image area in the electrophotographic printing machines illustrated in FIG. 1 after that image area has been charged; 
     FIG. 3 shows a typical voltage profile of the image area after being exposed; 
     FIG. 4 shows a typical voltage profile of the image area after being developed; 
     FIG. 5 shows a typical voltage profile of the image area after being recharged by a first recharging device; 
     FIG. 6 shows a typical voltage profile of the image area after being recharged by a second recharging device; 
     FIG. 7 shows a typical voltage profile of the image area after being exposed for a second time; 
     FIG. 8 is a schematic elevational view showing the development apparatus used in the FIG. 1 printing machine. 
     FIG. 9 shows a voltage profile of the electrode; and 
     FIG. 10 shows a voltage profile of the donor member. 
    
    
     In as much as the art of electrophotographic printing is well known, the various processing stations employed in the printing machine will be shown hereinafter schematically and their operation described briefly with reference thereto. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Referring to FIG. 1, there is shown an illustrative electrophotographic machine having incorporated therein the development apparatus of the present invention. An electrophotographic-printing machine creates a color image in a single pass through the machine and incorporates the features of the present invention. The printing machine uses a charge retentive surface in the form of an Active Matrix (AMAT) photoreceptor belt  10  which travels sequentially through various process stations in the direction indicated by the arrow  12 . Belt  10  travel is brought about by mounting the belt about a drive roller  14  and two tension rollers  16  and  18  and then rotating the drive roller  14  via a drive motor  20 . 
     As the photoreceptor belt  10  moves, each part of it passes through each of the subsequently described process stations. For convenience of explanation, a span of the photoreceptor belt  10 , contains three sections referred to as document sections  110   a ,  110   b ,  110   c , which will be discussed in more detail (FIG.  8 ). The document sections  110   a ,  110   b ,  110   c  are that part of the photoreceptor belt  10  that receive the toner powder images that, after being transferred to a substrate, produce the final image. While the photoreceptor belt  10  may have numerous document sections  110   a ,  110   b ,  110   c , each document section is processed in the same way, a description of the typical processing of one document section  110   a  suffices to fully explain the operation of the printing machine. The document sections  110   a ,  110   b ,  110   c  are separated by interdocument or inter-image regions or areas  112   a ,  112   b  that will be explained here below FIGS. 8,  9 , and  10 . Note that since the belt  10  rotates continuously the number of consecutive document sections  110   a ,  110   b ,  100   c  and interdocument areas  112   a ,  112   b  are unlimited and not constrained by the circumference of the belt (FIG.  8 ). 
     As the photoreceptor belt  10  moves, the document section passes through a charging station A. At charging station A, a corona generating device, indicated generally by the reference numeral  22 , charges the document section to a relatively high and substantially uniform potential. FIG. 2 illustrates a typical voltage profile  68  of a document section  110   a  after the document section  110   a  has left the charging station A. As shown, the document section  110   a  has a uniform potential of about −500 volts. In practice, this is accomplished by charging the document section  110   a  slightly more negative than −500 volts so that any resulting dark decay reduces the voltage to the desired −500 volts. While FIG. 2 shows the document section  110   a  as being negatively charged, it could be positively charged if the charge levels and polarities of the toners, recharging devices, photoreceptor, and other relevant regions or devices are appropriately changed. 
     After passing through the charging station A, the now charged document section  110   a  passes through a first exposure station B. At exposure station B, the charged document section  110   a  is exposed to light which illuminates the document section  110   a  with a light representation of a first color (say black) image. That light representation discharges some parts of the document section  110   a  so as to create electrostatic latent images or image areas (not shown) within the document sections  110   a ,  110   b ,  110   c  (FIG.  8 ). While the illustrated embodiment uses a laser-based output scanning device  24  as a light source, it is to be understood that other light sources, for example an LED printbar, can also be used with the principles of the present invention. FIG. 3 shows typical voltage levels, the levels,  72  and  74 , which might exist, on the document section  110   a  after exposure. The voltage level  72 , about −500 volts, exists on those parts of the document section  110   a , which were not illuminated, while the voltage level  74 , about −50 volts, exists on those parts which were illuminated. Thus after exposure, the document section  110   a  has a voltage profile comprised of relative high and low voltages. 
     After passing through the first exposure station B, the now exposed document section  110   a  passes through a first development station C which is identical in structure with development system E, G, and I. The first development station C deposits a first color, say black, of negatively charged toner  31  onto the document section  110   a . That toner is attracted to the less negative sections of the document section  110   a  and repelled by the more negative sections. The result is a first toner powder image on the document section  110   a . It should be understood that one could also use positively charged toner if the exposed and unexposed areas of the photoreceptor are interchanged, or if the charging polarity of the photoreceptor is made positive. 
     For the first development station C, development system includes a donor roll  40 . As illustrated in FIG. 8, electrode wires or a grid  42  is electrically biased with an AC voltage relative to the donor roll  40  for the purpose of detaching toner therefrom. This detached toner forms a toner powder cloud in the gap between the donor roll  40  and the photoconductive surface. Both electrode grid  42  and donor roll  40  are biased with DC sources  102  and  92  respectively for discharge area development (DAD). The discharged photoreceptor image attracts toner particles from the toner powder cloud to form a toner powder image thereon. 
     FIG. 4 shows the voltages on the document section  110   a  after the document section  110   a  passes through the first development station C. Toner  76  (which generally represents any color of toner) adheres to the illuminated part of the document section  110   a . This causes the voltage in the illuminated part of the document section  110   a  to increase to, for example, about −200 volts, as represented by the solid line  78 . The unilluminated parts of the document section  110   a  remain at about the level −500 volts  72 . 
     Referring back to FIG. 1, after passing through the first development station C, the now exposed and toned image area passes to a first recharging station D. The recharging station D is comprised of two corona recharging devices, a first recharging device  36  and a second recharging device  37 . These devices act together to recharge the voltage levels of both the toned and untoned parts of the document section  110   a  to a substantially uniform level. It is to be understood that power supplies are coupled to the first and second recharging devices  36  and  37 , and to any grid or other voltage control surface associated therewith, so that the necessary electrical inputs are available for the recharging devices to accomplish their task. 
     FIG. 5 shows the voltages on the document section  110   a  after it passes through the first recharging device  36 . The first recharging device overcharges the image area to more negative levels than that which the image area is to have when it leaves the recharging station D. For example, as shown in FIG. 5 the toned and the untoned parts of the document section  110   a , reach a voltage level in the range of about −700 volts  80  to about −500 volts  82 . The first recharging device  36  is preferably a DC scorotron. 
     After being recharged by the first recharging device  36 , the document section  110   a  passes to the second recharging device  37 . Referring now to FIG. 6, the second recharging device  37  reduces the voltage of the document section  110   a , both the untoned parts and the toned parts (represented by toner  76 ) to a level  84  which is the desired potential of −500 volts. 
     After being recharged at the first recharging station D, the now substantially uniformly charged document section  110   a  with its first toner powder image passes to a second exposure station  38 . Except for the fact that the second exposure station illuminates the document section  110   a  with a light representation of a second color image (say yellow) to create a second electrostatic latent image, the second exposure station  38  is the same as the first exposure station B. FIG. 7 illustrates the potentials on the document section  110   a  after it passes through the second exposure station. As shown, the non-illuminated areas have a potential about −500 as denoted by the level  84 . However, illuminated areas, both the previously toned areas denoted by the toner  76  and the untoned areas are discharged to about −50 volts as denoted by the level  88 . 
     The document section  110   a  then passes to a second development station E. Except for the fact that the second development station E contains a toner  40  which is of a different color (yellow) than the toner  31  (black) in the first development station C, the second development station is substantially the same as the first development station. Since the toner  40  is attracted to the less negative parts of the document section  110   a  and repelled by the more negative parts, after passing through the second development station E the document section  110   a  has first and second toner powder images which may overlap. 
     The document section  110   a  then passes to a second recharging station F. The second recharging station F has first and second recharging devices, the devices  51  and  52 , respectively, which operate similar to the recharging devices  36  and  37 . Briefly, the first corona recharge device  51  overcharges the document section  110   a  to a greater absolute potential than that ultimately desired (say −700 volts) and the second corona recharging device, comprised of coronodes having AC potentials, neutralizes that potential to that ultimately desired. 
     The now recharged document section  110   a  then passes through a third exposure station  53 . Except for the fact that the third exposure station illuminates the document section  110   a  with a light representation of a third color image (say magenta) so as to create a third electrostatic latent image, the third exposure station  38  is the same as the first and second exposure stations B and  38 . The third electrostatic latent image is then developed using a third color of toner  55  (magenta) contained in a third development station G. 
     The now recharged document section  110   a  then passes through a third recharging station H. The third recharging station includes a pair of corona recharge devices  61  and  62  that adjust the voltage level of both the toned and untoned parts of the document section  110   a  to a substantially uniform level in a manner similar to the corona recharging devices  36  and  37  and recharging devices  51  and  52 . 
     After passing through the third recharging station the now recharged document section  110   a  then passes through a fourth exposure station  63 . Except for the fact that the fourth exposure station illuminates the document section  110   a  with a light representation of a fourth color image (say cyan) so as to create a fourth electrostatic latent image, the fourth exposure station  63  is the same as the first, second, and third exposure stations, the exposure stations B,  38 , and  53 , respectively. The fourth electrostatic latent image is then developed using a fourth color toner  65  (cyan) contained in a fourth development station I. 
     To condition the toner for effective transfer to a substrate, the document section  110   a  then passes to a pretransfer corotron member  50  which delivers corona charge to ensure that the toner particles are of the required charge level so as to ensure proper subsequent transfer. 
     After passing the corotron member  50 , the four toner powder images are transferred from the document section  110   a  onto a support sheet  57  at transfer station J. It is to be understood that the support sheet is advanced to the transfer station in the direction  58  by a conventional sheet feeding apparatus which is not shown. The transfer station J includes a transfer corona device  54 , which sprays positive ions onto the backside of sheet  57 . This causes the negatively charged toner powder images to move onto the support sheet  57 . The transfer station J also includes a detack corona device  56  which facilitates the removal of the support sheet  57  from the printing machine. 
     After transfer, the support sheet  57  moves onto a conveyor (not shown) which advances that sheet to a fusing station K. The fusing station K includes a fuser assembly, indicated generally by the reference numeral  60 , which permanently affixes the transferred powder image to the support sheet  57 . Preferably, the fuser assembly  60  includes a heated fuser roller  67  and a backup or pressure roller  64 . When the support sheet  57  passes between the fuser roller  67  and the backup roller  64  the toner powder is permanently affixed to the sheet support  57 . After fusing, a chute, not shown, guides the support sheets  57  to a catch tray, also not shown, for removal by an operator. 
     After the support sheet  57  has separated from the photoreceptor belt  10 , residual toner particles on the document section  110   a  are removed at cleaning station L via a cleaning brush contained in a housing  66 . The document section  110   a  is then ready to begin a new marking cycle. 
     The various machine functions described above are generally managed and regulated by a controller which provides electrical command signals for controlling the operations described above. 
     Referring now to FIG. 8 in greater detail, development system  38  includes a donor roll  40  that may be considered a donor member. The donor member is shown as a roll, but may be any other suitable structure or member suited for transporting toner  82  to the development zone. The development system  38  advances developing material into development zone. The development system or development unit  38  is scavengeless. By scavengeless it is meant that the developing material or toner  82  of system  38  do not interact with an image already formed on the image receiver. Thus, the system  38  is also known as a non-interactive development system. The donor roll  40  conveys a toner layer to the development zone, which is the area between the photoreceptor belt  10  and the donor roll  40 . The toner layer  82  can be formed on the donor roll  40  by either a two-component developer (i.e. toner and carrier  82 ), as shown in FIG. 8, or a single component developer deposited on member  40  via a combination single-component toner metering and charging device. The development zone contains an AC biased electrode structure  42  self-spaced from the donor roll  40  by the toner layer. The single-component toner, developing material, or marking particles  82  may comprise positively or negatively charged toner. The electrode structure or terminal  42  may be coated with TEFLON-S (trademark of E. I. DuPont De Nemours) loaded with carbon black. 
     For donor roll  40  loading with two-component developer, a conventional magnetic brush  46  is used for depositing the toner layer  82  onto the donor roll  40 . The magnetic brush includes a magnetic core enclosed by a sleeve  86 . 
     With continued reference to FIG. 8, auger  76 , is located in housing  44 . Auger  76  is mounted rotatably to mix and transport developing material  48 . The augers have blades extending spirally outwardly from a shaft. The blades are designed to advance the developing material  48  in the axial direction substantially parallel to the longitudinal axis of the shaft. The developer-metering device is designated  88 . As successive electrostatic latent images  110   a ,  110   b ,  110   c  are developed; the toner particles  82  within the developing material are depleted. A toner dispenser (not shown) stores a supply of toner particles  82 . The toner dispenser is in communication with housing  44 . As the concentration of toner particles in the developer material  48  is decreased, fresh toner particles are furnished to the developer material  48  in the chamber from the toner dispenser. The augers in the chamber of the housing mix the fresh toner particles with the remaining developer material so that the resultant developer material therein is substantially uniform with the concentration of toner particles being optimized. In this manner, a substantially constant amount of toner particles are maintained in the chamber of the developer housing  44 . 
     In the preferred embodiment shown in FIG. 8, the electrode structure  42  may be comprised of one or more thin (i.e. 50 to 100 microns diameter) conductive wires which are lightly positioned against the toner  82  on the donor roll  40 . Although the electrode  42  is shown as conductive wires, it could encompass plates, supplemental or ancillary wires or any other electrical elements or members as one skilled in the art could devise. The distance between the wires and the donor roll  40  is self-spaced by the thickness of the toner layer, which is approximately 25 microns. End blocks (not shown) support the extremities of the wires at points slightly above a tangent to the donor roll  40  surface. A suitable scavengeless development system for incorporation in the present invention is disclosed in U.S. Pat. No. 4,868,600 and is incorporated herein by reference. As disclosed in the &#39;600 patent, a scavengeless development system may be conditioned to selectively develop one or the other of the two document section  110   a  (i.e. discharged and charged document section  110   a ) by the application of appropriate AC and DC voltage biases to the wires  42  and the donor roll  40 . 
     According to the present invention, and referring again to FIG. 8, the developer unit preferably includes a DC voltage source  102  to provide proper bias to the wires  42  relative to the donor roller  40 . The wires  42  receive AC voltages from sources  103  and  104 . These sources may generate different frequencies, and the resultant voltage on the wire  42  is the instantaneous sum of the AC sources  103  and  104  plus the DC source  102 . AC source  103  is often chosen to have the same frequency, magnitude, and phase as AC source  96 , which supplies the donor roll  40 . Then, the voltage of the wires  42  with respect to the donor roll  40  is just the AC source  104  plus the difference or offset between the two DC sources  102  and  92 . The DC voltage source  102  may be separate from the DC voltage sources  92  and  98  as shown in FIG. 8 or share a common voltage source. Further, the AC voltage source  104  may be separate from the AC voltage sources  96 ,  103 , and  100  as shown in FIG. 8 or share a common voltage source. 
     The electrical sections of FIG. 8 are schematic in nature. Those skilled in the art of electronic circuits will realize there are many possible ways to connect AC and DC voltage sources to achieve the desired voltages on electrodes  42 , donor roll  40 , and magnetic brush roll  46 . 
     Scavengeless developer systems such as shown in FIG. 8 exhibit an image quality defect known as “wire history”. In this defect either toner or some other particulate or component of the developer material  48  is non-uniformly attached to the electrodes  42 . The attachment of this material to the electrodes decreases the developability characteristics of the development system electrodes. If this attachment is non-uniform along the axial length of the development system then the developability performance of the development system along its axial length will be non-uniform and this will cause an undesired image quality defect. 
     To first order, the effects of the DC bias components of the electrode  42  and donor  40  can be understood best by convolving the bias sources as the difference ( 102  minus  92 ). Then the DC effects on the developability of toner to the photoconductor in the intentional image areas, e.g.  74 , by the difference ( 102 − 92 ) and in the unintended “background” areas  72  by the donor bias  92 , where the difference voltage ( 102 − 92 ) of a magnitude more toward the toner polarity with inhibit toner development in the intended areas and a donor bias  92  magnitude more toward the toner polarity will encourage toner development in the unintended areas. 
     It has been found that the “wire history” may be reduced by applying a shifting of the electrode or wire DC bias  102  relative to the donor DC bias  92  (i.e.  102 − 92 ) to a value more toward the polarity of the toner (e.g. more negative in our example). Additionally it has been found that shifting the donor DC bias  92  to a voltage more toward the polarity of the toner will also reduce wire history. Combining these two effects has been found to be the most effective method of reducing wire history defects. However it can be seen that whereas these two shifts result in improved wire history performance they tend to reduce intended toner development and increase unintended toner development. Accordingly the resolution of this is to provide for the wire and donor bias shifts only during otherwise unused interdocument zones or  112   a ,  112   b  (inter-imaging areas or inter-imaging zones) on the photoreceptor belt  10  without any loss in overall developability (FIGS. 8,  9 ,  10 ). This shift of voltage optimizes wire conditions for developability during document sections  110   a ,  110   b ,  110   c  while allowing the unused interdocument areas  112   a ,  112   b  to utilize a donor roll  40  and wire development electrical bias, perhaps even to the point of developing some toner  82  in the interdocument areas  112   a ,  112   b . Note that some printing machines utilize certain of the interdocument zones to print test patches for control of various process elements or for other purposes. The described bias shifts would only be applied in the otherwise unused interdocument zones. 
     An explanation of how wire history can be reduced or eliminated can be found by focusing on the photoreceptor belt  10  as it travels past or through the development zone in FIG.  8 . FIG. 8 shows the areas on the belt  10  where the electrical bias shift is performed. As discussed above, as the photoreceptor belt  10  moves, the charged document section  110   a ,  110   b ,  110   c  through the development zone in the direction indicated  16  and the charged toner particles  82  are attached to the voltage regions  74 ,  88 , etc. within the image areas  110   a ,  110   b ,  110   c . Next, the interdocument areas  112   a ,  112   b  pass through the development zone. 
     Specifically, while the unused interdocument area  112   a ,  112   b  is in the development zone the following events occur: 
     The power supply controller  94  supplies a DC component of an electrical bias through DC source  102  to the electrode  42 . This supply of power provides a burst of voltage that shifts the electrical bias of the electrode  42  during the passing of the unused interdocument area  112   a ,  112   b  on the photoreceptor belt  10  so as to reduce the accumulation of wire history forming particles on the electrode  42 . The electrical bias shift of the electrode  42  is relative to nominal in the D.C. component of the electrical bias of the donor roll  40  as maintained by the donor roll  40  during the imaging document section  110   a ,  110   b ,  110   c . During this instance the donor roll  40  is covered with toner  82 . The electrical bias shift of the electrode  42  has a polarity equal the polarity of the developing toner material  82 . Also, during the passing of the interdocument areas  112   a ,  112   b  the toner  82  remains on the donor roll  40 . 
     FIG. 8 shows the areas on the belt  10  including the portions of the interdocument areas  112   a ,  112   b  where the electrical bias shift is produced. FIG. 9 is a graph that illustrates a preferred electrical bias shift during the passage of part of the interdocument area  112   a ,  112   b  of the belt  10  past the electrode  42 . To shift the electrical bias of the electrode  42 , a variety of voltages and sources may be used. As illustrated in FIG.  9 , the DC  102  component of the electrical bias of the electrode  42  is shifted between about 25 volts and about 250 volts. 
     Wire history may also be reduced from the electrode  42  by an electrical bias shift of the donor roll  40  while the unused interdocument areas  112   a ,  112   b  are in the development zone. 
     Again FIG. 8 is useful to illustrate the areas and timing of the donor roll  40  electrical bias shift. During the passage of the document section  110   a  on the belt  10  through the development zone the voltage is supplied to the donor roll  40  from the AC  96  and DC  92  components, as discussed before, so that toner  82  is deposited directly on the belt  10  document section  110   a . First, the document section  110   a  passes through the development zone, second the unused interdocument zone  112   a  passes into the development zone. 
     During the time the unused interdocument area  112   a  passes into the development zone a shift of voltage is sent from the DC voltage source  92  to provide a shift in the DC component of the electrical bias of donor roll  40 . The electrical bias shift of the donor roll  40  is offset relative to the electrical potential of the photoreceptor belt  10 . The preferred polarity shift for the donor roll  40 , during the passage of the unused interdocument zone  112   a ,  112   b  through the development zone, is one that would attract toner  82  to the photoreceptor belt  10 . 
     A variety of voltages and sources may be used to shift the electrical bias of the donor roll  40 . FIG. 10 is a graph that illustrates a preferred voltage shift in the electrical bias of the donor roll  40 . Specifically, FIG. 10 shows a shift in the DC component of the electrical bias of between about 25 volts and about 100 volts. 
     As can be realized from FIGS. 9 and 10 both the electrical bias of the electrode  42  and the electrical bias of the donor roll  40  are shifted basically simultaneously during the passage of the unused interdocument areas  112   a ,  112   b  through the development zone. 
     Although, any combination of polarities and voltage sources may be used with the electrode  42  and the donor roll  40 , the preferred polarities, being the polarities that make the toner move in the directions described above, are as follows: the polarity of the electrical bias of the electrode  42  is equal to the polarity of the toner  82  and would repel toner  82  from the electrode; and the polarity of the electrical bias shift of the donor roll  40  is arranged with a polarity or charge that would repel toner  82  from the donor roll and attract it to the belt  10 . 
     In the alternative embodiments, the bias shift in the electrode  42  may be performed independent from a shift in bias of the donor roll  40 . For example, the bias shift of the electrode  42  may be performed prior to commencing the bias shift of the donor roll  40 . In other embodiments the bias shift of the donor roll  40  may be performed prior to the bias shift of the electrode  42 . 
     The electrical bias shifts of the electrode  42  and the donor roll  40  may be performed in an alternating sequence during the passage of the unused interdocument zones  112   a ,  112   b . Also, the electrical bias shifts of the electrode  42  and the donor roll  40  may be alternated or interspersed with the preferred embodiment of electrically biasing both the donor roll  40  and the electrode  42 . 
     In conclusion, this invention provides a successful way of reducing or eliminating significant wire history. To reduce wire history, electrical bias shifts in the form of a burst mode are applied during the unused interdocument zones  112   a ,  112   b  so that there is no loss in developability in the document sections  110   a ,  110   b ,  110   c . First, the DC component of the electrical bias on the electrode  42  may be shifted relative to the electrical bias on the donor roll  40 . Next, the DC component of the electrical bias on the donor roll  40  may be shifted relative to the electrical bias on the photoreceptor belt  10 . Also, the DC component of the electrical bias of both the electrode  42  and the donor roll  40  may be shifted. Any of these techniques keeps the electrode cleaner and enhances the robustness of the developer unit. The present invention as described above protects the developer unit from mechanical, electrical and moisture degradation, therefore, extends the dependability and durability of the developer unit. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. For example, in place of the photoreceptor belt  10 , the present invention may be used on an imaging apparatus having a photoreceptor drum or any other type of desired electrostatically charged receiver. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.