Patent Publication Number: US-7711296-B2

Title: Pretransfer toner treatment in an electrostatographic printer

Description:
TECHNICAL FIELD 
   The present disclosure relates generally to transfer efficiency in an electrostatographic imaging device, and, in particular, to leading edge smearing during transfer in an electrostatographic imaging device. 
   BACKGROUND 
   In high-speed reproduction machines, such as electrostatographic copiers and printers, a photoconductive member (or photoreceptor) is charged to a uniform potential and then a light image of an original document is exposed onto a photoconductive surface, either directly or via a digital image driven laser. Exposing the charged photoreceptor to a light image discharges the photoconductive surface thereof in areas corresponding to non-image areas in the original document while maintaining the charge on the image areas to create an electrostatic latent image of the original document on the photoconductive surface of the photoreceptor. A developer material is then brought into contact with the surface of the photoconductive member to transform the latent image into a visible reproduction. The developer material includes toner particles with an electrical polarity opposite that of the photoconductive member, causing them to be naturally drawn to it. A blank print substrate such as a sheet of paper is brought into contact with the photoconductive member and the toner materials are transferred to it by electrostatic charging of the substrate. The substrate is subsequently heated and pressed to permanently bond the reproduced image to the substrate, thus producing a hard print reproduction of the original document or image. Thereafter, the photoconductive member is cleaned and reused for subsequent print production. 
   The process of transferring charged toner particles from an image bearing member, such as the photoreceptive member, to an image support substrate, such as a print sheet, is accomp lished at a transfer station. In a conventional electrostatographic machine, transfer is achieved by transporting an image support substrate into the area of the transfer station where electrostatic force fields sufficient to overcome the forces holding the toner particles to the photoconductive surface are applied to the substrate to attract and transfer the toner particles to the image support substrate. In general, such electrostatic force fields are generated via electrostatic induction using a corona generating device. The reverse side of the print sheet is exposed to a corona discharge while the front of the print sheet is placed in direct contact with the developed toner image on the photoconductive surface. The corona discharge generates ions having a polarity opposite that of the toner particles, thereby electrostatically attracting and transferring the toner particles from the photoreceptive image bearing member to the print sheet. 
   The interface between the image bearing surface and the print sheet, however, is not always optimal. In particular, non-flat or uneven image support substrates, such as copy sheets that have been mishandled, paper that has been left exposed to the environment, or substrates that have previously passed through a fixing operation (for example, heat and/or pressure fusing) often tend to yield imperfect contact with the photoconductive surface. Some printing applications require imaging onto high quality papers having surface textures which prevent intimate contact of the paper with the developed toner images. In duplex printing systems, even initially flat paper can become cockled or wrinkled as a result of paper transport and/or the first side fusing step. Also, color images can contain areas in which intimate contact of toner with paper during the transfer step is prevented due to adjacent areas of high toner pile heights. The lack of uniform intimate contact between the belt and the copy sheet in these situations can result in spaces or air gaps between the developed toner powder image on the selectively charged imaging surface and the copy substrate. When spaces or gaps exist between the developed image and the copy substrate, various problems may result. For example, there is a tendency for toner not to transfer across gaps, causing variable transfer efficiency and, under extreme circumstances, creating areas of low toner transfer or even no transfer, resulting in a phenomenon known as image transfer deletion. 
   In order to minimize transfer deletions, transfer assist blades (TABs) have been utilized to press the back of the copy substrate against the imaged area of the charged imaging surface. The transfer assist blade is typically moved from a non-operative position spaced from the copy substrate, to an operative position in contact with the copy substrate. A mechanism supporting the TAB is operable to press the TAB against the copy sheet with a typically pre-determined force sufficient to press the copy substrate into contact with the developed image on the photoconductive or other charged imaging surface in order to substantially eliminate any spaces therebetween during the transfer process. 
   While the transfer assist apparatus of the type described above may be used to improve transfer efficiency, it may also induce copy quality defects in the lead edge area of the copy sheet. For instance, when the developed toner image extends to the lead edge of the imaged area on the photoreceptor, there may be a loss of toner electrostatic tack force in the lead edge region. As a result, drag force on the image substrate caused by pressing the TAB onto the substrate may be greater than the tack force between the substrate and the photoreceptor, which, in turn, generates a velocity mismatch between the copy sheet and the photoreceptor, manifesting itself as a smeared image on the lead edge of the copy sheet. This lead edge image defect is unacceptable in most high speed environments where customers demand lead edge to trail edge copy quality as the electrostatographic printing process penetrates further into the offset printing market. 
   One method that has been utilized to minimize the leading edge smear defect is delaying the engagement of the TAB to allow the electrostatic tacking force to increase by providing a timing delay between sensing of the leading or trailing edge of a copy substrate and actuation of the mechanism that urges the TAB toward the copy substrate. Delaying engagement of the TAB, however, may result in spaces or air gaps between the developed toner powder image on the selectively charged imaging surface at the lead edge resulting in transfer deletions at the lead edge. 
   SUMMARY 
   A system for reducing lead edge (LE) smearing in an electrostatographic machine is provided. The system comprises a pretransfer corotron positioned between a developing station and a transfer station in an electrostatographic imaging device. The pretransfer corotron is configured to apply a predetermined charge to at least a portion of a developed toner image on a photoreceptive surface. The predetermined charge is for increasing an electrostatic tacking force that tacks the toner of the developed toner image to the photoreceptive surface. The system further includes a controller operably associated with the pretransfer corotron. The controller is configured to activate the pretransfer corotron to apply said predetermined charge to the developed toner image at a lead edge region of the developed toner image when leading edge smearing is detected during imaging operations. 
   In another embodiment, a method for reducing LE smear in an electrostatographic imaging device is provided. The method comprises detecting if LE smear is indicated during imaging operations. If LE smear is indicated, tacking force for tacking toner of a developed toner image to a photoreceptive member is increased at approximately the lead edge region of the developed toner image on the photoreceptive surface prior to transfer. Increasing the tacking force may comprise applying a charge to the lead edge region of developed toner image using a pretransfer corotron. 
   In yet another embodiment, an electrostatographic machine operable to reduce LE smearing is provided. The electrostatographic machine comprises a circulating photoreceptive member; a charging station for uniformly charging the photoreceptive member; an imaging station for selectively discharging the photoreceptive member to form latent images thereon; a developing station for depositing toner on the latent images; a transfer station for transferring the developed toner image from the photoreceptive member to an image substrate; a pretransfer corotron for applying a predetermined charge to a lead edge region of the developed toner image prior to transfer, the predetermined charge for tacking toner of the lead region of the developed toner image to the photoreceptive member; and a system controller operably associated with the pretransfer corotron, the system controller for activating the pretransfer corotron to apply the predetermined charge to the lead edge region of the developed toner image when lead edge smearing is detected during imaging operations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects and features of the present embodiments will become apparent as the following description proceeds and upon reference to the drawings, in which: 
       FIG. 1  is a schematic elevational view of an illustrative electrostatographic machine. 
       FIG. 2  is a schematic elevational view of a transfer station of the electrostatographic machine of  FIG. 1 . 
       FIG. 3  is a flowchart of an exemplary method for reducing leading edge smearing in the electrostatographic machine of  FIG. 1 . 
   

   DESCRIPTION 
   For a general understanding of the present embodiments, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. 
   An exemplary imaging system is a multifunctional printer with print, copy, scan, and fax services. Such multifunctional printers are well known in the art and may comprise print engines based upon liquid or solid ink jet, electrophotography, other electrostatographic technologies, and other imaging technologies. The general principles of electrophotographic imaging are well known to many skilled in the art and are described above as an exemplary embodiment of an imaging system to which the present disclosure is applicable. 
   Moving now to a description of  FIG. 1 , there is shown an elevational view of an electrostatographic printing apparatus  10 , such as a printer or copier, including a feeder unit  14 , an imaging unit  18 , and an output unit  20 . The feeder unit  14  houses supplies of media sheets and substrates onto which document images are transferred by the printing unit  18 . Sheets to which images have been fixed are delivered to the output unit  20  for correlating and/or stacking in trays for pickup. 
   The imaging unit  18  employs an image-retentive member, such as photoreceptor belt  14 . The belt  14  includes a photoconductive surface deposited on an electrically grounded conductive substrate. Photoreceptor  14  continuously travels the circuit depicted in the figure in the direction indicated by the arrow advancing successive portions of the photoconductive surface of the belt  14  through various processing stations, disposed about the path of movement thereof, as will be described. While a photoreceptor belt  14  is shown, other types of image-retentive members may be used, such as an intermediate belt or drum as used in a color electrophotographic machine, offset printing apparatus, or ink-jet printer. 
   Initially, a segment of belt  14  passes through charging station  18 . At charging station  18 , a corona generating device (not shown) or other charging apparatus, charges photoreceptor belt  14  to a relatively high, substantially uniform potential which is typically a negative voltage between −600V and −800V. Once charged, the photoreceptor belt  14  is advanced to imaging station  20 . 
   At imaging station  20 , a raster output scanner (ROS) (not shown) discharges selectively those portions of the charge corresponding to the image portions of the document to be reproduced. In this way, an electrostatic latent image is recorded on the photoconductive surface. An electronic subsystem (ESS) (not shown) controls the ROS. The ESS is adapted to receive signals from a system controller  100  and transpose these signals into suitable signals for controlling the ROS so as to record an electrostatic latent image corresponding to the document to be reproduced by the printing machine  10 . Other types of imaging systems may also be used employing, for example, a pivoting or shiftable LED write bar or projection LCD (liquid crystal display) or other electro-optic display as the “write” source. When exposed at the imaging station  20 , the photoreceptor surface is selectively discharged to a level of about −60V to −80V. 
   After the electrostatic latent image is recorded on photoconductive surface of belt  14 , belt  14  advances to development station  28  where toner material is deposited onto the electrostatic latent image. In the development station  28 , 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 developing material is brought into contact with the photoreceptor belt  14  such that the latent image thereon attracts the toner particles from the developing material to develop the latent image into a visible image. 
   Subsequent to image development, a substrate (not shown) is moved into contact with toner images at transfer station  30 . The substrate is obtained from a supply and advanced to transfer station  30  by sheet feeding unit  14 . The substrate is then brought into contact with the photoconductive surface of photoreceptor belt  14  in a timed sequence so that the toner powder image developed thereon contacts the advancing substrate at transfer station  30 . Transfer station  30  preferably includes a corona-generating device, such as a corotron, for charging the copy sheet to a proper potential so that the sheet is electrostatically secured or “tacked” to belt  10  and the toner image thereon is attracted to the copy sheet. As previously discussed, it is not uncommon for air gaps or spaces to exist between the copy sheet and the surface of the belt  14  at the transfer station. Thus, the interface between the sheet feeding apparatus and transfer station  30  may include a transfer assist apparatus, such as transfer assist blade  20  for applying uniform contact pressure to the sheet as it is advanced onto belt  14 . 
   After transfer, the substrate continues to advance toward fuser station  50 . The toner image is thereby forced into contact with the substrate  68  between fuser rollers  54  and  58  to permanently affix the toner image to substrate  68 . After fusing, the print substrate  68  is advanced to receiving tray  60  for subsequent removal by an operator. 
   After the substrate is separated from photoconductive surface of photoreceptor belt  14 , the residual toner particles carried by the non-image areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station  70 , using, for example, a cleaning brush or plural brush structure or any number of well known cleaning systems. 
   The various machine functions are regulated by a system controller  100  contained within control panel  24 . The controller  100  is preferably a programmable controller, such as a microprocessor, which controls all of the machine functions hereinbefore described. The controller may be programmed to monitor various operating parameters of the electrostatographic machine such as print substrate type, the number of documents being recirculated, the number of print sheets selected by the operator, time delays, and jam indications, among other various functions including transfer assist actuation. Conventional sheet path sensors or switches may be utilized to keep track of the types and position of documents and print substrates in the machine. The system controller may include a pixel counter (see  FIG. 2 ) for counting the number of pixels to be imaged with toner on each sheet or page of the job, for each color. The pixel count information is stored in a memory of the system controller  100 . A memory may be provided to store data necessary for the controller such as, for example, the various component control protocols. The memory may be a non-volatile memory such as a read only memory (ROM) or a programmable non-volatile memory such as an EEPROM or flash memory. Of course, memory  90  may be incorporated into the controller  100 , or may be externally located. 
   The operation of all of the exemplary systems described hereinabove may be accomplished by conventional user interface control. The user interface  68  is configured to display the available features and programming options, such as media trays, the type of media in each tray, the size of media in the trays, colors of ink or toner, and the like, and may be used to obtain the print job parameters for a print job so the MFD driver is able to generate the print job and send it to the MFD for processing. 
   The foregoing description should be sufficient for purposes of illustrating the general operation of an electrostatographic printing machine incorporating an exemplary embodiment of an apparatus for reducing transfer deletions. As described, an electrostatographic printing machine 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 exemplary embodiment. 
   Referring to  FIG. 2 , there is shown a magnified view of the transfer station  30  of an electrostatographic imaging device. A transfer assist blade (“TAB”)  200  is shown engaged with the back of copy substrate  204 , thereby pressing copy substrate  204  onto an image bearing member, such as photoreceptor belt (“PR”)  14 , as the copy substrate is driven in the direction of arrow P by pinch rollers  25 . As noted above, many varieties of TAB systems are possible, and this embodiment is exemplary only. Corotron  208  charges copy substrate sufficiently to urge toner particles to transfer from PR  14  to copy substrate  204 . Upon exiting the transfer section, corotron  210  provides an opposite charge, thereby aiding the detacking of copy substrate  204  from PR  14 . In a typical embodiment, activation and deactivation of TAB  200  is induced by rotation of cam  214  which acts upon lever  218 . TAB  200  is attached to the other end of lever  218 . A controller  100  cooperates with a leading and trailing edge sensor system comprised of light emitter  224  and sensor array  228 . In particular, the controller  100  determines the timing for activating a stepper motor  230  that controls the rotation of cam  214  in order that TAB  200  may be in contact with the back of copy substrate  204  as near as possible to both the leading and the trailing edges of the substrate. 
   As mentioned above, when the image to be reproduced is to the lead edge (LE) of the imaged area on the photoreceptor, there may be a loss of electrostatic toner tack force between the imaged area of the photoreceptor and the LE region of the substrate. As a result, drag force on the image substrate caused by pressing the TAB onto the substrate may be greater than tack force between the substrate and the photoreceptor, which, in turn, generates a velocity mismatch between the copy sheet and the photoreceptor, manifesting itself as a smeared image on or near the LE region of the substrate. The LE region may be assigned a predetermined distance from the leading edge of the substrate in accordance with typical image transfer protocols. For instance, in one embodiment, the LE region may be from 0 to about 5.0 mm from the leading edge of the substrate based upon the assumption that most copying or printing does not occur in that region of the substrate. 
   LE smearing may be reduced by increasing a tacking force that tacks the toner of the developed toner image to the photoreceptor at the lead edge region of the developed toner image. In one embodiment, the tacking force may be increased by applying a charge to the lead edge of the developed toner image. Referring again to  FIG. 2 , there is shown an apparatus for reducing leading edge (LE) smear in an electrostatographic machine comprising a pretransfer corotron  234  for applying a charge to the lead edge of the developed toner image prior to transfer. The pretransfer corotron  234  includes a generally U-shaped shield  236  partially surrounding an elongated electrode wire  232  that is connected to a power supply (not shown). The pretransfer corotron  234  is disposed traversely to the photoconductive belt  14  in the electrostatic imaging device at a position between the developing station  28  and the transfer station  30  to expose the photoconductive belt  14  to a corona discharge across its width. The pretransfer corotron  234  is configured to generate ions having the same polarity as the toner particles, thereby electrostatically increasing the adhesion of the toner particles to the photoreceptive member  14 . 
   The pretransfer corotron  234  may be controlled by controller  100  which may be configured to control the corona discharge of the corotron in a known manner, such as by controlling the corotron (or coronode) current to the corotron. A memory is provided to store data necessary for the controller  100  to implement the corotron control protocols. In one embodiment, the bias applied to the corotron to produce the LE tacking force comprises a fixed pre-determined value. The predetermined bias or charge may be pre-programmed or set into the controller during manufacturing, input through the user interface or supplied over a network. The predetermined charge is calculated to increase the tacking force that tacks the toner of the developed toner image to the photoreceptor in order to overcome or compensate for the drag force that may be introduced by the application of the transfer assist blade to the substrate. The magnitude of the bias or charge applied to the lead edge is generally greater than that applied to the rest of the imaged area and may be approximately 1600V. Although, the actual current and voltage ranges, polarities, and nominal settings may depend on the specific system components, photoreceptors, imaging materials, copy members, etc. 
   In one embodiment, the timing sequence for activation of the pretransfer corotron  234  to treat the toner in the LE region involves cooperation between controller  100  and a location indicator  238  associated with the photoreceptor belt  14 . In this timing sequence, a synchronizing sensor  240  detects when a location indicator  238  on the belt  14  passes the sensor location and relays a synchronization signal to controller  100 . The location indicator  238  may be a hole in the PR  14 . Since the rate of rotation or travel of PR  14  in the direction P is known, controller  100  is able to determine when the beginning of the LE region of the image bearing surface of the photoreceptor is in an operative position beneath the pretransfer corotron  234 . The controller may then activate the pretransfer corotron for a predetermined duration that corresponds to the time that it takes for the LE region to pass beneath the corotron  234 . Thus, the controller is configured to coordinate the activation and deactivation of the pretransfer corotron  234  in order to apply the predetermined charge to the LE region of the imaged area of the photoreceptor. Although the use of a location indicator  238  on the photoreceptor has been described, any suitable method for detecting the LE region may be implemented. 
   By increasing the tacking force at the lead edge to overcome the drag force of the TAB, LE smearing is less likely to occur because the toner particles at the lead edge adhere more strongly to the photoreceptor. The drag force caused by the TAB engaging the substrate may then be less likely to overcome the increased LE tacking force. Because increasing the tacking force may result in a slight loss of transfer efficiency resulting in a “lighter” or “hazier” image, the tacking force should only be increased at the lead edge where it is necessary to reduce LE smearing. Therefore, the image may only be lightened at the lead edge and not for the remainder of the image. Pretransfer switching need only be enabled, or “switched on”, when LE smearing is detected during imaging operations. Thus, in one embodiment, the pretransfer switching system may be configured to be enabled or disabled in response to user input. For example, pretransfer switching may be included as an item that may be selectively controlled through the user interface. Thus, if an operator notices LE smear occurring during printing operations, pretransfer switching may be enabled manually such as, for example, by activating pretransfer switching through the user interface. For instance, a button may be provided on the interface that allows the selection of, for example, pretransfer toner treatment, LE smear reduction, etc. 
   In another embodiment, pretransfer toner treatment may be an automated function of imaging device  10  by determining the solid area coverage (SAC) in the leading region of the substrate after transfer to determine if LE smearing is occurring. The SAC may be determined in any acceptable manner that is capable of ascribing a quantitative value to the SAC for the leading region. For instance, in one embodiment, a pixel counter  244  of the controller  100  can provide information regarding the pixel count at each scanned line in the LE region. Since each pixel corresponds to an area of the substrate which will receive toner or other image transfer marking material, a count of the pixels in the leading region is representative of the solid area coverage. Various techniques may be employed to evaluate the pixel count, and ultimately the SAC, in the leading region. For instance, every image pixel may be counted and compared to a known value for the total number of pixels available in the leading region to produce an SAC ratio. Alternatively, only pixels in certain areas or in a certain pattern within the LE region may be evaluated to minimize the number of pixels that must be examined. As a further alternative, a random sampling pattern may be employed to provide a snapshot of the solid area coverage for the LE region. 
   Regardless of the smear detection approach, an SAC value is generated that is indicative of the image area transferred to the LE region. Armed with a solid area coverage value, the controller  100  is able to determine whether LE smear is occurring and whether pretransfer toner treatment is required to reduce the smearing. In one embodiment, the system controller may make a determination whether LE smearing is occurring by comparing the SAC value, or pixel count, of the LE region to a threshold coverage area value. This threshold value may be pre-defined to correspond to a certain coverage value that has been determined to indicate smearing. In this embodiment, the controller  100  receives the image pixel information produced in the pixel counter  244 . A memory is provided to store data necessary for the controller such as, for example, lead edge SAC threshold values, corotron control protocols, etc. 
     FIG. 3  shows a flowchart of a method for reducing LE smear in an electrostatographic imaging device is provided. The method comprises detecting if LE smear is indicated during imaging operations (block  300 ). If LE smear is indicated, tacking force for tacking toner of a developed toner image to a photoreceptive member is increased at approximately the lead edge region of the developed toner image on the photoreceptive surface prior to transfer (block  304 ). The tacking force may be increased by applying a charge to the lead edge region of developed toner image prior to transfer by using pretransfer corotron (block  308 ). This increased tack level charge or current may be a fixed pre-determined level selected and pre-programmed into the memory of the controller during manufacturing. Alternatively, the charge may at first be a minimum value that may be incrementally increased based continuous monitoring of the SAC in the LE region, thus assuring that the minimum required charge to reduce LE smearing is utilized. 
   Detecting if LE smear is indicated may comprise determining a solid area coverage value (SAC) of at least a portion of the lead edge region (block  310 ). Once the SAC is determined, the SAC is compared to a threshold value (block  314 ). This threshold value may be pre-defined to correspond to a certain coverage value that has been determined to indicate smearing. If the SAC determined is less than the threshold value, then LE smearing is not indicated and pretransfer toner treatment is not required. If the SAC is found to be approximately greater than or equal to the threshold value then LE smearing is indicated and pretransfer toner treatment is necessary. 
   While various exemplary embodiments have been described and illustrated, it is to be understood that many alternatives, modifications and variations would be apparent to those skilled in the art. Accordingly, Applicants intend to embrace all such alternatives, modifications and variations that follow in the spirit and scope of this disclosure.