Abstract:
An apparatus for removing charged particles from a surface, the surface being capable of movement, including: a preclean corotron having a first polarity; and a first cleaning brush for cleaning charged particles from the surface, having a second polarity different from the first polarity of the preclean corotron; a second cleaning brush for cleaning the charged particles from the surface, having a predefined polarity, the second cleaning brush being located downstream from the first cleaning brush, in the direction of motion of the surface; and a controller for changing the predefined polarity of the second cleaning brush from the first polarity to the second polarity.

Description:
This invention relates to an electrostatographic printer or copier, and more particularly concerns a cleaning apparatus for removing toner from an imaging surface. 
     Electrostatic brush (ESB) cleaners are designed to satisfy a requirement of cleaning a maximum toner mass entering the cleaner in a given number of passes through the cleaner. Generally these requirements are a maximum single pass cleaning requirement and a maximum two pass cleaning requirement. The single pass cleaning requirement is typically a residual toner mass on a photoreceptor belt following transfer under conditions of the highest developed mass (DMA) with the lowest transfer efficiency (TE). In some machines a mark-to-edge, or bleed edge, requirement raises the single pass cleaning requirement to the highest DMA level. The two pass cleaning requirement is typically cleaning of untransferred control patches and/or untransferred images in jam recovery. These input densities are equal to the highest DMA. It has been demonstrated that a two pass cleaning requirement is equivalent to cleaning half of the required toner mass in a single pass. 
     The two pass cleaning requirement, except in the case of mark-to-edge machines, is much more stressful than the single pass cleaning requirement. Therefore, the cleaning brushes are designed to clean the two pass requirement. Half of the toner is cleaned in each pass through the cleaner. In designing the cleaner the speed of the brushes, the number of fibers on the brushes, the interference of the brushes to the photoreceptor belt, the electrical bias on the brushes and the number of brushes are chosen to clean the equivalent single pass toner input. 
     Conventional multiple electrostatic brush cleaners consist of two or more brushes electrically biased to remove toner and other debris from the photoreceptor surface of the photoreceptor belt. Prior to the brushes a preclean charge device adjusts the toner charge of the incoming toner to a natural tribo charging polarity of the toner. This is known as right sign toner. Toner that does not charge to the polarity of the majority of the toner in the preclean charging step is known as wrong sign toner. The first brushes are biased opposite to the polarity of the right sign toner so that this toner can be removed. The last cleaning brush is biased opposite to the first brushes so that the wrong sign toner can be removed. Since there is only a small percentage of the toner that is wrong sign only a single brush is ever needed to clean the wrong sign toner mass. 
     Conventional multiple electrostatic brush cleaners have their single pass toner cleaning capacity limited by the amount of right sign toner that can be cleaned by the first brushes and the amount of wrong sign toner that can be cleaned by the last brush. As more cleaning capacity is required, such as for an increase in machine process speed, additional right sign cleaning brushes or additional cleaning passes must be added. These additions to the cleaning system are undesirable. Additional cleaning brushes increase the size and cost of the cleaner and may not fit in the available machine space. Additional cleaning passes decrease the productivity of the machine by requiring a longer recovery from paper jams. Additional cleaning passes impact the xerographic control of the machine by requiring a longer time to clean process control patches. 
     The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows: 
     U.S. Pat. No. 5,729,815 to Lindblad et al. discloses an apparatus and method for cleaning charged triboelectric negative toner residual particles from a photoreceptor surface. A positive bias is applied to two electrostatic brushes in a dual cleaning system or to a single electrostatic brush. The rotational speed of the single electrostatic brush is increased, over that of the individual brushes in a dual brush cleaner, to clean charged triboelectric negative toner particles. 
     U.S. Pat. No. 5,257,079 to Lange et al. discloses a cleaning brush electrically biased with an alternating current to remove discharged particles from an imaging surface. The particles on the imaging surface are discharged by a corona generating device. A second cleaning device including an insulative brush, a conductive brush or a blade, located upstream of the first mentioned brush, in the direction of movement of the imaging surface, further removes redeposited particles therefrom. 
     U.S. Pat. No. 4,545,669 to Hays et al. discloses an apparatus for simultaneously charging, exposing, and developing imaging members at low voltages which comprises a semi-transparent deflected flexible imaging member, an electronic imaging source means, a light beam deflector member, a means containing magnets therein, a development roll means containing magnets therein, a voltage source means for sensitizing roll means, a voltage source for the development roll means, a developer supply reservoir containing conductive developer particles therein comprised of insulating toner resin particles and conductive carrier particles, a sensitizing nip situated between the flexible imaging member and the sensitizing roll means, a development nip situated between the flexible imaging member and the development roll means, the sensitizing roll means and development roll means moving in the same direction of movement as the semitransparent deflected flexible imaging member, the voltage being generated by the voltage source means with the sensitizing nip being of an opposite polarity of the voltage generated by the voltage source means for the development roll means, wherein an electric field of a predetermined polarity is established between the semi-transparent deflected flexible imaging member and the sensitizing roll means, which field exerts in the sensitizing nip an electrostatic force on the charged toner particles causing these particles to uniformly migrate toward the flexible imaging member, subsequently subjecting the deflected flexible imaging member to the electronic imaging source means whereby the electrostatic force exerted on the toner particles adjacent the light struck areas of the flexible imaging member are increased thereby causing toner particles to be deposited on the deflected flexible imaging member, and wherein toner particles are removed from the deflected flexible imaging member in areas not exposed to light by the development roll means and developed in the areas exposed to light. 
     Briefly stated, and in accordance with one aspect of the present invention, there is provided a multiple electrostatic brush cleaner the first brushes are biased to clean right sign toner and the last brush is biased to clean wrong sign toner. The highest cleaning stress occurs when untransferred toner is cleaned following a machine jam or control patches are cleaned. Generally two or more passes are allowed to clean these very high density inputs to the cleaner. The present invention biases all brushes for the first cleaning pass to clean right sign toner. On the second cleaning pass the brushes are biased normally. This new bias switching arrangement results in more efficient use of the cleaning brushes and allows existing cleaners to be used at higher process speeds than with conventional multi-pass cleaner biasing. Bias switching is especially advantaged with air detoning of the electrostatic brush cleaner since little toner accumulates in the brushes. Since there are opportunities for any toner redeveloped to a photoreceptor during the bias switching to be cleaned in the following passes, switching may also be useful in electrostatically detoned cleaners. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features of the present invention will become apparent as the following description proceeds and upon reference to the drawings, in which: 
     FIG. 1 is a schematic illustration of a first mode of operation of the present invention using a positively biased first brush and a negatively biased second brush; 
     FIG. 2 is a schematic illustration of a second mode of operation of the present invention using all positively biased brushes; and 
     FIG. 3 is a schematic illustration of a printing apparatus incorporating the inventive features of the present invention. 
    
    
     While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     For a general understanding of a color electrostatographic printing or copying machine in which the present invention may be incorporated, reference is made to U.S. Pat. Nos. 4,599,285 and 4,679,929, whose contents are herein incorporated by reference, which describe the image on image process having multi-pass development with single pass transfer. Although the cleaning method and apparatus of the present invention is particularly well adapted-for use in a color electrostatographic printing or copying machine, it should become evident from the following discussion, that it is equally well suited for use in a wide variety of devices and is not necessarily limited to the particular embodiments shown herein. 
     Referring now to the drawings, where the showings are for the purpose of describing a preferred embodiment of the invention and not for limiting the same, the various processing stations employed in the reproduction machine illustrated in FIG. 3 will be briefly described. 
     A reproduction machine, from which the present invention finds advantageous use, utilizes a charge retentive member in the form of a photoconductive belt  10  consisting of a photoconductive surface  11  and an electrically conductive, light transmissive substrate mounted for movement pass charging station A, and exposure station B, developer station C, transfer station D, fusing station E and cleaning station F. Belt  10  moves in the direction of arrow  16  to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. Belt  10  is entrained about a plurality of rollers  18 ,  20  and  22 , the former of which can be used to provide suitable tensioning of the belt  10 . A motor  23  rotates roller  20  to advance belt  10  in the direction of arrow  16 . Roller  20  is coupled to motor  23  by suitable means such as a belt drive. 
     As can be seen by further reference to FIG. 3, initially successive portions of belt  10  pass through charging station A. At charging station A, a corona device such as a scorotron, corotron or dicorotron indicated generally by the reference numeral  24 , charges the belt  10  to a selectively high uniform positive or negative potential. Any suitable control, well known in the art, may be employed for controlling the corona device  24 . 
     Next, the charged portions of the photoconductive surface  11  are advanced through exposure station B. At exposure station B, the uniformly charged, photoconductive belt, photoreceptor or charge retentive surface  10  is exposed to a laser based input and/or output scanning device  25  which causes the charge retentive surface  10  to be discharged in accordance with the output from the scanning device  30  (for example a two level Raster Output Scanner (ROS)). 
     The photoreceptor  10 , which is initially charged to a voltage, undergoes dark decay to a voltage level. When exposed at the exposure station B it is discharged to near zero or ground potential for the image area in all colors. 
     The system shown is a multiple pass—single transfer system. The cleaner is retracted for the development of each color,  4  passes in the shown configuration. After the final color is developed, the four layer image is transferred to paper and the cleaner cammed in to remove any transfer residual toner. The description of the process implies that all colors are developed following a single charge and exposure step. 
     At development station C, a development system, indicated generally by the reference numeral  30 , advances development materials into contact with the electrostatic latent images. The development system  30  comprises first  42 , second  34 , third  31  and fourth  32  developer apparatuses. (However, this number may increase or decrease depending upon the number of colors, i.e. here four colors are referred to, thus, there are four developer housings.) The first developer apparatus  42  comprises a housing containing a donor roll  47 , a magnetic roller  48 , and developer material  46 . The second developer apparatus  34  comprises a housing containing a donor roll  43 , a magnetic roller  44 , and developer material  45 . The third developer apparatus  31  comprises a housing containing a donor roll  37 , a magnetic roller  38 , and developer material  39 . The fourth developer apparatus  32  comprises a housing containing a donor roll  35 , a magnetic roller  36 , and developer material  33 . The magnetic rollers  36 ,  38 ,  44 , and  48  develop toner onto donor rolls  35 ,  37 ,  43  and  47 , respectively. The donor rolls  35 ,  37 ,  43 , and  47  then develop the toner onto the photoconductive or imaging surface  11 . It is noted that development housings  32 ,  34 ,  39 ,  42 , and any subsequent development housings must be scavengeless so as not to disturb the image formed by the previous development apparatus. All four housings contain developer material  33 ,  39 ,  45 ,  46  of selected colors. Electrical biasing is accomplished via bias sower supply  41 , electrically connected to developer apparatuses  32 ,  34 , 39  and  42 . 
     Sheets of substrate or support material  58  are advanced to transfer station D from a supply tray, not shown. Sheets are fed from the tray by a sheet feeder, also not shown, and advanced to transfer station D through a corona charging device  60 . After transfer, the sheet continues to move in the direction of arrow  62 , to fusing station E. 
     Fusing station E includes a fuser assembly, indicated generally by the reference numeral  64 , which permanently affixes the transferred toner powder images to the sheets. Preferably, fuser assembly  64  includes a heated fuser roller  66  adapted to be pressure engaged with a back-up roller  68  with the toner powder images contacting fuser roller  66 . In this manner, the toner powder image is permanently affixed to the sheet. 
     After fusing, copy sheets are directed to a catch tray, not shown, or a finishing station for binding, stapling, collating, and removal from the machine by the operator. Alternatively, the sheet may be advanced to a duplex tray (not shown) from which it will be returned to the processor for receiving a second side copy. A leading edge to trailing edge reversal and an odd number of sheet inversions is generally required for presentation of the second side for copying. However, if overlay information in the form of additional or second color information is desirable on the first side of the sheet, no leading edge to trailing edge reversal is required. Of course, the return of the sheets for duplex or overlay copying may also be accomplished manually. Residual toner and debris remaining on photoreceptor  10  after each copy is made, may be removed at cleaning station F with a brush or other type of cleaning system  70 , after the particles are charged a preclean corotron  96 . The cleaning system  70  is supported under the photo receptor  10  by two backers  160  and  170 . 
     Reference is now made to FIG. 1, which shows the conventional brush bias polarity for a DESB (i.e., dual electrostatic brush) cleaner to remove residual triboelectric negative toner particles from an imaging surface. A negative preclean corotron  96  provides negative charge to the residual triboelectric negative toner particles  95  remaining on the photoreceptor  10  (e.g. imaging surface) after transfer. A residual toner particle patch or toner patch G carries predominantly a high negative charge after preclean (although a small amount of low positive charge is present). The triboelectric negative toner particles  95  accept negative charge from the negative preclean corotron  96 . This is an inherent toner characteristic that allows the triboelectric negative toner particles  95  to have a high negative charge value in the toner patch G. Thus, first cleaner brush  100 , rotating against the direction of motion (shown by arrow  16 ) of the photoreceptor  10 , is positively biased to attract the predominantly negatively charged toner particles G from the photoreceptor  10 . With typical post transfer residual toner input the positively biased first cleaner brush  100  removes almost all of the negatively charged toner in toner patch G that is later detoned from the first cleaner brush  100 . However, a small portion of the toner patch G is often not cleaned by the first cleaner brush  100 , (i.e. a small portion passes under the first cleaner brush  100  and a small amount may be redeposited from the first cleaner brush  100  onto the photoreceptor  10 ) and remains on the photoreceptor  10 , after the first cleaner brush  100 , as a toner patch H. For typical post transfer residual toner input the toner patch H of triboelectric toner  95  is predominantly positively charged after contact with the positively biased first cleaner brush  100  and of very low density. For high density inputs, such as experienced during jam recovery, control patch cleaning and other cases, toner patch G may not be substantially cleaned by the first cleaner brush  100 . In this case toner patch H will consist largely of negatively charged toner particles at a relatively high density. 
     With continuing reference to FIG. 1, the second cleaner brush  105 , rotating against the direction of motion (shown by arrow  16 ) of the photoreceptor  10 , is negatively biased. For the typical post transfer residual toner input case toner patch H is cleaned by the second cleaner brush  105 , due to the positive charge on the toner, the low toner density of the patch and the negative bias of the second cleaner brush  105 . In this case a residual toner patch I following the second cleaner brush  105  is typically less than 30 toner particles per mm 2 . For the high input density case (e.g., jam recovery) toner patch H is poorly cleaned by the second cleaner brush  105 , due to the negative charge on the toner, the relatively high toner density of the patch and the negative bias of the second cleaner brush  105 . For this case the residual toner patch I following the second cleaner brush  105  is nearly as high a density as the toner patch H entering the second cleaner brush  105 . A second cleaning pass through the conventionally biased DESB shown in FIG. 1 will remove substantially the same quantity of toner from the photoreceptor  10  as was removed during the first cleaning pass. More than two cleaning passes may be required to remove very high toner input densities. More than two cleaning passes is generally undesirable due to a decrease in machine productivity for paper jam recovery and a longer time between process control patch readings. 
     Reference is now made to FIG. 2, which shows the preferred embodiment for the first of two cleaning passes of the present invention using dual electrostatic cleaner brushes for cleaning high toner density inputs to a cleaner. These high input densities are greater than the toner density that can be cleaned in two passes through a conventionally biased cleaner. The residual toner patch K of high density charged triboelectric negative toner particles  95  is negatively charged by a negative preclean corotron  96 . The first cleaner brush  100 , rotating against the direction of motion, shown by arrow  16 , of the photoreceptor  10 , is positively biased to remove negatively charged toner particles  95  in residual patch K from the photoreceptor  10 . Toner cleaned from toner patch K is detoned from the first cleaner brush  100  by a detoning roll  101 . (Other means of detoning not shown include air detoning and flicker bars.) The toner particles not removed by the first positively biased cleaner brush  100 , on the photoreceptor  10 , are shown by toner patch L. The second cleaner brush  105  rotating against the direction of motion of the photoreceptor  10 , shown by arrow  16 , is also positively biased. The second positively biased cleaner brush  105  removes toner particles from toner patch L from the photoreceptor  10 . The toner cleaned from toner patch L is then removed from the second cleaner brush  105  by a detoning roll  107 . The toner particles not removed by the second positively biased cleaner brush  105 , on the photoreceptor  10 , are shown by toner patch M. 
     In the second pass of the two pass cleaning process the brush biases revert to the conventional polarities by controller  200  as shown in FIG. 1. A negative preclean corotron  96  provides negative charge to the triboelectric negative toner particles  95  in toner patch M that were not cleaned from the photoreceptor  10  (e.g. imaging surface) in the first cleaning pass. The toner patch G carries predominantly a high negative charge after preclean (although a small amount of low positive charge is present). The first cleaner brush  100 , rotating against the direction of motion (shown by arrow  16 ) of the photoreceptor  10 , is positively biased to attract the predominantly negatively charged toner particles from the photoreceptor  10 . Because the toner input density to the positively biased first cleaner brush  100  has been reduced by a prior cleaning pass under both brushes biased positively almost all of the negatively charged toner in toner patch G is cleaned and later detoned from the first cleaner brush  100 . However, a small portion of wrong sign toner in the toner patch G is often not cleaned by the first cleaner brush  100 , (i.e. a small portion passes under the first cleaner brush  100  and a small amount may be redeposited from the first cleaner brush  100  onto the photoreceptor  10 ) and remains on the photoreceptor  10 , after the first brush  100 , as a toner patch H. Because of three cleaning passes under positively biased brushes the toner density of toner patch H is very low. 
     With continuing reference to FIG. 1, the second cleaner brush  105 , rotating against the direction of motion (shown by arrow  16 ) of the photoreceptor  10 , is negatively biased. Toner patch H is cleaned by the second cleaner brush  105 , due to the positive charge on the toner, the low toner density of the patch and the negative bias of the second cleaner brush  105 . The residual toner patch I following the second cleaner brush  105  is typically less than 30 toner particles per mm 2 . 
     It is, therefore, apparent that there has been provided in accordance with the present invention, bias switching of dual electrostatic brushes with a negative preclean corotron for negatively charged triboelectric toner that fully satisfies the aims and advantages hereinbefore set forth. While this, invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.