Patent Publication Number: US-8121523-B2

Title: Developer station with tapered auger system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to commonly assigned, copending U.S. application Ser. No. 12/415,380, filed Mar. 31, 2009, entitled: “DEVELOPER STATION FOR AN ELECTROGRAPHIC PRINTER HAVING REDUCED DEVELOPER AGITATION”, U.S. application Ser. No. 12/1415,439, filed Mar. 31, 2009, entitled: “DEVELOPER STATION WITH AUGER SYSTEM” and U.S. application Ser. No. 12/415,476, filed Mar. 31, 2009, entitled: “DEVELOPER STATION FOR AN ELECTROGRAPHIC PRINTER WITH MAGNETICALLY ENABLED DEVELOPER REMOVAL.” 
     FIELD OF THE INVENTION 
     This invention generally relates to electrographic printers, and is particularly concerned with a developer station and methods that improve the mixing and feed of a magnetic developer from a sump to a rotating magnetic brush. 
     BACKGROUND OF THE INVENTION 
     Electrographic printers that use a rotating magnetic brush to apply a dry, particulate developer to a photoconductor member are known in the art. In such electrographic printers, the rotating magnetic brush includes a rotatable magnetic core surrounded by a rotatable, cylindrical toning shell that is eccentrically mounted with respect to the axis of rotation of the magnetic core. The magnetic brush is mounted adjacent to a developer sump that holds a reservoir of dry, two-component developer including a mixture of ferromagnetic carrier particles and toner particles capable of holding an electrostatic charge. The eccentric mounting of the toning shell defines an area of relatively strong magnetic flux where the shell comes closest to the magnetic core, and an area of relatively weak magnetic flux 180° opposite to the area of strongest magnetic flux where the shell is farthest away from the core. The area of strongest magnetic flux also contains a line of closest approach between the toning shell of the magnetic brush and the photoconductor member. This line of closest approach defines a “nip” between these two components where the particulate toner component of the developer is transferred to the photoconductor member as a result of electrostatic attraction between the toner particles and the electrostatic field from the photoconductor member. The combination of the magnetic brush and the developer sump is referred to as the developer station in this application. 
     In operation, the photoconductive member is moved past a pre-cleaner and a cleaning station to remove any residual toner that might be on the surface of the member after the previous image transfer. A corona charger then imparts a uniform static charge on to the surface of the member. The photoconductive member is next moved past an image writing station (which may include an LED bar) that writes a latent, electrostatic image on the member by exposing it to a pattern of light. Next, the exposed photoconductor member is moved past the developer station, where the magnetic brush develops the latent electrostatic image on the member by continuously applying a uniform layer of developer at the nip between the toning shell and the photoconductor member. At the nip, toner particles in the developer are transferred to the photoconductor member in a pattern commensurate with the electrostatic image on the member. The developed image on the photoconductor member is then transferred to, for example, an intermediate transfer web for subsequent transfer to a final receiver. The developer that remains on the toning shell downstream of the nip is removed by a skive and deposited back in the developer sump. The used, toner-depleted developer is replenished as needed with additional toner particles in the sump. Replenished developer is continuously applied downstream of the skive far from the toning nip at or near the area of weakest magnetic flux on the toning shell of the magnetic brush, where it is moved back toward the area of strongest magnetic flux and the nip. 
     In color printing, a series of electrographic printers arranged in tandem are used to create image separations in different primary colors (i.e. cyan, magenta, yellow, and black) which are superimposed over one another to create a final color image. To this end, each printer prints its particular primary color image on an intermediate transfer web which resembles a conveyor belt. The conveyor-belt movement of the intermediate transfer belt is synchronized with the printing by the photoconductor members of the in tandem printers such that the images are superimposed in alignment with one another, creating a final color image. 
     It is highly desirable for the intermediate transfer web to be horizontally oriented so the height of the resulting color printing assembly is less than a standard room ceiling height. As a consequence, the intermediate transfer web should engage the photoconductor element of each printer at either the 6 o&#39;clock position in a “process-over-image” configuration, or in a 12 o&#39;clock position in an “image-over-process” configuration. As a further consequence, the nip between the toning shell and the photoconductor member should be located at one or the other of the sides of the photoconductor member, preferably near the 9 o&#39;clock or 3 o&#39;clock position. 
     It is further desirable to use a photoconductor that is as small in diameter as possible to reduce cost and overall printer size. The pre-clean, clean, charge, expose, develop, and transfer stations must all be positioned adjacent to the photoconductor member. If a small photoconductor member is used, all of these systems must also be as small as possible so as not to interfere with each other or the intermediate transfer web, yet still produce adequate images. Hence there are limitations on, in particular, the height of developer station positioned at the 9 o&#39;clock or 3 o&#39;clock position relative to the photoconductor member. 
     It is also desirable to print images as quickly as possible, requiring faster printer speeds. The combination of small size and high process speed is technologically demanding. From a fundamental point of view, large fluxes of charge, light, or particles are needed due to the high rates required for the short time allowed for each process step. This means in general that, as speed is increased and size is decreased, larger concentrations, intensities, and driving forces are used. 
     Faster printing can be accomplished by increasing the rotational speed of the magnetic brush. However, the inventors have observed that increasing the rotational speed of the magnetic core can produce undesirable effects, such as embedment of toner and heating of carrier particles that ages the developer. Also, increasing the rotational speed of the magnetic core can cause toner particles to fracture and produce small particles, or fines. To fully appreciate the first-mentioned problem, some explanation of the constitution of the toner particles is in order. 
     A widely practiced method of improving the transfer of the toner particles is by use of so-called surface treatments. Such surface treated toner particles have adhered to their surfaces sub-micron particles, e.g., of silica, alumina, titania, and the like (so-called surface additives or surface additive particles). Surface treated toners generally have weaker adhesion to a smooth surface than untreated toners, and therefore surface treated toners can be electrostatically transferred more efficiently from a photoconductor member to another member. Such surface treated toners also advantageously maintain the same amount of electrostatic attractive force with respect to the photoconductor member despite variations in the ambient humidity. 
     In particular, the inventors have observed that, when the rotational speed of the rotating magnetic core is increased beyond a certain limit, the carrier particles become excessively heated as a result of hysteresis of the magnetization of the carrier particles caused by the rapidly changing magnetic field of the rotating core. The resulting heat is transferred from the carrier particles to the toner particles, which in turn softens them. The rapidly changing magnetic field of the rotating core also creates excessive mechanical agitation in the toner. The resulting heating, softening, and mechanical impact between the carrier particles and the toner particles causes the sub-micron surface-treatment particles of silica, alumina, titania, and the like to embed into the toner particles, thereby diminishing the ability of the toner particles to hold the static charges necessary for reliable and consistent transfer to the photoconductor member. 
     It is also desirable to improve the efficiency of the process of skiving the developer that remains on the toning shell downstream of the nip and depositing that developer back in the developer sump. It is also desirable to reduce the expense of the precision required for a straight, thin skive spaced close to the toning shell with a small spacing tolerance and to generally improve the removal of developer from the magnetic brush without interfering with other aspects of the development system. 
     In addition, a developer station of relatively small size capable of high printing speed in a printer of compact size necessarily has a small, narrow developer sump. For a small sump, a significant fraction of the developer in the sump can be removed from the sump and applied to the toning shell of the magnetic brush. This can result in poor feed of the developer to portions of the toning shell if the volume of developer in the sump is not constant adjacent the toning shell. It is desirable to improve the mixing of developer and the feed of a magnetic developer from the sump to the rotating magnetic brush. 
     SUMMARY OF THE INVENTION 
     The invention is a developer station and method for an electrographic printer that improves the mixing and feed of developer from the sump to the toning shell during the printing process. The developer station includes a sump for holding a reservoir of magnetic developer including toner and carrier and a magnetic roller mounted above said sump and including a rotatable magnetic core surrounded by a substantially cylindrical rotatable toning shell rotatably mounted with respect to the core, said shell being adjacent to the photoconductor drum and defining a nip and a conveyance device for transporting developer in the developer station in a flow direction. The conveyance device has a tapered auger including a shaft and one or more blades such that the developer volume in the flow direction is controlled to maintain a uniform developer level in the sump as well as a conveyance controller for controlling the conveying device, including the tapered auger such that the tapered auger preferentially creates an uniform layer of developer on the toning shell. 
     When a relatively high speed printing operation is carried out such that magnetic carrier particles on the toning shell are subjected to at least about 190 pole flips per second as a result of relative rotation between the magnetic core and the toning shell, developer is preferably delivered to the toning shell at an angular distance no more than about 120° from the tangent line between the toning shell and the photoconductor member to reduce the residence time that the developer stays on the developer shell prior to transfer of toner particles from the toning shell to the photoconductor element. 
     In a method of the invention, for relatively high speed printing operations in which a large proportion of developer from the sump is fed at a high rate from the sump to the toning shell and returned from the toning shell to the sump, auger assemblies and sump features of specific construction are used to provide a uniform level of developer in the sump adjacent the conveyor roller or toning shell and to provide uniform feed of developer to the toning shell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1A  is a schematic side view of a typical electrographic printing assembly in process-over-image configuration suitable for use with the developer station of the invention; 
         FIG. 1B  is a schematic side view of a typical electrographic printing assembly in image-over-process configuration suitable for use with the developer station of the invention; 
         FIG. 2  is a side view, in partial cross section, of one of the printing modules used in the printing assembly of  FIG. 1A , on an enlarged scale; 
         FIG. 3A  is a cross sectional side view of a first embodiment of the developer station of the invention which may be used in the printing module illustrated in  FIG. 2  and which employs two conveyor rollers; 
         FIG. 3B  is a schematic view of the magnetic brush, photoconductor drum and second conveyor roller of the developer station of  FIG. 3A , illustrating the angular relationship between the delivery point of the developer on the toning shell of the magnetic brush and the nip between the toning shell and the photoconductor drum; 
         FIG. 3C  is a schematic view of the magnetic brush, photoconductor drum and second conveyor roller of the developer station of  FIG. 3A , illustrating the angular relationship between the nip between the toning shell and the photoconductor drum and the closest line between the toning shell and the rotating magnetic core of the magnetic brush; 
         FIG. 4  is a cross sectional side view of second embodiment of the developer station of the invention which does not employ any conveyor rollers, 
         FIG. 5  is a cross sectional side view of third embodiment of the developer station of the invention which employs a single conveyor roller; and 
         FIG. 6  is a graph of the force produced on a carrier particle by the magnetic field of the development roller for several different magnetic cores and toning shells. 
         FIG. 7A  is a schematic top view of auger configurations; and 
         FIG. 7B  is a schematic top view of tapered auger configurations 
         FIG. 7C  is a schematic top view of intermeshed auger configurations 
         FIG. 7D  is a schematic top view of intermeshed tapered and notched auger configurations 
         FIG. 8  is a side elevational view, in cross-section, of a reproduction apparatus magnetic brush developer station according to this invention. 
         FIG. 9  shows a tapered auger of the magnetic brush development station of  FIG. 8 . 
         FIG. 10  shows a tapered auger of the magnetic brush development station of FIG.  8 ., particularly showing other embodiments according to this invention. 
         FIG. 11  shows a tapered auger of the magnetic brush development station of  FIG. 8 , particularly showing other embodiments according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1A , an electrographic printing apparatus  1  has a number of tandemly arranged electrostatographic image-forming printers in the form of printer modules  3   a ,  3   b ,  3   c ,  3   d , and  3   e . Each of the printer modules  3   a - 3   e  is disposed over a horizontally-oriented intermediate transfer web  5  in process-over-image configuration, although the invention is equally applicable to a printing apparatus  1  wherein the intermediate transfer web is disposed above the printer modules  3   a - 3   e  in image-over-process configuration, as shown in  FIG. 1B . Each printer module  3   a - 3   e  includes a photoconductor element which may take the form of a photoconductor drum  7 . In  FIG. 1A , the top portion of the intermediate transfer web  5  is moved from left to right by rollers  8   a ,  8   b  in conveyor-belt fashion immediately beneath the photoconductor drum  7  of each printer modules  3   a - 3   e  while the photoconductor drums rotate counterclockwise at the same speed. Each of the printer modules  3   a - 3   d  includes a developer station  10  that develops a single-color toner image such as black (K), cyan (C), magenta (M), or yellow (Y) onto the photoconductor drum  7 . Printer module  2   e  may include an additional color toner or a clear toner for transfer of clear images to the intermediate transfer web  5 . A backer bar  11  having an electrostatic voltage transfers the toner image off of the drum  7  of each of the printer modules  3   a - 3   e  and onto the web  5 . In operation, the intermediate transfer web  5  is first passed through module  3   a , where it receives a first toner image. Subsequent toner images are superimposed in registry with this first toner image as it passes through printer modules  3   b - 3   e  in order to form a single pentachrome image, and one clear toner image. The single pentachrome image is ultimately transferred to a receiver such as a sheet of paper  100  and then fused into a permanent color image in a fuser assembly  120  in a manner well-known in the art. 
     In  FIG. 1B , for the image-over-process configuration, the bottom portion of the intermediate transfer web  5  is moved from right to left by rollers  8   a ,  8   b  in conveyor-belt fashion immediately above the photoconductor drum  7  of each printer modules  3   a - 3   e  while the photoconductor drums  7  rotate counterclockwise at the same speed. For the image-over-process configuration shown in  FIG. 1B , each of the printer modules  3   a - 3   d  perform the same functions as for the process-over-image configuration shown in  FIG. 1A . 
     With reference now to  FIG. 2 , each of the printer modules  3   a - 3   e  includes a pre-cleaner unit  12  having a corona charger and a lamp for recharging, exposing and discharging residual electrostatic charge on the photoconductor drum  7  that remains after the transfer of the toner image  13  onto the web  5 . Such electrostatic neutralizing of the drum  7  facilitates the removal of residual toner particles by the toner cleaner  14  located downstream of the pre-cleaner unit. A corona charger  16  is located downstream of the toner cleaner  14 . Charger  16  imparts a negative charge of between about  600  and  700  volts to the surface of the photoconductor drum  7 . An optical image writer  18  is located downstream of the corona charger  16 . Writer  18  includes a digitally-controlled LED bar that exposes the surface of the photoconductor drum  7  to a modulated light signal, which in turn selectively discharges portions of the charged surface of the photoconductor drum such that a latent electrostatic image is written across the surface of the photoconductor drum  7 . 
     With reference now to  FIGS. 2 and 3A , the developer station  10  includes a housing  20 , a magnetic brush  22 , and a developer sump  23 . The magnetic brush  22  is also known as the combination of the developer and the toning roller. The toning roller includes a magnetic core  24  having a plurality of magnets  25  arranged around its outer periphery and a toning shell. The core  24  is rotatably mounted with respect to the housing  20 . While not expressly shown, the north-south magnetic axes of the magnets are radially-oriented with respect to the cylindrically-shaped core  24 , and the magnets  25  are arranged with alternating north and south magnetic poles around the outer periphery of the core  24  such that “U” shaped lines of magnetic flux interconnect adjacent magnets. The magnetic core  24  is surrounded by a rotatably mounted, cylindrically shaped toning shell  26 . Toning shell  26  may be eccentrically mounted with respect to the magnetic core as shown. The axes of rotation of the toning shell  26  and the photoconductor drum  7  are parallel as indicated, and a first nip  27  is defined at the line of closest approach between the cylindrically-shaped toning shell  26  and the photoconductor drum  7 . The axes of rotation of the magnetic core  24  and the toning shell  26  are also parallel, and the line of closest approach between these two components defines the location  28  where the magnetic field on the toning shell  26  is generally greatest in magnitude. 
     With reference to  FIG. 3A , the sump  23  contains a reservoir  29  of two-component developer  30  formed from a dry mixture of magnetic carrier particles and toner particles. Preferably, the carrier particles are hard magnetic ferrite particles having high coercivity. The carrier particles may have a volume-weighted diameter of approximately 26 mu.m. The dry toner particles are preferably substantially smaller, (on the order of 6.mu.m to 15.mu.m in volume-weighted diameter). The toner particles are removed from the carrier particles during the development operation that occurs at the nip  27  between the toning shell  26  and the photoconductor drum  7 . The toner particles are polymeric or resin-based, and are electrostatically chargeable. The toner particles are created by blending various components, which can include binders, resins, pigments, fillers, and additives, for example, and processing the components by heating and milling, for example, whereupon a homogeneous mass is dispensed by an extruder. The mass is then cooled, crushed into small chips or lumps, and then ground into a powder. As previously mentioned, a widely practiced method of improving the transfer of the toner particles is by use of so-called surface treatments. Such surface treated toner particles have adhered to their surfaces sub-micron particles, e.g., of silica, alumina, titania, and the like which in turn improves the electrostatic properties of the toner particles. 
     The sump  23  of the developer station  10  functions to continuously provide a supply of developer  30  to the toning shell  26  of the magnetic brush  22  having a correct proportion of toner particles relative to magnetic carrier particles. As is well known in the art, when developer  30  is used to develop a latent electrostatic image on the photoconductor drum  7 , the toner particles in the developer are electrostatically transported from the toning shell  26  to the drum  7 , while the magnetic carrier particles remain on the toning shell  26 . These remaining magnetic carrier particles and unused developer are removed from the toning shell by a skive  31  and are re-deposited back into the right-hand side of the reservoir  29  of developer  30 . The area of the magnetic brush where the developer is removed and returned to the sump is referred to as the strip zone. The skive  31  is located in the strip zone of magnetic brush  22 . The strip zone is above the sump. In order to maintain a correct proportion of carrier and toner particles in the developer conveyed to the toning shell  26 , a toner replenisher tube  32  conveys toner particles to the right-hand side of the developer reservoir  29  as needed. Sump  23  further includes a pair of return augers  33   a ,  33   b  having left-handed screw blades  34   a ,  34   b  in the augers and/or auger assemblies for simultaneously conveying the developer particles stripped away from the developing shell  26  by the skive  31  and the toner particles added by the replenisher tube  32  (along with other developer in the sump  23 ) to a front mixing chamber (not shown)  35  where flippers on the return augers  33   a ,  33   b  mix the carrier particles and toner particles to form a replenished developer which is conveyed from the front mixing chamber to feed augers  38   a ,  38   b . The feed augers  38   a ,  38   b  have left-handed screw blades  40   a ,  40   b  which convey the replenished toner down the length of the sump  23 . Flippers at the rear ends of feed augers  38   a  and  38   b  (not shown) convey the developer to return augers  33   a  and  33   b . Augers are also referred to as auger assemblies and an auger assembly can have one or more augers. In this example of the invention, the return augers  33   a ,  33   b  turn counterclockwise while the feed augers  38   a ,  38   b  turn clockwise, thereby causing the developer to circulate around the sump  23  in a clockwise direction when viewed from above. 
     With reference again to  FIG. 3A , the developer station  10  further includes first and second conveyor rollers  50 ,  63  for conveying developer to the toning shell  26 . Conveyor rollers are also referred to as transport rollers. The first conveyor roller  50  includes a stationary magnetic core  53  surrounded by a rotatable cylindrical shell  55 . The rotatable shell  55  of the roller  50  is located above the feed augers  38   a ,  38   b  adjacent with the developer  30  in the reservoir  29  so that it can pick up developer material. The magnetic core  53  preferably a plurality of magnets  59  for conveying the developer over the 12 o&#39;clock position of the roller  50 . As was the case with the magnets  25  in the core  24  of the magnetic brush  22 , all of the magnets  59  of the first conveyor roller are arranged to present alternating north and south magnetic poles around the circumference of the rotating shell  55  that are magnetically linked by U-shaped flux lines. The cylindrical shell  55  rotates clockwise and carries developer to the second conveyor roller  63 . 
     The second conveyor roller  63  likewise includes a fixed magnetic core  64  having a plurality of magnets  65  that is surrounded by a rotatable cylindrical conveyor shell  66 . Like the shell  55 , the shell  66  also rotates clockwise. The magnets  65  of the second conveyor roller produce a magnetic field at the nip  67  between rollers  50  and  63  such that developer is transferred from roller  50  to roller  63  at the nip  67  between the rollers. The clockwise rotation of both of the rollers  50 ,  63  causes the developer to make a U-shaped turn at the nip  67  as it is transferred to the second roller  63 . As a result of its continued clockwise rotation after receiving developer from the first conveyor roller  50 , the second conveyor roller  63  delivers developer to the toning shell  26  at the nip  70 . The area on the magnetic brush where developer is applied to the brush from the sump is referred to as the feed zone. Here, the developer makes another U-shaped turn and travels over the upper portion of the toning shell  26  through a metering skive  72  and into the nip  27  between the shell  26  and the photoconductor drum  7 . 
       FIG. 3B  illustrates the preferred orientation of a tangent line T that is tangent to the nip  27  between the toning shell  26  and the photoconductor drum  7  with respect to a vertical axis V. Preferably, the line T is oriented at an angle A between about +45 20  and −45° with respect to vertical axis V. In  FIG. 3B , this angle is about +20° and the toning shell  26  is illustrated as contacting the photoconductor drum  7  at about the 10 o&#39;clock position. Angle A would be −20° if the toning shell were illustrated as contacting the photoconductor drum at the 8 o&#39;clock position. More preferably, the angle that the tangent line T makes with the vertical axis V is preferably between about +10° and −10°. Most preferably, the tangent line T is substantially aligned with the vertical axis V, as is shown in  FIG. 3C . Such a tangent line orientation allows the developer station  10  to be positioned at one of the sides of the photoconductor drum. 
       FIG. 3B  also illustrates the preferred angular distance θ between the developer delivery point on the toning shell (which in this embodiment corresponds to the nip  70  and is also referred to as the feed zone) and the nip  27  between the toning shell  26  and the photoconductor drum  7  which is preferably less than 120°. Even more preferably, this angular distance θ is 100°. Most preferably, this angular distance θ is 90°. Such an arrangement shortens the residence time of the developer on the toning shell  26 , which advantageously reduces both the amount of hysteresis-generated heating of the magnetic carrier particles (which in turn heats the fusible toner particles), as well as the mechanical agitation of the mixture of carrier and toner particles. The lower amount of heating and agitation advantageously avoids embedment of the surface treatments applied to the toner particles, which in turn allows them to maintain their enhanced ability for efficient transport between the toning shell and the photoconductive drum. The lower amount of agitation also reduces the generation of fines and undesirable “dusting” of the toner as it is conveyed by the toning shell  26 . Dusting refers to a smoke-like, uncontrolled release of toner particles from the magnetic carrier particles prior to the arrival of the developer at the nip  27 . Such dusting can cause an unwanted toner deposition in the light portions of the printed image. In this particular embodiment of the invention, the relatively short angular distance θ is achieved by the use of a second conveyor roller  63  having horizontal and vertical components of spacing with respect to the first roller  50  such that the developer is applied above the center line of the toning shell  26 . It should further be noted that the use of two conveyor rollers  50 ,  63  having a horizontal component of spacing provides the developer station  10  with a relatively short height dimension, which allows it to be positioned adjacent to a side of the photoconductor  7  without mechanical interference with the intermediate transfer web  5  or other components of the printer module. 
       FIG. 3C  illustrates the preferred angular distance β between the line of closest approach  28  of the rotating magnetic core  24  and the toning shell  26  of the magnetic brush  22 , and the nip  27  of the toning shell  26  and the photoconductor drum  7 . In all embodiments of the developer station of the invention, angle β, the angle between the line of closest approach  28  of the rotating magnetic core  24  and the toning shell  26  and the line through nip  27 , is preferably less than between about +30° and −30°. More preferably, angle β is less than between about +10° and −10°. Most preferably, angle β is about 0° such that the nip  27  and the line  28  are horizontally aligned with one another. Such an alignment positions the strongest portion of the magnetic field of the brush  22  at the nip  27  which helps to secure the carrier particles onto the toning shell  26  during toner development, and further positions the weakest part of the magnetic field of the brush  22  on the portion of the toning shell facing away from the photoconductor drum  7 , thereby facilitating the removal of residual carrier particles on the shell  26  by skiving. 
     The operation of the developer station  10  will now be described with reference to  FIGS. 3A ,  3 B, and  3 C. The shells  55  and  66  of the first and second conveyor rollers  50 ,  63  rotate clockwise around their stationary magnetic cores  53  and  64 . The magnets  59  in the core  53  of the first conveyor roller  50  attract developer  30  from the reservoir  29  onto the shell  55 . The rotating shell  55  conveys this developer  30  to the rotating shell  66  of the second conveyor roller  63 . The developer  30  is transferred to the rotating conveyor shell  66  of the second conveyor roller  63  at the nip  67  between the first and second conveyor rollers due to the magnetic field of the magnets  65  in the magnetic core  64  of the second conveyor roller  63 . At the nip  67 , the layer of developer makes a U-shaped turn as it moves from the first to the second conveyor roller, and continues to move over the top of the second conveyor roller  63 . The layer of developer next makes a second U-shaped turn at the nip  70  between the second conveyor roller  63  and the toning shell  26  of the magnetic brush  22  as a result of the greater magnetic strength of the rotating magnetic core  24 , where it is transferred to the toning shell  26 . As a result of the clockwise rotation of the toning shell  26 , the layer of developer  30  is conveyed under a metering skive  72  as insurance against non-uniformities in thickness in route to the nip  27  between the toning shell  26  and the photoconductor drum  7 . 
     In a typical printer module printing 70 pages per minute (PPM), the toning shell  26  may rotate clockwise at 82 rpm while the magnetic core rotates counterclockwise at 800 rpm. While such operational speeds allow a high rate of toner image developing on the photoconductor drum  7 , they also create substantial developer agitation and hysteresis-induced heating due to the rapid rate of magnetic flux changes the hard magnetic carrier particles are subjected to as a result of the rotating magnets  25  in the core  24 . As described in detail with respect to  FIG. 3B , such agitation and heating are substantially reduced by reducing the angular distance between the nip  70  and nip  27  to less than 120° to reduce the residence time of the developer  30  on the toning shell  26 . In this first embodiment of the developer station  10  of the invention, such a relatively small angular distance θ is achieved in a station having a magnetic brush capable of applying developer on a photoconductor drum  7  at a 9 o&#39;clock or 3 o&#39;clock position by the horizontally and vertically spaced apart conveyor rollers  50  and  63 . 
       FIG. 4  illustrates a second embodiment  80  of the developer station of the invention in use in a printer module  81  arranged in an image-over-process configuration as shown in  FIG. 1B  where the intermediate transfer web contacts the photoconductor drum  7  at the 12 o&#39;clock position. The cleaners  13 ,  14 , charger  16 , and writer  18  surrounding the photoconductor drum  7  are not shown for simplicity. In this embodiment, no conveyor rollers are used to transport developer to the toning shell  26  of the magnetic brush  22 . Instead, a lower portion of the toning shell  26  directly contacts developer  30  at the developer reservoir  26  contained within the sump  23 . A layer of developer is acquired onto the surface of the toning shell at the feed zone containing nip  70  adjacent feed auger  84  and is transported in a clockwise direction through a metering skive  86 . The resulting trimmed layer of developer then proceeds into the nip  27  between the toning shell  26  and the photoconductor drum  7 . The residual magnetic carrier particles which remain on the toning shell  26  after the transfer of the toner particles at the nip  27  are in turn removed by stripping skive  88  located in the strip zone close to 180° away from the nip  27 , where they are deposited over a return auger  82 . Return auger  82  mixes the residual magnetic carrier particles removed by the skive  88  with fresh toner particles received from the toner replenisher tube  32 , and conveys the reconstituted developer to a feed auger  84  which functions as part of the feed assembly containing two or more augers to supply an uniform layer of developer on the toning shell. 
     The feed auger is located in the feed channel, on the feed side of the station of  FIG. 4 . The feed auger is located in the feed channel, on the feed side of the station of  FIG. 4 . The return auger is located in the return channel, on the return side of the station. The return side is also referred to as the strip side of the developer station. The feed apparatus which moves the developer toward the toning shell has a housing with two or more channel profiles to support two or more auger assemblies. One auger assembly has a continuous auger and the other auger assembly has one or more auger segments that fit the channel profile. The auger segments have intermeshed blades with a helical portion and optional angled paddles. The paddle angles range from 20 to 40 degrees and can have orientation ranges from +/−90 to +/−180 degrees. In one embodiment the auger assembly has adjacent augers of the same handedness that rotate in opposite directions and/or adjacent augers of the opposite handedness that rotate in the same direction. In some embodiments a guide member is placed between the auger assemblies. The guide member can have a curved member with a top portion ending within a region of a magnetic field surrounding the toning shell that exerts a magnetic force greater than or equal to 1 g on the surrounding magnetic carrier particles. 
     The return and feed augers  82 ,  84  operate in essentially the same way as the augers  33   a ,  33   b  and  38   a ,  38   b  described with respect to the first embodiment in that they create a clockwise circulation of developer when viewed from above. However, as developer is fed from the feed auger  84  to the toning shell and circulated (in a direction that is into the plane of the cross section of  FIG. 4 ), there will tend to be less developer in the feed auger longitudinally along the shaft of the auger at the far end of the toning shell. Paradoxically, a countermeasure to depletion of the feed auger and the poor developer feed that can result from this is to implement a feed auger  84  that transports developer at a faster rate than the return auger  82 . This causes developer to back up at the far end of feed auger  84 , producing a uniform developer level in the sump. This can be done by, for example, by using a continuous helix for the feed auger, or auger segments that approximate a continuous helix, in combination with an interrupted helix for the return auger. For example, if each auger is constructed of auger segments (also referred to as paddles or blades) that are ¼ of a full 360° helix, the feed auger can be composed of a series of segments at 0°, 90°, 180°, and 270° to approximate each turn of the helix, and the return auger can preferably be composed of a series of segments at 0°, −90°, −180°, and −270° to form an interrupted helix. The angle of paddle orientation between two segments is measured clockwise for a right hand auger (counterclockwise for a left hand auger) from a first segment to a second segment that is located down the axis of the auger in the direction of developer flow from the first segment). Augers of this construction are shown in  FIG. 7A , where arrows  83  indicate the direction of developer flow for feed auger  84  and return auger  82 . Paddles  85  can have an angled paddle angle that preferably ranges from 20° to 40° from a plane perpendicular to the axis of the auger shaft, corresponding to a helix angle of 20° to 40° Flippers  87  are typically angled at an angle of 90° from a plane perpendicular to the axis of the auger shaft, but this angle can range from 45° to 90°. A continuous helix  89  consisting of a single helical web can be used on feed auger  84  instead of paddles approximating a helix. Other configurations can be used for the interrupted helix, as well. For example, paddles for the interrupted helix of this example can be at a relative orientation angle from each other of −90° or −180° (equals 180°) or any other suitable angle. 
     The full helix of the feed auger  84  conveys developer at a faster rate down the length of the auger than the interrupted helix of the return auger  82 . The interrupted helix of the return auger also produces more mixing of toner supplied by replenisher  32  into the developer removed from the toning shell. 
     Another means of producing a uniform level of developer is to implement a tapered shaft on feed auger  84  to compensate for the volume removed from auger  84  and fed to the toning shell. A similar taper can be implemented on return auger  82  so that its action on the developer is uniform down the length of the auger shaft. Augers of this construction are shown in  FIG. 7B  with the tapered shafts  105  shown in phantom 
     An additional means of producing uniform feed is to provide a guide member  68  that will feed developer from the sump to, in this case, toning shell  26 . To enable feeding of developer up the wall of guide member  68 , feed auger  84  must rotate counter clock wise if right handed and feed developer into the plane of the cross section of  FIG. 4 , or feed auger  84  must rotate clockwise if left handed and feed developer out of the plane of  FIG. 4 . This enables developer blocked by metering skive  86  to fall back into feed auger  84 . The gap  69  between the guide member and the toning shell can be large enough to allow for overflow of excess developer from the feed channel to the return channel, preferably between approximately 9/10 and 7/10 of the auger diameter that is wherein the guide member extends to within a distance from the toning shell that equals between 9/10 and 7/10ths the total distance. The return auger also feeds developer against the wall on the right as shown in  FIG. 4 . This enables overflow developer from the feed channel to enter the return channel. It also enables toner provided by toner replenisher tube  32  to mix with developer transported by return auger  82 . For the configuration shown in  FIG. 4 , if the feed auger is right handed and rotating counterclockwise, a return auger that is left-handed and rotating counterclockwise is required. This left handed auger will feed developer out of the plane of  FIG. 4  against the wall on the right. If the feed auger of  FIG. 4  is left-handed, rotating clockwise, and feeding developer out of the plane of  FIG. 4 , then a return auger that is right handed, rotating clockwise, and feeding developer into the plane of  FIG. 4  is required, and, when viewed from above, circulation in the sump will be counterclockwise. 
     In the  FIG. 4  embodiment  80  of the developer station, the direct engagement between the toning shell  26  and the developer  30  in the reservoir  29  also advantageously allows the angular distance θ to be shortened to about 80°, thereby substantially reducing the residence time of the developer on the toning shell  26  over the prior art. Moreover, the tangent line T at the nip  27  between the toning shell  26  and photoconductor drum  7  is substantially aligned with the vertical axis such that this embodiment may be easily arranged into either a 3 o&#39;clock position as shown or a 9 o&#39;clock position with respect to the photoconductor drum  7 . 
       FIG. 5  illustrates a third embodiment  90  of the developer station of the invention in use in a printer module  91  again arranged in an image-over-process configuration as shown in  FIG. 1B  where the intermediate transfer web  5  contacts the photoconductor drum  7  at the 12 o&#39;clock position. Again, the cleaners  13 ,  14 , charger  16 , and writer  18  surrounding the photoconductor drum  7  is not shown for simplicity. In this embodiment, the feed auger  84  and the return auger  82  are intermeshed as shown in  FIG. 7C . Intermeshed paddles or blades have an overlap range between 0.5x and 0.95x, where x represents the distance between two adjacent auger shafts in the auger assembly. 
     If a tapered shaft is used for each auger to compensate for the volume of developer removed from the feed auger or that has not been returned to the return auger the tapered shaft can be notched to allow for the passage of the adjacent auger as shown in  FIG. 7D , where the notched tapered shafts  110  are shown in phantom. Preferably, the auger with the tapered shaft in this case is assembled from individual auger segments and a series of individual shaft collar segments. These shaft collar segments are notched to allow for the passage of the adjacent auger. 
     In  FIG. 5 , a single, conveyor roller  92  is used to transport developer  30  to the toning shell  26  of the magnetic brush  22  from the reservoir  29  of the sump  23  to the nip  70  in the feed zone. This conveyor roller can be a magnetic roller similar to rollers  55  or  63  of  FIG. 2  and  FIG. 3A , or it can be a mechanical paddle-type roller as is known in the art. Skives  94  and  96  operate in the same manner described with respect to the skives  86  and  88  of the  FIG. 4  embodiment  80 . In both embodiments, developer is removed from the magnetic brush in a stripping zone and returned to the sump. The stripping zone contains skives  88  and  96 . In this embodiment  90  of the developer station, the positioning of the single conveyor roller  92  toward the photoconductor drum  7  and between the toning shell  26  and the developer  30  in the reservoir  29  advantageously allows the angular distance θ to be shortened to about 80°, thereby substantially reducing the residence time of the developer on the toning shell  26  over the prior art. Again, the tangent line T at the nip  27  between the toning shell  26  and photoconductor drum  7  is substantially aligned with the vertical axis such that this embodiment may be easily arranged into either a 3 o&#39;clock position as shown or a 9 o&#39;clock position with respect to the photoconductor drum  7 . The intermeshed auger configuration shown in  FIG. 5  can be used for feeding developer directly to the toning shell, as shown in  FIG. 4 . Similarly, the auger configuration of  FIG. 4  can be used to feed developer to a conveyor roller and ultimately to a toning shell as shown in  FIG. 5 . 
     As mentioned previously, it is desirable to print at high process speeds. The usefulness of the invention as described and also as shown in  FIG. 2  to  FIG. 5  can be explained by application of the following examples from U.S. Pat. No. 6,959,162 (also assigned to Eastman Kodak Company of Rochester, N.Y., the entire text of which is hereby expressly incorporated herein by reference) for process speeds ranging from 17.49 inches per second, the equivalent of 110 PPM, to 33.39 inches per second, the equivalent of 210 PPM, and extrapolation to faster speeds. The speed of the developer on the toning shell  26  can be estimated to be approximately equal to the process speed. For example, at 110 PPM or 17.49 inches per second process speed, magnetic core speeds for the magnetic brush  22  of approximately 877 RPM are used, corresponding to 205 pole flips per second for a 14 pole magnetic core. A toning shell speed of 125.5 RPM is used, corresponding for a 2 inch diameter shell to surface speeds of approximately 13.14 inches per second. The developer velocity on the toning shell is approximately 17.49 inches per second. For higher process speeds, the core speed and toning shell speed can be increased proportionally to the process speed. For example, at 150 PPM corresponding to a process speed of 23.85 inches per second, a core speed of 1196.5 RPM can be used, corresponding to approximately 279 pole flips per second, and a toning shell speed of 171.14 RPM is used, corresponding to approximately 17.92 inches per second surface speed. At 220 PPM or a process speed of 34.98 inches per second, a core speed of 1754 RPM can be used, corresponding to approximately 409 pole flips per second, and a toning shell speed of 251 RPM can be used, corresponding to approximately 26.28 inches per second surface speed. 
     The rate of kinetic energy generated per second contributing to embedment, dusting, and generation of fines is proportional to the square of the number of pole flips per second. For example, a printer that is producing images at 220 PPM will have 4 times the power contributing to embedment and the other problems mentioned than a 110 PPM printer. At a given process speed, the total amount of kinetic energy generated in the developer between transfer of the developer to the toning shell and the toning nip is proportional to the angle θ. For example, at a given process speed, a developer that is transferred to the toning shell within 90 degrees of the development nip will be exposed to only half the kinetic energy resulting from pole flips by the time it reaches the development nip as a developer that is transferred to the toning shell 180 degrees from the nip. 
     Heat generation in units of power or energy per unit time in the developer due to magnetic hysteresis in the carrier particles during magnetic pole flips is proportional to the number of pole flips per second of the development system. The total amount of heat generated is proportional to the distance traveled on the toning shell. For example, a printer that is producing images at 220 PPM will generate heat due to magnetic hysteresis at approximately 2 times the rate of a 110 PPM printer. The total amount of energy resulting from hysteresis is proportional to the distance traveled on the toning shell by the developer. For example, at a given process speed, a developer that is transferred to the toning shell within 90 degrees of the development nip will be exposed to only half the energy resulting from hysteresis by the time it reaches the development nip as a developer that is transferred to the toning roller 180 degrees from the nip. 
     Finally, the performance of the developer station is improved in another embodiment illustrated in  FIG. 6  by a graph of the force produced on a carrier particle by the magnetic field of the toning roller for several different magnetic cores and toning shells. When a developer station, such as shown in  FIG. 2  to  FIG. 5 , has a toning shell of a sufficiently large diameter mounted eccentric to the magnetic core, the large diameter toning shell can improve the performance of the skive  31 ,  88 , or  96  or, in a preferred embodiment, cause the developer to fall off the toning shell in the strip zone and return to the sump without requiring a skive. This obviates the need to produce a straight, thin skive  31 ,  88 , or  96  spaced close to the toning shell with a small spacing tolerance. A toning roller that allows the removal of developer by gravity and centrifugal force in the strip zone requires than the force of removal, which is the centrifugal force produced by the rotation of the shell and the force of gravity, be greater than the magnetic force attracting developer or a carrier particle. This requirement is satisfied if the magnetic force is less than the force of gravity, or, more preferably, less than ½ the force of gravity in the strip zone. For toning stations used in the image over process configuration, such as those shown in  FIGS. 4 and 5 , the force of magnetic attraction within 120° or within 90° of the toning nip is required to be greater than the force of gravity, and preferably at least twice the force of gravity. 
     In the method of the invention, the diameter of the toning shell and the eccentric offset of the toning shell from the rotating magnetic core are used in combination with the magnetic properties of the rotating magnetic core to improve the skiving and removal of developer from the toning shell after the developer has passed through the nip with the photoconductor element, while also enabling the application of developer to the toning shell at an angular distance of no more than about 120° from the nip, preferably no more than about 90° from the nip, and more preferably in the range 90° to 75° from the nip. 
     The magnetic field of a rotating magnetic core  24  having N pairs of alternating north and south poles that produce a sinusoidally-varying magnetic field is given by the solution of Laplace&#39;s Equation. For the region outside the magnetic core:
 
∇ 2 φ=0,  (Equation 1)
 
with the scalar potential
 
                     φ   ⁡     (     r   ,   ϕ     )       =       -     B   0       ⁢       R   C     N     ⁢       (       R   C     r     )     N     ⁢       cos   ⁡     (     N   ⁢           ⁢   ϕ     )       .               (     Equation   ⁢           ⁢   2     )               
In Equation 2, r is the radial distance from the center of the magnetic core in cm, R C  is the radius of the core in cm, B 0  is the magnetic field at the surface of the magnetic core in the center of a north or south pole in Gauss, N is the number of magnetic north-south pole pairs, and φ is the angle around the magnetic core from the center of one of the north poles arbitrarily taken as an origin. In the following, the north pole origin is also at the location of closest approach of the surface of the magnetic core to the toning shell. This potential corresponds to the magnetic field
 
                       B   →     ⁡     (     r   ,   ϕ     )       =             B   0     ⁡     (       R   C     r     )         N   +   1       ⁡     [         r   ^     ⁢     cos   ⁡     (     N   ⁢           ⁢   ϕ     )         +       ϕ   ^     ⁢     sin   ⁡     (     N   ⁢           ⁢   ϕ     )           ]       .             (     Equation   ⁢           ⁢   3     )               
The magnetic force F M  for a magnetic core with N pole pairs on a carrier particle with magnetization M emu/g and mass m is directed toward the center of the magnetic core, and F M  has magnitude in g&#39;s of
 
                         F   M     ⁡     (     in   ⁢           ⁢     g   ′     ⁢   s     )       =         F   M     mg     =         mM   ⁢           ⁢     ∇     ·     B   →           mg     =           MB   0     ⁢   N     g     ⁢       (       R   C     r     )     N     ⁢       R   C       r   2               ,           (     Equation   ⁢           ⁢   4     )               
The force in g&#39;s is a dimensionless number. The acceleration due to gravity g is taken to have the value of 981 cm/s 2 .
 
     Referring now to  FIGS. 3B and 3C , for an eccentrically-mounted toning shell of radius R S  offset a distance δ from the center of the magnetic core of radius R C , the distance r of a point on the toning shell where all distances are in centimeters (cm) from the center of the magnetic core is given by the law of cosines:
 
 r= (δ 2   +R   S   2 −2δ R   S  cos(θ−β)) 1/2 ,  (Equation 5)
 
with angles θ and β measured from the photoconductor nip with the toning roller in the direction toward the feed zone and all lengths in cm.
 
     For a carrier particle having magnetization M of 32 emu/g and a typical diameter of 22 to 28 microns, small compared to R C /N, the force F M  in g&#39;s on a carrier particle as a function of location on the toning shell is shown in  FIG. 6  for several different configurations of toning shell and magnetic core that can be implemented in toning systems of the present invention, as shown in  FIG. 3  to  FIG. 5 . The angle specified on the x axis of  FIG. 6  is the angle on the toning shell from the photoconductor nip in the direction toward the conveyor roller or feed zone. The characteristics of the toning rollers of  FIG. 6  are listed in Table 1. B 0  is 1554.65 Gauss. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Characteristics of toning rollers of FIG. 6 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 F M   
                 F M   
                 F M   
                 F M   
                 F M   
               
               
                   
                 No. of 
                   
                   
                   
                 β 
                 180° 
                 120° 
                 90° 
                 75° 
                 60° 
               
               
                 Roller 
                 poles 
                 R S  (cm) 
                 R C  (cm) 
                 δ (cm) 
                 (°) 
                 (g&#39;s) 
                 (g&#39;s) 
                 (g&#39;s) 
                 (g&#39;s 
                 (g&#39;s) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 14 
                 25.37 
                 21.56 
                 2.54 
                 0 
                 1.84 
                 2.71 
                 4.15 
                 5.26 
                 6.64 
               
               
                 2 
                 14 
                 27.91 
                 21.56 
                 5.08 
                 0 
                 0.41 
                 0.77 
                 1.59 
                 2.45 
                 3.80 
               
               
                 3 
                 14 
                 27.91 
                 21.56 
                 5.08 
                 30 
                 0.48 
                 1.60 
                 3.80 
                 5.77 
                 8.18 
               
               
                 4 
                 12 
                 25.37 
                 18.48 
                 5.62 
                 0 
                 0.31 
                 0.58 
                 1.26 
                 1.99 
                 3.24 
               
               
                 5 
                 12 
                 25.37 
                 18.48 
                 5.62 
                 30 
                 0.36 
                 1.26 
                 3.24 
                 5.19 
                 7.75 
               
               
                 6 
                 8 
                 25.37 
                 14.37 
                 9.73 
                 0 
                 0.08 
                 0.16 
                 0.39 
                 0.68 
                 1.31 
               
               
                   
               
            
           
         
       
     
     For the toning station of  FIG. 3  for process over image, the feed zone is approximately 120° to 90° from the photoconductor nip and the strip zone is as distant as approximately 180° from the photoconductor nip, or +/−120° from the photoconductor nip. Note that the nip location is the line of closest approach between the toning shell and the photoconductor member and parallel to the axis of rotation of the magnetic core. In the strip zone, the magnetic force for at least one location along the length of the toning shell should be less than 1 g, and preferably less than 0.5 g. For the toning station configuration of  FIG. 4  and  FIG. 5  for image over process, the feed zone is at approximately 80° from the photoconductor nip, within the range 75° to 90°. Developer removal occurs in a strip zone approximately 180° from the photoconductor nip, or +/−120° from the photoconductor nip. In the feed zone the magnetic force for at least one location along the length of the toning shell should be at least 1 g, and preferably at least approximately 2 g&#39;s. In the strip zone, the magnetic force for at least one location along the length of the toning shell should be less than 1 g, and preferably less than 0.5 g.  FIG. 6  shows that toning roller  1 , a 14 pole roller nominally 2 inches in diameter, does not satisfy strip zone conditions of F M &lt;1 g. Toning roller  6  does not satisfy feed zone conditions of F M &gt;1 g for the ranges 120° to 90° or 90° to 75° from the photoconductor nip. Rollers  2  to  5  satisfy the preferred strip zone condition of F M &lt;0.5 g as well as the feed zone condition of F M &gt;1 g over the range 90° to 75° from the photoconductor nip. Roller  3  and roller  5  also satisfy the feed zone condition of F M &gt;1 g over the range 120° to 90° from the photoconductor nip as well as satisfying the preferred feed zone condition of F M &gt;2 g over the range 90° to 75° from the photoconductor nip. 
     The invention as described herein enables improved feeding and mixing by utilizing a development station housing defining two channel profiles to support an improved powder conveyance device in a developer sump. The sump comprises two auger assemblies rotating in opposite directions if the augers have the same handedness or in the same direction if the augers have opposite handedness. The feed channel contains the feed auger primarily transporting developer that is fed to the toning shell directly or to a conveyor roller. The return channel contains the return auger primarily transporting developer that has been removed from the toning shell and is being replenished with toner. Preferably, the feed auger is a full helix or segments approximating a full helix. One or both augers can have a tapered shaft to maintain developer level in the sump. The taper increases in diameter in the direction of developer feed along the axis for the feed channel and decreases in diameter in the direction of developer feed along the axis for the return channel. Guide members can also be utilized to guide developer to conveyor rollers or toning rollers. If the guide member is between the feed and return channel, it can be spaced from the adjacent feed roller or toning shell to allow a gap for developer overflow. In the case that the feed and return augers are intermeshed, the tapered shaft can be notched to allow for the passage of the adjacent auger. Preferably, the auger with the tapered shaft in this case is assembled from individual auger segments and a series of individual shaft collar segments that take up the volume of developer that is removed from the feed auger or has not been returned to the return auger. These shaft collar segments are notched to allow for the passage of the adjacent auger. 
       FIG. 8  shows an electrostatic printer magnetic brush developer station, according to this invention, sometimes simply referred to as a developer station or toning station, designated generally by the numeral  210 . The development station housing  212  encloses a feed apparatus  214  and a powder conveyance device  216  and forms, in part, a reservoir  215  for developer material  217  including a powder and a carrier material. The developer level in the reservoir is shown schematically by dotted line  219 . The reservoir is also referred to as a sump. A development roller  218  is mounted within the development station housing  212 . The development roller  218  includes a rotating (shown as counterclockwise in  FIG. 8 ) fourteen-pole magnetic core  220  inside a rotating (shown as clockwise in  FIG. 8 ) toning shell  222 . The magnetic core  220  and the shell can have many other suitable relative rotations as is known in the art. 
     The quantity of developer material delivered from the reservoir  215  to the development zone  224  is controlled by a metering skive  226 , positioned parallel to the longitudinal axis of the development roller  218 , at a location upstream in the direction of shell rotation prior to the development zone. The metering skive  226  extends the length of the development roller  218  The magnetic core  220  does not extend the entire length of the development roller; as such, the developer nap on the shell  222  does not extend to the end of the development roller. The development station  212  may house one or more conveyor rollers  228  to move the developer material within the reservoir of the housing  212  from the mixing area to the toning shell. However, it is possible to feed developer directly from the reservoir to the toning shell. 
       FIGS. 9 and 10  show one or more tapered augers  282  and  284 , each having a shaft  250 . In  FIG. 9 , auger  284  has a tapered shaft. In  FIG. 10  auger  282  (or  284 ) has one or more variable height shaft collars  252 , each of a constant, specific diameter such that there is less volume for developer in the auger as the developer moves in the direction of flow (F) in the feed channel  244  and more volume for developer as the developer moves in the direction of flow (F) in the return channel  242 . In the feed channel,  244 , this change in the volume encompassed by the auger along the axis of the auger compensates for developer that is removed from the sump and applied to the toning roller. In the return channel  282 , this change in the volume encompassed by the auger compensates for developer that is removed from the toning roller and returned to the sump. Generally, each auger has paddles or blades  285  that can move some specific volume of developer  217  per revolution along the axis of the auger.  FIG. 10  shows shaft collars, which can also be referred to as sleeves, of diameter ‘d’ increasing in the direction of the developer flow (F) on the feed auger, and decreasing in the direction of developer flow (F) on the return auger. This is sometimes referred to as volume bias. These sleeves are notched to provide clearance for the blades of an adjacent, interleaved auger. Developer feed uniformity is improved by creating a variable shaft diameter ‘d’ on the augers. This can be accomplished by machining a taper on a series of shaft collars that are to be assembled on the auger shaft as shown in  FIG. 9  or by using a series of shaft collars of increasing diameter as shown in  FIG. 10 . 
     The magnetic brush development station  210 , shown in  FIG. 8  and  FIG. 9 , uses one conveyor roller, although more than one conveyor roller or none could be used in conjunction with the tapered augers. Controller  260  controls the development station including the tapered augers  282  and  284  as shown in  FIG. 9 . The controller also controls the powder-conveying device in the reservoir, such that the auger preferentially mixes in the mixing space  245  and transports in the second transport space  246  as the powder is conveyed toward the conveyor roller  228  as shown in  FIG. 8 . The tapered augers  282  and  284  described above allow a more uniform level of developer in the reservoir as the developer moves in the direction of flow (F) and circulates around the sump in a counter-clockwise direction as viewed from above. 
     Developer feed uniformity is improved by tapering the auger shafts. In one embodiment this is achieved using shaft collars of variable diameter ‘d’ on the auger shaft as shown in  FIG. 9  and discussed above to provide a taper angle α. This can also be accomplished by varying other features of the augers that result in developer feed uniformity and specifically encourage a higher developer level in areas of the sump where the amount of developer tends to be low, such as at the second end  262 .  FIG. 11  shows an auger  282  with an external blade taper angle γ and a shaft tilt angle ε as well as the variable shaft diameter ‘d’ approximating a taper angle α. Theses features could be combined or used separately to control the volume bias as required. The paddles  217  can also have one or more surface features, such as texture or pockets that might effectively create a bucket type effect, to further stabilize the volume of developer moved toward the toning shell. 
     The developer station includes a sump for holding a reservoir of magnetic developer including toner and carrier and a magnetic roller mounted above said sump and including a rotatable magnetic core surrounded by a substantially cylindrical rotatable toning shell rotatably mounted with respect to the core, said shell being adjacent to the photoconductor drum and defining a nip and a conveyance device for transporting developer in the developer station in a flow direction. The conveyance device has, in one embodiment, a tapered auger including a shaft and one or more blades such that the developer volume in the flow direction is controlled to maintain a uniform developer level in the sump as well as a conveyance controller for controlling the conveying device, including the tapered auger such that the tapered auger preferentially creates an uniform layer of developer on the toning shell. The auger can improve developer delivery by a number of embodiments including increasing a shaft diameter in the flow direction for a feed auger and decreasing the shaft diameter in the flow direction for a return auger. The tapered auger has the blade tapered angle γ tilted at shaft tilt angle ε in the direction of the toning shell to further control the level of volume of developer in the sump. The tapered auger taper angle α controls a developer volume in the feed channel within a range that results in an uniform layer of developer in the sump within +/−0.5 inches on average. In another embodiment the tapered auger angle is less then 5 degrees and the tapered feed roller is angled a shaft tilt angle ε between 0 and 5 degrees towards the toning shell. The conveyance controller can control the speed and the tilt angle of the auger in some embodiments. The conveyance controller can control a tilt angle to further control the volume of developer moved toward the feed apparatus. 
     Various embodiments can be used to compensate for the change in the relative volumes of developer traveling in direction F. These include tapering the shaft diameter or auger diameter and/or sloping the whole reservoir the required amount to effect the desired constant developer level by compensation for the volume of developer removed or returning to the sump, machining a taper on a solid auger shaft or blade made from a cylinder, or some other similar method of providing volume bias. This variable volume associated with each auger is oriented such that the effective blade height ‘a’ decreases in the direction of the developer flow (F) in the feed channel and increases in the direction of the developer flow (F) in the return channel. Since during operation there is normally more developer at the first or front end  261  of the conveyor roller  228  than at the second or rear end  262 , as shown in  FIGS. 9 to 11 , the tapered augers compensate for this effect. This is important since when there is less developer left in the feed channel the pick-up point at the surface of the developer in the channel becomes even further from the conveyor roller or development roller. The tapered auger allows the rear of reservoir to hold less developer and provides the same developer height in the rear as in the front of the reservoir, thereby resulting in more uniform pick-up by the conveyor roller or toning shell and thus more efficient and higher quality prints. 
     In this application, the term “electrographic printer” is intended to encompass electrophotographic printers and copiers that employ dry toner developed on any type of electrophotographic receiver element (which may be a photoconductive drum or belt), as well as ionographic printers and copiers that do not rely upon an electrophotographic receiver. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
     
         
           1 ) printing apparatus 
           3 ) printer modules a-e 
           5 ) intermediate transfer web 
           7 ) photoconductor element 
           8 ) web rollers 
           10 ) developer station 
           11 ) charged back-up bar 
           12 ) pre-cleaner 
           13 ) toner image 
           14 ) cleaning brush 
           16 ) corona charger 
           18 ) optical image writer 
           20 ) housing 
           22 ) magnetic brush 
           23 ) sump 
           24 ) rotatable magnetic core 
           25 ) magnets 
           26 ) toning shell 
           27 ) nip 
           28 ) line of closest approach 
           29 ) reservoir of developer 
           30 ) developer 
           31 ) skive 
           32 ) toner replenisher tube 
           33 ) return augers a, b 
           34 ) screw blades a, b 
           38 ) feed augers a, b 
           40 ) screw blades a, b 
           48 ) central portion of sump 
           50 ) first conveyor roller 
           53 ) magnetic core 
           55 ) rotating shell 
           59 ) small magnets 
           61 ) skive 
           63 ) second conveyor roller 
           64 ) magnetic core 
           65 ) magnets 
           66 ) rotating cylindrical conveyor shell 
           67 ) nip 
           68  guide member 
           69  gap 
           70 ) nip 
           72 ) skive 
           74 ) skive 
           80 ) second embodiment 
           81 ) printer module 
           82 ) return auger 
           83 ) direction of developer flow 
           84 ) feed auger 
           85 ) paddle 
           86 ) metering skive 
           87 ) flipper 
           88 ) stripping skive 
           89 ) continuous helix 
           90 ) third embodiment 
           92 ) single conveyor roller 
           94 ) metering skive 
           96 ) stripping skive 
           100 ) paper 
           105 ) tapered shaft 
           110 ) notched and tapered shaft 
           120 ) fuser apparatus