Abstract:
The invention relates generally to apparatus for electrostatic image development and, in particular, apparatus that implement a magnetic brush with a drum photoconductor. According to an aspect of the invention, a method and apparatus for developing an electrostatic image are provided implementing a drum photoconductor and a magnetic brush contacting the drum photoconductor. The magnetic brush includes a mixture of toner and hard magnetic carriers.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/204,881, filed May 17, 2000. 
    
    
     BACKGROUND 
     The invention relates generally to apparatus for electrostatic image development and, in particular, apparatus that implement a magnetic brush with a drum photoconductor. 
     Development apparatus and processes that implement a magnetic brush to deposit toner on a photoconductor have been known for many years. Development apparatus and processes that implement a magnetic brush having hard magnetic carriers are described in U.S. Pat. Nos. 4,473,029 and 4,546,060. The apparatus described in those patents implements a rotating shell coated with a mixture of hard magnetic carrier particles and toner, a rotating magnetic core having a multitude of magnetic poles disposed within the rotating shell, and a film (sheet-like) photoconductor. The rotating magnetic core causes the hard magnetic carrier particles to tumble on the surface of the shell, which provides increased agitation and toner contact with the film photoconductor. 
     The two-component dry developer composition of U.S. Pat. No. 4,546,060 comprises charged toner particles and oppositely charged, magnetic carrier particles, which (a) comprise a magnetic material exhibiting “hard” magnetic properties, as characterized by a coercivity of at least 300 gauss and (b) exhibit an induced magnetic moment of at least 20 EMU/gm when in an applied field of 1000 gauss, is disclosed. As described in the &#39;060 patent, the developer is employed in combination with a magnetic applicator comprising a rotatable magnetic core and an outer, nonmagnetizable shell to develop electrostatic images. When hard magnetic carrier particles are employed, exposure to a succession of magnetic fields emanating from the rotating core applicator causes the particles to flip or turn to move into magnetic alignment in each new field. Each flip, moreover, as a consequence of both the magnetic moment of the particles and the coercivity of the magnetic material, is accompanied by a rapid circumferential step by each particle in a direction opposite the movement of the rotating core. The observed result is that the developers of the &#39;060 flow smoothly and at a rapid rate around the shell while the core rotates in the opposite direction, thus rapidly delivering fresh toner to the film photoconductor and facilitating high-volume copy and printer applications. 
     Prior applications of hard magnetic carriers have implemented a flexible photoconductor in the form of a film. Flexible or film based photoconductors have several significant advantage for high speed photocopying. In a typical apparatus, the film photoconductor is positioned under moderate tension with a series of rollers. There is an inherent resilience in the film photoconductor that allows it to respond to variations in the thickness of the magnetic brush. 
     Drum photoconductors are also known in the art, but have not been used with magnetic brushes having hard magnetic carriers. At least part of the reason drum photoconductors have not been used is because they tend to be far more rigid than a film photoconductor, and increasing thickness of the magnetic brush can damage the surface of the drum photoconductor. Conversely, decreasing thickness of the magnetic brush can cause insufficient contact with the drum photoconductor and poor image development. 
     SUMMARY 
     According to an aspect of the invention, an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor and a magnetic brush contacting the drum photoconductor, a magnetic brush contacting said drum photoconductor, said magnetic brush comprising a mixture of toner and hard magnetic carriers. 
     According to a further aspect of the invention, a method for developing an electrostatic image is provided comprising applying a mixture of toner and hard magnetic carriers to a drum photoconductor with a magnetic brush. 
     According to a further aspect of the invention, a method for developing an electrostatic image is provided comprising applying a mixture of toner and hard magnetic carriers to a drum photoconductor with a magnetic brush comprising a magnetic core within a shell having a center of rotation, and the magnetic core is offset relative to the center of rotation toward the drum photoconductor. 
     According to a further aspect of the invention an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor, a magnetic. brush contacting the drum photoconductor, the magnetic brush comprising a mixture of toner and hard magnetic carriers, the magnetic brush comprising a rotatable magnetic core within a shell. 
     According to a further aspect of the invention an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor, a magnetic brush contacting the drum photoconductor, the magnetic brush comprising a mixture of toner and hard magnetic carriers, the magnetic brush comprising a rotatable magnetic core within a rotatable shell. 
     According to a further aspect of the invention an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor, a magnetic brush contacting the drum photoconductor, the magnetic brush comprising a mixture of toner and hard magnetic carriers, the magnetic brush comprising a magnetic core within a rotatable shell, the magnetic core being rotatable in a direction of rotation and the shell being rotatable in a direction opposite to the direction of rotation of the magnetic core. 
     According to a still further aspect of the invention a method for developing an electrostatic image is provided, applying a magnetic brush to a drum photoconductor, said magnetic brush comprising a mixture of toner and hard magnetic carriers with a mass flow rate, and limiting said mass flow rate of the mixture to be less than a limiting mass flow rate for which roll-back occurs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 presents an end view of a development apparatus according to an aspect of the invention, including a cross-sectional view of a toning station according to an aspect of the invention. 
     FIG. 2 presents an enlarged view of a development zone according to an aspect of the invention. 
     FIG. 3 presents an enlarged view of a development zone having a roll-back region. 
     FIG. 4 presents an exploded perspective view of the FIG. 1 toning station. 
     FIG. 5 presents an exploded perspective view of the FIG. 1 toning station from an opposite end thereof as that presented in FIG.  4 . 
     FIG. 6 presents an end view of a toning station carriage assembly according to an aspect of the invention. 
     FIG. 7 presents a perspective view of the carriage assembly of FIG.  6 . 
     FIG. 8 presents a side plan view of a sliding rail implemented in the carriage assembly of FIG.  6 . 
     FIG. 9 presents a perspective view of the carriage assembly of FIG. 6 from an opposite end thereof as that presented in FIG.  7 . 
     FIG. 10 presents an end view of the carriage assembly of FIG. 6 with the toning station of FIG. 1 registered with a drum photoconductor. 
     FIG. 11 presents an enlarged view of a registration shoe and registration pin in an unregistered state. 
     FIG. 12 presents an enlarged view of a registration shoe and registration pin in a registered state. 
    
    
     DETAILED DESCRIPTION 
     Various aspects of the invention are presented in FIGS. 1-12, which are not drawn to scale, and wherein components in the numerous views are numbered alike. Referring now to FIG. 1, a toning station  10  is presented, according to an aspect of the invention, comprising a drum photoconductor  12  and a magnetic brush  14  that carries a mixture of toner and hard magnetic carriers into contact with the external surface of the drum photoconductor  12 . As used herein, the term “hard magnetic carriers” means carriers having hard magnetic properties, as described in the Background section. The magnetic brush  14  operates according to the principles described in U.S. Pat. Nos. 4,473,029 and 4,546,060, the contents of which are fully incorporated by reference as if set forth herein. The magnetic brush  14  comprises a shell  16 , preferably of non-magnetizable material, and a magnetic core  18  comprising a rotating plurality of magnetic poles. The drum photoconductor  12  rotates in the direction indicated by the arrow  20  (drum photoconductor  12  direction of rotation), the shell  16  rotates in the direction indicated by the arrow  22  (shell  16  direction of rotation), and the magnetic core  18  rotates in the direction indicated by the arrow  24  (magnetic core  18  direction of rotation). 
     For the purposes of this description, the drum photoconductor  12  has a direction of rotation  20  that is opposite the direction of rotation  22  of the shell  16 , and drum photoconductor  12  has a direction of rotation  20  that is co-directional with the direction of rotation  24  of the magnetic core  18 . The various directions of rotation are not so limited in the practice of the invention, and may be changed as may be suited for a particular application. 
     The toning station  10  and the drum photoconductor  12  are mounted to a frame  28  that is configured, as needed, for a particular copy machine or printer application, as desired. The toning station  10  comprises a shell  16 , a sump  30 , quadruple augers  32  mounted for rotation in the sump  30 , a transport roller  34  above the augers  32  adjacent the shell  16 , and a replenishment unit  36  that comprises a fixed perforated tube  38 , and a replenishment brush  40  inside the tube  38  that rotates in the direction of arrow  42 . The replenishment unit  36  adds toner to the toner/carrier mixture in response to a toner concentration monitor  44  to maintain a weight ratio of toner to carrier, for example 1/10. More or less such components may be added or removed depending upon the particular application. 
     The additional mixing provided by a four auger system improves cross mixing and, therefore, cross track uniformity in a developed image. Toner concentration gradients are preferably minimized, and no more than a few percent in magnitude. 
     In use, the mixture of hard magnetic carriers and toner covers the augers  32  to a level about even with the bottom of the transport roller  34 . The augers  32  comprise a multitude of angled blades  46  mounted on shafts  48 . The blades mix the carriers and toners to create tribocharging and uniformity, and to provide a generally even level of toner/carrier mixture in the sump  30 . The transport roller  34  rotates in the direction of arrow  50  and lifts the mixture out of the sump  30  to the shell  16 . 
     Referring now to FIG. 2, an enlarged view of the nip area of FIG. 1 is presented, showing where a toner/carrier mixture  52  coating the shell  16  contacts the drum photoconductor  12 . The flow of toner/carrier mixture  52  has a thickness T, and contacts the drum photoconductor  12  over a length L. The flow presented in FIG. 2 is very desirable. Referring now to FIG. 3, an undesirable flow of the toner/carrier mixture  52  is presented wherein a roll-back region  54  has developed. The roll-back region  54  is believed to be caused by the flow of toner/carrier mixture  52  being greater than can flow through the nip between the shell  16  and the drum photoconductor  12 . The roll-back region may cause artifacts in the image development on drum photoconductor  12 , and may also physically damage the surface of the drum photoconductor  12 . The roll-back region  54  is not passive since the rotating magnetic core  18  tends to drive the mass of material in the roll-back region  54  into the nip with an active force. 
     According to an aspect of the invention, development of the roll-back region is avoided. According to a further aspect of the invention, development of the roll-back region is avoided at least in part by accurately and repeatably locating the shell  16  a predetermined distance from the drum  12 . Referring again to FIG. 1, the registration of the shell  16  to the drum  12  is controlled by providing a pair of registration pins  56  on either side of the drum  12 . The drum  12  comprises a shaft  58  that is mounted in a pair of bearings  60  that, in turn, are mounted to the frame  28 . 
     The toning station  10  comprises a pair of registration shoes  62  on either end that mate with the pins  56 . According to a preferred embodiment, the magnetic brush  14  is registered to the drum photoconductor  12 . For example, the toning station  10  is wedged into registration with the pins  56  thereby rendering it immovable during operation of the drum photoconductor  12  and magnetic brush  14 . The registration shoe  62  presented in FIG. 1 is mostly hidden behind the toning station  10 . A better representation is provided in FIG. 12, which will be discussed in more detail. 
     According to a further aspect of the invention, development of the roll-back region is avoided at least in part by accurately controlling the thickness of the toner/carrier mixture  52  in the nip between the shell  16  and the drum  12 . The center of rotation of the core  18  may be offset relative to the center of rotation of the shell toward the drum  12  (eccentric), thereby forming an area  64  wherein the inside surface of the shell  16  is closest to the outside surface of the core  18  (slightly below the point where the drum  14  and the shell are closest), referred to herein as the perigee. A metering skive  66  is located opposite the area  64  where the shell  16  is furthest from the core  18 , referred to herein as the apogee. The metering skive  66  meters a predetermined thickness of toner/carrier mixture onto the shell  16 . The magnetic field applied to the toner/carrier mixture is weakest at this point, so it is not fully compacted. The thickness of the mixture decreases as it is carried around the shell to the drum  12 . Therefore, any variation in the thickness decreases along with the thickness of the mixture, thereby providing a more precise thickness T. According to a preferred embodiment, a line drawn from the area  64  (the perigee) to the metering skive  66  is horizontal, and the two are 180° apart (one at the 9 o&#39;clock position, the other at the 3 o&#39;clock position). 
     According to a preferred embodiment, the flow of the toner/carrier mixture  52  is limited to be less than the limiting mass flow rate through the nip between the drum photoconductor  12  and the shell  16  for which roll-back occurs. This may be accomplished, at least in part by adjusting the skive gap (the distance between the skive  66  and shell  16 ) to achieve a predetermined mass flow of toner/carrier mixture  52  per unit length past the skive  66  that is less than the limiting mass flow rate. Mass flow rate of the toner/carrier mixture is also affected by numerous other parameters including speed and direction of rotation of the various rotating components, and the dimensions and physical properties of the toner/carrier mixture and various components of the development station, for example the magnetic field strength of the core. This list is not intended to be exhaustive since the effects and the limiting mass flow rate for a particular application may be determined by experimental observation and measurement. 
     Referring now to FIGS. 4 and 5, exploded isometric views of the toning station  10  from opposite ends are presented in order to show additional detail, and the relative relation of the components. 
     The rotating magnetic core  18  may develop eddy current losses in the drum photoconductor  12 . A combination of relatively fine magnetic pole frequency (for example a 14 pole, 1.6 inch diameter) and relatively thin, 4 to 8 mm, wall thickness for the drum photoconductor  12  reduces eddy current losses. Further, the material choice for the drum wall of a relatively hard aluminum (T3 or T6) minimizes the wall conductivity, and therefore, the eddy current losses. 
     According to a further aspect of the invention, the metering skive  66  is provided with wings  67  at both ends that locally reduce the flow of the toner/carrier mixture proximate the ends of the drum photoconductor  12 . Reducing the flow in such manner prevents physical damage that may occur at the very ends of the development zone due to edge effects that locally increase flow. Further reductions in the rate of formation of rollback regions at the ends of the development zone were obtained by placing small permanent magnets on the metering skive  66  at the wing locations  67 . 
     A DC bias is applied to the shell  16  in order to create an electric field that transports toner to the surface of the drum photoconductor  12 . An AC bias may also be implemented to improve the development rate, and therefore the level and consistency of image quality. In solid area development systems, an electrical bias is applied between the ground layer of the drum photoconductor and the shell of the development station. If a high frequency, high voltage, e.g. 1000 to 2000 hertz and 500 to 1500 volt rms, signal is added to the constant bias, the development rate is significantly increased. 
     Referring now to FIGS. 6-12, a carriage assembly  100  is presented according to a further aspect of the invention, that may be employed in combination with the toning station  10  (FIGS. 1 and 10) to provide precise registration of the toning station with the drum photoconductor  12  (FIGS.  1  and  10 ). The carriage assembly  100  compensates for skew in the drum photoconductor  12 . Referring now to FIGS. 6-9, the carriage assembly  100  comprises a support rail  102  and a sliding rail  104  suspended and guided by three rods  106 ,  108 , and  110 . The support rail  102  is rigidly attached to a structure, such as the frame  28  of FIG.  1 . As best shown in FIG. 8, the sliding rail  104  comprises two horizontally elongated holes  132  that receive the outer rods  106 , and  110 , and a slightly oversized hole  134 . The center rod  108  constrains movement of the sliding rail  104  in the longitudinal direction while allowing lateral movement of the sliding rail  104  along the rod  108 . The two outside rods  106  and  110  maintain levelness of the sliding rail  104 . 
     Referring again to FIGS. 6-9, the sliding rail  104  is attached to a side plate  116 . A camshaft  112  is driven by an electrical actuator motor  114 , and is captured between two components of the side plate  116  and provides the mechanism for positioning the sliding rail  104  relative to the photoconductor drum  12 . The electrical actuator motor  114  is rigidly mounted to the same support structure as the support rail  102 . The camshaft  112  comprises a cam bearing  113 . A load arm  122  is also attached to the side plate  116  and pivoted about a vertical axis at a load arm pivot  124 . The position of camshaft  112  is controlled through the use of two solid state micro switches  126  and a cam position coupling  118 . 
     As the camshaft  112  is rotated from a disengaged position to an engaged position it pushes against a detented cam retainer plate  120  attached to the side plate  116 . This motion pushes the sliding rail  104  into its engaged position, best shown in FIG. 6, as indicated by arrow  132 . As the sliding rail  104  travels to its engaged position, the load arm  122  mounted to the side plate  116  encounters the toning station  10  (FIGS. 1 and 10) and is deflected thereby creating a spring force that pushes the toning station  10  into registration with the registration pins  56  (FIGS.  1  and  10 ). The detented cam retainer plate  120  provides a nesting force so that the camshaft  112  does not rotate away when the mechanism is in the engaged position. 
     Still referring to FIGS. 6-9, a positive vertical lift force is achieved through the use of two angled push pads  128  mounted on the load arm  122  and a corresponding angled wedge  130  mounted to the toning station  10  (see FIGS.  1  and  10 ). The push pads  128  pass through a pair of windows provided in the side plate  116 . Referring now specifically to FIG. 10, an angled push pad  136  is mounted to the bottom of the toning station  10 , and a mating push pad  138  is mounted to the frame of the machine, such as frame  28  of FIG.  1 . The angled push pad  136  and mating push pad  138  provide additional vertical lift force. The net force is oriented toward the drum photoconductor  12  and up, as indicated by arrow  140 . As presented in FIGS. 11 and 12, the registration shoes  62  mounted at either end of the toning station  10  preferably comprise V-shaped notches  63 , and the registration pins  56  are received with the notches  63  upon forcing the toning station  10  in the direction of arrow  140 . The force holding the toning station  10  in place may exceed 100 lbf, and according to a certain embodiment is on the order of 160 lbf. 
     The sliding rail  104  comprises a track  105  that toning station  10  is received within and guided upon while being inserted into the machine until all electrical and mechanical interfaces are met. The sliding rail  104  and track  105  serve to accurately locate the toning station  10  in relation to the pins  56  so that, upon activation of the cam shaft  112 , the registration pins  56  are received within the notches. 
     Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the true scope and spirit of the invention as defined by the claims that follow. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.