Apparatus for developing a latent image

An apparatus for developing a latent image recorded on an imaging surface, including a housing defining a chamber storing a supply of toner; a donor member, spaced from the imaging surface, for transporting developer material on the surface thereof to a region opposed from the imaging surface, a transport member for transporting toner from said housing and loading toner onto the donor member, the transport member, further includes a mask layer adjacent to the photoconductive layer, the mask layer having a screen pattern; a charging device for charging the photoconductive layer and an exposure device for discharging unmasked portions of the transport member thereby generating the fringe field on the surface of the transport member to attract toner from the housing to the surface of the transport member.

This invention relates generally to a development apparatus for ionographic
 or electrophotographic imaging and printing apparatuses and machines, and
 more particularly is directed to an apparatus and method for loading dry
 Xerographic toner onto a donor member and developing a latent
 electrostatic image.
 BACKGROUND OF THE INVENTION
 Generally, the process of electrophotographic printing includes charging a
 photoconductive member to a substantially uniform potential so as to
 sensitize the surface thereof. The charged portion of the photoconductive
 surface is exposed to a light image from either a scanning laser beam or
 an original document being reproduced. This records an electrostatic
 latent image on the photoconductive surface. After the electrostatic
 latent image is recorded on the photoconductive surface, the latent image
 is developed. Two component and single component developer materials are
 commonly used for development. A typical two component developer comprises
 magnetic carrier granules having toner particles adhering
 triboelectrically thereto. A single component developer material typically
 comprises toner particles. Toner particles are attracted to the latent
 image forming a toner powder image on the photoconductive surface, the
 toner powder image is subsequently transferred to a copy sheet, and
 finally, the toner powder image is heated to permanently fuse it to the
 copy sheet in image configuration.
 The electrophotographic marking process given above can be modified to
 produce color images. One color electrophotographic marking process,
 called image on image processing, superimposes toner powder images of
 different color toners onto the photoreceptor prior to the transfer of the
 composite toner powder image onto the substrate. While image on image
 process is beneficial, it has several problems. For example, when
 recharging the photoreceptor in preparation for creating another color
 toner powder image, it is important to level the voltages between the
 previously toned and the untoned areas of the photoreceptor.
 In the application of the toner to the latent electrostatic images
 contained on the charge-retentive surface, it is necessary to transport
 the toner from a developer housing to the surface. A basic limitation of
 conventional xerographic development systems, including both magnetic
 brush and single component, is the inability to deliver toner (i.e.
 charged pigment) to the latent images without creating large adhesive
 forces between the toner and the conveyor, which transport the toner to
 latent images. As will be appreciated, large fluctuation (i.e. noise) in
 the adhesive forces that cause the pigment to tenaciously adhere to the
 carrier, severely limit the sensitivity of the developer system, thereby
 necessitating higher contrast voltages forming the images. Accordingly, it
 is desirable to reduce such noise, particularly in connection with latent
 images formed by contrasting voltages.
 Fluidized beds have been used to provide a means for storing, mixing and
 transporting toner in certain single component development systems and
 loading onto developer rolls. Efficient means for fluidizing toner and
 charging the particles within the fluidized bed are disclosed in U.S. Pat.
 No. 4,777,106 and U.S. Pat. No. 5,532,100, which are hereby incorporated
 by reference. In these disclosures, corona devices are embedded in the
 fluidized toner for simultaneous toner charging and deposition onto a
 receiver roll. While the development system as described has been found
 satisfactory in some development applications, it leaves something to be
 desired in the way applications requiring the blending of two or more dry
 powder toners to achieve custom color development. Also, it has been found
 in the above systems that there are frequently disturbances to the flow in
 the fluidized bed associated with charged particles in the high electric
 fields surrounding corona devices immersed in the reservoir. Also, wire
 contamination presents a reliability issue.
 Triboelectric charging (contact electrification) of dry toners is a
 standard method used to electrically charge toner particles for
 development of latent electrostatic images. An alternate method to charge
 toners is via ion bombardment (Ion Charging) which offers many advantages,
 especially in applications to custom color where "in-situ" toner mixing is
 advantageous. Triboelectric charging of colored toners requires different
 additives dependent on toner color to achieve stable charging, whereas ion
 charging of toners offers the advantage of charging toner particles based
 mainly on their size, independent of their intrinsic composition and
 surface structure. Triboelectric charging of toners also can create
 localized patches of charge on the toner particles which can lead to
 strong adhesion of these toners to various surfaces requiring special
 measures to remove them in the development, transfer and cleaning steps in
 the xerographic process. In the ion charging process, charged ions
 bombarding the toner particles are driven by the net field around the
 particles which tends to uniformly charge the toner, helping to decrease
 adhesion of these toners to donor or photoreceptor surfaces. One method to
 charge toner via ion bombardment involves fluidizing the toner and
 charging it using corona generation in close proximity to this fluidized
 bed.
 However, noting the issues above the achievement of high reliability and
 simple, economic manufacturability of the system continue to present
 problems.
 SUMMARY OF THE INVENTION
 Briefly, the present invention obviates the problems noted above by
 utilizing an apparatus for loading and charging toner and developing an
 image. An apparatus for developing a latent image recorded on an imaging
 surface, including a housing, defining a chamber, storing a supply of
 toner; a donor member, spaced from the imaging surface, for transporting
 developer material on the surface thereof, to a region opposed from the
 imaging surface, a transport member for transporting toner from said
 housing and loading toner onto said donor member, said transport member
 further includes a surface with uniformly distributed, high density fringe
 fields, to pickup and transport a toner layer and a corona device to
 ionicly charge the toner on said fringe field surface.
 In accordance with another aspect of this invention, the transport member
 further includes a mask layer adjacent to a photoconductive layer, said
 mask layer having a screen pattern; a charging device for charging said
 photoconductive layer and an exposure device for discharging unmasked
 portions of said transport member thereby generating said fringe field on
 the surface of the transport member to attract toner from said housing to
 the surface of the transport member.

Inasmuch as the art of electrophotographic printing is well known, the
 various processing stations employed in the printing machine will be shown
 hereinafter schematically and their operation described briefly with
 reference thereto.
 DETAILED DESCRIPTION OF THE INVENTION
 Referring initially to FIG. 1, there is shown an illustrative
 electrophotographic machine having incorporated therein the development
 apparatus of the present invention. An electrophotographic printing
 machine creates a color image in a single pass through the machine and
 incorporates the features of the present invention. The printing machine
 uses a charge retentive surface in the form of an Active Matrix (AMAT)
 photoreceptor belt 10 which travels sequentially through various process
 stations in the direction indicated by the arrow 12. Belt travel is
 brought about by mounting the belt about a drive roller 14 and two tension
 rollers 16 and 18 and then rotating the drive roller 14 via a drive motor
 20.
 As the photoreceptor belt moves, each part of it passes through each of the
 subsequently described process stations. For convenience, a single section
 of the photoreceptor belt, referred to as the image area, is identified.
 The image area is that part of the photoreceptor belt which is to receive
 the toner powder images that, after being transferred to a substrate,
 produce the final image. While the photoreceptor belt may have numerous
 image areas, since each image area is processed in the same way, a
 description of the typical processing of one image area suffices to fully
 explain the operation of the printing machine.
 As the photoreceptor belt 10 moves, the image area passes through a
 charging station A. At charging station A, a corona generating device,
 indicated generally by the reference numeral 22, charges the image area to
 a relatively high and substantially uniform potential. FIG. 2 illustrates
 a typical voltage profile 68 of an image area after that image area has
 left the charging station A. As shown, the image area has a uniform
 potential of about -500 volts. In practice, this is accomplished by
 charging the image area slightly more negative than -500 volts so that any
 resulting dark decay reduces the voltage to the desired -500 volts. While
 FIG. 2 shows the image area as being negatively charged, it could be
 positively charged if the charge levels and polarities of the toners,
 recharging devices, photoreceptor, and other relevant regions or devices
 are appropriately charged.
 After passing through the charging station A, the now charged image area
 passes through a first exposure station B. At exposure station B, the
 charged image area is exposed to light which illuminates the image area
 with a light representation of a first color (say black) image. That light
 representation discharges some parts of the image area so as to create an
 electrostatic latent image. While the illustrated embodiment uses a
 laser-based output scanning device 24 as a light source, it is to be
 understood that other light sources, for example an LED printbar, can also
 be used with the principles of the present invention. FIG. 3 shows typical
 voltage levels, the levels 72 and 74, which might exist on the image area
 after exposure. The voltage level 72, about -500 volts, exists on those
 parts of the image area which were not illuminated, while the voltage
 level 74, about -50 volts, exists on those parts which were illuminated.
 Thus after exposure, the image area has a voltage profile comprised of
 relative high and low voltages.
 After passing through the first exposure station B, the now exposed image
 area passes through a first development station C which is identical in
 structure with development station E, G, and I. The first development
 station C deposits a first color, say black, of negatively charged toner
 31 onto the image area. That toner is attracted to the less negative
 sections of the image area and repelled by the more negative sections. The
 result is a first toner powder image on the image area. It should be
 understood that one could also use positively charged toner if the exposed
 and unexposed areas of the photoreceptor are interchanged, or if the
 charging polarity of the photoreceptor is made positive.
 The first development station C includes a donor roll. As illustrated in
 FIG. 8, electrode grid 42 is electrically biased with an AC voltage
 relative to donor roll 40 for the purpose of detaching toner therefrom.
 This detached toner forms a toner powder cloud in the gap between the
 donor roll and photoconductive surface. Both electrode grid 42 and donor
 roll 40 are biased with DC sources 102 and 92 respectively for discharge
 area development (DAD). The discharged photoreceptor image attracts toner
 particles from the toner powder cloud to form a toner powder image
 thereon.
 FIG. 4 shows the voltages on the image area after the image area passes
 through the first development station C. Toner 76 (which generally
 represents any color of toner) adheres to the illuminated image area. This
 causes the voltage in the illuminated area to increase to, for example,
 about -200 volts, as represented by the solid line 78. The unilluminated
 parts of the image area remain at about the level -500.
 Referring back to FIG. 1, after passing through the first development
 station C, the now exposed and toned image area passes to a first
 recharging station D. The recharging station D is comprised of two corona
 recharging devices, a first recharging device 36 and a second recharging
 device 37. These devices act together to recharge the voltage levels of
 both the toned and untoned parts of the image area to a substantially
 uniform level. It is to be understood that power supplies are coupled to
 the first and second recharging devices 36 and 37, and to any grid or
 other voltage control surface associated therewith, so that the necessary
 electrical inputs are available for the recharging devices to accomplish
 their task.
 FIG. 5 shows the voltages on the image area after it passes through the
 first recharging device 36. The first recharging device overcharges the
 image area to more negative levels than that which the image area is to
 have when it leaves the recharging station D. For example, as shown in
 FIG. 5, the toned and the untoned parts of the image area reach a voltage
 level 80 of about -700 volts. The first recharging device 36 is preferably
 a DC scorotron.
 After being recharged by the first recharging device 36, the image area
 passes to the second recharging device 37. Referring now to FIG. 6, the
 second recharging device 37 reduces the voltage of the image area, both
 the untoned parts and the toned parts (represented by toner 76) to a level
 84 which is the desired potential of -500 volts.
 After being recharged at the first recharging station D, the now
 substantially uniformly charged image area with its first toner powder
 image passes to a second exposure station 38. Except for the fact that the
 second exposure station illuminates the image area with a light
 representation of a second color image (say yellow) to create a second
 electrostatic latent image, the second exposure station 38 is the same as
 the first exposure station B. FIG. 7 illustrates the potentials on the
 image area after it passes through the second exposure station. As shown,
 the non-illuminated areas have a potential about -500 as denoted by the
 level 84. However, illuminated areas, both the previously toned areas
 denoted by the toner 76 and the untoned areas are discharged to about -50
 volts as denoted by the level 88.
 The image area then passes to a second development station E. Except for
 the fact that the second development station E contains a toner 40 which
 is of a different color (yellow) than the toner 31 (black) in the first
 development station C, the second development station is substantially the
 same as the first development station. Since the toner 40 is attracted to
 the less negative parts of the image area and repelled by the more
 negative parts, after passing through the second development station E,
 the image area has first and second toner powder images which may overlap.
 The image area then passes to a second recharging station F. The second
 recharging station F has first and second recharging devices, the devices
 51 and 52, respectively, which operate similar to the recharging devices
 36 and 37. Briefly, the first corona recharge device 51 overcharges the
 image areas to a greater absolute potential than that ultimately desired
 (say -700 volts) and the second corona recharging device, comprised of
 coronodes having AC potentials, neutralizes that potential to that
 ultimately desired.
 The now recharged image area then passes through a third exposure station
 53. Except for the fact that the third exposure station illuminates the
 image area with a light representation of a third color image (say
 magenta) so as to create a third electrostatic latent image, the third
 exposure station 38 is the same as the first and second exposure stations
 B and 38. The third electrostatic latent image is then developed using a
 third color of toner 55 (magenta) contained in a third development station
 G.
 The now recharged image area then passes through a third recharging station
 H. The third recharging station includes a pair of corona recharge devices
 61 and 62 which adjust the voltage level of both the toned and untoned
 parts of the image area to a substantially uniform level in a manner
 similar to the corona recharging devices 36 and 37 and recharging devices
 51 and 52.
 After passing through the third recharging station the now recharged image
 area then passes through a fourth exposure station 63. Except for the fact
 that the fourth exposure station illuminates the image area with a light
 representation of a fourth color image (say cyan) so as to create a fourth
 electrostatic latent image, the fourth exposure station 63 is the same as
 the first, second, and third exposure stations, the exposure stations B,
 38, and 53, respectively. The fourth electrostatic latent image is then
 developed using a fourth color toner 65 (cyan) contained in a fourth
 development station I.
 To condition the toner for effective transfer to a substrate, the image
 area then passes to a pretransfer corotron member 50 which delivers corona
 charge to ensure that the toner particles are of the required charge level
 so as to ensure proper subsequent transfer.
 After passing the corotron member 50, the four toner powder images are
 transferred from the image area onto a support sheet 57 at transfer
 station J. It is to be understood that the support sheet is advanced to
 the transfer station in the direction 58 by a conventional sheet feeding
 apparatus which is not shown. The transfer station J includes a transfer
 corona device 54 which sprays positive ions onto the backside of sheet 57.
 This causes the negatively charged toner powder images to move onto the
 support sheet 57. The transfer station J also includes a detack corona
 device 56 which facilitates the removal of the support sheet 57 from the
 printing machine.
 After transfer, the support sheet 57 moves onto a conveyor (not shown)
 which advances that sheet to a fusing station K. The fusing station K
 includes a fuser assembly, indicated generally by the reference numeral
 60, which permanently affixes the transferred powder image to the support
 sheet 57. Preferably, the fuser assembly 60 includes a heated fuser roller
 67 and a backup or pressure roller 64. When the support sheet 57 passes
 between the fuser roller 67 and the backup roller 64 the toner powder is
 permanently affixed to the support sheet 57. After fusing, a chute, not
 shown, guides the support sheet 57 to a catch tray, also not shown, for
 removal by an operator.
 After the support sheet 57 has separated from the photoreceptor belt 10,
 residual toner particles on the image area are removed at cleaning station
 L via a cleaning brush contained in a housing 66. The image area is then
 ready to begin a new marking cycle.
 The various machine functions described above are generally managed and
 regulated by a controller which provides electrical command signals for
 controlling the operations described above.
 Referring now to FIG. 8 in greater detail, development station 38 includes
 a donor roll 40. A development apparatus advances developer materials into
 development zones. The development station 38 is scavengeless. By
 scavengeless is meant that the developer or toner of station 38 must not
 interact with an image already formed on the image receiver. Thus, the
 station 38 is also known as a non-interactive development station. The
 development station 38 comprises a donor structure in the form of a roll
 40. The donor structure 40 conveys a toner layer to the development zone
 which is the area between the photoreceptor belt 10 and the donor
 structure 40. The toner layer 82 can be formed on the donor structure 40
 by either a two-component developer (i.e. toner and carrier), as shown in
 FIG. 8, or a single-component developer deposited on donor structure 40
 via a combination single-component toner metering and charging device. The
 development zone contains an AC biased electrode grid 90 self-spaced from
 the donor roll 40 by the toner layer. The single-component toner may
 comprise positively or negatively charged toner. The electrode grid 42 may
 be coated with TEFLON-S (trademark of E. I. DuPont De Nemours) loaded with
 carbon black.
 For donor roll, loading with developer material is accomplished by the
 present invention. Transport member 200 in FIG. 8 is shown as a belt or
 can take the form of a drum as shown in FIG. 9. The transport member is
 charged by charging device 204. Transport member 200 as shown in FIG. 10
 is a schematic Active Matrix (AMAT) photoreceptor belt having a mask
 screen pattern 203 therein. Mask screen pattern 203 can be a fine line
 pattern or a random pattern. The transport member can take form as a belt
 entrained about roller 212 and 214 as shown in FIG. 8 or a drum as shown
 in FIG. 9.
 Transport member 200 is then exposed by a flood exposure device 215 which
 creates voltage differences on the surface of the transport member
 resulting in a fringe electric field pattern being generated on the
 surface of the transport member 200. The flood exposure can be from the
 inside of the belt or outside the belt. Preferably, the resultant fringe
 electric field pattern has a contrast greater than 100 Volts. Transport
 member 200 travels through a toner bed 300. The toner bed can be fluidized
 if desired by an air plenum (not shown). Toner is attracted to the surface
 of transport member 200 by the fringe field and a toner layer is formed
 thereon. Preferably, the toner layer has a thickness between 10 to 200
 .quadrature.m. Thereafter, the toner is ion charged by charging device
 210. Preferably, the toner layer is charged to about 10 to 100
 .quadrature.C/g. Next, the layer of toner is transferred to donor roll 40.
 Let's focus on the toner pickup and toner layer formation process. Fringe
 lines are the boundaries between areas with high and low voltages. Fringe
 fields exist along the fringe lines, The fields originate from the high
 voltage side of the fringe line and end on the low voltage side. The
 fringe fields are typically very strong due to a high voltage drop across
 a short distance and is spatially localized only in the neighborhood of
 the fringe lines. Because of its bi-directional nature (fields are in
 opposite directions on the high/low voltage sides), it can pickup toner of
 both polarities. Also due to its strong spatial gradient (fields decay
 away from the fringe area quickly), it can also pickup neutral toner due
 to polarization effect. According to the present invention, a surface with
 uniformly distributed high density fringe lines is used to load toner
 materials. The density of the boundary lines should be greater than 5
 mm/mm.sup.2 for uniform and effective toner loading. In a further
 preferred embodiment, the density is higher than 20 mm/mm.sup.2. The
 pattern can be regular patterns such as lines and dots or randomly but
 uniformly distributed fringe lines.
 An advantageous feature of the present invention is that the fringe fields
 can pick up both charge toner and neutral toner. Further, the charging
 characteristics of the transport member can be modified so that the
 desired rate (i.e. rotation speed of transport member) of loading of donor
 roll 40 and discharging rate of transport member 200 can be matched so
 that the fringe fields are minimized or zero at reload zone 220.
 Other embodiments of transport member 200 could take the form wherein the
 fringe field patterns are burned in the fringe field surface structure. As
 one preferred embodiment, the surface layer of the transport member has a
 fixed spatial conductivity pattern which consists of more conductive and
 less conductive regions so that when the transport member is charged, the
 more conductive regions discharge soon thereafter thereby causing a fringe
 field pattern to be generated on the surface of the transport member. The
 more conductive region can be conductive or semiconductive and the less
 conductive region can be semiconductive or insulative. Alternatively, the
 transport member can be made by doping of a photoconductive layer to
 create a conductivity pattern so that when the transport member is
 charged, the more conductive pattern discharges to a lower potential
 thereby causing fringe fields to be generated on the surface of the
 transport member.
 Other embodiments and modifications of the present invention may occur to
 those skilled in the art subsequent to a review of the information
 presented herein; these embodiments and modifications, as well as
 equivalents thereof, are also included within the scope of this invention.