Vacuum Assisted Bead Pick Off Apparatus Employing a Plural Level Surface-Hybrid Air Knife

This invention relates generally to a bead pick off device of a development system for an electrophotographic printing machine adapted to produce highlight color and monochromatic copies, and more particularly concerns a bead pick off apparatus for removing ferromagnetic carrier beads and non ferromagnetic particles from a photoconductive surface delivered by a developer unit prior to transfer to a copy sheet.

The relevant parts of U.S. Pat. No. 4,829,338 and U.S. Pat. No. 4,811,046 are incorporated herein by reference.

The features of the present invention are useful in the printing arts and more particularly in electrophotographic printing. In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is image wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image in a region known as the development zone. Toner particles are attracted from beads of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a copy sheet and permanently affixed thereto. The foregoing generally describes a typical mono-color electrophotographic copying machine.

Recently, electrophotographic printing machines have been developed which produce highlight color copies. A typical highlight color printing machine records successive electrostatic latent images on the photoconductive surface. When combined, these electrostatic latent images form a total latent image corresponding to the entire original document being reproduced. Thus, one latent image is usually developed with black toner particles. The other latent image is developed with color highlighting toner particles, e.g. red toner particles. These developed toner images are transferred in registration to the copy sheet to form the color highlighted copy.

In tri-level electrophotographic printing, the charge on the photoconductive surface is divided in three, rather than two, ways as is the case in mono-color printing. The photoconductive surface is charged, typically to about 900 volts. It is exposed image wise, such that one image corresponding to charged image areas remains at the full potential of 900 volts. The other image, which corresponds to discharged image areas is exposed to discharge the photoconductive surface to its residual potential of typically about 100 volts. The background areas are exposed to reduce the photoconductive surface potential to about halfway between the charged and discharged potentials, (typically about 500 volts). The developer unit arranged to develop the charged image areas, is typically biased to about 600 volts, and the developer unit, arranged to develop the discharged image areas, is biased to about 400 volts.

The single pass nature of this system dictates that the electrostatic latent image pass through the developer units in a serial fashion. The electrostatic forces and adhesion forces within the developer units and between the photoconductive surface contribute to a condition where the beads are carried out of the development unit. Bead removal devices (BRDs) are well known and commonly used to pick off any beads which are carried out of the development zone of a development unit.

Generally BRDs operate by generating a strong magnetic field in the area between the photoconductive surface and the BRD to attract free beads to the shell of the BRD. The beads are typically made from ferromagnetic material, while the toner in the developed image is made from a non-ferro-magnetic material. The BRD can generally remove the magnetic beads, while not disturbing the latent toner image on the photoreceptor belt. These captured beads are then deposited in a sump or developer receiver as the shell of the BRD is rotated. This arrangement, however, renders release of beads from the BRD more difficult, e.g. gravity and centripetal forces often are insufficient to achieve release of the beads from the magnetic field as the shell of the BRD rotates. That is the strong magnetic field necessary to attract beads from the photoconductive surface to the BRD shell are sufficiently strong around the shell itself to retain some beads as the shell rotates.

As disclosed in U.S. Pat. No. 4,829,338, the magnetic field from the magnet positioned in the shell of the BRD can be directed by use of a ferromagnetic shunt to promote bead removal from the photoconductive surface while enhancing the field between the photoconductive surface and the BRD to attract free beads. Nevertheless, this solution has not overcome all problems associated with BRDs.

For example, a condition associated with BRD rollers is that during ordinary operation of the BRD rollers some beads fail to move with the circumference of the BRD rollers and tend to ride or "walk" over the shell of the roller such that these beads remain in a substantially constant position as the roller rotates under them. Generally, the attraction of gravity and the magnetic force are sufficient to hold most beads to the roller for movement to the release zone. However, in many cases, particularly on longer print jobs, not all beads are transported to the release area. In this case, bead groups form along the surface of the BRD roller and do not move to the release area.

It is believed that this "walking" phenomena is due to the magnetic fields generated along the surface of the roller which are greatest in the pick off zones of the BRD roller and lessen around the circumference. Thus, as the roller rotates, beads attracted to the BRD at the pick off zone are magnetically urged back toward the pick off zone so that they remain in a relatively constant position. In fact, over time this can involve a substantial number of beads.

FIGS. 2, 3, and 4, although not entirely identical, illustrate aspects of the problem. In essence, as in FIG. 2, the magnetic field generated by magnet 12 along the surface of roller 13 is tangent along the roller in the area 14. As better seen in FIG. 3 and FIG. 4, beads tend to conglomerate in the area 14. As further demonstrated in FIG. 4, the relatively stationary beads in the area 14 tend to disturb toner 16 riding on the surface of the BRD 17 to form a toner cloud. Some of the toner in the cloud has been found to settle on the moving photoconductive surface. This settled toner can cause a print defect, referenced hereinafter as a "smudge", which appears as a background colorant in a localized area of a printed copy sheet. Generally, smudges tend to vary in size but are generally 3-4 mm long by 2 mm wide. In any event, a smudge is a defect which impacts particularly upon high quality graphic printing. Thus, controlling smudging of images is critical to the utility of printing machines in many applications. Thus, the beads build up along the surface of the BRD roller and the smudging of the printed copy sheets in many cases are related.

Various techniques have heretobefore been used to develop electrostatic latent images as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention:

U.S. Pat. No. 4,292,924 Patentee: Lindblad et al. Issued: Oct. 6, 1981

U.S. Pat. No. 4,466,730 Patentee: Jugle, et al. Issued: Aug. 21, 1984

Co-pending U.S. application Ser. No. 07/757,093, now U.S. Pat. No. 5,225,880, Applicant: Shehata et al. Filed: Sep. 10, 1991

Co-pending U.S. application Ser. No. 07/942,866, now U.S. Pat. No. 5,283,617, Applicant: Benedict et al. Filed: Sep. 10, 1992

Co-pending U.S. application Ser. No. 07/814,171, now U.S. Pat. No. 5,280,323, Applicant: Alvarez et al. Filed: Dec. 30, 1991

The relevant portions of the foregoing patents may be briefly summarized as follows:

U.S. Pat. No. 4,292,924 discloses a magnetic brush apparatus for cleaning photoconductive surfaces and suggests its application to magnetic brush development mechanisms. The disclosure device has a series of magnetics disposed within a rotating cylindrical sleeve disposed within a cleaning housing. The cleaning housing has an aperture which is proximate a photoconductive surface so that the surface is cleaned as it passes the aperture. The sleeve is provided with field shaping devices at the extreme ends of the cleaning roller so that developer material attracted to the sleeve does not build at the extreme ends of the cylindrical sleeve.

U.S. Pat. No. 4,466,730 discloses a magnetic brush developer unit having a housing in which a magnet is disposed within a rotating tubular member for transporting beads with toner adhering thereto to a photoconductive surface. The area adjacent the tubular member and within the housing has a negative air pressure as air is withdrawn from the housing so that airborne toner and toner on the metering blade are attracted to the bottom of the housing.

U.S. Ser. No. 07/757,093 also discloses a system for removing agglomerates from a developed image on a photoreceptor in an electrophotographic printing machine. The disclosed apparatus uses a vacuum source to draw the agglomerates from the developed image.

U.S. Ser. No. 07/814,171 discloses a development system which includes an apparatus for removing beads from a photoconductive surface. The disclosed apparatus includes magnets which attract the beads to a bead removal roller and magnetic shunts to shape the generated fields.

U.S. Ser. No. 07/942,866 discloses a development system which includes a similar apparatus of that disclosed by U.S. Ser. No. 07/814,171, which further includes a vacuum source to generate an overflow to attract beads from the photoconductive surface and to the BRD. The airflow also aides in the cleaning of the BRD roller.

In accordance with one aspect of the present invention, there is provided an apparatus for removing ferro-magnetic carrier beads from a moving imaging surface in a region between a developer station and a transfer station. The apparatus includes a member, interposed between the developer station and the transfer station. The member has a plurality of grooves in the exterior surface thereof. Each of the grooves have a width greater than each portion of the member between adjacent grooves. The apparatus also has a magnetic member operatively associated with the member. The magnetic member generates a magnetic field to attract the magnetic granules from the imaging surface to the member. The member and the magnetic member are adapted to move relatively to one another.

In accordance with another aspect of the present invention, there is provided an apparatus for removing ferro-magnetic particles from a moving imaging surface in a region between a developer station and a transfer station. The apparatus comprises a member, interposed between the developer station and the transfer station, which defines a plurality of grooves in the exterior surface thereof. The apparatus also comprises a housing defining an aperture proximate the moving imaging surface. The housing defines a cavity which has a shape substantially larger than and generally conforming to the member. The housing and the member define an air passageway therebetween. An air flow generator coupled to the housing is included for generating an air flow through the air passageway. The particles are attracted toward the aperture. The apparatus also includes means for attracting the particles from the imaging surface to the member.

Other features of the present invention will become apparent as the following description proceeds and upon reference to the drawings, in which:

FIG. 1 is an elevational view, partially in section, of a bead removal device of an embodiment of the present invention;

FIG. 2 is an elevational view of a prior art bead removal device;

FIG. 3 is an elevational view, partially in section, of another prior art bead removal device incorporating islands;

FIG. 4 is an elevational view, partially in section, of another prior art bead removal device with toner adhering to the device;

FIG. 5 is a schematic elevational view of an illustrative electrophotographic printing machine incorporating the developer units and bead removal device of the present invention therein;

FIGS. 6 is an elevational view, partially in section, of an embodiment of a developer unit of the FIG. 5 electrophotographic printing machine;

FIG. 7 is an enlarged portion of the BRD sleeve, partially in section, of the FIG. 6 developer unit;

FIG. 8 is an elevational view, partially in section, of another embodiment of the developer unit;

FIG. 9 is an elevational view, partially in section, of a bead removal device of another embodiment of the present invention and a portion of the printing machine of FIG. 5; and

FIG. 10 is an elevational view, partially in section, of a bead removal device of another embodiment of the present invention.

While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

For a general understanding of the illustrative electrophotographic printing machine incorporating the features of the present invention therein, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. FIG. 5 schematically depicts the various components of an electrophotographic printing machine incorporating the developer units of the present invention therein. Although the developer units of the present invention are particularly well adapted for use in the illustrative printing machine, it will become evident that these developer units are equally well suited for use in a wide variety of printing machines and are not necessarily limited in their application to the particular embodiments shown herein.

Referring now to FIG. 5, the electrophotographic printing machine employs a belt 18, i.e., a charge retentive member, having a photoconductive surface deposited on a conductive substrate. Belt 18 moves in the direction of arrow 19 to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. Belt 18 is entrained about drive roller 21, tensioning roller 20 and stripping roller 22. Motor 23 rotates roller 21 to advance belt 18 in the direction of arrow 19. Roller 21 is coupled to motor 23 by suitable means such as a belt drive.

Initially successive portions of belt 18 pass through charging station A. At charging station A, a corona generating device, indicated generally by the reference numeral 24, charges the belt 18 to a selectively high uniform electrical potential, preferably negative. Any suitable control, well known in the art, may be employed for controlling the corona generating device 24.

Next, the charged portions of the photoconductive surface are advanced through exposure station B. At exposure station B, the uniformly charged photoconductive surface or charge retentive surface is exposed to a laser based raster output scanning device 25 which causes the charge retentive surface to be selectively discharged in accordance with the output from the scanning device 25. Preferably, the scanning device is a three level laser Raster Output Scanner (ROS). The output scanning device 25 is driven by an input signal from an electronic subsystem (ESS) 27, which would serve as the interface between the device 25 and an input signal generator (not shown). Thus, in this embodiment, the photoconductive surface, which is initially charged to a high charge potential, is discharged image wise in the background (white) image areas and to near zero or ground potential in the highlight (i.e. color other than black) color parts of the image.

At development station C, a magnetic brush development system, indicated generally by the reference numeral 30 advances developer materials into contact with the electrostatic latent images. The development system 30 comprises first and second developer units 32 and 34. Preferably, each magnetic brush developer units includes a pair of magnetic brush developer rollers mounted in a housing. Thus, developer unit 32 contains a pair of magnetic brush rollers 35, 36 with developer unit 34 containing a pair of magnetic brush rollers 37, 38. Each pair of rollers advances its respective developer material into contact with the latent image. Appropriate developer biasing is accomplished via first and second power supplies 41 and 43, respectively, electrically connected to respective developer units 32 and 34.

Color discrimination in the development of the electrostatic latent image is achieved by moving the latent image recorded on the photoconductive surface past two developer units 32 and 34 in a single pass with the magnetic brush rolls 35, 36, 37 and 38 electrically biased to voltages which are offset from the background voltage, the direction of offset depending on the polarity of toner in the housing. The first developer unit 32, in the direction of movement of belt 18 as indicated by arrow 19, develops the discharged image areas of the photoconductive surface. This developer unit contains, for example, red developer material 40 having triboelectric properties such that the red toner is driven to the discharged image areas of the latent image by the electrostatic field between the photoconductive surface and the electrically biased developer rolls, which are electrically connected to the first bias power supply 41. Conversely, the second developer unit 34, in the direction of movement of belt 18 as indicated by arrow 19, develops the highly charged image areas of the latent image. This developer unit contains black developer, for example, material 42 having a triboelectric charge such that the black toner is urged towards highly charged areas of the latent image by the electrostatic field existing between the photoconductive surface and the electrically biased developer rolls in the second developer unit which are connected to the second bias power supply 43. Further, the first and second developer units 32 and 34 have bead removal devices 44 and 46 disposed therein and augers 47 for mixing and charging the developer material. Because the composite image developed on the photoreceptor consists of both positive and negative toner, a negative pre-transfer corona generating device 56 is provided to condition the toner for effective transfer of a developed toner image to a substrate using positive corona discharge.

A sheet of support material 58 is moved into contact with the toner image at transfer station D. The sheet of support material is advanced to transfer station D by conventional sheet feeding apparatus, not shown. Preferably, the sheet feeding apparatus includes a feed roll contacting the uppermost sheet of a stack of copy sheets. Feed rolls rotate so as to advance the uppermost sheet from the stack into a chute which directs the advancing sheet of support material into contact with the photoconductive surface of belt 18 in a timed sequence so that the toner powder image developed thereon contacts the advancing sheet of support material at transfer station D.

Transfer station D includes a corona generating device 60 which sprays ions of a suitable polarity onto the backside of sheet 58. This attracts substantially simultaneously the black and non-black portions of the toner powder image from the belt 18 to sheet 58. After transfer, the sheet continues to move, in the direction of arrow 62, onto a conveyor (not shown) which advances the sheet to fusing station E.

Fusing station E includes a fuser assembly, indicated generally by the reference numeral 64, which permanently affixes the transferred powder image to sheet 58. Preferably, fuser assembly 64 comprises a heated fuser roller 66 and a pressure roller 68. Sheet 58 passes between fuser roller 66 and pressure roller 68 with the toner powder image contacting fuser roller 66. In this manner, the toner powder image is permanently affixed to sheet 58. After fusing, a chute, not shown, guides the advancing sheet 58 to a catch tray, also not shown, for subsequent removal from the printing machine by the operator. It will also be understood that other post-fusing operations can be included, for example, binding, inverting and returning the sheet for duplexing and the like.

After the sheet of support material is separated from the photoconductive surface of belt 18, the residual toner particles carried by image and the non-image areas on the photoconductive surface are charged to a suitable polarity and level by a preclean charging device 72 to enable removal therefrom. These particles are removed at cleaning station F. The vacuum assisted, electrostatic, fur brush cleaner unit 70 is disposed at the cleaner station F. The cleaner unit has two fur brush rolls that rotate at relatively high speeds which creates mechanical forces that tend to sweep the residual toner particles into an air stream (provided by a vacuum source), and then into a waste container. Subsequent to cleaning, a discharge lamp or corona generating device (not shown) dissipates any residual electrostatic charge remaining prior to the charging thereof for the next successive imaging cycle.

It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an electrophotographic printing machine incorporating the development apparatus of the present invention therein.

The embodiments of the present invention illustrated within this application will now be further discussed with reference to FIG. 6 which shows the second developer unit 34. Developer rollers 37 and 38 of the developer unit 34 are substantially identical to one another so only developer roller 37 will be described.

Developer roller 37 advances, for example, the black developer material 42, including both toner and beads, so that the toner will develop the electrostatic latent image recorded on the photoconductive surface of belt 18. Developer roller 37 is electrically biased so that the highly charged image areas of the latent image attract toner thereto. Developer roller 37 includes a non-magnetic tubular member or sleeve 80 preferably made from aluminum. Sleeve 80 generally rotates in the direction of arrow 82. A magnet assembly 84 is mounted interiorly of sleeve 80 and spaced therefrom. Magnet assembly 84 is stationary and positioned to attract the developer material to the lower exterior circumferential surface of sleeve 80. In this way, as sleeve 80 rotates in the direction of arrow 82, beads are attracted to the exterior circumferential surface by the pick-up magnet 90. Thus, beads with toner triboelectrically attracted thereto are magnetically attracted to the sleeve 80 which rotates to mechanically transport developer material (beads and toner) into the development zone 92.

As the beads and remaining toner pass the development zone, the beads carryout pole or magnet 102 acts to strongly attract the beads to the sleeve 80. But, as the roller continues to rotate, the beads reach an area 104, where the gravitational force and the centrifugal forces generally overcome the magnetic attraction holding the beads to the roller. This area 104 is often referenced as the bead release zone or point. The bead release zone is further enhanced by the use of a shunt 106 which attenuates the generated magnetic field in the release zone. The magnetic units 84 within the sleeves 80 can be formed in a variety of manners including, by way of example, that disclosed in U.S. Pat. Nos. 4,517,719 and 4,640,808.

Continuing to reference FIG. 6, the second developer unit 34 is shown with a BRD (bead removal device) 46 having a sleeve 120. The sleeve is formed with a plural level exterior surface comprising raised portions forming islands 130 separated by lowered portions forming troughs 132 (see FIG. 7). The islands 130 and troughs 132 are connected by generally extending radial portions 134 which extend generally radially from the troughs 132. It is preferred that the bead removal device 46 generally rotates in the direction indicate by arrows 149 (i.e., opposite the direction of the developer rollers 37 and 38). Magnet 150 is disposed in a fixed relationship with the belt 18 so that it is stationary within rotating sleeve 120. Shunt 155 is likewise disposed within sleeve 120 to shape the magnetic fields generated by the magnet 150 in selected directions. Further, shunt 159 is positioned in conjunction with the magnet 150 and shunt 155 so the magnetic field from the magnet is minimized along the sleeve 120 in all but those directions generally radiating toward the belt 18.

Applicants have found that the provision of the plural surface sleeve 120 eliminates substantially all smudging, when the arc defined by each of the islands 130 are substantially smaller than the arc defined by each of the troughs 132. Additionally, Applicants have found that the height of the radial portions 134 are also significant to the performance of the present invention. For example, in the BRD reduced to practice, for trapping beads of approximately 0.1 mm, the radius of the sleeve 120 as measured from the surface of the islands 130 to the center of the sleeve 120 ranges between approximately 12 mm and 17 mm with 15 mm being preferred. The radial extensions 134 range between approximately 0.45 mm and 0.70 mm high with 0.55 mm being preferred. In this embodiment, there are approximately thirty seven of the raised portions 130 with each defining an arc between approximately 1.5 and 1.6 degrees or approximately 0.4 mm wide in the preferred BRD construction with a 15 mm radius. Thus, there are also thirty seven of the trough portions 132 which define an arc between approximately 8.10 degrees and 8.25 degrees, of the type illustrated in FIG. 7. As also illustrated by FIG. 7, the radial extensions 134 are preferred to be substantially perpendicular to the surface of the troughs 132 and the islands 130 between which they extend.

Applicants believe that an arc defined by one of the troughs 132 should be at least four times larger than the arc defined by the islands 130. Additionally, Applicants further believe that the radial extensions 134 connecting the troughs 132 and the islands 130 extend at an angle no greater than approximately 105.degree. with respect to the trough surface at their intersection and no less than 75.degree.. This is because obtuse angles encourage the "walking" phenomena previously discussed and acute angles will not capture beads as well as more open angles and may trap beads.

In any event, the BRD 46 operates in an improved fashion over those of the prior art, since the BRD now functions to prevent bead buildup along the surface of the sleeve 120. In this regard, the raised portions due to their height relative to the troughs inhibit the buildup by urging beads through the tangential magnet fields. Further, beads tending to "walk" while the surface of one of the islands will fall into the adjacent trough for transport by the rotation of the sleeve 120 to the release zone.

While, in most applications, the BRD 46 will perform sufficiently as previously described, it may become advantageous to supplement the BRD 46 with an air flow device to draw beads away from the belt 18 and toward the BRD 46. Further, while ferro-magnetic carrier beads 15 are attracted by magnetic fields, non-ferro-magnetic particles and contaminants (designated 157) are not so attracted. The air flow device serves to remove these items 157 from the belt 18. The BRD is located downstream of the developer rolls 38 and 39 which have formed the developed image on the photoreceptor belt 18. The BRD 46, when an air flow device is added, must be so designed to remove non-ferromagnetic particles 157 while avoiding disturbing the toner forming the latent image. A vacuum pump 161 may be used to generate such an air flow.

For example, as shown in FIG. 6, a vacuum port 160, may be positioned in the wall of the housing of the development unit 34. The port 160 is connected to the vacuum pump 161 via a conduit 162 to urge release of materials 15 retained on the sleeve 120. These materials may either be permitted to pass through an opening 169 (shown in phantom) in wall 170 onto auger 47 or to be deposited in a sump 163. If the materials are not permitted to pass through opening 169 in wall 170, a second port 165 is also positioned in the housing to allow materials not passing through the vacuum port 160 to sump 163 to pass to the sump 166 through conduit 167. It will be recognized that in many applications the sumps 163 and 166 can be combined into a single sump so that materials passing through ports 160 and 165 ultimately end in the single sump and/or that both ports 160 and 165 could be connected to the vacuum pump 161.

To assure proper air flow around the sleeve 120 the housing of the development unit 34 surrounding the sleeve 120 needs to closely conform to the sleeve to assist the proper flow of the air. It may be advantageous to position the sleeve eccentrically in the opening of the housing to provide for directional air flow.

The shunts 155 and 159 of BRD 46 are preferably formed of a ferromagnetic material (e.g. cold rolled steel with a thickness greater than approximately 1 mm) with a preferred thickness of between approximately 1.5 mm and 1.6 mm. Generally, Applicants have found that the shunt 159 should be formed of cold rolled steel with a permeability of greater than 1,000 gauss. Applicants have also found that the forming of the shunt 159, as arc shunts with a radius proportional to the BRD's shell provides greater effectiveness, although ferromagnetic strips have also been employed. The magnet 150 is a preferable so-called rare earth magnet. The magnet and shunts may be held in place by foam in a fashion similar to that discussed with magnetic assembly 84. With the disclosed arrangement of shunts and magnets, beads will be attracted to the BRD from the belt 18 and, then due to the rotation of the sleeve 120 and the mechanical forces acting on such beads, moved to the release zone for deposit in a sump or pass through opening 169 to be returned to the reservoir of developer material 42.

Applicants have also discovered that in some instances application of an AC or DC voltage from an AC or DC power source 168 to the sleeve 120 of the bead removal device 46 increases the efficiency of such devices. Applicants have found that the voltage applied should be between 1000 and 3000 volts peak to peak AC or between 500 and 1500 volts DC and, preferably, approximately 2000 volts of a low amperage AC voltage or 1000 volts DC. Nevertheless, Applicants have also found that application of such a voltage to the BRD sleeve 120, which in this case is electrically isolated from housing 172 of the developer unit 34, is of limited value. In some applications, over time, the buildup of material including toner along the sleeve may foul it and minimize its effectiveness. Thus, Applicants have also found it may be appropriate to include a vacuum means as embodied in the port 160 and vacuum pump 161 to assist removal of developer material including beads from the housing.

Further, as will be appreciated, except for the portions of the sleeve proximate the photoconductive surface and the portions immediately adjacent thereto the magnetic field strength along the bead removal device's sleeve 120 is minimized by the disclosed arrangement. The optimal arrangement is dependent, in part, upon the characteristics of the developer material and ambient conditions. However, as indicated, the disclosed plural surface sleeve overcomes the problems associated with failure of prior arrangements to move substantially all beads to the bead release area.

Referring to FIG. 6, the structure and function of the present invention are illustrated in greater detail. The beads 15 are attracted to the bead pick off zone and then fall into trough areas where they are held to the sleeve as it rotates toward the release area and are inhibited from walking over the raised portions due to the depth of the trough and the relatively perpendicular nature of the radially extending portions.

Applicants have further found that overall performance of the BRD may be improved, if the captured beads are directed to a sump for removal rather than returned to the developer material in the developer housing 172 for reuse. It appears that recirculation of the beads captured by the BRD into the developer housing tend to be prone to carryout. Thus, in the preferred embodiment, the sump means as embodied in sump 163 and 166 is provided for the developer material captured by the BRD, including particularly beads. Nevertheless, satisfactory results have also been achieved with a recirculating system.

FIG. 8 is another embodiment of the developer unit in accordance with the present invention. Identical portions to FIG. 6 are commonly numbered for ease of understanding. For example, the developer rollers 37 and 38 of FIG. 8 are identical to those of FIG. 6. Bead removal devices 246 as shown with the developer unit 34 of FIG. 8 is not, however, identical to the device 46 of FIG. 6. Nevertheless, similar to the embodiment of FIG. 5, the plural level sleeve 220 is similarly constructed to that of FIG. 7 and, the BRD 246 is rotationally driven by a motor (not shown), preferably in the opposite rotational direction as the rotation of the developer rollers 37 and 38 as indicated by arrow 249. The magnetic field generator of the BRD 246, however, comprises two magnets, magnets 250 and 251, disposed in a fixed relationship with the belt 18. The magnets 250 and 251 are stationary within the plural level sleeve 220. Shunt 255 is disposed within the sleeve 220 to shape the magnetic fields generated by the magnets 250 and 251 in selected directions. Further, shunt 257 is positioned in conjunction with the magnets 250 and 251 and shunt 255 so the magnetic field from the magnets is attenuated in all but those directions generally radiating toward the belt 18. The plural level sleeve 220 is substantially identical to the sleeve 120 of FIG. 6 and operates to move beads in substantially identical fashion.

Shunts 255 and 257 of the bead removal devices 246 are preferably formed of a ferromagnetic material (e.g., low carbon cold rolled steel with a thickness greater than 1 mm and preferably approximately 1.6 mm thick). The permeability of the material used is approximately 1000 gauss and preferably between 180 and 2000 gauss. The magnets 250 and 251 are of substantially opposite polarity relative to their positioning within the sleeve 220. The magnets are preferably a Neodymium Iron Boron alloy (NIB) available from Delco-Remy (a division of General Motors Corporation). It will be appreciated that other materials can be used to accomplish substantially the same results. The magnets and shunts may be held in place by foam in a fashion similar to that discussed with magnetic unit 84, previously.

As stated above, the magnets 250 and 251 are arranged in approximate opposite polarity orientation. That is, one magnet has a north pole of approximately opposite orientation of the north pole of the other so that the magnets 250 and 251 do not repel but rather attract each other. Applicants have found that the maximum force across the nip between the shell and the surface is yielded by magnetic vectors of the two magnets being offset between approximately 140.degree. to 100.degree. and preferably 120.degree.. (See FIG. 8, wherein the north poles of magnet 250 and 251 are at 53.degree. and 293.degree., respectively, so that in the example only 60.degree. separate the opposite poles adjacent the belt 18.) Further, it has been found that magnetic fields radiating in directions away from the belt 18 in the shown configuration are minimized when the walls 258 and 259 of magnets 250 and 251, respectively, are formed approximately parallel to the magnetic polar axis of the magnets 250 and 251, respectively.

In some cases, arc shunt 257 should extend in a direction further toward the photoconductive belt 18. It has been found that, due to the proximity of the developer rollers 37 and 38 to bead removal device 246 the magnetic fields between them can inhibit and affect their respective functions. Generally, in such cases the shunt 257 would extend in an arc between 5.degree. and 20.degree., as shown by phantom section 265. This and the shaped walls 258 and 259 substantially eliminate magnetic interference or cross talk between the bead removal device and developer roller. Additionally, in some instances, application of an AC or DC voltage to the sleeve 220 from an AC or DC power source 268 substantially increases the bead removal efficiency of the bead removal device 246. Applicants have found that the voltage applied should be between 1000 and 3000 volts peak to peak AC or 500 and 1500 volts DC and, preferably, approximately 2000 volts of a low amperage AC voltage or 1000 volts of a DC voltage. A vacuum pump may also be provided to assist release of beads and toner particles from the sleeve 220. A vacuum port 260 connected to a vacuum pump 261 via conduit 262 urges developer material from the sleeve to a sump 263. Alternately, as described earlier with FIG. 6, the beads 15 may pass through a hole 269 (shown in phantom) in wall 270 into development housing 172.

Where the vacuum port 160--(FIG. 6) and vacuum port 260 (FIG. 8) are employed, it is preferred that the ports are between 0.25 and 1.50 mm away from the sleeve 120 and sleeve 220 of the BRD 46 and 246 in FIGS. 6 and 8, respectively, with 0.75 mm being preferred. Further, Applicants have used a flow rate of approximately 30 liters/sec into a nozzle which is 2 mm wide and approximately 360 mm long. An appropriate flow rate is believed to be between 15 and 90 liters/sec into a nozzle between 0.75 and 4.00 mm wide. To avoid the air flow affecting the development of the image on the belt 18, the vacuum port is disposed adjacent the sleeve and the air passage around the sleeve adjacent the belt 18 is substantially open.

While the bead removal devices 46 and 246 as shown in FIGS. 6 and 8, respectively, may be in many cases sufficient for effective bead removal, in certain applications, involving very high speed belt movement, beads may still remain on belt 18 after the belt 18 passes by development unit 34. In such applications, in addition to BRD 46 or BRD 246, an additional device is necessary to assure complete bead removal.

Another embodiment of the present invention utilizing this additional device is shown in FIG. 9. Specifically, referring to FIG. 5, a magnetic air bead pick off device 300 is shown positioned between a developer unit 34 and a transfer station D. It is believed that use of such a device improves efficient operation of an electrophotographic printing machine, including, particularly, by substantially reducing the number of beads carrying through to the transfer station and thus, substantially reduces bead deletions being visible in produced copies.

Again referring to FIG. 5, preferably the pick off device is positioned between the negative pre-transfer corona generating device 56 and the transfer station D. None the less the device 300 may also be effective between the developer unit 34 and the corona generating device 56. When positioned between the pre-transfer corona device 56 and the transfer station D, the charging of the developed image is critical. The charging is critical because the charging effects three critical portions of the xerographic process, cleaning, transfer and deletion, each with conflicting requirements. A high current charging of approximately 130 .mu.A assist the transfer of the latent image at the transfer station D. An intermediate current charging of approximately 110 .mu.A assist the cleaning of the belt 18 at the cleaning station F. A low current charging below 70 .mu.A assist the deletion of beads and contaminants at the pick off device 300.

To control the critical developed image charging, preferably, the device 56 is a dicorotron with a glass coated wire 280 surrounded on three sides by a shield 282. A current control device 278 is attached to the device 56. The device 278 includes an alternating current power supply 284 which is electrically connected to the wire 280 and a direct current (DC) constant current source 286 which is electrically connected to the shield 282. The current control device 278 for the pre-transfer corona generating device 56 has been found to more accurately control charging current than the commonly used voltage control device. Therefore, the pre-transfer corona generating device 56 preferably utilizes the current control device 278. Applicants have found that by utilizing the current control device 278 and by setting the current at approximately 70 .mu.A, the conflicting current charging needs of the cleaning station F, pick off device 300 and the transfer station D can be met, thereby permitting the placement of the bead removal device between the developer unit 34 and the transfer station D.

Again referring to FIG. 9, the magnetic air bead pick off device 300 is shown with a housing 301 which has a rotating sleeve 310 mounted therein. The sleeve 310 is formed of a non-magnetic material, such as aluminum, and is preferred to be of a plural level structure somewhat similar to the sleeves 120 and 220 of FIGS. 6 and 8, respectively.

The Applicants have found that unlike sleeves 120 and 220, as shown in FIGS. 6 and 8, respectively, on sleeve 310 the arc defined by islands 330 may be similar to the arc defined by troughs 332 and still effectively remove the beads. Sleeve 310 may, however, be effective with narrow islands and wide troughs, such as in sleeves 120 and 220. The relative size of the troughs 332 are none the less important in the effectiveness of the device. For example, in the BRD reduced to practice, for trapping beads of approximately 0.1 mm, the radius of the sleeve 310 as measured from the surface of the islands 330 to the center of the sleeve 310 ranges between approximately 12 mm and 17 mm with 15 mm being preferred. The radial extensions 334 range between approximately 0.1 mm and 0.4 mm with 0.2 mm being preferred. In this embodiment, there are approximately forty seven of the raised portions 330 with each defining an arc between approximately 3.8 and 3.9 degrees or approximately 2.0 mm wide in the preferred BRD construction with a 15 mm radius. Thus, there are also forty seven of the trough portions 332 which define an arc between approximately 3.8 degrees and 3.9 degrees. The troughs 332 serve to transport the beads and contaminants from the sleeve 310 near the belt 18 to the lowest portion of the sleeve. The sleeve is driven by a any suitable means, such as a belt 316 and motor 318.

A magnet 320 and a shunt 340 are fixedly mounted within the sleeve 310 which is mounted for rotation around them. The magnet 320 is positioned in the sleeve 310 so that a pick up zone 312 is defined on the shell of the sleeve 310 adjacent the surface of the belt 18. The positioning of the shunt 340 and the magnet 320 define a release area 314 where the magnetic field strength generated by the magnet 320 along the sleeve 310 is minimized or attenuated by the shunt 340. It will also be recognized and understood that the magnet 320 and shunt 340 can be replaced by a set of magnets and shunts similar to that disclosed in the BRD 246 of FIG. 8.

To remove non-ferromagnetic particles 157 from the belt 18, the BRD 300 preferably utilizes air flow to remove these particles. To avoid disturbing the developed toner image on the photoreceptor belt 18, the air flow must be carefully controlled. The developed toner image particles are smaller than non-ferromagnetic particles 157 and, thus, not as affected by the air flow. Vacuum is preferably used to generate the air flow. The vacuum portion of the combination magnetic air bead pick off device 300 includes a vacuum pump 361, a vacuum port 360 to receive captured beads and other materials within the inner walls of the housing 301; a conduit 362 connecting the vacuum pump 361 with port 360, and a sump 363 for receiving materials passing from the inner walls of the housing 301 through port 360. The walls of the housing 301 form a venturi thin plate nozzle 370 adjacent the belt 18 opposite magnet 320 so that the air flow generated through the nozzle by the vacuum and the magnetic field generated by the magnet 320 attract materials both on and near the belt 18 toward the pickup zone. Thus, loose beads, free beads, and agglomerate materials are urged toward the inlet nozzle.

In employing the bead pick off device of this invention such as that disclosed in FIG. 9, it is preferred that the nozzle 370 should be spaced between 0.25 and 1.75 mm away from belt 18 with 0.75 mm being preferred. Preferably, the nozzle 370 should also be between 0.25 and 3.00 mm wide with a flow rate through the nozzle that is between 10 and 20 liters/sec. A nozzle 370 that is approximately 1 mm wide in the direction of belt rotation and 360 mm long with an air flow through the nozzle of 14 liters/sec. has been employed. Variations in these preferred parameters can be made according to the details and conditions of the printing machines and material in which the device is used as well as the ambient conditions. It will be further understood that the described embodiment of FIG. 9 may be placed, as appropriate, along the photoconductive surface between a developer unit and a transfer station.

It will be now recognized that in certain applications, the applying of an AC or DC bias to the sleeve 310 by an AC or DC power source 380 may enhance the performance of the device 300, as previously discussed with respect to the bead removal devices 46 and 246 of FIGS. 6 and 8, respectively.

A further embodiment of an alternate bead removal device 300 of the present invention is shown in FIG. 1. While the embodiments illustrated in FIGS. 6-9 may result in effective bead removal, the applicant has found that the embodiment utilizing the device 300 as illustrated in FIG. 1 is particularly effective in obtaining proper bead removal where particularly high belt speeds are encountered. Again, referring to FIG. 5, the magnetic air bead pick off device 300 is positioned between the developer unit 34 and the transfer station D. Preferably, to take advantage of the inertial effects of the beads and to closely control the spacing of the belt 18 to housing 434 by providing belt stability, the bead removal device 300 is preferably located adjacent an area of the belt 18 where the belt 18 turns. Such a position is adjacent the drive roller 21.

Now, referring to FIG. 1, the bead removal device 300 includes the housing assembly 434. The housing assembly 434 generally conforms to the contour of the belt 18 as it is in contact with the drive roller 21. The housing 434 preferably includes a left housing portion 410 located on the upstream side of the belt 18 and a right housing portion 412 located on the downstream side of the belt 18.

A bead removal sleeve 420 is located within the housing 434. The sleeve 420 is similar to sleeve 310 as shown in FIG. 9. Preferably the sleeve 420 rotates in the direction of arrow 423 which is the same direction as the direction of belt 18 as shown by arrow 19. The sleeve 420 includes islands 430 and troughs 432 located about the periphery of the sleeve 420. The islands 430 and troughs 432 are similar to the islands 330 and the troughs 332 as illustrated in FIG. 9. The islands 430 typically have an arc of approximately three and three fourth degrees.

The bead removal device 300 of FIG. 1 includes a magnetic field generator 414 which is similar to the configuration as shown in FIG. 8. The magnetic field generator 414, includes left and right magnets 450 and 451, respectively, which are similar to magnets 250 and 251, as shown in FIG. 8. The field generator 414 also includes shunts 455 and 457 which are similar to the shunts 255 and 257, respectively, of FIG. 8. The magnetic field generator 414 further includes an arcuate shunt 453 which is located between shunt 455 and shunt 457. The shunt 453 serves to further improve the operation of the bead removal device 300 by assisting in the release of the beads.

Unlike the bead removal device 300 as shown in FIG. 9, the sleeve 420 is not centrally located within the housing 434. In fact, the sleeve 420 is significantly biased toward the left housing portion 410. This eccentric configuration is of great assistance in the effectiveness of this device to remove beads as will be later discussed. This configuration provides for a left air channel 415 located between the sleeve 420 and the left housing portion 410 which is significantly narrower than a right air channel 416 located between the sleeve 420 and the right housing portion 412.

To assist in the bead removal process, an air flow generator is preferably incorporated into the device to provide an air flow to blow beads from the belt 18. To provide this flow, the bead removal device 300 also includes a vacuum pump 461 which is connected to a sump 463. The vacuum pump 461 and the sump 463 are similar to the vacuum pump 261 and sump 263 as shown in FIG. 8. The vacuum pump 461 is connected to the right air channel 416 by means of a conduit 462. The conduit 462 connects the left air channel 415 and the right channel 416 at a vacuum port 460 located within the housing 434 opposite the photoreceptor belt 18.

Since the left air channel 415 has a left air channel gap 421 which is significantly smaller than an right air channel gap 422 of the right air channel 416, air flow created by the vacuum pump 461 flows primarily through the right air channel 416. The left housing portion 410 has a left housing edge 424 adjacent the photoreceptor belt 18 and the right housing portion 412 has a right housing edge 426 adjacent the photoreceptor belt 18. The left housing edge 424 and the right housing edge 426 define an aperture 428 which is located in a nip area 436 between the photoreceptor belt 18 and the sleeve 420.

The left housing portion 410 includes a lip 438 which extends from the left housing edge 424 downwardly and toward the left away from the nip area 436 and away from the belt 18. The lip 438 includes an outer face 440 adjacent the photoreceptor belt 18. The face 440 is inclined downwardly away from the nip area 436 at an angle to the photoreceptor belt 18. The face 440, thus, defines an entry slope angle 442 between the face 440 and the photoreceptor belt 18. While any of a variety of entry slope angles 442 may be satisfactory for the practice of this invention, preferably the entry slope angle 442 is approximately 15 degrees. The face 440 and the belt 18 form an air guide 444 for directing the air flow into the nip area 436. The edge 424 and the photoreceptor belt 18 define a lip entry gap 448 which size is influential in the proper flow of air through the bead removal device 300. Preferably, the lip entry gap 448 is approximately between 0.8 mm and 1.5 mm. The space between the right housing edge 426 and the belt 18 defines an exit lip gap 466. The thickness of the exit lip gap 466 influences the proper air flow through the bead removal device 300 and the applicants have found that the exit lip gap 466 is preferably approximately 0.5 mm.

To isolate the air flow in the bead removal device 300 from the operation of other components within the machine, it may be desirable to include an air turbulence shield 470. The shield 470 avoids turbulence of paper within the machine paper path 59 (see FIG. 5). The air turbulence shield 470 may be positioned wherever suitable, but, preferably extends from the right housing portion 412 and has an outer face 472 conforming to and spaced from the photoreceptor belt 18. The face 472 and the belt 18 define a paper guide gap 474 which the applicants have found to preferably be approximately 0.63 mm.

The eccentric mounting of the sleeve 420 within the housing 434 provides for an air flow which significantly improves bead and particle removal. Air flow enters at the air guide 444 and is concentrated at the nip 436 where the air blows most beads and particles that have not been removed by magnets 450 and 451 or by AC/DC bias from the belt 18. The air continues to flow generally into right air channel 416 in a direction colinear to the belt 18 as it enters the nip area 436. The inertia of the beads 15 and particles 157 urging them to continue to travel in a direction parallel to the belt as it enters the nip area and the colinear air flow work in harmony to separate the beads 15 and particles 157 from the belt 18. The position of the bead removal device 300 below the belt allows gravity to further assist the beads and particles.

In order to optimize the performance of the bead removal device 300, preferably, the photoreceptor belt 18 is movable relative to the sleeve 420. Applicants have found that a belt to sleeve spacing 476 is critical for the performance of the bead removal device 300. A belt to sleeve spacing 476 of approximately 0.63 mm has been found to be optimal for the performance of the device 300. Manufacturing tolerances and idiosyncrasies in the operation of the bead removal device 300 may render a fixed belt 18 and sleeve 420 arrangement not conducive for obtaining an optimum belt to sleeve spacing 476. During assembly of the bead removal device 300, adjustments in the spacing between the belt 18 and sleeve 420 may be made to obtain optimum performance. Further, during servicing of the device 300, movement of the belt 18 to the sleeve 420 may be useful to test the operation of the bead removal device 300.

Air drawn by the vacuum pump 461 enters the bead removal device 300 at the lip entry gap 448 being directed by the air guide 444. Beads 15 and particles 157 adhering to the belt 18 are drawn from the belt 18 by the combination of the magnetic field generated by magnets 450 and 451 and air flow passing by the nip area 436. The beads and particles continue through the right air channel 416 and out the vacuum port 460. From there the beads and particles progress down the conduit 462, into the vacuum pump 461 and into the sump 463.

It will be now recognized that in certain applications, the applying of an AC or DC bias to the sleeve 420 by an AC or DC power source 468 may enhance the performance of the device 300, as previously discussed with respect to the bead removal device 300 of FIG. 9.

While the bead removal device 300 illustrated in FIG. 1, depicts an embodiment of the bead removal device which may be preferred where high belt speeds are present, the invention may be practiced by the embodiments as illustrated in FIG. 9. A further embodiment is illustrated in FIG. 10. The bead removal device 300 comprises a housing 534 which is similar to housing 434 of FIG. 1. The housing 534 is attached at a vacuum port 560 to a vacuum pump 561 and a sump 563 via conduit 562. A sleeve 520 is located within the housing 534 in an eccentric position similar to that of bead removal device 300 as shown in FIG. 1. The sleeve 520 may have any outer surface condition including but not limited to knurls or splines and be rough or smooth. A magnetic field generator 514 of any suitable configuration is stationary with respect to the housing 534 and located within the rotating sleeve 520.

It will be now recognized that in certain applications, the applying of an AC or DC bias to the sleeve 520 by an AC or DC power source 568 may enhance the performance of the device 300, as previously discussed with respect to the bead removal device 300 of FIGS. 9 and 1.

It will be understood that the plural surface magnetic bead pick off devices of this invention, as described in illustrative detail in conjunction with FIGS. 6-11, can be used singularly or in conjunction with the other embodiments of this invention effectively, as well as with other devices, as necessitated by the constraints of the applications in which employed. The present invention, as previously indicated is not limited to use in single-pass highlight color electrophotographic printing machines but would be useful in single, two-pass and multi-pass electrographic printing machines generally. Applicants have found satisfactory results with the employment of two embodiments of the invention concurrently (e.g., a plural surface magnetic bead pick off device in the developer housing itself and another magnetic air bead pick off device position downstream therefrom prior to the transfer station.). But in many circumstances, as indicated, employment of a single embodiment of the present invention achieves the aims and goals of this application.

The bead removal device is effective for both the ferro-magnetic and non ferro-magnetic particles as well as for contaminants. The BRD combines a directed air flow steam and magnetics to remove the beads and particles from the belt. Electrostatics can also be used to remove the beads and particles from the belt. The special rotating surface with islands and troughs, inertia and gravity can assist in the transport the removed beads and particles through the bead removal device.

In recapitulation, a latent image recorded on the photoconductive surface has areas imaged according to its charge state which are developed by toner. A developer unit, in the direction of movement of the photoconductive surface, develops the imaged areas of the latent image with toner. The developer unit has at least one developer roller which has magnetic means positioned therein for generating a magnetic field extending therefrom and shaped by the positioning of magnets and magnetic field shaping shunts. Further, a BRD is provided with an outer plural level sleeve downstream from the developer roller relative to the direction of belt travel. The BRD may be located within the developer housing or be positioned between the developer housing and the transfer station. A combination of BRDs may also be employed. A magnetic field generator operates to attract beads adhering to the photoconductive surface to the BRD sleeve. The magnetic fields and the plural level surface of the sleeve cooperate to carry the beads to a bead release area for deposit in a sump. The BRD preferably includes a shunt to shape the generated magnetic fields to encourage release of beads into the sump. Additionally, an air flow generator for generating can be employed to generate an air flow which also attracts material to the sleeve roller and/or encourage release of material from the roller housing and deposit of the beads in a sump, as well as an AC or DC power source connected to the BRD sleeve further improves the performance of the BRD under appropriate conditions.

It is, therefore, apparent that there has been provided in accordance with the present invention, a BRD for use in an electrophotographic printing machine that fully satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.