US 3724422 A
A magnetic brush developing apparatus for use in an electrostatic printing machine having a plurality of brushes arranged with their axes in parallel and in a plane inclined relative to the horizontal and at such an angle as to allow gravitational forces to return the developing material to a point wherein the magnets associated with the brushes may continuously form bristles thereon.
Description (OCR text may contain errors)
United States Datent 1 Latone et a1.
 MAGNETIC BRUSH DEVELOPING APPARATUS  Inventors: Salvatore Latone, Rochester; Michael R. Stanley, Pittsford, both of N.Y.
 Assignee: Xerox Conn.
 Filed: Dec. 14, 1970  App1.No.: 97,856
 U.S. Cl. ..118/637,117/17.5, 118/623, 118/636, 222/DIG. l  int. Cl ..G03g 13/00, BOSb 5/02  Field of Search ..118/636, 637, 623, 624, DIG.
24; 117/175; 222/56, 57, DIG. 1; 355/3  References Cited UNITED STATES PATENTS 3,543,720 12/1970 Drexler et al ..1 18/637 3,575,139 4/1971 Nuzum ..1 18/637 1 Apr. 3, 1973 6/1968 Shelfio etal. ..118/637 3,584,601 6/1971 Turner ....118/637 3,572,289 3/1971 Maksymiak ..118/637 OTHER PUBLICATIONS H. C. Medley, Development of Electrostatic Images, IBM Digest, Vol. 2, No. 2, August 1959.
Primary Examiner-Mervin Stein Assistant Examiner-Leo Millstein Attorney.lames .l. Ralabate, Norman E. Schrader and Bernard A. Chiama  ABSTRACT A magnetic brush developing apparatus for use in an electrostatic printing machine having a plurality of brushes arranged with their axes in parallel and in a plane inclined relative to the horizontal and at such an angle as to allow gravitational forces to return the developing material to a point wherein the magnets associated with the brushes may continuously form bristles thereon.
5 Claims, 6 Drawing Figures PATENTEUAPR3 I975 3,724,42
SHEET 1 BF 4 INVENTORS' SALVATORE LATONE MICHAEL R. STANLEY BY @QC/Qm A T TORNEY MAGNETIC BRUSH DEVELOPING APPARATUS This invention relates to improvements in developing apparatus for electrostatic printing machines, and particularly to improvements in magnetic brush developing devices for accomplishing highly efficient, fast speed development during the printing of electrostatically produced reproductions.
The invention has particular advantages for an electrostatic copying machine of the type employing a photoreceptor in the form of an endless belt. Generally, in endless belt printing machine configurations which employ a plurality of magnetic brushes, the brushes are arranged for developing purposes with a run of the belt in the horizontal orientation. With the housing for the developing apparatus having a plurality of magnetic brushes arranged in a line generally parallel to the plane of the belt run, both of which are in horizontal attitude, development material must be moved from the finishing end of the housing to the starting end by means of special conveying mechanisms such as conveying belts, augers, material-throwing impellers and the like as well as special arrangements for maintaining proper material mix and triboelectric characteristics. These special devices also require drive mechanisms for imparting the desired action. For large developing apparatus employing two or more magnetic brushes, these special provisions add considerable cost to the developing apparatus, take up much needed space and require extra maintenance and cost.
Therefore, it is the principal object of the present invention to improve electrostatic printing machines of the type utilizing an endless belt photoreceptor and a plurality of magnetic brushes.
Another object of the invention is to maximize the use of development material presented to an image to be developed in electrostatic printing machines which employ magnetic brush development.
These and other objects of the present invention are acquired by the use of a photoreceptor endless belt having its development run at an inclined plane and a plurality of magnetic brushes having their axes arranged in parallel and in a plane generally parallel to the plane of the photoreceptor belt at the zone of development therefor. The brushes are arranged so that the developing material in bristle form is moved up the incline plane whereupon, after moving past the last brush in the string of brushes, the material is returned for reuse to the beginning end of the string by gravity forces. Enhancement is further provided by arranging the movement of the electrostatic belt in an upward direction along this incline plane and to impart a direction of rotation to the magnetic brushes which is in the same direction as the movement of the belt.
A preferred form of the invention is shown in the accompanying drawings in which:
FIG. 1 is a schematic sectional view of an electrostatic reproduction machine embodying the principles of the invention;
FIG. 2 is an enlarged elevational view of a magnetic brush developing apparatus utilized in the machine shown in FIG. 1, with parts broken away;
FIG. 3 is a partial sectional view of the magnetic brush developing system showing the recirculation flow path of developing material;
FIG. 4 is an elevational view of another side of the housing shown in FIG. 3 and showing the drive mechanism for the magnetic brushes;
FIG. 5 is an isometric view of a cross-mixing baffle device utilized in the flow path of the developing material; and
FIG. 6 is a top view of the cross-mixing baffle in relation to one of the magnetic brushes.
For a general understanding of an electrostatic processing system in which the invention may be incorporated, reference is had to FIG. 1 in which various components of a system are schematically illustrated. As in all electrostatic systems such as a xerographic machine of the type illustrated, a light image of an original to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material comprising carrier beads and smaller toner particles triboelectrically adhering thereto to form a xerographic powder image, corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be fixed by a fusing device whereby the powder image is caused permanently to adhere to the support surface.
The electrostatically attractable developing material commonly used in magnetic brush developing apparatus comprises a pigmented resinous powder referred to here as toner and a carrier of larger granular beads formed with steel cores coated with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the toner powder and the granular carrier. The magnetizable carrier also provides mechanical control for the formation of brush bristles by virtue of magnetic fields so that the toner can be readily handled and brought into contact with the exposed xerographic surface. The toner is then attracted to the electrostatic latent image from the carrier bristles to produce a visible powder image on an insulating surface.
In the illustrated machine, an original D to be copied is placed upon a transparent support platen P fixedly arranged in an illumination assembly generally indicated by the reference numeral 10. While upon the platen, an illumination system flashes light rays upon the original thereby producing image rays corresponding to the informational areas on the original. The image rays are projected by means of an optical system 11 to an exposure station A for exposing the photosensitive surface of a moving xerographic plate in the form of a flexible photoconductive belt 12. In moving in the direction indicated by the arrow, prior to reaching the exposure station A, that portion of the belt being exposed would have been uniformly charged by a corona device 13 located at a belt run extending between belt supporting rollers 14 and 15. The exposure station extends between the roller 14 and a third support roller 16, and the belt run between these rollers is encompassed entirely by the exposure station for minimizing the space needed for the belt and its supporting rollers.
The exposure of the belt surface to the light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the belt a latent electrostatic image in image configuration corresponding to the light image projected from the original on the supporting platen. As the belt surface continues its movement, the electrostatic image passes around the roller 16 and through a developing station lB located at a third run of the belt and in which there is positioned a developing apparatus generally indicated by the reference numeral 117. Suitable means (not shown) such as, vacuum panels or tensioning means may be utilized for maintaining the belt flat in all three belt runs, and additionally, the belt run related to the development zone B is maintained at an inclined plane. The developing apparatus 17 comprises a plurality of magnetic brushes which carry developing material to the adjacent surface of the upwardly moving inclined photoconductive belt 12 in order to provide development of the electrostatic image.
As the developing material is applied to the xerographic belt, toner particles in the development material are attracted electrostatically to the belt surface to form powder images. As toner powder images are formed additional toner particles are supplied to the developing material in proportion to the amount of toner deposited on the belt during xerographic processing. For this purpose, a toner dispenser generally indicated by reference numeral lift is used to accurately meter toner, upon demand, to the developer material in the developing apparatus 17.
The developed electrostatic image is transported by the belt 12 to a transfer station C located at a point of tangency on the belt as it moves around the roller 15 whereat a sheet of copy paper is moved at a speed in synchronism with the moving belt in order to accomplish transfer of the developer image. There is provided at this station a transfer roller 119 which is arranged on the frame of the machine for contacting the nontransfer side of each sheet of copy paper as the same is brought into transfer engagement with the belt 112. The roller 19 is electrically biased with sufficient voltage so that a developed image on the belt 12 may be electrostatically transferred to the adjacent side of a sheet of paper S as the same is brought into contact therewith, and also for tacking the same on the roller l9. A stripping finger or air puffing device 21 utilized for stripping the sheet from the roller is provided to permit pick-up and continued movement of the sheet by a vacuum conveying system 22. In tacking on the roller 19, each sheet of paper travels only a short distance before being stripped therefrom by the stripper 21. Devices such as gripper bars and release elements mounted on the roller 19 may be utilized instead of the stripper 2H for gripping the leading edge of each sheet of copy paper to ensure proper positioning thereon and to effect the release of a copy sheet at a precise time so as to strip the same for pick-up by a conveying system. The timing of the release of each edge relative to the sheet separation from the supply stack of sheets may be for the same period of time.
There is also provided a suitable sheet transport mechanism adapted to transport sheets of paper seriatim from a paper handling mechanism generally indicated by the reference numeral 23 to the developed image on the belt as the same is carried around the roller 15. A programming device operatively connected to the mechanism 23, the stripper device 21 and the illumination device for producing an electrostatic latent image on the belt 12 is effective to present a developed image at the transfer station C in timed sequences with the arrival of a sheet of paper and being coordinated with the activation of the stripper 21 at the precise time that these elements are to function for their intended purpose.
After the sheet is stripped from the belt 12, it is conveyed by the conveying system 22 into a fuser assembly generally indicated by the reference numeral 24 wherein the developed and transferred xerographic powder image on the sheet material is permanently affixed thereto. After fusing, the finished copy is discharged from the apparatus at a suitable point for collection externally of the apparatus. The remaining toner particles remaining as residue on the developed images, background particles and those particles otherwise not transferred are carried by the belt 12 to a cleaning apparatus 25 positioned on the run of the belt between the rollers l4, 15 adjacent the charging device 113. The cleaning device comprises a rotating brush, a corotron for neutralizing charges remaining on the particles and discharge lamp for discharging any remaining electrostatic charges on the belt. It will be appreciated that the run of the belt adjacent the cleaning device is at an incline angle relative to the horizontal as this run leaves the uppermost roller 15 where a developed image is transferred. Such an arrangement maintains the relatively straight line of copy sheet movement which operatively cooperates with the printing belt 12 at its highest point.
From the foregoing, it will be apparent that the configuration and positioning of the processing devices for electrostatic reproductions and the inclined angles of two of the runs are such as to utilize fully all three runs of the photoconductive belt 12 so as to minimize its size as well as to optimize efficiency and the utilization of machine space. The arrangement also results in a machine which has a height that is ideal for operator that is, waist high, especially in the provision of a paper path of movement above the image processing apparatus. in this manner paper jams can be attended to easily, without dismantling or involvement with other apparatus in the machine, and without having to resort to activities in the vicinity of the floor supporting the machine or the lower regions thereof.
As shown in FIGS. 2 and 3, the developing apparatus comprises a housing 26 having a generally rectangular cross section and a length extending beyond the width of the belt 12. The housing 26 is substantially closed except for an opening adjacent the photoconductive belt 12 whereat development of the latent image is effected. This housing serves as a container, closed at its ends, by end walls 27 and 22B and supporting an inclined bottom wall 30 for containing developing material comprising carrier beads from magnetizable material and colored electrostatic toner particles which adhere electrostatically in great numbers to the carrier beads.
Mounted for rotation within the developer housing are four magnetic brushes 311, 32, 33, and 34 positioned with their axes in parallel and below the selenium belt 12. The magnetic brush Elli, comprising outer cylinder 35, made of non-magnetizable material and extending almost the length of the housing 26, is mounted for rotation by and between the end walls 27, 28.
One end of the cylinder 35 is closed by a cap 36 which supports a drive shaft 38 in axial alignment with the cylinder and is mounted in suitable bearings on the end plate wall 27. The other end of the cylinder is similarly supported by the wall 23. Within the cylinder 35 there is positioned elongated bar magnets 40, 41 extending nearly the full length of the cylinder and being mounted therein by means of suitable shafts (not shown) rotatably supported in the end caps for the cylinder. These shafts on the magnets 40, 41 may be adapted to be rotated by an external control device as will be described hereinafter for rotating the bar magnets. ln operation during a development cycle, the
brush cylinder 35 is rotated by way of the drive shaft 38 and the magnets 40, 41 remain stationary.
The second, third and fourth magnetic brushes 32, 33 and 34 each comprise a cylinder 42, 43, 44 respectively, end caps for the respective cylinders, complement. bar magnets and rotatably supporting shafts therefor. Since each of the brushes 32, 33, 34 is similar to the structure of the brush 31, further details are not necessary to describe these brushes. Their only distinction lies in the relative orientation of magnet polarities (see FIG. 3 for preferred arrangement) and the inclusion of a pick-up magnetic device 45 for the first brush 31. The pick-up device 45 includes bar magnets supported by the magnets 4t), 41 for the magnetic brush 31 for transporting developing material from the lower sump portion of the housing 26 and to the adjacent periphery of the cylinder 35.
As shown in FIG. 3, the peripheral walls of the brush cylinder 35 and those for the other brushes are relatively close to each other. During a development cycle when all cylinders are rotating in unison in the same direction and with their respective magnetic bars held stationary, the brush bristles produced by the influence of the magnetic field emanating from the bar magnets acting upon the magnetizable carrier beads in the developing material will form on the upper region of the cylinder 35 between this cylinder and the undersurface of the selenium belt 112. Bristles remain formed during the developing cycle, being initiated by the influence of the pick-up device 45 and maintained, during rotation of the cylinder 35 produced by the magnetic field of the magnets 40, 31. When bristles are moved out of the influence of the magnets 40, 41 beyond the closest distance between the belt 112 and the cylinder 35, they maintain formation by the influence of the magentic field for the magnets associated with the brush 32, which influence is stronger at this point than the diminished strength of the magnetic field attributed to the magnets for the brush 3E. The developing material is carried during rotation of the cylinder 42 until they in turn reach the stronger influence of the magnetic brush 33, which effects the continued formation of bristles and movement of the bristles until the influence of the last brush 341 is stronger than the diminishing influence of the brush 33. After passing the last brush 34, the cylinder 44 thereof transports the remaining developing material around the same from which they will be directed eventually to the sump of the housing 26 by gravity.
During movement of the carrier beads and toner through the development zone B, the magnetic bristles and, therefore, the development material, is in the form of a magnetic blanket extending continuously over all of the brushes 31-34 for the entire width of the development zone B wherein the material is disposed or available to some degree for developing purposes. Control means, not shown, may be operatively connected to the shafts associated with each of the bar magnet pairs 40, 41 for each of the magnetic brushes for rotating the pairs in unison in order to terminate the formation of bristles on each of the brushes. In this manner, the magnetic blanket" over all of the brushes may be quickly eliminated during the operating program for the reproduction machine. Upon a restart of the machine, the control means may be set to be activated as the machine is placed in standby condition wherein all magnet pairs are oriented to operative positions. It will be apparent that the width of the development zone B is larger than the sum of the individual development zones for each of the magnetic brushes. Further details regarding the formation and effect of the magnetic blanket are described in the copending application Ser. No. 830,285 assigned to the same assignee as the present application.
Also mounted within the development housing 26 and below the magnetic brush 31 is a paddle wheel impeller 46 having a plurality of blades radially extending therefrom and having its ends rotatably mounted in the end walls 27, 28 by means of a drive shaft 47. During a development cycle, the impeller 46 is rotated in the direction shown by the arrow in FIG. 3 and serves to transport development material toward the pick-up device adjacent the lower surface of the magnetic brush 31 independent of the state of levelness and the amount of carrier beads in the system. The development material in this vicinity is picked up by the pickup magnet which commences the formation of bristles on the cylinder 35. As this cylinder rotates, the newly formed bristles come under the influence of the magnets 40, 41 and the magnetic blanket for the assembly 17 will be initiated.
The rotational motion for all of the rotary components of the developing apparatus 17, as illustrated in FIG. 4, is derived by a motor M-1 and a drive system comprising a pulley 418 secured to the shaft of the motor, a smaller pulley 56D also secured to the shaft, and timing belts 51, 52 for connecting the pulleys 48, 50 respectively, to the rotary components. Specifically, the belt Si is drivingly engageable with suitable pulleys 53, 54 mounted on the drive shafts for each of the magnetic brush cylinders 311, 32 respectively, an idler pulley S5 and pulleys 56, 5'7 secured to the drive shafts for the magnetic brush cylinders 33, 343 respectively. With this arrangement, the four magnetic brush cylinders rotate with the same peripheral speeds, in the same direction and in a direction which moves the magnetic blanket comprising magnetic brush bristles upwardly in an inclined plane arranged at the same angle as the angle of the plane which the belt 12 assumes in the development run. The timing belt 52 connects the drive pulley 50 with a driven pulley 58 secured to the shaft 47 for the impelier 4% thereby assuring that the impeller and the magnetic brush cylinders move in unison. The relative speed of the impeller 46 is slightly less than the peripheral speeds of the magnetic brush cylinders but incorporates a surface capacity which provides an excess of development material being transported by the impeller which cannot be picked up by the pick-up device 45 and will be carried around the impeller and back into the sump of housing 26.
In order to optimize the length of the bristles during the formation of the magnetic blanket upon the four cylinders, the cylinder 35 has associated therewith a trimming blade 60 secured to the end walls 27, 28 of the housing 26. This blade extends radially toward the cylinder 35 and being spaced from the periphery thereof a short distance equal to the desired length of the bristles to be formed on each of the magnetic brush 31. The blade also smoothes the developing material and contains the powder cloud in the housing 26.
The depleted developing material which is carried beyond the magnetic brush 34 or the last brush in the chain of magnetic brushes that comprise the developing device, is conveyed back into the housing 26 in order to be reused for development purposes. As this material is moved by the cylinder 44, away from the development zone B before being directed back into the sump of the housing 26, it is directed by a longitudinal planar baffle plate 61 secured at its ends on the end walls 27, 26 for the developer housing 26 to the upper end of a cross-mixing baffle generally indicated by the numeral 62. Another baffle plate 63 mounted within the housing 26 controls the movement of the development material leaving the cross-mixing baffle 62 into working engagement with the paddle wheel 46 for presenting the development material to the pick-up magnet 45.
The cross-mixing baffle 62 is illustrated in FIGS. and 6 and is preferably formed from a single piece of metallic or plastic material as by a single molding process. The baffle is formed with a centrally disposed, longitudinal wall element 6 1 secured to and spaced from the rear wall 30 of the developer housing 26 at an angle slightly inclined to the vertical. The front side of the central wall 64, or that side facing the magnetic brushes, is formed with a plurality of deflector plates 65 which project perpendicularly from the wall and are angled downwardly and laterally relative to the vertical. In the illustrated baffle, six of these deflector plates are provided and, in effect, form six open-sided chutes for the flow of development material through this side of the cross-mixing baffle. Similarly, the rear side of the wall 64 is also formed with deflector plates, labeled 66, which extend downwardly and laterally at an angle to the vertical that is equal and opposite to that of the plates 65. Preferably, there are enough deflectors 66 formed so as to provide seven conduits on the rear of the wall 64 defined by the deflectors, the wall 66 and the housing wall 36 so as to equalize cross flow in both directions.
At the upper input end of the baffle 62 there is also formed as integral structure a plurality of U-shaped guide elements which serve as means for deflecting some developing material into the front side of the baffle 62. Specifically, one of these guide elements 67 receives some of the development material flowing downwardly along the baffle M and directs this material along the chute E defined by the two adjacent plates 65 near the end wall 27. Similarly, another guide element 66 is arranged on the upper end of the baffle 62 for directing development material down a chute identified by the letter F. Extending along from the wall 27 toward the wall 26, the cross-mixing baffle is also formed with additional U-shaped guide elements 70, 71, 72, 73 which serve to collect and direct development material into and downwardly into corresponding chutes G, H, J and K. Within the confines of the guide element '73, the end wall 28 and the housing wall 30 is an opening 76 provided which serves to direct some development material into the chute defined by the two adjacent plates 66 and the wall 30. This flow of development material will be from left to right as viewed in FIGS. 5 and 6, as distinguished from the movement from right to left produced by the chutes at the front side of the cross-mixing baffle. Similarly, between the guides 72-73, 71-72, 70-71, 68-70 and 67-68, there are openings functionally similar to opening 74l for allowing the passage of development materi al therethrough for directing this material into the chutes formed by two opposed plates 66.
From the foregoing, it will be apparent that the development material cascading down the baffle 61 will fall along the upper longitudinal edge of the crossmixing baffle 62 and that this material will fall at random in fairly equal amounts through the guides 67, 68, 70, 71, 72, 73 and through the openings therebetween so that quantities of development material will be cascaded through the baffle 62 on either side of the wall 64. That material which falls through the baffle by virtue of the deflectors 65 on the front side of the wall 641 will be moved or carried in successive steps of movement from the right to the left, under action of gravity. On the other hand, that development material which falls between the wall 64 and the end wall 30 along the chutes defined by the deflectors 66 will be carried along from the left to the right in successive increments of movement.
From the illustrated cross-mixing baffle in FIGS. 5 and 6 with six chutes on the front side of the wall 64 and seven chutes on the back side of this wall so that exits of chutes are lined up with the entrances of the next desired chutes, it will be appreciated that during continuous recirculating movement of development material throughout its flow path in the housing 26 comprising movement around the paddle wheel 46, over the magnetic brushes 31, 32, 33, 34, across the baffle plate 61, and through the baffle 62, 13 such cycles of such movement of the material will result in any quantity of the material being moved as viewed in FIG. 6, from the housing wall 27 to the left to the wall 28 and back to the end wall 27 again. In FIG. 6, one of the magnetic brushes, labeled R, is schematically illustrated in position relative to the baffle 62. In this manner all of the development material utilized in the housing 26 may be carried from one side of the housing to the other continuously thereby insuring at all times a homogenous mixture of toner particles and carrier, and also to assure the presence of fairly equal amounts of toner particles for each carrier particle. Toner starvation, therefore, for any particular zone in a transverse direction of the flow path in the developer housing will be minimized. Such continuous cross-mixing of the developer mixture will also enhance the triboelectric relationship between the particles and carrier beads. It will also be appreciated that the entire process of crossmixing is accomplished by the action of gravity upon the cross-mixing baffle 62 and without the need for moving structural parts. By utilizing magnetic brushes having their axes of rotation on an inclined plane and in parallel with the inclined plane of a run of an endless photoreceptor belt, recirculation of developing material in its continuous flow path is accomplished by utilizing gravity instead of conveying devices, mixing augers throwing impellers, etc.
While the invention has been described with reference to the structure disclosed, it is not confined to the details set forth; but is intended to cover such modifications, or changes as may come within the scope of the following claims.
What is claimed is:
l. A magnetic brush apparatus for applying developing material to electrostatic latent images in an electrostatic printing machine of the type utilizing a moving, flexible photoreceptor plate having a development portion thereof disposed at an angle relative to the horizontal plane and wherein movement of the plate along the portion is inclined upwardly comprising a housing having a sump for containing developing material,
a plurality of magnetic developing brushes in said housing, each having a roller with a periphery upon which bristles of magnetic developing material are adapted to be formed.
said rollers each having an axis of rotation, said axes being substantially parallel to the latent images surface of said plate along said development portion,
a first of said brushes being arranged at the lower side of said portion for receiving developing material from said sump and the last of said brushes arranged at the upper side of said portion and being adapted to direct unused material in said housing,
said brushes including means for forming a continuous path of movement of the developing material from said first brush to the last brush between said brushes and the photoreceptor plate, and
means for directing the movement of said unused developing material as it flows under the influence of gravity to the sump to be received by said first brush.
2. The apparatus as claimed in claim 1 wherein said rollers for said magnetic brushes are rotated in the same direction.
3. The apparatus in claim 2 wherein said rollers are rotated in the direction of movement of the upwardly moving photoreceptor plate.
4. A magnetic brush apparatus for applying developing material to electrostatic latent images in an electrostatic printing machine of the type utilizing a moving, flexible photoreceptor plate having a development portion thereof disposed at an angle relative to the horizontal plane and wherein movement of the plate along the portion is inclined upwardly comprising a housing having a sump for containing developing material,
a plurality of solely magnetic developing brushes in said housing, each having a roller with a periphery upon which bristles of magnetic developing material are adapted to be formed,
said rollers each having an axis of rotation, said axes being substantially parallel to the latent image surface of said plate along said development ortion, a first of sa1d brushes eing arranged at t e lower side of said portion for receiving developing material from said sump and the last of said brushes arranged at the upper side of said portion and being adapted to direct unused material in said housing,
means defining a path of flow for said unused developing material between said last brush and said first brush, said path defining means being formed to permit the flow of the material along said path of flow under the influence of gravity, and
means positioned in said path of flow for subjecting the material to a mixing action as the material moves under the influence of gravity.
5. The apparatus of claim 4 wherein said development portion is flat and wherein said roller axes lie substantially in a plane.