US2843084A - Xerographic apparatus with endless development electrode - Google Patents

Description

July 15, 1958 R. E. HAYFORD XEROGRAPHIC APPARATUS WITH ENDLESS DEVELOPMENT ELECTRODE A 2 Sheets-Sheet 1 Filed June 16, 1955 VACUUM HIGH POTENTlAL SOURCE CLEANER DEVELOPER'VZS SUPPLY INVENTOR RICHARD E. HAYFORD ATTORNE July 15, 1958 RD 2,843,084 XEROGRAPHIC APPARATUS WITH ENDLESS 7 DEVELOPMENT ELECTRODE Filed June 16, 1955 2 Sheets-Sheet 2 DEVELOPER 6 CLOUD 55 62 i s u P P LY vAcuuM VACUUM 12 CLEANER CLEANER E m A a; A f

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RICHARD E. HAYFORD BYRMPAJS+ i ATTORNEY United States Patent XEROGRAPHIC APPARATUS WITH ENDLESS DEVELOPMENT ELECTRODE Richard E. Hayford, Pittsford, N. Y., assignor to The Haloid Company, Rochester, N. Y., a corporation of New York Application June 16, 1955, Serial No. 515,886

4 Claims. (Cl..118-637) This invention relates in general to xerography and more particularly to automatic xerographic equipment.

formed on the surface of the insulating layer by exposing this layer, while sensitive, to a light pattern. Areas of the .photoconductive insulating layer exposed to light become more conductive than areas not exposed to light and charges on the surface of areas exposed are rapidly dissipated, whereas charges on the surface of areas not exposed remain in place. The charge pattern formed by exposure of the sensitive plate may be developed by bringing electrostatically charged particles .into contact with the surface area carrying the pattern, or the charges may be otherwise utilized. A developed image composed of particles deposited in conformity with the electrostatic charge pattern may be transferred from the surface of the plate to sheets or webs through the use of electrostatics or other techniques known to the art, and developed images, whether transferred or not, may be permanently afiixed to the surface on which they reside.

Xerography as described above may be adapted for automatic operation by cycling the xerographic plate through the various steps which produce the developed image, which may then be transferred. The cycle to produce the developed and transferred image may be repeated automatically.

A problem which has been encountered in automatic continuously operating machines involves an electrode used in the development stage. This electrode, which will be referred to in this application as a development electrode, is closely positioned while at a spaced distance from the surface of the photoconductive insulating layer during development.

Basic to the use of the development electrode is the desire to reproduce copies of high quality and without distortion of the original image.. Electrostatic lines of force exist between the electrostatic charges on the photoconductive insulating layer and areas of different charge potential. When large areas carrying electrostatic charges exist the lines of force which are present due to charges in the central area of the large area tend to run inward through the photoconductive insulating layer to the conductive backing member which is the nearest surface carrying a different patential. Lines of force running from electrostatic charges near the external boundaries of this large area will tend to extend outward and around the outside border of the large area at which point their paths will extend inw-ardto the photoconductive insulating layer to an area carrying a different potential. De-

velopment of such an electrostatic image creates deposition which relates to the paths taken by the electrostatic lines of force or development of the electrostatic fields.

Therefore, development of a large area as hasjust been 'ing member.

Patented July 15, 1958 described will. produce copies with hollow centers and emphasized edges. To prevent such development an equipotential surface is positioned at a slight distance from :the photoconductive insulating layer during development. composed of a conductive material and is either biased :or maintained at the potential found on the plate back- Such an equipotential surface will cause .an increase in the .lines of force extending outwardly from'the plate member creating electrostatic fields which when developed will produce distortion free and fringe free high quality copy.

During development fine charged powder particles are brought into the field of influence of the charges on the surface and are deposited on charged areas. The development electrode which is spaced at a slight distance from the sensitive layer, tends to build up on its surface a coating of developer material. While this coating remains thin the image which is developed is generally not alfected. However, a coating of developer material on the development electrode interposes a new surface be- .tween the sensitive layer and the development electrode. If the particles carry electrostatic charge, this coating may effectively changethe bias potential on the electrode and .quickly cause the electrode to be no longer an equipotential surface which substantially decreases beneficial aspects of the development electrode during development.

This coating also adds additional and uncontrollable problems. The lack of control which follows heavy deposition is to some extent attributed to the fact that the coating which forms is generally uneven and follows unknown and different patterns. A heavy coating may :drop agglomerates of developerpowder to the plate surface resulting in a nonuniform distorted development of the electric image pattern. Also, such a coating may cause irregular and uncontrolled flow patterns of the developer material being presented to the plate surface which will cause streaking of the developed image.

, In continuously operating. automatic machines, development is constantly taking place, in that a new portion of a. plate carrying an electrostatic charge pattern is continuously presented to the development zone for development purposes. It, therefore, becomes desirable to present a development electrode above the area to be developed with a minimum coating of developer particles to facilitate'high quality true reproductions in continuously operating automatic machines.

It is an object of this invention to improve the art of xerography by providing new means, methods and apparatus which relate to automatic xerographic equipment.

It is another object of this invention to improve the art of xerography by providing new means, methods and apparatus which relate 'to the development electrode.

It is a'further object of this invention to devise and disclose mechanism to provide for presentation of a development electrode carrying a minimum coating of developer particles over areas to be developed on a xerographic plate.

It is yet a further object of this invention to devise .new methods to clean all areas of the developmentelectrodeused in a machine for continuous operation during each successive cycle of movement of the development electrode.

Other objects and advantages of this invention will be apparent in view of the following description considered in the light .of the attached drawings, wherein:

Fig 1 is. aschematic view of one embodiment of an automatic electrophotographic device adapted for continuous operation.

Fig. 2 is a diagrammatic view of another embodiment of exposure according to this invention.

This'surface, the development electrode, is

Fig. 3 is a diagrammatic view of one embodiment of a development zone according to this invention.

Fig. 4 is a diagrammatic view of another embodiment of a development zone according to this invention.

Fig. 5 is a cross'section viewtaken along line 55 of Fig. 4.

Referring more specifically to the drawings, in Fig. 1 is shown an embodiment of a continuously operating xerographic machine. A xerographic plate generally designated formed in the shape of a drum is adapted to be rotated in the direction indicated. Rotation may be brought about through the use of normal driving techniques, such as motors or the like. In this figure, driving is accomplished by motor 14 through belt 19. The plate 10 is composed of a conductive backing member 11 which, as is illustrated, is held at a ground potential, and a photoconductive insulating layer 12. on the inner surface of the drum. A uniform electrostatic charge is placed on layer 12 by corona discharge electrode 13 supplied with high potential corona generating voltage from high potential source 15. Generally from 6000 to 8000 volts will be sufiicient to create corona discharge. It is to be realized, of course, that other means for placing an electrostatic charge on the surface of the photoconductive insulating layer 12 may be used and are intended to be encompassed by this invention as for example radioactive charging, frictional charging, or the like. The drum is moved following charging to an exposure station generally designated 16. In this embodiment, a projector 17 throws light from light 18 through film 20. The pattern of light passing through the film is then projected through lens 21 and onto the surface of the photoconductive insulating layer 12. The film is supplied from film supply spool 25 and is moved, as indicated, at a proper linear speed through slit area 23 and to film wind-up spool 22. Slit projection is used in this embodiment to avoid distortion in forming the image on the curved surface of photoconductive insulating layer 12, but other techniques known to those in the art may be used. The film 20 is driven by normal driving mechanism, such as a motor or the like, synchronized to the linear speed of the drum or otherwise controlled to bring about proper speed for continuous projection of continuously moving copy to be reproduced on a continuously moving drum. Although projection type of exposure is shown in this figure, it is to be realized that other types of exposure may be used, as for example reflex exposure, contact exposure, and the like, and such means of exposure are intended to be included in this invention. Also it is noted that since exposure is to the inner surface of the drum, it may be desirable to create the slit light pattern to be projected to the inner surface of the drum at an area external of the drum. This may be accomplished through the use of prisms positioned to focus the slit light pattern on the inner surface of the drum, whereas the projector or reflex mechanism or the like is positioned externally.

Next in position of movement of the inner surface of the drum is development station 26. This station comprises development electrode 27, developer supply 28, and conveying means to convey the developer supply to the area between developement electrode 27 and the surface of photoconductive insulating layer 12. At development station 26 development of the electrostatic charge pattern formed at exposure station 16 takes place. Developer supply 28 may supply a cloud of developer particles in gas to the area carrying the charge pattern for development purposes, or other known techniques of development may be used, as for example, it is intended to include within the scope of this invention development of the electrostaic charge pattern through the use of cascade development commonly used in the art of xerography. Cascade developing is disclosed in Walkup U. S. Patent 2,618,551.

Following development, the plate 10 is next moved 4 to a transfer station generally designated 31. Various transfer techniques may be used. In this embodiment, adhesive type of transfer is illustrated. A supply roll 32 of adhesive coated transfer paper or other material supplies the transfer material. The material lS passed over roller 33, which is positioned to bring pressure to bear against the back side of transfer material 35 to thereby press the front side of transfer material 35 into contact with the surface of photoconductive insulating layer 12 carrying the developed xerographic image. The image adheres to the transfer material because of the adhesive coating on its surface. Roller 33 may be any type of roller which will bring pressure to bear against transfer material 35, as for example, a rubber coated roller or the like. The transfer material 35 is then covered in this embodiment with a thin, transparent cover material 36, such as cellophane or the like, supplied from cover supply spool 37. The cover material 36 and the transfer material 35 are pressed together between rollers 38 and 4t) and the two materials adhere to one another because of the adhesive coating on the surface of transfer material 35. The covered transfer material is next fed from between rollers 38 and 40 to copy takeup spool 41. Driving means, such as motors or the like, drive the transfer material 35 and the cover mate rial 36 at a rate which is synchronized with the linear speed of the movement of plate 10.

Although an adhesive type of transfer mechanism has been shown in this embodiment, it is to be realized that other transfer means may be used, as for example, the developed image may be transferred through the use of corona transfer as is described, for example, in Schaffert U. S. Patent 2,576,047. The image may also be made permanent in other ways, such as for example, through vapor fusing, heat fusing, coating or the like.

Next in the path of movement following transfer station 31 is cleaning station 42, whereat the surface of the photoconductive insulating layer 12 on which the image was formed is brushed clean by rotating brush 43 or the like. Surrounding rotating brush43 is a hood 45 connected to a vacuum cleaner 46, which acts to draw off removed developer or dust particles. Plate 10 is then ready for recycling. It is also to be realized that although the cleaning means in this embodiment has been described in terms of a rotating brush, other means such as a wiper blade, air knife, or the like may be used, and are intended to be encompassed by this invention.

Another exposure technique is shown in Fig. 2. In this figure, as throughout this application, like numerals designate like elements. In this embodiment, drum 10 comprises a conductive backing member 47 and photoconductive insulating layer 12. At exposure station 16, lamps 48 project light to copy 50 which is reflected back through slit 51 and through lens 21. In this embodiment, conductive backing member 47 is a transparent conductive material, as for example, conductive transparent glass. To carry out the xerographie process, the photoconductive insulating material must be affected by an activating pattern, as for example, a pattern of light and shadow to be recorded. In the embodiment of this figure the photoconductive insulating layer 12 receives radiation through the conductive backing member which is transparent and the light pattern affects the photoconductive insulating layer 12 at the interface between layer 12 and the conductive backing member 47, thus bringing about exposure of the photoconductive insulating layer 12 to the pattern of light and shadow to be recorded. As has been indicated previously, other techniques of exposure are also intended to be encompassed by this invention.

Reference is now had to Fig. 3 wherein is shown in greater detail an embodiment of development stage 26. As is more clearly shown in this figure, the development electrodes'27 are in the shape of endless belts and spaced so ,that the surfaces near the sensitive layer conform, as

would concentric cylinders, with the curve of the surface of photoconductive insulating layer 12. Thus, when portions of the endless belt development electrodes 27 are between positioning plates 53 and the surface of photoconductive insulating layer 12, they conform to the shape of the near surface of the photoconductive insulating layer 12.

The endless belt development electrodes 27 are driven by rollers 52, which may be rubber rollers or the like. Rollers 52 are rotated for example, by motor 55 through driving belt 56. Rollers 52 press against the back side of endless belt development electrodes 27 and thus transpose their rotation into movement of endless belt development electrodes 27. At the axle of one of the rollers 52 gearing means may be provided to bring about motion of the endless belt development electrodes in the directions indicated in this figure. Other means for driving the rollers or the electrodes generally known to those in the art are also intended to be included in this invention.

Positioned along the path of movement of endless belt development electrodes 27 are cleaning means, as for example, rotating brushes 57 driven by driving belts 58 and motors 66. Surrounding the rotating brushes 57 are hoods 61 connected to vacuum cleaners 62 through connecting tubes 63. The brushes '7 rotate against development electrodes 2'7 and brush particles deposited during development from the surface of the development electrode into hood 61. Particles are removed from the hood into vacuum cleaners 62 through connecting tubes 63. Additional connecting tubes 65 are connected to hoods 66 and negative pressure is thus supplied to the areas defining the extremes of the development station 26. A cloud of developer particles in gas is supplied from developer cloud supply 67 through conveying means 68 to the area defined by development electrodes 27 and stops 7b, which may comprise felt or the like, acting to close oh? the development zone and to prevent backfire of cloud supplied out the output end of conveying means 68. Although the cloud is illustrated as fed normal to the plate, there is no intention to be limited thereto. It is to be realized that this illustration is intended to be pictorial in this respect and that various means of supplying the cloud that would occur to those skilled in the art are intended to be included herein. The cloud of particles in gas supplied from developer cloud supply 67 is conveyed to the surface of photo-conductive insulating layer 12 beneath portions of development electrodes 27. The portions of development electrodes 27 which define the areas to which the cloud of particles in gas is supplied are those portions between positioning plates 53 and the surface of photoconductive insulating layer 12. The cloud which is supplied to the plate passes along between those portions of development electrodes 27 and the surface of the photoconductive insulating layer to hoods 66, whereat particles not deposited are removed into vacuum cleaners 62. Particles, as they travel along their path of movement, deposit on the electrostatic charge patterns carried by the surface'of the photoconductive insulating layer 12, thereby creating the developed xerographic image. The output feed of developer supply, as is illustrated in this figure, is at a distance from the surface of the photoconductive insulating layer. It is a distance defined on two sides by development electrodes 27. The other two sides or edges are preferably blocked off with similar stops such as stops 70 or the like, and the area from the output end of conveying means 68 to the surface of the photoconductive insulating layer 12 defines in effect a diffusion area wherein particles in gas which may be .fed in streams or the like from tubes or the like diffuse to create a uniform aerosol for presentation to the surface of the plate. Optionally thtre may be added additional air or the like to diffuse streams of cloud or to perfect the aerodynamics of the cloud being presented to the image carrying surface.

Although a specific direction of movement of the development electrodes is shown in this figure, there is no intention to be limited to such motion. Such motion, however, is preferred in that after the development electrodes are cleaned by brushes 57, they are moved against stops 70 whereat they are again brushed clean, and a clean electrode is then presented for development purposes. This electrode is presented first at the diffusion area and then to the area where development takes place.

In Fig. 4 is shown another embodiment of development zone 26 according to this invention. In this figure, a single positioning plate 5.3 is used to position the endless belt development electrode 27 at a proper distance from the surface of the xerographic plate, generally designated 16, which comprises backing member 11 and photoconductive insulating layer 12. The belt drive may be a roller 52 similar to the roller shown in Fig. 3, or it may comprise other types of friction drive generally known to the art. Roller 52 may be driven as is the roller in Fig. 3, or it may be driven in other ways generally known in the art. In this embodiment developer is supplied in aerosol form from developer cloud supply 67 through conveying tube 71 to output head 72. Output head 72 comprises a chamber and a tube or nozzle 73 extending into the area whereat development of images on the surface of photoconductive insulating layer 12 takes place. A plurality of nozzles or tubes may be used. Positioned at the other end of development zone 26 is a hood 66connected to negative pressure generator 75 to create a negative pressure and to define a path of movement for the particles in gas following passage over the area'of the plate surface to be developed. In this embodiment, the cleaning means is illustrated as a wiper blade 76 which brushes against the surface of the development electrode 27 and removes the particles deposited thereon during development. The wiper blade 76 is surrounded by a hood 61 and is connected through connecting means 63 to negative pressure generator 75. Negative pressure generator 75 supplies negative pressure to draw off particles'from hood 61 removed by wiper blade 76.

Reference is now had to Fig. 5 wherein is shown a cutaway cross-section taken along line 5-5 of Fig. 4. The plate designated 10 comprising conductive backing member 11 and photoconductive insulating layer 12 is spaced from the surface of development electrode 27 by a gas gap, generally air, and development electrode 27 is positioned in physical contact with support plate 53. The width of the electrode is at least as Wide as the surface on which. the charge pattern is formed. Extending to the edge of the photoconductive insulating layer 12 is a stop 77 to prevent powder cloud leakage out the edges at the development zone. Such a stop 77 acts also to create an enclosed area at the development zone, thereby allowing the creation of flow patterns which are regular during continuous development. Stop 77 is permanently fastened to positioning plate 53 and may comprise felt or a similar material which will allow plate 10 to rotate freely while contacting stop 77.

When in position the development electrode should be taut. The use of a flexible material to make up the endless belt development electrode of this invention allows proper shaping and proper movement as the belt moves through its cycle, which includes turns and changes of direction. By holding the electrode taut and .by using a flexible material, proper spacing and positioning of the belt near the surface of the plate to be developed is accomplished as the belt moves through its cycle.

The surface material of the endless belt development electrode should be at least a fair conductor of electricity, in that generally the electrode would be biased to the potential of the plate backing member or at a potential raised therefrom during development. Such an electrode would include, but is not limited to, a cloth material carrying a metal layer, cloth having interspersed throughout sputtered metal or the like, a plastic belt carrying a metal layer, foil, or any flexible conductive material such as conductive rubber.

To assure that the development electrode is held taut according to this invention, while avoiding the use of other mechanism such as vacuums, magnets, or the like which would further complicate the development electrode mechanism, the positioning plate which acts to hold the electrode at a proper distance from the surface to be developed is preferably convex. In some instances, a flat positioning plate may be used, as for example, when rollers or the like are positioned at the edges of the positioning plate to facilitate movement of the development electrode. However, even when a flat positioning plate is used, it is desirable to position the rollers or the like at the edges slightly behind the surface of the positioning plate, and it is also desirable to use a short rather than a long plate over which the electrode travels. When using a fat positioning plate, the image carrying layer will also be flat.

The development electrode should be dimensionally stable and should be polished when possible, so as to present a smooth, even equidistant surface to the photoconductive insulating layer. Preferably the thickness of the development electrode is uniform throughout.

The positioning and drive mechanism for an electrode according to this invention preferably should have means to vary spacing from the surface of the photoconductive insulating layer and means to vary the speed of movement of the electrode as it passes through its cycle. Generally the electrode is spaced from .1 to .001 inch from the photoconductive insulating layer and screw adjustments or the like may be connected to the supporting elements of the positioning plate and driving roller to vary spacing from the surface'to be developed. Other means generally known to the art may also be used. A variable speed motor will allow sufiicient control over the speed of movement of the belt.

It is to be realized that the linear speed of the endless belt development electrode of this invention is dependent on a number of factors, such as the linear speed of the plate, the quality of copy desired, and the like. Generally faster speeds are preferred in that portions of the plate would be carried through the entire cycle with greater rapidity than if slower speeds were used. Preferred timing for movement of a portion of the development electrode from the point when that portion enters the zone in which development will take place to the point when that portion leaves the development zone is less than three minutes for high quality continuous tone xerographic reproductions. For line copy work, slower speeds are possible.

When two endless belt development electrodes are used, as for example, as in the embodiment shown in Fig. 3, it is preferred that the aerosol of powder particles is sprayed as near to the center as possible. This will allow even distribution of aerosol or cloud under each of the development electrodes and equally against areas of the surface to be developed.

Erushes or wiper blades used to clean the electrodes may be types generally known to those in the art. For example, nylon brushes, fur brushes, rubber wipers, and the like have been found to produce results desired according to this invention.

While there has been described what at present is considered to be a preferred embodiment of the invention, it is to be understood that many changes and modifications may be made therein without departing from the essen- 3 tial spirit of the invention. It is intended, therefore, that the appended claims cover all modifications as fall within the scope of this invention.

What is claimed is:

1. In a xerographic apparatus, the combination comprising a circular cylindrical xerographic plate comprising an outer layer of conductive material and an inwardly facing inner layer of photoconductive insulating material, means to rotate the cylindrical xerographic plate about its axis, and a development station within said xerographic plate, said development station comprising a development electrode positioning plate having dimensions corresponding to an area to be developed and having a convex surface positioned and conforming to the arc of a cylinder concentric to and located within the cylindrical xerographic plate, a conductive endless belt development electrode positioned around the development electrode positioning plate and in area contact therewith, means to drive the endless belt development electrode to cause continuous movement of the electrode during development while maintaining a portion of the electrode against the convex surface of the positioning plate, the electrode at said positioning plate being spaced apart by a gap from said photoconductive insulating layer of said xerographic plate, means to supply developer powder to the gap between the surface of the photoconductive insulating layer and'the development electrode at the development station, and cleaning means to clean powder from the endless belt development electrode positioned at a point removed from the surface of the xerographic plate.

2. In a xerographic apparatus, the combination comprising a cylindrical xerographic plate comprising an outer layer of conductive material and an inwardly facing inner layer of photoconductive insulating material, a first and a second positioning plate each positioned at a slight distance from the photoconductive insulating inner layer of the xerographic plate and each adjacent the other, said positioning plates having convex surfaces conforming to the arc of a cylinder concentric to and positioned within the cylindrical xerographic plate while spaced apart therefrom, said positioning plates having dimensions corresponding to an area of said xerographic plate to be developed, a first conductive endless belt development electrode positioned to move through a path which includes area contact with the convex surface of the first positioning plate, a second conductive endless belt development electrode positioned to move through a path which includes area contact with the convex surface of the second positioning plate, said electrodes against said positioning plates being spaced apart by a gap from said photoconductive insulating layer of said xerographic plate, drive means to move said electrodes through their respective paths, means to supply developer powder to the gap between the surface of the photoconductive insulating layer and the development electrodes at the development station, and cleaning means positioned along the path of movement of each endless belt development electrode at a point removed from the surface of the xerographic plate to clean powder from the endless belt development electrodes.

3. In a xerographic apparatus, the combination comprising a cylindrical xerographic plate comprising an outer layer of conductive material and an inwardly facing inner layer of photoconductive insulating material, a first and second positioning plate positioned at a slight distance from the photoconductive insulating inner layer of the xerographic plate and each adjacent the other, said positioning plates having convex surfaces conforming to the arc of a cylinder concentric to and positioned within the cylindrical xerographic plate and spaced apart from said xerographic plate, said positioning plates having dimensions corresponding to an area of said xerographic plate to be developed, a first conductive endless belt development electrode positioned to move through a path which includes area contact with the convex surface of the first positioning plate, a second endless belt development electrode positioned to move through a path which includes area contact with the convex surface of the second positioning plate, said electrodes against said positioning plates being spaced apart by a gap from said photoconductive insulating layer of said xerographic plate, the paths of said development electrodes further including a region extending from the adjacent ends of said positioning plates and in which the electrodes are in a spaced apart relation defining a channel extending to the gap between the development electrodes and said photoconductive insulating layer of said xerographic plate, drive means to move each of said electrodes through their respective paths, powder spray means to spray an aerosol of powder particles into said channel toward said plate, and cleaning means positioned along the path of movement of each endless belt development electrode at a point removed from the surface of the xerographic plate to clean powder from each of said endless belt development electrodes.

4. In a Xerographic apparatus, the combination comprising a cylindrical xerographic plate comprising an outer layer of conductive material and an inwardly facing inner layer of photoconductive insulating material, means to rotate the cylindrical xerographic plate through various stations including a charging station to sensitize the photoconductive insulating material, an exposure station to form an electrostatic charge pattern on the surface of the photoconductive insulating material, a development station to develop the electrostatic charge pattern with powder particles and a transfer station to transfer the developed image to a transfer surface, said development station comprising at least one development electrode positioning plate positioned within the Xerographic plate and having a convex surface shaped to conform to the arc of a cylinder concentric to and positioned within the cylindrical Xerographic plate while spaced apart therefrom, the positioning plate area corresponding in dimensions to an area of said xerographic plate to be developed, an endless belt development electrode positioned around each development electrode positioning plate, means to drive each endless belt development electrode to cause continuous movement of each electrode during development while maintaining a portion of each electrode in area contact with the convex surface of its development electrode positioning plate, means to hold each electrode taut to thereby maintain the electrode in close area contact with its positioning plate, each electrode against each positioning plate being spaced apart by a gap from said photoconductive insulating layer of said Xerographic plate, means to spray developer powder material to the gap between the photoconductive insulating layer and each development electrode, and cleaning means positioned to clean powder from each endless belt development electrode at a point removed from the surface of the xerographic plate.

References Cited in the file of this patent UNITED STATES PATENTS

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