US3696783A

US3696783A - Automated touchdown developement system

Info

Publication number: US3696783A
Application number: US98372A
Authority: US
Inventors: Joseph Fantuzzo
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1970-12-15
Filing date: 1970-12-15
Publication date: 1972-10-10
Anticipated expiration: 1989-10-10
Also published as: DE2162086A1; CA949822A; GB1373666A; JPS559711B1

Abstract

Xerographic apparatus employing a donor member adapted to retain toner on its periphery and a number of processing stations arranged around its periphery. The donor member has a plurality of electrically isolated screen patterns thereon which are in association with programmed means adapted to permit selective independent variation of the electrical potential of one or more of the screen patterns within the range of ground potential to charge potential as the donor member surface is transported past the several processing stations.

Description

United States Patent Fantuzzo [54] AUTOMATED TOUCHDOWN DEVELOPEMENT SYSTEM [72] Inventor: Joseph Fantuzzo, Webster, NY [73] Assignee: Xerox Corporation, Stamford, Conn.

[22] Filed: Dec. 15, 1970 [21] Appl. No.: 98,372

[52] US. Cl ..1l8/637, 117/175 [51] Int. Cl. ..G03g 13/00 [58] Field of Search ..118/7, 637; 117/175; 101/D1G. 13

[56] References Cited UNITED STATES PATENTS 3,405,682 10/1968 King et al. ..118/637 2,880,699 4/1959 Hayford ..118/637 3,203,394 8/1965 Hope et a1. ..118/637 IACUUM VACUUM [151 3,696,783 1451 Oct. 10,1972

3,234,019 2/1966 Hall .1l7/17.5

Primary Examiner-Mervin Stein Assistant Examiner--Leo Millstein Attorney-James J. Ralabate, James Paul OSullivan and Donald F. Daley ABSTRACT or more of the screen patterns within the range of ground potential to charge potential as the donor member surface is transported past the several processing stations.

10 Claims, 2 Drawing Figures PAIENTEUUBT l0|972 3.696.783

VACUUM VACUUM FIG.

INVENTOR. JOSEPH FANTUZZO ATTORNEY BY na/z ifigb Qaw v AUTOMATED TOUCHDOWN DEVELOPEMENT SYSTEM BACKGROUND OF THE INVENTION This apparatus relates to xerography and more particularly to an improved apparatus for the development of electrostatic images by which a toner layer is presented to the latent image for development thereof.

In the reproduction process of xerography, a photoconductive surface is charged and then exposed to a light pattern of the information to be reproduced,

thereby forming an electrostatic latent image on the photoconductive surface. Toner particles, which may be finely divided, pigmented, resinous material are presented to the latent image where they are attracted to the photoconductive surface. The toner image can be fixed and made permanent on the photoconductive surface or it can be transferred to another surface where it is fixed.

One known method of developing latent electrostatic images is by a process called transfer development. Transfer development broadly involves bringing a layer of toner to an imaged photoconductor where toner particles will be transferred from the layer to the imaged areas. In one transfer development technique, the layer of toner particles is applied to a donor member which is capable of retaining the particles on its surface and then the donor member is brought into close proximity to the surface of the photoconductor. In the closely spaced position, particles of toner in the toner layer on the donor member, are attracted to the photoconductor by the electrostatic charge on the photoconductor so that development takes place. In this technique the toner particles must traverse an air gap to reach the imaged regions of the photoconductor. In two other transfer techniques the toner-laden donor actually contacts the image photoreceptor and no air gap is involved. In one such technique the toner-laden donor is rolled in non-slip relationship into and out of contact with the electrostatic latent image to develop the image in the single rapid step. In another such technique, the toner-laden donor is skidded across the xerographic surface. Skidding the toner by as much as the width of the thinnest line will double the amount of toner available for development of a line which is perpendicular to the skid direction, and the amount of skidding can be increased to achieve greater density or greater area coverage.

It is to be noted, therefore, that the term transfer development is generic to development techniques where (l) the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to effect development (2) the toner layer is brought into rolling contact with the imaged photoconductor to effect development, and (3) the toner layer is brought into contact with the imaged photoconductor and skidded across the imaged surface to effect development. Transfer development has also come to be known as touchdown development.

In a typical transfer development system, a cylindrical or endless donor member is rotated so that its surface can be presented to the moving surface of a photoconductive drum bearing an electrostatic latent image thereon. Positioned about the periphery of the donor member are a number of processing stations including, a donor loading station, at which toner is retained on the donor member surface; and agglomerate removal station at which toner agglomerates are removed from the toner layer retained on the surface of the donor member; a charging station at which a uniform charge is placed on the particles of the toner retained on the donor surface; a clean-up station at which the toner layer is converted into one of uniform thickness and at which any toner agglomerates not removed by the agglomerate removal station are removed; a development station at which the toner particles are presented to the imaged photoconductor for image development; and a cleaning station at which a neutralizing charge is placed upon the residual toner particles and at which a cleaning member removes residual toner from the peripheral surface of the donor. In this manner, a more or less continuous development process is carried out.

Among the donor members employed in the process and apparatus described is that embodying the principles described in US. Pat. No. 3,203,394. Such a donor includes, an electrically conductive support member in the form of a cylinder, a thin electrically insulating layer overlying the support member, and a continuous, electrically conductive screen pattern overlying the insulating layer. The screen pattern is provided with an electrical connection to a slip ring so that its potential may be varied between ground potential and a charge potential at different stages in the process. A multitude of high fringe fields or microfields are created at the surface of this type of donor member. When this type of donor member is brought into contact with toner particles, the toner particles assume triboelectric charges so that the toner is charged with the same polarity as that being applied to the screen and the toner is repelled into the interstices of the screen. The donor member is in this manner loaded with toner.

A shortcoming of this technique, employing a microfield donor of the type described, is that different potentials or voltages are required at the several different treating stations and the donor cannot function at different potentials at the same time. For example, if a potential of +300 volts is required on the microfield donor at the toner loading station, while at the charging station the microfield donor must be at ground potential, it means these two operations cannot be carried on at the same time. Thus, while the microfield donor member is at the potential of +300 volts it must continue at this potential while the donor member makes at least one complete revolution in order to load the entire periphery of the donor member. When this is accomplished, then the potential on the grid or screen pattern of the donor member must be changed to ground potential so that the charging operation at the charging station can take place while the donor member makes another complete revolution. The charging means applies a uniform charge on the layer of retained toner particles. After this has been accomplished, then the potential on the screen pattern must again be changed to one appropriate for the operation taking place at the clean-up station. At this station the potential on the clean-up roll is adjusted to be approximately the same as the background potential on the imaged photoconductor. If the background potential on the image photoconductor is say approximately +200 then the potential on the clean-up roll can be approximately +100 while the potential on the microfield donor screen pattern is approximately +300. The voltage or potential on the microfield donor is adjusted to this value and the donor member rotated at least one complete revolution so that the clean-up roll may remove toner which would otherwise become background on the imaged photoconductor. Thereafter, the potential on the microfield donor screen pattern must be adjusted as it approaches the imaged photoconductor so that the potential will be that most appropriate for transfer development. If the imaged areas of the photoconductor are for example +700 volts, then the potential of the screen pattern of microfield donor can appropriately be +300 volts. After development, the surface of a microfield donor must be treated to remove the ghost pattern or residual image on the microfield donor surface. This is accomplished by impressing a neutralizing charge on the toner particles via a charging device. This necessitates once again that the potential on the microfield donor screen pattern be changed to that of ground potential. When this is accomplished the donor member is permitted to revolve at least one revolution while all other operations are held in abeyance.

It is thus seen that the individual processing stations operate sequentially and because of the structure of the donor member, cannot be operating simultaneously.

The art of xerographic development and in particular transfer development would be significantly advanced if the functions involved in transfer development as described, could all be carried out more or less simultaneously.

Accordingly, it is an object of the invention to improve donor member apparatus for developing latent electrostatic images.

It is a further object of the invention to improve apparatus for accomplishing transfer development of xerographic images.

It is still a further object of the invention to permit multiple functions involved in transfer development to be accomplished simultaneously.

Still another object of the invention is to improve upon known transfer development processes.

SUMMARY OF THE INVENTION This invention is directed to a xerographic microfield donor member which is adapted to transport toner particles to a latent electrostatic image on the surface of the xerographic plate for development of the image. The donor member comprises, an endless electrically conductive support member carrying on the surface thereof a continuous dielectric layer, a plurality of electrically conductive screen patterns in contact with the dielectric layer and spaced by the dielectric layer from the conductive support. At least two of the screen patterns are electrically isolated from each other and in association therewith is means adapted to maintain the conductive support member at ground potential. Also in association therewith is programmed means adapted to permit selective independent variation of the electrical potential of each of the screen patterns within the range of ground potential to a charge potential.

The invention is also directed to a xerographic apparatus for developing a latent electrostatic image formed on the surface of a xerographic plate. The apparatus includes a means for developing a latent image and the means includes:

a. a microfield donor member adapted to transport toner particles to the latent image comprising, and endless electrically conductive support member carrying on the surface thereof a continuous dielectric layer, and a plurality of electrically conductive screen patterns in contact with the dielectric layer and spaced by the dielectric layer from the conductive support, with at least two of the screen patterns being electrically isolated from each other;

b. means adapted to maintain the conductive support member at ground potential;

c. means to continuously advance the donor member past the plurality of treating station including: l) a tonar loading station at which toner particles are contacted and a layer of toner particles retained by said donor member in response to microfields set up between said screen patterns and the grounded support member; and (2) a developing station at which said layer of toner particles is presented in developing relation to a latent image on the xerographic plate;

d. programmed means adapted to permit selective independent variation of the electrical potential of each of said screen patterns within the range of ground potential to a charge potential, said means being programmed in accordance with the potential appropriate for each of said stations; and

e. a ground and charge source in association with said programmed means and being responsive in accordance with said program.

The apparatus of the present invention may also include several other treating stations located about the periphery of the donor member. A charging station may be located between the toner loading station and the developing station. The charging station includes a charging means adapted to place a uniform charge on the toner particles retained by the donor member of a polarity opposite to that of the latent image. A toner agglomerate removal station may be located between the charging station and the toner loading station and is operative to vacuum remove toner agglomerates and loosely adhering toner particles from the retained layer of toner particles. A clean-up station may be located between the charging station and the developing station and includes a rotatable cylindrical clean-up member located adjacent the donor member so as to contact the outermost region of the layer of toner particles. The clean-up member is so constructed and designed to convert the layer of retained toner particles to one of comparatively uniform thickness and to remove any remaining toner agglomerates therefrom. Located between the developing station and the toner loading station may be a charging means in association with a cleaning means both of which are operative after development. They are adapted respectively to neutralize the charge on the toner particles to facilitate removal of toner by the cleaning means.

The present invention is also directed to a xerographic developing step. A latent image is formed on the surface of a xerographic plate. The surface of a donor member is transported past a plurality of treating stations. The donor member is of the type described in the preceding paragraph and more particularly described below. The treating stations include a toner loading station, having a-supply of toner particles, at which toner particles are adhered to the surface of said donor member, and a developing station at which toner particles carried by said donor member are presented in developing relation to the latent image. The support member of the donor is maintained at ground potential. A means adapted to permit independent variation of the electrical state of each of the screen patterns of the donor, with respect to the support member, within the range of ground potential to a charge potential, is programmed in accordance with the potential appropriate for each of the treating stations. Electrical contact is maintained between the programmed means and each of the screen patterns. A ground and a charge source is provided in association with the programmed means and are responsive in accordance with the program. The respective functions at the treating stations are then simultaneously performed during transport of the surface of the donor member.

Other processing steps may be added to the basic process. For example, a toner agglomerate removal step may precede the uniform charging step, a clean-up step may follow the uniform charging step and a residual or ghost image removal step may follow the development step.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the invention as well as other objects and further features thereof, reference is made to the accompanying drawing, wherein:

FIG. I is a sectional view of xerographic apparatus in accordance with the present invention; and

FIG. 2 is an isometric view of one section of a microfield donor in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is a transfer development system in which toner particles are applied to an electrostatic latent image on a photoconductive plate to develop the image. Although the system is described herein as part of a xerographic copier, it can be utilized in conjunction with any reproduction system wherein a latent image is to be developed by applying toner thereto.

Referring to FIG. 1, there is shown a xerographic reproduction system utilizing the concept of the present invention. In this apparatus a xerographic plate is in the form of a drum which passes through stations A-E in the direction shown by the arrow. The drum has a suitable photosensitive surface, such as one including selenium overlying a layer of conductive material, on which a latent electrostatic image can be formed. The various stations about the periphery of the drum which carry out the reproduction process are: charging station A, exposing station B, developing station C, transfer station D, and cleaning station E. Stations A, B, D, and E represent more or less conventional means for carrying out their respective functions. Apart from their association with the novel arrangement to be described with respect to station C they form no part of the present invention.

At station A, a suitable charging means 12, e.g., a corotron, places a uniform electrostatic charge on the photoconductive material. As the drum rotates, a light pattern, via a suitable exposing apparatus 14, e.g., a projector is exposed onto the charged surface of drum 10. The latent image thereby formed on the surface of the drum is developed or made visible by the application of a finely divided pigmented, resinous powder, called toner, at developing station C, which is described in greater detail below. After the drum is developed at station C, it passes through transfer station D, comprising copy sheet 16, corona charging device 18 and fuser device 20. Following transfer and fixing of the developed image to the copy sheet, the drum rotates through cleaning station E, comprising cleaning device 22, e.g. a rotating brush.

At developing station C, the apparatus includes a donor member 24 (more particularly described below) rotatably mounted adjacent a toner reservoir26 containing a supply of toner 28. The donor roll 24 is positioned so that a portion of its periphery comes into contact with toner 28. The donor roll is also located so as to provide a small gap between the surface of drum 10 and the outer surface of a toner layer carried by donor roll 24. This gap can be approximately 1 to 10 mils.

Because of conditions difficult to control, some of the toner particles tend to agglomerate and be deposited on the surface of the roll and protrude well above the mean thickness of toner. In addition, if some provision is not made for controlling the thickness of the toner layer carried by the donor roll, thicker regions of the toner layer will be compacted between the donor roll surface and the surface of the photoconductive layer in the development zone, also producing agglomerates.

In order to assure removal of agglomerates and also to control the thickness of the toner layer presented to the imaged photoconductive drum, a vacuum means 30 is positioned adjacent the periphery of the donor drum at a point between the toner reservoir and the development zone. The vacuum means can conveniently be a narrow slit of a length slightly wider than the width of the toner layer, in a vacuum manifold. The vacuum manifold is spaced a short distance from the toner layer surface and sufficient force is exerted via the vacuum so that toner agglomerates and loosely adhering toner will be pulled from the toner layer. The force can be carefully adjusted so that only an even layer of particulate toner, needed for development, remains on the donor surface.

Located between vacuum means 30 and the development zone is a charging means 32, such as a corona charging device, which is adapted to place a uniform charge on the toner particles of a polarity opposite to the polarity of the latent image on the photoconductive drum.

It is sometimes found that the vacuum means 30 does not completely remove all agglomerates not result in the desired degree of evenness in the toner layer. This may be the result of employing a particular type of toner, or a particular donor loading technique. When such a condition exists, agglomerate removal and toner layer evenness can be assured by employing a clean-up member 34 positioned between the charging means 32 and the development region. The clean-up roll 34 may be an electrically biased roll which removes agglomerates and loosely adhering toner by electrostatic attraction. Roll 34 can be smooth-surfaced and can conveniently rotate at a rate approximately the same as that of the donor roll. A bias of +100 to +200 volts has been found satisfactory to remove any remaining agglomerates and loosely adhering toner which would otherwise be attracted to the background region of the imaged photoreceptor. When using a biased clean-up roll of this type the spacing of this roll from the donor member must be such as to only remove agglomerates and loosely adhering particles from the toner layer. This spacing can conveniently be approximately 1-10 mils. In association with clean-up member 34 it is contemplated to use a cleaning mechanism designed to remove toner particles and agglomerates which are dislodged or electrostatically attracted from the toner layer of the donor roll and adhered to the surface of the clean-up roll. This cleaning mechanism can be a brush 36, employed in association with a vacuum system 38, which draws the removed toner particles from the cleaning brush.

After being rendered agglomerate-free and of comparatively uniform thickness, the uniformly charged toner layer is presented to the imaged regions of the photoconductor and touch-down development as described above, takes place. Because the toner layer is of uniform thickness, no toner compaction is experienced. This, coupled with an absence of agglomerates, results in copy of excellent quality.

Following development, the donor roll is prepared for toner reloading by exposing the residual toner thereon to a neutralizing charge means 40, e.g., a corotron, to make easier the removal of the residual toner by way of a suitable cleaning means, e.g., a rotating brush 42, equipped with a vacuum means 44. The donor roll is thus freed of any image history or ghost image from the developed region and is prepared to pick up a new supply of toner.

Referring now to FIG. 2 of the drawing, there is shown part of a donor member of the type contemplated by the present invention. Member 46 represents part of a metal drum, e.g., an aluminum drum, over the surface of which is coated a dielectric layer 48, which can be, for example, a dielectric enamel. Positioned over the dielectric layer 48 is a conductive grid pattern 50. The material employed for this grid pattern can be any conductive material, for example a pattern of silver, copper, etc. In this manner exposed dielectric regions 52 will be surrounded by the conductive material of the grid. In any suitable manner the electrically conductive support member 46 is electrically grounded via ground means 58.

The donor member of the present invention can be prepared in any manner, and, the following technique is merely illustrative. An aluminum drum was coated with a thin layer of a dielectric enamel. A thin layer of copper was applied over all of the surface of the dielectric enamel. A thin layer of a proprietary photoresist was coated over the copper surface. Employing well known photographic techniques, a segmented gravure grid pattern was exposed onto the surface of photoresist. This pattern can be of a plurality of the type of grid pattern shown in FIG. 2. That is, there may be two or more such grid patterns on the surface of the donor member. Each grid pattern will be electrically isolated from one another on the surface of the donor member. Thereafter, the exposed pattern is developed and the pattern is etched into the copper film. The remaining photoresist is removed in any suitable manner, e.g., by an appropriate solvent. A thin dielectric layer, about 0.25 to 0.5 mils., can be placed over the grid pattern for protection purposes. Ground contact to the aluminum drum can be made in any manner within the skill of the art. For example, a brush in contact with the inner sur face of the aluminum drum and to an electrical lead, can make contact to ground. Next, individual electrical connections are made from grid pattern 50 to a programmed means which will permit each grid pattern to be placed at ground potential or to some desired charge potential. The electrical contact 60 and the segmented slip ring 62 are merely shown schematically in FIG. 2 for convenience. In actual practice, contacts 60 could be located inside of the donor member so as to slidingly contact members insulated from the conductive support but in electrical communication with each grid pattern. Likewise, the other ends of contact 60 could be in sliding contact with segments 64 of segmented slip ring 62. A voltage source 66 can supply the appropriate potential to each segment of the slip ring. Thus, electrical contact 60 extending from each grid pattern can be placed in electrical connection with a segment of a segmented slip ring 62, which serves as the programmed means. Depending upon the position of each grid pattern relative to the several stations located about the periphery of the donor member, they can be automatically placed either at ground potential or at the appropriate charge potential.

Thus in a developing system having six processing stations located about the donor member, i.e., (l) a toner loading station, (2) an agglomerate removal station, (3) a uniform charging station, (4) a cle an-up station, (5) a developing station and (6) a cleaning station, the donor member can have six electrically isolated grid patterns and the electrically segmented slip ring can be programmed according to the potential appropriate to the respective station e.g., (1) +300 volts, (2) +300 volts, (3) ground potential, (4) +200, (5) +350 volts and (6) ground potential.

It will be appreciated that by this arrangement as each individual grid pattern is adjacent one of the several processing stations, the potential appropriate for that station is automatically achieved and simul taneously the potential appropriate for all other stations are achieved. The slip ring can be programmed to place the grid patterns at the appropriate potential in any convenient manner, e.g., via individual potential sources and grounded members.

It is to be understood that while for purposes of illustration the donor member has been described basically as a cylinder, it may be an endless belt adapted to deliver toner from the toner source to the several stations.

Thus, it can be seen that by the particular construc tion and method of use of the donor member described, it is no longer necessary to operate the various functions at their respective stations sequentially but can be operated more or less simultaneously. This gives greater control of the entire developing system and cuts down on the time necessary for development of each particular image.

Conventional drive means, e.g., motors, belts, etc. are employed to drive the several movable members all in a manner well within the skill of the art.

Since many changes can be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof, it is intended that all matter contained in the drawing and specification should be interpreted illustratively and not in a limited sense.

What is claimed is:

l. A xerographic microfield donor member adapted to transport toner particles to a latent electrostatic image on the surface of a xerographic plate for development of said image, said donor member comprising: an endless electrically conductive support member carrying on a surface thereof a continuous dielectric layer, a plurality of electrically conductive screen patterns in contact with said dielectric layer and spaced by said dielectric layer from said conductive support, at least two of said screen patterns being electrically isolated from each other, means adapted to maintain said conductive support member at a reference potential; and means adapted to permit selective independent variation of the electrical potential of each of said screen patterns.

2. In a xerographic apparatus for developing a latent electrostatic image formed on the surface of a xerographic plate, means for developing said latent image, said means including:

a. a microfield donor member adapted to transport toner particles to said latent image comprising, an endless electrically conductive support member carrying on a surface thereof a continuous dielectric layer, and a plurality of electrically conductive screen patterns in contact with said dielectric layer and spaced by said dielectric layer from said conductive support, at least two of said screen patterns being electrically isolated from each other;

b. means adapted to maintain said conductive support member at a reference potential;

c. means to continuously advance said donor member past a plurality of treating stations, said treating stations including:

. a toner loading station including a supply of toner particles at which toner particles are contacted and a layer of toner particles retained on said donor member in response to microfields set up between said electrically isolated screen patterns and the grounded support member; and

2. a developing station at which said layer of toner particles is presented in developing relation to a latent image on a xerographic plate, and

10 d. programmed means adapted to permit independent variation of the electrical potential of each of said screen patterns, said means being programmed in accordance with the potential appropriate for each of said stations.

3. The apparatus of claim 2 wherein a charging station at a point in advance of said developing station is located between said toner loading station and said developing station and includes a charging means adapted to place a uniform charge on said toner particles retained by said donor member of a polarity opposite to that of said latent imagg.

e apparatus of claim wherein a toner agglomerate removal station is located between said charging station and said toner loading station and is operative to vacuum remove toner agglomerates and loosely adhering toner particles from said retained layer of toner particles.

5. The apparatus of claim 3 wherein a clean-up station is located between said charging station and said developing station and includes a rotatable, cylindrical clean-up member located adjacent said donor member so as to contact the outermost region of said layer of toner particles, said clean-up member being so constructed and designed to convert the layer of retained toner particles to one of comparatively uniform thickness and to remove toner agglomerates therefrom.

6. The apparatus of claim 4 wherein a clean-up station is located between said charging station and said developing station and includes a rotatable, cylindrical clean-up member located adjacent said donor member so as to contact the outermost region of said layer of toner particles, said clean-up member being so constructed and designed to convert the layer of retained toner particles to one of comparatively uniform thickness and to remove toner agglomerates therefrom.

7. The apparatus of claim 6 wherein said clean-up member is an electrically biased, comparatively smooth-surfaced cylindrical member.

8. The apparatus of claim 7 having a toner removal means in association with said clean-up member adapted to remove toner particles from the surface of said clean-up member.

9. The apparatus of claim 3 wherein a charging means and a cleaning means are located between said developing station and said toner loading station, said charging means being operative after development and adapted to neutralize the charge on the toner particles to facilitate removal of toner by said cleaning means.

10. The apparatus of claim 6 wherein a toner removal means in association with said clean-up member is adapted to remove toner particles from the surface of said clean-up member.

Claims

1. A xerographic microfield donor member adapted to transport toner particles to a latent electrostatic image on the surface of a xerographic plate for development of said image, said donor member comprising: an endless electrically conductive support member carrying on a surface thereof a continuous dielectric layer, a plurality of electrically conductive screen patterns in contact with said dielectric layer and spaced by said dielectric layer from said conductive support, at least two of said screen patterns being electrically isolated from each other, means adapted to maintain said conductive support member at a reference potential; and means adapted to permit selective independent variation of the electrical potential of each of said screen patterns.

2. a developing station at which said layer of toner particles is presented in developing relation to a latent image on a xerographic plate, and d. programmed means adapted to permit independent variation of the electrical potential of each of said screen patterns, said means being programmed in accordance with the potential appropriate for each of said stations.

2. In a xerographic apparatus for developing a latent electrostatic image formed on the surface of a xerographic plate, means for developing said latent image, said means including: a. a microfield donor member adapted to transport toner particles to said latent image comprising, an endless electrically conductive support member carrying on a surface thereof a continuous dielectric layer, and a plurality of electrically conductive screen patterns in contact with said dielectric layer and spaced by said dielectric layer from said conductive support, at least two of said screen patterns being electrically isolated from each other; b. means adapted to maintain said conductive support member at a reference potential; c. means to continuously advance said donor member past a plurality of treating stations, said treating stations including:

3. The apparatus of claim 2 wherein a charging station at a point in advance of said developing station is located between said toner loading station and said developing station and includes a charging means adapted to place a uniform charge on said toner particles retained by said donor member of a polarity opposite to that of said latent image.

4. The apparatus of claim 3 wherein a toner agglomerate removal station is located between said charging station and said toner loading station and is operative to vacuum remove toner agglomerates and loosely adhering toner particles from said retained layer of toner particles.