Development Apparatus For Latent Electrostatic Images

Abstract

Powder cloud development apparatus for developing latent electrostatic images including an ion generator adjacent one wall of the development chamber. A cloud of toner particles is introduced into the development chamber through a port in the wall directly opposite the ion generator wall. Above the powder cloud port and the apertures in the ion generator wall, and extending completely between the opposed walls, is a baffle under which the powder and toner clouds meet and are thoroughly mixed. A grid electrode, positioned above the baffle and spaced much further from the photoconductive surface than a conventional powder cloud development electrode, is utilized to control image quality and contrast.

Description

BACKGROUND OF THE INVENTION

Xeroradiography, as disclosed in U.S. Pat. No. 2,666,144, is a process wherein an object is internally examined by subjecting the object to penetrating radiation. A uniform electrostatic charge is deposited on the surface of a xerographic plate and a latent electrostatic image is created by projecting the penetrating radiation, such as X-rays or gamma rays, through the object and onto the plate surface. The latent electrostatic image may be made visible by contacting the latent electrostatic image on the plate surface with fine powdered particles electrically charged opposite to the latent electrostatic image pattern on the plate. The visible image may be viewed, photographed or transferred to another surface where it may be permanently affixed or otherwise utilized. The entire processing is dry and no dark room is necessary.

Xeroradiography in recent years has been utilized to detect breast cancer in women. In examination of breasts wherein soft tissue comprises most of the breast area, xeroradiography, or xeromammography as it is generally called, provides greater resolving power than the conventional roentgenographic film and greater image detail is achieved. A wide range of contrast is seen on the xeroradiographic plate as compared to the conventional roentgenographic films so that all the structures of the breast from the skin to the chest wall and ribs may be readily visualized. Besides providing better contrast, xeromammography detects small structures like tumer calcification and magnifies them more than conventional film, is quicker, less expensive, gives greater detail and requires less radiation than X-ray techniques.

The technique of powder cloud development, as disclosed in U.S. Pat. No. 2,711,481, has been utilized to develop xeroradiography plates. This development technique is preferred in xeromammography because discontinuities in the object being examined are readily developed. The charged surface of the plate is disposed facing a chamber area -inch cloud of powder particles are introduced. The particles must be charged opposite to the polarity of the charge on the plate so that the particles may deposit upon the surface of the plate in an image configuration due to the action of the electrostatic forces of the latent electrostatic image on the plate acting on the charges on the particles in the powder cloud. Various prior art techniques for charging the powder cloud include turbulently flowing the powder particles in air through a nozzle, tube or the like to triboelectrically charge the particles or by passing the particles through a corona discharge area comprising a fine needle or fine wire and a grounded electrode as disclosed in U.S. Pat. No. 2,725,304. The corona method of charging the particles has been found to provide more uniform charging of the powder cloud, thereby producing high resolution images when the xerographic plate is developed. Prior are devices which utilized a corona discharge to charge the powder particles, such as that set forth in U.S. Pat. No. 2,725,304, have disadvantages associated therewith. The powder particles, when introduced into the chamber and charged, may accumulate around the corona discharge electrode such that the electrode ceases to charge powder particles entering the chamber area. Although the electrode may be cleaned periodically to remove the powder particles therefrom the necessity of cleaning the corona electrode is time consuming, may increase the cost of development, the complexity of the development operation is increased and the particles may deposit on the person manually cleaning the electrode.

In application Ser. No. 832,697 filed June 12, 1969, and now abandoned, and assigned to the assignee of the present invention, there is described apparatus and technique for substantially eliminating the contamination of the ion generators by the toner particles blown into the development chamber. This is achieved by placing the ion generator in its own housing and by maintaining a suitable positive pressure differential between the ion generator housing and the development chamber proper, whereby toner particles are substantially prevented from entering the ion generator housing.

While the invention described in the aforementioned copending application does in fact solve the problem of contamination of the ion generator, the apparatus described therein, where a manifold extends along the longitudinal axis of the development chamber, is not totally satisfactory inasmuch as toner buildup on the manifold itself affects the electrostatic fields in the development chamber. This, in turn, can cause undesired artifacts to be imposed upon the latent electrostatic image and eventually developed into the final reproduction. Under prolonged use, uneven powder cloud distribution results from the clogging of the toner ports in the manifold thereby resulting in improper mixing of the ion and powder clouds. Finally, the open chamber is detrimental to the development of optimum image quality in that no means are provided to control free ions which can migrate to the plate surface causing discharge of the latent electrostatic image thereon. These factors which, in general, do not affect the initially developed images, do have a significant effect with continued use and therefore affect the overall usefulness of the described development chamber in automated xerographic processing equipment.

OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to provide improved powder cloud apparatus for developing latent electrostatic images.

It is a further object of the present invention to provide novel powder cloud apparatus for developing latent electrostatic images wherein the ion cloud and the powder cloud are introduced into the development chamber through opposite walls and meet under a baffle, extending between the opposed walls, where the clouds are thoroughly mixed.

It is an additional object of the present invention to provide improved development apparatus for use in xeromammography.

These and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed disclosure.

BRIEF SUMMARY OF THE INVENTION

The above and still further objects of the present invention are achieved, in accordance therewith, by providing, as in the aforementioned copending application, an ion generator and a powder cloud generator, the generators serving to respectively introduce a cloud of ions and a cloud of toner particles into the development chamber. The ion generator is enclosed in a housing which has a plurality of openings along a wall thereof adjacent the development chamber. Air introduced into the housing adjacent the energized high-voltage wire or wires carries ions into the development chamber through the openings. The airflow is chosen such that a positive pressure differential is maintained between the ion generator housing and the development chamber. This pressure differential prevents the flow of toner particles into the ion generator housing such that the possibility of contamination of the high-voltage wire in the housing is substantially minimized.

In the invention described herein, the ion generator is adjacent one sidewall of the development chamber. A cloud of toner particles is introduced into the development chamber through a port in the sidewall opposite from the ion generator, the cloud of ions and the cloud of toner particles meeting under a baffle, extending between the opposed walls, where they thoroughly mix to form a cloud of charged toner particles. To insure that the ion cloud and toner cloud meet under the baffle, the ion generator egress ports and the powder cloud port are positioned only under the baffle whereby thorough mixing of the clouds is assured. Because of the flow rates associated with the powder cloud generator and the ion generator, the charged powder cloud is caused to swirl out from under the baffle toward the upper portion of the development chamber where the charged toner particles pass through and around a grid electrode, and are attracted to the latent electrostatic image, whereby the latent image is made visible. It is essential that the baffle extend the entire distance between the opposed sidewalls, whereby discharge of portions of the latent image by free ions and over-development of portions of the latent electrostatic image by toner particles carried to the top of the development chamber in the absence of the proper baffle, as described herein, is avoided.

To enable operational control of image contrast and quality, a grid electrode is spaced above the baffle at a distance of about 1 to about 2 inches, generally on the order of about 11/4 inches, from the photoconductive surface. By appropriately biasing the grid electrode during the development cycle, the grid electrode separates particles charged to an undesired polarity. In addition, since the electrode is further spaced from the photoconductive surface than is normal for a powder cloud development electrode and the charged powder cloud is caused to enter into the zone above the electrode, the toner particles, charged to a polarity for developing the latent electrostatic image, will be accelerated toward the latent image and away from the grid electrode which is biased to the same polarity.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of the invention will be more easily understood when it is considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view of a powder cloud development apparatus incorporating the teachings of the present invention; and

FIG. 2 is a top plan view, partially in section, of the development apparatus of FIG. 1.

Referring to FIGS. 1 and 2, development means 100 includes a rigidly mounted backing plate 102 positioned above development chamber 104 and slightly above the path of xerographic plate travel therebetween. The xerographic plate is shown schematically as having a conductive backing member 106 and a downwardly facing photoconductive layer 108 thereon. As described in application Ser. No. 874,834, filed concurrently herewith, the development means of FIGS. 1 and 2 herein is part of an automated flat-plate xerographic processing system. As more fully described in the aforementioned concurrently filed copending application, means are provided to advance a latent electrostatic image-bearing xerographic plate into the development means and to advance the xerographic plate out of the developing means after the latent electrostatic image has been converted to a corresponding xerographic powder image. Portions of said copending application which are necessary for a complete understanding of the present invention, or to provide sufficient disclosure to understand the more fully automated operation of the development chamber described herein, are incorporated herein by reference.

Development chamber 104 rests on inflatable elements 110 supported, in turn, by brackets 112. The walls 114 of chamber 104 terminate, about the upper periphery thereof, in outwardly extending lip 116. Attached to lip 116 is a gasket supporting member 118 having gasket 120 on the upper portion thereof. After a xerographic plate is properly positioned within the development station with the development means in the down position, the development chamber is raised by applying pressurized gas to inflatable elements 110. As shown in FIG. 1, gasket 120 is caused to seat against nonphotoconductive coated portions 122 of conductive backing member 106 whereby a leaktight development chamber is defined.

Within the development chamber, and positioned approximately 1-2 inches from the photoconductive surface of the xerographic plate, there is a grid electrode 124 mounted on support bracket 126. Grid electrode 124 can be, for example, 0.010 -inch diameter wire strung on a frame at approximately 1/4 -inch spacing and is typically biased on the order of about .+-.1,000 volts. Below the grid electrode and mounted on support bracket 128 is a canopy shaped baffle 130 made of insulator material. Extending substantially across the width of the development chamber (i.e., in a direction transverse to the direction of xerographic plate travel through the development means) is an ion generator 132 comprising housing 134, a single corotron wire 136 therein and air inlet ports 138, coupled to a pressurized air source 139 by conduit 140. On the opposite wall of housing 134 from the pressurized air entrance ports, but through only that portion of the housing wall directly beneath baffle 130, is a plurality of ionized air egress ports 142 through which ionized air passes into the development chamber 104. Air inlet ports 138 are adjacent the ends of corotron wire 136 whereas ionized air egress ports 143 are adjacent the middle of the wire. During airflow through the ion generator, on the order of about 1 cubic foot/minute for about 1/2 cubic foot development chamber, and upon energization of the corotron wire, the air entering through ports 138 scavenges the ions created by the corotron wire as the air flows from ports 138 through the ion generator to ports 142, where the ionized air exits to that portion of the development chamber beneath baffle 130. Extending through the sidewall of the development chamber directly opposite ion generator 132 is a toner entrance port 144, also positioned beneath baffle 130. Port 144 is connected to toner aerosol generator 146, including source 149 of pressurized gas, by way of flexible coupling 148. Adjacent at least a portion of the side of development chamber 104 are porous pads 150, securely mounted to the external sides of the development chamber. Pads 150 are adapted to permit the passage of air therethrough whereby suitable minimum pressure differential can be maintained between development chamber 104 and the external environment outside the chamber. This prevents pressure buildup in the chamber which would cause seal leakage with attendant leakage of toner to the internal portions of the surrounding enclosure. Pads 150 are of limited porosity such that while airflow can be maintained therethrough, toner particles cannot pass therethrough into the internal portions of surrounding enclosure. It should be noted that the ion generator 132 may be located external to development chamber 104 if so desired with appropriate entrance points made through the walls of the development chamber to enable the ions to enter and mix with the powder cloud under baffle 130.

At the bottom of the development chamber, in the bottom wall thereof, there is a port 152 through which unused toner is withdrawn during the purge cycle. Port 152 is connected to conduit 154 by means of flexible coupling 156. Inside conduit 154, there is a flapper valve 158 hinged for rotational movement, in the direction as shown by the arrow, about hinge 160. Conduit 154 is connected to toner filter means 162 which, in turn, is connected to blower means 164. At the beginning of the purge cycle, valve 158 is moved downward and to the left under the action of rotary solenoid (not shown). This movement of flapper valve 158 places the blower in communication with the development chamber through toner filter means 162, conduit 154 and port 152, whereby unused toner is withdrawn from the development chamber. After the purge cycle, the valve is released from its open position and rotated back into the closed position under urging of a spring (not shown).

Backing plate 102 also has a plurality of apertures 171, 173, 175 and 177 therethrough adapted to receive pins 179 extending upwardly from the movable development chamber 104, the pins serving to guide the chamber into the proper sealing position about the photoconductive surface. Mounted on top of plate 102 is switch S1 having an arm depending downwardly through a slot in the backing plate. Positioned adjacent aperture 175 is a switch S2 which is actuated when a pin passes through the aperture. Adjacent aperture 171 is a switch S3 which is actuated when the development chamber is lowered and the pin which had passed through aperture 171 is no longer in contact therewith.

In operation, as the xerographic plate is inserted into the development means, the leading edge thereof comes into contact with the arm on switch S1 which extends through the slot in development chamber backing plate 102. With continued insertion of the xerographic plate into the development means, the arm associated with switch S1 drops off the trailing edge of the xerographic plate, deactuates the drive motor, whereby the plate is properly positioned within the development means, and initiates elevation of the development chamber.

Pressurized gas is admitted to inflatable elements 110 whereby the development chamber 104 is raised into the upper position where it seats against the xerographic plate to provide a leaktight development chamber suitable for the development of the latent-electrostatic image residing on the downward-facing photoconductive surface. As the chamber is elevated, a pin 179 passes through aperture 175 in backing plate 102 and comes into contact with switch S2, indicating that the chamber is in the upper position. The movement of the development chamber to the upper position is confirmed by switch S4 (not shown) which monitors the pressure applied to inflatable elements 110. When both switches S2 and S4 confirm that the development chamber is in the upper position, the development cycle is initiated.

Development means 100 is of the powder cloud type wherein a fine cloud of charged toner particles is created by powder cloud generator 146, for example, as shown in U.S. Letters Pat. Nos. 2,812,883 or 2,862,646, and blown into development chamber 104 through port 144. The powder cloud so generated by the powder cloud generator is then mixed with an ion cloud produced by passing pressurized air over corotron wire 136 and then through egress ports 142 in housing 134. The powder cloud and the ion cloud meet under baffle 130 and are thoroughly mixed. Because of the flow rates of the powder cloud generator and the ion generator, the charged powder cloud within the development chamber is caused to swirl out from under baffle 130 toward the upper portion of the development chamber where the charged toner particles are attracted to the latent electrostatic image, whereby the latent image is made visible.

As set forth above, the positive pressure differential maintained between ion generator 132 and development chamber 104 prevents the toner particles from entering the ion generator housing and contaminating or clogging the corotron wire or the ion generator egress ports 142. This pressure differential is created and maintained by limiting the number and size of ports 142 in relation to the pressure applied to housing 134 through pressure controlled communication with the pressurized air source. A pressure differential in the range from about 4 inches of water to about 18 inches of water has been successfully utilized to prevent such contamination.

Grid electrode 124, which is biased oppositely from the polarity of the latent-electrostatic image, when it is desired to form a positively sensed reproduction, is utilized to remove particles or ions which have the same polarity as the latent electrostatic image and to establish field lines normal to the photoconductive surface whereby the phenomenon of edge deletion can be controlled, as desired. In xeromammography, for example, where it is advantageous to develop discontinuities in the object being examined, substantial, or at least partial, retention of edge deletion (i.e., edge enhancement) will be, in many cases, the desired condition whereby maximum information can be obtained in the developed image. The grid electrode also serves to accelerate the movement toward the photoconductor surface of those particles, of like polarity to the bias potential, which are between the grid and the photoconductor surface.

When switches S2 and S4 initiate the development cycle, they do so by actuating master timer means (not shown) which controls the development operations broadly described above in accordance with the following sequence. Initially, pressurized air is continuously applied to the corotron housing and the grid electrode is appropriately biased. The positive pressure differential, on the order of about 4-18 inches of water, thereby established between the ion generator housing and the development chamber proper serves to prevent toner particles, subsequently blown into the development chamber, from entering the ion generator housing and adversely affecting the operational characteristics of the ion generator. Then, the toner powder cloud generator is pulsed one or more times to fill the development chamber with a charge of toner particles, the air applied to the corotron housing being at sufficient pressure to prevent toner particles from entering the housing. Typically, pulsation of the toner feed mechanism is on the order of about 0.25-0.6 second. The powder cloud generator is initially pulsed by itself to avoid discharge of the latent electrostatic image by charged ions blown into the development chamber in the absence of toner particles. Thereafter, the powder cloud generator and the ion generators are simultaneously pulsed a plurality of times, for example, 6 times. The pulsation of the ion generator, by energizing corotron wire 136, is on the order of about 1-3 seconds. A typical pulse-off time period is on the order of about 5 seconds, after which the powder cloud and ion generators are pulsed together (for their respective pulsing periods) and then off a plurality of times, as indicated above, in successive 5 second intervals. The toner cloud and the ion cloud, injected into the development chamber from opposite sides, meet under baffle 130, where they are thoroughly mixed. Due to the flow rates chosen for the ion and powder clouds, the now charged toner cloud swirls out from under the baffle, rises through and around the biased grid electrode to the top of the development chamber where toner particles, normally charged to a polarity opposite that of the polarity of the latent electrostatic image, are drawn thereto whereby the latent electrostatic image is made visible. It should be understood that the aforementioned operating characteristics can be varied by the operator or radiologist to give the desired developmental results, e.g., best images for viewing, most efficient use of toner, etc. For example, by providing an appropriately biased grid electrode and by establishing the proper toner and ion generator pulsing periods, the presence of free ions (i.e., ions unattached to toner particles) and their discharge of portions of the latent electrostatic image can be, to a great extent, controlled, such that the final reproduction is not adversely degraded.

At the end of the pulsed development cycle, during which the latent-electrostatic image has been made visible by the attraction of oppositely charged toner particles, the master timer means actuates the rotary solenoid associated with flapper valve 158 such that the valve is caused to rotate to the position where the development chamber is in communication with blower 164 via filter means 162. Air is drawn into the chamber via filter pads 150 and helps to entrain unused toner. In this manner, unused toner is purged from the development chamber. After the purge is completed, as determined by the purge duration timer normally set for a purge duration on the order of about 6 seconds, the rotary solenoid is deactuated, whereby flapper valve 158 is spring urged back to the position as shown in FIG. 1. Additionally, at the end of the purge cycle the three-way valve to inflatable elements 110 is opened to the atmosphere, thereby causing the pressurized gas to be released from the inflatable elements whereby the development chamber is lowered from the leaktight position under the combined influence of gravity and spring means (not shown).

As the development chamber is lowered, a second pin 179, passing through aperture 171 in backing plate 102, drops out of contact with switch S3. This actuates a motor (not shown) which causes initiation of the movement of the xerographic plate out of the development means.

To complete the xerographic processing cycle, it is necessary to withdraw a single support sheet from a supply tray, transport it to a point where it is in registration with the xerographic plate having the powder image thereon, transfer the powder image to the support sheet as the plate and the support sheet are moved in synchronization, strip the image-bearing support sheet from its position adjacent the xerographic plate and transport it to a fuser where the powder image is permanently bonded to the adjacent support sheet surface. Simultaneously, the xerographic plate is cleaned and processed for subsequent reuse.

The baffle which defines the zone in which the toner cloud and the ion cloud initially mix is dimensioned and shaped to provide a uniform distribution of toner in the upper section of the development chamber as each cloud is moved out from under it during succeeding pulses of the toner and ion generator apparatus. In general, this movement of the charged powder cloud is affected by the kinetic energy imposed on the toner particles from the powder cloud generator system and by the convection currents set up by the continuous flow of air from the ion generator system. The shape of the baffle should be such as to provide for proper charged powder cloud movement to the development zone adjacent the charged xerographic plate.

The bias applied to the grid electrode attracts to the grid toner particles and free ions charged to the opposite polarity as the bias potential. In this manner, it suppresses the movement of free ions which might adversely affect image quality through image discharge. The electrode also accelerates the movement toward the photoconductive surface of those toner particles between the grid and the photoconductive surface which are charged to the same polarity as the polarity of the grid bias potential. It is this accelerated cloud of particles that is to be used in the development of the latent image. Finally, the grid electrode assists in controlling the contrast of the developed image by providing an electrostatic field to counteract the fringing fields associated with the edges between adjacent areas of varying charge density. This field causes the toner particles having the desired polarity of charge to move toward the photoconductive surface thereby increasing the effectiveness of the development process. In controlling the contrast of the image, the grid electrode also serves to increase the exposure latitude of a xerographic system. This is, the grid electrode provides the radiologist with a more versatile system which will give him essentially the same information content, in the final image, in a wider variety of exposure conditions with a minimum change in his exposure technique.

Additionally, the grid electrode makes it possible to develop both positive and negative images. In a conventional xerographic processing system, the selenium photoconductor is positively charged. The latent image remaining after exposure is therefore of positive potential. In order to develop an image of the positive sense, it is necessary to provide a preponderance of negatively charged toner particles in the cloud approaching the plate. These negatively charged particles are attracted to the latent image and collect thereon, creating a visible image of deposited toner. To increase the effectiveness of this developing apparatus for positive image development, the grid electrode is provided with a negative potential bias. As indicated above, the negatively biased grid attracts positively charged particles (not ions) and at the same time causes the negatively charged particles, in the zone between the grid and the plate, to be accelerated toward the plate surface. The field lines in this system generally exist between those portions of the grid opposite those areas on the plate which contain fairly high positive potential in the latent image. In order to increase development in plate areas of lesser plate charge, it is necessary to increase the bias potential on the grid, thus creating a more effective field to bring additional toner in these areas.

To develop toner images having a negative sense, however, the grid bias is of positive potential. In this situation the stronger field lines occur in the areas of the plate that have been more heavily exposed and therefore show a much lower positive charge potential. The positive potential on the grid tends to attract the negatively charged toner particles and ions and at the same time causes the positively charged toner particles, between the grid and the plate surface, to be accelerated toward the uncharged or relatively uncharged areas of the plate surface. By varying the magnitude of the bias applied to the grid, similar control of the positively charged toner particles, to that described above in the preceding paragraph with regard to the negatively charger toner particles, can be achieved.

To partially counteract the effect of toner accumulated at the bottom of the development chamber which is not completely removed by the purge system, there can be provided a grounded grid electrode spaced from the bottom wall of the development chamber. Though relatively closely spaced to the bottom wall, the grid should not extend under baffle 130 so as to adversely affect the powder cloud-ion cloud mixing process, nor should it be so close to the bottom wall as to be easily and rapidly covered by accumulated toner.

Initially, since the charged powder cloud must pass adjacent to the grounded grid electrode as it exists from beneath the baffle, the electrode serves to collect free ions not attached to toner particles in the charged cloud. Additionally, since the electrode is in grid form and toner particles call fall therethrough, the electrode, to some extend, minimizes the field effect of toner accumulated at the bottom of the development chamber beneath the grid. In this regard, the electrode serves to isolate any charge which might be residing on the insulative toner particles which have fallen through the grid.

Prolonged deposition of toner particles on either or both of the grid electrodes can significantly affect, if not removed, the operational characteristics of the development chamber. Accordingly, mechanical cleaning of the electrodes is periodically required. This can be achieved, for example, by causing a brush to traverse the grid electrode between development cycles, preferably in an automated mode after each complete development cycle. By limiting toner buildup on the grid electrode in this manner, the adverse affects, including possible breakdown with free ion generation between the grid and the plate surface which might cause localized discharge of the plate, can be virtually eliminated. Where such traversing brush cleaning is provided, a grid of parallel conductors, as opposed to an intersecting grid of parallel and perpendicular conductors, is preferred since essentially total toner removal is more easily achieved.

Since it has been found that accumulated toner at the bottom of the development chamber does affect the electrostatic force field lines existing in the development chamber, other means, such as scrapper means or brush cleaner means, can be provided, in addition to or in place of, the purge system already described to remove accumulated toner from the bottom of the development chamber. In this manner, the adverse affect of having charged, insulating toner particles at the bottom of the development chamber can be minimized or substantially eliminated.

In addition, the baffle and grid electrode described above can be utilized in a system where the developer particles are charged by triboelectrification (i.e., in the absence of charging by means of the ion generator described above). In such a system, the grid electrode functions to separate the toner particles of undesired charge and to accelerate the toner particles of desired charge, between the grid and the latent electrostatic image-bearing surface, towards the latent image. This is in addition to the previously stated function of modifying the electrostatic field lines associated with the latent electrostatic image.

While the invention has been described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. For example, although the present invention has been described herein with reference to an automated development technique wherein the xerographic plate is advanced into the development station, the development chamber moved upwardly to seat against the xerographic plate and, after the development cycle completed, the chamber lowered, and the xerographic plate advanced from the development station, the invention is also susceptible to manually operated development chambers or development chambers wherein the xerographic plate is lowered to seat against the upper edge of the sealing means defining the upper bounds of the leaktight development chamber. In addition, many modifications may be made to adapt a particular situation, material or structural design, to the spirit of the present invention without departing from its essential teachings.

Claims

What is claimed is:

1. An apparatus for developing a latent electrostatic image on a surface comprising a chamber having a pair of opposed sidewalls, support means for supporting said surface with its latent electrostatic image-bearing side facing said chamber, means for supplying a cloud of developer particles to said chamber through a port in one of said opposed sidewalls, ion generator means for supplying a cloud of ions to said chamber through at least one port in the other of said opposed sidewalls, means to maintain said ion generator means at a higher pressure than said chamber whereby developer particles are substantially prevented from entering said ion generator means, a baffle overlying said ports through which said developer particles and said ions are introduced into said chamber, said baffle extending completely from one of said opposed sidewalls to the other of said opposed sidewalls whereby said ion cloud and said developer particle cloud initially meet and mix under said baffle, a grid electrode positioned between said latent electrostatic image-bearing surface and said baffle, and means to bias said grid electrode, said grid electrode being spaced from the latent electrostatic image-bearing surface a distance sufficient to control the contrast and quality of the developed image.

2. The apparatus of claim 1 wherein said ion generator means is positioned external to said chamber.

3. The apparatus of claim 1 wherein said ion generator means is positioned within said chamber, said ion generator means having a housing having at least one corona emitting wire therein, that portion of said ion generator housing beneath said baffle and opposite said developer particle port at least partially defining the opposed sidewall of said chamber through which said cloud of ions is introduced into said chamber

4. The apparatus of claim 1 further including means for introducing under pressure an ionizable gas to said ion generator means whereby the flow of said pressurized gas carries ions formed within said ion generator means through said ion generator ports into that portions of said chamber beneath said baffle.

5. The apparatus of claim 1 wherein said ion generator means comprises a housing having at least one corona emitting wire therein, and means to introduce under pressure an ionizable gas to said housing adjacent only the ends of said corona emitting wire, whereby the normal flow of the ionizable gas is along the length of said corona wire to about the midpoint thereof where the ion cloud exits through said ion generator ports into that portion of said chamber beneath said baffle.

6. The apparatus of claim 5 further including means to energize said corona wire.

7. The apparatus of claim 6 wherein said corona wire is energized intermittently.

8. The apparatus of claim 1 wherein said means for supplying a cloud of developer particles to said chamber is pulsed intermittently.

9. The apparatus of claim 1 wherein said grid electrode is spaced on the order of about 1 inch to about 2 inches from the latent electrostatic image-bearing surface.

10. The apparatus of claim 1 further including means for causing relative movement of said development chamber and said latent electrostatic image-bearing surface whereby a leaktight development chamber is defined.

11. The apparatus of claim 1 wherein said baffle has an upper flat portion and downwardly depending side portions, whereby there is defined a zone within said chamber beneath said baffle where said ion cloud and said toner cloud initially meet and mix prior to movement of the charged toner cloud to the top of said chamber adjacent said latent electrostatic image-bearing surface.

12. The apparatus of claim 11 wherein said side portions are angled outwardly from said upper flat portion.

13. The apparatus of claim 1 further including means to remove unused toner from said chamber.

14. The apparatus of claim 1 further including a grid electrode adjacent the bottom wall of said chamber in those areas not directly beneath said baffle.

15. The apparatus of claim 1 further including means adjacent the walls defining said chamber for permitting the establishment of a pressure differential between the inside of said chamber and the outside of said chamber, said means assisting in the maintenance of the pressure differential established between said ion generator and said chamber.

16. The apparatus of claim 15 wherein said means comprises porous pads of limited porosity such that air but not developer particles can pass therethrough.

17. The apparatus of claim 1 further including delay means connected to said ion generator means and said developer cloud generator for enabling developer particles to be supplied to said chamber before free ions.

18. The apparatus of claim 1 wherein said ionizable gas is continuously introduced into said ion generator housing during the development of the latent electrostatic image.

19. An apparatus for developing a latent electrostatic image on a surface comprising a chamber having a pair of opposed sidewalls; means for supporting said surface with its latent electrostatic image-bearing side facing inwardly toward said chamber; a baffle extending completely from one of said opposed sidewalls to the other of said opposed sidewalls, said baffle being spaced from the remaining sidewalls whereby material introduced into said chamber under said baffle can move from beneath said baffle to the top of said development chamber; means for supplying a cloud of developer particles to said chamber through a port in one of said opposed sidewalls, said port being positioned in said sidewall beneath said baffle; ion generator means for supplying a cloud of ions to said chamber through at least one port in the other of said opposed sidewalls, all of said ion generator ports being positioned beneath said baffle, said ion generator means comprising a housing having at least one corona emitting means therein, means to energize said corona emitting means, means to introduce under pressure an ionizable gas to said housing whereby the flow of said pressurized gas carries ions formed within said ion generator means through said ion generator ports into that portion of said chamber beneath said baffle; means to maintain said ion generator means at a higher pressure than said chamber whereby developer particles are substantially prevented from entering said ion generator means; means adjacent the walls defining said chamber for permitting the establishment of a pressure differential between the inside of said chamber and the outside of said chamber, said means assisting in the maintenance of the pressure differential established between said ion generator and said chamber; a grid electrode positioned between said latent electrostatic image-bearing surface and said baffle, said grid electrode being spaced from the latent electrostatic image-bearing surface a distance sufficient to control the contrast and quality of the developed image; and means to bias said grid electrode.

20. The apparatus of claim 19 wherein said ion generator means is positioned external to said chamber.

21. The apparatus of claim 19 wherein that portion of said ion generator housing beneath said baffle and opposite said developer particle port at least partially defines the opposed sidewall of said chamber through which said cloud of ions is introduced into said chamber

22. The apparatus of claim 19 wherein said pressurized gas is introduced to said ion generator housing only adjacent the ends of said corona emitting means, whereby the flow of ionizable gas is along the length of said corona emitting means to about the midpoint thereof where the ion cloud exits through said ion generator ports into that portion of said chamber beneath said baffle.

23. The apparatus of claim 19 wherein said corona emitting means is energized intermittently.

24. The apparatus of claim 19 wherein said means for supplying a cloud of developer particles to said chamber is pulsed intermittently.

25. The apparatus of claim 24 further including means to cause said means for supplying a cloud of developer particles to said chamber to be pulsed at least once before said corona emitting means is energized.

26. The apparatus of claim 19 wherein said grid electrode is spaced on the order of about 1 inch to about 2 inches from the latent electrostatic image-bearing surface.

27. The apparatus of claim 19 wherein said baffle has an upper flat portion and outwardly and downwardly depending side portions, whereby there is defined a zone within said chamber beneath said baffle where said ion cloud and said developer cloud initially meet and mix prior to movement of the charged developer cloud to the top of said chamber adjacent said latent electrostatic image-bearing surface.

28. The apparatus of claim 19 wherein said means for permitting the establishment of a pressure differential between the inside and outside of said chamber comprises porous pads of limited porosity such that air but not developer particles can pass therethrough.

29. An apparatus for developing a latent electrostatic image on a surface comprising a chamber having a pair of opposed sidewalls, support means for supporting said surface with its latent electrostatic image-bearing side facing said chamber, means for supplying a cloud of charged developer particles to said chamber through a port in one of said opposed walls, a baffle overlying said port through which said developer particles are introduced into said chamber, said baffle extending completely from one of said opposed sidewalls to the other of said opposed sidewalls, a grid electrode positioned between said latent electrostatic image-bearing surface and said baffle, and means to bias said grid electrode to a desired potential and polarity, said grid electrode being spaced from the latent electrostatic image-bearing surface a distance sufficient to control the contrast and quality of the developed image.

30. The apparatus of claim 29 wherein said means for supplying a cloud of charged developer particles to said chamber is pulsed intermittently.

31. The apparatus of claim 29 wherein said grid electrode is spaced on the order of about 1 inch to about 2 inches from the latent electrostatic image-bearing surface.