US3893413A - Xerographic developing apparatus - Google Patents

Abstract

A xerographic development apparatus for enhancing the quality of the images developed. A dielectric screen grid is interposed between a conventional xerographic developing apparatus and a photosensitive plate to be developed. The grid is arranged to move in synchronous contact with the photosensitive surface through an active development zone wherein the recorded image is made visible. Preferably, the grid has a triboelectric characteristic such that it will assume a charge potential similar to that of the potential of the images recorded on the plate. Alternatively, the grid can be charged to a desired potential by means of a corona generator prior to entering the development zone.

Description

United States Patent 11 1 Weiler 1 July 8, 1975 41 XEROGRAPHIC DEVELOPING APPARATUS 3,647,291 3/1972 Pressman et al 355/3 [75] Inventor: Ernest A. H. Weiler, Rochester,

Primary Examiner-Ronald Feldbaum Attorney, Agent, or Firm.lames J. Ralabate; Earl T. [73] Ass1gnee: gerox Corporatlon, Stamford, Reichen onn.

[22] Filed: Sept. 21, 1972 57 S C [21] Appl. No.: 291,111 A xerographic development apparatus for enhancing the quality of the images developed. A dielectric 52 US. Cl 118/637- 117 175 screen grid is interpmd a 51 1111. C1. B05b 5/02- sha 13/06 gtaphic devebping f a f 58 Field of Search 118/629 637' 117/17.5- Plate be devebped' T f R 1 synchronous contact mm the photosensmve surface through an active development zone wherein the re- [56] References Cited corded image is made visible. Preferably, the grid has a triboelectric characteristic such that it will assume a UNITED STATES PATENTS charge potential similar to that of the potential of the 3,011,473 12/1961 Gundlach 118/637 images recorded on the plate. Alternatively, the grid 3.203.394 8"965 p 81 ill 3/637 can be charged to a desired potential by means of a l 2:32;? 32 corona generator prior to entering the development 3,237,150 11/1966 Lehmann... 117/175 3,518,969 7/1970 Snelling 118/637 6 Claims, 3 Drawing Figures 3,592,167 7/1971 Blow 118/637 1 XEROGRAPHIC DEVELOPING APPARATUS This invention relates to a xerographic developing system and, in particular, to a developing system for enhancing the development of half-tone images and for continuous solid area images.

The development of half-tone and solid area images has long been a problem in the xerographic art because of certain inherent characteristics found in the xerographic process. Conventionally, in xerography, a light image of the original subject matter to be reproduced is recorded upon a photosensitive surface in the form of a latent electrostatic image. The latent image is developed or made visible by applying charged toner particles to the plate where the particles are captured within the electrostatic force fields associated with the recorded imaged regions. As explained in greater detail in Dessauer and Clarks text Xerography and Related Processes" published by Focal Press in those plate regions where a defined electrical contrast exists, as for example along the edge regions of the electrostatic images, the electrical force fields extend outwardly from the photoreceptor surface and therefore are capable of readily encapturing and holding the charged toner particles. On the other hand, where little or no electrostatic contrast exists, as for example in the interior regions of a relatively large image, the force fields remain close to the plate surface and, as a consequence, little or no development takes place.

Development electrodes, such as disclosed by Walkup in U.S. Pat. No. 2,784,109, have been devised to provide the xerographic process with a solid area developing capability. Typically, the electrode is constructed of a conductive material and is positioned in close proximity to the image bearing plate surface within the development zone. The electrode is electrically biased to provide an electrical contrast over the entire imaged region whereby the lines of force extend outwardly from the plate towards the electrode. Development is accomplished by passing developer material through the force field, that is, between the electrode and the plate surface. Although the development electrode, as attested to by its wide commercial acceptance, has proven to be a useful device in enhancing xerographic development, it nevertheless has certain inherent physical drawbacks in that it limits the type of development that might be employed.

A screen-like conductive electrode has also been developed which also has the capability of enhancing xerographic development by promoting both solid area and half-tone image development. This type of development technique is the subject matter of a U.S. Pat. No. 3,01 1,479 to Gundlach. The conductive grid has the further advantage over the development electrode in that it allows the developer material to pass freely through the screen into the development zone thereby providing greater flexibility in the type of development that can be used. In practice, an extremely fine grid is placed in direct overlying contact with the photoreceptor surface to be developed and a charge is placed on the conductive member. As such, the grid dissipates the charge on the localized plate regions coming in contact therewith thereby electrically dividing the plate into a vast number of small electrical islands. Each island is small enough so that the total edge development thereof serves to completely develop the island region. The conductive screen grid, like the development electrode has proven itself to be the useful tool in enhancing xerographic development, but like the electrode it too has certain disadvantages. Because the conductive grid is brought into direct contact with the photoconductive surface there is an ever present danger of completely shorting out the photoconductive drum which could damage the photoreceptor and, of course, degrade the copy produced.

It is therefore an object of the present invention to provide an unique development system for enhancing the solid area and tonal reproducing capabilities of a xerographic developing system.

It is a further object of the present invention to provide a relatively inexpensive and simple development system having the capability of reproducing solid area and tonal images.

These and other objects of the present invention are attained by means of a xerographic developing apparatus adapted to deposit charged toner particles upon the surface of a photoreceptor surface thereby rendering the recorded latent electrostatic images thereon visible and interposing a foraminous member between the plate and the developing means within the developing zone, the foraminous member being in contact with the photoreceptor and being fabricated of a dielectric material capable of supporting a triboelectric charge thereon for enhancing the development systems solid area and half-tone developing capability.

For a better understanding of the invention as well as other objects and further features thereof reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings wherein:

FIG. I is a schematic representation of an automatic xerographic reproducing machine employing the developing apparatus of the present invention;

FIG. 2 is a partial perspective view showing a throw away developing system embodying the teachings of the present invention;

FIG. 3 is also a partial perspective view showing a second embodiment of a throw away container utilizing the teachings of the present invention.

Referring now to FIG. 1, there is illustrated an automatic xerographic reproducing machine incorporating the developing system of the present invention. As is illustrated therein, and as is widely practiced in the art, the automatic xerographic processing stations are positioned about the periphery of a xerographic drum 10 which is composed of a conductive substrate 11 upon which is placed a layer of photoconductive material 12. The drum is mounted upon a support shaft 13 and is arranged to rotate in the direction indicated so as to advance the photoconductive surface through the processing stations. Although not shown, it should be understood that drive means are herein provided for both moving and coordinating the various machine functions involved to produce a xerographic copy of the original input scene information in the manner to be explained in greater detail below.

The processing stations include a charging station A wherein a corona generator 14 is positioned so as to uniformly charge the drum surface to a relatively high potential. The charged plate surface is then moved through an exposure station B wherein a flowing light image of the input scene information is recorded upon the photoreceptor in the form of a latent electrostatic image. As the drum continues to rotate in the direction indicated, the imaged drum surface is caused to be passed through a developing station C embodying the teachings of the present invention. As will be explained in greater detail below, a charge particulate material, herein referred to as toner, is applied to the image drum surface thus rendering the latent electrostatic image visible.

The now visible image, supported on the drum surface, is brought into synchronous moving contact with a cut sheet of final support material within a transfer station D. Here, the back side of the copy sheet is sprayed with a corona discharge, provided by a second corona generator 16, of a polarity and a potential sufficient to electrostatically transfer the toner image from the photoreceptor plate surface to the support sheet.

As shown in FIG. 1, the sheets of final support material are initially stored within the machine in a stack configuration by means of storage trays 17. A feed roller 18, located above the stack, functions to both separate the uppermost sheet therefrom and advance the sheet in proper registration into the transfer zone. Upon the completion of the transfer operation, the image bearing copy sheet is stripped from the drum surface and transported via vacuum transport 19 through a fusing station F wherein the toner images are affixed to the sheet; the sheets then being delivered into a collecting tray 20.

Any residual toner remaining on the photoreceptor surface after completion of the transfer step is removed therefrom within a cleaning station E. A cleaning brush 23 is rotatably mounted within the cleaning station and is arranged to both mechanically and triboelectrically dislodge residual toner particles from the drum surface. The residual toner material collected on the brush is exhausted from the system through a vacuum tube 24.

Referring more specifically to the developing station C illustrated in FIG. 1, there is provided a magnetic brush developing system which is generally referenced 25. Magnetic brush development, as disclosed by Greig in U.S. Pat. No. 2,874,063, basically involves the creation of a brush-like structure of magnetic developer material, the fibers of which are brought into light rubbing contact with the imaged photoreceptor to achieve the desired development. The brush forming structure 26 utilized herein is located within a new developer housing 27 and is made up of a multiple pole magnet 28 mounted in a stationary condition within a rotating nonpermeable applicator roller 29. The roller is adapted to continually move through the development zone in close proximity with the photoreceptor surface.

In operation, the magnetizable developer material generally made up of charged toner particles electrostatically adhering to a permeable carrier material, is stored in the lower sump region of the developer housing. A lifting mechanism 30, having vanes formed about the outer periphery thereof, rotates through the sump area and carries developer material into operative communication with an intermediate metering roll 31. The metering roll, which is similar in construction to the magnetic developer roller, picks up a predetermined quantity of material and transports the material into the flux field associated with the developer roll assembly 26. This magnetic material is then formed into a developing brush with the fibers of the brush extending outwardly in the radial direction from the applicator roll surface. As the fibers are drawn through the development by the moving applicator roller, the toner in the brush fibers is brought into contact with the image plate surface causing the latent electrostatic image recorded thereon to be developed.

In the developing apparatus of the present invention, a screen-like foraminous member 32 is interposed between the magnetic brush developing means and the photoreceptor surface within the development zone. The foraminous member is electrically isolated from the other machine components and is placed in an endless loop configuration over a series of four pulleys. A continuous run 33 of the belt, extending between pulleys 35 and 36, is forced into intimate physical contact against the photoreceptor surface within the developing zone. A motor means 40, actively engaging a drive pulley, provides a drive input to the endless member causing the member to move through the development zone in the same direction as the xerographic plate surface. The speed of the screen-like member is controlled through the drive system whereby there is no relative motion between the contacting members as they pass through the development zone.

The foraminous member is constructed of a dielectric material capable of being charged by triboelectrification to a desired level as it moves over the roller guiding it through its endless path of travel. In this manner, the member is caused to assume a charge having a polarity similar to the charge recorded on the photoreceptor. Conventionally, in the xerographic process, the photoreceptor is charged to a positive potential and negatively charged toner particles utilized to develop the images. In this particular case, the screen would be fabricated of a material which would assume a positive charge somewhat close to the background image potential recorded on the drum surface. The term background potential, as herein used refers to the voltage recorded on the photoreceptor after exposure in those plate regions containing very weak or no input scene information, as for example, the voltage produced by a white sheet of paper. It has been found that a finely woven silk fabric can be successfully employed in most conventional xerographic processes of this nature to attain the desired results. It should be clear, however, that any type of dielectric material, both of a natural and synthetic nature, can be herein utilized without departing from the teachings of the present invention.

The dielectric material is fabricated into a fine screen-like structure having a grid size through which the toner particles can pass freely during the developing operation. A grid size of between and 300 mesh can be used with most commercially available toners within a particle size range of between 5 and 20 microns.

As previously noted, the force field normally associated with a recorded latent electrostatic image is composed of strong outwardly reaching lines of force in and about the edge area where an electrical contrast exists between the image and background areas. However, where the electrical contrast is not so pronounced, as for example, the internal regions of relatively large recorded images (solid areas), the outwardly reaching force fields are weak and development normally re mains incomplete unless aided by some outside instrumentality. In the present invention, development of this type of image, as well as half-tone images, is enhanced by placing the triboelectrically charged dielectric screen in actual physical contact with the photoreceptor surface. It has been found that this electrically isolated dielectric screen, although triboelectrically charged, does not act to dissipate, or bleed off, the existing image charge on the photoreceptor. However, the screen serves to physically divide the imaged regions on the plate into a number of relatively small islands. Furthermore, the effect of placing the charged grid in contact with the photoreceptor within the development zone, because of the charge relationship involved, causes the force lines of each island to reach outwardly through the grid openings into the development zone thus enhancing the ability of the more highly charged image region to capture toner particles. Because the grid is charged to the same polarity as the photoreceptor, the combined effect of the two force fields is such as to pull the toner particles into close physical proximity with the photoreceptor where the particles can be more efficiently utilized in the developing process.

It should be further pointed out that the screen also serves to prevent background development. The screen, because it is charged to about the same potential as the background recorded on the plate, will capture toner particles migrating towards the plate before they can come into contact with the background regions. In this sense, the screen functions as both an electrical and a mechanical barrier to toner which normally would be deposited in the background regions.

Alternatively, a corona generator 45, similar to that disclosed by Vyverberg in U.S. Pat. No. 2,836,725, may be employed to place the foraminous member at a predetermined charge potential. As shown in FIG. 1, the generator is positioned to spray a corona discharge onto the screen surface prior to the screen entering the development zone. Although not shown, further means may also be provided beyond the development zone to neutralize the initial charge on the screen thereby preventing a successive charge build up from occurring. This neutralization means can take the form of a second corona generator arranged to spray a neutralizing charge of corona upon the member surface as it leaves the active development zone.

Although a megnetic brush developing system is herein shown, it should be clear to one skilled in the art that any known developing mechanism can be employed in conjunction with the dielectric screen without departing from the teachings of the present invention. Similarly, the invention, because of its unique characteristics, lends itself to use in novel system, as for example, the throw away development units illustrated in FIGS. 2 and 3.

Referring now more specifically to FIG. 2 there is illustrated one form of a throw away unit. The unit, generally referenced 50, is fabricated in the form of a cyl inder which is arranged to ride in moving contact with the surface of the photoconductive drum 10. The cylinder basically consists of an internal brushJike element 51 over which is secured a fine screen grid member 52 having the properties of the foraminous member described in reference to developing station C illustrated in FIG. I. The brush consists of an axially aligned shaft 53 having a central hub 54 upon which is secured a number of radially extending brush-like fibers 55. Although not shown, end caps are also provided which function to seal both ends of the cylindrical member thus forming the cylinder into a self-contained unit. The unit is initially charged with a quantity of toner mamanner described above thus terial which is supported therein between the brushlike fibers.

In order to prevent the screen from slipping over the brush fibers and to provide the cylinder with the desired structural form, the fibers of the brush possess a cross-sectional dimension which is relatively large in comparison to the screen grid openings. The brush fibers are constructed of a resilient material which imparts a deformable quality to the cylinder. Preferably the fibers are constructed of or coated with a material capable of triboelectrically interacting with the toner material stored therein to charge the toner to a developing potential. The fibers may be composed of any suitable material including metallic strands such as steel, natural resins, thermoplastic resins, partially cured thermoplastic resins, solvents and/or combinations of solute and solvents. For further information concerning these materials and means for applying them to a support material, reference is herein had to US. Pat. No. 3,507,686. Because of the electrical relationship involved, the brush and the screen will combine to hold the toner within the container under normal conditions but will release the toner when the unit is brought into contact with a xerographically recorded image.

In operation, the shaft 53 of the throw away unit 50 is operatively coupled to a gear box 56 which, in turn, is a motor means 57. When the unit is mounted in the machine in the manner illustrated in FIG. 2, the outer surface of the cylinder is deformed against the photoreceptor surface so as to provide an extended development zone in which the toner particle contained within the unit can be applied to the image recording surface. The movement of the drum surface is co-ordinated with that of cylindrical unit via gear box 56 and shaft 59 whereby the peripheral surfaces of the two contacting members move at the same relative speed. As can be seen, development takes place, within the contact development zone, through the dielectric screen in the providing a relatively cheap and compact system capable of delivering a high quality image development. Because of its simple and inexpensive construction it is herein contemplated that the unit will be disposed of once the toner supply contained therein becomes depleted.

Similarly, the throw away unit can take other forms. One such form is illustrated in FIG. 3. Here the unit 60 is formed about a relatively rigid skeleton 61 constructed of plastic or the like having a series of helically aligned slotted openings 62 formed thereon. Over the skeleton is placed a complementary layer of resilient elastomeric material 63 upon which is placed the outer dielectric screen grid 64 having the properties described above. Contained within the cylindrical member is conventional xerographic two component developer material made up of toner particles and relatively coarser carrier beads. As the cylindrical member is rotated in the direction indicated through the development zone, the resilient elastomeric material is caused to deform against the photoreceptor surface thereby providing an extending development zone.

During the rotation of the unit the two component developer material is sufficiently agitated to triboelec trify the toner particles to a desired level as is normally accomplished in cascade development. The developer material flows through the slotted openings in the skeletal member and contacts the back side of the foraminous member. As the screen moves in contact with the plate in the development zone, the developer material gives up toner particles to the imaged regions on the plate in the manner described in greater detail above. It should be understood that the granular carrier beads are chosen having a size greater than the screen grid size wherein the carrier materials will be retained within the unit during the developing process.

While this invention has been described with reference to the structure disclosed herein. it is not confined to the details as set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

What is claim is:

1. Apparatus for developing a latent electrostatic image recorded upon the charged surface of a photosensitive plate including means to apply charged particulate developer material to the image bearing plate surface whereby the latent electrostatic image recorded thereon is rendered visible, and

a foraminous shield interposed between the developing means and the plate, the shield being in contact with the imaged plate surface and being formed of a dielectric material having a triboelectric characteristic such that the shield is charged to a polarity similar to the polarity of the charged plate surface and a potential about the same as that of the background potential recorded on the plate surface.

2. The apparatus of claim I wherein the foraminous shield is comprised of a silk screen.

3. The apparatus of claim 1 wherein the developing means comprises a magnetic brush developing unit.

4. Apparatus for enhancing the development of a latent electrostatic image recorded upon a moving charged photosensitive plate including means to apply charged particulate material to the imaged plate within a development zone whereby latent electrostatic images recorded thereon are rendered visible,

a foraminous shild of dielectric material mounted in electrical isolation and interposed between the developing means and the plate,

means for moving the shield through the development zone in contact with the image plate so that there is no relative motion between the plate and the shield whereby the plate is developed through the shield. and

means to electrically bias the shield to a polarity similar to the polarity of the charged plate and to a potential about the same as that of the background potential recorded on the plate.

5. The apparatus of claim 4 wherein the means to electrically bias the shield comprises a corona generator arranged to spray corona upon the shield prior to the shield entering the development zone.

6. The apparatus of claim 4 wherein the openings formed in the shield are of a size to allow a quantity of developer material to pass therethrough.

* l l =l'

Claims (6)

1. Apparatus for developing a latent electrostatic image recorded upon the charged surface of a photosensitive plate including means to apply charged particulate developer material to the image bearing plate surface whereby the latent electrostatic image recorded thereon is rendered visible, and a foraminous shield interposed between the developing means and the plate, the shield being in contact with the imaged plate surface and being formed of a dielectric material having a triboelectric characteristic such that the shield is charged to a polarity similar to the polarity of the charged plate surface and a potential about the same as that of the background potential recorded on the plate surface.

2. The apparatus of claim 1 wherein the foraminous shield is comprised of a silk screen.

3. The apparatus of claim 1 wherein the developing means comprises a magnetic brush developing unit.

4. ApparaTus for enhancing the development of a latent electrostatic image recorded upon a moving charged photosensitive plate including means to apply charged particulate material to the imaged plate within a development zone whereby latent electrostatic images recorded thereon are rendered visible, a foraminous shild of dielectric material mounted in electrical isolation and interposed between the developing means and the plate, means for moving the shield through the development zone in contact with the image plate so that there is no relative motion between the plate and the shield whereby the plate is developed through the shield, and means to electrically bias the shield to a polarity similar to the polarity of the charged plate and to a potential about the same as that of the background potential recorded on the plate.

5. The apparatus of claim 4 wherein the means to electrically bias the shield comprises a corona generator arranged to spray corona upon the shield prior to the shield entering the development zone.

6. The apparatus of claim 4 wherein the openings formed in the shield are of a size to allow a quantity of developer material to pass therethrough.

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