US3893419A

US3893419A - Background reduction technique

Info

Publication number: US3893419A
Application number: US428539A
Authority: US
Inventors: Herbert L Bresnick
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1973-12-26
Filing date: 1973-12-26
Publication date: 1975-07-08
Anticipated expiration: 1992-07-08
Also published as: CA1076791A

Abstract

In an uphill electroded cascaded development station, background toner deposits are reduced by substantially discharging background areas or recharging background areas reversely as by subjecting the partially developed image to illumination or appropriate corona charging. Further development of the latent electrostatic image bearing surface occurs with the background areas thereof being less attractive for toner.

Description

United States Patent Bresnick 1 July 8, 1975 [54] BACKGROUND REDUCTION TECHNIQUE 3,654,893 4/1972 Piper et a1. v. 1 18/637 3,685,488 8/1972 Stover 118/637 [75] inventor: Herbert L. Bresnlck, Rochester, 3,722,994 3,1973 Tanaka et 8/637 N 3,741,156 6/1973 .10 eta]. [18/637 [73] Assignee: Xerox Corporation, Stamford,

Conn. Primary ExammerMervm Stem Assistant ExaminerDouglas Salser [22] Filed: Dec. 26, 1973 [21] App]. No.: 428,539 [57] ABSTRACT In an uphill electroded cascaded development station, 52 us. c1 ll8l637- 117/175 background depsits are reduced by Substan- 511 1111. C1 603 15/08 dischargir'g backgmund areas recharging 58 Field of Search 118/637- 117/175 backgmmd reversal) as by subjecting the tially developed image to illumination or appropriate [56] References Cited corona charging. Further development of the latent electrostatic image bearing surface occurs with the UNITED STATES PATENTS background areas thereof being less attractive for 3,411,482 11/1968 Brodie 118/637 tenet 3,611,992 10/1971 Lyles et a1. 3,628,950 12/1971 Wirley 117 175 6 Claims, 3 Drawing Figures 38 BIAS I4 46 4 souac: 42

2 I2 32 ans sounc:

1 BACKGROUND REDUCTION TECHNIQUE This invention relates to xerographic development wherein background toner depositions are minimized in an uphill electroded cascaded development zone by reducing background charges during development.

In the process of xerography as described in 1.1.8. Pat. No. 2,297,69! to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support latent electrostatic images. In the usual manner of carrying out the process, the xerographic surface is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern of the original being reproduced.

The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as powder. The powder is held in image areas by the electrostatic charge field gradiant on the xerographic surface. Where the charge field gradiant is greatest, the greatest amount of toner materials is deposited. Where the charge field grandiant is the least, little or no toner material is deposited. Thus a visible toner image is produced in conformity with the light image of the copy being reproduced. The toner is subsequently transferred to a sheet of paper or other surface and suitably affixed thereto to form a permanent print.

During normal downhill two component cascade development of electrostatic images, developer is dropped on a xerographic plate to be developed. The plate is moved downwardly in the direction of developer flow. Upon being dropped onto the xerographic plate, toner is jarred loose from the carrier granules. These free floating toner particles are then deposited on the image bearing surface to develop not only image areas but also, to a lesser degree, backgroundor non image areas. As the partially denuded carrier granules flow across the latent image within the development zone, they scavenge toner from non-image areas where it had been weakly held thereto triboelectrically.

An alternate approach to downhill two component xerographic cascaded development is described in US. Pat. No. 3,638,610 issued Feb. 1, 1972 to James M. Lyles, et al. According to that disclosure, the latent electrostatic image bearing surface is transported upwardly against the flow of cascading two component developer, with the development zone being heavily electroded along the length. By this it is meant that closely spaced, electrically conductive members are placed close to the photoreceptor so that developer can flow therebetween. The conductive elements hold the charge pattern and field gradiants outwardly so that central portions of large solid areas on the electrostatic images present field lines for attracting toner thereto. In this manner, large solid areas may be developed. Without the development electrodes, no field gradiants would have existed and, in these large centrally charged areas, no development would have occurred thereat.

Further, according to the Lyles, et al disclosure, the electrodes are biased whereby toner particles within the development zone tend to be more attracted to the development electrode than non image area portions of the photoreceptor. This biasing results in the reducing of background toner depositions which would otherwise appear as dark particles on the final copy.

In the apparatus described in the Lyles et al disclosure, the upper-most portion of the development zone, that portion that the latent image sees last before leaving the development zone is highly biased whereby toner particles would rather be electrostatically attracted thereto than to the image bearing surface. In the lower portion of the development zone, a bias only slightly above the non-image areas is utilized to permit the developing of the latent electrostatic images.

The present invention utilizes a charge dissipating device such as a lamp or corotron at an intermediate position along in the development zone to reduce the charge in non-image areas after the image has been substantially developed and before it is subjected to the turbulent area of high powder clouds in the upper region on the development zone. In this manner, development of non image areas thereof prior to the xerographic plate leaving the development zone is further minimized.

It is therefore an object of the instant invention to increase the efficiency of xerographic development systems.

It is a further object of the instant invention to minimize the deposition of unwanted toner particles on a latent electrostatic image bearing surface moving in an electroded zone, upwardly against the flow of two component developer.

It is a further object of the instant invention to reduce background charge patterns in a central portion of a two component development system whereby deposition of background toner will be minimized.

A further object of the instant invention is to create xerographic imagesiwith minimized toner deposition in non-image areas.

These and other objects of the instant invention are attained by a charge reducing device in a central portion of a two component, cascade development system wherein the image bearing surface is moved upwardly against the flow of developer while the image bearing surface is subjected to the effects of a development electrode.

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

FIG. 1 is a schematic side elevation of a continuous and automatic reproducing machine constructed in accordance with the instant invention; and

FIGS. 2 and 3 are enlarged side elevational views of the development zone shown in FIG. 1.

Referring now to the drawings, there are shown embodiments of the subject invention in a suitable environment such as an automatic xerographic reproducing machine. The machine as shown in FIG. 1 includes a xerographic plate 10 or surface formed in the shape of a drum. The plate has a photoconductive layer or light receiving surface carried on a conductive backing, journaled on the shaft in a frame to rotate in the direction indicated by the arrow. The rotation will cause the plate surface to sequentially pass a series of xerographic processing stations. For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the plate surface may be described functionally as follows:

A charging station A at which a uniform electrostatic charge is deposited on the photoconductive plate;

An exposure station E at which light or radiation pattern of copy to be reproduced is projected onto the plate surface to dissipate the charge in the exposed areas thereof to thereby form a latent electrostatic image of the copy to be reproduced;

A developing station C at which xerographic developing material, including carrier granules and toner particles having an electrostatic charge opposite to that of the electrostatic image, is cascaded over the plate surface whereby the toner particles adhere to the latent electrostatic image to form a toner powder image in a configuration of the copy being reproduced;

A transfer station D at which the toner powder image is electrostatically transferred from the plate surface to a transfer material or a support surface such as paper;

A drum cleaning and discharge station E at which the plate surface is brushed to remove residual toner particles remaining thereon after image transfer and exposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon; and

A fixing station F, remote from the plate wherein the paper and toner particles are subjected to heated pressure rolls to fuse the toner particles into the support material to create a permanent copy of the original.

Referring now specifically to FIGS. 2 and 3, the ap paratus of the present development system basically comprises a series of

conductive development electrodes

12 and 14 or control members, separated by insulating blocks 16 which are supported in close parallel relationship with the moving xerographic surface so as to form a continuous and closed flow path therebetween. This flow path is herein referred to as the development zone 18. Positioned at the upper entrance to the enclosed development zone is an entrance chute 20 through which a continuous flow of two component developer material is introduced into the development zone. The series of

development electrodes

12 and 14 form the backwall of the development zone and function to control the distribution of developer material in the flow stream of development. The front wall of the development zone is defined by the upwardly moving drum surface. It should be noted that in this particular development apparatus, the drum surface is moving upwardly in opposition to the downwardly moving developer flow stream. This particular flow relationship between developer and plate is opposite that utilized in most conventional cascade development zones as described, for example, in US. Pat. No. 3,062,109 to Mayo et al. Therefore, the carrier beads do not operate in the classical sense to first give up toner during the initial portion of the development process and then, when partially denuded, scavenge the weakly held background toner from the non-image areas.

The development electrode segments and the insulating blocks separating each one from another are mounted on a nonconductive rigid support frame 22 and the frame is affixed to the sidewall of the developer housing. An opening 24 is provided in one end wall of the housing through which the rotating drum surface is allowed to pass in close proximity to the electrodes. The electrodes and the insulating support blocks both extend horizontally across the drum surface and have end seals in contact with the extreme ends of the drum surface to enclose the development zone.

Two component developer material is transported from a storage and mixing area in the sump 26 of the developer housinginto an entrance chute 20 by means of a conveyor 28. The conveyor is made up of a series of horizontally extending elongated buckets affixed to endless belts which pass over pulley assemblies. As the buckets are transported in the direction indicated, through the developer sump area, the buckets become loaded with developer material. The continuous movement of the buckets through the developer mix sufficiently agitates the developer mix to produce triboelectric charging of the materials. The loaded buckets, upon leaving the sump area are raised to the top of the developer housing where the developer is discharged into the entrance chute thus applying a continuous flow of developer material to the development zone.

The electrodes of the instant system include the lower or development electrode 12 and the upper or clean up electrode 14. The development electrode is the main electrode and it is isolated from the adjacent electrode by means of dielectric blocks 16. A suitable biasing source 30 is connected to the main development electrode [2 by means of wiring 32 and an electric terminal 34. The adjacent upper electrode is of substantially equal thickness. The development electrode is biased to the same potential as that on the image bearing surface at a magnitude about or slightly greater than that of the non-image areas of the latent electrostatic image being developed. When the developer material is cascading across the latent image first entering the development zone at the bottom, the development electrode permits the good development of image areas throughout its entire extent. The conductivity of this electrode permits the development of large solid image areas while its bias reduces toner deposition in background areas.

The upper electrode or clean up electrode 14 then acts upon the already partially developed latent image in the upper or final portion of the development zone adjacent the opening through which fresh developer is first introduced into the development zone. This is the last area of development that the latent image will see prior to leaving the development zone. It is physically positioned immediately above the development or lower electrode 12 and is electrically isolated therefrom by means of a dielectric block 16. The clean up electrode, by virtue of a high potential, primarily functions to establish an extremely high directional field force capable of attracting toner to the electrode side of the system. This will control the movement of free or weakly held toner particles in the upper part of the development zone. The clean up electrode further functions to condition the carrier beads moving into contact with the plate in this region to clean unwanted background toner from the xerographic plate surface.

The clean up electrode is connected to a suitable bias source 36 by means of wire 38 and an electrical terminal 40. The biasing source 36 functions to place the cleanup electrode at an extremely high potential and of a polarity the same as that found on the drum surface. The strong direction force field produced in this upper region is of a strength sufficient to force freely or weakly held toner particles away from the drum surface. Any free toner material being present within the region of the cleanup electrode, is therefore, moved under control conditions along the backside of the system. The charge field established in this region is of sufficient strength to also strip some of the toner particularly weakly attracted toner, from the beads flowing in contact with the drum and move this toner toward the back or electroded side of the development zone. With the preponderance of toner material concentrated on the backside of the flow stream, carrier beads moving in contact with the drum surface are relatively depleted of toner particles and therefore can readily function to scrub and electrostatically attract weakly held background development from the drum surface. A potential of about 1200 volts positive on this electrode has been found suitable when acting upon a photoreceptor which has about 750 volts in image areas and 250 volts in background areas.

In testing the development system of the instant invention, it was found that background toner deposition could be further minimized by the developer action adjacent the cleanup electrode if the non-image area charges were reduced to thereby further reduce the tendency of non-image area toners to deposit upon the non-image areas of the photoreceptor. This in effect strengthened the electrostatic action of the cleanup electrode on the developer.

One way to reduce unwanted background development is to completely discharge background areas or recharge them negatively to thereby repel toner. In the alternative, a more practical approach is merely to minimize the toner attracting charge from the background areas as through a lamp or corona source. Shown in the figures is hardware to achieve this end. The lamp 42 of FIG. 2 is placed between the cleanup and

development electrodes

12 and 14 at a point whereafter the majority of the development has occurred. When the lamp is appropriately placed in this zone, it can shine light onto the photoreceptor to therby discharge non image areas of the charge pattern significantly as from 250 to 100 volts. While there is, in fact, a light impeding flow of developer between the lamp and the photoreceptor, enough light can pour through this veil to cause this desired charge reducing result. The illumination source is preferably constructed with a surface material having triboelectric properties sending to repel the toner, to thus prevent the lamp from getting dirty by the toner. Further, the flow of developer also acts to strike toner on the lamp to move it back into the developer flow.

The lamp would at first appear to have an effect of also reducing the charge in image areas. But since these areas are substantially completely covered by toner as caused through the development of images adjacent the development electrode, no undesirable effect occurs in minimizing the image area charges. If image area discharge occurred, it would undesirably reduce the toner to photoconductor bond.

As shown, the source of illumination of the instant invention of FIG. 2 is illustrated as an electroluminescent panel where the lumin output can be readily modified through varying the voltage from its potential source 44 to the source of illumination by wires 46 and terminal 48.

Shown in the FIG. 3 embodiment is a corotron 50 biased negatively to effect the same result of the lamp. The corotron is biased by variable potential source 52 via wires 54 and terminal 56 with the corotron wire being shielded by member 58. The field pattern of the photoreceptor coming adjacent this corotron will cause non image areas to attract the negative emissions. In this manner, the background areas of the latent image will be reduced to minimize the toner attracting bond thereof.

In the alternative, the alternating source of emissions could be utilized to act upon the photoconductive surface and as in the above described embodiment. such emissions act as the negative bias described above with the positive emissions being repelled by the like positive charges on the photoreceptor.

In either of the embodiments, however, the result is the same in that a partially, or nearly completely developed image is subjected to the effect of a charge reducing device which will minimize the charge in nonimage areas. This reduces the tendency of non-image areas to attract toner from the further or clean up section of the development zone. Development of non image areas will thus be minimized and the overall effect will be a more faithful copy of the image being reproduced inasmuch as toner depositions in non image areas has been minimized.

While the instant invention as to its objects and advantages has been described as being carried out in specific embodiments, it is not intended to be so limited, but it is intended to cover the invention broadly within the scope of the appended claims.

What is claimed is:

1. In a xerographic development zone wherein relative motion exists between the developing material and the surface to be developed, the improvement comprising means adjacent an intermediate portion of the development zone to reduce the charge on non-image areas of the surface being developed.

2. The development system of claim I wherein said last mentioned means is a source of illumination.

3. The development system of claim 1 wherein said last mentioned means is a source of corona emissions.

4. In a xerographic development system wherein two component developer including, carrier granules and triboelectrically adhering toner particles, is cascaded down a photoconductive surface support latent electrostatic images to be developed, and wherein the surface is moved upwardly against the flow of two component developer, and wherein the lower portion of the development zone is provided with a low potential development electrode and wherein the upper portion of the development zone is provided with a high potential clean up electrode, the improvement comprising means within the development zone to reduce the charge on the photoreceptor in non-image areas to thereby minimize the deposition of background toner in non-image areas.

5. The development system of claim 4 wherein said last mentioned means is an electroluminescent light emitting panel.

6. The development system of claim 4 wherein said last mentioned means is a source of corona emissions opposite the charge in non-image areas of the latent electrostatic image.

* II II!

Claims

2. The development system of claim 1 wherein said last mentioned means is a source of illumination.

4. In a xerographic development system wherein two component developer including, carrier granules and triboelectrically adhereing toner particles, is cascaded down a photoconductive surface support latent electrostatic images to be developed, and wherein the surface is moved upwardly against the flow of two component developer, and wherein the lower portion of the development zone is provided with a low potential development electrode and wherein the upper portion of the development zone is provided with a high potential clean up electrode, the improvement comprising means within the development zone to reduce the charge on the photoreceptor in non-image areas to thereby minimize the deposition of background toner in non-image areas.