US3828728A

US3828728A - Xerographic development system

Info

Publication number: US3828728A
Application number: US00197874A
Authority: US
Inventors: F Yang
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1971-11-11
Filing date: 1971-11-11
Publication date: 1974-08-13
Anticipated expiration: 1991-08-13
Also published as: CA980635A; DE2241468A1; GB1413337A; FR2160200A5; JPS4856141A; NL7213635A; IT970350B; SE372831B; BE791193A

Abstract

An apparatus for maintaining a substantially uniform distribution of toner particles within the developer flow in a latent electrostatic image development system in which an electrically conductive member positioned adjacent the flow path of the developer material is connected to a source of electrical potential and biased to a polarity opposite from the polarity on the toner particles in the developer flow. The electrically conductive member is moved in a direction generally transverse to the direction of developer flow. Toner particles are attracted to the conductive member from developer material in portions of the developer flow having greater quantities of toner particles dispersed therein and as the conductive member is transported across the developer flow, toner particles are attracted from the conductive member to portions of the developer flow having lesser quantities of toner particles dispersed therein.

Description

United States Patent 1191 Yang 1451 Aug. 13, 1974 XEROGRAPHIC DEVELOPMENT SYSTEM Primary Examiner-Robert R. Mackey 75 Inventor: Frank Y. Yang, Webster, NY. Aswan Mnstem [73] Assignee: Xerox Corporation, Stamford, [57] ABSTRACT Conn. An apparatus for maintaining a substantially uniform Flledi NOV-11,1971 distribution of toner particles within the developer [21] AWL Na: 197,874 flowin a latent electrostatic image development sys tem 1n which an electrlcally conductive member positioned adjacent the flow path of the developer matel 1 rial is connected to a source of electrical potential and [51] Int. Cl G03g 13/00 bi d t a polarity opposite from the polarity on the Field of Search 1 18/621, 623, 627, 637, toner particles in the developer flow. The electrically 117/175, A, 93-42 conductive member is moved in a direction generally transverse to the direction of developer flow. Toner References Cited particles are attracted to the conductive member from UNITED STATES PATENTS developer material in portions of the developer flow 3,357,403 12/1967 Donalies 118/637 having greater quantities of toner Particles dispersed 3,330,437 4/1968 Swylcr 118/637 therein and as the Conductive member is transported 3,396,700 8/1968 Donalies 1 18/637 across the developer flow, toner particles are attracted 3,575,139 4/1971 Nuzum 1 18/637 from the conductive member to portions of the devel- 3,645,770 2/1972 Flint ll7/l7.5 per flow having lesser quantities of toner particles 3,680,779 8/l972 Reilly 239/3 di d h i 3,707,390 l2/l972 Sullivan, Jr. l17/l7.5

9 Claims, 3 Drawing Figures PAIENIE mm 31am saw 1 or 2 INVENTOR, FRANK Y. YANG ATTORNEY XEROGRAIHIC DEVELOPMENT SYSTEM BACKGROUND OF THE INVENTION This invention relates in general to the development of latent electrostatic images and more particularly to maintaining a substantially uniform toner distribution ina developer flow.

In the practice of electrophotography or xerography, a photosensitive surface such as xerographic plate comprising a layer of photoconductive insulating material supported on a conductive backing is uniformly electrostatically charged over its surface and then exposed to a light pattern of the image to be reproduced, thereby discharging the photoconductor in the illuminated areas. The areas of the layer that thereafter remain charged thus form an electrostatic latent image in conformity with the configuration of the subject matter to be reproduced.

The latent electrostatic image may then be developed by contacting it with a finely divided electrostatically attractable material such as a resinous powder. The powder is held in image areas by the electrostatic charges on the layer. Where the charge is greatest, the greatest amount of material is deposited; and where the charge is least, little, if any, material is deposited. Reversal development techniques familiar to those skilled in the art wherein charged areas that remain on the layer after exposure represent non-image areas, may also be employed to develop the latent electrostatic image. Thereafter, the developed xerographic powder image is usually transferred to a support surface, such as a sheet of copy paper to which it may be fixed by any suitable means.

A commonly used developing material in electrostatic copying devices consists of a pigmented resinous powder referred to hereinafter as toner" and a coarse granular material hereinafter referred to as carrier." The carrier is removed in the triboelectric series from the toner so a charge is generated between the powder and carrier upon mutual interaction. Such charge causes the toner to adhere to the carrier. The carrier, besides providing a charge to the toner, permits mechanical control so the toner may readily be brought into contact with the exposed xerographic surface. As noted hereinabove, the toner particles are attracted to the electrostatic latent image to produce a visible powder image. The carrier may also take the form of brushlike fibers having properties and functions similar the the granular carrier material. Such a brush-like carrier is described, for example, in U.S. Pat. No. 3,25l,706. The brush may also be formed of fibers formed of magnetic carrier granules formed into a brush-like formation through the use of a magnetic field as described, for example, in U.S. Pat. No. 2,832,311.

The quality of the developed image may be affected by the ratio of toner particles to carrier granules commonly referred to as toner concentration. For example, if there is a deficiency of toner, the image areas will be unable to attract sufficient toner to fully develop the image resulting in light copies. On the other hand, if there is an excess amount of toner in a developer material, the image areas become overly dark, with quantities of toner becoming attracted to non-image areas.

The reason for this phenomenon is the attractive force holding toner particles on a carrier granule is reduced as the number of toner particles on the particular carrier granule is increased. Conversely, as the number of toner particles on a carrier granule is decreased, the attractive force holding the particles on the carrier is increased. Therefore, with fewer toner particles on a carrier granule, the particles will not be readily given up to the electrostatic charge of the latent image. In the case of a high number of toner particles on a carrier granule, the force of attraction therebetween is markedly decreased, often to the point where even residual charges on non-image areas is sufficient to attract the toner particles.

In the past, efforts have been made to control the amount of toner dispensed to the developer mixture by various methods and means. Normally, this has been accomplished by adding toner intermittently on the basis of a timed cycle or by a device which measures either the toner concentration directly or the density of the developed image. While such systems are generally satisfactory, they do present some obvious shortcomings. In particular, these systems need time to react to localized depletion of toner caused by the development of dense image areas.

For example, an original may have large non-image areas upon which are solid-area objects to be developed. Obviously, depletion of the toner particles from the developer mixture will be localized. The indiscriminate addition of toner particles to the developer mixture, as has been the common practice heretofore employed in the art, has sometimes caused the problems hereinabove described.

It has been proposed in the past to use mechanical means, such as augers, disposed within the developer material sump, to cross mix the developer material in an attempt to obtain uniform toner distribution. Such mechanical means require ample space for its utilization. In addition, since it is only the toner that is required to be re-distributed, it is somewhat inefficient to move the entire developer mixture in order to obtain optimum toner distribution, since the developer mixture is formed of a relatively small amount of toner particles relative to the amount of carrier granules. Furthermore, such mixing of the toner particles has heretofore been accomplished in the developer housing which is generally remote from the development zone of the system. Thus, there is a time lag between the occurrence of localized depletion of toner particles at the development zone, and the occurrence of reuniform toner distribution therein.

SUMMARY OF THE INVENTION It is therefore an object of the invention to improve development systems employed in xerographic reproducing apparatus.

Another object of this invention is to control the toner distribution in a developer mixture by selfregulating means.

' Another object of this invention is to eliminate development defects characterized by localized developer conditions.

A still further object of this invention is to regulate the toner distribution of developer in an automatic copying machine so as to maintain high quality prints on a continuous basis.

It is still a further object of this invention to minimize the required number of parts and to increase the efficiency of obtaining uniform toner distribution in development zones.

A still further object of this invention is to provide a toner distributing device adjacent the development zone of the xerographic apparatus.

These and other objects of the invention are obtained by providing an electrically conductive member adjacent the flow path of the developer material. The member is connected to a source of electrical potential and is biased to a polarity opposite from the polarity on the toner particles in the developer flow. An electric field is thus created which extends from the electrically conductive member to the developer material. The electric field attracts toner particles to the conductive member from portions of the developer material having greater quantities of toner particles dispersed therein. The electrically conductive member is moved in a direction generally transverse to the direction of movement of developer material to transport the toner particles attracted to the conductive member so they are attracted by portions of the developer mixture having lesser quantities of toner particles dispersed therein.

Other objects of the invention and further features thereof shall become apparent to those skilled in the art in view of the following detailed disclosure and description of a preferred embodiment of the invention, particularly when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view of a xerographic reproducing machine embodying the principles of the present invention;

FIG. 2 is an enlarged view ofa portion of the machine illustrated in FIG. I; and

FIG. 3 is a frontal elevation view of substantially the enlarged portion ofthe xerographic machine illustrated in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 shows a typical xerographic machine in which the present invention may be employed. As shown therein, a photosensitive member 11 is supported on drum 12 which is driven about shaft 14 by a motor (not shown). Member 11 is covered with layer 13 of electrically conductive material which, in turn, is covered on its outer surface with layer 15 of photoco'nductive insulating material such as vitreous selenium. An electrostatic latent image is formed on member II by passing it under charging station A and exposin station E.

The charging station includes any suitable means for placing a uniform electrostatic charge on layer 15 such a corona charging device 16.

Exposing station B includes a source of light 17 and lens 18. An original subject to be reproduced. such as transparency I9, is disposed between the light source and lens. Transparency 19 is fed from reel 20 through the beam of light generated by source 17 and then rewound on reel 21 after being exposed. The light. transparency and lens, working on combination. project on the photosensitive member an optical image. the movement of which is synchronized with the movement of the drum. The projected optical image causes selective charge dissipation in illuminated areas of layer l5, thus forming an electrostatic latent image. Other means of forming such images including means for forming images on ordinary insulating surfaces are known in the art and may be used in lieu of the one shown.

After the charging and exposing steps are completed. the photosensitive member passes by development station C wherein the latent electrostatic image is ren dered visible. Station C will be described more fully in conjunction with FIGS. 2 and 3. Following image development, the next step in the typical xerographie process is the transfer step which is accomplished at transfer station D. The image is transferred from photosensitive member 111 to a web of paper 22 or the like which is fed between guide rolls 23 and 24. Second corona device 27 cffects the electrostatic transfer of the developed latent image from member 11 to web 22. Web 22 is fed from reel 25 before transfer and is rcwound on reel 26 after transfer and fusing. Fusing element 28 is positioned to heat the transferred image and thus fix and permanently bond the image to paper web 22.

Cleaning station E includes a rotary cylindrical brush 29 which contacts the photosensitive member subsequent to image transfer and removes any residual image material from layer 15, thereby readying it for reuse. The above described process and apparatus are familiar to those skilled in the xerographic arts. Any of the many known equivalents of process or apparatus elements may be employed in connection with the present invention.

Now referring to FIGS. 2 and 3, the details of the present invention shall be more fully described. Development station C includes a developer housing 30 comprising side walls 36 and 37 (shown in FIG. 3). A developer mixture is contained in the lower portion of housing 30 which functions as a sump therefor. The developer material comprises a mixture of charged toner particles and oppositely charged carrier granules. The carrier granules, which function to carry the toner particles and to generate a triboelectric charge on the toner particles, normally comprise a core. base or interior composed of any solid material which may be of high specific gravity such as glass or steel and nickel beads. covered with or encased in a suitable covering which imparts the necessary triboelectrie properties to the granular carrier material. The triboelectric relationship between the toner particles and granular carriers depends on the respective relative positions in the triboelectric series. In triboelectrification theory. materials are arranged in a column in such a way that each material is charged to a positive polarity when contacted with any material below it in the series and to a negative polarity when contacted with any material above it in the series. It is desirable to select the toner particles and granular carrier materials so their mutual electrification is considerable. The degree of such electrification is governed in most cases by the distance between the positions in the triboelectric series; that is, the greater distance they are removed from one another the greater the mutual electrification and the closer they are together in the series the less the mutual electrification. For a better description of how the triboelectric charge is generated on the toner particles, reference may be had to United States Pat. No. 2.6l8.55 l.

Rotatably disposed within housing 30 is developer distributing member 31, having vanes 31' extending therefrom. Member 31 rotates about shaft 38 which is suitably journaled by bearings 39 in

side walls

36 and 37 of the housing.

A magnetic brush is disposed in housing 30 above member 31. The magnetic brush includes a stationary magnet 33 and a roller or applicator member 32. Roller 32 is rotatably disposed about magnet 33. The magnetic field of magnet 33 attracts the magnetically attractable developer mixture to roller 32. The roller thereafter conveys the developer mixture so that the toner is attracted by the latent electrostatic image on layer 15. The roller is mounted on shaft 40, which is suitably journaled in walls 36 and 37.by bearings 41.

Shafts

38 and 40 are connected to a source of power (not shown).

In operation, as member 31 rotates, vanes 31 sweep the developer disposed in the bottom of housing 30 upward, so that the developer is brought within the magnetic field of magnet 33. The distance between member 31 and roller 32 of the magnetic brush should be limited so as to maintain the strength of the magnetic field at a sufficient level to attract the developer onto roller 32.

The developer attracted to roller member 32 by magnet 33 adheres thereto the roller rotates. The magnetic carrier-toner mixture orients itself along the lines of magnetic force, thereby forming a structure which is similar to the bristles of a brush. The developer mixture is thus rotated into close proximity with the latent electrostatic image formed on layer of member 11. The latent electrostatic image attracts toner particles form the developer mixture, to thereby develop the latent image. It should be noted, magnet 33 will be oriented so one of its poles is positioned to influence the developer mixture on roller 32 so the mixture will be erect when brought into close proximity with the latent electrostatic image.

As noted hereinbefore, it is of extreme importance that the toner distribution within the developer mixture be uniform to obtain copies having desired visual characteristics.

A conductive member 34 such as a wire, is provided in the flow path of the developer mixture. The member moves transversely to the movement of the developer mixture. One end of member 34 is opcratively connected to a coiled spring 43 which is disposed within housing 42. The other end of member 34 is looped over a suitable guide 44 and is affixed to one end of a rotatable member or other suitable means 45. Rotatable member 45 is connected to shaft 38 and will rotate therewith. Member 45 functions as an eccentric to transform the rotational movement of shaft 38 into a reciprocal or translational movement for member 34. As member 45 rotates, member 34 will move in a reciprocal member through the flow path of developer mixture. Spring 43 is provided to return member 34 to its initial position and in addition to provide a force to maintain member 34 in a taut manner.

One end of spring 43 is joined to a shaft 49. Shaft 49 is journaled within side walls 48, only one of which is shown in FIG. 3. The opposite end 50 of spring 43 is rigidly affixed to housing 42. One end of member 34 is also joined to shaft 49. As member 34 moves through the flow path of developer material, spring 43 is compressed. By utilizing the force produced by the compression of the spring, member 34 is maintained substantially taut and is returned to its initial position during the continued rotation of shaft 38.

Member 34 is connected via brush 46 to a source of electrical potential 47. The member is biased to a suitable potential, for example, 500 volts. An electric field is thus created about member 34. The source of electrical potential should be of the same polarity as the charge on the carrier material. The potential of a bare carrier granule should be greater than 500 volts for a reason that shall become more apparent hereinafter.

As noted hereinbefore, the attractive force holding toner particles on carrier granules is proportional to the number of particles on the particular carrier. Therefore, with fewer toner particles on a carrier granule, these particles will be more strongly attracted thereto and will not not be readily given up to the electrostatic charge of the latent image. Conversely, where a high number of toner particles are attracted to a carrier granule, the force of attraction therebetween is markedly decreased, often to the point where even residual charges on non-image areas are sufficient to attract the toner particles.

Member 34 moving within the flow path of the developer mixture will attract toner particles from carrier granules having a high number of toner particles attached thereto. The particles are attracted from the carrier granules to member 34 because the attractive force holding the toner particles on the toner granules is less than the attractive force between the toner particles and the field of the conductive member. Thereafter, when the toner particles are brought into contact with carrier granules having relatively fewer toner particles attached thereto, the attractive force between such toner particles and carrier granule is greater than the attractive force between the toner particles and the field of the conductive member. Uniform toner distribution within the developer mixture may be readily achieved.

Member 34 is preferably positioned substantially adjacent to one of the poles of magnet 33 so the developer particles are erected when brought into contact with the conductive member.

By positioning the conductive member close to the development zone of a xerographic apparatus, uniform toner distribution may be readily achieved. Localized toner depletion due to the reproduction of solid-area images is readily compensated for without the time lag heretofore encountered.

It should be specifically understood, member 34 may take other forms without departing from the scope of this invention. For example, member 34 may be an endless chain or belt, driven by a plurality of wheels or pulleys. Other arrangements for member 34 that may readily occur to one skilled in the arts may be also employed.

In a preferred embodiment, the surface area of member 34 may be increased by providing thereon projections 34'. Other suitable means for increasing the surface area of member 34 may be substituted in lieu of projections 34. By increasing the surface area of conductive member 34, a greater number of toner particles may be attracted thereto for each pass of the member through the development zone.

While the present invention is carried out in a specific embodiment, it is not intended to be limited thereby but it is intended to be covered broadly within the scope of the appended claims.

What is claimed is:

l. A development system for developing latent electrostatic images on a photosensitive surface comprismg:

a housing including a sump portion containing a quantity of magnetically attractable developer material comprising carrier granules and toner particles triboelectrically attracted to said carrier granules,

a generally cylindrical applicator member supported for rotation within said housing adjacent said photosensitive surface,

magnetic field producing means supported within said applicator member, for supporting quantities of said developer material on the surface of said up plicator member for contact with said photosensitive surface,

a developer distributing member supported for rotation within said housing having a plurality of vanes extending therefrom for transporting developer material from said sump portion into operative contact with said cylindrical applicator member,

an electrically conductive member disposed adjacent the path of movement of developer material supported on said applicator member,

a source of electrical potential connected to said electrically conductive member to bias said conductive member to a polarity opposite from the polarity of the toner particles in said developer material on said applicator member to create an electric field extending from said electrically conductive member to the developer material on said applicator member, said electric field attracting toner particles to said conductive member from said applicator member from portions of said developer material having greater quantities of toner particles dispersed therein, and

means for moving said electrically conductive member in a direction generally transverse to the direction of movement of said developer material on said applicator member to transport toner particles on said conductive member proximate to portions of developer material on said applicator member having lesser quantities of toner particles dispersed therein to permit an attraction of the transported toner particles from said conductive member to said portions of developer material on said applicator member having lesser quantities of toner particles dispersed therein.

2. The development system in accordance with claim 1 wherein said conductive member is positioned substantially adjacent one of the poles of said magnetic field producing means.

3. The development system in accordance with claim 2 wherein said conductive member includes means associated therewith to increase the surface area thereof.

4. The development system in accordance with claim 1 wherein said conductive member includes means associated therewith to increase the surface area thereof.

5. A development system for developing latent electrostatic images carried by a photoconductive surface: a sump having developer material therein, means including a magnetic transport means for moving said developer material from said sump along a predetermined path to the photoconductive surface, said developer material including magnetic carrier granules and toner particles triboelectrically attracted to said carrier granules, a member disposed adjacent said magnetic transport means, means for reciprocally moving said member in a transverse direction to said predetermined path for moving toner particles from portions of said developer material on said magnetic transport means having greater quantities of toner particles dispersed therein to portions of developer material on said magnetic transport means having lesser quantities of toner particles dispersed therein.

6. The structure as recited in claim 5 wherein said magnetic transport means is located to present developer to the latent image.

7. In a development system for developing latent electrostatic images carried by a photoconductive surface: a sump having developer material therein, means including magnetic transport means for moving said developer material from said sump along a predetermined path to the photoconductive surface. said developer material including magnetic carrier granules and toner particles triboelectrically attracted to said carrier granules, an electrically conductive member. an electrically conductive member disposed adjacent said magnetic transport means. means for moving said conductive member in a transverse direction to said predetermined path. a source of electrical potential connected to said electrically conductive member to bias said conductive member to a polarity which is the same as the polarity of the electrostatic image, said electrically conductive member being so located relative to said magnetic transport means that during movement thereof toner particles are attracted thereto from portions of developer material on said magnetic transport means having a greater quantity of toner particles dispersed therein for movement to portions of developer material on said magnetic transport means having lesser quantities of toner particles dispersed therein to permit attraction thereto of the toner particles on said conductive member.

8. The structure as recited in claim 7 wherein the movement of said conductive member is reciprocal.

9. The structure as recited in claim 7 wherein said magnetic transport means is located to present developer to the latent image.

Claims

1. A development system for developing latent electrostatic images on a photosensitive surface comprising: a housing including a sump portion containing a quantity of magnetically attractable developer material comprising carrier granules and toner particles triboelectrically attracted to said carrier granules, a generally cylindrical applicator member supported for rotation within said housing adjacent said photosensitive surface, magnetic field producing means supported within said applicator member, for supporting quantities of said developer material on the surface of said applicator member for contact with said photosensitive surface, a developer distributing member supported for rotation within said housing having a plurality of vanes extending therefrom for transporting developer material from said sump portion into operative contact with said cylindrical applicator member, an electrically conductive member disposed adjacent the path of movement of developer material supported on said applicator member, a source of electrical potential connected to said electrically conductive member to bias said conductive member to a polarity opposite from the polarity of the toner particles in said developer material on said applicator member to create an electric field extending from said electrically conductive member to the developer material on said applicator member, said electric field attracting toner particles to said conductive member from said applicator member from portions of said developer material having greater quantities of toner particles dispersed therein, and means for moving said electrically conductive member in a direction generally transverse to the direction of movement of said developer material on said applicator member to transport toner particles on said conductive member proximate to portions of developer material on said applicator member having lesser quantities of toner particles dispersed therein to permit an attraction of the transported toner particles from said conductive member to said portions of developer material on said applicator member having lesser quantities of toner particles dispersed therein.

7. In a development system for developing latent electrostatic images carried by a photoconductive surface: a sump having developer material therein, means including magnetic transport means for moving said developer material from said sump along a predetermined path to the photoconductive surface, said developer material including magnetic carrier granules and toner particles triboelectrically attracted to said carrier granules, an electrically conductive member, an electrically conductive member disposed adjacent said magnetic transport means, means for moving said conductive member in a transverse direction to said predetermined path, a source of electrical potential connected to said electrically conductive member to bias said conductive member to a polarity which is the same as the polarity of the electrostatic image, said electrically conductive member being so located relative to said magnetic transport means that during movement thereof toner particles are attracted thereto from portions of developer material on said magnetic transport means having a greater quantity of toner particles dispersed therein for movement to portions of developer material on said magnetic transport means having lesser quantities of toner particles dispersed therein to permit attraction thereto of the toner particles on said conductive member.