US3645770A

US3645770A - Improved method for developing xerographic images

Info

Publication number: US3645770A
Application number: US33117A
Authority: US
Inventors: Thomas J Flint
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1968-04-22
Filing date: 1970-04-16
Publication date: 1972-02-29
Anticipated expiration: 1989-02-28

Abstract

An improved method for developing latent electrostatic images by forming a surface coating of developer material including a magnetic component and an electroscopic component, transporting the coating of developer material along a path past latent electrostatic images to be developed, and uniformly charging the outer surface of the coating of a polarity opposite that of the latent electrostatic images. At development an outer layer portion of the coating is deflected into close proximity with the latent images to be developed and oscillating toward and away from the images in the development zone to effect a continuous undulating flow pattern of the outer layer of the coating.

Description

States Patent Unite 1' Flint '[54] IMPROVED METHOD FOR DEVELOPING XEROGRAPHIC IMAGES [72] lnventori Thomas J. Flint, l-lerkimer, N.Y.

[73] Assignee: Xerox Corporation, Rochester, N.Y.

[22] Filed: Apr. 16, 1970 [211 Appl. No.: 33,117

Related US. Application Data [62] Division of Ser. No. 723,041, Apr. 22, 1968, Pat. No.

1 Feb. 29, 1972 Swyler ..1 17/1 7.5 Lehmann.. 18/637 3,357,399 12/1967 Fisher ..1 18/637 3,058,444 10/1962 Sugarman, Jr. et al. 18/637 3,152,924 10/1964 Wanielista et a1. ..1 18/637 [57 ABSTRACT An improved method for developing latent electrostatic images by forming a surface coating of developer material including a magnetic component and an electroscopic component, transporting the coating of developer material along a path past latent electrostatic images to be developed, and uniformly charging the outer surface of the coating of a polarity opposite that of the latent electrostatic images. At development an outer layer portion of the coating is deflected into close proximity with the latent images to be developed and oscillating toward and away from the images in the development zone to effect a continuous undulating flow pattern of the o'uterlayer of the coating.

1 Claims, 5 Drawing Figures PATENTEDFEBZB 1972 645.770

sum 1 BF 2 INVENTOR. THOMAS J. FLINT BY WM; Am

AT'IDRAEV PATENTEDFEB 2 9 I972 SHEET 2 [IF 2 IMPROVED METHOD FOR DEVELOPING XEROGRAPHIC IMAGES This application, is a division of U.S. Pat. No. 3,552,355. This invention relates to electrostatographic copying and, particularly, to an improved method and apparatus for the deposition of visible powder material on an electrostatic latent image as in the development of a xerographic image or the like.

In xerography, it is usual to form an electrostatic image on a sensitized surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation as set forth, for instance, in U.S. Pat. No. 2,297,691 to Chester F. Carlson. Whether formed by this means or any other, the resulting electrostatic charge pattern is conventionally developed by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic image.

A common processof applying the developer to the electrostatic image described in U.S. Pat. No. 2,618,552 to E. N. Wise involves cascading a finely divided colored material called a toner deposited on a slightly more coarsely divided material called a carrier across the electrostatic image areas. The toner and carrier being rubbed against each other while cascading, impart an electrostatic charge to each other by triboelectric charging. When a carrier particle, bearing on its surface oppositely charged particles of toner, across an area on the image surface having an electrostatic charge, the

charge on the surface exerts greater attraction for the toner than does the carrier and retains the toner in the charged areas and separates it from the carrier particles. The carrier particles, being oppositely charged and having greater momentum, will not be retained by the charged areas of the plate. When a toned carrier particle passes over a noncharged area of the plate, the electrostatic attraction of the carrier particles for the toner particles is sufficient to retain .the toner on the carrier preventing deposition in such areas as the carrier particles momentum carries both toner and carrier past. I

The above process, referred to as cascade carrier development," has a high-development latitude and is particularly noteworthy in freedom from background deposition. Further, the process is dependable, operates with high efficiency under extreme humidity conditions and is easily converted to give either positive or reverse reproduction of the original to be copied. The process also has certain limitations. Thus, cascade carrier development gives little or no solid area coverage, that is, solid colored areas such as those presented by block letters develop only around the periphery leaving a white or undeveloped area in the center. Again, relying largely on gravity to move the carrier across the image-bearing surface, the process requires relatively large carrier particle sizes for best efficiency. As a result, using cascade development at high speeds places undue frictional stress upon the photoconductor surface and the developing materials as well as the equipment necessary to produce cascade movement of developing material. In other words at high speeds, the use of two-component developer material requires low impact of developing materials on photoreceptors and tightly sealed developer housings in order to prevent scattering and loss of toner particles and the usual carrier beads. Then, too, there is a tendency for smaller carrier particles to be retained on the plate thereby interfering with transfer of the toner image. Closely related to the cascade carrier development is magnetic brush development as disclosed in U.S. Pat. No. 2,832,311. In this process a granular carrier is selected having ferromagnetic properties and selected relative to the toner in a triboelectric series so as to impart the desired electrostatic polarity to the toner and carrier as in cascade carrier development. On inserting a magnet into such a mixture of toner and magnetic granular material the carrier particles align themselves along the lines of force of the magnet to assume a brushlike array. The toner particles are electrostatically coated on the surface of the granular magnetic carrier particles. Development proceeds as in regular cascade carrier development on moving the magnet over the surface bearing the electrostatic image so that the bristles of the magnetic brush contact the electrostatic imagebearing surface.

Magnetic carrier development gives good coverage of solid areas and is eminently suitable for machine application by reason of the greater compactness of the developer system and freedom from dependence on gravity which limits the placement of a cascade'carrier system around a rotary drum. Against these advantages, magnetic development is inherently less efficient than cascade development. In magnetic development only part of the brush contacts the image-bearing surface. ln-addition, the magnetic field restricts the motion of the carrier particles interfering with the individual toner particles smoothly rolling across the image surface. As one consequence of this, a higher concentration of toner is generally essential in magnetic carrier development. By reason of this and the electrical characteristics which result in solid area coverage,the process gives a high-background deposition and is generallycharacterized by poor development latitude.

As a consequence of these development techniques, toner powder images are formed and the toner consumed must be replenished to the developer mixture substantially in proportion to the amount consumed by complicated dispensing devices. Various attempts have been made to devise a single component development system in which toner particles are used without carrier beads, but thus far, none have been entirely successful.

It is therefore an object of the invention to improve the development of electrostatic latent images.

It is another object of the invention to provide method and apparatus for the development of electrostatic latent images utilizing a magnetic toner material.

It is another object of the invention to quality at very high development speeds.

It is a further object of the invention to effect optimum solid image quality with minimum background conditions during electrostatic development processing.

It is still a further object of the invention to produce solid area images while at the same time effecting line copy images at very high speeds using a minimum of developing materials and mechanical parts and equipment and thus extensively reducing the impact and frictional wear on the photoreceptor and the developing materials.

It is still a further object of the invention to obviate the need for regulating toner concentration in a developer mixture in proportion to the amount consumed.

These and other objects of the invention are attained by utilizing a magnetically controlled toner which is applied to an electrostatic latent image in an undulating pattern at the development station of an electrostatic reproduction machine. Means are provided for imparting a uniform charge of proper polarity to the toner to effect high quality development for both line and solid images.

A preferred form of the invention is shown in the accompanying drawings, wherein:

FIG. 1 is a schematic sectional view of a typical xerographic reproduction machine embodying the principles of the invention;

FIG. 2 is a side-sectional view of the development apparatus according to the present invention;

FIG. 3 is an end-sectional view of the development apparatus taken along line 3-3 ofFIG. 2;

FIG. 4 is an enlarged view ofa circled portion of FIG. 3, and

FIG. 5 is an isometric view partly broken away of the development apparatus.

For a general understanding of a typical xerographic processing system in which the invention may be incorporated, reference is made to FIG. 1 in which various components of a typical system are schematically illustrated. As in all xerographic systems, a light image of an original to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon.

enable high image Thereafter, the latent image is developed with the same or an oppositely charged developing toner material, depending upon negative-to-positive or positive-to-positive mode of reproduction, to form a xerographic powder image corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface such as a sheet of paper or the like to which it may be fused by a fusing device whereby the powder image is caused permanently to adhere to the support surface.

For purposes of the present disclosure, the xerographic reproduction machine includes an exposure station at which a light or radiation pattern of a document to be reproduced is projected by a lens 11 onto an electrostatographic surface, such as a xerographic drum 12.

The xerographic drum 12 is detachably secured to a shaft 13 mounted in suitable bearings in the frame of the machine and is driven in a counterclockwise direction by a motor at a constant rate that is proportional to the scan rate for the document being reproduced whereby the peripheral rate of the drum surface is identical to the rate of movement of the projected light image of the document. The drum surface com prises a layer of photoconductive material on a conductive backing that is sensitized prior to exposure by means of a corona generating device 14.

The exposure of the drum to the document light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the drum an electrostatic latent image in configuration corresponding to the light image projected from the document. As the drum surface continues, the electrostatic latent image passes through a developing station in which there is positioned a developer apparatus 16 in accordance with the present invention as will be described hereinafter.

Positioned next and adjacent to the developing station is the image transfer station which includes a pair of rollers 18 for holding a support material in the form of paper web P against the surface of the drum to receive the developed xerographic powder image therefrom. The web P is moved in synchronism with the rotation of the drum by means of a takeup roll 20 which drives the support material P from a supply roll 22. A suitable drive mechanism (not shown) is connected to the drum 12 for imparting rotation thereto at a continuous speed. This drive mechanism may be connected to the takeup roll 20 for imparting rotation thereto thereby producing movement of the web material P in the same peripheral direction and at the same speed as the peripheral surface of the drum. In order to insure identical movement of the two coating surfaces, a suitable programming device may be utilized to effect continuous synchronous movement of these surfaces.

The transfer of the xerographic powder image from the drum surface to the transfer material is effected by means of a corona transfer device 23 that is located at place of contact between the transfer material and the rotating drum. The corona transfer device 23 is substantially similar to the corona discharge device 14 in that it includes an array of one or more corona discharge electrodes that are energized from a suitable high-potential source and extend transversely across the drum surface and are substantially enclosed within a shielding member.

In operation, the electrostatic field created by the corona discharge device 23 of appropriate polarity is effective to attract the toner particles comprising the xerographic powder image from the drum surface and cause them to adhere electrostatically to the surface of the transfer material.

Immediately subsequent to the image transfer station, the

transfer material is carried to a fixing device in the form of a fuser assembly 25 whereby the developed and transferred xerographic powder image on the sheet material P is permanently fixed thereto. After fusing, the finished copy is preferably discharged from the apparatus as a suitable point for collection externally of the apparatus.

The nextand final station in the device is a drum cleaning station having positioned therein a corona precleaning device 26 similar to the corona charging device 14 of appropriate polarity, negative for positive-to-positive mode of reproduction and positive for negative-to-positive mode of reproduction, to impose. an electrostatic charge on the drum and residual powder adherent thereto to aid in effecting removal of the powder and a drum cleaning device under suction in the form of a rotary brush 27 adapted to remove any powder remaining on the xerographic drum.

In general the electrostatic charging of the xerographic drum in. preparation for the exposure step and the electrostatic charging of the support surface to effect toned image transfer are accomplished by means of corona generating devices whereby electrostatic charge on the order of from 700 to 1,000 volts is measured on the respective surface in each instance. Although any one of a number of types of corona generating devices may be used, a corona charging device of the type disclosed in Vyverberg U.S. Pat. No. 2,836,725 is used for both the corona charging device 14 and the corona transfer device 23, each of which is secured to suitable frame elements of'the apparatus and connected to a suitable electrical circuit.

Referring now to FIGS. 2-5, there is shown in greater detail the development apparatus 16 according to the present invention. Development apparatus 16 comprises a frame 50 on which there is a trough 51 for containing a supply of magnetic toner material 53. Magnetic toner material is made up of two components, one of which is magnetic particle and the other which is an electroscopic marking resin powder. Any suitable electroscopic marking resin powder can be used such as those described in US. Pat. No. 2,618,551 to Walkup, US. Pat. No. 2,618,552 to Wise and US. Pat, No. 2,638,416 to Walkup and Wise.

The magnetic component should be a material which will respond to a lowor high-frequency magnetic field so that it will readily transfer the electroscopic binder and preferably can be heated, thereby causing the electroscopic component of the developer to melt or flow and become attached to the transferred material. Magnetic materials suitable for the purposes of the present invention are magnetic iron and its alloys, such as nickel-iron alloys, nickel-cobalt-iron alloys, and magnetic oxides, such as hematite (Fe O and magnetite (Fe;,0,,) and ferromagnetic ferrites. Cobalt and its alloys are also useful, such as, for example, aluminum-nickel-cobalt, coppernickel-cobalt, and cobalt-platinummanganese alloys. Moreover, other alloys, such as certain magnetic alloys of aluminum, silver, copper, magnesium and manganese can likewise be used with satisfactory results. These materials can be addedsingly or in mixtures to the electroscopic powder component. A preferred magnetic material comprises iron oxide particles under the trademark of I.R.N. No. manufactured by C. K. Williams Division of Charles Phizer Co. V The magnetic component should be finely divided as this enables it to be readily mixed or coated with the electroscopic binder component and greatly increases its pigment value. Also the magnetic component should be substantially coated or firmly attached to a relatively larger amount, areawise, of the electroscopic component in order that the powder will readily be influenced by and develop electrostatic images since the magnetic component itself may not be susceptible to electrostatic charges and not, by itself, developed. Particles sizes of 1 to 20 microns have been found satisfactory for producing good, clear dense pictures.

There should be sufficient resin present in the composition so that the resin containing the magnetic component will respond to the electrical charges on the plate and thereby develop a picture even if the magnetic component not be electroscopic. Also, there should be sufficient resin present to hold the magnetic portion when the powder is transferred and fixed. The magnetic material should be present in an amount sufiicient to respond to the electromagnetic field and to carry the resin through such a field, as well as to have a mass or volume to provide, under the influence of a high-frequency electromagnetic field, sufficient heat to fuse or flow the resin attached to it. It has been found that the ratio of binder or resin to the magnetic component can vary from 19 to 2 to 3. For the best results, there should be at least 20 percent by weight of the magnetic particles, but not over 70 percent by weight, as the higher amounts may contain too little binder to satisfactorily secure the magnetic portion of the transferred media.

Magnetic toner material 53 can be readily prepared by first finely dividing or crushing the resin material, after which it is mixed with the magnetic material. Thorough mixing is necessary in order to insure that the magnetic particles have been entirely encased with the binder. The mixed resin and magnetic powders are melted and stirred to thoroughly disperse the magnetic powder in the resin. The mass is then permitted to cool, and preferably-is mixed on a rubber mill where the heated rollers assure sufficient plasticity to blend the components thoroughly, after which it is broken into small chunks and again subdivided. it is then-micronized and sieved to size Obviously, other methods can readily be devised by those skilled in the art for the production of extremely fine pigmented resin powders of this type where the pigment particles are magnetic in character.

Journaled for rotation, as by ball bearings 55, mounted on frame 50, is a transport roll 57, which serves to move'the toner material from trough 51 into the image development zone. Roll 57 comprises alternately spaced magnetic field producing members or ring magnets 59 which are annular in shape and which are alternately spaced by magnetic insulating members 61 for a purpose to be described. Magnets 50 and magnetic insulating members 61 are held tightly together by a pair. of end plates 63,64 which are received in frame 50. A sleeve 65 is wrapped about the outer periphery of the transport roll. Sleeve 65, end plates 63,64 and magnetic insulating members 61 are made from any suitable nonmagnetic material. Typical materials comprise glass, or any of the'nonmagnetic metals, such as, brass, aluminum or copper and mixturesthereof.

It is to be understood that the ring magnets 59 conveniently comprise permanent magnets which exhibit polarities indicated by letters N and S in FIG. 2 showing north and south poles, respectively. Thus, magnetic fields are produced which result in lines of flux passing through sleeve 65 and forming flux concentrations such that brushlike tufts of magnetic toner material are formed in projecting relationship to the peripheral surface in a somewhat undulating pattern due to the flux patterns being formed. It is desirable to provide independent magnets which are spaced in the arrangement shown since the flux produced from magnetic pole to magnetic pole is relatively constant across the face of the transport roll, thereby overcoming any disadvantages of long pole pieces where flux distribution may be difficult to control. To rotate transport roll 57 there is fixed to end plate 64 one end of a shaft 67 connected at the other end thereof to a driving pulley 69 which can be driven from any suitable power source.

As transport roll 57 is moved through the supply of the magnetic toner material, magnetic field producing members 59 on the roll surface which is trimmed to a uniform thickness by a doctor blade 71. Typically the thickness of the developer coating after trimming ranges from about 0.050 inches to about 0.100 inches. After being trimmed to a uniform thickness on the transport roll, the developer coating is moved past a corona charging device 73 similar to the corona charging devices previously described at which time a uniform charge of a polarity opposite to that of the electrostatic latent image is applied to the developer coating. Charging potentials ranging from about 4,500 to about 7,500 volts are suitable for the development of latent electrostatic images. .An insulating block 74 serves to insulate charging device 73 from housing 50. The charging causes the surface of the developer coating to expand or spread slightly from its position prior to charging. In order to pack down the coating, a baffle element 75 is positioned adjacent to corona charging device 73 so that there is a smoothened uniform layer of developer coating presented to the latent image to be developed.

At the topmost position in the path of transport roll 57, there is positioned one or more wave forming elements around which development of the latent image takes place'as will become more apparent. Wave forming elements 80 desirably have an arcuate shape and are positioned sufficiently close to the transport roll surface such that a charged layer of the developer coating is deflected upwardly into close proximity with the latent image to be developed due to the rotational movement of the roll.

At the same time wave forming elements 80 are oscillated in a direction transverse to the rotational movement of the transport roll 57 causing an undulation of the developer coating in the vicinity of development. As a result, the latent image is completely submerged in a flowing developer material in an undulating pattern resulting in optimum development of the latent image.

Wave forming elements 80 are desirably made out ofa conductive material so as to serve as an electrode to strengthen the electrostatic fields emanating from the latent image. Hence the solid area development portion of the image is greatly enhanced. Typically each of the elements 80 may comprise ribbon shaped steel which is approximately 0.250 inches wide and about 0.030 to about 0.075 inches thick. The waveforming elements 80 are shaped tubular at their ends where they are received in openings 81 formed in housing 50. To adjust the tension of the wave-forming elements, tensioning screws 83 are threadingly received at one end of these elements.

In order to oscillate the wave forming elements 80 magnetic fields are utilized from a rotatable linear magnet disposed on the interior of transport roll 57 on a concentric axis therewith. Rotatable magnet 85 is supported by ball bearings 87 and driven by any suitable drive as by shaft 89 driven by a pulley 91. It will be appreciated that when magnet 85 is in the vertical position the magnetic fields directed toward field elements 80 are greatest and thus the field elements are oscillated or vibrated due to pulsing magnetic forces acting upon them.

It has been found that a speed ratio ranging from about 75 to about times of linear magnet 85 to the transport roll 57 results in developed images of very high quality. Both the ring magnets and linear magnet 85 may be made from any suitable material, such as, alnico.

It is preferred that the surface of trough 51 and the outer surface oftransport roll 57 and field elements be coated with a suitable electrically insulating material, as, for example, ethyl cellulose so that the charged toner particles do not stick to these surfaces or become grounded.

in operation, the transport roll rotates in the same direction as the travel of the xerographic photoreceptor but at 1.5 to about 5 times the photoreceptor speed so that a renewed portion of the roll continually contacts the latent image. The transport roll continually picks up magnetic toner material from the trough which is at a level slightly less than the outside periphery of the roll. Any suitable means may be used for periodically replenishing the trough with a new supply of magnetic toner material as it is consumed. Since all of the magnetic toner material is utilized in the development of the image, there is no problem of insuring proper metering and proportionality between carrier and toner particles as in the prior art development devices. As the transport roll moves past the magnetic toner material, the magnetic forces emanating from the ring magnets draw a sufficient amount of the material to form a developer coating on the periphery which is then reduced to a uniform thickness by knife blade element 71. The topmost portion of the coating is charged by charging device 73 and smoothened by baffle elements 75 for development in the vicinity of the wave-forming elements 80. Due to the pulsing action of the wave forming elements, an undulating pattern of developer is flowed across the latent image resulting in high-quality development. Since wave forming elements 80 are conductive, image fields are strengthened and solid area development is effected as well as the line copy. Also due to the magnetic attraction of the layer of developer material to the transport roll 57, the background deposited on the photoreceptor is minimized. If.desired, an electrical bias may be applied to the transport-roll to suppress low electrostatic fields in the background areas.

While the present invention as to its objects and advantages has been described herein as carried out in a specific embodiment, it is not desired to be limited thereby; but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. An improved method for (xerographically reproducing document information) developing latent electrostatic images comprising forming a surface coating of developer material ineluding a magnetic component and an electroscopic com ponent, transporting the coating of developer material along a predetermined path past latent electrostatic images to be developed, applying a uniform charge to the outer surface of said coating of a polarity opposite that of the latent electrostatic images physically deflecting by wave forming means an outer portion of the coating into close proximity with the latent images to be developed, and simultaneously oscillating the deflected portion of the coating toward and away from the images in the development zone to effect a continuous undulating flow pattern of the outer layer of the coating.

* t I i UNITED STATES PATENT OFFICE CERTIFICATE O CORRECTION Patent O, 3, 645, 770 Dated 2/29/72 Inventor(s) Thomas nt It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I Claim 1, column 7, lines ll and 12, delete (xerographic ally reproducing document information) Signed and sealed this 20th day of June 1972.

Attest:

EDWARD M.FLETCEER,JR. ROBERT GOTTSCHALK Attesting @ificar- Commissioner of Patents PO-WW UNITED STATES PATENT OFFICE b 9 CERTIFICATE OF CORRECTION Patent No. 3,645,770 Dated 2/29/72 lnventofls) Thomas J. Flint Q It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, column 7, lines ll and 12, delete "(xerographically reproducing document information) Signed and sealed this 20th day of June 1972.

{SEAL} Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTISCHALK Attesting Qfi'icer Commissionerof Patents