IL31757A - Electrostatic copying method and apparatus - Google Patents

Description

Electrostatic copying method and apparatus RANK XEROX LIMITED C. 30088 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. Patent 2,297,691 to Chester P. 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 process of applying the developer to the electrostatic image described in U.S. Patent 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 ele.ctrostatic 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 charce 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 non-charged area of the plate, the electrostat c 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..

• ,: . 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 . ej, 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 fractional stress upon the photoconductor surface and the developing materials as well as the equipment neces sary .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. Patent 2,832,311. In-this process a granular carrier is selected having ferromagnetic properties and selected relative to the toner in a tr iboelectric 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 brush-like 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 image-bearing 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 place- . ment 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. In 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 essentia in magnetic carrier development. By reason of this and the electrical characteristics which result in solid area coverage, the process gives a high background deposi tion and is generally characterized 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 developmen of electrostatic latent images utilizing a magnetic toner material. . . · It is another object of the invention to enable high image 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 v/ear 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 elect ostat c 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 of Fig. 2; Fig. 4 is an enlarged view of a 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 t ■ 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.' Thereafter, the latent image is developed with the same or an oppositely charged developing toner material, depending upon negative-to-positive or pos tive-to-positi e 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 10 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 comprises 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 electrostat c latent image in conf guration 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 v/eb 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 take-up 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 take-up 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 d um surface to the transfer material is effected by means of a corona transfer device 23 that is located -at the 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 par c les. 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 at a suitable point for collection externally of the apparatus.

The next and 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 posit ive-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 xerographj.c drum.

In general the electrostatic charging of the xero- ■ graphic 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 1000 volts is measured en the respective surface in each instance. Although any one of a number of types of corona generating devices may be used, a corona charging United States device of the type disclosed in Vyverberg/ Patent 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 Figures 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 particles and the oth 'er/qfwhich is an electroscopi.c m'arking resin powder.

Any suitable electroscopic marking resin powder can be used such as those described in U.S. Patent 2,618,551 to Walkup, U.S. Patent 2,618,552 to Wise and U.S. Patent 2,638,416 to , Walkup and Wise.

The magnetic component should be a material which will respond to a low or 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 rnelt 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 (E^O-j) and magnetite (Fe3C>4) and ferromagnetic ferrites. Cobalt and its alloys are also useful, such as, for example, aluminum-nickel-cobalt, copper-nickel-cobalt , and, cobalt-platinum-manganese alloys.

Moreover, other alloys, such as certain magnetic alloys of aluminum, silver, copper, magnesium and manganese can likev/ise be used with satisfactory results. These materials can be added singly or in mixtures to the electroscopic powder component. A preferred magnetic material comprises iron oxide particles under the trademark of I.R.N. #100 manufactured by C.K. Williams Division of Charles Phizer Co.

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, areav/ise, 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. Pcirticle 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 electro- , scopic. 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 sufficient 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 1 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 4 ιρ neceKsary jn 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 froi,-, trough 51 into the image development zone. Roll 57 compris1 - alternately spaced magnetic field producing members or rii.g magnets 59 which are annular in shape and which are alternately .spaced by magnetic insulating members 61 for a purpose to be described. Magnets 59 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 non-magnetic material.

Typical materials comprise glass, or any of the non-magnetic metals, such as, brass, aluminum or copper and mixtures thereof. .

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 brush-like 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 v/here flux •distribution may be difficult to control. To rotate transport l-oll 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 produce a developer coating 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 .050 inches to about .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 w ich 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 eibout 4500 to about 7500 volts are suitable for tVe development of latent electrostatic images. An insulating block 74 serves to insulate charging device 73 from housing 50.' T'ne 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 80 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 of a 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 .250 inches v/ide and about .030 to about .075 inches thick. The wave forming 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 thread ingly 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- hen 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 125 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 of transport 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 renev.'ed 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 transp ort 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 element 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 th transport. oll 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 scope1 of the appended claims.

Claims (8)

1. Apparatus for developing an electrostatio latent image comprising a trough for containing a supply of magnetic toner material, magnetic transport means adapted for movement in a path past aaid trough drive means for moving said magnetic transport means past said trough whereby a developer coating of magnetic toner material can be formed on the surface thereof, charging means positioned in the path of oaid transport means to apply an electrostatic charge on such a developer coating, deflection means positioned adjacent the path of said transport means at a development zone to deflect at least a portion of a charged developer coating when formed on said transport means into close proximity v?ith an electrostatic latent image when placed in the development zone, and means for oscillating said deflection means in a direction towards and away from said transport means to effect undulation of the developer coating during development of the latent image.

2. Apparatus according to Claim 1 wherein said deflection means comprises at least one elongated, member made up of electrically conductive material coated with an electrically insulating material.

3. Apparatus according to Claim 1 or 2, wherein said magnetic transport means comprises a plurality of ring. shaped magnet members arranged in a longitudinal rov? and a layer of non-magnetic material wrapped around the outer periphery thereof.,

4. Apparatus according to any of Claims 1 to 3 wherein said means for oscillating said deflection means comprises a rotatable linear m gnet member disposed in the interior of said - 18 - <*

5. Apparatus according to any one' of Claims 1 to 4 including a doctor blade element and a baffle element positioned in the path of said transport means before and after said charging means, respectively, to cause a developer coating when formed on the transport means to be doctored to a uniform thickness prior" to charging and then smoothed subsequent to charging.

6. A method for forming an electrostatic latent image comprising forming a coating of magnetic developer material on a magnetic transport device, said developer being in the form of a toner including a magnetic component and an electro-seopic component transporting said developer coating by means of said device along a p&th which passes adjacent to the electrostatic latent image, applying a uniform charge to the surface of said coating, deflecting a portion of the charged developer coatin into close proximity with the latent image by means of a deflector, and osoillating the deflector thereby to effect undulation of the developer coating during development.

7. Apparatus for developing an electrostatic latent image substantially as hereinbefore described with reference to, and ag illustrated in, the accompanying drawings,

8. . . A method according to Claim 6 substantially as hereinbefore described. For the Applicants I3:CB