US 2911330 A
Description (OCR text may contain errors)
Nov. 3, 1959 H. E. CLARK MAGNETIC BRUSH CLEANING Filed April 11, 1958 F G J INVENTOR.
Harold E. Clark ATTORNEY Patented Nov. 3, 1959 2,911,330 MAGNETIC BRUSH CLEANING Harold E. Clark, Penfield, N.Y.-, assignor to Haloid Xerox Inc., a corporation of New York Application April 11, 1958, Serial No. 728,004 9 Claims. c1. 134-1) This invention relates generally to electrostatic techniques and more particularly to an improved process for removing residual electroscopic powder from a surface.
Several processes exist for forming electrostatic images on insulating surfaces. Probably the most widely used commercially today is the xerographic process discovered by C. F. Carlson and disclosed in his U.S. Patent 2,297,691. This process makes use of the unique properties of a structure termed a xerographic plate. This plate comprises a photoconductive insulating layer on an electrically conductive backing. The photoconductive insulating layer may be a continuous layer of a suitable material such as anthracene, sulphur, vitreous selenium, etc. or may be a finely-divided photoconductive pigment such as an oxide, sulfide, or selenide of zinc or cadmium dispersed in an insulating resinous binder (whence the term binder plate). Suitable conductive backings include aluminum, brass, paper, conductively coated glass, etc. Xerographic plates are described in more detail in U.S. Patent 2,753,278 to W. E. Bixby et al., U.S. 2,803,542 to O. Ullrich, U.S. 2,803,541 to B. Paris and U.S. 2,663,636 to A. E. Middleton. In the xerographic process the xerographic plate is first electrostatically charged and then exposed to an image of light and shadow to be reproduced whereby the electrostatic charge is selectively dissipated to produce an electrostatic image corresponding to the pattern of light and shadow.
Other means of forming electrostatic images are set forth in U.S. Patent 2,647,464 to J. P. Ebert and U.S. patent application Ser. No. 368,408, filed July 16, 1953, by L. E. Walkup, now Patent No. 2,833,648. The end result of these processes is the production of an electrostatic image which comprises a pattern of electrostatic charges on an insulating surface.
This electrostatic image is developed, ile., made visible, by contacting the insulating surface with an electroscopic powder. The powder selectively deposits in accordance with the electrostatic image thereby rendering the electrostatic image visible. The resulting powder image may be either permanently affixed in place or may be transferred to a final image support member as of paper, plastic, metal, etc. and affixed thereto. Depending on the nature of the electroscopic powder, the support base and the nature of the resulting image, afiixing may be done by any of a variety of means such as heating, using a solvent vapor, using a transparent plastic overlay, spraying with a fixative lacquer, etc.
A number of processes exist for contacting surfaces bearing electrostatic images with electroscopic marking particles. These processes include liquid spray and powder cloud both described, for example, in U.S. 2,784,109 to L. E. Walkup and simply flowing the electroscopic powder over the image-bearing surface as described by Carlson. The usual process of applying the developer to the electrostatic image is set forth in U.S. 2,618,552 to E. N. Wise and involves the use of a finelydivided colored material called a toner deposited on a slightly more coarsely divided material called a carrier.
This two component developer is cascaded across the electrostatic image, the toner and carrier being rubbed against each other while cascading to impart an electrostatic charge to each other by triboelectric charging. To produce a positive of the electrostatic image (that is such that pigmented areas on the copy correspond to charged areas on the plate), the toner and carrier are selected such that the toner will be charged to a polarity opposite to that of the electrostatic image, the carrier being charged to the same polarity as the electrostatic image. When a carrier particle bearing on its surface oppositely charged particles of toner crosses an area on the image surface having an electrostatic charge, the charge on the image surface asserts a 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 are not retained by the charged areas of the plate. When a toned carrier particle (i.e., a carrier particle bearing toner particles on its surface) passes over a non-charged area of the plate,-the electrostatic attraction of the carrier particles for the toner particles is suflicient to retain the toner on the carrier preventing deposition in such areas as the carrier particles momentum carries both toner and carrier past. By this mechanism the image is developed.
It has long been known that when a magnet is inserted into a quantity of finely-divided ferromagnetic material that streamers are formed as the powder material, is attracted to the magnet and orients itself therefrom along the lines of force of the magnetic field. The streamers so formed constitute a brush-like mass, whence the term magnetic brush. In one of the early descriptions of the cascade development process, i.e., U.S. 2,618,551 to L. E. Walkup, it was disclosed that the granular carrier used in the development process may be a ferromagnetic material. Recently it has been discovered in the carrier development process described above with a toned ferromagnetic carrier that the use of a magnet to form a magnetic brush with the toned ferromagnetic carrier permits the motion of the carrier over the image-bearing surface to be controlled by the mechanical motion of the magnet. Thus the magnet can be used to give a greater degree of control over the movement of the electroscopic powder over the xerographic plate while still obtaining high quality image development. In this process thedeveloper is both triboelectrically charged and deposited on the electrostatic image in a manner similar to that wherein the toner and carrier mix is cascaded across the image bearing surface by the force of gravity.
After the development of the electrostatic image, the resulting powder image is generally transferred to a final image support material and the initial surface having thereon the electrostatic image is reused. The transfer operation is not percent efficient so that there remains on the initial image-bearing surface a residual deposit of finely-divided toner material. Before this surface can be reused it is necessary that this residual toner be removed without harmful effect to the surface. In those cases wherein it is desired to aflix the powder image to the initial image surface, i.e., to omit the transfer step, it is often desired to make spot deletions or erasures to remove part but not all of the powder image without smearing or lowering the quality of the resulting permanently fixed image. The most common problem is the complete cleaning of the surface of the xerographic plate in the xerographic process wherein it is particularly necessary to clean the xerographic plate thoroughly to remove therefrom any residual powder so as to prepare the plate for'another electrostatic charge and exposure. To this end it is conventional to provide a cleaning powder and to cause this powder to pass back and forthacross the electrophotographic plate, this being accomplished by a rocking or oscillatory motion of the plate in a device and manner similar to that used in cascade development. It hasgbeen found that the mater s. used as a ri i the development step do. not constitute eflicient cleaning agents. Accordingly, the cleaning process requires the use of a second cascade development apparatus containing .a supply of a suitable cleaning material. Among the drawbacks attending the use of such cleaners, in ad-, dition to the difliculty of storage and added handling of the plate, have been that some cleaners adversely efiect the photoconductive layer of the plate, others create an unpleasant dirtiness when in use in a dry atmosphere, still others are toxic in nature and have a relatively brief effective life. Accordingly, it is the principal object of this invention to provide an improved c eaning process for removing clectroscopic powder from a xerographic plate which process is free from the drawbacks attending the use of known cleaners.
In automatic zerographic machines such as described in US. 2,781,705 to H. E. Crurnrine et al., the high speed of operation combined with design limitations imposed in a cascade system based on a gravity feed do not permit the use of cascade cleaning. Accordingly a rapidly rotating fur brush is used to remove residual toner as illustrated in US. 2,751,616 to M. I. Turner et al. and 2,752,271 to L. E. Walkup et al. When such a brush becomes saturated with :toner necessitating replacement, valuable down-time is lost while the change is made. Accordingly, it is a further object of the invention to provide a cleaning device useful on automatic xerographic machines which may be easily and quickly reconstituted thereby reducing down-time.
Additional advantages and objects of the invention will in part be obvious and will in part become apparent from the following description of the invention and from the drawings in which:
Fig. 1 is a semidiagrammatic view of a cleaning mechanism according to one embodiment of this invention; and
Fig. 2 is a diagrammatic view of a cleaning mechanism according to another embodiment of the invention.
In accordance with the invention residual powder is cleaned from the ,xerographic plate by passing a magnetic brush over the surface of the plate whereby the streamers of ferromagnetic material contact the surface. This is illustrated in Fig. 1 wherein a magnetic brush 10 is formed by inserting a magnet .11 in a suitable non mag netic shield 12 as of glass, plastic, brass, aluminum, etc. When the device is contacted with a supply of ferromagnetic carrier particles, the particles form streamers 13 from the outer surface of shield 12 due .to the lines of force from magnet 11. Suitable means 14 as a rod are secured to magnet 11 to facilitate removal from shield 12. Projecting lip 15 or similar means restrains carrier 13 from traveling up shield 12 whenever magnet 11 is removed, thus assuring easy and quick reformation of the brush 10 without contamination. The tumbling action of the ferromagnetic particles in the magnetic brush intimately and thoroughly contact the electrically insulating surface as the surface of the xerographic plate 16 comprising a photoconductive insulating layer 17 on a conductive backing 18 and the toner powder particles '19 thereon. As a result of the intimate rolling contact therebetwcen, the ferromagnetic carrier particles clean the toner particles from the surface of the xerographic plate by both 'triboelectrically and mechanically attracting the toner particles and the efficiency of the cleaning action of the ferromagnetic particles is high on both ac counts. By positioning a flux intensifying means (as a soft iron member) on the side of the xerographic plate opposite from that to be cleaned, it is Possible to'vary the force of the contact between the brush and the plate.
In order to obtain efficient cleaning either with the granular materials of the prior art or by means of a 7 dr p d y rotating fur brush, the prior art frequently sprayed reverse polarity electrostatic charges on the surface bearing the residual toner. These residual toner particles were bound to the surface of the xerographic plate primarily by means of electrostatic attraction. Spraying this surface with reverse polarity charges reduced this attraction by neutralizing at least a portion of this attractive force thereby increasing the efficiency of the cleaning operation. Using ferromagnetic carrier pa ti le w ich are t ea li t y electrically n ctive to clean .a Xcrographic plate in accordance with the instant invention acts to discharge the electrostatic charges on contact thus eliminating the need for such a reverse charging operation with no decrease in cleaning efliciency by reason of this omission.
In automatic Xerographic machines design limitations effectively prevent the use of cascade cleaners requiring the use of fur brush cleaning devices. When such a brush becomes saturated with toner so as to reduce its cleaning efiiciency the entire fur brush must be discarded. The use of a magnetic brush to clean residual powder, as in the instant invention, permits the same flexibility in design as the fur brush with the added advantage that the entire brush need not be discarded when saturated with toner. Thus, a magnetic brush is, in eifect, a brush where the fibers themselves are replaceable. In addition, if desired for purposes of economy, the ferromagnetic carrier particles saturated with toner need not be discarded but may be recycled to use as carrier in the development process. The most important advantage for magnetic brush cleaning in automatic xerographic machines is that by using the process of the instant invention it is not necessary to disassemble the brush to replace the cleaning device. The simple replacement of the supply of ferromagnetic material quickly and simply reconstitutes the magnetic brush to full cleaning strength. This saving of down-time on expensive automatic xerographic equipment is a critical factor in the utility of such devices.
The use .of a revolving cylindrical magnetic brush to clean a xerographic plate in an automatic xerographic machine is illustrated in Fig. 2 wherein a xerographic plate 16 in the form of a cylindrical drum rotating on its longitudinal axis passes sequentially through the successive steps of the xerographic process: charging, 20; exposure, 21; development, 22; transfer, 23; and cleaning, 24. This overall process is described in more detail for example, in US. 2,756,676 to F. A. Steinhilper and in US. 2,784,694 to H. E. Crumrine et al. At cleaning station 24 cylindrical drum magnet 11 rotating on its longitudinal axis dipping into a supply of magnetic carrier particles 26 retained in contact with magnet 11 by suitable means as a trough 25. Desirably agitating means 27 as a solenoid are provided to agitate the trough 25 and thereby the carrier particles 26 to prevent caking. As magnet 11 rotates through carrier supply 26, the magnetic flux emanating from magnet 11 forms streamers 13 from particles 26 thus constituting a magnetic brush. Magnet 11 is positioned and adapted so that streamers 13 contact the surface of plate 16. The tumbling action of the ferromagnetic particles 26 intimately and thoroughly contact plate 16 as plate 16 and magnet 11 rotate on their respective axes cleaning any residual toner by a combination of triboelectric and mechanical action. By reason of the strong triboelectric attraction of the carrier for the toner, dusting of the toner (dusting refers to the tendency of toner to form a cloud of toner particles when agitated) is held to a minimum. As the magnet 11 rotates, streamers 13 are reformed with evcry rotation. The large supply .of carrier particles 26 in trough 25 permits long operation without significant decrease in cleaning efliciency. Merely replenishing the carrier supply 26 in trough 25 restores the brush to full cleaning efliciency.
Another equally important application of magnetic brush cleaning is in spot cleaning binder plates. As stated above, a binder plate comprises a finely-divided photoconductive pigment in an insulating binder on a conductive backing. The most common commercial type of binder plate comprises a zinc oxide pigment in a silicone resin. By reason of its physical structure, that is, pigment particles dispersed in a resin binder, the surface of a binder plate is toothed as contrasted to the glossy, specular surface of a vitreous selenium film. This toothed surface makes the problem of cleaning a binder plate particularly troublesome as the teeth act to retain toner. There has recently been discovered a process whereby the surface of a binder plate may be converted into a lithographic master. t is often necessary in the lithographic process to make deletions or changes on the lithographic master. In addition, to obtain the highest quality of work it is desirable to clean up any background powder which might have accidentally deposited on the plate in the course of the xerographic process. Because of the toothed nature of the binder plate itself, the problem of making deletions or changes to the master is exceedingly difiicult, particularly as it is desired to do so without any smearing or decrease in image quality. The use of a magnetic brush as described herein affords an easy efficient and practicable means for making such spot deletions and erasures on the surface of the binder plate.
The ferromagnetic material used to form a magnetic brush for use in cleaning residual powder as described herein may comprise any finely-divided ferromagnetic material such as magnetic iron and its alloys, such as carbonyl iron, alcoholized iron, nickel-iron alloys, nickelcobalt-iron alloys, and magnetic oxides, such as hematite (Fe O magnetite (Fe O and ferromagnetic ferri-tes. Cobalt and its alloys are also useful, such as, for example, aluminum-nickel-cobalt, copper-nickel-cobalt, and cobaltplatinum-manganese alloys. Moreover, other alloys, such as certain magnetic alloys of aluminum, silver, copper, magnesium, and manganese can likewise be used with satisfactory results. It is particularly preferred to use ferromagnetic materials which are also electrically conductive. It has been found best to use granular ferromagnetic particles of a size larger than about 100 mesh usually between about 20 and about 60 mesh. If desired, slightly smaller ferromagnetic particles may be used. Desirably, however, for efiicient cleaning, the carrier particles should not be smaller than about 180 mesh.
The number of times that the magnetic brush may be used to efliciently clean a xerographic plate is dependent on the quantity of ferromagnetic materials in the brush and the amount of toner powder to be removed on an average plate. The efiiciency of the magnetic brush in cleaning toner from a xerographic plate is high permitting a given mix of ferromagnetic material to be used a number of times before there is any detectable decrease in cleaning efliciency.
1. The method of removing electroscopic powder from a xerographic plate which consists in brushing the plate with granular ferromagnetic particles oriented in bristle formation with respect to magnetic field producing means.
2. The method of removing electroscopic powder retained on an electrically insulating surface by electrostatic attraction which consists of contacting a magnetic field producing means with granular ferromagnetic particles whereby said particles are oriented along the lines of flux of said field producing means to thereby constitute a magnetic brush and moving said magnetic field producing means relative to said insulating surface whereby said ferromagnetic particles intimately contact said surface.
3. A process according to claim 2 wherein said ferromagnetic particles are electrically conductive.
4. A process according to claim 2 wherein a flux concentrating means is positioned on the side of said insulating surface opposite from that contacting said particles to thereby vary the force of contact between said particles and said surface.
5. A process according to claim 2 wherein said ferromagnetic particles are no smaller than about mesh.
6. The method of removing electroscopic powder retained on an electrically insulating surface by electrostatic attraction which consists of contacting a cylindrical magnetic structure producing lines of magnetic flux externally of said cylindrical structure with ferromagnetic particles whereby said particles are oriented along said lines of flux in bristle formation with respect to the cylindrical magnetic structure, rotating said cylindrical structure on its longitudinal axis, and moving said cylindrical structure relative to said insulating surface whereby said rotating bristle formation intimately contacts said surface thereby removing electroscopic powder therefrom.
7. A process according to claim 6 wherein said ferromagnetic particles are electrically conductive.
8. A process according to claim 6 wherein a flux concentrating means is positioned on the side of said insulating surface opposite from that contacting said particles to thereby vary the force of contact between said particles and said surface.
9. A process according to claim 6 wherein said ferromagnetic particles are no smaller than about 180 mesh.
References Cited in the file of this patent UNITED STATES PATENTS 2,751,616 Turner June 26, 1956 2,752,271 Walkup June 26, 1956 2,756,676 Steinhilper July 31, 1956 2,784,694 Crumrine Mar. 12, 1957