US2880696A - Apparatus for developing an electrostatic latent image - Google Patents

Description

April 7, 1959 H. E. CLARK ETAL APPARATUS FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE Filed May 2, 1956 Fig 5 s1 IIIJIIlIl/I Fiy 4 IIVVENTORS HAROLD E. CLARK ROBERT W. GUNDLACH United States Patent APPARATUS FOR DEVELOPING AN ELECTRO- STATIC LATENT IMAGE Harold E. Clark, Penfield, and Robert W. Gundlach, Spencer-port, N.Y., assignors to Haloid Xerox, inc., a corporation of New York Application May 2, 1956, Serial No. 582,224

3 Claims. (Cl. 118637) This invention relates to developing and developers for electrostatic images, and, more particularly, to magnetic core developers having a high specific inductive capacitance for use in conjunction with apparatus for developing a powder image on an insulating image layer of a xerographic plate, wherein the developer is cascaded under control through a channel formed between said image layer and a conductively bodied equipotential surface of a developer electrode.

In the formation of powder images on xerographic plates in accordance with the electrostatic charge pattern thereon it has been usual to develop the charge pattern with materials consisting of a combination of finely divided, pigmented, electroscopic powder with coarser carrier ma terial. The carrier material is triboelectrically charged with a polarity opposite to that of the electroscopic powder upon frictional engagement therewith and acts to retain the electroscopic powder which is attracted to and surrounds the particle of carrier material. For development, particles are cascaded across the plate or image bearing surface and the electroscopic powder deposits in conformity with the charge pattern on the image bearing surface. Cascade developers and the technique of cascade development are described in United States Patent 2,618,- 551, Walkup, Developer for Electrostatic Images, United States Patent 2,618,552, Wise, Development of Electrophotographic Images, and United States Patent 2,638,416, Walkup and Wise, Developer Composition for Developing an Electrostatic Latent Image, and the disclosure of these patents is incorporated herein by reference.

It has also been found in zerography that placing an equipotential surface or development electrode closely adjacent to the image bearing surface during development improves the quality of the developed image. This electrode may be maintained at a potential or grounded, and its effect on the developed image includes preventing deposition of electroscopic particles in areas of background where deposition is not desired, full development of large areas, removal of halo, and the like as is well known in the art. In order to significantly improve image quality the electrode should be as close to the image bearing surface as possible and generally because of practical limitations the electrode is positioned about /3 inch and preferably effectively about ,4 inch from the image surface.

The effectiveness of the developer electrode depends upon the field strength of the electrostatic field established thereby, this strength being a function of the spacing between the xerographic plate and the developer electrode. When using a rigid developer electrode, for example, as illustrated and described in United States Patent 2,573,881 issued to Walkup et al., experience has shown that the xerographic plate is subject to damage if the electrode is closely spaced thereto, for the powder tends to pile up and distort or crush the delicate photoconductive insulating surface or layer of the xerographic plate, as well as to leave a wake of undeveloped areas due to the block formed by the powder pile-up. 0n the other hand, if the developer electrode is withdrawn somewhat from the xerographic plate to minimize pile-up effects, then the electrostatic field strength is reduced, thereby impairing the effectiveness of the developer electrode. Thus, in practice the electrode spacing has been adjusted to effect a compromise between optimum spacing for good field strength and optimum spacing for unimpeded fiow.

There has recently been disclosed a flexible developer electrode in patent application Serial No. 485,866, filed February 3, 1955, entitled Developer Electrode for Electrophotographic Apparatus. The present application is a continuation-in-part of said co-pending application on the flexible developer electrode (Serial No. 485,866). This electrode may be brought into closer relationship to the xerographic plate without resulting in damage to the sensitive surface of the plate. By reason of the close spacing, however, it has been difiicult to force the relatively light powder under the flexible electrode without the powder being trapped before it completes its course, thus resulting in undeveloped blocked areas and carrier particles in the developed image on the image bearing surface.

There has also recently been found a technique and device to bring about the effects of virtual contact between the developer electrode and the image bearing surface by developing the image bearing surface with cascading particles as discussed above including carrier particles of high specific inductive capacitance. These particles act during development to bring about the electrostatic field effects of a development system in which the development electrode is in contact with the plate surface. This invention is described in patent application Serial No. 482,- 757, filed January 18, 1955, and entitled Electrophotographic Development, and now abandoned. The present application is a continuation-in-part of patent application Serial No. 482,757.

Another difiiculty present in all cascade developing systems and especially noticeable in connection with flat plate development in xerography is the lack of uniformity across the surface of the image bearing layer and, specifically, at the last edge developed by the cascading particles at the end of the cascade cycle as that edge relates to the remainder of the developed image areas. It is presently believed that the cause of this lack of uniformity in development is due to the acceleration of the cascading particles which takes place as they move across the surface being developed. The particles accelerate to their greatest speed at the end of each pass over the surface being developed and at the high rates of speed which are reached at the end of a pass sweep the area being developed resulting in particle removal rather than particle deposition. Thus, in the usual case of flat plate development the last segment (which may involve a few inches) of the plate surface is developed as an image of decreasing quality to an area where all image particles are removed. This invention overcomes this problem associated with cascade development.

Accordingly, it is the principal object of the present invention to provide an invention with a developer electrode, a developer material having a core including a magnetic portion of high specific inductive capacitance whereby the field strength established between the developer electrode and the xerographic plate surface is intensified without the need to decrease the spacing therebetween and means for controlling movement of the cascading particles across the plate surface whereby the image is uniformly developed.

It is also an object of the invention to provide, in combination with the developer electrode and developer material having a core of magnetic material, magnetic controlled flow means to cause uniform movement of the particles over the image bearing surface and to prevent trapping of the developer in the channel between the plate surface and the developer electrode.

More specifically, it is an object of this invention to provide in combination with a developer electrode a developer comprising a mixture of loose, moveable particles of electrostatically attractable powder and separate granular carrier material composed of particles each of which includes a core having a magnetic portion and being of a high specific inductive capacitance, and magnetic control means to control the flow of the cascading particles as they move across the surface being developed.

In accordance with the invention the core may consist entirely of a magnetic material having a high specific inductive capacitance or it may be formed of solid insulating beads having a magnetic coating thereon of high specific inductive capacitance. Alternatively, the core may be constituted by 'a hollow magnetic body of hgh specific inductive capacitance or it may comprise a magnetic body with an overlying conductive coating.

' For a better understanding of the invention as well as other objects and further features thereof, reference is bad to the following detailed description thereof to be read in conjunctiton with the accompanying drawings, wherein:

Figure 1 is a schematic showing of a developer apparatus for use in conjunction with the invention;

Figure' 2 is a schematic illustration of another embodiment of a device (and a detail drawing of a portion there of) to control the magnets movement of the apparatus as illustrated in Figure 1;

Figure 3 is a schematic diagram of a charged xerographic plate and a grounded electrode illustrative of the theory underlying the invention, and;

Figure 4 is a schematic representation of a charged xerographic plate and a grounded electrode with a developer interposed between in accordance with the invention.

Referring now to the drawings, and more particularly, to Figure 1, there is shown a developer apparatus including a developer electrode structure 11 of the general character described above as a flexible developer electrode and including a xerographic plate, generally designated by the numeral 10. Electrode structure 11 is disposed in parallel spaced relationship to the xerographic plate on support means 26 to form a channel therebetween through which is cascaded a developer 12 in accordance with the invention. The cascading operation is carried out by operating the apparatus alternatively in opposite directions in a manner similar to that disclosed in United States Patent 2,600,580, dated June 17, 1952.

proper direction and movement at a uniform rate of speed of the magnet along the apparatus takes place. The motor 8 is caused to operate by switch 9 to bring about movement of magnet 7 in a downward direction when the apparatus illustrated in this figure is tilted as described above and as illustrated herein. At the end of rods 6 limiting switches 4 stop the movement of magnet 7. 7

Developer electrode structure 11 consists of a frame 13 and a flexible electrode sheet 14 mounted thereunder. Sheet 14 has a conductive surface or layer 15 or, alternatively, the flexible electrode sheet 14 may be conductive and layer 15 may be omitted. Xerographic plate 10 consists of a photoconductive layer 16, formed, for example, of amorphous selenium and a backing 17 which may, for example, be of conductive material such as metal or the like. Thus, when the developer powder cascades over the image surface of the xerographic plate controlled by the magnetic control system described above, the powder travels at a uniform rate in the space between the image surface and the flexible electrode plate 14, which plate is arranged in close proximity to the xerographic plate during the development operation. It is to be understood that the invention is applicable to rigid electrode structures as well as to rotary development apparatus and when used with rotary development apparatus the magnetic flow control means may comprise sta tionary magnets of varying field strength to cause the moving surface of the plate to make uniform contact with the powder being flowed over the surface of the rotary drum.

The developer in accordance with the invention comprises carriers each surrounded by electroscopic particles held to the carriers by electrostatic forces. The core, base or interior of the carrier includes a magnetic portion, and preferably is encased in a covering which imparts the necessary triboelectric properties to the granular carrier material so that it will properly charge the electroscopic powder when mixed therewith. The other requirement of the core particles is that they be of high specific inductive capacitance, i.e. dielectric constant. A granular carrier of this type may be produced by adhering an outer covering consisting of a selected triboelectric resinous or other suitable substance to the core particles by first coating the core particles with an intermediate bonding layer, for example, a liquid resin which contains a hydrochloric acid catalyst. The coated core particles are then added to the dry triboelectric resinous substance which adheres thereto and constitutes a covering that is fused or otherwise affixed to the core particles, any remaining fines being separated by screening. Such coated granular material is then mixed with the electroscopic powder to form the developer. The nature of the electroscopic powder, as well as that of the core casing, as well as methods of applying to the core, is more fully dis closed in the aforementioned patents, United States Patent 2,618,551 and United States Patent 2,618,552, issued on November 18, 1952.

The core in accordance with the invention must include a magnetic portion and for this purpose the core may be made entirely of a ferromagnetic material such as iron, steel, or ferrites. Alternatively, the core may be formed by an insulating material such as glass or plastic bead,

' coated with a magnetic material. In still another modification, the core may consist of a hollow magnetic ball. In addition, the core will include a portion of high specific inductive capacitance, the function of which will be explained in connection with Figures 3 and 4.

In Figure 2 there is a schematic representation of another device to control movement of the magnet so that the particles are made to move at a uniform rate across the surface of the plate. The magnet 7 is attached to chain 18 which extends around pulleys 19 and 20 Attached to pulley 20 is governor 21 shown in position and in detail also in this figure. Governor 21 is fixed in position and allows rotation of the elements attached to shaft 22 which shaft is connected to pulley 20. The elements of governor 21 comprise arms 26, each having at their extremities fly arms 23. Fly arms 23 are connected by springs 24 to the central portion of the governor mechanism and each fly arm has a friction coated tip 25. Positioned on arms 26 are stops 27 to prevent the fly arms 23 from being thrown inwardly an undesired amount. As rotation of pulley 20 takes place, the shaft connecting pulley 20 to governor 21 causes rotation of the elements in governor 21 and, accordingly, causes fly arms 23 to be reacted on by centrifugal force because of the rotation and friction tips 25 brush against the outer rim of governor 21 thereby regulating the freedom of motion of pulley 20 and thus regulating the speed of movement of magnet 7. The governor 21, such as the one illustrated, allows movement of the magnet in either direction and allows movement of the magnet 7 in accordance with the gravitational pull on the magnet controlled by the angle of tilt of the developing apparatus illustrated, for example, in Figure 1. The governor may be so adjusted as to quickly come into play so that the particles cascading across the surface are controlled throughout their entire pass across the surface.

Figure 3 is a schematic representation of a charged xerographic plate at a distance d from a grounded electrode. The selenium film thickness is represented by the value d It is assumed that the charge area is large as compared to the values d and d Accordingly, the relative field strengths in the regions indicated :by the subscripts are as follows:

where K is the specific inductive capacitance of the slab material and the specific inductive capacitance of air is taken as unity. From this equation it can be seen that the unit area capacitance, and hence the field strength for a given charge on the plates surface will increase with increasing values of K For glass or plastic carriers K z2; for metal core carriers K =00. In these cases and respectively. Clearly, then, a stronger field will result from a carrier core having a conductive portion, since In actual development, the space d of Figure 4 is substantially filled with developer with the result that the effectiveness of the electrode is increased as if it were moved into virtual contact with the image surface. Thus,

the preferred core material is a conductive magnetic particle such as iron particles.

The advantage of a magnetic core is that it permits control of the flow of powder. Further, in the past it has been usual to use carrier particles which are large in mass and/or size in order to prevent their deposition on the image and so that they may roll freely over the plate surface during the cascading cycle. The use of a magnetically controlled system with magnetically controlled carrier particles now allows the use of fine and substantially smaller particles than it has been the custom in the past to employ in cascade systems. This new freedom allows, for example, the use of fine iron filings or the like which are caused to move across the plate surface partially by gravity and partially by the forces exerted by the magnet moving in relation to the plate surface. Further, the use of magnetic fields to control developer flow is readily adapted to retard the flow of powder if in a particular instance slow movement is desired or by adjusting the movement of the magnet the developer may be made to move rapidly over the image surface. A choice of a particular speed will depend on such factors as the image being developed, the developer materials, the number of passes involved, and the like. In the specific embodiment of fiat plate development illustrated in Figure 1 rocking of the plate to bring about development has been included. However, it is to be realized that movement of the developer may be completely controlled by magnetic fields of force and it is intended to include within the scope of this invention development of a stationary plate and developer movement controlled by moving fields and the like as will readily occur to those skilled in the art.

While the invention has been described in connection with certain specific materials, it is not confined to the details herein disclosed. Accordingly, it is intended in the accompanying claims to cover all such modifications as fall within the true scope of the invention.

What is claimed is:

1. Apparatus to develop an electrostatic latent image on the surface of an image bearing layer comprising support means adapted to support and position an image bearing layer for development, means to cascade a developer mix comprising toner particles, releasable to the electrostatic latent image, electrostatically adhering to carrier particles of magnetic material across the image bearing layer on said support means, magnetic field means positioned and disposed adjacent to the image bearing layer on said support means in magnetic field contact with the developer mix during development while physically separated therefrom by the image bearing layer on said support means, and means during development to move at a uniform rate of speed said field means relative to said image bearing layer.

2. Xerographic apparatus to develop an electrostatic latent image on the surface of a xerographic plate comprising a photoconductive insulating layer overlying a conductive backing member, said apparatus comprising support means adapted to support and position the xerographic plate for development, the backing member of the plate being positioned and disposed on said support means, means to cascade a developer mix comprising toner particles, releasable to the electrostatic latent image, electrostatically adhering to carrier particles of magnetic material across the photoconductive insulating layer of the plate on said support means, magnetic field means positioned and disposed in proximity to the backing member of the plate in magnetic field contact with the developer mix during development while separated therefrom by the plate, and means during development to move said field means at a uniform rate of speed relative to said plate.

3. Apparatus according to claim 2 including a development electrode positioned in relation to said support means providing a narrow and uniform channel in which the developer mix moves during development while between the plate being developed and the development electrode.

References Cited in the file of this patent UNITED STATES PATENTS 109,354 Smith Nov. 15, 1870 1,074,999 Zander Oct. 7, 1913 2,184,348 Kirchner et a1 Dec. 26, 1939 2,376,342 Carlton May 22, 1945 2,573,881 Walkup et al Nov. 6, 1951 2,618,551 Walkup Nov. 18, 1952 2,676,100 Huebner Apr. 20, 1954 2,786,439 Young Mar. 26, 1957 2,786,440 Giaimo Mar. 26, 1957 2,786,441 Young Mar. 26, 1957 2,836,147 McComb May 27, 1958

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