US3176652A

US3176652A - Xerographic developing apparatus

Info

Publication number: US3176652A
Application number: US163288A
Authority: US
Inventors: George R Mott; Harold E Clark
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1960-09-26
Filing date: 1961-12-29
Publication date: 1965-04-06
Anticipated expiration: 1982-04-06
Also published as: GB1006078A

Description

April 6, 1965 G. R. MOTT ETAL 3,

XEROGRAPHIC DEVELQPING APPARATUS Filed D90- 29, 1961 Fig.5a Fig.5!)

INVENTORS. GEORGE R. MOTT BY HAROLD E. CLARK ATTORNEY United States Patent Ofilice 3,l'i,ti52 Patented Apr. 6, 1965 3,176,652 XEROGRAPEHC DEVELOPING APPARATUS eorge R. Mott, Rochester, and Harold E. Clark, Penfield, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 29, 1961, Ser. No. 163,288 4 Claims. (Cl. 118-637) This invention relates to xerography and more particularly to improved apparatus for developing electrostatic images.

In xerography, it is usual to form an electrostatic latent image on a 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 example, in US. 2,297,691 to Chester F. Carlson. Other means of forming electrostatic latent images are set forth in U.S. 2,647,464 to James P. Ebert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized 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 latent image. Alternatively, the electrostatic charge pattern may be transferred to an insulating filmand the electroscopic particles deposited thereon to form the visible image. In any case this visible image, in turn, may be transferred to a second surface to form a xerographic print or may be afiixed directly to the surface on which it is developed.

The usual process of applying the developer to the latent electrostatic image is set forth in US. 2,618,552 to E. N. Wise and involves the use of a finely-divided colored material called a toner deposited on a slightly more coarsely divided material called a carrier. This twocomponent developer is cascaded 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. To produce a positive of the electrostatic image, a 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 exerts a greater attraction for the toner than the carrier and retains the toner in the charged area 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 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. By this mechanism, the image is developed, i.e., made visible.

Another form of developer mix is composed of a toner and a ferro-magnetic carrier material controlled by means of a magnet. This forms streamers of the developer which constitute a brush-like mass and the brush is passed over the surface bearing the electrostatic latent image whereby the brush contacts the image-bearing surface. The developer is both triboelectrically charged and dcposits on the electrostatic latent image in a manner similar to that wherein the toner and carrier mix is cascaded across the image-bearing surface. Variations on the use of ferro-magnetic developer are set forth, for example,

ill

in US. 2,8 6,333 to Wilson and US. 3,015,305 to Hall et al.

An object of the present invention is to provide a novel means for applying electroscopic developer powder to an electrostatic image-bearing member.

Another object of this invention is to provide suitable means as set forth, wherein the developer mix is applied as a brush under the influence of a magnetic field.

A further object is to provide developing means as set forth, wherein the brush can be easily and quickly cleaned, permitting the use of different developer powder without contamination from the development powder previously used by the brush.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description in connection with the accompanying drawings in which:

PEG. 1 is a top plan view partly in section substantially of all embodiments of apparatus in accordance with the invention;

FIG. 2 is a side sectional view according to one embodiment of the invention;

FIG. 3 is a side sectional view according to a second embodiment of the invention;

FIG. 4 is a side sectional view according to a third embodiment of the invention; and

FlGS. 5(a) and (b) are typical rough surface patterns for the apparatus shield.

The apparatus to be described, embodying the invention in illustrative form, may be operated with a developer comprising a toner powder and a carrier material. The carrier material consists of magnetically attractable powders such as magnetic ferrites as described in US. patents to I. L. Snoeck Nos. 2,452,529; 2,452,530 and 2,452,531 all dated October 26, 1948, iron carbonyl, alco holized iron, etc. while any of the large number of toner materials known to those skilled in the art such as those described in US. 2,618,551 to L. E. Walkup, 13.5. 2,618,552 to E. N. Wise and US. 2,753,308 to R. B. Landrigan are suitable for mixing with the magnetic carrier to form a developer to be employed in the present invention. The ferrites developed by Snoeck described above may also be used as single component magnetic developers, i.e., the ferrite may act as both carrier and toner.

If alcoholized iron is used as the magnetic carrier, it is possible to develop a reversed image or a direct image by selecting a toner above or below iron in the tribeelectric series. As examples, powdered shellac or rosin, each pigmented with a suitable material such as carbon black, when used with alcoholized iron give a direct image with a negative charged surface and a reversed image with a positively charged surface. Similarly, Vinsol resin (an extract from long leave yellow pine stumps composed principally of an oxidized form of abietic acid and manufactured by Hercules l owder Company, Wilmington, Delaware) colored with a suitable dye such as nigrosine gives a direct image with a positively charged surface and a reversed image with a negatively charged surface. Alternatively, the toner may be kept the same and the magnetic carrier varied to obtain a direct or reversed reproduction of the electrostatic image. Thus, using a polystyrene resin with a suitable pigment as carbon black for the toner one can obtain a direct image of a negatively charged surface when using PMC Z270, a powdered iron produced by the Xerox Corporation, Rochester, New York, as the magnetic carrier and a reversed reproduction of the negatively charged image when using Carbonyl L, a powdered iron made by the reduction of an iron carbonyl by Antara Chemicals Division of General Aniline and Film Corporation, New York, New York, as the magnetic carrier.

The improved magnetic brush apparatus of the present invention is formed of an elongated magnet held stationary inside a rotating cylindrical shield. The shield encloses at least that portion of the magnet that is effective to form the brush and may be removable from the magnet. At least a portion of the exterior surface of the shield, and at least a portion effective in forming a brush, presents developer to the surface of an image-bearing member. The shield surface is characterized as having a high coefiicient of friction relative to the developer or otherwise having the capability to retain developer substantially relatively stationary on its surface While rotating relative to the elongated magnet. Prior to presenting developer to the image-bearing member, the shield is continuously supplied from a supply source with a quantity of developer of substantially uniform toner concentration that is magnetically attracted to the shield. By maintaining developer relatively stationary to the surface of the shield, there is assured the continuous presentation of a uniform mix of developer to the surface to be developed.

Referring now to FIGS. 1 and 2, there is illustrated a first embodiment of a magnetic brush device constructed in accordance with the present invention. As illustrated, an image-bearing plate 10, which may be a xerographic plate having a photoconductive surface on a conductive substrate, is continuously advanced in a predetermined path. The surface of the plate contains an electrostatic latent image prepared by any of the means known to those skilled in the art as for example, those set forth in US. 2,297,691 to Chester F. Carlson; Along the path of travel, the plate may be passed through the usual xerographic processing stations including sensitizing exposure and fixing, known to the art and not illustrated in the figures. shown to be constituted of flexible material issuing from supply reel 11 from which it is continuously drawn onto take-up reel 12 driven by motor 13. Suitable means such as a slip-clutch or the like (not shown), may be provided between the motor and take-up reel in order to maintain the velocity of the plate substantially constant. As the plate advances to the take-up reel, it is supported on its rear or lower surface by means of a support platen 14 that is suitably grounded.

The image-bearing member or plate with which this invention is used may comprise any of a variety of forms known in the art. Thus, at least the surface layer supporting the image may be comprised of an insulating material, such as polystyrene capable of retaining a charge pattern for a reasonable period of time, or it may comprise a photoconductive insulating material such as vitreous selenium, a dispersion of zinc oxide in an insulating binder, or various other materials known in xerography. When light sensitive layers bearing charge patterns are developed, then the developer should be presented in the absenceof activating radiation. The backing substrate may comprise any suitable mechanical support, whether rigid or flexible such as metal, conductive paper, or the like. Optionally, the substrate may be dispensed With where the insulating layer itself has adequate strength. p

The developing apparatus of the invention is generally designated and is supported adjacent the path of movement of the xerographic plate to present developing particles to the plate surface during the course of plate movement. As illustrated, the developing apparatus is comprised of an elongated permanent magnet 21 extending width-wise of the plate and secured to stationary rod supports 22 and 23 in turn secured to stationary support blocks 24 and 25.

Mounted as to substantially enclose the magnetic member and for rotation thereabout is a cylindrical shield 26 which in the embodiment of FIG. 2 is non-magnetic and may be of aluminum, brass, glass, or the like. It may also be desired in this embodiment of the invention that For purposes of illustration, the plate is and 31, respectively, which in turn press-fit onto

rods

22 and 23, respectively. For the purpose of mounting or removing the shield onto the bearings, the shield is formed of two semi-cylindrical mating sections connected by means of two resilient spring hasps 32 and 33. By drawing at the opposite sections, the individual sections are separable to enable convenient removal of the shield from the bearings and likewise they may be joined. in assembled relation by pressing them together when ap propriately arranged. As an alternative, the sleeve can be mounted on one end instead of two. This latter method permits an integral shield which can he slipped onto the single mount.

Rotation of the shield is elfected by means of a motor 49 (or alternately motor 13) the output shaft of which securely supports a pulley 41 driving a timing belt 42 connected to a toothed or ribbed portion 43 on sleeve 29. As the motor operates, there is continuous rotation of the shield during which it isthe purpose of the exterior surface of the shield to carry with it 'a quantity of developing material for presentation to the surface of the image-bearing member 10. The rotational speed is dependent on several factors including developer being employed, plate being developed, etc. Speeds on the order of approximatelyVz 'to 2 /2 inches per second are usually suitable with up to 1 /2 inches per second being preferred. 1

Thus the outer or exterior surface ofshield 26, and designated 34, is not smooth but rather is roughened in a random or regular pattern such that the surface roughness serves to hold magnetic developer 27 in place and prevent its sliding along the surface as the, shield rotates upwardly in this embodiment as viewed in FIG. 2. The texture of the surface 34 may be inherently rough as by using the uniform checkerboard rough pattern on the backside of fiberboard such as Masonite (a trademark of the Masonite Corporation) or like material. Otherwisethe texture may be formed rough as by molding a plastic shield against a wire screen and removing the Wire screen. once the shield material has hardened. Sandblasting has also been found a convenient method of forming a roughened surface as long as provisions are taken to provide for uniform roughness along the top surface and such roughness provides a suificient friction coefficient to carry developer particles during rotation of the shield. Thus, uniformityv in roughness is desired so that uniformity of presentation results, thereby constantly supplying during operation a uniform supply of developer particles to the surface of the image-bearing member. Therefore any known means for accomplishing uniform roughness of the shield surface is intended Referring particularlyto FIG. 2, the shield 26 is I mounted to rotate about its axis in the direction shown by the arrow and eccentrically about magnet 21. The magnet 21, which may be of alnico, remains stationary with one of its poles (shown as the north pole as close' to the inner surface of shield 26 as possible to produce a magnetic field thereabout while the south magnetic pole is spaced from the inner surface of the shield to produce a weakened or no magnetic field at the shield surface. By this arrangement, the field strength generated by the pole close to the inner surface and to the left of line A-A assures that the magnetic field. strength For this pur-l to the left of the line is sufficient to cause developer to cling to the surface while to the right of the line A-A, there is insufiicient field strength to retain developer on the shield surface. With this arrangement the developing material 27, which is contained in a trough 47, is magnetically picked up by the rotating shield as the periphery of the shield moves over the trough. Thereafter, the developer particles are arranged into a brushlike array and carried upwardly or downwardly as the case may be, to be brushed across the electrostatic latent image on surface 10, shown moving a direction opposite to the direction of shield rotation. This then causes the image to be developed, after which the developing particles still remaining on the surface 34- drop off as the surface continues to rotate beyond the magnetic field. By this means, there is produced a continuous cycle of attracting developer, brushing the developer across the image, and then returning the developer to its source of supply. It has also been noticed that the brush apparently varies during rotation of the shield helping to create a changing mix in the brush for development.

Furthermore, with the continuous cycle of operation of development, developer pick up and drop off to the trough causes a continuous mixing of the developer particles and maintains uniform toner concentration.

In order to re lenish the developer with toner consumed in developing, a toner supply 48 is maintained in toner dispenser 49 which dispenses a controlled quantity of toner into the trough. As the shield rotates, the developer in the trough is constantly agitated and mixed such that the brush continually presents replenished developer to the electrostatic image to assure uniform development density of each succeeding image.

In operation, therefore, a quantity of magnetic developer 27 is continuously removed from the trough 4'7 as the shield 26 rotates towards the north pole as illustrated in FIG. 2. The developer forms a brush-like array on the moving surface 34 of the shield that is brushed against the surface of the plate 1'9. After developing the electrostatic image, continued shield rotation causes the developer to be separated from the shield by dropping off to trough 4. The developed image may therefore be utilized in any of a variety of ways known in the art. Any means of afiixing the developed image known to those skilled in the art may be employed such as those set forth in US. 2,297,691 to Chester F. Carlson. Alternately, the developed image may be trans ferred to a second surface to which the transferred image may be affixed.

If it is desired to change the composition of developer, the apparatus can be simply adapted by merely separating the two semi-cylindrical sections of the shield main tained together by spring hasps 32 and The trough as shown is conveniently provided with a hinge bottom Wall St) supported by a hinge 51 and a hasp 52 such that developer contained therein may be easily and simply dumped by opening the hasp. Subsequently a developer of a different type may be substituted while a new shield 26 is mounted about the magnetic member 21. Optionally, dispenser 49 may also have means provided for conveniently dumping the toner 4S and substituting a toner of a different composition assu of course that developer is employed including toner as a separate component. Alternately, by supporting the magnet removably independent of the shield, removing the magnet will cause developer drop off to the trough.

As stated above, a particularly preferred form of the instant invention is that wherein the non-magnetic shield is also electrically insulating. By the term non-magnetic as used herein is meant a material which does not interfere with the lines of force of a magnetic field, i.e., a material having a permeability not substantially different from that of free space. Magnetic brush development is similar in some respects to cascade development.

As used in the instant invention, it differs primarily in that a magnet is used to effect adherence to the presenting surface which carries the carrier-toner mixture over the surface of a xerographic plate, rather than the force of gravity. The carrier in magnetic brush development therefore is ferro-magnetic. This magnetic carrier is itself conductive and hence the mixture of a carrier and toner is somewhat conductive, rather than purely insulating. The mass of developer, i.e. toner-carrier mixture, thus acts very much like a grounded conductor and tends to establish strong and uniform fields above the electrostatic image as it passes by. The deposition of toner is thus uniform over large image areas. One disadvantage of this system, however, is its reduced latitude of exposure. Careful measurements show that exposure latitude with cascade development is about 2-3 times that using magnetic brush, other conditions remaining the same. One result of this is the difficulty obtained with background control. Unless a xerographic plate has con exposed so as to reduce the non-image areas completely to zero, the result will be black on gray reproduction of the original, rather than a black on white as desired. Regulating exposure to this extent, particularly when the copy to be reproduced consists of variable quality subjects, is very difficult.

it has been found that using a non-magnetic shield which is also electrically insulating eliminates this problem. In effect, such a non-magnetic electrically insulating shield isolates the magnetic brush. The average potential of the brush determined by the average potential it sees on the surface bearing the electrostatic image, so that the brush will float to that potential approxirnating the background potential. Thus, using a shield which is not only non-magnetic, but electrically insulating not only permits mixing new toner, clean operation etc., but also enables magnetic brush development to be used with an exposure latitude comparable to that obtained with cascade development. However, under cer tain circumstances as discussed below, an electrically conductive shield may be preferred.

Referring now to FIG. 3, a second embodiment of the invention is illustrated constituting a variation of the embodiment of FIG. 2. As such, the apparatus of FIG. 3 combines the operational characteristics of the apparatus described in FIG. 2 with the further advantage of a positive control to cause agitation of the particles of the developer held by magnetic force during the development step. Thus in the development zone where the particles brush across the surface of plate 1d, the magnetic member (shown solid) is generally cylindrical in shape but with an even number of alternating longitudinal flutes and ribs. As shown, each rib constitutes a magnetic pole of polarity opposite to that of the next adjacent rib. By this means, as the shield rotates, the developer particles carried on the shield surface are agitated by the magnetic effects of the opposite poles to further assure uniformity of development by causing mixing of the developer during the time it is being actively used. This result is achieved by the multi-polar structure having adjacent alternate poles of opposite magnetic polarity, such that the magnetic field emanating therefrom is repeatedly varied in direction as the shield rotates therepast. In order to render the fields effective the space at the development zone between the poles of the magnet and the inner surface of the shield 25 is made as small as possible. This then achieves strong magnetic fields existing on the left side of line B-B to enhance developer attraction and agitation on the shield s rface while to the right of the line, there is little or no magnetic field existing to permit developer to fall off and return to trough 47. Thus as the shield rotates, developer is picked up from trough 47 and held to the surface 34-. However, due to the polar configuration of the magnetic member 21, the developer particles constantly re-orient themselves in the changing fields of force, thus agitating the developer and insuring the contact of fresh developer presented to the imagebearing surface of the plate. To the right of line B-B, the developer drops to the trough as in'FIG. 2 above. Thus in the embodiment of FIG. 3, there is achieved two separate and distinct types of developer mixing which add to give more reliable and uniform performance of the system As a further aid to enhanced development, a flexible plate could be at least partially wrapped about the shield to accord increased areas of developer contact.

Optionally, as shown in dashed outline in FIG. 3, the alternating longitudinal flutes and ribs may extend about the entire circumference of the magnetic member. With this latter arrangement, there is accorded continuous agitation of developer not only in the development zone but in the non-development zone as well. Because of the continuity of magnetic fields, drop off does not occur as readily as in the absence of the continuity except to the extent that centrifugal force causes throw-off as the shield surface advances downward toward the trough. Even in this latter instance, the optional embodiment of FIG. 3 has been found toeffect sufiicient developer agitation because of the continued agitation created by the additional poles. Therefore the mixing effect of the optional embodiment has been found adequate to insure uniform toner concentration and mixing with toner 43 being dispensed from toner dispenser 49.

Referring now to FIG. 4, there is illustrated a still further embodiment and variation of the embodiment of FIG. 3. In FIG. 4, the shield rather than being wholly non-magnetic is constructed instead of materials in secr tions which are alternately magnetic, designated 53, and non-magnetic, designated 54. The circumferential extent of each of these sections corresponds with the periodicity of the pole pieces of the magnetic member 21 and in addi-' at the shield surface, thus affording more reliable developer re-orientation.

With reference to each of the above figures, the magnetic member 21 has been regarded as being a permanent type magnet. It is not intended, however, to be so limited, since with respect to all the above described embodiments, electromagnets of similar geometrical configuration could be substituted for each of the permanent magnets described. Furthermore, whereas the shield 26 has been described preferably as being electrically insulating, it may optionally be either electrically insulating or electrically conductive depending on other factors of its utility. For example, frequently, as is well known in the art, it is desired to alter the electrostatic fields contained on the image-bearing surface either to effect a reversal development or to suppress background potentials in order to avoid background deposition of developer. Under those circumstances, electrical conductivity of the shield may be preferred in order to apply a bias potential between the magnetic brush and the electrostatic imagebearing surface being developed. Furthermore, the rotational direction of the shield need not be opposite to the direction of plate movement but could just as well be in a like direction. As another alternative, the plate could be stationary while the brush is axially advanced simultaneously with rotation.

There has thus been described an improved and novel apparatus for applying electroscopic developer powder to an electrostatic image by means of a device that is simple, and efiicient as well as according greater facility than heretofore in utilizing a variety of carrier-toner com binations. The apparatus provides means for making the exposure latitude of magnetic brush development comparable to thatof cascade development without increasing the complexity of the device. The instant developer apparatus accords a minimum 'of moving parts and is therefore substantially service free.

Since many changes could be made in the above construction and many apparently Widely different embodi previously formed on a support member, the apparatus comprising in combination:

(a) a stationary magnetic field producing means of predetermined length,

(b) means to support said field producing means,

(c) a cylindrical shield formed of alternating circumferential sections of magnetic and nonmagnetic material substantially enclosing the length of said field producing means and spaced at least partially within the effective magnetic field of said field producing means to physically prevent magnetic particles from attaching to said field producing means,

(d) said shield having an exterior surface patterned to retain a quantity of magnetic powder developer particles attracted thereto by the field producing means enclosed therewithin and substantially stationary relative to said shield during rotation, the magnetic particles being caused to form a brushlike array on said exterior surface,

(e) a supply source of magnetic powder developer particles supported at least partially within the field produced by said field producing means,

(1) means to rotate said shield'about said field producing means and between said supply source and said field producing means whereby developer particles are attracted from said source to said shield surface, and

(3) means to effect relative movement between a image-bearing member and a rotating shield to continuously present a brush-like array of developer on said shield surface into contact with an image to be developed on the image-bearing member.

2. Apparatus for developing electrostatic latent images previously formed on a support member, the apparatus comprising in combination:

(a) a stationary magnetic field producing means of predetermined length and comprising a permanent magnet formed generally at least part cylindrical, the periphery of which contains longitudinally extending alternating flutes and ribs, with adjacent of said ribs constituting magnetic poles of opposite polarity,

(b) means to support said field producing means,

- (d) said shield having an exterior surface patterned to retain a quantity ofmagnetic powder developer particles attracted thereto by the field producing means enclosed therewithin and substantially stationary relative to said shield during rotation, the magnetic particles being caused to form a brush-like array on said exterior surface,

(e) a supply source of magnetic powder developer particles supported at least partially within the fieldproduced by said field producing means, (1) means to rotate said shield about said fieldproducing means and between said supply source and said field producing means whereby developer particles are attracted from said source to said shield surface, and

(g) means to move an image-bearing member relative to said shield to continuously present a brushlike array of developer on said shield surface into contact with an image to be developed on the imagebearing member.

3. Apparatus according to claim 2 in Which said circumferential shield sections correspond substantially With the periodicity of the rib sections of said magnet 4. Apparatus according to claim 2 in which said shield is formed of separable semi-spherical sections securable to each other by a latch means.

References Cited by the Examiner UNITED STATES PATENTS WILLIAM D. MARTIN, Primary Examiner.

Claims

1. APPARATUS FOR DEVELOPING ELECTROSTATIC LATENT IMAGES PREVIOUSLY FORMED ON A SUPPORT MEMBER, THE APPARATUS COMPRISING IN COMBINATION: (A) A STATIONARY MAGNETIC FIELD PRODUCING MEANS OF PREDETERMINED LENGTH, (B) MEANS TO SUPPORT SAID FIELD PRODUCING MEANS, (C) A CYLINDRICAL SHIELD FORMED OF ALTERNATING CIRCUMFERENTIAL SECTIONS OF MAGNETIC AND NONMAGNETIC MATERIAL SUBSTANTIALLY ENCLOSING THE LENGTH OF SAID FIELD PRODUCING MEANS AND SPACED AT LEAST PARTIALLY WITHIN THE EFFECTIVE MAGNETIC FIELD OF SAID PRODUCING MEANS TO PHYSICALLY PREVENT MAGNETIC PARTICLES FROM ATTACHING TO SAID FIELD PRODUCING MEANS, (D) SAID SHIELD HAVING AN EXTERIOR SURFACE PATTERNED TO RETAIN A QUANTITY OF MAGNETIC POWDER DEVELOPER PARTICLES ATTRACTED THERETO BY THE FIELD PRODUCING MEANS ENCLOSED THEREWITHIN AND SU BSTANTIALLY STATIONARY RELATIVE TO SAID SHIELD DURING ROTATION, THE MAGNETIC PARTICLES BEING CAUSED TO FORM A BRUSHLIKE ARRAY ON SAID EXTERIOR SURFACE, (E) A SUPPLY SOURCE OF MAGNETIC POWDER DEVELOPER