Magnetic brush belt development

United States Patent £23,584,601

<td> [72]</td><td> Inventor</td><td> Lyman H. Turner Pittsford, N.Y.</td>

<td> [21]</td><td> Appl. No.</td><td> 771,219</td>

<td> [22]</td><td> Filed</td><td> Oct. 28,1968</td>

<td> [45]</td><td> Patented</td><td> June IS, 1971</td>

<td> [73]</td><td> Assignee</td><td> Xerox Corporation</td>

<td></td><td></td><td> Rochester, N.Y.</td>

3,455,276 7/1969 Anderson.................... 118/637

3.457,900 7/1969 Drexler........................ 118'637

Primary Examiner—Peter Feldman

Attorneys— Paul M. Enlow, Norman F. Schrader, James J.

Ralabate, Ronald Zibelli and Michael J. Colitz. Jr.

[54] MAGNETIC BRUSH BELT DEVELOPMENT 6 Claims, 2 Drawing Figs.

152] U.S.C1...................... 118/637,

117/17.5

[51] Int.CI......................................................... B05c5/00

[50] Field of Search............................................ 118/637,

623; 117/17.5

[56] References Cited

UNITED STATES PATENTS 3,117,891 1/1964 Lehmann..................... 117/17.5 X

3,145,122 8/1964 Streich......................... 118/637

3,208,296 9/1965 Baermann.................... 118/637 UX

ABSTRACT: Xerographic apparatus for the magnetic brush development of latent electrostatic images with a two-component magnetic developer. The apparatus includes a nonmagnetic shield with a roughened face spaced from a parallel stretch of photoconductive material in a xerographic development zone. A rotatable array of spaced magnets is moved on the side of the plate remote from the photoconductor to move developer in brushing contact with the latent electrostatic image being developed. The magnetic array has alternating north and south faces extending transverse to the direction of movement of the magnets so that the developer alternately collapses and reforms during movement of the magnets to thereby move the developer in a direction opposite to the direction of movement of the magnets to develop images in the photoreceptor.

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PATENTED JUN 151971

3,584,601

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ATTORNEY

3,584,601

MAGNETIC BRUSH BELT DEVELOPMENT

This invention relates in general to xerographic development and in particular to apparatus for the magnetic brush development of latent electrostatic images.

In the practice of xerography as described in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as a resinous powder. The powder is held in image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited; and where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image or of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably affixed to thereby form a permanent print.

The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred to here as “toner” and a “carrier” of larger granular beads formed with glass, sand or steel cores located with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the toner powder and the granular carrier. Such charge causes the toner to adhere to the carrier. The carrier also provides mechanical control so that the toner can be readily handled and brought into contact with the exposed xerographic surface. The toner is then attracted to the electrostatic image from the carrier to produce a visible powder image on the xerographic surface.

One common technique employed today for bringing developer into contact with a latent electrostatic image-bearing surface for its development is the magnetic brush system. In the magnetic brush system, the carrier employed is magnetic in nature, that is, it responds when placed in a magnetic field to form a brushlike array with bristles corresponding to the magnetic lines of the field producing means. Thus, when a quantity of two-component developer mix having magnetic carrier granules is entrained by a magnet, movement of the magnet adjacent the latent electrostatic image may move the entrained developer across the image to deposit the charged toner in a configuration corresponding to the image being developed. This occurs because the charge on image area portions of the photoconductor exceeds the electrostatic attraction of the carrier coating for the oppositely charged toner.

Typical of magnetic brush development systems employed today is that disclosed in U.S. Pat. No. 3,003,462 to Streich. According to that disclosure, a cylinder, formed with magnetic washers having alternate north and south poles, is positioned adjacent the surface being developed. The lower portion of the magnetic cylinder is in contact with a reservoir or sump of magnetic developer material while the upper portion of the magnetic cylinder is located adjacent the latent electrostatic image-bearing surface. When the magnetic cylinder is rotated and the image being developed is moved across the cylinder, magnetic bristles are formed on the surface of the cylinder which contact the image-bearing surface for its development. Other types of magnetic brush development systems are those disclosed in U.S. Pat. No. 2,975,758 to Bird and U.S. Pat. No. 2,874,063 to Greig.

In each of these prior art systems a magnetic brush is formed under the influence of a magnetic field producing means. Movement of the magnetic field producing means causes concurrent movement of the brush of magnetic developer. Consequently, that portion of the magnetic developer near the base of the bristle of the brush, adjacent the magnetic producing means is never employed for development since that portion of developer never contacts the photoconductor. Furthermore, each of the prior art systems employs a single extended bristle contacting the image-bearing surface along a small line portion of the xerographic surface transverse to their direction of relative movement.

The magnetic brush development apparatus of the instant invention is adapted to form a magnetic brush with many bristles extending over a large surface area whereby image developing material may be contacted to a large surface area of a photoconductive surface. This increases the time and area of the xerographic surface being developed to increase development speed. Furthermore, the apparatus of the instant invention is designed so that the natural movement of the magnetic field producing means will cause a constant erection and disruption of the magnetic bristles so that the entire mass of two-component developer in the brush may be employed for developing images. This feature insures that freshly toned developer will be contacting the xerographic surface along its path of movement.

It is therefore an object of the present invention to develop latent electrostatic images with magnetic developer.

It is a further object of the instant invention to develop latent electrostatic images with magnetic brush development apparatus over an extended area of the xerographic surface.

It is a further object of the instant invention to cause turbulence within image developing magnetic brushes for the presentation of fresh developer within the image developing zone.

It is a further object of the instant invention to disrupt and reform magnetic brushes during the development of images.

It is a further object of the instant invention to employ the movement of magnetic field producing means to cause developer turbulence within a magnetic brush development system.

These and other objects of the instant invention are obtained by xerographic development apparatus including a shield having a flat portion positioned in spaced relationship from a parallel stretch of photoconductive material in a xerographic development zone. The shield has a roughened surface facing the photoconductor. A cylindrically shaped rotatable array of magnets having a flat stretch corresponding to the photoreceptor is movable on the side of the plate remote from the photoconductor to move developer in brushing contact with the image being developed. The magnetic array has alternating north and south faces extending transverse to the direction of movement of the magnets and photoconductor so that the developer alternately collapses and reforms during movement of the magnets to thereby move the developer in a direction opposite to the direction of movement of the magnets.

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

FIG. 1 is a diagrammatic sectional view illustrating a xerographic reproducing machine adapted for continuous and automatic use and which employs the magnetic brush development apparatus of the instant invention, and

FIG. 2 is an enlarged perspective showing of the magnetic brush development apparatus shown in FIG. 1 with parts broken away for clarity.

Shown in FIG. 1 is a xerographic machine employing the magnetic brush development apparatus of the instant invention. The elements of this machine, which are constructed for continuous and automatic operation, are all conventional in the xerographic art except for the novel development apparatus, which forms the basis of the instant application. For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the xerographic surface may be briefly described as follows:

3,584,601

A charging station A, at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic surface,

An exposure station B, at which the light or radiation pattern of copy to be reproduced is projected onto the xero- 5 graphic surface to dissipate the charge in the exposed areas thereof to thereby leave a latent electrostatic image of the copy to be reproduced:

A developing station C, at which a xerographic developing material, including toner particles having an electrostatic 10 charge opposite to that of the electrostatic latent image, are moved into contact with the image bearing surface whereby the toner particles adhere to the electrostatic latent image to form a xerographic powdered image in the configuration of the copy being reproduced; 15

A transfer station D, at which the xerographic powder image is electrostatically transferred from the xerographic surface to a transfer material or a support surface; and

A cleaning and discharge station E, at which the xerographic surface is brushed to remove residual toner particles 20 remaining thereon after image transfer, and at which the surface is exposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.

The latent electrostatic images to be developed are formed on a xerographic surface 10 formed in the shape of an endless belt. The belt is adapted to be rotated about three supporting rollers 12,14 and 16, two of which, 12 and 14, are idler rollers and the third of which, 16, is a power imparting roller driven by any convenient power source 18. The movement of the ³⁰xerographic belt permits its surface to be moved past the various xerographic processing stations including the development zone C.

The xerographic developing apparatus is provided within a developer housing 20 formed of a nonmagnetic materia! such ³⁵as of metal, plastic or the like. The lower portion of the housing is troughlike in configuration to define a sump 22 to support a quantity of magnetic developer. The developer employed includes image developing toner particles and larger ₄₀carrier granules having magnetic properties. The carrier granules may be metallic or other materials having magnetic properties either uncoated or coated with a carrier coating material so that a triboelectric charge is generated between it and the toner upon mutual interaction. This charge is necessa- 45 ry to effect the xerographic development of the images. Examples of such developers are described in U.S. Pat. No. 2,874,063 to Greig. Although developer is generally two-component, it could readily be formed of one-component magnetic toner. ⁰

The housing is supported on its sides by support members 24 which are part of the xerographic reproducing machine and are spaced a distance which is large than the width of the photoconductor.

Also supported within the housing are bearings for support- 55 ing shafts upon which the magnet driving rollers 26, 28 and 30 are supported. Two of these rollers 26 and 28 may be idler rollers while the third 30, is a power driven member positively driven by power source 32. If desired, one of the idler rollers may be spring biased to provide tension within the magnetic 60 assembly. Endless belts 34 are mounted on the rollers for rotation therewith. The belts 34 could readily be formed on any cross-sectional configuration as could the xerographic surface 10. The surface of the belts support the magnetic members 36 in their proper orientation with respect to each other and with 65 respect to the photoconductive surface to be developed. The magnets may be glued or otherwise attached to the belts. Each of the magnets extends the full width of the photoconductor surface of the xerographic belt. The magnets are placed with alternating north and south faces extending transverse to the 70 direction of their movement with spacings 38 therebetween for a purpose to be later described.

Surrounding the major extent of the magnetic belt assembly and extending the full width between the side support members 24 is a shield 40 of a nonmagnetic material such as metal 75 or plastic arranged in a flat extent 42 to closely conform to the path of movement of the magnets within the development zone. This shield extends from an area to extend between rollers 26 and 30. It also has an extension portion 44 so that magnetic developer passed above roller 26 will be directed beyond the magnetic field of magnets 36 and cascade down the extension portion 44 for being redeposited in the sump 22 for recirculation by the magnetic belt assembly. The exterior surface 46 of the shield facing the photoreceptor is slightly roughened within the development zone as by etching or sandblasting so that movement of developer therealong, as caused by the movement of the magnets, will prohibit a smooth sliding on the bristles of the magnetic brushes along the shield within the development zone.

In operation, roller 30 is driven by power source 32 to rotate on its axis in a counterclockwise direction, as viewed in FIGS. I and 2 causing the endless belts 34 and magnets 36 to move downwardly from roll 26 to and around roll 30, upwardly to and around roll 28, and then to and around roll 26. This action will be continually carried on during development of an image while the image supporting photoconductive belt is moved in a counterclockwise direction by power source 18 in a direction of the arrow as shown in the arrow in FIGS. 1 and 2.

Due to the orientation of the magnets within the magnetic belt assembly magnetic lines of force extend from the faces of the magnets through the flat portion of the shield at 42. These force lines are substantially at right angles to the faces of the magnets in a relatively wide area at the center of the magnets extending at least to the photoreceptor. The lines of force gradually incline toward the adjacent magnets when adjacent the edges thereof until the force field at the edge of the magnets is substantially parallel to their faces across spaces 38. Very good results were attained when the spaces between the magnets were slightly less than half the cross-sectional lengths of the magnets.

The effect of the lines of force emanating from the faces of the magnets and passing through the nonmagnetic material of the shield at 42 will cause the magnetic developer mixture to arrange itself in the form of bristles conforming to the shape of the magnetic field lines which extend at right angles to the surface of the shield in areas corresponding to the center areas of each magnet while the developer mixture is held to the surface of the enclosure in a loose mass in areas corresponding to the spaces between the magnets.

As the magnets are moved in relation to the shield by the rotation of the belts 34, the roughened surface 46 of the shield prevents the developer mix from smoothly sliding thereon under the influence of the moving magnetic force lines. Thus as each magnet moves in relation to the mass of developer mixture on the shield, the bristles adjacent the magnetic force lines corresponding to the approximate center areas of the magnet will first incline toward the next approaching magnet and then collapse in a loose mass under the varying position of the magnetic field producing means. Virtually simultaneously therewith, the mass of loose developer mixture in the areas corresponding to the spaces between the magnets will now be influenced by the lines of force from the center of the approaching magnet and will be erected as new bristles. The alternating erection and collapse of the whiskers in end-overend fashion causes the entire mass of developer mixture on the surface of the enclosure to be moved in a tumbling manner in a direction opposite to the direction of movement of the magnets.

As the developer mixture moves to that portion of the shield around roll 26 and is removed from the influence of the magnetic lines of force by reason of the shape of the shield in this area, the developer mixture gravity falls down extension 44 to the sump for remixing with the developer already there. In this manner, it is ready for recirculation under the influence of the magnets for reuse in developing images on the xerographic surface in a continuous fashion. Positioned adjacent this portion of the developer housing is a toner dispenser 50 for the

3,584, addition of supplemental toner particles for replacing that toner lost from the mixture due to the development of images.

The shield is held in position within the housing so as to be closely spaced from the surface 10 being developed. This distance should be substantially equal throughout the mutual 5 extents of the flat portion of the shield and photoconductor at a distance slightly less than the average length of the bristles of the magnetic brush so that the tips of the brushes will contact the surface of the image bearing photoconductor. As the tips of the brushes move over the xerographic surface, toner parti- 1 θ cles are transferred to image area portions of the photoconductor from the developer mix since the individual toner particles have a greater electrostatic attraction for the high charge on the image areas than the relatively low charge on the associated carrier granules. 15

The collapse of each bristle of the magnetic brushes into a loose mass will occur through the movement of the magnets after the tip of each brush has contacted the xerographic surface and thus different alignments of the developer mixture will present different portions of the different portions of the ²⁰developer mixture to the photoconductor upon reerection of the developer. Thus each magnetic brushing of the surface of the photoconductor member will present the maximum available toner particles for complete development of the latent „ electrostatic image. Beyond this, the extended length of the ²⁵development zone insures complete development of the images.

While the instant invention as to its objects and advantages has been described herein as carried out in a specific embodi- ,₀ment thereof, it is not desired to be limited thereby; but it is intended to cover the invention broadly within the scope of the appended claims.