|Número de publicación||US3906121 A|
|Tipo de publicación||Concesión|
|Fecha de publicación||16 Sep 1975|
|Fecha de presentación||23 Nov 1973|
|Fecha de prioridad||25 May 1971|
|Número de publicación||US 3906121 A, US 3906121A, US-A-3906121, US3906121 A, US3906121A|
|Inventores||Fraser Lawrence J, Parker Delmer G|
|Cesionario original||Xerox Corp|
|Exportar cita||BiBTeX, EndNote, RefMan|
|Citas de patentes (10), Citada por (12), Clasificaciones (6)|
|Enlaces externos: USPTO, Cesión de USPTO, Espacenet|
United States Patent Fraser et al.
ELECTROSTATIC DEVELOPMENT METHOD USING MAGNETIC BRUSH CONFIGURATION TRANSPORT Inventors: Lawrence .1. Fraser; Delmer G.
Parker, both of Rochester, NY.
Xerox Corporation, Stamford, Conn.
Filed: Nov. 23, 1973 Appl. No.1 418,497
Related U.S. Application Data Continuation of Scr. No. 146,704, May 25, 1971. abandoned.
References Cited UNITED STATES PATENTS 6/1962 Bliss 117/175 12/1963 Hall.... 1l7/l7.5 11/1965 King ll7/l7.5
" 14 1 Sept. 16, 1975 Primary ExaminerMichael Sofocleous Attorney, Agent, or FirmJames J. Ralabate; Donald C. Kolasch; Ernest F. Chapman  ABSTRACT An electrostatographic developing technique for applying developer material to a developer receiving surface in conformity with an electrostatic charge pattern wherein the developer is transported from the developer supply to a development zone while in a magnetic brush configuration and thereafter, transported through the development zone in magnetically unconstrained blanket contact with the developer receiving surface. The magnetic brush configuration is formed by subjecting the developer mixture to a field formed by stationary multipole magnet members wherein each member is affixed to generate an opposite polarity with respect to an adjacent magnetic pole member.
12 Claims, 4 Drawing Figures ELECTROSTATIC DEVELOPMENT METHOD USING MAGNETIC BRUSH CONFIGURATION TRANSPORT CROSS REFERENCES TO RELATED APPLICATIONS This is a continuation of application Ser. No. l46,7(]4, filed May 25, 1971, now abandoned.
BACKGROUND OF THE INVENTION This invention relates to imaging systems and more particularly, to improved electrostatographic development methods and apparatus.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in US. Pat. No. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow pattern to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper and may be subsequently permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow pattern, one may form the latent image directly by charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Several techniques are known for applying electroscopic marking particles to the electrostatic latent image to be developed. Included within this group are the cascade" development technique disclosed by E. N. Wise in US. Pat. No. 2,618,552, the powder cloud technique disclosed by C. F. Carlson in US. Pat. No. 2,221,776 and the "magnetic bruch" process disclosed, for example, in U.S. Pat. No. 2,874,063. The magnetic brush development technique has proven to be particularly satisfactory in the development of large solid areas. In this technique, a fine marking powder called toner is mixed with a relatively coarse magnetically attractable carrier material and, through the phenomenon of triboelectrifieation, the toner particles become triboelectrically charged and cling to the surface of the carrier material. In magnetic brush development, the brush may consist of a magnet with the magnetically attractable carrier such as iron filings or other ferromagnetic powder attached to it by magnetic attraction in a chainlike arrangement simulating the fibers of a brush. When the toner material is applied to the brush, the toner particles cling to the ferromagnetic fibers by triboelectric attraction. Image development is accomplished by brushing the imaging surface with the brush to enable the toner particles to be electrostatically attracted by a charge of opposite polarity on the imaging surface to develop the electrostatic latent image.
In its most suitable and practical configuration, the magnetic brush development system comprises a rotatable cylindrical shell or sleeve surrounding a stationary bar magnet to provide the necessary magnetic field. Upon rotation, the developer is magnetically attracted to the rotatable shell from the developer reservoir and formed into a brush and upon being rotated is delivered to the development zone while in a brush configuration. Typically, the magnetic field is produced from a single large heavy expensive permanent bar magnet of high energy product material which must be placed off axis inside the cylindrical sleeve thereby requiring awkward mountings and minimizing freedom of structural design. In addition, when only a single magnetic field generating means is used, the single magnet must be strong enough that the holding force at the most distant point on the cylindrical roller be adequate. When the magnetic field is produced by a single magnet, the magnetic attractive force cannot be adjusted at different points on the roller circumference to perform the various functions of pickup, holding, erection of a brush and release of the carrier in an optimum manner. Furthermore, the field with the bar magnet configuration extends far beyond the roller surface where it is needed to perform the attractive function and may interact with the field of adjacent rollers or interfere with other operations. Additionally, stray fields can permeate the flowing developer where they tend to increase friction and interfere with crossmixing and may also cause eddy current losses in nearby moving parts.
In these magnetic brush development systems, it is also generally desirable to regulate or control the thickness of the developer layer carried on the roller by moving the roller past a metering blade, which in addition to requiring fairly sensitive adjustment may abrade the moving developer and increase the number of collisions between adjacent carrier particles contributing to increased impaction of toner material on the carrier beads with a consequent reduction in developer life. Toner impaction in the carrier is generally to be avoided since the impaction of any material on the surface of the carrier will alter the triboelectric relationship between the carrier and the toner particles and may contribute to the subsequent flaking off of impacted material from the carrier leading to uneven development on the imaging surface. Further areas of toner impaction on carrier beads may be present in magnetic brush development systems where the flow of developer is interrupted and started by mechanical scapers of gates or by rotating the bar magnet inside the roller. The gates contribute to carrier wear and impaction of toner material into the carrier particles. In addition, in the actual development zone where the magnetic brush is brushed across the imaging surface, the developer must be passed through a development zone defined by a narrow restrictive opening between the roller and the imaging surface so that the toner particles may be electrostatically attracted from the carrier particles to the imaging surface. To provide sufficient toner particles to the imaging surface, it is generally necessary to compress the developer bristles thereby making toner adhering to carrier particles remote from the edges of the bristles available for development. To accomplish this compression in the development zone, it is generally necessary to force the developer through the development zone with a consequent increase in the energy or powder required during development.
An additional problem encountered with magnetic brush development is in the scratchy prints which are frequently obtained as a result of the magnetically attractable carrier particles being pulled or pushed through developed solid areas as magnetically linked chain segments. Furthermore, with a magnetic field generating structure comprising a single dipole bar magnet, after the formation of the developer in a brush configuration on an applicator surface, little mixing of the carrier and toner is accomplished on the applicator surface prior to reaching the development zone. As a result, charging of the toner material may be inadequate in places and the toner may not be evenly distributed throughout the developer both resulting in lower quality development.
Various attempts have been made to solve or minimize the difficulties described above. In each of these proposals, however, while certain problems may be alleviated to one extent or another, deficiencies still exist in the remaining areas of difficulty. There is a continuing need for improvements minimizing or eliminating all of the foregoing mentioned and other difficulties.
SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a development method of apparatus which overcomes the above noted deficiencies.
It is another object of this invention to provide a development technique and apparatus requiring reduced power to accomplish development.
It is another object of this invention to provide a development technique and apparatus capable of reduced carrier impaction.
It is another object of this invention to provide a development technique and apparatus capable of providing increased developer life.
It is another object of this invention to provide a development technique and apparatus of increased flexibility and simplicity of design.
It is another object of this invention to provide a development technique and apparatus which limits the presence of magnetic fields to only those areas where required.
It is another object of this invention to provide a development technique and apparatus capable of producing improved solid area development.
It is another object of this invention to provide a development technique and apparatus with automatic selfmetering of the developer to the developer applicator.
It is another object of this invention to provide a development technique and apparatus capable of producing prints with more uniform solid areas.
The above objects and others are accomplished, generally speaking, by providing an electrostatographic developing technique and apparatus for applying developer material to a developer receiving surface in conformity with an electrostatic charge pattern wherein the developer is transported from a developer supply to a development zone while present in a brush configuration and the developer is transported through the development Zone in a magnetically unconstrained blanket contact with the developer receiving surface. More specifically. the development zone is substantially free of the influence of the magnetic field generating means used to maintain the developer in a brush configuration during the transport of developer to the development zone. Since'in the development zone the developer is not magnetically constrained and there are substantially no magnetic cohesive forces between adjoining magnetically attractable beads the developer will fall off the developer donor member by its own weight when placed upside down. Typically, the developer is present on a developer donor member in the development zone in a brushless, dense closely packed blanket contact with the developer receiving surface.
While the magnetic field present in the development zone is insufficient to maintain the developer in the form of a magnetic brush, the magnetic field generating means are sufficient to maintain the developer during transport from the developer supply to the development zone in the form of a magnetic brush. The use of small multipole magnetic field generating structures such as commerically available flexible magnets and ceramic magnets enables the design and control of the magnetic field in such a manner that the effectively short range magnetic fields can be tailored to perform certain functions while at the same time not influencing other nearby functions. This degree of control over the individual toner and carrier particles in the developer enables the regulation of the contacts or collisions between particles and with external surfaces and enables the transport of developer material only to desired areas.
Further, the removal of the magnetic field from the development zone reduces the forces between the carrier beads permitting them to roll more freely in contact with the developer receiving member thereby reducing friction between the developer receiving member and the developer donor member. Surprisingly with this degree of control, the power required in the development zone and toner impaction on the carrier are significantly reduced. In addition, copies of improved print quality may be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS The novel features and advantages of the present invention may be further illustrated by reference to the accompanying drawings in which:
FIG. 1 is a schematic illustration of development apparatus according to the present invention.
FIG. 2 is a schematic illustration of an alternative embodiment according to the present invention.
FIG. 3 is a schematic illustration of an alternative embodiment according to the invention.
FIG. 4 is a graphical comparison of the power requirements for development with three different apparatus.
In FIG. 1, an electrostatographic imaging surface 11 such as a photoconductive insulating layer in the configuration of a drum, only portion of which is illustrated, and which may have been previously uniformly charged and exposed to a light and shadow pattern in conventional manner is passed through a development zone defined by the space between the drum surface and the developer applicator or donor member 15. Developer applicator 15 comprises a stationary shaft 13 of high magnetic permeability which may function as a magnetic shunt or keeper and affixed to the stationary shaft about at least a portion thereof a plurality of stationary magnetic members 14 which may be individual small bar magnets or small strips or sectors of flexible rubber magnetic material or a sector of a ceramic magnet which has been magnetized in a manner to provide a plurality of magnetic pole members. Adjacent magnetic pole members are of opposite polarity on the outer surface to provide the desired magnetic field generating structure. The individual magnetic members 14 may be positioned adjacent to one another or if desired, separated by a small distance. During operation, the magnetic field produced by the plurality of stationary magnetic members 14 should be sufficient to magnetically support the developer in the form of a brush while being transported from the developer supply or reservoir along the surface of rotatably mounted conductive cylindrical sleeve member 12 as the sleeve member is rotated within the reservoir to the development zone defined by the gap between imaging surface ll and the surface of sleeve 12. The lines of force of the magnetic field producing means are insufficient to maintain the brush configuration of the developer in the development zone and outside this zone there is substantially no contact between the developer and the developer receiving member. If desired, uniformity of composition of the developer may be maintained in the developer reservoir 10 by means of rotating vaned mixing members 16.
FIG. 2 illustrates an alternative embodiment according to the present invention wherein the developer donor member comprises an endless movable transport surface such as belt 21 supported and driven around cylindrical driving rollers and 29. Within the driving roller which advances the belt into the development zone is placed an annular multipole magnetic field generating structure concentric with the axis of the roller and along that portion of the roller which transports developer to the development zone. Typically, the magnetic field generating structure comprises a plurality of magnetic members 23 fixedly positioned interior of roller 20 about shaft 28 with, if desired, a magnetic shunt or keeper 22 positioned inbetween the shaft and the magnetic members. In this embodiment developer will be magnetically attrached to the nonmagnetic conductive belt 21 and transported into the development zone defined by the space between the belt 21 and an image bearing surface, for example, an insulating material 25 driven through the development zone by means of positioning and driving rollers 24. With this structures and due to the comparatively small size of the magnetic field the developer may be transported while in the bristle configuration of a magnetic brush from the developer supply to the development zone and thereafter transported through the development zone in a magnetically unconstrained configuration. Following development, the developer may be discharged from the development zone and fall by gravity into developer reservoir 26. If desired vaned mixing members 27 may be employed to move the developer from the discharge portion to the pickup area of the developer reservoir. Alternatively, following discharge from the development zone the developer may be magnetically attracted to donor belt and conveyed from the discharge portion through the developer supply and into the development zone while magnetically constrained in a brush configuration.
ln FIG. 3 is depicted an alternative embodiment where the developer may be transported from a developer supply while the magnetically constrained in a brush configuration and thereafter pitched or catapulted into a magnetic field free region development zone and subsequently discharged from the development zone and magnetically recaptured and returned to the developer reservoir. In this embodiment, the development zone is generally defined by that gap between the imaging surface 30 and a conductive member 12 serving as a development electrode in well known manner. Developer in the developer reservoir 31 is picked up from the reservoir onto rotatably mounted cylindrical roller 33 containing a concentric multipole magnetic field generating means 34 housed within in a manner described with respect to either FIGS. 1 or 2 and the developer is transported around roller 33 while in a brush configuration and pitched or catapulted into the above described development zone. The peripheral speed of the developer donor member is such that at the top of the roller when the influence of the magnetic field is terminated the developer is thrown from the surface of the roller 33 and into the development zone. Depending on the specific configuration of the developer donor member and the structure of the magnetic field generating means the speed with which the roller 33 must be moved to transport developer from the developer supply to the development zone and catapult or pitch the developer therein may be readily determined by the artisan. Upon discharge of the developer from the development zone, the developer may be magnetically recaptured in the form of a magnetic brush on cylindrically mounted rotatable roller 35 having concentrically mounted within a stationary multipole magnetic field means 36 similar to that described in FIGS. 1 and 2. If desired, the developer may then be transported back to the developer supply.
it should be understood that the above described three figures of the drawings are intended to be exemplary only of the development technique and apparatus of the present invention. The basic principals of this invention may be easily modified to provide the requisite control of developer in any particular development scheme. For example, it may be desirable in certain instances to employ a plurality of developer donor rollers similar to roller 15 in FIG. 1 in the development zone, particularly when high speed development is desired. Alternatively, developer may be conveyed to the transporting and donor roller 15 of FIG. 1, for example, by means of a second roller containing multipole magnetic field generating means. In addition, with the use of adjacent roller developer donor members rotating in opposite directions, both rollers containing annular multipole magnetic field generating means, simultaneous development with motion of the developer with and against the motion of the imaging surface may alsobe obtained.
Any suitable multipole magnetic field producing means of any suitable structure may be employed in the practice of the invention. Typical materials may be readily selected by the artisan from the commercially available ceramic magnets and flexible magnets. In general, the ceramic magnets are prepared by extruding a composition comprising ferrite powder and a ceramic binder and the resulting shape is then sintered at high temperatures, cooled and magnetized. If required, the ceramic magnets may be finished by grinding but it quite difficult to drill or machine them. The flexible magnets ingeneral, consist of finely divided ferrite particles such as barrium ferrite uniformly dispersed in a generally nonmagnetic rubber or plastic binder. These materials are generally prepared by extruding a large magnetic matrix containing the ferrite particles into the desired shape and permitting the resinous plastic or elastomeric material to harden or cure to a solid state. The binder material may contain any additional agent such as vulcanizing agents, plasticizers and the like to aid in the compounding of the matrix. In addition to barrium ferrite. lead and strontium ferrite are also particularly effective particles which form good permanent magnets.
The multipole magnetic field producing means may be of any suitable shape or configuration. They may. for example. be in the form of a plurality of strips of the flexible rubber magnets with each strip representing a single dipole magnet or alternatively may be in the form of a flexible sheet which is appropriately shaped and which may contain a plurality of individual dipole magnetic field producing members. In addition, the use of shaped ceramic magnets of any desired configuration may be employed. Particularly satisfactory results may be had with extruded annular sections made of ferrite loaded rubber which may be adequately magnetized along a portion of its perimeter.
The multipole magnetic field producing materials may be magnetized in any suitable manner. Typically, they are magnetized with a standard impulse discharge type or DC type magnetizer. as is well known in the art. Since the magnetic field produciing means of this invention may be magnetized prior to assembly of the magnets in their appropriate place and since several poles may be placed on one face or surface of these magnets, the flexibility and design in fabricating is vastly improved. The multipole magnetic field producing means may produce a magnetic field of any desired strength. Typically, the peak magnetic field at the surface of the developer donor member in the transporting zone is of the order of from about 200 to about L000 oersted. Generally, the lower value is characterized by the magnetic field which is capable of holding the developer on the surface of the donor member in a brush configuration while the upper value is a practical value established by the geometry and material characteristics. the short range nature of the fields produced from these multipole magnetic field producing members assures that the fields are confined to the immediate region near the magnetic members and accordingly, the fields can be eliminated at any given point if desired.
The developer donor member may be of any suitable size, structure or configuration. including that of a cylindrical roller or belt and may generally be described as an endless movable transport surface. With the appropriate use and placement of a short range magnetic field producing means, control of the magnetic field in the substantially field free areas is readily facilitated. Typically, cylindrical roller donor members having a diameter of from about 1 inch to several inches may be employed. Positioned interior of this roller may be a plurality of a single annular magnet or rectangular bar magnets or rubber magnets having widths of the order of about one quarter to about I inch and thicknesses between about 1/16 to about 1/2 inch.
The donor member which may. for example, be the rotatable cylindrical sleeve of FIG. I may be of any suitable conductive material, rough or smooth surfaced. Since the multipole magnetic field producing means employed herein are relatively low energy product materials with short range field configurations it is generally preferred to keep this donor member quite close to the magnetic material. To insure that the magnctic field does not interfere with other magnetic fields in any particular applicator configuration. and to provide the best use ofthe magnetic material. it may be desirable to employ a magnetic shunt or keeper in the manner depicted in FIG. 1. Any suitable high magnetic permeability material of suitable configuration may be employed for this purpose.
In operation. the flow of developer from the developer reservoir to the development zone may be readily interrupted by the rotation of the otherwise stationary multipole magnet and particularly the annular multipole magnet about an axis. Since the fields are short ranged and well defined. the magnet need not be moved very far to cut the developer flow. When the developer flow is to be interrupted. the magnet may be rotated in the direction of the developer flow so that all the developer then present on the roller may be transported through the development zone and no additional developer will be transported thereby leaving the roller clean. A simple cam mechanism on the shaft of the magnetic support member may be provided for this rotation.
With the relatively low energy short range field producing magnets, the developer will generally be present on the donor member in a brush thickness of the order of the pole spacing between the poles of the magnetic field producing structure. Typically, this is of the order of from about one eighth to about three eights of an inch. The developer is held on the surface of the donor member in the form of a brush by the high gradient short range magnetic field and as the donor member moves past the several poles of the magnetic field producing structure, the individual developer particles tumble under the effects of the changing direction of the magnetic field while moving as a mass in the direction of movement of the moving donor surface. It is currently believed that this tumbling contributes to increased uniformity of developer and thereby more uniform development. The magnetic field in the transport zone is generally sufficient to form and maintain this brush against the influence of gravity, inertia and centrifugal force while the developer is transported from a suitable supply to the development zone. The donor member transports the developer through the development zone in a magnetically unconstrained configuration so that the developer has freedom of movement within the development zone.
With the use of a roller donor member and an arcuate imaging surface as depicted in FIG. 1, development will take place in the center portion of the nip formed between the roller applicator and the imaging surface and in this zone substantially no magnetic field is present. in the interior of the donor member. the magnetic field producing structure is so positioned that the developer may be transported in the brush configuration from the developer supply to a position on the donor member where the mere physical movement of the donor member enables the developer to be further transported to and through the development zone. In the case of a cylindrical donor member. for example, this may be readily accomplished by positioning the multipole magnetic field producing structure interior of that arcuate portion of the cylindrical roller required in transporting the developer from the developer supply to the development zone. Within the development zone. any magnetic field present is insufficient to restrict the movcment of the developer material.
While the development zone may be described as being substantially free of magnetic field this does not mean that the field is strictly zero oersteds. Clearly the effects of the earths magnetic field which is about 0.5 oersted are present. It is intended that the magnetic field present in the developer zone be significantly reduced such that it does not affect the structure of the developer during development. Typically, the magnetic field in the development zone will be reduced to less than about 10 percent of the field applied for forming the developer brush. This generally will provide a magnetic field within the development zone of about 50 oerstcds or less.
Any suitable spacing between the donor member and the imaging surface within a development zone may be employed. Typically, the spacing is generally dependent upon the quantity of developer desired to be present during development. Typically, development zone spacings of from about 0.020 to about 0.120 inches have proven to be adequate. With such spacing, the movable donor member with the developer material physically adhering to the surface moves through the development zone and forces the developer to pass through the development zone creating the desired intimate flowing contact between the developer and the imaging surface to provide uniform development.
Generally, both the donor member and the surface upon which an electrostatic charge pattern is to be developed are both moved through the development zone and may be moved either in the same direction or in opposite directions. While it is currently believed that there is wide latitude in the choice of speed and direction of movement of the developer donor member and the speed of development, it is generally preferred to move the developer donor member at a rate faster than the surface upon which to develop the electrostatic charge pattern in order to insure an adequate supply of developer for development. Typically, a ratio of about 3 to l of the peripheral speed of the developer donor to the speed of the imaging surface in the development zone has been found to be effective. Generally, the individual speed of the donor member is such that the magnetic brush formed by the multipole magnetic field producing structure is undisturbed merely be the effects of centrifugal force. If desired, the size of the development zone may be increased with the use of a belt type donor member which may be adjacent a greater portion of the imaging surface, and thereby insure more uniform development. As will be more fully apparent hereinafter, development in an essentially field free region reduces the power required to force the developer present on the donor member through the development zone. It is currently believed that in the absence of a strong magnetic field, there are no significant magnetic linkages between adjoining magnetically attractable particles and as such, there is freedom of motion of the developer particles within the development zone and reduced friction between the imaging surface and the donor member. An additional advantage of this development technique is that the developer donor member may function as a development electrode in conventional manner to provide improved solid area coverage and if desired. reversal development. Typically, reversal development may be obtained by applying to the electrode a potential of the same polarity and about the same magnitude as that in the charged areas of the imaging surface.
The development technique and apparatus according to the present invention may be employed to develop an electrostatic charge pattern present on any suitable electrostatographic imaging surface. Basically, any surface upon which an electrostatic charge pattern may be formed and maintained for a short period of time may be employed. Typical electrostatographic imaging surfaces include dielectrics such as plastic coated papers and photoconductors. Typical photoconductors include photoconductive materials on an electrically conductive support member such as brass, aluminum, nickel, steel or the like. The support member may be of any convenient thickness, rigid or flexible, and may be in any desired form such as sheet, web, plate, cylinder, drum or the like and may also comprise other materials such as metalized paper and plastic coated sheets.
Development may be accomplished with any suitable developer materials. Typically, as is well known in the art, development may be accomplished with electroscopic marking particles referred to in the art as toner and grossly larger magnetically attractable carrier beads which are selected in accordance with their triboelectric properties so that when they are brought into contact each material becomes electrically charged to a polarity opposite to that of the other. Thus, one material may be charged positively if the other material is below it in a triboelectric series and negatively if the other material is above it in a triboelectric series. To provide a magnetically attractable developer some portion of the developer is made of magnetically attractable material. Typically, the granular carrier material may, therefore, comprise iron filings or other ferromagnetic powder such that when under the influence of a magnetic field, a chainlike arrangement of magnetic particles simulating the fibers of a brush is formed. The toner material clings to the ferromagnetic fibers by triboelectric attraction. Typically, the grossly larger carrier particles which may have average particle diameters of from about 50 microns to about 600 microns may be coated with a material to provide the desired triboelectric relationship between the carrier and the toner material. The carrier material is generally of sufficient mass to avoid adherence to the electrostatic image during development. Any suitable dyed or pigmented electroscopic marking material may be employed. Typical well known materials include gum copal, gum sandarac, rosin, phenolformaldehyde resins, methylacrylic resins, polystyrene resins, polypropylene resins, epoxy resins, polyethylene resins and mixtures thereof. These toners generally have average particles diameters of from about 1 and about 30 microns.
As discussed above, when development is accomplished in a region essentially free of the eostraining effects of the magnetic field, the power necessary to accomplish development is surprisingly found to be markedly reduced as will be apparent from the following nonlimiting comparative examples which describe and compare preferred methods and apparatus of the present invention. Example I is a practice according to the present invention while Examples II and Ill and presented for comparative purposes only. Unless otherwise specified, all amounts, proportions and percentages are by weight.
' EXAMPLE I An electrophotographic plate comprising a surface layer of selenium, 60 microns thick, on a conductive substrate is positively charged to 900 volts and exposed to a light and shadow pattern in conventional manner. Development of the electrostatic latent image present on the selenium plate is obtained with a developer applicator similar to that depicted in FIG. 1 by placing a plurality of about one quarter inch square bar magnets cut from a piece of Plastiform (available from Leyman Corp. Cincinnati, Ohio) around a steel keeper such that the pole closest to the vertical from the advancing side of the roller applicator to the development zone is approximately 30 to the left of the vertical and that the last bar magnet placed adjacent to the discharge portion of the development zone is approximately 60 to the right of the vertical. The rotatable roller is a W; inch diameter stainless steel roller and development is obtained with a developer comprising 250 micron steel carrier beads and Xerox 914 toner, the toner being present in the concentration of about l k percent. by weight, of the developer. The plate is moved through the development zone at a spacing of about 0.05 inches in a direction both with and against the direction of the developer donor roller. The power dissipated in the development zone is determined by measuring the torque on the applicator roller necessary to maintain a constant roller surface speed of about 20 inches per second. A reference zero power level is that required to rotate the roller at a surface speed of 20 inches per second while the roller is carrying developer but is not in contact with the plate. Developer is loaded onto the ro tating roller at a rate of about 21 grams per inch second. With the field free zone created by terminating the EXAMPLE ll The procedure of Example I is repeated except that the multipole magnetic field generating means is rotated about the axis of the roller such that the bar magnets extend around the roller through the entire development zone and the field free region is present where developer is thrown from the roller. Curve B of FIG. 4 graphically illustrates the power requirements in this development technique with the magnetic field present in the development zone with various speeds of development with the photoreceptor moving with and against the direction of rotation of the roller. Comparison of the print quality of Examples l and ll reveals that both techniques provided prints of equal quality even though the energy dissipated in the development zone differed by an average factor of about 3 or more.
EXAM PLE lll For further comparison the annular magnetic field producing means of Examples l and ll is replaced by a dipole magnet which produces a peak magnetic field at the roller surface of about 450 gauss. Contrasted to the rate of passing developer through the development zone in Examples l and II ofonly 2l grams per inch second. the developer flow rate is about 45 grams per inch second in this example due to the longer range nature of the magnetic field. Development is obtained at various development speeds with the photoreceptor being moved with and against the direction of rotation of the applicator. Comparison of the print quality of prints produced in this mode indicates that they are comparable in quality to those produced in Examples l and ll. Curve C of Flg. 4 graphically illustrates the power requirements in the development zone and clearly demonstrates the extraordinary reduction in power which may be achieved in practicing the present invention lt is currently believed that this reduction in power is achieved since the magnetic brush does not have to be counteracted or collapsed while forcing the developer through the development zone. Further, extended life study tests of the developer in development configurations where development is achieved in a magnetic field region and in a nonmagnetic field region indicate that the developer employed in a nonmagnetic field region is believed to enjoy a greater life span due to reduced toner impaction of the carrier. In addition, since the short range fields of the annular multipole magnetic field producing means of this invention during the transport portion and recapture portion, if used, produce magnetic brushes of very small dimensions, there is no need for the use of leveling blades and other devices to regulate the thickness of developer on the applicator surface. Accordingly, impaction between the carrier particles is reduced and developer life extended. In addition, reduced streaking in the development of solid areas when developing in an essentially field free region has been observed. The increase in power as the imaging surface and applicator surface approach synchronous speed as shown in FIG. 4 is believed to be due to frictional losses since they approach a maximum because static friction losses which are higher than sliding friction losses are possible between the developer and both the applicator surface and imaging surface.
The development technique and apparatus described herein provide a relatively small compact development system of great versatility and flexibility in operation. Thus, the development system may be tailored to produce strong magnetic field influences in areas where desired and weaker or essentially no magnetic field influences in other areas. The use of magnetic fields to hold the developer in the form of a brush from the de veloper supply to the development zone and the ability to exclude the magnetic field from areas not desired permit development in a zone where the developer is magnetically unconstrained resulting in reduced power requirements for development. In addition, the increased mixing of developer material during the transport from the developer supply into the development zone is believed to contribute to a more uniform development in the development zone. Since the mass of material that must be moved during each development cycle may be markedly reduced, the collisions between neighboring carrier particles and toner particles are markedly reduced and the impaction of toner on the carrier particles is also reduced resulting in increased developer life.
Although specific techniques, apparatus and materials are set forth in the foregoing disclosure and comparative examples using the development apparatus and technique of this invention, there are other materials and techniques and modifications of the present invention which will occur to those skilled in the art upon a reading of the present disclosure which techniques, modifications and materials are intended to be included within the scope of this invention.
What is claimed is:
l. A method of developing an electrostatic charge pattern with a dry developer comrisirig a mixture of electrostatically attractable toner particles and magnetically attractable carrier particles. said carrier particles and said toner particles having a triboelectric relationship of opposite polarity and said toner particles being charged to a polarity opposite that of the charge pattern to be developed. said method comprising providing a development zone between a movable developer donor member and a developer receiving member upon which to develop said charge pattern, transporting the developer mixture from a developer supply to the development zone while present in a magnetic brush configuration on the movable developer donor member, said magnetic brush configuration being formed by stationary multipole magnet members, each member being affixed to generate an opposite polarity with respect to an adjacent magnetic pole member, transporting said developer while on said movable donor member through the development zone in which the magnetic field in the development zone is insufficient to maintain the developer in a brush configuration, said developer mixture being in substantially uniform brushless, blanket contact with said developer receiving member in said development zone, and terminating contact between said developer receiving member and said brushless blanket of developer following development.
2. The method of claim 1 wherein said developer receiving member and said developer donor member are passed through the development zone in the same direction.
3. The method of claim 2 wherein the surface speed of said developer donor member through the development zone is faster than that of said developer receiving member.
4. The method of claim 1 wherein said developer receiving member and said developer donor member are passed through the development zone in opposite directions.
5. The method of claim 1 wherein during the transporting of developer from the supply to the development zone the individual developer particles are under the influence of the lines of force of a multipole magnetic field and tumble on the surface of the movable donor member while moving as a mass in the direction of movement of the movable donor member.
6. The method of claim 1 wherein said developer donor member comprises an endless movable surface.
7. The method of claim 1 wherein said magnetic field in the transport zone is from about 200 to about 1,000 oersteds and the magnetic field in the development zone is less than about 50 oersteds.
8. The method of claim 1 wherein following development, the developer is magnetically recaptured in the form of a magnetic brush and returned to the developer supply.
9. The method of claim 1 wherein said carrier particles have a particles size of from about 50 to about 600 microns and said toner particles have a particle size of from about 1 to about 30 microns.
10. The method of claim I wherein said developer receiving member comprises a photoconductive insulating layer bearing an electrostatic latent image.
11. A method of developing an electrostatic charge pattern with a dry developer comprising a mixture of electrostatically attractable toner particles and magnetically attractable carrier particles, said carrier particles and said toner particles having a triboelectric relationship of opposite polarity and said toner particles being charged to a polarity opposite that of the charge pattern to be developed, said method comprising providing a development zone between a movable developer receiving member upon which to develop said charge pattern and a stationary conductive surface, transporting the developer mixture from a developer supply while in a magnetic brush configuration on a developer transport surface, said magnetic brush configuration being formed by stationary multipole magnet members. each member being affixed to generate an opposite polarity with respect to an adjacent magnetic pole member. catapulting the developer from the transport surface into the development zone in which the magnetic field is insufficient to maintain the developer in a brush configuration, said developer mixture being in substantially uniform brushless, blanket contact with said developer receiving member in said development zone, terminating contact between said developer receiving member and said brushless blanket of developer following development, and magnetically recapturing the developer in the form of a magnetic brush on a developer recapture surface following development.
12. A method of developing an electrostatic charge pattern comprising providing a development zone between a movable endless developer donor member and a developer receiving member upon which to develop said charge pattern, the magnetic field in said development zone being insufficient to maintain developer in a brush configuration, transporting a dry electrostatically attractable developer from a developer reservoir to the development zone while in a magnetic brush configuration on the movable endless donor member, said magnetic brush configuration being formed by stationary multipole magnet members, each member being affixed to generate an opposite polarity with respect to an adjacent magnetic pole member, transporting said developer while on said movable endless donor member through the development zone in a brushless blanket magnetically unconstrained configuration while said developer is in substantially uniform contact with said developer receiving member and terminating contact between said developer receiving member and said developer.
UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. ,9 ,121
DATED September 16, 1975 INVENTOR(S) 1 Lawrence J. Fraser and Delmer G. Parker It is certified that error appears in the above-identified patent and that sa'rd Letters Patent are hereby corrected as shown below:
Column 1, line 46, the word "bruch" should be deleted and insert therefor -brush--.
Column 2, line 50, delete the words "scapers of" and insert therefor scrapers or.
Column 2, last line, delete the word "powder" and insert therefor --power.
Column 3, line 26, delete the word "of" and substitute therefor and.
Column 4, line 54, insert --aafter the word "only".
Column 5, line 44, delete the word "structures" and substitute therefor structure.
Column 5, line 65, delete the word "the" and insert therefor --being--.
Column 6, line 5, the numeral "12" should be deleted and insert therefor the numeral 32.
Column 7, line 41, capitalize the word "The" in the sentence.
Column 9, line 44, delete the word "be" and substitute-by-.
Column 10, line 57, delete the word "costraining" and substitute therefor --constraining-.
Column 14, line 7, Claim 9, the word "particles" should be deleted and substitute therefor -particle.
Signed and Sealed this twentieth Day of April1976 [SEAL] AHCSI.
RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummissrum'r uflarems and Trademarks
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