US3396700A - Xerographic toner dispensing apparatus - Google Patents

Description

Aug. 13, 1968 D. J. DONALIES 3,396,700

I XEROGRAPHIC TONER DISPENSING APPARATUS Filed Aug. 9, 1-967 INVENTOR. DANIEL J. DONALI ES BY qz A T TORNE Y5 United States Patent 0 3,396 700 XEROGRAPHIC TONER D ISPENSHNG APPARATUS Daniel J. Donalies, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Aug. 9, 1967, Ser. No. 659,519 10 Claims. (Cl. 118-637) ABSTRACT 0F THE DISCLGSURE Apparatus for dispensing toner particles to undertoned carrier granules in a xerographic development system, as of the fluidized bed type. A quantity of carrier granules to be toned is passed through a toning zone bounded by a pair of screens, at least one of which is electrically biased. Charged toner particles are held in suspension between the screens as by vibration. The carrier granules to be toned triboelectrically attract and retain toner particles from the toning zone, only up to the point of optimum toner concentration since the electric field within the zone acts as a competing force against which the carrier granules rnust act in order to attract and retain additional toner particles.

Background of the invention This invention relates in general to controlling the amount of toner particles on carrier beads in two component xerographic development systems and in partic- I No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material afiixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic surface is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the 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 electrostatical-ly attractable material such as a powder. The powder is held in image areas by the electrostatic charges on the layer. Where the charge is greatest, the greatest amount of material is deposited; Where the charge is least, little or no material is deposited. Thus a powder image is produced in conformity with the light image of copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably affixed thereto to form a permanent print.

The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred to herein as toner and a coarse granular material called carrier. The carrier is coated with a material removed in the triboelectric series from the toner so that a charge is generated between the powder and the granular carrier upon mutual interaction. Such charge causes the powder to adhere to the carrier. The carrier, besides providing a charge to the toner, permits mechanical control so that the toner can readily be brought into contact with the exposed xerographic surface. The powder particles are attracted to the electrostatic image from the granular material to produce a visible powder image on the xerographic surface.

The quality of the developed image may be aifected by the ratio of toner particles to carrier beads used for development, or, as this ratio is commonly called, the toner concentration. For example, if there is a deficiency of toner, the image areas will be unable to attract sufiicient toner to fully develop the image, henoe, undesirably light images result. On the other hand, if there is an excess amount of toner in a development system, the image areas become overly dark with quantities of toner becoming attached to non-image areas. This is because the attractive force holding the toner particles on a carrier head is inversely proportional to the number of particles on such bead. Consequently, if there are few toner particles on a carrier bead, such particles will be strongly attracted thereto, and will not be readily given up to the charge of the image areas. If excess toner particles are on a carrier bead, the mutual attraction is decreased often to the point where the residual charge on non-image areas is sufficient to attract the toner particles from their associated carrier beads.

In the past, various attempts have been made to control the ratio of toner to carrier in the developer. These attempts have generally have been directed to the periodic addition of raw toner into the developer mix to supplant the toner lost from the system by its deposition on the image. Various systems have accomplished this by adding toner periodically to the developer mixture on the basis of a time :cycle or some suitable apparatus which automatically measures the developer characteristics directly or the density of the developed image. Then in response to the information measured, a suitable amount of toner may be added to the system.

While such systems are somewhat satisfactory, they have certain obvious shortcomings. For example, when toner is added to the developer it is generally added equally across the length of the development zone. If, however, the image being reproduced has large dark sections in one area, as the middle, then the central portions of the developer mix would be reduced to a lesser toner concentration than the side portion. The addition of toner to the developer evenly across the length of the development zone would not rectify this imbalance. Furthermore, systems of the known type which are somewhat self-regulating generally need time to react to a sudden depletion of toner caused by the development of large dense image areas. Hence, this time delay might cause sporadic density of development. This problem can be aggravated in some types of the time regulated toner dispensers. While these problems are not overly critical today it appears that they will become magnified in the future due to increasing speeds and the advent of improved solid area capabilities.

Known xerographic toner dispensing mechanisms generally permit carrier granules to accept toner particles until the individual carrier granules can triboelectrically attract and retain no more toner. As carrier granules triboelectically attract and retain additional toner to their surfaces, their electrostatic attraction for additional toner particles decreases in proportion to the amount of toner already being held on their surfaces. Once a carrier granule can electrostatically accept no more toner, it is said to be saturated. This saturation level is usually considered greater than the optimum toner concentration level desired in two component development systems. Consequently, latent electrostatic images developed with saturated two component developer appear to be darker than desired. It has been found, however, that by providing a supplemental electrostatic force against which carrier granules must compete in order to attract and retain toner patricles, toner concentration of a two component developer mix can be reduced from the saturation level to a level optimum for good development results. A supplemental force which can be varied, as a variable potential source, will consequently permit selective control and regulation of toner concentration.

Summary of the invention It is, therefore, an object of the instant invention to dispense toner into a two component xerograp'hic development system.

It is also an object of the instant invention to dispense toner particles into a xerographic development system equal to the amount of toner depleted from the system through the development of images.

A further object of the invention is to electrostatically control the toner concentration of developer in a xerographic development system.

A further object of the invention is to maintain optimum toner concentration in xerographic development zones.

A further object of the instant invention is to maintain equal toner concentration across xerographic development zones.

It is a further object of the present invention to add toner particles to carrier granules in a two component development system in response to the electrostatic properties of the developer.

These and other objects of the instant invention are obtained by apparatus for maintaining a quantity of charged toner particles between a pair of screens or networks. At least one of the networks is electrically biased to retain an electric field within the toning zone defined by the network. When carrier granules are moved through the zone, they may triboelectrically attract and retain toner particles from within the zone. In attracting toner particles, however, the carrier granules must compete against the electrostatic field within the zone. Consequently, individual carrier granules may only attract and retain toner which would bring the concentration up to an optimum degree. This optimum degree is less than their saturation level which would be achieved if the biased screens were not employed.

Brief description of the drawings For a better understanding of the nature 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 claims wherein:

FIG. 1 is a sectional view of a xerographic reproducing machine embodying the principles of the instant invention with parts of the developing and toner dispensing instrumentalities broken away for clarity and FIG. 2 is a sectional view of a reproducing machine similar to that shown in FIG. 1 with modified toner dispensing elements.

FIG. 3 is a sectional view of a reproducing machine similar to that shown in FIGS. 1 and 2 with further modified toner dispensing elements.

Description of the preferred embodiments The illustrated embodiment as represented by FIGURE 1 shows a xerographic surface It formed in the shape of a drum and mounted for movement upon a main dnrm drive shaft 12 for rotation in direction of the arrow. The drum and main drum drive shaft may be moved by any conventional power source, not shown. The motion of the surface of the drum is such as to sequentially and continuously move the surface past the several xerographic processing stations as illustrated in FIG. 1. The elements of this machine are all conventional in the xerographic art with the exception of those at the developing station which form the basis of the present invention. 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:

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

An exposure station B at which a light or radiation 3.- pattern of copy to be reproduced is projected onto the drum surface to dissipate the drum charge in the exposed areas thereof to thereby form a latent electrostatic image of the copy to be reproduced;

A developing station C at which a xerographic development material including toner particles having an electrostatic charge opposite to that of the electrostatic image, are moved into Contact with the drum surf-ace, whereby the toner particles adhere to the electrostatic image to form the xerographic powdered image in the configuration of the copy to be reproduced, and at which toner is dispensed into the developer material to replace that depleted from the developer mixture through the develop ment of images;

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

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

Development takes place adjacent a main developer rocker assembly 14 supported by opposed end faces 16. The end faces are each provided with apertures 18 for mounting the rocker assembly on the main drum drive shaft 12 adjacent the ends of the xerographic drum. A lower support plate 20 interconnects the end faces adjacent the xerographic development zone 22. A reciprocatory motion is imparted to the rocker assembly by a motor 24- and pivoted linkage means 25 which interconnects one of the end faces with the motor for imparting a rapid oscillation to the rocker assembly. The end of the linkage assembly connected to the end face should be secured thereto at the center of percussion of the. rocker assembly for minimizing extraneous vibration.

Insofar as has been described, the development instrumentalities are substantially as described in my aforementioned copending application Ser. No. 566,944, filed July 21, 1966.

When a quantity of two-component developer is located upon the support plate 20 and a rapid reciprocation is impaited thereto by motor 24 through linkage assembly 26, the vibrated developer will suspend itself in a fluidized state throughout the development zone for contacting the surface and developing latent electrostatic images which have been formed thereon. A development electrode 28 is provided to define the lower face of the development zone 22. The development electrode as shown here takes the form of a screen of such mesh as to permit the developer to freely flow therethrough. The development electrode is biased to a potential slightly greater than that on background areas of the image to be developed for the minimization of background toner deposition. This would be positive in conventional systems. The development electrode 23 serves another function in the instant device as will be later explained.

In the above-referred to former application, developer was gravity fed through the development zone for maintaining adequate toner concentration within the development zone. According to the instant invention, however, this problem is overcome by virtue of the novel toner dispenser.

According to the instant invention, the rocker assembly 14- is formed with a supplemental screen electrode 30. This electrode is a support means to retain a quantity of developer mix adjacent the image for its development. It is preferably constructed concentric with respect to the support plate 20, xerographic surface 10 and development electrode 28. The supplemental screen electrode 30 is of such mesh as to permit free flow of toner particles, but not carrier granules therethrough. Like the development electrode 28, the supplemental electrode is constructed of an electrically conductive material for supporting an elcctric potential thereon. The area beneath the supplemental electrode 30 and above the support plate constitutes a sump 32 for the retention of a supply of two-component developer mix having an excess quantity of toner particles. The region between the development electrode 28 and supplemental electrode 30 is the toning zone 34. Since no continuing flow of developer through the development zone is shown, as there was in my aforementioned copending application, retaining lips 36 are provided on the supporting plate 20 between the end faces 16 for restricting the movement of the fluidized developer from beyond the development zone.

When the developer mixture is vibrated into a fluidized state during the development of images, toner is continuously being lost from the fluidized developer material by the development of xerographic images. It is intended that a quantity of toned developer be located above the supplemental electrode 30 for movement between the supplemental electrode 30 and xerographic surface 10. This developer is used for development of images and is retained within the toning zone 34 and development zone 22 since the mesh of the supplemental screen 30 will not permit the movement of carrier thcrebeneath.

As the developer rocker assembly 14 vibrates during normal operation, a quantity of toner particles from within the sump will be charged due to interaction with the quantity of carrier within the sump 32. The vibrations act to permit a quantity of these charged toner particles to rise through the supplemental electrode 30 into the toning zone 34. It is this same vibrational force which moves the two-component developer above the supplemental electrode 30 into and out of contact with the xerographic surface 10.

In order to eifect the optimum toning of carrier particles trapped within the zoning and development zones 34 and 22, it is necessary to electrically bias the electrodes 28 and 30. This is done by means of a first variable potential source 38 adapted to hold the development electrode 28 at a potential slightly above the potential on background areas of the surface to be developed. This would be approximately one hundred volts positive in conventional xerographic systems. The second variable potential source is adapted to hold the lower or supplemental electrode 310 at a higher positive potential with respect to the development electrode 28. In the instant system, wherein a spacing is employed between the xerographic surface and the development electrode 28 with a A" spacing between the two electrodes, a 4,000 volt bias on the supplemental electrode has been found suitable for producing the desired toner migration.

When such a system is employed, and negatively charged toner particles are sifting into the toning zone 34 from the sump 32 for developing positive images, the development electrode will exhibit an electrically repulsive force on the charged toner particles. This will keep toner particles, unassociated with carrier granules, from entering the development zone 22. As undertoned carrier granules pass into the vicinity of the charged toner particles, they will attract the charged toner particles and retain them up to the point where the carrier granules are optimumly toned. They will not attract toner particles up to their saturation level inasmuch as the force of the development electrode 28 acts against the force of undertoned carrier granules, tending to urge the toner particles back toward the sump. It can thus be seen that the variable ability of potential sources 38 or 40 could vary the repulsive force on the toner particles to control the amount of toner particles the carrier granules can accept. This in turn could be employed to control image density. This process is in continual effect as carrier granules are constantly moved in their fluidized state from the xerographic surface where they deposit toner on the image being developed and back to the toning zone for attracting additional toner for subsequent image development.

The purpose of the sump is to provide a large quantity of toner particles for replacing that lost from the system through the development of images. It has been determined, however, that using conventional two-component developer comprising carrier granules and toner particles in the sump region has met with certain limiting factors. When an overly large quantity of toner is positioned within the sump, there is a tendency for the heavier carrier granules to work their way toward the bottom of the developer of the sump upon extended agitation. When this happens, there will be an upper layer of toner particles entering the toning zone from the sump in an unchanged condition due to inadequate contact with the carrier granules. This can be overcome by using a lesser quantity of toner or by replacing the conventional carriers in the sump with larger pieces of material capable of imparting a triboelectric charge to the toner moving into the toning zone. Another manner in which this could be done would be to employ an additional screen within the sump slightly beneath the supplemental screen. By making this additional screen with a surface of a carrier coating material, toner sifting upward toward the toning zone would contact the screen surface for charging the toner to thereby insure triboelectric charging of all toner particles entering the toning zone.

Shown in FIGURE 2 is a second machine which is similar to that shown in FIGURE 1 except for a modified lower or supplemental screen. The purpose of this modication is to insure the complete charging of all toner particles entering the toning zone. All of the elements of this second embodiment are designated by the same reference numerals of the first embodiment except those which structurally differ from the first embodiment.

The supplemental electrode screen 30 of the first embodiment is replaced in the second or FIG. 2 embodiment by an undulating supplemental screen 42 with electrically insulating caps 44 covering the upper portions of the screen 42. As the developer rocker assembly 14 is vibrated causing toner particles to sift upwardly from the sump 32 into the toning zone 34, the toner-charging carrier granules within the sump are caused to seep downwardly within the developer mix in the sump. These carrier granules thus congregate in a region close to the suppor plate 20 in the lowermost regions of the sump 32. This action is accompanied by a relative rising of the toner particles into the upper portions of the sump 32 which is adjacent the caps 44. The caps 44 prohibit the movement of uncharged toner particles from the upper regions of the sump to the toning zone. Thus, the toner which will be continually rising upwardly from the sump to the toning zone 34 will be those toner particles adjacent the lower regions of the sump which are in an agitating relationship with the carrier granules from the lower regions of the sump. This embodiment thus insures more complete charging of those toner particles rising into the toning region than does the first embodiment.

In order to operate the machine of either of the disclosed embodiments, it is first necessary to activate a general cycle initiating means, not shown. This will act to energize the various processing stations as described above and to move the xerographic drum surface past each station in a continuing and automatic cycle of operation. The general cycle initiating means Will also establish the biases on the various electrode elements as well as cause the rapid oscillation of the developer rocker assembly 14.

The developer rocker assembly 14 is set into a vibrating state with a quantity of overtoned developer in the sump 32 and a quantity of two-component developer in the toning zone. The developer within the toning zone will be agitated into a fluidized state for contacting and developing latent electrostatic images on the xerographic surface. Concurrently therewith, the carrier within the sump will agitate the toner therewith to thereby eifect a charging of that toner within the sump. The same vibration will also cause the charged toner within the sump to move upwardly through the supplemental electrode screen 30 or 42 to suspend itself in the toning zone. A portion of the charged toner particles will also electrostatically attach itself to the supplemental screen but can be removed therefrom by a tendency to triboelectrically attach themselves to undertoned carrier granules.

When negative particles are employed to develop positive images on the drum, a positive bias on the development electrode 28, lower than that on the supplemental electrode 30, will act as a repulsive force against the toner in the toning zone to prohibit its movement into the development zone 22. When, however, undertoned carrier granules are moved through the toning zone, they will attract additional toner particles to their surface up to the point that they will become optimumly toned. This degree is somewhat less than the saturation toner concentration for the carrier granules since the development electrode is exhibiting a force tending to repel the charged toner particles back to the supplemental electrode and sump.

As toner is lost from the developer mix through the development of images, the carrier granules are moving into and out of the toning zone for attracting and retaining additional charged toner particles. These additional charged toner particles are in suspension within the ton ing zone during vibration. A large quantity of such toner particles, however, remains electrostatically attached to the intermediate screens 36 or 42. The charge from the undertoned carrier granules is suflicient to attract these toner particles from the screen as needed.

The second embodiment operates in the same fashion as the first embodiment except that the toner rising from the sump will be more uniformly and completely charged triboelectrically due to being contacted by the heavier concentration of charging carrier granules which tend to congregate in the lower region of the sump 32.

In either embodiment additional toner can be added to the systems as by providing a removable section in one of the end faces 16 of the rocker assembly. Thus, when the rocker assembly is at rest, the end face can be opened adjacent the sump for manually inserting additional toner thereto. It is also possible to add additional toner to the lower sump region by a flexible hose coupling linking a raised toner supply to the sump. Furthermore, it is possible to create a flow of developer across the development zone in a manner similar to the developer flow disclosed in my aforementioned copending application.

The third or FIG. 3 embodiment of the instant invention involves a modification of the FIG. 2 embodiment. In this particular embodiment, the support plate 20 is mounted onto retaining lips 36 and end faces 16 by suitable electrically insulating barriers 418. The upper face of the plate 20 is covered with an insulating layer 50 as of any of the conventional carrier coating materials described in US. Patent No. 2,618,551. Development electrode wires 52 running across the length of the development zone are used in place of the development electrode screen 28 of the first two embodiments. A suitable source of potential 38 creates an electrical bias between the substrate of the xerographic surface and the development electrode wires 52. A second source of potential 4-0 creates an electrical bias between the development electrode wires 52 and plate 20.

The intermediate screen 42 of the first two embodiments is replaced by a stiff electrically insulating screen 54, as of saran or the like. This screen, like the intermediate screens of the first two embodiments, is apertured sufficiently to permit the ilow of toner, but not carrier, therethrough. The cap portion 44 may be formed integrally with such a screen in a manner similar to the FIG. 2 embodiment. The toning zone of this embodiment extends between electrode wires 52 and insulating screen 54. The electrical field within the toning zone of this third embodiment is not created by the intermediate screen. Rather, it is created between the development electrode wires 52 and plate 20. The field within the toner loading zone between screen 54 and the drum surface 10 is achieved by a proper high voltage, as for example, 5,000 volts positive on the lower plate 20. The insulating layer 50 on the lower support plate 20 assists in charging the toner and also serves as a dielectric barrier to permit the use of high field generating potentials between wires 52 and plate 20 to eliminate the chance of electrical breakdown therebetween.

The third embodiment has an advantage of permitting a flow of developer from the development electrode wires back to the sump zones. This is because the field in the toning zone extends from the development electrode wires down to the lower plate 20 rather than merely stopping at the intermediate screen. The electrostatic retention of charged toner particles on a charged intermediate screen is eliminated.

In all three embodiments, the variable potential source permits adjustment of the potential on the various con ductive elements to thus permit variations in the rate of toning and consequently the image density of developed xerographic copy.

It has been disclosed that the present invention may be readily carried out in a fluidized bed development systern of the oscillatory type. It should also be understood that the principles herein are equally applicable to other development systems including fluidized beds of the horizontally or vertically reciprocating as well as orbital types. Other types of conductive electrode networks suitably apertured could be employed as substitutes for the electrode screens to effect the same result. And although specific operational values have been referred to in order to illustrate voltages, distances, and polarities, capable of permitting the practice of the instant invention, these values should be considered as illustrative only inasmuch as the invention is operable through a wide range of operating parameters.

While the present invention as to its objects and advantages has been described herein as carried out in specific embodiments 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.

What is claimed is:

1 In a xerographic development system of the type having support means to retain a quantity of developer material including carrier granules and charged toner particles adjacent a surface bearing a latent electrostatic image to be developed, means to vibrate the support means to thereby suspend the developer material on the support means in a fluidized state in contact with the image for the development thereof, an apertured development electrode in operative proximity to the image and means to electrically bias the development electrode to the same polarity as that on the image, improved toner dispensing apparatus comprising means to create an electrical bias between the support means and the development electrode tending to repel charged toner particles from the development electrode towards the support means and means to introduce toner particles, charged to a polarity opposite from that on the image to be developed, into a toning zone between the development electrode and the support means so that undertoned carrier granules vibrating between the image and support means can triboelectrically attract and retain additional toner particles up to an optimum toner concentration by overcoming the electrostatic force of the development electrode tending to repel the charged toner particle towards the support means.

2. The apparatus as set forth in claim 1 wherein the support means is an electrically conductive member apertured to permit the movement of toner particles but not carrier granules therethrough.

3. The apparatus as set forth in claim 1 wherein the support means is an electrically conductive member and the means to create an electrical bias is a source of potential electrically interconnecting the development electrode and the support means.

4. The apparatus as set forth in claim 1 wherein the support means is an electrically insulating screen apertured to permit the flow of toner but not carrier therethrough.

5. The apparatus as set forth in claim 4 and further including an electrically conductive underplate capable of supporting a supply of toner particles and wherein the means to create an electrical bias is a source of potential electrically coupling said development electrode and said underplate.

6. The apparatus as set forth in claim 2 and further including sump means on the side of the support means remote from the development electrode for feeding charged toner particles into the toning zone.

7. The apparatus as set forth in claim 6 and further including means to vibrate the sump means for fluidizing the charged toner particles to thereby effect the migration of charged toner particles from the sump to the toning zone.

8. The apparatus as set forth in claim 7 and further including means interconnecting the support means and sump means for concurrent vibration.

9. The apparatus as set forth in claim 8 wherein the sump means contains a supply of toner particles as well as means to triboelectrically impart a charge to the toner particle.

10. The apparatus as set forth in claim 9 wherein the support means is a screen with an undulating cross-sectional configuration and further including imperforate means over the portions of the undulating screen closest to the development electrode so that toner particles entering the toning zone from the sump enter from the lower region of the supply of toner particles.

References Cited UNITED STATES PATENTS CHARLES A. WILLMUTH, Primary Examiner.

P. FELDMAN, Assistant Examiner.

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