US3428025A

US3428025A - Xerographic development apparatus

Info

Publication number: US3428025A
Application number: US604725A
Authority: US
Inventors: William A Sullivan Jr
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1966-12-27
Filing date: 1966-12-27
Publication date: 1969-02-18
Anticipated expiration: 1986-02-18

Description

Feb. 18, 1969 w. A. SULLIVAN, JR 3,

XEROGRAPH IC DEVELOPMENT APPARATUS Filed D90. 27, 1966 INVENTOR.

WILLIAM A SULL IVAN,JR.

A TTORNEV United States Patent 3,428 025 XERO-GRAPHIC DEVELbPMENT APPARATUS William A. Sullivan, Jr., Webster, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 27, 1966, Ser. No. 604,725 US. Cl. 118-637 Int. Cl. G03g 13/06, 15/08 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates in general to apparatus for developing electrostatic images, and in particular to apparatus for developing electrostatic images with a cloud of powder material.

In the practice of xerography, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to create and 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 dissipate 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 with a finely divided electrostatically attractable material such as a resinous 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; and where the charge is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder may subsequently be transferred to a sheet of paper or other surface and suitably aflixed to thereby form a permanent print.

The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred to herein as toner. In order for the toner to attach itself to the electrostatic charge corresponding to the image areas, the toner particles are charged to a polarity opposite from the charge on the image areas.

One method for developing latent electrostatic images is the powder cloud system which is the system employed in the instant invention. In powder cloud development, a quantity or cloud of toner particles is brought into an area adjacent a latent electrostatic image bearing surface. In order for the toner to deposit itself on image areas, a triboelectric charge must first be imparted to the individual toner particles.

Charging of the toner can be accomplished by impacting the particles against the walls of a tube or other container. By properly selecting the material from the triboelectric series, triboelectric charging of the powder will occur during impact. The charged cloud of toner parti- 3,428,025 Patented Feb. 18, 1969 ice cles may then be moved adjacent to a latent electrostatic image for development thereof.

In another method of charging toner the charge may be generated by agitating a quantity of carrier granules with the toner. The toner is usually a pigmented resinous powder. The carrier is usually a glas or sand bead coated with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the powder and the granular carrier upon mutual agitation. The toner particles thus become properly charged for development of images due to their interaction with the carrier. The triboelectrically charged toner particles then can develop latent electrostatic images by contacting surfaces containing the images.

In powder cloud development there is a natural tendency of random toner particles to become charged oppositely from the polarity intended. When such occurs, the oppositely charged toner particles become attracted to non-image areas and consequently manifest themselves as unwanted background.

The inability to create a cloud wherein all of the toner particles are of like polarity is thought to be due to mechanical shortcomings of the apparatus. In triboelectric toner charging there are invariably some toner particles which receive incomplete or inadequate rubbing contact with the charge generating carrier or tube walls. This incomplete contact and interaction results in incomplete charging of all toner particles which make up the cloud.

Regardless of the cause of improperly charged toner particles within the cloud, it is an observable fact that such errant toner particles do exist and their presence is a cause of undesired background in developed images and final xerographic copy. The instant invention is directed to a development system capable of creating a powder cloud consisting of unipolarly charged toner. Such unipolar charging can be achieved by subjecting the toner to a sustained electrical discharge in the nature of a high density ion flow rather than by triboelectrification.

Therefore, it is an object of the present invention to develop latent electrostatic images.

It is a further object of the present invention to improve powder cloud development apparatus in xerographic machinery.

A further object of the present invention is to prohibit the generation of improperly charged toner particles in image developing powder clouds.

A further object of the invention is to form powder clouds wherein all toner particles are charged to a like polarity.

A further object of the invention is to charge quantities of toner to an image developing polarity by means which are simultaneously capable of impelling the toner towards an electrostatic image to be developed.

Still a further object of the present invention is to create -a unipolar powder cloud by introducing toner to an area of a sustained electrical breakdown whereby the toner particles become charged by the high density ion flow within the breakdown.

These and other objects of the present invention are achieved by powder cloud development apparatus employing an apertured member positioned between an electrode and a latent electrostatic image bearing surface to be developed. A high electrical bias between the apertured member and electrode causes a sustained electrical breakdown therebetween. When toner is brought into this area it becomes charged due to the high density ion flow within the breakdown. As the toner is accepting the charge, it exhibits a repulsive force with respect to the electrode. Because of this force, the breakdown may be simultaneously utilized to move the charged toner away from the electrode towards the image bearing surface for development thereof.

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 illustrates schematically a side cross-sectional view of a xerographic machine employing the development instrumentalities of the instant invention.

FIG. 2 is another side cross-sectional view with the development instrumentalities enlarged for greater clarity.

Shown in the figures is an embodiment of the present invention constructed for continuous and automatic operation. The elements of this machine, as shown schematically in the figures, are all conventional in the xerographic art with the exception of the development station which forms 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 the light or radiation 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 toner particles are charged oppositely to the charge of the electrostatic latent image and at which the charged toner particles are moved into contact with the drum 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;

A transfer station D, at which the xerographic powder 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.

The xerographic development apparatus is adapted to develop latent electrostatic images which are formed in the conventional manner on a drum shaped xerographic surface 10. While the xerographic surface is shown as drum shaped, this is merely done for illustrative purposes since it is readily understood by anyone versed in the art that any similar xerographic surface such as a flexible belt or a rigid flat plate could readily be employed in the practice of the instant invention.

The development apparatus of the disclosed embodiment is generally contained with a housing 12. A bucket conveyor system disposed within the housing acts to lift a quantity of two component developer from the sump area 16 to an elevated location and then gravity drop the developer adjacent the development instrumentalities. Any suitable source of power, not shown, may be employed to move the conveyor along its path of travel for lifting the developer.

Upon being dropped, the developer falls into a constricting set of plates which begin to guide the developer through its path of travel. This set of plates may comprise an upper developer chute 18 in proximity to a guide plate 20. Positioned downstream from the upper guide plate 18 is a lower guide plate 22 which further continues to guide the developer flow. As developer flows beyond the lower portion of the lower guide plate 22, it drops by gravity into the sump for further recirculation through the system. The

chutes

18 and 22 and guide plate 20 extend for a length at least equal to the length of the drum. The sump and conveyor buckets should also be of the same length to assure the flow of developer across a space equal to the length of the drum.

Spanning the space between the upper and lower guide chutes is a grounded conductive screen portion 24. The

mesh of the screen should be such that the individual toner particles of a two component developer will be capable of passing therethrough. The mesh of the screen must, however, be fine enough so that the carrier particles of the developer cannot pass therethrough. Positioned in proximity to the grounded screen 24 is a row of pin type electrodes 26. The electrodes are spaced from one another across the width of the screen and drum so that When a potential is applied thereto a high density flow of ions is caused to span the space between the electrodes and screen across the entire length of the drum. So that the individual pin type electrodes may be provided with a source of potential independent of each other, the pins are mounted in an insulated block 28 constructed, as for example, of any rigid dielectric or the like.

A source of potential 30 is electrically connected to the individual electrodes for energization thereof. Since the electrodes must be maintained at a high potential, a limiting resistor 32 may be inserted into the circuitry between the electrode 26 and potential source 30 for sustaining the electrode at high potential.

In operation, when development is to be accomplished, latent electrostatic image on a drum surface 10 is continuously moved adjacent the development instrumentalities for sequential development of the image. A quantity of two component developer material is then cascaded down the chute 18, across the screen 24, down chute 22 and back to the sump for recirculation. When the potential is applied to the pin electrodes 26 of sufiiciently high magnitude an electrical breakdown is caused to occur between the electrodes and grounded screen. As the developer is moved within this area, the high density ion flow caused by the electrical breakdown achieves virtually complete charging of all the toner particles in the path of the ion flow and repels them from the high potential source, through the screen and to the latent electrostatic image to be developed.

It should be noted that the charging of the toner, according to the instant invention, is not accomplished by a corona discharge. In a corona discharge a potential difference creates a limited flow of ions and is evidenced by a bluish glow adjacent the corona emitting device. Corona emissions of this type are generally used to charge the xerographic surface and can in some instances cause a charging of toner by the building up of a charge thereon. The charging of toner by corona discharge has in the past produced less than desirable results. This is though to be due to the properties of the toner in the path of a corona discharge. If the toner is too conductive, it will dissipate the charge from the corona too quickly. If the toner is not sufiiciently conductive it cannot take on the charge because of the low current output of a corona. Furthermore, corona charging of toner is limited in effect due to the suppression of the corona dis charge by the toner after it has been brought up to an equilibrium voltage with respect to the corona emitting device. The equilibrium voltage for a system is generally significantly below the voltage of the emission source. The toner may be brought up to such an equilibrium value but, thereafter, continued corona emissions are repelled from the toner by the toner charge which has already reached its peak potential for the system. In high density ion flow charging, as contemplated herein, the equilibrium potential is never reached by the toner. Hence, the toner particles never cease accepting charge from the emission source until they are moved outside of the region of the ion flow.

According to the instant invention, however, a potential, higher than that used in corona charging, is applied in the charging area to cause a true breakdown of the entire air space between the charge creating elements. As such, the flow of ions is sufiiciently high to bombard the toner and to create a virtually unipolar charge distribution on the toner of significantly high magnitude to carry out high quality development. An ion flow of this nature is also sutficient to charge the toner to the point wherein it exhibits a repulsive force with respect to the charging electrode for impelling itself towards the surface to be developed.

Inasmuch as the electrical breakdown adjacent the pin electrodes and screen creates a glowing spark in the vicinity, it is necessary to light baflle this glow from the photoconductive surface so that the light does not dissipate the image-representing charge on the surface 10. This is done by providing

parallel plates

34 and 36 as extensions of the

chute members

18 and 22. By angling these baflles with respect to the straight line path between the electrodes 26 and drum surface 10, the light caused by the sparking electrodes is in effect shielded and does not reach the charge pattern on the xerographic surface 10. Since the baffles will also act to deflect the flow of toner from its straight line path to the drum surface 10, a supplemental toner directing electrode 38 may be employed. The electrode 38 extends the length of the drum and is adapted to be biased by a source of potential 40 to the same potential as that on the toner particles. As such, when the stream of toner particles approaches the drum surface 10 it will be deflected until it strikes the drum substantially perpendicular to the surface 10 or radially with respect to the drum. This direction of toner movement against the drum has been found to offer the best development results. A suitable insulating barrier 42 may be positioned between the electrode 38 and baffle 34 for the sustaining of the potential on the the electrode 38.

While the apparatus of the instant invention is operable within a wide range of variables, one set of parameters which may be employed to carry out the instant invention includes a positive 7500 volt DC. potential on the potential source 30. A limiting resistor of approximately one megohm may be employed to hold the pin electrodes somewhere between 0.50 and milliamps. Since the charge breakdown is dependent upon the distance from which it is held from a grounded surface, a inch distance between the pin electrode and grounded screen has been found satisfactory.

While pin electrodes have been shown as the desired breakdown creating means, any similar electrode could be employed. It has been found, however, that a row of pin electrodes are more desirable than a conventional wire electrode since the pins are more susceptible for creating and maintaining the high potentials for an extended duration needed to practice the instant invention. Since each pin electrode under these conditions is operative within a distance dependent radius, it has been found that a M; inch spacing of the pin electrodes across the development zones works very well to keep a current flow across the length of the drum for the inch electrode to screen spacing. Since the system is not dependent upon the triboelectric creation of charge on the toner particles, a two component developer with an extremely high toner concentration may be employed. A developer with 3% to 4% toner has been found suitable. The carrier is added merely to facilitate handling of the toner.

It is obvious that this entire development system is readily adapted for incorporation in commercial xerographic machines.

While the present invention, as to its objects and advantages, has been described herein as carried out in a specific embodiment 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. Apparatus for developing latent electrostatic images formed on a xerographic surface with charged toner particles including a conductive member positioned adjacent a xerographic surface to be developed, the conductive member being apertured to permit the movement of toner particles but not carrier granules therethrough,

toner charging means positioned in spaced proximity to the conductive member, on the side thereof remote from the xerographic surface, means to apply a potential to the charging means suflicient to cause a sustained electrical breakdown from the charging means to the conductive member, and

means to introduce a flow of two-component developer including carrier granules and charged toner particles into the space between the charging means and conductive member whereby the toner will become additionally charged by the high density ion flow created by the electrical breakdown and due to such charging, be electrostatically repelled from the charging means towards the xerographic surface for development thereof.

2. The apparatus as set forth in claim 1 wherein the conductive member is a screen and further including means to ground the conductive member.

3. The apparatus as set forth in claim 1 wherein the charging means is a series of pin shaped electrodes and further including means to secure the pin shaped electrodes in a fixed position with respect to the conductive member.

4. The apparatus as set forth in claim 1 and further including baflle means positioned adjacent the side of the conductive member remote from the charging means, the baffle means being positioned to light shield the latent electrostatic image bearing surface being developed from the electrical breakdown.

5. The apparatus as set forth in claim 4 and further including a supplemental electrode positioned adjacent the side of the baffle means remote from the conductive member and means to electrically bias the supplemental electrode to thereby deflect the flow of charged toner particles into a path substantially normal to the image bearing surface being developed.

References Cited UNITED STATES PATENTS 3,306,193 2/1967 Rarey et al. 118-637 XR 3,372,675 3/1968 Tressel 118-637 2,918,900 12/1959 Carlson 118-637 3,263,649 8/1966 Heyl et al 118-637 3,295,440 1/1967 Rarey et al. 118-637 XR PETER FELDMAN, Primary Examiner.

US Cl. X.R. 117-175