US3448724A - Developing apparatus - Google Patents

Description

P. P. CHAWDA ETAL.

DEVELOPING APPARATUS June 10, 1969 Sheet Filed Jan. 11, 1968 INVENTOR. JOHN P. CALDWELL HULAL F! CHAWDA ATTURNEKS June 10, 1969 Filed Jan. 11, 1968 P. P. CHAWDA ET AL DEVELOPING APPARATUS Sheet 3 of 2 PRIOR ART INVENTOR. JOHN P. CALDWELL ,arrome'rs United States Patent US. Cl. 118637 4 Claims ABSTRACT OF THE DISCLOSURE Apparatus to develop a latent image on a photoconductive surface in which the moving photoconductive surface is moved in contact with a quantity of developer material supported in a housing or the like. The moving surface causes a flow of developer material at the interface so that developer is moved more or less in contact with the surface in an uphill direction. A steep return geometry is provided in the housing to rapidly return developer material to the start of the flow-contact zone and further means are provided to replenish developer material in the housing.

This invention relates in general to xerography and, in particular, apparatus for developing a xerographic image.

In the art of xerography, as originally disclosed by Carlson in US. Patent No. 2,297,691, a plate, comprising a conductive backing upon which is placed a photoconductive insulating material, is charged uniformly and the photoconductive surface then exposed to a light image of an original object to be reproduced. The photoconductive coating is caused to become conductive under the influence of the light image so as to selectively dissipate the electrostatic charge found thereon to produce what is known as a latent electrostatic image. The development of the latent image is generally effected by electrostatically attracting a pigmented resin to the image areas on the plate. The amount of charge found in the image areas determines the amount of resin material attracted thereto. The amount of charge can be said to be proportional to image density and therefor areas of small charge concentration become areas of low toner density while the areas of greater charge concentration become proportionally more dense. A permanent record of the original object is obtained by transferring the developed image to a final support material and fixing the developed image thereto.

A wide variety of pigmented resins have been'developed for the purpose of developing a latent electrostatic image, these resins being cornmonly referred to as toners. The toner material is' generally transported to the image areas by means of a relatively coarser material konwn as carrier, the carrier being adapted to support a quantity of toner upon its surface. The two-component, that is, the toner and carrier, are selected so that the materials interact eleotrostatically when placed in close rubbing contact to cause a triboelectric attraction therebetween. This two-component material is known in the xerographic art as developer material and this term is used herein to denote a two-component developer comprising carrier and toner material.

Although many workable xerograpihic development systems are known in the art, most of these systems have been found to be impracticable, in a commercial sense, because they are either too slow, too inefficient, or too complex to readily lend themselves to use in automatic xerographic machines. Cascade development, as illustrated in US. patents to Walkup et al., 2,573,881 and to Carlson, 2,990,278, because of its many advantages, has become one of the most prevalent methods for develop- 3,448,724 Patented June 10, 1969 ICC ing a latent electrostatic image. [[n a cascade developing system, two-component developer material is conveyed, as for example by buckets, to a point above an image bearing xerographic plate and the developer material poured or cascaded over the plate surface. Through the combined mechanical and electrostatic forces involved, toner is dislodged from the carrier material and attracted to the image areas on the photoconductive plate. The cascade system, however, has proven to be space consuming. The need for bulky conveyors or the like add greatly to the size of the Xerographic reproducing apparatus. Unwanted powder clouds also result due to the relatively violent cascade action which, in turn, deposits unwanted toner material in background areas. Furthermore, dropping or cascading the developer material on the plate surface causes plate abrasions resulting in a relatively high rate of plate failures. A high rate of developer material failure due to bead fracture is also noted in the cascade system.

'In order to overcome some of the disadvantages found in a prior art and, particularly, those associated with cascade development, a new method of xerographic development was devised in which a moving photoconductive plate surface is brought into contact with a quantity of developer material contained in a housing or the like. Fundamentally, an uphill flow of developer material is established at the plate-developer interface due to the (frictional forces involved. These frictional forces are suf- 'ficient to carry the developer material along in contact with the moving surface at approximately drum speed. Although not clearly understood, it is believed that development is effected during the period of flowing contact by means of the classical development-scavenging technique as disclosed in the previously mentioned Walkup and Carlson patents. The developer material, upon being released from the plate surface, returns to the backside of the developer housing Where it is replenished before once again being returned to the Zone of active development. This flow-contact type system is disclosed in a copending application to Gundlach, Ser. No. 528,846.

Many geometric configurations are discussed and disclosed in the Gundlaoh application, however, they all employ the same basic development principle of development. Although the basic flow-contact system disclosed by Gundlach overcomes some of the previously mentioned disadvantages found in the prior art, the flow-contact system has certain inherent disadvantages. The flow-contact system is basically a slow system in that a relatively small volumes of material is moved through the system during each developing cycle. Addition of new toner material to basic flow-contact housing has also proven to be a major problem primarily due to the extremely low flow rates maintained therein. It has been found that a prep-onderence of new toner added to the housing stagnates in voids and pockets and never finds its way into the flow stream. This stagnation is believed to be due to the lack of agitation [found in the slow, gentle developer flow.

It is therefore a primary object of this invention to improve xerographic development apparatus.

Another object of this invention is to improve the efi'iciency of a xerographic development apparatus.

It is another object of this invention to increase the flow rate in a flow-contact development apparatus.

A further object of this invention is to provide apparatus for introducing new developer material into flowcontact development housing.

It is a still further object of this invention to provide apparatus capable of introducing new toner material directly into the flow stream of. a flow-contact development system.

These and other objects of the present invention are attained by means of a housing having an opening in the top thereof for receiving a portion of a moving xerographic drum therein and being capable of holding a quantity of developer material in contact with the moving drum surface whereby an uphill flow of developer material is established at the drum-developer interface, a vertical plate positioned in the housing along which developer leaving the drum contacting flow moves through the backside of the system, an opening in said plate through which developer flows in communication with a developer dispenser, and dispensing means to add new developer material into the flow of material through the backside of the housing.

: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 connection with the accompanying drawings, wherein:

FIG. 1 is an isometric view of a development system suitable for use with a xerographic drum embodying the present invention and having portions broken away to show the internal construction of the apparatus;

FIG. 2 illustrates a front view of the preferred embodiment of the development apparatus of the instant invention shown in partial section; and

FIG. 3 is an illustration of a basic flow-contact apparat-us found in the prior art.

A brief discussion of the basic flow-contact development system, and the flow mechanism associated therewith, is believed warranted at this time in order to more fully understand the teachings of the present invention. In the basic flow-contact development system, as shown in FIG. 4, a drum is rotatably mounted so as to move in the direction indicated through a clam-shell shaped housing 13. The housing contains suflicient developer material so that the drum surface continually moves in contact with the developer material as it moves through the housing. The frictional forces established between the moving drum surface and the developer material cause a thin layer of developer material adjacent to the drum surface to move in an uphill direction at approximately drum speed. The upward movement of developer material creates a void at the bottom of the housing which is filled as the entire back layer of developer moving down more or less as a unit. A flow of developer material is thus established within the housing.

The path followed by the uphill flow of developer material as it moves in contact with the drum surface describes the active development zone. Theoretically, properly charged and toned developer material is delivered to the start of the active development zone from the supply of developer material found on the backside of the system. The developer material develops a latent image on the drum as it flows along in contact with the drum through this active development zone. At this time, toner is electrostatically attracted from the carrier material to the more highly charged image areas. Because of the relatively gentle action of the flow established with in the housing, little or no toner is mechanically dislodged from the carrier material. The developer material continues to move along in contact with the moving drum surface until the frictional forces holding it to the drum are overcome, as for example by the drum moving out of contact with the developer material. The relatively deplete developer material returns to the backside of the system where it is replenished as it moves down to fill the void left by the continuous how of upwardly moving material.

The flow rate in the basic flow-contact development apparatus is not limited by the drum-developer interface flow through the active development zone but rather by the time required to return developer material to the active development zone through the backside of the system. Because this is a closed system, t volume rate of flow through the active development zone must be equal to the volume rate of flow found on the backside of the system. Tests have shown that a flow rate 8-10 times greater than that found in the basic system (shown in FIG. 4) can be supported by the forces acting at the drum-developer interface. From this discussion, it should be clear that the flow rate in the basic flow-contact system is limited by the backside flow rate and the backside fiow rate is, in turn, restricted by the return geometry. Ideally, the backside of the developer system should have the steepest possible return geometry in order to provide the most rapid flow rate possible.

Despite the relatively low volulmmetric flow rate found in the basic flow-contact development apparatus it is nevertheless the highly efficient development system. High efficiency is evidenced by the fact that good xerographic copy can be produced on an apparatus utilizing a flowcontact development system having a volumetric developer flow rate which is 200 to 400 times less than the flow found in comparable developer systems utilizing cascade flow mechanisms. This extremely high efliciency is further evidenced by the starved condition of the carrier beads noted leaving the active development zone indicating that the carrier material has given up a great preponderance of its toner during the development operation.

Although the apparatus of the present invention is well adapted for use in any suitable xerographic reproduction apparatus, it is shown in FIGS. 1 and 2 embodied in a drum-type xerographic apparatus for purposes of illustration. As shown in FIG. 2, a drum 10 is mounted on shaft 11 and the shaft rotatably supported in the side frames of a machine (not shown). The major xerographic processing components are mounted around the drum periphery so that they are able to act thereon as the drum continually rotates through the various stations.

In general, the several xerographic processing stations in the path of movement of the drum surface may be described functionally as follows: a charging station A, at which a uniform electrostatic charge is deposited on the photoconductive surface; an exposure station B, at which a light or radiation pattern of a copy to be reproduced is projected onto the xerographic drum to dissipate the charge in the exposed areas to form a latent electrostatic image thereon of the copy to be reproduced; a development station C, at which a xerographic developing material, including toner particles having an electrostatic charge opposite to that of the electrostatic image, are placed in contact with the moving drum surface whereby the toner particles are caused to adhere to the electrostatic latent image found thereon; a transfer station D, where the developed electrostatic image is transferred from the plate surface to a final support material; and a drum cleaning and discharge station E, at which the plate surface is brushed to remove residual toner particles found thereon and the drum exposed to a high intensity light source to dissipate any charge found thereon.

Housing 20', which is capable of holding a quantity of two component developer material, therein has an opening in the upper portion thereof arranged to receive the rotatably mounted drum 10. The housing, and the drum opening therein, extend at least the longitudinal length of the drum so that the drum can be rotated through the developer housing. Drum 10 is driven by drive means, as for example a drive motor, through the housing in the direction indicated. The housing and drum are arranged so that the rotating drum is moved through the developer material supported in the housing. As can be seen in FIG. 2, drum and developer contact is maintained in a moving relationship along a zone extending from approximately point M to point N, the points marking the approximate beginning and end of the acti e development zone respectively.

As previously noted, the rotating drum moving through the developer material mechanically drives a thin layer of developer material adjacent to the drum surface in an upward direction and a flow pattern is established with the housing.

Although the basic flow-contact development system is an extremely efiicient system, replenishing the starved or denuded carrier beads leaving the active development zone with new toner is extremely diflicult. It has been found that fresh toner material that is simply deposited within the housing does not readily find its way into the developer flow pattern established within the housing. A large percentage of new toner material so charged into the housing stagnates in non-flow areas. On the other hand, toner which does find its way into the developer flow pattern oftentimes is not properly mixed with the carrier material because of insufficient agitation due to slow flow rates. Ineffective toner charging results in excessive toner being deposited on the drum surface in the background areas.

In the instant invention, the steepest possible return geometry is provided within developer housing 20 in order to increase the flow rate on the backside of the system. As shown in FIGS. 1 and 2, a vertical backplate or batlle 22 is positioned within the housing substantially opposite to the moving drum surface. The backplate is supported between two end plates, one of which is shown in FIG. 1 and numerically referred to as 25. Baffle 22 extends deep into the sump area of housing 20 so that developer material leaving the active development zone at point N is forced to flow downwardly in a vertical direction along the vertical backplate on its return to the start of the active development zone. The steep, or vertical, return path results in the most rapid return flow possible being established on the backside of the system.

A toner hopper, generally designated 21, is positioned adjacent to the development housing 20. The backplate 22 of the developer housing is also used to form the front wall of the hopper. The back wall of the hopper is formed by an inclined plate 23 which runs the entire length of the developer housing and the hopper is closed at both ends by means of the developer housing end plates. Positioned in the bottom portion of the toner hopper, and running the entire length thereof, is toner dispensing apparatus generally designated 24. The toner dispensing apparatus basically comprises a base plate or dispensing platform 30 which is arranged to enclose the bottom portion of the toner hopper. As shown in- FIG. 1, the dispensing plate has a series of elongated slots 31 through which toner material in the hopper is dispensed. A metering element 32 is positioned adjacent to and directly below the dispensing plate 30. The metering element is movably supported on parallel guide rails 39 or rod members which are anchored in the end plates of the developer housing. The metering element has longitudinal slots positioned therein which complement the slots found in the dispensing plate so that toner is dispensed from the hopper when the slots in the respective plates are placed in alignment. Control of the dispensing of toner is effected by moving the metering element in a lateral direction along the guide rails to either align or misalign the slotted holes. It is believed that the above description of the toner dispenser is sufficient for purposes of this disclosure, however, for further information concerning this type of toner dispenser, reference is had to US. Patent 3,013,703 issued in the name of R. A. Hunt.

The vertical backplate is positioned in the housing so that an opening exists between the plate and the bottom of the housing through which granular developer material can pass. In operation, the developer material moves down the vertical back-plate and is caused to pileup at the bottom of the housing at the opening. The pile of granular material delivered at the opening acts much in the same manner as sand being dispensed from a dump truck in that the material will continue to fan-out or slide in all directions as the angle of repose of the material is exceeded. As can be seen, in a continuous flow system as herein disclosed, granular material continually delivered adjacent to the opening will slide or flow under the backplate 22 into the bottom of the toner hopper.

In the instant invention, the toner dispensing apparatus is positioned high enough in the hopper to allow the developer material to continually slide thereunder in the manner described above. As can be seen, the path of travel of developer material through the backside of the system is controlled so that toner is dispensed directly into the developer flow path. New toner introduced into this area of constant movement is readily mixed with the developer material and rapidly carried off in the main flow stream established on the backside of the system. It should be also clear that the teachings of the present invention not only make it possible to replenish a flowcontact development system directly with new toner but also provide apparatus having the steepest possible return geometry thus establishing the most rapid possible return flow without restoring to mechanical aids or the like.

While this invention has been described with reference to the structure disclosed herein, it is not confined to the details as set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

What is claimed is:

1. Apparatus for developing a latent electrostatic image on a photoconductive surface including:

a rotatably mounted xerographic drum,

a housing being capable of holding a quantity of particulate developer material and being arranged to place developer material in contact with a portion of said drum surface,

means to rotate said drum so that the portion of the drum in contact with the developer material moves in an uphill direction such that the developer material in contact therewith moves upwardly at about drum speed,

a vertical baflle plate positioned in said housing and extending along the width of said housing adjacent to said drum surface to guide the developer material moving down to fill the void left in said housing by the upward moving developer material,

a second housing positioned in juxtaposition to said first housing being adapted to hold a quantity of developer material,

an opening in'the lower portion of said baffie plate be ing arranged so that the flow of material moving down said plate is directed through said second housing, and

dispensing means associated with said second housing to dispense new toner material into the flow of continually moving developer material.

2. Apparatus for developing a latent electrostatic image on a photoconductive drum surface including:

a housing for holding a quantity of developer material,

a rotatively mounted photoconductive drum, means to move the image bearing photoconductive surface upwardly and outwardly through said housing in contact with the developer material whereby developer material along the zone of contact moves in an upward direction and the remaining developer material moves downwardly to fill the void left by the upwardly moving material, and

a vertical baffle plate positioned in said housing adacent to the portion of the moving photoconductive surface leaving said housing to direct the developer material moving out of contact with said photoconductive surface back to the start of the contact zone.

3. The apparatus of claim 1 having further means to dispense fresh developer into the flow of material moving downwardly through said housing.

4. Apparatus for developing a latent electrostatic image on a photoconductive surface including a rotatably mounted xerographic drum,

a housing being capable of holding a quantity of particulate developer material and being arranged to place developer material in contact with a portion of said drum surface,

means to rotate said drum so that the portion of the drum in contact with the developer material moves in an uphill direction such that the developer material in contact therewith moves upwardly at about drum speed,

a vertical baffle plate positioned in said housing and extending along the width of said housing adjacent to said drum surface to guide the developer material References Cited UNITED STATES PATENTS PETER FELDMAN, Primary Examiner.

U.S. Cl. X.R.

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