US 3550556 A
Description (OCR text may contain errors)
United States Patent Inventor Appl. No. Filed Patented Assignee Prabhulal P. Chawda Rochester. N.Y.
Jan. 11, 1968 Dec. 29, 1970 Xerox Corporation Rochester, N.Y.
a corporation of New York DEVELOPMENT APPARATUS 13 Claims, 4 Drawing Figs.
B05b 5/00 Field of Search LX, Miss. 638. 636, 629. 612; 117/1 7.5; 346/74 References Cited UNITED STATES PATENTS 1/1964 Greig 3,333,566 8/1967 Kent 355/3X ass/3x 2,551,582 5/1951 Carlson 118/637X 2,690,394 9/1954 Carlson 118/637X 2.753.796 7/1956 Wood et a1. 118/637LX 2,856,848 10/1958 Pritchard 118/637LX 3,003,462 10/ 1961 Streich,.1r. 118/637 3,377,988 4/1968 Zawiski 1 18/637 3,426,729 2/1969 Hawkins 1 18/312 FORElGN PATENTS 2,043 1/ 1908 Great Britain 1 18/312 Primary Examiner-Morris Kaplan Attorneys-Norman E. Schrader, James .1. Ralabate and Ronald Zibelli ABSTRACT: A development area is defined by a horizontally disposed drum bearing an electrostatic latent image and rotating in close proximity to a plate element generally concentric therewith and located on the upwardly moving side thereof. Separate auger means located at the entrance and exit of said area comprise means to circulate developer material therethrough. The entrance is located at the bottomside of the upwardly directed development area.
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SHEET 2 0? 3 INVENTOR. PRAB HULAI F? CHAWDA ATTORNEYS DEVELOPMENT APPARATUS This invention relates in general to xerography and, in particular, to method and apparatus for developing a xerographic image.
In the art of xerography, as originally disclosed by Carlson in US. Pat. 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 commonly referred to as"toners." The toner material is generally transported to the image areas by means of a relatively coarser material known as carrier, the carrier being adapted to support a quantity of toner upon its surface. The two components, that is, the toner and carrier, are selected so that the materials interact electrostatically 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 xerographic 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 U.S. 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 developing a latent electrostatic image. In the cascade developing system, a two-component component 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 photoconductiveplate. 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 platedeveloper interface due to the frictional forces involved. These frictional forces are sufficient to carry the developer material along at approximately drum speed in contact with the moving plate surface. Although not clearly understood, it is believed that development is effected during the period of flowing contact by means of the classical developmentscavenging technique as disclosed in the previously mentioned Walkup and Carlson patents. The developer materialfupon 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, and now US. Pat. No. 3,503,776.
Many geometric configurations are discussed and disclosed in the Gundlach 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 also has certain inherent disadvantages. The flow-contact system is basically a slow system in that a relatively small volume of material is moved through the system during each developing cycle. This low volumetric flow rate, in turn, results in a slow rate of developer mixing and triboelectrification and, therefore, the basic system does not lend itself for use in high-speed automatic machines. Furthermore, a relatively long response time after startup is needed in order to bring the system up to optimum operating conditions.
Carrier material sticking to the photoconductive plate surface, commonly referred to as bead stickage, is another problem associated with the basic flow-contact system. A triboelectric relationship, much like that found between toner and carrier, exists between the photoconductive surface and the carrier material. The carrier material, upon becoming toner starved exhibits a charge relationship with the plate of a polarity and magnitude such that the carrier material is electrostatically attracted to the plate surface in the nonimage areas. The bond created, although weak, is sufficient to cause the carrier material to be pulled out of the relatively gentle developer flow pattern by the moving plate surface. This carrier, sticking to the moving plate, is capable of being transported to subsequent processing stations where it can cause damage.
A space charge build up on the carrier material has also been found to occur in the basic flow-contact developing system. It is found that a preponderence of the individual carrier beads are electrostatically isolated from the neutralizing effect of the grounded housing simply by other carrier beads which surround it. Although electrically-isolated, in a sense, the carrier material nevertheless continues to electrostatically build up an excessive charge about its surface which is confined to an infinitely small area or space around the bead. This space charge in effect monopolizes the region around the individual carrier beads and has a deleterious effect upon the triboelectric balance of the system.
It is therefore a primary object of this invention to improve xerographic development apparatus.
Another object of this invention is to improve flow-contact development apparatus.
It is yet another object of this invention to increase the flow rate found in a flow-contact type development apparatus.
A further object of this invention is to eliminate space charge buildups in a flow-contact development system.
Yet another object of this invention is to increase developer availability in a flow-contact development apparatus.
Still another object of this invention is to improve developer distribution and mixing throughout a flow-contact development apparatus.
Another object of this invention is to reduce carrier bead stickage in a xerographic development apparatus and, in particular, in a flow-contact type development apparatus.
Yet another object of this invention is to provide aflowcontact development apparatus adapted to continually produce a uniform quality of development adaptable for use in an automatic xerographic machine.
It is still another object of this invention to reduce the amount of time required to bring a flow-contact development apparatus to optimum operating conditions.
A further object of this invention is to eliminate plate abrasion and carrier bead failure in a xerographic development apparatus.
These and other objects of the present invention are attained by means of a developer apparatus having a housing with an opening provided therein through which a xerographic plate can be moved therethrough, said housing being adapted to hold a quantity of developer material therein such that a plate moving through said housing is brought into contact with the developer and causes the developer to flow in contact with the plate in an uphill direction through a development zone, a delivery auger positioned at the start of the development zone, a return auger positioned at the end of the development zone, and means by which the delivery and return augers communicate to supply a continuous flow of developer through the development apparatus.
For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following description of the invention to be read in connection with the drawings, wherein:
FIG. 1 is an isometric view of a developer housing and a xerographic drum embodying the present invention which is suitable for use in an automatic xerographic machine and having portions broken away to show the internal construction of the apparatus;
FIG. 2 illustrates schematically a preferred embodiment of the development apparatus of the instant invention shown adapted for continuous use in an automatic xerographic machine;
FIG. 3 is a sectional view of the development apparatus taken along lines 33 of FIG. 2 with the xerographic drum shown invisible; and
FIG. 4 is an illustration of the basic flow-contact development system as disclosed 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 I is rotatably mounted so as to move in the direction indicated through a clamshell-shaped housing 13. The housing contains sufficient developer material so that the drum surface eontinually 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 within 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 flow of upwardly moving material.
The flow rate in the basic flow-contact development apparatus is not limited by the drum-developer 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, the volume rate of flow through the active development zone must be equal to the volume rate of flow found on the backside ofthe system. Tests have shown that a flow rate 8l0 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 flow 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.
However, in the basic flow-contact apparatus the angle of repose of the developer material acts to limit the return geometry of the system. The angle of repose is the maximum angle with the horizontal at which a quantity of loose particulate material, such as developer material, will retain its natural position before sliding. As shown in FIG. 4, the angle of repose of the developer in the clamshell-shaped housing 12 precludes the developer from contacting the drum surface for a considerable distance as the drum rotates through the housing. For most commercially available developer materials, drum contact cannot be made with the developer material before the drum has rotated through about 45of are from the vertical. The angle of repose of the material severely restricts the return geometry of the system and thus limits the rate of flow that can be maintained in a basic flow-contact apparatus.
Despite the relatively low volumetric 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 flow-contact develop ment system having a volumetric developer flow rate which is to 400 times less then the flow found in comparable developer systems utilizing cascade flow mechanisms. This extremely high efficiency 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 preponderence of its toner during the development operation.
The apparatus of the present invention is shown in FIGS. 1, 2 and 3. Although the instant invention is well adapted for use in any automatic xerographic reproduction apparatus, it is shown, for illustrative purposes, herein embodied in a drumtype xerographic apparatus. As shown in FIG. 2, a drum I0 is mounted on shaft 11 and the shaft adapted to be supported in the side frames of an automatic xerographic 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 drum 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 reproduces;
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 remaining thereon after image transfer and at which time the plate surface is exposed to a relatively intense light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.
Basically, the apparatus of the present invention comprises a housing having an opening in the upper portion to receive, in moving relation therein, a rotatable xerographic drum 10. A section of the lower portion of the housing is formed into an arcuate-shaped shell or plate, generally designated 21, which is arranged to be positioned adjacent to, and running transverse along, the length of the drum surface. As can be seen in FIG. 2, an opening, or gap, 22 is formed when the arcuate shell or plate and the drum surface are positioned in operative relation. Functionally, opening 21 is the zone of active development, the operation of which will be explained in further detail below.
The remaining section of the lower housing is shaped to form two elongated troughs or-sumps which extend longitudinally along the length of the drum surface. 'A supply trough 23 is located at the start of the active development zone, that is, at the upstream entrance to the active zone in regard to the direction of drum travel and a return trough 24 similarly positioned at the opposite end or downstream exit of the active development zone. It should be noted that both sump areas are arranged to communicate with the active development zone along the length of the drum surface through means of openings 29 and 30 which are provided.
The two sump areas and the active development zone are closed at both ends by means of end plates 31 and 32, respectively. The end plates are provided with end seals 47 to prevent developer material from escaping from the developer housing as well as effectively preventing extraneous matter, such as dirt or grim which could mar the drum surface, from entering the system. Although end seals are utilized in this embodiment, it should be clear that any type of seal, as for example radial seals, could be employed to effectively isolate the developing zone. I
At one end of the drum the housing is extended to form an inclined return chute 40. The return chute is closed at the extended or extreme end by means of plate 42. As can be seen in FIG. 1, the other end of the inclinedreturn chute is closed by means of housing end plate 31 so as to form a reservoirlike area being capable of holding a quantity of developer material. End plate 31 has two circular apertures 43 and 44 therein by which supply trough 23 and return trough 24, respectively, communicate with the inclined return chute 40.
A tapered delivery auger joumaled for rotation in ball bearings mounted in bearing blocks 33. The bearing blocks are, in turn, mounted in plates 32 and 42. The delivery auger is arranged so that when it is rotated in the direction indicated developer material is transported from the lower end of the inclined chute through aperture 43 into the supply trough. The shaft portion of auger 25 extends beyond main housing 20 and has securely affixed thereto drive sprocket 48 and the drive sprocket, in turn, is driven by any suitable drive means, as for example motor means 50 (FIG. 2). The delivery auger is tapered in relation to the preselected rotational speed at which it is driven so that in operation developer material delivered from inclined chute 40 is evenly distributed along the length of the supply sump. A kicker flight 37 (FIG. 3), mounted on the far end of the supply sump area, is employed to prevent developer material from building up against end plate 32. v
In a like manner, return auger 26 is rotatably mounted in bearing blocks 33 positioned in plates 32 and 42. Again, the shaft portion of return auger 26 extends beyond the main developer housing and has affixed thereto a drive sprocket 49. The drive sprocket is driven by any suitable drive means, as
for example motor means 51, (FIG. 2) so as to rotate the return auger in the direction indicated in FIG. 2 to empty developer material in the return trough into the return chute.
In operation, the apparatus of the present invention is first charged with sufficient developer material to sustain continu' ous operation and then the drum and the supply and return augers brought to operating speeds. The tapered delivery auger is caused to rotate in the direction indicated so that the helical feed threads thereon deliver a continuous supply of developer mixture to the supply trough 23. As explained above, developer material is circulated evenly throughout the supply trough due to the mixing action of the tapered helical feed threads. Sidewall 46 (FIG. 2) of trough 23, that is. the sidewall opposite to the entrance to the active development zone, is extended in a vertical direction enabling the trough to support developer at a height whereby the granular developer material will slide into opening 29 when the angle of repose of the material is exceeded. The sliding action of the developer material at the entranceto the active development zone exerts a horizontal pressure at this point. This force insures that a rapid, continuous, trouble-free, flow of material moves from the trough 23 into the entrance of the active development zone. The developer material so delivered into the active development zone fills the entrance area 29 allowing the rotating drum 10 to contact and carry along the developer material. The developer material flows through the active development zone under the action of the rotating drum surface and then passes through opening 30 into return trough 24. Return auger 26, rotating in the direction indicated, moves the mass or body of developer mixture laterally in the trough through aperture 44 in end plate 31 into inclined chute 40. The developer material falls to the bottom of the chute area where it once again can be acted upon by tapered supply auger 25.
Additional developer material canbe, from time to time, added to the system through the open top of the inclined chute to replenish the developer material deposited on the photoconductive insulating material passing through the developing apparatus. For this purpose, any of a number of known devices for automatically feeding developer material which are known in the art may be provided.
As can be seen, the apparatus of the present invention provides a closed flow system or loop utilizing the highly efficient flow-contact mechanism and in which the flow found in the backside or return portion of the system is controllable. That is, the flow of material through the system is not restricted by the system's return geometry. In fact, the flow through the system can be maintained at any desired rate by controlling the amount of material delivered by the delivery auger.
In this preferred embodiment, the delivery auger and the return auger are preselected such that both are capable of moving approximately the same amount of material in a linear direction per revolution. However, the return auger is adapted to rotate at a slightly high speed in the delivery auger The system therefore does not have to be compensated for slight changes in the flow rate through the active development zone. Slight changes in the flow rate through the active development zone occur with changes in humidity or when developer of different materials are employed. The delivery auger is insensitive to these changes and therefore continues to deliver the same quantity of material to the start of the active development zone. By rotating the return auger at a higher rate than the delivery auger, increases in the flow rate will not materially effect the systems performance. In practice, it has been found that a 1.4 to 1 ratio between delivery and return auger speeds is satisfactory to attain the desired results.
It should be further noted that the apparatus of the present invention is not severely restricted onthe positioning of the augers and associated supply and return troughs. However, because this is an uphill development system, the return trough must be at a higher elevation than the supply trough. As can be seen, a wide latitude as to the length and positioning of the active development zone is afforded by the present apparatus so that it is readily adaptable for use in a wide variety of machine environments. For example, the active development zone can be lengthened or shortened so as to optimize development in machines operating at varying speeds or having different size drums. it should also be clear to one skilled in the art that the dual auger arrangement of the present invention is not restricted to use in conjunction with a cylindrical drum but can be readily adapted to be used with a wide variety of endless belt photoconductor configurations as well as with moving flat plate photoconductors.
The dual auger arrangement, as herein disclosed, isolates the active development zone from the backside or return portion of the system thus giving the apparatus greater flexibility than afforded by the basic flow-contact system. The arcuate plate 21 which forms the bottom wall of the active development zone is positioned in close proximity to the moving photoconductive surface and is capable of acting much in the same manner as a biased electrode in conventional development systems when a DC potential is applied thereto. As shown in FIG. 2, the arcuate plate is electrically isolated from housing and is biased with a sufficient DC potential to suppress background development and also to further enhance the solid area coverage capability of the development apparatus. For further information concerning the operation of development electrodes, reference is had to Walkup, US. Pat. No. 2,573,881; and Landrigan et al., US. Pat. No. 2,725,304.
Sufficient agitation of developer material to produce effective developer mixing and triboelectrification has been found to occur over the entire operating range of the present apparatus. That is, the present system is capable of delivering properly mixed and charged developer material to the entrance of the active development zone regardless of the flow rate being maintained in the system. The system is not dependent on the drum-developer interface flow for developer mixing but rather upon the agitation produced by the continuously rotating augers.
Both the delivery and return augers serve second functions other than moving and mixing developer material. Delivery auger 25, which contacts an extremely high percentage of particulate developer material prior to the materials delivery to the active development zone, is placed at a ground potential to bleed off any space charge build up which might occur about the carrier material. The return auger is used to suppress bead stickage. This auger is electrically isolated from the developer housing and a DC bias placed thereon. The auger is positioned in close proximity to the drum surface and biased with a charge having a polarity and magnitude sufficient to attract carrier beads triboelectrically adhering to the drum surface. In operation, it is preferred to rotate the biased return auger in a counterclockwise direction so that the carrier beads coming under its influence, that is, the beads electrostatically dislodged from the drum surface, are pulled downwardly towards the sump area of trough 24 thereby preventing the beads from escaping from the system through the opening between the housing and the drum surface.
While this invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims.
1. Apparatus for developing an electrostatic latent image including:
a photoconductive surface being adapted to support an electrostatic latent image thereon:
means to move said photoconductive surface whereby at least a portion of said surface is moving in an uphill direction;
a plate positioned adjacent to the upwardly moving portion of said photoconductive surface and extending transverse to the direction of surface movement to form an opening therebetween. the opening being at least equal to the image area on said surface;
a delivery auger positioned transverse to the direction of surface travel at the lower entrance to the opening and being arranged to deliver a continuous supply of particulate developer material to said opening;
a return auger positioned transverse to the direction of surface travel at the uphill exit to said opening and being arranged to continuously remove the developer material flowing from the exit of the opening;
means by which the return auger communicates with the delivery auger whereby developer material leaving said opening is returned to the entrance to said opening; and
means to rotate the augers whereby the latent image on the photoconductive surface moved intermediate the aforementioned augers is continuously developed by developer material circulating therebetween.
2. The apparatus of claim 1 having further means to replenish developer material as the developer material returns to the entrance of the opening.
3. The apparatus of claim 2 wherein said delivery auger is placed at a ground potential.
4. The apparatus of claim 3 having further means to place the plate at a bias potential.
5. The apparatus of claim 4 wherein said return auger is rotated at a higher speed than said delivery auger.
6. The apparatus of claim 5 wherein the ratio of return auger speed to delivery speed is about L4 to l.
7. The apparatus of claim 1 wherein the return auger communicates with the delivery auger by means of a inclined chute.
8. The apparatus ofclaim 6 having further means to bias the return auger.
9. Apparatus to develop an electrostatic latent image on a photoconductive surface including:
a xerographic drum adapted to support a latent electrostatic image thereon;
means to rotate said drum at a predetermined speed;
a housing having an opening to receive a portion of said rotating drum therein;
said housing being adapted to hold a quantity of developer material in contact with the surface of said drum along a zone of active development so that a flow of developer moves in contact with the drum surface as the drum rotates through said housing;
auger means positioned at the entrance of the active development zone adjacent to the drum surface and arranged to convey a continuous supply of developer material transverse to the direction of rotation of said drum;
second auger means positioned at the exit of the active development zone adjacent to said drum surface and arranged to convey a continuous supply of developer material transverse to the drum surface;
the entrance to the active development zone being at a lower elevation than the exit of said active zone;
return means being adapted to operatively communicate with the delivery and return augers whereby the developer material leaving the active zone is returned to the entrance of said zone; and
means to rotate said augers in relation to the rotating xerographic drum whereby a continuous flow of developer material is moved through the active development zone sufficient to effect development of a latent image on said xerographic drum.
it). The apparatus of claim 9 having further means to replenish developer material used to develop an electrostatic latent image.
ll. The apparatus of claim 10 wherein the delivery auger is placed at a ground potential.
12. The apparatus of claim 11 having a biasing electrode positioned in close proximity to the rotating drum surface in the zone of active development.
R3. The apparatus of claim i2 wherein the return auger is rotated at a greater speed than the delivery auger.