US3336905A

US3336905A - Xerographic developer apparatus

Info

Publication number: US3336905A
Application number: US419343A
Authority: US
Inventors: Ernest H Lehmann
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1964-12-18
Filing date: 1964-12-18
Publication date: 1967-08-22
Anticipated expiration: 1984-08-22
Also published as: DE1497227B2; GB1123059A; DE1497227A1

Description

22, 1967 E. H. LEHMANN 3,336,905

XEROGRAPHIC DEVELOPER APPARATUS Filed Dec. 18, 1964 3 Sheets-Sheet 1 INVENTOR. ERN EST H EN RY LEHMANN A TTORNEYS 22, 1967 E. H. LEHMANN 3,336,905

XEROGRAPHIC DEVELOPER APFARATUS Filed Dec. 18, 1964 3. Sheets$heet ii INVENTOR. ERNEST HENRY LEHMANN A T TORNE Y5 1967 E. H. LEHMANN 3,336,905

XEROGRAPH I C DEVELOPER APPARATUS Filed Dec. 18, 1964 5 Sheets-Sheet 3 INVENTOR. ERNEST HENRY LEHMANN ATTORNEYS United States Patent 3 336 905 xnnocnarrnc nEvnLoPEn APPARATUS Ernest H. Lehmann, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 18, 1964, Ser. No. 419,343 8 Claims. (Cl. 118--637) ABSTRACT OF THE DISCLOSURE Background of the invention This invention relates to xerography and, in particular, to apparatus for improving cascade development of latent electrostatic images.

In the process of xerography, for example, as disclosed in Carlson Patent No. 2,297,691 issued Oct. 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them and thereby creates an electrostatic latent image on or in the photoconductive layer. Development ofthe latent image is effected with an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in the pattern corresponding to the latent electrostatic image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be afiixed by any suitable means.

The system of cascade development has been extensively employed to develop latent electrostatic images and generally consists of gravitationally flowing developer mate-rial consisting of a two-component material of the type disclosed in Walkup Patent No. 2,638,416 over the xerographic plate bearing the latent image. The two components consist of an electroscopic powder termed toner and a granular material termed carrier and which by mixing acquire triboelectric changes of opposite polarities. In development the toner component, usually oppositely charged to the latent image, is deposited on the latent electrostatic image to render that image visible.

The quality of cascade developed images is dependent on the relative concentration of carrier and toner present in the developer and on the extent of image contact with the toner material. For optimum development, about the same developer concentrations must be contacted with all image areas.

Most commercially marketed xerographic reproducing machines employ a xerographic plate in the form of a rotating drum operating at surface speeds of about 10 feet per minute such that developer velocity exceeds the drum peripheral velocity by about a factor of 13. With recently developed relatively higher speed machines, it has become necessary to increase the drum speed which decreases the ratio of developer velocity/ drum velocity to a factor of about 4 and lower.

When this ratio is reduced to about 8 or less, a print defect in the final copy has been found to appear as to render the reproductions of substandard quality. This defect is evidenced by localized image areas of reduced developed density appearing as fadeout and termed starvation, usually being found immediately following image areas which consume large quantities of toner. Following is a relative term denoting that developer in its flow first contacts the high density region before contacting the starved image region. This defect may occur, for example, in image areas following such high density regions as solid area, half tone, continuous tone or the like.

The reason for the print defect is not completely understood but is believed to be attributed to the essentially laminar flow characteristics of the developer stream and reduced available development time as associated with the higher drum speeds of the high speed xerographic reproducing machines. These characteristics are believed contrasted to the more agitated type flow and longer development times resulting when developer is cascaded over the drums in the lower speed machines. Because of the limited and laminar flow associated with the high speed units, only the toner in the developer layer adjacent the drum is permitted to contact the latent electrostatic image. Accordingly, the bottom-most layer of developer becomes detoned after being subjected to high density images which consume excessive amounts of toner. Since the developer velocity is still relatively greater than that of the drum, it is believed that initially, adequately developed image areas are forced to compete with the detoned carrier particles passing subsequently over these areas. As the detoned carrier beads pass the previously developed image, they scavenge toner from the image to satisfy their own unbalanced charge condition. This scavenging etiect coupled with the shorter development times, leaves the scavenged image with a low density development appearance, hence the name starvation. Accordingly, it is believed that this print defect, occurring in the high speed machines, is dependent on the development history of the developer, which in turn is dependent on the history of the density and area of latent electrostatic image contacted by the developer before reaching the starved image portions. Prints bearing defects Now, in accordance with the present invention, there is provided apparatus whereby the cascade starvation defect associated with high speed xerographic reproducing machines is overcome. That is, by the invention hereof, it is now possible in reproducing machines having a developer velocity/ drum surface velocity ratio varying from about 8 or less to consistently produce prints without the localized low density, starved, image areas.

This problem has been overcome by the improved developer apparatus of this invention in which a series of properly oriented obstructions are supported in the developer stream to disturb the flow as the developer flows in contact with the image bearing drum surface. These obstructions are effective to create a high degree of developer turbulence bringing increased toner into contact with the drum surface. In a preferred embodiment, the apparatus comprises a plurality of spaced spoiler Wires or cord-like members extending the axial length of the drum closely spaced to its peripheral surface. The developer flows through and about the members prior to and during contact with the xerographic plate.

Thus, it is an object of this invention to provide improved cascade development apparatus, whereby the image starvation defect associated with high xerographic drum speeds is overcome.

For a better understanding of the invention, as well as the other objects and further features thereof, reference is made to the following detailed description of the invention to be read in connection with the accompanying drawings.

Brief description of the drawings FIG. 1 is a schematic illustration of a xerographic reproducing machine incorporating an embodiment of the invention;

FIG. 2 is an isometric view of a first apparatus embodiment shown in its relation to the drum;

FIG. 3 is an isometric view of a second apparatus embodiment shown in its relation to the drum; and

FIG. 4 is a sectional, side view of the second apparatus embodiment shown in its relation to the drum.

Brief description of the preferred embodiments For a general understanding of the Xerographic processing system in which the invention is incorporated, reference is had to FIG. 1 in which the various system components are schematically illustrated. As in all xerographic systems based on the concept disclosed in the above-cited Carlson patent, a radiation image of copy to be reproduced is projected onto the sensitized surface of a Xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material to form a xerographic powder image corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be aflixed by a fusing device whereby the powder image is caused permanently to adhere to the support surface.

In the xerographic apparatus illustrated, original copy to be reproduced is placed on a support tray from which it is fed onto a feed apparatus generally designated 11. On the feed apparatus, an original is moved on an endless belt 12, driven by motor 13, to pass the optical axis of projection lens system 14 that is illuminated by a projection lamp LMP-l. The image of the original is reflected by mirror 15 through an adjustable objective lens 16 and then reflected by mirror 17 downwardly through a variable slit aperture assembly 18 and onto the surface of a xerographic plate in the form of a drum 19.

Xerographic drum 19 includes a cylindrical member mounted in suitable bearings in the frame of the machine and is driven in a clockwise direction by a motor 24 at a constant rate that is proportional to the transport rate of the original, whereby the peripheral rate of the drum surface is identical to the rate of movement of the projected radiation image. The drum surface comprises a layer of photoconductive insulating material, which is commonly vitreous selenium on a conductive backing, that is sensitized prior to exposure by means of a screened corona generating device 25, which may be an adaptation of the type disclosed in Walkup Patent No. 2,777,957, that is energized from a suitable high potential source.

The exposure of the drum to the radiation image discharges the photoconductive layer in the area struck by radiation, whereby there remains on the drum a latent electrostatic image in image configuration corresponding to the radiation image projected from the original. As the drum surface continues its movement, the latent electrostatic image passes through a developing station 26 in which a two-component developer material 27, which may be of the type disclosed in Walkup Patent No. 2,638,416, is cascaded over the drum surface by means of developing apparatus 28.

In the developing apparatus, developer material is carried up by conveyor 29, driven by suitable drive means from motor 30, and is then released onto chute 31 wherefrom, as it flows from the chute and cascades down over the drum surface, it passes over, around and through an embodiment of the improved developer apparatus of the invention. As illustrated and as further described below, the embodiment comprises a plurality of spoiler wire, cord-like members extending essentially parallel to the drum axis across the drum surface. The toner component 32 of the developer that is consumed in developing is stored in dispenser 33 and is dispensed in amounts controlled by gate 34.

After developing, the xerographic powder image passes a discharge station 41 at which the drum surface is illuminated by lamp LMP-Z, whereby residual charges on the non-image areas of the drum surface are completely discharged. Thereafter, the powder image passes through an image transfer station 42 at which the powder image is electrostatically transferred to a moving support surface 43 by means of a second corona generating device 44 similar to corona charging device 25, mentioned above.

The moving support surface to which the powder image is transferred may be of any convenient type, such as paper, and may be obtained from supply roll 45, fed over guide rolls 46 and 60 and over suitable tensioning rolls being directed into surface contact with the drum in the immediate vicinity of transfer corona generating device 44. After transfer, the support surface is separated from the drum surface and guided through a fusing apparatus 48 whereby the powder image is permanently affixed to the support surface. Thereafter, the support surface may be fed over a further system of guide and tensioning rolls including guide roll 47 and onto a take-up roll 52 that is driven by motor 53.

After separation of the support surface from the drum, a corona generating device 54 directs electrostatic charge to a residual powder image on the drum surface. Thereafter, the xerographic drum surface passes through a cleaning station 55 at which its surface is brushed by a cleaning brush assembly 56, rotated by a motor 57, whereby residual developing material remaining on the drum is removed. The drum surface then passes through a second discharge station 58 at which it is illuminated by fluorescent lamp LMP-3 whereby the drum surface in this region is completely flooded with light to remove any electrostatic charge that may remain thereon.

In FIG. 2, there is illustrated a first embodiment of the invention comprising an array of spoiler wire, cord-like members 36 of plastic, metal or the like strung in parallel relation to each other and extending transversely across the developer path uniformly spaced from the drum surface. The members span the drum and are attached to opposite adjustable tension screws 64 mounted on slide means 68 on each side of the support frame 62. The support frame in turn is mounted to the frame 73 of the copying apparatus by pivotal bolt means 74 to allow the distance of the members from the drum surface to be varied and preset for optimum operation. The screws 64 provide a convenient means for adjusting and maintaining members 36 under proper tension which should be sufficient to prevent sag during development, yet not so great as to break the members. Slide means 68 are bolted to the support frame 62, and may be manually moved along slot 69 to vary inter-member spacing. Preferably the members are supported to interfere in essentially normal fashion with developer on its initial gravity release from chute 31 to the image bearing drum surface 20 and as it continues its flow across the image surface.

It has been found with a wire size of about 0.005-(1015 inch diameter best results are obtained when the wires are arranged with the spacing of the first wire closest to the chute 31 approximately /4 inch to 1 inch from the chute, wire-to-wire spacing of about A inch to 1 inch, and supported in the developer stream up to about inch above the drum surface 20.

Electrically conductive spoiler members, such as wires, may be biased, as shown, by connection to high voltage source 70, and ground 71, with frame 62 being of a suitable dielectric material. By biasing thin wires collectively or individually just above their corona threshold in air, corona emission is effected such that in regions Where the developer has a positive space charge corona ionization will take place. This has been found unessential but helpful in removing excessive carrier charges present in regions of the developer which have justpassed over and developed areas of high image density as to aid in the removing of the image starvation defect.

For the arrangement described, starvation is found to be reduced to a degree somewhat dependent on the variables just described and also on the total number of wires employed. Five wires are shown for illustrative purposes. With eight wires strung transversely across the length of the drum in the manner shown in FIGS. 1 and 2, starvation effects are held to a minimum and even eliminated. It was also found, however, that the eight member configuration apparently produced excessive turbulence to the extent that scattered toner was deposited in the nonimage areas as to effect undesirablebackground deposit on the finished print. Five membersgave acceptable image density with little or no background. More than 8, up to 18 members at inch inter-member spacing, eliminated starvation entirely, but under ordinary toner-carrier proportions overall image development density was found to be decreased to an undesirable degree. An increase in toner concentration was found to partially remedy the density deficiency. The embodiments, as described above, with a member to drum spacing above about & inch reduced or eliminated starvation without causing developer to back up or bind, which would likely cause abrasive scoring of the drum surface by the relatively hard carrier beads.

In FIGS. 3 and 4 there is illustrated a second embodiment of the invention. In contrast to horizontal obstructions strung in the developer path, this embodiment includes a geometrically spaced pattern of vertical posts 77 suspended substantially normal to the plate surface. The posts may conveniently be formed by fastening rivets to a backing sheet 78 punched to receive the rivets. The posts, protruding downward from the sheet 78 are supported in the developer path by support frame 62 which is mounted to the frame 73 of the copying apparatus by pivotal bolt means 74. These same bolts can also be used for varying the distance of the sheet and the postsfrom the drum surface. Slot 69 permits locating the posts in the developer stream at a desirable position relative to chute I The numerous small posts 77 may be of any suitable size and in the embodiment being described are about 1/I1(;1/8 inch in diameter, about A1-% inch center-to-center. Their individual length is sulficient to inhibit extensive contact with the underside of plate 78 being about 4 inch in length. Optimum terminal spacing was found to be about to about inch from the drum surface. This embodiment mixes the developer in essentially lateral fashion with some vertical mixing, as compared to the previous embodiment in which the plurality of wire-like members, mixes in essentially vertical and circular motion. A single row of staggered posts about 1% inches from the chute and extending the length of the drum gave excellent image starvation reduction. As like the first embodiment, the electrically conductive backing sheet and parts may be biased by connection to voltage source 70 and ground 71 to aid in the removal of excess carrier charges present in regions of the developer which have just passed over and developed areas of high image density.

By the above description there is disclosed novel apparatus for developing xerographic images and which overcomes the image starvation defect associated with cascade development in high speed xerographic machines. By means thereof these machines are rendered able to produce commercially acceptable prints with the same stand- 6 ard of quality now produced with the relatively lower speed machines. With the members 36 placed, as illustrated in FIG. 2, just beyond the chute 31 taken in the direction of developer flow and above the selenium drum 19 at the developing station 26, the developer stream flows through and past the members in an interfered with relation. By passing through these members, the laminar flow in the developer stream is disturbed and converted to a turbulent or agitated type of flow, mixing the developer as to remove detoned, charged carrier particles from the bottommost layer of developer and replacing these particles with more developer 27. This allows new developer comprised of toned carrier particles to pass in contact with latent electrostatic images being carried on the rotating drum.

While the invention has been described with reference to the details and constructions herein illustrated, it is not intended to be confined to the exact mechanism shown. This application is intended to cover such modifications or departures as may come within the purposes of the invention and the scope of the following claims.

What is claimed is: 1. In a xerographic reproducing apparatus wherein latent electrostatic images of copy are formed on the surface of a moving xerographic plate, an improved apparatus for developing the latent electrostatic images comprising in combination,

means for continuously discharging a developer mix onto the image bearing surface of a moving xerographic plate, said xerographic plate forming a path of flow over which said developer mix is cascaded.

spaced support means at the opposite ends of said xerographic plate and extending from said developer discharge means in the direction of flow along said path of flow, and

flow obstructing means suspended between said spaced support means and extending into said path of developer flow at close proximity to said xerographic plate whereby to create an increased agitated and turbulent flow of said developer mix over said xerographic plate.

2. Apparatus according to claim 1 in which said flow obstructing means comprises a plurality of uniformly positioned spaced apart substantially parallel wire or cordlike members.

3. Apparatus according to claim 2 in which said wire or cord-like members extend transversely to said developer path of flow and being spaced longitudinally therealong.

4. Apparatus according to claim 3 in which said wire or cord-like members are electrically conductive and further including means for electrically biasing said wire or cord-like members. to aid in the removal of unbalanced developer charges.

5. Apparatus according to claim 1 in which said flow obstructing means comprises a plurality of uniformly positioned spaced apart substantially parallel post members.

6. Apparatus according to claim 5 in which said post members being suspended from above extend normal into said developer path of flow and are spaced longitudinally therealong.

7. Apparatus according to claim 6 in which said post members are electrically conductive and further including means for electrically biasing said post members to aid in the removal of unbalanced developer charges.

8. In a xerographic reproducing apparatus wherein latent electrostatic images are formed on a rotating xerographic drum and there is included cascade developer means for continuously discharging a cascade developer mix over the image bearing drum surface with a ratio of developer to drum surface velocity of less than at least eight, an improved developing apparatus comprising in combination:

(a) a support frame adjacent the paths of the xerographic plate and the developer flow,

7 8 (b) a plurality of spaced-apart posts supported and References Cited depending from said frame from above said paths UNITED STATES PATENTS and extending into a partially interfering relation with said developer as it cascades in contact with the 21808328 10/1957 9 plate, said posts being about to /8 inch in diarn- 5 3011474 12/1961 Ulrich 118*637 eter and with a center-to-center spacing of from 3147147 9/1964 Carlson A to in and terminating about %4 ch to CHARLES A. WILLMUTH, Primary Examl'ner.

34 u 0 {F 1 2 55 55 the drum Surface and Contact PETER FELDMAN, Alysistant Examiner.

Claims

1. IN A XEROGRAPHIC REPRODUCING APPARATUS WHEREIN LATENT ELECTROSTATIC IMAGES OF COPY ARE FORMED ON THE SURFACE OF A MOVING XEROGRAPHIC PLATE, AN IMPROVED APPARATUS FOR DEVELOPING THE LATENT ELECTROSTATIC IMAGES COMPRISING IN COMBINATION, MEANS FOR CONTINUOUSLY DISCHARGING A DEVELOPER MIX ONTO THE IMAGE BEARING SURFACE OF A MOVING XEROGRAPHIC PLATE, SAID XEROGRAPHIC PLATE FORMING A PATH OF FLOW OVER WHICH SAID DEVELOPER MIX IS CASCADED. SPACED SUPPORT MEANS AT THE OPPOSITE ENDS OF SAID XEROGRAPHIC PLATE AND EXTENDING FROM SAID DEVELOPER DISCHARGE MEANS IN THE DIRECTION OF FLOW ALONG SAID PATH OF FLOW, AND FLOW OBSTRUCTING MEANS SUSPENDED BETWEEN SAID SPACED SUPPORT MEANS AND EXTENDING INTO SAID PATH OF DEVELOPER FLOW AT CLOSE PROXIMITY TO SAID XEROGRAPHIC PLATE WHEREBY TO CREATE AN INCREASED AGITATED AND TURBULENT FLOW OF SAID DEVELOPER MIX OVER SAID XEROGRAPHIC PLATE.