US3682538A

US3682538A - Xerographic pick-off plate

Info

Publication number: US3682538A
Application number: US20985A
Authority: US
Inventors: Ronald L Cade; Stewart W Volkers
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1970-03-19
Filing date: 1970-03-19
Publication date: 1972-08-08
Anticipated expiration: 1989-08-08
Also published as: DE2113504A1; BE764562A; SU426387A3; NL7103695A; FR2087878A5; GB1349272A; CA935635A

Abstract

A pick-off plate is positioned adjacent a continuous xerographic surface at a point where cascading developer would normally tend to leave the xerographic surface by inertia and gravity. The developer includes toner and carrier particles and the pick-off plate isolates toner powder clouds from the xerographic surface in areas where carrier particles are not moving over the surface to clean it with a scavenging action.

Description

United States Patent 15] 3,682,538 Cade et al. [451 Aug. 8, 1972 [54] XEROGRAPHIC PICK-OFF PLATE 3,424,131 1/1969 Aser et a1 ..1 l7/ 17.5 X 72 m rs: R mm L C d F t 3,430,606 3/1969 Pease et al. ..ll7/l7.5 X l 1 6 volkers, fi m 51,31; 3,147,147 9/1964 Carlson ..1 18/637 3,367,307 2/1968 Lawes et al. ..l17/l7.5 X 3,303,817 2/1967 Cranch etal. ..1l7/l7.5 X [73] Ass gnee e o Corpo a o Roch te NY. 3,415,224 12/1968 Hudson ..117/17.5 X

22 F'led: M h 19 1970 I 1 l m Primary Examiner-William D. Martin PP 20,985 Assistant ExaminerM. sofocleous Attorney-James J. Ralabate, David C. Petre and Related US. Application Data Michael H. Shanahan [63] Continuation-impart of Ser. No. 789,031, Dec. 31, 1968. [57] ABSTRACT A pick-off plate is positioned adjacent a continuous 12?} lif'oif'.:11:311313331111332351 1% 863; i233; xemgraphic at a Point where cascading 58 Field of Search. 117/175 19- 118/637 355/15 devel9pe wuld P 9 leave the 355/3 graph1c surface by mertla and gravity. The developer includes toner and carrier particles and the pick-off plate isolates toner powder clouds from the xero- [56] References C'ted graphic surface in areas where carrier particles are not UNI E STATES PATENTS moving over the surface to clean it with a scavenging t 3,412,710 11/1968 Robinson ..l18/637 3,347,691 10/1967 Lyles l l7/17.5 8 Claims, 2 Drawing figures PATENTEDAuc 8 m2 3,682,538

H 0RZ. DRUM AXIS INVENTORS RONALD L. CADE STEWART ATTORNEY 1 XEROGRAPHIC PICK-OFF PLATE CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 789,031, filed Dec. 31, 1968, in the name of the present inventors and titled Imaging System.

BACKGROUND OF THE INVENTION graphic plate is electrostatically charged unifonnly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as a powder. The powder is held in image areas by the electrostatic charge 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 confonnity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably affixed thereto to form a permanent print.

The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred to herein as toner and a coarse granular material called carrier. The carrier is usually a glass, sand or steel bead coated with and encased in 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 interaction. Such charge causes the toner powder to adhere to the carrier. When the toner latent carrier is moved into contact with a latent electrostatic image, the charge of the image areas attracts the toner from the carrier to the image areas to thus render the latent electrostatic image visible.

Many development systems have been proposed for bringing the charged toner into contact with the image bearing surface. One of such systems in wide use is the cascade development system. In cascade development the two component developer is poured or cascaded across a xerographic surface under the action of gravity. The transfer of toner to the image bearing surface, in theory at least, is achieved by the electrostatic attraction of the image areas for the toner particles overcoming the electrostatic attraction of the carrier for the toner. In practice, however, appreciable quantities of toner particles are jarred loose from their associated carrier granules when dropped onto, or while cascading across, the surface being developed. Such unassociated toner particles accumulate into powder clouds and deposit themselves randomly on xerographic surfaces. The toner particles of the clouds often attach themselves to portions of the xerographic surface representing non-image areas, which if left there and transferred to the final copy sheet would exhibit themselves as unwanted background. Background can also occur from toner particles attaching themselves to non-image areas merely by the mechanical attraction between toner and non-image portions of the surface.

Not all toner particles which come to rest on nonimage areas remain there for transfer to the final copy. A great percent of such toner is removed by the scavenging effect of the carrier granules. Scavenging occurs when carrier particles move across a toner bearing surface after depositing their previously associated toner particles onto image areas. As the carrier granules continue their movement across the surface, carrier to toner contact can loosen and subsequently attract those toner particles from the surface which are held thereto by weak electrostatic forces. Such forces are characteristic of the forces holding toner particles to non-image areas. Consequently, scavenging carrier granules tend to remove toner particles which have been undesirably deposited in non-image areas.

In the usual practice of cascade development, as practiced on continuous and automatic machinery wherein the xerographic surface is fonned as a rotary drum, belt or the like, the developer is dropped onto the xerographic surface somewhere adjacent the upper portion of the surface. The developer then cascades down the drum until it arrives at approximately the horizontal center line of the drum at which time it drops from the drum under the influence of gravity. The internal turbulence of the cascading developer, as mentioned above, gives rise to an agitated cloud of freely floating toner along its path which deposits toner onto all areas of the xerographic surface including the area beneath the line where the carrier has fallen from the drum. Since the carrier has gravity fallen from the drum surface at the area adjacent the horizontal center line, there is no chance to remove the background toner from this part of the drum by the scavenging action, i.e., scavenge cleaning. It has thus been the practice to retain this background toner on the drum. Attempts have been made to minimize the background created by this cloud by closely regulating the toner concentration viz., the percentage of toner to carrier.

US Pat. No. 3,412,710 discloses method and apparatus for eliminating background as caused by uncontrolled powder clouds in the area where the developer falls away from the xerographic surface under gravity. In that patent the powder cloud is electrically suppressed. An electrode is placed adjacent the powder cloud and it is coupled to a bias that attracts the charged toner away from the xerographic surface. In the present invention, background is eliminated by preventing generation of the powder cloud rather than suppressing the cloud after it has been generated.

Accordingly, it is an object of the present invention to overcome the above noted difficulties encountered in development of latent electrostatic images.

Specifically, it is an object of this invention to reduce background in xerographically formed images.

Yet another object of the invention is to prevent generation of powder clouds in xerographic machinery.

Even a further object of the invention is to restrict development on xerographic surfaces to areas being cleaned by the scavenging action of developer carrier particles.

, Another object of the present invention is the improvement of the two component cascade development systems.

A further object of this invention is to devise means for diverting the flow of two component developer materials.

Another object of the present invention is to create a seal across a xerographic surface effective to reduce the flow of two component developer.

These and other objects of the present invention are accomplished by a pick-off plate or electrode having a blunt end closely spaced to a xerographic surface near the point where developer would normally fall away under gravitational forces. The pick-off plate is electrically biased to enable development and scavenge cleaning to occur between it and the xerographic surface. lnaddition, it is shaped and spaced from the xerographic surface so as to encourage the collection of developer therebetween to create a seal for diverting the oncoming toner. The pick-off plate is preferably used with a development electrode having a shape to complement the diversion of the developer from the xerographic surface.

DESCRIPTION OF THE DRAWINGS Other objects and features of the invention will be apparent upon a further reading of the present description and from the drawings wherein:

FIG. 1 is a schematic cross-sectional elevation view of a xerographic machine adapted for continuous and automatic operation utilizing the teachings of the instant invention.

FIG. 2 is an enlarged sectional view taken from FIG. 1 illustrating the relation between a xerographic surface, development electrode and pick-off plate.

DESCRIPTION OF THE INVENTION Shown in FIG. 1 is a xerographic machine constructed for continuous and automatic operation and embodying the principles of the instant invention. All of the processing stations referred to by letters are conventional in the xerographic art except for the development station C which forms the basis of the instant invention. For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the xerographic drum may be 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 charge in the exposed areas thereof to thereby form a latent electrostatic image of the copy to be reproduced;

A development station C, at which a xerographic developing'material including toner powder having an electrostatic charge opposite to that of the latent electrostatic image, is moved into contact with the drum surface, whereby the toner powder adheres to the latent electrostatic image to form a xerographic powdered image in the configuration of the copy being reproduced;

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

A drum cleaning station E, at which the drum surface is brushed to remove residual toner particles remaining thereon after image transfer.

As stated above, all of these stations are conventional in the xerographic art except for the development station C.

The developing instrumentalities are positioned adjacent the xerographic surface 10 upon which the images are adapted to be formed. The xerographic surface is shown in the illustrated embodiment as a rotating drum movable to sequentially move its surface portions in a path through the various processing stations for an automatic and continuous cycle of operation. Power may be applied for rotating the drum, as well as for initiating the cycle of operation for the various processing stations by any conventional power source, not shown. For automatic operation, the xerographic surface should be continuous such as a belt or drum but it need not be circular in cross section.

The developing instrumentalities are housed within a general developer housing 12. The lower or sump portion of the developer housing is adapted to be filled with a quantity of two component developer material. The developer may be raised to an elevated position for cascading down the xerographic surface by a series of buckets 14 movable on a belt 16 and guided for its motion by rollers 18. Power may be imparted to the rollers by any conventional power source, not shown, to move the buckets in the direction as indicated by the arrows.

As the buckets reach their uppermost position they are adapted to drop the developer between the upper housing wall 20 and the plate 22 which together guide the developer onto the drum surface. The sump, buckets, walls, and plates extend a length approximately equal to the length of the drum to insure the cascading of developer across the length of the drum. As the developer cascades down the arc of the drum it develops the latent electrostatic images on the drum surface which it contacts. As the developer falls past the horizontal center line of the drum it drops under gravitational force onto the bottom housing wall 24 and is returned to the sump for recycling through the development zone. A toner dispenser may be incorporated into the developer housing for supplementing the toner given up by the system through development of images.

When cascading down a curved sector of the drum surface, the carrier granules follow a path defined by the curvature of the drum. They then fall away from the drum under the influence of gravity. Since the carrier granules have a component of motion in the horizontal direction, they leave the drum surface slightly above the horizontal center line of the drum due to the combination of gravity and inertia action on the moving carrier granules. Their high mass prevents them from granules. Thus the movement of the toner clouds is over an extended path which contacts the drum in a toner depositing manner including an area beneath the line where the carrier granules drop away from the rum.

Positioned adjacent the drum are the development electrode 26 and the pick-off plate 28. The development electrode includes a cylindrical surface 30 concentric with the cylindrical xerographic surface and beveled upper 32 and lower 34 surfaces that are sloped relative to vertical. The upper surface 32 is beveled in order to act like a funnel in directing the developer into the space between the development electrode and drum. Similarly, the bottom surface 34 is beveled to create, along with the blade portion 36 of the pick-off electrode, a passageway for exiting the developer from the surface of the drum.

The development electrode is made of a conductive material such as aluminum or other suitable metal and is coupled to an appropriate voltage source (not shown) to establish the required electric field for image development between it and the conductive backing supporting the xerographic surface 10. By way of example, the drum 10 is held at ground potential and the xerographic surface is charged positive by the coratron charging apparatus at station A. The bias on the development electrode is normally slightly greater than the magnitude of the charge on surface 10 in nonimage (exposed, discharged) areas for attracting comparatively negatively charged toner particles away from surface 10 in the background or non-image areas. Of course other potential schemes may be employed and the foregoing example is only given for illustrative purposes. Development electrodes are discussed in Ulrich US. Pat. No. 3,011,474, Clark et al US. Pat. No. 2,952,241 and Tandrigan et al US. Pat. No. 2,752,304 and the disclosures of those patents are incorporated by reference herein.

At this point, the motion of the carrier particles is once again noted. The carrier flows downward under gravity from a conveyor bucket 14 onto the entrance or leading edge of the development electrode. The developer (carrier and toner) flows through the space between the drum surface 10 and electrode 26 to the terminating or trailing edge of the development electrode. The trailing edge, represented by the lower bevel surface 34, is positioned generally at a gravity tangent line to the xerographic surface 10.

The gravity tangent line is a line extending the width or length of the drum 10 substantially parallel to horizontal. It marks the altitude at which the carrier particles begin to leave the drum surface 10 under gravitational and inertial forces. The gravity tangent line is perpendicular to the plane of FIG. 1 and is located generally at point 38. Point 38 also marks the intersection of a vertical plane tangent to drum 10. The gravity tangent line may be higher than point 38 because of the horizontal inertia component of the developer, especially the carrier. However, a relative low drum velocity minimizes the contribution of inertia on where the developer leaves the drum surface and the concentric shaped development electrode tends to direct the developer downward along the vertical plane tangent to the drum. The gravity tangent line is intended to include that line, point or area in any xerographic system where the developer is separated from the xerographic surface whether by gravity, inertia or other force or combination thereof. The gravity tangent line is that line on a xerographic surface after which carrier particles are no longer available for scavenge cleaning. The intent of this invention is to insure that the toner particles are separated from the xerographic surface at substantially the same location that the carrier particles are separated from the xerographic surface. The reason being to eliminate background.

The pick-off electrode 28 of the present invention is positioned with its blade 36 closely spaced from the drum at generally the gravity tangent line 38. FIG. 2 is an enlarged view of electrode 28 and its blade 36 and divider 40 portions or elements. Both the blade and divider are electrically conductive being made from aluminum or other metal. They are electrically coupled to an appropriate voltage source, e.g., by battery 28a to establish electric fields between the plate 28 and drum 10 and plate 28 and electrode 26 that draw or attract the charged toner particles away from the drum 10, especially in areas below the gravity tangent line 38.

The blade portion 36 has a blunt end 42, i.e., a finite end surface, (about 0. 10 inch thick) rather than a sharp edge such as a knife edge, to expose a definite surface area to drum 10. This surface area 42 permits the pickoff electrode to act as a developing-cleaning electrode in the space opposite to it similar to the development electrode 26. The blade also includes a notch surface 44 that is generally perpendicular to the flow path of the developer, or at least protrudes into the path of the flow. The notch surface tends to cause an accumulation of developer between the blade 36 and drum surface 10 that effectively creates a seal between the two surfaces. This seal helps divert the flow of developer away from the drum. The notch surface in FIG. 2 extends beyond the surface 30 of electrode 26.

The blade 36 slopes at an angle to vertical generally parallel to the lower bevel surface 34 of the development electrode and as such defines, along with bevel surface 34, an exit passage for the developer. The divider portion 40 of the pick-off electrode extends from the blade downward to the lower wall 24 of the housing. The divider defines left 46 and right 48 powder cloud chambers within the housings. The right chamber 48 is the one in which the largest powder cloud appears because the majority of the developer is diverted into this volume. Of particular note is the fact that the powder cloud in chamber 48 is physically separated from the drum surface 10 by divider 40 thereby preventing background areas from being developed where carrier particles are not being cascaded over the surface 10 to scavenge clean the surface, i.e., at points below the gravity tangent line. Any powder cloud that appears in chamber 46 due to leakage of developer through the space between the blade 36 and surface 10 is suppressed by the electric field between the pick-off electrode and surface 10. The space between divider 40 and bottom wall 24 pemrits any toner in chamber 26 to return to the sump. In view of the foregoing remarks, the pick-off plate electrode 28 (referred to as an electroded developer control baffle in the parent application) is clearly designed to control carrier flow and toner action, such as toner powder clouds, near the exit of the development-scavenge cleaning electrode 26.

The distance 50 between the development electrode and drum may be varied over a wide range. The size of the toner and carrier particles is a consideration, however. Toner particles are generally less than 30 microns in diameter whereas carrier particles generally vary from 30 to 1,000 microns in diameter (i.e., their largest dimension). Excellent results have been obtained with a dimension 50 of 0.08 inch with carrier particles ranging in size from about 200 to 850 microns. The distance 52 from the blade to the drum is preferably from about 0.02 to about 0.06 inch with excellent results at spacings of about 0.045 inch. It is presently preferred to have the width of the notch surface 44 such that it extends from the blunt end of the blade to at least the edge of the electrode 26.

The voltage potentials coupled to

electrodes

26 and 28 need not be the same. Since the distance of the divider 40 from drum is clearly greater than the distance of the development electrode from the drum, it is customary to place the pick-off electrode at a higher potential (assuming the earlier example of positive charging and positively biased development electrode). The reason of course is that a higher potential is needed to establish a desired field strength over the greater distances. Also, the higher potential on the plate 28 (again assuming the polarities of the example) causes the negative toner to tend to move toward the pick-off plate rather than the development electrode.

Modifications to the present invention can be made while staying within the scope of the basic concept; namely, the physical separation of the developer at the point where scavenge cleaning no longer occurs. By way of example, the shape of electrode 26 near the lower bevel surface 34 can be varied as long as room is provided for the developer to exit from the vicinity of the drum. This and other similar modifications will occur to those skilled in the art and are intended to be within the scope of the present invention.

What is claimed is:

l. Xerographic apparatus comprising,

a continuous xerographic surface coupled to drive means for circulating the surface past charging and exposing apparatus for creating latent electrostatic images on the xerographic surface,

development means for depositing developer including toner and carrier particles onto the xerographic surface with toner particles developing the latent image and carrier particles scavenge cleaning the xerographic surface,

an electrically biased development electrode uniformly spaced from the xerographic surface to allow developer flow therebetween for developing and cleaning and terminating adjacent a gravity tangent line to permit the developer to leave the xerographic surface and an electrically biased pick-off plate positioned adjacent the gravity tangent line for mechanically deflecting developer away from the xerographic surface to isolate toner powder clouds from the xerographic surface where carrier particles are unable to scavenge clean, 1

said pick-off plate being spaced from the development electrode to permit deflected developer to flow therebetween and including a blunt end surface spaced closer to the xerographic surface than th I l trod ter l fl gfi' ll perperf cular to the pat developer flow for accumulating developer to form a seal between the pick-off electrode and xerographic surface. 2. The apparatus of claim 1 wherein said development means includes a housing having top and bottom walls closely spaced from the xerographic surface above and below a gravity tangent line at which carrier particles leave said xerographic surface, and I means for gravity dropping developer onto the xerographic surfaceabove said gravity tangent line and for recycling developer dropped onto said surface and deflected therefrom by said pick-off means wherein said pick-off means extends from said gravity tangent point to a closely spaced position with the bottom wall of said housing.

3. The apparatus of claim 1 wherein said development electrode surface is spaced about 0.08 inch from the xerographic surface, the blunt end of said pick-off plate is spaced about 0.045 inch from said xerographic surface and said notch surface of the pick-off electrode extends from said blunt end to at least about 0.080 in ch from said xerographic surface.

4. The apparatus of claim 1 including an electrical bias means coupled to the pick-off plate for electrically biasing the plate with a potential to effect development of a latent image with developer between the blunt end and xerographic surface. s

5. The apparatus of claim 1 wherein said continuous xerographic surface is a cylindrical surface.

6. The apparatus of claim 5 wherein said development electrode includes a cylindrical surface concentric with and spaced from said cylindrical xerographic surface and a beveled surface at a terminating end sloping away from the xerographic surface to permit passage of deflected developer under gravity between said development electrode and pick-off plate.

7. The apparatus of claim 1 wherein said development means includes a housing having a bottom wall closely spaced to said xerographic surface at a point below agravity tangent line on said xerographic surface for collecting developer not deflected by said pick-off electrode.

8. The apparatus of claim 7 wherein said pick-off plate extends from said gravity tangent point on said xerographic surface to a closely spaced position adjacent said housing bottom wall defining toner powder cloud chambers on both sides thereof with the cloud in the chamber adjacent said xerographic surface being suppressed by an electric field established between the xerographic surface and pick-off plate and the cloud in the remaining chamber being isolated from the xerographic surface by the pick-off plate.

d a notch surface ex-

Claims

1. Xerographic apparatus comprising, a continuous xerographic surface coupled to drive means for circulating the surface past charging and exposing apparatus for creating latent electrostatic images on the xerographic surface, development means for depositing developer including toner and carrier particles onto the xerographic surface with toner particles developing the latent image and carrier particles scavenge cleaning the xerographic surface, an electrically biased development electrode uniformly spaced from the xerographic surface to allow developer flow therebetween for developing and cleaning and terminating adjacent a gravity tangent line to permit the developer to leave the xerographic surface and an electrically biased pick-off plate positioned adjacent the gravity tangent line for mechanically deflecting developer away from the xerographic surface to isolate toner powder clouds from the xerographic surface where carrier particles are unable to scavenge clean, said pick-off plate being spaced from the development electrode to permit deflected developer to flow therebetween and including a blunt end surface spaced closer to the xerographic surface than the development electrode and a notch surface extending generally perpendicular to the path of developer flow for accumulating developer to form a seal between the pick-off electrode and xerographic surface.

2. The apparatus of claim 1 wherein said development means includes a housing having top and bottom walls closely spaced from the xerographic surface above and below a gravity tangent line at which carrier particles leave said xerographic surface, and means for gravity dropping developer onto the xerographic surface above said gravity tangent line and for recycling developer dropped onto said surface and deflected therefrom by said pick-off means wherein said pick-off means extends from said gravity tangent point to a closely spaced position with the bottom wall of said housing.

3. The apparatus of claim 1 wherein said development electrode surface is spaced about 0.08 inch from the xerographic surface, the blunt end of said pick-off plate is spaced about 0.045 inch from said xerographic surface and said notch surface of the pick-off electrode extends from said blunt end to at least about 0.080 inch from said xerographic surface.

4. The apparatus of claim 1 including an electrical bias means coupled to the pick-off plate for electrically biasing the plate with a potential to effect development of a latent image with developer between the blunt end and xerographic surface.

7. The apparatus of claim 1 wherein said development means includes a housing having a bottom wall closely spaced to said xerographic surface at a point below a gravity tangent line on said xerographic surface for collecting developer not deflected by said pick-off electrode.