Corona Discharge Device

Abstract

A corona generator for providing a uniform and highly efficient corona discharge for use in such apparatus as electrophotographic printers.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates specifically to corona discharge devices for generating an ion flow particularly adapted to charge an electrophotographic plate used in electrophotography Xerography.

In electrophotographic copiers and other such printing devices, it is common to use corona charging devices to charge the electrophotographic plate surface. The charge is effected by imposing a high-voltage potential on a conductor spaced from the surface to impart a high-field intensity on the surrounding air. The voltage potential causes the air to break down resulting in a migration of ions to the wire with ions of opposite potential migrating to the electrophotographic surface. The migration of ions must be sufficient to deposit a predetermined charge on the electrophotographic plate for satisfactory operation of the electrophotography process. Additionally, the charge must be uniformly distributed across the plate.

In the past, the primary method of equalizing the charge distribution across the surface has been to position a ground shield on the opposite side of the charged conductors from the plate. It has also been common practice to drive the emitting conductors at a very high potential because, if not so driven, the phenomena occurs which is referred to as beading. The effect of such beading is a streaking of the charge pattern deposited on the electrophotographic surface. The shield acts to raise the total ion flow along the corona wire length and thus at least partially prevents the beading.

While solving some of the more immediate problems, the ground shield also presents disadvantages. One disadvantage is that the shield is not transparent and therefore cannot be used where the charging and exposure of the electrophotographic plate must be concurrent. Additionally, the ground shield serves as a conductor for bleeding off ions which strike it and thereby limits the ion flow to the plate and decreases the efficiency of the corona unit. In the past, less than 10 percent of the ions actually reach the plate and more than 90 percent migrate to the ground shield thereby making the corona unit operate at a very low efficiency.

SUMMARY OF THE INVENTION

It is the primary object of the subject invention to provide a corona charging unit which operates at a higher efficiency than prior art devices and distributes a more uniform charge on the surface being charged.

An additional object of the present invention is to provide a simple and relatively inexpensive corona charging unit.

In accordance with these and other objects, the invention is embodied in a charging apparatus for use in electrophotographic devices to deposit increments of charge on a charge-bearing surface and comprises a corona discharge wire held in spaced relationship to the surface to be charged, a bias rod positioned in spaced relationship on each side of said wire, and means to impress an electrical voltage potential on said wire and rods whereby said rods effect a virtual ground plane extending to the surface to be charged on either side of the wire to enhance the charge flow from the wire to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagrammatic schematic view of an electrophotographic process in which the subject invention can be used;

FIG. 2 illustrates the corona charging unit embodying the subject invention with the primary components disassembled;

FIG. 3 is an enlarged cross-sectional view of the corona charging unit embodying the subject invention in assembled form;

FIG. 4 is an enlarged schematic view of a corona wire plus bias rods showing the approximate path of the ions;

FIG. 5 illustrates an alternate embodiment of the corona charging unit embodying the subject invention with primary components disassembled;

FIG. 6 is an enlarged cross-sectional view of the corona charging unit of FIG. 5 embodying the subject invention in assembled form.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One example of a particular apparatus in which the subject invention is adapted for use is the electrophotographic apparatus shown in FIG. 1. In this apparatus, a rotatable drum 1 carries around its periphery an electrophotographic photoconductive member 2 on which is directed an image which it is desired to reproduce on the paper sheets 3 stored in a hopper 4. A document having the image to be copied is placed upon a transparent plate 5 mounted within a table 6 and the light sources 7 are energized which direct light onto the image for reflection back through a lens 8 onto the surface of the photoconductor 2. The document is moved at a speed synchronized with the surface speed of drum 1 in a manner known in the art. The surface of the photoconductor has been electrostatically charged in a manner to be described later and where illuminated by the image, is discharged leaving a charged pattern in a form of the image to be copied.

Thereafter, a drum passes a development station 10 at which a toner-carrier mixture 11 is cascaded across the electrostatic image on the surface of the photoconductor 2. The toner, having a charge opposite from the polarity of the electrostatic image charge, is attracted to the drum surface to render the image visible. An endless conveyor belt 12 carries the toner-carrier mixture to a position for gravity feeding across the surface of the photoconductive member.

Continuing the clockwise rotation of the drum 1, a copy paper 3 is fed into contact with the developed electrostatic image by the rollers 14. Preferably a corona unit 15 is disposed beneath the paper at the area of contact with a polarity opposite that of the toner thereby attracting the toner to the copy paper. After this so-called corona transfer, the paper 3 is separated from the drum and fed past a heating element 16 which serves to fuse and permanently fix the toner to the paper. The drum continues to rotate past a cleaning brush 17 which wipes the surface of the photoconductor and removes any excess toner which drops into a receptacle 18. With the exception of the charging station, this describes the complete cycling of the drum for reproducing the image desired.

In accordance with the present invention, the corona unit 20 comprises at least one corona discharge wire held in spaced relationship to the photoconductor surface with a bias rod positioned to either side and approximately an equal distance from the photoconductor surface such that by impressing a high voltage on the wire and a lower voltage on the rod, the rods form a virtual ground plane extending normal to the photoconductor surface and through the rods so as to guide most of the resulting ions from the wire to the photoconductor surface for the charging of the photoconductor.

Accordingly, in FIGS. 2 and 3, is shown the wire support unit comprising a plurality of small diameter corona wires 21 mounted on a pair of insulator blocks 22 and 24 having extending fingers 25 contacting the wires. The wires are supported in tension between a pair of spring fingers 26 made of a conductive metal and fixed to the end of a mounting block 27 by screws 28.

Similarly, the rod support unit holds the bias rods 30 mounted to extend between a pair of insulator blocks 31 and 32 held in parallel spaced relationship by a pair of support rods 34 at each end. The inner side of each of the mounting blocks includes a concave inset portion 35 sized to permit the entry of the wire support unit so that the corona wires 21 fit between and extend parallel to the bias rods 30. Terminals 36 and 37 connect through the conductor plate to the bias rods for supplying a reference potential V.sub.R to the rods 30. A terminal 38 extends through a slot 39 in the insulator block 31 when the support units are assembled for supplying a high-voltage potential V.sub.H to the corona wires through the spring fingers 26.

The voltage to the corona unit 20 is provided by power supply 33. The voltage is usually referenced to the support member for photoconductor 2 which is generally maintained at ground potential. The voltage V.sub.R applied to bias rods 30 may be selected at a value from ground potential to about 25 percent V.sub.H and provide a substantial increase in efficiency over the type of corona unit which utilizes a ground shield. However, the preferred potential for V.sub.R is ground potential. The magnitude of the potential V.sub.H selected for the corona wires may vary over a fairly wide range dependent upon many design factors known to those in the art. However, the preferred range is a potential of 6 to 8 kilovolts and a polarity the same as that desired for the charge to be produced on the surface of photoconductor 2.

As shown in FIG. 3, the support units are held together by a plurality of spring biased buttons 40 which snap between the support rods 34. This construction facilitates the servicing of the unit since the parts can be easily disassembled by snapping the supports apart. This construction also gives access to the individual parts for cleaning, adjusting or replacing any of the individual parts of the corona unit.

The corona unit is suitable for those applications in which charging and exposure of the photoconductor surface must be concurrent since the area where the wires run is substantially free of interfering material as shown in FIG. 1. The embodiment shown in FIGS. 2 and 3 also possesses this advantage since support 27 can be constructed of a transparent material or alternatively support 27 can be constructed as a frame-shaped member which is open in the area where the wires run.

The schematic view of FIG. 4 shows the approximate path of the ions from the corona wires 21 to the photoconductive member 2. It can be shown mathematically that the effect of the bias rods 30 on the electrostatic field is similar to that of two grounded shields whose planes are perpendicular to the surface of the photoconductive member 2 and parallel to rods 30, and this effect is essential to establish a high-corona current. The ions tend to migrate to the ground planes but, inasmuch as the region from the rods 30 toward the photoconductor is essentially at ground potential and no conductor is present there, a large percentage of the ions continue on to the photoconductor surface thereby resulting in an efficient transfer of the high-corona current to charge on the photoconductor surface.

In a particular embodiment the bias rods 30 were constructed of metal rods having a diameter of 0.050 of an inch. The corona wires were polished tungsten wires 0.002 of an inch in diameter. The adjacent bias rods 30 as shown in FIG. 4 were approximately 1.5 centimeters apart. The bias rods and the corona wires were in a plane substantially parallel to the photoconductive member and spaced approximately 0.65 centimeter from the photoconductor member in one operation. This structure was energized with a potential V.sub.H of -7 kilovolts applied to the corona wire and a reference potential V.sub.R coupled to bias rods 30 of ground potential. The voltage of the photoconductor support member was also ground potential. This configuration provided a charge current approximately 10 times as high as conventional corona units having ground shields and supplying the same total current. In another embodiment the plane including the bias rods and the corona wires was spaced from the photoconductive member by 0.5 centimeter, and this embodiment showed a further four-fold increase in the efficiency of transferring charge to the photoconductive surface as compared to a conventional corona unit having ground shields.

Thus, the disclosed construction provides greatly increased corona current as well as a higher efficiency in transfer of charge to the photoconductive surface. This advantage results in a decreased high-voltage power supply capacity requirement for a given corona requirement, thereby resulting in a reduced cost for the system. This construction has the added advantage of being an easily iterated structure. Additional individual units of corona wires and adjacent bias rods can be added without appreciably influencing the operation of adjacent units. Thus, the disclosed construction has the advantage of not only increased corona current and a higher efficiency in transfer of charge to the photoconductive member, but also the construction facilitates the servicing of the unit since the individual parts are easily accessible.

In some cases the high-current density provided by the corona charging device described above may cause damage to the photoconductor in places in which small pinholes or depressions exist in the photoconductive coating. The irregularities in the photoconductive coating give rise to strong local fields in the area of the photoconductive member surrounding the pinhole. These strong local fields cause the onset of a glow discharge which eventually results in electrical breakdown and arc-over to the corona wire. To significantly reduce the change of electrical breakdown in the corona charging unit, a suppressor screen is added between the corona wire and the photoconductive member and energized with a relatively low potential. The addition of the suppressor screen not only reduces the chance of arc-over, but also leads to a charge limiting effect so that the charge on the photoconductor is uniformly placed.

With the addition of the suppressor screen a somewhat different mode of operation can be used for the corona charging unit. By applying to the bias rods an intermediate level voltage, the corona wires can be operated at a higher voltage thereby operating at a greater efficiency due to the field gradient being greater between the corona wires and the photoconductive member.

The embodiment of the invention which includes the suppressor screen is shown in FIGS. 5 and 6. In these drawings components which are substantially the same as the embodiment shown in FIGS. 2 and 3 are assigned the same reference number followed by a prime ('). Suppressor screen 42 is added in a plane parallel to the photoconductive member between the plane which includes corona wires 21' and the bias rods 30'. The suppressor screen comprises a plurality of spaced wires attached at the ends to mounting plates 43. Plates 43 are attached to insulator blocks 31' by screws 44, for example, to maintain screen 42 in a plane. Suppressor screen 42 is energized by a suitable voltage V.sub.1 from power supply 45. In a particular embodiment that works well the photoconductive member 2' is maintained at a reference voltage V.sub.R of ground potential and the suppressor screen is maintained at a voltage V.sub.1 of -1 kilovolts. In this case bias rods 30' are energized with a potential V.sub.2 of -2 kilovolts and corona wires 21' are energized with a potential V.sub.3 of -7 kilovolts. This corona device produces a uniform charge on the photoconductive member without damaging the photoconductor surface and also produces an efficient utilization of current to produce charge on the photoconductive surface. This has the practical result of requiring a smaller size corona device for a specified charging current and a smaller capacity power supply to produce the required charge density.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A charging apparatus for use in such apparatus as electrophotographic mechanisms for depositing increments of charge on a charge-bearing surface, said apparatus comprising

a corona discharge wire held in spaced relationship to said surface;

a pair of bias rods each positioned in substantially the same spaced relationship to said surface as said corona discharge wire; means for holding one of said bias rods in fixed position on each side of said wire; and

means to impress an electrical voltage potential on said wire and rods whereby said rods effect a virtual ground plane extending to the surface to enhance the charge flow from said wire to said surface.

2. The charging apparatus according to claim 1 wherein said bias rods are positioned parallel to said corona discharge wire.

3. The charging apparatus according to claim 2 wherein said bias rods and said corona discharge wire are included in a plane substantially parallel to said charge-bearing surface.

4. The charging apparatus according to claim 1 wherein said electrical voltage potential on said corona discharge wire comprises a potential in the range of 6 to 8 kilovolts and the electric voltage potential on said bias rods comprises a range from substantially ground potential to one quarter of the corona wire potential.

5. The charging apparatus according to claim 1 wherein said electrical voltage potential on said corona wire comprises a potential of 7 kilovolts and the electrical voltage potential on said bias rods comprises substantially ground potential.

6. A charging apparatus for use in such apparatus as electrophotographic mechanisms for depositing increments of charge on a charge-bearing surface, said apparatus comprising:

a corona discharge wire held in spaced relationship to said surface;

a pair of bias rods positioned in substantially the same spaced relationship to said surface as said corona discharge wire;

means for holding one of said bias rods in fixed position on each side of said wire;

a suppressor screen positioned in spaced relationship intermediate said wire and said charge-bearing surface; and

means to impress a first relatively high electrical voltage potential on said wire, a second relatively lower electrical voltage potential on said rods and a third electric voltage potential relatively lower than said second potential on said suppressor screen, whereby the charge flow from said wire to said surface is enhanced.

7. The charging apparatus according to claim 6 wherein said bias rods are positioned parallel to said corona discharge wire.

8. The charging apparatus according to claim 7 wherein said bias rods and said corona discharge wire are included in a first plane substantially parallel to said charge-bearing surface and spaced a first distance therefrom; and said suppressor screen is included in a second plane parallel to said first plane and said charge-bearing surface, said second plane being located a closer distance to said charge-bearing surface than said first distance.

9. A charging apparatus for use in such apparatus as electrophotographic mechanisms for depositing increments of charge on a charge-bearing surface, said apparatus comprising:

a plurality of corona discharge wires held in spaced relationship to said surface;

a plurality of bias rods each positioned in substantially the same spaced relationship to said surface as said corona discharge wires; means for holding one of said bias rods in flanking relation on each side of each of said wires; and

means to impress an electrical voltage potential on said wires and rods whereby said rods effect a virtual ground plane extending to the surface to enhance the charge flow from said wires to said surface.