GB2139428A - Corona generating apparatus - Google Patents

Abstract

A miniaturized corona generator that is energy efficient and minimizes ozone emissions is adapted for charging or discharging a receiver surface. The corona element, such as a wire (210), is spaced from 1 mm to 6mm from the surface (12) to be charged, and from 1 mm to 6 mm from the shield (201) which is formed by a groove in a conductive block (250). In one embodiment the corona generator is self-limiting and includes a plurality of corona emitting wires (210, 211, 212, 213) housed in respective conductive shields (201, 202, 203, 204) with the wires being spaced farther from the receiver surface (12) than the wire-to- shield spacing in order to provide self-limiting of surface potential on the receiver surface. <IMAGE>

Description

SPECIFICATION Corona generating apparatus This invention relates to a corona generating apparatus which is particularly, aithough not exclusively, useful for charging or discharging a photosensitive surface in an electrophotographic apparatus.

Many methods and devices have been disclosed in the prior art for producing a uniform electrostatic charge upon a photosensitive member. One such charging device is disclosed by Vyverberg in U. S. Patent 2,836,725, issued May 27, 1958, wherein an electrode in the form of a wire partially surrounded by an electrically grounded conductive shield is placed adjacent to a grounded receiving surface and a high voltage source connected to the wire wherein a corona discharge is produced. The corona discharge, in close proximity to the photosensitive member cause charged ions formed around the corona generator to flow to the grounded photosensitive member surface, and are deposited thereon to raise the surface potential to a relatively high level.

Historically, corona generators have been evaluated at wire to plane spacings of 6.4 mm or greater. This is shown throughout the literature as in Charging Compendium of Xerography by O. A.

Ullrich and L. E. Walkup, December 1963 (K6631) of Battelle Memorial Institute.

Most recent literature still discusses theory and experiments employing wire to plane spacings of 6.4 to 12.7 mm. Also, plane to wire to plane spacing of 6.4 mm is disclosed in a paper presented at the 1976 Electrophotography Conference by B. E. Springett entitled "Threshold Voltages and lonic Mobilities in a Corona Discharge". The mini-corotron of the present invention employs a plane to wire to plane distance of from as small as 1.1 to 3.2 mm.

In the art of xerography, it has been found that consistent reproductive quality can only be maintained when a uniform and constant charge potential is applied to the photoconductive surface. In many automatic machines of this type, a single wire generator, generally referred to as a "corotron" is employed. Generally, the efficiency of the corotron is dependent on many factors including the gap distance between the wire and the photosensitive member surface, the nature of the generating wire material, the diameter of the wire and other physical features thereof and the amount of energy supplied to the corona emitter.

Heretofore, these corona devices required large power supplies to meet high current and voltage requirements, were costly and took up a large area of machine space. Such units are designed for use with thin (0.09 mm) wire or wires located approximately 12.7 mm from a grounded photosensitive member or shield. Typically, corona wire voltages for charging are near 7KV with a bare plate receiver current of 66 juA for a 38 cm long wire (1.77 yA/cm). The cross sectional area of such a unit is near 6.5 cm2. As Neblette's Handbook of Photography and Reprography states in the Seventh Edition published in 1977, page 348, "In practical corotron devices the wires are maintained at a potential above 6000V, usually charging the photoconductor surface to several hundred volts".These units were adequate in the past, but with present need for copiers that emit less ozone, use less energy, are less costly and take up less space, changes in corona generating devices are required. This was thought to be impossible because conventional thinking on corona generators and experience had taught that reducing the cavity partly surrounding the corotron and bringing the corotron closer to a receiver surface would cause arcing to occur and burn out the wire corotron. Also, it was thought that the use of long thin wire (0.038 mm) and small radius cavities would cause singing and sagging in the wires.

According to the present invention, there is provided a corona generating apparatus comprising an elongate corona element, a conductive shield, and a surface to be charged, the conductive shield comprising a longitudinal groove extending parallel with the corona element, and the corona element being located such that its distance from the surface to be charged and its distance from the groove are each between 1 mm and 6 mm.

Despite the conventional teachings to the contrary, the present invention includes a small mini-corona generating device that is energy efficient, useful in confined spaces and charges over a narrow region instead of a spread out area.

In another embodiment, a miniaturized selflimiting corona generator is disclosed.

Accordingly, in one aspect of the present invention, there is provided a miniaturized apparatus for charging a photosensitive surface which includes a corona generating wire, a source of electrical energy being operatively connected to the generating wire to cause the wire to emit a corona discharge, the wire generator being constructed with the standard 0.089 mm wire, spaced approximately 1.6 mm from the photosensitive surface and requiring only a cigarette pack sized power supply.

In another aspect of the invention, it has been found that by reducing voltage and, wire diameter and by increasing impedance, arcing is suppressed to the point where a 0.038 mm wire can be located 1.1 mm from a shield and produce corona without arcing.

In yet another aspect of the present invention, there is provided an apparatus for discharging a dielectric body as the body is moved relative to said apparatus. Electrical discharge from the apparatus causes the dielectric body to be discharged as it passes the apparatus.

In another embodiment of the present invention, there is provided a miniaturized apparatus for charging a photosensitive surface with self limiting control of receptor potential which includes a corona generating wire, a source of electrical energy being operatively connected to the generating wire to cause the wire to emit a corona discharge, the wire generator being constructed with a 0.038 mm to 0.051 mm diameter wire, spaced approximately 1.6 mm from a biased conductive shield and approximately 6.4 mm from the photosensitive surface and requiring only a miniature power supply. The mini-corotron provides a lower potential source with scorotron like characteristics of limiting surface potential.

The foregoing and other features of the instant invention will be more apparent from a further reading of the specification and claims and from the drawings in which: Figure 1 is a schematic elevational view of an electrophotographic printing machine incorporating the features of the present invention.

Figure 2 is an enlarged partial side view of the mini-corona unit that comprises the present invention.

Figure 3 is a chart showing improved bare plate current obtained with less potential with use of the device of the present invention.

Figure 4 is an enlarged partial side view of the self limiting mini-corona unit that comprises an aspect of the present invention.

For a general understanding of an electrophotographic printing machine in which the features of the present invention may be incorporated, reference is made to Figure 1 which depicts schematically the various components thereof. Hereinafter, like reference numerals will be employed throughout to designate identical elements. Although the apparatus of the present invention is disclosed as a means for charging a photosensitive member or for discharging a dielectric body, it should be understood that the invention could be used in an electrophotographic environment as a transfer device also.

Since the practice of electrophotographic printing is well known in the art, the various processing stations for producing a copy of an original document are represented in Figure 1 schematically. Each process station will be briefly described hereinafter.

As in all electrophotographic printing machines of the type illustrated, a drum 10 having a photoconductive surface 12 entrained about and secured to the exterior circumferential surface of a conductive substrate is rotated in the direction of arrow 14 through the various processing stations.

By way of example, photoconductive surface 12 may be made from selenium of the type described in U.S. Patent 2,970,906 issued to Bixby in 1961.

A suitable conductive substrate is made from aluminum.

Initially, drum 10 rotates a portion of photoconductive surface 1 2 through charging station A. Charging station A employs a corona generating device in accordance with the present invention, indicated generally by the reference numeral 16, to charge photoconductive surface 12 to a reiatively high substantially uniform potential.

Thereafter drum 10 rotates the charged portion of photoconductive surface 1 2 to exposure station B. Exposure station B includes an exposure mechanism, indicated generally by the reference numeral 18, having a stationary, transparent platen, such as a glass plate or the like for supporting an original document thereon. Lamps illuminate the original document. Scanning of the original document is achieved by oscillating a mirror in a timed relationship with the movement of drum 10 or by translating the lamps and lens across the original document so as to create incremental light images which are projected through an apertured slit onto the charged portion of photoconductive surface 12.Irradiation of the charged portion of photoconductive surface 12 records an electrostatic latent image corresponding to the information areas contained within the original document.

Drum 10 rotates the electrostatic latent image recorded on photoconductive surface 1 2 to development station C. Development station C includes a developer unit, indicated generally by the reference numeral 20, having a housing with a supply of developer mix contained therein. The developer mix comprises carrier granules with toner particles adhering triboelectrically thereto.

Preferably, the carrier granules are formed from a magnetic material with the toner particles being made from a heat settable plastic. Developer unit 20 is preferably a magnetic brush development system. A system of this type moves the developer mix through a directional flux field to form a brush thereof. The electrostatic latent image recorded on photoconductive surface 12 is developed by bringing the brush of developer mix into contact therewith. In this manner, the toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image on photoconductive surface 1 2.

With continued reference to Figure 1, a copy sheet is advanced by sheet feeding apparatus 35 to transfer station D. Sheet feed apparatus 35 advances successive copy sheets to forwarding registration rollers 23 and 27. Forwarding registration roller 23 is driven conventionally by a motor (not shown) in the direction of arrow 38 thereby also rotating idler roller 27 which is in contact therewith in the direction of arrow 39. In operation, feed device 35 operates to advance the upeprmost substrate or sheet from stack 30 into registration rollers 23 and 27 and against registration fingers 24. Fingers 24 are actuated by conventional means in timed relation to an image on drum 12 such that the sheet resting against the fingers is forwarded toward the drum in synchronism with the image on the drum. A conventional registration finger control system is shown in U.S. Patent 3,902,715 which is incorporated herein by reference to the extent necessary to practice this invention. After the sheet is released by finger 24, it is advanced through a chute formed by guides 28 and 40 to transfer station D.

Continuing now with the various processing stations, transfer station D includes a corona generating device 42 which is the same as corona device 1 6 and applies a spray of ions to the back side of the copy sheet. This attracts the toner powder image from photoconductive surface 12 to the copy sheet.

After transfer of the toner powder image to the copy sheet, the sheet is advanced by endless belt conveyor 44, in the direction of arrow 43, to fusing station E.

Fusing station E includes a fuser assembly indicated generally by the reference numeral 46.

Fuser assembly 46 includes a fuser roll 48 and a backup roll 49 defining a nip therebetween through which the copy sheet passes. After the fusing process is completed, the copy sheet is advanced by conventional rollers 52 to catch tray 54.

Invariably, after the copy sheet is separated from photoconductive surface 12, some residual toner particles remain adhering thereto. Those toner particles are removed from photoconductive surface 1 2 at cleaning station F. Cleaning station F includes a corona generating device (not shown) adapted to neutralize the remaining electrostatic charge on photoconductive surface 12 and that of the residual toner particles. The neutralized toner particles are then cleaned from photoconductive surface 1 2 by a rotatably mounted fibrous brush (not shown) in contact therewith. Subsequent to cleaning, a discharge lamp (not shown) floods photoconductive surface 1 2 with light to dissipate any residual electrostatic charge remaining thereon prior to the charging thereof for the next successive imaging cycle.

It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an electrophotographic printing machine. Referring now to the subject matter of the present invention, Figure 2 depicts the corona generating device 1 6 in greater detail. Corona generating units 1 6 and 42 are constructed similarly except for particularized dimensions that will be explained hereinafter.

Even though corona generator 1 6 is shown having one corotron, a plurality of corotrons could be employed if desired.

Referring now specifically to Figure 2, the detailed structure and operation of an aspect of the present invention will be described. The corona generating unit, generally referred to as 16, is positioned above the photosensitive surface 12 and is arranged to deposit an electrical charge thereon as the surface 1 2 moves in a clockwise direction. The corona unit includes a block member about 6 mm in height that has a shield member 1 7 which is curvilinear in shape and encloses a substantial portion of corona generator wire or coronode 1 9. The shield is preferably made up of an electrically conductive material that is placed at ground potential.A slit or opening 13 is formed in the bottom of the shield opposite the moving photosensitive member and provides a path by which a flow of ions discharged by the generator are directed towards and deposited upon photosensitive surface 12. For further details regarding the structure of a conventional corona unit, reference is had to the disclosure in U. S.

Patent 2,836,725 issued to Vyverberg in 1 958.

The corona generating wire 1 9 is connected by suitable means such as an electrical connector to a power high potential source or power supply 90.

This power supply gives off a much lower voltage than conventional corona generator power supplies and as a result aids in reducing arcing.

Also, a resistor or impedance device 95 is provided in line to coronode 1 9 as a current limiting means in order to further reduce the current to the coronode and reduce arcing possibilities. Thus, only the amount of current necessary to produce the desired corona is allowed to the wire. If desired, edge-on metal foils or metalized plastics could be used as an alternative corona source. The corona wire utilized in the present embodiment is connected directly to the positive terminal of the power source whereby positive ion discharge is placed on the photosensitive surface. The wire could be made from stainless steel, platinum or oxidized tungsten or the like. However, it should be clear that an opposite arrangement can be employed to obtain negative discharge. Conventionally, as in U. S.

Patent 2,836,725, corona generators have been designed with a cross sectional area of 6.5 cm2 and use thin wire (0.089 mm) located about 12 mm from a shield surrounding the wire and 12 mm from a receiver surface. Thinner wires could be used if desired. Large power supplies for high charging voltages of near 7KV with a 38 cm long wire are required for such devices in order to get a current of 66 A or 1.77 A/cm. In accordance with the present invention the corona generator 16 is fashioned with the standard 0.089 mm wire corotron spaced 1.6 mm from receiver surface 1 2 and requires only a cigarette pack sized power supply to get the similar ion output as conventional corona generators.A surface current of 300 yA was obtained with the present invention from a 21.6 cm corona wire (35.3 jumAlin) at (+)2.8KV. It should be understood that the wire could extend to any length desired, such as 38 cm. Increasing the spacing to 3.2 mm between the corona wire and surface 1 2 results in 50yA (2.32,uAlcm) of current. Additional current increases result when thinner corona wire is employed. For example, with 0.038 mm oxidized tungsten wire as the corotron, a spacing of 1.6 mm produces 440 yA (20.4 uAlcm) and 3.2 mm produces 130 yA (6.02 uAlcm). An applied corona potential of 2.7KV yielded the above results. Near 2.1 KV will result in order of magnitude reduction in current flow. Thus, this small and compact mini-corona generator can produce typical receiver surface currents at low KV with a very small power supply.

Mini-corona generator 1 6 has several advantages over conventional corona generators.

For example, it is useful in confined spaces, it can charge over a sharp radius, is useful at the transfer station of a copier as a sheet stripping device, charges over a narrow region, i.e., it does not spread the ions out, requires lower current and voltages and give off less ozone than conventional devices since a lower KV is required for efficient operation. Corona generator 1 6 comprises corotron 19 that is about 21.6 cm long and located at a distance (b) of approximately 1.0 to 6.4 mm away from both shield 17 and receiver surface 12, but preferable distance (b) is 2.5 cm.

Preferably, the height (c) of shield housing 1 7 is 6.4 mm while the diameter (d) of the semi-circular groove in the shield is about 4.8 mm.

In Figure 3, a chart is shown where proper voltages and distances can be determined for a given size wire. The chart shows bare plate current vs corona wire potential for various diameter corona wires spaced at 1.5 mm and 3.2 mm from the bare plate. The wire to shield spacing is 3.2 mm with wire diameters of 0.038 mm, 0.064 mm, and 0.089 mm shown in solid lines. The dashed lines represent data derived from spacing the wires 3.2 mm from the bare plate. As shown in the chart, in contrast to conventional corotrons that require 7 KV with a bare plate current of 66,uA for a 0.089 mm diameter wire 38 cm in length (1.77 juA/cm), the present mini-corotron device can produce typical bare plate currents at low coronode potential.For example, a coronode potential of 2.12KV provides a bare plate current of 1.97 yA/cm from a coronode of 0.038 mm in diameter spaced 1.5 mm from the bare plate.

The mini-corona generator of the present invention can also be used as a discharging unit as shown at 70 in Figure 1. Such a unit is preferably constructed similarly to unit 1 6 and includes a conductive housing having a 4.8 mm radius groove, about 12 mm longer than the width of the photosensitive member, with a 0.038 mm oxidized tungsten corotron wire running down the center axis of the groove. A 2.5 mm distance from the wire to a surface to be discharged, such as, a sheet on conveyor 44 is preferable. The unit is energized from a suitable power source to a voltage sufficient to discharge a dielectric body or some other body passing under it. This unit uses an AC voltage and can be used for discharging at preclean or detacking stations or simply to discharge a photoreceptor to zero.The 4.8 mm groove radius appears to be a minimum radius since a 3.2 mm radius causes arcing before corona. It should be understood that a mini-corona generating device in accordance with the present invention can be fashioned in a common block so that a receiver may be discharged and then charged to an appropriate level within a confined space.

The mini-corotron device of the present invention is usable as a hand held charging or discharging device. For example, the device could take the shape of a flashlight like handle housing one or two 12 volt D size batteries to supply power. Connected to the handle at one end of a high voltage housing which includes a potentiometer to allow the selection of the required voltage, i.e., depending on what voltage is desired, either DC positive or negative voltages are available through the use of a simple double pole, double throw switch to select polarity. The coronode housed in a shield is electrically connected to the high voltage housing. Spacing between the hand held charging unit and a photosensitive member is accomplished by proper fastening of end blocks holding the coronode to the shield.

Referring now to Figure 4, the detailed structure and operation of another aspect of the present invention will be described. The self limiting corona generating unit, generally referred to as 200, could be positioned above the photosensitive surface 1 2 and arranged to deposit an electrical charge thereon as the surface 1 2 moves in a clockwise direction. The corona unit includes a series of shield members 201 through 204 which are curvilinear in shape and enclose a substantial portion of corona generator wires 210 through 213 respectively. The shields are preferably made from an electrically conductive material that is biased with respect to ground.A slit or opening is formed in the bottom of each shield opposite the moving photosensitive member and provides a path by which a flow of ions discharged by the generator are directed towards and deposited upon photosensitive surface 12.

The corona generating wires are connected by suitable means such as an electrical connector to a high potential source or power supply 220. If desired, edge-on metal foils or metalized plastics could be used as alternative corona sources. The corona wire utilized in the present embodiment is connected directly to the positive terminal of the power source whereby positive ion discharge is placed on the photosensitive surface. However, it should be clear that an opposite polarity can be employed to obtain negative discharge.

Conventionally, as in U. S. Patent 2,836,725, corona generators have been designed with a cross sectional area of 6.5 cm2 and use thin wire (0.089 mm) located about 12 mm from a shield surrounding the wire and 12 mm from a receiver surface. Large power supplies for high charging voltages of near 7KV with a 38 cm long wire are required for such devices in order to get a current of 22 ssuA or 0.59 yA/cm. In accordance with this embodiment, the corona generator 17 is fashioned with 0.038 mm diameter wire coronodes spaced at a distance b of approximately 6.4 mm from receiver surface 12 and requires only a small sized power supply to get the similar ion output as conventional corona generators. The preferred coronode is a corosion resistant wire of 0.038 mm diameter.A coronode of up to 0.089 mm diameter can be used but every 0.025 mm increase in diameter requires an increase of about 200 volts to achieve the same corona output current. These high voltages make suppression of arcing a greater problem.

Mini-corona generator 200 has several advantages over conventional corona generators.

For example, it is useful in confined spaces, it can charge over a sharp radius, is useful at the transfer station of a copier as a sheet stripping device, charges over a narrow region, i.e., it does not spread the ions out, requires lower current and voltages and gives off less ozone than conventional devices since a lower KV is required for efficient operation and provides control of surface potential on the receptor.

Corona generator 200 enables positive corona generation at about 2.6KV and comprises a block housing 250 that has a series of semi-cylindricai shields 201, 202, 203 and 204 therein. The shields are about 8-21.6cm long and are connected to an adjustable electrical energy source 260 that is adapted to bias the shields. The shields have a radius of about 1.6 mm and have corona wires 210,211,212 and 213 positioned adjacent to and spaced an equal distance from respective shield surfaces while being spaced a distance of approximately 6.4 mm from photosensitive member 12. Resistors 270, 271, 272 and 273 are provided in the wires as a current limiting means in order to further reduce the current to the wires and thereby reduce arcing possibilities. The wires are biased by battery 220 and are adapted to apply an ion charge to the photosensitive surface that is self limiting. For example, if the shields are biased to 1 KV and the wires are biased to 4KV the surface 12 of the photosensitive member will receive a uniform charge of 1400KV. To accomplish this control the plurality of shields are spaced much further from the receptor surface 12 than the wire-to-wire shield spacing. Because the surface area of the wires is very small compared to the area of the shields as seen by the receptor, and because the wires are much closer to the shields, the ions between the charge unit and receptor will respond primarily to the fields generated by the potential difference between the shield and the charge receptor. This limits the final surface potential of the charge receptor to a little more than the potential of the shield.

Claims (13)

1. A corona generating apparatus comprising an elongate corona element, a conductive shield, a surface to be charged, the conductive shield comprising a longitudinal groove extending parallel with the corona element, and the corona element being located such that its distance from the surface to be charged and its distance from the groove are each between 1 mm and 6 mm.

2. The apparatus of Claim 1 wherein the distances of the corona element from said surface and from said groove are substantially equal.

3. The apparatus of Claim 2 wherein said distances are each about 2.5 mm.

4. The apparatus of any one of Claims 1 to 3, wherein said groove is semi-circular in shape with a diameter of between 2.4 and 6.4 mm and said shield is in block form having a height of 6.4 mm.

5. The apparatus of any one of Claims 1 to 3, wherein the diameter of said groove in said shield is 4.8 mm.

6. The apparatus of any one of Claims 1 to 5, wherein the voltage to said corona means is approximately 2.9KV.

7. The apparatus of any one of Claims 1 to 6, wherein said corona element is a wire.

8. The apparatus of Claim 7, wherein said wire is tungsten.

9. The apparatus of any one of Claims 1 to 6, wherein said corona element is one or more edgeon metal foils.

10. The apparatus of any one of Claims 1 to 6, wherein said corona element comprises metalized plastics.

11. A self limiting corona generating apparatus according to any one of claims 1 to 10 comprising a plurality of said corona elements associated respectively with a corresponding number of said grooves.

12. The apparatus of claim 11 wherein the distance of each corona element from said surface is greater than its distance from its associated shield, so that ions from the corona elements that travel between the shields and the surface will respond primarily to the fields generated by the potential difference between the shields and the surface to thereby limit the surface potential of the surface.

13. A corona generating apparatus substantially as herein described with reference to the accompanying drawings.