Corona generator cleaning apparatus

Abstract

A corona generating apparatus in which the electrode and grid are cleaned to remove particle contamination therefrom. The cleaning device flexes and scrapes the grid while substantially simultaneously therewith wiping the electrode to remove contaminants adhering to either of the foregoing. This maintains the electrical characteristics of the corona generating apparatus substantially constant eliminating any variations introduced by the accumulation of contaminating particles on the grid or electrode. The foregoing abstract is neither intended to define the invention disclosed in the specification, nor is it intended to be limiting as to the scope of the invention in any way.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to an electrophotographic printing machine, and more particularly concerns a corona generating apparatus adapted to be cleaned prior to charging a photoconductive surface to a substantially uniform potential.

Typical electrophotographic printing machines include a photosensitive surface which is electrostatically charged substantially uniformly thereover. The surface is then irradiated with a light pattern of an image to be reproduced. This light image discharges the electrostatic charge in the irradiated areas of the photoconductive surface. As a consequence thereof, the remaining charge on the photoconductive surface forms an electrostatic charge pattern in image configuration, i.e., an electrostatic latent image. The electrostatic latent image is then developed by contacting it with finely divided electrostatically attractable material, i.e. toner particles. The toner particles adhere electrostatically to the photoconductive surface in a pattern corresponding to the latent image recorded thereon. Thereafter, the developed image is transferred to a sheet of support material, e.g. paper or any other suitable material, such as a thermoplastic sheet. The powder image transferred to the sheet of support material is subsequently suitably affixed thereto forming a permanent print thereon. This process is more fully disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1947.

Multi-color electrophotographic printing is similar to the heretofore discussed black and white process, however, each image represents a particular color contained in the original document. Thus, multi-color electrophotographic printing employs a plurality of single color images each adapted to be reproduced with its correspondingly colored toner particles. Color reproduction requires a plurality of differently colored toner powder images which are superimposed onto the sheet of support material to create a color copy therefrom. The formation of the color copy from a colored original requires substantially more toner particles than has heretofore been employed in the creation of black and white copies. Therefore, it is evident that there is an increased risk of contamination from dust and toner particles.

A uniform potential is applied to the photoconductive surface by a corona generating device. A typical corona device is described in U.S. Pat. No. 2,836,725 issued by Vyverberg in 1958. Corona generating devices generally include a corona discharge electrode surrounded by a conductive shield. The corona discharge electrode is a coronode wire adapted to be supplied with a DC voltage of sufficient magnitude to create a corona current flow from the electrode to the photoconductive surface closely spaced thereto. Another type of corona generating apparatus particularly suitable for use in multi-color electrophotographic printing is disclosed in U.S. Pat. No. 2,778,946 issued to Mayo in 1957. An apparatus of this type includes a corona discharge electrode such as a coronode wire, surrounded by a conductive shield with a grid interposed between the coronode wire and the photoconductive surface. The shield is normally at ground potential and the grid is operated at some preselected intermediate potential between the discharge electrode voltage and the ground voltage. This enables the grid to control the charge applied to the photoconductive surface.

Although the corona generating devices heretofore described may be advantageously utilized to substantially uniformly charge a photoconductive surface, these devices are also suitably adapted for various other applications. For example, they can electrostatically transfer a powder image from a photoconductive surface to a sheet of support material as well as remove background. Furthermore, corona generating devices can neutralize the charge on toner particles adhering to the photoconductive surface, after the transfer of the powder image to the support material, facilitating the removal thereof. However, a disadvantage of the foregoing type of device is its sensitivity to the accumulation of dust and toner particles. Dust or toner particles adhering to the coronode wires will decrease the corona current generated therefrom as the density of particle accumulation increases. Contrawise, dust or toner particles adhering to the grid wires will increase the corona current generated therefrom as the density of particle accumulation increases. It should be noted, however, that the increase in current produced by particle accumulation on the grid wires is not inversely proportional to the decrease in current produced by particle accumulation on the coronode wires. Thus, the performance of the corona generating device is sensitive to dust and toner particles and the corona current produced thereby is a variable dependent upon the dust and toner particle accumulation.

Hereinbefore, various prior art devices have been developed for cleaning corona generating devices. By way of example, IBM Technical Disclosure Bulletin Volume 11, No. 8 of January 1969 describes cleaning pads engaging the coronode wire of a corona generating device. The cleaning pads are reciprocated along the length of the coronode wire to remove dust and toner particles accumulated thereon. Similarly, copending application Ser. No. 235,306, filed in 1972, now abandoned, describes cleaning pads surrounding the coronode wire and contacting the interior surface of the shield surrounding the coronode wire. Once again, the cleaning pads are reciprocated along the length of the coronode wire and shield to remove dust and toner particles accumulated thereon. In addition, co-pending application Ser. No. 307,250 filed in 1972 describes cleaning pads adapted to be reciprocated along the coronode wire and grid wires so as to remove particle accumulation thereon. Moreover, co-pending application Ser. No. 380,302 filed in 1973, now U.S. Pat. No. 3,842,273, describes scraping the coronode wire and/or shield to remove particle accumulations therefrom. Various other techniques have been devised to reduce or remove particle contamination. These techniques are exemplified by U.S. Pat. Nos. 3,324,291, 3,339,069, 3,382,360, 3,471,965, 3,483,372, 3,496,352 and 3,499,143. However, none of the foregoing prior art devices describe flexing and scraping of the grid wires while wiping the coronode wires remove particles adhering thereto.

Accordingly, it is a primary object of the present invention to improve cleaning of a corona generating apparatus to remove particle accumulation therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which:

FIG. 1 is a schematic perspective view depicting a color electrophotographic printing machine employing the corona generating apparatus of the present invention therein;

FIG. 2 is a perspective view of the corona generating apparatus used in the FIG. 1 printing machine;

FIG. 3 is a fragmentary side elevational view, partially in section, showing scraping of the grid wires; and

FIG. 4 is a fragmentary front elevational view, partially in section, showing scraping of the grid wires and wiping the coronode wires.

While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims.

SUMMARY OF THE INVENTION

Briefly stated, and in accordance with the present invention, there is provided an apparatus for cleaning a corona generator employed in an electrophotographic printing machine.

This is achieved in the present instance by a corona generating apparatus having a shield, grid means, electrode means and cleaning means. The shield defines an elongated open ended chamber adapted to house the grid means in the open end thereof. The grid means extends in a substantially longitudinal direction along the length of the shield with the electrode interposed therebetween. As the grid means is flexed, contaminating particles are scraped therefrom by the cleaning means. In addition, the cleaning means wipes the electrode means to remove particles therefrom. This enables the corona generating apparatus to be maintained substantially free from contaminating particles.

DETAILED DESCRIPTION OF THE INVENTION

For a general understanding of the disclosed electrophotographic printing machine in which the present invention may be incorporated, continued reference is had to the drawings wherein the like reference numerals have been used throughout to designate like elements. FIG. 1 schematically illustrates the various components of a color electrophotographic printing machine arranged to produce copies in color. Although the corona generating apparatus of the present invention is particularly well adapted for use in an electrophotographic printing machine, it should become evident from the following discussion that it is equally well suited for use in a wide variety of electrostatographic printing machines and is not necessarily limited to the particular embodiment shown herein.

Referring now to FIG. 1, there is shown a printing machine embodying a photoconductive member having a drum 10 mounted rotatably on a shaft (not shown) of the printing machine. Photoconductive surface 12 is entrained about and secured to drum 10. As drum 10 rotates in the direction of arrow 14, photoconductive surface 12 passes sequentially through a series of processing stations. A timing disc (not shown) is positioned in the region of one end of the shaft supporting drum 10. The timing disc rotates with drum 10 and triggers the machine logic to coordinate the sequence of events at the various processing stations through which photoconductive surface 12 passes.

Drum 10 initially rotates photoconductive surface 12 through charging station A. The corona generating apparatus of the present invention, indicated generally at 16, is positioned at charging station A. Corona generating apparatus 16 extends in a generally transverse direction across photoconductive surface 12. This enables corona generating apparatus 16 to charge photoconductive surface 12 to a relatively high substantially uniform potential. Corona generating apparatus 16 will be described in greater detail hereinafter with reference to FIGS. 2 through 4, inclusive.

Continuing now with the description of the various processing stations through which photoconductive surface 12 rotates, drum 10 rotates charged photoconductive surface 12 to exposure station B. At exposure station B, a color filtered light image of the original document is projected onto charged photoconductive surface 12. Preferably, exposure station B includes a moving lens system, generally designated by the reference numeral 18 and a color filter mechanism shown generally at 20. A suitable moving lens system is described in U.S. Pat. No. 3,062,108 issued to Mayo in 1962. As shown in FIG. 1, an original document 22, such as a paper, book or the like is disposed face down on transparent viewing platen 24. Lamp assembly 26, lens system 18 and filter mechanism 20 move in timed relation with drum 10 to scan successive incremental areas of original document 22 disposed upon platen 24. This enables a flowing light image of original document 22 to be projected onto charged photoconductive surface 12. During exposure, filter mechanism 20 interposes selected color filters into the path of the flowing light image created by lens 18. The color filter operates on the light ray to produce a single color light image which records an electrostatic latent image on photoconductive surface 12 corresponding to a preselected spectral region of the electromagnetic wave spectrum, hereinafter referred to as a single color electrostatic latent image.

The single color electrostatic latent image recorded on photoconductive surface 12 is next rotated to development station C. Development station C includes three individual developer units, generally designated by the reference numerals 28, 30 and 32, respectively. A suitable development station utilizing a plurality of such developer units is described in co-pending application Ser. No. 255,259 filed in 1972. Preferably, the developer units are all of a magnetic brush type. A typical magnetic brush system employs a magnetizable developer mix of carrier granules and toner particles. The developer mix is continually brought through a directional flux field to form a brush thereof. As drum 10 rotates, photoconductive surface 12 passes through the brush of developer mix, and the electrostatic latent image recorded thereon contacts the developer mix. Each of the respective developer units 28, 30 and 32, contain discretely colored toner particles corresponding to the complement of the spectral region of the wave length of light transmitted through filter 20. For example, a green filtered electrostatic latent image is rendered visible by depositing green absorbing magenta toner particles thereon, blue and red latent images are developed with yellow and cyan toner particles, respectively.

After the electrostatic latent image recorded on photoconductive surface 12 is developed, drum 10 rotates to transfer station D. At transfer station D, the toner powder image adhering electrostatically to photoconductive surface 12 is transferred to a sheet of final support material 34. Final support material 34 may be, amongst others, plain paper or a sheet of thermoplastic material. Sheet 34 is secured releasably to a transfer roll, shown generally at 36. As indicated in FIG. 1, transfer roll 36 rotates in the direction of arrow 38 to move support material 34 therewith in a recirculating path. An electrical bias is applied to transfer roll 36 having sufficient magnitude and the proper polarity to attract electrostatically toner particles from the electrostatic latent image recorded on photoconductive surface 12 to support material 34. A suitable electrically biased transfer roll is described in U.S. Pat. No. 3,612,677 issued to Langdon et al. in 1971. Transfer roll 36 rotates in synchronism with drum 10 (in this case at substantially the same angular velocity.) Inasmuch as support material 34 is secured releasably thereon for movement in a recirculating path therewith, successive toner powder images may be transferred from photoconductive surface 12 to support material 34 in superimposed registration with one another.

Prior to proceeding with the remaining processing stations through which photoconductive surface 12 passes as drum 10 rotates in the direction of arrow 14, a brief description will be given of the sheet feeding arrangement.

Support material 34 is advanced from stack 40 thereof. Stack 40 is disposed upon tray 42. Feed roll 44 in operative communication with retard roll 46 advances and separates the uppermost sheet from stack 40. The advancing sheet moves into chute 48 which directs the sheet between the nip of register rolls 50. Thereafter, gripper fingers 52 mounted on transfer roll 36 secure releasably thereto support material 34 for movement therewith in a recirculating path.

Returning now to the printing sequence, with continued reference to FIG. 1, gripper fingers 52 release sheet 34 and stripper bar 54 separates sheet 34 from transfer roll 36. Sheet 34 is, thereafter, advanced on endless belt conveyor 56 to fixing station E.

At fixing station E, a suitable fuser, indicated generally at 58, applies sufficient heat to the toner powder images transferred to sheet 34 so as to permanently affix them thereto. A suitable fuser is described in U.S. Pat. No. 3,498,592 issued to Moser et al. in 1970. After the fixing process, sheet 34 is advanced by endless belt conveyors 60 and 62 to catch tray 64 for subsequent removal by the machine operator.

Although a preponderance of the toner particles are transferred to support material 34, invariably some residual toner particles remain on photoconductive surface 12 after the transfer of the powder image to support material 34. The residual toner particles are removed from photoconductive surface 12 of drum 10 as it passes through cleaning station F. Here, the residual toner particles are first brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining on the residual toner particles and photoconductive surface 12. The neutralized toner particles are then removed from photoconductive surface 12 by a rotatably mounted fibrous brush 65 in contact therewith. A suitable brush cleaning device is described in U.S. Pat. No. 3,590,412 issued to Gerbasi in 1971.

It is believed that the foregoing description is sufficient for purposes of the present application to show the general operation of a multi-color electrophotographic printing machine embodying the teachings of the present invention therein.

Referring now to the specific subject matter of the present invention, FIG. 2 depicts corona generating apparatus 16 in detail. Corona generating apparatus 16 includes an elongated conductive shield 66 defining an open ended chamber opposed from and closely spaced to photoconductive surface 12. Shield 66 is a U-shaped conductive housing which is preferably made from an aluminum extrusion. Grid means or a plurality of substantially parallel spaced, fine conductive wires 68 (in this case 10) extend in a longitudinal direction from one end of shield 66 to the other end thereof and across about threequarters of the open end of the chamber therein. An insulating plate 74 is affixed permanently to either end of shield 66 by suitable means (not shown) e.g. fasteners or adhesive. Grid wires 68 are preferably formed from a single wire attached to fasteners 70 and entrained about uprights 72 on insulating plate 74. Interposed between grid wires 68 and back wall 76 of shield 66 is corona discharge electrode or a pair of coronode wires 78 and 80, respectively. Coronode wires 78 and 80 are suitably secured to insulating plate 74, preferably by fasteners (not shown). Both grid wires 68 and coronode wires 78 and 80, respectively, are preferably made from a conductive material, for example, platinum. Insulating plate 74 is, preferably made from a dielectric material such as a glass alkyd, Lucite Plexiglass, Lexan or the like. As depicted in FIG. 2, coronode wire 78 is positioned in the portion of the chamber of shield 66 that is not covered by grid wires 68, i.e. grid wires 68 do not extend over this portion of the open end of shield 66. As previously mentioned, grid wires 68 extend only across about three-quarters of the open end of shield 66. This permits rapid and roughly controlled charging of photoconductive surface 12 in the lead section or the portion of shield 66 not covered by grid wires 68. Slow and well controlled charging is obtained over the trailing section or the portion of shield 66 covered by grid wires 68. Energizing means or a suitable high voltage source (not shown) excites coronode wires 78 and 80 to a voltage preferably ranging from about 6000 volts to about 8000 volts, the coronode wire current ranging from about 200 to about 500 micro amperes. Field regulating means or a low voltage source excites grid wires 68 to, preferably, about 925 volts. In operation, drum 10 rotates in the direction of arrow 14 permitting coronode wire 78, i.e., the coronode wire positioned in the one-quarter of shield 66 not covered by grid wires 68, to roughly charge photoconductive surface 12 to a substantially uniform potential. Thereafter, slow and well-controlled charging of photoconductive surface 12 is obtained from coronode wire 80 and grid wires 68. Grid wires 68 have the bias voltage thereof set at about the desired final voltage of photoconductive surface 12. In this manner, the final charging is field sensitive. For example, as photoconductive surface 12 moves under grid wires 68, it acquires a charge which increases up to about the voltage of grid wire 68. Further charging thereof is suppressed by grid wires 68. That is, when photoconductive surface 12 is charged to a voltage substantially the same as that of grid wire 68, most of the corona current under grid wire 68 is conducted thereto rather than to photoconductive surface 12.

In order to reduce the sensitivity of corona generating apparatus 16 to dirt, cleaning means 82 is provided therein. Deposits of toner particles and dust collected on coronode wires 78 and 80, respectively, and grid wires 68 are removed by cleaning means 82. Cleaning means 82 includes a scraping member depicted generally by the reference numeral 84 and a wiping member depicted generally by reference numeral 85. Scraping member 84 and wiping member 85 will be described hereinafter in greater detail with reference to FIGS. 3 and 4. Moving means, indicated generally by the reference numeral 86, is associated with cleaning means 82. Moving means 86 comprises an elongated rod 88 attached to cleaning means 82. Rod 88 extends through insulating plate 74 affixed to the end portions of shield 66. Preferably, rod 88 extends longitudinally through the center of shield 66. Handle 90 is attached to rod 88 and is external to one end portion of shield 66 in the region of insulating plate 74 permitting an operator to grasp handle 90 to reciprocate rod 88 and, in turn, move cleaning means 82. Guide means or block 92 is, preferably, integral with insulating plate 74 and affixed to one end portion of shield 66 with a channel 94 therein adapted to receive elongated rod 88. Channel 94 is arranged to guide rod 88 in the longitudinal movement thereof. In this manner, cleaning means 82 is reciprocated in the direction of arrow 96 along grid wires 68 and coronode wires 78 and 80, respectively, so as to remove contaminating particles adhering thereto.

Turning now to FIG. 3, there is shown the detailed structural configuration of scraping member 84. As shown therein, grid wires 68 pass through slots 98 in upper housing 100 which, in turn, is secured to lower housing 102. Upper housing 100 is U-shaped and slidably mounted on lower housing 102. Slots 98 in upper housing 100 are arranged to provide a simple support for grid wires 68 at either end of housing 100. Upper surface 104 of lower housing 102 is positioned to engage grid wires 68 in substantially the mid-region of upper housing 100. Upper surface 104 deflects grid wires 68 in an upwardly direction as it moves therealong. Thus, as scraping member 84 traverses grid wires 68, grid wires 68 are repeatedly flexed. This tends to brake the surface of the contaminating particles baked onto grid wires 68 so as to loosen them therefrom. This aids the cleaning action in that the loosened contaminating particles are more readily scraped therefrom. Upper surface 104 of lower housing 102 is beveled forming a knife edge which engages the interior surface of grid wires 68 opposed from coronode wires 78 and 80. As scraper member 84 traverses grid wires 68, contaminating particles adhering thereto are loosened and scraped therefrom.

Referring now to FIG. 4, there is shown once again cleaning means 82 in detail. As shown therein, upper housing 100 includes a plurality of slots 98 therein. The slots encompass grid wires 68 to provide a support therefore as upper surface 104 traverses grid wire 68 scraping particle contamination therefrom. Wiper 106 is located in the lower region of upper housing and arranged to engage coronode wires 78 and 80. Similarly, lower wiping member 85 has wiper 108 located in the upper region thereof engaging coronode wires 78 and 80. Thus, coronode wires 78 and 80 are interposed between wipers 106 and 108 which remove contaminating particles therefrom as cleaning means 82 traverses coronode wires 78 and 80. Preferably, wipers 106 and 108 are made from suitable felt pads. Lower wiping member 85 is preferably molded from a suitable plastic. Similarly, lower and upper housing 102 and 104, respectively, of scraping member 84 are molded from a suitably rigid plastic material.

Corona generating apparatus 16 is received by a supporting arm (not shown) of the electrophotographic printing machine depicted in FIG. 1. Shield 66 is affixed to the frame of the printing machine such that it extends in a longitudinal direction transversely across photoconductive surface 12. To clean coronode wires 78 and 80, and grid wires 68, the operator grasps handle 90 and moves it coaxially in the direction of arrow 96. This advances scraper member 84 along the length of grid wires 68. As scraping member 84 traverses grid wires 68 grid wires 68 flex loosening the baked particles adhering thereto. This facilitates the scraping action of scraping member 84. Substantially simultaneously therewith, wipers 106 and 108 are contacting coronode wires 78 and 80 to wipe particles therefrom. Cleaning means 82 is then returned to its normal position adjacent the other end of shield 66 where it remains until the next subsequent cleaning operation. This relative movement of cleaning means 82 within shield 66 causes scraping member 84 to traverse the inner surface of grid wires 68 opposed from coronode wires 78 and 80, thereby scraping solid materials, e.g. dust and toner particles, from the contacted surfaces. Moreover, particles adhering to coronode wires 78 and 80 are wiped therefrom by wipers 106 and 108.

In recapitulation, it is evident that there has been provided a corona generating apparatus adapted to have toner particles and dust cleaned therefrom in order to maintain the charging voltage produced on the photoconductive surface substantially constant. This is achieved in the present invention by advancing a scraping member along the interior surface of the grid wires opposed from the coronode wires and wipers along the coronode wires to remove particles therefrom. As the scraping member and wipers advance, they contact the foregoing wires and remove toner particles and dust therefrom.

Thus, it is apparent that there has been provided in accordance with the present invention, a corona generating apparatus that fully satisfies the objects, aims and advantages set forth above. While this invention has been described in connection with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.

Claims

What is claimed is:

1. A corona generating apparatus, including:

a shield defining an elongated open ended chamber;

grid means disposed in the open end of said shield extending substantially in a longitudinal direction along the length of said shield;

electrode means interposed between said shield and said grid means; and

means for cleaning said electrode means and said grid means, said cleaning means having a first portion engaging said grid means to flex said grid means loosening particles adhering thereto and scraping the loosened particles therefrom, said cleaning means having a second portion engaging said electrode means to wipe particles therefrom.

2. An apparatus as recited in claim 1, further including:

means for energizing said electrode means to generate a corona discharge therefrom; and

means for applying a field regulating potential to said grid means for controlling the corona discharge generated by said electrode means.

3. An apparatus as recited in claim 2, wherein:

said electrode means includes at least a pair of spaced substantially parallel conductive coronode wires extending substantially in a longitudinal direction along the length of said shield; and

said grid means includes a plurality of spaced substantially parallel longitudinally extending grid wires mounted in said shield partially enclosing the open end thereof with one of said coronode wires being disposed therebeneath and the other of said coronode wires being disposed in the unenclosed portion of the chamber of said shield.

4. An apparatus as recited in claim 3, wherein the first portion of said cleaning means includes a scraping member having a first member with a plurality of open ended slots therein adapted to support said grid wires and a second member having a beveled edge arranged to engage the interior surface of said grid wires opposed from said pair of coronode wires.

5. An apparatus as recited in claim 4, further including means for moving said cleaning means along said pair of coronode wires and said grid wires to remove particles therefrom.

6. An apparatus as recited in claim 5, wherein said moving means includes:

guide means affixed to at least one end portion of said shield; and

a rod member secured to said cleaning means, said rod member being disposed in said guide means to reciprocate said cleaning means along the length of said pair of coronode wires and said grid wires.

7. An apparatus as recited in claim 6, wherein said shield includes a substantially U-shaped conductive housing.

8. An electrophotographic printing machine of the type having a photoconductive member adapted to be charged to a substantially uniform potential by a corona generating device, wherein the improvement in said corona generating device includes:

a shield defining an elongated open ended chamber;

grid means disposed in the open end of said chamber extending substantially in a longitudinal direction along the length of said shield;

electrode means interposed between said shield and said grid means; and

means for cleaning said electrode means and said grid means, said cleaning means having a first portion engaging said grid means to flex said grid means loosening particles adhering thereto and scraping the loosened particles therefrom, said cleaning means having a second portion engaging said electrode means to wipe particles therefrom.

9. A printing machine as recited in claim 8, further including:

means for energizing said electrode means to generate a corona discharge therefrom; and

means for applying a field regulating potential to said grid means for controlling the corona discharge generated by said electrode means.

10. A printing machine as recited in claim 9, wherein:

said electrode means includes at least a pair of spaced substantially parallel conductive coronode wires extending substantially in a longitudinal direction along the length of said shield; and

said grid means includes a plurality of spaced substantially parallel longitudinally extending grid wires mounted in said shield partially enclosing the open end thereof with one of said coronode wires being disposed therebeneath and the other of said coronode wires being disposed in the unenclosed portion of the chamber of said shield.

11. A printing machine as recited in claim 10, wherein the first portion of said cleaning means includes a scraping member having a first member with a plurality of open ended slots therein adapted to support said grid wires and a second member having a beveled edge arranged to engage the interior surface of said grid wires opposed from said pair of coronode wires.

12. A printing machine as recited in claim 11, further including means for moving said cleaning means along said pair of coronode wires and said grid wires to remove particles therefrom.

13. A printing machine as recited in claim 12, wherein said moving means includes:

guide means affixed to at least one end portion of said shield; and

a rod member secured to said scraping member, said rod member being disposed in said guide means to reciprocate said cleaning means along the length of said pair of coronode wires and said grid wires.

14. A printing machine as recited in claim 13, wherein said shield includes a substantially U-shaped conductive housing.