US3578970A

US3578970A - Variable width corona discharge apparatus with means to shield or vary a predetermined length of a corona discharge wire

Info

Publication number: US3578970A
Application number: US726386A
Authority: US
Inventors: David E Michaud; Nils L Hakanson
Original assignee: Plastic Coating Corp
Current assignee: Plastic Coating Corp
Priority date: 1968-05-03
Filing date: 1968-05-03
Publication date: 1971-05-18
Anticipated expiration: 1988-05-18

Abstract

Variable width corona discharge apparatus for applying an electrostatic charge to a transverse dimension of variable width and position of the surface of strip material moving relative to the apparatus. One embodiment includes a movable shield adapted to mask the corona discharge wires along a predetermined portion of their length. Another embodiment includes a torque biased rotatable element transversely movable relative to the strip material and adapted to wind up the corona discharge wire along a predetermined portion of its length.

Description

rm m 1a, 1971 v 3,578,970

3 sheets sheet 1 INVENTORS.

DAVID E. MICHAUD BY NILS HAKANSON hunted May 18, 1971 3 Sheets-Sheet z on QM QDW QM INVENTORS. DAVID E. MICHAUD NILS L HAKANSON ATT RNEY.

VARIABLE WIDTH CORONA DISCHARGE APPARATUS WITH MEANS TO SHIELD OR VARY A PREDETERMINED LENGTH OF A CORONA DISCHARGE WIRE BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to corona discharge apparatus and, more particularly, to new and improved apparatus for-applying an electrostatic charge to a transverse dimension of variable width and position of a surface such as that of a photoconductive insulating layer on an electrophotographic recording element in either single sheet or continuous strip form.

2. Description of the Prior Art In all of the wide variety of electrostatic copying processes, a uniform charge on the order of several hundred volts must be applied to one surface of a recording element. Generally. this has been done by one or more corona-generating strands or wires disposed adjacent'the surface and moved relative thereto. Often, a screen is employed between the coronadischarge wires and the surface, and serves as a stabilizing grid, sometimes being connected to a charge potential lower than that of the corona discharge wires but of the same polarity. This process and apparatus have proven to be quite satisfactory for most operations and areextensively used on a commercial basis.

All of the corona discharge apparatus described heretofor have utilized a corona discharge wireof a fixed :lengthand have been arranged so that, when moved relative to a surface to be charged, a transverse portion of the surface having a fixed width and position has been'charged. Recently, the need has arisen for corona discharge apparatus capable of charging a variable width surface and in a manner such that the width of surface to be charged can be accurately controlled. Thus, where preprinted forms are used as recording elements and are electrostatically charged, imagedand. developed by electrophotographie means to receive additional information, it is desired to charge only those portions of the form which must receive information in this manner. This avoids degradation of the remaining portions of the printed form due to background caused by residual charges on the sheet and toner applied thereto during the development of the charge image.

It is an object of this invention to provide variable width corona discharge apparatus.

In the past, apparatus has been devised for varying the spacing of portions of a corona discharge wire or electrode from a surface to be charged so that only the desired portion of a surface moving relative to the electrode are treated. In this regard, see US. .Pat. No. 3,334,227 issued'on Aug. 1, I967 to Mario Cipriani. However, that apparatus was disclosed primarily for the electrical surface treatment of polymeric materials such as polyethylene to render the surface printable. This requires close proximity of a high potential electrode to the surface in order to achieve the corona discharge intensity required to accomplish the desired surface treatment. The electrostatic charging of photoconductive insulating layers on recording elements does not require electrodes charged at such high potentials or spaced so closely to the surface in order to be effective, at least asvv far as causing residual developer or toner to be retained on the surfaces beneath the more distantly spaced portions of the electrode is concerned in causing undesirable background on the recording element.

Therefore, the apparatus disclosed by the above-referenced patent maybe satisfactory for the variable width treatment of a polymeric surface to render it printable, but it is not suitable for preventing background from forming when only a portion of: the width of an electrostatic recording element is concerned. In this latter instance, the portions of -the corona discharge wireadjacent portions of the surface not to be charged must be rendered completely ineffective as far as their ability to transmit charge even of reduced intensity to these portions of the surface.

It is a further object of the invention to provide variable width corona discharge apparatus in which charging width control means are employed for varying-the length of the corona discharge wire which is effective to apply an electrostatic charge to a surface moving relative thereto.

SUMMARY OF THE INVENTION The corona discharge apparatus of this invention is adapted to apply an electrostatic charge to a surface moving relative to the corona discharge apparatus. It includes two spaced-apart insulating members, at least one corona discharge wire extending from oneof the insulating members to the other, a corona-generating potential source connected to the corona discharge wire, and ch'argingwidth control means for varying the length of the corona discharge wire which is effective to apply an electrostatic charge to the surface. In a preferred embodiment, the two insulating members are arranged for movement relatively toward and away from one another and in a direction generally parallel to the plane of the surface to be charged and substantially normal to the direction of movement of that surface relative to the corona discharge apparatus. Adjustment means are preferably provided for effectingthe relative movement of the insulating members. At least one corona discharge wire extends from one insulating member to theother, and. a corona-generating. potential source is connected to the corona discharge wire. Takeup means associated .with at least one of the insulating members is adapted to maintain the portion of the wire extending between the insulating members taut while varying'the length of the portion in response to movement of the members relative to one another. In another embodiment, a shield is provided and adapted to mask a predetermined portion of thelength of the corona discharge wire.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of one embodiment of apparatus of the invention disposed relative to the surface of a moving sheet to be charged;

FIG. 2 is a sectional view-taken along line 2-2 of FIG. 1;

FIG. 3 is an exploded perspective view of one form of takeup means of the invention;

FIG. 4 is a sectional view taken along line 4-4 of FIG. 1;

FIG. 5 is a sectional view illustrating a modified form of corona discharge apparatus of the invention;

FIG. 6 is a sectional view of another form of corona discharge apparatus of the invention;

FIG. 7 is a plan view of a modified form of corona discharge apparatus of the invention;

FIG. 8 is a front elevation view of the apparatus shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show a preferred embodiment of corona discharge apparatus of the invention arranged 'to apply an electrostatic charge to the bottom surface of a sheet 10, which may be in the form of a single sheet or a continuous strip, moving in the direction of the arrow 11. Sheet 10 is a recording element comprising a sheet of paper or other electrically conductive backing having on one surface thereof a coating or film of an insulating. matrix which has dispersed therein a photoconductive pigment usually in the form of fine particles. The photoelectrostatic coating is normally directed toward the corona discharge apparatuswhile the'electrically conductive backing is normally directed away from the corona discharge apparatus.

The corona discharge apparatus includes a conductive shield l2'in the form of an elongated channel having a bottom 13 and sides 14; Insulating

members

15 and 16 are disposed at opposite ends of the conductive shield I2; Insulating member 15 is secured to shield I2 as by bolts (not shown) passing through the bottom 3 and into insulating member 15: Insulating: member 16 is slidably disposed within the conductive shield 12 and is arranged to move therealong relative to the insulating memberlS between juxtaposition and spaced-apart position, that is, generally from one end of the shield 12 to the other.

A pair of corona discharge wires 17 extend between insulating

members

15 and 16 in parallel relationship to one another and in a plane generally parallel to the plane of the surface of sheet to be charged. The corona discharge wires 17 lead into a recess 18 in the insulating member and are connected to a conductive strip 20 by means of screws 21 which fasten the conductive strip 20 to the insulating member 15. A conductive wire 22 passes through an opening 23 in the side 14 of conductive shield 12 and connects the conductive strip 20 with a corona-generating potential source (not shown). The corona discharge wires 17 are connected at their opposite ends to one .fonn of takeup means (indicated generally by reference numeral 24) carried by insulating member 16. Takeup means 24 are adapted to maintain the corona discharge wires 17 taut while the length of each wire 17 extending between the

insulating members

15 and 16 varies upon movement of the insulating member 16 along the conductive shield 12. Takeup means 24 are disposed in a recess V 25 opening on the top surface of the insulating member 16 and the corona discharge wires 17 pass through slots 26 in the insulating member 16.

Cover plates

27 and 28, preferably made of an insulating or dielectric material, are secured to the upper surface of the insulating

members

15 and 16, respectively, by screws 30 in a manner such that they are disposed between portions of the bottom surface of sheet 10 and the corona discharge wires 17. These

cover plates

27 and 28 accurately delineate the side boundaries of, or the limits of the transverse dimension of, the portion of the bottom surface of sheet 10 which is to be charged. Thus, electrostatic charge emanating from the portions of the corona discharge wires 17 beneath the

cover plates

27 and 28 is not effective to charge the portions of the surface of the sheet 10 covered or masked by the cover plates as this electrostatic charge is effectively blocked by the cover plates. In accordance with the invention, the side boundary delineated by the cover plate 28 secured to the insulating member 16 changes in a manner corresponding directly to a change of position of insulating member 16 along conductive shield 12.

FIG. 2 illustrates one type of adjusting means for moving insulating member 16 apparatus conductive shield 12 relative to insulating member 15 to vary the distance therebetween. A threaded element 31 in the form of a threaded cylindrical shaft is rotatably carried beneath the shield 12 and secured to the bottom 13 of shield 12 by spaced supports 32 secured to the bottom 13 of shield 12 by bolts (not shown). A cooperating threaded element 33 is secured to the bottom of insulating member 16 by bolts 34 extending through a slot 35 in the bottom 13 of conductive shield 12. The slot 35 extends over a major portion of the length of the shield 12. Upon rotation of the through-extending end 36 of the rotatably mounted threaded element 31, the cooperating threaded element 33 is forced to move along the length of the shield 12 and, correspondingly, to move the insulating member 16.

The details of construction of the member 16 and the takeup means 24 associated therewith are more clearly shown in FIGS. 3 and 4. The takeup means 24 associated with each corona discharge wire 17 includes a stationary portion 37 and a rotatable portion 38. The stationary portion 37 is mounted on a shaft 40 within the recess 25. One end of shaft 40 is held within a slot 41 in the sidewall of insulating member 16 and the other end is supported in a slot 43 in an upstanding rib 42 in the center of recess 25 in insulating member 16. Stationary portion 37 is prevented from rotating on shaft 40 by a rib 44 tion 45. The opposite end of the torsional spring 46 passes through a hole 48 in the rotatable portion 38 of the takeup means 24 so that torsion in the spring 46 is increased or decreased upon rotation of rotatable portion 38. The rotatable portion 38 has a peripheral wire winding groove 50 in which the corona'discharge'wire 17 is wound upon movement of the insulating member 16 toward the insulating member 15.

In operation, the torsional spring 46 is arranged to exert a slight torque when the insulating member 16 is adjacent the insulating member 15 so tat upon movement of the insulating member 16 away from the insulating member 15, the torsion spring 46 winds up as the corona discharge wire 17 unwinds, increasing the tension on the corona discharge wire 17 and tending to pull the insulating

members

15 and 16 together.

- FIG. 5 illustrates an alternative embodiment of corona discharge apparatus of the invention in which a conductive screen 51 is included. The conductive screen 51 is mounted upon the conductive shield 12 and extends from one side thereof to the other across thecorona discharge'wires 17 and between the corona discharge wires 17 and the surface of sheet 10 to be charged. It is insulated from the conductive shield 12 by mounting strips 52 of an insulating material. In this embodiment, the screen 51 serves as a stabilizing grid. In some instances, the stabilizing grid 51 may be used uncharged and will receive an induced charge from the corona discharge wires 17. Often, however, it is desirable to increase the charge applied to the corona discharge wires 17 and to apply a lower potential of the same polarity to the conductive screen 51. In either instance, the conductive screen 51 need not change in length as long as the effective charging length of the corona discharge wires beneath it is changed in the manner desired and in accordance with the invention.

FIG. 6 illustrates an alternative embodiment of the invention wherein a pair of corona discharge units of the type described with reference to FIGS. 1-4 are employed opposite one another with the sheet to be charged passing between them. This arrangement is used where charging of both sides of the sheet is desired and, advantageously, one unit is arranged to receive a corona-generating potential of one polarity while the other unit is arranged to receive a corona-generating potential of the opposite polarity.

Any conventional type of corona-generating potential source may be employed to supply voltage to the corona discharge wires through the conductive wire 22. A potential of at least about 4,000 volts is usually required between the corona discharge wires 17 and the nearest conductors or electrodes in order to generate a useful corona discharge and it is preferable that the voltage be less than 10,000 volts in order to avoid sparking or other problems in structures of conventional size. Either the positive or the negative terminal of the coronagenerating potential source may be connected to the corona discharge wire 17 depending upon the polarity of charge desired on the surface to be electrostatically charged. A DC potential which is constant bay be used. A pulsating DC potential such as that obtained by half-wave rectification of alternating current may also be used. An AC potential source may also be used where arranged to make ions of both polarities available in alternating half-cycles.

Where the conductive screen 51 is employed, the corona discharge emitted by the apparatus is stabilized to a greater degree and results in greater uniformity of charging of a surface. This conductive screen 51 is preferably held at a predetermined potential below the voltage of the corona discharge wires 17. Another voltage source (not shown) such as a battery may be used if desired to supply this potential to the conductive screen 51. Alternatively, conductive screen 51 may be connected to ground through a resistor of predetermined resistance value in order to maintain the charge received on the conductive screen 51 through induction at the desired level. Typically, the voltage level of the conductive screen 51 will be from abo i; 200 volts to about 1,000 volts and of the same polarity as that of the corona discharge wires 17 most immediately adjacent thereto.

'strip 53 on core.54. In this FIGS. 7 and 8 illustrate an alternative embodiment of of apparatus of the invention wherein the charging width control means include a strip 53 of insulating or dielectric material wound upon a spring-biased core 54 rotatably mounted adjacent one of said insulatingmembers and adapted to wind up embodiment, the insulating

members

15 and 16 are fixed in position relative to one another. A means is provided for drawing one end of the strip 53 across a I portion of the length'of the corona discharge wires 17 extending between the insulating

members

15 and 16 to prevent effective charging of the surface of sheet disposed opposite that portion of the corona discharge wires 17. In the embodiment shown, a pair of rods 55 are secured to a bar 56 across one end of the strip 53 and are secured at their opposite end to a bar 57. The bar 57 is positioned in one of a number of slots 58 in a holding member 60. Bar 57 is disposed in the desired slot 58 so that strip 53 covers the desired portion of the length of the corona discharge wires 17. The front edge 61 of the strip 53 delineates one side boundary of the transverse portion of the surface of sheet 10 to be charged while a stationary plate 62 secured to insulating member delineates the opposite side boundary of the transverse portion of the surface of sheet 10 to be charged, Thus, the portions of discharge wires 17 covered by strip 53 are rendered completely ineffective as far as their ability to apply an electrostatic charge to portions of the surface of sheet 10 moving adjacent thereto.

From the above, it can be seen that the corona discharge apparatus of the invention is capable of applying an electrostatic charge to a surface moving relative thereto in a manner whereby the charge can be effectively applied to'the desired portions of the surface. Thus, the apparatus includes means for varying the length of the corona discharge wire which is effective to apply an electrostatic charge to the surface moving relative thereto. While a number of specific embodiments have been described with reference to the drawings, it will be apparent the various modifications to the apparatus may be made within the scope of the invention. For example, other types of shielding means could be employed such as extensible tubular insulators about the discharge wires. Therefore, the invention is not intended to be limited to the specific details described herein except as may be required by the following claims. We claim: 1. In corona discharge apparatus adapted to apply an electrostatic charge to a surface moving relative thereto, including two spaced-apart insulating members, at least one corona discharge wire extending from one of said insulating members to the other, and a corona-generating potential source connected to said corona discharge wire, the improvement comprising charging width control means for varying the length of said corona discharge wire which'is effective to apply an electrostatic charge to said surface, said charging width control means comprising a strip of flexible insulating material, roll means on which at least a portion of the length of said strip may be wound, and means to unwind a portion of said strip from said roll means and to draw it between said corona discharge wire and said surface along a predetermined portion of the length of said corona discharge wire. 2. The improvement in corona discharge ing to claim 1, wherein said insulating members are arranged for movement relatively toward and away from one another and in a direction generally parallel to the plane of said surface and substantially normal to the direction of relative movement of said surface, and said charging width control means comprises takeup means associated with at least one of said insulating members and adapted to maintain the portion of said wire extending between said insulating members taut while the length of said portion varies in response to said movement of said members relative to one another.

apparatus accord- 3. The improvement in corona discharge apparatus according to claim 4, wherein said apparatus includes adjustment means for effecting said movement of said insulating members relative to one another.

4. A corona discharge device adapted to apply an electrostatic charge to a surface moving relative to said corona discharge device, comprising two insulating members, said members being arranged for movement relatively toward and away from one another and in a direction generally parallel to the plane of said surface and substantially normal to the direction of movement of said surface relative to said corona discharge device, I

at least one corona discharge wore extending from one insulating member to the other,

takeup means associated with at least one of said insulating members and adapted to maintain the portion of said wire extending between said insulating members taut while varying the length of said portion in response to said movement of said members relative to one another, said takeup means comprising a rotatable element upon which said wire is wound, and

torque biasing means operatively connecting said rotatable element to said one insulating member, whereby the portion of said wire extending between said insulating members is maintained taut while its length changes in response to a change in the position of one of said insulating members relative to the other, and a corona-generating potential source connected to said wire.

5. A corona discharge device according to claim 4, includmg a stabilizing grid disposed between said corona wire and said surface, and

a source of electrical potential connected to said stabilizing discharge grid, said source having a potential lower than the potential of said corona-generating potential source. ,I 6. A corona discharge device according to claim 4, including a stabilizing grid disposed between said corona wire and said surface, and a resistor connecting said stabilizing grid to ground. 7. A corona discharge device according to claim 4, includdischarge ing adjustment means for effecting said movement of said inmove the last mentioned insulating member having said threaded portion relative to the other insulating member.

10. A corona discharge device according to claim 4, includlng a shaft rotatably supporting at least one end of said rotatable element, and

1 wherein said torque biasing means comprise a torsion spring, one end of said spring being fixed relative to said one insulating member in a manner preventing relative rotation therebetween, the other end of said spring being fixed to said rotatable member in a manner preventing relative rotation therebetween, said spring being arranged to apply winding torque to said rotatable element throughout the extent of said relative movement of said insulating members.

11. A corona discharge device according to claim 4, including a conductive shield partially surrounding said corona discharge wire so that said wire is shielded on the side most remote from said surface.

12. A corona discharge device according to claim 11, wherein said conductive shield is connected to ground.

13. A corona discharge device according to claim 11, wherein said conductive shield comprises an elongated channel member, and wherein said insulating members are disposed therein, at least one of said insulating members being 7

Claims

1. In corona discharge apparatus adapted to apply an electrostatic charge to a surface moving relative thereto, including two spaced-apart insulating members, at least one corona discharge wire extending from one of said insulating members to the other, and a corona-generating potential source connected to said corona discharge wire, the improvement comprising charging width control means for varying the length of said corona discharge wire which is effective to apply an electrostatic charge to said surface, said charging width control means comprising a strip of flexible insulating material, roll means on which at least a portion of the length of said strip may be wound, and means to unwind a portion of said strip from said roll means and to draw it between said corona discharge wire and said surface along a predetermined portion of the length of said corona discharge wire.

2. The improvement in corona discharge apparatus according to claim 1, wherein said insulating members are arranged for movement relatively toward and away from one another and in a direction generally parallel to the plane of said surface and substantially normal to the direction of relative movement of said surface, and said charging width control means comprises takeup means associated with at least one of said insulating members and adapted to maintain the portion of said wire extending between said insulating members taut while the length of said portion varies in response to said movement of said members relative to one another.

3. The improvement in corona discharge apparatus according to claim 4, wherein said apparatus includes adjustment means for effecting said movement of said insulating members relative to one another.

4. A corona discharge device adapted to apply an electrostatic charge to a surface moving relative to said corona discharge device, comprising two insulating members, said members being arranged for movement relatively toward and away from one another and in a direction generally parallel to the plane of said surface and substantially normal to the direction of movement of said surface relative to said corona discharge device, at least one corona discharge wore extending from one insulating member to the other, takeup means associated with at least one of said insulating members and adapted to maintain the portion of said wire extending between said insulating members taut while varying the length of said portion in response to said movement of said members relative to one another, said takeup means comprising a rotatable element upon which said wire is wound, and torque biasing means operatively connecting said rotatable element to said one insulating member, whereby the portion of said wire extending between said insulating members is maintained taut while its length changes in response to a change in the position of one of said insulating members relative to the other, and a corona-generating potential source connected to said wire.

5. A corona discharge device according to claim 4, including a stabilizing grid disposed between said corona discharge wire and said surface, and a source of electrical potential connected to said stabilizing grid, said source having a potential lower than the potential of said corona-generating potential source.

6. A corona discharge device according to claim 4, including a stabilizing grid disposed between said corona discharge wire and said surface, and a resistor connecting said stabilizing grid to ground.

7. A corona discharge device according to claim 4, including adjustment means for effecting said movement of said insulating members relative to one another.

8. A corona discharge device according to claim 4, wherein said surface is the surfacE of a photoconductive layer of an electrophotographic element upon which an image is to be recorded.

9. A corona discharge device according to claim 7, wherein at least one of said insulating members has a threaded portion affixed thereto, and said adjustment means includes a elongate threaded member adapted to engage said threaded portion so as to move the last mentioned insulating member having said threaded portion relative to the other insulating member.

10. A corona discharge device according to claim 4, including a shaft rotatably supporting at least one end of said rotatable element, and wherein said torque biasing means comprise a torsion spring, one end of said spring being fixed relative to said one insulating member in a manner preventing relative rotation therebetween, the other end of said spring being fixed to said rotatable member in a manner preventing relative rotation therebetween, said spring being arranged to apply winding torque to said rotatable element throughout the extent of said relative movement of said insulating members.

13. A corona discharge device according to claim 11, wherein said conductive shield comprises an elongated channel member, and wherein said insulating members are disposed therein, at least one of said insulating members being adapted for relative movement therealong, whereby said channel member supports and guides the last-mentioned insulating member throughout its movement relative to the other insulating member.