US3555378A

US3555378A - Charging xerographic images

Info

Publication number: US3555378A
Application number: US729094A
Authority: US
Inventors: Ian E Smith; Kenneth A Metcalfe
Original assignee: Commonwealth of Australia; SEC DEP OF SUPPLY
Current assignee: Commonwealth of Australia; SEC DEP OF SUPPLY
Priority date: 1967-05-15
Filing date: 1968-05-14
Publication date: 1971-01-12
Anticipated expiration: 1988-01-12
Also published as: DE1772432C3; GB1211422A; AU412176B2; FR1578969A; AU2171867A; DE1772432B2; NL145686B; NL6806873A; DE1772432A1; BE715208A

Abstract

APPARATUS FOR CHARGING SURFACES, PARTICULARLY XEROGRAPHIC SURFACES, USING A RELATIVE MOTION BETWEEN A POINT AND A SURFACE TO PROVIDE A RADIAL SWEEP OF A SLOPE SUCH THAT RELATIVELY UNIFORM CHARGE ACCEPTANCE TAKES PLACE OVER EACH INCREMENTAL AREA.

D R A W I N G

Description

Jan. 12, 1971 I sMlTH ETAL 3,555,378

CHARGING XEROGRAPHIC IMAGES Filed Bay 14, 1968 2 Sheets-Sheet I Jan. 12, 1971 s y ETAL 3,555,378 r CHARGING XEROGRAPHIC IMAGES Filed Hay, 1968 2 Sheets-Sheet 2 United States Patent Oflice 3,555,378 Patented Jan. 12, 1971 3,555,378 CHARGING XEROGRAPHIC IMAGES Ian E. Smith and Kenneth A. Metcalfe, Lockleys, South Australia, Australia, assignors to The Commonwealth of Australia, The Secretary Department of Supply, Parkes, Australian Capital Territory, Australia Filed May 14, 1968, Ser. No. 729,094 Claims priority, application Australia, May 15, 1967, 21,718/ 67 Int. Cl. H01t 19/04 US. Cl. 317262 7 Claims ABSTRACT OF THE DISCLOSURE \Apparatus for charging surfaces, particularly xerographic surfaces, using a relative motion between a point and a surface to provide a radial sweep of a slope such that relatively uniform charge acceptance takes place over each incremental area.

This invention relates to apparatus for charging surfaces, such as xerographic surfaces or insulating membranes, and in particular it relates to the charging of surfaces by means of a corona.

It is customary in charging surfaces to provide a series of points, or -a wire or the like, to which a sufficiently high voltage is applied to produce a corona discharge, and to pass this over the surface which is to be charged.

It has been shown that it is important to use the correct part of such a discharge for the actual charging of the surface, because different areas of such a discharge have a different charging effect, and also, when using a series of points as is customary there is a lack of uniformity in the field produced within each of the corona fields, and also between each of the corona fields generated by each point. Thus each corona pattern spreads outwards from the point at which it is generated, with a maximum field density effect at the center of the discharge and a gradually decreasing intensity outwards therefrom, and for this reason, When using a series of corona discharge points, there are a number of independent patterns generated, one for each point, which do not overlap due to repulsion effects, and therefore it is necessary to move the medium being charged in relation to the points to try to cause averaging of the charge at all areas and to avoid absence of charging at fringe areas.

Whichever way this movement is effected, it is found impossible to obtain a completely uniform charge acceptance on the entire area of the medium which is subjected to such charging.

For this purpose, instead of using a series of points, it is the practice to use a wire or other laterally extending member so that there is a concentrated effect along a plane, but if the section of the wire or member varies, or if there is variation of its distance fromany part of the surface being charged, there will be a difference in charge intensity and effect, and although there is relative movement of the wire and surface at right angles to the plane of the wire, charging bands are produced along which the intensity can vary. Although, it has been proposed heretofore to so arrange the corona that the selected part or band of the field produced by such a discharge is actually effective on the surface, the above problems make uniform charging very difficult if not impossible, and the primary object of this invention therefore is to allow a more uniform charge to be obtained than was heretofore possible, and secondly to achieve this in a simpler manner.

The objects of the invetion are achieved by moving a. single corona point relative to the work piece being charged in such a manner that the required band selectively sweeps the surface about a radius to produce a relatively uniform charge acceptance.

The use of a single point removes the problem of multiple non-overlapping fields or varying fields over a length, but this then introduces a further problem, namely, that the charge levels vary from a maximum central zone to a minimum fringe zone, and thus if relatively uniform charge areas are required, only small areas are available, and this effect will be demonstrated later herein with reference to the illustrations, but according to this invention this problem is overcome by using a single basic corona discharge point so arranged that its field extends sufficient- 1y to cover at least slightly more than one half of the area to be charged, and this area or the point is moved the one in relation to the other, with the result that relatively uniform charge acceptance is obtained over a large area.

It is possible to move both the workpiece and the discharge member, or to move only the discharge member in relation to a workpiece in such a manner that this uniform charging results, but it will be realized also that by suitable positioning, a corona needle or the like outside of the normal area which is being charged, the unwanted part of the discharge zone can be kept outside of the actual charge area of the required surface.

To enable the invention to be fully understood, an embodiment will now be described with reference to the accompanying drawings which are illustrative only and are not to be taken as limiting the invention, the scope of which is defined in the claims herein.

In the drawings:

'FIG. 1 is a side elevation view of the apparatus,

FIG. 2 is a plan view of same, indicating the corona field density,

FIG. 3 is a graph showing the charge density effect at different corona distances from a workpiece, and

FIG. 4 shows a modified apparatus.

Referring first to FIGS. 1 to 3, the sheet 1 to be charged is placed on a rotating disc 2 with a charging point 3 positioned outside the periphery of the disc 2, and a high voltage is applied between the charging point 3 and the disc 2. It will then be appreciated readily, that in the central zone 4 of the corona 5 emanating from the point 3, the charging rate is highest and the incremental area of the disc 2 traverse is greatest, whereas in the outer corona zone 6 the charging rate is least where the incremental area traversed is least. A uniform charge is generated on the sheet using such an arrangement.

T o understand this reference should be made to FIG. 3 where the X axis shows distance or extension of the corona while the Y axis shows the field intensity at the surface member being charged along the extension when measured at various heights above the work piece, from which it will be seen that at a height of one inch there is a high field intensity at the Y axis but a short extension along the X axis, whereas at a height of six inches the field intensity is relatively lower at the Y axis but extends considerably along the X axis, the actual distance usiug 15 kv. direct current being about eight or nine 3 inches. By selecting an appropriate part of such a zone, say the part designated R, and revolving the work piece 1 through this about the axis A, a relatively uniform charge acceptance is obtainable over an area of some 12 or more inch circle.

It will of course 'be appreciated that because of incremental area involved the denser field at the Y axis is effective at each area only for a short time during each revolution of the disc 2 whereas at the axis A the charge is continuously maintained. Thus, as said, by appropriately selecting the slope as indicated in FIG. 3 to be in the ratio of the radius, which can be done by a correct selection of the height of the point to the spread required and varying the voltage or corona shape accordingly.

It will be realized that this overcomes the problem which has existed heretofore where a single point to plane corona produced a circular charge pattern on an insulating sheet, with the maximum density at the center decreasing radially outward in the method used heretofore, and the maximum charge density increasing with charge interval. When the charge density at the center reaches a critical value, a change occurs which is characterized by a marked reduction in electroradiographic speed and quality. The diameter of this central zone increases with time. Optimum image quality and X-ray response occurs at the outer edge of the central zone, and image density decreases radially outward.

The present invention however, produces a uniform charge density over a large area, this charge being limited to a maximum just below the critical value which causes the central zone type of charging. On the other hand with the disc charging device referred to above, when the disc or baseplate rotates, the rate of charge acceptance is reduced at the center of the corona and increased in the outer zone of the corona. The corona envelope, of charge density decreasing from the pattern center outwards, is matched to the moving electroradiographic member in such a manner that the charge acceptance increases from the corona center outwards. Charge uniformity and charging time for a particular size of electrophotographic member is adjusted experimentally by varying the height and the distance of the corona point from the center of the disc for a given applied voltage as stated.

It will be realized that the invention can be variously applied but in each case there will be a relative motion between the corona producing device and the surface on which the charge is being developed and this will be so arranged that there is a scanning of the whole of the area by a part of the corona discharge which will produce a relatively uniform charge over the required area.

It is possible to use a shield as 7, see FIG. 1, which can be held at a voltage to modify the pattern or projecting of the corona by introducing a repulsion or attraction effect.

In the drawings the high voltage generator 13 and the driving motor 14 for the disc 2 are housed in the base 8, voltage controls 9 being provided, the point 3 being adjustable up and down on the stem 10, by means of collar 11 with locking knob 12 of conventional construction and if required the point 3 may be axially adjustable in collar 11 to allow a required pattern to be achieved. Generally it is advisable to avoid the center of the corona discharge and to select the starting point some distance in from the Y axis as shown by the left limit of R in FIG. 3 as this tends to have the wrong slope and characteristic for charging purposes, and of course the right limit of R must fall Within the charging density line.

While generally a single point has been referred to, it is possible within the spirit of the invention to use more than one point provided the repulsion effect is maintained so that it does not prevent the extension of at least one of the fields over the axis of rotation, this being usable where perhaps an edge portion requires to be filled in with a heavier charge.

Also instead of charging a single work piece extending across the axis of rotation, a number of work pieces could be placed around the outer part of the disc for successive charging, in which case the field need not necessarily extend over the axis of rotation.

Such an embodiment is shown in FIG. 4 where 15 represents a disc rotating about the axis 16 and carrying a series of work pieces 17 which pass through the corona zones 18 generated from the points 19. In this embodiment the work pieces 17 can be placed into position at S on the disc 15 and are then carried through the charge zones 18 to be finally removed at F.

From the foregoing it will be appreciated that the corona, or a series of coronas, are directed to charge a xerographic surface which has relative rotation to the point about an axis perpendicular to the surface, and when an area of charge is required which is longer than the radius of the corona at the xerographic surface the point, or one of the points only, is arranged to have its field extended from an area outside of the xerographic surface inwards to beyond the axis of rotation, with the center of the corona. preferably just outside of the edge of the area being charged.

We claim:

1. Apparatus for charging surfaces comprising: a disc on which a member having an insulating layer can be carried, at least one corona discharge point disposed above said disc, said disc being rotatable about an axis perpendicular to the plane of the disc, means to rotate the disc, and means to apply a voltage between the discharge point and the said disc, said point being so disposed in relation to the said rotational axis that the center of the corona is disposed near the outer edge of the area being charged with part of the corona extending inward toward the axis of rotation, whereby a relatively uniform charging per incremental area is attained during rotation.

2. Apparatus according to claim 1 wherein a single point is used which is disposed approximately over the edge of the said disc and faces generally toward the said disc.

3. Apparatus according to claim 1 wherein the discharge point is variable at least in its distance above the said support, and the means to apply the voltage between the discharge point and the disc is variable whereby the pattern of the corona may be selected.

4. Apparatus according to claim 1 comprising a shield disposed rearwardly of the said point to assist control of the corona pattern.

5. Apparatus for charging surfaces comprising: a support on which a member having an insulating layer can be carried, said support being planar and having an edge, a single corona discharge point disposed above the said support approximately at the edge thereof and facing said support, said discharge point and support being arranged to allow relative rotational motion about an axis perpendicular to the plane of the support, means to cause the relative motion, and means to apply a voltage between the discharge point and the said support, said point being so disposed in relation to the said rotational axis that the center of the corona is disposed near the outer edge of the area being charged with part of the corona extending inward toward the axis of rotation, whereby a relatively uniform charging per incremental area is attained during rotation.

6. Apparatus for charging surfaces comprising: a sup port on which a member having an insulating layer can be carried, at least one corona discharge point disposed above the said support arranged to allow relative rotational motion about an axis perpendicular to the plane of the support, means to cause the relative motion, and means to apply a voltage between the discharge point and the said support, said point being so disposed in relation to the said rotational axis that the center of the corona is disposed near the outer edge of the area being charged with part of the corona extending inward toward the axis of rotation and said discharge point being so positioned in relation to the edge of the area being charged that the center part of the corona is outside of the charging area whereby a relatively uniform charging per incremental area is attained during rotation.

7. Apparatus according to claim '6 wherein the discharge point is so positioned in relation tothe edge of the area being charged that the center corona extends inwards to just beyond the rotational axis.

References Cited UNITED STATES PATENTS 3,241,466 3/1966 Clark 355-3 1,250,140 12/1917 Chapman 317262 3 3,146,687 9/1964 Donelson et al. 355-3 2,043,217 6/1936 Yaglou 317-4 J D MILLER, Primary Examiner C. L. YATES, Assistant Examiner US. 01. X.R.