US3758779A

US3758779A - Electrostatic charging apparatus

Info

Publication number: US3758779A
Application number: US00191799A
Authority: US
Inventors: E Thicthener
Original assignee: Savin Business Machines Corp
Current assignee: Savin Business Machines Corp
Priority date: 1971-10-22
Filing date: 1971-10-22
Publication date: 1973-09-11
Anticipated expiration: 1990-09-11

Abstract

I provide charging apparatus for an electrostatic copying machine of the type in which a photoconductive material is charged electrostatically and exposed to a radiation pattern to form a latent image which is developed by the application of suitable ink, ink particles or colored toner. The charge retained by the material is a function of two major variables, the strength of the field reaching the photoconductive material and the duration of its exposure to the charging field. In my apparatus the charge is emitted by the charger at or near saturation level to produce a strong field. The area of the material exposed to this field at any time is limited in proportion to the field strength by shielding the material from the charge either physically, electrostatically or both.

Description

0 United States Patent 1 [111 3,758,779

Thicthener Sept. 1 1, 1973 [54] ELECTROSTATIC CHARGING APPARATUS 3,146,687 9/1964 Donelson et al 250/495 X 746 I2 1964 Cl k {75] Inventor: Edward P. Thicthener, Hove, at 250/49 5 A "mama I Primary ExaminerWilliam F; Lindquist [73] Assignees Savin Business Machines A m L,'Shenier Corporation, New York, NY.

[22] Filed: Oct. 22, 1971 [57] ABSTRACT [21 1 Appl. No.: 191,799 I provide charging apparatus for an electrostatic copying machine of the type in which a photoconductive Relflted Application Data material is charged electrostatic-ally and exposed to a [63] g" 12988 1970' radiation pattern to form alatent image which is develabandone oped by the application of suitable ink, ink particles or colored toner. The charge retained by the material is a 2% 5 function of two major variables, the strength of the d g 5 ZC field reaching the photoconductive material and the l 1 0 care 50/49 To 5 duration of its exposure to the charging field. in my app I paratus the charge is emitted by the charger at or near saturation level to produce a strong field. The area of [56] References cued the material exposed to this field at any time is limited UNITED STATES PATENTS in proportion to the field strength by shielding the ma- 3,603,851 9/1971 Metc'alfe et a1 317/262 terial from the charge either physically, electrostati- 3,233,l56 2/1966 Jarvis et a1 317/262 all r b th 3,038,073 6/1962 Johnson g 250/495 y I 3,651,323 3/1972 Tanaka et a1. 250/495 8 Claims, 6 Drawing Figures DC. oi 50w 4 .120 /5 mo p K li- PATENTED W73 3.758.779

52 I /////f% -3/ T '53 INVENTOR HTTORNEV$ ELECTROSTATIC CHARGING APPARATUS This is a continuation of application Ser. No. 12,088 filed Feb. 10, 1970 now abandoned.

BACKGROUND OF THE INVENTION In the art of electrostatic copying, an electrostatic charge is applied to a photoconductive surface or layer. When a pattern of radiation is applied to the surface, a latent image is formed thereon which is developed and made visible by the suitable application of toner material such as visible liquid or powdered developer.

- The specific field to which my invention is directed is that of providing a uniformly distributed charge on the photoconductive surface at an intensity level which will produce an accurate copy free from imperfections.

Electrostatic charging is effected by energizing an electrode above its discharge threshold, with the electrode either perpendicular to the plane of movement of the layer (point-to-pla'ne charging) or in a wire parallel to the layer (line-to-planecharging). With either approach to charging, uniformity of the electrostatic field thefirst charge. In some instances, the second charge is of lower value than the first, and, in other instances, the charging source'is physically positioned in various ways relative to the photoconductive layer. While such processes have produced the acceptable line copies,

they have not succeeded for half-tone and continuous tone reproduction. I

It is well known that energizing the charger to a value near the saturation level of the photoconductive surface increases the allowable speed of the charging process and produces an acceptable line copy. For continuous tone and half-tonereproduction such charging causes a number of imperfections, such as minute flashes or pinholes, and may cause entire bands of dark or light areas on completion of development.

Another procedure used with point-to-plane charging uses only the outer less intense rings generated within the charging field and overlaps enough rings to cover the entiresurface to be charged. 7

Still another process used a grid shielding pattern in which the grid is energized and performs a function analogous to the grid in a triode vacuum tube.

While all of these methods were steps forward in the art, they do not achieve a uniform charging level productive of clean copy, free of imperfections, most especially where half-tone or continuous-tone prints are being produced.

SUMMARY OF THE INVENTION One object of my invention is to provide an improved charging apparatus for an electrostatic copying machine which produces consistent copies of good clarity and high contrast, freeof imperfections.

A further object of my invention is to provide an apparatus which subjects a photoconductive layer to an electrostatic charge of comparatively low intensity over the entire lateral extent of the layer.

Another object of my invention is to provide an electrostatic charging apparatus which shields portions of the medium being charged at the highest level from prolonged exposure to the full intensity of the charge.

An additional object of my invention is to provide an electrostatic charger which emits a saturating level of charge, but in which a portion of the charge is intercepted before reaching the photoconductive layer to shape the charge reaching the layer.

Still another object of my invention is to provide an electrostatic charger which distributes the charge uniformly over a photoconductive surface by varying the duration of the charge over an increment of the surface as a function of the intensity of the charge at that increment. V

Other and further objects of my invention will appear from the following description viewed in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings which form a part of the instant specification and which areto be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:

FIG. 1 is a sectional elevation of an electrostatic copying machine employing my invention; I

' FIG. 2 is a fragmentary elevation, drawn on an enlarged scale, of the charging apparatus shown in FIG. I with the charging circuit shown diagrammatically.

FIG. 3 is a bottom plan view of the charging apparatus shown in FIG. 2 with portions of the copy paper removed. 1

FIG. 4 is a fragmentary elevation, similar to FIG. 2, showing another embodiment of my invention.

FIG. 5 is a bottom plan view of the apparatus in FIG. 4.

FIG.6 is a fragmentary elevation, drawn on enlarged scale, similar to FIG. 2, showing another embodiment of my invention.

DESCRIPTION OF THE PREFERRED Y EMBODIMENT Referring now to the drawings, in FIG. 1, I show one form of electrostatic copying machine 10, which has a generally rectangular enclosing housing 12 in the top surface of which there is a glass exposure window 14 on which the object to be copied is placed. Window 14 has a sight area large enough to accommodate at least an 8% inches by 13 inches sheet flat on the glass surface. An opaque cover sheet 16, which may be rigid or flexible is hinged adjacent one window edge to normally cover the window and is arranged to be raised to permit the original to be placed on the window. Spaced below the window is a scanner system 20 which includes horizontally disposed laterally extending, spaced-apart slide rails 22 (only one of which is visible in FIG. 1). Mounted for transverse travel along the length of slide rails 22 is scanner carriage 30. The scanner carriage 30 is driven by a motor (not shown) which tracks an endlessdrive chain 26 about sprockets 28 at each travel limit'of the rails to allow reciprocatory movement of the scanner carriage 30.

Scanner carriage 30 is comprised of a common enclosing frame structure-bearing operatively positioned components, such as a lamp 32, mirrors 34, reflective lens network 36, shutter 38, and charging apparatus 100. Scanner carriage 30 operates in generally known manner to sweep across the underside of an object resting on glass window 14, electrostatically charge the photoconductive layer, and thereafter transmit the radiation pattern reflected from the object through a moving slit to the photoconductive layer.

At one end of the housing 12 mounted for rotation about a horizontal axis there is a feed roll 50 of sheet photoconductive material, generically called copy paper herein. This roll contains copy paper having a photoconductive surface layer 52 on the inner side of the paper such that on unrolling, the layer is on the upper side. A conductive base or support layer 53 is disposed on the other side of the assembly to receive a positive charge from the charging source. Positioned adjacent the roll is a feed solenoid 54. The feed solenoid on energization draws a length of copy paper from the roll. The length of copy paper being fed'passes through feed rollers 56 and straightening rollers 58.

The length of copy paper being unrolled passes onto the upper platform 60 of an endless belt transport mechanism 62 and rests there while being charged and while a latent image is formed thereon. Also positioned adjacent the length of copy paper being unrolled is a cutter 64 reciprocable to sever the length of copy paper. This cutter is located'between the feed rollers and straightening rollers 58 and is actuatable to cut the unrolled copy paper to a sufficient length to receive the entire transmitted image.

Adjacent the opposite end of the housing is the developing trough 70 with its paper drive rollers72. Dispersed within trough 70 is a liquid dispersion including the developing powder or toner particles which are circulated to adhereto charged portions of the copy paper and develop the latent image. Further downstream in the path of movement of the copy paper is the output chute 74 extending from the machine housing 12 to carry the copy for removal by the machine user.

Automatic controls (not shown) actuate and program'the automatic cycle of the machine to perform the steps of feeding the copy paper from the roll onto the platform, cutting alength of copy paper, actuating the lamp, starting the traverse of the scanner carriage to charge and expose the copy paper, and returning the scanner carriage, initiatingthe platform movement to feed the copy paper to and through the developing trough, and to complete the final movement of the copy paper through the finished copy chute.

The structure and process described above is generally known in the art and is described herein as the environment within which the charger forming the present invention has been fitted. 1

In FIG. 1, I show an electrostatic charging apparatus indicated generally by the numeral 100 and embodying one form of my invention. Charger 100 is mounted rigidly by means of a suitable bracket 102 on the underside of the scanner carriage 30 at the forward end of the scanner. in this way the charger passes over a length of copy paper resting on endless belt copy platform 60 during passage of scanner assembly and thereby charges the copy paper before the exposure of the paper length to the radiation pattern.

In FIGS. 2 and 3, I show the charger 100 with a topmounting plate 110, rectangular in extent and fastened to the scanner carriage by bracket 102 for movement with the carriage -30. This plate 110 may be fabricated of a suitable dielectric material such as "FLEXI- GLAS", which is the registered trademark of Roem & Hass Company of Philadelphia, Pennsylvania, for acrylic resin plastic sheets and molding powders. Secured to the underside of the plate are two spaced

struts

114 and 115 of dielectric material such as that used for plate 110. The struts preferably are cylinders of identical size and shape, advantageously approximately one inch in diameter. The axis of each cylinder is aligned above the lateral center of the copy paper length 117.

Cylinders

114 and 115 serve as electrostatic charge deflectors, as will be explained hereinafter. Midway between the

cylinders

114 and 115, and in the plane of the axis of the cylinders, I position a charging electrode 120. Electrode is a single needlepoint directed downwardly from its mounting on mounting plate 110. Point electrode 120 emits a corona dis charge on the application of a sufficiently high voltage. With a corona point of 0.02 inch diameter tungsten, l have found that a voltage of 10,000 volts produces a field strong enough to approximate the saturation charge on photoconductive surface approximately 2 inches below the corona point and traveling at a speed of 0.2 feet per second.

Cylinders

114 and 115 and the point electrode 120 all are directed downwardly, perpendicularly to the surface of the photoconductive surface, the cylinders 114 andllS reach considerably below the point electrode 120. Atthe base of each of the

cylinders

114 and 115 is secured an identically shaped shielding template or shoe referred to as 122 and 124. These two templates'are fabricated of aluminum or similar electrical conducting material. Each template or

shoe

122 and 124 is formed in the shape of a segment 126 of a circle, the templates being spaced apart and extending horizontally in a common plane. The adjacent segmental edges 126 of the templates are convexly curved over an arc of approximately 150 with the center of the arc of each templatealigned collinearly in the path of movement of the paper. The template segments extend beyond the walls of

cylinders

114 and 115, and the segments terminate at their remote ends in parallel rectilinear edges normal to the path of movement of the copy paper.

To provide an idea of the proportions involved in the charger construction, 1 find it advantageous to space the bottom plane of the

templates

122 and 124 approximately A inch above the photoconductive surface 52 of the copy paper 117. The point electrode may be about 2 to 3 inches above the surface, while the two

cylinders

114 and 115 are spaced from one another by l to 1% inches and the templates may be spaced from three-fourths to 1 inch apart. A speed of relative movement which 1 have found satisfactory with the parameters listed is approximately 0.20 feet per second in the direction of arrow 125.

The corona point electrode 120 is connected to the negative lead of a high voltage direct current source 130. This source may be any conventional source of direct current with output in the range of 7,500 to 12,000 volts. The positive lead 156 from the source 154 is connected to the conducting support for the photoconductive layer 52. The conductive support may be an external support, conductive substrate, or as shown, a support layer 53. The positive lead from source 130 is also connected in multiple to the conducting

templates

122 and 124.

The corona point 120 on energization above its threshold level (which may approximate 10,000 volts for the parameters described) emits an electrostatic field which is strongest in a circular zone concentric to the downward projection of the corona point. Successivcly weaker annularzones surround the zone of maximum field strength.

When a photoconductive surface passes beneath an unshielded corona point electrode, the relative charge which the photoconductive surfaceaccepts and retains is proportional to the field strength. When such a charged surface is exposed to a radiation pattern to produce a latent image on the surface, and the latent image is developed, a dark zone appears on the print. The dark zone is in the shape of a rectangle defined by the passage of the zone of high field strength across the developed copy. This dark band results from the attraction of too much developing toner to this band of the print. The excess of toner in turn results from the excess of retained electrostatic charge on the photoconductive surface in this hand area.

My shielding structure prevents this dark rectangle on the copy by limiting the duration of exposure of the photoconductive surface to'the zone of intense field strength. I accomplish this in at least two ways. First, I utilize the

resistive cylinders

114 and 115 extending parallel to the corona point. These-cylinders deflect and repel the emitted field to constrict the shape of the field reaching the photoconductive surface below The resulting zone of maximum field strength is constricted to an area of short length along the center line of passsage of the surface under the corona point electrode. This zone is symmetrical about this line.'Thus, asthe surface passesthrough the zone of maximum field strength at a constant speed, the duration of exposure of the surface to the maximum field strength is minimized, especially along this line. The annular bands of lesser field strength are successively allowed greater duration of exposure to the field reaching these areas. By interrelating the emitted field strength and duration of exposure to the field, I develop a uniform charge over the entire photoconductive surface.

As a further measure I employ the

templates

122 and 124. In the embodiment shown in FIGS. 2 and 3, these templates are mounted on the underside of

theinsulating cylinders

114 and 115 and exteind over a larger area than the cylinders. These templates intercept the emitted field and physically shield the surface especially within the zone of maximum field strength. The exposed area beneath the corona point electrode is further lessened-by the use of these shaped templates; and the physical shielding'adds to the effect-of the electrostatic shielding. The curvature of edges 126 is symmetrical about the center line of the photoconductive surface providing a minimum of exposed surface under the center line. The amount of exposed surface increases as the, distance from the center line increases. Also, to alleviate other possible problems, the templates are energized to the same potentialas the conductive support layer. This energizing of the templates minimizes the possibility of arcing between the templates and the support layer in the form of either secondary corona emission orintermittent spark breakdown. B the combined use of both of these shielding means, I find that the charge level of the photoconductive surface can be maintained at a consistent and substantially uniform level over the area'of the surface. As

a result of this uniformity of charge distribution, the developed print has a uniform density.

Automatic controls (not shown) program the machine of FIG. ll, through its cycle to perform the steps of feeding copy paper from roll to the copying platform 60, cutting the paper in length, bringing the paper to rest on the copy platform 60, actuating the charger 100 and moving scanner carriage 30 to charge the paper and to expose it to the radiation pattern of the object being copied to produce a latent image, and thereafter carry the copy paper to the developing trough 70 for application of toner to develop the latent image. The finished copy is then carried from the trough and fed to the output chute 74.

In FIGS. 4- and 5, I show an alternative charging apparatus 200 using a single corona wire 202 extending parallel to the plane of the copy paper 204 (comprising photoconductive surface 52 and conductive layer 53) and parallel to the path of travel of the copy paper 204, as indicated by the arrow 206 (FIG. 4). The corona wire is backed by a conductive flat backing plate 208 spaced therefrom a distance sufficient to prevent arcing. Backing plate 208 has short side walls 211 extending downwardly from the plate on both outward sides, the side walls terminating in a plane above the level of the corona wire. The backingplate is supported by a top mounting plate 209 of insulating material similar to that described for the prior embodiment.

Extending from the mounting plate 209 are insulating

cylinders

210 and 212; These insulating cylinders, similar to those described previously, extend perpendicular to the plane of the photoconductive surface-and terminate a short distance fromthe surface. Secured to the outer tips of 'these cylinders are enlarged conducting shielding templates 214 and-216 of the size, shape and material as described previously relative to FIGS. 2 and 3. The templates are parallel to the plane of the photoconductive surface and are connected to the positive source lead 219. The axis of the cylinders is coplanar with the axis of the corona wire above the center line of the photoconductive surface to pass thereunder.

The corona wire 202 is connected to the negative terminal 219 of a high voltage direct current source 210, through mounting supports 225 embedded in the

cylinders

210 and 212. The positive lead 220 from source 219 is connected in multiple to the backing plate 208,

- the

conductive templates

214 and 216 and the conducting support layer 53 for the copy' paper 204. The

wire corona emits a field of maximum intensity in a band parallel to and below the wire. The intensity of the generated field lessens with increased lateral distance from the wire. Discontinuities in the corona wire 202 would normally cause discontinuities in the charge pattern. These charge discontinuities would lead to blemishes in the developed print if not compensated for. The conductive .backing plate, I have found, eliminates the effect of discontinuities in the emitted field and in the retained charge. The field is shaped and constricted by the

electrostatic shielding cylinders

210 and 212 and the

conductive templates

214 and 216. In this way I shield the photoconductive surface and'reduce the period that the photoconductive surface is exposed to the zone of maximum intensity and successively provide greater exposure to areas of lesser intensities. In

this way also, I produce a print of uniform charge level and uniformv developed density.

' known by the trademark PLEXIGLAS," as previously described. In this embodiment, no conductive shielding templates are used. Electrostatic cylinders 306 and 307 alone are used to constrict and shape the emitted field. The insulating cylinders repel the field and deflect its ions to a pattern below the corona point. The resulting charge pattern reaching the photoconductive surface 52 of copy paper 308 is that of a circle with deep indentations caused by the shielding cylinders. The charge on the photoconductive surface is of maximum intensity in the central area as the photoconductive surface passes beneath the charging apparatus, on movement in the direction of arrow 310 in a plane parallel to the cylinders, charger andbacking plate. The duration of exposure of the zone of maximum field intensity to the photoconductive surface is again limited during the traverse of the photoconductive surface at a constant speed. l i

in all the embodiments shown, I have employed a charge emitted toward a photoconductive surface at high field strength, and have shielded portions of the photoconductive surface from the charge. The shape of the shielding members controls the duration and area of the exposure of the surface to the emitted field. The duration of exposure of the zone of maximum field intensity is limited. Zones of lesser intensity are allowed longer periods in communication with the photoconductive surface. In this way, I produce a charge level on the photoconductive surface which is substantially uniform about the extent of the photoconductive surface.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. it is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. Apparatus for producing a uniform electrostatic charge on the surface of a length of photoconductive material having a longitudinal certerline including in combination, a corona discharge source for producing an electric field, means mounting said source above said supporting means, means moving said supporting means relative to said source to carry said length of photoconductive material past said source with the centerline thereof generally aligned with said source, a pair of elongated elements of dielectric material, and means mounting said elements respectively at locations in said field spaced fore and aft from said source with reference to the direction of movement of said supporting means relative to said source and with the longitudinal axes thereof intersecting said centerline and with the upper ends thereof adjacent to said source and with the lower ends thereof relatively closely spaced from said supporting means, said elements having configurations generally symmetrical with respect to said centerline to modify said field to produce a uniform charge over the surface of a length of photoconductive material being carried past said source by said supporting means.

2. Apparatus as in claim 1 including respective conductive plates carried by said elements at-the lower ends thereof.

3. Apparatus as in claim 2 including means for biasing said plates oppositely'to said source.

- 4. Apparatus as in claim 1 in which said elements are generally cylindrical.

5. Apparatus as in claim 1 including respective conductive plates carried by said elements at the lower ends thereof, said plates having facing circular edge portions.

6. Apparatus as in claim 1 including respective conductive plates carried by said elements at the lower ends thereof, said plates having facing circular edge portions symmetrical with respect to said centerline and in which said elements are generally cylindrical.

7. Apparatus as in claim 6 in which said source is a point source located between said elements adjacent to the upper ends thereof.

8. Apparatus as in claim 6 in which said source is a wire extending in the direction of said centerline, said wire being located between said elements adjacent to the upper ends thereof.

Claims

2. Apparatus as in claim 1 including respective conductive plates carried by said elements at the lower ends thereof.

3. Apparatus as in claim 2 including means for biasing said plates oppositely to said source.

4. Apparatus as in claim 1 in which said elements are generally cylindrical.