US3037478A

US3037478A - Apparatus for developing electrophotographic sheet

Info

Publication number: US3037478A
Application number: US691924A
Authority: US
Inventors: Melvin A Lace
Original assignee: American Photocopy Equipment Co
Current assignee: American Photocopy Equipment Co
Priority date: 1957-10-23
Filing date: 1957-10-23
Publication date: 1962-06-05
Anticipated expiration: 1979-06-05

Description

June 5, 1962 M. A. LACE 3,037,478

APPARATUS FOR DEVELOPING ELECTROPHOTOGRAFHIC SHEET Filed Oct. 23, 1957 H; 5 H; I

2. Sheets-Sheet 1 39 INvENToR MELVIN A. LACE by: v

ATTY.

M. A: LACE 3,037,478

APPARATUS FOR DEVELOPING ELECTROPHOTOGRAPHIC SHEET June 5, 1962 2 Sheets-Sheet 2 Filed Oct. 23, 1957 INVENTOR MELVIN A. LACE ATTY.

Patented June 5, 1962 3,037,478 APPARATUS FOR DEVELOPING ELECTRO- PHOTOGRAPHIC SHEET Melvin A. Lace, Chicago, Ill., assignor to American Photocopy Equipment Company, Chicago, Ill., a corporation of Illinois Filed Oct. 23, 1957, Ser. No. 691,924 8 Claims. (Cl. 118-637) This invention relates to a novel apparatus and method for magnetic brush development of latent electrostatic images on electropho-tographic sheets or plates.

In the electrophotographic process the surface of a photoconductive insulating material is first provided with a uniform electrostatic charge. The charged surface of the electrophotographic sheet or plate is then exposed to a light image, thereby discharging the charges on those areas of the surface irradiated by light while leaving the remainder of the surface in a charged state so as to form a latent electrostatic image substantially corresponding to the light image. The latent electrostatic image is then developed by applying a powdered toner which adheres to the charged areas of the surface. The powder may then be fixed directly to the electrophotographic sheet or may be transferred to another surface and fixed thereon.

The so-called magnetic brush system is one of the presently preferred techniques for developing the latent electrostatic image, that is, for applying the toner particles to the surface of the sheet bearing the electrostatic image. The magnetic brush comprises a magnet to which is adhered a developer mix including iron filings and toner particles held to the magnet in the form of a fibrous mass by the magnetic field. When the mass is wiped over the surface bearing the latent electrostatic image, the toner particles are retained on the charged areas of the sheet surface by electrostatic attraction due to the electrostatic charge developed upon the toner particles by a triboelectric reaction with the iron filings.

It has heretofore been discovered that improved results can be obtained by applying a bias potential or unidirectional electric field between the electrostatic image and the magnetic brush during the development step. The contrast of the developed image may be controlled by varying the voltage of the applied potential. Also, either a direct or reversed image may be developed, as desired, in accordance with the direction of the applied electrostatic field.

As heretofore practiced, the electrophotographic sheet bearing the electrostatic i age on its front surface is placed with its rear surface against a metal back-up plate, and the bias potential is applied between the plate and the magnetic brush. This technique involves a serious disadvantage when developing the marginal areas of an electrophotographic sheet having a width less than the width of the metal back-up plate. More specifically, when the magnetic brush contacts such a sheet along an edge thereof so as to overlap said edge, at least part of the developer mass is in electrical contact with the metal back-up plate, thereby short-circuiting the applied bias potential and drawing sufiicient current to cause the bias voltage to drop substantially below the desired value.

A primary object of the present invention is to provide a novel apparatus and method which enable the electrophotographic sheet to be developed by the ma netic brush technique along the marginal edge areas of the sheet while a bias potential is applied between the magnetic brush and the back-up plate, while preventing any substantial reduction in the bias voltage due to contact of the metal brush with the back-up plate.

In the drawings:

FIG. 1 is a schematic perspective view illustrating a first embodiment of the apparatus and method of the present invention;

FIG. 2 is a schematic perspective view illustrating a second embodiment of the invention;

FIG. 3 is a schematic perspective view illustrating a third embodiment of the invention;

FIG. 4- is a schematic perspective view illustrating a fourth embodiment of the invention;

FIG. 5 is a schematic sectional view of a system utilizing a rotary magnetic brush and which may embody the invention in any of the four forms thereof illustrated in FIGS. 1 to 4 inclusive; and

FIG. 6 is a sectional view taken substantially on line 6-6 of FIG. 5.

Referring first to FIG. 1, the reference numeral 11 designates generally a back-up plate adapted to contact the rear surface of the electrophotographic sheet bearing on its front surface a latent electrostatic image to be developed. The back-up plate 11 comprises a substantially rectangular insulating plate or block 12 composed of an electrical insulating material such as polystyrene, Lucite, Bakelite or similar phenol-formaldehyde resin, Wood or the like. Secured to or imbedded within the upper surface 13 of the insulating plate 12 are a plurality of rectangular conductive strips 14. The upper surfaces of the latter may be coplanar With, or eX- tend above, the upper surface of plate 12 so as to contact the rear surface of the electrophotographic sheet.

The strips 14 may be in the form of thin foil or thick plates and may be composed of any suitable conductive metal such as copper, bronze or aluminum. The strips 14 are in parallel contiguous relation, and have their respective

adjacent edges

15, 16 spaced from each other whereby each of the strips 14 is electrically insulated from the respective strips adjacent thereto.

The width of each conductive strip 4 is preferably about 2 inch, but may be considerably greater as shown in the drawing for clarity in illustration. The width of each space between adjacent strips 14 is preferably about .030 inch. In the event that the paper is moved longitudinally of the backup plate 11 during the development step, as shown in the embodiment of FIGURES 5 and 6, the conductive strips 14 may be oriented angularly with respect to the direction of paper movement; that is, diagonally with respect to the longitudinal axis of back-up plate 11 rather than transversely thereto as shown in FIGURE 1. This angular orientation of the strips 14 will eliminate any undesirable effect of discontinuity due to the insulating spaces between the spaced strips 14.

The reference numeral 17 designates a series of conventional resistors which may be of the carbon-deposited or Wire-wound or other suitable type. Each of the resistors 17 is of a high resistance value, preferably in the range between 100,000 ohms to 50 megohms. One end of each of the resistors 17 is connected by a lead 18 to a respective one of the conductive strips 14. The opposite ends of all the resistors 17 are connected by loads 20 to a common lead 21 which is in turn connected to the terminal 22 of a double-pole double-throw switch indicated generally at 23. The other terminal 24 of the latter is connected by a lead 25 to the magnet 26 of a magnetic brush 27 of any conventional or suitable type. The switch 23 is also connected by leads 28, 29 to the

terminals

30, 31 of any suitable source of direct-current electric potential indicated schematically by the symbol for a battery and designated generally by the reference numeral 32.

It will thus be seen that upon closing of the double-pole double-throw switch 23, the voltage of the electric potential source 32 will be applied between the magnetic brush 2'7 and the conductive strips 14, with the resistors 17 in series between the respective conductive strips 14 and one process.

of the terminals of the electric potential source 32. Upon reversing the switch 23, the polarity of the applied bias potential will be reversed. The voltage between the

terminals

30, 31 of the electric potential source 32 may be in the range of 50 to 1500 volts, with the range of 200 to 800 volts being preferred for typical electrophotographic techniques such as those employed with the well-known zinc oxide resin-coated paper.

After the electrophotographic sheet has been exposed to a light image so as to form a latent electrostatic image on the front surface thereof, the sheet is placed with its rear surface against the back-up plate 11 and in contact with the conductive metal strips 14. The switch 23 is closed so as to apply the voltage of the electric potential source 32 between the strips 14 and the magnetic brush 27. The latter may be of either the rotary. type shown in FIGURES 5 and 6 and to be described in detail below, or may be of the conventional manual type which is held in the hand and manually applied during the development As the magnetic brush 27 moves over and in contact with the sheet front surface bearing the latent electrostatic image, the developer powder or toner particles, electro-statically charged by triboelectric interaction with the iron filings in the mass of developer mix 33 magnetically held to one end of the magnet 26, are wiped off and adhere to the charged areas of the front surface of the electrophotographic sheet so as to form a visible image thereon.

It will be apparent that as the magnetic brush 27 is wiped over, or otherwise moves with respect to, the marginal areas of the electrophotographic sheet so as to overlap an edge of the latter, a portion of the mass of developer mix 33 will contact the back-up plate 11. If the latter were simply in the form of a unitary conductive metal plate, as heretofore employed in the art before the advent of the present invention, and if this metal plate were connected directly to one of the

terminals

30, 31 0f the electric potential source 32 Without the interposi tion of the series resistors 17, as has heretofore been the practice, it will be apparent that the potential source 32 would be short-circuited at the point where the magnetic brush developer mass 33 were to contact the back-up plate. This would cause a large current to be drawn from the electric potential source 32 so as to reduce the applied bias potential below the desired value, and the entire backup would then be at the same potential as the brush.

However, in the present invention the conductive strips 14 are spaced and electrically insulated from each other and are connected to the electric potential source 32 through relatively high value resistors 17. It will thus be seen that when the magnetic brush 27 overlaps an edge of the electrophotographic sheet, the developer mass 33 will contact one or a few of'the'conductive strips 14 and a short circuit of the potential source 32 will be prevented by reason of the fact that the resistors 17 are in series with the conductive strips 14 and the potential source 32. As a result, the voltage of the applied bias potential is maintaind at substantially the same value during development of the marginal edge areas' of the electrophotographic sheet as during development of the intermediate areas thereof. Furthermore, only those few conductive strips 14 which actually contact the brush will be at zero potential with respect to the brush, and all of the other strips 14, constituting practically the entire area of the back-up plate 11 will be maintained at the full bias potential with respect to the brush.

Referring to FIGURE 2 wherein is shown a second embodiment of the invention, the reference numeral 11a indicates generally a back-up plate having a somewhat different construction than the back-up plate 11 of FIG- URE 1. The back-up plate 11a comprises a lower rectangular plate 34 which is made of a highly conductive material such as copper, bronze or aluminum. Superimposed upon the conductive plate 34 is a plate 35 composed of a high resistance semiconduetive material, such as any of the well-known conductive rubbers. As will be understood by those skilled in the art, the latter are rubber compositions to which a conductive material such as carbon black or superconductive carbon black has been added so as to enable the resistivity of the rubber to be reduced to a value as low as approximately ohms per cm. The resistivity of the conductive rubber employed for the semi-conductive plate 35 should be preferably in the range from 10 to 10 ohms per cmfi. Instead of conductive rubber, the plate 35 may be composed of any other semi-conductive material having suitable mechanical characteristics and a resistivity in the above-noted range.

The term semi-conductive, as used throughout this specification and the appended claims, will be understood to refer to the range of electrical resistivity between approximately 1() ohms per cm. to approximately 10 ohms per cmfi, in accordance with the accepted nomenclature as exemplified in the publication Battelle Technical Review, August 1957, page 9.

Secured to or embedded within the upper surface of the semi-conductive plate 35 are a series of spaced contiguous conductive strips 14 which may be identical to those previously described with respect to the embodiment shown in FIGURE 1. The embodiment of FIGURE 2 also comprises a double-pole double-throw switch 23, a direct-current source of electric potential 32 and a magnetic brush 27 arranged in the manner described above with respect to FIGURE 1. A lead 36 has one end connected to the conductive metal plate 34 and its opposite end to the terminal 22 of the'switch 23.

It will be seen that a portion of the semi-conductive material of plate 35 extends in series between the conductive plate 34 and each of the conductive strips 14. That is, each of the portions of the semi-conductive material below a respective one of the strips 14 is in effect a component having a substantial electrical resistance of a value in the semi-conductive range. Each of these portions or components of plate 35 has the equivalent effect of a respective one of the resistors 17 of FIGURE 1.

When the magnetic brush 27 overlaps an edge of the electrophotographic sheet having its rear surface in contact with the upper surface of back-up plate 11a", whereby a portion of the magnetic brush is in direct contact with one or two of the conductive strips 14, said portions or components of the semi-conductive material of plate 35 will be in series with the by-pass circuit thus established around the edge of the sheet, so as to prevent said circuit from constituting a low-resistance shortcircuit across the

terminals

30, 31 of the electric potential source 32. This prevents the drawing of a large current which would cause the bias potential to be reduced below the desired value during development of the marginal areas of the electrophotog'raphic sheet when the magnetic brush 27 overlaps the edges of the latter. Furthermore, as noted above with respect to the embodiment of FIGURE 1, all of the strips 14, with the exception of the one or few strips in contact with the brush, are maintained at the desired potential with respect to the brush.

Referring now to FIGURE 3, there is shown a third embodiment of the invention wherein the reference numeral 11b indicates generally a back-up plate comprising a metal conductive plate 34 upon which is superimposed a semi-conductive plate 37 composed of wear-resistant conductive rubber. The plate 37 of FIGURE 3 preferably has substantially the same electrical characteristics as the plate 35 of FIGURE 2, except that the latter may be composed of soft rubber whereas plate 37 of the present embodiment is preferably composed of hard rubber re- 7 quired for wear-resistant characteristics. This is advantageous in the event that the embodiment shown in FIG- URE 3 is employed in an arrangement wherein the elec trophotographic sheet slides against the back-up plate 11b during the development step in the manner shown in FIGURES 5 and 6. Since rubber. is normally a highly I insulating medium, the relatively low resistivity (preferably in the range of ohms per cm. to 10 ohms per cm?) of the material of plate 37 is provided by incorporating carbon black or super-conductive carbon black therein in the manner Well-known in the art.

Although the upper surface of plate 37 may be uniform and continuous, it comprises in effect a plurality of contiguous adjoining areas each composed of a material having an electrical conductivity in the semi-conductive range as this range is normally defined in the art and defined hereinabove. The component or portion of the material of plate 37 immediately below each of said contiguous surface areas is in effect a component having a substantial electrical resistance of a value within the semiconductive range. That is, each of said components is in effect connected in series between a respective one of said contiguous surface areas and the conductive late 34, whereby each component has the equivalent function of a respective one of the resistors 17 of the embodiment shown in FIGURE 1.

The conductive plate 34 of FIGURE 3 is connected by a lead 36 to a double-pole double'throw switch 23 and associated with the latter are a magnetic brush 27 and a direct-current source of electrical potential 32 in the manner described above with respect to the embodiments of FIGURES l and 2.

The mode of operation of the embodiment shown in FIGURE 3 is similar to that of the embodiments of FIGURES l and 2. The electrophotographic sheet is placed with its rear surface in contact with the upper surface of the semi-conductive plate 37 and the magnetic brush 27 is wiped across the sheet front surface bearing the electrostatic image. When the fibrous mass 33 composed of developer mix adhered to the magnet 26 of the magnetic brush 27 overlaps the edge of the electrophoto graphic sheet and contacts the upper surface of the semiconductive plate 37, a low-resistance short-circuit is prevented by reason of the fact that a portion or component of the material of plate 37 will be in series between the brush 27 and the conductive plate 34, thereby maintaining the desired bias voltage during the development of the marginal areas of the electrophotographic sheet.

Referring now to FIGURE 4, there is shown a fourth embodiment of the invention comprising a backup plate 110 comprising an insulating block or plate 12 similar to the plate 12 of the embodiment shown in FIGURE 1. Mounted in the plate 12 are a plurality of circular conductive elements 38. These may be in the form of thin circular discs secured to or imbedded within the upper surface 13 of plate 12, or they may be in the form of cylindrical plugs extending from the surface 13 down through the plate 12 to a substantial depth therein. The elements 38 may be composed of any highly conductive metal such as copper, bronze or aluminum, and may be in the form of thin foil, or an electrolytic or otherwise coated form of these metals. The conductive elements 38 are illustrated as circular and are preferably of about one-eighth inch in diameter. The center-to-center spacing of adjacent elements 38 is preferably about .180 to .200 inch.

There are provided a plurality of resistors 17, similar to those described above with respect to the embodiment of FIGURE 1, and each of the resistors 17 is connected by a lead 18 to a respective one of the conductive elements 38., The opposite end of each of the resistors 17 is connected by a lead 20 to a conductive metal plate 39 which is in turn connected by a lead 40 to the terminal 22 of a doublepole double-throw switch 23 associated with a direct-current source of electrical potential 32 and a magnetic brush 27 in the same manner described above with respect to the first three embodiments of the invention.

The mode of operation of the embodiment of FIGURE 4 will be apparent from the above description of the mode of operation of the embodiment shown in FIGURE 1, the conductive elements 38 being the equivalent and functioning in substantially the same manner as the conductive strips 14. The conductive plate 39 merely serves as a common lead connecting one end of each of the resistors 17 of FIGURE 4 to the lead 40 which is in turn connected by the switch 23 to one of the

terminals

30, 31 of the direct-current electric potential source 32.

Referring now to FIGURES 5 and 6, there is shown an arrangement wherein any of the embodiments shown in FIGURES 1 to 4 inclusive may be utilized with a rotary magnetic brush. A longitudinal container or trough 42 is provided with opposite end walls 43, 44 having openings therein for rotatably mounting a longitudinal shaft 45 formed of steel or any other ferromagnetic material. The

shaft is provided with a pair of shoulders or collars 46,

47 adapted to abut the exterior surfaces of the end walls 43, 44 so as to limit longitudinal movement of shaft 45. Mounted on the latter and fixedly secured thereto are a plurality of spaced parallel elliptical discs 48 also composed of a ferromagnetic material.

One end 49 of shaft 45 is rotatably mounted within an opening formed in the leg 50 of an L-shaped element 51 having another leg 52 to which is secured an element 53 of permanently magnetized material, such as Alnico or the like. Secured to element 53 is a planar pole piece 54. The latter is adjacent to a back-up plate 11, but is spaced therefrom by an air gap indicated at 55. It will thus be seen that a continuous magnetic circuit is established from element 53 through pole piece 54, air gap 55,

back-up plate 11, discs 48, shaft 45, and then through L-shaped element 51 to complete the circuit.

A gear 56 is fixedly secured to shaft 45 and is engaged by a pinion 57 secured to the shaft 58 of a conventional electric motor 59, whereby the latter may drivingly rotate the shaft 45.

The developer mix comprising toner particles and iron filings is placed within the trough 42 and is caused to adhere to the peripheries of the discs 48, as indicated at 60, by means of the magnetic field. The electrophotographic sheet, indicated generally at 61 and comprising a layer of backing paper 62 having a coating of photoconductive material 63 is fed vertically (as viewed in FIGURE 5) between the back-up plate 11 and the elliptical discs 48 while the latter are rotated about the axis of shaft 45 and through the developer mix in the trough 42. The sheet 61 may be in the form of a continuous web wound in a roll, or a rigid plate, or a single sheet of document size. The rear surface 64 of the electrophotographic sheet 61 contacts the back-up plate 11 and the front surface comprising the photoconductive layer 63 is wiped by the masses of developer mix clinging to the peripheries of the rotating discs 48.

In the event that the embodiment of the invention shown in FIGURE 1 is employed in the type of apparatus shown in FIGURES 5 and 6, the back-up plate 11 will comprise a plurality of conductive strips 14 imbedded in the surface of an insulating block 12 and in contact with the rear surface 64 of sheet 61. Each of the conductive strips 14 is connected by a lead 18 to a respective one of a plurality of resistors 17 (only one being shown in FIG- URE 5 for simplicity in illustration). The opposite end of each resistor 17 is connected by a common lead 21 to the terminal 65 of the arm 66 of a potentiometer indicated generally at 67. The latter is connected to a double -pole double-throw switch 23 and electric potential source 32 arranged as heretofore described. One terminal 68 of switch 23 is connected by a lead 69 to ground and shaft 45 is also grounded by a lead 70 to complete the circuit for the bias potential.

As the sheet 61 is moved with respect to the back-up plate 11 and the rotating discs 48, the toner particles will be deposited on the charged areas of the latter as indicated at 71, thereby forming a visible image. It will be understood that the rotary magnetic brush apparatus of FIGURES 5 and 6 may be employed with the inventive embodiments of FIGURES 2, 3 or 4 by substituting the 7 back-up plate 11a, l lb or 110 respectively for the back-up plate 11 shown in FIGURES and 6.

The unidirectional field established between the magnetic brush and the back-up plate, as a result of the application of the bias potential, will add to or subtract from the electric field due to the latent electrostatic image, depending upon the polarity of the applied bias potential. When the two electric fields are properly additive, the physical image formed by the deposited toner powder has a very high contrast characteristic and a minimum amount of spurious deposit. If it is desired to reduce the contrast, the potentiometer 67 may be adjusted to reduce the voltage of the applied bias potential. If it is desired to produce a reverse image during the development step, the switch 23 may be reversed to cause the electric field of the applied bias potential to buck or oppose the electric field of the latent electrostatic image.

Although the application of a bias potential to obtain these advantages and modes of operation during development of the intermediate areas of the electrophotographic sheet, was known prior to the present invention, the novel apparatus and method disclosed and claimed herein enable the bias potential to be maintained at the desired voltage during development of the marginal areas of the electrophotographic sheet.

The specific embodiments described in the above specification and illustrated in the accompanying drawings are merely illustrative of several forms which the invention may take in practice, and there will readily occur to those skilled in the art numerous modifications with-in the scope of the invention as delineated by the appended claims which are to be construed as broadly as possible in view of the prior art.

I claim:

1. An apparatus for developing an electrophotographic sheet having a previously applied latent electrostatic image on the front surface thereof, said apparatus comprising means adapted to contact the rear surface of said sheet, said means comprising a plurality of contiguous areas ach composed of a material having an electrical conductivity of a value at least in the semi-conductive range, and a plurality of spaced conductive elements disposed in spaced electrically insulated relation to each other and positioned adjacent said means adapted to contact the rear surface of said sheet, a magnetic brush adapted to contact the front surface of said sheet, a source of electrical potential having a pair of terminals of opposite polarity, means connecting said magnetic brush to one of said terminals, and means connect-ingtsaid areas to the other of said terminals, said last-recited means having a substantial electrical resistance of a value no greater than that of the semi-conductive range. I

2. An apparatus as recited in claim 1 wherein said means having a substantial electrical resistance comprises a plurality of resistors, each of said resistors being connected to a respective one of said conductive elements,

each of said resistors being in series between its respective conductive element and said other terminal.

3. An apparatus as recited in claim 1 wherein said last-recited means comprises a plurality of components each having a substantial electrical resistance of a value no greater than that of the semi-conductive range, each of said components being connected to a respective one of said areas, each of said components being in series between its respective area and said other terminal,

4. An apparatus as recited in claim 1 wherein said means adapted to contact the rear surface of said sheet comprises a plate composed of electrically insulating material, each of said contiguous areas comprising a metallic conductive element, said conductive elements being secured to the surface of said plate and being in spaced configuous relation to each other.

5. An apparatus as recited in claim 4 wherein said means having a substantial electrical resistance comprises a plurality of resistors each connected in series with a respective one of said conductive elements.

6. An apparatus as recited in claim 1 and comprising a receptacle for containing a mixture of toner particles and iron filings, means mounting said magnetic brush for rotation in contact with said mixture, said means adapted to contact the rear surface of said sheet being spaced from said 'magnetic brush whereby the electro-photographic sheet may pass through the space therebetween,

and drive means for rotating said brush to cause the latter to convey said mixture from the receptacle to said front surface of the sheet.

7. An apparatus as recited in claim 6 wherein said means adapted to contact the rear surface of said sheet comprises a plate composed of electrically insulating material, each of said contiguous areas comprising a metallic conductive element, said conductive elements being secured to the surface of said plate and being in spaced contiguous insulated relation to each otherr 8. An apparatus as recited in claim 7 wherein said means having a substantial electrical resistance comprises a plurality of resistors, each of said resistors being connected to a respective one of said conductive elements, each of said resistors being in series between its respective conductive element and said other terminal of said source of electrical potential.

References Cited in the file of this patent UNITED STATES PATENTS Young Mar. 26, 1957 Giaimo June 16,

and