US3456109A

US3456109A - Method and means for photoelectrostatic charging

Info

Publication number: US3456109A
Application number: US592561A
Authority: US
Inventors: Stanley A Gawron
Original assignee: Multigraphics Inc
Current assignee: AB Dick Co
Priority date: 1966-11-07
Filing date: 1966-11-07
Publication date: 1969-07-15
Anticipated expiration: 1986-07-15
Also published as: NL140633B; GB1201042A; BE705817A; DE1597806A1; DE1597806B2; NL6714843A

Description

July 15, 1969 s. A. GAWRON METHOD AND MEANS FOR PHOTOELECTROSTATIC CHARGING Filed Nov. 7. 1966 F. .E. 2H er/M 16% BR MW .5 CS L L m Em WT vm nm We 6 T S T E0 00 NP PD I1\/V1N"/UR STANLEY A. GAWR ON sail W ATTORNEY United States Patent 3,456,109 METHOD AND MEANS FOR PHOTOELECTRO- STATIC CHARGING Stanley A. Gawron, Mount Prospect, 11]., assignor to Addressograph-Multigraph Corporation, Mount Prospect, Ill., a corporation of Delaware Filed Nov. 7, 1966, Ser. No. 592,561 Int. Cl. H01t 19/00; G03g 13/02 U.S. Cl. 250--49.5 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to photoelectrostatic imaging, and more particularly, to means for and methods of uniformly charging a photoelectrostatic member.

In the art of photoelectrostatic imaging, it is conventional to reproduce a graphic subject on a photoelectrostatic member which is composed of a photoconductive layer applied to a base support. The process involves imparting a blanket electrostatic charge to the photoconductive layer through the use of a corona discharge electrode or the like. This step of charging the photoconductive layer renders it sensitive to radiation of a particular wavelength, generally in the ultraviolet and visible wave bands.

While in this sensitized condition, the member is exposed to a radiation pattern produced by illuminating the graphic subject to be reproduced. On such exposure the charge is dissipated in the area where the photoconductive layer is struck by light, thereby forming an electrostatic charge pattern. This pattern is then developed by the application of an electroscopic powder forming a visible image which may be fixed by various known techniques.

It is conventional in photoelectrostatic imaging equipment to impart the electrostatic charge, that is, the charge that sensitizes the member, by passing the member between two spaced apart opposing corona discharge devices, each being connected to a DC. corona generating source of opposite polarity. One of the corona discharge devices emits negative charges which are conducted to one surface of the member while the other electrode produces a positive corona discharge conducting positive charges to the opposite surface of the member.

The effect of such a charging arrangement is to pass the member between a high potential field in which the opposite sides of the member are oppositely charged. In this type of operation there results a zero potential at the interface between the photoconductive layer and the base support. The same effect can be realized by replacing one of the electrodes with a conductive support placed against the base support of the photoelectrostatic member.

The above-described charging techniques are not entirely satisfactory because of the nonuniformity of charge deposition. This is particularly true where the photoconductive layer is comprised of finely divided photoconductive particles such as zinc oxide dispersed in an insulating resin binder. However, the problem of nonuniform charge deposition has been observed to occur on other photoelectrostatic members such as the conventional selenium plate.

Another shortcoming of the heretofore known techniques of sensitizing the photoelectrostatic member resides in the high charge levels applied to the photoconductive layer because of the high potentials necessary to be supplied to the emission electrode in order to reach and overcome the ionic emission threshold of these corona devices. At the high charge levels encountered it has been found that the exposure to the light pattern does not adequately dissipate the high charge and consequently the photoconductive layer in the non-image layer will tend to attract electroscopic material producing what is known as background.

In liquid development systems the latent image-bearing copy sheet is particularly receptive to the deposition of toner along the edge of the copy first to be immersed in the developing liquid. The attraction of toner to the leading edge is believed caused by the copy sheet appearing to have an overall charge polarity the same as the sensitizing charge which is opposite in polarity to the particles dispersed in the insulating liquid. This deposition of toner along the lead portion is undesirable because it obscures the legibility of the information and detracts from the appearance of the copy.

The use of corona-type discharge devices offers the advantage of imparting a charge to the surface without having to make physical contact with the photoconductive layer.

It is a general object of the" present invention to provide improved photoelectrostatic reproductions in which the attraction of electroscopic powder is controlled to be uniformly deposited in the image area and minimizing the deposition in the background.

It is a further object of this invention to provide improved means for imparting a controlled blanket electrostatic charge to a photoelectrostatic member at a charge level that is optimum for producing a copy having a uniformly dense image and, at the same time, a low level of background.

It is another object of this invention to provide improved charging means for controlling the net charge applied to the photoelectrostatic member which charging means is simple in its construction and operation.

In achieving the foregoing objects, there has been provided a modulating discharge electrode acting on the photoconductive layer in addition to the conventional pair of electrodes which are disposed in spaced apart opposing relation to one another forming an accessway through which the photoelectrostatic member may pass during the charging operation.

The movement of the photoelectrostatic member relative to the pair of electrodes produces a first charge on the photoconductive layer and a charge of opposite polarity on the base support. The resulting charge renders the photoconductive layer sensitive to electromagnetic radiation. Immediately after the application of the sensitizing charge a modulating charge is applied to the photoconductive layer which is opposite in polarity to the first charge. The modulating corona discharge electrode establishes a field between the electrode and the base support which is at some potential above ground.

A better understanding of the invention may be had from the detailed description which follows when read together with the drawing in which:

FIGURE 1 is a schematic of the apparatus embodying this invention;

FIGURE 2 is a perspective view of a charging unit constructed in accordance with the present invention.

Referring to FIG. 1, there is shown a photoelectrostatic member 10 comprising a photoconductive layer 12 applied to a base support 14 being processed through the charging assembly identified generally as 16. A wide variety of materials may be used for the base support material 14 such as metal foils, plastic materials, glass and the like. The preferred material for the base support is usually paper.

The photoconductive layer 12 may be formed by vacuum deposition of amorphous selenium onto a metal backing plate or it may comprise finely divided photoconductor particles such as zinc oxide dispersed in an insulating resin binder and applied to a paper base. The preparation and formation of photoelectrostatic members is now conventional in the art and need not be treated here.

The photoelectrostatic member is arranged to have relative movement with respect to the charging assembly 16. The chargeable member may be supported in a fixed position and the charging assembly be made to traverse the length of the member.

For purposes of the instant description the photoelectrostatic member will be considered to move relative to the charging assembly it being understood that it is necessary to effect only relative movement between the charging assembly and the member.

The charging assembly 16 is comprised of a pair of primary charging electrodes 18 and 20 which are disposed in spaced apart relation on either side of the path taken by the photoelectrostatic member 10. The first corona electrode 18 is comprised of at least one fine conductive wire 22 which is mounted parallel to the photoelectrostatic member and extends in a direction transverse the direction of movement. The conductive wire 22 is connected to a high potental D.C. corona generating source 22 which generates a high negative potential causing the wire 22 to emit negatively charged ions to the surface of the photoconductive layer 12. The wire 22 is surrounded by a conductive shield 24 which directs the ions in a direction generally perpendicular to the path of travel of the member. The second electrode is similarly comprised of a fine conductive wire 26 which extends transverse the direction of movement of the photoelectrostatic member and is likewise contained in a shield 28. The wire 26 is connected to a D.C. corona generating high potential source which generates a positive potential causing positive ions to be sprayed on the surface of the base support 14. As part of the charging assembly and connected therewith is a third electrode 30 comprised of a fine conductive wire 32 contained in a conductive shield 34 which electrode functions to modulate the electrostatic charge which has been deposited on the photoconductive layer 12 by the primary electrode. The fine conductive wire 32 of the modulating electrode 30 is connected to the positive potential source 25 so that the photoconductive layer as it passes through the assembly first receives a saturation charge of one polarity and thereafter passes beneath the modulating electrode to have a charge of lesser potential than and opposite in polarity to the charge deposited on the photoconductive layer 12 by the primary electrode arrangement.

It will be appreciated that the pair of electrodes 18 and 20 which deposit the primary electrostatic charge on the photoelectrostatic member 12 creates a high potential field between the

conductive wires

22 and 26 which are respectively connected to a negative D.C. corona generating source and positive D.C.

corona generating source

23 and 25, respectively. In the circumstance that the D.C. generating source is in the range of 4,000 to 6,000 volts, the wire 22 will carry a corresponding voltage of, for example, minus 6,000, and the wire 26 a potential of plus 6,000, producing a total field of 12,000 volts. Such a high field causes the level of charge deposition on the layer 12 to rapidly reach a saturation level in the range of 400 to 600 volts.

Understandably, the efficiency of the charging arrangement of the primary electrodes is desirable since in photocopying procedures it is desirable that the sensitization step as well as all subsequent steps be accomplished as efficiently and in as short a time as possible.

The modulating electrode 20 is disposed so that its modulating charge is applied shortly after the primary sensitizing charge has been deposited taking its power from the positive D.C. potential source 25. This electrode is intentionally arranged so that the field between the fine conductive wire 32 and the photoelectrostatic member 10 used as the reference for the electrode is of a sufiicient strength to properly modulate or alter in part the surface charge of the member. Understandably, the degree of modulation may also be regulated byincreasing or decreasing the distance between the electrode 30 and the charged surface of the member 10, by adjusting the potential applied to the electrode 30, and by varying the speed at which the member is moved past the electrode. The above factors may be employed to adjust the modulating effect of the electrode 30 without the deleterious effect of neutralizing the primary charge.

The charge levels necessary to achieve high quality images of proper density are known to be in the range of 400 to 600 volts, particularly in the circumstance when the member is of the zinc oxide type. This charge level is achieved in fractions of a second as it passes through the primary electrode. The measured effect of the modulating electrode is to neutralize areas of peak charge resulting from the primary charging and also to generally lower the overall negative charge that has been applied.

The precise mechanism by which the benefits of the modulating charge are realized is not fully understood. However, some theories have been advanced and their expression here is merely for purposes of contributing to a better understanding of the invention without limiting same.

The photoconductive surface of any photoelectrostatic member, whether it be produced by the vacuum deposition of metal or by the incorporation of photoconductive particles in an insulating resin binder will be somewhat irregular, probably more so in the latter situation, because of the irregularity of the pigment particles. The passage of the photoelectrostatic members between a pair of spaced apart primary electrodes such that the photoconductive layer is exposed to either a high negative or high positive potential, the high points of the irregularities on the surface will tend to serve as sites for charge concentrations. In other words, a measurement on the surface of the photoconductive layer after being exposed to a primary charge of about 500 volts may vary between different points anywhere from 400 to 600 volts. Understandably, this nonuniformity of charge deposition across the surface of the photoconductive member will result in a nonuniform electrostatic image after it has been exposed to a pattern of light and shadow. Development of such a nonuniform electrostatic image will, of necessity, produce a nonuniform material image since the force of the attraction exerted by the areas of charge concentration upon the mass of electroscopic powder will be greater than in other areas giving an irregular material image.

As explained earlier, exposure to a light image will cause the charged areas that are light-struck to be dissipated in those areas corresponding to the background or nonimage portions of the graphic subject matter. When in the first instance the photoconductive layer is charged to a high level, exposure to light does not consistently discharge the layer in the light-struck areas to a level where there will not be attraction of electroscopic particles. It is suggested here that a particular photoelectrostatic member has a threshold voltage below which electroscopic powder will not adhere to its surface. These voltages are usually very nominal and for the most part are less than 50 volts.

Thus, the application of a modulating voltage to the primary sensitizing voltage, that is, one that is opposite in sign and which does not operate as efficiently to deposit its charge on the photoelectrostatic member accomplishes the following:

(1) Neutralizes peak concentrations of charge on the photoelectrostatic member to correct for uneven attraction of electroscopic powder to the image area,

(2) Has an overall effect of lowering the primary voltage so that upon exposure to electromagnetic radiation the exposed areas reach a lower charge level minimizing the attraction of electroscopic powder to the background area, and

(3) Causes the photoelectrostatic member to take an overall charge having the same polarity as the toner particles, hence repelling toner from the leading edge without interfering with the attractive force that the charges in the image areas have for the toner particles.

It has been suggested that the primary charge be reduced in the first instance so as to avoid the necessity of the modulating charge. This has been found unsuccessful since the potentials necessary to achieve corona discharge or ionization of the atmosphere in the vicinity of the corona wire results in charge levels on the photoconductive members that are quite high and do not provide an opportunity for a sufliciently fine adjustment. The photoconductive layer in a dark adapted condition has a resistivity in the range of to 10 ohm centimeters so that it readily accepts a charge. In the charged condition, that is, after it immediately leaves the primary electrode, its resistivity is in the range of 10 to 10 ohm centimeters. It will readily be appreciated that when it has passed out of the range of the primary electrode the electrical environment has changed to one of greater resistivity so that its capacity to accept a charge is substantially decreased. Hence, electrodes which emit charged ions acting into a high resistance medium will reduce the current flow and substantially curtail the rate of charging as compared to the application of the primary charge. Accordingly, the photoelectrostatic member with its higher resistivity and the electrode 30 operating against a highly insulating surface tends to result in producing a self-regulating condition in which the modulating charge gives just the precise effect.

Referring now to FIGURE 2, there is illustrated an embodiment of the charging device according to this invention. The fine conductive wires are represented by the

numerals

22, 26 and 32 and correspond to the wires shown in the

electrodes

18, and 30 illustrated in FIG- URE 1. The electrodes shown in the embodiment of FIG- URE 2 employ multiple strands of conductive wires for each electrode.

The charging assembly 16 is formed by mounting the electrodes in a frame 40 made of an insulating material such as an acrylic polymer, with the primary electrodes 42 and 44 arranged opposite one another and the modulating electrode 46 located adjacent to and spaced from the electrode 42 so as to provide a dielectric medium therebetween. The

conductive wires

22, 26 and 30 are stretched inside the

conductive shields

48, 50 and 52 being secured to the shield structure by insulating connectors 54. The conductive shields are essentially of a shallow box-like construction enclosing the wires on five sides with one face of the enclosure being open. The electrodes are positioned with the open faces parallel to the surface of the photoelectrostatic member 10 so that the discharge from the corona wires is generally perpendicular to the path of travel of the member 10 through the charging assembly.

Over the open face of each of the shield structures there is provided an upper and

lower support member

60 and 62, respectively, for maintaining the member 10 in a supported position, out of contact from either of the energized conductive wires and guiding the sheet along a path uniformly distant from each of the electrodes. The

support members

60 and 62 comprise frames 64 over which are laced strands of an insulating filament material 66 such as a polyamide plastic or fluorocarbon plastic material. The lacings 66 are stretched over the frames 64 so as to present an oblique pattern to the direction of the member 10 and not interfere with the overall deposition of charge.

The electrodes 44 and 46 are connected to a high positive potential corona generating power source 68 operating the range of 4 kv. to 6 kv., preferably about 5 kv., and at a slightly lower potential than the high negative potential corona generating source 70 which supplies energy to the electrode 42 and operates at 5.5 to 6 kv., preferably at 5.5 kv. The modulating electrode 46 is connected to the high positive potential source that powers the electrode 44 being supplied a potential of about 5 kv. The electrode may be spaced from /2" to 2 /2" from the photoconductive member 10 at a distance that will lay down a charge less than the primary charge. The charge laid down during the time of exposure is rather small, somewhere in the range of 50-150 volts. The copy sheet passes through the electrode assembly at a rate ranging from 10 to 30 feet per minue. Passage of the member 10 through the charging assembly 16 with the electrodes connected to their respective power supplies as above described results in imparting a primary electrostatic charge, negative in polarity on the photoconductive layer in the range of 400 to 550 volts. The exact saturation charge level which is deposited by the primary electrodes 42 and 46 will depend on a number of factors such as the kind of photoconductor, the thickness of the photoconductive layer on the base support, the ratio of the photoconductive particles to the insulat' ing resin binder in the circumstance where the photoconductive layer is made up of photoconductive pigments, and also in the type of resin binder. The modulating electrode 46 when connected to a 5 kv. power source will modulate the primary charge lowering it approximately 10 to 20% from its original level.

It is to be 'understood that means other than double corona wires may be employed for imparting the primary sensitizing charge to the photoconductive layer. For example, the primary sensitizing charge may be accomplished by roller charging such as disclosed in Tregay et al. US. Patent No. 2,980,834, entitled Charging of Photoconductive Insulating Material, or by a single corona wire in conjuction with a conductive surface or a metal plate as a ground plane.

It will be appreciated that the polarity of the modulating potential must be opposite to the polarity of the primary potential applied to the photoconductive layer. Reference in his description to a primary charge of a negative potential to the photoconductive layer is merely exemplary and the polarity of the primary charge may be positive as in the case of sensitizing a selenuim photoelectrostatic member in which case the modulating electrode would then be connected to a high negative corona generating power source.

The particular configuration of the electrodes as shown herein represents a preferred embodiment of the invention but it is understood that many changes and modifications may be made by one skilled in the art without departing from the essential concept of the invention as covered in the appended claims.

What is claimed is:

1. A charging assembly for imparting uniformly distributed electrostatic charges to an electrophotographic member comprising first, second and third corona discharge electrodes, each including a plurality of spaced parallel, fine conductive wires carried on a non-conductive support, said second electrode mounted adjacent said first electrode and said third electrode mounted in opposed spaced parallel relation directly opposite said first electrode, means for applying a high electrical potential to said first and third electrodes opposite in polarity, means for applying a high electrical potential to said second electrode lower than the potential applied to said first electrode and opposite in polarity, conductive shielding means connected to a reference potential partially surrounding the conductive wires whereby the corona emitted from said second electrode is of a lesser intensity than the corona emission from said first electrode without interfering with the sensitizing effect of said first and third electrodes.

7 2. The charging apparatus as claimed in claim 1 wherein said D.C. source supplies a high negative potential to said first electrode and a high positive potential to said second and third electrodes.

charges to a photoelectrostatic member having a photoconductive layer applied to a base support to render said layer responsive to electromagnetic radiation comprising a pair of corona discharge electrodes each including at least one fine conductive wire carried on a nonconductive support, said electrodes being located in spaced relation directly opposite one another, means for locating said photoelectrostatic member between said charging electrodes, a third charging electrode comprising at least one fine conductive wire carried on a nonconductive support and adjacent one of said pair of electrodes facing the photoconductive layer, means for moving said member sequentially past said pair of electrodes and said third electrode, a high energy source for supplying oppositely poled D.C. potentials to said pair of electrodes and a DC. potential to said third electrode poled oppositely and lower in potential than the potential applied said adjacent electrode, whereby the intensity of the corona emitted from said third electrode is of a lesser intensity than the emission from either of said pair of electrodes without interfering with the sensitizing effect of said paired electrodes.

6. The method of sensitizing a photoelectrostatic member comprising a photoconductive layer on a base support comprising the steps of:

(a) sensitizing said photoconductive layer by locating the member between a first and second corona discharge electrodes located in spaced relation directly opposite each other, spraying oppositely poled charges onto the photoconductive layer and the base support respectively;

(b) spraying another charge onto said layer subsequent to the charging action of said second electrode from a third electrode which charge is opposite in polarity to the charge sprayed by said first electrode and of lower potential,

whereby the corona emission from said third electrode is of a lower intensity than the corona emission from said first electrode.

7. The method of sensitizing a photoelectrostatic member as claimed in claim 6 wherein said superimposed charge is applied to the charged portionts of the photoconductive layer concurrent with said portions of said layer passing between said first and second corona electrodes.

8. The method as claimed in claim 6 wherein said first electrode deposits a high negative charge to the photoconductive surface and said second and said third electrodes apply a high positive potential.

9. In electrostatic photography, means for imparting electrostatic charges to a photoelectrostatic member having a photoconductive layer applied to a carrier to establish therein a resistivity in the range of 10 -10 ohmcentimeters comprising primary charging electrode means for applying to the layer a primary sensitizing charge of one polarity and potential; modulating electrode means adjacent said primary charging electrode means for applying to the layer subsequent to the application of the primary charge a modulating charge of a polarity opposite that of the primary charge, said modulating charge being at a potential less than the potential of the primary charge to affect modulation of the primary charge without altering the sensitizing character of said primary charge; means for first locating the member in charging relationship with said primary charging electrode means and then locating the member in charging relationship With said modulating electrode means whereby the corona emission from said modulating electrode is of a lower intensity than the corona emission from said primary electrode means.

References Cited UNITED STATES PATENTS 2,790,082 4/ 1957 Gundlach 250-495 2,965,481 12/1960 Gundlach 961 3,244,083 4/ 1966 Gundlach 951.7

WILIIAM F. LINDQUIST, Primary Examiner US. Cl. X.R. 250