GB2079067A

GB2079067A - Apparatus and method for generating ions

Info

Publication number: GB2079067A
Application number: GB8021086A
Authority: GB
Original assignee: Dennison Manufacturing Co Ltd; Dennison Manufacturing Co
Current assignee: Dennison Manufacturing Co Ltd; Dennison Manufacturing Co
Priority date: 1977-10-25
Filing date: 1978-10-25
Publication date: 1982-01-13
Also published as: GB2087312A; GB2087312B; GB2079067B

Abstract

Apparatus and method for generating ions, for example, for use in electrostatic printing and copying. An ion generator comprises two electrodes separated by a solid dielectric 213, with a high frequency alternating potential used to create an air gap breakdown in a region at a junction of one of the electrodes and the solid dielectric. A direct current extraction potential applied to the junction electrode 215 is used to extract ions from the discharge thus created. The generator may also include a third screen electrode 219 which is separated from the junction electrode 215 by a solid dielectric 217 and which serves to control the ion flow. <IMAGE>

Description

SPECIFICATION Apparatus and method for generating ions Background of the Invention This invention relates to a method and apparatus for generating ions, for example for use in electrostatic printing and photocopying, particularly at high speeds.

Electrostatic printers and photocopiers share a number of common features as a rule, although they carry out different processes. Electrostatic printers and photocopiers which are capable of producing an image on plain paper may generally be contrasted in terms of the method and apparatus used to create a latent electrostatic image on an intermediate member. Copiers generally do so by uniformly charging a photoconductor electrostatically in the dark, and optically exposing the charged photo-conductor to an image corresponding to the image to be reproduced. Electrostatic printers use non-optical means to create a latent electrostatic image on a dielectric surface, in response to a signal indicative of an image to be created.In theory, after creation of the electrostatic latern image, the same apparatus could be used to carry out the common steps of toning the imaging, transferring it to plain paper, and preparing the member bearing the electrostatic latent image for a subsequent cycle, usually by erasure of a residual latent electrostatic image. It would, in.fact, be desirable to standardize the apparatus to perform these functions.

The toning of the electrostatic latent image on a photo-conductor and subsequent process steps raise problems not present in electrostatic printers. The residual toner must be removed from the photo-conductor generally by a cleaning brush. The cleaning process, frequently repeated, can damage the delicate photo-conductor surface. Furthermore, the numerous process steps lead to a costly and complex photocopying system.

A solution to this problem, known in the art, involves a transfer of the recorded latent electrostatic image from the photo-conductive member to a more durable dielectric member, where development, transfer, and cleaning occurs. This confines the photo-conductor to a recording function, perhaps with post transfer erasure of any residual electrostatic image.

A system utilizing this concept is described by G. Krulik and H. Sable in U.S. Patent No.

3,937,571, and by H. Sable in U.S. Patent No. 3,907,560. Here, the latent electrostatic image on an image drum is formed by means of an ion modulating screen, which allows the ions to pass in a pattern corresponding to the original image, and thence onto the image drum. Use of such a screen is awkward, however, and in particular results in an excessive first-copy time.

Another electrophotographic system of this nature is disclosed by W. R. Buchan et al in U.S.

Patent No. 3,947,11 3, and U.S. Patent No. 4,015,017. In this method, toner is transferred from a photoconductive drum to an intermediate silicone transfer belt. This apparatus is similarly cumbersome, and does not completely avoid the necessity of cleaning residual toner from the photoconductive member.

Systems utilizing charge transfer between two insulating sheets have been analyzed, and in the realm of photocopying, this phenomen has been given the acronym T.E.S.I, standing for Transfer of Electrostatic Image. This process is described in Xerography and Related Process, edited by John H. Desrauer and Harold E. Clark, The Focal Press, London and New York, 1965, at page 432. T.E.S.I. relies on an air gap breakdown in the region between the two insulating members, which results in a transfer of charge from one member to another through an ionization of the intervening air. The special problem which is associated with the transfer of charge upon the approach of two insulating sheets with an external applied potential is that of disruptive transfer of charge. Disruptive charge transfer typically results in a mottling of the transferred image.

A problem which often occurs in conventional electrophotographic apparatus is that of undesirable photo-conductor discharge characteristics. Between uniform charging and exposure of the photoconductor, there is invariably some loss of potential due to so-called dark discharge.

During exposure to the light and shadow image, the photoconductor theoretically loses its charge according to the intensity of light exposure and the length of time of such exposure.

Discharge curves (plots of photoconductor potential as a function of time), however, invariably do not show a linear function of photoconductor potential with respect to time; the rate of discharge generally decreases with time, and the curve levels off at a residual potential, below which no discharge occurs. These characteristics result in a smaller contrast potential-the difference between the residual potential and the potential immediately before exposure--which decreases the toner image contrast. Furthermore, non-linearity in the high voltage region of the discharge curve results in a loss of fidelity for the electrostatic counterpart of the original optical image.The presence of a residual potential in a high speed photocopying device leads to the further problem of residual potential buildup, which occurs when there is insufficient erasure of the residual image between cycles.

Numerous approaches have been taken in the formation of an electrostatic latent image for electrostatic printing. Common techniques include the use of air gap breakdown, corona discharges, and spark discharges. Other proposed techniques employ tribo-electricity, radiation (Alpha, Beta and Gamma, as well as x-rays and ultraviolet light) and microwave breakdown.

A particular previously proposed approach utilizes metal styli at minute distances from the surface of the dielectric transfer drum. The styli are electrically pulsed to provide a latent electrostatic image by air gap breakdown. This technique has the disadvantage of not allowing for multiplexing of the charging styli. In addition, the necessity for maintaining a very small air gap breakdown distance requires extremely close tolerances which limits the practicability of this technique. It is necessary that the gap spacing be maintained between about 0.005 and 0.02 mm in order to be able to operate with applied potentials at reasonable levels, and maintain charge image integrity. Even then, the latent charge image is not uniform, so that the resultant electrostatically toned image lacks good definition and dot fill.

Another type of electrostatic printer found in the prior art employs an ion source in the form of a corona point or wire used together with an image defining mask. Corona discharges are also used almost exclusively in electrostatic photocopiers to charge the photoconductive surface prior to exposure. Unfortunately, standard corona discharges provide limited currents. The maximum discharge current density heretofore obtained has been of the order of 10 microamperes per square centimetre. This can impose a severe printing speed limitation. In addition, coronas can create significant maintenance problems. Corona wires are small and fragile and easily broken. Because of their high operating potentials, they collect dirt and dust and must frequently be cleaned or replaced.

An alternative technique for forming high density corona discharges is to use high velocity air streams. For example, if high pressure air is employed with a small orifice at the corona discharge point, current densities as high as 1000 microamperes per square centimetre are reportedly obtainable (Proceedings of the Conference on Static Electrification, London 1967, Page 1 39 of The Institute of Physics and Physical Society, London SW1). This technique is awkward, however, and requires both a pressurized air source and critical geometry in order to prevent premature electrical breakdown.

Another method of forming ions, which is particularly useful in electrostatic applications, uses an electrical spark discharge. Representative U.S. Patents are B.E. Byrd 3,321,768; H. Epstein 3,335, 322, C.D. Hendricks Jr. 3,545,374; and W.P. Foster 3,362,325. A low energy spark discharge technique is described by Krekow and Schram in IEEE transactions on Electronic Devices, E.D.-21 No. 3, Page 189, March, 1974. The electrical spark discharge is objectionable, however, where uniform ion currents are desired or required. This is particularly true where the discharge occurs over the surface of a dielectric.

After formation of the latent electrostatic image on a dielectric member in electrostatic printing, or after a transfer thereto by TESI in a photocopier, it is desirable to employ a dielectric surface of sufficient resistivity to retain the latent electrostatic image until toning. Furthermore, in the case of a system involving a pressure transfer of the toned image onto a further member, it is preferable that the dielectric surface possesses the smoothness and hardness properties which facilitate a proper transfer.

According to the invention described and claimed in the present Applicants' co-pending British Patent Application No. 41862/78 from which the present Application is divided, electrophotographic apparatus employing a double transfer of an image comprises: a photoconductor member containing a photoconductive surface layer and a conducting inner substrate; means for uniformly charging said photoconductive surface layer; means for exposing the uniformly charged photoconductive surface layer to a pattern of light and shadow representing an original to be reproduced, whereby the surface layer is selectively discharged and a latent electrostatic image is produced thereon;; a dielectric image drum containing an insulating surface and conducting substrate, onto which said latent electrostatic image is transferred by means of the ionization of air in an air gap between said image drum and said photoconductive member; a direct current potential difference is applied between the conducting inner substrate of the photoconductor member and the conducting substrate of the dielectric image drum, which induces an electrical stress in the air gap and enhancing the ionization of air therein; means for toning said latent electrostatic image to form a visible counterpart; and means for transferring the toned, visible image to a receptor.

It is an object of the present invention to provide a system for generating ions suitable for use in such apparatus for the ionisation of air.

According to one aspect of the present invention, a method of generating ions comprises applying an alternating potential between first and second electrodes separated by a solid dielectric member, with an air gap region at a junction of the first electrode and the solid dielectric member, to cause an electrical discharge in the air gap region.

According to a second aspect of the invention, apparatus for generating ions comprises a solid dielectric member; a first electrode on one side of said solid dielectric member with an air gap region at a junction of the first electrode and solid dielectric member; a second electrode on an opposite side of said solid dielectric member; and means for applying an alternating potential between said electrodes to produce an electrical discharge in said air gap region.

According to a third aspect of the invention, apparatus according to the second aspect is in combination with a cylindrical dielectric member; means for extracting ions from said apparatus for generating ions to form a latent electrostatic image on said cylindrical dielectric member, means for toning said latent electrostatic image to form a visible counterpart thereof, and means for transferring the toned, visible image by pressure to a receptor to provide electrostatic printing.

According to a fourth aspect of the invention apparatus according to the second aspect is in combination with: a rotatable imaging drum having as conductive core and a dielectric layer thereon, means for extracting ions from said apparatus for generating ions to form a latent electrostatic image on said dielectric layer, means for toning said latent electrostatic image, and a rotatable pressure drum in contact with said imaging drum for transferring and fusing the toned image on an image receiving medium which passes between the imaging drum and the pressure drum at the point of tangency thereof to provide electrostatic printing.

Further details of the invention are set forth in the subsidiary claims but there will now follow a broad discussion of certain preferred embodiments and applications of the invention and there will subsequently be given a detailed description of the embodiments.

The preferred ion generator of the invention may be used to precharge a photoconductor and form an electrostatic latent image and may be used in other applications, but is particularly adapted for use in both photocopying and electrostatic printing.

The preferred photocopier and the preferred electrostatic printer use different means to create a latent electrostatic image on a dielectric member, but thereafter may employ identical apparatus to tone the electrostatic latent image and to effect subsequent processing. In the photocopier, an electrostatic image is formed on a photoreceptor member by conventional optical means, and is transferred to a dielectric member by TESI. In the electrostatic printer, an electrostatic latent image is formed on a dielectric member by an ion generator.

The preferred electrophotographic apparatus comprises a photoconductor member, a dielectric image drum, and various process stations. The photoconductor member contains a photoconductive surface and a conducting inner substrate, while the dielectric image drum contains an insulating surface layer and a conducting substrate. The above members preferably are cylindrical drums. In this embodiment, a latent electrostatic image is formed by uniformly charging the photoconductive surface in the dark and exposing it to a pattern of light and shadow corresponding to the original image to be reproduced. The latent electrostatic image is next transferred to the surface of the dielectric image drum. An erase lamp may be used to discharge a residual latent image on the photoconductive surface after image transfer.The latent electrostatic image is transferred from the photoconductor member to the dielectric image drum by bringing the surface of the latter into either contact or close prioximity with the image bearing region of the former. An external bias potential may be introduced between the conducting substrates of these members. Charge transfer is effected by means of an air gap breakdown, upon achieving a threshold potential using an ion generating system in accordance with the invention. The photoconductor member may contain a semiconducting layer between the photoconductive surface and the conducting substrate. This construction prevents a disruptive charge transfer from such member to the dielectric image drum, and enhances the quality of the transferred latent electrostatic image.

In the preferred electrostatic printer, a latent electrostatic image is formed in a dielectric image member by an ion generator in accordance with the invention which applies a potential between two electrodes separated by a solid dielectric member to cause an electrical air gap breakdown in fringing field regions. Ions thus produced can then be extracted from the discharge and applied to a conductive support with a dielectric coating. The discharge initiating potential is a high frequency alternating voltage, and the extraction is accomplished using a direct voltage.

The extracted ions can be used directly or applied to particulate matter which is moved under the action of an electric field. Such charged particles can be used in forming an electrostatic pattern using, for example, a discharge electrode with a gap patterned in accordance with the configuration of a character or symbol for which a charged image is desired.

The electrodes can be multiple electrodes forming cross points in a matrix array. Ions are extracted from electrode apertures at selected matrix crossover points by simultaneously providing both an electrical discharge at the selected apertures and an external ion extraction field.

The extracted ions can be used to form an electrostatic latent image which is subsequently toned and fused. The image can be formed on the dielectric layer and transferred to plain paper.

Alternatively, charged particulate matter can be deposited on plain paper to form a visible image, or collected on a conducting surface. In one form of ion generator, the apparatus is formed by a solid dielectric member which separates two electrodes, at least one of which has an edge on the surface of the dielectric member. When a voltage is applied between the electrodes, for example an alternating voltage in the frequency range fro 60 Hertz to 4 Megahertz, an electrical discharge is produced between one of the electrodes and the dielectric surface. The electrodes, which can be alike or different, can take a wide variety of forms, including an open mesh woven metallic screen.

In an alternative form of ion generator, a third electrode is used to control the discharge of the ions generated as described above. A high frequency alternating potential is applied between the first, "driver" electrode and the second, "control" electrode The third, "screen" electrode is separated from the control electrode by a second layer of dielectric. Ions produced by the air gap breakdown can be extracted subject to the influence of the screen electrode and applied to a further member. The screen electrode may be used to prevent an undesired image erasure when a previously formed latent electrostatic image is present under the ion generator, and no extraction voltage is applied to the control electrode.Alaternatively, the screen electrode may provide an electrostatic lensing action which may be used to control the size and shape of electrostatic images formed by the ion generator.

In both the preferred applications of the invention, the latent electrostatic image on the dielectric image member is toned to form a visible counterpart. The toned image is then transferred to a receptor medium. Provision may be made for cleaning the surface of the dielectric image member, and for discharging any residual image thereon.

In a preferred version of both these applications, the dielectric image member includes a surface layer consisting of impregnated anodized aluminium oxide. Such a member may be formed by the preliminary dehydration of the anodized aluminium member following which surface apertures of the dehydrated member are impregnated to achieve enhanced resistivity and dielectric properties. The surface apertures of the dielectric member may be pores, such as those present in the oxide coating produced by the anodization of the conductive member. The dehydration may take place by heating the dielectric member in a vacuum, or in a desicant dry box with a relatively humidity below 10 percent.Heating desirably takes place at a temperature in a range from 60"C to 1 80 C with the preferred temperature being about 1 00 C. The period of the heating desirably is about eight hours.

The dielectric member may be impregnated with an organic resin selected from a class consisting of ultra-violet (UV) curable resins, polyamide resins, UV acrylated resins, and thermally cured epoxy resins. The dielectric member may be a conductive substrate underlying a dielectric layer having substantially moistrue free sealed pores.

In accordance with a particular version of both the preferred applications of the invention, the tones visible image on the dielectric image member may be transferred to a receptor medium with simultaneous pressure fixing. Pressure is applied when a receptor web or sheet passes between the dielectric image drum and a backup roller at a point of tangency of the two members.

Again, in both these applications, two metal scrapers may be disposed respectively adjacent to the dielectric image drum and the backup roller to clean the surface of the drum and roller after image transfer. Any residual image on the dielectric image drum can be erased by electrodes on both sides of the dielectric layer, between which high frequency AC discharges are produced. Erasure can also be effected by a grounded conductor or semiconductor maintained in intimate contact with the surface of the dielectric layer. The grounded conductor can be a heavily loaded metal scraper blade, and the grounded semiconductor can be a semiconducting roller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention may be carried into practice in various ways but various embodiments will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an entire electrophotographic apparatus; Figure 2 is a partial sectional view of the region of proximity of the photoconductor member and dielectric image drum of the apparatus; Figure 3 is a schematic view of a belt photoconductor member and a dielectric image drum which may be substituted for the corresponding parts in the apparatus shown in Figures 1 and 2; Figure 4 is a perspective sectional view of a charge transfer member employing a photoreceptor assembly; Figure 5 is a perspective sectional view of a charge transfer member employing an alternative photoreceptor assembly;; Figure 6 is a schematic view of an electrostatic transfer printer; Figure 7 is a partial sectional view of a charge eraser unit for an electrostatic printer or photocopier; Figure 8 is a partial sectional view of a charge eraser unit for an electrostatic printer or photocopier; Figure 9 is a schematic, sectional view of an ion generator in accordance with the invention; Figure 10 is a schematic, sectional view of an ion generator and extractor in accordance with the invention; Figure 11 is a plan view of an ion generator for use in electrostatic printing; Figure 1 2 is a plan view of a matrix ion generator for an electrostatic dot matrix printer; Figure 1 3 is a partial perspective view of a physical model of an ion generator in accordance with the invention;; Figure 14 is a schematic view of an illustrative copier utilizing the ion generator of Figure 13; Figure 1 5 is a sectional view of an alternative ion source in accordance with the invention; Figure 1 6 is a sectional view of an aerosol charging system for high speed dot matrix printing in accordance with the ion generator of the invention; Figure 1 7 is a sectional view of a line scan printing system utilizing the ion generation method for the invention; Figure 1 8 is a sectional view of an electrostatic precipitator in accordance with the ion generator method of the invention; Figure 1 9 is a graph illustrating the relationship between electrode voltage and paper voltage in accordance with the ion generation method of the invention;; Figure 20 is a perspective view of a toned electrographic image on a conductor backed dielectric member, as produced by the matrix ion generator of Figure 12; Figure 21 is a schematic sectional view of an ion generator in accordance with an alternative embodiment of the invention; Figure 22 is a schematic, sectional view of an ion generator and extractor in accordance with the embodiment of Figure 21; and Figure 23 is a schematic view of an alternative circuit to be employed in the ion generator and extractor of Figure 22.

I. INTRODUCTION.

Two main applications in which the ion generating system of the invention may be used will be described, namely the double transfer electrophotographic apparatus which is the subject of Section I, and an electrostatic transfer printer which is the subject of Section IV. These two applications differ in the means by which a latent electrostatic image is created on a dielectric imaging roller; thereafter, identical apparatus may be employed.

The three layer photoreceptor of Section III is profitably used when it is desired that a latent electrostatic image formed on a photo-conductive member be transferred to a further dielectric member. The ion generator and extractor of Section V forms the subject of the present invention and may be used in either of the above applications, however, it may be applied as well to electrostatic discharging, precipitation, separation and coating, and in general to electrostatic printing and office copying. The impregnated anodized aluminium members of Section VI are suitable for applications requiring good dielectric properties and a hard, smooth surface. These are qualities which are preferred in the imaging roller of both basic applications.

II. DOUBLE TRANSFER ELECTROPHOTOGRAPHIC SYSTEM.

Figures 1 to 3 show double transfer electrophotographic apparatus 10 comprised of three cylinders, and various process stations.

The upper cylinder is a photoconductive member 11, which includes a photoconductor coating 1 3 supported on a conducting substrate 17, with an intervening semiconducting substrate 15, as discussed in detail in Section Ill below. Advantageous materials for the photoconductor surface layer 1 3 include cadmium sulphide powder dispersed in a resin binder (photoconductive grade CdS is employed, typically doped with activating substances such as copper and chlorine), cadmium sulphoselenide powder dispersed in a resin binder (defined by the formula CdSxSey, where x + y = 1), or organic photoconductors such as the equimolar complex of polyvinyl carbazole and trinitrofluorene.

The photoconductor is electrostatically charged at charging station 1 9 and then exposed at exposing station 21 to form on the surface of the photoconductor an electrostatic latent image of an original. The photoconductor may be charged employing a conventional corona wire assembly, or alternatively it may be charged using the ion generating scheme described in subsection VA below (Figure 14). The optical image which provides the latent image on the photoconductor may be generated by any of several well known optical scanning schemes. This latent image is transferred to a dielectric cylinder 25 formed by a dielectric layer 27 coated on a metal substrate 29. The latent electrostatic image on the dielectric cylinder 25 is toned and transferred by pressure to a receptor medium 35 which is fed between the dielectric cylinder 25 and a transfer roller 37.There are means 43, 45, 47 to remove residual toner from cylinder 25 and roller 37 and to erase any electrostatic image remaining on cylinder 25 after transfer.

Apparatus for effecting toning and subsequent steps, shown generally at 30 in Figure 1, is discussed in detail in sub-section IVB below.

The method by which a latent electrostatic image is transferred from the photoconductive cylinder 11 to the dielectric cylinder 25 employs a charge transfer by air gap breakdown. The process of uniformly charging and exposing the surface of the photoconductor coating 1 3 results in a charge density distribution corresponding to the exposed image, and a variable potential pattern of the surface of the photoconductor coating 1 3 with respect to the grounded conductive substrate 1 7. With reference to Figure 2, the charged area of the photoconductor 11 is rotated to a position of close proximity (less than 0.05 mm) to the dielectric surface.An external potential 33 is applied between electrodes in the conductive substrate of the photoconductive cylinder 11 and the metal substrate 29 of the dielectric cylinder 25, with a typical initial charge of about 1 ,000 volts on photoconductive layer 13, to which an additional 400 volts are added by the externally applied potential 33. The aggregate charge of 1,400 volts is decreased by about 800 volts during the exposing process.

The charge transfer process requires that a sufficient electrical stress be present in the air gap to cause ionization of the air. The required potential depends on the thickness and dielectric constants of the insulating materials, as well as the thickness of the air gap (see Dessauer and Clark, Xerography and Related Processes, the Focal Press, London and New York, 1965, at 427). Electrical stress will vary according to the local charge density, but if sufficient to cause an air gap breakdown it will result in a transfer of charge from photoconductor surface 1 3 to dielectric surface 27, in a pattern duplicating the latent image. This means that a certain threshold potential must be generated across the air gap. Roughly half the charge will be transferred, leaving a potential of around 600 volts on the dielectric surface 27.

The necessary threshold potential may exist as a result of the uniform charging and exposure of the photoconductor surface or an externally applied potential may be employed in addition.

Image quality is generally enhanced through the use of an external potential.

It is important to maintain the integrity of the latent electrostatic image, in the face of disruptive charge transfer, which occurs under certain conditions when charge transfer is effected on the approach of the two insulating surfaces. It has been observed that the addition of a semiconducting layer 1 5 between the photoconductive surface layer 1 3 and the conductive substrate 1 7 considerably reduces this effect as compared with using the usual two-layer photoconductor. Although the phenomenon by which the semiconducting layer eliminates the disruptive breakdown is not completely understood, it is believed that the time constant introduced by this semiconducting layer has the effect of smoothing or reducing the precipitous behaviour otherwise associated with disruptive breakdown.Suitable layer characteristics and materials are set out in Section Ill below. The employment of this preferred construction of the photoconductor member 11 avoids a mottling and blurring of detail in the transferred image. A typical range of air gap distances for charge transfer using this configuration would be of the order of 0.0125 to 0.0375 mm.

The use of this method of charge transfer alleviates some of the problems resulting from undesirable discharge characteristics of the photoconductive member. The employment of an external potential in achieving a threshold potential leaves a higher voltage on the dielectric cylinder than would be the case for a single transfer system relying on the constrast potential of the photoconductor surface. This, in turn, results in a greater contrast between the light and dark portions of the toned, visible image.

In order to provide uniformity from copy to copy, particularly with certain photoconductors which exhibit fatigue, it is advantageous to discharge the residual latent image remaining on the photoconductor after latent image has been transferred to the dielectric surface 27. This erasure may be conveniently carried out by an erase lamp 23 which provides sufficient illumination to discharge the photoconductor below a required level. The erase light 23 may be either fluorescent or incandescent.

In a specific operative example of an electrophotographic system of the construction described, the cylindrical conducting core 29 of the dielectric cylinder 25 was machined from 7075-T6 aluminium to a diameter of 76 mm. The length of this cylindrical core, excluding machined journals, was 230 mm. The journals were masked, and the aluminium anodized by use of the Sanford process (see S. Wernick and R. Pinner, "The Surface Treatment and Finishing of Aluminium and its Alloys", Robert Draper Ltd., 4th Edition 1971/72, Vol. 2, Page 567). The finished aluminium oxide layer was 60 micrometers (ym) in thickness. The conducting core 29 was next heated in a vacuum oven at a temperature of 150"C for twelve hours and then permitted to cool to 50"C. After removal from the oven, the cylindrical core was brush-coated with a low viscosity epoxy (Hysol Co. R9-2039 resin 00 parts by weight; H23404 hardener 11 parts by weight). The epoxy was allowed to impregnate the pores, and the excess on the surface then wiped off. The epoxy was cured at 78"C for eighteen hours in a vacuum oven, thereby forming the dielectric surface layer 27. The surface 27 of the dielectric cylinder 25 was then finished to 0.1 25 to 0.25 lim rms using 600 grit silicon carbide paper.

The pressure roller 37 consisted of a solid machined 50 mm diameter core 41 over which was press fitted a 50 mm inner diameter, 62.5 mm outer diameter polysulphone sleeve 39.

The conducting substrate 1 7 of the photoconductor member 11, comprising an aluminium sleeve, was fabricated of 6061 aluminium tubing with a 3 mm wall and 50 mm outer diameter.

The outer surface was machined and the aluminium anodized (again, using the Sanford process) to a thickness of 50cm. In order to provide the proper level of oxide layer conductivity, nickel sulphide was precipitated in the oxide pores by dipping the anodized sleeve in a solution of nickel acetate (50 g/l, pH of 6) for 3 minutes. To form the semiconducting layer 15, the sleeve was then immediately immersed into concentrated sodium sulphide for 2 minutes and then rinsed in distilled water. This procedure was repeated three times. The impregnated anodic layer was then sealed in water (92"Celsius, pH of 5.6) for ten minutes.The semiconducting substrate 1 5 was spray coated with a binder layer, the photoconductor coating 1 3 consisting of photoconductor grade cadmium sulphoselenide powder milled with a heatset DeSoto Chemical Co. acrylic resin, diluted with methyl ethyl ketone to a viscosity suitable for spraying. The dry coating thickness was 40 Ism, and the cadmium pigment concentration in the resin binder was 18% by volume. The resin was crosslinked by firing at 180"C for three hours.

The dielectric cylinder 25 was gear driven from an AC motor to provide a surface speed of twenty cms per second. The pressure roller 37 was mounted on pivoted and spring-loaded side frames, causing it to press against the dielectric cylinder 25 with a pressure of 55 kg per linear cm of contact.

Strips of tape 0.025 mm thick and 3 mm wide were placed around the circumference of the photoconductor sleeve 11 at each end in order to space the photoconductor at a small interval from the oxide surface of the dielectric cylinder 25. The photoconductor sleeve was freely mounted in bearings and friction driven by the tape which rested on the oxide surface.

The photoconductor charging corona station 19, single component latent image toning apparatus 31, and optical exposing station 21 were essentially identical to those employed in the Develop KG Dr. Eisbein BCo. (Stuttgart) No. 444 copier.

The remove and erase means 43 and 45 comprised flexible stainless steel scraper blades and were employed to maintain cleanliness of both the oxide cylinder 25 and the polysulphone pressure roll 37. With reference to the electrostatic image erasing embodiment shown at 93 in Figure 7, the residual latent image was erased using an AC corona wire 97 in combination with a 42% open area 90-mesh screen 95, which was maintained at ground potential and pressed into light contact with the oxide surface 27. A 0.075 mm diameter tungsten corona wire 97 was spaced 4.75 mm from the screen. This corona wire was operated at an AC 60 Hertz potential with a peak of 9 kilovolts.

With reference to the photoconductor-dielectric cylinder embodiment of Figure 2, a DC power supply 33 was employed to bias the photoconductor sleeve 11 to a potential of minus 400 volts relative to the dielectric cylinder core 29, which was maintained at ground potential. The photoconductor surface 1 3 was charged to a potential of minus 1 ,000 volts relative to its substrate 1 7. An optical exposure of 25 lux-seconds was employed in discharging the photoconductor in highlight areas. In undischarged areas, a latent image of minus 400 volts was transferred to the oxide dielectric 27. This image was toned, and then transferred to a plain paper receptor medium 35 which was injected into the pressure nip, at the appropriate time, from a sheet feeder.

Copies were obtained at a rate of 30 per minute, having clean background, dense black images, and a resolution in excess of twelve line pairs per millimetre. No image fusing, other than that occurring during pressure transfer, was required.

In another embodiment of Ihe double transfer copier, the photoconductor sleeve 11 was replaced with a flexible belt photoconductor 11', as shown in Figure 3. The photoconductor 11' was comprised of a photoconductor layer 13' which was formed from a one to one molar solution of polyvinyl carbazole and trinitrofluorenone dissolved in tetrahydrafuran, and coated onto a conducting paper base 15' (West Virginia Pulp and Paper 45 No. LTB base paper) to a dry thickness of 30 lim. The photoconductor belt 11 I was supported by two conducting rollers 1 7'a and 17'b and friction driven from the dielectric cylinder 25. The lower roller 1 7'b was biased to minus 400 volts. The photoconductor was charged to 1,000 volts with the double corona assembly 19' shown in Figure 3.The electrostatic latent image was generated by a flash exposure 21' so that the entire image frame was generated without the use of scanning optics.

The rest of the system was identical to the previous example with the exception of the dielectric cylinder 25, which was fabricated from non-magnetic stainless steel coated with a 1 5 ym layer of high density aluminium oxide. The coating was applied using a Union Carbide Corp.

(Linde Division) plasma spray technique. After spraying, the oxide surface was ground and polished to a 0.25 jum rms finish. Again, high quality copies were obtained, even at operating speeds as high as 75 cms per second.

III. THREE LAYER PHOTORECEPTOR.

The interposition of a semiconducting substrate between the photoconductive surface layer and the conducting core of a photoreceptor assembly confers considerable advantages when employing the assembly in charge transfer imaging. The photoreceptor embodiments of Figures 4 and 5, for example, are incorporated in the double transfer electrophotographic apparatus shown in Figures 1 and 3, respectively.

In the particular embodiment of Figure 4, the photoreceptor assembly 50 is a drum 60 with a photoconductive layer 61 overlaying a semiconductor layer 63 on a conducting substrate 65.

In the conventional transfer process, the presence of the electric field associated with the charges of the electrostatic field image formed on the drum 60 results in image degradation in the transfer process. The effect of such image degradation is mitigated by the inclusion of the semiconductor 63 between the conducting substrate 65 and the photoconductor 61.

Other forms of photoreceptor assembly can be provided, for example, by the flexible belt 60' of Figure 5 in which a photoconductive layer 61' overlies a semiconductive layer 63' which is in turn positioned on a conductive substrate 65'. In order to achieve the desired conductive substrate 65' a conductive coating may be applied to a plastics film or the substrate may be a thin metallic foil-for example, nickel.

The conductive substrate 65 of the drum 60 in Figure 4 is illustratively of aluminium, but any combination of materials which provides the desired conductivity may also be employed.

It has been empirically discovered that the semiconducting layers 63 and 63' preferably have a thickness in the range from 0.125 mm to 95 mm. The resistivity of the semiconductive layer much be such that charge will pass through the layer in a reasonable time. Accordingly the resistivity is advantageously less than 1012 ohm centimetres.

On the other hand the resistivity must be sufficiently high to provide a time constant for smoothing the charge transfer and thus reduce the degradation of the transfer image as heretofore encountered. The lower level of resistivity for the semiconductive layers 63 and 63' depends on the thickness of the superimposed photoconductive layer, and the operating speed.

It has been discovered generally that a resistivity of more than 103 ohm centimetres is suitable.

The semiconductive layer may be realized in a variety of ways. It may be formed by a semiconductive plastics or a semiconductive elastomer. A suitable conducting agent is carbon black, while a suitable matrix for receiving the carbon black is an epoxy resin. Thus the semiconductor layer may be formed by dispersing carbon black in a resin matrix to achieve a resistivity within the range set forth above. Similarly a wide variety of rubbers can be used with carbon black to obtain the desired resistivity.

The photoconductor may be of the type generally employed in electrostatic imaging. Materials which have been found to function satisfactorily with the semiconductive layer 63 or 63' include polyvinylcarbazole complexed with trinitrofluorenone; cadmium sulphide dispersed in a variety of binders including epoxies, silicones and thermoplastics; selenium and selenium alloys, including amorphous selenium, and low fatigue zinc oxide.

In general, for binder layer photoconductors, the semi-conducting layer may also be formed of the same material as the photoconductor, but with a higher photosensitive element concentration; thus a photoconductive layer of cadmium sulphide in epoxy with an 18% concentration behaves as an insulator in the dark, while the same layer with a 30% cadmium sulphide concentration behaves as a semiconductor in the dark.

The above described three layer photoreceptor is profitably employed wherever it is desirable to transfer a latent electrostatic charge image to any dielectric member, as for example an intermediate dielectric member which is subsequently toned, and the image produced by toning then transferred to a plain paper copy or a dielectric sheet which is itself toned to produce a copy.

IV. ELECTROSTATIC TRANSFER PRINTING.

The electrostatic transfer printing apparatus to be described includes apparatus for forming a latent electrostatic image on a dielectric surface (e.g. an imaging roller) and means for accomplishing subsequent process steps.

A. Latent Electrostatic Image Formation.

Apparatus for generating charged particles and for extracting them to be applied to a further surface is disclosed in detail in section V below. Any of the embodiments of such apparatus which are suitable for forming a latent electrostatic image on a dielectric surface may be employed in the electrostatic printing apparatus discussed in this section; for example, see the embodiments of Figures 11, 12, 1 3 and 1 5 and particularly the preferred matrix printing apparatus of Figure 12, which may be employed in multiplex printing. The three electrode embodiment of subsection V-B results in additional control over image size and shape, and avoids undesirable image erasures in matrix printing under conditions discussed in that subsection.

B. Subsequent Processing.

Identical apparatus may be employed to carry out process steps subsequent to the formation of a latent electrostatic image (e.g. by the electrographic device of section V) for both photography and printing (compare Figures 1 and 6).

In Figure 6, the dielectric layer 75 of the dielectric cylinder 73 should have sufficiently high resistance to support a latent electrostatic image during the period between formation of the latent image and toning, or, in the case of electrophotographic apparatus, between image transfer and toning. Consequently, the resistivity of the layer 75 must be in excess of 1012 ohm centimetres. The preferred thickness of the insulating layer 75 is between 0.025 and 0.075 mm. In addition, the surface of the layer 75 should be highly resistant to abrasion and relatively smooth, with a finish that is preferably better than 0.25 ym rms, in order to provide for complete transfer of toner to the receptor sheet 81. The dielectric layer 75 additionally has a high modulus of elasticity so that it is not distorted significantly by high pressure in the transfer nip.

A number of organic and inorganic dielectric materials are suitable for the layer 75. Glass enamel, for example, may be deposited and fused to the surface of a steel or aluminium cylinder. Flame or plasma sprayed high density aluminium oxide may also be employed in place of glass enamel. Plastics materials, such as polyamides, polyimides and other tough thermoplastic or thermosetting resins are also suiable. However, the preferred dielectric coating is impregnated anodized aluminium oxide as described in section VI, infra.

The latent electrostatic image on dielectric surface 75 is transformed to a visible image at toning station 79. While any conventional electrostatic toner may be used, the preferred toner is of the single component conducting magnetic type described by J.C. Wilson, U.S. Patent No.

2,846,333, issued August 5, 1 958. This toner has the advantage of simplicity and cleanliness.

The toned image is transferred and fused onto a receptive sheet 81 by high pressure applied between rollers 73 and 83. The bottom roller 83 consists of a metallic core 87 which may have an outer covering of engineering plastics 85. The pressure required for good fusing to plain paper is governed by such factors as, for example, roller diameter, the toner employed, and the presence of any coating on the surface of the paper. Typical pressures ran from 1 8 to 1 25 kg per linear cm of contact. The function of the plastics coating 85 is to absorb any high stresses introduced into the nip in the case of a paper jam or wrinkle. By absorbing stress in the plastics layer 85, the dielectric coated roller 73 will not be damaged during accidental paper wrinkles or jams.Coating 85 is typically a nylon or polyester sleeve having a wall thickness in the range of 3 to 1 2.5 mm. This coating need not be used, for example, if a highly controlled web is printed with which paper wrinkles and jams are not likely to occur.

Scraper blades 89 and 91 may be provided in order to remove any residual paper dust, toner accidentally impacted on the roll and airborne dust and dirt from the dielectric pressure cylinder and the back-up pressure roller. Since substantially all of the toned image is transferred to the receptor sheet 81, the scraper blades are not essential, but they are desirable in promoting reliable operation over an extended period.

The small residual electrostatic latent image remaining on the dielectric surface 75 after transfer of the toned image may be neutralized at the latent image discharge station 93. The action of toning and transferring a toned latent image to a plain paper sheet reduces the magnitude of the electrostatic image, typically from several hundred volts to several tens of volts. In some cases where the toning threshold is too low, the presence of a residual latent image will result in ghost images on the copy sheet, which are eliminated by the discharge station 93. Such erasure may be performed with arrangement of Figure 7. In Figure 7, the dielectric cylinder 73, with a dielectric coating 75, is maintained in contact with, or a short distance from an open mesh screen 95, maintained at substantially the same potential as the conducting cylinder 77.The screen is mounted on holder 99, and an AC corona wire 97 is positioned behind the screen at a distance of typically 6 to 12.5 mm. A high voltage alternating potential, illustratively 60 Hertz, is applied to the wire 97. The screen 95 establishes a reference ground plane near the dielectric surface and the AC corona wire 97 supplies both positive and negative ions. Any local field at the screen 95 due to a latent electrostatic image on the dielectric surface 75 attracts ions generated by the corona wire 97 onto the dielectric layer, thus neutralizing the majority of any residual charge. At very high surface velocities of dielectric coating 75, the remaining charge can again result in ghost images. In this case, multiple discharge stations will further reduce the residual charge to a level below the toning threshold.

Alternatively, erasure of any latent electrostatic image can be accomplished by using a high frequency AC discharge between electrodes separated by a dielectric as described in section V below.

The latent residual electrostatic image may also be erased by contact discharging. The surface of the dielectric must be maintained in intimate contact with a grounded conductor or grounded semiconductor in order effectively to remove any residual charge from the surface of the dielectric layer 75, for example, by a heavily loaded metal scraper blade. The charge may also be removed by a semiconducting roller which is pressed into intimate contact with the dielectric surface. Figure 8 shows a partial sectional view of a semiconductor roller 98 in rolling contact with dielectric surface 75. Roller 98 advantageously has an elastomer outer surface.

V. ION GENERATION AND EXTRACTION.

Subsection A discusses an ion generator and extractor in its basic form and variations thereof, involving two electrodes separated by a solid dieletric member. Subsection B describes the addition of a third electrode.

A. Two Electrode Embodiments.

Figure 9 depicts an ion generator 100, which produces an air gap breakdown between a dielectric 101 and respective conducting electrodes 102-1 and 102-2 using a source 103 of alternating potential. When electric fringing fields EA and EB in the air gap 104-a and 1 04b exceed the breakdown field of air, an electric discharge occurs which results in the charging of the dielectric 101 in regions 101-a and 101-b adjacent the electrode edges. Upon reversal of the alternating potential of the source 103, there is a charge reversal in the breakdown regions 101-a and 101-b. The generator 100 of Figure 9, therefore, produces an air gap breakdown twice per cycle of applied alternating potential from the source 103 and thus generates an alternating polarity supply of ions.

The extraction of ions produced by the generator 100 of Figure 9 is illustrated by the generator-extractor 110 of Figure 1 0. The generator 11 0A includes a dielectric 111 between conducting electrodes 11 2-1 and 11 2-2. In order to prevent air gap breakdown near electrode 112-1, the electrode 11 2-1 is encapsulated or surrounded by an insulating material 11 3.

Alternating potential is applied between the conducting electrodes 11 2-1 and 11 2-2 by a source 1 4A. The second electrode 1 12-2 has a hole 1 2-h where the desired air gap breakdown occurs relative to a region 11 1-rof the dielectric 111 to provide a source of ions.

The ions formed in the gap 1 1 2-h may be extracted by a direct current potential applied from a source 114-B to provide an external electric field between the electrodes 11 2-2 and a grounded auxiliary electrode 11 2-3. An illustrative insulating surface to be charged by the ion source in Figure 10 is a dielectric (electrographic) paper 11 5 consisting of a conducting base 11 5-p coated with a thin dielectric layer 11 5-d.

When a switch 11 6 is switched to position X and is grounded as shown, the electrode 1 12-2 is also at ground potential and no external field is present in the region between the ion generator 110A and the dielectric paper 11 5. However, when the switch 11 6 is switched to position Y, the potential of the source 1 4B is applied to the electrode 1 12-2. This provides an electric field between the ion reservoir 111-4 and the backing of dielectric paper 115. The ions extracted from the air gap breakdown region then charge the surface of the dielectric layer 11 5- d.

A number of materials may be used for the dielectric layer 111. Possible choices include aluminium oxide, glass enamels, ceramics, plastics films, and mica. Aluminium oxide, glass enamels and ceramics present difficulties in fabricating a sufficiently thin layer (i.e. around 0.025 mm) to avoid undue demands on the driving potential source 1 1 4A. Plastics films, including polyimides such as that known by the Trade Mark Kapton, and Nylon, tend to degrade as a result of exposure to chemical byproducts of the air gap break-down process in aperture 11 2-h (notably ozone and nitric acid). Mica avoids these drawbacks, and is thefore the preferred material for dielectric 111. Especially preferred is Muscovite mica, H2 KA13 (six4)3.

The generator and ion extractor 110 of Figure 10 is readily employed, for example, in the formation of characters on dielectric paper in high speed electrographic printing. Illustrative sources for the electrographic printing of characters are shown in Figures 11 and 1 2.

In Figure 11 a character generator 120 is formed by a dielectric member 1 21 which is sandwiched between an etched conductive sheet 122-1 and a set of counter-electrodes 122-2, 122-3 and 122-4.

The etched or mask electrode 122-1 illustratively is shown with etched characters, A, B and C. The fringing fields at the edges of the etched characters provide a high density source of ions when an air gap breakdown is produced by an alternating potential applied between the etched electrode 122-1 and the counterelectrodes. Thus when it is desired to generate ions for printing a selected character, such as the letter B, a source of high frequency alternating voltage (not shown) is applied between the etched electrode 122-1 and the associated counterelectrode 1223. This provides a high density supply of ions in the region of the dielectric 121 at the edges of the etched character B in the mask 122-1. The ions are then extracted and transferred to a suitable dielectric surface, for example the dielectric coated paper 11 5 of Figure 10, by the application of a direct voltage between the paper backing and the mask 122-1, resulting in the formation of the electrographic latent image B on the dielectric surface of the paper 1 5.

To employ ion extraction in the formation of dot matrix characters on dielectric paper, the matrix ion generator 1 30 of Figure 1 2 may be employed. The generator 1 30 makes use of a dielectric sheet 1 31 with a set of apertured air gap breakdown electrodes 132-1 to 132-4 on one side and a set of selector bars 133-1 to 133-4 on the other side, with a separate selector 1 33 being provided for each different aperture 1 35 in each different finger electrode 1 32.

When an alternating potential is applied between any selector bar 1 33 and ground, ions are generated in apertures at the intersections of that selector bar and the finger electrodes. Ions can only be extracted from an aperture when both its selector bar is energized with a high voltage alternating potential and its finger electrode is energized with a direct current potential applied between the finger electrode and the counter-electrode of the dielectric surface to be charged. Matrix location 13523, for example, is printed by simultaneously applying a high frequency potential between selector bar 133-3 and ground and a direct current potential between finger electrode 132-2 and a dielectric receptor member's counterelectrode. Unselected fingers as well as the dielectric member's counterelectrode are maintained at ground potential.

By multiplexing a dot array in this manner, the number of required voltage drivers is significantly reduced. If, for example, it is desired to print a dot matrix array across an area 200 mm wide at a dot matrix resolution of 80 dots per cm, 1 600 separate drivers would be required if multiplexing were not employed. By utilizing the array of Figure 1 2 with, for example, alternating frequency driven fingers, only 80 finger electrodes would be required and the total number of drivers is reduced form 1 600 to 100.

In order to prevent air gap breakdown from electrodes 132 to the dielectric member 131 in regions not associated with apertures 135, it is desirable to coat the edges of electrodes 1 32 with an insulating material. Unnecessary air gap breakdown around electrodes 1 32 may be eliminated by potting these electrodes.

In constructing and operating a matrix ion generator of this construction, it is desirable that the ion currents generated at various matrix crossover points be maintained at a substantially uniform level. Thickness variations in the dielectric layer 131 will result in commensurate variations in the dielectric layer 131 will result in commensurate variations in the ion current output, in that a lower ion current will be produced at an aperture 1 35 at which the dielectric 1 31 is thicker.It is a particularly advantageous property of mica that it has a natural tendency to cleave along planes of extremely uniform thickness, making it especially suitable for the matrix ion generator illustrated in Figure 1 2. In this regard, the uniformity of thickness of layer 131 is much more important than the actual value of that thickness.

The invention may be employed to form a rectangular area of charge using geometry of the module 140 shown in Figure 13. Charging electrodes 142-1 and 142-2 are separated from the electrode 142-3 by a dielectric member 1 41, with the electrode 142-3 potted in an insulator 145. The region between the electrodes 142-1 and 142-2 provides a slot in which an air gap discharge is formed when a high frequency alternating potential is applied between electrodes 142-1 and 142-2 and electrode 142-3.

The charging array of Figure 1 3 may be employed in a plain paper copier to replace the coronas normally found in such a copier.

Figure 1 4 illustrates schematically a plain paper copier employing charging arrays of the kind shown in Figure 13. A copier drum 151 is charged using a charging element 152-1, having the configuration shown in Figure 1 3. If the drum is selenium or a selenium alloy and it is desired to charge the surface, for example, to a positive potential of 600 volts, then the slotted electrode 142-1 is maintained at 600 volts. After charging, the drum 151 is discharged with an optical image provided by a scanner at station 1 53. The resulting latent electrostatic image is toned at station 1 56 and the toner is transferred to a plain paper sheet 158, using a transfer ion generator 152-2 according to Figure 13, with the slotted electrode again maintained at a positive potential.The latent residual electrostatic image in the surface of the drum and any uncharged toner may be electrically discharged by employing a discharge unit 152-3, also of the kind shown in Figure 1 3. Here the slotted electrode is maintained at ground potential and any residual charge on the surface of the drum and toner causes ions to be extracted from the air gap break-down in the slot, thus effectively discharging the surface. A cleaning brush 1 54 is employed to remove residual toner remaining on the surface and the drum is then ready to be recharged.

Also shown in Figure 1 4 is a dot matrix charging head 1 55 which may be configured according to Figure 1 2. This permits a plain paper copier to be employed as a printer. In that event the drum 151 is discharged at station 153 and recharged by the dot matrix printing head 155, permitting the machine 1 50 to function both as a copier and as a printer. In addition, the apparatus 1 50 may function simultaneously as a copier and a printer where overlaps are desired. Thus, an ion generator and extractor in accordance with Figure 10 may be employed as image forming and residual discharging means in the printer of Section IV, as well as precharging and discharging means in the electrophotographic apparatus of Section II.

Figure 1 5 illustrates an alternative ion generator 1 60 for use in charging or discharging an insulating surface. In Figure 1 5 the slotted electrode 142-1, 142-2 of Figure 1 3 is replaced by an open mesh screen 162-2 with longitudinal elements 162-a and cross members 162-b.

Discharge electrodes 162-1 and 162-2 are separated by dielectric sheet 1 61 and the air gap breakdown potential provided by alternating potential 1 63.

Figure 1 6 illustrates an apparatus 1 70 for applying a multiplexed dot matrix charging head 1 71 of the construction shown in Figure 1 2 in a system for high speed dot matrix printing on plain paper. The charging head 1 71 charges an aerosol 175, consisting of a dye dissolved in an appropriate solvent, which is carried by a low velocity airstream introduced through a slot 1 76.

The aerosol particles are charged by the ion generating system and enter an electric field region established by a direct potential supplied between electrodes 1 73 and 1 74. This field directs the charged aerosol particles onto a plain paper sheet 1 72 which moves through the apparatus at approximately the same speed as the velocity of the aerosol.

Figure 1 7 illustrates mechanical line scan printing system. A slotted electrode 1 86 is employed with a dielectric film 1 85 and a rapidly moving conducting head 1 87 to form a travelling air gap breakdown region. The bead 1 87 is mounted on a wire 1 88 and is driven by pulleys from a high speed motor (not shown). A high frequency alternating current source 1 83 supplies the potential necessary to break down the air gap in the slot of the electrode 186.In this example, a dielectric paper 181 is charged by a charging potential supplied by an amplifier 1 84 whose output is connected between the dielectric paper conductor support 1 82 and the slotted electrode 186. The line scan is effected by the mechanical motion of the bead 187 and selected areas are printed by applying a potential between the conducting sheet and the slotted electrode. As in the previous cases, the latent electrostatic image that is formed may be toned and fused using any conventional technique. Continuous tone images may be formed in this manner since the quantity of ions extracted from the discharge is dependent upon the extraction potential supplied by the amplifier 1 84.

Figure 1 8 illustrates the use of an ion generating system as an electrostatic precipitator 1 90.

A tubular electrode 1 92 is separated from a segmented electrode 1 94 by a dielectric 1 91. An air gap breakdown is produced in the open areas of the segmented electrode 1 94 through application of a high voltage alternating potential by a generator 1 96. The segmented electrode 1 94 is also biased by a direct potential source 1 98. A central ground wire 1 99 is mounted at the centre of the tube 192. Stack gases or other aerosols may be cleaned through electrostatic precipitation by passage along the tube. The high current ion density from the air gap breakdown regions charges solid particles in the aerosol and causes them to be attracted to the central electrode 199.

In general, the relationship between the electrode voltage and that of the ion receiving surface, for example, paper, is typically that shown in Figure 1 9 for charging systems of the constructions shown in Figures 10, 11, 1 2 and 1 3. The electrode voltage is the direct potential impressed between the apertured electrode and the counterelectrode of the dielectric surface being charged. The paper voltage is the electrostatic latent image potential of the charged dielectric membersdielectric (electrographic) paper in the example.

The foregoing examples of the use of the ion generating system illustrate its wide applicability. In general, the corona wires or points of any existing system may be replaced by one of the forms of apparatus described. In addition to the illustrated applications, the methods and apparatus described may be used in numerous other applications, not illustrated, such as those dealing with electrostatic separation and coatings.

EXAMPLES The foregoing description illustrates the general principles and features of the invention. The following specific examples illustrate specific applications of the invention.

EXAMPLE V-l A stainless steel foil 0.025 mm in thickness was laminated on both sides of 1 mil Muscovite mica. The stainless foil was coated with resist and photo etched with a pattern similar to that shown in Figure 12, with holes or apertures in the fingers approximately 0.1 5 mm in diameter.

This provides a charging head which can be employed to generate latent electrostatic dot matrix character images on dielectric paper according to Figure 10. Charging occurs only when there is simultaneously a potential of negative 400 volts on the fingers containing the holes and an alternating potential of 2 kilovolts peak to peak at a frequency of 500 kilohertz supplied between the finger and the counter electrode. A spacing of 0.2 mm is maintained between the print head assembly and the dielectric surface of the elecrographic sheet. The duration of the print pulse is 20 microseconds. Under these conditions, it is found that a latent electrostatic image of approximately 300 volts is produced on the dielectric sheet. The image is subsequently toned and fused to provide a dense dot matrix character image.The ion current extracted from this charging head, as collected by an electrode spaced 0.2 mm away from the head, is found to be 1 miliampere per square centimetre. The charging head enjoyed a service life of approximately 2000 hours.

EXAMPLE V-2 Example 1 was repeated employing a polyimide dielectric rather than Muscovite mica. A stainless steel foil 0.025 mm in thickness was laminated to a film of the polyimide sold under the Trade Mark KAPTON and having a thickness of 0.025 mm. Results equivalent to those of Example 1 were obtained at an applied high frequency potential of 1.5 kilovolts peak. The charging head yielded a service life of approximately 50 hours.

EXAMPLE V-3 An electrostatic charging head of the construction shown in Figure 11 was fabricated employing a stainless steel foil 0.025 mm in thickness laminated to both sides of a 0.025 mm polyimide sheet. In order to print fully formed characters on a dielectric surface, 2.5 mm high characters were etched in the foil on one side of the sheet, while fingers covering each character were etched on the other side of the foil as indicated in Figure 11. In order to establish conductivity within normally isolated areas of the characters, bridges 0.025 to 0.05 mm in thickness were left unetched. The character stroke width was etched to 0.1 5 mm. Printing was carried out by applying the potentials of Example V-2 with a pulse width of 40 microseconds.

The toned images exhibited sharp edges and high optical density. The character stroke width in the image was 0.3 mm.

EXAMPLE V-4 The invention was applied to provide continuous tone imagery by extracting a number of ions from the charging head per unit time in proportion to the applied ion extraction potential. This is illustrated in Figure 1 9 where the apparent surface potential on a dielectric surface is plotted as a function of the potential difference between the ion generating electrode and the dielectric counter electrode. The ion generating electrode dielectric surface spacing is 0.15 mm and the charging time is 50 microseconds.

B. Three Electrode Embodiments.

The matrix image generator 1 30 shown in Figure 1 2 is advantageously incorporated in electrostatic printing apparatus of the construction described in section IV. As was observed in connection with Figure 14, however an ion generator and extractor 110, as shown in Figure 10, may be used both to create an electrostatic image on a dielectric surface, and to discharge such an image. Thus, further referring to Figure 10, if switch 11 6 is closed at Y, electrode 11 2-2 is maintained at a positive potential V and a positive latent electrostatic image of lesser magnitude V is formed on the surface 11 5-d.If, however, switch 11 6 is at position X and a previously formed latent electrostatic image is under aperture 11 2-h, generator 11 0A will behave as an erasing unit. This phenomenon is further illustrated with respect to the dot matrix printing embodiment of Figure 12 at 200 in Figure 20. At a time t1, a given aperture 1 3523 on matrix ion generator 1 30 (Figure 12) is energized by a direct current pulse which creates a negative potential on a finger electrode 132-2, while a high frequency potential is applied to selector bar 133-3. This causes the formation of an electrostatic dot image which is negative in polarity, occupying regions 203 and 204 on dielectric surface 201 with backing electrode 202.At a later time t2, aperture 1 3523 is over region 204 and 205, selector bar 133-3 is still energised, but as charging is not desired, no negative pulse is applied to finger electrode 132-2. The presence of negative electrostatic image in region 204, however, attracts positive ions from the aperture 13523, erasing the previously created image in this region.

It has been discovered that the addition of a third electrode to the two electrode structure described above alleviates this problem, and offers additional benefits in terms of controlling the size and shape of an electrostatic image formed by an ion generator of this type. An ion generator 210 in accordance with this three electrode embodiment is shown in the sectional view of Figure 21. The ion generator 210 includes a "driver" electrode 211 and a "control" electrode 215, separated by a solid dielectric layer 213. A source 212 of alternating potential is used to provide an air gap breakdown in aperture 214.

A third, "screen" electrode 219 is separated from the control electrode by a second dielectric layer 21 7. The nomenclature adopted for the three electrodes draws an analogy to vacuum tube theory. The terms "driver" and "control" electrode may be validly extended to the corresponding electrodes in the basic two electrode embodiments. The second dielectric layer 217 has an aperture 216 which advantageously is substantially larger than the aperture 214 in the control electrode. This is necessary to avoid wall charging effects. The screen electrode 219 contains an aperture 218 which is at least partially positioned under the aperture 214. In an electrographic matrix printer, for example, the driver and control electrodes may be the selector bar and finger electrodes of Figure 12, and the screen electrodes may consist of either additional finger electrodes with apertures matching the pattern of the control with electrodes or a continuous apertured metal plate or other member with its openings adjacent to all printing apertures. The latter embodiment of the screen electrodes may take the form, for example, of an open mesh screen - The application of the above ion generator in electrographic matrix printing is illustrated in Figure 22. Figure 22 shows the ion generator 210 of Figure 21 used in conjunction with dielectric paper 220 consisting of a conducting base 223 coated with a dielectric layer 221, and backed by a grounded auxiliary electrode 225.When switch 222 is closed at position y, there is simultaneously an alternating potential across dielectric layer 213, a negative potential Vc on control electrode 215, and a negative potential on screen electrode 219. Negative ions in aperture 214 are subjected to an accelerating field which causes them to form an electrostatic latent image on dielectric surface 221, as in the two electrode embodiment. The presence of negative potential Vs on the screen electrode 219, which is chosen so that V5 is smaller than Vc in absolute value, does not prevent the formation of the image which will have a negative potential V, (smaller than Vc in absolute value).

With switch 222 at x, and a previously created electrostatic image of negative potential V, partially under aperture 214, a partial erasure of the image would occur in the absence of screen electrode 219. Screen potential Vs, however, is chosen so that it is greater than Vl in absolute value, and the presence of electrode 219 therefore prevents the passage of positive ions from aperture 214 to dielectric surface 221. See Example V-5.

The inclusion of screen electrode 219 in the ion generator confers advantages beyond the prevention of image discharge under the conditions discussed above. The screen electrode may be used alone or in connection with the control electrode to control matrix image formation.

With Vs = O, no latent image is produced due to the above discharge phenomenon. Thus, three level matrix image control is possible in an electrographic matrix printer.

Screen electrode 21 9 provides control over image size. Using the dot matrix print configuration shown in Figure 22 with overlaid finger screen electrodes, image size may be controlled by varying the size of screen apertures 218. See example V-6 infra. Furthermore, using such a configuration, with all variables constant except the screen potential 226, a larger screen potential has been found to produce a smaller dot diameter. See Example V-7. This technique may be used for the formation of fine or bold images. It has also been found that proper choices of Vs and Vc will allow an increase in the distance between ion generator 210 and dielectric surface 221 while retaining a constant dot image diameter. This is accomplished by increasing the absolute value of Vs while keeping the potential difference between V5 and Vc constant.See -Example V-8.

Image shape may be controlled by using a given screen electrode overlay in a matrix electrographic printer. See Example V-9. Screen apertures 218 may, for example, assume the shape of fully formed characters which are no larger than the corresponding round or square control apertures 214.

The electronic configuration used to control the electrographic printer of Figure 22 may be modified to allow the possibility of biasing the system, as shown in the circuit schematic of Figure 23. Element 231 is a pulse generator. The magnitude of the control pulse may be varied to produce a desired Vc and Vs by choosing an appropriate bias potential.For example, the following combinations will all produce Vs = - 700 volts, Vc = - 800 volts: 1. VgjaS = - 600 volts; Vs = - 100 volts; Vc = - 200 volts 2. VBjas = - 500 volts; Vs = - 200 volts; Vc = - 300 volts 3. VBjas = - 400 volts; Vs = - 300 volts Vc = - 400 volts 4. VBjas = - 300 volts; Vs = - 400 volts; Vc = - 500 volts 5. V5ias = - 200 volts; V5 = - 500 volts; Vc = - 600 volts The above advantages are further illustrated with reference to the following examples: EXAMPLE V-5 A 0.025 mm stainless steel foil is laminated to both sides of a sheet of 0.025 mm Muscovite mica.The foil is coated with a resist and photoetched with a pattern similar to that shown in Figure 12, with holes or apertures approximately 0.15 mm in diameter. A second mica layer 0.15 mm in thickness, is bonded to the foil in accordance with Figure 21. A screen electrode wit apertures of 0.38 mm diameter in the same pattern as those of the fingers is photoetched from 0.025 mm stainless steel, and bonded to the second mica layer with the finger and screen apertures being concentric.This construction provides a charging head which is used to provide a latent electrostatic image on dielectric paper as illustrated in Figure 22, with Vc = - 500 volts, is = 400 volts and an alternating potential 212 of 1 kilovolt peak at a frequency of 590 kilohertz. A spacing of 0.15 mm is maintained between the print head assembly and the dielectric surface 221. Vc takes the form of a print pulse 20 microseconds in duration. Under these conditions, a latent image in the form of a dot of approximately - 300 volts is produced op the dielectric sheet. This image is subsequently toned and fused to provide a dense dot matrix character image.The ion current extracted from discharge head as collected by an electrode 0.15 mm away from the head is found to be 0.5 milliamperes square centrimetre.

With the screen electrode 21 omitted, however, any electrostatic image under the control aperture will be erased when no print pulse is applied.

EXAMPLE V-6 The electrographic printer of Example V-5 was tested with a variety of diameters for screen aperture 218, and the size of resulting electrostatic dot image measured. The following results are representative: Screen Aperture Dot Image Diameter (mm) Diameter (mm) 0.38 0.38 0.25 0.30 0.20 0.25 It was found, in general, that a reduction in the size of the screen apertures caused a corresponding reduction of latent image size, without any compromise in image charge.

EXAMPLE V-7 The electrographic printer of Example V-5 was tested with a variety of screen potentials, Vs, and the size of the resulting electrostatic dot measured. The following results are representative.

Screen Potential Dot Image (volts) Diameter (mm) - 300 0.55 - 400 0.43 - 500 0.30 - 600 0.20 It was found, in general, that by increasing the potential on the screen, the latent image size was reduced without any compromise in image charge.

EXAMPLE V-8 The electrographic printer of Example V-5 was tested using a variety of spacings between the print head assembly and the dielectric surface 221. By varying the screen potential, Vs, and holding the potential difference between V5 and Vc constant, the size of the resulting electrostatic dot image was held constant. The following results are representative: Dot Image Separation V5 (volts) Vc (volts) Diameter 0.15 -400 -500 0.38 0.25 - 500 - 600 0.38 0.33 - 600 - 700 0.38 It was found in general, that with increasing print head assembly to dielectric surface spacing, an increase in screen potential, V5, provides constant dot image diameter without any compromise in image charge.

EXAMPLE V-9 The electrographic printer of Example 1 was modified so that the screen had apertures 48 in the form of slots instead of holes. The resulting toned latent electrostatic images were oval in shape.

Vl IMPREGNATION OF ANODIZED ALUMINIUM MEMBERS As discussed above, it is preferable to employ a dielectric material for the surface 27 of the image roller 25 (in the electrophotographic system of Figure 1) and for the surface 75 of the image roller 73 (in the electrostatic printing system of Figure 6 that satisfies certain criteria.

These critiera include a high resistivity, high abrasion resistance, a smooth finish, and a high modulus of elasticity. The preferred material is anodized aluminium oxide, impregnated by the method disclosed below.

Removal of absorbed water from the oxide layer of an anodized aluminium structure may be realized by using either heat, vacuum or storage of the article is a desicator. Although all three techniques are effective, best results are realized by heating in a vacuum, for example in a vacuum oven. Alternately, the article to be treated may be stored for several hours in a dry box containing a desicant. It is preferable that any thermal treatment of the oxide prior to impregnation be carried out at temperatures lower than 1 50"C and preferably not over 1 00"C.

At higher temperatures some cracking of the oxide coating can occur because of the high thermal coefficient of expansion of the aluminium substrate.

After removal of absorbed water from the oxide coating it may be impregnated with any organic resin. It is preferable, however, to employ a completely solid system since the use of solvent coatings leaves residual solvent in the pores. Thus, liquid resins which can be crosslinked to provide hard solid coatings are particularly advantageous materials. Such resins can be cured either by thermal cross-linking or by radiation cross-linking. It is desirable also that the resins have a low shrinkage and low moisture absorption after curing. In order to allow the organic resin to diffuse into the porous structure within a reasonable period of time, it is advantageous to employ liquid cross-linkage systems having viscosities lower than 500 centipoises.

These foregoing techniques can be used for the processing of a solid aluminium cylinder with an impregnated oxide coating for use in electrostatic imaging. In such a system, an electrostatic charge is placed on the insulating surface of the cylinder. The image is then toned as disclosed, for example, in U.S. patent 3,662,395 and the toned image transferred to plain paper. Table 2 of that patent indicates that a porous aluminium oxide surface sealed with polytetrafluoroethylene is not satisfactory for electrostatic imaging due to the low breakdown voltage and low pore insulation resistance and coating hardness.

A drum coated with an insulating film capable of supporting an electrostatic charge is disclosed in U.S. patent 3,907,560. The dielectric surface is a barrier layer aluminium oxide film since it is stated that the porous anodized aluminium oxide layer functions as a conductor rather than a dielectric. Although a barrier layer of anodized aluminium film is a good insulator, being non-porous, the maximum thickness of barrier layer films is restricted to the region of 0.012 to 0.025 mm. At this thickness, the maximum voltage the layer will support is limited and the surface is not hard in a conventiinal sense since any localized strains are transmitted through the thin film with subsequent deformation of the aluminium substrate.

The limitations of the thin barrier film can be overcome in the manner described in U.S.

patents 3,937,571 and 3,940,270 by the use of a duplex anodized aluminium coating. The coating is prepared by electrolytically oxidizing an aluminium surface and thereafter continuing the electrolytic oxidation under conditions which produce a barrier aluminium oxide layer. Not only does this increase the complexity of fabricating the anodized layer, but the limiting thickness is approximately 20 mms and the surface potential to which the oxide layer may be charged is a maximum of 620 volts.

By contrast with the prior art, this technique provides a simple and reliable technique for fabricating oxide films having a thickness as great as 100 mms and capable of supporting several thousand volts. The advantages will be further apparent from the following examples.

PRIOR ART EXAMPLE A cylinder, fabricated of aluminium alloy 7075-T6, was hard coat anodized in sulphuric acid according to the teachings of Wernick and Pinner. The final thickness of the anodic layer was 60 mm. The anodized cylinder was sprayed with an ultra-violet curable resin system in accordance with formula 1 of Table II below. The low viscosity liquid appeared to impregnate the pores within a period of one minute. After several minutes, the surface of the cylinder was wiped clean of excess liquid and the impregnated cylinder cured by exposing the cylinder to radiation from a medium pressure mercury arc lamp. After radiation curing, the cylinder was polished with 600 grit abrasive paper to a 0.25 mm finish with the removal of 3 or 4 mm of oxide. A test rig was employed to generate an electrostatic charge on the surface of the aluminium oxide layer.A corona source of ions was employed to charge the surface and the surface potential measuring employing a feedback electrometer. At the highest charging levels employed, a voltage of only a few volts was apparent on the surface. This potential decayed to zero within a second or so. The dielectric strength of this layer was determined by placing a lightly weighted electrical contact having a radius or curvature of 1 2.5 mm on the surface of the aluminium oxide and gradually increasing the applied potential between this electrode and the aluminium substrate until high currents were drawn through the layer. Using both D.C. and A.C.

and determining the breakdown potential in a number of locations, it was determined that the breakdown voltage was in the range of 900 to 1 200 volts.

Thus, both the charging levels and dielectric strength attained with this prior art example was unsatisfactory.

The following examples illustrating the advantages over the prior art are presented in tabular form below: TABLE VI-I EXAMPLES Example Aluminium Anodic Layer Pre-Impregnation Impregnation Breakdown Charging No. Alloy Thickness Treatment Formula* Voltage Voltage** 1 7075-T6 60 mm none 1 900-1200 5-15 2 " " Heated to 100 C for 8 hrs. 1 1800-2000 400-500 3 Heated to 100 C for 8 hrs. 1 2200-2400 1200-1400 4 " " Stored 24 hrs. in desicant 1 1400-1600 300-400 dry box 5 2024-T351 80 mm Heated to 120 C for 8 hrs. 1 1600-1800 400-500 6 6061-T351 75 " Heated to 120 C for 8 hrs. 1 1800-2000 400-500 7 7075-T6 87 " Heated to 120 C for 8 hrs. 1 1400-2000 500-600 8 " 62 " Heated to 100 C for 8 hrs. 2 2100-2300 900-1000 in vacuum over 9 61 ' Heated to 100 C for 8 hrs. 3 2000-2200 950-1200 in vacuum over 10 60 " Heated to 100 C for 8 hrs. 4 1800-2000 1100-1200 in vacuum over 11 " 58 " Heated to 100 C for 8 hrs. 5 2400-2500 1200-1400 in vacuum oven 12 7075-T6 57 " Heated to 120 C for 8 hrs. 6 1100-1300 800-850 13 " 56 " Heated to 120 C for 8 hrs. 7 1200-1400 900-1100 * See Table 2 ** Dielectric layer surface potential after charging with corona source of ions.

TABLE VI-2 IMPREGNATING RESINS USED IN THE EXAMPLE OF TABLE I Formula No. Material Parts (by Weight) 1. Celrad 3700 radiation curable resin (Celanese Co., N.Y. USA) 45 Neopentyl Glycol Diacrylate 45 Benzoin methyl ether 10 2. Celrad 3700 47 1,6 Hexanediol Diacrylate 47 Benzophenone 3 Dimethylaminoethanol 3 3. Acrylated polyester 78-3770 (National Starch Co.) 40 1,6 Hexanediol Diacrylate 60 Benzoin methyl ether 5 4. R8-2038 Epoxy (Hysol Div. of Dexter Corp., Olean, N.Y., U.S.A) 100 H2-3403 Hardener (Hysol Div. of Decter Corp., Olean, N.Y., U.S.A) 11 5. Eccoseal W19 Epoxy (Emerson # Cuming. Inc., Canton MA. U.S.A.) 100 Catalyst 11 (Emerson # Cuming, Inc. Canton MA., U.S.A.) 17 6. Dow Corning Q2-7044 Solventless silicone 10 Dow Corning Q2-7045 catalyst cured at 120 C for one half hour 1 7. Hercules. Inc. Piccolastic A50 polystyrene copolymer heated to 120 C (Piccolastic is a Trade Mark) (Where A50 viscosity is 100 Cp)-A50 poured over anodic layer while said layer is maintained at 120 C.

In all of the examples of Table I involving heating of the aluminium, the impregnation was carried out with the aluminium still warm (40-55"C). The vacuum oven treated samples were allowed to cool to the treating temperature while maintained in the vacuum.

Although vacuum impregnation with organic resins were not attempted, it is 3apparent that heating in a vacuum oven followed by vacuum impregnation of the anodic coating would provide the excellent electrical properties exhibited by the above examples employing the method described.

Claims

1. A method of generating ions which comprises applying an alternating potential between first and second electrodes separated by a solid dielectric member, with an air gap region at a junction of the first electrode and the solid dielectric member, to cause an electrical discharge in the air gap region.

2. A method as claimed in Claim 1 for generating electrostatic images in which the first electrode comprises a "control" electrode and the second electrode comprises a "driver" electrode and the ions are extracted by a direct voltage Vc on the control electrode, further comprising the steps of controlling the extraction of ions by providing an apertured "screen" electrode which is separated from the control electrode by an apertured solid dielectric member and which lies between the control electrode and the dielectric surface, and applying a screen voltage Vs between the screen electrode and the conductive backing; and forming an electrostatic image with the extracted ions.

3. A method as claimed in Claim 2 in which Vs is smaller than Vc in absolute value, whereby the application of screen voltage Vs does not prevent the extraction of ions.

4. A method as claimed in Claim 3 which includes providing relative motion between the ion generating assembly and the dielectric surface, and regulating the formation of an electrostatic image on the dielectric surface by selective application of direct voltage Vc, said electrostatic image having potential V, with respect to the conductive support, wherein the screen voltage Vs is larger in magnitude and equal in polarity to the image potential Vl in order to prevent undesired image erasure.

5. A method as claimed in Claim 2 in which a multiplicity of driver and control electrodes form cross points in a matrix array configured such that the control electrodes contain openings at matrix electrode crossover regions, the controlling step being performed by modulating the extraction of ions from said openings by means of a multiplicity of screen electrodes containing apertures corresponding to said openings.

6. A method of generating ions substantially as described herein with reference to Figures 9 to 23 of the accompanying drawings.

7. Apparatus for generating ions which comprises a solid dielectric member a first electrode on one side of said solid dielectric member with an air gap region at a junction of the first electrode and solid dielectric member; a second electrode on an opposite side of said solid dielectric member; and means for applying an alternating potential between said electrodes to produce an electrical discharge in said air gap region.

8. Apparatus as claimed in Claim 7 in which the solid dielectric member comprises a plastics film, glass, a ceramic or mica.

9. Apparatus as claimed in Claim 7 or Claim 8 in which the solid dielectric member comprises Muscovite mica.

1 0. Apparatus as claimed in Claim 7 for generating electrostatic images, in which the first electrode comprises a "control" electrode and said second electrode comprises a "driver" electrode and which includes: a further member, a source of direct voltage Vc between said control electrode and said further member, a third electrode (a "screen" electrode), a solid dielectric layer separating the screen electrode from the control electrode and the solid dielectric member, and a source of direct voltage Vs between the screen electrode and the said further member.

11. Apparatus as claimed in Claim 10 in which the said further member comprises a dielectric record member

1 2. Apparatus as claimed in Claim 10 or Claim 11 in which the control electrode, screen electrode, and solid dielectric layer contain corresponding discharge apertures.

1 3. Apparatus as claimed in Claim 1 2 in which the discharge apertures in the solid dielectric layer are larger in diameter than the corresponding discharge apertures in the control electrode.

14. Apparatus as claimed in any of Claims 6 to 1 3 in which the screen electrode consists of an open mesh woven metal screen.

1 5. Apparatus as claimed in any of Claims 6 to 1 3 in which the screen electrode comprises an apertured mask.

1 6. Apparatus for generating electrostatic images constructed and arranged substantially as herein described with reference to and as shown in FIGURES 9 to 23 of the accompanying drawings

14. Apparatus as claimed in Claim 11 in which the direct voltage Vs is equal in polarity to and smaller in absolute value than the direct voltage Vc.

1 5. Apparatus as claimed in Claim 14 which includes means for providing a relative motion between the apparatus for generating electrostatic images and said further member, and means for modulating said direct voltage Vc in order selectively to form an electrostatic pattern on the dielectric record member of voltage V, with respect to its conductive backing, the direct voltage Vs being larger in magnitude and equal in polraity to the direct voltage V1 in order to prevent undesired image erasure 16.Apparatus as claimed in Claim 10 in which the driver and control electrodes each comprises a plurality of sub-electrodes so as to form cross points in a matrix array configured such that the control electrodes contain openings at matrix electrode crossover regions, the solid dielectric layer containing apertures corresponding to said openings, and said screen electrode comprising a multiplicity of electrodes matching the control electrodes and containing apertures corresponding to said openings.

1 7. Apparatus as claimed in Claim 10 in which the driver and control electrodes each comprises a plurality of sub-electrodes so as to form cross paints in a matrix array configured such that the control electrodes contain openings at matrix cross over regions, said solid dielectric layer containing apertures corresponding to said openings and said screen electrode comprising a conducting member containing a series of apertures corresponding to said openings.

1 8. Apparatus as claimed in any of Claims 10 to 1 7 in which the screen electrode consists of an open mesh woven metal screen.

1 9. Apparatus as claimed in any of Claims 10 to 1 7 in which the screen electrode comprises an apertured mask.

20. Apparatus as claimed in Claim 7 in combination with a cylindrical dielectric member, means for extracting ions from said apparatus for generating ions to form a latent electrostatic image on said cylindrical dielectric member, means for toning said latent electrostatic image to form a visible counterpart thereof, and means for transferring the toned, visible image by pressure to a receptor to provide electrostatic printing.

21. A combination as claimed in Claim 20 in which the apparatus for generating ions and the extracting means are spaced from the cylindrical dielectric member by more than 0.025 mm.

22. A combination as claimed in Claim 20 or Claim 21 in which the transferring means comprises a transfer roller which contacts the cylindrical dielectric member with the receptor fed therebetween.

23. A combination as claimed in Claim 22 in which the transfer roller is coated with a stress absorbing plastics material.

24. A combination as claimed in Claim 23 in which the stress absorbing material is nylon or polyester.

25. A combination as claimed in Claim 22 or Claim 23 or Claim 24 in which the cylindrical dielectric member has a smoothness in excess of 0.5 jilm rms and a resistivity in excess of 1012 ohm centimetres.

26. A combination as claimed in any of Claims 20 to 25 in which the cylindrical dielectric member is made of aluminium oxide, glass enamel, or a resin.

27. A combination as claimed in any of Claims 20 to 25 in which the cylindrical dielectric member is made of polyamide or nylon.

28. Apparatus as claimed-in any of Claims 20 to 27 which includes a scraper blade for scraping residual toner from said cylindrical dielectric member.

29. Apparatus as claimed in any of Claims 20 to 28 which includes means for erasing any remaining electrostatic image after transfer printing has been completed.

30. Apparatus as claimed in Claim 29 in which the erasure means comprises a screen member and an AC-corona-generated source of ions.

31. Apparatus as claimed in Claim 7 in combination with: a rotatable imaging drum having a conductive core and a dielectric layer thereon, means for extracting ions from said apparatus for generating ions to form a latent electrostatic image on said dielectric layer, means for toning said latent electrostatic image, and a rotatable pressure drum in contact with said imaging drum for transferring and fusing the toned image on an image receiving medium which passes between the imaging drum and the pressure drum at the point of tangency thereof to provide electrostatic printing.

32. A combination as claimed in Claim 31 in which the dielectric layer has a thickness of more than 0.025 mm, a resisitivity of more than 1012 ohm centimetres, and a smoothness of more than 0.5 pm rms.

33. A combination as claimed in Claim 31 or Claim 32 in which a metal scraper blade is disposed adjacent to each drum in order to clean the surfaces of the drums after image transfer.

34. A combination as claimed in Claim 31 or Claim 32 or Claim 33 which includes a device adjacent to the image drum to erase a latent residual electrostatic image after image transfer.

35. A combination as claimed in Claim 34 in which the image erasing device comprises an electrode on each side of the dielectric layer, and means for effecting high frequency AC discharges between such electrodes to erase the image.

36. A combination as claimed in Claim 34 in which the image erasing device comprises a grounded conductor or grounded semiconductor which is maintained in intimate contact with the surface of the dielectric layer.

37. A combination as claimed in Claim 36 in which the grounded conductor consists of a heavily loaded metal scraper blade.

38. A combination as claimed in Claim 36 in which the grounded semiconductor consists of a semiconducting roller.

39. Apparatus for generating ions substantially as described herein with refence to Figures 9 to 23 of the accompanying drawings.

40. Apparatus as claimed in Claim 30 or Claim 31 and substantially as described herein with reference to the accompanying drawings.

CLAIMS (2 Mar 1981)

1. A method for generating electrostatic images which comprises the steps of applying a varying potential between a "driver" electrode in contact with one side of a solid dielectric member and a "control" electrode in contact with an opposite side of the solid dielectric member, the control electrode having an edge surface disposed opposite the driver electrode to define an air region at the junction of the edge surface and the solid dielectric member, to induce ion producing electrical discharges in the air region; extracting the ions by a direct voltage Vc on the control electrode; controlling the extraction of ions by providing an apertured "screen" electrode which is isolated from the control electrode by a solid dielectric member which provides an air gap between them, which screen electrode lies between the control electrode and an imaging surface, and applying a screen voltage Vs to the screen electrode; and forming an electrostatic image on the imaging surface with the extracted ions.

2. A method as claimed in Claim 1 in which Vs is smaller than Vc in absolute value, whereby the application of screen voltage Vs does not prevent the extraction of ions.

3. A method as claimed in Claim 2 which includes providing relative motion between the ion generating assembly and the image surface, and regulating the formation of an electrostatic image on the dielectric surface by selective application of direct voltage Vc, said electrostatic image having potential V1, wherein the screen voltage V5 is larger in magnitude and equal in polarity to the image potential V, in order to prevent undesired image erasure.

4. A method as claimed in Claim 1 in which a multiplicity of driver and control electrodes form cross points in a matrix array configured such that the control electrodes contain openings at matrix electrode crossover regions, the controlling step being performed by modulating the extraction of ions from said openings by means of a multiplicity of screen electrodes containing apertures correspdnding to said openings.

5. A method of generating ions substantially as described herein with reference to Figures 9 to 23 of the accompanying drawings.

6. Apparatus for generating electrostatic images which comprises: a solid dielectric member; a "driver" electrode in contact with one side of the solid dielectric member; control electrode substantially in contact with an opposite side of the solid dielectric member, with an edge surface of the control electrode disposed opposite the driver electrode to define an air region at the junction of the said edge surface and the solid dielectric member; means for applying a varying potential between the driver and control electrodes of sufficient magnitude to induce ion producing electrical discharges in the air region between the solid dielectric member and the edge surface of said control electrode; means for applying an ion extraction potention Vc to the control electrode to extract ions from the air region and to apply these ions to an imaging surface to form an electrostatic image thereon; a third, "screen" electrode; a solid dielectric layer isolating the screen electrode from the control electrode and the solid dielectric member, which provides an air gap between them; and a source of "screen" voltage Vs to the screen electrode.

7. Apparatus as claimed in Claim 6 in which the imaging surface comprises a dielectric surface.

8. Apparatus as claimed in Claim 6 or Claim 7 in which the control electrode, screen electrode, and solid dielectric layer contain corresponding discharge apertures.

9. Apparatus as claimed in Claim 8 in which the discharge apertures in the solid dielectric layer are larger in diameter than the corresponding discharge apertures in the control electrode.

10. Apparatus as claimed in Claim 7 in which the direct voltage Vs is equal in polarity to and smaller in absolute value than the direct voltage Vc.

11. Apparatus as claimed in Claim 10 which includes means for providing a relative motion between the apparatus for generating electrostatic images and said further member, and means for modulating said direct voltage Vc in order selectively to form an electrostatic pattern on the dielectric record member of voltage V, with respect to its conductive backing, the direct voltage Vs being larger in magnitude and equal in polarity to the direct voltage V, in order to prevent undesired image erasure.

1 2. Apparatus as claimed in Claim 6 in which the driver and control electrodes each comprises a plurality of sub-electrodes so as to form cross points in a matrix array configured such that the control electrodes contain openings at matrix electrode crossover regions, the solid dielectric layer containing apertures corresponding to the said openings, and said screen electrode comprising a multiplicity of electrodes matching the control electrodes and containing apertures corresponding to said openings.

1 3. Apparatus as claimed in Claim 6 in which the driver and control electrodes each comprises a plurality of sub-electrodes so as to form cross points in a matrix array configured such that the control electrodes contain openings at matrix crossover regions, said solid dielectric layer containing apertures corresponding to said openings and said screen electrode comprising a conducting member containing a series of apertures corresponding to said openings.