EP0058182B1

EP0058182B1 - Electrostatic printing and copying

Info

Publication number: EP0058182B1
Application number: EP81902352A
Authority: EP
Inventors: Richard A. Fotland; Leo A. Beaudet; Richard L. Briere; Jeffrey J. Carrish; Donald J. Lennon; Casey S. Vandervalk
Original assignee: Dennison Manufacturing Co
Current assignee: Dennison Manufacturing Co
Priority date: 1980-08-21
Filing date: 1981-08-17
Publication date: 1987-03-04
Also published as: PT73549B; EP0140399A1; AU4092589A; JPS57501348A; IT1139412B; EP0140399B1; EP0166494A1; EP0266823A2; WO1982000723A1; MX151040A; EP0166494B1; EP0266823A3; EP0265994A2; DE3177224D1; BR8108750A; ES8301037A1; IL63583A0; EP0058182A1; PT73549A; NZ198031A

Abstract

An electrostatic printing device in which an electrostatic latent image is formed on an imaging or dielectric roller (25), toned, and transferred by pressure to plain paper (35). A problem which is typically encountered in transferring a toner image solely by use of pressure is the existence of a residual toner image on the dielectric member after image transfer, due to inefficiencies in toner transfer. The residual toner particles require scraper blades or other removal devices, and accumulate over time at the various process stations associated with the dielectric member, including the apparatus for forming the latent electrostatic image. These toner accumulations decrease the reliability of the apparatus, necessitating service at intervals. Furthermore, inefficiencies in toner transfer may lead to mottling of the images formed on the image receptor sheets. These problems have not been overcome in the prior art through the use of extremely high pressures at the transfer nip. Toner transfer efficiency is improved in the device of the present invention by skewing dielectric roller (25) and pressure roller (37). In the printer, the latent image is formed by an ion generator (100) using two electrodes (102-1, 102-2) that are separated by a solid dielectric (101). A varying high frequency potential (103) is used to create an air gap breakdown in a region at a junction of one of the electrodes and the solid dielectric (101). The ion generator (100) is fabricated by laminating a metal foil (174, 175) to mica (171) using pressure sensitive adhesive (172, 173), and etching the foil to form electrodes. An alternative ion generator is formed using a dielectric-coated wire (197) and a series of transverse conductors (184, 186) A preferred method of fabricating the dielectric roller involves anodizing an aluminum cylinder (25), and impregnating the surface pores with a metallic salt of a fatty acid while maintaining the pores in a substantially moisture-free state.

Description

This invention relates to electrostatic printing, particularly at high speeds.
One commonly employed principle for generating ions is the corona discharge from a small diameter wire or a point source. Illustrative U.S. Patent Nos. are P. Lee 3,358,289; Lee F. Frank 3,611,414; A. E. Jvirblis 3,623,123; P. J. McGill 3,715,762, H. Bresnik 3,765,027; and R. A. Fotland 3,961,564. Corona discharges are used almost exclusively in electrostatic photocopiers to charge photoconductors prior to exposure, as well as for discharging. These applications require large area blanket charging/discharging, as opposed to formation of discrete electrostatic images. Unfortunately, standard corona discharges provide limited currents. The maximum discharge current density heretofore obtained has been on the order of 10 microamperes per square centimeter. 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 be frequently cleaned or replaced.
Corona discharge devices which enjoy certain advantages over standard corona apparatus are disclosed in Sarid et al., U.S. Patent No. 4,057,723; Wheeler et al. 4,068,284; and Sarid 4,110,614. These patents disclose various corona charging devices characterized by a conductive wire coated with a relatively thick dielectric material, in contact with or closely spaced from a further conductive member. A supply of positive and negative ions is generated in the air space surrounding the coated wire, and ions of a particular polarity are extracted by a direct current potential applied between the further conductive member and a counterelectrode. Such devices overcome many of the above-mentioned disadvantages of prior art corona charging and discharging devices but are unsuitable for electrostatic imaging. This limitation is inherent in the feature of large area charging, which does not permit formation of discrete, well-defined electrostatic images. This prior art corona device requires relatively high extraction potentials due to greater separation from the dielectric receptor.
Further types of known charging and discharging devices suitable for use with photocopying apparatus are described in US-A- . 4,155,093 and DE-A-2,846,474. Each of these specifications describes a large area charging array comprising parallel first and second elongate electrodes separated by a dielectric member. The first electrode is entirely surrounded by an insulating member, but the second electrode includes an area open to the air. When an alternating voltage is applied between the first and second electrodes a glow discharge occurs in the air in this region, and ions are produced which can be extracted by means of a secondary DC potential. Each of these specifications also discloses a laminar ion generator in the form of a matrix array, suitable for directly forming an image,. by means of a series of dots, in a printing apparatus. The array comprises a flat dielectric sheet having a parallel series of flat selector bars mounted on one side thereof,, and a series of flat finger electrodes mounted transversely on the other side, so .that the crossing points of the finger electrodes and the selector bars form a matrix. An AC potential supplied selectively between the selector bars and the electrodes causes a pool of ions to be generated of selected cross-over points, and these ions can then be transferred by a secondary applied DC potential to an imaging member to form a latent electrostatic image.
Various toner image transfer methods are known in the art. The transfer may be accomplished electrostatically, by means of a charge of opposite polarity to the charge on the toner particles, the former charge being used to draw the toner particles off the dielectric member and onto the image receptor. Patents illustrative of this transfer method include U.S. Patents Nos. 2,944,147; 3,023,731; and 3,715,762. Alternatively, the image receptor medium may be passed between the toner-bearing dielectric member and transfer member, and the toner image transferred by means of pressure at the point of contact. Patents illustrative of this method include U.S. Patent Nos. 3,701,966; 3,907,560; and 3,937,571. Usually, the toner image is fused to the image receptor subsequently to transfer of the image, at a further process station. Postfusing may be accomplished by pressure, as in U.S. Patent No. 3,874,894, or by exposure of the toner particles to heat, as in U.S. Patent No. 3,023,731, and Re. No. 28,693.
It is possible, however, to accomplish transfer and fusing of the image simultaneously, as shown for example in the patents cited above as illustrative of pressure transfer. This may be accomplished by a heated roller, as in Re. No. 28,693, or simply by means of high pressure between the image-bearing dielectric member and a transfer member, between which the image receptor passes.
A problem which is typically encountered in transferring a toner image solely by means of pressure is the existence of a residual toner image on the dielectric member after image transfer, due to inefficiencies in toner transfer. The residual toner particles require scraper blades or other removal means, and accumulate over time at the various process stations associated with the dielectric member, including the apparatus for forming the latent electrostatic image. These toner accumulations decrease the reliability of the apparatus, necessitating service at intervals. Furthermore, inefficiencies in toner transfer may lead to mottling of the images formed on the image receptor sheets. These problems have not been overcome in the prior art through the use of extremely high pressures at the transfer nip.
A phenomenon which is commonly observed when subjecting rollers to high pressures is that of "bowing" of the rollers. This phenomenon occurs when the rollers are subjected to a high compressive force at the ends, thereby imparting a camber to each roller. The effect is to have high pressure at the ends of the rollers but lower pressure at the center. It is known in the prior art to alleviate this problem when encountered in pressure fusing apparatus by skewing the pressure rollers, i.e. by adjusting the mounting of the rollers to create an oblique orientation of the roller axes. Representative United States patents include U.S. Patent Nos. 3,990,391; 4,188,104; 4,192,229; and 4,200,389. This technique has the disadvantage of causing "walking" of a receptor sheet fed between the rolls. In addition, this apparatus commonly encounters the problem of wrinkling of the receptor sheets.
Hardcoat anodization of aluminum and aluminum alloys is an electrolytic process which is used to produce thick oxide coatings with substantial hardness. Such coatings are to be distinguished from natural films of oxide which are normally present on aluminum surfaces and from thin, electrolytically formed barrier coatings.
The anodization of aluminum to form thick dielectric coatings takes place in an electrolytic bath containing an oxide, such as sulfuric or oxalic acid, in which aluminum oxide is slightly soluble. The production techniques, properties, and applications of these aluminum oxide coatings are described in detail in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Robert Draper Ltd., Paddington, England (chapter IX page 563). Such coatings are extremely hard and mechanically superior to uncoated aluminium. However, the coatings contain pores in the form of fine tubes with a porosity on the order of 6.4516x10'⁴ to 6.4516x10'^s pores per square meter (10¹⁰ to 10¹¹ pores per square inch). Typical porosities range from 10 to 30 percent by volume. These pores extend through the coating to a very thin barrier layer of aluminium oxide, typically 3x10-^B to 8x10-⁸m (300 to 800 Angstroms).
U.S. Patent No. 3664300 discloses a process for surface treatment of xerographic imaging cylinders wherein the surface is coated with zinc stearate to provide enhanced surface lubrication and improved electrostatic toner transfer. This treatment technique does not, however, result in a permanent dielectric surface or requisite hardness and smoothness for pressure transfer and fusing of a toner image.
For improved mechanical properties as well as to prevent staining, it is customary practice to seal the pores. One standard sealing technique involves partially hydrating the oxide through immersion in boiling water, usually containing certain nickel salts, which form an expanded boehmite structure at the mouths of the pores. Oxide sealing in this manner will not support an electrostatic charge due to the ion conductivity of moisture trapped in the pores.
The critical mechanical tolerances in providing a latent electrostatic image in an electrostatic printer may be reduced. Thus, the maintenance problems associated with the formation of such an image are also reduced. The specific image generators described are suitable particularly at high current densities, for use in electrostatic printing, as well as other applications. The ion generator design described is based upon a corona electrode, which achieves high current densities with an easily controllable source of ions. This apparatus does not require the critical periodic maintenance normally characteristic of such corona devices, and avoids the objectionable operational characteristics of corona wires.
The invention involves the use of an ion generator which produces a glow discharge to generate a pool of positive and negative ions, which may be extracted for application to a further member. A varying potential is applied between an elongate conductor having a dielectric sheath and a transverse conductive member in order to generate ions at a crossover point of these structures.
According to the present invention, electrostatic printing apparatus comprises an imaging roller having a conductive core and a dielectric surface layer; means for generating a latent electrostatic image on the dielectric surface layer; means for toning the latent electrostatic image; and a transfer roller which contacts the imaging member under pressure, with an image receptor fed therebetween; the means for generating a latent electrostatic image comprising elongate control electrodes and at least one elongate transversely orientated driver electrode separated by a dielectric member, with a varying potential applied between the electrodes to create a glow discharge, and means for extracting ions from the glow discharge; characterised in that the driver electrode comprises a wire, the dielectric member comprises a sheath surrounding the wire, and an insulating support member is provided which supports the sheath.
Preferably, the insulating support member includes a slot, the driver electrode and sheath being received within the slot, and the control electrodes being transversely mounted on the insulating support member across the slot.
The surface of the imaging roller may have a porous anodized aluminium dielectric surface layer impregnated with a metallic salt of a fatty acid. It may also have a smoothness in excess of 5.08x1^-7 metres rms (20 microinch rms), and/or may have a resistivity in excess of 10¹⁰ ohm metres.
In a preferred embodiment of the invention, the pressure transfer of the toner image effected by the dielectric and transfer rollers may be enhanced by providing a skew between the dielectric and transfer rollers. In the nip between the rollers, the ratio of the dielectric surface speed to the image receptor speed is advantageously in the range of about 1.01 to 1.1, most advantageously between 1.02 and 1.04. Best results are achieved where the dielectric surface has a smoothness in excess of 0.508 mrms (20 microinch rms), and a high modulus of elasticity. The transfer roller is preferably coated with a stress-absorbing plastics material. The roller materials are advantageously chosen so that the image receptor will have a tendency to adhere to the surface of the transfer roller in preference to that of the dielectric roller. The apparatus provides effective toner transfer and fusing without wrinkling of the receptor medium.
The above and additional aspects of the invention are illustrated, for the sake of example, with reference to the detailed description which follows, taken in conjunction with the drawings in which:

Figure 1 is a sectional schematic view of electrophotographic apparatus in accordance with a preferred embodiment of the invention;
Figure 2 is a partial sectional schematic view of the nip area of the upper rotters of Figure 1;
Figure 3 is a sectional- schematic view of electrophotographic apparatus in accordance with an alternative embodiment of the invention;
Figure 4 is a sectional schematic view of electrostatic printing apparatus in accordance with a preferred embodiment of the invention;
Figure 5 is a partial sectional schematic view of an illustrative charge neutralizing device for the dielectric roller of Figure 4;
Figure 6 is an elevation view of a preferred mounting arrangement for electrostatic printing apparatus of the type illustrated in Figure 4;
Figure 7 is a schematic view of the rollers of Figure 7 as seen from above;
Figure 8 is a geometric representation of the contact area of the rollers of Figure 6;
Figure 9 is a plot of residual toner as a function of end to end skew for the apparatus of Example: IV-3;
Figure 10 is a partial perspective view of an electrostatic imaging device for use with an alternative embodiment of the invention;
Figure 11 is a schematic sectional view of the apparatus of Figure 10, further including ion extraction apparatus and an ion receptor member;
Figure 12 is a cutaway perspective view of an alternative version of the imaging apparatus for Figure 10;
Figure 13 is a cutaway perspective view of a further alternative version of the electrostatic imaging apparatus of Figure 10;
Figure 14 is a plan view of matrix imaging apparatus of the type shown in Figure 10;
Figure 15 is a sectional schematic view of a three electrode embodiment of the imaging device of Figure 12;
Figure 16 is a perspective view of an electrostatic imaging device for use with yet another embodiment of the invention; and
Figure 17 is a plan view of a serial printer incorporating an electrostatic imaging device of the type illustrated in Figure 11.

I. Introduction

Two main embodiments of the invention are described namely the double transfer electrophotographic apparatus which is the subject of Section II, and the electrostatic transfer printer which is the subject of Section III. These two embodiments differ in the means by which a latent electrostatic image is created on a dielectric imaging roller; thereafter, identical apparatus may be employed.
The skewed roller apparatus of Section IV is profitably employed to provide enhanced toner transfer and fusing in either of the main embodiments. Section VI discloses an alternative ion generator and extractor which may be incorporated in the printing apparatus of Section III. The impregnated anodized aluminium members of Section VII 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 embodiments.

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 13 supported on a conducting substrate 17, with an intervening semiconducting substrate 15. Advantageous materials for the photoconductor surface layer 13 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 CdS_xSey, where x+y=1), or organic photoconductors such as the equimolar complex of polyvinyl carbazole and trinitrofluorenone.
The photoconductor is electrostatically charged at charging station 19 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 V below (Figure 14). The optical image which provides the latent image on the photoconductor may be generated by any of several wall 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 subsection IIIB 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 13 results in a charge density distribution corresponding to the exposed image, and a variable potential pattern of the surface of the photoconductor coating 13 with respect to the grounded conductive substrate 17. 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.
It is possible to maintain the photoreceptor 11 in direct contact with the dielectric roller 25, an arrangement which provides the advantage of simplicity in mounting and driving the cylinders. An effective TESI process may be achieved under these conditions, but this will result in toner transfer to the upper cylinder and therefore will require additional cleaning apparatus.
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 width of the air gap (see Dessauer and Clark, Xerography and Related Processes, the Focal Press, London and New York, 1865, 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 13 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 15 between the photoconductive surface layer 13 and the conductive substrate 17 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 precipitious behavior otherwise associated with disruptive breakdown. 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 on 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 of a single transfer system relying on the contrast 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 the 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.

Example 11-1

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 aluminum to a diameter of 76 mm. The length of this cylindrical core, excluding machined journals, was 230 mm. The journals were masked, and the aluminum anodized by use of the Sanford process (see S. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminum and its Alloys, Robert Draper Ltd., 4th Edition 1971/72, Vol. 2, Page 567). The finished aluminum oxide layer was 60 pm (micrometres) in thickness. The cylinder 25 was then placed in a vacuum oven at 101.5917 kPa (30 inches mercury). After half an hour, the oven temperature was set at 150°C. The cylinder was maintained at this temperature and pressure for four hours. The heated cylinder was brush-coated with melted zinc stearate and returned to the vacuum oven for a few minutes at 150°C, 101.59 kPa (30 inches mercury). The cylinder was removed from the oven and allowed to cool. The impregnated surface 27 of the dielectric cylinder 25 was then finished to 0.125 to 0.25 11m 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 17 of the photoconductor member 11, comprising an aluminum sleeve, was fabricated of 6061 aluminum tubing with a 3 mm wall and a 50 mm outer diameter. The outer surface was machined and the aluminum anodized (again, using the Sanford process) to a thickness of 50 m. 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/I, 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° Celcius, pH of 5.6) for ten minutes. The semiconducting substrate 15 was spray coated with a binder layer, the photoconductor coating 13 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 pm, 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. The side frames were machined to provide a 1.10 end-to-end between rollers 25 and 37.
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 & Co. (Stuttgart) No. 444. copier.
The toner removal 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. The residual latent image was erased using a semiconducting rubber roller in contact with the dielectric surface 27 (see Fig. 5).
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 13 was charged to a potential of minus 1,000 volts relative to its substrate 17. 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.

Example 11-2

In another embodiment of the 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 pm. The photoconductor rollers 17'a and 17'b were friction driven from the dielectric cylinder 25. The lower roller 17'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 15 11m 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 µm rms finish. Again, high quality copies were obtained, even at operating speeds as high as 75 cms per second.

III. 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. A latent electrostatic image formation

The printing apparatus may incorporate any embodiment of the electrostatic imaging device discussed in Section VI below.
All of the above charging devices are characterized by the production of a "glow discharge", a silent discharge formed in air between two conductors separated by a solid dielectric. Such discharges have the advantage of being self-quenching, whereby the charging of the solid dielectric to a threshold value will result in an electrical discharge between the solid dielectric and the control electrode. By application of a time-varying potential, glow discharges are generated to provide a pool of ions of both polarities.
It is useful to characterize all of the charging device embodiments in terms of a "control electrode" and a "driver electrode". The control electrode is maintained at a given DC potential in relation to ground, while the driver electrode is energized around this value using a time-varying potential such as a high voltage AC or DC pulse source. In the illustrated embodiments of section VI, the coated conductor or wire constitutes the driver electrode. In an alternative driving scheme for the latter device, the coated conductor may be employed as the control electrode.

B. Subsequent processing

Identical apparatus may be employed for both electrophotography and printing to carry out process steps subsequent to the creation on the dielectric cylinder of a latent electrostatic image (compare Figures 1 and 4). The apparatus of Figure 4 will be considered for illustrative purposes.
In Figure 4, 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 10" 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 11m rms, in order to provide for complete transfer of toner to the receptor sheet 81. The smoothness of dielectric surface 75 contributes to the efficiency of toner transfer to the receptor sheet 81 by enhancing the release properties of this surface. The dielectric layer 75 additionally has a high modulus of elasticity, typically on the order of 6.89476 x 107 kPa (10⁷ PSI), so that it is not distorted significantly by high pressures 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 aluminum cylinder. Flame or plasma sprayed high density aluminum 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 suitable. A preferred dielectric coating is anodized aluminum oxide impregnated with a metal salt of a fatty acid, as described in section VII, 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, 1958. 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. It has been observed that providing a non-parallel orientation, or skew, between the rollers of Figure 4 has a number of advantages in the transfer/fusing process. An image receptor 81 such as plain paper has a tendency to adhere to the compliant surface of the pressure roller 83 in preference to the smooth, hard surface of the dielectric roller 73. Where rollers 73 and 83 are skewed, this tendency has been observed to result in a "slip" between the image receptor 81 and the dielectric surface 75. The most notable advantage is a surprising improvement in the efficiency of toner transfer from dielectric surface 75 to image receptor 81. This efficiency may be expressed in percentage terms as the ratio of the weight of toner transferred to that present on the dielectric roller before transfer. Apparatus of this nature is disclosed in section IV.
The bottom roller 83 consists of a metallic core 87 which may have an outer covering of engineering plastics 85. The surface material 85 of roller 83 typically has a modulus of elasticity on the order of 1378952 to 3102642 kPa (200.000-450,000 PSI). The image receptor 81 will tend to adhere to the surface 85 in preference to the dielectric layer 75 because of the relatively high smoothness and modulus of elasticity of the latter surface. In the embodiment of section IV, one function of this surface 85 is to bond image receptor 81 when the latter is subjected to a slip between the roller surfaces. Another 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 12.5 mm.
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. It has been discovered, in addition, that the skewing of rollers 73 and 83 will decrease the transfer pressure requirements. See section IV, below. Typical pressures run from 18 to 125 kg per linear cm of contact.
Scraper baldes 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 quantity of residual toner is markedly reduced in the embodiments of section IV, infra.
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 tones 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.
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. Erasure of any latent electrostatic image can be accomplished by using a high frequency AC potential between electrodes separated by a dielectric.
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 5 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.

Example 111-1

In a specific operative example of an electrographic printing in accordance with the invention, the cylindrical conducting core 5 of the dielectric cylinder 1 was machined from 7075-T6 aluminium to a 76.2 mm (3 inch) diameter. The length of the cylindrical core, excluding machined journals, was 228.6 mm (9 inches). 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 Aluminum and its Alloys, Robert Draper Ltd. fourth edition, 1971/72 volume 2, page 567). The finished aluminum oxide layer was 60 microns in thickness. The conducting core was then heated in a vacuum oven, 101.5917 kPa (30 inches mercury), to a temperature of 150°C which temperature was achieved in 40 minutes. The cylinder was maintained at this temperature and pressure for four hours prior to impregnation.
A beaker of zinc stearate was preheated to melt the compound. The heated cylinder was removed from the oven and coated with the melted zinc stearate using a paint brush. The cylinder was then placed in the vacuum oven for a few minutes at 150°C, 101.5917 kPa (30 inches mercury), thereby forming dielectric surface layer. The cylinder was removed from the oven and allowed to cool. After cooling, the member was polished with successively finer SiC abrasive papers and oil. Finally, the member was lapped to a 0.1143 µm (4.5 microinch) finish.
The pressure roller 11 consisted of a solid machined two inch diameter aluminum core 12 over which was press fit a 50.8 mm (two inch) inner diameter, 63.5 mm (2.5 inch) outer diamter polysulfone sleeve 13. The dielectric roller was gear driven from an AC motor to provide a surface speed of 304.8 mm/s (12 inches per second). The transfer roller 11 was rotatably mounted in spring-loaded side frames, causing it to press against the dielectric cylinder with a pressure of 5337.4 kg/m (300 pounds per linear inch) of contact. The side frames were machined to provide a skew of 1.1° between rollers 1 and 11.
A charging device of the type described in U.S. Patent No. 4,160,257 was manufactured as follows. A 25.4 pm (1 mil) stainless steel foil was laminated on both sides of a 25.4 um (1 mil) sheet of Muscovite mica. The bonding material and technique is detailed in Example V-1, infra. The stainless foil was coated with resist and photoetched with a pattern similar to that shown in Figure 12, with holes or apertures in the fingers approximately 0.1524 mm (.006 inch) in diameter. The complete print head consisted of an array of 16 drive lines and 96 control electrodes which formed a total of 1536 crossover locations capable of placing 1536 latent image dots across 195.072 mm (7.68 inch) length of the dielectric cylinder. Corresponding to each crossover location was a 0.1524 mm (.006 inch) diameter etched hole in the screen electrode. Bias potentials of the various electrodes were as follows (with the cylinder's conducting core maintained at ground potential):
The DC extraction voltage was supplied by a pulse generator, with a print pulse duration of 10 microseconds. Charging occurred only when there was simultaneously a pulse of negative 400 volts to the fingers 44, and an alternating potential of 2 kilovolts peak to peak at a frequency of 1 Mhz supplied between the fingers 44 and selector bars 43. The print head was maintained at a spacing of 203.2 mm (8 mils) from dielectric cylinder.
Under these conditions it was found that a 300 volt latent electrostatic image was produced on the dielectric cylinder in the form of discrete dots. The image was toned using single component toning apparatus essentially identical to that employed in the Develop KG Dr. Eisbein and Company (Stuttgart) No. 444 copier. The toner employed was Hunt 1186 of the Phillip A. Hunt Chemical Corporation.
The printing apparatus 70 included user- actuatable sheet-feeding apparatus (not shown) for feeding individual sheets 81 of paper between cylinders 73 and 83. The paper feed, toning apparatus, and cylinder rotation were driven from a unitary drive assembly (not shown). Paper feed was synchronized with the rotation of dielectric cylinder 73 to ensure proper placement of the toned image.
Digital control electronics and a digital matrix character generator, designed according to principles well known to those skilled in the art, were employed in order to form dot matrix characters. Each character had a matrix size of 32 by 24 points. A shaft encoder mounted on the shaft of the dielectric cylinder was employed to generate appropriate timing pulses for the digital electronics.
Flexible steel scraper blades 89 and 91 were employed to maintain cleanliness of dielectric cylinder 73 and transfer cylinder 83. With reference to the electrostatic image erasing embodiment shown at 98 in Figure 5 the residual latent image was erased using a semiconducting rubber roller in contact with the dielectric surface 75.

IV. Toner transfer apparatus with skewed rollers

Figure 6 shows in a plan view illustrative transfer printing apparatus 70 of the type shown schematically in Figure 4, including details of a preferred mounting arrangement. Side frames 59 and 69 house bearing retainers 57 and 67, which are fitted to rollers 73 and 83 in order to allow the rotation of the rollers while constraining their horizontal and vertical movement. Substantially identical side frames and bearing retainers are located at the other end of rollers 73 and 83. Bearing retainers 57 and 67, which advantageously are of the type known as "self- aligning", fit within lips 51 and 61 on the respective side frames, and against shoulders (not shown) on the respective rollers. The side frames are mounted on one side to superstructure 55, and are mounted on the other end in spring-loaded journals 58 in order to provide a prescribed upward pressure against roller 73. Roller 73 is driven at a desired rotational velocity by means not shown, while roller 83 is frictionally driven due to the contact of the rollers at the nip.
The mounting illustrated in Figure 6 is machined in order to provide a specified "skew", or deviation of the axis of rollers 73 and 83 from a parallel orientation. Rollers 73 and 83 may be adjustable around a pivot point at one end, by varying the angular relationship (in the vertical plane) of the rollers at the other end. Alternatively, the rollers may pivot around a central point of contact, by adjusting the offset of one of the rolls about the axis of the other, this adjustment being equal at both ends. This latter, "end-to-end" skew will be assumed hereinafter for illustrative purposes.
The mounting arrangement shown in Figure 6 may be easily adapted to electrophotographic apparatus of the type shown in Figure 1. In a further embodiment, the dielectric imaging roller (upper roller) may comprise a photoconductive surface layer over a conducting substrate. With reference to the sectional view of Figure 4, the imaging apparatus 71 may be replaced with any suitable apparatus known in the art for depositing a uniform charge on surface 75, and for exposing the surface to a pattern of light and shadow whereby the charge is selectively dissipated to form a latent electrostatic image. As in the dielectric embodiment, photoconductive surface 75 is advantageously smooth and abrasion resistant, with a high modulus of elasticity. See Example IV-4.
As shown in Figure 6, axle 50A is disposed in end-to-end skew, which may be measured as an offset L in the plane of side frame 59. A more significant measure of skew, however, is the angle between the projected axes of rollers 73 and 83 as measured in the horizontal plane, or plane or paper feed. An illustrative value of skew to effect the objects of the invention is 0.10 inch (2.54 mm), measured at the center of roller bearings 57 and 67, which are separated by a distance of 263,525 mm (10.375 inch) for 228.6 mm (9 inch) long rollers. This represents an angle of roughly 1.1°.
Figure 7 schematically illustrates skewed rollers 73 (with axis B-B) and 83 (with axis C-C) as seen from above. Roller 83 is skewed at the bearing mounts by horizontal offset L from the vertically projected axis B'-B' of roller 73. This corresponds to an angle between axes B-B and C-C. Axis B-B is perpendicular to the direction A of paper feed.
Figure 8 is a geometric representation of the surface of contact of the rollers at the nip, showing the direction of paper feed before and after engagement by the rollers. As a sheet of paper 81 travelling in direction A enters the nip, it is subjected to divergent forces in direction D (perpendicular to the projected axis C'-C' or roller). Because of the relatively high smoothness and modulus of elasticity of the surface 75 of roller 73, the paper will tend to adhere to the lower roller, and therefore to travel in direction E. This results in a surface speed differential or "slip" between the surface of paper and roller.
Due to the compression of the lower roller 83 at the nip, paper 81 will contact both roller surfaces over a finite distance M in direction D. The width of the contact area, M, can be calculated using a formular found in Formulas for Stores and Strain (4th edition) by Ronald J. Roark, published by McGraw-Hill Book Company. The formula for the case of two cylinders in contact under pressure with parallel axes can be found on page 320 of the Roark Text, table XIV, section 5.
The transaxial width in metres of the contact of the two cylinders is given by
where P represents the cylinder loading in Nm-';

D', D² represent the diameters of the cylinders in m;
V, and V₂ represent Poissons ratio in compression for the materials of the cylinders; and
E,', E₂' represents the modulus of elasticity in compression for the materials of the cylinders in Pa. (with W expressed in inches thus 4s:-
where
P is in pounds per linear inch;
D₁, D₂ are in inches;
V, and V₂ as above;
E, and E₂ in pounds per square inch).

With reference to the resultant triangle in Figure 8, the surface of receptor 81 will undergo a proportional side travel N with respect to the surface of roller 73, the factor of proportionally being the surface speed differential.
The skewing of rollers 73 and 83 in the above described manner results in a surprising improvement in the efficiency of toner transfer from dielectric surface 73 to image receptor 81. This efficiency may be expressed in percentage terms as the ratio of the weight of toner transferred to that present on the dielectric roller before transfer. This bears a complementary relationship to the weight of residual toner on the dielectric roller after transfer. The increase in transfer efficiency, which is the most notable advantage of the invention, minimizes the service problems attributable to the accumulation of residual toner at the process stations associated with the image roller 73, including scraper blades 89 and 91, erase head 98 and image generator 71. This effect depends on the choice of surface material 75 and toner.
It is another surprising advantage of this technique that this enhanced toner transfer is achieved without wrinkling of the receptor medium 81. These advantages accrued even in the case of nonfibrous substrates 81, such as Mylar film.

Example IV-1

Apparatus of the type illustrated in Figures 4 and 6 incorporated a 228.6 mm (9 inch long), 101.6 mm (4 inch) outer diameter roller 73 having a dielectric surface 75 of anodically formed porous aluminum oxide, which had been dehydrated and impregnated with zinc stearate (see section VII) and then surface polished. The dielectric surface of roller 73 was polished to obtain a finish of better than 0.254 m rms (10 microinch rms).
The pressure cylinder 83 included a 228.6 mm (9 inch) long steel mandrel with an outer diameter of 79.375 mm (3.125 inches) over which was pressed a 9.525 mm (0.375 inch) thick sleeve of polyvinylchloride. The rollers were pressed together at 6250,3 Kg/m (350 pounds of pressure per linear inch) of nip.
A latent electrostatic image was formed on the dielectric surface of roller 73. The various voltages to the ion generator 71 were maintained at constant values. The tests were conducted under the same ambient conditions throughout.
The toner employed was Hunt 1186 of Phillip A. Hunt Chemical Corporation. The single component latent image toning apparatus was essentially identical to that employed in the Develop KG Dr. Eisbein & Co., (Stuttgart) No. 444 copier.
The toner was transferred onto Finch white bond paper, 60 vellum of Finch, Pruyn and Co. This paper was fed into the nip between the dielectric and pressure rollers at a constant speed throughout the tests.
Using the above specifications, the apparatus was operated at 0° skew, .55° skew, and 1.1° skew, where the skew was measured as a 2.54 mm (0.10 inch) offset at the bearing retainers of the 228.6 mm (9 inch) long pressure roll. The results shown in Table IV-A were obtained by collecting the residual toner and comparing its weight to the known weight of toner before transfer. No after transfer printing was present on the upper cylinder during the tests with 0.55° and 1.1° skew. However, transfer was so poor during the test without skew that printing was plainly visible on the upper cylinder after transfer.

Example IV-2

The apparatus of Example IV-1 was employed with Desoto toner 2949-5 of Desoto Inc. The toner was transferred onto coated OCR Imagetroll paper, manufactured by S. D. Warren. The rollers were pressed together without skew at 7500.36 Kg/m (420 pounds per linear inch), resulting in a transfer efficiency of 92.6 percent, measured by comparing the weight of toner before image transfer to the weight of residual toner. The rollers were then pressed together at 1.1° skew, with a pressure of 3571.6 Kg/m (200 pounds per linear inch), and all other parameters unchanged, resulting in a transfer efficiency of 99.95 percent.

Example IV-3

The apparatus of Example IV-1 was employed with the following modifications. The pressure cylinder 83 comprised a 228.6 mm (9 inch) long steel mandrel with a 1.945 inch outer diameter, over which was pressed a 228.6 mm (9 inch) long Celcon sleeve with a 88.9 (3.50 inch) outer diameter. (Celcon is a trademark of Celanese Chemical Co. for thermoplastic linear acetal resins). The two rollers were pressed together at 3571.6 Kg/m (200 pounds of nip pressure per linear inch) of nip.
The toner employed was Coates RP0357 of the Coates Bros. and Co., Ltd. The toner was transferred onto Finch white bond paper, #60 vellum.
Using the above specifications, the apparatus was operated with end-to-end skew, varied over a range of angles from 0.0° to 1.1°. The apparatus was operated using a constant weight of toner prior to transfer, and the residual toner present on dielectric roller 73 was collected and weighed. The results are shown in Table IV-B, and are graphed in Figure 9. In the case of the test using no skew, the residual toner was visible as printing remaining on the upper roller.
These tests showed a dramatic improvement in the efficiency of toner transfer when the skew was increased from 0.0° to .42°; this resulted in a decrease in the weight of residual toner by a factor of 53. Increases in skew from .42° to .85° and from .55° to 1.1° further reduced the weight of residual toner by factors of somewhat better than 2.

Example IV-4

The apparatus of Example IV-4 was employed with the modification that the imaging roller 73 comprised a photoconductive roller. An aluminium sleeve was fabricated of 6061 aluminum tubing with 3.175 mm (1/8") wall and 101.6 mm (4") outer diameter. The sleeve was spray coated with a binder layer photoconductor consisting of photoconductor grade Sylvania PC-100 cadmium sulfide pigment of Sylvania Comp. Electronics Corp., dispersed in a melamine-acrylic resin, diluted with methyl ethyl ketone to a viscosity suitable for spraying. The resin was crosslinked by firing at 600° for three hours.
A photoconductor charging corona and optical exposing system were essentially identical to those employed in the Develop KG Dr. Eisbein & Co. (Stuttgart) No. 444 Copier. The toner transfer efficiency underwent improvements comparable to those of Example IV-1 for increasing skew angles of 0.0°, 0.55°, and 1.1°.

V. (Section V was deleted during prosecution)

VI. Electrostatic imaging device using dielectric coated wire

Figure 10 shows in perspective a basic embodiment of an electrostatic imaging device which may be utilized, for example, in the printing apparatus of Figure 4. Print device 180 includes a series of parallel conductive strips 184, 186, 188, etc. laminated to an insulating support 181. One or more dielectric coated wires 193 are transversely oriented to the conductive strip electrodes. The wire electrodes are mounted in contact with or at a minute distance above (i.e. less than 50.8 urn=2 mils) the strip electrodes. Wire electrodes 193 consists of a conductive wire 197 (which may consist of any suitable metal) encased in a thick dielectric material 195. In the preferred embodiment, the dielectric 195 comprises a fused glass layer, which is fabricated in order to minimize voids. Other dielectric materials may be used in the place of glass, such as sintered ceramic coatings. Organic insulating materials are generally unsuitable for this application, as most such materials tend to degrade with time due to oxidizing products formed in atmospheric electrical discharges.
Crossover points 185, 187, 189, etc. are found at the intersection of coated wire electrodes 193 and the respective strip electrodes 184, 186, 188, etc. An electrical discharge is formed at a given crossover point as a result of a high voltage varying potential supplied by a generator 192 between wire 197 and the corresponding strip electrode. Crossover regions 185, 187, 189, etc. are preferably positioned between 127 and 508 µm (5 and 20 mils) from dielectric receptor 200 (see Figure 11).
The currents obtainable from an ion generator of the type illustrated in Figure 10 may be readily determined by mounting a current sensing probe at a small distance above one of the crossover locations 185, 187, 189, etc. Current measurements were taken using an illustrative AC excitation potential of 2000 volts peak to peak at a frequency of 1 MHz., pulse width of 25 microseconds, and repetition period of 500 microseconds. A DC extraction potential of 200 volts was applied between the strip electrode and a current sensing probe spaced 203.2 um (8 mils) above the dielectric coated wire 193. Currents in the range from about .03 to .08 microamperes were measured at AC excitation potentials above the air gap breakdown value, which for this geometry was approximately 1400 volts peak to peak. At excitation voltages above the breakdown value, the extraction current varied linearly with excitation voltage. The extraction current varied linearly with extraction voltage, as well. For probe-wire spacings in the range of 101.6 pm to 508 pm (4-20 mils), the extraction current was inversely proportional to the gap width. Under 101.6 µm (4 mils), the current rose more rapidly. With the above excitation parameters, the imaging device was found to produce latent electrostatic dot images in periods as short as 10 microseconds.
In the sectional view of Figure 16, ions are extracted from an ion generator of the type shown in Figure 10 to form an electrostatic latent image on dielectric receptor 200. A high voltage alternating potential 192 between elongate conductor 197 and transverse electrode 184 results in the generation of a pool of positive and negative ions as shown at 194. These ions are extracted to form an electrostatic image non- dielectric surface 200 by means of a DC extraction voltage 198 between transverse electrode 184 and the backing electrode 205 of dielectric receptor 200. Because of the geometry of the ion pool 194, the extracted ions tend to form an electrostatic image on surface 200 in the shape of a dot.
A further imaging device embodiment is illustrated in Figure 12 showing a print head 210 similar to that illustrated in Figure 10, but modified as follows. The dielectric coated wire 213 is not located above the strip electrodes, but instead is embedded in a channel 219 in insulating support 211. The geometry of this arrangement may be varied in the separation (if any) of dielectric coated wire 213 from the side walls 212a and 212b of channel 219; and in the protrusion (if any) of wire electrode 213 from channel 219.
Figure 13 is a perspective view of ion generator 220 of the same type as that illustrated in Figure 12 with the modification that the strip electrodes 224; 226, and 228 are replaced by an array of wires. In this embodiment wires having small diameters are most effective and best results are obtained with wires having a diameter between 25.4 pm and 101.6 µm (1 and 4 mils).
The air breakdown in any of the above embodiments occurs in a region contiguous to the junction of the dielectric sheath and transverse conductor (see Figure 11). it is therefore easier to extract ions from the print heads of Figures 11 and 13 than that of Figure 10 in that this region is more accessible in the former embodiments. The ion pool may extend as far as 101.6 pm (4 mils) from the area of contact, and therefore may completely surround the dielectric sheath where the latter has a low diameter.
As the separation of the transverse conductors and the dielectric sheath has a critical effect on ion current output, they are placed in contact in order to maintain consistent outputs among various crossover points. This also has the benefit of minimizing driving voltage requirements.
It is useful to characterize all of the above embodiments in terms of a "control electrode" and a "driver electrode". The electrode excited with the varying potential is termed the driver electrode, while the electrode supplied with an ion extraction potential is termed the control electrode. The energizing potential is generically described herein as "varying", referring to a time-varying potential which provides air breakdown in opposite directions, and hence ions of both polarities. This is advantageously periodically varying potential with a frequency in the range 60 Hz.-4 MHz. In any of the illustrated, preferred embodiments, the coated conductor or wire constitutes the driver electrode, and the transverse conductor comprises the control electrode. Alternatively, the coated conductor could be employed as the control electrode.
Figures 10,12, and 13 illustrate various embodiments involving linear arrays of crossover points or print locations. Any of these may be extended to a multiplexible two-dimensional matrix by adding additional dielectric-coated conductors. With reference to the plan view of Figure-14, a two-dimensional matrix print head is shown utilizing the basic structure shown in Figure 10, with a multiplicity of dielectric-coated conductors. A matrix print head 230 is shown having a parallel array of dielectric-coated wires 231A, 231 B, 231 C etc. mounted above a crossing array of finger electrodes 232A, 232B, 232C, etc. A pool of ions is formed at a given crossover location 233_x,_Y when a varying excitation potential is applied between coated wire 231X and finger electrode 232Y. Ions are extracted from this crossover location to form an electrostatic dot image by means of an extraction potential between finger electrode 232Y and a further electrode (see Figure 11).
In any of the two-dimensional matrix print heads, there is a danger of accidentically erasing all or part of a previously formed electrostatic dot image. This occurs in the ion generator illustrated in Figure 14 when a crossover location 233 is placed over a previously deposited dot image, and a high voltage varying potential is supplied to the corresponding coated wire electrode 231. If in such a case no extraction voltage pulse is supplied between the corresponding finger electrode 232 and ground, the previously established dot image will be totally or partially erased. In any of the embodiments of Figure 10-13, this phenomenon may be avoided by the inclusion of an additional, apertured "screen" electrode, located between the control electrode and the dielectric receptor surface 200. The screen electrode acts to electrically isolate the potential on the dielectric receptor 200, and may be additionally employed to provide an electrostatic lensing action.
Figure 15 shows in section an ion generator 240 of the above-described type. The structure of Figure 12 is supplemented with a screen electrode 255, which is isolated from control electrode 244 by a dielectric spacer 252. The dielectric spacer 252 defines an air space 253 which is substantially larger than the crossover region 245 of electrodes 242 and 244. This is necessary to avoid wall charging effects. The screen electrode 255 contains an aperture 257 which is at least partially positioned under the crossover region 245.
The ion generator 240 may be utilized for electrographic matrix printing onto a dielectric receptor 258, backed by a grounded auxiliary electrode 259. When the switch is closed at a position Y, there is simultaneously an alternating potential across dielectric sheath 242, a negative potential V_c on control electrode 244, and a negative, potential V_S on screen electrode 255. Negative ions at crossover region 245 are subjected to an accelerating field which causes them to form an electrostatic latent image on dielectric surface 258. The presence of negative potential V_s on screen electrode 255, which is chosen so that V_S is smaller than Vein absolute value, does not prevent the formation of the image, which will have a negative potential V_i (smaller than V_c in absolute value).
When the switch is at X, and a previously created electrostatic image of negative potential V_i partially under aperture 257, a partial erasure of the image would occur in the absence of screen electrode 255. Screen potential V_S, however, is chosen so that V_s is greater than V_i in absolute value; and the presence of electrode 255 therefore prevents the passage of positive ions from aperture 257 to dielectric surface 258.
Screen electrode 255 provides unexpected control over image size, by varying the size of screen apertures 257. Using a configuration such as that shown in Figure 15 a larger screen potential has been found to produce a smaller dot diameter. This technique may be used for the formation of fine or bold images. It has also been found that proper choices of V_s and V_c will allow an increase in the distance between ion generator 240 and dielectric surface 258 while retaining a constant dot image diameter. This is done by increasing the absolute value of V_s while keeping constant the potential difference between V and V_c.
Image shape may be controlled by using a given screen electrode overlay. Screen apertures 257 may, for example, assume the shape of fully formed characters which are no larger than the corresponding crossover regions 245. This technique would advantageously utilize larger crossover regions 245. The lensing action provided by the apertured screen electrode generally results in improved image definition, at the cost of decreased ion current output.
Figure 16 illustrates yet another electrostatic imaging device 260 for use in a high speed serial printer. An insulating drum 261 is caused to rotate at a high rate of speed, illustratively around 1200 rpm. This drum is bonded a dielectric-coated conductor 262 in the form of a helix. The drum is disposed over an array of parallel control wires which are held rigid under spring tension. The dielectric-coated wire is maintained in gentle contact with or closely spaced from the control wire array. By rotating the drum, the helical wire provides a serial scanning mechanism. As the helix scans across the wires with a high frequency high voltage excitation applied to dielectric-coated wire 262, printing is effected by applying an extraction voltage pulse to one of the control electrode wires 263.
Figure 17 illustrates an alternative scheme for providing a relative motion between the print device of the invention and a dielectric receptor surface. A charging head 270 in accordance with Figure 14 is slidably mounted on guide bars 275. Any suitable means may be provided for reciprocating print head 270, such as a cable drive actuated by a stepping motor. This system may be employed to form an electrostatic image on dielectric paper, a dielectric transfer member, etc.
The electrostatic printing device of the invention is further illustrated with reference to the following specific embodiments.

Example VI-1

An imaging device of the type illustrated in Figure 10 was fabricated as follows. The insulating support 181 comprised a G-10 epoxy fiberglass circuit board. Control electrodes 184, 186, 188, etc. were formed by photoetching a 25.4 pm (1 mil) stainless steel foil which had been laminated to insulating substrate 181, providing a parallel array of 101.6 pm (4 mil) wide strips at a separation of 254 µm (10 mils). The driver electrode 193 consisted of a 127 µm (5 mil) tungsten wire coated with a 38.1 pm (1.5 mil) layer of fused glass to form a structure having a total diameter 203.2 um (8 mils).
AC excitation 192 was provided by a gated Hartley oscillator operating at a resonant frequency of 1 MHz. The applied voltage was in the range of 2000 volts peak-to-peak with a pulse width of 3 microseconds, and a repetition period of 500 microseconds. A 200 volts DC extraction potential 198 was applied between selected control electrodes and an electrode supporting a dielectric charge receptor sheet. The ion generating array was positioned 0.254 mm (0.01 inches) from the dielectric-coated sheet.
This apparatus was employed to form dot matrix characters in latent electrostatic form on dielectric sheet 2000. After conventional electrostatic toning and fusing, a permanent high quality image was obtained.

Example VI-2

An ion projection print device of the type illustrated in Figure 12 was fabricated as follows. A channel 219 of 127 µm (5 mils) depth and 254 µm (10 mils) width was milled in a 3.175 mm (0.125 inch) thick G-10 epoxy fiberglass circuit board. A driver electrode 213 identical to that of Example VI-1 was laid in the channel. Photoetched stainless steel foil electrodes 214, 216, 218, etc. were laminated to circuit board 211, contacting dielectric 215. The device exhibited equivalent performance to the imaging device of Example VI-1 when excited at the same potential.

Example VI-3

The electrostatic print device of Example VI-2 was modified to provide imaging apparatus of the type shown in Figure 13. The control electrodes comprised a series of 76.2 pm (3 mil) diameter tungsten wires cemented to support 221. This device achieved approximately double the ion current output as compared with the devices of Examples VI-1 and VI-2.
In all three examples, the glass coated wire was not firmly bonded in place, but was allowed to move freely along its axis. This provided a freedom of motion to allow for thermal expansion when operating at high driving potentials.

VII. Fabrication of dielectric members

This section describes a series of steps for fabricating and treating anodized aluminum members which results in members particularly suited to electrostatic imaging. The treated member is adapted to receive an electrostatic latent image, to carry the image with minimal charge decay to a toning station, and to impart the toned image to a further member preferably by pressure transfer. A number of properties of particular concern in this utilization are the hardness and abrasion resistance of the oxide surface; the potential acceptance and dielectric strength of the dielectric layer; the resistivity of the dielectric layer; and the release properties of the surface with respect to electrostatic toner.
This method is advantageously employed in fabricating the dielectric cylinders of the apparatus described above in sections II and III. This method provides a simple and reliable technique for fabricating aluminum oxide layers of a thickness as great as 101.6 pm (4 mils) and capable of supporting several thousand volts. Such cylinders are characterized by a hard, smooth surface which is suitably employed in the simultaneous pressure transfer and fusing of a toner image.
In order to provide a member of suitable configuration, an initial step entails the fabrication of an aluminum member of desired form. In the preferred embodiment, the member consists of a cylinder of aluminum or aluminum alloy, machined to a desired length and outside diameter. The surface is smoothed preparatory to the second step of hardcoat anodization.
In the second processing stage, the machined aluminum member is hardcoat anodized preferably according to the teachings of Wernick and Pinner; see The Surface Treatment and Finishing of Aluminum and its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Robert Draper Ltd., Paddington, England. The anodization is carried out to a desired surface thickness, typically 25.4-50.8 um (1-2 mils). This results in a relatively thick porous surface layer of aluminum oxide characterized by the presence of a barrier layer isolating the porous oxide from the conductive substrate. Following anodization, the member's surface is thoroughly rinsed in de-ionized water in order to remove all anodizing bath and other residual substances from the surface and the pores. The rinsed surface may be wiped dry to minimize surface moisture.
After anodizing the member, and prior to impregnating of the pores with a sealing material, the method of the invention requires a thorough dehydration of the porous surface layer. For best results, the dehydration is accomplished immediately after anodization. If there is a long delay between these two steps, however, it is advisable to maintain the member in a moisture- free environment in order to avoid a reaction with ambient moisture which leads to the formation of boehmite [AIO(OH)_2] at pore mouths, effectively partially sealing the porous oxide so that subse-_' quent impregnation is incomplete and dielectric properties degraded. This partial sealing can occur at room temperature in normal ambient humidity in a period of several days.
Removal of absorbed water from the oxide layer of an anodized aluminum structure may be realized by using either heat, vacuum, or storage of the article in a dessicator. The dehydration step requires thorough removal of water from the pores. Although all three techniques are effective, best results are realized by heating in a vacuum, for example in a vacuum oven. A preliminary step of dehydrating the member in a vacuum oven is especially preferred where the member has been stored in a moist environment for a period after anodization. Heating of the member in air, as compared with vacuum heating, results in only a slightly lower level of charge acceptance. It is preferable that any thermal treatment of the oxide prior to impregnation be carried out at a temperature in the range from about 80°C to about 300°C, with the preferred temperature being about 150°C. Where precautions have been taken after anodizing to minimize the retention and accumulation of moisture, the dehydration step may be accomplished in conjunction with the impregnation step, as explained below.
After removal of absorbed water from the oxide coating it is sealed with an impregnant material. In the present invention, the impregnant material consists essentially of a compound of a Group 11 or III metal with a long chain fatty acid. It has been discovered that a particularly advantageous class of materials includes the compounds of Group II metals with fatty acids containing between 8 and 32 carbon atoms saturated or unsaturated. The impregnant materials may comprise either a single compound or a mixture of compounds. Due to the water repellant nature of these alakaline earth derivatives, the product of the invention has superior dielectric properties at high humidities.
In order to avoid introduction of moisture into the dehydrated porous surface layer, the member should be maintained in a substantially moisture- free state during impregnation. This will occur as a natural consequence of the preferred method of applying the impregnant materials of the invention. At room temperature these materials take the form of powders, crystalline solids, or other solid forms. In the preferred embodiment of the invention, the member is maintained at an elevated temperature (above the melting point of the impregnant material) during the impregnation step in order to melt the material or to avoid solidifying premelted material. These materials have sufficiently low viscosity after melting to readily impregnate the pores of the oxide surface layer. In this embodiment the period of heating the member from room temperature to the impregnating temperature may provide the preliminary dehydration which is required to avoid trapped moisture in the pores, often without a prior separate dehydrating step. This preheating stage may take minutes or hours depending on the mass and volume of the aluminum member. See Examples 1, 2. In the alternative embodiment of the invention discussed below, in which the impregnant materials are applied in solution to the anodized member, it is advisable to heat the member or take other steps in order to avoid reintroduction of moisture during the impregnation process.
It has generally been found unnecessary to maintain the heated member in a vacuum environment during impregnation, either to avoid absorption of moisture or to assist the impregnation of the pores through capillarity. In the preferred embodiment, the impregnant material may be applied to the oxide surface under moist ambient conditions because the heating of the aluminum member will tend to drive off any absorbed moisture from the oxide surface. Optionally, a vacuum may be employed in order to provide an extra precaution against reintroduction of moisture. Special measures may be required, however, in the alternative embodiment in which the impregnant material is dissolved prior to application to the anodized member.
In the preferred embodiment of the invention, the impregnant material is applied to the surface of the aluminum member after heating the member to a temperature above the melting point of the material. In one version of this embodiment, the material is applied to the surface in solid form (as by dusting or blowing it onto the surface), whereupon the material will melt. In an alternative version, the material is premelted and applied to the oxide surface in liquid form (as by brushing the material onto the member or immersing the member in melted material). In either case, the material should then be allowed to spread over the oxide surface layer.-This may be done by permitting a flow of the melted material, or by manually spreading the material over the surface using a clean implement. The member should be maintained at this elevated temperature for a period of time sufficient to allow the melted material to completely impregnate the pores of the oxide surface layer. This period will be shorter when using a vacuum to assist impregnation.
In the preferred embodiment, if the member is allowed to cool prior to complete filling of the pores with the impregnant material, the material will tend to solidify leaving undesirable air pockets in the pores. It is a particularly advantageous aspect of this method that this problem may be remedied simply by reheating the aluminum member and allowing a more complete filling of the pores. The member may be reheated for a subsequent impregnation step at any time subsequent to the initial impregnation, as the impregnant material of the invention is not permanently cured.
In an alternative embodiment of the invention, the impregnant material is dissolved prior to application of the oxide surface layer. Materials of the invention susceptible to application in this manner include the compounds of Group III metals with fatty acids, as well as the compounds of Group II metals with some of the larger chain fatty acids (those having around 32 carbon atoms). Solvents which are suitable for this purpose include, for example, benzene, and butyl acetate. After the material is dissolved, it may be applied to the member by spraying or brushing it onto the oxide surface layer. The solution is allowed to penetrate the pores. Any excess impregnant is removed by wiping the members surface. In order to avoid reintroduction of moisture into the dehydrated porous surface layer, the member may be impregnated in a vacuum oven or in air at a temperature in the range from about 40°C to 55°C. Alternatively, the member may be impregnated in a dessicant dry box. Advantageously, this method would reflect that employed in the prior dehydration step.
It is desirable subsequent to precipitation of the impregnant material in the alternative embodiment to heat the member to a temperature above the melting point of the material. This fuses the material in the pores, and minimizes the occurrence of air pockets which are deleterious to dielectric properties. The member may be reheated as in the preferred embodiment in order to provide a more complete impregnation.
Subsequent to impregnation of the pores, the aluminium is allowed to cool. The member is then treated (as by wiping or scraping) to remove any excess material from the surface.
The advantages of this method will be further apparent from the following non-limiting examples.

Example VII-1

A series of panels 38.1 mmx38.1 mmxl.7018 mm (1.5 insxl.5 insx.067 ins) fabricated of aluminium alloy 7075-T6 were hard-coat anodized in sulphuric acid by the Sanford "Plus" process^* to a depth of 38.1 pm (1.5 mil). The panels were rinsed with deionized water and wiped free of surface moisture. They were then wrapped in moisture absorbent paper and stored for about one day.
The anodized panels were unwrapped and heated to a temperature above the melting point of the material to be applied (see Table Vil) and

^* Sanford Process Corp.; 65 North Avenue, Natick, MA.

The coated member was maintained at the elevated temperature for another minute, and then allowed to cool to room temperature. This process was repeated with a number of different impregnant materials including in one case a mixture of two different compounds-see Table VII.
After cooling, the samples were ground with 240 grit sandpaper and water to a thickness of between 40 and 45 microns. They were then heated on a hot plate at 150°C for approximately 30 seconds in order to rapidly evaporate the surface moisture, and then allowed to cool.
The plates were placed over a negative ion discharge and charged to a maximum voltage. This voltage was measured by a Monroe Electronics electrostatic voltmeter.

Example VII-2

A hollow aluminum cylinder of extruded 7075-T651 alloy was machined to an outer diameter of 101.6 mm (4 inches) and 228.6 mm (9 inch) length, with 19.05 mm (0.75 inch) wall thickness. The cylinder was machined to a 7.62x10-⁷ m (30 microinch) finish, then polished to a 5.715x10^-8 m (2.25 microinch) finish. The cylinder was hardcoat anodized by the Sandford "Plus" process to a thickness between 42 and 52 microns, then rinsed in deionized water and packed in plastic bags.
On the following day, the cylinder was unpacked and placed in a vacuum oven at 101.5917 kPa (30 inches mercury). After half-an- hour, the oven temperature was set at 150°C., which temperature was achieved in a further forty minutes. The cylinder was maintained at this temperature and pressure for four hours prior to impregnation.
A beaker of zinc stearate was preheated to melt the compound. The heated cylinder was removed from the oven, and coated with the melted zinc stearate using a paint brush. The cylinder was then placed back in the vacuum oven for a few minutes at 150°C., 101.5917 kPa (30 inches mercury). The cylinder was removed from the oven and allowed to cool.
After cooling, the member was polished with successively finer SiC abrasive papers and oil. Finally, the member was lapped to a 11.43x10-B m (4.5 microinch) finish by application of a lapping compound and oil with a cloth lap.
Using the testing method of Example VII-1, the cylinder's charge acceptance was measured at 980 volts.

Claims

1. Electrostatic printing apparatus comprising an imaging roller (25) having a conductive core (29) and a dielectric surface layer (27); means (19) for generating a latent electrostatic image on the dielectric surface layer; means for toning (31) the latent electrostatic image; and a transfer roller (37) which contacts the imaging member under pressure, with an image receptor (81) fed therebetween; the means for generating a latent electrostatic image comprising elongate control electrodes and at least one elongate transversely orientated driver electrode separated by a dielectric member, with a varying potential applied between the electrodes to create a glow discharge, and means for extracting ions from the glow discharge; characterised in that the drive electrode comprises a wire (197, 217, 227, 243), the dielectric member comprises a sheath (5, 195, 215, 225) surrounding the wire, and an insulating support member (181, 211, 221, 241, 261) is provided which supports the sheath.

2. Electrostatic printing apparatus as claimed in Claim 1 in which the insulating support member (211, 221) includes a slot (212b, 222b), the driver electrode (217, 227) and sheath (215, 225) being received within the slot, and the control electrodes being transversely mounted on the insulating support member (221, 211) across the slot.

3. Electrostatic printing apparatus as claimed in Claim 1 in which the control electrodes comprise conductive strips (184, 186, 188) mounted on the insulating support member (181) and the driver electrode (197) and the sheath (5) are transversely mounted over the conductive strip.

4. Electrostatic printing apparatus as claimed in any one of Claims 1 to 3 further including an apertured screen electrode (255), a spacer (252) separating the screen electrode from the control electrodes (244), and means for providing a DC voltage between the screen electrode (255) and the conductive core (29; 259).

5. Electrostatic printing apparatus as claimed in Claim 1 in which the insulating support member comprises a rotatable drum (261), the driver electrode (262) being disposed in a helical fashion around the drum (261); and including means for rotating the drum.

6. Electrostatic printing apparatus as claimed in Claim 5 in which the control electrodes comprise a plurality of wires (263) disposed parallel to the axis of rotation of the drum (261).

7. Electrostatic printing apparatus as claimed in any one of Claims 1 to 4 in which the insulating support member (181, 211, 221, 241) also supports the control electrodes.

8. Electrostatic printing apparatus as claimed in any one of the preceding claims in which the means for generating a latent electrostatic image are spaced from the imaging roller by more than 0.025 mm (1 mil).

9. Electrostatic printing apparatus as claimed in any one of Claims 1 to 8 in which the transfer roller (37) and imaging roller (25) are maintained in a non-parallel axial orientation with the image receptor fed between the rollers to receive the toner image from the imaging roller.

10. Electrostatic printing apparatus as claimed in any one of the preceding claims in which the imaging roller (25) has a porous anodized aluminium dielectric surface layer impregnated with a metallic salt of a fatty acid.

11. Electrostatic printing apparatus as claimed in any one of the preceding claims in which the imaging roller has a smoothness in excess of 5.08x10-' metres rms (20 microinch rms).

12. Electrostatic printing apparatus as claimed in any one of the preceding claims in which the imaging roller has a surface with a resistivity in excess of 10¹⁰ ohm-metres.

13. Electrostatic printing apparatus as claimed in any one of the preceding claims in which the dielectric surface layer (27) of the imaging roller (25) comprises a porous aluminium oxide layer impregnated with zinc stearate or a compound of Group II metals with fatty acids containing between 8 and 32 carbon atoms, saturated or unsaturated.