Electrostatic printers ana 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 dc so by uniformly charging a photoconductor electrostatically in the dark, and optically exposing the charged photoconductor 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 latent image, the same apparatus could be used to carry out the common steps of toning the image, 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 standardise the apparatus to perform these functions .

One commonly employed principle for generating ions is the corona discharge from a small diameter wire or a point source.

Illustrative U.S. Patent Mos . are ?. Lee 3,353,239; lee ?.

Frank 3,511,41 ; A.E. Jvirblis 3,623,123; ?.J. McC-iil

3,715,762; H. Bresnik 3,765,027; and R.A. Fotiand

3,961,564. Corona discharges are used almost exclusively in electrostatic copier-s 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 maxiumum discharge current density heretofore obtained has been on the order of 10 microamperes -per square centimeter. This can impose a seve printing speed limitation. In addition, coronas can create significant maintenance problems. Corona wires are small and fragile and easily broken. Because of -heir 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-, .S .* Patent No. 4,057,723; wheeler az al. 4,0 ^β a,234.; and Sarid 4,llC,.6l4.. These patents disclose various corona charging devices characterised by a conductive wire coated with a relatively thick dielectric material, in contact with or closely soaced from a further conductive member. A supply of positive and negative ions is generated in the air space surrounding t e 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.

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. Patent 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 a 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, inef iciencies 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 thrσug-h 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.

Hardcαat 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

MPI

uncσated aluminum. However, the coatings contain pores in the form of fine tubes with a porosity on the order of 10-° to iO ¹² pores Ώ~ Γ 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 aluminum oxide, typically 300 to 300 Angstroms.

U.S. Patent No. 3,664,300 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 of 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 oiiin-g water, usually containing- certain nickel salts, which form an expanded- c-ehmite" structure- t the mouths of the pores-. Oxide sealing in this manner will not support an electrostatic charge due to the ionic conductivity of moisture trapped in the pores.

It is often desirable in electrostatic printing and copying to create an image on both sides of a sheet of paper or other receptor. In electrophotography, the most accurate reproduction of a two-sided original document

would require this faculty. In electrographic printing, duplex imaging affords significant savings in paper costs and permits a greater flexibility in printing formats.

A criterion which should be considered in modifying an existing single-sided printing or copying system to permit dupleximaging is the extent to which the system must be modified or supplemented. It is advantageous to employ a system which is structurally compatible with two-sided image production requiring only minor changes.

Another factor of some importance is the speed and efficiency with which the system transfers the two images. In particular, it is desirable that such a system allow the simultaneous fusing of the two images onto a receptor medium.

The invention provides compatibility of design for electrostatic printing and photocopying apparatus. It also provides high speed printing and photocopying with excellent image quality.

The invention further provides a plain paper photocopying- system which is simple, ^" compact, and low in cost. The photocopying' system requires fewer processing steps than those of conventional copying systems, with an extremely short and simple paper path.

The invention is further able to reduce critical mechanical tolerances in providing a latent electrostatic image in an electrostatic printer. It thus 'reduces the maintenance problems associated with the formation of such an image, and it can facilitate the generation of ions,

- ~\} ~_Z _

ClΛ~l

particularly at high current densities, for use in electrostatic printing and photcopying, as well as other applications.

A particular fabrication technique is given for ion generators characterized by a laminate of mica and foil electrodes. This design is durable, resisting dela ination due to moisture and erosion due to ozone, nitric-acid and other environmental substances. Such a laminate is physically stable over a wide range of temperatures, and can carry high peak voltage RF signals over a long service life.

The invention also provides an alternative ion generator design 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 aintainance normally characteris ic of such corona devices, and avoids the objectionable operational characteristics of corona wires.

The invention provides electrostatic imaging apparatus for pressure transfer o -a toner image from a dielectric surface to plain paper and the like. Such apparatus effects simultaneous fusing of the toner image, and is characterized by a high efficiency of toner transfer.

A preferred embodiment of the invention incorporates an impregnated aluminum layer for the dielectric member. This dielectric surface possesses smoothness and hardness properties which facilitate toner transfer, while

possessing sufficient resistivity to obtain a latent electrostatic image until toning. The dielectric surface created by this preferred method maintains the above properties at elevated humidities.

The apparatus of the invention may be employed in duplex imaging onto plain paper and the like. This duplex imaging enjoys the advantage of the avoidance of offset images and other problems often associated with duplex imaging. It also achieves a simultaneous transfer and fusing of two images onto a receptor medium.

SU?1?TARY OF THE INVENTION

The invention encompasses both electrophotography and electrostatic printing, as well as preferred components to be employed in these processes. The invention also encompasses two alternative ion generator designs, the first of which may be used to precharge a photoconductor or to form a latent electrostatic image, as well as other applications . The second ion generator is speci ically adapted to the formation of an electrostatic image.

A first aspect of the invention relates to the structure of the ion generators, which are characterized ^' oj the use of a glow discharge to generate a pool of positive and negative ions, which may be extracted for application to a further member. In the first ion generator, a varying potential is applied between two electrodes separated by a solid dielectric member to cause an electrical air gap breakdown adjacent the junction of the edge surface of at least one of the electrodes and the solid dielectric member. In the second ion generator, a varying potential is applied betweeϋ an elongate conductor having a dielectric sheath and a transverse conductive member in order to generate ions at a crossover point of these structures. 3oth ion generator embodiments may be characterized as including a control electrode and a driver electrode; an extraction potential applied to the former electrode is used to extract ions from the glow discharge created by the varying potential.

Another aspect of the invention is seen in the shared processing stages in the electrostatic copier and printer apparatus of the invention. After an electrostatic latent image has been formed on a dielectric cylinder, the image is toned and pressure transferred to plain paper or any suitable image receptor. Preferably, this transfer is achieved by inserting the image receptor between the dielectric cylinder and a transfer roller under high pressure. Advantageously, this pressure transfer is effected with simultaneous fusing of the toner image. Provision may be made for cleaning the surface of the dielectric cylinder and transfer roll, and for discharging any residual electrostatic image on the dielectric surface.

In a preferred embodiment of the invention, the pressure transfer of the toner image effected by dielectric and transfer rollers may be enhanced by providing a skew between the dielectric and transf r 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 20 icroinch r s, 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

BAD ORIGINAL

^~ in preference to that of the dielectric roller. The apparatus provides effective toner transfer and fusing without wrinkling of the receptor medium.

In the preferred version of the first ion generator, this device comprises a plurality of foil electrodes bonded to opposite faces of a mica dielectric sheet. The invention provides a preferred method for fabricating laminations of mica and conductive materials, which technique may be advantageously employed to produce such an ion generator. Such laminations include a sheet of mica, one or more metallic sheets-, and bonding layers of pressure sensitive adhesive. The conductive laye.r or layers may be selectively removed as by etching to create a desired electrode pattern.

Another aspect of the invention relates to a preferred method of fabricating a dielectric member having a smooth, hard surface with a resistivity in excess of 10 ^* ^ ohm- centimeters; such a technique may be employed to advantage in producing a suitable dielectric cylinder. This method provides- for the preliminary dehydration of an anodic aluminum member followed by impregnation of surface pores of the dehydrated member with a metallic salt of a fatty acid. After completion of the impregnating stage, any excess imp-regnant is removed from the member's surface. In the preferred version of this technique, the surface is then polished to a better than 20 icroinch finish. The impregnant material consists essentially of a Group II

TE SHEET O PI

metal with a fatty acid containing between 3 and 32 carbon atoms, saturated or unsaturated.

3RIEF DESCRIPTION 0? THE DRAWINGS

The above and additional aspects of the invention are illustrated with reference to the detailed description which follows, taken i 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 rollers of Figure 1;

FIGURE 3 is a sectional schematic view of electrophotograhic 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 o 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 sectional view of ion generating apparatus in accordance with the preferred embodiment;

FIGURE 11 is a sectional view of the ion generating apparatus of Figure 10, further showing ion extraction apparatus and an ion receptor member;

FIGURE 12 is a plan view of dot matrix printing apparatus of the type illustrated in Figure 11;

FIGURE 13 Is a schematic sectional view of a mica foil laminate in accordance with the invention;

FIGURE 14 is a partial perspective view of an electrostatic imaging device in accordance with an alternative embodiment of the invention;

FIGURE 15 is a schematic sectional view of the apparatus of Figure 14, further including ion extraction apparatus and an ion receptor member;

FIGURE Iβ is a cutaway perspective view of an alternative version of the imaging apparatus of Figure 14;

FIGURE 17 is a cutaway perspective view of a further alternative version of the electrostatic imaging apparatus of Figure 14;

FIGURE 18 is a plan view of matrix imaging apparatus of the type shown in Figure 14;

FIGURE 19 is a sectional schematic view of a three electrode embodiment of the imaging device of Figure Iβ;

15

FIGURE 20 is a perspective view of an electrostatic imaging device in accordance with yet another embodiment of the invention;

FIGURE 21 is a plan view of a serial printer incorporating" an electrostatic imaging device of the type illustrated in Figure 15;

FIGURES 22-27 are sequential schematic views of electrostatic imaging apparatus of the type illustrated in Figure 4, adapted to duplex imaging in accordance with the invention;

FIGURES 28-32 are partial perspective views of electrostatic imagin ^• apparatus of the type illustrated in Figure 4, showing an electrostatic latent image and a resulting toner image for various stages of the duplex transfer process in accordance with the invention;

DETAILED DESCRIPTION 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 . The ion generator and extractor of Section V may be used 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 aluminum 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 . The apparatus of either main embodiment may be modified to provide duplex imaging capability, as disclosed in Section VIII.

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

OMP

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 _χ Se , where x+y ^e l), or organic photoconductors such as the equimolar complex of poiyvinyl carbazole and trinit o luoreπone.

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 well known optical scanning schemes. This latent image i3 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 a-nd 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 π ) 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 ab-σut 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 e f ctive TEST process may be achieved under these conditions, but this will result in toner transfer to the upper cylinder and there ore 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 Clar , Xerography and Related Processes, the Focal Press, London and New lork, 1 65, 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 500 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 precipitous 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 C.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. Thε 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 cf the photoconductor surface. This, in turn, results in a greater contrast between the light and dark portions of the toned, visible image.

' Ur ^' Λ

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' om 7075-Tβ 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 ^' ay 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. Z . Page 5β7) . The finished aluminum oxide layer was ό ^" 0 micrometres ^{ ) in thickness. The cylinder 25 was then placed in a vacuum oven at 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

O at 150°C, 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 0.25Mm 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 βOβl 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 . 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/1, pH of β) for 3 minutes. Tc 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, pK of 5.6) for ten minu es. The semiconducting substrate 15 s 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 BAD ORIGINAL ^ ^τ ££

_/ , and the cadmium pigment concentration in the resin binder was 182 by volume. The resin was crosslinked by firing at l8θ°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 pejr 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. Ξisbein & Co. (Stuttgart) No. 444 copier.

The toner removal means 43 and comprised flexible stainless steel scraper blades and were employed to maintain cleanliness of both the oxide cylinder 25 and the poiysulphone 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-dieleσtric 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 II-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. LT3 base paper ⁾ to a dry thickness of 30A- m. 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 layer of high density aluminum 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 m r s 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. 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, and 13 and particularly the preferred matrix printing apparatus of Figure 13, which may be employed for multiplex printing. Alternatively, 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 apparatus of section V, the apertured

conductor comprises the control electrode; 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.

3. 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 I 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 ¹² ohm centimeters. 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 i rms, in order to provide for complete transfer of toner to the receptor sheet 31. The smoothness of dielectric surface 75 contributes to the efficiency of toner transfer - to the receptor sheet 8l by enhancing the release properties of this surface. The dielectric layer 75

additionally has a high modulus of elasticity, typically on the order of 10 ⁷ 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 pol ami es, 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 VTI, 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 33 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 37 which may have an outer covering- of engineering plastics 85. The surface material 85 or roller 83 typically has a modulus of elasticity on the order of 200,000-450,000 PSI. The image receptor 81 will tend to adhere to the surface 35 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 35 is to bond image receptor 81 when the latter is subjected to a slip between the roller surfaces. Another function of the plastics- coating S5 is to absorb any high stresses introduced into the nip in the case of a paper jam or wrinkle. 3y absorbing stress in the plastics layer 85, the dielectric coated roller 73 will not be damaged during accidental paper wrinkles or ams. Coating 85 is typically a nylon or polyester sleeve having a wall thickness in the range of 3 to 12.5 mm.

OMPI

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 blades 89 and 91 may be provided in order to remove any residual paper dust, toner accidentally impacted on the roil, 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 ^' imajge 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.

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, 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 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 III-l In a specific operative example of an electrographic printer in accordance with the invention, the cylindrical conducting core 5 of the dielectric cylinder 1 was machined from 7075-T ^β aluminum to a 3 inch diameter. The length o^ the cylindrical core, excluding machined journals, was 9 inches. The journals were masked and the aluminum anodized

- H

by use of the the Sa.nf ord 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 5β7). The finished aluminum oxide layer was 60 microns in thickness . The conducting core 5 was then heated in a vacuum oven, 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, 30 inches mercury, thereby forming dielectric surface layer 3. 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 4.5 microlnch finish.

.The pressure roller 11 consisted of a solid machined two inch diameter aluminum core 12 over which was press fit a two inch inner diameter, 2.5 inch outer diameter polysulfone sleeve 13. The dielectric roller 1 was gear driven from an AC motor to provide a surface speed of 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 300 pounds per linear inch of contact. The side frames were

BAD ORIGINAL

machined to provide a skew of 1 .1° be tween rollers 1 and

11 .

A charging device of the type described in U.S. Patent No. 4,160,257 was manufactured as follows. A 1 mil stainless steel foil was laminated on both sides of a 1 mil sheet of-Muscovite mica. The bonding material and technique is detailed in Example V-I, infr . The stainless foil was coated with resist and ph.otoetched with a pattern similar to that shown in Figure 22, with holes or apertures in the fingers approximately .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 a 7.68 inch length of the dielectric cylinder. Corresponding to each crossover location was a .006 inch diameter etched hole in the screen electrode. 3ias potentials of the various electrodes were as follows (with the cylinder's conducting- core maintained at ground potential) :

screen, potential -6C0 volts

control electrode potential -4QO volts (during the application of-a -400 volts print pulse, this voltage becomes -700 volts)

driver electrode bias ÷300 volts with respect to Screen Potential

The DC extraction voltage was supplied by a pulse generator, with a print pulse duration of 10 microseconds. Charging occured 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 3. The print head was maintained at a spacing of 8 mils from dielectric cylinder 3.

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. Ξisbein and Company.. (Stuttegart) 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 en 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 semiconduc ing 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 3hown 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 33.. Searing retainers 57 and 67, which advantageously are of the type known as. ^* 'self-aligning*, ^' it 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,

" ^* "" ^~ - ^~ -

i f while roller 83 is frictionaily driven due to the contact of the rollers at the nip.

The mounting illustrated in Figure 9 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.

-r

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 _y the imaging apparatus 71 may be replaced with any suitable apparatus known in the _. art for depositing a uniform charge en 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 of paper feed. An illustrative value of skew to effect the objects of the invention is 0.10 inch, measured at the center of roller bearings 57 and 67, which are separated by a distance of 10.375 inch for 9 inch long rollers. This represents an angle of roughly 1.1°.

Figure 7 schematically illustrates skewed rollers 73 (with axis 3-3) 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 3'-3 ^? of roller 73. This corresponds to an angle θ between axes 3-3 and C-C. Axis 3-3 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 αf paper feed be ore 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 3"-3" of roller 3) and Ξ ⁽ perpendicular to the projected axis C-C of roller 21). 3ecause 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 roil, and therefore to travel in direction Ξ. -This results in a surface soeed

„ _ -± .

OMPI

di _f ferential or "slip" between the surfaces 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, ?ϊ, can be calculated using a formula found in Formulas For Stress and Strain (4th edition) by Ronald J. Roark, published by McGraw-Hill 3ook 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 inches of the contact area of the two cylinders is given by:

wnere

P represents the cylinder loading in pounds per linear inch;

Dτ_ and Dg represent the diameters of the cylinders in inches;

. ^ and g. represent Pois.son's ratio in compression for the materials of the cylinders; and

. E^ and Eg represent the modulus of elasticity in compression for the materials of the cylinders, 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 proportionality 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 a ter 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 8 and 91, erase head 93, 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 8l. These advantages accrued even in the case of nonfibrous substrates 31, such as Mylar film.

Example IV-1

Apparatus of the type illustrated in Figures 4 and 9 incorporated a 9 inch long, 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 10 microinch r s .

The pressure cylinder 83 included a 9 inch long steel mandrel with an outer diameter of 3*125 inches over which was pressed a 0.375 inch thick sleeve of polyvinylchloride . Th rollers were pressed together at 350 pounds of pressure e per linear inch of nip.

A latent electrostatic image was formed on the dielectric surface of roller 73 by means of an ion generator of the type disclosed in section V. 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 the Phillip A. Hunt Chemical Corporation. The single component latent image toning apparatus was essentially identical to that employed in the Develop KG Dr.-Eisbein S- 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 0.10 inch offset at the bearing retainers of the 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.

TA3LE IV-A

PERCENTAGE OF ΞND-TO-ΞND SKEW TONER NOT TRANSFERRED none 12.60

_* 55 ^c . .10

1.1° .10

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 Imagetroil paper, manufactured by S.D. Warren. The- rollers were pressed together without skew at 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 200 pounds per linear inch, and ail other parameters unchanged, resulting in a transfer efficiency of 99.95 percent.

BAD ORIGINAL

Example IV-3 The apparatus of Example IV-1 was employed with the following modifications. The pressure cylinder 83 comprised a 9 inch long steel mandrel with a 1.945 inch outer diameter, over which was pressed a 9 inch long Celcon sleeve with a 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 200 pounds of nip pressure per linear inch of nip.

The toner employed was Coates RP0357 of the Coates 3ros. 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.

TABLE IV-3

TONER TRANSFER EFFICIENCY, EXAMPLE IV-3 END-TO-END SKEW RESIDUAL TONER (GRAMS)

0° 6.034

.42° 0.114

.55° 0.066

.85° 0.050

.97° 0.036

1 . 1*3 ^' 0.031

Example IV-4 The apparatus of Example IV-4 was employed with the modification that the imaging roller 73 comprised a photoconductive roller. An aluminum sleeve was fabricated of 6θ6l aluminum tubing wit ^' a 1/3" wall and 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 ZG Dr. Ξisbein & Co. (Stuttgart) Mo. 44ϋ Copier. The toner transfer efficiency underwent improvements comparable to those of Example 17-1 for increasing skew angles of 0.0°, 0.55°, and 1.1°.

V. Ion Generation and Extraction

Figure 10 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 time-varying potential, illustratively a periodically alternating potential. When electric fringing fields S _A and E _B in the air gap 104-a and 104-b 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 19 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 10 is illustrated by the generator-extractor 110 of Figure 11. The generator 110A includes a dielectric 11 between conducting electrodes 112-1 and 112-2. In order to prevent air gap- trreakdσwn near electrode- 112-1, the electrode 112-1 is encapsulated or surrounded by an insulating material 113. Alternating potential is applied between the conducting electrodes 112-1 and 112-2 by a source 11 ^. The second electrode 112-2 has a hole 112-h where the desired air gap breakdown occurs relative to a region 111-r of the dielectric 111 to provide a source of ions.

BAD ORIGINAL

The ions formed in the gap 112-h may be extracted by a direct current potential applied from a source 114-3 to provide an external electric field between she electrode 112-2 and a grounded auxiliary electrode 112-3. An illustrative insulating surface to be charged by the ion source in Figure 20 is an electrographic paper 115 consisting of a conducting base 115-ρ- coated with a thin dielectric layer 115-d.

_

When a switch llβ is switched to position X and is grounded as shown, the electrode 112-2 is also at ground potential and no external field is present in the region between the ion generator 1IQ _A and the dielectric paper 115. However, when the switch liβ is switched to position ... the potential of the source li g is applied to the electrode 112-2. This provides an electric field between the ion reservoir 1II-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 II5-d.

A number of materials may be used for the dielectric layer ill.- Possible, choices- include aluminum oxide, glass enamels, ceramics, plastics films, and mica. Aluminum oxide, glass enamels and ceramics present difficulties in fabricating a sufficiently thin layer (i.e. around 0.025 rπm) to avoid undue demands on the driving potential source 114A. Plastics ilms, including pαlyimides such as that known by the Trade Mark Kapton, and Mylon, tend to degrade as a result of exposure to chemical bjrproducts of the air gap breakdown process in aperture 112-h (notably osone and

and nitric acid). Mica avoids these drawbacks, and is therefore the preferred material for dielectric 111. Especially preferred is Muscovite mica, HgKAl^ (Si0 ₄ ) ₃ .

The generator and ion extractor 110 of Figure 11 is readily employed, for example,in the formation of - characters on dielectric paper in high speed electrographic printing. Devices embodying this principle may be used for charging and discharging a photoconductor as in the apparatus of section II; suitable embodiments are disclosed in U.S. Patent No. 4,155,093. To employ ion extraction in the formation of dot matrix characters on dielectric paper, the matrix ion generator 130 of Figure 12 may be employed. The generator 130 makes use of a dielectric sheet 131 with a set of apertured ^' -alr gap breakdown electrodes 132-1 to 132-4 on one side and a set of selector bars 133-1 to 133-^ on the other side, with a separate selector 133 being provided for each different aperture 135 in each different finger electrode 132.

When ^' n alternating potential- is applied, between any ^" selector bar 133 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 counterelectrode of

43 the dielectric surface to be charged. Matrix location 135 ₂ - ₃ , 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. Unseiected fingers as well as the dielectric member's counterelectrode are maintained at ground potential.

3y multiplexing a dot matrix 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, IβOO separate drivers would be required if multiplexing were not employed. 3 utilizing the array of Figure 12 with, for example, alternating frequency driven fingers, only 80 finger electrodes would be required and the total number of drivers is "reduced from IβOO to 100.

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

In constructing and operating a aatrix 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 ion current output, in that a lower ion current will be produced at an aperture 135 at which the dielectric 131 is thicker. It is a particuarly 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 12. In this regard, the uniformity of thickness of layer 131 is much more important than the actual value of that thickness.

Ion generators of the type illustrated in Figures 11 and 12 may be fabricated using a layer of mica laminated to thin sheets of metallic foil, by etching the foil to create an array of electrodes on each side of the mica. Electrodes 102-1 and 102-2 (Fig. 11) are formed by laminating a thin sheet of conductive foil to each face of the mica sheet 101. With reference to the sectional view of Figure 25, a mica sheet 171 of uniform thickness is bonded to two layers of foil 174 and 175. The bonding is achieved using thin layers of pressure sensitive adhes-ive 172 and 173.

The preferred dielectric material is Muscovite mica, K ₂ Al-2(SiO] _j )^ . It is desirable to have a sheet of uniform thickness in the range from about - 7 _/ , most preferably lθX - - The thinner mica sheets are generally harder to handle and more expensive, while the thicker mica requires higher RF voltages between electrodes

^•■ > ^■ '- i -

102-1 and 102-2 (see Figure II). The mica should be free of cracks, fractures, and similar defects.

The foil layers 174 and 175 advantageously comprise a metal which may be easily etched in a pattern of electrodes 132, 133. Illustrative materials include nickel, copper, tantalum, and titanium; the preferred material, however, is stainless steel. A foil having a thickness from about βjn - ^(is desirable, with the preferred thickness being around

A wide variety of pressure sensitive adhesives are suitable for layers 172 and 173. A number of characteristics should be considered in choosing an appropriate pressure sensitive adhesive.-. The adhesive should be thermoplastic, and be resistant to moisture and chemicals. It should be able to withstand the high- temperatures resulting from high voltage alternating potentials, on the order of kilovoits. The adhesive should be suitable for bonding of metal to mica. Illustrative adhesive formulations which satisfy the above criteria include solutions of organopolysiloxane resins , as well as pressure sensitive adhesives.

^■ The mica is coated with a pressure sensitive adhesive formulation using any well known technique which permits precise control over the coating thickness . The adhesive layers desirably have a thickness in the range" 0 . ~_M - 5 Λ , most preferably in the range G.β/-(- - 2.5 _/ { . The

c

thickness may be determined after lamination by subtracting the known thickness of the mica and foil sheets from the total thickness of the laminate. The adhesive may be applied manually, as by brush coating, spraying, and dipping. The preferred method of coating is that of dipping the mica into a bath of pressure sensitive adhesive, followed by withdrawal of the mica at a calibrated speed. Generally, a faster speed of withdrawal results in a thicker pressure sensitive adhesive coating on each side of the mica sheet 171.

In the preferred embodiment of the invention, the pressure sensitive adhesive is applied to the mica in solution. The resin may be diluted to a desired viscosity using a variety of solvents, well known to those skilled in the art. In general, higher viscosity formulations will result in a thicker layer of pressure sensitive adhesive for a given method of application. Advantageously, the pressure sensitive adhesive formulation has a viscosity in the range from about 10 cps. - 100 cps . The mixture advantageously is filtered prior to coating- onto the mica sheet 171.

The coating of mica sheet 171 preferably involves dipping the sheet into the pressure sensitive adhesive bath to completely cover both sides; it is not necessary, however, to coat the edges of the mica sheet in the preferred embodiment, which calls for a separate protective medium for the edges of the- lamination. In lieu of or in - addition to a protective coating around the edges of the

^" '

πiica sheet 171, a protective layer of tape may be applied to the edges of the mica-foil lamination. The tape provides protection against migration of moisture between layers of the mica. Alternatively, the tape may be removed after processing of the mica, during which it provides a protective layer, as further discussed herein. Preferably, ^• the tape is coated on one face with pressure sensitive adhesive which may be the same type as used to bond the mica-foil layers.

In the case of certain pressure sensitive adhesives, the adhesive coating is cured in order to cross-link the formulation and thereby enhance its adhesive character _* This may be done using any suitable technique for the given adhesive formulation, such as heat or radiation curing.

The foil sheets 174 and 175 are cut to desired dimensions, and cleaned prior to application to the mica sheet 171. Each sheet is placed in registration with one ace, of the mica sheet, and then bonded to the mica by applying, pressure evenly over the foil layers.

^• After lamination of the ^' roil layers 174 and 175 to mica sheet 17 l _x the foil is selectively removed to create a desired pattern, as for example the pattern of electrodes 132 and 133 shown in Figure 12. In the preferred embodiment, the desired pattern is created by a photoetching process. This involves coating the foil with a photoresistant material; covering the coated foil with a photomask to create the desired patterns; exposing the masked laminate to ultraviolet radiation; and etching the

*-

T - :

irradiated foil in order to remove those portions which have been rendered soluble during the preceding steps . The preferred versions of this process uses a ^p ositive photoresist, which is characterized in that those areas which are exposed to ultraviolet radiation will be rendered soluble and later dissolved.

In the case of solvent based photoresist, there is a tendency of the solvent to leach out the pressure sensitive adhesive around the edges of the lamination. In addition, the photoresist will not coat well due to edge effects, creating a danger of etch-through. For these reasons, it is advisable to tape the edges o provide a protective layer during these processing steps; the tape may be removed after etching. Alternatively, one may employ a dry film photoresist, which will adequately protect the edges of the lamination if applied in a thickness of around 35/4*

In accordance, with a particular embodiment, a heat sink may be appended to the mica-foil laminate. The heat sink is applied to the ^" lamination face containing selector bars 133 in order to absorb heat resulting from high voltage alternating potentials. A variety of materials are suitable as well known in the art; in the case of electrically conductive materials, an insulating layer must be included to isolate the heat sink from selector bars 133.

In the examples which follow, all proportions given are by weight unless otherwise noted.

EXAMPLE V-l

220 parts Methylphenyl polysiloxane resin solution 1 part 2,4 DichlorobenzoyI peroxide 1 part Dibutyl phthalate

A pressure-sensitive adhesive composition as set forth in the above table was formulated, then diluted to 90 cps . with butyl acetate. The resulting liquid was filtered under a pressure of approximately 30 ?SI, and poured into a graduate.

The following steps were carried out in a dust-free environment. A sheet of mica having a thickness in the range -20-25 microns was cleaned using lint- ee tissues and methyl ethyl ketone (MΞK) . After drying, the mica sheet was suspended from a dipping fixture and lowered into the pressure-sensitive adhesive formulation until all but two millimeters was submersed. The mica was then withdrawn from the adhesive bath at a speed of 2 cm/minute, providing a laye of adhesive approximately 3 microns in thickness. The= coated mica was- stored in a dust-free jar and placed in a 150 C. oven for five minutes in order to cure the pressure-sensitive adhesive.

Two sheets of stainless steel 25 microns thick were cut to the desired dimensions and cleaned using MEK and lint-free tissues. One of the sheets was placed in a registration fixture, followed by the coated mica and the secon ^d foil sheet. Bonding was effected by application of

light finger pressure from the middle out to the edges, followed by moderate pressure using a rubber roller. Any adhesive remaining on exposed mica surfaces was removed using MEK and lint-free tissues. The edges of the lamination were then covered with a .6 mm wide Kapton Tape coated with the above pressure sensitive adhesive formulation.

The foil layers were respectively etched in the patterns of electrodes 132 and 133 (Figure 22) using -a positive photoresist.

EXAMPLE V-2 An ion generator was fabricated in accordance with Example V-1, modified as follows: The pressure sensitive adhesive was formulated from an acrylic cooolymer of vinyl acetate. The adhesive was diluted to 50 cps. using butyl acetate.

EXAMPLE V-3 ^{' '} An ion generator was fabricated in accordance with Example V-I, and placed in a mounting fixture with the selector bars 23 upward. A capacitor glass mounting block of dimensions compatible with the mica was prepared for mounting by application of a layer of silicon adhesive resin in accordance with the table of Example V-1, followed by smoothing of the adhesive using a metering blade. The

mounting block was clamped in registration with the laminate, and any excess adhesive at the edges was removed using cotton swabs. The completed structure was -set aside for 24 hours to allow the adhesive to set.

VI. Electrostatic Imaging Device Using Dielectric- Coated Wire

Figure 14 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 I81, Cne 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 2 mils) the strip electrodes. Wire electrode 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. Although a dielectric-coated

cylindrical wire is illustrated in the preferred embodiment, the electrode 193 is more generally defined as an elongate conductor of indeterminate cross-section, with a dielectric sheath.

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 5 and 20 mils, from dielectric receptor 200 (see Fig. 15) .

The currents obtainable from an ion generator of the type illustrated in Figure 14 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 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 4-20 mils, the extraction current was inversely proportional to the gap width. Under 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 15, ions are extracted from an ion generator of the type shown in Figure 14 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 on dielectric surface 200 by means of a DC extraction voltage 198 between transverse electrode 194 and the backing, electrode 205 of dielectric receptor 200. Because of the geometry of the ion pool 1 , 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 I ^β showing a print 'head 210 similar to that illustrated in Figure 14, 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 17 is a perspective view of ion generator 220 of the same type as that illustrated in Figure 16 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 1 and 4 mils.

The air breakdown in any of the above embodiments occurs in a region continguous to the junction of the dielectric sheath and transverse conductor (see Fig. 15) • It is therefore easier to extract ions from the print heads of Figs. 14 and 17 than from that of Fig. 14 in that this region is more accessible in the former embodiments. The ion pool may extend as far as 4 mils from the area of contact, and therefore- may completely surround the dielectric sheath where the latter has a low diameter.

In the preferred embodiment, the transverse conductors contact the dielectric sheath. As the separation of these members 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

feasible , however to separate these s tructures by as much as 1-2 mil .

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 a 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 14,. 16, and 17 illus.trate various embodiments involving, linear arrays of crossover points or print . locations. Any of these may be extended to a uitiplexible two-dimensional matrix by adding additional dielectric- coated conductors. With reference to the plan view of Figure 18, a two-dimensional matrix print head is shown utilizing the basic structure shown in Figure 1 , with a multiplicity of dielectric-coated conductors. A matrix print head 230 is shown having a parallel array of

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dielectric-coated wires 231A, 2313, 23IC 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,, , _τ 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 2321 and a further electrode (see

Figure 15) .

In any of the two-dimensional matrix- print heads, there is a danger of accidentally erasing all or part of a previously formed electrostatic dot image. This occurs in the ion generator illustrated in Figure 18 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 correspondng finger electrode 232 and ground, the previously established dot image will be totally or partially erased. In any of the embodiments of Figures 14- 17, 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.

__ι - -

Figure 19 shows in section an ion generator of the a _b ove-described type. The structure of Figure l ^β is supplemented with a screen electrode 255, which is isolated from control electrode 244 by a dielectric spacer 252. The dielectric spacer 252 de ines an air space 253 which is substantially larger than the crossover region 45 of electrodes 242 and 244. This is necessary to avoid wall charging e fects. 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 utilised for electrographic matrix printing onto a dielectric receptor 253, backed by a grounded auxiliary electrode 259. When the switch is closed at position ϊ, there is simultaneously an alternating potential across dielectric sheath 2, a negative potential V _c on control electrode 24 ₃ and a negative potential _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 253 ^' . The presence of negative potential V_ σn ^" screen- electrode 255, which is chosen so that V_ is smaller than V„ in absolute value, does not prevent the formation of the image, which will have a negative potential 7 ₁ (smaller than V _c in absolute value) .

When the switch is at X, and a previously created electrostatic image of negative potential _ - partially

under aperture 257, a partial erasure of the image would occur in the absence of screen electrode 255. Screen potential V , however, is chosen so that _s is greater than V-, 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 19, 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_ and V_ will allow an increase in the distance between iαπ generator 240 and dielectric surface 258 while retaining a constant dot image diameter. This is cone by increasing the absolute value of _s while keeping constant the potential difference between V _s and V .

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 20 illustrates yet another electrostatic imaging device 260 for use in a high speed serial printer.

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An insulating drum 251 is caused to rotate at a high rate of speed, illustratively around 1200 rpm. To 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. 3y 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 2δ2, printin-g is effected by applying an extraction voltage pulse to one of the control electrode wires 263.

Figure 21 illustrates an alternative scheme for providing a relative motion between the print device of ths invention and a dielectric receptor surface. A σharging- head 270 in accordance with Figure 13 is slidably mounted on guide bars 275. Any suitable means may be provided for reciprocating print head 270, such as a cable drive. actuated y a stepping. moto ► 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.

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EXAMPLE VI-1 An imaging device of the type illustrated in Figure 14 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 photoβtching a 1 mil stainless steel foil which had been laminated to insulating substrate 181, providing a parallel array of 4 mil wide strips at a separation of 10 mils. The driver electrode 193 consisted of a 5 mil tungsten wire coated with a 1.5 mil layer of fused glass to form a structure having a total diameter of 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 th-e 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 0.01. inches- from the _. dielectric-coated sheet. ^' -

This apparatus was employed to form dot matrix characters in latent electrostatic form on dielectric sheet 200. 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 16 was fabricated as follows. A channel 219 of 5

EXAMPLE VI- mils depth and 10 mils width was milled in a 0.125 inch thick 0-10 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, 213, etc.- ere 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 71-2 was modified to provide imaging apparatus of the type shown in Figure 17. The control electrodes comprised a series of 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 71-1 and 71-2.

In all three examples, the glass coated wire was not irmly-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

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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 4 mils and capable of supporting several thousand volts. Such cylinders are charactrized 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.

6S

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 3. Wernick and H. Ε±nne r, fourth edition, 1972, published by Robert Draper Ltd., Paddington, England. The anodization is carried out to a desired surface thickness, typically one to two mils. This results" ina 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 sur ce 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- ee environment in order to avoid a reaction with ambient moisture which leads to the formation of boehmite [AIO(OH)23 at pore mouths, effectively partially sealing the porous oxide so that subsequent impregnation is incom ^p lete 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 desicator. 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 8θ°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 im regnant material. In the present invention, the imp egnant material consists essentially of a compound of a Group II or III metal with a long chain fatty acid. It has been discovered that a particularly

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advantageous class of materials includes the compounds of Group II metals with fatty acids containing between 3 and 32 carbon atom ^' s saturated or unsaturated. 'The i pregnant materials may comprise either a single compound or a mixture of compounds. Due to the water repellant nature of these alkaline 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 pre elted material. These materials have sufficiently low viscosity after- elting 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

BAD ORIGINAL

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 extr -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

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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 subs-equent 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 susce ^p tible 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 longer 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 member's 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 desicant 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. oint 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 prove a more complete impregnation.

Subsequent to impregnation of the pores, the aluminum 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 (1.5 inch X 1.5 inch X .067 inch) fabricated of aluminum alloy 7075-Tβ were hard-coat anodized in sulphuric acid by the Sanford "Plus" process* to a depth of 1.5 mil. The panels were rinsed with deionized water and wiped free of surface moisture. They were then wrapped in moisture absorbant 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 VII) and maintained at this temperature for one minute prior to application of the impregnant material. The material was dusted onto the heated panel where it melted rapidly and was allowed to flow over the oxide surface layer.

Sanfαrd- Process Corp.; 65 North Avenue, Natick, MA.

TABLE VII

IMPREGNANT IMPRΞGNATING CHARGE MATERIAL TΞMPΞRATϋRΞ(°C) (Volts/micron) ACCEPTANCE

Barium Stearate 300° 22

Zinc Stearate 150° 34.5

Magnesium Stearate 150° 25

Zinc Octanoate 150° 33

Zinc Behe ate 150° 41.5

Zinc Oleate 150° 7

Zinc Octanoate: 300° 19

Barium Stearate

The coated member was maintained at the elevated temperature for another minute, and then allowed to cool to room temperature. Thi3 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 4 inches and 9 inch length, with 0.75 inch wall thickness. The cylinder was machined to a.30 microinch finish, then polished to a 2.25 microinc-h finish.. The cylinder was hardcoat anodized by the Sanford ^' "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 30 inches mercury. After half an hour, the even temperature was set at 150°C, which temperature was achieved in a further fortv minutes. The

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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, 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 4.5 mlcroinch finish by. application ^" of a lapping compound and oil with a cloth lap.

Using the testing method of Example 7TI-l,the cylinder's charge acceptance was measured at 980 volts.

VIII. Duplex Imaging

This section describes a duplex imaging technique employing either the electrophotographic apparatus of Figure 1 or the electrostatic printing apparatus of Figure 4. The apparatus of either of these embodiments may be adapted as discussed below to effect simultaneous pressure transfer and fusing of toner images to opposite sides of an image receptor medium. Reference should be had to Figure 4 and to the discussion at section III3. In the duplex imaging method utilizing this apparatus, receptor sheet 81 is inserted between rollers 73 and 83 only during the

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second of two toner image transfers. An initial transfer takes place directly from first image drum 73 to second image drum 83, with no receptor inserted between the two. Such transfer should be substantially complete, leaving a -toned image on second image drum 83 which is the mirror image of that formed on first imaging drum 73 during previous processing stages.

Second image roller 83 serves a number of functions in the duplex imaging process. Initially, it receives and carries the toned image transferred from roller 73. During the second transfer, it should effect as complete as possible a transfer of toner to receptor sheet 81. It is therefore desirable that bottom roller 33 have a relatively smooth surface, advantageously better than 0.25 microinch rms. In a preferred embodiment, the second, two-sided transfer to receptor sheet 81 is accomplished simultaneously with a fusing of the toned image due to high ^p ressure applied between the two rollers. Such pressure may be provided by pressure drum 83 comprising a metallic co-re -87- having an. outer- coating- of' engineering plastic.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 run from 18 to 125 kg per linear cm of contact. Roller 83 desirably has a surface 85 of engineering thermoplastic or

thermoset material, which will absorb any high stresses in the transfer nip in the case of a paper Jam or wrinkle. 3y absorbing stress in the plastics layer, the dielectric coated roller will not be damaged during accidental paper wrinkles or jams. Surface 85 preferably has a relatively low modulus of elasticity as compared with dielectric 75, in order to provide efficient toner transfer from roller 73 to roller 83. Illustrative values are a modulus of elasticity on the order of 10^ PSI for dielectric 75, and approximately 400,000 PSI for layer 85. Illustratively, surface 85 comprises a nylon or polyester sleeve having a wall thickness in the range 3 to 12.5 nun.

The efficiency of toner transfer from surface 75 to surface 85 depends primarily on the relative modulus of elasticity of the two surfaces, as discussed above. A second factor to be considered in choosing suitable materials is the relative roughness of the two surfaces . Advantageously, roller 73 has a relatively smooth surface as- compared with roller 83. Exemplary values would be a roughness of around 30- microinch rms for surface 85, as compared with around 10 microinch rms. for surface 75. Drums 73 and 83 are advantageously rotated from a common drive source. First image drum 73-. for example, may be directly driven at a given angular velocity, and second image drum 83 friction driven by contact with the first image roller. Due to the high pressure with which the drums are held together, they move at virtually the same linear

surface velocity with or without a receptive sheet inserted between them.

The various stages of the two-sided imaging process are illustrated in the schematic views of FIGURES 22 through 27. In FIGURE 22, a first latent electrostatic image I^is formed on first image drum 73 by image generating station 71. Image 1^ is toned at toning station 79 (FIGURE 23) , and rotated to a position of contact with second- image drum 83 to which it is pressure transferred (FIGURE 24). The first image, now- inverted

(-1^), continues to rotate on second image drum while a second latent electrostatic image I ₂ is formed on first image drum 73 (FIGURE 25) . During this period, any residual electrostatic image on first image drum 73 ma be erased at erasing station 93. This second image I ₅ is toned (FIGURE 25), and the two toned images are rotated to the nip, where they are pressure transferred to receptive sheet 81 (FIGURE 27). If it is desired to match the positions of images -I^ and I ₂ on receptive sheet 31, it is necessary to time _. the formation of image I ₂ .so. hat the circum erential distance from the nip on roller 73 of leading edge of image I ₂ equals the circumferential distance from the nip on roller 33 of the leading ed_z_e of image -I-, . The time interval between successive image formations should equal the period of rotation of bottom roller 83. 'This is calculable by the formula

T * Roller 33 Diameter

Surface Soeed of Rollers

, .-- m. _* --

In order to counteract the mirror reversal of first image I-, that results from the double transfer of the image, it is necessary to provide an inverted latent electrostatic image at image generating station 71. FIGURE 28 shows the case of one-sided printing from the top roller 73. In order to transfer a row of toned characters onto receptor 81, image generating station 71 forms an inverted row of latent electrostatic characters along the circumference of roller 73. In FIGURE 29, the toned characters have been transferred to bottom roller 83. In FIGURE 30, the toned characters have been further transferred to the bottom side of receptive sheet 81. As a result of the double transfer, they are printed in an inverted orientation. Thus, as shown in FIGURE 31, it is necessary to reverse the orientation (i.e. back to normal orientation) of the latent characters on drum 73 for transfer to the second side of receptor 81.

Image generating station 71 may comprise a photoconductor member on which a latent electrostatic image is formed corresponding to a ^" scanned optical image, with a transfer of the latent image to image roller 20 ^' by TESI (Figure 1) . As will be apparent to skilled artisans, the scanning optics 21 may be simply modified to provide an inversion of alternate images.

In the case of electrographic printing apparatus, the latent electrostatic image on image roller 73 is formed by ion generating means in response to a signal' indicative of

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the desired image. Image generating station 71 may comprise, for example, the ion generator and extractor discussed in section V. FIGURE 32 shows in a plan view a multiplexed ion generator of this type. The ion generator 130 includes a series of finger electrodes 132 and a crossing series of selector bars 133 with an intervening dielectric layer 131. Ions are generated at apertures 135 in the finger electrodes at matrix crossover points. Ions can only be extracted from an aperture 135 when both its selector bar is energized by a high voltage alternating potential supplied by one of gated oscillators 137, and its finger electrode is energized by a direct current potential supplied by one of pulse generators 13β. The timing of gated oscillators is advantageously controlled by a counter 138.

If axis — of the print head is oriented along the circumference of upper roller 73, one may invert the latent electrostatic image as required by the invention by reversing the order of signals to selector bars 133 from gated oscillator 137.- This may be ^' done by reversing the sequence of actuating signals from counter 133.