Electrostatic printing and copying

Description

[0001] This invention relates to a method of manufacturing an aluminium member for use in electrostatic printing and photocopying, particularly at high speeds.

[0002] Electrostatic printers and photocopiers share a number of common features as a rule, although they carry out different processes. Electrostatic printers and photocopiers which are capable of producing an image on plain paper may generally be contrasted in terms of the method and apparatus used to create a latent electrostatic image on an intermediate member. Copiers generally do so by uniformly charging a photoconductor electrostatically in the dark, and optically exposing the charged 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 standardize the apparatus to perform these functions.

[0003] 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 US-A-2,944,147; US-A-3,023,731; and US-A-3,715,762. Alternatively, the image recetor 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 US―A―3,701,966; US-A-3,907,560; and US-A-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 US-A-3,874,894, or by exposure of the toner particles to heat, as in US-A-3,023,731, and US re-issue patent 28,693.

[0004] 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 US re-issue patent 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.

[0005] Hardcoat anodization of aluminum and aluminum alloys is an electrolytic process which is used to produce thick oxide coatings with substantial hardness. Such coatings are to be distinguished from natural films of oxide which are normally present on aluminum surfaces and from thin, electrolytically formed barrier coatings.

[0006] The anodization of aluminum to form thick dielectric coatings takes place in an electrolytic bath containing an oxide, such as sulfuric or oxalic acid, in which aluminum oxide is slightly soluble. The production techniques, properties, and applications of these aluminum oxide coatings are described in detail in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Robert Draper Ltd. Paddington, England (chapter IX page 563). Such coatings are extremely hard and mechanically superior to uncoated aluminum. However, the coatings contain pores in the form of fine tubes with a porosity on the order of 6.4516x 10'⁴ to 6.4516×10¹⁶ pores per square meter (10¹⁰ to 10¹² pores per square inch). Typical porosities range from 10 to 30 percent by volume. These pores extend through the coating to a very thin barrier layer of aluminium oxide, typically 3x 10-⁸ to 8x10-⁸m (300 to 800 Angstroms).

[0007] For improved mechanical properties as well as to prevent staining, it is customary practice to seal the pores. One standard sealing technique involves partially hydrating the oxide through immersion in boiling water, usually containing certain nickel salts, which form an expanded boehmite structure at the mouths of the pores. Oxide sealing in this manner will not support an electrostatic charge due to the ionic conductivity of moisture .trapped in the pores.

[0008] GB-A-2007157 discloses a method of manufacturing an aluminium member having a dielectric surface layer with a resistivity in excess of 10" ohm-centimetres. This document discloses the features set out in the precharacterising portion of Claim 1.

[0009] US-A-3664300 discloses a process for surface treatment of xerographic imaging cylinders wherein the surface is coated with zinc stearate to provide enhanced surface lubrication and improved electrostatic toner transfer. This treatment technique does not, however, result in a permanent dielectric surface of requisite hardness and smoothness for pressure transfer and fusing of a toner image.

[0010] According to the present invention, a method of manufacturing an aluminum member having a dielectric surface layer is characterised by the features set out in Claim 1.

[0011] The invention thus provides a method of manufacturing a dielectric surface layer on an aluminium member which allows compatibility of design for electrostatic printing and photocopying apparatus. It also provides high speed printing and photocopying with excellent image quality.

[0012] The dielectric surface produced by the method of the invention possesses smoothness and hardness properties which facilitate toner transfer, while possessing sufficient resistivity to obtain a latent electrostatic image until toning. The dielectric surface maintains the above properties at elevated humidities.

[0013] The technique of the invention may be employed to advantage in producing a dielectric cylinder. Preferably, the surface is polished to a better than 2.54x10-⁸m (20 microinch) finish. The impregnant material consists essentially of a Group II metal with a fatty acid which may contain, for example, between 8 and 32 carbon atoms, saturated or unsaturated.

[0014] The invention may be carried into practice in various ways and one specific method of treating an aluminium member, along with some ways such a member can be used, will now be described, by way of example, with reference to the drawings, in which:

Figure 1 is a sectional schematic view of an electrophotographic apparatus having a roller which has been constructed using one specific example of the method 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 another electrophotographic apparatus;

Figure 4 is a sectional schematic view of an electrostatic printing apparatus;

Figure 5 is a partial sectional schematic view of an illustrative charge neutralizing device for the dielectric roller of Figure 4; and

Figure 6 is an elevation view of a preferred mounting arrangement for electrostatic printing apparatus of the type illustrated in Figure 4.

1. Introduction

[0015] Two main examples of apparatus incorporating aluminium members treated according to a specific example of the method 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 examples differ in the means by which a latent electrostatic image is created on a dielectric imaging roller; thereafter, identical apparatus may be employed.

II. Double transfer electrophotographic system

[0016] Figures 1 to 3 show double transfer electrophotographic apparatus 10 comprised of three cylinders, and various process stations.

[0017] The upper cylinder is a photoconductive member 11, which includes a photoconductor coating 13 supported on a conducting substrate 17, with an intervening semiconducting substrate 15. Advantageous materials for the photoconductor surface layer 13 include cadmium sulphide powder dispersed in a resin binder (photoconductive grade CdS is employed, typically doped with activating substances such as copper and chlorine), cadmium sulphoselenide powder dispersed in a resin binder (defined by the formula CdS_xSey, where x+y= 1), or organic photoconductors such as the equimolar complex of polyvinyl carbazole and trinitrofluorenone.

[0018] 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 conventional corona wire assembly, or alternatively it may be charged using the ion generating scheme described in the parent application. The optical image which provides the latent image on the photoconductor may be generated by any of several well known optical scanning schemes. This latent image is transferred to a dielectric cylinder 25 formed by a dielectric layer 27 coated on a metal substrate 29. The latent electrostatic image on the dielectric cylinder 25 is toned and transferred by pressure to a receptor medium 35 which is fed between the dielectric cylinder 25 and a transfer roller 37. There are means 43, 45, 47 to remove residual toner from cylinder 25 and roller 37 and to erase any electrostatic image remaining on cylinder 25 after transfer. Apparatus for effecting toning and subsequent steps, shown generally at 30 in Figure 1, is discussed in detail in subsection IIIB below.

[0019] The method by which a latent electrostatic image is transferred from the photoconductive cylinder 11 to the dielectric cylinder 25 employs a charge transfer by air gap breakdown. The process of uniformly charging and exposing the surface of the photoconductor coating 13 results in a charge density distribution corresponding to the exposed image, and a variable potential pattern of the surface of the photoconductor coating 13 with respect to the grounded conductive substrate 17. With reference to Figure 2, the charged area of the photoconductor 11 is rotated to a position of close proximity (less than 0.05 mm) to the dielectric surface. An external potential 33 is applied between electrodes in the conductive substrate of the photoconductive cylinder 11 and the metal substrate 29 of the dielectric cylinder 25, with a typical initial charge of about 1,000 volts on photoconductive layer 13, to which an additional 400 volts are added by the externally applied potential 33. The aggregate charge of 1,400 volts is decreased by about 800 volts during the exposing process.

[0020] It is possible to maintain the photoreceptor 11 in direct contact with the dielectric roller 25, an arrangement which provides the advantage of simplicity in mounting and driving the cylinders. An effective TESI process may be achieved under these conditions, but this will result in toner transfer to the upper cylinder and therefore will require additional cleaning apparatus.

[0021] The charge transfer process requires that a sufficient electrical stress be present in the air gap to cause ionization of the air. The required potential depends on the thickness and dielectric constants of the insulating materials, as well as the width of the air gap (see Dessauer and Clark, Xerography and Related Processes, the Focal Press, London and New York, 1965, at 427). Electrical stress will vary according to the local charge density, but if sufficient to cause an air gap breakdown it will result in a transfer of charge from photoconductor surface 13 to dielectric surface 27, in a pattern duplicating the latent image. This means that a certain threshold potential must be generated across the air gap. Roughly half the charge will be transferred, leaving a potential of around 600 volts on the dielectric surface 27.

[0022] 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.

[0023] 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 precipituous behavior otherwise associated with disruptive breakdown. The employment of this preferred construction of the photoconductor member 11 avoids a mottling and blurring of detail in the transferred image. A typical range of air gap distances for charge transfer using this configuration would be on the order of 0.0125 to 0.0375 mm.

[0024] The use of this method of charge transfer alleviates some of the problems resulting from undesirable discharge characteristics of the photoconductive member. The employment of an external potential in achieving a threshold potential leaves a higher voltage on the dielectric cylinder than would be the case of a single transfer system relying on the contrast potential of the photoconductor surface. This, in turn, results in a greater contrast between the light and dark portions of the toned, visible image.

[0025] 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

[0026] In a specific operative example of an electrophotographic system of the construction described, the cylindrical conducting core 29 of the dielectric cylinder 25 was machined from"7075-T6 aluminum to a diameter of 76 mm. The length of this cylindrical core, excluding machine journals, was 230 mm. The journals were masked, and the aluminum anodized by use of the Sanford process (see S. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminum and its Alloys, Robert Draper Ltd., 4th Edition 1971/72, Vol. 2, Page 567). The finished aluminum oxide layer was 60 um (micrometres) in thickness. The cylinder 25 was then placed in a vacuum oven at 101.5917 kPa (30 inches mercury). After half an hour, the oven temperature was set at 150°C. The cylinder was maintained at this temperature and pressure for four hours. The heated cylinder was brush-coated with melted zinc stearate and returned to the vacuum oven for a few minutes at 150°C, 101.59 kPa (30 inches mercury). The cylinder was removed from the oven and allowed to cool. The impregnated surface 27 of the dielectric cylinder 25 was then finished to 0.125 to 0.25 um rms using 600 grit silicon carbide paper.

[0027] 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.

[0028] The conducting substrate 17 of the photoconductor member 11, comprising an aluminum sleeve, was fabricated of 6061 aluminum tubing with a 3 mm wall and a 50 mm outer diameter. The outer surface was machined and the aluminum anodized (again, using the Sanford process) to a thickness of 50 m. In order to provide the proper level of oxide layer conductivity, nickel sulphide was precipitated in the oxide pores by dipping the anodized sleeve in a solution of nickel acetate (50 g/l, pH of 6) for 3 minutes. To form the semiconducting layer 15, the sleeve was then immediately immersed into concentrated sodium sulphide. for 2 minutes and then rinsed in distilled water. This procedure was repeated three times. The impregnated anodic layer was then sealed in water (92° Celcius, pH of 5.6) for ten minutes. The semiconducting substrate 15 was spray coated with a binder layer, the photoconductor coating 13 consisting of photoconductor grade cadmium sulphoselenide powder milled with a heatset DeSoto Chemical Co. acrylic resin, diluted with methyl ethyl ketone to a viscosity suitable for spraying. The dry coating thickness was 40 um, and the cadmium pigment concentration in the resin binder was 18% by volume. The resin was crosslinked by firing at 180°C for three hours.

[0029] The dielectric cylinder 25 was gear driven from an AC motor to provide a surface speed of twenty cms per second. The pressure roller 37 was mounted on pivoted and spring-loaded side frames, causing it to press against the dielectric cylinder 25 with a pressure of 55 kg per linear cm of contact. The side frames were machined to provide a 1.10 end-to-end between rollers 25 and 37.

[0030] 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.

[0031] The photoconductor charging corona station 19, single component latent image toning apparatus 31, and optical exposing station 21 were essentially identical to those employed in the Develop KG Dr. Eisbein & Co. (Stuttgart) No. 444 copier.

[0032] The toner removal means 43 and 45 comprised flexible stainless steel scraper blades and were employed to maintain cleanliness of both the oxide cylinder 25 and the polysulphone pressure roll 37. The residual latent image was erased using a semiconducting rubber roller in contact with the dielectric surface 27 (see Fig. 5).

[0033] With reference to the photoconductor-dielectric cylinder embodiment of Figure 2, a DC power supply 33 was employed to bias the photoconductor sleeve 11 to a potential of minus 400 volts relative to the dielectric cylinder core 29, which was maintained at ground potential. The photoconductor surface 13 was charged to a potential of minus 1,000 volts relative to its substrate 17. An optical exposure of 25 lux-seconds was employed in discharging the photoconductor in highlight areas. In undischarged areas, a latent image of minus 400 volts was transferred to the oxide dielectric 27. This image was toned, and then transferred to a plain paper receptor medium 35 which was injected into the pressure nip at the appropriate time from a sheet feeder.

[0034] Copies were obtained at a rate of 30 per minute, having clean background, dense black images, and a resolution in excess of twelve line pairs per millimetre. No image fusing, other than that occurring during pressure transfer, was required.

Example 11-2

[0035] 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 tetrahydrofuran, and coated onto a conducting paper base 15' (West Virginia Pulp and Paper 45 No. LTB base paper) to a dry thickness of 30 um. 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.

[0036] The rest of the system was identical to the previous example with the exception of the dielectric cylinder 25, which was fabricated from non-magnetic stainless steel coated with a 15 pm 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 rms finish. Again, high quality copies were obtained, even at operating speeds as high as 75 cms per second.

Ill. Electrostatic Transfer Printing

[0037] 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

[0038] Apparatus for generating charged particles and for extracting them to be applied to a further surface is disclosed in detail in the parent application. 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.

[0039] All of the above charging devices are characterised by the production of a "glow discharge", a silent discharge formed in the 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.

[0040] 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.

B. Subsequent processing

[0041] Identical apparatus may be employed for both electrophotography and printing to carry out process steps subsequent to the creation on the dielectric cylinder of a latent electrostatic image (compare Figures 1 and 4). The apparatus of Figure 4 will be considered for illustrative purposes.

[0042] 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 10¹¹ 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 0.025 m rms, in order to provide for complete transfer of toner to the receptor sheet 81. The smoothness of dielectric surface 75 contributes to the efficiency of toner transfer to the receptor sheet 81 by enhancing the release properties of this surface. The dielectric layer 75 additionally has a high modulus of elasticity, typically on the order of 6.89476_X10⁷ kPa (10⁷ PSI), so that it is not distorted significantly by high pressures in the transfer nip.

[0043] A number of organic and inorganic dielectric materials are suitable for the layer 75. Glass enamel, for example, may be deposited and fused to the surface of a steel or aluminum cylinder. Flame or plasma sprayed high density aluminum oxide may also be employed in place of glass enamel. Plastics materials, such as polyamides, polyimides and other tough thermoplastic or thermosetting resins, are also suitable. A preferred dielectric coating is anodized aluminum oxide impregnated with a metal salt of a fatty acid, as described in the parent application.

[0044] 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 JC Wilson, US Patent No. 2,846,333, issued August 5, 1958. This toner has the advantage of simplicity and cleanliness.

[0045] 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 ahere to the compliant surface of the pressure roller 83 in preference to the smooth, hard surface of the dielectric roller 73. Where rollers 73 and 83 are skewed, this tendency has been observed to result in a "slip" between the image receptor 81 and the dielectric surface 75. The most notable advantage is a surprising improvement in the efficiency of toner transfer from dielectric surface 75 to image receptor 81. This efficiency may be expressed in percentage terms as the ratio of the weight of toner transferred to that present on the dielectric roller before transfer.

[0046] The bottom roller 83 consists of a metallic core 87 which may have an outer covering of engineering plastics 85. The surface material 85 of roller 83 typically has a modulus of elasticity on the order of 1378952 to 3102642 kPa (200,000-450,000 PSI). The image receptor 81 will tend to adhere to the surface 85 in preference to the dielectric layer 75 because of the relatively high smoothness and modulus of elasticity of the latter surface. One function of the plastics coating 85 is to absorb any high stresses introduced into the nip in the case of a paper jam or wrinkle. By absorbing stress in the plastics layer 85, the dielectric coated roller 73 will not be damaged during accidental paper wrinkles or jams. Coating 85 is typically a nylon or polyester sleeve having a wall thickness in the range of 3 to 12.5 mm.

[0047] 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. Typical pressures run from 18 to 125 kg per linear cm of contact.

[0048] Scraper blades 89 and 91 may be provided in order to remove any residual paper dust, toner accidentally impacted on the roll, and airborne dust and dirt from the dielectric pressure cylinder and the back-up pressure roller. Since substantially all of the toned image is transferred to the receptor sheet 81, the scraper blades are not essential, "but they are desirable in promoting reliable operation over an extended period. The quantity of residual toner is markedly reduced in the embodiment disclosed in the parent application.

[0049] The small residual electrostatic latent image remaining on the dielectric surface 75 after transfer of the toned image may be neutralized at the latent image discharge station 93. The action of toning and transferring a toned latent image to a plain paper sheet reduces the magnitude of the electrostatic image, typically from several hundred volts to several tens of volts. In some cases where the toning threshold is too low, the presence of a residual latent image will result in ghost images on the copy sheet, which are eliminated by the discharge station 93.

[0050] 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.

[0051] The latent residual electrostatic image may also be erased by contact discharging. The surface of the dielectric must be maintained in intimate contact with a grounded conductor or grounded semiconductor in order effectively to remove any residual charge from the surface of the dielectric layer 75, for example, by a heavily loaded metal scraper blade. The charge may also be removed by a semiconducting roller which is pressed into intimate contact with the dielectric surface. Figure 5 shows a partial sectional view of a semiconductor roller 98 in rolling contact with dielectric surface 75. Roller 98 advantageously has an elastomer outer surface.

Example 111-1

[0052] 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-T6 aluminium to a 76.2 mm (3 inch) diameter. The length of the cylindrical core, excluding machined journals, was 228.6 mm (9 inches). The journals were masked and the aluminum anodized by use of the Sanford Process (see S. Wernick and R. Pinner, The Surface Treatment and Finishing of Aluminum and its Alloys, Robert Draper Ltd. fourth edition, 1971/72 volume 2, page 567). The finished aluminum oxide layer was 60 microns in thickness. The conducting core was then heated in a vacuum oven, 101.5917 kPa (30 inches mercury), to a temperature of 150°C which temperature was achieved in 40 minutes. The cylinder was maintained at this temperature and pressure for four hours prior to impregnation.

[0053] A beaker of zinc stearate was preheated to melt the compound. The heated cylinder was removed from the oven and coated with the melted zinc stearate using a paint brush. The cylinder was then placed in the vacuum oven for a few minutes at 150°C, 101.5917 kPa (30 inches mercury), thereby forming dielectric surface layer. The cylinder was removed from the oven and allowed to cool. After cooling, the member was polished with successively finer SiC abrasive papers and oil. Finally, the member was lapped to a 0.1143 urn (4.5 microinch) finish.

[0054] The pressure roller 11 consisted of a solid machined two inch diameter aluminum core 12 over which was press fit a 50.8 mm (two inch) inner diameter, 63.5 mm (2.5 inch) outer diameter polysulfone sleeve 13. The dielectric roller was gear driven from an AC motor to provide a surface speed of 304.8 mm/s (12 inches per second). The transfer roller 11 was rotatably mounted in spring-loaded side frames, causing it to press against the dielectric cylinder with a pressure of 5337.4 kg/m (300 pounds per linear inch) of contact. The side frames were machined to provide a skew of 1.1° between rollers 1 and 11.

[0055] A charging device of the type described in US Patent No. 4,160,257 was manufactured as follows. A 25.4 µm (1 mil) stainless steel foil was laminated on both sides of a 25.4 um (1 mil) sheet of Muscovite mica. The stainless foil was coated with resist and photoetched with a pattern having holes or apertures in the fingers approximately 0.1524 mm (.006 inch) in diameter. The complete print head consisted of an array of 16 drive lines and 96 control electrodes which formed a total of 1536 crossover locations capable of placing 1536 latent image dots across 195.072 mm (7.68 inch) length of the dielectric cylinder. Corresponding to each crossover location was a 0.1524 mm (.006 inch) diameter etched hole in the screen electrode. Bias potentials of the various electrodes were as follows (with the cylinder's conducting core maintained at ground potential):

screen potential -600 volts

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

driver electrode bias with +300 volts

respect to screen potential

[0056] The DC extraction voltage was supplied by a pulse generator, with a print pulse duration of 10 microseconds. Charging occurred only when there was simultaneously a pulse of negative 400 volts to the fingers 44, and an alternating potential of 2 kilovolts peak to peak at a frequency of 1 Mhz supplied between the fingers 44 and selector bars 43. The print head was maintained at a spacing of 203.2 mm (8 mils) from dielectric cylinder.

[0057] Under these conditions it was found that a 300 volt latent electrostatic image was produced on the dielectric cylinder in the form of discrete dots. The image was toned using single component toning apparatus essentially identical to that employed in the Develop KG Dr. Eisbein and Company (Stuttgart) No. 444 copier. The toner employed was Hunt 1186 of the Phillip A. Hunt Chemical Corporation.

[0058] 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.

[0059] Digital control electronics and a digital matrix character generator, designed according to principles well known to those skilled in the art, were employed in order to form dot matrix characters. Each character had a matrix size of 32 by 24 points. A shaft encoder mounted on the shaft of the dielectric cylinder was employed to generate appropriate timing pulses for the digital electronics.

[0060] Flexible steel scraper blades 89 and 91 were employed to maintain cleanliness of dielectric cylinder 73 and transfer cylinder 83. With reference to the electrostatic image erasing embodiment shown at 98 in Figure 5 the residual latent image was erased using a semiconducting rubber roller in contact with the dielectric surface 75.

IV. Fabrication of dielectric members

[0061] This section describes a series of steps for fabricating and treating anodized aluminum members which results in members particularly suited to electrostatic imaging. The treated member is adapted to receive an electrostatic latent image, to carry the image with minimal charge decay to a toning station, and to impart the toned image to a further member preferably by pressure transfer. A number of properties of particular concern in this utilization are the hardness and abrasion resistance of the oxide surface; the potential acceptance and dielectric strength of the dielectric layer; the resistivity of the dielectric layer; and the release properties of the surface with respect to electrostatic toner.

[0062] This method is advantageously employed in fabricating the dielectric cylinders of the apparatus described above in sections II and III. This method provides a simple and reliable technique for fabricating aluminum oxide layers of a thickness as great as 101.6 um (4 mils) and capable of supporting several thousand volts. Such cylinders are characterized by a hard, smooth surface which is suitably employed in the simultaneous pressure transfer and fusing of a toner image.

[0063] 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.

[0064] In the second processing stage, the machined aluminum member is hardcoat anodized preferably according to the teachings of Wernick and Pinner; see The Surface Treatment and Finishing of Aluminum and its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Rober Draper Ltd., Paddington, England. The anodization is carried out to a desired surface thickness, typically 25.4-50.8 µm (1-2 mils). This results in a relatively thick porous surface layer of aluminum oxide characterized by the presence of a barrier layer isolating the porous oxide from the conductive substrate. Following anodization, the member's surface is thoroughly rinsed in de-ionized water in order to remove all anodizing bath and other residual substances from the surface and the pores. The rinsed surface may be wiped dry to minimize surface moisture.

[0065] After anodizing the member, and prior to impregnating of the pores with a sealing material, the method of the invention requires a thorough dehydration of the porous surface layer. For best results, the dehydration is accomplished immediately after anodization. If there is a long delay between these two steps, however, it is advisable to maintain the member in a moisture-free environment in order to avoid a reaction with ambient moisture which leads to the formation of boehmite [AIO(OH)₂₁ at pore mouths, effectively partially sealing the porous oxide so that subsequent impregnation is incomplete and dielectric properties degraded. This partial sealing can occur at room temperature in normal ambient humidity in a period of several days.

[0066] 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 desiccator. 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 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 change acceptance. It is preferable that any thermal treatment of the oxide prior to impregnation be carried out at a temperature in the range from about 80°C to about 300°C, with the preferred temperature being about 150°C. Where precautions have been taken after anodizing to minimize the retention and accumulation of moisture, the dehydration step may be accomplished in conjunction with the impregnation step, as explained below.

[0067] After removal of absorbed water from the oxide coating it is sealed with an impregnant material. In the present invention, the impregnant material consists essentially of a compound of a Group II or III metal with a long chain fatty acid. It has been discovered that a particularly advantageous class of materials includes the compounds of Group II metals with fatty acids containing between 8 and 32 carbon atoms saturated or unsaturated. The impregnant materials may comprise either a single compound or a mixture of compounds. Due to the water repellant nature of these alkaline earth derivatives, the product of the invention has superior dielectric properties at high humidities.

[0068] 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 consequent of the preferred method of applying the impregnant materials of the invention. At room temperature these materials take the form of powders, crystalline solids, or other solid forms. In the preferred embodiment of the invention, the member is maintained at an elevated temperature (above the melting point of the impregnant material) during the impregnation step in order to melt the material or to avoid solidifying premelted material. These materials have sufficiently low viscosity after melting to readily impregnate the pores of the oxide surface layer. In this embodiment the period of heating the member from room temperature to the impregnating temperature may provide the preliminary dehydration which is required to avoid trapped moisture in the pores, often without a prior separate dehydrating step. This preheating stage may take minutes or hours depending on the mass and volume of the aluminum member. See Examples 1, 2. In the alternative embodiment of the invention discussed below, in which the impregnant materials are applied in solution to the anodized member, it is advisable to heat the member or take other steps in order to avoid reintroduction of moisture during the impregnation process.

[0069] It has generally been found unnecessary to maintain the heated member in a vacuum environment during impregnation, either to avoid absorption of moisture or to assist the impregnation of the pores through capillarity. In the preferred embodiment, the impregnant material may be applied to the oxide surface under moist ambient conditions because the heating of the aluminum member will tend to drive off any absorbed moisture from the oxide surface. Optionally, a vacuum may be employed in order to provide an extra precaution against reintroduction of moisture. Special measures may be required, however, in the alternative embodiment in which the impregnant material is dissolved prior to application to the anodized member.

[0070] In the preferred embodiment of the invention, the impregnant material is applied to the surface of the aluminum member after heating the member to a temperature above the melting point of the material. In one version of this embodiment, the material is applied to the surface in solid form (as by dusting or blowing it onto the surface), whereupon the material will melt. In an alternative version, the material is premelted and applied to the oxide surface in liquid form (as by brushing the material onto the member or immersing the member in melted material). In either case, the material should then be allowed to spread over the oxide surface layer. This may be done by permitting a flow of the melted material, or by manually spreading the material over the surface using a clean implement. The member should be maintained at this elevated temperature for a period of time sufficient to allow the melted material to completely impregnate the pores of the oxide surface layer. This period will be shorter when using a vacuum to assist impregnation.

[0071] In the preferred embodiment, if the member is allowed to cool prior to complete filling of the pores with the impregnant material, the material will tend to solidify leaving undesirable air pockets in the pores. It is a particularly advantageous aspect of this method that this problem may be remedied simply by reheating the aluminum member and allowing a more complete filling of the pores. The member may be reheated for a subsequent impregnation step at any time subsequent to the initial impregnation, as the impregnant material of the invention is not permanently cured.

[0072] In an alternative embodiment of the invention, the impregnant material is dissolved prior to application of the oxide surface layer. Materials of the invention susceptible to application in this manner include the compounds of Group III metals with fatty acids, as well as the compounds of Group II metals with some of the 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 desiccant dry box. Advantageously, this method would reflect that employed in the prior dehydration step.

[0073] It is desirable subsequent to precipitation of the impregnant material in the alternative embodiment to heat the member to a temperature above the melting point of the material. This fuses the material in the pores, and minimizes the occurrence of air pockets which are deleterious to dielectric properties. The member may be reheated as in the preferred embodiment in order to prove a more complete impregnation.

[0074] 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.

[0075] The advantages of this method will be further apparent from the following non-limiting examples.

Example IV-1

[0076] A series of panels 38.1 mmx38.1 mmxl.7018 mm (1.5 insx1.5 insx.067 ins) fabricated of aluminium alloy 7075-T6 were hard-coat anodized in sulphuric acid by the Sanford "Plus" process^* to a depth of 38.1 pm (1.5 mil). The panels were rinsed with deionized water and wiped free of surface moisture. They were then wrapped in moisture absorbent paper and stored for about one day.

[0077] The anodized panels were unwrapped and heated to a temperature above the melting point of the material to be applied (see Table I) 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.

[0078] The coated member was maintained at the elevated temperature for another minute, and then allowed to cool to room temperature. This process was repeated with a number of different impregnant materials including in one case a mixture of two different compounds-see Table I.

[0079] 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.

[0080] 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 IV-2

[0081] A hollow aluminum cylinder of extruded 7075-T651 alloy was machined to an outer diameter of 101.6 mm (4 inches) and 228.6 mm (9 inch) length, with 19.05 mm (0.75 inch) wall thickness. The cylinder was machined to a 7.62x10^-7 m (30 microinch) finish, then polished to a 5.715x10^-8 m (2.25 microinch) finish. The cylinder was hardcoat anodized by the Sanford "Plus" process to a thickness between 42 and 52 microns, then rinsed in deionized water and packed in plastic bags.

[0082] On the following day, the cylinder was unpacked and placed in a vacuum oven at 101.5917 kPa (30 inches mercury). After half an hour, the oven temperature was set at 150°C., which temperature was achieved in a further forty minutes. The cylinder was maintained at this temperature and pressure for four hours prior to impregnation.

[0083] A beaker of zinc stearate was preheated to melt the compound. The heated cylinder was removed from the oven, and coated with the melted zinc stearate using a paint brush. The cylinder was then placed back in the vacuum oven for a few minutes at 150°C, 101.5917 kPa (30 inches mercury). The cylinder was removed from the oven and allowed to cool.

[0084] After cooling, the member was polished with successively fine SiC abrasive papers and oil. Finally, the member was lapped to a 11.43×10^-8 m (4.5 microinch) finish by application of a lapping compound and oil with a cloth lap.

[0085] Using the testing method of Example IV-1, the cylinder's charge acceptance was measured at 980 volts.

Claims

1. A method of manufacturing an aluminium member having a dielectric surface layer with a resistivity in excess of 10¹² ohm-centimetres, comprising the steps of: hardcoat anodizing an aluminium member to form an oxide surface layer having a plurality of pores, dehydrating the oxide surface layer to thoroughly remove water from the pores, impregnating the pores of the oxide surface layer while maintaining the member in a moisture-free state, and removing any excess material from the member's surface; characterised in that the pores are impregnated with a material consisting of compounds of Group II or Group III metals with fatty acids.

2. A method as claimed in Claim 1 further comprising the step of polishing the impregnated member's surface to a finish better than 5.1 10-⁷ m rms (20 microinch rms).

3. A method as claimed in Claim 1 or Claim 2 in which the dehydrating and impregnating steps comprise heating the member to an elevated temperature above the melting point of the impregnant material, applying the impregnant material to the oxide surface layer, and maintaining the member at a temperature above the melting point of the impregnant material to allow impregnation of the pores with melted material.

4. A method as claimed in Claim 1 in which the member is impregnated with a material consisting of compounds of Group II metals with fatty acids.

5. A method as claimed in Claim 4 in which the member is impregnated with a material consisting of compounds of Group II metals with fatty acids containing between 8 and 32 carbon atoms, saturated or unsaturated.

6. A method as claimed in Claim 5 in which the member is impregnated with zinc stearate.

7. A method as claimed in any one of the preceding claims in which the impregnating step comprises the steps of dissolving the material to form an impregnant solution, and applying the impregnant solution to the oxide surface layer, while maintaining the member in a moisture-free state.

8. A method as claimed in Claim 7 further comprising the step of heating the impregnated member to a temperature above the melting point of the impregnant material.

9. A method as claimed in any one of the preceding claims in which the dehydrating step comprises heating the anodized member.

10. A method as claimed in Claim 9 in which the heating is carried out in a vacuum.

11. A method as claimed in Claim 9 or Claim 10 in which the member is heated to a temperature in the range from about 80°C to about 300°C.

Ansprüche

1. Verfahren zur Herstellung eines Aluminiumteils, das eine dielektrische Oberflächenschicht mit einer Widerstandsfähigkeit von über 10¹² Ohm-cm aufweist, welches die Schritte umfaßt: Hartschicht-Eloxieren eines Aluminiumteils, um eine Oxidschicht an der Oberfläche mit einer Vielzahl von Poren zu bilden, Entwässern der Oxidschicht, um das Wasser aus den Poren vollständig zu entfernen, Imprägnieren der Poren der Oxidschicht, wobei das Teil in einem wasserfreien Zustand gehalten wird und Entfernen aller Restmaterialien von der Oberfläche des Teils, dadurch gekennzeichnet, daß die Poren mit einem Material, bestehend aus Verbindungen von Metallen der Gruppe 11 oder der Gruppe III mit Fettsäuren, imprägniert werden.

2. Verfahren nach Anspruch 1, gekennzeichnet durch den weiteren Schritt des Polierens der Oberfläche des imprägnierten Teils bis auf einen Schliff von besser als 5.1 x 10-⁷ m in quadratischem Mittelwert (20 microinch in quadratischem Mittelwert).

3. Verfahren nach Anspruch 1 oder 2, in dem die Schritte zum Entwässern und Imprägnieren umfassen: Erwärmen des Teils auf eine erhöhte Temperatür oberhalb des Schmelzpunktes des Imprägniermaterials, Aufbringen des Imprägniermaterials auf die an der Oberfläche befindliche Oxidschicht und Halten des Teils bei einer Temperatur oberhalb des Schmelzpunktes des Imprägniermaterials, um die imprägnierung der Poren mit dem geschmolzenen Material zu ermöglichen.

4. Verfahren nach Anspruch 1, in dem das Teil mit einem Material bestehend aus Verbindungen von Metallen der Gruppe II mit Fettsäuren imprägniert wird.

5. Verfahren nach Anspruch 4, in dem das Teil mit einem Material bestehend aus Verbindungen von Metallen der Gruppe II mit gesättigten oder ungesättigten Fettsäuren, die zwischen 8 und 32 Kohlenstoffatomen aufweisen, imprägniert wird.

6. Verfahren nach Anspruch 5, in dem das Teil mit Zinkstearat imprägniert wird.

7. Verfahren nach einem der vorhergehenden Ansprüche, in dem der Schritt zum Imprägnieren die Schritte aufweist: Lösen des Materials, um eine Imprägnierlösung zu bilden und Aufbringen der Imprägnierlösung auf die an der Oberfläche befindliche Oxidschicht, wobei das Teil in einem wasserfreien Zustand gehalten wird.

8. Verfahren nach Anspruch 7, mit dem weiteren Schritt des Erwärmens des imprägnierten Teils auf eine Temperatur oberhalb des Schmelzpunktes des lmprägniermaterials.

9. Verfahren nach einem der vorhergehenden Ansprüche, in dem der Schritt zum Entwässern das Erwärmen des eloxierten Teils umfaßt.

10. Verfahren nach Anspruch 9, in dem das Erwärmen in Vakuum ausgeführt wird.

11. Verfahren nach Anspruch 9 oder 10, in dem das Teil auf eine Temperatur im Bereich von etwa 80°C bis etwa 300°C erwärmt wird.

Revendications

1. Procédé de fabrication d'un élément en aluminium muni d'une couche superficielle diélectrique d'une résistivité supérieure à 10¹¹ ohm-centimètres, comprenant les étapes suivantes: anodiser en un revêtement dur un élément d'aluminium pour former une couche superficelle d'oxyde munie d'une pluralité de pores, déshydrater la couche superficielle d'oxyde pour enlever de façon approfondie l'eau des pores, imprégner les pores de la couche superficielle d'oxyde tout en maintenant l'élément dans un état exempt d'humidité, et enlever tout matériau en excès de la surface de l'élément; caractérisé en ce que les pores sont imprégnés d'un matériau comprenant des composés de métaux du groupe Il et du groupe III avec des acides gras.

2. Procédé selon la revendication 1, comprenant en outre l'étape consistant à polir la surface de l'élément imprégné en un fini meilleur que 5,1 10-' m moyens (20 micropouces moyens).

3. Procédé selon l'une des revendications 1 ou 2, dans lequel les étapes de déshydratation et d'imprégnation comprennent les étapes consistant à chauffer l'élément à une température élevée au-dessus du point de fusion du matériau imprégnant, appliquer le matériau imprégnant à la couche superficielle d'oxyde, et maintenir l'élément à une température supérieure au point de fusion du matériau imprégnant pour permettre l'imprégnation des pores par le matériau fondu.

4. Procédé selon la revendication 1, dans lequel l'élément est imprégné d'un matériau comprenant des composés de métaux du groupe Il et d'acides gras.

5. Procédé selon la revendication 4, dans lequel l'élément est imprégné d'un matériau comprenant des composés de métaux du groupe Il et d'acide gras contenant entre 8 et 32 atomes de carbone, saturés ou insaturés.

6. Procédé selon la revendication 5, dans lequel l'élément est imprégné de stéarate de zinc.

7. Procédé selon l'une quelconque des revendications précédentes dans lequel l'étape d'imprégnation comprennent les étapes consistant à dissoudre le matériau pour former une solution d'imprégnation, et appliquer la solution d'imprégnation à la couche superficielle d'oxyde, tout en maintenant l'élément dans un état exempt d'humidité.

8. Procédé selon la revendication 7, comprenant en outre l'étape consistant à chauffer l'élément imprégné à une température supérieure au point de fusion du matériau d'imprégnation.

9. Procédé selon l'une quelconque des revendications précédentes dans lequel l'étape de déshydratation comprend un chauffage de l'élément anodisé.

10. Procédé selon la revendication 9, dans lequel le chauffage est effectué sous vide.

11. Procédé selon la revendication 9 ou 10, dans lequel l'élément est chauffé à une température dans la gamme allant d'environ 80°C à environ 300°C.

Drawing