US 3552848 A
Description (OCR text may contain errors)
mm 5, N71 E. M. VAN WAGNER 9 XEROGRAPHI C PLATE Oyiginal Filed Sept. 25, 1965 INVENTOR. EDWARD M. VAN WAGNER f gag ,4 TTOfP/VEV U.S. Cl. 3553 1 Claim ABSTRACT OF THE DISCLOSURE A xerographic apparatus having a photosensitive member in the form of a xerographic drum, said drum having a conductive substrate, a thin layer of chromium contained on said substrate, and a layer of photoconductive insulating amorphous selenium covering the central portion of said chromium coated drum leaving the two peripheral edges of said drum coated only with a chromium layer, means for forming a latent electrostatic image on the photoconductive insulating layer, developer means including a developer housing for developing said latent electrostatic image, means adapted to form a contact seal between a portion of said developer housing and a portion of the chromium coated edges of the xerographic drum, and means to move said xerographic drum with respect to said imaging and developer means.
This invention relates in general to xerography and in particular to an improved xerographic plate; and this application is a division of application Ser. No. 311,476, filed Sept. 25, 1963 (now abandoned).
In the art of xerography as originally disclosed by Carlson in U.S. Pat. 2,297,691 and as further amplified by many related patents in the field, an electrostatic latent image is formed on a photoconductive insulating layer and is developed through the deposition thereon of finely divided electroscopic material. The image may be fixed in place or transferred to a sheet of copy paper where it is permanently fixed. In most applications, the photoconductive insulating layer, which is referred to in the art as a plate regardless of its shape or flexibility, is first charged to sensitize it and is then exposed to a light image or other pattern of activating electromagnetic radiation which serves to dissipate the charge in radiation struck areas, thus forming a charge pattern which conforms to the electromagnetic radiation pattern which impinges upon the plate. This charge pattern is then developed or made visible by the chargewise deposition on the plate of an electroscopic or electrostatically attract-able, finely divided, colored material which is referred to in the art as toner.
The system of reusable plate xerography described above has enjoyed very wide commercial success pri marily because it is capable of producing extremely high quality copies and because the cost of the xerographic plate may be amortized over the many thousands of copies that it is capable of producing. As a matter of fact, in many prior art systems where the xerographic plate is in the form of a circular cylinder, the plates have been refurbished by turning them down on a lathe to remove the depleted photoconductive material and a portion of the underlying metallic support layer, heat treated to form an aluminum oxide surface coating and then recoated with a new layer of photoconductive material so that they are capable of producing many thousands of additional copies. Although most cylindrical xerographic plates of the type now in commercial use may be refurbished in this manner once or twice, the third or some 'i United States Patent additional refurbishing of the plate generally serves to reduce its dimensions to such an extent that it may no longer be used in the precision built xerographic copying devices in which such plates are generally employed. In addition to the problems encountered in refurbishing the xerographic plates described above, other problems arise because the drums are subjected to rather heavy wear by virtue of the fact that portions of their surfaces are in constant contact with other parts of the xerographic apparatus during operation of the device, thus requiring additional turning down of the drums when they are refurbished to provide a smooth drum surface.
Although it might appear at first blush that the apparatus could be redesigned to prevent wear so that different materials might be employed in the fabrication of the xerographic drums to reduce wear on the drums and facilitate the refurbishing operation, it has generally been found that these changes cannot be made because of great losses in system efiiciency which result from them. Thus, for examples, the material employed as the substrate for the xerographic plate has been found to have critical effects on the electrical properties and consequently upon the xerographic operability of the xerographic plate.
Accordingly, it is an object of this invention to define a novel and improved xerographic plate.
Another object of this invention is to define a xerographic plate which is capable of being easily refurbished without significant changes in its final dimensions and without loss in its xerographic image forming capabilities.
A still further object of this invention is to define a novel xerographic plate which is much less subject to wear in commercially available xerographic equipment.
The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of specific embodiments of the invention, especially when taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a side cross sectional view of a reusable plate type xerographic copying apparatus.
FIG. 2 is a partially cut away isometric view of the xerographic plate and developing elements of the apparatus illustrated in FIG. 1.
FIG. 3 is a sectional view of the improved xerographic plate according to this invention extending to the end of the xerographic drum as seen in FIG. 2.
Referring now to FIG. 1 there is illustrated an exemplary type of automatic, continuous, xerographic copier of a type now in wide commercial use utilizing a cylindrical plate generally designated 11 made up of a grounded substrate layer 12, an interfacial barrier layer 13 and a photoconductive insulating layer 14. It is to be noted that although the plate 11 is illustrated in the form of a rigid cylinder it may take many other shapes including that of a flat plate, a polygon, or an ellipse, or the like and may be flexible as well as rigid. Thus, for example, it might take the form of a flexible endless belt. In the operation of the apparatus shown in FIG. 1 the xerographic plate 11 is first charged to sensitize it. This is accomplished with a charging unit 16 connected to a source of high potential 17. The charging unit 16 contains one or more wire filaments which are connected to the potential source and operate on the corona discharge technique as described in U.S. Pats. 2,588,699 to Carlson and 2,777,957 to Walkup. Essentially, this technique consists of spacing a filament slightly from the surface of the xerographic plate having its conductive base grounded and applying a high potential to the filament so that a corona discharge occurs between the filament and the plate, thereby serving to deposit charge on the plate surface to raise its level of potential with respect to ground. Other charging techniques known in the xerographic art such as induction charging as described, for example, in U.S. Pat. 2,833,930 to Walkup may also be employed for the purpose of sensitizing the xerographic plate. The cylindrical xerographic plate is generally rotated at a uniform velocity in a direction indicated by the arrow in FIG. 1 so that after portions of the drum periphery pass beneath the charging unit 16 and have been uniformly charged, they come beneath a projector 18 or other means for exposing the charged plate to the image to be reproduced. This exposure step serves to dissipate charge in surface areas of the cylindrical xerographic plate which are exposed to light from the image exposure resulting in a residual charge pattern on the xerographic plate 11 corresponding to the original image to be reproduced. Subsequent to the formation of this residual charge pattern at the charging and exposure stations of the apparatus, the drum surface moves past a developing unit generally designated 19. The illustrated developing unit is of the cascade type which includes an outer container or cover 21 with a trough at its bottom containing a supply of developing material 22. The developing material is picked up from the bottom of the container 21 and dumped or cascaded over the drum surface by a number of buckets 23 on an endless driven conveyor belt 24. This development technique which is more fully described in U.S. Pat. 2,618,- 552 to Wise and 2,618,551 to Walkup utilizes a two element developing mixture including finely divided, colored, marking particles or as it is known in the art, toner and grossly larger carrier beads. The carrier beads serve both to deagglomerate the toner particles and to charge the toner by virtue of the rubbing together of the carrier and toner in the apparatus and their relative positions of the toner and carrier materials in the triboelectric series. When the carrier beads with toner particles clinging to them are cascaded over the drum surface, the electrostatic fields from the charge pattern on the drum pull toner particles off the carrier beads serving to develop the residual charge pattern making up the image. The carrier beads, along with any toner particles not used to develop the image, then fall back into the bottom of container 21. As a general rule, the toner and carrier materials are selected so that the charge triboelectrically imparted to them is opposite in polarity to the residual charge pattern on the xerographic plate 11. The result of this material selection is that the toner particles are deposited upon those areas of the xerographic plate surface which bear the residual charge pattern. Again, it is to be noted that the xerographic plate fabricated according to this invention may be used in xerographic copying apparatus employing development systems other than the illustrated cascade system, although it is of particular value when employed in conjunction with a cascade development system or other systems employing friction seals because of its resistance to wear and erosion caused by the developer seals as will be more fully explained in connection with FIG. 2. Thus, for example, magnetic brush development as described in US. Pat. 3,015,305 to Hall, skid development as described in U.S. Pat. 2,895,847 to Mayo, or powder cloud development as described in U.S. Pat. 2,918,910 to Carlson, may be substituted along with any one of the number of other development techniques well known in the art for the cascade development unit 19 illustrated and described above. Once the residual charge pattern on the xerographic plate 11 has been developed with toner particles by the development unit 19, the plate hearing this developed powder image moves around until it comes into contact with a copy web 26 which is pressed up against the drum surface by two idle rollers 27 so that the web moves at the same speed as the periphery of the drum. A transfer unit 28 is placed behind the web and spaced slightly from it between the rollers 27. This unit is similar in nature to the plate charging mechanism 16, 17 and also operates on the corona discharge principle. The transfer unit is connected to a source of high potential 29 of the same polarity as that employed in the charging device so that it deposits charge on the back of web 26 which is of the same polarity as the charge on the xerographic plate and is opposite in polarity to the toner particles utilized in developing the drum. As more fully described in U.S. Pat. 2,576,047 to Schaifert the application of this corona discharge to the back of the web 26 serves to transfer the developed toner particle image from the surface of the drum to the web 26. It should be noted at this point that may other transfer techniques may be utilized with this invention. For example, a roller connected to a source of high potential opposite in polarity to the toner particles may be placed immediately behind the copy web or the copy web may itself be adhesive to the toner particles. After transfer of the toner particle image to web 26, the web moves beneath a fixing unit 31 which serves to fuse or permanently fix the toner image to the web. In this case a resistance heating type fixer is illustrated for use with a toner including a resinous component; however other techniques known in the xerographic arts may also be utilized including the subjection of the toner particle image to a solvent vapor or the spraying over the toner image with an adhesive overcoating. After fixing the web is rewound on a coil 32 for later use. Once the cylindrical xerographic plate or drum has passed the transfer station it continues in its rotation, coming into contact with a cleaning brush 33 which removes any residual toner particles preparing it for new cycle of operation. A complete xerographic apparatus operating according to the above principles is described in great detail in U.S. Pat. 3,062,- 109 to Mayo.
As should be apparent from the preceding description of the process the xerographic plate 11 must have certain electrical properties. Thus, for example, the xerographic plate must be capable of holding charge from the time when it is sensitized by the corona generating unit 16, at least until the time when it is developed by the cascade developing unit 19. Or, in the terminology of the art, the xerographic plate must have low charge dark decay. In addition, the plate must be capable of dissipating charge relatively rapidly upon exposure to the light or other activating electromagnetic radiation to which it is sensitive so that non-image areas of the plate will not contain developable residual charge when the plate passes beneath the developing unit 19.
Although the amorphous form of elemental selenium as described in U.S. Pat. 2,970,906 to Bixby has been found to be an excellent material for use as the photoconductive insulating layer on a xerographic plate it has been found that it does not have the desired combination of electrical properties for use in the process when it is merely deposited upon a cleaned conductive metallic substrate such as a aluminum, steel, or the like. Rather it has generally been found that deposition of the selenium in its amorphous form directly upon such a conductor results in a xerographic plate with an unacceptably high level of dark decay. In order to solve this problem, xerographic plates in commercial use today generally employ a substrate such as aluminum bearing a thin interfacial barrier layer such as aluminum oxide between the luminum and selenium layers. Although the theory of operation of the barrier layer is imperfectly understood it to presently believed that this barrier layer is of sufficiently insulating character to prevent the injection of charge from the conductive plate substrate prior to exposure, thereby avoiding the problem of dark decay in the xerographic plate. The interfacial barrier layer is of such a nature as to allow charge dissipation from the plate upon exposure of the charged plate to activating electromagnetic radiation. It is presently thought that dissipation of charge through this thin barrier interface upon illumination of the charged plate is caused by a tunneling of the charge through the barrier layer once the charge carriers move through the bulk of the selenium to the interface as more fully described in U .8. Pat. 2,901,348 to Dessauer. In the past, aluminum oxide has proved to be one of the most acceptable barrier layer materials and has enjoyed widespread commercial use whereas most other materials which might be expected to operate as good barrier materials have proved to produce electrically unacceptable plates, thus making the selection of barrier materials a most perplexing problem.
In FIG. 2 there is illustrated in isometric, a portion of the FIG. 1 apparatus including the cylindrical xerographic drum 11 and the cascade developing unit 19. As best seen in this figure the conductive drum substrate 12 and the interface layer 13 extend out beyond the photoconductive insulating layer 14 and the end seals 34 of the developer housing 21 make sliding contact with the end portion of the drum. Seals of the type shown in FIG. 2 are provided at both ends of the drum although only one seal is shown in FIG. 2. Since the lower portion or collecting lip on the trough of the developer housing 21 is below the point where residual developing material falls ofl? the drum surface and since the developing material first strikes the drum surface at a point below the uppermost position in its path of rotation these end seals 34 may be said to complete the isolation of the developing system from all portions of the copying apparatus except for a part of the drum surface. This isolation of the developing system is necessary to prevent stray developing material from falling into or being blown into motors, drives, clutches, scanning systems, and other portions of the xerographic apparatus whose operation might be severely impaired by either the carrier beads or the toner particles in the developing mixture. Some appreciation of the large number of precision components which go into such an apparatus and which, therefore, must be protected from the granular material may be gained from a study of US. Pat. 3,062,109 to Mayo. Although the end seals 34 are usually fabricated of some foam material such as foamed polyurethane or other materials capable of forming a good seal with the surface of the drum substrate, carrier beads and/or toner particles sometimes become lodged between these end seals and the surface of the drum and in past instances when the drum and interface layer were formed of aluminum and aluminum oxide respectively, these particles have caused significant scoring of the drum surface as the drum rtates with these developing materials between it and the end seals. Since these carrier beads generally take the form of coated glass, this scoring of the drums is frequently severe enough to impair the effectiveness of the seal over long periods of drum use.
Even though amorphous selenium as described in greater detail in the above referenced patent to Bixby is an extremely rugged, high quality photoconductive insulator as is widely used in xerographic plates, it sufiers a constant battering from contact with the developing mixture, the paper to which the developed image is transferred and the cleaning brush in a xerographic copying apparatus so that after thousands of cycles, surface defects such as scratches, holes, and the like eventually tend to appear on the selenium of the xerographic drum requiring its replacement. Since the cylindrical substrate is a precision made component which is machined specifically for mounting in the xerographic copying apparatus, large savings can be made if the selenium photoconductive insulating layer can be stripped from the drum and a new coating of amorphous selenium can be reapplied to the drum surface rendering it ready for reuse in thousands of additional cycles of machine operation. Several disadvantages have been found when it was attempted to refurbish the commercially utilized aluminum based xerographic drums for more than a few times. Thus, for example, the bond between the evaporated amorphous selenium photoconductive insulating layer and the substrate and interface layers of aluminum and aluminum oxide was found to be so good that removal of the selenium could only be readily accomplished by machining the drums on a lathe and even in those occasional instances where the selenium can be removed by other techniques, machining of the drum blank is required to remove grooves, pits, and the like caused by scoring of the ends of the drum by developer caught between the end seals and the drum surface. Since the diameter of the drum blank is reduced by a certain finite amount during each machining operation, aluminum drums could be refurbished on average only about twice before the drum blanks became unacceptably out of tolerance requirements for use in the machine. In addition, once the aluminum drum has been machined down to remove the selenium and any surface defects such as scoring in the aluminum blank, the drum must then be cleaned to remove any irregular surface oxides as by dipping it in a nitric acid solution whereupon a new and highly uniform layer of aluminum oxide is then formed on the drum surface by heating over an extended period or by any one of a number of various chemical treatments. This prior art type aluminum drum blank is then ready for recoating with amorphous selenium which constitutes the final step in the refurbishing process.
It has now been found that a novel xerographic drum constructed according to the sectional view in FIG. 3 is operable in a highly efficient manner in the xerographic copying process, is much less susceptible to wear from contact with developer end seals in the apparatus and can be recoated with selenium more times and with greater facility than can prior art xerographic plates. As seen in FIG. 3 this xerographic plate consists of a supporting or substrate layer 36 which is preferably at least one or two orders of magnitude more conductive than amorphous selenium layer 38 and may advantageously be formed of a conductive metal or alloy such as steel. A thin but uniform layer of chrome 37 overlies the substrate below selenium layer 38 which covers only the center of the chrome layer. Surprisingly this chrome layer acts as an excellent xerographic plate interface. Except where very hard substrate materials are employed, the chromium layer should preferably be at least thick enough to impart hardness to the surface of the drum blank and thereby prevent abrasion or erosion of the ends of the drum when they come into contact with the developer seals and trapped carrier beads in the developer seals during operation of the xerographic apparatus which employs the drum. To provide this hardness, chrome layers on the order of from about .0005 to about .001 inch are required. When the drum is to be utilized in a xerographic apparatus in which serious abrasion or erosion problems are not envisioned, thinner layers of chrome which do not range up into the hard chrome coating region may be employed to satisfy the electrical requirements of the interfacial layer. The chrome may be deposited by any of the known chrome surface coating techniques such as, for example, electroplating. If electroplating is em ployed, nickel or copper undercoating may be used to provide good adhesion between the chromium and its steel substrate and to facilitate the electroplating. When electroplating is utilized to apply the chromium layer in thicknesses in the hard chromium range the plating operation should be followed by heat treatment at about from 300 to about 500 F. for from about 1 to 5 hours to outgas the chromium layer and prevent hydrogen embrittlement which might otherwise result from the electrolytic coating process. Once the chromium layer has been applied to the substrate and outgassed as described above, a layer of amorphous selenium is applied to the chromium to serve as the photoconductive insulating layer of the plate. This layer will generally range in thickness from about 10 to about 200 microns depending upon its mode of operation, the type of activating electromagnetic radiation to which it is exposed and similar factors.
Actual testing of a cylindrical xerographic plate constructed in accordance with the FIG. 3 description in a xerographic office copier of the type described in the above referenced U.S. Pat. 3,062,109 to Mayo showed that the wear on the drum surface in the area of the developer seals was very much less than that obtained with the standard aluminum drum commonly used, while print quality was found to be at least equal to that of a standard aluminum base drum even after a 20,000 print test indicating that the electrical properties of the chrome interface were fully adequate for the xerographic process.
One of the most important advantages of the chrome interface drum described in connection with FIG. 3 above is that it may be refurbished by a method which is both fast and simple as compared to the refurbishing method of the prior art type drum. In addition, refurbishing may be carried out a great number of times without degradation of the drum blank since it need not be turned down on a lathe. Basically, this refurbishing process involves merely stripping the exhausted selenium from the chrome plated drum by bringing a strong adhesive tape into contact with the selenium surface and then pulling the adhesive tape away from the drum to demove the selenium followed by a recoating of a fresh layer of selenium onto the chrome surface. It has also been found that the selenium may be removed by a jet spray process which involves causing a very high pressure spray of water to impinge on the selenium. It has been found that the chrome to selenium bond is not strong enough to withstand these types of removal processes; however, it is sufficiently strong so that the selenium stays on the chrome base without flaking off or otherwise being removed during operation in the xerographic copying apparatus described. Tests to 20,000 cycles and more of drum operation in this type of copying device have failed to disclose one instance of selenium-chrome bond failure.
Owing to the fact that there is little or no scratching of the chrome base drum at the location of the developer seals and the fact that turning down is not required to remove exhausted selenium from the drum blank, the blank is virtually as good as a new blank once the exhausted selenium has been removed. It is also to be noted that the heat treatment described above in connection with the chrome base is only required prior to the first coating of selenium on the drum and only when the chrome has been applied by electroplating.
Any residual small feathers of selenium which may be left by the stripping process can be removed by a hot sodium sulfite bath.
The present invention has been described with reference to certain specific embodiments which have been presented in illustration of the invention. It is to be understood, however, that numerous variations of the invention may be made and that it is intended to encompass such variations within the scope and spirit of the invention as described by the following claim.
What is claimed is:
1. A xerographic apparatus comprising a photosensitive member in the form of a cylindrical drum, said drum having a conductive substrate coated with a thin uniform layer of chromium on the outer surface of said substrate, a photoconductive insulating layer of amorphous selenium covering the major portion of said chromium layer leaving the two peripheral edges of said drum coated only with chromium, means for forming a latent electrostatic image on the photoconductive insulating surface of said drum, developer means including a developer housing to develop said latent electrostatic image, said developer housing including a contact seal, with said seal being in sliding contact with said chromium coated edges, and means to move said xerographic drum with respect to said imaging and developer means.
References Cited UNITED STATES PATENTS 2,892,973 6/1959 Straughan 355-3(X) 2,987,660 6/1961 Walkup 355-3 (X) 3,053,962 9/1962 Cerasani 355-3(X) 3,105,770 10/1963 Lehmann 3553 (X) 3,174,855 3/1965 Gray 96-1 3,234,020 2/1966 Stockdale 961 JOHN M. HORAN, Primary Examiner R. P. GREINER, Assistant Examiner U.S. Cl. X.R. 35515