US 3063859 A
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Description (OCR text may contain errors)
Nov. 13, 1962 H. HECKSCHER 3,063,859
METHOD AND APPARATUS FOR TRANSFERRING IMAGES FROM XEROGRAPHIC TO METALLIC PLATES Filed March 1, 1955 2 SheetsSheet 1 INVENTOR. Helmut Heckscher nwxpzi w ATTORNEYS.
H. HECKSCHER PARATUS FOR T FROM XEROGRAPHIC TO M 3,063,859 RANSFERRING IMAGES ETALLIC PLATES 2 Sheets-Sheet 2 Nov. 13, 1962 METHOD AND AP Filed March 1, 1955 Transformer HOV INVENTOR Helmut Heckscher BY F ATTORNEY United States atent Qfiice 3,063,859 1 Patented Nov. 13, 1962 3,063,859 METHOD AND APPARATUS FOR TRANSFERRING IMAGES FROM XEROGRAPHIC T METALLIC PLATES Helmut Heckscher, Cambridge, Mass., assignor, by mesne assignments, to Xerox Corporation, a corporation of New York Filed Mar. 1, 1955, Ser. No. 491,343 5 Claims. (Cl. 117-175) This invention relates to the production of high quality permanent images through xeropraphy and, more partic ularly, to methods and apparatus for the transfer and fixing of developed xerographic images onto metallic plates.
In the art of xerography it is usual to place an electrostatic charge upon the surface of a xerographic plate which is composed of a conductive backing and a photoconductive insulating coating, for example, of selenium. The coating is a good insulator in darkness and will retain any electrostatic charge placed thereon. However, any area of the charge surface will be quickly discharged upon its exposure to light. When the charge plate is exposed to a pattern of light an electrostatic latent image is produced on the coating. This latent image may then be developed by cascading powder particles across the surface of the coating of the xerographic plate. These particles readily adhere to the charged areas on the plate surface and render the latent image visible.
Usually this developed image of powder particles is then transferred electrostatically to another surface and is fixed on the latter, for example, by heat fusing. However, this method of transfer and fixing is subject on occasion to certain limitations and is not always the method of choice, particularly for high quality photographic reproduction. It is, therefore, an object of this invention to provide method and apparatus for the production of high quality transfers of xerographic images and additionally to provide method and apparatus for fixing and fusing the transferred image.
Heretofore the transfer of a developed xerographic image to a conductive metallic plate such as a lithographic plate, for example, of zinc, has involved procedures as embodied, for example, in the Copley Patent No. 2,637,651 granted May 5, 1953. In effecting such transfer heretofore, the developed xerographic image has first been transferred from the xerographic plate to a non-conductive sheet, such as a piece of paper. Thereafter the image transferred to the sheet of paper has in turn been transferred to the conductive lithographic plate. This two-stage transfer procedure can in some instances be replaced by a single-stage transfer by the practice of the instant invention.
An object, therefore, of the present invention is to provide method and apparatus for direct single-stage transfer of a developed xerographic image to a conductive metallic surface, such as a zinc, aluminum or other metallic plate, without loss of quality, or dimensional changes. The transfer when practicing the instant invention is effected from one dimensionally stable surface to another.
In the transfer of a developed xerographic image directly to a conductive surface problems of fusing or fixing the transferred image permanently to the conductive surface are encountered. It is, therefore, still another object of this invention to provide novel methods and apparatus for permanently fusing the transferred image to the conductive surface. In the instant embodiment the fusion is effected by heat fixation during the transfer operation.
Further objects and features of the invention will become apparent from the following specification and the accompanying drawing wherein:
FIG. 1 is a plan view of a xerographic powder image on a photoconductive insulative coating of a xerographic plate;
FIG. 2 is diagrammatic view of transfer apparatus illus-' trative of a manner of transferring the powder image of FIG. 1 directly to a conductive metallic plate and of permanently fusing the transferred xerographic image thereto; 7
FIG. 3 is a longitudinal section taken along line. 33 of FIG. 2;
FIG. 4 is an elevation showing a lithographic plate to which an image has been transferred and fused.
Normally the transfer of a developed xerographic powder image from the xerographic plate to a piece of paper, or other non-conductive medium is accomplished by placing the latter in contact with the coating of the xero:v graphic plate bearing the powder image and then applying a charge to the paper. This results in transfer of the powder image from the coating to the paper by electrostatic attraction. Thereafter the transferred powder image may be fixed to the paper sheet or else in turn it may be transferred to a conductive sheet or plate.
The transfer to a paper sheet involves no diificult prob-' lems. If an attempt, however, is made to substitute a conductive sheet, for example, of zinc or other metal for the paper sheet, the transfer has not heretofore been found to be satisfactory principally because of the conductivity of the zinc or metallic plate. This conductivity" makes it impossible to isolate an electric charge at ,any one localized area of the metal plate. If, therefore, for example, on point or area in the xerographic plate coating is a little too thin or if there happen to be pin holes or other similar defects in the coating, or if for any other reason electric charges can pass freely from the metal plate surface to the conductive backing'rnember of the xerographic plate, then any charge applied to the zinc surface in its entirety sparks through to the xefographic backing member.
This problem is not material where the transfer plate is paper and non-conductive because any local discharges from the charged paper to the backing of the photo conductive coating is kept localized. Accordingly, in transfer to paper localized break-down points do not seriously affect the quality of the transferred image.
On the other hand, where the transfer plate is conductiv'e a single point of electric break-down would result in failure of transfer over the entire area of the conductive transfer plate in contact with the xerographic plate. In addition, each point of local break-down results in permanent destruction of the photoconductive coating at that point.
It is to be noted also that a zinc or other metallic plate superposed upon the photoconductive selenium layer or coating of a xerographic plate in combination with the conductive backing member of the latter provides a com paratively high capacity condenser. Thus, if electric charge leaks through the photoconductive selenium layer or coating at any point a great deal of such charge can leak through because theentire condenser is discharged.
A basic principle utilized in the instant invention to" avoid such problem is the maintenance merely of tangent line contact between the metallic lithographic plate and the xerographic plate during transfer of the developed powder image on the latter to the conductive metallic. plate. This line contact is in substitution for area contact and this reduces the capacity of the condenser to a minimum. This reduction in capacity reduces the leakage charge available and also reduces the severity of electrical breakdowns when they occur.
In brief, the desired result is. accomplished by applying the zinc or other conductive metallic plate to the surface of a drum and by feeding the drum-borne conductive plate to the xerographic plate surface which is positioned merely in tangent line contact under controlled pressure with the drum-borne plate. During such feeding the conductive plate and the xerographic backing plate are maintained with an electrical potential difference between them to cause transfer of the powder image from the xerographic plate to the conductive plate. In the present embodiment of the invention, improved results are achieved, particularly when using large sized plates, by heating of the conductive plate while maintaining a potential difference between it and the xerographic plate. Fusion then occurs during transfer to form a lithographic master plate as shown in FIG. 4, and no subsequent fusion treatment is needed. In particular, Where transfer to large plates, i.e. plates upwards of 27%. x 30", is effected, such transfer may be spotty or result in hollow image lines unless additional precautions and expedients are undertaken. These include the improvement of heating the metallic plate and preferably backing it up with a resilient material, such as sponge rubber, during transfer.
Referring now particularly to the drawing and particularly to FIG. 1, the reference character denotes a xerographic plate bearing a photoconductive surface or coating 11. A developed powder image I is shown upon the coating 11. The image I on the surface of the Xerographic plate or element 10 may be composed of any material which will fuse to a solid and permanent image body upon transfer and exposure to heat. This includes, but is in no way limited to known xerographic developers such as pigmented resins including rosin modified phenol formaldehyde resin-s such as are disclosed in Copley U.S. Patent No. 2,659,670 issued November 17, 1953. One such preferred image material of this kind is available from The Haloid Comp-any, Rochester, N.Y., under the name of XeroX Toner.
According to one embodiment of the invention, shown in FIGS. 2 and 3, in order to transfer the developed image I from the surface or coating 11 of the xerographic plate 10 to a metallic conductive plate 12, for example, of Zinc, the conductive plate 12 is mounted in any desired manner, for example, by masking tape T, on the periphery of a transfer drum 13. If the plate 12 is of zinc, its grain surface is faced outermost. The masking tape T may be replaced by suitable quick acting. release clamps (not shown) or by other mechanism. The drum 13 is rotated by any conventional means such as motor M. A smaller diametered pressure drum or roller 14 is supported in adjustable pressure relationship relative to the surface of the drum 13. The roller 14 is preferably of metal and is covered with a layer 15 of rubber or the like.
In order to transfer the powder image I of the xero' graphic plate 10 to the metal plate 12, the pressure of the roller 14 against the roller 13 is adjusted to a desirable figure by regulation of tension on springs 16 located at opposite ends of the shaft of roller 14, and the xerogiaphie plate 10 with its developed powder image facing the outermost surface of metal plate 12 on the drum 13 is fed between the rollers 13 and 14 'While an electrical potential difference, for example, from a battery 13, is applied between the metal plate 12 and the xerographie plate 10. When dealing with negatively charged xerographic image materials as is usual in the commercial practice of xerography, the negative terminal of battery B is connected to a contact spring 17 that engages the conductive backing surface of the xerographic element 10. The positive terminal of battery B is connected through a resistor R and a contact spring 18 to the conductive plate 12 on drum 13.
Since the conductive plate 12 along any particular xerographic plate 10 are merely in momentary line contact along any particular portion during image transfer, the break-down difficulties and electric discharges from plate 10 to plate 12 mentioned above have less likelihood of occurring. The speed of rotation of the drum 13 is selected for the most effective transfer of the image to the zinc surface 12.
In effecting transfer, particularly to large dimensioned metallic plates, for example, to plates whose dimensions are 27%" x 30 or larger, spotty transfer and hollow image lines may occur. In a presently preferred embodiment, the metallic plate is heated during application of electric potential difference to effect transfer, and, in addition, a resilient backing, for example, of sponge rubber sheeting is provided for the metallic plate. The backing assures more uniformity of contact and better control of pressure. A preferred arrangement for effecting transfer to large dimensioned plates is illustrated diagrammatically in the figures. In the figures a cushion 22 of sponge rubber or like resilient cushioning material is placed around drum 13 with a conventional electric blanket or like heating means 23 overlying the cushion.
In practicing the invention as just described, it is found that the effectiveness of the transfer depends upon the applied pressure, the temperature, and the electrical potential between the plates. If excessive pressure is applied by the pressure roller 14, the usual result is hollow lines in images transferred to the lithographic plates. If insufficient pressure is applied, incomplete transfer is the result. Transfer forces of the order of /3, es and 1 pound per linear inch of pressure roller 14 in contact with the xerographic plate 10 during transfer have been used. The force of pound per linear inch gives the best over-all results. The springs 16 whose tensions may be adjusted serve to adjust the pressure of roller 14.
The value of the electrical potential difference to be applied by battery B between the plates 10 and 12 is governed by the frequency of occurrence of electrical breakdown and that value which gives the best image transfer. In practice it is found that the best over-all results occur when the voltage applied during transfer is of the order of from 250 volts to 335 volts and preferably is about 315 volts DC. with a protective resistor R of the order of approximately 200,000 ohms between the battery B supplying the electrical potential difference and the trans fer plate 12 when the thickness of the coating on the xerographic plate or element 10 is approximately 20 mi crons thick. It has been found in practice that increase of the resistor value to 40 megohms and of the applied voltage to approximately 405 volts has no beneficial effect.
The requirement for heating the drum-mounted plate 12 so that it acquires a desired elevated temperature during the application of electric potential difference and pressure between the roller 14 and the surface of plate 12 on drum 13 may be satisfied in any number of ways. The needed heat may be supplied, for example, by supplying electric heating current to the blanket 23 below the plate 12 from an electric source via transformer 24 and slip rings 25. The heat could be supplied without use of an electric blanket, in a number of Ways. One Way would be by directing the heat rays of infra red radiation lamps or other heat lamps (not shown) against the surface of plate 12 directly just in advance of its rotary movement into contact with the surface of plate 10 under roller 14. Other ways of heating the plate 12 may be utilized. For example, the ambient air may be heated directly by gas, coal, oil or other fired flame or even indirectly as by heat exchange, to heat the plate 12. Liquids such as water, molten sodium, eutectic mixtures, for example, of diphenyl and diphenyl oxide (Dowtherm), mercury, or other suitable liquids heated in any suitable manner may be supplied to the interior of drum 13 to heat the latter and the plate 12 borne thereby to the desired elevated temperature. In fact, any practicable way of heating the plate 12 to desired temperature may be utilized.
The optimum temperature to which plate 12 should be heated while application of electrical potential difference and transfer is being effected depends largely upon the nature of the resin content of the electroscopic powder.
With the powders hereinbefore recited a temperature of approximately 200 C. was found effective. The particular temperature in any event which is selected is the lowest practicable one which will render the particular resin content of the electroscopic powder sufiiciently tacky to adhere to the plate 13. On the other hand, this temperature should not be so high as to melt the resin completely as this would cause running of the resin on the surface of metal plate 12 with consequent blurring of the image produced. While the temperature of 200 C. noted herein is a practicable one for the resins named, it is evident that for any particular resin, the temperature may vary over fairly wide limits as a high temperature for a relatively short time may produce the same good results as a lower temperature for a longer period of time.
In general, the device shown in the figures is large enough to accommodate plates of a size of about 27% by 30" or upwards and is operated at a peripheral speed of about 15 feet per minute, although the speed may be either increased or decreased as desired.
The use of heat and pressure simultaneously with electric potential during transfer as practiced with the apparatus of this invention eliminates any necessity for vapor fusion or other means or methods of image fixation, as permanent fusion occurs at the instant of electrical and pressure transfer from xerographic plate to heated transfer plate 12 as they come into tangential line contact in passing between roller 14 and drum 13.
After simultaneous heat and electric transfer have been completed, the metallic plates 12 are stripped from drum 13 and are ready for use. Such plates '12, if lithographic plates, may be used literally for thousands of impressions without deterioration of the transferred Xerographic image.
The resilient backing sheet or pad 22 assures maximum uniformity of line contact between plates 10 and 12 during transfer and excellent control of pressure which is preferably from /3 pound to 1 pound pressure per linear inch of roller contact. This together with simultaneous heat and electric transfer prevents spotty transfer or hollow image lines in the transfer. While the apparatus of FIGS. 2 and 3 is particularly applicable to large dimensioned transfers, it is applicable as well where necessary for transferring to smaller dimensioned metallic plates whenever particular conditions show that combined heat and electrical transfer are desirable.
While specific embodiments of method and apparatus have been described and shown, variations within the scope of the claims are possible and are contemplated. There is no intention, therefore, of limitation to the exact details shown and described.
What is claimed is:
1. The method of transferring a powder image from a photoconductive surface of a Xerographic plate to the surface of a conductive plate comprising the steps of forming one of said plates in an arc of substantially uniform radius, contacting the convex surface of the curved plate in a rolling motion against the surface of the other plate with the powder image therebetween, whereby the respective plate surfaces are progressively brought into tangential contact and, simultaneously with such rolling motion, applying an electrical potential between the Xerographic plate and the conductive plate, heating the conductive plate during the entire period of powder image transfer, and applying substantially uniform pressure to maintain the plates in surface contact along the line of tangential contact.
2. The method of claim 1 wherein the pressure applied to maintain the plates in surface contact is in the range from /3 1b. to 1 lb. per linear inch of line contact.
3. The process of claim 1 wherein the electric potential applied between the plates is of the order of 250 volts D.C. to 335 volts DO, and wherein the conductive plate is positively charged and the xerographic plate is negatively charged.
4. The process of claim 1 wherein the conductive plate is heated to approximately 200 C. during transfer of the powder image thereto.
5. An apparatus for transferring a powder image from a Xerographic plate to a conductive plate, said apparatus including a cylindrical element for supporting a conductive plate, means to support the cylindrical element for rotational movement, a heating element arranged on the periphery of the cylindrical element over an area to accommodate a conductive plate, clamp means on the cylindrical element for securing a conductive plate in superposed relation on the heating element on the cylindrical element, pressure applying means arranged in operative relation to the cylindrical element for urging a Xerographic plate into tangential line contact with a conductive plate supported on the cylindrical element, and means to apply an electric potential between a Xerographic plate and a conductive plate when arranged in said apparatus.
References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Schaifert et al.: Xerography, A New Principle of Photography and Graphic Reproduction, J. Optical Society of America, vol. 38, No. 12, December 1948, pp. 991 to 998, only p. 996 made of record. Copy available in Div. 25.