IMAGE-FORMING ELEMENT FOR AN ELECTROSTATIC PRINTER, AND A PRINTER IN WHICH AN ELEMENT OF THIS KIND IS USED

Abstract: An image-forming element for an electrostatic printer, consisting of an endless support (1) with thereon a dielectric surface layer and beneath the surface of said layer electrodes (4) which are insulated from one another and extend in the peripheral direction of the sup­ port (1) in the form of endless paths, parallel to one another. Beneath the electrodes (4) electrodes (2) are provided and are also insulated from one another and extend from one end of the support (1) in the direction of the other end of the support. In each case, one of the electrodes (2) is electrically conductive connected to one of the electrodes (4). Further, the electrodes (2) are connected to means (3) for supplying voltage to said electrodes. A printing device in which the image-forming element is used, is also described.

Description: This invention relates to an image-forming element for an electrostatic printer, consisting of an endless support with a dielectric surface layer thereon and first electrodes being provided beneath the dielectric surface layer, said electrodes being insulated from one another and extending in the form of endless paths parallel to one another in the peripheral direction, i.e. the direction of the endless path of the support. US Patent 3 816 840 describes an electrostatic printing process and printer in which a dielectric image-receiving material is fed between a first and a second electrode which are disposed a short distance apart and one of which is covered with a layer of magnetically attractable electrically conductive toner powder, while voltage pulses are applied between the said electrodes so that toner powder is deposited on the image-receiving material in the form of an information pattern. A disadvantage of this process is that only dielectric image-receiving material can be used, thus restricting the choice of image-receiving materials. US Patent 3 946 402 describes an electrostatic printer comprising a rotatable drum provided with a dielectric layer on which a uniform layer of electrically conductive magnetically attractable toner powder is applied. A magnetic roller is disposed in an image-forming zone near the drum surface covered with toner powder and has a stationary non-magnetic sleeve and a rotatable magnet system mounted inside the sleeve. A large number of magnetic electrodes in the form of rods each connected to a voltage supply is disposed axially on the sleeve of this magnetic roller. When the electrodes are not energised, toner powder is attracted from the drum surface to the magnetic roller while no toner powder is attracted when the electrodes are energised. By energising the electrodes pulse-wise according to an information pattern, a toner image corresponding to the information pattern is formed on the drum and can then be transferred to a receiving support. Since the electrodes are conductive they must be insulated from one another. A disadvantage of this known device is that the conductive toner powder can short-circuit some electrodes, thus disturbing the image formation. It is also a very complex and expensive matter to construct a row of fine magnetic electrodes in rod form. According to the invention, an image-forming element for an electrostatic printer is provided with which the above disadvantages can be obviated. According to the invention this object is attained by providing an image-forming element of the kind referred to in the preamble, characterised in that second electrodes are provided beneath said first electrodes and are also insulated from one another and extend each from a point near one lateral edge of the support in the direction of the other lateral edge of the support, each one of said second electrodes being electrically conductively connected to a respective first electrode and to means for supplying voltage to said electrodes. In the image-forming element according to the invention the electrodes are completely insulated from one another so that short-circuiting of one or more electrodes by the applied electrically conductive toner is obviated. Since the image-forming electrodes are disposed in the image-forming element itself, a conventional magnetic roller can be used in the image forming process. This results in a simpler and cheaper construction apart from better copy quality. According to a preferred embodiment of the invention the electronic facilities for energising the electrodes in accordance with an information pattern requiring to be printed are disposed near one or both edges of the support on the periphery thereof. As a result these electronic facilities can be installated fairly simply, and, what is particularly important, they are readily accessible for maintenance or for the replacement of faulty components. The invention also provides a device for printing information using an image-forming element according to the invention. The invention and its advantages will be explained in detail hereinafter with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic view of a part of an image-forming element according to a preferred embodiment of the invention. Fig. 2 is a drawing representing the principle of an electrostatic printer equipped with an image-forming element according to the invention. The image-forming element according to Fig. 1 comprises a drum 1 having an insulating surface on which a plurality of (second) electrodes 2 are disposed, which extend axially to the drum 1. Each electrode 2 is connected to one of the blocks 3 which are disposed on one side of the drum 1 and which contain the electronic facilities for selectively applying voltage to the electrodes 2 in accordance with an information pattern. The electrodes 2 are covered with an insulating layer which, however, has been omitted from Fig. 1 for the sake of clarity. Electrodes 4 (first electrodes) are disposed on this insulating layer and extend in the direction of the periphery of the drum 1 in the form of endless paths parallel to one another equidistantly. One first electrode 4 is conductively connected to one second electrode 2 via perforations in the intermediate insulating layer, such perforations being filled with conductive material. The conductive connections are indicated in the form of dots 5 in Fig. 1. That part of the drum 1 which is covered with the electrodes 4 is covered with a dielectric layer which again has been omitted in Fig. 1. Thus apart from the conductive connections 5 the electrodes 2 and 4 are completely insulated from one another. The number of electrodes 2 on the drum 1 is equal to the number of electrodes 4, one electrode 2 in each case being conductively connected to one electrode 4 in each case. The quality of the images formed on the image-forming element depends, inter alia on the number of electrodes 4. As the electrode density increases so the image quality improves. Preferably the number of electrodes 4 is at least ten per millimetre, and preferably fourteen to twenty per millimetre. According to a preferred embodiment, the number of electrodes 4 is equal to sixteen per millimetre, the electrodes 4 having a width of about 40 micrometers and the distance between the electrodes being about 20 micrometers. The electronic control blocks 3 each comprise a plurality of integrated circuits known, for example, from video display techniques, comprising a serial-in parallel-out shift register, an output register and connected thereto drivers with a voltage range of 15 to 25 volts for example. Each electrode 2 is connected to a driver of one of the integrated circuits provided. The image-forming element according to the invention can be made by applying an electrically conductive metal layer, e.g. copper, to a drum having an insulating surface, or having a conductive surface provided with an insulating layer, in known manner, e.g. by vapour-coating or electroplating, and then converting this metal layer to a pattern of electrodes 2 extending transversely, e.g. by the use of a known photo-etch technique. That part of the drum surface, on which the peripherally extending electrodes 4 should be disposed is then covered with an insulating layer and perforations are formed in this insulating layer, e.g. by burning-in with a laser beam, at the place where the electrically conductive connections 5 are to be formed between the electrodes 2 and the electrodes 4 still to be applied. The perforations may alternatively be formed photographically by covering the drum surface provided with the electrodes 2 with a light-sensitive layer of varnish, exposing this layer of varnish to light except for the places where the perforations 5 are to be formed, and removing the unexposed parts of the layer of varnish by means of a suitable solvent. The exposed layer of varnish then acts as an insulating intermediate layer. After the perforations have been formed in the insulating layer, a conductive metal layer is applied over this insulating layer, the perforations being filled at the same time. This metal layer can be applied in the same way and can consist of the same material as the metal layer from which the electrodes 2 were formed. The peripherally extending electrodes 4 are then formed from this metal layer, e.g. again by using a known photo-etch technique, each electrode 4 of course being formed where an electrical connection is achieved between the metal layer and one of the electrodes 2 situated there-beneath. Finally, that part of the drum 1 provided with the electrodes 4 is covered with a smooth dielectric layer so that the electrodes 4 are completely insulated from one another. The electronic blocks 3 for selectively controlling the electrodes 2 are then secured to the side of the drum by fixing techniques known per se. The insulating layer which separates the electrodes 2 from the electrodes 4 is, for example, of a thickness of at least 5 micrometers and has a breakdown voltage of, for example, of 100 V or more. The layer can be formed by means of known insulating materials. A suitable material for forming this insulating layer is epoxy resin, e.g. Epo-tek type 360 or 353 ND made by Messrs. Epoxy Technology Inc. The dielectric top layer applied over the electrodes 4 preferably has a thickness of just a few tenths of a micrometer, (e.g. 0.2 to 0.8 micrometers). Suitable dielectric materials for forming this layer are known, inter alia , from microelectronics. In the illustrated embodiment of the invention the electronic blocks 3 for controlling the electrodes 2 are disposed along one side of the drum. It will be apparent that these blocks can be distributed also over both sides of the drum 1. The fact that the electronic components are disposed on the outer surface of the drum 1 has the advantage that they are readily accessible and can therefore readily be replaced in the event of a fault. It is also possible to dispose the electronics for controlling the electrodes 2 inside the drum 1 and to connect the electrodes 2 to the electronics via the sides of the drum by separate connecting leads. The electrodes 2 extending transversely to the drum 1 need not extend axially nor need they cover all the complete working width of the drum 1. Of course they need only extend to the place where the electrically conductive connection is established with the electrode 4 thereabove. Fig. 2 diagrammatically illustrates a printer equipped with an image-forming element according to the invention, which element has the reference 10 in this Figure. In an image-forming station 11 a magnetic roller 12 is disposed a short distance from the surface of the image-forming element 10 and comprises a rotatable electrically conductive non-magnetic sleeve and an internal stationary magnet system. The rotatable sleeve of the magnetic roller 12 is covered with a uniform layer of electrically conductive and magnetically attractable toner powder which in an image-forming zone 13 is in contact with the image-forming element 10. By applying a voltage between the magnetic roller 12 and one or more of the selectively controllable electrodes of the image-forming element 10 a powder image is formed on the image-forming element 10. This powder image is transferred by pressure to a heated rubber-covered roller 14. From a stock pile 26 a sheet of paper is taken by roller 25 and fed via guideways 24 and rollers 22 and 23 to a heating station 19. The heating station 19 comprises a belt 21 trained about a heated roller 20. The sheet of paper is heated by contact with the belt 21. The sheet heated in this way is then fed through rollers 14 and 15, the softened image present on roller 14 being completely transferred to the sheet of paper. The temperatures of the belt 21 and the roller 14 are so adjusted to one another that the image fuses on the sheet of paper. The sheet of paper provided with an image is fed via conveyor rollers 17 to a tray 18. Unit 30 comprises an electronic circuit which converts the optical information of an original into electrical signals which are fed, via leads 31 having slide contacts, and conductive tracks 32 in the insulating side wall of image-forming element 10, to the electronic blocks 3 connected to the tracks 32. The information is fed serially line by line to the shift register of the integrated circuits on the blocks 3. If the shift registers are completely filled in accordance with the information of one line, that information is put into the output register and via the drivers the electrodes 2, 4 are actuated or not dependent on the signal. While this line is being printed the information of the next line is being fed to the shift registers. Apart from optical information originating from an original, electrical signals originating from a computer or a data processing device can also be converted in unit 30 to signals which are fed to the electronic blocks 3. In the printer represented in Fig. 2, the electrically conductive magnetically attractable toner powder is fed to the image-forming zone 13 by the magnetic roller 12. It will also be clear that the toner powder can also be applied in a uniform layer to the image-forming element 10 and then be selectively removed therefrom in the image-forming zone 13 as described in the above-mentioned US Patent 3 946 402. Other variants of the invention will be apparent to the skilled addressee but they all come under the invention as described in the following claims.

Claims: An image-forming element for an electrostatic printer, consisting of an endless support (1) with a dielectric surface layer thereon first electrodes (4) being provided beneath the dielectric surface layer, said electrodes being insulated from one another and extending in the form of endless paths parallel to one another in the peripheral direction i.e. the direction of the endless path of the support, characterised in that second electrodes (2) are provided beneath the said first electrodes (4) and are also insulated from one another and extend each from a point near one lateral edge of the support (1) in the direction of the other lateral edge of the support (1), each one of said second electrodes (2) being electrically conductively connected to a respective first electrode (4) and to means (3) for supplying voltage to said electrodes. An image-forming element according to claim 1, characterised in that the second electrodes (2) extend in a direction perpendicular to the peripheral direction of the support. An image-forming element according to claim 1 or 2, characterised in that the second electrodes (2) are embedded in an insulating layer. An image-forming element according to claim 1, characterised in that the means (3) for supplying voltage to the electrodes are disposed on one or both edges of the support (1). A device for printing information, comprising a movable image-forming element (10) with a dielectric surface, an image-forming station (11) situated along the trajectory of the image-forming element, in which a magnetic roller (12) having an electrically conductive sleeve is disposed near the surface of the image-forming element (10) and means (3) for generating an electric field according to an information pattern between the image-forming element (10) and the magnetic roller (12), while electrically conductive magnetically attractable toner powder is fed to the zone between the image-forming element (10) and the magnetic roller (12), characterised in that the image-forming element is provided according to one of the preceding claims 1 to 4.

Inventors: LA VOS, PETER GEORGE, PANNEKOEK, REINDER, VAN GENUCHTEN, ADRIANUS JOHANNES MARIA
Applicants: OCE-NEDERLAND B.V.
Application year: 1987
Main classification: G03G17
IPC codes: G03G17, G03G15, B41C1, B41J2