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 diëlectric surface layer and a number of image-forming electrodes (2) beneath the surface of said layer. In the zone of the image-forming element that is used for the image formation, the electrodes (2) extend parallel to one another, each according to a helix having a pitch which is at least equal to the working width of the image-forming element. Each electrode (2) is connected to means (5) for supplying voltage to the electrodes (2), the means (5) being disposed on the support (1) out­ side the zone used for the image formation. An electrostatic printer 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 image forming electrodes provided beneath the dielectric surface layer and extending parallel to one to another in accordance with a helix. 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. Japanese Application 59-224369 discloses an image-forming element having a cylindrical support with a dielectric surface layer and image-forming electrodes beneath the dielectric surface layer which electrodes extend parallel to one another in accordance with a helix or as endless circular paths in the peripheral direction of the support. As disclosed in this application the electrodes are connected with means for supplying a voltage thereto, which means are disposed within the interior of the cylindrical support. A disadvantage of this construction is that the voltage supplying means are not readily accessible for maintenance or replacement of faulty components. 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 in the zone used for image-formation, the helix according to which the electrodes extend has a pitch that is at least equal to the working width of the image-forming element and in that the electrodes are connected to means for supplying voltage thereto, said means being disposed on the support outside the zone used for image formation. The term "working width" of the image-forming element denotes the width of that part of the image-forming element which is used for image formation as considered in a direction perpendicular to the direction of movement. In the image-forming element according to the invention the electrodes are completely insulated from one another so that shortcircuiting 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 the invention, the electric facilities for energising the electrodes in accordance with an information pattern to be printed are disposed outside the zone used for the image formation near one or both ends of the support and on the periphery thereof. The electronic facilities can thus be secured fairly simply, and, what is particularly important, they are now readily accessible for maintenance or for the replacement of faulty components. The following relation applies to the angle of inclination (α) of the helix along which the image-forming electrodes extend, the working width (W) of the image-forming element and its diameter (d): in which F is the ratio between the helix pitch and the working width of the image-forming element. The values of F, W and d are preferably so selected that the angle of inclinations α is between 35° and 55° and is preferably about 45°. An angle of about 45° is preferred from the aspect of forming image characters (fonts). In practical situations the working width W is determined by the requirement made in respect of the largest (widest) image format that is to be reproduced, while the diameter d is frequently determined from constructional considerations. To achieve the preferred angle of inclination of, for example, 45°, the value F then has to be calculated in accordance with the above formula. Thus in the case of an image-forming element having a working width of 330 mm and a diameter of 110 mm, the value of F must be 1.05 to achieve an angle of inclination of about 45°. 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 an image-forming element according to a preferred embodiment of the invention. Fig. 2 is a diagrammatic representation of the way in which images are printed by means of the image-forming element according to the invention. Fig. 3 is a drawing representing the principle of an electrosta-tic 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 electrodes 2 are disposed, which extend in parallel relationship. In that part of the drum surface which is situated between the dotted lines 3 and 4, i.e. the part of the drum 1 which is used for image-formation, each electrode 2 extends along a helix whose pitch is equal to the working width of the image-forming zone on the drum 1, i.e. equal to the distance between the dotted lines 3 and 4. In Fig. 1, for the sake of clarity, only a few electrodes 2 are shown completely. The angle of inclination (α) of the helix followed by the electrodes 2 is about 45° in the embodiment illustrated. The electrodes 2 are covered with a dielectric layer which, however, has been omitted from Fig. 1 for the sake of clarity. Each electrode 2 is connected to one of the blocks 5 disposed on one side, outside the image-forming zone, on the drum surface and which contain the electronic facilities for applying voltage to the electrodes selectively in accordance with an information pattern. The blocks 5 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 quality of the images formed on the image-forming element depends, inter alia , on the number of electrodes 2. As the electrode density increases so the image quality improves. The number of electrodes as considered in a direction perpendicular to the circumferential direction of drum 1 is preferably at least ten per millimetre, preferably fourteen to twenty per millimetre. 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 extending helically, e.g. by the use of a known photo-etch technique or by burning-in with a laser beam. The drum surface covered with the electrodes, or at least that part of the drum surface which forms the image-forming zone, is then provided with a dielectric layer, which 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 5 for controlling the electrodes 2 are disposed along one side of the drum outside the image-forming zone. 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. Fig. 3 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 roller 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 5 connected to the tracks 32. The information is fed serially, line by line, to the shift register of the integrated circuits on the blocks 5. 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 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. Since the electrodes 2 extend in accordance with a helix in the image-forming zone of the image-forming element 10, the helix pitch being equal to the working width of the image-forming element, only one small zone of each electrode 2 is ever present in the image-forming zone 13. During the printing of a line of information each electrode 2 will therefore print one image-dot of that line. Because of the helical path of the electrodes 2 the image dot originating from the same electrode 2 will be pushed up one place in two lines written in succession. The direction in which the image dot is shifted depends on the direction of rotation of the helix. For example, the electrode 2 which provides the first image dot during the printing of the first line will successively provide the second, the third, the fourth ... and the last image dot during the printing of the following lines. The electrode 2 which provides the last image dot of the first line then provides the first, the second ... and the last image dot but one. After each complete revolution of the image-forming element 10 the situation is again the same as that at which the first image line was written. Fig. 2 diagrammatically represents how the image dots of the electrodes 2 which provided the first and last image dots in the first line shift in successive lines. The successively printed lines of information are represented as zones 40-48. Line 51 represents the collection of image dots provided by the electrode 2, which provided the first image dot of the first line and line 52 and point 53 represent the image dots which were provided by the electrode 2 which provided the last image dot in the first line. After each revolution of the image-forming element 10 the situation is again equal to the starting situation. On the start of each printing cycle (i.e. when the first line of information is to be printed) the unit 30 must be informed about the position of the image-forming element 10 with respect to the image-forming zone 13 to be able to fill the shift registers correctly. In the device represented in Fig. 3 each printing cycle is started when the image-forming element 10 has reached a fixed starting position. The starting signal for writing the first line is derived by unit 30 from a detection signal originating from a detector not shown in Fig. 3, which detects a mark 33 provided on the image-forming element 10. The electronic circuit of unit 30 also ensures that the shift registers of the blocks 5 are correctly filled when the consecutive lines are written in. 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 5. In the printer represented in Fig. 3, 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 and image-forming electrodes (2) provided beneath the dielectric surface layer and extending parallel to one another in accordance with a helix characterised in that in the zone used for image formation, the helix according to which the electrodes (2) extend has a pitch that is at least equal to the working width (W) of the image-forming element and in that the electrodes (2) are connected to means (5) for supplying voltage thereto, said means (5) being disposed on the support (1) outside the zone used for image-formation. An electrostatic printer 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 (5) for generating an electric field, in accordance with 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 (13) between the image-forming element (10) and the magnetic roller (12), characterised in that an image-forming element according to claim 1 is provided.

Inventors: PANNEKOEK, REINDER, VAN STIPHOUT, JOHANNES G. V.
Applicants: OCE-NEDERLAND B.V.
Application year: 1987
Main classification: G03G17
IPC codes: G03G15, B41J2