A PRINTING DEVICE WITH AN IMAGE-RECEIVING SUPPORT AND A NUMBER OF IMAGE-FORMING STATIONS DISPOSED ALONG THE PATH OF ROTATION OF THE IMAGE-RECEIVING SUPPORT

Abstract: Printing device comprising a cylindrical image-receiving support (1) and a number of image-forming stations (4,5,6) which are disposed along the rotational path of the image-receiving support and which each generate a separation image of the required image and transfer said separation image to the image-receiving support (1) in an image-­ transfer zone and each comprise a rotatable cylindrical image-­ registration element (7) and imaging means (8) to form the separation image on the image-registration element (7). As considered along the circumference of the image-registration ele­ ment (7) and in the direction of movement thereof, there is an equal distance in each image-forming station (4,5,6) between the place where the separation image is formed on the image-registration element (7) and the centre of the image-transfer zone in which said separation image is transferred to the image-receiving support (1). A control means (28,10) derive the start time of the image registra­ tion in each image-forming station (4,5,6) from the position of the image-receiving support (1) with respect to that image-forming sta­ tion, as a result of which the separation images are transferred in correct register to the image-receiving support (1). Another control means controls the temperature of each image-registration element.

Description: This invention relates to a printing device with an image-receiving support in the form of a rotatable cylinder and a number of image-forming stations which are disposed along the rotational path of the image-receiving support and which each generates a separation image of the required image and transfers said separation image to the image-receiving support in an image transfer zone and each comprises a rotatable cylindrical image-registration element and imaging means to form the associated separation image on the image-registration element. A printing device of this kind is known from Japanese Patent Application No. 58/44459. In this know printing device, there is an increasing distance, in the successive image-forming stations, between the place where a separation image is formed on the image-registration element and the place where that separation image is transferred to the image-receiving support. The difference in the distance at two successive image-forming stations is equal to the distance between the image-transfer places of those image-forming stations measured along the circumference of the image-receiving support. As a result of this arrangement, the start signal for forming the separation images can be given simultaneously for all the image-forming stations. A disadvantage of the device is that its various parts, and particularly the image-registration elements and the image-receiving support, must satisfy high accuracy requirements to obtain good register accuracy in the transfer of the separation images to the image-receiving support. UK Patent 1 277 233 describes a printing device which comprises only one image-forming station for forming the separation images. The various separation images are produced in consecutive cycles and transferred successively to the image-receiving support. Although high register accuracy can be obtained with this device it has the disadvantage that the separation images are formed consecutively so that the printing process is time-consuming. Another disadvantage is that the device cannot be used to print images longer than the circumference of the image-receiving support. The object of the invention is to obviate the above disadvantages, and to this end the invention provides a device according to the preamble, characterised in that the imaging means are so disposed in the image-forming stations with respect to the image-registration element that there is an equal distance in each image-forming station, as considered along the circumference of the image-registration element and in the direction of movement thereof, between the place where the separation image is formed on the image-registration element and the centre of the image-transfer zone between the image-registration element and the image-receiving support, and in that control means are provided which derive the start time of the image registration in each image-forming station from the position of the image-receiving support with respect to that image-forming station. According to the invention, high register accuracy is obtained on the transfer of the separation images to the image-receiving support, while it is also possible to print images which are longer than the circumference of the image-receiving support. It has surprisingly been found that inaccuracies in the shape of the image-receiving support do not affect the register accuracy and that the image-registration elements are not required to satisfy any extreme requirements in respect of accuracy of shape and the tolerance range required for these components is acceptable in practice. Another advantage of the printing device according to the invention is that the register accuracy is independent of the location of the imaging means around the image-receiving support and of the relationship between the diameter of the image-receiving support and the diameter of the image-registration elements and of the mutual relationship of the diameters of the image-registration elements. As a result, there are wide choices open for the printing device construction. According to a preferred embodiment of the invention, the cylindrical image-registration elements have the same diameter, are driven at the same circumferential speed and the signals for controlling the imaging means are generated by a pulse transmitter connected to the image-receiving support. This gives a simple printing device construction. According to another preferred embodiment of the invention, each image-registration element is driven by a gearwheel directly coupled to a gearwheel on the shaft of the image-receiving support, and the gearwheels coupled to the image-registration elements are so secured that the tooth having the largest eccentricity in each gearwheel occupies the same position with respect to the gearwheel on the shaft of the image-receiving support. This gives a further improvement of register accuracy. According to yet another preferred embodiment of this latter embodiment, the image-registration elements are in pressure contact with the image-receiving support and the transmission ratio in the drive between the image-receiving support and the image-registration elements is somewhat greater than the transmission ratio which would occur if the image-registration elements were driven solely by friction as a result of the pressure contact with the image-receiving support. As a result, the same gearwheel always operates as the driving gearwheel, thus obviating speed varations due to tooth clearance. This invention and its advantages will be explained in greater detail with reference to the accompanying drawing which diagrammatically illustrates a printing device according to a preferred embodiment of the invention. The printing device as illustrated in the drawing comprises a cylindrical image-receiving support 1, the metal sleeve of which (e.g. of aluminium) is coated with a layer of silicone rubber 2. The image-receiving support 1 can be driven in the direction of arrow 3 by drive means (not shown). Image-forming stations 4, 5 and 6 are disposed along the path of rotation of the image-receiving support 1. Each of these image-forming stations comprises a cylindrical image-registration element 7 and imaging means 8 to form a separation image on the image-registration element 7. The image-registration elements 7 have the same diameter (± 0.1 mm) and are all in pressure contact with the image-receiving support 1, the pressure in the three pressure zones being substantially identical and such that a separation image formed on the image-registration elements is transferred, by this pressure, to the silicone rubber surface 2 of the image-receiving support 1. Each image-registration element 7 consists of a cylinder having an insulating surface layer on which a large number of electrodes is disposed, the electrodes being insulated from one another and extending as endless paths in parallel relationship in the circumferential direction of the cylinder. A thin closed dielectric top layer is applied over the electrodes and the parts of the insulating layer situated therebetween. All the electrodes are connected via perforations in the cylinder wall to an electronic circuit 9 mounted inside the cylinder for supplying a voltage to the electrodes in accordance with an information pattern to be printed. The electronic circuit 9 in each image-registration element 7 is in turn connected to a central control unit 10 which feeds the information concerning the separation image to be printed to each electronic circuit 9 line by line. The imaging means 8 each comprise a magnetic roller disposed at a small distance (± 0.2 mm) from the circumference of the imageregistration element 7, said roller having a rotatable sleeve 11 of electrically conductive diamagnetic material (e.g. copper). A stationary magnetic system is disposed inside the sleeve 11 and consists of magnets 12, 13, 14, 15, 16 and a soft-iron blade 17 clamped between like poles of the magnets 15 and 16. The point of the soft-iron blade 17 is located at the place where there is the minimum distance between the sleeve 11 and the surface of the image-registration element 7. This place is the place where the separation engage is formed on the image-registration element 7. (Image-registration elements and imaging means as described hereinbefore are illustrated in detail in European Patent Application No. 0 191 521). As considered along the circumference of the image-registration element 7 and in the direction of movement thereof, the distance between the place where the separation image is formed on the image-registration element (and hence the place on the image-registration element situated opposite the point of the blade 17) and the centre of the pressure contact zone between the image-registration element 7 and the image-receiving support 1 is identical in all the image-forming stations. At the sleeve 11 of each magnetic roller there is disposed a reservoir 18 filled with electrically conductive magnetically attractable toner powder. A stripper 19 is provided at each reservoir 18 to see that an even layer of toner powder is applied to the sleeve 11 of the magnetic roller. Also disposed along the path of rotation of the image-receiving support 1 are a radiant heater 20, means for supplying a sheet of image-receiving material, such means consisting of co-operating conveyor rollers 21 and a guide plate 22, a pressure roller 23, discharge means for the sheet of image-receiving material, consisting of co-operating conveyor belts 24 en 25, and a cleaning device 30. Each image-registration element 7 is driven by a gearwheel 26 mounted on the rotational shaft of the image-registration element 7 and engaging in a gearwheel 27 secured to the driven shaft of the image-receiving support 1. (In the drawing, the gearwheels 26 and 27 are shown in the form of interrupted circles, these circles indicating the pitch circle of each gearwheel). According to a preferred embodiment of the invention, the gearwheels 26 are so mounted that the tooth having the largest eccentricity in each gearwheel 26 occupies the same position with respect to the gearwheel 27 so that these teeth always engage identically in the gearwheel 27. The transmission ratio between the gearwheel 27 and the gearwheel 26 is so selected that it is somewhat greater than the transmission ratio that would occur if the image-registration elements 7 were driven by friction as a result of the pressure contact with the image-receiving support. This somewhat greater transmission ratio is obtained by making the diameter of the pitch circle of gearwheel 27 slightly larger than the outside diameter of the image-receiving support 1 and the diameter of the pitch circle of the gearwheels 26 slightly smaller than the outside diameter of the image-registration elements. For example, the diameter of the image-receiving support 1 and the image-registration elements 7 are 269 mm and 177 mm respectively and those of the pitch circles of the gearwheels 27 and 26 are 270 mm and 176 mm respectively. A pulse generator 28 is connected to the image-receiving support 1 and generates pulses in relation to the angular rotation of the image-receiving support 1. The angular rotation between consecutive pulses corresponds to a movement of the surface of the image-receiving support 1 over the width of one image line. The recording of the consecutive image lines on the image-registration elements 7 by the imaging means 8 can thus be controlled by the control unit 10 by means of the pulses which are delivered by the pulse generator 28 and are fed to the control unit 10 via the connection 29. When the printing device is in operation, the image-receiving support 1, the image-registration elements 7 and the sleeves 11 of the imaging means 8 are driven in the direction indicated by the arrows 3, 31 and 32. The sleeves 11 are provided with a thin even layer of toner powder. In the zone between the sleeves 11 and the image-registration elements 7, at the soft-iron blade 17, as a result of the strong magnetic field emerging there, a compact toner brush is formed which comes into contact with the image-registration element 7. If no voltage is applied to the electrodes beneath the surface of the image-registration elements 7, then no toner powder is deposited on the image-registration elements. By the selective application of a voltage to the electrodes in accordance with an image pattern, a powder image pattern is formed on the image-registration elements 7. The information concerning the image lines of the various separation images which are to be recorded is fed line by line serially to a shift register of the electronic circuits 9 by the control unit 10. On the subsequent reception of the next pulse from the pulse generator 28 the information stored in the shift register of the first image-forming station 4 is transferred to an output register and certain electrodes receive voltage via drivers in accordance with the image line which is to be recorded. In the meantime the shift register is filled with the information of the next image line. On reception of the next pulse from the pulse generator 28 this image line is recorded. On reception of a given pulse from pulse generator 28 the imaging means of the second image-forming station 5 are also activated and a number of pulses thereafter those of the next image-forming station 6 too. The number of pulses after which the imaging means of the second and the next image-forming station or stations are activated is predetermined from the distance between the image-forming stations as considered along the circumference of the image-receiving support 1. The correct number of pulses is fixed in a control programme stored in a memory of the control unit 10. The separation images formed on image-registration elements 7 are transferred to the image-receiving support 1 in register in the various pressure contact zones. The image-receiving support 1 provided with the powder image then passes through a heating zone where the powder image is softened by the radiant heater 20. The softened powder image is then transfered to an image-receiving material (e.g. a sheet of paper) in the pressure zone between the image-receiving support 1 and the pressure roller 23, the said image-receiving material having been fed via the feed means 21, 22 at the correct time. The printed image-receiving material is discharged by the co-operating conveyor belts 24 and 25. The image-receiving support 1 then moves past the cleaning device 30. Various variants are possible in respect of the above-described preferred embodiments of the invention. For example, a toothed belt or chain drive or a drive using friction rollers can be used instead of the gearwheel drive with directly coupled gearwheels (26, 27). The gearwheel drive illustrated is preferred because of the simplicity of construction and the greater accuracy and durability in comparison particularly with a drive using friction rollers. The image-registration elements 7 are preferably so mounted in the printing device in a manner known per se that when the printing device is switched off or is in the standby position said elements are released from the image-receiving support 1 but the gearwheels 26 and 27 continue to engage in order to maintain the correct positioning of the gearwheels 26 with respect to the gearwheel 27. When an image is required to be printed, the image-registration elements 7 are again brought into pressure contact with the image-receiving support 1, a possible construction being one in which all the image-registration elements 7 are pressed into contact or only those which are required to come into operation in image forming. Instead of image-registration elements 7 with the same diameter it is possible to use image-registration elements with different diameters driven at the same circumferential speed. There must then also be the same distance, at all the image-forming stations, as considered along the circumference of each image-registration element and in the direction of movement thereof, between the place where the image is formed on the image-registration element and the centre of the image-transfer zone. The time at which image formation is to be started at each imaging station can also be determined by means of detectors disposed along the path of rotation of the imaging-receiving support, such detectors detecting a marking on the image-receiving support. Recording of the consecutive image lines can then be further controlled by means of pulses generated by pulse generators connected to the rotating image-registration elements. Other imaging means known per se can also be used instead of the imaging means illustrated hereinbefore. For example, it is possible to use an electrode system which applies an electrostatic charge pattern to the image-registration elements, the charge pattern then being developed with toner powder to form a toner image. When the printing device is in operation for a relatively long period, the image register in the direction perpendicular to the direction of conveyance may become less accurate due to a difference in thermal expansion of the image-registration elements 7 as a result of unequal heating of the image-registration elements by the radiant heater 20. To obviate this register error, the printing device can be provided with a temperature control which prevents an excessive temperature difference between the image-registration elements 7 or eliminates it if it occurs. The temperature control comprises temperature sensors to measure the temperature of each image-registration element and a measuring and control unit which determines the temperature difference between the hottest and coldest image-registration element and compares it with a reference value. If the difference is greater than the reference value, the temperature of the image-registration elements is adapted to bring the difference beneath the reference value. To control the temperature of the image-registration elements, cooling means are provided, for example, whose cooling intensity can be varied separately for each image-registration element. For example, a fan with an (infinitely) variable speed or a fan (possibly with a fixed speed) which is switched on when the associated image-registration element requires cooling, can be provided at each image-registration element. The temperature control is carried out by bringing the temperature of (excessively hot) image registration elements back to the lowest measured temperature value. It is also possible to provide a circuit which releases the start of a printing process only if the temperature difference between the image-registration elements 7 is below the reference value. Other variants of the invention will be apparent to the skilled addressee. They all, however, will come under the invention as defined in the following claims.

Claims: A printing device with an image-receiving support (1) in the form of a rotatable cylinder and a number of image-forming stations (4, 5, 6) which are disposed along the rotational path of the cylinder and which each generates a separation image of the required image and transfers said separation image to the image-receiving support (1) in an image transfer zone and each comprises a rotatable cylindrical image-registration element (7) and imaging means (8) to form the associated separation image on the image-registration element (7), characterised in that the imaging means (8) are so disposed in the image-forming stations (4, 5, 6) with respect to the image-registration element (7) that there is an equal distance in each image-forming station, as considered along the circumference of the image-registration element (7) and in the direction of movement thereof, between the place where the separation image is formed on the image-registration element (7) and the centre of the image-transfer zone between the image-registration element (7) and the image-receiving support (1), and in that control means (28, 10) are provided which derive the start time of the image registration in each image-forming station (4, 5, 6) from the position of the image-receiving support (1) with respect to that image-forming station. A printing device according to claim 1, characterised in that the cylindrical image-registration elements (7) have the same diameter and are driven at the same circumferential speed and in that a pulse generator (28) is connected to the image-receiving support (1) to generate signals for controlling the imaging means (8). A printing device according to claim 1 or 2, characterised in that each image-registration element (7) is driven by a gearwheel (26) directly coupled to a gearwheel (27) on the rotational shaft of the image-receiving support (1). A printing device according to claim 3, characterised in that the gearwheels (26) coupled to the image-registration elements (7) are so secured that the tooth having the largest eccentricity in each gearwheel (26) occupies the same position with respect to the gearwheel (27) on the shaft of the image-receiving support (1). A printing device according to claim 3, characterised in that the image-registration elements (7) are in pressure contact with the image-receiving support (1) and in that the transmission ratio in the drive between the image-receiving support (1) and the image-registration elements (7) is somewhat greater than the transmission ratio which would occur if the image-registration elements (7) were driven solely by friction as a result of the pressure contact with the image-receiving support (1).

Inventors: KLEIN MEULEMAN, PETER, LA VOS, PETER GEORGE
Applicants: OCE-NEDERLAND B.V.
Application year: 1988
Main classification: G03G15
IPC codes: G03G15