Image-forming device

Abstract: An image-forming device for recording images on a dielectric surface (41) of an image-recording medium (10), in which the dielectric surface (41) is in contact, in an image-forming zone (90), with developing powder bound to a powder support (84) by a magnet system (87). The magnet system (87) comprises two oppositely magnetised areas (85, 88) separated by a gap and, as considered in the direction of transport of the image-recording medium (10), disposed consecutively. The distance between at least one of the magnetised areas (85, 88) and the surface of the powder support (84) is less than 150 µm and the distance between the same magnetised area (85, 88) and the surface of the image-recording medium (10) is less than 600 µm. This raises the background-free level while less magnetic material is required for the magnet system (87) than in comparable prior-art devices.

Description: The invention relates to an image-forming device for recording images on an image-recording medium having a dielectric surface, the device comprising an image-forming zone in which the dielectric surface is in contact with developing powder bound to a powder support by the magnetic attraction of a magnet system co-operating with the powder support and in which a powder image is recorded on the dielectric surface by applying an electric field across the image-forming zone in accordance with an image pattern. Image-forming devices of the above kind are described, inter alia, in European patent applications 0 191 521 and 0 304 983. As explained in these patent applications, to achieve good image quality it is important that the developing powder mass (hereinafter referred to as the "toner brush") which, in the image-forming zone, is bound by magnetic attraction to the powder support acting as the powder transport means, should have a shape substantially remaining constant. A constant brush shape is important particularly on the side where the image-recording medium leaves the image-forming zone. A constant toner brush can be produced by a magnet system of the kind described in the above-mentioned European application 0 304 983. The construction of the magnet system described there has the disadvantage that to enable use to be made of developing powder having a low percentage of magnetic pigment by volume, it is necessary to use very strong magnets in order to achieve an adequately strong magnetic field in the image-forming zone via the knife blade. The strong magnets required, which in addition have to be used in duplicate (one on each side of the knife blade) make the magnet system expensive and bulky. In addition, the device has the disadvantage that if toner powders are used which contain only little magnetic pigment there is a fairly low background-free level. The term "background-free level" denotes the maximum voltage across the image-forming zone at which no toner deposition on the image-recording medium is observed. The disadvantage of a low background-free level is that, for example, as a result of low tribo-electric charging effects between the toner brush and the dielectric surface of the image-recording medium, the finer toner particles from the toner brush are deposited on the image-recording medium, so that images are recorded with a background. The object of the invention is to provide an image-forming device of the type referred to in the preamble equipped with a simpler and hence cheaper magnet system and having a higher background-free level, under comparable conditions, than the prior art devices, more particularly as described in the above-mentioned European patent applications. According to the invention this object is attained in that the magnet system comprises two magnetised areas which opposite to the image-forming zone, are separated by a gap and disposed consecutively as considered in the direction in which the image-recording medium is transported through the image-forming zone, which areas have opposite magnetic poles at their ends terminating in the gap and which extend substantially parallel to one another over the entire working width of the image-forming zone, the distance between at least one of said magnetised areas and the surface of the powder support being less than 150 micrometers and the distance between the same magnetised area and the surface of the image-recording medium being less than 600 micrometers. Thus according to the invention a higher background-free level is achieved than in comparable devices as described in the above-mentioned European patent applications, while a more compact and cheaper construction of the magnet system is obtained, which for an optimum result in respect of constant shape of the toner brush and maximum background-free level may comprise much smaller magnets than are required in the prior-art devices to achieve an optimum result. The result obtained according to the invention is surprising, because it would be expected that a magnet system made up of a knife blade directed towards the image-forming zone and magnetised by strong magnets to saturation would generate a higher magnetic force in the image-forming zone than a magnet system constructed from two opposite magnetic poles separated by a gap, with which it would be expected that the magnetic field would extend primarily in the gap and have hardly any effect at a short distance above the gap. According to a preferred embodiment of the invention, the width of the gap between the magnetised areas is 0.5 to 20 times the distance between the magnetised area closest to the powder support and the surface of the image-recording medium. Preferably, the gap width is 1 - 2 times this distance, because then the most compact construction can be obtained for the magnet system with optimum image quality and maximum background-free level. Preferably, the magnetised areas consist of ferromagnetic material and are preferably magnetised by magnetic connection to the north and south poles respectively of a permanent magnet being a constituent of the magnet system. The form and magnetisation of the magnetised areas are preferably such that their ends terminating in the gap are magnetically saturated. Instead of a magnet system made up of a permanent magnet whose poles are connected to magnetisable elements terminating in the gap, equally good results can be obtained with a yoke of ferromagnetic material magnetised by energisation of a coil wound around the yoke. This electromagnetic embodiment, however, requires a fairly high current to achieve the same magnetisation, and undesirable heat evolution may occur as a result. For this reason and because of the lower cost price, the embodiment with a permanent magnet is preferred. The invention is explained in detail below with reference to the following description and the accompanying drawings wherein: Fig. 1 is a diagram showing the principle of an image-forming device according to the invention, Fig. 2 is a cross-section of a preferred embodiment of an image-forming zone according to the invention, Figs. 3 to 6 are sections of different embodiments of magnet systems for use in the image-forming device according to the invention. Fig. 1 is a diagram showing the principle of an electrostatic printing device having an image-forming medium in the form of a rotating drum 10 provided with an electrostatic layer made up of a number of controllable electrodes in and beneath a dielectric layer. The image-forming medium may, for example, be constructed as described in one of the European applications 0 191 521, 0 247 694 or 0 247 699. The term "working width" as used hereinafter has the meaning as defined in European application No. 0 247 694, column 2, lines 19-23. In an image-forming station 11 a magnetic roller 12 is disposed a short distance from the surface of the image-forming medium 10 an comprises a rotatable electrically conductive 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 medium 10. By applying a voltage between the magnetic roller 12 and one ore more of the selectively controllable electrodes of the image-forming element 10 a powder image is formed on the image-forming medium 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 powder image present on roller 14 being completely transferred onto 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 medium 10, to the controllable electrodes (not shown). Fig. 2 is a cross-section of an image-forming medium 10 in the form of a drum 36 rotatable in the direction of arrow 35 and provided with an insulating layer 43 on which there are disposed a large number of electrodes 42 which are disposed next to one another, are insulated from one another, extend endlessly in the direction of movement of the drum, and are covered by a dielectric layer 41. Magnetic roller 84 comprises an earthed electrically conductive sleeve 92 rotatable in the direction of arrow 89 about a stationary magnet system 87. A uniform layer of conductive magnetic toner is applied to the dielectric layer 41 by means of a toner feed device. This feed takes place by means of a magnetic cylinder 130. The latter comprises a sleeve 131 of non-magnetisable material, e.g. aluminium, brass or stainless steel. This sleeve 131 is mounted on a shaft 132 in known manner so as to be rotatable and can be driven in the direction of arrow 133 by drive means (not shown). A number of magnets 135 are mounted on the shaft 132 of the magnetic cylinder 130, said shaft 132 being fixed in the frame of the printing device. Under the influence of the magnets 135 a magnetic field is produced at the surface of the sleeve 131. Magnetically attractable toner powder is applied from a reservoir 136 to the sleeve 131 of the magnetic cylinder 130 and retained thereon by the magnetic field. On rotation of the sleeve 131 in the direction of arrow 133, a layer of toner powder restricted to a specific layer thickness by a scraper 137, is conveyed to a transfer zone between the image-forming medium 10 and the magnetic cylinder 130. A uniform layer of toner powder is then transferred to the dielectric layer 41 under the influence of an electric field applied in known manner across the transfer zone. The magnets 135 of the magnetic cylinder 130 must on the one hand satisfy the requirement that the magnetic induction should be high enough to generate a magnetic field at the surface of the sleeve 131 such that a layer of toner powder be retained and be driven by the rotating sleeve 131 without causing dust problems. The magnetic induction is therefore determined by toner powder parameters and the speed of revolution of the magnetic cylinder 130. On the other hand, the magnetic induction of the magnets should not be too high in order that the layer of toner powder may be easily transferred in the transfer zone to the dielectric layer 41 without the need to apply a very strong electric field. These contradictory requirements can be met in two ways. Firstly, by selecting an optimum magnetic induction for the transfer function for the magnet 135 which determines the field strength in the transfer zone, and an optimum magnetic induction in respect of the toner transport function for all the other magnets. Of course, a compromise can be selected in which the same magnetic induction forming a compromise for both functions is selected for all the magnets 135. A third function of the magnetic cylinder 130 is that toner powder remaining on the sleeve 92 of the magnetic roller 84 after passing the image-forming zone 90 is attracted by the magnetic field of the magnetic cylinder 130 and received in the toner powder layer on cylinder 130. As described above, a layer of toner powder is transported to the image-forming zone 90 via the image-forming medium 10, and forms a narrow toner brush there under the influence of the magnetic field of the magnet system 87. The toner brush in the image-forming zone 90 is formed by the magnet system 87, which is shown to an enlarged scale in Fig. 3. The magnet system 87 comprises a permanent magnet 86 consisting, for example, of an alloy of neodynium - iron - boron or samarium - cobalt. Magnetisable elements 85 and 88 are fixed against the poles of the magnet, and their ends not connected to the magnet 86 terminate in a gap 93 and gradually narrow in the direction of gap 93. The magnet 86 and the magnetisable elements 85 and 88 are preferably so dimensioned that the ends of the elements 85 and 88 terminating in the gap 93 are magnetically saturated. The magnetisable material of the elements 85 and 88 consists of ferromagnetic material. Preferably, a material having high saturation magnetisation and high permeability is selected, such as iron - cobalt. Other suitable materials are iron and iron - nickel. In the embodiment of Fig. 2, the magnet system 87 is so positioned in the sleeve 92 that the gap 93, which in this embodiment has a width of 300 micrometers, is situated in the centre of the image-forming zone 90 and the inside of the sleeve 92 touches the magnetisable elements 85 and 88. There is no need for symmetrical positioning of the magnet system 87 with respect to the centre of the image-forming zone 90. It has been found that to obtain good image quality at least one of the magnetised areas 85 or 88 must be situated in the developing zone at a distance of no more than 150 micrometers from the surface of the sleeve 92. Also, the distance between the same magnetised area and the surface of the image-forming medium 10 should not be more than 600 micrometers, preferably about 200 micrometers. If larger distances are applied, then the attraction exerted by the magnet system 87 on the toner particles in the image-forming zone 90 is inadequate to take the toner powder (if no voltage is applied to the image-forming electrodes 42) completely from the image-forming medium, so that images with a background are obtained. The width of the gap 93 between the magnetised areas 85 and 88 is preferably in the range from 1 to 2 times the shortest distance in the image-forming zone between the sleeve of the magnetic roller 92 and the surface of the image-forming medium 10. A wider gap (up to about 20 x this distance) can be used, but a wider gap offers no advantages in respect of the image-forming result obtained and the size of the background-free level, and does have the effect that a larger stronger magnet must be used to achieve a sufficiently strong magnetic field at the ends of the magnetised areas. An arrangement as shown in Fig. 2, in which the magnet 86 consists of a bar magnet of neodynium - iron - boron having a magnetic induction of 1.1 T and a rectangular cross-section of 6 x 4 mm, the elements 85 and 88 consist of iron, the gap 93 has a width of 300 micrometers, and the distance in the image-forming zone between the sleeve 92 and the image-forming medium 10 is set to 200 micrometers, was compared with a device according to Fig. 2 of European patent application 0 304 983, which was set to give optimum image-forming result and background-free level. The device according to the application 0 304 983 was for this purpose equipped with a knife blade of iron having a thickness of 1.5 mm and two bar magnets of the same neodynium - iron - boron alloy as that used for the magnet 86 in the device according to the invention. The distance between the image-forming medium and the magnetic roller in the image-forming zone in the device according to European patent application 0 304 983 was also 200 micrometers. The developing powder was applied to the image-forming medium in the same way in both devices, to wit in the way as described hereinbefore for the device according to the invention. The developing powder used in the comparison had the following composition: thermoplastic polyester resin type Atlac 500T (ICI, England) derived from oxypropylated bisphenol A and fumaric acid. (Atl ac is a trademark), 1% by volume carbonyl iron with a particle size of approximately 2 micrometers (type HS, BASF, Germany), 3% by weight red dye (Basonyl Rot 560-C.I. Basic Violet 11:1) in the form of the perchlorate (Basonyl Rot is a trademark) and had a specific resistance of 10 5 ohm. meter; obtained by coating the powder particles with fluorine-doped tin oxide in accordance with the process of European patent application 0 441 426, and a particle size of 10 - 20 micrometers. Background-free images were formed in the device according to the invention, the background-free level measured being ± 5 volts. In the device according to European application 0 304 983, the optimum setting gave background-free images, but with a background-free level of ± 2. 5 volts. The knife angle α (see European application 0 304 983) at the optimum setting was 10°, while two magnets having a rectangular cross-section of 6 x 15 mm were necessary to achieve the optimum magnetic field. This means that the magnet volume is reduced by a factor of 7.5 in the device according to the invention. In addition, it was even found possible to form background-free images in the device according to the invention with a measured background-free level of ± 5 volts by using a bar magnet of a less strong magnetic material, such as strontium ferrite or barium ferrite, with a rectangular cross-section of 6 x 15 mm. Although in this case the reduction of the magnet volume only has a factor of 2 with respect to the device according to European application 0 304 983, the materials for these permanent magnets are very much cheaper than neodynium - iron - boron. The rise in the background-free level as described hereinbefore using the device according to the invention additionally offers the advantage that toner particles of smaller particle size can be used, so that higher resolution can be achieved in the image forming. Figs. 4, 5 and 6 show a number of other embodiments of magnet systems for use in the image-forming device according to the invention. In all the embodiments, permanent magnets 142; 152, 153; 162 are used which as far as materials and magnetic dimensions are concerned correspond to the permanent magnet 86 in Figs. 2, 3, and magnetisable elements 141, 143; 151, 154; 161, 163 are used which consist of the same materials as (and are similarly dimensioned to) the magnetisable elements 85 and 88. The magnet system 140 according to Fig. 4 differs from the magnet system 87 only in respect of geometry, in that the magnetisable elements 141 and 143 terminate at their ends not connected to the magnet 142 in the form of a point in the gap 144. In the magnet systems 150 and 160 as shown in Figs. 5 and 6 respectively, there is an addition with respect to the magnet systems 87 and 140, in that permanent-magnetic material is present in the gap (155, 164) between the magnetisable elements (151, 154; 161, 163). In the case of magnet system 150, this is done by means of an extra permanent magnet 152 in the gap 155 between the magnetisable elements 151, 154 while in the case of the magnet system 160 the space between the magnetisable elements 161, 163, which space gradually narrows in the direction of the gap 164, is completely filled by the permanent magnet 162.

Claims: An image-forming device for recording images on an image-recording medium (10) having a dielectric surface (41), the device comprising an image-forming zone (90) in which the dielectric surface (41) is in contact with developing powder bound to a powder support (92) by the magnetic attraction of a magnet system (87; 140; 150; 160) cooperating with the powder support (92) in the image-forming zone (90), and in which a powder image is recorded on the dielectric surface (41) by applying an electric field across the image-forming zone (90) in accordance with an image pattern, characterised in that the magnet system (87; 140; 150; 160) comprises two oppositely magnetised areas (85; 88; 141; 143; 151; 154; 161; 163) which opposite to the image-forming zone (90), are separated by a gap (93; 144; 155; 164) and which are disposed consecutively as considered in the direction in which the image-recording medium (10) is transported, said areas (85; 88; 141, 143; 151, 154; 161; 163) extending substantially parallel to one another over the working width of the image forming zone (90), the distance in the image-forming zone (90) between at least one of said magnetised areas (85, 88; 141, 143; 151, 154 ; 161, 163) and the surface of the powder support (92) being less than 150 µm and the distance between the same magnetised area and the surface of the image-recording medium (10) being less than 600 µm. An image-forming device according to claim 1, characterised in that the width of the gap (93; 144; 155; 164) is 1 to 20 times the shortest distance in the image-forming zone (90) between the surface of the powder support (92) and the surface of the image-recording medium (10). An image-forming device according to one or more of the preceding claims, characterised in that the magnetised areas (85, 88; 141, 143; 151, 154; 161, 163) consist of ferromagnetic material, whose ends situated adjacent the gap (93; 144; 154; 164) are magnetised to saturation. An image-forming device according to claim 3, characterised in that the ferromagnetic material is magnetised by magnetic contact with unlike poles of a permanent magnet (86; 142; 153; 162), said permanent magnet being a constituent of the magnet system (87; 140, 150; 160).

Inventors: KLERKEN PIERRE ANTONIUS MARIE, KLERKEN, PIERRE ANTONIUS MARIE
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1993
Main classification: G03G15
IPC codes: G03G15, G03G19