Printing device

Abstract: A printing device for reproducing information, comprising a movable image-forming element (10) having a dielectric surface, an image-forming station (11) in which a magnetic roller (84) is disposed with a rotatable electrically conductive sleeve (92), first means to generate an electric field between the image-forming element (10)and the magnetic roller (84) in accordance with an information pattern, while an electrically conductive magnetically attractable toner powder is present in the image-forming zone (90), and second means consisting of a magnetic system (85) which generate a magnetic field in the image-forming zone (90). A force is applied to the sleeve (92) to press the latter, at the image-forming zone (90), against spacer means disposed inside the sleeve (92) or against the magnetic system (85). As a result, a well-defined space is created in the image-forming zone (90) between the sleeve (92) and the image-forming element (10) so that no distortion of the toner brush occurs in the image-forming zone (90).

Description: Printing device The invention relates to a printing device for reproducing information, comprising a movable image-forming element having a dielectric surface, an image-forming station in which a magnetic roller is disposed with a rotatable electrically conductive sleeve near the surface of the image-forming element, so as to form an image-forming zone, first means to generate an electric field between the image-forming element and the magnetic roller in accordance with an information pattern, while an electrically conductive magnetically attractable toner powder is present in the image-forming zone, and second means which generate a magnetic field in the image-forming zone, which second means comprise a magnetic system disposed stationary within the sleeve of the magnetic roller. A printing device of this kind is known from US patent 4 884 188. The wall thickness of the sleeve of the magnetic roller of the printing device must be as thin as possible in order to minimise any distortion of the magnetic field in the image-forming zone. However, a thin wall-thickness for the magnetic roller sleeve means that the roller is of relatively low rigidity so that the sleeve vibrates during rotation. Such vibrations cause changes in the distance between the surface of the sleeve and the image-forming element in the image-forming zone so that the toner brush formed there does not remain satisfactorily in position. This results in lack of uniformity in image-formation. Also, the degree of sleeve vibration varies over the length of the magnetic roller so that the toner brush does not extend rectilinearly transversely of the direction of transport of the image-forming element, but extends along a curved line continually changing shape. As a result, toner particles do not reach the image-forming element at the correct location, and this is visible as image defects on the copy. The object of the invention is to provide a printing device of the type referred to in the preamble without the above-mentioned disadvantages. This object is attained in that means are provided for applying a force to said electrically conductive sleeve so that the sleeve bears against spacer means disposed within the sleeve, thereby establishing in the image-forming zone a static distance between the sleeve en the dielectric surface of the image-forming element. According to the one embodiment of the invention, the sleeve of the magnetic roller comprises magnetisable components. As a result, the sleeve of the magnetic roller is attracted against an edge of the magnetic system under the influence of the magnetic field generated by the magnetic system. As a result, a well-defined distance is created in the image-forming zone between the sleeve of the magnetic roller and the image-forming element so that no distortion of the toner brush occurs. According to a further embodiment of the printing device according to the invention, one or more spacer means are disposed inside the sleeve of the magnetic roller near the image-forming zone at a given distance from the surface of the image-forming element, so that the sleeve of the magnetic roller is pulled against these spacer means. The magnetic components of the sleeve of the magnetic roller may be provided, for example, by arranging the sleeve to have a layer of a soft-magnetic material or consist completely of such material. In another embodiment of the printing device according to the invention, one or more contact-pressure elements are provided to exert a normal force on the sleeve of the magnetic roller, so that the sleeve is brought into contact with the magnetic system in the image-forming zone. In this embodiment, also, one or more spacer means can be disposed inside the sleeve of the magnetic roller near the image-forming zone at a given distance from the surface of the image-forming element. As a result of the normal force exerted on the sleeve by the contact-pressure elements, said sleeve is brought into contact with the spacer means in the image-forming zone. The invention is explained in detail with reference to the following description and the accompanying drawings wherein: Fig. 1 is a drawing showing the principle of an electrostatic printing device, Fig. 2 is a cross-section of a first embodiment of a printing device according to the invention, Fig. 3 is a cross-section of a second embodiment of a printing device according to the invention, and Fig. 4 is a cross-section of a third embodiment of a printing device according to the invention. Fig. 1 is a drawing showing the principle of an electrostatic printing device having an image-forming element in the form of a rotating drum 10, provided with an electrostatic layer built up from a number of controllable electrodes in and beneath a dielectric layer. A magnetic roller 12 is disposed in an image-forming station 11 at a short distance from the surface of the image-forming element 10 and comprises a rotatable electrically conductive sleeve and an internal stationary magnetic system. The rotatable sleeve of the magnetic roller 12 is covered with a uniform layer of electrically conductive and magnetically attractable toner powder, which toner powder is in contact with the image-forming element 10 in an image-forming zone 13. A powder image is formed on the image-forming element 10 by the application of a voltage between the magnetic roller 12 and one or more of the selectively controllable electrodes of the image-forming element 10. This powder image is transferred by the application of pressure to a heated rubber-covered roller 14. A sheet of paper is taken by roller 25 from the stock pile 26 and is fed via guideways 24 and rollers 22 and 23 to a heating station 19. Heating station 19 comprises a belt 21 running 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 between the rollers 14 and 15, the softened powder image present on the roller 14 being completely transferred to the sheet of paper. The temperatures of the belt 21 and the roller 14 are so adapted to one another that the image fuses to the sheet of paper. The sheet of paper provided with an image is fed via the transport rollers 17 to a collecting tray 18. Unit 30 comprises an electronic circuit which converts the optical information of an original into electrical signals which are fed to the controllable electrodes, which are not shown in detail, via wires 31 having trailing contacts and conductive tracks 32 disposed in the insulating side wall of image-forming element 10. Fig. 2 is a cross-section of an image-forming element 10 in the form of a drum 36 rotatable in the direction of arrow 35 and provided with an insulating layer 43 on which are disposed a large number of adjacent and mutually insulated electrodes 42 which extend endlessly in the direction of movement of the drum, said electrodes 42 being covered by a dielectric layer 41. Magnetic roller 84 comprises an earthed sleeve 92 rotatable in the direction of arrow 89 about a magnetic knife 85 consisting of a ferromagnetic blade 88 held between two magnets 86 and 87. The thickness of the ferromagnetic blade 88 is at least 0.4 mm in order to achieve the optimal magnetic flux in the material, while a maximum thickness of about 4 mm is used for constructional reasons. The magnets 86 and 87, which are in contact with the blade 88 by like poles, generate a narrow magnetic field in the image-forming zone 90, said field emerging from the end of the blade 88 disposed at a short distance from the sleeve 92. A uniform layer of conductive magnetic toner is applied to the dielectric layer 41 by means of a toner feed device. This feed is effected by means of a magnetic roller 130. The latter comprises a sleeve 131 of diamagnetic material, e.g. aluminium, brass or stainless steel. Sleeve 131 is mounted in known manner for rotation about a shaft 132 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 roller 130, said shaft 132 being fixed in the frame of the printing device. A homogeneous magnetic field is obtained at the surface of the diamagnetic sleeve 131 under the influence of the magnets 135. Magnetically attractable toner powder is applied to the sleeve 131 of the magnetic roller 130 from a reservoir 136 and is 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 given thickness by a scraper 137 is transported to a transfer zone between the image-forming element 10 and the magnetic roller 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 over the transfer zone. The magnets 135 of the magnetic roller 130 must, on the one hand, satisfy the requirement that the magnetic induction must be sufficiently high to generate a magnetic field on the surface of the sleeve 131 such that a layer of toner powder is retained and is entrained by the rotating sleeve 131 without causing dust problems. The magnetic induction is thus determined by toner powder parameters and the speed of revolution of the magnetic roller 130. On the other hand, the magnetic induction of the magnets must not be too high to enable the layer of toner powder to be readily transferred to the dielectric sleeve 41 in the transfer zone without a very strong electric field being required. These two contradictory requirements can be met in two ways. Firstly, by using an optimal magnetic induction for the transfer function for the magnet 135 which determines the field strength in the transfer zone, and an optimal magnetic induction in respect of the toner transport function for all the other magnets. Of course, a compromise can be made in which case the same magnetic induction is used for all the magnets 135, this magnetic induction being a compromise for both functions. A third function of the magnetic roller 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 roller 130 and is included in the layer of toner powder on roller 130. As described above, a layer of toner powder is transported to the image-forming zone 90 via the image-forming element 10 in order to form a very narrow toner brush there under the influence of the directional magnetic field. To obtain the sharpest possible toner brush, the strongest possible magnetic field is required with a high magnetic gradient certainly on that side where the image-forming element 10 leaves the image-forming zone 90. To this end, the assembly comprising the blade 88 and magnets 86,87 is disposed at an angle α with respect to the line connecting the centres of the drum 36 and sleeve 92. The angle α is between 0° and 20° and is preferably 10°. An additional way of achieving a sharp toner brush is for the magnets 86, 87 to be disposed in mutually offset positions with respect to the blade 88. In that case the magnet 87 is positioned much closer to the end of the blade 88 than the magnet 86. It has been found that a very strong magnetic field is obtained by using, for magnets 86, 87, permanent magnets having a magnetic energy product B x H of at least 246 kJ/m³, so that excellent results are obtained even using toners having weak magnetic properties. A material which satisfies this requirement for a suitable magnet is a neodynium-iron-boron alloy. In order to limit any distortion of the magnetic field in the image-forming zone 90 as much as possible, the wall thickness of the sleeve 92 must be fairly thin (e.g. 40-100 µm). A sleeve 92 having such a thin wall thickness may, however, vibrate during rotation so that distortion of the toner brush, which has been sharply defined by the steps indicated hereinabove, occurs and image errors may arise. The solution to this is to produce a well-defined distance between the sleeve 92 and the image-forming element 10 by applying a force to the sleeve 92 so that in the image-forming zone 90 sleeve 92 bears against the end of the blade 88. A first embodiment of a contact-pressure means of this kind against the blade 88 is shown in Fig. 2. In this embodiment the sleeve 92 comprises magnetisable material so that said sleeve 92 is magnetised in the image-forming zone under the influence of the magnetic field applied by the magnetic knife 85. The magnetised sleeve 92 experiences a force in the magnetic field in the image-forming zone 90, the force pressing said sleeve 92 against the end of the magnetic blade 88 and holding it pressed against the same. The magnetisable components of the sleeve 92 preferably consist of a layer of soft-magnetic material, e.g. nickel, since soft-magnetic material is, on the one hand, rapidly magnetised under the influence of the magnetic field and, on the other hand, is also rapidly magnetically saturated, so that the configuration of the field lines of the magnetic field is no longer distorted. If a sleeve 92 is used which consists completely of nickel, with a wall thickness between 40 and 100 µm, good contact pressure is obtained against the end of the blade 88 with the embodiment described hereinbefore (as regards construction and magnetic specifications) for the magnetic knife 85. The skilled addressee, of course, will very readily determine by experiment a different combination of the determining parameters (magnetisable material, wall thickness and single-layer or multi-layer construction) for the sleeve 92 to give a sleeve 92 which is satisfactorily pressed against the blade 88 in a given magnetic field and is sufficiently rapidly magnetically saturated not to distort the configuration of the field lines. Fig. 3 shows a second embodiment of the printing device according to the invention with another embodiment of the contact-pressure means to press the magnetic roller sleeve against the blade. Like parts in Figures 2 and 3 have like references. This second embodiment comprises a magnetic roller 100 consisting of an electrically conductive non-magnetic sleeve 102 rotatable in the direction of arrow 103 about the magnetic knife 85. Contact-pressure means in the form of a cylinder segment 105 are disposed inside the sleeve 102, said segment 105 being pressed outwardly by means known in the art so that the sleeve 102 is held against the end of the magnetic blade 88 in the image-forming zone 90. The cylinder segment 105, which may consist of one complete segment or a number of segmental parts, can, for example, be pressed outwards by spring action or pneumatically. Fig. 4 shows a third embodiment of the printing device according to the invention with yet another embodiment of the contact-pressure means for pressing the magnetic roller sleeve against the blade. Parts being identical to parts in Fig. 2 and Fig. 3 have like references in this third embodiment. This third embodiment comprises a magnetic roller 120 consisting of an electrically conductive non-magnetic sleeve 121 rotatable in the direction of arrow 122 about the magnetic knife 85. As considered in the direction of rotation, contact-pressure means 125, for example in the form of a curved strip, pressed by means (not shown) against the inner wall of the sleeve 121, are provided within the sleeve 121 just before the image-forming zone 90. In this case, any means known in the art can be used to achieve this contact pressure. The normal force exerted on the sleeve 121 by means of the contact-pressure means 125 results in a frictional force between the contact-pressure means 125 and the sleeve 121. This tangentially directed frictional force on the sleeve 121 acts in the opposite direction to the driving force on the sleeve 121, so that the sleeve 121 of the magnetic roller 120 is pulled against the end of the magnetic blade 88 at the image-forming zone 90. Although the foregoing description relates to the application of a normal force to the sleeve of the magnetic roller from the inside in order to hold the sleeve against the blade, it will be clear to the skilled addressee that a force can also be applied to the sleeve from outside to give the same result. In order to avoid unnecessary distortion of the sleeve 102, 121 during the time when the printing device is inoperative, the contact-pressure means 105, 125 are preferably released from the sleeve 102, 121 in that position. This can be achieved by means not shown in detail but known in the art. Instead of using the blade 88 directed towards the image-forming element 10 as an element against which the sleeve 92, 102, 121 bears, it is also possible to use separate spacer means. Such spacer means (not shown in detail) may, for example, consist of elongate integral or divided support shafts or support members extending in the axial direction of the magnetic roller 84, 100, 120. Such spacer means are then disposed near the end of the blade 88 within the sleeve of the magnetic roller at a predetermined distance from the surface of the image-forming element 10.

Claims: A printing device for reproducing information, comprising a moving image-forming element (10) having a dielectric surface, an image-forming station (11) in which a magnetic roller (12; 84; 100; 120) is disposed with a rotatable electrically conductive sleeve (92; 102; 121) near the surface of the image-forming element (10), so as to form an image-forming zone (90), first means to generate an electric field between the image-forming element (10) and the magnetic roller (12; 84; 100, 120) in accordance with an information pattern, while an electrically conductive magnetically attractable toner powder is present in the image-forming zone (90), and second means which generate a magnetic field in the image-forming zone (90), which second means comprise a magnetic system (85) disposed stationary within the sleeve of the magnetic roller (12; 84; 100; 120), characterised in that means are provided for applying a force to said electrically conductive sleeve (92; 102; 121) so that said electrically conductive sleeve (92; 102; 121), bears against spacer means disposed within the sleeve, thereby establishing in the image-forming zone (90) a static distance between said electrically conductive sleeve (92; 102; 121) and the dielectric surface of said image-forming element (10). A printing device according to claim 1, characterised in that the sleeve (92) of the magnetic roller (84) comprises magnetisable components such that said magnetised sleeve in combination with said magnetic system (85) provides said force means. A printing device according to claim 2, characterised in that the sleeve (92) of the magnetic roller (84) comprises a layer of soft-magnetic material. A printing device according to claim 2, characterised in that the sleeve (92) of the magnetic roller (84) consists of soft-magnetic material. A printing device according to claim 1, characterised in that the force means comprise one or more contact-pressure elements (105; 125) which exert a normal force on the sleeve (102; 121) of the magnetic roller (100;120). A printing device according to claim 5, characterised in that the contact-pressure elements (105; 125) are disposed inside the sleeve (102; 121) of the magnetic roller (100; 120) and exert an outwardly directed normal force on the sleeve (102; 121). A printing device according to claim 1, characterised in that the spacer means are formed by an edge of the magnetic system (85). A printing device according to claim 1, characterised in that the spacer means are disposed inside the sleeve (92; 102; 121) of the magnetic roller (84; 100; 120), near the image forming zone (90) at a given distance from the surface of the image-forming element (10).

Inventors: CORVER JOZEF ANTONIUS WILLEM M, HOEP ANTOON LEENDERT, QUIRIJNEN ROBERTUS PETRUS CORN, CORVER, JOZEF ANTONIUS WILLEM MARIA, HOEP, ANTOON LEENDERT, QUIRIJNEN, ROBERTUS PETRUS CORNELIS
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1992
Main classification: G03G15
IPC codes: B41J2, G03G15