DEVICE FOR TRANSFERRING A POWDER IMAGE TO A RECEIVING MATERIAL AND FIXING THE POWDER IMAGE THEREON

Abstract: A device for transferring a powder image from an image support (1) to a receiving material (19) by means of a rotatable transfer roller (25) and fixing the powder image on the receiving material. A photoconductive drum (1) presses against the transfer roller (25) in a first contact zone (25a) to transfer the powder image from the image-supporting drum (1) to the transfer roller (25) and in a second contact zone (25b) a pressure roller (31) presses the receiving material (19) against the transfer roller (25) with a force greater than the force with which the drum (1) presses against the transfer roller (25), in order to transfer the powder image from the transfer roller (25) to the receiving material. In a third contact zone (25c) a third roller (34) also presses against the transfer roller (25) and exerts a compensating force on the transfer roller (25) in such manner that the resulting force exerted on the transfer roller (25) is substantially 0. The photoconductive drum (1) is of rigid construction, the com­ pensating third roller (34) is cambered and the pressure roller (31) is pressed into contact by cambered support rollers (32, 33) so that straight contact zones (25a, 25b, 25c) can form on the transfer roller (25). In the case of a distribution of the contact zones (25a, 25b, 25c) over the transfer roller (25) such that the contact zones (25a, 25b, 25c) are not situated close together and an associated setting of the magnitude of the contact pressure forces, not only a regular image transfer is obtained but also a setting at which the magnitude of the compensating force does not become extremely high.

Description: This invention relates to a device for transferring a powder image from an image support to a receiving material and fixing the powder image thereon, comprising a rotatable transfer roller and a first roller, a second roller, and a third roller, which are pressed against the transfer roller with the axes substantially parallel to the axis of rotation of the transfer roller, so that a first contact zone, a second contact zone and a third contact zone are formed respectively, the first contact zone being intended for the transfer of the powder image from the image support to the transfer roller and the second contact zone being intended for the transfer of the powder image to the receiving material, the first contact zone and the second contact zone being not situated directly opposite one another on the transfer roller . A device of this kind is known from US-A-4 645 327 and JP-A-59 168 482. A further device is known from European Patent Application 0 149 860 which describes a contact fixing device in which a photoconductive element and a pressure roller for pressing the receiving material into contact, are both pressed against the transfer roller. In this case the nip between the photoconductive element and the transfer roller and the nip between the transfer roller and the pressure roller are situated directly opposite one another on the transfer roller. In a device of this kind, the force with which the photoconductive element is pressed against the transfer roller generally differs from the force with which the pressure roller is pressed against the transfer roller (as described inter alia in Netherlands Patent Applications 8301978 and 8402912), so that a resultant force is exerted on the transfer roller. This resultant force causes the transfer roller to sag if it is conventionally secured at the ends, the result being that the pressure in the transfer nip is not sufficiently uniform, so that the transfer is irregular. Irregular transfer is further aggravated by the presence of a cleaning roller in the device known from European Patent Application 0 149 860, which roller causes extra sagging in a direction other than that already referred to. If receiving material is passed through a transfer nip in which the pressure is irregular due to sagging, the receiving material can also easily crease. The said sagging phenomena are at a maximum in devices equipped with a long transfer roller for copying on large receiving material formats. The object of the invention is to provide a device of the kind referred to in the preamble without this disadvantage. In a device according to the invention this object is attained in that the resultant of the forces by which all the rollers are pressed against the transfer roller is substantially equal to 0 at the axis of rotation of the transfer roller. Preferably, all the rollers pressed against the transfer roller are so shaped and pressed against the transfer roller that they form a substantially straight contact zone as considered in the longitudinal direction of the rollers. The term substantially straight contact zone in this context denotes a contact zone of which the width, i.e. the distance between its edges measured in the circumferential direction of the rollers, is substantially the same over the entire length of the zone. Preferably, at least one of the rollers pressed against the transfer roller is cambered and the force with which the roller concerned is pressed into contact is exterted on the roller ends, the surface of the roller being straightened in the contact zone. As a result, the roller in question need not be made stiff and hence need not be made heavy and/or thick to form a substantially straight contact zone. The invention is explained by reference to the accompanying drawings wherein: Fig. 1 is a diagrammatic cross-section of an electrophotographic copying machine in which a device according to the invention can be used, Fig. 2 is a diagrammatic cross-section of an electrophotographic copying machine according to Fig. 1 in which a device according to the invention is applied. Fig. 3 is a graphic representation of the relationship between the magnitude and the direction of the forces which can be exerted on the transfer roller in a device according to the invention. The electrophotographic copying machine represented in Fig. 1 comprises a photoconductive drum 1 which can rotate in the direction of the arrow. A glass plate 2 is disposed some distance above the drum. By means of pairs of rollers 3 an original 4 can be fed over the glass plate 2 in the direction of an arrow. An imaging unit 10 is disposed between the glass plate 2 and the drum 1 and comprises a glass fibre objective 5 consisting of one or more arrays of image-forming glass fibres. The optical axes of the glass fibres intersect the drum axis at some distance. On the side of the glass fibre objective 5 remote from the glass plate 2 a mirror 6 is disposed at an angle such that said mirror projects perpendicularly on to the photoconductive drum 1 the beam of light formed by the glass fibre objective. A tubular light source 7 is disposed next to the glass fibre objective 5. A reflector 8 which partially surrounds the light source 7, and a mirror 9 disposed on the side of the glass fibre objective 5 situated opposite the light source 7, concentrate the light emitted by the light source 7 in the field of view of the glass fibre objective 5. With the imaging unit 10 described above, an air feed duct 11 is provided from which cooling air can flow through the exposure unit to dissipate the heat produced by the light source 7. The rotating photoconductive drum 1 successively passes: a charging station 12 constructed with two rows of corona pins 13 with earthed wires 14 therebetween for the uniform charging of the photoconductive surface of the drum 1, the above-described imaging unit 10 for the image-wise discharge of the charged surface, a developing station 15 with a magnetic developing roller 16 and a feed system 17, for developing the resulting charge image with a single-component magnetizable developing powder, a transfer and fixing station 18 which will be described hereinafter, for the transfer of the resulting powder image to a receiving material 19, and a cleaning station 20 for removing remaining toner from the photoconductive drum 1, with a first magnetic roller 21 for neutralizing the charge on the toner and a second magnetic roller 22 for removing the discharged toner from the photoconductive drum 1. The transfer and fixing station 18 comprises an image transfer roller 25 covered with a layer of silicone rubber and internally provided with a heating element to heat said silicone rubber layer. In a first contact zone 25a the drum 1 is pressed against the image transfer roller 25 in the manner to be explained hereinafter. The force required for the contact pressure may vary within rough limits but is preferably kept as low as possible in order to obtain maximum rubber life. The minimum force required for transferring the powder image from drum 1 to the transfer roller 25 depends on various factors, including the deformability of the silicone rubber. A force of 750 N per metre roller length is usually ample. The receiving material 19 is fed by a pair of transport rollers 26 between the top wall of a reservoir 27 and a contact-pressure member 28. Reservoir 27 contains a quantity of water 29 which can be brought to the boil by a heating element 30, the resulting steam uniformly heating the top wall of the reservoir. The receiving material 19 is also heated uniformly as a result. This material is then pressed against the image transfer roller 25 by means of a pressure roller 31 in contact zone 25b with a much higher force than that with which the image transfer roller 25 is pressed against the photoconductive drum 1. The magnitude of the force required depends inter alia on the temperature of the powder image and on the melting temperature of the developing powder. A force of about 1500 N per metre roller length is usually sufficient to fix a powder image which has been softened to near melting temperature by a temperature rise. The pressure roller 31 is pressed against the image transfer roller 25 by means of two support rollers 32 and 33. A cleaning roller 34 for the removal of toner and receiving material dust remaining on the image transfer roller 25 after fixing is in contact with the image transfer roller 25 in a contact zone 25c in a part of the circumference which, as considered in the direction of rotation of the image transfer roller 25, is situated past the fixing nip. As will be explained hereinafter, the cleaning roller 34 is so positioned and pressed into contact that the resulting force on the shaft ends of the image transfer roller 25 is substantially equal to 0. Heat-insulating elements 35 are disposed between the heat-producing parts of the copying machine such as the imaging station 10 and the transfer and fixing station 18, and the rest of the machine, in order to keep the rest of the machine cool, particularly the photoconductive drum 1. The mechanical structure of a device according the the invention will now be explained with reference to Fig. 2, at least insofar as this structure is relevant to the operation of the device according to the invention. The image transfer roller 25 is provided with journals 36 at the ends, which are mounted in the frame of the copying machine. The imaging unit 10 with the parts shown in Fig. 1 is also fixedly secured to the frame of the copying machine. Support rollers 37 are secured so as to be freely rotatable on either side of the imaging unit 10. The axes of rotation of the support rollers 37 are situated on a line passing through the plane that intersects the image formed by the glass fibre objective 5 perpendicularly. Apart from a strip at the two edges , the cylindrical surface of the photoconductive drum 1 is coated with a photoconductive layer. The support rollers 37 are in contact with the non-coated strips of the photoconductive drum 1 and keep the glass fibre objective 5 at a constant distance from the drum 1 as required for the image formation. In the copying state of the copying machine, represented in Figs. 1 and 2, the photoconductive layer of the photoconductive drum 1 is in contact with the silicone rubber layer of the image transfer roller 25. In this copying state the extension of the incident light beam passes through the axis of the drum 1. The charging device 12 is slidably secured in the copying machine by means not shown, for movement towards and away from the photoconductive drum 1 in a radial plane of the drum. Support rollers 38 are mounted to be freely rotatable on either side of the charging device 12. The support rollers 38 are pressed against those strips of the photoconductive drum 1 which are free from photoconductor by means of springs 39 pressing against the charging device 12, the drum in turn pressing both against the support rollers 37 rotatable about fixed axes and against the image transfer roller 25 which is rotatable about a fixed axis. The developing unit 15 and the cleaning unit 20, like the charging unit 12, are slidably secured in the copying machine for movement towards and away from the photoconductive drum 1 and are each provided with support rollers 40 and 41 respectively, which are in contact with those strips of the photoconductive drum 1 which are free from photoconductor. The developing unit 15 and the cleaning unit 20 respectively are pressed against the photoconductive drum 1 by springs 42 and 43 respectively, by a force which is considerably less than the force with which the charging unit 12 is pressed against the photoconductive drum 1. The forces with which the support rollers 40, 41 and 38 are pressed against the photoconductive drum 1 are so set that the force with which the drum presses against the image transfer roller 25 is 750 N per metre roller length. This force is exterted primarily by the relatively strong spring 39 because the relatively slack springs 42 and 43 substantially cancel out the effect of one another. A felt strip 49 moistened with silicone oil is in contact with each of the non-photoconductive strips of the photoconductive drum 1 with which the support rollers 37, 38, 40 and 41 come into contact. The oil spread on these contact surfaces mixes with any toner present there and prevents any build-up of a layer of toner on the running surfaces for the rollers 37,38,40 and 41, in order that the position of the units 10,12,15 and 20 pressed against the photoconductive drum 1 may be kept exactly equal with respect to the drum. Since the support rollers 40 and 41 are situated approximately opposite one another, the force with which support rollers 38 are pressed against the photoconductve drum 1 by springs 39 is substantially the sole determining factor for the pressure occurring in the transfer nip between the photoconductive drum 1 and the image transfer roller 25. If the photoconductive drum 1 is made sufficiently stiff it will transmit the force of the springs 39 to the transfer nip between the photoconductive drum 1 and the image transfer roller 25, substantially without itself undergoing any deformation. The support rollers 32 and 33 are mounted at the ends in a subframe 43 rotatable about a spindle 44 secured immovably to the frame of the copying machine. A prestressed compression spring 45 disposed between the subframe 43 and the main frame of the copying machine presses the support rollers 32 and 33 against the pressure roller 31, which in turn presses against the image transfer roller 25. The cleaning roller 34 is mounted at the ends in a subframe 46 rotatable about a spindle 47 secured immovable to the frame of the copying machine. A prestressed compression spring 48 disposed between the subframe 46 and the main frame of the copying machine presses the cleaning roller 34 against the image transfer roller 25. The support rollers 32 and 33 and the likewise supporting cleaning roller 34 are made considerably thinner than the photoconductive drum 1 and have a much lower resistance to flexure than the photoconductive drum 1. The pressure of the compression springs on the ends of the rollers 32,33 and 34 will cause these rollers to be sagged slightly. In order that a straight contact zone despite this sagging may nevertheless be obtained between the support rollers 32,33 and the pressure roller 31, between the pressure roller 31 and the image transfer roller 25, and between the supporting cleaning roller 34 and the image transfer roller 25, the rollers 32,33 and 34 are barrel-shaped, the difference in diameter between the middle and the ends of these rollers being such that with the spring forces applied the surface of these rollers is straightenend in the contact zone. The difference in diameter required between the middle and the ends of the support rollers 32,33 and 34 is dependent upon the diameter, wall thickness, length, type of material, etc., of the rollers. In the case of a steel roller of a diameter of 55 mm, a wall thickness of 5 mm, and a length of 930 mm, the diameter of the roller is 0.45 mm larger in the middle than at the ends. A cam disc 50 is mounted rotatably on each journal 36 of the image transfer roller 25. The cam discs 50 are interconnected by a yoke 51 extending over part of the cylindrical surface of the image transfer roller 25. Near each end of the image transfer roller 25 the yoke 51 is provided with two freely rotatable rollers 52 which project slightly above the yoke and which can be brought into contact with the non-photoconductive strips of the photoconductive drum 1 by rotation of the cam discs 50. Rollers 56 and 57 respectively are mounted in the subframes 43 and 46. These rollers 56 and 57 can be in contact with the cam discs 50 depending upon the position of these discs. The cam discs 50 can be rotated together with the yoke 51, by means of a drive 53, and be stopped in four different angle positions. In the position of the cam discs 50 represented in Fig. 2 the copying machine is in the copying mode. In this position the rollers 56 and 57 are both at a slight distance from a part of the circumference of the cam disc 50 which has a small diameter. In this position springs 45 and 48 respectively hold the pressure roller 31 and cleaning roller 34 respectively with the required force against the image transfer roller 25, while the photoconductive drum 1 is pressed against the transfer roller 25 mainly by springs 39. In the position of the cam discs 50 illustrated the image transfer roller 25 can be driven by a suitable drive (not shown) which engages journal 36. When the image transfer roller 25 is driven, the photoconductive drum 1 and the rollers 31 and 34 are rotated by the frictional contact between the silicone rubber layer of the image transfer roller 25, on the one hand, and rollers 31 and 34 and drum 1, on the other hand. If the other copying conditions are satisfied, e.g. the temperature of the image transfer roller 25 and of the paper pre-heating 27-30, an image of an original 4 can be transferred to a receiving material 19. On completion of the copying operation the copying machine is brought into a position of readiness by rotation of the cam discs 50 in the clockwise direction with respect to Fig. 2, until all the rollers 52 are in contact with the photoconductive drum 1. In doing so the drum is pressed up against the action of springs 39 in particular, the drum rotating about the support rollers 37. At the same time, yoke 51 is pushed into the space created between the photoconductive drum 1 and the image transfer roller 25 to form a heat-insulating shield between the relatively hot image transfer roller 25 and the photoconductive drum 1 which must be kept cooler. This shield largely fills the space between the heat-insulating elements 35. On further rotation of the cam discs 50, from the position illustrated in Fig. 2 and in the clockwise direction, raised parts of the cam discs 50 situated opposite one another also come into contact with rollers 56 and 57 respectively. Subframes 43 and 46 are thus pressed outwards against the action of compression springs 45 and 48 respectively, so that the support rollers 32 and 33 with the pressure roller 31 resting thereon, and the cleaning roller 34 are moved away from the image transfer roller 25. This position of the cam discs 50 is set when the copying machine is completely switched off. From the position illustrated in the drawing the cam discs 50 can also be rotated in the anticlockwise direction into a position in which only rollers 56 are pressed outwards by raised parts of the cam discs 50 as a result of which only the pressure roller 31 is released from the image transfer roller 25. In this state of the copying machine a layer of toner can be transferred from the developing station 15 via the surface of the photoconductive drum 1 and the silicone rubber surface of the image transfer roller 25 to the cleaning roller 34. In the embodiment of the cleaning roller described in the above-mentioned European Patent Application 0 149 860, this should be done periodically to give a good cleaning effect. In a suitable embodiment of a copying machine provided with the device according to the invention the photoconductive drum has a diameter of 200 mm. This is the minimum diameter required to enable there to be placed around a photoconductive drum image-forming units for charging, imaging, developing, transfer and cleaning, of a size normal in electrophotographic copiers. In that suitable embodiment, the image transfer roller provided with a compressible silicone rubber coating has a diameter of 100 mm. This is the maximum diameter which can still give a roller that can be readily handled in terms of weight, a feature which is important when a worn roller is replaced by a service engineer. The pressure roller, which is provided with a coating relatively less compressible and which co-operates with the image transfer roller, has a diameter of 25 mm. Since the diameter of the "hard" pressure roller is considerably smaller than the diameter of the "soft" image transfer roller, a receiving material fed through the nip between the image transfer roller and the pressure roller is readily released from the image transfer roller after passing through the nip. In the above-described embodiment and with the diameters selected for the photoconductive drum and the image transfer roller, the angle between the radial plane of the image transfer roller which passes through the first contact zone 25a and the radial plane of the image transfer roller which passes through the second contact zone 25b must be approximately 120° minimum. With a smaller angle there is insufficient space to pass receiving material to the fixing nip. The "angle" between the photoconductive drum and the cleaning roller supporting the image transfer roller is preferably 75° minimum in order to provide space for, for example, a heat-insulating element between the photoconductive drum and the supporting cleaning roller. In the above suitable embodiment, the "angle" between the photoconductive drum and the pressure roller is set to 135° and the "angle" between the photoconductive drum and the cleaning roller is set to 75°. Also, as already stated, the force per meter between the photoconductive drum and the image transfer roller is set to 750 N and the force per meter between the transfer roller and the pressure roller 1500 N. The relationship between the magnitude and direction of the forces which can be exerted on the image transfer roller is represented in Fig. 3. This shows a system of co-ordinates with the centre point of the image transfer roller as the centre and the radial line of the image transfer roller which passes trough the contact zone between this roller and the photoconductive drum as the y-axis. The vectors 60 and 61 of the forces exerted in this embodiment of the device according to the invention on the image transfer roller by the pressure roller and the cleaning roller respectively are represented in the system of co-ordinates at angles of 135° (angleα ) and 75° (angle β). The force vector that the cleaning roller 34 must exert in the contact zone 25c and which is required to obtain a zero resultant of the forces exerted on the image transfer roller 25 at the axis of rotation thereof is determined from the magnitude and direction of the force vector 62 in the first contact zone 25a and the magnitude and direction of the force vector 60 in the second contact zone 25b. Hatching in Fig. 3 shows the area in which there are no usable directions of the force vectors 60 and 61. Starting from a force vector 60 of 1500 N/m and a force vector 62 of 750 N/m, a good setting at which no extremely high compensating force vector 61 has to be applied is obtained by setting the angle α between 130° and 140° and the angleβ to 75°. With a transfer/fixing force of 1500 N/m the magnitude of the compensating force that the cleaning roller must apply to achieve equilibrium of forces is then between about 1050 and 1150 N/m.

Claims: A device for transferring a powder image from an image support to a receiving material and fixing the powder image theron, comprising a rotatable transfer roller (25) and a first roller (1), a second roller (31), and a third roller (34), which are pressed against the transfer roller (25) with the axes substantially parallel to the axis of rotation of the transfer roller (25), so that a first contact zone (25a), a second contact zone (25b) and a third contact zone (25c) are formed respectively, the first contact zone (25a) being intended for the transfer of the powder image from the image support (1) to the transfer roller (25) and the second contact zone (25b) being intended for transfer of the powder image to the receiving material (19), the first contact zone (25a) and the second contact zone (25b) being not situated directly opposite one another on the transfer roller (25), characterised in that the resultant of the forces by which all the rollers (1, 31 and 34) are pressed against the transfer roller (25) is substantially equal to 0 at the axis of rotation of the transfer roller (25). A device according to claim 1, characterised in that all the rollers (1, 31 and 34) pressed against the transfer roller (25) are so shaped and pressed against the transfer roller (25) that they form a substantially straight contact zone as considered in the longitudinal direction of the rollers. A device according to claim 2, characterised in that at least one of the rollers (34) pressed against the transfer roller (25) is cambered and in that the force with which the roller (34) concerned is pressed into contact is exerted on the roller ends, the surface of the roller (34) being straightened in the contact zone. A device according to claim 2 or 3, characterised in that at least one support roller (32, 33) is provided which presses one of the rollers (31) against the transfer roller (25), which support roller (32, 33) is cambered, and in that the force with which the support roller (32, 33) is pressed into contact is exerted at the roller ends, the surface of the support roller (32, 33) being straightened in the contact zone. A device according to any one of the preceding claims, in which the first roller is a photoconductive drum (1) on which a powder image can be formed and the second roller is a pressure roller (31) which presses the receiving material against the transfer roller (25), characterised in that the angle between the radial plane of the transfer roller (25) which extends through the axis of rotation of the drum (1) and the radial plane of the transfer roller (25) which extends through the axis of rotation of the pressure roller (31) is between 130° and 140° and in that the angle between the radial plane of the transfer roller (25) which extends through the axis of rotation of the drum (1) and the radial plane of the transfer roller (25) which extends through the axis of rotation of the third roller (34) is approximately 75°. A device according to claim 5, characterised in that the photoconductive drum (1), the pressure roller (31) and the third roller (34) are pressed against the transfer roller (25) by means of springs (39,42,43; 45; 48) and in that a rotatable element (50,51,52) is mounted on the transfer roller (25), which element (50, 51, 52) is provided with individual cams which, on rotation of the element (50, 51, 52) to a first position, bring the drum (1), the pressure roller (31) or the third roller (34) respectively out of contact with the transfer roller (25) against the action of the springs. A device according to claim 6, characterised in that the cams are so formed on the rotatable element (50, 51, 52) that, depending upon the position of the rotatable element (50, 51, 52), only the pressure roller (31), or only the photoconductive drum (1), or both the pressure roller (31), the photoconductive drum (1) and the third roller (34) are out of contact with the transfer roller (25).

Inventors: D'HONDT, ARNOLD AUGUST PETRONELLA MARIE, GIEZEMAN, PAULUS GERARDUS
Applicants: OCE-NEDERLAND B.V.
Application year: 1988
Main classification: G03G15
IPC codes: G03G15