Radiant fusing station capable of fixing duplex-copies for use in an electrophotographic apparatus

Abstract

 There has been found a fusing station (14) suitable for fixing duplex copies and comprising a radiant source (15) of energy mounted in a reflector cavity, and a roller (16), so positioned in relation to said source of radiant energy as to provide an opening through which a sheet of support material (19) carrying toner images can be moved for thermal exposure of the toner images of the upper surface of the support material to said radiant source of energy.
Said roller (16) guides and supports the sheet (19) of support material through the fusing station (19) in such a way that its outer surface moves synchronuously with the speed of advancement of the sheet of support material through the fusing station.

Description

[0001] This invention relates to an electrophotographic copying or printing apparatus, capable of producing duplex or recto-verso copies. More particularly, this invention relates to a fixing-system to be used within said electrophotographic copying or printing apparatus capable of fusing electroscopic toner material to both sides of a support member.

[0002] In the process of electrophotography, a light image of an original document to be copied is recorded in the form of a latent electrostatic image upon a photosensitive member. For the purposes of the present invention the latent electrostatic image may however be formed by means other than by the exposure of an electrostatically charged photosensitive member to a light image of an original document. For example, the latent electrostatic image may be generated by exposing the photosensitive member to a plurality of appropriately activated discrete spotlike sources of radiation. Said discrete spotlike sources of radiation may be constituted by a linear array of light emitting diodes or by a scanner, e.g. a laser scanner, the beam of which is modulated to determine during each scan movement a plurality of elementary image sites that may receive radiation or not depending on the modulation of the radiation beam. The generated electrostatic latent image is subsequently rendered visible by the application of electroscopic particles, commonly called toner. The visual image is then transferred from the photosensitive member to a support such as a sheet of plain paper. Since the toner image is then in a loose powdered form which may be easily disturbed or destroyed, it finally has to be permanently fixed or fused upon said support in a fusing or fixing device, a new and improved form of which is the primary object of the present invention.

[0003] In order to permanently fix a toner image to a support member, such as e.g. white bond paper, the application of thermal energy is well known. By elevating the temperature of tne toner material to a point at which the constituents of the toner coalexe and become tacky or melt, the toner is absorbed into the fibers of the support member. As thereafter the toner cools, solidification causes it to be firmly bonded to the support member.

[0004] Several approaches to thermal fusing of electroscopic toner images are described in the prior art. One commonly used fusing process con-GV.1271 sists in passing the support material with the toner images thereon past a source of radiant energy such that the image bearing side of the support material is facing the source of radiant energy while the reverse side thereof is moving in contact with a heated platen. A more detailed description of such a fusing device may be found in the German 'Offenle- gungsschrift' 27 53 625.

[0005] As in said fusing station the surface of the support material contacting the heated platen is devoid of toner material, the problem of toner offset to said platen does not occur, at least as far as said fusing station is used for the production of simplex copies only, i.e. copies with images on only one side.

[0006] The production of duplex or recto/verso copies, i.e. copies where images are formed on both sides of the support material, will however pose proolems due to a severely occurring offset-problem : when a sheet of support material, one side of which already has been provided with a fused toner image due to a first passage through the electrophotographic apparatus, has to pass for the second time through the fusing station in order to fix a toner image on the reverse side of said support material, the side of the sheet with the previously fixed toner image will contact the heated platen whilst passing the fusing station, which inevitably causes toner particles of said side to offset to said platen.

[0007] In view of the above, fusing stations of the type described above are unsuitable for being installed in electrophotographic apparatus designed for fixing recto/verso copies ; up till now these apparatus have been provided with another type of fusing station, obviously more adapted for the fusing of recto/verso copies, namely a fusing station whereby the toner images are fused on a sheet of support material under the combined influence of heat and pressure by passing said sheet through a pair of heated rollers. U.S. patent 4,269,594 gives an example of such a fusing station.

[0008] Apart from tne appearance of wrinckles in the support material, which is one disadvantage of said type of fusing stations - especially when said apparatus are used for the production of duplex copies -, said fusing devices also present to some extent the problem of toner offset from the sheet of support material to the rollers and vice versa, which reduces the quality of the final copy.

[0009] Many attempts have been made to overcome said offset problem, e.g. by the use of release agents.

[0010] Instead now of searching for remedies for the above mentioned disadvantages occurring in heated roller pair fusing stations, it has been our aim to conceive a new type of fusing station for use in an electrophotographic apparatus capable of satisfactorily fusing recto/verso copies that would not present the typical drawbacks of the heated roller pair fusing station.

[0011] In accordance with the present invention there is provided a fusing station comprising a radiant source of energy, e.g. an infrared quartz lamp, mounted in a reflector cavity, and a means for guiding and supporting a sheet of support material through said fusing station, said means being so positioned in relation to said source of radiant energy as to provide an opening through which said sheet of support material carrying toner images can be moved for thermal exposure of the toner images of the upper surface of the support material to said radiant source of energy, said fusing station _Deing characterised in that said guiding and supporting means consists of a roller, the outer surface of which is moving synchronuously with the speed of advancement of the sheet of support material through the fusing station.

[0012] The fusing station according to the present invention will be further described hereinafter and illustrated by means of the accompanying figure. Figure 1 is a schematic cross-sectional representation of an electrophotographic apparatus incorporating the novel fusing station according to the present invention.

[0013] Since the practice of electrophotography is well known to those skilled in the art, the various processing stations are represented in Fig. 1 as blocks and a brief description will hereinafter be given for the purpose of illustrating the general operation of an electrophotographic apparatus which can embody the teachings of the present invention.

[0014] A drum-like member 1, coated with a suitable photoconductive material, moves photoconductive surface through a charging station 2 by rotation by means of a shaft 3. Preparatory to imaging, a uniform electrostatic charge is placed over the photoconductive surface of the drum 1, e.g. by a corona generating device 4. Thereafter, the drum 1 is rotated to exposure station 5 where the charged photoconductive layer is exposed to a light image generated by an optical device of a conventional copying machine, or an opto/electronical device converting digitally stored information to alphanumeric images.

[0015] The selective dissipation of the charge in the light exposed regions causes the original input scene to be recorded as a latent electrostatic image.

[0016] Thereafter the rotation of the drum causes the photoconductive surface carrying the recorded electrostatic latent image to pass through a development station 6, using e.g. magnetic brush development. In said magnetic brush development system the recorded electrostatic latent image is developed by bringing it into contact with a brush of developer mix, brought about by applying a directional flux field to a magnetizable developer mix of carrier granules and toner.

[0017] After development, the toner image is transferred in a transfer station 7, e.g. by transfer corona's 8, from the photoconductive surface to the contacting side of a sheet of final support material 9 such as plain paper, labels, or transparancies, as desired.

[0018] Said sheet of support material 9 may be provided by a transport roller pair 10 and may be supplied to said transport roller pair by conventional sheet dispensing devices 11 wherein each sheet is caused to slide off a stack of sheets by use of feed rollers or sucker-cups or by peeling action.

[0019] Invariably, although a preponderance of the toner powder is separated from the photoconductive surface of the drum-like member 1 by transfer to the final sheet of support material 9, some residual toner particles remain thereto. These particles are cleaned from the photoconductive surface at cleaning station 10. At said cleaning station the residual toner particles are first brought under the influence of a cleaning corona 11 generating device adapted to neutralize the electrostatic charge remaining on the toner particles. The neutralized toner particles may then be cleaned from the photoconductive surface by conventional mechanical means as for example the use of mechanical brushes 12 or a web.

[0020] The photoconductive surface may then be used for the next successive imaging cycle.

[0021] Finally the sheet carrying the toner image is advanced e.g. by a belt transport mechanism 13 to a suitable fusing device 14 to permanently fix the toner image on said support material.

[0022] In figure 1 the support material 9 bearing a toner image on its upper surface is seen passing through the fusing station 14, which comprises basically two parts.

[0023] The portion of the fusing station 14 above support material 9 is made of a housing comprising a radiant source of energy 15, e.g. an infrared or halogen quartz lamp, mounted in a reflector cavity. A shield for the quartz lamp, such as a quartz shield, that is substantially transparent to the infrared radiation, may be provided to shield the lamp and the reflector means from the support material, debris and other machine impurities.

[0024] The portion of the fusing station 14 below support material 9 is made of a housing comprising a roller 16 for guiding and supporting the sheet of support material 9 through the fusing station 14. Said roller 16 is characterised, as will be described hereinafter, by the fact that its outer surface, contacting the sheet of support material 9, moves synchronuously with the speed of advancement of said sheet of support material 9 through the fusing station 14. Roller 16 is so positioned in relation to said radiant source of energy 15 as to provide an opening through which support material 9 carrying toner images can be moved for thermal exposure of the toner images of the upper surface of the support material 9 to the radiant energy emitted from the source 15.

[0025] It may be observed here that fusing systems capable of fusing duplex copies and using radiant sources of energy are comprised in the prior art; all of these systems however use a belt as guiding and supporting means through the radiant fusing station. Dutch patent n° 7414930 e.g. cites the use of one and the same belt transport mechanism as well for moving the sheet of support material from the transfer station of the photoconductive drum to the fusing station as for moving said sheet of support material tnrough the fusing station itself. The German 'Offenle- gungsschrift' n° 25 11 423, as well as n° 25 11 424, also teaches the use of a belt transport mechanism for guiding and supporting the support material though the fusing station.

[0026] Although from the above prior art, it may be deduced at first sight that the use of a roller instead of a band seems inappropriate, this solution still offers, as well be seen hereinafter, several practical advantages.

[0027] An auxiliary heating device 17, consisting of an infrared or halogen quartz lamp may advantageously be incorporated within the roller 16 : it has indeed been found that without the use of such an auxiliary contact heating means, it seems to be difficult to uniformly fix an image com- sisting of e.g. very fine lines : in such a case the additional heat supplied by conduction through the sheet of support material 9 through contact with the roller 16 is beneficial for compensating the rather low energy absorbed by the fine toner powder pattern from the radiant fuser means 15. In view hereof, the roller core should be made from a good thermal conducting material such as copper or aluminium.

[0028] As has been put forward hereinbefore, it is important to ensure that no offset of the toner particles from the support material to the contacting fuser members of the fusing station, i.e. the roller 16, takes place and vice versa. Now, when a blanc sheet of support material passes for the first time through the electrophotographic apparatus in order to produce a simplex copy, no offset problem is likely to occur within the fusing station, as the toner image to be fused is situated opposite the side of the support material contacting the roller. When said sheet with a fused toner image on one side has to repass the electrophotographic apparatus in order to fix an image on its other side so as to produce a duplex copy, it is important to ensure that also then no offset of the toner particles from the support material 9 to the roller 16 takes place. Now in order to ensure that no offset due to friction between the underside of said sheet of support material 9 and the roller 16 would occur, said roller 16 is driven by a suitable motor through suitable belts or gears (not shown), so that the outer surface of said roller 16 advances synchronuously to the advancement of the sheet of support material 9 through the fusing station. In this way no offset due to friction occurs between said roller 16 and said sheet of support material 9.

[0029] In order to prevent any offsetting from toner image from the underside of said sheet of support material 9 to roller 16 due to adhesion of toner particles to said roller, it is known for those skilled in the art, to cover the roller with a surface layer of a release material such as polytetrafluoro ethylene, silicone rubber or the like.

[0030] As tnese materials are heat insulators and as we have seen the roller 16 may serve as an auxiliary heating means, the thickness of the layer of these materials on the roller must be kept thin since heat conductance decreases with increased thickness.

[0031] It may be pointed out here that in a type of fusing station using a pair of heated rollers through which the support material passes, even when said neated rollers are covered with a release material of the kind described above and even wnen an additional release agent such as silicone oil is used to reduce the offset problem due to the adhesion of toner material to said heated rollers, said offset problem can never be fully avoided.

[0032] Contrary to a heated roller pair fusing station whereby said toner offset problem arises due to the intimate contact between the roller pair and the support material, same will not occur in a radiant type of fusing station used for the production of simplex copies, as said fusing station doesn't imply any contact at all between the radiant source of energy and the toner material to be fused.

[0033] When now in the type of radiant fusing station according to the present invention duplex copies are produced, contact between toner material and roller 16 will occur, but this contact is for a quite different purpose and consequently of a quite different scope as compared to the corresponding contact occurring in a heated roller pair fusing station and as will be explained hereinafter, this difference will imply that the residual offset problem in a kind of fusing station according to the present invention, will be less as compared to a heated roller pair contact fusing station, which in its turn will yield a improved quality of the final recto/verso copy.

[0034] In a heated roller pair contact fusing station intimate contact between the support material and the heated roller pair is essential for a effective fusing of the toner material on the support material, since all the heat needed for fusing the toner material has to be passed on to said material through heat conductance from the heated rollers to said toner material. This implies that, during fixing, the toner being fused will be in direct contact with the heated roller and subjected simultaneously to pressure.

[0035] In the kind of fusing station according to the present invention contact between the support material 9 and roller 16 is required primarily for guiding the support material 9 through the fusing station, and only in the second place, and if need be, for passing on, some additional heat to the support material for fusing toner material on the opposite side of said support material. This mode of operation consequently implies no direct contact between the toner being fused and the roller 16.

[0036] In a practical embodiment, the foregoing advantage of the fusing station according to the present invention for making duplex copies, over the known heated roller contact fusing station, may be realised by setting the temperature of the outer surface of roller 16 to a value beneath the temperature at which toner material commences to become tacky or melt.

[0037] In view of the above it may be seen that the overall temperature control of the varying parts of the fusing station according to the present invention will be an important point for assuring the good functioning of said fusing station for making duplex copies. The temperature of that side of the support material carrying the unfixed toner image clearly must be raised above the fusing temperature of said toner material, whereas the temperature of the fused toner material on the side of the support material in contact with the roller 16, must be kept beneath the temperature at which said toner material becomes tacky or melt.

[0038] In practice the temperature of the roller 16 may be kept constant at a predetermined value by introducing a thermistor, e.g. a bimetal within said roller and connecting said thermistor to an thermostatic control circuit (not shown). Same may be provided for controlling the temperature of radiant fuser 15.

[0039] It may be pointed out here that the use of a simple roller instead of a continuous belt transport mechanism as referred to above, is much more adapted for the required stringent temperature control of the outer surface of the guiding and supporting means.

[0040] The sheet of support material 9 as it leaves the fusing station 14 may be taken by a pair of rollers 18 for further transport to a copy paper tray 19 and for subsequent removal therefrom.

[0041] Although the problem of offset of toner particles may have been efficiently solved according to the present invention in the fusing station of an electrophotographic apparatus itself, said problem causing consequently spoiled copies may arise in said copy paper tray 19 due to the stacking of multiple copies therein. It will be clear that said problem will in particular arise for electrophotographic apparatus with a high output rate of copies and/or when using toner material characterised by a rather low melting and/or weakening point, e.g. around 55°C. The combined effect of the residual heat of the support material as it finally leaves the fusing station of the electrophotographic apparatus on the one hand and the pressure resulting from the stacking of e.g. 100 copies in the copy paper tray on the other hand causes the toner of the finished copies to become tacKy and as a result thereof to partially adhere to the adjacent copies in said copy paper tray.

[0042] It is clear however that a person skilled in the art and taking into account that this problem arises due to the too high temperature of the copies leaving the electrophotographic apparatus, will cause said copies to cool before leaving said apparatus. In a practical embodiment, this may be realised e.g. by cooling the rollers 18 by blowing fresh air over them by cooling devices, such as ventilators 20.

[0043] In particular circumstances, e.g. when an image of rather large black and white areas has been fixed to the one side of a support material, and when thereafter said support material passes for the second time through the electrophotographic apparatus in order to fix an image on the other side of said support material so as to produce a duplex or recto/verso copy, a deficient image may be formed on said other side. Indeed as the fixation of the toner-image comes about by absorption of radiant heat, emitted by the radiant fuser 15, the powdered ("black") areas of said support material 9 will absorb much more energy than the non-powdered, ("white") areas of said support material, causing a localised curling of said support material due to an increased local evaporation of moisture from the paper.

[0044] When said support material 9 with a fixed image on one side passes for the second time through the electrophotographic apparatus for the production of a recto/verso copy, a deficient image transfer will occur in the transfer station of said electrophotographic device since the curling of the support material 9 prevents a uniform intimate contact between said support material 9 and the photosensitive surface of the drum 1, which is a prerequisite for a faultless transfer of the toner image from the photoconductive surface unto said support member.

[0045] Since this problem arises by a curling of the support material occurring in the fusing station of the electrophotographic apparatus, it may be overcome by exercising a sufficient pressure power upon said support material leaving the fusing station in order to smooth out the uneven- esses of said support material. In practice this pressure power may be exercised by the roller pair 18 which then will serve at the same time as an output device of the support material 9 to a copy paper tray 19 for subsequent removal therefrom, and as a pair of mechanical flat-rollers.

[0046] When using in particular a light material such as plain paper with a weight of less than e.g. 100 g/m² as support material, it has been found that the copying quality of the last two or three centimeters of a sheet of such a support material as it leaves the electrophotographic apparatus is rather poor, i.e. all the information of the original to be copied is not faithfully reproduced on the corresponding part of said support material.

[0047] A deficient transfer process in transfer station 7 of the toner image from the photoconductive surface to said sheet of support material 9 appears to be the cause of said phenomenon; in view hereof the cited problem may be overcome by providing a device, such as a flap (not shown in figure 1) situated after transport roller pair 10 and just beneath the path to be followed by the support material 9, which pushes softly the end of said sheet of support material 9 onto the photoconductive surface of the drumlike member 1. The action of said flap may be regulated by a spring, which in its turn is controlled by a control means (not shown) which directs the action of said spring as a function of e.g. the length of the sheets of support material.

Claims

1. A fusing station of an electrophotographic apparatus for fixing a resinous powder image to a support material, comprising a radiant source of energy mounted in a reflector cavity, and a means for guiding and supporting a sheet of support material through said fusing station, said means being so positioned in relation to said source of radiant energy as to provide an opening through which said sheet of support material carrying toner images can be moved for thermal exposure of the toner images of the upper surface of the support material to said radiant source of energy, characterised in that said guiding and supporting means consists of a roller, the outer surface of which is moving synchronuously with the speed of advancement of the sheet of support material through the fusing station.

2. A fusing station according to claim 1, wherein an auxiliary heating device is incorporated within said roller.

3. A fusing station according to claim 1 or 2, wherein said auxiliary heating device consists of an infrared or halogen quartz lamp.

4. A fusing station according to any of claims 1 to 3, wherein said roller is made of a suitable heat conducting material and is covered with a suitable surface layer of a release material.

5. A fusing station according to any of claims 1 to 4, wherein said heat conducting material is copper or aluminium.

6. A fusing station according to any of claims 1 to 4, wherein said release material is silicone rubber or polytetrafluoroethylene.

7. A fusing station according to any of claims 1 to 6, wherein, by means of a thermistor connected to a thermostatic control circuit, the temperature of the roller is kept constant at a predetermined value, said value being set between the temperature at which the resinous toner powder becomes tacky or melts and the fusing temperature of said toner.

8. A method for fixing duplex or recto/verso copies using a fusing apparatus according to any of claims 1 to 4.

Drawing