Low mass heat and pressure fuser
This invention relates to the art of forming powder images and, more particularly, to heat and pressure fuser apparatus for fusing such images to substrates. 
     In the art of xerography or other similar image reproducing arts, a latent electrostatic image is formed on a charge-retentive surface which may comprise a photoconductor which generally comprises a photoconductive insulating material adhered to a conductive backing. When the image is formed on a photoconductor, the photoconductor is first provided with a uniform charge after which it is exposed to a light image of an original document to be reproduced. The latent electrostatic images, thus formed, are rendered visible by applying any one of numerous pigmented resins specifically designed for this purpose. 
     It should be understood that for the purposes of the present invention, which relates to rendering permanent powder or toner images, the latent electrostatic image may 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 from information electronically stored or generated, and the digital information may be converted to alphanumeric images by image generation electronics and optics. However, such image generation electronic and optic devices form no part of the present invention. 
     In the case of a reusable photoconductive surface, the pigmented resin, more commonly referred to as toner which forms the visible images is transferred to a substrate such as plain paper. After transfer the images are made to adhere to the substrate by a fuser apparatus. To date, the use of simultaneous heat and contact pressure for fusing toner images has been the most widely accepted commercially. Heretofore, it has been necessary with the foregoing type of fuser to heat the fuser not only when images are being fused but also during standby when images are not being fused. This is because of the long delay that would be required to elevate the fuser to a proper operating temperature if the heat supply were turned off during standby, the long delay being due to the relatively large mass that has to be brought up to the fusing temperature. Such delays would not be tolerated by the user even though operating the fuser in such a manner would eliminate a substantial waste of energy and therefore money. Along with this saving of energy, there would also be a reduction in heat loading to the environment. 
     Elimination of fuser standby power has been accomplished in prior art devices such as flash fusers and cold pressure fusers. Both of these types of fusers, however, exhibit other drawbacks. For example, cold pressure fusers exhibit poor quality images. Flash fusers create undesirable effluents and they work very poorly with colored toners, especially the lighter colored ones. Also, the optical density of flashed fused images is unsatisfactory. 
     Accordingly, we have provided, as disclosed herein, a heat and pressure fuser than can be satisfactorily operated without the employment of standby power. To this end, our fuser comprises a low mass endless belt which is entrained about a mandrel. A pressure roll is supported for pressure engagement with an area of the belt to provide the necessary pressure for fusing and also to effect movement of the belt. 
     A heat source for elevating the temperature of the belt is operatively supported at a predetermined distance from the area of contact between the belt and pressure roll, the distance being such that the belt has sufficient time to rise to the proper fusing temperature prior to contacting the toner images. Thus, when copy substrates carrying toner images thereon pass through this area the images are subjected simultaneously to heat and pressure. 
     An important aspect of our invention lies in the separation of the heat and pressure functions. By separating the two functions it is possible to optimize each of them whereas if, as in prior art devices, they are integrated, design criteria that must be satisfied for optimum heating of the belt are counter-productive to optimization of the pressure function and vise versa. For example, in an integrated arrangement, since pressure creating application requires a relatively large rigid mass the heating of such a mass would require standby energy to be used in order to minimize the total time required for making copies. On the other hand, if the mass is substantially reduced in order to accommodate fast heating then the requisite mass for applying the required pressure would not be present. 
    
    
     FIG. 1 is a side view depicting a xerographic reproduction machine or printer of the type adapted to incorporate the present invention; 
     FIG. 2 is a perspective view of one embodiment of a fuser apparatus incorporating the inventive features of the invention; and 
     FIG. 3 is a side elevational view of a preferred embodiment of a fuser apparatus incorporating the inventive features of the invention. 
    
    
     Referring to FIG. 1 of the drawings, there is shown by way of example an automatic xerographic reproduction or printing machine, designated generally by the numeral 10 incorporating a fuser device 99 of the present invention. 
     The reproduction machine 10 depicted in FIG. 1 illustrates the various components utilized in machines of this type for producing copies of a document original 14. Although the device 99 of the present invention is particularly well adapted for use in reproduction machine 10, it should become evident from the following description that it is equally well suited for use in a wide variety of other reproduction and printing machine types and systems and is not necessarily limited in application to the particular embodiment of embodiments shown herein. 
     Reproduction machine 10 has an image recording photoreceptor 15 in the form of a drum, the outer periphery of which has a suitable photoconductive material 16. Photoreceptor 15 is suitably journaled for rotation within the machine frame (not shown) as by means of shaft 17. A main drive motor 19 is drivingly coupled to photoreceptor 15, motor 19 rotating photoreceptor 15 in the direction indicated by arrow 18 to ring the photoconductive surface 16 of photoreceptor 15 past a series of xerographic processing stations. A suitable controller 21 with microprocessor 22 and memory 23 is provided for operating in predetermined timed relationship the various components that comprise machine 10 to reproduce the document original 14 upon a sheet of final support material such as copy sheet 20. As will be understood by those familiar with the art, memory 23 may comprise suitable read only memory (ROM), random access memory (RAM), and/or non-volatile memory (NVM), memory 23 serving to store the various operating parameters for reproduction machine 10 and the copy run information programmed by the machine user or operator. 
     Initially, the photoconductive surface 16 of photoreceptor 15 is uniformly charged by a suitable charging device such as scorotron 25 at charging station 24. The uniformly charged photoconductive surface 16 is exposed at exposure station 26 to create a latent electrostatic image of the document original 14 on photoreceptor 15. For this purpose, a suitable supporting surface or platen 28 for document original 14 is provided having a scan aperture or slit 30 therethrough. A suitable document transport, depicted herein as inlet and outlet constant velocity roll pairs 32, 33 is provided for transporting the document original past scan slit 30. Roll pairs 32, 33 are drivingly coupled to main drive motor 19, roll pair 32 being coupled through an electromagnetically operated clutch 34. A suitable document sensor (not shown) is provided at the inlet to platen 28 for sensing the insertion of a document original 14 to be copied and initiating operation of the reproduction machine 10. 
     A lamp 35, which is disposed below platen 28, serves to illuminate scan slit 30 and the line-like portion of the document original 14 thereover. A suitable fiber optic type lens array 37, which may, for example, comprise an array of gradient index fiber elements, is provided to optically transmit the image ray reflected from the line-like portion of the document original being scanned to the photoconductive surface 16 of photoreceptor 15 at exposure station 26. 
     Following exposure, the latent image of the photoconductive surface 16 of photoreceptor 15 is developed at a development station 40. There, a suitable developer such as magnetic brush roll 41, which is drivingly coupled to main drive motor 19, brings a suitable developer mix in developer housing 43 into developing elevation with the latent image to develop the image and render the same visible. 
     Copy sheets 20 are supported in stack-like fashion on base 44 of copy sheet supply tray 45. Suitable biasing means are provided to raise base 44 of tray 45 and bring the topmost copy sheet 20 in the stack of sheets 47 into operative relationship with segmented feed rols 49. Feed rolls 49 are driven by main drive motor 19 through an electromagnetically operated clutch 51. Rolls 49 serve upon actuation of clutch 51 to feed the topmost copy sheet forward into the nip of a registration roll pair 50 which register the copy sheet with the image on the photoconductive surface 16 of photoreceptor 15. Registration roll pair 50 advance the copy sheet to transfer station 52. There, suitable transfer/detack means such as transfer/detack corotrons 53, 54 bring the copy sheet into transfer relation with the developed image on photoconductive surface 16 and separate the copy sheet therefrom for fixing and discharge as a finished copy. 
     Following transfer station 52, the image bearing copy sheet is transported to fuser 57 where the image is permanently fixed to the copy sheet. Following fusing, the finished copy is transported by roll pair 56 to a suitable receptacle such as an output tray (not shown). Registration roll pair 50 and transport roll pair 56 are driven by main drive motor 19 through suitable driving means such as belts and pulleys. 
     Following transfer, residual developer remaining on the photoconductive surface 16 of photoreceptor 15 is removed at cleaning station 62 by means of cleaning blade 63 (FIG. 2). Developer removed by blade 63 is deposited into a suitable collector 64 for removal. 
     While a drum type photoreceptor is shown and described herein, it will be understood that other photoreceptor types may be employed such as belt, web, etc. 
     To permit effective and controlled charging of the photoconductive surface 16 by scorotron 25 to a predetermined level necessitates that any residual charges on the photoconductive surface 16 or trapped in the photoreceptor be removed prior to charging. An erase device 69 is provided for this purpose. 
     At the cleaning station 62, the cleaning blade 63 is supported in contact with the photoreceptor 15 such that residual toner is chiselled therefrom. 
     The toner and debris that are removed from the photoreceptor 15 fall into the collector 64 and are transported by means of an auger 72 disposed in the bottom of the collector 64. It is moved toward the back of the machine where it falls through an opening in the bottom of the collector 64. The residual toner and debris fall downwardly via conduit 71 into a receptacle (not shown) which serves to store the residual toner until the receptacle is full after which it is removed from the machine. 
     The inventive aspects of our invention will become apparent from a detailed discussion of FIGS. 2 and 3. 
     The fuser apparatus 57 disclosed in FIG. 2 comprises a relatively thin fuser belt structure 80 comprising a base member 82 preferably fabricated from a metal which is sufficiently stiff to be dragged across a non-rotating mandrel. To this end, the base member is fabricated from nickel by a conventional electroforming process which provides a seamless belt of uniform thickness in the order of 2-3 mils. The outer surface of the base member is coated with a conformable layer 84 which preferably comprises silicone or Viton (trademark of E. I. Dupont) rubber. The inner surface of the base member 82 is preferably coated with a low friction material such as polytetrafluoroethylene commonly known by the tradename Teflon (registered trademark of E. I. DuPont). The thickness of the conformable layer is preferably at least 5 mils, depending on modulus, in this embodiment. 
     The belt structure is heated by a radiant heater 86 to a temperature suitable for fusing toner images carried by copy sheets 20. The radiant heated 86 is positioned at a predetermined distance away from a nip area 88 through which the copy sheets pass with the conformable layer 84 contacting the toner images on the sheets. This distance between the nip area and the fuser is such that the heated portion of the belt contacts the toner images before the temperature of the belt has time to drop to a non-fusing temperature. 
     Because the belt structure is relatively thin, it is incapable of creating adequate nip pressures for fusing by the simultaneous application of heat and pressure. Accordingly, there is provided a rigid pressure mandrel 92 for creating the required pressure in the nip area. The belt structure 80 is entrained about the mandrel 92 and a roller 94. A suitable force applying device such as a cam 96 is provided for effecting pressure engagement of the rod 90 and the mandrel 92 which, in turn, cooperate with pressure roll 100 to create the desired pressure on the belt and toner images sandwiched between the mandrel 92 and the pressure roll. The cam is designed to apply a loading in the nip area 88 of approximately 200 pounds or 70-100 PSI. A suitable drive train represented schematically by the reference character 101 serves to drive the pressure roll 100 which, in turn, frictionally effects movement of the belt about the mandrels. 
     The belt structure 80 and radiant heater 86 form a low (i.e. less than 150 grams and preferably 80 grams) mass fuser which can be elevated to an operating level in 6-8 seconds while operating at fusing speeds from 10-12 in/sec or any other desired speed. For such operating conditions, the power rating of the radiant energy source 86 is in the order of 1500-2000 watts. The belt structure in its non-tensioned condition has a diameter of 21/2 inches and a width of 13 inches or greater. 
     Another embodiment 99 of the fuser apparatus disclosed in FIG. 3 comprises a fuser belt structure 80. The belt structure is entrained about a stationary mandrel 102 and a thin-walled, rotationally supported tube heater 104, the latter of which has an internal source of energy 106 for elevating the temperature of the belt. A nip 108 is formed between the belt surface and a pressure roll 110. The nip is effected through rotation of cam member 118 and its engagement with a cam follower 120, the former of which is rotated by a drive system indicated schematically by reference character 116. The mandrel has appended thereto a plurality of insulating nubs 112 to minimize the heat loss from the belt. Rotation of the pressure roll in a manner similar to that for rotating conventional roll fusers causes the belt to move about the mandrel whereby a heated portion of the belt is brought into the nip for fusing in toner images. In this embodiment, the belt structure 80, tube heater 104 and the internal heat source 106 form the low mass fuser. 
     As may now be appreciated from the foregoing, by separating the heating and pressure application functions we have provided a fuser that has a very fast warmup time. Consequently, the fuser apparatus representing our invention can be operated at relatively high (i.e. 10-12 inches/sec.) speeds without the use of standby power.