Patent Publication Number: US-7899353-B2

Title: Method and apparatus for fusing toner onto a support sheet

Description:
BACKGROUND 
     Fuser assemblies, electrophotographic apparatuses, and methods of fusing toner on support sheets in electrophotographic processes are disclosed. 
     In a typical electrophotographic printing process, a photoconductive member having a photoconductive layer is substantially uniformly charged. The photoconductive member is then exposed to selectively discharge areas of the photoconductive layer, while charge in other areas corresponding to image areas of an original document is maintained, so as to record an electrostatic latent image of an original document on the photoconductive layer. The latent image is then developed by depositing developer material including toner on the photoconductive layer. The developer material is attracted to the charged image areas to produce a visible toner image on the photoconductive layer. The toner image is then transferred from the photoconductive member to a support sheet. 
     To fuse (i.e., fix) the toner onto the support sheet, the toner is heated. The toner then cools and solidifies, resulting in the toner being bonded to the support sheet. 
     One process for the thermal fusing of toner onto support sheets involves passing a support sheet having a toner image thereon between rolls of a fuser with a nip between them. Belt fusers include a pressure roll, a fuser roll and a fuser belt positioned between the rolls. During operation, the support sheet with a toner image is passed to a nip between the rolls, and the pressure roll presses the support sheet onto the fuser roll. The fusing temperature for the toner image is controlled based on the temperature of the fuser belt. 
     It would be desirable to provide belt fusers that have a suitably long service life and are energy efficient. 
     SUMMARY 
     According to aspects of the embodiments, there are provided fuser assemblies for fusing toner on support sheets, electrophotographic apparatuses and methods of fusing toner on support sheets. Embodiments of the fuser assemblies include a fuser belt; a thermally-insulated enclosure surrounding at least a portion of the fuser belt; a conveyor for conveying the support sheet to a nip at which the fuser belt contacts the support sheet and the toner is fused onto the support sheet; a pre-heater; and a heat transfer system for transferring heat from inside of the enclosure to the pre-heater, the pre-heater using the heat to pre-heat the support sheet before the support sheet is conveyed to the nip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embodiment of an electrophotographic apparatus; 
         FIG. 2  illustrates an embodiment of a fuser assembly including a continuous fuser belt and a support sheet pre-heater; 
         FIG. 3  illustrates a portion of an embodiment of a fuser assembly including a non-continuous fuser belt; 
         FIG. 4  illustrates another embodiment of a fuser assembly including a continuous fuser belt and a support sheet pre-heater; 
         FIG. 5  shows a calculated isothermal temperature versus distance profile for the nip region of a fuser assembly at a fuser belt temperature of 204° C. without pre-heating of a support sheet; and 
         FIG. 6  shows a calculated isothermal temperature versus distance profile for the nip region of a fuser assembly at a fuser belt temperature of 192° C. for a support sheet pre-heated to a temperature of 40° C. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the embodiments disclosed herein relate to fuser assemblies, electrophotographic apparatuses including the fuser assemblies, and methods of fusing toner on support sheets using the fuser assemblies. 
     The disclosed embodiments include a fuser assembly for fusing toner onto a support sheet, which comprises a fuser belt; a thermally-insulated enclosure surrounding at least a portion of the fuser belt; a conveyor for conveying the support sheet to a nip at which the fuser belt contacts the support sheet and the toner is fused onto the support sheet; a pre-heater; and a heat transfer system for transferring heat from inside of the enclosure to the pre-heater, the pre-heater using the heat to pre-heat the support sheet before the support sheet is conveyed to the nip. 
     The disclosed embodiments further include a fuser assembly for fusing toner onto a support sheet, which comprises an endless fuser belt; a thermally-insulated enclosure surrounding at least a portion of the fuser belt; a conveyor including an endless conveyor belt for conveying the support sheet to a nip at which the fuser belt contacts the support sheet and the toner is fused onto the support sheet; a pre-heater; and an air circulation system for circulating hot air from inside of the enclosure to the pre-heater, wherein the pre-heater comprises a heat exchanger heated by the hot air circulated from the enclosure, the heat exchanger including a heating member for conductively heating the conveyor belt, which conductively pre-heats the support sheet before the support sheet is conveyed to the nip. 
     The disclosed embodiments further include a method of fusing toner onto a support sheet having toner thereon. The method comprises containing heat emanated by a fuser belt contained within a thermally-insulated enclosure at least partially surrounding the fuser belt; transferring heat from inside of the enclosure to a pre-heater; pre-heating a first support sheet supported on a conveyor with the pre-heater using heat transferred from the enclosure; and conveying the pre-heated first support sheet on the conveyor to a nip and fusing the toner onto the first support sheet. 
       FIG. 1  illustrates an exemplary electrophotographic apparatus (digital imaging system) in which embodiments of the disclosed fuser assembly can be used. Such digital imaging systems are disclosed in U.S. Pat. No. 6,505,832, which is hereby incorporated by reference in its entirety. The imaging system is used to produce an image, such as a color image output in a single pass of a photoreceptor belt. It will be understood, however, that embodiments of the fuser assemblies can be used in other imaging systems. Such systems include, e.g., multiple-pass color process systems, single or multiple pass highlight color system, or black and white printing systems. 
     As shown in  FIG. 1 , an output management system  660  can supply printing jobs to a print controller  630 . Printing jobs can be submitted from the output management system client  650  to the output management system  660 . A pixel counter  670  is incorporated into the output management system  660  to count the number of pixels to be imaged with toner on each sheet or page of the job, for each color. The pixel count information is stored in the output management system  660  memory. The output management system  660  submits job control information, including the pixel count data, and the printing job to the print controller  630 . Job control information, including the pixel count data and digital image data are communicated from the print controller  630  to the controller  490 . 
     The printing system can use a charge retentive surface in the form of an active matrix (AMAT) photoreceptor belt  410  supported for movement in the direction of arrow  412 , for advancing sequentially through the various xerographic process stations. In the embodiment, the photoreceptor belt  410  is a continuous (endless) belt provided on a drive roll  414 , tension roll  416  and fixed roll  418 . The drive roll  414  is operatively connected to a drive motor  420  for moving the photoreceptor belt  410  sequentially through the xerographic stations. 
     During the printing process, a portion of the photoreceptor belt  410  passes through a charging station A including a corona generating device  422 , which charges the photoconductive surface of photoreceptor belt  410  to a relatively high, substantially uniform potential. 
     Next, the charged portion of the photoconductive surface of the photoreceptor belt  410  is advanced through an imaging/exposure station B. At the imaging/exposure station B, a controller  490  receives image signals from the print controller  630  representing the desired output image, and processes these signals to convert them to signals transmitted to a laser-based output scanning device, which causes the charged surface to be discharged in accordance with the output from the scanning device. In the exemplary system, the scanning device is a laser raster output scanner (ROS)  424 . 
     The photoreceptor belt  410 , which is initially charged to a voltage V 0 , undergoes dark decay to a level equal to about −500 volts. When exposed at the exposure station B, the photoreceptor belt  410  is discharged to a voltage level equal to about −50 volts. After exposure, the photoreceptor belt  410  contains a monopolar voltage profile of high and low voltages, with the high voltages corresponding to charged areas and the low voltages corresponding to discharged or developed areas. 
     At a first development station C, comprising a developer structure  432  utilizing a hybrid development system, a developer roll is powered by two developer fields. The first field is the AC field, which is used for toner cloud generation. The second field is the DC developer field which is used to control the amount of developed toner mass on the photoreceptor belt  410 . The toner cloud causes charged toner particles to be attracted to the electrostatic latent image. Appropriate developer biasing is accomplished via a power supply. This type of system is a non-contact type in which only toner particles (black, for example) are attracted to the latent image and there is no mechanical contact between the photoreceptor belt  410  and a toner delivery device to disturb a previously developed, unfixed image. A toner concentration sensor  200  senses the toner concentration in the developer structure  432 . 
     The developed image is then transported past a second charging device  436  where the photoreceptor belt  410  and developed toner image areas are recharged to a predetermined level. 
     A second exposure/imaging is performed by device  438  including a laser-based output structure, which selectively discharges the photoreceptor belt  410  on toned areas and/or bare areas, pursuant to the image to be developed with the second color toner. At this point of the process, the photoreceptor belt  410  contains toned and untoned areas at relatively high voltage levels, and toned and untoned areas at relatively low voltage levels. These low voltage areas represent image areas, which are developed using discharged area development (DAD). A negatively-charged, developer material  440  comprising color toner is employed. The toner, e.g., yellow toner, is contained in a developer housing structure  442  disposed at a second developer station D and is transferred to the latent images on the photoreceptor belt  410  using a second developer system. A power supply (not shown) electrically biases the developer structure to a level effective to develop the discharged image areas with negatively charged yellow toner particles. Further, a toner concentration sensor can be used to sense the toner concentration in the developer housing structure  442 . 
     The above procedure is repeated for a third image for a third suitable color toner, such as magenta (station E), and for a fourth image and suitable color toner, such as cyan (station F). The exposure control scheme described below may be utilized for these subsequent imaging steps. In this manner, a full-color composite toner image is developed on the photoreceptor belt  410 . In addition, a mass sensor  110  measures developed mass per unit area. 
     In case some toner charge is totally neutralized, or the polarity reversed, thereby causing the composite image developed on the photoreceptor belt  410  to consist of both positive and negative toner, a negative pre-transfer dicorotron member  450  is provided to condition the toner for transfer to a support sheet using positive corona discharge. 
     Subsequent to image development, a support sheet  452  (e.g., paper) is moved into contact with the toner images at transfer station G. The support sheet  452  is advanced to transfer station G by a sheet feeding apparatus  500 . The support sheet  452  is then brought into contact with the photoconductive surface of photoreceptor belt  410  in a timed sequence so that the toner powder image developed on the photoreceptor belt  410  contacts the advancing support sheet  452  at the transfer station G. 
     The transfer station G includes a transfer dicorotron  454 , which sprays positive ions onto the backside of the support sheet  452 . The ions attract the negatively charged toner powder images from the photoreceptor belt  410  to the support sheet  452 . A detack dicorotron  456  is provided for facilitating stripping of support sheets from the photoreceptor belt  410 . 
     After transfer of the toner images, the support sheet continues to move, in the direction of arrow  458 , onto a conveyor  600 . The conveyor  600  advances the support sheet to a fusing station H. The fusing station H includes a fuser assembly  460  for permanently affixing the transferred powder image to the support sheet  452 . The fuser assembly  460  comprises a heated fuser roll  462  and a pressure roll  464 . The support sheet  452  passes between the fuser roll  462  and pressure roll  464  with the toner powder image contacting the fuser roll  462 , causing the toner powder images to be permanently affixed to the support sheet  452 . After fusing, a chute (not shown) guides the advancing support sheet  452  to a catch tray, stacker, finisher or other output device (not shown), for subsequent removal from the printing apparatus by the operator. The fuser assembly  460  can be contained within a cassette, and can include additional elements not shown in  FIG. 1 , such as a belt around the fuser roll  462 . 
     After the support sheet  452  is separated from the photoconductive surface of the photoreceptor belt  410 , residual toner particles carried by the non-image areas on the photoconductive surface are removed from the photoconductive surface. These toner particles are removed at cleaning station I using a cleaning brush structure contained in a housing  466 . 
     The controller  490  is operable to regulate the various printer functions. The controller  490  can be a programmable controller operable to control printer functions described above. 
       FIG. 2  illustrates an exemplary embodiment of a fuser assembly  800  constructed to provide improved thermal efficiency in different types of electrophotographic apparatuses. For example, in the electrophotographic apparatus shown in  FIG. 1 , the fuser assembly  800  can be used in place of the fuser assembly  460  at station H. 
     The fuser assembly  800  further includes a conveyor  810  with an endless (continuous) conveyor belt  812 . A fuser roll  814  and a pressure roll  816  are located near the downstream end of the conveyor belt  812 . The fuser roll  814  and pressure roll  816  define a nip  818  between them. In the embodiment, an endless fuser belt  820  is provided on the fuser roll  814  and on a belt roll  822 . A tensioning roll  824  is arranged to tension the fuser belt  820 . The fuser belt  820  can be driven in the counter-clockwise direction by a stepper motor or the like (not shown). 
     The conveyor belt  812  is driven in the clock-wise direction by a motor (not shown) to convey a support sheet  825  with a toner image to the nip  818 . At the nip  818 , the fuser belt  820  contacts the support sheet  825  and sufficient heat and pressure are applied to fuse the toner on the support sheet  825 . Typically, the fusing temperature used to fuse the toner on the support sheet at the nip  818  is in the range of about 180° C. to about 200° C. The glass transition temperature of toner is typically in the range of about 55° C. to about 65° C. 
     In the embodiment, the fuser belt  820  can be longer than a typical fuser belt. For example, the fuser belt  820  can have a length of at least about 350 mm, such as at least about 500 mm, 600 mm, 700 mm, 800 mm, 900 mm, 1000 mm, or even longer. 
     The primary failure modes of belt fusers, which represent the largest contribution to fuser run cost, are typically attributed to the life of the fuser belt. The fuser belt comes into contact with the toner during the fusing process, and largely influences the final quality of prints. The longer fuser belt  820  used in the fuser assembly  800  can provide a relatively longer service life than shorter belts because the longer fuser belt  820  has more total surface area available for wear. 
     The greater total exposed surface area of the longer fuser belt  820  causes it to emanate significant heat during fusing. Accordingly, it would be desirable to provide fuser assemblies that include longer fuser belts to utilize the advantage of increased belt service life (and thus also increased fuser assembly service life), without comprising thermal efficiency. The fuser assembly  800  is constructed to reclaim heat emanated by the longer fuser belt  820  so that this heat is not lost within the fuser assembly as waste heat. 
       FIG. 3  depicts another embodiment of a fuser assembly including a non-continuous (i.e., non-endless) fuser belt  1020 . During operation, the fuser belt  1020  is unspooled from the roll  1022  onto the roll  1024  as indicated by arrows A, B, and then unspooled from the roll  1024  onto the roll  1022  by rotation of the rolls  1022 ,  1024  in the reverse direction. A support sheet  825  is shown entering the nip  818  between the fuser roll  814  and the pressure roll  816 . The fuser assembly including the fuser belt  1020  can be constructed to reclaim heat emanated by the fuser belt  1020 . 
     As shown in  FIG. 2 , the fuser assembly  800  further includes a thermally-insulated enclosure  830 . The enclosure  830  includes an open end  832  and an interior space  834 . The enclosure  830  is constructed to confine heat emanated by the fuser belt  820 , as well as by other components of the fuser assembly  800  that are enclosed by the enclosure  830 , inside of the enclosure during operation of the fuser assembly  800 . In the embodiment, the enclosure  830  is configured to surround at least a portion of the fuser belt  820 , such as a significant portion of the fuser belt as shown in  FIG. 2 , and also surround a portion of the fuser roll  814 . It is desirable that the enclosure  830  have a small size so that the volume of the space  834  for confining heat is small. 
     The enclosure  830  is comprised partially or entirely of at least one thermal insulator material. The thermal insulator material used to form the enclosure  830  can be any material having the desired thermal insulating properties and which is compatible for use within the environment of the fuser assembly  800 . For example, the enclosure can be constructed entirely of at least one ceramic, polymeric (e.g., plastic) or composite material. In embodiments, these materials can be formed in the desired configuration of the enclosure  830  by a molding process, for example. Alternatively, the enclosure  830  can be made from two or more pieces of such materials, which are joined together using an adhesive, fasteners, or the like. In other embodiments, the enclosure can be made from at least one thermal insulator material and at least one other material that is not used for its thermal insulating properties. For example, a fiberglass material or like thermal insulator can be provided on a plastic, metallic or composite substrate. In other embodiments, a sheet of a thermal insulator material can be secured to a sheet of a metal or plastic to form a laminate structure. The enclosure  830  can be fixedly mounted in an electrophotographic apparatus in any suitable manner, such as by attachment to the mainframe. 
     The ability of the enclosure  830  to confine heat emanated by the fuser belt  820  and other components located within the space  834  can be increased by, for example, increasing the thickness of the thermal insulator material(s), using thermal insulator materials having a reduced thermal conductivity value (i.e., k value), and/or decreasing the size of the open end  832  of the enclosure  830  to control air flow into and out of the enclosure. By increasing the heat confinement efficiency of the enclosure  830 , a greater percentage of the heat emanated from the fuser belt  820  and other components, which otherwise would be waste heat, can be reclaimed and used to preheat support sheets prior to fusing toner on the sheets using the fuser belt  820 . By using this pre-heating, the temperature to which the fuser belt  820  needs to be heated in order to effectively fuse toner on support sheets using the fuser belt  820  can be reduced as compared to not reclaiming this heat. Consequently, the total amount of energy consumption by the fuser assembly  800  can be reduced by using pre-heating. 
     Heat emanated by the fuser belt  820  and other components inside of enclosure  830  heats the air within the enclosure to an elevated temperature. By thermally insulating the fuser belt  820  within the enclosure  830 , the air temperature within the space  834  is increased relative to the air temperature (i.e., ambient air temperature) outside of the enclosure  830 . The configuration of the enclosure  830  and the materials used to form the enclosure  830  can be selected to control the heat confinement efficiency of the enclosure  830 , and thereby control the maximum air temperature that is reached within the space  834  during operation of the fuser assembly  800 . The enclosure  830  can be constructed so that internal electrical components, such as sensors, electrical wiring and the like, are not exposed to temperatures that can cause heat-related damage to these components. For example, the enclosure can be constructed so that the maximum air temperature reached within the space  834  during operation of the fuser assembly  800  is about 120° C., 130° C., 140° C., or 150° C. A temperature sensor (not shown) can optionally be provided in the fuser assembly  800  to monitor the air temperature within the space  834 , to ensure that the maximum air temperature is not exceeded. 
     In the embodiment, the fuser assembly  800  includes a heat transfer system  840  for transferring heat from the space  834  inside of the enclosure  830  to a pre-heater  850  for heating support sheets. The heat transfer system  840  includes an air-circulating system for circulating hot air from the enclosure  830  to the pre-heater  850 . The air circulating system includes a flow passage  842  extending from the enclosure  830  to the pre-heater  850 , and a blower  844  operable to circulate the hot air through the flow passage  842 . The flow passage  842  is desirably thermally insulated to minimize cooling of the hot air within the flow passage. As indicated by arrow  843 , the blower  844  also re-circulates ambient air into the enclosure  830  through the open end  832 . In the space  834 , this re-circulated air is heated by heat emanated by the fuser belt  820  and other components. This heated air is circulated to the pre-heater  850  through the flow passage  842  by operation of the blower  844 . 
     As indicated by arrows  851 , hot air supplied to the pre-heater  850  via the flow passage  842  is applied to the support sheet  825  being conveyed by the conveyor  810 . The hot air pre-heats the support sheet  825  primarily by convection before the support sheet  825  reaches the nip  818 , where it is subjected to sufficient heat and pressure via the fuser belt  820  and pressure roll  816  to fuse the toner onto the support sheet  825 . In the fuser assembly  800 , heat emanated by the fuser belt  820  and other components confined by the enclosure  830  is reclaimed and used as the primary heat source to pre-heat support sheets before fusing toner on the support sheets. 
     By pre-heating the support sheet  825  using the hot air distributed by the pre-heater  850 , the amount of additional heat that needs to be supplied to the support sheet  825  at the nip  818  via the fuser belt  820  (and optionally the pressure roll  816 ) to effect fusing of the toner on the support sheet  825  can be reduced significantly as compared to not pre-heating the support sheet  825  prior to fusing. The amount of additional heat applied to the support sheet  825  at the nip  818  by the fuser belt  820  is controlled by the fuser temperature set-point. As the amount of energy that needs to be applied to the fuser roll  814  (and optionally also to the pressure roll  816 ) in order to heat the fuser belt  820  to a sufficiently-high temperature to fuse toner onto the support sheet  825  can be reduced in the fuser assembly  800 , the fuser temperature set-point can be reduced. Accordingly, using the pre-heater  850  to pre-heat the support sheet  825  with the reclaimed heat from the enclosure  830  enhances the energy efficiency of the fuser assembly  800 . 
     In the embodiment, the pre-heater  850  is positioned to distribute the hot air from the enclosure  830  directly onto the support sheet  825  being conveyed on the conveyor belt  812 . The pre-heater  850  comprises a housing  852  defining a plenum  854 . The housing  852  is desirably thermally insulated to minimize cooling of the hot air within the plenum  854 . The pre-heater  850  also includes a porous member  856  positioned adjacent the conveyor  810  for distributing the hot air onto the support sheet  825  to pre-heat the support sheet. The porous member  856  can be located close to the conveyor belt  812  (e.g., within a distance of about 50 mm) to minimize cooling of the hot air reaching the support sheet  825 . 
     It is desirable that the pre-heat temperature of the support sheet  825  be below the glass transition temperature for the toner on the support sheet. For example, for a duplex (two-sided) printing process, it is desirable to limit the maximum temperature to which the support sheet  825  is heated by the hot air typically to a temperature of about 60° C. to 70° C., in order to avoid fused toner being subject to image quality (IQ) defects on the support sheets. The pre-heat temperature of the support sheet  825  can be controlled by adjusting the flow of the hot air from the enclosure  830  to the pre-heater  850 . 
       FIG. 4  depicts a fuser assembly  900  according to another embodiment. In this embodiment, the fuser assembly  800  includes a fuser belt  920 , fuser roll  914 , pressure roll  916 , roll  922 , roll  924 , enclosure  930  and conveyor  910  having a conveyor belt  912 . These components can have the same structures as the corresponding components included in the fuser assembly  800 . 
     As shown in  FIG. 4 , the enclosure  930  includes an open end  932  and an interior space  934 . The enclosure  930  can surround at least a portion of the fuser belt  920  and the fuser roll  914 , as shown. 
     The fuser assembly  900  includes a heat transfer system  940  with an air circulating system for circulating hot air from within the enclosure  930  to a pre-heater  950 . In this embodiment, the air circulating system includes a flow passage  942  extending from the enclosure  930  to the pre-heater  950 , and a blower  944 . The flow passage  942  is desirably thermally insulated to minimize cooling of the hot air within the flow passage. The blower  944  is operable to circulate the hot air through the flow passage  942 . As indicated by arrow  943 , the blower  944  also re-circulates air from the pre-heater  950  to the open end  932  of the enclosure  930  via a flow passage  943 . In the space  934 , the re-circulated air is heated by heat emanated by the fuser belt  920  and other components, and this heated air is transported to the pre-heater  950  through the flow passage  942 . 
     In the embodiment, the pre-heater  950  is constructed to directly heat the conveyor belt  912  by conduction, which, in turn, directly heats the support sheet  925  by conduction. In the illustrated configuration of the fuser assembly  900 , the pre-heater  950  heats the bottom portion of the rotating conveyer belt  912 . Heat is conducted from the conveyor belt  912  to the support sheet  925  supported on the top portion of the conveyor belt  912 . Accordingly, the pre-heater  950  indirectly pre-heats the support sheet  925  before toner is fused onto the support sheet at the nip  918 . 
     In the embodiment, the pre-heater  950  includes a housing  952  defining a space  954 , and a heat exchanger  958 . The heat exchanger  958  is heated by hot air circulated from the space  934  within the enclosure  950  to the space  954  within the housing  952  via the flow passage  942 . The housing  952  can be thermally insulated to reduce cooling of the hot air in the space  954  to allow the heat exchanger  958  to be heated to a desirable temperature. 
     The heat exchanger  958  can heat the conveyor belt  912  to a desired temperature to effect pre-heating of support sheets. The temperature to which the conveyor belt  912  is heated by the heat exchanger  958  can be selected based on various factors including, for example, the thickness of the support sheet  925 , the thermal conductivity of the support sheet  925 , and the toner composition (and corresponding glass transition temperature and thermal conductivity). The enclosure  930 , heat transfer system and pre-heater  950  are constructed to allow control of the temperature to which the heat exchanger  958  is heated by the hot air transferred from the enclosure  930 . For example, the configuration and materials of the enclosure  930 , the heat insulating characteristics of the flow passage  912  and pre-heater  950 , the heat transfer characteristics of the heat exchanger  958 , and the blower  944  can be selected to control heat transfer from the enclosure  930  to the pre-heater  950  and heating of the conveyor belt  912  by the pre-heater. 
     It is desirable that the pre-heat temperature of the support sheet  925  be less than the glass transition temperature for the toner. For example, for a duplex (two-sided) printing process, it is desirable to limit the maximum temperature to which the conveyor belt  912  is heated typically to a temperature of about 60° C. to 70° C. to avoid fused tuner being subject to image quality (IQ) defects on the support sheets. To avoid heating the support sheet  925  to a temperature above the glass transition temperature of the toner, the temperature of the conveyor belt  912  can be maintained no higher than slightly above the glass transition temperature of the toner. 
     In the embodiment, the heat exchanger  958  includes a plurality of fins  960  to provide a high effective surface area for convective heat transfer from the hot air. By increasing the effective surface area of the fins, the amount of air flow of the hot air needed to heat the fins  960  to a desired temperature can be decreased. The fins  960  are in thermal contact with a heating member  962 , such as a metallic plate. The heating member  962  can have a width as large as that of the conveyor belt  912  to allow the metallic plate to directly heat the entire width of the conveyor belt  912 . 
     The temperature of the conveyor belt  912  can be controlled by adjusting the flow of the hot air from the enclosure  930  to the pre-heater  950 . 
     In an embodiment, the heating member  962  can be selectively movable toward and away from the conveyor belt  912  to control heating of the conveyor belt  912 . This movement of the heating member  962  can be provided, for example, by a mechanism and a motor (not shown) operatively connected to the heating member  962 . The motor can be controlled by a controller. The heating member  962  can be automatically moved into contact with the conveyor belt  912  to heat the conveyor belt  912 , or moved away from the conveyor belt  912  to discontinue heating of the conveyor belt  912 . The surface of the heating member  962  facing the conveyor belt  912  can optionally be coated with a thermally-conductive, lubricating substance, effective to reduce wear of the conveyor belt  912  caused by contact with the heating member  962 . The lubricating substance that is used should be chemically compatible with the support sheets and toner. 
     The fuser assembly  900  can be used for fusing toner on support sheets having a range of thicknesses. During operation of an electrophotographic apparatus, a user may produce copies using support sheets all of the same thickness, or from support sheets having different thicknesses. For example, a user may make copies using support sheets having a first thickness and then make copies from support sheets having a greater second thickness. The amount of heat that needs to be supplied to thicker support sheets to fuse toner on the sheets generally is greater than the amount of heat that needs to be supplied to thinner support sheets of the same material to fuse the same toner composition on the thinner sheets. In order to heat thicker sheets to a sufficiently-high temperature to fuse toner on the sheets, the fuser assembly typically heats the fuser belt to a higher temperature than used for thinner support sheets in order to supply an increased amount of heat to the thicker support sheets to effect fusing of toner on the sheets. 
     Increasing the temperature of the fuser belt (i.e., the fuser temperature set point) during operation of the fuser assembly requires increasing the amount of heat supplied to the fuser belt. Heating the fuser belt from one set point to a higher set point can cause a time delay in the printing process. To reduce this time delay, the apparatus can be programmed to begin to increase the temperature set point of the fuser belt before thicker support sheets are printed. This approach may result in thinner support sheets being subjected to a higher fuser temperature set point than needed to fuse toner on the thinner sheets. 
     Embodiments of the disclosed fuser assemblies, such as the fuser assembly  900 , can be used to fuse toner on both thinner and thicker sheets while keeping the temperature set point of the fuser belt  920  more uniform. For example, to fuse toner on thinner support sheets using the fuser assembly  900 , the heating member  962  can be moved away from contact with the conveyor belt  912  so that the support sheet  925  is not subjected to pre-heating. The temperature set point of the fuser belt  920  can be selected such that the fuser belt  920  supplies sufficient heat to the thinner support sheet  925  in the nip  918  to fuse toner on the support sheet. When a thicker support sheet  925  is to be printed using the fuser assembly  900 , the heating member  962  can be moved into contact with the conveyor belt  912  to effect pre-heating of the thicker support sheet  925  so that the fuser belt  920  supplies sufficient additional heat to the support sheet  925  in the nip  918  to fuse toner on the thicker support sheet. The heating member  962  can be used to heat the conveyor belt  912  to the desired temperature to pre-heat the thicker support sheet more quickly than heating the fuser belt  920  to a higher temperature set point. Due to the amount of heat needed to heat the fuser belt  920 , which desirably has a longer length, to a higher set point, it can also be more energy efficient to pre-heat the support sheet  925  as compared to not pre-heating the support sheet  925 , but instead increasing the temperature set point of the fuser belt  920 . Accordingly, the fuser assembly  900  (and other embodiments of the disclosed fuser assemblies) can provide improved time and energy efficiency when used for printing thinner and thicker support sheets in the same apparatus. 
     Aspects of heat transfer that occurs in embodiments of the disclosed fuser assemblies can be estimated by thermal modeling. When the fuser belt of a fuser assembly is at an elevated temperature, T belt , and exposed to ambient temperature, T amb , the rate of heat loss from the belt, {dot over (Q)} belt     —     loss , is:
 
 {dot over (Q)}   belt     —     loss =α amb   A   belt ( T   belt   −T   amb ),  (1)
 
where α amb  is the convective heat transfer coefficient from the belt surface to the ambient environment.
 
     The amount of heat, {dot over (Q)} paper , that needs to be supplied to a sheet of paper (with toner on the paper) to heat the paper from ambient temperature to the fusing temperature for the toner, T paper     —     out , is given by:
 
 {dot over (Q)}   paper   ≈{dot over (m)}   paper   Cp   paper (   T     paper     —     out   —T   amb ),  (2)
 
where {dot over (m)} paper  is the mass rate of the paper, Cp paper  is the specific heat of the paper, and  T   paper     —     out  is the paper average output temperature.
 
     When the paper is heated by the fuser belt, the heat supplied from the fuser belt to the paper, {dot over (Q)} paper , is approximately equal to: 
                         Q   .     paper     ≈       (         T   _     belt     -       T   _     paper       )       R     belt   ⁢   _   ⁢   paper           ,           (   3   )               
where  T   belt  is the average fuser belt temperature within the nip,  T   paper  is the average paper temperature within the nip, and R belt-paper  is the thermal resistance between the fuser belt and the paper.
 
     When the paper enters the nip at a pre-heat temperature that exceeds T amb  by an amount ΔT, the amount of heat effective to heat the pre-heated paper to the toner fusing temperature, T paper     —     out , is reduced by an amount equal to the product {dot over (m)} paper Cp paper ΔT, as follows: 
     
       
         
           
               
             
               
                 
                   
                     
                       
                         
                           
                             
                               Q 
                               . 
                             
                             paper 
                           
                           = 
                             
                           ⁢ 
                           
                             
                               
                                 m 
                                 . 
                               
                               paper 
                             
                             ⁢ 
                             
                               
                                 Cp 
                                 paper 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     T 
                                     
                                       paper 
                                       ⁢ 
                                       _ 
                                       ⁢ 
                                       out 
                                     
                                   
                                   - 
                                   
                                     ( 
                                     
                                       
                                         T 
                                         amb 
                                       
                                       + 
                                       
                                         Δ 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         T 
                                       
                                     
                                     ) 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                           ⁢ 
                           
                             
                               
                                 
                                   m 
                                   . 
                                 
                                 paper 
                               
                               ⁢ 
                               
                                 
                                   Cp 
                                   paper 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       T 
                                       
                                         paper 
                                         ⁢ 
                                         _ 
                                         ⁢ 
                                         out 
                                       
                                     
                                     - 
                                     
                                       T 
                                       amb 
                                     
                                   
                                   ) 
                                 
                               
                             
                             - 
                             
                               
                                 
                                   m 
                                   . 
                                 
                                 paper 
                               
                               ⁢ 
                               
                                 Cp 
                                 paper 
                               
                               ⁢ 
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 T 
                                 . 
                               
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
     By pre-heating the paper to a temperature above ambient temperature, a lower average belt fusing temperature,  T ′ belt , can be used to heat the paper to the toner fusing temperature.  T ′ belt  is approximated as follows: 
     
       
         
           
             
               
                 
                   
                     
                       T 
                       _ 
                     
                     belt 
                     ′ 
                   
                   ≈ 
                   
                     
                       
                         T 
                         _ 
                       
                       belt 
                     
                     - 
                     
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           T 
                         
                         2 
                       
                       ⁢ 
                       
                         
                           ( 
                           
                             
                               2 
                               ⁢ 
                               
                                 R 
                                 
                                   bel 
                                   ⁢ 
                                   t 
                                   ⁢ 
                                   _ 
                                   ⁢ 
                                   paper 
                                 
                               
                               ⁢ 
                               
                                 
                                   m 
                                   . 
                                 
                                 paper 
                               
                               ⁢ 
                               
                                 Cp 
                                 paper 
                               
                             
                             - 
                             1 
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     When the paper is pre-heated by direct convection (such as with the fuser assembly  800  shown in  FIG. 2 ), hot air at a temperature, T hot     —     air , heats the paper and exits warm (T warm     —     air ). It can be estimated that an average air temperature,  T   preheat     —     air , heats the paper:
 
 {dot over (Q)}   preheat ≈α preheat   A   preheat (   T     preheat     —     air −( T   amb   +ΔT/ 2)),  (6)
 
where α preheat  is the convective heat transfer coefficient between the pre-heat air and the paper.
 
     When the hot air used to pre-heat the paper is supplied from the insulated enclosure containing the belt fuser, the air is heated inside of the enclosure as follows:
 
 {dot over (m)}   air   Cp   air ( T   hot     —     air   −T   warm     —     air )=α cavity   A   belt ( T′   belt   −  T     preheat     —     air ),  (7)
 
where α cavity  is the convective heat transfer coefficient between the fuser belt and the air in the insulated enclosure.
 
     By heating the paper by conduction (i.e., by contact between the heated conveyor belt and the paper) instead of by convection (i.e., by flowing hot air over the paper), the thermal efficiency of the pre-heating process is significantly increased. Also, by using a heat exchanger with a large amount of convective heat transfer surface area (such as a heat exchanger including fins), lower hot air temperatures and lower hot air flow rates can be used to heat the heat exchanger to a temperature effective to heat the paper, as compared to convectively heating the paper by flowing hot air over it: 
                         Q   .     preheat     =         (         T   _       preheat   ⁢   _   ⁢   air       -     (       T   amb     +     Δ   ⁢           ⁢     T   /   2         )       )         1       α   fins     ⁢     A   fins         +     R     fins   ⁢   _   ⁢   belt       +     R     belt   ⁢   _   ⁢   paper           -       Q   .       belt   ⁢   _   ⁢   loss           ,           (   8   )               
where α fins  is the convective heat transfer coefficient between the fins and the hot air, R fins     —     belt  is the thermal resistance between the fins and the conveyor belt, and R belt     —     paper  is the thermal resistance between the conveyor belt and the paper.
 
     As α fins A fins &gt;&gt;α preheat A preheat , and 
                   R     fins   ⁢   _   ⁢   belt       +     R     belt   ⁢   _   ⁢   paper         ⪡     1       α   fins     ⁢     A   fins           ,         
then the equivalent thermal resistance to heat paper by conduction, R eq     —     conduction , compares to the equivalent thermal resistance to heat paper by convection, R eq     —     convection , as follows:
 
     
       
         
           
               
             
               
                 
                   
                     
                       
                         
                           
                             R 
                             
                               eq 
                               ⁢ 
                               _ 
                               ⁢ 
                               conductio 
                               ⁢ 
                               n 
                             
                           
                           = 
                             
                           ⁢ 
                           
                             
                               1 
                               
                                 
                                   1 
                                   
                                     
                                       α 
                                       fins 
                                     
                                     ⁢ 
                                     
                                       A 
                                       fins 
                                     
                                   
                                 
                                 + 
                                 
                                   R 
                                   
                                     fins 
                                     ⁢ 
                                     _ 
                                     ⁢ 
                                     belt 
                                   
                                 
                                 + 
                                 
                                   R 
                                   
                                     belt 
                                     ⁢ 
                                     _ 
                                     ⁢ 
                                     paper 
                                   
                                 
                               
                             
                             ⪡ 
                           
                         
                       
                     
                     
                       
                         
                             
                           ⁢ 
                           
                             1 
                             
                               1 
                               
                                 
                                   α 
                                   preheat 
                                 
                                 ⁢ 
                                 
                                   A 
                                   preheat 
                                 
                               
                             
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                           ⁢ 
                           
                             R 
                             
                               eq 
                               ⁢ 
                               _ 
                               ⁢ 
                               convection 
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
     According to Equation (9), there is a significantly lower thermal resistance for pre-heating paper when using an embodiment of the fuser assembly constructed to heat the paper by conduction (e.g., the embodiment shown in  FIG. 4 ), as compared to using an embodiment of the fuser assembly that is constructed to heat the paper by convection by blowing hot air onto the paper (e.g., the embodiment shown in  FIG. 2 ). Accordingly, embodiments of the fuser assembly that pre-heat support sheets by conduction can provide still higher energy efficiency. 
     EXAMPLES 
     The Table below shows calculated energy consumption and efficiency values: (i) using a fuser assembly without pre-heating capabilities for pre-heating a support sheet, and (ii) using a fuser assembly to conductively pre-heat a support sheet with a heated conveyor belt, such as the embodiment of the fuser assembly shown in  FIG. 4 . A thermal balance was determined using equations (1) through (8) for cases (i) and (ii). For the calculations, the fuser belt was assumed to be heated by lamps inside the heating rolls. 
     As shown in the Table, the amount of power consumed by the lamps to heat the fuser belt to a temperature effective to fuse toner on the support sheet can be reduced significantly by pre-heating the support sheet with a pre-heater before fusing the toner at a nip. Consequently, the total power consumption by the fuser assembly including the pre-heater is significantly lower as compared to a fuser assembly without pre-heating capabilities. 
     The energy efficiency, E, of the respective fuser assemblies used for the fusing processes with and without pre-heating can be expressed as: E=(1−((pre-heat blower power consumption+heat loss)/total power consumption)). As shown in the Table, the energy efficiency using pre-heating is significantly higher than the energy efficiency without using pre-heating. More particularly, the fuser assembly power consumption is reduced by about 35% (i.e., 5138 W−3380W/5138 W) by using pre-heating of the paper, and the energy efficiency, E, is increased from about 53% to 86%. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Without Pre-heating [W] 
                 With Pre-heating [W] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Fuser Lamps Power 
                 5138 
                 3380 
               
               
                 Consumption 
               
               
                 Pre-heat Blower Power 
                 0 
                 150 
               
               
                 Consumption 
                   
                   
               
               
                 Total Power 
                 5138 
                 3530 
               
               
                 Consumption 
               
               
                 Pre-heating Power 
                 0 
                 900 
               
               
                 Consumption 
               
               
                 Heat Loss 
                 2438 
                 350 
               
               
                 Efficiency 
                 53% 
                 86% 
               
               
                   
               
            
           
         
       
     
     Additional calculations demonstrate that by pre-heating the paper prior to fusing toner on the paper, the fuser belt can be operated at a lower temperature set point to heat the paper to a selected toner fusing temperature as compared to fusing the toner without pre-heating the paper. The calculations are for a fuser assembly without pre-heating and a fuser assembly including a pre-heater for conductively heating the paper (such as shown in  FIG. 4 ). In the calculations, the fuser belt includes an outermost layer of perfluoroalkoxy (PFA) and an adjacent underlying layer of silicone. The paper includes toner on its outer surface. The k (thermal conductivity) values for the different materials used in the calculations are shown in  FIG. 5 .  FIG. 5  shows a temperature versus distance (Y) curve at the nip region using a 702 mm/s process speed and a 26 ms dwell time for a fuser assembly including a fuser belt at a temperature set point of 204° C. with the paper entering at an ambient temperature of 25° C. (i.e., without pre-heating of the paper). A curve is also shown in  FIG. 5  for a dwell time of 0 ms (i.e., immediately before the fuser belt and paper came into contact). As indicated in  FIG. 5 , the temperature, T t/f , reached at the toner/fuser belt interface is 129.3° C. 
       FIG. 6  shows a temperature versus distance curve at the nip region using the same 702 mm/s process speed and 26 ms dwell time for a fuser assembly including a continuous fuser belt with a length of 1 m and having a pre-heater for conductively pre-heating the paper (such as the fuser assembly  900 ). A curve is also shown for a dwell time of 0 ms. As shown in  FIG. 6 , the paper and fuser belt structures and materials are the same as those used for the example of  FIG. 5 . In the example shown in  FIG. 6 , the paper is pre-heated to a temperature of 40° C.  FIG. 6  demonstrates that by pre-heating the paper, a lower fuser belt temperature of 192° C. can be used to heat the toner to the same temperature of 129.3° C. By pre-heating the paper, a significant increase in energy efficiency and reduction in energy consumption by the fuser assembly can be achieved. 
     It will be appreciated that various ones of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.