Patent Publication Number: US-9417572-B2

Title: Fuser heating element for an electrophotographic imaging device

Description:
BACKGROUND 
     1. Technical Field 
     The present application relates generally to an electrophotographic imaging device and more particularly to a fuser for an electrophotographic imaging device. 
     2. Description of the Related Art 
     In the electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image and thereafter the toner image is transferred to the media intended to receive the image. The toner is fixed to the media by a combination of heat and pressure applied by a fuser. 
     The fuser may include a belt fuser that includes a fusing belt and an opposing backup member, such as a backup roll. The belt and the backup member form a nip therebetween. The media with the toner image is moved through the nip to fuse the toner to the media. Belt fusers allow for “instant-on” fusing where the fuser has a relatively short warm up time thereby reducing electricity consumption. Fusing speed is a function of the width of the fuser nip and the belt surface temperature, among other things. A fuser with a relatively wide nip is able to fuse toner to media moving at higher speeds through the nip than a comparable fuser with a relatively narrow nip. Further, a fuser with a higher belt surface temperature is able to fuse toner to the media faster than a fuser with a lower belt surface temperature. Higher fusing speeds in turn lead to higher print speeds. 
     Fusers in laser printers are designed to bond toner to the entire width of media by using heat and pressure. In most fusers, heat is generated by either by a halogen lamp or a ceramic heater. In the case of the halogen lamp fuser, heat is transferred radiantly from the lamp to the black coated inside of an aluminum tube. For monochrome printers, the aluminum tube may have a release layer of either a perfluoroalkoxy (PFA) or polytetrafluoroethylene (PTFE) coating. For color printers, the aluminum tube may be first coated with silicone rubber and then a perfluoroalkoxy (PFA) sleeve. In the cases of the fuser with ceramic heater, heat is transferred conductively from the ceramic heater to either a polyimide tube with a PFA and/or PTFE release layer (for a monochrome fuser), a stainless steel tube with a PFA and/or PTFE release layer (also a monochrome fuser), or a stainless steel tube with a silicone layer and a PFA sleeve (for a color fuser). The release layer coated surface of these tubes applies the heat to the surface of the media that has toner. The pressure is produced by a rubber coated steel or aluminum shaft that is pressed against the coated tube. The media passes between the coated tube and the rubber coated steel shaft. The rubber coated steel shaft typically has a PFA sleeve placed over the rubber coating. This rubber coated steel shaft is commonly called a backup roll. The length of the heating region is typically about 2 to 3 mm longer than the widest media that the laser printer is designed to print. An overheating problem occurs when narrow media is printed in the laser printer. In regions of the fuser nip where the media does not pass through the fuser, the tube and backup roll become very hot and may be damaged due to the high temperature. 
     In particular, in this case heat generated by the ceramic heater is not removed from those regions of the fuser nip which fail to contact media sheets passing through the nip. The heat generated in such regions heats the tube and the backup roll as a result. Because laser printers are designed to have a very small first copy time, the thermal mass of the heater and of the tube is very small. Because of the small thermal mass, the axial heat conduction from hot regions of the tube and heater to cooler regions is very small. This causes the amount of heat to build up relatively rapidly in the heater and tube in such fuser nip regions not contacting the passing media sheets. The heat build up is not significant for fuser nip regions contacting the media sheets because energy is removed from the system by the sheets and toner fixing. In addition, to achieve the very small first copy time and fix the toner to the media, the backup roll surface needs to become very hot without conducting heat to the steel or aluminum shaft. This is achieved because the rubber is a thermal insulator. However, this also means the heat conducted away from the coated tube and heater by the backup roll in regions not contacting the media sheets is very small. 
     One other possible mechanism to remove heat from the coated tube and backup roll in such overheating fuser nip regions is by convection into the air. Unfortunately, the amount of heat removed by convection is very small because in order to meet the very small first copy time, the heat lost to the air is minimized by enclosing the coated tube and backup roll in plastic covers to keep the air still. However, the plastic covers are designed to act as a heat insulating surface, thereby providing little if any opportunity to reduce heat to the overheating fuser nip regions via air convection. 
     A current solution to prevent overheating is to reduce the velocity of the media sheets traveling through the laser printer and increase the distance between media sheets. Reducing the velocity of the media allows the temperature of the coated tube to be reduced. The reduced temperature produces less heat in the overheating regions. The distance between sheets is increased so that, with the reduced media velocity, the time between media sheets becomes large enough for the small heat conduction to cool the overheating regions and prevent overheating. As the media widths become smaller, the amount of time needed to cool the overheating fuser nip regions becomes larger because the size of such regions becomes larger. The overall result of increasing sheet spacing between narrow media is that narrow width media is printed very slowly. For example, existing laser printers may reduce printing speeds for some media sheets of a print job more than 50%. 
     Accordingly, it will be appreciated that an efficient belt fuser with enhanced heating performance is desired. 
     SUMMARY 
     Example embodiments of the present disclosure overcome at least some of the shortcomings in prior fuser heaters and thereby satisfy a significant need for a fuser heater for effectively controlling heat within the fuser. According to an example embodiment, there is shown a heating member for a fuser of an electrophotographic imaging device, including a panel of positive thermal coefficient (PTC) material having a first surface and a second surface; and first and second conductor members. The PTC material exhibits a first electrical resistance within a predetermined range of operating temperatures and a near exponential increase in resistance at temperatures greater than the predetermined temperature range. The first conductor member is electrically coupled to the first surface of the panel of PTC material and the second conductive member is electrically coupled the second surface thereof so that the first and second conductive members support a voltage differential to be placed across the panel of PTC material. Application of an AC voltage between the first and second conductive members results in the PTC material having a temperature falling within the predetermined temperature range. 
     When used in an electrophotographic device, the fuser heating member provides the temperature falling within the predetermined temperature range so that the electrical resistance of the PTC material is at about the first electrical resistance. Passing media sheets through the fuser that are substantially narrower than the fuser nip width causes the portion of the fuser nip region which does not contact the sheets to increase in temperature. If such temperature increase is sufficiently beyond the predetermined temperature range, the electrical resistance of the PTC material increases substantially exponentially within the portion of the fuser nip region which does not contact the media sheets. This increase in electrical resistance within the portion of the fuser nip region not contacting the media sheets results in the temperature of the PTC material corresponding to such portion of the fuser nip region to decrease. This temperature decrease of the PTC material corresponding to the portion of the fuser nip region not contacting the media sheets serves to stabilize the material at around a temperature level greater than the temperature of the fuser nip region that contacts the media sheets but less than a temperature which may adversely affect the performance or longevity of the fuser components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of the various embodiments, and the manner of attaining them, will become more apparent and will be better understood by reference to the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a heating element for an electrophotographic imaging device according to an example embodiment; 
         FIG. 2  is a side view of the heating element of  FIG. 1  according to an example embodiment; 
         FIG. 3  is a plot showing the relationship between electrical resistance and temperature of a component of the heating device of  FIG. 1 ; 
         FIG. 4  is a side view of the heating element of  FIG. 1  according to another example embodiment; 
         FIG. 5  is a side view of a fuser assembly incorporating the heating element of  FIG. 1 ; 
         FIG. 6  is a perspective view of components of the fuser assembly of  FIG. 5  together with a corresponding plot of temperature; 
         FIG. 7  is a perspective view of a heating element for an electrophotographic imaging device according to another example embodiment; 
         FIG. 8  is a perspective view of a heating element for an electrophotographic imaging device according to another example embodiment; 
         FIG. 9  is a side view of the heating element of  FIG. 8 ; and 
         FIG. 10  is a side elevational view of an imaging device having a fuser assembly incorporating heating elements of the example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following description and drawings illustrate embodiments sufficiently to enable those skilled in the art to practice it. It is to be understood that the subject matter of this application is not limited to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The subject matter is capable of other embodiments and of being practiced or of being carried out in various ways. For example, other embodiments may incorporate structural, chronological, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of the application encompasses the appended claims and all available equivalents. The following description is, therefore, not to be taken in a limited sense, and the scope of the present application as defined by the appended claims. 
     Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
       FIGS. 1 and 2  illustrate a heating element  400  for a fuser assembly of an electrophotographic device according to an example embodiment of the present disclosure. Heating element  400  may include a panel member  402  to which electrodes  404  are attached. Accordingly to the example embodiment, panel member  402  is formed from a PTC material such that within a predetermined temperature range, the electrical resistivity of panel member  402  varies very little and is otherwise substantially constant. However, at temperatures above the predetermined temperature range, the electrical resistivity of panel member  402  rises markedly.  FIG. 3  shows an approximate relationship between the electrical resistivity of panel member  402  to temperature, with the reference TR denoting the above-mentioned predetermined temperature range. The predetermined temperature range TR may be the operating temperatures of the fuser assembly at which toner is fused to media. For example, the predetermined temperature PT may be between about 220 degrees F. and about 230 degrees F. 
     Panel member  402  may be, for example, shaped as a rectangular prism having substantially the same rectangular cross section along its length L 402 . As shown in  FIGS. 1 and 2 , panel member  402  may have two opposed major surfaces  402   a  and four sides  402   b . In the example embodiment, panel member  402  may have a length L 402  between about 250 mm and about 280 mm, and in an example embodiment is between about 260 mm and about 270 mm. A width W 402  of panel member  402  may be between about 5 mm and about 25 mm, and in an example embodiment may be between about 7 mm and 18 mm. A thickness T 402  of panel member  402  may be between about 0.5 mm and about 2.0 mm, and in an example embodiment may be between about 1 mm and about 1.5 mm. The PTC material of panel member  402  may have a Perovskite ceramic crystalline structure. In an example embodiment, the PTC material may be a barium titanate (BaTiO 3 ) composition. Barium titanate compositions have been used in the production of piezoelectric transducers, multilayer capacitors and PTC thermistors due to their ferroelectric behavior that exhibits spontaneous polarization at temperatures below a corresponding Curie temperature (approximately 120° C.). Pure barium titanate ceramic is an insulator but can be made a semiconductor by controlled doping. The barium titanate composition of the PTC material of panel member  402  may be doped with strontium (Sr) and/or lead (Pb), wherein strontium is used to lower the Curie point of the material and lead is used to increase the Curie point. Doping the barium titanate composition in this manner changes grain boundary conditions such that above the Curie point, the resistance increases substantially in the PTC material. The effect of such doping is known as the positive temperature coefficient of resistivity (PTCR) effect. For the PTC material of panel member  402 , lead doping percentages may be between about 12 and about 20 percent, yielding a Curie point between about 180° C. and about 220° C. 
     Conventional ceramic fabrication processes may be utilized to produce the doped barium titanate composition of panel member  402 . Example processes may include tape casting, roll compaction, slip casting, dry pressing and injection molding. Though the selection of a particular process may be based upon a number of factors, tape casting is believed to include economic benefits over some other processes for the production of fuser heating elements for electrophotographic imaging devices. 
     Each electrode  404  may be mechanically, thermally and electrically coupled to a distinct major surface  402   a . For example, electrodes  404  may be attached to panel member  402  using a ceramic glass cement or other adhesive. A width W 404  and length L 404  of electrode  404  may be sized to extend substantially along the fuser nip region in a paper feed direction and a direction substantially orthogonal thereto, respectively. In one example embodiment, the length L 404  of electrode  404  may be between about 200 and about 230 mm, and in an example embodiment may be between about 208 mm and about 222 mm. Width W 404  of each electrode  404  may be between about 5 mm to about 20 mm, and in one example embodiment may be between about 6.5 mm to about 15 mm. Each electrode  404  may be coupled to at least one wire or the like for providing a voltage across panel member  402 . 
     As shown in  FIG. 4 , heating element  400  may further include a protective coating  406 , such as a glass insulative coating, which covers substantially all of panel member  402  and electrodes  404 . 
       FIG. 5  illustrates a cross elevational view of a portion of a fuser assembly  500  according to an example embodiment of the present disclosure. Fuser assembly  500  may include heating element  400 , a heater housing  502  which maintains heating element  400  in a substantially fixed position within fuser assembly  500 , tubular belt  504  which is disposed around heater housing  502 , and backup roll  506  which is positioned relative to and provides pressure against belt  504 , heater housing  502  and heating element  400  so as to form a fuser nip N therewith. Heating element  400  may be disposed within fuser assembly  500  such that a major surface  402   a  is immediately adjacent to and/or contacts the inner surface of belt  504  so that heating element  400  provides sufficient heat at fuser nip N to facilitate toner fusing to a sheet of media S as the sheet is passed through fuser nip N. 
     Because heater housings, tubular belts and backup rollers of fuser assemblies are well known, such components will not be discussed in detail herein for reasons of simplicity. 
     Conductors  404  of heating element  400  may be coupled, either directly or indirectly, to an AC power source  510  which may be controlled by a controller  512 . In this way, an AC voltage, such as a 120 v or 240 v, may be applied across panel member  402 . 
     The operation of fuser assembly  500  will now be described. During a fusing operation, backup roll  506  rotates about its axis, which causes belt  504  to rotate due to contact with backup roll  506 . An AC voltage from AC power source  510  is applied across panel member  402 , which causes a certain current to flow between conductors  404  and heat to be generated by panel  402  as a result. The voltage across panel member  402  may fall within temperature range TR having little variation in electrical resistivity. 
     Referring to  FIG. 6 , sheets of media S having unfused toner particles are passed through fuser nip N in direction D. Media sheets S have a narrow width, noticeably narrower than the length L 404  of electrodes  404 . Because of the narrower sheet width, the regions A of belt  504  and backup roll  506  which do not contact media sheets S increase in temperature due to the absence of sheets to dissipate heat. However, because panel member  402  is formed from PTC material, any temperature increase of panel member  402  in portions corresponding to regions A results in an increase in electrical resistance of such portions. This may be seen in the graph of  FIG. 3  in which the temperature of the portions of panel member  402  corresponding to regions A increase above temperature range TR and falls within the resistance-temperature curve that shows a more exponential relationship between electrical resistance and temperature. The increase in electrical resistance reduces the current passing through panel member  402  in the portions corresponding to regions A, which thereby results in such portions of panel member  402  to decrease in temperature. The result is that the PTC material of panel member  402  performs self-regulation and allows for heater element  400  to reach a steady state temperature within those portions of panel member  402  that are part of the fuser nip region failing to contact the media sheets S, with the steady state temperature being above the predetermined temperature range TR but less than a temperature that has been seen to cause significant damage to belt  504  and backup roll  506  over the useful life of fuser assembly  500 . 
       FIG. 6  further shows a resulting temperature along fuser nip N relative to the temperature measured across a fuser nip of a conventional fuser assembly. As can be seen, the self-regulating characteristic of heating element  400 , due to use of PTC panel member  402 , results in markedly reduced temperatures along regions A of belt  504  and backup roll  506  which do not contact media sheets S. Temperatures in regions A have been seen to be about 20 degrees C. to about 50 degrees C. below temperatures in existing fuser assemblies. 
       FIG. 7  shows a heating element  700  according to another example embodiment. Heating element  700  may include a plurality of individual sections  702  of PTC material. Each section  702  may be, for example, about 4.3 cm by about 1.1 cm and have a thickness of about 0.2 cm. Heating element  700  may further include a substrate  704  having a first side  704   a  along which sections  702  of PTC material may be arranged in a side-by-side arrangement. The thickness of substrate  704  may be about 0.64 mm. Each section  702  may contact an adjacent section  702  and be attached to substrate surface  704  using a cement such as potting cement or other adhesive. It is understood that the cement or adhesive used to secure sections  702  to side  704   a  of substrate  704  may be thermally conductive. In this way, the temperature of substrate  704  may substantially follow the temperature of sections  702  of PTC material. 
     Further, heating element  700  may include a protective layer  706  disposed over a second side  704   b  of substrate  704 , as shown in  FIG. 7 . Protective layer  706  may be a glass layer, for example. Heating element  700  may be held within a heater housing such that protective layer  706  is disposed adjacent and in contact with the inner surface of a tubular belt  504  of a fuser assembly and the fuser nip. Heating element  700  may further include a plurality of conductive wires or traces  708 , each one of which is electrically and mechanically connected to each section  702  of PTC material. One wire  708  is disposed along a top surface of sections  702  and the other wire  708  disposed along a bottom or opposed surface thereof. Wires  708  may be coupled to sections  702  of PTC material by spot welding or other methods. Wires  708  may be coupled to an AC voltage source, such as AC source  510 , so that an AC voltage may be applied across each section  702 . In this way, application of an AC voltage across sections  702  of PTC material creates a current through and a temperature to develop across sections  702 . 
     Operation of a fuser assembly having heating element  700  follows the operation of fuser assembly  500  described above. An AC voltage applied across sections  702  creates a temperature falling within a predetermined operating range, such as temperature range TR, for the fuser assembly. When narrower media sheets S are passed through the fuser assembly, regions of the fuser belt and corresponding backup roller increase in temperature, which increases the temperature of sections  702  adjacent thereto. The increase in temperature of such sections  702  above the predetermined temperature range TR and into the portion of the electrical resistance-temperature curve corresponding to an approximately exponential relationship, results in an increase in the electrical resistivity of sections  702  having increased temperatures. The increase in resistivity causes the amount of current through and hence the temperature of such sections  702  to decrease, thereby serving to stabilize the temperature of the sections  702  at a steady state temperature value that is less than a temperature that would otherwise be experienced. As a result of experiencing reduced temperatures, the belt and backup roller will not substantially overheat and become damaged. 
       FIGS. 8 and 9  illustrate a heating element  800  according to another example embodiment. Heating element  800  may include a panel member  810  constructed from PTC material having the characteristics and general shape as described above with respect to panel member  402 . Panel member  810  may extend across fuser nip N. A width W of panel member  810  may be between about 9 mm and about 24 mm, and in particular between about 10.5 mm and about 19 mm. A height H of panel member  810  may be between about 0.5 mm to about 4 mm, and in particular between about 1 mm and about 3 mm. 
     Heating element  800  may further include electrodes  820  which extend along length L of panel member  810 . As shown in  FIGS. 8 and 9 , electrodes  820  are disposed on the same surface of panel member  810  at opposite longitudinal sides thereof. Electrodes  820  may have a spacing S from each other that is between about 5 mm and about 20 mm, and in particular about 6.5 mm to about 15 mm. Each electrode  820  is mechanically, thermally and electrically coupled to panel member  810  along length L. Each electrode  820  may be between about 200 mm to about 230 mm in length, and in particular between about 208 mm to about 222 mm. Wires are connected to electrodes  820  to facilitate application of an AC signal thereto. Application of an AC signal across electrodes  820  causes a current to flow between the electrodes through panel member  810 , which causes panel member  810  to become heated. An insulator layer  830  is disposed along the surface of panel member  810  opposite the surface against which electrodes  820  are attached. This surface, the surface that is opposite the surface against which electrodes  820  are attached, is disposed adjacent tubular belt  504  at fuser nip N in the fuser assembly. 
       FIG. 10  depicts an electrophotographic imaging device  10  having a fuser assembly incorporating the heating elements of the example embodiments described above. Imaging device  10  may include a main body  12 , a media tray  14 , a pick mechanism  16 , an intermediate transfer member  18 , a plurality of image forming units  20   y ,  20   c ,  20   m , and  20   k , a second transfer area  22 , a fuser assembly  500 , exit rollers  26 , an output tray  28 , a print head  30 , and a duplex path  32 . An auxiliary feed  34  allows a user to manually feed print media into the image forming apparatus  10 . 
     The intermediate transfer member  18  is formed as an endless transfer belt supported about a plurality of support rollers  36 . During image forming operations, transfer member  18  moves in the direction of arrow  38  past the plurality of image forming stations  20   y ,  20   c ,  20   m , and  20   k  for printing with yellow, cyan, magenta, and black toner, respectively. Each image forming stations  20   y ,  20   c ,  20   m , and  20   k  applies a portion of an image on the transfer member  18 . The moving transfer member  18  conveys the image to a print media at the second transfer area  22 . 
     The media tray  14  is positioned in a lower portion of the main body  12  and contains a stack of media. The media tray  14  is removable for refilling. Pick mechanism  16  picks print media from top of the media stack in the media tray  14  and feeds the print media into a primary media path  40 . The print media is moved along the primary media path  40  and receives the toner image from the transfer member  18  at the second transfer area  22 . 
     Once the toner image is transferred, the print media is conveyed along the primary media path  40  to the fuser assembly  500 , having heating elements  400  or  700 . The fuser assembly  500  fuses the toner to the print media and conveys the print media towards the exit rollers  26 . Exit rollers  26  either eject the print media to the output tray  28 , or direct it into the duplex path  32  for printing on a second side of the print media. In the latter case, the exit rollers  26  partially eject the print media and then reverse direction to invert the print media and direct it into the duplex path  32 . A series of rollers in the duplex path  32  return the inverted print media to the primary media path  40  upstream from the second transfer area  22  for printing on the second side of the media. The image forming apparatus  10  also includes a controller  42  that provides instructions to the imaging device  10  for performing imaging. 
     The foregoing description of multiple embodiments has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the application to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is understood that the subject matter of the present application may be practiced in ways other than as specifically set forth herein without departing from the scope and essential characteristics. It is intended that the scope of the application be defined by the claims appended hereto.