Abstract:
An electrophotographic imaging device includes a print media transport assembly and a fuser positioned in association with the print media transport assembly. The fuser includes a heater assembly having a housing carrying a heater and a resilient pad. The resilient pad extends from the housing. A flexible belt is positioned around the heater assembly and adjacent to the resilient pad. A backup member is positioned in opposition to the heater assembly.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to electrophotographic imaging devices and, more particularly, to fusers of electrophotographic imaging devices.  
         [0003]     2. Description of the Related Art  
         [0004]     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 final permanent image. The toner image is fixed to the media by the application of heat and pressure in a fuser. A fuser may include a heated roll and a backup roll forming a fuser nip through which the media passes. A fuser may also include a fuser belt and an opposing backup member, such as a backup roll.  
         [0005]     As the regulation of office/home printers becomes focused more on the conservation of energy and the environmental impacts of energy usage, the need for so called “instant-on” fusers becomes critical. Instant on fusers do not require idle modes where the fuser is maintained at an elevated temperature through periodic applications of power. For typical color machines the power usage in idle or standby mode is 150-200 watts. The fuser is the main source of power usage in a printing device, typically around 120-170 watts in an idle mode. Currently the Energy Star/Blue Angel certifications allow for such a mode, but in the future there is discussion of eliminating the idle mode feature and requiring energy usage of less than 45 watts when not printing. This will prevent any current fixing roller fuser from being able to either pass Energy Star/Blue Angel certification or have a quick warm up time. Current color fixing roller fusers have a cold start warm up time in the range of 2 to 4 minutes.  
         [0006]     Prevailing instant on fuser technology uses either a ceramic heater or an inductive heating system. For maintaining print quality in color printing, instant on fusers require an expensive, elastomer coated belt. In color applications, elastomer coatings are required to generate compliance in the fusing nip to produce high print quality on images with multiple layers of toner. To minimize thermal constraints, these elastomer coated belts are typically made of a metallic base layer (required for inductive heating), rather than the standard polyimide belts used in mono ceramic heater applications. Metallic, elastomer coated, and often PFA sleeved belt costs may be up to five times more than a standard polyimide based belt with only a PFA/PTFE coating (PFA is a perfluoroalkyl vinyl ether copolymer, and PTFE is polytetrafluoroethylene), What is needed in the art is a fuser which allows for the use of a polyimide base layer belt for color printing, provides improved gloss and transparency quality for high speed printing with a ceramic heater, and provides improved print media release properties with less print artifacts.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a fuser with a heater assembly having a resilient pad which extends above the heater surface to apply a differential pressure to the print media in the fusing nip.  
         [0008]     The invention comprises, in one form thereof, an electrophotographic imaging device including a print media transport assembly and a fuser positioned in association with the print media transport assembly. The fuser includes a heater assembly having a housing carrying a heater and a resilient pad. The resilient pad extends from the housing. A flexible belt is positioned around the heater assembly and adjacent to the resilient pad. A backup member is positioned in opposition to the heater assembly.  
         [0009]     The invention comprises, in another form thereof, a method of operating a fuser of an electrophotographic imaging device, including the steps of: transporting a print medium to the fuser; carrying the print medium through a fuser nip between a flexible belt and a backup member; heating toner particles on the print medium using a heater assembly positioned on a side of the flexible belt opposite the print medium, the heater assembly having a housing carrying a heater and a resilient pad, the resilient pad extending from the housing; and exerting a nip pressure on the print medium in the fuser nip using the resilient pad which is different than a nip pressure on the print medium in the fuser nip adjacent the heater.  
         [0010]     An advantage of the present invention is that a higher differential pressure is exerted on the print media in the fusing nip using the resilient pad.  
         [0011]     Another advantage is that the resilient pad allows for the use of a polyimide base layer belt for color printing.  
         [0012]     Yet another advantage is that use of the resilient pad provides improved gloss and transparency quality for high speed printing with a ceramic heater.  
         [0013]     A still further advantage is that the resilient pad provides improved print media release properties with less print artifacts.  
         [0014]     A still further advantage is that the resilient pad allows the use of a polyimide belt with no elastomer coating; thus, creating an instant on ceramic color fuser that still performs as well as much more expensive fusing systems with elastomer coated belts. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0016]      FIG. 1  is a schematic view of an imaging device, in the form of a printer, incorporating a fuser of the present invention;  
         [0017]      FIG. 2  is an exploded, perspective view of an embodiment of a portion of a fuser of the present invention;  
         [0018]      FIG. 3  is an assembled, end view of the portion of the fuser shown in  FIG. 2 ;  
         [0019]      FIG. 4  is an end view of the resilient pad shown in  FIGS. 2 and 3 , taken at detail  4  shown in  FIG. 2 ; and  
         [0020]      FIG. 5  is a graphical illustration of the print media velocity as the media is advanced past the resilient pad. 
     
    
       [0021]     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Referring now to the drawings and particularly to  FIG. 1 , there is shown an embodiment of an EP printer  10  of the present invention. Paper supply tray  12  contains a plurality of print media (not shown), such as paper, transparencies or the like. A print medium transport assembly (not numbered) includes a plurality of rolls and/or transport belts for transporting individual print media through EP printer  10 . For example, in the embodiment shown, the print medium transport assembly includes a pick roll  14  and a paper transport belt  16 . Pick roll  14  picks an individual print medium from within paper supply tray  12 , and the print medium is transported past an intermediate transfer member (ITM) in the form of an ITM belt  18 . A plurality of color imaging stations  20 ,  22 ,  24  and  26  apply toner particles of a given color to ITM belt  18  at selected pixel locations. The toner particles are then transferred from ITM belt  18  to the print medium in nip  28 . In the embodiment shown, color imaging station  20  is a black (K) color imaging station; color imaging station  22  is a magenta (M) color imaging station; color imaging station  24  is a cyan (C) color imaging station; and color imaging station  26  is a yellow (Y) color imaging station.  
         [0023]     Paper transport belt  16  transports an individual print medium to fuser  30  ( FIGS. 1-3 ) where the toner particles are fused to the print medium through the application of heat and pressure. Fuser  30  includes a heater assembly  32 , flexible belt  34  carried by heater assembly  32 , and backup member in the form of a backup roll  36 . In the embodiment shown, backup roll  36  is a driven roll and flexible belt  34  is an idler belt; however, the drive scheme may be reversed depending upon the application. Belt  34  and backup roll  36  define a fuser nip  37  therebetween.  
         [0024]     Backup roll  36  has a metallic core and an elastomeric covering, but may be differently configured. Techniques for the general concept of rotatably driving backup roll  36  using gears, belts, pulleys and the like (not shown) are conventional and not described in detail herein.  
         [0025]     Heater assembly  32  includes a high temperature housing  38  (liquid crystal polymer or the like) carrying a ceramic heater  40 . Ceramic heater  40  includes a ceramic substrate (alumina, aluminum nitride, etc.), a resistive ink pattern screened onto the substrate, and a glass protective layer. Other types of ceramic heaters may also be used. Housing  38  includes a small slot cut in a longitudinal direction at the nip exit side of the housing. A resilient pad  42  of a defined thickness and hardness is placed within this longitudinal slot.  
         [0026]     The shape of resilient pad  42 , preferably formed from an elastomeric material, has been shown to affect release characteristics. Rather than a standard rectangle cross-section, it has been found that a trapezoidal shape is preferred ( FIG. 4 ). This shape shows an improvement in release and reduction in curl when compared to a similar rectangle shaped pad.  
         [0027]     The height differential between elastomeric pad.  42  and the heater surface (unloaded), should be in the range of 0.5 to 3 mm. A height differential in the range of between 0.5 to 3 mm has been found to be effective, with a smaller height differential resulting in no effect being seen, and a greater height differential resulting in the paper being bent at an angle such that +W curl is imparted to the print media.  
         [0028]     The needed height difference may change depending on the location of the pad within the fusing nip and size of the backup roll. Moving the pad towards the entry side reduces the needed height, whereas moving it towards the exit requires a more extreme height difference. The radius of the backup roll is a consideration in that a smaller roll has a tighter radius and thus a larger height pad may be needed to generate enough contact between the backup roll and pad to create the needed pressure differential.  
         [0029]     The hardness of the elastomer used in resilient pad  42  is proportional to transmittance and curl; that is, the harder the elastomer the greater gains seen in transmittance and the worse the paper curl imparted. In one embodiment, resilient pad  42  has a hardness ranging from 10 to 50 Shore A. Testing has shown that a hardness over 50 Shore A results in unacceptable levels of curl and a hardness under 10 Shore A results in no significant improvement in gloss or transmittance.  
         [0030]     Flexible belt  34  is an idler belt, not a driven belt, and thus friction between belt  34  and resilient pad  42  should be minimized. This may be done by either covering resilient pad  42  with a low-friction material like a silicon oil impregnated teflon film or by simply coating resilient pad  42  in the grease normally used in a ceramic heater system.  
         [0031]     Resilient pad  42  creates a raised, differential pressure region (may be higher or lower than heater nip depending on elastomer type) at the exit of fuser nip  37  that creates an optimum exit condition for color printing. Typically with polyimide belt systems, there is some amount of mottling when toned images release off the belt. This mottling is an undesirable print defect for color printers, especially if high gloss is desired. As the page enters the fusing nip, the toner changes to it&#39;s molten state and is pressed into the paper fibers by the pressure of the ceramic heater loaded against the backup roller. The nip of a typical ceramic system is a flat nip with an even pressure distribution throughout. As the page exits the nip the speed of the top surface of the page and the belt are nominally the same. This creates a release issue, where release of the toner from the belt is not clean, resulting in a phenomenon termed “taffy pull”. For current ceramic systems the lack of compliance as in the roller system and flat nip result in the toner surface being uneven with various high and low spots. This lack of surface uniformity manifests itself in visible gloss differential and unacceptably high surface roughness.  
         [0032]     By adding resilient pad  42 , media release properties are improved through the creation of a pressure zone at the exit side of fuser nip  37 . This pressure zone results in a more roller like nip exit leading to clean release and reduced mottling. The pressure difference manifests itself in a peripheral velocity change in the top surface of the media passing through fuser nip  37  ( FIG. 5 ). As the media travels past resilient pad  42 , the top surface of the media must travel a longer distance than the bottom surface of the media. Hence, the velocity of the top surface of the sheet is greater than the bottom surface, resulting in a clean release of the toned surface from flexible belt  34 . In contrast, a standard flat nip ceramic system has no velocity differential between the top and bottom surface of the media. It is important to optimize the pressure between too high a pressure that generates belt stalls and creating enough of a pressure differential to benefit from the advantages of this fuser system.  
         [0033]     Resilient pad  42  also results in improved transparency quality in a standard polyimide belt system. Transmittance is a metric to measure transparency quality: it is a ratio of the amount of light able to pass through a transparency measured at two different locations between an emitter and receiver. If the top surface is uneven (as in the case of a standard polyimide belt system), light does not pass through cleanly. Instead the light is scattered, leading to less vibrant colors and thus unacceptable transparencies. A polyimide belt without the inclusion of a resilient pad  42  is able to sufficiently fuse toner onto transparencies. However, the surface roughness is such that light is scattered when projected through. Adding a thin layer of silicone oil fills in the valleys and evens out the surface of the transparency resulting in acceptable transmittance. This is not an acceptable solution, however, as the belt is an idler belt and requires friction between it and the backup roll and media to drive the belt. Placing any type of oil or liquid in the system may result in belt stalls. The use of resilient pad  42  with a polyimide belt adds compliance to the ceramic system at the nip exit. This compliance and the clean release that results from the surface velocity differential previously described results in a smoother layer of fused toner on the surface of the transparency. Thus, light is able to pass through in a manner which results in more vibrant colors and higher transmittance values.  
         [0034]     The use of resilient pad  42  in a ceramic system also reduces cost and increases function of a metal belt configuration. Currently ceramic heater fusers with a metal belt use a thick layer of elastomer (330 to 350 μm) and still show some slight mottling defects. By including resilient pad  42  in a ceramic heater fuser, a differential pressure region created at the fuser nip exit results in cleaner release of the print media from the metal belt. Since resilient pad  42  is only a few millimeters wide (3 to 4 mm optimum), rather than a thick coating around the circumference of the metal belt, less elastomer is used. Also, since the elastomer is on the inside surface of the belt, a low thermal conductivity elastomer is preferred. If the elastomer is made of the same high thermal conductivity elastomer as the thick belt coating layer, it would act as a heat sink, creating problems with dynamic temperature droop. Using a low thermal conductivity elastomer also provides a cost benefit as it does not require expensive doping agents. Thus, a cost benefit is realized by not only using less elastomer, but using a lower thermal conductivity elastomer as well.  
         [0035]     As compared to a polyimide belt, the pressure increase of a similar sized and hardness pad is less due to the increase in stiffness of the metal belt. Therefore, a larger height or harder elastomer may need to be used. The stiffness of the belt is determined by the thickness of the metal layer and the diameter of the belt. The proper size and hardness of elastomer pad  42  may be determined empirically. Currently, an elastomer pad  42  with a durometer of 30 Shore A and a width of 3 to 4 mm has been shown to provide increase in gloss and transmittance when used with a metal belt with 250 μm of elastomer and an inner diameter of 30 mm.  
         [0036]     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.