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 ceramic substrate and an endless flexible belt positioned around the heater assembly. The flexible belt includes an inner base layer comprised of a polyimide with a thermally conductive filler; a metallic layer adjacent the base layer; a first primer layer adjacent the metallic layer; a thermally conductive elastic coating adjacent the first primer layer; a second primer layer adjacent the thermally conductive elastic coating; and an outer release layer adjacent the second primer layer.

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]     In color EP imaging, time to first print from cold start is an important factor. If the time to first print is short enough, the printer need not use standby mode, and therefore significantly reduces power usage. The environmental impact of reducing power usage has led to the “Energy Star Program” developed by the “Sustainable Energy Development Authority” (SEDA). SEDA is a New South Wales government agency that runs in conjunction with the USA Environmental Protection Agency (EPA). These governmental agencies promote the reduction in power usage by setting the Energy Star rating for low power.  
         [0006]     Traditional two soft roller fusing systems have a long warm up time, as a result of the high thermal mass inherent in the elastomer covered metal rolls. Power usage in standby modes for typical color machines using a roller system is 150 to 200 watts, which meets the current Energy Star certification. There is discussion to change the Energy Star certification requirement to  45  watts maximum usage when not printing, which would prevent fixing roller fusers from obtaining Energy Star certification. Belt fuser systems typically have a much lower thermal mass and therefore a significantly reduced warm up time so that standby mode is not required. Print quality of transparencies, and gloss variation in paper printing, is still a problem in color belt fusing systems. These problems result from a lack of conformity to the changing toner pile heights experienced in color fusing.  
         [0007]     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  
       [0008]     The present invention provides a fuser with a six layer endless flexible belt resulting in improved gloss in paper printing and improved transmittance in transparency printing.  
         [0009]     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 ceramic substrate and an endless flexible belt positioned around the heater assembly. The flexible belt includes an inner base layer comprised of a polyimide with a thermally conductive filler; a metallic layer adjacent the base layer; a first primer layer adjacent the metallic layer; a thermally conductive elastic coating adjacent the first primer layer; a second primer layer adjacent the thermally conductive elastic coating; and an outer release layer adjacent the second primer layer.  
         [0010]     The endless flexible belt design for the fuser is large in diameter, has a thin resin base, a thin metal layer, a thick elastic layer, and a release layer. The thin base layers in conjunction with the thick elastic layer allows the belt to conform to the difference in toner pile heights experienced in color fusing. The large belt size allows a large nip for high residence time, and the filler that is added to the base resin layer increases the thermal conductivity of the layer, improving heat conduction. The increased heat conductivity of the base resin and metal layer combination provides a belt fuser system capable of running at higher speeds than traditional thick polyimide belts.  
         [0011]     In a color system employing the fuser belt of the present invention, speeds of 28 ppm and higher can be achieved with excellent print quality; and therefore, this fuser belt design can be considered for use in higher end machines. To keep the fuser low in cost, a ceramic heating system is used as opposed to a high cost induction heating system. A rubber pad underneath the fusing belt on the exit of the nip area is also used to increase the exit pressure to improve print quality. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     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:  
         [0013]      FIG. 1  is a schematic view of an imaging device, in the form of a printer, incorporating a fuser of the present invention;  
         [0014]      FIG. 2  is an exploded, perspective view of an embodiment of a portion of a fuser of the present invention;  
         [0015]      FIG. 3  is an assembled, end view of the portion of the fuser shown in  FIG. 2 ; and  
         [0016]      FIG. 4  is a sectional, end view of the endless belt shown in  FIGS. 2 and 3 . 
     
    
       [0017]     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  
       [0018]     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.  
         [0019]     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.  
         [0020]     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.  
         [0021]     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 one or more glass protective layers. 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.  
         [0022]     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. This shape shows an improvement in release and reduction in curl when compared to a similar rectangle shaped pad.  
         [0023]     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.  
         [0024]     The needed height difference may change depending on the location of the pad within the fusing nip and size of the backup roller. 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 roller plays a roll 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 roller and pad to create the needed pressure differential.  
         [0025]     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.  
         [0026]     Referring now to  FIG. 4 , belt  34  will be described in greater detail. Belt  34  includes an inner base layer  50  which is a thermally conductive Upilex-S polyimide. The high thermal conductivity of the Upilex-S polyimide is achieved by the addition of boron nitride at a rate of 10 to 50% by weight. Base layer  50  is a thin layer with a thickness of between 5 to 50 microns. Base layer  50  prevents wear of ceramic heater  40  and provides electrical insulation properties and flexibility to belt  34 . Belt  34  needs to be stiff enough to prevent buckling yet flexible enough to conform in the nip to the change in the toner pile heights.  
         [0027]     Metallic layer  52  is radially adjacent to base layer  50 . In the embodiment shown, base layer  50  is formed on the inside of a drawn stainless steel tube  52  by a specially designed spin coating method followed by imidization.  
         [0028]     Another belt type considered consists of a thicker Upilex-S polyimide belt which could be made by dispensing the polyamic acid, made from BPDA (bisphenyl dianhydride) and PDA (phenylene diamine), on a spinning mandrel and imidizing. This is a method used for making polyimide belts for mono printers. This belt of approximately 20 to 70 microns in thickness could then be coated with a metallic layer of approximately 10 to 70 microns again providing a thermally conductive, yet flexible belt.  
         [0029]     Fuser belts of known design may have a metallic layer with electroformed nickel which is known to have cracking issues at temperatures near or above 200° C. On the other hand, metallic layer  52  does not use nickel but rather stainless steel or copper. These metals do not have the cracking issues observed in nickel belts and therefore 100% imidization is carried out. By fully imidizing the polyamic acid, a coating with higher modulus is obtained. A primer layer  54  is then applied to metallic layer  52  for sufficient adhesion of the next layer to be applied. Primer layer  54  is between 1 to 5 microns in thickness and is applied, e.g., by spray coating or dip coating. A primer such as Shin-Etsu primer X-33-156-20 is suitable for this metallic to elastic adhesion.  
         [0030]     An elastomer layer  56  having a thickness of between 150 to 600 microns is spray coated on the surface of primer  54 . Elastomer  56  is preferably a silicone rubber with a thermal conductivity of between 0.6 to 2.0 W/m° k, preferably between 0.8 to 1.2 W/m° k. Elastomer  56  has a durometer of less than 60 shore A, preferably between 5 to 35 shore A. Shin-Etsu manufactures at least three different silicone rubbers that are suitable for this layer, X-34-2451, X-34-2452 and X-34-2453. Elastomer layer  56  is thick enough and soft enough to conform to the changing pile heights of the color toner, yet thermally conductive enough to be used in a high speed, low thermal mass fuser. Elastomer layer  56  is heated prior to the application of the next primer layer  58 .  
         [0031]     Primer layer  58  is applied by spray coating or dip coating, again 1 to 5 microns thick. This is for the adhesion of elastomer layer  56  to a release layer  60 . Release layer  60  can either be a fluororesin sleeve or a fluororesin coating of 5 to 100 microns in thickness, preferably between 10 to 30 microns in thickness. Spray coating or dip coating is suitable for application of the fluororesin coating. Release layer  60  is coalesced in conjunction with the final curing step of primer layer  58 .  
         [0032]     A fluororesin sleeve (not shown) may be applied over elastomer layer  56  prior to curing, and elastomer layer  56  is then cured while in contact with the sleeve.  
         [0033]     In fuser  30 , belt  34  slides over high temperature heater housing  38  containing ceramic heater  40  and resilient pad  42 . Ceramic heater  40  is located on the inside of belt  34 , opposite backup roll  36  located on the outside of belt  34 . Backup roll  36  is pushed against belt  34  in fusing nip  37  encompassing the width of ceramic heater  40  and resilient pad  42 . Ceramic heater  40  is therefore under high pressures that has lead to heater cracking with conventional designs.  
         [0034]     The careful design of belt  34  as well as the use of resilient pad  42  on the exit side of fuser nip  37  allows a suitable pressure to achieve high gloss. The design of a stronger ceramic heater has enabled the system to run at a higher pressure without the risk of breaking ceramic heater  40 . Ceramic heater  40  consists of a ceramic substrate, a resistive ink pattern on the substrate followed by one or more glass protective layers. The substrate is an alumina ceramic, which has two opposed concave regions that are laminated together. The two layers sintered together under pressure and heat with no adhesive layer, form a flat ceramic substrate. Thick film printing is then used to apply the electrical resistive elements for heating on the surface of the substrate.  
         [0035]     Careful selection of the durometer, shape, height, and thickness of resilient pad  42  on the exit side of fuser nip  37  provides a pressure profile that is suitable for high gloss images and transparencies with high transmittance. The relatively large nip is provided by the wide heater trace and the large diameter of belt  34  (at least 24 mm diameter). The relatively large diameter backup roll  36  also provides a high residence time which further improves the gloss on images and transmittance on transparencies.  
         [0036]     Current color belt fusing systems use expensive belts and heating methods, yet still suffer from significant print quality issues. Although time to first print has been reduced dramatically providing quick heating fusing systems, high gloss images cannot be achieved, and significant gloss variation is observed. Also the transmittance on transparencies is poor and could be considered unacceptable. High speeds over 28ppm have not been achieved in ceramic heated color belt fusing systems. The use of belt  34  described above provides an instant-on belt fusing system that achieves high speeds, and high print quality such as uniform high gloss and high transmittance on transparencies, with a low-cost heating system.  
         [0037]     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.