Patent Abstract:
A printer includes, in its fixing device operable to fix toner particles onto a recording medium, a heating roller containing magnetic metal, a fixing roller disposed parallel to the heating roller, an endless belt containing magnetic metal bridged across the heating roller and the fixing roller. The printer also includes a press roller pressed to the fixing roller via the endless belt and the recording medium, and a coil core being operable to produce magnetic fields so as to cause both of the heating roller and the endless belt to generate heat with the magnetic metals contained therein.

Full Description:
This application is a divisional application of Ser. No. 10/022,621, filed Dec. 20, 2001, now U.S. Pat. No. 6,591,082. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a printer, or a fixing device used for image forming devices such as copying machines, facsimiles and printers. 
     BACKGROUND OF THE INVENTION 
     Demands for faster and more energy-efficient image forming devices such as printers, copying machines and facsimiles have been increasing in the market. To satisfy such demands, it is critical to improve the thermal efficiency of fixing devices used in the image forming devices. 
     During image forming processes such as electro-photographic recording, electrostatic recording and magnetic recording, an image forming device forms an unfixed toner image on recording media such as recording sheets, photosensitive paper and electrostatic recording paper by an image transfer method or a direct method. The unfixed toner image is fixed, in general, by a fixing device based on contact heating methods such as a hot roller method, a film heating method, or an electromagnetic induction heating method. 
     The fixing device of the hot roller method comprises, as a basic construction, a pair of rollers including a temperature regulated fixing roller having a heat source such as a halogen lamp and a press roller pressing against the fixing roller. A recording medium is inserted into and carried through a section where the fixing roller and press roller come into contact, a so-called fixing nip portion, so that the unfixed toner image is melted and fixed by heat and pressure applied by the rollers. 
     The fixing device of the film heating method is disclosed, for example, in the Japanese Patent Laid-Open Publications S63-313182 and H01-263679. 
     In the case of the foregoing fixing device, a recording medium is positioned into a close contact with a heater which is tightly fixed to a supporting member via a thin heat-resistant fixing film. The fixing film is slid against the heating body and the heat is transferred from the heating body to the recording medium via the film. 
     International Publication WO 00/52534 A1 discloses a fixing device based on the electromagnetic induction heating method. According to the method, Joule heat produced by an eddy current generated in a magnetic metal member by an alternating field heats up a heater, including the metal members, by an electromagnetic induction. A heating roller is heated by electromagnetic induction heating, and the heat is transferred to a thin heating medium made of a heat-resistant resin by thermal conduction. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide a printer in which a stable temperature for fixing a toner image can be maintained stable. 
     The printer of the present invention comprises an exposure device for generating a light beam corresponding to an image information, a photosensitive body on which a latent image is formed based on the light beam delivered from the exposure means, a charger for charging the photosensitive body, a developer for converting the latent image formed on the photosensitive body into a visible image using toner particles, a belt on which the visible toner image is transferred, and a fixing device for fixing the toner image on the belt onto a recording medium. 
     The foregoing fixing device comprises a heating roller containing a magnetic metal, a fixing roller disposed parallel to the heating roller, an endless belt bridging the heating roller and the fixing roller, a press roller pressed to the fixing roller via the endless belt and recording medium, and a device for producing magnetic fields disposed adjacent to the heating roller. 
     The endless belt contains magnetic metal or the belt is made of materials that can be heated by magnetic induction heating. The device for producing magnetic fields causes both of the heating roller and the endless belt to generate heat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an outline concept of a printer in accordance with an exemplary embodiment of the present invention. 
     FIG. 2 shows a fixing device of the printer in accordance with a preferred embodiment of the present invention. 
     FIG. 3 is a cross sectional view showing an arrangement of an induction coil used in a printer of the present invention. 
     FIG. 4 is a side view showing an arrangement of a coil and an induction heater, used in a printer of the present invention. 
     FIG. 5 is a schematic view showing an alternating magnetic field and a generation of eddy current in a printer of the present invention. 
     FIG. 6 shows a fixing device in accordance with another exemplary embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Exemplary embodiments of the present invention are described with reference to the drawings, using a printer comprising a color image forming device as an example. 
     The elements commonly shown in the drawings are shown with the same numerals, and redundant descriptions are omitted. 
     Referring to FIG. 1, a color image forming device comprises four image stations  1   a ,  1   b ,  1   c    1   d . Each of the respective image stations has a photosensitive drum (photosensitive body), or an image bearer,  2   a ,  2   b ,  2   c ,  2   d , respectively, accompanied by charging means or chargers  3   a ,  3   b ,  3   c ,  3   d  for electrostatically charging the surface of the drum homogeneously, developing means or developers  4   a ,  4   b ,  4   c ,  4   d  for converting an electrostatic latent image into a visible image, and cleaning means or cleaners  5   a ,  5   b ,  5   c ,  5   d  for removing residual toner particles staying on the drum surface. Exposure means or exposure devices  6   a ,  6   b ,  6   c ,  6   d , which are a scanning optical system, irradiate light on the photosensitive drums  2   a ,  2   b ,  2   c ,  2   d , respectively, in accordance with information corresponding to an image. Image transfer means or an image transfer device  7  comprises an intermediary transfer belt (transfer member)  12  and transfer means or transfer devices  8   a ,  8   b ,  8   c ,  8   d  for transferring a toner image on the transfer belt. 
     At each of the respective image stations  1   a ,  1   b ,  1   c ,  1   d , an image is reproduced in terms of yellow, magenta, cyan and black color components, respectively. 
     Each of the exposure means or the exposure devices  6   a ,  6   b ,  6   c ,  6   d  outputs a light beam  9   a ,  9   b ,  9   c ,  9   d  that corresponds to the yellow, magenta, cyan and black components, respectively. 
     Under the image stations  1   a ,  1   b ,  1   c ,  1   d , an intermediary transfer belt  12  in the form of an endless belt is provided bridging the rollers  10  and  11 . The endless belt travels in a direction as indicated with an arrow A. 
     Pattern detection means or pattern detector  14  is provided facing towards the intermediary transfer belt  12  for detecting a resist pattern generated from resist pattern generating means or a resist pattern generator  13 . Further, dislocation correction means or dislocation corrector  15  is provided for correcting dislocation in each of the colors, based on detection results delivered from the pattern detection means or pattern detector  14 . The pattern detection means or pattern detector  14  is disposed at both ends of the transfer belt  12  in the width direction. 
     Sheets  17  stored in a dispensing cassette  16  are supplied by a paper feed roller  18   a , and discharged to a discharge tray (not shown) via a transferring roller and fixing means or a fixing device  19 . 
     In the above-configured color image forming device, a latent image corresponding to the black component is formed on the photosensitive drum  2   d  at the image station  1   d  by a known electro-photographic process using the charging means or charger  3   d  and the exposure means or exposure device  6   d . The latent image is made into a visible black toner image at the developing means or developer  4   d  using a developer containing black toner particles. The black toner image is transferred at the transfer means or transfer device  8   d  to the intermediay transfer belt  12 . 
     When the black toner image is being transferred to the intermediary transfer belt  12 , a latent image corresponding to the cyan component is formed at the image station  1   c . This latent image is made into a cyan toner image at the developing means or developer  4   c  and transferred at the transfer means or transfer device  8   c  to be overlaid on the black toner image which had been transferred to the intermediary transfer belt  12 . 
     The magenta toner image and the yellow toner image are likewise processed. When all of the four toner images are overlaid on the intermediary transfer belt  12 , paper or the like sheet  17  is delivered by a paper supply roller  18   a  from the dispensing cassette  16 . The overlaid toner images are printed altogether on the sheet material by a transfer-printing roller  18   b , and fixed by heating at the fixing means or fixing device  19  to yield a full-color image on the sheet material  17 . 
     After the printing process is finished, respective photosensitive drums  2   a ,  2   b ,  2   c ,  2   d  have their surfaces cleaned to remove residual toner particles at the cleaning means or cleaners  5   a ,  5   b ,  5   c ,  5   d  in preparation for the next image formation. This completes a printing operation. 
     The process of fixing a color image in the present embodiment is described more in detail, referring to FIG.  2 -FIG.  6 . 
     The fixing device in FIG. 2 comprises a heating roller  21  heated by electromagnetic induction of an induction heating means or an induction heater  26 , a fixing roller  22  disposed in parallel to the heating roller  21 , a heat-resistant endless belt (toner heating medium belt)  23  bridging across the heating roller  21  and the fixing roller  22 , wherein the belt  23  is heated by the heating roller  21  and rotated by the rotation of one of the rollers in the direction shown by an arrow A, and a press roller  24  which is pressed to the fixing roller  22  via the belt  23  and rotates in the same direction as the belt  23 . 
     The heating roller  21  is made of a hollow cylindrical magnetic metal such as iron, cobalt or nickel, and alloys of those metals. In this embodiment, the external diameter of the heating roller  21  is 20 mm and the thickness is 0.3 mm, and its temperature rises rapidly due to its low beat capacity. 
     The fixing roller  22  comprises a metallic core  22   a  made of such metals as stainless steel, and a resilient member  22   b  coating the metallic core  22   a . The resilient member  22   b  is made of solid or foamed heat-resistant silicon rubber. The external diameter of the fixing roller  22  is 30 mm, and it is set larger than the heating roller  21  so that the press roller  24  and the fixing roller  22  come in contact at a predetermined width when pressed by the pressure of the press roller  24 . The thickness of the resilient member  22   b  is 3-8 mm and the hardness is 15-50° (Asker hardness: hardness measured by JIS (Japan Industrial Standard) A is 6-25°). This configuration makes the heat capacity of the heating roller  21  smaller than that of the fixing roller  22  so as to heat the heating roller  21  rapidly, thereby shortening the warm-up time. 
     The belt  23  bridging the heating roller  21  and the fixing roller  22  is heated at a position W 1  where it comes in contact with the heating roller  21  heated by the induction heating means or induction heater  26 . As the rollers  21  and  22  rotate, the inner surface of the belt  23  is heated continuously, and in this manner, the entire belt is heated. 
     As FIG. 5 shows, the belt  23  is a composite layer belt which comprises a heating layer  23   a  made of magnetic metal such as iron, cobalt or nickel, or alloys of such metals as a base material, and a releasing layer  23   b  made of a resilient member such as silicon rubber and fluorocarbon rubber. The belt  23  is formed of the heating layer  23   a , a resilient layer  23   c  and a the releasing layer  23   b  stacked together in that order. 
     The composite layer helps to stabilize the temperature of the belt  23  and improves reliability even when a foreign object gets in between the belt  23  and the heating roller  21  and makes a gap. This is because heat from the heating layer  23   a  generated by the electromagnetic induction heats up the belt  23 . 
     The thickness of the heating layer  23   a  is preferably 20-50 μm and in the present embodiment it is about 30 μm. If the heating layer  23   a  is thicker than 50 μm, distortion stress generated during the rotation of the belt becomes large. Consequently, shear force causes cracks and in some cases, lowers the mechanical strength significantly. When the heating layer  23   a  is thinner than 20 μm, thrust load generated by meandering of the belt during rotation is applied on the ends of the belt, causing cracks or fissures to develop in the composite layer belt. 
     The preferable thickness of the releasing layer  23   b  is between 100 and 300 μm and in the present embodiment it is around 200 μm. When the thickness is within this range, the toner image T formed on the recording medium  21  can be sufficiently enclosed by the surface layer of the belt  23 , thus the toner image T can be heated and melted evenly. 
     When the releasing layer  23   b  is thinner than 100 μm, the thermal capacity of the belt  23  becomes small. As a consequence, the temperature on the surface of the belt drops significantly during the fixing process of the toner so that sufficient fixing cannot be maintained. On the other hand, if the releasing layer  23   b  is thicker than 300 μm, the heat capacity of the belt  23  becomes larger, extending the warm-up time. Furthermore, since the temperature of the surface of the belt does not drop quickly during the toner fixing process, solidification of the melted toner near the exit of the fixing section is hindered. As a result, so-called hot offset is triggered, lowering the releasing ability of the belt and allowing the toner to stick to the belt. 
     The inner surface of the heating layer  23   a  may be coated with resin in order to prevent oxidization of the metal and improve contact conditions with the heating roller  21 . 
     As the base material of the belt  23 , the heating layer  23   a  made of the above metals can be replaced with a heat-resistant resin layer made of such resins as fluorocarbon resins, polyimide resin, polyamide resin, polyamideimide resin, PEEK, PES, and PPS. 
     When the base material is made of a resin layer with a high heat-resistance, the belt  23  can easily fit on the heating roller  21  according to its curvature, and the heat from the heating roller  21  can be transferred to the belt  23  effectively. 
     In this case, the resin layer is preferably 20-150 μm and in the present embodiment it is around 75 μm in thickness. When the resin layer is thinner than 20 μm, sufficient mechanical strength against meandering during the rotation of the belt cannot be obtained. On the other hand, when the resin layer is thicker than 150 μm, the heat is not effectively transferred from the heating roller  21  to the releasing layer  23   b  of the belt  23 , since heat conductivity of the resin becomes small. As a result, the fixing condition deteriorates. 
     The base material can be made of an electro-conductive composite resin which can be heated by an electromagnetic induction heating. The resin materials for the electro-conductive composite resin may preferably include heat-resistant resins. 
     Referring to FIG. 2, the press roller  24  comprises a metal tube core  24   a  made of a metal with high heat conductivity such as copper and aluminum, and on the surface of the core  24   a , a resilient member  24   b  having high heat-resistance and toner releasing ability. The metallic core  24   a  may be made of stainless steel in the place of the foregoing metals. 
     The press roller  24  presses the fixing roller  22  via the belt  23  and forms the fixing nip portion N. However, in the present embodiment, since the press roller  24  is harder than the fixing roller  22 , the press roller  24  presses into the fixing roller  22  (and the belt  23 ). Due to this, the medium  17  follows the outer periphery of the press roller  24 , improving the releasing ability of the medium  17  from the belt  23 . The external diameter of the press roller  24  is approximately 30 mm, almost the same as that of the fixing roller  22 . However, the thickness of resilient member  24   b  is about 2-5 mm, thinner than the fixing roller  22 , and surface hardness is 20-60° (Asker hardness: hardness measured by JIS A is 6-25°), harder than the fixing roller  22  as mentioned previously. 
     FIG. 3 shows a cross sectional view in part of the induction heating means or induction heater  26 , while FIG. 4 shows a side view in part of the induction heating means or induction heater  26 . 
     As shown in FIG.  3  and FIG. 4, the induction heating means or induction heater  26 , which heats the heating roller  21  by electromagnetic induction, comprises a coil  27 , a magnetization means or a magnetizer, and a coil guiding plate  28  on which the magnetizing coil  27  is wound. The coil guiding plate  28  is half-cylindrical, and is disposed in the vicinity of the outer periphery of the heating roller  21 . As FIG. 4 shows, the coil  27  is manufactured by alternately winding a long wire around the coil guiding plate  28 , in a direction of the axis of the heating roller  21 . The length of the coil is the same as the area where the belt  23  and the heating roller  21  come in contact. 
     This construction allows the heating roller  21  to have the largest possible area to be heated by the electromagnetic induction of the induction heating means or induction heater  26 . Furthermore, the contact time between the heated surface of the heating roller  21  and belt  23  becomes as large as possible. Thus, the heat conduction efficiency to the belt  23  is increased. 
     The coil  27  is connected to a driving power source with a variable frequency oscillator. 
     Adjacent to the coil  27  is a half-cylindrical coil core  29  made of a ferromagnetic material such as ferrite, fixed on a coil core supporting member  20 . In the present embodiment, the coil core  29  has a relative permeability of 2500. 
     The coil  27  is supplied with a high-frequency alternating current of 10 kHz-1 MHz, preferably 20 kHz-800 kHz from the driving power source, thereby the coil  27  generates an alternating field. At and around the contacting position W 1  of the heating roller  21  and the heat resistant belt  23 , the alternating field affects the heating roller  21  and the heating layer  23   a  of the belt  23 , causing an eddy current I to flow in the heating roller  21  and the heating layer  23   a  in the direction B, a direction which prevents the alternating field from changing. 
     The eddy current I generates Joule heat according to the resistance of the heating roller  21  and the heating layer  23   a , and, via the electromagnetic induction, heats up mainly at and around their contacting portion of the heating roller  21  and the belt  23  having the heating layer  23   a.    
     The temperature of the inner surface of the belt  23  heated in the foregoing manner is measured in the vicinity of the entrance of the fixing nip portion N by a temperature sensor  25  made with high heat-responsive, temperature sensitive elements such as a thermistor disposed in contact with the inner surface of the belt  23 . 
     With this construction, since the temperature sensor  25  does not damage the outer surface of the belt  23 , a stable fixing capacity can be maintained and the temperature of the belt  23  just before entering in the fixing nip portion N can be detected. Based on the output signals providing the temperature information, the power input into the induction heating means or induction heater  26  can be controlled, thereby securely maintaining the temperature of the belt  23  at, for example, 180° C. 
     According to the present embodiment, since the fixing nip portion N is formed with the belt  23  which is heated by the heating roller  21  heated by the induction heating means or induction heater  26 , and the press roller  24 , differences in temperatures between the outer and inner surfaces of the belt  23  are restricted when the toner image T formed on the medium  17  in the image forming section (not illustrated) enters the fixing nip portion N. Therefore, so called overshoot, in which the temperature on the surface of the belt becomes excessively high compared with the set temperature, can be prevented. Thus, temperature of the belt  23 , a toner heating medium, can be controlled in a stable manner. 
     Therefore, in the fixing process, the belt  23  whose temperature is a tightly controlled constant comes in contact with the toner image T, securing a high fixing quality. 
     The fixing device of a second exemplary embodiment is described below. As FIG. 6 shows, in the second embodiment of the fixing device, an induction heating means or induction heater  32  comprises a coil  33 , a coil guiding plate  34  on which the coil  33  is wound, and a coil core  35  fixed by a coil core supporting member  36 , which is disposed adjacent to the coil  33 . 
     In this device, the heating area W 2  is approximately half of the contact area of the half-cylindrical induction heating means or induction heater  32 , since the induction heating means or induction heater  32  is a quarter-cylindrical. The other constituent components of the present fixing device remain the same as those in the previous embodiment. 
     As shown in FIG. 6, the centers of the fixing roller  22 , the coil  33 , the coil guiding plate  34  and the coil core  35  are located on a substantially straight line. 
     With such a construction, the size of the induction heating means or induction heater  32  can be made small, which leads to a fixing device that is compact in dimensions and lower in parts cost. 
     According to the present invention, the fixing nip portion comprises a toner heating medium which is heated by the heating roller heated by the induction heating means or induction heater, and a press roller. Due to this construction, temperatures of the outer and inner surfaces of the toner heating medium are kept almost the same when entering the fixing nip portion. Therefore, temperatures of the toner heating medium can be controlled in a stable manner. Thus, the printer of the present invention provides quality prints on stable basis.

Technology Classification (CPC): 6