Patent Publication Number: US-6661991-B2

Title: Induction heating and fixing device for fixing toner on a recording medium in an image forming apparatus

Description:
RELATED APPLICATION 
     The present invention is based on Japanese Patent Application No. 2001-25591, the content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTIOJN 
     1. Field of the Invention 
     The present invention relates to an induction heating and fixing device for fixing toner on a recording medium in image forming apparatuses such as copiers, printers, and facsimile machines. 
     2. Description of the Related Art 
     In recent years fixing devices using the induction heating method have been used as fixing devices in image forming apparatuses. 
     For example, U.S. Pat. No. 4,570,044 discloses an induction heating and fixing device wherein a part of an iron core forming a closed magnetic circuit is inserted into the hollow space within a cylindrical fixing roller heated by induction, and a coil is wound around the iron core to generate a magnetic field, such that the fixing roller is heated when an alternating current flows through the coil. 
     Disadvantages arise, however, in that since this induction heating and fixing device is closed at both ends of the fixing roller heated by induction, the temperature distribution in the axial direction of the fixing roller produces a higher temperature in the center area and lower temperature at both ends due to the greater radiant heat at the ends which easily reduces the temperature and causes a large temperature gradient between the center part and the end parts. 
     SUMMARY 
     An object of the present invention is to provide an induction heating and fixing device which suppresses temperature drop at the bilateral ends of the fixing roller, and maintains uniform temperature distribution in the axial direction of the fixing roller. 
     The present invention relates to an induction heating and fixing device comprising: 
     a hollow cylindrical fixing member heated by induction; 
     a core for forming a closed magnetic circuit, a part of the core being inserted onto the hollow cylindrical fixing member; 
     a ring member formed of magnetic material and provided on the core in the vicinity of the bilateral ends of the hollow cylindrical fixing member; 
     a coil wound around the core; and 
     a pressure member disposed so as to press against the hollow cylindrical fixing member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a brief structural view of a copier incorporating the induction heating and fixing device of the present invention; 
     FIG. 2 is a brief cross section view showing the main part of the induction heating and fixing device of FIG. 1; 
     FIG. 3 briefly shows the iron core of the induction heating and fixing device; 
     FIG. 4 is a side view of the ring member provided on the iron core; 
     FIG. 5 is a perspective view of the ring member provided on the iron core; 
     FIG. 6 is a cross section view along the A—A line shown in FIG. 3; and 
     FIG. 7 shows the condition of a band-like silicon steel plate shaped in a spiral as another mode of the ring member provided on the iron core. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiments of the present invention are described hereinafter with reference to the accompanying drawings. 
     FIG. 1 is a brief structural view of a copier as an image forming apparatus incorporating the induction heating and fixing device of the present invention. 
     As shown in the drawing, a copier  100  of an embodiment of the present invention has an image scanning unit  101  for reading a document image, a signal processing unit  102  for executing signal processing, a printer unit  103  for printing on a sheet an image read by the image scanning unit  101 , and a casing  104  for layout and housing each of these units. 
     In the image scanning unit  101 , a document placed upon a glass platen  105  is pressed down by a platen cover  106 , however, when an automatic document feeder is installed, the automatic document feeder replaces the platen cover  106 . 
     A document placed on the glass platen  105  is illuminated by a lamp  107 , and the light reflected by the document is directed by mirrors  108   a,    108   b,    108   c,  and condenser lens  109  and forms an image on a line image sensor  110 , which converts the light image to image information and transfers this information to the signal processing unit  102 . A first slider  112  is mechanically moved at a speed V, and a second slider  113  is mechanically moved at a speed V/ 2  in a perpendicular direction (subscan direction) relative to the electrical scanning direction (main scan direction) of the image sensor via the drive of a scanner motor  111 , so as to scan the entire surface of a document. 
     The signal processing unit  102  is provided with a laser generator  115 , and a photosensitive drum  116  as an image-carrying body; and arranged around the periphery of the rotating photosensitive drum  116  are a charging roller  117  as a charging device, developing device  118 , transfer roller  119  as a transfer device, discharge needle  120  as a sheet  10  discharger and separation device, and cleaning device  121  for removing residual toner from the surface of the photosensitive drum  116 . 
     The laser generator  115  modulates a semiconductor laser in accordance with the level of the image signal sent from the signal processing unit  102 . The laser light is directed by a polygonal mirror, f-θ lens, and deviating mirror not shown in the drawing, and illuminates the photosensitive drum  116  at a position between the charging roller  117  and the developing device  118 . The electrostatic latent image formed on the surface of the photosensitive drum  116  is developed by toner in the developing device  118 . 
     On the other hand, a plurality of sheets  10  are stacked in a paper cassette  125  removably mounted in the casing  104 . The sheets  10  within the paper cassette  125  are fed sheet by sheet via a feed roller  126 , and transported toward a transfer position between the photosensitive drum  116  and the transfer roller  119  by a timing roller  127  with a specific timing. 
     An image developed on the surface of the photosensitive drum  116  is transferred onto the sheet  10  by the transfer roller  119 . After transfer, the sheet  10  is separated from the photosensitive drum  116  and transported by a transport belt  129  toward a fixing device  128 . 
     The sheet  10  is transported to the induction heating and fixing device  128 , and the unfixed toner transferred onto the sheet  10  is fixed by the induction heating and fixing device, whereupon the sheet  10  bearing the fixed toner is ejected to a discharge tray  130 . 
     When the transfer of the image to the sheet  10  by the transfer roller  119  is completed, the surface of the photosensitive drum  116  is charged to a negative polarity by a pre-cleaning charger not shown in the drawing, the residual toner is removed by the cleaning device  121 , and the residual charge is then removed by an eraser. Thereafter, the surface of the photosensitive drum  116  is again charged by the charging roller  117 , subjected to latent image formation by the laser light, the formed latent image is developed by the developing device  118 , and the charge of the non-image region is discharged by the pre-cleaning eraser not shown in the drawing. 
     FIG. 2 is a brief cross section view of the main part of the fixing device of FIG. 1, and FIG. 3 briefly shows the iron core of the fixing device. 
     First, referring to FIG. 2, the induction heating and fixing device  128  is a fixing device which thermally fuses a toner  11  maintained on the surface of a sheet  10  so as to fix the toner onto the sheet  10 , and is provided with an induction-heated fixing roller  12 , a pressure roller  13  which presses against the fixing roller  12 , and a coil  14  for inductively heating the fixing roller  12 . 
     The fixing roller  12  is mounted so as to be powered to rotate in the arrow “a” direction in FIG. 2, and the pressure roller  13  is driven in rotation in conjunction with the rotation of the fixing roller  12 . 
     The fixing roller  12  is a conductive pipe having the shape of a hollow cylinder, e.g., a carbon steel tube, stainless steel alloy tube, or aluminum tube, or formed of iron or the like, and has a wall thickness sufficient to ensure thermal capacity within a range capable of ensuring sufficient mechanical strength relative to the pressure force received from the pressure roller  13 . The exterior surface of the roller is coated with a fluororesin for ease of separation of the sheet  10  and this coating forms a release layer having excellent release characteristics relative to the toner as well as excellent heat resistance. 
     The fixing roller  12  has a drive gear not shown in the drawing mounted at one end, and is rotated by a power source not shown in the drawing such as a motor or the like connected to this drive gear. 
     The pressure roller  13  comprises a shaft core  15 , and a silicone rubber layer  16  formed on the exterior surface of the shaft core  15 . The silicone rubber layer  16  is a rubber layer having heat resistance and release characteristics for easy separation of the sheet  10  from the surface of the roller. In the mode shown in the embodiment in the drawing, a halogen lamp  28  is installed within the shaft core  15 . 
     The pressure roller  13  is pressed toward the fixing roller  12  by a spring not shown in the drawing, so as to form a nip  19  through which the sheet  10  passes in the contact region with the fixing roller  12 . 
     The induction heating and fixing device  128  has an iron core  17  in a rectangular shape forming a closed magnetic circuit, and part of this iron core  17  is inserted into the hollow center part  12   a  within the fixing roller  12 , as shown in FIG. 3. A coil  14  is wound around the iron core  17 . The coil  14  is connected to a power source not shown in the drawing. 
     The iron core  17  may be such as is normally used in a transformer, e.g., it is desirable that the iron core  17  has a high magnetic permeability such as a silicon steel plate laminate iron core. A normal single wire having on its surface an adhesion layer and an insulation layer may be used as the coil  14 . 
     A ring member  30  which is thicker than the magnetic circuit is provided on the iron core  17  in the vicinity of the bilateral ends in the axial direction (length direction of the iron core) of the fixing roller  12 . 
     The ring member  30  is formed of a magnetic material such as ferrite, pure iron, silicon steel or the like, and part of the ring member  30  is segmented with a slit  31  in one part. Furthermore, the ring member  30  is formed with a taper  32  at both ends. 
     The iron core  17  has an approximately circular cross section shape formed by the laminate layers of the silicon steel plate  18 , as shown in FIG. 6, for fitting the ring member  30 . FIG. 6 is a cross section view along the A—A line shown in FIG.  3 . 
     The fixing operation in a copier having the above-described structure is started when the copier  100  is turned ON (or standby is cancelled), whereupon an alternating current of approximately 50˜60 Hz is applied to the coil  14  from a power circuit (not shown in the drawing) until the temperature of the fixing roller  12  is heated to a suitable fixing temperature (e.g., 150˜200° C.) by induction heating. 
     Then, when a copy start key (not shown in the drawing) is pressed on the copier  100 , a document image is read and the transport of a sheet  10  is started from the paper cassette  125 . In this way, a sheet  10  bearing the unfixed toner  11  is transported from the left as indicated by the arrow “b” in FIG. 2, and fed toward the nip  19  in the contact part between the fixing roller  12  and the pressure roller  13 . 
     The sheet  10  is transported through the nip  19  while receiving the heat of both the heated fixing roller  12  and the pressure roller  13  and the pressure exerted by the pressure roller  13 . In this way, the unfixed toner  11  is fixed on the surface of the sheet  10  to form a fixed toner image on the sheet  10 . Of the two sides of the sheet  10 , the toner  11  is maintained on the side of the sheet  10  which contacts the fixing roller  12 . The sheet  10  which has passed through the nip  19  naturally separates from the curvature of the fixing roller  12  by the resilience/stiffness of the sheet itself, and is transported to the right in FIG.  2 . This sheet  10  is transported by a discharge roller and ejected onto a discharge tray  130 . 
     The effects of the embodiment are described below. 
     The temperature distribution of the fixing roller  12  is normally lower at the bilateral end parts. When the coil  14  is provided on the exterior of the fixing roller  12  as in the present embodiment, the temperature drop at the bilateral end parts becomes greater. The ring member  30  is provided to suppress the temperature drop at the bilateral end parts of the fixing roller  12 . 
     The magnetic bond between the fixing roller  12  and the iron core  17  is increased near the bilateral ends of the fixing roller by providing the ring member  30  on the iron core  17  near the end parts of the fixing roller, thereby increasing the induced electromotive force generated at the bilateral ends of the fixing roller  12  so as to be greater than the induced electromotive force generated at other part of the fixing roller  12 . For this reason, the induction current flowing within the fixing roller  12  is greater near the bilateral ends of the fixing roller  12  than at the other part of the fixing roller  12  even when the wall thickness of the fixing roller  12  is uniform, and the temperature rise near the bilateral end parts of the fixing roller  12  is also greater than at the other part of the fixing roller  12 . Accordingly, the temperature drop near the bilateral end parts of the fixing roller  12  is corrected, and the temperature distribution is uniform in the axial direction of the fixing roller  12 . 
     The taper  32  provided at both ends of the ring member  30  increases the magnetic bond between that part and the fixing roller  12  such that the magnetic field generated within the iron core  17  which forms a closed magnetic circuit readily enters the ring member  30 . 
     Furthermore, providing the slit  31  in the ring member  30  along the axis of the fixing roller  12  to segment the ring eliminates conductivity in the ring member  30  in directions intersecting the magnetic field generated in the iron core  17 . In this way, the flow of induction current in the ring member  30  is prevented, thereby preventing the ring member  30  from generating heat. Accordingly, energy loss is prevented by providing the ring member  30 . 
     In the present embodiment, the cross section of the iron core  17  has an approximately circular shape, thereby increasing the contact surface area between the iron core  17  and the ring member  30 , and increasing the magnetic bond. A cross section approximately circular in shape may be provided, for example, by cutting after the iron core has been formed by laminating the silicon steel plates  18 . This cross section shape of the iron core  17  is not limited to a laminated iron core, and also may be applied to an iron core formed as an integrated single part. 
     Although the present invention has been described by way of the embodiments mentioned above, the present invention is not limited to these embodiments. 
     For example, a band-like silicon steel plate  35  may be wound in a spiral shape and used as a ring member  30 ′, as shown in FIG.  7 . In this case, the spirally wound silicon steel plate  35  is provided with an insulation layer  36  to prevent the windings from making mutual contact. In this way, an induction current is prevented from flowing in the spiral-shaped wound silicon steel plate  35  in a direction intersecting the magnetic field generated within the iron core. The insulation layer  36  may be a space such that the windings of the silicon steel plate  35  do not make mutual contact, or an insulation material may be wrapped around the silicon steel plate  35 . The example shown in FIG. 7 provides a space as the insulation layer  36 . 
     Furthermore, while the cross section of the iron core is approximately circular in the above embodiments to increase the contact surface area between the iron core and the ring member, the present invention is not limited to a circular shape inasmuch as a multisided shape also may be used. 
     Moreover, while a copier has been used as an example of an image forming apparatus in the above embodiments, the present invention is not limited use in a copier, inasmuch as the present invention may be suitably adapted for use as the fixing device in various image forming apparatuses such as, for example, printers, facsimile machines and the like. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. 
     Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.