Abstract:
A fusing device of an image forming apparatus and method thereof are provided. The fusing device and method include a conductive member having a linear portion for contacting a printing medium, a fusing film for sliding on a circumference of the conductive member, a pressing roller for contacting the fusing film in the linear portion, forming a fusing nip area, and rotating the fusing film, and an induction heating unit for heating the conductive member by induction and generating heat. The thickness of the conductive member in the fusing nip area is smaller than the thickness of the conductive member in other areas.

Description:
PRIORITY 
   This application claims the benefit under 35 U.S.C. 119(a) of Korean Patent Application No. 2004-363, filed on Jan. 5, 2004, in the Korean Intellectual Property Office, the entire contents of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a fusing device of an image forming apparatus and method thereof. More particularly, the present invention relates to a fusing device of an image forming apparatus and method thereof which locally heats a fusing nip area. 
   2. Description of the Related Art 
   In general, a copier, a printer, a facsimile, and a multifunctional device which provides all functions of the copier, the printer, and the facsimile, have a printing function in common. These devices are referred to as an image forming apparatus. 
   The image forming apparatus includes a fusing device which heats and presses a sheet of paper onto which a toner image is transferred, melts the powdery toner image on the sheet of paper, and fuses the melted toner image on the sheet of paper. The fusing device includes a heating unit which generates heat, and a pressing roller which forms a fusing nip with the heating unit, applies pressure thereto, and helps the toner melt. 
   The fusing unit melts the toner when the fusing unit is heated to a predetermined temperature, for example, 180° C. Means for heating the fusing unit include a halogen lamp, a resistive coil, or an induction heating coil. 
     FIGS. 1 and 2  are cross-sectional views illustrating a conventional induction heating-type fusing device according to an embodiment disclosed in U.S. Pat. No. 6,341,211. 
   Referring to  FIGS. 1 and 2 , the fusing device includes a conductive member  12  which is a hollow structure fixed in an unrotated state and melts toner  11  on a sheet of paper  10  thermally, a pressing roller  13  which closely presses the sheet of paper  10  having the toner  11  toward the conductive member  12 , a traveling belt  20  which is interposed between the fixed conductive member  12  and the pressing roller  13  and transfers the sheet of paper  10 , and a coil  14  which inductively heats the conductive member  12 . The pressing roller  13  moves in a direction of an arrow B, and the traveling belt  20  is rotated in a direction of an arrow A as the pressing roller  13  moves in the direction of the arrow B. 
   The conductive member  12  comprises a hollow pipe and comprises one of a carbon steel pipe, a stainless alloy pipe, an aluminum pipe, and iron. 
   The pressing roller  13  includes an axial core  15  and a silicon rubber layer  16  formed at a circumference of the axial core  15 . The pressing roller  13  is pressed in a direction of the conductive member  12  using a spring member (not shown). 
   A rectangular core  17  forms a closed magnetic circuit, and a part thereof perforates a hollow portion  12   a  of the conductive member  12 . The coil  14  is wound around the core  17 . When a current flows through the core  17 , magnetic flux by which an inductive current is generated along a circumferential direction of the conductive member  12  is produced. The core  17  is an iron core used in a general transformer. An insulating layer  22  electrically insulates the coil  14  from the core  17 . 
     FIG. 3  shows the structure of a conventional induction heating-type fusing device according to another embodiment disclosed in U.S. Pat. No. 6,341,211. The same reference numerals are used for the same elements as those of the conventional fusing device, and detailed descriptions thereof will be omitted. 
   The difference between the embodiment of  FIGS. 1 and 2  and the embodiment of  FIG. 3  is that the core  17  and the conductive member  12  in the embodiment of  FIGS. 1 and 2  have a rectangular shape and, in the embodiment of  FIG. 3 , a conductive member  12 ′ is a cylindrical roller. Since the other elements are substantially the same, the same reference numerals are used, and detailed descriptions thereof will be omitted. 
   In the conventional fusing device, since the conductive members  12  and  12 ′ are uniformly heated by induction, a heating unit is heated to maintain a temperature required for a fusing operation, and the heat loss is significant. In addition, it is difficult to obtain a uniform fusing property due to heat loss at both ends of the conductive members. 
   SUMMARY OF THE INVENTION 
   The present invention provides a fusing device of an image forming apparatus and method in which heat generated at a conductive member by inductive heating is densely concentrated in a fusing nip area. 
   The present invention also provides a fusing device of an image forming apparatus and method in which a fusing unit is uniformly heated in a lengthwise direction. 
   According to an aspect of the present invention, there is provided a fusing device of an image forming apparatus and method thereof. The fusing device and method comprise a conductive member having a linear portion for contacting a printing medium; a fusing film for sliding on a circumference of the conductive member; a pressing roller for contacting the fusing film in the linear portion, forming a fusing nip area, and rotating the fusing film; and an induction heating unit for heating the conductive member by induction and generating heat, wherein the thickness of the conductive member in the fusing nip area is smaller than the thickness of the conductive member in other areas. 
   The induction heating unit can comprise a core perforating a hollow of the conductive member and forming a magnetic circuit; a coil surrounding an outer circumference of the core spirally; and an AC voltage source applying a predetermined AC voltage to both ends of the coil. 
   The fusing device can further comprise an insulating layer formed between the coil and the core. 
   A number of turns of the coil at both ends of the conductive member can be greater than a number of turns of the coil in a central portion of the conductive member. 
   The fusing device can further comprise a coating layer formed on a circumference of the conductive member to reduce a frictional force between the fusing film and the conductive member. 
   The thickness of the conductive member at both ends of the fusing nip area can be smaller than the thickness in a central portion of the conductive member. 
   The width of the conductive member at both ends of the fusing nip area can be greater than the width in a central portion of the conductive member. 
   A temperature measuring sensor can be installed to contact an upper portion of the conductive member in the fusing nip area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above aspects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the accompanying drawings in which: 
       FIGS. 1 and 2  are cross-sectional views illustrating a conventional induction heating-type fusing device disclosed in U.S. Pat. No. 6,341,211; 
       FIG. 3  illustrates a conventional induction heating-type fusing device according to another embodiment disclosed in U.S. Pat. No. 6,341,211; 
       FIG. 4  is a partial cross-sectional view schematically illustrating a fusing device of an electrophotograhpic image forming apparatus according to an embodiment of the present invention; 
       FIG. 5  is a longitudinal cross-sectional view of the fusing device of  FIG. 4 ; 
       FIG. 6  illustrates a fusing device of an electrographic image forming apparatus according to an exemplary embodiment of the present invention. 
       FIG. 7  illustrates a fusing device of an electrographic image forming apparatus according to another exemplary embodiment of the present invention. 
       FIG. 8  illustrates a fusing device of an electrographic image forming apparatus according to yet another exemplary embodiment of the present invention. 
   

   Throughout the drawings, it should be noted that the same or similar elements are denoted by like reference numerals. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the drawings, the thicknesses of layers or regions are exaggerated for illustrative purposes. 
     FIG. 4  is a cross-sectional view schematically illustrating the structure of a fusing device of an electrophotograhpic image forming apparatus according to an embodiment of the present invention, and  FIG. 5  is a longitudinal cross-sectional view of the fusing device of  FIG. 4 . 
   A fusing device  100  includes a pressing roller  130  which is rotated in a direction of an arrow C, and a heating unit  120  which is installed to be opposite to the pressing roller  130  and fuses the toner image  111  onto the sheet of paper  110  passing between the pressing roller  130  and the heating unit  120  at a fusing nip N formed between the heating unit  120  and the pressing roller  130 . 
   The heating unit  120  includes a fixing portion having both ends fixed and a heating element therein, and a fusing film  121  which is slid on the surface of the fixing portion. The fusing film  121  can comprise polyimide having a thickness of 50–1000 μm, and a Teflon coating (not shown) which is a toner protective layer, can be formed on a surface contacting the toner image  111 . 
   The fixing portion includes a conductive member  122  and an induction heating part. The conductive member  122  includes a linear part  122   a  formed on one side thereof in an area corresponding to the fusing nip N and a cylindrical area having a hollow structure. The induction heating part heats the conductive member  122  by induction. 
   The induction heating part includes a core  123  which perforates a hollow of the conductive member  122 , a coil  125  which is wound in an outer circumference of the core  123  and inductively heats the conductive member  122 , and an AC voltage source as illustrated in  FIG. 6  which applies a predetermined AC voltage to both ends of the coil  125 . 
   The conductive member  122  comprises conductive metal, such as a carbon steel pipe, a stainless alloy pipe, an aluminum pipe, or iron. A coating layer  122   c  which reduces a frictional force against the fusing film  121 , can be formed at a circumference of the conductive member  122 . The coating layer  122   c  comprises fluoric resin, such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), or tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or silicon resin to a thickness of about 0.1 mm. 
   The core  123  forms a closed magnetic circuit. The coil  125  is wound on a portion of the circumference of the core  123  inside a hollow section  122   b  of the heating unit  120 , several hundreds or thousands of times, and an insulating layer  124 , for example, mica sheet is wound between the core  123  and the coil  125 . The insulating layer  124  prevents electrical connection between the core  123  and the coil  125 . 
   When an AC voltage is applied from an AC voltage source (not shown) to the coil  125 , magnetic flux by which an inductive current is generated along a circumferential direction of the conductive member  122 , is produced. The core  123  can comprise an iron core used in a conventional transformer and has high magnetic permeability. The conductive member  122  is heated by the inductive current. 
   The fusing film  121  is rotated in a direction of an arrow D. The fusing film  121  can be driven and rotated by the pressing roller  130  due to a frictional force between the pressing roller  130  and the fusing film  121  rather than by an additional driving unit. 
   The pressing roller  130  includes an elastic roller  131  which contacts the fusing film  121  and forms the fusing nip N, and a shaft  132  which supports the elastic roller  131  at the center of the elastic roller  131  and is rotated by a driving unit (not shown). The shaft  132  is elastically biased toward the opposite heating unit  120  using a spring member  133 . The elastic roller  131  can be formed of heat-resistant silicon rubber. Due to rotation of the elastic roller  131 , the fusing film  121  is driven and rotated on the circumference of the conductive member  122 . 
   A thermistor  127  which measures a temperature of the fusing nip N, is installed above the linear portion  122   a  of the fusing nip N. The temperature of the fusing nip N is determined by the number of turns of the coil  125  and frequency and voltage from the AC voltage source. 
   Meanwhile, a thickness t 1  of an area corresponding to the fusing nip N of the conductive member  122  is different from a thickness t 2  of another area of the conductive member  122 . When an AC of several tens to hundreds of Hz is applied from the AC voltage source to the coil  125 , an AC magnetic field is formed in an axial direction around the core  123  and the coil  125 , and an inductive current flows in a circumferential direction of the conductive member  122 . In this case, a skin depth at which the current is generated can be given by Equation 1. 
   
     
       
         
           δ 
           = 
           
             
               2 
               ωμσ 
             
           
         
       
     
   
   Here, δ is a skin depth, ω is an angular frequency, μ is magnetic permeability, and σ is an electrical conductive constant. When a thickness (t 1  of  FIG. 4 ) of the fusing nip N of the conductive member  122  is smaller than the skip depth δ, a significant amount of an inductive current flowing in the circumferential direction is generated at the fusing nip N rather than in another area of the conductive member  122 . The current flowing in the circumferential direction flows to a small cross section of the fusing nip N. Since resistance at the fusing nip N increases in inverse proportion to the cross section and Joule&#39;s heat at the fusing nip N is in proportion to the resistance, the thickness t 1  of the fusing nip N is adjusted so that the temperature of the fusing nip N can be locally increased. 
   When an AC having frequency of several tens or hundreds of Hz is used, the skin depth is 2–20 mm. Thus, the thickness of the fusing nip N is less than the skip depth, and the thicknesses of other areas are greater than the skin depth so that stiffness of the heating unit  120  is maintained. 
   Meanwhile, the thickness ti of the fusing nip N can be gradually reduced from the center toward both ends. The thickness of the conductive member  122  at the both ends of the fusing nip N is reduced so that heat loss at the both ends is compensated for and the temperature in the lengthwise direction of a fusing nip area is maintained at a constant level. 
   An operation of the fusing device having the above structure will be described with reference to the accompanying drawings. 
   First, when an AC having a frequency of several tens or hundreds of Hz is applied from an AC voltage source (not shown) to the coil  125 , AC magnetic flux is generated in the core  123  wound by the coil  125 . Due to the magnetic flux, an inductive current is generated in a circumferential direction of the conductive member  122  which is an adjacent conductor, and Joule&#39;s heat is generated by the inductive current. In this case, the thickness of the conductive member  122  of the fusing nip N is less than a skin depth and the fusing nip N having a thickness smaller than the thickness t 2  of other areas is locally and further heated and is rapidly heated to a temperature appropriate for a fusing operation, for example, 150–200° C. In addition, since the thickness of both ends of the fusing nip N is smaller than the thickness of the center of the fusing nip N, both ends of a heating unit  120  is further heated, and the temperature at both ends of the heating unit  120  is prevented from being lowered. 
   When a sheet of paper  110  on which toner  111  which has not yet been fused is fed into the fusing device maintained at a predetermined fusing temperature, the sheet of paper  110  enters between the heating unit  120  and a pressing roller  130 , the unfused toner  111  is heated at the fusing nip N and is melted, pressed by the pressing roller  130 , and is fused onto the sheet of paper  110 . 
   A surface temperature of the fusing nip N of the conductive member  122  can be adjusted using a thermistor  127  by controlling the AC voltage and the frequency applied to the coil  125 . 
   According to another embodiment of the present invention as illustrated in  FIG. 7 , the thickness of the conductive member  122  of the fusing nip N is maintained at a constant level, and the width of the conductive member  122  is gradually increased from the center toward both ends so that heat loss at both ends of the fusing device can be compensated for. 
   According to another embodiment of the present invention as illustrated in  FIG. 8 , the number of turns of the coil  125  is gradually increased from the center toward both ends so that the temperature at both ends of the fusing device can be prevented from being lowered 
   As described above, in the fusing device of the image forming apparatus according to an embodiment of the present invention, the thickness of a conductive member at a fusing nip is less than a skin depth and is locally heated such that heat loss in other areas can be reduced. In addition, the thickness and width of a conductive layer in an area corresponding to the fusing nip are adjusted to compensate for heat loss at both ends of the fusing device such that temperatures in a lengthwise direction of the fusing device are maintained at a constant level and an image quality is improved. 
   While this invention has been particularly shown and described with reference to certain embodiments thereof, it should be understood by those skilled in the art that various changes in form and details can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.