Patent Publication Number: US-2007122215-A1

Title: Fixing device for an image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 2005-113600, filed Nov. 25, 2005, the entire contents of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an image forming apparatus. More particularly, the present invention relates to a fixing device for an image forming apparatus that fuses and fixes a toner image onto a paper.  
      2. Description of the Related Art  
      Generally, image forming apparatuses, such as printers, scanners, copiers, and multifunction peripherals (MFP), include a fixing device for fixing developer, for example, a toner image, transferred onto the paper by a general transferring device.  
       FIG. 1  is a sectional view schematically showing a conventional fixing device of an image forming apparatus.  
      As shown in  FIG. 1 , the conventional fixing device includes a heating roller  10  and a pressing roller  20  rotated in contact with each other. The heating roller  10  includes therein a heat source  11 , such as a heating coil for generating heat for fixing the toner image onto the paper permanently. Additionally, a temperature sensor (not shown) is contactedly or non-contactedly formed on the surface of the heating roller  10  to detect temperature of the heat source  11 .  
      While paper  1  passes through a nip a formed by contact between the heating roller  10  and the pressing roller  20 , the above-structured fixing device heats and presses an unfixed toner image  1 ′ transferred on the paper  1 , thereby fixing the unfixed toner image  1 ′ onto the paper. The temperature sensor mounted on the outside of the heating roller  10  detects the temperature of the heating roller  10 . A control unit (not shown) controls the temperature of the heating roller  10  by turning on and off the heat source  11 .  
      In response to a demand for high-speed operation of the image forming apparatus, an outer diameter of a fixing roller needs to be increased. This expands the width of the nip because the fixing quality deteriorates due to a decreased staying time of the paper at the nip as the printing speed increases. However, increasing the diameter of the roller is limited. Also, as the roller diameter increases, warming-up time and the manufacturing costs also increase.  
      Even in the high-speed image forming apparatus, the fixing quality may be ensured by increasing the pressure applied to the nip. However, too much pressure at the nip deteriorates the durability of the fixing device. Furthermore, paper jams will occur more frequently because the paper cannot move smoothly. Moreover, as driving torque increases, the apparatus may be damaged. Referring to the curve identified by {circle around (1)} of  FIG. 3 , distribution of the pressure at the nip in the conventional fixing device is symmetrical between a nip inlet and a nip outlet. Therefore, when the paper whereon the image is fixed passes through the nip outlet, a printed side of paper may roll up adhering to the surface of the heating roller  10 , which is called a ‘wrap jam’.  
      Accordingly, a need exists for an image forming apparatus having an improved fixing device in which the width of the fixing nip is increased to improve print quality.  
     SUMMARY OF THE INVENTION  
      Accordingly, an aspect of the present invention is to provide a fixing device for an image forming apparatus adapted to improve the fixing quality according to high-speed printing by increasing the width of a fixing nip.  
      Another aspect of the present invention is to provide a fixing device for an image forming apparatus that substantially prevents wrap jams from occurring at a nip outlet after image fixing.  
      A fixing device for an image forming apparatus includes a heating roller, a pressing roller rotated in tight contact with the heating roller, and a pressing member mounted corresponding to the heating roller and contacted with the heating roller. A fixing belt winds around the pressing member and moves along a caterpillar track.  
      The pressing member is disposed at a predetermined distance from a rear portion of the heating roller.  
      The pressing member is configured in a manner that one side surface thereof facing and contacting the heating roller has a corresponding form to a surface of the heating roller and the other side surface thereof is bent to be inscribed in the fixing belt to apply tension to the fixing belt.  
      The pressing member includes a protrusion part formed on a rear end of the one side surface thereof.  
      The pressing member includes a resilient member contacting an inner surface of the fixing belt. A support member supports the resilient member.  
      According to a first exemplary embodiment of the present invention, the support member may be a bent member having a clamp form, and the bent portion protrudes to press the heating roller.  
      According to a second exemplary embodiment of the present invention, the support member may include a first member facing the heating roller by one side surface thereof; and a second member connected to a rear end of the first member and having a protruded end for pressing the heating roller.  
      A fixing device according to another exemplary embodiment of the present invention includes a heating roller, a pressing roller forming a first fixing nip by contacting the heating roller, and a pressing member disposed at a lower stream of the heating roller and forming a second fixing nip by contacting the heating roller. A fixing belt runs along a caterpillar track, and winds around the pressing roller and the pressing member. The pressing member includes a protrusion part for pressing the paper passing through the second fixing nip with a greater pressure at a nip outlet than at the second fixing nip.  
      The pressing member includes a resilient member contacting an inner surface of the fixing belt. A support member supports the resilient member.  
      The support member is bent to include first and second members arranged substantially perpendicularly to each other. The protrusion part is formed at a corner between the first and the second members.  
      The support member includes first and second members connected substantially perpendicularly to each other. The protrusion part is formed at a corner between the first and the second members. The protrusion part is formed by connecting the first and second members such that one end of the first and second members deviate from each other.  
      The resilient member includes one of silicon rubber, urethane, and foamed resin.  
      The resilient member is applied with lubricant on one side contacting the fixing belt.  
      The support member includes metal.  
      Other objects, advantages and salient features of the invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
      The above aspect and other features of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawing figures, wherein;  
       FIG. 1  is a sectional view schematically showing a conventional fixing device of an image forming apparatus;  
       FIG. 2A  is an elevational view in partial cross section of a fixing device of an image forming apparatus according to a first exemplary embodiment of the present invention;  
       FIG. 2B  is an elevational view in partial cross section of a fixing device of an image forming apparatus according to a second exemplary embodiment of the present invention; and  
       FIG. 3  is a graph illustrating pressure distribution at a fixing nip of the fixing device according to an exemplary embodiment of the present invention. 
    
    
      Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawing figures.  
      The matters defined in the description, such as a detailed construction and elements thereof, are provided to assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention may be carried out without those defined matters. Also, well-known functions or constructions thereof are omitted to provide a clear and concise detailed description.  
       FIG. 2A  is an elevational view in partial cross section of a fixing device of an image forming apparatus according to a first exemplary embodiment of the present invention.  
      As shown in  FIG. 2A , the fixing device of the image forming apparatus includes a heating roller  100 , a pressing roller  200 , a pressing member  300 , and a fixing belt  400 .  
      Being substantially shaped as a cylindrical roller, the heating roller  100  includes therein a heat source  110 , such as a heating coil, for generating heat for fixing a toner image  1 ′ onto paper  1  permanently.  
      The pressing roller  200  is substantially formed as a cylindrical roller and arranged substantially parallelly with the heating roller  100 . The pressing roller  200  rotates in contact with the heating roller  100 , thereby forming a first fixing nip a.  
      Since structure and operation of the heating roller  100  and the pressing roller  200  are generally known, a detailed description thereof is omitted.  
      The pressing member  300  is mounted at a predetermined distance from a rear portion of the pressing roller  200  and contacts the heating roller  100 , thereby forming a second nip b. The pressing member  300  includes a protrusion part  300 ′ formed at a rear end of one side surface thereof that faces and contacts the heating roller  100 . Also, the pressing member  300  is bent so that the other side surface thereof is disposed within the fixing belt  400  and thereby applies tension to the fixing belt  400 . The pressing member  300  includes a resilient member  320  contacting an inner surface of the fixing belt  400 , and a support member  310  supporting the resilient member  320 . Preferable, the resilient member  320  and the fixing belt  400  are applied with a lubricant, respectively, on a surface for contacting with each other.  
      The support member  310  is a bent member having a clamp form and pressed by a spring (not shown). The bent portion protrudes to press the heating roller  100 . The support member  310  has one side curved substantially corresponding to the circumference of the heating roller  100  to contact the circumference of the heating roller  100 , thereby securing a certain width of the second fixing nip b. According to a second exemplary embodiment of the present invention, as shown in  FIG. 2B , a support member  311  includes a first member  311   a  facing the heating roller  100  with one side surface thereof, and a second member  311   b  connected to a rear end of the first member  311   a  and having a protruded end for pressing the heating roller  100 . Preferably, the resilient member  320  has an arc section  320 ′ behind the second fixing nip b to smoothly guide running of the fixing belt  400 . At the second fixing nip b, the resilient member  320  is curved to substantially correspond to the surface of the heating roller  100  to secure width and pressure of the second fixing nip b. The support member  310  may be formed of a robust material, such as general carbon steel, stainless steel, aluminum, aluminum alloy, or ceramics. The resilient member  320  may be formed of silicon rubber, urethane, or foamed resin.  
      The fixing belt  400  runs along a caterpillar track and winds around the pressing roller  200  and the pressing member  300 .  
      Although not shown, the fixing device according to the exemplary embodiments of the present invention may include a temperature sensor for detecting the temperature of the heating roller  100  and a control unit for controlling the temperature of the heating roller  100  detected by the temperature sensor.  
      The fixing device of the image forming apparatus, according to the exemplary embodiments of the present invention includes the pressing member  300  disposed behind the heating roller  200 , which rotates in tight contact with the heating roller  100  and pressingly contacts the heating roller  100 . According to this configuration, the first fixing nip a is formed by contact between the heating roller  100  and the pressing roller  200 , and the second nip b is formed between the heating roller  100  and the pressing member  300 . Accordingly, the width of the fixing nip may be efficiently increased. Therefore, as paper  1  is fed to the fixing device passing through the first fixing nip a and the second fixing nip b, an unfixed toner image  1 ′ transferred on the paper  1  by a transferring device is heated and pressed, thereby being fixed onto the paper  1 . Here, with the same printing conditions as in the conventional fixing device, because the widened fixing nip increases the staying time of the paper  1  at the fixing nip, printing quality may be improved even during high-speed printing.  
      As shown by the curve identified by {circle around (2)} in  FIG. 3 , pressures applied to the first and the second fixing nips a and b are distributed unsymmetrically. Especially, because the pressing member  300  includes the protrusion part  300 ′ formed on the rear end of one side surface thereof that contacts the heating roller  100 , the paper  1  that has passed through the second nip b is applied with greater pressure at the nip outlet b′ than at the second nip b. As a consequence, the paper  1  passed through the fixing device is forwarded to the fixing belt  400  and accordingly, wrap jams may be substantially prevented.  
      As may be appreciated from the above description of the exemplary embodiments of the present invention, as the width of the fixing nip is increased, the staying time at the fixing nip increases, thereby improving printing quality according to performance of high-speed printing. Additionally, wrap jams of the paper  1  may be substantially prevented from occurring at the nip outlet b.  
      While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.