Abstract:
A fixing device is provided, which has a fixing roller and a sheet. The recording paper where the toner image has been transcribed thereon is passed through a nip portion between the fixing roller and the sheet. By pressing and heating, the toner image is fixed on the recording paper. A heating line is wound on a surface opposite to a sheet&#39;s surface that is in contact with the fixing roller. By applying a power source to the heating line, the sheet is heated. When the recording paper enters the nip, the cause of the temperature suddenly lowering can be prevented.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the priority benefit of Japanese application serial no. 2001-235852, filed on Aug. 3, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates in general to a fixing device and an image forming device, which can be used in a facsimile device, a printer, a copier, or a multi-function machine with the above functions.  
           [0004]    2. Description of Related Art  
           [0005]    [0005]FIG. 7 is a side view showing an internal structure of a conventional fixing device. The item  100  is a fixing roller. A pressing roller  101  is pressed to be in contact with the fixing roller  100 . A fixing heater  102  is arranged inside the fixing roller  100  for heating the fixing roller  100 . A temperature detecting device  103  is used for detecting the temperature of the fixing roller  100 . A separation claw  104  is used for separating the recording paper wound on the fixing roller  100  from the fixing roller  100 . A cleaning pad  105  is in contact with the fixing roller  100  for cleaning the fixing roller  100 . A cleaning pad holder  106  is used for supporting the cleaning pad  105 . A cleaning pad invasion-regulating unit  107  is installed at a downstream side of the rotational direction of the fixing roller  100  with respect to the cleaning pad  105 . An entrance guiding plate  108  is installed at a recording paper entrance of the fixing device  100 . A paper-ejecting roller is installed at a recording paper exit of the fixing device  100 .  
           [0006]    The recording paper which the toner image has been transcribed thereon enters a nip portion between the fixing roller  100  and the pressing roller  101  through the entrance guiding plate  108 . By pressing and heating at the nip portion, the toner image is fixed on the recording paper, and then the recording paper is ejected to external through a paper-ejecting roller  109 . When the recording paper passes the nip portion, the toner attached on the fixing roller  100  is removed by the cleaning pad  105 .  
           [0007]    The surface temperature of the fixing roller  100  is detected by the temperature detecting device  103 . According to a detected result of the temperature detecting device  103 , a heating control of the fixing heater is performed by a controlling device (not shown).  
           [0008]    Conventionally, an infrared heater or a halogen heater is used as the fixing heater in the fixing device, and a structure constituted of the fixing roller and the pressing roller pair is most common. The pressing roller is a structure that is made by forming silicon rubber on a core bar and then a mold-releasing layer (such as Teflon, registered Trademark) is formed on the silicon rubber, so that the heat capacity of the pressing roller becomes larger. At the beginning of heating, the pressing roller does not get warm easily, causing incomplete fixing problems. Furthermore, there is also a demerit that parts have high cost. In addition, in order to assure a suitable nip width of the nip portion between the fixing roller and the pressing roller, a large pressing force (pressure) is required. Because of the pressure, stress will act on the recording paper, causing fixing wrinkle problems.  
           [0009]    Instead of the pressing roller, a nip portion is formed by pressing a sheet to be in contact with the fixing roller. For example, a fixing device, where a non-fixed image is fixed on the recording paper by making the recording paper pass the nip portion, is well studied and researched. By the above structure, the sheet can be easily warmed up, and the nip width can be also assured.  
           [0010]    However, regarding the fixing device using the sheet material, the heat capacity of the nip portion is small. Therefore, when the recording paper enters the nip portion, the heat will dissipate suddenly. At this time, the temperature detected by a thermistor might cause the temperature at the nip portion to be lowered and incomplete fixing might occur at the rear end of the recording paper because the position of the thermistor is located outside the nip portion and the response time of the detection is slow, etc.  
         SUMMARY OF THE INVENTION  
         [0011]    According to the foregoing description, an object of this invention is to provide a fixing device and an image forming device. The fixing device is composed of a heat conducting unit and a sheet. When the recording paper enters the nip portion, the heat from the sheet being suddenly dissipated can be prevented.  
           [0012]    According to the objects mentioned above, the invention provides a fixing device, comprising a heat conducting unit, having a heating source, and being heated by the heating source; and a sheet that is in contact with and presses the heat conducting unit. One end of the sheet is mounted on an installing fitment that is located at upstream side of a transporting direction of a recording paper, and the other end of the sheet is in contact with the heat conducting unit under a pressing status. When the recording paper onto which a toner image has been transcribed is passed through a nip portion between the sheet and the heat conducting unit, by pressing and heating, the toner image is fixed on the recording paper. Furthermore, a heating unit of line shape is laid on a back surface of the sheet at a region in the vicinity of the nip portion. According to the above structure, because the sheet is heated by the heating unit, the fixing device can be in a fixable status quickly. Additionally, when the recording paper passes the nip portion between the heat conducting unit and the sheet, the temperature of the nip portion can be prevented from being suddenly reduced.  
           [0013]    In the above fixing device, the heating unit can be laid parallel to a transporting direction of the recording paper. According to this structure, the strength of the sheet becomes robust and the pressure against the heat conducting unit can become more stable. Furthermore, because the central portion of the sheet can be bent in the up-and-down direction, the sheet can be changed to a suitable shape to meet the shape of the heat conducting unit.  
           [0014]    Alternatively, the heating unit can be laid in a grid shape. According to this structure, the strength of the entire sheet becomes more robust and the pressure against the heat conducting unit can become more stable.  
           [0015]    The invention further provides an image forming device, comprising a fixing device that has the aforementioned structure and features. According to the above structure, the fixing property can be improved 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:  
         [0017]    [0017]FIG. 1 is a side view showing an internal structure of a facsimile apparatus that has a fixing device and an image forming device;  
         [0018]    [0018]FIG. 2 is a side view substantially showing the structure of the fixing device;  
         [0019]    [0019]FIG. 3 is a perspective view of FIG. 2;  
         [0020]    [0020]FIG. 4 shows a back surface of the sheet;  
         [0021]    [0021]FIG. 5 shows a back surface of the sheet according to the second embodiment of the invention;  
         [0022]    [0022]FIG. 6 shows a back surface of the sheet according to the third embodiment of the invention; and  
         [0023]    [0023]FIG. 7 is a side view showing an internal structure of a conventional fixing device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    The embodiment of the invention is described in detail with reference of the attached drawing.  
         [0025]    [0025]FIG. 1 is a side view showing an internal structure of a facsimile apparatus that has a fixing device and an image forming device. The facsimile apparatus is composed of a facsimile main body  1 , a process cartridge  2 , a photo-sensing unit  3  having the process cartridge, an optical writing device  4 , a paper-feeding cassette  5  storing the recording paper, a paper-feeding roller  6  for feeding the recording paper from the paper-feeding cassette  5 , a transcribing roller  7 , a fixing device  8  for fixing the toner image on the recording paper, an ejecting roller, a sealed sensor  10 , and a document stage  11  formed above the facsimile main body  1 . The process cartridge  2  contains various processing devices for performing the image formation by the electronic photographic process, and is detachable from the facsimile main body  1 . The optical writing device  4  makes the photo-sensing unit  3  scan by a laser beam that is modulated based on the image data. The transcribing roller  7  is in contact with the photo-sensing unit  3  for transcribing the toner image formed on the photo-sensing unit  3  to the recording paper.  
         [0026]    The document set on the document stage  11  is passed through the sealed sensor  10  by a transferring system, and then ejected to external When the document passes the sealed sensor  10 , the image on the document is optically read by the sealed sensor  10 . The image data, which is read by the sealed sensor  10  or input from external, is transmitted to the optical writing device  4 . Based on the image data, the optical writing device  4  emits a modulated laser beam to the surface of the uniformly charged photo-sensing unit  3 , so that an electrostatic latent image is formed on the surface of the photo-sensing unit  3 . Then, toner is made to adhere to the electrostatic latent image to form a toner image, and the toner image is transcribed onto the recording paper by the transcribing roller  7 . By pressing and heating using the fixing device, the toner image is fixed on the recording paper, and the recording paper is ejected to external by the paper-ejecting roller  9 .  
         [0027]    [0027]FIG. 2 is a side view substantially showing the structure of the fixing device. FIG. 3 is a perspective view of FIG. 2. The fixing device consists of a cylindrical fixing roller  20 , a fixing heater  21 , a sheet  22 , and a fixing frame  23 . The fixing heater  21  is arranged inside the fixing roller  20  for heating the fixing roller  20 . The sheet  22  is in contact with the fixing roller  20  and therefore presses against the fixing roller  20 . The fixing frame  23  is used to affix one end of the sheet  22  so as to support the sheet  22  in a cantilever manner.  
         [0028]    [0028]FIG. 4 shows a back surface of the sheet, in which item  24  is a heating line. As shown in FIG. 4, the heating line  24  is wound on the back surface, i.e., opposite to the surface in contact with the fixing roller  20 , of the sheet  22 . The heating line  24  is a single line, and is straightly extended from an end in the longitudinal direction of the sheet to the other end, at which the heating line  24  is bent and then the heating line  24  is straightly extended again from the other end to the first end. By alternatively repeating the above manner, the heating line  24  is arranged on the back surface of the sheet  22  so that the lines parallel to the longitudinal direction of the sheet are arranged in parallel.  
         [0029]    The timing to apply a power to the heating line  24  can be at a time when the image forming device is powered on, within an interval that the temperature of the fixing roller  20  is raised from a warm-up status to a fixable temperature, or an interval that the recording paper passes the nip portion between the fixing roller  20  and the sheet.  
         [0030]    According to the above structure, it is possible to make the fixing device be in a fixable status quickly. Furthermore, when the recording paper passes a nip portion between the fixing roller  20  and the sheet  22 , even though the sheet  22  is cooled by the recording paper, the temperature of the nip portion can be maintained at a fixable temperature.  
         [0031]    [0031]FIG. 5 shows a back surface of the sheet according to the second embodiment of the invention. In FIG. 4, as regarding the back surface of the sheet  22  in the first embodiment, the heating line  24  is wound on the back surface of the sheet  22  in a manner such that the lines parallel to the longitudinal direction of the sheet are arranged in parallel. With respect to this, the second embodiment in FIG. 5 arranges the heating line  24  on the back surface of the sheet  22  in a manner that lines parallel to the recording paper transporting direction are arranged in parallel.  
         [0032]    In the fixing device using the sheet  22 , the nip pressure at each portion in the longitudinal direction of the fixing roller  20  is easily varied according to the shape of the fixing roller  20 . For example, when the fixing roller is a drum type, because the pressing roller is made of an elastic body, the nip pressure is undoubtedly acted on the central portion, but applying the pressure by the sheet is difficult to apply the nip pressure to the central portion, so that the fixing property is reduced.  
         [0033]    As shown in FIG. 5, by winding the one heating line  24  on the back surface of the sheet  22 , the strength of the sheet  22  becomes robust and the pressure against the fixing roller  20  can become more stable. Furthermore, because the central portion of the sheet  22  can be bent in the up-and-down direction, the sheet  22  can be changed to a suitable shape to meet the shape of the fixing roller  20 .  
         [0034]    [0034]FIG. 6 shows a back surface of the sheet according to the third embodiment of the invention. First, in the third embodiment, as the first embodiment in FIG. 4, the heating line  24  is arranged on the back surface of the sheet  22  in the manner that the lines parallel to the longitudinal direction of the sheet are arranged in parallel. Next, as the second embodiment in FIG. 5, the same heating line  24  is further wound on the back surface of the sheet  22  in the manner that lines parallel to the recording paper transporting direction are arranged in parallel. In this way, as shown in FIG. 6, the heating line  24  is wound in a grid shape on the back surface of the sheet  22 .  
         [0035]    According to the above structure, the strength of the entire sheet  22  becomes robust. Additionally, the pressure against the fixing roller  20  can become more stable.  
         [0036]    In addition, the foregoing embodiments describe a fixing device whose sheet is made to contact with fixing roller by pressure. However, fixing devices of other types can be also suitable for the invention.  
         [0037]    For example, the sheet of the invention can be applied to a fixing device, in which a fixing belt is suspended between a fixing roller with a heating source and a tension roller, and the fixing belt is made in contact with the sheet. As the fixing belt is rotated, the recording paper that the toner image is transcribed thereon passes the nip portion between the fixing belt and the sheet, so that the toner image is fixed on the recording paper. Furthermore, the sheet of the invention can be applied to another fixing device, in which a heating source is mounted inside a cylindrical fixing film, and the sheet is in contact with the heating source through the fixing film. By rotating the fixing film, the recording paper that the toner image is transcribed thereon passes the nip portion between the heating source and the sheet, so that the toner image is fixed on the recording paper.  
         [0038]    According to the aforementioned structure of the invention, because the sheet is heated by a heating unit, it is possible to make the fixing device be in a fixable status quickly. In addition, when the recording paper passes the nip portion between the heat conducting unit and the sheet, the temperature of the nip portion can be prevented from being suddenly reduced. As a result, the time of starting the image recording can be reduced, and an excellent image such that the entire recording paper has no incomplete fixing portions can be obtained.  
         [0039]    While the present invention has been described with a preferred embodiment, this description is not intended to limit our invention. Various modifications of the embodiment will be apparent to those skilled in the art. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.