Abstract:
An image forming apparatus, an image fixing apparatus, and an image fixing method that fix a toner image onto an recording medium in an electrophotographic process capable of effectively controlling an image fixing temperature. In one example, when a narrower recording medium, narrower than a maximum allowable size, is continuously fed at an image fixing temperature, a gap between the conveyed recording mediums is controlled to be longer than a default gap corresponding to a wider recording medium in a feeding direction to avoid an increase of the temperature of the ends of the fixing member in the longitudinal direction, to thereby maintain an allowable temperature limit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an image forming apparatus such as a printer, a copying machine, a facsimile, etc., which forms an image on a recording medium such as paper, an OHP sheet, cloth, etc., and to an image fixing apparatus and an image fixing method in which the image on the recording medium is fixed by heat and pressure in a nip press region between a fixing member and a pressing member. The present invention more particularly relates to an image forming apparatus, an image fixing apparatus, and an image fixing method capable of effectively controlling an image fixing temperature. 
   2. Discussion of the Background 
   A background image forming apparatus such as a printer, a copying machine, a facsimile, etc. mostly adopts an electrophotographic process from the considerations of speed, image quality, cost, etc. The electrophotographic process includes operations of charging a photo-conductor, forming an electrostatic image by irradiating, developing to form a toner image, transferring the toner image onto a recording medium, and fixing the toner image on the recording medium. 
   Accordingly, such a kind of image forming apparatus using an electrophotographic process is equipped with an image fixing apparatus for fixing the toner image. As the fixing apparatus, a hot pressing type apparatus is mostly adopted from the consideration of safety. The hot pressing type fixing apparatus has a fixing nip press region between a fixing member, which is a drum or a belt to be heated, and a pressing member, which is also a drum or a belt for pressing. The toner image on a recording medium is fixed by heat and pressure in the nip press region. 
     FIG. 1  illustrates a cross sectional view of an exemplary configuration of a background fixing apparatus. A rotating fixing member  1  includes a heater  2  inside. A temperature sensor  3  is provided on the surface of the rotating fixing member  1 . A rotating pressing member  4  presses the rotating fixing member  1  to form a fixing nip press region n. When a recording medium P passes through the fixing nip press region n, a toner image may be fixed by heat and pressure on the recording medium P.  FIG. 2  illustrates another cross sectional view of the fixing apparatus of  FIG. 1 . The detected result by the temperature sensor  3  is input into a central processing unit (CPU)  7  through an input circuit  6 . The CPU  7  controls the heater  2  through a driver  8  based on the detected result by the temperature sensor  3 . 
   When a recording medium p 2 , which has a narrower width than a recording medium p 1 , is continuously fed, non-feeding areas m on both sides in the longitudinal direction may have a high temperature because heat in these areas is not absorbed in the recording medium p 2 . This may cause a problem. The problem is a decrease of the fixing apparatus life due to the higher temperature of the ends of the rotating fixing member  1  that exceeds an allowable temperature limit. 
   To avoid the problem, some image forming apparatuses have a fixing apparatus in which a fixing roller has two heaters, one for heating the center part of the fixing roller in the longitudinal direction, and the other for heating the end parts of the fixing roller. When using the narrower recording medium, only the heater for heating the center part of the fixing roller in the longitudinal direction is turned on, so the temperature of the ends of the rotating fixing member may rise but stays below the allowable temperature limit. 
   To avoid the problem in other examples, some image forming apparatuses have a fixing apparatus in which a temperature of the end parts of the fixing roller is detected, and a feeding timing of the recording medium may be controlled based on the detected temperature. For example, if the end parts of the fixing roller are expected to have a high temperature, the gap between the conveying recording mediums may be longer than a normal gap, and then the temperature of the ends of the rotating fixing member may rise but still be less than the allowable temperature limit. 
   SUMMARY OF THE INVENTION 
   The present invention sets forth a novel image forming apparatus, a novel image fixing apparatus, and a novel image fixing method that fix a toner image onto an recording medium in an electrophotographic process capable of effectively controlling an image fixing temperature. In one example, when a narrower recording medium, narrower than a maximum allowable size, is continuously fed at an image fixing temperature, a gap between the conveyed recording mediums is controlled to be longer than a default gap corresponding to a wider recording medium in a feeding direction to avoid an increase of the temperature of the ends of the fixing member in the longitudinal direction for maintaining an allowable temperature limit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is an illustration illustrating a cross sectional view of a background image fixing apparatus; 
       FIG. 2  is another cross sectional view of the image fixing apparatus of  FIG. 1 ; 
       FIG. 3  is an illustration illustrating an exemplary configuration of an image forming apparatus according to an exemplary embodiment of the present invention; 
       FIG. 4A  illustrates an exemplary configuration of the image fixing apparatus of  FIG. 3 ; 
       FIG. 4B  illustrates an exemplary configuration of the image fixing apparatus of  FIG. 3 ; 
       FIG. 4C  illustrates an exemplary configuration of the image fixing apparatus of  FIG. 3 ; 
       FIG. 5A  is a timing chart showing a relation of time and heat power of the image fixing apparatus of  FIG. 3 ; 
       FIG. 5B  is a timing chart showing a relation of time and heat power of the image fixing apparatus of  FIG. 3 ; 
       FIG. 6A  is a graph showing a relation of time and temperature of the fixing belt of  FIG. 3 ; and 
       FIG. 6B  is a graph showing a relation of time and temperature of the fixing belt of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 3 , an illustration illustrating an exemplary configuration of an image forming apparatus according to an embodiment of the present invention is provided. 
   The image forming apparatus has a tandem color electrophotographic printer engine  10 . The tandem color electrophotographic printer engine  10  includes four image forming stations  12 , which correspond to four colors of yellow, magenta, cyan, and black, respectively. Each image forming station  12  includes a charger  14 , a developer  15 , a first transferring member  16 , and a first cleaner  17  around a photo-conductor drum  13 . An exposure device  18  for four colors is provided under the tandem color electrophotographic printer engine  10 . A transferring unit  20  is provided upside of the tandem color electrophotographic printer engine  10 . 
   The transferring unit  20  includes an endless belt as a transferring belt  26 , which is tensed by rollers  21 ,  22 ,  23 ,  24 , and  25 . The transferring belt  26  rotates counterclockwise in  FIG. 3 . The transferring belt  26  passes through a position of the first transferring between the photo-conductor drum  13  and the transferring member  16  at each image forming station  12 . 
   Further, the transferring belt  26  passes through a position of the second transferring between a second transferring roller  21  and a second transferring member  27 . Furthermore, waste toner on the transferring belt  26  is cleaned by a second cleaner  29 . 
   A paper cassette  30 , which is detachable, is provided at a lower part of the printer. Paper as the recording medium is put into the paper cassette  30 . The printer has a paper conveying area  32  at a right side of the printer in  FIG. 3 . Paper is fed by a feeding roller  33 , and held by resist rollers  34 , and passes through the position of the second transferring between the transferring belt  26  and the second transferring member  27  in the paper conveying area  32 . Further, the paper passes through a fixing apparatus  35 , and is then output by paper exit rollers  36 . 
   When an image is to be formed on the backside of the paper, the paper output from the fixing apparatus  35  is switch-backed in a paper switchback area  37 , and the paper is conveyed to the resist rollers  34  again. 
   Toner bottles  38 , which supply toner for the image forming stations  12 , are set at an upper part of the printer. The toner bottles  38  correspond to the four colors of yellow, magenta, cyan, and black, respectively. 
   When a color image forming is performed with this printer, motors (not shown) are driven based on an image signal from a host computer. Then, the photo-conductor drums  13  rotate clockwise, one of the rollers  21 ,  22 ,  23 ,  24 , and  25  rotates for driving, the other rollers rotate by being driven, and the transferring belt  26  rotates counter clockwise in  FIG. 3 . 
   At each image forming station  12 , the surface of the photo-conductor drum  13  is evenly charged by a charger  14 . The exposure device  18  exposes the surface of each photo-conductor drum  13  based on the image signal to form electrostatic images. The electrostatic images on the surfaces of the photo-conductor drums  13  are developed to form visible toner images by the developers  15 . Then, black, yellow, magenta, and cyan images are formed on the photo-conductor drums  13 , respectively. Each toner image on the photo-conductor drums  13  is transferred color by color to the transferring belt  26  to form a four color image in an electric field by the first transferring member  16  at the position of the first transferring. 
   Meanwhile, the feeding roller  33  rotates counter clockwise based on the image signal, and feeds the paper from the paper cassette  30 . The paper is conveyed into the paper conveying area  32 , and is stopped with the resist rollers  34 . The resist rollers  34  rotate based on the conveying timing of the four color image on the transferring belt  26 , and convey the paper to the position of the second transferring. The four color image on the transferring belt  26  is transferred to the paper in an electric field by the second transferring roller  21 . 
   After the second transferring, the paper is conveyed into the fixing apparatus  35 , and the transferred image is fixed on the paper by heat and pressure. After fixing, the paper is output onto a paper stack  31  by paper exit rollers  36 , or when an image is to be formed on the backside of the paper, the paper output from the fixing apparatus  35  is switch-backed in paper switch-back area  37 , and the paper is conveyed to the resist rollers  34  again, and after another transferring and fixing, the paper is output onto a paper stack  31  by paper exit rollers  36 . 
   After the first transferring, each photo-conductor drum  13  is cleaned by its respective first cleaner  17 , and the waste toner is removed. Then, each photo-conductor drum  13  is set to standby for a next image. Also, after the second transferring, the transferring belt  26  is cleaned by the second cleaner  29 , and the waste toner is removed. Then, the transferring belt  26  is set to standby for the next image. 
     FIGS. 4A to 4C  illustrate an exemplary configuration of the fixing apparatus  35  of  FIG. 3 . The fixing apparatus  35  includes a fixing belt  42  which is tensed by a fixing roller  40  and a heating roller  41 , and a pressing roller  43  which is pushed by a pushing member (not shown). A fixing nip is formed between the fixing belt  42  and the pressing roller  43 . 
   The fixing roller  40  may have a structure in which a metal roller is covered with silicone rubber. The heating roller  41  may be made of aluminum, iron, etc. with a structure in which the roller  41  has an internal heat source  44 . As a heat source  44 , a halogen heater or an induction heating (IH) are typically used. The fixing belt  42  may have a structure in which a base layer made of nickel, polyimide, etc. is covered with a surface layer so it is easy to release the paper, such as perfluoro-alkoxyalkane (PFA), polytetrafluoroethylene (PTFE), etc. The fixing belt  42  may have an intermediate layer made of silicone rubber, etc. between the surface layer and the base layer. The pressing roller  43  may have a structure in which a metal roller made of aluminum, iron, etc. is covered with an elastic layer, such as silicone rubber, and the elastic layer is covered with a surface layer so it is easy to release the paper, such as PFA, PTFE, etc. 
   In  FIG. 4A , a pushing member  45  pushes a tension roller  46  at an outside of the fixing belt  42  causing tension in the fixing belt  42 . In  FIG. 4B , a pushing member  45  pushes a tension roller  46  at an inside of the fixing belt  42  causing tension in the fixing belt  42 . In  FIG. 4C , a pushing member  47  pushes the heating roller  41  causing tension in the fixing belt  42 . A thermo sensitive register  48  is provided in contact with the fixing belt  42 . A separator plate  49  is provided in contact with the fixing belt  42  near the exit portion of the nip. 
   When a fixing process is carried out, the heat source  44  is turned on, the fixing roller  40  rotates by action of a motor (not shown), and the heating roller  41 , the fixing belt  42 , and the pressing roller  43  are driven to rotate. The heat source  44  heats the heating roller  41 , and the heating roller  41  heats the fixing belt  42 . When paper  50 , on which toner is transferred in the second transferring, is passed through the fixing nip region, the toner image on the paper is fixed by heat and pressure. Further, the paper  50  is conveyed by rotating of the fixing belt  42  and the pressing roller  43 . Furthermore, the paper  50  is peeled off from the fixing belt  42  by the separator plate  49 , and is guided upward, and is output to the paper stack. 
     FIG. 5A  and  FIG. 5B  are timing charts showing a relation of time and heat power of the heat source  44 . In the printer, when using paper  50  that has the maximum width for feeding, the heat source  44  is turned on at the timing of  FIG. 5A . There is a difference between the turning on time of the heat source  44  when the paper  50  is fed and the turning on time of the heat source  44  when the paper  50  is not fed due to an intermission of feeding. This causes a balance between the heat quantity which is supplied and the heat quantity which is taken by the paper  50  and is lost by heat dispersion. 
   When using paper  50  that is narrower than the maximum width for feeding, a non-feeding area on both sides of heating roller  41  and fixing belt  42  may have a high temperature because heat in this area is not absorbed in the paper  50  and a supplied heat quantity is larger than a lost heat quantity by heat releasing. Therefore, when continuously feeding the narrower paper into the fixing apparatus  35 , the intermission period between feeding may be prolonged, as shown in  FIG. 5B . As shown in  FIG. 5B , the intermission period is longer compared with the feeding period, so that the temperature of both sides of the fixing belt  42  is controlled for preventing an excessive high temperature, by heat releasing in the intermission period. 
   When the detected fixing temperature of the fixing apparatus  35  using the thermo-sensitive register  48  is the same, the intermission period may be made longer based on and proportional to the length of the paper  50  in the feeding direction. That is, when the length of the paper  50  in the feeding direction is expressed by a, the length between paper to paper in feeding is expressed by b, a/b may be nearly the same as a predetermined value c. Therefore, when the length of the paper  50  in the feeding direction is longer and is capable of causing a higher temperature at the non-feeding areas on both sides of the fixing belt  42 , an even longer intermission period may be used to reduce the temperature of the fixing belt  42 . 
   Thereby, when continuously feeding the narrower paper  50  into the fixing apparatus  35 , with consideration of the length of the paper  50 , the intermission period between feeding may be set effectively to prevent the temperature of non-feeding areas on both sides of the fixing belt  42  from increasing to be excessively high. 
   When the detected fixing temperature of the fixing apparatus  35  by the thermo-sensitive register  48  is high enough to be capable of increasing the temperature of non-feeding areas on both sides of the fixing belt  42 , the intermission period between feeding may made be longer. 
   Further, when continuously feeding the narrower paper  50  into the fixing apparatus  35 , with consideration of the length of the paper  50  in the feeding direction and the fixing temperature, the intermission period between feeding may be set effectively to prevent the temperature of non-feeding areas on both sides of the fixing belt  42  from increasing to be excessively high further considering such cases that the thickness or type of the paper  50  affects the fixing temperature. 
     FIG. 6A  is a graph showing relation of time and temperature of the fixing belt  42  at normal environmental temperatures.  FIG. 6B  is a graph showing relation of time and temperature of the fixing belt  42  at high environmental temperatures. The printer can be equipped with a temperature detector to detect environmental temperature (although not shown). 
   As shown in  FIG. 6A  and  FIG. 6B , when the environmental temperature is high, the temperature of non-feeding areas on both sides of the fixing belt  42  may more easily increase rapidly. Therefore, when the detected environmental temperature of the fixing apparatus  35  is higher, the intermission period between feeding may be made even longer. Thereby, when continuously feeding the narrower paper  50  into the fixing apparatus  35 , with consideration of the length of the paper  50  in the feeding direction and the environmental temperature, the intermission period between feeding may be set effectively to prevent the temperature of non-feeding areas on both sides of the fixing belt  42  from increasing to be excessively high in such a case that the environmental temperature is high. 
   When continuously feeding the narrower paper  50  into the fixing apparatus  35 , the printer may have a first control operation to prolong the intermission period between feeding and a second control operation to suspend the feeding, and also may have a selector to select the first control operation or the second control operation. 
   A user may also preferably select the first control operation or the second control operation through such as an operation panel. When the first control operation is selected and the narrower paper  50  is continuously fed into the fixing apparatus  35 , the intermission period between feeding may be prolonged to prevent the temperature of non-feeding areas on both sides of the fixing belt  42  from increasing to be excessively high. When the second control operation is selected and the narrower paper  50  is continuously fed into the fixing apparatus  35 , the feeding may be suspended until the temperature of non-feeding areas on both sides of the fixing belt  42  decreases to about the same temperature of the feeding area of the fixing belt  42  to prevent the temperature of the non-feeding areas on both sides of the fixing belt  42  from increasing to be excessively high. 
   When the user selects the second control operation and feeding the narrower paper  50  is suspended due to the high temperature of non-feeding areas on both sides of the fixing belt  42 , rotating the fixing belt  42  and the pressing roller  43  decreases the temperature of the non-feeding areas on both sides of the fixing belt  42  rapidly due to the heat transmission from the fixing belt  42  to the pressing roller  43 . So, an electromagnetic clutch etc. can be used to intercept other roller&#39;s rotation and the fixing belt  42  and the pressing roller  43  are rotated in free time. More particularly, rotating the fixing belt  42  and the pressing roller  43  faster than at a normal printing speed is effective to reduce a waiting time. 
   In the fixing apparatus  35 , when the thickness of the heating roller  41  is not greater than 1.0 mm, the temperature of non-feeding areas on both sides of the fixing belt  42  may easily increase rapidly due to the small heat capacity of the heating roller  41 . In this case, when continuously feeding the narrower paper  50  into the fixing apparatus  35 , with consideration of the length of the paper  50  in the feeding direction and the width of the paper  50 , the intermission period between feeding may be set effectively to prevent the temperature of non-feeding areas on both sides of the fixing belt  42  from increasing to be excessively high. 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
   This patent specification is based on Japanese patent applications, No. JPAP2005-263515 filed on Sep. 12, 2005 in the Japan Patent Office, the entire contents of which are incorporated by reference herein.