Patent Publication Number: US-10768561-B2

Title: Fixing device and image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent application No. 2018-133243 filed on Jul. 13, 2018, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a fixing device fixing a toner image on a sheet and an image forming apparatus including this fixing device. 
     As one manner heating a fixing belt of a fixing device, a manner using a flat heater is known. The flat heater has a heat generating part having a longitudinal direction orthogonal to a conveying direction of a sheet, and the cylindrical fixing belt is sandwiched between the flat heater and a pressuring roller. When the pressuring roller is driven and rotated, the fixing belt is co-rotated with the pressuring roller, and the sheet on which the toner is transferred is sandwiched between the fixing belt and the pressuring roller and conveyed, and then, the toner is fixed on the sheet. 
     The above-mentioned heat generating part has a length corresponding to a sheet of a maximum size as a size in a direction orthogonal to the conveying direction, but in a case where a sheet with a size smaller than it is used, heat consumption of both ends in the longitudinal direction through which the sheet does not pass is decreased. Thereupon, a configuration is proposed, in which heat generating part is divided into a plurality of heat generating parts and the heat generating part according to the size of the sheet is heated. However, in a case providing a safety device (e.g. thermostat) interrupting power supply to the heat generating part when abnormal temperature rise occurs, if the safety devices are provided for each of the heat generating parts, cost is increased. Moreover, because wirings are increased, compact design is difficult. 
     However, in the fixing device having an above-described configuration, because the thermostat is provided only to the heating area at the center in the longitudinal direction, even if abnormal temperature rise occurs at the other heating area, power supply cannot be interrupted. 
     SUMMARY 
     In accordance with the present disclosure, a fixing device includes a cylindrical rotatable fixing belt, a flat heater, a safety device, a heater holding part and a pressuring member. The flat heater has a plurality of heat generating parts. The safety device is arranged across two adjacent heat generating parts to interrupt power supply to the heat generating parts in a case where temperature reaches a threshold value. The heater holding part holds the flat heater so that the flat heater comes into contact with an inner circumference face of the fixing belt. The pressuring member sandwiches the fixing belt with the flat heater to form a pressuring area for sandwiching and conveying a sheet between the fixing belt and the pressuring roller. 
     In accordance with the present disclosure, an image forming apparatus includes an image forming part forming a toner image on the sheet, and the fixing device as described above to fix the toner image on the sheet. 
     The above and other objects, features, and advantages of the present disclosure will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present disclosure is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically showing an internal structure of a printer according to an embodiment of the present disclosure. 
         FIG. 2  is a sectional view showing a fixing device according to the embodiment of the present disclosure. 
         FIG. 3  is a lower face view showing a flat heater in the fixing device according to the embodiment of the present disclosure. 
         FIG. 4  is an upper face view showing the flat heater in the fixing device according to the embodiment of the present disclosure. 
         FIG. 5  is a sectional view showing the flat heater in the fixing device according to the embodiment of the present disclosure. 
         FIG. 6  is a circuit diagram showing the flat heater in the fixing device according to the embodiment of the present disclosure. 
         FIG. 7  is a graph plotting results of temperature rise test of the flat heater in the fixing device according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, with reference to the accompanying drawings, embodiments of an image forming apparatus and a fixing device of the present disclosure will be described. 
     First, the entire structure of a printer  1  as the image forming apparatus will be described with reference to  FIG. 1 .  FIG. 1  is a sectional view schematically showing an internal structure of the printer  1 . Hereinafter, it will be described so that the front side of the color printer  1  is positioned at a near side on a paper sheet of  FIG. 1  and that left and right directions is defined as seen from the front side of the color printer  1 . Arrows U, Lo, L, R, Fr and Rr in each of the drawings respectively indicate an upper side, a lower side, a left side, a right side, a front side and a rear side of the printer  1 . 
     In an apparatus body  2  of the printer  1 , a sheet feeding cartridge  3  storing sheets S, a sheet feeding device  5  feeding the sheet S from the sheet feeding cartridge  3 , an image forming part  7  forming a toner image on the sheet S, a fixing device  9  fixing the toner image on the sheet S, a sheet ejecting device  11  ejecting the sheet S, and an ejected sheet tray  13  receiving the ejected sheet S are provided. Further, in the apparatus body  2 , a conveying path  15  of the sheet S is arranged so as to run from the sheet feeding device  5  to the sheet ejecting device  11  through the image forming part  7  and the fixing device  9 . 
     The sheet S fed from the sheet feeding cartridge  3  by the sheet feeding device  5  is conveyed to the image forming part  7  along the conveying path  15  and the toner image is formed on the sheet S. The sheet S is conveyed to the image fixing device  9  along the conveying path  15  and the toner image is fixed on the sheet S. The sheet S having the fixed toner is ejected from the sheet ejecting device  11  to the ejected sheet tray  13 . 
     Next, the fixing device  9  will be described with reference to  FIG. 2 .  FIG. 2  is a sectional view showing the fixing device  9 . The fixing device  9  includes a cylindrical shaped rotatable fixing belt  21 , a flat heater  23 , thermostats  71  and  72  (an example of a safety device), a heater holding member  25  and a pressuring roller  27 . The flat heater  23  has a plurality of heat generating parts  41  to  43 . Each of the thermostats  71  and  72  is arranged across two adjacent heat generating parts to interrupt power supply to the heat generating parts  41  to  43  in a case where temperature reaches a threshold value. The heater holding member  25  holds the flat heater  23  so that the flat heater  23  comes into contact with an inner circumference face of the fixing belt  21 . The pressuring roller  27  sandwiches the fixing belt  21  with the flat heater  23  to make the fixing belt  21  co-rotate with the pressuring roller  27 . In the following description, an “axial direction X” indicates an axial direction of the pressuring roller  27  (forward and backward directions). Although, in the present embodiment, an example of the fixing device  9  arranged in a posture that the pressuring roller  27  is located below the fixing belt  21 , the fixing device  9  may be arranged in any of various postures. 
     The fixing belt  21  is a cylindrical belt having a longitudinal direction along the axial direction X, and has a predetermined inner diameter, and its length in the longitudinal direction is longer than a width of the sheet S. The fixing belt  21  is made of material having flexibility, and has a base material layer, an elastic layer provided around an outer circumference face of the base material layer, and a release layer provided around an outer circumference face of the elastic layer. The base material layer is made of metal, such as stainless steel or nickel alloy. The elastic layer is made of silicone rubber or the like. The release layer is made of PFA tube or the like. On an inner circumference face of the base material layer, a sliding layer may be formed. The sliding layer is made of polyamide-imido, PTFE or the like. 
     In a hollow portion of the fixing belt  21 , a pressing member  24  having a longitudinal direction along the axial direction X is penetrated, and both ends of the pressing member  24  are fixed to a housing of the fixing device  9 . The pressing member  24  is made of metal, such as stainless steel or aluminum alloy. The fixing belt  21  is supported by an arc-shaped belt guide (not shown) supported by the pressing member  24 , and is rotatable along the belt guide. 
     Next, with reference to  FIGS. 3 to 6 , a configuration of the flat heater  23  will be described.  FIG. 3  is a lower face view showing the flat heater  23 .  FIG. 4  is an upper face view showing the flat heater  23 .  FIG. 5  is a sectional view along an I-I line in  FIG. 3 .  FIG. 6  is a circuit diagram showing the flat heater  23 . 
     The flat heater  23  is formed in a roughly rectangular plate shape having a longitudinal direction along the axial direction X. The flat heater  23  has a base material  30 , a heat insulation layer  31 , and a coat layer  32 . Incidentally, for convenience of illustration, the coat layer  32  is omitted in  FIG. 3 , and the base material  30  is omitted in  FIG. 4 . 
     The base material  30  is made of material, such as ceramic, with electrical insulation property, and is formed in a roughly rectangular plate shape having a longitudinal direction along the axial direction X. 
     The heat insulation layer  31  is made of material, such as ceramic or glass, with electrical insulation property and low heat conductivity, and is laminated on a lower face of the base material  30 . The heat insulation layer  31  restrains conducting of heat generated by the heat generating parts  41  to  43  to a side of the base material  30 . 
     The heat generating parts  41  to  43  are made of material, such as metal, with electrical conductivity having a resistance value higher than feeders  61  to  64 , and are formed on a lower face of the heat insulation layer  31 . The heat generating parts  41  to  43  are arranged along one line parallel to the axial direction X. Each of the heat generating parts  41  to  43  has a plurality of resistance heat generating elements  40  arranged along one line parallel to the axial direction X. 
     The heat generating part  41  is arranged within a range corresponding to a length of a longer side of the sheet S of a small size (e.g. JIS A5 size). The heat generating parts  42  and  43  are arranged within a range corresponding to a length of a longer side of the sheet S of a large size (e.g. JIS A4 size) where the heat generating part  41  is not arranged. The heat generating parts  42  and  43  are respectively located at a front side and a rear side of the heat generating part  41 . 
     The feeders  61  to  64  are made of material, such as metal, with electrical conductivity having a resistance value lower than the resistance heat generating element  40 , and are formed on the lower face of the heat insulation layer  31 . The feeder  61  is connected to right ends (ends at an upstream side in a conveying direction Y of the sheet S) of the plurality of resistance heat generating elements  40  included in the heat generating part  41 . The feeder  62  is connected to right ends of the plurality of resistance heat generating elements  40  included in the heat generating part  42 . The feeder  63  is connected to right ends of the plurality of resistance heat generating elements  40  included in the heat generating part  43 . On the other hand, the feeder  64  is connected to left ends (ends at a downstream side in the conveying direction Y) of the plurality of resistance heat generating elements  40  included in the heat generating parts  41  to  43 . The electrode parts  51  to  54  are located at a front side portion from the heat generating part  42  on the lower face of the heat insulation layer  31  in order of the electrode parts  53 ,  51 ,  52 ,  54  from the front side. The feeders  61 , and  63  are respectively connected to the electrode parts  51 ,  52  and  53 . Incidentally, on the lower face of the heat insulation layer  31 , in portions in which the heat generating parts  41  to  43 , the feeders  61  to  64  and the electrode parts  51  to  54  are not formed, the heat insulation layer  31  is laminated so as to form a flat face together with the heat generating parts  41  to  43 , the feeders  61  to  64  and the electrode parts  51  to  54 . 
     The coat layer covers a range in the longitudinal direction of the heat insulation layer  31  where the heat generating parts  41  to  43  are formed. The coat layer is made of, for example, material, such as ceramic, with electrical insulation property and small sliding friction force with respect to the fixing belt  21 . A lower face of the coat layer comes into contact with the inner circumference face of the fixing belt  21 . 
     As shown in  FIGS. 4 and 5 , the thermostat  71  is arranged across an upper face of the heat generating part  41  and an upper face of the heat generating part  42 . The thermostat  72  is arranged across an upper face of the heat generating part  41  and an upper face of the heat generating part  43 . Each of the thermostats  71  and  72  has, for example, a switch using a bimetal to interrupt power supply by deforming the bimetal when temperature of the bimetal is risen by thermal conduction from the outside to reach the threshold value. The thermostats  71  and  72  are connected in series by the feeder  65 . 
     As shown in  FIGS. 4 and 5 , in a rear end of the heat insulation layer  31 , a through hole  34  is formed and, near the electrode part  54  of the heat insulation layer  31 , a through hole  35  is formed. The feeder  64  is connected to one end of the feeder  65  via the through hole  34  and the other end of the feeder  65  is connected to the electrode part  54  via the through hole  35 . 
     A heat conduction member  73  is connected between the thermostat  71  and the heat generating parts  41  and  42 . A heat conduction member  74  is connected between the thermostat  71  and the heat generating parts  41  and  43 . Each of the heat conduction members  73  and  74  has an area larger than an area of each of the thermostats  71  and  72  facing to the heat generating parts  41  to  43 . The heat conduction members  73  and  74  are sheet-shaped members, such as copper, aluminum or graphite, having high heat conductivity to promote heat conduction from the heat generating parts  41  to  43  to the thermostats  71  and  72  as the safety device. Between the thermostats  71  and  72  and the heat conduction members  73  and  74 , fluorine grease for promoting heat conduction is applied. 
     As shown in  FIG. 6 , to the electrode part  51 , a triac  81  is connected and, to the electrode parts  52  and  53 , a triac  82  is connected. The triacs  81  and  82  are connected to one terminal of an AC power source  85  via a relay  83 . The electrode  54  is connected to the other terminal of the AC power source  85  via a relay  84 . 
     The triacs  81  and  82  control power supply from the AC power source to the heat generating parts  41  to  43  in accordance with the size of the sheet. In a case of the sheet of the small size, the triac  81  supplies power to the heat generating part  41 , and thereby, the heat generating part  41  generates heat. In a case of the sheet of the large size, the triac  81  supplies power to the heat generating part  41  and the triac  82  supplies power to the heat generating parts  42  and  43 , and thereby, the heat generating parts  41  to  43  generate heat. 
     As shown in  FIG. 2 , the heater holding member  25  holds the flat heater  23  so that the flat heater  23  comes into contact with the inner circumference face of the fixing belt  21 . The heater holding member  25  is a member having the same length as a width X in the axial direction X of the fixing belt  21 . The heater holding member  25  is made of, for example, heat resistant resin, such as liquid crystal polymer. The heater holding member  25  is supported by the pressing member  24 . In a lower portion of the heater holding member  25 , a recessed portion  251  having a longitudinal direction along the axial direction X is formed. The flat heater  23  is fitted into the recessed portion  251  in a posture that the coat layer is positioned at a lower side. The heater holding member  25  has curved portions  253  with curvature slightly larger than curvature of the fixing belt  21 , and the curved portions  253  and the flat heater  23  compose a smooth face along the inner circumference face of the fixing belt  21 . 
     The pressuring roller  27  has a core metal, an elastic layer provided around an outer circumference face of the core metal, and a release layer provided around an outer circumference face of the elastic layer. The elastic layer is made of silicon rubber or the like. The release layer is made of PFA tube or the like. The pressuring roller  27  sandwiches the fixing belt  21  with the flat heater  23  to form a pressuring area N between the fixing belt  21  and the pressuring roller  27 . The pressuring roller  27  is driven by a motor  28  to make the fixing belt  21  co-rotate with pressuring roller  27 , and sandwiches and conveys the sheet S. 
     A fixing operation of the fixing device  9  having the above-described configuration will be described. When the pressuring roller  27  is driven and rotated in a predetermined rotating direction Z, the fixing belt  21  is co-rotated with the pressuring roller  27  in an opposite rotating direction to the rotating direction Z of the pressuring roller  27 , and the inner circumference face of the fixing belt  21  is slide with respect to the flat heater  23 . When power is supply to the heat generating parts  41  to  43  of the flat heater  23 , the heat generating parts  41  to  43  generate heat to heat the fixing belt  21 . After temperature of the fixing belt  21  reaches predetermined temperature, the sheet S on which the toner is transferred is conveyed to the pressuring area N. In the pressuring area N, the sheet S is sandwiched between the fixing belt  21  and the pressuring roller  27  and conveyed. At this time, the toner is heated and pressured by the fixing belt  21 , and them, the toner is fixed on the sheet S. The sheet S on which the toner is fixed is separated from the fixing belt  21  and conveyed along the conveying path  15 . 
     Here, in a case where any of the triacs  81  and  82  is broken down to disable control of power supply, abnormal temperature rise occurs in the heat generating part  41 ,  42  or  43  connected to the broken-down triac  81  or  82 . In a case where temperature of the thermostat  71  or  72  risen by heat conduction from the heat generating part  41 ,  42  or  43  due to abnormal temperature rise reaches the threshold value, the thermostat  71  or  72  interrupts power supply to the heat generating part  41 ,  42  or  43 . For instance, if the triac  81  is broken down, abnormal temperature rise occurs in the heat generating part  41 , and then, if temperature of the thermostat  71  or  72  reaches the threshold value, the thermostat  71  or  72  with temperature reaching the threshold value interrupts power supply to the heat generating part  41 ,  42  or  43 . On the other hand, if the triac  82  is broken down, abnormal temperature rise occurs in the heat generating parts  42  and  43 , and then, if temperature of the thermostat  71  or  72  reaches the threshold value, the thermostat  71  or  72  with temperature reaching the threshold value interrupts power supply to the heat generating part  41 ,  42  or  43 . 
     Next, with reference to  FIG. 7 , effect of the present embodiment will be described.  FIG. 7  is a graph plotting results of temperature rise test of the flat heater  23 . Heater temperature of a horizontal axis is temperature of the flat heater  23  and thermostat temperature of a vertical axis is temperature of the thermostat  71 . White dots indicate results simulating a case of both sides broken-down, i.e. a case where both the triacs  81  and  82  are broken down and the heat generating parts  41  and  42  fall into abnormal temperature rise. While stepwisely increasing power supply to the heat generating parts  41  and  42 , temperature of heat generating parts  41  and  42  and temperature of the thermostat  71  are measured, and temperature of the thermostat  71  is plotted whenever temperature of heat generating part  41  or  42  is 200, 250, 300 or 350 degrees centigrade. However, in this case, the heat conduction member  73  is not provided. Black triangles indicate results simulating a case of one side broken-down, i.e. a case where the triac  81  is broken down and the heat generating part  41  falls into abnormal temperature rise. In this case, similarly, the heat conduction member  73  is not provided. Black dots indicate results simulating a case where the heat conduction member  73  is provided and one side is broken down. 
     Firstly, in comparison with the case of both sides broken-down (without the heat conduction member) and the case of one side broken-down (without the heat conduction member), although thermostat temperature is comparatively close to heater temperature in the case of both sides broken-down, thermostat temperature is lower by 20 to 25 degrees centigrade than heater temperature in the case of one side broken-down. This is caused because, by rising temperature of both of the heat generating parts  41  and  42  in the case of both sides broken-down, on the other hand, by rising temperature of both of the heat generating part in the case of one side broken-down, heat quantity conducted to the thermostat  71  in the case of one side broken-down is small in comparison with the case of both sides broken-down. It is considered that thermostat temperature in the case of both sides broken-down is close to thermostat temperature in a case where the thermostat is provided for each heat generating part and broken-down occurs. Therefore, in the present embodiment, although thermostat temperature is slightly lowered in comparison with the case where the thermostat is provided for each heat generating part, it is considered that sufficient accuracy can be achieved for practical use. 
     On the other hand, a difference between thermostat temperature and heater temperature in the case of one side broken-down (with the heat conduction member) is restrained by approximately 10 degrees centigrade, and the difference between thermostat temperature and heater temperature is small in comparison with the case without the heat conduction member. It is considered that this result is caused because heat conduction from the heat generating parts  41  and  42  to the thermostat  71  is promoted by the heat conduction member  73 . 
     As described above, in accordance with the present embodiment, even if the flat heater  23  having the plurality of heat generating parts  41 ,  42  and  43  in the longitudinal direction does not include the thermostat  71  or  72  arranged for each of the heat generating parts  41 ,  42  and  43 , it is possible to interrupt power supply in abnormal temperature rise occurs in any of the heat generating parts  41 ,  42  and  43 . 
     Moreover, in accordance with the present embodiment, since the heat conduction member  73  or  74  promotes heat conduction from the heat generating part  41 ,  42  or  43  to the thermostat  71  or  72 , it is possible to improve accuracy of temperature of the thermostat  71  or  72 . Further, in accordance with the present embodiment, in comparison with a case where the heat conduction member  73  or  74  has the same area as the area of each of the thermostats  71  and  72  facing to the heat generating parts  41  to  43 , since a large quantity of heat is conducted from the heat generating part  41 ,  42  or  43  to the thermostat  71  or  72 , it is possible to further improve accuracy of temperature of the thermostat  71  or  72 . 
     In a modified example, as the safety device instead of the thermostats  71  and  72  using the bimetal, thermostats using shape memory alloy may be applied. Alternatively, in a modified example, as the safety device instead of the thermostats  71  and  72 , fuses may be applied. 
     Although, in the present embodiment, an example that the flat heater  23  has three heat generating parts  41 , and  43  is described, the present disclosure may be applied in a case where the flat heater  23  has two heat generating parts or four or more heat generating parts. 
     Although, in the present embodiment, a case where the present disclosure is applied to the printer  1  has been described as one example, the disclosure is not restricted by this, but may be applied to a copying machine, a facsimile, a multifunction peripheral or the like. 
     The above-description of the embodiment of the present disclosure was described about a preferable embodiment of the fixing device and the image forming apparatus according to the disclosure. However, the technical scope of the present disclosure is not limited to the embodiments.