Patent Publication Number: US-8532530-B2

Title: Fixing device for fixing on a recording material a toner image formed on the recording material including a fixing roller and heating and pressing members

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a fixing device to be mounted in an image forming apparatus, such as printer, a copying machine or a facsimile machine, using electrophotography or electrostatic recording technology. 
     A heat fixing device as a conventional heat fixing means in the image forming apparatus using an electrophotographic process fixes, on a recording material, an unfixed image (toner image) formed on the recording material by an image forming means, such as an electrophotographic process means. As a fixing type, a heating-roller type using a known halogen heater as a heat source or a film-heating type using a ceramic heater as the heat source has been employed. 
     Further, in recent years, a fixing device for heating a fixing roller, provided with no heat source therein, from an outer peripheral surface of the fixing roller by bringing a heating member into surface contact with the outer peripheral surface of the fixing roller to form in a heating press-contact portion has been proposed. For example, a heating device (fixing device) for heating only the outer peripheral surface of the fixing roller with the heating member by bringing the heating member, such as the ceramic heater or a small-diameter heat roller, into contact with the outer peripheral surface of the fixing roller has been proposed (e.g., Japanese Laid-Open Patent Application (JP-A) Hei 6-75491, JP-A 2004-317788 and JP-A 2002-123117). 
     These fixing devices are, similar to the heat-roller-type fixing device, excellent in stability and durability. Further, the fixing roller is externally heated by the heating member having a low thermal capacity and therefore, the surface of the fixing roller can be abruptly increased in temperature, so that the warm-up time is reduced compared with the heat-roller-type fixing device. 
     In a fixing device in which a recording material P is guided into a fixing nip and is heated while being nipped and conveyed, in the case where the recording material P is, e.g., thin paper with low rigidity, the melted toner image on the recording material acts as an adhesive material, so that the recording material is wound about the fixing member in some cases. In the case where a paper jam (jamming), such as winding of the recording material, occurs, a user removes the wound recording material by grasping a trailing end portion of the recording material P at which the recording material P is not wound about the roller and then by pulling out the recording material nipped in the fixing nip (jam clearance). 
     However, as shown in  FIG. 13 , in a constitution in which a film  16  is contacted to an outer peripheral surface of a fixing roller  30  and a contact portion N 2  other than a fixing nip N 1  is provided, the wound recording material is moved from the fixing nip N 1  and then reaches the contact portion N 2  other than the fixing nip N 1  in some cases. 
     The user intends to pull out from the contact portion N 2  in a direction, indicated by an arrow A as shown in (a) of  FIG. 13 , the recording material P by holding one end of the recording material P but the recording material P is nipped at a plurality of portions consisting of the fixing nip N 1  and the contact portion N 2 , so that a brake is applied to the recording material P at the portions N 1  and N 2  in different directions with different forces. 
     When the recording material P is not moved at the contact portion N 2  but is pulled out from only the fixing nip N 1 , the recording material P is placed in a state in which the fixing roller  30  is tightened as shown in (b) of  FIG. 13 , so that an excessive load is exerted on the fixing roller  30  in some cases. Further, when the recording material P was broken (torn), the fixing device was placed in some cases in a state in which it was difficult to remove the recording material P. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in view of the above problems. A principal object of the present invention is to provide a fixing device capable of easily removing a jammed recording material even when a winding jam, such that the recording material is wound about a fixing roller, occurs. 
     According to an aspect of the present invention, there is provided a fixing device for fixing on a recording material a toner image formed on the recording material, the fixing device comprising: 
     a fixing roller; 
     a heating member for heating the fixing roller in contact with a surface of the fixing roller; and 
     a pressing member for forming a fixing nip between itself and the fixing roller, 
     wherein the recording material on which the toner image is formed is subjected to fixing by being nipped and conveyed in the fixing nip, and 
     wherein when the recording material which has been jammed is removed by pulling a trailing end of the recording material in a winding jam state in which a leading end of the recording material has passed through the fixing nip to reach a contact portion between the heating member and the fixing roller, a drawing force F 1  for drawing the recording material, toward an upstream side with respect to a rotational direction of the fixing roller, nipped in the fixing nip between the fixing roller in a rest state and the pressing member and a drawing force F 2  for drawing the recording material, toward the upstream side with respect to the rotational direction of the fixing roller, nipped between the fixing roller in the rest state and the heating member are set so that the drawing force F 2  is not more than the drawing force F 1 , so as to prevent the fixing roller from being tightened by the recording material. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an image forming apparatus. 
         FIG. 2  is a schematic sectional view showing a schematic structure of a fixing device in First Embodiment. 
         FIG. 3  is a partial perspective view showing a schematic structure of a ceramic heater. 
         FIG. 4  is a block diagram of a connection relationship among the ceramic heater and its peripheral means. 
         FIG. 5  is a schematic view for illustrating a measuring method of drawing forces (pulling-out forces). 
         FIG. 6  is a schematic sectional view for illustrating a jam clearance method in the image forming apparatus. 
         FIG. 7  is a table showing a result of confirmation of effects in First Embodiment. 
         FIGS. 8 to 12  are schematic sectional views showing schematic structures of fixing devices in Third Embodiment to Seventh Embodiment, respectively. 
       Parts (a) and (b) of  FIG. 13  are schematic sectional views for illustrating a winding jam clearance method with respect to a fixing member. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinbelow, embodiments for carrying out the present invention will be described with reference to the drawings. 
     First Embodiment 
     Image Forming Portion 
       FIG. 1  is a schematic illustration of an image forming apparatus using an electrophotographic process in First Embodiment of the present invention and shows the case of, e.g., a laser (beam) printer. 
     The structure and operation of the image forming apparatus will be described below. A laser printer main assembly  101  as the image forming apparatus includes a sheet feeding cassette  102  for accommodating a recording material P, and the sheet feeding cassette  102  is provided with feeding rollers  105  for feeding the recording material P. Further, the main assembly  101  also includes an externally insertable sheet feeding tray  103  which is provided with feeding rollers  104  for feeding the recording material P. 
     On a downstream side of the feeding rollers  105 , a registration roller pair  106  for 
     synchronization-conveying the recording material P, and a top sensor  151  are provided. A signal detected by the top sensor  151  is transmitted to a CPU  23  described later, so that the CPU  23  detects passing of the recording material P at a pressure of the registration roller pair  106 . 
     Further, downstream of the registration roller pair  106 , an image forming portion  108  for forming a toner image on the recording material P on the basis of laser light from a laser scanner portion  107  is provided. That is, in the laser printer in this embodiment, an image forming means is constituted by this laser scanner portion  107  and the image forming portion  108 . 
     Further, downstream of the image forming portion  108 , a fixing device  109  as a heating device for heat-fixing the toner image formed on the recording material P is provided. Downstream of the fixing device  109 , a first sheet discharging roller pair  111 , a second sheet discharging roller pair  140  and a stacking tray  112  for stacking sheets of the recording material P on which recording has been completed are provided. 
     The image forming portion  108  is constituted by members necessary for the electrophotographic process, such as a photosensitive drum  117 , a charging roller  119 , a developing device  120 , a primary transfer roller  124 , a secondary transfer roller  121 , a cleaner  122 , an intermediary transfer member  123 , and the like. 
     (Fixing Device) 
     Next, the fixing device  109  provided in the printer  1  will be described with reference to  FIG. 2 . Incidentally,  FIG. 2  is a schematic sectional view showing a schematic structure of the fixing device  109  in First Embodiment. 
     The fixing device  109  includes a rotatable fixing roller  30 . On the fixing roller  30 , an endless belt-like film  16  for forming a contact portion N 2  in press-contact with the outer peripheral surface of the fixing roller  30  is provided so as to contact the fixing roller  30 . Inside the film  16 , a ceramic heater  15  and a thermistor temperature sensor (hereinafter referred to as a thermistor)  18  are provided. Under the fixing roller  30  and at an opposing position through the recording material P, a rotatable pressing roller  17 , which is a pressing member and is formed in a cylindrical or substantially cylindrical shape, is provided. 
     The fixing roller  30  and the pressing roller  17  are press-contacted by a first pressing mechanism  41 , such as a known spring, to form a fixing nip N 1 . Further, the fixing roller  30  and the film  16  contacted to the ceramic heater  15  are press-contacted by a second pressing mechanism  42 , such as a known spring, to form a contact portion N 2 . 
     As shown in  FIG. 1 , on the downstream side of the fixing nip N 1  of the fixing device  109 , the first sheet discharging roller pair  111  and a sheet discharging sensor  152  are provided. The sheet discharging sensor  152  is connected to the CPU  23 . For this reason, passing of the recording material P can be detected by the sensor  152 . 
     (Parts of Fixing Device) 
     The ceramic heater  15  provided in the fixing device  109  is supported by a holder  19  attached to the main assembly of the fixing device  109 . The ceramic heater  15  will be described.  FIG. 3  is a partial perspective view showing a schematic structure of the ceramic heater  15 . 
     As shown in  FIG. 3 , the ceramic heater  15  is constituted by a substrate  15 A, a heat generating resistor  15 B and a protective layer  15 C. 
     The substrate  15 A is principally formed of a ceramic, such as alumina or aluminum nitride, in a thin plate shape. The heat generating resistor  15 B is principally formed of silver, palladium or the like on one surface of the substrate  15 A. On the other surface of the substrate  15 A, the thermistor  18  is provided in contact with the substrate  15 A. Further, the protective layer  15 C is principally formed of glass or a heat resistant resin material, such as fluorine-containing resin material or polyimide, and is coated so as to protect the surface, at which the heat generating resistor  15 B is provided, from sliding contact with the film  16  or the like. 
       FIG. 4  is a block diagram showing a connection relationship among the ceramic heater  15  and its peripheral means. 
     As shown in  FIG. 4 , the heat generating resistor  15 B constituting the ceramic heater  15  is supplied with electric power from a commercial power source  21  via a triac  20 . Therefore, by heat generation of the heat generating resistor  15 B, which has been supplied with electric power from the commercial power source  21 , heating by the ceramic heater  15  is performed. Further, a signal from the thermistor  18  is transmitted to the CPU  23  via an A/D conversion circuit  22 . 
     The film  16  provided in the fixing device  109  is, as shown in  FIG. 2 , formed in an inner circumferential length which is longer than an outer circumferential length of the holder  19  by a predetermined length. For this reason, the film  16  is externally engaged with the holder  19  with no tension and is guided in a predetermined direction by the holder  19  while being moved by the rotation of the fixing roller  30 . Incidentally, the fixing roller  30  is rotated by a driving mechanism (not shown) provided outside the main body of the fixing device  109 . 
     Further, the film  16  employs a two-layer structure such that an endless belt-like member principally formed of PFA is coated on the outer peripheral surface of an endless belt-like member principally formed of polyimide. 
     The fixing roller  30  includes a metal core  30 A, an elastic layer  30 B and a parting layer  30 C. The metal core  30 A is a rotatable cylindrical member which is formed, in a cylindrical or substantially cylindrical shape, of iron, SUS, aluminum or the like. The elastic layer  30 B is formed, on the outer peripheral surface of the metal core  30 A, principally of a silicone rubber or the like. Further, the parting layer  30 C is formed as an outermost layer of the fixing roller  30 . The parting layer  30 C is principally formed of PTFE, PFA, FEP or the like. The thus-constituted fixing roller  30  is rotationally driven by receiving a driving force from a driving mechanism to an end portion of the metal core  30 A with respect to a roller axis direction thereof. 
     The pressing roller  17  as the pressing member includes a metal core  17 A, an elastic layer  17 B and a parting layer  17 C. The metal core  17 A is a rotatable cylindrical member which is formed, in a cylindrical or substantially cylindrical shape, principally of aluminum or the like. The elastic layer  17 B is formed, on the outer peripheral surface of the metal core  17 A, principally of a silicone rubber or the like. Further, the parting layer  17 C is formed as an outermost layer of the pressing roller  17  and is principally formed of PTFE, PFA, FEP or the like. The thus-constituted fixing roller  17  is rotated by the rotation of the fixing roller  30  which has been rotationally driven. 
     (Measuring Method of Drawing Force F) 
     A measuring method of a fixing device (pulling-out force) F is as follows.  FIG. 5  is a schematic view for illustrating the measuring method of the drawing force F. 
     As shown in  FIG. 5 , in the case where a first drawing force F 1  in the fixing nip N 1  is measured, the recording material P is nipped in the fixing nip N 1  at a position of 50 mm from a leading end thereof and then a wire bonded to a longitudinal central portion of a trailing end of the recording material P is drawn (pulled out). Then, a force at the time when the recording material P is drawn in a tangential direction of the outer circumferential surface of the fixing roller  30  (with respect to the fixing nip N 1 ) is measured by a digital force gauge  150 . Here, as the recording material P, LTR (letter)-sized paper (“Xerox 4200 copy paper”, basis weight: 85 g/m 2 ) was used. As the digital force gauge  150 , a digital force gauge (“MODEL FGN 10”, mfd. by NIDEC-SIMPO Corp.) was used. A second drawing force F 2  at the contact portion N 2  is also measured similarly by drawing the recording material P from the contact portion N 2 . 
     In the fixing device  109  in this embodiment, a relationship between these drawing forces F 1  and F 2  is set as follows. That is, the first drawing force F 1  necessary to draw the recording material P from the fixing nip N 1  is set so as to be not less than the second drawing force F 2  necessary to draw the recording material P from the contact portion N 2  (F 1 ≧F 2 ). 
     The drawing forces F 1  and F 2  can be adjusted by changing the pressure of the film on the fixing roller  30 , the pressure of the pressing roller  17  on the fixing roller  30 , and the materials and surface properties of the surface layers of the film  16  and the pressing roller  17 . 
     For example, the pressure of the film  16  on the fixing roller  30  may be equal to or less than the pressure of the pressing roller  17  on the fixing roller  30 . 
     Further, in general, of the same fluorine-containing resin material, PTFE is excellent in its sliding property compared with PFA, so that the drawing force can be made small. For this reason, the surface of the film  16  may also be coated with a blend resin material of PTFE and PFA, and the surface of the pressing roller  17  may also be coated with the PFA resin material. 
     (Operation) 
     Operations of the fixing device and the image forming apparatus will be described with reference to  FIG. 1 . 
     The main assembly  101  receives a print signal by an unshown controller. Then, the fixing roller  30  is driven and rotated, so that the film  16  and the pressing roller  17  are also rotated together by the rotation of the fixing roller  30 . 
     The electric power supply to the ceramic heater  15  is started, the temperature of the ceramic heater  15  detected by the thermistor  18  and is controlled so that the temperature reaches a predetermined target temperature. 
     The surface of the fixing roller  30  is heated by the ceramic heater  15  through the film  16  to be increased in temperature up to a predetermined temperature. The recording material P fed from the sheet feeding cassette  102  by the feeding rollers  105  with predetermined timing is sent to the image forming portion  108  by the registration roller pair  106 . 
     When the leading end of the recording material P reaches the registration roller pair  106 , the top sensor  151  disposed at the registration-roller position detects passing of the recording material P. The recording material P on which an unfixed toner image has been transferred is guided into the fixing nip N 1  of the fixing device  109 . 
     The recording material P guided into the fixing nip N 1  is semi-permanently fixed on the recording material P by melting the unfixed toner image while being nip-conveyed between the pressing roller  17  and the fixing roller  30  which has been heated by the ceramic heater  15  through the film  16 . 
     The recording material P fixed in the fixing nip N 1  passes through the first sheet discharging roller pair  111  and the second sheet discharging roller pair  140 , thus being discharged on the stacking tray  112 . 
     When the leading end of the recording material P reaches the first sheet discharging roller pair  111 , the sheet discharging sensor  152  disposed at the charging-roller-pair position detects the passing of the recording material P. 
     In the case where a series of printing operations are normally performed, after a lapse of a predetermined time (a movement time calculated from a distance between the top sensor  151  and the sheet discharging sensor  152 ) from the detection of the leading end of the recording material P by the top sensor  151 , the sheet discharging sensor  152  detects the leading end of the recording material P. Similarly, after a lapse of a predetermined time from the detection of the trailing end of the recording material P, the sheet discharging sensor  152  detects the trailing end of the recording material P. 
     In the case where the sheet discharging sensor  152  cannot detect the leading end of the recording material P after the lapse of the predetermined time from the detection of the leading end of the recording material P by the top sensor  151 , it is determined that the recording material P has caused jamming such as winding of the recording material P about the fixing roller  30  or a paper jam. In this case, the image formation operation is stopped, and the driving of the fixing roller  30  and the first sheet discharging roller pair  111  and the electric power supply to the ceramic heater  15  are stopped. 
     Here, a winding jam will be described. 
     At the surface of the fixing roller  30 , the surface layer of the fluorine-containing resin material with a good parting property is formed. Further, when the toner is melted in the fixing nip N 1 , wax contained in the toner bleeds out, thus assisting the separation of the recording material P from the fixing roller  30 . Generally, based on this force, the recording material P is separated from the fixing roller  30  after passing through the fixing nip N 1  by rigidity of the recording material P, thus being discharged in safety. 
     However, when the toner is excessively melted in the fixing nip N 1 , the surface tension of the toner becomes small and the toner is liable to adhere, so that adherence of the toner to the surface layer of the fixing roller  30  is increased. Further, on the other hand, when the toner is not melted sufficiently, the wax does not sufficiently bleed out. In the case where the unfixed toner image is intended to be fixed at an improper temperature, the adherence between the toner on the recording material P and the surface of the fixing roller  30  exceeds the degree of the rigidity of the recording material P in some cases. In this case, the recording material P is rotated together with the fixing roller  30  while being adhered to the fixing roller  30  to enter the contact portion N 2 , so that there is a possibility of an occurrence of the winding jam. 
     The temperature and heat quantity of the surface of the fixing roller  30  suitable for melt-fixing the toner image on the recording material P to be separated from the fixing roller in safety are largely changed depending on a basis weight of the recording material P, i.e., thermal capacity. Therefore, it is difficult to fix various recording materials, ranging from thin paper with the basis weight of about 50 g/m 2  to thick paper with the basis weight of about 200 g/m 2 , at the same temperature. 
     It is difficult to select a fixing mode with a proper fixing temperature automatically or by the user with respect to the various recording materials which can be used by the user. Therefore, it is difficult to completely prevent the winding jam about the fixing roller  30 . 
     It is desirable that the jam is determined to occur before the leading end of the recording material P which has been wound about the fixing roller  30  reaches the contact portion N 2  and then the drive of the fixing roller  30  is stopped. However, there arises the following problem in this case, so that it is actually difficult to completely prevent this winding jam. 
     First, the distance from the fixing nip N 1  to the sheet discharging sensor  152  is required to be shorter than that from the fixing nip N 1  to the contact portion N 2 , but there is a possibility that the sheet discharging sensor  152  does not operate normally as a result of the heat generated by the fixing roller  30 , so that the sheet discharging sensor  152  cannot be disposed so close to the fixing nip N 1 . Further, depending on detection accuracy of the top sensor  151  and the sheet discharging sensor  152 , it takes much time until the recording material P is determined to cause the jam. Further, by idling of a gear or a motor, it takes much time from the intention to stop the driving of the fixing roller  30  until the fixing roller  30  is actually stopped. For these reasons, in this embodiment jam clearance is performed in the following manner.  FIG. 6  is a schematic sectional view for illustrating a jam-clearance method in the image forming apparatus. 
     In the case where the jam has occurred, the CPU  23  notifies the user of the occurrence of the jam by, e.g., turning on a warning LED of the jam occurrence. In this case, as shown in  FIG. 6 , the user opens a jam clearance door  40  provided between the image forming portion  10   f  and the fixing device  109  and draws the recording material P by pulling the trailing end of the recording material P in an outward direction (indicated by an arrow A in  FIG. 6 ). Thus, the jam clearance is performed. 
     In this embodiment, the first and second drawing forces F 1  and F 2 , which are the forces for drawing the recording material P from the fixing nip N 1  and the contact portion N 2 , respectively, are set to satisfy: F 1 ≧F 2 . 
     In this constitution, even when the winding of the recording material P about the fixing roller  30  has occurred and the recording material P has reached the contact portion N 2 , by the first drawing force F 1 , the recording material P can be pulled out from not only the fixing nip N 1  but also the contact portion N 2 . Further, the recording material P can be drawn while rotating the fixing roller  30  by the pulling-out operation of the recording material P without being deviated from the surface of the fixing roller  30  in the fixing nip N 1 . 
     Further, when the user draws the recording material P, and thus the recording material P is gradually pulled out, as shown in  FIG. 5 , the speed S 2  at which the recording material P is gradually pulled out from the contact portion N 2  is not more than the speed S 1  at which the recording material P is gradually pulled out from the fixing nip N 1 . 
     In contrast with this embodiment, in the case where the drawing forces F 1  and F 2  satisfy: F 1 &lt;F 2 , the recording material P can be drawn from the fixing nip N 1  by the drawing force F 1  but cannot be drawn from the contact portion N 2  by the drawing force F 2 . In this case, when the recording material P does not slip on the fixing roller  30  in the fixing nip N 1 , the fixing roller  30  is to be rotated by being pulled by the recording material P. However, at the contact portion N 1 , the brake is applied. As a result, there is a possibility that wrinkles occur on the surface of the fixing roller  30  or that an excessive load is exerted on the elastic layer  30 B. Further, the elastic layer  30 B is elastically deformed and the recording material P is drawn from only the fixing nip N 1 , so that the fixing roller  30  is tightened by the recording material P. Further, even when the recording material P has slipped on the fixing roller  30  in the fixing nip N 1 , the recording material P is drawn from only the fixing nip N 1 , so that the fixing roller  30  is tightened by the recording material P. When the fixing roller  30  is tightened by the recording material P, pressure is applied to the surface of the fixing roller  30  at the tightened portion, so that a force necessary for the jam clearance is increased. As the user draws the recording material P in an increasing distance, an increasing drawing force is generated, so that it is difficult to perform the jam clearance. Further, when the user intends to forcedly pull out the recording material P, there is a possibility that an overload is exerted on the tightened fixing roller  30 . Further, when the recording material P is broken or torn, it becomes further difficult to perform the jam clearance. 
     (Effect) 
     The effect of the fixing device in this embodiment was confirmed through an experiment. First, a constitution of the fixing device used in the experiment in this embodiment will be described. 
     The ceramic heater  15  includes the substrate  15 A formed of aluminum in a thickness of 1.0 mm and a width of 7.0 mm and includes the heat generating resistor  15 B formed of silver and palladium in the thickness of 10 μm and the width of 4.0 mm. The resistor  15  is coated with a 60 μm-thick glass layer as the protective layer  15 C. The film  16  includes a 30 μm-thick film base layer of SUS and a 20 μm-thick parting layer formed of PFA resin on the base layer. 
     The fixing roller  30  is prepared by forming a 3.0 mm-thick elastic layer  30 B of a silicone rubber having a thermal conductivity of 0.2 W/m·K on the metal core  30 A which is formed of aluminum and has an outer diameter of 14 mm and then by providing a 20 μm-thick parting layer  30 C of the PFA resin as the outermost layer. 
     The pressing roller  17  is prepared by forming a 3.0 mm-thick elastic layer  17 B of a silicone rubber having a thermal conductivity of 0.2 W/m·K on the metal core  17 A which is formed of aluminum having an outer diameter of 14 mm and then by providing a 20 μm-thick parting layer  17 C of the PFA resin as the outermost layer. 
     Each of the first drawing force F 1  necessary to pull out the recording material P from the fixing nip N 1  and the second drawing force necessary to pull out the recording material P from the contact portion N 2  was measured. During the measurement, each of the pressure between the ceramic heater  15  and the fixing roller  30  and the pressure between the fixing roller  30  and the pressing roller  17  was changed. In each of resultant constitutions, a jam-clearance test was conducted. 
     The jam-clearance test was conducted in the following manner. The LTR-sized paper (“Xerox 4200 copy paper”, basis weight: 75 g/m 2 ) was nipped in the fixing nip N 1  and then nipped in the contact portion N 2  while being wound about the fixing roller  30  so that the recording material P for the jam-clearance test was set so as to be protruded from the contact portion N 2  by 10 mm. Then, the user held (grasps) the recording material P with one hand at a position of about 50 mm from the trailing end of the recording material P, which was then is pulled out in a straight line in the tangential direction of the outer circumferential surface of the fixing roller  30  from the fixing nip N 1 . This operation was performed 10 times. 
     As a result, the jam clearance was evaluated as “o” (Good) when there was of no problem, and was evaluated as “x” (No good) when there arose the problem in at least one pulling-out operation. 
     The result is shown in  FIG. 7 , which is a table showing the set pressure values, measured values of the drawing forces, and the evaluation results of the jam-clearance test. 
     As the result of the jam-clearance test, as shown in  FIG. 7 , in study  1  to study  4  in which F 1 &lt;F 2  was satisfied, the fixing roller  30  was tightened by the recording material P and thus it was difficult to draw the recording material P. When the recording material P was further pulled, the recording material P was broken (torn) and remained in a jam-clearance disable state. Further, in study  1  and study  2 , the surface layer of the fixing roller  30  was broken. In study  5  to study  7  in which F 1 ≧F 2  was satisfied, it was possible to perform the jam clearance in safety without tightening the fixing roller  30  by the recording material P. 
     As described above, in this embodiment, in the heat-fixing device including the plurality of the press-contact portions, the relationship between the second drawing force F 2  at the contact portion N 2  and the first drawing force at the fixing nip N 1  was F 1 ≧F 2 . As a result, even in the case where the jam occurs in the fixing device  109 , the fixing roller  30  was not tightened by the recording material P. For this reason, the wound recording material P could be easily removed and thus the fixing performance could be easily restored. 
     Second Embodiment 
     In this embodiment, a constitution including a pressure-reducing(releasing) mechanism for adjusting press-contact forces of members will be described. Constituent elements similar to those described above are represented by the same reference numerals or symbols and will be omitted from description. 
     In this embodiment, the relationship between the first drawing force F 1  and the second fixing device F 2  during a normal operation is not particularly limited, but a first pressing mechanism  41  and a second pressing mechanism  42  are used as a pressing mechanism including the pressure-reducing mechanism which may be a known mechanism for moving an end of the spring or the like in a pressure reducing (releasing) direction, for example. 
     The first pressing mechanism  41  and the second pressing mechanism  42  include the pressure-reducing mechanism which not only increases but also decreases the pressures to the fixing roller  30  and the pressing roller  17 . For this reason, the first pressing mechanism  41  and the second pressing mechanism  42  can change the pressure to an increased-pressure side and a decreased-pressure side. The adjustment of the pressure is performed in accordance with instructions from the CPU  23 . 
     By this constitution, in the case where a jam occurrence is detected, by the action of the first pressing mechanism  41  and the second pressing mechanism  42 , a first pressure to be applied to the fixing nip N 1  and a second pressure to be applied to the contact portion N 2  can be weakened, respectively. 
     The first pressing mechanism  41  and the second pressing mechanism  42  are reduced in pressure at a stage in which the jam clearance is performed. Specifically, their pressures are reduced so that a first drawing force F 1   b  necessary to pull out the recording material P from the fixing nip N 1  and a second drawing force F 2   b  necessary to pull out the recording material P from the contact portion N 2  satisfy a relationship of F 1   b ≧Fb 2 . 
     In the case where the winding jam of the recording material P occurs, the sensor detects the jam occurrence. Then, the CPU  23  stops the image formation operation and stops the driving of the fixing roller  30  and the sheet discharging roller pair  111  and electric power supply to the ceramic heater  15 . Further, by the first pressing mechanism  41  and the second pressing mechanism  42 , the pressure applied to the fixing nip N 1  and the pressure applied to the contact portion N 2  are weakened, respectively. As a result, based on the relationship between of F 1   b ≧F 2   b , the recording material P can be easily pulled out. 
     Third Embodiment 
     In this embodiment, a constitution including a pressure-reducing mechanism for adjusting press-contact forces of members will be described. Constituent elements similar to those described above are represented by the same reference numerals or symbols and will be omitted from description.  FIG. 8  is a schematic sectional view showing a schematic structure of a fixing device in Third Embodiment. 
     In this embodiment, different from First Embodiment, the film  16  is not externally engaged with the ceramic heater  15  but as shown in  FIG. 8 , the ceramic heater  15  is slidably contacted to the fixing roller  30  directly or via the protective member or the like to form the contact portion N 2 . 
     Specifically, on the protective layer  15 C of the ceramic heater  15 C, a sliding layer  21 D (press-contact member) is formed for the purposes of enhancing the sliding property relative to the fixing roller  30  and of preventing the adherence of the toner. For example, the sliding layer  21 D is formed of a material such as the fluorine-containing resin material with a good sliding property and parting property in the thickness from about 10 μm to about 30 μm. Or, on the heat-generating resistor layer, the protective layer of the fluorine-containing resin material or the like is directly formed in the thickness from about 10 μm to about 100 μm so as to be configured to perform the functions as an insulating protective layer for the heat generating member and the sliding layer. 
     Further, the contact portion N 2  may also be formed by providing a protective sheet on the fixing roller  30  side (front surface side) of the ceramic heater  15  and by interposing the protective sheet between the ceramic heater  15  and the fixing roller  30 . 
     The protective sheet is formed with a metal member such as stainless steel (SUS), nickel (Ni), titanium (Ti) or copper (Cu). Or, the protective sheet may also be a sheet-like protective member formed on a sheet base layer on the fixing roller side. In this case, the sheet base layer is formed in the thickness of 10-30 μm by mixing a large amount of high heat-resistant filler, such as powders of metal particles, metal oxide, artificial diamond, graphite and the like, in the heat-resistant resin material such as polyimide (PI). Further, the sheet-like protective member is formed as a 10-30 μm thick parting layer of, e.g., the fluorine-containing resin material or formed of the fluorine-containing resin material alone in a sheet shape having the thickness of 30-100 μm. 
     Also in this embodiment, the relationship between the first drawing force F 1  at the fixing nip N 1  and the second drawing force F 2  at the contact portion N 2  is set at F 1 ≧F 2 . 
     At the contact portion N 2 , the fixing roller  30  and the ceramic heater  15  are press-contacted while sliding on each other and therefore the drawing force in the case where the recording material P is nipped at the contact portion N 2  is liable to be increased. However, the relationship of F 1 ≧F 2  is kept by changing the pressure, the material and the surface property with respect to the fixing roller  30 . 
     Specifically, e.g., the pressure of the film  16  on the fixing roller  30  is made small so as to be 10% to 20% of the pressure of the pressing roller  17  on the fixing roller  30 . Further, e.g., the sliding property is enhanced by changing the fluorine-containing resin material for the ceramic heater  15  slidably contacted to the fixing roller  30  into a blend resin material of PTFE and PFA. Further, into the fluorine-containing resin material, an inorganic filler such as graphite for enhancing the sliding property is dispersed. 
     In this way, by providing the drawing forces with the relationship of F 1 ≧F 2 , during the jam clearance, the recording material P can be easily pulled out without tightening the fixing roller  30 . 
     Fourth Embodiment 
     In this embodiment, a constitution using a heat roller  51  as the heating member will be described. Constituent elements similar to those described above are represented by the same reference numerals or symbols and will be omitted from description.  FIG. 8  is a schematic sectional view showing a schematic structure of a fixing device in Fourth Embodiment. 
     In this embodiment, as shown in  FIG. 9 , as the heating member for forming the contact portion N 2  with the fixing roller  30 , the heat roller  51  (press-contact member) containing a halogen heater  50  is used. The heat roller  51  is, e.g., a 0.3-3 mm thick hollow cylindrical member including a metal core  51 A principally formed of SUS, iron, aluminum or the like. As the outermost layer of the heat roller  51 , a parting layer  51 C principally formed of the fluorine-containing resin material such as PTFE or PFA is provided. 
     Also in this embodiment, the relationship between the first drawing force F 1  at the fixing nip N 1  and the second drawing force F 2  at the contact portion N 2  is set at F 1 ≧F 2 . 
     The adjustment of the drawing forces F 1  and F 2  is, e.g., performed by making the pressure of the heat roller  51  on the fixing roller  30  smaller than the pressure of the pressing roller  17  on the fixing roller  30 . Further, e.g., the parting layer  51 C of the heat roller  51  is formed of a blend resin material of PTFE and PFA, and the parting layer of the pressing roller  17  is formed of the PFA resin material. Further, into the fluorine-containing resin material of the parting layer  51 C, an inorganic filler such as graphite for enhancing the sliding property is dispersed. 
     In this way, by providing the drawing forces with the relationship of F 1 ≧F 2 , during the jam clearance, the recording material P can be easily pulled out without tightening the fixing roller  30 . 
     Fifth Embodiment 
     In this embodiment, a constitution using a plate-like pressing member  96  as the pressing member will be described. Constituent elements similar to those described above are represented by the same reference numerals or symbols and will be omitted from description.  FIG. 8  is a schematic sectional view showing a schematic structure of a fixing device in Fifth Embodiment. 
     In this embodiment, as shown in  FIG. 10 , as the pressing member, the plate-like pressing member  96  to be slidably contacted to the fixing roller  30  is used. The plate-like pressing member  96  is, constituted by forming a heat insulating member or layer  91  on a supporting member of, e.g., SUS material and then by forming a sliding parting layer  92  on the heat insulating layer  91 . 
     The sliding parting layer  92  of the plate-like pressing member  96  may preferably be formed of a material with an excellent sliding property so as not to prevent conveyance of the recording material P and may preferably be formed of a material having an excellent parting property so as to prevent the transferred toner to adhere to the fixing roller  30 . For example, it is suitable that a sheet of the fluorine-containing resin material is formed as the sliding parting layer by bonding or coating. 
     In the fixing nip N 1 , the fixing roller  30  and the plate-like pressing member  96  are contacted while sliding on each other. 
     Also in this embodiment, the relationship between the first drawing force F 1  at the fixing nip N 1  and the second drawing force F 2  at the contact portion N 2  is set at F 1 ≧F 2 . 
     In order to realize the above-described relationship between the drawing forces F 1  and F 2 , e.g., the pressure of the film  16  on the fixing roller  30  is made smaller than the pressure of the pressing roller  17  on the fixing roller  30 . Further, e.g., the surface of the film  16  is coated with a blend resin material of PTFE and PFA, and the surface of the plate-like pressing member is coated with the PFA resin material. 
     In this way, by providing the drawing forces with the relationship of F 1 ≧F 2 , during the jam clearance, the recording material P can be easily pulled out without tightening the fixing roller  30 . 
     Sixth Embodiment 
     In this embodiment, a constitution using a heat roller containing a halogen heater  50  as the fixing roller  30  will be described. Constituent elements similar to those described above are represented by the same reference numerals or symbols and will be omitted from description.  FIG. 8  is a schematic sectional view showing a schematic structure of a fixing device in Sixth Embodiment. 
     In this embodiment, shown in  FIG. 11 , the fixing roller  30  contains the halogen heater  50  and is internally and externally heated to shorten the warm-up time. 
     The fixing roller  30  is, e.g., a 0.3-3 mm thick hollow cylindrical member including a metal core  51 A principally formed of SUS, iron, aluminum or the like. On the metal core  51 A, an elastic layer  51 B principally formed of the silicone rubber is provided. As the outermost layer of the heat roller  51 , a parting layer  51 C principally formed of the fluorine-containing resin material such as PTFE or PFA is provided. 
     Also in this embodiment, the relationship between the first drawing force F 1  at the fixing nip N 1  and the second drawing force F 2  at the contact portion N 2  is set at F 1 ≧F 2 . 
     In order to realize the above-described relationship between the drawing forces F 1  and F 2 , e.g., the pressure of the film  16  on the fixing roller  30  is made smaller than the pressure of the pressing roller  17  on the fixing roller  30 . Further, e.g., the surface of the film  16  is coated with a blend resin material of PTFE and PFA, and the surface of the plate-like pressing member is coated with the PFA resin material. 
     In this way, by providing the drawing forces with the relationship of F 1 ≧F 2 , during the jam clearance, the recording material P can be easily pulled out without tightening the fixing roller  30 . 
     Seventh Embodiment 
     In this embodiment, a constitution in which the heating member is press-contacted to also the pressing roller  17  will be described. Constituent elements similar to those described above are represented by the same reference numerals or symbols and will be omitted from description.  FIG. 8  is a schematic sectional view showing a schematic structure of a fixing device in Seventh Embodiment. 
     In this embodiment, as shown in  FIG. 12 , the heating member is press-contacted to also the pressing roller  17  to form a press-contact portion N 3 . In addition to the fixing roller  30 , the pressing roller  17  is also externally heated, so that the warm-up time is further shortened. The heating member is the heat roller  51  containing the halogen heater  50 . 
     Also in this embodiment, the relationship between the first drawing force F 1  at the fixing nip N 1  and the second drawing force F 2  at the contact portion N 2  is set at F 1 ≧F 2 . 
     Further, the relationship between a third drawing force F 2  at the press-contact portion N 3  and the first drawing force F 2  at the fixing nip N 1  is set at F 1 ≧F 3 . 
     In order to realize the above-described relationship between the drawing forces F 1  and F 2  and the relationship between the drawing forces F 1  and F 3 , e.g., the pressure of the heat roller  51  on the fixing roller  30  and the pressure of the heat roller  51  on the pressing roller  17  are made smaller the pressure of the pressing roller  17  on the fixing roller  30 . Further, e.g., the parting layer  51 C of the heat roller  51  is formed of a blend resin material of PTFE and PFA, and the parting layers of the fixing roller  30  and the pressing roller  17  are formed of the PFA resin material. Further, into the fluorine-containing resin materials of the parting layers  51 C of the heat rollers  51  an inorganic filler, such as graphite for enhancing the sliding property, is dispersed. 
     In this way, by providing the drawing forces F 1  and F 2  with the relationship of F 1 ≧F 2 , during the jam clearance, the recording material P can be easily pulled out without tightening the fixing roller  30 . 
     Further, by providing the drawing forces F 1  and F 3  with the relationship of F 1 ≧F 3 , during the jam clearance, the recording material P wound about the fixing roller  17  can be easily pulled out. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 071466/2010 filed Mar. 26, 2010, which is hereby incorporated by reference.