Patent Publication Number: US-8126346-B2

Title: Apparatus and method for fixing an image

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 11/128,363 filed May 13, 2005 now U.S. Pat. No. 7,620,336, the entire contents of which are incorporated herein by reference. U.S. application Ser. No. 11/128,363 is based on and claims priority to Japanese patent application No. 2004-142992, filed on May 13, 2004, the entire contents of which are hereby incorporated herein by reference. 
    
    
     FIELD 
     The following disclosure relates generally to an apparatus and method for fixing an image. 
     BACKGROUND 
     An image forming apparatus is usually provided with a fixing device for fixing a toner image on a recording medium by heat and pressure. For example, a fixing roller having a heater inside and a pressure roller are provided to form a nip. When a recording medium passes through the nip, a toner image is heated by the heater through the fixing roller, and fixed onto the recoding medium by a pressure generated at the nip. 
     Recently, to reduce a warm-up time, a fixing roller having a low heat capacitance is provided with an external heater. The external heater heats up the surface of the fixing roller, which constantly rotates, at a position away from the nip. 
     However, the heat applied to the fixing roller may be transmitted to the other member, such as the pressure roller in contact with the fixing roller, thus causing a large amount of energy loss. 
     Further, the rotation of the fixing roller may accelerate wear of the surface of the fixing roller, or it may increase the amount of electric consumption. 
     SUMMARY 
     Exemplary embodiments of the present invention include a fixing device for use in an image forming apparatus. 
     In an exemplary embodiment, the fixing device includes a fixing member, a pressure member, a heater, and a pressure controller. The fixing member and the pressure member face with each other to form a nip. The heater heats a surface of the fixing member when the fixing member rotates. The pressure controller changes a pressure generated at the nip, according to an operation of the image fixing device. 
     In an exemplary embodiment, the fixing device includes a controller, a fixing member, a pressure member, a heater, and a pressure controller. 
     The controller switches operation modes of the fixing device, including a waiting mode and an operating mode. The fixing member rotates in the operating mode. The pressure member, facing the fixing member, forms a nip with the fixing member. The heater heats a surface of the fixing member in the operating mode. The pressure controller changes a pressure generated at the nip when the operation modes are switched. 
     In addition to the above-described fixing devices, this patent specification may be implemented in many other ways, as will be apparent to those skilled in the art, without departing from the spirit or scope of the appended claims and the following disclosure. 
    
    
     
       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 a schematic side view illustrating a part of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a schematic side view illustrating a fixing device according to an exemplary embodiment of the present invention; 
         FIG. 3  is a perspective view illustrating a part of the fixing device shown in  FIG. 2 ; 
         FIG. 4  is a schematic side view illustrating an exemplary structure of the fixing roller shown in  FIG. 2 ; 
         FIG. 5  is a schematic side view illustrating an exemplary structure of the pressure roller shown in  FIG. 2 ; 
         FIG. 6  is a schematic side view illustrating a fixing device according to an exemplary embodiment of the present invention; 
         FIG. 7  is a schematic side view illustrating a fixing device according to an exemplary embodiment of the present invention; 
         FIG. 8  is a schematic side view illustrating a fixing device according to an exemplary embodiment of the present invention; 
         FIG. 9  is a schematic side view illustrating a fixing device according to an exemplary embodiment of the present invention; 
         FIG. 10  is a schematic side view illustrating a fixing device according to an exemplary embodiment of the present invention; and 
         FIG. 11  is a schematic side view illustrating a fixing device according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology selected and it is to be understood that each specific element includes all 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,  FIG. 1  illustrates an image forming apparatus  100  according to an exemplary embodiment of the present invention. 
     In  FIG. 1 , a selected portion of the image forming apparatus  100 , including an image forming device  1  and a fixing device  2 , is shown. 
     The image forming device  1  forms a toner image on a recording medium. The image forming device  1  includes a writing unit  1 , first to fourth image carriers  3 Y,  3 M,  3 C, and  3 K, an intermediate transfer belt  4 , a first roller  5 , and a second roller  6 . 
     The first to fourth image carriers  3 Y to  3 K are arranged side by side. The intermediate transfer belt  4  is provided in parallel to the first to fourth image carriers  3 Y to  3 K. The first roller  5  and the second roller  6  drive the intermediate transfer belt  4  in the direction indicated by the arrow A. 
     Each of the first to fourth image carriers  3 Y to  3 K forms a toner image in a substantially similar manner. As a way of example, an image forming operation for forming a yellow toner image is explained. 
     The first image carrier  3 Y, which rotates clockwise, is uniformly charged by a charging roller  7 . The writing unit  8  irradiates a modulated laser beam L onto the charged surface of the first image carrier  3 Y. This forms a latent image on the first image carrier  3 Y. The latent image is developed by a developer  9  into a yellow toner image. 
     The intermediate transfer belt  4  receives a recording medium, such as paper P, which is transferred in the direction indicated by the arrow B. The paper P is further carried by the intermediate transfer belt  4  in the direction A. When the paper passes through a nip formed between the first image carrier  3 Y and a transfer roller  10 , which faces the first image carrier  3 Y, a voltage having the polarity opposite to the charged polarity is applied. As a result, the yellow toner image formed on the first image carrier  3 Y is transferred onto the paper P. The residual toner remained on the first image carrier  3 Y is removed by a cleaner  11 . 
     A magenta toner image, a cyan toner image, and a black toner image are formed respectively on the second image carrier  3 M, the third image carrier  3 C, and the fourth image carrier  3 K. Each of the toner images is transferred to the paper P in a sequentially manner. The paper P having the composite toner image of four colors is further transferred toward the fixing device  2  in the direction indicated by the arrow C. 
     The fixing device  2  fixes the toner image onto the paper P. After this fixing operation, the paper P is transferred to a discharging roller (not shown) to be discharged onto an output tray (not shown). Alternatively, the paper P may be reversed to the other side by a reversing unit (not shown), for another image forming operation. 
     Now, referring to  FIGS. 2 to 11 , exemplary structures of the fixing device  2  is explained. 
     As shown in  FIGS. 2 and 3 , the fixing device  21  includes a heater  52 , a controller  42 , a fixing roller  28 , a pressure roller  30 , and a pressure controller  60 . 
     The heater  52  preferably has a length of about 70 mm in the direction nearly parallel to the circumferential direction of the fixing roller  28 . The heater  52  has a sleeve-like shape, extending in the axial direction of the fixing roller  28  ( FIG. 3 ). The heater  52  includes a coil supporter  54 , and a coil  56  wound around the coil supporter  54 . The coil supporter  54  has a rod-like shape, and fixed at a predetermined position of the fixing device  21 . The coil  56  is preferably implemented by a litz wire. 
     The controller  42  includes any kind of processor capable of controlling the fixing device  21 . For example, the controller  42  controls the heater  52 , or a drive source (not shown), such as a motor or an actuator, for driving the fixing roller  28  or the pressure controller  60 . In this exemplary embodiment, the controller  42  is implemented by a printer controller of the image forming apparatus  100 , which is a microcomputer including a CPU (central processing unit), a ROM (read only memory), a RAM (random access memory), and an I/O (input/output) interface. 
     The fixing roller  28  transmits a heat from the heater  52  to the paper P passing through the nip SN formed between the fixing roller  28  and the pressure roller  30 . The fixing roller  28  is made of a plurality of layers formed one above the other. In this exemplary embodiment, as shown in  FIG. 4 , the fixing roller  28  includes a core  28   a , a heat absorbing layer  28   b , a heat emitting layer  28   c , an elastic layer  28   d , and a releasing layer  28   e , from the inner side to the outer side, as indicated by the arrow r. 
     The core  28   a  is preferably made of metal, such as aluminum or steel, having a strength sufficient to prevent deflection of the fixing roller  28 . Alternatively, the core  28   a  may be made of glass or ceramics. The thickness of the core  28   a  is preferably 2 mm to 3 mm, however, it is not limited to this example. Further, the core  28   a  has an outer radius of 50 mm, however, it is not limited to this example. 
     The heat absorbing layer  28   b  prevents a heat to transmit from the heat emitting layer  28   c  to the core  28   a . The heat absorbing layer  28   b  is preferably made of foamed silicone rubber having a hardness of 5 to 50 based on the JIS-A standard. Alternatively, any kind of heat resistance material, such as elastomeric material including fluorocarbon rubber, may be used. Further, the thickness of the heat absorbing layer  28   b  is preferably around 4 mm, however, it is not limited to this example. 
     The heat emitting layer  28   c  is made of magnetic or nonmagnetic metal. Preferably, magnetic stainless such as SUS430 and SUS410, iron, or nickel may be used. Alternatively, alloy based on any one of the above-mentioned metals may be preferably used. The thickness of the heat emitting layer  28   c  is preferably between 0.05 mm and 0.5 mm. 
     The elastic layer  28   d  is made of heat resistance eralstomeric material, such as silicon rubber or fluorocarbon rubber, for example. Preferably, any kind of material capable of transmitting a heat from the heat emitting layer  28   c  to the surface of the fixing roller  28  is used. To increase heat conductivity, filler metal may be combined. The thickness of the elastic layer  28   d  is preferably between 0.2 mm to 2 mm. The hardness of the elastic layer  28   d  is preferably below 30 based on the JIS-A standard. 
     The releasing layer  28   e  is optionally provided to increase releasability of the fixing device  28 , and is preferably made of fluorocarbon resin such as PFA (Perfluoroalkoxy) and PTFE (PolyTetraFluoroEthylene), silicon resin, or silicon rubber. The thickness of the releasing layer  28   e  is preferably between 10 μm and 80 μm. 
     The pressure roller  30 , which faces the fixing roller  28 , forms the nip SN with the fixing roller  28 . The pressure roller  30  is made of a plurality of layers formed one above the other. In this exemplary embodiment, as shown in  FIG. 5 , the pressure roller  30  includes a core  30   a , an elastic layer  30   b , and a releasing layer  30   c , from the inner side to the outer side. 
     The core  30   a  is made of metal, such as aluminum or steel. The thickness of the core  30   a  is preferably between 0.4 mm and 0.8 mm. The core  30   a  has an outer radius of 30 mm to 40 mm, however, it is not limited to this example. 
     The elastic layer  30   b  is made of silicon rubber, having a hardness of 30 to 60 based on the JIS-A standard, for example. The thickness of the elastic layer  30   b  is preferably between 0.2 mm and 1 mm. 
     The releasing layer  30   c  is optionally provided to increase releasability of the pressure roller  30 , and is preferably made of fluorocarbon resin, having a thickness of about 50 μm. 
     The pressure controller  60  is capable of controlling a pressure generated at the nip SN. Further, the pressure controller  60  may control a distance of the nip SN, i.e., a distance between the fixing roller  28  and the pressure roller  30 . 
     As shown in  FIG. 2 , the pressure controller  60  includes a pressure spring  61   a , a roller supporter  62 , and a cam  63 . The cam  63  is rotatable in the direction indicated by the arrow. The roller supporter  62  moves the pressure roller  30  upward and downward, according to the position of the cam  63 . The pressure spring  61   a , which is attached to the roller supporter  62 , extends or compresses along with the movement of the roller supporter  62 . 
     In an exemplary operation, when the image forming apparatus  100  is in a waiting mode, the cam  63  is rotated at a first position. When the cam  63  is in the first position, the pressure roller  30  is positioned away from the fixing roller  28 . 
     When the image forming apparatus  100  is activated, or switched from the waiting mode to an operating mode, such as by a user, the controller  42  sends a control signal to the driving source for rotating the fixing roller  28 . 
     At the same time, the coil  56  of the heater  52  applies a current having a high frequency of about 20 kHz to 60 kHz to the surface of the fixing roller  28 , which is rotatably driven. The heat emitting layer  28  of the fixing roller  28  is self heated by the Joule heat caused by the eddy current. 
     By applying a high frequency current of about 20 to 60 kHz to the coil  56 , an eddy current is generated at the heating layer  28   c  of the fixing roller  28 . With this Joule heat, the temperature of the heating layer  28   c  is increased. With this induction heating, the surface temperature of the fixing roller  28  can be raised to a temperature sufficient for melting the toner. Using the induction heating, the heating layer  28   c , which is provided near the surface layer of the fixing roller  28 , can be directly heated, thus reducing the start-up time. 
     When a predetermined time period passes, the controller  42  sends a control signal to the driving source. With this control signal, the cam  63  is rotated to a second position. When the cam  63  is in the second position, the roller supporter  62  moves the pressure roller  30  toward the fixing roller  21 . The pressure spring  61   a  extends due to the reduced pressure from the roller supporter  62 . As a result, the nip SN sufficient for fixing a toner image is formed between the fixing roller  28  and the pressure roller  30 . Further, with the rotation of the fixing roller  28 , the pressure roller  30  is rotated in the direction opposite to the direction of the fixing roller  28 . The controller  42  then sends a control signal to start an image fixing operation. The paper P is then transferred to the nip SN. 
     In this exemplary embodiment, the above predetermined time period is a time needed for the surface of the fixing roller  28  to raise to a temperature for melting a toner. Information regarding this time period may be stored in the ROM of the controller  42 , for example. 
     Further, in this exemplary embodiment, the surface of the pressure roller  30  is made harder than the surface of the fixing roller  28 . Thus, as shown in  FIG. 2 , the surface of the fixing roller  28  is deformed under the pressure from the pressure roller  30  at the nip SN. With this deformation, the paper P passing through the nip SN is curved to form a convex shape. With this convex shape, the paper P can be easily separated from the fixing roller  28  after the image fixing operation. 
     In another exemplary operation, the controller  42  may wait for a predetermined time period, after the cam  63  is rotated at the second position and before the image fixing operation. 
     For example, the pressure roller  30 , which is brought in contact with the fixing roller  28 , is heated by the fixing roller  28 . When a predetermined time period passes, the controller  42  sends a control signal to start an image fixing operation. 
     The above predetermined time period is a time needed for the surface of the pressure roller  30  to raise to a temperature substantially equal to the surface temperature of the fixing roller  28 . Information regarding this time period may be stored in the ROM of the controller  42 , for example. 
     The fixing device  22  of  FIG. 6  is substantially similar in structure to the fixing device  21  of  FIG. 2 . The differences include the heater  53  and the pressure roller  31 . 
     The heater  53  is curved along the circumferential direction of the fixing roller  28 . Further, the coil  56  is wound around the coil supporter  54 , having a plate-like shape, in the axial direction of the fixing roller  28 . 
     The pressure roller  31  is made of a plurality of layers, including the core  30   a , the elastic layer  30   b , and the releasing layer  30   c , as shown in  FIG. 5 . However, the elastic layer  30   b  of the pressure roller  31  has a thickness of about 5 mm to 10 mm. The thicker elastic layer  30   b  may suppress a heat to transmit from the surface of the pressure roller  31  to the core  30   a.    
     The fixing device  23  of  FIG. 7  is substantially similar in structure to the fixing device  21  of  FIG. 2 . The differences include the fixing roller detector  34 , the pressure roller detector  36 , and the controller  43 . 
     The fixing roller detector  34  detects a surface temperature of the fixing roller  28 . As shown in  FIG. 7 , the fixing roller detector  34  is provided remote from the surface of the fixing roller  28 . This requires the fixing roller detector  34  to detect a surface temperature without contacting the surface of the fixing roller  28 . For this reason, the fixing roller detector  34  is preferably implemented by an infrared detector, such as a thermopile, for example. 
     Alternatively, the fixing roller detector  34  may be provided in contact with the surface of the fixing roller  28 . However, this may accelerate wear of the fixing roller  28 . 
     The pressure roller detector  36  detects a surface temperature of the pressure roller  30 . As shown in  FIG. 7 , the pressure roller detector  36  may be provided in contact with the surface of the pressure roller  36 , since the pressure roller  36  is made harder in this exemplary embodiment. 
     Alternatively, the pressure roller detector  36  may be provided remote from the surface of the pressure roller  36 , as long as it is capable of detecting the surface temperature. 
     In this exemplary embodiment, one fixing roller detector  34  and one pressure roller detector  36  are provided. However, the number of detectors is not limited to this example, as long as at least the surface of the fixing roller  28  can be measured. Further, the position of the detector  34  or  36  is not limited to the position shown in  FIG. 7 . 
     The controller  43  is substantially similar in structure to the controller  42 . However, the controller  43  may operate differently from the controller  42 . 
     In an exemplary operation, when the image forming apparatus  100  is in the waiting mode, the pressure roller  30  is positioned away from the fixing roller  28 . 
     When the image forming apparatus  100  is activated, or switched from the waiting mode to the operating mode, the controller  43  sends a control signal to the driving source for rotating the fixing roller  28 . 
     At the same time, the heater  56  applies a heat to the fixing roller  28 , which is rotatably driven, in a substantially similar manner as described referring to  FIG. 2 . 
     The fixing roller detector  34  constantly measures a surface temperature of the fixing roller  28 , and the measured temperatures are checked by the controller  43 . When the surface temperature reaches a predetermined temperature, the controller  43  sends a control signal to rotate the cam  63  to the second position. As a result, the pressure roller  30  moves upward toward the fixing roller  28 , and forms the nip SN for an image fixing operation. 
     In this exemplary embodiment, the predetermined temperature is a temperature sufficient for melting a toner. Information regarding this temperature may be stored in the ROM of the controller  43 , for example. 
     In addition, the controller  43  may additionally check a surface temperature of the pressure roller  30 . 
     In an exemplary operation, the pressure roller detector  36  constantly measures a surface temperature of the pressure roller  30 , and the measured temperatures are checked by the controller  43 . When the surface temperature of the pressure roller  36  reaches a predetermined temperature, which is substantially equal to the predetermined temperature of the fixing roller  28 , the controller  43  sends a control signal to start an image fixing operation. 
     The fixing device  24  of  FIG. 8  is substantially similar in structure to the fixing device  22  of  FIG. 6 . The differences include the pressure controller  65 . 
     The pressure controller  65  is capable of controlling a pressure generated at a nip formed between the fixing roller  28  and the pressure roller  31 . As shown in  FIG. 8 , the pressure controller  65  includes a pressure spring  61   b , the roller supporter  62 , the cam  63 , and a pressure lever  64 . 
     The cam  63  is rotatable in the direction indicated by the arrow. The pressure lever  64  is moved upward or downward, according to the position of the cam  63 . The pressure spring  61   b , which connects the pressure lever  64  and the roller supporter  62 , extends or compresses along the movement of the pressure lever  64 . The roller supporter  62  moves upward or downward, according to the extension or compression of the pressure spring  61   b.    
     When the cam  63  is moved to the first position upon receiving a control signal from the controller  42 , the pressure lever  64  is moved downward, and compresses the spring  61   b . The compressed spring  61   b  moves the pressure roller  31  slightly away from the fixing roller  28 . 
     When the cam  63  is moved to the second position upon receiving a control signal from the controller  42 , the pressure lever  64  is moved upward, and extends the spring  61   b . The extended spring  61   b  moves the pressure roller  31  slightly toward the fixing roller  28 . 
     In this exemplary embodiment, the fixing roller  28  and the pressure roller  30  may not be separated to have a large distance, as long as the pressure generated at the nip SN is reduced. 
     The fixing device  25  of  FIG. 9  is substantially similar in structure to the fixing device  23  of  FIG. 7 . The differences include the separator  65 . 
     The separator  65  separates the paper P, which has passed through the nip SN, from the fixing roller  28 . As shown in  FIG. 9 , the separator  65  is provided remote from the surface of the fixing roller  28  and in parallel to the nip SN. 
     Alternatively, the separator  65  may be provided in contact with the surface of the fixing roller  28 . However, this may accelerate wear of the fixing roller  28 . 
     The fixing device  26  of  FIG. 10  is substantially similar to the fixing device  23  of  FIG. 7 . The differences include the releasing agent applying member  70 , which applies a releasing agent to the surface of the fixing roller  28 . In this exemplary embodiment, the releasing layer  28   e  may not be provided. 
     As shown in  FIG. 10 , the applying member  70  includes a frame  71 , a spring  72 , a solenoid  73 , a swinging member  74 , and a casing  78  having a tank  75 , a supplier  76 , and an applying roller  77 . 
     The tank  75  stores a releasing agent, such as a releasing agent having silicon oil. The supplier  71 , which is made of felt, has one end dipped into the tank  75  and the other end contacting the surface of the applying roller  77 . The applying roller  77  applies the releasing agent, supplied by the supplier  76 , to the surface of the fixing roller  28 . The tank  75 , the supplier  76 , and the applying roller  77  are accommodated in the casing  78 . 
     The frame  71  is fixed at a predetermined position in the fixing device  26 . 
     The spring  72  has one end attached to the frame  71  and the other end attached to the casing  78 . 
     The solenoid  73  has one end surface attached to the frame  71 , and the other end connected to the swinging member  74  via a flexible member, such as a spring. 
     The swinging member  74 , which is attached to the casing  78 , swings at its center. 
     When the solenoid  73  has no current flowing in, the swinging member  74  moves upward, while compressing the spring  72 . The compressed spring  72  and the swinging member  74  keep the position of the casing  78  to be away from the surface of the fixing roller  28 . 
     When the solenoid  73  has a current flowing in, the swinging member  74  moves downward, while extending the spring  72 . The extended spring  72  and the swinging member  74  move the position of the casing  78  toward the surface of the fixing roller  28 . 
     In an exemplary operation, when the image forming apparatus  100  is in a waiting mode, the solenoid  73  has no current flowing in. Thus, the applying roller  77  is kept away from the surface of the fixing roller  28 . 
     When the image forming apparatus  100  is activated, or switched from the waiting mode to an operating mode, such as by a user, the controller  42  causes the fixing roller  28  to rotate, as described referring to  FIG. 2 , for example. At the same time, the controller  42  sends a control signal for sending a current to the solenoid. The applying roller  77 , which is moved to a position in contact with the surface of the fixing roller  28 , can apply a releasing agent to the fixing roller  28 . 
     The above-described fixing devices or other fixing devices of the present invention may be implemented to have a fixing belt, for example, as illustrated in  FIG. 11 . 
     The fixing device  27  of  FIG. 11  includes a fixing belt  128 , a roller  129 , an elastic roller  127 , the pressure roller  31 , the heater  53 , and the separator  65 . In this exemplary embodiment, the fixing belt  128  is heated by the heater  53 , while rotating around the roller  129  and the elastic roller  127 . 
     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. 
     For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     Further, in any one of the above-described exemplary embodiments, the controller may control an operation of the fixing device, by switching operation modes of the fixing device. 
     For example, when the fixing device is in a waiting mode, the cam is at the first position. Accordingly, the pressure roller is kept away from the fixing roller. 
     The fixing device is then switched from the waiting mode to a warm-up mode. In the warm-up mode, the fixing roller is rotated, and heated by the heater. 
     The fixing device is switched from the warm-up mode to a fixing mode, when a predetermined time period passes or when a surface temperature of the fixing roller reaches a predetermined value. In the fixing mode, the cam is rotated to the second position. Accordingly, the pressure roller is moved toward the fixing roller. Subsequently, an image fixing operation is performed. 
     In another example, the fixing device may be switched from the waiting mode to a first warm-up mode. In the first warm-up mode, the fixing roller is rotated, and heated by the heater. 
     The fixing device is then switched from the first warm-up mode to a second warm-up mode, when a predetermined time period passes or when a surface temperature of the fixing roller reaches a predetermined value. In the second warm-up mode, the cam is rotated to the second position. Accordingly, the pressure roller is moved toward the fixing roller, and starts rotating along with the rotation of the fixing roller. 
     The fixing device is switched from the second warm-up mode to a fixing mode, when a predetermined time period passes or when a surface temperature of the pressure roller reaches a predetermined value. In the fixing mode, an image fixing operation is performed. 
     Furthermore, in any one of the above-described exemplary embodiments, the pressure controller may be provided to move the position of the fixing roller. 
     In addition to the embodiments described above, other examples of the invention are provided in the following description. 
     As described above, when using the method of partially heating a fixing member having low heat capacity by heating means located outside of the nip, it is necessary to rotate the fixing member when heating. Since the fixing member has low heat capacity, if the fixing member is heated while it is not rotated, the temperature of the fixing member will reach abnormally high temperature of equal to or greater than 200 degree C. in one or two seconds. 
     In light of the above, the fixing member needs to be rotated when being heated during the start-up time. While heating only the belt is appropriate, the heat may be lost to the pressure roller or the elastic layer inside the fixing member due to the rotation of the fixing member. In order to reduce the start-up time, it is important to consider how the temperature of the fixing member can be increased without heating other members. 
     When the method of fixing at the nip after heating the fixing member having low heat capacity by outside heating means is used, the start-up time period is not 0 such that about a few or several seconds to 30 seconds are needed. 
     In order to reduce electric power consumption, ideally, the electric power supply to the fixing device should be 0 at the time of waiting. However, it is necessary to provide, and it is generally provided, a mode that allows the fixing device to return to the normal state during the start-up time period that is sufficiently short not to cause much stress for the user. Since this usability is prioritized, the electric power is supplied to the fixing device during the waiting state to keep the fixing member at a predetermined temperature. 
     As described above, when using the method of partially heating the fixing member by the outside heating means, the fixing member should be rotated when being heated such that the fixing member needs to be constantly rotated during the waiting state. When constantly rotating the fixing member at the time of waiting, the releasing layer provided on the surface of the fixing member is degraded due to the friction caused at the time of rotation such that life of the fixing member may be shortened. 
     In view of the above, an object of the present invention is to provide a fixing device having longer life while reducing the start-up time period and improving usability of the user. Another object of the present invention is to provide an image forming apparatus provided with such fixing device. 
     The above-described objectives of the present invention can be achieved by the following means. According to a first aspect of the present invention, a fixing device, which fixes an unfixed image formed on a recording medium at a fixing nip, includes a fixing member, a pressure rotator facing the fixing member configured to form the fixing nip with the fixing member, a heating source configured to partially heat the fixing member at a position other than the fixing nip, and controlling means. The fixing device further includes a pressure rotator separating mechanism for moving the pressure rotator away from the fixing member. During the waiting state, the controlling means heats the fixing member while rotating the fixing member and keeping the pressure rotator away from the fixing member, and keeps the fixing member at a predetermined temperature. 
     According to a second aspect of the present invention, a fixing device, which fixes an unfixed image formed on a recording medium at a fixing nip, includes a fixing member, a pressure rotating facing the fixing member configured to form the fixing nip with the fixing member, a heating source configured to partially heat the fixing member at a position other than the fixing nip, and controlling means. The fixing device further includes a pressure adjusting mechanism for reducing a pressure generated by the pressure rotator against the fixing member. During a waiting state, the fixing device heats the fixing member while rotating the fixing member, while reducing the pressure generated by the pressure rotator against the fixing member, and keeps the fixing member at a predetermined temperature. 
     According to a third aspect of the present invention, the fixing device according to the first or second aspect of the present invention further includes means for detecting a surface temperature of the fixing roller. The means for detecting detects the surface temperature while being in non-contact with the fixing member. 
     According to a fourth aspect of the present invention, the fixing device according to the first or second aspect of the present invention further includes means for separating transfer paper and the fixing member, which does not contact the fixing member. 
     According to a fifth aspect of the present invention, the fixing device according to the first or second aspect of the present invention further includes a mechanism configured to separate a releasing agent applying member, which is in contact with the fixing member in a normal state, from the fixing member to make the releasing agent applying member in non-contact with the fixing member. During the waiting time, the controlling means separates the releasing agent applying member from the fixing member in conjunction with operation of separating the pressure rotator or reducing the pressure generated by the pressure rotator. 
     According to a sixth aspect of the present invention, the fixing device, which fixes an unfixed image formed on a recording medium at a fixing nip, includes a fixing member, a pressure rotator facing the fixing member configured to form the fixing nip with the fixing member, a heating source configured to partially heat the fixing member at a position other than the fixing nip, and a controlling means. The fixing device further includes a pressure rotator separating mechanism for moving the pressure rotator away from the fixing member. During a start-up state, the controlling means heats and rotates the fixing member the pressure rotator is positioned away from the fixing member, and brings the pressure rotator into pressure contact with the fixing member after the fixing member reaches a predetermined temperature. 
     According to a seventh aspect of the present invention, the fixing device, which fixes an unfixed image formed on a recording medium at a fixing nip, includes a fixing member, a pressure rotator facing the fixing member configured to form the fixing nip with the fixing member, a heating source configured to partially heat the fixing member at a position other than the fixing nip, and a controlling means. The fixing device further includes a pressure adjusting mechanism for adjusting a pressure generated by the pressure rotator against the fixing member. During a start-up state, the controlling means heats the fixing member while rotating the fixing member, and brings the pressure generated by the pressure rotator to the pressure generated at the normal state after the fixing member reaches a predetermined temperature. 
     According to an eighth aspect of the present invention, the fixing device according to the sixth aspect of the present invention completes the start-up state by performing two step rotating modes, which includes a first start-up mode, in which, while the pressure rotator is positioned away from the fixing member, the fixing member is heated and rotated until the fixing member reaches the predetermined temperature or the fixing member is heated and rotated for a predetermined time period; and a second start-up mode, performed after the first start-up mode, in which the fixing member is heated and rotated while the pressure rotator is brought into contact with the fixing member. 
     According to a ninth aspect of the present invention, the fixing device according to the seventh aspect of the present invention completes the start-up state by performing the two step rotation modes, which includes a first start-up mode in which, while the pressure generated by the pressure rotator against the fixing member is reduced, the fixing member is heated and rotated until the fixing member reaches the predetermined temperature or the fixing member is heated and rotated for a predetermined time period; and a second start-up mode, performed after the first start-up mode, in which the fixing member is heated and rotated while the pressure rotator is brought into pressure contact with the fixing member. 
     According to a tenth aspect of the present invention, an image forming apparatus is provided, which includes the fixing device according to any one of the first to ninth aspects of the present invention. 
     According to the fixing device of one aspect of the invention, during the waiting state, the fixing member is heated and rotated while the fixing member is positioned away from the pressure rotator. Accordingly, it is possible to return from the waiting state to the fixing state in which fixing is possible, without spending the substantial amount of time for waiting, and without shortening life of the fixing member. 
     According to the fixing device of another aspect of the invention, during the waiting state, the fixing member is heated and rotated while the pressure generated by the pressure rotator against the fixing member is reduced. Accordingly, it is possible to return from the waiting state to the fixing state in which fixing is possible, without spending the substantial amount of time for waiting, and while minimizing shortening of life of the fixing member. 
     Another aspect of the fixing device further includes means for detecting that detects a surface temperature of the fixing member. Since the means for detecting detects the temperature while being in non-contact with the fixing member, shortening of life of the fixing member may be prevented, which may be caused by partial degradation of the surface of the fixing member due to the friction generated between the fixing member and the means for detecting. 
     Another aspect of the fixing device includes means for separating (for example, a separating pawl), which separates the transfer paper from the fixing member. Since the means for separating is in non-contact with the fixing member, shortening of life of the fixing member may be prevented, which may be caused by partial degradation of the surface of the fixing member due to the friction generated between the fixing member and the means for separating. 
     Another aspect of the fixing device further includes a member (for example, a silicon oil applying roller), which is in contact with the fixing member in a normal state, is brought into non-contact with the fixing member in conjunction with operation of moving the pressure rotator or reducing the pressure generated by the pressure rotator. Thus, shortening of life of the fixing member may be prevented, which may be caused by partial degradation of the surface of the fixing member due to the friction generated at the fixing member. Additionally, applying an excess amount of silicon oil during the start-up time is prevented. 
     In another aspect of the fixing device, during the start-up time, the fixing member is heated and rotated while the fixing member is positioned away from the pressure rotator. When the fixing member reaches a predetermined temperature, the pressure rotator is brought into pressure contact with the fixing member. Accordingly, time it takes for increasing the temperature of the fixing member can be reduced while minimizing the heat loss to the pressure rotator during the start-up time. Thus, ununiformed fixed state caused due to the ununiformed temperature of the fixing member or the pressure rotator is prevented while minimizing the heat loss to the pressure rotator during the start-up time. 
     In another aspect of the fixing device, during the start-up time, the fixing member is heated and rotated while the pressure generated by the pressure rotator is reduced. When the fixing member reaches a predetermined temperature, the pressure generated by the pressure rotator is brought to the pressure generated at the normal state. Accordingly, time it takes for increasing the temperature of the fixing member can be reduced while minimizing the heat loss to the pressure rotator during the start-up time. When compared with the case of moving the pressure rotator away from the fixing member, the time it takes for increasing the temperature of the fixing member can be reduced with minimized work. 
     In another aspect of the fixing device, the start-up state is completed by performing two step rotation modes, which includes: a first start-up mode in which, while the pressure rotator is positioned away from the fixing member, the fixing member is heated and rotated until the fixing member reaches the predetermined temperature or the fixing member is heated and rotated for a predetermined time period; and a second start-up mode, performed after the first start-up mode, in which the fixing member is heated and rotated while the pressure rotator is brought into contact with the fixing member. Thus, ununiformed fixed state caused due to the ununiformed temperature of the fixing member or the pressure rotator is prevented while minimizing the heat loss to the pressure rotator during the start-up time. 
     According to another aspect of the fixing device, the start-up state is completed by performing two step rotation modes, the modes comprising: a first start-up mode in which, while the pressure generated by the pressure rotator against the fixing member is reduced, the fixing member is heated and rotated until the fixing member reaches the predetermined temperature or the fixing member is heated and rotated for a predetermined time period; and a second start-up mode, performed after the first start-up mode, in which the fixing member is heated and rotated while the pressure rotator is brought into pressure contact with the fixing member. Thus, ununiformed fixed state caused due to the ununiformed temperature of the fixing member or the pressure rotator is prevented while minimizing the heat loss to the pressure rotator during the start-up time. When compared to the case of moving the pressure rotator away from the fixing member, the time it takes for increasing the temperature can be reduced with minimized work. 
     According to another aspect of the fixing device, since any one of the above-described fixing devices is provided, an image forming apparatus can be returned to the normal state without spending the substantial amount of waiting time, while improving usability of the user without shortening life of the fixing member. 
     Detailed description of certain examples of the invention are provided below with reference to the figures. 
     The pressure roller  30 , which functions as the pressure rotator, includes a metal core  30   a  of aluminum or iron having the outer diameter of 30 to 40 mm and the thickness of 0.4 to 0.8 mm; and an elastic layer  30   b  that covers the surface of the metal core  30   a . The elastic layer  30   b  is formed of silicon rubber having the JIS-A hardness of 30 to 60 and has the thickness of 0.2 to 1 mm. Preferably, a surface releasing layer  30   c  is formed on the outer side of the elastic layer  30   b , which is made of fluorocarbon resin with the thickness of about 50 μm, in order to increase releasability. The pressure roller  30  is brought into pressure contact with the fixing roller  28  by biasing means, not illustrated. 
     As illustrated in  FIG. 5 , the pressure roller  30  may have the structure having a surface releasing layer  30   c  having the thickness of about 50 μm at the outer side of the elastic layer  30   b  having the thickness of 5 to 10 mm. 
     Referring to  FIGS. 2 ,  4 , and  5 , the fixing roller  28  is made of the structure softer than the surface hardness of the pressure roller  30 . For this reason, as illustrated in figures, at the fixing nip portion, the pressure roller  30  is pressed against the fixing roller  28  such that the elastic layer  28   d  and the heat insulating layer  28   b  are deformed. With this structure, the transfer paper P is curved at the fixing nip portion SN so as to form the convex shape against the fixing roller  28 , thus making the transfer paper P to be easily separated from the fixing roller  28  after fixing. Referring to  FIG. 2 , the fixing nip portion SN corresponds to the concaved portion of the fixing roller caused by the pressure roller. 
     The fixing roller  28  is driven by a motor and a transmission gear, not illustrated, and drives the pressure roller  30 . 
     According to this example, as illustrated in  FIG. 2 , a mechanism (pressure rotator separating mechanism) for moving the fixing member  28  away from the pressure roller  30  is provided. The controlling means rotates a cam  63  by a drive source not illustrated, and moves a pressure lever  62  supporting the pressure roller  30  upward or downward, thus moving the pressure roller  30  away from the fixing member  28 . 
     According to this example, when the method of heating the fixing member  28  having low heat capacity outside the nip formed between the fixing member  28  and the pressure roller  30  is used, the fixing member  28  needs to be rotated while being heated. Since the fixing member  28  has low heat capacity, if it is partially heated while being unrotated, the heated portion reaches 200 degree C. or greater for 1 or 2 seconds. By rotating the fixing member  28  while rotating, the entire circumference of the fixing member  28  is heated. If the fixing member  28  is heated while being rotated, heat may be lost to the pressure roller  30  or the inner side of the fixing member  28  due to the rotation, while it is preferable to heat only the surface of the fixing member  28 . 
     According to this example, during the start-up time, the cam  63  is rotated such that the pressure roller  30  is positioned away from the fixing member  28 , and heating and rotating is performed. When the fixing member  28  reaches a predetermined temperature or a predetermined time period passes, the cam  63  is rotated such that the pressure roller  30  is brought into pressure contact with the fixing member  28 , while heating and rotating. 
     The start-up is completed by performing a first start-up mode in which the fixing member  28  is heated and rotated while the pressure roller  30  is positioned away from the fixing member  28 , and a second start-up mode in which the fixing member  28  is heated and rotated while the pressure roller  30  is made in contact with the fixing member  28 . If the second start-up mode is not performed, fluctuation in temperature between the fixing member  28  and the pressure rotator  30  in the direction of rotating may be high when performing fixing. As a result, the unevenness in gloss may be caused or fixing may be partially insufficient. By performing the start-up state through two modes, fluctuations in temperature between the fixing member  28  and the pressure rotator  30  may be suppressed while minimizing heat loss from the fixing member  28  to the pressure roller  30  during the start-up time. 
     In order to keep the waiting state in which the fixing device can quickly return to the normal state, it may be necessary to heat the fixing member while rotating the fixing member. If the fixing member is heated and rotated while the pressure roller is in pressure contact with the fixing member, the surface of the fixing member may be degraded due to the friction generated with the pressure roller, thus shortening life of the fixing member. According to this example, for the user who prefers usability (waiting time is 0) over reduced power consumption, it may be necessary to heat the fixing member during the waiting time such that the fixing device can quickly returns to the fixing operation after the waiting time. For this reason, during the waiting time, the fixing member is heated and rotated while the pressure roller is positioned away from the fixing member to keep the fixing member at a predetermined temperature, while extending life of the fixing member. 
     The fixing member is provided with means for detecting, which detects a surface temperature of the fixing member, and controls heating. According to this example, the thermopile  34  is provided, which detects infrared rays from the surface of the fixing member  28  to measure the temperature. When heating and rotating the fixing member  28 , partial degradation of the surface of the fixing member caused due to friction generated with the temperature detecting means is prevented. Since the temperature detecting means is provided in non-contact with the fixing member, life of the fixing member can be extended, as it is generally known. Especially in this example, in which the fixing member  28  needs to be rotated even during the start-up time or waiting time, the time in which the fixing device rotates may be increased by 10 times or more when compared with the generally-used fixing device, depending on how the user operates. For this reason, the temperature detecting means needs to be provided in this example. By providing the temperature detecting means, the fixing device can be provided with reduced start-up time, improved usability, and improved durability. 
     In a vicinity of the nip of the fixing member  28 , separating means  65  is provided. In order to prevent the transfer paper P after fixing from being sticked to the fixing member or wound around the fixing member due to the melted toner, the separating means  65  is provided to mechanically separate the transfer paper P from the fixing member  28 . According to this example, the separating means  65  is provided in the vicinity of the fixing member  28 , but it is not in contact with the fixing member. In order to improve separability, a separating sprawl may be made in contact with the fixing member. For the same reason described above referring to the case of the temperature detecting means, it is necessary to make the separating means  65  in non-contact with the fixing member  28 , thus preventing degradation of the surface of the fixing member  28  due to friction generated at the time of rotation. 
     The fixing member is further provided with an applying roller, which functions as applying means, for applying silicon oil, which is the releasing agent, to the surface of the fixing member. By lightly and uniformly applying silicon oil to the surface of the fixing member, releasability between the fixing member and the melted toner is improved, thus preventing offset of the toner to the fixing member or the transfer paper from wounding around the fixing member. According to this example, since the fixing member is heated and rotated at the waiting time, the releasing agent may be consumed despite the number of sheets passing through if the releasing agent is applied during the waiting time. In order to prevent this, a separating mechanism is provided to separate the releasing agent applying roller from the fixing member. Since the applying roller is moved away in conjunction with operation of positioning the pressure roller away from the fixing member during the waiting time, consumption of the releasing agent may be minimized. 
     According to another example embodiment of the present invention, as illustrated in  FIG. 8 , the mechanism for adjusting the pressure generated by the pressure roller  31  (the pressure adjusting mechanism) may be provided. The pressure adjusting mechanism is controlled by the control means. The control means adjusts the pressure by controlling a drive source (such as a motor or an actuator) of a cam such that the cam  63  is rotated, moving the pressure adjusting lever  64 , and expanding or contracting the pressure spring  61   b . Unlike the above-described example embodiment, the pressure roller  31  is not moved in a distance from the fixing member  28 . However, the size of the nip formed between the fixing member  28  and the pressure roller  31  is reduced by reducing the amount of pressure. As described above referring to the example embodiment, at the time of start-up, the amount of pressure of the pressure roller  31  is reduced. When the fixing member  28  reaches a predetermined temperature, the amount of pressure is returned to the normal state. By reducing the size of the nip at the time of start-up, heat loss to the pressure roller  31  may be suppressed, thus reducing the start-up time period. During the waiting time, the pressure is reduced such that degradation caused by contact friction between the fixing member  28  and the pressure roller  31  may be reduced. While this example is not effective compared to the above-described example of positioning the pressure roller  31  away from the fixing member  28 , a distance or drive power it takes for the pressure roller  31  to move from the fixing member  28  may be made smaller. Accordingly, a low-cost mechanism, such as a low torque motor, may be used to achieve the objectives of the present invention. 
       FIG. 11  illustrates other example embodiments of the present invention. The fixing member is implemented by a thin belt  128  provided with a releasing layer on its surface. After heating the belt  128  by the coil  56 , fixing is performed at the nip. On the inner side of the fixing belt  128 , a ferrite roller  129  for increasing heating efficiency of the fixing belt, and an elastic roller  127  facing the pressure roller  31  via the fixing member  128  that forms the nip, are provided. With this structure, if the fixing belt  128  is heated while the fixing member  128  is not rotated, the fixing belt  128  quickly reaches a high temperature. For this reason, the fixing belt  128  needs to be rotated when being heated. By positing away from the pressure roller  31  or reducing pressure generated by the pressure roller  31  during the start-up time or waiting time, the start-up time may be reduced without causing heat loss to the pressure roller  131 , thus preventing degradation of the surface of the fixing belt  128  that may be caused due to the rotation during the waiting time. 
     Furthermore, any one of the image fixing operations mentioned above may be embodied in the forms of a computer program. In such a case, the computer program is preferably stored in a storage device readable to the CPU of the controller. The storage device includes any kind of memory, such as a built-in memory installed inside an image forming apparatus or a removable memory separable from the image forming apparatus. Alternatively, the computer program may downloaded via a network to be stored in the storage device.