Patent Publication Number: US-11385582-B2

Title: Image forming device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of International Application No. PCT/JP2019/030789 filed on Aug. 5, 2019, and claims priority from Japanese Patent Application No. 2019-28810 filed on Feb. 20, 2019. 
    
    
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming device. 
     Related Art 
     An electrophotographic system disclosed in JP-H08-50429 sets a printing speed used in a case where plural electrophotographic devices are connected to perform printing and a printing speed used in a case where plural electrophotographic devices are made independent of each other and printing is performed by separate electrophotographic devices, and includes a printing speed switching unit that switches between the printing speeds. 
     SUMMARY 
     In the related art, when forming a toner image on both surfaces of a recording medium, the toner image is first transferred onto a first surface of the recording medium. Next, a preheating unit heats the recording medium having the toner image transferred onto the first surface thereof. Further, a main heating unit comes into contact with the recording medium to heat the recording medium, and fixes the toner image onto the first surface of the recording medium heated by the preheating unit. Next, the toner image is transferred onto a second surface of the recording medium having the toner image fixed onto the first surface thereof. Next, the preheating unit heats the recording medium having the toner image transferred onto the second surface thereof. Further, the main heating unit comes into contact with the recording medium to heat the recording medium, and fixes the toner image onto the second surface of the recording medium heated by the preheating unit. 
     Here, since the preheating unit heats the recording medium having the toner image transferred onto the second surface thereof, a temperature of the first surface of the recording medium becomes equal to or higher than a softening point of a toner. Thereafter, when the toner image on the first surface is heated a second time by the main heating unit, glossiness of the toner image on the first surface becomes higher than glossiness of the toner image on the second surface. That is, a glossiness difference (=gloss difference) occurs between the toner image on the first surface and the toner image on the second surface. 
     Aspects of non-limiting embodiments of the present disclosure relate to reducing the difference between the glossiness of the toner image on the first surface and the glossiness of the toner image on the second surface, as compared with the case where, before the main heating unit heats the recording medium having the toner image transferred onto the second surface thereof, the temperature of the first surface of the recording medium is equal to or higher than the softening point of the toner since the recording medium having the toner image transferred onto the second surface thereof is heat by the preheating unit. 
     Aspects of certain non-limiting embodiments of the present disclosure address the above advantages and/or other advantages not described above. However, aspects of the non-limiting embodiments are not required to address the advantages described above, and aspects of the non-limiting embodiments of the present disclosure may not address advantages described above. 
     According to an aspect of the present disclosure, there is provided an image forming device including: a transfer unit that transfers a toner image onto a recording medium being conveyed; a main heating unit that is arranged downstream of the transfer unit in a conveyance direction of the recording medium, comes into contact with the recording medium to heat the recording medium, and fixes the toner image onto the recording medium; a reversing unit that reverses front and back of the recording medium having a first toner image fixed onto a first surface of the recording medium by the main heating unit, and sends the recording medium to the transfer unit; and a preheating unit that is arranged between the transfer unit and the main heating unit in the conveyance direction of the recording medium and heats the recording medium having a second toner image transferred onto a second surface of the recording medium, wherein the preheating unit heats the recording medium so that a temperature of the first surface of the recording medium is lower than a softening point of a toner before the recording medium is heated by the main heating unit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Exemplary embodiment(s) of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a configuration diagram illustrating a fixing device provided in an image forming device according to an exemplary embodiment; 
         FIG. 2  is a cross-sectional view illustrating a main fixing portion of the fixing device provided in the image forming device according to the exemplary embodiment; 
         FIG. 3  is a table showing evaluation results of evaluating the image forming device according to the exemplary embodiment and an image forming device according to a comparative embodiment; 
         FIG. 4  is a perspective view illustrating the main fixing portion of the fixing device provided in the image forming device according to the exemplary embodiment; 
         FIG. 5  is a cross-sectional view illustrating the main fixing portion of the fixing device provided in the image forming device according to the exemplary embodiment; 
         FIG. 6  is a perspective view illustrating a conveying mechanism provided in the image forming device according to the exemplary embodiment; 
         FIG. 7  is a cross-sectional view illustrating a cooling unit provided in the image forming device according to the exemplary embodiment; 
         FIG. 8  is a configuration diagram illustrating a toner image forming unit provided in the image forming device according to the exemplary embodiment; 
         FIG. 9  is a configuration diagram illustrating the image forming device according to the exemplary embodiment; and 
         FIG. 10  is a configuration diagram illustrating a fixing device provided in an image forming device according to a comparative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An example of an image forming device according to an exemplary embodiment will be described with reference to  FIGS. 1 to 10 . In the drawings, an arrow H indicates a device vertical direction (perpendicular direction), an arrow W indicates a device width direction (horizontal direction), and an arrow D indicates a device depth direction (horizontal direction). 
     (Image Forming Device  10 ) 
     An image forming device  10  according to the exemplary embodiment is an electrophotographic image forming device that forms a toner image on a sheet member P. As illustrated in  FIG. 9 , the image forming device  10  includes a control unit  160 , an accommodating portion  50 , a discharge portion  52 , an image forming unit  12 , a conveying mechanism  60 , a reversing mechanism  80 , a fixing device  100 , and a cooling unit  90 . 
     [Control Unit  160 ] 
     The control unit  160  is configured with a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and a hard disk drive (HDD) (none is shown). The CPU executes a processing program. The ROM stores various programs, various tables, parameters, and the like. The RAM is used as a work area or the like when the CPU executes various programs. 
     [Accommodating Portion  50 ] 
     The accommodating portion  50  has a function of accommodating the sheet member P as a recording medium. The image forming device  10  may include plural (for example, two) accommodating portions  50 , and may selectively send out the sheet member P from the plural accommodating portions  50 . 
     [Discharge Portion  52 ] 
     The discharge portion  52  is a portion from which the sheet member P on which a toner image is formed is discharged. Specifically, after the toner image is fixed by the fixing device  100 , the sheet member P cooled by the cooling unit  90  is discharged to the discharge portion  52 . 
     [Image Forming Unit  12 ] 
     The image forming unit  12  has a function of forming a toner image on the sheet member P by an electrophotographic method. Specifically, the image forming unit  12  includes a toner image forming unit  20  that forms a toner image, and a transfer device  30  that transfers the toner image formed by the toner image forming unit  20  to the sheet member P. 
     Plural toner image forming units  20  are provided so as to form a toner image for each color. The image forming device  10  includes toner image forming units  20  of a total of four colors of yellow (Y), magenta (M), cyan (C), and black (K). The (Y), (M), (C), and (K) shown in  FIG. 9  show constituent portions corresponding to the respective colors. 
     -Toner Image Forming Unit  20 - 
     The toner image forming units  20  of these colors have basically the same configuration except for toner to be used. Specifically, as illustrated in  FIG. 8 , the toner image forming unit  20  of each color includes a photosensitive drum  21  (photoconductor) that rotates in a direction indicated by an arrow A in  FIG. 8 , and a charger  22  that charges the photosensitive drum  21 . Further, the toner image forming unit  20  of each color includes an exposure device  23  that exposes the photosensitive drum  21  charged by the charger  22  to light to form an electrostatic latent image on the photosensitive drum  21 , and a developing device  24  that uses toner to develop the electrostatic latent image, which is formed on the photosensitive drum  21  by the exposure device  23 , so as to form a toner image. 
     -Transfer Device  30 - 
     The transfer device  30  has a function of primarily transferring toner images of the photosensitive drums  21  of the respective colors onto an intermediate transfer body in a superimposed manner, and secondarily transferring the superimposed toner images onto the sheet member P. Specifically, as illustrated in  FIG. 9 , the transfer device  30  includes a transfer belt  31  as the intermediate transfer body, a primary transfer roll  33 , and a transfer unit  35 . 
     The primary transfer roll  33  has a function of transferring a toner image formed on the photosensitive drum  21  to the transfer belt  31  at a primary transfer position T (see  FIG. 8 ) between the photosensitive drum  21  and the primary transfer roll  33 . 
     The transfer belt  31  has an endless shape, and is wound around plural rolls  32  to determine a posture thereof. When at least one of the plural rolls  32  is driven to rotate, the transfer belt  31  rotates in a direction indicated by an arrow B, and conveys the primarily transferred toner image to a secondary transfer position NT to be described later. 
     The transfer unit  35  has a function of transferring the toner image, which is transferred onto the transfer belt  31 , to the sheet member P. Specifically, the transfer unit  35  includes a secondary transfer unit  34  and a facing roll  36 . 
     The facing roll  36  is disposed below the transfer belt  31  so as to face the transfer belt  31 . The secondary transfer unit  34  is disposed on an inner side of the transfer belt  31  such that the transfer belt  31  is disposed between the secondary transfer unit  34  and the facing roll  36 . Specifically, the secondary transfer unit  34  is configured with a corotron. In the transfer unit  35 , the toner image transferred onto the transfer belt  31  is transferred onto the sheet member P passing through the secondary transfer position NT by a electrostatic force generated due to electricity-discharge of the secondary transfer unit  34 . Here, the secondary transfer position NT is a position where the transfer belt  31  and the facing roll  36  are in contact with each other. 
     [Conveying Mechanism  60 ] 
     The conveying mechanism  60  has a function of conveying the sheet member P accommodated in the accommodating portion  50  to the secondary transfer position NT. Further, the conveying mechanism  60  has a function of conveying the sheet member P from the secondary transfer position NT to the main heating unit  120  to be described later. The conveying mechanism  60  will be described in detail later. 
     [Reversing Mechanism  80 ] 
     The reversing mechanism  80  has a function of reversing the front and back of the sheet member P. The reversing mechanism  80  will be described in detail later. 
     [Fixing Device  100 ] 
     The fixing device  100  has a function of fixing the toner image, which is transferred onto the sheet member P by the transfer device  30 , onto the sheet member P. The fixing device  100  will be described in detail later. 
     [Cooling Unit  90 ] 
     The cooling unit  90  has a function of cooling the sheet member P heated by the fixing device  100 . As illustrated in  FIG. 9 , the cooling unit  90  is disposed downstream of the main heating unit  120  of the fixing device  100  in a conveyance direction of the sheet member P. The cooling unit  90  includes two cooling rolls  92  arranged in the device width direction. Since the two cooling rolls  92  have the same configuration, one of the cooling rolls  92  will be described. 
     As illustrated in  FIG. 7 , the cooling roll  92  includes a pair of rolls  92   a  and  92   b  that sandwich the sheet member P in a conveyance path thereof. The roll  92   a  is disposed above the conveyance path of the sheet member P. The roll  92   b  is disposed below the conveyance path of the sheet member P. 
     The rolls  92   a  and  92   b  have cylindrical base members  94   a  and  94   b  respectively that extend in the device depth direction. The base members  94   a  and  94   b  are, for example, aluminum tubes. An air blowing mechanism (not shown) is configured to generate an air flow inside the base members  94   a  and  94   b . Due to the air flow, a temperature of surfaces of the rolls  92   a  and  92   b  is lower than a temperature thereof in a case where the air flow is not generated. 
     In this configuration, the roll  92   b  is rotated by a rotational force transmitted from a driving member (not shown). Further, the roll  92   a  is driven to rotate by the roll  92   b . The rolls  92   a  and  92   b  convey the sheet member P while sandwiching the sheet member P therebetween, and cool the sheet member P. 
     (Operation of Image Forming Device) 
     In the image forming device  10  illustrated in  FIG. 9 , a toner image is formed as follows. 
     First, the charger  22  (see  FIG. 8 ) of each color, to which a voltage is applied, uniformly charges a surface of the photosensitive drum  21  of each color at a predetermined negative potential. Subsequently, based on image data input from the outside, the exposure device  23  irradiates the charged surface of the photosensitive drum  21  of each color with exposure light to form an electrostatic latent image. 
     Accordingly, electrostatic latent images corresponding to the image data are formed on respective surfaces of the photosensitive drums  21 . Further, developing devices  40  of the respective colors develop the electrostatic latent images to form toner images on the respective surfaces of the photosensitive drums  21  of the respective colors. The transfer device  30  transfers the toner images formed on the surfaces of the photosensitive drums  21  of the respective colors onto the transfer belt  31 . 
     Meanwhile, the sheet member P is sent out from the accommodating portion  50  illustrated in  FIG. 9  to the conveyance path of the sheet member P by the conveying mechanism  60  to be described later. The sheet member P being conveyed along the conveyance path is sent out to the secondary transfer position NT where the transfer belt  31  and the facing roll  36  are in contact with each other. At the secondary transfer position NT, the sheet member P is conveyed while being sandwiched between the transfer belt  31  and the facing roll  36 , and accordingly the toner images on a front surface of the transfer belt  31  are transferred onto a first surface (=front surface) of the sheet member P. 
     Further, the fixing device  100  fixes the toner images, which are transferred onto the first surface of the sheet member P, onto the sheet member P, and the sheet member P is conveyed to the cooling unit  90 . The cooling unit  90  cools the sheet member P onto which the toner images are fixed, and discharges the sheet member P to the discharge portion  52 . 
     On the other hand, in a case of forming a toner image on a second surface (=back surface) of the sheet member P, the sheet member P that passes through the fixing device  100  by being conveyed by the conveying mechanism  60  is conveyed to the reversing mechanism  80 . The sheet member P conveyed to the reversing mechanism  80  has the front and back thereof reversed by a reversing device  84 . Conveying rolls  86  convey the sheet member P, whose front and back are reversed, to the conveying mechanism  60 . In order to form a toner image on the second surface of the sheet member P, conveyance of the sheet member P to the secondary transfer position NT, transfer of the toner image onto the second surface of the sheet member, and fixing of the toner image onto the second surface are performed in the same manner as described above. 
     (Configuration of Main Part) 
     Next, the conveying mechanism  60 , the reversing mechanism  80 , and the fixing device  100  will be described. 
     [Conveying Mechanism  60 ] 
     As illustrated in  FIG. 9 , the conveying mechanism  60  includes a sending-out roll  62 , plural conveying rolls  64 , and a chain gripper  66 . The conveying mechanism  60  is an example of a conveying unit. 
     The sending-out roll  62  is a roll that sends out the sheet member P accommodated in the accommodating portion  50 . The plural conveying rolls  64  include rolls that convey the sheet member P sent out by the sending-out roll  62  to the chain gripper  66 , and rolls that convey the sheet member P conveyed by the chain gripper  66  to the cooling unit  90 . The chain gripper  66  has a function of conveying the sheet member P while holding a leading end portion of the sheet member P. Specifically, as illustrated in  FIG. 6 , the chain gripper  66  has a pair of chains  72 , and grippers  76  serving as holding portions (=gripping portions). 
     The pair of chains  72  is formed in an annular shape. The chains  72  in a pair are disposed at an interval in the device depth direction. The pair of chains  72  is wound around a pair of sprockets (not shown) disposed at both ends in an axial direction of the facing roll  36  (see  FIG. 9 ), a pair of sprockets  71  (see  FIG. 4 ) disposed at both ends in an axial direction of a pressing roll  140  to be described later, and a pair of sprockets  74  (see  FIG. 9 ). When one pair of the sprockets rotates, the chain  72  rotates in a direction indicated by an arrow C. 
     Plural attachment members  75  extend between the pair of chains  72  along the device depth direction, at predetermined intervals along a circumferential direction (rotational direction) of the chains  72 . The grippers  76  are attached. 
     Plural grippers  76  are attached to each attachment member  75  at predetermined intervals along the device depth direction. Each gripper  76  has a function of holding a leading end portion of the sheet member P. Specifically, each gripper  76  has a claw  76   a . In addition, the attachment member  75  is formed with a contact portion  75   a  (see  FIG. 5 ) with which the claw  76   a  comes into contact. 
     When the leading end portion of the sheet member P is sandwiched between the claw  76   a  of the gripper  76  and the contact portion  75   a  of the attachment member  75 , the sheet member P gets held. For example, the claw  76   a  of the gripper  76  is pressed against the contact portion  75   a  of the attachment member  75  by a spring or the like, and the claw  76   a  is brought into contact with or separated from the contact portion  75   a  by an action of a cam or the like. 
     In the chain gripper  66 , the chain  72  rotates in the direction indicated by the arrow C in a state where the leading end portion of the sheet member P is held between the gripper  76  and the attachment member  75 , thereby conveying the sheet member P. The chain gripper  66  conveys the sheet member P conveyed by the plural conveying rolls  64  to the secondary transfer position NT, and further conveys the sheet member P to the main heating unit  120  described later after passing the sheet member P through a preheating unit  102  described later. A part of the conveyance path in the conveying mechanism  60  along which the sheet member P is conveyed is indicated by a one-dot chain line in  FIG. 9 . 
     In this configuration, at least from the secondary transfer position NT to the main heating unit  120 , the conveying mechanism  60  conveys the sheet member P with both sheet surfaces (=recording surfaces) thereof facing in the vertical direction. In other words, at least from the secondary transfer position NT to the main heating unit  120 , the conveying mechanism  60  conveys the sheet member P with a surface having an unfixed toner image being directed upward. 
     Further, there is case where toner images are formed on the first surface (=front surface) and the second surface (=back surface) of the sheet member P. Hereinafter, this is referred to as “the case of double-sided printing”. In this case, when the sheet member P having an image transferred onto the second surface thereof is heated by the preheating unit  102  to be described later, the conveying mechanism  60  conveys the sheet member P with an image forming region of the first surface of the sheet member P in a state of not being in contact with other members. In other words, arrangement of other members is determined in consideration of a conveyance posture of the sheet member P being conveyed by the conveying mechanism  60 . The “image forming region” refers to a portion other than an outer peripheral portion of the sheet member P where an image cannot be formed, and is a region where an image is formed when a solid image is formed on the sheet member P. 
     [Reversing Mechanism  80 ] 
     As illustrated in  FIG. 9 , the reversing mechanism  80  includes plural conveying rolls  82 , the reversing device  84 , and plural conveying rolls  86 . The reversing mechanism  80  is an example of a reversing unit. 
     The plural conveying rolls  82  convey the sheet member P sent from the fixing device  100  to the reversing device  84 . The reversing device  84  is, for example, a device that conveys the sheet member P while rotating the sheet member P plural times such that the conveyance direction of the sheet member P changes by, for example, 90 degrees each time, thereby twisting the sheet member P like a mevius band to reverse the front and back of the sheet member P. The plural conveying rolls  86  are rolls that convey the sheet member P whose front and back are reversed by the reversing device  84  to the chain gripper  66 . 
     In this configuration, in the case of double-sided printing, the reversing mechanism  80  reverses the front and back of the sheet member P having the toner image fixed onto the first surface thereof. Then, the reversing mechanism  80  sends the sheet member P again to the secondary transfer position NT through the conveying mechanism  60 . 
     [Fixing Device  100 ] 
     As illustrated in  FIG. 1 , the fixing device  100  includes the preheating unit  102  that heats the sheet member P being conveyed in a state of not being in contact with the sheet member P, the main heating unit  120  that comes into contact with the sheet member P to heat and press the sheet member P, and a blowing unit  170 . 
     [Preheating Unit  102 ] 
     As illustrated in  FIG. 1 , the preheating unit  102  is disposed downstream of the secondary transfer position NT (see  FIG. 9 ) in the conveyance direction of the sheet member P and above the conveyance path of the sheet member P. In other words, the preheating unit  102  is disposed on a side of the unfixed toner image transferred onto the sheet member P. That is, the preheating unit  102  is provided on the same side as the transfer unit  35  with respect to the conveyance path of the sheet member P. The preheating unit  102  includes a reflecting plate  104 , plural infrared heaters  106  (hereinafter referred to as “heaters  106 ”), and a wire mesh  112 . 
     -Reflecting Plate  104 - 
     The reflecting plate  104  is formed of an aluminum plate, and has a shallow bottomed box shape opened on a side of the sheet member P being conveyed. In the present exemplary embodiment, as viewed from above, the reflecting plate  104  covers, in the device depth direction, the sheet member P being conveyed. 
     -Heater  106 - 
     The heater  106  has a cylindrical shape extending in the device depth direction, and plural heaters  106  are accommodated inside the box-shaped the reflecting plate  104 . In the present exemplary embodiment, as viewed from above, the heaters  106  cover, in the device depth direction, the sheet member P being conveyed. Each heater  106  is separated from the conveyance path of the sheet member P by 30 mm in an upward direction. 
     The plural heaters  106  are arranged at intervals in the device width direction. In the present exemplary embodiment, as viewed from above, a region where the plural heaters  106  are arranged covers, in the device width direction, one sheet member P being conveyed. In other words, the plural heaters  106  heat at one time the entire sheet member P being conveyed. 
     In the above configuration, from the heater  106 , an infrared ray having a maximum spectral radiance at a wavelength of 3 μm or more and 5 μm or less is emitted. A surface temperature of the heater  106  is a predetermined temperature of 300° C. or higher and 1175° C. or lower. 
     -Wire Mesh  112 - 
     The wire mesh  112  is fixed to an edge portion of an opening of the box-shaped reflecting plate  104  by a fixing member (not shown), and partitions the inside of the box-shaped reflecting plate  104  from the outside of the reflecting plate  104 . Accordingly, the wire mesh  112  prevents the sheet member P being conveyed from coming into contact with the heater  106  inside the box-shaped reflecting plate  104 . 
     In this configuration, the preheating unit  102  heats the sheet member P in a non-contact state from the unfixed toner image side. In other words, the preheating unit  102  functions as a softening unit that softens an unfixed toner. 
     Further, in a case of single-sided printing, when heating the sheet member P having the toner image transferred onto the first surface thereof, the preheating unit  102  heats the sheet member P so that a temperature of the first surface of the sheet member P is equal to or higher than a softening point of the toner or close to the softening point before the sheet member P is heated by the main heating unit  120 . 
     In the case of double-sided printing, when heating the sheet member P having the toner image transferred onto the second surface thereof, the preheating unit  102  heats the sheet member P so that the temperature of the first surface of the sheet member P is lower than the softening point of the toner before the sheet member P is heated by the main heating unit  120 . 
     Here, the “temperature of the first surface of the sheet member P before the sheet member P being heated by the main heating unit  120 ” is, as will be described later, the temperature of the first surface of the sheet member P at a position S 01  separated from an upstream end of a nip portion N of the main heating unit  120  in the conveyance direction toward an upstream side in the conveyance direction by 100 mm. That is, when facing the second surface to heat the toner image transferred onto the second surface, the preheating unit  102  heats the sheet member P such that the temperature of the first surface at the position S 01  is lower than the softening point of the toner. In other words, an output of the preheating unit  102  is adjusted such that the temperature of the first surface at the position S 01  is lower than the softening point of the toner. Specifically, even when the toner image transferred onto the second surface is a black solid image, the output of the preheating unit  102  is adjusted such that the temperature of the first surface at the position S 01  is lower than the softening point of the toner. 
     In the present exemplary embodiment, as an example, an output condition of the preheating unit  102  under which the temperature of the first surface of the sheet member P is lower than the softening point of the toner is obtained in advance for each paper type or size by a test, and an output table of the preheating unit  102  is stored in the control unit  160 . Then, the control unit  160  adjusts the output of the preheating unit  102  based on information of the paper type or size input by a user. Accordingly, the preheating unit  102  heats the sheet member P such that the temperature of the first surface of the sheet member P is lower than the softening point of the toner. The temperature of the first surface at the position S 01  may be measured by a temperature sensor, and the control unit  160  may adjust the output of the preheating unit  102  based on a measurement result, thereby heating the sheet member P such that the temperature of the first surface of the sheet member P is lower than the softening point of the toner. 
     Here, “the softening point of the toner (=glass transition temperature of the toner)” is a ½ descent rate measured under conditions of a die pore diameter of 0.5 mm, a pressure load of 0.98 MPa, and a temperature increase rate of 1° C./min in a temperature increase test using a flow tester (CFT500, manufactured by Shimadzu Corporation). Note that the ½ descent rate is a temperature corresponding to ½ of a height obtained from an outflow start point to an end point when a toner sample is melted and flowed out. 
     [Blowing Unit  170 ] 
     As illustrated in  FIG. 1 , the blowing unit  170  is disposed so as to face the preheating unit  102  in the vertical direction, and the sheet member P being conveyed passes between the blowing unit  170  and the preheating unit  102 . The blowing unit  170  includes plural fans  172  arranged in the device width direction and the device depth direction. The fan  172  is an example of a blowing unit. 
     In this configuration, when the plural fans  172  blow air toward the sheet member P passing between the blowing unit  170  and the preheating unit  102 , the conveyance posture of the sheet member P conveyed with the leading end portion thereof being held is stabilized. The fan  172  is an example of a stabilizing unit. 
     Here, “the conveyance posture of the sheet member P is stabilized” means that a distance from a rear end portion of the sheet member in a state of being bent by gravity to the preheating unit  102  is smaller than when the stabilizing unit is not provided. A distance from the sheet surface of the sheet member P to the preheating unit  102  may be longer than a distance from the preheating unit  102  to the gripper  76  and variation in the distance to the preheating unit  102  depending on a position of the sheet surface may be suppressed. In other words, the distance from the sheet surface of the sheet member P to the preheating unit  102  may be longer than the distance from the preheating unit  102  to the gripper  76 , and a difference between a longest distance from the sheet surface of the sheet member P to the preheating unit  102  and a shortest distance may be reduced. Here, an output of the fan  172  may be adjusted. In the present exemplary embodiment, as an example, an output condition of the fan  172  is obtained for each paper type or size, and an output table of the fan  172  is stored in the control unit  160 . Then, the control unit  160  adjusts the output of the fan  172  based on the information of the paper type or size input by the user. For example, when a paper thickness input by the user is larger than a predetermined value or a size is larger than a predetermined value, the output of the fan is increased. The distance from the sheet surface of the sheet member P to the preheating unit  102  may be measured by an optical sensor, and the control unit  160  may adjust the output of the fan  172  based on a measurement result thereof. 
     Further, when the plural fans  172  blow air toward the sheet member P, the temperature of the sheet surface of the sheet member P on a side to which the air is blown decreases. In this way, the fan  172  functions as a temperature reducing unit. 
     [Main Heating Unit  120 ] 
     As illustrated in  FIG. 1 , the main heating unit  120  is disposed downstream of the preheating unit  102  in the conveyance direction of the sheet member P. The main heating unit  120  includes a heating roll  130  that comes into contact with the sheet member P being conveyed to heat the sheet member P, a pressing roll  140  that presses the sheet member P toward the heating roll  130 , and a driven roll  150  that is driven to rotate by the rotating heating roll  130 . 
     -Heating Roll  130 - 
     As illustrated in  FIG. 1 , the heating roll  130  is disposed so as to come into contact with a surface facing upward of the sheet member P being conveyed and extend in the device depth direction with an axial direction thereof serving as the device depth direction. The heating roll  130  includes a cylindrical base member  132 , a rubber layer  134  formed so as to cover an entire circumference of the base member  132 , a release layer  136  formed so as to cover an entire circumference of the rubber layer  134 , and a heater  138  accommodated in the base member  132 . An outer diameter of an outer circumferential surface of the release layer  136  of the heating roll  130  is, for example, 80 mm. 
     The base member  132  is an aluminum tube and has a thickness of 20 mm, for example. The rubber layer  134  is made of silicone rubber, and has a thickness of 6 mm, for example. Further, the release layer  136  is made of a copolymer of tetrafluoroethylene and perfluoroethylene (PFA resin), and has a thickness of 50 μm, for example. 
     As illustrated in  FIG. 4 , shaft portions  139   a  extending in the device depth direction are formed at both end portions of the heating roll  130  respectively in the device depth direction. Each shaft portion  139   a  is supported by a support member  139   b . The heating roll  130  is rotatably supported by support members  139   b  at both end portions of the heating roll  130 . 
     -Driven Roll  150 - 
     As illustrated in  FIGS. 1 and 4 , the driven roll  150  is disposed so as to extend in the device depth direction with an axial direction thereof serving as the device depth direction, on an opposite side of the heating roll  130  than the sheet member P being conveyed. The driven roll  150  includes a cylindrical base member  152  and a heater  154  accommodated in the base member  152 . An outer diameter of an outer circumferential surface of the base member  152  of the driven roll  150  is, for example, 50 mm. 
     The base member  152  is an aluminum tube and has a thickness of 10 mm, for example. The driven roll  150  is rotatably supported by a support member (not shown) at both end portions of the driven roll  150 . 
     In this configuration, the driven roll  150  is driven to rotate by the heating roll  130 . The driven roll  150  heats the heating roll  130 . As described above, since the heating roll  130  is heated by the driven roll  150  and the heating roll  130  itself has the heater  138 , a surface temperature of the heating roll  130  becomes a predetermined temperature of 180° C. or higher and 200° C. or lower. 
     -Pressing Roll  140 - 
     As illustrated in  FIGS. 1 and 4 , the pressing roll  140  is provided on an opposite side of the sheet member P being conveyed than the heating roll  130 , and is disposed so as to be in contact with a surface facing downward of the sheet member P being conveyed and extend in the device depth direction with an axial direction thereof serving as the device depth direction. The pressing roll  140  includes a cylindrical base member  142 , a rubber layer  144  formed so as to cover the base member  142 , a release layer  146  formed so as to cover the rubber layer  144 , and a pair of shaft portions  148  (see  FIG. 4 ) formed at both end portions in the device depth direction. An outer diameter of an outer circumferential surface of the release layer  146  of the pressing roll  140  is, for example, 225 mm. Thus, an outer diameter of the pressing roll  140  is larger than an outer diameter of the heating roll. 
     The base member  142  is an aluminum tube and has a thickness of 20 mm, for example. The rubber layer  144  is made of silicone rubber and has a thickness of 1 mm, for example. Further, the release layer  146  is made of a copolymer of tetrafluoroethylene and perfluoroethylene (PFA resin), and has a thickness of 50 μm, for example. 
     A recessed portion  140   a  extending in the device depth direction is formed in an outer circumferential surface of the pressing roll  140 . As illustrated in  FIG. 5 , when the sheet member P passes between the pressing roll  140  and the heating roll  130 , the gripper  76  that grips the leading end portion of the sheet member P is accommodated in the recessed portion  140   a.    
     As illustrated in  FIG. 4 , the pair of shaft portions  148  are formed at both end portions of the pressing roll  140  in the device depth direction, have a diameter smaller than that of the outer circumferential surface of the release layer  146  of the pressing roll  140 , and extend in the axial direction. 
     In this configuration, the pressing roll  140  is rotated by a rotational force transmitted from a driving member (not shown). Further, the heating roll  130  is driven to rotate by the rotating pressing roll  140 , and the driven roll  150  is driven to rotate by the rotating heating roll  130 . When conveying the sheet member P while sandwiching the sheet member P onto which the toner image is transferred, the heating roll  130  and the pressing roll  140  fix the toner image onto the sheet member P. 
     -Others- 
     As illustrated in  FIG. 4 , the main heating unit  120  includes a support member  156  that supports the pressing roll  140 , and a biasing member  158  that biases the pressing roll  140  toward the heating roll  130  side via the support member  156 . 
     A pair of support members  156  is disposed so as to rotatably support the pair of shaft portions  148  of the pressing roll  140  from below. 
     Biasing members  158  in a pair are compression springs, and are disposed on an opposite side of the support members  156  than the shaft portions  148 . 
     In this configuration, the pair of biasing members  158  bias the pressing roll  140  toward the heating roll  130  side, so that the pressing roll  140  presses the sheet member P toward the heating roll  130 . Then, as illustrated in  FIG. 2 , a portion of the heating roll  130  that is biased by the pressing roll  140  is deformed, and the nip portion N that is a region where the heating roll  130  and the pressing roll  140  are in contact with each other is formed. 
     (Operation of Configuration of Main Part) 
     Next, an operation of the image forming device  10  will be described in comparison with an image forming device  510  according to a comparative embodiment. First, with respect to a configuration of the image forming device  510  according to the comparative embodiment, portions different from those of the image forming device  10  will be described mainly. Note that with respect to an operation of the image forming device  510 , portions different from those of the image forming device  10  will also be described mainly. 
     [Image Forming Device  510 ] 
     The image forming device  510  includes the accommodating portion  50 , the discharge portion  52 , the image forming unit  12 , the conveying mechanism  60 , the reversing mechanism  80 , a fixing device  600 , and the cooling unit  90 . As illustrated in  FIG. 10 , the fixing device  600  includes a preheating unit  602  that heats the sheet member P being conveyed in a state of not being in contact with the sheet member P, and the main heating unit  120 . 
     In this configuration, the preheating unit  602  heats the sheet member P in a non-contact state from an unfixed toner image side. In the case of double-sided printing, before the main heating unit  120  heats the sheet member P, the temperature of the first surface of the sheet member P is equal to or higher than the softening point of the toner since the preheating unit  602  heats the sheet member P having a toner image transferred onto the second surface thereof. 
     (Operation of Image Forming Devices  10 ,  510 ) 
     In the image forming device  10  illustrated in  FIG. 9 , the conveying mechanism  60  conveys the sheet member P to the secondary transfer position NT, and at the secondary transfer position NT, the transfer belt  31  and the facing roll  36  convey the sheet member P while sandwiching the sheet member P. Accordingly, a toner image on a front surface of the transfer belt  31  is transferred onto the first surface (=front surface) of the sheet member P. 
     Thereafter, while the conveying mechanism  60  conveys the sheet member P with both sheet surfaces thereof facing in the vertical direction, the preheating units  102  and  602  illustrated in  FIGS. 1 and 10  heat the sheet member P in a non-contact state from the first surface side (=unfixed toner image side) of the sheet member P being conveyed by the conveying mechanism  60 . When the preheating unit  102  illustrated in  FIG. 1  heats the sheet member P, the fan  172  blows air toward the second surface (=back surface) of the sheet member P. Accordingly, the conveyance posture of the sheet member P in a state of being heated by the preheating unit  102  is stabilized. 
     Further, the main heating units  120  illustrated in  FIGS. 1 and 10  fix the toner image onto the first surface of the sheet member P when the sheet member P is conveyed in a sandwiched manner by the heating roll  130  and the pressing roll  140 . In addition, the reversing mechanism  80  receives the sheet member P having the toner image fixed onto the first surface thereof from the main heating unit  120 , conveys the sheet member P, and reverses the front and back of the sheet member P. Further, the conveying mechanism  60  receives the sheet member P whose front and back are reversed from the reversing mechanism  80  and conveys the sheet member P. 
     The conveying mechanism  60  conveys the sheet member P again to the secondary transfer position NT, and at the secondary transfer position NT, the transfer belt  31  and the facing roll  36  convey the sheet member P while sandwiching the sheet member P. Accordingly, a toner image on the front surface of the transfer belt  31  is transferred onto the second surface (=back surface) of the sheet member P. 
     Thereafter, while the conveying mechanism  60  conveys the sheet member P with both sheet surfaces thereof facing in the vertical direction, the preheating units  102  and  602  heat the sheet member P in a non-contact state from the second surface side (=unfixed toner image side) of the sheet member P being conveyed by the conveying mechanism  60 . When the preheating unit  102  illustrated in  FIG. 1  heats the sheet member P, the fan  172  of the blowing unit  170  blows air toward the first surface (=front surface) of the sheet member P. Accordingly, a posture of the sheet member P in a state of being heated by the preheating units  102  and  602  is stabilized. 
     When the preheating units  102  and  602  heat the sheet member P having the image transferred onto the second surface thereof, the conveying mechanism  60  conveys the sheet member P with an image forming region on the first surface of the sheet member P in a state of not being in contact with other members. 
     Further, the main heating unit  120  fixes the toner image onto the second surface of the sheet member P when the sheet member P is conveyed in a sandwiched manner by the heating roll  130  and the pressing roll  140 . In this way, the toner image on the first surface is sandwiched twice by the heating roll  130  and the pressing roll  140 . In addition, the cooling unit  90  cools the sheet member P having the toner images fixed onto both surfaces thereof, and discharges the sheet member P to the discharge portion  52 . 
     Here, before the main heating unit  120  heats the sheet member P, the temperature of the first surface of the sheet member P becomes equal to or higher than the softening point of the toner since the sheet member P having the toner image transferred onto the second surface thereof is heated by the preheating unit  602  illustrated in  FIG. 10 . In other words, since the preheating unit  602  heats the sheet member P having the toner image transferred onto the second surface thereof, the temperature of the first surface at the position S 01  illustrated in  FIG. 2  becomes equal to or higher than the softening point of the toner. As described above, when the temperature of the first surface at the position S 01  is equal to or higher than the softening point of the toner, the sheet member P is heated by the main heating unit  120  in a state where the temperature of the first surface is equal to or higher than the softening point of the toner. 
     In contrast, when heating the sheet member P having the toner image transferred onto the second surface thereof, the preheating unit  102  illustrated in  FIG. 1  heats the sheet member P so that the temperature of the first surface of the sheet member P is lower than the softening point of the toner before the sheet member P is heated by the main heating unit  120 . In other words, when heating the toner image transferred onto the second surface, the preheating unit  102  heats the sheet member P such that the temperature of the first surface at the position S 01  illustrated in  FIG. 2  is lower than the softening point of the toner. As described above, since the temperature of the first surface at the position S 01  is lower than the softening point of the toner, the sheet member P is heated by the main heating unit  120  in a state where the temperature of the first surface is lower than the softening point of the toner. 
     -Evaluation of Glossiness- 
     Next, since the glossiness (=gloss) of the toner images output by the image forming devices  10  and  510  is evaluated, this evaluation will be described. Specifically, toner images were formed on both surfaces of the sheet member P, and a difference between the glossiness of the first surface and the glossiness of the second surface was evaluated. In this evaluation, a toner having a softening point temperature of 75° C. was used. 
     1. Evaluation Specifications (see  FIG. 3 ) 
     (a) In Example 1, the preheating unit  102  heated, from a second surface side, the sheet member P having a toner image transferred onto a second surface thereof such that a temperature of a first surface thereof at the position S 01  was 60° C. and a temperature of the second surface was 65° C. 
     (b) In Example 2, the preheating unit  102  heated, from a second surface side, the sheet member P having a toner image transferred onto a second surface thereof such that a temperature of a first surface thereof at the position S 01  was 65° C. and a temperature of the second surface was 70° C. 
     (c) In Example 3, the preheating unit  102  heated, from a second surface side, the sheet member P having a toner image transferred onto a second surface thereof such that a temperature of a first surface thereof at the position S 01  was 70° C. and a temperature of the second surface was 75° C. 
     (d) In Comparative Example 1, the preheating unit  602  heated, from a second surface side, the sheet member P having a toner image transferred onto a second surface thereof such that a temperature of a first surface thereof at the position S 01  was 80° C. and a temperature of the second surface thereof was 85° C. 
     (e) In Comparative Example 2, the preheating unit  602  heated, from a second surface side, the sheet member P having a toner image transferred onto a second surface thereof such that a temperature of a first surface thereof at the position S 01  was 90° C. and a temperature of the second surface thereof was 95° C. 
     (f) In Comparative Example 3, the sheet member P having a toner image transferred onto a second surface thereof was heated by the preheating unit such that a temperature of a first surface thereof at the position S 01  was 75° C. and a temperature of the second surface thereof was 70° C. In Comparative Example 3, differently from Examples 1 to 3 and Comparative Examples 1 and 2, the preheating unit heated, from a first surface side, the sheet member P having the toner image transferred onto the second surface thereof. That is, the preheating unit is disposed below the sheet member P being conveyed. 
     The details other than those described above were all similar to each other. 
     2. Evaluation Method 
     As the sheet member P, an OS coated paper of A4 size (manufactured by Fuji Xerox Co., Ltd., basis weight: 127.9 g/m 2 ) was used. Then, a black solid image (an image of 100% black area coverage) was formed on both surfaces of the sheet member P. 
     The surface temperature of the heating roll  130  was set to 200° C., and pressure at the nip portion N where the heating roll  130  and the pressing roll  140  were in contact with each other was set to 250 KPa. 
     3. Evaluation Items 
     The difference (=gloss difference) between the glossiness of the first surface and the glossiness of the second surface of the sheet member P, which was output, was evaluated. The glossiness was evaluated using a gloss meter (AG-4430 manufactured by BYK-Gardner). Specifically, this gloss meter was used to perform measurement at an incident angle of 60 degrees in a specular gloss measurement method (JIS Z 8741), and a measured value was defined as the glossiness. 
     4. Evaluation Criteria and Evaluation Results 
     When a difference between glossiness of a toner image on a first surface and glossiness of a toner image on a second surface is large even though the toner images are formed by the same image forming device, a user feels uncomfortable. In this evaluation, when the difference between the glossiness of the toner image on the first surface and the glossiness of the toner image on the second surface is 10 or less, it is considered that the user does not feel uncomfortable, and the evaluation is “good”; when the difference between the glossiness of the toner image on the first surface and the glossiness of the toner image on the second surface is larger than 10, the evaluation is “poor”. 
     The evaluation results are shown in a table of  FIG. 3 . As shown in the table of  FIG. 3 , in Examples 1 to 3 in which the temperature of the first surface was lower than the softening point of the toner, the evaluation result was “good”. In contrast, in Comparative Examples 1 to 3 in which the temperature of the first surface was equal to or higher than the softening point of the toner, the evaluation result was “poor”. 
     5. Consideration 
     In the case of double-sided printing, the toner image on the first surface is sandwiched twice by the heating roll  130  and the pressing roll  140 , and the toner image on the second surface is sandwiched once by the heating roll  130  and the pressing roll  140 . 
     Here, in Comparative Examples 1 to 3, the temperature of the first surface at the position S 01  is equal to or higher than the softening point of the toner. Therefore, when the toner image formed on the first surface is sandwiched by the heating roll  130  and the pressing roll  140  for the second time, surface roughness of a surface of the toner image is reduced than that in a case where the toner image is sandwiched only once, and the glossiness is improved. 
     On the other hand, in Examples 1 to 3, the temperature of the first surface at the position S 01  is lower than the softening point of the toner. Therefore, when the toner image formed on the first surface is sandwiched by the heating roll  130  and the pressing roll  140  for the second time, the occurrence that the surface roughness of the surface of the toner image is reduced and the glossiness is improved is suppressed. 
     Therefore, as shown in the table of  FIG. 3 , in Comparative Examples 1 to 3, the difference between the glossiness of the first surface and the glossiness of the second surface was large, and the evaluation result was considered to be “poor”. On the other hand, in Examples 1 to 3, the difference between the glossiness of the first surface and the glossiness of the second surface was small and the evaluation result was considered to be “good”. 
     (Summary) 
     As described above, when heating the sheet member P having the toner image transferred onto the second surface thereof, the preheating unit  102  heats the sheet member P so that the temperature of the first surface of the sheet member P is lower than the softening point of the toner before the sheet member P is heated by the main heating unit  120 . Therefore, as seen from the evaluation results described above, the difference between the glossiness of the toner image on the first surface and the glossiness of the toner image on the second surface is smaller than that in the case of using the image forming device  510  according to the comparative embodiment. 
     The preheating unit  102  heats the sheet member P in a non-contact state from the unfixed toner image side. That is, when heating the sheet member P having the toner image transferred onto the second surface thereof, the preheating unit  102  heats the sheet member P from the second surface side. Therefore, the temperature of the second surface of the sheet member P onto which the unfixed toner image is transferred is higher than the temperature of the first surface of the sheet member P onto which the toner image is fixed. 
     In addition, since the temperature of the second surface of the sheet member P onto which the unfixed toner image is transferred is higher than the temperature of the first surface, the glossiness of the toner image on the second surface is improved as compared with a case where the temperature of the second surface is lower than the temperature of the first surface. 
     When the sheet member P is to be heated by the preheating unit  102 , the conveying mechanism  60  conveys the sheet member P with the sheet surface thereof facing in the vertical direction. Further, the preheating unit  102  heats the sheet member P from above the sheet member, and the fan  172  blows air to the sheet member P from below the sheet member P. As described above, when the sheet member P having the toner image transferred onto the second surface thereof is heated by the preheating unit  102 , the fan  172  blows air to the sheet member P from the first surface side of the sheet member P. Therefore, the temperature of the first surface is easily lower than the softening point of the toner, as compared with a case where air on the first surface side is stagnant. 
     When the sheet member P is to be heated by the preheating unit  102 , the conveying mechanism  60  conveys the sheet member P with the sheet surface thereof facing in the vertical direction. Further, the preheating unit  102  heats the sheet member P from above the sheet member, and the fan  172  blows air to the sheet member P from below the sheet member P. Therefore, the conveyance posture of the sheet member P being conveyed is stabilized as compared with a case where the sheet member P is conveyed in a state of being bent by gravity. 
     In addition, since the conveyance posture of the sheet member P is stabilized, a temperature of the rear end portion of the sheet member P is prevented from being lower than a temperature of the leading end portion of the sheet member P as compared with a case where the sheet member P is conveyed in a state of being bent by gravity. 
     When the preheating unit  102  heats the sheet member P having the image transferred onto the second surface thereof, the conveying mechanism  60  conveys the sheet member P with the image forming region on the first surface of the sheet member P in a state of not being in contact with other members. Therefore, as compared with a case where the image forming region on the first surface of the sheet member P is in contact with another member, occurrence of a difference in temperature in one sheet member P is suppressed, and thus a difference in glossiness of the toner image formed on the first surface of the sheet member P occurs. 
     When the sheet member P is to be heated by the preheating unit  102 , the conveying mechanism  60  conveys the sheet member P with the sheet surface thereof facing in the vertical direction. In addition, the preheating unit  102  heats the sheet member P from above the sheet member. Therefore, as compared with a case where the conveying mechanism conveys the sheet member P with the sheet surface thereof being directed in a horizontal direction and the preheating unit heats the sheet member P from the horizontal direction, hot air generated by the preheating unit is suppressed from rising and escaping from between the sheet member P and the preheating unit. 
     In the above-described exemplary embodiment, the preheating unit  102  heats the sheet member P from the unfixed toner image side in a non-contact state, and alternatively, the preheating unit may heat the sheet member P in a state of being in contact with the sheet member P, for example. However, in this case, an effect obtained when the preheating unit  102  heats the sheet member P in a non-contact state is not obtained. 
     Although not particularly described in the above-described exemplary embodiment, a temperature detection member may be provided at the position S 01 , and the output of the preheating unit  102  may be adjusted based on a detection result thereof. 
     Although not particularly described in the above-described exemplary embodiment, a cooling member may be provided that cools the first surface of the sheet member P before the toner image is fixed onto the second surface by the main heating unit  120 . 
     In the above-described exemplary embodiment, when the preheating unit  102  heats the sheet member P having the image transferred onto the second surface thereof, the conveying mechanism  60  conveys the sheet member P with the image forming region on the first surface of the sheet member P in a state of not being in contact with other members, and alternatively, the image forming region on the first surface may contact with other members. However, in this case, an effect produced by the non-contact state is not produced. 
     When the temperature of the first surface at the position S 01  is lower than the softening point of the toner, the preheating unit  102  may cause the temperature of the second surface to be lower than the softening point of the toner, or to be equal to or higher than the softening point of the toner. However, in order to easily fix the toner onto the sheet member P with the main heating unit  120 , the temperature of the second surface may be equal to or higher than the softening point of the toner. 
     It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.