Patent Publication Number: US-2011076067-A1

Title: Fixing apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO THE RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2009-226422 which was filed on Sep. 30, 2009. The entire discloser of Japanese Patent Application No. 2009-226422 is hereby incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to fixing apparatuses and image forming apparatuses that handle electrophotography. 
     2. Related Art 
     A fixing device, in an image forming apparatus, that includes a first fixing unit fixing an image transferred onto a transfer material, a second fixing unit that fixes after the first fixing, and a bypass transport path in which the transfer material that has passed through the first fixing unit does not pass through the second fixing unit, has been disclosed in the past. Japanese Patent Application Publication No. 2006-308889 discloses such fixing device. 
     In addition, an image forming apparatus that includes a first fixing unit and a second fixing unit and that adjusts the temperature of a heating roller of the first fixing unit based on transfer material information has been disclosed as well. Japanese Patent Application Publication No. 2008-90115 discloses such image forming apparatus. 
     However, the image forming apparatus according to Japanese Patent Application Publication No. 2006-308889 also includes a curl application unit that corrects curls arising due to heat contraction of the toner and the like and a flapper that switches transport paths. Consequently, the image forming apparatus increases in the size and complexity of the apparatus. Furthermore, there have been situations where rollers, the flapper, and so on have made contact with the image surface immediately after the fixing, when the image surface is still in a high-temperature state. This results in having different glossiness in the areas of the image where the contact was made from the rest of the image; the quality of the image has thus dropped as a result. 
     Meanwhile, with the image forming apparatus according to Japanese Patent Application Publication No. 2008-90115, adjusting the temperature of the fixing device takes time, particularly when decreasing the temperature thereof. Consequently, the productivity of the apparatus drops. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a fixing apparatus for reducing the influence of heat from a second fixing roller on the surface of an image while employing a simple structure, and an image forming apparatus capable of forming a favorable image. 
     A fixing apparatus for forming an image on a transfer material is provided to solve the above mentioned problems. 
     The fixing apparatus includes first and second fixing roller units, a suction guide, and a movement member. The first fixing roller unite includes a first heating roller that heats the transfer material bearing the image and a first pressure roller that is in contact with the first heating roller. 
     The suction guide guides the transfer material. The suction guide includes a suction surface that vertically upwardly sucks the transfer material after being heated by the first fixing roller unit with a surface of the transfer material that bears the image facing vertically down. 
     The second fixing roller unit fixes the transfer material after being guided by the suction guide. The second fixing roller unit includes a second heating roller that heats the transfer material, and a second pressure roller that is disposed above the second heating roller in the vertical direction and is in contact with the second heating roller. 
     The movement member causes the second pressure roller to move vertically upwardly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings. The same reference numbers apply to similar elements. 
         FIG. 1  is a diagram illustrating a first embodiment of an image forming apparatus. 
         FIG. 2  is a diagram illustrating an example of the vicinity of a second fixing roller pair of a fixing unit according to the first embodiment. 
         FIG. 3  is a diagram illustrating a state in which a second pressure roller in a second fixing roller pair has moved upward in the vertical direction, according to the first embodiment. 
         FIG. 4  is a cross-sectional view taken perpendicular to the axial direction of a fixing unit that is in the state shown in  FIG. 2 . 
         FIG. 5  is a cross-sectional view taken perpendicular to the axial direction of a fixing unit that is in the state shown in  FIG. 3 . 
         FIG. 6  is a cross-sectional view illustrating an example of the vicinity of a fixing unit according to a second embodiment. 
         FIG. 7  is a cross-sectional view illustrating a state in which a second pressure roller in a second fixing roller pair has moved upward in the vertical direction, according to the second embodiment. 
         FIGS. 8A to 8C  are diagrams illustrating examples in which the shape of a fixing unit suction surface in a fixing unit suction guide has been changed, according to the second embodiment. 
         FIG. 9  is a diagram illustrating a fixing unit suction guide according to a third embodiment. 
         FIG. 10  is a plan view illustrating the fixing unit suction guide according to the third embodiment. 
         FIG. 11  is a diagram illustrating a fixing unit suction guide according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the invention will now be described in terms of the explanatory embodiment with reference to the drawings. 
       FIG. 1  is a diagram illustrating the primary constituent elements of which an image forming apparatus according to a first embodiment of the invention is configured. Developing units  30 Y,  30 M,  30 C, and  30 K serving as developing units are disposed in a lower portion of the image forming apparatus, opposing an intermediate transfer belt  40  and serving as a transfer medium. The developing units  30 Y,  30 M,  30 C, and  30 K are disposed in a central portion of the image forming apparatus. A secondary transfer unit  60  serving as a transfer unit, a fixing unit  90  serving as a fixing unit, and so on are disposed in an upper portion of the image forming apparatus. In particular, the fixing unit  90  is configured above the intermediate transfer belt  40 , and thereby makes it possible to reduce the installation footprint of the image forming apparatus as a whole. 
     First, units surrounding photosensitive members will be described. Photosensitive members  10 Y,  10 M,  10 C, and  10 K, are provided. Each serving as a latent image bearing member and each configured of a cylindrically-shaped member on the outer circumferential surface of which a photosensitive layer such as an amorphous silicon photosensitive member is formed. And the following elements are disposed in the respective surrounding areas thereof. In the rotational direction of the outer circumference of the photosensitive members  10 Y,  10 M,  10 C, and  10 K, the elements are: corona charging units  11 Y,  11 M,  11 C, and  11 K; exposure units  12 Y,  12 M,  12 C, and  12 K; developing rollers  20 Y,  20 M,  20 C, and  20 K serving as developer holding members for the developing units  30 Y,  30 M,  30 C, and  30 K; first photosensitive member squeeze rollers  13 Y,  13 M,  13 C, and  13 K; second photosensitive member squeeze rollers  13 Y′,  13 M′,  13 C′, and  13 K′; primary transfer units  50 Y,  50 M,  50 C, and  50 K; discharge units (not shown); and photosensitive member cleaning blades  18 Y,  18 M,  18 C, and  18 K. Note that in an image formation process, elements disposed earlier in the order from the corona charging units  11 Y,  11 M,  11 C, and  11 K to the photosensitive member cleaning blades  18 Y,  18 M,  18 C, and  18 K are defined as being upstream from elements disposed later in the stated order. 
     With the units surrounding the photosensitive members, first, the photosensitive members  10 Y,  10 M,  10 C, and  10 K are uniformly charged by the corona charging units  11 Y,  11 M,  11 C, and  11 K, which are disposed further upstream in the rotational direction of the photosensitive members  10 Y,  10 M,  10 C, and  10 K than nip portions formed between the photosensitive members  10 Y,  10 M,  10 C, and  10 K and the developing rollers  20 Y,  20 M,  20 C,  20 K. Then, the exposure units  12 Y,  12 M,  12 C, and  12 K, which are disposed downstream from the corona charging units  11 Y,  11 M,  11 C, and  11 K and upstream from nip portions formed between the photosensitive members  10 Y,  10 M,  10 C, and  10 K and the developing rollers  20 Y,  20 M,  20 C, and  20 K carry out exposure based on inputted image signals to form electrostatic latent images by irradiating the surfaces of the charged photosensitive members  10 Y,  10 M,  10 C, and  10 K. 
     Next, the developing units  30 Y,  30 M,  30 C, and  30 K will be described. In the developing units  30 Y,  30 M,  30 C, and  30 K, augers  34 Y,  34 M,  34 C, and  34 K that agitate and transport liquid developers of various colors held within developer reservoirs  31 Y,  31 M,  31 C, and  31 K supply the liquid developers to anilox rollers  32 Y,  32 M,  32 C, and  32 K serving as developer supply members. Next, the anilox rollers  32 Y,  32 M,  32 C, and  32 K apply the liquid developers of the respective colors to the developing rollers  20 Y,  20 M,  20 C, and  20 K. The amount of the liquid developers having been regulated by regulating blades  33 Y,  33 M,  33 C, and  33 K. 
     The liquid developers held on the developing rollers  20 Y,  20 M,  20 C, and  20 K are put into a state of compaction by compaction corona generators  22 Y,  22 M,  22 C, and  22 K. Then, the electrostatic latent images formed upon the photosensitive members  10 Y,  10 M,  10 C, and  10 K are developed and toner images are formed upon the photosensitive members  10 Y,  10 M,  10 C, and  10 K. Liquid developer that has remained on the developing rollers  20 Y,  20 M,  20 C, and  20 K is cleaned off by developing roller cleaning blades  21 Y,  21 M,  21 C, and  21 K. 
     The liquid developer held in the developer reservoir  31 Y is a non-volatile liquid developer, which is non-volatile at normal temperatures, and which has a high concentration and high viscosity rather than a volatile liquid developer that uses Isopar (an Exxon brand) as its carrier, which is volatile at normal temperatures. The volatile liquid developer with Isopar has a low concentration (approximately 1-3 wt %), and has a low viscosity, as it has generally been used in the past. In other words, the liquid developer in the invention is a high-viscosity liquid developer (that is, a viscoelasticity of approximately 30 to 300 mPa·s at a shear rate of 1000 (1/s) at 25° C., measured using a HAAKE RheoStress RS600) with a toner solid content concentration of approximately 15 to 25%, in which solid particles of a colorant such as a pigment having an average particle diameter of 1 μm are dispersed within a thermoplastic resin and are added to a liquid carrier such as an organic carrier, silicon oil, mineral oil, or cooking oil along with a dispersant. 
     The toner images formed upon the photosensitive members  10 Y,  10 M,  10 C, and  10 K are squeezed by the first photosensitive member squeeze rollers  13 Y,  13 M,  13 C, and  13 K and the second photosensitive member squeeze rollers  13 Y′,  13 M′,  13 C′, and  13 K′. 
     The intermediate transfer belt  40  is a belt, formed of a seamless elastic member such as rubber, which is stretched across a belt driving roller  41  and a tension roller  42 . The intermediate transfer belt  40  is rotationally driven by the belt driving roller  41  while making contact with the photosensitive members  10 Y,  10 M,  10 C, and  10 K at the primary transfer units  50 Y,  50 M,  50 C, and  50 K. The primary transfer units  50 Y,  50 M,  50 C, and  50 K form a full-color toner image by sequentially transferring the developed toner images of the stated colors upon the photosensitive members  10 Y,  10 M,  10 C, and  10 K onto the intermediate transfer belt  40  by superimposing the toner images on one another. The primary transfer units  50 Y,  50 M,  50 C, and  50 K use the positions where the intermediate transfer belt  40  makes contact with the photosensitive members  10 Y,  10 M,  10 C, and  10 K. The intermediate transfer belt  40  is located between the photosensitive members  10 Y,  10 M,  10 C, and  10 K and primary transfer rollers  51 Y,  51 M,  51 C, and  51 K as the transfer positions. 
     Liquid developer that has remained on the photosensitive members  10 Y,  10 M,  10 C, and  10 K after passing through the primary transfer units  50 Y,  50 M,  50 C, and  50 K is removed by using a discharge unit (not shown), the photosensitive member cleaning blades  18 Y,  18 M,  18 C, and  18 K, and so on. 
     Note that the disposition order of the photosensitive members, the elements of the developing units, and so on with respect to the Y, M, C, and K colors is not limited to the order illustrated in  FIG. 1 . The order may be set as desired. 
     Next, the transfer of a toner image from the intermediate transfer belt  40  to a transfer material S will be described. 
     The transfer material S is supplied to the image forming apparatus by a paper supply unit. The transfer material S, which has been set in a first paper supply tray  101   a  or a second paper supply tray  101   b , is transported to a paper supply transport path La at a predetermined timing, one respective sheet at a time, by a first paper supply roller  102   a  or a second paper supply roller  102   b . Meanwhile, in a transfer material transport path Lab, the transfer material S is transported to a secondary transfer position by a pair of gate rollers  104  and  104 ′ and a transfer material guide  105 . 
     The secondary transfer unit  60  includes a secondary transfer roller  61  serving as a transfer member, and a secondary transfer roller cleaning blade  85  that cleans the secondary transfer roller  61 . The secondary transfer roller  61  rotates along with the belt driving roller  41  in the direction indicated by the arrow, and a transfer bias is applied thereto. 
     The leading edge of the transfer material S transported by the transfer material guide  105  is caught by a gripper  64  serving as a transfer material catching unit provided within a concave portion  63  extending in the axial direction of the secondary transfer roller  61 , and a gripper support portion  65  serving as a transfer material catching unit receiving portion on which the gripper  64  is mounted; thus the transfer material S is positioned relative to the secondary transfer roller  61  and moves with certainty toward a transfer nip as the secondary transfer roller  61  rotates. 
     The toner image on the intermediate transfer belt  40  is transferred to the transfer material S, which is paper, film, cloth, or the like, at the transfer nip. When a catching portion of the gripper  64  and the leading edge of the transfer material S pass through the transfer nip, the gripper  64  commences movement in a direction away from a claw support member  65 , and the leading edge of the transfer material S is released as a result. Then, a protruding claw  79  is set to a protruding position as the secondary transfer roller  61  rotates further, and thus the transfer material S separates from the secondary transfer roller  61 . 
     Meanwhile, the transfer material S that was released from the gripper  64  is lightly pressed against the secondary transfer roller  61  by air blown from a blowing unit  400  that expels air, in the direction indicated by an arrow A, from an opening portion  402  in a housing unit  401 . The air is expelled through the operation of an airflow generation unit  405 ; the transfer material S is also pressed in a direction away from the secondary transfer roller  61  by the protruding claw  79 . 
     In this manner, the transfer material S pinched by the nip portion between the belt driving roller  41  and the secondary transfer roller  61  moves to a transfer material transport unit  200 , in the order from the leading edge to the trailing edge, due to further rotation of the belt driving roller  41  and the secondary transfer roller  61 . In other words, the portion of the transfer material S for which transfer is complete separates while the toner image on the intermediate transfer belt  40  undergoes the secondary transfer onto the transfer material S. 
     According to this configuration, it is possible to prevent the trailing edge of the transfer material S from making contact with the intermediate transfer belt  40  or the like and the image formed thereupon from being soiled, when the trailing edge of the transfer material S is discharged from a secondary transfer nip. Note that the air blowing performed by the blowing unit  400  may be omitted, if where the transfer material S has a low elastic restitution force and is flimsy. 
     Next, transport of the post-secondary transfer transfer material S will be described. 
     A first suction device  210 , a transfer material transport device  230 , and a second suction device  270  are disposed in that order, as the transfer material transport unit  200 , downstream from the secondary transfer unit  60  in the transfer material transport path Lab, and serve to transport the transfer material S to the fixing unit  90 . 
     The post-secondary transfer material S is held on a suction surface  212  of a housing portion  211 , without falling, as a result of a suction force B from the suction surface  212  that is generated through the operation of an airflow generation unit  215  of the first suction device  210 , and is transported along the suction surface  212 , by the force of a feeding operation, from the side of the secondary transfer unit  60 . 
     As a result of the force of the feeding operation from the side of the secondary transfer unit  60 , the transfer material S transported along the suction surface  212  of the first suction device  210  reaches the side of the transfer material transport device  230 . Next, the transfer material S is held against a transport surface by a suction force C, from a suction surface  232  of a housing portion  231 , that is generated through the operation of an airflow generation unit  235  of the transfer material transport device  230 . Along with this, the transfer material S advances along the transport surface toward the fixing unit  90  as a result of a transfer material transport member  250 , which is wound upon a transfer material transport member driving roller  251  and transfer material transport member tension rollers  252  and  253 , while the transfer martial transport member driving roller  251  performs moving operations due to driving force exerted by the transfer material transport member driving roller  251 . 
     The transfer material S that has been transported along the transport surface of the transfer material transport device  230  is sucked as a result of a suction force D, from a suction surface  272  of a housing portion  271 . The suction force D is generated through operations performed by an airflow generation unit  275  of the second suction device  270 . And thus the transfer material S is transported. 
     Next, fixing of the toner image onto the transfer material S will be described. 
     In the fixing unit  90 , a single-color toner image, a full-color toner image, or the like that has been transferred onto the transfer material S, such as paper or the like, is melted and fixed to the transfer material S, such as paper or the like. 
     The fixing unit  90  includes a first fixing roller pair  91  and a second fixing roller pair  92  located downstream from the first fixing roller pair  91 . The first fixing roller pair  91  includes a first heating roller  91   a  that has an internal heater and a first pressure roller  91   b  that is biased at a predetermined pressure toward the first heating roller  91   a . The transfer material S enters into a nip formed by these rollers, where a single-color toner image, a full-color toner image, or the like transferred onto the transfer material S is melted and fixed to the transfer material S, which is paper or the like. Likewise, the second fixing roller pair  92  includes a second heating roller  92   a  that has an internal heater and a second pressure roller  92   b  that is biased at a predetermined pressure toward the second heating roller  92   a . The transfer material S enters into a nip formed by these rollers, where the single-color toner image, the full-color toner image, or the like transferred onto the transfer material S is melted and fixed with further strength to the transfer material S, which is paper or the like. 
     Furthermore, a fixing unit suction guide  300  is disposed between the first fixing roller pair  91  and the second fixing roller pair  92  of the fixing unit  90 . The fixing unit suction guide  300  includes a housing portion  301 , a fixing unit suction surface  302  disposed on the transfer material transport path Lab side of the housing portion  301 , and an airflow generation unit  305  disposed in the housing portion  301  on the side opposite to the transfer material transport path Lab. The transfer material S is held on the fixing unit suction surface  302  without falling by a suction force E, from the fixing unit suction surface  302  of the housing portion  301 . The suction force E is generated through the operation of the airflow generation unit  305 , and is transported along the fixing unit suction surface  302 , while being held thereon, toward the second fixing roller pair  92  by the force of the feeding operation from the first fixing roller pair  91 . 
     By using the fixing unit suction guide  300  in this manner, the amount of moisture contained in the transfer material S is rapidly reduced by subjecting the transfer material S to a large amount of air. This can result in correcting curling. Accordingly, it is no longer necessary to provide a curl roller pair as was done in the past. And thus it is possible to reduce the size of the image forming apparatus. Furthermore, collisions between the leading edge of the transfer material S and the second heating roller  92   a  can be reduced by sucking the transfer material S using the fixing unit suction guide  300 . 
     After the fixing, if it is single-sided printing, the transfer material S is transported along a discharge transport path Lc and is discharged. 
     On the other hand, if it is double-sided printing, the transfer material S is guided by a first flapper  121 , and is transported to a first double-sided transport path Lb 1 . After this, the transport direction of the transfer material S is inverted in a second double-sided transport path Lb 2  having a first inversion transport roller pair  111  and  111 ′, a second inversion transport roller pair  112  and  112 ′, and a third inversion transport roller pair  113  and  113 ′, after which the transfer material S is guided by a second flapper  122  and transported to a third double-sided transport path Lb 3 . 
     In the third double-sided transport path Lb 3 , the transfer material S passes through a curl application roller pair  114  and  114 ′ serving as a curl application unit, and then passes through a first double-sided transport roller pair  115  and  115 ′, a second double-sided transport roller pair  116  and  116 ′, a third double-sided transport roller pair  117  and  117 ′, and a fourth double-sided transport roller pair  118  and  118 ′. Next, the inverted transfer material S is transported from the pair of gate rollers  104  and  104 ′ and the transfer material guide  105  to the transfer material transport path Lab. After this, the transfer material S passes through the fixing unit  90 , is transported to the discharge transport path Lc, and is discharged. 
     Detailed descriptions of the fixing unit  90  will be given hereinafter. 
     The first heating roller  91   a  and the second heating roller  92   a  of this embodiment according to the invention are 60 mm in diameter. The metal cores thereof are 2 mm in thickness, and the material of the metal cores is aluminum, iron, stainless steel, brass, or the like. An elastic layer is formed around the circumference of the respective metal cores. The thickness of the elastic layer is 2 mm, and the material of the elastic layer is silicon rubber, fluorocarbon rubber, urethane rubber, or the like. Furthermore, a delaminating layer is formed around the elastic layer. The delaminating layer is 30 μm thick, and the material of the delaminating layer is PFA, PTFE, FEP, ETFE, or the like. 
     Furthermore, the first pressure roller  91   b  and the second pressure roller  92   b  of this embodiment according to the invention are 60 mm in diameter; the metal cores thereof are 2 mm in thickness, and the material of the metal cores is aluminum, iron, stainless steel, brass, or the like. An elastic layer is formed around the circumference of the respective metal cores. The thickness of the elastic layer is 2 mm, and the material of the elastic layer is silicon rubber, fluorocarbon rubber, urethane rubber, or the like. Furthermore, a delaminating layer is formed around the elastic layer. The delaminating layer is 30 μm thick, and the material of the delaminating layer is PFA, PTFE, FEP, ETFE, or the like. 
     In addition, the first heating roller  91   a  and the second heating roller  92   a  are driven by a driving member (not shown), and the first pressure roller  91   b  and the second pressure roller  92   b  rotate as slaves based on the rotation of the first heating roller  91   a  and the second heating roller  92   a , respectively. 
     Note that if a rigid material such as iron, aluminum, stainless steel, brass, or the like is used for the first pressure roller  91   b  and the second pressure roller  92   b , and a soft material such a rubber layer made from silicon rubber, urethane rubber, or natural rubber, or a sponge layer made from a silicon sponge, a urethane sponge, or a melamine sponge is provided on the first heating roller  91   a  and the second heating roller  92   a , curling in the direction of the image surface caused by heat can be corrected. In addition, it is not necessary for the second heating roller  92   a  and the second pressure roller  92   b  to be separate from each other completely. The rollers may be formed so that the applied pressure is reduced, rather than separating from each other completely. 
       FIG. 2  is a diagram illustrating an example of the vicinity of the second fixing roller pair  92  of the fixing unit  90  according to the first embodiment. 
     Second heating roller anchor plates  94  that are disposed on a base  93  and that support the second heating roller  92   a  in a rotatable state and second pressure roller anchor plates  95  that support the second pressure roller  92   b  in a rotatable state are present in the vicinity of the second fixing roller pair  92 . The second heating roller anchor plates  94  and the second pressure roller anchor plates  95  are linked on one side by a side plate linking shaft  96  so as to be capable of pivoting relative to a plane that contains the axis of the second heating roller  92   a  and the second pressure roller  92   b , and are linked on the other side by a spring  97 . 
     Furthermore, a cam shaft  98  is supported in a rotatable state by through-holes in a cam shaft support member such as a main body case or the like (not shown), and cams  99  are anchored to the cam shaft  98 . The cams  99  make contact with cam contact portions  95   a  in the respective second pressure roller anchor plates  95 . 
     The base  93 , the second heating roller anchor plates  94 , the second pressure roller anchor plates  95 , the side plate linking shaft  96 , the springs  97 , the cam shaft  98 , and the cams  99  configure a movement member. Note that the movement member is not limited to the configuration used in the first embodiment, and may be any mechanism as long as that mechanism moves the second pressure roller  92   b  upwardly in the vertical direction. 
     When the cam shaft  98  rotates in the direction of an arrow R 1  starting from the state shown in  FIG. 2 , the cams  99  also rotate in the direction of the arrow R 1 . Pressure is applied to the cam contact portions  95   a  by the cams  99  rotating, and the second pressure roller anchor plates  95  rotate central to the side plate linking shaft  96  in the direction of an arrow R 2  against the biasing force of the springs  97 . 
       FIG. 3  is a diagram illustrating a state in which the second pressure roller  92   b  of the second fixing roller pair  92  has moved upwardly in the vertical direction, according to the first embodiment. 
     In  FIG. 2 , when the second pressure roller anchor plates  95  rotate in the direction of the arrow R 2 , the second pressure roller  92   b  supported by the second pressure roller anchor plates  95  moves upwardly in the vertical direction, as shown in  FIG. 3 . Note that in  FIG. 3 ,  94   a  indicates concave portions formed so that the second pressure roller  92   b  and the second heating roller anchor plates  94  do not interfere with each other. 
     When the cam shaft  98  rotates in the direction of an arrow R 4  starting from the state shown in  FIG. 3 , the cams  99  also rotate in the direction of the arrow R 4 . The cam contact portions  95   a  move downwardly due to the rotation of the cams  99 , and the second pressure roller anchor plates  95  rotate central to the side plate linking shaft  96  in the direction of an arrow R 5  due to the biasing force of the springs  97 . 
       FIG. 4  is a cross-sectional view taken perpendicular to the axial direction of the fixing unit  90  that is in the state shown in  FIG. 2 , and  FIG. 5  is a cross-sectional view taken perpendicular to the axial direction of the fixing unit  90  that is in the state shown in  FIG. 3 . 
     Next, a working example of the fixing unit  90  according to the first embodiment will be described. In the working example, the movement of the second pressure roller  92   b  is carried out in accordance with the paper type. 
     When printing onto heavy paper, and when performing single-sided printing, the paper passes through the first fixing roller pair  91  and the second fixing roller pair  92 , and the fixing is carried out at a high strength, as shown in  FIG. 4 . Meanwhile, in double-sided printing, when printing onto a first surface of heavy paper, the paper passes through the first fixing roller pair  91  and the second fixing roller pair  92 , as shown in  FIG. 4 . Then, when printing onto a second surface, which is the reverse surface of the first surface, the second pressure roller  92   b  is moved upwardly in the vertical direction, away from the second heating roller  92   a , This movement reduces the pressure, as shown in  FIG. 5 . By reducing the amount of heat applied at a second fixing nip portion between the second pressure roller  92   b  and the second heating roller  92   a  in this manner, the process of melting of the toner that has formed an image due to the applied heat can be slowed. 
     Next, when printing onto light paper, heat has a strong influence, and thus the second pressure roller  92   b  is moved upwardly in the vertical direction, away from the second heating roller  92   a . This movement reduces the pressure, as shown in  FIG. 5 , for both single-sided printing and double-sided printing. By reducing the amount of heat applied at the second fixing nip portion between the second pressure roller  92   b  and the second heating roller  92   a  in this manner, the process of melting of the toner that has formed an image due to the applied heat can be slowed. 
       FIGS. 6 and 7  are diagrams illustrating an image forming apparatus according to a second embodiment.  FIG. 6  is a cross-sectional view illustrating an example of the vicinity of the fixing unit  90  according to the second embodiment, and  FIG. 7  is a cross-sectional view illustrating a state in which the second pressure roller  92   b  of the second fixing roller pair  92  has been moved upwardly in the vertical direction, according to the second embodiment. 
     In the image forming apparatus according to the second embodiment, the end of the fixing unit suction surface  302  of the fixing unit suction guide  300  on the side toward the second fixing roller pair  92  in the transport direction of the transfer material S is located higher in the vertical direction than the second fixing nip portion formed by the second heating roller  92   a  and the second pressure roller  92   b  of the second fixing roller pair  92  making contact with each other. Furthermore, the side of the fixing unit suction surface  302  toward the second fixing roller pair  92  in the transport direction of the transfer material S is located higher in the vertical direction than the side toward the first fixing roller pair  91  in the transport direction of the transfer material S. 
     With the fixing unit suction guide  300  according to the second embodiment, the fixing unit suction surface  302  includes a first fixing unit suction surface  302   a  and a second fixing unit suction surface  302   b . On the side toward the first fixing roller pair  91 , the first fixing unit suction surface  302   a  is formed so as to follow a line that connects the nip portions of the first fixing roller pair  91  and the second fixing roller pair  92 . Meanwhile, the second fixing unit suction surface  302   b  is formed as a slope continuing from the side of the first fixing unit suction surface  302   a  onto the side toward the second fixing roller pair  92 , so that the location of the second fixing unit suction surface  302   b  on the side toward the second fixing roller pair  92  is higher than a highest point P of the second heating roller  92   a  in a vertical direction G. 
     In this manner, part of the fixing unit suction surface  302  is formed as a slope so that the location of the second fixing unit suction surface  302   b  on the side toward the second fixing roller pair  92  is higher than the highest point P of the second heating roller  92   a . Thus, impact of collisions between the leading edge of the transfer material S and the second heating roller  92   a  can be reduced even more in the current embodiment than in the first embodiment. The size of the image forming apparatus can be reduced as well. 
       FIGS. 8A to 8C  are diagrams illustrating examples in which the shape of the fixing unit suction surface  302  of the fixing unit suction guide  300  has been changed, according to the second embodiment.  FIG. 8A  is an example in which the fixing unit suction surface  302  of the fixing unit suction guide  300  has been formed so as to slope upwardly in the vertical direction, from the side toward the first fixing roller pair  91  to the side toward the second fixing roller pair  92 , relative to the transport direction of the transfer material. Meanwhile,  FIG. 8B  is an example in which the fixing unit suction surface  302  slopes from the side toward the first fixing roller pair  91  to the side toward the second fixing roller pair  92  as an upward curve. Finally,  FIG. 8C  is an example in which the fixing unit suction surface  302  is formed from a first fixing unit suction surface  302   a  and a second fixing unit suction surface  302   b  that have slopes of different angles from the side toward the first fixing roller pair  91  to the side toward the second fixing roller pair  92 . 
       FIGS. 9 and 10  are diagrams illustrating a third embodiment.  FIG. 9  is a diagram illustrating the fixing unit suction guide  300  according to the third embodiment, and  FIG. 10  is a plan view illustrating the fixing unit suction guide  300  according to the third embodiment. 
     An image forming apparatus according to the third embodiment includes: a fixing unit transport belt  310  that forms a suction surface  302  and transports the transfer material S that has undergone fixing in the fixing unit  90 ; a driving roller  311  upon which the fixing unit transport belt  310  is wound and that drives the fixing unit transport belt  310 ; and a first slave roller  312 , a second slave roller  313 , and a third slave roller  314 , upon which the fixing unit transport belt  310  is wound. The fixing unit transport belt  310  is wound around the driving roller  311 , the first slave roller  312 , the second slave roller  313 , and the third slave roller  314 , and is rotated in a direction that transports the transfer material S as the result of a driving unit (not shown) driving the driving roller  311 . 
     In addition, holes  310   a  are provided in the fixing unit transport belt  310 , and thus the suction force E acting at the fixing unit suction surface  302  due to airflow generated by the airflow generation unit  305  is not inhibited. 
     In this manner, by providing the fixing unit transport belt  310  in the periphery of the fixing unit suction guide  300 , the transfer material S sticks to the fixing unit transport belt  310  and the transfer material S and the fixing unit transport belt  310  move together; as a result, the transfer material S can be transported from the first fixing roller pair  91  toward the second fixing roller pair  92  in a stable manner. 
       FIG. 11  is a diagram illustrating the fixing unit suction guide  300  according to a fourth embodiment. 
     In the image forming apparatus according to the fourth embodiment, the end of the fixing unit suction surface  302  of the fixing unit suction guide  300  on the side toward the second fixing roller pair  92  in the transport direction of the transfer material S is located higher in the vertical direction than the second fixing nip portion formed by the second heating roller  92   a  and the second pressure roller  92   b  of the second fixing roller pair  92  making contact with each other. Furthermore, the side of the fixing unit suction surface  302  toward the second fixing roller pair  92  in the transport direction of the transfer material S is located higher in the vertical direction than the side toward the first fixing roller pair  91  in the transport direction of the transfer material S. 
     With the fixing unit suction guide  300  according to the fourth embodiment, the fixing unit suction surface  302  includes the first fixing unit suction surface  302   a  and the second fixing unit suction surface  302   b . On the side toward the first fixing roller pair  91 , the first fixing unit suction surface  302   a  is formed so as to follow a line that connects the nip portions of the first fixing roller pair  91  and the second fixing roller pair  92 . Meanwhile, the second fixing unit suction surface  302   b  is formed as a slope from the side of the first fixing unit suction surface  302   a  to the side toward the second fixing roller pair  92 , so that the location of the second fixing unit suction surface  302   b  on the side toward the second fixing roller pair  92  is higher than the highest point P of the second heating roller  92   a  in the vertical direction G. Note that a fourth slave roller  315  is disposed between the first fixing unit suction surface  302   a  and the second fixing unit suction surface  302   b.    
     In this manner, the fixing unit suction surface  302  is formed as a slope so that the location of the second fixing unit suction surface  302   b  on the side toward the second fixing roller pair  92  is higher than the highest point P of the second heating roller  92   a . Thus, impact of collisions between the leading edge of the transfer material S and the second heating roller  92   a  can be reduced even more than in the third embodiment. 
     Note that the shape of the fixing unit suction surface  302  in the fixing unit suction guide  300  according to the fourth embodiment can also be altered as per the examples illustrated in  FIG. 8 . 
     In this manner, according to the fixing unit  90  of this embodiment, the influence of heat from the second heating roller  92   a  on the image surface can be reduced by using a simple configuration. In addition, according to the image forming apparatus of this embodiment, the influence of heat from the second heating roller  92   a  on the image surface can be reduced by using a simple configuration. It is possible to form a favorable image. In addition, the end of the fixing unit suction surface  302  of the fixing unit suction guide  300  on the side toward the second fixing roller pair  92  in the transport direction of the transfer material S is located higher in the vertical direction than a fixing nip portion formed by the second heating roller  92   a  and the second pressure roller  92   b  of the second fixing roller pair  92  making contact with each other, and it is thus possible to further reduce the influence of heat from the second heating roller  92   a  on the image surface. 
     Furthermore, the end of the fixing unit suction surface  302  is located higher in the vertical direction on the side toward the second fixing roller pair  92  in the transport direction of the transfer material S than on the side toward the first fixing roller pair  91  in the transport direction of the transfer material S, and it is thus possible to move the transfer material S in a smooth manner. In addition, because the fixing unit suction surface  302  progresses upwardly and continuously in the vertical direction, the transfer material S can be moved in an even smoother manner. 
     Meanwhile, because the image forming apparatus includes the fixing unit transport belt  310  that forms the suction surface  302  and transports the transfer material S that has undergone fixing in the fixing unit  90 , the driving roller  311  upon which the fixing unit transport belt  310  is wound and that drives the fixing unit transport belt  310 , and the slave rollers  312  to  315  upon which the fixing unit transport belt  310  is wound, the transfer material S sticks to the fixing unit transport belt  310  and the transfer material S and the fixing unit transport belt  310  move together. As a result, the transfer material S can be transported from the first fixing roller pair  91  toward the second fixing roller pair  92  in a stable manner. 
     Furthermore, because the fixing unit transport belt  310  includes the holes  310   a  through which air flows, the transfer material S can be transported in an even more stable manner. 
     GENERAL INTERPRETATION OF TERMS 
     In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Finally, terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.