Abstract:
A fusing unit includes a heat roller and a press roller, which fuse a toner image fused to a recording sheet; a first separating pawl contacting a circumferential surface of the heat roller and separating the recording sheet from the heat roller; second separating pawls that are spaced from the first separating pawl in an axial direction to contact the circumferential surface of the heat roller and separate the recording sheet from the heat roller; a first rotatable guide in the first separating pawl that guides the recording sheet to a sheet discharge guide of an exit feed path while changing its orientation; a second rotatable guide provided in each of the second separating pawls which guides the recording sheet to a sheet discharge guide of an exit feed path while changing an orientation of the recording sheet; and a fusing feed path that reaches the sheet discharge guide.

Description:
[0001]    This application claims priority under 35 U.S.C. 119 to Japanese Patent Application No. 2010-246023, filed on Nov. 2, 2010, which application is hereby incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a fusing unit for an image forming apparatus such as a copying machine, a facsimile machine, or a printer, for example, and more particularly to an improvement of a structure of a separating pawl that forcedly separates a sheet from a circumferential surface of a heat roller. 
         [0004]    2. Description of the Related Art 
         [0005]    In a fusing unit of an image forming apparatus of the related art, the separating pawls are disposed at six points in a circumferential surface of a heat roller. The six separating pawls are constructed by three groups of pairs of separating pawls. In the first group, the separating pawls are disposed at two points on the right and left in the center of the heat roller. In the second group, the separating pawls are disposed outside the separating pawls of the first group. In the third group, the separating pawls are disposed on both side ends of the heat roller. The separating pawls of the third group are pressed against the circumferential surface of the heat roller by a spring having an elastic force larger than that of the separating pawls of the first and second groups. An image forming area of the sheet is separated from the heat roller by the separating pawls of the first and second groups, which are biased by a force that is smaller than that of the separating pawls of the third group, so that the fused image can be prevented from being damaged by the separating pawls. 
         [0006]    In another fusing unit of the related art, the separating pawls are disposed at five points along the circumferential surface of the heat roller. The five separating pawls are constructed by central separating pawls that are disposed at three points in the center of the heat roller and two side separating pawls that are disposed on both side ends of the heat roller. Although the central separating pawls are substantially identical to the separating pawls in a basic configuration, the central separating pawls differs from the separating pawls in a structure of a rear end portion of a guide surface that is continuously provided adjacent to a tip end of the separating pawl. The rear end portion of the guide surface of the central separating pawl is drawn from the rear end portion of the guide surface of the side separating pawl with respect to the sheet. Therefore, the central separating pawl is prevented from providing a large frictional force, and generation of a streak flaw can be prevented in the central portion in a width direction of the sheet. 
         [0007]    In still another fusing unit of the related art, the sheet is guided toward a sheet discharge guide by a rotatable guide provided in the separating pawl. The rotatable guide is constructed by a sprocket. 
         [0008]    Generally, five or six separating pawls are provided in the fusing unit of the image forming apparatus of the related art, and the post-fusing sheet can stably be fed toward the sheet discharge guide near an exit roller because the sheet can be guided by more separating pawls as a sheet size is enlarged. There is no problem in a finishing state of the post-fusing sheet. However, in the case of a small-size sheet such as a postcard, because the sheet is guided by one or two separating pawls provided in the center in the circumferential surface of the heat roller, a large pressure is applied to the sheet, and sometimes a problem is generated in the finishing of the post-fusing sheet. 
         [0009]    Particularly, in the case that a feed path is largely flexed between a nip portion of the fusing unit and the sheet discharge guide, a portion of a length in a feed direction of the small-size sheet such as the postcard is flexed, and thus, a sheet plane is not free from application of a large amount of pressure. In the case that the sheet is guided while an orientation of the sheet is changed by the sprocket provided in the separating pawl, because the pressure applied to the sheet plane is concentrated, a broken-line pattern matched with a pitch of the teeth of the sprocket is formed in a rear surface of the sheet, thereby degrading the finishing state of the sheet. 
         [0010]    For example, when the central separating pawl that separates the small-size sheet is eliminated, the pressure applied to the sheet plane can be reduced to relax a sheet feeding condition, and the same finishing state as the large-size sheet is obtained. However, in fusing a thin and weak sheet, it is difficult to stably feed the sheet, and there is generated a new problem in that a corner portion at a start end edge in the feed direction of the sheet is folded or a wrinkle. 
       SUMMARY OF THE INVENTION 
       [0011]    In order to overcome the problems described above, preferred embodiments of the present invention provide a fusing unit for an image forming apparatus, which can relax a small-size sheet feeding condition to improve a finishing state of the small-size sheet to the same degree as a large-size sheet and stably feed a thin and weak sheet. 
         [0012]    A fusing unit for an image forming apparatus according to a preferred embodiment includes a heat roller and a press roller, which fuse a toner image to a recording sheet. The fusing unit also includes a plurality of separating pawls that are disposed to come into contact with a circumferential surface of the heat roller in order to separate the recording sheet from the heat roller. The separating pawls are disposed at predetermined points in a front and rear direction that corresponds to an axial direction of the heat roller. Rotatable guides that guide the recording sheet passing through a nip portion of the fusing unit to a sheet discharge guide of an exit feed path while an orientation of the recording sheet is changed is provided in each of the separating pawls. In a fusing feed path from the nip portion of the fusing unit to the sheet discharge guide through the rotatable guides, a flexion angle of a central feed path that passes by the rotatable guide of the first separating pawl disposed in the axial direction of the heat roller is larger than a flexion angle of a lateral feed path that passes by the second rotatable guide of the second separating pawl disposed in the axial direction of the heat roller. 
         [0013]    Specifically, assuming that a triangle is defined by the lateral feed path and a straight line connecting the nip portion of the fusing unit and an introduction start end of the sheet discharge guide, a position in which the recording sheet is in contact with the first rotatable guide of the first separating pawl disposed in the axial direction of the heat roller is located inside a vertex of the triangle. 
         [0014]    The sheet discharge guide includes a central sheet discharge guide that is provided opposite a central area in a width direction of the exit feed path and a lateral sheet discharge guide that is provided opposite an area near the central area. Assuming that a triangle is defined by the lateral feed path and a straight line connecting the nip portion of the fusing unit and an introduction start end of the lateral sheet discharge guide, an introduction start end of the central sheet discharge guide is located outside an oblique-side portion on a side of the lateral sheet discharge guide of the triangle. 
         [0015]    In the present preferred embodiment, the flexion angle of the central feed path through which the minimum-size sheet is fed is preferably larger than the flexion angle of the lateral feed path through which the larger-size sheet is fed, so that the degree of the curvature of the minimum-size sheet can be reduced during feeding the minimum-size sheet. Therefore, the minimum-size sheet feeding condition can be relaxed to reduce the pressure applied to the sheet plane, and the same finishing state as the large-size sheet is obtained. Particularly, in a case in which the minimum-size sheet is guided while deflected by the first sprocket provided with teeth, a flexion reactive force of the teeth acting on the sheet plane of the small-size sheet can be significantly reduced and minimized to eliminate the formation of the broken-line pattern matched with the pitch of the teeth in the rear surface of the sheet. Even in fusing the thin and weak sheet, a feed surface of the sheet can stably be guided toward the sheet discharge guide while being deflected by the rotatable guides of the plural separating pawls. Accordingly, the deformations of the sheet plane such as the folding of the corner portion at the start end edge in the feed direction of the sheet and the generation of the wrinkle in the sheet plane, which are caused by the feeding failure, are not generated. 
         [0016]    When the position in which the recording sheet is in contact with the first rotatable guide of the first separating pawl is located inside the vertex of the triangle that is defined by the nip portion, the introduction start end, and the lateral feed path, the flexion angles can be separated from each other only by partially changing a structure of the fusing unit. Specifically, the flexion angles of the feed paths can be separated from each other only by changing a position of the central rotatable guide. Accordingly, the structure change that reduces the small-size sheet feeding condition is minimized, and a cost increase associated with the structural change can be reduced. Because the flexion angles of the feed paths can be separated from each other only by changing the position of the central rotatable guide, advantageously the present preferred embodiment can be applied to existing image forming apparatus. 
         [0017]    When the introduction start end of the central sheet discharge guide is located outside the oblique-side portion on the side of the lateral sheet discharge guide of the triangle that is defined by the nip portion, the introduction start end, and the lateral feed path, the feed paths from the nip portion to the rotatable guides can be matched with each other. The same position in the feed direction of the sheet can be guided while deflected by the rotatable guides. Accordingly, the sheet can stably be fed from the nip portion to the rotatable guides, and particularly the thin and weak sheet can properly be fed. Therefore, deformations of the sheet plane such as folding of the corner portion at the start end edge in the feed direction of the sheet and the generation of the wrinkle in the sheet plane, which are caused by the feeding failure, can more securely be eliminated. 
         [0018]    The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a view illustrating a principle of a fusing feed path in a fusing unit according to a first preferred embodiment of the present invention. 
           [0020]      FIG. 2  is a schematic front view of an image forming apparatus. 
           [0021]      FIG. 3  is a vertical sectional view illustrating the fusing unit and a peripheral structure thereof. 
           [0022]      FIG. 4  is a plan view illustrating alignments of the fusing unit and a separating pawl. 
           [0023]      FIG. 5  is a sectional view taken on a line A-A of  FIG. 4 . 
           [0024]      FIG. 6  is a view illustrating a principle of a fusing feed path according to a second preferred embodiment of the present invention. 
           [0025]      FIG. 7  is a vertical sectional view illustrating the fusing unit of the second preferred embodiment of the present invention and a peripheral structure thereof. 
           [0026]      FIG. 8  is a sectional view taken on a line B-B of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]      FIGS. 1 to 5  illustrate a fusing unit according to a first preferred embodiment of the present invention that is preferably applied to a multi-function peripheral (image forming apparatus) having a copy function and a facsimile function. In the drawings, front and rear, right and left, and up and down are subject to cross arrows illustrated in  FIGS. 2 and 4  and signs of front and rear, right and left, and up and down expressed near the arrows. 
         [0028]    Referring to  FIG. 2 , a multi-function peripheral  1  includes an image forming unit  2 , a main body unit  4  in which a fusing unit  3  is disposed, and an image reader  5  that is located in an upper portion of the main body unit  4 . In the multi-function peripheral  1 , a sheet feed path  8  is provided between a paper cassette  6  disposed in a lower portion of the main body unit  4  and an exit unit  7  disposed in the upper portion of the main body unit  4 . The image forming unit  2  is disposed below the feed path  8  while the fusing unit  3  is disposed above the feed path  8 . Openings and covers that open and close the openings are provided in surrounding walls of the main body unit  4 , which faces the fusing unit  3 . Maintenance of the fusing unit  3  can be performed by opening the covers, and a paper jam can be released in the fusing unit  3  and the feed path  8  around the fusing unit  3 . 
         [0029]    The image forming unit  2  includes a developing unit  10 , a photosensitive drum  11 , and a toner cartridge  12 , and forms an image by transferring a toner image to a sheet fed from the paper cassette  6 . An operational panel  13  including various buttons is provided in a front surface of the image reader  5 , and an Auto-Document Feeder (ADF)  14  is provided on an upper surface of the image reader  5 . The images of a bundled document such as a book can be read while the bundled document is placed on a platen glass of the upper surface of the image reader  5 , and a sheet-like document can be read by passing through the ADF  14 . 
         [0030]    Referring to  FIG. 3 , the fusing unit  3  includes a heat roller  15 , a press roller  16  that is in contact with a circumferential surface of the heat roller  15 , and a plurality of separating pawls  17  and  18  that circumscribe the circumferential surface of the heat roller  15 . The heat roller  15  is preferably an aluminum-alloy tube, and a heater  20  is provided in the heat roller  15 . Front and rear ends of the heat roller  15  are supported on bearings, and the heat roller  15  is rotated in a counterclockwise direction of  FIG. 3  by a motor (not illustrated). 
         [0031]    The press roller  16  includes a roll layer  21  that constitutes a majority of the press roller  16  and a roll axis  22  on which the roll layer  21  is supported. The roll layer  21  is preferably made of rubber or synthetic-resin foam. Vicinities of ends of the roll axis  22  are rotatably supported in a guide frame (not illustrated), the guide frame is guided while being able to reciprocate with respect to the heat roller  15 , and the guide frame is pressed against the heat roller  15  by a press spring (not illustrated) constructed by a coil spring. The press roller  16  is brought into close contact with the heat roller  15  to define a sheet passing nip portion  23  between the rollers  15  and  16 . The sheet on which the toner image is formed by the image forming unit  2  is caused to pass through the nip portion  23  to heat and pressurize the toner image, which allows the toner image to be fused on the sheet. 
         [0032]    The sheet that passes through the nip portion  23  is fed to a fusing feed path  25  that is provided downstream from the fusing unit  3  and the exit unit  7  through an exit feed path  26 . Sheet discharge guides  27  and  28  that include rib walls are provided above and below an introduction start end of the exit feed path  26 , and a feed roller  29  is provided adjacent to an end portion on an upstream side of the upper sheet discharge guide  27 . A pair of exit rollers  30  and  31  is disposed above and below the exit feed path  26 . The feed roller  29  and exit roller  31  rotate to feed the sheet in a direction in which the sheet exits. 
         [0033]    As illustrated in  FIG. 4 , the first separating pawl  17  and the second separating pawls  18  are disposed preferably at five points along the circumferential surface of the heat roller  15  in a front and rear direction. Particularly, one first separating pawl  17  is disposed in the center of the heat roller  15  in the front and rear direction (center axis direction), and each two of four second separating pawls  18  are provided on the front side and the rear side of the heat roller  15 , respectively. As to the sheet that can be used in the multi-function peripheral  1  of the first preferred embodiment, the multi-function peripheral  1  can accommodate anything from a minimum postcard (100×148 mm) to an A3-sheet, a postcard-size sheet S 1  is guided while separated by the central first separating pawl  17 . A B5-size or A4-size sheet S 2  is guided while being separated by the total of three separating pawls of the first separating pawl  17  and the second separating pawls  18  on the first separating pawl  17  and the front side and the rear side of the first separating pawl  17 , and a B4-size or an A3-size sheet S 3  are guided while being separated by the five separating pawls  17  and  18 .  FIG. 4  illustrates the state in which a short edge side of each of the sheets S 1  to S 3  is located above the first separating pawl  17  and the second separating pawl  18 . 
         [0034]    As illustrated in  FIG. 3 , the first separating pawl  17  and the second separating pawl  18  are constructed by plastic moldings each of which integrally includes a boss  34 , a separating arm  35 , and a spring receiver arm  36 . Specifically, each of the separating pawls  17  and  18  includes the boss  34  that includes a rocking shaft  37  in the center in the right and left direction, the wedge-shaped separating arm  35  is obliquely arranged to extend from the boss  34  toward the circumferential surface of the heat roller  15 , and the spring receiver arm  36  is provided on the other side of the boss  34 . The rocking shaft  37  of each of the separating pawls  17  and  18  is supported in a pawl support frame  38  while being able to rock. At this point, a lower surface of the spring receiver arm  36  of each of the separating pawls  17  and  18  is biased upward by a coil spring  41 , whereby pawl tips  42  and  43  of the separating arms  35  are brought into contact with the circumferential surface of the heat roller  15  as illustrated in  FIG. 3 . The center positions of the rocking shafts  37  of the separating pawls  17  and  18  are aligned with each other. The group of the sheet discharge guides  28  is provided in an outer surface of the pawl support frame  38  (see  FIG. 4 ). Similarly the group of the upper sheet discharge guides  27  is provided in a guide wall  53  while facing the lower sheet discharge guides  28  (see  FIG. 3 ). 
         [0035]    A first sprocket (rotatable guide)  45  and second sprockets  46  are provided on the front surface side in an upper corner of the separating arm  35  in order that the sheet separated from the circumferential surface of the heat roller  15  is deflected and guided toward the sheet discharge guide  27  of the exit feed path  26 . Each of the first sprocket  45  and the second sprockets  46  preferably includes a spur structure in which a group of teeth  47  is circumferentially arranged, and each of the first sprocket  45  and the second sprockets  46  is rotatably supported by a shaft  48  provided in the separating arm  35 . 
         [0036]    The fusing feed path  25  includes a feed path that reaches an introduction start end  49  of the upper sheet discharge guide  27  from the nip portion  23  through the first sprocket  45  and the second sprockets  46 . In the first preferred embodiment, each of the separating pawls  17  and  18  is disposed such that the circumferential surface of each of the sprockets  45  and  46  projects toward the side of the feed roller  29  from a straight line connecting the nip portion  23  and the introduction start end  49 , whereby the fusing feed path  25  is flexed into a reverse L-shape. A guide surface  50  that guides the sheet toward the introduction start end  49  is arranged to extend obliquely upward between the introduction start end  49  and the feed roller  29 . 
         [0037]    As described above, in the flexed fusing feed path  25 , the small-size sheet is fed while largely curved along the surrounding of the first sprocket  45 , and a large pressure is applied to a sheet plane. In order to reduce the pressure associated with the curvature, the central separating pawl  17  and the first sprocket  45  provided in the central separating pawl  17  are provided as follows. The separating arm  35  of the central separating pawl  17  is preferably smaller than that of the second separating pawl  18 , and the pawl tip of the first separating pawl  17  is circumscribed on the circumferential surface of the heat roller  15  downstream from the pawl tip  43  of the second separating pawl  18  in the rotating direction of the heat roller  15 . A rotation center P 1  of the first sprocket  45  of the first separating pawl  17  is located closer to the rocking shaft  37  than a rotation center P 2  of the second sprocket  46  of the second separating pawl  18 . 
         [0038]    As described above, the sprockets  45  and  46  differ from each other in the positions of the rotation centers P 1  and P 2 , whereby the center and the front and rear ends of the heat roller  15  differ from each other in the flexion angle of the fusing feed path  25  as illustrated in  FIG. 1 . Particularly, a flexion angle θ 2  of a lateral feed path  252  passing by the second sprockets  46  of the four second separating pawls  18 , which are disposed in front and rear of the central first separating pawl  17 , is preferably set to about 142 degrees, for example, when a flexion angle θ 1  of a central feed path  251  passing by the first sprocket  45  of the central first separating pawl  17  is set to about 152 degrees, for example. 
         [0039]    When the flexion angle θ 1  of the central feed path  251  is smaller than the flexion angle θ 2  of the lateral feed path  252 , the large pressure applied to the sheet plane of postcard-size sheet fed along the central feed path  251  can be eliminated while the postcard-size sheet is prevented from being largely curved. That is, the small-size recording sheet feeding condition can be relaxed to reduce the strong action of the flexion reactive force, generated by the teeth  47  of the first sprocket  45 , on the sheet plane of the small-size recording sheet. Accordingly, the formation of the broken-line pattern matched with the pitch of the teeth  47  can be eliminated on the rear surface of the sheet to obtain the same finishing state as the large-size sheet. In a case in which the thin and weak sheet is fused, the surface of the sheet can securely and stably be guided toward the sheet discharge guide  27  by the sprockets  45  and  46  of the five separating pawls  17  and  18 . Therefore, the deformations of the sheet plane such as the folding of the corner portion at the start end edge in the feed direction of the sheet and the generation of the wrinkle in the sheet plane, which are caused by the feeding failure, are not generated. 
         [0040]    In this preferred embodiment, there is the following positional relationship between the first separating pawl  17  and the first sprocket  45 , which are disposed in the center, and the lateral feed path  252 . Assuming that a triangle is defined by the lateral feed path  252  and a straight line connecting the nip portion  23  of the fusing unit  3  and the introduction start end  49  of the sheet discharge guide  27 , a partial arc position in which the sheet comes into contact with the sprocket  45  of the separating pawl  17  disposed in the central portion is located inside a vertex of the triangle. As used herein, the vertex of the triangle is a partial arc position in which the sheet comes into contact with the sprockets  46  of the separating pawls  18  disposed in the front and rear of the heat roller  15 . 
         [0041]    In the above-described preferred embodiment, the rotation center P 1  of the first sprocket  45  of the first separating pawl  17  located in the center of the heat roller  15  is shifted toward the side of the rocking shaft  37 , whereby the flexion angles θ 1  and θ 2  of the central feed path  251  and the lateral feed path  252  differ from each other. However, this is not necessary. As illustrated in  FIGS. 6 to 8 , the central feed path  251  differs from the lateral feed path  252  in the position of the sheet discharge guide  27 , the flexion angles θ 1  and θ 2  of the feed paths  251  and  252  may differ from each other by separating the position of the sheet discharge guide  27  of the central feed path  251  from the position of the sheet discharge guide  27  of the lateral feed path  252 . 
         [0042]    Particularly, as illustrated in  FIG. 6 , the rib-shaped sheet discharge guide  27  provided in the guide wall  53  includes four central sheet discharge guides  271  provided opposite a central area in the width direction of the exit feed path  26  and a group of lateral sheet discharge guides  272  provided in areas in the front and rear of the central area. Additionally, as illustrated in  FIG. 8 , a projection length H 1  from the guide wall  53  of the central sheet discharge guide  271  is preferably smaller than a projection length H 2  from the guide wall  53  of the lateral sheet discharge guide  272 . Therefore, a position of an introduction start end  492  of the central sheet discharge guide  271  is brought closer onto the side of the guide wall  53  than a position of an introduction start end  492  of the lateral sheet discharge guide  272 , and the flexion angle θ 1  of the central feed path  251  can be increased larger than the flexion angle θ 1  of the lateral feed path  252 . In this preferred embodiment, the flexion angle θ 1  preferably is about 148 degrees, and the flexion angle θ 2  preferably is about 142 degrees, for example. 
         [0043]    In this preferred embodiment, the central separating pawl  17  is identical to the front and rear separating pawls  18  except the central separating pawl  17  in a shape and a size, the sprockets  45  and  46  provided in the separating pawls  17  and  18  are identical to each other in a center position. The postcard-size sheet can pass through the areas where the central sheet discharge guides  271  are provided, and the sheet having the size larger than the postcard-size sheet can pass through the areas where the lateral sheet discharge guides  272  are provided (see  FIG. 8 ). Because other configurations of the second preferred embodiment are preferably identical to those of the first preferred embodiment, the same component is designated by the same numeral, and the description thereof is omitted. 
         [0044]    In the preferred embodiment of  FIGS. 6 to 8 , a relationship between the central sheet discharge guide  271  and the lateral sheet discharge guide  272  is as follows. Assuming that a triangle is defined by the lateral feed path  252  and a straight line connecting the nip portion  23  of the fusing unit  3  and the introduction start end  492  of the sheet discharge guide  272 , the introduction start end  491  of the central sheet discharge guide  271  is located outside an oblique-side portion on the side of the lateral sheet discharge guide  272  of the triangle. 
         [0045]    In the preferred embodiments, the sprockets  45  and  46  are preferably used as the rotatable guide by way of example. Alternatively, the rotatable guide may be constructed by a roller in which a knurling is located in a circumferential surface thereof or a fan-wheel-shaped rotating body of a centrifugal fan. It is not necessary that one central separating pawl  17  and one sprocket  45  be disposed opposite the central feed path  251 , but two or three central separating pawls  17  and two or three sprockets  45  may be disposed. 
         [0046]    Alternatively, the rotation center P 1  and P 2  of the sprockets  45  and  46  are separated from each other, and the positions of the sheet discharge guide  271  and  272  of the central feed path  251  and the lateral feed path  252  are separated from each other, whereby the flexion angles θ 1  and θ 2  of the feed paths  251  and  252  may be separated from each other. Alternatively, a diameter of the sprocket  45  provided in the central separating pawl  17  is preferably set larger than diameters of the sprockets  46  provided in the front and rear separating pawls  18 , and the whereby the flexion angles θ 1  and θ 2  of the feed paths  251  and  252  may be separated from each other. The rotation centers P 1  and P 2  of the sprockets  45  and  46  may be located on an arc that is coaxial with the heat roller  15 . 
         [0047]    While the present invention has been described with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically set out and described above. Accordingly, the appended claims are intended to cover all modifications of the present invention that fall within the true spirit and scope of the present invention.