Patent Publication Number: US-9411301-B2

Title: Frame for image forming apparatus and manufacturing method thereof

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a frame for an image forming apparatus and a manufacturing method of the frame. 
     In recent years and in order to meet demands for downsizing and price reduction, designed cost reduction efforts have been made repeatedly to the image forming apparatus. In a conventional image forming apparatus, a metal plate-combined frame, which is a combination of simple metal plates, has been employed as a frame (casing) for mounting parts of an entirety of a main assembly. The metal plate-combined frame has advantages, compared with a large-sized integral mold frame or the like, such that a material cost is stable, accuracy is easily ensured, mold investment is less, parts are flat plates, and therefore, a distribution cost is low. 
     The metal plate-combined frame is constituted by left and right side plates, flat metal plates such as an optical stay for mounting an optical scanning unit using a laser or an LED, a main stay for mounting a paper feeding system and a process unit, and a bottom plate for reinforcing a bottom portion of the main assembly. The optical stay, the main stay, and the bottom plate fixed at their ends to the left and right side plates by a fastening member such as screws or by a fixing method such as welding. 
     In the metal plate-combined frame, the parts constituting the frame are flat plates, and therefore there is a problem such that rigidity with respect to directions of bending and torsion is weak. When the rigidity of the frame is weak, an entirety of the image forming apparatus contorts and thus positional relationships among various parts held by the frame are disordered, so that distance accuracy between respective parts cannot be maintained. Particularly, between the optical scanning unit and a photosensitive member, in the case where the distance accuracy cannot be ensured, there is a liability that major defect, in terms of an image quality, such as image distortion or color misregistration generates. 
     In order to compensate for insufficient strength of the frame as described above, changes are made, such as the thickness of the metal plate is increased, a reinforcing member is added, increasing the number of parts, weight, cost, and the like, thus being contrary to downsizing and low price. In order to solve this problem, there are constructions in which the frame rigidity is ensured by a shape such as drawing of the frame and in which arrangement of frames is devised. For example, Japanese Laid-Open Patent Application (JP-A) 2003-237176 discloses a construction in which the optical stay and the bottom plate are substantially horizontally disposed and the main stay is obliquely disposed along a paper feeding guide or a feeding guide at an angle of 30 degrees to 60 degrees with respect to a substantially horizontal surface. In this embodiment, even when a force is applied from any direction, bending (distortion) of the frame can be obviated by supporting components of the image forming apparatus by associated one of the flat metal plate parts. 
     A means of ensuring distance accuracy between a fastening portion and a side plate in the metal plate-combined frame of an image forming apparatus in a conventional example will be described with reference to  FIGS. 9 - 12 .  FIG. 9  is a perspective view of an outer appearance of a metal plate-combined frame in the conventional example.  FIG. 10  is a schematic sectional view of the metal plate-combined frame in the conventional example.  FIG. 11  is a perspective view of an outer appearance of an optical stay in the conventional example.  FIG. 12  is a schematic view for illustrating fastening between the optical stay and the side plate in the conventional example. 
     As shown in  FIGS. 9 and 10 , in a conventional metal plate-combined frame  100 , a supporting stay such as an optical stay  101  or a main stay  102  which are a flat metal plate includes a flat plate portion  105  and a bent portion  106  vertically bent from the flat plate portion  105 . The bent portion  106  is constituted so as to be parallel to left and right side plates  103  and  104 . The supporting stays such as the optical stay  101  and the main stay  102  are fastened to the left and right side plates  103  and  104  at the bent portions  106  which are end portions thereof by fastening means  107  such as screws. 
     Further, as shown in  FIG. 11 , the supporting stay such as the optical stay  101  or the main stay  102  is provided with abutment portions  108  at end portions of the flat plate portion  105 . As shown in  FIG. 9 , the abutment portions  108  abut against the left and right side plates  103  and  104 , so that the conventional metal plate-combined frame  100  is capable of ensuring the press accuracy belt the left and right side plates  103  and  104 . 
     Further, as shown in  FIG. 12 , in the conventional metal plate-combined frame  100 , the abutment portions  108  somewhat project relative to the bent portion  106 . This is because even when the bent portions  106  of the supporting stays such as the optical stay  101  and the main stay  102  open within a variation range of an angular tolerance, the abutment portions  108  are abutted against the left and right side plates  103  and  104  with reliability. The reason why this constitution is employed is that the abutment portion  108  is better in distance accuracy than the bent portion  106 . 
     However, in the metal plate-combined frame of the conventional image forming apparatus shown in  FIGS. 9-12 , the following problem occurred. In a constitution in which the supporting stay as a first plate member is provided with the abutment portions  108 , as shown in  FIG. 12 , before the left and right side plates  103  and  104  as a second plate member and the bent portions  106  are fastened, a spacing (gap)  109  is formed between the left and right side plates  103  and  104  and the bent portions  106 . 
     On the other hand, when the left and right side plates  103  and  104  and the bent portions  106  are fastened, and the left and right side plates  103  and  104  and the bent portions  106  are not closely contacting each other at peripheries of fastening means  107 , e.g., in the case where screws are used as the fastening means  107 , a loosening torque of the screws after the fastening lowers. In a state in which the loosening torque is low, screw deviation is caused due to impact (shock) or the like during transportation of the image forming apparatus, so that the entirety of the image forming apparatus is distorted in some cases. Also in the case where a fixing method using, e.g., welding is used as the fastening means  107 , improper welding is caused in some cases when the left and right side plates  103  and  104  and the bent portions  106  are not closely contacted to each other, and similarly as in the case of the screws, the entirety of the image forming apparatus is distorted in some cases. 
     In order to solve this problem, as shown in  FIG. 13 , there is a constitution in which a slit  110  is provided at a base portion or the like of each of the bent portions  106  in the neighborhood of a fastening portion in order to bring the left and right side plates  103  and  104  and the bent portions  106  described above in close contact with each other with reliability. By this constitution using the slit  110 , rigidity of the bent portion  106  lowers. As a result, e.g., in the case where the screws are used as the fastening means  107 , in a fastening step using the screws, the bent portions  106  can be reliably deformed by being pulled in the directions of the left and right side plates  103  and  104 . However, in this constitution, the slit  110  is provided at the base portion of each of the bent portions  106 , and therefore the frame rigidity as the entire of the metal plate-combined frame becomes low. 
     In place of the slit  110 , there is also a constitution in which a projected portion  111  projected in a free end direction by bending a part of the bent portion  106  as shown in  FIG. 14 . By such a constitution, the fastening means  107  is spaced from the base portion of the bent portion  106 , whereby the left and right side plates  103  and  104  and the bent portions  106  can be closely contacted to each other with reliability. When the fastening means  107  is spaced from the base portion of the bent portion  106  and a width of the projected portion  111  is narrow, the rigidity with respect to a bending direction at a periphery of the fastening means  107  lowers. For that reason, e.g., in the case where the screws are used as the fastening means  107 , the bent portions  106  are easily pulled in the directions of the left and right side plates  103  and  104 . However, the part of the bent portions  106  projects, and therefore the supporting stay is upsized correspondingly. As a result, there is a liability that the supporting stay itself increases in cost. Further, there is a need to provide a fastening position of the fastening means  107  in a position spaced from a body portion of the supporting stay to some extent, and therefore there is a liability that the image forming apparatus is upsized correspondingly. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to maintain and improve rigidity of a frame for an image forming apparatus, while suppressing an increase in cost and upsizing of the image forming apparatus. 
     According to an aspect of the present invention, there is provided a frame for an image forming apparatus, comprising: a first plate member and a second plate member; a bent portion provided as a part of the first plate member and bent along the second plate member; a portion-to-be-fixed provided as a part of the bent portion and to be fixed to the second plate member; a positioning portion, provided on the first plate member, for positioning the second plate member relative to the first plate member by being abutted vertically against the second plate member, wherein a free end of the positioning portion is disposed in a position where the free end projects toward the second plate member more than the portion-to-be-fixed; and a fixing portion, provided as a part of the second plate member, to which the portion-to-be-fixed is fixed, wherein the second plate member is deformed to contact and deform the fixing portion to said portion-to-be-fixed while the second plate member is in contact to the positioning portion, so that the first plate member and the second member are fixed to each other. 
     According to another aspect of the present invention, there is provided a manufacturing method of a frame for an image forming apparatus, wherein the frame includes a first plate member, a second plate member, a positioning portion, provided on the first plate member, for positioning the second plate member relative to the first plate member by being abutted vertically against the second plate member, a bent portion provided as a part of the first plate member and bent along the second plate member, a portion-to-be-fixed provided as a part of the bent portion and to be fixed to the second plate member, and a fixing portion, provided as a part of the second plate member, to which the portion-to-be-fixed is fixed, and wherein a free end of the positioning portion is disposed in a position where the free end projects toward the second plate member more than the portion-to-be-fixed, the manufacturing method comprising: a positioning step of positioning the second plate member relative to the first plate member by vertically abutting the positioning portion against the second plate member; and a fixing step of fixing the first plate member and the second plate member to each other by deforming the second plate member to contact and deform the fixing portion to the portion-to-be-fixed while the second plate member is in contact to the positioning portion. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an outer appearance of an image forming apparatus in an embodiment. 
         FIG. 2  is a schematic sectional view showing a structure of the image forming apparatus in the embodiment. 
         FIG. 3  is a perspective view of an outer appearance of a metal plate-combined frame in the embodiment. 
         FIG. 4  is a schematic sectional view of the metal plate-combined frame in the embodiment. 
         FIG. 5  is an enlarged view showing a periphery of a bent portion and a projected portion of a supporting stay. 
       In  FIG. 6 , (a) and (b) are schematic views for illustrating a fixing method of left and right side plates and the supporting stay in the embodiment. 
         FIG. 7  is a schematic view for illustrating a distance between a portion-to-be-fixed and the projected portion. 
         FIG. 8  is a schematic view showing a fixed state of the left and right side plates and the supporting stay. 
         FIG. 9  is a perspective view of an outer appearance of a metal plate-combined frame in a conventional example. 
         FIG. 10  is a schematic sectional view of the metal plate-combined frame in the conventional example. 
         FIG. 11  is a perspective view of an outer appearance of an optical stay in the conventional example. 
         FIG. 12  is a schematic view for illustrating fastening between the optical stay and a side plate in the conventional example. 
       In  FIG. 13 , (a) and (b) are schematic views for illustrating fastening between a bent portion and the side plate in the conventional example. 
       In  FIG. 14 , (a) and (b) are schematic views for illustrating fastening between a bent portion and the side plate in the conventional example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinbelow, embodiments of the present invention will be specifically described with reference to the drawings. 
     However, dimensions, materials and shapes of constituent elements and their relative arrangements and the like described in the following embodiments should be changed appropriately depending on structures and various conditions of apparatuses (devices) to which the present invention is applied, and the scope of the present invention is not intended to be limited to the following embodiments. 
     (Image forming apparatus) 
     First, with reference to  FIGS. 1 and 2 , a general structure of an image forming apparatus in an embodiment of the present invention will be described.  FIG. 1  is a perspective view of an outer appearance of the image forming apparatus in this embodiment.  FIG. 2  is a schematic sectional view showing a structure of the image forming apparatus in this embodiment. 
     An electrophotographic image forming apparatus  1  in this embodiment is a four-color based full-color laser printer using an electrophotographic process. However, the present invention is not limited thereto, but may also be, e.g., an electrophotographic copying machine, an electrophotographic printer (such as a color LED printer), a facsimile machine and a word processor. The image forming apparatus  1  employs a process cartridge type in which a process cartridge P is detachably mounted into an apparatus main assembly  2  through an apparatus openable door  3 . The apparatus main assembly  2  is a portion of the image forming apparatus  1  from which the process cartridge P is removed. The image forming apparatus  1  in this embodiment forms a color image on a recording material S. On the recording material S, the image is formed by the image forming apparatus  1 , and examples of the recording material S may include a sheet, an OHP sheet and the like. 
     As shown in  FIG. 2 , in the apparatus main assembly  2 , four cartridges P consisting of a first cartridge PY, a second cartridge PM, a third cartridge PC and a fourth cartridge PK are provided horizontally. The first to fourth cartridges P (PY, PM, PC, PK) have a similar electrophotographic process mechanism and are different from each other in color of developers (toners) accommodated in the cartridges. 
     The first cartridge PY accommodates the toner of yellow (Y) and forms a developer image (toner image) of yellow (Y) on the surface of a photosensitive drum  40 . The second cartridge PM accommodates the toner of magenta (M) and forms a toner image of magenta (M) on the surface of a photosensitive drum  40 . The third cartridge PC accommodates the toner of cyan (C) and forms a toner image of cyan (C) on the surface of a photosensitive drum  40 . The fourth cartridge PK accommodates the toner of black (K) and forms a toner image of black (K) on the surface of a photosensitive drum  40 . Each of the cartridges includes an unshown charging means and an unshown developing means. 
     To each of the first to fourth cartridges P (PY, PM, PC, PK), from a drive output portion (not shown) of the apparatus main assembly  2 , a rotational driving force is transmitted. In each of the first to fourth cartridges P (PY, PM, PC, PK), from the apparatus main assembly  2 , bias voltages (charging bias and developing bias) are supplied to the charging means and the developing means (not shown). 
     Above the first to fourth cartridges P (PY, PM, PC, PK), a laser scanner unit LS as an exposure unit is provided. This laser scanner unit LS outputs laser light Z corresponding to image information. The surface of the photosensitive drum  40  is scanned and exposed to the laser light Z passing through an exposure window portion of each cartridge P. 
     A toner image forming process is as follows. First, each photosensitive drum  40  is electrically charged by the charging means and then is exposed to light by the laser scanner unit LS to form a latent image thereon. Then, the toner is deposited on the photosensitive drum  40  by the developing means, so that the toner image is formed on the photosensitive drum  40 . 
     Below the first to fourth cartridges P (PY, PM, PC, PK), an intermediary transfer belt unit  11  as a transfer unit is provided. The intermediary transfer belt unit  11  includes a tension roller  13 , a driving roller  17  and an assist roller  15  by which a flexible transfer belt  12  is extended and stretched. 
     Each of the photosensitive drums,  40  of the first to fourth cartridges P (PY, PM, PC, PK) contacts a peripheral surface of the transfer belt  12  at a lower surface thereof. A contact portion therebetween is a primary transfer portion. Inside the transfer belt  12 , a primary transfer roller  16  is provided opposed to the photosensitive drum  40 . By applying a primary transfer voltage to the primary transfer roller  16 , at the primary transfer portion, the toner image is transferred from the photosensitive drum  40  onto the transfer belt  12 . A secondary transfer roller  14  is contacted to the transfer belt  12  toward the driving roller  17 . A contact portion between the transfer belt  12  and the secondary transfer roller  14  is a secondary transfer portion. 
     Below the intermediary transfer belt unit  11 , a feeding unit  18  is provided. This feeding unit  18  includes a paper feeding tray  19  in which the recording material S is stacked and accommodated and includes a paper feeding roller  20 . The recording material S is fed from the paper feeding tray  19  to the secondary transfer portion by the paper feeding roller  20 . 
     At an upper left portion inside the apparatus main assembly  2  in  FIG. 2 , a fixing unit  21  and a discharging unit  22  are provided. An upper surface of the apparatus main assembly  2  constitutes a discharge tray  23 . On the recording material S on which the toner image is transferred at the secondary transfer portion, the toner image is fixed by a fixing means provided in the fixing unit  21 , and then the recording material S is discharged onto the discharge tray  23 . 
     (Metal Plate-combined Frame) 
     Next, with reference to  FIGS. 3 and 4 , a metal plate-combined frame  50  in this embodiment will be described.  FIG. 3  is a perspective view of an outer appearance of the metal plate-combined frame  50  in this embodiment.  FIG. 4  is a schematic sectional view of the metal plate-combined frame  50  in this embodiment. 
     An image forming apparatus frame of the image forming apparatus in this embodiment is constituted by the metal plate-combined frame  50  shown in  FIG. 3 . The metal plate-combined frame  50  includes side plates (left side plate  52  and right side plate  53 ) as a pair of second plate members, an optical stay  54  as a first plate member, and a main stay  55 . The first plate member and the second plate member are members manufactured by processing a metal-made plate member (metal plate) into a desired shape by press machining or the like. 
     As shown in  FIGS. 3 and 4 , the left side plate  52  and the right side plate  53  are provided in parallel to each other, and between these side plates, the optical stay  54  and the main stay  55  are provided. The optical stay  54  positions and supports optical parts such as the laser scanner unit LS described above, and the main stay  55  positions and supports a frame of a transfer device such as the intermediary transfer belt unit  11  described above and an unshown manual feeding unit. Hereinafter, the optical stay  54  and the main stay  55  are referred to as a supporting stay. The left and right side plates  52  and  53  and the supporting stays  54  and  55  are provided perpendicularly (vertically) to each other and are fixed. A frame for the intermediary transfer belt unit  11  supports and positions the first to fourth cartridges P (PY, PM, PC, PK). 
     Each of the supporting stays  54  and  55  includes a flat plate portion  56  provided vertically to the left and right side plates  52  and  53 . At an edge portion of the flat plate portion  56  fixed to each of the left and right side plates  52  and  53 , the supporting stays  54  and  55  are provided with projected portions  58  as positioning portions for positioning the left and right side plates  52  and  53  relative to the supporting stays  54  and  55  by vertically abutting against the left and right side plates  52  and  53 . Further, the supporting stays  54  and  55  are provided with bent portions  57  including portions-to-be-fixed  61   b  (described later specifically) to be fixed to the left and right side plates  52  and  53  while extending along the left and right side plates  52  and  53 . Each of the projected portions  58  positions an associated one of the left and right side plates  52  and  53  relative to an associated one of the supporting stays  54  and  55 , and ensures distance accuracy between the left and right side plates  52  and  53 . 
     (Bent Portion and Projected Portion) 
     Next, with reference to  FIG. 5 , details of the bent portions  57  and the projected portions  58  of the supporting stays  54  and  55  will be described.  FIG. 5  is an enlarged view showing a periphery of the bent portion and the projected portion of the supporting stay. 
     As shown in  FIG. 5 , a projection amount of the projected portion  58  from a base portion  57   a  of the bent portion  57  is L 1 . A projection amount of a free end portion  57   b  of the bent portion  57  from the base portion  57   a  of the bent portion  57  is L 2 . Further, a (bending) length from the base portion  57   a  to the free end portion  57   b  of the bent portion  57  is H. 
     The projection amounts L 1  and L 2  are required to satisfy a relationship of L 1 ≧L 2 . This is because the projected portion  58  is caused to abut against the left side plate  53  or the right side plate  53  with reliability. When the relationship between L 1  and L 2  is reversed (L 1 &lt;L 2 ), the free end portion  57   b  of the bent portion  57  abuts against the left side plate  52  or the right side plate  53 , so that a spacing (gap) is formed between the projected portion  58  and the associated one of the left and right side plates  52  and  53 . The reason why the projected portion  58 , not the bent portion  57 , is caused to abut against the associated one of the left and right side plates  52  and  53  is that the projected portion  58  is capable of improving the distance accuracy between the left and right side plates  52  and  53  more than the bent portion  57 . In this way, with respect to a direction in which the flat plate portion  56  extends, a free end of the projected portion  58  is disposed at a position closer to the left and right side plates  52  and  53  than from the free end portion  57   b  of the bent portion  57  to the left and right side plates  52  and  53 . 
     (Fixing Method of Left and Right Side Plates and Supporting Stays) 
     Next, with reference to  FIGS. 6 to 8 , as a manufacturing method of the metal plate-combined frame, a fixing method of the left and right side plates and the supporting stays will be described. In this embodiment, welding was used as the fixing method of the left and right side plates  52  and  53  and the supporting stays  54  and  55 . The fixing method of the left and right side plates and the supporting stays includes a positioning step and a fixing step. 
     In  FIG. 6 , (a) and (b) are schematic views for illustrating the fixing method of the left and right side plates and the supporting stays. In  FIG. 6 , (a) shows a state before the left and right side plates are fixed by the welding, and (b) shows a state in which the left and right side plates are deformed for fixing the left and right side plates by the welding. As shown in (a) of  FIG. 6 , a plate thickness of each of the left and right side plates  52  and  53  is t 1 , and a plate thickness of each of the supporting stays  54  and  55  is t 2 .  FIG. 7  is a schematic view for illustrating a distance between the portion-to-be-fixed and the projected portion.  FIG. 8  is a schematic view showing a fixed state, between the left or right side plate and the associated supporting stay, as seen from above the metal plate-combined frame  50 . 
     First, the projected portions  58  of the supporting stays  54  and  55  are abutted against the left and right side plates  52  and  53  to perform positioning of the left and right side plates  52  and  53  relative to the supporting stays  54  and  55  (developing step). 
     Then, the fixing step will be described. In the state in which the left and right side plates  52  and  53  are positioned relative to the supporting stays  54  and  55 , a pair of electrode members  62  and  63  for sandwiching the left or right side plate  52  or  53  and the associated one of the supporting stays  54  and  55  is provided. An electrode member contacting the left or right side plate  52  or  53  is the electrode member  62 , and an electrode member contacting the associated one of the supporting stays  54  and  55  is the electrode member  63 . 
     Of the portions sandwiched between the pair of electrode members  62  and  63 , the portion positioned in the side of the left or right side plate  52  or  53  is a fixing portion  61   a,  and the portion positioned in the side of the associated one of the supporting stays  54  and  55  is a portion-to-be-fixed  61   b.  The fixing portion  61   a  and the portion-to-be-fixed  61   b  which are sandwiched between the electrode members  62  and  63  are locally pressed. Then, the fixing portion  61   a  is deformed toward the portion-to-be-fixed  61   b  of the bent portion  57  and thus is closely contacted to the portion-to-be-fixed  61   b,  so that a current generates between the electrode members  62  and  63 . By the generation of the current, the left and right side plates  52  and  53  are heated and melted. As a result, the fixing portion  61   a  and the portion-to-be-fixed  61   b  are welded to each other. In this way, the left and right side plates  52  and  53  and the supporting stays  54  and  55  are welded to each other. In this embodiment, as shown in  FIG. 6 , a free end of each of the electrode members  62  and  63  has a roller shape so that pressure is easily concentrated. 
     Further, details of the fixing step will be described. As shown in  FIG. 6 , the projected portion  58  projects from the edge portion of the supporting stay  54  or  55  in the projection amount L 1 . Therefore, as shown in (a) of  FIG. 6 , before the fixing portion  61   a  and the portion-to-be-fixed  61   b  are heated and pressed by the electrode members  62  and  63 , a gap corresponding to approximately the projection amount L 1  is formed the fixing portion between  61   a  of the left or right side plate  52  or  53  and the portion-to-be-fixed  61   b  of the bent portion  57 . This is because as described above, the free end of the projected portion  58  is disposed at the position where the projected portion free end projects toward the left or right side plate  52  or  53  (the second plate member) more than the portion-to-be-fixed  61   b  of the bent portion  57 . Further, the above-described current cannot be generated when the fixing portion  61   a  and the portion-to-be-fixed  61   b  are not closely contacted to each other. 
     In the fixing step in this embodiment, the electrode member  62  in the side of the left or right side plate is urged toward the associated supporting stay with an urging force f 1  indicated by an arrow in  FIG. 6  by an unshown urging means. As a result, the electrode member  62  locally deforms the fixing portion  61   a  of the left or right side plate  52  or  53  in the projection amount L 1 , so that the fixing portion  61   a  is contacted to the portion-to-be-fixed  61   b  of the bent portion  57 . 
     At this time, the electrode member  63  in the supporting stay side contacts the bent portion  57  is urged (pressed) from a direction, opposite to the direction in which the electrode member  62  is urged, with an urging force f 2  to the extent such that the electrode member  62  is not pushed by the urging force f 1  of the electrode member  62 . The urging force f 1  losses in an amount such that the urging force f 1  of the electrode member  62  in the side of the left or right side plate deforms the fixing portion  61   a  of the left or right side plate  52  or  53 , and therefore the urging force f 2  of the electrode member  63  in the supporting stay side is set so as to be weaker than the urging force f 1 . 
     In the fixing step in this embodiment, both of the electrode member  62  for the left or right side plate and the electrode member  63  for the supporting stay are urged, but a constitution in which only the electrode member  62  for the left or right side plate is urged while fixing the electrode member  63  for the supporting stay at a position where the electrode member  63  contacts the bent portion  57  may also be employed. 
     In the fixing step in this embodiment, the urging force f 1  of the electrode member  62  for the left or right side plate was about 30 kgf, and the urging force f 2  of the electrode member  62  for the supporting stay was about 20 kgf. The plate thickness t 1  of each of the left and right side plates  52  and  53  is set to satisfy a range of 0.6 mm or more and 1.2 mm or less, and the plate thickness t 2  of each of the supporting stays  54  and  55  is set to satisfy a range of 0.6 mm or more and 1.2 mm or less. 
     Further, a constitution in which the position of the portion-to-be-fixed  61   b  and the position of the projected portion  58  are spaced and shifted by a predetermined distance with respect to a direction (longitudinal direction of the bent portion  57 ) in which an edge line of a bent portion extends when the bent portion  57  is provided by bending the associated one of the supporting stays  54  and  55  was employed. Specifically, as shown in  FIG. 7 , the positions of the portion-to-be-fixed  61   b  and the projected portion  58  closest to the portion-to-be-fixed  61   b  are shifted so that a distance X between the portion-to-be-fixed  61   b  and the projected portion  58  is 15 mm or more. In the case where the press X is set so as to be shorter than 15 mm, the urging force f 1  for bringing the fixing portion  61   a  for the left or right side plate  52  or  53  into contact with the portion-to-be-fixed  61   b  is required to be considerably increased because the projected portion  58  exists in the opposite side. In this embodiment, the plate thickness t 1  of each of the left and right side plates  52  and  53  is 0.6 mm or more and 1.2 mm or less, and therefore in the case where the distance X is made shorter than 15 mm, the urging force f 1  for each of the left and right side plates  52  and  53  is required to be set at 30 kgf or more in some cases. This is because it becomes difficult to ensure a space in which an urging means for urging the electrode member  63  is to be disposed. 
     As described above, in the case where the distance X is set so as to be 15 mm or more, the metal plate-combined frame  50  in this embodiment has the following shape in the state in which the left and right side plates  52  and  53  and the supporting stays  54  and  55  are fixed. That is, as shown in  FIG. 8 , in the case where a portion of the left and right side plates  52  and  53  of the metal plate-combined frame  50  is seen from above, the left and right side plates  52  and  53  are deformed so as to contact the projected portions  58  and the portions-to-be-fixed  61   b  of the supporting stays  54  and  55  to assume a wavy shape such that the projected portions  58  and the portions-to-be-fixed  61   b  constitute a deformed portion (belly portion). In this way, in this embodiment, a constitution in which the left and right side plates  52  and  53  are positively deformed (curved) so as to bring the projected portions  58  and the portions-to-be-fixed  61   b  of the supporting stays  54  and  55  into contact with the left and right side plates  52  and  53  was employed. As a result, there is no need to employ a constitution, as in the conventional example, in which the bent portions  57  provided with the portions-to-be-fixed  61   b  for the supporting stays  54  and  55  are positively deformed. 
     In order to realize close contact between the fixing portion  61   a  and the portion-to-be-fixed  61   b,  a constitution in which the portion-to-be-fixed  61   b  of the bent portion  57  is deformed by urging of the electrode member  63  for the supporting stay would be also considered, but the following problem generates. As in this embodiment, the portion-to-be-fixed  61   b  is close to the base portion  57   b  of the bent portion  57 , and a shape, for decreasing rigidity, such as a slit or hole does not exist at a periphery of the portion-to-be-fixed  61   b.  For this reason, rigidity against the deformation of the metal plate-combined frame  50  is high. In this embodiment, a distance h ((a) of  FIG. 6 ) between the base portion  57   a  and the portion-to-be-fixed  61   b  is 4 mm or more and 10 mm or less. Therefore, compared with the urging force f 1  when the fixing portion  61   a  of each of the left and right side plates is deformed, the urging force f 2  for the supporting stay is required to be considerably increased. In this embodiment, the plate thickness t 2  of each of the supporting stays  54  and  55  is 0.6 mm or more and 1.2 mm or less, and therefore in order to deform the portion-to-be-fixed  61   b,  the urging force f 2  of the electrode member  63  for each of the supporting stays  54  and  55  is required to be set at 30 kgf or more in some cases. In this case, it becomes difficult to ensure a space in which an urging means for urging the electrode member  63  is to be disposed. 
     Further, when the urging force f 2  of the electrode member  63  for the supporting stay is set so that the urging force f 2  is set to push the electrode member  62  urged by the urging force f 1 , the left and right side plates  52  and  53  are deformed toward an outside the main assembly constituting frame. At this time, the projected portions  58  of the supporting stays  54  and  55  are spaced from the left and right side plates  52  and  53  in some cases, and therefore there is a liability that the distance accuracy between the side plates cannot be ensured. 
     In order to facilitate the deformation of the portion-to-be-fixed  61   b  of the bent portion  57  by urging of the electrode member  63  for the supporting stay, compared with the constitution in which the bent portion  57  is provided with the slit or the like as in the constitution in the conventional example, when the constitution in this embodiment is employed, frame rigidity of the frame as a whole can be maintained. Further, compared with the constitution in which the narrow projected portion is provided at the bent portion  57  as in the conventional example, when the constitution in this embodiment is employed, there is no need to provide the narrow projected portion at the bent portion  57 , so that it is possible to suppress upsizing of the supporting stays  54  and  55 . 
     As described above, in the metal plate-combined frame  50  in this embodiment, it is possible to maintain rigidity of the metal plate-combined frame  50  while suppressing the increase in cost of the product and the upsizing of the image forming apparatus, and it is possible to fix the left and right side plates  52  and  53  with the supporting stays  54  and  55  with high positional accuracy. 
     In the metal plate-combined frame  50 , it is preferable that the projection amount L 1  is 0.1 mm or more and 0.3 mm or less, the projection amount L 2  is 0±about 0.1 mm, and the bending length H is 8 mm or more and 20 mm or less. The projection amount L 2  becomes larger with a longer bending length H, and therefore if the bending length H is made longer than 20 mm, the projection amount L 2  is required to be set so as to be larger than a value of the projection amount L 1 . At this time, there is a need to increase also a deformation amount of the fixing portion  61   a  of the left and right side plates  52  and  53 , and therefore the urging force of the electrode member  62  is required to be made high, so that it is difficult to ensure the space in which the urging means is disposed. On the other hand, in the case where the bending length H is made smaller than 8 mm, bending rigidity and torsional rigidity of the supporting stays  54  and  55  themselves lower, so that the frame rigidity becomes insufficient. Further, there is no space in which the electrode members  62  and  63  are disposed, and therefore it is difficult to employ the fixing method in this embodiment. 
     In order to bring the projected portions  58  and the portions-to-be-fixed  61   b  of the supporting stays  54  and  55  into contact with the left and right side plates  52  and  53 , if the left and right side plates  52  and  53  are deformed, the fixing method between the fixing portions  61   a  and the portions-to-be-fixed  61   b  is not limited to the welding, but may also be fixing using fastening with screws. The flat plate portions  56  of the supporting stays  54  and  55  are vertically to the left and right side plates  52  and  53 , but may only be required to satisfy a crossing relationship. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 016830/2014 filed Jan. 31, 2014, which is hereby incorporated by reference.