Abstract:
An image forming apparatus, includes: a mainframe having an opening portion; an electrostatic latent image carrier being loadable in and unloadable from the main frame through the opening portion; a transport path; a first opening and closing member provided openably and closably on the mainframe so as to open and close the opening portion; and a second opening and closing member disposed on a side opposite to the electrostatic latent image carrier with respect to the first opening and closing member and provided openably and closably on the mainframe so as to open and close the transport path. The first opening and closing member and the second opening and closing member are openable and closable integrally with each other. The second opening and closing member is openable and closable independently of the first opening and closing member.

Description:
[0001]     This is a Continuation of application Ser. No. 10/801,797 filed Mar. 17, 2004. The entire disclosure of the prior application is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an image forming apparatus such as a printer which forms a color image by means of superimposition of toner images respectively formed on plural electrostatic latent image carriers.  
         [0004]     2. Background Art  
         [0005]     As a color image forming apparatus such as a printer or a facsimile apparatus, known is a tandem image forming apparatus in which process units each including: a photosensitive member serving as an electrostatic latent image carrier; a charging device and an exposing device that form an electrostatic latent image on the photosensitive member; and a developing device that develops the electrostatic latent image formed on the photosensitive member by a toner are disposed respectively for colors (for example, four colors of cyan, magenta, yellow, and black), and the color toner images respectively formed on the photosensitive members are transferred so as to be superimposed on one another, thereby forming a color image.  
         [0006]     In some of such tandem image forming apparatuses, plural process units are horizontally arranged (for example, see JP-A-10-307439 (Page 4)).  
         [0007]     However, such a so-called lateral tandem image forming apparatus has a problem in that the installation area is large. Therefore, a so-called vertical tandem system in which plural process units are vertically arranged so as to reduce the installation area has been proposed.  
       SUMMARY OF THE INVENTION  
       [0008]     Such a vertical tandem image forming apparatus may be contemplated to have a configuration in which a recording medium (for example, a paper sheet) is transported by a transporting section along plural process units that are vertically arranged, and toner images are transferred to the recording medium by a transferring section during the vertical transportation.  
         [0009]     In the thus configured vertical tandem image forming apparatus, the transport path for a recording medium is long, and hence a jam easily occurs. When a jam once occurred, the transporting section is opened so that the jammed recording medium can be removed away. In this configuration, when a photosensitive member is to be replaced with another one, however, the photosensitive member cannot be taken out unless the transporting section is opened and the transferring section is then opened, thereby causing a problem in that the work of replacing the photosensitive member is cumbersome. During the work of replacing the photosensitive member, the transporting section must be once opened. When the worker accidentally touches the transporting section, finger marks are formed on the transporting section, and there arises the possibility that a printing failure may occur. When the transporting section is stained by a toner or the like, the clothes or the hands may be soiled.  
         [0010]     An image forming apparatus is disclosed herein, in which works of removing a jammed sheet and replacing an electrostatic latent image carrier can be easily conducted.  
         [0011]     According to one aspect of the invention, an image forming apparatus, including: a mainframe having an opening portion; an electrostatic latent image carrier on which an electrostatic latent image formed to be developed with a developer, the electrostatic latent image carrier being loadable in and unloadable from the mainframe through the opening portion; a transport path through which a recording medium is transported to be formed image thereon; a first opening and closing member provided openably and closably on the mainframe so as to open and close the opening portion; and a second opening and closing member disposed on a side opposite to the electrostatic latent image carrier with respect to the first opening and closing member and provided openably and closably on the mainframe so as to open and close the transport path. The first opening and closing member and the second opening and closing member are openable and closable integrally with each other. The second opening and closing member is openable and closable independently of the first opening and closing member.  
         [0012]     According to another aspect of the invention, the electrostatic latent image carrier includes a plurality of electrostatic latent image that are disposed respectively for colors and vertically arranged.  
         [0013]     In the thus configured image forming apparatus, the second opening and closing member can be opened without opening the first opening and closing member, and in addition the first and second opening and closing members can be integrally opened.  
         [0014]     When a recording medium is jammed in the transport path, therefore, the second opening and closing member can be opened without opening the first opening and closing member, and the recording medium can be then removed away. When the electrostatic latent image carriers are to be attached or detached, moreover, the first and second opening and closing members can be integrally opened from the same side as that in the case of removal of a jammed recording medium, so that the opening portion can be opened. Therefore, the opening portion can be easily opened in order to attach or detach the electrostatic latent image carriers.  
         [0015]     According to another aspect of the invention, an image forming apparatus, includes: a mainframe having an opening portion; an electrostatic latent image carrier loadable in and unloadable from the mainframe through the opening portion; a first opening and closing member provided rotatably on the mainframe and disposed exterior to the electrostatic latent image carrier so as to open and close the opening portion; a transport path through which a recording medium is transported, the transport path disposed exterior to the first opening and closing member; a second opening and closing member provided rotatably on the mainframe and disposed exterior to the transport path so as to cover and uncover the transport path; and a coupling member that couples the first opening and closing member with the second opening and closing member so that the first opening and closing member and the second opening and closing member are rotatable integrally. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The present invention may be more readily described with reference to the accompanying drawings:  
         [0017]      FIG. 1  is a schematic side section view of a color laser printer of Embodiment 1.  
         [0018]      FIG. 2  is a schematic view of the color laser printer of Embodiment 1.  
         [0019]      FIG. 3  is a diagram of an opening and closing mechanism of the color laser printer of Embodiment 1.  
         [0020]      FIGS. 4A and 4B  are diagrams of an opened state of the color laser printer of Embodiment 1.  
         [0021]      FIGS. 5A and 5B  are diagrams of the opened state of the color laser printer of Embodiment 1.  
         [0022]      FIGS. 6A and 6B  are diagrams of the opened state of the color laser printer of Embodiment 1.  
         [0023]      FIG. 7  is a schematic side section view of a color laser printer of Embodiment 2.  
         [0024]      FIGS. 8A and 8B  are diagrams of an opened state of the color laser printer of Embodiment 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Embodiment 1  
       [0025]     Hereinafter, Embodiment 1 of the invention will be described with reference to the accompanying drawings.  
         [0026]      FIG. 1  is a schematic side section view of a color laser printer  1  that is an image forming apparatus to which the invention is applied. The color laser printer  1  which is exemplarily shown in  FIG. 1  comprises an image forming unit  10 , a belt-like intermediate transfer member  20 , a fixing unit  30 , a sheet feeding unit  40 , a sheet discharging tray  50 , and an inverting and transporting unit  60 .  
         [0027]     For steps of forming visual images by respective toners serving as developers of magenta (M), cyan (C), yellow (Y), and black (K), the image forming unit  10  comprises: developing devices  11 M,  11 C,  11 Y,  11 K; photosensitive drums  14 M,  14 C,  14 Y,  14 K; cleaning rollers  16 M,  16 C,  16 Y,  16 K; charging devices  15 M,  15 C,  15 Y,  15 K; and exposing devices  17 M,  17 C,  17 Y,  17 K.  
         [0028]     The developing device, the photosensitive drum, the cleaning roller, and the charging device for each of the colors are integrated to one another to constitute a process unit. Each process unit is attachable to and detachable from the color laser printer  1 .  
         [0029]     Hereinafter, each of the components will be described in detail.  
         [0030]     The developing devices  11 M,  11 C,  11 Y,  11 K comprise developing rollers  12 M,  12 C,  12 Y,  12 K, respectively. Each of the developing rollers  12 M,  12 C,  12 Y,  12 K uses electrically conductive silicone rubber as a base material, and is formed into a columnar shape. A coating layer made of a material containing fluorine such as a resin or a rubber material is formed on the surface of the roller.  
         [0031]     The developing devices  11 M,  11 C,  11 Y,  11 K comprise supply rollers  13 M,  13 C,  13 Y,  13 K, respectively. The supply rollers  13 M,  13 C,  13 Y,  13 K are electrically conductive sponge rollers, and placed so as to be in press contact with the developing rollers  12 M,  12 C,  12 Y,  12 K by the elasticities of the sponges, respectively.  
         [0032]     A toner container which accommodates a positively chargeable polymer toner is formed in each of the developing devices.  
         [0033]     For example, a drum in which a positively chargeable photosensitive layer is formed on a base member made of aluminum is used as each of the photosensitive drums  14 M,  14 C,  14 Y,  14 K.  
         [0034]     The cleaning rollers  16 M,  16 C,  16 Y,  16 K are rollers each formed by an elastic member such as electrically conductive sponge, and placed below the photosensitive drums  14 M,  14 C,  14 Y,  14 K so as to be in sliding contact with the photosensitive drums  14 M,  14 C,  14 Y,  14 K, respectively. A voltage the polarity of which is opposite to that of a toner or is negative is applied by a power source (not shown) to the cleaning rollers  16 M,  16 C,  16 Y,  16 K so that residual toners on the photosensitive drums  14 M,  14 C,  14 Y,  14 K are removed away by the functions of sliding frictional forces and the electric fields due to the voltages on the photosensitive drums  14 M,  14 C,  14 Y,  14 K. In the embodiment, a so-called cleaner-less developing system is employed. When, in a predetermined cycle after a developing step, a voltage which causes removed residual toners to return to the respective photosensitive drums is once applied to the cleaning rollers  16 M,  16 C,  16 Y,  16 K, therefore, the removed residual toners are returned toward the photosensitive drums  14 M,  14 C,  14 Y,  14 K, respectively.  
         [0035]     The charging devices  15 M,  15 C,  15 Y,  15 K are scorotron charging devices, and placed below the photosensitive drums  14 M,  14 C,  14 Y,  14 K and downstream of the cleaning rollers  16 M,  16 C,  16 Y,  16 K in the rotation direction of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, so as to be opposed to the surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, respectively. Alternatively, roller charging devices which are in contact with the photosensitive drums  14 M,  14 C,  14 Y,  14 K may be used as the charging devices  15 M,  15 C,  15 Y,  15 K.  
         [0036]     The exposing devices  17 M,  17 C,  17 Y,  17 K are configured by well-known laser scanner units, and expose the surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K to laser beams at positions downstream of the charging devices  15 M,  15 C,  15 Y,  15 K in the rotation direction of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, respectively. The surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K are illuminated with laser beams corresponding to image data by the exposing devices  17 M,  17 C,  17 Y,  17 K, and electrostatic latent images for the colors are formed in the surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, respectively.  
         [0037]     In the above configuration, at portions where the developing rollers  12 M,  12 C,  12 Y,  12 K are in contact with the photosensitive drums  14 M,  14 C,  14 Y,  14 K, the electrostatic latent images having the plus polarity (positively charged) can be developed with the positively charged toners by the reversal developing method, so that images of a very high quality can be formed.  
         [0038]     The belt-like intermediate transfer member  20  is configured by forming an electrically conductive sheet of polycarbonate, polyimide, or the like into a belt-like shape. As shown in  FIG. 1 , the belt-like intermediate transfer member  20  is looped around one driving roller  21  and two driven rollers  22 ,  23 . Intermediate transfer rollers  24 M,  24 C,  24 Y,  24 K are disposed in the vicinities of positions where the intermediate transfer member is opposed to the photosensitive drums  14 M,  14 C,  14 Y,  14 K, respectively. As shown in  FIG. 1 , the surface of the intermediate transfer member  20  on the side opposed to the photosensitive drums  14 M,  14 C,  14 Y,  14 K is set so as to be moved from the vertically upper side to the lower side.  
         [0039]     When a predetermined voltage is applied by a power source (not shown) to the intermediate transfer rollers  24 M,  24 C,  24 Y,  24 K, the toner images formed on the photosensitive drums  14 M,  14 C,  14 Y,  14 K are transferred to the intermediate transfer member  20 . A secondary transfer roller  25  is opposed to the driven roller  23  which is placed at a position where the toner images are to be transferred to a sheet P (corresponding to a recording medium). When a predetermined potential is applied by a power source (not shown) to the secondary transfer roller  25 , the toner images of the four colors carried on the belt-like intermediate transfer member  20  are transferred to the sheet P which is transported between the secondary transfer roller  25  and the intermediate transfer member  20 .  
         [0040]     The fixing unit  30  is configured by a first heating roller  31  and a second heating roller  32 . The sheet P to which the four-color toner images have been transferred is heated and pressed by the first and second heating rollers  31  and  32  while being clamped and transported, thereby the toner images are fixed to the sheet P.  
         [0041]     The sheet feeding unit  40  is disposed in the lowermost portion of the apparatus, and configured by: a storage tray  41  which accommodates plural sheets P in a stacked manner; and a pick-up roller  42  which separates the stacked sheets P and feeds the sheets one by one. The sheet feeding unit  40  feeds the sheet P in predetermined timing with the image forming step which is conducted by the image forming unit  10  and the intermediate transfer member  20 . The sheet P fed from the sheet feeding unit  40  is transported by a transport roller pair  70  to a transferring position in a sheet transport path  71 , or specifically to a pressure contact portion between the intermediate transfer member  20  and the secondary transfer roller  25 .  
         [0042]     The sheet transport path  71  is a path along which the sheet P fed from the storage tray  41  of the sheet feeding unit  40  is sent to the fixing unit  30  through the transport roller pair  70  and the pressure contact portion between the intermediate transfer member  20  and the secondary transfer roller  25 .  
         [0043]     The sheet P which has passed through the fixing unit  30  is transported by a discharge roller pair  61  toward the sheet discharging tray  50 .  
         [0044]     The sheet discharging tray  50  is disposed in the uppermost portion of the apparatus, and accommodates the sheet P which is transported by the discharge roller pair  61  to be discharged.  
         [0045]     The inverting and transporting unit  60  is configured by the discharge roller pair  61 , a flapper  62 , an inversion and transport path  63 , an inversion and transport path  64 , and inverting and transporting roller pairs  65 ,  66 ,  67 . The discharge roller pair  61  can be switched over between forward and reverse rotations. When the sheet P is to be discharged to the sheet discharging tray  50 , the roller pairs are forward rotated, and, when the sheet P is to be transported to the inversion and transport path  63 , the roller pairs are reversely rotated.  
         [0046]     The flapper  62  is disposed in a portion branching in the direction along which the sheet P is to be discharged to the sheet discharging tray  50 , and that along which the sheet is to be transported to the inversion and transport path  63 . Depending on excitation and non-excitation of a switching solenoid which is not shown, the flapper can be swung between a position where the sheet P is discharged to the sheet discharging tray  50  (the position indicated by the solid line in  FIG. 1 ), and that where the sheet is transported to the inversion and transport path  63  (the position indicated by the broken line in  FIG. 1 ).  
         [0047]     The inverting and transporting roller pairs  65 ,  66 ,  67  are disposed in the inversion and transport path  64  so that the sheet P transported from the inversion and transport path  63  to the inversion and transport path  64  is transported from the vertically upper side to the lower side to again transport the sheet to the transport roller pair  70 .  
         [0048]     Next, the operation of the thus configured color laser printer  1  of the embodiment will be described. First, the surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K are uniformly charged by the charging devices  15 M,  15 C,  15 Y,  15 K, and then exposed by the exposing devices  17 M,  17 C,  17 Y,  17 K in accordance with magenta, cyan, yellow, and black images, respectively. The magenta, cyan, yellow, and black developing devices  11 M,  11 C,  11 Y,  11 K causes magenta, cyan, yellow, and black toners to adhere to electrostatic latent images formed in the surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, thereby developing magenta, cyan, yellow, and black images. The magenta, cyan, yellow, and black toner images which are formed as described above are once transferred onto the surface of the intermediate transfer member  20 .  
         [0049]     After the transfer process, residual toners on the photosensitive drums  14 M,  14 C,  14 Y,  14 K are temporarily held by the cleaning rollers  16 M,  16 C,  16 Y,  16 K. The toner images of the colors are formed with producing a small time difference therebetween in accordance with the movement speed of the intermediate transfer member  20  and the positions of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, and then transferred so as to overlap with one another on the intermediate transfer member  20 .  
         [0050]     The four-color toner images which are formed on the intermediate transfer member  20  as described above are transferred to the sheet P fed from the sheet feeding unit  40 , at the pressure contact portion between the secondary transfer roller  25  and the intermediate transfer member  20 . The transferred toner images are fixed to the sheet P by the fixing unit  30 , and the sheet is then discharged onto the sheet discharging tray  50 . As a result, a four-color image is formed.  
         [0051]     In the case where images are to be formed respectively on both the faces of the sheet P, the printer operates in the following manner. When the sheet P in which an image has been formed on one face is sent to the discharge roller pair  61 , the discharge roller pair  61  is forward rotated while clamping the sheet P, to once transport the sheet P toward the outside (the sheet discharging tray  50 ). Immediately after a major portion of the sheet P is sent to the outside and the rear end of the sheet P is detected by a discharge sensor  68 , the forward rotation is stopped, and the transportation direction is switched over by the flapper  62  so that the sheet P can be transported to the inversion and transport path  63 .  
         [0052]     Then, the discharge roller pair  61  is reversely rotated to transport the sheet P into the inversion and transport path  63 . The sheet P which is transported to the inversion and transport path  63  is further transported to the transport roller pair  70  by the inverting and transporting roller pairs  65 ,  66 ,  67 .  
         [0053]     The sheet P which is transported to the transport roller pair  70  is in the reversed state, and again transported by the transport roller pair  70  toward the pressure contact portion between the secondary transfer roller  25  and the intermediate transfer member  20 . Toner images are then transferred to the back face, and then fixed thereto. As a result, images are formed respectively on both the faces of the sheet P.  
         [0054]     Next, an opening and closing mechanism of the color laser printer  1  for opening and closing an opening portion  190  (see  FIG. 6 ) through which the process units are to be attached and detached, and the sheet transport path  71  or the inversion and transport path  64  will be described.  FIG. 2  is a schematic view of the color laser painter  1 , and  FIG. 3  is a view showing the configuration of an opening and closing mechanism.  
         [0055]     As shown in  FIG. 2 , a front cover  110 , a frame  120 , a frame  130 , and a frame  140  are disposed in a main unit casing  100  of the color laser printer  1 . The inversion and transport path  64  is formed between the front cover  110  and the frame  120 , and the sheet transport path  71  is formed between the frame  120  and the frame  130 . When the frame  130  is opened, the opening portion  190  (see  FIG. 6 ) for allowing the process units to be attached or detached is opened.  
         [0056]     Driven rollers  65   a ,  66   a ,  67   a  of the inverting and transporting roller pairs are supported by the front cover  110 . Driving rollers  65   b ,  66   b ,  67   b  of the inverting and transporting roller pairs, the transport roller pair  70 , and the secondary transfer roller  25  are supported by the frame  120 .  
         [0057]     The intermediate transfer member  20 , the intermediate transfer rollers  24 M,  24 C,  24 Y,  24 K, the driving roller  21 , and the driven rollers  22  and  23  are supported by the frame  130 . The image forming unit  10 , the fixing unit  30 , and a part of the inverting and transporting unit  60  are supported by the frame  140 .  
         [0058]     The front cover  110 , the frame  120 , and the frame  130  are swingable about a rotation shaft  101 , so as to be supported openably and closably by a support frame  150  which is secured to the frame  140  (see  FIG. 3 ). Support shafts  111  are disposed on both the side faces of the front cover  110 , respectively (in  FIG. 3 , only one side face is shown). Lock levers  112  are supported so as to be swingable about the support shafts  111 . At one end, the lock levers  112  have hook portions  112   a  which are engageable and disengageable with projections  128  disposed on the side faces of the frame  120 , respectively. The lock levers are urged by springs  113  attached to the front cover  110 , in the direction along which the hook portions  112   a  are to be engaged with the projections  128 , respectively. A handle  117  is attached to the other ends of the lock levers  112  so as to connect the other ends together.  
         [0059]     In a state where the hook portions  112   a  are caused by the springs  113  to be engaged with the projections  128 , the tip ends of the hook portions  112   a  are engaged with the projections  128 , and, when the handle  117  is operated, the lock levers  112  can be moved in the urging direction of the springs  113 .  
         [0060]     Support shafts  121  are disposed on both the side faces of the frame  120 , and lock levers  122  are supported so as to be swingable about the support shafts  121 , respectively. At one end, the lock levers  122  have hook portions  122   a  which are engageable and disengageable with projections  138  disposed on the side faces of the frame  130 , respectively. The lock levers are urged by springs  123  attached to the frame  120 , in the direction along which the hook portions  122   a  are not engaged with the projections  138 , respectively. The other ends of the lock levers  122  are opposed to the hook portions  112   a  of the lock levers  112 .  
         [0061]     Support shafts  114  are disposed on both the side faces of the front cover  110 , and engaging levers  115  are supported so as to be swingable about the support shafts  114 , respectively. At one end, the engaging levers  115  have engaging pawls  115   a  which are engageable and disengageable with projections  127  disposed on the side faces of the frame  120 , respectively. The engaging levers are urged by springs  116  attached to the front cover  110 , in the direction along which the engaging pawls  115   a  are to be engaged with the projections  127 , respectively.  
         [0062]     A positioning projection  161  which protrudes toward the frame  120  is disposed at each of two or upper and lower positions of the both sides of the front cover  110 . The positioning projections  161  can be fitted into grooves  162  disposed in front faces of the both sides of the frame  120 , respectively. When the projections are fitted into the grooves, the position in the case where the front cover  110  is closed is determined.  
         [0063]     Support shafts  124  are disposed on both the side faces of the frame  120 , and engaging levers  125  are supported so as to be swingable about the support shafts  124 , respectively. At one end, the engaging levers  125  have engaging pawls  125   a  which are engageable and disengageable with projections  137  disposed on the side faces of the frame  130 , respectively. The engaging levers are urged by springs  126  attached to the frame  120 , in the direction along which the engaging pawls  125   a  are to be engaged with the projections  137 , respectively.  
         [0064]     A positioning projection  163  which protrudes toward the frame  130  is disposed at each of two or upper and lower positions of the both sides of the frame  120 . The positioning projections  163  can be fitted into grooves  164  disposed in front faces of the both sides of the frame  130 , respectively. When the projections are fitted into the grooves, the position in the case where the frame  120  is closed is determined.  
         [0065]     Support shafts  134  are disposed on both the side faces of the frame  130 , and engaging levers  135  are supported so as to be swingable about the support shafts  134 , respectively. At one end, the engaging levers  135  have engaging pawls  135   a  which are engageable and disengageable with projections  147  disposed on the side faces of the frame  140 , respectively. The engaging levers are urged by springs  136  attached to the frame  130 , in the direction along which the engaging pawls  135   a  are to be engaged with the projections  147 , respectively.  
         [0066]     A positioning projection  165  which protrudes toward the frame  140  is disposed at each of two or upper and lower positions of the both sides of the frame  130 . The positioning projections  165  can be fitted into grooves  166  disposed in front faces of the both sides of the frame  140 , respectively. When the projections are fitted into the grooves, the position in the case where the frame  130  is closed is determined.  
         [0067]     The urging forces of the springs  116 ,  126 ,  136  become larger in this sequence.  
         [0068]     Next, an operation in the case where only the front cover  110  is to be opened will be described with reference to  FIG. 4 .  
         [0069]     When the front cover  110 , the frame  120 , and the frame  130  are in the closed state, the hook portions  112   a  of the lock levers  112  are engaged with the projections  128 , the hook portions  122   a  of the lock levers  122  are not engaged with the projections  138 , the engaging pawls  115   a  of the engaging levers  115  are engaged with the projections  127 , the engaging pawls  125   a  of the engaging levers  125  are engaged with the projections  137 , and the engaging pawls  135   a  of the engaging levers  135  are engaged with the projections  147 .  
         [0070]     As shown in  FIG. 4A , the handle  117  is swung against the urging force of the springs  113  in the direction A, specifically in a downward direction, and the engagement between the hook portions  112   a  and the projections  128  is then canceled. When the handle  117  is thereafter pulled in the direction B, specifically in a forward direction, the engagement between the engaging pawls  115   a  and the projections  127  is canceled because the urging forces of the springs  116  are smaller than those of the springs  126  and  136  and the engaging pawls  115   a  elongate at an obtuse angle with respect to the direction along which the front cover  110  is to be opened (the direction which is substantially identical with the direction B in the projections  127 ) and butt against the projections  127  to be engaged therewith. As shown in  FIG. 4B , therefore, only the front cover  110  is opened, and the inversion and transport path  64  is opened.  
         [0071]     When, in the state shown in  FIG. 4B , a handle  129  disposed on a side face of the frame  120  is pulled in the direction B, the engagement between the engaging pawls  125   a  and the projections  137  is canceled because the hook portions  122   a  of the lock levers  122  are not engaged with the projections  138 , the urging forces of the springs  126  are smaller than those of the springs  136 , and the engaging pawls  125   a  elongate at an obtuse angle with respect to the direction along which the frame  120  is to be opened (the direction which is substantially identical with the direction B in the projections  137 ) and butt against the projections  137  to be engaged therewith. Therefore, the frame  120  is opened, and the sheet transport path  71  is opened. When, in the state where the frame  120  is opened, a handle  139  disposed on a side face of the frame  130  is pulled in the direction B, the engagement between the engaging pawls  135   a  and the projections  147  is canceled because the engaging pawls  135   a  elongate at an obtuse angle with respect to the direction along which the frame  130  is to be opened (the direction which is substantially identical with the direction B in the projections  147 ) and butt against the projections  147  to be engaged therewith. Therefore, the frame  130  is opened, and the opening portion  190  for enabling the process units to be attached and detached is opened.  
         [0072]     When, in the state shown in  FIG. 4B , the handle  139  of the frame  130  is pulled in the direction B, the frame  120  and the frame  130  are integrally opened because the engaging pawls  125   a  of the engaging levers  125  are engaged with the projections  137 . As a result, the opening portion  190  is opened.  
         [0073]     Next, an operation in the case where the front cover  110  and the frame  120  are to be integrally opened without opening the frame  130  will be described with reference to  FIG. 5 .  
         [0074]     As shown in  FIG. 5A , when, in the state where the front cover  110 , the frame  120 , and the frame  130  are closed, the handle  117  is pulled in the direction B, the engagement between the hook portions  112   a  and the projections  128  is not canceled, and the front cover  110  is integrated with the frame  120  because the hook portions  112   a  elongate at a right angle with respect to the direction along which the front cover  110  is to be opened (the direction which is substantially identical with the direction B in the projections  128 ) and butt against the projections  128  to be engaged therewith, and the engagement between the engaging pawls  125   a  and the projections  137  is canceled because the urging forces of the springs  126  are smaller than those of the springs  136  and the engaging pawls  125   a  elongate at an obtuse angle with respect to the direction along which the frame  120  is to be opened (the direction which is substantially identical with the direction B in the projections  137 ) and butt against the projections  137  to be engaged therewith. As shown in  FIG. 5B , therefore, the front cover  110  and the frame  120  are integrally opened, and the sheet transport path  71  is opened.  
         [0075]     When, in the state shown in  FIG. 5B , the handle  139  of the frame  130  is pulled in the direction B, the engagement between the engaging pawls  135   a  and the projections  147  is canceled because the engaging pawls  135   a  elongate at an obtuse angle with respect to the direction along which the frame  130  is to be opened (the direction which is substantially identical with the direction B in the projections  147 ) and butt against the projections  147  to be engaged therewith. As a result, the frame  130  is opened, and the opening portion  190  for enabling the process units to be attached and detached is opened.  
         [0076]     Next, an operation in the case where the front cover  110 , the frame  120 , and the frame  130  are to be integrally opened will be described with reference to  FIG. 6 .  
         [0077]     As shown in  FIG. 6A , when, in the state where the front cover  110 , the frame  120 , and the frame  130  are closed, the handle  117  is swung in the direction C, specifically in an upward direction, the hook portions  112   a  downward push the other ends of the lock levers  122  while the engagement between the hook portions  122   a  and the projections  128  is maintained. Therefore, the lock levers  122  are swung to cause the hook portions  122   a  to be engaged with the projections  138 . When the handle  117  is thereafter pulled in the direction B, the hook portions  112   a  of the lock levers  112  are engaged with the projections  128  because the hook portions  112   a  elongate at a right angle with respect to the direction along which the front cover  110  is to be opened (the direction which is substantially identical with the direction B in the projections  128 ) and butt against the projections  128  to be engaged therewith; the hook portions  122   a  of the lock levers  122  are engaged with the projections  128  because the hook portions  122   a  elongate at a right angle with respect to the direction along which the frame  120  is to be opened (the direction which is substantially identical with the direction B in the projections  138 ) and butt against the projections  138  to be engaged therewith; and the engagement between the engaging pawls  135   a  and the projections  147  is canceled because the engagement between the hook portions  122   a  and the projections  138  is held, the front cover  110 , the frame  120 , and the frame  130  are integrated with one another, and the engaging pawls  135   a  elongate at an obtuse angle with respect to the direction along which the front cover  130  is to be opened (the direction which is substantially identical with the direction B in the projections  147 ) and butt against the projections  147  to be engaged therewith. As shown in  FIG. 6B , therefore, the front cover  110 , the frame  120 , and the frame  130  are integrally opened, and the opening portion  190  for enabling the process units to be attached and detached is opened.  
         [0078]     In Embodiment 1 described above, the frame  140  and the support frame  150  function as the mainframe, the frame  130  functions as the first opening and closing member, the frame  120  functions as the second opening and closing member, the front cover  110  as the third opening and closing member, and the photosensitive drums  14  function as the electrostatic latent image carriers.  
         [0079]     The support shafts  121 , the lock levers  122 , the hook portions  122   a , the springs  123 , the projections  138 , and the handle  117  function as the first coupling section and the first canceling section, and the support shafts  111 , the lock levers  112 , the hook portions  112   a , the springs  113 , the projections  128 , and the handle  117  function as the second coupling section and the second canceling section.  
         [0080]     The projections  147  function as the first engaged member, the projections  137  as the second engaged member, and the projections  127  as the third engaged member.  
         [0081]     The engaging levers  135  function as the first engaging member, the engaging levers  125  as the second engaging member, and the engaging levers  115  as the third engaging member.  
         [0082]     The springs  136  function as the first urging section, the springs  126  as the second urging section, and the springs  116  as the third urging section.  
         [0083]     The front cover  110  functions as the cover member, and the handle  117  as the operating portion.  
         [0084]     The series of operations consisting of the operations on the handle  117  in the directions A and B shown in  FIGS. 4A and 4B  functions as the third operation, the operation on the handle  117  in the direction B shown in  FIG. 5  functions as the fourth operation, and the series of operations consisting of the operations on the handle  117  in the directions C and B shown in  FIGS. 6A and 6B  functions as the fifth operation.  
         [0085]     The intermediate transfer rollers  24  function as the intermediate transferring section, the secondary transfer roller  25  functions as the recording medium transferring section in the invention, and the discharge roller pair  61  and the flapper  62  function as the inverting section.  
         [0086]     In the thus configured color laser printer  1 , it is possible to open only the front cover  110  with respect to the frame  120 , and the front cover  110  and the frame  120  can be integrally opened without opening the frame  130 . Furthermore, the front cover  110 , the frame  120 , and the frame  130  can be integrally opened.  
         [0087]     When the sheet P is jammed in the inversion and transport path  64 , therefore, the inversion and transport path  64  is opened by opening only the front cover  110 , so that the sheet P jammed in the inversion and transport path  64  can be removed away. When the sheet P is jammed in the sheet transport path  71 , moreover, the sheet transport path  71  can be opened by integrally opening the frame  120  and the front cover  110 . Therefore, the sheet P jammed in the sheet transport path  71  can be easily removed away. When the process units are to be attached or detached, moreover, the opening portion  190  for enabling the process units to be attached or detached can be opened by integrally opening the frame  130 , the frame  120 , and the front cover  110 . Therefore, the opening portion  190  can be easily opened.  
         [0088]     After only the front cover  110  is opened, the frame  120  can be opened without opening the frame  130 ; after the frame  120  is opened, the frame  130  can be opened; and, after only the front cover  110  is opened, the frame  120  and the frame  130  can be integrally opened.  
         [0089]     In the case where the sheet transport path  71  is to be opened after the inversion and transport path  64  is opened, it is not required to perform a procedure in which the front cover  110  is once closed and the front cover  110  and the frame  120  are then integrally opened. Consequently, the sheet transport path  71  can be easily opened. In the case where the opening portion  190  is to be opened after the sheet transport path  71  is opened, similarly, it is not required to perform a procedure in which the front cover  110  and the frame  120  are once closed and the front cover  110 , the frame  120 , and the frame  130  are then integrally opened. Consequently, the opening portion  190  can be easily opened.  
         [0090]     In the case where the opening portion  190  is to be opened after the inversion and transport path  64  is opened, similarly, it is not required to perform a procedure in which the front cover  110  is once closed and the front cover  110 , the frame  120 , and the frame  130  are then integrally opened. Consequently, the opening portion  190  can be easily opened.  
         [0091]     By the series of operations consisting of the operations on the handle  117  in the directions A and B shown in  FIG. 4 , the engagement between the hook portions  112   a  and the projections  128  is canceled, and only the front cover  110  is opened, so that the inversion and transport path  64  can be opened. By the operation on the handle  117  in the direction B shown in  FIG. 5 , the front cover  110  and the frame  120  are integrally opened while the engagement between the hook portions  112   a  and the projections  128  is maintained, so that the sheet transport path  71  can be opened. By the series of operations consisting of the operations on the handle  117  in the directions C and B shown in  FIG. 6 , the hook portions  122   a  are engaged with the projections  138 , and the front cover  110 , the frame  120 , and the frame  130  are integrally opened while the engagement between the hook portions  112   a  and the projections  128  is maintained, so that the opening portion  190  can be opened.  
         [0092]     In accordance with an operation on the handle  117 , therefore, it is possible to select whether only the front cover  110  is opened to open the inversion and transport path  64 , the front cover  110  and the frame  120  are integrally opened without opening the frame  130  to open the sheet transport path  71 , or the front cover  110 , the frame  120 , and the frame  130  are integrally opened to open the opening portion  190 . Since the place to be opened can be changed by an operation on the single handle  117 , the operator can easily operate the printer.  
         [0093]     Since the fixing unit  30  is placed in a level higher than the photosensitive drums  14 , the photosensitive drums  14  can be prevented from being heated by the heat of the fixing unit  30 . Since the photosensitive drums  14 , the sheet feeding unit  40 , and the sheet discharging tray  50  are placed in a vertically stacked manner, the installation area of the color laser printer  1  can be reduced.  
       Embodiment 2  
       [0094]     Hereinafter, Embodiment 2 of the invention will be described with reference to the accompanying drawings.  
         [0095]      FIG. 7  is a schematic side section view of a color laser printer  2  that is an image forming apparatus to which the invention is applied. The color laser printer  2  which is exemplarily shown in  FIG. 7  comprises an image forming unit  10 , a sheet transport belt  20 A, a fixing unit  30 , a sheet feeding unit  40 , a sheet discharging tray  50 , and an inverting and transporting unit  60 .  
         [0096]     These components other than the sheet transport belt  20 A are identical with those of Embodiment 1, and therefore the description of the components other than the sheet transport belt  20 A is omitted.  
         [0097]     As shown in  FIG. 7 , the sheet transport belt  20 A is looped around one driving roller  21 A and two driven rollers  22 A,  23 A. Transfer rollers  24 AM,  24 AC,  24 AY,  24 AK are disposed in the vicinities of positions where the belt is opposed to photosensitive drums  14 M,  14 C,  14 Y,  14 K, respectively. As shown in  FIG. 7 , the surface of the sheet transport belt  20 A on the side opposed to the photosensitive drums  14 M,  14 C,  14 Y,  14 K is set so as to be moved from the vertically lower side to the upper side.  
         [0098]     Next, the operation of the thus configured color laser printer  2  of the embodiment will be described.  
         [0099]     First, the surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K are uniformly charged by charging devices  15 M,  15 C,  15 Y,  15 K, and then exposed by exposing devices  17 M,  17 C,  17 Y,  17 K in accordance with magenta, cyan, yellow, and black images, respectively. Magenta, cyan, yellow, and black developing devices  11 M,  11 C,  11 Y,  11 K causes magenta, cyan, yellow, and black toners to adhere to electrostatic latent images formed in the surfaces of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, thereby developing magenta, cyan, yellow, and black images.  
         [0100]     The sheet feeding unit  40  feeds the sheet P in predetermined timing with the image forming step which is conducted by the image forming unit  10 . The sheet P fed from the sheet feeding unit  40  is transported by a transport roller pair  70  to pressure contact portions which are formed respectively between the photosensitive drums  14 M,  14 C,  14 Y,  14 K on a sheet transport path  72 , and portions of the sheet transport belt  20 A where the transfer rollers  24 AM,  24 AC,  24 AY,  24 AK are placed.  
         [0101]     The sheet transport path  72  is a path along which the sheet P fed from the storage tray  41  of the sheet feeding unit  40  is to be sent to the fixing unit  30  through the pressure contact portions. The toner images of the colors are formed with producing a small time difference therebetween in accordance with the movement speed of the sheet transport belt  20 A and the positions of the photosensitive drums  14 M,  14 C,  14 Y,  14 K, and then transferred so as to overlap with one another on the sheet P on the sheet transport belt  20 A.  
         [0102]     The transferred toner images are fixed to the sheet P by the fixing unit  30 , and the sheet is then discharged onto the sheet discharging tray  50 . As a result, a four-color image is formed.  
         [0103]     In the case where images are to be formed respectively on both the faces of the sheet P, the printer operates in the following manner. When the sheet P in which an image has been formed on one face is sent to a discharge roller pair  61 , the discharge roller pair  61  is forward rotated while clamping the sheet P, to once transport the sheet P toward the outside (the sheet discharging tray  50 ). Immediately after a major portion of the sheet P is sent to the outside and the rear end of the sheet P is detected by a discharge sensor  68 , the forward rotation is stopped, and the transportation direction is switched over by a flapper  62  so that the sheet P can be transported to an inversion and transport path  63 .  
         [0104]     Then, the discharge roller pair  61  is reversely rotated to transport the sheet P into the inversion and transport path  63 . The sheet P which is transported to the inversion and transport path  63  is further transported to the transport roller pair  70  by inverting and transporting roller pairs  65 ,  66 ,  67 .  
         [0105]     The sheet P which is transported to the transport roller pair  70  is in the reversed state, and again transported by the transport roller pair  70  toward the pressure contact portions between the photosensitive drums  14  and the portions of the sheet transport belt  20 A where the transfer rollers  24 A are placed. Toner images are transferred to the back face, and then fixed thereto. As a result, images are formed respectively on both the faces of the sheet P.  
         [0106]     The color laser printer  2  comprises a front cover  210 , a frame  220 , and a frame  230  in a main unit casing  200  (see  FIG. 7 ). An inversion and transport path  64  is formed between the front cover  210  and the frame  220 , and the sheet transport path  72  is formed between the frame  220  and the frame  230 . When the frame  220  is opened, an opening portion  190  for enabling the process units to be attached or detached, and the sheet transport path  72  are opened.  
         [0107]     Driven rollers  65   a ,  66   a ,  67   a  of the inverting and transporting roller pairs are supported by the front cover  210 . Driving rollers  65   b ,  66   b ,  67   b  of the inverting and transporting roller pairs, the sheet transport belt  20 A, the transfer rollers  24 AM,  24 AC,  24 AY,  24 AK, the driving roller  21 , and the driven rollers  22  and  23  are supported by the frame  220 . The image forming unit  10 , the fixing unit  30 , and a part of the inverting and transporting unit  60  are supported by the frame  230  (see  FIG. 7 ).  
         [0108]     The front cover  210  and the frame  220  are swingable about a rotation shaft  201 , so as to be supported openably and closably by the frame  230 .  
         [0109]     The opening and closing mechanism of the color laser printer  2  of Embodiment 2 of the invention can be easily configured with reference to that of the color laser printer  1  of Embodiment 1. Therefore, the opening and closing mechanism of the color laser printer  2  will be described with reference to  FIG. 3 .  
         [0110]     In the same arrangement as in the front cover  110 , support shafts  111 , lock levers  112 , springs  113 , a handle  117 , engaging levers  115 , support shafts  114 , and springs  116  are disposed on both the side faces of the front cover  210  (see  FIG. 3 ).  
         [0111]     In the same arrangement as in the frame  120 , a handle  129 , projections  127 , support shafts  124 , engaging levers  125 , springs  126 , and projections  128  are disposed on both the side faces of the frame  220 . In the same arrangement as in the frame  140 , projections  147  are disposed, and a support frame  150  is secured (see  FIG. 3 ).  
         [0112]     The lock levers  112  are supported on the side faces of the front cover  210  so as to be swingable about the support shafts  111 , respectively. At one end, the lock levers  112  have hook portions  112   a  which are engageable and disengageable with the projections  128  disposed on the side faces of the frame  220 , and are urged by the springs  113  attached to the front cover  210 , in the direction along which the hook portions  112   a  are to be engaged with the projections  128 , respectively. The handle  117  is attached to the other ends of the lock levers  112  so as to connect the other ends together.  
         [0113]     In a state where the hook portions  112   a  are caused by the springs  113  to be engaged with the projections  128 , the tip ends of the hook portions  112   a  are engaged with the projections  128 , and, when the handle  117  is operated, the lock levers  112  can be moved in the urging direction of the springs  113 .  
         [0114]     The engaging levers  115  are supported on the side faces of the front cover  210  so as to be swingable about the support shafts  114 , respectively. At one end, the engaging levers  115  have engaging pawls  115   a  which are engageable and disengageable with the projections  127  disposed on the side faces of the frame  220 , and are urged by the springs  116  attached to the frame  210 , in the direction along which the engaging pawls  115   a  are to be engaged with the projections  127 , respectively.  
         [0115]     The engaging levers  125  are supported on the side faces of the frame  220  so as to be swingable about the support shafts  124 , respectively. At one end, the engaging levers  125  have engaging pawls  125   a  which are engageable and disengageable with the projections  147  disposed on the side faces of the frame  230 , and are urged by the springs  126  attached to the frame  220 , in the direction along which the engaging pawls  125   a  are to be engaged with the projections  147 , respectively.  
         [0116]     The urging force of the springs  126  is larger than that of the springs  116 .  
         [0117]     Next, the operation in the case where only the front cover  210  is to be opened will be described with reference to  FIG. 3 .  
         [0118]     When the front cover  210  and the frame  220  are in the closed state, the hook portions  112   a  of the lock levers  112  are engaged with the projections  128 , the engaging pawls  115   a  of the engaging levers  115  are engaged with the projections  127 , and the engaging pawls  125   a  of the engaging levers  125  are engaged with the projections  147 .  
         [0119]     When the handle  117  is swung against the urging force of the springs  113  in a downward direction, the engagement between the hook portions  112   a  and the projections  128  is canceled. When the handle  117  is there after pulled in a forward direction (the leftward direction in  FIG. 3 ), the engagement between the engaging pawls  115   a  and the projections  127  is canceled because the urging force of the springs  116  is smaller than that of the springs  126 . As shown in  FIG. 8A , therefore, only the front cover  210  is opened, and the inversion and transport path  64  is opened.  
         [0120]     When, in the state where only the front cover  210  is opened, the handle  129  disposed on a side face of the frame  220  is pulled in a forward direction (the leftward direction in  FIG. 3 ), the engagement between the engaging pawls  125   a  and the projections  147  is canceled. As shown in  FIG. 8B , therefore, the frame  220  is opened, and the opening portion  190  for enabling the process units to be attached or detached, and the sheet transport path  72  are opened.  
         [0121]     Next, an operation in the case where the front cover  210  and the frame  220  are to be integrally opened will be described with reference to  FIG. 3 .  
         [0122]     When, in the state where the front cover  210  and the frame  220  are closed, the handle  117  is pulled in a forward direction (the leftward direction in  FIG. 3 ), the front cover  210  is integrated with the frame  220  because the hook portions  112   a  of the lock levers  112  are engaged with the projections  128 , and the engagement between the engaging pawls  125   a  and the projections  147  is canceled. As shown in  FIG. 8B , therefore, the front cover  210  and the frame  220  are integrally opened, and the opening portion  190  for enabling the process units to be attached or detached, and the sheet transport path  72  are opened.  
         [0123]     In Embodiment 2 described above, the frame  230  and the support frame  150  function as the apparatus main unit, the frame  220  functions as the first opening and closing member, and the front cover  210  to the second opening and closing member.  
         [0124]     The transfer rollers  24 A function as the recording medium transferring section in the invention, and the discharge roller pair  61  and the flapper  62  function as the inverting section in the invention.  
         [0125]     The series of operations consisting of the operations on the handle  117  in the downward and forward directions functions as the first operation in the invention, and the operation on the handle  117  in the forward direction in the case where the front cover  210  and the frame  220  are to be integrally opened functions as the second operation in the invention.  
         [0126]     In the thus configured color laser printer  2 , it is possible to open only the front cover  210  so as to open the inversion and transport path  64 , and the front cover  210  and the frame  220  can be integrally opened so as to open the sheet transport path  72  and the opening portion  190 .  
         [0127]     When the sheet P is jammed in the inversion and transport path  64 , therefore, the inversion and transport path  64  is opened by opening only the front cover  210  without opening the frame  220 , so that the sheet P jammed in the inversion and transport path  64  can be removed away. When the sheet P is jammed in the sheet transport path  72  or the process units are to be attached or detached, moreover, the sheet transport path  72  and the opening portion  190  can be opened by integrally opening the frame  220  and the front cover  210 . Therefore, the sheet transport path  72  and the opening portion  190  can be easily opened.  
         [0128]     In the above, the embodiments of the invention have been described. It is a matter of course that the mode for carrying out the invention is not restricted to the embodiments, and may be variously modified as far as belonging to the technical scope of the invention.  
         [0129]     In Embodiment 1 described above, the printer comprises the intermediate transfer member, the sheet transport path, and the inversion and transport path so that printing can be conducted on both faces of a sheet. For example, such a printer may comprise an intermediate transfer member and a sheet transport path so that printing can be conducted only on one face of a sheet. In this case, two opening and closing members are used as in Embodiment 2.  
         [0130]     In Embodiment 1, the front cover  110  and the frame  120  can be integrally opened simply by operating the handle  117  in the direction B. Alternatively, the front cover  110 , the frame  120 , and the frame  130  may be configured so as to be integrally opened by the operation. The combinations of the kind of operation and opening and closing members are not restricted to those described above.  
         [0131]     The plural opening and closing members are swingable about the common shaft. Alternatively, the opening and closing members may have respective swing centers.  
         [0132]     The front cover  110  may be swingably disposed on the frame  120 , the frame  120  may be swingably disposed on the frame  130 , and the front cover  110  and the frame  120  may be swingably disposed on the frame  130 .  
         [0133]     In each of the process units, a photosensitive member and a developing device are integrated with each other. Alternatively, a photosensitive member and a developing device may be formed so as to be separately attached to and detached from the color laser printer  1 .  
         [0134]     While the invention has been described in conjunction with the specific embodiments described above, many equivalent alternatives, modifications and variations may become apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention as set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.