Patent Publication Number: US-8528896-B2

Title: Image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2010-244465, filed on Oct. 29, 2010, the entire subject matter of which is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     An aspect of the present invention relates to an image forming apparatus having a pair of frames to hold an image forming unit therein. 
     2. Related Art 
     An image forming apparatus containing an image forming unit to form an image on a recording sheet and a pair of frames to hold the image forming unit in there-between in an exterior covering is known. Inside the covering, the frames may be fixed to each other by purpose-made top and bottom joint members, which are in shapes of plates to connect top portions and bottom portions of the frames respectively. 
     SUMMARY 
     When the purpose-made joint members in the shapes of plates are attached to large areas of the top portions and the bottom portions of the frames, cost for manufacturing the image forming apparatus may increase. In order to reduce manufacturing cost, sizes of the plates being the joint members may be reduced, and the joint members may partially be attached to smaller areas of the top portions and the bottom portions of the frames and may partially connect the frames. In other words, the frames may be fixed to each other by the joint members at smaller areas in the top portions and the bottom portions. Therefore, when the sizes of the joint members are reduced, the frames may be deformed to be separated apart from each other at the non-attached areas when force to widen stretch the frames  21  outward is applied, and rigidity of the frames may be undesirably decreased. 
     In view of the above difficulties, the present invention is advantageous in that an image forming apparatus having a pair of frames, which are restricted from being separated apart from each other even without the purpose-made joint members, is provided. 
     According to an aspect of the present invention, an image forming apparatus is provided. The image forming apparatus includes an image forming unit, which is configured to form an image on a recording sheet having a width and a length, a pair of frames, which are arranged to face each other and hold the image forming unit interposed in there-between, and a sheet cassette, which is configured to store the recording sheet therein and to be movable between an operable position, in which the sheet cassette is settled in between the pair of frames, and a removed position, which is out of the operable position. The sheet cassette is formed to have a pair of lateral walls on widthwise sides of the sheet cassette along a direction of width of the recording sheet, a first engagement portion, which is arranged on a widthwise outer side of one of the lateral walls with clearance being maintained from an outer surface of the one of the lateral walls, and a first joint portion, which connects the first engagement portion and the one of the lateral walls. Space enclosed by the first engagement portion, the first joint portion, and the outer surface of the one of the lateral walls is exposed toward an installation side, from which the sheet cassette is installed in the image forming apparatus. The pair of frames is formed to have a second engagement portion, which is arranged on an inner surface of one of the frames along the direction of width of the recording sheet, and a second joint portion, which connects the second engagement portion and the inner surface of the one of the frames. Space enclosed by the second engagement portion, the second joint portion, and the inner surface of the one of the frames is exposed toward a side opposite from the installation side. When the sheet cassette is in the operable position, the first engagement portion is placed in an engagement position on widthwise outer side with respect to the second engagement portion to be engaged with the second engagement portion. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a cross-sectional side view of a laser printer according to an embodiment of the present invention. 
         FIG. 2  is an exploded view of exterior coverings and a pair of frames in the laser printer according to the embodiment of the present invention. 
         FIG. 3  a perspective view of first engagement claws and a part of a sheet-feed tray with second engagement claws in the laser printer according to the embodiment of the present invention. 
         FIG. 4  is a diagram to illustrate the sheet-feed tray in a removed position in the laser printer according to the embodiment of the present invention. 
         FIG. 5  is a horizontally-sectioned view of the first engagement claws formed in the frames and the second engagement claws formed in the sheet-feed tray in the laser printer according to the embodiment of the present invention. 
         FIG. 6  is a horizontally-sectioned view of first locks and surroundings in the laser printer according to the embodiment of the present invention. 
         FIG. 7  is a cross-sectional side view of a laser printer with a return-conveying unit according to the embodiment of the present invention. 
         FIG. 8  is a diagram to illustrate the return-conveying unit removed out of the laser printer according to the embodiment of the present invention. 
         FIG. 9  is a horizontally-sectioned view of second locks and surroundings in the laser printer according to the embodiment of the present invention. 
         FIG. 10  is a perspective view of first engagement claws and a part of a sheet-feed tray with second engagement claws in a different example according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment according to an aspect of the present invention will be described with reference to the accompanying drawings. In the following description, firstly, an overall configuration of a laser printer  1  being an image forming apparatus will be described. Secondly, more specific configuration of the laser printer  1  will be described in detail. In the present embodiment, directions concerning the laser printer  1  will be referred to based on the orientation of the laser printer  1  shown in  FIG. 1 . Therefore, for example, a viewer&#39;s left-hand side appearing in  FIG. 1  is referred to as a front side of the laser printer  1 , and a right-hand side in  FIG. 1  opposite from the front side is referred to as rear. A side which corresponds to the viewer&#39;s nearer side is referred to as right, and an opposite side from the right, which corresponds to the viewer&#39;s further side, is referred to as left. The up-down direction in  FIG. 1  corresponds to a vertical direction of the laser printer  1 . The right-left direction may be referred to as a widthwise direction, and the front-rear direction may be referred to as a direction of depth. Further, directions of the drawings in  FIGS. 2-10  are similarly based on the orientation of the laser printer  1  as defined above and correspond to those with respect to the laser printer  1  shown in  FIG. 1  even when the laser printer  1  and components thereof are viewed from different angles. 
     Overall Configuration of Laser Printer 
     The laser printer  1  includes an image forming unit  4  and a feeder unit  3 . The image forming unit  4  forms an image on a sheet P of paper being a recording medium. The feeder unit  3  feeds the sheet P to the image forming unit  4 . 
     The feeder unit  4  includes sheet-feed tray  31  being a sheet cassette, which stores the sheet P therein and is detachably attached to a main body  2  of the laser printer  1 , and a feeder system  32 , which conveys the sheet P from the sheet-feed tray  31  along a sheet path to the image forming unit  4 . The sheet P is set in the sheet-feed tray  31  with its width aligned to the widthwise direction of the laser printer  1  and with its length aligned orthogonally to the widthwise direction. 
     The image forming unit  4  includes a scanner unit  5 , a processing cartridge  6 , a transfer roller TR, and a fixing unit  7 . 
     The scanner unit  5  is arranged in an upper section in the main body  2  and includes a laser emitter, polygon mirrors, lenses, and reflection mirrors, which are not shown. The scanner unit  5  emits laser beam to scan a surface of a photosensitive drum  81 , which will be described in detail below. 
     The processing cartridge  6  is detachably attached to the main body  2 . The processing cartridge  6  includes a drum cartridge  8  with a photosensitive drum  81 , and a developer cartridge  9  including a developer roller  91  and toner. 
     In the processing cartridge  6 , a peripheral surface of the photosensitive drum  81  being rotated is evenly charged by a charger (not shown) and exposed to the laser beam selectively emitted from the scanner unit  5 . Thus, potential of the exposed areas is lowered, and a latent image is formed in the lower-potential areas on the surface of the photosensitive drum  81 . 
     Meanwhile, the developer roller  91  being rotated carries the toner contained in the developer cartridge  9  and supplies the toner to the latent image on the surface of the photosensitive drum  81 . When the toner adheres to the surface of the photosensitive drum  81 , a toner image is formed on the surface of the photosensitive drum  81 . When the sheet P is carried in a position between the photosensitive drum  81  and the transfer roller TR, the toner image on the photosensitive drum  81  is transferred onto the surface of the sheet P. Thus, the image is formed on the surface of the sheet P. 
     The fixing unit  7  includes a heat roller  71  and a pressure roller  72 . The pressure roller  82  is arranged in an opposite position from the heat roller  71  to be pressed against the heat roller  71 . When the sheet P with the toner image formed thereon is carried in a position between the heat roller  71  and the pressure roller  72 , the toner image is fixed thereto by the heat and the pressure. 
     The sheet P with the thermally-fixed image is conveyed further by a discharge roller R, which is arranged on a downstream side of the fixing unit  7  along the sheet path, to be ejected out of the main body  2 . 
     Configuration of the Main Body 
     Configuration of components in the main body  2  will be described below in detail. As shown in  FIG. 2 , the main body  2  includes a pair of frames  21 , which are arranged to oppose to each other to hold the image forming unit  4  interposed there-between, and exterior coverings  22 , which cover the image forming unit  4  and the frames  21 . 
     The exterior coverings  22  are made of resin and includes a top panel  221 , which is arranged on top of the pair of frames  21 , a side panels  222 , which are arranged on right and left outer sides of the frames  21 , a front cover  223 , which is arranged on an outer front position of the frames  21 , and a rear cover  224 , which is arranged on an outer rear position of the frames  21 . 
     The top panel  221  is formed to have a discharge tray  221 A, in which the sheet P ejected by the discharge roller R (see  FIG. 1 ) is settled. The top panel  221  is attached to top edges of the frames  21  via adhesive agent. 
     The side panels  222  are fixed to a right-side and left-side outer surfaces of the frames  21 . Each of the side panels  222  is formed to have a front bearing (not shown), which rotatably supports the front cover  223 , and a rear bearing  222 A, which rotatably supports the rear cover  224 . 
     The front cover  223  is formed to have a shape and a size to cover a front opening  2 A (see  FIG. 1 ) of the main body  2 . The front opening  2 A is defined by a front edge of the top panel  221 , front edges of the side panels  222 , and an upper edge of a front surface of the sheet-feed tray  31 . The front cover  223  is rotatably supported by the side panels  222  via the front bearings at a lower edge thereof to expose and cover the front opening  2 A. When the front cover  223  is in an open position (not shown) to expose the front opening  2 A, the processing cartridge  6  can be installed in or removed from the main body  2  through the front opening  2 A. 
     The rear cover  224  is formed to have a shape and a size to cover a rear opening  2 B (see  FIG. 1 ) of the main body  2 . The rear opening  2 B is defined by a rear edge of the top panel  221  and rear edges of the side panels  222 . The rear cover  224  is rotatably supported by the side panels  222  via the rear bearings  222 A at a lower edge thereof to expose and cover the rear opening  2 B. When the rear cover  223  is in an open position (not shown) to expose the rear opening  2 B, the sheet P jammed in the sheet path can be removed therefrom through the rear opening  2 B. 
     The frames  221  may be made of metal or resin and are attached to each other at top portions thereof via the scanner unit  5 . Therefore, the frames  221  are restricted by the scanner unit  5  from being moved in the widthwise (right-left) direction and from being separated from each other at the top portions. It is to be noted that the fixing unit  7  or the processing cartridge  6  (not shown in  FIG. 2 ) does not necessarily restrict the frames  21  from being moved in the widthwise direction. Rather, in order to correctly hold the thermally distensible fixing unit  7 , the fixing unit  7  is supported by the frames  21  via pinholes (not shown), which are elongated in the widthwise direction, and pins (not shown) to be inserted in the pinholes. 
     On lower edges of the frames  21  and in vicinity of the front edges of the frames  21 , a joint beam  214  to connect the side frames  21  at lower front sections thereof is provided. Therefore, the side frames  21  are restricted from being separated apart from each other in the widthwise direction by the joint beam  214  at the lower front sections. 
     Meanwhile, at the lower side and in the vicinity of the rear edges, the frames  21  are attached to each other via first engagement claws  316  (see  FIG. 3 ) in the sheet-feed tray  31  and are restricted from being moved a from each other in the widthwise direction. The attachment of the frames  21  via the sheet-feed tray  31  will be described later in detail. 
     On inner surfaces of the frames  21 , in lower positions, rails  215  are formed. The rails  215  have cross-sectional shapes of “brackets” and extend in the rear-front direction along the inner surfaces of the frames  21  to slidably guide the sheet-feed tray  31  along the direction of depth. Therefore, the sheet-feed tray  31  is movable between an operable position (see  FIG. 1 ), in which the sheet-feed tray  31  is settled in the position between the frames  21 , and a removed position, which is out of the operable position (e.g.,  FIG. 4 ) to allow the sheet P to be removed from the sheet-feed tray  31 . 
     Inside each of the rails  215 , a second engagement claw  216  and a first lock  23  are arranged. The second engagement claw  216  is formed to be engageable with a first engagement claw  316  (see  FIG. 3 ) of the sheet-feed tray  31 , and the first lock  23  is arranged to place the sheet-feed tray  31  in the operable position. The first engagement claw  316  of the sheet-feed tray  31  will be described later in detail. It is to be noted, in the drawings of perspective views such as  FIG. 2  (cf.  FIG. 6 ), that the first locks  23  are drawn in simplified box-shaped figures for convenience. 
     The second engagement claws  216  are arranged along the widthwise direction of the laser printer  1  and extend in parallel with the frames  21  in inner positions with respect to the frames  21  (see  FIG. 3 ). Each of the second engagement claws  216  is arranged to maintain clearance from an inner surface  21 A of the frame  21  and connected with the inner surface  21 A via a second joint piece  217  at a rear end thereof. Therefore, whilst the second joint piece  217  connects the second engagement claw  216  and the inner surface  21 A, space enclosed by the second engagement claw  216 , the second joint piece  217 , and the inner surface  21 A is exposed toward front, i.e., toward a side opposite from an installation side. The installation side refers to a side, from which the sheet-feed tray  31  is installed (e.g., the rear side). 
     Meanwhile, the sheet-feed tray  31  is formed to be smaller in width at a rear end section, and inner-side lateral walls  31 A are formed in the smaller-width section along the widthwise direction. Further, the sheet-feed tray  31  is formed to have first engagement claws  316  and first joint pieces  317  (see  FIG. 3 ). The inner-side lateral walls  31 A are formed on right and left sides of the sheet-feed tray  31  to face each other and to extend in parallel with the direction of depth of the sheet-feed tray  31 . The first engagement claws  316  are arranged to extend in parallel with the inner-side lateral walls  31 A in outer positions with respect to the inner-side lateral walls  31 A. Each of the first engagement claws  316  is arranged to maintain clearance from an outer surface of the inner-side lateral wall  31 A and connected with the inner-side lateral wall  31 A via the first joint piece  317  at a front end thereof. Therefore, whilst the first joint piece  317  connects the first engagement claw  316  and the inner-side lateral wall  31 A, space enclosed by the first engagement claw  316 , the first joint piece  317 , and the inner-side lateral wall  31 A is exposed toward rear, i.e., toward the installation side, from which the sheet-feed tray  31  is installed. 
     When the sheet-feed tray  31  is installed in the operable position (see  FIG. 5 ), the first engagement claws  316  are placed in engagement positions on outer sides with respect to the second engagement claws  216  to be engaged with the second engagement claws  216 . The engagement restricts lower rear portions of the frames  21  from being separated apart from each other in the widthwise direction. 
     It is to be noted that rear edges of the first engagement claws  316  are beveled inwardly, and front edges of the second engagement claws  216  are beveled outwardly in order to lead the first engagement claws  316  smoothly to the outer-side engagement positions with respect to the second engagement claws  216 . 
     As shown in  FIG. 6 , each of the first locks  23  arranged in a position between the sheet-feed tray  31  in the operable position and the frame  21 . The first lock  23  includes a locking arm  231  and a coil spring  232 . 
     The locking arm  231  is formed to have a shape similar to an “L” in a top view. A front end portion  231 A of the locking arm  231  is rotatably attached to a part of the rail  215 , and a rear end portion  231 B is swingable in the widthwise direction within a swingable range. Further, a roller  231 C is rotatably attached to a widthwise tip end of the rear end portion. When the sheet-feed tray  31  is installed in and removed from the operable position, the roller  231  C being in contact with the sheet-feed tray  31  is rotated. Therefore, friction between the locking arm  231  and the sheet-feed tray  31  is reduced, and the sheet-feed tray  31  can be smoothly installed in and removed from the main body  2  of the laser printer  1 . 
     The coil spring  232  is arranged in a position between the frame  21  and the locking arm  231  to press the rear end portion  231 B of the locking arm  231  inwardly against the sheet-feed tray  31 . 
     The sheet-feed tray  31  is formed to have a positioning dent  312  (see  FIG. 3 ), which is inwardly dented in the widthwise direction, in a lower position with respect to the first engagement claw  316  on each outer side of the inner-side lateral wall  31 A. The positioning dents  312  are placed in positions to become in contact with the rollers  231 C of the first locks  23  when the sheet-feed tray  31  is in the operable position. More specifically, the feeder unit  3  is provided with a restricting member (not shown), which restricts the sheet-feed tray  31  in the operable position from being moved rearward. Therefore, when the first locks  23  are urged against rear parts of curvature in the positioning dents  312 , the sheet-feed tray  23  is urged rearward against the restricting member by the first locks  23  and placed in the correct operable position. In other words, the first locks  23 , which apply expanding force to separate the sheet-feed tray  31  and the frames  21  apart from each other to the sheet-feed tray  31 , places the sheet-feed tray  31  in the correct operable position. 
     Further, with a set of the first lock  23 , the first engagement claw  316 , and the second engagement claw  216  being arranged in a coincident position at least partially with reference to the direction of depth (i.e., the direction of installation and removal of the sheet-feed tray  31 ), the first engagement claws  316  and the second engagement claws  216  arranged in vicinities of the first locks  23  can effectively absorb the expanding force from the first locks  23  and restrict the frames  21  from being separated apart from each other whilst the first locks  23  tend to separate the frames  21  apart from each other. 
     It may be noted in the present embodiment that, within a set of the first engagement claw  316 , the second engagement claw  216 , and the first lock  23 , the first engagement claw  316  and the second engagement claw  216  may not necessarily be in the coincident position with the first lock  23 . For example, one of the first engagement claw  316  and the second engagement claw  216  may be partially in the coincident position with the first lock  23 , and the other of the first engagement claw  316  and the second engagement claw  216  may be in a different position. 
     Further, the first lock  23 , the first engagement claw  316 , and the second engagement claw  216  may not necessarily be arranged in the rear positions with reference to the direction of depth. For example, the first lock  23  may be arranged in the rear position, and the first and second engagement claws  316 ,  216  may be arranged in positions closer to the front. 
     Furthermore, a quantity of set of the first engagement claw  316  and the second engagement claw  216  may not necessarily be one. That is, a plurality of sets of the first engagement claw  316  and the second engagement claw  216  may be provided with respect to one first lock  23 . For example, two sets of the first engagement claw  316  and the second engagement claw  216 , one in an upper position and the other in a lower position with respect to the first lock  23 , may be provided on each widthwise side. In this configuration, four sets of the first engagement claw  316  and the second engagement claw  216  are provided in total. Thus, the frames  21  can be restricted from being separated apart from each other even more securely. 
     With the above-described configuration, when the sheet-feed tray  31  is in the operable position, the first engagement claws  316  of the sheet-feed tray  31  are engaged with the second engagement claws  216  of the frames  12 . Therefore, the frames  21  are restricted from being separated apart from each other without employing a specific purpose-made joint member. Specifically, when the first locks  23  apply the expanding force, which may tend to separate the frames  21  apart from each other, the engagement of the first engagement claws  316  and the second engagement claws  216  effectively restrict the frames  21  from being separated apart. 
     According to the above-described configuration, a set of the first lock  23 , the first engagement claw  316 , and the second engagement claw  216  is arranged in the coincident position with reference to the direction of depth. Therefore, the expanding force from the first lock  23  can be absorbed promptly in the first engagement claw  316  and the second engagement claw  216 , which are in the vicinity of the first lock  23 . 
     Although an example of carrying out the invention has been described, those skilled in the art will appreciate that there are numerous variations and permutations of the image forming apparatus that falls within the spirit and scope of the invention as set forth in the appended claims. It is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or act described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. 
     For example, the frames  21  may not necessarily be restricted from being separated apart from each other by the expanding force applied from the first locks  21  of the sheet-feed tray  31  but may be restricted by expanding force applied from other locking mechanisms. 
     More specifically, an image forming apparatus  1  with a return-conveyer  400  will be described below with reference to  FIGS. 7-9 . The return-conveyer  400  enables the sheet P with an image formed on one side thereof to be reversed and returned to the image forming unit  4  in order to allow images to be formed on both sides of the sheet P. The return-conveyer  400  includes a discharge roller R, a sheet guide (not shown), and a return-conveying unit  410 . The discharge roller R conveys the sheet P having been conveyed thereto backwards, and the sheet guide guides the backward sheet P to the return-conveying unit  410 . In this regard, whilst the sheet P is guided, the sheet P is reversed (turned over). The return-conveying unit  410  conveys the sheet P with the image formed on one side thereof in the reversed orientation to the image forming unit  4 . 
     The return-conveying unit  410  includes a plurality of return-conveyer rollers  411 , which carry the reversed sheet P toward the image forming unit  4 , and a sheet guide (not shown). The return-conveying unit  41  is installable in and removable from the main body  2  through the rear opening  2 B. Therefore, the return-conveying unit  410  is movable between an operable inner position (see  FIG. 7 ), in which the return-conveying unit  410  is settled a position between the frames  21 , and a removed outer position, which is out of the operable inner position (e.g.,  FIG. 8 ). 
     In a position between the return-conveying unit  410  settled in the operable inner position and the frame  21 , a second lock  500  to place the return-conveying unit  410  in the correct operable inner position is provided on each widthwise side of the return-conveying unit  410 . The second lock  500  applies expanding force, which tends to separate return-conveying unit  410  apart from the frame  21 , to the return-conveying unit  410 . The second locks  500  are arranged inside rails  218 , which are formed on inner surfaces of the frames  21  and allow the return-conveying unit  410  to slidably move along the direction of depth. Each of the second locks  500  includes a locking arm  231  and a coil spring  232 , which are similar to those in the aforementioned first lock  23 . Further, the return-conveying unit  410  is formed to have positioning dents  412 , which are similar to the aforementioned positioning dents  312  in the sheet-feed tray  31 . 
     The return-conveyer  400  is provided with a restricting member (not shown), which restricts the return-conveying unit  410  in the operable inner position from being moved rearward. Therefore, when the second locks  500  are urged against rear parts of curvature in the positioning dents  412 , the return-conveying unit  410  is urged rearward against the restricting member by the second locks  500  and placed in the correct operable inner position. In other words, the second locks  500 , which apply expanding force to the return-conveying unit  410  to separate the return-conveying unit  410  and the frames  21  apart from each other, places the return-conveying unit  410  in the correct operable inner position. 
     In this regard, it is preferable that the second lock  500  is arranged in a position to coincide with at least one of the first engagement claw  316  and the second engagement claw  216 . With the second lock  500  in the coincident position, the expanding force from the second lock  500  can be absorbed promptly in one of the first engagement claw  316  and the second engagement claw  216 , which is in the vicinity of the second lock  500 . Therefore, the frames  21  are effectively restricted from being separated from each other. 
     According to the present invention, the first joint piece  317  may not necessarily be arranged at the front end of the first engagement claw  316 , or the second joint piece  217  may not necessarily be arranged at the rear end of the second engagement claw  216  as long as the space between the first engagement claw  316  and the inner-side lateral wall  31 A is exposed toward the installation side, from which the sheet-feed tray  31  is installed, and the space between the second engagement claw  216  and the inner surface  21 A is exposed toward the side opposite from the installation side. For example, as shown in  FIG. 10 , a first joint piece  617  may be arranged at an upper end of a first engagement claw  616 , and a second joint piece  717  may be arranged at a lower end of a second engagement claw  716 . 
     For another example, the joint beam  214  may be arranged at lower rear positions of the frames  21 , and the first locks  23  may be arranged in front positions. In this configuration, the first engagement claws may be arranged in front sections of the sheet-feed tray  31 , and the second engagement claws may be arranged in lower front sections in the frames  21 . 
     Further, the first locks  23  and the second locks  500  may not necessarily be provided on both of the right and left sides, but a single lock may be provided in a position between one of the frames  21  and the sheet-feed tray  31  (or the return-conveying unit  410 ). In this case, the sheet-feed tray  31  (or the return-conveying unit  410 ) may be urged against the other of the frames  21  by the single lock to be placed in the correct operable position. 
     For another example, the sheet-feed tray  31  may not necessarily be removable from the laser printer  1  as long as the sheet-feed tray  31  is movable with respect to the main body  2 . That is, once the sheet-feed tray  31  is installed in the operable position, the sheet-feed tray  31  may be fixed to the position, for example, via screws and by use of a tool (e.g., a screw driver). 
     The sheet P may not necessarily be paper (e.g., cardboard, postcard, and thinner paper) but may be, for example, an OHP film sheet. 
     For another example, the scanner unit  5  in the image forming unit  4  may be replaced with an LED head. Further, the photosensitive drum  81  in the image forming unit  4  may be replaced with a photosensitive belt. Further, the heat roller  71  may be replaced with a cylindrically-rolled fixing film, which is slidably supported in the fixing unit  7 . Furthermore, the transfer roller TR may be replaced with, for example, conductive brushes or conductive blade springs, as long as the photosensitive drum  81  is capable of bearing the applied transfer bias from the conductive brushes, blade springs, etc. 
     For another example, the frames  21  to hold the image forming unit  4  may not necessarily be arranged on the widthwise sides of the main body  2  but may be arranged on the front and rear sides when the sheet-feed tray  31  is movable in the laser printer  1  along the direction of width. 
     The embodiments described above may not necessarily be applied to a laser printer, but may be employed in, for example, a copier and a multifunction peripheral device. 
     Further, the frames  21  may be restricted from being separated apart from each other by other removable unit, which is installable in and removable from the laser printer  1 , as long as the removable unit is provided with first engagement claws to be engaged with second engagement claws, which are formed in the frames  21 .