Tandem-type process unit removably loaded in image forming device

A tandem type of process unit removably loaded in an image forming device has a plurality of process cartridges, a pair of lower supporting plates, and a pair of upper supporting plates. Each of the plurality of process cartridges has a photosensitive drum rotatably about an axis extending in a first direction. Each of the plurality of process cartridges has two side faces facing each other in the first direction and a cartridge electrode provided on the side face. The plurality of process cartridges is aligned in a second direction perpendicular to the first direction. Each of the lower supporting plates supports a lower part of the side face of each of the plurality of process cartridges. The pair of upper supporting plates is positioned upward and separately from the pair of lower supporting plates. Each of the upper supporting plates supports an upper part of the side face of each of the plurality of process cartridges. The pair of upper support plates and the pair of lower support plates provide a side opening facing the side face of each of the plurality of process cartridges to expose the electrode of each of the plurality of process cartridges toward outside through the side opening.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No. 2008-168155 filed Jun. 27, 2008. The entire content of this priority application is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus such as a color printer, and a tandem-type process unit attached thereto.

BACKGROUND

In electrophotographic color printers, so-called tandem-type color printers have become the mainstream, in which photosensitive drums corresponding to colors of yellow, magenta, cyan, and black, are arranged in parallel.

Among these tandem-type color printers, Japanese Patent Application Publication No. 2007-313033 discloses a printer in which process cartridges corresponding to the respective colors as a whole are detachably attached to the main unit of the printer. Each of the process cartridges includes a photosensitive drum and a developer which are contained together in a case for each color. The process cartridges of the respective colors are placed on a tray which is detachably attached to the main unit. On a side of the tray, intermediate electric contacts corresponding to the respective process cartridges are provided. In a state that the process cartridges are placed on the tray, the cartridge electric contact of each process cartridge is connected to the corresponding intermediate electric contact. The printer main unit includes main-unit electric contacts to be connected to the respective intermediate electric contacts in a state that the tray has been attached to the main unit. The respective cartridge electric contacts and the respective intermediate electric contacts are mechanically connected, and the respective intermediate electric contacts and the main-unit electric contacts are mechanically connected at the same time. As a result, the respective cartridge electric contacts and the respective main-unit electric contacts are electrically connected through the respective intermediate electric contacts. In the state that the respective cartridge electric contacts and the main-unit electric contacts are electrically connected, power can be supplied from the main unit to the respective process cartridges.

However, the structure of the tray side becomes complicated when the above intermediate electric contacts are provided.

Therefore, an object of the present invention is to provide a tandem-type process unit having a simple structure.

SUMMARY

The present invention features a tandem type of process unit removably loaded in an image forming device, having a plurality of process cartridges, a pair of lower supporting plates, and a pair of upper supporting plates. Each of the plurality of process cartridges has a photosensitive drum rotatably about an axis extending in a first direction. Each of the plurality of process cartridges has two side faces facing each other in the first direction and a cartridge electrode provided on the side face. The plurality of process cartridges is aligned in a second direction perpendicular to the first direction. Each of the lower supporting plates supports a lower part of the side face of each of the plurality of process cartridges. The pair of upper supporting plates is positioned upward and separately from the pair of lower supporting plates. Each of the upper supporting plates supports an upper part of the side face of each of the plurality of process cartridges. The pair of upper support plates and the pair of lower support plates provide a side opening facing the side face of each of the plurality of process cartridges to expose the electrode of each of the plurality of process cartridges toward outside through the side opening.

The present invention features an image forming device having a main unit, and a tandem type of process unit removably loaded in the main unit. The tandem type of process unit further includes a plurality of process cartridges, a pair of lower supporting plates, and a pair of upper supporting plates. Each of the plurality of process cartridges has a photosensitive drum rotatably about an axis extending in a first direction. Each of the plurality of process cartridges has two side faces facing each other in the first direction and a cartridge electrode provided on the side face. The plurality of process cartridges is aligned in a second direction perpendicular to the first direction. Each of the lower supporting plates supports a lower part of the side face of each of the plurality of process cartridges. The pair of upper supporting plates is positioned upward and separately from the pair of lower supporting plates. Each of the upper supporting plates supports an upper part of the side face of each of the plurality of process cartridges. The pair of upper support plates and the pair of lower support plates provide a side opening facing the side face of each of the plurality of process cartridges to expose the cartridge electrode of each of the plurality of process cartridges toward outside through the side opening.

The present invention features a tandem type of process unit removably loaded in an image forming device having a plurality of process cartridge, a pair of lower supporting plates, and a pair of upper supporting plates. Each of the plurality of process cartridge has a photosensitive drum rotatably about an axis extending in a first direction, the rotating shaft having two ends. Each of the plurality of process cartridge has two side faces facing each other in the first direction. The plurality of process cartridge is aligned in a second direction perpendicular to the axial direction. Each of the lower supporting plates supports a lower part of the side face of each of the plurality of process cartridge. The pair of upper supporting plates is positioned separately and upward from the pair of lower supporting plates. Each of the upper supporting plates supports an upper part of the side face of each of the plurality of process cartridges.

DETAILED DESCRIPTION

Embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description to follow, the expressions “front”, “rear”, “up”, “left” and “right” are used throughout the description to define the various parts when the printer is disposed in an orientation in which it is intended to be used. Specifically, the side where the front cover4is provided (right side inFIG. 1) is “front” and the opposite side (left side inFIG. 1) is “rear”. The right-left direction is synonymous with the width direction. From a directional point of view, the tandem-type process unit3is described in a state that the tandem-type process unit3has been attached to the main unit2unless explicitly written otherwise.

1. Structure of Color Printer

An image forming apparatus according to one embodiment of the present invention is provided as a tandem-type color printer1. The color printer1has a main unit2. The main unit2includes a tandem-type process unit3. The main unit2has a front cover4on one side thereof so that the tandem-type process unit3is loaded and unloaded in the main unit2through the front cover4.

The tandem-type process unit3includes four process cartridges5. The process cartridges5are aligned in parallel at regular intervals in the front-rear direction. The process cartridges5(5K,5Y,5M,5C) correspond to colors of black, yellow, magenta, and cyan arranged from the front in the mentioned order. Above the tandem-type process unit3, an exposure unit6is provided to emit four beams of laser light corresponding to the respective colors. Instead of the exposure unit6, four LED arrays corresponding to the respective process cartridges5may be provided.

Each of the process cartridges5includes a photosensitive drum7, a developing roller8, and a scorotron charger10. As the photosensitive drum7rotates about an axis, the surface of the photosensitive drum7is evenly electrified by the scorotron charger10. The surface of the photosensitive drum7is then selectively exposed to the laser beams emitted from the exposure unit6. This exposure forms an electrostatic latent image on the surface of the photosensitive drum7. When the photosensitive drum7further rotates and the electrostatic latent image faces the developing roller8, the developing roller8supplies toner to the electrostatic latent image so as to form a toner image on the surface of the photosensitive drum7.

The main unit2further includes a sheet cassette11for holding recording sheets P, at a bottom portion thereof. Each recording sheet P held in the sheet cassette11is fed onto a paper-feeder belt12by a plurality of rollers. The paper-feeder belt12faces the four photosensitive drums7from the lower side of the drums. A transfer roller13is provided to face each photosensitive drum7through the paper-feeder belt12. As the paper-feeder belt12runs, the recording sheet P on the paper-feeder belt12sequentially passes between the paper-feeder belt12and the respective photosensitive drums7. When the respective photosensitive drums7face the recording sheet P, the toner images carried on the surfaces of the photosensitive drums7are transferred onto the recording sheet P by transfer bias applied to the transfer roller13.

A fixing unit14is disposed downstream of the paper-feeder belt12in a feeding direction of the recording sheet P. The recording sheet P on which the toner images have been transferred is fed to the fixing unit14. In the fixing unit14, the toner images are fixed onto the recording sheet P by heat and pressure. The recording sheet P on which the toner images have been fixed is ejected by a plurality of rollers to a sheet discharge tray15provided at a top portion of the main unit2.

2. Tandem Type of Process Unit

(1) Process Cartridge

Referring toFIG. 2, each process cartridge5is formed into a substantial L-shape as viewed from a side. Each process cartridge5includes toner corresponding to the respective colors, in addition to the corresponding photosensitive drum7and the corresponding developing roller8(seeFIG. 1). An opening is provided at a bottom portion of each process cartridge5so that a peripheral surface of the photosensitive drum7is partially exposed from the opening.

A development driving input portion23for transferring drive force to the developing roller8is provided on the left side wall of each process cartridge5.

On the right side wall of each process cartridge5, a cartridge electrode25is provided.

On both side walls of each process cartridge5, projecting parts27are formed respectively as cylindrical lower supported portions. The projecting parts27have a center axis which is on the axis of the photosensitive drum7. The respective projecting parts27project outward from the respective side walls of the process cartridge5in the width direction. The distance between the end surfaces27aof the pair of projecting parts27is larger than the distance between the outer surfaces32aof a pair of lower supporting plates32which will be described later. Alternatively, the distance between the end surfaces27aof the pair of projecting parts27is larger than the distance between the inner surfaces32bof the pair of lower supporting plates32.

As shown inFIG. 2, an input gear28for transferring a drive force to the photosensitive drum7is provided on the projecting part27on the left side wall of each process cartridge5.

Boss members30project outward in the width direction from top end portions of both side walls of each process cartridge5. The boss member30is used for supporting the process cartridge5to upper supporting plates31, which will be described later. The distance between the end surfaces30aof the pair of boss members30is larger than the distance between inner surfaces31bof the pair of upper supporting plates31.

(2) Unit Frame

Referring toFIG. 2, the tandem-type process unit3includes a unit frame20which supports the four process cartridges5together.

The unit frame20includes the pair of upper supporting plates31facing each other in the width direction; the pair of lower supporting plates32arranged at distances and downward from the pair of upper supporting plates31; and a pair of joint plates33facing each other in the front-rear direction.

Each upper supporting plate31is formed into a rectangular shape extending in the front-rear direction as viewed from a side. The front end of each upper supporting plate31has a shorter length than a length of a middle portion thereof in the vertical direction. The rear end of each upper supporting plate31has a longer length than that of the middle portion thereof in the vertical direction, thereby forming a stopper portion34extending in the vertical direction.

Each upper supporting plate31has four notches35whose upper portions are open, formed into substantial U-shapes as viewed from a side. The four notches35are arranged at regular intervals in the front-rear direction. The boss member30of each process cartridge5is engaged in t h e corresponding notch35. As a result, each boss member30is supported by the corresponding notch35in the front-rear direction. The respective boss members30of the four process cartridges5are supported by the respective notches35so that the four process cartridges5are supported together by the pair of upper supporting plates31.

Each lower supporting plate32is formed into a rectangular plate extending in the front-rear direction as viewed from a side. Each lower supporting plate32is shorter than each upper supporting plate31in the front-rear direction. The pair of lower supporting plates32faces each other at a distance shorter than the distance between the pair of upper supporting plates32. In another embodiment, the pair of lower supporting plates32may be positioned at a first center-to-center distance which is shorter a second center-to-center distance between the upper supporting plates31.

Each lower supporting plate32has four notches36whose upper portions are open, formed into substantial V-shapes as viewed from a side. The four notches36are arranged at regular intervals in the front-rear direction. The projecting part27of each process cartridge5is engaged in the corresponding notch36from above. As a result, each projecting part27is supported by the corresponding notch36from beneath. The respective projecting parts27of the four process cartridges5are supported by the respective notches36so that the four process cartridges5are supported together by the pair of lower supporting plates32.

The joint plates33are formed into substantially rectangular plates extending both in the vertical direction and in the width direction as viewed from the rear. Each joint plate33is jointed to the pair of upper supporting plates31and the pair of lower supporting plates32from inside in the width direction.

The front joint plate33is positioned in front of the four process cartridges5. A unit handle37formed into a substantial L-shape as viewed from a side is provided at a front portion of the front joint plate33.

The rear joint plate33is positioned behind the four process cartridges5. As shown inFIG. 3, the rear joint plate33has a narrower middle portion61which is positioned between the portion joined with the pair of upper supporting plates31and the portion joined with the pair of lower supporting plates32. The portion61has a shorter width than the distance between the inner surfaces of the pair of upper supporting plates31in the width direction. As a result, the rear joint plate33is constricted in the middle in the vertical direction due to the middle portion61.

(3) Attachment of Process Cartridge

Each process cartridge5is attached to and removed from the tandem-type process unit3, in a state that the tandem-type process unit3has been removed from the main unit2.

In the state that the tandem-type process unit3has been removed from the main unit2, each process cartridge5is movable in the tandem-type process unit3without being fixed thereto. Specifically, as shown inFIG. 2, the boss member30of each process cartridge5is engaged in the corresponding notch35of each upper supporting plate31of the tandem-type process unit3. Each projecting part27of each process cartridge5is also engaged in the corresponding notch36of each lower supporting plate32of the tandem-type process unit3. As a result, each process cartridge5is removably supported both on the pair of upper supporting plate31and on the pair of the lower supporting plates32.

When each process cartridge5is attached to the tandem-type process unit3, each boss member30and each projecting part27are engaged in the corresponding notch35and the corresponding notch36from above respectively, while the process cartridge5is being guided between the pair of upper supporting plates31. As a result, each process cartridge5is supported by the pair of upper supporting plates31and the pair of lower supporting plates32, thereby finishing an attachment to the tandem-type process unit3.

3. Internal Structure of Main Casing

As shown inFIG. 4, the main unit2includes a pair of main-unit side plates50facing each other at a distance therebetween in the width direction (only the right main-unit side plate50is shown inFIG. 4). Each main-unit side plate50includes a guiding rail51extending in the front-rear direction; a pair of positioning members52for positioning each process cartridge5with respect to the main unit2; and pressing members53for pressing each process cartridge5to the corresponding positioning member52.

For example, each guiding rail51is formed by deforming the corresponding main-unit side plate50outward in the width direction. As shown inFIG. 7, each guiding rail51is open to the front, and the front end of the guiding rail51has a narrower length than the length of rear end thereof in the vertical direction. The rear end of each guiding rail51includes an inclined face54having a lower edge inclined downward and rearward; and a flat face62extending from the edge of the inclined face54to the rear edge. The rear end of each guiding rail51has a longer length than the length of the stopper portion34of each upper supporting plate31in the vertical direction. The middle portion of each guiding rail51between the front and rear ends thereof has the almost same length as that of the stopper portion34of each upper supporting plate31in the vertical direction.

As shown inFIG. 5, the pair of positioning members52face each other at a distance in the width direction. As shown inFIG. 4, each positioning member52is formed into a substantially rectangular plate extending in the front-rear direction. Each positioning member52is positioned outside of the corresponding lower supporting plate32in the front-rear direction, in a state that the tandem-type process unit3has been attached to the main unit2.

Each positioning member52has four positioning grooves55whose upper portions are open, formed into a substantial V-shape as viewed from a side. The four positioning grooves55are arranged at regular intervals in the front-rear direction, specifically, at the same interval as the notches36so that each positioning groove55faces the corresponding notch36in the width direction when the tandem-type process unit3has been loaded in the main unit2.

As shown inFIG. 7, pressure members53are formed from coil springs and provided to correspond to the respective process cartridges5. One end of the pressure member53is fixed to the top panel of the main unit2. The other lower end of the pressing member53is in contact with the corresponding process cartridge5from above to press the process cartridge5downward when the tandem-type process unit3has been loaded in the main unit2.

As shown inFIGS. 5 and 8, the main unit2includes development driving output portions24and drum driving output portions29so as to transfer a drive force to the process cartridges5. Each development driving output portion24and each drum driving output portion29correspond to each process cartridge5. In the state that the tandem-type process unit3has been loaded in the main unit2, the development driving output portion24and the drum driving output portion29corresponding to each process cartridge5face the corresponding development driving input portion23and the corresponding the drum development driving input portion28of the process cartridge5, respectively. Each development driving output portion24and each drum driving output portion29are joined with the corresponding development driving input portion23and the corresponding drum development driving input portion28respectively, so that drive force for rotating the photosensitive drum7and the developing roller8is transferred to the corresponding process cartridge5through the joined portions of the development driving output portion24and the drum driving output portion29.

The main unit2also includes main-unit electrodes26for supplying electric power to the process cartridges5. Each main-unit electrode26corresponds to the cartridge electrode25of each process cartridge5. As shown inFIGS. 5 and 8, the end of each main-unit electrode26is positioned inside of the pair of upper supporting plates31in the width direction, as the main unit2is viewed from above. In the state that the tandem-type process unit3has been loaded in the main unit2, each main-unit electrode26comes into contact with the corresponding cartridge electrode25of the process cartridge5from the right in the width direction. As a result, each main-unit electrode26and the corresponding cartridge electrode25are electrically and mechanically connected, thereby supplying electric power to the process cartridge5through the main-unit electrode26and the cartridge electrode25.

4. Attachment of Tandem-Type Process Unit

As shown inFIG. 4, in the state that the tandem-type process unit3has been removed from the main unit2, the stopper portion34of each upper supporting plate31is positioned on a front end portion of the guiding rail51. Each guiding rail51has the front end portion shorter than each stopper portion34in the vertical direction. As a result, when each stopper portion34is drawn up to the front end portion of the corresponding guiding rail51, the stopper portion34comes into contact with the front end face of the guiding rail51, thereby preventing the tandem-type process unit3from being drawn out any further. In order to remove the tandem-type process unit3from the main unit2, both stopper portions34are removed from the respective guiding rails51.

As the tandem-type process unit3moves to the rear from the position shown inFIG. 4, the stopper portions34of the respective upper supporting plates31slide along the corresponding guiding rails51so that the tandem-type process unit3is guided into the main unit2as shown inFIG. 6.

As the stopper portions34move downward to the rear along the inclined faces54of the corresponding guiding rails51, the tandem-type process unit3as a whole moves to one level lower downward to the rear, the projecting parts27of the process cartridges5are engaged in the corresponding positioning grooves55of the positioning members52. As shown inFIG. 7, as the stopper portions34comes on the flat faces62, the tandem-type process unit3moves to one more level lower downward to the rear, and the projecting parts27are removed from the corresponding notches36of the lower supporting plates32. As a result, the projecting parts27of the process cartridges5are passed from the corresponding notches36of the lower supporting plates32to the corresponding positioning grooves55of the positioning members52.

On the other hand, the pressing member53is in contact with the corresponding process cartridge5from above to press the process cartridge5from above. As a result, the projecting parts27of the process cartridges5are pressed into the corresponding positioning grooves55, thereby positioning the respective process cartridges5with respect to the main unit2. The loading of the tandem-type process unit3in the main unit2has thus completed.

The lower end of each pressing member53has a member having a face convex downward, for example. As a result, as the tandem-type process unit3slides along the guide rails51, the lower end of each pressing member53smoothly gets on the top face of the corresponding process cartridge5, and smoothly gets off the top face thereof.

As mentioned above, the four process cartridge5including the respective photosensitive drums7are arranged in parallel in the direction orthogonal to the axial directions of the photosensitive drums7(front-rear direction). The upper supporting plates31are arranged on both sides of the process cartridges5in the width direction. The lower supporting plates32are arranged below the respective upper supporting plates31. The four process cartridges5are supported together both by the pair of upper supporting plates31and by the pair of lower supporting plates32.

Each upper supporting plate31is positioned at a distance from the corresponding lower supporting plate32in the vertical direction. Therefore, each side of each process cartridge5in the width direction exposes outside through a side opening between the corresponding supporting plate31and the lower supporting plate32. Therefore, by using the opening between the upper supporting plate31and the corresponding lower supporting plate32, the cartridge electrode25of each process cartridge5and the corresponding main-unit electrode26of the main unit2are exposed toward outside from the opening so as to be directly connected mechanically and electrically. This configuration can remove an intermediate electrode for relaying the electric connection between the cartridge electrode25and the corresponding main-unit electrode26. Furthermore, by using the side opening between each upper supporting plate31and the corresponding lower supporting plate32, the drum development driving input portion28provided in each process cartridge5is exposed toward outside from the opening so as to be directly connected to the corresponding drum driving output portion29. This configuration can remove a through-hole provided on each side of the tandem-type process unit3in the width direction for passing the drum driving output portion29therethrough. Therefore, the configuration of each side of the tandem-type process unit3becomes simpler.

In this embodiment, no intermediate electrodes between the process cartridge and the main unit are required. Accordingly, a design for the positions of the cartridge electrode25and the corresponding main-unit electrode26has more flexibility, if the cartridge electrode25and the corresponding main-unit electrode26satisfy a positional relationship required to establish an electric connection.

When the mechanical connection between each cartridge electrode25of the process cartridge5and the corresponding main-unit electrode26of the main unit2is achieved, the electrical connection between the cartridge electrode25and the main-unit electrode26is also achieved. This configuration can prevent troubles from occurring about the electrical connection between each process cartridge5and the main unit2.

Since the distance between the inner surfaces31bof the pair of upper supporting plates31is longer than the distance of between the inner surfaces32bof the pair of lower supporting plates32, the upper supporting plates31do not interfere with the attachment of the process cartridge5to the pair of lower supporting plates32. This configuration achieves a smooth attachment of the process cartridge5to the pair of lower supporting plates32. In other words, since the distance of the inner surfaces32bof the pair of lower supporting plates32is shorter than the distance of the inner surfaces321bof the pair of upper supporting plates31, the pair of lower supporting plates32can support each process cartridge5stably.

Each process cartridge5includes the pair of boss members30supported by the corresponding pair of upper supporting plates31, and the pair of projecting parts27supported by the corresponding pair of lower supporting plates32. Since the distance between the end surfaces of the pair of projecting parts27is longer than the distance between the outer surfaces32aof the pair of lower supporting plates32, the pair of lower supporting plates32can support the pair of projecting parts27stably. Furthermore, since the distance between the end surfaces of the pair of boss members30is equal to or longer than the distance between the inner surfaces of the pair of upper supporting plates31, the pair of upper supporting plates31can support the pair of boss members30stably. Specifically, the pair of upper supporting plates31and the pair of lower supporting plates32can support the process cartridges5stably from below. Still furthermore, the distance between the inner surfaces of the pair of upper supporting plates31is longer than the distance between the end surfaces of the pair of projecting parts27. Therefore, each projecting part27does not interfere with the corresponding upper supporting plate31when the process cartridge5is attached to the unit frame20, thereby achieving a smooth attachment of the process cartridge5to the unit frame20. As a result, this configuration can achieve both the stable support of the respective process cartridges5by the unit frame20and the smooth attachment of the process cartridge5to the unit frame20at the same time.

Each input gear28is positioned on the axis of the photosensitive drum7. When the drive force is transferred to each input gear28to rotate the corresponding photosensitive drum7about the axis thereof, the photosensitive drum7is not eccentrically rotated. Therefore, the pair of lower supporting plates32can keep supporting the respective process cartridge5stably.

The joint plates33extending both in the vertical direction and in the width direction connect the pair of upper supporting plates31with the pair of lower supporting plates32. Accordingly, the positional relation between the pair of upper supporting plates31, the positional relation between the pair of lower supporting plates32, and the positional relation between each upper supporting plate31and the corresponding lower supporting plate32are maintained constant.

In the rear joint plate33, the portion61positioned between the portion held between the pair of upper supporting plates31and the portion held between the pair of lower supporting plates32is narrower then the distance between the inner surfaces of the pair of upper supporting plates31. Even when the tandem-type process unit3is loaded in the main unit2from the front, each main-unit electrode26projecting to the corresponding process cartridge5rather than and below the corresponding upper supporting plate31in the main unit2does not interfere with the rear joint plate33.

In the state that the tandem-type process unit3has been loaded in the main unit2, the positioning members52provided in the main unit2support the process cartridges5and position the process cartridges5with respect to the main unit2. As a result, the process cartridges5can be precisely positioned with respect to the main unit2.

Each process cartridge5includes the cartridge electrode25. On the other hand, the main unit2includes the main-unit electrodes26. In the state that the tandem-type process unit3has been loaded in the main unit2, each main-unit electrode26is directly connected to the corresponding cartridge electrode25from the outside of the unit frame20in the width direction. The portion where each main-unit electrode26is connected to the corresponding cartridge electrode25is positioned inside of the pair of upper supporting plates31in the width direction. Therefore, each main-unit electrode26can be connected to the corresponding cartridge electrode25easily without any intermediate electrode.

The next description will explain the second embodiment according to the present invention, referring toFIGS. 9-14. Referring toFIGS. 9-14, like elements inFIGS. 1 to 8are identified by the same reference numerals, and their explanation will be omitted.

(1) Drum Lock Lever

Referring toFIGS. 9 and 10, each lower supporting plate32of the unit frame20includes drum lock levers101provided behind the respective notches36. Each drum lock lever101is a bar substantially bent into the shape of a crank and extending in the direction orthogonal to the width direction. Each drum lock lever101is pivotably supported to a supporting shaft104extending in the width direction and provided at a center of the lever101. Each supporting shaft104is supported to the lower supporting plates32. Each drum lock lever101is urged by a spring (not shown) in the counterclockwise direction as viewed from the left. In a state that no external force except the urging force is applied to the lever101, each drum lock lever101is standing erect almost in the vertical direction.

(2) Cam Member

As shown inFIG. 11, the main unit2includes cam members102instead of the positioning members52. Each cam member102is formed from a plate extending in the front-rear direction. Each cam member102overlaps with the lower end of each drum lock lever101in the width direction. In other words, the cam member102is positioned under the drum lock lever101.

Each cam member102has four cam portions103formed into a substantial trapezoid as viewed from a side. The cam portions103are arranged at regular intervals in the front-rear direction. As shown inFIG. 12, the top face of each cam portion103does not interact with the corresponding drum lock lever101, while the tandem-type process unit3is being loaded in or unloaded out of the main unit2. As shown inFIG. 13, each cam portion103has a height so as to be in contact with the lower end of the drum lock lever101in the state that the tandem-type process unit3has been loaded in the main unit2.

(3) Loading of Process Unit in Main Unit

As shown inFIG. 11, in the state that the tandem-type process unit3has been removed from the main unit2, the drum lock lever101keeps standing erect almost in the vertical direction.

As the tandem-type process unit3moves to the rear from the position shown inFIG. 11, the stopper portion34of each upper supporting plate31slides along the guiding rail51so that the tandem-type process unit3is guided into the main unit2as shown inFIG. 12.

As the stopper portion34then moves downward to the rear along the inclined face54of the guiding rail51, the tandem-type process unit3as a whole moves to one level lower downward to the rear, each drum lock lever101comes into contact with the corresponding cam portion103of each cam member102from above. When each drum lock lever101comes in contact with the corresponding cam portion103, the drum lock lever101is rotated in the clockwise direction against the urging force applied in the counterclockwise direction.

As the stopper portion34then reaches the flat face62as shown inFIG. 13, the tandem-type process unit3moves to one more level lower downward to the rear. The drum lock lever101is further rotated in the clockwise direction due to the cam portion103, so that one end of the drum lock lever101comes in contact with the corresponding projecting part27from above to press the corresponding projecting part27to the corresponding notch36. The process cartridges5can thus be positioned with respect to the tandem-type process unit3stably and precisely.

As has been described above, when each drum lock lever101is operated by the corresponding cam portion103of the cam member102, each projecting part27is supported and fixed to the corresponding lower supporting plate32. Each projecting part27is supported and fixed to the corresponding notch36of the lower supporting plate32, thereby positioning the respective process cartridges5to the lower supporting plates32accurately.

It is understood that the foregoing description and accompanying drawings set forth the embodiments of the invention at the present time. Various modifications, additions and alternative designs will, of course, become apparent to those skilled in the art in light of the foregoing teachings without departing from the spirit and scope of the disclosed invention. Thus, it should be appreciated that the invention is not limited to the disclosed embodiments but may be practiced within the full scope of the appended claims.