Image forming unit and image forming apparatus having the same

An image forming unit includes an image forming device that forms an image on a recording medium disposed at a lower portion thereof, a device frame that supports the image forming device at the lower portion, having a first attaching portion to which to attach a first storage frame and a second attaching portion to which to attach a second frame. Either a first storage unit or a second storage unit is attached to the image forming unit. The first storage unit includes a first storage tray having a first storage capacity, a first guide member and the first storage frame. The second storage unit includes a second storage tray having a second storage capacity larger than the first storage capacity, the second guide member and the second storage frame.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming unit for forming an image on a recording medium supplied from a storage tray, and an image forming apparatus having the image forming unit, and particularly relates to an image forming unit to which storage trays having various storage capacities can be removably attached in accordance with the need of users in spite of easiness in design or molding and reduction in cost, and an image forming apparatus having the image forming unit.

2. Background Art

An image forming apparatus such as a printer includes a process unit, a fixing unit, a scanner, and so on as a printing device. Under the printing device, a storage tray for storing sheets of paper as recording media is provided removably. The sheets of paper in the storage tray are fed one by one to an printing device by a paper supply roller disposed above the storage tray or other rollers. Thus, printing is performed on the sheets of paper.

In some of the printers configured thus, another optional tray unit can be additionally attached in accordance with the need of a user (see JP-A-5-17034, page 3 and FIG. 1). That is, the tray unit has another storage tray separate from the storage tray provided in the printer. When the tray unit is additionally attached to the printer, the paper storage capacity is increased so that the frequency of paper replenishment can be reduced.

In such a manner, the frequency of paper replenishment can be reduced by adding a tray unit suitably in accordance with the use conditions. To this end, however, users have to purchase tray units and attach them severally. To solve such labor, it can be therefore considered to manufacture and provide printers for a plurality of models different only in the paper storage capacity of the storage tray. Thus, users can choose printers in accordance with their needs respectively. This solution is very user-friendly because users do not have to purchase tray units and attach them severally.

Here, in order to design printers for a plurality of models different only in the paper storage capacity of the storage tray, configurations as shown inFIGS. 8A and 8Bmay be considered.FIGS. 8A and 8Bare diagrams showing the overall configurations of the printers for a plurality of models different only in the paper storage capacity of the storage tray. First, a printer500shown inFIG. 8Ahas a printing device50such as a process unit, and a storage tray506lying under the printing device50and having a certain storage capacity. Both the printing device50and the storage tray506are supported by frames552from their opposite (left and right) sides. Tray guides561for supporting the storage tray506from the opposite (left and right) sides of the paper ofFIG. 8Aand supporting the weight of the printing device50are disposed between the storage tray506and the frames552respectively. A cover553is provided to cover the outsides of the frames552.

On the other hand, a printer600shown inFIG. 8Bhas a storage tray606having a paper storage capacity larger than that of the storage tray506depicted inFIG. 8A. Further, the printer600has tray guides661, a printing device50and frames652and a cover653in the same manner as the printer500depicted inFIG. 8A. That is, the tray guides661support the storage tray606from the opposite (left and right) sides of the paper ofFIG. 8Band support the weight of the printing device50. The frames652support the printing device50and the tray guides661. The cover653is located outermost.

SUMMARY OF THE INVENTION

However, when printers for a plurality of models different only in the paper storage capacity of the storage tray are designed using the overall configurations shown inFIGS. 8A and 8B, the dimensions of each frame552,652differ from one model to another. Thus, the frame552,652has to be designed for each model or manufactured for each model. As a result, there occurs a problem that the manufacturing cost of the printers increases or the inventory control of the frames becomes complicated. The same things apply to the cover553,653.

An image forming unit is disclosed herein, to which storage trays having various storage capacities can be removably attached in accordance with the need of users in spite of easiness in design or molding and reduction in cost, and an image forming apparatus having the image forming unit.

According to one aspect of the invention, an image forming unit includes: an image forming device that forms an image on a recording medium supplied from a storage tray for storing recording media; a device frame that supports the image forming device; a first attaching portion that attaches a first storage frame of a first storage unit to the device frame, the first storage unit including a first storage tray, a first guide member and the first storage frame, the first storage tray having a first storage capacity and serving as the storage tray, the first guide member guiding the first storage tray, the first storage frame supporting the first guide member; and a second attaching portion that attaches a second storage frame of a second storage unit to the device frame, the second storage unit including a second storage tray, a second guide member and the second storage frame, the second storage tray having a second storage capacity larger than the first storage capacity and serving as the storage tray, the second guide member guiding the second storage tray, the second storage frame supporting the second guide member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a laser printer according to a first embodiment of the invention will be described with reference toFIGS. 1A,1B,2,3A,3B,4A,4B,5A and5B.FIG. 1Ais a schematic perspective view showing the laser printer in which a small tray has been attached to a printer unit, as an example of an image forming unit, according to the first embodiment of the invention.FIG. 1Bis a schematic sectional view taken on line Ib-Ib inFIG. 1A. As shown inFIGS. 1A and 1B, a laser printer1X according to this embodiment is constituted by a printer unit1aand a small tray unit1bdisposed under the printer unit1a.

The printer unit1ahas a printing device50, as an example of an image forming device, which will be described later with reference toFIG. 2, device frames52supporting the printing device50from the left and right sides of the paper ofFIG. 1B, and a cover53disposed outside the device frames52.

FIG. 2is a sectional view taken on line II-II inFIG. 1A. The printing device50has a process unit18, a scanner unit17, a fixing unit19and so on as shown inFIG. 2. The process unit18forms a given image on a sheet of paper3supplied thereto. The fixing unit19fixes the image on the sheet of paper3. Sheets of paper stored in the small tray unit1bare taken out one by one by a paper supply roller9provided at the lower part of the printing device50, and supplied to the process unit18and so on.

A feed path7of the sheet of paper3shown by the chain line inFIG. 2is formed between the paper supply roller9and an image forming position P of the process unit18(a contact portion between a photoconductor drum23and a transfer roller25which will be described later, that is, a transfer position where a toner image on the photoconductor drum23is transferred to the sheet of paper3). Between the paper supply roller9and the image forming position P, a feed roller pair11and a resist roller pair12are disposed in turn at a distance along the feed path7. The sheets of paper3are sent to the feed roller pair11and the resist roller pair12in turn one by one, and corrected for skewing by the resist roller pair12as will be described later. After that, each sheet of paper3is sent to the image forming position P of the process unit18.

On the downstream side of the feed roller pair11in the feed direction, a manual paper feed tray13for feeding the sheet of paper3manually is attached to be collapsible.

The scanner unit17is disposed above the process unit18. The scanner unit17has a laser beam emitting portion (not shown), a polygon mirror20to be driven to rotate, lenses21aand21b, a reflecting mirror22, etc. Then, a laser beam emitted from the laser beam emitting portion in accordance with given image data is passed through or reflected on the polygon mirror20, the lens21a, the reflecting mirror22and the lens21bin that order. Thus, the surface of the photoconductor drum23serving as a photoconductor in the process unit18is scanned and irradiated with the laser beam at a high speed.

The process unit18is constituted by a drum cartridge, a developing cartridge24, and so on. The drum cartridge includes the photoconductor drum23, a Scorotron type charger37serving as a charging unit, the transfer roller25serving as a transfer unit, and so on. The developing cartridge24can be removably attached to the drum cartridge. The developing cartridge24has a toner storage portion26, a developing roller27serving as a developing unit, a layer thickness limiting blade (not shown), a toner feed roller29, etc.

The toner storage portion26is filled with polymer toner of one positive non-magnetic component as a developer. The toner is fed to the developing roller27by the toner feed roller29. In this event, the toner is positively charged by friction between the toner feed roller29and the developing roller27. Further, the toner fed onto the developing roller27is held as a thin layer with a constant thickness on the developing roller27due to rubbing of the layer thickness limiting blade with the rotation of the developing roller27. On the other hand, the rotating photoconductor drum23is disposed to face the developing roller27, and the drum body is grounded. In addition, the surface of the photoconductor drum23is formed out of a positive photoconductor layer made from an organic photoconductor material such as polycarbonate.

The Scorotron type charger37serving as a charging unit is disposed above the photoconductor drum23at a predetermined distance therefrom so as not to touch the photoconductor drum23. The Scorotron type charger37is a positive charging Scorotron type charger that generates corona discharge from a charging wire of tungsten or the like. The Scorotron type charger37is designed to charge the surface of the photoconductor drum23positively uniformly.

The surface of the photoconductor drum23is, first, positively charged uniformly by the Scorotron type charger37with the rotation of the photoconductor drum23. After that, the surface of the photoconductor drum23is exposed to a laser beam emitted from the scanner unit17and scanning the surface at a high speed. Thus, an electrostatic latent image is formed in accordance with given image data. Then, as soon as the toner held on the developing roller27and charged positively touches the photoconductor drum23in opposition thereto due to the rotation of the developing roller27, the toner is supplied to the electrostatic latent image formed on the surface of the photoconductor drum23, that is, to the exposed portion which is part of the surface of the photoconductor drum23positively charged uniformly and whose potential has been lowered due to the exposure to the laser beam. As a result, the toner is held selectively to be visualized. Thus, a toner image is obtained.

The transfer roller25is disposed under the photoconductor drum23so as to face the photoconductor drum23. The transfer roller25has a roller shaft made from metal and coated with a roller made from an ion-conductive rubber material. A transfer bias (transfer forward bias) is applied from a transfer bias applying power source to the transfer roller25at the time of transfer. Thus, the toner image held on the surface of the photoconductor drum23is transferred to the sheet of paper3when the sheet of paper3passes between the photoconductor drum23and the transfer roller25.

The fixing unit19is disposed on the downstream side of the process unit18along the feed path7. The fixing unit19has a heating roller30, a pressure roller31disposed to press the heating roller30, and a feed roller pair32provided on the downstream side of the rollers30and31. The heating roller30is made from metal such as aluminum, and provided with a heater such as a halogen lamp for heating so that the toner transferred onto the sheet of paper3in the process unit18is fixed thermally when the sheet of paper3passes between the heating roller30and the pressure roller31. After that, the sheet of paper3is carried to the position of a paper delivery roller pair35by the feed roller pair32. The sheet of paper3subjected to print processing completely is delivered onto a paper outlet tray36by the rotation of the paper delivery roller pair35.

The laser printer1according to this embodiment can perform double-sided printing on the sheet of paper3. A choice of delivering the sheet of paper3fed to the paper delivery roller pair35as described above or performing print processing on the other side of the sheet of paper3again can be made on the laser printer1.

Here, description will be made on the double-sided printing by the laser printer1. When the double-sided printing is set, the front and back of the sheet of paper fed to the paper delivery roller pair35after single-sided printing are reversed due to the reverse rotation of the paper delivery roller pair35. In this state, the sheet of paper is fed toward the resist roller pair12again along a reverse path41and a refeed path40afollowing the reverse path41. In the refeed path40a, the sheet of carried is fed while being held between a plurality of pairs of refeed rollers43aand43bdisposed at a distance from one another, and fed to the resist roller pair12through a refeed guide45. Then sheet of paper is subjected to printing on the other unprinted side thereof by the process unit18. The sheet of paper after the double-sided printing is delivered onto the paper outlet tray36due to the rotation of the paper delivery roller35as described above.

Incidentally, the device frames52shown inFIG. 1Bare disposed on the opposite sides of the printing device50in the direction vertical to the paper ofFIG. 2, so as to support the printing device50.

Each device frame52is made of a plate-like member, whose opposite end portions in the height direction are bent outside at right angles so as to make the length of the device frame52in the height direction is substantially equal to that of the printing device50. Thus, as shown inFIG. 1B, the device frame52is formed into a U-shape in sectional view. Then, in the lower bent surface of the device frame52, three threaded holes54x,54yand54zare provided to extend in the direction vertical to the paper ofFIG. 1B(seeFIG. 4A). The threaded holes54x,54yand54zare used for attaching a guide frame of a tray unit to the device frame52. Further in the lower bent surface, a protrusion portion56is provided to project downward in the vertical direction.

The cover53is made of a plate-like member, whose upper end is bent inside at right angles so as to cover the outsides of the device frames52, while the lower end of the cover53is formed not to project from the printing device50or the device frames52. That is, the length of the cover53in the height direction is substantially equal to that of each device frame52(SeeFIG. 4A).

In addition, the small tray unit1bdisposed under the printer unit1aincludes a small tray6b, small tray guides61b, small guide frames62b, and a small unit cover63b. The small tray6bcan store 250 sheets of paper as recording media. The small tray guides61bsupport the small tray6bfrom the left and right sides of the paper ofFIG. 1B. The small guide frames62bsupport the small tray guides61bfurther outside the small tray guides61brespectively. The small unit cover63bis disposed outermost.

The small tray6bis a box-like tray open at the top. The small tray6bis supported under the printing device50and between the two tray guides61bremovably in the downward direction of the paper ofFIG. 1B. As shown inFIG. 2, a paper pressure plate8, a separation pad unit10, and soon, are provided in the small tray6b. The separation pad unit10is provided in a lower portion of one side end portion of the small tray6b.

The paper pressure plate8has a top on which the sheets of paper3can be stacked, and a bottom urged upward by a spring8a. In addition, the paper pressure plate8is supported swingably at one end more distant from the paper supply roller9provided in the printer unit1a. Thus, the other end closer to the paper supply roller9can move in the up/down direction.

The separation pad unit10is disposed to face the paper supply roller9. A separation pad (not shown) made from a member having a high friction drag is pressed toward the paper supply roller9by a spring10bdisposed on the back side of a pad backing10cin the separation pad unit10.

The separation pad and the paper supply roller9provided in the printer unit1aare formed so that the width in a direction perpendicular to the feed direction of the sheet of paper3is shorter than the width of the sheet of paper3. In addition, the separation pad and the paper supply roller9are disposed to come in contact with only the substantially widthwise central portion of the sheet of paper3when the sheet of paper3is fed.

Incidentally, the small tray guides61bor the small guide frames62bshown inFIG. 1Bare disposed on the opposite sides of the small tray6bin the direction vertical to the paper ofFIG. 2, so as to support the small tray6b.

Each small tray guide61bis constituted by two substantially rectangular parallelepiped members disposed at a distance in the direction vertical to the paper ofFIG. 1B. The small tray guides61bsupport the small tray6bfrom the left and right sides of the paper ofFIG. 1B, while bearing the weight of the printer unit1ain the height direction.

Each small guide frame62bis made of a plate-like member, whose opposite end portions in the height direction are bent outside at right angles in the same manner as the device frame52. Thus, the small guide frame62bis formed into a U-shape in sectional view, while the height of the small guide frame62bis substantially equal to that of each small tray guide61bor the small tray6b. Then, in the upper bent surface of the small guide frame62b, two threaded holes64xand64zare provided to extend in the direction vertical to the paper ofFIG. 1B(seeFIG. 4A). The bent surface of the small guide frame62bprovided with the threaded holes64xand64zis disposed to face the lower bent surface of the device frame52provided with the threaded holes54x,54yand54zso that the threaded holes54xand54zof the device frame52correspond to the threaded holes64xand64zof the small guide frame62brespectively. Further, in the upper bent surface of the small guide frame62b, an insertion hole66bto which the protrusion portion56formed in the lower bent surface of the device frame52can be inserted is provided (SeeFIG. 4A).

The small unit cover63bis made of a plate-like member substantially as long in the height direction as each small guide frame62b, and disposed under each cover53as shown inFIG. 1B.

That is, the laser printer1xhas a printer unit1aand a small tray unit1bsupported by different frames respectively, and the small tray unit1bis removable from the printer unit1a. Accordingly, when the small tray unit1bin the laser printer1X is replaced by a large tray unit1chaving a larger storage capacity (capable of storing 500 sheets of paper), the large tray unit1ccan be attached to the printer unit1a(seeFIGS. 3A and 3B). That is, the laser printer1X having a paper storage capacity of 250 sheets as shown inFIGS. 1A and 1Band a laser printer1Y having a paper storage capacity of 500 sheets as shown inFIGS. 3A and 3Bcan be provided by use of the printer unit1aaccording to this embodiment.

FIG. 3Ais a schematic perspective view showing a laser printer in which a large tray having a storage capacity larger than that of the small tray has been attached to the printer body inFIG. 1A.FIG. 3Bis a schematic sectional view taken on line III-III inFIG. 3A. The laser printer1Y shown inFIGS. 3A and 3Bis constituted by the printer body1aand a large tray unit1cdisposed under the printer body1a. Members constituting the large tray unit1care similar to those of the small tray unit1bdescribed above, but different therefrom in size. That is, the heights of a large tray6c, large tray guides61c, large guide frames62cand a large unit cover63cconstituting the large tray unit1care substantially twice as large as the heights of the small tray6b, the small tray guides61b, the small guide frames62band the small unit cover63bconstituting the small tray unit1bshown inFIG. 1B, respectively. As should be appreciated, the small tray guide61band the large tray guide61care examples of a first guide member and a second guide member, respectively. As should be appreciated, the small guide frame62band the large guide frame62care examples of a first storage frame and a second storage frame, respectively.

In addition, while the two threaded holes64xand64zare provided in the upper bent surface of each small guide frame62b, three threaded holes74x,74yand74zare provided in the upper bent surface of each large guide frame62c(seeFIGS. 4A and 4B). As shown inFIG. 4B, the threaded holes54x,54yand54zof the device frame52correspond to the threaded holes74x,74yand74zof the large guide frame62crespectively. Further, in the upper bent surface of the large guide frame62c, an insertion hole66cto which the protrusion portion56formed in the lower bent surface of the device frame52can be inserted is provided.

FIG. 4Ais a perspective view showing a method for attaching the small guide frame62bto the device frame52inFIG. 1B.FIG. 4Bis a perspective view showing a method for attaching the large guide frame62cto the device frame52inFIG. 3B.

To attach the small guide frame62bto the device frame52, as shown inFIG. 4A, the protrusion portion56of the device frame52is inserted into the insertion hole66bof the small guide frame62bwhile the threaded holes54xand54zare aligned with the threaded holes64xand64zrespectively. Then, while the lower bent surface of the device frame52is brought into contact with the upper bent surface of the small guide frame62b, screws94xand94zare screwed down to the threaded holes54xand54zrespectively. Thus, the device frame52and the small guide frame62bare fixed to each other.

To attach the large guide frame62cto the body frame52, as shown inFIG. 4B, the protrusion portion56of the body frame52is inserted into the insertion hole66cof the large guide frame62cwhile the threaded holes54x,54yand54zare aligned with the threaded holes74x,74yand74zrespectively. Then, while the lower bent surface of the body frame52is brought into contact with the upper bent surface of the large guide frame62c, screws94x,94yand94zare screwed down to the threaded holes54x,54yand54zrespectively. Thus, the body frame52and the large guide frame62care fixed to each other. As should be appreciated, the screws94x-94zare an example of an attaching member.

That is, of the three threaded holes54x,54yand54zin the body frame52, the two threaded holes54xand54zexcluding the central threaded hole54yare used for attaching the small guide frame62bwhile all the three threaded holes54x,54yand54zare used for attaching the large guide frame62c. This is because the large tray unit1cis larger in weight than the small tray unit1bso that the large tray unit1cmust be able to bear its own gravity when the laser printer1Y is lifted up. As should be appreciated, the holes54x,54zare an example of a first attaching portion and the holes54x,54yand54zare an example of a second attaching portion.

Incidentally, another optional tray unit can be additionally mounted on the laser printer1X,1Y according to this embodiment.FIG. 5Ais a schematic sectional view showing the state where an optional small tray unit70bhas been mounted on the laser printer1X inFIG. 1B.FIG. 5Bis a schematic sectional view showing the state where an optional large tray unit70chas been mounted on the laser printer1Y inFIG. 3B. As should be appreciated, the small tray unit70bis an example of a first storage unit and first storage section and the large tray unit70cis an example of a second storage unit and a second storage section

The optional large tray unit70cmounted under the laser printer1Y as shown inFIG. 5Bhas a large tray7c, large tray guides71c, large guide frames72c, and an optional large unit cover73c. The large tray7ccan store 500 sheets of paper in the same manner as the large tray6cof the large tray unit1c. The large tray guides71csupport the large tray7c. The large guide frames72csupport the large tray guides71crespectively. The optional large unit cover73cis disposed outermost. The two large trays6cand7care supported by the frames62cand72cdifferent from the body frames52respectively, and the tray guides61cand71cor the covers63cand73care also separate from each other. As should be appreciated, the small tray7band the large tray7care examples of a first storage tray and a second storage tray, respectively

As described above, the printer unit1aaccording to this embodiment does not include any tray6b,6cas its constituent component, but has the threaded holes54xto54zsuch that the small tray6band the large tray6chaving storage capacities different from each other can be selectively mounted on the printer unit1a. Accordingly, only by attaching the small tray6bor the large tray6cto the printer unit1a, it is possible to provide the laser printers1X and1Y for a plurality of models different only in the paper storage capacity of the tray while curbing the cost. That is, it is possible to realize the laser printers1X and1Y to which the trays6band6chaving various storage capacities can be removably attached in accordance with the need of users in spite of easiness in design or molding and reduction in cost.

In addition, of the three threaded holes54x,54yand54zprovided in the body frame52, the two threaded holes54xand54zare used for attaching the small guide frame62bof the small tray unit1bwhile all the three threaded holes54xto54zare used for attaching the large guide frame62cof the large tray unit1c. That is, the threaded holes for the small guide frame62bare included in the threaded holes for the large guide frame62c. As should be appreciated, the holes54x,54zare an example of a first attaching portion and the holes54x,54yand54zare an example of a second attaching portion. As should also be appreciated, the second attaching portion has more attaching portions than the first attaching portion because the second attaching portion has three holes54x,54yand54z(i.e., attaching portions) and the first attaching portion has two holes54xand54z. Because holes54xand54zare a part of both the first and second attaching portions, the first attaching portion can be included in the second attaching portion. In this case, the work for attaching the frame when the laser printer1X,1Y is manufactured is easier than in the case where threaded holes for the small guide frame62bare provided separately from threaded holes for the large guide frame62c.

In addition, since the device frame52has the protrusion portion56capable of being engaged with the guide frame62b,62c, the work for attaching the frame when the laser printer1x,1Y is manufactured is easier than in the case where only the threaded holes54xto54zare provided.

In addition, the height-direction length of the cover53disposed outside the device frames52is substantially equal to that of each device frame52, and the cover53is included in the printer unit1awithout reaching the area of the tray unit1b,1c. It is therefore unnecessary to exchange the cover53when the tray6b,6cis replaced. For example, assume that the cover is longer than the device frame52. In such a case, when the small tray6bis replaced by the large tray6c, the cover must be exchanged for another larger one correspondingly to the tray size. In the configuration according to this embodiment, however, the cover53is designed to cover the whole of each device frame52. When the size of a tray is larger than that of the cover53, the shortage of the length of the cover53may be covered by the cover63b,63cprovided additionally. That is, the labor or time for exchanging the cover whenever the tray6b,6cis changed is saved. Thus, according to the printer unit1aincluding the cover53in this configuration, it is possible to provide the laser printers1X and1Y for a plurality of models different only in the paper storage capacity of the tray6b,6cwhile curbing the cost further.

In addition, the printer unit1aaccording to this embodiment further includes the paper supply roller9for making contact with the sheet of paper3stored in the tray6b,6c, and feeding the sheet of paper3in contact to the process unit18by means of axial rotation. It is therefore unnecessary to provide the paper supply roller9for each individual tray6b,6cthat can be removably attached to the printer unit1a. Accordingly, by use of the printer unit1aconfigured thus, it is possible to provide the laser printer1X,1Y while curbing the cost.

Next, a laser printer according to the second embodiment of the invention will be described with reference toFIGS. 6A and 6B. Incidentally, parts similar in structure to those in the first embodiment will be denoted by the same reference numerals correspondingly, and their description will be omitted.FIG. 6Ais a schematic sectional view corresponding toFIG. 1Band showing a laser printer in which a small tray has been attached to a printer unit according to the second embodiment of the invention.FIG. 6Bis a schematic sectional view corresponding toFIG. 3Band showing a laser printer in which a large tray having a storage capacity larger than that of the small tray has been attached to the printer unit inFIG. 6A.

As shown inFIG. 6A, a laser printer101X according to this embodiment is constituted by a printer unit101aand a small tray section101bdisposed under the printer unit101a. The small tray section101bis constituted by a small tray6band small tray guides61b. That is, the small tray section101bis equivalent to one obtained by omitting the small guide frames62band the small unit cover63bfrom the small tray unit1baccording to the first embodiment shown inFIG. 1B.

Each device frame152of the printer unit101aaccording to this embodiment is different from the device frame52in the first embodiment shown inFIG. 1B. The device frame152projects downward from the printing device50and reaches the bottom portion of the small tray6bor the small tray guide61b. That is, the device frame152supports not only the printing device50but also the small tray section10b. More in particular, the device frame152has at least one threaded hole156in lower portion of the support surface supporting the printing device50and so on and facing the small tray guide61b. A screw (not shown) is screwed down to the threaded hole156. Thus, the small tray guide61bis attached to the device frame152.

The lower bent surface of each device frame152has at least one threaded hole154similar to the threaded hole54x-54zin the first embodiment shown inFIGS. 4A and 4B. The threaded hole154is used for attaching a large guide frame162cof a large tray unit101cshown inFIG. 6B.

The large tray unit101cin this embodiment is constituted by a large tray6cand large tray guides61csimilar to those of the large tray unit1cin the first embodiment shown inFIG. 3B, and large guide frames162cand a large unit cover163cwhich are approximately half as high as the large guide frames62cand the large unit cover63c. The upper bent surface of each large guide frame162cis provided with at least one threaded hole174similar to the threaded hole74x-74zin the first embodiment.

Thus, the printer unit101aand the large tray unit101care fixed to each other by screwing a screw (not shown) down to the threaded hole156to thereby attach each large tray guide61cto the corresponding device frame152, and screwing a screw (not shown) down to the threaded holes154and174to thereby attach each large guide frame162cto the corresponding device frame152. Incidentally, the large guide frame162cand the large tray guide61cmay be fixed to each other or not fixed to each other.

That is, since the printer unit101aaccording to this embodiment has the threaded holes156and154, the small tray guides61bof the small tray section101bcan be attached to the printer unit101a, or the large tray guides61cand the large guide frames162cof the large tray unit101ccan be attached to the printer unit101a. Thus, the laser printer101xhaving a paper storage capacity of 250 sheets as shown inFIG. 6Aand a laser printer101Y having a paper storage capacity of 500 sheets as shown inFIG. 6Bcan be provided by use of the printer unit101aaccording to this embodiment.

Incidentally, another optional tray unit can be additionally attached to the laser printer101X,101Y according to this embodiment, in the same manner as that shown inFIGS. 5A and 5B.

As described above, the printer unit101aaccording to this embodiment does not include any tray6b,6cas its constituent component, but has the threaded holes154and156such that the small tray6band the large tray6chaving storage capacities different from each other can be selectively mounted. Accordingly, only by attaching the small tray6bor the large tray6cto the printer unit101a, it is possible to provide the laser printers101X and101Y for a plurality of models different only in the paper storage capacity of the tray while curbing the cost. That is, in the same manner as in the first embodiment, it is possible to realize an image forming apparatus to which storage trays having various storage capacities can be removably attached in accordance with the need of users in spite of easiness in design or molding and reduction in cost.

Generally, in order to secure rigidity in a printer unit, it is preferable that the size of each device frame is increased. As another effect of this embodiment, it can be therefore noted that the embodiment is advantageous in securing rigidity because the size of each device frame152becomes large enough to support the small tray section101b.

Next, modifications of the laser printer inFIG. 6Bwill be described with reference toFIGS. 7A and 7B.FIGS. 7A and 7Bare schematic sectional views showing modifications of the laser printer inFIG. 6B. Each laser printer201Y,301Y shown inFIG. 7A,7B has a large tray6chaving a paper storage capacity of 500 sheets.

The laser printer201Y inFIG. 7Ahas a printer unit201asubstantially similar to the printer unit101ashown inFIGS. 6A and 6B. Threaded holes254and256similar to the threaded holes154and156shown inFIGS. 6A and 6Bare provided in each device frame252of the printer unit201a. While the threaded hole154is used for attaching the large guide frame162c, the threaded hole254is used for attaching a large tray guide261c. The threaded hole256is used for attaching the small tray guide61bor the large tray guide261cin the same manner as inFIGS. 6A and 6B.

A large tray section201cinFIG. 7Ais equivalent to one obtained by omitting the large guide frames162cfrom the large tray section101cdepicted inFIG. 6B, but including large tray guides261ceach formed into an L-shape in sectional view so as to cover the area of the corresponding large guide frame162c. Each large tray guide261cis attached to the corresponding device frame252by screwing screws (not shown) down to the threaded holes254and256. Incidentally, to attach a small tray section101bto the printer unit201a, the small tray guides61bare attached to the device frames252only through the threaded holes256respectively in the same manner as inFIG. 6A.

The laser printer301Y inFIG. 7Bhas a printer unit301ahaving a configuration different from that of the printer unit101ashown inFIGS. 6A and 6B. In each device frame352of the printer unit301a, the threaded hole154provided in the lower bent surface inFIG. 6Bis omitted, but only a threaded hole356is provided in a portion of the guide support surface corresponding to the large tray guide361c.

A large tray section301cinFIG. 7Bis equivalent to one obtained by omitting the large guide frames162cfrom the large tray unit101cinFIG. 6B. Each large tray guide361cis attached to the corresponding device frame352only through the threaded hole356. Also when the small tray section101bis attached to the printer unit301a, each small tray guide61bis attached to the corresponding device frame352through the threaded hole356.

Incidentally, each tray guide also serves to bear the weight of the printing device50and so on. Accordingly, it is necessary to pay attention to the necessity that the large tray guides361chave to have enough strength or rigidity to support the printing device50in comparison with the large tray guides61csupported by the large guide frames162cinFIG. 6Bor the large tray guides261ceach having an L-shape in section inFIG. 7A.

Incidentally, the threaded hole256inFIG. 7Aand the threaded hole356inFIG. 7Bare used for attaching any one of the small tray guide61bfor guiding the small tray6band the large tray guide261c,361cfor guiding the large tray6c. However, in the same manner as in the first embodiment, the number of threaded holes used for attaching the small tray guide61bis smaller than the number of threaded holes used for attaching the large tray guide261c,361c(seeFIGS. 4A and 4B). That is, the threaded holes for the small tray guide61bare included in the threaded holes for the large tray guide261c,361c.

As described above, the printer unit201a,301adoes not include any storage tray as its constituent component, but has the threaded holes254,256,356such that the small tray6band the large tray6chaving storage capacities different from each other can be selectively mounted to the printer unit201a,301a. Accordingly, only by attaching the small tray6bor the large tray6cto the printer unit201a,301a, it is possible to provide laser printers for a plurality of models different only in the paper storage capacity of the tray while curbing the cost. That is, in the same manner as in the first embodiment, it is possible to realize laser printers to which the trays6band6chaving various storage capacities can be removably attached in accordance with the need of users in spite of easiness in design or molding and reduction in cost.

In addition, when the small tray section101bor the large tray section201c,301cis attached to the printer unit201a,301a, each tray guide61b,261c,361cis attached to the corresponding device frame252,352. Due to such a configuration, each tray guide61b,261c,361cis supported by the device frame252,352. Accordingly, the small guide frames62b(seeFIG. 1B) or the large guide frames62c(seeFIG. 3B) used in the first embodiment are dispensable. Thus, it is possible to obtain an effect in which the embodiment is advantageous in securing rigidity in the printer unit201a,301a.

In addition, since the threaded holes for the small tray guides61bare included in the threaded holes for the large tray guides261c,361c, the work for attaching the tray guides when the laser printer is manufactured is easier than in the case where the threaded holes for the small tray guides61bare provided separately from the threaded holes for the large tray guides261c,361c.

The preferred embodiments of the invention have been described above. However, the invention is not limited to the embodiments, but various changes in design can be made on the invention without departing from the claimed scope thereof.

For example, although two trays having different paper storage capacities, that is, the small tray6band the large tray6ccan be removably attached in the embodiments, the invention is also applicable to the configuration in which three or more trays having different paper storage capacities can be removable attached.

In addition, the protrusion portion56provided in the lower bent surface of each device frame52so as to project downward in the vertical direction is dispensable.

The threaded holes for the small guide frames62bare included in the threaded holes for the large guide frames62cin the embodiments. In the modifications of the second embodiment, the threaded holes for the small tray guides61bare included in the threaded holes for the large tray guides261c,361c. However, the invention is not limited to such configurations. For example, the threaded holes for the small tray guides62band the threaded holes for the large guide frames62cmay be provided individually, or the threaded holes for the small tray guides61band the threaded holes for the large tray guides261c,361cmay be provided individually.

In addition, the attaching portions used for attaching guide frames or tray guides to device frames are not limited to threaded holes. Various configurations including concave and convex engagement portions may be provided.

In the embodiments, the cover53,153is also included as a constituent component of the printer unit1a,101a,201a,301a, which is designed so that only the tray unit or the tray section under the printer unit is replaced without replacing the cover. However, the cover may be made long enough to cover the area of respective tray units to be mounted. On this occasion, the cover is replaced whenever any tray unit is replaced. Also in this case, laser printers having various paper storage capacities can be provided sharing the device frames52,152,252,352and so on excluding the cover53,153.

In addition, the cover153may be divided into a part for the area of the printing device50and the other part for the area of the small tray unit101bin the second embodiment shown inFIGS. 6A and 6B.

In addition, the paper supply roller9may not have to be provided in the printer unit1a,101a,201a,301a, which are examples of an image forming unit. In this case, however, paper supply rollers9must be provided for the trays6band6cto be mounted on the printer unit1a,101a,201a,301a, respectively.

Although the embodiments have been described on the case where a laser printer is adopted as an example of an image forming apparatus according to the invention, the invention is also applicable to various image forming apparatuses including other printers of an inkjet type and the like, copying machines, facsimile machines, and so on.

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.