Apparatus for forming an image on a sheet of paper.

0001This invention concerns an apparatus for forming an image on a sheet of paper, the apparatus being of the type comprising a casing having a paper insertion opening and a paper discharge opening, transport means within the casing for causing the sheet of paper to be transported along a paper transport path from the paper insertion opening to the paper discharge opening, and electrophotographic means for imparting the image to the sheet of paper while the latter is on the paper transport path.

0002Apparatus of the above-mentioned type may either be arranged for "face-up" or for "face down" delivery, in which the surface of the sheet of paper having the image thereon faces upwardly or downwardly respectively. Apparatus arranged for face-up delivery requires a floor area which is about three times the area of the sheet of paper on which the image is to be formed, whereas apparatus arranged for face-down delivery is liable to cause the paper to curl since it is turned back on itself, there is an increase in the rate of jamming due to an increase in the length of the paper transport path, the said transport means is complicated and expensive, and printing is slow.

0003According, therefore, to the present invention there is provided an apparatus for forming an image on a sheet of paper comprising a casing having a paper insertion opening and a paper discharge opening which is adjacent the bottom of the casing; transport means within the casing for causing the sheet of paper to be transported along a paper transport path from the paper insertion opening to the paper discharge opening; and electrophotographic means for imparting the image to the sheet of paper while the latter is on the paper transport path, characterised in that the casing has a height which is greater than its depth, the paper insertion opening being adjacent the top of the casing and the paper discharge opening being at the front of the casing.

0004Preferably the said depth is not greater than 1.5 times the width of the paper insertion opening.

0005Preferably the width of the casing is not greater than twice its depth.

0006The apparatus may be constituted by a desk-top apparatus.

0007In the preferred embodiment of the present invention, the apparatus has a reduced production cost and a reduced installation space and is capable of printing thick paper, envelopes and the like using a face-down delivery mechanism alone without the need to provide a face-up delivery mechanism and associated switching means and with a reduced risk of causing a jam.

0008It is preferably so designed that the fast print time (the period of time which begins at the time when a print command is given and which ends at the time when a printed sheet of paper has completely been discharged) is reduced by a large margin and the operability is improved.

0009It is also preferably such as to have a reduced production cost and a reduced installation space and may be compact in size and light in weight. It is preferably so designed that any jam can readily be cleared even by an inexperienced user in a natural posture without any fear of his hands being stained and with no risk of the machine being damaged. It is also preferably such that the user can reliably exchange a cartridge employed in the apparatus in a natural posture, this being achieved by performing almost the same operation as a jam removing operation from the front side of the apparatus from which paper is discharged in normal use.

0010Preferably the angle between the plane of the sheet of paper at any position on the paper transport path and its plane at the paper insertion opening does not exceed 90°, the sheet of paper being discharged from the paper discharge opening with its printed surface facing downwardly.

0011The electrophotographic means preferably comprises a photosensitive member, an optical signal generator for irradiating the photosensitive member with a light pattern in accordance with the image to be produced, a developing device for developing a latent image on the photosensitive member, and a transfer device for transferring the developed image from the photosensitive member to the sheet of paper. Moreover, a fixing device may be provided for fixing on the sheet of paper the developed image which has been transferred thereto.

0012The optical signal output from the optical signal generator preferably enters the photosensitive member at an angle not exceeding 60° to the horizontal line which passes through a central axis of the photosensitive member.

0013The developing device is preferably disposed above an optical signal incident portion of the photosensitive member.

0014The casing may have an immovable part; a movable part which may be moved into and out of a closed position in which it is disposed alongside the immovable part so as to close the casing, the paper transport path being disposed between the immovable and movable parts; and the said photosensitive member, the said optical signal generator, and the said developing device being disposed on the immovable part.

0015The transfer device may be carried either by the movable part or by the immovable part.

0016The movable part may be moved out of the said closed position by being pivoted around an axis parallel to the axis of the photosensitive member.

0017The fixing device may be provided on the immovable part or on the movable part or may comprise a pair of rollers between which the paper transport path extends, the construction being such that the nip pressure between the rollers is automatically cancelled when the movable part is moved out of the said closed position.

0018There may be storage means for storing and supplying sheets of paper to said paper transport path, the said storage means being disposed above the movable part.

0019Preferably at least a part of the paper transport path is defined by an outer casing of the movable part.

0020The electrophotographic means may comprise a cartridge which is provided with the photosensitive member, the cartridge being disposed in front of a light-emitting device of the optical signal generator when viewed in the direction in which the movable part is moved out of the said closed position.

0021The construction may be such that the cartridge is unloaded from the apparatus in the direction in which the movable part is moved out of the said closed position and is loaded into the apparatus in the opposite direction.

0022The cartridge may carry a charging device for charging a portion of the photosensitive member and a cleaning device for cleaning toner from the photosensitive member.

0023The cartridge may also carry the said transfer device and the latter may be detachable from the cartridge.

0024The transfer device may be pivotally connected at one end thereof to the cartridge so that it can be moved pivotally about said end in a direction in which the movable part is moved out of the said closed position.

0025Alternatively, the arrangement may be that the opposite ends of the transfer device are pivotally connected to and supported by the cartridge.

0026The cleaning device may have a waste toner receptacle, the cleaning device having a flat bottom surface or at least three projections on its bottom surface which define a ground plane, the centre of gravity of the cartridge being within the ground plane.

0027The cartridge may be provided with a handle by means of which the cartridge may be maintained in the same posture while being loaded into and out of the apparatus and while being transported. The cleaning device may be arranged to be supported in the cartridge by said handle in such a way that a first straight line which intersects the handle and the centre of gravity of the cartridge when there is no waste toner in said receptacle is disposed adjacent a second straight line which intersects the said handle and the centre of gravity of the cartridge when there is a substantial quantity of toner in the said receptacle.

0028Preferably, the lower front part of the casing has a tray portion which may be moved into and out of a position in which it is arranged to receive printed sheets of paper.

0029The transfer device may be formed integrally with the cartridge, thereby eliminating the need to carry out maintenance such as cleaning of the transfer device and also enabling the transfer device to be readily moved away from the photosensitive member in order to facilitate, for example, removal of jamming paper. This construction allows the transfer device to be readily cleaned if it needs to be cleaned when trouble occurs.

0030According to another aspect of the present invention, there is provided an apparatus for forming an image on a sheet of paper comprising a paper insertion region, a paper discharge region; transport means for causing the sheet of paper to be transported along a paper transport path from the paper insertion region to the paper discharge region; and electrophotographic means for imparting the image to the sheet of paper while the latter is on the paper transport path, characterised in that the angle between the plane of the sheet of paper at any position on the paper transport path and its plane at the paper insertion region does not exceed 90°, the sheet of paper being discharged from the paper discharge region with its printed surface facing downwardly.

0031According to yet a further aspect of the present invention there is provided an apparatus for forming an image on a sheet of paper comprising a casing having a paper insertion opening and a paper discharge opening; transport means within the casing for causing the sheet of paper to be transported along a paper transport path from the paper insertion opening to the paper discharge opening; and electrophotographic means for imparting the image to the sheet of paper while the latter is on the paper transport path, the electrophotographic means comprising a photosensitive member, an optical signal generator for irradiating the photosensitive member with a light pattern in accordance with the image to be produced, a developing device for developing a latent image on the photosensitive member, and a transfer device for transferring the developed image from the photosensitive member to the sheet of paper, characterised in that the casing has an immovable part; a movable part which may be moved into and out of a closed position in which it is disposed alongside the immovable part so as to close the casing, the paper transport path being disposed between the immovable and movable parts; and a detachable cartridge which is carried by the immovable part and which is provided at least with the said photosensitive member.

0032According to still a further aspect of the present invention, there is provided an apparatus for forming an image on a sheet of paper comprising a casing having a paper insertion opening and a paper discharge opening; transport means within the casing for causing the sheet of paper to be transported along a paper transport path from the paper insertion opening to the paper discharge opening; and electrophotographic means, for imparting the image to the sheet of paper while the latter is on the paper transport path, the electrophotographic means comprising a photosensitive member, an optical signal generator for irradiating the photosensitive member with a light pattern in accordance with the image to be produced, a developing device for developing a latent image on the photosensitive member, and a transfer device for transferring the developed image from the photosensitive member to the sheet of paper characterised in that the developing device is disposed above an exposure portion of the surface of the photosensitive member.

0033If the developing device is disposed above the photosensitive member included in the cartridge in order to arrange the electrophotographic means vertically, the movable part of the casing can be opened outwardly from the front side thereof, thus enabling cartridges to be exchanged and jammed paper to be removed from the opening provided between the immovable and movable parts.

0034The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- <ul id="ul0001" list-style="none"><li>Figure 1 is a sectional view of one embodiment of an apparatus according to the present invention;</li><li>Figure 2 is a perspective view which shows the external appearance of one embodiment of an apparatus according to the present invention in which a delivery tray can be accommodated on the outer side of the casing of the apparatus;</li><li>Figure 3 shows one example of an apparatus according to the present invention in which a laser diode is employed as an optical signal generator;</li><li>Figure 4 shows one example of an apparatus according to the present invention in which a reflecting mirror is employed in the laser optical path;</li><li>Figure 5 shows a movable part of one embodiment of the present invention in its opened state and also shows the direction in which a cartridge is unloaded;</li><li>Figures 6(a) and 6(b) show in combination the way in which the nip pressure applied by the rollers of a fixing device in one embodiment of the present invention is automatically cancelled by opening the movable part;</li><li>Figures 7(a) and 7(b) show in combination the way in which the degree of opening of the movable part in one embodiment of the present invention is limited to a predetermined angle by an abutment member and, even when the movable part is in the fully opened position, the angle to which it is opened is regulated so that the position of centre of gravity thereof is not offset from the bottom of the apparatus;</li><li>Figure 8 shows one embodiment of the present invention in which the fixing device is installed on the immovable part;</li><li>Figures 9(a) and 9(b) show in combination one embodiment of the present invention which a damping means is provided for preventing the movable part from gravitationally opening suddenly and in which the movable part is opened to such an extent that it comes into contact with the surface of the place on which the apparatus is installed;</li><li>Figures 10(a₁), 10(a₂), 10(b₁), 10(b₂), 10(b₃), and 10(c₁), 10(c₂) show various examples of the arrangement of a cartridge of an apparatus according to the present invention;</li><li>Figure 11 shows one example of a cartridge which may be used in an apparatus according to the present invention, the cartridge being provided with projections;</li><li>Figure 12 is a sectional view showing the positional relationship between a handle and a cleaning device provided on the cartridge;</li><li>Figure 13 shows a prior art apparatus in which paper is discharged in a face-up delivery manner; and</li><li>Figure 14 shows another prior art apparatus in which paper is fed from the front side of the apparatus and is discharged onto the upper side thereof in a face-down delivery manner.</li></ul>

0035Terms such as "right" and "left", as used in the description below, are to be understood to refer to directions as seen in the respective drawings.

0036Figure 13 schematically shows the arrangement of a prior art electrophotographic image forming apparatus. In the apparatus of Figure 13, a paper stacker 8 and a delivery tray 18 are disposed on the right- and left-hand sides, respectively, as viewed from the front side of the apparatus. A sheet of paper is fed from the right-hand side as viewed in Figure 13, passed through the inside of an image forming apparatus and discharged onto the tray 18 in such a manner that the surface of the sheet of paper having an image formed thereon faces upwardly. When the discharge of the paper is carried out in this way it is generally known as "face-up delivery". In such arrangement, the width W of the image forming apparatus needs to be about three times the length ℓ of the paper, and the depth D of the apparatus needs to be equal to the sum of the width <u style="single">w</u> of the paper and some additional minimum length.

0037Figure 14 shows another prior art apparatus in which paper is discharged in such a manner that the image forming surface thereof faces downwardly. When the discharge of the paper is carried out in this way it is generally known as "face-down delivery". When the face-down delivery is adopted, output sheets of paper are stacked in the order, page 1, page 2, page 3 ..., from the bottom toward the top of the stack of sheets. Therefore, when the stack of output sheets is turned upside down so that the image forming surface of the uppermost sheet can be seen, the uppermost sheet is page 1 and the following sheets are page 2, page 3..., which means that it is unnecessary to rearrange the stack of sheets in the paginal order. In the case of the face-up delivery, on the other hand,when the stack of output sheets is placed in the state wherein the image forming surface of the uppermost sheet of the stack can be seen, the uppermost sheet is the final page and the lowermost sheet is page 1, which means that it is necessary to rearrange the stack of sheets in the paginal order.

0038In the prior art apparatus shown in Figure 14, the width Wʹ of the image forming apparatus is the same as the depth D of the apparatus shown in Figure 13, whereas the depth Dʹ of the apparatus shown in Figure 14 is about twice the length ℓ of the paper.

0039Reference numerals shown in Figures 13 and 14 and not referred to above correspond to like numbered parts of Figures 1 and 2 which are described below.

0040Thus, the prior art image forming apparatuses having the above-described arrangements suffer from the following disadvantages. Namely, when the apparatus employing face-up delivery is installed, there is a need for a floor area which is about three times the area of a sheet of paper on which an image is to be formed, whereas when the apparatus employing face-down delivery is installed, a floor area which is about twice the area of said paper is needed. In particular, the arrangement shown in Figure 14, which is adopted to achieve face-down delivery, has the disadvantages that there is an increase in the degree to which the paper curls, since the paper is turned back, and that there is an increase in the rate of jamming due to an increase in the length of the paper transport path. In addition, since the paper transport mechanism is complicated, the production costs are raised and there is a considerable increase in the "first print time" (the period of time which begins at the time when a print command is given and which ends at the time when the first printed sheet of paper has been completely discharged from the machine body. The quality of the printing may be judged by the length of the first print time).

0041Further, unlike the face-up delivery, the face-down delivery involves a structure in which a paper transport path 21 is twice bent at approximately 90° in its course, which means that it is impossible to prevent thick paper, envelopes and the like from causing a jam or from becoming wrinkled.

0042In order to enable envelopes and the like to be printed without such problems, it is necesary to increase the radius of a circle defined by a bent portion of the paper transport path 21 to about 5 cms, and this leads to a considerable increase in the size of the apparatus.

0043Accordingly, an apparatus which is stated to be capable of printing envelopes and the like is provided with a mechanism in which a paper path switching lever is provided immediately downstream of a fixing device 11 so that paper is not bent but is discharged as it is in the face-up delivery manner, which means that the size of this type of apparatus is increased correspondingly and the number of required parts is also increased, resulting in a rise in the production cost. In addition, it is necessary to switch over the face-up and face-down delivery modes from one to the other according to need, and if the operator forgets to conduct this changeover operation, a jam may occur, and this may lead to a failure of the apparatus.

0044The prior art further involves the problem that it is complicated and difficult to exchange cartridges, which get used up, and the apparatus can only be installed in a position which gives free access to the relevant side of the apparatus which must be accessible to enable parts to be exchanged, and this requires an exceedingly large installation space. In addition, since a clamshell system is adopted in order to overcome jamming, there is a need to provide a strong spring for raising almost all the elements constituting a heavy electrophotographic system and an optical writing system, and the machine frame therefore needs to have great strength so that it is not deformed by the force from the strong spring, and this produces an increase in production costs. Further, it is necessary, when removing a cause of a paper jam, to conduct an operation in which the operator stoops down to look into the interior of the machine from an opening which is located at a relatively low position and looks like an open mouth of a shellfish, which is an awkward operation. In addition, since the inside of the apparatus cannot be seen very clearly, an essential member of the electrophotographic system may be damaged when the jammed paper is pulled out.

0045The arrangement and features of an apparatus according to the present invention will therefore now be described hereinunder with reference to the accompanying drawings.

0046Figure 1 shows the arrangement of an apparatus in accordance with one embodiment of the present invention.

0047A cylindrical photosensitive member or drum 1 is rotatably mounted so as to be rotatable in the direction of an arrow A. A charging device 2, an optical signal generator 3 for irradiating the photosensitive member 1 with a light pattern in accordance with the image to be produced, a developing device 4 for developing a latent image on the photosensitive member 1, a transfer device 5 for transferring the developed image from the photosensitive member 1 to the sheet of paper, a cleaning device 6 for cleaning toner from the photosensitive member 1, and an erasing device 7 are disposed around the photosensitive member 1. A paper stacker 8, a paper feed roller 9 and paper register rollers 10 are disposed above the photosensitive member 1, while a fixing device 11 for fixing on the sheet of paper the developed image which has been transferred thereto, a paper guide plate 12, delivery rollers 13, an exhaust fan 14 and a power supply 15 are disposed below the photosensitive member 1. A control board 16 is disposed on the rear side of the body of the apparatus. The apparatus is provided with an outer casing 17.

0048A delivery tray 18 is, as shown in Figure 2, provided so as to be capable of being pivoted in the direction of an arrow B about a pivot axis 18<u style="single">a</u> so that it can be accommodated on the outer side of the casing 17 when the apparatus is in an inoperative state.

0049The casing 17 has a paper insertion opening 19 adjacent the top of the casing 17 and a paper discharge opening 20 which is adjacent to the bottom of the casing 17 and is at the front of the casing 17. The paper feed roller 9, the paper register rollers 10 and the delivery rollers 13 constitute transport means for causing the sheet of paper (not shown) to be transported along a paper transport path 21 from the paper insertion opening 19 to the paper discharge opening 20, the parts 1-7, 11 constituting electrophotographic means for imparting the image to the sheet of paper while the latter is on the paper transport path 21.

0050The image forming process carried out by the apparatus according to the present invention will be explained below.

0051The photosensitive member 1 has a surface layer which has photo-semiconductor characteristics such that it shows a relatively high electrical resistance in a dark place (i.e. the surface layer becomes an insualtor), whereas, when light is applied thereto, the electrical resistance of the irradiated portion lowers (i.e. the irradiated portion becomes an electrical conductor).

0052The charging device 2 generates a corona discharge when a high voltage, i.e. several kilovolts, is applied between a fine metal wire (not shown) and a ground electrode (not shown), thus causing a positive or negative electrical charge to be generated near the charging device 2.

0053The optical signal generator 3 is constituted by a device having a relatively short optical path, such as a liquid crystal shutter array or an LED array, and is adapted to convert electrical image information produced at the control board 16 into optical signals (ON and OFF signals in the form of light) and to output the converted signals. It should be noted that a laser scanner (not shown) may also be employed as the optical signal generator 3, although in such a case the optical path becomes relatively long.

0054The ambient light is shut off by means of the outer casing 17, and when the inside of the outer casing 17 is dark, the photosensitive member 1 is constituted by an insulator. When, in this state, the photosensitive member 1 is rotated at a constant speed, the positive or negative electrical charge generated by means of the charging device 2 adheres to the surface of the photosensitive member 1 (i.e. the member 1 is electrically charged). The charged photosensitive member 1 is irradiated with light in accordance with the image information from the optical signal generator 3. In consequence, the irradiated portion is changed to an electrical conductor and the charge on the surface of the irradiated portion is grounded. More specifically, the surface of the photosensitive member 1 in this state has a portion on which a charge corresponding to the image information is present and a portion having no charge (i.e. a latent image portion is formed).

0055The developing device 4 contains toner (not shown) which is charged positive or negative. The developing device 4 brings the toner into contact with the said latent image portion through a developing sleeve 4<u style="single">a</u>. Whether the toner is to be attached to a charged portion of the surface of the photosensitive member 1 or to the non-charged portion is determined in accordance with the polarity of the charge on the surface of the photosensitive member 1 and the polarity of the charged particles of the toner. More specifically, when the charge on the surface of the photosensitive member 1 is positive, if the polarity of charged particles of the toner is positive, the toner adheres to the non-charged portion of the photosensitive member 1, whereas, if the polarity of the charged particles of the toner is negative, the toner adheres to the charged portions of the surface of the photosensitive member 1. On the other hand, when the charge on the surface of the photosensitive member 1 is negative, if the polarity of the charged particles of the toner is positive, the toner adheres to the charged portion of the surface of the photosensitive member 1, whereas, if the polarity of the charged particles of the tones is negative, the toner adheres to the non-charged portion of the surface of the photosensitive member 1.

0056The uppermost one of the sheets of paper stored in the paper stacker 8 is fed by the action of the paper feed roller 9 to the paper register rollers 10 to stand by at this position.

0057The register rollers 10 rotate in synchronism with the rotation of the photosensitive member 1 developed by the operation of the developing device 4 having the toner adhering to the surface thereof, and the paper is thereby advanced to an intermediate position between the photosensitive member 1 and the register rollers 10.

0058The transfer device 5 has two constituent members, namely a fine metal wire (not shown) and a ground electrode (not shown) in the same way as the charging device 2 and is adapted to charge the rear side of the paper by means of positive or negative ions which are generated by a relatively high voltage applied between the two consitutuent members. The toner developed on the surface of the photosensitive member 1 is attracted to the obverse surface of the paper by means of the positive or negative charge on the reverse surface of the paper, thus effecting the transfer. The paper subjected to the transfer operation is passed through the area between fixing rollers of the fixing device 11 and, while doing so, the toner on the surface of the paper is fixed thereto. Although there are known fixing methods employing heat and pressure rollers, respectively, this embodiment adopts a fixing method employing a heated roller for the purpose of minimizing the size of the apparatus. A heat source such as an infra-red lamp is disposed in the centre of a roller 11<u style="single">a</u>, thereby effecting temperature control so that the surface temperature of the roller 11<u style="single">a</u> is maintained at a constant level. The surface temperature of the roller 11<u style="single">a</u> depends upon the kind of toner used, but it is generally set at from about 140°C to 200°C. The toner on the paper comes into contact with the surface of the roller 11<u style="single">a</u> and receives heat from the latter, thus causing a resin contained in the toner as a component thereof to be fused so as to penetrate the fibres of the paper. The fused toner is cooled and fixed to the surface of the paper at the time the paper is fed out from the area between the fixing rollers.

0059In this embodiment, the transfer device 5 is disposed on the horizontal line which passes through the centre of the photosensitive member 1 and on the front side of the image forming apparatus. The optical signal generator 3 is disposed on said horizontal line and on the rear side of the apparatus. When the image forming apparatus is used for a long period of time, paper dust and toner accumulate inside the apparatus, and the amount of accumulated dust and toner is largest at the gravitationally lower portion, that is, the lower portion of the apparatus. If toner or paper dust adheres to the transfer device 5, a corona discharge may not occur in a normal state when a high voltage is applied; in such a case, transfer cannot satisfactorily be effected. Any toner or paper dust adhering to the optical signal generator 3 blocks the passage of light and thus prevents the photosensitive member 1 from becoming electrically conductive, so that the charge on the surface will not escape sufficiently.

0060To solve these problems, the disposition of the transfer device 5 and the optical signal generator 2 in the lower portion inside the apparatus is avoided in this embodiment.

0061When an optical system employing a laser diode (not shown) is adopted as the optical signal generator 3, the length of the optical path ℓ (Figure 3) needs to be about 300 mm at the minimum, which means that it is difficult to reduce the depth D of the image forming apparatus as shown in Figure 3. It should be noted that ℓ in this embodiment is about 300 mm. If, in order to minimize the depth D, an optical system employing a laser diode is provided with one reflecting portion (or mirror 19<u style="single">b</u>) as shown in Figure 4, a polygon scanner 19<u style="single">a</u> which rotates at high speed must be disposed in such a manner that its longitudinal axis extends horizontally, which means that a very heavy load is imposed on a means for supporting the rotation of the polygon scanner 19<u style="single">a</u> and this makes it difficult to ensure that it has a long life. The light generated from the laser diode is reflected by the polygon scanner 19<u style="single">a</u> which is rotated at high speed and which transfers the latent image to the photosensitive member 1 by way of the mirror 19<u style="single">b</u>. In this embodiment, a liquid crystal shutter array or an LED array (the optical path of which devices is 70 mm or less) is employed to constitute the optical signal generator 3, thereby allowing a reduction in the depth D of the image forming apparatus. However, if a reflecting mirror is additionally provided, the optical signal generator 3 can also be constituted by a laser scanner although the cost is relatively high.

0062The paper passing through the fixing device 11 is discharged to the outside of the image forming apparatus by the action of the delivery rollers 13. In this embodiment, the paper is discharged to the front side of the apparatus in such a manner that the printed surface thereof faces downwardly. Further, in this embodiment the paper register rollers 10 which are defined by a pair of rollers and the fixing device 11 which is also defined by a pair of rollers are disposed in such a manner that the line which intersects the area of contact between the former pair of rollers and that between the latter pair or rollers (said line defining the paper transport path 21), extends substantially vertically, and the photosensitive member 1 is disposed so as to be in contact with said vertical line at the transfer position.

0063The photosensitive member 1, after the completion of the transfer, has a slight amount of toner remaining on its surface, said toner having failed to be transferred. The remaining toner is scraped off by means of the cleaning device 6.

0064Further, the surface of the photosensitive member 1 is uniformly irradiated with light by means of the erasing device 7 in order to allow the charge to escape reliably from the surface of the photosensitive member 1 and to make the surface condition of the latter uniform.

0065The exhaust fan 14 is disposed near and below the fixing device 11 to discharge the air heated by the power supply 15 and by the fixing device 11.

0066The power supply 15 supplies electrical power to each of the units in the image forming apparatus. Since the power supply 15 includes a transformer (not shown) and other heavy members as its constituent elements, it is disposed in the lowermost part of the apparatus. In the case of an image forming apparatus having a relatively small bottom area and a relatively large height, it is very desirable to dispose the power supply 15 in the lowermost part of the apparatus.

0067The control board 16 is arranged to control a signal to be sent to the optical signal generator 3, to interface the image forming apparatus with a host computer not shown, and to effect sequence control of each of the elements in the apparatus. In this embodiment, since the control board 16 has a size which is approximately equal to the paper size A4, it is disposed on the rear side of the image forming apparatus.

0068With the above-described contents, an image forming apparatus having a width W of 300 mm, a depth D of 170 mm and a height H of 260 mm was experimentally produced. Thus the height H is greater than the depth D. The paper transport path 21 was not bent to a substantial degree and the greatest angle of bend was about 20 to 30° as described above. Thus, it was possible to print envelopes and the like without the occurrence of any jam and without the generation of wrinkles. In the experiments, a structure in which the paper transport path 21 was bent at about 90° was examined and almost no problem was found.

0069However, when the paper transport path 21 was bent at 120° or more, problems such as jamming and wrinkles occurred considerably in the case of certain kinds of paper, and when the angle of bend exceeded 180°, a relatively large number of different kinds of envelope became wrinkled almost every time they were printed.

0070Thus, the most preferable angle at which the paper transport path 21 may be bent is about 20° to 30° at the maximum, the preferred range of angles being from about 60° to about 80°.

0071In the case where the apparatus is arranged so as to minimize the degree to which the paper transport path 21 is bent as described above and is also arranged to employ face-down delivery, the paper transport path 21 may be arranged so as to extend substantially vertically as in the case of this embodiment, or may be such as to have a structure obtained by turning the arrangement of the prior art shown in Figure 13 upside down. In the latter case, there are problems such as an increase in the amount of toner adhering to the charging device 2 and to the optical signal generator 3 due to the gravity drop, complication of the toner transport mechanism of the developing device 4, difficulty in employing a blade system which is suitable for reducing the size of the cleaning device 6, and damage to the image caused by the contact of the toner which has not yet been fixed with the paper transport path 21 due to the fact that the printed surface of the paper faces upwardly In addition, the size of the apparatus is undesirably increased in the same way as in the case of the prior art shown in Figure 13.

0072Accordingly, it is preferable to provide a paper transport path 21 which extends substantially vertically as in the case of this embodiment, and it is very desirable, in order to realize this structure, to dispose the optical signal generator 3 so as to emit an optical signal toward the centre of the photosensitive member 1 from a position which is within a range of 60° around the horizontal line which passes through the centre of the photosensitive member 1. It is preferable for the optical signal generator 3 to emit an optical signal from a position which is substantially on said horizontal line. Further, it is very desirable to dispose the developing device 4 above the optical signal generator 4 from the viewpoint of the above-described structural features. It is preferable to dispose the developing device 4 within a range of 10° to 90° from said horizontal line. More specifically, it is preferable to dispose the developing sleeve 4<u style="single">a</u> at the lowermost portion of the developing device 4 with a view to realizing a structure which enables the toner to be most efficiently transported by means of gravity.

0073When this embodiment was experimentally run at a print speed of 8 ppm. the fast print time was able to be shortened to only 12 seconds, whereas the fast print time in the prior art apparatus shown in Figure 14 is generally 30 to 40 seconds because of a relatively long paper transport path.

0074In this embodiment, among the process elements, the photosensitive member 1, the charging device 2 and the cleaning device 6 are formed as a unit to constitute a cartridge 22 which is adapted to be detachable with respect to the apparatus body, as shown in Figure 5 which illustrates the way in which the cartridge 22 is loaded or unloaded with a movable part 23 of the image forming apparatus opened. In Figure 5, the same members as those shown in Figure 1 are denoted by the same reference numerals. The transfer device 5 and the fixing device 11 are rigidly secured to the inner side of a movable part 23 and constitute the movable part 23 as a whole. The movable part 23 is supported by a pivot shaft 24 mounted in an immovable part 25 of the image forming apparatus. When the cartridge 22 is to be unloaded the movable part 23 is pivoted in the direction of an arrow C and the cartridge 22, which is guided and supported by a guide member (not shown) which is rigidly secured to the immovable part 25, is unloaded in the direction of an arrow G.

0075As shown in Figures 6(a) and 6(b), the fixing device 11 is constituted by two roller sections 31 and 32 which include the rollers 11 and 11<u style="single">a</u> and resilient members, for example, springs 34, are provided so as to urge the roller sections 31 and 32 away from each other and thus urge them in the direction in which the nip pressure is cancelled.

0076Accordingly, when the movable part 23 is opened, the nip pressure is cancelled, whereas, when it is closed, the roller section 32 is pressed by an abutment member 33, and a normal nip pressure for fixing is thus obtained.

0077When the movable part 23 is opened, the centre of gravity is, as a matter of course, moved toward the front side of the apparatus, and in the worst case the apparatus may fall down toward the front side. However, in the embodiment illustrated in Figures 7(a) and 7(b), an abutment member 35 is provided so that the movable part 23 can be opened only to an angle 0 beyond which the position of the centre of gravity is no longer within the bottom of the apparatus, thereby making it possible to prevent the apparatus from falling down.

0078When, for example, the fixing device 11 is provided on the movable part 23, the movable part 23 becomes very heavy, and if the movable part 23 is opened carelessly, a harmful impact may be applied to the whole of the apparatus in addition to the impact to the abutment member 35 and the support shaft 24. There is also a risk that the apparatus will be forced to fall down or will be forced to collide with another object. The occurrence of such accidents is prevented by the action of a damping means 36 which enables the movable part 23 to be opened smoothly and slowly at a predetermined speed.

0079It is also effective practice to provide the fixing device 11 on the immovable part 25 as shown in Figure 8 for the purpose of facilitating removal of a jam caused by paper passing through the fixing device 11. As shown in Figure 8, the movable part 23 is provided with one of the register rollers 10 and with paper guides 40. In such a case, it is necessary to arrange the apparatus so that the cartridge 22 can be loaded from or unloaded toward the front side of the apparatus without being obstructed by the fixing device 11. In this arrangement, particularly, as shown in Figures 9(a), 9(b), since no heavy member is provided on the movable part 23, the structure may be greatly simplified. That is to say, it may be possible to omit the abutment member 35 and the damping means 36 and allow the movable part 23 to be opened until it comes into contact with the surface of the place on which the apparatus is installed. However, employment of the damping means 36 of course enables the movable part 23 to be opened and closed even more smoothly.

0080Referring next to Figures 10(a) to 10(c2), the transfer device 5 may be provided on the cartridge 22, and may further be made detachable, as best shown in Figure 10(a₂), or the transfer device 5 may be made pivotal, as in Figures 5(b₁) and 5(b₂) about either one or both of two regions where it is supported by the cartridge 22, thereby facilitating cleaning of the transfer device 5 and removal of jamming paper. Figure 5(b₃) illustrates how the lower end of the transfer device 5 may be received in a catch 5<u style="single">a</u> in the cartridge 22.

0081The cartridge 22 may have a flat bottom surface or may have a plurality of projections 41 to 44 provided on its bottom surface as shown in Figure 11. More specifically, the cartridge 22 may be arranged such that it is supported by at least three projections which define in combination a polygonal stable plane and the centre of gravity of the cartridge 22 is perpendicularly upwardly thereof. In such a case, it is possible to dispose the cartridge 22 on even a non-flat plane without any fear of the cartridge 22 oscillating unstably. Employment of rubber leg members to define the above-described projections improves the stability and prevents generation of noise. It is also possible to increase the degree of accuracy in the mounting of the cartridge 22 by employing the projections as positioning members when the cartridge 22 is loaded into the apparatus body.

0082It should be noted that the position of the centre of gravity of the cartridge 22 changes as the amount of waste toner changes, but it is preferable to meet the above-described conditions concerning the centre of gravity irrespective of the amount of toner.

0083The cartridge 22 may be loaded or unloaded using a handle 26 as shown in Figure 5. The handle 26 is defined by a member which is formed integrally with the casing of the cartridge 22 so as to project from the casing and extend in the direction of the axis of rotation of the photosensitive member 1. The handle 26 is formed at such a position that it is possible to draw out the cartridge 22 from the image forming apparatus and transport the cartridge 22 without any need to change its posture and shift it from one hand to the other. In other words, when the cartridge 22 is to be unloaded, the user opens the movable part 23, draws out the cartridge 22 in the direction of the arrow G and then, for example, transports it to another place. Since the handle 26 is provided on the uppermost part of the cartridge 22 as viewed when it is loaded inside the apparatus body, there is apparently no fear that the posture of the cartridge 22 provided with such a handle 26 will be changed during the unloading operation. Accordingly, the waste toner which is collected within the cleaning device 6 provided below the photosensitive drum 1 is left as it is during the unloading operation and there is therefore no risk of the waste toner being moved to one side so as to scatter or leak out of the cartridge 22. Further, the cartridge 22 according to this embodiment has a bottom surface which is so shaped and a centre of gravity which is so positioned that the cartridge 22 will maintain the above-described posture without any change even when it is left outside the image forming apparatus for some reason.

0084Toner has an angle of repose in the range from 40° to 50° with respect to the horizontal and when the angle of inclination exceeds this range, the toner flows. Accordingly, any change in posture of the cartridge 22 during the unloading operation and transportation, or when it is temporarily left outside the image forming apparatus, should be kept at a level which is less than the above-described angle of repose. It is desirable that any change in the posture of the cartridge 22 should be much smaller than the angle of repose of the toner. In order to achieve this, the cartridge 22 according to this embodiment is arranged as shown in Figure 12 which is a sectional view thereof. More specifically, when the cartridge 22 has not yet been used, no waste toner is collected in a waste toner box 50 provided inside the cleaning device 6 and the centre of gravity of the cartridge 22 is therefore at the point E, so that the point E and the handle 26 are connected together by the straight line EH. As the cartridge 22 is used, waste toner is collected in the waste toner box 15, resulting in the centre of gravity of the cartridge 22 moving to the point F, the handle 26 and the point F being connected by the straight line FH. The cleaning device 6 is, however, disposed at such a position that the straight lines EH and FH are adjacent to each other, thereby preventing, as much as possible, an undesirable degree of inclination of the cartridge 22 during its use.

0085As has been described above, the image forming apparatus has a reduced bottom area and therefore occupies a minimized area on the top of a desk. In particular, the above-described embodiment has a depth D of about 170 mm and therefore, when this apparatus is placed on an ordinary desk having a depth of 700 mm and sheets of paper of A4 size are discharged from the front side of the apparatus, the sum total of the length of the paper, i.e., 300 mm, and the depth of the apparatus, i.e. 170 mm, is 470 mm, which means that a sufficiently large area is left on the top of the desk. Further, since paper is discharged from the front side of the apparatus, it is easy for the operator to handle the sheets of paper coming from the machine. The arrangement wherein paper is inserted from the upper side of the apparatus and discharged from the front side thereof enables the face-down delivery system to be achieved with a paper transport path which is much simpler than that of the prior art. Accordingly, envelopes and the like can be printed without the need to additionally provide a face-up delivery mechanism and its associated switching means and with substantially reduced problems such as jamming and the generation of wrinkles.

0086In addition, it is possible, according to this embodiment, to shorten the fast-print time to 12 seconds in the case of paper of A4 size, whereas the fast-print time of conventional 8 ppm machines is 25 to 30 seconds.

0087It is possible for the operator to readily exchange cartridges and to remove jamming paper in a natural posture and also possible for him to see his hand during such an operation. There is therefore no fear of an electrophotographic process member being accidentally damaged.

0088Since exchange of cartridges is carried out from the front side of the apparatus, there is no need to provide any free space for this purpose as a part of the installation space, which has heretofore been required for the prior art apparatus wherein exchange of cartridges is conducted from one lateral side of the apparatus.

0089Although the prior art which adopts the clamshell system in order to cancel jamming needs a strong spring for supporting a movable part having a weight of several tens of kilograms in close proximity with the support point and also needs a high-strength frame for preventing deformation, the embodiments of the present invention described above eliminate the need to provide such members.

0090Further, since the transfer device 5 is incorporated in the cartridge 22, it is unnecessary for the user to clean the transfer device (i.e. the transfer device is disposable). The embodiments of the present invention described above enable the transfer device to be readily cleaned in case of occurrence of any abnormal operation.

0091It is possible for the user to readily load and unload the cartridge by holding the handle provided thereon and by placing it on the top of a desk or the like with ease and without any fear of waste toner collected therein moving to one side to fall.