Abstract:
A method of forming an image, including emitting a plurality of light beams from a plurality of vertically arranged light emitting devices; passing the plurality of light beams through corresponding slots of a vertical stay; receiving the plurality of light beams which have passed through the corresponding slots of the vertical stay by a plurality of photosensitive surfaces contained in a corresponding plurality of vertically arranged detachable cartridges, thereby forming a plurality of latent images; developing the plurality of latent images into a corresponding plurality of different colored toner images using different colored toner contained in the plurality of detachable cartridges; and transferring each of the plurality of different colored toner images onto a medium by moving the medium in a vertical direction.

Description:
This application is a Continuation of application Ser. No. 10/426,871 Filed on May 1, 2003, which is a division of 10/160,066, filed Jun. 4, 2002 now U.S. Pat. No. 6,580,887, which is a continuation of Ser. No. 09/805,246, filed Mar. 14, 2001 now U.S. Pat. No. 6,400,917, which is a division of Ser. No. 09/305,275, filed May 5, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus including a plurality of image forming cartridges arranged one above the other and a plurality of optical writing means arranged one above the other or a single optical writing means. 
     There has been known an image forming apparatus of the type including an apparatus body and a plurality of image forming cartridges removably mounted to the apparatus body one above the other, or stacked, in the direction of gravity. This type of image forming apparatus forms an image with image forming means when the image forming cartridges are mounted to the apparatus body. Photoconductive elements each are supported by either one of the respective image forming cartridge or the apparatus body beforehand. In the case where the photoconductive elements are supported by the apparatus body, the image forming means arranged on the cartridges contact the photoconductive elements when the cartridges are mounted to the apparatus body. 
     The prerequisite with the image forming apparatus of the type described is that the image forming cartridges removable from the apparatus body be stably positioned on the apparatus body. Should the cartridges be unstable in position, so-called banding would occur in an image due to the vibration of a driveline. Further, optical writing means are stacked one above the other and respectively associated with the cartridges. The optical writing means are also susceptible to the vibration of the driveline, aggravating the banding. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an image forming apparatus capable of obviating banding ascribable to the vibration of image forming cartridges and that of optical writing means. 
     In accordance with the present invention, an image forming apparatus for forming an image on a photoconductive element with image forming means includes an apparatus body, a plurality of image forming cartridges removably mounted to the apparatus body in the form of a stack, and a structural member for partitioning off the space between nearby image forming cartridges mounted to the apparatus body. A of photoconductive elements each are supported by the respective image forming cartridge beforehand, or the photoconductive elements are supported by the apparatus body beforehand such that when the image forming cartridges are mounted to the apparatus body, the image forming means supported by the image forming cartridges beforehand each partly contact the associated photoconductive element. 
     Also, in accordance with the present invention, an image forming apparatus includes an apparatus body, and a plurality of optical writing means stacked one above the other and each being mounted on a respective base member supported by the apparatus body. Adjusting means is included in at least one of the optical writing means for correcting the shift of a scanning line relative to the scanning lines of the other optical writing means. A structural member partitions off the space between the optical writing means including the adjusting means and the optical writing means adjoining it. The structural member is affixed to the apparatus body at a part thereof. 
     Further, in accordance with the present invention, an image forming apparatus includes an apparatus body, and a plurality of photoconductive elements mounted on the apparatus body one above the other. A plurality of optical writing means each form a latent image on a respective photoconductive element. The optical writing means are constructed into a single box-like writing unit for emitting a plurality of light beams toward the photoconductive elements. The writing unit is spaced from the photoconductive elements by a preselected distance. 
     Moreover, in accordance with the present invention, an image forming apparatus for forming an image on a photoconductive element with image forming means includes an apparatus body, a plurality of image forming cartridges removably mounted to the apparatus body in the form of a stack, and a plurality of optical writing means each for forming a latent image on a photoconductive element associated therewith. A plurality of photoconductive elements each are supported by a respective one of the plurality of image forming cartridges beforehand, or the photoconductive elements are supported by the apparatus body beforehand such that when the image forming cartridges are mounted to the apparatus body, the image forming means supported by the image forming cartridges beforehand each partly contact associated one of the photoconductive elements. The optical writing means are constructed into a single box-like writing unit for emitting a plurality of light beams toward the photoconductive elements in a stacking direction of the image forming cartridges. The writing unit is spaced from the photoconductive elements by a preselected distance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1A is a fragmentary front view showing an image forming apparatus representative of a first example of a first embodiment; 
     FIG. 1B is a fragmentary side elevation of the first example; 
     FIG. 2A is a fragmentary front view showing an image forming apparatus representative of a second example of the first embodiment; 
     FIG. 2B is a fragmentary side elevation of the second example; 
     FIG. 3A is a fragmentary front view showing an image forming apparatus representative of a third example of the first embodiment; 
     FIG. 3B is a fragmentary side elevation of the third example; 
     FIG. 4A is a fragmentary front view showing an image forming apparatus representative of a fourth example of the first embodiment; 
     FIG. 4B is a fragmentary side elevation of the fourth example; 
     FIG. 5A is a fragmentary front view showing an image forming apparatus representative of a fifth example of the first embodiment; 
     FIG. 5B is a fragmentary side elevation view of the fifth example; 
     FIG. 6 is a perspective view of a horizontal stay; 
     FIG. 7 is a perspective view of a vibration-proof rubber block; 
     FIG. 8 is a perspective view of a vertical stay; 
     FIG. 9 is a fragmentary front view showing a first example of a second embodiment of the present invention; 
     FIG. 10 is a plan view of the first example shown in FIG. 9; 
     FIG. 11 is a side elevation of the first example shown in FIG. 9; 
     FIG. 12 is a fragmentary plan view showing a second example of the second embodiment; 
     FIG. 13 is a side elevation of the second example shown in FIG. 12; 
     FIG. 14 is a fragmentary view showing a third example of the second embodiment; 
     FIG. 15 is a side elevation of the third example shown in FIG. 14; 
     FIG. 16 is a fragmentary view showing a first example of a third embodiment of the present invention; 
     FIGS. 17 and 18 are fragmentary side elevation of the first example shown in FIG. 16; 
     FIG. 19 is a fragmentary front view showing a modification of the first example shown in FIG. 16; 
     FIG. 20 is a fragmentary view showing a second example of the third embodiment; 
     FIG. 21 is a fragmentary front view showing a modification of the second example shown in FIG. 20; 
     FIG. 22 is a fragmentary front view showing an image forming cartridge representative of a third example of the third embodiment; 
     FIGS. 23 and 24 are respectively a perspective view and a front view showing how the inclination of a scanning line is corrected; 
     FIG. 25A is a perspective view showing holding means assigned to a mirror; 
     FIG. 25B is a fragmentary sectional view of the holding means; 
     FIG. 26 is a fragmentary front view showing a modification of the third example shown in FIG. 22; 
     FIG. 27 is a fragmentary front view showing another modification of the example shown in FIG. 22; 
     FIG. 28 is a perspective view showing an apparatus body representative of a fourth example of the third embodiment; 
     FIG. 29 is a perspective view showing a modification of the fourth example shown in FIG. 28; 
     FIG. 30 is a perspective view showing an apparatus body representative of a fifth example of the third embodiment: 
     FIG. 31 is a perspective view showing a modification of the fifth example shown in FIG. 30; 
     FIG. 32 is a fragmentary view showing a sixth example of the third embodiment; 
     FIG. 33 is a fragmentary front view showing the sixth example shown in FIG. 32; 
     FIG. 34A is a sectional view showing the structure of a writing unit included in a seventh example of the third embodiment and a positional relation between it and photoconductive elements; 
     FIG. 34B is a fragmentary sectional view showing a dust-proof glass included in the seventh example shown in FIG. 34A; 
     FIG. 35 is a fragmentary plan view showing a ninth example of the third embodiment; 
     FIG. 36 is a fragmentary front view of the ninth example shown in FIG. 35; 
     FIG. 37 is a fragmentary sectional view showing a portion for mounting an optical writing unit included in the ninth example of FIG. 35; 
     FIG. 38 is a view similar to FIG. 37, showing a modification of the portion of FIG. 37; 
     FIG. 39 is a perspective view showing how an optical writing unit is mounted in a tenth example of the third embodiment; 
     FIG. 40 is a fragmentary plan view showing an eleventh example of the third embodiment; 
     FIG. 41 is a front view of the eleventh example shown in FIG. 40; 
     FIG. 42 is a front view showing a twelfth example of the third embodiment; 
     FIGS. 43A-43D are front views each showing a particular image forming cartridge not including a photoconductive element; 
     FIG. 44 is a fragmentary front view of a conventional image forming apparatus; 
     FIG. 45 is an external perspective view of the conventional image forming apparatus; 
     FIG. 46 is a section along line J—J of FIG. 45; 
     FIGS. 47 and 48 are respectively a plan view and a side elevation showing an image forming cartridge included in the conventional apparatus; 
     FIG. 49 shows the image forming cartridge of the conventional apparatus mounted to an apparatus body; 
     FIG. 50 is a view showing a spacing member for providing a preselected space between a developing roller and a photoconductive element 
     FIG. 51 is a front view showing a part of an image forming apparatus of the type having photoconductive elements mounted on its body beforehand; 
     FIGS. 52A-52D are front views each showing a particular image forming cartridge not including a photoconductive element; 
     FIG. 53A is a view showing an image forming cartridge vibrating in the up-and-down direction; 
     FIG. 53B is a view similar to FIG. 53A, showing the cartridge vibrating in the torsional direction; 
     FIG. 54 is a section along line Q—Q of FIG. 45; 
     FIG. 55 is a section along line W—W of FIG. 54; and 
     FIGS. 56A and 56B are views respectively showing a vertical vibration mode and a torsional vibration mode. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To better understand the present invention, reference will be made to a conventional image forming apparatus capable of forming a full-color image with a plurality of image forming cartridges, shown in FIGS. 44-46. As shown in FIG. 44, an image transfer belt (simply belt hereinafter)  1  is passed over rollers  2  and  3  and extends in the up-and-down direction. At the time of image formation, the belt  1  turns in such a direction that its surface for retaining a paper or similar recording medium moves upward, as indicated by an arrow in FIG.  44 . 
     Four image forming cartridges (simply cartridges hereinafter)  4 ,  5 ,  6  and  7  are arranged one above the other and face the above surface of the belt  1  moving upward. The cartridges  4 - 7  are assumed to store black (K) toner, cyan (C) toner, magenta (M) toner and yellow (Y) toner, respectively. The cartridges  4 - 7  are identical in mechanical construction and therefore in members constituting them. Let the following description concentrate on the cartridge  5  by way of example. The other cartridges  4 ,  6  and  7  are simply distinguished from the cartridge  5  by suffices Y, M and K attached to the reference numerals. 
     The cartridge  5  includes a photoconductive element in the form of a drum  8 C and image forming means for forming an image on the drum  8 C. The image forming means includes a charge roller  9 C, a developing roller  10 C and a cleaning blade  12 C arranged around the drum  8 C. The charge roller  9 C plays the role of charging means. The developing roller or developing means feeds toner to the drum  8 C. The cleaning blade  12 C removes toner left on the drum  8 C after image transfer. 
     A supply roller  11 C is associated with the developing roller  10 C for supplying a developer to the roller  10 C. Rotary bodies  13 C and  14 C convey the developer toward the supply roller  11 C while agitating it. Optical writing means  104 C, which will be described later, emits a light beam Lb to an image writing position on the drum  9 C between the charge roller  9 C and the developing roller  10 C. 
     As shown in FIG. 45, the cartridges  4 - 7  are removably mounted to an apparatus body  22  for maintenance including the replacement of various image forming members each having a particular life. Specifically, as shown in FIG. 44, lock pins or positioning and supporting means  16 C and  17 C extend in the direction in which the cartridge  5  is mounted and dismounted, i.e., the direction perpendicular to the sheet surface of FIG.  44 . Further, as shown in FIGS. 46-48, a drive joint or drive inputting means  15 C is provided for transferring a driving force to the above image forming means. 
     As shown in FIG. 45, the apparatus body  22  is implemented as a hexahedral box-like frame. Specifically, the apparatus body  22  has a front wall  22   a  through which the cartridge  5  is mounted and dismounted, a rear wall  22   b  facing the front wall  22   a , a right side wall  22   c , a left side wall  22   d , a top wall  22   e , and a bottom wall  22   f . While the walls  22   a - 22   f  are shown as each having a simple configuration, they are in practice provided with notches, bent portions, holes and so forth for mounting various parts. 
     A wide opening is formed in the front wall  22   a  in the up-and-down direction for receiving the cartridges  4 - 7  in the axial direction of the drums. As shown in FIGS. 47 and 48, a rectangular window is formed in one side of the cartridge  5 , so that the drum  8 C is partly exposed to the outside through the window. The shaft of the drum  8 C is journal led to the case of the cartridge  5 . The drive joint  15 C mentioned earlier is tapered and mounted on one end of the shaft of the drum  8 C. 
     As shown in FIG. 49, holes  16 C′ and  17 C′ are formed in the front wall  22   a  for receiving the lock pins  16 C and  17 C. As shown in FIGS. 46 and 49, a prime joint  15 C′ is mounted on the rear wall  22   b  and mates with the drive joint  15 C. 
     To mount the cartridge  5  to the apparatus body  22 , the cartridge  5  is inserted into the apparatus body  22  in the mounting and dismounting direction in FIGS. 45,  47  and  48 . At the same time as the lock pins  16 C and  17 C mate with the holes  16 C′ and  17 C′, respectively, the drive joint  15 C mates with the tapered bore of the prime joint  15 C. In this manner, the cartridge  5  is locked to the apparatus body  22  mainly at three points, i.e., by the drive joint  15 C mating with the prime joint  15 C′ mounted on the back of the rear wall  22   b  and the lock pins  16 C and  17 C mating with the holes of the front wall  22   a . The prime joint  15 C′ is connected to a drive source not shown. Such a configuration is also applied to the other cartridges  4 ,  6  and  7 . 
     As shown in FIG. 44, a pair of registration rollers  18  are positioned in the vicinity of the lower end of the belt  1 . In a full-color mode, the cartridges  4 - 7  respectively form toner images on their photoconductive drums in black, cyan, magenta and yellow. A paper or similar recording medium is conveyed by the registration roller  18  toward the top of the belt  1  along an inlet passage indicated by an arrow in FIG.  44 . While the belt  1  conveys the paper upward, a Y, an M, a C and a K toner image are sequentially transferred from the drums of the cartridges  7 - 4  one above the other. The paper with the resulting full-color image is driven out of the apparatus via a fixing device not shown. 
     Assume that any one of the cartridges  4 - 7  runs out of toner or reaches a time for maintenance. Then, only the cartridge needing maintenance is pulled out of the apparatus body  22 , maintained, and again mounted to the apparatus body  22 , or replaced with a new cartridge. 
     The cartridge  5 , for example, is removably supported at three points by the lock pins  16 C and  17 C and drive joint  15 C. The charge roller  9 C, developing roller  10 C and so forth each are supported by the cartridge  5  at axially opposite ends thereof. To insure accuracy, the lock pins  16 C and  17 C and drive joints  15 C supporting the cartridge  5  on the apparatus body  22  are positioned on the side walls of the cartridge  5  supporting the opposite ends of the above rollers  9 C and  10 C. 
     As stated above, the cartridge  5  is supported by the apparatus body  22  at its opposite ends in the lengthwise direction in a so-called bridge structure. As a result, the vibration of the apparatus body  22  ascribable to, e.g., the drive of the belt  1  and paper and the drive of the fixing device causes the cartridge  5  to vibrate. 
     Basically, the cartridge  5  is caused to vibrate either in the vertical direction, as indicated by an arrow in FIG. 53A, or in the torsional direction, as indicated by arrows of different directions in FIG.  53 B. Let the vibration modes shown in FIGS. 53A and 53B be referred to as a vertical mode and a torsional mode, respectively. When the cartridge  5  bodily vibrates in either one of the above modes, the vibration is directly transferred to the drum  8 C supported by the cartridge  5 . Also, the vibration of the cartridge  5  is transferred to the drum  8 C via the charge roller  9 C, developing roller  10 C, cleaning blade  12 C and other image forming means. As a result, a displacement mainly ascribable to the drum  8 C itself shifts the image writing position and an image transferring position. This makes the scanning pitch irregular in the subscanning direction (the direction of movement of the belt  1 ) in accordance with the resonance frequency. The irregular scanning pitch causes the density of an image to be periodically irregular in the subscanning direction (so-called banding). This is also true with the other cartridges  4 ,  6  and  7 . 
     Another conventional type of image forming apparatus has photoconductive drums not mounted on the cartridges, but journalled to its body beforehand. In this type of apparatus, each cartridge includes a developing roller and a toner hopper for feeding toner to the developing roller and is mounted to the apparatus body by members similar to the lock pins and drive joint of FIGS. 46-49. For example, when the C cartridge  5  is mounted to the apparatus body  22 , the developing roller  10 C is brought into contact with the drum  8 C mounted on the apparatus body  22  beforehand. 
     FIGS. 50,  51  and  52 B show another specific configuration. As shown, when a C cartridge  5 ″ is mounted to the apparatus body  22 , a developing roller  10 C″ mounted on the cartridge  5 ″ is spaced from a photoconductive drum  8 C″ by a small gap. As shown in FIG. 50, to maintain the above small gap, rings  10 C″- 1  and  10 C″- 2  are mounted on the axially opposite ends of the developing roller  10 C″; the rings  10 C″- 1  and  10 C″- 2  are greater in diameter than the developing roller  10 C″. The drum  8 C″ is mounted on the apparatus body  22  beforehand. When the cartridge  5 ″ is mounted to the apparatus body  22 , the rings  10 C″- 1  and  10 ″- 2  abut against the axially opposite ends of the drum  8 C″ and thereby form the above gap. 
     The above relation also applies to the other cartridges  4 ″,  6 ″ and  7 ″. Specifically, as shown in FIG. 51, photoconductive drums  8 K″,  8 M″ and  8 Y″ are mounted on the apparatus body  22  beforehand. As shown in FIGS. 52A,  52 C and  52 D, developing rollers  10 K″,  10 M″ and  10 Y″ each having rings corresponding to the rings  10 C″- 1  and  10 C″- 2  are mounted on the cartridges  4 ″,  6 ″ and  7 ″, respectively. When the cartridges  4 ″,  6 ″ and  7 ″ are mounted to the apparatus body  22 , the developing rollers  10 K″,  10 M″ and  10 Y″ are respectively spaced from the drums  8 K″,  8 M″ and  8 Y″ by the preselected small gap. 
     In the above apparatus, the developing roller  10 C″ journalled to the cartridge or the rings or spacing members  10 C″- 1  and  10 C 2 - 2  abut against the drum  80 ″ mounted on the apparatus body  22  beforehand. Consequently, when the cartridge vibrates, the drum  8 C″ vibrates via the developing roller or developing means  10 C″ or the rings  10 C″- 1  and  10 C″- 2 . This results in banding in the same manner as with the cartridge  5  including the drum  8 C. Specific cases in which such banding occurs are as follows. 
     (1) In the apparatus wherein the drum  8 C is mounted on the cartridge  5 , more specifically the case of the cartridge  5 , when the cartridge  5  is mounted to the apparatus body  22  for image formation, the vibration of the cartridge  5  is transferred to the drum  8 C via the charge roller, developing roller  10 C, cleaning blade  12 C and other image forming means, resulting in banding. More specifically, the drum  8 C and developing roller  10 C are supported by a single member (cartridge  5 ) and can therefore be accurately spaced from each other without resorting to the rings  10 ″- 1  and  10 ″- 2 , FIG.  7 . However, the vibration of the cartridge  5  is transferred to the drum  8 C and additionally transferred to the drum  8 C via the charge roller  9 C, cleaning blade  12 C and other image forming means mounted on the cartridge  5 . 
     (2) As shown in FIGS. 50-53, assume the configuration wherein when the cartridge is mounted to the apparatus body, the developing means (developing roller  10 C″ or the rings  10 ″C- 1  and  10 ″C- 2 ) mounted on the cartridge or one or more of the charging means and cleaning means abut against the drum  8 C″ mounted on the apparatus body. Even in this configuration, the vibration of the cartridge is transferred to the drum  8 C″ and brings about banding. 
     In any case, banding ascribable to the vibration of the cartridge is extremely conspicuous at and around a pitch of 0.5 mm, but it is not noticeable when the vibration frequency and therefore the pitch on an image decreases. It follows that when the resonance frequency is low in the previously mentioned modes, banding is conspicuous and often degrades an image to a critical degree. This is particularly true with an image forming apparatus including a plurality of cartridges that are driven by a sophisticated mechanism. 
     Conventional arrangements for supporting an image forming unit removably mounted to an apparatus body may be generally classified into the following three types: 
     (a) an arrangement wherein a process cartridge including four developing units arranged side by side and a photoconductive belt is removably mounted to the apparatus body; the process cartridge is supported by a resilient member affixed to a push-up member mounted on the apparatus body (Japanese Patent Laid-Open Publication No. 5-313425) 
     (b) an arrangement wherein a plurality of toner cartridges are removably mounted to a developing device facing an image carrier; nearby toner cartridges are formed-with projections and recesses mating with each other and prevented from shaking thereby (Japanese Patent Laid-Open Publication No. 6-148968); and 
     (c) an arrangement wherein a toner cartridge for replenishing toner is mounted to a process cartridge including a photoconductive drum and removable from the apparatus body; a guide member restricts the position of the toner cartridge being pushed into toner storing means included in the process cartridge (Japanese Patent Laid-Open Publication No. 10-20647). 
     Referring again to FIG. 44, four optical writing means  104 K.  104 C,  104 M and  104 Y are stacked one above the other in the direction of gravity and correspond to the four cartridges  4 ,  5 ,  6  and  7 , respectively. Because the writing means  104 K- 104 Y are identical in mechanical arrangement and therefore in members constituting them, let the following description concentrate on the writing means  104 C by way of example. The other writing means  104 K,  104 M and  104 Y are simply distinguished from the writing means  14 C by suffixes Y, M and K added to the reference numerals. Also, only the operation of the writing means  104 C and that of the cartridge  5  will be described because the operations of the others will be understood by analogy. 
     The writing means  104 C scans the drum  8 C with the light beam Lb in order to form a latent image on the drum  8 C. Specifically, in the writing means  10 C, a laser beam issuing from a laser diode, not shown, is steered by a polygonal mirror  106 C and then focused on the drum  8 C in the form of a beam spot via a first f-θ lens  108 C, mirrors  110 C and  111 C, and a second f-θ lens  112 C. 
     The cartridge  5  includes, in addition to the drum  8 , the cleaning means, charging means, developing means, toner and others necessary for image formation and each having a particular life. 
     In the above apparatus, the cartridges  4 - 7  are stacked one above the other at intervals, which are too small to position the writing means  104 K- 104 Y therebetween. This is why the writing means  104 K- 104 Y are located at positions relatively remote from the drums  8 K- 8 Y in the horizontal direction. 
     When the writing means  104 C, for example, vibrates, the beam spot on the drum  8 C is noticeably displaced and apt to bring about banding. 
     The apparatus body  22  is basically made up of the front wall  22   a , rear wall  22   b , side walls  22   c  and  22   d , top wall  22   e , and bottom wall  22   f , as described with reference to FIG.  45 . As shown in FIGS. 54 and 55, the writing means  104 C is mounted on a flat base member  328 C extending between the front wall  22   a  and the rear wall  22   b . The base member  328 C is affixed to the rear wall  22   b  at the rear end and supported by the front wall  22   a  via adjusting means  330 C at the front end. The base member  328 C and adjusting means  330 C form a bridge structure. 
     The adjusting means  330 C is used to move the front end of the base member  328 C upward or downward, i.e., in the subscanning direction in order to adjust the inclination of the light beam Lb issuing from the writing means  104 C. By so adjusting all the writing means, it is possible to prevent four images of different colors from being inclined by different angles when superposed. 
     Specifically, as shown in FIG. 54, the base member  328 C is formed with a slit-like notch  328   a  at its rear end, so that it can be moved in the above direction on a hinge basis. While adjusting means  330 KI,  330 C,  330 M and  330 Y are assigned to all of the different colors, the base member of one writing means assigned to one reference color may be directly affixed to the front wall  22   a  and-rear wall  22   b  without the intermediary of the adjusting member. This allows one of such adjusting means to be omitted. 
     Technologies for adjusting the position of optical writing means or for preventing it from being displaced are also disclosed in Japanese Patent Laid-Open Publication Nos. 5-6071, 7-104545, and 6-34901. In Laid-Open Publication No. 5-6071, optical writing means is adjustably mounted on a structural body via a spring, a screw, etc. In Laid-Open Publication No. 7-104545, a structural body is formed of ceramics or similar material having a small coefficient of thermal expansion in order to obviate the dislocation of colors ascribable to thermal expansion. In Laid-Open Publication No. 6-34901, an elastic member is interposed between the housing of optical writing means and a cover for reducing the vibration of the cover which would effect optical writing. 
     The cartridges  4 - 7  and optical writing means  104 K- 104 Y arranged one above the other in the direction of gravity, as stated above, promote the miniaturization of the apparatus. However, because the base members  328 K- 328 Y and adjusting means  330 K- 330 Y are provided in a bridge structure, the vertical mode shown in FIG.  56 A and torsional mode shown in FIG. 56B basically exist with, e.g., the writing means  104 C. This is also true with the other writing means  104 K,  104 M and  104 Y. 
     Assume that the vibration of, e.g., the drive source is imparted to the writing means  104 C via the front wall  22   c  and rear wall  22   b , causing the writing means  104 C to bodily vibrate. Then, the beam spot on the drum  8 C is periodically displaced with the result that the scanning pitch in the subscanning direction becomes irregular in accordance with the resonance frequency. The irregular scanning pitch causes the image density to become periodically irregular in the subscanning direction and thereby brings about banding, as discussed earlier. 
     Banding is more conspicuous with an image forming apparatus including a plurality of optical writing means than with a single-color image forming apparatus. This is because the apparatus with a plurality of optical writing means needs a sophisticated driveline apt to increase the vibration level, requires each writing means to have a small cross-sectional area for miniaturization which is apt to aggravate vibration, and makes it difficult to arrange a strong structural body around the writing means due to the limited space. 
     As stated above, banding ascribable to the vibration of the image forming cartridges and that of the optical writing means is the problem with the conventional technologies. 
     Preferred embodiments of the image forming apparatus in accordance with the present invention will be described hereinafter. 
     1st Embodiment 
     Basically, this embodiment constitutes an improvement mainly over the conventional image forming cartridge described with reference to FIGS. 44-52. Briefly, the illustrative embodiment is constructed to obviate banding ascribable to the vibration of the photoconductive elements caused by the vibration of the image forming cartridges. Therefore, the embodiment is applicable to both of the construction wherein the photoconductive elements are mounted on the cartridges, more particularly the cases of the cartridges, and the construction wherein when the cartridges supporting the photoconductive elements are mounted to the apparatus body, one or more of the charge rollers, developing means with the developing rollers or the spacing members, and cleaning blades abut against the associated photoconductive elements. 
     The following description will concentrate on the construction described with reference to FIGS. 44-49 and  53 , i.e., the apparatus of the type including the photoconductive elements mounted on the cartridges. However, the illustrative embodiment is similarly applicable to the apparatus described with reference to FIGS. 50-52 wherein the photoconductive elements are mounted on the apparatus body. 
     EXAMPLE 1 
     FIGS. 1A and 1B show a first example of the first embodiment. To reduce the size of an image forming apparatus, it is preferable to stack a plurality of image forming cartridges one above the other in the direction of gravity at a small distance or pitch. In this example, structural members (horizontal stays hereinafter)  25  each are interposed between nearby ones of a plurality of cartridges  4 - 7  arranged at a small pitch. Horizontal stays  25  similar to the above stays  25  are also positioned above the top cartridge  4  and below the bottom cartridge  7 , respectively. 
     The horizontal stays  25  each are implemented as a plate bent upward at its opposite ends in the direction perpendicular to the cartridge mounting and dismounting direction. The stays  25  are affixed to the front wall  22   a  in the vicinity of the cartridge mounting and dismounting opening and the rear wall  22   b  by fastening means not shown. 
     The cartridges  4 - 7  each are supported by the upper surface of the associated stay  25 . Because the stays  25  are fastened to the front wall  22   a  in the vicinity of the opening and the rear wall  22   b , as stated above, the two walls  22  and  22   b  are connected together by the stays  25  in the vicinity of the cartridges  4 - 7 . 
     As for the cartridge  5 , the vibration of the lock pins  16 C and  17 C and drive joint  15 C can be effectively reduced because they rest on the front wall  22   a  and rear wall  22   b . This is also true with the other cartridges  4 ,  6  and  7 . Particularly, as for a vibration mode in which the front wall  22   a  and rear wall  22   b  perform planar vibration, the stays  25  are configured to just halve the plane. This successfully obviates a low frequency resonance mode undesirable from the banding standpoint and thereby allows only a high frequency resonance mode to occur. In addition, the stays  25  positioned above the top cartridge  4  and below the bottom cartridge  7  increase the rigidity of the entire cartridge support structure and thereby further promote the obviation of banding. 
     The stays  25  may be formed with holes and notches for implementing cooling passages and for an assembly purpose so long as they do not reduce strength. At the opening for mounting and dismounting the cartridges, the edges of the stay  25  are exposed to the outside and should preferably be bent or folded for safety and greater strength. 
     The cartridges  4 - 7  have substantially the same sectional shape and extend in the axial direction of, e.g., the photoconductive drums  8 K- 8 Y. Therefore, so long as the cartridges  4 - 7  are mounted and dismounted in the axial direction of the drums  8 K- 8 Y, as in this example, the stays  25  may be formed with projections and recesses complementary to the sectional shape of the cartridges  4 - 7 . Such projections and recesses increase the strength of the structural body and save space without interfering with the cartridges  4 - 7  at the time of mounting or dismounting. 
     Further, the cartridges  4 - 7  each storing a developer of particular color are identical in mechanical arrangement and can therefore be produced with identical specifications. This promotes the efficient production of the cartridges  4 - 7  on a quantity basis. 
     Preferably, the members needing accurate positioning relative to the apparatus body  22 , e.g., the drums  8 Y- 8 K have their shafts supported by bearings with play (margin) relative to the associated cartridges in the direction perpendicular to the shafts. Then, the cartridges each are positioned on a preselected part of the associated stay  25 . In this configuration, when each cartridge is affixed to the apparatus body  22 , the shaft of the drum mounted on the cartridge with the above play moves within the range of the play. As a result, the drive joint  15 C, FIG. 49, mates with the prime joint  15 C′ mounted on the apparatus body  22 , setting up a drive transmission path. 
     As stated above, each photoconductive drum is supported by the associated cartridge in, so to speak, a floating manner. Therefore, when the cartridge is positioned relative to the apparatus body  22  via the associated stay  25 , the drive joint mounted on the shaft of the drum is brought into engagement with the prime joint. As a result, the drum is accurately positioned on the apparatus body  22 . Further, the cartridge does not need a support structure for accurately positioning the drum relative to the cartridge. In addition, the cartridge supported by the stay  25  vibrates little. That is, both of the accurate positioning of the drum relative to the apparatus body  20  and the reduction of vibration of the cartridge are achievable at the same time. Because a plurality of stays  25  are arranged one above the other in association with the cartridges, there can be effectively suppressed vibration in the vertical direction and therefore banding. 
     EXAMPLE 2 
     FIGS. 2A and 2B show a second example of the first embodiment. As shown, the bottom of. e.g., the cartridge  5  is curved in the form of a letter W complementarily to the curvatures of nearby rotary bodies  13 C and  14 C. The boundary between the two downwardly convex curved portions is implemented as a recess  26 C extending in the mounting and dismounting direction of the cartridge  5 . 
     In this example, a guide  27 C implemented as a flat plate stands upright from the upper surface of each horizontal stay  25  of Example 1 and is received in the recess or portion to be guided  26 C of the cartridge  5  above the stay  25 . In this condition, the guide  27 C guides the cartridge  5 . The other cartridges are also provided with such guides  27 C. The stay  25  above the top cartridge  4  is not provided with the guide  27 C because it has nothing to guide. 
     The guide  27 C received in and extending along the recess  26 C of the cartridge positioned above the guide  27 C prevents the cartridge being mounted to or dismounted from the apparatus body  2  from being displaced in the direction perpendicular to the mounting or dismounting direction or from being rotated to hit against the surrounding members. 
     As shown in FIG. 2B, the guide  27 C, as well as guides  27 K,  27 M and  27 Y, is increased in height halfway. This configuration is successful to reduce the clearance between the guide and the portion to be guided at the last stage of mounting and therefore to guide the cartridge with accuracy. 
     The guides  27 K- 27 Y may be respectively molded integrally with the stays  25  or may be produced independently of the stays  25  and then affixed to the stays  25 . Moreover, the upright guides  27 K- 27 Y increase the bending rigidity of the stays  25  in the up-and-down direction and thereby increase mechanical strength and obviates banding. 
     EXAMPLE 3 
     FIGS. 3A and 3B show a third example of the illustrative embodiment. As shown, among the stays included in Example 1, the stay  25  between the cartridges  4  and  5 , the stay  25  between the cartridges  5  and  6  and the stay  25  between the cartridges  6  and  7  each are provided with resilient pressing means for pressing the overlying and underlying cartridges. 
     Specifically, as shown in FIGS. 3A,  3 B and  6 , the pressing means is implemented by leaf springs  28 U and  28 D each having a flat portion  28   a  and a curved portion  28   b . The leaf spring  28 U has its flat portion  28   a  affixed to the upper surface of the stay  25  with the curved portion  28   b  being convex upward. The leaf spring  28 D has its flat portion  28   a  affixed to the lower surface of the stay  25  with the curved portion  28   b  being convex downward. 
     The leaf springs  28 U and  28 D are respectively affixed to the intermediate portion of the upper surface and the intermediate portion of the lower surface of the stay  25 . The leaf spring  28 U resiliently presses the cartridge  4  overlying the stay  25  upward while the leaf spring  28 D resiliently presses the cartridge  5  underlying the stay  25  downward. Paying attention to the leaf springs  28 U and  28 D on the stay  25  intervening between the cartridges  4  and  5 , the curved portion  28   b  of the spring  28 U presses the cartridge  4  upward while the curved portion  28   b  of the spring  28 D presses the cartridge  5  downward. This is also true with the leaf springs  28 U and  28 D affixed to the stay  25  between the cartridges  5  and  6  and the stay  25  between the cartridges  6  and  7 . The leaf springs  28 U and  28 D resiliently support the antinode portions of the cartridges  4 - 7  as to the amplitude of vibration and thereby effectively suppress vibration. 
     Assume that the guides  27 K- 27 Y shown in FIGS. 2A and 2B are applied to this example. Then, the leaf springs  28 U are so positioned as to respectively contact the two convex portions of the bottom of the overlying cartridge, so that the springs  28 U do not interfere with the above guide. This configuration will be described specifically later with reference to FIG.  4 A. 
     The leaf springs  28 U and  28 D pressing the bottom of the overlying cartridge and the top of the underlying cartridge, respectively, may be positioned face-to-face and provided with the same resilient force. This arrangement is advantageous in that the resilient forces of the leaf springs  28 U and  28 D cancel each other and do not bend the entire cartridges. Such leaf springs or similar biasing parts may also be provided above the top cartridge and below the bottom cartridge for the same purpose. 
     Each cartridge may be formed with recesses such that the leaf springs  28 U and  28 D click into the recesses when the cartridge is inserted into the apparatus body  22  as far as a preselected position. The clicking action of the leaf springs  28 Y and  28 D will allow the operator to surely feel the insertion of the cartridge. 
     Further, the above recesses for the clicking action may be configured to more firmly mate with the leaf springs  28 U and  28 D. This allows the cartridges to be fixed in place without resorting to lock levers or similar extra affixing means and thereby reduces the cost of the apparatus. This example may be combined with the guides of Example 2 in order to promote easy mounting and dismounting of the cartridges. The leaf springs  28 U and  28 D may be replaced with any other suitable resilient members, if desired. 
     EXAMPLE 4 
     FIGS. 4A and 4B show a fourth example of the illustrative embodiment. As shown, a vibration-proof rubber block  29  is fitted on the lower surface of the stay  25  overlying the cartridge  4 . The rubber block  29  contacts the upper surface of the cartridge  4  and exerts a viscoelastic pressing force between the stay  25  and the cartridge  4 . Such rubber blocks  29  are also fitted on the lower surfaces of the stays  25  overlying the other cartridges  5 ,  6  and  7 , respectively. As shown in FIG. 7, each rubber block  29  has a rectangular configuration. 
     Two leaf springs  28 U each having the configuration shown in FIG. 6 are affixed to the upper surface of the stay  25  between the cartridges  4  and  5  at positions around a position facing the rubber block  29 . The leaf springs  28 U are also affixed to the upper surface of the stay  25  between the cartridges  5  and  6  and the upper surface of the stay  25  between the cartridges  6  and  7  in exactly the same manner as the above leaf springs  28 U. 
     As shown in FIG. 4A, at the position facing the rubber block  29 , the bottom of the casing of the cartridge is recessed. The two leaf springs  28 Y are respectively positioned to face the two convex portions of the casing on both sides of the above recess. The leaf springs  28 U and rubber block  29  constitute vibration proofing means. 
     The leaf springs  28 U bias the overlying cartridge upward. The cartridge is therefore pressed against the overlying rubber block  29  with the result that the rubber block  29  exerts a viscoelastic force on the cartridge. The rubber block  29  enhances vibration proofing based on the thermal conversion of vibration energy making the most of the viscoelastic characteristic. 
     In this example, even leaf springs exerting a relatively small resilient force can implement the above vibration proofing, so that the force to at on each cartridge is reduced. That is, this example causes a minimum of deformation to occur despite the use of the leaf springs and is therefore desirable from the accuracy standpoint as well. 
     With the combination of the leaf springs and rubber blocks, it is possible to effectively generate the force for pressing each cartridge against the overlaying rubber block. Further, by additionally using the guide arrangement of Example 2 and so configuring the guide as to increase the frictional force of the rubber block  29  just before the completion of the insertion of the cartridge, it is possible to reduce the manual force required to slide the cartridge on the rubber block  29  to an adequate degree. 
     EXAMPLE 5 
     FIGS. 5A and 5B show a fifth example of the illustrative embodiment. As shown in FIGS. 5A and 8, a flat vertical stay  30  is mounted on the left ends of the stays  25  and faces the left side wall  22   d  (FIG.  45 ). As shown in FIG. 8, the vertical stay  30  includes mounting portions  30   b  positioned to face the scanning direction of the light beams Lb. The mounting portions  30   b  are affixed to the front wall  22   a  and rear wall  22   b , respectively. The stay  30  is affixed to the top wall  22   e  at its upper end and affixed to the bottom wall  22   f  at its lower end. The vertical flat portion of the stay  30  is fastened to the horizontal stays  25  by screws  210 . 
     In the above configuration, the horizontal stays  25  are firmly affixed to the apparatus body via the vertical stay  30  and reduce the planar vibration mode of the front wall  22   a  and rear wall  22   b  more positively. In addition, the stays  25  and stay  30  substantially perpendicular to each other realize an extremely great sectional moment and thereby provides the structural body with great bending rigidity. 
     Particularly, the improved bending rigidity is successful to reduce the vibration of the horizontal stays  25  themselves in the event of suppression of vibration, as described in relation to Examples 3 and 4. This example may therefore be combined with the configurations of Examples 3 and 4. 
     Optical writing devices, not shown, are located at the left-hand side of the cartridges  4 - 7  shown in FIG.  5 A and respectively emit the light beams Lb toward the drums  8 K- 8 Y. The writing devices may also be supported by a structural body similar to the structural body including the vertical stay  30 . In such a case, the stay  30  bears a compression stress (buckling load) ascribable to the weights of the cartridges and those of the writing devices in the vertical direction. This condition increases strength, reduces deformation and suppresses resonance more positively than a condition wherein the cartridges and writing devices are arranged on horizontal plates. This will be described more specifically in conjunction with Example 1 of 3rd Embodiment. 
     As shown in FIG. 8, the vertical stay  30  is formed with slots  30   d  each extending in the scanning direction of the light beam Lb with a width corresponding to the diameter of the light beam Lb. The light beams Lb issuing from the writing devices are respectively passed through the slots  30   d . That is, each slot  30   d  has a minimum necessary length and a minimum necessary width for allowing the light beam Lb to pass therethrough. This minimizes a decrease in the rigidity of the stay  30  as a structural body and serves to obviate banding. 
     The vertical stay  30  may be additionally formed with holes and notches so long as they do not reduce the strength of the stay  30 . For example, as shown in FIG. 8, holes  30   c  positioned above and below each slot  30   d  are used to affix the horizontal stays  20  to the vertical stay  30 . It should be noted that any suitable number of holes  30   c  may be formed in the stay  30 . While the stays  20  are fastened to the stay  30  by the screws  210 , the screws  210  will be replaced with, e.g., soldering when use is made of metal or replaced with, e.g., injection molding when use is made of resin. 
     Examples 1-5 shown and described may be suitably combined not only to obviate banding but also to promote easy operation and reduce the cost. 
     2nd Embodiment 
     This embodiment mainly constitutes an improvement over the construction of the conventional optical writing means described with reference to FIGS. 54 and 55. The structural parts of this embodiment identical with the structural parts of the conventional arrangement are designated by like reference numerals and will not be described specifically in order to avoid redundancy. 
     EXAMPLE 1 
     As shown in FIGS. 9-11, this example is implemented as a full-color image forming apparatus including four image forming cartridges  4 - 7  stacked one above the other in the direction of gravity. Four optical writing means  104 K- 104 Y are also arranged one above the other in the direction of gravity and associated with the cartridges  4 - 7 , respectively. The writing means  104 K- 104 Y respectively include the adjusting means  330 K- 330 Y stated earlier. 
     As shown in FIG. 11, a flat structural member  202  is positioned between nearby ones of the writing means  104 K- 104 Y, i.e., between the base member  328 K and the writing means  104 C underlying the base member  328 K. The structural member  202  partitions off the space between the nearby writing means. The structural member  202  is affixed to the front wall  22   a  and rear wall  22   b  by fastening means, not shown, at opposite ends thereof. 
     Structural members  202  are also provided between the writing means  104 C and  104 M and between the writing means  104 M and  104 Y in exactly the same manner as the above structural member  202 . In FIG. 9, the base members  328 K- 328 Y included in the writing means  104 K- 104 Y are not shown. 
     The structural members  202  between the consecutive writing means  104 K- 104 Y increase the structural strength of the front wall  22   a  and rear wall  22   b , among others. This is successful to suppress the vibration of the portions around the positions where the writing means  104 K- 104 Y are affixed to the walls  22   a  and  22   b . Particularly, as for the planar vibration mode of the walls  22  and  22   b , the structural members  202  divide the plane of vibration and eliminates a low frequency resonance mode apt to result in banding. 
     As shown in FIG. 11, the structural members  202  represented by dash-and-dot lines P1 and P2 may also be positioned above the top writing means  104 K and below the bottom writing means  104 Y. Such structural members  202  further increase the total strength of the apparatus body and enhance the anti-banding function. 
     The structural members  202  may be formed with holes and notches for cooling and mounting purposes so long as they do not reduce the strength implementing the above anti-banding function. Further, the structural members  202  may be suitably bent or folded. The cartridges  4 - 7  and writing means  104 K- 104 Y should preferably be arranged at a small pitch in order to further miniaturize the apparatus. 
     EXAMPLE 2 
     As shown in FIGS. 6,  12  and  13 , a leaf spring or pressing means  280 D is mounted on the lower surface of, e.g., the structural member  202  between the writing means  104 C and  104 M for pressing the writing means  104 M downward. Likewise, a leaf spring or pressing means  280 U is mounted on the upper surface of the structural member  202  for pressing the writing means  104 C upward. This configuration is also applied to the other structural members  202 . 
     The leaf springs  280 U and  280 D are identical in shape and material with the leaf springs  28 U and  28 D described with reference to FIG.  6 . The leaf springs  280 U and  280 D are affixed to the intermediate portion of the upper surface and the intermediate portion of the lower surface of the structural body  202 . In FIG. 13, the curved portion  28   b  of the leaf spring  280 U and the curved portion  28   b  of the leaf spring  280 D are shown as having different curvatures. This stems from a difference in the distance to the base member of the structural body  202  or distance to the optical writing means. In FIG. 12, the base members  328 K- 328 Y are not shown. In this manner, the leaf springs  280 U and  280 D each resiliently press associated one of the writing means  104 K- 104 Y upward or downward. 
     The writing means  104 C, for example, is expected to be displaced by the adjusting means  330  together with the base member  328 C (movable member) and cannot therefore be directly affixed to the structural member  202 . This is also true with the other writing means  104 K,  104 M and  104 Y. 
     The leaf springs or pressing means  280 U and  280 D allow the structural members  202  to support the writing means  104 C while maintaining the writing means  104 C movable. Assume the vibration mode of FIG. 56A having nodes at opposite ends of the writing means  104 C and an antinode at the intermediate portion of the writing means  104 C. Then, the leaf springs  280 U and  280 D exert forces in such a manner as to suppress the antinode of the amplitude of the above vibration mode. This further enhances the anti-vibration function available with the structural members  202 . This is also true with the other writing means  104 K,  104 M and  104 Y. 
     The leaf springs  280 U and  280 D may advantageously exert the same pressing force, so that the resilient forces acting on the top and bottom of each writing means can cancel each other. This prevents the writing means from being bent. 
     In this example, the leaf springs  280 U and  280 D are also positioned on the upper surface of the top structural members  202  and the lower surface of the bottom structural members  202 , respectively. Although these leaf springs  280 U and  280 D do not actually exhibit their pressing function, they are significant for the following reasons. The structural members  202  all having the leaf springs  280 U and  280 D promote standardization, i.e., general-purpose application and can readily cope with an increase in the number of writing means. Further, the top and bottom structural members  202  increase the mechanical strength of the entire structural body. The leaf springs  280 U and  280 D are a specific form of pressing means and may be replaced with any other suitable resilient means. 
     EXAMPLE 3 
     FIGS. 7,  14  and  15  show a third example of the illustrative embodiment. As shown, a vibration-proof rubber block  29 D is fitted on the lower surface of the structural member  202  between the writing means  104 C and  104 M. Likewise, a vibration-proof rubber block  29 U is fitted on the upper surface of the above structural member  202 . This is also true with the other structural members. 
     The rubber blocks or vibration proofing means  29 U and  29 D are identical in shape and material with the rubber blocks  29  of FIG. 7 having a viscoelastic characteristic. The rubber blocks  29 U and  29 D each having a suitable size are respectively adhered to the intermediate portion of the upper surface and the intermediate portion of the lower surface of the structural member  202 . In FIG. 14, the base members  328 K- 328 Y are not shown. The vibration proofing means implemented by the rubber blocks  29 U and  29 D proof vibration based on the thermal conversion of vibration energy and thereby effectively suppress the previously stated vibration mode. 
     The rubber blocks or vibration proofing means  29 U and  29 D are capable exhibiting their effect based on viscosity even when their elasticity is low, compared to the leaf springs or resilient pressing means  280 U and  280 D. Therefore, the forces to act on the writing means  104 K- 104 Y and therefore the deformation of the writing means  104 K- 104 Y can be reduced, insuring the accuracy of the structural body. 
     The rubber blocks  29 U and  29 D are also fitted on the upper surface of the top structural member  202  and the lower surface of the bottom structural member  202 , respectively, for the reasons described with reference to FIGS. 6,  12  and  13 . 
     The rubber blocks  29 U and  29 D may abut against the base members  328 K- 328 Y or the writing means  104 K- 104 Y via leaf springs or similar resilient members, if desired. In this case, the adjusting means  130 K- 130 Y can function without resorting to the great deformation of the rubber blocks  29 U and  29 D. 
     EXAMPLE 4 
     FIGS. 8,  16  and  17  show a fourth example of the illustrative embodiment. As shown in FIG. 16, each structural member  202  has vertical walls  202   a  and  202   b  at its right and left edges. The left vertical wall  202   a  is affixed to the left side wall  22   b  by fastening means. The right vertical wall  202   b  is directly affixed to a vertical stray or structural member  300  extending in parallel to the direction of arrangement of a plurality of optical writing means and substantially perpendicularly to each structural member  202 . 
     The vertical stay  300  may be provided with the same shape and same size as the vertical stay  30  shown in FIG.  8 . The various portions of the stay  300  are designated by the same reference numerals as the portions of the stay  30 . Specifically, the stay  300  includes the portions  30   a  to be affixed to the top wall  22   e  and bottom wall  22   f , portions  30   b  to be affixed to the front wall  22   a  and rear wall  22   b , and holes  30   c  for affixing the stay  30  to the structural members  202 . In addition, four slots  30   d  are formed in the stay  300  in order to allow the light beams Lb issuing from the writing means  104 K- 104 Y to pass therethrough. 
     As shown in FIG. 17, the right wall  202   b  of each structural member  202  is formed with screw holes  202   c  corresponding in position to the holes  30   c  of the stay  300 . Each structural member  202  and stay  300  are fastened together by screws or fastening means  210 ′ shown in FIG.  8 . 
     The stay  300  further promotes the suppression of the planar vibration mode achievable with the front wall  22   a  and rear wall  22   b . Further, the horizontal structural members  202  and stay  300  substantially perpendicular to each other implement an extremely great sectional moment and provide the structural body with great bending rigidity. 
     In this example, the writing means  104 K- 10 Y are arranged one above the other in the direction of gravity. The stay  300  therefore bears a compression force ascribable to its own weight and the weights of the structural members  202  in the direction perpendicular to the direction of thickness. Such an arrangement therefore increases strength, reduces deformation and obviates the resonance mode, compared to an arrangement wherein writing means are arranged in the horizontal direction. 
     The stay  300  formed with the slots  30   d  may be additionally formed with holes and notches for cooling and mounting purposes so long as they do not reduce strength. While the structural members  202  and stay  300  are shown as being connected together by the screws  210 , they may be, e.g., welded together when use is made of metal or may be implemented by a single molding by injection molding. 
     3rd Embodiment 
     This embodiment obviates banding by using all or part of the configurations of the examples of the foregoing embodiments. 
     EXAMPLE 1 
     In Example 5 of 1st Embodiment shown in FIGS. 5A,  5 B and  8 , the horizontal stays  25  are connected to the vertical stay  30 . In Example 4 of 2nd Embodiment shown in FIGS. 8 and 17, the structural members  202  are connected to the vertical stay  300 . The vertical stays  30  and  300  have been shown and described as being separate members having the same shape and same size. 
     In this example, the vertical stays  30  and  300  shown in FIGS. 5A and 5B and FIG. 16, respectively, are implemented as a single member. Specifically, as shown in FIGS. 18 and 19, this example includes a single vertical stay  30  to which both the horizontal stays  25  and structural members  202  are connected. In this sense, the vertical stay  30  plays the role of a shared structural member. 
     In the above configuration, the horizontal stays  25 , vertical stay  30 , structural members  202  and apparatus body  22  are constructed into a single structural body. This increases the rigidity of the entire structure and thereby obviates banding. In addition, the stay  30  serves to reinforce the structural members  202  and horizontal stays  25  and thereby enhances simplification and miniaturization. 
     In FIG. 19, the left ends of the structural members  202  are spaced from the left side wall  22   d  for the layout reason. That is, the space is used to accommodate electrical parts and other parts for image formation. Even this configuration is capable of obviating banding because the structural members  202  are affixed to the front wall  22   a  and rear wall  22   b  at their front and rear ends. As shown in FIG. 18, the left ends of the structural members  202  may be affixed to the left side wall  22   d , depending on the layout. In FIG. 19, the horizontal stays  25 , vertical stay  30  and structural members  202  are indicated by bold lines to show that they constitute a single structural body. 
     EXAMPLE 2 
     In FIG. 1, the cartridges  4 - 7  are separated from each other by the structural members or partitions  25 . In the example to be described, the image forming means is received in a casing separate from the image forming cartridge. The casing plays the role of the structural member separating nearby cartridges. 
     Specifically, as shown in FIG. 20, casings  35  indicated by bold lines each accommodate the respective image forming means. In this example, as for the cartridge  4 , the developing roller  10 K, supply roller  11 K and rotary bodies  13 K and  14 K are the image forming means received in the casing  35 . On the other hand, the charge roller  9 K and cleaning blade  12 K are mounted on the cartridge  4  as the other image forming means. Because the developing roller  10 K, supply roller  11 K and rotary bodies  13 K and  14 K are positioned below the charge roller  9 K and cleaning blade  12 K, the casing  35  effectively separates the cartridges  4  and  5  from each other. This is also true with the other cartridges  6  and  7 . 
     Because the charge roller  9 K and cleaning blade  12 K include parts that should be replaced at relatively short intervals, they are constructed into the cartridge  4  removable from the apparatus body  22 . By contrast, the developing roller  10 K, supply roller  11 K and rotary bodies  13 K and  14 K withstand repeated use over a relatively long period of time. These members  10 K,  11 K,  13 K and  14 K can therefore be fixedly connected to the apparatus body  22  only if means for replenishing toner from the outside is provided. This is true with the casings  35  associated with the other cartridges  5 ,  6  and  7 . By using the casing  35  as partitions, it is possible to reinforce the structural body and prevent the cartridges  4 - 7  from vibrating. 
     The casings  35  each have a roll-like configuration surrounding the developing means, e.g., the developing roller  10 K, supply roller  11 K and rotary bodies  13 K and  14 K. Each casing  35  extends in the front-and-rear direction and has its front end and rear end affixed to the front wall  22   a  and rear wall  22   b , respectively. The casings  35  are therefore implemented as a single structural body together with the apparatus body. Such a structural body has sufficient strength and prevents the cartridges  4 - 7  from vibrating more positively. 
     The casings  35  intervening between the cartridges  4 - 7  not only separate the cartridges  4 - 7  from each other, but also serve as casings surrounding the image forming means. This configuration further enhances the simple and miniature construction while obviating banding, compared to the configuration using the structural members  25  for partition. 
     FIG. 21 shows a modification of the above example. As shown, each casing  35  has an extension  35   a  affixed to the vertical stay  30  shown in FIGS. 5A,  5 B and  19 . This modification further increases the strength of the structural body. 
     While the casings  35  each accommodate the respective developing means, they may accommodate any other suitable image forming means. 
     EXAMPLE 3 
     In the examples shown in FIGS. 9-18, the optical writing means  10 K- 104 Y are respectively provided with the adjusting means  330 K- 330 Y for correcting the shift of scanning lines. The adjusting means  330 K- 330 Y each are positioned outside of the respective housing accommodating the writing means and operated to move the housing. The problem with this configuration is that the housings themselves cannot be used as the structural members  202 . A third example to be described accommodates each adjusting means in the housing so as to use the housing as the structural member  202 . Let the writing means each including the respective adjusting means and accommodated in the respective housing be labeled  104 K′,  104 C′,  104 M′ and  104 Y′. Because the writing means  104 K′- 104 Y′ are identical in construction, the following description will concentrate on the writing means  104 K′ by way of example. 
     As shown in FIG. 22, the housing of the writing means  104 K′ accommodates the polygonal mirror  106 K, first f-θ lens  108 K and mirrors  110 K and  111 K, as stated earlier. As shown in FIGS. 23 and 24, one end  37  of the mirror  111 K in the lengthwise direction corresponding to the main scanning direction of the light beam Lb is movable by any desired angle about the other end  36 . When the mirror  111 K is so moved, the scanning line formed by the light beam Lb on the drum  8 K is shifted in the subscanning direction at a position corresponding to the above end  37  of the mirror  111 K; the entire scanning line is inclined by, e.g., an angle θ. Holding means that will be described holds the mirror  111 K at such an adjusted position. The holding means constitutes the adjusting means. 
     As shown in FIG. 25A, one surface of the mirror  111 K is supported by a knife edge  38  in the vicinity of the end  36  in such a manner as to be movable while maintaining a beam reflection angle. The above surface is constantly biased by a compression spring or resilient means  40  in the vicinity of the other end  37 . The other surface of the mirror  111 K is pressed by a moving member  41 . As shown in FIG. 25, the moving member  41  is a kind of a nut and held in threaded engagement with a screw  43  rotatable coaxially with the output shaft of a motor  42 . A groove  45  is formed in the side of the moving member  41  and elongate in the axial direction of the member  41 . A detent  44  is received in the groove  45 . 
     The knife edge  38 , spring  40 , moving member  41 , motor  42 , screw  43  and detent  44  constitute the holding means mentioned earlier and playing the role of the adjusting means. When the motor  42  is driven, the mirror  111 K is angularly moved about the knife edge  38  and then locked at the adjusted position. 
     The above adjusting means associated with the mirror  111 K can be received in the housing of the writing means  104 K′. Therefore, the housing of the writing means  104 K′ can be bodily mounted to the apparatus body  22  in a static condition and can therefore replace the structural member  202  for partition. 
     FIG. 26 shows the writing means  104 K′- 104 H′ each having the adjusting means arranged in the respective housing. As shown, the housings each have a bottom plate  47  having a greater size or grater rigidity than the usual bottom plate and connected to the front wall  22   a  and rear wall  22   b  at opposite ends. With this configuration, this example realizes a structure simpler and smaller than the structures of the examples shown in FIGS. 9-19. 
     As shown in FIG. 27, the bottom plates  47  of the writing means  104 K′- 104 Y′ may be connected to the vertical stay  300  in the same manner as in FIGS. 8 and 16. The stay  300  is connected to the top wall  22   e  at the upper end, connected to the bottom wall  22   f  at the lower end, connected to the front wall  22   a  at the front end, and connected to the rear wall  22   b  at the rear end. If desired, the structural members  25  shown in FIG. 18 may also be connected to the stay  300 . 
     EXAMPLE 4 
     FIG. 28 shows a fourth example of the illustrative embodiment using the horizontal stays  25  described with reference to FIGS. 1A-6. As shown, the apparatus body or frame  22  has the front wall  22   a , rear wall  22   b , right side wall  22   c , left side wall  22   d , top wall  22   e , and bottom wall  22   f . The stays  25  are arranged one above the other in the apparatus body  22  for separating the cartridges  4 - 7 . The drums  8 K- 8 Y included in the cartridges  4 - 7 , respectively, extend perpendicularly to the front wall  22   a . A single opening  50  is formed in the front wall  22   a  and broad enough to accommodate the cartridges  4 - 7 , so that the cartridges  4 - 7  can be mounted and dismounted in the axial direction of the drums  8 K- 8 Y. The front ends of the stays  25  are affixed to the edges of the opening  50  by screws or fastening means  51  while traversing the opening  50  in the right-and-left direction. 
     The stays  25  traversing the opening  50  of the front wall  22   a  reinforce the front wall  22   a . This prevents the rigidity of the front wall  22   a  and therefore the rigidity of the entire frame from decreasing and thereby obviates banding. 
     FIG. 29 shows a modification of the above example. As shown, the front wall  22   a  of the frame is formed with openings  54 ,  55 ,  56  and  57  in place of the single opening  50  of FIG.  29 . The openings  54 - 57  are assigned to the cartridges  4 - 7 , respectively. Part of the front wall  22   a  are left in the form of ribs between the openings  54 - 57 , as illustrated. The front ends of the stays  25  are respectively affixed to the ribs by the screws  51 . The rigidity of such a front wall  22   a  decreases little because each opening is small and because a rib intervene between nearby openings, compared to the front wall  22   a  shown in FIG.  28 . This, coupled with the fact that the stays  25  reinforce the front wall  22   a , insures the rigidity of the frame and obviates banding more positively. 
     EXAMPLE 5 
     FIG. 30 shows a fifth example of the illustrative embodiment also using the horizontal stays  25  described with reference to FIGS. 1A-6. As shown, the stays  25  for separating the cartridges  4 - 7  are arranged one above the other in the frame also made up of the six walls  22   a - 22   f . The right side wall  22   c  extends perpendicular to the axial direction of the drums  8 K- 8 Y in a horizontal plane. The transfer belt  1  shown in FIG. 9 is disposed in the side wall  22   c . The entire side wall  22   c  is implemented as a cover  58  surrounding the belt  1  and openable away from the frame. 
     Specifically, the lower end of the cover  58  is connected to the bottom wall  22   f  by a hinge or a shaft. As shown in FIG. 30, when the cover  58  is opened away from the frame, the entire area corresponding to the side wall  22   c  is uncovered and allows the cartridges  4 - 7  to be easily mounted and dismounted therethrough. FIG. 30 shows the cartridge  4  pulled out of the frame. 
     FIG. 31 shows a modification of the above example. In the foregoing examples, the writing means  104 K- 0104 Y or  104 K′- 104 Y′ and vertical stay  30  or  300  are arranged at the left-hand side of the cartridges  4 - 7 , so that the cartridges  4 - 7  cannot be mounted or dismounted via the position where the left side wall  22   d  is present. The modification of FIG. 31 is constructed to allow the cartridges  4 - 7  to be mounted and dismounted via the above position. 
     Specifically, in the modification, a single optical writing unit  100  in the form of a flat box is substituted for the writing means  104 K- 104 Y or  104 K′- 104 Y′. The writing unit  100  is arranged in a cover  59  mainly constituted by the left side wall  22   ds . The cover  59  is openable away from the frame about a shaft  60 . When the cover  59  is opened, as indicated by a dash-and-dots line in FIG. 31, it uncovers the area corresponding to the left side wall  22   d  and allows the cartridges  4 - 7  to be easily mounted and dismounted. 
     In any case, the side wall of the frame extending perpendicularly to the axial direction of the drums in a horizontal plane is bodily implemented as an openable cover. It is therefore not necessary to form the front wall  22   a  with an opening or openings (FIG. 28 or  29 ) which would reduce the rigidity of the structural body and result in banding. 
     EXAMPLE 6 
     This example, like the above example, includes the box-like writing unit  100 . As shown in FIGS. 32 and 33, the writing unit  100  is affixed to a structural body  102  which is affixed to the front wall  22   a  and rear wall  22   b  at its opposite ends. The cartridges  4 - 7  are stacked one above the other and affixed to the apparatus body  22 . 
     The writing unit  100  is formed with openings  100 K,  100 C,  100 M and  100 Y respectively aligning with the drums  8 K- 8 Y of the cartridges  4 - 7  for passing the light beams Lb therethrough. The writing unit  100  is located at a preselected distance from the drums  8 K- 8 Y. 
     The single writing unit  100  is easier to position than the four writing means  104 K- 104 Y shown in FIG.  9  and reduces the overall size of the apparatus. Further, the single writing unit  100  allows reinforcing members to be easily added for increasing rigidity. In addition, the flat writing unit  100  reduces the space to be occupied to the apparatus. 
     EXAMPLE 7 
     FIGS. 34A and 34B show a seventh example of the illustrative embodiment and relating to the configuration of the writing unit  100  described with reference to FIGS. 31-33. As shown in FIG. 34A, a polygonal mirror  70  is positioned at the center of the writing unit  100  and constitutes a polygon scanner. A motor  72  causes the polygonal mirror  70  to rotate. The mirror  70  has an axis of rotation extending perpendicularly to the axial direction of the drums  8 K- 8 Y. 
     Four light sources, not shown, are arranged in the writing unit  100 . The light sources are respectively modulated by image signals representative of cyan, magenta, yellow and black. The resulting light beams issuing from the light sources are incident to four points on the polygonal mirror  70 . The mirror  70  steers the incident light beams in the direction perpendicular to its axis of rotation. The drums  8 K- 8 Y are stacked in the direction in which the mirror  70  steers the incident light beams. 
     The light beam representative of a black component and steered by the polygonal mirror  70  is incident to the drum  8 K via an f-θ lens  73 , mirrors  74  and  75 , an elongate lens  76 , a mirror  77  and the opening  100 K. The light beam representative of a cyan component and steered by the polygonal mirror  70  is incident to the drum  8 C via the f-θ lens  73 , mirrors  78  and  79 , an elongate lens  80 , a mirror  81  and the opening  100 C. The light beam representative of a magenta component and steered by the polygonal mirror  70  is incident to the drum  8 M via an f-θ lens  83 , mirrors  84  and  85 , an elongate lens  86 , a mirror  87  and the opening  100 M. Further, the light beam representative of a yellow component and steered by the polygonal mirror  70  is incident to the drum  8 Y via the f-θ lens  83 , mirrors  88  and  89 , an elongate lens  90 , a mirror  91  and the opening  100 Y. As shown in FIG. 34B, the openings  100 K- 100 Y each are covered with a dust-proof glass  130 . 
     As stated above, in the writing unit  100 , the polygonal mirror  70  steers the incident light beams in the same direction as the direction in which the drums  8 K- 8 Y are stacked. The writing unit  100  can therefore be implemented as a single horizontally flat box and can reduce the space requirement, compared to the four writing means  104 K- 104 Y shown in FIG.  9 . Moreover, the number of polygonal mirrors that generate heat is reduced from four to one, so that temperature inside the apparatus can be maintained low. 
     EXAMPLE 8 
     FIGS. 35 and 36 show an eighth example of the illustrative embodiment relating to an arrangement for mounting the writing unit of FIGS. 34A and 34B to the apparatus. As shown, a flat structural member  92  for supporting the writing unit  100  extends in parallel to the direction in which the cartridges  4 - 7  are stacked, i.e., in the up-and-down direction. The structural member  92  is affixed to the front wall  22   a , rear wall  22   b , top wall  22   e  and bottom wall  22   f.    
     The structural member  90  includes four seats  92   a . The writing unit  100  is mounted to the seats  92   a  by bolts or mounting means  94 . In this configuration, the writing unit  100  and drums  8 K- 8 Y are held at a preselected distance from each other. The seats  92   a  may be omitted, if desired. 
     The structural member  92  affixed to the walls  22   a ,  22   b ,  22   e  and  22   f  of the frame increases the rigidity of the entire apparatus body  22 . This, coupled with the fact that the writing unit  100  is mounted on the structural member  92 , effectively obviates banding. 
     EXAMPLE 9 
     In the example shown in FIGS. 35 and 36, the structural member  92  is usually formed of metal while the frame of the writing unit  100  is formed of resin. The polygonal scanner included in the writing unit  100  and constituting a heat source causes the structural member  92  and frame to expand due to heat during operation. When the writing unit  100  thermally expands, the structural member  92  also thermally expands. Because the frame of the writing unit  100  and structural body  92  are different in material and therefore in the coefficient of thermal expansion, the writing unit  92  is apt to deform, i.e., to curve in its intermediate portion without its affixed ends being displaced. 
     For example, in FIGS. 34A and 34B, assume that the writing unit  100  tends to expand in the up-and-down direction with its upper end lower end being restricted by the structural member  92 . Then, the intermediate portion of the writing unit  100  in the up-and-down direction curves away from the drum side. As a result, the mirror  77 , for example, is displaced due to the deformation of the writing unit  100 , shifting the path of the light beam Lb by an angle β. Although the angle β itself is not great, it is magnified before reaching the drum. Because the shift of the light beam Lb differs from one drum to another drum, image components of different colors expected to form a full-color image are brought out of register and lower image quality. The ninth example to be described is constructed to reduce the displacement of the writing unit  100  as far as possible. 
     Briefly, in this example, the upper and lower ends of the writing unit  100  each are retained by the structural member  92  via a resilient member with a margin with respect to movement in the up-and-down direction. Specifically, as shown in FIG. 37, the writing unit  100  is formed with a seat  100   a  at its upper end. A hole  140  is formed throughout the seat  100   a . A bolt  94  is passed through the opening  140  with the intermediary of a resilient washer  96  and screwed into the structural member  92 . A compression spring  95  is loaded between the structural member  92  and the seat  100   a . The hole  140  has a diameter D greater than the diameter d of the bolt  94 , implementing a margin for the writing unit  100  to move up and down. The above configuration is also applied to the lower end of the writing unit  100 . 
     In the above construction, when the writing unit  100  thermally expands during operation, it is capable of moving in the up-and-down direction within the range of the difference between the diameters D and d. It follows that the writing unit does not curve, as indicated by a dash-and-dots line in FIG. 37, but simply expands in the up-and-down direction. This is successful to reduce the displacement of the light beam Lb. 
     FIG. 38 shows a modification of the above example. As shown, a bolt  97  is screwed into the seat  92  included in the structural member  92 . A spring or resilient member  98  is loaded between the seat  100   a  and the head of the bolt  94 . Again, the hole  140  has a greater diameter than the bolt  97  so as to provide the writing unit  100  with a margin with respect to movement in the up-and-down direction. 
     The above example and its modification each elastically fasten the structural member  92  and writing unit  100  and provide the writing unit  100  with the above margin, thereby reducing the displacements of the light beams which would bring colors out of register. 
     EXAMPLE 10 
     The configurations described with reference to FIGS. 35 and 38 free the writing unit  100  from curve-like deformation, but cannot fully obviate the displacement in the up-and-down direction. A tenth example to be described further reduces the displacement in the up-and-down direction. 
     Specifically, as shown in FIGS. 38 and 39, the intermediate portion of the writing unit  100  in the up-and-down direction are supported by the structural members  92  at two horizontally spaced points, i.e., via two pins  99 . In this condition, the displacement of the writing unit  100  ascribable to thermal expansion is divided into the upper half and lower half. This further reduces irregularity in color ascribable to thermal expansion. 
     EXAMPLE 11 
     This example is similar to the example of FIG.  19  and connects the horizontal stays  25  shown in FIGS. 1A-6 and assigned to the cartridges  4 - 7  to the structural member  92  described with reference to FIGS. 35-39. Specifically, the stays  25  effectively obviating the vibration of the cartridges  4 - 7  are connected to the structural member  92  perpendicular to the stays  25  and supporting the writing unit  100 . The resulting apparatus body  22  achieves greater rigidity and obviates banding more positively. 
     EXAMPLE 12 
     As shown in FIG. 42, photoconductive drums  8 K″,  8 C″,  8 M″ and  8 Y″ are supported beforehand. As shown in FIGS. 43A-43D, cartridges  4 ″,  5 ″,  6 ″ and  7 ″ do not support any drum. As shown in FIG. 50, when the cartridges  4 ″- 7 ″ are mounted to the apparatus body  22 , a part of the image forming means, e.g., the rings  10 C″- 1  and  10 C″- 2  (FIG. 50) contact the drum  8 C″. Even with this type of apparatus, it is possible to increase the rigidity of the apparatus body  22  to thereby obviate banding by connecting the horizontal stays  25  to the structural member  92  of FIGS. 35-39, as shown in FIG.  42 . 
     EXAMPLE 13 
     This example applies the guides  27 K- 27 Y shown in FIGS. 2A to  2 B to the cartridges shown in FIGS. 41-43D. 
     EXAMPLE 14 
     This example applies the leaf springs  28 U and  28 D shown in FIGS. 3A,  3 B,  4 A,  4 B and  6  to the cartridges shown in FIGS. 41-43D. 
     EXAMPLE 15 
     This example provides the stays  25  of FIGS. 41-43D with the vibration-proof rubber blocks shown in FIGS. 4A,  4 B and  7  and exerting viscoelastic pressing forces. 
     While the above description has concentrated on the characteristic configurations of the illustrative embodiments, the characteristic configurations may be combined as far as possible in order to further enhance the anti-vibration function. 
     In summary, it will be seen that the present invention provides an image forming apparatus capable of effectively obviating banding ascribable to the vibration of image forming cartridges and optical writing means and members to which they are affixed. In addition, the image forming apparatus of the present invention is miniature, low cost and easy to operate. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.