# Patent

## Title
Image formation apparatus including an overlap between portions of a photosensitive drum and an adjacent developer unit

## Abstract

A color image formation apparatus includes a plurality of stages of photosensitive drums provided for respective developing agents, the photosensitive drums arranged in a predetermined direction; and developer units which are provided for respective photosensitive drums and which develop electrostatic latent images formed on the photosensitive drums, wherein an overlap exists in the predetermined direction between a portion of each photosensitive drum and a portion of the developer unit provided so as to correspond to an adjacent photosensitive drum.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a color image formation apparatus which forms a synthetic image by superimposing image information by utilization of the electrophotography technique.
2. Description of the Related Art
In an image formation apparatus adopting the electrophotography technique, an electrophotographic photosensitive member serving as an image carrier is charged by a charger, and the photosensitive member is exposed to radiation in accordance with image information, to thereby form a latent image. A toner image into which the latent image is developed by a developer is transferred onto a sheet material or the like, to thereby produce an image.
In association with colorization of an image, a plurality of pieces of tandem image formation apparatus have hitherto been proposed. Such an apparatus is provided with a plurality of image carriers to be subjected to such a round of image formation processes. Color images; i.e., a cyan image, a magenta image, a yellow image, and preferably a black image, are formed on corresponding image carriers. Respective color images are transferred onto sheet material at respective transfer positions of the respective image carriers in a superimposed manner, thereby producing a full-color image. Such a tandem multiple-image formation apparatus is said to be advantageous in achieving speedup, because image formation sections are provided for respective colors.
As described in, e.g., Japanese Patent Publication No. 2000-284592, a tandem multiple-color image formation apparatus has an image formation unit for forming toner images of yellow (Y), magenta (M), cyan (C), and black (K) colors; an exposure device for forming an electrostatic latent image by outputting an image signal; an intermediate transfer belt which is made in the form of a closed loop and travels; and a fuser. A toner image formed on the intermediate transfer belt is transferred onto paper P supplied from a paper cassette, and the toner image is fixed on the paper by means of the fuser.
The yellow (Y) image formation unit, the magenta (M) image formation unit, the cyan (c) image formation unit, and the black (K) image formation unit share a common structure. One of the image formation units is shown in FIG. 9, which is a schematic front view of the principal section showing an example configuration of a related-art development unit and that of a related-art photosensitive unit. As illustrated, the image formation unit is a combination of a photosensitive unit and a development unit. The photosensitive unit has: a photosensitive drum 101, on a circumferential surface of which an electrostatic latent image is to be made by means of a laser beam emitted from an exposure device (not shown); a charger roller 102 for charging the latent image; and a cleaning blade 103. The development unit includes a toner feed tank 104, a toner feed roller 105, and a developer roller 106 to be brought into contact with the photosensitive drum 101, and a toner thin-layer blade 107 which comes into contact with the developer roller 106, to thereby make a toner layer a uniform thin layer, and which charges the layer with a predetermined potential.
In such an image formation unit, an electrostatic latent image of image information is formed on the photosensitive drum 101 by means of the exposure device. The electrostatic latent image is made visible in the form of a toner image by the developer roller 106 and transferred onto the intermediate transfer belt. In a four-color image formation apparatus, toner images of respective colors are transferred onto the intermediate transfer belt, to thereby finally produce a multiple color image.
However, as illustrated, the cleaning blade 103, the developer roller 106, and the toner feed roller 105 are arranged in this order with the photo sensitive drum 101 interposed between the blade 103 and the developer roller 106. Hence, the intermediate transfer belt in one image formation unit becomes longer in the direction in which the belt is to travel (i.e., a horizontal direction in the drawing). For this reason, an interval between the photosensitive drums 101 of the four-color image formation units tend to become greater.
As mentioned above, if the interval between the photosensitive drums 101 is excessive, a limitation is imposed on miniaturization of the apparatus. Further, an interval between transfer positions on the intermediate transfer belt also becomes wider. In the case of formation of a multiple color images, the transfer accuracy of color is deteriorated, thereby inducing color misregistration and resulting in a failure to form a superior color image. In addition, the transfer belt must be made longer, and hence a color image becomes apt to color misregistration in the same manner, thus presenting a problem of the apparatus becoming bulky and expensive.
SUMMARY OF THE INVENTION
The invention aims at providing a color image formation apparatus which can be miniaturized by rendering an arrangement pitch between photosensitive drums short and can maintain high accuracy in transfer of an image onto an intermediate transfer belt.
In one aspect of the invention, there is provided a color image formation apparatus including: a plurality of stages of photosensitive drums provided for respective developing agents; developer units which are provided for respective photosensitive drums and which develop electrostatic latent images formed on the photosensitive drums, wherein an overlap in an arrangement direction of the drums exists between a portion of each photosensitive drum and a portion of the developer unit provided to correspond to an adjacent photosensitive drum. By means of such a configuration, a dead space is eliminated, thereby scaling down the width of the apparatus.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic front view showing the configuration of a color image formation apparatus according to a first embodiment of the invention;
FIG. 2 is a longitudinal front cross-sectional view of the principal section obtained when a developer unit is fitted to a housing having a photosensitive drum and a charging bracket;
FIG. 3 is a perspective view of a housing;
FIG. 4 is a perspective view of a cleaning blade retained by a support;
FIG. 5 is a perspective view showing one developer unit along with a photosensitive drum;
FIG. 6 is a perspective view showing a yellow developer unit when the unit is built into the housing;
FIG. 7 is a perspective view showing all developer units when the units are built into the housing;
FIG. 8 is a longitudinal front cross-sectional view of a color image formation apparatus according to the second embodiment of the invention; and
FIG. 9 is a schematic front view of the principal section showing an example configuration of a related-art developer unit and that of a photosensitive unit.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Embodiments of the invention will be described hereinbelow by reference to the drawings.
(Embodiment 1)
FIG. 1 is a schematic front view showing the configuration of a color image formation apparatus according to a first embodiment of the invention.
As shown in FIG. 1, the color image formation apparatus is substantially constructed such that an intermediate transfer belt 51 which is driven by drive pulleys 51a in the direction of the arrow is provided in the form of a closed loop and such that paper P is fed to a fuser 53 from a paper cassette 52 located at a position below the intermediate transfer belt 51 by way of a space between a transfer roller 51b and the intermediate transfer belt 51. In short, a color toner image formed on the intermediate transfer belt 51 is transferred onto the paper P to be nipped by the transfer roller 51b, and the transferred toner image is fixed on the paper P by means of the fuser 53. An exposure device (not shown) which radiates a laser beam in accordance with image information is disposed at a position above the intermediate transfer belt 51. Disposed between the exposure and the intermediate transfer belt 51 is an image formation unit A for sequentially producing a yellow (Y) image, a magenta (M) image, a cyan (C) image, and a black (K) image.
FIG. 2 is a longitudinal front cross-sectional view of the principal section obtained when a developer unit is fitted to a housing having photosensitive drums and a charging bracket. FIG. 3 is a perspective view of the housing.
As shown in FIG. 3, the housing 1 is a box-shaped housing whose top and bottom are opened. Photosensitive drums 2a, 2b, 2c, and 2d shown in FIG. 2, which come into contact with the transfer belt 51 to transfer a toner image, are supported at a lower end of the housing such that the drums 2a to 2d can be rotated in a clockwise direction by an external drive mechanism. The photosensitive drums 2a to 2d are arranged so that an electrostatic latent images can be formed on respective circumferential surfaces of the drums by means of a laser beam output from the exposure device (not shown). The photosensitive drums 2a to 2d can be removed from removal holes 1a formed in one end wall of the housing 1 shown in FIG. 3. Further, charger brushes 3a, 3b, 3c, and 3d which come into contact with and charge the circumferential surfaces of the photosensitive drums 2a to 2d are rotatively provided in the vicinities of the photosensitive drums 2a to 2d. Cleaning blades 4a, 4b, 4c, and 4d are arranged to be used for exfoliating and removing the toner that remains after the toner image has been transferred onto the intermediate transfer belt 51. The photosensitive drums 2a to 2d, the charger brushes 3a to 3d, and the cleaning blades 4a to 4d constitute a photosensitive unit.
FIG. 4 is a perspective view of the cleaning blade 4a (4b 4d) retained by a support. As is evident from FIG. 2, the cleaning blade 4a (4b to 4d) which is formed as rubber as raw material is integrally provided on a metal plate support 5 having a substantially horseshoe shaped cross-sectional profile. As shown in FIG. 3, the supports 5 are caused to butt against corresponding holding seats 1d which are obliquely formed on exterior surfaces of both end walls of the housing 1. The supports 5 are fastened to the holding seats 1d by insertion of small screws into holes 5a-1 formed in the supports 5.
A pair of mount grooves 1b, 1c are formed at four locations on an interior surface of the end wall of the housing 1. A yellow (Y) developer unit 6, a magenta (M) developer unit 7, a cyan (C) developer unit 8, and a black (K) developer unit 9 are removably inserted into the housing 1 from above while being guided by the mount grooves 1b, 1c. As shown in FIG. 1, toner feed channels of the developer units 6 to 9 can be connected to toner tanks 10a, 10b, 10c, and 10d which house color toners and are removably attached to a main unit of the image formation apparatus.
The respective developer units 6 to 9 are provided with casings 6a, 7a, 8a, and 9a which are housed in the housing 1 so that toner flow channels of the casings can be connected to the toner tanks 10a to 10d. Agitators 6b, 7b, 8b, and 9b for agitating toner, toner feed rollers 6c, 7c, 8c, and 9c, developer rollers 6d, 7d, 8d, and 9d, and thin-layer rollers 6e, 7e, 8e, and 9e are rotatably housed in the casings 6a, 7a, 8a, and 9a, respectively. The toner feed rollers 6c to 9c cause toner to adhere to the developer rollers 6d to 9d; and the thin-layer rollers 6e to 9e make toner layers of the developer rollers 6d to 9d thinner and charge the developer rollers 6d to 9d with a predetermined potential. The developer rollers 6d to 9d are intended for forming toner images by causing toner to adhere to the electrostatic latent images formed on the photosensitive drums 2a to 2d by means of charging. Provided at lower ends of the casings 6a to 9a are recovery sheets 6f, 7f, 8f, and 9f which come into contact with the developer rollers 6d to 9d, to thereby cause the toner feed rollers 6c to 9c to recover adhering toner and prevent transfer of toner to the intermediate transfer belt 51. Further, scrapers 6g, 7g, 8g, and 9g, which come into contact with circumferential surfaces of the thin-layer rollers 6e to 9e to remove adhering toner, are provided in the vicinity of the thin-layer rollers 6e to 9e. 
FIG. 5 is a perspective view showing one developer unit 6 along with the corresponding photosensitive drum 2a. FIG. 6 is a perspective view showing a yellow developer unit when the unit is built into the housing. FIG. 7 is a perspective view showing all developer units when the units are built into the housing. As illustrated, a spring 6h is provided on either end of the casing 6a. As shown in FIG. 6, when the casing 6a is inserted into the housing 1, the springs 6h engage with receiving seats (not shown) provided on the internal wall of the housing 1, thereby forcing the casing 6a in a downwardly oblique direction. As a result, the developer unit can be assembled such that the developer roller 6d remains in contact with the circumferential surface of the photosensitive drum 2a with appropriate pressing force. The same also applies to assembly of the casings 7a to 9a of the other developer units 7 to 9. FIG. 7 shows that all developer units 6 to 9 are set in the housing 1.
Turning back to FIGS. 1 and 2, discharge units 5a, each having a built-in screw to be used for recovering toner, are provided at locations below the respective supports 5 of the cleaning blades 4a to 4d. The discharge units 5a have the function of recovering the toner scraped by the cleaning blades 4a to 4d and discharging the thus-recovered toner.
Since the toner still remaining on the circumferential surfaces of the photosensitive drums 2a to 2d adheres to the charger brushes 3a to 3d, pads 11a, 11b, 11c, and 11d are provided for removing the residual toner. The pads 11a, 11b, and 11c, are provided on exterior walls of the casings 7a, 8a, and 9a provided on the right side. The remaining pad 11d is provided on the interior wall of the housing 1. The casings 7a, 8a, and 9a are formed so as to assume such a cross-sectional profile that the casings pass by the vicinities of the charger brushes 3a, 3b, and 3c of the developer units 6, 7, and 8 provided on the left side. Portions of the circumferential surfaces of the charger brushes 3a to 3c can be covered by means of the shapes of the casings 7a to 9a. Accordingly, the casings 7a to 9a can block splashing of toner which would be caused at the time of rotation of the charger brushes 3a to 3c. In this way, the casings 7a to 9a act as covers for the charger brushes 3a to 3c of the units located on the left side of the casings. Simultaneously, the casings 7a to 9a can also be utilized as members for mounting the pads 11a to 11c for removing adhering toner. Consequently, there is obviated a necessity for members specifically designed for use as covers of the charger brushes 3a to 3c or for mounting the pads 11a to 11c, thereby enabling narrowing of surrounding space occupied by charger brushes 3a to 3c. 
Here, the cleaning blades 4a to 4c are located between the adjacent photosensitive drums 2a to 2c, and the supports 5, each having a substantially horseshoe shaped cross-sectional profile, support the rear surfaces of the cleaning blades 4a to 4c. As a result, the supports 5 are slightly, downwardly tilted toward the right while surfaces of the supports 5 facing the developer rollers 7d, 8d, and 9d are recessed. Therefore, the toner that still adheres to the developer rollers 7d, 8d, and 9d and could not be recovered by the recovery sheets 7f, 8f, and 9f can be received by the supports 5. Only the supports 5 and the cleaning blades 4a to 4c held thereby are interposed between the four photosensitive drums 2a to 2d. As a result, the intervals between the photosensitive drums 2a to 2d can be made narrow. Further, as mentioned previously, the supports 5 can also be utilized as members for receiving toner. Hence, the developer rollers 7d to 9d which most easily release toner are preferably located at positions above the supports 5, and the supports 5 are preferably located at positions below the recovery sheets 7f to 9f on which toner falls from the circumferential surfaces of the toner feed rollers 7c to 9c. According to the invention, shaft cores of the developer rollers 7d to 9d of the developer units 7 to 9 are aligned with lines located at higher left positions relative to the centers of the photosensitive drums 2b to 2d. Such an arrangement is also applicable to the developer roller 6d of the developer unit 6. Falling toner can be received and recovered by a lower end of the housing 1.
By virtue of the positional relationship between the photosensitive drums 2a to 2d and the developer rollers 6d to 9d, the photosensitive drums 2a to 2d are not susceptible to interference which is induced in a direction of arrangement by the developer rollers 7d to 9d. Consequently, the intervals between the photosensitive drums 2a to 2d can be made shorter than those required by the construction described in connection with the related-art color image formation apparatus. The developer rollers 6d to 9d are not positioned at locations immediately above the photosensitive drums 2a to 2d, but at positions located leftwardly oblique with respect to the photosensitive drums 2a to 2d. Hence, the heights of the developer rollers 6d to 9d and those of the photosensitive drums 2a to 2d can also be reduced. As mentioned previously, the developer rollers 7d to 9d are located at positions above the supports 5. Hence, the toner discharged from the developer rollers 7d to 9d can be received by the supports 5, thereby preventing adhesion of useless toner to the intermediate transfer belt 51.
A process of transferring toner images from the photosensitive drums 2a to 2d to the intermediate transfer belt 51 and operation for transferring the toner image from the intermediate transfer belt 51 to the paper. P by means of the transfer rollers 51b are the same as those in the related-art tandem image formation apparatus.
(Embodiment 2)
A second embodiment of the invention will now be described.
FIG. 8 is a longitudinal front cross-sectional view of a color image formation apparatus according to the second embodiment of the invention. Those constituent elements which are the same as those described by reference to FIGS. 2 through 7 are assigned the same reference numerals, and their repeated explanations are omitted.
As shown in FIG. 8, the cleaning blades 4a to 4c are located between the adjacent photosensitive drums 2a to 2c, and the supports 5, each having a substantially horseshoe shaped cross-sectional profile, support the rear surfaces of the cleaning blades 4a to 4c. As a result, the supports 5 are slightly, downwardly tilted toward the right while surfaces of the supports 5 facing the developer rollers 7d, 8d, and 9d are recessed. Therefore, the supports 5 can receive the toner that still adhere to the developer rollers 7d, 8d, and 9d and cannot have been recovered by the recovery sheets 7f, 8f, and 9f. Only the supports 5 and the cleaning blades 4a to 4c held thereby are interposed between the four photosensitive drums 2a to 2d. As a result, the intervals between the photosensitive drums 2a to 2d can be made narrow. Further, as mentioned previously, the supports 5 can also be utilized as members for receiving toner. Hence, the developer rollers 7d to 9d which most easily release toner are preferably located at positions above the supports 5, and the supports 5 are preferably located at positions below the recovery sheets 7f to 9f on which toner falls from the circumferential surfaces of the toner feed rollers 7c to 9c. According to the invention, shaft cores of the developer rollers 7d to 9d of the developer units 7 to 9 are aligned with lines located at higher left positions relative to the centers of the photosensitive drums 2b to 2d. Such an arrangement is also applicable to the developer roller 6d of the developer unit 6. Falling toner can be received and recovered by a lower end of the housing 1.
The developer units 6, 7, 8, and 9 are arranged close to each other. As indicated by a hatched area shown in FIG. 8, a portion of the photosensitive drum 2a and that of the developer unit 7 are positioned such that an overlap exists between the same in a direction parallel to that in which the photosensitive drums 2a, 2b, 2c, and 2d are arranged. Similarly, as indicated by the hatched area shown in FIG. 8, a portion of the photosensitive drum 2b and that of the developer unit 8, and a portion of the photosensitive drum 2c and that of the developer unit 9 are respectively positioned such that an overlap exists between the same in a direction parallel to that in which the photosensitive drums 2a, 2b, 2c, and 2d are arranged.
Put it another way; as indicated by the hatched area shown in FIG. 8, a portion of the developer unit 7 is superimposed above a portion of the photosensitive drum 2a. Similarly, as indicated by the hatched area shown in FIG. 8, a portion of the developer unit 8 is superimposed above a portion of the photosensitive drum 2b, and a portion of the developer unit 9 is superimposed above a portion of the photosensitive drum 2c. 
Preferably, at least a portion of the developer units 7, 8, 9 and at least one-third of the photosensitive drums 2a, 2b, 2c are respectively positioned such that an overlap exists between the developer units 7, 8, 9 and the photosensitive drums 2a, 2b, 2c in a direction parallel to that in which the photosensitive drums 2a, 2b, 2c, and 2d are arranged
Such a configuration enables elimination of dead space, thereby reducing the width of the color image formation apparatus.
The photosensitive drum 2a, the developer roller 6d, and the toner feed roller 6c are arranged such that their shaft cores are arranged in substantially a straight line. Preferably, the developer roller 6d is arranged so as to oppose the photosensitive drum 2a at an angle of about 45° from a direction in which the photosensitive drums 2a to 2d to the toner feed roller 6c. Needless to say, the same relationship can also be applied to the four stages.
In another configuration, the photosensitive drum 2a and the developer roller 6d are arranged such that their shaft cores are arranged in substantially a straight line. The shaft core of the toner feed roller 6c may be provided at a position closer to the thin-layer roller 6e with reference to a line connecting the shaft core of the photosensitive drum 2a to that of the developer roller 6d. Needless to say, the same relationship can also be applied to the four stages.
Alternatively, put it in another way; the toner feed roller 6c can be provided at a position where an angle defined between the normal (or a direction perpendicular to a direction in which the photosensitive transfer member drums 2a to 2d) and a line connecting together the shaft core of the photosensitive drum 2a and that of the developer roller 6d becomes greater than that defined between the normal (or a direction perpendicular to a direction in which the photosensitive transfer member drums 2a to 2d) and a line connecting together the shaft core of the photosensitive drum 2a and that of the toner feed roller 6c. Needless to say, this relationship can also be applied to the four stages.
By means of such a configuration, the volume and width of each of the developer units 6, 7, 8, and 9 can be reduced. Further, a total volume and dimension of the photosensitive drums 2a to 2d and the developer units 6 to 9 can also be reduced.
According to the invention, only cleaning blades held by supports are interposed between photosensitive drums. Developer rollers are aligned with lines which are located at positions higher than the supports and leftwardly tilted relative to the axes of the respective photosensitive drums. Hence, an interval between the photosensitive drums can be made narrower than that required by the photosensitive drums having the related-art structure. Consequently, miniaturization of the image formation apparatus and a reduction in intervals between transfer positions on an intermediate transfer belt can be achieved simultaneously, thereby enabling formation of a superior color image. Moreover, the intermediate transfer belt is also shortened, and hence a time required for a transfer process is also shortened, thus significantly enhancing operability of the image formation apparatus.
The volume and width of the developer units can also be reduced. In addition, the total volume and dimension of the photosensitive drums and the developer units can also be reduced.

## Claims

1. An image formation apparatus comprising:
first and second photosensitive drums arranged in a predetermined direction; 
first and second developer rollers which are respectively provided to correspond to the first and second photosensitive drums and which cause toner to adhere to electrostatic latent images formed on the first and second photosensitive drums by means of charging for forming toner images on the respective first and second photosensitive drums; and 
first and second toner feed rollers which are respectively provided to correspond to the first and second developer rollers and feed toner to the first and second developer rollers, 
wherein the first toner feed roller and the second photosensitive drum are arranged such that an overlap exists between the first toner feed roller and the second photosensitive drum in the predetermined direction.

2. The image formation apparatus according to claim 1, wherein the first developer roller is arranged so as to oppose the first photosensitive drum at an angle of about 45° from the predetermined direction to the first toner feed roller.

3. The image formation apparatus according to claim 1, further comprising:
a first developer unit housing the first developer roller and the first toner feed roller, 
wherein an overlap exists in the predetermined direction between at least a portion of the first developer unit and at least one-third of the second photosensitive drum.

4. The image formation apparatus according to claim 3, further comprising
a second developer unit housing the second developer roller and the second toner feed roller; and 
a third photosensitive drum arranged in the predetermined direction with respect to the first and second photosensitive drums, 
wherein an overlap exists in the predetermined direction between at least a portion of the second developer unit and at least one-third of the third photosensitive drum.

5. The image formation apparatus according to claim 4, wherein the first developer unit houses a first agitator which is provided so as to correspond to the first toner feed roller and agitates toner, and the second developer unit houses a second agitator which is provided so as to correspond to the second toner feed roller and agitates toner.

6. The image formation apparatus according to claim 3, wherein the first developer unit houses a first agitator which is provided so as to correspond to the first toner feed roller and agitates toner.

7. The image formation apparatus according to claim 1, further comprising:
a third photosensitive drum arranged in the predetermined direction with respect to the first and second photosensitive drums, 
wherein the second toner feed roller and the third photosensitive drum are arranged such that an overlap exists between the second toner feed roller and the third photosensitive drum in the predetermined direction.

8. The image formation apparatus further comprising:
first and second photosensitive drums arranged in a predetermined direction; 
first and second developer rollers which are respectively provided to correspond to the first and second photosensitive drums and which cause toner to adhere to electrostatic latent images formed on the first and second photosensitive drums by of charging for forming toner images on the respective first and second photosensitive drums; and 
first and second toner feed rollers which are respectively provided to correspond to the first and second developer rollers and feed toner to the first and second developer rollers, 
wherein the first toner feed roller and the second photosensitive drum are arranged such that an overlap exists between the first toner feed roller and the second photosensitive drum in the predetermined direction; and 
first and second agitators which are provided to correspond to the first and second toner feed rollers and which stir toner, 
wherein the first agitator and the second photosensitive drum are arranged such that an overlap exists between the first agitator and the second photosensitive drum in the predetermined direction.

9. The image formation apparatus according to claim 8, wherein the first developer roller is arranged so as to oppose the first photosensitive drum at an angle of about 45° from the predetermined direction to the first toner feed roller.

10. The image formation apparatus according to claim 8, further comprising:
a third photosensitive drum provided in the predetermined direction with respect to the first and second photosensitive drums, 
wherein the second agitator and the third photosensitive drum are arranged such that an overlap exists between the second agitator and the third photosensitive drum in the predetermined direction.