JP2748403B2 - Color image forming equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus such as a color copying machine or a color printer, and in particular, includes a plurality of independent image forming units for appropriate color components. To improved color image forming apparatuses.

2. Description of the Related Art Conventionally, as this type of color image forming apparatus, there are those described in, for example, JP-A-62-141574 and JP-A-62-148268.

For example, as shown in FIG. 11, four image forming units 1 (specifically, 1a to 1d) of cyan, magenta, yellow, and black are appropriately arranged in a horizontal direction, and each image forming unit 1 is The image carrier sheet 8 is sequentially conveyed to the toner image transfer portion of the forming unit 1 at an appropriate timing. The image carrier sheet 8 on which the toner image is placed is stably transported in the lateral direction, and each toner image transfer process is performed stably. It is intended to be performed.

In such an image forming apparatus, each of the image forming units 1 includes a photosensitive drum 2, a charger 3 for charging the surface of the photosensitive drum 2 in advance, and a latent image corresponding to image information on the charged surface of the photosensitive drum 2. An optical system for writing an image, a latent image writing means 4 such as a laser, a developing device 5 for developing the latent image formed on the photosensitive drum 2 with toner of each color, and an image carrying sheet 8 for transferring the toner image on the photosensitive drum 2 to the image carrying sheet 8 And a cleaner 7 for removing residual toner on the photosensitive drum 2 after the transfer process. Further, as shown in FIGS. 11 and 12, the sheet conveying means 10 includes a pair of endless chains which are wound around the driving sprocket 12 and the driven sprocket 13 and pass through each transfer portion of each of the image forming units 1. 11, and a sheet holding mechanism 14 provided at an appropriate interval between the pair of endless chains 11, and each sheet holding mechanism 14 includes, for example, a moving bar 14a that is bridged over the pair of endless chains 11, An appropriate number of grippers provided on the moving bar 14a to hold the leading end of the image bearing sheet 8
14b. In FIGS. 11 and 12, reference numeral 16 denotes sheet feeding means for supplying the image carrying sheet 8 to the sheet conveying means 10, and reference numeral 17 denotes an image which is disposed immediately before each transfer site and intrudes into each transfer site. A guide plate for maintaining the posture of the carrying sheet 8, a fixing device 18 for fixing an unfixed toner image on the image carrying sheet 8 after each transfer process, and a discharge / conveying device 19 for the image carrying sheet 8 having passed through the fixing device 18. It is a discharge tray.

[Problems to be Solved by the Invention] In such a conventional color image forming apparatus, since the endless chain 11 of the sheet conveying means 10 is arranged in the lateral direction, the endless chain 11 The endless chain 11 may bend downward and vibrate due to its own weight or the own weight of the plurality of sheet holding mechanisms 14 attached to the endless chain 11. At this time, since the position of the image carrying sheet 17 by the guide plate 17 is not regulated at each transfer site, the distance between the photosensitive drum 2 and the image carrying sheet 8 is not maintained constant, and the photosensitive drum 2 and the image carrying sheet 8 and the process direction (image carrying sheet 8
(Corresponding to the conveyance direction of the photosensitive drum 2), the contact of the image bearing sheet 8 with the photosensitive drum 2 is poor, and the transfer of the toner image is poor. In particular, in the case of a color image forming apparatus, a situation may occur in which toner images formed on the respective photosensitive drums 2 of the respective image forming units 1 are sequentially superimposed and transferred. When the transfer failure based on the image occurs, an image is lost or a color shift occurs, which causes a problem that the quality of the color image is deteriorated.

In order to avoid such a situation, it is necessary to apply a relatively large tension to the endless chain 11, but it is necessary to apply a large load torque to the drive motor (not shown) of the drive sprocket 12. And that much,
There is a problem that the size of the driving motor is increased.

Further, in the conventional color image forming apparatus, when the position of each of the sheet holding mechanisms 14 changes along with the rotational movement of the endless chain 11, or when the gripper 14b opens and closes, it acts on the driving motor. Since the load fluctuates, the conveyance operation of the endless chain 11 tends to be unstable, and a problem arises in that the high rotation accuracy of the sheet conveyance means 10 cannot be obtained.

Further, when the photosensitive drum 2 and the latent image writing means 4 of each image forming unit 1 are installed, parts such as the photosensitive drum 2 must be appropriately moved in the vertical direction and positioned at a predetermined position. However, there is also a problem that the installation work and the maintenance and inspection work of each component become troublesome from the working direction.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and suppresses an image forming operation defect at a portion where each color component image is formed while simplifying installation and maintenance work of each image forming unit. Further, the present invention provides a color image forming apparatus that stabilizes the operation of transporting the image bearing sheet.

[Means for Solving the Problems] That is, the present invention provides a plurality of image forming units that are arranged at appropriate intervals along a vertical direction and form images of appropriate color components on an image bearing sheet, A sheet conveying means for conveying the image bearing sheet from the lower side to the upper side to each image forming portion arranged in the vertical direction of the unit, and a driving section in which the sheet conveying means is arranged on the upper side; And a driven part disposed at the second position.

In such technical means, the image forming unit includes a drum-shaped or belt-shaped latent image carrier made of a photoreceptor, a dielectric, or the like, and a latent image forming unit that forms a latent image on the latent image carrier. Developing means for developing the latent image on the latent image carrier with toner, transfer means for transferring the toner image on the latent image carrier to the image carrying sheet,
Various types such as those in which an image is formed on an image bearing sheet using a thermal recording head or an inkjet recording head are included. Here, as shown in the embodiment, the electrophotographic image forming unit includes a latent image carrier (for example, a photosensitive drum 21) and a latent image writing unit (for example, a laser scanning system 23) of a latent image forming unit. Are preferably arranged horizontally. Further, the number and arrangement of the image forming units can be appropriately selected as long as they can form an image of an appropriate color component such as cyan, magenta, yellow and black, and the selected ones are arranged in an arbitrary combination. However, as shown in the embodiment, it is preferable that the uppermost image forming unit among the image forming units is one that forms a black color component image.

Further, as the sheet conveying means, an endless sheet conveying and transmitting system which circulates and moves through each image forming portion lined up and down, and a sheet which is disposed in the sheet conveying and transmitting system and selectively holds the image bearing sheet And holding means. In this case, as the sheet conveyance transmission system, a system in which an endless chain or an endless belt is wound between a driving member and a driven member, and the endless chain or the like is appropriately circulated by rotating the driving member. Can be

Further, as the sheet holding means, it is possible to appropriately change the design as long as it can selectively hold the image bearing sheet and can maintain the conveying posture of the image bearing sheet in a favorable state. At this time, as a means for selectively holding the image carrying sheet, a gripper that can be opened and closed is provided on a member that moves together with the sheet conveyance transmission system, and the gripper is opened and closed appropriately to grip an end of the image carrying sheet. Alternatively, a positioning member that regulates the leading end position of the image carrying sheet may be provided, and the image carrying sheet positioned in contact with the member may be designed to be electrostatically attracted and held. In addition, in order to maintain a good conveying posture of the image carrying sheet, basically, it can be realized by holding the leading end of the image carrying sheet and carrying the image carrying sheet from the lower side to the upper side. In order to improve the conveying posture of the image carrying sheet, a sheet placing portion on which the image carrying sheet is placed in a predetermined posture is provided, and the image carrying sheet can be held by this sheet placing portion. Good.

[Operation] According to the above-described technical means, the sheet conveying means is rotated by the driving unit disposed on the upper side and the driven unit disposed on the lower side, and the image forming unit of each image forming unit is rotated. The image bearing sheet is conveyed from the lower side to the upper side to the site.

At this time, the driving unit disposed on the upper side pulls the sheet conveying unit in a direction to stretch the portion facing each image forming unit, and the portion on the opposite side of the sheet conveying unit is extended without being bent by gravity. .

For this reason, even if a relatively high tension is not applied to a sheet conveyance transmission system such as a chain or a belt of the sheet conveyance unit, the sheet conveyance unit does not bend under its own weight, and the sheet conveyance unit is connected to each of the image bearing sheets. The image carrying sheet is stably conveyed while keeping the relative positional relationship between the image forming unit and each image forming portion constant.

Hereinafter, the present invention will be described in detail based on an embodiment shown in the accompanying drawings.

FIG. 1 shows an embodiment in which the present invention is applied to a full-color printer. This color printer is a four-color printer arranged in order to form images of respective color components of cyan, magenta, yellow and black in order from the bottom along a vertical direction. The image forming unit 20 includes two image forming units 20 (specifically, 20a to 20d) and a sheet conveying unit 40 that sequentially conveys the image bearing sheet 28 to an image forming portion of each image forming unit 20 arranged in the vertical direction. .

In this embodiment, each image forming unit 20 includes a photosensitive drum 21, a charger 22 for previously charging the surface of the photosensitive drum 21, a laser 24 for irradiating a laser beam corresponding to image information, and a beam from the laser 24. A polygon mirror 25 for deflecting in the scanning direction and an imaging lens 26 for imaging the beam deflected by the polygon mirror 25 on the photosensitive drum 21,
A laser scanning system 23 for writing a latent image corresponding to image information on a charged surface of the photosensitive drum 21; a developing device 27 for developing the latent image formed on the photosensitive drum 21 with toner of each color;
A transfer device 29 such as a corotron for transferring the upper toner image onto the image carrying sheet 28 and a cleaner 30 for removing residual toner on the photosensitive drum 21 after the transfer process are provided. A sheet feeding cassette 31 for accommodating the image carrying sheet 28 is provided at a machine portion corresponding to the lower side of the sheet conveying means 40, and the image carrying sheet 28 from the sheet feeding cassette 31 is fed on a feeding roll. The sheet is fed out at a predetermined timing by the sheet feeder 32, and is supplied to a sheet supply portion A located at the lowermost part of the sheet conveying means 40 via a supply side guide chute 33. Further, a storage tray 34 for the image carrying sheet 28 is provided at the machine portion corresponding to the upper side of the sheet conveying means 40, and the image carrying sheet 28 separated at the sheet separating section B located at the uppermost part of the sheet conveying means 40. After passing through the fixing device 36 via the discharge side guide chute 35, the paper is discharged to the storage tray 34.

Further, as shown in FIG. 2 to FIG. 5, the sheet conveying means 40 is provided with an endless sheet conveying and transmitting system 41 and the image carrying sheet 28 disposed at four positions of the sheet conveying and transmitting system 41. And sheet holding means 46 for temporarily holding.

That is, as shown in FIG. 5 in particular, the sheet conveying transmission system 41 includes a pair of driving sprockets 43 disposed on the upper side of the apparatus and appropriately rotated by the driving motor 42 and disposed on the lower side of the apparatus. The driven sprocket 44 includes a pair of driven sprockets 44 that rotate naturally and an endless chain 45 that extends between the driving sprocket 43 and the driven sprocket 44.

On the other hand, the sheet holding means 46 has a pair of holder bars 47, 48 which are installed on the endless chain 45 before and after in the traveling direction of the sheet conveyance transmission system 41, as shown in particular in FIGS. An appropriate number of openings 49 are formed in the front holder bar 47, and the openings 49 have cam projections 50a on the back side.
The gripper 50 provided with is mounted in a state of being biased in the closing direction by a spring member (not shown). The front end is fixed, and a locking bar is
Attach the locking bar 52 and the rear holder bar
48 has a pair of insertion holes 53 and 54, respectively, and each insertion hole 53, 54 has a locking rod 55 formed at one end with a locking portion 55a capable of locking with the locking bar 52. , A weight 56 is fixed to the tip of the locking rod 55, and a coil-shaped balance spring 57 is interposed between the weight 56 and the rear holder. In this embodiment, as shown in FIG. 4, the distances L1 to L4 between the leading ends of the sheet holding means 46 are set to be equal to each other.

Further, the driving sprocket 43 and the driven sprocket
5 and 6, the drive-side guide drum 61 is coaxial with the sprockets 43 and 44, as shown in FIGS.
And a guide drum 62 on the driven side, and in the longitudinal direction of the circumferential surface of each guide drum 61, 62, a groove 63 for preventing interference when the front and rear holder bars 47, 48 of the sheet holding means 46 pass. 63a and 63b) are formed appropriately, and the grippers 50 of the sheet holding means 46 are provided on the peripheral surfaces of the respective guide drums 61 and 62.
Depending on the position, a plurality of cut grooves 64 (specifically, 64a, 64
b) is formed. A rotatable cam shaft 65 (specifically, 65a, 65b) is provided in the vicinity of each of the guide drums 61, 62. Each cam shaft 65 has a cam on which one end is placed at the position of the cut groove 64. 66 (specifically, 66a and 66b) are attached, and each cam 66 is formed with a cam surface 67 for engaging with the cam projection 50a of the gripper 50 to temporarily open the gripper 50.

Furthermore, in this embodiment, before the image holding sheet 28 is held by the sheet holding means 46, a sheet screen is used.
A screen charger 71 for pre-charging 51 is provided, and the image bearing sheet 28 is electrostatically attracted to the sheet screen 51. Each of the transfer steps behind each transfer portion of each image forming unit 20 is provided. A static eliminator 72 (specifically, 72a to 72d) for removing the residual charge of the image bearing sheet 28 is provided. Further, the charge state of the sheet screen 51 is determined at the position where the last transfer step is completed. A screen static eliminator 73 to be removed is provided.

Next, the operation of the color printer according to this embodiment will be described.

When a start switch (not shown) is turned on to operate the color printer, each image forming unit 20 starts operating at a desired timing, and the sheet conveying means 40 starts operating.

At this time, first, the image-bearing sheet 28 in the sheet feeding cassette 31 is sent out at a predetermined timing by the feeding roll 32 and is supplied to the sheet feeding portion A. Then, as shown in FIG. 7, the cam projection 50a of the gripper 50 of one sheet holding means 46 circulating and moving by the sheet conveyance transmission system 41 slides on the cam surface 67 of the cam 66b at the sheet supply site A. When the gripper 50 is temporarily released, the leading end of the image carrying sheet 28 is guided to the gripping position of the gripper 50, and when the cam projection 50a passes over the cam 66b, the gripper 50 is The leading end of the sheet 28 is held. Then, the image-carrying sheet 28 held by the gripper 50 is sequentially electrostatically attracted to the previously charged sheet screen 51, and the sheet holding means 46 securely holds the image-carrying sheet 28.

In this state, the front and rear holder bars 4748 pass through the driven guide drum 62, but the respective holder bars 47 and 48 fit into the interference preventing groove 63b of the driven guide drum 62. 46 and driven side guide drum 62
Do not interfere with each other. The gripper 50 is temporarily released at the sheet feeding portion A. However, since the releasing force F (see FIG. 1) of the gripper 50 acts in the vertical direction of the endless chain 45, the endless chain 45 is not opened. It does not vibrate as needed. Further, since the sheet screen 51 is guided and conveyed on the peripheral surface of the driven-side guide drum 62, as shown by the phantom line in FIG.
The sheet screen 51 that has passed through is immediately installed in the vertical direction. Therefore, the image holding sheet 28 sucked and held by the sheet screen 51 is also set to the vertical posture immediately after passing through the driven guide drum 62, and the cyan image forming unit 20a is moved to the driven guide drum. Even if it is arranged close to 62, the image carrying sheet 28 will rush into the transfer site T in a vertical normal posture, and accordingly, the driven sprocket 44 of the sheet transporting transmission system 41 and the cyan image forming unit 20a The distance D between them can be set to a short dimension.

Then, as shown in FIG. 1, the sheet holding means 46 holding the image carrying sheet 28 is attached to each image forming unit 20a.
In each of the transfer sites T, the toner images of the respective color components on the photosensitive drum 21 are transferred to the image carrying sheet 28 at the respective transfer sites T.

In such an operation process, as shown in FIG. 1, the endless chain 45 wound around the driving sprocket 43 is basically arranged vertically below by its own weight. The vertical portion of the endless chain 45 is not unnecessarily bent without applying a relatively large tension to the endless chain 45. For this reason, there is no concern that a large load torque acts on the drive motor 42, and a small drive motor 42 for small torque can be used.

The sheet screen 51 is basically arranged along its own weight direction (vertical downward direction).
Seat screen for forcibly pulling vertically downward by the weight of 56 and the biasing force of the balance spring 57
The posture of 51 is securely held in the vertical direction. For this reason,
The posture of the image carrying sheet 28 sucked and held on the sheet screen 51 into each transfer portion is always maintained in a normal posture, and the image carrying sheet 28 and the photosensitive drum 21
Is kept constant. Therefore, in each transfer portion T, slippage in the process direction between the photosensitive drum 21 and the image bearing sheet 28 does not occur, and poor contact between the image bearing sheet 28 and the photosensitive drum 21 does not occur. The toner image on the drum 21 is transferred to an image carrying sheet 28.
The transfer image is uniformly transferred in a good condition, and a transfer image free from position shift and color shift can be obtained.

Further, in this embodiment, since the respective sheet holding means 46 are arranged at regular intervals, the right and left weight balance with respect to the driving sprocket 43 of the sheet conveying means 40 is kept substantially constant. Therefore, each sheet holding means
Even if 46 moves sequentially, the load on the drive motor 42 does not fluctuate.

Further, in this embodiment, since the image carrier sheet 28 that has passed through each transfer site is each neutralized by the static eliminator 72, the image carrier sheet 28 is excessively charged in the subsequent transfer process and the transfer condition is impaired, A situation in which charges remain on the image bearing sheet 28 discharged after each transfer step is effectively avoided.

Further, the sheet holding means 46 which has passed the transfer portion T of the black image forming unit 20d moves to the sheet separation portion B as shown in FIG. 9 in particular. At this time, the charged charges on the sheet screen 51 of the sheet holding means 46 are removed by the screen discharger 73, and thereafter, the cam projection 50a of the gripper 50 of the sheet holding means 46 is
And the gripper 50 is temporarily released. Then, the image-carrying sheet 28 released from being held by the gripper 50 moves to the discharge-side guide chute 35, and is discharged and stored in the discharge tray 34 after passing through the fixing device 36.

In such an operation process, the front and rear holder bars
47 and 48 pass through the drive-side guide drum 61, but since each holder bar 47 and 48 fits in the interference preventing groove 63a of the drive-side guide drum 61, the sheet holding means 46 and the drive-side guide drum 61 61 does not interfere. The gripper 50 is temporarily released at the sheet separating portion B. Since the releasing force F '(see FIG. 1) of the gripper 50 acts in the vertical direction of the endless chain 45, the endless chain 45 It does not vibrate unnecessarily. Further, since the sheet screen 51 is guided and conveyed to the peripheral surface of the driving guide drum 61 in order from the front end side, the driving guide drum 61
The rear end side of the sheet screen 51 that does not reach the position is maintained in a vertical posture. For this reason, the image bearing sheet 28 at a position close to the driving side guide drum 61 is set in the vertical posture, and the uppermost black image forming unit 20d is set.
Even if it is arranged close to the drive side guide drum 61, the rear end side of the image bearing sheet 28 will pass through the transfer portion T in a vertical normal posture, and accordingly, the drive sprocket 43 of the sheet conveyance transmission system 41 The distance E between the image forming unit 20d and the black image forming unit 20d can be set to a short dimension.

Further, in this embodiment, since the image forming units 20 are arranged in the vertical direction, it is possible to secure a work space for disposing unit components such as the photosensitive drum 21 and the laser scanning system 23 in the horizontal direction. In addition, installation work and maintenance work of the unit components of the image forming unit 20 can be easily performed.

In the above embodiment, as shown in FIG. 10, the sheet screen 51 of the sheet holding means 46 is made of a rectangular frame 58 made of a thin insulating resin having a certain degree of rigidity, for example, a polyethylene terephthalate (trade name Mylar) film. , The slack of the sheet screen 51 can be further reduced.

[Effects of the Invention] As described above, according to the first aspect of the present invention, the following basic effects can be obtained.

That is, first, the sheet conveying means is rotated by the driving unit disposed on the upper side and the driven unit disposed on the lower side, and transfers the image bearing sheet from the lower side to each image forming portion of each image forming unit. Since the sheet is conveyed toward the upper side, the upper driving unit pulls the sheet conveying unit in a direction to stretch the portion facing each image forming unit,
The portion on the opposite side of the sheet conveying means can be maintained in an extended state without being bent by gravity.

Therefore, it is possible to prevent the sheet conveyance system from bending under its own weight without applying a relatively high tension to the sheet conveyance system of the sheet conveyance unit, and each image forming unit is disposed at a position opposite to the image forming unit. On the other hand, it is possible not only to stably convey the image carrying sheet while maintaining the posture of the image carrying sheet in the vertical direction, but also to extend the life of the sheet carrying means.

Therefore, not only can the drive source be reduced in size by reducing the load torque of the sheet conveyance transmission system, but also the gap between the image forming portion of each image forming unit and the image bearing sheet (relative positional relationship) ) Can be made constant, and accordingly, the image forming operation in each image forming unit can be stabilized, and a high-quality color image can be obtained.

Second, the load acting on the driving source of the sheet conveyance transmission system due to the movement of the holding means for the image carrying sheet is considered by considering the right and left weight balance based on the position of the driving unit of the sheet and the conveyance transmission system. Since the setting can be made so as not to fluctuate, it is possible to effectively prevent the unstable operation due to the load fluctuation of the sheet conveying means.

Third, since the installation and maintenance and inspection work direction of the main parts of the image forming unit can be set to the horizontal direction, the installation and maintenance and inspection workability of the image forming unit can be improved.

According to the second aspect of the present invention, the sheet conveying means includes an endless sheet conveying and transmitting system that circulates through each image forming portion arranged in the vertical direction, and a predetermined interval between the sheet conveying and transmitting system. And a plurality of sheet holding means arranged to selectively hold the image bearing sheet, and each sheet holding means is provided with a sheet placing portion on which the image bearing sheet is placed, It is possible to pass the image carrying sheet to the image forming portion of each image forming unit in an extremely stable state, and accordingly, the transfer image quality of each color component can be kept very good.

Further, according to the third aspect of the present invention, as the image forming unit positioned at the uppermost position among the plurality of image forming units, the one that forms a black color component image is used. When forming a monochromatic black image, it is possible to avoid a situation where the black image passes through the image forming portion of another image forming unit.

For this reason, if the black image forming unit is arranged at a position other than the uppermost portion, when the image bearing sheet is transported in the vertical direction, the black image passes through the image forming portion of another image forming unit. Thus, there is a concern that an image defect will occur in the conveyance path of the image image sheet. In the present invention, however, a concern that such an image defect occurs can be effectively avoided. Further, since the driving source is used in the upper portion, the portion facing the uppermost image forming unit can be driven at a more stable speed.

According to the fourth aspect of the present invention, the latent image carrier and the latent image writing means for writing an electrostatic latent image on the latent image carrier are horizontally arranged. The optical path between the latent image carrier and the latent image writing means can be simplified.

For this reason, each image forming unit can be reduced in size, and the writing accuracy of the electrostatic latent image on the latent image carrier can be improved. In the above, the quality of an image formed on the image bearing sheet can be further improved.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an embodiment of a color image forming apparatus according to the present invention, FIG. 2 is a perspective view showing details of a sheet holding means used in the embodiment, and FIG. FIG. 4 is a sectional view taken along line III, FIG. 4 is an explanatory view showing the positional relationship of the sheet holding means used in the embodiment, FIG. 5 is an explanatory view showing details of a gripper opening / closing mechanism of the sheet holding means used in the embodiment, FIG. 6 is a view taken in the direction of arrow VI in FIG. 5, FIGS. 7 to 9 are cross-sectional explanatory views showing the operation process of the color image forming apparatus according to the embodiment, and FIG. FIG. 11 is an explanatory view showing a modification of the sheet holding means to be used, FIG. 11 is an explanatory view showing an example of a conventional color image forming apparatus, and FIG.
FIG. 11 is a detailed view of an XII part in FIG. [Explanation of Symbols] 20 image forming unit 28 image carrying sheet 40 sheet conveying means

Claims (4)

(57) [Claims] 1. A plurality of image forming units which are arranged at appropriate intervals along the vertical direction and form images of appropriate color components on an image bearing sheet, and each image forming portion arranged in the vertical direction of each image forming unit A sheet conveying means for conveying the image-bearing sheet from the lower side to the upper side, and the sheet conveying means is rotated by a driving unit disposed on the upper side and a driven unit disposed on the lower side. A color image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the sheet conveying means includes an endless sheet conveying transmission system that circulates through image forming portions arranged in a vertical direction, and a predetermined interval between the sheet conveying transmission system. And a plurality of sheet holding means for selectively holding the image bearing sheet, wherein each sheet holding means is provided with a sheet placing portion on which the image bearing sheet is placed. Color image forming apparatus. 3. The color image forming apparatus according to claim 1, wherein the uppermost image forming unit among the plurality of image forming units forms a black color component image. Image forming device. 4. A color image forming apparatus according to claim 1, wherein each image forming unit horizontally arranges a latent image carrier and a latent image writing means for writing an electrostatic latent image on the latent image carrier. A color image forming apparatus.