JP2000187367A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color image forming device making correct color reproduction feasible by optimally controlling replenishment of toner. SOLUTION: This image forming device is, allowed to form the color image by successively transferring the toner image respectively formed by plural image forming units 23Y, 23M, 23C and 23K on a paper sheet fed from a resist roller in order. The toner density sensor of the developing device on the respective image forming unit is, allowed to monitor the toner density after the elapse of the specified timing tA, tB and tC from the time point when the paper sheet is transported by the resist roller. In such a manner, the device is allowed to obtain a color image stabilized in image density of each color independently formed by respective image forming unit, and free from the change of the color balance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus for copying an image of an object to be copied onto a material to be transferred using an electrostatographic process.

[0002]

2. Description of the Related Art An image forming apparatus using an electrostatographic process uses a light-dark pattern of light reflected from a reading object obtained by illuminating a reading object such as a sheet-shaped original or a book as an image signal. It has an image reading unit to take in, an image forming unit that forms an image corresponding to the image signal obtained by the image reading unit, and the like.

[0005] The image forming section is provided with a photosensitive member for holding an electrostatic image corresponding to an image signal, a developing device for developing the electrostatic image held on the photosensitive member, and a developer image supplied from a paper supply portion. And a fixing device for fixing the developer image to the recording paper, and outputs an image corresponding to the image signal to the recording paper. In addition, many image forming apparatuses capable of outputting a color image in which a predetermined number of single-color image forming units including a photoreceptor and a developing device are arranged in series by a subtractive color mixture method have been put to practical use.

[0004]

In a color image forming apparatus capable of outputting a color image, a single color developer image formed by each single color image forming unit is sequentially printed on a recording sheet conveyed by a sheet conveying mechanism. Transcribed. In addition,
The developing device is a two-component developing device that uses a two-component developer including a toner (visualizing material) and a carrier (charge control and toner conveyance), and has a toner density sensor that monitors the ratio of the carrier to the toner (toner density). Then, the control is constantly performed so that the toner density becomes constant based on the output of the toner density sensor.

The timing of monitoring the toner density by the toner density sensor is immediately after the electrostatic image formed on the photosensitive member is developed, and the amount of toner consumed by the development is fed back into the developing device. A predetermined timing is preferable. In many cases, the toner concentration is monitored in accordance with the timing of turning on the aligning roller, which is the timing for feeding the next recording sheet (the predetermined timing when the number of copies is one). Is done.

For this reason, as described above, in a color image forming apparatus in which a plurality of developing devices are arranged in series along the direction in which a sheet is conveyed, a developing device located downstream (downstream) in the sheet conveying direction. In the apparatus, when the toner is replenished or the toner concentration is monitored even though the toner is not consumed, there is a problem that the toner concentration is reduced due to the fact that the toner is not consumed.

[0007] This causes a problem that the color balance fluctuates and a predetermined color cannot be reproduced, especially when a color image is displayed.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus for forming a color image by superimposing a single color image formed by using a plurality of single color image forming units, thereby optimally controlling the replenishment of each single color toner to achieve accurate printing. It is an object of the present invention to provide a color image forming apparatus capable of reproducing various colors.

[0009]

SUMMARY OF THE INVENTION The present invention has been made on the basis of the above problems, and has a first image carrier for holding an electrostatic image corresponding to a first image, and a first image carrier. A first developing means for supplying a developer to the electrostatic image formed on the body to form a first developer image, and the first developing means reduces the first developer image by forming the first developer image; A first developer monitoring means for monitoring the amount of the developer, and a transfer target material holding the first developer image formed on the first image carrier by the first developing means at a predetermined timing And a second image carrier provided downstream of the first image carrier in the direction in which the material to be transferred is transported with respect to the first image carrier, and holding an electrostatic image corresponding to the second image. And a second developer for supplying a developer to the electrostatic image formed on the second image carrier to form a second developer image A developing unit, in the second developing means, and a second developer monitoring means for monitoring the amount of reduced developer by the formation of the second developer image, the second
And a control means for operating the developer monitoring means in association with the start of the transfer of the material to be transferred by the transfer means.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing an example of a color image forming apparatus according to an embodiment of the present invention.

As shown in FIG. 1, the color image forming apparatus, that is, the digital copying apparatus 1 has a scanner section 10 as image reading means and a printer section 20 as image forming means.

A scanner unit 10 includes a document glass 11 for holding a document D to be copied, and a document D
Is pressed, and the document D set in advance is conveyed one by one toward the document glass 11.
An automatic document feeder (hereinafter, abbreviated as ADF) 12 for discharging the document D from 1 to a predetermined discharge position, and image information of the document D held on the document glass 11 are taken in as light and dark, and photoelectrically converted to an image signal CCD sensor 13 that generates
Etc.

The printer unit 20 has a CC of the scanner unit 10.
An image that converts an image signal photoelectrically converted by the D sensor 13 (or supplied from an external device not shown) into a print signal (laser exposure control data) that can be converted into an electrostatic image by an image forming unit described below. Processing unit 21, laser exposure unit 22 that emits laser light in response to a print signal supplied from image processing unit 21, laser exposure unit 2
Image forming unit 23, which receives the laser light from 2 and generates an electrostatic image corresponding to the laser light, and visualizes the generated electrostatic image with a developer.
A transfer device 24 for transferring the developer image visualized by the above to recording paper supplied from a paper feeding unit (not described in detail), and a fixing device for fixing the developer image transferred to the paper by the transfer device 23 to the paper 25. It should be noted that the laser exposure unit 22 performs C-color separation based on the subtractive color mixture method.
(Cyan), M (magenta) and Y (yellow)
Four sets of laser beams corresponding to K (black) for correcting the primary color and the density are emitted. Further, the image forming unit 23 is provided in proximity to the first to fourth photoconductors for receiving the respective laser beams of Y, M, C, and K described above, and each of the photoconductors. It includes four sets of single-color image forming units including a developing device for selectively supplying a toner as a visualizing agent to the generated electrostatic image and developing the toner.

FIG. 2 shows the image forming unit 2 shown in FIG.
FIG. 5 is a partially enlarged view illustrating the vicinity of a single-color image forming unit of No. 3;

As shown in FIG. 2, the image forming unit 23 includes a first single-color image forming unit 23Y for forming a color-separated Y component image and a second single-color image for forming an M component image. The image forming apparatus includes a forming unit 23M, a third image forming unit 23C for forming an image of the C component, and a fourth image forming unit 23K for forming an image of the K component.

Each monochrome image forming unit 23
(Y, M, C, and K) are a drum-shaped photosensitive drum 30 that generates an electrostatic latent image corresponding to the laser light from the laser exposure unit 22, and the toner is applied to the electrostatic latent image generated on the drum 30. The developing device 31 to be supplied supplies the developer image (toner image) developed on the surface of the drum 30 by the developing device 31 to a recording sheet fed at a predetermined timing from a sheet feeding unit (not shown). It has a transfer roller 32 (transfer device 24) and a cleaning device 33 for removing residual charges and transfer residual toner on the surface of the drum 30, respectively. The transfer roller 32 and the drum 30
A transfer belt 24a (transfer device 24) formed so as to be able to convey the recording paper toward the fixing device 25 is disposed between the transfer belt 24a and the transfer belt 24a.

As shown in FIG. 3, the developing device 31 includes a housing 31 for storing a developer, that is, a carrier and a toner.
a, a developing roller 31b that guides a developer charged to a predetermined potential by being stirred by the housing 31a to a developing position opposed to the drum 30, and is consumed by developing the electrostatic latent image on the drum 30 at the developing position. A toner density sensor 31c for monitoring the toner density in the developer in order to replenish the toner, and a predetermined amount of toner is supplied from a toner storage unit (not shown) into the housing 31a based on the detection result of the toner density by the toner density sensor 31c. And a toner supply motor 31e for rotating the toner supply roller 31d.
A continuous (monochromatic) color image is provided while constantly replenishing the toner consumed as a result of developing the electrostatic latent image so that the toner density of the toner image developed on the toner image becomes constant.

FIG. 4 is a schematic diagram for explaining an example of a control system of the digital copying apparatus (color image forming apparatus) shown in FIG.

As shown in FIG. 4, the digital copying apparatus 1 includes a main control unit 100 including a system-CPU 101 as a main control unit, and is connected to the main control unit 100, and includes a scanner unit 10 and a printer unit 20, respectively. Has a mechanism control unit 110 for controlling the operation of.

The system CPU 101 converts the image signal photoelectrically converted by the CCD sensor 13 of the scanner unit 10 into a print signal for forming an electrostatic latent image on the photosensitive drum 30 by the laser exposure unit 22. Processing circuit 102 (image processing unit 21), system-CPU 10
A program memory (ROM) 103 in which a program for operating the CPU 1 is stored; a random access memory (RAM) 104 used for image processing by the image processing circuit 102 and temporary storage of input numerical data; The data is backed up by a battery (not shown) so that data is not lost even when the power supply is interrupted. Non-volatile NVRAM (non-volatile memory) 105 to hold,
Also, communication hardware 131 for exchanging signals with the mechanism control unit 110 and the like are connected.

The mechanism control section 110 includes a mechanical control CPU 111 for controlling the mechanism control section 110, the photosensitive drum 30 of each of the single-color image forming units 23 (Y, M, C, and K) of the printer section 20 and the development of the developing device 31. Roller 31b
A drive control unit 112 for controlling many driving elements such as a motor for driving the like, a predetermined voltage is applied to a charging device (not shown) of the image forming unit 23, a transfer roller 32 of the transfer device 24, or a developing roller 31b of the developing device. An output signal can be directly input to the high-pressure process system control unit 113 and the mechanical controller-CPU 111, and a sensor and an actuator 114 (including the toner density sensor 31c) for detecting jamming of the sheet inside the copying apparatus 1 and the temperature of the fixing device 25 are included. )group,
The light intensity of the laser light emitted by a semiconductor laser element (not shown) of the exposure unit 22 is converted into a printing signal (laser modulation signal).
A laser drive circuit 115 for changing the CCD sensor 13 of the scanner unit 10, a motor (not shown) for reciprocating a carriage (reading mirror and lamp) (not shown) of the scanner unit 10 at a predetermined speed. Scanner driving unit 117 for driving the
An input / output circuit (I / O) 118 used for passing control signals between a sheet feeding unit and a reversing mechanism (not described in detail), a ROM 119 a storing a program for operating the mechanical control-CPU 111 and the ROM 119 a described above. It is interconnected with a RAM 119b that temporarily stores many numerical data required for the operation of each unit, and is also connected with the system-CPU 101 via the communication hardware 131.

Next, the operation of the digital copying apparatus 1 shown in FIGS. 1 to 4 will be described.

Operation panel (or external device) not shown
When image formation is instructed from the system-CPU 101
Control, the laser drive circuit 115 is energized from the mechanical controller-CPU 111, the intensity is modulated from the laser exposure unit 22 in accordance with the above-described print signal (laser modulation signal), and the laser light is emitted to a predetermined position on each drum 30. Is irradiated.

Thus, each single-color image forming unit 23
An electrostatic latent image corresponding to the image data is formed on the outer periphery of each of the (Y, M, C, and K) drums 30.

The electrostatic latent images formed on the outer periphery of each drum 30 are developed with toner from a developing device 31 of each image forming unit, and are transferred to a transfer belt 24a at a position facing a corresponding transfer roller 32. Is transferred by the electric field from the transfer roller 32 onto the recording paper conveyed by the printer. The sheet conveyed by the transfer belt 24a is transferred from a sheet feeding unit (not described in detail) to the transfer belt 24a.
a when the electrostatic latent image is formed by the laser exposure unit 22 on the drum 30 of the first single-color image forming unit 23Y, which is positioned closest to the registration roller 34, when the transfer is performed. Sent.

On the other hand, the drum 30 after the transfer of the toner to the recording paper by the transfer device 24 is opposed to the cleaning device 33 as a result of the subsequent rotation, and the transfer residual toner (residual toner) remaining on the outer periphery is removed. Thus, the state before the surface potential is supplied by the charging device (not shown) is returned to the initial state, and the next image formation becomes possible.

By repeating the above process, a continuous image forming operation becomes possible.

Incidentally, the single-color image forming units 23Y, 23Y,
Since each of 23M, 23C and 23K is arranged in series along the direction in which the transfer belt 24a is rotated, as described with reference to FIG. 5, each of the single-color image forming units 23Y, 23M, 23C and At 23K, the timing at which the electrostatic latent image is generated (on the drum 30), that is, the timing at which the laser exposure unit 22 emits laser light for each color component is different.

More specifically, a printing signal (laser light) corresponding to the Y image is exposed from the laser exposure unit 22 to the drum 30 of the first image forming unit 23Y disposed closest to the registration roller 34. Timing Ey
Indicates that the leading end of the sheet temporarily stopped by the registration roller 34 is the drum 30 of the unit 23Y and the transfer roller 3
2 is reached at the timing of reaching the first transfer area defined by the second transfer area.

That is, the drum 30 of the unit 23Y
The time when the laser light exposed to the first transfer area reaches the first transfer area is Sy, and the paper stopped by the registration roller 34 is conveyed at a predetermined speed by turning on the registration roller 34 to the first transfer area. Assuming that the arrival time is Sl, Ey = Sy + α = Sl−α α: the registration roller 34 and the first image forming unit 23
And a constant term defined by the distance from Y.

On the other hand, the second monochrome image forming unit 23M
Timing Em at which the laser exposure unit 22 irradiates the drum 30 with laser light corresponding to the image of the M component.
Is that the time Sm during which the laser beam exposed to the drum 30 reaches the second transfer area is the same as Sy, and the distance between the first image forming unit 23Y and the second image forming unit 23M (No. The distance between the first transfer area defined by one image forming unit and the second transfer area defined by the second image forming unit) is d, and the time for moving the section is Sd. When indicated, Em = Sm + Sd + α = S1−Sy−αα: registration roller 34 and first image forming unit 23
It is a constant term defined by the distance from Y.

The third monochrome image forming unit 23C
The laser exposure unit 22 irradiates the drum 30 with laser light corresponding to the image of the C component.
Is that the time Sc at which the laser beam exposed to the drum 30 reaches the third transfer area is the same as Sy, and the distance (second distance) between the second image forming unit 23M and the third image forming unit 23C. The distance between the second transfer area defined by the second image forming unit and the third transfer area defined by the third image forming unit) is also d, so that the section is moved. Ec = Sc + 2Sd + α = S1−Sy−Sd−αα: registration roller 34 and first image forming unit 23
It is a constant term defined by the distance from Y.

Similarly, the timing Ek at which the laser beam corresponding to the K component image is irradiated on the drum 30 of the fourth image forming unit 23K by the laser exposure unit 22 is
The time Sk during which the laser beam exposed to the drum 30 reaches the fourth transfer area is the same as Sy, and is defined by the third transfer area defined by the third image forming unit and the fourth image forming unit. Since the distance from the fourth transfer area to be transferred is also d, the time for moving the section is defined by Sd, and Ek = Sy + 3Sd + α = S1−Sy−2Sd−αα: Registration roller 34 And first image forming unit 23
It is a constant term defined by the distance from Y.

Incidentally, the first image forming unit 23Y
The timing at which the toner concentration sensor 31c of the developing device 31 monitors the toner concentration is determined based on the accumulation of the control method in the known device and many experiments, and the visualization of the electrostatic image on the photoconductor by the developing device has been completed. It is preferable that the second sheet conveyed following the first sheet is transferred by the registration roller 34 to the transfer belt 24a.
, Ie, the timing at which the second and subsequent sheets are fed from the registration rollers 34 when the image forming operation is continued twice or more.

Accordingly, in each of the second to fourth single-color image forming units 23M, 23C and 23K located on the downstream side in the direction in which the transfer belt 24a rotates, the amount of toner consumed by image formation In order to accurately monitor the toner density in which the toner density is reflected, the timing at which the toner density is monitored by the toner density sensor 31c is determined by the timing at which the leading edge of the paper conveyed toward the transfer area of each image forming unit is It is necessary to set the timing so as to be the same as the relationship between the distance between the first transfer area and the registration roller 34 in one image forming unit 22.

That is, the second image forming unit 23M
The timing of monitoring the toner concentration of the toner concentration sensor 31c in the developing device 31 is such that the leading edge of the sheet conveyed by the rotation of the registration roller 34 is positioned at the first image forming unit 22 This is a point in time when a position corresponding to the distance 1 between the transfer region 1 and the registration roller 34 is reached (point A in FIG. 5). At this position (point A), the timing Em at which the image of the M component is exposed in the second image forming unit 23M is, as described above, Em = Sm + Sd + α = S1−Sy−α. The time during which the paper is transported over the distance between the transfer area and the registration roller 34 is Sd + Sl, and therefore, the timing at which the toner density sensor 31c in the developing device 31 of the second image forming unit 23M should monitor the toner density Is defined as Sd after the registration roller 34 is turned on.

From the above, the third image forming unit 2
The timing of monitoring the toner concentration of the toner concentration sensor 31c in the developing device 31 of 3C is determined by the registration roller 3
The leading end of the sheet conveyed by the rotation of No. 4 is moved from the first image forming unit 22 in the first image forming unit 22 to the third transfer area.
(Point B in FIG. 5) when the position reaches the position corresponding to the distance 1 between the transfer area and the registration roller 34.
This position (point B) corresponds to the timing Ec at which the image of the C component is exposed in the third image forming unit 23C.
As described above, Ec = Sc + 2Sd + α = Sl−Sy (Sd) −Sd−α, and the time during which the sheet is transported over the distance between the third transfer area and the registration roller 34 is 2Sd + Sl. Therefore, the timing at which the toner concentration sensor 31c of the developing device of the third image forming unit 23C should monitor the toner concentration is determined after the registration roller 34 is turned on.
It is defined that Sd + Sd = 2Sd has elapsed.

The timing of monitoring the toner density of the toner density sensor 31c of the developing device 31 of the fourth image forming unit 23K is similarly set by the registration roller 34.
Has reached the position corresponding to the distance 1 between the first transfer area in the first image forming unit 22 and the registration roller 34 with respect to the fourth transfer area. It is time (point C in FIG. 5). In this position (point C), the timing Ek at which the K component image is exposed in the fourth image forming unit 23K is, as described above, Ek = Sy + 3Sd + α = S1−Sy−2Sd−α. Since the time during which the sheet is transported over the distance between the fourth transfer area and the registration roller 34 is 3Sd + S1, the toner density sensor 31c of the developing device of the fourth image forming unit 23K should monitor the toner density. The timing is set after the registration roller 34 is turned on.
It is defined that Sd + Sd + Sd = 3Sd has elapsed.

FIG. 6 shows each of the image forming units 23 described above.
6 is a timing chart for explaining the timing at which the toner density sensor 31c of each of the (Y, M, C, and K) developing devices 31 monitors the toner density, together with the operation of the other main components of the copying apparatus 1; In FIG. 6, it is assumed that time elapses from the left side to the right side of the horizontal axis, and the registration roller 34 is used as an example in a case where any two sheets are conveyed during a continuous copying operation. The timing of turning on, the timing of monitoring of the image area in the exposure and developing device and the toner density by the toner density sensor are shown.

As shown in FIG. 6, the mechanism control unit 110
The drive control unit 11 is controlled by the mechanical control CPU 111.
3 and the high-pressure process system control unit 113 are operated to apply a predetermined surface potential to the photosensitive drums 30 of the image forming units 23Y, 23M, 23C and 23K rotated at a predetermined speed. Before the laser beam is irradiated, the sheet that has been conveyed from the sheet feeding unit to the registration roller 34 in advance is
Is turned on (start of rotation) and conveyed again toward the transfer belt 24a, a laser beam corresponding to the Y image is applied to the drum 30 of the first image forming unit 23Y after a predetermined time has elapsed since the registration roller 34 was turned on. , From the laser exposure unit 22. The laser exposure unit 2
5, the timing at which the Y image is exposed is the time when the sheet is conveyed by the transfer belt 24a by “α” after the registration roller 34 is turned on, as already described with reference to FIG. When the registration roller 34 is turned on, the toner density sensor 31c of the developing device 31 of the image forming unit 23Y for the Y image
Energized by the PU 111, the toner density is monitored.

Thereafter, after a predetermined time, the electrostatic latent image of the Y image formed on the drum 30 by the developing device 31 is developed and visualized. At this point, the next succeeding sheet is stopped by the registration roller 34.

Subsequently, after a predetermined time has passed since the registration roller 34 was turned on, as shown in FIG. 5, the first image forming unit 23 was turned on after the registration roller 34 was turned on.
After Sy + α = S1, which is the time required for the Y image exposed in Y to reach the first transfer position, a laser beam corresponding to the M image is supplied from the laser exposure unit 22 to the drum 30 of the second image forming unit M. Irradiated. Meanwhile, the first
The Y image formed by the image forming unit 23Y of
In the first transfer area, the image is transferred to a sheet.

Thereafter, after a predetermined time, the electrostatic latent image of the M image formed on the drum 30 by the developing device 31 is developed and visualized. Further, the developed M image is transferred over the sheet on which the Y image has already been transferred after Sm has elapsed from the start of exposure.

Subsequently, after a predetermined time has passed since the registration roller 34 was turned on, that is, as shown in FIG. 5, the first image forming unit 23 was turned on after the registration roller 34 was turned on.
In Sy + α = Sl, which is the time when the Y image exposed in Y reaches the first transfer position, and Sm, which is the time when the M image exposed in the second image forming unit 23M reaches the second transfer position. After a lapse of time equal to the sum of = Sd, the laser beam corresponding to the C image is emitted from the laser exposure unit 22 to the drum 30 of the third image forming unit C. On the other hand, the M image formed by the second image forming unit 23M is transferred in the second transfer area on the sheet on which the Y image has already been transferred after Sm has elapsed from the start of exposure. Further, after a predetermined time has passed since the registration roller 34 was turned on, that is, as shown in FIG.
At Sy + α = S1, which is the time for the Y image exposed in 3Y to reach the first transfer position, and at Sm, which is the time for the M image exposed at the second image forming unit 23M to reach the second transfer position. = Sd and after the time of adding Sc = Sd, which is the time required for the C image exposed in the third image forming unit 23C to reach the second transfer position, is applied to the drum 30 of the fourth image forming unit K. Laser light corresponding to the K image is emitted from the laser exposure unit 22. On the other hand, the C image formed by the third image forming unit 23 </ b> C is transferred in the third transfer area on the sheet on which the M image and the Y image have already been transferred after Sc has elapsed from the start of exposure.

Subsequently, after a predetermined time, the electrostatic latent image of the K image formed on the drum 30 is developed and visualized by the developing device 31. In the fourth transfer area, the C image has already been formed after the lapse of Sk from the start of exposure. , M image and Y image are transferred in a superimposed manner on the sheet on which the M image and the Y image are transferred.

By the way, the first image forming unit 23Y
When the Y image formed by the above is transferred onto the sheet, the registration roller 34 is rotated after a predetermined time elapses in order to feed the sheet holding the next image. At this time, the toner density is monitored by the toner density sensor 31c of the developing device 31 of the first image forming unit 23Y.

On the other hand, each of the developing devices 31 of the second to fourth image forming units 23M, 23C and 23K
As shown in FIG. 6, the timing (time interval) at which the toner concentration is monitored by the toner concentration sensor 31c is from the time when the rotation of the registration roller 34 is turned on.
After t _A , t _B and t _C respectively.

The time intervals t _A , t _B and t _C shown in FIG. 6 correspond to the points A, B and C shown in FIG. , Match the time between reaching.

From this, each time interval t _A ,
t _B and t _C are, as already described with reference to FIG.
For the developing device 31 of the second image forming unit _{23M, t A = Sd + Sl} , with respect to the developing device 31 of the third image forming units _{23C, t B = 2Sd + Sl} , with respect to the developing device 31 of the fourth image forming units 23K Can be expressed as t _C = 3Sd + Sl.

The above time intervals t _A , t _B and t _C are, for example, determined by the ROM 103 or the NVRAM 105.
, The timing sensor or the like that may be required to operate the toner density sensor 31c, that is, to set the timing for monitoring the toner density, becomes unnecessary, and the component cost of the copying apparatus 1 can be reduced.

That is, the timing at which the toner density is to be monitored by the toner density sensor 31c of the developing device 31 of each of the second to fourth image forming units 23M, 23C and 23K is determined regardless of which image forming unit is used. It is defined based on the timing at which the registration roller 34 is turned on and the conveyance of the sheet is started.

As described above, in a color image forming apparatus in which a plurality of developing devices (monochromatic image forming units) are arranged in series along the direction in which a sheet is conveyed, a toner density sensor monitors the toner density in the developing device. The second to fourth monochrome image forming units 23M are set such that the toner concentration is monitored when the decrease in the amount of toner consumed in image formation is reflected on the toner concentration in the developer. , 23C and 23K can be accurately monitored.

Thus, the density of the image for each color component independently formed by each image forming unit is stabilized, and a color image in which the color balance does not change is provided.

[0055]

As described above, in the image forming apparatus of the present invention, in each of the developing devices of two or more monochromatic image forming units, the toner concentration of the developer in the developing device is consumed in image formation. Since the monitoring is performed when the decrease in the toner amount is reflected on the toner concentration in the developer, the amount of the supplied toner is accurately monitored.

As a result, the color balance is prevented from undesirably fluctuating, and a color image forming apparatus having high color reproducibility is provided.

Further, since a mechanism (timing sensor or the like) for setting the timing for monitoring the toner density by the toner density sensor is unnecessary, the cost of the entire apparatus can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a color image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating an image forming unit of the image forming apparatus shown in FIG.

FIG. 3 is a schematic diagram illustrating an example of a developing device incorporated in each single-color image forming unit in the image forming unit illustrated in FIG. 2;

FIG. 4 is a schematic diagram illustrating an example of a control system of the image forming apparatus illustrated in FIGS. 1 to 3;

FIG. 5 is a schematic diagram illustrating exposure timing of image light in each single-color image forming unit in the image forming unit shown in FIG. 2;

6 is a timing chart for explaining the relationship between the exposure timing shown in FIG. 5 and the timing for monitoring the toner density of the developing device in each monochrome image forming unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Color image forming apparatus (digital copying apparatus), 20 ... Image forming part (printer part), 21 ... Image processing part, 22 ... Laser exposure unit, 23 ... Image forming unit ( 23Y ... Y image forming unit (single color), 23M ... M image forming unit (single color), 23C ... C image forming unit (single color), 23K ... K image forming unit (single color) , 24 ... Transfer device, 24a ... Transfer belt, 25 ... Image color converter, 30 ... Photoconductor drum, 31 ... Developing device, 31b ... Developing roller, 31c ... Toner concentration sensor 31e toner supply motor 32 transfer roller 33 cleaning device 34 registration roller 101 system CPU 102 · An image processing circuit, 103 ... ROM, 105 ... NVRAM (nonvolatile memory), 111 ... mechanical control -CPU, 112 ... drive control unit, 115 ... laser drive circuit.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshiyuki Fukuda 70 Yanagimachi, Saiwai-ku, Kawasaki-shi, Kanagawa F-term (reference) 2H027 DA32 DA38 DC03 DD07 DE09 EA06 EB04 EC06 EC18 ED10 EE02 EE06 EF09 2H030 AB02 AD01 AD17 BB23 2H077 AD02 AD06 DA15 DA42 DA81 DB04 EA01 GA13

Claims (5)

[Claims] A first image carrier that holds an electrostatic image corresponding to the first image; and a developer that supplies a developer to the electrostatic image formed on the first image carrier to form a first image carrier. A first developing means for forming a developer image; a first developer monitoring means for monitoring the amount of developer reduced by the formation of the first developer image in the first developing means; Transport means for transporting, at a predetermined timing, a transfer material holding the first developer image formed on the first image carrier by the first developing means; A second image carrier that is provided on the downstream side in the direction in which the transfer material is conveyed and holds an electrostatic image corresponding to the second image; and a second image carrier that is formed on the second image carrier. A second developing means for supplying a developer to form a second developer image; and Second developer monitoring means for monitoring the amount of developer reduced by the formation, and control means for operating the second developer monitoring means in association with the start of the transfer of the material to be transferred by the transfer means. An image forming apparatus comprising: 2. An image forming apparatus according to claim 1, wherein said control means operates said second developer monitoring means after a lapse of a predetermined time from the start of conveyance of the transfer material by said conveyance means. . 3. The method according to claim 2, wherein the control unit supplies the developer to the electrostatic image formed on the second image carrier and forms the second developer image at a predetermined timing after forming the second developer image. 2. The image forming apparatus according to claim 1, wherein the agent monitor is operated. 4. A first image carrier for holding an electrostatic image corresponding to a first image, and a developer is supplied to the electrostatic image formed on the first image carrier by supplying a developer to the first image carrier. A first developing means for forming a developer image; a first developer monitoring means for monitoring the developer reduced by the formation of the first developer image in the first developing means; Transport means for transporting, at a predetermined timing, a transfer material holding a first developer image formed on the first image carrier by a developing means; and a transfer material for the first image carrier. A second image carrier that is provided on the downstream side in the direction in which the image is conveyed, and that holds an electrostatic image corresponding to the second image; and a developer on the electrostatic image that is formed on the second image carrier. A second developing means for supplying a second developer image to form a second developer image; and forming a second developer image in the second developing means. A second developer monitoring means for monitoring the reduced developer; a second developer monitoring means provided downstream of the second image carrier in the direction in which the transfer material is conveyed; A third image carrier for holding an electric image, third developing means for supplying a developer to the electrostatic image formed on the third image carrier to form a third developer image, A third developer monitor for monitoring the developer reduced by the formation of the third developer image in the third developer; and a third developer monitor and the second developer monitor. And a control unit that operates after a lapse of a predetermined time from the start of conveyance of the transfer material by the conveyance unit. 5. The method according to claim 1, wherein said control means operates at a predetermined timing after supplying the developer to the electrostatic images formed on the respective image carriers and forming the respective developer images. The image forming apparatus according to claim 4.