JPH0375678A - Image forming device - Google Patents

Abstract

PURPOSE:To keep toner replenishing ability and toner content inside a developing device irreducible minimum and to miniaturize the device by replenishing before the finish of the next developer concentration detecting action when the replenishing time becomes greater than the maximum time for replenishing after the toner concen trating detection. CONSTITUTION:For example, a tranveling body is which is capable of traveling approxi mately horizontally, and a magenta developing device 1M, a cyan developing device 1C, a yellow developing device 1Y, and a black developing device 1BK over the travel ing body 1a are provided in a horizontal travel method developing device 1. Then a replenishing means 400 which replenishes each toner, a concentration detecting means 500 which detects the toner concentration, a calculating means which calculates the replenishing time from this concentration detecting means 500, and a storing means which stores the replenishing time are provided. When the replenishing time becomes greater than the maximum time for replenishing after the toner concentration detection, it is replenished before the finish of the next toner density detecting action. Thus, toner replenishing ability and the margin quantity of the toner content inside the developing devices 1M, 1C, 1Y, 1BK are made to irreducible minimum and the device is miniaturized.

Description

[Detailed Description of the Invention] This invention is generally produced using an electrophotographic method or an electrophotographic method.
It relates to a vehicle recording force type image forming device, and in particular,
The present invention can be conveniently applied to a multicolor image forming apparatus having a plurality of developing devices, each of which is supplied with toner.

A latent image is formed on the L, △, and blue image bearing pair, and the WI image is developed with a developing device that uses a two-component developer made of a mixture of magnetic particles (carrier) and toner particles. In an image forming apparatus using an electrophotographic method or an electrostatic recording method, toner particles in the developer are consumed during repeated development operations and magnetic particles remain, so the toner concentration in the developer varies depending on the development operation. Therefore, in order to keep the toner concentration in the developer at a constant value, it is necessary to always accurately detect the toner concentration of the developer being used in the developing device. Toner must be replenished according to the results.

In particular, in a multicolor image forming apparatus, a full color image is formed by aligning basic color toners such as magenta, cyan, yellow, and black toners, so the C degree reproducibility and color reproducibility are ensured. In order to achieve this, it is required to reliably control the WJ image forming conditions and development operating conditions.

Therefore, in the latent image forming system, one potential sensor is installed in the main body of the apparatus to reliably control the dark area potential and the capital potential. Further, in the developing system, the bias potential is controlled to match the latent image forming conditions, and the toner density is controlled. In order to control this toner concentration, it is important to accurately detect the toner concentration and accurately replenish toner stars according to the detection signal.

Conventionally, a developing device for forming a multicolor image has one developing device for each color of developing toner, and the developing device for each color is moved to a developing section of a photoreceptor that is a latent image carrier. There are mobile developing devices that develop and form multicolor images. Compared to a device-type developing device in which developing units for each color are arranged side by side around a photoconductor, such a mobile developing device has the advantages of a smaller photoconductor, which is usually a cylinder, and a smaller device itself, as well as lower cost. It has the advantage of being able to achieve this, and is widely used.

This mobile developing device is generally one in which a developing device and a toner hopper for storing replenishment toner are integrated. However, this type has the disadvantage that the device becomes larger or more complicated.

For this reason, in recent years, the hopper and the developing device have been completely separated, and the movement of the developing device and toner replenishment have been controlled so that replenishment toner is supplied from the toner container to the developing device only when the developing device moves to a certain position. A device has been proposed.

In such devices, the toner replenishment mechanism is equipped with a pipe with a screw inside and rotates the screen and knee at a constant rotation speed for the required time to supply the required amount of toner to the developing device. It is common to supply to Furthermore, in such an apparatus, toner is replenished only when the developing device moves to a certain position, so an upper limit is set on the toner replenishment time. Furthermore, in order to accurately detect toner concentration, toner C degree detection is performed multiple times,
Since the toner concentration is detected after waiting until the agitation state of the toner in the developing device becomes constant, the above-mentioned toner replenishment time has historically been shortened.

Therefore, if the toner replenishment capacity is set sufficiently high and within the upper limit time,
A method is used to replenish toner in excess of the amount consumed, but if the screw rotation speed is not increased too much in order to increase the replenishment capacity, toner will aggregate in the pipe and cause toner jams. In particular, toner having a small average particle size of 3 to 8 Bm, so-called small particle size, was particularly prone to agglomeration and clogging.

Therefore, an object of the present invention is to perform control to replenish toner fields that could not be replenished last time using the time until toner concentration is detected, thereby minimizing the toner replenishment capacity and the amount of free space in the toner container in the developing device. Another object of the present invention is to provide an image forming apparatus which is capable of downsizing the apparatus, performs real-time toner concentration control, enables highly accurate toner concentration control, and can always obtain high-quality images.

Therefore, the above objects are achieved in an image forming r& mold apparatus according to the present invention. To summarize, the present invention includes at least one developing device, a replenishing means for replenishing the developer to the developing device, a degree C detection means for detecting the concentration of the developer, and a replenishment time determined by the concentration detecting means. It is equipped with a calculation means for calculating and a storage means for storing replenishment time, and if the replenishment time becomes longer than the eel-sized time that can be replenished after detecting the developer concentration, the next developer concentration detection operation is terminated. This image forming apparatus is characterized in that it is replenished up to According to a preferred embodiment, in an image forming apparatus equipped with a plurality of developing devices, the maximum replenishable time is the time until the developing device is moved for development of the next color, or the time until development of the next color starts. It is said to be the time of

DESCRIPTION OF THE PREFERRED EMBODIMENTS The image forming apparatus according to the present invention will now be described in more detail with reference to the drawings.

FIG. 1 shows the overall configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus shown in FIG. 1 is a full-color, multicolor image forming apparatus that employs an electrophotographic method in the image forming process, and although its overall configuration is already well known, its aff. The explanation is as follows.

That is, in FIG. 1, a transfer material holder, that is, a transfer drum 13, is disposed approximately at the center of the image forming apparatus main body 40. As shown in FIG. The transfer drum 13 is supported rotatably in the arrow direction (clockwise) in FIG.
On the inner circumferential side, a transfer material adsorption means, that is, an adsorption charger 1 is provided.
Start with 1.

A transfer charger 14, a neutralization charger 17, and a separation pressing roller 23 are provided. Moreover, on the outer peripheral side of the transfer drum 13,
A charger for neutralization 18 is disposed opposite to the charger for neutralization 17, and a charger 19 and a separation claw 20 are disposed opposite to the separation pressing roller 23. Further, a transfer drum cleaner 34 is disposed on the outer circumferential side of the transfer drum 13, and a suction residual current '
The transfer material adsorption means, that is, the contact roller 12 faces A! A photosensitive drum 2 is disposed facing the transfer charger 14 .

The downstream end of the paper feed guide 9 on the right side of the registration roller pair 10 faces the contact roller 12. A pair of paper feed rollers 24, a pickup roller 25, a pair of paper feed rollers 26, and a pickup roller 27 are provided at the upstream end of the paper feed guide 9, which branches in two directions.

A fixing roller 31. pressure roller 30,
Fixing device 2 having oil sponge 32, web 33, etc.
2 is provided. The fixing 'Jji 22 and the separation claw 20
A transfer material conveying belt 21 is disposed between the two, and a motor 3i is disposed above the transfer material conveying belt 21.

In the upper region of the image forming apparatus main body 40, a document illumination lamp 4, a first mirror 5, a folding mirror 6, a lens system 7, and a color image sensor 8 are arranged.

Image formation? In the lower area of the main body 40, a developing means, that is, a horizontally movable developing device 1 is disposed in close proximity to the outer peripheral surface of the photosensitive drum 2. Water f mobile developing device 1
The details will be described later.

The photoreceptor drum 2 described above is rotatably supported in the direction of the arrow in FIG. Various equipment necessary for carrying out the operation include the already described transfer drum 13, transfer charger 14, and horizontally moving developing mold 211, as well as the primary charger 3, potential sensor 28, charger 15 for static elimination, and cleaning member. 16, Charger for static elimination 1
5, a laser beam scanner unit 29 and the like are arranged.

To be more specific about the water f moving type developing mold 21t, the horizontally moving type developing mold 22i includes a movable body 1a which can move substantially horizontally in the lateral direction in FIG. It is equipped with a magenta developer 1M, a cyan developer IC, a yellow developer IY, and a black developer IBK.

Four toner hoppers 400 shown in FIG.
01, it is arranged on the front side in the first U54.

That is, in FIG. 4, the hoppers 400 include a magenta toner hopper 402M, a cyan toner hopper 402C, and a yellow toner hopper 402M.
-Toner hopper 402Y, black toner hopper 402
BK is on the right.

Each hopper has supply connection parts 403M and 403C1403Y.
, 403BK are connected, and the supply port 404C1404Y, 404BK (supply connection part 403
There is a replenishment port 404M at the tip (not shown in FIG. 4), and only when each developer reaches the developing position, toner is transferred to the third hopper shutter clutch (not shown).
The toner is configured so that it can be supplied to the toner supply port 305 of the developer 1300 shown in the figure.

The toner supplied to the toner supply port 305 of the developer 3300 is circulated in the direction of the arrow in FIG. 3 by two screws 303 arranged parallel to each other and rotated by the drive of a sleeve motor (not shown). ing.

In the image forming apparatus configured as described above, a case where image formation is performed in full color mode will be described below.

First, the photoreceptor drum 2 is
The outer peripheral surface of the mirror 5 is charged approximately evenly, and then a document (not shown) is scanned while being irradiated with the document illumination lamp 4, and the reflected light from the scanning is transmitted to the first mirror 5. The light is applied to a color image sensor 8 via a folding mirror 6 and a lens system 7, and an image is formed by the color image sensor 8.

When the reflected light is color-separated by the color image sensor 8 and a magenta image signal is generated by the color separation, the magenta image signal is transmitted to the laser beam scanner unit 2.
The laser beam E emitted from the laser beam scanner unit 71/29 is modulated by the laser beam E modulated with the magenta image signal, and image exposure is performed by the laser beam E emitted from the laser beam scanner unit 71/29. An electrostatic PfII image is formed.

The static 'itt N image formed on the photoreceptor drum 2 comes to the developing position by the simultaneous rotation of the photoreceptor drum 2.

The magenta developer 'aIM moves in accordance with the leading edge of this electrostatic latent image, and a lifter mechanism (not shown) causes the drum abutting roller 302 shown in FIG. The configuration is such that the distance between the outer periphery of the drum 2 and the developing sleeve 301 is kept constant. Simultaneously with this abutting operation, a sleeve motor clutch (not shown) for transmitting the drive of a sleeve motor (not shown) to the sleeve 301 and screw 303 is turned on. Four sleeve motor clutches are provided for each color.

Next, toner concentration detection and toner replenishment operations according to the present invention will be explained using FIGS. 5 to 8.

FIG. 5 is a schematic cross-sectional view of the developing device shown in FIG. A toner concentration detection section 50'O is arranged at a position where the toner concentration is obtained.

The toner concentration detection section 500 has a configuration as shown in FIG. 6, and includes a detection window 5oi made of a transparent member. The detection window 501 is configured so that toner and carrier do not adhere to the side that comes into contact with the developer. Further, the toner concentration detection section 500 has an auto toner regulator lamp (hereinafter referred to as "rATR lamp") 503, which irradiates the developer with light from the lamp 503, and the reflected light is received by the light receiving section 502. Toner density is detected.

Furthermore, the light receiving section 502 also receives direct light from the ATR lamp 503, and by comparing it with the initial state value of the ATR lamp 503, the signal amount of the light receiving section 502 can be corrected for changes over time. It has become.

The screw 303 is also rotated by the ON operation of the sleeve clutch, which is activated in synchronization with the leading edge of the electrostatic latent image formed on the photosensitive drum, and a stirring operation is performed within the developing device. In order to eliminate the influence of toner concentration in the stopped state, the ATR lamp 503 is turned on after a while, and after the toner concentration detection operation is performed several times, the ATR lamp 503 is turned on.
Wo? 1 itr, the amount of toner replenishment is calculated, and a toner replenishment operation is performed. Development is also performed simultaneously with these toner concentration detection toner replenishment operations, and the static Mls image just mentioned is developed by the magenta developer 41M.

On the other hand, referring to FIG.
The transfer material sent in the direction is electrostatically wound around the outer peripheral surface of the transfer drum 13 by the attraction charger 11 and the contact roller 12.

The rotational speed of the transfer drum 13 is normally set to be synchronized with the rotational speed of the photosensitive drum 2.

The toner image formed on the outer peripheral surface of the photosensitive drum 2 by the magenta developing device 1M is transferred onto the transfer material by the transfer charger 14.

The transfer drum 13 continues to rotate at the above-mentioned rotational speed, and continues to transfer the toner image formed on the outer peripheral surface of the photoreceptor drum 2 with a new color toner (cyan color in FIG. 1). Prepare for transcription.

On the other hand, the photoreceptor drum 2, which has finished transferring the magenta image,
The static electricity is removed by the static electricity removal charger 15, and the cleaning member 16
After the toner remaining on the photoreceptor drum z is cleaned by the primary charger 3, the toner is charged again approximately evenly by the primary charger 3, and subjected to image exposure by the laser beam E modulated by the cyan image signal. becomes.

Further, in the developing device, the cyan developing device IC is the first one of the moving body 1a.
By moving in the direction of the arrow in the figure, it waits at the above-mentioned developing position, and the static 'fr! , the latent image is developed to form a cyan toner image. at the same time,
- The toner e-degree detection operation is performed in the same way as when developing the magenta image, and the toner replenishment operation is also performed if necessary.It should be noted that, as a matter of course, the toner e-degree detection section 500 is arranged for each developing device. .

The same process is continued for the original image information corresponding to the yellow color and the original image information corresponding to the black color, and when the toner images for the four colors are transferred to the transfer material, the toner images are transferred to the transfer material H. 17. The four-color toner image is removed by the charger for static electricity removal.
After being neutralized at step 18 and reloaded by a charger 19, the transfer material is separated from the transfer drum 13 by a separating claw 20.

The transfer material separated from the transfer drum 13 by the separation claw 20 is sent to the fixing device 22 by the transfer material conveying belt 21, and the image forming sequence is completed and a desired full-color printed image is obtained.

FIG. 2 shows the configuration of an embodiment of a control system in which the image forming apparatus according to the present invention is in AW mode.

As is clear from FIG. 2, the control system of this embodiment includes the CPU 100, ROM10I, RAM102,
110103, a transfer drum home position sensor (hereinafter referred to as "transfer drum HP sensor j") 104, a pulse encoder 105, and the like.

The configuration of the control system mentioned above will be further explained as follows. That is, the ROM10I contains a control program and the like, and also stores non-volatile identification data. The RAM 102 stores various data given to the CPU 100 from outside via the I10103, and is also read out by the CPU 100. l10103 is CPU10
This is for connecting between 0 and various external devices.
Examples of external equipment include a sleeve motor 107 and sleeve clutches 108 for each gender. ATR lamp 109,
Toner supply clutch 110, hopper shutter clutch 1
There are 12 etc.

The transfer drum HP sensor 104 is disposed within a region where one detection target (not shown) provided on the rotating transfer drum 13 can be detected. Transfer drum H
The P sensor 104 outputs a predetermined pulse signal as a detection signal of a reference position set on the transfer drum 13 each time the object to be detected passes through the detection area of the HP sensor 104 due to the rotation of the transfer drum 13. do.

The pulse encoder 105 detects the number of rotations of the motor 35 that rotates the transfer drum 13 and outputs a predetermined pulse signal, and this pulse is sent to the CPU 10.
0 generates the timing to control various Ilo by counting, and the toner purpurity receiving light i'1ll11 for each sex is connected to the A/D converter 106, A/D compass is performed at the necessary timing, and the control data is RAM10
2 is stored and used.

Next, the control operation of the control system shown in FIG. 2 will be explained mainly with reference to the flowcharts of FIGS. 9 to 11. The flowcharts in FIGS. 9 and 10 show only the control from the start to the end of the developing operation, and the entire image forming operation will be supplemented in the main text.

When the operator turns on the drive power of the image forming apparatus, the CPU 100 starts the control operation and recognizes that the start key of the operation unit (not shown) provided in the main body 40 of the image forming apparatus has been operated. A series of image forming operations is started at the inner point.

When the start key is pressed, the size of the transfer material to be used is received from a paper size switch (not shown), and the RAM
1 as paper size information. After that, the transfer drum home position sensor 104 and the pulse encoder 10
By counting the pulse signals from 5, various l10
103 to control various devices necessary to execute the image forming operation.

Based on the output signal from the potential sensor 28, the output of the primary charger 3 is controlled so that the outer peripheral surface of the photoreceptor drum 2 is uniformly charged, and the pickup roller 25 or 27 is moved from a designated paper feed cassette. The transfer material is supplied to the transfer drum 13 through the transfer drum 13.

At this time, in the case of one sheet [I] in the continuous copy operation, the transfer drum cleaner 34 cleans the outer periphery of the transfer drum 13 and the charger 17 performs a charge removal operation.

The color image sensor 8 modulates the laser light emitted from the laser beam scanner unit 29 using the color-separated image signals to form an electrostatic latent image on the outer periphery of the photosensitive drum 2. Drive of the pair of registration rollers 10 is controlled so that the skew of the transfer material is removed and the leading edge of the transfer material in the moving direction reaches a predetermined position n on the outer peripheral surface of the transfer drum 13 in synchronization with the transfer drum HP signal. do.

In order to attract the transfer material supplied at a predetermined timing from the pair of registration rollers 10 to the transfer sheet of the transfer drum 13, the drive of the attraction charger 11, which is a corona (1) electrical device with a polarity opposite to that of the developer, is controlled. , a contact roller 1 which is a grounded conductive roller and serves as a counter electrode of the adsorption charger ll.
2, charges are injected into the transfer material.

Adsorption charger 11. When the transfer material is attracted onto the outer circumferential surface of the transfer drum 13 by the contact roller 12, the developing operation starts almost simultaneously (3700).Since the developing operation for each color is almost the same, the developing operation for magenta will be explained in detail only.

Electrostatic PP! The developing unit IM, which has moved so that it can develop the leading end of the image, turns on the sleeve clutch 108 in order to transmit the drive of the sleeve motor 107 for developing (57
01), the time this sleeve clutch is ON is the same as the paper size to be developed, and this time is T, 5ize
shall be. If the time from detecting the toner concentration to starting toner replenishment is Tgtart, the toner replenishment time is maximum (Tsize - Tstart )L or replenishment is not possible.To solve this problem, increase the toner replenishment capacity per fixed interval. It would be fine if it were opened, but this would cause problems as it would increase the size of the machine and cause toner clogging.

To this end, according to the invention, T 5tart in FIG.
To use the time effectively, that is, the time when replenishment could not be done last time (replenishment time)
By performing this during 5 tarts, real-time and highly accurate toner concentration control can be performed even with the minimum toner replenishment capacity.

Figures 7 and 8 show the Nth <(
This figure schematically shows the N+1)th toner replenishment operation. Assuming that the toner replenishment operation at the time Tstart described above is pre-replenishment, FIG. 7 shows the case where there is no pre-replenishment.

Returning to the flowchart in FIG. 9 and continuing the explanation, immediately after the sleeve clutch is turned on (S701), it is confirmed whether there is a replenishment stiffness time Tremain (S702). If there is a replenishment stiffness time, this time is the previous replenishment time. Check whether Tstart is greater than the maximum value Tstart (370
3) If it is large, the stiffness replenishment time Tremain (T rem
ain -T 5tart) (37
04), if it is smaller than this, all remaining replenishment time can be replenished, so the period between Tremains is cleared ((3705), and if there is a replenishment time Tremain in step 5702, the unillustrated The hopper shutter is opened and pre-supply is started (S706).

The replenishment operation is automatically completed, the hopper shutter is closed, and the operation is stopped.

Before turning on the sleeve clutch 108, screw 3
03 was in a stopped state, so after a predetermined period of time (
S707), the ATR lamp 109 is turned on, and toner density sampling is performed multiple times (5708).

Next, from the measured results, the toner replenishment time T5upp1
However, since the required replenishment amount is determined based on the toner replenishment capacity for a certain period of time and the toner concentration information converted by the A/D converter 106, the Tsupp
ly can be determined (S709). However, since the maximum toner supply time is (Tgize - Tstart), if Tsupply is larger than this, (371
0), the remaining time is added to the toner replenishment time Tremain (S711), and the maximum time (T 5ize - T 5tart ) is set for Tsupply (S712), this determined toner replenishment time T 5u
After opening the hopper shutter by ppD (3723
),) While performing the toner replenishment operation and the development operation at the same time, after the replenishment operation is completed (3715), close the hopper shutter (
3716), and if the development operation is finished (5717),
Set the sleeve clutch to 0FFI (3718) and end the operation.

Further, the transfer drum 13 is rotated simultaneously by the motor 35, and when the transfer material reaches the part where the adsorption charger 11 is disposed, an electric charge of opposite polarity to that of the developer is applied to the back surface of the transfer sheet, and the first Perform the second transcription.

As described above, the developing device moves the developing device IM and positions the other developing device at the predetermined position. As a result, a visible image is formed on the photoreceptor drum 2 using a developer of another color.

In this way, the visible images are superimposed and transferred onto the transfer material by rotating the transfer drum 13, and when the four transfer steps in this embodiment are completed, the charger for weight removal 17 and 18 is transferred.
is driven to eliminate static electricity from the transfer material and weaken the attraction force of the transfer material to the transfer sheet, and the transfer material is separated from the transfer sheet and discharged outside the machine after being fixed.

In the embodiment described above, the maximum time during which toner can be replenished is until the end of the developing operation, but according to another embodiment, the hopper replenishment port and the developing Since the replenishment ports overlap, the object of the present invention can also be achieved by setting the maximum time during which toner can be replenished in the above-described embodiment to the time when the developer starts moving to the development position of the next color.

Historically, in the embodiments described above, the toner replenishment operation could only be performed when the developing device came to the developing position, but this is not limited to this. For example, the supply ports 404M and 40 of the connection part shown in FIG.
4G, 404Y, and 4048K are configured so that they always overlap the supply port 305 on the developing device side regardless of the position of the developing device. That is, the connecting portion is a flexible hose that can accommodate changes in the mutual positional relationship due to movement of the current R unit.

Therefore, according to this embodiment, as an extreme example, magenta toner replenishment operation is also possible during cyan development. However, in this case, a drive source for the toner replenishment system is required to have a large torque margin. Also, if the screws in the four developing units are driven by one drive source (sleeve motor), magenta toner supply is stopped during cyan development! Load fluctuations may occur when the cyan color developing operation is changed. In order to solve this problem, it is necessary to prepare a corresponding sleeve motor for each developing device.
This is not desirable as it causes an increase in costs.

Therefore, even in a configuration in which the hopper and the developing device are connected by a flexible hose, the maximum toner replenishment time is from the time the toner concentration is detected until the time when the next color's developing operation is started. FIG. 12 shows these relationships in a simple timing chart.

In this embodiment, unlike the first embodiment, the toner replenishment time includes the moving operation of the developing device, so there is no need to increase the toner replenishment capacity.

In addition to this, the time from the start of the development operation to the end of toner concentration detection can be used as the toner replenishment margin, that is,
Since the toner can be replenished during the next development, it is possible to further reduce the toner replenishment capacity.

As described above, the image forming apparatus according to the present invention uses the toner concentration detection time, which was not included in the conventional replenishment time, to absorb the previous toner replenishment time, thereby replenishing the toner supply time. The replenishment capacity and toner capacity in the developing device can be minimized, the device can be made more compact, and toner density control can be performed almost in real time, making it possible to perform highly accurate toner density control. It has the advantage of being able to form high-quality images.

[Brief explanation of drawings]

The first factor is a schematic cross-sectional view of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram of the control system of the apparatus shown in FIG. FIG. 3 is a perspective view of the developing device. FIG. 4 is a perspective view of the replenishment means. FIG. 5 is a cross-sectional view of the developing device of FIG. 3. FIG. 6 is a sectional view of the toner concentration detection section. FIG. 7 is a timing chart illustrating a conventional toner replenishment operation. FIG. 8 is a timing chart illustrating a toner replenishment operation according to the present invention. FIG. 9, FIG. 10, and FIG. 11 are flowcharts illustrating the toner replenishment operation according to the present IJ+. FIG. i2 is a timing chart illustrating a toner replenishment operation according to another embodiment of the present invention. l: Developing? C position 2: Photosensitive drum 301: Developing sleeve 400: Toner replenishment means 500: Toner density detection section

Claims (1)

[Scope of Claims] 1) At least one developing device, a replenishing means for replenishing developer to the developing device, a concentration detecting means for detecting the concentration of the developer, and a replenishment time determined by the concentration detecting means. It is equipped with a calculation means for calculating and a storage means for storing the replenishment time, and if the replenishment time is longer than the maximum replenishable time after the developer concentration is detected, the replenishment time is stored until the end of the next developer concentration detection operation. An image forming apparatus characterized in that: 2) The image forming apparatus according to claim 1, wherein in an image forming apparatus equipped with a plurality of developing devices, the maximum replenishable time is the time until the developing devices are moved for development of the next color. 3) The image forming apparatus according to claim 1, wherein in an image forming apparatus equipped with a plurality of developing units, the maximum replenishable time is the time until the start of development of the next color.