JPH03260668A - Image forming device - Google Patents

Abstract

PURPOSE:To faithfully reproduce density and a thin line by discriminating whether the cause of the lowering of image density is the set condition of a developing device or a pattern latent image forming condition and controlling regulating gap or the electrostatic charging potential of a photosensitive body. CONSTITUTION:A pattern latent image is formed on a photosensitive body 1 and developed by a 1st developing device 10 so as to form a 1st toner pattern image TP1. Then, the density D1 of the TP1 is detected by a density sensor 17. Next, an electrostatic charger 2 is turned off and the bias of the developing device 10 is switched to +500V to form a 2nd toner pattern image TP2, and the density D2 is read by the sensor 17. When D1<DA and D2<DB hold between 1st reference density DA and 2nd reference density DB, one or both of the regulat ing gaps Db1 and Db2 of the developing devices 10 and 11 are widened. When D1<DA and D2>DB hold, the electrostatic charging potential of the photosensi tive body 1 is made high. Thus, the density and the thin line are faithfully reproduced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an image forming apparatus such as a copying machine or a printer using an electrophotographic copying process.

(Prior Art) Conventionally, with respect to image forming apparatuses such as copying machines and printers that form toner images using a two-component developer consisting of toner and carrier, the following two methods have been used to optimize image density. is proposed.

First, a pattern latent image formed on the surface of a photoconductor is visualized as a toner pattern image using a developing device, and the image density of this toner pattern image is read by a reflective photosensor. If the density is lower than a predetermined standard density, there is a method of replenishing toner to the developing device (hereinafter referred to as rA I).
It is called the DC method. ).

Another method is to install a magnetic sensor in the developing device to measure the toner concentration of the developer, that is, the weight mixing ratio of toner to carrier, and if the measured toner concentration is less than the standard toner concentration, toner is replenished into the developing device. method (hereinafter referred to as “ATDC
It is called "law". ).

When comparing the two, the ATDC method calculates the image density indirectly by measuring the toner density, whereas the ATDC method calculates the image density indirectly by measuring the toner density.
The method measures the image density of the actual image formed on the photoreceptor,
Since toner density control is performed based on this, it can be said that this method is highly reliable.

By the way, conventionally, as an image forming apparatus,
In Japanese Patent No. 7783, two magnetic plan developing devices containing toner of the same color are provided on the side of a photoreceptor, and these developing devices are driven almost simultaneously, and the electrostatic latent image developed by the first developing device is transferred to the second developing device. It has been proposed that the image be developed again using two developing devices.

In this image forming apparatus, the first developer ensures image density, the second developer faithfully reproduces fine lines to the original, and an image with excellent density reproducibility and fine line reproducibility is formed.

Therefore, strict image density control is not necessarily required for the second developer, which aims to improve the reproducibility of fine lines, but highly accurate image density control is performed for the first developer, which aims to ensure image density. In this case, it is preferable to manage the image density by the ATDC method, which is more reliable.

(Problem to be Solved by the Invention) However, the image density is determined by the formation conditions of the electrostatic latent image and the setting conditions of the developing device. That is, a decrease in image density is not caused only by a decrease in toner concentration of the developer, but may also occur due to poor bias conditions or charging conditions.

For this reason, even if the toner density is normal, if the image density has decreased due to bias failure, for example, if toner is replenished to the developing device according to the ATDC method, the toner density of the developer will become higher than necessary. (However, there were problems such as background fogging on the image and toner scattering, leading to contamination of peripheral equipment.

In order to solve this problem, it may be possible to use the ATDC method in combination and regulate the upper limit so that the toner concentration of the developer does not exceed a predetermined value, but the cause of the decrease in image density may be caused by something other than the toner concentration. In this case, a problem arises in that the necessary image density cannot be guaranteed.

(Means for Solving the Problems) Accordingly, the present invention provides an image forming apparatus that develops electrostatic latent images multiple times by driving the plurality of developing devices simultaneously, which detects the cause of image density reduction and corrects the problem. An object of the present invention is to provide an image forming apparatus that recovers image density through appropriate control.

In view of the above object, the present invention includes a photoreceptor (1), a magnetic plan type first developer 2g (10) and a second developer (11), which are formed on the photoreceptor (1)-h. The electrostatic latent image
After developing with the developing device (10), the second developing device (11)
), the image forming apparatus comprises: a pattern latent image forming means (2, 9) for forming a pattern latent image (1), ) on the photoreceptor (1); a first pattern image forming mode (step 521) in which a first toner pattern image (T P , ) is developed by the developing device (10), and iii. a second pattern image forming mode (step 524) in which a second toner pattern image (T P t) is formed on the photoreceptor by applying a bias of the same polarity; The image density measuring means (17)
The image density (Dυ, (D
, ) is detected in the image i8 degree detection mode (step S21
．． 524), a first comparison mode (step 523) for comparing the image density (Dl) of the first toner pattern image (T P , ) and a first base 1 density (DA), vii
Image density (D) of the second toner pattern image (TP, )
a second comparison mode (step 525) in which the image density (Dl) of the first toner pattern image (TP, ) is compared with a second reference density (D, ); (DA), and the image density (Dt) of the second toner pattern image (T P t) is the second reference density (
D, ), the gap adjustment mode (step 826) widens the regulation gap (D, , D□) of at least one of the first developing device (10) and the second developing device (l l). , 1x The image density (Dl) of the first toner pattern image (T P , ) is lower than the first base 1 density tffi (DA), and the image density (Dl) of the second toner pattern image (TP
1) a charge potential change mode (step 527) in which the charge potential (V O ) of the photoreceptor (1) is increased when the charge potential (D, ) is higher than the second reference density (D, );

(Embodiments) Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

1. Configuration of image forming device FIG. 1 shows the main parts of an image forming apparatus.

In the figure, a photoreceptor 1 is a cylindrical body having an organic photoreceptor layer on its outer periphery, and is designed to rotate at a speed of 150 am/sec in the direction of arrow a.

Around the photoconductor 1, along the direction of rotation, there are a charger 2, an inter-image eraser 9, a first developer lO1, a second developer 11, a transfer charger 12, a separation charger 13, a cleaning device 14, and a main eraser 15. It is arranged.

The charger 2 includes a charge stabilizing plate 3 arranged parallel to the photoconductor l, a charge wire 4 stretched over the charge stabilizing plate 3, and a grid covering an opening of the charge stabilizing plate 3 facing the photoconductor l. , Tome.

7[j6] is connected to the charge wire 4, and another power source 7 is connected to the grid meson unit 5.

The first developing device 10 is a magnetic brush type developing device using a two-component developer consisting of toner and carrier.

In the first developing device 10, the developing roller 20a is composed of a fixed magnet body 21a and a cylindrical sleeve 22a.

The sleeve 22a faces the photoreceptor 1 with a developing gap D81, and a regulating plate 23a is provided on the upper outer circumferential surface of the sleeve 22a so that the regulating gap D81 is provided.
b+, and the regulation gap D bl can be adjusted by a gap adjustment device 40 shown in FIGS. 2-4. Further, the sleeve 22a is connected to the power source 24a, and the sleeve 22a is connected to the power source 24a, and is
It is designed to rotate at a speed of pm.

The rear space of the developing roller 20a is connected to the front conveyance path 2 by a wall 25a.
6a and a rear conveyance path 27a, which are connected to each other via passages (not shown) provided on the prefecture side and the front side of the wall 25a, and a bucket roller 28a and a conveyance blade 29a can be driven to rotate in the conveyance paths 26a and 27a, respectively. It is located in Further, developer is stored in the transport paths 26a and 27a, and the toner haze is measured by a toner concentration detection sensor 30a provided at the bottom of the front transport path 26a.

The toner supply section 31a stores toner, and the toner is supplied to the rear conveyance path 27a by driving the supply roller 32a with a motor 33a.

In the cap adjustment device 40, the regulation plate 23a is supported by a guide portion 45 so as to be movable forward and backward relative to the sleeve 22a. The drive shaft 41 is disposed along the regulation plate 23a, supported by a bearing (not shown), and driven by a motor 43 to rotate in forward and reverse directions. In addition, the drive shaft 4
1 are provided with gears 42, 42 at both ends thereof, and these gears 42, 42 mesh with a rack 44 provided on the regulating plate 23a. Therefore, by driving the motor 43, the regulation plate 23a moves in the directions of arrows c and c' via the drive shaft 41, gears 42, 42, and rack 44, and the regulation gap D between the regulation plate 23a and the sleeve 22a, is adjusted.

The second developing device 11 has almost the same configuration as the first developing device 10, or has a fixed regulation gap without the cap adjustment device 40, and has a photo sensor on the top facing the photoreceptor 1. This is different from the first developing device 10 in that I7 is provided.

Note that 20a is a developing roller, 21b is a magnet, 22b is a sleeve, 23b is a regulating plate, 24b is a power source, 25b is a wall, 26b is a front transport path, 27b is a rear transport path, 28b is a bucket roller, and 29b is a transport 30b is a toner concentration detection sensor, 31b is a toner replenishment unit, 32b is a replenishment roller, 33b is a motor, Db2 is a regulation cap, Ds
t indicates the development gap.

Further, a photosensor 17 is provided above the second developing device 11 so as to face the photoreceptor 1 .

FIG. 5 shows the circuit configuration of the image forming apparatus. This circuit is composed of a control device 100, a comparator +02,
Signals from the toner concentration detection sensors 30a, 30b and other signals are input, and operation signals are output to the power supplies 6.72/Ia, 24b, motors 33a, 33b, and motor 43, as well as other signals. ing.

4i, , , , , : image, image, shape, formation The processing contents of the control device +00 will be explained below.

First, the operation of the first developing device 10 will be explained.

In the first developing device 10, the developer is transferred to the packet roller 28a.
It is circulated and conveyed through the conveyance paths 26a and 27a by the conveyance blade 29a. That is, the developer in the front transport path 26a is transported from the back side to the front side, and is sent to the rear transport path 27a via the communication path at the front end.

Further, the developer in the rear conveyance path 27a is conveyed from the front side to the back side, and at the rear end, it passes through the connecting 1M path to the front conveyance path 27a.
Sent to a. The developer is conveyed through the transport paths 26a and 27a.
are mixed as they are transported, and the toner is triboelectrically charged to a positive polarity and the carrier to a negative polarity.

Further, the developer transported through the front transport path 26a is supplied to the developing roller 20a by a bucket roller 28a. The developer supplied to the developing roller 20a is attracted by the magnetic force of the magnet 21a and held on the outer periphery of the sleeve 22a, and is conveyed in the direction of the arrow in the state of a magnetic brush based on the rotation of the sleeve 22a, until it reaches the regulating gear knob Db+. Only the developer that has passed through the sleeve 22a and the photoreceptor 1 face each other (hereinafter referred to as "
"Development area". ) X4, passes through the development gap Dsl while contacting the photoreceptor 1, and then falls into the front conveyance path 26a at the portion facing the packet roller 28a and is mixed with the developer.

The toner concentration of the developer is detected by the toner concentration detection sensor 30a, and when it is determined that it is lower than a predetermined standard vP4 degree, the motor 33a is driven, and the toner is replenished from the toner replenishing section 31a to the rear conveyance path 27a, and the developing The toner concentration of the agent is maintained almost constant.

The second developing device 11 performs the same processing as the first developing device 10. ], and the explanation will be omitted.

Next, a description will be given of multiple development processing in which an electrostatic latent image formed on the outer periphery of the photoreceptor 1 is developed by rotating the first developing device 10 and the second developing device 11 substantially simultaneously. At this time, the first
In the developing device 10 and the second developing device 11, the developing cap D8.
= 0.6 mm, Dst = 1.0 mm, regulation cap D Ill + D bt = 0, set to 5 mm, and the biases V B l + V B t applied to the sleeves 23 a, 23 b from the power supplies 24 a, 24 b are both -150. ■It is set to.

While the photoreceptor 1 is rotating in the direction of arrow a, the charger 2 has a charge wire 4 connected to the power source 6 by approximately -
Voltage ■ is applied to 6, and power supply 7 is applied to grid main 4.
A bias of about -600 .mu. is applied. the result,
The surface of the photoreceptor 1 passing through the charging area X is charged to -600V.

The charged surface of the photoreceptor 1 is exposed to the image light 8 in the exposure area X2, and an electrostatic latent image corresponding to the image to be reproduced is formed.

The surface of the photoreceptor 1 on which the electrostatic latent image has been formed is illuminated with an inter-image eraser 9 in the static elimination area x3, and the charges in the non-image area are erased.

The electrostatic latent image that has passed through the static elimination area X3 is transferred to the first development area x
4, toner is supplied from the developer passing through the developing area X4 as the sleeve 22a rotates, based on the electrostatic contrast between the bias ■8I (150V) and the potential of the electrostatic latent image image area. Developed as an image.

The electrostatic latent image that has passed through the first development area X4 is conveyed to the second development area Toner is again supplied from the second developing device 11 to reproduce the image.

As shown above, the toner image formed in the development areas X, , X, is transferred to the transfer area
The image is transferred onto the recording medium 16 carried by the transfer char/+12 discharge.

The recording medium 16 to which the toner image has been transferred is stored in a separate charger 1.
The toner image is separated from the surface of the photoreceptor 1 at step 3, and the toner image is heated and fixed by a fixing device.

On the other hand, the cleaning device 14 removes residual toner from the surface of the photoreceptor 1 that has passed through the portion facing the separation charger 13, and the main eraser 15 erases the residual charge in preparation for the next charging.

Here, in both the first developing device 10 and the second developing device 11, the regulation gear +'yb Dbl + Dbt is the same (-
0.5 mm) and development area X4. The amount of developer transported per unit time to X and X is set to be the same.

On the other hand, the development camp DS+ of the first development area X4 is 0.
6 mm, and the development gap Dst of the second development area It is higher than X.

Did you do this?), the static TX passing through the first development area X4?
Sufficient toner is supplied to the Fi image, and solid images and character images of normal thickness are visualized with the necessary density.

However, due to the high frequency of contact with the developer, the toner adhering to the surface of the photoreceptor 1 is often scraped off, and the reproducibility of fine lines is not necessarily good.

In the subsequent second development area
Here, the toner adhering to the thin line electrostatic latent image passes through the development area X5 without being scraped off.

Therefore, on the image reproduced on the recording medium 16, the rough image and the normal character image are reproduced with sufficient density, and even the fine lines are reproduced faithfully to the original.

■1 Image density control 6. Image density control of the image forming apparatus. This will be explained with reference to the flowcharts in FIGS.

(1) Copy control routine (see FIG. 6) In this routine, in step Sll, it is determined whether or not a print key (not shown) has been pressed, and if it is determined that the print key has been pressed, the process advances to step S12. An image density control subroutine to be described later is executed.

When the execution of the image density control subroutine is completed, a paper feed conveyance control subroutine, an optical system control subroutine, and a photoconductor rotation subroutine are respectively executed in steps S13 to 315, and then a toner replenishment control routine is executed in step S16 (not shown). Return to main routine.

(11) Image density control subroutine (see FIG. 7) In this subroutine, in step S2+, a toner pattern image for image density detection is created and the image density is detected. Specifically, as shown in FIG. 8, the outer periphery of the photoreceptor l rotating in the direction of arrow a is charged by the charger 2 (step 534). Next, as shown in FIG. 1O, the inter-image eraser 9 is blinked to form a pattern latent image P3 (step 535), and the first developer 10 is driven to form the pattern latent image P1 using the toner particles shown in the figure. pattern image TP,
Visualize as. Further, the image density of the toner pattern image TP is read by the photosensor 17 (step 5).
36).

In step S22, the toner pattern image TP on the photoreceptor 1 is
, the bias voltage VB of the second development Wll(7) development sleeve 22b.

Change to -500V.

As a result, as shown in FIG. 12, the positively charged toner constituting the toner pattern image TP is collected into the sleeve 22b by electric force.

Therefore, the toner of the toner pattern image TP is not wasted and is used for development in the second developing device 11.

Subsequently, in step (S23), a signal corresponding to the image density is input from the photosensor 17 to the comparison input terminal of the comparator 102. Also, the power supply 10 is connected to the comparator +02.
1 to a voltage signal corresponding to the first reference concentration DA is input. Note that the first base $ concentration DA is a concentration that regulates the lower limit. Then, the comparator 102 compares the voltages of the respective input signals.

As a result, if Dl<DA, that is, the image density, or the first reference density r)A, the comparator 102 outputs a signal to the control device lOO, and the control device 100 performs step (S
24) is executed.

In step (S24), another image density detection pattern is formed and its image density is read.

Specifically, as shown in FIG. 9, the charger 2 is turned off while the photoreceptor is rotating (step 537).
, the surface of the photoreceptor 1 is maintained in an uncharged state. That is,
The resulting image density does not include any factors that may cause deterioration of the charger 2 or the photoreceptor 1. Next, the bias VB+ of the first developing device 10 is switched to +500V (step 538), and the positively charged toner is electrically repelled to fly toward the photoreceptor l, thereby forming a toner pattern image TP shown in FIG. The photo sensor 17 reads the image density (step 539). In addition,
Similar to the toner pattern image TP, the toner constituting the toner pattern image TP is electrically collected by the sleeve 22b to which -500 cm is applied in step S22.

Therefore, toner is not wasted.Subsequently, in step S25, the second reference density DB and the haze level of the image l are compared.

Then, D, <Da, that is, the toner pattern image T)),
If the image density is lower than the second base dJ', in degree DR, the cause of the decrease in image density is the setting conditions of the developing device 10. This is because, as described above, the toner pattern image TP is created with the photoreceptor 1 kept in an uncharged state, eliminating factors that affect the deterioration of the photoreceptor 1 and charging.

Therefore, in step S26, the motor 13 of the cap adjustment device 40 rotates the drive shaft 41 in the direction of the arrow n to move the regulation plate 23a in the direction of the arrow C' force, and the regulation gear knob D5.
The image density is restored by increasing the width of the image.

On the other hand, if D≧D8, that is, the image l haze of the toner pattern image TP2 is higher than the second reference density D8, the image density D1 of the toner pattern image TP is lower than the reference value [)A. Contrary to the above case, it is determined that the cause is the latent image forming conditions such as deterioration of the photoreceptor or charging.

Therefore, in step S27, the output of the filter source 7 of the grid unit 5 is increased.

As a result, the charged potential of the photoreceptor l becomes ■. It is attempted to improve the image density and restore the image density.

As mentioned above, whether the cause of the decrease in image density or the first development mH]0
If the decrease in image density is due to the setting conditions, the image density can be restored by driving the motor 43 and spreading the regulation cap Db, and if the decrease in image density is due to deterioration or charging of the photoconductor, the charging Rank V.

is adjusted. That is, the image density can be recovered by eliminating the cause of the decrease in image density.

If it is determined in step S23 that DA≦D1, that is, the image density D1 satisfies the first reference density DA, the process advances to step S29.

In step S29, the image density is compared with a third reference density DC. This third reference l emptiness Dc is a concentration that regulates the upper limit.

As a result of the comparison, D1≦DC1, that is, the image density is the lower limit regulation density DA. If the concentration is at an appropriate concentration (DA<DI≦D,), step 32
Proceed to step 8.

If D + > D c, that is, the image density D1 exceeds the upper limit regulation density Dc and is too dark, the process advances to step S30;
It is determined whether the motor 43 rotates normally (rotation in the direction of arrow n), that is, whether the developing gap D5I is widened. If the developing cap I)s+ is wide, then in step S31, the motor 43 is reversed, the drive shaft 41 is rotated in the direction of the arrow n, the regulating plate 23a is brought close to the photoreceptor 1, and the regulating caps D and 1 are closed. Narrow it to reduce image density. On the other hand, the motor 43 is not rotating normally, and the regulation gear and pulley
) 51 is in a narrow state, it is no longer possible to narrow the regulation cap 〕ゎ, so step S32
Then, the image density is lowered by adjusting the charging conditions.

In step S32, it is determined whether or not the output of the power supply 17 is in a high voltage output state, and if it is in a high voltage output state, step S32 is performed.

In step S33, the output of the power source 7 is lowered to reduce the charged potential of the photoreceptor 1 (■). is adjusted so that it is low, and the process proceeds to step S28.
On the other hand, if the output of the power supply 7 is in a low voltage output state, step 328
Proceed to.

In step (828), the output of the electric FA 24a is changed to return the developing bias to VB-1-+50V, resulting in a normal image forming state.

,! ! ! ,,,,,fl! In the above explanation, when it is assumed that the cause of the decrease in image density is in the first developing device 10, the regulation gear Dbl is adjusted.
A gap adjustment device 40 is also provided in the second developing device 11, and the second developing device 11
The image density may be restored by adjusting the regulation cap Db2 of the developing device 11 or by adjusting both the development gap D s + and OSt.

Furthermore, in the above description, the toner pattern image T))TP,
may be collected in the second developing device 11 or may be collected in the cleaning device 14. In this case, especially for the second toner pattern image T゛P, the time for setting the developing bias VB+ to 500■ is as short as possible, and the pattern image T
Reduce Pt!・Try to reduce wasteful consumption of ner.

Furthermore, although the photosensor 17 is provided at the second portion of the second developer 2H11, it may be provided at the bottom of the cleaning device 14.

Furthermore, in the embodiment, in order to make the developer filling rate of the developing area where the developing sleeve of the developing device faces the electrostatic latent image carrier higher in the first developing device 10 than in the second developing device 11. , an example in which the developing gear knob DS+ of the first developing device 10 is made smaller than the developing cap D5 of the second developing device 11 has been described; The amount of developer conveyed to the developing area X4 of the first developing device 10 by making it larger than Dbt or by making the peripheral speed of the sleeve of the first developing device 10 higher than the peripheral speed of the sleeve of the second developing device 11. may be made larger than the amount of developer conveyed to the developing area X5 of the second developing device 11.

(Effects of the Invention) As is clear from the following explanation, the image forming apparatus according to the present invention develops the same electrostatic latent image several times using a plurality of developing units. In image density control, determine whether the cause of the decrease in image density is due to the developing device setting conditions or the pattern latent image formation conditions, and if the cause is due to the developing conditions, adjust the regulation cap and remove the latent image. If the formation conditions are the cause, the charged potential of the photoreceptor is adjusted.

Therefore, even though the toner concentration of the developer is maintained at an appropriate level, toner is not further replenished to the developer, and a high-quality image without background fog can be obtained.

[Brief explanation of drawings]

Fig. 1 is a sectional view of the main parts of the image forming apparatus, Fig. 2 is a sectional view of the gap adjustment device, Fig. 3 is an enlarged side view of the cap adjustment device, and Fig. 4 is a front view showing the overall configuration of the gear knob adjustment device. , FIG. 5 is a control circuit diagram, FIG. 6 is a flowchart of a copy control routine, FIG. 7 is a flowchart of an image density control subroutine, FIG. 8 is a flowchart of a subroutine for forming a toner pattern image, etc., and FIG. 9 is a flowchart of a subroutine for forming a toner pattern image. is a flowchart of a subroutine such as the shape of a toner pattern image,
FIG. 10 is a perspective view showing a pattern latent image formed on a photoconductor and a toner pattern image developed therefrom; FIG. 11 is a perspective view showing a toner pattern image formed on a photoconductor;
FIG. 12 is an explanatory diagram showing the scraping action of the second developing device. DESCRIPTION OF SYMBOLS 1... Photoreceptor, 2... Charger, 9... Inter-image eraser, 10... First developer, 11... Second developer, 20a, 20b... Developing roller, 22a, 22b
- Sleeve, 23a, 23b--Restriction member, 24a. 24b...Power supply, 40...Gap adjustment device, D9
+D st...Development gap, D III+ I)
by...regulatory cap, p,,p,...pattern latent image, TP,,TP,...toner pattern image.

Claims (1)

[Claims] (1) Two-component developer is held on the outer periphery of a sleeve that rotates around the photoreceptor and the magnet, and the amount of developer conveyed to the development area is adjusted by a regulation plate that faces the sleeve with a regulation gap. The first developing device and the second developing device contain toner of the same color, and the electrostatic latent image formed on the photoreceptor is transferred to the second developing device. An image forming apparatus configured to perform development with one developer and reproduce the image with a second developer, comprising: a pattern latent image forming means for forming a pattern latent image on the photoconductor; and a pattern latent image forming means for forming the pattern latent image on the first developer. a first pattern image forming mode in which a first toner pattern image is formed by developing the toner pattern image, and a second toner pattern image is formed on the photoreceptor by applying a bias having the same polarity as the toner charging polarity to the first developing device. a second pattern image forming mode in which the first toner pattern image and the second toner pattern image are formed; an image density measuring means; and an image density detection mode in which the image density measuring means detects the image density of the first toner pattern image and the second toner pattern image. , a first comparison mode for comparing the image density of the first toner pattern image and a first reference density; and a second comparison mode for comparing the image density of the second toner pattern image and a second reference density. when the image density of the first toner pattern image is lower than the first reference density and the image density of the second toner pattern image is lower than the second reference density;
a gap adjustment mode that widens the regulation gap of at least one of the first developing device and the second developing device; the image density of the first toner pattern image is lower than the first reference density;
An image forming apparatus further comprising: a charging potential change mode for increasing the charging potential of the photoreceptor when the image density of the second toner pattern image is higher than the second reference density.