JP2011053392A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus wherein excessive deterioration of a developer is suppressed without formation of as toner band, even when a low-coverage continuous printing state continues over a long period. <P>SOLUTION: The image forming apparatus includes a developing device which is allowed to select one of two image forming operation modes of a cost-per-print priority mode and an image quality priority mode. In the cost-per-print priority mode, a fundamental amount B (mg) of carrier is supplied at intervals of a prescribed time. In the image quality priority mode, if the amount of toner supplied to a developer storage section for a prescribed time is smaller than a prescribed amount A(mg), an amount C (mg) of carrier calculated by formula (1): (the amount C of carrier to be supplied) = ä(A-(amount (mg) of toner supplied to the developer storage section))<SP>2</SP>/k}+B (wherein k is a coefficient selected to be in the range of 0.2Dw≤k≤2Dw, wherein Dw(g) is the amount of the two-component developer in the developer storage section) is supplied at intervals of a prescribed time. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus that performs development using a two-component developer.

Conventionally, as an electrophotographic image forming apparatus, a two-component developer (hereinafter also simply referred to as “developer”) composed of a toner and a carrier is used. A developing device that forms a toner image by adhering the toner to an electrostatic latent image formed on an image bearing member, and replenishing new toner and carrier continuously or intermittently. There is known an image forming apparatus equipped with a so-called trickle developing type developing device that gradually discards used developer.
In general, a two-component developer is stressed by an agitating operation, and an external additive such as a lubricant added to the surface of toner particles constituting the toner is detached from the toner particles, or conversely buried in the toner particles. As a result, the chargeability of the developer decreases and the charge amount of the toner becomes low, or the fluidity of the developer decreases and the coefficient of friction between the carrier and the toner becomes high. When the carrier contamination phenomenon occurs where the toner adheres to the carrier, new toner is replenished continuously or intermittently by using this trickle development method, thereby supplying both external additives. Therefore, excessive deterioration of the developer is suppressed. Even if the external additive alone is replenished into the developing device, it is difficult to bring the toner and carrier into uniform contact.
In a general trickle developing type developing device, a predetermined amount of carrier is put in a toner bottle or toner cartridge in which toner for replenishment is stored, so that the carrier is also supplied when the toner is replenished. The composition to be replenished is adopted.

However, in such a trickle development type developing device, a state in which a document having a low printing rate (coverage) such as a character document is continuously printed (hereinafter also referred to as “low coverage continuous printing state”) is long. When the toner is used for image formation, the toner density in the developing device is hardly lowered due to a very small amount of toner. As a result, the developer is excessively deteriorated without replenishment of new toner, In the formed visible image, image fogging or toner scattering due to a decrease in chargeability and fluidity of the developer occurs.
On the other hand, if the toner is replenished even though the toner concentration in the developing device hardly decreases, the toner concentration in the developing device becomes excessively high, resulting in image fogging and toner scattering. There is a problem that.

  In order to solve such a problem, conventionally, when a low coverage continuous printing state continues for a long time, a solid image having a high printing rate, for example, solid black, is formed in a non-image forming area between the toner images on the image carrier. The toner band is formed and the toner is forcibly consumed (see, for example, Patent Document 1 and Patent Document 2).

  However, the toner band formed for forcibly consuming the toner in this way is cleaned as it is without being transferred to the recording material at all. Since the transfer residual ratio is about 10 wt%, a considerable amount of untransferred toner has to be cleaned per unit area, which causes a problem that a large load is applied to the cleaning member.

Japanese Patent Laid-Open No. 11-2946 JP 2008-309921 A

The present invention has been made on the basis of the above circumstances, and the object of the present invention is also when a low coverage continuous print state continues for a long time without forming a toner band for forcibly consuming toner. It is an object of the present invention to provide an image forming apparatus capable of forming a good image free from image fogging and toner scattering over a long period of time while suppressing excessive deterioration of the developer.
Another object of the present invention is to provide an image forming apparatus capable of suppressing the consumption of an unnecessarily large amount of carriers as a whole.

The image forming apparatus of the present invention is an image forming apparatus including a developing device, and the developing device includes:
A developer containing portion containing a two-component developer comprising a toner and a carrier for developing an electrostatic latent image carried on the image carrier;
Surplus developer discharging means for discharging surplus two-component developer from the developer containing portion;
A toner concentration detecting means for detecting the toner concentration of the two-component developer in the developer accommodating portion;
Toner replenishing means for replenishing the developer containing portion with toner;
A carrier replenishing means for replenishing the developer container with a carrier;
A toner replenishment amount control means for controlling the amount of toner replenished to the developer accommodating portion by the toner replenishment means in accordance with the toner density detected by the toner density detection means; and
Developer replenishment having toner replenishment amount counting means for cumulatively measuring the amount of toner replenished to the developer accommodating portion by the toner replenishment amount control means at a predetermined time set within the driving time of the developing device Having a mechanism,
An operation mode selection means for selecting one of the two image forming operation modes of the cost per print priority mode and the image quality priority mode;
In the cost per print priority mode, the carrier replenishing means replenishes a basic amount B (mg) each time the predetermined time elapses,
In the image quality priority mode, the amount of toner replenished in the developer container at the predetermined time measured by the toner replenishment amount counting means is consumed by development when the printing rate is within an appropriate range. When the predetermined amount A (mg) is equal to or greater than the predetermined amount A (mg), the carrier replenishing unit replenishes the basic amount B (mg) every time the predetermined time elapses, and the predetermined amount measured by the toner replenishment amount counting unit. When the amount of toner to be replenished in the developer containing portion is less than the prescribed amount A (mg) at the time of the time, it is calculated by the following formula (1) every time the predetermined time elapses by the carrier replenishing means. The amount C (mg) of the carrier is replenished.
Formula (1): Amount of carrier to be replenished C = {[A−amount of toner to be replenished in developer accommodating portion (mg)] ² / k} + B
[In the above formula (1), k is a coefficient selected in the range of 0.2Dw ≦ k ≦ 2Dw, where Dw (g) is the amount of the two-component developer in the developer container. ]

  In the image forming apparatus of the present invention, it is preferable that the coefficient k is selected to be a smaller value as the cumulative driving time of the developing device becomes longer.

  In the image forming apparatus of the present invention, it is preferable that the coefficient k is selected to be smaller as the usage environment becomes higher.

  In the image forming apparatus of the present invention, it is preferable that the specified amount A and the basic amount B have a relationship of 0.05 ≦ B / A ≦ 0.2.

  Furthermore, in the image forming apparatus of the present invention, it is preferable that the appropriate range of the printing rate is 4 to 6%.

  According to the image forming apparatus of the present invention, in the trickle developing type developing device, the amount of toner replenished to the developer accommodating portion during a predetermined time is consumed by the development when the printing rate is within an appropriate range. If the amount is less than the prescribed amount A (mg), that is, when development is performed with low coverage, in addition to the amount of carrier replenished when development is performed with normal coverage each time a predetermined time elapses Therefore, since a specific amount of carrier is replenished, the toner is replenished so as to maintain a constant toner density in the developer accommodating portion, and the amount of toner consumed by the development is long. The deterioration of the developer is suppressed over a period of time, and therefore, even when the low coverage continuous print state continues for a long time without forming a toner band because the toner is forcibly consumed. Degree of degradation can be suppressed, as a result, it is possible to form a good image free from an image fogging and toner scattering were obtained good chargeability over a long period of time.

  Further, according to the image forming apparatus of the present invention, since the user can select the cost-per-print priority mode or the image-quality priority mode according to the use situation, the cost-per-print priority mode is selected when the image quality is not prioritized. As a result, it is possible to suppress an unnecessarily large amount of carriers from being consumed as a whole.

It is explanatory drawing which shows an example of a structure of the image forming apparatus of this invention. FIG. 2 is an explanatory diagram illustrating an example of a configuration of a developing device in the image forming apparatus of FIG. 1. FIG. 3 is a plan view schematically showing a part of a developing roller and a developer container in the developing device of FIG. 2. 6 is a graph showing a transition of a charge amount of toner in a developer container according to Examples 1 and 2 and Comparative Examples 1 and 2. 6 is a graph showing the relationship between the total amount of new toner replenished during a drive time of 30 seconds of the developing devices according to Examples 1 and 2 and Comparative Examples 1 and 2, and the amount of carrier to be replenished after the 30 seconds.

  Hereinafter, the present invention will be specifically described.

<First Embodiment>
The image forming apparatus according to the present invention includes a cost per print priority mode in which the amount of carriers used per visible image is suppressed, and the number of carriers used per visible image than the cost per print priority mode. Although there is a large amount, it has an operation mode selection means for selecting one of two image forming operation modes of an image quality priority mode that suppresses the deterioration of the developer in the developing device and realizes a good image quality. A developing device having a developer replenishing mechanism described below is provided.

  Specifically, the image forming apparatus of the present invention is configured such that, for example, four sets of image forming units 100Y, 100M, 100C, and 100K are arranged in the vertical direction along the intermediate transfer belt 17 that is an intermediate transfer member. Tandem color image forming apparatus.

  Each of the image forming units 100Y, 100M, 100C, and 100K is stretched around the rollers 17a to 17i, and is driven by an intermediate transfer belt 17 that circulates in a direction indicated by an arrow by a drive source (not shown), and is grounded. In this state, the photosensitive drums 10Y, 10M, 10C, and 10K that are image carriers that are rotated counterclockwise and parallel to the rotational axes of the photosensitive drums 10Y, 10M, 10C, and 10K that include the scorotron charger. And charging means 11Y, 11M, 11C, and 11K that apply a uniform potential to the surfaces of the photosensitive drums 10Y, 10M, 10C, and 10K by corona discharge having the same polarity as the toner, and a polygon mirror, for example. The photoconductive drums 10Y, 10M, 10C, and 10K that are scanned in parallel with the rotation axes of the photoconductive drums 10Y, 10M, 10C, and 10K are uniformly charged. Exposure means 12Y, 12M, 12C, and 12K for forming an electrostatic latent image by performing image exposure on the surface of 0M, 10C, and 10K, and toner, and photosensitive drums 10Y, 10M, 10C, and 10K. Development devices 20Y, 20M, 20C, and 20K having a developer replenishment mechanism according to the present invention that are conveyed to the surface of the toner and visualize the electrostatic latent image.

  The photoconductor drums 10Y, 10M, 10C, and 10K are obtained by forming an organic photoconductor layer (OPC) provided with an overcoat layer (protective layer) on the outer peripheral surface of a cylindrical metal base made of an aluminum material. For example, the outer diameter is 100 mm.

  In FIG. 1, 16Y, 16M, 16C, and 16K are cleaning units that collect residual toner remaining on the photosensitive drums 10Y, 10M, 10C, and 10K after the primary transfer.

  Here, the image forming unit 100Y forms a yellow toner image, the image forming unit 100M forms a magenta toner image, and the image forming unit 100C forms a cyan toner image. According to the unit 100K, a black toner image is formed.

  In such an image forming apparatus, the toner images of the respective colors formed on the photosensitive drums 10Y, 10M, 10C, and 10K of the image forming units 100Y, 100M, 100C, and 100K are circulated on the intermediate transfer belt 17. Further, for example, a color toner image is formed by being sequentially transferred and superimposed by primary transfer means 14Y, 14M, 14C, and 14K made of, for example, a transfer roller. In the secondary transfer means 18 made of, for example, a transfer roller, a paper feed tray is formed. The recording material P transferred from (not shown) is collectively transferred, fixed by a fixing device (not shown), and then discharged outside the apparatus.

  Specific examples of the recording material P include plain paper from thin paper to thick paper, high-quality paper, coated printing paper such as art paper or coated paper, commercially available Japanese paper or postcard paper, and plastic film for OHP. Various kinds of cloth and the like can be mentioned, but the invention is not limited to these.

[Development equipment]
The developing devices 20Y, 20M, 20C, and 20K provided in the image forming apparatus of the present invention have the same configuration except that the colors of toners to be filled are different from each other. In the following, the developing device 20Y will be described as a representative.
FIG. 2 is an explanatory diagram showing an example of the configuration of the developing device in the image forming apparatus of the present invention.
In the developing device 20Y, the toner and the carrier are replenished continuously or intermittently, and a part of the developer in the developing device 20Y is continuously or intermittently discharged little by little and is developed while being discarded. The trickle development method.

  As shown in FIGS. 2 and 3, the developing device 20Y contains a two-component developer composed of a toner and a carrier for developing the electrostatic latent image carried on the photosensitive drum 10Y as an image carrier. The developer container 21, the developer replenishment mechanism for replenishing the developer container 21 with toner and carrier, and the surplus developer discharging means for discharging the excess developer from the developer container 21 ( And a toner concentration detecting means 25 for detecting the toner concentration of the developer in the developer accommodating portion 21.

Specifically, the developing device 20Y is opposed to the photosensitive drum 10Y via the developing region R and is counterclockwise as indicated by an arrow so that the developing device 20Y is moved in the opposite direction to the photosensitive drum 10Y in the developing region R. A developing roller 22 rotated in the direction, and a supply collecting member 24 that rotates counterclockwise and supplies developer to the developing roller 22 on the upper side and collects developer returned from the developing roller 22 on the lower side. The agitating members 23 that are rotated in the opposite direction to the supply and recovery member 24 and mix and agitate while circulating and conveying the developer extend in the axial direction (perpendicular to the paper surface in FIG. 2) and are parallel to each other. The developer accommodating portion 21 is arranged in this order in the front-rear direction (left-right direction in FIG. 2), and is arranged in the supply / recovery area 21y and the agitation area 21x respectively partitioned by the partition walls 29. It is provided to. The supply / recovery member 24 and the stirring member 23 are driven to rotate in synchronization with the driving of the developing roller 22.
Reference numeral 28 denotes, for example, a rod-like or plate disposed with a predetermined gap from the developing roller 22 for regulating the thickness of the developer layer to be formed on the outer peripheral surface of the developing roller 22 to a predetermined size. It is a developer layer thickness regulating member made of a magnetic material.

The toner concentration detection means 25 is not particularly limited, and various known ones can be used. For example, a so-called magnetic type that detects a current value that changes according to the magnetic permeability by applying a voltage to the coil. A sensor can be used. When such a magnetic sensor is used, this magnetic sensor is disposed at a position facing the bottom of the agitating member 23 of the developer accommodating portion 21, for example.
Further, as the toner density detection means 25, for example, a system that indirectly detects the toner density by detecting the toner adhesion amount based on the reflection density of the patch image formed on the photosensitive drum 10Y may be used. it can.

The developing roller 22 is, for example, a cylindrical developing sleeve made of a nonmagnetic material such as aluminum, which is rotatably provided, and a fixing roller 22 that is fixed inside the developing sleeve so as to be separated in the circumferential direction. And a cylindrical magnet body having magnetic poles. The outer diameter of the developing roller 22 is, for example, φ30 mm.
The stirring member 23 and the supply / recovery member 24 are, for example, screw-shaped having spiral blades formed so as to extend spirally at a pitch of a uniform size in the axial direction over the entire outer periphery of the cylindrical shaft member. Can be.
The outer diameters of the stirring member 23 and the supply / recovery member 24 are each, for example, φ27 mm.

In the developing device 20Y, a developer agitation circuit is formed by the agitation zone 21x and the supply / recovery zone 21y of the developer accommodating portion 21, and the agitating member 23 in the roof plate 21a of the developer accommodating portion 21 is installed. On the stirring area 21x and on the upstream side (left side in FIG. 3) in the transport direction, along the transport direction, in the toner replenishing means, a toner transport member (for example, a screw) is formed from a toner storage portion (not shown). For example, a toner supply port 27a for supplying new toner (hereinafter also referred to as “new toner”) into the developer storage unit 21 by the carrier supply unit (not shown) from the carrier storage unit (not shown), for example. A new unused carrier (hereinafter also referred to as “new carrier”) is formed by a carrier conveying member (not shown) made of a screw. Carrier supply opening 27b for replenishing the developer accommodating section 21 is formed side by side in this order.
By providing the toner replenishing port 27a and the carrier replenishing port 27b on the upstream side in the transport direction on the agitation zone 21x, the replenished new toner and the new carrier are not sufficiently agitated with the already-stored developer, and the developing roller 22 is prevented from being supplied.

Further, the position immediately upstream of the position where the toner supply port 27a and the carrier supply port 27b are formed in the developer agitation circulation path, that is, the supply / recovery area 21y where the supply / recovery member 24 is installed in the developer container 21. A developer discharge port 26 for discharging excess developer is formed at the most downstream position.
Since the developer discharge port 26 is provided at a position immediately upstream of the position where the toner supply port 27a and the carrier supply port 27b are formed, before the new toner and the new carrier are mixed with the already stored developer. It is suppressed that it is discharged immediately.
The developer discharging method is not particularly limited and various methods can be adopted. Specifically, for example, the height of the wall surface of the developer containing portion 21 is set only at the portion of the developer discharging port 26. By keeping the fixed amount low, the excess developer in the agitation circuit is automatically overflowed, or, for example, a spiral blade is partially reversed as the supply / recovery member 24 or the agitation member 23 For example, a method in which the developer is prevented from being transported by using and the surplus developer in the stirring circulation path is automatically overflowed from the wall surface of the developer container 21.

  The amount of developer relating to yellow that is accommodated in the developer accommodating portion 21 of the developing device 20Y is, for example, 600 g.

The developer replenishing mechanism has a toner replenishing means (not shown) for replenishing toner to the developer containing portion 21 and a carrier replenishing means (not shown) for replenishing the developer containing portion 21 with carrier. A toner replenishment amount control means (not shown) for controlling the amount of toner replenished to the developer accommodating portion 21 by the replenishment means in accordance with the toner density detected by the toner density detection means 25, and the driving time of the developing device 20Y And a toner replenishment amount counting means (not shown) for cumulatively measuring the amount of toner replenished to the developer containing portion 21 by the toner replenishment amount control means at a predetermined time set inside the apparatus. .
The driving time of the developing device 20Y is substantially measured as the driving time of the developing roller 22, for example.

In the developing device 20Y as described above, the developing operation is performed as follows.
That is, the two-component developer filled in the developer accommodating portion 21 is mixed, agitated, and supplied as described in detail below, and conveyed to the outer peripheral surface of the developing roller 22 by the supply and recovery member 24. Thereafter, the developer is adhered to the outer peripheral surface of the developing roller 22 by a magnetic force, and the developer layer thickness regulating member 28 regulates the thickness thereof to form a developer layer by a magnetic brush, which is conveyed to the developing region R, and the developing region In R, reversal development is performed by applying a development bias voltage in which an alternating current (AC) voltage is superimposed on a direct current (DC) voltage as necessary between the developing roller 22 and the photosensitive drum 10Y. The electrostatic latent image on the photosensitive drum 10Y is visualized.
On the other hand, the two-component developer that has not visualized the electrostatic latent image is peeled off from the developing roller 22 by the action of the repulsive magnetic field of the magnet body, recovered by the supply recovery member 24, and mixed, stirred, and supplied again. To be served.

The mixing / stirring / feeding operation is performed as follows.
That is, the developer is mixed, stirred and frictionally charged while being transported in one direction along the axial direction by the stirring member 23 in the stirring area 21x, and is transported by the stirring member 23 by the supply and recovery member 24 in the supply and recovery section 21y. The toner is frictionally charged by being mixed and stirred while being conveyed in the same size in the opposite direction and supplied to the developing roller 22. The developer is circulated and conveyed in the stirring zone 21x and the supply / recovery zone 21y.

In the image forming apparatus of the present invention, the developer replenishing mechanism performs a developer replenishing operation in which the following new toner and new carrier are separately controlled and replenished respectively.
As for toner replenishment, the toner concentration in the developer accommodating portion 21 is detected by the toner concentration detecting means 25 every time a specified time elapses, and an amount of new toner controlled by the toner replenishment amount control means based on this is detected. Will be replenished after the prescribed time has elapsed.
The specified time can be 0.5 to 10 seconds, for example, and is preferably 1 second.

  In the toner replenishment amount control means, the toner concentration in the developer accommodating portion 21 is maintained at a toner concentration at which a desired image density can be obtained, for example, 4 to 8%, particularly 6%. The replenishment amount is controlled.

Specifically, for example, the toner density detecting unit 25 detects the toner density every time one second elapses, and based on this, for example, the amount of new toner shown in Table 1 below is replenished after the one second elapses. . Note that the specific amount of new toner to be replenished varies depending on the specific configurations of the image forming apparatus and the developing apparatus and the usage state of the developer.
In this example, for example, when the output value of the toner density detecting means 25 is 2.02 V, the toner density is converted to 6%.

On the other hand, with respect to carrier replenishment, when the user selects the cost-per-print priority mode, the basic amount B (mg) every time a predetermined time set within the drive time of the developing device 20Y by the carrier replenishment unit elapses. New career is replenished.
The predetermined time set within the driving time of the developing device 20Y can be, for example, 10 to 60 seconds, and is preferably 30 seconds.
The basic amount B varies depending on the specific configuration of the image forming apparatus and the developing device 20Y and the usage state of the developer, but is 10 to 100 mg, for example, and is a predetermined time set within the driving time of the developing device 20Y. Is preferably 50 mg in the case of 30 seconds.
When the basic amount B is too small, the replacement of the carrier in the developer becomes insufficient, and there is a possibility that excessive deterioration of the carrier cannot be sufficiently suppressed, while the replenishment amount of the carrier is excessive. If a large amount of carrier is replenished at one time, the replenished carrier may not be mixed with the developer already stored, which may cause problems, and an unnecessarily large amount of new carrier should be used. Therefore, the use cost is high.

  Further, when the user selects the image quality priority mode, [1] the amount of toner replenished to the developer accommodating portion 21 at a predetermined time measured by the toner replenishment amount counting means is within an appropriate printing rate range. For example, when the amount is not less than a specified amount A (mg) consumed by development when the amount is 4 to 6%, the carrier replenishing means replenishes the basic amount B (mg) after the predetermined time has elapsed, [2] If the amount of toner replenished to the developer storage unit 21 at a predetermined time measured by the toner replenishment amount counting means is less than the prescribed amount A (mg), the carrier replenishing means After the elapse of time, a new carrier of an amount C (mg) calculated by the following formula (1) is supplied.

The prescribed amount A and the basic amount B are preferably in a relationship of 0.05 ≦ B / A ≦ 0.2.
When the relationship B / A between the prescribed amount A and the basic amount B is less than 0.05, a large amount of carrier is replenished at a time, and the replenished carrier is difficult to be mixed with the already-stored developer. In addition, since a large amount of new carriers are used unnecessarily, the use cost is high. On the other hand, the relationship B / A between the prescribed amount A and the basic amount B is 0. When it exceeds 2, replacement of the carrier in the developer becomes insufficient, and excessive deterioration of the carrier may not be sufficiently suppressed.

Formula (1): Amount of carrier to be replenished C = {[A−amount of toner to be replenished in developer accommodating portion (mg)] ² / k} + B

  In the above formula (1), k is a coefficient selected in the range of 0.2 Dw ≦ k ≦ 2 Dw, where Dw (g) is the amount of developer in the developer containing portion 21. It is a numerical value appropriately set according to the use environment from the range of 2Dw ≦ k ≦ 2Dw.

As this coefficient k, it is preferable to select a smaller value as the cumulative driving time of the developing device becomes longer.
By performing the image forming operation according to the above equation (1) with the coefficient k selected in this way, it is possible to form an image with high image quality over a long period from the initial use to the end of its life.

Further, as the coefficient k, it is preferable that a smaller value is selected as the use environment becomes higher humidity.
By performing the image forming operation according to the above equation (1) with the coefficient k selected in this way, a sufficient amount of new carriers can be supplied even in a high humidity environment, and a high quality image can be formed. .

  The control in the developing device 20Y described above is similarly performed in each of the developing devices 20Y, 20M, 20C, and 20K of the image forming units 100Y, 100M, 100C, and 100K. Control in each of these developing devices 20Y, 20M, 20C, and 20K is performed independently.

[Two-component developer]
The toner constituting the two-component developer used in the image forming apparatus of the present invention is not particularly limited, and various known toners can be used. For example, the volume-based median diameter is 3 to 9 μm. It is preferable to use a so-called polymerized toner obtained by a polymerization method. By using the polymerized toner, high resolution and stable image density can be obtained in the formed visible image, and the occurrence of image fog is extremely suppressed.

  Further, the carrier constituting the two-component developer used in the image forming apparatus of the present invention is not particularly limited, and various known ones can be used. For example, the volume-based median diameter is 30 to 65 μm. And a ferrite carrier made of magnetic particles having a magnetization of 20 to 70 emu / g is preferable. When a carrier with a volume-based median diameter of less than 30 μm is used, carrier adhesion may occur and a white-out image may be generated. When a carrier with a volume-based median diameter of greater than 65 μm is used, There may occur a case where an image having a uniform image density is not formed.

  According to the image forming apparatus as described above, in the trickle developing type developing device 20Y, when the amount of toner replenished to the developer accommodating portion 21 within a predetermined time is within the proper range of the printing rate. When the amount is less than the prescribed amount A (mg) consumed by development, that is, when development is performed with low coverage, the amount replenished when development is performed with normal coverage every time a predetermined time elapses. Since a specific amount of carrier is replenished in addition to the carrier, the toner is replenished so as to keep the toner concentration in the developer accommodating portion 21 constant. Regardless of the condition, the deterioration of the developer is suppressed over a long period of time, and therefore the low coverage continuous print state continues for a long time without forming a toner band because the toner is forcibly consumed. Also suppressed excessive deterioration of the developer when, as a result, it is possible to form a good image free from an image fogging and toner scattering were obtained good chargeability over a long period of time.

  Further, according to the image forming apparatus as described above, since the user can select the cost-per-print priority mode or the image-quality priority mode according to the use situation, the cost-per-print priority mode is selected when the image quality is not prioritized. By this, it can suppress that a large amount of carriers are consumed unnecessarily as a whole.

  Although the image forming apparatus of the present invention has been described above, the present invention is not limited to the above-described configuration, and various changes can be made.

  For example, in the developing device, the new toner and the new carrier are not limited to being replenished independently. For example, in the developer replenishment mechanism, the new toner and the new carrier are conveyed from the toner accommodating portion and the carrier accommodating portion. Once mixed and stirred in the intermediate tank, the developer container may be replenished as a new developer.

  Further, for example, the image forming apparatus of the present invention is not limited to being configured as a color image forming apparatus, and may be a monochrome image forming apparatus.

  Specific examples of the present invention will be described below, but the present invention is not limited to these examples.

<Example 1: Image quality priority mode>
Using the image forming apparatus having the configuration shown in FIG. 1, under the following replenishment conditions, continuously, 50,000 sheets using an A4 halftone document with 5% coverage and 5 using an A4 halftone document with 1% coverage. 50,000 sheets using an A4 halftone original with 5% coverage, 50,000 using an A4 halftone original with 0.1% coverage, 50,000 using an A4 halftone original with 5% coverage, A print test was performed to obtain a total of 250,000 halftone images. The uniformity of the obtained halftone images was observed, and changes in the toner charge amount were observed. The transition of the toner charge amount is shown in FIG. In addition, in FIG. 4, it shows with a line (1).
-Replenishment conditions-
Toner replenishment The toner concentration is detected every second by the toner concentration detecting means, and new toner is replenished every second according to Table 1 above.
・ Carrier replenishment The cumulative amount of new toner replenished during the development device drive time of 30 seconds is
(1) When the amount is 500 mg (the prescribed amount A, which is the amount of toner consumed by development when the coverage is 5%) or more, a carrier of 50 mg (basic amount B) is replenished every 30 seconds.
(2) When the amount is less than 500 mg, the following formula (I) shown in the graph of FIG. 5 is obtained every 30 seconds: {[500-cumulative amount of new toner replenished in the 30 seconds (mg)] carrier amount is calculated by ^{2/1000} +50 C (mg} ) 210mg, i.e., 50 mg every 30 seconds when using the coverage 5% A4 document, when using a 1% A4 document coverage every 30 seconds (Amount of toner consumed by development: 100 mg) When using an A4 original with a coverage of 0.1%, 290 mg (amount of toner consumed by development: 10 mg) of carrier was replenished every 30 seconds.

  The halftone images obtained in Example 1 described above all had good uniformity. Since a higher charge amount can be obtained over a longer period than in FIG. 4, it is considered that the deterioration of the developer accommodated in the developer accommodating portion is suppressed. However, a total of 1 kg of carrier is supplied while obtaining 250,000 halftone images, which is much more than that when obtaining 250,000 halftone images in Example 2 (supplementing a total of 380 g of carriers). Was expensive.

<Example 2: Cost-per-print priority mode>
The carrier replenishment condition in the replenishment condition is that 50 mg (basic amount B) of carrier is replenished every 30 seconds regardless of the cumulative amount of new toner replenished during the developing device drive time of 30 seconds. Except for the change, a print test was performed in the same manner as in Example 1, and the uniformity of the halftone image was observed and the change in the charge amount of the toner was observed. The transition of the toner charge amount is shown in FIG. In addition, in FIG. 4, it shows with a line (2).

  The halftone image obtained in Example 2 described above was inferior in uniformity to that of Example 1, but had practically sufficient image quality. From FIG. 4, although there is a slight variation depending on the coverage of the document, it can be considered that the deterioration of the developer accommodated in the developer accommodating portion is suppressed because an approximately high charge amount can be obtained over a long period of time. .

<Comparative Example 1: Formation of a toner band between toner images>
A print test is performed while forming a toner band between toner images on which a visible image is to be formed on the photosensitive drum, and the toner density is detected every second by the toner density detecting means in the developer container. A print test was conducted in the same manner as in Example 1 except that the developer for replenishment with a trickle rate of 10 wt% was changed so that the toner concentration of the developer was maintained at 6%. Since a cleaning failure occurred when 10,000 halftone images were formed, the width of the toner band was reduced to 1/3, but the cleaning failure occurred again when a total of 200,000 halftone images were formed. The print test was interrupted because the wear state of the cleaning blade could not be used. In addition, a change in the charge amount of the toner was observed. The transition of the toner charge amount is shown in FIG. In addition, in FIG. 4, it shows with a line (3).

  The halftone image obtained in the above Comparative Example 1 had good uniformity up to the 100,000th sheet, but the subsequent halftone image was slightly more than that up to the 100,000th sheet. However, it was superior to that of Comparative Example 2 described later. As shown in FIG. 4, since a high charge amount can be obtained during the print test while forming the toner band, the toner is forcibly consumed by the formation of the toner band. At this time, it is considered that the new toner was replenished and the deterioration of the developer accommodated in the developer accommodating portion was suppressed. However, if the size of the toner band is reduced, the amount of toner that is forcibly consumed is reduced accordingly, and the effect is impaired.

<Comparative Example 2>
A print test was conducted in the same manner as in Comparative Example 1 except that no toner band was formed, and the uniformity of the halftone image was observed and the change in the charge amount of the toner was observed. The transition of the toner charge amount is shown in FIG. In addition, in FIG. 4, it shows with a line (4).

  In the halftone images obtained in Comparative Example 2 described above, noticeable image defects occurred in the 50,000th and subsequent images. As shown in FIG. 4, when the coverage of the document is 1% and when it is 0.1%, a significant decrease in the charge amount occurs, so that the developer accommodated in the developer accommodating portion is deteriorated. It seems to have been caused.

<Example 3: Image quality priority mode>
An image forming apparatus having the configuration shown in FIG. 1 and having a developer amount of 1000 g accommodated in the developing device is used, except that the carrier supply conditions in the supply conditions are changed as follows. A print test was carried out in the same manner as in Example 1, and the uniformity of the halftone image was observed and the change in the charge amount of the toner was observed. As a result, all of the obtained halftone images had good uniformity. In addition, a high charge amount was obtained over a long period of time.
-Replenishment conditions-
・ Carrier replenishment The cumulative amount of new toner replenished during the development device drive time of 30 seconds is
(1) When the amount is 833 mg (a prescribed amount A, which is the amount of toner consumed by development when the coverage is 5%) or more, a carrier of 50 mg (basic amount B) is replenished every 30 seconds.
(2) If the amount is less than 833 mg, the following formula (I) shown in the graph of FIG. 5 is obtained every time 30 seconds have elapsed: {[833-Cumulative amount of new toner replenished in 30 seconds (mg)] carrier amount is calculated by ^{2/1500} +50 C (mg} ) 408mg, i.e., 50 mg every 30 seconds when using the coverage 5% A4 document, when using a 1% A4 document coverage every 30 seconds When using an A4 manuscript with a coverage of 0.1%, 500 mg of carrier was replenished every 30 seconds.

10Y, 10M, 10C, 10K Photosensitive drums 11Y, 11M, 11C, 11K Charging means 12Y, 12M, 12C, 12K Exposure means 14Y, 14M, 14C, 14K Primary transfer means 16Y, 16M, 16C, 16K Cleaning means 17 Intermediate transfer Belts 17a to 17i Roller 18 Secondary transfer means 20Y, 20M, 20C, 20K Developing device 21 Developer container 21a Roof plate 21x Stirring area 21y Supply / recovery area 22 Developing roller 23 Stirring member 24 Supply / recovery member 25 Toner density detecting means 26 Developer discharge port 27a Toner supply port 27b Carrier supply port 28 Developer layer thickness regulating member 29 Partition 100Y, 100M, 100C, 100K Image forming unit P Recording material R Development area

Claims (5)

An image forming apparatus including a developing device, wherein the developing device includes:
A developer containing portion containing a two-component developer comprising a toner and a carrier for developing an electrostatic latent image carried on the image carrier;
Surplus developer discharging means for discharging surplus two-component developer from the developer containing portion;
A toner concentration detecting means for detecting the toner concentration of the two-component developer in the developer accommodating portion;
Toner replenishing means for replenishing the developer containing portion with toner;
A carrier replenishing means for replenishing the developer container with a carrier;
A toner replenishment amount control means for controlling the amount of toner replenished to the developer accommodating portion by the toner replenishment means in accordance with the toner density detected by the toner density detection means; and
Developer replenishment having toner replenishment amount counting means for cumulatively measuring the amount of toner replenished to the developer accommodating portion by the toner replenishment amount control means at a predetermined time set within the driving time of the developing device Having a mechanism,
An operation mode selection means for selecting one of the two image forming operation modes of the cost per print priority mode and the image quality priority mode;
In the cost per print priority mode, the carrier replenishing means replenishes a basic amount B (mg) each time the predetermined time elapses,
In the image quality priority mode, the amount of toner replenished in the developer container at the predetermined time measured by the toner replenishment amount counting means is consumed by development when the printing rate is within an appropriate range. When the predetermined amount A (mg) is equal to or greater than the predetermined amount A (mg), the carrier replenishing unit replenishes the basic amount B (mg) every time the predetermined time elapses, and the predetermined amount measured by the toner replenishment amount counting unit. When the amount of toner to be replenished in the developer containing portion is less than the prescribed amount A (mg) at the time of the time, it is calculated by the following formula (1) every time the predetermined time elapses by the carrier replenishing means. An image forming apparatus that replenishes an amount C (mg) of carrier.
Formula (1): Amount of carrier to be replenished C = {[A−amount of toner to be replenished in developer accommodating portion (mg)] ² / k} + B
[In the above formula (1), k is a coefficient selected in the range of 0.2Dw ≦ k ≦ 2Dw, where Dw (g) is the amount of the two-component developer in the developer container. ]   The image forming apparatus according to claim 1, wherein the coefficient k is selected to be a smaller value as the cumulative driving time of the developing device becomes longer.   The image forming apparatus according to claim 1, wherein the coefficient k is selected to be a smaller value as the use environment becomes higher humidity.   The image forming apparatus according to claim 1, wherein the prescribed amount A and the basic amount B have a relationship of 0.05 ≦ B / A ≦ 0.2.   The image forming apparatus according to claim 1, wherein an appropriate range of the printing rate is 4 to 6%.