JP2009181114A - Image forming apparatus and density control method for the image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can suppress the occurrence of developing ghosts, over a long period of time. <P>SOLUTION: The image forming apparatus is provided with: a magnetic roller 27 for carrying two component developer on its surface; a developing roller 28 on which a toner layer is formed by having only toner transferred from the two component developer carried by the magnetic roller 27; a photoreceptor 18 on which a toner image is formed by the toner in the toner layer formed on the developing roller 28, on the basis of the electrostatic latent image formed on the surface; a power source control circuit 33 for applying DC bias to the magnetic roller 27 and the developing roller 28; a density sensor 36 for detecting the image density of the toner image; and a control circuit 35 for comparing an image density for the toner image which is formed first, with the image density of the toner image which is formed afterwards and then lowering the applied DC bias to the developing roller 28, by controlling the power source control circuit 33, when the difference in the densities is equal to threshold set beforehand or higher. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunction machine functionally equipped with an electrophotographic method, and more particularly, two-component development for charging nonmagnetic toner using a magnetic carrier. The present invention relates to an image forming apparatus that uses an agent, holds only charged toner on a developing roller, and flies to an electrostatic latent image for development, and a density control method for the image forming apparatus.

  2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunction machine equipped with these functions, a one-component developing method is well known as a developing method using dry toner as a developer.

  The one-component development method does not include a carrier in the developer, so that the electrostatic latent image formed on the image carrier (for example, a photoconductor) is less likely to be disturbed and is suitable for high image quality. I have.

  However, the one-component development method has a problem that it is difficult to stably maintain the charge amount of the developer (dry toner).

  In addition, since color toners used in full-color image forming devices require non-magnetic toner because of its transparency, a two-component development method using a carrier as a medium for charging and transporting toner is adopted. There are many cases to do.

  This two-component development method is suitable for extending the life because a stable charge amount can be obtained for a long period of time.

  However, in the two-component development method, since the developer contains a carrier, the electrostatic latent image formed on the image carrier is likely to be disturbed, which is disadvantageous in terms of improving the image quality.

  Therefore, in order to take advantage of the contradictory characteristics in each of these development methods, a two-component development method that takes into account a long life is adopted in the toner charging region, and a one-component development method that takes high image quality into consideration in the development region. A so-called hybrid development system has attracted attention.

  In particular, this hybrid development system can fully exhibit its characteristics in a full-color image forming apparatus in which both high image quality and long life are important.

  In the hybrid development system, a two-component developer containing toner and carrier is supported on the surface of a developer carrier (for example, a magnetic roller) to form a magnetic brush, and only the toner is transferred from the magnetic brush to the toner carrier (for example, The toner layer is transferred to the surface of the developing roller to form a toner layer, and the toner is ejected from the toner layer to the surface of the photoreceptor (electrostatic latent image carrier) to visualize the electrostatic latent image as a toner image (development). ).

  However, in the hybrid development method, it is necessary to supply the toner from the magnetic brush to the developing roller at the same time while removing the residual toner on the developing roller with the magnetic brush of the magnetic roller. However, the residual toner is not sufficiently removed. If there is insufficient toner supply to the developing roller, a so-called development ghost (history phenomenon) is likely to occur, in which afterimages after toner flies from the developing roller to the photosensitive member appear during subsequent image formation. A problem has arisen.

  Therefore, as a measure for preventing the development ghost in the hybrid development system, for example, the magnetic brush density is increased by low magnetization of the carrier of the two-component developer to be used, the toner stripping effect on the development roller is improved, and the development ghost is improved. A technique for suppressing the occurrence of this is known (for example, see Patent Document 1).

In addition, a method for promoting toner replacement on the developing roller by applying an AC bias having an opposite phase to the AC component of the developing roller to the magnetic roller is disclosed (see Patent Document 2).
JP 2003-295613 A JP-A-2005-242281

  However, in the image forming apparatus configured as described above, for example, in the technique disclosed in Patent Document 1, if the density or uniformity of the magnetic brush is impaired due to the deterioration of the carrier, the toner peeling effect is also obtained. Therefore, there is a problem that it may be difficult to suppress the development ghost from occurring for a long period of time.

  Further, in the technique disclosed in Patent Document 2, not only complicated control is required in order to cope with changes in the generation level of development ghosts and development leaks accompanying the deterioration of the developer, but also development ghosts are generated for a long period of time. There has been a problem that it is difficult to suppress it over time.

  In view of the above circumstances, an object of the present invention is to provide an image forming apparatus and a density control method for the image forming apparatus that can suppress the development ghost from occurring for a long period of time.

  The image forming apparatus according to the present invention includes a developer carrying member that carries a two-component developer composed of toner and a carrier on the surface, and only toner is transferred from the two-component developer carried on the developer carrying member. A toner carrier on which a layer is formed; an electrostatic latent image carrier on which a toner image is formed by toner of a toner layer formed on the toner carrier based on an electrostatic latent image formed on a surface; In an image forming apparatus comprising a developer carrier and a power supply control circuit that applies a DC bias to the toner carrier, a density sensor that detects an image density of the toner image, and an image of the toner image previously formed The density is compared with the image density of the toner image formed later, and if the density difference is equal to or greater than a preset threshold, the power supply control circuit is controlled to lower the DC bias applied to the developer carrier. control Characterized in that it comprises a road, a.

  The image forming apparatus density control method according to the present invention includes a reference bias setting step for setting a reference applied DC bias to be applied to the developer carrier and the toner carrier, and driving the toner carrier to remove the toner. A toner peeling step for peeling, a toner layer forming step for driving the toner carrier to form a toner layer, a patch forming step for patching a first solid image on the image carrier, and the toner carrier A second toner peeling step for driving the toner to peel off the toner, a second toner layer forming step for driving the toner carrier to form a toner layer, and a second solid image on the image carrier. A second patch forming step for forming a patch, a density detecting step for detecting each density of the first solid image and the second solid image, and a first solid image. A density difference calculating step for calculating a density difference from the second solid image, and if the calculated density difference is less than a preset threshold value, an actual DC bias applied to the developer carrier at that time is actually An applied DC bias control step for lowering the applied DC bias to the developer carrier when the density difference calculated and determined as the applied DC bias is equal to or greater than a preset threshold value. It is characterized by.

  The image forming apparatus of the present invention can suppress the development ghost from occurring for a long period of time.

  Next, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is an explanatory diagram of a printer as an image forming apparatus according to an embodiment of the present invention. FIG. 2 is an explanatory diagram of a main part of the image forming apparatus according to an embodiment of the present invention.

(Overall configuration of color printer 1)
In FIG. 1, a color printer 1 as an image forming apparatus according to an embodiment of the present invention is turned upside down on a paper feed cassette 3 that can be pulled out from one surface (for example, the front surface) of a printer main body 2 and one surface of the printer main body 2. A manually feed tray 4 that can be provided, a transport path 5 that transports transfer paper (not shown) set in the paper feed cassette 3 or the manual feed tray 4, and an upstream end of the transport path 5 are configured. A sheet feeding unit 6, a secondary transfer roller 7 disposed in the middle of the conveyance path 5, and an endless belt-shaped intermediate for forming a toner transfer image on the surface of the transfer paper in cooperation with the secondary transfer roller 7 A plurality of colors (for example, yellow (Y), magenta (M), cyan (C), and black (K)) toners that are transferred onto the surface of the transfer belt 8 and the intermediate transfer belt 8 as necessary. An image forming unit 9; A fixing unit 10 that is arranged near the end of the feeding path 5 and fixes a toner transfer image formed on the surface of the transfer paper, and a paper discharge tray portion of the printer main body 2 from the downstream end of the transport path 5 for the fixed transfer paper. 11 is provided.

  The intermediate transfer belt 8 has a function as an image carrier, and an elastic intermediate transfer belt using at least one elastic layer is adopted for the layer structure, and includes a driving roller 13, a driven roller (tension roller) 14, an additional roller. Rotating movement between the pressure rollers 15 (see the arrow in the figure).

  The pressure roller 15 is disposed opposite to the secondary transfer roller 7 with the intermediate transfer belt 8 interposed therebetween, and the toner is applied to the surface of the transfer paper conveyed to the conveyance path 5 by applying a transfer bias to the secondary transfer roller 7. The image is transferred to form a toner transfer image.

(Configuration of the image forming unit 9)
In the present embodiment, the image forming units 9 are arranged at four locations in a tandem manner along the rotational movement direction of the intermediate transfer belt 8, and those having substantially the same configuration are arranged. In addition, the image forming unit 9 includes a toner container 16 that stores toner of each color, a supply unit 17 that supplies toner stored in the toner container 16, and a drum-like electrostatic that is supplied with toner from the supply unit 17 on the surface. A photosensitive member 18 as a latent image carrier, a transfer roller 19 disposed opposite to the photosensitive member 18 with the intermediate transfer belt 8 interposed therebetween, a cleaning device 20 for removing residual toner and the like adhering to the surface of the photosensitive member 18, and A neutralization device 21 that removes the electric charge of the photoconductor 18, a charging device 22 that charges the surface of the photoconductor 18, and an exposure device 23 that forms a predetermined electrostatic latent image on the surface of the photoconductor 18 are provided. .

  In the present embodiment, each image forming unit 9 includes a magenta image forming unit 9 (M), a cyan image forming unit 9 (C), and a yellow along the rotational movement direction of the intermediate transfer belt 8. The image forming unit 9 (Y) for black is arranged in this order, and the image forming unit 9 (K) for black is arranged at the right end in the figure.

(Configuration of supply unit 17)
As shown in FIG. 2, the supply unit 17 includes a stirring mixer 24 that stirs and charges the toner supplied from the toner container 16 together with a carrier, and a paddle mixer that stirs and conveys the two-component developer supplied from the stirring mixer 24. 25, a partition plate 26 interposed between the mixers 24, 25, and a magnetic roller 27 as a sleeve-like developer carrier having a plurality of fixed magnets arranged therein and rotatable around the periphery. A developing roller 28 as a toner carrier that forms a toner layer (not shown) by a magnetic brush (not shown) formed on the magnetic roller 27; and the height of the magnetic brush formed on the magnetic roller 27. A constant blade 29 for maintaining a constant current, a direct current power source 30 for applying a direct current (DC) bias to the magnetic roller 27, and a direct current (DC) bias to the developing roller 28. Based on the control data stored in the ROM 34, the AC power source 32 that applies an alternating current (AC) bias to the developing roller 28, the power source control circuit 33 that controls the power sources 30, 31, and 32. And a control circuit 35 that controls the entire printer 1.

  The ROM 34 also stores control data related to development ghost generation suppression control according to the present invention. Thus, the ROM 34 and the control circuit 35 constitute a microcomputer that executes development ghost generation suppression control according to the present invention. The density detection signal output from the density sensor 36 is input to the control circuit 35, and the power supply control circuit is based on the control data relating to the development ghost occurrence suppression control according to the present invention stored in the ROM 34 according to the input result. 33 is controlled.

(Image formation processing)
In the color printer 1 having such a configuration, when print data is output from a personal computer (not shown) or the like, the surface of the photoconductor 18 is charged by the charging device 22 (charging process), and then exposed by the exposure device 23. The surface of the body 18 is exposed to form an electrostatic latent image (exposure process).

  On the other hand, in the supply unit 17, the two-component developer is carried on the surface of the magnetic roller 27 to form a magnetic brush, and only the toner is transferred from the magnetic brush on the surface of the magnetic roller 27 to the surface of the developing roller 28. A toner layer is formed on the surface of the developing roller 28.

  Further, the toner is ejected from the toner layer of the developing roller 28, and the toner is attached to the electrostatic latent image on the photosensitive member 18 to develop the electrostatic latent image as a toner image (developing process), and then the toner image is transferred. The roller 19 performs primary transfer from the photoreceptor 18 to the intermediate transfer belt 8 (primary transfer process).

  Next, the toner image primarily transferred to the intermediate transfer belt 8 is conveyed to the secondary transfer roller 7 by the rotational movement of the intermediate transfer belt 8, and the transfer paper is applied by the transfer bias applied to the secondary transfer roller 7. (Secondary transfer process).

  The toner remaining on the surface of the intermediate transfer belt 8 after the toner image is secondarily transferred by the secondary transfer roller 7 is disposed on the downstream side of the secondary transfer roller 7 in the rotational movement direction. Removed by.

(Development ghost generation mechanism)
Next, a development ghost generation mechanism will be described.

  In the development ghost, for example, when an image forming process is continuously performed in the circumference of the next developing roller 28 after the image forming process is performed on a certain printing area, the previous image printing remains on the surface of the developing roller 28, and the next This phenomenon appears as a density variation (afterimage) during the image forming process, and particularly when the background of the next image forming process is a half density.

  One of the causes of the development ghost phenomenon is that the toner layer on the developing roller 28 is lost due to image printing, and the toner is sufficiently peeled off by the magnetic brush carried by the magnetic roller 27 thereafter. Is not possible. The other is that the toner cannot be supplied enough to fill the missing portion in the previous image forming process.

  In particular, the second factor is important, and the fact that the toner cannot be sufficiently supplied from the magnetic roller 27 to the developing roller 28 in one round means that two or more rounds are required for forming the toner layer on the developing roller 28. become.

  Therefore, if two or more laps are spent for forming the toner layer on the developing roller 28, the image is output with the toner layer formed while the developing roller 28 makes one turn, and the developing roller 28 has two laps. As a result, when the toner layer is formed and the image is output, the image density is higher when the toner layer is formed by rotating the developing roller 28 twice.

  Therefore, it is desirable that the toner layer is formed on the developing roller 28 while the developing roller 28 makes one round as much as possible. Therefore, the saturation amount of the toner to be transferred from the magnetic roller 27 to the developing roller 28 It is important to reduce as much as possible.

  Here, the amount of toner transferred from the magnetic roller 27 to the developing roller 28 is determined by a difference in direct current (DC) bias between the magnetic roller 27 and the developing roller 28, a so-called potential difference.

  That is, if a sufficient amount of toner is supplied from the magnetic roller 27 to the developing roller 28 in one round, the development ghost is prevented from being generated, so that the direct current (DC) bias between the magnetic roller 27 and the developing roller 28 is suppressed. The smaller the potential difference is, that is, the smaller the amount of saturated toner is, the more difficult the development ghost is.

  At this time, since the image density is insufficient when the toner layer is small, it is necessary to increase the toner layer of the developing roller 28 to such an extent that the image density is not impaired.

  Therefore, in order to maintain the image density while avoiding the development ghost, it is necessary to set an optimum potential difference.

  As for the toner supply performance from the magnetic roller 27 to the developing roller 28 and the ability to peel off the toner on the developing roller 28, the distance between the developing roller 28 and the magnetic roller 27, the layout of the magnetic poles arranged on the magnetic roller 27, and the like. It depends on the height of the head of the magnetic brush (the gap between the tip of the regulating blade 29 and the magnetic roller 27), the particle size of the carrier contained in the developer, the saturation magnetization, and the like. Further, the optimum amount of the toner layer supplied to the developing roller 28 is very likely to fluctuate throughout the endurance, depending on the toner concentration in the two-component developer and the change in the toner charge amount due to carrier deterioration.

  Therefore, it is appropriate to consider that the potential difference at which development ghosts are likely to occur is changed each time. Therefore, the potential difference at which development ghosts are generated is detected in advance, and the potential difference is avoided and image density is ensured. It is desirable to set a developing bias that can be used.

(Development ghost detection method)
Next, a method for detecting a development ghost will be described.

  The image density can be detected by, for example, using a reflection type density sensor 36 to calculate the toner amount from the reflected light with respect to the background surface and the reflected light of the toner layer surface, and converting it to the image density.

  Also, the image density detection timing needs to be implemented after the primary transfer is completed and before the secondary transfer is started.

  Therefore, it is desirable to install the density sensor 36 between the primary transfer and the secondary transfer. In the color printer 1 employing the tandem method shown in FIG. 1, a plurality of image forming units 9 are arranged in parallel along the rotational movement direction of the intermediate transfer belt 8. Therefore, as shown in FIG. 1, the density sensor 36 is disposed between the image forming unit 9 (K) and the secondary transfer roller 7 disposed on the most downstream side in the rotational movement direction of the intermediate transfer belt 8. The

  In such an arrangement, first, for example, a first solid image is printed on the toner layer on the developing roller 28 after the developing roller 28 has made one turn, and a peeling bias is applied while the developing roller 28 makes one turn. The toner layer is removed by application, and then the toner layer is formed on the developing roller 28. After the developing roller 28 makes two turns, a second solid image is printed.

  The smaller the difference in image density between the first solid image and the second solid image at this time, the less the development ghost. In practice, if there is a difference of 0.1 or more in image density, it is sufficiently recognized as a density difference, so that a development ghost becomes obvious. Therefore, if the potential difference between the magnetic roller 27 and the developing roller 28 is set so that the image density difference between the first solid image and the second solid image is less than 0.1, the development ghost can be avoided. .

  Hereinafter, an example of control from detection of image density by the density sensor 36 to development condition setting will be described in detail with reference to the flowchart shown in FIG.

(Step S1)
In step S1, the control circuit 35 sets each applied DC bias of the magnetic roller 27 and the developing roller 28 (for example, the applied DC bias of the magnetic roller 27 is 400V, and the applied DC bias of the developing roller 28 is 100V). The setting signal is output to the power supply control circuit 33, and the process proceeds to step S2.

(Step S2)
In step S2, the control circuit 35 drives the developing roller 28 (one round) to perform toner peeling, and proceeds to step S3.

(Step S3)
In step S3, the control circuit 35 drives the developing roller 28 (one turn) to form a toner layer, and proceeds to step S4.

(Step S4)
In step S4, the control circuit 35 rotates and moves the intermediate transfer belt 8, and for example, forms a black solid image (1) on the intermediate transfer belt 8 and proceeds to step S5.

(Step S5)
In step S5, the control circuit 35 drives the developing roller 28 again (one turn) to remove the toner, and proceeds to step S6.

(Step S6)
In step S6, the control circuit 35 drives the developing roller 28 (two rounds) to form a toner layer, and proceeds to step S7.

(Step S7)
In step S7, the control circuit 35 rotates and moves the intermediate transfer belt 8, and for example, forms a black solid image (2) on the intermediate transfer belt 8, and proceeds to step S8.

(Step S8)
In step S8, the control circuit 35 causes the density sensor 36 to detect the densities of the solid image (1) and the solid image (2), and stores the respective detection results in the storage unit 37 such as a RAM, and then proceeds to step S9. And migrate.

(Step S9)
In step S9, the control circuit 35 calculates an image density difference from the densities of the solid image (1) and the solid image (2) respectively stored in the storage unit 37, and the density difference is set to a preset threshold value (for example, If it is less than 0.1), the process proceeds to step S10.

(Step S10)
In step S10, since the density difference between the solid image (1) and the solid image (2) is less than a preset threshold value, the control circuit 35 sets the applied DC bias of the magnetic roller 27 at that time to the actual applied DC. Determine as bias.

(Step S11)
In step S11, since the density difference between the solid image (1) and the solid image (2) is equal to or greater than a preset threshold value, the control circuit 35 supplies power to reduce the applied DC bias of the magnetic roller 27 at that time. A command is output to the control circuit 33, and thereafter, the process loops to step S1 in order to perform detection again with the lowered (for example, −50V) applied DC bias.

  Hereinafter, every time it is determined that the threshold value is greater than or equal to the threshold value in step S9, the applied DC bias is decreased by a predetermined unit (for example, −50 V) in step S11, and the applied DC bias of the magnetic roller 27 is determined.

  In this way, when the difference in image density between the solid image (1) and the solid image (2) is detected and the density difference is equal to or greater than a predetermined threshold (0.1), the applied DC bias of the developing roller 28 is The applied DC bias of the magnetic roller 27 is lowered while being fixed, the solid image (1) and the solid image (2) are printed again, and the image density difference is detected.

  By repeating this, by setting the DC bias applied to the developing roller 28 and the magnetic roller 27 when the image density difference becomes less than the threshold value (0.1) as a developing condition, the development ghost is suppressed and the image density is reduced. It is possible to set an optimum developing bias that can be secured.

  The flow from the detection of the image density by the density sensor 36 to the setting of the development conditions has been described in the case of a single color. However, as shown in FIG. 4, patch image formation when a plurality of colors (for example, four colors) are used is used. Even in the case of.

  At this time, by measuring the image density difference between the first solid image (solid image (1)) and the second solid image (solid image (2)), it is possible to efficiently avoid development ghosts even for a plurality of colors. Image density adjustment is possible.

  In addition, it is important to adjust the development conditions using the density sensor 36 described above before printing an image on a transfer sheet so as not to cause a development ghost in an actual printed matter.

  Of course, it is ideal to carry out before every printing, but every time it leads to a decrease in printing efficiency and an increase in toner consumption, so every fixed driving time of the developer, for example, every 500 sheets printing. It only needs to be done once. It is also preferable to adjust the development conditions because the image quality is likely to change even when the printing environment has changed significantly or when it has been left undriven for a long time.

(Example)
Hereinafter, the present invention will be described in detail with reference to examples. In addition, this invention is not limited only to a following example. As an image evaluation machine, a color printer FS-C5016N (manufactured by Kyocera Mita) was used.

  As an evaluation method, a solid image (1) and a solid image (2) were printed by FS-C5016N according to the procedure shown in FIG. For the development ghost image, as shown in FIG. 5, a solid image having a length within one rotation of the developing roller is formed at the leading end of the transfer paper, and the length after the length of one developing roller from the leading end of the transfer paper, that is, Evaluation was performed using an image pattern that forms a half image G2 (for example, a dot pattern with a printing rate of 25%) after the solid image G1.

  The DC bias applied to the developing roller 28 is fixed at 100 V, the DC bias applied to the magnetic roller 27 is used as a reference shown in FIG. 6, and the solid image (1) and the solid image obtained from the development ghost detection pattern (FIG. 4). The image density of (2) was measured using a Macbeth reflection densitometer (RD914), and a case where the difference was less than 0.1 was determined to be acceptable. For the development ghost, the pass / fail judgment was made based on whether or not the development ghost pattern G1 ', which is a residual image history of the solid image G1 on the development roller, can be visually discriminated in the half image G2.

  The measurement results are shown in FIG. In the development ghost determination of FIG. 6, the case where there is no afterimage in the half image G2 is indicated as “◯” (determination: pass <OK>), and the case where there is an afterimage in the half image G2 is indicated as “X” (determination: failure <NG>). And evaluated.

  As a result, when the potential difference ΔV of the DC bias applied between the magnetic roller 27 and the developing roller 28 is 250 V or less, the image density difference is less than 0.1, and the potential difference ΔV also in the development ghost pattern G1 ′. It can be seen that the development ghost is determined to be acceptable at 250 V or less.

  It can also be seen that the potential difference ΔV is required to be 200 V or more in order to secure an image density of 1.2 or higher than that of the solid image (1).

  From the results obtained by these experiments, by setting the potential difference ΔV as the developing bias between 200 V and 250 V, it becomes possible to suppress the development ghost and ensure the image density.

  FIG. 7 is a graph showing the relationship between the toner layer potential (V) and the rotation speed (circumference) of the developing roller 28 when ΔV = 200V and when ΔV = 300V.

  In the above embodiment, the image forming apparatus according to the present invention is described as being mounted on the color printer 1, but the present invention is applicable to all full-color and monochrome image forming apparatuses such as copying machines and multifunction machines. Of course you can.

  In the above embodiment, the toner image formed on the photosensitive member 18 as the electrostatic latent image carrier is primarily transferred to the intermediate transfer belt 8 and then the toner image is secondarily transferred to the transfer paper. Although the apparatus has been disclosed, the present invention can also be applied to an image forming apparatus that transfers a transfer paper toner image directly from the photoreceptor 18.

1 is an explanatory diagram of an image forming apparatus according to an embodiment of the present invention. 1 is an explanatory diagram of a main part of an image forming apparatus according to an embodiment of the present invention. FIG. 6 is a flowchart of a control example from image density detection to development condition setting by a control circuit in the image forming apparatus according to the embodiment of the present disclosure. FIG. 6 is a timing diagram of an example of developing ghost detection pattern creation in the image forming apparatus according to the embodiment of the present invention. FIG. 6 is an explanatory diagram of a development ghost pattern in the image forming apparatus according to an embodiment of the present invention. 5 is a comparative chart of development ghost determination in the image forming apparatus according to the embodiment of the present invention. FIG. 6 is a graph showing the relationship between the toner layer potential (V) and the rotation speed (circumference) of the toner carrier in a state where the applied voltage to the toner carrying low is different in the image forming apparatus according to the embodiment of the present invention. is there.

Explanation of symbols

1. Color printer (image forming device)
8 ... Intermediate transfer belt (image carrier)
9. Image forming unit 18. Photoconductor (electrostatic latent image carrier)
27 ... Magnetic roller (developer carrier)
28: Developing roller (toner carrier)
33 ... Power supply control circuit 35 ... Control circuit (bias polarity control unit)
36 ... Concentration sensor

Claims (2)

A developer carrier that carries on its surface a two-component developer comprising toner and carrier, and a toner carrier that forms a toner layer by transferring only toner from the two-component developer carried on the developer carrier An electrostatic latent image carrier in which a toner image is formed by toner in a toner layer formed on the toner carrier based on the electrostatic latent image formed on the surface, the developer carrier, and the toner carrier In an image forming apparatus comprising a power supply control circuit for applying a DC bias to a body,
When the density sensor for detecting the image density of the toner image is compared with the image density of the toner image formed earlier and the image density of the toner image formed later, and the density difference is equal to or greater than a preset threshold value. The image forming apparatus further comprises: a control circuit that controls the power supply control circuit to lower the DC bias applied to the developer carrying member. A reference bias setting step for setting a reference applied DC bias to be applied to the developer carrier and the toner carrier;
A toner peeling step of driving the toner carrier to remove the toner;
A toner layer forming step of driving the toner carrier to form a toner layer;
A patch forming step of patching the first solid image on the image carrier;
A second toner peeling step for driving the toner carrier to remove the toner;
A second toner layer forming step of driving the toner carrier to form a toner layer;
A second patch forming step of patching the second solid image on the image carrier;
A density detection step for detecting each density of the first solid image and the second solid image;
A density difference calculating step for calculating a density difference between the first solid image and the second solid image;
If the calculated density difference is less than a preset threshold value, the DC bias applied to the developer carrier at that time is determined as the actual applied DC bias, and the calculated density difference is a preset threshold value. In the above case, an applied DC bias control step for lowering the applied DC bias to the developer carrier at that time,
A density control method for an image forming apparatus, comprising:

Images