Image forming apparatus, method for determining cover margin, and program for determining cover margin

Abstract

To determine an appropriate fogging margin without increasing performance to detect the density of toner.SOLUTION: An image forming apparatus comprises: a density detection unit 53 that detects the density of a fogging image formed on an image carrier; a temporary determination unit 57 that, based on the density detected by the density detection unit 53, determines a temporary fogging margin that is a fogging margin indicating the difference between a developing bias applied to a toner carrier and an electrification bias applied to an electrifier electrifying the image carrier; and a correction unit 59 that corrects the temporary fogging margin based on the rate of change of the density detected by the density detection unit 53 to a change in the fogging margin.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to an image forming apparatus, an overlapping margin determination method, and an overlapping margin determination program, and more particularly to an image forming apparatus that forms a toner image on an image carrier, an overlapping margin determination method executed by the image forming apparatus, and an overlapping margin determination program.

Background Art

[0002] An image forming apparatus typified by an MFP (Multi Function Peripheral) includes a photosensitive drum and a developing roller. The developing roller carries a developer containing toner. The toner carried by the developing roller is transferred to the electrostatic latent image formed on the photosensitive drum, thereby forming a toner image on the photosensitive drum. After the toner image carried by the photosensitive drum is transferred to a recording medium or the like, the residual toner remaining on the photosensitive drum without being transferred is removed by a blade member. At this time, the toner particles act as a lubricant, and the frictional force between the edge of the blade and the photosensitive drum is reduced.

[0003] It is preferable that the portion corresponding to the background of the image is not originally transferred with toner to the recording medium. On the other hand, in order to reduce the frictional force between the edge of the blade and the photosensitive drum, it is necessary to carry a predetermined amount of toner on the portion corresponding to the background of the image on the photosensitive drum by the photosensitive drum. The amount of toner carried by the photosensitive drum on the portion corresponding to the background of the image on the photosensitive drum is referred to as the overlapping amount. Increasing this overlapping amount increases the toner consumption. Conversely, decreasing the overlapping amount reduces the cleaning performance of the photosensitive drum.

[0004] According to Patent Document 1, an optical sensor for measuring the density of a control toner image formed on a photoreceptor is used to measure the density of the fouling toner on the photoreceptor. If the fouling toner density exceeds a predetermined value, the transfer bias is increased by a predetermined amount within a range where a decrease in transfer efficiency is acceptable. This reduces the transfer efficiency of the fouling toner, thereby suppressing noticeable background staining caused by toner adhering to the white areas of the image. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-271240 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, supplying toner to the surface of the photoreceptor as a fouling image increases toner consumption, so it is desirable to supply as little toner as possible while minimizing friction. Thus, the amount of fouling toner supplied to the surface of the photoreceptor must be minute, and as described in Patent Document 1, detecting the fouling toner on the surface of the photoreceptor using an optical sensor is difficult due to its sensitivity.

[0007] On the other hand, it is known that the amount of fogging is affected by the fogging margin, which is the voltage difference between the development bias applied to the developing roller and the charging bias that charges the photoreceptor drum. Conventionally, this fogging margin was determined in advance by experimentation. However, since the appropriate value of the fogging margin changes depending on the state of the image forming apparatus, it is difficult to determine an appropriate fogging margin in response to changes in the state of the image forming apparatus.

[0008] One of the objectives of this invention is to provide an image forming apparatus, a method for determining a cover margin, and a program for determining a cover margin that can determine an appropriate cover margin without improving the performance of detecting toner density. [Means for solving the problem]

[0009] According to one aspect of this invention, an image forming apparatus comprises: a density detection means for detecting the density of a haze image formed on an image carrier; a provisional determination means for determining a provisional haze margin, which is a haze margin indicating the difference between a development bias applied to a toner carrier and a charging bias applied to a charger that charges the image carrier, based on the density detected by the density detection means; and a correction means for correcting the provisional haze margin based on the rate at which the density detected by the density detection means changes in relation to the change in the haze margin. The correction means predicts the charge distribution of the toner based on the rate at which the concentration detected by the concentration detection means changes in relation to the change in the fouling margin, and determines a first correction value for correcting the provisional fouling margin based on the predicted charge distribution of the toner. ru.

[0010] According to another aspect of this invention, a method for determining a fouling margin involves having an image forming apparatus perform the following steps: a density detection step of detecting the density of a fouling image formed on an image carrier; a provisional determination step of determining a provisional fouling margin, which is the difference between the development bias applied to the toner carrier and the charging bias applied to the charger that charges the image carrier, based on the density detected in the density detection step; and a correction step of correcting the provisional fouling margin based on the rate at which the density detected in the density detection step changes with respect to the change in the fouling margin. The correction step predicts the charge distribution of the toner based on the rate at which the concentration detected in the concentration detection step changes in relation to the change in the fouling margin, and determines a first correction value to correct the provisional fouling margin based on the predicted charge distribution of the toner. ru.

[0011] According to yet another aspect of this invention, a fouling margin determination program causes a computer to perform the following steps: a density detection step of detecting the density of a fouling image formed on an image carrier; a provisional determination step of determining a provisional fouling margin, which is the fouling margin representing the difference between the development bias applied to the toner carrier and the charging bias applied to the charger that charges the image carrier, based on the density detected in the density detection step; and a correction step of correcting the provisional fouling margin based on the rate at which the density detected in the density detection step changes with respect to the change in the fouling margin. The correction step predicts the charge distribution of the toner based on the rate at which the concentration detected in the concentration detection step changes in relation to the change in the fouling margin, and determines a first correction value to correct the provisional fouling margin based on the predicted charge distribution of the toner. ru. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic cross-sectional view showing the internal structure of an MFP in one embodiment of the present invention. [Figure 2] This is a cross-sectional view showing the details of the developing unit. [Figure 3] This is a block diagram illustrating the hardware configuration of the MFP in the first embodiment. [Figure 4] This is a block diagram illustrating the drive control unit. [Figure 5] This block diagram shows an example of the functions of the CPU included in the MFP in this embodiment. [Figure 6] The first graph shows an example of corresponding data. [Figure 7] This is a diagram showing a portion of the charge distribution of degraded toner. [Figure 8] This figure shows a portion of the charge distribution of undegraded toner. [Figure 9] This is the second graph, which shows an example of corresponding data. [Figure 10] This figure shows an example of the first correction table. [Figure 11] This is a flowchart illustrating an example of the overlap margin setting process. [Figure 12] This block diagram shows an example of the functions of the CPU in the MFP in this embodiment in the first modified example. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described below with reference to the drawings. In the following description, identical parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions of them will not be repeated.

[0014] FIG. 1 is a schematic cross-sectional view showing the internal configuration of an MFP in one embodiment of the present invention. In FIG. 1 and predetermined figures described later, arrows indicating the X direction, Y direction, and Z direction orthogonal to each other are attached to clarify the positional relationship. The X direction and the Y direction are orthogonal to each other in the horizontal plane, and the Z direction corresponds to the vertical direction. Referring to FIG. 1, an MFP (Multi Function Peripheral) 100 is an example of an image forming apparatus, and includes a document reading unit 130 that reads a document, an automatic document feeder 120 that conveys the document to the document reading unit 130, an image forming unit 140 that forms an image on a sheet based on image data, and a paper feeding unit 150 that supplies paper to the image forming unit 140.

[0015] The document reading unit 130 exposes an image of a document set on the document glass 11 by the automatic document feeder 120 with an exposure lamp 13 attached to a slider 12 that moves below it. The reflected light from the document is guided to a lens 16 by a mirror 14 and two reflecting mirrors 15, 15A, and forms an image on a CCD (Charge Coupled Devices) sensor 18.

[0016] The reflected light that forms an image on the CCD sensor 18 is converted into image data as an electrical signal within the CCD sensor 18. The image data is converted into printing data for cyan (C), magenta (M), yellow (Y), and black (K), and output to the image forming unit 140.

[0017] The image forming unit 140 has developing units 20Y, 20M, 20C, 20K corresponding to yellow, magenta, cyan, and black respectively, and toner bottles 41Y, 41M, 41C, 41K. Here, "Y", "M", "C", and "K" represent yellow, magenta, cyan, and black respectively.

[0018] Since the developing units 20Y, 20M, 20C, 20K and the toner bottles 41Y, 41M, 41C, 41K only differ in the color of the toner they handle, the developing unit 20Y and the toner bottle 41Y for forming a yellow image will be described here.

[0019] The toner bottle 41Y contains yellow toner. The developer contains non-magnetic toner and a magnetic carrier. The toner bottle 41Y rotates using a toner bottle motor as the driving source and discharges toner to the outside. The toner discharged from the toner bottle 41Y is supplied to the developer unit 24Y. The toner bottle 41Y supplies developer to the developer unit 24Y in accordance with the amount of toner remaining in the developer unit 24Y falling below a predetermined lower limit.

[0020] The intermediate transfer belt 30 is suspended by a drive roller 33 and a driven roller 34 to prevent slack. When the drive roller 33 rotates counterclockwise in Figure 1, the intermediate transfer belt 30 rotates counterclockwise in the figure at a predetermined speed. As the intermediate transfer belt 30 rotates, the driven roller 34 rotates counterclockwise.

[0021] The developing unit 20Y contains the developer. The developer contains a non-magnetic toner and a magnetic carrier. The developing unit 20Y receives toner from the toner bottle 41Y and agitates the carrier and toner. The developing unit 20Y forms a toner image with the toner contained in the developer and transfers the toner image to the intermediate transfer belt 30. The timing of the developing unit 20Y transferring the toner image onto the intermediate transfer belt 30 is adjusted by detecting a reference mark attached to the intermediate transfer belt 30.

[0022] When forming a full-color image, the MFP100 drives all of the development units 20Y, 20M, 20C, and 20K. This superimposes the yellow, magenta, cyan, and black toner images onto the intermediate transfer belt 30. When forming a monochrome image, the MFP100 drives one of the development units 20Y, 20M, 20C, or 20K. It can also form an image by combining two or more of the development units 20Y, 20M, 20C, and 20K.

[0023] Each of the paper feed cassettes 35, 35A, and 35B is loaded with paper of a different size. The paper stored in each of the paper feed cassettes 35, 35A, and 35B is supplied to the transport path by the ejection rollers 36, 36A, and 36B, which are attached to each of the paper feed cassettes 35, 35A, and 35B, respectively, and then sent to the timing roller 31 by the paper feed roller 37.

[0024] The timing roller 31 transports the paper conveyed by the paper feed roller 37 to the nip section between the intermediate transfer belt 30 and the secondary transfer roller 26, which is a transfer member. The secondary transfer roller 26 generates an electric field in the nip section. Due to the action of the electric field force in this nip section, the toner image formed on the intermediate transfer belt 30 is transferred to the paper conveyed by the timing roller 31. The paper with the transferred toner image is transported to the fuser roller 32, where it is heated and pressurized. This melts the toner and fixes it to the paper. After that, the paper is discharged to the output tray 39. A belt cleaning blade 29 is provided upstream of the developing device 24Y on the intermediate transfer belt 30. The belt cleaning blade 29 removes any toner remaining on the intermediate transfer belt 30 that was not transferred to the paper.

[0025] In this explanation, the MFP100 uses a tandem system equipped with 24Y, 24M, 24C, and 24K developing units to form four different toners on the paper. However, a four-cycle system in which a single photoreceptor drum transfers all four toners to the paper sequentially may also be used.

[0026] Figure 2 is a cross-sectional view showing details of the developing unit. Figure 2 is a cross-sectional view of the developing unit 20Y cut in a plane perpendicular to the rotation axis of the photoreceptor drum. Referring to Figure 2, the developing unit 20Y includes a developing device 24Y, a photoreceptor drum 23Y, a charging roller 22Y, an exposure unit 21Y, a primary transfer roller 25Y, a drum cleaning blade 27Y, and a detection sensor 29Y.

[0027] The photoreceptor drum 23Y is a cylindrical image carrier with a photoconductive layer formed on the outer circumference of a conductive substrate such as aluminum. The photoreceptor drum 23Y is pivotally supported in the housing of the MFP 100 so as to be rotatable on an axis of rotational symmetry. Around the photoreceptor drum 23Y, a charging roller 22Y, an exposure unit 21Y, a developing device 24Y, a detection sensor 29Y, a primary transfer roller 25Y, and a drum cleaning blade 27Y are arranged in order along the rotational direction of the photoreceptor drum 23Y. The primary transfer roller 25Y is positioned above the photoreceptor drum 23Y, sandwiching the intermediate transfer belt 30. The charging roller 22Y uniformly charges the surface of the photoreceptor drum 23Y, which is the image carrier. The drum cleaning blade 27Y removes residual toner from the photoreceptor drum 23Y.

[0028] The photoreceptor drum 23Y is charged by the charging roller 22Y, and then irradiated with laser light emitted by the exposure unit 21Y. The exposure unit 21Y exposes the image-corresponding portion of the surface of the photoreceptor drum 23Y. An electrostatic latent image is formed on the exposed portion of the photoreceptor drum 23Y.

[0029] The developing device 24Y uses a developer consisting of a carrier and toner to form a toner image on the photoreceptor drum 23Y. The developing device 24Y comprises a housing 200Y, a first screw 201Y, a second screw 203Y, a developing roller 221Y, and a regulating blade 223Y.

[0030] The housing 200Y is a casing that houses the developer, the first screw 201Y, the second screw 203Y, the developing roller 221Y, and the regulating blade 223Y. A sensor is attached to the housing 200Y to detect the amount of developer inside the housing 200Y. If the amount of developer detected by the sensor is less than a predetermined value, developer is supplied from the toner bottle 41Y to the housing 200Y.

[0031] Within the housing 200Y, the developing roller 221Y, the first screw 201Y, and the second screw 203Y are arranged side by side and are rotatably supported by the housing 200Y. The direction in which the developing roller 221Y, the first screw 201Y, and the second screw 203Y extend is the Y direction.

[0032] The housing 200Y is a container extending in the Y direction and has two spaces, a first circulation tank Sp1 and a second circulation tank Sp2, separated by a partition wall 205Y extending in the Y direction. The first circulation tank Sp1 is provided with a first screw 201Y, and the second circulation tank Sp2 is provided with a second screw 203Y. The first screw 201Y and the second screw 203Y each have a cylindrical rotating shaft extending in the Y direction with spiral blades on the outer surface, and convey the developer by rotating. The first circulation tank Sp1 and the second circulation tank Sp2 are storage spaces for containing the developer.

[0033] Openings are provided at both ends of the partition wall 205Y in the Y direction, connecting the first circulation tank Sp1 and the second circulation tank Sp2. As the first screw 201Y rotates, the developer in the first circulation tank Sp1 is transported to the positive side in the Y direction, and the developer transported to the end of the partition wall 205Y enters the second circulation tank Sp2 through the opening. As the second screw 203Y rotates, the developer in the second circulation tank Sp2 is transported to the negative side in the Y direction, and the developer transported to the end of the partition wall 205Y enters the first circulation tank Sp1 through the opening. In this way, the developer is circulated between the first circulation tank Sp1 and the second circulation tank Sp2 by the first screw 201Y and the second screw 203Y.

[0034] The developing roller 221Y is installed in the first circulation tank Sp1, facing the first screw 201Y. Furthermore, the developing roller 221Y is exposed from the housing 200Y. The portion of the developing roller 221Y exposed from the housing 200Y faces the photoreceptor drum 23Y. Specifically, the developing roller 221Y's rotation axis is rotatably supported in the housing 200Y so that a small gap is maintained between it and the photoreceptor drum 23Y. The developer contains a magnetic carrier and a non-magnetic toner. The developing roller 221Y uses the magnetic force of the roll section 225Y located inside it to attract the magnetic carrier together with the non-magnetic toner, and carries the developer that has been transported by the first screw 201Y. Hereinafter, the collection of developer carried by the developing roller 221Y is referred to as the developing brush.

[0035] A regulating blade 223Y is positioned near the developing roller 221Y. The regulating blade 223Y is supported at both ends by the housing 200Y. The end of the regulating blade 223Y facing the developing roller 221Y is located circumferentially upstream of the part of the developing roller 221Y's surface that is closest to the photosensitive drum 23Y.

[0036] Therefore, the amount of developer carried by the developing roller 221Y is limited by the regulating blade 223Y. Specifically, as the developing roller 221Y rotates, the amount of developer carried by the developing roller 221Y that comes into contact with the regulating blade 223Y is no longer carried by the developing roller 221Y. The developer passing through the gap between the regulating blade 223Y and the developing roller 221Y reaches the developing region where the distance between the developing roller 221Y and the photosensitive drum 23Y is minimized.

[0037] The developing roller 221Y develops the electrostatic latent image by applying toner to the photoreceptor drum 23Y. Specifically, a developing bias is applied to the developing roller 221Y. As a result, the potential of the surface of the developing roller 221Y is lower than the potential of the area on the surface of the photoreceptor drum 23Y where the electrostatic latent image is formed (approximately 0V), and higher than the potential of the area on the photoreceptor drum 23Y where the electrostatic latent image is not formed. The toner in the developer carried by the developing roller 221Y is negatively charged, so it adheres to the area on the surface of the photoreceptor drum 23Y where the electrostatic latent image is formed. As a result, a toner image is formed on the area on the surface of the photoreceptor drum 23Y where the electrostatic latent image is formed by the negatively charged toner.

[0038] The detection sensor 29Y detects the density of the toner image formed on the photoreceptor drum 23Y. Specifically, the detection sensor 29Y is a reflective optical sensor that irradiates the surface of the photoreceptor drum 23Y with light of a predetermined wavelength. The light irradiated towards the surface of the photoreceptor drum 23Y is diffusely reflected by the surface of the photoreceptor drum 23Y or the toner image formed on the surface of the photoreceptor drum 23Y. The detection sensor 29Y detects the light that is reflected in the direction of the detection sensor 29Y from the diffusely reflected light. The detection sensor 29Y outputs a detection value indicating the amount of light detected. The amount of light received by the detection sensor 29Y varies depending on the density of the toner image formed on the photoreceptor drum 23Y. The higher the density of the toner image, the smaller the amount of light. Therefore, the density of the toner image supported by the photoreceptor drum 23Y is detected from the output value of the detection sensor 29Y.

[0039] The toner image formed on the photoreceptor drum 23Y is transferred onto the intermediate transfer belt 30, which is the image carrier, by the action of an electric field by the primary transfer roller 25Y. Toner that remains on the photoreceptor drum 23Y without being transferred is removed from the photoreceptor drum 23Y by the drum cleaning blade 27Y. The toner contains cleaning and lubricating components. Therefore, the portion of the photoreceptor drum 23Y that carries the toner is cleaned while the frictional force is reduced by the toner removed by the drum cleaning blade 27Y.

[0040] Figure 3 is a block diagram illustrating the hardware configuration of the MFP in the first embodiment. Referring to Figure 3, the MFP 100 includes a main circuit 110, a document reading unit 130 for reading documents, an automatic document transport device 120 for transporting documents to the document reading unit 130, an image forming unit 140 for forming an image on paper or the like based on image data output by the document reading unit 130, a paper feeding unit 150 for supplying paper to the image forming unit 140, and an operation panel 160 as a user interface.

[0041] The main circuit 110 includes a CPU 111, a communication interface (I / F) unit 112, a ROM 113, a RAM 114, a hard disk drive (HDD) 115 as a mass storage device, a facsimile unit 116, and an external storage device 117 on which a CD-ROM 118 is installed. The CPU 111 is connected to the automatic document transport device 120, the document reading unit 130, the image forming unit 140, the paper feeding unit 150, and the operation panel 160, and controls the entire MFP 100.

[0042] ROM 113 stores the program executed by CPU 111, or the data necessary to execute that program. RAM 114 is used as a workspace when CPU 111 executes the program. Furthermore, RAM 114 temporarily stores image data that is continuously sent from document reading unit 130.

[0043] The communication interface 112 is an interface for connecting the MFP 100 to a network. The CPU 111 communicates with the PC 200 via the communication interface 112 to send and receive data. The communication interface 112 can also communicate with computers connected to the Internet 5 via the network.

[0044] The facsimile unit 116 is connected to the Public Switched Telephone Network (PSTN) and transmits facsimile data to or receives facsimile data from the PSTN. The facsimile unit 116 stores the received facsimile data in the HDD 115 or outputs it to the image forming unit 140. The image forming unit 140 prints the facsimile data received by the facsimile unit 116 onto paper. The facsimile unit 116 also converts the data stored in the HDD 115 into facsimile data and transmits it to a facsimile device connected to the PSTN.

[0045] The external storage device 117 is equipped with a CD-ROM 118. The CPU 111 can access the CD-ROM 118 via the external storage device 117. The CPU 111 loads the program recorded on the CD-ROM 118 installed in the external storage device 117 into the RAM 114 and executes it. Note that the medium for storing the program executed by the CPU 111 is not limited to the CD-ROM 118, but may also be a flexible disk, cassette tape, optical disc (MO (Magnetic Optical Disc) / MD (Mini Disc) / DVD (Digital Versatile Disc)), IC card, optical card, mask ROM, EPROM (Erasable Programmable ROM), or other semiconductor memory.

[0046] Furthermore, the programs executed by the CPU 111 are not limited to those recorded on the CD-ROM 118; programs stored on the HDD 115 may be loaded into the RAM 114 and executed. In this case, other computers connected to the network may overwrite programs stored on the MFP 100's HDD 115 or add and write new programs. In addition, the MFP 100 may download programs from other computers connected to the network and store those programs on the HDD 115. The term "program" here includes not only programs that the CPU 111 can directly execute, but also source programs, compressed programs, encrypted programs, and so on.

[0047] The control panel 160 is provided on the top surface of the MFP 100 and includes a display unit 161 and an operation unit 163. The display unit 161 is, for example, a liquid crystal display (LCD) or an organic LE (electroluminescence) display, and displays instruction menus for the user, information about acquired image data, etc. The operation unit 163 includes a touch panel 165 and a hard key unit 167. The touch panel 165 is provided on the top or bottom surface of the display unit 161, superimposed on the display unit 161. The hard key unit 167 includes a plurality of hard keys. The hard keys are, for example, contact switches. The touch panel 165 detects the position indicated by the user on the display surface of the display unit 161.

[0048] The image forming unit 140 includes a drive control unit 141 that controls the development bias and the charging bias.

[0049] Figure 4 is a block diagram illustrating the drive control unit. Referring to Figure 4, the drive control unit 141 controls the development bias and charging bias for each of the Y, M, C, and K color developing units 24Y, 24M, 24C, and 24K. Here, the control of the development bias and charging bias for the developing unit 24Y will be described. The drive control unit 141 controls the DC power supply circuit 142 to apply a DC voltage as the charging bias to the charging roller 22Y. The drive control unit 141 also controls the DC power supply circuit 143 to apply a DC voltage as the development bias to the developing roller 221Y. In some cases, the drive control unit 141 may also control the developing roller 221Y to apply a voltage obtained by superimposing an AC voltage on a DC voltage as the development bias. In this case, the drive control unit 141 controls the AC power supply circuit 144 to apply an AC voltage as the development bias to the developing roller 221Y in parallel with the output of the DC power supply circuit 143.

[0050] Figure 5 is a block diagram showing an example of the functions of the CPU in the MFP in this embodiment. The functions shown in Figure 5 are functions realized by the CPU 111 of the MFP 100 when the CPU 111 executes an image forming program stored in the ROM 113, HDD 115, or CD-ROM 118.

[0051] Referring to Figure 5, the CPU 111 includes a fringe image forming unit 51, a density detection unit 53, a corresponding data acquisition unit 55, a preliminary determination unit 57, and a correction unit 59.

[0052] The fouling image forming unit 51 controls the image forming unit 140 to form a fouling image on the photoreceptor drum 23Y. The fouling image forming unit 51 forms a fouling image on the photoreceptor drum 23Y that corresponds to the fouling margin. The fouling margin is the voltage difference between the charging bias applied to the charging roller 22Y and the developing bias applied to the developing roller 221Y, and is the value obtained by subtracting the developing bias from the charging bias. If the developing bias includes an AC component, it is the value obtained by subtracting the DC component of the developing bias from the charging bias. The fouling image is a toner image formed on the photoreceptor drum 23Y when the image forming unit 140 is operated with the developing bias determined by the fouling margin applied to the developing roller 221Y and the charging bias determined by the fouling margin applied to the charging roller 22Y. Furthermore, while the fouling image formation unit 51 is forming a fouling image on the photoreceptor drum 23Y, the photoreceptor drum 23Y is not exposed by the exposure unit 21Y.

[0053] The fouling image forming unit 51 causes multiple fouling images corresponding to multiple fouling margins to be formed on the photoreceptor drum 23Y. The fouling image forming unit 51 outputs the values ​​of the multiple fouling margins to the corresponding data acquisition unit 55 and density detection unit 53. In this embodiment, an example will be described in which 21 fouling images corresponding to a total of 21 fouling margin values ​​are formed on the photoreceptor drum 23Y, with the fouling margins set in 10V increments between -50V and 150V.

[0054] The density detection unit 53 controls the detection sensor 29Y and acquires the output of the detection sensor 29Y, which detects light reflected from the fouling image formed on the photoreceptor drum 23Y. The density of the toner in the fouling image is calculated from the output of the detection sensor 29Y. The output value of the detection sensor 29Y is smaller the higher the toner density. The density detection unit 53 outputs the output value of the detection sensor 29Y to the corresponding data acquisition unit 55.

[0055] The corresponding data acquisition unit 55 generates corresponding data that associates the fouling margin with the output value of the detection sensor 29Y. The corresponding data acquisition unit 55 associates the output value input from the detection sensor 29Y at the timing when the fouling image formed on the photoreceptor drum 23Y by the fouling image forming unit 51 reaches the detection range in which the detection sensor 29Y can detect toner with the value of the fouling margin used to form that fouling image. The same number of corresponding data is generated as the number of fouling images that the fouling image forming unit 51 causes to form on the photoreceptor drum 23Y. In this embodiment, 21 corresponding data are generated.

[0056] The provisional determination unit 57 determines a provisional cover margin based on multiple corresponding data. The larger the cover margin, the smaller the density of the cover image. On the other hand, the detection accuracy of the detection sensor 29Y has limits. When the density of the cover image exceeds the detection accuracy range of the detection sensor 29Y and falls below a predetermined value, the output value of the detection sensor 29Y becomes a value within the predetermined range. Therefore, it is not possible to accurately determine whether a cover image has been formed based solely on the output of the detection sensor 29Y.

[0057] Figure 6 is the first graph showing an example of corresponding data. In Figure 6, the horizontal axis represents the fouling margin (V), and the vertical axis represents the output value of the detection sensor 29Y. Curve 201 shows the change in the output value of the detection sensor 29Y as the fouling margin changes. The output value of the detection sensor 29Y is the difference from the output value when there is no obstruction (i.e., fouling toner) on the surface of the photosensitive drum 23Y, which is the target of detection by the detection sensor 29Y.

[0058] According to Figure 6, when the fouling margin is small, the output value of the detection sensor 29Y is small, and as the fouling margin increases, the output value of the detection sensor 29Y also increases. In the unstable region where the fouling margin is between -50V and 20V, the smaller the fouling margin, the greater the amount of toner in the fouled image, and the larger the fouling margin, the less toner in the fouled image. In the stable region where the fouling margin is 20V or higher, the output value of the detection sensor 29Y falls within a predetermined range and is saturated.

[0059] Returning to Figure 5, the provisional determination unit 57 determines a provisional cover margin based on the rate of change, which is the ratio of the change in the output value of the detection sensor 29Y to the change in the cover margin.

[0060] The provisional determination unit 57 calculates the rate of change for each overlap margin using the corresponding data. Here, the overlap margin is denoted by M(i), and the output value of the detection sensor 29Y corresponding to the overlap margin M(i) is denoted by V(i). i is a positive integer and is a value that identifies the overlap margin. Here, the overlap margin for i=1 is -50V, and the overlap margin for i=21 is +150V. An overlap margin that is 10V larger than the overlap margin M(i) is denoted by M(i+1). The rate of increase ΔV(i) of the output value of the detection sensor 29Y with respect to the overlap margin M(i) is calculated using the following equation (1). i is an integer greater than 1. In addition, the rate of change δ(j), which shows the change in the rate of increase ΔV(j), is calculated using the following equation (2). j is an integer greater than 2.

[0061] The rate of increase ΔV(i) = (V(i) - V(i-1)) / (M(i) - MI(i-1)) ... (1) Fluctuation rate δ(j)=(ΔV(j)-ΔV(j-2)) / (M(j)-M(j-2))…(2) The provisional determination unit 57 determines the overlap margin M(J) in which the variable j has the smallest difference between the rate of change δ(j) and the rate of change δ(j-1) is less than or equal to a predetermined threshold, among a plurality of rate of change δ(j) values. J is the suffix of the smallest rate of change δ(j) among a plurality of rate of change δ(j) values ​​less than or equal to the predetermined threshold. The provisional determination unit 57 sets the overlap margin M(J) as the provisional overlap margin. The provisional determination unit 57 outputs the provisional overlap margin to the correction unit 59.

[0062] The provisional determination unit 57 may calculate the rate of change δ(j) from the growth rate ΔV(j) and the growth rate ΔV(j-1). Alternatively, the provisional determination unit 57 may calculate the rate of change δ(i) from the growth rate ΔV(j) and the growth rate ΔV(j-3). Alternatively, the provisional determination unit 57 may calculate the rate of change δ(j) from the ratio of the growth rate ΔV(j) and the growth rate ΔV(j-2). Alternatively, the provisional determination unit 57 may calculate the rate of change δ(j) from the ratio of the growth rate ΔV(j) and the growth rate ΔV(j-1).

[0063] The correction unit 59 corrects the provisional cover margin input from the provisional determination unit 57. The correction unit 59 includes a first correction value determination unit 61. As described above, the provisional cover margin is the minimum density of the cover image that the detection sensor 29Y can detect. The correction unit 59 corrects the provisional cover margin for the portion that exceeds the detection accuracy of the detection sensor 29Y using the output value output by the detection sensor 29Y within the range of its detection accuracy.

[0064] Here, we will explain the relationship between the degradation state of the developer stored in the developing unit 24Y and the fouling margin. As the toner degrades, the charging performance of the toner decreases. It is known that this difference in toner charging performance affects the density of the fouling image relative to the fouling margin. Generally, when comparing the charging distribution of undegraded toner and degraded toner, the dispersion of undegraded toner is greater than that of undegraded toner.

[0065] Figure 7 shows a portion of the charge distribution of degraded toner. Figure 8 shows a portion of the charge distribution of undegraded toner. In both Figures 7 and 8, the horizontal axis shows the charge amount of the toner. The further to the right on the horizontal axis, the smaller the charge amount. The vertical axis shows the number of toner particles. In Figures 7 and 8, the low-charge portion of the toner's charge distribution is shown. Also, the area of ​​the two portions with the same hatching is the same. When the toner stored in the developer unit 24Y is degraded, the charge distribution has a large standard deviation, and when the toner stored in the developer unit 24Y is undegraded, the charge distribution has a small standard deviation.

[0066] Furthermore, in Figures 7 and 8, a provisional fouling margin and target fouling amount are shown for each toner charge level. The charge level relative to the provisional fouling margin indicates the charge level of toner that does not form a fouling image. Toner with a charge level greater than the provisional fouling margin does not form a fouling image. Toner with a charge level less than the provisional fouling margin may or may not form a fouling image.

[0067] Here, if the charge distribution of the toner is known, it is possible to predict the amount of charge that will not form a fouling image for toner whose charge is less than the charge corresponding to the provisional fouling margin. This is called the target fouling margin, which corresponds to the predicted charge of toner that does not form a fouling image. Specifically, the amount of charge of toner that becomes the target fouling margin can be determined from the ratio of the change in the number of toners to the change in the charge of toners whose charge is less than the charge corresponding to the provisional fouling margin. For example, the charge for the target fouling margin is determined such that the number of toners whose charge is less than or equal to the target fouling margin and greater than or equal to the charge corresponding to the provisional fouling margin is the same as the number of toners which is less than or equal to a predetermined number of toners which is less than the provisional fouling margin. In the figure, the difference between the charge for the target fouling margin and the charge for the provisional fouling margin is shown as the first correction amount equivalent to the first correction amount that corrects the provisional fouling margin. The predetermined number can be determined by experiment.

[0068] Referring to Figures 7 and 8, in the charge distribution of undegraded toner shown in Figure 8, the change in the number of toner particles higher than the charge amount relative to the hypothetical fouling margin is steeper than in the charge distribution of degraded toner shown in Figure 7. Therefore, the amount equivalent to the first correction amount is smaller for undegraded toner than for degraded toner.

[0069] Figure 9 is a second graph showing an example of corresponding data. In Figure 9, the horizontal axis represents the fouling margin (V), and the vertical axis represents the output value of the detection sensor 29Y. Curve 201 shows the change in the output value of the detection sensor 29Y in relation to the change in fouling margin for non-degraded toner. Curve 203 shows the change in the output value of the detection sensor 29Y in relation to the change in fouling margin for degraded toner.

[0070] Comparing curves 201 and 203, the slope of curve 201 is greater than the slope of curve 203 in the unstable region. It has been found that the difference in charge distribution due to toner degradation correlates with the slopes of the unstable regions of curves 201 and 203, respectively. When curve 203, which has a small slope in the unstable region, is obtained, the standard deviation of the charge distribution of the toner stored in the developing unit 24Y is large, and when curve 201, which has a large slope in the unstable region, is obtained, the standard deviation of the charge distribution of the toner stored in the developing unit 24Y is small.

[0071] Returning to Figure 5, the first correction value determination unit 61 defines a normally assumed distribution and uses the offset margin to the target value of the fogging margin in that fogging development characteristic as the correction center value, correcting the change in the fogging development characteristic as variability. Here, the fogging development characteristic S(i) is the value calculated by the following equation (3). S(i)=(V(i)-V(i-3)) / (M(i)-MI(i-3))…(3) However, i > 3 and i = J.

[0072] The first correction value determination unit 61 pre-determines a standard correction value for correcting a provisional fogging margin to a target fogging margin for a standard toner charge distribution, and determines the first correction value by feeding back the fogging margin due to variations in the toner charge distribution to the standard correction value. Specifically, the first correction value determination unit 61 stores in advance a first correction table obtained experimentally from the relationship between fogging development characteristics and the amount of correction. In the experiment to create this table, the amount of toner in the fogging image is measured by collecting the toner in the fogging image actually formed on the photoreceptor drum 23Y.

[0073] Furthermore, the first correction value determination unit 61 sets the first correction value to a predetermined upper limit if the fogging development characteristics fall below a predetermined threshold. If the fogging development characteristics fall below a predetermined value, it is expected that the toner charge distribution will not be a normal distribution and will include two or more normal distributions. In this case, the fogging development characteristics will not represent the toner charge distribution.

[0074] Figure 10 shows an example of a first correction table. In Figure 10, the horizontal axis represents the fogging characteristic, and the vertical axis represents the first correction value. Here, the points plotted from the table values ​​are shown as an approximate graph. Referring to Figure 10, the toner charge distribution when the fogging characteristic is "8" is considered the standard toner charge distribution. It is also shown that the first correction value in this case is "30V". Furthermore, for the toner charge distribution with a fogging characteristic of "5", the first correction value of "60V" is set. Furthermore, for the toner charge distribution with a fogging characteristic of "11", the first correction value of "20V" is set. In the correction table, the first correction value is set so that it increases as the fogging characteristic decreases. Note that here, the case of determining the first correction value using a table is explained as an example, but the calculation formula showing the relationship between the fogging characteristic and the first correction value shown in the graph in Figure 10 may also be used.

[0075] Figure 11 is a flowchart showing an example of the overlap margin setting process. The overlap margin setting process is performed by the CPU 111 of the MFP 100 when the CPU 111 executes an overlap margin setting program stored in the ROM 113, HDD 115, or CD-ROM 118.

[0076] Referring to Figure 11, the CPU 111 of the MFP 100 determines whether or not image formation is in progress. If image formation is in progress, it enters a standby state (YES in step S01); if image formation is not in progress (NO in step S01), the process proceeds to step S02. If the process proceeds to step S02, it is set to a mode for setting the overlap margin. In the mode for setting the overlap margin, image formation processing is prohibited.

[0077] In step S02, a fogging margin is selected, and the process proceeds to step S03. A fogging margin to be processed is selected from among a predetermined number of fogging margins. In step S03, a fogging image is formed, and the process proceeds to step S04. The voltages for the development bias and charging bias, determined by the fogging margin, are determined, and the development process is performed without exposing the photoreceptor drums 23Y, 23M, 23C, and 23K. As a result, fogging images are formed on each of the photoreceptor drums 23Y, 23M, 23C, and 23K.

[0078] In step S04, the density of the fringe image is detected, and the process proceeds to step S05. The output values ​​of each detection sensor 29Y, 29M, 29C, and 29K are acquired. In step S05, corresponding data is generated, and the process proceeds to step S06. Corresponding data is generated that includes the fringe margin selected in step S02 and the output values ​​of each detection sensor 29Y, 29M, 29C, and 29K acquired in step S04, and is stored in HDD 115. Corresponding data is generated for each developing unit 20Y, 20M, 20C, and 20K.

[0079] In step S06, it is determined whether or not there are overlap margins that have not been selected for processing. If there are unselected overlap margins, the process returns to step S02; otherwise, the process proceeds to step S07. By repeating the process from step S02 to step S06, corresponding data is generated for each of the pre-prepared overlap margins.

[0080] In step S07, a provisional cover margin is determined, and the process proceeds to step S08. A provisional cover margin is determined for each developing unit 20Y, 20M, 20C, and 20K. Using equation (1), the increase rate ΔV(i) for each of the multiple cover margins is determined, and using equation (2), the fluctuation rate δ(j) for each of the multiple cover margins is determined. Then, the cover margin M(J) in which the variable j is smallest among the multiple fluctuation rate δ(j) in which the difference between the fluctuation rate δ(j) and the fluctuation rate δ(j-1) is less than or equal to a predetermined threshold is determined as the provisional cover margin. J is the suffix of the smallest fluctuation rate δ(j) among the multiple fluctuation rates δ(j) less than or equal to the predetermined threshold.

[0081] In step S08, the correction value is determined, and the process proceeds to step S09. A correction value is determined for each developing unit 20Y, 20M, 20C, and 20K. Based on the charge distribution of the toner stored in each developing device 24Y, 24M, 24C, and 24K, a provisional fogging margin correction value is determined.

[0082] In step S09, the fringe margin is determined and set, and the process proceeds to step S10. The fringe margin is determined and set for each developing unit 20Y, 20M, 20C, and 20K. The temporary fringe margin is corrected by the correction value determined in step S08, and the corrected temporary fringe margin is determined as the fringe margin. Then, the determined fringe margin is set.

[0083] In step S10, the mode for setting the overlap margin is deactivated, and the process ends. This sets the system to a state where image formation is possible.

[0084] <First variation> Figure 12 is a block diagram showing an example of the functions of the CPU in the MFP in this embodiment of the first modified example. Referring to Figure 12, the differences from the functions shown in Figure 5 are the addition of a film thickness detection unit 71 and the change of the correction unit 59 to a correction unit 59A. The other functions are the same as those shown in Figure 5, so they will not be explained again here. The correction unit 59A includes a first correction value determination unit 61, a second correction value determination unit 63, and a third correction value determination unit 65.

[0085] The film thickness detection unit 71 detects the film thickness of the photoreceptor drum 23Y and outputs the detected film thickness to the second correction value determination unit 63. The film thickness detection unit 71 determines that the film thickness of the photoreceptor drum 23Y is correlated with the charging current flowing through the charging roller 22Y. Therefore, the film thickness detection unit 71 converts the charging current into the film thickness of the photoreceptor drum 23Y by detecting the charging current flowing through the charging roller 22Y.

[0086] The second correction value determination unit 63 determines a second correction value based on the film thickness of the photoreceptor drum 23Y and corrects the first correction value with the second correction value. When the film thickness of the photoreceptor drum 23Y changes, the film thickness resistance decreases, and as a result, the strength of the electric field generated at the developing nip between the photoreceptor drum 23Y and the developing roller 221Y increases. This causes the fogging development characteristics to change in a direction that increases.

[0087] The fogging development characteristics used by the first correction value determination unit 61 when determining the first correction value are based on the film thickness of the photoreceptor drum 23Y in its initial state, where the film thickness is at its maximum. Therefore, if the film thickness of the photoreceptor drum 23Y changes from its initial state, it is necessary to convert the fogging development characteristics to those corresponding to the initial film thickness of the photoreceptor drum 23Y. Experiments have confirmed that even with the same charge bias voltage, the actual electric field strength at the development nip changes depending on the film thickness of the photoreceptor drum 23Y, which accelerates the convergence of the output value of the detection sensor 29Y and results in a smaller provisional margin.

[0088] The second correction value determination unit 63 determines the fogging development characteristics of the photoreceptor drum 23Y at its initial film thickness by converting the voltage of the charging bias into a ratio of the film thickness change, since the film thickness of the photoreceptor drum 23Y and the electric field strength at the developing nip are inversely proportional. Specifically, the second correction value determination unit 63 corrects the first correction value with a second correction value, which is the increase in electric field strength determined by the difference between the film thickness of the photoreceptor drum 23Y input from the film thickness detection unit 71 and the initial film thickness. The second correction value determination unit 63 has prepared a second correction table in advance that defines the relationship between the decrease in film thickness of the photoreceptor drum 23Y and the second correction value. The second correction value determination unit 63 determines the second correction value by determining the difference between the film thickness of the photoreceptor drum 23Y input from the film thickness detection unit 71 and the initial film thickness of the photoreceptor drum 23Y, and by referring to the second correction table. The second correction value determination unit 63 corrects the first correction value with the second correction value according to the film thickness of the photoreceptor drum 23Y, so that the fogging margin can be determined with high accuracy.

[0089] The third correction value determination unit 65 corrects the first correction value based on the distance between the photoreceptor drum 23Y and the developing roller 221Y. Hereinafter, the distance between the photoreceptor drum 23Y and the developing roller 221Y is referred to as the development gap. The development gap may differ due to individual differences between the photoreceptor drum 23Y and the developing roller 221Y. Variations in the development gap cause variations in the electric field strength at the development nip, which in turn affects the fogging development characteristics.

[0090] <In case of simultaneous replacement> First, let's explain the case where the photoreceptor drum 23Y and the developing roller 221Y are replaced simultaneously. The fogging development characteristics used by the first correction value determination unit 61 to determine the first correction value are based on a standard development gap. Therefore, the fogging development characteristics determined by the first correction value determination unit 61 can be assumed to have been detected for toner with a predetermined standard deviation distribution relative to new toner.

[0091] The third correction value determination unit 65 calculates the development gap by comparing the fogging development characteristics at a provisional fogging margin determined by the first correction value determination unit 61 with the standard fogging development characteristics, using the fogging development characteristics at a standard development gap as a reference. Specifically, it is known that the development gap and the fogging development characteristics are proportional. The third correction value determination unit 65 has prepared a development gap table in advance that associates the ratio of the fogging development characteristics at the provisional fogging margin determined by the first correction value determination unit 61 to the standard fogging development characteristics with the development gap. The third correction value determination unit 65 finds the ratio of the fogging development characteristics at the provisional fogging margin determined by the first correction value determination unit 61 with the fogging development characteristics at the reference development gap, and determines the development gap by referring to the development gap table.

[0092] Furthermore, the third correction value determination unit 65 has prepared a third correction table in advance, which defines the relationship between the ratio of the calculated development gap to the reference development gap and the third correction value. The third correction value determination unit 65 finds the ratio of the calculated development gap to the reference development gap, and determines the third correction value by referring to the third correction table. The third correction value determination unit 65 corrects the first correction value with the third correction value. Since the third correction value determination unit 65 corrects the first correction value with the third correction value according to the development gap, the fogging margin can be determined with high accuracy.

[0093] <When replacing only the photoconductor drum> Next, we will explain the case where the photoreceptor drum 23Y is replaced, but the developing unit 24Y is not. In this case, the electric field strength fluctuates due to the change in the developing gap caused by replacing the photoreceptor drum 23Y. For this reason, the third correction value determination unit 65 determines the fourth correction value, assuming that there is no change in the charge distribution of the toner stored in the developing unit 24Y before and after the replacement of the photoreceptor drum 23Y.

[0094] The fogging development characteristics for the temporary fogging margin last detected before the photoreceptor drum 23Y is replaced include the change in electric field strength due to the decrease in film thickness of the photoreceptor drum 23Y. Therefore, the third correction value determination unit 65 converts the fogging development characteristics for the temporary fogging margin last detected before the photoreceptor drum 23Y is replaced, based on the film thickness of the photoreceptor drum 23Y before the replacement, to the fogging development characteristics for the initial film thickness of the photoreceptor drum 23Y. The fogging development characteristics converted from the fogging development characteristics for the temporary fogging margin last detected before the photoreceptor drum 23Y is replaced are called the pre-replacement development characteristics.

[0095] The third correction value determination unit 65 determines the development gap based on the ratio of the fogging development characteristics to the provisional fogging margin calculated by the first correction value determination unit 61 immediately after the photoreceptor drum 23Y is replaced, and the development characteristics before replacement.

[0096] The third correction value determination unit 65 determines the ratio of the calculated development gap to the reference development gap, and determines the fourth correction value by referring to the third correction table. The third correction value determination unit 65 corrects the first correction value with the fourth correction value. Since the third correction value determination unit 65 corrects the first correction value with the fourth correction value according to the development gap, the fogging margin can be determined with high accuracy.

[0097] <If only the developing unit is replaced> Next, we will explain the case where the developing unit 24Y is replaced, but the photoreceptor drum 23Y is not. In this case, the electric field strength fluctuates due to the change in the development gap caused by replacing the developing unit 24Y. The fogging development characteristics immediately after replacing the developing unit 24Y are affected by the change in electric field strength due to the reduction in the film thickness of the photoreceptor drum 23Y. For this reason, the third correction value determination unit 65 converts the fogging development characteristics with respect to the temporary fogging margin, based on the film thickness of the photoreceptor drum 23Y after replacement, to the fogging development characteristics with respect to the film thickness of the photoreceptor drum 23Y in its initial state. The development characteristics obtained by converting the fogging development characteristics with respect to the temporary fogging margin detected after replacing the photoreceptor drum 23Y are called the post-replacement development characteristics.

[0098] The third correction value determination unit 65 determines the development gap based on the ratio of the fogging development characteristics to the provisional fogging margin calculated by the first correction value determination unit 61 immediately after the photoreceptor drum 23Y is replaced, and the development characteristics after replacement.

[0099] The third correction value determination unit 65 determines the ratio of the calculated development gap to the reference development gap, and determines the fifth correction value by referring to the third correction table. The third correction value determination unit 65 corrects the first correction value with the fifth correction value. Since the third correction value determination unit 65 corrects the first correction value with the fifth correction value according to the development gap, the fogging margin can be determined with high accuracy.

[0100] <Second variation> Although the correction unit 59 is shown as including a second correction value determination unit 63 and a third correction value determination unit 65 in addition to the first correction value determination unit 61, the present invention is not limited thereto. The correction unit 59 may include only the first correction value determination unit 61 and not include the second correction value determination unit 63 and the third correction value determination unit 65. Alternatively, the correction unit 59 may include the first correction value determination unit 61 and the second correction value determination unit 63 and not include the third correction value determination unit 65. Furthermore, the correction unit 59 may include the first correction value determination unit 61 and the third correction value determination unit 65 and not include the second correction value determination unit 63.

[0101] Furthermore, if there are other factors that affect the cover margin depending on the model, or if there are factors that require statistical correction of the cover margin based on data collected from devices released to the market, the cover margin may be corrected based on those factors.

[0102] As described above, the MFP100 in this embodiment includes a detection sensor 29Y that detects the density of a fringe image formed on the photoreceptor drum 23Y when a development bias is applied to the developing roller 221Y and a charging bias is applied to the charging roller 22Y that charges the photoreceptor drum 23Y, and

[0103] Furthermore, the MFP100 predicts the toner's charge distribution based on the fogging and development characteristics, and determines a first correction value to correct the provisional fogging margin based on the predicted toner's charge distribution. Since the fogging and development characteristics indicate differences in the toner's charge distribution, the charge bias and development bias are determined according to the toner's charge distribution. Therefore, it is possible to set a fogging margin according to the toner's charge distribution.

[0104] Furthermore, the MFP100 determines a second correction value based on the difference between the film thickness detected by the detection sensor 29Y, which detects the film thickness of the photoreceptor drum 23Y, and a reference film thickness defined for the first correction value, and corrects the first correction value with the second correction value. Therefore, it is possible to set a cover margin according to the change in the film thickness of the photoreceptor drum 23Y.

[0105] Furthermore, when the developing roller 221Y and photoreceptor drum 23Y are replaced, the MFP100 determines a third correction value based on the ratio of the fogging development characteristics detected in the replaced state to the fogging development characteristics detected for the standard developing roller 221Y and photoreceptor drum 23Y defined for the first correction value, and corrects the first correction value with the third correction value. This makes it possible to determine a fogging margin that corresponds to individual differences between the developing roller 221Y and photoreceptor drum 23Y.

[0106] Furthermore, when the photoreceptor drum 23Y is replaced, the MFP100 determines a fourth correction value based on the ratio of the fogging characteristics detected after correcting the fogging characteristics detected for a standard image carrier (defined for the first correction value) to the fogging characteristics detected after replacement, and then corrects the first correction value with the fourth correction value. This allows for the determination of a fogging margin that corresponds to individual differences in the photoreceptor drum 23Y.

[0107] Furthermore, when the developing roller 221Y is replaced, the MFP100 determines a fifth correction value based on the ratio of the fogging characteristics detected after the replacement to the fogging characteristics detected for a standard image carrier defined for the first correction value, and the fogging characteristics detected after the replacement, and then corrects the first correction value with the fifth correction value. This makes it possible to determine a fogging margin that corresponds to the individual differences of the developing roller 221Y.

[0108] <Summary of Embodiments> (Article 1) A density detection means for detecting the density of a hazy image formed on an image carrier, A provisional determination means for determining a provisional fouling margin, which is a fouling margin that indicates the difference between the development bias applied to the toner carrier and the charging bias applied to the charger that charges the image carrier, based on the density detected by the density detection means, An image forming apparatus comprising: a correction means for correcting the provisional cover margin based on the rate at which the concentration detected by the concentration detection means changes in relation to the change in the cover margin.

[0109] In this scenario, a provisional cover margin is determined based on the density detected by the density detection means, and the provisional cover margin is corrected based on the rate at which the density detected by the density detection means changes in relation to the change in the cover margin. Therefore, it is possible to provide an image forming apparatus that can determine an appropriate cover margin without improving the performance of detecting toner density.

[0110] (Clause 2) The image forming apparatus according to Clause 1, wherein the correction means predicts the charge distribution of the toner based on the rate at which the concentration detected by the concentration detection means changes with respect to the change in the cover margin, and determines a first correction value for correcting the set candidate value based on the predicted charge distribution of the toner.

[0111] Following this process, the rate of density change indicates differences in the toner's charge distribution, thus determining the charge bias and development bias according to the toner's charge distribution. Therefore, it is possible to set a fouling margin according to the toner's charge distribution.

[0112] (Clause 3) The image forming apparatus according to Clause 1, further comprising a detection means for detecting the film thickness of an image carrier, wherein the correction means determines a second correction value based on the difference between the detected film thickness and a reference film thickness defined for a first correction value, and corrects the first correction value with the second correction value.

[0113] Following this approach, it is possible to set a cover margin that corresponds to the change in the film thickness of the image carrier.

[0114] (Clause 4) The image forming apparatus according to paragraph 2 or 3, wherein, when the toner carrier and the image carrier are replaced, the correction means determines a third correction value based on the ratio of the rate of change in density detected in the state after replacement to the rate of change in density detected for the toner carrier and image carrier that are standards set for the first correction value, and corrects the first correction value with the third correction value.

[0115] Following this approach, it is possible to determine the cover margin according to the individual differences between the toner carrier and the image carrier.

[0116] (Clause 5) When an image carrier is replaced, the correction means determines a fourth correction value based on the ratio of the percentage obtained by correcting the percentage change in density detected in the state before replacement with the percentage change in density detected for a standard image carrier defined with respect to the first correction value, and the percentage change in density detected in the state after replacement, and corrects the first correction value with the fourth correction value, as described in any of paragraphs 2 to 4.

[0117] Following this approach, it is possible to determine the cover margin according to the individual differences in the image carrier.

[0118] (Clause 6) When a toner carrier is replaced, the correction means determines a fifth correction value based on the ratio of the percentage after correcting the percentage of the percentage change in density detected in the replaced state with the percentage of the percentage change in density detected in a standard image carrier defined with respect to the first correction value, and the percentage of the percentage change in density detected in the replaced state, and corrects the first correction value with the fifth correction value, as described in any of Clauses 2 to 4.

[0119] Following this approach, it is possible to determine the cover margin according to the individual differences in the toner carrier.

[0120] (Clause 7) A method for determining a fouling margin, comprising: a density detection step for detecting the density of a fouling image formed on an image carrier; a provisional determination step for determining a provisional fouling margin, which is a fouling margin indicating the difference between a development bias applied to a toner carrier and a charging bias applied to a charger that charges the image carrier, based on the density detected in the density detection step; and a correction step for correcting the provisional fouling margin based on the rate at which the density detected in the density detection step changes in relation to the change in the fouling margin.

[0121] Following this approach, it is possible to provide a method for determining an appropriate cover margin without increasing the accuracy of detecting toner concentration.

[0122] (Clause 8) A fouling margin determination program that causes a computer to perform the following steps: a density detection step for detecting the density of a fouling image formed on an image carrier; a provisional determination step for determining a provisional fouling margin, which is a fouling margin indicating the difference between the development bias applied to the toner carrier and the charging bias applied to a charger that charges the image carrier, based on the density detected in the density detection step; and a correction step for correcting the provisional fouling margin based on the rate at which the density detected in the density detection step changes in relation to the change in the fouling margin.

[0123] Following this approach, it is possible to provide a cover margin determination program that can determine an appropriate cover margin without increasing the accuracy of toner concentration detection.

[0124] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of symbols]

[0125] 100 MFP, 111 CPU, 113 ROM, 114 RAM, 115 HDD, 116 Facsimile unit, 117 External storage device, 118 CD-ROM, 140 Image forming unit, 141 Drive control unit, 142 DC power supply circuit, 143 DC power supply circuit, 144 AC power supply circuit, 221Y Developing roller, 20Y, 20M, 20C, 20K Developing unit, 21Y Exposure unit, 22Y Charging roller, 23Y Photoreceptor drum, 24Y Developing device, 25Y Primary transfer roller, 29Y Detection sensor, 41Y Toner bottle, 51 Image forming unit, 53 Density detection unit, 55 Corresponding data acquisition unit, 57 Provisional determination unit, 59 Correction unit, 59A Correction unit, 61 First correction value determination unit, 63 Second correction value determination unit, 65 Third correction value determination unit, 71 Film thickness detection unit, M: fogging margin, S: development characteristics, V: increase rate, ΔV: increase rate, δ: fluctuation rate.

Claims

1. A density detection means for detecting the density of a hazy image formed on an image carrier, A provisional determination means for determining a provisional fouling margin, which is a fouling margin that indicates the difference between the development bias applied to the toner carrier and the charging bias applied to the charger that charges the image carrier, based on the density detected by the density detection means, The system includes a correction means for correcting the provisional cover margin based on the rate at which the concentration detected by the concentration detection means changes with respect to the change in the cover margin, The correction means predicts the charge distribution of the toner based on the rate at which the concentration detected by the concentration detection means changes with respect to the change in the cover margin, and determines a first correction value for correcting the provisional cover margin based on the predicted charge distribution of the toner.

2. The system further includes a detection means for detecting the film thickness of the image carrier, The image forming apparatus according to claim 1, wherein the correction means determines a second correction value based on the difference between the detected film thickness and a reference film thickness determined for the first correction value, and corrects the first correction value with the second correction value.

3. The image forming apparatus according to claim 1, wherein, when the toner carrier and the image carrier are replaced, the correction means determines a third correction value based on the ratio of the rate of change in density detected in the state after replacement to the rate of change in density detected for a toner carrier and an image carrier that serve as a standard for the first correction value, and corrects the first correction value with the third correction value.

4. The image forming apparatus according to claim 1, wherein when the image carrier is replaced, the correction means determines a fourth correction value based on the ratio of the percentage obtained by correcting the percentage change in density detected in the state before replacement with the percentage change in density detected for a reference image carrier set with respect to the first correction value, and the percentage change in density detected in the state after replacement, and corrects the first correction value with the fourth correction value.

5. The image forming apparatus according to claim 1, wherein when the toner carrier is replaced, the correction means determines a fifth correction value based on the ratio of the percentage obtained by correcting the percentage change in density detected in the state after replacement with the percentage change in density detected for a reference image carrier defined with respect to the first correction value, and the percentage change in density detected in the state after replacement, and corrects the first correction value with the fifth correction value.

6. A density detection step for detecting the density of the haze image formed on the image carrier, A preliminary determination step in which a provisional cover margin is determined, which is a cover margin that indicates the difference between the development bias applied to the toner carrier and the charging bias applied to the charger that charges the image carrier, based on the density detected in the density detection step, The image forming apparatus is instructed to perform a correction step, which corrects the provisional cover margin based on the rate at which the concentration detected in the concentration detection step changes with respect to the change in the cover margin, The correction step is a method for determining a fouling margin, which predicts the charge distribution of the toner based on the rate at which the concentration detected in the concentration detection step changes with respect to the change in the fouling margin, and determines a first correction value for correcting the provisional fouling margin based on the predicted charge distribution of the toner.

7. A density detection step for detecting the density of the haze image formed on the image carrier, A preliminary determination step in which a provisional cover margin is determined, which is a cover margin that indicates the difference between the development bias applied to the toner carrier and the charging bias applied to the charger that charges the image carrier, based on the density detected in the density detection step, The computer is instructed to perform a correction step, which corrects the provisional cover margin based on the rate at which the concentration detected in the concentration detection step changes with respect to the change in the cover margin, The correction step is a fouling margin determination program that predicts the charge distribution of the toner based on the rate at which the concentration detected in the concentration detection step changes with respect to the change in the fouling margin, and determines a first correction value for correcting the provisional fouling margin based on the predicted charge distribution of the toner.

Images