Image forming apparatus and image forming method that ensure improved measurement sensitivity of patch density with optical sensor, and recording medium therefor

An image forming apparatus includes a photoreceptor, an exposure unit, a developing unit, and a control unit. The control unit sets the toner-layer-forming electric potential difference lower than an electric potential when forming the image on the print medium, forms a combined patch for calibration by the set toner-layer-forming electric potential difference for calibration, so as to adjust the developing-bias potential by measuring a print density of the formed combined patch. The combined patch includes a first patch and a plurality of second patches. The plurality of second patches have a width within a range of 0.2 mm to the predetermined value in the peripheral-velocity direction of the photoreceptor, extend with a length equal to or more than the predetermined value in the rotation-shaft direction of the photoreceptor, and are arranged at a predetermined interval in the peripheral-velocity direction of the photoreceptor.

INCORPORATION BY REFERENCE

This application is based upon, and claims the benefit of priority from, corresponding Japanese Patent Application No. 2016-137196 filed in the Japan Patent Office on Jul. 11, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND

To ensure accurate color reproduction, there is proposed a technique that prints color patches, detects the printed color patches by a sensor, and then calibrates an image formation process. As a technique related to this, there is proposed a technique that uses a ladder patch as a color patch. The ladder patch has a plurality of thin, line-shaped patches, which are adjacently arranged. Assuming that print densities of respective straight-line portions of the ladder patch are identical to the print density of a solid patch, the technique enables detection in a print density region (a region with a large reflection-light amount by a combination with a background portion) where a detection sensitivity of an optical sensor is high. On the other hand, there is also proposed a technique that reduces generation of a transfer scattering (diffusion of toner to an outside of an image range) caused by edge effect, by reducing toner attachment amount in a contour portion of a toner image due to image processing or an adjustment of a gap.

SUMMARY

An image forming apparatus according to one aspect of the disclosure forms an image on a print medium. The image forming apparatus includes a photoreceptor, an exposure unit, a developing unit, and a control unit. The exposure unit exposes the photoreceptor to form an electrostatic latent image. The developing unit includes a magnetic roller and a development roller, forms a toner layer with a thickness corresponding to a toner-layer-forming electric potential difference between the magnetic roller and the development roller on the development roller, so as to attach a toner to the photoreceptor from the toner layer based on a developing-bias potential as an electric potential of the development roller and the electrostatic latent image. The control unit sets the toner-layer-forming electric potential difference lower than an electric potential when forming the image on the print medium, forms a combined patch for calibration by the set toner-layer-forming electric potential difference for calibration, so as to adjust the developing-bias potential by measuring a print density of the formed combined patch. The combined patch includes a first patch and a plurality of second patches. The first patch has a width larger than a predetermined value in a peripheral-velocity direction of the photoreceptor and extends with a length equal to or more than the predetermined value in a rotation-shaft direction of the photoreceptor. The plurality of second patches have a width within a range of 0.2 mm to the predetermined value in the peripheral-velocity direction of the photoreceptor, extend with a length equal to or more than the predetermined value in the rotation-shaft direction of the photoreceptor, and are arranged at a predetermined interval in the peripheral-velocity direction of the photoreceptor. The control unit measures the print density of the first patch and the print densities of the second patches to adjust a dot-area rate using the measured values.

DETAILED DESCRIPTION

The following describes configurations for implementing the disclosure (hereinafter referred to as “embodiment”) in the following order with reference to the drawings.A. First embodimentB. Second embodimentC. Modifications

A. First Embodiment

FIG. 1illustrates a block diagram illustrating a functional configuration of an image forming apparatus1according to a first embodiment of the disclosure. The image forming apparatus1includes a control unit10, an image forming unit20, a storage unit40, an image reading unit50, and a fixing unit80. The image reading unit50reads an image from a document and generates image data ID as digital data.

The image forming unit20includes a color conversion processing unit21, a halftone processing unit22, a calibration-print-density sensor28, an exposure unit29, photoreceptor drums (image carriers)30cto30kthat are amorphous silicon photoreceptors, developing units100cto100k,and charging units25cto25k.The color-conversion processing unit21performs color conversion to convert the image data ID, which is RGB data, into CMYK data. The half-tone processing unit22performs a half-tone process on the CMYK data to generate half-tone data of CMYK.

The control unit10includes a main storage unit such as a RAM and a ROM, and a control unit such as a micro-processing unit (MPU) or a central processing unit (CPU). The control unit10has a controller function related to an interface such as various kinds of I/Os, a universal serial bus (USB), a bus, and other hardware and controls the whole image forming apparatus1.

The storage unit40is a storage device constituted of a hard disk drive, which is a non-transitory recording medium, a flash memory, and similar memory and stores control programs and data for processes executed by the control unit10.

The storage unit40, in this embodiment, further stores a CMYK-tone-calibration adjustment patch and calibration data CD for forming a CMYK half patch. The half patch has a dot-area rate less than 100% and is a patch that expresses a solid image (an image with a solid print density). Since the amorphous silicon photoreceptors are employed for the photoreceptor drums30cto30k,a solid image is expressed by a half patch, which will be described later in detail.

The dot-area rate means an area rate that a dot occupies when each dot is assumed to have a preliminarily set area. In practice, for example, since a dot size varies corresponding to a toner-layer-forming electric potential difference ΔV (described below) between a developing-bias potential Vslvand a magnetic-roller potential Vmagor similar electric potential difference, the dot-area rate is different from an area rate in an optical aspect. On the other hand, the solid print density means a print density defined as a print density where a print medium appears to be gaplessly covered with dots in an optical aspect.

FIG. 2illustrates a cross-sectional view illustrating an overall structure of the image forming apparatus1according to the first embodiment. The image forming apparatus1in the embodiment is a tandem-type color printer. The image forming apparatus1includes the photoreceptor drums (image carriers)30m,30c,30y,and30karranged in a row corresponding to respective colors of magenta, cyan, yellow, and black, in its housing70. The developing units100m,100c,100y,and100kare arranged adjacent to the respective photoreceptor drums30m,30c,30y,and30k.

The exposure unit29irradiates (exposure) the photoreceptor drums30m,30c,30y,and30kwith laser beams Lm, Lc, Ly, and Lk for the respective colors. This irradiation forms electrostatic latent images on the photoreceptor drums30m,30c,30y,and30k.The developing units100m,100c,100y,and100kattach toners to the electrostatic latent image formed on the surfaces of the photoreceptor drums30m,30c,30y,and30kwhile stirring the toners. This completes a development process, thus ensuring the formed toner images of the respective colors on the surfaces of the photoreceptor drums30cto30k.

The image forming apparatus1includes an endless intermediate transfer belt27. The intermediate transfer belt27is stretched by a tension roller24, a driving roller26a,and a driven roller26b.The intermediate transfer belt27is circularly driven by a rotation of the driving roller26a.

For example, the photoreceptor drum30kand a primary transfer roller23ksandwich the intermediate transfer belt27, and then the intermediate transfer belt27is circularly driven. This causes a black toner image on the photoreceptor drum30kto be primarily transferred onto the intermediate transfer belt27. The same applies to the other three colors of cyan, yellow, and magenta. The intermediate transfer belt27has the surface on which the primary transfers are performed and mutually superimposed at predetermined timings, and then a full-color toner image is formed. Then, the full-color toner image is secondarily transferred to a printing paper sheet P supplied from a sheet feed cassette60, and is fixed on the printing paper sheet P as a print medium by a fixing roller pair81of the fixing unit80.

FIG. 3illustrates a cross-sectional side view illustrating a structure of the developing unit100kaccording to the first embodiment of the disclosure. The developing units100m,100c,and100yhave the configurations identical to the developing unit100kand are also simply referred to as a developing unit100. The developing unit100includes two stir conveying members141and142, a magnetic roller143, a development roller (developer carrier)144, a developing container145, and a regulating blade146.

The developing container145constitutes an outside of the developing unit100. In a lower portion of the developing container145, a partition portion145bis located. The partition portion145bseparates an inside of the developing container145into a first conveying chamber145aand a second conveying chamber145c.The first conveying chamber145aand the second conveying chamber145cextend in a columnar shape in a direction perpendicular toFIG. 3, house a two-component developer (also simply referred to as developer) made of magnetic carrier and black toner.

The developing container145further holds the magnetic roller143and the development roller144. The developing container145has an opening147that exposes the development roller144toward the photoreceptor drum30(30k).

The two stir conveying members141and142cyclically move the developer while stirring the developer inside the first conveying chamber145aand the second conveying chamber145c,respectively. The stir conveying member142, as a magnetic brush, supplies the electrostatically charged developer to the magnetic roller143. The magnetic roller143includes a non-magnetic rotation sleeve143aand a stationary magnet body143bsecured inside the rotation sleeve143a.The magnetic roller143and the development roller144face with a predetermined clearance. The regulating blade146adjusts the magnetic brush to a predetermined height preliminarily set.

The development roller144includes a rotatable, non-magnetic development sleeve144aand a development-roller-side magnetic pole144bsecured to inside of the development sleeve144a.The magnetic-roller potential Vmagis applied to the magnetic roller143. The developing-bias potential Vslvis applied to the development roller144.

In the embodiment, a surface potential of the photoreceptor drum30is set to 20 V and forms a development field with the development roller144. On the other hand, an alternating bias, where a DC potential of 20 V to 80 V as the developing-bias potential Vslvand a sinusoidal potential with a frequency of 2 kHz and a peak-to-peak value of 2000 V are superimposed, is applied to the development roller144. To the magnetic roller143, a DC potential of 200 V as the magnetic-roller potential Vmagis applied during development, and a DC potential of −200 V is applied during non-development.

This causes the time during which the developing-bias potential Vslv< the magnetic-roller potential Vmag(an electric-potential state where the toner is supplied to the development roller144) to be longer to cause the supply time of the toner to the development roller144to be longer during development, and causes the time during which the developing-bias potential Vslv> the magnetic-roller potential Vmag(an electric-potential state where the toner is recovered from the development roller144) to be longer to cause the recovery time of the toner from the development roller144to be longer during non-development.

Furthermore, adjusting the magnetic-roller potential Vmagapplied to the magnetic roller143during development and non-development enables changing the toner-layer-forming electric potential difference ΔV between the developing-bias potential Vslvand the magnetic-roller potential Vmagduring development. This forms a thin toner layer (also simply referred to as a toner layer) with a thickness that corresponds to the toner-layer-forming electric potential difference ΔV between the developing-bias potential Vslvand the magnetic-roller potential Vmag, on the development roller144.

The development roller144attaches the toner to the photoreceptor drum30via a facing portion (a development nip) having a predetermined clearance with the photoreceptor drum30and then forms a toner image on the surface of the photoreceptor drum30. The toner image is formed based on an electric potential difference between the electric potential of the electrostatic latent image on the surface of the photoreceptor drum30and the developing-bias potential Vslvapplied to the development roller144.

The amorphous silicon photoreceptor has a relative dielectric constant about three times higher compared with an organic photoreceptor (OPC) and has a feature that the holdable toner amount is large with respect to a development-contrast potential. In view of this, the amorphous silicon photoreceptor ensures holding more toner than the solid print density, which is ordinarily used. Consequently, using the amorphous silicon photoreceptor in a saturation state holds a toner amount, which exceeds a required amount for the solid print density. Therefore, the embodiment uses the amorphous silicon photoreceptor in a non-saturation state even in the solid print density and uses the amorphous silicon photoreceptor to determine the solid print density by completing the development when approximately all the toner formed on the development roller144is developed to the photoreceptor.

FIGS. 4A to 4Cillustrate conceptual diagrams that illustrate and compare the development processes according to a comparative example and the first embodiment.FIG. 4Aillustrates a state where the image is formed by a toner-layer-forming electric potential difference ΔVi for image formation (also simply referred to as an image-formation electric potential difference ΔVi) in a front-end portion and a center portion of the image.FIG. 4Billustrates a state where the image is formed by the image-formation electric potential difference ΔVi in the rear-end portion of the image. In this description, the front-end portion, the center portion, and the rear-end portion are based on a running direction of the photoreceptor drum30, and the front-end portion, the center portion, and the rear-end portion are defined in order from the running direction.

In this embodiment, as illustrated inFIG. 4A, the photoreceptor drum30receives supply of the toner from the development roller144while neutralizing the electric potential of a latent image. In this case, the development process is configured to complete by the thin toner layer formed on the development roller144to be consumed in the non-saturation state, not by the saturation of the electric potential. The thickness of the thin toner layer is set to have a thickness T1for achieving the highest print density during the solid development in the image formation.

As illustrated inFIG. 4B, the development roller144has a peripheral velocity Vs and is configured to form the image while overtaking the photoreceptor drum30that has a peripheral velocity Vd. In view of this, in the proximity of the solid rear-end portion during the solid development, the toner-unconsumed surface of the development roller144is present. The toner-unconsumed surface overtakes the rear-end portion of the solid latent image in the amorphous silicon photoreceptor30.

In this case, since the photoreceptor drum30as the amorphous silicon photoreceptor is in the non-saturation state, the toner is further developed from the toner-unconsumed surface of the development roller144. This development actualizes a rear-end accumulation (thickness T2) as a higher solid print density than a preliminarily assumed print density.

FIG. 4Cillustrates a state where an image is formed by a toner-layer-forming electric potential difference ΔVp for combined-patch formation (also simply referred to as a combined-patch-formation electric potential difference ΔVp) in the rear-end portion of the image. In this embodiment, the combined-patch-formation electric potential difference ΔVp is set to be 100 V lower than 200 V, which is set for the image formation, by 100 V.

Thus, during the image formation, a toner layer with a thickness D (seeFIG. 4A), corresponding to the image-formation electric potential difference ΔVi, is formed on the development roller144, and during the combined patch, a toner layer with a thickness Da (seeFIG. 4C), corresponding to the combined-patch-formation electric potential difference ΔVp, is formed on the development roller144.

FIG. 5illustrates contents of a combined-patch-calibration processing procedure of the image forming apparatus1according to the first embodiment. The combined-patch-calibration processing procedure is processing for calibrating a solid image density by adjusting the developing-bias potential Vslv. The developing-bias potential Vslvis an electric potential of the development roller144. The higher the developing-bias potential becomes, the more the toner attachment amount to the photoreceptor drum30from the development roller144increases, and the darker the image becomes. In this embodiment, assume that the combined-patch-calibration processing procedure is automatically executed during activation in the morning in an office.

At Step S110, the control unit10performs an initial setting of the developing-bias potential Vslv. The initial setting of the developing-bias potential Vslvsets, for example, three initial values centered around a final calibration value of a previous day. Specifically, for example, when the final calibration value of the developing-bias potential Vslvof the previous day was 60 V, three electric potentials of 50 V, 60 V, and 70 V are set.

At Step S120, the control unit10sets the magnetic-roller potential Vmagas the electric potential of the magnetic roller143. The magnetic-roller potential Vmagis set to be higher by 100 V than the three developing-bias potential Vslvof 50 V, 60 V, and 70 V, such that the combined-patch-formation electric potential difference ΔVp becomes lower by 100 V than the image-formation electric potential difference ΔVi (during development).

At Step S130, the control unit10forms the combined patch on the intermediate transfer belt27. In this embodiment, the combined-patch-formation electric potential difference ΔVp causes the combined patch to be formed in a print density region where the calibration-print-density sensor28has good sensitivity (a print density region where a reflection-light amount is large).

FIG. 6illustrates an explanatory diagram illustrating an exemplary combined patch according to the first embodiment. A combined patch CP1is a patch formed with the developing-bias potential 50 V. A combined patch CP2is a patch formed with the developing-bias potential 60 V. A combined patch CP3is a patch formed with the developing-bias potential 70 V. The combined patch CP1is constituted of a solid patch (solid filled patch) SP1and a ladder patch LP1. The combined patch CP2is constituted of a solid patch SP2and a ladder patch LP2. The combined patch CP3is constituted of a solid patch SP3and a ladder patch LP3. The combined patches CP1to CP3are formed for each toner color and thus totaled12.

In this embodiment, each of the solid patches SP1to SP3has a square shape of 5 mm square, and each of the ladder patches LP1to LP3is formed as eight patches having a straight-line shape of 1 mm×5 mm. A count of the patches of the ladder patches LP1to LP3is preferably five or more, and more preferably eight or more. The solid patch SP is also referred to as a first patch. The ladder patch LP is also referred to as a second patch.

It is only necessary that the solid patches SP1to SP3have a width larger than 2 mm in a peripheral-velocity direction of the photoreceptor drum30and extend in a length equal to or more than 5 mm in a rotation-shaft direction of the photoreceptor drum30. The ladder patches LP1to LP3include at least eight straight-line-shaped patches. It is only necessary that each straight-line-shaped patch have a width in a range of 0.2 mm to 2 mm in the peripheral-velocity direction of the photoreceptor drum30and extend in a length equal to or more than 5 mm in the rotation-shaft direction of the photoreceptor drum30. At least eight straight-line-shaped patches are preferably arranged at a preliminarily set interval (for example, at an equal interval identical to the width of the straight-line-shaped patch) in the peripheral-velocity direction of the photoreceptor drum30and are preferably arranged at a position approximately identical to a position of the solid patch SP in the rotation-shaft direction of the photoreceptor drum30.

At Step S140, the control unit10measures the patch density for each color (for example, cyan (C)) using the calibration-print-density sensor28. The calibration-print-density sensor28measures six print densities as the print densities of the solid patches SP1to SP3and the ladder patches LP1to LP3in total. The print densities of the ladder patches LP1to LP3are measured as an average value of the measured print densities of the eight patches. On the other hand, in the embodiment, the print densities of the solid patches SP1to SP3are measured at the position equal to or more than 2 mm apart from the rear-end portion in the peripheral-velocity direction of the photoreceptor drum30to avoid the region of the rear-end-accumulation.

FIGS. 7A to 7Cillustrate explanatory diagrams illustrating a measuring method of the reflection-light amount according to the comparative example and the first embodiment.FIG. 7Aillustrates a toner attachment state (a stacked state) in the solid patch SP1and the ladder patch LP1. Both the solid patch SP1and the ladder patch LP1have the rear-end portion where the toner layer is raised. The raised-toner layers are the rear-end accumulations. The rear-end accumulations are similarly generated in both the solid patch SP1and the ladder patch LP1. However, since the ladder patch LP1is shortly formed in the peripheral-velocity direction of the photoreceptor drum30, most of the region becomes the rear-end accumulation.

FIG. 7Billustrates the reflection-light amounts of the solid patch SP1and the ladder patch LP1in the comparative example (an image-formation setting). Since having the solid print densities, both the solid patch SP1and the ladder patch LP1have the significantly low reflection-light amounts, and thus, are the print density regions where the measurement sensitivity of the calibration-print-density sensor28is low. The toner layer of the solid patch SP1generates a light-amount difference DL1(light-amount reduction) in the region where the rear-end accumulation is not generated. The toner layer of the ladder patch LP1generates a light-amount difference DL2(light-amount reduction) in the whole region including the rear-end accumulation.

FIG. 7Cillustrates the reflection-light amount of the solid patch SP1and the ladder patch LP1in the first embodiment. Although the solid patch SP1and the ladder patch LP1are both solid, forming with the combined-patch-formation electric potential difference ΔVp causes the thin toner layer on the development roller144to have a thin thickness, thus having received less toner supply from the development roller144. This increases the reflection-light amount and enables measurement in the print density region where the measurement sensitivity of the calibration-print-density sensor28is high.

In the embodiment, the toner layer of the solid patch SP1generates a light-amount difference DL1a(light-amount reduction) in the region where the rear-end accumulation is not formed. The toner layer of the ladder patch LP1generates a light-amount difference DL2a(light-amount reduction) in the whole region including the rear-end accumulation.

In the comparative example, a rear-end-accumulation ratio, which will be described later in detail, has a positive correlation with the light-amount difference DL2/the light-amount difference DL1. On the other hand, in the embodiment, the rear-end-accumulation ratio has the positive correlation with the light-amount difference DL2a/the light-amount difference DL1a.

Here, as will be appreciated from the forming mechanism of the rear-end accumulation, the difference between the light-amount difference DL2aand the light-amount difference DL1adoes not significantly become smaller than the difference between the light-amount difference DL2and the light-amount difference DL1. This is because the rear-end accumulation is generated caused by the development roller144overtaking of the photoreceptor drum30and is not reduced even when the thickness of the thin toner layer on the development roller144become thin. Consequently, because the light-amount difference DL1as a denominator becomes small to the light-amount difference DL1a,the light-amount difference DL2a/the light-amount difference DL1asignificantly indicates the influence of the rear-end accumulation with respect to the light-amount difference DL2/the light-amount difference DL1, and thus contributing to the improved-measurement sensitivity.

In the embodiment, the calibration-print-density sensor28, for example, emits infrared light from an LED (not illustrated), transmits the light through a polarization filter, which transmits only P-wave, to irradiate the P-wave of the infrared light to the patch, and then detects the print density based on a ratio of the P-wave and S-wave of the reflection light detected by a light receiving element. Some calibration-print-density sensors28employ a regular reflection method detecting regular reflection light from a patch and others employ a diffuse reflection method detecting diffuse reflection light from a patch. The calibration-print-density sensor28may measure, for example, a light amount of reflected red light that has a complementary-color relationship with cyan (C).

At Step S150, the control unit10executes a toner-attachment-amount calculation process. In the toner-attachment-amount calculation process, the control unit10calculates the toner attachment amount (unit: g/m2) to the intermediate transfer belt27based on a preliminarily prepared approximate curve or table from the reflection-light amount measured in the solid patches SP1to SP3and the ladder patches LP1to LP3.

At Step S160, the control unit10executes a rear-end-accumulation-ratio calculation process. In the rear-end-accumulation-ratio calculation process, the control unit10calculates the rear-end-accumulation ratio in the combined patch CP1(50 V), the combined patch CP2(60 V), and the combined patch CP3(70 V).

The rear-end-accumulation ratio in the combined patch CP1is calculated as a ratio R1(=TL1/TS1) of a toner attachment amount TS1of the solid patch SP1to a toner attachment amount TL1of the ladder patch LP1. The rear-end-accumulation ratio in the combined patch CP2is calculated as a ratio R2(=TL2/TS2) of the toner attachment amounts of the solid patch SP2and the ladder patch LP2. The rear-end-accumulation ratio in the combined patch CP3is calculated as a ratio R3(=TL3/TS3) of the toner attachment amounts of the solid patch SP3and the ladder patch LP3.

At Step S170, the control unit10compares the ratio R1, the ratio R2, and the ratio R3with a threshold value Th. Assume that the threshold value Th is 1.2 in the embodiment. The control unit10selects the ratio that is smaller than the threshold value Th (1.2) and closest to the threshold value Th among the ratio R1, the ratio R2, and the ratio R3. Specifically, when the ratio R1, the ratio R2, and the ratio R3are, for example, 1.3, 1.1, and 1.0, respectively, the ratio R2is selected.

When a ratio smaller than the threshold value Th (1.2) does not exists, or a ratio closest to the threshold value Th is a maximum value, a plurality of combined patches CP1to CP3may be formed again, by appropriately readjusting the developing-bias potential Vslv.

At Step S180, the control unit10executes a developing-bias-potential setting process. In the developing-bias-potential setting process, the control unit10, for example, sets 60 V, which is the developing-bias potential Vslvcorresponding to the ratio R2, as the developing-bias potential Vslvfor the image formation.

The developing-bias potential Vslvmay be set by an interpolation calculation or an extrapolation calculation using data of the ratio R1, the ratio R2, and the ratio R3. In setting of the developing-bias potential Vslv, a minimum electric potential may be preliminarily set. That is, when an adjusting value of the developing-bias potential Vslvbecomes equal to or less than the minimum electric potential by the interpolation calculation or similar calculation, the minimum electric potential may be set to be the developing-bias potential Vslv.

At Step S190, the control unit10stores the developing-bias potential Vslv(60V) as the calibration data CD in the storage unit40.

Thus, in the calibration of electrophotography that achieves the solid print density by development with the photoreceptor in the non-saturation state, the image forming apparatus1according to the first embodiment ensures the enhanced sensitivity when measuring the patch density with the calibration-print-density sensor28as an optical sensor. This enables the image forming apparatus1according to the first embodiment to adjust the developing-bias potential Vslvwith high accuracy.

B. Second Embodiment

FIG. 8illustrates contents of a dot-area-rate-calibration processing procedure of the image forming apparatus1according to a second embodiment. At Step S210, the control unit10executes the initial setting of the developing-bias potential Vslv. In the initial setting, for example, 60 V is set as the final calibration value of the previous day. At Step S220, the control unit10, similarly to Step S120, sets the magnetic-roller potential Vmagas the electric potential of the magnetic roller143.

At Step S230, the control unit10, similarly to Step S130, forms the combined patch on the intermediate transfer belt27. In this embodiment, only a single combined patch is consequently formed. At Step S240, the control unit10, similarly to Step S140, measures the print density of the combined patch using the calibration-print-density sensor28.

At Step S250, the control unit10, similarly to Step S150, executes the toner-attachment-amount calculation process. At Step S260, the control unit10, similarly to Step S160, executes the rear-end-accumulation-ratio calculation process.

At Step S270, the control unit10executes a dot-area-rate setting process. In the dot-area-rate setting process, the dot-area rate is set using the rear-end-accumulation ratio calculated in the rear-end-accumulation-ratio calculation process.

FIGS. 9A and 9Billustrate explanatory diagrams illustrating a setting method of the dot-area rate and the half patch according to the second embodiment of the disclosure.FIG. 9Ais a graph illustrating the relationship between the rear-end-accumulation ratio and the dot-area rate (the printing rate of the solid patch). The graph indicates a curved line CV generated based on, for example, an experiment and a simulation and represents a preferred dot-area rate at each rear-end-accumulation ratio. Specifically, the graph indicates that a dot-area rate of 90% is desirable when each rear-end-accumulation ratio is 1.5.

FIG. 9Billustrates a half patch HP. The half patch HP is constituted of oblique screen lines that are densely formed. Since each screen line is constituted of dots formed with toner, strictly speaking, it is not solid in enlarged view. However, each screen line appears to be a solid patch in an ordinary state when a human views a printed matter. Using the half patch HP instead of the solid patch ensures eliminating the problem of the rear-end accumulation. Since each screen line is formed with the print density of the rear-end accumulation portion, the whole patch becomes the uniform print density.

At Step S280, the control unit10executes a magnetic-roller-potential adjustment process. The magnetic-roller-potential adjustment process is a process where the control unit10applies, for example, 250 V, 300 V, and 350 V instead of DC potential 200 V as the magnetic-roller potential Vmagto form the combined patches CP1to CP3and adjusts the half patch HP with a dot-area rate of 90% to be the desirable solid print density, similarly to the developing-bias potential Vslv.

At Step S290, the control unit10, similarly to Step S190, stores the magnetic-roller potential Vmagas the calibration data CD in the storage unit40. The developing-bias potential Vslvmay be adjusted again after the adjustment of the magnetic-roller potential Vmag, or the developing-bias potential Vslvmay be adjusted instead of the adjustment of the magnetic-roller potential Vmag.

Thus, in the calibration of the electrophotography that achieves the solid print density by the development with the photoreceptor in the non-saturation state, the image forming apparatus1according to the second embodiment ensures adjustment of, for example, the dot-area rate with high accuracy, in a state where the sensitivity in measuring the patch density by the calibration-print-density sensor28is enhanced.

The disclosure is not limited to the above-described embodiment and embodied as the following modifications.

While in the above-described embodiments the solid patch has the square shape of 5 mm square, it is only necessary that the solid patch have a width larger than a predetermined value, which is preliminarily set, and extend with a length equal to or more than a predetermined value in the rotation-shaft direction of the photoreceptor (for example, it may have a square shape of 10 mm square), such that the print density of the region with small influence of the rear-end accumulation is measurable in the peripheral-velocity direction of the photoreceptor. On the other hand, in consideration of, for example, the optical measurement, it is preferred that the ladder patch have a width equal to or more than 0.2 mm, and have a width equal to or less than a predetermined value such that the influence of the rear-end accumulation becomes dominant in the toner thickness.

The toner thickness of the ladder patch is likely to become non-uniform due to the influence of the rear-end accumulation and thus, it is preferred that a plurality of ladder patches be formed to ensure performing statistical processing such as an average value or a median value. Furthermore, it is preferred that the ladder patch be arranged at a position approximately identical to a position of the solid patch in the rotation-shaft direction of the photoreceptor. This forms the patches at identical portions in the axial direction of the photoreceptor drum and thus, even if the variations of the characteristics in the axial direction of the photoreceptor drum exist, thus ensuring the eliminated influence of the variations.

While in the above-described embodiments the above-described respective embodiments use the rear-end-accumulation ratio (division), a difference may be used. That is, the process may be performed similarly to the rear-end-accumulation ratio by subtracting the average toner attachment amount of the solid patch from the toner attachment amount of the ladder patch.

While in the above-described embodiments the amorphous silicon photoreceptor is used, the disclosure is not limited to use of the amorphous silicon photoreceptor. The disclosure is applicable to an image forming apparatus that, in general, reproduces a solid print density in a photoreceptor in a non-saturation state.