JP2021179478A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that has a developing device applied with a two-component developing system, the image forming apparatus capable of obtaining stable image density and image quality by setting a DC bias of a developing bias and a peak-to-peak voltage of an AC bias both affecting a same image defect, while associating them with each other.SOLUTION: A bias condition determination unit executes bias calibration including an approximate expression setting mode S01 and a bias setting mode S02. In the approximate expression setting mode, information on the relationship between a DC bias and a saturated peak-to-peak voltage is acquired. The saturated peak-to-peak voltage is a peak-to-peak voltage at which density is saturated with a predetermined DC bias. In the bias setting mode, the DC bias and the peak-to-peak voltage are changed with the relationship information being satisfied, and a developing bias is set with which target density can be obtained.SELECTED DRAWING: Figure 4

Description

The present invention relates to an image forming apparatus provided with a developing apparatus to which a two-component developing method is applied.

Conventionally, as an image forming apparatus for forming an image on a sheet, an image forming apparatus including a photoconductor drum (image carrier), a developing apparatus, and a transfer member is known. When the electrostatic latent image formed on the photoconductor drum is manifested by the toner by the developing device, the toner image is formed on the photoconductor drum. The transfer member transfers the toner image to the sheet. As a developing apparatus applied to such an image forming apparatus, a two-component developing technique using a developer containing a toner and a carrier is known.

In the two-component developing technique, the developing apparatus has a developing roller, and a developing bias in which an AC bias is superimposed on a DC bias is applied to the developing roller to form a suitable toner image. Conventionally, there is known a technique of measuring the image density of a halftone image while changing the DC bias and selecting a DC bias that can obtain a target image density from the characteristics. On the other hand, when Vpp (inter-peak voltage) of the AC bias is set high, the image density is increased and the texture of the halftone image is improved, and the half image pitch unevenness that tends to occur in the rotation cycle of the developing roller tends to be improved. However, if Vpp is set too high, a leak may occur in the developing nip where the photoconductor drum and the developing roller face each other, and the print history one round before the developing roller appears on the image, so-called development. Ghost gets worse. Further, if Vpp is set too low, an image density change (half image pitch unevenness) corresponding to the circumferential runout of the developing roller or the photoconductor drum occurs on the halftone image. Therefore, it is necessary to appropriately set not only the DC bias of the development bias but also the Vpp of the AC bias.

In Patent Document 1, a development current when a test electrostatic latent image is developed is detected, and an image formation including a surface potential of a photoconductor drum and a development bias Vpp according to the detected development current is formed. The technology whose conditions are changed is disclosed. Further, Patent Document 2 describes a technique in which a toner charge amount is estimated from a developing current and a toner adhesion amount on a photoconductor drum, and at least one of Vpp and duty of AC bias is adjusted based on the toner charge amount. Is disclosed.

Japanese Unexamined Patent Publication No. 5-107835 Japanese Unexamined Patent Publication No. 2015-203731

In the conventional technique as described above, Vpp of AC bias and the like are adjusted in order to suppress image defects. Here, if the difference between the above-mentioned DC bias and the background potential of the photoconductor drum becomes too large, the development ghost deteriorates, but the half image pitch unevenness improves. As described above, since the DC bias of the development bias and the Vpp of the AC bias affect the same image defect, it is difficult to obtain a stable image even if only the Vpp is adjusted. That is, even if the Vpp of the AC bias is appropriately adjusted, the image defect to be eliminated may be deteriorated depending on the value of the DC bias. In addition, when the development current is used as a reference when optimizing these development biases, changes in carrier resistance, toner concentration, etc. will affect the development current, so the measured values are likely to vary and stable bias settings can be obtained. There was the problem of becoming difficult.

The present invention is for solving the above-mentioned problems, and in an image forming apparatus having a developing apparatus to which a two-component developing method is applied, DC bias and alternating current of development bias affecting the same image defect are applied. The purpose is to obtain stable image density and image quality by setting the peak voltage of the bias while associating them with each other.

The image forming apparatus according to one aspect of the present invention is an image forming apparatus capable of performing an image forming operation for forming an image on a sheet, and allows rotation to form an electrostatic latent image. An image carrier having a surface on which the electrostatic latent image carries a toner image manifested by toner, a charging device that charges the image carrier to a predetermined charging potential, and an image carrier rather than the charging device. An exposure apparatus that forms the electrostatic latent image by exposing the surface of the image carrier, which is arranged on the downstream side in the rotation direction of the body and is charged with the charge potential, according to predetermined image information, and the exposure apparatus. A developing apparatus arranged to face the image carrier at a predetermined developing nip portion on the downstream side in the rotational direction, and has a peripheral surface that is rotated and carries a developer composed of a toner and a carrier. A developing device including a developing roller that forms the toner image by supplying toner to the carrier, a transfer unit that transfers the toner image supported on the image carrier to a sheet, and an AC voltage in the DC voltage. The development bias application unit capable of applying the superimposed development bias to the development roller, the density detection unit capable of detecting the density of the toner image, and the image formation operation in which the image formation operation is executed are different from each other. When the development bias is applied to the developing roller corresponding to a predetermined latent image for measurement formed on the image carrier during non-image formation, the latent image for measurement is developed into a toner image for measurement with toner. Based on the density of the measurement toner image detected by the density detector, the reference DC voltage that serves as a reference for the DC voltage applied to the developing roller during the image forming operation and the AC voltage of the developing bias. A bias condition determination unit for executing a bias condition determination mode for determining each reference peak voltage, which is a reference of the peak voltage of the above, is provided. The bias condition determination unit is a mode for developing a preset first measurement latent image into a first measurement toner image as the bias condition determination mode, and the plurality of preset DC voltages. The peak-to-peak voltage of the AC voltage is changed for each of the above, and the saturated peak-to-peak voltage, which is the peak-to-peak voltage at which the concentration of the first measurement toner image is saturated, is acquired for each of the DC voltages. , A relational information acquisition mode for acquiring the relational information between the DC voltage and the saturation peak voltage, and a mode for developing a preset second measurement latent image into a second measurement toner image. The DC voltage and the peak voltage are changed based on the relation information acquired in the relation information acquisition mode, and the combination of the DC voltage and the saturation peak voltage corresponding to the preset target image density is obtained. It is possible to execute the reference DC voltage and the bias setting mode acquired as the reference peak voltage, respectively.

According to this configuration, even if each image forming condition such as the distance between the developing roller and the image carrier (DS gap), the amount of toner charged, and the resistance of the carrier changes, the bias condition determining unit biases as necessary. By executing the condition determination mode, it is possible to set the DC voltage of the development bias and the peak-to-peak voltage of the AC voltage according to each image formation condition. As a result, it is possible to stably set the DC voltage of the development bias and the peak-to-peak voltage of the AC voltage, which affect the same image defect, at appropriate timings, and to stabilize and improve the image quality. In particular, in the relational information acquisition mode, the DC voltage and the saturation peak voltage are associated with each other as relational information, and then in the bias setting mode, the bias condition corresponding to the target image density is set based on the relational information to develop. It is possible to maintain a balanced state of toner movement (development, recovery) between the roller and the image carrier, and it is possible to obtain stable image quality.

In the above configuration, the bias condition determination unit sets the bias condition determination unit so that the density of the first measurement toner image saturated at the saturation peak voltage is smaller than the target image density in the relational information acquisition mode. It is desirable to form a first latent image for measurement.

According to this configuration, by setting the density of the first measurement toner image to be smaller than the target image density, the DC voltage and the saturation peak voltage can be set in the region where the detection sensitivity of the optical density detector is high. Related information can be obtained in a stable manner.

In the above configuration, the storage unit capable of storing the related information, the reference DC voltage, and the reference peak voltage is further provided, and the bias condition determination unit is the reference acquired in the bias setting mode. When the absolute value of the difference between the DC voltage and the reference DC voltage acquired in the previous bias setting mode and stored in the storage unit exceeds a preset threshold voltage, the related information acquisition mode is set. It is desirable that the bias setting mode is executed again after the related information acquisition mode is executed.

According to this configuration, it is possible to accurately determine whether or not the related information acquisition mode should be executed by estimating changes in toner chargeability and the like from changes in the reference DC voltage acquired in the bias setting mode.

In the above configuration, the storage unit capable of storing the related information, the reference DC voltage, and the reference peak voltage is further provided, and the bias condition determination unit is the reference acquired in the bias setting mode. When the DC voltage is below the preset lower limit threshold, or when the reference DC voltage exceeds the preset upper limit threshold, the relational information acquisition mode is executed and the relational information acquisition mode is executed. It is desirable that the bias setting mode is re-executed after the execution of.

According to this configuration, it is estimated that the chargeability of the toner changes when the reference DC voltage acquired in the bias setting mode exceeds the preset allowable range, so that the related information acquisition mode can be used. It is possible to accurately determine whether or not execution is necessary.

In the above configuration, it is desirable that the bias condition determining unit execute the related information acquisition mode when the preset mode start condition is satisfied.

According to this configuration, when there is a possibility that the chargeability of the toner has changed, the relational information between the DC voltage and the saturation peak voltage is reacquired by executing the relational information acquisition mode. A stable bias setting can be performed.

In the above configuration, the mode further comprises at least an apparatus main body for accommodating the image carrier and the developing apparatus, and a toner container arranged in the apparatus main body and accommodating toner supplied to the developing apparatus. The start condition is a first condition satisfied when the power of the apparatus main body is turned on, a second condition satisfied when the amount of change in absolute humidity around the apparatus main body exceeds a predetermined humidity threshold, and the image. At least one of the third condition satisfied when the number of sheets for which the forming operation is executed exceeds the preset number threshold and the fourth condition satisfied when the toner container is replaced is satisfied. It is desirable to include it.

According to this configuration, when any one of the first, second, third and fourth conditions is satisfied and there is a possibility that the chargeability of the toner has changed, the related information acquisition mode is executed. By doing so, it is possible to reacquire the relational information between the DC voltage and the saturation peak voltage and execute stable bias setting.

According to the present invention, in an image forming apparatus having a developing apparatus to which a two-component developing method is applied, the DC bias of the development bias and the peak voltage of the AC bias that affect the same image defect are set while being associated with each other. It is possible to obtain stable image density and image quality.

It is sectional drawing which shows the internal structure of the image forming apparatus which concerns on one Embodiment of this invention. It is sectional drawing of the developing apparatus which concerns on one Embodiment of this invention, and is the block diagram which showed the electric structure of the control part. It is a schematic diagram which shows the development operation of the image forming apparatus which concerns on one Embodiment of this invention. It is a schematic diagram which shows the magnitude relation of the potential of the image carrier and the developing roller which concerns on one Embodiment of this invention. It is a schematic diagram which shows the relationship between the DC bias and the AC bias of the development bias which concerns on one Embodiment of this invention. It is a flowchart of bias calibration which concerns on one Embodiment of this invention. It is a flowchart of the approximate expression setting mode of bias calibration which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the peak voltage of AC bias and image density in a plurality of DC biases in the approximate setting mode which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the DC bias and the voltage between saturation peaks in the approximate setting mode which concerns on one Embodiment of this invention. It is a flowchart of the bias setting mode of the bias calibration which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the DC bias and the peak voltage referred to in the bias setting mode of the bias calibration which concerns on one Embodiment of this invention, and the image density. It is a flowchart of the bias calibration which concerns on the modification embodiment of this invention. It is a schematic diagram which shows the relationship between the reference DC voltage and the reference peak voltage in the bias calibration which concerns on the modification of this invention.

Hereinafter, the image forming apparatus 10 according to the embodiment of the present invention will be described in detail based on the drawings. In this embodiment, a tandem color printer is exemplified as an example of the image forming apparatus. The image forming apparatus may be, for example, a copying machine, a facsimile apparatus, a multifunction device thereof, or the like. Further, the image forming apparatus may form a monochromatic (monochrome) image. The image forming apparatus 10 is capable of performing an image forming operation of forming an image on the sheet P.

FIG. 1 is a cross-sectional view showing the internal structure of the image forming apparatus 10. The image forming apparatus 10 includes an apparatus main body 11 having a box-shaped housing structure. In the apparatus main body 11, the paper feed unit 12 for feeding the sheet P, the image forming unit 13 for forming the toner image to be transferred to the sheet P fed from the paper feed unit 12, and the toner image are primarily transferred. The intermediate transfer unit 14 (transfer unit), the toner supply unit 15 (toner container) that supplies toner to the image forming unit 13, and the unfixed toner image formed on the sheet P are fixed to the sheet P. The fixing portion 16 is incorporated. Further, the upper part of the apparatus main body 11 is provided with a paper ejection portion 17 from which the sheet P subjected to the fixing treatment by the fixing portion 16 is discharged.

An operation panel (not shown) for inputting and operating output conditions and the like for the seat P is provided at an appropriate position on the upper surface of the apparatus main body 11. This operation panel is provided with a power key, a touch panel for inputting output conditions, and various operation keys.

A sheet transport path 111 extending in the vertical direction is further formed in the apparatus main body 11 at a position on the right side of the image forming unit 13. The sheet transport path 111 is provided with a transport roller pair 112 that transports the sheet in a suitable place. Further, a resist roller pair 113 that performs skew correction of the sheet and feeds the sheet to the nip portion of the secondary transfer described later at a predetermined timing is provided on the upstream side of the nip portion in the sheet transport path 111. The sheet transport path 111 is a transport path for transporting the sheet P from the paper feed unit 12 to the paper discharge unit 17 via the image forming unit 13 and the fixing unit 16.

The paper feed unit 12 includes a paper feed tray 121, a pickup roller 122, and a paper feed roller pair 123. The paper feed tray 121 is removably mounted at a lower position of the apparatus main body 11 and stores a sheet bundle P1 in which a plurality of sheets P are laminated. The pickup roller 122 feeds out the uppermost sheet P of the sheet bundle P1 stored in the paper feed tray 121 one by one. The paper feed roller pair 123 feeds the sheet P unwound by the pickup roller 122 to the sheet transport path 111.

The paper feed unit 12 includes a manual paper feed unit attached to the left side surface of the apparatus main body 11 as shown in FIG. The manual paper feed unit includes a manual feed tray 124, a pickup roller 125, and a paper feed roller pair 126. The manual feed tray 124 is a tray on which the manually fed sheet P is placed, and is opened from the side surface of the apparatus main body 11 as shown in FIG. 1 when the sheet P is manually fed. The pickup roller 125 pays out the sheet P placed on the manual feed tray 124. The paper feed roller pair 126 feeds the sheet P unwound by the pickup roller 125 to the sheet transport path 111.

The image forming unit 13 forms a toner image to be transferred to the sheet P, and includes a plurality of image forming units that form toner images of different colors. As this image forming unit, in the present embodiment, magenta (M) colors are sequentially arranged (from the left side to the right side shown in FIG. 1) from the upstream side to the downstream side in the rotation direction of the intermediate transfer belt 141 described later. A magenta unit 13M using a developer, a cyan unit 13C using a cyan (C) color developer, a yellow unit 13Y using a yellow (Y) color developer, and a black (Bk) color developer are used. A black unit 13Bk is provided. Each unit 13M, 13C, 13Y, 13Bk includes a photoconductor drum 20 (image carrier), a charging device 21, a developing device 23, a primary transfer roller 24, and a cleaning device 25 arranged around the photoconductor drum 20, respectively. To prepare for. Further, an exposure apparatus 22 common to each unit 13M, 13C, 13Y, and 13Bk is arranged below the image forming unit.

The photoconductor drum 20 is rotationally driven around its axis to allow the formation of an electrostatic latent image and has a cylindrical surface on which the electrostatic latent image carries a toner image manifested by toner. .. As the photoconductor drum 20, as an example, a known amorphous silicon (α-Si) photoconductor drum or an organic (OPC) photoconductor drum is used. The charging device 21 uniformly charges the surface of the photoconductor drum 20 to a predetermined charging potential. The charging device 21 includes a charging roller and a charging cleaning brush for removing toner adhering to the charging roller. The exposure device 22 is arranged on the downstream side in the rotation direction of the photoconductor drum 20 with respect to the charging device 21, and has various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror. The exposure device 22 exposes the surface of the photoconductor drum 20 uniformly charged to the charging potential by irradiating the surface with light modulated based on image data (predetermined image information) to expose an electrostatic latent image. Form.

The developing device 23 is arranged facing the photoconductor drum 20 at a predetermined developing nip portion NP (FIG. 3A) on the downstream side in the rotation direction of the photoconductor drum 20 from the exposure device 22. The developing apparatus 23 includes a developing roller 231 that has a peripheral surface that is rotated and carries a developer composed of toner and a carrier, and forms the toner image by supplying toner to the photoconductor drum 20.

The primary transfer roller 24 forms a nip portion with the photoconductor drum 20 by sandwiching the intermediate transfer belt 141 provided in the intermediate transfer unit 14. Further, the primary transfer roller 24 primary transfers the toner image on the photoconductor drum 20 onto the intermediate transfer belt 141. The cleaning device 25 cleans the peripheral surface of the photoconductor drum 20 after the toner image is transferred.

The intermediate transfer unit 14 is arranged in a space provided between the image forming unit 13 and the toner replenishing unit 15, and includes an intermediate transfer belt 141 and a drive roller 142 rotatably supported by a unit frame (not shown). , A driven roller 143, a backup roller 146, and a density sensor 100. The intermediate transfer belt 141 is an endless belt-shaped rotating body, and is bridged over a drive roller 142, a driven roller 143, and a backup roller 146 so that the peripheral surface side thereof abuts on the peripheral surface of each photoconductor drum 20. Has been done. The intermediate transfer belt 141 is orbitally driven by the rotation of the drive roller 142. In the vicinity of the driven roller 143, a belt cleaning device 144 for removing the toner remaining on the peripheral surface of the intermediate transfer belt 141 is arranged. The density sensor 100 (concentration detection unit) is arranged to face the intermediate transfer belt 141 on the downstream side of the units 13M, 13C, 13Y, and 13Bk, and determines the density of the toner image formed on the intermediate transfer belt 141. Detected by reflected light (reflection type). Specifically, the density sensor 100 includes an irradiation unit (not shown) capable of irradiating the detection light and a light receiving unit (not shown) capable of receiving the reflected light reflected by the intermediate transfer belt 141. It is possible to detect the density of the toner image according to the reflected light. In another embodiment, the density sensor 100 may detect the density of the toner image on the photoconductor drum 20 or may detect the density of the toner image fixed on the sheet P.

A secondary transfer roller 145 is arranged on the outside of the intermediate transfer belt 141 so as to face the drive roller 142. The secondary transfer roller 145 is pressed against the peripheral surface of the intermediate transfer belt 141 to form a transfer nip portion with the drive roller 142. The toner image primaryly transferred onto the intermediate transfer belt 141 is secondarily transferred to the sheet P supplied from the paper feed section 12 at the transfer nip section. That is, the intermediate transfer unit 14 and the secondary transfer roller 145 function as a transfer unit that transfers the toner image supported on the photoconductor drum 20 to the sheet P. Further, a roll cleaner 200 for cleaning the peripheral surface of the drive roller 142 is arranged on the drive roller 142.

The toner supply unit 15 stores toner used for image formation, and in the present embodiment, it includes a magenta toner container 15M, a cyan toner container 15C, a yellow toner container 15Y, and a black toner container 15Bk. These toner containers 15M, 15C, 15Y, and 15Bk each store replenishment toner of each color of M / C / Y / Bk. From the toner discharge port 15H formed on the bottom surface of the container, toner of each color is supplied to the developing apparatus 23 of the image forming units 13M, 13C, 13Y, 13Bk corresponding to each color of M / C / Y / Bk.

The fixing portion 16 includes a heating roller 161 having a heating source inside, a fixing roller 162 arranged to face the heating roller 161 and a fixing belt 163 stretched on the fixing roller 162 and the heating roller 161. It is provided with a pressurizing roller 164 which is arranged to face the fixing roller 162 via 163 and forms a fixing nip portion. The sheet P supplied to the fixing portion 16 is heated and pressurized by passing through the fixing nip portion. As a result, the toner image transferred to the sheet P at the transfer nip portion is fixed to the sheet P.

The paper ejection portion 17 is formed by recessing the top of the apparatus main body 11, and a paper ejection tray 171 for receiving the ejected sheet P is formed at the bottom of the recess. The sheet P that has been subjected to the fixing process is discharged toward the output tray 151 via the sheet transport path 111 extending from the upper part of the fixing portion 16.

<About the developing device>
FIG. 2 is a cross-sectional view of the developing apparatus 23 according to the present embodiment and a block diagram showing an electrical configuration of the control unit 980. The developing apparatus 23 includes a developing housing 230, a developing roller 231, a first screw feeder 232, a second screw feeder 233, and a regulation blade 234. A two-component developing method is applied to the developing device 23.

The developing housing 230 is provided with a developing agent accommodating portion 230H. The developer accommodating portion 230H accommodates a two-component developer composed of a toner and a carrier. Further, the developer accommodating portion 230H conveys the developer in the first transport direction (direction orthogonal to the paper surface in FIG. 2, direction from rear to front) in which the developer is directed from one end side to the other end side in the axial direction of the developing roller 231. The first transport section 230A to be processed and the second transport section 230B that communicates with the first transport section 230A at both ends in the axial direction and transports the developer in the second transport direction opposite to the first transport direction. include. The first screw feeder 232 and the second screw feeder 233 are rotated in the directions of arrows D22 and D23 in FIG. 2, and transport the developer in the first transport direction and the second transport direction, respectively. In particular, the first screw feeder 232 supplies the developing agent to the developing roller 231 while conveying the developing agent in the first conveying direction.

The developing roller 231 is arranged facing the photoconductor drum 20 in the developing nip portion NP (FIG. 3A). The developing roller 231 includes a rotated sleeve 231S and a magnet 231M fixedly arranged inside the sleeve 231S. The magnet 231M comprises S1, N1, S2, N2 and S3 poles. The N1 pole functions as a main pole, the S1 pole and the N2 pole function as a transport pole, and the S2 pole functions as a peeling pole. Further, the S3 pole functions as a pumping pole and a regulating pole. As an example, the magnetic flux densities of S1 pole, N1 pole, S2 pole, N2 pole and S3 pole are set to 54 mT, 96 mT, 35 mT, 44 mT and 45 mT. The sleeve 231S of the developing roller 231 is rotated in the direction of the arrow D21 in FIG. The developing roller 231 is rotated to receive the developing agent in the developing housing 230, support the developing agent layer, and supply toner to the photoconductor drum 20. In this embodiment, the developing roller 231 rotates in the same direction (with direction) at a position facing the photoconductor drum 20. Further, in the axial direction (width direction) of the developing roller 231 the range in which the magnetic brush of the two-component developer is formed is, for example, 304 mm.

The regulating blade 234 is arranged on the developing roller 231 at a predetermined interval, and regulates the layer thickness of the developer supplied from the first screw feeder 232 onto the peripheral surface of the developing roller 231.

The image forming apparatus 10 including the developing apparatus 23 further includes a developing bias applying unit 971, a driving unit 972, and a control unit 980. The control unit 980 is composed of a CPU (Central Processing Unit), a ROM (Read Only Memory) for storing a control program, a RAM (Random Access Memory) used as a work area of the CPU, and the like.

The development bias application unit 971 is composed of a DC power supply and an AC power supply, and based on a control signal from the bias control unit 982 described later, the development roller 231 of the development device 23 has a DC voltage (DC bias) and an AC voltage (AC). Bias) is superimposed on the development bias.

The drive unit 972 includes a motor and a gear mechanism for transmitting the torque thereof, and in response to a control signal from the drive control unit 981 described later, the developing roller 231 in the developing device 23 is added to the photoconductor drum 20 during the developing operation. The first screw feeder 232 and the second screw feeder 233 are rotationally driven.

The control unit 980 includes a drive control unit 981, a bias control unit 982, a storage unit 983, and a calibration execution unit 984 (bias condition determination unit) by executing a control program stored in the ROM by the CPU. Function.

The drive control unit 981 controls the drive unit 972 to rotationally drive the developing roller 231 and the first screw feeder 232 and the second screw feeder 233. Further, the drive control unit 981 controls a drive mechanism (not shown) to rotationally drive the photoconductor drum 20.

The bias control unit 982 controls the development bias application unit 971 during the development operation (during the image formation operation) in which the toner is supplied from the development roller 231 to the photoconductor drum 20 to form the photoconductor drum 20 and the developing roller 231. A potential difference between the DC voltage and the AC voltage is provided between the two. Due to the potential difference, the toner is moved from the developing roller 231 to the photoconductor drum 20.

The storage unit 983 stores various information referred to by the drive control unit 981, the bias control unit 982, and the calibration execution unit 984. As an example, the rotation speed of the developing roller 231 and the value of the developing bias adjusted according to the environment are stored. Further, the storage unit 983 stores the print rate and the number of line lines set according to each toner image when a plurality of measurement toner images are formed on the photoconductor drum 20. The data stored in the storage unit 983 may be in a format such as a graph or a table. Further, the storage unit 983 can store the relational information described later, the reference DC voltage Vdct, and the reference peak-to-peak voltage Vppt, respectively.

The calibration execution unit 984 executes a bias calibration (bias condition determination mode) including the approximate expression setting mode and the bias setting mode described later. The calibration execution unit 984 corresponds to a predetermined latent image for measurement formed on the photoconductor drum 20 at the time of non-image formation different from the time of image formation in which the image formation operation is executed in the bias calibration. By applying a development bias to the developing roller 231 to develop the measurement latent image into a measurement toner image with toner, and based on the density of the measurement toner image detected by the density sensor 100, during the image forming operation. The reference DC voltage Vdct, which is the reference of the DC voltage applied to the developing roller 231 and the reference peak voltage Vpt, which is the reference of the peak-to-peak voltage of the AC voltage of the development bias, are determined.

More specifically, the calibration execution unit 984 forms a plurality of measurement toner images on the photoconductor drum 20 while controlling the photoconductor drum 20, the charging device 21, and the developing device 23 in the approximate setting mode. More specifically, the calibration execution unit 984 applies the development bias to the developing roller 231 corresponding to the predetermined latent image for measurement formed on the photoconductor drum 20 to obtain the latent image for measurement with toner. After developing into a measurement toner image, it is transferred from the photoconductor drum 20 to the intermediate transfer belt 141, and the density of the measurement toner image is measured by the density sensor 100. At this time, the density of the toner image for measurement is changed by changing the voltage between peaks of the AC bias of the development bias. Then, the calibration execution unit 984 executes the above operation for a plurality of DC biases (Vdc), and extracts the peak-to-peak voltage (saturation-peak-to-peak voltage) that maximizes the concentration in each DC bias. do. As a result, the calibration execution unit 984 acquires the relationship information indicating the relationship between the DC bias (DC voltage) and the saturation peak voltage. In the approximate setting mode, the electrostatic latent image and the toner image formed on the photoconductor drum 20 are defined as the first measurement latent image and the first measurement toner image, respectively. The first latent image for measurement is formed by the potential difference between the photoconductor drum 20 and the developing roller 231 without using the exposure apparatus 22.

Further, the calibration execution unit 984 changes the DC bias Vdc (DC voltage) and the peak-to-peak voltage Vpp, respectively, based on the related information in the bias setting mode, and the DC voltage corresponding to the preset target image density. And the combination of the saturation peak voltage is acquired as the reference DC voltage Vdct and the reference peak voltage Vpt. In the bias setting mode, the electrostatic latent image and the toner image formed on the photoconductor drum 20 are defined as the second measurement latent image and the second measurement toner image, respectively. The second latent image for measurement is formed in a dot shape or a line shape on the photoconductor drum 20 by being irradiated with the exposure light from the exposure apparatus 22.

<Development operation>
FIG. 3A is a schematic diagram of the developing operation of the image forming apparatus 10 according to the present embodiment, and FIG. 3B is a schematic diagram showing the magnitude relationship between the potentials of the photoconductor drum 20 and the developing roller 231. FIG. 3C is a schematic diagram showing the relationship between the DC bias and the AC bias of the development bias. With reference to FIG. 3A, a developing nip portion NP is formed between the developing roller 231 and the photoconductor drum 20. The toner TN and carrier CA supported on the developing roller 231 form a magnetic brush. In the developing nip portion NP, the toner TN is supplied from the magnetic brush to the photoconductor drum 20 side, and the toner image TI is formed. With reference to FIG. 3B, the surface potential of the photoconductor drum 20 is charged to the background potential V0 (V) by the charging device 21. After that, when the exposure light is irradiated by the exposure apparatus 22, the surface potential of the photoconductor drum 20 changes from the background potential V0 (non-image forming portion) to the maximum image potential VL (V) according to the image to be printed. It is changed to (image forming part). On the other hand, with reference to FIG. 3C, a DC voltage Vdc (DC bias) of the development bias is applied to the development roller 231 and an AC voltage (AC bias) is superimposed on the DC voltage Vdc. As an example, as shown in FIG. 3C, the AC bias consists of a periodic square wave, and its peak-to-peak voltage (Vpp) has an amplitude exceeding the background potential V0 and the image potential VL of the photoconductor drum 20. ..

In the case of such a reverse development method, the potential difference between the surface potential V0 and the DC component Vdc (DC bias) of the development bias is the potential difference that suppresses toner fog on the background portion of the photoconductor drum 20. On the other hand, the potential difference between the surface potential VL after exposure and the DC component Vdc of the development bias becomes the development potential difference that moves the positive polarity toner to the image portion of the photoconductor drum 20. Further, the AC component (AC bias) of the development bias applied to the developing roller 231 promotes the transfer of toner from the developing roller 231 to the photoconductor drum 20.

<About bias calibration>
Conventionally, there is known a technique of measuring the image density of a halftone image while changing the DC bias as described above and selecting a DC bias capable of obtaining a target image density from the characteristics. On the other hand, when Vpp (inter-peak voltage) of the AC bias is set high, the image density increases, the texture of the halftone image improves, and the half image pitch unevenness that tends to occur in the rotation cycle of the developing roller 231 tends to improve. be. However, if Vpp is set too high, a leak may occur in the developing nip portion NP where the photoconductor drum 20 and the developing roller 231 face each other, and the printing history of the developing roller 231 one round before is displayed on the image. The so-called development ghost that appears worsens. Further, if Vpp is set too low, an image density change (half image pitch unevenness) corresponding to the circumferential runout of the developing roller 231 and the photoconductor drum 20 occurs on the halftone image. Therefore, it is necessary to appropriately set the Vpp of the AC bias among the development biases. Further, if the difference between the above-mentioned DC bias Vdc and the background potential V0 of the photoconductor drum 20 becomes too large, the development ghost is deteriorated, while the half image pitch unevenness is improved. As described above, since the DC bias of the development bias and the Vpp of the AC bias affect the same image result, it is difficult to obtain a stable image even if only the Vpp is adjusted. That is, even if the Vpp of the AC bias is appropriately adjusted, the image defect to be eliminated may be deteriorated depending on the value of the DC bias. Therefore, it is desirable that both are set in association with each other.

Therefore, the inventor of the present invention can stably set the DC bias of the development bias and the peak voltage of the AC bias in the image forming apparatus 10 having the developing apparatus 23 to which the two-component developing method is applied. Newly discovered "bias calibration". In the bias calibration according to the present embodiment, the density sensor 100 made of an optical sensor is used to set the DC bias (Vdc) and the AC bias peak-to-peak voltage (Vpp) for achieving high image quality.

Here, unlike Vdc, Vpp promotes the reciprocating motion of the toner in the developing nip portion, and therefore has the characteristic that the image density increases once and then decreases as the output is increased. When the output of Vpp is increased, the movement of toner from the developing roller 231 to the photoconductor drum 20 becomes more active than the recovery of toner from the photoconductor drum 20 to the developing roller 231. The movement of the toner reaches a plateau, and the recovery of toner from the photoconductor drum 20 to the developing roller 231 becomes dominant, so that the image density decreases. From this point of view, when the peak voltage at the time of image formation is set based on Vpp (saturation peak voltage) at which the image density peaks, the present inventor balances the development of the toner and the recovery of the toner. However, we paid a new attention to the fact that a certain image quality can be obtained. However, this saturation peak voltage changes according to the amount of charge of the toner. Therefore, it is necessary to appropriately set Vdc and Vpp according to the amount of charge of the toner.

In general, the density sensor 100 made of an optical sensor has low measurement sensitivity in a region where the image density is high, so that it is difficult to accurately extract the saturation peak voltage. In view of such a problem, the present inventor extracts the saturation peak voltage under a low Vdc condition, that is, an image density in which the sensitivity of the density sensor 100 is sufficiently secured, and the relationship (relationship) between this Vdc and the saturation peak voltage. Information) was newly found to determine Vdc and Vpp corresponding to the target concentration. The bias calibration according to this embodiment will be described in more detail below.

FIG. 4 is a flowchart of bias calibration executed by the calibration execution unit 984 in the image forming apparatus 10 according to the present embodiment. The bias calibration is performed at the time of non-image formation in which no image is formed on the sheet P.

Specifically, when executing the bias calibration, the calibration execution unit 984 determines whether or not the preset mode start condition is satisfied. As an example, the mode start condition is the first condition satisfied when the power of the apparatus main body 11 of the image forming apparatus 10 is turned on, and the amount of change in the absolute humidity around the apparatus main body 11 exceeds a predetermined humidity threshold value. The second condition satisfied in the case, the third condition satisfied when the number of sheets P for which the image forming operation is executed in the image forming apparatus 10 exceeds the preset number threshold value, and the toner replenishing unit 15 have been replaced. Includes a fourth condition that is met in the case. The calibration execution unit 984 executes bias calibration when at least one of the above-mentioned plurality of conditions is satisfied.

When the bias calibration is started, the calibration execution unit 984 executes the approximate expression setting mode (relationship information acquisition mode) (step S01). Further, when the approximate expression setting mode is completed, the calibration execution unit 984 executes the bias setting mode (step S02). The approximate setting mode and the bias setting mode will be described in more detail below.

FIG. 5 is a flowchart of the approximate expression setting mode of the bias calibration according to the present embodiment. FIG. 6 is a graph showing the relationship between the peak voltage of AC bias and the image density (ID: Image Density) in a plurality of DC biases in the approximate setting mode according to the present embodiment. FIG. 7 is a graph showing the relationship between the DC bias and the saturation peak voltage in the approximate expression setting mode according to the present embodiment.

When the calibration execution unit 984 starts the approximate expression setting mode, the calibration execution unit 984 controls the charging device 21 to charge the surface of the photoconductor drum 20 to a predetermined charging potential. Further, the calibration execution unit 984 controls the development bias application unit 971 to apply the development bias to the development roller 231. At this time, the calibration execution unit 984 sets the DC bias (DC voltage) to Vdc (n) (step S11). Note that n is a natural number, and the maximum number N thereof is preset. As an example, N = 3 is set. The calibration execution unit 984 sets the DC bias to Vdc1 in the first step (n = 1) of step S11. Further, the calibration execution unit 984 changes (increases) Vpp in a state where parameters other than Vpp of the AC bias are fixed. At this time, it is desirable that the application time of each Vpp is set to the time during which the developing roller 231 is rotated one or more times. When Vpp is increased in this way, as shown in FIG. 6, the potential difference between the surface potential of the photoconductor drum 20 and the DC bias applied to the developing roller 231 under the condition of Vdc = Vdc1 (first). The image density of the measurement toner image (first measurement toner image) is based on the effect of the measurement latent image) and the AC bias (Vpp). Data can be obtained in which D rises and then falls. As a result, the calibration execution unit 984 acquires the saturated peak-to-peak voltage Vpp1 (Vpp (n)), which is the peak-to-peak voltage corresponding to the maximum concentration, from the data of FIG. 6 (step S12). The saturation peak voltage Vpp (n) may be a point (maximum value) at which the slope of the graph in FIG. 6 becomes zero.

Next, the calibration execution unit 984 determines whether or not the set Vdc matches Vdc (N), that is, whether or not n = N (step S13). Then, while increasing n one by one (step S16) until n = N (YES in step S13), steps S11 to S13 are repeated. As shown in FIG. 6, when the saturation peak voltage Vpp2 and Vpp3 are obtained for Vdc = Vdc2 and Vdc3, respectively, the calibration execution unit 984 performs Vdc (n) and Vpp as shown in FIG. From the graph showing the relationship with (n), an approximate expression (a linear expression of the graph of FIG. 7) showing the relationship between the two is determined (step S14). The approximate expression is stored in the storage unit 983. As a result, the calibration execution unit 984 ends the approximate expression mode (step S15).

The calibration execution unit 984 sets the density of the measurement toner image saturated at the saturation peak voltage Vpp (n) to be smaller than the target density (target image density) described later in the approximate expression setting mode. , A potential difference (first latent image for measurement) between the surface potential of the photoconductor drum 20 and the DC bias applied to the developing roller 231 is formed. By setting the density of the toner image for measurement smaller than the target density in this way, the relationship between Vdc (n) and Vpp (n) (relationship information) in the region where the detection sensitivity of the optical density sensor 100 is high. ) Can be obtained stably.

When the approximation formula setting mode ends, the calibration execution unit 984 starts the bias setting mode (step S02 in FIGS. 4 and 8). When the bias setting mode is started, the calibration execution unit 984 receives preset Vdc = Vd (n) and the corresponding Vpp = Vp (N) based on the approximate expression of FIG. 7 stored in the storage unit 983. n) (Saturation peak voltage) are acquired (step S21). Note that n is a natural number, and the maximum number N thereof is preset in the same manner as in the above-mentioned approximate expression setting mode. As an example, N = 3. As a result, three combinations of (Vd1, Vp1), (Vd2, Vp2), and (Vd3, Vp3) are determined.

Next, the calibration execution unit 984 acquires the image density of the toner image for measurement under each of the above combination conditions (step S22). Specifically, the calibration execution unit 984 controls the charging device 21 to charge the surface of the photoconductor drum 20 to a predetermined charging potential, and controls the exposure device 22 to charge the surface of the photoconductor drum 20 to a predetermined value. A latent image (second measurement latent image) is formed. Further, the calibration execution unit 984 controls the development bias application unit 971 to apply the development bias Vdc (= Vd (n)) to the development roller 231. At this time, the calibration execution unit 984 sets Vpp to Vp (n) (saturation peak voltage) with parameters other than Vpp fixed in the AC bias.

As a result, the image density I. Corresponding to each condition as shown in FIG. The data of D is acquired. The calibration execution unit 984 stores the linear expression (relational expression) of the graph shown in FIG. 9 in the storage unit 983 (step S23).

Next, the calibration execution unit 984 determines from the graph of FIG. 9 the reference DC voltage Vdct and the reference peak-to-peak voltage Vpt corresponding to the preset target concentrations as indicated by the arrows of FIG. 9 (step S24). If the reference DC voltage Vdct is determined from the graph (relational expression) of FIG. 9, the reference peak voltage Vppt can be determined from the graph (approximate expression) of FIG. 7. As a result, the calibration execution unit 984 ends the bias setting mode (step S25). Then, in the subsequent image forming operation, a development bias is applied to the developing roller 231 based on Vdct and Vppt.

When the Vdct obtained in step S24 is less than the preset lower limit threshold value (VdcL: for example, 40V) of Vdc, it is replaced with Vdct = VdcL. Similarly, if Vdct exceeds a preset upper threshold of Vdc (VdcH: for example 200V), it is replaced with Vdct = VdcH. The graphs of FIGS. 7 and 9 may be drawn with the horizontal axis as ΔV (Vdc-VL).

As described above, in the present embodiment, the calibration execution unit 984 executes the approximate expression setting mode and the bias setting mode as the bias calibration (bias condition determination mode), respectively. In the approximate expression setting mode, the calibration execution unit 984 is a mode for developing a preset first measurement latent image into a first measurement toner image, and is a mode in which a plurality of preset DC voltages are used. The inter-peak voltage of the AC voltage is changed for each, and the saturated peak-to-peak voltage, which is the peak-to-peak voltage at which the concentration of the first measurement toner image is saturated, is acquired for each of the DC voltages. The relationship information between the DC voltage and the saturation peak voltage is acquired. On the other hand, the bias setting mode is a mode in which the calibration execution unit 984 develops a preset second measurement latent image into a second measurement toner image, and is acquired in the related information acquisition mode. The DC voltage and the peak-to-peak voltage are changed based on the related information, and the combination of the DC voltage and the saturated peak-to-peak voltage corresponding to the preset target image density is combined with the reference DC voltage Vdct and the reference peak. Acquired as an inter-voltage Vppt.

According to such a configuration, even if each image forming condition such as the distance (DS gap) between the developing roller 231 and the photoconductor drum 20, the amount of toner charged, and the resistance of the carrier changes, calibration is performed as necessary. By executing the bias calibration by the execution unit 984, it is possible to set the DC bias of the development bias and the peak-to-peak voltage of the AC bias according to each image formation condition. As a result, it is possible to stably set the DC bias of the development bias and the peak voltage of the AC bias, which affect the same image defect, at appropriate timings, thereby stabilizing and improving the image quality. In particular, in the approximate expression setting mode, the DC bias and the voltage between saturation peaks are associated with each other as relational information, and then in the bias setting mode, the bias condition corresponding to the target density is set based on the relational information. It is possible to maintain a balanced state of toner movement (development, recovery) between the 231 and the photoconductor drum 20, and it is possible to obtain stable image quality. Further, as shown in FIG. 6, in each of the DC biases Vdc1, Vdc2, and Vdc3, in the vicinity of the saturation peak voltage Vpp1, Vpp2, and Vpp3, the concentration is unlikely to change even if Vpp fluctuates. Therefore, even if a Vpp setting error occurs due to the performance of the development bias application unit 971, it is possible to prevent the density fluctuation from occurring on the image.

Further, in the present embodiment, when any of the first, second, third and fourth conditions is satisfied and there is a possibility that the chargeability of the toner has changed, the calibration execution unit 984 By executing the approximate expression setting mode, the relationship information between the DC bias Vdc (n) (DC voltage) and the saturation peak voltage Vpp (n) can be reacquired, and a stable bias setting can be executed.

Although the embodiment of the present invention has been described above, the present invention is not limited to this, and for example, the following modified embodiment can be adopted.

(1) In the above embodiment, the embodiment in which the surface of the developing roller 231 is knurled and blasted has been described, but the surface of the developing roller 231 has a concave shape (dimple) + blasting. It may be blasted only, knurled groove only, concave shape (dimple) only, or plated.

(2) When the image forming apparatus 10 has a plurality of developing devices 23 as shown in FIG. 1, the AC calibration according to the above embodiment is performed by one or two developing devices 23, and the result is obtained by another developing device 23. It may be used in.

(3) The calibration execution unit 984 may continuously execute the approximate expression setting mode and the bias setting mode as bias calibration as in the above embodiment, or may execute each mode independently. good. Further, the calibration execution unit 984 may execute the approximate expression setting mode as necessary after executing the bias setting mode. FIG. 10 is a flowchart of bias calibration according to another embodiment of the present invention.

(4) The saturated peak-to-peak voltage, which is the peak-to-peak voltage at which the concentration of the first measurement toner image is saturated, is acquired for each of the DC voltages, and the relationship information between the DC voltage and the saturated peak-to-peak voltage is obtained. May be used as information related to the DC voltage after multiplying the saturation peak voltage by a predetermined coefficient. Further, this coefficient is not a constant value, and may have a slope (difference) such that it is low when the peak voltage is high and high when the peak voltage is low.

In this modified embodiment, when any of the above-mentioned first to fourth conditions is satisfied, the calibration execution unit 984 executes the bias setting mode (step S31). At this time, the related information stored in the storage unit 983 in advance is referred to by the calibration execution unit 984. Then, the calibration execution unit 984 determines the reference DC voltage Vdct and the reference peak-to-peak voltage Vpt corresponding to the target concentration, respectively, as in the previous embodiment (step S32).

Next, the calibration execution unit 984 determines the magnitude relationship between the determined Vdct and the preset lower limit threshold value VdcL of Vdc and the upper limit threshold value VdcH of Vdc (step S33). Here, when Vdct is equal to or higher than the lower limit threshold value VdcL (for example, 40V) and equal to or less than the upper limit threshold value VdcH (for example, 200V) (YES in step S33), the calibration execution unit 984 has the Vdct determined in step S32 and the previous bias. The magnitude relationship between the absolute value of the difference from Vdct (= Vdc0) determined in the setting mode and the preset threshold voltage k (for example, 30V) is determined (step S34). Here, when the absolute value is equal to or less than the threshold voltage k (YES in step S34), the calibration execution unit 984 determines that the reference DC voltage Vdct determined in step S32 is appropriate, and ends the bias calibration. do.

On the other hand, when Vdct is less than the lower limit threshold value VdcL or exceeds the upper limit threshold value Vdc in step S33 (NO in step S33), or the absolute value of the difference between Vdct and Vdc0 determined in the previous bias setting mode in step S34. When exceeds the threshold voltage k (NO in step S34), the calibration execution unit 984 executes the above-mentioned approximate expression setting mode (step S35). In such a case, it is highly possible that the current chargeability of the toner and the relational information stored in the storage unit 983 do not sufficiently correspond to each other, so it is desirable to execute the approximate expression setting mode. Then, the calibration execution unit 984 acquires the approximate expression of Vdc (n) and Vpp (n) again in the approximate expression setting mode (step S36), and similarly to the bias setting mode, the reference DC corresponding to the target concentration. The voltage Vdct and the reference peak voltage Vpt are determined respectively (step S37).

After that, the calibration execution unit 984 executes the same processing as in steps S33 and S34 in steps S38 and S39. Here, when Vdct is equal to or higher than the lower limit threshold value VdcL and equal to or lower than the upper limit threshold value VdcH (YES in step S38), and the absolute value is equal to or less than the threshold voltage k (YES in step S39), the calibration execution unit 984 It is determined that the reference DC voltage Vdct determined in step S37 is appropriate, and the bias calibration is terminated.

On the other hand, when Vdct is less than the lower limit threshold value VdcL or exceeds the upper limit threshold value Vdc in step S38 (NO in step S38), or the absolute value of the difference between Vdct and Vdc0 determined in the previous bias setting mode in step S39. When exceeds the threshold voltage k (NO in step S39), the calibration execution unit 984 executes the refresh mode of the developing device 23 and the bias calibration again.

Here, in the refresh mode, it is considered that the chargeability of the toner in the developing device 23 is lowered, so that an image for forcibly consuming the toner is formed on the photoconductor drum 20 and the toner is formed from the developing device 23. Is discharged, and the toner replenished for the amount consumed by the developing device 23 is replenished from the toner replenishing unit 15. After that, bias calibration is performed according to the chargeability of the newly replenished toner. At this time, it is desirable that the approximate expression setting mode and the bias setting mode are executed, respectively. If the conditions of steps S38 and S39 are not satisfied even though the flow of FIG. 10 is repeated, the calibration execution unit 984 displays calibration error information on a display unit (not shown) of the image forming apparatus 10. It is desirable that the maintenance work be carried out by the maintenance worker.

In another embodiment, only one of the determination processes of steps S33 or S34 in FIG. 10 may be performed. Similarly, only one of the determination processes of steps S38 or S39 in FIG. 10 may be performed.

As described above, in the present modification embodiment, the calibration execution unit 984 has the reference DC voltage Vdct acquired in the bias setting mode and the reference DC voltage Vdc0 acquired in the previous bias setting mode and stored in the storage unit 983. When the absolute value of the difference from and exceeds the preset threshold voltage k, the approximation formula setting mode is executed and the bias setting mode is executed again after the execution of the approximation formula setting mode. There is.

According to such a configuration, it is possible to accurately determine whether or not the approximate setting mode is to be executed by estimating the change in the chargeability of the toner from the change in the reference DC voltage Vdct acquired in the bias setting mode. can.

Further, in the present modification embodiment, the calibration execution unit 984 has a case where the reference DC voltage Vdct acquired in the bias setting mode is lower than the preset lower limit threshold value VdcL, or the reference DC voltage Vdct is set in advance. When the upper limit threshold value VdcH is exceeded, the approximate expression setting mode is executed and the bias setting mode is executed again after the approximate expression setting mode is executed.

According to such a configuration, it is estimated that the chargeability of the toner changes when the reference DC voltage Vdct acquired in the bias setting mode exceeds a preset allowable range, and an approximate expression is used. It is possible to accurately determine whether or not the setting mode needs to be executed.

(5) Further, FIG. 11 is a schematic diagram showing the relationship between the reference DC voltage Vdct and the reference peak voltage Vppt in the bias calibration according to another embodiment of the present invention. As shown in FIG. 11, a value within a range including a predetermined tolerance with reference to the reference DC voltage Vdct and the reference peak voltage Vpt determined in the bias setting mode is used as a part of the development bias during the subsequent image formation operation. It may be set. In particular, the peak voltage Vpp may be corrected to a certain extent based on Vppt according to the preference of image quality. For example, if it is desired to particularly improve the texture of the halftone image, Vpp = Vppt + α (V) may be set. Further, if it is desired to increase the image density a little, Vdc = Vdct + β (V) may be set.

The above-mentioned experimental data were performed under the following conditions.

<Common conditions>
-Printing speed: 55 sheets / minute-Photoreceptor drum 20: Amorphous silicon photoconductor (α-Si)
・ Development roller 231: Outer diameter 20 mm, surface shape knurled groove processing + blast processing (80 rows of recesses (grooves) are formed along the circumferential direction),
-Regulatory blade 234: Made of SUS430, magnetic, thickness 1.5 mm
-Developer transport amount after regulation blade 234: 250 g / m ²
Peripheral speed of the developing roller 231 with respect to the photoconductor drum 20: 1.8 (in the trail direction at the opposite position)
Distance between the photoconductor drum 20 and the developing roller 231: 0.25 mm
-White background (background) potential V0: + 250V of the photoconductor drum 20
-Image potential VL of the photoconductor drum 20: + 10V
Development bias of developing roller 231: frequency = 10 kHz, duty = 50% AC voltage square wave (Vpp is adjusted according to each experimental condition), Vdc (direct current voltage) = 150 V
-Toner: Positively charged polar toner, volume average particle size 6.8 μm, toner concentration 6%
-Carrier: Volume average particle diameter 35 μm, ferrite resin coated carrier

<About the developer>
The same effect has been confirmed regardless of whether the toner is a pulverized toner or a toner having a core-shell structure. It was also confirmed that the same effect was exhibited in the toner concentration in the range of 3% to 12%. Since the movement of toner due to an AC electric field is more likely to occur as the magnetic brush is finer, the volume average particle size of the carrier is preferably 45 μm or less, and more preferably 30 μm or more and 40 μm or less. Further, a resin carrier having a smaller true specific density than a ferrite carrier is more preferable.

<About career>
The carrier is a ferrite core having a volume average particle diameter of 35 μm coated with silicon, fluorine, or the like, and was specifically prepared by the following procedure. A coating liquid is prepared by dissolving 20 parts by mass of silicon resin KR-271 (manufactured by Shin-Etsu Chemical Co., Ltd.) in 200 parts by mass of toluene in 1000 parts by weight of carrier core EF-35 (manufactured by Powdertech). Then, after spray-coating the coating liquid with a fluidized bed coating device, heat treatment was performed at 200 ° C. for 60 minutes to obtain carriers. Resistance adjustment and charge adjustment are performed by mixing and dispersing a conductive agent and a charge control agent in the coating liquid in the range of 0 to 20 parts with 100 parts of each coating resin.

10 Image forming device 11 Device body 100 Concentration sensor (concentration detection unit)
141 Intermediate transfer belt (image carrier)
20 Photoreceptor drum (image carrier)
21 Charging device 22 Exposure device 23 Developing device 231 Development roller 971 Development bias application unit 972 Drive unit 980 Control unit 981 Drive control unit 982 Bias control unit 983 Storage unit 984 Calibration execution unit (bias condition determination unit)

Claims (6)

An image forming apparatus capable of performing an image forming operation for forming an image on a sheet.
An image carrier having a surface that allows the electrostatic latent image to be rotated to form an electrostatic latent image and that carries the toner image manifested by the toner.
A charging device that charges the image carrier to a predetermined charging potential, and
The electrostatic latent image is formed by exposing the surface of the image carrier, which is arranged on the downstream side in the rotation direction of the image carrier from the charging device and is charged with the charging potential, according to predetermined image information. Exposure device and
A developing device arranged facing the image carrier at a predetermined developing nip portion on the downstream side in the rotation direction of the exposure device, and has a peripheral surface that is rotated and carries a developer composed of toner and a carrier. A developing apparatus including a developing roller that forms the toner image by supplying toner to the image carrier, and
A transfer unit that transfers the toner image supported on the image carrier to a sheet, and a transfer unit.
A development bias application unit capable of applying a development bias in which an AC voltage is superimposed on a DC voltage to the development roller, and a development bias application unit.
It includes an irradiation unit capable of irradiating the detection light and a light receiving unit capable of receiving the reflected light reflected by the image carrier, and can detect the density of the toner image according to the reflected light. Concentration detector and
By applying the development bias to the developing roller corresponding to a predetermined latent image for measurement formed on the image carrier at the time of non-image formation different from the time of image formation in which the image forming operation is executed. The measurement latent image is developed into a measurement toner image with toner, and the DC voltage applied to the developing roller during the image forming operation is based on the density of the measurement toner image detected by the density detection unit. A bias condition determination unit that executes a bias condition determination mode for determining a reference peak-to-peak voltage as a reference for the reference DC voltage as a reference and the peak-to-peak voltage of the AC voltage of the development bias, respectively.
Equipped with
The bias condition determination unit is set as the bias condition determination mode.
In a mode for developing a preset first measurement latent image into a first measurement toner image, the peak-to-peak voltage of the AC voltage is applied to each of the plurality of preset DC voltages. The saturated peak-to-peak voltage, which is the peak-to-peak voltage at which the concentration of the first measurement toner image is saturated, is acquired for each of the DC voltages, and the relationship information between the DC voltage and the saturated peak-to-peak voltage is obtained. Relationship information acquisition mode to acquire, and
In a mode for developing a preset second measurement latent image into a second measurement toner image, the DC voltage and the peak-to-peak voltage are based on the relational information acquired in the relational information acquisition mode. And a bias setting mode in which the combination of the DC voltage and the saturation peak voltage corresponding to the preset target image density is acquired as the reference DC voltage and the reference peak voltage.
An image forming apparatus capable of performing each of the above. The bias condition determining unit is used for the first measurement so that the density of the first measurement toner image saturated at the saturation peak voltage is smaller than the target image density in the relationship information acquisition mode. The image forming apparatus according to claim 1, which forms a latent image. Further, a storage unit capable of storing the related information, the reference DC voltage, and the reference peak voltage is further provided.
The bias condition determining unit presets the absolute value of the difference between the reference DC voltage acquired in the bias setting mode and the reference DC voltage acquired in the previous bias setting mode and stored in the storage unit. The invention according to claim 1 or 2, wherein when the threshold voltage is exceeded, the relational information acquisition mode is executed and the bias setting mode is executed again after the relational information acquisition mode is executed. Image forming device. Further, a storage unit capable of storing the related information, the reference DC voltage, and the reference peak voltage is further provided.
In the bias condition determination unit, when the reference DC voltage acquired in the bias setting mode is below the preset lower limit threshold value, or the reference DC voltage exceeds the preset upper limit threshold value. The image forming apparatus according to claim 1 or 2, wherein the relation information acquisition mode is executed and the bias setting mode is executed again after the relation information acquisition mode is executed. The image forming apparatus according to claim 1 or 2, wherein the bias condition determining unit executes the related information acquisition mode when a preset mode start condition is satisfied. At least the device main body that houses the image carrier and the developing device, respectively.
Further, a toner container arranged in the apparatus main body and accommodating toner to be replenished in the developing apparatus is provided.
The mode start condition is a first condition that is satisfied when the power of the apparatus main body is turned on, and a second condition that is satisfied when the amount of change in the absolute humidity around the apparatus main body exceeds a predetermined humidity threshold value. At least one of a third condition satisfied when the number of sheets on which the image forming operation is executed exceeds a preset number threshold value and a fourth condition satisfied when the toner container is replaced. The image forming apparatus according to claim 5, which includes conditions.

Images