JP7414488B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunctional device having multiple functions thereof.

The image forming apparatus has a configuration in which, when developing an electrostatic latent image formed on an image carrier with toner, a developing bias in which a DC voltage and an AC voltage are superimposed is applied to a developer carrier carrying a developer. Are known.

Patent Document 1 proposes a configuration in which image roughness is suppressed by using, as such a developing bias, a blank pulse having a blank portion in which only a DC voltage is applied without applying an AC voltage. .

Furthermore, Patent Document 2 discloses that image graininess is reduced by setting the duty ratio of a component of an AC component having a polarity opposite to the normal charge polarity (negative polarity) of the toner (positive polarity) to 94% or more. A configuration has been proposed to suppress this decrease.

Japanese Patent Application Publication No. 7-271161 Unexamined Japanese Patent Publication No. 2016-11981

However, if the developing bias is set to a blank pulse as in Patent Document 1, or the duty ratio is changed as in Patent Document 2, although it is possible to improve the roughness of the image, toner remains in the non-image area of the image carrier. There is a possibility that fogging may easily occur.

SUMMARY OF THE INVENTION An object of the present invention is to provide a configuration that can suppress the occurrence of fog while improving image roughness.

Ｖｇｏ：前記パルス部における前記交流電圧のうち、前記直流電圧に対してトナーの正規帯電極性と同極性側の電圧
Ｖｒｅ：前記パルス部における前記交流電圧のうち、前記直流電圧に対してトナーの正規帯電極性と逆極性側の電圧
Ｖ１ ：前記パルス部における前記交流電圧のうち、前記所定の帯電電位に対してトナーの正規帯電極性と同極性側の電圧
Ｖ２ ：前記パルス部における前記交流電圧のうち、前記所定の帯電電位に対してトナーの正規帯電極性と逆極性側の電圧
Ｔ_Ｐ ：前記パルス部の時間
Ｔ_Ｂ ：前記ブランク部の時間 The image forming apparatus of the present invention includes an image bearing member, a charging unit that charges the surface of the image bearing member to a predetermined charging potential, and an electrostatic latent material on the surface of the image bearing member charged to the predetermined charging potential. a latent image forming section that forms an image; a developer carrier that carries a developer containing toner; and a developer carrier that develops the electrostatic latent image formed on the image carrier by the toner carried on the developer carrier. a developing bias applying section that applies a developing bias to the developer carrier, the developing bias is a bias in which a DC voltage and an AC voltage are superimposed, and the waveform of the developing bias is equal to the DC voltage. A pulse part in which the AC voltage and the AC voltage are superimposed, and a blank part in which only the DC voltage is present are periodically repeated, and the following formula is satisfied.

Vgo: Of the AC voltage in the pulse section, a voltage that has the same polarity as the normal charging polarity of the toner with respect to the DC voltage. Voltage on the opposite polarity side to the charging polarity V1: Voltage on the same polarity side as the normal charging polarity of the toner with respect to the predetermined charging potential among the AC voltages in the pulse section V2: Among the AC voltages in the pulse section , a voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the predetermined charging potential T _P : Time of the pulse portion T _B : Time of the blank portion

Further, the image forming apparatus of the present invention includes an image bearing member, a charging unit that charges a surface of the image bearing member to a predetermined charging potential, and an electrostatic charge applied to the surface of the image bearing member charged to the predetermined charging potential. a latent image forming section that forms an electrostatic latent image; a developer carrier that carries a developer containing toner; and an electrostatic latent image formed on the image carrier by the toner carried on the developer carrier. a developing bias applying unit that applies a developing bias to the developer carrier for development, the developing bias is a bias in which a DC voltage and an AC voltage are superimposed, and the waveform of the developing bias is the same as that described above. A pulse portion in which a DC voltage and the AC voltage are superimposed, and a blank portion in which only the DC voltage is present are periodically repeated, and is characterized by satisfying the following equation.

Vgo: Among the alternating current voltages in the pulse section, a voltage having the same polarity as the normal charging polarity of the toner with respect to the direct current voltage.
Vre: Of the AC voltage in the pulse section, the voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the DC voltage.
V1: Of the AC voltage in the pulse section, a voltage on the same polarity side as the normal charging polarity of the toner with respect to the predetermined charging potential.
V2: Among the AC voltages in the pulse section, a voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the predetermined charging potential.
T _P : Time of the pulse part
T _B : Time of the blank section

Further, the image forming apparatus of the present invention includes an image bearing member, a charging unit that charges a surface of the image bearing member to a predetermined charging potential, and an electrostatic charge applied to the surface of the image bearing member charged to the predetermined charging potential. a latent image forming section that forms an electrostatic latent image; a developer carrier that carries a developer containing toner and carrier; and an electrostatic latent image formed on the image carrier by the toner carried on the developer carrier. a developing bias applying section that applies a developing bias to the developer carrier in order to develop an image, the developing bias is a bias in which a direct current voltage and an alternating current voltage are superimposed, and the waveform of the developing bias is , a pulse part in which the DC voltage and the AC voltage are superimposed, and a blank part in which only the DC voltage is present are periodically repeated, and of the AC voltage in the pulse part, the DC voltage is The blank portion is set to exist after a voltage having the same polarity as the normal charging polarity of the toner is applied, and is characterized in that the following two equations are satisfied.

Vgo: Among the alternating current voltages in the pulse section, a voltage having the same polarity as the normal charging polarity of the toner with respect to the direct current voltage.
V1: Of the AC voltage in the pulse section, a voltage on the same polarity side as the normal charging polarity of the toner with respect to the predetermined charging potential.
Tgo: application time of the Vgo in one cycle of the pulse section
T _B : Time of the blank section
d: Distance between the image carrier and the developer carrier at the closest position
Dc: Volume average particle diameter of carrier
q: average charge amount of the toner carried on the developer carrier
m: average mass of toner

Ｖｇｏ：前記パルス部における前記交流電圧のうち、前記受け渡しバイアス印加部から前記現像剤担持体に印加された前記直流電圧に対してトナーの正規帯電極性と同極性側の電圧
Ｖ１ ：前記パルス部における前記交流電圧のうち、前記現像バイアス印加部から前記トナー担持体に印加された前記直流電圧に対してトナーの正規帯電極性と同極性側の電圧
Ｔｇｏ：前記パルス部の１周期における前記Ｖｇｏの印加時間
Ｔ_Ｂ ：前記ブランク部の時間
ｄ ：前記トナー担持体と前記現像剤担持体の最近接位置における距離
Ｄｃ ：キャリアの体積平均粒径
ｑ ：前記現像剤担持体に担持されたトナーの平均電荷量
ｍ ：トナーの平均質量 Further, the image forming apparatus of the present invention includes an image bearing member, a charging unit that charges a surface of the image bearing member to a predetermined charging potential, and an electrostatic charge applied to the surface of the image bearing member charged to the predetermined charging potential. a latent image forming section that forms an electrostatic latent image; a developer carrier that carries a developer containing toner and carrier; a toner carrier that receives toner from the developer carrier and carries the toner; a transfer bias applying section that applies a transfer bias to the developer carrier in order to transfer the toner from the developer carrier to the toner carrier; a developing bias applying section that applies a developing bias including at least a DC voltage to the toner carrier in order to develop the electrostatic latent image, and the transfer bias is a bias that is a combination of a DC voltage and an AC voltage. The waveform of the transfer bias periodically repeats a pulse part in which the DC voltage and the AC voltage are superimposed, and a blank part in which only the DC voltage is present, and the AC voltage in the pulse part is The blank portion is set to exist after a voltage having the same polarity as the normal charging polarity of the toner is applied to the DC voltage, and the blank portion satisfies the following two equations. .

Vgo: Of the AC voltage in the pulse section, the voltage on the same polarity side as the normal charging polarity of the toner with respect to the DC voltage applied from the delivery bias application section to the developer carrier V1: The voltage in the pulse section Among the AC voltages, a voltage on the same polarity side as the normal charging polarity of the toner with respect to the DC voltage applied from the development bias application unit to the toner carrier Tgo: application of the Vgo in one cycle of the pulse unit Time T _B : Time of the blank portion d: Distance between the toner carrier and the developer carrier at the closest position Dc: Volume average particle diameter of the carrier q: Average charge of the toner carried on the developer carrier Amount m: Average mass of toner

According to the present invention, it is possible to suppress the occurrence of fog while improving image roughness.

1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment; FIG. FIG. 1 is a schematic cross-sectional view of a developing device according to a first embodiment. A schematic diagram showing the potential relationship between a solid area and a highlight area. FIG. 3 is a schematic diagram showing a waveform of a developing bias according to the first embodiment. FIG. 3 is a schematic diagram for explaining the duty ratio of developing bias. A schematic diagram regarding measurement of the duty ratio of the developing bias. FIG. 7 is a schematic diagram showing a waveform of a developing bias according to a second embodiment. FIG. 7 is a schematic cross-sectional view of a developing device according to a third embodiment. FIG. 7 is a schematic diagram of a developing section shown to explain the configuration according to the fourth embodiment. FIG. 2 is a schematic perspective view of a toner charge amount measuring device. FIG. 3 is a schematic cross-sectional view of a developing device according to a fifth embodiment.

<First embodiment>
A first embodiment of the present invention will be described using FIGS. 1 to 6. First, a schematic configuration of an image forming apparatus 100 of this embodiment will be described using FIG. 1. In this embodiment, a case will be described in which a tandem-type full-color printer is used as an example of the image forming apparatus 100.

[Image forming device]
The image forming apparatus 100 of this embodiment is a full-color image forming apparatus 100 that employs an electrophotographic method, and includes four image forming sections Pa, Pb, Pc, and Pd. Note that the configurations of each image forming section are substantially the same except for the different developing colors. Therefore, in the following, unless a particular distinction is required, the image forming section Pa will be described as a representative, and other image forming sections will be described with subscripts b, c, and d indicating the configuration of the image forming section. is added to the reference numeral, and detailed explanation will be omitted.

The image forming section Pa includes a photosensitive drum 1a as an image carrier that carries a toner image. The photosensitive drum 1a is an example of a photosensitive member for electrophotography, and is formed in a cylindrical shape. Such a photosensitive drum 1a rotates in the direction of the arrow in FIG. 1 (counterclockwise). A charger 2a as a charging section, a laser beam scanner 3a as a latent image forming section, a developing device 4a, a primary transfer roller 6a, a cleaning device 19a, and the like are arranged around the photosensitive drum 1a.

Next, the overall image forming sequence in the normal mode of the image forming apparatus 100 having the above configuration will be described. First, the surface of the photosensitive drum 1a is uniformly charged to a predetermined charging potential by the charger 2a. In the normal mode, the photosensitive drum 1a rotates counterclockwise as indicated by the arrow at a process speed (circumferential speed) of, for example, 273 mm/sec. The photosensitive drum 1a charged by the charger 2a is then scanned and exposed by a laser beam modulated by an image signal by a laser beam scanner 3a, which is an example of an exposure device.

The laser beam scanner 3a has a built-in semiconductor laser, and this semiconductor laser is controlled based on input image data and emits a laser beam. For example, laser light is controlled in response to a document image information signal (image data) input from a document reading device having a photoelectric conversion element such as a CCD, or in response to an image information signal input from an external terminal. Inject. As a result, the surface potential of the photosensitive drum 1a charged by the charger 2a changes in the image area, and an electrostatic latent image is formed on the photosensitive drum 1a.

The electrostatic latent image thus formed on the photosensitive drum 1a is reversely developed with toner by the developing device 4a to form a visible image, that is, a toner image. In this embodiment, the developing device 4a uses a two-component developing method that uses a developer containing toner and carrier as a developer. That is, each developing device 4a, 4b, 4c, and 4d contains two-component developer containing toner of each color. Specifically, the developing device 4a is loaded with yellow (Y) toner, the developing device 4b is loaded with magenta (M) toner, the developing device 4c is loaded with cyan (C) toner, and the developing device 4d is loaded with black toner. (K) toner is stored respectively. Therefore, by performing the above-mentioned steps for each image forming section Pa, Pb, Pc, and Pd, toner images of four colors of yellow, magenta, cyan, and black are formed on the photosensitive drums 1a, 1b, 1c, and 1d, respectively. It is formed.

Further, an intermediate transfer belt 5, which is an intermediate transfer member, is arranged below each image forming portion Pa, Pb, Pc, and Pd. The intermediate transfer belt 5 is suspended by rollers 61, 62, and 63, and is movable in the direction of the arrow. The toner images on each of the photosensitive drums 1a to 1d are sequentially transferred onto the intermediate transfer belt 5 by primary transfer rollers 6a to 6d as primary transfer members. As a result, the four-color toner images of yellow, magenta, cyan, and black are superimposed on the intermediate transfer belt 5 to form a full-color image. Further, toner remaining on the photosensitive drum 1a without being transferred onto the intermediate transfer belt 5 is collected by the cleaning device 19a.

The full-color image on the intermediate transfer belt 5 is taken out from the feeding cassette 12 and transferred to a recording material S such as a sheet (paper, OHP sheet, etc.) via a feeding roller 13 and a feeding guide 11. The image is transferred by the action of the next transfer roller 10. Toner remaining on the surface of the intermediate transfer belt 5 without being transferred to the recording material S is collected by the intermediate transfer belt cleaning device 18. On the other hand, the recording material S onto which the toner image has been transferred is sent to the fixing device 16, where the image is fixed, and then discharged onto the discharge tray 17.

[Developing device]
Next, the developing device 4a will be explained with reference to FIG. The other developing devices 4b to 4d have the same configuration as the developing device 4a, so the description thereof will be omitted. The developing device 4a includes a developing container 22 containing a two-component developer, a developing sleeve 28 as a developer carrier, and first and second conveying screws 25 and 26 as conveying members. Further, in the developing device 4a of the present embodiment, the inside of the developing container 22 is partitioned into a developing chamber 23 which is a housing section and a stirring chamber by a partition wall 27 whose substantially central portion extends along the rotational axis direction of the developing sleeve 28. It has a vertical stirring type structure in which it is divided into upper and lower sections. The developer is contained in a developing chamber 23 and a stirring chamber 24.

First and second conveying screws 25 and 26 are arranged in the developing chamber 23 and the stirring chamber 24, respectively. The first conveying screw 25 is disposed at the bottom of the upper developing chamber 23 substantially parallel to the rotational axis direction of the developing sleeve 28, and rotates clockwise in FIG. The agent is conveyed while being stirred in one direction along the axis of rotation. Further, the second conveyance screw 26 is disposed at the bottom of the lower stirring chamber 24 substantially parallel to the first conveyance screw 25, and is rotated counterclockwise in FIG. 2 in the opposite direction to the first conveyance screw 25. Rotate to . The second conveyance screw 26 stirs and conveys the developer in the stirring chamber 24 in the opposite direction to the first conveyance screw 25 along the rotational axis direction.

In this way, the developer is circulated between the developing chamber 23 and the stirring chamber 24 through the openings (not shown) at both ends of the partition 27 by being conveyed by the rotation of the first and second conveying screws 25 and 26. Ru.

Here, a two-component developer containing a non-magnetic toner and a magnetic carrier used in this embodiment will be described. The toner is made up of colored resin particles containing a binder resin, a colorant, and, if necessary, other additives, to which an external additive such as colloidal silica fine powder is externally added. The toner used in this embodiment is a negatively chargeable polyester resin, and preferably has a volume average particle diameter of 3 μm or more and 8 μm or less. Therefore, the normal charging polarity of the toner in this embodiment is negative.

Further, as the carrier, surface oxidized or unoxidized metals such as iron, nickel, cobalt, manganese, chromium, rare earths, alloys thereof, or oxide ferrites can be suitably used, and these magnetic The method for producing the particles is not particularly limited. The carrier has a volume average particle diameter of 20 to 50 μm, preferably 25 to 45 μm, and a resistivity of 10 ⁷ Ωcm or more, preferably 10 ⁸ Ωcm or more. In this embodiment, one with a resistance of 10 ⁸ Ωcm was used.

An opening is formed in the developing container 22 at a position corresponding to the developing position facing the photosensitive drum 1a, and the developing sleeve 28 is rotatably disposed in this opening so as to be partially exposed on the photosensitive drum 1a side. has been done. The developing sleeve 28 carries and conveys the developer contained in the developing container 22, and supplies the developer to the developing position of the photosensitive drum 1a.

The length (coating amount) of the developer ears (magnetic brush) carried by the developing sleeve 28 is regulated by a regulating blade 29 which is an ear cutting member. Here, the diameter of the developing sleeve 28 is, for example, 20 mm, the diameter of the photosensitive drum 1a is, for example, 80 mm, and the distance d between the developing sleeve 28 and the photosensitive drum 1a at the closest position is, for example, about 250 μm. As a result, the spikes of developer carried by the developing sleeve 28 and conveyed to the developing position with the length regulated by the regulating blade 29 are brought into contact with the photosensitive drum 1a in the developing section, and are brought into contact with the photosensitive drum 1a. It is set so that electrostatic latent images can be developed.

The regulating blade 29 is made of a plate-shaped non-magnetic member made of stainless steel or the like and extends along the rotational axis of the developing sleeve 28, and is located upstream in the rotational direction of the developing sleeve 28 from the position facing the photosensitive drum 1a. It is located in Then, both the developer toner and the carrier pass between the tip of the regulating blade 29 and the developing sleeve 28 and are sent to the developing section.

The amount of developer conveyed to the developing section is regulated by adjusting the gap between the regulating blade 29 and the surface of the developing sleeve 28 to control the amount of developer carried on the developing sleeve 28 that is trimmed by the magnetic brush. is adjusted. In this embodiment, the regulating blade 29 regulates the amount of developer coated per unit area on the developing sleeve 28 to, for example, 30 mg/cm ² . Further, the gap between the regulating blade 29 and the developing sleeve 28 is set to 200 to 1000 μm, preferably 300 to 700 μm. In this embodiment, it is set to 400 μm. Note that in this embodiment, a non-magnetic member is used as the regulating blade 29, but a magnetic member may also be used.

The developing sleeve 28 is made of a non-magnetic material such as aluminum or stainless steel, and a magnet roller 28m serving as a magnetic field generating section is installed inside the developing sleeve 28 in a non-rotating state. This magnet roller 28m has a development pole S2 disposed facing the photosensitive drum 1a in the development section. Further, the magnet roller 28m has a magnetic pole S1 disposed facing the regulating blade 29, a magnetic pole N2 disposed between the magnetic poles S1 and S2, and a magnetic pole N2 disposed facing the developing chamber 23 and the stirring chamber 24, respectively. It has magnetic poles N1 and N3.

As described above, the developing sleeve 28 having the magnet roller 28m therein carries and conveys the developer by rotating in the direction of the arrow in FIG. 2 (clockwise) during development. Then, the developer whose layer thickness is regulated by cutting the magnetic brush by the regulation blade 29 is conveyed to a developing section facing the photosensitive drum 1a, and the developer is applied to the electrostatic latent image formed on the photosensitive drum 1a. and develop the latent image.

At this time, in the developing section facing the photosensitive drum 1a, the developing sleeve 28 moves in the forward direction of the moving direction of the photosensitive drum 1a, and moves at a circumferential speed ratio of, for example, 1.75 times that of the photosensitive drum 1a. are doing. This circumferential speed ratio may be any number of times as long as it is set between 0 and 3.0 times, preferably between 1.2 and 2.0 times. As the moving speed ratio increases, the developing efficiency increases, but if it is too large, problems such as toner scattering and developer deterioration may occur, so it is preferable to set it within the above range.

[Development bias]
Next, the developing bias applied to the developing sleeve 28 of the developing device 4a will be explained. In this embodiment, by applying a developing bias to the developing sleeve 28 from a power source 30 (FIG. 2) serving as a developing bias applying section, an electrostatic latent image is formed on the photosensitive drum 1a by the toner carried on the developing sleeve 28. Develop. The developing bias is a bias voltage in which a DC voltage and an AC voltage are superimposed in order to improve the development efficiency, that is, the rate of applying toner to the electrostatic latent image.

FIG. 3 schematically shows the latent image potential of a solid area where the image density is maximum and a highlight area where the image density is low. The dot latent image forming the highlight area with low image density has a small maximum development contrast Vcont, and especially when aiming for high resolution, the latent image potential Vcont formed by the direct current component (DC component) of the development bias and the exposure may end up being about the same. As shown in FIG. 3, the development contrast Vcont is the difference between the DC component Vdc of the development bias and the latent image potential _VL due to exposure. The development contrast of such a dot latent image in the highlight area is a very unstable contrast, depending on whether the toner adheres to the photosensitive drum 1a side or to the developing sleeve 28 side. Therefore, when developing the latent image, dots are likely to be missing due to various types of unevenness such as unevenness in the latent image, unevenness in developer coating, and variations in particle size of toner, and the image is likely to be rough. Therefore, depending on the waveform of the developing bias, the roughness of the highlight portion varies. For this reason, a developing bias waveform that improves the roughness of highlight areas is desired.

FIG. 4 is a diagram showing the waveform of the developing bias output by the power supply 30 according to the present embodiment. In FIG. 4, the vertical axis is voltage, the horizontal axis is time, and the vertical axis is displayed so that the upper side is negative. The power supply 30 applies a developing bias in which an AC component and a DC component of a rectangular wave are superimposed to the developing sleeve 28 . In this embodiment, a DC voltage of -500V and an AC voltage with a peak-to-peak voltage Vpp of 1160V and a frequency f of 12.6kHz are applied.

Generally, in the two-component development method, application of an alternating current voltage increases development efficiency and produces high-quality images, but on the other hand, fogging is more likely to occur. Therefore, there is a potential difference Vback (=|Vd-Vdc|, also referred to as fog removal potential) between the DC voltage Vdc applied to the developing sleeve 28 and the charged potential of the photosensitive drum 1a (that is, the surface potential of the non-image area) Vd. By providing this, fogging can be suppressed. In this embodiment, Vback is set to 150V. That is, the charging potential Vd of the photosensitive drum 1a is -650V, and the DC voltage Vdc of the developing bias is -500V.

Note that the charging potential Vd of the photosensitive drum 1a differs immediately after charging and in the developing section because of dark decay. In this embodiment, since the phenomenon in the developing section is targeted, the charging potential Vd of the photosensitive drum 1a in this specification refers to the value at the developing section. This also applies to other potentials such as the latent image potential VL of the photosensitive drum 1a.

The waveform of the developing bias according to the present embodiment periodically repeats a pulse portion in which an alternating current voltage is superimposed on a direct current voltage and a blank portion in which only the direct current voltage is present. That is, the waveform of the developing bias is a blank pulse waveform in which the alternating current voltage is intermittently thinned out to include only direct current voltage. In this specification, the portion where the AC voltage is superimposed is called the pulse portion, and the portion where the AC component is thinned out and becomes only the DC voltage is called the blank portion. Incidentally, a developing bias waveform without thinning of the AC voltage (no blank portion) is called a rectangular waveform. By using a blank pulse waveform, the application ratio of the toner return side component of the AC voltage, that is, the toner normal charging polarity (negative polarity in this embodiment) and the opposite polarity (positive polarity) side component to the rectangular waveform. lower. Furthermore, it is possible to improve the reproducibility of the shallow highlight portion of the latent image and improve roughness.

In this embodiment, as shown in FIG. 4, the waveform of the developing bias is a double blank pulse (WBP) waveform in which a blank section is provided after a pulse section consisting of a two-cycle rectangular wave AC voltage. The length of the blank portion is set to be equivalent to one cycle of the rectangular wave. When transitioning from the pulse section to the blank section, immediately after the voltage application of the developing drive side component of the AC voltage, that is, the voltage application of the same polarity (minus polarity) as the normal charging polarity of the toner (minus polarity in this embodiment) is completed. The blank section continues. This is because the blank area is easily affected by the previous state, and it is more effective to increase the reproducibility of the shallow highlight area of the latent image if the blank area continues after the voltage application of the development drive side component ends. This is because roughness can be further improved. On the other hand, if a blank area continues immediately after the voltage application of the toner return side component is finished, the reproducibility of the shallow highlight area of the latent image will decrease, and roughness will tend to occur.

Here, Vgo, Vre, V1, V2, Tgo, and Tre of the developing bias waveform shown in FIG. 4 are defined as follows.
Vgo: Of the AC voltage in the pulse section, the voltage that has the same polarity as the normal charging polarity of the toner with respect to the DC voltage.Vre: The voltage of the AC voltage in the pulse section that has the opposite polarity to the normal charging polarity of the toner with respect to the DC voltage. Voltage V1: Of the AC voltage in the pulse section, the voltage on the side of the same polarity as the normal charging polarity of the toner with respect to a predetermined charging potential. Voltage on the opposite polarity side to charging polarity Tgo: Application time of Vgo in one cycle of the pulse section Tre: Application time of Vre in one cycle of the pulse section

Vgo and Vre are the development drive side component and toner return side component based on the DC voltage Vdc of the developing bias, and V1 and V2 are the development drive side component and toner return side component based on the charging potential Vd of the photosensitive drum 1a. It is a side component. Further, Vgo and V1 are development drive side components, and Vre and V2 are toner return side components. Note that although the normal charge polarity of the toner is negative in this embodiment, it may be positive. Further, when the normal charging polarity of the toner is negative polarity, even if it is said that the polarity is opposite to the normal charging polarity of the toner, this does not mean that the polarity is positive. That is, it means that the voltage is on the positive side with respect to the reference (Vdc or Vd), and when a negative polarity developing bias is applied as in this embodiment, it is opposite to the normal charging polarity of the toner. Even if it is on the polar side, it is negative polarity. However, it may also have positive polarity.

In this embodiment, the duty ratio is changed so that the applied voltage of the toner return side component of the AC voltage is smaller than the applied voltage of the development drive side component with the DC voltage Vdc as a reference. Here, a duty waveform with a changed duty ratio will be explained using FIG. 5. First, as mentioned above, when the applied voltage of the development drive side component of the AC voltage is Vgo and the applied voltage of the toner return side component is Vre, the ratio of the development drive side component to the whole, Vgo/(Vgo+Vre), is the duty ratio. It is called.

At this time, the application time Tgo of the development drive side component and the application time Tre of the toner return side component are changed so that they have approximately the opposite ratio to the applied voltage (Tgo:Tre≈Vre:Vgo). Therefore, the integral value of the development drive side component and the integral value of the toner return side component within one cycle of the AC voltage are approximately constant. That is, the area Vgo×Tgo of the hatched portion above Vdc in FIG. 5 and the area Vre×Tre of the hatched portion below Vdc in FIG. 5 satisfy Vgo×Tgo≈Vre×Tre. By setting in this way, the effective voltage level can be made substantially constant in the pulse portion where AC voltage and DC voltage are superimposed and the blank portion consisting of only the DC component. Note that the duty ratio may be defined as Tre/(Tgo+Tre) based on the application time.

In this embodiment, the duty ratio is changed so that the applied voltage of the toner return side component of the AC voltage is smaller than the applied voltage of the development drive side component. Specifically, the duty ratio was set to 75%. By setting the duty ratio higher than 50% and reducing the voltage applied to the toner return side component, it is possible to improve the reproducibility of the shallow highlight portion of the latent image and improve roughness.

As described above, the developing bias waveform of this embodiment is a duty blank pulse waveform that is the sum of a duty waveform in which the duty ratio is greater than 50% and a blank pulse waveform in which a blank portion is provided. In other words, the ratio of the applied voltage of the toner return side component of the AC voltage is reduced, and at the same time, the total application time of the voltage of the toner return side component of the AC voltage is also reduced. Therefore, it is possible to more effectively improve the reproducibility of the highlight portion with a shallow latent image and improve roughness.

However, if the duty ratio is made larger than 50% and the applied voltage ratio of the toner return side component of the AC voltage is made small, toner is likely to adhere to the non-image area of the photosensitive drum 1a, and fogging is likely to occur. There are concerns. Further, even if a blank portion is provided and the voltage application time of the toner return side component of the AC voltage is reduced, there is a concern that fogging will be more likely to occur. Therefore, with a duty blank pulse waveform that is a combination of both, there is a concern that toner may adhere to non-image areas and fog may occur.

In other words, by creating a duty blank pulse waveform with a duty ratio higher than 50% and an additional blank section, it is possible to more effectively improve the reproducibility of the shallow highlight section of the latent image and improve roughness. It is. However, in the case of such a duty blank pulse waveform, it is necessary to adjust the duty ratio and the length of the blank portion to suppress toner from sticking to the non-image portion and fogging.

If the duty ratio is relatively low and the applied voltage ratio of the toner return component is high, set a longer blank time and shorten the voltage application time of the toner return component to improve the reproducibility of highlighted areas. It's better to On the other hand, if the duty ratio is relatively high and the applied voltage ratio of the toner return side component is low, the occurrence of fogging is more of a concern than the reproducibility of highlighted areas, so the blanking time is increased. It's better not to be too much.

In order to express the above in an equation, let us consider a latent image in which the potential Vl of the latent image is approximately the same as the DC component Vdc of the developing bias, as shown in FIG. 4, as a shallow latent image in the highlight portion. Note that the depth of this latent image is a typical example and does not necessarily have to be this value. Even though it is simply called a highlight part, each latent image and latent image potential are different depending on the image density. Furthermore, the depth of the latent image within a dot is not uniform, and the depth of the latent image differs between the center and the periphery of the dot. Therefore, in this embodiment, a latent image in which the potential Vl of the latent image is approximately the same as the DC component Vdc of the developing bias is considered. According to the inventors' study, regarding the latent image in the highlight area with a reflection density of 0.6 or less, which is shallower than the latent image potential VL in the solid area, the potential Vl of the latent image is completely equal to the DC component Vdc of the developing bias. It has been found that the effects of this embodiment can be obtained to a certain extent even if the values do not match. Note that the reflection density was measured using a spectrodensitometer X-Rite 504/508 (manufactured by X-Rite Co., Ltd.).

The applied voltage ratio of the toner return side component to Vl≈Vdc of the shallow latent image portion of the highlight portion can be expressed as Vre/(Vgo+Vre). That is, since Vl≈Vdc, the applied voltage ratio can be expressed as described above using the development drive side component and toner return side components Vgo and Vre with reference to the DC voltage Vdc of the development bias. Further, as shown in FIG. 4, in the waveform of the developing bias, if the application time of the pulse part is T _P and the application time of the blank part is T _B , the ratio of the application time of the blank part is T _B /(T _P + T _B ).

Here, when the value of Vre/(Vgo+Vre) is small, problems with the reproducibility of the highlight section are unlikely to occur even if the blank section time is short, but when this value is large, the blank section time should be set longer. You can improve the reproducibility of highlighted areas. Therefore, in the present embodiment, in order to improve the reproducibility of the highlight portion, the ratio of the blank portion T _B /(T _P +T _B ) is higher than the applied voltage ratio Vre/(Vre+Vgo) of the toner return side component to Vdc. The blank portion time T _B is set so that it is large. If the above is expressed in a formula, it becomes the following formula (1).

On the other hand, the fogging of the non-image area is related to the applied voltage ratio V2/(V1+V2) of the toner return side component to the potential Vd of the non-image area. When this value is large, fogging is unlikely to occur even if the blanking period is long, but when this value is small, fogging can be suppressed by setting the blanking period to be short. Therefore, in this embodiment, in order to suppress fogging in non-image areas, the blank area ratio T _B /(T _P The blank portion time T _B is set so that +T _B ) is small. If the above is expressed in a formula, it becomes the following formula (2).

If the above equations (1) and (2) are combined, the following equation (3) is obtained.

Therefore, by adjusting the blank portion time T _B so as to satisfy this equation (3), it is possible to achieve both highlight reproducibility and fog suppression.

Note that, as is clear from FIG. 4, there is a relationship of V2=Vre+Vback. Also, since Vgo+Vre=V1+V2, equation (3) can also be written as equation (4) below.

From this, the relationship in equation (3) can be satisfied by adjusting not only _TB but also Vback. Regarding Vback, although it is set to 150V in this embodiment, it is preferable to set it in the range of 50V to 200V. The reason why Vback is set to 50 V or more is that when considering the stability of the potential in the non-image area, fogging may easily occur if it is less than 50 V (for example, 0 V). Furthermore, if Vback is set higher than 200V, there is a concern that carriers may adhere to non-image areas, so Vback is set to 200V or less.

More preferably, as shown in the following equation (5), the blank portion ratio T _B /(T _P +T _B ) is smaller than the applied voltage ratio V2/(V1+V2) of the toner return side component to Vd. It is preferable to set the blank portion time T _B as follows. By setting in this way, it is possible to further suppress fogging.

Further, as described above, the duty ratio Vgo/(Vgo+Vre) is preferably greater than 50% from the viewpoint of highlight reproducibility. However, if the duty ratio is too high, there is a concern that fogging will occur significantly, so it is preferable that the following formula (6) be satisfied. That is, the duty ratio is preferably 90% or less.

This equation (6) can also be written as the following equation (7).

Therefore, it is preferable that the following formula (8) be satisfied. If this formula is satisfied, formula (6) is naturally satisfied.

More preferably, the following equation (9) is used.

As described above, in this embodiment, by satisfying Expression (3), it is possible to suppress the occurrence of fog while improving the roughness of the image. Further, preferably the formula (5) is satisfied, more preferably the formula (8) is satisfied, and even more preferably the formula (9) is satisfied.

[Example]
Next, a description will be given of an experiment conducted regarding the occurrence level of roughness in highlight areas and fogging in non-image areas. In the experiment, in the image forming apparatus 100 described above, the occurrence of roughness in highlight areas and fogging in non-image areas was evaluated while mainly changing the length of the blank area. Roughness was evaluated using a line image with a reflection density of around 0.4 and 212 lines/inch. Note that the reflection density was measured using a spectrodensitometer X-Rite 504/508 (manufactured by X-Rite Co., Ltd.).

It has already been known from previous sensitivity evaluations that roughness is particularly noticeable in the vicinity of reflection density 0.3 to 0.5 due to the sensitivity of the human eye. A smooth surface with no roughness was rated as ○, a smooth surface with no noticeable roughness was rated as △, and a surface with noticeable roughness was rated as ×.

Fog was evaluated by outputting a solid white image using the image forming apparatus 100 in an environment of 30° C. and 80% RH. The evaluation was performed by visual inspection, and a case where the fog was hardly noticeable was rated ○, a case where the fog was little was rated △, and a case where the fog was a little large was rated ×. Table 1 shows the waveforms used in the experiment and the evaluation results.

The waveform “WBP” indicates that the developing bias waveform is a double blank pulse waveform in which a blank portion is provided after a pulse portion consisting of two cycles of rectangular wave AC voltage. Further, the "rectangular" waveform indicates a developing bias waveform without thinning of the AC voltage (no blank portion).

Furthermore, in Examples 1-1 to 1-4, the waveform of the developing bias satisfied the conditions described in the present embodiment, and in Comparative Examples 1-1 to 1-2, it did not. in particular,
The waveform of Example 1-1 satisfies equations (5) and (9).
The waveform of Example 1-2 satisfies equations (3) and (8), but does not satisfy equations (5) and (9).
On the other hand, the waveforms of Comparative Example 1-1 and Comparative Example 1-2 do not satisfy Expression (3) or Expression (8).
Further, Example 1-3 has the same waveform as Example 1-1, but Vpp is set larger than that of Example 1-1. This waveform satisfies equations (5) and (9) similarly to the waveform of Example 1-1.
Further, in Example 1-4, the duty ratio is 65%, in other words, Vre/(Vre+Vgo)=0.35, and the waveform of Example 1-4 is based on equation (3) and equation (8). is satisfied, but Equation (5) and Equation (9) are not satisfied.

As is clear from Table 1, when a blank portion for one wavelength was provided as in Example 1-1, good results were obtained regarding both roughness and fogging. In Example 1-1, since the waveform of the developing bias satisfies equations (5) and (9) as described above, the length of the blank portion was set appropriately while reducing the toner return side component. It is thought that this makes it possible to suppress both roughness in highlight areas and fog in non-image areas.

Also, when a blank portion for two wavelengths was provided as in Example 1-2, good results regarding roughness were obtained. Regarding fogging, although it was slightly lower than in Example 1-1, relatively good results were obtained. As mentioned above, unlike Example 1-1, the waveform of Example 1-2 does not satisfy equations (5) and (9), so the result was slightly inferior to Example 1-1 in terms of fogging. Conceivable.

On the other hand, when the blank portion was provided with a length corresponding to three wavelengths as in Comparative Example 1-1, good results were obtained regarding roughness, but fogging became noticeable. The waveform of Comparative Example 1-1 did not satisfy equations (3) and (8), and the length of the blank section was set to be long while the toner return side component was made small, so the evaluation of roughness was good. , it is thought that the evaluation of fog has decreased.

Further, in the case of a rectangular waveform without a blank as in Comparative Example 1-2, good results were obtained regarding fogging, but roughness was noticeable. In Comparative Example 1-2, although the toner return component was reduced, no blank portion was provided, so it is considered that although the evaluation of fogging was good, the evaluation of roughness was lower.

So far, it has been described that roughness can be improved by reducing the ratio of the applied voltage of the toner return side component to Vdc of the AC voltage. In order to improve the roughness evaluation, it seems possible to reduce the applied voltage intensity rather than the applied voltage ratio of the toner return side component. On the other hand, in this embodiment, the applied voltage ratio is reduced by simply reducing the development drive side component, such as by lowering Vpp. This is because there is a concern that the concentration may become unstable and fluctuations in concentration may occur. Therefore, by reducing the applied voltage ratio of the toner return side component, the voltage intensity of the development drive side component can be maintained.

However, the applied voltage strength of the toner return side component may be effective in some cases. Example 1-3 has the same waveform as Example 1-1, but Vpp is set larger than that of Example 1-1. In Example 1-3, the roughness was good, but the result was slightly inferior to the waveform of Example 1-1. This seems to be because the applied voltage intensity Vre of the toner return side component with respect to Vdc was 290V in the waveform of Example 1-1, whereas it was as large as 500V in Example 1-3. Therefore, the applied voltage intensity Vre of the toner return side component with respect to Vdc is preferably set to Vre≦500V, more preferably Vre≦400V, and still more preferably Vre≦300V as in Example 1-1. is good. However, if the applied voltage intensity Vre of the toner return side component with respect to Vdc is too low, there is a concern that fogging will easily occur, so it is preferable to set Vre≧100V.

Furthermore, in this embodiment, the duty ratio is set to 75%, but the duty ratio is not limited to this. However, as mentioned earlier, by increasing the duty ratio and decreasing the applied voltage ratio of the toner return side component, the applied voltage intensity of the toner return side component can be increased while maintaining the voltage intensity of the development drive side component. Can be made smaller. In this way, it is possible to maintain the image quality of solid areas while also improving the roughness of highlighted areas. Therefore, the duty ratio is preferably set to 65% or more, preferably 70% or more, and more preferably 75% or more as in Example 1-1. However, if the duty ratio is too high, the applied voltage intensity of the toner return side component becomes too small, and there is a concern that fogging may easily occur. Therefore, the duty ratio is preferably 90% or less.

Example 1-4 is an experimental result when the duty ratio is 65%, and although fogging is slightly worse than in Example 1-1, relatively good results were still obtained. Since the waveform of Example 1-4 did not satisfy Equation (5) or Equation (9), it is thought that the result was slightly inferior to Example 1-1 in terms of fog.

On the other hand, in Example 1-4, although relatively good in terms of roughness, the results were slightly worse than in Example 1-1. This is because the applied voltage ratio of the toner return side component to the latent image depth Vdc of the highlight area is kept relatively low at 0.35, but the applied voltage ratio of the toner return side component to the non-image area Vd is kept relatively low. This is thought to be due to the fact that 0.48 and 0.5 are quite close to each other. That is, in Example 1-4, Vre/(Vre+Vgo) is low at 0.35, but V2/(V1+V2) is 0.48, which is higher than Example 1-1, so the evaluation of roughness is lower. It is thought that this has decreased.

The deepest part of the latent image of the dot in the highlight area has a latent image depth of approximately Vdc, but in reality, as schematically shown in Figure 4, the potential of the latent image has a distribution, and the depth is approximately Vdc. The following parts occupy the majority. This is caused by the spot intensity distribution of the light source of the exposure device such as a laser or LED, the scattering of exposure light on the photosensitive drum, and the diffusion of electrons generated during exposure. Therefore, even if the applied voltage ratio of the toner return side component to Vdc is small, if the applied voltage ratio of the toner return side component to Vd becomes 0.5 or more, the latent image of the dot in the highlight area will be slightly There is a possibility that you will be affected. Therefore, as already shown in equation (9), the applied voltage ratio V2/(V1+V2) of the toner return side component to Vd is preferably set to be less than 0.5.

In this experiment, the developing bias waveform was measured using an oscilloscope "model number DPO2014B" manufactured by Tektronix, and the dark area potential Vd on the photosensitive drum 1a was measured using a surface electrometer (model 344) manufactured by TREK. . In both cases, there is some noise during measurement, so in that case, the average value will be used.

As described above, the potential on the photosensitive drum 1a is the potential at the development position. The potential at the development position can be measured by installing a surface electrometer probe at the development position, but if installation is difficult, a method of predicting the potential at the development position from the results measured at another position may be used.

Regarding the developing bias waveform, it is easily affected by rounding due to changes in the capacitance between the developing sleeve 28 and the photosensitive drum 1a during output. Therefore, in this experiment, when calculating the duty ratio by actually measuring the developing bias waveform, it was decided to calculate the duty ratio based on the application time rather than the applied voltage because it is particularly susceptible to the effects of rounding. FIG. 6 shows an example of the developing bias waveform in the case of distortion. The rounding of the developing bias waveform is a delay in potential response caused by a transient phenomenon that occurs when the potential is changed. Therefore, although the state of the potential is easily influenced by capacitance and the like, the timing at which the potential is changed is not easily affected. Therefore, in this experiment, it was decided to calculate the duty ratio based on the potential change start timing shown in a circle in FIG.

<Second embodiment>
A second embodiment will be described using FIG. 7. In the first embodiment, a case has been described in which a double blank pulse (WBP) waveform is used as the waveform of the developing bias. In contrast, in this embodiment, a single blank pulse (SBP) waveform is used as the waveform of the developing bias. Other configurations and operations are the same as those in the first embodiment, so similar configurations are designated by the same reference numerals, explanations and illustrations are omitted or simplified, and hereinafter, parts different from those in the first embodiment will be mainly focused. explain.

In this embodiment, as shown in FIG. 7, a single blank pulse waveform (hereinafter referred to as SBP) in which a blank portion is arranged after a pulse portion of a one-cycle rectangular wave is used as the waveform of the developing bias. The same concept as in the first embodiment also holds true for SBP. That is, in the case of this embodiment as well, each equation shown in the first embodiment is satisfied.

[Example]
Next, a description will be given of an experiment conducted regarding the occurrence level of roughness in highlight areas and fogging in non-image areas. The experiment was conducted in the image forming apparatus 100 using a method similar to that described in the example of the first embodiment, while mainly changing the length of the blank area and reducing the roughness of the highlight area and the non-image area. The state of fogging was evaluated. These evaluation methods are also the same as those in the first embodiment. Table 2 shows the waveforms used in the experiment and the evaluation results.

In Examples 2-1 and 2-2, the waveform of the developing bias satisfies the conditions described in this embodiment, and in Comparative Example 2-1, it does not. in particular,
The waveform of Example 2-1 satisfies equations (5) and (9).
The waveform of Example 2-2 satisfies equations (3) and (8), but does not satisfy equations (5) and (9).
On the other hand, the waveform of Comparative Example 2-1 does not satisfy Equation (3) or Equation (8).

As is clear from Table 2, when a blank portion for half a wavelength was provided as in Example 2-1, good results were obtained regarding both roughness and fogging. The waveform of Example 2-1 satisfies equations (5) and (9), so while reducing the toner return side component, by setting the length of the blank section appropriately, the roughness of the highlight section can be reduced. It is thought that both this and the suppression of fog in non-image areas were achieved.

Also, when a blank portion for one wavelength was provided as in Example 2-2, good results regarding roughness were obtained. Regarding fogging, although it was slightly lower than in Example 2-1, relatively good results were obtained. As mentioned above, unlike Example 2-1, the waveform of Example 2-2 does not satisfy formulas (5) and (9), so the result was slightly inferior to Example 2-1 in terms of fogging. Conceivable.

On the other hand, when the blank portion was provided with a length corresponding to 1.5 wavelengths as in Comparative Example 2-1, good results were obtained regarding roughness, but fogging became noticeable. The waveform of Comparative Example 2-1 did not satisfy Equations (3) and (8), and the length of the blank section was set to be long while the toner return side component was made small, so the evaluation of roughness was good. , it is thought that the evaluation of fog has decreased.

<Third embodiment>
A third embodiment will be described using FIG. 8. This embodiment differs from each of the above-described embodiments in the configuration of the developing device. The illustrations and descriptions of other configurations and operations, as well as configurations with common functions of the developing device, will be omitted or simplified, and the following description will focus on the differences from the first embodiment.

The developing device 4A of this embodiment employs a touchdown development method (also referred to as a hybrid development method) among image forming methods using two-component development methods. In the touch-down development method, in addition to the supply roller 28A as a developer carrier that includes a magnetic roller that carries a two-component developer, a toner carrier that carries the toner received from the supply roller 28A at a position P1 in FIG. A developing roller 31 is provided.

Specifically, the developing device 4A of this embodiment includes a developing container 22A containing a two-component developer, a supply roller 28A as a developer carrier, a developing roller 31 as a toner carrier, and a conveying member. It has first and second conveyance screws 25A and 26A. Further, in the developing device 4A of the present embodiment, the inside of the developing container 22A is partitioned into a developing chamber 23A, which is a housing section, and a stirring chamber, by a partition wall 27A whose substantially central portion extends along the rotational axis direction of the supply roller 28A. The structure is divided into 24A and 24A on the left and right. The developer is contained in a developing chamber 23A and a stirring chamber 24A.

In the developing chamber 23A and the stirring chamber 24A, the configuration in which the developer is stirred and conveyed to the first and second conveying screws 25A and 26A is the same as that of the developing device of the first embodiment. On the other hand, like the developing sleeve 28 of the first embodiment, the supply roller 28A has a magnet roller disposed therein and rotates in the direction of the arrow while supporting the developer in the developing chamber 23A on its surface. The layer thickness of the developer carried on the supply roller 28A is regulated by the regulation blade 29A, and the developer is conveyed to a position P1 facing the developing roller 31.

The developing roller 31 receives toner from the supply roller 28A at the position P1, carries the toner, and rotates in the direction of the arrow. At this time, a transfer bias is applied to the supply roller 28A from a power source 32 serving as a transfer bias applying section, so that the toner is transferred from the supply roller 28A to the developing roller 31. The transfer bias is, for example, a voltage obtained by superimposing a DC voltage and an AC voltage, and its waveform may be a blank pulse waveform having a blank portion as described in the developing bias of the first embodiment, or a blank pulse waveform having a blank portion as described in the developing bias of the first embodiment. It may also be a rectangular waveform that does not have . Note that it is preferable that the waveform of the transfer bias has a different pulse phase from the waveform of the developing bias described below.

By applying the transfer bias to the supply roller 28A in this manner, the toner of the developer containing toner and carrier carried on the supply roller 28A is transferred to the developing roller 31. The developing roller 31 rotates while carrying the transferred toner, and after the layer thickness is regulated by the regulating blade 29A, the developing roller 31 conveys the toner to a position facing a photosensitive drum (not shown). At this time, a developing bias similar to that of the first embodiment is applied to the developing roller 31 from a power source 30A serving as a developing bias applying section. That is, the waveform of the developing bias of this embodiment also satisfies any of the equations (3), (5), (8), and (9) described in the first embodiment. Note that the waveform of at least one of the transfer bias and the developing bias may be a single blank pulse waveform, similar to the waveform of the developing bias in the second embodiment.

By applying a developing bias to the developing roller 31 in this manner, the electrostatic latent image formed on the photosensitive drum is developed with the toner carried by the developing roller 31. In the case of this embodiment as well, since the same developing bias as in the first embodiment or the second embodiment is applied, roughness and fogging can be suppressed in the same manner as in these embodiments.

In addition, in such a touchdown development method, when an AC voltage is applied to the supply roller 28A and the developing roller 31, the toner moves back and forth at the position P1 in FIG. 8, which is the toner transfer position between the supply roller 28A and the developing roller 31. It will be repeated. Therefore, toner is likely to scatter at the position P1. On the other hand, if a blank pulse waveform with a blank portion is used as the developing bias applied to the developing roller 31 as in the present embodiment, the reciprocating motion can be reduced compared to the case of a rectangular waveform without a blank portion. Therefore, there is an advantage that toner scattering can be reduced.

<Fourth embodiment>
The fourth embodiment will be described using FIGS. 9 and 10 while referring to FIGS. 1 to 5. In the first embodiment described above, the waveform of the developing bias is made to satisfy any one of equations (3), (5), (8), and (9). On the other hand, in this embodiment, the waveform of the developing bias satisfies the following equation. Other configurations and operations are the same as those of the first embodiment described above, so similar configurations are given the same reference numerals, explanations and illustrations are omitted or simplified, and parts different from the first embodiment will be described below. I will mainly explain.

Also in this embodiment, as described in the first embodiment, the duty ratio of the waveform of the developing bias is greater than 50% and less than or equal to 90%. Also in this embodiment, roughness can be improved by changing the waveform of the developing bias to a duty blank pulse waveform in which a blank portion is further provided in a duty waveform with a duty ratio higher than 50%. However, in the case of such a duty blank pulse waveform, it is necessary to adjust the duty ratio and the length of the blank portion to suppress toner from sticking to the non-image portion and fogging.

Toner adhesion to the non-image area mainly occurs when the developing bias potential is more negative than the charging potential (also referred to as dark area potential) Vd of the photosensitive drum 1a. That is, it occurs when the development drive side component of the pulse portion of the development bias is applied. When the toner adhering to the non-image area generated in this manner is not successfully collected by the toner return side component of the pulse area or the blank area, fogging occurs.

As described above, in order to improve the reproducibility of the highlight portion, the duty ratio of the developing bias waveform is made larger than 50%, and the applied voltage ratio of the toner return side component of the AC voltage is made small. At the same time, a blank section is provided to reduce the voltage application time of the toner return side component of the AC voltage. In the case of such a developing bias waveform, if too much toner adheres to the non-image area of the photosensitive drum 1a when the developing drive side component of the pulse portion is applied, the toner cannot be completely collected and fogging tends to occur. Therefore, in order to suppress fog, it is desirable to reduce the amount of toner that adheres to the non-image area when the development drive side component is applied.

According to studies by the inventors, the amount of toner adhering to non-image areas tends to increase when the absolute value of the voltage applied to the development drive side component of the AC voltage is large or when the application time is long. This can be explained by the distance that the toner travels when the development drive side component is applied. The longer this distance is, the more likely it is that toner present in a wide area of the development area will adhere to the photosensitive drum. Therefore, it is preferable not to make the distance that the toner travels too long when the developing drive side component of the AC voltage is applied. According to the inventors' studies, it was found that toner adhesion to non-image areas can be effectively suppressed by setting the distance that the toner travels when the development drive side component is applied to the volume average particle diameter Dc of the carrier or less. Ta.

The above-mentioned contents will be explained below while explaining the movement of toner. FIG. 9 is a diagram showing the state of the developer in the developing section. Actually, during development, the carrier forms spikes as shown in the figure between the carrier and the photosensitive drum 1a, and more than half of the space is in a state where toner can freely reciprocate. Therefore, in this specification, the description will be made while ignoring the existence of the carrier.

Here, setting the distance that the toner travels when the development drive side component is applied to be equal to or less than the volume average particle diameter of the carrier means the following. That is, when the development drive side component is applied, only the toner on the photosensitive drum 1a side from the broken line α, which corresponds to the distance of the carrier average particle diameter Dc from the photosensitive drum 1a shown in FIG. 9, can adhere to the photosensitive drum 1a. be. In other words, the toner that has adhered to the carrier that is in contact with the photosensitive drum 1a will adhere to the photosensitive drum 1a even if the application time of the development drive side component is shortened. For this reason, by setting the distance that the toner travels when applying the development drive side component to a value equal to or less than the volume average particle diameter of the carrier, the toner that has adhered to the carrier on the developing sleeve 28 side is more concentrated on the photosensitive drum than the carrier that has adhered to the photosensitive drum 1a. I try not to attach it to 1a. As a result, the amount of toner adhering to the non-image area is limited, so that most of the toner is recovered by the fog removal potential Vback when the toner return side component is subsequently applied or when the blank area is applied.

In FIG. 9, d is the gap between the photosensitive drum 1a and the developing sleeve 28. That is, d is the distance at the closest position between the photosensitive drum 1a as an image carrier and the developing sleeve 28 as a developer carrier. Further, let q be the average charge amount of the toner carried on the developing sleeve 28, and m be the average mass of the toner. In this case, in the non-image area where the surface potential of the photosensitive drum 1a is the dark area potential Vd, the distance that the toner travels during the application time Tgo (see FIG. 4) of the development drive side component can be expressed as the following equation (10). .

First, the force that the toner with charge q receives from the electric field E is |q|E. The difference between the potential Vd of the photosensitive drum 1a in the non-image area and the potential applied to the development drive side component is V1 (see FIG. 4). Therefore, since E=V1/d, the force that the toner with charge q receives from the electric field E can be written as |q|×(V1/d). Therefore, under the condition that the initial velocity is 0, the distance that the toner travels during the application time Tgo of the development drive side component while receiving the force of q × (V1/d) can be written as the above equation (10). .

Setting the distance that the toner travels when the development drive side component is applied to be equal to or less than the volume average particle diameter of the carrier means that the distance shown in equation (10) should be equal to or less than the carrier volume average particle diameter Dc. That is, it is sufficient if the following equation (11) is satisfied.

Therefore, by setting Tgo of the waveform of the developing bias so as to satisfy Equation (11), toner adhesion to the non-image area can be effectively suppressed. Tgo can be controlled by frequency and duty ratio. In addition, in equation (11), the reason why the distance shown in equation (10) is made larger than Dc/10 is that even if this distance is short, there is no problem with fogging, but it may affect the dot reproducibility of the image area. That's because there isn't.

According to the inventors' studies, the distance that the toner travels during the application time Tgo of the development drive side component should be kept to 1.5 times or less the carrier volume average particle diameter Dc (corresponding to the dotted line β in FIG. 9). However, it was possible to maintain a state with little fogging. Until now, we have been discussing this while ignoring the existence of carriers, but in reality, carriers do exist, so even if the carrier volume average particle diameter Dc is set to 1.5 times or less, the actual distance traveled will be approximately This is because it will be less than that. Therefore, it is preferable to satisfy formula (11), but even if formula (12) below is satisfied, the above-mentioned effects can be obtained to some extent.

As described above, regarding the non-image area, fogging can be suppressed if the distance traveled during the application time Tgo of the development drive side component of the AC voltage is shorter than 1.5 times the carrier volume average particle diameter. On the other hand, considering the reproducibility of highlight areas, it is better for the distance that toner travels in image areas to be longer than the carrier volume average particle diameter. If both can be set at the same time, it is possible to achieve both fog and highlight reproducibility. In this embodiment, both can be achieved by making good use of the blank portion.

To express this in a formula, we will consider a latent image in which the potential Vl of the latent image is approximately the same as the DC component Vdc of the developing bias, as shown in FIG. 4, as a shallow latent image in the highlight portion. Note that the depth of this latent image is a typical example and does not necessarily have to be this value. Even though it is simply called a highlight part, each latent image and latent image potential are different depending on the image density. Furthermore, the depth of the latent image within a dot is not uniform, and the depth of the latent image differs between the center and the periphery of the dot. Therefore, in this embodiment, a latent image in which the potential Vl of the latent image is approximately the same as the DC component Vdc of the developing bias is considered. According to the inventors' study, regarding the latent image in the highlight area with a reflection density of 0.6 or less, which is shallower than the latent image potential VL in the solid area, the potential Vl of the latent image is completely equal to the DC component Vdc of the developing bias. It has been found that the effects of this embodiment can generally be obtained even if the values do not match. Note that the reflection density was measured using a spectrodensitometer X-Rite 504/508 (manufactured by X-Rite Co., Ltd.).

In a shallow latent image area where the surface potential of the photosensitive drum 1a corresponding to the highlight area is about Vl≈Vdc, the toner receives force in the direction toward the photosensitive drum 1a only when the development drive side component of the pulse area is applied. It is. However, if a blank section continues after the application of the development drive side component as in the present embodiment, the toner continues to advance toward the photosensitive drum 1a due to inertia during that period. That is, regarding the highlighted portion where the surface potential is about Vl≈Vdc, the blank portion has the same potential as the DC voltage Vdc, so if we simply think about it, there is no potential difference and no electric field is generated. Therefore, the toner accelerated by the development drive side component of the immediately preceding pulse portion continues to advance in the blank portion due to inertia. For this reason, in this embodiment, the distance that the toner travels, including this blank area, is made longer than the carrier volume average particle diameter, thereby improving the reproducibility of the highlighted area.

The difference between the potential of the highlighted portion of the photosensitive drum 1a where the surface potential is Vl=Vdc and the potential applied to the development drive side component of the photosensitive drum 1a is Vgo (see FIG. 4). At this time, the force that the toner with charge q receives from the electric field E can be written as |q|×(Vgo/d). Under the condition of initial velocity 0, while receiving a force of q×(Vgo/d), the application time Tgo (see FIG. 4) of the development drive side component and the subsequent blank portion time T _B (see FIG. 4) are applied. The distance that the toner travels during this time can be written as the following equation (13).

In order to make the distance that the toner travels including the blank portion longer than the carrier volume average particle diameter, it is sufficient that the distance shown in equation (13) is larger than the carrier average particle diameter Dc. That is, it is sufficient to satisfy the following equation (14).

The reason for setting the upper limit to 2d in equation (14) is that if the blank portion time _TB is unnecessarily long, excess toner tends to stay near the photosensitive drum 1a, and some of that toner becomes non-image. This is because there is a concern that it may adhere to the parts and cause fogging. If the distance that the toner travels including the blank area is equal to or greater than the distance d between the photosensitive drum 1a and the developing sleeve 28, sufficient amount of toner is supplied to reproduce the highlight area, and the blanking time T _B is Even if you extend it, the effect will become saturated. However, in an actual system, it is also necessary to consider the adhesion force between the toner and the carrier, which has been omitted here for the sake of simplicity. Therefore, as the distance that the toner travels according to the above equation (13), about 2d is sufficient. Therefore, the upper limit was set to 2d.

More preferably, the distance traveled by the toner including the blank portion is set to be greater than half the distance d between the developing sleeve 28 and the photosensitive drum 1a. With this setting, more than half of the toner supplied between the developing sleeve 28 and the photosensitive drum 1a can contribute to development, and highlight reproducibility can be further improved. That is, it is preferable that formula (15) is satisfied.

By setting the developing bias waveform, especially the blank portion application time _TB , so as to satisfy the above equation (15), it is possible to maintain the reproducibility of the highlight portion. Note that in this embodiment as well, the waveform of the developing bias may be the double blank pulse waveform described in the first embodiment, or the single blank pulse waveform described in the second embodiment.

As described above, in this embodiment, by satisfying equations (12) and (14), it is possible to suppress the occurrence of fog while improving the roughness of the image. Moreover, it is more preferable to satisfy formula (11) or formula (15). Note that it is preferable that the waveform of the developing bias also satisfies any one of equations (3), (5), (8), and (9) described in the first embodiment. That is, the waveform of the developing bias satisfies equations (12) and (14), preferably equations (11) and (15), and also satisfies equations (3), (5), (8), and (9). By satisfying either of the expressions, the effect of suppressing the occurrence of fogging while improving the roughness of the image can be obtained.

[Example]
Next, a description will be given of an experiment conducted regarding the occurrence level of roughness in highlight areas and fogging in non-image areas. The experiment was conducted in the image forming apparatus 100 using a method similar to that described in the example of the first embodiment, while mainly changing the frequency and the length of the blank part to reduce the roughness of the highlight part and the non-image. The occurrence of fogging was evaluated. These evaluation methods are also the same as those in the first embodiment. Table 3 shows the waveforms used in the experiment and the evaluation results. In the experiment of this example, the distance (gap) d between the developing sleeve 28 and the photosensitive drum 1a was 2.5×10 ⁻⁴ m. Further, the volume average particle diameter Dc of the carrier used in the experiment was 5.0×10 ⁻⁵ m. Further, the triboelectric charge amount |q|/m of the toner was 3.0×10 ⁻² C/kg.

In Examples 3-1 to 3-3, the waveform of the developing bias satisfies the conditions described in this embodiment, and in Comparative Examples 3-1 to 3-2, it does not. in particular,
The waveform of Example 3-1 satisfies equation (11), equations (14), and (15).
The waveform of Example 3-2 satisfies equations (12), (14), and (15), but does not satisfy equation (11).
The waveform of Example 3-3 satisfies equations (11) and (14), but does not satisfy equation (15).
On the other hand, the waveform of Comparative Example 3-1 does not satisfy Equations (11), (12), and Equation (14).
The waveform of Comparative Example 3-2 satisfies equation (11) but does not satisfy equation (14).

As is clear from Table 3, when the frequency of the pulse section is set to 12.6 kHz as in Example 3-1, and a blank section for one wavelength is provided, good results are obtained regarding both roughness and fogging. Obtained. As described above, the waveform of Example 3-1 satisfies equations (11) and (14). Specifically, in the case of Example 3-1, it is thought that fogging in non-image areas was suppressed by increasing the frequency and increasing the duty ratio, and setting the application time Tgo of the development drive side component to be short. It will be done. On the other hand, it is thought that by providing a blank area for one wavelength, it was possible to maintain the toner flying to the highlight area, thereby suppressing the occurrence of roughness.

Even when the frequency of the pulse portion was set to 9 kHz as in Example 3-2, good results regarding roughness were obtained. Regarding fogging, although it occurred slightly more easily than in Example 3-1, relatively good results were obtained. The waveform of Example 3-2 satisfies equation (14) as described above, but does not satisfy equation (11), but instead satisfies equation (12). Therefore, it is thought that the result was slightly inferior to Example 3-1 in terms of fog.

On the other hand, when the frequency of the pulse portion was lowered to 6 kHz as in Comparative Example 3-1, both roughness and fogging were more likely to occur. In the case of Comparative Example 3-1, the waveform does not satisfy equations (11), (12), or (14). Specifically, it is considered that as a result of lowering the frequency, the application time Tgo of the development drive side component becomes longer, making it easier for toner to adhere to non-image areas. Furthermore, as a result of lowering the frequency, the length of the blank portion also becomes longer, and as a result, toner tends to adhere to the highlighted portion. However, the blanking time becomes longer than necessary (to the extent that the upper limit of equation (14) is exceeded), and from this point of view as well, toner tends to adhere to non-image areas. As a result, it is thought that fogging is more likely to occur.

In the case of a rectangular waveform without a blank as in Comparative Example 3-2, good results were obtained regarding fogging, but roughness was noticeable. As described above, the waveform of Comparative Example 3-2 satisfies formula (11) but does not satisfy formula (14). Specifically, it is thought that because no blank area was provided, the fogging was good, but the reproducibility of the highlighted area was reduced and roughness was more likely to occur.

In Example 3-3, only the blank length is shorter than that in Example 3-1 by half a wavelength. In this case, good results regarding fogging were obtained. Regarding roughness, although it was slightly more likely to occur than in Example 3-1, relatively good results were obtained. The waveform of Example 3-3 satisfies equations (11) and (14) as described above, but does not satisfy equation (15). Therefore, with regard to roughness, it is considered that the results were slightly inferior to the settings of Example 3-1, in which a wider range of toner contributes to highlight reproducibility.

In this experiment, the developing bias waveform was measured using an oscilloscope "model number DPO2014B" manufactured by Tektronix, and the dark area potential Vd on the photosensitive drum 1a was measured using a surface electrometer (model 344) manufactured by TREK. . In both cases, there is some noise during measurement, so in that case, the average value will be used.

As described above, the potential on the photosensitive drum 1a is the potential at the development position. The potential at the development position can be measured by installing a surface electrometer probe at the development position, but if installation is difficult, a method of predicting the potential at the development position from the results measured at another position may be used.

[Measurement of toner charge amount]
Further, in the present embodiment, the triboelectric charge amount |q|/m of the toner was measured using the measuring device shown in FIG. First, the toner whose triboelectric charge amount is to be measured is mixed with carrier to form a two-component developer, placed in a polyethylene bottle with a capacity of 50 to 100 ml, and shaken by hand for about 10 to 40 seconds. Next, about 0.1 to 0.5 g of this developer is placed in a metal measuring container 142 whose bottom is a 635-mesh conductive screen 143, and the measuring container 142 is covered with a metal lid 144. The entire weight of the measurement container 142 in this state is measured, and this is defined as W1 (kg).

Next, the measuring container 142 is installed in the suction machine 141, suction is taken from the suction port 147, and the air volume control valve 146 is adjusted to set the pressure of the vacuum gauge 145 to 150 mmAq. Note that at least the portion of the suction device 141 that comes into contact with the measurement container 142 is made of an insulator. In this state, suction is performed for sufficient time, preferably 2 minutes, to remove the toner by suction. At this time, the potential indicated by the electrometer 149 connected to the measurement container 142 is read, and this is defined as Vt (V). Furthermore, the entire weight of the measuring container 142 after suction is measured, and this is defined as W2 (kg). Assuming that the capacitance of the capacitor 148 connected in parallel with the electrometer 149 in the measurement container 142 is C(F), the amount of triboelectric charge of the toner is calculated by the following formula:
Amount of triboelectric charge of toner |q|/m (C/kg)=C×Vt/(W1-W2)
It is calculated by

As the electrometer 149, the inventors used a low-current electrometer model 6514 manufactured by Keithley. Since this electrometer has a charge measurement function, the inventors directly measured the amount of charge |q|=C×Vt in the above formula using this electrometer.

In the above experiment, toner with a triboelectric charge amount |q|/m of 3.0×10 ⁻² C/kg was used, but the triboelectric charge amount of the toner is not limited to this value. For example, this embodiment is preferably applicable within the range of 1.0×10 ⁻² C/kg or more and 7.0×10 ⁻² C/kg or less. If it is less than 1.0×10 ⁻² C/kg, the electric field force applied to the toner becomes weak, making it difficult to control the flight of the toner. Moreover, if it is larger than 7.0×10 ⁻² C/kg, the electric charge amount of the toner is large, and the electric field adhesion force between the toner and the carrier becomes large, making it difficult to control the flight of the toner. More preferably, it is within the range of 1.0×10 ⁻² C/kg or more and 5.0×10 ⁻² C/kg or less.

The amount of triboelectric charge |q|/m of the toner tends to change depending on the usage environment. Therefore, values in an environment of 23° C. and 60% humidity were used as representative values. Problems such as image roughness and fogging are more likely to occur when the amount of triboelectric charge |q|/m of the toner is small. Therefore, if a value in which |q|/m is not too high and has moderate humidity, such as an environment with a temperature of 23° C. and a humidity of 60%, is used as a representative value, the effects of the invention can generally be obtained in other environments.

[Adjustment of the gap d between the developing sleeve 28 and the photosensitive drum 1a]
The gap d between the developing sleeve 28 and the photosensitive drum 1a was adjusted as follows. That is, by inserting a gap gauge with a desired thickness between the developing sleeve 28 and the photosensitive drum 1a, and using the gap gauge as a spacer, the distance between the centers of the photosensitive drum 1a and the developing sleeve 28 can be adjusted. d was set to a desired value. In the case of this embodiment, adjustment was made using a 250 μm gap gauge.

Note that by irradiating and scanning the gap with a laser beam and detecting the reflected light, this gap is measured, and the distance between the centers of the photosensitive drum 1a and the developing sleeve 28 is adjusted to match the predetermined facing distance. But it doesn't matter.

[Measurement of volume average particle diameter of carrier]
Next, a method for measuring the volume average particle size (D50) of the carrier of the developer will be described. First, the particle size distribution and the like were measured using a laser diffraction/scattering type particle size distribution measuring device (trade name: Microtrac MT3300EX, manufactured by Nikkiso Co., Ltd.).

The volume average particle diameter (D50) of carriers and the like was measured using a sample feeder for dry measurement (trade name: one-shot dry sample conditioner Turbotrac, manufactured by Nikkiso Co., Ltd.). The supply conditions for Turbotrac were as follows: a dust collector was used as a vacuum source, an air flow rate was 33 l/sec, and a pressure was 17 kPa. Control was performed automatically on software. For the particle size, the 50% particle size (D50), which is the cumulative volume average value, was determined. Control and analysis were performed using the attached software (version 10.3.3-202D). The measurement conditions are as follows.

Set Zero time: 10 seconds Measurement time: 10 seconds Number of measurements: 1 time Particle refractive index: 1.81%
Particle shape: non-spherical Measurement upper limit: 1408μm
Measurement lower limit: 0.243μm
Measurement environment: temperature 23℃/humidity 50%RH

<Fifth embodiment>
A fifth embodiment will be described using FIG. 11. The developing device 4B of this embodiment is a touchdown type developing device similar to the developing device 4A shown in FIG. 8, and has the same basic configuration as the developing device 4A. In addition, in the case of this embodiment, the waveform of the transfer bias applied to the supply roller 28A satisfies equations (12) and (14) similarly to the waveform of the developing bias described in the fourth embodiment. Preferably, formula (11) or formula (15) is satisfied. Therefore, the illustrations and explanations of other configurations and operations will be omitted or simplified, and the following explanation will focus on the points that are different from the third embodiment.

As described above, the developing device 4B of this embodiment employs the touchdown development method, and the developing roller 31 as a toner carrier is moved from the supply roller 28A as a developer carrier to the position P1 in FIG. It carries the toner received by the printer. At this time, a transfer bias is applied to the supply roller 28A from a power source 30B serving as a transfer bias applying section, so that the toner is transferred from the supply roller 28A to the developing roller 31. The developing roller 31 to which the toner has been delivered is applied with a developing bias from a power source 33 serving as a developing bias applying section, so that an electrostatic charge formed on a photosensitive drum (not shown) by the toner carried on the developing roller 31 is applied. Develop the latent image.

The above-mentioned transfer bias and development bias are, for example, voltages obtained by superimposing an alternating current voltage on a direct current voltage. Further, the waveforms of the transfer bias and the developing bias are the blank pulse waveforms having the blank portion described in the developing bias of the first embodiment. Note that it is preferable that the waveform of the transfer bias and the waveform of the developing bias described below have different pulse phases. Further, the waveform of the developing bias may be a rectangular waveform having no blank portion.

Here, when toner is moved from the supply roller 28A to the developing roller, if toner is insufficiently supplied to the developing roller 31, this may affect the development of the toner onto the latent image on the photosensitive drum. Normally, in order to move the toner from the supply roller 28A to the development roller 31, a potential difference is provided in the DC voltage (DC component) of the bias applied to both. If the potential difference is increased, the amount of toner movement can be increased, but this may cause discharge or the like to occur between the supply roller 28A and the developing roller 31.

Therefore, in this embodiment, a bias having a waveform similar to the developing bias described in the fourth embodiment is applied as the transfer bias. That is, as shown in FIG. 4, the waveform of the transfer bias periodically repeats a pulse portion in which an AC voltage is superimposed on a DC voltage and a blank portion in which only the DC voltage is present. In addition, the waveform of the transfer bias is set such that a blank portion exists after a voltage of the same polarity as the normal charging polarity of the toner is applied to the DC voltage among the AC voltage in the pulse portion. The waveform of the transfer bias satisfies the aforementioned equations (12) and (14), and preferably satisfies equations (11) and (15).

In this embodiment, Vgo, Vre, V1, V2, Tgo, and Tre of the developing bias waveform shown in FIG. 4 are defined as follows.
Vgo: Among the AC voltages in the pulse section, voltages that have the same polarity as the normal charging polarity of the toner with respect to the DC voltage applied from the power supply 30B to the supply roller 28A.Vre: Among the AC voltages in the pulse section, voltages supplied from the power supply 30B. Voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the DC voltage applied to the roller 28A V1: Of the AC voltage in the pulse section, the normal charging polarity of the toner with respect to the DC voltage applied from the power supply 33 to the developing roller 31 Voltage on the side of the same polarity as the charging polarity V2: Voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the DC voltage applied from the power supply 33 to the developing roller 31 among the AC voltage in the pulse portion Tgo: 1 of the pulse portion Vgo application time in a cycle Tre: Vre application time in one cycle of the pulse section

Further, the distance between the developing roller 31 and the supply roller 28A at the closest position is d, the volume average particle diameter of the carrier is Dc, the average charge amount of the toner carried on the supply roller 28A is q, and the average mass of the toner is m. . Further, T _P is the time of the pulse portion, and T _B is the time of the blank portion.

By setting the waveform of the transfer bias applied to the supply roller 28A in this way, it is possible to move the toner from the supply roller 28A to the developing roller 31 even with a small potential difference. If the potential difference can be reduced, the occurrence of the above-mentioned discharge etc. can be suppressed.

Here, in the fourth embodiment, the gap at the closest position between the developing sleeve 28 and the photosensitive drum 1a is d, but in this embodiment, as described above, the gap at the closest position between the supply roller 28A and the developing roller 31 is d. Let the gap be d. Further, the charging potential Vd of the photosensitive drum 1a in the fourth embodiment corresponds to the potential (or its DC component in the case of an alternating electric field) Vt of the developing roller 31. That is, the above-mentioned V1 and V2 are set to values based on the DC voltage applied to the developing roller 31 from the power source 33. In short, the developing sleeve 28 of the fourth embodiment corresponds to the supply roller 28A of this embodiment, and the photosensitive drum 1a of the fourth embodiment corresponds to the developing roller 31 of this embodiment. As a result, each equation can be used in the same manner as described in the fourth embodiment, and the same effect as the developing bias of the fourth embodiment can be obtained.

In addition, in such a touchdown development method, when an AC voltage is applied to the supply roller 28A and the developing roller 31, the toner moves back and forth at the position P1 in FIG. 8, which is the toner transfer position between the supply roller 28A and the developing roller 31. It will be repeated. Therefore, toner is likely to scatter at the position P1. On the other hand, if a blank pulse waveform with a blank portion is used as the bias applied to the supply roller 28A and the developing roller 31 as in this embodiment, the reciprocating motion will be reduced more than in the case of a rectangular waveform without a blank portion. You can do less. Therefore, there is an advantage that toner scattering can be reduced.

As described above, in this embodiment, the waveform of the transfer bias is made to satisfy Expression (12) and Expression (14), preferably Expression (11) and Expression (15). Note that it is preferable that the waveform of the transfer bias also satisfies any one of equations (3), (5), (8), and (9) described in the first embodiment. Furthermore, the waveform of the transfer bias may be the double blank pulse waveform described in the first embodiment, or the single blank pulse waveform described in the second embodiment.

<Other embodiments>
Each of the embodiments described above can be implemented in appropriate combinations. For example, in the first, second, and third embodiments, the waveform of the developing bias satisfies any one of formulas (3), (5), (8), and (9), and You may make it satisfy|fill Formula (12) and Formula (14) demonstrated in the embodiment. Furthermore, equation (11) or equation (15) may be satisfied.

Furthermore, the image forming apparatus 100 is not limited to a full-color printer, but may be a monochrome or monochrome printer. Further, the image forming apparatus 100 may be a printer, various printing machines, a copying machine, a FAX, a multifunction device having multiple functions thereof, or the like.

Further, in each of the above-described embodiments, the photosensitive drums 1a to 1d, which are drum-shaped organic photoreceptors, are used as image carriers, but of course, inorganic photoreceptors such as amorphous silicon photoreceptors can also be used. It is also possible to use a belt-shaped photoreceptor. The charging method, transfer method, cleaning method, and fixing method are not limited to the above methods.

Further, in the first, second, and fourth embodiments described above, the case where the present invention is applied to the developing device 4 in which the developing chamber 23 and the stirring chamber 24 are arranged vertically has been described. is not limited. For example, the present invention is applicable to a conventionally used developing device in which a developing chamber and a stirring chamber are arranged horizontally, or to developing devices of other types.

1a, 1b, 1c, 1d...Photosensitive drum (image carrier)/2a, 2b, 2c, 2d...Charger (charging section)/3a, 3b, 3c, 3d...Laser beam scanner (latent) Image forming section)/4a, 4b, 4c, 4d, 4A, 4B...Developing device/28...Developing sleeve (developer carrier)/28A...Supply roller (developer carrier)/30, 30A...Power supply (developing bias application section)/30B...Power supply (delivery bias application section)/31...Developing roller (toner carrier)/32...Power supply (delivery bias application section)/33. ...Power supply (developing bias application section)/100...Image forming device

Claims (16)

an image carrier;
a charging unit that charges the surface of the image carrier to a predetermined charging potential;
a latent image forming section that forms an electrostatic latent image on the surface of the image carrier charged to the predetermined charging potential;
a developer carrier that carries a developer containing toner;
a development bias applying section that applies a development bias to the developer carrier in order to develop the electrostatic latent image formed on the image carrier by the toner carried on the developer carrier,
The developing bias is a bias obtained by superimposing a DC voltage and an AC voltage,
The waveform of the developing bias periodically repeats a pulse portion in which the DC voltage and the AC voltage are superimposed, and a blank portion in which only the DC voltage is present, and satisfies the following formula:
An image forming apparatus characterized by:

Vgo: Of the AC voltage in the pulse section, a voltage that has the same polarity as the normal charging polarity of the toner with respect to the DC voltage. Voltage on the opposite polarity side to the charging polarity V1: Voltage on the same polarity side as the normal charging polarity of the toner with respect to the predetermined charging potential among the AC voltages in the pulse section V2: Among the AC voltages in the pulse section , a voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the predetermined charging potential T _P : Time of the pulse portion T _B : Time of the blank portion The waveform of the developing bias is set so that the blank portion exists after the Vgo is applied.
The image forming apparatus according to claim 1, characterized in that: The developer carried on the developer carrier includes toner and carrier,
The waveform of the developing bias satisfies the following two equations:
The image forming apparatus according to claim 2, characterized in that:

Tgo: Application time of the Vgo in one cycle of the pulse section d: Distance between the image carrier and the developer carrier at the closest position Dc: Volume average particle diameter of the carrier q: Carrier carried on the developer carrier Average charge amount of toner m: Average mass of toner When the duty ratio of the waveform of the developing bias is Vgo/(Vgo+Vre),
The duty ratio is greater than 50% and less than or equal to 90%.
The image forming apparatus according to any one of claims 1 to 3 , characterized in that: The waveform of the developing bias satisfies the following formula:
The image forming apparatus according to any one of claims 1 to 4 , characterized in that:

an image carrier;
a charging unit that charges the surface of the image carrier to a predetermined charging potential;
a latent image forming section that forms an electrostatic latent image on the surface of the image carrier charged to the predetermined charging potential;
a developer carrier that carries a developer containing toner;
a development bias applying section that applies a development bias to the developer carrier in order to develop the electrostatic latent image formed on the image carrier by the toner carried on the developer carrier,
The developing bias is a bias obtained by superimposing a DC voltage and an AC voltage,
The waveform of the developing bias periodically repeats a pulse portion in which the DC voltage and the AC voltage are superimposed, and a blank portion in which only the DC voltage is present, and satisfies the following formula:
An image forming apparatus characterized by:

Vgo: Among the alternating current voltages in the pulse section, a voltage having the same polarity as the normal charging polarity of the toner with respect to the direct current voltage.
Vre: Of the AC voltage in the pulse section, the voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the DC voltage.
V1: Of the AC voltage in the pulse section, a voltage on the same polarity side as the normal charging polarity of the toner with respect to the predetermined charging potential.
V2: Among the AC voltages in the pulse section, a voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the predetermined charging potential.
T _P : Time of the pulse part
T _B : Time of the blank section The waveform of the developing bias is set so that the blank portion exists after the Vgo is applied.
The image forming apparatus according to claim 6, characterized in that: The developer carried on the developer carrier includes toner and carrier,
The waveform of the developing bias satisfies the following two equations:
The image forming apparatus according to claim 2, characterized in that:

Tgo: application time of the Vgo in one cycle of the pulse section
d: Distance between the image carrier and the developer carrier at the closest position
Dc: Volume average particle diameter of carrier
q: average charge amount of the toner carried on the developer carrier
m: average mass of toner When the duty ratio of the waveform of the developing bias is Vgo/(Vgo+Vre),
The duty ratio is greater than 50% and less than or equal to 90%.
The image forming apparatus according to any one of claims 6 to 8. an image carrier;
a charging unit that charges the surface of the image carrier to a predetermined charging potential;
a latent image forming section that forms an electrostatic latent image on the surface of the image carrier charged to the predetermined charging potential;
a developer carrier carrying a developer containing toner and carrier;
a development bias applying section that applies a development bias to the developer carrier in order to develop the electrostatic latent image formed on the image carrier by the toner carried on the developer carrier,
The developing bias is a bias obtained by superimposing a DC voltage and an AC voltage,
The waveform of the developing bias periodically repeats a pulse part in which the DC voltage and the AC voltage are superimposed, and a blank part in which only the DC voltage is present, and a part of the AC voltage in the pulse part is , the blank portion is set to exist after a voltage having the same polarity as the normal charging polarity of the toner is applied to the DC voltage, and satisfies the following two equations,
An image forming apparatus characterized by:

Vgo: Of the alternating current voltage in the pulse section, a voltage that has the same polarity as the normal charging polarity of the toner with respect to the direct current voltage.V1: Of the alternating current voltage in the pulse section, the voltage that voltage on the same polarity side as the normal charging polarity Tgo: application time of the Vgo in one cycle of the pulse section T _B : time of the blank section d: distance between the image carrier and the developer carrier at the closest position Dc: Volume average particle diameter of carrier q: Average charge amount of toner carried on the developer carrier m: Average mass of toner The waveform of the developing bias satisfies the following formula:
The image forming apparatus according to claim 10 .

The waveform of the developing bias satisfies the following formula:
The image forming apparatus according to claim 10 or 11 , characterized in that:

Among the alternating current voltages in the pulse section, a voltage with a polarity opposite to the normal charging polarity of the toner with respect to the direct current voltage is set as Vre,
When the duty ratio of the waveform of the developing bias is Vgo/(Vgo+Vre),
The duty ratio is greater than 50% and less than or equal to 90%.
The image forming apparatus according to any one of claims 10 to 12 . The waveform of the developing bias is a double blank pulse waveform in which the blank portion is provided after the pulse portion consisting of the alternating current voltage of a two-cycle rectangular wave.
The image forming apparatus according to any one of claims 10 to 13, characterized in that: an image carrier;
a charging unit that charges the surface of the image carrier to a predetermined charging potential;
a latent image forming section that forms an electrostatic latent image on the surface of the image carrier charged to the predetermined charging potential;
a developer carrier carrying a developer containing toner and carrier;
a toner carrier that receives toner from the developer carrier and carries the toner;
a transfer bias applying unit that applies a transfer bias to the developer carrier in order to transfer the toner from the developer carrier to the toner carrier;
a development bias applying section that applies a development bias including at least a DC voltage to the toner carrier in order to develop the electrostatic latent image formed on the image carrier by the toner carried on the toner carrier. ,
The delivery bias is a bias obtained by superimposing a DC voltage and an AC voltage,
The waveform of the transfer bias periodically repeats a pulse part in which the DC voltage and the AC voltage are superimposed, and a blank part in which only the DC voltage is present; , the blank portion is set to exist after a voltage having the same polarity as the normal charging polarity of the toner is applied to the DC voltage, and satisfies the following two equations,
An image forming apparatus characterized by:

Vgo: Of the AC voltage in the pulse section, the voltage on the same polarity side as the normal charging polarity of the toner with respect to the DC voltage applied from the delivery bias application section to the developer carrier V1: The voltage in the pulse section Among the AC voltages, a voltage on the same polarity side as the normal charging polarity of the toner with respect to the DC voltage applied from the development bias application unit to the toner carrier Tgo: application of the Vgo in one cycle of the pulse unit Time T _B : Time of the blank portion d: Distance between the toner carrier and the developer carrier at the closest position Dc: Volume average particle diameter of the carrier q: Average charge of the toner carried on the developer carrier Amount m: Average mass of toner The waveform of the transfer bias satisfies the following formula:
The image forming apparatus according to claim 15 .

Vre: Among the AC voltages in the pulse section, the voltage has a polarity opposite to the normal charging polarity of the toner with respect to the DC voltage applied from the delivery bias application section to the developer carrier.V2: The voltage in the pulse section Among the AC voltages, a voltage on the opposite polarity side to the normal charging polarity of the toner with respect to the DC voltage applied from the development bias application unit to the toner carrier T _P : time of the pulse part

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