JP3715856B2 - Image forming apparatus - Google Patents

Description

表１に示されるように、Ａ＝｜Ｖpp−２Ｖcont｜・Ｔｂ²／４、Ｂ＝ｄ²／｜Ｑ｜として、これらの関係がＡ＜Ｂとなる場合のみ、ベタにおいて高濃度を維持し、さらにハイライトの再現性が良好であった。
【００６１】
前述したように、図５および図６は、現像スリーブ上のトナー粒子１個にかかる力を示した図で、図中ｑはトナーの電荷量、ｍは質量、ａは加速度、ΔＶは感光ドラムと現像スリーブ間の電位差、ｄは感光ドラムと現像スリーブ間のギャップである。
【００６２】
感光ドラムから現像スリーブへトナーが移動できる距離Ｘは、以下のようにして求められる。トナーに対して、現像スリーブに引戻す剥ぎ取り電圧が振動電界印加中に最大時間としてＴｂ秒間印加される。この間にトナーが移動できる距離Ｘは、下記の式（１）：
Ｘ＝｜Ｑ｜・｜１／２・Ｖpp−Ｖcont｜・Ｔｂ²／２ｄ ・・・（１）
で求められる。
【００６３】
ここで、感光ドラム上に現像されたトナーが、剥ぎ取り電圧が最大時間だけ印加された場合の移動距離Ｘでは、現像スリーブに戻されない条件を設定してやることにより、トナーは感光ドラム上に偏った振動を繰り返す。このときの条件は、Ｘが感光ドラムと現像スリーブ間のギャップｄよりも小さくなるときである。
【００６４】
式で表すと、
｜Ｑ｜・｜１／２・Ｖpp−Ｖcont｜・Ｔｂ²／２ｄ＜ｄ
∴ ｜Ｖpp−２Ｖcont｜・Ｔｂ²／４＜ｄ²／｜Ｑ｜ ・・・（２）
このような条件下において現像を行うと、剥ぎ取り電圧が最大時間の電界によっても、Ｓ−Ｄ間を十分に往復運動することができない上に、前述したように交番電圧が立たれる直前の現像促進側の電圧の印加時間を長くすることにより、Ｖppが低い場合でもＤＣ成分が潜像電位に見合った量のトナーを感光ドラムに引き付けるように働くため、ドットの欠落が発生しなくなる。また感光ドラム上において、断続的に振動を繰り返すことにより、潜像部にトナーが集中し、１つ１つのドットが忠実に再現されるため、従来の矩形バイアスでは不均一な画像しか得られないような浅い潜像でも、均一な画像が出力できるようになる。
【００６５】
本実施例では、印加する交互電界、すなわちＢＰバイアスを、図１２に示すようにしたが、本発明はこれに限られず、たとえば図１３に示すように、２波長印加、５波長休止、あるいは図１４に示すように、３波長印加、１０波長休止としてもよい。またこれらの振動部の波形は矩形波でなくても、三角波やサイン波など様々な波形が印加でき、複写速度や現像条件に応じて最も適切な波形を選ぶことができる。またバイアス印加時間と休止時間の比は１：１／２〜１：１５が好ましく、この範囲で良好な結果が得られた。
【００６６】
以上、本発明の実施例を説明したが、本発明はこれらの実施例に限定されるものではなく、本発明の技術思想内でのあらゆる変形が可能である。
【００６７】
【発明の効果】
以上説明したように、本発明の画像形成装置では、バイアス印加手段により現像剤担持体に印加する現像バイアス電圧を、波形の１周期に振動部と休止部とを備えるブランクパルスバイアスとし、その振動部から休止部に変化する直前の振動バイアスがトナーを像担持体へと飛翔させる方向であり、その印加時間がＴ１で、さらにその直前の振動バイアスがトナーを現像剤担持体への引き戻す方向であり、その印加時間がＴ２で、これらの振動バイアスにＴ１＞Ｔ２の関係を有するようにさせたので、周波数が高くピークツウピーク電圧が低い条件でも、画像の均一性が高く、濃度が十分な画像が得られ、異常放電による画像欠陥もなく、高画質の画像を安定して得られるようになった。
【図面の簡単な説明】
【図１】本発明の画像形成装置の一実施例を示す概略構成図である。
【図２】図１の実施例で使用したＢＰバイアスを示す波形図である。
【図３】従来のＢＰバイアスを示す波形図である。
【図４】図２、図３のバイアスを用いて同一条件で画像を出力した場合のＶ−Ｄ曲線を示す図である。
【図５】現像スリーブ上のトナー粒子１個にかかる力を示した図である。
【図６】図５の拡大図である。
【図７】本発明の他の実施例で使用したＢＰバイアスを示す波形図である。
【図８】本発明の画像形成装置のさらに他の実施例を示す構成図である。
【図９】図８の画像形成装置の画像処理部を示すブロック図である。
【図１０】図８の画像処理部のタイミングチャートである。
【図１１】２成分現像剤におけるトナーの摩擦帯電量を測定するのための測定装置を示す斜視図である。
【図１２】図８の実施例で使用したＢＰバイアスを示す波形図である。
【図１３】本発明で使用することができるＢＰバイアスの他の例を示す波形図である。
【図１４】本発明で使用することができるＢＰバイアスのさらに他の例を示す波形図である。
【図１５】矩形バイアスを示す波形図である。
【図１６】従来のＢＰバイアスを示す波形図である。
【符号の説明】
１３１ 感光ドラム
１３２ 一次帯電器
１３３ 現像器
１３３ａ 現像スリーブ
１３４ 転写帯電器
１３５ 定着器
１３８ 現像電源[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic or electrostatic recording image forming apparatus such as a copying machine or a printer, and more particularly to an image forming apparatus characterized by its developing device.
[0002]
[Prior art]
In an image forming apparatus of an electrophotographic system or an electrostatic recording system, development is performed by making a developing sleeve (developer carrying body) carrying a developer of a developing device correspond to an image carrying body carrying an electrostatic latent image. In general, a developing bias is applied to the developing sleeve in order to form a developing electric field between the image carrier and the developing sleeve.
[0003]
As the developing bias, as shown in FIG. 15, a DC component superimposed with an AC component has been used. Conventionally, the AC component rectangular wave shown in FIG. 15 has a frequency of 2 kHz (1/2 period = 1/2 cycle). A peak-to-peak voltage (Vpp) of about 2 kV was used, and a good developed image was obtained.
[0004]
[Problems to be solved by the invention]
However, the development bias as shown in FIG. 15 has a drawback that the highlight portion of the image is “craggy” and has a sufficient image density when the toner particle size of the developer is reduced. There was a drawback that it could not be obtained.
[0005]
In order to improve this drawback, the present applicant has proposed to use a developing bias in which an AC component is intermittently superimposed on a DC component as shown in FIG. 16, for example. However, this developing bias has the following problems.
[0006]
  The developing bias as shown in FIG. 16 (referred to as a blank pulse bias or BP bias in this specification) is more developable than the developing bias as shown in FIG. 15 (referred to as a rectangular bias in this specification). However, as a condition for eliminating the roughness of the highlight part, the frequency of the pulse part is 4 kHz (1/2 period = 125μIt was indispensable to raise more than a second). Further, as a condition for producing the concentration, it was necessary to set the Vpp of the pulse part to 2 kV or more like the conventional rectangular wave.
[0007]
When Vpp of the pulse part exceeds 2 kV and the frequency is 4 kHz or more, if conductive foreign matter is mixed in the developing part, abnormal discharge occurs in that part, and ring-shaped spots are formed on the image, There was a problem that the image quality was remarkably deteriorated.
[0008]
Therefore, when the BP bias is used, it is required to suppress Vpp smaller than in the case of the conventional rectangular bias.
[0009]
An object of the present invention is to provide an image forming apparatus that performs good development with an appropriate development bias, prevents the highlight portion from being crushed, provides a sufficient image density, and does not cause image defects due to abnormal discharge. That is.
[0010]
[Means for Solving the Problems]
  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention includes an image carrier that carries an electrostatic latent image, and a developer that develops the electrostatic latent image on the image carrier, and the developer includes the image carrier and the image carrier. A developer carrying member carrying a two-component developer having a toner and a carrier facing each other, and bias applying means for applying a developing bias to the developer carrying member, and development applied by the bias applying means The period of the waveform of the bias voltage includes a vibration part and a rest part, and the vibration bias immediately before changing from the vibration part to the rest part is a direction in which the toner flies to the image carrier, and the application time is T1. Further, the vibration bias immediately before that is the direction in which the toner is pulled back to the developer carrying member, and when the application time is T2, the relationship of T1> T2 is established.Have
  Furthermore, the following formula,
    ｜ V pp -2V cont ｜ ・ Tb ² / 4 <d ² / | Q |
  However,
    V pp : Development bias peak-to-peak voltage [ V ] ,
    Tb: The toner is pulled back to the developer carrying member of the alternating voltage applied to the developer carrying member.
          Maximum time [ Second ] ,
  V cont : Image contrast potential [ V ] ,
      Q: Average triboelectric charge amount of toner [ C / kg ] ,
      d: Distance between the image carrier and the developer carrier [ m ]
MeetAn image forming apparatus characterized by the above.
[0011]
  According to the present invention, it is preferable that the vibration portion of the developing bias voltage is a substantially rectangular wave. In the vibration portion of the developing bias voltage, the integral value of the vibration portion when the rest portion is the vibration center is substantially zero. The time ratio between the vibration part and the rest part of the developing bias voltage is 1: 1/2 to 1:15.It is.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments according to the present invention will be described in more detail with reference to the drawings.
[0013]
Example 1
FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus of the present invention.
[0014]
In FIG. 1, a photosensitive drum 131 as an electrostatic latent image carrier rotates in the direction of arrow R in the drawing, and the surface thereof is uniformly charged by a primary charger 132. Thereafter, the photosensitive drum 131 is exposed to laser light L corresponding to the image, and an electrostatic latent image is formed on the surface. The electrostatic latent image is developed by a developing unit 133 using a two-component developer composed of toner and carrier under application of a developing bias, and visualized as a toner image. In this embodiment, the toner is negatively charged. The obtained toner image is transferred onto a paper conveyed by a paper conveying device (not shown) by a transfer charger 134, and then fixed by a fixing device 135 and output as an image.
[0015]
In the transfer step, untransferred toner remaining on the photosensitive drum 131 is cleaned by a cleaning device 136 having a cleaning blade 161 pressed against the photosensitive drum 131. Thereafter, the photosensitive drum 131 is neutralized by the pre-exposure device 137 and used for image formation again.
[0016]
The present invention is characterized in that a BP bias is applied as a developing bias to the developing sleeve (developer carrying member) 133a of the developing device 133 from the developing power source 138 as a bias applying means at the time of development, and the BP bias is devised. is there. FIG. 2 shows the developing bias used in this example. This developing bias voltage is a BP bias (blank pulse bias) composed of a pause portion (blank portion) and a vibration portion, and the vibration portion is formed of four pulses. In FIG. 2, Tb ≧ T1, where Tb is the maximum voltage application time in the direction in which the toner is pulled back to the developing sleeve.
[0017]
This vibration unit applies a voltage + 900V for a time Tb = T1 with respect to the potential of the resting portion as an electric field for pulling the toner back to the developing sleeve of the developing device 133 immediately after the resting portion, and then applies a voltage −900V for a time T2. Further, a voltage + 900V is applied for a time T2, and finally, as an electric field in the direction in which the toner is allowed to fly to the photosensitive drum 131, -900V is applied for a time T1 with respect to the potential of the resting portion, and then the resting portion is formed. .
[0018]
In the present embodiment, the time of the pause portion is 6 × (T1 + T2) time. The waveform was selected so that T1 = 1.5 × T2. Since one cycle of T1 + T2 is selected to be 8 kHz, in this embodiment, the vibration part is 250 μs (microseconds), the pause part is 750 μs, and T1 = 75 μs and T2 = 50 μs, and the bias is paused with the vibration part. One period including the parts was 1000 μs, and a BP bias with a frequency of 1 kHz was obtained.
[0019]
A conventionally proposed BP bias waveform is shown in FIG. The bias of FIG. 3 set the peak-to-peak voltage Vpp to 2 kV (the bias of FIG. 2 set Vpp to 1.8 kV). FIG. 4 shows a VD (development contrast vs. image density) curve when an image is output under the same conditions using the biases of FIGS.
[0020]
As shown in FIG. 4, by using the BP bias of FIG. 2 which is the developing bias in the present invention, the concentration that could not be obtained until the Vpp of the BP bias was raised to 2 kV in the past, the Vpp was increased to 1.8 kV. Even if it is lowered, it can be obtained sufficiently.
[0021]
In this embodiment, the BP bias Vpp can be suppressed to 1.8 kV as described above. As a result, even if conductive foreign matter is mixed into the developing portion, an image caused by abnormal discharge which has been a problem in the past is used. A high-density image can be stably obtained without causing defects.
[0022]
The following can be considered as a reason why the concentration is sufficiently obtained. FIG. 5 is a view showing the force applied to one toner particle on the developing sleeve, and FIG. 6 is an enlarged view thereof. In the figure, q is the charge amount of toner, m is mass, a is acceleration, ΔV is a potential difference between the photosensitive drum and the developing sleeve, and d is a gap between the photosensitive drum and the developing sleeve.
[0023]
Here, consider the force that the toner receives during development. The force in the direction in which the toner flies from the developing sleeve to the photosensitive drum is maximized when an electric field in the direction in which the toner flies to the photosensitive drum is formed while the bias of the vibrating portion is applied. . The amount of toner adhering to the image area is considered to depend on the speed of the toner accelerated by the electric field when changing from the vibration part to the blank part (resting part). When the voltage of the vibration part is applied, and the voltage that causes the toner to fly from the developing sleeve toward the photosensitive drum immediately before moving to the resting part, the speed at which the toner that is stationary on the developing sleeve is accelerated, The present invention is compared with the conventional case.
[0024]
The accelerated toner speed is
V = | q | / m × ΔV / d × TL
Can be expressed as Assuming that the contrast required for development is 350 V, ΔV is 1350 V with the conventional bias. On the other hand, in the bias of the present invention, ΔV is 1250V. Other values are the same for the conventional bias and the bias of the present invention. Therefore, when the speed of the toner at the moment of becoming a blank portion by applying a conventional bias is V2, and the speed of the toner by applying a bias according to the present invention is V1,
V1 = 1.23 × V2
Therefore, even if Vpp is low, the flying speed of the toner is high, and as a result, it seems that a sufficient image density can be obtained even if Vpp is low.
[0025]
Example 2
In the first embodiment, the BP bias shown in FIG. 2 is used as the developing bias. However, the bias effective in the present invention is not limited to the bias shown in FIG.
[0026]
FIG. 7 shows the BP bias used for the developing bias in this embodiment. In the BP bias of the present embodiment, the vibration unit has a plurality of vibrations. In the figure, T1 = 50 μs, T2 = 25 μs, and the waveform was selected so that T1 = 2 × T2. Similar to the BP bias in FIG. 2, the BP bias has the same effect as that of the first embodiment even if the vibration is performed a plurality of times by setting the integral value to zero.
[0027]
According to the BP bias of FIG. 7, a sufficient density could be obtained even when Vpp was 1.6 kV, and no image defect due to abnormal discharge occurred. Further, by using such a BP bias, it was possible to obtain an image with little roughness and image irregularity as in Example 1.
[0028]
Example 3
Another embodiment of the image forming apparatus of the present invention will be described with reference to FIG.
[0029]
The image forming apparatus of this embodiment is a color copying machine having a digital color image reader unit at the top and a digital color image printer unit at the bottom.
[0030]
In the reader unit, the reflected light image from the document 30 obtained by placing the document 30 on the document glass table 31 and performing exposure scanning by the exposure lamp 32 is condensed on the full color sensor 34 by the lens 33, and the color color separation image is obtained. Get a signal. This color-separated image signal is amplified by an amplifying circuit (not shown), processed by a video processing unit (not shown), and sent to a printer unit.
[0031]
  In the printer unit, the photosensitive drum 1 serving as an image carrier is a photosensitive member having a surface protective layer, and is installed so as to be rotatable in the direction of an arrow in the figure. Around the photosensitive drum 1, there are a pre-exposure lamp 11 for initializing the surface of the photosensitive drum 1, a charger 2 for uniformly charging the surface of the photosensitive drum 1 (corona charger in this example), and a photosensitive drum 1. Laser exposure optical system 3 for forming an electrostatic latent image corresponding to image information on the surface, and four developers (toners) of different colors for developing the electrostatic latent image formed on the photosensitive drum 1 A developing device having a fixed arrangement composed of a developing device 4 (4Y, 4C, 4M, 4K), a light detecting means 13 for detecting the amount of toner on the photosensitive drum 1, and remaining on the photosensitive drum 1didA cleaner 6 and the like for removing the developer are respectively disposed.
[0032]
In this example, the laser exposure optical system 3 includes a polygon mirror 3a, a lens 3b, a mirror 3c, and the like. The laser exposure optical system 3 is modulated by the color-separated image signal from the reader unit, and is converted into an optical signal for image scan exposure at the laser output unit. The converted laser beam E is reflected by the polygon mirror 3a and projected onto the surface of the photosensitive drum 1 through the lens 3b and the mirror 3c to form an electrostatic latent image corresponding to the color image signal of each color.
[0033]
At the time of image formation at the printer unit, the photosensitive drum 1 is rotated in the direction of the arrow in the figure. First, the surface of the photosensitive drum 1 is neutralized and initialized by the pre-exposure lamp 11, and then the surface of the photosensitive drum 1 is minus by the charger 2. The surface of the photosensitive drum 1 is sequentially irradiated with laser light E corresponding to the image signals of the respective colors that are uniformly charged and separated by the exposure optical system 3 to form an electrostatic latent image in a predetermined color order.
[0034]
Next, a predetermined developing device is operated in the order of cyan (C), magenta (M), yellow (Y), and black (K) in a predetermined developing order to develop the latent image on the photosensitive drum 1 and to perform photosensitive. Toner images are sequentially formed on the drum 1 with a negative toner based on a resin. Here, the developing devices 4C, 4M, 4Y, and 4K of the developing device are selectively selected according to the color of the formed latent image by the operation of the eccentric cams 24C, 24M, 24Y, and 24K. The developing operation is performed close to the photosensitive drum 1.
[0035]
On the other hand, a recording material such as transfer paper fed from a recording material cassette 7a, 7b or 7c (which may be manually fed) by a conveying system including a pickup roller, a paper feed guide, a paper feed roller, etc. Is wound around the transfer device 5.
[0036]
In this example, the transfer device 5 includes a transfer drum 5a having a diameter of 180 mm as a recording material carrier, a transfer corona charger 5b for transferring the toner image on the photosensitive drum 1 to the recording material, and the recording material to the transfer drum 5a. A transfer drum 5a having an adsorption charger 5c serving as an adsorption charging means, an adsorption roller 5g as an opposite pole, an inner corona charger 5d, and an outer corona charger 5e, which are axially supported so as to be rotationally driven. A recording material carrying sheet 5f made of a dielectric material, which is a recording material carrying means, is integrally stretched in a cylindrical shape in the peripheral opening area. The recording material carrying sheet 5f is a dielectric sheet such as a polycarbonate film.
[0037]
The transfer drum 5a is rotationally driven in the direction of the arrow in the figure in synchronization with the photosensitive drum 1, and a cyan toner image obtained by developing with the cyan developing device 4C is transferred onto the recording material carried on the recording material carrying sheet. Transfer is performed by the transfer charger 5b at the transfer portion. The transfer drum 5a continues to rotate to prepare for the transfer of the next color (for example, magenta) image.
[0038]
Also, the photosensitive drum 1 to which the toner image has been transferred is cleaned of deposits such as residual toner by the cleaner 6, is uniformly charged again by the charger 2, and is modulated by laser light modulated by the next magenta image signal. The image exposure as described above is performed. This latent image is developed by a magenta developing device and visualized as a magenta toner image. This magenta toner image is superimposed on the recording material on the recording material carrying sheet 5f from above the cyan toner image by the transfer charger 5b in the transfer portion. Is transcribed. The transfer drum 5a continues to rotate to prepare for the transfer of the next color (for example, yellow) image.
[0039]
Subsequently, the image formation and transfer processes described above are performed for yellow and black, and when the four color toner images are superimposed and transferred onto the recording material, the recording material is discharged by the separating charger 5h and then pressed. The toner image is separated from the transfer drum 5a by the action of the raising roller 8b and the separation claw 8a, sent to the fixing device 9 (heat roller fixing device in this example) by the conveying means, and the toner image is fixed and discharged onto the external tray 10 Is done. Thus, a series of full-color print sequences is completed, and a required full-color print image is obtained.
[0040]
In the case of forming images on both sides of the recording material, the recording material drives the conveyance path switching guide 19 immediately after exiting the fixing device 9, and the recording material is once guided to the reverse path 21a through the conveyance vertical path 20. Thereafter, the reversing roller 21 is rotated in the reverse direction so that the recording material is withdrawn in the direction opposite to the feeding direction starting from the trailing edge when the feeding roller 21 is fed and stored in the intermediate tray 22. Thereafter, the recording material is conveyed again from the intermediate tray 22 to the transfer device 5, and an image is formed on the other surface of the recording material by the image forming process described above.
[0041]
The transfer drum 5a after separating the recording material is opposed to the recording material carrying sheet 5f through the recording material carrying sheet 5f in order to prevent powder from adhering to the recording material carrying sheet 5f and oil from adhering to the recording material. Cleaning is performed by the fur brush 14 and the backup brush 15, and the oil removing roller 16 and the backup brush 17. Such cleaning is performed before or after image formation, and at any time when a jam (paper jam) occurs.
[0042]
In this example, the gap between the recording material carrying sheet 5f and the photosensitive drum 1 is arbitrarily set by operating the eccentric cam 25 at a desired timing and operating the cam follower 5i integrated with the transfer drum 5a. It can be configured. For example, the distance between the transfer drum 5a and the photosensitive drum 1 is increased during standby (standby) or when the power is turned off.
[0043]
Next, the processing process of the image processing unit will be described with reference to the block diagram of FIG.
[0044]
In the image processing unit 40 of FIG. 9, the digital image data 42 is converted into an analog image signal 403 by a D / A converter 402 and input to one terminal of a comparator 411. The timing signal generation circuit 407 generates and outputs the pixel clock 404 and the screen clock 408 to the pattern signal generator 409 based on the input reference clock signal 43. The pattern signal generator 409 outputs a pattern signal 410 based on the screen clock 408 and inputs it to the other terminal of the comparator 411.
[0045]
The digital pixel signal 42 is input in synchronization with the pixel clock 404, and the D / A converter 402 outputs an analog image signal 403 in synchronization with the pixel clock 404. The screen clock 408 is a clock signal obtained by multiplying the pixel clock 404 by an integer, and defines the cycle of the pattern signal 410 that is, for example, a triangular wave.
[0046]
The analog image signal 403 and the pattern signal 410 are compared by the comparator 411, and the binary image data 41 subjected to pulse width modulation is generated and output as 0 when the analog image signal 403 is larger and 1 when smaller. The
[0047]
FIG. 10 shows a timing chart of each part in FIG. Thus, the image processing process will be further described.
[0048]
Here, the screen clock 408 is a clock having a cycle twice that of the pixel clock 404. When the digital image signal changes stepwise from hexadecimal 00 (white) to FF (black), the pulse waveform of the binarized image data 41 modulated by the pattern signal 410 is shown. Thus, by changing the amplitude of the pattern signal, the relationship between the input level of the digital image data 42 and the pulse width of the binarized image data 41 can be changed.
[0049]
The binarized image data 41 is input to the image forming unit of the full-color copying machine, controls the exposure width by laser light, projects a laser spot having an exposure width according to the image data on the photosensitive drum, and generates a latent image. It is formed. These latent images are developed by the above-described developing devices 4Y, 4C, 4M, and 4K.
[0050]
Next, the developing process which is a characteristic part of the present invention will be described in detail. As the developer, a two-component developer composed of a non-magnetic toner and a magnetic carrier (magnetic particles) is used. The mixing ratio was such that the nonmagnetic toner was about 5% by weight. The non-magnetic toner has a volume average particle size of about 8 μm. The magnetic carrier consists of resin-coated ferrite particles (maximum magnetization 60 emu / g), the weight average particle diameter is 50 μm, and the resistance value is 10⁸A value of Ωcm or more is shown. The magnetic carrier has a magnetic permeability of about 5.0.
[0051]
In the developing container of the developing device 4 (4Y to 4K), an opening is provided in a portion close to the photosensitive drum 1, and a developing sleeve as a developer carrying member protrudes outside from the opening. The developing sleeve is rotatably incorporated in the developing container, and is arranged so that the distance from the photosensitive drum 1 is 500 μm. The developing sleeve has an outer diameter of 25 mm and a peripheral speed of 280 mm / sec.
[0052]
  In this example,
    Vpp: Peak-to-peak of alternating voltage of developing bias applied to developer carrier
          Voltage [V],
    Tb: To be applied to the developer carrier having an alternating voltage of the developing bias applied to the developer carrier.
          Maximum time to pull back
  Vcont: image contrast potential [V] (maximum image from DC voltage of development bias
          Potential difference from latent image potential when outputting density),
      Q: the average triboelectric charge amount [C / kg] of the toner
      d: Distance between image carrier and developer carrier [m]
As a development bias,
    ｜ Vpp-2Vcont ｜・ Tb ² / 4<D²/ | Q |
An alternating voltage is applied so that an alternating electric field satisfying the above condition is intermittently formed.
[0053]
The toner used in this example has a triboelectric charge amount of about −2.0 × 10.^-2For C / kg, about -3.0x10^-2Two types of C / kg were used.
[0054]
Next, a method for measuring the triboelectric charge amount (two-component developer) of the toner will be described with reference to FIG. FIG. 11 shows an apparatus for measuring the triboelectric charge amount (tribo) of toner.
[0055]
First, the toner to be measured for triboelectric charge is combined with a carrier to form a two-component developer, placed in a polyethylene bottle of 50-100 ml capacity, and shaken by hand for about 10-40 seconds. About 0.5 to 1.5 g of developer is placed in a metal measuring container 142 that is a conductive screen 143 having a bottom of 500 mesh, and the container 142 is covered with a metal lid 144. The entire measurement container 142 in this state is weighed, and this is defined as W1 (kg).
[0056]
Next, the measurement container 142 is installed in the suction machine 141 (at least the part of the suction machine 141 that is in contact with the measurement container 142 is an insulator), sucked from the suction port 147, the air volume control valve 146 is adjusted, and the vacuum gauge 145 The pressure is 250 mmAq. In this state, suction is sufficiently performed, preferably for 2 minutes, and the toner is removed 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 V1 (V). Further, the weight of the entire measurement container 142 after the suction is measured, and this is defined as W2 (kg). When the capacitance of the capacitor 148 connected in parallel to the electrometer 149 in the measurement container 142 is C1 (μF), the triboelectric charge amount of the toner can be expressed by the following formula:
Toner triboelectric charge amount (μC / kg) = C1 × V1 × 10^-3/ (W1-W2)
It is calculated as follows.
[0057]
In this example, a highlight halftone image and a solid image having an image density of about 0.2 were output, and the smoothness of the highlight halftone image and the density of the solid image were evaluated.
[0058]
Here, formation of an electrostatic latent image for image output is as follows. First, when the photosensitive drum is uniformly charged to -650V and a highlight halftone image is output, PWM (pulse width modulation) is performed by a semiconductor laser, the surface potential is lowered to about -450V, and a solid image is output. In this case, the voltage was dropped to about −150V (Vcont = 350V).
[0059]
Next, the latent image was developed by the reversal development method using the developer having the above-described configuration and the toner having the charge amount. In this embodiment, the DC voltage of the developing bias is set to -500 V, the amplitude Vpp of the alternating voltage applied intermittently is fixed to 1800 V, and the maximum time Tb of the pullback time is changed. At this time, as shown in FIG. 12, the time for passing through the alternating electric field was set to two periods for every period of the alternating electric field. The developer is a two-component developer, and the toner includes the above-mentioned two types of toners (the triboelectric charge amount is about −2.0 × 10 × 10).^-2C / kg and about -3.0 × 10^-2C / kg). The results are shown in Table 1.
[0060]
[Table 1]

As shown in Table 1, A = | Vpp−2Vcont | · Tb²/ 4, B = d²Only when these relations satisfy A <B as / | Q |, solid density was maintained at a high level and the reproducibility of highlights was good.
[0061]
As described above, FIGS. 5 and 6 are views showing the force applied to one toner particle on the developing sleeve, where q is the charge amount of the toner, m is the mass, a is the acceleration, and ΔV is the photosensitive drum. And d is a gap between the photosensitive drum and the developing sleeve.
[0062]
The distance X that the toner can move from the photosensitive drum to the developing sleeve is obtained as follows. A stripping voltage to be pulled back to the developing sleeve is applied to the toner for Tb seconds as the maximum time during the application of the oscillating electric field. The distance X during which the toner can move is expressed by the following formula (1):
X = | Q |. | 1 / 2.Vpp-Vcont | .Tb²/ 2d (1)
Is required.
[0063]
Here, the toner developed on the photosensitive drum is biased on the photosensitive drum by setting a condition that the toner is not returned to the developing sleeve at the moving distance X when the stripping voltage is applied for the maximum time. Repeat the vibration. The condition at this time is when X is smaller than the gap d between the photosensitive drum and the developing sleeve.
[0064]
  Expressed as a formula:
    | Q | ・ | 1/2 ・ Vpp−Vcont | ・ Tb²/ 2d<D
    ｜｜ Vpp-2Vcont ｜ ・ Tb²/ 4 <d²/ | Q | (2)
  If the development is performed under such conditions, even if the stripping voltage is a maximum time electric field, the SD cannot sufficiently reciprocate between SD and the development just before the alternating voltage is established as described above. By increasing the voltage application time on the accelerating side, even when Vpp is low, the DC component works to attract the amount of toner corresponding to the latent image potential to the photosensitive drum, so that no dot loss occurs. In addition, by repeating vibration on the photosensitive drum intermittently, toner concentrates on the latent image portion and each dot is faithfully reproduced. Therefore, only a non-uniform image can be obtained with the conventional rectangular bias. Even with such a shallow latent image, a uniform image can be output.
[0065]
In this embodiment, the alternating electric field to be applied, that is, the BP bias is set as shown in FIG. 12, but the present invention is not limited to this. For example, as shown in FIG. As shown in FIG. 14, it is good also as a 3 wavelength application and a 10 wavelength rest. Further, even if the waveform of these vibration parts is not a rectangular wave, various waveforms such as a triangular wave and a sine wave can be applied, and the most appropriate waveform can be selected according to the copying speed and development conditions. The ratio between the bias application time and the rest time is preferably 1: 1/2 to 1:15, and good results were obtained in this range.
[0066]
As mentioned above, although the Example of this invention was described, this invention is not limited to these Examples, All the deformation | transformation within the technical thought of this invention are possible.
[0067]
【The invention's effect】
As described above, in the image forming apparatus of the present invention, the developing bias voltage applied to the developer carrying member by the bias applying unit is the blank pulse bias having the vibration part and the resting part in one cycle of the waveform, and the vibration. The vibration bias immediately before the change from the first portion to the rest portion is the direction in which the toner is caused to fly to the image carrier, the application time is T1, and the vibration bias immediately before that is the direction in which the toner is pulled back to the developer carrier. The application time is T2, and these vibration biases have a relationship of T1> T2. Therefore, even in a condition where the frequency is high and the peak-to-peak voltage is low, the uniformity of the image is high and the density is sufficient. An image can be obtained, and a high-quality image can be stably obtained without image defects due to abnormal discharge.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus of the present invention.
FIG. 2 is a waveform diagram showing a BP bias used in the embodiment of FIG.
FIG. 3 is a waveform diagram showing a conventional BP bias.
4 is a diagram showing a VD curve when an image is output under the same conditions using the biases of FIGS. 2 and 3. FIG.
FIG. 5 is a diagram illustrating a force applied to one toner particle on a developing sleeve.
6 is an enlarged view of FIG.
FIG. 7 is a waveform diagram showing a BP bias used in another embodiment of the present invention.
FIG. 8 is a configuration diagram showing still another embodiment of the image forming apparatus of the present invention.
9 is a block diagram illustrating an image processing unit of the image forming apparatus in FIG. 8;
10 is a timing chart of the image processing unit in FIG. 8. FIG.
FIG. 11 is a perspective view showing a measuring device for measuring the triboelectric charge amount of toner in a two-component developer.
12 is a waveform diagram showing a BP bias used in the embodiment of FIG.
FIG. 13 is a waveform diagram showing another example of a BP bias that can be used in the present invention.
FIG. 14 is a waveform diagram showing still another example of the BP bias that can be used in the present invention.
FIG. 15 is a waveform diagram showing a rectangular bias.
FIG. 16 is a waveform diagram showing a conventional BP bias.
[Explanation of symbols]
131 Photosensitive drum
132 Primary charger
133 Developer
133a Development sleeve
134 Transfer charger
135 Fuser
138 Development power supply

Claims (5)

An image carrier that carries an electrostatic latent image; and a developing unit that develops the electrostatic latent image on the image carrier. The developer unit includes a toner and a carrier facing the image carrier. A developer carrying body carrying the two-component developer, and bias applying means for applying a developing bias to the developer carrying body, and the period of the waveform of the developing bias voltage applied by the bias applying means is: The vibration bias immediately before the change from the vibration part to the pause part is a direction in which the toner flies to the image carrier, the application time is T1, and the vibration bias immediately before the vibration bias is provided. Is the direction in which the toner is pulled back to the developer carrying member, and when the application time is T2, the relationship is T1> T2 ,
Furthermore, the following formula,
^{| V pp -2V cont | · Tb} 2/4 <d 2 / | Q |
However,
V pp : Development bias peak-to-peak voltage [ V ] ,
Tb: The toner is pulled back to the developer carrying member of the alternating voltage applied to the developer carrying member.
The maximum of the time [s],
V cont : Image contrast potential [ V ] ,
Q: average amount of triboelectric charge [ C / kg ] of toner ,
d: Distance [ m ] between image carrier and developer carrier
An image forming apparatus characterized by satisfying the above.   The image forming apparatus according to claim 1, wherein the vibration portion of the developing bias voltage is substantially a rectangular wave.   The image forming apparatus according to claim 1, wherein the vibration portion of the developing bias voltage has substantially two cycles or more.   2. The image forming apparatus according to claim 1, wherein, in the vibration portion of the developing bias voltage, an integral value of the vibration portion when the rest portion is a vibration center is substantially zero.   2. The image forming apparatus according to claim 1, wherein a ratio of a time of the developing bias voltage vibration portion to a rest portion is 1: 1/2 to 1:15.

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