JP2004029166A - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus of tandem system which can eliminate image density difference in an image of toner color at the most upstream part occurred at the time between color image formation and monocolor image formation and further, which can always assure proper image density in any image formation mode. <P>SOLUTION: At the time of monocolor image formation performed by using only an image forming station located at the most upstream in a transfer material carrying direction, a transfer material carrying belt for carrying the transfer material can be switched into a refuge state to other image forming stations which do not participate in image formation. Further a control means 33 for changing duty ratio of AC components of developing bias voltage at the time of color image formation using all image forming stations and at the time of monocolor image formation, is provided. <P>COPYRIGHT: (C)2004,JPO

Description

【００８９】
以上のように、カラー画像形成をおこなう場合には、現像ローラ４へ、上記現像抑制側の電位の印加時間Ａが、上記現像促進側の電位の印加時間Ｂと略同じである現像バイアス電圧が印加され、単色画像形成をおこなう場合には、上記現像抑制側の電位の印加時間Ａが、上記現像促進側の電位の印加時間Ｂよりも長い現像バイアス電圧が印加される。
【００９０】
これにより、上記各画像形成モードの間で現像が促進される時間の長短（あるいは現像が抑制される時間の長短）が発生し、感光体へのトナー付着量を制御することができ、最上流の画像形成段のトナー色に関して画像濃度差が発生するのを最小限に抑制することができる。
【００９１】
【発明の効果】
以上の説明から明らかなように、本発明は、以下の効果を奏する。
【００９２】
（１）全ての画像形成ステーションをしようするカラー画像形成時と、単色画像形成時とで、現像バイアス電圧の交流成分のデューティ比を変更するための制御手段を設けているので、カラー画像形成時および単色画像形成時それぞれの場合に応じて、最上流の画像形成ステーションにて形成される画像の濃度を、適宜調整することができ、濃度差の発生を防止できる。
【００９３】
また、単色画像形成時には、転写材搬送ベルトを、黒色以外の画像形成ステーションから離間させるので、転写材搬送ベルトと他色の感光体ドラムとの不要な接触に伴う両者の損耗を回避することができる。
【００９４】
（２）制御手段は、最上流の画像形成ステーションの現像装置に対して印加する現像バイアス電圧の現像促進方向のデューティ比を、単色画像形成時よりもカラー画像形成時の方を大きく設定するので、カラー画像形成時と、転写材搬送ベルトの最上流に位置する画像形成ステーションのみが関与する単色画像形成時の画像濃度差を解消することができる。
【００９５】
（３）制御手段は、現像バイアス電圧のデューティ比の変更を、現像バイアス電圧の交流成分の周波数ｆおよび振幅Ｖｐ−ｐを固定し、パルス幅を変化させることによっておこなうので、正負のパルス幅（時間配分）の比のみを独立的に変えることによって、容易に現像バイアス電圧のデューティ比を変更し、画像濃度の調整をおこなうことができる。
【００９６】
（４）最上流に位置する画像形成ステーションを、黒色トナー用としたので、カラー画像形成時に転写材搬送ベルトの最上流の黒色トナーの画像形成ステーションに関して得られる画像濃度と、黒色トナーの画像形成ステーションのみが関与する黒色単色画像形成時に得られる画像濃度との間に発生する濃度差が解消され、カラー画像形成時および黒色単色画像形成時の両方において、高い画質の画像を得ることができる。
【００９７】
また、カラー画像形成時に、黒色トナー用の画像形成ステーションを、最上流に配置しているため、カラー画像形成時に黒色トナーが最下層となり、他色のトナー像への影響を最も小さくすることができ、画像品位が向上する。
【図面の簡単な説明】
【図１】従来の単一ドラム型のカラー画像形成装置の構成の一例を示す説明図である。
【図２】同タンデム方式のカラー画像形成装置の構成の一例を示す説明図である。
【図３】本発明のカラー画像形成装置の構成説明図である。
【図４】同カラー画像形成装置の現像装置の構成説明図である。
【図５】同現像バイアス電圧の電圧波形を示すグラフである。
【図６】同デューティ比と画像濃度との関係を示すグラフである。
【図７】同制御手段の制御系統を示すブロック図である。
【図８】同現像時の現像バイアス電圧を示すグラフである。
【符号の説明】
３３−制御手段
２００ＢＫ，２１０Ｃ，２２０Ｍ，２３０Ｙ−画像形成ステーション
２０４ＢＫ−現像装置
２５０−転写材搬送ベルト
Ｄ２−デューティ比[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a color image forming apparatus that forms a toner image corresponding to each color and black for forming a color image or a monochromatic image, and transfers the formed toner image of each color to a recording medium, and particularly relates to a color image forming apparatus. And a color image forming apparatus in which the toner image forming means is arranged in a tandem system.
[0002]
[Prior art]
Various color image forming apparatuses that include a plurality of developing devices, form visible images (toner images) of different colors by each developing device, and finally transfer these toner images in a superimposed manner on the same transfer sheet are proposed. Have been. Generally, color image forming apparatuses are roughly classified into a single drum type and a multi-stage drum type (tandem type).
[0003]
FIG. 1 schematically shows a main configuration of a single-drum type color image forming apparatus that performs a multi-rotation process. This color image forming apparatus is initialized along a peripheral surface of a single photosensitive drum 101. A charger 102, a recording head 103 for irradiating a laser beam, and four developing devices 104 a, 104 b, 104 c, and 104 d are arranged along the photosensitive drum 101. A sheet winding semiconductive drum 105 to be formed is arranged.
[0004]
A corona discharger 106 that discharges a transfer current is disposed in the cylinder of the paper winding semiconductive drum 105 in close proximity to the transfer portion. The paper winding semiconductive drum 105 wraps the transfer paper carried in from the lower left as shown by the arrow M in the drawing and makes a round in the counterclockwise direction as shown by the arrow N in the drawing. The photosensitive drum 101 rotates at a peripheral speed equal to the peripheral surface speed of the paper winding semiconductive drum 105 in a clockwise direction as indicated by an arrow C in FIG. The toner image formed by the developing device 4a is carried, and the toner image is transferred to a transfer sheet by the discharge current of the corona discharger 106.
[0005]
Next, the paper winding semiconductive drum 105 makes one rotation again, and accordingly, the photosensitive drum 101 carries the toner image formed by the developing device 104b of C (cyan: greenish blue). Is transferred onto a transfer sheet by a corona discharger 106 and overlapped. Further, the sheet winding semiconductive drum 105 makes one rotation again, and accordingly, the photosensitive drum 101 carries the toner image formed by the Y (yellow: yellow) developing device 104c, and the toner image is transferred to the corona discharger 106. Is transferred onto the transfer paper and overlapped.
[0006]
Finally, the paper winding semiconductive drum 105 makes one rotation again, and accordingly, the photosensitive drum 101 carries the toner image formed by the developing device 104d of Bk (black: black), and the toner image is transferred to the corona discharger 106. Is transferred onto the transfer paper and overlapped. When the transfer (overlapping) of the four color toner images is completed, the winding of the transfer sheet is released, and the transfer sheet is transported by the transport belt 107 to the fixing unit 108 disposed on the left side. The transferred four-color transfer toner images are heat-fixed to the paper surface.
[0007]
As described above, the single-drum type color image forming apparatus applies M (magenta) toner, C (cyan) toner, and Y (yellow), which are three primary colors of subtractive color mixing, to one sheet (one page) of transfer paper. Printing (exposure recording, development, and transfer) is performed individually for each toner in order to superimpose and transfer a total of four types of toner, a toner and a Bk (black) toner dedicated to printing of a black portion. Therefore, the printing process is repeated four times for one page of the transfer paper, and the printing process requires a long time.
[0008]
On the other hand, the tandem-type color image forming apparatus is provided with an image forming station for each of the four different color toners, and transfers four kinds of toners onto a sheet sequentially in one process. Therefore, the tandem-type color image forming apparatus has a processing speed almost four times that of the single-drum type image forming apparatus. For this reason, in recent years, tandem type color image forming apparatuses have become mainstream, in combination with the fact that internal apparatuses have been miniaturized and assemble apparatuses (units) have become relatively inexpensive.
[0009]
However, in the tandem type color image forming apparatus, the first color toner image formed in the developing process of the uppermost image forming station is transferred onto the transfer material, and then formed in the developing process of the next image forming station. When the transferred second color toner image is superimposed and transferred onto the transfer material, a reverse transfer phenomenon occurs in which a part of the previously transferred first color toner image is reversely transferred to the photoconductor side.
[0010]
Due to this reverse transfer phenomenon, unfixed toner once transferred onto the transfer material at the image forming station on the upstream side is robbed at the station on the downstream side, and a good color image cannot be obtained. The reverse transfer phenomenon also occurs when the third and fourth color toner images are transferred onto a transfer material.
[0011]
For this reason, after the transfer of the fourth color toner image is completed at the most downstream image forming station, the toner adhesion amount of the first color toner image to the transfer material is several tens of percent of the initial adhesion amount. Will decrease. The same phenomenon occurs in the second color toner image and the third color toner image.
[0012]
Therefore, for example, when the toner image formation and transfer processes are performed four times, the final amount of each toner image adhered to the transfer material is the smallest for the first color toner image. The color arrangement is out of balance such that the first color adhesion amount <the second color adhesion amount <the third color adhesion amount <the fourth color adhesion amount. For this reason, there is a problem that the color density becomes higher as the color becomes more downstream, resulting in a color image having a distance from a correct hue.
[0013]
Usually, in the image forming step, when the toner adheres to the photoconductor from the developing roller according to the image potential of the photoconductor (drum or belt), the toner does not necessarily have a uniform negative charge. Some have a weak negative polarity and some have a positive charge. Further, in some cases, a discharge is generated in a step of peeling off from the photoreceptor after the transfer and the polarity becomes positive later.
[0014]
It is said that the above-mentioned reverse transfer phenomenon is a phenomenon that is caused by a small amount of reverse transfer to the photoreceptor each time a positive transfer charge is received in a subsequent transfer step. Therefore, in consideration of such a reverse transfer phenomenon, a tandem type color image forming apparatus is generally set to supply a larger amount of toner in an upstream image forming station than in a downstream image forming station. Have been.
[0015]
By the way, in a tandem type color image forming apparatus, there is a case where a single color image is formed in which only the most upstream image forming station is involved. An image forming apparatus capable of inclining the loop of the transfer material transport belt downward and separating the photosensitive drum and the transfer material transport belt without substantially changing the belt loop shape during the formation of the single color image. And JP-A-6-258914 and JP-A-10-198121.
[0016]
The above-described image forming apparatus separates the photosensitive drum and the transfer conveyance belt during the formation of a single-color image, so that, for example, the transfer material conveyance belt and the photosensitive material are transferred to an image forming station that is not involved in image formation during the formation of a single-color image. It is possible to maximally avoid wear of both members due to contact with the body. Further, it is possible to prevent the image quality from deteriorating due to unnecessary color toner remaining on the photoreceptor that is not involved in image formation and adhering to the transfer material.
[0017]
In JP-A-6-258914 and JP-A-10-198121, the loop of the transfer material transport belt on the upstream side is inclined downward and separated during image formation in which only the image forming station at the most downstream position is involved. Although a configuration is adopted, the loop of the transfer material conveying belt on the downstream side can be similarly inclined downward and separated at the time of monochrome image formation involving the image forming station at the most upstream position.
[0018]
[Problems to be solved by the invention]
However, similar to the above-described JP-A-6-258914 and JP-A-10-198121, in the image forming apparatus, when the belt loops other than the image forming station at the most upstream position can be separated, the color Image forming conditions are slightly different between the time of image formation and the time of single-color image formation involving only the most upstream image forming station. Therefore, an image density difference may occur between the toner colors arranged at the uppermost stream position. This occurs because the conditions set for the color image formation are shared without changing the independent toner adhesion amount for the monochrome image formation without any change during the monochrome image formation. .
[0019]
That is, a large amount of toner (developer) of the image forming station at the uppermost stream is supplied in anticipation of the reverse transfer phenomenon at the time of forming a color image, but this condition is used as it is at the time of forming a monochromatic image. Therefore, the reverse transfer phenomenon is avoided by retreating the image forming stations other than the uppermost stream, so that the supply amount of the toner (developer) for forming a monochromatic image becomes too large.
[0020]
The present invention has been made in view of such circumstances, and eliminates the image density difference of the image of the most upstream toner color generated between the time of forming a color image and the time of forming a single-color image, and in any image forming mode. Another object of the present invention is to provide a tandem-type color image forming apparatus capable of always ensuring an appropriate image density.
[0021]
[Means for Solving the Problems]
According to the present invention, means for solving the above-described problem are configured as follows.
[0022]
(1) A plurality of image forming stations having different color components provided with a developing device for developing an electrostatic latent image on a photoconductor by applying a developing bias voltage in which an AC component and a DC component are superimposed are transferred. In a color image forming apparatus that is arranged along the material transport direction and forms one image by sequentially superimposing the color images formed at the respective image forming stations and transferring them to a transfer material,
At the time of monochrome image formation using only the image forming station located at the most upstream position in the transfer material transport direction, the transfer material transport belt for transporting the transfer material is switched to another image forming station not involved in image formation by the switching unit. In addition to being configured to be able to switch to the evacuation state,
Control means is provided for changing the duty ratio of the AC component of the developing bias voltage between the time of forming a color image using all the image forming stations and the time of forming the single-color image.
[0023]
In this configuration, a plurality of image forming stations having different color components are arranged along the transfer material transport direction, and images (color images) of each color formed at each image forming station are transported by the transfer material transport belt. An image is formed by sequentially superimposing and transferring the image on a transfer material such as a sheet to be printed.
[0024]
Further, by using only the image forming station located at the uppermost stream of the transfer material conveying belt, a single-color image can be formed. At this time, the other image forming stations not involved in the image formation are switched to the retracted state separated from the transfer material conveying belt.
[0025]
Normally, in a color image forming apparatus, when forming a color image, an unfixed toner once transferred onto a sheet at an upstream image forming station is robbed at the downstream image forming station. cause. Due to the above reverse transfer phenomenon, the density of the image formed in the image forming station on the upstream side tends to decrease.
[0026]
On the other hand, at the time of monochromatic image formation, since the configuration relating to development and transfer in the image forming station not involved in image formation is not functioned, the reverse transfer phenomenon does not occur and toner is not deprived. Therefore, the density does not decrease, and therefore, a difference occurs in the density of the image formed in the most upstream image forming station between the time of forming a color image and the time of forming a single color image.
[0027]
In general, when the duty ratio of the AC component of the developing bias voltage changes, the amount of toner adhered to the photoconductor changes accordingly. Therefore, the image density can be adjusted by changing the duty ratio of the developing bias voltage.
[0028]
In this configuration, control means for changing the duty ratio of the AC component of the developing bias voltage is provided in order to adjust the image density formed in the most upstream image forming station. The density of an image formed in the most upstream image forming station can be appropriately adjusted according to the case of forming a single-color image, and the occurrence of a density difference can be prevented.
[0029]
Further, for example, when the DC component of the developing bias voltage is changed for density adjustment, even a slight change is minus Vd (dark portion potential) and VI (light portion potential) of the photoconductor and Vp-p of the developing bias voltage. The balance between the side peak and the plus side peak of Vp-p is lost. As a result, the image quality becomes unstable.
[0030]
On the other hand, when changing the duty ratio of the AC component, the effect on the image quality can be minimized even if the adjustment width is widened. In addition, controlling the time (duty ratio) is less burdensome to the power supply than the direct control of the voltage, and it is easy to perform stable and accurate control. Therefore, the image density can be easily and appropriately adjusted by changing the duty ratio of the AC component.
[0031]
When forming a single-color image, the transfer material transport belt is separated from the image forming station other than black, so that the transfer material transport belt can be prevented from being worn due to unnecessary contact between the transfer material transport belt and the photosensitive drum of another color. it can.
[0032]
(2) The control means may set the duty ratio of the developing bias voltage applied to the developing device of the most upstream image forming station in the development promoting direction to be higher in the color image formation than in the single color image formation. Is set to be large.
[0033]
When the DC voltage and the AC voltage are superimposed on each other to form a developing bias voltage, by changing the duty ratio, it is possible to adjust the toner adhesion amount, and thus the image density. That is, when the duty ratio of the developing bias voltage applied to the developing device in the development promoting direction is increased, the amount of toner adhered to the photoconductor increases (image density increases), and when the duty ratio is decreased, toner adheres to the photoconductor. The amount decreases (image density decreases).
[0034]
When forming a color image, it is preferable to consider the occurrence of the reverse transfer phenomenon in the downstream image forming station. Accordingly, the duty ratio of the developing bias voltage applied to the developing device of the most upstream image forming station in the development promoting direction is set to be larger than in the case of forming a single color image in which it is not necessary to consider the loss due to the reverse transfer phenomenon. Is preferred.
[0035]
With such a measure, it is possible to eliminate an image density difference between when forming a color image and when forming a single-color image in which only the image forming station positioned at the uppermost stream of the transfer material conveying belt is involved.
[0036]
(3) The control means changes the duty ratio of the developing bias voltage by fixing the frequency f and the amplitude Vp-p of the AC component of the developing bias voltage and changing the pulse width. .
[0037]
The image density can be adjusted by, for example, changing the frequency f of the AC component or the amplitude Vp-p. However, in this method, a load on a power supply for applying a voltage is increased.
[0038]
Therefore, in this configuration, the duty ratio of the developing bias voltage can be easily set by keeping the frequency f and the amplitude Vp-p fixed at constant values and independently changing only the ratio of the positive and negative pulse widths (time distribution). To adjust the image density.
[0039]
(4) The uppermost image forming station is for the black toner.
[0040]
In general, since the color used for forming a single-color image is black in many cases, and since black is a color having a strong concealing color, as shown in this configuration, the most upstream image forming station uses a black toner. It is preferable to use
[0041]
According to this configuration, the image density obtained with respect to the black toner image forming station at the uppermost stream of the transfer material conveying belt at the time of color image formation and the image density obtained at the time of black monochrome image formation involving only the black toner image forming station Is eliminated, and a high-quality image can be obtained both when forming a color image and when forming a black monochrome image.
[0042]
In addition, when forming a color image, the image forming station for the black toner is arranged at the uppermost stream, so that the black toner becomes the lowermost layer during the formation of the color image, and the influence on the toner image of other colors can be minimized. Image quality can be improved.
[0043]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a color image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.
[0044]
(Schematic Configuration of Image Forming Section of Color Image Forming Apparatus)
FIG. 2 is a sectional view schematically showing an image forming section 100 of a color image forming apparatus to which the tandem system according to the present invention is applied. As shown in the figure, the image forming unit 100 includes four image forming stations 200BK, 210C, 220M, and 230Y, a paper feeding unit 240, a transfer material conveying belt 250, a registration roller 255, and a fixing device 260.
[0045]
The four image forming stations 200BK, 210C, 220M, and 230Y store black (BK), cyan (C), magenta (M), and yellow (Y) toners, respectively, and transfer toner images of each color onto transfer paper. Is formed. The paper supply unit 240 includes transfer paper in a tray, and supplies the transfer paper one by one.
[0046]
The transfer material conveying belt 250 is provided below each of the image forming stations 200BK, 210C, 220M, and 230Y, and electrostatically attracts the transfer paper and sequentially conveys the inside of each image forming station. The registration roller 255 sends the transfer paper to the transfer material transport belt 250 (in the direction of the arrow F in the figure) in synchronization with the image formation. The fixing device 260 presses and fixes the toner image formed by being superimposed on the transfer paper by each image forming station at a high temperature.
[0047]
The black (BK) image forming station 200BK is provided with a photoconductor drum (photoconductor) 201BK rotating in the direction of arrow E in the figure, and a charger 202BK is written around the photoconductor drum 201BK in the rotation direction. An optical system 203BK, a developing device 204BK, and a cleaning device 205BK are arranged. In addition, a transfer brush 206BK is disposed on a surface (rear surface) of the transfer material transport belt 250 which corresponds to the back side of the side on which the photosensitive drum 201BK is provided.
[0048]
On the photosensitive drum 201BK, an electrostatic latent image is formed on the photosensitive member on the surface. The charger 202BK uniformly charges the photosensitive drum 201BK. The writing optical system 203BK irradiates the photosensitive drum 201BK with laser light to form an electrostatic latent image. The developing device 204BK includes a black toner, and forms a toner image of an electrostatic latent image formed on the photosensitive drum 201BK.
[0049]
The transfer brush 206BK transfers the toner image onto the transfer paper at a contact position (transfer section) between the photosensitive drum 201BK and the transfer paper by an electric field formed by the applied transfer bias and the potential difference between the photoconductor drum 201BK. I do. The transfer brush 206BK may be, for example, a roller-shaped transfer device. The cleaning device 205BK collects residual toner remaining on the photosensitive drum 201BK without being transferred by the transfer unit.
[0050]
The image forming stations 210C, 220M, and 230Y equivalent to the above-described image forming station 200BK are juxtaposed along a transfer material transport belt 250 that is a transfer material transport unit. The transfer material conveying belt 250 is in contact with the photoconductor between the developing device and the cleaning device of each image forming station. Each image forming station has the same configuration except that the toner color inside the developing device is different.
[0051]
That is, in each of the image forming stations 210C, 220M, and 230Y, the photosensitive drums 211C, 221M, and 231Y, the chargers 212C, 222M, and 232Y, the writing optical systems 213C, 223M, and 233Y, and the developing devices 214C, 224M, and 234Y, A cleaning device 215C / 225M / 235Y and a transfer brush 216C / 226M / 236Y are provided.
[0052]
With the above configuration, the transfer paper is sent from the paper supply unit 240 to the transfer material transport belt 250 in the direction of arrow E in the figure. The transfer paper held on the transfer material transport belt 250 is transported, and the transfer of the toner image of each color is performed at a transfer portion with each photoconductor. Then, the recording paper on which the toner images of four colors are superimposed through the four transfer units is conveyed to the fixing device 260, where the toner is fixed, and is discharged to a discharge unit (not shown).
[0053]
As shown in FIG. 2, in this embodiment, the image forming stations are arranged in the order of BK, C, M, and Y from the upstream side to the downstream side in the transfer paper transport direction. The order is not limited. The color order of the image forming stations is arbitrarily set, but it is preferable that the most upstream image forming station is for black (BK) toner.
[0054]
As described above, each image forming station has the same configuration except that the toner colors inside the developing devices 204BK, 214C, 224M, and 234Y are different. Therefore, for example, the rotation speed of the developing device of each image forming station is also set to the same rotation speed.
[0055]
Further, the image forming apparatus according to the present invention can form a single-color image using black toner using only the image forming station 200 </ b> BK located at the uppermost stream of the transfer material conveying belt 250. In this case, the loop of the transfer material transport belt 250 on the downstream side is inclined downward by switching means (not shown), and the image forming stations 210C, 220M, and 230Y other than the image forming station 200BK in the image forming unit 100 are transferred to the transfer material transport belt. Separated from 250.
[0056]
Although not shown in detail, the transfer material transport belt 250 is stretched over a driving pulley and a driven pulley supported at both ends of a frame, and the frame is disposed in the image forming station 200BK on the most upstream side. A switching means which is supported by the apparatus main body so that the downstream side thereof can move up and down with a position near a contact point between the photosensitive drum upper drum 201B and the transfer brush 206BK as a rotation fulcrum, and has a driving source and a transmission mechanism. Accordingly, the transfer material transport belt 250 is configured to be able to switch its posture between two positions shown in FIGS. 3A and 3B.
[0057]
The switching unit is, for example, a cam mechanism, a link mechanism, a ball screw mechanism, a gear mechanism, or the like, which is rotationally driven by a driving source such as a motor, a solenoid, or a combination thereof, which supports the transfer material transport belt 250. May be configured to be able to be moved up and down on the downstream side, and the switching means may be operated via a control device such as a controller when the image forming mode is switched. It is needless to say that, depending on the arrangement of the image forming stations, a rotation fulcrum may be provided on the downstream side of the transfer material conveying belt 250 and the upstream side may be moved up and down.
[0058]
FIG. 3A shows a state during color image formation, in which all image forming stations 200BK, 210C, 220M, and 230Y are in contact with the transfer material transport belt 250. On the other hand, FIG. 3B shows a state when a single-color image is formed, in which the downstream side of the transfer material transport belt 250 is inclined downward, and the image forming stations 210C, 220M, and 230Y are separated from the transfer material transport belt 250, and Only the forming station 200BK is in contact with the transfer conveyance belt 250.
[0059]
As described above, when forming a single-color image, the transfer material transport belt 250 is separated from the image forming stations 210C, 220M, and 230Y other than the image forming station 200BK, so that the transfer material transport belt 250 and the photosensitive drums 211C, 221M, and 231Y are Can be prevented from being worn due to unnecessary contact between the two.
[0060]
(Outline configuration of developing device)
Next, the developing device 204BK in the image forming station 200BK will be described. FIG. 4 is a cross-sectional view illustrating a schematic configuration of the developing device 204BK. As illustrated, a developing device 204BK is disposed on the right side of the photosensitive drum 201BK. For example, when forming a color image, the photosensitive drum 201BK is driven to rotate in the direction of arrow E at a peripheral speed of about 120 mm / sec. Although not shown, a charger 202BK, a writing optical system 203BK, a cleaning device 205BK, a static eliminator, and a transfer brush 206BK are arranged around the photosensitive drum 201BK.
[0061]
The developing device 204BK has a casing 3, a developing roller 4, a toner supply roller 5, an agitator 6, a non-magnetic one-component developer (hereinafter, referred to as toner) 7, and a doctor blade 8. Further, the developing device 204BK includes a bias power supply 21 and a DC power supply 22 for rotationally driving the developing roller 4 and the toner supply roller 5, respectively.
[0062]
The casing 3 has an opening facing the surface of the photosensitive drum 201BK, and contains the toner 7 in a hopper 3a formed on the right side. The developing roller 4 is partially exposed from the opening, and is driven to rotate in the direction of arrow G at a predetermined peripheral speed by a bias power supply 21.
[0063]
The toner supply roller 5 is disposed in pressure contact with the developing roller 4 on the right side in the figure, and is driven to rotate in a direction indicated by an arrow H by a DC power supply 22. The agitator 6 supplies the toner 7 contained in the hopper 3a to the surface of the toner supply roller 5 and agitates the toner 7 in the hopper 3a. The doctor blade 8 performs an operation of making the thickness of the toner layer on the developing roller 4 conveyed to the developing region I, which is a portion facing the photosensitive drum 201BK, uniform by the rotation of the developing roller 4.
[0064]
The developing device 204BK controls the developing bias voltage applied by the bias power source 21 to adjust the amount of toner 7 adhered to the photosensitive drum 201BK, thereby adjusting the image density. it can.
[0065]
As shown in FIG. 4, the developing roller 4 may be arranged so as to perform non-contact development in a developing region I, facing the surface of the photosensitive drum 201BK with a predetermined gap therebetween. May be arranged so that the toner layer of the photosensitive drum 201BK contacts the surface of the photosensitive drum 201BK to perform contact development.
[0066]
Further, to the developing roller 4, an appropriate developing bias voltage, for example, DC, AC, DC superimposed AC, pulse voltage or the like is applied by the bias power supply 21. In particular, in the case of non-contact development, it is desirable to apply a voltage (alternating current, alternating current of direct current superimposition, pulse voltage, or the like) having an alternating component with good toner 7 flight conditions.
[0067]
(Duty ratio)
Next, the duty ratio D of the AC component of the developing bias voltage will be described. The duty ratio D is represented by the following relational expressions (1) and (2) in the voltage waveform (rectangular wave) of the developing bias voltage, as shown in FIG.
[0068]
Duty ratio: D1 = A / (A + B) (1)
Duty ratio: D2 = B / (A + B) (2)
The duration (pulse width) from when the voltage waveform starts rising to the positive electrode side (point a in the figure) to when it falls to the negative electrode side (point b in the figure) is A, and the voltage waveform falls to the negative electrode side. The continuation time (pulse width) from the first time to the rising to the positive electrode side is B.
[0069]
As described above, there are two types of duty ratio D indicating the ratio of the duration A to the duration (A + B) of one cycle and the percentage of the duration B to the duration (A + B) of one cycle. The former expressed by the expression (1) is called a duty ratio D1, and the latter expressed by the expression (2) is called a duty ratio D2.
[0070]
Further, the waveform of the developing bias voltage may be a waveform other than a rectangular wave as shown in FIG. In such a case, for the above-described duration A (pulse width) and duration B (pulse width), the start point (ie, the fall start point) when the voltage switches from negative polarity to positive polarity is represented by b1. B2, when the start point (that is, the rising start point) when the voltage switches from the positive polarity to the negative polarity is a, A is the time from b1 to a, and B is the time from a to b2. .
[0071]
In FIG. 5A, the positive peak voltage is indicated by Va, the negative peak voltage is indicated by Vb, and the peak-to-peak voltage (that is, the amplitude) is indicated by Vp-p. In the relationship between the bright portion potential VI, which is the potential of the electrostatic latent image on the photoreceptor, and the peak voltage Vb on the negative side of Vp-p, the higher the electric field intensity, the higher the reproducibility of the image portion.
[0072]
(Relationship between duty ratio and image density)
FIG. 6 shows the relationship between the duty ratio D2 and the image density. As shown in the figure, the image density increases as the duty ratio D2 increases. When the duty ratio D2 becomes about 0.6, the image density is saturated. That is, the image density increases as the ratio of the duration B (duty ratio D2) on the negative electrode side increases, and reaches the maximum density when the ratio between A and B becomes about 2: 3. Therefore, it can be said that the duty ratio D1 is a duty ratio in the development suppression direction and the duty ratio D2 is a duty ratio in the development promotion direction.
[0073]
(Control block diagram)
The color image forming apparatus according to the present invention includes control means for controlling the developing bias voltage applied by the bias power supply 21. Hereinafter, the control means will be described.
FIG. 7 is a block diagram showing the control means for controlling and applying the developing bias voltage to the developing roller 4. As shown in FIG. 7, the control means includes a rotation drive device 31 as a drive means for driving the development roller 4 to rotate, a rotation control device 32 as a drive control means for the development roller 4, and application of a development bias voltage to the development roller 4. A bias power supply 21, a bias control device (control means) 33 as a developing bias voltage control means, and a mode switching means 34 are provided.
[0074]
The rotation control device 32 controls the peripheral speed of the developing roller 4 with respect to the peripheral speed of the photosensitive drum 201BK via the rotation driving device 31. The mode switching unit 34 switches an image forming mode such as color (multicolor) image formation or single-color image formation.
[0075]
The mode switching means 34 includes a mode control device 41, a main control unit 42 including a CPU, a memory, and the like, and an I / O unit 43. The mode input device 41 inputs the configured image forming mode by a user-operable push button or the like provided at an appropriate position on the operation panel surface of the image forming apparatus main body.
[0076]
When performing color image formation, the user operates the mode input device 41 to select a color image formation mode. A signal of the setting data corresponding to the color image forming mode is input to the main control unit 42 via the I / O unit 43. Then, a color control signal corresponding to the color image forming mode is sent from the main control unit 42 to the rotation control device 32 via the I / O unit 43, and the driving conditions and the image forming conditions of the rotation driving device 31 are set. You. The color control signal is also sent to the bias control device 33 to control the developing bias voltage applied to the developing roller 4 by the bias power supply 21.
[0077]
On the other hand, when a single-color image is to be formed, similarly to the above-described color image formation, the rotation control device 32 is controlled by the rotation control device 32 from the main control unit 42 via the I / O unit 43 for the single-color image forming mode. A control signal is sent, and drive conditions and image forming conditions of the rotary drive device 31 are set. The control signal for single color is also sent to the bias control device 33 to control the developing bias voltage applied by the bias power supply 21 to the developing roller 4.
[0078]
The control means (bias control device) 33 applies the voltage to the developing roller 4 which is one of the developing conditions, in accordance with the switching of the color image forming mode or the monochrome image forming mode by the mode switching means 34. The type of developing bias voltage to be used can be switched. That is, the bias power supply 21 of the developing roller 4 has a configuration in which a voltage, a pulse voltage, or the like in which an AC component and a DC component are superimposed can be freely applied to the developing roller 4.
[0079]
Further, the control means 33 can control the type of the developing bias voltage such as a DC voltage and an AC voltage applied to the developing roller 4 and, in one cycle of the developing bias voltage, the direction in which the toner is pulled back to the developing roller 4 ( That is, the voltage application time (A in FIG. 5A) for generating an electric field in the development suppression direction and the voltage application time for generating an electric field in the direction in which the toner moves to the photosensitive drum side (ie, the development promotion direction). (Ie, B in FIG. 5A) (ie, duty ratio) can be controlled.
[0080]
Accordingly, the control unit 33 adjusts the image density in the image forming station 200BK located at the uppermost stream by changing the duty ratio according to the case of forming a color image and the case of forming a monochromatic image. Can be. It is preferable that the change of the duty ratio is performed without changing the frequency and the peak-to-peak voltage.
[0081]
(Setting example of duty ratio by image forming mode)
A method of setting the duty ratio (D2) will be described with an example.
When the user operates the mode input device 41 to select the color image forming mode, a signal of the setting data corresponding to the color image forming mode is input to the main control unit 42 via the I / O unit 43. You. Then, a color control signal corresponding to the color image forming mode is sent from the main control unit 42 to the bias control device 33 via the I / O unit 43.
[0082]
Thereafter, as shown in FIG. 8A, the developing bias voltage at the time of development changes between the image portion (bright portion potential VI) and the non-image portion (dark portion potential Vd) of the electrostatic latent image on the photosensitive drum 201BK. The bias application condition of the bias power supply 21 is set such that the voltage periodically changes around a substantially intermediate potential.
[0083]
Specifically, when the peak-to-peak voltage Vp-p = 1000 V and the frequency f = 1200 Hz, the application time A of the potential on the development suppression side is substantially the same as the application time B of the potential on the development promotion side (duty ratio D2 is 0.5 The conditions for applying the bias of the bias power supply 21 are set so that the pulse-like voltage is obtained. When an image is formed, the developing bias voltage is applied to the developing roller 4 by the bias power supply 21 to perform development.
[0084]
On the other hand, when the user operates the mode input device 41 to select the monochrome image forming mode, a signal of the setting data corresponding to the monochrome image forming mode is sent to the main control unit 42 via the I / O unit 43. Is entered. Then, a single-color control signal corresponding to the single-color image forming mode is sent from the main control unit 42 to the bias control device 33 via the I / O unit 43.
[0085]
Thereafter, as shown in FIG. 8B, the developing bias voltage at the time of development is periodically changed around a substantially intermediate potential between the image portion and the non-image portion of the electrostatic latent image on the photosensitive drum 201BK. The bias application condition of the bias power supply is set so as to obtain the required voltage.
[0086]
Specifically, when the peak-to-peak voltage Vp-p = 1000 V and the frequency f = 1200 Hz, the application time A of the potential on the development suppression side is longer than the application time B of the potential on the development promotion side, for example, the application time A and the application time The bias application condition of the bias power supply is set so that the time ratio with B becomes a pulse-like voltage of 3: 2 (the duty ratio D2 is 0.4). Then, at the time of image formation, the developing bias voltage is applied to the developing roller by a bias power supply to perform development.
[0087]
Table 1 lists the peak-to-peak voltage, frequency, time ratio A: B, and duty ratio D2 in each of the above-described image forming modes.
[0088]
[Table 1]

[0089]
As described above, when a color image is formed, the application time A of the potential on the development suppression side to the development roller 4 is substantially equal to the application time B of the potential B on the development promotion side. When a single color image is to be formed, a development bias voltage is applied in which the application time A of the potential on the development suppression side is longer than the application time B of the potential on the development promotion side.
[0090]
As a result, the length of time during which the development is promoted (or the length of time during which the development is suppressed) occurs between the image forming modes, and the amount of toner adhering to the photoconductor can be controlled. It is possible to minimize the occurrence of an image density difference with respect to the toner color of the image forming stage.
[0091]
【The invention's effect】
As is clear from the above description, the present invention has the following effects.
[0092]
(1) Since control means for changing the duty ratio of the AC component of the developing bias voltage is provided between the time of color image formation using all image forming stations and the time of monochrome image formation, a color image formation is performed. The density of an image formed in the most upstream image forming station can be appropriately adjusted according to the case of forming a single-color image, and the occurrence of a density difference can be prevented.
[0093]
In addition, when forming a single-color image, the transfer material transport belt is separated from the image forming station other than black, so that the transfer material transport belt and the photosensitive drum of another color can be prevented from being abraded due to unnecessary contact between the two. it can.
[0094]
(2) The control means sets the duty ratio of the developing bias voltage applied to the developing device of the most upstream image forming station in the development promoting direction to be larger in the color image formation than in the monochromatic image formation. In addition, it is possible to eliminate an image density difference between when forming a color image and when forming a single-color image in which only the image forming station positioned at the uppermost stream of the transfer material conveying belt is involved.
[0095]
(3) The control means changes the duty ratio of the developing bias voltage by fixing the frequency f and the amplitude Vp-p of the AC component of the developing bias voltage and changing the pulse width. By independently changing only the ratio of (time distribution), it is possible to easily change the duty ratio of the developing bias voltage and adjust the image density.
[0096]
(4) Since the most upstream image forming station is for black toner, the image density obtained with the most upstream black toner image forming station of the transfer material conveying belt during color image formation and the black toner image formation The density difference between the image density obtained at the time of forming a black single-color image involving only the station is eliminated, and a high-quality image can be obtained both at the time of forming a color image and at the time of forming a black single-color image.
[0097]
In addition, when forming a color image, the image forming station for the black toner is arranged at the uppermost stream, so that the black toner becomes the lowermost layer during the formation of the color image, and the influence on the toner image of other colors can be minimized. Image quality can be improved.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram illustrating an example of a configuration of a conventional single-drum type color image forming apparatus.
FIG. 2 is an explanatory diagram showing an example of a configuration of the tandem type color image forming apparatus.
FIG. 3 is an explanatory diagram of a configuration of a color image forming apparatus of the present invention.
FIG. 4 is an explanatory diagram of a configuration of a developing device of the color image forming apparatus.
FIG. 5 is a graph showing a voltage waveform of the developing bias voltage.
FIG. 6 is a graph showing the relationship between the duty ratio and the image density.
FIG. 7 is a block diagram showing a control system of the control means.
FIG. 8 is a graph showing a development bias voltage during the development.
[Explanation of symbols]
33-Control means
200BK, 210C, 220M, 230Y-image forming station
204BK-Developing device
250-transfer material transport belt
D2-Duty ratio

Claims (4)

A plurality of image forming stations having different color components are provided with a developing device for developing an electrostatic latent image on a photoconductor by applying a developing bias voltage in which an AC component and a DC component are superimposed. A color image forming apparatus is arranged along the color image forming apparatus that forms one image by sequentially superimposing the color images formed in each image forming station and transferring the color image to a transfer material.
At the time of monochrome image formation using only the image forming station located at the most upstream position in the transfer material transport direction, the transfer material transport belt for transporting the transfer material is switched to another image forming station not involved in image formation by the switching unit. In addition to being configured to be able to switch to the evacuation state,
A color image forming apparatus comprising: a control unit that changes a duty ratio of an AC component of the developing bias voltage between a time of forming a color image using all image forming stations and a time of forming the single-color image. The control means sets a duty ratio in the development promoting direction of the developing bias voltage applied to the developing device of the most upstream image forming station to be larger in the color image formation than in the single color image formation. The color image forming apparatus according to claim 1, wherein: 2. The method according to claim 1, wherein the control unit changes the duty ratio of the developing bias voltage by fixing a frequency f and an amplitude Vp-p of an AC component of the developing bias voltage and changing a pulse width. Or the color image forming apparatus according to 2. 4. The color image forming apparatus according to claim 1, wherein the uppermost image forming station is for a black toner.

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