JP3634401B2 - Development device - Google Patents

Description

【００３８】
表１に示されるように、現像スリーブ３と感光ドラム４ＫのＳＤ間隙ｒと現像穂高ｈとの関係ｒ／ｈが０．２未満では、現像部で現像スリーブ３と感光ドラム４との間にメカニカルな圧縮力が働いて、現像穂にトナーの融着が発生する。ｒ／ｈが０．７よりも大きい場合は、著しい画質の劣化が発生した。又ＳＤ間隙ｒが０．２ｍｍ未満では、現像穂による感光ドラム上の摺擦が生じ、得られる画像に掃き目状の画質劣化が発生した。
【００３９】
以上から、本発明では、ＳＤ間隙ｒと現像穂高ｈとが、
０．２≦ｒ／ｈ≦０．７ 且つ ０．２ｍｍ≦ｒ
を満たし、前述した現像バイアスを用いることを条件とした。
【００４０】
以上は、第４の画像形成ステーションＰ_Ｋ の現像装置９Ｋについて説明したが、第１〜第３の画像形成ステーションＰ_Ｍ 〜Ｐ_Ｙ の現像装置９Ｍ〜９Ｙについても同様にされる。
【００４１】
これによりアナログ潜像の画像部のガサツキは勿論のこと、ドット潜像によるハイライト部が滑らかでガサツキがなく、且つ現像剤の濃度変動による濃度変動を抑えた高画質のカラー画像を得ることが可能になった。
【００４２】
実施例２
先の実施例では、現像バイアスとして、図２に示すように、５００ＶのＤＣ電圧に、ピークツウピーク電圧が２ｋＶで１周期が１４Ｈｚの矩形波を２周期重畳させた波形を用いたが、本実施例では、矩形波を３周期設けたことが特徴である。その他は実施例１と同様にした。
【００４３】
本実施例においても、同様に、ハイライト部が滑らかでガサツキがなく、且つ現像剤の濃度変動による濃度変動を抑えた高画質のカラー画像を得ることができた。
【００４４】
本発明で用いる現像バイアスは、一般に１セットのパターン中にＤＣ電圧のみが存在する区間を有していれば、１セット中に矩形波は何周期分重畳してもよい。このような現像バイアスによれば、ピークツウピーク電圧２ｋＶでプラスの高電界及びマイナスの高電界が交互に印加され、１周期が１４ｋＨｚの矩形波が数周期形成されるので、ハイライト部が滑らかでガサツキをなくす効果がある。
【００４７】
本発明においては、一般に現像バイアスの交番電圧の１周期の長さは、周波数表示で６ｋＨｚから１６ｋＨｚを採用することができる。交番電圧の１周期が６ｋＨｚ〜１６ｋＨｚの間であれば、その交番電界を複数回印加する間に、摩擦帯電電荷が高くキャリアに付着していたトナーも電界で振動を受けて、現像に関与するようになるので、感光ドラム上の低電位潜像部であるハイライト部に対しても十分なトナーを供給でき、ハイライト部の滑らかさに優れたカラー画像を提供することを可能にできた。
【００４８】
【発明の効果】
以上説明したように、本発明は、「０．２≦ｒ／ｈ≦０．７、 且つ ０．２ｍｍ≦ｒ」の構成により、磁気ブラシの像担持体への接触状態を良好にして「トナー融着」や「磁気ブラシの掃き目による画質劣化」を防止し、さらに、「直流電界部を矩形波状の飛翔電界部の次に形成する」構成により、現像時（直流電界形成時）、キャリア（磁気ブラシ）の像担持体への接触状態をより一層軽くして、つまり、矩形波にしたことによりキャリアが現像剤担持体側に付勢される状態、一方、トナーは像担持体側に存在する状態が長くなり、このような状態にしてから直流電界部を形成するので、トナーの帯電状態に関わらず高帯電トナーをも現像に寄与させることができ、その結果、ドットの欠落を防止してハイライト部のガサツキのない高画質の画像を得ることができる。
更に、本発明では、「交番電圧の１周期の長さは周波数表示で６ｋＨｚ〜１４ｋＨｚである」構成とすることにより、交番電界を印加する間に、摩擦帯電電荷が高くキャリアに付着していたトナーも電界で振動を受けて、現像への寄与度が高くなり、ハイライト部に対して十分なトナーの供給が行え、滑らかなハイライト部の再現が可能となる。
即ち、本発明によれば、上記構成により、ハイライト部のガサツキをなくして高画質の画像を得ることができる、といった相乗効果を得ることができる。
【図面の簡単な説明】
【図１】本発明の現像装置を備えた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の画像形成装置に設置した現像装置で使用した現像バイアスを示す波形図である。
【図３】現像剤の磁気ブラシの現像穂高の測定方法を示す説明図である。
【図４】従来の画像形成装置の一実施例を示す概略構成図である。
【図５】図４の画像形成装置に設置した現像装置を示す概略構成図である。
【図６】図５の現像装置で使用した現像バイアスを示す波形図である。
【符号の説明】
３ 現像スリーブ
４Ｍ、４Ｃ、４Ｙ、４Ｋ 感光ドラム
９Ｍ、９Ｃ、９Ｙ、９Ｋ 現像装置
１３ 磁石
１５ 現像バイアス電源
１６ 転写ベルト[0001]
[Industrial application fields]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to a developing device in an image forming apparatus using an electrophotographic system that forms a latent image on an image carrier and develops and visualizes it, and in particular, a two-component developer having toner and carrier for development The present invention relates to a developing device that uses.
[0002]
[Prior art]
An example of a conventional electrophotographic color laser beam printer is shown in FIG. This color printer includes first, second, third and fourth image forming stations P _M , P _C , P _Y and P _K in the printer body, and one side thereof, that is, the right side of FIG. In addition, a paper feeding section (only the paper feeding cassette 30 is shown for the sake of simplicity) is provided on the opposite side, that is, on the left side of FIG. Further, an endless transfer material conveying means for conveying a transfer material, for example, a transfer belt 16 is installed between a plurality of rollers in a well-known manner below the path from the paper feeding unit to the fixing unit in the printer body. It is stretched. The transfer belt 16 is driven in the direction indicated by the arrow in the figure, carries a transfer material fed through the paper feeding unit, and is sequentially conveyed to the image forming stations P _M , P _C , P _Y and P _K described above. To do.
[0003]
Each of the image forming stations P _M , P _C , P _Y and P _K has substantially the same configuration, and is a photosensitive drum 4M, 4C, 4Y which is an image carrier that is driven to rotate in the direction of the arrow as usual. And 4K, and at the periphery of each photosensitive drum are primary chargers 22M, 22C, 22Y and 22K for uniformly charging the photosensitive drum, and developing units 9M, 9C for developing the electrostatic latent image formed on the photosensitive drum. , 9Y and 9K, transfer chargers 17M, 17C, 17Y and 17K for transferring a visible toner image obtained by development to a transfer material, drum cleaners 25M, 25C, 25Y for removing toner remaining on the photosensitive drum, and 25K are sequentially arranged in the drum rotation direction. Image exposure devices 23M, 23C, 23Y and 23K are provided above the photosensitive drums 4M, 4C, 4Y and 4K, respectively.
[0004]
The developing device 9M contains magenta toner, the developing device 9C contains cyan toner, the developing device 9Y contains yellow toner, and the developing device 9K contains black toner. In this example, the image exposure devices 23M, 23C, 23Y, and 23K are composed of a semiconductor laser, a polygon mirror, an fθ lens, and the like. Is scanned in the drum bus direction between the primary chargers 22M, 22C, 22Y and 22K and the developing devices 9M, 9C, 9Y and 9K to expose the drum surface, and an electrostatic latent image of a color corresponding to the drum surface Is supposed to form. The image exposure device 23M has a pixel signal corresponding to the magenta component image of the color image, the image exposure device 23C has a pixel signal corresponding to the cyan component image, and the image exposure device 23Y has a pixel signal corresponding to the yellow component image. The pixel signal corresponding to the black component image is input to the image exposure device 23K.
[0005]
Also between the first image forming station P _M and the sheet feeding section, a metal roller (not shown) that is grounded and the adsorption charger constituting the transfer material attracting means is opposed disposed across the transfer belt 16 In order to reliably attract the transfer material supplied from the paper feeding unit to the transfer belt 16, corona discharge and charge injection are performed. On the other hand, a charge-removing charger (not shown) is provided between the fourth image forming station _PK and the fixing unit, and the transfer material adsorbed to the transfer belt 16 is provided in this charge-removing charger. An AC voltage is applied to isolate
[0006]
In the color printer configured as described above, when the transfer material is fed onto the transfer belt 16 from the paper feed cassette 30, the transfer material is reliably attracted onto the transfer belt 16 by the action of the above-described adsorption charger and roller. Is accompanied with the movement of the arrow direction of the transfer belt 16, a magenta toner image on the photosensitive drum 4M of the first image forming station P _M is, the photosensitive drum 4C of the second image forming station P _C Cyan Toner image is a yellow toner image on the photosensitive drum 4Y of the third image forming station P _Y is, and the photosensitive drum 4K of the fourth image forming station P _K black toner image is formed is shared respectively.
[0007]
The toner images of these colors are transferred by the transfer belt 16 and the transfer material sequentially passes under the photosensitive drums 4M to 4K of the first to fourth image forming stations P _{M to} P _K toward the fixing unit. While being conveyed, the transfer chargers 17M, 17C, 17Y, and 17K of each image forming station sequentially transfer and superimpose on the transfer material to synthesize a color image. The transfer material passes through the fourth image forming station P _K, it is discharged by an AC voltage is applied the charge eliminating charger is separated from the transfer belt 16. The transfer material separated from the transfer belt 16 is sent to the fixing unit. After the image transferred in the fixing unit is fixed, the transfer material is discharged from the transfer material discharge port to the tray, thus completing one copy cycle.
[0008]
By the way, in a color image forming apparatus for forming a full color or multicolor image such as an electrophotographic color laser beam printer or a copying machine, a two-component developer is often used in the developing device from the viewpoint of the color of the image. Yes. As is well known, the toner concentration of the two-component developer (that is, the ratio of the toner particle weight to the total weight of the carrier particles and toner particles) is an extremely important factor in stabilizing the image quality. The toner particles of the developer are consumed during development, and the toner density changes. For this reason, the developer concentration control device (ATR) is used to accurately detect the toner concentration of the developer in a timely manner, and the toner is replenished in accordance with the change, so that the toner concentration is always controlled to be constant and the image quality is improved. Need to hold.
[0009]
For example, FIG. 5 shows an example of a developing device provided with a conventional developer concentration control device used in each image forming station of the above-described electrophotographic color laser beam printer. As a representative example, the developing device 9K of the fourth image forming station that forms a black toner image is illustrated, but a developing device having a similar configuration is also used in the first to third image forming stations.
[0010]
The developing device 9K is arranged to face the photosensitive drum 4K, and the inside thereof is divided into a first chamber (developing chamber) 11 and a second chamber (stirring chamber) 12 by a partition wall 10 extending in the vertical direction. Has been. A nonmagnetic developing sleeve 3 that rotates in the direction of the arrow is disposed in the first chamber 11, and a magnet 13 is fixedly disposed in the developing sleeve 3. The developing sleeve 3 carries and transports a layer of a two-component developer (including a magnetic carrier and a non-magnetic toner) whose layer thickness is regulated by a blade 14, and the developer is transferred to the photosensitive drum 4K in a developing region facing the photosensitive drum 4K. Supply and develop electrostatic latent image. In order to improve the developing efficiency, that is, the application rate of toner to the latent image, a developing bias voltage in which an AC voltage is superimposed on a DC voltage is applied from the power source 15 to the developing sleeve 3.
[0011]
Developer stirring screws 41 and 42 are disposed in the first chamber 11 and the second chamber 12, respectively. The screw 41 stirs and conveys the developer in the first chamber 11, and the screw 42 develops toner already supplied in the developing device 9K with toner supplied by rotation of the conveying screw 43 from a toner discharge port of a toner supply tank (not shown). The agent 44 is agitated and conveyed to make the toner density uniform. The partition wall 10 is formed with a developer passage (not shown) that allows the first chamber 11 and the second chamber 12 to communicate with each other at the front and back end portions in FIG. The developer in the first chamber 11 in which the toner density is reduced due to the transport force of the toner and the toner density is lowered moves from one passage into the second chamber 12, and the toner density is recovered in the second chamber 12. The developer is configured to move into the first chamber 11 from the other passage.
[0012]
On the other hand, the developer concentration control device is a so-called reflected light amount detection method, and when operated at a predetermined timing, a predetermined patch-like electrostatic latent image formed on the photosensitive drum 4K is subjected to a predetermined condition. A reference image 20 having a standard density in a patch shape formed by developing with the developing device 9K is irradiated with light from a bidirectional light emitting LED (light emitting diode) 1 and the reflected light is received by the photodiode 2. The current toner density is detected from the amount of reflected light and is compared with a reference level signal corresponding to the reference density to determine whether the toner is oversupplied or insufficiently supplied. Based on the above, an appropriate amount of toner is replenished to the developing device 9K, and the toner density is appropriately controlled.
[0013]
[Problems to be solved by the invention]
However, when an image is output by the above-described conventional printer, roughness occurs in a halftone region having a reflection density of 0.3 or less. Although this roughness does not occur relatively in character originals or the like, it often occurs in low-density images such as photographs.
[0014]
Then, when the cause of the occurrence of roughness was examined, the following was found. Normally, when forming a latent image of a highlight portion with a dot latent image, when viewed microscopically, the latent image on the photosensitive drum is not a broad latent image like an analog latent image but a local latent image. Yes. If an attempt is made to reproduce a lower density, the latent image is blurred due to the influence of the photosensitive drum film thickness of the photosensitive drum. For example, if an image having a reflection density of about 0.2 is to be reproduced, the maximum contrast V ₀ of the latent image at that time is about 150 to 250V. In the case of reversal development, the surface potential of the non-image area is set to be 100 to 200 V higher than the DC component of the developing bias in order to remove the fog, so that the developing bias DC when V ₀ is 150 to 250 V is set. The potential difference Vcont from the component is about 0 to 100V.
[0015]
The Vcont of 0 to 100 V is a very unstable contrast whether the toner reaches the photosensitive drum side or the developing sleeve side. Therefore, when developing the latent image with a two-component developer, the contact state of the magnetic brush of the developer greatly contributes to the development efficiency, and it is easy to cause roughness due to missing dots corresponding to unevenness of the magnetic brush head. Become.
[0016]
If a rectangular wave alternating voltage as shown in FIG. 6 is superimposed on the developing bias for the purpose of eliminating the roughness of the image, the toner sufficiently reciprocates between the developing sleeve and the photosensitive drum (between SD). However, the latent image potential of the highlight portion is shallow, and the toner having a low triboelectric charge has a low electrostatic adhesion, and is easily pulled back to the developing sleeve side by reciprocating motion. For this reason, only a toner having a high triboelectric charge is used for development, and a toner having a low triboelectric charge does not contribute to the development, so that it is still impossible to prevent the occurrence of roughness.
[0017]
An object of the present invention is to provide a developing device that can eliminate the roughness of highlight portions and obtain a high-quality image by good development.
[0018]
[Means for Solving the Problems]
The above object is achieved by the developing device according to the present invention. In summary, the present invention includes a developer carrying member that carries a developer including a non-magnetic toner and a magnetic carrier and conveys the developer to a developing unit, and a magnetic field generating unit built in the developer carrying member. In a developing device for developing with a developing electric field by bringing a magnetic brush of developer formed on the developer carrier into contact with an electrostatic latent image formed on an image carrier,
In the developing unit, when the gap between the image carrier and the developer carrier is r (mm) and the height of the magnetic brush head is h (mm),
0.2 ≦ r / h ≦ 0.7 and 0.2 mm ≦ r
Satisfy the relationship
During one cycle of the developing electric field, a return electric field part for urging toner from the image carrier to the developer carrier and a flying electric field part for urging toner from the developer carrier to the image carrier are alternated. A rectangular wave-shaped alternating electric field portion that repeats a plurality of times, and a direct-current electric field portion formed subsequent to the alternating electric field portion, the direct-current electric field portion is formed next to the flying electric field portion,
The length of one cycle of the alternating voltage is 6 kHz to 14 kHz in frequency display.
[0019]
【Example】
Example 1
FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus provided with the developing device of the present invention. In the present embodiment, the image is applied to the developing units of the developing devices 9M, 9C, 9Y, and 9K installed in the image forming stations P _M , P _C , P _Y , and P _K of the image forming apparatus, and the developing sleeves thereof. Characterized by development bias. The configuration of the image forming apparatus and the developing apparatus itself of the present embodiment is basically the same as that of the conventional image forming apparatus shown in FIG. 4 and the developing apparatus shown in FIG. The same members as those in FIG. 4 are denoted by the same reference numerals and the description thereof is omitted, and the developing device will be described with reference to FIG. 5 as much as necessary.
[0020]
The electrostatic latent images formed on the photosensitive drums 4M, 4C, 4Y, and 4K at the image forming stations P _M , P _C , P _Y , and P _K in FIG. 1 are respectively developed by the developing devices 9M, 9C, 9Y, and 9K. By applying a developing bias in sequence from a common power source 15 to the developing sleeves, the developing sleeve is developed into a toner image. The developer used in each of the developing devices 9M, 9C, 9Y, and 9K is a two-component developer having a nonmagnetic toner that is negatively charged and a carrier (magnetic particles) that is positively charged.
[0021]
In this embodiment, the mixing ratio of the non-magnetic toner and the carrier is such that the toner concentration of the developer is about 5% by weight. The non-magnetic toner has a volume average particle size of about 8 μm. The carrier is resin-coated with ferrite particles (maximum magnetic susceptibility 60 emu / g), and its weight average particle diameter is 50 μm and its resistance value is 10 ⁸ Ω · cm or more. The permeability of the carrier is about 5.0.
[0022]
Hereinafter, as a representative of a developing device used in each image forming station, it will be described by taking the developing device 9K of the fourth image forming station P _K for forming a black toner image previously shown in FIG. 5 as an example The same applies to the developing devices 9M to 9Y of the first to third image forming stations P _{M to} P _Y.
[0023]
As shown in FIG. 5, an opening is provided in the vicinity of the photosensitive drum 4K of the developing device 9K in the fourth station so that the nonmagnetic developing sleeve 3 protrudes from the opening toward the photosensitive drum 4K. The developing device 9K is rotatably installed. A two-component developer 44 containing black toner is accommodated in the developing device 9K.
[0024]
In this embodiment, the developing sleeve 3 has an outer diameter of 32 mm and a peripheral speed of 280 mm / second. The developing sleeve 3 is disposed with respect to the photosensitive drum 4K so that a gap (SD gap) r between the developing sleeve 3 and the photosensitive drum 4K is 0.5 mm (500 μm). In the developing sleeve 3, a magnet 13 as a magnetic field generating means is fixedly installed so as not to rotate with respect to the rotation of the developing sleeve 3.
[0025]
Magnet 13 is conveyed developing magnetic pole N ₁ of the developing sleeve 3 and the photosensitive drum 4K is positioned at the developing portion opposed, the magnetic pole S ₃ downstream of the magnetic poles N ₁ with respect to the rotational direction of the developing sleeve 3, a developer Magnetic poles S ₂ , N _2, and S ₁ . Magnet 13 is disposed to developing magnetic pole N ₁ is opposed to the photosensitive drum 4K. Developing magnetic pole N ₁ forms a magnetic field in the vicinity of the developing unit, a magnetic brush in the two-component developer carried on the developing sleeve 3 by the magnetic field is formed.
[0026]
Above the developing sleeve 3, a blade 14 is fixedly installed on the developing device 9K. The blade 14 is arranged with a predetermined gap from the developing sleeve 3, and in this embodiment, the gap is set to 800 μm. The blade 14 is made of a nonmagnetic material such as aluminum or SUS316, and regulates the layer thickness of the developer on the developing sleeve 3.
[0027]
Developer stirring screws 41 and 42 are installed in the first chamber (developing chamber) 11 and the second chamber (stirring chamber) 12, respectively. The screw 41 is rotationally driven in the direction of the arrow in the figure, thereby conveying the developer in the first chamber 11 toward the longitudinal direction of the developing sleeve 3, and the screw 42 is rotationally driven in the opposite direction to the above. As a result, the developer in the second chamber 12 is conveyed in the longitudinal direction of the developing sleeve 3 in the opposite direction. At this time, the toner replenished into the stirring chamber 12 by the conveying screw 43 from a toner replenishing device (not shown) is agitated and mixed with the developer conveyed by the screw 42.
[0028]
The developing sleeve 3 carries a developer in the vicinity of the magnetic pole S ₂ , and the carried developer is conveyed toward the developing unit as the developing sleeve 3 rotates. When the developer reaches the vicinity of the developing portion, the developer carrier (magnetic particles) rises from the surface of the developing sleeve 3 while being linked by the magnetic force of the magnetic pole N ₁ , and a magnetic brush is formed on the surface of the developing sleeve 3. . The tip of the magnetic brush rubs the surface of the photosensitive drum 4K and develops the latent image on the photosensitive drum 4K. At this time, in order to increase the development efficiency, a development bias in which an alternating voltage is superimposed on a DC voltage is applied between the development sleeve 3 and the photosensitive drum 4K, and development with a magnetic brush is performed under the application of the development bias.
[0029]
In the present invention, at this time, an image having a smooth highlight portion, no roughness, and a small density fluctuation due to a change in T / T + C ratio (toner density of developer; T: toner weight, C: carrier weight) is obtained. In order to obtain this, a developing bias is used in which an alternating voltage having a predetermined length in one cycle is superimposed on the DC voltage by a predetermined number of cycles. This developing bias will be described later.
[0030]
Furthermore, in the present invention, in order to further suppress fluctuations in image density due to fluctuations in the T / T + C ratio, the developing efficiency is lowered by widening the SD gap distance between the developing sleeve 3 and the photosensitive drum 4K. And to compensate for the decrease in the developing efficiency was increased height of the magnetic brush formed by the developing magnetic pole N ₁ of the developing portion (developing Hotaka). The relationship between the SD gap and the development head height will also be described later.
[0031]
FIG. 2 shows a waveform diagram of the developing bias used in the present invention. This developing bias has a pattern in which an alternating voltage having a predetermined length of one cycle is superimposed on the DC voltage by a predetermined number of cycles and has a section of the DC electric field portion after the alternating electric field portion. One set of these is applied and a developing bias is applied to the developing sleeve 3 in the same pattern. In this embodiment, a rectangular wave having a peak-to-peak voltage of 2 kV (2 kV _pp ) and a period of 14 kHz in terms of frequency is superimposed on a DC voltage of 500 V for two periods, and a 2 kHz DC voltage is placed after the alternating electric field portion. The developing bias has a boundary portion.
[0032]
First, in the part a of the waveform in FIG. 2, an alternating voltage of 2 kV _pp is applied to the developing sleeve 3 with respect to the photosensitive drum 4K, and a plus (+) high electric field (return electric field) is high frequency ( 28 kHz). At that time, the negatively charged toner in the magnetic brush on the developing sleeve 3 remains as it is without being separated from the carrier. Next, in the portion b of the waveform, a negative (−) high electric field (flying electric field) is applied to the developing sleeve 3 at a high frequency (28 kHz). At that time, the negatively charged toner moves away from the carrier toward the surface of the photosensitive drum 4K. Next, in the portion c of the waveform, a return electric field similar to that in the portion a is applied again, and the toner is returned to the developing sleeve 3 side, but the returned distance is a DC component rather than the flying distance due to the flying electric field in the portion b. It is shorter by (DC) and does not completely return to the developing sleeve 3. Next, in the portion d of the waveform, the toner again moves toward the surface of the photosensitive drum 4K by the same flying electric field as in the portion b. Thereafter, the toner moves at a constant speed by a DC electric field at a portion e of the waveform, and reaches a latent image on the photosensitive drum 4K. That is, while the alternating electric field is applied for two cycles, the toner having a high triboelectric charge and adhering to the carrier is also vibrated by the electric field, thereby contributing to development.
[0033]
As described above, the developing bias according to the present invention has a sufficient developing time since the toner pull-back time is shorter than that of the rectangular wave, and can contribute to the development regardless of the charging property of the toner. Therefore, it is considered that sufficient toner can be supplied to the highlight portion which is a low potential latent image portion on the photosensitive drum.
[0034]
Next, the relationship between the SD gap distance between the developing sleeve 3 and the photosensitive drum 4K and the spike height (developing spike height) at the developing portion of the magnetic brush will be described. In this embodiment, development is performed by variously changing the SD gap distance (this is represented by r (mm)) and the development brush height (this is represented by h (mm)) of the magnetic brush. The smoothness of the light part and toner fusion were evaluated.
[0035]
For example, the height of the developing brush of the magnetic brush was measured as shown in FIG. As shown in FIG. 3A, a stage 52 whose height z is known in advance is placed on the developing sleeve 3. Next, when an appropriate length of the glass plate 51 is stood between the stand 52 and the developing sleeve 3, the glass plate 51 comes into contact with the developing ear 53 formed on the surface of the developing sleeve 3, and FIG. ), The toner in the developing head 53 adheres to the glass plate 51. The length x of the portion of the glass plate 51 where the toner is adhered is measured. Further, the length k on the developing sleeve 3 from the contact point between the glass plate 51 and the developing sleeve 3 to the position where the glass plate 51 and the base 52 are in contact is measured. When the angle formed by the developing sleeve 3 and the glass plate 51 is α, the height h of the developing ear 53 on the surface of the developing sleeve 3 is
Developing Hotaka ^{h = xsinα, α = tan -} z / k
It is.
[0036]
Table 1 shows the results of evaluating the smoothness of the highlight portion of the image and the toner fusion.
[0037]
[Table 1]

[0038]
As shown in Table 1, when the relationship r / h between the SD gap r between the developing sleeve 3 and the photosensitive drum 4K and the developing head height h is less than 0.2, there is a gap between the developing sleeve 3 and the photosensitive drum 4 in the developing portion. The mechanical compression force acts to cause toner fusion on the developing ears. When r / h was greater than 0.7, significant image quality degradation occurred. On the other hand, when the SD gap r is less than 0.2 mm, rubbing on the photosensitive drum is caused by the developing spikes, and a sweep-like image quality deterioration occurs in the obtained image.
[0039]
From the above, in the present invention, the SD gap r and the development head height h are
0.2 ≦ r / h ≦ 0.7 and 0.2 mm ≦ r
And the development bias described above was used.
[0040]
Above it has been described developing device 9K of the fourth image forming station _{P K,} is the same for the developing device 9M~9Y of the first to third image forming station _P M to P _Y.
[0041]
As a result, it is possible to obtain a high-quality color image in which the highlight portion due to the dot latent image is smooth and free from roughness, and the density fluctuation due to the density fluctuation of the developer is suppressed, as well as the roughness of the image area of the analog latent image. It became possible.
[0042]
Example 2
In the previous embodiment, as shown in FIG. 2, a waveform obtained by superposing two periods of a rectangular wave having a peak-to-peak voltage of 2 kV and a period of 14 Hz on a DC voltage of 500 V was used as the developing bias. The embodiment is characterized in that three periods of rectangular waves are provided. Others were the same as in Example 1.
[0043]
In the present embodiment as well, a high-quality color image in which the highlight portion is smooth and free from roughness and the density fluctuation due to the density fluctuation of the developer is suppressed can be obtained.
[0044]
As long as the developing bias used in the present invention generally has a section in which only a DC voltage exists in one set of patterns, the rectangular wave may be superimposed for any number of periods in one set. According to such a developing bias, a positive high electric field and a negative high electric field are alternately applied at a peak-to-peak voltage of 2 kV, and a rectangular wave having one period of 14 kHz is formed, so that the highlight portion is smooth. This has the effect of eliminating rust.
[0047]
In the present invention, generally, the length of one period of the alternating voltage of the developing bias can be 6 kHz to 16 kHz in terms of frequency. If one cycle of the alternating voltage is between 6 kHz and 16 kHz, the toner having a high triboelectric charge and adhering to the carrier is also subjected to vibration by the electric field and is involved in development while the alternating electric field is applied a plurality of times. As a result, sufficient toner can be supplied to the highlight portion, which is a low-potential latent image portion on the photosensitive drum, and a color image having excellent smoothness of the highlight portion can be provided. .
[0048]
【The invention's effect】
As described above, according to the present invention, the configuration of “0.2 ≦ r / h ≦ 0.7 and 0.2 mm ≦ r” improves the contact state of the magnetic brush with the image carrier, “Fusion” and “Image quality degradation due to magnetic brush sweep” are prevented, and the “DC electric field part is formed next to the rectangular wave-like flying electric field part” structure enables development (DC electric field formation), carrier The contact state of the (magnetic brush) with the image carrier is further reduced, that is, the carrier is urged toward the developer carrier by the rectangular wave, while the toner is present on the image carrier. Since the DC electric field part is formed after such a state becomes longer, highly charged toner can contribute to the development regardless of the charged state of the toner, and as a result, dot loss is prevented. High image quality without blurry highlights Images can be obtained.
Furthermore, according to the present invention, by adopting a configuration in which “the length of one cycle of the alternating voltage is 6 kHz to 14 kHz in terms of frequency”, the frictional charge is high and the carrier is attached to the carrier while the alternating electric field is applied. The toner is also vibrated by the electric field and contributes to the development. Thus, a sufficient amount of toner can be supplied to the highlight portion, and the smooth highlight portion can be reproduced.
That is, according to the present invention, the above configuration can provide a synergistic effect that a high-quality image can be obtained without the roughness of the highlight portion.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus provided with a developing device of the present invention.
2 is a waveform diagram showing a developing bias used in a developing device installed in the image forming apparatus of FIG.
FIG. 3 is an explanatory view showing a method for measuring a developing head height of a magnetic brush of a developer.
FIG. 4 is a schematic configuration diagram illustrating an example of a conventional image forming apparatus.
FIG. 5 is a schematic configuration diagram showing a developing device installed in the image forming apparatus of FIG. 4;
6 is a waveform diagram showing a developing bias used in the developing device of FIG. 5. FIG.
[Explanation of symbols]
3 Developing sleeve 4M, 4C, 4Y, 4K Photosensitive drum 9M, 9C, 9Y, 9K Developing device 13 Magnet 15 Developing bias power supply 16 Transfer belt

Claims (1)

A developer carrying member that carries a developer including a non-magnetic toner and a magnetic carrier and conveys the developer to a developing unit; and a magnetic field generating unit that is built in the developer carrying member. In a developing device that develops a developing electric field by bringing a magnetic brush of developer formed into contact with an electrostatic latent image formed on an image carrier,
In the developing unit, when the gap between the image carrier and the developer carrier is r (mm) and the height of the magnetic brush head is h (mm),
0.2 ≦ r / h ≦ 0.7 and 0.2 mm ≦ r
Satisfy the relationship
During one cycle of the developing electric field, a return electric field part for urging toner from the image carrier to the developer carrier and a flying electric field part for urging toner from the developer carrier to the image carrier are alternated. A rectangular wave-shaped alternating electric field portion that repeats a plurality of times, and a direct-current electric field portion formed subsequent to the alternating electric field portion, the direct-current electric field portion is formed next to the flying electric field portion,
The length of one cycle of the alternating voltage is 6 kHz to 14 kHz in frequency display.

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