JP4074744B2 - Image forming apparatus - Google Patents

Description

【００６３】
評価画像には、露光手段の露光量を変化させる方式により作成した２５６段パッチ画像を採用した（解像度：６００dpi）。その理由はこの方式で作成した２５６段パッチ画像のハイライト部は、交番電界のＤｕｔｙやＶpp電圧の変化にも影響を受けやすく、ハーフトーン部のザラツキ感を評価するのに最も適していることを挙げることができる。なぜならばハイライト部の潜像は露光後の電位が浅いため、潜像のコントラストが小さく１ドットあたりに現像されるトナー量が必然的に少なくなる。そのため若干のトナー付着量変動がドット抜けにつながり、ザラツキが目立ちやすいからである。また交番電界に限って言うと、露光電位が浅いため現像を行う際、トナーを現像スリーブ側に引き戻す電界が作用する時間内にトナーが受ける静電力が大きくなる。そのため感光体上潜像に静電的に付着したトナー、または感光体上潜像へ飛翔途中のトナーが現像スリーブに引き戻される確率が大幅に増加しザラツキが低下する。
【００６４】
２５６段パッチ画像において、ザラツキを評価する場所は比較的トナー付着量の少ない部分であり、明度で言うと６０〜８０の領域で画像評価を行う（特に明度７０付近でザラツキ感が悪くなる傾向がある）。これらの明度において、ランク見本に基づいて目視によりランク評価を行った。ここでザラツキランクは、後端白抜けランクと同様に、最もハーフトーン部の均一感に優れたものをランク５と定義し、最も均一感に欠けるものをランク１とした（表中の条件ではランク１は存在しない．例えばＤｕｔｙ＝６０％、Ｖpp：１．２ｋＶ、Ｖoff：−５００Ｖの条件はランク１である）。ザラツキはＤｕｔｙが５０％以下でランク４以上と良好であるが、Ｄｕｔｙ＝５０％を超える領域では低下する。
【００６５】
細線再現性の評価には２値的に書き込んだ１ドット縦ラインを用いた。その理由として、１ドット縦ラインは現像γの変化に比較的敏感であり、現像γ変動による細線再現性の低下を容易に判断できるためである。このライン画像をＣＣＤカメラで読み込み、その後２値化処理した画像の線幅を実際に計測するという手法で評価した。８５μｍを狙いの中心値とし、実測１００μｍ以上、７０μｍ以下のサンプルに対して×と評価し表中に記した。
【００６６】
図２１に代表的なＤｕｔｙによる現像γ曲線（出力パッチ数に対する感光体上トナー付着量曲線）を示した。図示のようにＤｕｔｙ＝５０％を超える領域では、現像γが上昇する。それに起因する影響は細線再現性に欠け、階調性の乏しい画像となってしまうことである。更には地肌汚れレベルが著しく低下し、基本画像品質を満たせなくなる。Ｄｕｔｙ＝５０％以下では地肌汚れなどの基本品質を劣化させることない画像が得られるが、逆にＤｕｔｙ＝１０％では現像能力が低くなってしまっている。
【００６７】
周波数とザラツキの関係は、図２２のような結果となった。この時のトナーの帯電量はブローオフ法の測定で−２０μｃ／ｇであった。図示したように２．５ｋHZ〜５．０ｋHZの範囲で良好なハーフトーンが得られた。より低周波領域では、交番電界を重畳しない直流バイアス的な画像となり、ＡＣバイアスの効果が得られず、ハーフトーンのザラツキが向上しない。これは潜像に付着したトナーを振動させることにより忠実に潜像を再現させるＡＣ電界の効果が得られにくいためである。また、より高周波側では、ハイライト部のトナー付着量が２．５ｋHZ〜５．０ｋHZの周波数領域と比較して減少し、ドット抜けが多くなる。その結果として、ザラツキが低下する。この実験で使用したトナーの帯電量分布を図２３に示す。図に示すように、縦軸はトナーの個数であり、横軸はＱ/ｄ（femtＣ／１０μｍ）である。なお本測定にはホソカワミクロン社製Ｅスパートアナライザを用いた。
【００６８】
次の実験として、トナーの帯電量分布を上記より狭くしたトナーを用いて同様に周波数によるハーフトーン部のザラツキを評価した。帯電量分布を図２４に示し、実験結果を図２５に示す。このトナーは上記実験で使用したものと同処方で生成されたものであるが、分布範囲を狭くすることでブローオフ法での帯電量は−２５μｃ／ｇとなった。図２３で示された分布の場合の結果とは異なり高周波側でのザラツキ低下は確認されなかった。その理由はＡＣバイアスを高周波側へ振った際には、現像電界に対するレスポンスの悪いトナー（低帯電トナーであり現像されにくいトナー）では現像されないが、このように帯電量分布を狭くし、低帯電トナーをカットした系では十分に現像されるためである。つまり、ＡＣバイアスを高周波で使用するためには、帯電量分布を狭くすることが必須といえる。
【００６９】
以上は、現像主磁極に隣接させ当該主磁極の磁力と半値幅を調整するための補助磁極を設けた構成の磁石ローラを用いた場合の例において説明したが、これに限定されるものではなく、現像主磁極の法線方向磁束密度の減衰率が４０％以上であるような磁石ローラであれば、同様な効果が得られる。
【００７０】
【発明の効果】
本発明によれば、温度２３℃、相対湿度６５％の標準環境にて、オイラー法で測定した上記像担持体の紙に対する静止摩擦係数μが０．０２以上０．６未満であり、現像主磁極の法線方向磁束密度の減衰率が４０％以上であり、かつ現像領域に交番電界を形成することで、後端白抜けレベルを高く維持したまま、ハーフトーンのザラツキを向上することができる。交番電界の周波数が２．５〜５．０ｋＨｚであることで、ハーフトーンにトナーを均一に付着することができる。
【００７１】
直流バイアスに交流バイアスを重畳することで形成される交番電界として矩形波のものを用いる場合、矩形波１周期当たりにおいて、トナーを現像剤担持体から像担持体へ移動させる電界の印加時間Ｔ１と、同トナーを像担持体から現像剤担持体へ移動させる電界の印加時間Ｔ２とが、Ｔ１≦Ｔ２の関係を満たす、言い換えれば、交流バイアスのＤｕｔｙを５０％以下にすることにより、請求項１の効果に加えて形成画像上のトナー濃度（ＩＤ）の確保が容易である。またトナー帯電量分布のピーク位置から±５femtＣ／１０μｍ以内に全トナーが収まれば、経時的にもハーフトーンにトナーを均一に付着することができ、ザラツキを向上することができる。潤滑剤塗布機構を付設していれば、像担持体μが変化した際にも適正なμに補正することができ、安定した画像を経時的にも得ることができる。像担持体の初期摩擦係数μがμ≦０．４であれば、像担持体の膜削れを防ぎ、長寿命化することができる。
【図面の簡単な説明】
【図１】本発明に係る画像形成装置としてのカラー複写機の概略構成図である。
【図２】図１のカラー複写機に装着されるリボルバ現像ユニットの部分概略構成図である。
【図３】図２のリボルバ現像ユニットでの現像ローラの磁力分布とその大きさ程度を示す図である。
【図４】１ドット縦横ラインの線幅の比率と磁石ローラの主磁極法線方向磁束密度の減衰率との関係を示すグラフである。
【図５】塗布部材の構成概略図である。
【図６】図５に対する代替例としての塗布部材構成図である。
【図７】塗布量と感光体表面の摩擦係数の関係を示すグラフである。
【図８】感光体摩擦係数を測定するための測定装置の概略図である。
【図９】感光体上のトナー付着量と反射型フォトセンサ出力の関係を示すグラフである。
【図１０】潤滑剤塗布量の変化と現像γカーブの関係を表すもので、縦軸に画像濃度、横軸に現像ポテンシャルをプロットする。
【図１１】現像剤塗布量の制御を説明するフローチャートである。
【図１２】潤滑剤塗布後の感光体表面摩擦係数に対する後端白抜けのレベルをランク評価したグラフである。
【図１３】本発明に関わる磁石ローラでの感光体摩擦係数０．１以下の後端白抜けレベルのランク評価のグラフである。
【図１４】対感光体現像スリーブ線速比と後端白抜けの関係を説明するグラフである。
【図１５】対感光体現像スリーブ線速比と文字周辺抜けの関係を説明するグラフである。
【図１６】試薬と被測定材料表面の位置関係を示す概略図である。
【図１７】中間転写ベルト上の画像における虫食いの、温度・湿度変化における差異を表すグラフである。
【図１８】図１７に関わる実験における評価画像を示す図である。
【図１９】ランニング枚数による虫食い状態の変化を示すグラフである。
【図２０】現像電界として交番電界を用いる場合の現像バイアスの変化を説明する図である。
【図２１】代表的なＤｕｔｙによる現像γ曲線を示すグラフである。
【図２２】周波数とザラツキの関係を示すグラフである。
【図２３】図２２に関わる実験で使用したトナーの帯電量分布を示すグラフである。
【図２４】周波数とザラツキの関係を示す別のグラフである。
【図２５】図２４に関わる実験で使用したトナーの帯電量分布を示すグラフである。
【符号の説明】
１ 感光体ドラム
４ 現像装置
４１ 現像ローラ
４３ 現像スリーブ
４４ 磁石ローラ
４５ ドクタブレード
４７ スクリュー[0001]
BACKGROUND OF THE INVENTION
In the present invention, development is performed in a so-called development region (region where a developable electric field is secured between the developer carrier and the image carrier) on the surface of the developer carrier (raising the head). The present invention relates to an image forming apparatus including a developing device that increases image density and improves low-contrast images during processing.
[0002]
[Prior art]
In general, in various image forming apparatuses such as copying machines, printers, facsimiles, and the like using an electrophotographic method or an electrostatic recording method, a latent image carrier (also referred to as an image carrier) composed of a photosensitive drum or a photosensitive belt is used. An electrostatic latent image corresponding to the image information is formed, and a developing operation is executed by the developing device to obtain a visible image. In carrying out the developing operation in this way, from the viewpoint of transferability, halftone reproducibility, stability of development characteristics with respect to temperature and humidity, etc., there is a magnetic brush development system using a two-component developer composed of toner and carrier. It is becoming mainstream. In this type of developing device, the two-component developer is held on the developer carrier in the form of a brush chain, and is transported to the development area facing the latent image carrier, so that it is contained in the developer. The toner is supplied to the electrostatic latent image portion on the latent image carrier.
[0003]
The developer carrying member is usually configured to include a sleeve (developing sleeve) formed in a cylindrical shape, and a magnet body (magnet roller) that forms a magnetic field so that the developer can be raised on the sleeve surface. Has inside the sleeve. At the time of the spike, the carrier is spiked on the sleeve so as to follow the lines of magnetic force generated by the magnet roller, and charged toner adheres to the spiked carrier. The magnet roller is provided with a plurality of magnetic poles and is formed in a rod shape or the like, and in particular, a development main magnetic pole is provided for causing the developer to spike in the development region portion on the surface of the developer carrying member. By moving at least one of the sleeve and the magnet roller, the developer spiked on the sleeve surface moves. The developer transported to the development area spikes along the lines of magnetic force emitted from the development main pole, and in the case of contact development, the chain chain of the developer contacts the surface of the latent image carrier so as to bend and contact. The toner is supplied while the chain chain of the developed developer rubs against the electrostatic latent image based on the relative linear velocity difference from the latent image carrier.
[0004]
On the other hand, as one means for stably providing a high-quality image over a long period of time, applying a lubricant to a latent image carrier or an image forming process around the latent image carrier is widely performed. If the coefficient of friction of the photosensitive member as a latent image carrier is high, the cleaning blade is caught in the cleaning process, the photosensitive member cannot be cleaned, black streaks are generated, and the cleaning blade itself is abnormally worn. By applying a lubricant to the photoconductor, etc., the frictional force between the photoconductor and the cleaning blade, the photoconductor and the transfer member, etc. is reduced, and the amount of film scraping of the photoconductor due to friction is reduced. The problem was avoided and the long life of the photoreceptor was achieved. At the same time, the generation of “squeaking noise” due to friction could be suppressed, and the generation of unpleasant noise could be prevented. Also, by reducing the coefficient of friction of the photosensitive member, it was possible to suppress the hollowing out phenomenon (so-called insect erosion) that tends to occur in a portion with a large amount of toner adhesion such as the center of the line portion in the transfer step.
[0005]
[Problems to be solved by the invention]
However, the amount of toner adhering to the photoreceptor is reduced due to the reduction in μ by applying lubricant, making it difficult to obtain a sufficient image density, and it is necessary to perform gradation correction by changing the developing bias and writing beam power. . These controls are very complicated, leading to an increase in cost. Further, in the halftone area, when the balance between the adhesion force of the toner to the photoconductor and the rubbing force of the magnetic brush is lost, dot missing occurs, resulting in an image with an increased feeling of roughness of the image. In particular, in the halftone portion of the photosensitive member, counter charge and the rubbing of the magnetic brush act on the carrier to cause a so-called white-out phenomenon, so that the linear velocity ratio of the developing sleeve to the photosensitive member is increased. Was impossible.
[0006]
The present invention provides a high-uniformity image of a halftone portion even in a system in which low friction is achieved by an image carrier or an image forming device around the image carrier, and white spots occur at the rear end of the image. Image with excellent horizontal line reproducibility, improved halftone dot omission around the character area, improved color reproducibility in overlapping areas of each color toner, and image carrier An object of the present invention is to provide an image forming apparatus in which the above-mentioned image quality can be maintained satisfactorily even when the friction coefficient of the lens changes from a low state to a high state.
[0007]
[Means for Solving the Problems]
  According to the present invention, the object is to provide an image having a developing device having a developing main magnetic pole for magnetically adsorbing the developer on the outer peripheral surface of the developer carrying member to form a magnetic brush, and an image carrier facing the developing device. In the forming apparatus, the static friction coefficient μ with respect to the paper of the image carrier measured by the Euler method in a standard environment at a temperature of 23 ° C. and a relative humidity of 65% is 0.02 or more and less than 0.6, and the development main pole The normal direction magnetic flux density attenuation rate is 40% or more, and an alternating electric field is applied to the development area.The frequency of the alternating electric field is 2.5-5.0 kHzIt is solved by.
[0008]
  It is preferable to provide a magnet for assisting magnetic force formation of the developing main magnetic pole. When a rectangular wave is used as the alternating electric field, an electric field application time T1 for moving the toner from the developer carrier to the image carrier per period of the rectangular wave, and the toner from the image carrier to the developer carrier. It is convenient if the application time T2 of the electric field to be moved satisfies the relationship of T1 ≦ T2.. AlsoIt is preferable that all toners are contained within ± 5 femt C / 10 μm from the peak position of the toner charge amount distribution. It is preferable to provide a lubricant application mechanism. The initial friction coefficient μ of the image carrier is preferably μ ≦ 0.4.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The details of the present invention will be described based on examples shown in the drawings.
First, an electrophotographic color copying machine (hereinafter referred to as a color copying machine) as an image forming apparatus according to the present invention will be described with reference to FIG. The color copying machine includes a color image reading device (hereinafter referred to as a color scanner) 11, a color image recording device (hereinafter referred to as a color printer) 12, a paper supply bank 13, and the like.
[0010]
The color scanner 11 forms an image of the document 10 on the contact glass 101 on the color sensor 105 via the illumination lamp 102, the mirror groups 103a, 103b, and 103c and the lens 104, and the color image information of the document 10 is obtained. For example, red, green, and blue (hereinafter referred to as R, G, and B) color separation lights are read and converted into electrical image signals. In this example, the color sensor 105 includes R, G, and B color separation means and a photoelectric conversion element such as a CCD, and simultaneously reads three color images obtained by color separation of the image of the document 10. . Then, based on the R, G, and B color separation image signal intensity levels obtained by the color scanner 11, color conversion processing is performed by an image processing unit (not shown) to obtain black (hereinafter referred to as Bk), cyan (hereinafter referred to as “Bk”). , C), magenta (hereinafter referred to as M), and yellow (hereinafter referred to as Y) color image data.
[0011]
The operation of the color scanner 11 for obtaining the Bk, C, M, and Y color image data is as follows. In response to a scanner start signal that takes the operation and timing of the color printer 12 to be described later, the optical system including the illumination lamp 102 and the mirror groups 103a, 103b, and 103c scans the document 10 in the left direction of the arrow, and scans once. One color image data is obtained every time. By repeating this operation a total of four times, color image data of four colors is sequentially obtained. Each time, the color printer 12 sequentially visualizes the images and superimposes them to form a final four-color full-color image.
[0012]
The color printer 12 includes a photosensitive drum 20 as an image carrier, a writing optical unit 22, a revolver developing unit 23, an intermediate transfer device 26, a fixing device 27, and the like. The photosensitive drum 20 rotates counterclockwise as indicated by an arrow, and around the photosensitive drum cleaning device 201, the charge eliminating lamp 202, the charger 203, the potential sensor 204, the selected developer of the revolver developing unit 23, and the development A density pattern detector 205, an intermediate transfer belt 261 of the intermediate transfer device 26, and the like are arranged. A lubricant application member is built in the charger 203, and the application member will be described later.
[0013]
Further, the writing optical unit 22 converts the color image data from the color scanner 11 into an optical signal, performs optical writing corresponding to the image of the document 10, and forms an electrostatic latent image on the photosensitive drum 20. The writing optical unit 22 includes a semiconductor laser 221 as a light source, a laser light emission drive control unit (not shown), a polygon mirror 222 and its rotation motor 223, an f / θ lens 224, a reflection mirror 225, and the like.
[0014]
The revolver developing unit 23 includes a Bk developing unit 231K, a C developing unit 231C, an M developing unit 231M, and a Y developing unit 231Y, and a revolver rotation driving unit described below that rotates each developing unit counterclockwise as indicated by an arrow. It is configured. Each developing device includes a developing sleeve that rotates by contacting a developer ear with the surface of the photosensitive drum 20 in order to develop an electrostatic latent image, a developer paddle that rotates to pump up and stir the developer, and the like. It consists of The toner in each developing device 231 is negatively charged by stirring with a ferrite carrier, and each developing sleeve has a developing bias in which an AC voltage Vac is superimposed on a negative DC voltage Vdc by a developing bias power source (not shown). The developing sleeve is biased to a predetermined potential with respect to the metal substrate layer of the photosensitive drum 20 when applied (in order to use an alternating electric field as the developing electric field). In the standby state of the copying machine main body, the revolver developing unit 23 has the Bk developing unit 231K set at the developing position, and when the copying operation is started, the color scanner 11 starts reading Bk color image data from a predetermined timing. Then, based on the color image data, optical writing with a laser beam and formation of an electrostatic latent image start (hereinafter, an electrostatic latent image based on Bk image data is referred to as a Bk latent image. The same applies to C, M, and Y). The Bk developing sleeve is started to rotate before the leading edge of the electrostatic latent image reaches the Bk developing position so that the leading edge of the Bk latent image can be developed, and the Bk latent image is developed with BK toner. When the developing operation of the Bk latent image area continues and the trailing edge of the electrostatic latent image passes the Bk developing position, the developing device for the next color (normally the C developing device in this example) immediately comes to the developing position. Until the revolver developing unit 23 rotates. This is completed at least before the leading edge of the electrostatic latent image by the next image data arrives.
[0015]
The revolver developing unit 23 will be described in detail later.
The intermediate transfer device 26 includes an intermediate transfer belt 261, a belt cleaning device 262, a paper transfer corona discharger (hereinafter referred to as a paper transfer device) 263, and the like. The intermediate transfer belt 261 is stretched around a drive roller 264a, a transfer counter roller 264b, a cleaning counter roller 264c, and a driven roller group, and is driven and controlled by a drive motor (not shown). The belt cleaning device 262 including a lubricant application member includes an inlet seal, a rubber blade, a discharge coil, an inlet seal, a rubber blade contact / separation mechanism, and the like. The first color Bk image is transferred to the intermediate transfer belt 261. During the transfer of the second, third and fourth color images after the transfer, the entrance seal and the blade are separated from the surface of the intermediate transfer belt 261 by the contact / separation mechanism. The application member will be described later. The paper transfer unit 263 applies an AC voltage + DC voltage or a DC voltage by a corona discharge method, and collectively transfers the superimposed toner images on the intermediate transfer belt 261 onto the recording paper.
[0016]
The recording paper cassette 207 in the color printer 12 and the recording paper cassettes 30a, 30b, and 30c in the paper supply bank 13 store recording papers of various sizes. The paper is fed and conveyed in the direction of the registration roller pair 29 by the paper feed rollers 28, 31a, 31b, and 31c. Further, a manual feed tray 21 for manual paper feed such as OHP paper or thick paper is provided on the right side in the drawing of the printer 12.
[0017]
In the color copying machine having the above configuration, when an image forming cycle is started, first, the photosensitive drum 20 is rotated counterclockwise as indicated by an arrow, and the intermediate transfer belt 261 is rotated clockwise as indicated by an arrow by a drive motor (not shown). As the intermediate transfer belt 261 rotates, Bk toner image formation, C toner image formation, M toner image formation, and Y toner image formation are performed, and finally, on the intermediate transfer belt 261 in the order of Bk, C, M, and Y. Overlaid, a toner image is formed.
[0018]
The Bk toner image is formed as follows. The charger 203 uniformly charges the photosensitive drum 20 to about −700 V with a negative charge by corona discharge. The semiconductor laser 221 performs raster exposure based on the Bk color image signal. When this raster image is exposed, the charge proportional to the exposure light amount disappears in the exposed portion of the photosensitive drum 20 that is initially uniformly charged, and a Bk latent image is formed. When the negatively charged Bk toner on the Bk developing sleeve comes into contact with the Bk latent image, the toner does not adhere to the remaining portion of the photosensitive drum 20, and the portion without charge, that is, the exposed portion is exposed. The Bk toner is attracted to the portion, and a Bk toner image similar to the electrostatic latent image is formed. The Bk toner image formed on the photosensitive drum 20 is transferred by the belt transfer unit 265 to the surface of the intermediate transfer belt 261 that is driven at a constant speed in contact with the photosensitive drum 20 (hereinafter referred to as a photosensitive drum). Transfer of the toner image from the body drum 20 to the intermediate transfer belt 261 is referred to as belt transfer).
[0019]
Some untransferred residual toner on the photoconductor drum 20 is cleaned by the photoconductor cleaning device 201 in preparation for reuse of the photoconductor drum 20. The collected toner is stored in a waste toner tank (not shown) via a collection pipe.
[0020]
On the photosensitive drum 20 side, the process proceeds to the C image forming process after the Bk image forming process, and C image data reading by the color scanner 11 is started at a predetermined timing, and C latent image formation is performed by laser beam writing using the C image data. Done. Then, after the rear end portion of the previous Bk latent image passes and before the front end portion of the C latent image arrives, the revolver developing unit 23 is rotated, and the C developing unit 231C is set at the developing position. The C latent image is developed with C toner. When the development of the C latent image area is continued and the rear end of the C latent image passes the development position, the revolver developing unit 23 is rotated in the same manner as in the case of the previous Bk developing unit 231B, and the next M The developing device 231M is moved to the developing position. This is also completed before the leading edge of the next M latent image reaches the developing position.
[0021]
The description of the M and Y image forming steps will be omitted because the operations of reading color image data, forming an electrostatic latent image, and developing are the same as those in steps Bk and C described above.
[0022]
On the intermediate transfer belt 261, Bk, C, M, and Y toner images sequentially formed on the photosensitive drum 20 are sequentially aligned on the same surface to form a four-color superimposed toner image. In the transfer process, the four color toner images are collectively transferred to the recording paper by the paper transfer unit 263.
[0023]
At the time when the image forming operation is started, the recording paper is fed from either the recording paper cassette or the manual feed tray and is waiting at the nip of the registration roller pair 29. When the leading edge of the toner image on the intermediate transfer belt 261 approaches the paper transfer unit 263, the registration roller pair 29 is driven so that the leading edge of the recording paper coincides with the leading edge of the toner image, and the recording paper and toner Registration with the image is performed. Then, the recording paper is superimposed on the toner image on the intermediate transfer belt 261 and passes over the positive potential paper transfer device 263. At this time, the recording paper is charged with a positive charge by the corona discharge current, and the toner image is transferred onto the recording paper. Subsequently, the recording paper is neutralized and peeled off from the intermediate transfer belt 261 when passing through a portion facing a separation static eliminator by an AC + DC corona (not shown) that should be arranged on the left side in the drawing of the paper transfer unit 263. Move to conveyor belt 211.
[0024]
Then, the recording paper onto which the four-color superimposed toner images are collectively transferred from the surface of the intermediate transfer belt 261 is conveyed to the fixing device 27 by the conveying belt 211, and the nip portion between the fixing roller 271 and the pressure roller 272 controlled to a predetermined temperature. Then, the toner image is melted and fixed, sent out of the apparatus main body by the discharge roller pair 32, and stacked on the copy tray (not shown) so as to obtain a full color copy.
[0025]
On the other hand, the surface of the photosensitive drum 20 after the belt transfer is cleaned by the photosensitive member cleaning device 201 (brush roller, rubber blade), and is uniformly discharged by the discharging lamp 202. Further, the surface of the intermediate transfer belt 261 after the toner image is transferred to the recording paper is cleaned by pressing the blade of the belt cleaning device 262 again by the blade contact / separation mechanism.
[0026]
Next, the revolver developing unit 23 will be described. FIG. 2 is a sectional view showing the internal structure of the revolver developing unit 23 in which the developing units 231K, 231C, 231M, and 231Y are integrated. The developing devices 231K, 231C, 231M, and 231Y of the revolver developing unit 23 are respectively supported by a hollow square cylindrical stay member 242 provided between front and rear end plates (not shown). The developing units 231K, 231C, 231M, and 231Y are provided with the same type of developing unit casing portions 283K, 283C, 283M, and 283Y, respectively. Each of the developing device casing portions 283K, 283C, 283M, and 283Y contains a carrier as a developer and a two-component developer composed of toner of each color. In the illustrated example, the Bk developing unit 231K containing black toner and carrier is located at the developing position facing the photosensitive drum 20, and the Y developing unit 231Y containing yellow toner and carrier in the counterclockwise order in FIG. An M developing unit 231M containing magenta toner and a carrier and a C developing unit 231C containing cyan toner and a carrier are provided.
[0027]
Here, since the internal structure of each of the four developing devices is exactly the same, the internal structure will be described below by taking the Bk developing device 231K at the developing position in FIG. 2 as an example, and the internal structures of the other developing devices are As the reference numerals of the members to be used, the same numerals as those in the Bk developing unit are attached with the suffixes Y, M, and C to distinguish the yellow, magenta, and cyan developing units, and the description thereof is omitted. .
[0028]
In the Bk developing device 231K, a developing roller 284, which is a developer carrying member, is disposed so as to be close to the photosensitive member 20, and a developing region is formed at both opposing portions. The developing roller 284 is provided with a developing sleeve 285 formed of a non-magnetic material such as aluminum, brass, stainless steel, or conductive resin in a cylindrical shape so as to be rotated in a clockwise direction by a rotation driving mechanism (not shown). ing. In this example, the drum linear speed of the photosensitive drum 20 is set to 200 mm / second, and the sleeve linear speed of the developing sleeve 43 is set to 360 mm / second. Therefore, the ratio of the sleeve linear velocity to the drum linear velocity is 1.8. The development gap, which is the distance between the photosensitive drum 20 and the development sleeve 43, is set to 0.35 mm.
[0029]
In the developing sleeve 285, a magnet roller 286 that forms a magnetic field is provided in a fixed state so as to cause the developer to stand up on the peripheral surface of the developing sleeve 285. The carrier constituting the developer is spiked in a chain shape on the developing sleeve 285 so as to follow the normal magnetic field lines emitted from the magnet roller 286, and the carrier that has spiked in the chain shape is charged. Toner is attached to form a magnetic brush. The magnetic brush is transferred in the same direction as the developing sleeve 285 (clockwise as viewed in the figure) by the rotational transfer of the developing sleeve 285. The magnet roller 286 includes a plurality of magnetic poles. Specifically, as shown in detail in FIG. 3, a developing main magnetic pole P1b for causing the developer to stand up in the developing region, main magnetic pole forming auxiliary members P1a and P1c for assisting the formation of the developing main magnetic pole, and a developing sleeve 285 Magnetic poles P4 and P5 for pumping up the developer, magnetic poles P6, P7 and P8 for transporting the pumped developer to the development area, and magnetic poles P2 and P3 for transporting the developer in the developed area are provided. These magnetic poles P1b, P1a, P1c, P4, P5, P6, P7, P8, P2 and P3 are arranged toward the radial direction of the developing sleeve 285. The measurement of the magnet roller was performed using the ADS TS-10A probe and Gauss meter HGM-8900S described above. The position of the Hall element for measuring the magnetic flux density in the normal direction and the tangential direction was set to 0.5 mm from the sleeve surface.
[0030]
The magnet roller 286 is composed of 10 poles. However, in order to improve the drawability and black solid image followability, the magnet roller 286 may be composed of 12 poles by further increasing the number of magnetic poles from the P3 pole to the doctor blade. Regarding the shape of the magnets, the sectional shape of each magnet inside the magnet roller may be a square, but other shapes such as a fan shape and a bamboo ring shape may be considered.
[0031]
As can be recognized in FIG. 2, the development main pole group P1 is composed of magnets having small cross sections arranged in the order of P1a, P1b, and P1c from the upstream side. These magnets having a small cross section are made of a rare earth metal alloy, but samarium alloy magnets, particularly samarium cobalt alloy magnets, can also be used. Among rare earth metal alloy magnets, the typical iron neodymium boron alloy magnet has a maximum energy product of 358 kJ / m.³The maximum energy product of iron neodymium boron alloy bonded magnet is 80 kJ / m³Before and after. Unlike conventional magnets, such a magnet can ensure the required developing roller surface magnetic force even if the size is considerably reduced. For conventional normal ferrite magnets and ferrite bonded magnets, the maximum energy product is 36 kJ / m.³Before and after, 20kJ / m³Before and after. When it is acceptable to increase the sleeve diameter, it is possible to narrow the half-value width by forming the tip of the magnet facing the sleeve side using a conventional ferrite magnet or ferrite bonded magnet. Here, the full width at half maximum is the maximum normal magnetic force (vertex) of the magnetic distribution curve in the normal direction or half the peak magnetic flux density (for example, the maximum normal magnetic force of a magnet made of N poles is 120 mT (millitesla)). In this case, the half value is 60 mT). It may be called a half-value central angle and a half-value center angle width.
[0032]
Returning to the normal magnetic force pattern by the magnet roller in the present invention shown in FIG. The solid line is a pie chart graph measuring the magnetic flux density in the normal direction on the surface of the developing sleeve. The scale is every 20 mT. This is compared with the magnetic flux density in the normal direction at a distance of 1 mm from the surface of the developing sleeve. These are the measurement device used, the ADS Gauss meter (HGM-8300), and the ADS A1 type axial probe. In the magnet roller in this example, the magnetic flux density in the normal direction on the sleeve surface of the main magnetic pole P1b is 108.1 mT, and the normal direction magnetic flux density at a portion 1 mm away from the sleeve surface is 50.2 mT. The change in magnetic flux density was 57.9 mT. Attenuation rate of normal direction magnetic flux density at this time (the difference between the normal direction magnetic flux density peak value on the sleeve surface and the normal direction magnetic flux density peak value at a distance of 1 mm from the sleeve surface The ratio divided by the peak value of the magnetic flux density in the linear direction) is 53.6%. The normal magnetic flux density on the sleeve surface of the main magnetic pole forming auxiliary magnet P1a located upstream of the main magnetic pole P1b is 100.1 mT, and the normal magnetic flux density at a portion 1 mm away from the sleeve surface is 53 The magnetic flux difference was 46.7 mT, and the magnetic flux difference was measured. The attenuation factor of the normal direction magnetic flux density at this time is 46.7%. The normal magnetic flux density on the sleeve surface of the main magnetic pole magnetic force forming auxiliary magnet P1c located downstream of the main magnetic pole P1b is 108.0 mT, and the normal magnetic flux density at a portion 1 mm away from the sleeve surface is 60 The magnetic flux difference was 47.3 mT, and the magnetic flux difference was 47.3 mT. The attenuation factor of the normal direction magnetic flux density at this time is 43.8%. In this example, the magnetic brush formed of the developer along the magnetic force lines generated on the magnet roller is in contact with only the brush portion formed on the main magnetic pole P1b, and the electrostatic latent image on the photoconductor is exposed. Image. At this time, if the photoconductor is not in contact, the length of the magnetic brush at that location is about 1 mm, and the head is shorter than the magnetic brush formed by the conventional magnet roller, and a dense state is created. Became possible. When the distance between the developer regulating member and the developing sleeve is the same as the conventional one, since the amount of developer passing through the developer regulating member is the same, the magnetic brush in the development area is short and dense. It was confirmed that This phenomenon can be understood from the normal magnetic force pattern of FIG. 3, and the magnetic flux density in the normal direction at 1 mm away from the developing sleeve surface is greatly reduced. It cannot be formed, and the magnetic brush is short and densely formed on the surface of the developing sleeve. Experimentally, it has been found that there is a correlation between the increase / decrease of the normal magnetic flux density attenuation rate at a certain magnetic pole and the increase / decrease of the half-value width, and that the attenuation rate increases by reducing the half-value width. is doing. In order to set the attenuation factor of the normal direction magnetic flux density to 40% or more, the full width at half maximum should be 22 ° or less, preferably 18 ° or less. Incidentally, in the conventional magnet roller, the normal magnetic flux density on the sleeve surface of the main pole is 90 mT, and the normal magnetic flux density at a portion 1 mm away from the sleeve surface is 63.9 mT. Measured a magnetic force difference of 26.1 mT. The attenuation factor of the normal direction magnetic flux density at this time is 29%.
[0033]
FIG. 4 shows the relationship between the ratio of the line width of 1 dot vertical and horizontal lines and the attenuation factor of the normal direction magnetic flux density of the main magnetic pole (P1b). If the ratio of the line widths of 1 dot vertical and horizontal lines on the vertical axis is 1, the widths of the vertical and horizontal lines are equal. The 80% line is a border line where the horizontal line is conspicuous below this. In other words, if it is above the line, there is no practical problem. From this graph, it can be seen that the horizontal thinning is eliminated in the magnet roller used in this example. Therefore, if the attenuation factor of the normal direction magnetic flux density is 40% or more, the whiteout at the rear end of the image and the thinning of the horizontal thin line are eliminated. Further, it was confirmed that the magnetic pole adjacent to the main magnetic pole was also free from the trailing edge of the image and the thinning of the horizontal thin line due to the attenuation factor of the normal direction magnetic flux density of 40% or more.
[0034]
The measuring method which measured the said magnetic flux density is shown. Measured with an ADS Gauss meter (HGM-8300) and an ADS A1 type axial probe, it was recorded with a pie chart recorder. When measuring the normal direction magnetic flux density on the surface of the developing sleeve, attach an axial probe so that it contacts the developing sleeve, rotate the magnet roller, measure 360 degrees in steps of 0.1 degree, Record on chart recorder. Next, the tip of the axial probe was fixed 1 mm higher than the surface of the developing sleeve, and the magnetic roller was rotated 360 degrees in the same manner as described above to measure the magnetic flux density.
[0035]
By configuring the normal magnetic force as described above, it is possible to form a state in which the development nip width is narrow, suppress the occurrence of developer accumulation on the upstream side of the development nip, and to form a stable development nip. An image having no white spots or thin horizontal line images can be obtained, and as a result, a beautiful image with high dot uniformity can be provided.
[0036]
In the developing device having the developing roller having the above-described configuration, a doctor blade that regulates the amount of developer carried on the developing roller 284 and conveyed to a portion facing the photosensitive drum 20 in the developing device casing portion 283K. 287, a first conveying screw 288 that conveys the negative portion of the developer, which is regulated by the doctor blade 287 and pressed into the developing device casing, along the central axis direction from the back to the front, and along the central axis direction. A second conveying screw 289 that conveys the developer in a direction opposite to the first conveying screw 288 is disposed. In the developing device casing portion 283K below the second conveying screw 289, a toner concentration sensor for detecting the toner concentration of the developer contained in the developing device casing portion 283K is installed.
[0037]
Further, as shown in FIG. 5, the coating member 209 built in the charging device and disposed on the surface of the photosensitive member 20 has a solid lubricant 424 disposed on a fixing jig 421 via a spring. It is also possible to incorporate the coating member in a cleaning device or another device which is another image forming apparatus. A similar coating member is also disposed in the intermediate transfer device 26.
[0038]
The solid lubricant 424 fixed by the fixing jig 421 is in contact with the brush portion 422 by the pressure of a pressing member (not shown), and the lubricant is scraped off by the brush 422. At this time, the pressing pressure of the pressing member is variable, and the application amount is changed by a change in the pressure pressing the lubricant against the brush 422. By changing the coating amount, the friction coefficient on the surface of the photoreceptor and the surface of the transfer member is changed.
[0039]
As a lubricant material, it is desired to be chemically inactive and to have high thermal stability, such as low surface energy, and specifically zinc stearate, barium stearate, stearic acid. Examples thereof include fatty acid metals such as iron, nickel stearate, and magnesium stearate, and fluorine-based polymers such as polytetrafluoroethylene (PTFE) and PFA. In this example, zinc stearate was used. The reason is that a molding process is easy and a stable image can be obtained without adversely affecting image formation.
[0040]
As another method for applying the lubricant to the photosensitive member, there is a means of changing the coating amount by changing the linear speed ratio of the brush to the photosensitive member. As shown in FIG. 6, the solid lubricant 432 can be directly pressed against the photoconductor 20 by a fixing jig 431 that also serves as a pressing member, and the application amount and the friction coefficient can be changed by changing the pressure.
[0041]
In this example, in order to reduce the friction coefficient μ of the photoconductor 20, a mechanism for applying a lubricant to the photoconductor 20 is provided. However, in a system where the friction coefficient is as low as this example due to the difference in the photoconductor composition and the manufacturing method. Also, the effect of the present invention has been confirmed.
[0042]
The relationship between the coating amount and the coefficient of friction of the photoreceptor surface will be described. FIG. 7 shows the relationship between the coating amount and the friction coefficient (hereinafter referred to as μ). By applying a lubricant to the photoconductor, the photoconductor μ changes with a gradient as shown in the figure. The photosensitive member μ converges to a certain value rather than infinitely close to zero. This value depends on the composition of the photoconductor before coating, the surface condition, and the environmental conditions (especially humidity). In the present invention, the photoreceptor μ was measured by the Euler method. An outline of this measuring apparatus is shown in FIG. In this measurement, the photosensitive member μ converged without limitation to 0.02 (the photosensitive member μ before applying the lubricant was 0.7). However, this value differs depending on the case where other measurement methods are used and the measurement environment. This measurement was performed in a standard environment of RH 65% and temperature 23 ° C.
[0043]
Returning to FIG. 2, a photosensor 510 composed of a light emitting element and a light receiving element is arranged in the vicinity of the developing unit 23. The density of the reference pattern is measured by installing a sensor at such a position shown in the figure, and this information is input to a control means having a CPU to control the lubricant application amount and parameters related to development.
[0044]
FIG. 9 shows the relationship between the toner adhesion amount on the photoconductor and the output of the reflection type photosensor. For example, a latent image having a size of about 2 cm × 2 cm is created on the photoconductor as the reference pattern. Since this pattern is sensitive to changes in conditions, a halftone such as a halftone is desirable, but it can also be controlled by a black solid image.
[0045]
The change in the amount of lubricant applied also affects the development γ curve as shown in FIG. In this graph, the image density is plotted on the vertical axis and the development potential is plotted on the horizontal axis. As shown in this graph, when the amount of applied lubricant is small, it becomes higher than the designed development γ. On the other hand, when the application amount is large, the value is low. Therefore, it is necessary to immediately correct the development γ to the design value. In the present invention, for example, the developer application amount is controlled as shown in the flowchart of FIG. That is, a reference density pattern is formed after setting the application brush linear speed as a standard, and the reflection density of the reference pattern is read. If the reference brush pattern is higher than a certain reference value (upper limit side), the application brush linear speed is increased. If it is lower than another reference value (lower limit side), the reflection brush density is controlled within the appropriate range by controlling the application brush linear velocity to be reduced. By controlling in this way, a stable image can always be obtained.
[0046]
The inventors of the present invention have a magnet roller (magnet roller of the first example) provided with an auxiliary magnetic pole for adjusting the magnetic force and the half-value width of the main magnetic pole adjacent to the main magnetic pole adjacent to the image carrier, and a conventional type. The experiment was conducted in detail using a magnet roller (with a half-width of the developing magnetic pole of 48 °). The relationship between the trailing edge blank and the photoreceptor μ is shown below. The data of the conventional type magnet roller is also attached as a reference.
[0047]
FIG. 12 shows the rank evaluation of the level of trailing edge blanking with respect to the photoreceptor surface μ after applying the lubricant. The rank evaluation of the trailing edge blank level is a rank 5 where no gap is present at the trailing edge of the halftone image, and a rank 1 is the most missing (for example, at the rank 1 level, the photosensitive member). In a system using a conventional magnet roller with a linear speed of 200 mm / s, a developing gap of 0.35 mm, a linear speed ratio of the photosensitive member developing sleeve of 1.8, an AC bias frequency of 9 kHz, and a photosensitive member μ of 0.2, after image An image loss of 4.2 mm occurred from the end). The photoreceptor surface friction coefficient is plotted up to μ: 0.7 in the figure. However, when μ is 0.6 or more, the cleaning blade is worn due to running of about 10K, and its function is remarkably deteriorated. Further, the toner causes filming on the surface of the photosensitive member, thereby degrading the image quality. As shown in the figure, the conventional type magnet roller has a photoreceptor μ of 0.2 and a trailing edge blank rank of rank 1. On the other hand, when the magnet roller according to the present invention is used, the trailing edge blank rank is maintained at 5 even if the photosensitive member μ is changed from 0.5 to 0.1. With respect to the magnet roller according to the present invention, an experiment was conducted on a smaller photosensitive member μ. As shown in FIG. 13, the rear end blank rank when the photosensitive member μ is 0.1 or less shows a satisfactory level when the photosensitive member μ is 0.02 or more. When the photoreceptor μ is less than 0.02, the trailing edge blank rank is remarkably lowered. This is because a toner having a particle diameter of 6.5 μm was used in the experiment, and in the system using a toner having a smaller particle diameter, the decrease in the trailing edge white-out level is small.
[0048]
Further, the magnet roller according to the present invention is particularly excellent in the system in which the photosensitive member μ is 0.4 or less as compared with the white level at the rear end of the conventional type magnet roller. When the initial photoreceptor μ is 0.4 or less, it has been impossible to maintain a sufficient trailing edge blanking level with a conventional type magnet roller. Generally, in a system that does not have a lubricant application system, the photosensitive member μ increases with time (due to scraping of the surface of the photosensitive member due to contact with a cleaning blade or a magnetic brush). There is a recovery of the trailing edge blank level.
[0049]
Next, the relationship between the photosensitive member developing sleeve linear velocity ratio and the trailing edge white spot will be described. As in the case of the relationship between the photoconductor μ and the trailing edge white spot, a conventional magnet roller is added to FIG. 14 as a reference. As shown in the figure, in the magnet roller according to the present invention, it was possible to maintain a very good rank with respect to the trailing edge white spot up to a linear speed ratio of 4.0. On the other hand, in the case of the conventional magnet roller, the trailing edge blank rank decreased at a line speed ratio of 1.5 and decreased to rank 1 at a line speed ratio of 2.0.
[0050]
Furthermore, the blank rank around characters was evaluated for the magnet roller of the present invention. In the rank evaluation of the white space around the character, a character having no missing character at the boundary between the character and the surrounding halftone image is defined as a rank 5, and the character having the most missing character is defined as a rank 1. The result of this experiment is shown in FIG. Like this graph, the rank was improved by increasing the linear speed ratio. As described above, by using the developing roller having a magnet configuration as in the present invention, it is possible to achieve both white spots at the rear end and missing letters at the periphery of the character.
[0051]
Next, a mechanism for applying the lubricant to the intermediate transfer belt (primary transfer process) will be described. A lubricant application brush 266 for the intermediate transfer belt 261 was installed at a position in a belt cleaning device (not shown) shown in FIG. Since the intermediate transfer belt 261 is in contact with the photoreceptor 20 to form a nip, it is desirable to use the same lubricant material as that of the photoreceptor coating apparatus. Similar to the application of the lubricant to the photosensitive member, the application amount can be changed by changing the contact pressure of the application brush (not applied when the contact pressure is 0) or the linear velocity of the brush. Since this is the same mechanism as that for applying the lubricant to the photoconductor (FIGS. 5 and 6), a further illustration is omitted. The material of the application brush is conductive acrylic fiber (20,000 / inch)²= 3100 / cm²)It was used.
[0052]
By applying a lubricant to the intermediate transfer belt 261, the frictional force between the photoconductor 20 and the intermediate transfer belt 261 is reduced, so that the life of the photoconductor and the intermediate transfer belt is dramatically increased. In addition, since toner filming on the belt can be prevented, the surface energy of the primary transfer is reduced in the secondary transfer process in which the toners of the respective colors are stacked in the primary transfer process and then transferred to the transfer material in a batch. The image can be suppressed, transferability is improved, and an image with no omission in the image area can be obtained even with lapse of time.
[0053]
By the way, the surface energy (surface tension) can be expressed by the following equation using the contact angle θ with respect to the reagent on the surface.
W = γ (1 + cos θ) (1)
In the equation (1), W is the surface tension of the material to be measured, γ is the surface tension of the reagent, and θ is the contact angle between the material to be measured and the reagent. FIG. 16 shows an outline of the reagent and the material to be measured. Surface tension is commonly used as a surrogate property of surface energy.
[0054]
As the reagent, a pure substance such as pure water is used. A change in surface tension is measured by measuring the wettability of a material to be measured using a reagent having the same surface tension. As the surface tension increases, the adhesion between two different substances increases. This equation is used to determine the surface tension (critical surface tension) of the material to be measured with respect to the reagent (liquid), but it is widely used as a method to know the change in the adhesion force of the powder adhering to the surface of the object to be measured. It has been.
[0055]
Experiments were performed to determine the difference between images on the intermediate transfer belt in three environments by changing the surface energy of the photoreceptor 20 and the intermediate transfer belt 261. The three environments are related to temperature and humidity, and are HH (30 ° C., 90%), MM (23 ° C., 65%), and LL (10 ° C., 15%). The experimental results are shown in FIG. However, each experimental data is an initial tendency, and does not represent image fluctuations due to changes over time. The horizontal axis represents each environment, and the vertical axis represents the worm-eating rank. Here, as for the worm-eating rank, as in the case of the rear end blank dropout rank, a dropout is defined as rank 5 and the dropout rank is rank 1. Since it cannot be confirmed visually at rank 4 or higher, all images were taken into the CCD camera and binarized, and then judged from changes in the area ratio of the image area. However, the solid image part was used for the evaluation image. The outline is shown in FIG.
[0056]
As a result of the experiment, as shown in FIG. 17, when the condition of the surface energy of the photoconductor <the surface energy of the intermediate transfer belt was satisfied, the image could be stably transferred without being affected by environmental fluctuations. Under other conditions, images of rank 4 or higher were not obtained in any environment. In particular, in an HH environment, a large number of insect eaters occurred, and image quality was not acceptable at all.
[0057]
Next, a short running test up to 3K (3000 sheets) was performed under the above-mentioned surface energy conditions. The result is shown in FIG. However, the running environment is MM (23 ° C., 65%). As shown in the figure, the condition of the surface energy of the photoconductor <intermediate transfer belt did not decrease the worm-eating rank even when running. Under other conditions, at 0.5K and 1K, filming occurred in which the toner adheres to the photosensitive member in a film form, so that it was impossible to continue running due to significant deterioration in image quality. Under the condition of the surface energy of the photoreceptor <intermediate transfer belt, the running was continued up to 20K, but no problem occurred.
[0058]
Next, the alternating electric field in the present invention will be described. As described above, in order to use an alternating electric field as the developing electric field, in the present invention, an AC bias superimposed on a DC bias is used as the developing bias. As shown in FIG. 20, this developing bias is applied to the developer carrying member from the photosensitive member side with a voltage application time T1 for forming an electric field in the direction in which the toner on the developer carrier is caused to fly to the latent image on the photosensitive member. Application time of voltage for forming an electric field acting in the direction of pulling back in the body direction: T2 is alternately repeated every cycle. By applying this developing bias to the developer carrying member for a certain period of time, the toner is developed on the photoreceptor. The generation time of the electric field acting in the direction of developing the toner on the photoconductor per one AC bias cycle is called Duty, and is represented by the following equation.
Duty = T1 / (T1 + T2) × 100 (%) (2)
[0059]
Further, the DC bias component Voff at the time of AC bias superposition was set as in the following equation so that the time-average development bias was constant.
Voff = Vb + (1 / 2− (Duty / 100)) × Vpp (V) (3)
Here, Vb is a DC component when no AC component is superimposed, and Vpp is a peak-to-peak voltage of AC bias. Voff was appropriately corrected within a range of ± 20V.
[0060]
The experimental conditions are
Development gap Gp: 0.35 mm
Developer pumping amount ρ: 0.065 mg / cm²
Photoconductor linear velocity Vs: 200 mm / sec
Photoconductor sleeve linear speed ratio: 1.3
Vpp: 0.8kV
It is.
[0061]
In the system of the present invention, the output image difference depending on the duty of the AC bias and the frequency was evaluated. The evaluation results are shown in Table 1. In the present invention, an image in which the duty was changed from 10% to 70% was evaluated.
[0062]
[Table 1]

[0063]
As the evaluation image, a 256-step patch image created by a method of changing the exposure amount of the exposure means was adopted (resolution: 600 dpi). The reason is that the highlighted part of the 256-step patch image created by this method is easily affected by changes in the duty of the alternating electric field and the Vpp voltage, and is most suitable for evaluating the roughness of the halftone part. Can be mentioned. This is because the latent image in the highlight portion has a shallow potential after exposure, so that the contrast of the latent image is small and the amount of toner developed per dot is inevitably reduced. For this reason, a slight change in the toner adhesion amount leads to missing dots, and the roughness is easily noticeable. Speaking only in the alternating electric field, since the exposure potential is shallow, the electrostatic force received by the toner increases during the time when the electric field that pulls the toner back to the developing sleeve acts when developing. For this reason, the probability that the toner electrostatically attached to the latent image on the photosensitive member or the toner flying to the latent image on the photosensitive member is pulled back to the developing sleeve is greatly increased, and the roughness is reduced.
[0064]
In a 256-level patch image, the roughness is evaluated at a portion where the toner adhesion amount is relatively small, and image evaluation is performed in the region of 60 to 80 in terms of brightness (especially, the roughness is liable to deteriorate near brightness 70). is there). At these lightnesses, rank evaluation was performed visually based on rank samples. Here, the roughness rank is defined as rank 5 with the most uniform halftone part, and rank 1 with the least uniform feeling (as in the conditions in the table). There is no rank 1. For example, the conditions of Duty = 60%, Vpp: 1.2 kV, Voff: −500 V are rank 1). The roughness is good with a duty of 50% or less and a rank of 4 or more, but decreases in a region where the duty exceeds 50%.
[0065]
For evaluation of fine line reproducibility, a 1-dot vertical line written in binary was used. The reason is that the 1-dot vertical line is relatively sensitive to changes in the development γ, and it is easy to determine a reduction in fine line reproducibility due to development γ fluctuations. This line image was read by a CCD camera and then evaluated by a method of actually measuring the line width of the binarized image. A center value of 85 μm was set as the target, and a sample of 100 μm or more and 70 μm or less of actual measurement was evaluated as “x” and described in the table.
[0066]
FIG. 21 shows a typical development γ curve (duty toner adhesion amount curve with respect to the number of output patches) by Duty. As shown in the figure, the development γ increases in an area exceeding Duty = 50%. The resulting effect is that the fine line reproducibility is lacking and the image has poor gradation. Furthermore, the background dirt level is remarkably lowered and the basic image quality cannot be satisfied. When Duty = 50% or less, an image that does not deteriorate the basic quality such as background stains can be obtained. Conversely, when Duty = 10%, the developing ability is low.
[0067]
The relationship between the frequency and the roughness is as shown in FIG. At this time, the charge amount of the toner was −20 μc / g as measured by the blow-off method. As shown in the figure, a good halftone was obtained in the range of 2.5 kHz to 5.0 kHz. In a lower frequency region, the image becomes a DC bias-like image in which an alternating electric field is not superimposed, an AC bias effect cannot be obtained, and the halftone roughness is not improved. This is because it is difficult to obtain an AC electric field effect that faithfully reproduces the latent image by vibrating the toner attached to the latent image. On the higher frequency side, the toner adhesion amount in the highlight portion is reduced as compared with the frequency region of 2.5 kHz to 5.0 kHz, and dot missing increases. As a result, roughness is reduced. The charge amount distribution of the toner used in this experiment is shown in FIG. As shown in the figure, the vertical axis represents the number of toners, and the horizontal axis represents Q / d (femt C / 10 μm). For this measurement, an E-spart analyzer manufactured by Hosokawa Micron Corporation was used.
[0068]
As the next experiment, the roughness of the halftone portion due to the frequency was similarly evaluated using a toner having a toner charge amount distribution narrower than that described above. The charge amount distribution is shown in FIG. 24, and the experimental results are shown in FIG. This toner was produced with the same formulation as that used in the above experiment, but the charge amount in the blow-off method was -25 μc / g by narrowing the distribution range. Unlike the result in the case of the distribution shown in FIG. 23, no reduction in roughness on the high frequency side was confirmed. The reason for this is that when the AC bias is moved to the high frequency side, it is not developed with a toner that has a poor response to the developing electric field (a toner that is lowly charged and difficult to develop). This is because the system in which the toner is cut is sufficiently developed. That is, in order to use the AC bias at a high frequency, it can be said that it is essential to narrow the charge amount distribution.
[0069]
The above has been described with respect to an example in which a magnet roller having a configuration in which an auxiliary magnetic pole for adjusting the magnetic force and the half-value width of the main magnetic pole is provided adjacent to the developing main magnetic pole, but is not limited thereto. The same effect can be obtained if the magnet roller has a normal magnetic flux density attenuation rate of 40% or more of the developing main pole.
[0070]
【The invention's effect】
  According to the present invention, the static friction coefficient μ with respect to the paper of the image carrier measured by the Euler method in a standard environment at a temperature of 23 ° C. and a relative humidity of 65% is 0.02 or more and less than 0.6. When the attenuation rate of the magnetic flux density in the normal direction of the magnetic pole is 40% or more and an alternating electric field is formed in the development region, it is possible to improve the halftone roughness while maintaining the trailing edge white level high. .When the frequency of the alternating electric field is 2.5 to 5.0 kHz, the toner can be uniformly attached to the halftone.
[0071]
  When a rectangular wave is used as an alternating electric field formed by superimposing an AC bias on a DC bias, an electric field application time T1 for moving the toner from the developer carrier to the image carrier per one period of the rectangular wave; The electric field application time T2 for moving the toner from the image carrier to the developer carrier satisfies the relationship of T1 ≦ T2, in other words, by setting the duty of the AC bias to 50% or less. In addition to the above effect, it is easy to secure the toner density (ID) on the formed image.. AlsoIf all the toner falls within ± 5 femt C / 10 μm from the peak position of the toner charge amount distribution, the toner can be uniformly attached to the halftone over time, and the roughness can be improved. If a lubricant application mechanism is provided, it can be corrected to an appropriate μ even when the image carrier μ changes, and a stable image can be obtained over time. If the initial coefficient of friction μ of the image carrier is μ ≦ 0.4, the film of the image carrier can be prevented from being scraped and the life can be extended.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a color copying machine as an image forming apparatus according to the present invention.
FIG. 2 is a partial schematic configuration diagram of a revolver developing unit installed in the color copying machine of FIG. 1;
3 is a diagram showing a magnetic distribution of a developing roller in the revolver developing unit of FIG. 2 and a magnitude thereof. FIG.
FIG. 4 is a graph showing the relationship between the ratio of the line width of 1-dot vertical and horizontal lines and the attenuation factor of the magnetic flux density in the normal direction of the main magnetic pole of the magnet roller.
FIG. 5 is a schematic configuration diagram of an application member.
6 is a configuration diagram of an application member as an alternative to FIG. 5. FIG.
FIG. 7 is a graph showing the relationship between the coating amount and the coefficient of friction of the photoreceptor surface.
FIG. 8 is a schematic view of a measuring apparatus for measuring a photoconductor friction coefficient.
FIG. 9 is a graph showing the relationship between the toner adhesion amount on the photoconductor and the output of the reflective photosensor.
FIG. 10 represents the relationship between the change in the amount of lubricant applied and the development γ curve, with the vertical axis representing image density and the horizontal axis plotting development potential.
FIG. 11 is a flowchart illustrating control of a developer application amount.
FIG. 12 is a graph showing a rank evaluation of the level of trailing edge blanking relative to the photoreceptor surface friction coefficient after application of a lubricant.
FIG. 13 is a graph of the rank evaluation of the trailing edge white spot level of the photoconductor friction coefficient of 0.1 or less in the magnet roller according to the present invention.
FIG. 14 is a graph for explaining the relationship between the photosensitive member developing sleeve linear velocity ratio and the trailing edge white spot;
FIG. 15 is a graph for explaining the relationship between the photosensitive member developing sleeve linear velocity ratio and missing characters.
FIG. 16 is a schematic view showing the positional relationship between the reagent and the surface of the material to be measured.
FIG. 17 is a graph showing differences in temperature and humidity changes of worm-eaten in an image on an intermediate transfer belt.
18 is a diagram showing an evaluation image in the experiment related to FIG.
FIG. 19 is a graph showing changes in worm-eating state depending on the number of running sheets.
FIG. 20 is a diagram illustrating a change in developing bias when an alternating electric field is used as the developing electric field.
FIG. 21 is a graph showing a development γ curve by a typical duty.
FIG. 22 is a graph showing the relationship between frequency and roughness.
23 is a graph showing a charge amount distribution of toner used in the experiment relating to FIG.
FIG. 24 is another graph showing the relationship between frequency and roughness.
FIG. 25 is a graph showing a charge amount distribution of toner used in the experiment relating to FIG. 24;
[Explanation of symbols]
1 Photosensitive drum
4 Development device
41 Developing roller
43 Development Sleeve
44 Magnet roller
45 Doctor blade
47 screw

Claims (7)

In an image forming apparatus comprising: a developing device having a developing main magnetic pole for magnetically adsorbing a developer on the outer peripheral surface of the developer carrying member to form a magnetic brush; and an image carrier facing the developing device.
The static friction coefficient μ of the image carrier to the paper measured by the Euler method in a standard environment at a temperature of 23 ° C. and a relative humidity of 65% is 0.02 or more and less than 0.6, and the normal direction of the developing main pole An image forming apparatus, wherein an attenuation rate of magnetic flux density is 40% or more, an alternating electric field is formed in a developing region, and a frequency of the alternating electric field is 2.5 to 5.0 kHz .   The image forming apparatus according to claim 1, further comprising a magnet for assisting magnetic force formation of the developing main magnetic pole.   The image forming apparatus according to claim 1, wherein a rectangular wave is used as the alternating electric field.   An electric field application time T1 for moving the toner from the developer carrier to the image carrier and an electric field application time T2 for moving the toner from the image carrier to the developer carrier per period of the rectangular wave are T1. The image forming apparatus according to claim 3, wherein a relationship of ≦ T 2 is satisfied. The image forming apparatus according to any one of claims 1 to 4, characterized in that all the toner falls from the peak position of the toner charge distribution within ± 5femtC / 10μm. The image forming apparatus according to any one of claims 1 to 5, characterized in that annexed lubricant application mechanism. The image forming apparatus according to any one of claims 1 to 6, wherein the initial coefficient of friction of the image carrier mu is mu ≦ 0.4.

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