JP3721282B2 - Image forming apparatus - Google Patents

Description

【００３４】
すると、バンディングの目立ち易さは、ギアの歯ピッチ（モジュールＭ）の大きさに加えて、潜像担持体上の光ビーム照射位置における曲率半径の２倍（＝感光ドラム１の直径Ｄｐ）、ギアの直径（Ｄｇ）、ビーム強度のピーク値が１／ｅとなる領域のビーム径（Ｌｂ）などとも相関があることがわかった。
【００３５】
図２は、上記表１の条件において、バンディングの発生周期Ｌｇ（π×Ｄp×Ｍ／Ｄｇ）と、ビーム強度が１／ｅとなる領域のビーム径Ｌｂ（以下、１／ｅ径Ｌｂという）との関係を示すグラフである。図２のグラフにおいて、直線Ａは方程式Ｌb／０．０６＋Ｌｇ／２＝１［ｍｍ］の解を示し、方程式Ｌb／０．０５＋（π×Ｄp×Ｍ／Ｄg）／１．５＝１［ｍｍ］の解を示している。また、×、△及び○印は、それぞれバンディングの目立ち易さの評価結果を示している。図示のように、直線Ａよりも下側の領域においては、バンディングが目立ち難くなることがわかる。更に、直線Ｂよりも下側の領域においては、より確実にバンディングが目立ち難くなることがわかる。
【００３６】
上述した各方程式の解を１［ｍｍ］より小さくしてバンディングを目立ち難くするためには、各方程式の左辺第一項の１／ｅ径Ｌｂを小さくしたり、左辺第二項の感光ドラムの直径Ｄｐ、ギアの直径Ｄｇ、ギアのモジュールＭなどを小さくすればよい。
【００３７】
従来、ビーム径としては、ビーム強度のピーク値の１／ｅ^２になる領域の副走査方向における径（以下、１／ｅ^２径という）が一般に用いられてきたが、本発明者の実験によれば、この１／ｅ^２径とバンディングの目立ち易さとには明確な相関は認められなかった。例えば、所定のＬＤ光学系を用い、このＬＤ光学系のパラメータを最適条件よりずらしてビームスポットのビーム強度分布を変化させた場合、１／ｅ^２径が変化してもバンディングの目立ち易さはあまり変わらなかった。また、光学系としてＬＤではなく、ＬＥＤ光学系を用い、ＬＥＤの発光部の大きさを変えて１／ｅ^２径を変化させても、この変化に伴うバンディングの目立ち易さの変化量は測定誤差中に含まれるくらい小さいものであった。
【００３８】
一方、それぞれパラメータの異なる複数のＬＤ光学系を用い、各光学系における１／ｅ^２径を最小にするパラメータ条件で実験した場合には、各光学系の１／ｅ径Ｌｂとバンディングの目立ち易さとに良好な相関が認められた。また、ＬＥＤ光学系においては、主走査方向のドット形成数に対応した個数のＬＥＤとレンズとを有するＬＥＤアレイとセルフォックスレンズアレイとの組み合わせを使用することになり、各ＬＥＤからのビーム強度分布を統一することには技術上の限界がある。このため、各ＬＥＤにおいて１／ｅ^２径や１／ｅ径Ｌｂにはバラツキがあるが、バンディングの目立ち易さに対して、１／ｅ^２径については相関が認められなかったが、１／ｅ径Ｌｂについては良好な相関が認められた。即ち、光書き込みユニットの光学系がＬＤであるかＬＥＤであるかにかかわらず、１／ｅ径Ｌｂとバンディングの目立ち易さとに良好な相関が認められた。
【００３９】
このように、１／ｅ^２径ではなく１／ｅ径Ｌｂとバンディングの目立ち易さとに良好な相関が認められる理由については定かではないが、次のようなことが関連していると考えられる。即ち、図３に示すように、１／ｅ径Ｌｂはビーム強度がピーク値の約３６．８［％］以上となる領域であり、ビーム強度分布曲線の変化が急峻に（測定点の変化に対する光量の変化が大きく）なる領域である。これに対し、１／ｅ^２径はビーム強度がピーク値の約１３．５［％］以上となる領域であり、ビーム強度分布曲線の裾引き部分になる領域である。ここで、光学系のパラメータを僅かに変化させた場合、１／ｅ径Ｌｂについてはそれほど変化させないが１／ｅ^２径については大きく変化させることになる。特に、ＬＤ光学系でビームスポットを小さく絞った場合には、この傾向が強くなる。これは、ＬＤ光の干渉の結果、いわゆるサイドローブと呼ばれる第二の山がビーム強度分布曲線のピーク値付近に生まれ、この山がビーム強度分布曲線の裾引き部分を持ち上げて、１／ｅ^２径を大きく変化させるためであると考えられる。静電潜像の形成には上記裾引き部分の強度のビーム部分はあまり寄与しておらず、専ら１／ｅ径Ｌｂ内のビーム部分で光書き込みして該静電潜像を形成しており、この１／ｅ径Ｌｂが大きくなると隣接画素に対する光書き込み量も大きくなる。逆に。１／ｅ径が小さくなると、隣接画素に対する光書き込み量が小さくなる。そして、このことにより、感光ドラムの速度変動により生ずるバンディングの濃度差も小さくなって、バンディングが目立ち難くなると考えられる。
【００４０】
図２に示したように、バンディングを目立ち難くするためには、１／ｅ径Ｌｂを小さくしたり、左辺第二項の感光ドラムの直径Ｄｐ、ギアの直径Ｄｇ、ギアのモジュールＭなどを小さくすればよい。しかし、１／ｅ径Ｌｂを小さくするためには、例えばＬＤ光学系のレンズ系を大きくしたり、ＬＤの発光波長を小さくしたりなどする必要があるが、これらはコストアップを招来するばかりでなく、感光ドラム上で安定的に３０［μｍ］以下の１／ｅ径Ｌｂを実現することが極めて困難である。また、感光ドラムの直径Ｄｐを小さくし過ぎると、帯電チャージャ２、光書き込みユニット３、現像装置４、一次転写装置５、除電装置７、クリーニングブレード８など、感光ドラム１の周囲における各ユニットの配設が困難になるので、直径Ｄｐの低減にも限界がある。また、ギアの直径Ｄｇには装置の大きさ上の制約があり、特に、中間転写ベルトの周囲に複数の感光ドラムを並列的に配設したいわゆるドラタンデム方式の画像形成装置の場合には、感光ドラム同士の間隔によりこのギアの直径Ｄｇが大きく制約される。また、ギアのモジュールＭを小さくし過ぎると、コストアップを招来するばかりでなく強度上の問題が生ずるので、モジュールＭの下限値は、実質的に０．５［ｍｍ］程度である。以上のことを考慮すると、１／ｅ径Ｌｂ、モジュールＭの下限値をそれぞれ３０［μｍ］、０．５［ｍｍ］とする条件で、感光ドラムに配設する各ユニットの大きさなどに応じて感光ドラムの直径Ｄｐやギアの直径Ｄｇを低減して、上述した各方程式の解を１［ｍｍ］より小さくすることが望ましい。
【００４１】
また、本発明者の実験によれば、感光ドラム２の上記感光層の厚みを１５［μｍ］以下に形成することにより、１／ｅ径Ｌｂを小さくしたときと同様の効果が認められた。このような効果が発揮される理由としては、上記感光層の厚みを小さくすることにより、該感光層内での電荷の拡散を小さくするためであると考えられる。また、上記感光層の厚みが薄いことにより、現像電界のＭＴＦ強調領域がより１［ｍｍ］よりも短い周期側にシフトし、小さなドットや細いラインを十分な濃度で現像できるようになったことも関与していると考えられる。
【００４２】
また、本発明者は次のような実験を行った結果、副走査方向における１／ｅ径Ｌｂと、上記感光層の厚みと、感光ドラム１のギアの噛み合わせによる速度変動とが同じであっても、転写紙１１の非画像部に適度なトナーチリを発生させた方が、バンディングを目立ち難くし得ることがわかった。即ち、図４に示すように、転写紙１１上における主走査方向（図面左右方向）に延在する各画素列Ｌ１、Ｌ２、Ｌ３、Ｌ４に対し、Ｌ１とＬ４とにライン画像を形成し且つＬ２とＬ３とを非画像部とするような条件（実際には、２つの画素列からなる非画像部を介してライン画像を形成する繰り返し）で作像を行った。そして、この転写紙１１の２００倍の顕微鏡写真を撮影し、非画像部の画素間に中心線Ａ−Ａ’を引き、トナーチリによってこの中心線Ａ−Ａ’上に掛かるトナーの割合を測定した。トナーチリの発生度合いを変化させながらこのような測定を繰り返し実行した結果、中心線Ａ−Ａ’対して全体の２０［％］以上の部分にトナーを付着させるようなトナーチリが生ずると、バンディングが目立ち難くなることがわかった。このようにバンディングが目立ち難くなる理由は定かでないが、ライン画像間の非画像部にある程度のトナーが存在しないと、該非画像部の白抜きラインが際立つためであると考えられる。
【００４３】
但し、副走査方向における非画像部の長さが１３０［μｍ］以下になるような場合に、該非画像部の中心線上に２０［％］を超える割合でトナーが存在する場合には、非画像部の白抜きラインにおける片幅６５［μｍ］以上の範囲にライン画像からのトナーが飛散・付着し、これにより該ライン画像が滲んだように見えて画像のシャープネスが低下することがわかった。具体的には、副走査方向における画素の長さ（画素の中心間隔）をＬとし、主走査方向に延在するｎ列の画素列における片端列に１本のライン画像を形成する動作を繰り返した場合において、Ｌ×（ｎ−１）<１３０［μｍ］となる条件で作像した場合に、画像のシャープネスが低下することがわかった。従って、副走査方向における非画像部の長さが１３０［μｍ］以下になるような場合に、該非画像部の中心線上に２０［％］以下の割合でトナーが存在させるようにトナーチリ量を制御する必要がある。例えば、６００［ｄｐｉ］の解像度で図４に示したようなライン画像を形成する場合には、副走査方向における画素の長さＬは、２５４００÷６００＝４２．３［μｍ］となる。このような場合に上記中心線上に２０［％］を超える割合でトナーが付着すると、白抜きラインの太さ４２．３×（３−１）＝８４．６［μｍ］が１３０［μｍ］よりも小さくなって画像のシャープネスが低下する。また例えば、１２００［ｄｐｉ］の解像度で２本のライン画像を形成する場合には、副走査方向における画素の長さＬは、２５４００÷１２００＝２１．２［μｍ］となる。このため、図４に示したように３つの画素列の片端にライン画像を形成する動作を繰り返す場合には、白抜きラインの太さ２１．２×（３−１）＝４２．４［μｍ］は１３０［μｍ］よりも小さくなる。但し、４、５、６、７つの画素列の片端にライン画像を形成する動作を繰り返す場合にも、白抜きラインの太さはそれぞれ６３．６、８４．８、１０６．０、１２７．２［μｍ］となり１３０［μｍ］よりも小さくなる。このような場合には、３、４、５、６、７つの画素列の片端にライン画像を形成する動作を繰り返す場合の何れかにおいて、上記中心線上に２０［％］以下の割合でトナーを付着させれば、画像のシャープネスの低下を回避することができる。
【００４４】
なお、トナーチリの発生度合いについては、例えば、添加剤との混合によるトナー（現像剤）流動性の調整、トナー粒径の調整、中間転写ベルトの電気抵抗値の調整、感光ドラムの直径（感光ベルトの場合にはベルト表面のビーム照射位置における曲率）の調整、一次転写ニップや二次転写ニップにおける転写電界領域の大きさの調整などにより調整が可能である。
【００４５】
次に、濃度階調表現のための画像データの処理方法について説明する。
従来、電子写真方式の画像形成方法で用いられてきた濃度階調表現のための一般的な画像データの処理方法としては、ディザ法や誤差拡散法などがある。
【００４６】
このディザ法は、マトリックス単位で階調を表現するものである。具体的には、複数の基本画素からなるマトリックスを設定するとともに、各基本画素毎に濃度の閾値を設定し、画像データでこの閾値以上の濃度が示される基本画素部分を画像部であるとみなし、この閾値よりも低い濃度が示される基本画素部分を非画像部（地肌部）であるとみなす。このようなディザ法において、濃度の閾値の設定方法としては、電子写真方式特有の現像特性にマッチングしたドット集中型が用いられることが多い。このドット集中型においては、中間電位になる感光層領域が少なくなるので、感光体の速度変動により生ずる中間電位の領域の電位変動に起因するバンディングは発生し難い。しかしながら、濃度階調表現のマトリックスの形状が際立って画像のザラツキとして認識され易いという欠点がある。
【００４７】
一方、誤差拡散法は、インクジェット方式などで広く用いられている処理法であり、形成すべき画素の濃度を複数画素にわたって累積していき、その濃度レベルが閾値を超える毎に一定あるいは数レベルの大きさと濃度のドットを形成していく方法である。この誤差拡散法においては、低濃度の画像部におけるドット間隔が大きくなり、高濃度の画像部におけるドット間隔が小さくなる。このため、ドットが視認できないものになっていればそのほんどが独立しているため、ザラツキを感じさせない良好な画像が得られる。しかしながら、ディザ法とは反対に中間領域の潜像が各ドットの周囲にできるため、バンディングが発生し易いという欠点がある。
【００４８】
そこで、濃度階調表現のための画像データの処理方法として誤差拡散法を採用し、且つ、上述のような各方程式、感光層の厚さ、あるいはトナーチリの発生度合いの要件を具備させることで、バンディングを目立ち難くしながらザラツキのない画像を得ることができる。
【００４９】
本発明は、以上の検討の中から生まれた新たな技術的思想であり、従来の電子写真に適用することで、バンディングの目立たない画像を形成することができる。
【００５０】
なお、実施形態において、移動力伝達手段としてギア列を備える画像形成装置について説明したが、例えば歯車としてのギアプーリとタイミングベルトからなるものなど、少なくとも歯車を有する移動力伝達手段を備える画像形成装置であれば本発明の適用が可能である。
【００５１】
また、潜像担持体として感光ドラムを備える画像形成装置について説明したが、他の潜像担持ドラムや潜像担持ベルトを備える画像形成装置についても本発明の適用が可能である。潜像担持ベルトを備える画像形成装置においては、該潜像担持ベルトの光ビーム照射位置での曲率半径をＤｐとすればよい。
【００５２】
【発明の効果】
請求項１、２又は３の発明によれば、潜像担持体の回転軸に固定された歯車の噛み合わせに起因して生ずるバンディングを目立ち難くすることができるという優れた効果がある。
【００５４】
また特に、請求項３の発明によれば、画像のザラツキ感を低減するので、該ザラツキ感を低減しながら、移動力伝達手段内で最も潜像担持体側に位置する歯車の噛み合わせに起因して生ずるバンディングを低減することができるという優れた効果がある。
【図面の簡単な説明】
【図１】実施形態に係るプリンタの概略構成図。
【図２】バンディングの発生周期Ｌｇ（π×Ｄp×Ｍ／Ｄｇ）と、１／ｅ径Ｌｂとの関係を示すグラフ。
【図３】ビーム強度分布曲線を示すグラフ。
【図４】転写紙上のライン画像を示す拡大図。
【図５】画像上に出現する濃度ムラの空間周波数（Ｘ軸）と、ヒトに対する該濃度ムラの許容濃度差（Ｙ軸）との関係を示すグラフ。
【符号の説明】
１ 感光ドラム
２ 帯電チャージャ
３ 光書き込みユニット
４ 現像装置
５ 一次転写装置
６ 二次転写ローラ
７ 除電装置
８ クリーニングブレード
９ 定着装置[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a facsimile, a copying machine, and a printer. More specifically, the present invention relates to a latent image carrier that carries a latent image, and a beam that carries the latent image on the latent image carrier by irradiation with a light beam. Irradiation means, visualization means for visualizing the latent image, movement force transmission means having a gear for transmitting movement force to the latent image carrier, and the latent force via the movement force transmission means The present invention relates to an image forming apparatus including a moving force applying unit that applies a moving force to an image carrier.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in a digital electrophotographic image forming apparatus, a surface of a photoreceptor, which is a latent image carrier, is uniformly charged while being moved endlessly, and then a laser diode (LD) or a light emitting diode (LED) is applied to the surface. ) And the like to form an electrostatic latent image by irradiation with a light beam emitted from the above.
[0003]
As a driving method of the photoconductor for realizing such endless movement, a DD motor driving method, a gear driving method, and the like are known. In the DD motor drive system, a photosensitive drum, a driving roller, and the like are rotated and driven by a motor directly connected to a shaft of a photosensitive drum, which is a photosensitive member, and a driving roller for driving a photosensitive belt, which is also a photosensitive member. It is a method to make it. Also, the gear drive system transmits the motor drive force to the photosensitive drum shaft, drive roller, etc. via a gear train as a moving force transmission means or a combination of a gear pulley and a timing belt. In this method, the shaft of the photosensitive drum, the driving roller, and the like are driven to rotate at a peripheral speed different from the speed.
[0004]
Among these driving methods, the DD motor driving method generally causes rotation unevenness with respect to the motor every rotation or every predetermined cycle due to the harmonic component of the drive power supply, and this rotation unevenness causes image position shift. Is likely to occur.
[0005]
In the gear drive system, a shading difference called banding is likely to occur on an image position having a period of about 1 [mm] corresponding to the tooth pitch of the gear as a gear. This banding is mainly caused by the engagement of gears and gear pulleys (hereinafter collectively referred to simply as gears) located on the most photoreceptor side in the moving force transmission means (drive transmission mechanism) with other members. Occur. For example, when the drive transmission mechanism is a gear train, banding due to the meshing of the intermediate gears hardly occurs due to the buffering action of the gaps between the intermediate gears, but the final stage that directly transmits the driving force to the photoconductor. Such a shock-absorbing effect is not exerted on the gear. For this reason, banding with a period corresponding to the number of teeth of the gear located closest to the photoreceptor in the gear train is likely to occur in the image. Further, even when the driving force transmission mechanism is a combination of a gear pulley and a timing belt, for the same reason, banding with a period corresponding to the number of teeth of the gear pulley positioned closest to the photoreceptor is likely to occur in the image.
[0006]
Therefore, conventionally, a shock absorber is provided on the gear closest to the photoconductor, or a flywheel is provided on the rotating shaft of the photoconductor to prevent banding due to meshing between the gear and another member. Measures to reduce were taken.
[0007]
However, such measures not only increase the cost, but also cannot completely eliminate fluctuations in the moving speed of the photoreceptor.
[0008]
In an image forming apparatus that reproduces the density gradation of an image by adjusting the irradiation conditions of the LD and LED, banding may occur even if the moving speed of the photosensitive member is slightly changed for the following reason. That is, in general, in an image forming apparatus using an LD or LED, an electrostatic latent image portion corresponding to a high density portion of an image by irradiating a photosensitive member with a light beam having an intensity capable of sufficiently attenuating the potential. And forming an electrostatic latent image portion having an intermediate potential corresponding to the halftone portion or highlight portion of the image by modulating the intensity, the irradiation time, etc. Is adjusted. However, when the movement speed of the photoconductor varies due to the meshing of the gears, a portion where the photoconductor is not irradiated in the normal irradiation time occurs at a period of about 1 [mm]. When this portion is an electrostatic latent image portion having an intermediate potential as described above, potential unevenness occurs. On the other hand, the Fourier component (MTF) of the developing electric field formed in the developing region between the photosensitive member and the developing device as the visualization means is enhanced with a period of about 1 [mm]. When the Fourier component is emphasized in this way, the potential unevenness that occurs in a period of about 1 [mm] is further emphasized. In addition, by enhancing the potential unevenness in this way, the development unevenness in the developing process is enhanced and banding occurs.
[0009]
It has also been found that the human visual detection sensitivity for density unevenness with a period of around 1 [mm] is very high. FIG. 5 is a graph showing the relationship between the spatial frequency (X axis) of density unevenness appearing on an image and the allowable density difference (Y axis) of the density unevenness with respect to humans. As shown in the figure, there is a correlation between the spatial frequency of the density unevenness and the human allowable density difference with respect to the density unevenness, and the human allowable density difference with respect to the density unevenness occurring in a cycle of the order of 1 [mm] It becomes lower than the one for the shading unevenness appearing at. For this reason, banding that occurs in a period of about 1 [mm] due to the gear tooth pitch becomes conspicuous even if the density difference is low.
[0010]
[Problems to be solved by the invention]
By the way, as a result of diligent research by the present inventors, it has been found that the conspicuousness of banding has a correlation with the beam diameter on the surface of the photoreceptor as well as the spatial frequency of the banding. Specifically, the present inventor has determined that the diameter in the photosensitive member moving direction (hereinafter referred to as the sub-scanning direction) of a region where the beam intensity is 1 / e of the peak value in the beam spot on the photosensitive member surface. And found that there is a correlation between the visibility of banding. Conventionally, as the beam diameter indicating the effective beam range in the beam spot on the surface of the photoreceptor, the beam intensity in the beam spot is 1 / e. ² The diameter in the sub-scanning direction of the region to be used has generally been used. However, according to the inventor's research, there is no clear correlation between the beam diameter and the conspicuousness of banding, and the conspicuousness of banding indicates that the beam intensity in the beam spot is 1 / (peak) of the peak value. It has been found that there is a correlation with the beam diameter in the sub-scanning direction of the region e. This seems to suggest that the conventional definition of the beam diameter was inappropriate.
[0011]
Furthermore, as a result of extensive research by the inventor, it has been found that the conspicuousness of banding has a correlation with the thickness of the photosensitive layer on the surface of the photosensitive member and the degree of contamination on the non-image area on the recording medium, such as the degree of toner dust. I found it.
[0012]
The present invention has been made in view of the above background, and an object of the present invention is to provide an image forming apparatus in which banding caused by meshing of gears can be made inconspicuous.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 is directed to a drum-like latent image carrier that carries a latent image on a surface that rotates about a rotation axis, and a light beam applied to the surface. A beam irradiating means for carrying the latent image, a developing means for developing the latent image, and a moving force having a gear fixed to the rotating shaft for transmitting the rotational force to the latent image carrier. In an image forming apparatus including a transmission unit and a moving force applying unit that applies a moving force to the latent image carrier via the moving force transmission unit, the diameter of the gear is Dg [mm], and the module is M [mm]. And the diameter of the latent image carrier is D p [ mm], the circumference ratio is π, and the diameter in the direction of movement of the surface of the region where the beam intensity in the beam spot on the surface is 1 / e of the peak value is Lb [mm]. A conditional expression of 0.05+ (π × Dp × M / Dg) /1.5 <1 [mm] is provided.
[0014]
This image forming apparatus uses Lb / 0.05 + (Π × Dp × M / Dg) / 1.5 <1 [mm] is satisfied. The present inventor has made such a conditional expression by earnest research, Fixed to the rotating shaft of the latent image carrier It has been found that the banding caused by the meshing of the gears can be made inconspicuous.
[0017]
Claim 2 According to the present invention, the latent image carrier is a photoconductor. 1's In the image forming apparatus, the thickness of the photosensitive layer on the surface of the photoreceptor is formed to 15 [μm] or less.
[0018]
In this image forming apparatus, the thickness of the photosensitive layer on the surface of the photosensitive member is 15 [μm] or less. The inventor has intensively studied to form the thickness of the photosensitive layer in this way, Fixed to the rotating shaft of the latent image carrier It has been found that the banding caused by the meshing of the gears can be made more inconspicuous.
[0021]
Claim 3 The invention of claim 1 further comprises image data processing means for performing halftone processing on the image data, and irradiating the latent image carrier with the light beam based on the halftone processed data. Or 2 In the image forming apparatus, the image data processing means is configured to perform halftone processing on the image data by an error diffusion method.
[0022]
In this image forming apparatus, halftone processing is performed on the image data by the error diffusion method, so that, for example, an image forming apparatus that uses a dither method as the halftone processing method is likely to have a matrix shape at the density gradation portion. Reduces the so-called roughness of the image.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment in which the present invention is applied to a digital electrophotographic printer as an image forming apparatus will be described below.
First, an outline of the entire printer will be described. FIG. 1 is a schematic configuration diagram of a printer according to the present embodiment. In FIG. 1, 1 is a photosensitive drum as a latent image carrier, 2 is a charging charger 2 for uniformly charging the photosensitive layer on the surface of the photosensitive drum 1, 3 is an optical writing unit as beam irradiation means, and 4 is an optical writing unit. Is a developing device as a visualization means, 5 is a primary transfer device as a transfer means, 6 is a secondary transfer roller as the transfer means, 7 is a static elimination device for neutralizing the photosensitive layer, and 8 is a photosensitive device. Each of the cleaning blades for cleaning the drum is shown.
[0024]
In this printer, a gear train as a moving force transmitting means and a drum motor as a moving force applying means (not shown) are connected to a rotating shaft (not shown) protruding from the end face of the photosensitive drum 1. A photosensitive layer is coated on the surface of the photosensitive member 1 rotated in the direction of the arrow (clockwise) in the figure via the gear train by the driving force of the drum motor. That is, in this printer, a gear drive system is used as a drive system for the photosensitive drum 1.
[0025]
First, the photosensitive layer of the photosensitive drum 1 is uniformly charged when passing through a position facing the charging charger 2 as the photosensitive drum 1 rotates. Next, when passing through a position facing the optical writing unit 3, a laser diode (not shown) of the optical writing unit 3 is irradiated with a light beam based on an image signal from an image processing unit (not shown). Carries, for example, an electrostatic latent image with a positive polarity charge.
[0026]
On the other hand, the developing device 4 contains toner as an image forming substance, and this is carried on the developing roller 4a that rotates in the arrow direction (counterclockwise) in the figure while being charged to, for example, a negative polarity by stirring or the like. . A developing bias is applied to the developing roller 4a, which is partially exposed from the opening of the developing device 4, by a bias applying means (not shown). By this application, the developing roller 4a is shown between the photosensitive drum 1 and the developing roller 4a. A developing electric field is not formed.
[0027]
The primary transfer device 5 is also counterclockwise while being stretched by the primary transfer roller 5b, the stretching rollers 5c and 5d, which are rotated counterclockwise in the drawing by a driving device (not shown), and these three rollers. It is composed of a flexible intermediate transfer belt 5a that is rotationally driven around. The primary transfer roller 5b is in contact with the photosensitive drum 1 via the intermediate transfer belt 5a on the downstream side of the developing device 4 in the rotation direction of the photosensitive drum 1, thereby forming a primary transfer nip. A primary transfer bias is applied to the primary transfer roller 5b by a bias applying means (not shown), and a primary transfer electric field is formed in the primary transfer nip by this application. Further, the stretching roller 5d is in contact with the secondary transfer roller 6 via the intermediate transfer belt 5a below the primary transfer device 5 to form a secondary transfer nip. A secondary transfer bias is applied to the secondary transfer roller 6 by a bias applying means (not shown), and a secondary transfer electric field is formed in the secondary transfer nip by this application.
[0028]
When the electrostatic latent image having a positive polarity carried on the photosensitive layer passes through the development position as the photosensitive drum 1 rotates, the toner charged to the negative polarity on the developing roller 4a has a developing electric field. It is moved and adhered by the influence, and the adhesion is visualized to become a toner image. Then, when passing through the primary transfer nip, the image is primarily transferred onto the intermediate transfer belt 5a under the influence of the pressing force in the primary transfer nip and the primary transfer electric field.
[0029]
In view of such primary transfer timing, a sheet feeding device (not shown) feeds the transfer sheet 11 as a recording member toward the secondary transfer nip. When the toner image transferred onto the intermediate transfer belt 5a passes through the secondary transfer nip with the rotation of the intermediate transfer belt 5a, the pressing force in the secondary transfer nip and the secondary transfer electric field are reduced. Under the influence, it is secondarily transferred onto the transfer paper 11.
[0030]
The transfer paper 11 on which the toner image is secondarily transferred is sent into the fixing device 9. In this fixing device, the toner image is sandwiched between the heating roller 9a and the pressure roller 9b, and after the toner image is fixed by heating, the toner image is discharged out of the printer.
[0031]
The photosensitive layer that has passed through the primary transfer nip is neutralized when it passes through a position facing the neutralization device 7 as the photosensitive drum 1 rotates. Then, when passing through the position facing the cleaning blade 8, the untransferred toner remaining without being primarily transferred is mechanically scraped off and prepared for the next image formation.
[0032]
In the printer having the above configuration, banding is performed on an image position having a period of about 1 [mm] corresponding to a tooth pitch of a gear (hereinafter simply referred to as a gear) directly connected to the photosensitive drum rotating shaft in the gear train (not shown). Is likely to occur. Thus, it is known that banding that occurs in a cycle of about 1 [mm] due to the gear tooth pitch is conspicuous even if the density difference is low. If banding is generated at different periods, the banding can be made inconspicuous. In a normal image forming apparatus, it is difficult to make the gear tooth pitch larger than the order of 1 [mm] due to the diameter and peripheral speed of the photosensitive drum. Will do. However, there is a limit to reducing the gear tooth pitch, and there is a cost problem such as the need to increase the strength of the gear.
[0033]
Therefore, the inventor conducted an experiment on the conspicuousness of banding that occurs when the printer is set under various conditions as shown in Table 1 below.
[Table 1]

[0034]
Then, the conspicuousness of banding is not only the size of the gear tooth pitch (module M), but also the radius of curvature at the light beam irradiation position on the latent image carrier (= the diameter Dp of the photosensitive drum 1), It has been found that there is a correlation with the diameter (Dg) of the gear, the beam diameter (Lb) in the region where the peak value of the beam intensity is 1 / e, and the like.
[0035]
FIG. 2 shows the banding generation period Lg (π × Dp × M / Dg) and the beam diameter Lb in the region where the beam intensity is 1 / e (hereinafter referred to as 1 / e diameter Lb) under the conditions shown in Table 1 above. It is a graph which shows the relationship. In the graph of FIG. 2, the straight line A represents the solution of the equation Lb / 0.06 + Lg / 2 = 1 [mm], and the equation Lb / 0.05 + (π × Dp × M / Dg) /1.5=1 [mm ] Shows the solution. Further, x, Δ, and ◯ indicate the evaluation results of the conspicuousness of banding, respectively. As shown in the figure, it can be seen that banding is less noticeable in the region below the straight line A. Furthermore, it can be seen that in the region below the straight line B, banding is more difficult to stand out.
[0036]
In order to make the solution of each equation described above smaller than 1 [mm] and make banding inconspicuous, the 1 / e diameter Lb of the first term on the left side of each equation is reduced, or the photosensitive drum of the second term on the left side is reduced. The diameter Dp, the gear diameter Dg, the gear module M, etc. may be reduced.
[0037]
Conventionally, the beam diameter is 1 / e of the peak value of the beam intensity. ² In the sub-scanning direction (hereinafter referred to as 1 / e ² Has been used in general, but according to the experiments of the present inventors, this 1 / e ² There was no clear correlation between the diameter and the conspicuousness of banding. For example, when a predetermined LD optical system is used and the beam intensity distribution of the beam spot is changed by shifting the parameters of the LD optical system from the optimum conditions, 1 / e ² Even if the diameter changed, the conspicuousness of banding did not change much. Also, instead of LD as the optical system, an LED optical system is used, and the size of the light emitting portion of the LED is changed to 1 / e. ² Even if the diameter was changed, the amount of change in the conspicuousness of banding accompanying this change was small enough to be included in the measurement error.
[0038]
On the other hand, a plurality of LD optical systems having different parameters are used, and 1 / e in each optical system is used. ² When the experiment was performed under the parameter conditions that minimize the diameter, a good correlation was found between the 1 / e diameter Lb of each optical system and the conspicuousness of banding. In addition, in the LED optical system, a combination of an LED array having a number of LEDs and lenses corresponding to the number of dots formed in the main scanning direction and a lens array is used, and a beam intensity distribution from each LED is used. There are technical limits to unifying For this reason, 1 / e in each LED ² The diameter and the 1 / e diameter Lb vary, but the 1 / e ² No correlation was observed for the diameter, but a good correlation was observed for the 1 / e diameter Lb. That is, regardless of whether the optical system of the optical writing unit is an LD or an LED, a good correlation was found between the 1 / e diameter Lb and the conspicuous banding.
[0039]
Thus, 1 / e ² The reason why there is a good correlation between the 1 / e diameter Lb, not the diameter, and the conspicuousness of the banding is not clear, but the following is considered to be related. That is, as shown in FIG. 3, the 1 / e diameter Lb is a region where the beam intensity is about 36.8 [%] or more of the peak value, and the change of the beam intensity distribution curve is steep (with respect to the change of the measurement point). This is a region where the change in the amount of light is large. In contrast, 1 / e ² The diameter is a region where the beam intensity is about 13.5 [%] or more of the peak value, and is a region that becomes the tail of the beam intensity distribution curve. Here, when the parameters of the optical system are slightly changed, the 1 / e diameter Lb is not changed so much. ² The diameter will change greatly. In particular, this tendency becomes stronger when the beam spot is narrowed down by the LD optical system. This is because, as a result of the interference of the LD light, a second peak called a so-called side lobe is generated near the peak value of the beam intensity distribution curve, and this peak lifts the tail part of the beam intensity distribution curve. ² This is thought to be due to a large change in diameter. In the formation of the electrostatic latent image, the beam portion having the intensity of the tail portion does not contribute so much, and the electrostatic latent image is formed by optical writing with the beam portion within the 1 / e diameter Lb exclusively. As the 1 / e diameter Lb increases, the amount of light writing to adjacent pixels also increases. vice versa. As the 1 / e diameter decreases, the amount of light writing to adjacent pixels decreases. As a result, it is considered that the banding density difference caused by the speed fluctuation of the photosensitive drum becomes small, and the banding becomes inconspicuous.
[0040]
As shown in FIG. 2, in order to make the banding inconspicuous, the 1 / e diameter Lb is reduced, or the photosensitive drum diameter Dp, the gear diameter Dg, the gear module M, etc. in the second term on the left side are reduced. do it. However, in order to reduce the 1 / e diameter Lb, for example, it is necessary to enlarge the lens system of the LD optical system or reduce the light emission wavelength of the LD. However, these only increase the cost. In addition, it is extremely difficult to stably realize a 1 / e diameter Lb of 30 [μm] or less on the photosensitive drum. If the diameter Dp of the photosensitive drum is too small, the units such as the charging charger 2, the optical writing unit 3, the developing device 4, the primary transfer device 5, the static eliminator 7, and the cleaning blade 8 are arranged around the photosensitive drum 1. Since the installation becomes difficult, there is a limit to the reduction of the diameter Dp. In addition, the gear diameter Dg has a restriction on the size of the apparatus. In particular, in the case of a so-called Dratandem type image forming apparatus in which a plurality of photosensitive drums are arranged in parallel around the intermediate transfer belt, The gear diameter Dg is greatly restricted by the interval between the photosensitive drums. Further, if the gear module M is made too small, not only the cost is increased, but also a problem in strength occurs. Therefore, the lower limit value of the module M is substantially about 0.5 [mm]. In consideration of the above, depending on the size of each unit arranged on the photosensitive drum under the condition that the 1 / e diameter Lb and the lower limit value of the module M are 30 [μm] and 0.5 [mm], respectively. Thus, it is desirable to reduce the diameter Dp of the photosensitive drum and the diameter Dg of the gear so that the solutions of the above equations are smaller than 1 [mm].
[0041]
Further, according to the experiment by the present inventor, the same effect as that obtained when the 1 / e diameter Lb was reduced by forming the photosensitive layer 2 of the photosensitive drum 2 to have a thickness of 15 [μm] or less was recognized. The reason why such an effect is exhibited is thought to be to reduce the diffusion of charges in the photosensitive layer by reducing the thickness of the photosensitive layer. In addition, since the photosensitive layer is thin, the MTF emphasis region of the developing electric field is shifted to a period shorter than 1 [mm], and small dots and thin lines can be developed with sufficient density. Is also considered to be involved.
[0042]
Further, as a result of conducting the following experiment, the inventor found that the 1 / e diameter Lb in the sub-scanning direction, the thickness of the photosensitive layer, and the speed fluctuation due to the meshing of the gear of the photosensitive drum 1 were the same. However, it was found that banding can be made inconspicuous if moderate toner dust is generated in the non-image area of the transfer paper 11. That is, as shown in FIG. 4, a line image is formed at L1 and L4 for each pixel row L1, L2, L3, L4 extending in the main scanning direction (left-right direction in the drawing) on the transfer paper 11, and Image formation was performed under conditions such that L2 and L3 are non-image portions (actually, a line image is formed through a non-image portion including two pixel columns). Then, a 200 × magnification micrograph of the transfer paper 11 was taken, a center line AA ′ was drawn between the pixels of the non-image area, and the ratio of the toner on the center line AA ′ was measured by toner dust. . As a result of repeatedly performing such measurement while changing the degree of occurrence of toner dust, if toner dust that causes toner to adhere to 20% or more of the entire center line AA ′ occurs, banding is conspicuous. I found it difficult. The reason why banding is not so conspicuous in this way is not certain, but it is considered that when a certain amount of toner is not present in the non-image portion between the line images, the white line in the non-image portion stands out.
[0043]
However, when the length of the non-image portion in the sub-scanning direction is 130 [μm] or less and the toner is present at a rate exceeding 20 [%] on the center line of the non-image portion, the non-image portion It was found that the toner from the line image scatters and adheres to a white width of 65 [μm] or more in the white line of the portion, thereby causing the line image to appear blurred and reducing the sharpness of the image. Specifically, the length of pixels in the sub-scanning direction (pixel center interval) is set to L, and the operation of forming one line image in one end column of n pixel columns extending in the main scanning direction is repeated. L × (n−1) It was found that the sharpness of the image was lowered when the image was formed under the condition of <130 [μm]. Accordingly, when the length of the non-image portion in the sub-scanning direction is 130 [μm] or less, the toner dust amount is controlled so that the toner is present at a ratio of 20 [%] or less on the center line of the non-image portion. There is a need to. For example, when a line image as shown in FIG. 4 is formed with a resolution of 600 [dpi], the pixel length L in the sub-scanning direction is 25400 ÷ 600 = 42.3 [μm]. In such a case, if toner adheres to the center line at a rate exceeding 20 [%], the thickness of the white line 42.3 × (3-1) = 84.6 [μm] is from 130 [μm]. And the sharpness of the image decreases. For example, when two line images are formed with a resolution of 1200 [dpi], the pixel length L in the sub-scanning direction is 25400 ÷ 1200 = 21.2 [μm]. Therefore, when the operation of forming a line image at one end of three pixel columns is repeated as shown in FIG. 4, the thickness of the white line 21.2 × (3-1) = 42.4 [μm ] Is smaller than 130 [μm]. However, even when the operation of forming a line image at one end of four, five, six, and seven pixel rows is repeated, the thickness of the white line is 63.6, 84.8, 106.0, 127.2, respectively. [Μm] and smaller than 130 [μm]. In such a case, in any of the cases where the operation of forming a line image at one end of three, four, five, six, and seven pixel rows is repeated, toner is applied at a rate of 20% or less on the center line. If it adheres, the fall of the sharpness of an image can be avoided.
[0044]
As for the degree of toner dust generation, for example, adjustment of toner (developer) fluidity by mixing with additives, adjustment of toner particle diameter, adjustment of electric resistance value of intermediate transfer belt, diameter of photosensitive drum (photosensitive belt) In this case, adjustment is possible by adjusting the curvature at the beam irradiation position on the belt surface, adjusting the size of the transfer electric field area in the primary transfer nip and the secondary transfer nip, and the like.
[0045]
Next, a method for processing image data for density gradation expression will be described.
Conventional image data processing methods for density gradation expression that have been used in electrophotographic image forming methods include a dither method and an error diffusion method.
[0046]
This dither method expresses gradation in matrix units. Specifically, a matrix composed of a plurality of basic pixels is set, a density threshold value is set for each basic pixel, and a basic pixel portion whose density is equal to or higher than this threshold value is regarded as an image portion in the image data. The basic pixel portion having a density lower than the threshold value is regarded as a non-image portion (background portion). In such a dither method, a dot concentration type that matches the development characteristics peculiar to an electrophotographic system is often used as a method for setting a density threshold. In this dot concentration type, since the photosensitive layer region having an intermediate potential is reduced, banding due to the potential variation in the intermediate potential region caused by the speed variation of the photosensitive member hardly occurs. However, there is a drawback in that the shape of the density gradation expression matrix is conspicuously easily recognized as roughness of the image.
[0047]
On the other hand, the error diffusion method is a processing method widely used in the ink jet method and the like. The density of pixels to be formed is accumulated over a plurality of pixels, and each time the density level exceeds a threshold value, a constant or several level is accumulated. This is a method of forming dots of size and density. In this error diffusion method, the dot interval in the low density image portion is increased, and the dot interval in the high density image portion is decreased. For this reason, if the dots are invisible, most of them are independent, and a good image that does not feel rough can be obtained. However, contrary to the dither method, a latent image in the intermediate area can be formed around each dot, so that there is a drawback that banding is likely to occur.
[0048]
Therefore, by adopting the error diffusion method as the image data processing method for density gradation expression, and having the above-mentioned equations, photosensitive layer thickness, or toner dust generation requirement, It is possible to obtain a rough image without making banding conspicuous.
[0049]
The present invention is a new technical idea born out of the above studies, and can be applied to conventional electrophotography to form an image with inconspicuous banding.
[0050]
In the embodiment, the image forming apparatus including the gear train as the moving force transmitting unit has been described. However, the image forming apparatus includes the moving force transmitting unit having at least a gear, such as a gear pulley and a timing belt as a gear. If it exists, the present invention can be applied.
[0051]
Further, the image forming apparatus including the photosensitive drum as the latent image carrier has been described. However, the present invention can also be applied to an image forming apparatus including another latent image carrier drum or a latent image carrier belt. In an image forming apparatus including a latent image carrying belt, the radius of curvature of the latent image carrying belt at the light beam irradiation position may be Dp.
[0052]
【The invention's effect】
Claims 1, 2 Or 3 According to the invention of Fixed to the rotating shaft of the latent image carrier There is an excellent effect that banding caused by the meshing of the gears can be made inconspicuous.
[0054]
In particular, the claims 3 According to the invention, since the feeling of roughness of the image is reduced, the banding caused by the meshing of the gear located closest to the latent image carrier in the moving force transmitting means is reduced while reducing the feeling of roughness. There is an excellent effect of being able to.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a printer according to an embodiment.
FIG. 2 is a graph showing a relationship between banding generation period Lg (π × Dp × M / Dg) and 1 / e diameter Lb.
FIG. 3 is a graph showing a beam intensity distribution curve.
FIG. 4 is an enlarged view showing a line image on transfer paper.
FIG. 5 is a graph showing a relationship between a spatial frequency (X axis) of density unevenness appearing on an image and an allowable density difference (Y axis) of the density unevenness with respect to a human.
[Explanation of symbols]
1 Photosensitive drum
2 Charger charger
3 Optical writing unit
4 Development device
5 Primary transfer device
6 Secondary transfer roller
7 Static eliminator
8 Cleaning blade
9 Fixing device

Claims (3)

A drum-like latent image carrier that carries a latent image on a surface that rotates about a rotation axis; beam irradiation means that irradiates the surface with a light beam to carry the latent image; and Visualizing means for visualizing, moving force transmitting means having a gear fixed to the rotating shaft for transmitting rotational force to the latent image carrier, and the latent image via the moving force transmitting means In an image forming apparatus comprising a moving force applying means for applying a moving force to a carrier,
The diameter of the gear is Dg [mm], the module is M [mm], the diameter of the latent image carrier is D p [ mm], the circumference is π, and the beam within the beam spot on the surface An image forming apparatus comprising the conditional expression expressed by the following formula 1 when the diameter in the surface moving direction of the region where the intensity is 1 / e of the peak value is Lb [mm] .

The image forming apparatus according to claim 1, wherein the latent image carrier is a photoconductor.
An image forming apparatus, wherein the thickness of the photosensitive layer on the surface of the photoreceptor is 15 [μm] or less.   3. The image forming apparatus according to claim 1, further comprising image data processing means for performing halftone processing on the image data, and irradiating the latent image carrier with the light beam based on the halftone processed data. An image forming apparatus comprising the image data processing means configured to perform halftone processing on image data.

Images