JP3678027B2 - Image forming apparatus - Google Patents

Description

【００４８】
また、二次転写ロール２５としては、接地された芯金上にカーボンを分散して１０⁷ Ω・ｃｍ以下の低抵抗に設定したウレタン発泡層を形成し、その外周にイオン導電性のウレタンゴム層（厚さ１ｍｍ程度）を形成し、さらにその外周にフッ素樹脂を含むコート層（厚さ数１０μｍ）を形成したものであって、その硬度（アスカーＣ）を４０°以下に設定したものを使用した。この二次転写ロール２４の軸方向の長さｗは、３５０ｍｍとした。なお、この試験における二次転写ロール２４ではクリーニングブレード２５ａを使用していない。
【００４９】
そして、記録用紙Ｐの裏面汚れについては以下のグレード（Ｇ）基準で評価した。なお、Ｇ２〜Ｇ４におけるかっこ書きの濃度は前記試験用パッチ像の濃度を「１」としたときの相対的な濃度である。また、このグレードＧ１〜Ｇ４のうち許容できる許容グレードは「Ｇ１」である。
Ｇ１：肉眼でわかる汚れはなし（ルーペでトナーの付着が観察できる程度）
Ｇ２：肉眼でわかる汚れが発生した（濃度０．０９）
Ｇ３：かなり目立つ汚れが発生した（濃度０．１５）
Ｇ４：ひどい汚れが発生した（濃度０．１９以上）
【００５０】
以上の試験による結果を表２及び３に示すとともに図４に示す。表中において横線「−」を付した項目部分は、グレードＧ１の評価結果が得られる可能性が低い等の予測的判断から試験を行っていない。
【００５１】
【表２】

【００５２】
【表３】

【００５３】
この試験によって得られた結果を、図４に示すような横軸を電荷密度ρ、縦軸を用紙裏面汚れのグレード（Ｇ）としたグラフ図に示した。電荷密度については、ρ＝Ｉ／（ｖ・ｗ）の式に、ｖ＝２２０ｍｍ／ｓｅｃ又は６０ｍｍ／ｓｅｃ、ｗ＝３５０ｍｍを代入するとともに、測定された電流Ｉの値を代入することにより算出している。また、図中のプロットした○印のいくつかは複数のデータが重なった状態で図示されている。
【００５４】
この図４に示す結果から明らかなように、電荷密度ρが０＜ρ＜０．３の範囲にあれば、用紙の裏面汚れのグレードが許容グレード「Ｇ１」になることがわかる。
【００５５】
また、表２や図３に示す結果から、中間転写ベルト２０の回転速度ｖと二次転写ロール２５の長さｗを先に設定した場合には、二次転写部４０に逆極性の電圧を印加したときに流れる電流Ｉを適切な値（汚れグレードがＧ１になるような値）に保ちさえすれれば、二次転写部の抵抗Ｒ_n にかかわらず、記録用紙の裏面汚れを確実に防止できることがわかる。従って、このことから、この画像形成装置では、転写装置５０の第２電源５２として定電流制御機能を備えた電源を使用して、その電源から逆極性の電圧を印加する際にその電流Ｉが上記した適切な値になるように制御する構成を採用することにより、二次転写部の合成抵抗Ｒ_n が変動することがあっても、二次転写ロール２５へのトナーの付着を確実に防止でき、ひいては記録用紙Ｐの裏面汚れを確実に防止できるようになる。
【００５６】
［実施の形態２］
この実施の形態は、電源装置５０として、図５に示すような構成のものを適用した以外は実施の形態１に係るカラー画像形成装置と同じ構成からなるものである。すなわち、この実施の形態における電源装置５０は、実施の形態１における電源装置５０と対比した場合、第２電源５２として定電圧制御がなされる電源を使用し、かつ、その第２電源５２と二次転写部（給電ロール２６）の間に付加抵抗５４を直列に接続した点で異なり、それ以外の点では共通するものである。
【００５７】
この場合、付加抵抗５４としては、少なくとも二次転写部４０の合成抵抗Ｒ_n と同等以上の抵抗値のものを使用すればよい。また、この電源装置５０の第２電源５２から逆極性の電圧を印加した際に、二次転写部４０に流れる電流Ｉから算出される電荷密度ρが前述の特定範囲を満たすような適切な当該電圧の値を選択し、その選択した値の電圧を第２電源５２から定電圧制御して二次転写部４０に印加すればよい。
【００５８】
そして、このような電源装置５０を適用した画像形成装置においては、その二次転写に際し、二次転写ロール２５と中間転写ベルト２０との間に記録用紙Ｐが存在しているときは、図６ａに示すように第１電源５１から二次転写電圧を給電ロール２６を介して二次転写部４０に印加する。これにより、正常な二次転写が行われる。一方、二次転写ロール２５と中間転写ベルト２０との間に記録用紙Ｐが存在していないときは、図６ｂに示すように、第２電源５２から逆極性の電圧を給電ロール２６を介して二次転写部４０に印加する。このとき、逆極性の電圧は、その電荷密度ρが前記した条件を満たすものであり、しかも、二次転写部４０の合成抵抗Ｒ_n が変動してもその適切な逆極性の電圧を二次転写部４０にほぼ安定して印加することができるため、実施の形態１の場合と同様に、二次転写部４０の合成抵抗Ｒ_n が環境条件等により変動しても、中間転写ベルト２０上のトナーが二次転写ロール２５側に静電的に転写されることはない。
【００５９】
従って、このような電源装置５０を適用した場合においても、実施の形態１の場合とほぼ同様に、二次転写部４０の合成抵抗Ｒ_n が変動することがあっても、二次転写ロール２５へのトナーの付着を確実に防止でき、ひいては記録用紙Ｐの裏面汚れを確実に防止できるようになる。
【００６０】
図７は、実施の形態１に係る試験で使用したカラー画像形成装置のうち二次転写部４０の合成抵抗Ｒ_n を低い二次抵抗（１６ＭΩ）と高い二次抵抗（４１ＭΩ）とに設定した各画像形成装置についてその二次転写部４０の電圧−電流特性を示すとともに、かかる各画像形成装置における電源装置５０の第２電源５２を「１ｋＶ」が定電圧制御されて出力される電源とし、かつ、その第２電源５２と二次転写部４０の間に「１００ＭΩ」の付加抵抗５４を接続した場合における当該二次転写部４０と付加抵抗部（５４）での分圧の状態を示すものである。
【００６１】
この図７に示す結果について表２のデータを参照しながら考察すると、まず、定電圧制御の第２電源５２及び付加抵抗５４を適用しない電源装置を使用した画像形成装置では、その二次転写部４０の合成抵抗が前記高抵抗（４１ＭΩ）時の場合には、図７をはじめ表２にも示されるとおり１ｋＶの電圧を印加すると１５μＡの電流が流れ、そのときの汚れグレードの結果が表２より「Ｇ１」であることから、肉眼で確認できるような用紙の裏面汚れが発生しない状態にあることがわかる。しかし、その二次転写部４０の合成抵抗が前記低抵抗（１６ＭΩ）時の場合には、図７や表２に示すとおり１ｋＶの電圧を印加すると９０μＡの電流が流れ、そのときの汚れグレードの結果が表２より「Ｇ３」となり、用紙の裏面汚れが発生する状態にあることがわかる。
【００６２】
これに対し、定電圧制御の第２電源５２及び付加抵抗５４を適用した画像形成装置では、図７に示されるように、交点Ｑの電圧値を境にして、定電圧電源５２から印加する１ｋＶの電圧のうち「約０．７ｋＶ」が付加抵抗部（５４）に分圧されて、その残りの「約０．３ｋＶ」が二次転写部４０に分圧されることになる。この結果、この画像形成装置では、その二次転写部４０の合成抵抗が前記低抵抗（１６ＭΩ）時の場合であっても、二次転写部４０に実際にかかる電圧が上記のとおり「約０．３ｋＶ」となり、かかる印加電圧値における汚れグレードの結果が表２中の０．３ｋＶに近い値の０．２５ｋＶの欄の結果をみると「Ｇ１」とあることから、用紙の裏面汚れが発生しない状態にあることがわかる。
【００６３】
従って、本実施の形態のように電源装置５０として定電圧制御の第２電源５２及び付加抵抗５４を適用したものを使用した場合には、二次転写部４０の合成抵抗が変動しても、二次転写ロール２５へのトナーの付着に起因した記録用紙Ｐの裏面汚れを防止することが可能になることがわかる。ちなみに、この画像形成装置においては、その付加抵抗５４として２０ＭΩ〜４００ＭΩ程度のものを使用することが有効である。
【００６４】
［他の実施の形態］
実施の形態１、２では、１つの感光体ドラム１１を４回転させることによりフルカラー画像の４色のトナー像を順次形成するタイプのカラー画像形成装置に基づいて説明したが、本発明はこれに限られるものではなく、例えば図８に示すように中間転写方式を利用したいわゆるタンデムタイプのカラー画像形成装置にも同様に適用することができる。図８中、符号１０Ｋ，１０Ｙ，１０Ｍ，１０Ｃはいずれも単色の作像システムであり、それぞれブラックＫ，イエローＹ，マゼンタＭ，シアンＣの色のトナー像を個別に形成する専用の画像形成部である。その各作像システム１０の構成は、実施の形態１等のような１つの作像システム１０とほぼ同様であり、現像装置１４が各システム１０毎に専用の色のトナーを収容した１つの現像器１４Ｋ，１４Ｙ，１４Ｍ，１４Ｃで構成されている点で異なるのみである。
【００６５】
そして、このようなタンデムタイプの画像形成装置の場合にも、その二次転写部４０の電源装置として、前記した各実施の形態と同様の構成からなる電源を採用することにより、二次転写部４０の合成抵抗の変動にかかわらず、二次転写ロール２５へのトナーの付着を確実に防止でき、ひいては記録シートＰの裏面汚れも確実に防止できる。
実施の形態１では、
【００６６】
【発明の効果】
以上説明したように、本発明の画像形成装置によれば、二次転写ロールと中間転写体との間に記録材が存在しないときにおいて、二次転写ロールへのトナーの付着を確実に防止することができる。しかも、このトナー付着の防止効果は、二次転写部の抵抗が変動しても確実に得られる。従って、この画像形成装置を使用した場合には、二次転写ロールへのトナー付着に起因した記録材の裏面汚れや転写抜け等のない良好な二次転写を行うことが可能となる。
【図面の簡単な説明】
【図１】 実施の形態１に係るカラー画像形成装置の主要な構成を示す概要図である。
【図２】 二次転写部とその電源装置の主要な構成を示す概要図である。
【図３】 図２の電源装置から二次転写部への電圧の印加状態を示す説明図である。
【図４】 実施の形態１における試験の結果を示すグラフ図である。
【図５】 実施の形態２に係るカラー画像形成装置における二次転写部とその電源装置の主要な構成を示す概要図である。
【図６】 図５の電源装置から二次転写部への電圧の印加状態を示す説明図である。
【図７】 実施の形態２における試験の結果を示すグラフ図である。
【図８】 他の実施の形態に係るカラー画像形成装置の主要な構成を示す概要図である。
【符号の説明】
１０…作像システム、１１…感光体ドラム（像担持体）、２０…中間転写ベルト（無端状の中間転写体）、２５…二次転写ロール、４０…二次転写部、５０…電源装置、５２…第２電源（逆極性の電圧を印加する電源）、５４…付加抵抗、Ｐ…記録シート（記録材）。[0001]
BACKGROUND OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, or a multi-function machine using an intermediate transfer method, and in particular, toner adhesion to a secondary transfer roll that comes into contact with an intermediate transfer member used in the intermediate transfer method The present invention relates to an image forming apparatus that can surely prevent the above-described problem.
[0002]
[Prior art]
In general, an image forming apparatus including an intermediate transfer member is configured to transfer a toner image formed by an electrophotographic process or the like on an image carrier such as a photosensitive member that is rotationally driven at a part of the image carrier (primary transfer position). By primary transfer to an endless intermediate transfer member in the form of a belt or drum that rotates synchronously in contact, and then secondary transfer from the intermediate transfer member to a recording material such as recording paper or OHP sheet Then, image formation is performed.
[0003]
For the secondary transfer, a recording material is supplied and sent between the intermediate transfer member and a secondary transfer roll that is in contact with the transfer member at least when the secondary transfer is performed. A secondary transfer voltage for forming an electric field in the direction in which the toner image on the intermediate transfer body moves toward the recording material between the roll (specifically, the polarity opposite to the toner charging polarity of the toner image) Voltage) is performed in an environment in which the voltage is applied from the power supply device. Further, with respect to the secondary transfer voltage, for example, the secondary transfer voltage is directly applied to the secondary transfer roll, or the secondary transfer voltage is disposed on the back surface (inner surface) side of the intermediate transfer body facing the secondary transfer roll and the transfer roll. A method of applying between the backup roll (electrode) is adopted.
[0004]
In the secondary transfer portion having such a configuration, the toner image on the intermediate transfer member is pressed and electrostatically applied to the recording material that passes while being pressed against the intermediate transfer member by the secondary transfer roll. Reliable secondary transfer is performed by transferring and adhering to the action.
[0005]
[Problems to be solved by the invention]
By the way, in the intermediate transfer type image forming apparatus using such a secondary transfer roll, when the recording material does not exist between the secondary transfer roll and the intermediate transfer body, the above-described secondary transfer voltage is used. If the toner is applied as it is, a part of the toner of the control toner image such as the density formed in the inter-image portion of the intermediate transfer body, or the toner adhering to the intermediate transfer body at the time of paper jam or the like is secondary. There has been a problem of adhering to the transfer roll. When toner adheres to the secondary transfer roll, the back surface of the recording material supplied thereafter is soiled by the toner adhering to the secondary transfer roll, or normal due to the secondary transfer voltage. There is a problem that a strong electric field is not formed between the secondary transfer roll and the intermediate transfer member, and a part of the transfer is generated.
[0006]
As a technique for preventing the toner from adhering to such a secondary transfer roll, for example, in Japanese Patent Application Laid-Open No. 8-63011, for a secondary transfer roll disposed so as to always contact an intermediate transfer member, There has been proposed an image forming apparatus in which a voltage having a polarity opposite to the secondary transfer voltage is applied when no recording material (transfer material) is present between the secondary transfer roll and the intermediate transfer member. . This publication also provides a power source for applying a positive polarity secondary transfer voltage (950V) and a power source for applying a negative polarity voltage (-950V) to the secondary transfer roll. An embodiment in which the switching element is used by switching at a predetermined timing is shown.
[0007]
However, as in the image forming apparatus disclosed in this publication, when a voltage having a polarity opposite to the secondary transfer voltage is applied as the voltage having the opposite polarity applied to the secondary transfer roll, the secondary transfer roll The toner may not be prevented from adhering to the toner. Even when such a reverse polarity voltage is applied, the adhesion of the toner is caused by the electrical resistance value (specifically, the secondary transfer portion) composed of the secondary transfer roll, the intermediate transfer member, and the like. When the set value of the combined resistance of the secondary transfer roll, intermediate transfer body, and backup roll constituting the secondary transfer section does not match the value of the reverse polarity voltage applied, or the secondary When the combined resistance in the transfer portion fluctuates with time or due to factors such as humidity, or when the rotational speed of the intermediate transfer member or the like is changed according to the type of recording material.
[0008]
The present invention has been made to solve the above problems, and in particular, when no recording material is present between the secondary transfer roll and the intermediate transfer member, the toner adheres to the secondary transfer roll. An object of the present invention is to provide an image forming apparatus capable of reliably preventing the above-described problem.
[0009]
[Means for Solving the Problems]
  An image forming apparatus of the present invention capable of achieving the above object includes an image forming system for forming a toner image on an image carrier, and the image carrier that rotates in contact with the image carrier of the image forming system. The upper toner image is primarily transferred., Conductive material containedEndless intermediate transferbeltAnd this intermediate transferbeltAt least during secondary transfer, and the intermediate transferbeltIntermediate transfer of recording material onto which the upper toner image is secondarily transferredbeltPress againstGroundedA secondary transfer roll;A backup roll disposed to support the intermediate transfer belt on the opposite side of the secondary transfer roll, and coated with a semiconductive film;Secondary transfer roll and intermediate transferbeltWhenWith the backup rollSecondary transfer roll and intermediate transferbeltWhen a recording material is present between them, a positive or negative secondary transfer voltage is applied, and the secondary transfer roll and intermediate transfer are applied.beltA power supply device that applies a voltage having a polarity opposite to the secondary transfer voltage when no recording material is present betweenA power supply roll for supplying a secondary transfer voltage by contacting a secondary transfer voltage from the power supply device with the backup roll;
In an image forming apparatus comprising:
The reverse polarity voltage applied from the power supply device is defined as I (mA), a current that flows when a voltage is applied to the secondary transfer portion, v (m / sec), the rotational speed of the intermediate transfer member, When the length in the axial direction is w (m), the charge density represented by ρ = I / (v · w) ρ (mC / m² ) Is set such that 0 <ρ ≦ 0.3.
[0010]
In this case, the image forming system generally has a configuration in which a (color) toner image is formed on a rotating drum or belt-shaped photosensitive member by an electrophotographic process or the like, but is not limited thereto. is not. Further, the quantity is not limited to one and may be plural. In the case where there is one image forming system, a configuration in which toner images of a plurality of colors are sequentially formed on the image carrier is usually used.
[0011]
  In addition, the intermediate transferbeltThe bell is a member that can hold the toner image transferred from the imaging system.GWhat(Intermediate transfer member)And rotates in synchronization with the image carrier of the image forming system. The secondary transfer roll is usually arranged so as to be able to contact and separate from the intermediate transfer body, but an image having a configuration in which an image forming operation is not required to separate the secondary transfer roll from the intermediate transfer body. In the case of a forming apparatus, the secondary transfer roll may be disposed so as to always contact the intermediate transfer member.
[0012]
  Furthermore, the power supply device,DuringWhen the intermediate transfer member is in the form of a belt, the secondary transfer is usually performed on a backup roll disposed so as to support the belt on the side opposite to the secondary transfer roll contacting the intermediate transfer belt.PhotocopyConfigured to apply pressure via a feed rollTheThe secondary transfer voltage may be any voltage that can form an electric field between the intermediate transfer member and the secondary transfer roll so that the toner image on the intermediate transfer member is electrostatically transferred to the recording material side. The polarity is appropriately selected to be either positive or negative depending on the toner charging polarity of the toner image and the condition of the target to which the secondary transfer voltage is applied.
[0013]
In addition, the power supply device has a function of converting a voltage obtained from one power supply into either a secondary transfer voltage or a voltage having a polarity opposite to the secondary transfer voltage and selectively outputting the voltage to the secondary transfer unit. A power supply control circuit having a power supply control circuit can be used, but in addition to this, a dedicated power supply for applying a secondary transfer voltage and a dedicated power supply for applying a reverse polarity to the secondary transfer voltage It is possible to use a power supply that is prepared separately and independently, and the voltage applied to the secondary transfer portion from the two power supplies is switched by the switching means.
[0014]
The reverse polarity voltage applied from the power supply device is set so that the charge density ρ is 0 <ρ ≦ 0.3, preferably 0 <ρ ≦ 0.12, as described above. When the charge density ρ is larger than “0.3”, a discharge is generated at the nip portion where the secondary transfer roll and the intermediate transfer member are in contact with each other, thereby reversing the charging polarity of the toner on the intermediate transfer member. The inverted toner is electrostatically transferred to the secondary transfer roll side due to the reverse polarity voltage applied.
[0015]
According to such an image forming apparatus, when a recording material exists between the secondary transfer roll and the intermediate transfer member, the secondary transfer voltage is applied from the power supply device. As a result, the toner image on the intermediate transfer member is secondarily transferred to the recording material by the applied pressure and electrostatic action. On the other hand, when there is no recording material between the secondary transfer roll and the intermediate transfer member, the voltage from the power supply device is opposite to the secondary transfer voltage and the charge density ρ is 0 <ρ ≦ 0.3. Is applied. As a result, an appropriate electric field is formed between the intermediate transfer member and the secondary transfer roll so that the toner on the intermediate transfer member is attracted in the direction toward the intermediate transfer member. The toner is held while adhering to the transfer body side, and such toner hardly transfers and adheres to the secondary transfer roll side.
[0016]
Incidentally, in this image forming apparatus, when a paper jam occurs, the toner may adhere to the intermediate transfer member or the toner may adhere to the intermediate transfer member. Is applied to the secondary transfer unit from the power supply device to transfer the toner adhering to the secondary transfer roll to the intermediate transfer member and return it to the intermediate transfer member. It is also effective to configure this. Specifically, for example, during a jam recovery operation (including toner removal work on the intermediate transfer member) performed after the operation of the image forming apparatus is stopped due to detection of occurrence of a paper jam and the jam removal work is completed. Then, the reverse polarity voltage is applied. As a result, the toner adhering to the secondary transfer roll is transferred to the intermediate transfer member and returned to the intermediate transfer member, and is then removed by the intermediate transfer member cleaning means.
[0017]
In this image forming apparatus, for example, a reverse polarity voltage applied from a power supply device may be applied from a constant current power supply.
[0018]
In this case, a power source provided with a known constant current control circuit can be used as the constant current power source. In the case of such a configuration, the above-described equation representing the charge density ρ is expressed as ρ = I / (v · w) = V_n/ (R_n(V · w)_nIs the combined resistance, V, which is the sum of the resistances of the parts that make up the secondary transfer section_nIndicates the voltage applied to the secondary transfer portion), and the charge density ρ has a constant current I, and therefore the combined resistance R of the secondary transfer portion._nIt can be seen that it can be easily set and controlled in accordance with the rotational speed v of the intermediate transfer member and the axial length w of the secondary transfer roll regardless of the value of. Therefore, even if the combined resistance R of the secondary transfer portion_nEven if the voltage fluctuates, the charge density ρ can be maintained within the specific range described above, and thus toner adhesion to the secondary transfer roll can be reliably prevented.
[0019]
Further, in this image forming apparatus, for example, a reverse polarity voltage applied from a power supply device is applied from a constant voltage power source, and at least a combined resistance of the secondary transfer unit between the constant voltage power source and the secondary transfer unit It is advisable to provide an additional resistance equal to or greater than that in series.
[0020]
In this case, a power source provided with a known constant voltage control circuit can be used as the constant voltage power source. Moreover, a general resistance element can be used as the additional resistance. The upper limit value of the additional resistance is preferably about 10 times the combined resistance of the secondary transfer portion. In the case of such a configuration, the above-described equation representing the charge density ρ is expressed as ρ = I / (v · w) = (R_n+ R_x) / (V_n・ V · w)} and transform (R_xIndicates the value of the additional resistance), voltage V_nAnd the combined resistance R of the secondary transfer part_nIf the value of the additional resistance is sufficiently large even if the value of R_n+ R_x) Value does not vary so much, so the combined resistance R_nIt can be seen that even if fluctuates, the change in charge density ρ can be reduced. Therefore, the combined resistance R of the secondary transfer portion_nEven if fluctuates, the charge density ρ can be maintained within the specific range described above, so that the toner can be reliably prevented from adhering to the secondary transfer roll.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0022]
[Embodiment 1]
FIG. 1 is a schematic diagram of a color image forming apparatus to which the present invention is applied. In the figure, reference numeral 10 denotes an image forming system. The image forming system 10 is disposed around a photosensitive drum (image carrier) 11 that rotates in the direction of arrow A and the photosensitive drum 11. The charging device 12, the laser writing device 13, the developing device 14, the drum cleaner 16, the static eliminator 17, and the like. Among these, the developing device 14 sensitizes four developing devices (specifically, 14Bk, 14Y, 14M, and 14C) corresponding to the respective colors of black (Bk), yellow (Y), magenta (M), and cyan (C). These are sequentially arranged around the body drum 11.
[0023]
In such an image forming system 10, basically, after the surface of the photosensitive drum 11 is uniformly charged by the charging device 12, the exposure beam Bm scans the charged drum surface from the laser writing device 13. By exposure, an electrostatic latent image corresponding to the image information is formed. Subsequently, the electrostatic latent image is developed with a predetermined color toner supplied from the developing device 14 to be visualized, whereby a toner image T is formed on the photosensitive drum 11.
[0024]
Reference numeral 20 denotes an intermediate transfer belt (intermediate transfer member) disposed so as to come into contact with the surface of the photosensitive drum 11 in the image forming system 10, and is stretched around a plurality of rolls 21 to 24 in the direction of arrow B. Rotate to. Here, the roll 21 is a drive roll for the intermediate transfer belt 20, the roll 22 is a driven roll, the roll 23 is a tension roll that controls the tension of the intermediate transfer belt 20 to be constant, and the roll 24 is a backup roll for secondary transfer. (Opposite roll).
[0025]
A primary transfer roll 15 as a primary transfer device is disposed on the back side of the portion (primary transfer position) of the intermediate transfer belt 20 facing the photosensitive drum 11. A primary transfer voltage having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roll 15, whereby the toner image T on the photosensitive drum 11 is electrostatically attracted to the intermediate transfer belt 20. ing.
[0026]
Further, on the surface side (secondary transfer position) supported by the backup roll 24 of the intermediate transfer belt 20, a secondary transfer roll 25 that can contact and separate from the intermediate transfer belt 20 is disposed. . The secondary transfer roll 25 is provided with a cleaning blade 25a for removing the toner adhering to the roll surface as necessary. The backup roll 24 functions as a counter electrode of the secondary transfer roll 25, and a secondary transfer voltage having the same polarity as the charging polarity of the toner is supplied via the power supply roll 26.
[0027]
Further, on the downstream side of the secondary transfer position of the intermediate transfer belt 20, a belt cleaner 27 that removes toner that remains on the intermediate transfer belt 20 and is not transferred after the secondary transfer is attached to the surface of the intermediate transfer belt 20. It is provided so that it can be touched and separated. A detection sensor 28 including a reflective photosensor is disposed downstream of the belt cleaner 27 and upstream of the primary transfer position. The detection sensor 28 is configured to detect a reference tape 20 a for detecting the writing timing of the laser writing device 13, which is affixed to an end region that is a non-image forming region of the intermediate transfer belt 20.
[0028]
As for the conveyance system of the recording paper (recording material) P, the recording paper P accommodated in the supply tray 30 is sent out by the field roll 31, temporarily stopped by the registration roll (registration roll) 32, and then the recording paper P Are fed to a nip where the secondary transfer roll 24 and the intermediate transfer belt 20 abut at a predetermined timing. Further, the paper P after the secondary transfer is guided to the belt conveyance device 33 through a paper conveyance guide (not shown), and is conveyed to the fixing device 34 by the belt conveyance conveyance 33.
[0029]
The secondary transfer roll 25 and the belt cleaner 28 described above are disposed so as to be freely contacted and separated from the surface of the intermediate transfer belt 20 by a retract device (not shown). When the primary transfer is performed so that the toner images of a plurality of colors are superimposed on the intermediate transfer belt 20, until the toner image before the final color passes through the secondary transfer roll 25 and the belt cleaner 28. Both of them are separated from the intermediate transfer belt 20.
[0030]
Further, in this color image forming apparatus, a patch image composed of a toner image for correcting an image density image forming condition is transferred to an image non-transfer area of the intermediate transfer belt 20 (an intermediate transfer belt on which a normal toner image T is formed). The patch image 28 is detected by a density detection detection sensor 39 disposed at a predetermined position around the intermediate transfer belt 20. It is like that. Further, the image forming process speed (the rotational speed of the photosensitive drum 11 and the intermediate transfer belt 20) can be changed according to the type of the recording paper P. For example, the recording paper P is a general paper such as plain paper. A “normal speed” in the case of a typical recording material and a “low speed” in the case of a special recording material such as an OHP sheet are prepared.
[0031]
In this color image forming apparatus, as shown in FIG. 2, the secondary transfer unit 40 including the secondary transfer roll 25, the intermediate transfer belt 20, the backup roll 24, and the power supply roll 26 has a secondary transfer. A predetermined voltage including the secondary transfer voltage described above is applied from the power supply device 50 for transfer via the power supply roll 26.
[0032]
The power supply device 50 in this embodiment is used for an image forming apparatus in which the toner charging polarity of the toner image T on the intermediate transfer belt 20 is set to “negative polarity”, and applies a secondary transfer voltage. The first power source 51, the second power source 52 that applies a voltage having a polarity opposite to the secondary transfer voltage and a charge density ρ described later, and the connection between the two power sources 51 and 52 and the power supply roll 26 are switched. And a switch 53. Then, when the secondary transfer is executed, the power supply device 50 operates the switch 53 at a timing to be described later, and connects the terminal a of the switch 53 to the terminal b so that the first transfer is made to the secondary transfer unit 40. A negative voltage equal to the toner charging polarity is applied as a secondary transfer voltage from the power supply 51, and a positive voltage is applied to the secondary transfer portion 40 from the second power supply 52 by connecting the terminal a to the terminal c. It is to be applied.
[0033]
The operation of the switch 53 is controlled by a control means (not shown). Specifically, when the recording paper P is between the secondary transfer roll 25 and the intermediate transfer belt 20, the secondary transfer voltage is set. The terminal a is connected to the terminal b so as to be applied, and the terminal a is connected to the terminal c so as to apply the reverse polarity voltage when there is no recording paper P between them. At this time, the presence / absence of the recording paper P between the secondary transfer roll 25 and the intermediate transfer belt 20 is known because the timing information about the recording paper P until it reaches and passes through the secondary transfer unit 40 is known. A prediction is determined based on the timing information.
[0034]
In addition to the voltage applied from the second power supply 52 having a polarity opposite to that of the secondary transfer voltage, the power supply device 50 generates a current that flows when a voltage is applied to the secondary transfer unit 40 as I ( mA), where the rotational speed of the intermediate transfer belt 20 is v (m / sec) and the length of the secondary transfer roll 24 in the axial direction is w (m), ρ = I / (v · w). Charge density ρ (mC / m²) Is set to satisfy 0 <ρ ≦ 0.3. In this embodiment, in order for the charge density ρ to satisfy the above condition, the rotational speed v of the intermediate transfer belt 20 and the length w of the secondary transfer roll 24 are appropriately set and fixed, and then 0 < Adjustment is made so as to apply a voltage such that a current I satisfying ρ ≦ 0.3 is obtained.
[0035]
Next, a basic image forming process by this color image forming apparatus will be described.
[0036]
First, in the image forming system 10, a toner image T of a predetermined color is formed on the photosensitive drum 11 as described above. For example, if the electrostatic latent image written on the photoconductive drum 1 corresponds to yellow image information, the latent image is developed by the developing device 14Y corresponding to yellow (Y) toner, and thereby the photoconductor. A yellow toner image T is formed on the drum 11.
[0037]
Subsequently, the toner image T is transferred to the surface of the intermediate transfer belt 20 by a primary transfer voltage applied to the primary transfer roll 15 at a primary transfer position where the photosensitive drum 11 and the intermediate transfer belt 20 are in contact with each other. On the other hand, the toner remaining on the photosensitive drum 11 after the primary transfer is removed by the drum cleaner 16, and the cleaned surface of the photosensitive drum 11 is neutralized by the static eliminator 17.
[0038]
At this time, when a single color image is formed, the single color toner image is secondarily transferred to the recording paper P immediately after the primary transfer. However, when a color image in which a plurality of color toner images are superimposed is formed. In the image forming system 10, the toner image formation and the primary transfer process of the toner image are repeated for the number of colors. For example, when a toner image (full color image) is formed by superimposing four color toner images, yellow, magenta, cyan, and black toner images T are formed on the photosensitive drum 11 for each rotation. Thereafter, each of the toner images T is primarily transferred to the intermediate transfer belt 20. On the other hand, the intermediate transfer belt 20 is rotated at the same cycle as that of the photosensitive drum 11 while retaining the yellow toner image that has been primarily transferred first, whereby magenta, Cyan and black toner images are transferred and formed so as to be superimposed.
[0039]
Subsequently, the toner image T primarily transferred to the intermediate transfer belt 20 is conveyed to the secondary transfer position with the rotation of the intermediate transfer belt 20, while the recording paper P is conveyed to a predetermined timing by the registration roll 32. (The secondary transfer roll 25 is in contact with the intermediate transfer belt 20) and is supplied to the secondary transfer position. Thereby, the secondary transfer of the toner image T onto the recording paper P is executed.
[0040]
In this secondary transfer, when the recording paper P exists between the secondary transfer roll 25 and the intermediate transfer belt 20, the secondary transfer voltage is supplied from the first power source 51 as shown in FIG. 26 is applied to the secondary transfer section 40 via the H.26. The recording paper P supplied to the secondary transfer position is brought into pressure contact with the intermediate transfer belt 20 supported by the backup roll 24 by the secondary transfer roll 25. As a result, the toner image T carried on the intermediate transfer belt 20 is transferred to the recording paper P side under electrostatic action and pressure action.
[0041]
On the other hand, when the recording paper P is not present between the secondary transfer roll 25 and the intermediate transfer belt 20, the secondary transfer roll 25 and the intermediate transfer belt 20 are in direct contact. As shown, a reverse polarity voltage is applied from the second power source 52 to the secondary transfer unit 40 via the power supply roll 26. At this time, since the reverse polarity voltage satisfies the above-described condition of the charge density ρ, even if the combined resistance R of the secondary transfer unit 40 is_nEven if the resistance value of the backup roll 24 and the intermediate transfer belt 20 is changed due to environmental conditions or the like, the toner on the intermediate transfer belt 20 is electrostatically transferred to the secondary transfer roll 25 side. Never happen.
[0042]
The recording paper P after the secondary transfer is transported to the fixing device 34 via the transport belt 33, and the toner image T subjected to the secondary transfer is fixed. On the other hand, the toner remaining on the intermediate transfer belt 20 after the secondary transfer is removed by the belt cleaner 28. Even if a small amount of toner or the like adheres to the surface of the secondary transfer roll 25, the toner image is removed by the cleaning blade 25a.
[0043]
Through the above process, a single color image or a color image made of toner is formed on the recording paper P. In this image forming apparatus, as described above, the toner does not adhere to the secondary transfer roll 25 and is not soiled. Therefore, when the subsequent secondary transfer is performed, the recording that passes through the secondary transfer unit 40 is performed. The back surface of the paper P is not soiled by the toner adhering to the secondary transfer roll 25. In addition, due to the toner adhering to the secondary transfer roll 25, a uniform transfer electric field is not formed, and no transfer breakage occurs. Therefore, in this image forming apparatus, a uniform and stable secondary transfer is performed without contamination of the back side of the recording paper.
[0044]
Next, the following tests were performed using this color image forming apparatus.
[0045]
That is, the test is configured such that the rotation speed v of the intermediate transfer belt 20 in the image forming apparatus can be changed between a normal speed of 220 m / sec and a low speed of 60 m / sec, and then the secondary transfer section 40 is configured. Composite resistance R_nAnd the reverse polarity voltage applied from the second power source 52 are changed as shown in Tables 2 and 3, the current I (μA) flowing through the secondary transfer unit 40 is measured, and the intermediate transfer belt 20 is also measured. A test patch image having a density of 50% was formed in the inter-image portion, and when the recording paper P was passed after passing through the secondary transfer portion 40, the state of toner contamination on the back surface of the paper was examined. Incidentally, the secondary transfer voltage applied in the image forming apparatus in this test is appropriately set within the range of −1 to −2.3 kV.
[0046]
At this time, the combined resistance R of the secondary transfer unit 40_nAbout the resistance R of the intermediate transfer belt 20₁And resistance R of the backup roll 24₂And set by combining them appropriately (R_n= R₁+ R₂). Among these, the intermediate transfer belt 20 is a single-layer belt formed by adding a suitable amount of a conductive material such as carbon black to polyimide and molding it to a thickness of 0.1 mm and a width of 320 mm. R₁10^9.8Ω · cm and 10^12.1The one set to Ω · cm was used. Further, as the backup roll 24, a semiconductive thin film is coated on an insulating roll, and its surface resistance R₂10^7.3Ω / □ and 10^8.2The one set to Ω / □ was used. Then, by combining the intermediate transfer belt 20 of each volume resistance and the backup roll 24 of each surface resistance as shown in Table 1, the combined resistance R of the secondary transfer unit 40 is obtained._n4 types of 16 MΩ, 23 MΩ, 34 MΩ, and 41 MΩ were prepared (this combined resistance R_nIs a value when a voltage that causes a current of 35 mA to flow through the secondary transfer portion 40 is applied).
[0047]
[Table 1]

[0048]
Further, as the secondary transfer roll 25, carbon is dispersed on a grounded cored bar to obtain 10%.⁷A urethane foam layer set to a low resistance of Ω · cm or less is formed, an ionic conductive urethane rubber layer (thickness of about 1 mm) is formed on the outer periphery, and a coat layer (thickness) containing a fluororesin on the outer periphery. (Several tens of μm) having a hardness (Asker C) set to 40 ° or less was used. The length w of the secondary transfer roll 24 in the axial direction was 350 mm. The secondary transfer roll 24 in this test does not use the cleaning blade 25a.
[0049]
The backside contamination of the recording paper P was evaluated according to the following grade (G) standard. The density in parentheses in G2 to G4 is a relative density when the density of the test patch image is “1”. Moreover, the allowable grade which can be accepted among the grades G1 to G4 is “G1”.
G1: No dirt visible to the naked eye (to the extent that toner adhesion can be observed with a magnifying glass)
G2: Dirt that was visible to the naked eye was generated (density 0.09)
G3: Conspicuous dirt was generated (density 0.15)
G4: Severe dirt was generated (concentration of 0.19 or more)
[0050]
The results of the above tests are shown in Tables 2 and 3 and shown in FIG. In the table, the items marked with a horizontal line “-” are not tested from a predictive judgment such as the low possibility of obtaining grade G1 evaluation results.
[0051]
[Table 2]

[0052]
[Table 3]

[0053]
The results obtained by this test are shown in a graph with the horizontal axis as the charge density ρ as shown in FIG. 4 and the vertical axis as the grade (G) of the paper backside stain. The charge density is calculated by substituting v = 220 mm / sec or 60 mm / sec, w = 350 mm into the equation of ρ = I / (v · w), and substituting the value of the measured current I. ing. In addition, some of the ◯ marks plotted in the figure are shown in a state where a plurality of data are overlapped.
[0054]
As is apparent from the results shown in FIG. 4, when the charge density ρ is in the range of 0 <ρ <0.3, it can be seen that the grade of the back surface stain of the paper is the allowable grade “G1”.
[0055]
Further, from the results shown in Table 2 and FIG. 3, when the rotational speed v of the intermediate transfer belt 20 and the length w of the secondary transfer roll 25 are set first, a voltage having a reverse polarity is applied to the secondary transfer unit 40. As long as the current I flowing when it is applied is kept at an appropriate value (a value such that the dirt grade becomes G1), the resistance R of the secondary transfer portion_nRegardless of this, it can be seen that the backside contamination of the recording paper can be reliably prevented. Therefore, in this image forming apparatus, when the power source having a constant current control function is used as the second power source 52 of the transfer device 50 and the reverse polarity voltage is applied from the power source, the current I is By adopting the above-described configuration for controlling to an appropriate value, the combined resistance R of the secondary transfer portion_nEven if the fluctuation occurs, it is possible to reliably prevent the toner from adhering to the secondary transfer roll 25 and thus to reliably prevent the back surface of the recording paper P from being stained.
[0056]
[Embodiment 2]
This embodiment has the same configuration as that of the color image forming apparatus according to the first embodiment except that the power supply device 50 having the configuration as shown in FIG. 5 is applied. That is, when compared with power supply device 50 in the first embodiment, power supply device 50 in this embodiment uses a power supply that is subjected to constant voltage control as second power supply 52, and is connected to second power supply 52 and second power supply 52. The difference is that an additional resistor 54 is connected in series between the next transfer portion (power feeding roll 26), and the other points are common.
[0057]
In this case, as the additional resistor 54, at least the combined resistance R of the secondary transfer unit 40_nA resistor having a resistance value equal to or greater than that may be used. Further, when a voltage having a reverse polarity is applied from the second power supply 52 of the power supply device 50, the charge density ρ calculated from the current I flowing through the secondary transfer section 40 is appropriate so as to satisfy the specific range described above. A voltage value may be selected, and the voltage of the selected value may be applied from the second power source 52 to the secondary transfer unit 40 under constant voltage control.
[0058]
In the image forming apparatus to which such a power supply device 50 is applied, when the recording sheet P is present between the secondary transfer roll 25 and the intermediate transfer belt 20 during the secondary transfer, FIG. As shown, the secondary transfer voltage is applied from the first power source 51 to the secondary transfer unit 40 via the power supply roll 26. Thereby, normal secondary transfer is performed. On the other hand, when the recording paper P is not present between the secondary transfer roll 25 and the intermediate transfer belt 20, a reverse polarity voltage is supplied from the second power source 52 through the power supply roll 26 as shown in FIG. Applied to the secondary transfer unit 40. At this time, the reverse polarity voltage is such that the charge density ρ satisfies the above-described condition, and the combined resistance R of the secondary transfer unit 40_nEven if the voltage fluctuates, the appropriate reverse polarity voltage can be applied to the secondary transfer unit 40 almost stably, so that the combined resistance R of the secondary transfer unit 40 is the same as in the first embodiment._nHowever, the toner on the intermediate transfer belt 20 is not electrostatically transferred to the secondary transfer roll 25 side even if it fluctuates due to environmental conditions or the like.
[0059]
Therefore, even when such a power supply device 50 is applied, the combined resistance R of the secondary transfer unit 40 is almost the same as in the first embodiment._nEven if the fluctuation occurs, it is possible to reliably prevent the toner from adhering to the secondary transfer roll 25 and thus to reliably prevent the back surface of the recording paper P from being stained.
[0060]
FIG. 7 shows the combined resistance R of the secondary transfer unit 40 in the color image forming apparatus used in the test according to the first embodiment._nThe voltage-current characteristics of the secondary transfer unit 40 for each image forming apparatus in which the low secondary resistance (16 MΩ) and the high secondary resistance (41 MΩ) are set, and the power supply device 50 in each image forming apparatus are shown. The second power source 52 is a power source that is output with the constant voltage controlled to “1 kV”, and the second power source 52 and the secondary transfer unit 40 are connected with an additional resistor 54 of “100 MΩ”. The state of the partial pressure in the next transfer part 40 and the additional resistance part (54) is shown.
[0061]
  Considering the results shown in FIG. 7 with reference to the data in Table 2, first, in the image forming apparatus using the power source device to which the second power source 52 and the additional resistor 54 of the constant voltage control are not applied, the secondary transfer unit When the combined resistance of 40 is the high resistance (41 MΩ), a current of 15 μA flows when a voltage of 1 kV is applied as shown in Table 2 as well as FIG. 7, and the result of the dirt grade at that time is shown in Table 2. Since it is “G1”, it can be seen that the back side of the paper is not smudged with the naked eye. However, when the combined resistance of the secondary transfer portion 40 is the low resistance (16 MΩ), FIG.2As shown in FIG. 2, when a voltage of 1 kV is applied, a current of 90 μA flows, and the result of the stain grade at that time becomes “G3” from Table 2, and it can be seen that the back side of the paper is in a state of being generated.
[0062]
On the other hand, in the image forming apparatus to which the second power source 52 and the additional resistor 54 of constant voltage control are applied, as shown in FIG. 7, 1 kV applied from the constant voltage power source 52 with the voltage value at the intersection Q as a boundary. “About 0.7 kV” is divided into the additional resistance portion (54), and the remaining “about 0.3 kV” is divided into the secondary transfer portion 40. As a result, in this image forming apparatus, even when the combined resistance of the secondary transfer unit 40 is the low resistance (16 MΩ), the voltage actually applied to the secondary transfer unit 40 is “about 0” as described above. .3 kV ”, and the result of the stain grade at the applied voltage value is“ G1 ”in the column of 0.25 kV, which is a value close to 0.3 kV in Table 2, so that the back surface of the paper is stained. It turns out that it is in the state which does not.
[0063]
Accordingly, when the power supply device 50 to which the second power source 52 and the additional resistor 54 to which the constant voltage control is applied is used as in the present embodiment, even if the combined resistance of the secondary transfer unit 40 fluctuates, It can be seen that the back surface of the recording paper P due to the toner adhering to the secondary transfer roll 25 can be prevented. Incidentally, in this image forming apparatus, it is effective to use an additional resistor 54 having a resistance of about 20 MΩ to 400 MΩ.
[0064]
[Other embodiments]
In the first and second embodiments, the color image forming apparatus of the type that sequentially forms four color toner images of a full color image by rotating one photoconductor drum 11 four times has been described. For example, as shown in FIG. 8, the present invention can be similarly applied to a so-called tandem type color image forming apparatus using an intermediate transfer method. In FIG. 8, reference numerals 10K, 10Y, 10M, and 10C are all monochromatic image forming systems, and dedicated image forming units for individually forming toner images of black K, yellow Y, magenta M, and cyan C, respectively. It is. The configuration of each image forming system 10 is substantially the same as that of one image forming system 10 as in the first embodiment, etc., and the developing device 14 has one development in which toner of a dedicated color is accommodated for each system 10. The only difference is that the devices 14K, 14Y, 14M, and 14C are configured.
[0065]
Even in the case of such a tandem type image forming apparatus, by adopting a power supply having the same configuration as that of each of the above-described embodiments as a power supply device for the secondary transfer unit 40, the secondary transfer unit Regardless of the variation in the combined resistance of 40, it is possible to reliably prevent the toner from adhering to the secondary transfer roll 25, and thus to reliably prevent the back surface of the recording sheet P from being stained.
In the first embodiment,
[0066]
【The invention's effect】
As described above, according to the image forming apparatus of the present invention, when no recording material is present between the secondary transfer roll and the intermediate transfer member, toner adhesion to the secondary transfer roll is reliably prevented. be able to. In addition, the effect of preventing toner adhesion can be reliably obtained even if the resistance of the secondary transfer portion varies. Therefore, when this image forming apparatus is used, it is possible to perform good secondary transfer without the backside contamination of the recording material or transfer omission due to toner adhesion to the secondary transfer roll.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating a main configuration of a color image forming apparatus according to a first embodiment.
FIG. 2 is a schematic diagram illustrating a main configuration of a secondary transfer unit and its power supply device.
3 is an explanatory diagram illustrating a voltage application state from the power supply device of FIG. 2 to a secondary transfer unit;
4 is a graph showing a result of a test in the first embodiment. FIG.
5 is a schematic diagram illustrating a main configuration of a secondary transfer unit and a power supply device thereof in a color image forming apparatus according to Embodiment 2. FIG.
6 is an explanatory diagram illustrating a voltage application state from the power supply device of FIG. 5 to the secondary transfer unit.
FIG. 7 is a graph showing a result of a test in the second embodiment.
FIG. 8 is a schematic diagram illustrating a main configuration of a color image forming apparatus according to another embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Image forming system, 11 ... Photosensitive drum (image carrier), 20 ... Intermediate transfer belt (endless intermediate transfer member), 25 ... Secondary transfer roll, 40 ... Secondary transfer part, 50 ... Power supply device, 52... Second power source (power source for applying reverse polarity voltage) 54... Additional resistance P. Recording sheet (recording material).

Claims (3)

An image forming system for forming a toner image on an image carrier;
An endless intermediate transfer belt containing a conductive material that rotates in contact with the image carrier of the image forming system and to which a toner image on the image carrier is primarily transferred;
A grounded secondary transfer roll that contacts the intermediate transfer belt at least during execution of secondary transfer and presses a recording material onto which the toner image on the intermediate transfer belt is secondarily transferred to the intermediate transfer belt ;
A backup roll disposed on the opposite side of the secondary transfer roll to support the intermediate transfer belt, and coated with a semiconductive film;
When a recording material is present between the secondary transfer roll and the intermediate transfer belt with respect to a secondary transfer portion including the secondary transfer roll, the intermediate transfer belt, and the backup roll, positive polarity or negative polarity A secondary transfer voltage and a power supply device for applying a voltage having a polarity opposite to that of the secondary transfer voltage when no recording material is present between the secondary transfer roll and the intermediate transfer belt ,
In an image forming apparatus comprising: a power supply roll that supplies a secondary transfer voltage by contacting a secondary transfer voltage from the power supply device with the backup roll ;
The reverse polarity voltage applied from the power supply device is defined as I (mA), a current that flows when a voltage is applied to the secondary transfer portion, v (m / sec), the rotational speed of the intermediate transfer belt , When the length in the axial direction is w (m), the charge density ρ (mC / m ² ) represented by ρ = I / (v · w) is set to satisfy 0 <ρ ≦ 0.3. An image forming apparatus.   The image forming apparatus according to claim 1, wherein a reverse polarity voltage applied from a power supply device is applied from a constant current power supply.   A reverse polarity voltage applied from the power supply is applied from the constant voltage power supply, and at least an additional resistance equal to or greater than the combined resistance of the secondary transfer section is placed in series between the constant voltage power supply and the secondary transfer section. The image forming apparatus according to claim 1.

Images