JP4283572B2 - Image forming method and apparatus - Google Patents

Description

「○」印は付着もしくは散りが全く見られなかったもの、「×」印は付着もしくは散りが多く観察されたもの、「△」印はごく少量の散りが見られたものである。
【０１１５】
図５に示す電位検出手段９０４２を用い、前述のようにして請求項２記載の方法により「カウンタバイアス：Ｖｔの決定」を行った。図６にフロー図を示す。このフロー図における「Ｔｈｒ」は、１色転写による電位変化量の上限であり、１色の電位変化量がこの値以上である場合、３色目以降における１次転写前の中間転写ベルト上の画像部電位をＶ１とすると、４色目を１次転写する際の転写部上流側での電位差が大きくなり、トナーの飛翔が生じた。Ｔｈｒ以下であれば画像部電位をＶ１として問題は生じなかった。
【０１１６】
「Ｔｈｒ」は５０〜７０Ｖ程度であるが実施例では７０Ｖとした。おおよそ決定した中間転写ベルト上の画像部電位に基づき以下のテーブルを参照して、カウンタバイアス値を決める。このテーブルはシステムで固定したものを予め記憶させたものである。画像形成はフルカラーの画像形成を行い、１色転写後、２色転写後、３色転写後、４色転写後のそれぞれのタイミングで装置を停止し、感光体上と中間転写ベルト上のトナーの散り（プレ転写）とトナー付着（逆転写）を観察した。
【０１１７】

【０１１８】
カウンタバイアスを印加しない場合と比較し、転写チリの少ない良好な画像が得られた。画像部電位の検出に際しては、前述の如く「専用のパターン」による作像を行った。
【０１１９】
図５に示す電位検出手段９０４２を用い、前述のようにして請求項３記載の方法により「カウンタバイアス：Ｖｔの決定」を行った。
図７に示すフロー図に従い、中間転写体ベルト上の画像部の電位を推定し、上記テーブルを参照してカウンタバイアスを決定した。
【０１２０】
感光体上画像部への逆転写による他色の混入、プレ転写による中間転写体ベルト上のトナー散りもなく、良好な画像が得られた。
【０１２１】
なお、各静電潜像担持体上のトナー画像の画像部電位と、中間転写媒体の各転写部の上流側での画像部電位とを各々電位検出して、これら電位を合致させるように各転写部におけるカウンタバイアスを定め得ることは勿論である。
【０１２２】
【発明の効果】
以上に説明したように、この発明によれば新規な画像形成方法および装置を実現できる。この発明の画像形成方法によれば上記の如く、複数のトナー画像を中間転写ベルト媒体上に１次転写する際のプレ転写・逆転写を有効に防止もしくは低減できる。従って、この発明の画像形成方法を実施する画像形成装置は、プレ転写や逆転写に起因する像質劣化を良好に防止もしくは低減して高品質の画像形成を行うことができる。
【図面の簡単な説明】
【図１】カウンタバイアスを説明するための図である。
【図２】カウンタバイアスを説明するための図である。
【図３】１次転写後の中間転写媒体上の画像部電位を説明するための図である。
【図４】画像形成装置の実施の１形態を説明するための図である。
【図５】図５の画像形成装置における転写部を説明するための図である。
【図６】実施例の画像部電位推定のフロー図である。
【図７】実施例の画像部電位推定の別のフロー図である。
【符号の説明】
ＰＣ 静電潜像担持体（光導電性の感光体）
ＢＬ 中間転写ベルト
Ｒ１ 転写ローラ
Ｒ２ カウンタバイアスローラ
ＴＹ イエロートナー[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming method and apparatus.
[0002]
[Prior art]
The negative latent image formed on the electrostatic latent image carrier is reversely developed to obtain a positive toner image, and this toner image is transferred and fixed onto a sheet-like recording medium such as transfer paper. It is widely implemented in various image forming apparatuses such as apparatuses and optical printers.
[0003]
Recently, color image formation and the like has been widely put into practical use, and a plurality of negative latent images formed on one or more electrostatic latent image carriers are separately reversed and developed, and each toner image is transferred onto an intermediate transfer medium such as an intermediate transfer belt. An image forming method is performed in which toner images are synthesized by primary transfer to each other and superimposed on each other, and the synthesized toner images are secondarily transferred and fixed on a sheet-like recording medium.
[0004]
Conventionally, a problem of “toner scattering” has been known in connection with transfer of a toner image. Toner scattering is a phenomenon in which toner adheres to a “portion that is not an original image” in an image that is finally carried on a sheet-like recording medium as if the toner is “scattered” and impairs the image quality of the formed image. This phenomenon is caused when a part of the toner particles constituting the toner image is in a state where the distance between the toner image and the transfer material (intermediate transfer medium in the intermediate transfer method) is narrowed when the toner image enters the transfer region. It is thought to be due to so-called “pre-transfer” that flies and adheres to the surface of the transfer material.
[0005]
Further, as described above, when a plurality of toner images are combined on the intermediate transfer medium, the toner image transfer (primary transfer) to the intermediate transfer medium is repeated, but the second and subsequent times in the primary transfer are repeated. When transfer is performed, the toner particles of the toner image previously transferred onto the intermediate transfer medium fly to the latent image carrier and are further transferred to the intermediate transfer medium (called “reverse transfer”) ) Occurs. Even when reverse transfer occurs, image degradation similar to the “toner scattering” described above occurs in the formed image.
[0006]
Various methods for preventing or reducing the toner scattering have been proposed (for example, Patent Documents 1 and 2), but currently, countermeasures are being sought for the "reverse transfer". It is.
[0007]
[Patent Document 1]
JP-A-8-166728
[Patent Document 2]
JP-A-10-186878
[0008]
[Problems to be solved by the invention]
An object of the present invention is to realize an image forming method and apparatus that can effectively eliminate or reduce the problem of reverse transfer as well as the problem of toner scattering.
[0009]
[Means for Solving the Problems]
  According to the image forming method of the present invention, “N (≧ 2) negative latent images are formed on one or more electrostatic latent image carriers, and each of these negative latent images is reversely developed to produce N positive toner images. These toner images are obtained from the intermediate transfer medium.On the surfaceThe image forming method in which the images are sequentially transferred to each other and superimposed on each other, and then secondarily transferred from the intermediate transfer medium onto the sheet-like recording medium.
[0010]
The “electrostatic latent image carrier” is a medium on which a desired electrostatic latent image is formed. The most common electrostatic latent image carrier is a “photoconductive photoreceptor”. In this case, the electrostatic latent image is formed by uniform charging and exposure of the photoreceptor.
[0011]
The electrostatic latent image carrier can also have a dielectric surface and can hold an electrostatic latent image as a charge distribution on the surface. In this case, the electrostatic latent image is positioned by transferring the electrostatic latent image formed on another electrostatic latent image carrier, or after the electrostatic latent image carrier is uniformly charged, by a needle electrode or the like. It can be formed by selectively removing electricity.
[0012]
A “negative latent image” is an electrostatic latent image in which the potential of a portion to be an image (a portion to which toner adheres) is attenuated. Positive toner is obtained by reversal development using toner having the same polarity as the electrostatic latent image charge. Visualized as an image. If the electrostatic latent image carrier is, for example, a photoconductive photoconductor, after the photoconductor is uniformly charged, the potential of the image area is attenuated by light writing such as optical scanning, or It can be formed by irradiating a negative image of a document as a light image.
[0013]
The N electrostatic latent images may be formed in different ways. For example, when two negative latent images are formed, one may be formed by optical scanning, and the other may be formed by irradiating a negative image of a document as a light image.
[0014]
Two or more negative latent images may be visualized with “same color toner” or may be visualized with different color toners. When visualization is performed using different color toners, a two-color image, a multicolor image, or a color image can be formed by combining the toner images.
[0015]
  The electrostatic latent image carrier has a drum shape or a belt shape.
  The intermediate transfer medium has a belt shape, and is hereinafter referred to as “intermediate transfer belt”. Of electrostatic latent image carrier and intermediate transfer beltAs a combination in form, a combination of a drum shape and a belt shape, or a belt shape and a belt shape is possible.
[0016]
  In such a morphological combination, a drum-like or belt-like electrostatic latent image carrier and a belt-like member are formed at the primary transfer portion.Intermediate transfer beltContact to form a “nip”.
  That is, the transfer roller sandwiches the intermediate transfer belt together with the electrostatic latent image carrier to form the nip portion as the primary transfer portion. That is, the transfer roller contacts the intermediate transfer belt on the back side of the intermediate transfer belt.
[0017]
  The image forming apparatus according to claim 1 has the following characteristics.
  That is, at the time of the primary transfer, a counter bias: Vt is applied to the intermediate transfer belt in the vicinity of the upstream side of the primary transfer portion, and the counter bias: Vt at the M (= 2 to N) th primary transfer is set. , The electrostatic latent image carrier having the M-th toner image in which “the potential of the toner image portion previously formed on the intermediate transfer belt” is M-1 times of primary transfer.UpOf toner imagesurfaceIt is set to be approximately the same potential as the potential.
  The counter bias is applied by bringing a counter bias roller into contact with the back surface of the intermediate transfer belt in the vicinity of the upstream side of the primary transfer portion.
[0018]
  The above “M−1 potential of the toner image portion formed on the intermediate transfer belt first by the primary transfer” is the primary transfer of the toner image formed by the exposure and development of the reference pattern to the intermediate transfer medium. The potential of the toner image portion after transfer can be known in advance by measuring it with a surface potential meter. This image portion potential is shifted by “counter bias: Vt, and the toner image portion on the electrostatic latent image carrier is changed.surfaceThe shift amount by the counter bias is set so that the potential is substantially the same as the potential ”.
[0019]
  The potential of the “image portion of the Mth toner image” can be known in advance from the electrostatic latent image formation conditions and the development conditions.
  "Upstream side of the transfer part" means intermediate transferbeltThis is the side where the toner image that is primarily transferred enters the transfer portion.
[0020]
  When counter bias Vt is applied as described above, intermediate transferbeltThe M-th toner image that is primarily transferred toPrimaryWhen entering the transfer section, the surface of the electrostatic latent image carrier and the intermediate transferBelt surfaceIs transferred to the toner constituting the toner image on the electrostatic latent image bearing member (M-th primary transferred toner image).beltLarge electric force that flies to the side does not work, intermediate transferbeltFormed ontonerA large electric force that reversely transfers the toner to the electrostatic latent image carrier does not act on the toner constituting the image (formed by the toner image that has been primarily transferred to 1 to M-1).
  Therefore, pre-transfer and reverse transfer can be satisfactorily prevented or reduced.
[0021]
  2. The image forming method according to claim 1, wherein the counter bias Vt in the M (= 2 to N) -th primary transfer is “the toner image portion formed on the intermediate transfer belt by the first primary transfer.Difference between surface potential and surface potential before first primary transfer; ΔVOr the toner image portion formed on the intermediate transfer belt by the Nth primary transfer.surfaceIt is also possible to estimate and set based on the potential (claim 3).
[0022]
  For example, when N = 4, four electrostatic latent images are visualized with toners of four colors of yellow, magenta, cyan, and black, and are primarily transferred onto the intermediate transfer belt in the above order. After the yellow toner image obtained according to the reference pattern is primarily transferred onto the intermediate transfer belt, the potential is shifted by applying a counter bias to the potential obtained by measuring the surface potential of the toner image portion. In the vicinity of the upstream side of the transfer portion, the yellow toner image portionsurfaceThe potential is “magenta toner image portion of the latent image carrier having a magenta toner image.surfaceIt should be approximately equal to “potential”.
[0023]
  Intermediate transferbeltSince the transfer conditions for each color toner image are known in advance, the intermediate transfer is performed as described above.beltIf the potential of the yellow toner image on the medium (the image that has been primarily transferred first) is known, the intermediate transfer by the primary transfer of each subsequent color toner imagebeltSince the upper image portion potential can be estimated empirically (by preliminary experiment), the counter bias: Vt can be set based on such an estimated value.
[0024]
  4. An image portion on an intermediate transfer belt in a state where four color toner images (all of which are formed by a reference pattern) are primarily transferred from a yellow toner image to a black toner image as in the case of claim 3.Surface ofAlternatively, the potential may be measured and the counter bias (Vt) at the time of primary transfer of each toner image may be estimated and determined from this value. In the case of claim 3, after the primary transfer of all four color toner imagestonerOf the image partsurfacePotential and yellow toner image after transfertonerOf the image partsurfaceBased on the potential, a counter bias at the time of primary transfer of magenta, cyan, and black toner images may be estimated.
[0025]
  4. The image forming method according to claim 1, wherein the image forming method is formed on an intermediate transfer belt.tonerOf the image partsurfaceA potential maximum value can be used as the potential. That is, in this case, the toner image formed on the intermediate transfer beltsurfaceThe maximum value of the potential is shifted so as to be substantially equal to the potential of the toner image portion on the electrostatic latent image carrier.
[0026]
  5. The image forming method according to claim 1, wherein intermediate transfer is performed.beltBased on the output voltage detected by the surface potential detection means to detect the surface potentialtonerOf the image partsurfaceThe potential can be determined. Thus, the counter bias: Vt is determined simultaneously with determining the potential of the image portion.
  Further, in the image forming method according to claim 1, the surface potential of the intermediate transfer belt before the primary transfer: Vb, the surface potential of the toner image portion after the first transfer: V1 are detected, and the difference between these potentials: | When Vb−V1 | is greater than a predetermined threshold value: Thr, the counter bias at the time of M (= 2 to N) -th primary transfer: Vt is set to Vb + (V1−Vb) × (M−1). When the difference: | Vb−V1 | is equal to or less than a predetermined threshold value: Thr, the counter bias Vt in the M (= 2 to N) -th primary transfer may be set to V1. (Claim 5).
[0027]
The image forming method according to any one of claims 1 to 5, wherein N = 3 or 4, and different electrostatic latent images are yellow, magenta, cyan or red, green and blue, or black on these. Development can be performed with the added four color toners. In this case, different electrostatic latent images can be “sequentially formed on the same electrostatic latent image carrier”, but can be “divided into two or more electrostatic latent image carriers” (claims). Item 7).
[0028]
  According to the image forming apparatus of the present invention, “N (≧ 2) negative latent images are formed on one or more electrostatic latent image carriers, and each of these negative latent images is reversely developed to produce N positive toner images. Obtain these toner images for intermediate transferBelt surfaceSequential primary transfer on top and overlay each other, then intermediate transferbeltAn image forming apparatus for secondary transfer onto a sheet-like recording medium from above.
[0029]
The image forming apparatus according to claim 8 has the following characteristics.
[0030]
  That is, the primary transfer means for performing the primary transfer applies a counter bias: Vt to the intermediate transfer medium in the vicinity of the transfer bias applying means for applying the transfer bias to the primary transfer portion and the upstream side of the primary transfer portion. It has “counter bias means”.
  "Transfer bias means"The image forming apparatus includes a transfer roller that sandwiches the intermediate transfer belt together with the electrostatic latent image carrier and uses the sandwiching portion as a primary transfer portion.
  "The “counter bias unit” includes a counter bias roller that is in contact with the back side of the intermediate transfer belt in the vicinity of the upstream side of the primary transfer unit.
[0031]
  Then, the counter bias Vt at the time of the M (= 2 to N) -th primary transfer is set to the toner image portion previously formed on the intermediate transfer belt by the M-1 primary transfer.surfaceIt has a “counter bias setting unit” that sets the potential substantially the same as the potential.
[0032]
  The counter bias setting unit estimates “counter bias (Vt) at the time of M (= 2 to N) primary transfer based on the potential of the image portion formed on the intermediate transfer belt by the first primary transfer. (The counter bias at the time of the M (= 2 to N) th primary transfer: Vt is formed on the intermediate transfer belt by the Nth primary transfer).tonerOf the image partsurfaceIt is also possible to estimate and set based on the potential ”(claim 10).
[0033]
  11. The image forming apparatus according to any one of claims 8 to 10, wherein the counter bias setting unit is used for setting a counter bias.tonerOf the image partsurfaceAs the potential, the maximum potential value of the image portion formed on the intermediate transfer belt can be used.
[0034]
  The image forming apparatus according to any one of claims 8 to 11, wherein "on the intermediate transfer belt after the primary transfer"tonerOf the image partsurfaceAnd a counter bias setting unit that determines a surface potential of the toner image portion on the basis of the detection potential detected by the surface potential detection unit, and based on the surface potential, the second potential in the primary transfer. The counter bias can be set in the subsequent transfer ".
[0035]
The image forming apparatus according to any one of claims 8 to 12, wherein N = 3 or 4 and different electrostatic latent images are added to three colors of yellow, magenta, cyan or red, green and blue, or black to these. Further, it can be configured to develop with only four color toners. In this case, different electrostatic latent images may be sequentially formed on the same electrostatic latent image carrier, but different electrostatic latent images are formed separately on two or more electrostatic latent image carriers. In addition, primary transfer from each electrostatic latent image carrier to the intermediate transfer medium may be performed.
[0036]
  The image forming apparatus according to claim 14, wherein “3 or 4 electrostatic latent image carriers are transferred intermediately.beltThe image forming apparatus can be configured as a tandem image forming apparatus arranged in the traveling direction of the surface of the image forming apparatus (claim 15).
[0037]
To add a little, in the present invention, the number of toner images N used for image formation of one image can be N = 2, 3 or 4 as described above, but is not limited thereto. The present invention can be implemented even when N is 5 or more. For example, when forming a color image, normally, N = 4, and four electrostatic latent images are visualized with toners of four colors of yellow, magenta, cyan, and black. A more precise color expression is possible in the formed image.
[0038]
  Here, the operation of the invention will be described.
  1 shows an intermediate transfer from an electrostatic latent image carrier.beltThe state of the primary transfer of the toner image to is schematically shown. The electrostatic latent image carrier is rotated in the direction of an arrow by a photoconductive photosensitive member PC formed in a drum shape. The intermediate transfer medium travels to the right in the figure by an intermediate transfer belt BL formed in an endless belt shape.
[0039]
A “transfer roller” indicated by reference numeral R1 is applied with a transfer voltage from a power source D1. Reference numeral R2 denotes a counter bias roller. Counter bias: Vt is applied from the power source D2 to the counter bias roller R2. The counter bias roller R2 and the power source D2 constitute “counter bias means”.
[0040]
Since the rotation direction of the photoconductor PC and the traveling direction of the intermediate transfer belt BL are as described above, the counter bias Vt by the counter bias roller R2 is intermediate on the upstream side of the position of the transfer roller R1 that is the “transfer portion”. It is applied from the back side of the transfer belt BL.
[0041]
FIG. 1 shows a state in which a yellow toner image formed with yellow toner TY is transferred. Here, it is assumed that the yellow toner image is first primarily transferred.
[0042]
As the photoconductor PC rotates in the direction of the arrow, the yellow toner image enters the transfer portion from the left side of the drawing and is transferred onto the intermediate transfer belt BL by the transfer voltage applied by the transfer roller R1.
[0043]
For the purpose of description, in the following description, an electrostatic latent image is a negative latent image formed by a negative potential distribution (the non-image portion has a negative potential uniformly, and the image portion is exposed to light. , The surface potential is light attenuated). The yellow toner TY is negatively charged with the same polarity as that of the electrostatic latent image and is captured by the image portion of the negative latent image where the potential is attenuated by reversal development to form a yellow toner image. Accordingly, in order to transfer the yellow toner image to the intermediate transfer belt BL, the transfer roller R1 applies a positive potential to the transfer portion.
[0044]
FIG. 2A schematically shows the potential relationship between the yellow toner image portion and the non-image portion on the photoreceptor PC. The potential changes in the vertical direction, and the upper side of the figure is the direction in which the positive potential increases, and GND indicates the ground level. The surface of the photoreceptor PC has a uniform negative potential: V₀In the negative latent image portion, the latent image potential is V due to light attenuation._LThe negative yellow toner TY is captured in this portion.
[0045]
Latent image potential: V_LIs 0 V if light attenuation is ideally performed, but in reality, a potential of about −100 to −150 V remains. The transfer voltage by the transfer roller R1 forms an “electric field from the intermediate transfer belt BL to the photoreceptor PC” at the transfer portion, and the yellow toner TY is transferred onto the intermediate transfer belt BL by the action of the electric field. That is, since the yellow toner TY has a negative polarity, the yellow toner TY moves to a higher potential side (a side where the potential increases in a positive direction, that is, upward in FIG. 2A).
[0046]
Considering the aforementioned “pre-transfer”, if the counter bias Vt by the counter bias roller R2 is set to 0, the image portion potential of the electrostatic latent image shown in FIG._LIs the ideal value: 0V, even if the surface of the photoreceptor PC and the surface of the intermediate transfer belt BL are close to each other in the vicinity of the upstream side of the transfer portion, the “yellow toner TY is transferred to the intermediate transfer belt BL in the gap portion between them. The “electric field flying to the side” does not substantially occur and pre-transfer does not occur.
[0047]
However, as described above, in reality, the image portion potential: V_LSince the counter bias: Vt is 0, an electric field from the photoreceptor PC toward the intermediate transfer belt BL is generated in the gap portion, and yellow toner TY is caused by the action of the electric field. It flies toward the intermediate transfer belt BL and generates “toner scattering” due to pre-transfer.
[0048]
In order to prevent such pre-transfer, counter bias: Vt is set to “absolute value is at least latent image potential: V_LIn this way, the yellow toner TY is repelled from the intermediate transfer belt BL in the gap between the photoconductor PC and the intermediate transfer belt BL in the vicinity of the upstream side of the transfer portion. An electric field is generated and pre-transfer is effectively prevented.
[0049]
FIG. 2B schematically shows a potential state on the intermediate transfer belt BL to which the yellow toner image is transferred by the yellow toner TY. In the state where the yellow toner image is transferred onto the intermediate transfer belt, the surface potential of the intermediate transfer belt BL is more positive than the ground level GND as shown in the figure. This is because positive charges are applied to the intermediate transfer belt from the transfer roller R1 during transfer.
[0050]
  The potential of the surface of the intermediate transfer belt is not uniform, and the “yellow toner image portion” where the yellow toner TY is attachedsurfaceThe potential is higher than the surface potential of the non-image area. This is because, when the transfer is performed, the negative charge constituting the non-image portion on the photoconductor PC is transferred by the peeling discharge to the non-image portion, and the potential is biased to the minus side from the image portion. is there.
[0051]
FIGS. 2C to 2E are explanatory diagrams showing the potential relationship when the magenta toner image is transferred by the magenta toner TM onto the intermediate transfer belt on which the yellow toner image is transferred by the yellow toner TY. .
[0052]
In the state of FIG. 2C, “the counter bias: the negative polarity of Vt is insufficiently positive” in the vicinity of the upstream side of the transfer portion of the magenta toner image, and the surface potential on the intermediate transfer belt BL is the image portion of the electrostatic latent image. Potential: when higher than VL. In this case, since the electric force F1 directed toward the photoconductor PC acts on the negative magenta toner TM constituting the magenta toner image on the photoconductor, toner scattering due to the pre-transfer of the magenta toner YM occurs. appear.
[0053]
On the contrary, the state of FIG. 2C shows a state in which the negative polarity of the counter bias Vt is too large, and the surface potential of the intermediate transfer belt is lower than the surface potential of the photosensitive member in the vicinity of the upstream side of the transfer unit. In this case, an electric field from the photoconductor to the intermediate transfer belt is generated between the surface of the adjacent photoconductor and the intermediate transfer belt, and the yellow toner TY transferred on the intermediate transfer belt is directed toward the photoconductor. Electric force: F2 is generated, and the action of this electric force: F2 causes “reverse transfer” of the yellow toner TY.
[0054]
FIG. 2E shows that the “image portion potential” of the yellow toner image formed on the intermediate transfer belt becomes equal to the image portion potential of the magenta toner image on the photosensitive member by an appropriate counter balance: Vt. This is the case. In this case, neither the force to reverse transfer the yellow toner on the intermediate transfer belt nor the force to pre-transfer the magenta toner on the photosensitive member acts, and the magenta toner image is transferred to the intermediate transfer medium without pre-transfer and reverse transfer. Can be transferred onto.
[0055]
Hereinafter, even when the cyan toner image and the black toner image are primarily transferred onto the intermediate transfer belt, the counter bias: Vt is appropriately set so that the image portion potential of the toner image on the intermediate transfer belt is on the photosensitive member. Each toner image can be appropriately primary-transferred by being substantially equal to the image portion potential of the toner image.
[0056]
  That is, generally, when there are N toner images to be subjected to primary transfer, the counter bias (Vt) at the time of M (= 2 to N) -th primary transfer is set first by M-1 primary transfers. Intermediate transferbeltFormed intonerOf the image partsurfaceThe potential is on the electrostatic latent image carrier having the Mth toner image.tonerOf the image partsurfaceAppropriate primary transfer can be performed by setting the potential to be substantially the same as the potential.
[0057]
2 (e), the potential of the image portion is “equal to the reference potential of the image portion of the yellow toner image on the intermediate transfer belt” during the primary transfer of the magenta toner image. However, in reality, it is difficult to make the two potentials “equal”.
[0058]
  However, even if these potentials do not completely match, if “the difference between the two potentials is small to some extent”, pre-transfer and reverse transfer due to toner flying do not occur. That is, on the intermediate transfer belttonerIt was experimentally obtained that the problem of “pre-transfer / reverse transfer” does not occur when the potential difference between the image portion potential and the image portion potential of the toner image on the photoreceptor is “about 200 V”.
[0059]
  Therefore, “formed on the intermediate transfer belttonerOf the image partsurfaceOn the electrostatic latent image carrier having the toner image newly transferred to the primary transfertonerOf the image partsurface“Substantially the same potential as the potential” means that the difference between these potentials is smaller than about 200V.
[0060]
  In addition, “on the image portion of the toner image on the intermediate transfer beltsurfacePotential "and the image part of the toner image on the photoreceptorsurfaceWhen comparing the potential difference with the potential as “same potential difference”, the flight from the intermediate transfer belt to the photoconductor (reverse transfer) is less likely to occur than the flight from the photoconductor to the intermediate transfer belt (pre-transfer). confirmed.
[0061]
  This is because the image on the photoreceptor is a negative / positive image, whereas the image on the intermediate transfer belt istonerThis is presumably because the image is a positive / positive image and the intermediate transfer belt “has a charge that electrostatically attracts toner”.
[0062]
  On the electrostatic latent image carrier near the upstream of the transfer unittonerImage portion surface potential: V_LAnd intermediate transferbeltOn the mediumtonerImage portion surface potential: V_tLFrom the above, in order to obtain the counter bias: Vt, the relationship: Vt_L= V_tL+ Vt can be used, intermediate transferbeltUpper surface potential: V_tLIs known, that is, when it is previously obtained as data by experiment, the calculation: Vt = V_L-V_tLIt can ask for.
[0063]
In practice, it is preferable to set a counter bias: Vt by constructing and referring to a table based on data experimentally obtained in advance.
[0064]
Thus, V_LThe value can be set as a “value stored in the table in advance”, but it is possible to add the “aging degradation change” predicted from the number of images formed or measure the residual potential in the image area of the electrostatic latent image It is preferable to provide a simple means and determine the value based on the output.
[0065]
  FIG. 2 shows an intermediate transfer in which a yellow toner image is primarily transferred.beltThe case where a magenta toner image is transferred has been described above. As described above, intermediate transfer after yellow toner image is transferredbeltThe surface potential of the image area is positive as a whole, and the potential of the image area is higher than that of the non-image area.
[0066]
  In FIG. 3, the vertical axis represents intermediate transfer.beltRepresents the surface potential. Reference numeral 3-1 indicates an intermediate transfer when the yellow toner image is first primarily transferred with the yellow toner TY as described above.beltThe upper surface potential is shown. Reference numeral 3-2 indicates an intermediate transfer in which a yellow toner image is primarily transferred.beltAbove, intermediate transfer after primary transfer of magenta toner image with magenta toner TMbeltThe upper surface potential is shown.
[0067]
Surface potential portion: A is a portion where both the yellow toner image and the magenta toner image are non-image portions. Surface potential portion: B is a portion where the yellow toner image is an image portion and the magenta toner image is a non-image portion. : C represents a portion in which both the yellow toner image and the magenta toner image are image portions, and a surface potential portion: D represents a portion in which the yellow toner image is a non-image portion and the magenta toner image is an image portion.
[0068]
In each surface potential portion, the potential is shifted to the negative side (downward in the figure) after the primary transfer of the yellow toner image. This is due to an increase in the negative charge of the magenta toner TM due to the transfer of the magenta toner image and the influence of “peeling discharge during transfer”.
[0069]
  Hereinafter, intermediate transfer when cyan toner image and black toner image are primarily transferredbeltThe potential distribution above isFIG.In this case, it changes in the same manner as described above and generally shifts to the negative side as the number of times of primary transfer increases.
[0070]
  When the conditions such as the charge amount and the proper adhesion amount of each color toner are substantially the same, “intermediate transfer by transferring one color”beltThe “change in potential of the upper image portion” can be estimated.
[0071]
  Intermediate transfer before the start of primary transferbeltIf the difference between the surface potential of the first color and the surface potential after the transfer of the first color (yellow toner image in the above example) is ΔV (this value is obtained experimentally in advance), this ΔV istonerIt can be estimated as “the potential change of the image portion”, and the counter bias: Vt can be set based on this ΔV (claim 2).
[0072]
  As described above, if the conditions such as the charge amount and the appropriate adhesion amount of each color toner are set to be substantially the same, the potential change in the image portion due to one-color transfer (primary transfer of one color toner image) can be estimated. It is possible, but as an alternative method, intermediate transferbeltIntermediate transfer of “after primary transfer” of yellow toner image to black toner image on topbeltIf the surface potential difference between the start of primary transfer and after the transfer of four colors: V is experimentally determined, the potential change for each color transfer: ΔV can be estimated as V / 4 Based on this V / 4, the counter bias: Vt can be set (Claim 3).
  Referring again to FIG. 3, intermediate transfer after the primary transfer of the magenta toner image.beltThe upper surface potential distribution 3-2 changes in a stepwise manner depending on the degree of overlap between the yellow toner image and the magenta toner image. In such a case, it becomes a problem how to determine the amount of shift of the surface potential by the counter bias: Vt during the primary transfer of the next cyan toner image.
[0073]
In this case, in the method according to claim 4, the maximum potential value V1 at the potential 3-2 after the primary transfer of the magenta toner image is used, and this is shifted by the counter bias Vt so that the cyan toner image on the photosensitive member is transferred. It is approximately equal to the image portion potential. That is, the potential of the surface potential portion: C in FIG. 3 is shifted to the negative polarity side by the counter bias to be equal to “the potential of the cyan toner image portion on the electrostatic latent image carrier”.
[0074]
  In this case, the cyan toner is intermediate transferred from the electrostatic latent image carrier in all of the surface potential portions A, B, C, and D in FIG.beltPotential state that does not move upward (pre-transfer) (intermediate transfer with electrostatic latent image carrier near the upstream side of the transfer unit)beltThe electric field in the vicinity of thebeltHey)
[0075]
  In this potential state, yellow toner and magenta toner are transferred intermediately.beltCan be moved from the electrostatic latent image carrier onto the electrostatic latent image carrier in terms of potential, but as described above, the intermediate transferbeltSince toner does not easily fly from the electrostatic latent image carrier to the electrostatic latent image carrier, the primary transfer of the cyan toner image is performed by effectively preventing or reducing reverse transfer while preventing pre-transfer as described above. it can.
[0076]
  Similarly, during the primary transfer of the last black toner image, the intermediate transferbeltThe reference potential obtained by shifting the maximum potential value of the image portion potential of the yellow, magenta, and cyan toner images by the counter bias: Vt is substantially equal to the image portion potential of the black toner image on the electrostatic latent image carrier. You can do it.
[0077]
When the polarity of the electrostatic latent image formed on the electrostatic latent image carrier is positive and reverse development is performed using positively charged toner, the above “maximum potential value” is “negative polarity”. Means the maximum absolute value of the potential.
[0078]
  The counter bias: Vt setting method described above is “intermediate transfer after primary transfer of each color toner image.beltTherefore, it is not necessary to individually detect the “potential of the image portion”, and an individual surface potential measuring means is not required, so that space saving and cost reduction can be realized.
[0079]
  In the actual operation of the image forming apparatus, when the intermediate transfer medium passes through the application portion of the counter bias: Vt, the intermediate transfer is performed.beltSurface potential is affected by secondary transfer, intermediate transferbeltLevel of static elimination, intermediate transferbeltThe effect of the counter bias differs depending on the potential state when passing through the counter bias application unit.
[0080]
  From this point of view, surface potential detecting means for detecting the potential of the surface of the intermediate transfer belt is provided on the upstream side of each transfer portion.tonerOf the image partsurfaceIt is preferable to set counter bias: Vt based on “potential”I can say that.
[0081]
  Note that pre-transfer occurs when the electrostatic latent image carrier and intermediate transferbeltWhen the balance between the transfer voltage and the counter bias is considered, the position where the counter bias: Vt is applied to the transfer portion on the upstream side of the transfer portion is considered to be “a region of several hundred μm or less”. It is preferable that the portion is as close as possible.
[0082]
For example, in the case shown in FIG. 1, when the diameter of the photosensitive member PC is 40 mm and the contact width (nip width) between the photosensitive member PC and the intermediate transfer belt BL is about 10 mm, the counter bias: Vt is set by the counter bias roller R2. The position to apply is preferably about 5 mm away from the upstream side of the nip portion where the photoreceptor PC and the intermediate transfer medium BL are in contact.
[0083]
  The means for applying the transfer voltage and the counter bias does not necessarily have a “roller shape”, but the transfer roller and the counter bias roller areThe transfer voltage and counter bias can be applied by contacting these with the back side of the intermediate transfer medium.it can.
[0084]
DETAILED DESCRIPTION OF THE INVENTION
With reference to FIGS. 4 and 5, one embodiment of an image forming apparatus embodying the present invention will be described. FIG. 4 is an overall view of the apparatus, and FIG. 5 is a partial view for explaining the characteristic part of the invention. An overview of the image forming process will be described first with reference to FIG.
[0085]
An image forming apparatus 900 shown in FIG. 4A is a “tandem type color image forming apparatus”. A color original is read by red, green, and blue by the reading unit 901, and image data for each color of black (B), yellow (Y), magenta (M), and cyan (C) is read based on the read information. Generated. The configuration of such a reading unit has been widely known.
[0086]
These image data are subjected to optical writing by the optical writing device 902 at the image forming stations 903B, 903Y, 903M, and 903C. Since the image forming stations 903B to 903C are structurally the same, the image forming station 903B will be described as an example.
[0087]
As shown in FIG. 4B, the image forming station 903B is charged around a photoconductive photosensitive member (electrostatic latent image carrier) 91B formed in a drum shape rotated in a counterclockwise direction. A charger 92, a developing device 93, a transfer means 94, and a cleaning device 95 are arranged.
[0088]
The transfer means 94 includes a transfer roller 94A, a counter bias roller 94B, and means (not shown) for applying a predetermined voltage to them.
[0089]
As shown in FIG. 4A, an intermediate transfer belt 9041 as an intermediate transfer medium runs in the direction of the arrow between the photosensitive member 91B and the transfer roller 94A / counter bias roller 94B. The photoconductor 91B is uniformly charged by the charging charger 92 while rotating counterclockwise, and B image data corresponding to a black image is written by the laser beam LBB to form a B latent image (negative latent image).
[0090]
The B latent image is reversely developed by the developing device 93 to become a “black toner image” using black toner, and the black toner image is primarily transferred onto the intermediate transfer belt 9041 by the transfer roller 94A. At this time, counter bias: Vt is applied by the counter bias roller 94B to prevent pre-transfer.
[0091]
The photoreceptor 91B after the toner image transfer is cleaned by a cleaning device 95.
[0092]
Similarly, in the image forming stations 903Y, 903M, and 903C shown in FIG. 4A, yellow, magenta, and cyan toner images are formed, and these color toner images are displayed on the intermediate transfer belt 9041 as “black toner images”. It is transcribed as “overlapping”. Thus, the color image formed by the black, yellow, magenta, and cyan toner images formed on the intermediate transfer belt 9041 is secondarily transferred onto the transfer sheet S as a “sheet-like recording medium”.
[0093]
The transfer sheet S is fed from a cassette 906 at the bottom of the image forming apparatus main body or manually fed from a manual feed unit 907, and is “secondary transfer unit”, that is, an intermediate transfer belt 9041 and a secondary transfer belt by a registration roller 909. The color image is transferred to the contact portion with the reference numeral 905 by timing of the movement of the color image, and the color image is transferred by the action of the secondary transfer bias applied to the secondary transfer belt 905 from a bias applying unit (not shown). The secondary transfer belt 905 and a bias applying unit (not shown) constitute a “secondary transfer unit”.
[0094]
The transfer sheet S on which the color image has been transferred is conveyed by the secondary transfer belt 905, neutralized by a neutralization charger (not shown), separated from the secondary transfer belt 905, the color image is fixed by a fixing device 910, and conveyed by a conveyance roller 911. It is conveyed and discharged out of the apparatus by a discharge roller 912.
[0095]
The intermediate transfer belt 9041 and the secondary transfer belt 905 after the secondary transfer are cleaned on the outer peripheral surface by a belt cleaning device (not shown).
[0096]
In the “double-sided image forming mode” in which image formation is performed on both sides of the transfer sheet S, the transfer path of the transfer sheet S on which the color image is fixed on one side is switched by a switching claw 915, and a transfer roller 911 and a guide (not shown) To the reversing unit 913, and the reversing unit 913 reverses the front and back, stacks them on the stacker 914 with “the surface on which the color image is formed facing upward”, transports again to the position of the registration roller 909, and the same as above. To transfer the color image to the back side. Thereafter, the color image on the back surface is fixed by the fixing device 910, conveyed by the conveyance roller 911, and discharged outside the device by the discharge roller 912.
[0097]
FIG. 5 is a view for explaining a transfer portion in the image forming apparatus shown in FIG. In FIG. 5A, reference numerals 91B to 91C denote photoconductive photoreceptors, respectively, a black toner image on the photoreceptor 91B, a yellow toner image on the photoreceptor 91Y, and a magenta toner image on the photoreceptor 91M. Cyan toner images are respectively formed on the photoreceptor 91C. Reference numerals 94BK to 94C denote "primary transfer means" for performing primary transfer on each photoconductor. In the figure, a transfer roller and a counter bias roller provided in the vicinity of the upstream side thereof are shown.
[0098]
The primary transfer performed in each primary transfer unit is as described above with reference to FIGS. 1 to 3, and the counter bias: Vt is applied to the counter bias roller in each primary transfer unit. The primary transfer is performed while effectively preventing or reducing pre-transfer and reverse transfer.
[0099]
If the electric resistance of the intermediate transfer belt is low, a large current flows through the intermediate transfer belt between the counter bias roller and the transfer roller at each transfer portion, so the volume resistance value of the intermediate transfer belt may be high to some extent. Necessary, specifically 10⁸-10¹⁰Ωm or so can be used, but preferably 1 × 10⁹Ωm is good.
[0100]
If the volume resistance value of the intermediate transfer belt is too high, the neutralization property of the intermediate transfer belt is deteriorated, so that it is necessary to neutralize with a dedicated neutralization device. Since the self-discharge is performed by the flow of electric charge through the bulk of the intermediate transfer belt, no current flows between the counter bias roller and the intermediate transfer roller, and the bulk of the intermediate transfer belt has a structure that allows the current to flow easily. preferable.
[0101]
For example, the intermediate transfer belt has a three-layer structure, and the volume resistivity of the first, second, and third layers from the contact side (back side) of the counter bias roller: ρ1, ρ2, ρ3, ρ1> ρ2> ρ3, or ρ1> ρ2 <ρ3, and the overall resistance value is 10⁸-10¹⁰What is necessary is just to comprise so that it may become (ohm) m.
[0102]
Referring to FIG. 5B, reference numerals 194B, 194Y, 194M, and 194C denote counter bias rollers in the transfer units 94BK to 94C, respectively, which are counter bias setting units CBS (configured by a microcomputer or the like). Thus, “counter bias set for each counter bias roller: Vt” is applied from the power sources BDB, BDY, BDM, and BDC.
[0103]
In FIG. 5A, reference numeral 9042 denotes a potential detecting means for detecting the surface potential of the intermediate transfer belt 9041.
[0104]
In the image forming apparatus shown in FIGS. 4 and 5, if the method according to claim 2 is realized, a negative latent image of a reference pattern is created on the most downstream side photoreceptor 91C in the transfer portion, and this is inverted. The cyan toner image obtained by development is transferred onto the intermediate transfer belt 9041. At this time, the counter bias may not be applied.
[0105]
The potential detecting means 9042 detects the image portion potential of the cyan toner image transferred onto the intermediate transfer belt 9041, and estimates and sets the counter bias value of the primary transfer for the second and subsequent colors as described above. The counter bias setting unit CBS in FIG. 5B performs the estimation and setting according to the detection potential of the potential detection means 9042.
[0106]
If the method according to claim 3 is realized, a negative latent image of a reference pattern is created on each of the photoconductors 91BK to 91C in the transfer section, and each color toner image obtained by reversal development is used as an intermediate transfer belt 9041. Sequential transfer up. At this time, the counter bias is not applied.
[0107]
The image portion potential of the color image obtained on the intermediate transfer belt in this way is detected by the potential detecting means 9042, and the counter bias value for the primary transfer of each color toner image as described above is obtained as the counter bias setting portion CBS. Estimate and set in. In any of the above cases, the surface potential: Vb of the intermediate transfer belt is detected prior to the primary transfer of the first toner image.
[0108]
【Example】
  Image formation was performed using the image forming apparatus described with reference to FIGS. Intermediate transferbeltIn the case of color image formation, the order of primary transfer to the medium is the order of black, yellow, magenta, and cyan toner images. However, in view of ease of evaluation, the two colors of yellow toner image and magenta toner image are used. Images were formed and evaluated.
[0109]
First, a yellow toner image was formed on the photoreceptor 91Y. The image portion of the yellow toner image has an image portion potential of the electrostatic latent image of about −100 V, and about −200 V with yellow toner attached.
[0110]
The surface potential on the intermediate transfer belt 9041 on which this yellow toner image was transferred “without applying a counter bias” was measured by the potential detection means 9042. The image portion potential at this time was + 500V. This potential varies depending on the bias condition during the secondary transfer. The secondary transfer bias condition is controlled to change according to environmental changes. The main purpose of this control is to correct the change in efficiency of the secondary transfer caused by the “change in resistance value of the transfer paper” due to the environment.
[0111]
When the yellow toner image of the first color is primarily transferred, a negative potential having a sufficiently large absolute value is applied as the counter bias: Vt because there is no toner image previously transferred on the intermediate transfer belt.
[0112]
In the example, vt = −700V. After the transfer of the yellow toner image, the second color magenta toner image was transferred. The latent image potential of the image portion of the magenta toner image on the photoreceptor 91M is about −100V, and about −200V with the toner attached. The image portion potential of the yellow toner image on the intermediate transfer belt before transferring the magenta toner image was measured and found to be + 430V.
[0113]
In this state, the value of the counter bias is variously changed, and after the primary transfer of the magenta toner image, the apparatus is stopped, whether yellow toner adheres due to reverse transfer on the photosensitive member 91M, and on the intermediate transfer belt. The following results were obtained when the state of magenta toner scattering by pre-transfer was observed.
[0114]

“◯” indicates that no adhesion or scattering was observed, “×” indicates that much adhesion or scattering was observed, and “Δ” indicates that only a small amount of scattering was observed.
[0115]
Using the potential detection means 9042 shown in FIG. 5, “counter bias: determination of Vt” was performed by the method described in claim 2 as described above. FIG. 6 shows a flowchart. “Thr” in this flowchart is an upper limit of the potential change amount due to one-color transfer. When the potential change amount of one color is equal to or larger than this value, the image on the intermediate transfer belt before the primary transfer for the third and subsequent colors. Assuming that the partial potential is V1, the potential difference on the upstream side of the transfer portion at the time of the primary transfer of the fourth color becomes large, and toner flying occurs. If Thr or less, the image portion potential was set to V1, and no problem occurred.
[0116]
“Thr” is about 50 to 70V, but in the example, it was set to 70V. The counter bias value is determined with reference to the following table based on the image portion potential on the intermediate transfer belt which is roughly determined. This table is pre-stored with what is fixed by the system. For image formation, full-color image formation is performed, the device is stopped at the timing of 1 color transfer, 2 color transfer, 3 color transfer, and 4 color transfer, and the toner on the photoreceptor and intermediate transfer belt is transferred. Scattering (pre-transfer) and toner adhesion (reverse transfer) were observed.
[0117]

[0118]
Compared to the case where no counter bias was applied, a good image with less transfer dust was obtained. When detecting the potential of the image portion, the image formation by the “dedicated pattern” was performed as described above.
[0119]
Using the potential detection means 9042 shown in FIG. 5, “counter bias: determination of Vt” was performed by the method described in claim 3 as described above.
According to the flowchart shown in FIG. 7, the potential of the image portion on the intermediate transfer belt was estimated, and the counter bias was determined with reference to the above table.
[0120]
A good image was obtained with no mixing of other colors by reverse transfer to the image area on the photoreceptor and no toner scattering on the intermediate transfer belt by pre-transfer.
[0121]
It is to be noted that the image portion potential of the toner image on each electrostatic latent image carrier and the image portion potential on the upstream side of each transfer portion of the intermediate transfer medium are respectively detected and matched so that these potentials are matched. Of course, the counter bias in the transfer portion can be determined.
[0122]
【The invention's effect】
  As described above, according to the present invention, a novel image forming method and apparatus can be realized. According to the image forming method of the present invention, as described above, a plurality of toner images are transferred intermediately.beltPre-transfer and reverse transfer during primary transfer onto a medium can be effectively prevented or reduced. Therefore, the image forming apparatus that implements the image forming method of the present invention can satisfactorily prevent or reduce image quality deterioration due to pre-transfer or reverse transfer and perform high-quality image formation.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a counter bias.
FIG. 2 is a diagram for explaining a counter bias.
FIG. 3 is a diagram for explaining an image portion potential on an intermediate transfer medium after primary transfer.
FIG. 4 is a diagram for explaining one embodiment of an image forming apparatus.
FIG. 5 is a diagram for explaining a transfer unit in the image forming apparatus of FIG. 5;
FIG. 6 is a flowchart of image portion potential estimation according to the embodiment.
FIG. 7 is another flowchart of image portion potential estimation according to the embodiment.
[Explanation of symbols]
PC electrostatic latent image carrier (photoconductive photoreceptor)
BL Intermediate transfer belt
R1 transfer roller
R2 counter bias roller
TY yellow toner

Claims (15)

N (≧ 2) negative latent images are formed on one or more electrostatic latent image carriers, and each of these negative latent images is reversely developed to obtain N positive toner images. In an image forming method in which primary transfer is sequentially performed on the surface of a transfer belt and superimposed on each other, and then secondarily transferred from the intermediate transfer belt onto a sheet-like recording medium.
The intermediate transfer belt is sandwiched between the transfer roller and the electrostatic latent image carrier, and the sandwiched portion is used as a transfer portion for primary transfer.
At the time of primary transfer, a counter bias roller is brought into contact with the back side of the intermediate transfer belt in the vicinity of the upstream side of the transfer portion of the primary transfer, and a counter bias: Vt is applied to the intermediate transfer medium.
Counter bias at the time of M (= 2 to N) -th primary transfer: Vt is the surface potential of the image portion due to the toner image formed on the intermediate transfer belt by M-1 primary transfer first. In the contact portion of the counter bias roller with the intermediate transfer belt, the surface potential of the toner image portion on the electrostatic latent image carrier having the Mth toner image is set to be substantially the same potential. An image forming method. The image forming method according to claim 1.
When the difference between the surface potential of the intermediate transfer belt before the start of the first primary transfer and the surface potential of the toner image portion after the first primary transfer is ΔV, the intermediate transfer belt by transferring one color. A change in the surface potential of the upper toner image portion is estimated as ΔV, and the counter bias (Vt) at the time of M (= 2 to N) -th primary transfer is set based on ΔV. Forming method. The image forming method according to claim 1.
When the surface potential of the toner image portion formed on the intermediate transfer belt by the Nth primary transfer is V, the change in the surface potential of the toner image portion due to one-color transfer is estimated as V / N, and M ( = 2 to N) An image forming method characterized in that a counter bias Vt in the first primary transfer is set based on the above V / N. The image forming method according to any one of claims 1 to 3,
An image forming method, wherein a maximum potential value is used as a surface potential of a toner image portion formed on an intermediate transfer belt. The image forming method according to claim 1.
The surface potential of the intermediate transfer belt before the primary transfer: Vb, the surface potential of the toner image portion after the first transfer: V1, and the difference between them: | Vb−V1 | is larger than a predetermined threshold value: Thr. Is set to Vb + (V1−Vb) × (M−1), and the above difference: | Vb−V1 | is predetermined. Image forming method, wherein the counter bias at the time of M (= 2 to N) -th primary transfer: Vt is set to V1. The image forming method according to any one of claims 1 to 5,
N = 3 or 4, and different electrostatic latent images are developed with toners of three colors of yellow, magenta, cyan or red, green and blue, or four colors obtained by adding black to these. Image forming method. The image forming method according to claim 6.
An image forming method, wherein different electrostatic latent images are formed separately on two or more electrostatic latent image carriers. N (≧ 2) negative latent images are formed on one or more electrostatic latent image carriers, and each of these negative latent images is reversely developed to obtain N positive toner images. In an image forming apparatus in which primary transfer is sequentially performed on the surface of a transfer belt and superimposed on each other, and then secondarily transferred from the intermediate transfer belt onto a sheet-like recording medium.
A primary transfer unit that performs primary transfer applies a counter bias: Vt to the intermediate transfer medium in the vicinity of the transfer bias applying unit that applies a transfer bias to the primary transfer unit and the upstream side of the primary transfer unit. Having counter bias means;
The transfer bias means includes a transfer roller that sandwiches the intermediate transfer belt together with the electrostatic latent image carrier and uses the sandwiching portion as a primary transfer portion.
The counter bias means has a counter bias roller that contacts the back side of the intermediate transfer belt in the vicinity of the upstream side of the primary transfer portion,
Counter bias at the time of M (= 2 to N) -th primary transfer: Vt, the surface potential of the toner image portion formed on the intermediate transfer belt by M-1 primary transfer first is Mth. An image forming apparatus comprising: a counter bias setting unit configured to set a potential substantially equal to a surface potential of a toner image portion on an electrostatic latent image bearing member having the toner image. The image forming apparatus according to claim 8.
The counter bias setting unit estimates the counter bias (Vt) during the M (= 2 to N) -th primary transfer based on the surface potential of the toner image portion formed on the intermediate transfer belt by the first primary transfer. An image forming apparatus characterized by being set. The image forming apparatus according to claim 8 or 9,
The counter bias setting unit estimates the counter bias (Vt) at the M (= 2 to N) -th primary transfer based on the surface potential of the toner image portion formed on the intermediate transfer belt by the N-th primary transfer. The image forming apparatus is characterized in that the image forming apparatus is set. The image forming apparatus according to any one of claims 8 to 10, wherein
An image forming apparatus using a maximum potential value of a toner image portion formed on an intermediate transfer belt as a surface potential of a toner image portion used by a counter bias setting portion for setting a counter bias. The image forming apparatus according to any one of claims 8 to 11,
Has a surface potential detecting means for detecting the surface potential of the toner image portion on the intermediate transfer belt after primary transfer, the counter bias setting portion, the surface potential of the toner image portion on the basis of the detection voltage by the surface potential detect means An image forming apparatus comprising: determining and setting a counter bias in second and subsequent transfers in primary transfer based on the potential. The image forming apparatus according to any one of claims 8 to 12, wherein
N = 3 or 4 and different electrostatic latent images are developed with toners of three colors of yellow, magenta, cyan or red, green and blue, or four colors obtained by adding black to these. Image forming apparatus . The image forming apparatus according to claim 13.
An image forming apparatus, wherein different electrostatic latent images are formed separately on two or more electrostatic latent image carriers and are primarily transferred from each electrostatic latent image carrier to an intermediate transfer belt. The image forming apparatus according to claim 14.
An image forming apparatus comprising a tandem type image forming apparatus in which three or four electrostatic latent image carriers are arranged in a running direction on a surface of an intermediate transfer belt.

Images