JP3584148B2 - Image forming device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a copying machine using an intermediate transfer member, a laser beam printer, and the like.PaintingThe present invention relates to an image forming apparatus.
[0002]
[Prior art]
FIG. 11 illustrates a schematic configuration of an electrophotographic four-color full-color laser beam printer (hereinafter, referred to as an “image forming apparatus”) using an intermediate transfer member.
[0003]
The photosensitive drum 1 as an image carrier rotates in the direction of arrow R1, and its surface is uniformly charged by a primary charger 2, and is exposed from an exposure unit 3 modulated based on an image information signal sent from a host computer. An electrostatic latent image is formed on the surface by the laser light L. The intensity and the irradiation spot diameter of the laser beam L are appropriately set according to the resolution of the image forming apparatus and the desired image density, and the portion of the electrostatic latent image on the photosensitive drum 1 irradiated with the laser beam has a bright portion potential. V_L  And the unirradiated portion is the dark portion potential V_D  (The potential charged by the primary charger 2).
[0004]
The electrostatic latent image reaches a portion facing the developing device 14 due to the rotation of the photosensitive drum 1, and powder (toner) charged to the same polarity is adhered and developed (developed). The four-color full-color image forming apparatus generally includes four developing units, that is, developing units 4a, 4b, 4c, and 4d that store yellow, magenta, cyan, and black toners, respectively.
[0005]
Here, in the image forming apparatus shown in FIG. 11, the transfer means 5 has an intermediate transfer belt 5a, and four-color toner images are synthesized on the intermediate transfer belt 5a. In this case, the electrostatic latent image is developed with a single color toner. The intermediate transfer belt 5a is stretched around a primary transfer roller 5b, a driving roller 5c, and a secondary transfer opposing roller 5d. First, the first (yellow) developing device 4a applies yellow toner to the electrostatic latent image on the photosensitive drum 1 and develops it as a toner image. At this time, the second to fourth developing units 4b, 4c, and 4d are off, and the yellow toner image of the first color is affected by the second to fourth developing units 4a, 4b, and 4c. Primary transfer nip N formed between the intermediate transfer belt 5a and the photosensitive drum 1 without receiving₁  To reach. The first toner image is applied to the primary transfer nip N by a voltage having a polarity opposite to that of the first color toner image applied to the primary transfer roller 5b in contact with the back surface of the intermediate transfer belt 5a.₁  Is primary-transferred onto the intermediate transfer belt 5a by the electric field formed in the intermediate transfer belt 5a. After the transfer of the first color toner image has been completed, the cleaning device 6 removes (cleans) the primary transfer residual toner remaining on the surface of the photosensitive drum 1, and then enters a second color (magenta) image forming process. .
[0006]
In the second color image forming process, only the second developing device 4b operates and the other developing devices 4a, 4c, and 4d are turned off. Hereinafter, the second color developing device 4b operates in the same procedure as described above. The toner image is primarily transferred onto the intermediate transfer belt 5a. Hereinafter, primary transfer is sequentially performed on the third color (cyan), the fourth color (black), and the intermediate transfer belt 5a, and a superimposed color image of four colors is formed on the transfer belt 5a. During this time, the transfer cleaning device 21 of the intermediate transfer belt 5a is released, so that the toner image on the intermediate transfer belt 5a is not disturbed.
[0007]
Next, one sheet of the transfer material P is taken out from a sheet feeding means (not shown), and a secondary transfer nip N between the intermediate transfer belt 5a and the secondary transfer roller 7 is taken out.₂  Is inserted through. At this time, a secondary transfer voltage having a polarity opposite to that of the toner is applied to the transfer roller 7, and the toner images of the first to fourth colors are collectively and secondarily transferred onto the transfer material P from the intermediate transfer belt 5a.
[0008]
The transfer material P after the secondary transfer is conveyed to the fixing device 10 with an unfixed toner image carried on the surface, where the toner image is fixed as a permanent fixed image by receiving heat and pressure. On the other hand, the secondary transfer residual toner remaining on the surface of the intermediate transfer belt 5 a after the secondary transfer is removed by the transfer cleaning device 21.
[0009]
As the above-mentioned intermediate transfer belt 5a, a dielectric film such as a fluororesin, polyester, polyethylene, nylon, or the like, a film in which carbon black or the like is dispersed to adjust the resistance, or a two-layer film in which a conductor is lined on the back surface is used. Things are known.
[0010]
As the intermediate transfer member, a drum-shaped intermediate transfer drum is also known in addition to the above-described belt-shaped intermediate transfer belt 5a.
[0011]
As a transfer cleaning device for removing the secondary transfer residual toner on the intermediate transfer member, in addition to the transfer cleaning device shown in FIG. 11 (mechanical scraping by a blade 21a), Japanese Patent Application Laid-Open No. 5-303310 discloses Means for cleaning the conductive member by applying a bias having a polarity opposite to that of the toner to the conductive member is disclosed.
[0012]
A method has also been devised in which a secondary brush is mechanically and electrically collected using a fur brush as a transfer cleaning member, and further collected by another removing member.
[0013]
Further, as disclosed in JP-A-1-105980, the secondary transfer residual toner on the intermediate transfer member is transferred to the primary transfer nip portion N.₁  A method is also known in which the photosensitive drum is returned to the photosensitive drum via a photoreceptor, and is removed by the photosensitive drum cleaning device 6.
[0014]
[Problems to be solved by the invention]
However, the image forming apparatus using the above-mentioned intermediate transfer member has the following disadvantages.
[0015]
The intermediate transfer member is a member on which a toner image is primarily transferred from a photosensitive drum, and also transfers the primary-transferred toner image onto a transfer material. However, it is difficult to configure the material with a low resistance.
[0016]
Therefore, the intermediate transfer member has a medium resistance or higher (a volume resistivity of about 10⁷  (Ω · cm or more).
[0017]
However, even if the resistance is higher than the medium resistance, the surface is easily charged when a high-resistance material is used. Due to its characteristics, it has been difficult to prevent electrification in all use environments.
[0018]
Such charging of the intermediate transfer body affects the transfer electric field, and thus hinders uniform transfer of the toner image, thereby causing deterioration of image uniformity. In particular, in an apparatus which needs to start the next image formation immediately after the surface potential is attenuated due to a recent demand for a higher printing speed, the non-uniform image due to the charging of the intermediate transfer body has become an important problem. .
[0019]
When an intermediate transfer drum or an intermediate transfer belt lined with a conductor is used as the intermediate transfer member, the back surface is always at the same potential. When an intermediate transfer member having a necessary and sufficient size is used, the electric field in the secondary transfer nip portion and other voltage supply portions is affected by the back surface potential. For example, since the voltage of the secondary transfer nip is required to have a value added to the back potential of the intermediate transfer body, a high voltage is required when the primary transfer voltage is supplied to the back potential. Thus, there is a problem that a leakage current easily flows to the transfer material and its surroundings, and secondary transfer becomes unstable.
[0020]
On the other hand, when a method of mechanically removing the secondary transfer residual toner on the intermediate transfer body by using a scraping member such as a blade or a fur brush is used as a transfer cleaning method, the blade is strongly applied to the intermediate transfer body. It is necessary to secure the scraping ability by pressing or increasing the rotation speed of the fur brush, but this has caused abrasion, breakage, etc. of the intermediate transfer member and the transfer cleaning member itself. More recently, toner particles have become finer, and devices for improving image quality with spherical toner such as polymerized toner have been increasing. However, when such a high-quality toner is used, these scraping members are used. It has been clarified that slippage occurs and the transfer cleaning property becomes insufficient.
[0021]
On the other hand, an apparatus disclosed in JP-A-5-303310 and JP-A-1-105980, which applies a voltage to the transfer cleaning member and electrically operates the secondary transfer residual toner, is not known. Although the transfer cleaning property can be greatly improved, it can be said that the structure is easy to charge the surface of the intermediate transfer member because it also has the effect of supplying electric charge to the intermediate transfer member. In this case, the charged state of the surface of the intermediate transfer member changes variously depending on the amount of the secondary transfer residual toner, the triboelectric charge, and the like. During the formation, there was a problem that the uniformity of the image could not be maintained due to the influence of the primary transfer electric field. In particular, there has been a problem that the image density changes between a case where a sufficient neutralization time is provided before image formation (for example, when power is turned on) and a case where continuous image formation is performed.
[0022]
In view of the above, the present invention has been made to prevent deterioration of image uniformity while maintaining good transfer cleaning performance for an intermediate transfer member.PictureIt is an object to provide an image forming apparatus.
[0023]
[Means for Solving the Problems]
The present invention according to claim 1 isAn image carrier and a movable intermediate transfer member, a toner image on the image carrier is primarily transferred to the intermediate transfer member at a primary transfer nip portion, and a toner image on the intermediate transfer member is transferred at a secondary transfer nip portion. A toner image secondary-transferred to a transfer material, a primary transfer member provided on the back surface of the intermediate transfer body at the primary transfer nip portion, and the secondary transfer nip portion with respect to a moving direction of the intermediate transfer body. A charging member provided on the surface of the intermediate transfer body at a downstream side of the primary transfer nip portion and an upstream side of the primary transfer nip portion, wherein a plurality of color toner images are sequentially superimposedly transferred onto the intermediate transfer body. In the device,
The distance from the charging member to the primary transfer nip along the surface of the intermediate transfer member is L, and the moving speed of the intermediate transfer member is V _P When a potential decay time when a charge is applied to the surface of the intermediate transfer member is τ,
τ> L / V _P
Holds, the surface of the intermediate transfer member is charged by the charging member prior to the primary transfer operation of the first color, and in the primary transfer operation of the second color and thereafter, the primary transfer member performs the primary transfer operation Prior to the charging, a potential higher than a charging potential obtained by charging the surface of the intermediate transfer member is applied.
[0026]
Claim2The present invention according to the aboveCharging memberAnd the voltage obtained by superimposing the AC voltage on the DC voltageApplyIt is characterized by the following.
[0027]
Claim3The present invention according to the aboveCharging memberContactable / separable from the intermediate transfer memberToIt has a contact / separation mechanism.
[0028]
ClaimTo fourAccording to the present invention,Charging memberIs,twoSecondary transfer residual toner remaining on the intermediate transfer body after the next transferContact withThe charge polarity of the next transfer residual toner is changed to the same polarity as the charge polarity of the intermediate transfer member.
[0029]
6. The secondary transfer residual toner on the intermediate transfer member after secondary transfer on the downstream side of the secondary transfer nip portion and on the upstream side of the charging member with respect to the moving direction of the intermediate transfer member. A cleaning member capable of recovering and applying a voltage to the cleaning memberVoltage supply meansAnd having the following.
[0030]
Claim6The present invention relates tosecondaryThe area corresponding to the toner image on the intermediate transfer body after the transfer isCleaning memberWhile in the positionCleaning memberThe voltage applied toCharging memberAnd the same polarity as the charging polarity of the intermediate transfer member.
[0031]
Claim7The present invention relates tosecondaryThe area corresponding to the toner image on the intermediate transfer body after the transfer isCleaning memberAfter passing through the position ofCleaning memberWherein the polarity of the voltage applied to is changed.
[0032]
Claim8The present invention according tosecondaryThe area corresponding to the toner image on the intermediate transfer body after the transfer isCharging memberAfter passing through the position ofCharging memberWherein the polarity of the DC voltage applied to is changed.
[0033]
ClaimTo 9The present invention relates tosecondaryThe area corresponding to the toner image on the intermediate transfer body after the transfer isCharging memberAfter passing through the position ofCharging memberIs separated from the intermediate transfer member.
[0034]
Claim10The present invention according to the aboveCleaning memberIsConductive fiberConsists of brushes,SaidbrushThe aboveA contact / separation mechanism for contacting / separating the intermediate transfer memberHave,It is characterized by the following.
[0038]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0039]
<Embodiment 1>
FIG. 1 shows an image forming apparatus according to the present invention. The image forming apparatus shown in FIG. 1 is an electrophotographic four-color full-color laser beam printer.
[0040]
The photosensitive drum 1 as an image carrier rotates in the direction of arrow R1, and its surface is uniformly charged by a primary charger 2, and is exposed from an exposure unit 3 modulated based on an image information signal sent from a host computer. An electrostatic latent image is formed on the surface by the laser light L. The exposure means 3 includes a laser oscillator 3a, a reflection mirror 3b, and the like. The intensity and the irradiation spot diameter of the laser beam L are appropriately set according to the resolution of the image forming apparatus and the desired image density, and the portion of the electrostatic latent image on the photosensitive drum 1 where the laser beam is irradiated has a bright portion potential. V_L  And the unirradiated portion is the dark portion potential V_D  (The potential charged by the primary charger 2).
[0041]
The electrostatic latent image reaches a portion facing the developing device 4 by the rotation of the photosensitive drum 1, and powder (toner) charged to the same polarity is adhered and developed (developed). The four-color full-color image forming apparatus generally includes four developing units, that is, developing units 4a, 4b, 4c, and 4d that store yellow, magenta, cyan, and black toners, respectively.
[0042]
Here, in the image forming apparatus shown in FIG. 1, the transfer means 5 has an intermediate transfer belt 5a, and the toner images of four colors are synthesized on the intermediate transfer belt 5a. In this case, the electrostatic latent image is developed with a single color toner. The intermediate transfer belt 5a is stretched around a primary transfer roller 5b, a driving roller 5c, and a secondary transfer opposing roller 5d. First, the first (yellow) developing device 4a applies yellow toner to the electrostatic latent image on the photosensitive drum 1 and develops it as a toner image. At this time, the second to fourth developing units 4b, 4c, and 4d are off, and the yellow toner image of the first color is affected by the second to fourth developing units 4a, 4b, and 4c. Primary transfer nip N formed between the intermediate transfer belt 5a and the photosensitive drum 1 without receiving₁  To reach. The first toner image is formed on the primary transfer nip N by the voltage applied to the primary transfer roller 5b in contact with the back surface of the intermediate transfer belt 5a and the surface potential of the intermediate transfer belt 5a.₁  Is primary-transferred onto the intermediate transfer belt 5a by the electric field formed in the intermediate transfer belt 5a. After the transfer of the first color toner image has been completed, the cleaning device 6 removes (cleans) the primary transfer residual toner remaining on the surface of the photosensitive drum 1, and then enters a second color (magenta) image forming process. .
[0043]
In the second color image forming process, only the second developing device 4b operates and the other developing devices 4a, 4c, and 4d are turned off. Hereinafter, the second color developing device 4b operates in the same procedure as described above. The toner image is primarily transferred onto the intermediate transfer belt 5a. Hereinafter, primary transfer is sequentially performed on the third color (cyan), the fourth color (black), and the intermediate transfer belt 5a, and a superimposed color image of four colors is formed on the transfer belt 5a.
[0044]
Next, one sheet of the transfer material P is taken out from a sheet feeding means (not shown), and a secondary transfer nip N between the intermediate transfer belt 5a and the secondary transfer roller 7 is taken out.₂  Is inserted through. At this time, a secondary transfer voltage having a polarity opposite to that of the toner is applied to the transfer roller 7, and the toner images of the first to fourth colors are collectively and secondarily transferred onto the transfer material P from the intermediate transfer belt 5a.
[0045]
The transfer material P after the secondary transfer is conveyed to the fixing device 10 with an unfixed toner image carried on the surface, where the toner image is fixed as a permanent fixed image by receiving heat and pressure. On the other hand, the secondary transfer residual toner remaining on the surface of the intermediate transfer belt 5 a after the secondary transfer is removed by the transfer charging roller 8 and the opposing roller 9.
[0046]
In this embodiment, the intermediate transfer belt 5a is made of PVDF resin having a thickness of 100 μm, and is hung on the next three rollers, namely, a primary transfer roller 5b, a driving knurl 5c, and a secondary transfer opposing roller 5d. Has been passed. The primary transfer roller 5b applies a potential from the back side of the intermediate transfer belt 5a when transferring the toner image on the photosensitive drum 1 onto the intermediate transfer belt 5a, and has a medium resistance (this embodiment) for forming a transfer electric field. The actual resistance in the nip formation when 1 kV is applied is 10⁷ΩthingIs formed of a conductive sponge. The primary transfer roller 5b presses the intermediate transfer belt 5a against the surface of the photosensitive drum 1 from the back side, and a primary transfer nip portion N between the surface of the photosensitive drum 1 and the surface of the intermediate transfer belt 5a.₁Is composed. The primary transfer roller 5b has a metal core (not shown), and a primary transfer bias power supply11 (FIG. 3), A primary transfer bias is applied. The drive roller 5c rotates (moves) the intermediate transfer belt 5a in the direction of arrow R5. A secondary transfer roller 7 is disposed at a position facing the secondary transfer opposing roller 5d with the intermediate transfer belt 5a interposed therebetween. The secondary transfer roller 7 is covered with an EPDM foam layer, and is brought into and away from the intermediate transfer belt 5a by a contact and separation mechanism (not shown). At the time of the secondary transfer, the secondary transfer roller 7 is brought into contact with the intermediate transfer belt 5a by the contact / separation mechanism, and the secondary transfer nip N₂And a secondary transfer bias is applied to the secondary transfer nip N₂The toner image is secondarily transferred to the transfer material P inserted through the transfer member P.
[0047]
In addition to these three rollers, a medium-resistance transfer charging roller 8 and a facing roller 9 are respectively disposed outside and inside the intermediate transfer belt 5a between the secondary transfer facing roller 5d and the primary transfer roller 5b. Have been. A voltage is applied to the transfer charging roller 8 by a transfer bias power supply 12 (see FIG. 3), and supplies a charge to the intermediate transfer belt 5a. The opposing roller 9 is formed of a conductive rubber roller and is electrically grounded. The transfer charging roller 8 has a role of charging the intermediate transfer belt 5a and, at the same time, giving a charge to the secondary transfer residual toner on the intermediate transfer belt 5a after the secondary transfer process.
[0048]
Since the intermediate transfer belt 5a needs to hold at least an image of one page of the transfer material P, the circumferential length of the transfer material P is the largest in the transport direction (the same as the moving direction of the intermediate transfer belt 5a). The length must be secured. In the present embodiment, since the maximum size of the transfer material P is A4 size (297 mm × 210 mm), the circumference of the intermediate transfer belt 5a needs to be about 300 mm or more, and the diameter of the intermediate transfer belt 5a is actually 100 mm, and thus the circumference is about 314 mm. Is formed into a cylindrical film.
[0049]
The resistance of the intermediate transfer belt 5a according to the present embodiment is a condition that can hold the electric charge on the surface.⁹  -10^FifteenΩ · cm is preferable, and in addition to the above-mentioned PVDF resin, urethane-based resin, fluorine-based resin, nylon-based resin, elastic material such as silicone rubber or hydrin rubber, or carbon or conductive powder dispersed therein. A resistance-adjusted one or the like can be used.
[0050]
FIG. 2 shows a typical state of charge potential decay when a charge is supplied to the surface of the intermediate transfer belt 5a by a charger. Charge supply potential V₀  (E is a natural logarithm base) is generally called a decay time constant τ, and is determined by the volume resistivity, thickness, and dielectric constant of the intermediate transfer belt 5a. You can think.
[0051]
The charging potential attributable to the charged charge on the front surface is the difference between the potential of the back electrode at the time of charging and the DC voltage applied to the charging member and has the above-described characteristics, but is observed as the surface potential. , And the sum of this and the potential of the back electrode at the time of measurement, and as shown in the present embodiment, the primary transfer nip N₁  When discussing the potential of the surface of the intermediate transfer belt 5a in (1), the sum of the above-described charged potential and the potential applied to the primary transfer roller 5b is an object.
[0052]
In the present embodiment, the decay time constant τ and the image forming speed V_P  (Equal to the rotational speed of the intermediate transfer belt 5a) and the primary transfer nip N₁  The actual running distance L up to
τ> L / V_P    ...... (1)
In such a way that the following relationship is satisfied, the charge by the transfer charging roller 8 is transferred to the primary transfer nip N₁  It is configured to work effectively by then.
[0053]
In this embodiment, the intermediate transfer belt 5a has a volume resistivity of about 10^ThirteenΩ · cm, a relative dielectric constant of 7, and a thickness of 100 μm are used for PVDF._P  = 120 mm / sec and L = 100 mm, satisfying the above expression.
[0054]
Next, a specific operation of the image forming apparatus having the above configuration will be described with reference to FIG.
[0055]
When a print command is input from the host computer to the image forming apparatus, the surface of the photosensitive drum 1 is uniformly charged by the primary charger 2, exposure is performed by the exposure unit 3 according to image information, and the photosensitive drum 1 is exposed. A first color electrostatic latent image is formed thereon. This electrostatic latent image is developed as a yellow toner image by the first color developing unit 4a. On the other hand, the transfer charging roller 8 is brought into contact with the intermediate transfer belt 5a, and a voltage in which an AC voltage having an amplitude of 3 kV is superimposed on the DC voltage (offset voltage) + 100V from the transfer bias power supply 12 is applied to the intermediate transfer belt 5a. The surface is uniformly charged to + 100V.
[0056]
The charged area is changed in accordance with the size of the transfer material and the image forming area instructed by the host computer, and the electric potential for the primary transfer, that is, the first color toner image Tx ( An electric field is formed for the primary transfer of the toner (having a negative polarity tribo) onto the intermediate transfer belt 5a. At this time, a ground potential is applied to the primary transfer roller 5b, and the surface potential of the intermediate transfer belt 5a and the surface potential (V_L  Is about -100 V) to form a primary transfer electric field.
[0057]
The transfer charging roller 8 is separated from the intermediate transfer belt 5a after completing the charging of the charging area.
[0058]
The primary transfer of the second, third, and fourth colors is performed by sequentially applying +200 V, +300 V, and +400 V to the primary transfer roller 5b in accordance with each color.
[0059]
The color image synthesized on the intermediate transfer belt 5a in the four primary transfer steps described above is transferred to the secondary transfer nip N by a bias applied to the secondary transfer roller 7.₂  Is secondarily transferred to the transfer material P inserted in the batch.
[0060]
The transfer charging roller 8 comes into contact with the intermediate transfer belt 5a before the area corresponding to the toner image on the intermediate transfer belt 5a after the secondary transfer reaches the contact position M, and the intermediate transfer belt 5a is driven by the applied voltage. Is charged again to a predetermined potential, and a charge having the same polarity as the charged polarity of the intermediate transfer belt 5a is given to the secondary transfer residual toner Ty.
[0061]
When the image formation is thereafter terminated, the secondary transfer residual toner Tz after the recharging is reverse-transferred to the photosensitive drum 1 due to the potential difference from the photosensitive drum 1, and even when the continuous image formation is subsequently performed, By performing the same operation as the above-described primary transfer process of the first color, the primary transfer of the toner image of the first color onto the intermediate transfer belt 5a is performed, and at the same time, the secondary transfer residual toner Tz after recharging is transferred to the photosensitive drum. The reverse transfer to 1 enables continuous operation.
[0062]
During the primary transfer process and the secondary transfer process, the intermediate transfer belt 5a tends to have various charging potentials depending on the image, environment, and type of the transfer material P due to the supply of charge to the surface. Therefore, when image formation is performed continuously, such a difference in charging potential affects the primary transfer electric field between the first and second sheets, and a uniform transferred image cannot be obtained. There was something.
[0063]
The feature of this embodiment is that the surface of the intermediate transfer belt 5a is uniformly charged to the primary transfer potential prior to the primary transfer step by utilizing the charge holding ability of the intermediate transfer belt 5a based on the above fact. In this case, the primary transfer of the first color is performed using this, and according to this, the image uniformity is improved.
[0064]
FIG. 4 shows the measurement of the surface potential (backside ground potential) of the intermediate transfer belt 5a before the primary transfer of the first color. The broken line indicates the case where charging before the primary transfer was not performed, and the solid line indicates the case of the present embodiment. It is.
[0065]
As can be seen from the broken line, when this pre-transfer pre-charging was not performed, the potential of the first sheet was the ground potential, and the influence of the charge supply action of the primary transfer and the secondary transfer was not applied to the second and subsequent sheets. The surface potential fluctuated greatly upon receiving. When the primary transfer voltage is supplied from the back side to the non-uniform surface potential, a non-uniform image due to this history occurs in the image on one transfer material P, and especially the first and second sheets It was confirmed that the image density fluctuated greatly. On the other hand, in the present embodiment, the surface potential of the intermediate transfer belt 5a can be uniformly set to a predetermined potential between the first and second sheets, and the primary transfer voltage V for the first color can be obtained._T  Can be set to Therefore, the non-uniform images in one transfer material P and among the plurality of transfer materials P do not occur.
[0066]
In this embodiment, the charging potential is set to the primary transfer voltage V for the first color._T  In the first color, the ground potential was applied to the primary transfer roller 5b. However, as described above, the primary transfer electric field is the sum of the potentials of the two, so that the primary transfer roller 5b is optional for forming the primary transfer voltage of the first color. It is possible to change the ratio between the two. For example, the surface potential is 0 V (ground potential), and the primary transfer voltage V of the first color is applied to the primary transfer roller 5b of the first color._T  Can also be applied.
[0067]
Although the image forming apparatus described here is based on reversal development using negative toner, it depends on the polarity of the toner and the developing method.Cleaning memberBy appropriately changing the polarity of the voltage applied to the image forming apparatus, it is possible to cope with image forming apparatuses of various types.
[0068]
<Embodiment 2>
Embodiment 2 will be described with reference to FIG. The same members as those in the first embodiment are denoted by the same reference numerals, and the description thereof will not be repeated.
[0069]
In the present embodiment, an intermediate transfer drum 13 is employed as an intermediate transfer member. The intermediate transfer drum 13 is formed by dispersing carbon on the surface of the aluminum cylinder 13a by 6 × 10⁷  The elastic resistance layer 13b is formed from NBR rubber having a medium resistance of Ω · cm, and a coating layer 13c is formed on the surface thereof with a urethane resin of about 30 μm. The decay time constant τ of the intermediate transfer drum 13 is determined by the dielectric constant, the volume resistivity, and the layer thickness of the elastic resistance layer 13b and the coating layer 13c. In the present embodiment, the volume resistivity of the coating layer 13c is 3 × 10¹⁴Ω · cm, which is high, is dominant in the decay time constant τ. When this decay time constant τ was actually measured, it was 5 seconds in an environment at a temperature of 35 ° C. and 85% humidity and 2000 seconds in an environment of 15 ° C. and 10% humidity.
[0070]
In the present embodiment, the same operation is enabled by directly supplying power to the aluminum cylinder 13a instead of the primary transfer roller 5b of the first embodiment. On the other hand, the aluminum cylinder 13a also serves as the secondary transfer facing roller 5d and the facing roller (electrode) 9 at the same time.
[0071]
The diameter of the intermediate transfer drum 13 is limited by the size of the entire image forming apparatus, and the size is usually determined based on the length of the transfer material P having the maximum sheet passing size in the transport direction. In some cases, the secondary transfer step and the primary transfer step simultaneously proceed during continuous image formation.
[0072]
In the present embodiment, the intermediate transfer drum 13 having a diameter of 140 mm is used and the primary transfer nip N₁  Secondary transfer nip N from downstream₂  The layout is set such that the upstream region is as long as 300 mm so that the primary transfer process and the secondary transfer process of the fourth color do not proceed at the same time (the primary transfer process and the secondary transfer process of the first color). May progress simultaneously).
[0073]
In the present embodiment, the intermediate transfer drum 13 that satisfies the above-described expression (1) is used as the attenuation time constant τ. Can be formed. Therefore, when the primary transfer process and the secondary transfer process of the first color proceed simultaneously during continuous image formation, the potential of the aluminum cylinder 13a is set to the ground potential, the primary transfer electric field is set by the surface potential of the intermediate transfer drum 13, and The secondary transfer voltage can be obtained from the potential supplied to the secondary transfer roller 7. This feature is such that in the configuration in which the primary transfer potential supply means, the secondary transfer opposing roller, and the transfer charging opposing roller also serve as the aluminum cylinder 13a as in the present embodiment, mutual interference can be eliminated, and the supply voltage can be reduced. It is excellent in that it can be determined independently and a voltage value having a low absolute value can be selected.
[0074]
In this embodiment, the decay time constant τ is obtained by the coating layer 13c. However, the volume resistivity and the film thickness of the elastic resistance layer 13b are increased, and the decay time constant τ is secured by the resistance and capacitance of these two layers. May be.
[0075]
<Embodiment 3>
Embodiment 3 will be described with reference to FIG. The same members as those in the first embodiment are denoted by the same reference numerals, and the description thereof will not be repeated.
[0076]
In the present embodiment, the intermediate transfer belt 5e in which the surface of the conductive rubber belt is coated with a high-resistance layer is used as the intermediate transfer member in the apparatus configuration described in the first embodiment, and the back surface of the intermediate transfer belt is used. By making the potential the same as the potential supplied by the primary transfer roller, a configuration is adopted in which an opposing potential of a member in contact with the surface is provided. The secondary transfer nip N₂Between the intermediate transfer belt 5 e and the transfer charging roller 8.Cleaning memberThe rotary brush 14 is arranged as a. The rotating brush 14 can be separated from and contacted with the intermediate transfer belt 5e similarly to the transfer charging roller 8, and as shown in FIG.⁵-10⁸150 to 300 wires / inch of conductive fiber of Ω · cm on metallic cored bar 14a²The hair is planted at a density ofPower supply 15To apply a DC voltage.
[0077]
Next, these operations will be described in detail with reference to FIGS.
[0078]
(Transfer process)
As in the first embodiment, when a print command from the host computer is input to the image forming apparatus, a toner image is formed on the photosensitive drum 1 by charging, exposure, development, and the like, and the transfer charging roller 8 Is brought into contact with the intermediate transfer belt 5e, and the surface of the intermediate transfer belt 5e is uniformly charged to + 100V.
[0079]
A ground potential is applied to the primary transfer roller 5b, and a primary transfer electric field is formed by a potential difference between the surface potential of the intermediate transfer belt 5e and the surface potential of the photosensitive drum 1, so that the primary transfer of the first color is performed.
[0080]
The transfer charging roller 8 is separated from the intermediate transfer belt 5e after the charging of the charging area is completed.
[0081]
The primary transfer from the second color is performed by sequentially applying +200 V, +300 V, and +400 V to the primary transfer roller 5b according to each color. The color image synthesized on the intermediate transfer belt 5e by the above-described four primary transfer steps is subjected to a constant current controlled voltage applied to the secondary transfer roller 7 so that the secondary transfer nip portion N₂  Is secondarily transferred to the transfer material P inserted in the batch.
[0082]
(Transfer cleaning process)
Before the leading end of the image on the intermediate transfer belt 5e after the secondary transfer reaches the contact portion of the rotating brush 14, the rotating brush 14 contacts the surface of the intermediate transfer belt 5e, and the image is formed on the metal core 14a of the rotating brush 14. A voltage (cleaning bias) having the same polarity as the primary transfer bias is applied from the forming apparatus main body. At this time, the primary transfer roller 5b is set to the ground potential, and a voltage having the same polarity as the primary transfer bias is applied as the above-described cleaning bias, and an electric field that can be selectively recovered by the polarity of the secondary transfer residual toner on the intermediate transfer belt 5e is generated. Has formed. 10⁵  -10⁸  With the rotating brush 14 of Ω · cm, good recoverability for negative secondary transfer residual toner can be obtained in the range of +500 to +2000 V (2 kV). After passing through the rotating brush 14, the secondary transfer residual toner Ty remaining on the intermediate transfer belt 5e has a tribo of the same polarity as the primary transfer bias (reverse toner). The transfer charging roller 8 comes into contact with the intermediate transfer belt 5e almost simultaneously with the rotating brush 14, and is supplied with the same voltage as the operation performed prior to the primary transfer of the first color, thereby cleaning the surface of the intermediate transfer belt 5e. Charge.
[0083]
The intermediate transfer belt 5e proceeds to the formation of the next image. In the primary transfer process of the first color, the above-described reversal toner Ty has a polarity opposite to the tribo of the original image forming toner (regular toner) Tx. Therefore, the reversal toner Ty is reversed on the photosensitive drum 1 simultaneously with the primary transfer of the image forming toner Tx.
[0084]
(Spitting process)
After the rear end portion of the image on the intermediate transfer belt 5e has passed the rotary brush 14, a voltage (a reverse polarity to the primary transfer bias of the first color) is applied to the core metal 14a of the rotary brush 14, as shown in FIG. (Discharge bias) is applied. With this bias, the toner Tz collected on the rotating brush 14 by the cleaning bias is discharged again to the non-image forming portion on the intermediate transfer belt 5e. At this time, the transfer charging roller 8 prevents the toner Tz discharged away from the intermediate transfer belt 5e from attaching thereto. The discharged toner Tz is in the primary transfer nip N₁  , A reverse bias is applied to force the reverse transfer to the photosensitive drum 1.
[0085]
After finishing the discharging process, the rotating brush 14 and the transfer charging roller 8 are separated from the intermediate transfer belt 5e before the leading end of the next image arrives, to prepare for the next transfer cleaning process.
[0086]
By the above-described transfer cleaning operation, the secondary transfer residual toner present on the intermediate transfer belt 5e is selectively recovered according to the polarity of the tribo, and the toner of any polarity is reversely transferred to the photosensitive drum 1 and removed. be able to. In particular, even when the tribo-distribution of the secondary transfer residual toner is largely fluctuated or the amount is large, by using the rotating brush 14, the charging ability of the transfer charging roller 8 can be sufficiently exhibited. Also, good image uniformity can be obtained even when the environment fluctuates.
[0087]
In the present embodiment, the intermediate transfer belt 5e having the resistance layer provided on the surface layer is used to obtain the attenuation time constant τ satisfying the expression (1). The same effect can be obtained for the intermediate transfer member (intermediate transfer belt 5a, intermediate transfer drum 13) used in the above.
[0088]
<Embodiment 4>
Next, another embodiment of the present invention will be described with reference to FIGS.
[0089]
The configuration of the image forming apparatus according to the present embodiment is substantially the same as that described in the third embodiment, except that the rotating brush 14 and the transfer charging roller 8 are integrated by a connecting arm 18 to form an intermediate transfer belt. The operation of contacting / separating from / to 5e is enabled by the same drive mechanism (not shown), and the configuration is simpler.
[0090]
In the “discharge process” of the present embodiment, the “discharge process” is performed with a delay corresponding to the interval between the rotating brush 14 and the transfer charging roller 8. However, unlike the above-described third embodiment, the transfer charge is performed as shown in FIG. Since the roller 8 is in contact with the intermediate transfer belt 5e, a discharge bias is also applied to the transfer charging roller 8 to prevent the toner Tz on the intermediate transfer belt 5e from adhering.
[0091]
Further, by supplying a potential from the same power source to the rotating brush 14 and the transfer charging roller 8 as in a power source 19 shown in FIGS. 9 and 10, the mutual potential relationship can be kept constant, and the control can be performed. It can be simplified.
[0092]
【The invention's effect】
As described above, according to the present invention, in the intermediate transfer member,Charging memberWhere L is the actual running distance of the intermediate transfer member from the position corresponding to the primary transfer nip portion, VP is the rotation speed of the intermediate transfer member, and τ is the potential decay time when a charge is applied to the surface of the intermediate transfer member. , These values are
τ> L / V_P
By setting so as to satisfy the condition, the surface potential of the intermediate transfer body can be maintained uniformly before the primary transfer, so that it is possible to perform uniform primary transfer and improve image uniformity. Can be done.
[0093]
Also,Cleaning memberEven when a brush to which a voltage is supplied is used, it is possible to erase the charging history such as sweeping unevenness by the brush, and to achieve compatibility with high transfer cleaning performance.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to a first embodiment.
FIG. 2 is a diagram illustrating a state of charge potential attenuation of an intermediate transfer belt.
FIG. 3 is a schematic sectional view for explaining the operation of the first embodiment;
FIG. 4 is a diagram for comparing charging states of the intermediate transfer belt according to the first embodiment and a conventional intermediate transfer belt.
FIG. 5 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to a second embodiment.
FIG. 6 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to a third embodiment.
FIG. 7 is a schematic sectional view for explaining the operation of the third embodiment;
FIG. 8 is a schematic sectional view for explaining the operation of the third embodiment.
FIG. 9 is a schematic sectional view for explaining the operation of the fourth embodiment.
FIG. 10 is a schematic sectional view for explaining the operation of the fourth embodiment.
FIG. 11 is a longitudinal sectional view showing a schematic configuration of a conventional image forming apparatus.
[Explanation of symbols]
1. Image carrier (electrophotographic photosensitive member, photosensitive drum)
2 Image forming means (primary charger)
3. Image forming means (exposure means)
4 Image forming means (developing device)
5 transfer means
5a, 5e Intermediate transfer member (intermediate transfer belt)
7 Secondary transfer roller
8Charging member(Contact charging member, transfer charging roller)
11Primary transfer bias power supply
13 Intermediate transfer body (transfer drum)
14Cleaning member(Rotating brush)
FifteenPower supply
L Actual running distance
N₁        Primary transfer nip
N₂        Secondary transfer nip
P transfer material
V_P        Moving Speed
τ Potential decay time

Claims (10)

An image carrier and a movable intermediate transfer member, wherein a toner image on the image carrier is primarily transferred to the intermediate transfer member at a primary transfer nip portion, and a toner image on the intermediate transfer member is transferred at a secondary transfer nip portion. A toner image secondary-transferred to a transfer material, a primary transfer member provided on a back surface of the intermediate transfer body in the primary transfer nip portion, and a secondary transfer nip portion with respect to a moving direction of the intermediate transfer body. A charging member provided on the surface of the intermediate transfer body at a downstream side of the primary transfer nip and a charging member provided on the surface of the intermediate transfer body. In the device,
Wherein the distance from said charging member along the surface of the intermediate transfer member to said primary transfer nip portion L, and the moving velocity V _P of the intermediate transfer _member, a potential decay time when the applied charge to the intermediate transfer member surface τ , And
τ> L / V _P
Holds, the surface of the intermediate transfer member is charged by the charging member prior to the primary transfer operation of the first color, and in the primary transfer operation of the second color and thereafter, the primary transfer member performs the primary transfer operation Prior to the application of a potential higher than the charging potential charged the intermediate transfer member surface,
An image forming apparatus comprising: Applying a voltage obtained by superimposing an AC voltage on a DC voltage to the charging member,
The image forming apparatus according to claim 1, wherein: Having a contact / separation mechanism that allows the charging member to contact / separate from the intermediate transfer member,
The image forming apparatus according to claim 1, wherein: The charging member contacts the secondary transfer residual toner remaining on the intermediate transfer body after the secondary transfer, and changes the charge polarity of the secondary transfer residual toner to the same polarity as the charge polarity of the intermediate transfer body.
The image forming apparatus according to claim 1, wherein: A cleaning member capable of collecting the secondary transfer residual toner on the intermediate transfer body after the secondary transfer on the downstream side of the secondary transfer nip portion and on the upstream side of the charging member with respect to the moving direction of the intermediate transfer body,
Voltage supply means for applying a voltage to the cleaning member,
The image forming apparatus according to claim 1, wherein: While the area corresponding to the toner image on the intermediate transfer member after the secondary transfer is at the position of the cleaning member, the voltage applied to the cleaning member has the same polarity as the charging polarity of the intermediate transfer member by the charging member. Is,
The image forming apparatus according to claim 5, wherein: The area corresponding to the toner image on the intermediate transfer body after the secondary transfer, after passing the position of the cleaning member, changes the polarity of the voltage applied to the cleaning member,
The image forming apparatus according to claim 6, wherein: The area corresponding to the toner image on the intermediate transfer body after the secondary transfer, after passing through the position of the charging member, changes the polarity of the DC voltage applied to the charging member,
The image forming apparatus according to claim 7, wherein: An area corresponding to the toner image on the intermediate transfer body after the secondary transfer passes the position of the charging member, and then separates the charging member from the intermediate transfer body.
The image forming apparatus according to claim 8, wherein: The cleaning member is made of a conductive fiber brush, and has a contact / separation mechanism for bringing the brush into contact with / separating from the intermediate transfer member.
The image forming apparatus according to claim 5, wherein:

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