JP2010164616A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve white fogging on transfer material even when using glossy paper and even in a recent image forming apparatus in which transfer efficiency is enhanced by decreasing toner fogging on a photoreceptor due to weakly electrified toner or inverted toner. <P>SOLUTION: The image forming apparatus includes: an image carrier; a developing means for developing an electrostatic latent image formed on the image carrier with toner; a transfer means for transferring a toner image developed by the developing means to transfer material; and a conductive member provided to be opposed to the image carrier on a downstream side of the developing means in a rotating direction of the image carrier and on an upstream side of the transfer means in the rotating direction of the image carrier, and electrically attracting toner having a reverse polarity to the toner on the image carrier by application of voltage having the same polarity as an electrifying polarity of the toner. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as an electrophotographic system or an electrostatic recording system copying machine, a printer, or the like.

  Conventionally, as an example of an electrophotographic image forming apparatus, for example, an image forming apparatus disclosed in Patent Document 1 has been proposed.

  However, such an image forming apparatus has a problem of “fogging”. This is because a small amount of toner adheres to the non-exposed portion of the photoreceptor, which should originally be a white background, and the toner should not adhere to the transfer material, and the white background portion of the transfer material is slightly stained with the toner. It is a phenomenon.

  The toner fog on the photosensitive member is that toner particles adhere to the surface of the photosensitive member with a mirror power when the photosensitive member and the toner come into contact with each other. For this reason, the white background potential of the photosensitive member is set with a difference so that the toner is peeled off from the surface of the photosensitive member with respect to the DC component effective value of the developing bias (minus side in the prior art). This difference is called “fogging guarantee potential”, “Vback”, or the like.

  However, even when the fogging guarantee potential is set in this way, the toner may still be covered on the photoreceptor. This is mainly due to the charge amount of the toner.

  The charge amount of each toner particle varies. Some of them have a slight amount of charge whose absolute value is close to 0 μC / g (hereinafter referred to as weakly charged toner), and those that are charged positively (hereinafter referred to as reverse toner). Exists. The weakly charged toner adheres to the surface of the photosensitive member due to van der Waals force, and the reverse toner adheres to the surface of the photosensitive member due to the electric field generated by the guaranteed fogging potential.

  Since the toner adhering to the photosensitive member is electrically transferred to the transfer material or intermediate transfer member, conventionally, the weakly charged toner or the reversal toner, which is the fog toner, is not transferred, and this is not a problem in practice. .

JP 2003-345209 A

  However, due to the recent improvement in transfer technology, transfer efficiency has increased, and such fog toner may be transferred onto a transfer material, which is becoming a problem in practical use.

  In addition, glossy paper with a glossy feeling like silver halide photography, which has been in increasing demand as a transfer material in recent years, probably has a high van der Waals force with toner. Is transferred even if it is a reversal toner.

  That is, in recent years, it has been demanded to reduce the amount of fog toner on the photoreceptor more than before.

  Therefore, the object of the present invention is to reduce the toner fog on the photosensitive member due to weakly charged toner or reversal toner, and in recent image forming apparatuses in which transfer efficiency has been increased, and even when using glossy paper, It is to improve the fogging of white background.

  A typical configuration of the present invention for achieving the above-described object includes an image carrier, a developing unit that develops an electrostatic latent image formed on the image carrier with toner, and a developing unit that develops the electrostatic latent image. A transfer means for transferring the toner image to a transfer material; and opposed to the image carrier on the downstream side in the rotation direction of the image carrier relative to the developing means and on the upstream side in the rotation direction of the image carrier relative to the transfer means. And a conductive member that electrically attracts toner having a polarity opposite to that of the toner on the image carrier when a voltage having the same polarity as the charged polarity of the toner is applied. .

  With the above-described configuration, it is possible to reduce toner fog on the photosensitive member due to weakly charged toner or reversal toner, and to improve white fog on the transfer material.

[First Embodiment]
The image forming apparatus according to the first embodiment will be described in detail below.

[Schematic Configuration and Operation of Image Forming Apparatus]
First, a schematic configuration and operation of the entire image forming apparatus will be briefly described. FIG. 1 is a schematic configuration diagram of an image forming apparatus according to the first embodiment.

  In FIG. 1, an electrostatic latent image is formed on the photosensitive drum 28 by exposing the surface of the photosensitive drum 28 as an image carrier charged by the primary charger 21 with a laser 22 from an exposure unit.

  This latent image is developed (developed) by a developing device 1 as developing means to obtain a toner image. This toner image is electrostatically transferred onto the surface of a transfer material 27 such as recording paper by a transfer charger 23. The transfer material 27 to which the toner image has been transferred is heated by the fixing device 25, and the toner image is fixed to the transfer material 27 to obtain a permanent image. Residual toner remaining on the photosensitive drum 28 after the transfer is removed by the cleaner 26.

  The photosensitive drum 28 will be described. The photosensitive drum 28 of the present embodiment is an OPC photosensitive member having a negatively charged polarity, and is obtained by sequentially providing functional layers mainly made of resin on a grounded aluminum drum base. The film pressure of the functional resin layer is 26 μm. This surface is uniformly charged to −600 V by the primary charger 21, and the potential of this portion is called a white background portion potential or Vd. If exposure with the laser 22 is performed on the basis of the image information, the potential of the portion (image portion) becomes −200V. This portion of the potential is called an image portion potential or Vl.

  The developing device 1 will be described. The image forming apparatus according to the present embodiment uses a magnetic one-component developing system using magnetic toner as a magnetic developer. The magnetic toner is a powder having a volume average particle size of about 8 μm obtained by pulverizing and classifying a kneaded polymer obtained by kneading and polymerizing a magnetic material and a coloring material such as magnetite to a resin mainly composed of polyester.

  The developing device 1 of the present embodiment uses a developing sleeve 3 in which a magnet 5 arranged in a fixed manner is included in a non-magnetic metal feeling as a developer carrying member. The magnetic toner accommodated in the developing device 1 is mainly negatively charged by rubbing against the surface of the developing sleeve 3 when the layer thickness is regulated by the blade 4. The developing sleeve 3 is provided in non-contact with the photosensitive drum 28, and the magnetic toner carried on the developing sleeve 3 approaches the photosensitive drum 28 in the state of a magnetic brush.

  A developing bias in which an AC component is superimposed on a DC component is applied to the developing sleeve 3 from a developing bias power source (not shown). The DC component of the developing bias is −450V. The AC component of the developing bias is a rectangular wave, the frequency is 3 kHz, and the peak-to-peak voltage is 1.5 kV.

  The magnetic toner reciprocates between the developing sleeve 3 and the photosensitive drum 28 by an alternating electric field due to a developing bias AC component, and moves from the developing sleeve 3 to the photosensitive drum 28 (this direction is set as a developing direction). Adhere to. Further, a fogging guarantee potential of 150 V is provided between the white background portion potential and the DC component of the developing bias, and the toner adhering to the photosensitive drum 28 by the mirror force during the reciprocation of the toner is opposite to the developing direction. An electric force (polarity) is applied in the direction to suppress toner adhesion to the white background.

  The toner image formed on the photosensitive drum 28 is transferred to the transfer material 27 by a transfer bias applied to a transfer charger 23 as a transfer unit. As the transfer material, there are used general papers such as acidic or neutral high-quality paper, neutral paper, renewed paper, recycled paper, and coated paper in which a coating layer is bonded using these as a base material.

[About coated paper]
Here, the coated paper will be described in detail. As described above, in recent years, there is an increasing demand for a transfer material having a glossy feeling like a silver salt photograph, and a coated paper that gives a glossy feeling by coating the surface of ordinary paper is often used.

  As the base material of the coated paper, general materials such as acid or neutral high-quality paper, neutral paper, reprinted paper, and recycled paper are used.

  For the coating layer to be coated, a general pigment can be used. For example, there are mineral pigments such as heavy calcium carbonate, light calcium carbonate, titanium dioxide, and aluminum hydroxide, polystyrene resin fine particles, urea formaldehyde resin fine particles, fine hollow particles, and other organic pigments. These can be used alone or in combination.

  As an adhesive in the coating layer, a water-soluble adhesive having a strong adhesive force to an additive such as a base material or a pigment, or an emulsion, latex, or the like can be used alone. For example, water-soluble resins such as polyvinyl alcohol and modified polyvinyl alcohol, acrylic emulsions, vinyl acetate emulsions, vinylidene chloride emulsions, polyester emulsions and the like are used.

  Moreover, you may use the general material used for the coating material as a coating layer. For example, specifically, a liquid paint mainly composed of a resin composition, a white pigment and an organic solvent is raised. In the colored base paint, a colored pigment may be used, and the white pigment is mainly composed of the pigment.

  As the resin composition, a normal thermosetting resin composition is preferable. For example, there are an acrylic resin, a vinyl resin, a polyester resin, an alkyd resin, a urethane resin, and the like having one or more functional groups selected from a hydroxyl group, a carboxyl group, and the like. One or more base resins selected from these, and one or more selected from melamine resins, urea resins, epoxy group-containing compounds, etc. that react with these functional groups in order to improve the paper strength of paper A mixture of two or more crosslinking agents can be used.

The coating amount is about 10 g / m ^{2. With} this amount, the surface can be sufficiently glossy, and an appropriate stiffness can be maintained with respect to the transportability in the image forming apparatus.

  As for the glossiness of the surface of the coated paper of this embodiment, the glossiness measured by a measuring device conforming to the standard for 75-degree specular glossiness of JISP-8142-1993 is about 40%.

  Here, the characteristic part of this embodiment is demonstrated in detail.

  As described in the prior art, even if the fogging guarantee potential is set, the toner is applied to the photosensitive drum 28 although it is very slight. This is mainly due to the charge amount of the toner. FIG. 2 is a diagram showing a toner charge amount distribution in the first embodiment. The horizontal axis represents the charge amount of each toner particle (μC / g), and the vertical axis represents the frequency.

  In the one-component development system as in this embodiment, the area of the friction partner (the surface of the development sleeve 3 in this embodiment) is limited with respect to the surface area of the toner. Charging is inevitable. For this reason, as shown in FIG. 2, the distribution has a shape with a peak on the minus side and a skirt. That is, some toners have a toner charge amount close to 0 (weakly charged toner) and those charged positively (reversed toner).

  The weakly charged toner adheres to the surface of the photosensitive drum 28 by van der Waals force, and the reversal toner adheres to the surface of the photosensitive drum 28 by an electric field due to the fogging guarantee potential.

  The fog toner adhered on the photosensitive member is electrically transferred to the transfer material 27. Here, in the case of generally used high-quality paper, neutral paper, etc., the weakly charged toner and the reversal toner are hardly transferred, and there is no practical problem.

  However, since the coated paper mentioned above has a high surface smoothness and good contact with the toner, the weakly charged toner is probably transferred by van der Waals force. Even if the toner is reversed, if the van der Waals force wins against the electric field due to the transfer bias, the toner is transferred onto the coated paper. As a result, the toner adheres to the white background on the coated paper and is recognized as a fog.

  Therefore, in the present embodiment, by adopting the following configuration, the weakly charged toner and the reversal toner are removed in advance before the transfer step to suppress the white background fog of the coated paper.

[Configuration of the cover roller 31]
As shown in FIG. 1, between the developing sleeve 3 and the transfer charger 23 facing the photosensitive drum 28, an elastic layer is provided around the core metal and a surface layer having releasability is coated on the surface layer. The fog removing roller 31 (conductive member) is provided. The fog removing roller 31 is provided so as to contact the photosensitive drum 28 and rotate according to the movement of the photosensitive drum 28.

  The fog roller 31 will be described in detail with reference to FIG. FIG. 3 is a cross-sectional view showing the configuration of the fog removing roller 31.

  As shown in FIG. 3, an elastic layer 31b made of an elastic member is coated around a cored bar 31a made of a stainless steel round bar. This elastic layer 31 b is for forming a contact nip between the photosensitive drum 28 and the fog removing roller 31.

  As the material of the elastic layer 31b, rubber materials such as ethylene-propylene-diene rubber (EPDM), urethane rubber, silicone rubber, epichlorohydrin rubber, and the like, which are made of an elastic sponge body (foamed structure), and the like are used. Moreover, in order to impart moderate conductivity, conductive particles such as carbon particles and metal oxides are dispersed.

  The elastic layer 31b is covered with an adhesive layer 31c made of urethane rubber, NBR rubber or the like.

  A surface layer 31d having releasability is coated around the adhesive layer 31c. The surface layer 31d is formed in order to improve surface releasability and substance transfer, and a coating film such as a fluororesin or a polyamide resin is used. Further, tin oxide, carbon, or the like may be dispersed for adjusting the resistance as appropriate. In addition, if the surface of the surface layer 31d has a ten-point average roughness Rz of about 0.1 μm to 4 μm, sufficient releasability can be ensured.

  The length of the cover roller 31 in the longitudinal direction (axial direction) is such that the length of the elastic layer 31b, the adhesive layer 31c, and the surface layer 31d is 320 mm, and the core metal 31a is 340 mm. The cored bar 31a is exposed by 10 mm on each side at both ends, and is supported by a conductive bearing (not shown). Further, this conductive bearing is supported by a metal spring, and the fog removing roller 31 is pressed against the photosensitive drum 28 by a force of 4.9 N from both ends, a total of 9.8 N. The fog removing roller 31 rotates as the surface of the photosensitive drum 28 moves.

  As described above, when the elastic member is brought into contact with the photosensitive drum 28, the weakly charged toner adhering to the photosensitive drum 28 in a narrow dot-shaped region is wrapped by the fog removing roller which is an elastic member, so that a wide area is obtained. With van der Waals force can be applied. By this action, the weakly charged toner is removed from the photosensitive drum 28 and is attracted to the fog removing roller 31.

Here, the electrical resistance of the fog removing roller 31 will be described. The resistance of the fog removing roller 31 can define the electrical characteristics in the cross-sectional direction of FIG. 1 by defining a value per unit length in the axial direction. The electrical resistance Rr per unit length in the axial direction is such that the cover roller 31 is pressed against the metal cylinder having the same outer diameter as that of the photosensitive drum 28 with the actual pressing force, and the metal cylinder is rotated at the same rotational speed as that of the photosensitive drum 28. Connect the power supply while rotating. Then, the current value was measured while applying a predetermined voltage, and the actual resistance value obtained from these values was divided by the axial length. The Rr of the present embodiment thus determined was 3 × 10 ⁹ (Ω / m).

  Further, a DC bias voltage of −750 V is applied from the fog removing bias power source 33 to the fog removing roller 31 via a spring and a conductive bearing. The reverse toner on the photosensitive drum 28 is attracted to the fog removing roller 31 by this electric force. Here, although the toner image is formed on the photosensitive drum 28, since the charging polarity of the toner particles in the toner image is negative, the toner image is pressed against the photosensitive drum 28 side. And between the fogging rollers 31. That is, the formed toner image is conveyed to the transfer region without being disturbed by the fog removing roller 31.

  In addition, a rotating brush 32 having a conductive brush implanted is in contact with the fog removing roller 31 so that the toner adhering to the fog removing roller 31 is scraped off.

  As described above, according to this embodiment, a conductive member whose surface layer is made of an elastic material is provided in contact between the development region and the transfer region on the surface of the photosensitive member, so that the weakly charged toner is exposed to light by van der Waals force. It can be removed from the body surface. Further, the conductive member is applied with a DC bias voltage from the bias power source, so that only a reversal toner is electrically attracted to the conductive member without disturbing a toner image charged with a normal polarity. Can be removed from above.

  An experiment was conducted to see how the fogging on the transfer material and the coated paper changes depending on whether or not the fogging roller 31 is actually mounted.

  The fog on the transfer material is measured as follows. First, the average reflectance Dr (%) of the transfer material before image formation is measured by a reflectometer (“REFLECTOMETER ODEL TC-6DS” manufactured by Tokyo Denshoku Co., Ltd.). On the other hand, an entire white background image is formed on the transfer material by the image forming apparatus of this embodiment, and the reflectance Ds (%) of this image is measured. This Dr (%) − Ds (%) is defined as the fogging amount (%).

  The measurement results are shown in FIG. FIG. 4 is a chart showing the measurement results for high-quality paper and coated paper when a fog removal roller is used.

  As shown in FIG. 4, even when a transfer material such as a coated paper having a high surface smoothness and a good contact property with the toner was used, the improvement of the white background fog could be achieved.

  Although the first embodiment has been described above, more detailed conditions will be described.

Preferably, the electric resistance Rr of the fog removing roller 31 is
1 × 10 ⁶ (Ω / m) ≦ Rr ≦ 1 × 10 ¹² (Ω / m) (Formula 1)
Satisfy the relationship.

If Rr is less than 1 × 10 ⁶ (Ω / m), the charge is likely to move, so that a bias is applied to the fog removal roller 31 to inject the charge onto the surface of the photosensitive drum 28 and the toner image. May end up. If charges are injected into the surface of the photosensitive drum 28, the electrostatic latent image is disturbed, and it is not preferable for electrically holding the toner image. Further, when electric charge is injected into the toner of the toner image, toner particles having an excessive charge amount are formed, and there is a possibility that the toner image is disturbed by causing transfer omission or the like during transfer in the next process.

When Rr exceeds 1 × 10 ¹² (Ω / m), the electric resistance of the fog removing roller 31 approaches the electric resistance of the photosensitive drum 28, and the surface potential difference between the fog removing roller 31 and the photosensitive drum 28 is small. Therefore, there is a possibility that the effect of this embodiment cannot be obtained with certainty.

Preferably, the DC bias voltage Vr applied to the conductive member by the bias power source is set so that Vr is opposite to Vl across Vd,
Vr at which discharge starts between the conductive member and the image portion of the photoconductor is Vrb,
(When the charging polarity of the toner is negative)
Vd−50 (V) ≧ Vr ≧ Vrb + 50 (V) (Formula 2)
(When the charging polarity of the toner is positive)
Vd + 50 (V) ≦ Vr ≦ Vrb−50 (V) (Formula 3)
Satisfy the relationship.

  Regarding the difference between Vr and Vd (the left side of Formula 2 and Formula 3), the following experiment was conducted to determine the condition.

  FIG. 5 is a chart showing the measurement results of the fog on the coated paper when Vd = −600V and the value of Vr is changed.

  As shown in FIG. 5, when the difference between Vr and Vd is 50 V or more, sufficient electric force is applied to the reversal toner on the white background, and the effect of this embodiment can be obtained with certainty. I understand.

  Further, regarding the maximum value of the absolute value of Vr (the right side of Equations 2 and 3), conditions were determined by conducting the following experiment.

  When the difference between Vr and the surface potential of the photosensitive drum 28 is increased, a discharge phenomenon occurs, and a phenomenon occurs in which the photosensitive drum 28 and the toner image are recharged. The inventors conducted an experiment to determine how the surface potential of the photoconductive drum 28 changes by applying the bias by applying the bias roller 31 to the photoconductive drum 28 charged to a predetermined surface potential. .

FIG. 6 is a graph showing how the surface potential of the photosensitive drum 28 is changed by the fog removing roller 31. Vr (V) is a bias applied to the fog removing roller 31. V ₁ (V) and V ₂ (V) are obtained by measuring the surface potential of the photosensitive drum 28 upstream and downstream of the fog removal roller 31, respectively. The measurement was performed with a surface potential meter (Model 344 manufactured by TREK, USA). The horizontal axis of the graph represents the difference (Vr−V ₁ ) between the bias Vr (V) and the surface potential V ₁ (V) upstream of the fog removal roller 31, and the vertical axis represents the downstream of the fog removal roller 31. The difference (V ₂ −V ₁ ) between the potential V ₂ (V) and the upstream surface potential V ₁ (V) is plotted.

The photosensitive drums A, B, and C have functional resin layer thicknesses of 16 μm, 21 μm, and 26 μm, respectively. For example, in the case of the photosensitive drum A, the discharge phenomenon does not occur until (Vr−V ₁ ) is about −550V, and the discharge phenomenon occurs when (Vr−V ₁ ) exceeds −550V. This means that the surface potential of the photosensitive drum changes.

Since the graph is not strictly bent at the discharge start portion, the value of (Vr−V ₁ ) at the point where the straight line portion with the slope 1 rising to the right in FIG. It will be called Vb. The values of the discharge start voltage Vb of the photosensitive drums A, B, and C were −550V, −640V, and −700V, respectively.

The value of the Note _V 1 (V) is -200 V, -400 V, although experiment varied between -600 V, either line also the graph of FIG. 6 cases were the same.

  As described above, when the difference between the surface potential of Vr and the photosensitive drum 28 becomes large and a discharge phenomenon occurs, the electrostatic latent image on the surface of the photosensitive drum 28 is disturbed. Further, the charge amount of the toner changes due to the discharge, and there is a possibility that the toner image may be disturbed by causing a transfer omission or the like during the transfer in the next process.

  In order not to cause such a discharge phenomenon, it is necessary that the difference between Vr and Vr with respect to the image portion potential Vl having the widest potential difference does not exceed the discharge start voltage Vb. The value of Vr at this time is assumed to be Vrb (= Vl + Vb). In the present embodiment, the actual Vr setting value is set with a margin of 50 V with respect to Vrb in consideration of disturbance factors and the like.

  The photosensitive drum 28 used in this embodiment is the photosensitive drum C of FIG. 6, Vd is −600 V, Vl is −200 V, and the discharge start voltage Vb is −700 V. Therefore, Vr is from −650 V. Can be set between -850V.

  By applying the conditions as described above, the effects of the present embodiment can be reliably obtained without disturbing the surface potential of the photosensitive drum 28 and the charge amount of the toner image.

  The image forming apparatus according to the present embodiment has been described above. The effects of the present embodiment will be described here.

  In this embodiment, a conductive member provided opposite to the image carrier is provided on the downstream side in the image carrier rotation direction with respect to the developing unit and on the upstream side in the image carrier rotation direction with respect to the transfer unit. The conductive member electrically attracts toner having a polarity opposite to that of the toner on the image carrier when a voltage having the same polarity as the charged polarity of the toner is applied.

  With such a configuration, the elastic member of the conductive member may be brought into contact with the photosensitive drum, and the configuration is simple. That is, as shown in Patent Document 1, carrier supplementing means that is considered to be a rigid body configured by fixing a magnet having a magnetic flux density of 40 to 100 mT inside a non-magnetic sleeve is brought close to 0.005 mm or 0.05 mm. It does not take a complicated configuration.

  Further, in a configuration in which foreign matter is removed or an electric field is applied between development and transfer, the member is of a non-contact type with the photosensitive drum. Then, even if an electric field is applied to the photosensitive drum, the weakly charged toner attached to the photosensitive drum by van der Waals force cannot be removed. Further, although an electric field is applied in the direction to the reversal toner, it is difficult to accurately narrow the gap between the photosensitive drum and the member in order to increase the electric field as described above.

  In this respect as well, the present embodiment can remove weakly charged toner with van der Waals force in the same manner because the elastic member is brought into contact. Further, since a bias is applied by contacting an elastic member having an appropriate resistance value, it is possible to efficiently remove the reverse toner by applying a strong electric field to the reversal toner with a simple configuration.

[Second Embodiment]
The image forming apparatus according to the second embodiment of the present invention will be described in detail below. FIG. 7 is a schematic configuration diagram of an image forming apparatus according to the second embodiment.

  As shown in FIG. 7, the image forming apparatus of this embodiment is provided with a plurality of stations that perform the toner image forming process described in the first embodiment. That is, stations corresponding to each of four stations of yellow, magenta, cyan, and black (referred to as Y, M, C, and K stations, respectively) are provided. The image forming apparatus obtains a full-color image by superimposing four color toner images on the intermediate transfer belt 24 and then collectively transferring the toner images onto the transfer material 27. This is a full-color image forming apparatus called a so-called tandem system. In the following description, what is simply indicated by reference numerals is a portion common to each station in FIG.

  The photosensitive drum 28 (Y, M, C, K) of this embodiment uses the photosensitive drum 28 described in the first embodiment, and the active resin layer has a film thickness of 16 μm. Since the electrostatic capacity of the photosensitive drum 28 increases as the film thickness is thinner than in the first embodiment, a sufficient amount of toner can be obtained even if the contrast potential difference (= difference between the Vl and the DC component of the developing bias) is small. It is done. In this embodiment, Vd is −400V, Vl is −150V, and the DC component of the developing bias is −280V.

  In this embodiment, a two-component development method is used as a development method, and a non-magnetic toner having a negatively charged polarity and a magnetic carrier are mixed to form a developer. The AC component of the developing bias is a rectangular wave, the frequency is 8 kHz, and the peak-to-peak voltage is 1.8 kV.

  FIG. 8 is a diagram showing a toner charge amount distribution according to the second embodiment. Compared to the charge amount distribution of the one-component system of the first embodiment (FIG. 2), the surface area of the carrier which is a frictional charging member is large, so the charge amount distribution of the toner is sharp and the weakly charged toner and the reversal toner The ratio is also small. On the contrary, in this embodiment, since the toner is nonmagnetic, the toner cannot be directly restrained by magnetism. Therefore, in this embodiment, it is necessary to take a considerable measure against the white background fog on the photosensitive drum 28.

  As the intermediate transfer belt 24, a polyimide resin base layer provided with an elastic layer of chloroprene rubber and a surface layer coated with a fluororubber paint was used. Such a material with very good transfer efficiency also transfers weakly charged toner and reversal toner in white areas, which could not be transferred conventionally. For this reason, there is a strict requirement for a white background fog on the photosensitive drum 28 immediately before transfer.

  Therefore, in the image forming apparatus according to the present embodiment, similarly to the first embodiment, the fog removing rollers 31 (Y, M, C, and K) are provided in contact with the photosensitive drums 28, so that the weakly charged toner or the reversal is provided. The toner is removed from the photosensitive drum 28 in advance before transfer. Thereby, a high-quality product with little fogging on white background can be obtained.

  FIG. 9 is a chart showing the measurement results of the fog on the coated paper when the value of Vr is changed in the second embodiment.

  From the results shown in FIG. 9, it can be seen that the effect of the present invention can be obtained with certainty if the difference between Vr and Vd is 50 V or more in this embodiment. Further, since the value of the discharge start voltage Vb of the photosensitive drum is -550V, Vrb = -700V when Vl is -150V. That is, under this condition, Vr may be selected in the range of -450V to -650V.

  In the above description, the various potential settings of each station are made common in order to simplify the explanation. However, the present invention is of course not limited to this, and an optimal setting value may be applied to each station. However, since there is a range in the settable range of Vr, for example, the fog removing bias power supply 33 may be common to all stations, that is, Vr may be common to all stations.

  By the way, in the tandem image forming apparatus as in the present embodiment, there is a great restriction on the arrangement configuration of each station, and it may be difficult to arrange the rotating brush 32 and the like as in the first embodiment.

  Therefore, in this embodiment, the toner removal from the fog removing roller 31 is performed by the magnetic brush on the developing sleeve 3.

  The characteristic part of this embodiment will be described with reference to FIG. FIG. 10 is an image forming apparatus according to the second embodiment shown in FIG. 7 and shows a common part of each station.

  In the present embodiment, as a means for removing weakly charged toner and reversal toner from the fog removal roller 31, a two-component developer that stands up as a magnetic brush near the S 1 magnetic pole on the developing sleeve 3 is applied to the fog removal roller 31. It is provided in contact. Specifically, the closest distance between the developing sleeve 3 and the fogging roller 31 is 2.5 mm, and it comes into contact with a magnetic brush having a head length of about 4 to 5 mm.

  The reason why the magnetic brush of this S1 magnetic pole is long is that the magnetic pole downstream in the rotation direction of the developing sleeve 3 is the S3 pole having the same polarity, and a region having no magnetic conveying force is created between them to retain the developer and develop. This is because the two-component developer is peeled off from the surface of the sleeve 3. Of course, if the pole downstream of the pole facing the fog roller 31 is a different pole, after the layer thickness is regulated by the blade 4, such a magnetic brush having a long head length is not obtained. For this reason, the closest distance between the developing sleeve 3 and the cover roller 31 may be made closer according to the head length. In short, it is important that the magnetic brush and the fog removing roller 31 are in reliable contact with each other and that the fog removing roller 31 does not hinder the conveyance of the developer.

  Returning to the present embodiment, the weakly charged toner adhering to the fog removing roller 31 comes into contact with the magnetic brush near the S1 magnetic pole. Thus, the weakly charged toner is removed from the surface of the fog removal roller 31 by the mechanical scraping force of the magnetic brush.

  Further, the weakly charged toner contacts the magnetic carrier and receives frictional charging, so that it is charged to the negative side and is electrically attracted to the surface of the magnetic carrier, or the force generated by the electric field between the developing sleeve 3 and the fogging roller 31. Or removed from the fog removal roller 31.

  Since the reversal toner has a slight charge, it adheres to the fog removal roller with a stronger electric force than the weakly charged toner.

  Here, it is noted that an AC component voltage (peak-to-peak voltage is 1.8 kV, that is, 900 V on one side) is applied to the developing sleeve 3. The developing sleeve 3 and the fog roller 31 are larger than the DC component. There is a timing when a potential difference is applied. For example, there is a timing at which a potential difference of maximum 600V-280V + 900V = 1220V is applied. In addition, an electric force is applied to the reversal toner that is positively charged at this timing in the direction from the fog removing roller 31 toward the developing sleeve 3. With this strong electric force, the reversal toner restrained on the surface of the fog removal roller 31 can be once released. The reversal toner once detached from the surface of the fog removal roller 31 can be easily collected with a magnetic brush.

  Thus, if the configuration as in the present embodiment is adopted, it is not necessary to separately provide a mechanism for cleaning the fog roller 31, and the object of the present invention can be achieved in a simpler form.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment. FIG. 6 is a diagram illustrating a toner charge amount distribution according to the first embodiment. Sectional drawing which shows the structure of the fog removal roller 31 of 1st Embodiment. The chart which shows the measurement result in a quality paper and a coated paper at the time of using the fog removal roller of 1st Embodiment. The chart which shows the measurement result of the fog on the coated paper when the value of Vr is changed at Vd = −600 V in the first embodiment. 6 is a graph showing how the surface potential of the photosensitive drum is changed by the fog removing roller 31 in the first embodiment. FIG. 5 is a schematic configuration diagram of an image forming apparatus according to a second embodiment. FIG. 6 is a diagram illustrating a toner charge amount distribution according to a second embodiment. The chart which shows the measurement result of the fog on the coated paper when the value of Vr is changed in the second embodiment. The figure which extracted and showed the common part of each station in the image forming apparatus of 2nd Embodiment.

DESCRIPTION OF SYMBOLS 1 ... Developer 3 ... Development sleeve 4 ... Blade 5 ... Magnet 21 ... Primary charger 22 ... Laser 23 ... Transfer charger 25 ... Fixer 26 ... Cleaner 27 ... Transfer material 28 ... Photoconductor drum 31 ... Fog removing roller 31a ... Core metal 31b ... elastic layer 31c ... adhesive layer 31d ... surface layer 33 ... bias power supply

Claims (4)

  An image carrier, a developing unit for developing the electrostatic latent image formed on the image carrier with toner, a transfer unit for transferring the toner image developed by the developing unit to a transfer material, and the developing unit A voltage having the same polarity as the charging polarity of the toner, provided on the downstream side in the rotation direction of the image carrier and further on the upstream side in the rotation direction of the image carrier than the transfer unit And an electrically conductive member that electrically attracts toner having a polarity opposite to that of the toner on the image carrier when applied to the image forming apparatus. When the electric resistance per unit length in the axial direction of the conductive member is Rr,
1 × 10 ⁶ (Ω / m) ≦ Rr ≦ 1 × 10 ¹² (Ω / m)
The image forming apparatus according to claim 1, wherein the relationship of the following formula is satisfied. The white background portion potential of the image carrier is Vd, the image portion potential of the image carrier is Vl,
The DC bias voltage Vr applied to the conductive member is set so that Vd is between Vl and Vr,
When Vr where discharge starts between the conductive member and the image portion of the image carrier is Vrb,
(When the charging polarity of the toner is negative)
Vd−50 (V) ≧ Vr ≧ Vrb + 50 (V),
(When the charging polarity of the toner is positive)
3. The image forming apparatus according to claim 1, wherein a relationship of an expression of Vd + 50 (V) ≦ Vr ≦ Vrb−50 (V) is satisfied.   The developing means includes a developer carrying member including a fixed magnet, and the magnetic developer accommodated in the developing means forms a magnetic brush by the magnetic pole of the magnet, and the magnetic brush contacts the conductive member. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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