JP2007279346A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which prevents the degradation of image quality like transfer dust resulting from the variance in Q/M of a toner image on a transfer object member due to a discharge caused when peeling the transfer object member, by surely removing residual charges on the transfer object member. <P>SOLUTION: In an image forming apparatus including a belt-like intermediate transfer member having a toner image thereon and a roller constituting a nip part together with the intermediate transfer member, the intermediate transfer member and the transfer object member are brought into close contact with each other in the nip part, and the toner image is transferred to the transfer object member by a bias. The image forming apparatus 150 includes a first electrode 100 for directly giving an electric field having a polarity opposite to that of toner to a rear side of the transfer object member during the close contact in the nip part and a second electrode 101 for directly giving an electric field having the same polarity as toner to a place which is a distance D<SB>0</SB>or less above an upstream side end of the nip part and is below the first electrode 100. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus having a secondary transfer process. The present invention also relates to an image forming apparatus such as a color analog / digital PPC, a color laser printer, or a monochrome machine.

  Normally, in order to create a transfer electric field, a certain amount of charge is moved to the main electrode by applying a bias. However, in an intermediate transfer belt, paper, or other transfer medium having high electric field-dependent resistance characteristics, current easily flows when voltage is applied under the main electrode, and the resistance increases after passing under the electrode, and the holding charge increases the potential. As a result, electric charge remains for a long time. When the transferred body after the toner image transfer is peeled off from the intermediate transfer belt after the secondary transfer, peeling discharge occurs from the transferred body to the intermediate transfer belt, and the Q / M of the toner becomes higher than that before the secondary transfer. . For this reason, the toners tend to repel each other, transfer dust is generated, and image quality is deteriorated.

Therefore, Patent Document 1 discloses an image forming apparatus in which a charge eliminating unit that prevents peeling discharge is placed so as to face a peeling start point between an image carrier and paper. Japanese Patent Application Laid-Open No. 2004-228561 prevents peeling discharge so as not to give peeling discharge noise to a latent image in a single latent image multiple transfer machine. However, the charge removal in Patent Document 1 is an AC corotron charge removal, and the charge is removed over a wide range in the vicinity of the peeling point in a non-contact manner. For this reason, there is a possibility that an abnormality in the toner charge amount occurs due to AC discharge, resulting in deterioration of image quality. In addition, the neutralization of the AC corotron generally increases the separation ability, but at the same time the Q / M of the toner is fluctuated, so there is a possibility that the image quality may be deteriorated.
Patent Document 2 discloses an image forming apparatus in which the neutralization bias of the paper from the intermediate transfer drum is performed with a DC bias, the neutralization blade is in direct contact with the back of the paper, and the neutralization device is separated during image formation. However, this method can be reduced in size and cost, but there is a problem that image abnormality countermeasures are not sufficient depending on the position of the static eliminator. In particular, the static eliminator is not close to the transfer device, preventing at the peeling point, and there is a problem that charge injection with the transfer bias polarity cannot be prevented.
Patent Document 3 also discloses an image forming apparatus having a charge removing / separating unit. However, this method also has a problem that image abnormality countermeasures are not sufficient depending on the position of the static eliminator.
Further, Patent Document 4 discloses a separation static eliminator and an image forming apparatus having first and second guide members on the downstream side of the separation static eliminator. Thereby, it is possible to prevent the transfer paper from entering the separation static eliminator. However, this method also has a problem that image abnormality countermeasures are not sufficient depending on the position of the static eliminator.

  As a conventional method, there is a counter bias method. In the counter bias system, a bias having the same polarity as the toner polarity is applied before nip and before contact, and a reverse electric field is applied before contact at the entrance to prevent toner transfer before contact. However, this method has a problem in that it does not extend to the exit side, and image abnormality countermeasures are not sufficient.

JP-A-61-153679 Japanese Patent Laid-Open No. 9-244423 JP-A-8-146777 JP-A-6-222682

  Accordingly, the present invention has been made in view of the above problems, and the problem is that the residual charge on the transfer medium is surely removed, and the Q / M of the toner image on the transfer medium is reduced by discharge caused at the time of peeling. An object of the present invention is to provide an image forming apparatus that prevents image quality deterioration such as transfer dust from fluctuating.

The features of the present invention, which is a means for solving the above problems, are listed below.
The present invention includes a belt-like intermediate transfer member on which a toner image is placed, and a roller that forms a nip portion with the intermediate transfer member, and the intermediate transfer member and the transfer target member are in close contact with each other at the nip portion. In the image forming apparatus for transferring the toner image to the transfer body with a bias, a first electrode for directly applying an electric field having a polarity opposite to that of the toner is provided on the back side of the transfer body when closely contacting in the nip portion. and a distance D ₀ within upstream side of the nip portion downstream end, is an image forming apparatus and having a second electrode applying an electric field of the same polarity as the toner directly downstream of the first electrode .
The present invention is the image forming apparatus, wherein the first electrode is in direct contact with the back side of the transferred body, and the second electrode is in direct contact with the back side of the transferred body.
Further, the present invention is characterized in that a contact portion between the second electrode and the transfer target is covered with a material having a volume resistance of 10 ⁶ to 10 ¹² Ω · cm. The volume resistance is preferably 10 ⁷ to 10 ¹¹ Ω · cm.
Further, according to the present invention, the displacement of the contact position in the thickness direction between the second electrode and the transfer object is swingable, and the contact between the second electrode and the transfer object in the transport direction is possible. It has the electrode support body which controls the shift | offset | difference of a position, It is characterized by the above-mentioned.
The present invention is the image forming apparatus characterized in that a spur is installed downstream of the second electrode.
The present invention is the image forming apparatus, wherein the second electrode is a blade type.
In the present invention, the blade-type second electrode includes a torsion spring.
The present invention is an image forming apparatus in which the intermediate transfer member and a transfer target are brought into close contact with an entrance contact blade, and the same bias power source as that of the second electrode is connected to the entrance contact blade.
The present invention is an image forming apparatus characterized in that a repulsive transfer bias is generated by stepping down from a bias power source between the second electrode and the second electrode facing roller, and a limiting resistor Rx is provided.
The present invention is characterized in that Rx> Rt when the load resistance value Rt obtained from the VI characteristic of the first electrode is used.

  According to the present invention, by the means for solving the above problem, the residual charge on the transfer medium is surely removed, and the Q / M of the toner image on the transfer medium fluctuates due to the discharge caused at the time of peeling, thereby deteriorating the image quality such as transfer dust. It has become possible to provide an image forming apparatus that prevents the occurrence of the above.

  The best mode for carrying out the present invention will be described below with reference to the drawings. Note that it is easy for a person skilled in the art to make other embodiments by changing or correcting the present invention within the scope of the claims, and these changes and modifications are included in the scope of the claims. The following description is an example of the best mode of the present invention, and does not limit the scope of the claims.

Embodiments of the present invention will be described below in detail with reference to the drawings.
FIG. 1 is a schematic diagram illustrating an example of an image forming apparatus. An image forming apparatus 150 shown here includes image carriers 40Y, 40C, 40M, and 40BK each including four drum-shaped photoreceptors 40. A yellow toner image, a cyan toner image, a magenta toner image, and a black toner image are formed on the peripheral surface of each image carrier 40, respectively. An intermediate transfer belt 4 composed of an endless belt is provided facing the image carriers 40Y to 40BK. The intermediate transfer belt 4 is wound around the support rollers 14, 15 and 16 and is driven to travel in the direction of the arrow while contacting the surface of the image carriers 40Y to 40BK, and the toner on the image carriers 40Y to 40BK. The image is primarily transferred while being superimposed on the intermediate transfer belt 4.
Since the configuration for forming the toner image on each of the image carriers 40Y to 40BK and the configuration for transferring the toner image to the intermediate transfer belt 4 are all the same, the toner image is formed on the image carrier 40Y and this is transferred to the intermediate transfer belt 4 Only the structure for transferring to the belt 4 will be described. The image carrier 40Y is driven to rotate counterclockwise in FIG. 1, and at this time, the image carrier 40Y is charged to a predetermined polarity by the charging roller 60. Here, it is assumed that the charging polarity is negative. Next, the charged surface of the image carrier 40Y is irradiated with light-modulated writing light L (laser light in the example in the figure) emitted from the exposure device 21, thereby forming an electrostatic latent image on the image carrier. The electrostatic latent image is visualized as a yellow toner image by the reversal developing type developing device 20. The developing device 20 shown in the figure has a developing roller to which a developing bias is applied, and the electrostatic latent image is visualized as a toner image by a dry developer carried on the developing roller. As the dry developer, a two-component developer having a toner and a carrier or a one-component developer not having a carrier is used. In any case, the toner has a normal charging polarity (negative polarity in the example in the figure). The toner is electrostatically transferred to the electrostatic latent image formed on the image carrier 40Y, and the electrostatic latent image is visualized.

Next, components for carrying out the present invention will be described with reference to FIGS.
Here, P indicates paper as a transfer target, and T indicates a toner image. Reference numeral 4 denotes an intermediate transfer member (intermediate transfer belt) in which carbon black such as a polyimide single layer belt is dispersed and has a thickness of 60 to 150 μm. As the volume resistance, one having a characteristic that the resistance decreases as the applied voltage increases (voltage dependency of resistance) is used. Here, a resistor whose resistance decreases by 4 digits or more when a voltage of 10 times is applied in the range of 25 to 1000 V is used. A toner image of 0 to 1 mg / cm ² and −15 to −35 μC / mg is formed on the intermediate transfer member 4.
Reference numeral 100 denotes a transfer electrode, which is the first electrode of the present invention. Specifically, a transfer bias application roller is used. In the present invention, a small diameter (eg, Φ8) or a normal diameter (eg, Φ16) is used. In general, a material having a volume resistance of 10 ⁶ to 10 ¹² Ω · cm, preferably 10 ⁷ to 10 ¹⁰ is coated. Examples of the material include low-hardness carbon-dispersed foamed EPDM (ethylene / propylene rubber) / foamed urethane and conductive ion (for example, Li) -dispersed foamed urethane. In the case of a small-diameter roller, it is difficult to reduce the hardness because the thickness is small, and a carbon-dispersed rubber (eg, hydrin, NBR, etc.) or a conductive ion-dispersed urethane layer is attached in terms of molding dimensional tolerance.
Reference numeral 101 denotes a second electrode of the present invention, which is a static elimination electrode. It is a member that removes residual charges by coming into contact with an object to be charged, and is made of a small diameter roller (Φ8) or a blade (for example, phosphor bronze plate 0.05 to 0.2 mm). The medium resistance region material is coated on the surface.
As a result, since air breakdown discharge can be used, charge supply unevenness due to contact unevenness is improved. In the primary transfer SEF method, the electric field dependency of the resistance in the thickness direction of the intermediate transfer body 4 is indispensable, but the limitation is eased in the present invention. This is because the paper P has a thickness direction resistance of 10 ¹⁰ Ω / cm or more. However, the more the electric field dependency of the intermediate transfer body 4 is, the easier it is to remove the sheet.
Further, the transfer power source 102 and the neutralizing bias power source 103 which are bias power sources are programmable and are mainly constant current power sources. These are controlled by an arithmetic unit (not shown). The opposing roller 104 serves as an internal shaft.

  FIG. 2 is a conventional example of a secondary transfer configuration. The paper is conveyed from left to right. First, after a transfer electric field is applied by the first electrode 100 at the nip, the paper P passes over the static elimination needle 110. The static elimination needle 110 is discharged after the paper P is peeled off. Here, AC discharge simultaneously neutralizes the back and front surface charges of the paper, but also changes the toner Q / M. For this reason, the toner on the paper P is likely to fluctuate and is shifted by a slight electric field, thereby generating dust. However, when there is no AC discharge, discharge occurs at the post nip portion (paper P and intermediate transfer belt 4). As a result, a peeling discharge charge is given to the toner image, and the toner Q / M becomes more (-). For this reason, the (−) reinforced toner is easily repelled toward the paper P, and the dust is deteriorated. The appropriate AC voltage is carefully set from the trade-off between these, however, however, the paper P and the like are also uneven from the beginning, and the unevenness due to the static elimination needle 110 is inevitable.

  FIG. 3 is a view showing a small-diameter two-roller type secondary transfer device. Here, the two rollers are an upstream transfer electrode roller that is the first electrode 100 and a downstream neutralization electrode roller that is the second electrode 101. The positions of these two rollers are fixed by a sliding member 120 and a support body 121. The support 121 is temporarily suspended by a rough positioning mechanism 125 with respect to the opposing roller 104, and is pressed against the opposing roller 104 by the leaf spring 122 and the cam operation arm 123, so that the intermediate transfer body 4, the paper P, and the toner image T are pressed. Adhere. Here, the cam operating arm 123 can be brought into and out of contact with the cam 124 and the spring 126, but the description thereof is omitted.

The paper P is wound around the nip N, and among these, a transfer electric field is applied with an interval of Dt from the start of winding. The second electrode 101 is a neutralizing electrode is at a distance of D ₀ from end winding, are arranged. Here, D ₀ is a distance necessary for discharging a material to be transferred such as paper and separating it, and is a distance upstream from the separation point of the nip portion.
The first electrode 100 and the second electrode 101 are deflected in the pressure direction. This can be dealt with by increasing the roller center diameter, but the shaft deforms when left for a long time and becomes pulsating the next time it is turned, which causes undesirable bias unevenness and poor contact, which is undesirable. . Further, as another direction, a deflection in the conveyance direction occurs. This may cause a stick-slip phenomenon between the frictional state and the state where the friction is lightened due to the curvature of the facing roller.
Therefore, in order to suppress these two problems, the present invention takes a method of restricting the movement of the entire circumference of the roller with the sliding member 120 which is a member having a particularly improved slidability.
Here, in the proposal of making each roller a foam type, it is difficult to form a sponge layer having an appropriate thickness because the roller is small in diameter, and a roller called a semi-hard roller is used in the present invention. In order to keep the pressure low with this roller, it is necessary to delicately manage the pressing force of the support. In the present invention, a positioning member 125, a cam operation arm 123, and the like are placed for that purpose. In addition, a leaf spring / lap leaf spring 122 that is easier to design the rigidity range than the spring is used.

FIG. 4 shows the electric potential of the image surface of the paper and the image surface of the intermediate transfer belt in the machine of FIG. Although accurate measurement is impossible, analogy is made by, for example, measuring the potential in contact with the toner Q / M before and after transfer and the internal electrodes.
What is important is that when a transfer electric field is received inside the nip, both the paper potential and the intermediate transfer belt potential rise to the (+) side, and there is no high potential at the exit peeling portion during static elimination. The absence of large potential fluctuations due to this stripping discharge results in no discharge.

The second electrode of the present invention, the downstream side nip distance D ₀ within the upstream side of the downstream end of the first electrode, applying an electric field of the same polarity as the toner directly.
Here, although the possibility of noise in the measurement system remains, the residual charge does not disappear immediately after the charge removal, and a charge removal time is required. This time is related to the static elimination time constant of the system as will be described later, but if it is at least 10 ¹² Ω or more and about 100 pF, the time constant is on the order of 1 second. For this reason, even if a static elimination electrode is placed, time is required for the effect to work sufficiently. In view of this delay, it is necessary to shift the neutralizing electrode on the upstream side only D ₀ of the separation starting point.
In the present invention, since the second electrode is disposed on the upstream side of the distance D ₀ from the paper peeling point, the residual charge on the paper is surely removed, and the Q / M of the toner image on the paper is discharged by the discharge caused at the time of peeling. To prevent image quality deterioration such as transfer dust from occurring.

Further, this delay time is determined in association with the static elimination time constant of the belt material / paper material. For example, the capacity of the paper / belt is about several tens of pF and is in the order of 10 pF because it is serially connected. The resistance of the belt material + paper is usually 10 ¹⁰ Ω or more. When these are integrated, a time constant appears, but it becomes 10 mS or more. As a result, if the transport speed is 100 mm / s, D ₀ needs to be 1 mm or more. Of course, higher speeds require longer intervals.

  FIG. 5 is a diagram showing the Chile rank and the Q / M on paper with respect to the bias applied to the static elimination electrode. As in the present invention, when the static elimination electrode was placed in the nip and a (−) bias was applied, the dust rank was improved after the static elimination bias was −400 V as shown in FIG. At this time, the Q / M on the paper P is lowered, and the high Q / M on the intermediate transfer belt 4 is prevented.

FIG. 6 shows an embodiment in which a repulsive force transfer method is adopted, and the cost can be reduced. The repulsive transfer method is a method in which a (+) bias is not applied to the first electrode 100 but is grounded, and (−) is applied to the opposing roller 104 side. In this method, current leakage to the conveyance path can be avoided.
In addition, since the (−) bias is applied to the second electrode 101 and the counter roller 104, respectively, a single polarity output power source can be used, which is economical. A Zener or a varistor can be used for the step-down. Thereby, it is possible to prevent application of an excessive charge elimination bias.

In the present invention, it is necessary to increase the former of the transfer electric field and the electric discharge electric field as absolute values, and it is necessary to reduce the amount of electric current without fail. Here, the limiting resistor Rx is placed. The limiting resistor Rx determines the charge removal and the transfer current distribution, and is determined so that Rx> Rt with respect to Rt (transfer system impedance) obtained from the IV characteristics of the transfer current.
The limiting resistance Rx varies depending on the actual transfer conditions and static elimination conditions. For example, when the resistance between the belts is low, a part of the current leaks immediately in the present invention, but this can be assumed in advance. In this case, the leakage current is subtracted to determine and distribute the total current. Here, 50 M to 100 MΩ is used as an example. In addition, this can be made more accurate by making it programmable by some means.
In FIG. 6, the current is indicated by a broken line. However, even when a large current is generated by direct leakage between the first electrode 100 and the second electrode 101, the bias is easily reduced by Rx.

FIG. 7 is a diagram showing a secondary transfer method using a normal diameter roller and a downstream static elimination blade. In this method, a normal diameter roller is used as the first electrode 100, and a downstream static elimination blade is used as the second electrode 101, and a medium resistance material is coated on the tip of the static elimination blade 101. This material preferably has good wear resistance. These electrodes are the same as the two-roller type of the present invention. In order to prevent the paper P from intruding, an inclination is given in FIG.
As a result, it is possible to prevent the occurrence of deflection caused by an increase in rotational load, which occurs with the roller type, and to prevent contact unevenness. In addition, in the roller type, rotation is stopped due to the magnitude of frictional force in a method such as follow rotation, and when the other party is a cylindrical roller, it may return, causing a problem in applying self-bias, The present invention can prevent these.
Further, the static elimination blade 101 of the present invention can use a torsion spring. As a result, the size can be reduced as compared with a spring or the like, and the pressing can be performed with a weak and stable pressure, which stabilizes the pressing force with time and improves the wear characteristics.

FIG. 8 is a diagram showing the appearance of the apparatus of FIG. In the present invention, the first electrode 100 (transfer roller), the insulating material 130, and the second electrode 101 are fastened by one member, and the paper P can be swung in the thickness direction, but the position in the transport direction is It is fixed. The arm is the contact / separation mechanism of the first electrode 100 itself. At the time of separation, the second electrode 101 is also separated. Further, here, a spur 133 is provided for fixing the peeling point so as not to peel off before the static elimination is completed.
Here, it is necessary to sufficiently weaken the pressing of the second electrode 101. Specifically, the wire 131 is hooked and the torsional stress of the wire 131 is used to realize stable low pressing. Economical with simple structure.
Note that the insulation distance is also important because a reverse bias is applied close to the transfer. Here, an insulating sheet 130 made of PET material is used. This can prevent air leaks. Further, there may be a leak through the paper P, but here, the leak prevention by the above-mentioned Rx can be performed.
By adopting the above configuration, it is possible to press with a small pressing force, and stable contact is possible. Furthermore, it is easy to cope with positional deviations such as torsion of the entire secondary transfer unit, and there is an advantage that pressure deviation is less likely to occur. Further, since the movement of the static elimination electrode is restricted with respect to the force in the conveyance direction, the static elimination can be performed by fixing the position of the static elimination electrode in the conveyance direction.
As a result, insect biting is improved and paper thickness compatibility is also increased.

FIG. 9 is an embodiment having an upstream blade 134 and a spur 133. A bias of the second electrode 101 is applied to the upstream blade 134 and the spur 133. The electrode for this acts as a counter bias on the entrance side, and the image quality is improved. Moreover, since the electric potential is the same when the spur 133 touches the paper P, the image of the contact portion is not scattered.
By the spur 133, the peeling position is extended, so that static elimination is completed before peeling, and the peeling position can be prevented from becoming unstable.
FIG. 10 shows an example in which the conveyor belt 132 is suspended from the first electrode 100 to the spur 133 position. The conveyance belt 132 is formed of a medium resistance member such as PI (polyimide), and is generally called a transfer conveyance belt.

1 is a schematic diagram illustrating an example of an image forming apparatus. This is a conventional example of a secondary transfer configuration. It is a figure which shows a small diameter 2 roller type secondary transfer apparatus. FIG. 6 is a diagram illustrating a potential of a paper image surface and an image surface of an intermediate transfer belt. It is the figure which showed Q / M on the rank and paper on the applied bias of a static elimination electrode. The Example which employ | adopted the repulsive transfer system is shown. It is a figure which shows the secondary transfer system using a normal diameter roller and a downstream static elimination blade. It is a figure which shows the external appearance of the apparatus of FIG. This is an embodiment having an upstream blade 134 and a spur 133. The example which suspended the conveyance belt 132 from the 1st electrode 100 to the spur 133 position is shown.

Explanation of symbols

4 Intermediate transfer belt 14 Support roller 15 Support roller 16 Support roller 20 Developing device 21 Exposure device 40 Photoconductor (image carrier)
60 Charging roller 100 First electrode (transfer electrode)
101 Second electrode (static charge electrode)
DESCRIPTION OF SYMBOLS 102 Transfer power supply 103 Static elimination bias power supply 104 Opposite roller 110 Static elimination needle 120 Sliding member 121 Support body 122 Leaf spring 123 Cam action arm 124 Cam 125 Positioning mechanism 126 Spring 130 Insulation material 131 Wire 132 Conveying belt 133 Spur 134 Upstream blade 150 Image forming apparatus P paper

Claims (10)

A belt-like intermediate transfer member carrying a toner image;
A roller constituting a nip portion with the intermediate transfer member;
Have
The intermediate transfer member and the transfer target are brought into close contact with each other at the nip portion,
In the image forming apparatus for transferring the toner image to the transfer target with a bias,
The image forming apparatus includes a first electrode that directly applies an electric field having a polarity opposite to that of the toner to the back side of the transfer object when closely contacting in the nip portion.
And wherein a nip distance D ₀ within the upstream side of the downstream end, the image forming apparatus characterized by having a second electrode applying an electric field of the same polarity as the toner directly downstream of the first electrode. The image forming apparatus according to claim 1.
In the image forming apparatus, the first electrode is in direct contact with the back side of the transferred body,
The image forming apparatus, wherein the second electrode is in direct contact with the back side of the transfer target. The image forming apparatus according to claim 2.
An image forming apparatus, wherein a contact portion between the second electrode and the transfer target is covered with a material having a volume resistance of 10 ⁶ to 10 ¹² Ω · cm. The image forming apparatus according to claim 2 or 3,
The displacement of the contact position in the thickness direction of the second electrode and the transfer object is swingable,
An image forming apparatus comprising: an electrode support that regulates displacement of a contact position of the second electrode and the transfer target in the transport direction. The image forming apparatus according to claim 1,
The image forming apparatus is characterized in that a spur is installed downstream of the second electrode. The image forming apparatus according to any one of claims 1 to 5,
The image forming apparatus, wherein the second electrode is of a blade type. The image forming apparatus according to claim 6.
The blade-type second electrode includes a torsion spring. The image forming apparatus according to any one of claims 1 to 7,
The image forming apparatus causes the intermediate transfer member and the transfer target to be in close contact with an entrance contact blade,
An image forming apparatus, wherein the same bias power source as that of the second electrode is connected to the inlet contact blade. The image forming apparatus according to claim 1,
A repulsive transfer bias is generated by stepping down from a bias power source between the second electrode and the second electrode facing roller,
An image forming apparatus having a limiting resistor Rx. The image forming apparatus according to claim 9.
When the load resistance value Rt is obtained from the VI characteristic of the first electrode,
Rx> Rt
An image forming apparatus.

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