JP2005099645A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus more surely preventing transfer memory from occurring on the surface of an image carrier and the back staining of a body to be transferred from occurring than ever. <P>SOLUTION: In the image forming apparatus, a control means C is provided with a transfer bias control part controlling transfer bias to be applied to a transfer roller 5 by a transfer bias applying means BS in constant-voltage control areas having fixed width before and behind the trailing edge P<SB>3</SB>of the body to be transferred when the trailing edge P<SB>3</SB>passes between the image carrier 1 and the roller 5 so as to turn it off at the point of time when the control area is finished or apply reverse bias thereto after operation to perform constant-voltage control at voltage value V<SB>2</SB>whose absolute value is lower than that at the normal transfer time is performed by at least one stage. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using a transfer roller as transfer means for transferring a toner image formed on an image carrier onto a transfer medium such as paper.

  In an electrostatic copying machine, a plain paper facsimile machine, a laser printer, and a complex machine using electrophotography, first, the surface of the image carrier is uniformly charged to a predetermined charging potential by a charging unit. Next, after exposing by an exposure means to form an electrostatic latent image, the electrostatic latent image is developed into a toner image by a developing means. Next, the toner image is transferred onto a transfer medium such as paper by a transfer bias having a polarity opposite to the charged polarity of the surface of the image carrier applied to the transfer unit, and then fixed on the transfer medium by a fixing unit. Thus, a series of image forming steps is completed.

  Among the above, the transfer means includes a transfer charger, a transfer roller, etc., but compared to the case where a transfer charger is used, the generation of ozone in the image forming apparatus is suppressed, and the generation of discharge products by the ozone and the surface of the photoconductor Since the formation of an oxide film on the surface can be suppressed, a transfer roller is preferably used. As the transfer roller, it is common to use a transfer roller having at least a surface having conductivity and connected to a predetermined potential.

Depending on the variation of the resistance value of the transfer roller, the increase in resistance value when the transfer roller is repeatedly used, the type (material or thickness) of the transfer object, the amount of charge applied to the transfer object is partially or In order to prevent the transfer characteristics from fluctuating as a whole, the transfer bias applied to the transfer roller is generally controlled at a constant current (see Patent Document 1).
Further, as described in Patent Document 1, the transfer bias is applied to the transfer roller only when the transfer target exists between the transfer roller and the image carrier, and the transfer target is transferred. The transfer bias must be turned off immediately before the rear edge of the body passes between the two, or switching must be performed so that a bias (reverse bias) having a polarity opposite to the transfer bias is applied.

  This is because when a transfer roller to which a transfer bias is applied is in direct contact with the image carrier, a transfer memory is generated on the surface of the image carrier. That is, the image carrier after the toner image transfer is prepared for the next image formation by removing the toner remaining on the surface by a cleaning means and then removing the charge by irradiating light using a charge eliminating lamp. If the area of the surface of the image carrier that is in direct contact with the transfer roller to which the transfer bias is applied is charged with a reverse polarity, the reverse polarity potential is removed by light irradiation from the discharge lamp. In that case, when the charging means is charged to perform the next image formation, the surface potential in the above range is locally larger than the predetermined value by subtracting the charging potential of the opposite polarity. This results in a lower transfer memory. In addition, the image density of the formed image in the range where the transfer memory is generated increases, particularly in a halftone image, or an image defect occurs in which the fog density other than the portion where the toner image is formed increases. .

On the other hand, as described above, the transfer bias applied to the transfer roller is turned off immediately before the trailing edge of the transfer object passes between the transfer roller and the image carrier, or the image carrier is charged. When switching is performed so that a reverse bias having the same polarity as that of the polarity is applied, the above-described transfer memory can be prevented from being generated.
Further, when the reverse bias is applied as described above in a state where the transfer roller and the image carrier are in direct contact, the toner attached to the surface of the transfer roller can be returned to the image carrier. Therefore, for example, even if image formation is repeatedly performed, even if the surface of the transfer roller is soiled by fog toner or the like transferred from the surface of the image carrier, it can be cleaned.
JP-A-8-160776 (Claim 1, columns 0026 to 0027)

  However, if the surface of the transfer roller is contaminated with fog toner as described above when switching is performed with the transfer target being sandwiched between the transfer roller and the image carrier, this occurs at the time of switching. Due to the sudden change in the electric field, the toner adhering to the surface of the transfer roller is transferred to the back surface of the transfer medium, causing a back-stain phenomenon. The backside dirt becomes a problem particularly when an image is formed on both sides of the transfer target.

In addition, if the above switching is performed with the rear edge of the transfer target being inclined, the back of the transfer target is stained in the range where the rear edge has not yet passed at the time of switching. In the range after the trailing edge has already passed at this point, there is a problem that a transfer memory is formed on the surface of the image carrier that is exposed and is in direct contact with the transfer roller.
Therefore, the inventor studied to gradually reduce the transfer bias by using a charge / discharge circuit or the like in order to suppress an abrupt change in the electric field in order to prevent the back surface of the transfer material from being soiled. In this case, if the position of the trailing edge of the transfer object varies and the transfer roller and the image carrier are in direct contact when the voltage value of the transfer bias has not decreased so much, the image carrier There is a problem that a transfer memory is generated on the surface of the substrate.

  The inventor has also studied stepwise reducing the constant current value of the transfer bias applied to the transfer roller in a region of a constant width in the front and rear of the trailing edge of the transfer object. However, in constant current control, for example, due to a partial variation in resistance value on the surface of the transfer roller, the current concentrates on a portion of the transfer roller where the resistance value is low, and a large amount of current is contacted with the surface of the image carrier in contact with this portion. Current (charge) flows in. This electric charge is held on the surface of the image carrier, and a transfer memory is generated on the surface of the image carrier.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a novel image forming apparatus that can more reliably prevent the generation of a transfer memory on the surface of an image carrier and the occurrence of stains on the back of a transfer target. There is to do.

  According to a first aspect of the present invention, an image carrier and a transfer roller for transferring the toner image formed on the image carrier to the member to be transferred by bringing the image carrier into contact with the image carrier while being conveyed. And a transfer bias applying means for applying a transfer bias for transferring the toner image to the transfer roller, and a control means for controlling the operation of each part. When the trailing edge passes between the image carrier and the transfer roller, the transfer bias applied to the transfer roller by the transfer bias applying means in a region having a constant width before and after the trailing edge is compared with that during normal transfer. It includes a transfer bias control unit that performs a constant voltage control operation with a voltage value having a low absolute value at least one stage and then turns off when the above region ends or applies a reverse bias. The image forming apparatus according to claim.

According to a second aspect of the present invention, the transfer bias control unit performs constant voltage control by setting the transfer bias in the region to a voltage value lower than a discharge start voltage between the transfer roller and the image carrier. The image forming apparatus according to 1.
According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the transfer bias control unit changes a voltage value for constant voltage control according to environmental conditions.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the first aspect, the transfer bias controller controls the transfer bias at a constant current during normal transfer.

According to the first aspect of the present invention, an operation is performed in which the transfer bias applied to the transfer roller is controlled at a constant voltage with a voltage value whose absolute value is lower than that at the time of normal transfer in a constant width region at the rear edge of the transfer object. At least one stage has been implemented.
For this reason, in the above region (hereinafter referred to as “constant voltage control region”), the transfer bias applied to the transfer roller is turned off or the charging polarity of the image carrier is provided in front of the rear edge of the transfer target. A sudden change in the electric field as in the case of switching so as to apply a reverse bias having the same polarity as can be suppressed. Therefore, even if the surface of the transfer roller is soiled with fog toner or the like, it is possible to prevent the back surface of the transfer body from being soiled.

In the constant voltage control, for example, even if there is a partial variation in resistance value on the surface of the transfer roller, the voltage value does not increase locally unlike the constant current control. Further, it is possible to prevent the transfer memory from being generated on the surface of the image carrier on the rear side of the rear end edge of the transfer object.
Therefore, according to the first aspect of the present invention, it is possible to more reliably prevent the generation of the transfer memory on the surface of the image bearing member or the occurrence of the back contamination of the transferred member. .

  According to the second aspect of the present invention, the transfer bias of the constant voltage control region is set between the two when the transfer roller and the image carrier are in direct contact with each other on the rear side of the rear end edge of the transfer target in the region. Since the discharge voltage is set to a voltage value lower than the threshold value at which discharge is started, the surface of the image carrier is not charged to a reverse polarity by discharge. Therefore, it is possible to more reliably prevent the transfer memory from being generated on the surface of the image carrier.

  According to the third aspect of the present invention, the voltage value of the transfer bias in the constant voltage control region is changed according to the threshold value that changes depending on the environmental conditions. Specifically, the voltage value of the transfer bias is lowered in a high humidity environment where the threshold value is lowered and discharge is likely to occur. For this reason, it is possible to more reliably prevent the surface of the image carrier from being charged to a reverse polarity due to discharge.

  Further, in the configuration of the fourth aspect, since the transfer bias at the time of normal transfer is controlled at a constant current, the resistance value of the transfer roller varies as described above, and the resistance value increases when the transfer roller is used repeatedly. Depending on the type of transfer object, the transfer amount can be prevented from changing due to partial or overall change in the amount of charge applied to the transfer object, and a good image can always be formed.

FIG. 1 is a view showing the periphery of an electrophotographic photosensitive member 1 as a main part in an example of an embodiment of an image forming apparatus of the present invention.
As shown in the figure, the image forming apparatus of this example is arranged around the drum-shaped electrophotographic photosensitive member 1 as an image carrier in order along the rotation direction indicated by the solid line arrow in the drawing. Charging means 2 for uniformly charging the surface of the body 1, exposure means 3 for exposing the surface of the charged electrophotographic photosensitive member 1 to form an electrostatic latent image, and the formed electrostatic latent image Developing means 4 for developing the toner image, a transfer roller 5 for transferring the developed toner image to the transfer member, and a toner for removing the toner remaining on the surface of the electrophotographic photosensitive member 1 after the transfer. A cleaning means 6 and a neutralizing lamp 7 for exposing and neutralizing the surface of the electrophotographic photosensitive member 1 after cleaning are disposed.

  Among these, as the charging means 2, a charger (charger) such as a corotron or a scorotron can be used as in the prior art. The example shown in the figure is a scorotron charger, which surrounds the discharge wire 21 stretched along the axial direction of the electrophotographic photosensitive member 1 and three sides thereof excluding the surface facing the electrophotographic photosensitive member 1. A shield 22 and a grid 23 provided in the opening of the shield 22 are provided.

As the exposure unit 3, for example, when the image forming apparatus is a laser printer, a digital electrostatic copying machine, a plain paper facsimile machine, or the like, the surface of the electrophotographic photosensitive member 1 is charged electrostatically based on input electronic data. A semiconductor laser array or the like that can form a latent image can be used.
As the developing means 4, various types of developing devices corresponding to various developing methods for developing the electrostatic latent image formed on the surface of the electrophotographic photosensitive member 1 into a toner image can be used. For example, in the figure, a developing roller 41 made of an elastic material such as a sponge disposed in contact with the surface of the electrophotographic photosensitive member 1, and a housing 42 for containing toner to be supplied to the developing roller 41, The developing means 4 provided with is used. However, depending on the type of developing device to be used, a developing sleeve arranged at a certain distance from the surface of the electrophotographic photosensitive member 1 may be used instead of the developing roller 41.

At least the surface of the transfer roller 5 has conductivity and is connected to a predetermined potential. This suppresses the generation of ozone in the image forming apparatus and suppresses the generation of discharge products due to the ozone and the formation of an oxide film on the surface of the photoreceptor as compared with the case where a transfer charger is used as described above. can do.
Further, the cleaning means 6 includes a cleaning blade 61 for scraping off toner remaining in contact with the surface of the electrophotographic photosensitive member 1 and a casing 62 for storing the scraped toner. Can be used.

In the next step, when a toner image is fixed on the surface of the transfer medium through a fixing means (not shown), a series of image formation is completed.
FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG.
As shown in the figure, the image forming apparatus includes a control unit C that operates each part of the apparatus.

The input to the control panel CP as the operation unit of the image forming apparatus and the output of the humidity sensor HS are given to the control means C. In the control means C, there are provided a timer C1 for defining the timing of various operations, and a memory C2 in which initial values and the like serving as a reference for various operations are registered.
The control means C performs various calculations based on the input to the control panel CP, the output of the humidity sensor HS, the timing defined by the timer C1, the initial value recorded in the memory C2, and the like, and the control signal is sent to the driver Give to D. Then, the driver D rotates the electrophotographic photosensitive member 1 on the basis of a given signal, and applies to the charging unit 2, the exposing unit 3, the developing unit 4, the transfer roller 5, the cleaning unit 6, the charge eliminating lamp 7, and the transfer roller 5 Drive control of the transfer bias applying means BS for applying the transfer bias is performed.

FIG. 3 is a graph showing the timing of changing the transfer bias applied to the transfer roller 5 by controlling the drive of the transfer bias applying means BS by the control means C.
First, the control unit C based on the information such as the size and orientation of the transfer member is set by an input from the control panel CP, and the rear edge P ₃ of the transfer member, the rear end of the material to be transferred of determining the boundary line P ₁ of the full image area and an incomplete image range (margin). Then, with reference to both positions P ₁ and P ₃ , an area having a constant width before and after the rear edge P ₃ is set as a constant voltage control area.

That start position P ₂ of the constant voltage control region, in order not to affect the image formed in the complete image formation range, behind the boundary line P _1, yet at the time of oblique conveyance within allowed standards on the transfer member , set as much as possible ahead of the position do not intersect the inclined boundary line P _1. The end position P ₄ of the constant voltage control region is set to a position that does not intersect the rear edge P ₃ of the transfer target body which is inclined at said oblique conveyor. In this example, the control unit C is set backward from the end position P ₄ of the constant voltage control region, a constant width until the reverse bias start position P _5, the range to be the OFF state a transfer bias.

Next, the control unit C sets the voltage value V ₂ to the constant voltage control in the constant voltage control region. The voltage value V ₂ may be lower than the range of the voltage value V ₁ that is constant-current controlled during normal transfer in the complete image forming region as described above, but the generation of the transfer memory can be prevented more reliably. considering that, when the transfer roller 5 and the electrophotographic photosensitive member 1 at the rear side of the rear end P ₃ of the transfer member is in direct contact with, than threshold discharge is started between them It is preferable to set a low voltage value.

However, when the transfer roller 5 and the electrophotographic photosensitive member 1 are in direct contact with each other, they are equal to or greater than a threshold value at which discharge is started between the two, and the surface of the electrophotographic photosensitive member 1 is charged to a reverse polarity by such discharge Even if it is possible to eliminate static electricity with the static elimination lamp 7 or the like, there is no possibility of generating a transfer memory.
The above threshold environmental conditions, particularly varies with the humidity, the control unit C, at the time of image formation, from time to time based on the output of the humidity sensor HS, the fine adjustment so that the voltage value V ₂ does not exceed the threshold It is preferable to carry out.

At the same time, the control means C also sets a reverse bias voltage value V ₃ applied to the transfer roller 5 in a range behind the reverse bias start position P ₅ . The voltage value V ₃ can return the toner adhering to the surface of the transfer roller 5 to the surface of the electrophotographic photosensitive member 1 in a state where the transfer roller 5 and the electrophotographic photosensitive member 1 are in direct contact as described above. The minimum voltage value should be exceeded.
In the state where the above settings are made, the control means C determines that the positions P _{1 to} P ₅ are the center of the transfer nip between the transfer roller 5 and the electrophotographic photosensitive member 1 in the transport direction of the transfer target. The transfer bias is controlled while obtaining the timing of passing through the time after the registration roller (not shown) starts rotating.

For example, the time T ₂ from when the registration roller starts rotating until the start position P ₂ of the constant voltage control region reaches the center of the transfer nip is equal to the distance A between the nip of the registration roller and the center of the transfer nip, and from the tip of the transfer member to the start position P _2, a length B of the conveying direction of the material to be transferred from the conveying speed R of the material to be transferred, wherein:
T ₂ = (A + B) / R
Can be obtained. The same applies to other positions.

First, the controller C applies a toner image formed on the electrophotographic photosensitive member 1 by a predetermined procedure while applying a constant current controlled transfer bias (voltage value V ₁ ) to the transfer roller 5. Are passed between the transfer roller 5 and the electrophotographic photosensitive member 1. As a result, the toner image can be satisfactorily transferred onto the surface of the transfer target as described above.
Next, the control means C, when the start position P ₂ has reached the constant voltage control region to the center of the transfer nip as, by switching the transfer bias applied to the transfer roller 5, the voltage value becomes V ₂ the constant voltage controlled transfer bias is applied to, and the end position P ₄ of the constant voltage control region to the center of the transfer nip at the time of reaching to turn off the transfer bias applied to the transfer roller 5.

Thus the constant voltage control region, the rear end P ₃ from the front side range of the transferred object (ranging from the start position P ₂ to the rear edge P _3), the backside contamination of the transfer object is prevented The The constant voltage control region, a range from the rear end edge P ₃ in the rear side (the range from the rear end edge P ₃ to the end position P _4), subsequent to the reverse bias start position P ₅ from the end position P ₄ In the range between, transfer memory is prevented from being generated on the surface of the electrophotographic photosensitive member 1 in direct contact with the transfer roller 5.

Note When the end position P ₄ reaches the constant voltage control region to the center of the transfer nip, may be a reverse bias is applied to the transfer roller 5, may ripple caused by abrupt voltage change in that case Since a transfer memory is generated when a ripple occurs, it is preferable to turn off the transfer bias as described above at this point in order to more reliably prevent the transfer memory from being generated.

In the illustrated example, a constant voltage control in the constant voltage control region, but the voltage value is performed only one stage is V _2, the constant voltage control of two or more stages which have different voltage values, the voltage values successively It can also be implemented step by step so as to approach zero. For this purpose, for example, the voltage value may be controlled by a limiter voltage.
Further, the control means C, when the center of the transfer nip reverse bias start position P ₅ arrives, the transfer roller 5, to apply a reverse bias voltage value is set to V _3. As a result, the toner adhering to the surface of the transfer roller 5 can be returned to the electrophotographic photoreceptor 1 to clean the transfer roller. The reverse bias may be constant voltage control or constant current control.

As described above, by performing a series of operations, the transfer memory is generated on the surface of the electrophotographic photosensitive member 1 and the backside contamination of the transfer object is more reliably prevented while preventing the transfer. The toner image can be satisfactorily transferred onto the surface of the transfer body.
Note that the configuration of the present invention is not limited to the example illustrated in the above description. For example, the example shown in the figure shows an image forming apparatus in which the image carrier is an electrophotographic photosensitive member, but the image carrier may be an intermediate in a full-color image forming apparatus, for example. In addition, various design changes can be made without departing from the scope of the present invention.

Hereinafter, the present invention will be described in more detail based on examples and comparative examples.
Prototype of Image Forming Apparatus A laser printer (model number FS-1020D) manufactured by Kyocera Mita Co., Ltd. having the structure shown in FIG. 1 is modified so that the transfer bias can be adjusted stepwise as shown in FIG. A prototype image forming apparatus was made. The specifications of the image forming apparatus are as follows.

Electrophotographic photoreceptor 1
Type: Positively charged single-layer organic photoreceptor Diameter: 30 mm
Photosensitive layer thickness: 30 μm
Rotational linear velocity: 120mm / sec Charging potential: + 400V
Transfer roller 5
Structure: A metal shaft penetrates and is integrated into the center of a roller body made of conductive foamed EPDM (ethylene-propylene-diene copolymer rubber).

Roller diameter: 16mm
Shaft diameter: 6mm
Volume resistivity of roller body: 10 ⁸ to 10 ¹⁰ Ω · cm
Then, constant current control (voltage value V ₁ = −1.5˜) is applied to the transfer roller 5 and the transfer bias at the time of toner image transfer before the start position P ₂ of the constant voltage control region in FIG. -2.5 kV), constant voltage control with a constant voltage value in the range of voltage value V ₂ = −0.5 to −1.0 kV for the transfer bias in the constant voltage control region from the start position P ₂ to the end position P _4. , by setting the reverse bias of the reverse bias start position P ₅ since the constant voltage control of the voltage value V _{3 =} + 550 V, after 50,000 sheets of continuous image formation was observed both the front and back surfaces of the end of the transfer member.

As a result, there was no increase in image density or fog due to the generation of the transfer memory in the formed image on the front side of the transfer object, and no backside contamination was seen on the back side.
For comparison, between the start position P ₂ of the constant voltage control region in FIG. 3 to the end position P _4, turn off the transfer bias applied to the transfer roller, without providing the constant voltage control region, the same number After the continuous image formation was performed, both the front and back surfaces of the final transferred body were observed.

As a result, there was no partial increase in image density or fogging due to the generation of the transfer memory in the formed image on the surface side of the transfer object, but backside contamination was seen on the back side.
Further in FIG. 3, to the end position P ₄ of the constant voltage control region, the transfer bias applied to the transfer roller, as the constant current control of the same current value -8μA that during the toner image transfer, without providing the constant voltage control region After the same number of continuous images were formed, both the front and back surfaces of the last transfer target were observed.

  As a result, no backside contamination was observed on the back side of the transfer object, but a partial increase in image density and fogging due to the generation of the transfer memory were observed in the formed image on the front side.

FIG. 2 is a diagram showing the periphery of an electrophotographic photosensitive member that is an image carrier as a main part of an example of an embodiment of the image forming apparatus of the present invention. FIG. 2 is a block diagram illustrating an electrical configuration of the image forming apparatus in FIG. 1. 7 is a graph showing timing for changing a transfer bias applied to a transfer roller by driving and controlling a transfer bias applying unit by a control unit.

Explanation of symbols

1 Electrophotographic photoreceptor (image carrier)
5 Transfer roller BS Transfer bias applying means C Control means

Claims (4)

  An image carrier and a transfer roller for transferring the toner image formed on the image carrier to the transfer body by bringing the image carrier into contact with the image carrier while transporting the transfer body, and a toner on the transfer roller A transfer bias applying means for applying a transfer bias for image transfer, and a control means for controlling the operation of each of the above-mentioned parts. When passing through the transfer roller, the transfer bias applied to the transfer roller by the transfer bias applying means is set to a voltage value having a lower absolute value than that during normal transfer in an area of a certain width before and after the trailing edge. An image forming apparatus comprising: a transfer bias control unit that controls to turn off or apply a reverse bias after the region is completed after performing at least one step of voltage control. .   The image forming apparatus according to claim 1, wherein the transfer bias control unit performs constant voltage control by setting the transfer bias in the region to a voltage value lower than a discharge start voltage between the transfer roller and the image carrier.   The image forming apparatus according to claim 2, wherein the transfer bias control unit changes a voltage value for constant voltage control according to environmental conditions.   The image forming apparatus according to claim 1, wherein the transfer bias controller controls the transfer bias at a constant current during normal transfer.

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