JP2007101980A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ensure stable primary transfer free from the effect of a change in a charge quantity of toner. <P>SOLUTION: A primary transfer bias is a constant voltage bias variably controlled based on ATVC control. A reference density pattern is formed on an intermediate transfer belt 11 and its density is detected by an image density sensor Sp. Based on the result of the detection, a control circuit 23 variably controls a charging bias and a development bias. Further, based on the result of the control of the development bias, a target current at primary transfer is controlled. Since the charge quantity of toner is reflected on the control of the target current, stable primary transfer free from the effect of a change in the charge quantity of toner is ensured. The density is detected and, based on the information about the density, the charging bias and the development bias are variably controlled. Based on the control result of the development bias, the target current for the primary transfer is variably controlled. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a printer, a copying machine, and a facsimile.

  FIG. 5 shows a schematic configuration of a conventional electrophotographic image forming apparatus. The surface of the photosensitive drum 1 is uniformly charged by the charging roller 2, and an electrostatic latent image is formed on the surface by receiving laser irradiation corresponding to image information by the exposure device 3. The electrostatic latent image is visualized by electrostatically adhering charged toner by the developing device developing means.

  The toner image on the photosensitive drum is electrostatically primary transferred onto the intermediate transfer belt 6 by the primary transfer roller. Further, the secondary transfer roller 7 electrostatically performs secondary transfer onto the transfer material P conveyed at a predetermined timing. The transfer material P onto which the toner image has been transferred is conveyed to the fixing device 8 and is subjected to heat fixing to form a permanent image.

  In the image forming apparatus as described above, a bias having a polarity opposite to that of the charged toner is applied to transfer the toner image from the photosensitive drum 1 to the intermediate transfer belt 6 or from the intermediate transfer belt 6 to the transfer material P. It is done electrostatically. At this time, since it is necessary to apply a charge amount equal to the total charge amount of the toner image, it is essential to appropriately control the bias.

  However, the primary transfer roller 5 is usually made of a resin, rubber, sponge or the like whose resistance value has been adjusted. However, variations in production, temperature and humidity, and durability fluctuations are used. The resistance value greatly fluctuates due to the influence of the above.

  Therefore, in order to always obtain good transferability, it is ideal to control the amount of charge to be applied to the transfer portion at the time of applying the transfer bias, and for example, constant current control in which a predetermined current is continuously applied is performed. It is possible to implement.

  In the constant current control, a local impedance difference occurs depending on the presence or absence of a toner image in the primary transfer portion and the presence or absence of a transfer material in the secondary transfer portion. At this time, current concentrates on the low impedance portion, and the amount of charge that can be supplied to the toner image portion or the transfer material P becomes smaller than the intended amount of charge, which may cause transfer failure.

  The phenomenon in which charge concentration occurs in the low impedance portion as described above occurs remarkably when a photosensitive body mainly composed of amorphous silicon is used as the photosensitive drum 1. This is because the impedance of the photoconductor itself is lower than that of other photoconductors mainly composed of OPC (organic photoconductor). When an amorphous silicon photoconductor is used, the current concentrates particularly outside the toner image area in the primary transfer. Therefore, in order to perform primary transfer of a wide variety of toner images having different toner image widths, different currents are used. There is a risk of having to set a value.

  On the other hand, by performing constant voltage control in the primary transfer portion, it is possible to apply a desired electric field to the toner image portion even if the above-described charge concentration on the low impedance portion occurs. Become.

  As the resistance value of the primary transfer roller varies, the constant voltage bias value needs to be varied as appropriate. For example, it is possible to maintain transferability by performing a known ATVC control in which a resistance value is detected every time a primary transfer operation is performed and an applied voltage value is optimally controlled (see, for example, Patent Document 1). .

Japanese Patent No. 2614309

  The ATVC control as described above measures the impedance of the primary transfer portion when a toner image is not formed, and applies it to the primary transfer roller so that a desired target current can be applied to the primary transfer portion. It is control which determines the voltage value to perform. For this reason, it is not possible to take into account fluctuations in the characteristics of the toner that is the target of primary transfer.

  The charge amount of the toner varies depending on various external conditions such as the temperature and humidity of the atmosphere or when used for a long time. For example, when a normal transfer bias is applied when the toner charge amount is lower than the assumed charge amount, a necessary transfer charge, that is, a charge amount larger than the total charge amount of toner is applied. Therefore, there is a possibility that an image defect may occur. On the other hand, when the total charge amount of the toner is higher than an assumed value, an image defect may occur even if the charge amount that can be applied by the primary transfer is insufficient. As described above, there has been a problem that transfer bias accuracy is insufficient due to a change in toner charge amount.

  Accordingly, the present invention provides an image forming apparatus capable of controlling a transfer bias with high accuracy when a toner image formed on an image carrier (for example, an amorphous silicon photoconductor) is transferred to an intermediate transfer member. Is intended to provide.

  The present invention relates to an image carrier, a charging unit for charging the surface of the image carrier, a latent image forming unit for forming an electrostatic latent image on the surface of the image carrier after charging, and an electrostatic latent image as a toner image. And detecting a density of a reference density pattern formed on the intermediate transfer body in an image forming apparatus including a developing means for developing the toner image and a transfer means for transferring a toner image on the image carrier to an intermediate transfer body. Based on the density detection unit, the first control for controlling the charging bias applied to the charging unit and the development bias applied to the development unit, and the first control based on the detection result of the density detection unit. Control means for performing a second control for controlling a transfer bias applied to the transfer means based on the developing bias, wherein the control means includes the image carrier and the transfer means in the second control. When Based on the current-voltage characteristics in the transfer portion formed between them, the constant voltage bias value applied to the transfer means is controlled so that a desired target current value can be applied to the transfer portion, and the development The target current value is changed based on a bias.

  As described above, according to the present invention, the density detector detects the density of the reference density pattern formed on the intermediate transfer member. As the first control, the control unit controls the charging bias applied to the charging unit and the developing bias applied to the developing unit based on the detection result of the density detecting unit. The control unit controls the transfer bias applied to the transfer unit based on the development bias determined by the first control as the second control. Therefore, the transfer bias reflects the state of the toner forming the reference density pattern, for example, the change in the toner charge amount. Further, the target current is changed based on the developing bias determined by the second control. This makes it possible to adjust the transfer bias with high accuracy suitable for the toner state at that time. That is, stable transfer can be performed without being affected by the toner state.

  Exemplary embodiments of the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. Absent. In addition, the materials, shapes, and the like of the members and the like once described in the following description are the same as those in the first description unless otherwise described.

<Embodiment 1>
FIG. 1 shows an image forming apparatus to which the present invention can be applied. The image forming apparatus shown in the figure is an electrophotographic four-color full-color laser printer, and the figure is a longitudinal sectional view schematically showing a schematic configuration thereof. In the following description, a case where four color full color images are formed by sequentially superposing toner images of each color of yellow (Y), magenta (M), cyan (C), and black (K) will be described as an example. To do.

  The image forming apparatus shown in FIG. 1 includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 as an image carrier. The photosensitive drum 1 is formed by forming a photosensitive body mainly composed of amorphous silicon on an aluminum drum base (cylinder) having an outer diameter of 84 mm. The photosensitive drum 1 is driven to rotate at a predetermined process speed (circumferential speed) in the direction of arrow R1 by a driving means (not shown).

  The surface of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by a charging roller 2 as a primary charger.

  The uniformly charged surface of the photosensitive drum 1 is exposed to laser light by a laser scanner 3 as an exposure unit, and the potential of the exposed portion is removed to form an electrostatic latent image.

  This electrostatic latent image is developed by a developing device 4 as developing means. The developing device 4 includes a rotary 4a that can rotate in the direction of arrow R4, and four color (four) developing devices 4Y, 4M, 4C, and 4K mounted on the rotary 4a. In this order, the developing devices 4Y, 4M, 4C, and 4K store toner of each color of yellow (Y), magenta (M), cyan (C), and black (K). In the present embodiment, first, the yellow developing device 4Y, which is the first color developing device, is disposed at a developing position facing the surface of the photosensitive drum 1 by the rotation of the rotary 4a. Then, by applying a developing bias to the developing device 4Y, yellow toner is attached to the electrostatic latent image on the photosensitive drum 1 and developed as a toner image.

  The yellow toner image thus formed on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 11 by the primary transfer roller (transfer means) 5 in the primary transfer portion T1. The photosensitive drum 1 on which the yellow toner image has been transferred onto the intermediate transfer belt 11 is subjected to removal of the toner remaining on the surface (transfer residual toner) by the cleaning device 12, and the surface is cleaned for use in the next image formation. The

  In the same process as the above-described yellow, toner images of the remaining three colors, that is, magenta, cyan, and black, are sequentially formed on the photosensitive drum 1. These toner images are sequentially primary-transferred onto the intermediate transfer belt 11 and superimposed on the intermediate transfer belt 11.

  The four color toner images superimposed on the intermediate transfer belt 11 are formed between a secondary transfer inner roller 7a and a secondary transfer portion T2 formed between the intermediate transfer belt 11 and the secondary transfer outer roller 7b. It is conveyed to. In synchronization with this, the transfer material is conveyed to the secondary transfer portion T2 by a feeding / conveying device (not shown). At this time, a secondary transfer bias is applied to the secondary transfer outer roller 7b. As a result, the four color toner images on the intermediate transfer belt 11 are secondarily transferred collectively onto the transfer material. The transfer material after the toner image is transferred is heated and pressurized by the fixing device 9 to fix the toner image on the surface. As a result, a four-color full-color image is formed on one transfer material.

  On the other hand, the intermediate transfer belt 11 after the toner image has been transferred is subjected to removal of secondary transfer residual toner by the belt cleaner 10 and the surface thereof is cleaned, and is used for the next and subsequent image formation.

  Hereinafter, the configuration of each member and the image forming conditions when a yellow toner image is formed will be described.

  The yellow developing device 4Y conveys yellow toner to the developing sleeve by a yellow toner conveying mechanism in a developing device (not shown), and a thin layer of yellow toner is formed on the outer periphery of the developing sleeve by a regulating blade pressed against the outer periphery of the developing sleeve. Apply. After the electric charge is applied to the yellow toner, development is performed on the electrostatic latent image formed on the photosensitive drum 1 by applying a development bias in which an AC bias is superimposed on a DC bias to the development sleeve. The developing sleeve is disposed at a position facing the photosensitive drum 1 with a minute interval (300 μm).

The intermediate transfer belt 11 uses a polyimide resin film having a thickness of 85 μm as a base material, disperses carbon black, and has a surface resistivity of 1 × 10 ¹² Ω / □ and a volume resistivity of 1 × 10 ^9.5 Ω · The resistance is adjusted to be cm. The peripheral length of the intermediate transfer belt 11 was 527.5 mm, and the process speed (driving speed) was 130 mm / sec.

The primary transfer roller 5 is a sponge roller having a foamed rubber layer made of EPDM that has been foamed on a steel core having an outer diameter of 8 mm, and has an outer diameter of 16 mm. The resistance value of the roller is adjusted by dispersing an ion conductive resistance adjusting agent so that it becomes 10 ^6.5 Ω (when 100 V is applied) in an environment of a temperature of 23 ° C. and a humidity of 50% Rh. It was.

The secondary transfer outer roller 7b is a sponge roller in which a foamed rubber layer based on foamed NBR (nitrile butadiene rubber) is provided on a steel core with an outer diameter of 12 mm, and includes an NBR rubber layer. The outer diameter was 24 mm. Disperse the ion conductive resistance adjusting agent so that the resistance value of the secondary transfer outer roller 7b is 10 ^7.5 Ω (when 2 kV is applied) in an environment of a temperature of 23 ° C. and a humidity of 50% Rh. The resistance was adjusted at.

  The primary transfer downstream roller 6 is a SUS roller having an outer diameter of 16 mm, and is grounded. The reason is that, in the primary transfer portion T1, the intermediate transfer belt 11 to which electric charges are applied from the primary transfer roller 5 is carried from the back surface. If the battery is charged up, it may leak to surrounding members and cause electrical stress to the image forming apparatus main body. Here, the intermediate transfer belt 11 is wound around rollers 8a, 8b, and 8c outside the primary transfer roller 5, the secondary transfer inner roller 7a, and the primary transfer downstream roller 6 described above.

  As shown in FIG. 1, an image density sensor Sp is disposed as a density detection means in the vicinity of the roller 8c. The image density sensor Sp is disposed so as to face a portion of the intermediate transfer belt 11 that is stretched over the roller 8c. The image density sensor Sp detects the optical density of the reference density pattern formed on the intermediate transfer belt 11. A potential sensor Sv is disposed on the downstream side of the developing device 4 along the rotation direction of the photosensitive drum 1 and on the upstream side of the primary transfer roller 5. The potential sensor Sv directly measures the surface potential of the photosensitive drum 1.

  A control circuit 21 is connected to the above-described image density sensor Sp. The control circuit 21 is for controlling the image density based on the density detection result of the image density sensor Sp. A control circuit 22 is connected to the potential sensor Sv. The control circuit 22 is for controlling the high voltage output based on the potential detection result of the potential sensor Sv. Further, a high voltage control circuit 23 for performing control based on output signals from the control circuits 21 and 22 is provided. The high-voltage control circuit 23 includes a charging bias applied to the charging roller 2, a developing bias applied to each color developing device 4 Y, 4 M, 4 C, and 4 K, and a primary transfer bias applied to the primary transfer roller 5. Take control.

  In the present embodiment, the image density sensor Sp is a specular reflection type sensor that detects reflected light, as shown in FIG. An LED light source 24a disposed at a predetermined angle φ with respect to a detection position on the surface of the intermediate transfer belt 11 stretched on a roller (belt stretching roller) 8c, and a phototransistor 24b as a light receiving element. It is configured. The reflected light emitted from the LED light source 24a to the reference density pattern is detected as a light quantity by the light receiving element 24b, so that the optical density of the reference density pattern can be estimated.

  In the image forming apparatus of the present embodiment configured as described above, a constant voltage bias is applied to the primary transfer roller 5 as the primary transfer bias. The bias amount at this time is optimally controlled so that a desired current amount (target current) can always be applied to the primary transfer portion T1 by ATVC control.

  Further, for the charging bias and the developing bias, optimum control (first control) is performed according to the image density information and the surface potential information of the amorphous silicon photoconductor, and the target current is linked with the control of the charging bias and the developing bias. Is also variable control (second control).

  Below, each more concrete control method is demonstrated.

  When performing the first control, the control circuit 21 first detects the surface of the intermediate transfer belt 11 with the image density sensor Sp in a state where the reference density pattern is not formed on the intermediate transfer belt 11 (see FIG. 3). S1) The image density output at this time is stored. Next, a reference density pattern of each color of yellow, magenta, cyan, and black is formed under the same charging, exposure, development, and transfer conditions as those in normal image formation. In this embodiment, as a reference density pattern, 100% density 20 mm × 20 mm square patches are continuously formed on the intermediate transfer belt 11. The optical density of the reference density pattern of each color is detected by the image density sensor Sp (S2). Here, 16 points of each color patch of the above-mentioned reference density pattern are detected at a pitch of 1 mm, the average value of the optical density is calculated, and the respective image density outputs are stored in the control circuit 21. In the control circuit 21, the image density information of each color is obtained by taking the image density of the patch and the image density ratio of the intermediate transfer belt 11 when no patch is formed for each color (S3). Here, the reason for taking the image density ratio is to correct fluctuations in the reflectance of the intermediate transfer belt 11 and fluctuations in the amount of LED light.

  As a result, the image density information is compared with a predetermined value for each color, and when the obtained density of the reference density pattern is lower than the predetermined density (YES in S4), the DC component of the developing bias according to the difference. The absolute value and the absolute value of the DC component of the charging bias are increased (S5). As a result, the developing electric field formed between the developing sleeve for each color component and the electrostatic latent image on the photosensitive drum 1 is increased, so that the amount of toner to be developed can be increased and the image density can be increased. become. After increasing the absolute values of the DC components of the developing bias and charging bias, the process returns to S2.

  On the other hand, when the obtained density of the reference density pattern is higher than the predetermined density (No in S4 and Yes in S6), the absolute values of the DC components of the developing bias and the charging bias are lowered (S7). The image density can be reduced. After reducing the absolute values of the DC components of the developing bias and the charging bias, the process returns to S2.

  On the other hand, when the obtained reference density is within the predetermined range (No in S4, No in S6), the developing bias and the charging bias are set to the current set values (S8). That is, the image density can be corrected and controlled to a predetermined value by continuously performing the above-described operations from S2 to S7 until the density of the reference density pattern of each color converges to a predetermined density range.

  Next, the second control will be described.

  The ATVC control of the primary transfer bias in the present embodiment is performed according to the following procedure. Several methods are known for the ATVC control. Here, while the photosensitive drum 1 is charged to a predetermined charging potential, the two stages of voltages V1 and V2 are each rotated by the primary transfer roller 5 once. Apply. The respective transfer currents I1 and I2 at this time are obtained, and from these relationships, the voltage Vb necessary to flow the optimum transfer current (target current) It for a predetermined density is a constant voltage obtained by linear interpolation. A method of performing control is used.

  In the present embodiment, the target current It is set so that toner transfer can be suitably performed, and is initially set to 15 μA, for example. However, as described above, the present invention is characterized in that when the development bias is changed according to the image density information, the target current is also variably controlled according to the change. A specific method for controlling the target current It in the present embodiment will be described below.

  In this embodiment, the target current It is initially set to 15 μA. This is because the transfer failure occurs at a current of 14 μA or less, and conversely, when a current of 16 μA or more is applied, retransfer occurs in which the charge polarity of some toner is reversed. ing. This setting is made in order to efficiently transfer toner of a predetermined density developed on the photosensitive drum, and an amount of charge that is substantially equal to the total amount of charge of the toner is applied to the intermediate transfer belt 11. It is intended to be set.

  However, in actuality, the charge amount of the toner increases or decreases depending on the temperature and humidity conditions of the atmosphere, or the charge amount decreases due to deterioration of the carrier or the toner itself that has a role of imparting charge to the toner by long-term use. I will do. For this reason, in order to continuously apply a charge amount equal to the total charge amount of the toner to the intermediate transfer belt 11, the target current needs to be variably controlled as the toner charge fluctuates.

  The total charge amount of the toner developed on the photosensitive drum corresponds to the difference between the potential of the photosensitive drum where the toner is developed and the DC component of the developing bias.

  As indicated by Yes in S4 of FIG. 3, the fact that the density of the reference density pattern developed with the set developing bias is lower than the expected density means that the charged charge amount of the toner is high. . In this case, control is performed to increase the image density by increasing the DC component of the developing bias and charging bias (S5) and increasing the difference from the potential of the photosensitive drum 1. This is synonymous with the control of increasing the DC component of the charging bias and the developing bias, that is, increasing the difference from the DC component of the photosensitive drum 1 when the charged charge amount of the toner increases. On the other hand, when the density of the reference density pattern is higher than the expected density (No in S4 and Yes in S6), it means that the charge amount of the toner is low. In this case, it is synonymous with the control for reducing the DC components of the developing bias and the charging bias (S7).

  FIG. 4 shows a flowchart of the second control. In addition, about S1-S8, since it is the same as that of FIG. 3, description is abbreviate | omitted.

  As described above, when the DC components of the developing bias and the charging bias are increased or decreased, the primary transfer target current It must also be increased or decreased according to the total charge amount of the toner. For this, referring to the setting of the DC component of the developing bias, variable control is performed. That is, when the density of the reference density pattern is lower than the expected density (Yes in S4 in FIG. 4), control is performed to increase the DC component of the developing bias (S5). In this case, since it means that the total charge amount of the toner is increased, control is performed to increase the target current in synchronism with this (S9). On the other hand, when it is detected that the density is high (No in S4 and Yes in S6), the total charge amount of the toner has decreased, and therefore when the control for reducing the DC component of the developing bias is performed. (S7) In conjunction with the control, the target current is lowered (S10).

  In the present embodiment, when the developing bias increases or decreases by 5 V, the target current can be increased or decreased in increments of 0.2 μA accordingly. For this reason, for example, when the DC component of the developing bias is controlled to be 5 V higher, the target current is controlled from 15 μA to 15.2 μA. Further, when the DC component of the developing bias is set to be 10V lower, the target current is controlled to be 14.6 μA.

  As described above, when the primary transfer target current value is increased (S9) or decreased (S10), the primary transfer application voltage is reset (S11) by ATVC control, and the process returns to S2. .

  As described above, according to the present embodiment, the control circuit (control unit) 23 sets the charging bias and the developing bias based on the output of the image density sensor Sp by the first control. Further, the control circuit 23 sets the transfer bias by the second control based on the development bias set by the first control. Further, the target current is changed based on the set development bias. Therefore, the transfer bias reflects the toner state such as the toner charge amount. Therefore, it is possible to adjust the transfer bias with high accuracy suitable for the toner state at that time. That is, stable transfer can be performed without being affected by the toner state.

1 is a diagram schematically illustrating a schematic configuration of an image forming apparatus to which the present invention can be applied. It is a figure explaining the structure of an image density sensor. 6 is a flowchart illustrating a flow for controlling a charging bias and a developing bias based on a reference density pattern formed on an intermediate transfer belt. 6 is a flowchart for explaining a flow of controlling a charging bias and a developing bias based on a reference density pattern formed on an intermediate transfer belt, and further changing a target current at the time of primary transfer based on the developing bias. It is a figure which shows typically schematic structure of the conventional image forming apparatus.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (primary charger, charging means)
3 Laser scanner (exposure means)
4 Developing device (Developing means)
5 Primary transfer roller (transfer means)
23 Control circuit (control means)
Sp Image density sensor (density detection means)
Sv potential sensor T1 primary transfer portion (transfer portion)

Claims (3)

An image carrier, a charging unit for charging the surface of the image carrier, a latent image forming unit for forming an electrostatic latent image on the surface of the image carrier after charging, and development for developing the electrostatic latent image as a toner image An image forming apparatus comprising: a transfer unit configured to transfer a toner image on the image carrier to an intermediate transfer member;
Density detecting means for detecting the density of a reference density pattern formed on the intermediate transfer member;
Based on the detection result of the density detecting means, a first control for controlling a charging bias applied to the charging means and a developing bias applied to the developing means, and the developing bias determined by the first control. And a control means for performing a second control for controlling a transfer bias applied to the transfer means based on,
In the second control, the control unit applies a desired target current value to the transfer unit based on a current-voltage characteristic in the transfer unit formed between the image carrier and the transfer unit. And controlling the constant voltage bias value applied to the transfer means, and changing the target current value based on the development bias.
An image forming apparatus. The control means performs constant voltage control on a transfer bias applied to the transfer means in the second control.
The image forming apparatus according to claim 1. The image carrier is an image carrier having an amorphous silicon photosensitive layer as a surface layer.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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