JP6304496B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

  2. Description of the Related Art Conventionally, an image forming apparatus including a photosensitive drum, a conveyance belt that conveys a sheet, and a transfer roller that is pressed against the photosensitive drum with the conveyance belt interposed therebetween is known (for example, see Patent Document 1). The sheet is conveyed downstream while being nipped by the conveyance belt and the photosensitive drum. When the sheet passes through the nip position, a transfer current having a polarity opposite to the charging polarity of the toner is applied to the transfer roller. As a result, the toner image carried on the photosensitive drum is transferred to the sheet. A stretching roller is provided downstream of the nip position in the sheet conveyance direction. The tension roller stretches the conveyance belt at a separation position where the sheet is separated from the conveyance belt.

JP 2002-031960 A

  However, if the tension roller is grounded in the above-described image forming apparatus using the conveyance belt, the charge (toner charge polarity) held on the sheet and the conveyance belt when the trailing edge of the sheet is separated from the conveyance belt. And a charge of the opposite polarity) is discharged to the ground through the stretching roller. When this discharge (so-called peeling discharge) occurs, the toner cannot be held on the sheet due to the charge having the opposite polarity to the charge polarity of the toner, and there is a problem that a part of the toner is scattered.

  In order to solve this problem, it is conceivable to apply a current (toner scattering suppression current) having a polarity opposite to the charging polarity of the toner to the tension roller. Thereby, it is possible to suppress peeling discharge when the trailing edge of the sheet is peeled from the conveying belt, and to suppress toner scattering.

  However, in this case, the following problems occur. That is, in general, in an image forming apparatus, a transfer current having a polarity opposite to the toner charging polarity is applied to the transfer roller during image formation, and after the trailing edge of the sheet passes the nip position, the toner is applied to the transfer roller. In order to prevent adhesion, a current having the same polarity as the charged polarity of the toner is applied. For this reason, after the trailing edge of the sheet passes through the nip position, electric charges (electric charges having a polarity opposite to the charging polarity of the toner) flow out from the stretching roller to the transfer roller through the conveyance belt. As a result, the scattering suppression current applied to the stretching roller is insufficient, and the above-described peeling discharge cannot be sufficiently suppressed.

  The present invention has been made in view of the above points, and an object thereof is to provide an image carrier, a conveyance belt that conveys a sheet, and a transfer that is pressed against the image carrier with the conveyance belt interposed therebetween. In an image forming apparatus including a roller, it is intended to reliably prevent peeling discharge when the trailing edge of the sheet is peeled from the conveyance belt, and to prevent toner scattering due to the peeling discharge. .

  An image forming apparatus according to one aspect of the present invention includes an image carrier that carries a toner image, a conveyance belt that contacts the image carrier to form a nip, and conveys a sheet via the nip position; A transfer roller that transfers the toner image onto the sheet at the nip position; and a tension that stretches the conveyance belt at a separation position that is downstream of the nip position in the sheet conveyance direction and where the sheet is separated from the conveyance belt. A transfer current having a polarity opposite to the charging polarity of the toner with respect to the transfer roller from the time when the front end of the sheet and the front end of the sheet reach the nip position to the time when the rear end of the sheet passes the nip position. And a first power supply unit for applying.

From the time when the front edge of the sheet reaches the peeling position to the time when the rear edge of the sheet passes through the peeling position, toner scattering with a polarity opposite to the charging polarity of the toner is applied to the stretching roller. The first power supply unit further includes a second power supply unit that applies a suppression current, and the first power supply unit is configured to move the back end of the sheet from the nip position to the time when the sheet passes the peeling position. Te, the and the absolute value the charging polarity opposite to the toner is configured to apply an absolute equal posttranscriptional current of the toner scattering suppression current, the absolute value of the post-transcriptional current than the absolute value of the transfer current Is also small .

  According to the present invention, it is possible to reliably prevent peeling discharge when the trailing edge of the sheet is peeled from the conveying belt, and to prevent toner scattering due to the peeling discharge.

FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment. FIG. 2 is a schematic diagram showing the configuration of the transfer device. FIG. 3 is a block diagram illustrating a configuration of a control system that controls the first and second power supply units connected to the transfer device. FIG. 4 is a flowchart showing the first half of the output current control of the first and second power supply units in the controller. FIG. 5 is a flowchart showing the latter half of the output current control of the first and second power supply units in the controller. FIG. 6 is table data used when determining the toner scattering suppression current and the transfer current in the controller. FIG. 7 is a time chart illustrating an example of output current control of the first and second power supply units in the first embodiment. FIG. 8 is a view corresponding to FIG. 7 showing a modification of the first embodiment. FIG. 9 is a view corresponding to FIG. FIG. 10 is a table showing the results of a print test using the image forming apparatuses according to Embodiments 1 and 2 and the modified example. FIG. 3 is a view corresponding to FIG. 2 showing another embodiment. FIG. 8 is a view corresponding to FIG. 7 showing another embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the following embodiment.

Embodiment 1
FIG. 1 shows a monochrome copying machine which is an example of an image forming apparatus 1 in the present embodiment. The image forming apparatus 1 includes a housing 2, an image reading unit 3, and an ADF (automatic document feeder) 4. The image reading unit 3 is attached to the upper end of the housing 2, and the ADF is disposed so as to cover the document placement surface of the image reading unit 3. The image reading unit 3 includes a light source, a reflection mirror, a CCD (Charge Coupled Device), and the like, and optically reads an image of a document set on a document placement surface or an image of a document supplied from an ADF. The image reading unit 3 converts the read document image into data and transmits it to the image forming unit 20 described later.

  In the housing 2, a paper feeding unit 10, an image forming unit 20, and a fixing unit 30 are accommodated. The paper feeding unit 10 is provided at the bottom of the housing 2. The paper feeding unit 10 includes a paper feeding cassette 11 that stores a plurality of sheets S that are stacked on each other, a pick roller 12 that takes out the sheets S in the paper feeding cassette 11 one by one, and a single sheet S that has been taken out. A feed roller 13 and a retard roller 14 that are separated and sent to the conveyance path T are provided.

  The image forming unit 20 is provided above the paper feeding unit 10 in the housing 2. The image forming unit 20 includes a photosensitive drum 21 that is an image carrier rotatably provided in the housing 2, a charger 22 disposed around the photosensitive drum 21, a developing device 23, a transfer device 24, and the like. A cleaning unit 25 and an optical scanning device 26 disposed above the photosensitive drum 21 are provided.

  The fixing unit 30 is disposed above the image forming unit 20. The fixing unit 30 includes a fixing roller 31 that is heated by a heating unit such as a heater, and a pressure roller 32 that is pressed against the fixing roller 31.

  In the image forming unit 20, when image data is received from the image reading unit 3, the photosensitive drum 21 is rotationally driven by a motor (not shown), and the surface of the photosensitive drum 21 is charged to a predetermined potential by the charger 22. . In the present embodiment, the charger 22 charges the surface of the photosensitive drum 21 to a positive polarity. Thus, after the surface of the photosensitive drum 21 is charged, laser light is emitted from the optical scanning device 26 to the photosensitive drum 21 based on the image data from the image reading unit 3. An electrostatic latent image is formed on the surface of the photosensitive drum 21 by irradiation with laser light. The electrostatic latent image formed on the photosensitive drum 21 is developed with toner charged by the developing device 23 and visualized as a toner image. In this embodiment, a toner image is formed by so-called reversal development, and the charging polarity of the toner charged by the developing device 23 is positive (the same polarity as the charging polarity of the surface of the photosensitive drum 21). . The toner image may be formed not only by reversal development but also by regular development. In this case, the toner charge polarity is negative.

  After the toner image is formed on the surface of the photosensitive drum 21, the sheet S sent from the registration roller pair 16 is pressed against the surface of the photosensitive drum 21 by the transfer device 24. Then, the transfer device 24 applies a transfer current having a toner polarity and a reverse charging polarity (negative polarity in the present embodiment) to the sheet S, whereby the toner image on the photosensitive drum 16 is transferred to the sheet S. . The sheet S on which the toner image is transferred is heated and pressed by the fixing roller 31 and the pressure roller 32 in the fixing unit 30. As a result, the toner image is fixed on the sheet S.

  As illustrated in FIG. 2, the transfer device 24 includes an endless conveyance belt 241, a transfer roller 242, an upstream roller 243, a downstream roller 244, a first power supply unit 246, and a second power supply unit 247. .

  The conveyance belt 241 conveys the sheet S while electrostatically attracting the sheet S to the outer peripheral surface. The outer peripheral surface of the conveyance belt 241 is in contact with the photosensitive drum 21 to form a nip with the photosensitive drum 21. The conveyor belt 241 is stretched around the upstream roller 243 and the downstream roller 244 with a predetermined tension. The transport belt 241 is driven in the direction of the arrow in FIG. 2 when the upstream roller 243 or the downstream roller 244 is rotationally driven by a motor (not shown). The conveyor belt 241 is formed by coating a fluororesin on the outer peripheral surface of a rubber belt-shaped member, for example.

  The transfer roller 242 is disposed to face the photosensitive drum 21. The outer peripheral surface of the transfer roller 242 is in contact with the inner peripheral surface of the transport belt 241. The transfer roller 242 is pressed against the photosensitive drum 21 with a predetermined load with the conveyance belt 241 interposed therebetween. As a result, the sheet S is nipped between the conveyance belt 241 and the photosensitive drum 21 at the nip position A1. The transfer roller 242 applies a transfer current C1 having a polarity opposite to the toner charging polarity to the sheet S passing through the nip position A1, thereby transferring the toner image to the sheet S.

  The upstream roller 243 stretches the conveyance belt 241 at a conveyance start position A2 that is upstream of the nip position A1 in the sheet conveyance direction and the front end of the sheet S reaches the conveyance belt 241. The upstream roller 243 is formed of a conductive member such as a metal and is grounded through a bearing or the like. In the present embodiment, the front end and the rear end of the sheet S mean the front end and the rear end of the sheet S in the sheet conveyance direction.

  The downstream roller (stretching roller) 244 stretches the conveyance belt 241 at a separation position A3 where the trailing edge of the sheet S is separated from the conveyance belt 241 on the downstream side of the nip position A1 in the sheet conveyance direction. The downstream roller 244 is formed of a conductive member such as metal.

  A guide member 245 is provided on the downstream side of the downstream roller 244 in the sheet conveyance direction. The guide member 245 guides the sheet S separated from the conveyance belt 241 to the fixing unit 30 at the peeling position A3. The guide member 245 is made of a conductive member such as metal. The guide member 245 is grounded in this embodiment. As will be described later, a current having a polarity opposite to the charging polarity of the toner may be applied to the guide member 245.

  The first power supply unit 246 applies any one of the transfer current C1, the post-transfer current C2, and the standby current C3 to the transfer roller 242. The negative terminal of the first power supply unit 246 is connected to the transfer roller 242 via the wiring L1, and the positive terminal of the first power supply unit 246 is grounded via the wiring L2. As shown in FIG. 3, the first power supply unit 246 includes a current detection circuit 246a, a constant current control circuit 246b, and a voltage control circuit 246c. The current detection circuit 246a detects an output current of the first power supply unit 246 (that is, a current applied to the transfer roller 242 via the wiring L1). The constant current control circuit 246b adjusts the output voltage of the voltage control circuit 246c so that the current detected by the current detection circuit 246a is equal to the target current determined by the controller 100. Then, the first power supply unit 246 applies the currents C1 to C3 to the transfer roller 242 by constant current control.

  Here, the transfer current C1 is a current having a polarity opposite to the charging polarity of the toner and is a current for transferring the toner image on the photosensitive drum 21 to the sheet S. The post-transfer current C <b> 2 is a current having a polarity opposite to the charging polarity of the toner, and is a current for preventing a toner scattering suppression current C <b> 4 (described later) from flowing out to the transfer roller 242 side through the conveyance belt 241. In the present embodiment, the absolute value of the post-transfer current C2 is smaller than the absolute value of the transfer current C1. The standby current C3 is a current having the same polarity as the charging polarity of the toner, and is a current for preventing the toner from adhering to the conveyance belt 241 during non-image formation. The controller 100 controls the absolute values and application timings of the transfer current C1, the post-transfer current C2, and the standby current C3.

  The second power supply unit 247 applies the toner scattering suppression current C <b> 4 to the downstream roller 244. The negative electrode side terminal of the second power supply unit 247 is connected to the downstream roller 244 via the wiring L3, and the positive electrode side terminal of the second power supply unit 247 is grounded via the wiring L4. Like the first power supply unit 246, the second power supply unit 247 includes a current detection circuit 247a, a constant current control circuit 247b, and a voltage control circuit 247c. Then, the second power supply unit 247 applies the toner scattering suppression current C4 to the downstream roller 244 by constant current control.

  The toner scattering suppression current C4 is a current having a polarity opposite to the charging polarity of the toner, and plays a role of holding the toner on the sheet S by suppressing the peeling discharge accompanying the peeling of the sheet S. The controller 100 controls the absolute value and application timing of the toner scatter suppression current C4.

  The controller 100 includes a microcomputer having a CPU 100a, a ROM 100b, a RAM 100c, and the like. The controller 100 controls a printing operation and a sheet conveying operation based on detection signals from various sensors. The controller 100 controls output currents of the first power supply unit 246 and the second power supply unit 247 based on detection signals from the temperature / humidity sensor and table data stored in the ROM (see FIG. 6).

  The temperature / humidity sensor 101 is provided, for example, around the image forming unit 20. The temperature / humidity sensor 101 detects atmospheric temperature, relative humidity, and absolute humidity, and outputs those detection signals to the controller 100.

  The table data (see FIG. 6) is obtained by setting the transfer current C1 and the toner scattering suppression current C4 in advance for each of the three atmospheric conditions K1 to K3 having different atmospheric temperatures, relative humidity, and absolute humidity. It has become. As can be seen from the table data, the absolute value of the toner scattering suppression current C4 is set to a higher value as the absolute humidity or relative humidity of the atmosphere is smaller (that is, the resistance of the sheet S is larger). In other words, the absolute value of the toner scattering suppression current C4 is set so as to increase as the occurrence probability of the peeling discharge of the sheet S increases. In the present embodiment, the current values are set for the three atmospheric conditions K1 to K3. However, the present invention is not limited to this. For example, the atmospheric conditions are further classified into four or more atmospheric conditions. You may make it set said each current value about conditions.

  Here, if the temperature, relative humidity, and absolute humidity detected by the temperature / humidity sensor 101 do not apply to any of the above atmospheric conditions 1 to 3, the controller 100 suppresses the transfer current C1 and toner scattering by linear interpolation. The current C4 is determined.

  For example, when the temperature is 16.5 ° C. (= median value between 10 ° C. and 23 ° C.), the relative humidity is 10%, and the absolute humidity is 1 kg / 1 kg, By linearly interpolating the current values of the atmospheric condition 1 and the atmospheric condition 2 of the data, the toner scattering suppression current C4 is calculated as -8 μA (= {(− 10) + (− 6)} / 2), and the transfer is performed. The bias current C1 is calculated as −110 μA (= {(− 70) + (− 150)} / 2).

  The table data does not include the post-transfer current C2 and the standby current C3. The post-transfer current C2 is set to the same value as the toner scattering suppression current C4 by the controller 100. The standby current C3 is set to a constant value (for example, 50 μA) regardless of the atmospheric conditions.

  The current control of the first and second power supply units 246 and 247 by the controller 100 will be described in detail with reference to the flowcharts of FIGS.

  In step S1, it is determined whether image data for printing (print data) has been received. If this determination is NO, the process returns. If YES, the process proceeds to step S2.

  In step S <b> 2, a paper feed operation is started by outputting a paper feed start signal to the paper feed unit 10.

  In step S3, a preset standby current (current having the same polarity as the toner charging polarity) C3 is applied to the transfer roller 242.

  In step S4, the detection signal from the temperature / humidity sensor 101 is read, and the atmospheric temperature, relative humidity, and absolute humidity are calculated based on the read detection signal.

  In step S5, the transfer current C1 and the toner scattering suppression current C4 are determined based on the table data stored in the ROM and the temperature, relative humidity, and absolute humidity calculated in step S4. This determination procedure is obtained by linearly interpolating the current values C1 and C4 described in the table data as described above.

  In step S6, it is determined whether or not the front end of the sheet S has reached the nip position A1 based on the elapsed time from the start of the sheet feeding operation in step S2 to the current time. When this determination is NO, the process returns to the step S4, and when it is YES, the process proceeds to the step S7.

  In step S7, a control signal is output to the first power supply unit 246 to apply the transfer current C1 determined in step S5 to the transfer roller 242, and the toner scattering suppression current C4 determined in step S5 is used as the downstream roller 244. A control signal is output to the second power supply unit 247 to be applied to the second power supply unit 247.

  In step S8 (see FIG. 5), based on the elapsed time from the time when the front edge of the sheet S reaches the nip position in step S6 to the present time, and the length information in the conveyance direction of the sheet S included in the print data, It is determined whether the trailing edge of the sheet S has passed the nip position A1. When this determination is NO, step S8 is executed again, while when it is YES, the process proceeds to step S9.

  In step S9, a control signal is sent to the first power supply unit 246 to apply the post-transfer current C2 having the same absolute value as the toner scattering suppression current C4 determined in step S5 and having a polarity opposite to the toner charging polarity to the transfer roller 242. Is output. Then, the current applied to the transfer roller 242 is switched from the transfer current C1 to the post-transfer current C2.

  In step S10, it is determined whether or not the trailing edge of the sheet S has passed the separation position A3 based on the elapsed time from the time when the trailing edge of the sheet S has passed the nip position A1 in step S8. When this determination is NO, the process of step S10 is executed again, and when it is YES, the process proceeds to step S11.

  In step S <b> 11, a control signal is output to the first power supply unit 246 to apply a preset standby current C <b> 3 to the transfer roller 242. In addition, a stop signal is output to the second power supply unit 247 to stop the application of the toner scatter suppression current C4 to the downstream roller 244.

  In step S12, it is determined whether or not there is print data to be printed. If this determination is YES, the process returns. If NO, the process proceeds to step S13.

  In step S13, a stop signal is output to the first power supply unit 246 to stop applying the current to the transfer roller 242, and then the process returns.

  FIG. 7 is a time chart showing an example of the above-described current control by the controller 100. This time chart shows an example in which continuous printing is performed on two sheets S. When the image forming apparatus 1 receives a print start request, the printing operation starts and a charging voltage is applied to the surface of the photosensitive drum 21, and then the developing voltage is applied in the developing unit 23. At time t1, the standby current C3 having the same polarity as the toner charging polarity (positive polarity in this embodiment) is applied to the transfer roller 242 at the same time when the image forming apparatus 1 starts the printing operation. At time t <b> 2, the transfer current C <b> 1 having a polarity opposite to the toner charging polarity (negative polarity) is applied to the transfer roller 242 at the same time when the front end of the sheet S reaches the nip position A <b> 1, and further to the downstream roller 244. A toner scattering suppression current C4 having a polarity opposite to the toner charging polarity (negative polarity) is applied. At time t2 ″, the front end of the sheet S has reached the separation position A3, and at this time, the toner scattering suppression current C4 has already been applied to the downstream roller 244. At time t3, the rear end of the sheet S is in the nip. Simultaneously with passing through the position A1, the applied current to the transfer roller 242 is switched from the transfer current C1 to the post-transfer current C2 (current having the opposite polarity to the toner charging polarity and the same absolute value as the toner scattering suppression current C4). At the same time as the trailing edge of the sheet S passes the peeling position A3, the current applied to the transfer roller 242 is switched from the post-transfer current C2 to the standby current C3, and the application of the toner scattering suppression current C4 to the downstream roller 244 is stopped. At time t5 to t7, the same processing as time t2 to t4 is executed when printing the subsequent sheet S. At time t8, transfer is performed. The application of current to the roller 242 is stopped, and the printing operation ends.

  As described above, in the above-described embodiment, at least from the time when the front end of the sheet S reaches the peeling position A3 to the time when the rear end of the sheet S passes the peeling position A3, the toner is applied to the downstream roller 244. The toner scattering suppression current C4 having a polarity opposite to the charging polarity is applied (steps S7 and S8). Therefore, when the trailing edge of the sheet S is peeled off at the peeling position A3, it is possible to prevent electric charges having a polarity opposite to the charging polarity of the toner from flowing out to the downstream roller 244 from the trailing edge of the sheet S and discharging. . As a result, part of the toner held on the sheet S can be prevented from scattering.

  Here, in the conventional image forming apparatus 1, a transfer current having a polarity opposite to the toner charging polarity is applied to the transfer roller 242 during image formation, and after the trailing edge of the sheet S passes the nip position A 1, the sheet is conveyed. In order to prevent toner adhesion to the belt 241, a current having the same polarity as the toner charging polarity is applied to the transfer roller 242. For this reason, after the trailing edge of the sheet S passes the nip position, a charge having a polarity opposite to the charging polarity of the toner flows out from the downstream roller 244 toward the transfer roller 242 through the conveyance belt 241. As a result, there is a problem that the toner scattering suppression current C4 applied to the downstream roller 244 is insufficient and the above-described peeling discharge cannot be sufficiently suppressed.

  On the other hand, in Embodiment 1 described above, until the trailing edge of the sheet S passes the nip position A1 and passes the peeling position A3, the transfer roller 242 has a polarity opposite to the charging polarity of the toner and A post-transfer current C2 having the same absolute value as the toner scattering suppression current C4 is applied. Accordingly, it is possible to prevent the toner scattering suppression current C4 from flowing out from the downstream roller 244 to the transfer roller 242 through the conveyance belt 241. As a result, the toner scattering suppression current C4 can be avoided from being insufficient, and the toner on the sheet S can be reliably prevented from scattering.

  In the first embodiment, the absolute value of the post-transfer current C2 is set lower than the absolute value of the transfer current C1. As a result, it is possible to prevent an excessive current from flowing through the conveyor belt 241 to reduce the durability life of the conveyor belt 241.

Further, in Embodiment 1, the controller 100 is to reduce the absolute value and the absolute value of the transfer current after C2 toner scattering suppression current C4 as atmospheric humidity is high (the same size as the toner scattering suppression current C4) It is configured (see FIG. 6).

  According to this, the peeling discharge from the rear end of the sheet S occurs, and the absolute value of the toner scattering suppression current C4 increases as the environment becomes smaller, so that the peeling discharge can be reliably suppressed. As a result, the post-transfer current C2 also increases, so that the toner scattering current C4 does not flow out to the transfer roller 242 side through the transport belt 241. In the environment where the peeling discharge of the sheet S is unlikely to occur, the absolute value of the toner scattering suppression current C4 becomes small, so that it is possible to prevent the toner scattering suppression current C4 and the post-transfer current C2 from being wasted, and after the transfer. It is possible to suppress a decrease in the life of the conveyor belt 241 due to the application of the current C2.

<Modification>
FIG. 8 shows a modification of the first embodiment. This modification differs from the first embodiment in that the application start time of the toner scatter suppression current C4 is set to a time t2 "that is later than the time t2. That is, in this modification, the front end of the sheet S is the conveying belt. Application of the toner scatter suppression current C4 is started when the separation position A3 from 241 is reached, thereby reducing the consumption of the toner scatter suppression current C4 as much as possible, and the same effects as the first embodiment. Can be obtained.

<< Embodiment 2 >>
FIG. 9 shows the second embodiment. This embodiment is different from the first embodiment in that the toner scattering suppression current C4 is applied to the guide member 245 in addition to the downstream roller 244. That is, in this embodiment, the guide member 245 is connected to the negative terminal of the second power supply unit 247 via the wiring L5. According to this configuration, a charge having a polarity opposite to the charging polarity of the toner can be supplied to the sheet S by a minute discharge from the downstream end of the guide member 245 in the sheet conveyance direction. Therefore, the scattering of toner from the sheet S can be further reliably suppressed.

<< Print Test Results of Each Embodiment and Modification >>
In the image forming apparatuses 1 of the first and second embodiments and the modified example described above, 600K (K = 1000) continuous printing tests were performed using the A4Y size sheet S. FIG. 10 is a table showing the test results. “◯” in the table indicates that the amount of scattered toner in the image forming apparatus 1 measured by the sensor is less than the reference value, and “X” indicates that the amount of scattered toner is greater than or equal to the reference value. . The “belt resistance value” is a value obtained by measuring the resistance value of the conveyance belt 241. “◯” in the “belt life” column means that the belt resistance value is not more than the reference value (7.85 logΩ in this test) and the belt life is sufficiently left. According to the table of FIG. 10, in the conventional example in which the post-transfer current C <b> 2 is not applied, the amount of scattered toner in the image forming apparatus 1 exceeds the reference amount after the completion of printing 600 K sheets, whereas in the first and second embodiments, In the modified example, it can be seen that the amount of scattered toner in the image forming apparatus 1 is suppressed to less than the reference amount.

<< Other embodiments >>
In each of the above-described embodiments and modifications, the transfer current C1, the post-transfer current C2, the standby current C3, and the toner scattering suppression current C4 are applied by constant current control. However, the present invention is not limited to this. The current may be applied by constant voltage control.

  In the second embodiment, the toner scattering suppression current C4 is applied to the downstream roller 244 and the guide member 245 by using a common power supply (second power supply unit 247). However, the present invention is not limited to this. Instead, for example, as shown in FIG. 11, a third power source 248 for applying the toner scattering suppression current C <b> 4 to the guide member 245 may be further provided.

  An example of current control in this case is shown in FIG. In this example, the application timing of the toner scattering suppression current C4 to the guide member 245 is delayed by ΔT (sec) more than the application timing of the toner scattering suppression current C4 to the downstream roller 244. This ΔT is equal to the time from when the front end of the sheet S passes the separation position A3 of the conveying belt 241 until it passes the upstream end of the guide member 245. The timing at which the toner scattering suppression current C4 is applied to the downstream roller 244 may be the timing t2 ″ when the front end of the sheet reaches the peeling position A3.

  In each of the above-described embodiments and modifications, the example in which the toner charging polarity is positive has been described. However, the present invention is not limited to this, and the present invention is an image forming apparatus 1 in which the toner charging polarity is negative. This also applies to

  As described above, the present invention is useful for an image forming apparatus, and particularly useful for an image forming apparatus that transfers a toner image carried on an image carrier to a sheet conveyed by a conveyance belt.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus A2 Nip position A3 Separation position 21 Photosensitive drum (image carrier)
100 controller (control unit)
101 Temperature / humidity sensor 241 Conveyor belt 242 Transfer roller 244 Downstream roller (stretching roller)
246 First power supply unit 247 Second power supply unit

Claims (2)

An image carrier for carrying a toner image;
A conveying belt that contacts the image carrier to form a nip and conveys the sheet via the nip position;
A transfer roller for transferring the toner image to the sheet at the nip position;
A tension roller that stretches the conveyance belt at a separation position where the sheet is separated from the conveyance belt on the downstream side in the sheet conveyance direction from the nip position;
From the time when the front edge of the sheet reaches the nip position to the time when the rear edge of the sheet passes through the nip position, a transfer current having a polarity opposite to the toner charging polarity is applied to the transfer roller. An image forming apparatus comprising a power supply unit,
From the time when the front edge of the sheet reaches the peeling position to the time when the rear edge of the sheet passes the peeling position, the toner scattering suppression current having a polarity opposite to the charging polarity of the toner is applied to the stretching roller. A second power supply unit for applying
The first power supply unit has a polarity opposite to the toner charging polarity and absolute with respect to the transfer roller from the time when the trailing edge of the sheet passes the nip position to the time when the sheet passes the peeling position. value is configured to apply an equal posttranscriptional current to the absolute value of the toner scattering suppression current,
The image forming apparatus, wherein an absolute value of the post-transfer current is smaller than an absolute value of the transfer current . The image forming apparatus according to claim 1 .
A humidity detector that detects atmospheric humidity;
Based on the atmospheric humidity detected by the humidity detection unit, further comprising a control unit for controlling the first power supply unit and the second power supply unit,
The controller controls the absolute value of the toner scattering suppression current applied to the tension roller by the second power supply unit and the first power supply unit as the humidity of the atmosphere detected by the humidity detection unit increases. An image forming apparatus configured to reduce an absolute value of the post-transfer current applied to the transfer roller.

Images