JP5821296B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus such as a printer or a copying machine that transfers a toner image onto a recording medium such as paper by an electrophotographic method.

  In general, a transfer voltage applied to a transfer roller is controlled at a constant voltage in order to transfer a toner image from an image carrier (photosensitive member or intermediate transfer belt) that carries a toner image to a recording medium such as paper. . In this case, in order to obtain a good transfer image, it is important to keep the transfer current within a certain range. The transfer current means a current flowing through the image carrier, the recording medium, the toner, and the resistor of the transfer roller.

  As such constant voltage control, Patent Document 1 stores an upper limit value and a lower limit value of the secondary transfer current, and monitors the transfer current so that the transfer current does not exceed the upper limit value. It is described that the applied voltage is controlled so as not to fall below the lower limit value.

  On the other hand, it is necessary to separate the recording medium from the intermediate transfer belt immediately after the secondary transfer, and in order to smoothly separate the recording medium, a neutralizing needle for removing charges from the recording medium is provided immediately after the transfer roller. A DC voltage is applied to. Patent Document 2 discloses that a bias voltage applied to a secondary transfer roller so that a transfer contribution current falls within a specified range using a value obtained by subtracting a detected neutralization current from a detected secondary transfer current. It is described to control.

  By the way, an ion conductive roller used as a transfer roller has temperature / humidity dependency, and its electric resistance largely fluctuates due to changes in temperature / humidity. If the fixing device is arranged close to the transfer roller as the image forming apparatus is downsized as in recent years, the transfer roller is heated by heat radiation from the fixing device, and the resistance value greatly fluctuates. That is, when the transfer unit and the fixing unit are close to each other, the resistance variation of the transfer roller easily occurs from the initial start of the printing operation to the middle of the printing operation. The transfer roller generates uneven resistance in the circumferential direction while it is stopped. If continuous printing is performed, the uneven resistance tends to be eliminated.

  Specifically, if the state in which the transfer roller is stopped during non-image formation is continued, the moisture on the roller surface volatilizes due to fixing heat on the side facing the fixing device of the transfer roller in a high temperature / high humidity environment. On the other hand, the resistance value does not increase on the opposite side of the fixing unit, and local resistance unevenness occurs in the circumferential direction of the transfer roller (see curve M1 in FIG. 4). On the other hand, in a low-temperature and low-humidity environment, on the side of the transfer roller facing the fixing device, the resistance value decreases due to activation of the ion conductive material by the fixing heat, and conversely on the opposite side of the fixing device, the resistance value decreases. Accordingly, local resistance unevenness occurs in the circumferential direction of the transfer roller (see curve M2 in FIG. 5). As a result, a partial current excess portion and a current shortage portion appear in the sheet passing direction, resulting in image defects. However, when image formation is continuously performed, the temperature of the roller surface is uniformly increased with the rotation of the transfer roller, and the uneven resistance is eliminated (see curve N1 in FIG. 4 and curve N2 in FIG. 5). ).

  In the constant voltage control described in Patent Documents 1 and 2, even the resistance unevenness in the circumferential direction of the transfer roller is not detected. Therefore, the voltage control is performed by unconsciously taking the resistance unevenness, and the transfer current is appropriately set. Therefore, an image with uneven density cannot be maintained. In particular, in Patent Document 1, since the upper limit value and the lower limit value are fixed, after the resistance unevenness of the transfer roller is eliminated during image formation, the transfer current is controlled by the fixed upper limit value / lower limit value. May not be within the preferred range.

  Japanese Patent Application Laid-Open No. H10-228561 describes that control for determining a transfer voltage at the time of image formation is performed by detecting resistance unevenness in the circumferential direction of the transfer roller. However, this control can cope with current fluctuations in the circumferential direction of the transfer roller. However, since the resistance of the recording medium is not detected, when a recording medium having a resistance value higher or lower than normal is passed. The transfer current becomes insufficient or excessive, resulting in an image defect.

JP 2008-275946 A JP 2010-249872 A Japanese Patent Laying-Open No. 2005-099217

An object of the present invention is to provide an image forming apparatus capable of preferable constant current control corresponding to uneven resistance in the circumferential direction of a transfer roller.

An image forming apparatus according to an aspect of the present invention is
An image carrier for carrying a toner image;
A transfer roller that conveys the recording medium together with the image carrier;
Voltage applying means for applying a bias voltage to the transfer roller;
Voltage control means for performing constant current control on the voltage application means;
Voltage detection means for detecting the voltage of the transfer roller;
Current detection means for detecting the current of the transfer roller;
Recording means for recording the voltage value detected when the transfer roller is rotated by the voltage detection means;
Storage means for storing a plurality of predetermined upper limit values and a plurality of predetermined lower limit values of the transfer current flowing through the transfer roller;
With
At the time of image formation, before entering the transfer operation, the detection voltage in the circumferential direction of the transfer roller detected by the voltage detection unit is recorded on the recording unit, and the maximum value, minimum value, and average value of the detection voltage are recorded. And select one upper limit value and one lower limit value from the storage means according to the difference ,
During image formation, during the transfer operation, when the transfer current detected by the current detection unit is larger than the selected upper limit value, the voltage is set so that the transfer current is less than or equal to the upper limit value. The voltage applying means controls the applying means and, when the transfer current detected by the current detecting means is smaller than the selected one lower limit value, the voltage applying means so that the transfer current becomes not less than the one lower limit value. To control,
It is characterized by.

  In the image forming apparatus, before starting the transfer operation, the detected voltage in the circumferential direction of the transfer roller detected by the voltage detecting unit is recorded on the recording unit. At the same time, one upper limit value and one lower limit value are selected from the storage means based on the detected voltage. During image formation, during the transfer operation, when the transfer current detected by the current detection unit is larger than the selected upper limit value, the voltage application unit is set so that the transfer current is equal to or less than the upper limit value. When the transfer current detected by the current detection means is smaller than the selected one lower limit value, the voltage application means is controlled so that the transfer current becomes equal to or higher than the one lower limit value. This makes it possible to cope with resistance irregularities in the circumferential direction of the transfer roller, and in particular, good constant voltage control even with respect to resistance irregularities of the transfer roller at the start of printing and resistance fluctuations during continuous printing. Is possible. As a result, toner scattering and printing bleeding are prevented.

According to the present invention, preferable constant current control corresponding to the uneven resistance in the circumferential direction of the transfer roller is possible, and good transfer can be maintained.

1 is a schematic configuration diagram illustrating an image forming apparatus. FIG. 4 is an explanatory diagram schematically showing a secondary transfer region in the image forming apparatus. FIG. 6 is a chart showing a secondary transfer current and voltage during a printing operation (first and second sheets). FIG. 7 is a chart showing uneven resistance in the circumferential direction of the transfer roller at high temperature and high humidity. FIG. 5 is a chart showing uneven resistance in the circumferential direction of the transfer roller at low temperature and low humidity.

  Embodiments of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. In addition, in each figure, the same code | symbol is attached | subjected regarding the same member and part, and the overlapping description is abbreviate | omitted.

(Schematic configuration of image forming apparatus, see FIG. 1)
As shown in FIG. 1, the image forming apparatus is configured as a tandem color printer, and is designed to form images of Y (yellow), M (magenta), C (cyan), and K (black). Image units 1Y, 1M, 1C, and 1K are juxtaposed directly below the intermediate transfer belt 10. Each of the image forming units 1Y, 1M, 1C, and 1K has a known configuration in which a developing device 3, a primary transfer roller 4 and the like are arranged around the photosensitive drum 2. The intermediate transfer belt 10 is stretched endlessly on the support rollers 11 and 12 and is driven to rotate in the direction of arrow a. The toner images formed on the respective photosensitive drums 2 are sequentially primary transferred onto the intermediate transfer belt 10 and combined with a color image.

  Sheets as recording media are stacked on a sheet feeding tray 15 and are fed one by one by a sheet feeding roller 16 and pass through a nip portion between an intermediate transfer belt 10 and a secondary transfer roller 18 via a timing roller pair 17. As a result, the color image is secondarily transferred from the intermediate transfer belt 10 by the electric field applied from the secondary transfer roller 18. Thereafter, the sheet is conveyed to the fixing unit 19 where the toner is heated and fixed, and is discharged from the discharge roller pair 5 to the upper surface of the main body.

(Configuration of secondary transfer unit, see FIG. 2)
In the secondary transfer portion, as schematically shown in FIG. 2, the intermediate transfer belt 10 and the secondary transfer roller 18 are transported in the direction of arrow b with the sheet S interposed therebetween, and from the transfer roller 18 to the intermediate transfer belt 10. A bias DC voltage (positive electric field with respect to the negative polarity of the toner) is applied toward the toner, and the toner T is transferred onto the paper S by this electric field. The intermediate transfer belt 10 is grounded via a support roller 11.

  A DC power supply 20 is connected to the transfer roller 18, and the DC power supply 20 is controlled at a constant voltage by a voltage control unit 21. Further, a current detector 22 that detects a current flowing from the DC power supply 20 to the transfer roller 18 as a transfer current is connected, and an output of the current detector 22 is input to the voltage control unit 21. A voltage detector 23 is connected between the output unit of the DC power supply 20 and the ground, and the output of the voltage detector 23 is input to the recording unit 24. The storage unit 25 stores a plurality of predetermined upper limit values and a plurality of predetermined lower limit values of the transfer current. In the present embodiment, the first upper limit value, the second upper limit value, and the first lower limit value are stored. And a second lower limit value.

  Hereinafter, the control of the secondary transfer voltage during the printing operation (first and second sheets) will be described with reference to FIG. In FIG. 3, the horizontal axis indicates the time (ms) after the start of the printing operation, the left vertical axis indicates the transfer current (μA), and the right vertical axis indicates the transfer voltage (V). During the period shown in FIG. 3, the transfer roller 18 continues to press / rotate against the intermediate transfer belt 10.

The period of the operation 1 shown in FIG. 3 is a start-up period, and a printing signal is issued to start the operation. However, the sheet S is not yet conveyed to the secondary transfer unit. In operation 1, normal constant current control is performed, and resistance unevenness in the circumferential direction of the transfer roller 18 is detected to select an upper limit value and a lower limit value for control.

In action 1,
(1) The transfer roller 18 is pressed / rotated to the intermediate transfer belt 10 and the supply of constant current from the DC power source 20 is started.
(2) A voltage equal to or more than one turn in the circumferential direction of the transfer roller 18 is detected, and the detected voltage is recorded in the recording unit 24.
(3) The supply of constant current is terminated while the transfer roller 18 is pressed / rotated.
(4) The average value of the detection voltages is calculated, and the bias voltage output from the DC power supply 20 during the transfer operation is determined.
The above is the constant current control normally performed.

(5) The differences ΔV1 and ΔV2 between the maximum value and minimum value of the detection voltage and the average value are calculated. ΔV1 and ΔV2 indicate resistance unevenness.
ΔV1 = Maximum voltage value−Average value ΔV2 = Average value−Minimum voltage value (6) The storage unit 25 includes the table shown in Table 1 below, and is based on Table 1 according to the values of ΔV1 and ΔV2. Select an upper limit and one lower limit. For example, when ΔV1 is larger than a predetermined value (10 in this example) and ΔV2 is larger than a predetermined value, the first lower limit value (see FIG. 3) and the first upper limit value are selected from Table 1. In FIG. 3, since the upper limit value is not used in the control, the illustration is omitted.

  The period of operation 2 shown in FIG. 3 is a secondary transfer process for the first sheet, and is a period during which the first sheet is conveyed through the secondary transfer unit. In the operation 2, the secondary transfer current is controlled based on the upper limit value and the lower limit value selected in the operation 1.

In action 2,
(1) The bias voltage determined in the operation 1 is applied to the transfer roller 18 by constant voltage control.
(2) While detecting the transfer current, when the detected current exceeds the first lower limit value, a constant voltage is maintained, and when the detected voltage becomes smaller than the first lower limit value, the bias voltage becomes equal to or higher than the first lower limit value. Raise to be.
(3) The control in (2) is continued until the first sheet is passed.
(4) The application of the bias voltage is terminated simultaneously with the end of the transfer of the first sheet.

  Incidentally, in FIG. 3, the hatched area is an appropriate transfer current area, and it is necessary to control the transfer current within this area.

The period of the operation 3 shown in FIG. 3 is a sheet passing interval period (between the first sheet and the second sheet), normal constant current control is performed, and uneven resistance in the circumferential direction of the transfer roller 18 is performed. And an upper limit value and a lower limit value for control are selected.

In action 3,
(1) Application of a constant current to the transfer roller 1 is started.
(2) A voltage equal to or more than one turn in the circumferential direction of the transfer roller is detected, and the detected voltage is recorded in the recording unit 24.
(3) The supply of constant current is terminated while the transfer roller 18 is pressed / rotated.
(4) The average value of the detection voltages is calculated, and the bias voltage output from the DC power supply 20 during the transfer operation is determined.
(5) The differences ΔV1 and ΔV2 between the maximum value and minimum value of the detection voltage and the average value are calculated. ΔV1 and ΔV2 indicate resistance unevenness as described above.
(6) With reference to the table shown in Table 1, one upper limit value and one lower limit value are selected according to the values of ΔV1 and ΔV2. For example, when ΔV1 and ΔV2 are equal to or less than a predetermined value, the second lower limit value (see FIG. 3) and the second upper limit value are selected from Table 1. In FIG. 3, since the upper limit value is not used in the control, the illustration is omitted.

  The period of the operation 4 shown in FIG. 3 is a secondary transfer process for the second sheet, and is a period during which the second sheet is conveyed through the secondary transfer unit. In the operation 4, the secondary transfer current is controlled based on the upper limit value and the lower limit value selected in the operation 3.

In action 4,
(1) The bias voltage determined in the operation 1 is applied to the transfer roller 18 by constant voltage control.
(2) While detecting the transfer current, a constant voltage is maintained when the detected current exceeds the second lower limit value, and when the detected voltage becomes smaller than the second lower limit value, the bias voltage is equal to or higher than the second lower limit value. Raise to be.
(3) The control in (2) is continued until the second sheet is passed.
(4) The application of the bias voltage is finished simultaneously with the end of the transfer of the second sheet.

  If the printing process is finished, the control of the transfer current is also finished. If the third and subsequent sheets are printed, the operations 3 and 4 are repeated.

  As described above, in this embodiment, the detected voltage in the circumferential direction of the transfer roller 18 detected by the voltage detector 23 is recorded on the recording unit 24 before entering the transfer operation. At the same time, one upper limit value and one lower limit value are selected from the storage unit 25 based on the detected voltage. During the transfer operation, when the transfer current detected by the current detector 22 is larger than the selected upper limit value, the DC power source 20 is controlled so that the transfer current is equal to or lower than the upper limit value. When the transfer current detected by the current detector 22 is smaller than the selected one lower limit value, the DC power supply 20 is controlled so that the transfer current becomes equal to or more than the one lower limit value. As a result, it is possible to cope with the resistance unevenness in the circumferential direction of the transfer roller 18, and in particular, the resistance unevenness of the transfer roller 18 at the time of printing start-up and the resistance unevenness during continuous printing can be well determined. Voltage control is possible. As a result, toner scattering and printing bleeding are prevented.

(Other examples)
Note that the image forming apparatus according to the present invention is not limited to the above-described embodiment, and can be variously modified within the scope of the gist thereof.

  In particular, the overall configuration of the image forming apparatus is arbitrary, and may be a 4-cycle system in addition to the tandem system. In addition to the intermediate transfer belt, the image carrier as a transfer source of the toner image may be a photosensitive drum.

As described above, the present invention is useful for an image forming apparatus, and is particularly excellent in that preferable constant current control corresponding to uneven resistance in the circumferential direction of the transfer roller is possible.

DESCRIPTION OF SYMBOLS 10 ... Intermediate transfer belt 18 ... Secondary transfer roller 20 ... DC power supply 21 ... Voltage control part 22 ... Current detector 23 ... Voltage detector 24 ... Recording part 25 ... Storage part

Claims (1)

An image carrier for carrying a toner image;
A transfer roller that conveys the recording medium together with the image carrier;
Voltage applying means for applying a bias voltage to the transfer roller;
Voltage control means for performing constant current control on the voltage application means;
Voltage detection means for detecting the voltage of the transfer roller;
Current detection means for detecting the current of the transfer roller;
Recording means for recording the voltage value detected when the transfer roller is rotated by the voltage detection means;
Storage means for storing a plurality of predetermined upper limit values and a plurality of predetermined lower limit values of the transfer current flowing through the transfer roller;
With
At the time of image formation, before entering the transfer operation, the detection voltage in the circumferential direction of the transfer roller detected by the voltage detection unit is recorded on the recording unit, and the maximum value, minimum value, and average value of the detection voltage are recorded. And select one upper limit value and one lower limit value from the storage means according to the difference ,
During image formation, during the transfer operation, when the transfer current detected by the current detection unit is larger than the selected upper limit value, the voltage is set so that the transfer current is less than or equal to the upper limit value. The voltage applying means controls the applying means and, when the transfer current detected by the current detecting means is smaller than the selected one lower limit value, the voltage applying means so that the transfer current becomes not less than the one lower limit value. To control,
An image forming apparatus.

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