JP5183323B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that transfers a toner image by applying a constant current to a transfer unit, and more specifically, a change in transfer efficiency when the leading edge of a recording material reaches a static elimination brush disposed downstream of a fixing device. It is related with the control which suppresses.

  Recording applied to the transfer portion formed between the image carrier (photoreceptor or intermediate transfer member) and the transfer member by applying a constant current controlled voltage to the transfer portion over the image carrier. Image forming apparatuses that transfer a toner image from an image carrier to a material are widely used.

  In such an image forming apparatus, the recording material onto which the toner image has been transferred by the transfer unit is sent to the fixing device and is heated and pressurized to fix the toner image on the surface. The recording material output from the fixing device is charged either positively or negatively according to the balance between the charged charge of the transferred toner image and the transferred charge injected into the recording material at the time of transfer. The subsequent smooth transportation and loading are hindered. For this reason, a neutralizing brush connected to the ground potential is disposed downstream of the fixing device to rub the image surface of the recording material, thereby removing unnecessary charges and reducing the charged state of the recording material.

  Patent Document 1 discloses an image forming apparatus that applies a constant current controlled voltage to a transfer member to transfer a toner image from a photosensitive drum to a recording material.

  Patent Document 2 discloses an image forming apparatus in which a static eliminating brush is disposed adjacent to an outlet of a fixing device.

  In Patent Document 3, at the timing when the recording material reaches the fixing roller to which a voltage is applied, the constant current control of the voltage applied to the transfer unit is switched to the constant voltage control using the voltage value taken in at the time of constant current control. An image forming apparatus is shown.

JP 2006-220974 A Japanese Patent Laid-Open No. 9-269685 JP 2006-153932 A

  In recent years, with the miniaturization of image forming apparatuses, the fixing apparatus has also been miniaturized, and products having a distance of 100 mm to 150 mm from the transfer unit to the static eliminating brush disposed downstream of the fixing apparatus have been developed.

  In such an image forming apparatus having a short distance from the transfer unit to the neutralization brush, the transfer of the toner image continues on the rear end side of the same recording material for a while after the leading edge of the recording material passes through the neutralization brush. .

  At this time, if the resistance value in the conveyance direction of the recording material is low corresponding to thick paper, colored paper using a conductive pigment, high humidity environment, etc., a part of the current to be supplied to the transfer part flows through the recording material. Leak from the static elimination brush to ground potential. As a result, before and after the leading edge of the recording material reaches the static elimination brush, the effective transfer current that actually flows through the transfer portion and participates in the movement of the toner image changes abruptly, and the transfer efficiency changes discontinuously. Therefore, an image density step is formed in the recording material conveyance direction.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus in which a change in transfer efficiency before and after the leading edge of a recording material reaches a static elimination brush is suppressed and an image density level difference is unlikely to appear in the recording material conveyance direction. .

In the image forming apparatus of the present invention, a toner image is formed between an image forming unit for forming a toner image and supporting the image carrier, and a transfer portion between the image carrier and the toner image from the image carrier to a recording material. The image forming apparatus includes a transfer member to be transferred, and a contact member that is connected to a ground potential and contacts a recording material to which a toner image is transferred and separated from the image carrier. A power source capable of switching between a constant-current controlled output voltage and a constant-voltage controlled output voltage to be output to the transfer unit, a detection unit for detecting an instantaneous current value flowing through the transfer member, and the transfer unit Control for controlling the power source so as to perform the constant current control for each position in the conveyance direction of the recording material, using a current value for each position in the conveyance direction of the recording material detected by applying a constant voltage to Means.

  In the image forming apparatus of the present invention, in the constant current control until the leading edge of the recording material contacts the contact member during the period when the leading edge of the recording material contacts the contact member and the current leaks to the ground potential through the contact member. Also increases the current output from the power supply. It is not only simply increased, but is increased so that the change of the output voltage after the recording material reaches the contact member relative to the constant current controlled output voltage before the recording material reaches the contact member is suppressed. As a result, the change in the effective transfer current that actually flows through the transfer portion and is involved in the movement of the toner image is suppressed.

  Therefore, a change in transfer efficiency due to a difference in effective transfer current before and after the leading edge of the recording material reaches the static elimination brush is suppressed, and an image density step is less likely to appear in the recording material conveyance direction.

  Hereinafter, some embodiments of the present invention will be described in detail with reference to the drawings. The present invention replaces part or all of the configuration of the embodiment as long as the current supplied to the transfer unit increases after reaching the contact member than the constant current control before the leading edge of the recording material reaches. Other embodiments replaced with a typical configuration can also be implemented.

  Accordingly, the contact roller may be a conveyance roller and a conveyance guide that are arranged downstream of the fixing device and directly connected to the ground potential, and that are connected to the ground potential through a medium to high resistance.

  In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general matter of the image forming apparatus shown by patent documents 1-3, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted. In addition, the reference symbols in parentheses shown in the configuration names used in the claims are examples for assisting understanding of the invention, and are not intended to limit the configuration to the corresponding members in the embodiments. Absent.

<Image forming means>
FIG. 1 is an explanatory diagram of a configuration of the image forming apparatus according to the first embodiment.

  As shown in FIG. 1, the image forming apparatus 100 is a tandem full-color copying machine in which image forming portions Pa, Pb, Pc, and Pd of yellow, magenta, cyan, and black are arranged along the intermediate transfer belt 1.

  In the image forming portion Pa, a yellow toner image is formed on the photosensitive drum 11 a and is primarily transferred to the intermediate transfer belt 1. In the image forming portion Pb, a magenta toner image is formed on the photosensitive drum 11b, and is primarily transferred onto the yellow toner image on the intermediate transfer belt 1. In the image forming portions Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 11c and 11d, respectively, and similarly, the toner images on the intermediate transfer belt 1 are sequentially superimposed and sequentially transferred.

  The four-color toner images carried on the intermediate transfer belt 1 are conveyed to the secondary transfer portion T2 as shown in FIG. 2, and the recording material is nipped and conveyed on the intermediate transfer belt 1 at the secondary transfer portion T2. Batch transfer to P. As shown in FIG. 1, the recording material P on which the toner image has been secondarily transferred is separated from the intermediate transfer belt 1 by the curved surface of the separation roller 1b and sent to the fixing device 17, and is heated and pressurized to the surface. The toner image is fixed. In this way, the recording material P on which the full-color image is formed is discharged by the discharge brush 19 and discharged onto the discharge tray 18 by the discharge roller 19 and stacked.

  The image forming portions Pa, Pb, Pc, and Pd are substantially the same except that the toner colors used in the attached developing devices 14a, 14b, 14c, and 14d are different from yellow, magenta, cyan, and black. Hereinafter, the image forming unit Pa will be described, and the other image forming units Pb, Pc, and Pd will be described by replacing “a” at the end of the reference numerals with “b”, “c”, and “d”.

  In the image forming section Pa, a charging roller 12a, an exposure device 13a, a developing device 14a, a primary transfer roller 15a, and a cleaning device 16a are arranged around the photosensitive drum 11a.

  The photosensitive drum 11a is formed with an organic photoconductor layer (OPC) having a negative polarity on the outer peripheral surface of an aluminum cylinder, and rotates in the direction of arrow R1 at a process speed of 100 mm / sec.

  The charging roller 12a is driven to rotate while being pressed against the photosensitive drum 11a in a state where an oscillating voltage obtained by superimposing an alternating voltage on a direct current voltage is applied, and charges the surface of the photosensitive drum 11a to a uniform dark negative potential VD. .

  The exposure device 13a scans the scanning line image data obtained by developing the yellow separation color image with a rotating mirror, and writes an electrostatic image of the image on the surface of the charged photosensitive drum 11a. The intensity of the laser beam and the irradiation spot diameter are appropriately set according to the resolution of the image forming apparatus 100 and the desired image density. In the electrostatic image formed on the photosensitive drum 11a, the portion irradiated with the laser beam has the bright portion potential VL (about −100 V), and the portion not irradiated with the dark portion potential VD (about −700 V).

  The developing device 14a stirs and charges the two-component developer, and holds the charged two-component developer in a spiked state on the developing sleeve S that rotates in the counter direction around the fixed magnetic pole. The drum 11a is rubbed. By applying an oscillating voltage, in which an AC voltage is superimposed on a negative DC voltage, to the developing sleeve S, the electrostatic image of the photosensitive drum 11a having a relatively positive polarity relative to the developing sleeve S is negatively charged. Move to reversely develop the electrostatic image.

The primary transfer roller 15a is configured by covering an elastic layer having a medium resistance (volume resistivity of 10 ⁴ to 10 ¹⁰ Ωcm) on the central axis, and is driven to rotate while being in pressure contact with the intermediate transfer belt 1 to be intermediate between the photosensitive drum 11a and the intermediate transfer roller 15a. A primary transfer portion is formed between the transfer belt 1 and the transfer belt 1. The primary transfer roller 15 a is applied with a positive DC voltage having a polarity opposite to the charging characteristic of the toner image, thereby charging the toner image negatively charged and carried on the photosensitive drum 11 a to the intermediate transfer belt 1. Make primary transfer.

  The cleaning device 16 a slides a cleaning blade on the photosensitive drum 11 a to remove transfer residual toner that is not transferred to the intermediate transfer belt 1 and adheres to the photosensitive drum 11 a.

<Image carrier>
The intermediate transfer belt 1 is formed by forming a surface layer of 10 ¹³ Ωcm fluororesin with a thickness of 20 μm on a 0.5 mm thick base layer in which carbon is dispersed in hydrin rubber and the volume resistivity is adjusted to 10 ⁷ Ωcm. Composed.

  The intermediate transfer belt 1 is supported around a driving roller 1a, a separation roller 1b, and a tension roller 1c, is driven by the driving roller 1a, and rotates in the arrow R2 direction at a process speed of 100 mm / second. The tension of the intermediate transfer belt 1 is preferably set so that the elongation rate is within 1% so that the belt material does not break or become permanently set. Here, the tension is set to 150 N. It is.

The base layer of the intermediate transfer belt 1 is a material in which volume resistivity is adjusted to about 10 ⁶ to 10 ¹² Ωcm by dispersing carbon or conductive powder on a base material of urethane resin, fluorine resin, nylon resin, or polyimide resin. Can be used. As the surface layer, an elastic layer of an elastic material such as silicon rubber or hydrin rubber may be formed.

The secondary transfer roller 3 is configured by covering the outer periphery of a metal central shaft 3a with an elastic layer 3b of an EPDM foam layer having a resistance value of medium resistance (volume resistivity 10 ⁴ to 10 ¹⁰ Ωcm). The secondary transfer roller 3 is pressed against the intermediate transfer belt 1 formed from a metal cylinder and supported from the inside by a separation roller 1b connected to the ground potential, and between the intermediate transfer belt 1 and the secondary transfer roller 3. Then, the secondary transfer portion T2 is formed.

  The power source D2 applies a positive voltage having a polarity opposite to the charging polarity of the toner image to the secondary transfer roller 3, and feeds the toner image carried on the intermediate transfer belt 1 to the secondary transfer portion T2. Electrostatically transferred to the recording material P.

  As the primary transfer of each color toner image to the intermediate transfer belt 1 progresses, the recording material P is taken out from the recording material storage cassette 20 one by one by the paper feeding device 21, and is inserted into the secondary transfer portion T2 by the registration roller 22. Is done.

  The fixing device 17 presses the pressure roller 24 heated from the inside by the heater 24 h to the fixing roller 23 heated from the inside by the heater 23 h to form a high-temperature and high-pressure fixing nip for heating and pressing the recording material P. To do. In the fixing roller 23, an elastic layer 23b of silicon rubber is provided on the outer periphery of a base material layer 23a of an aluminum pipe, and a release layer of a fluororesin material is formed on the surface of the elastic layer 23b. In the pressure roller 24, an elastic layer 24b of silicon rubber is provided on the outer periphery of a base material layer 24a of an aluminum pipe, and a release layer of a fluororesin material is formed on the surface of the elastic layer 24b. Since the surfaces of the fixing roller 23 and the pressure roller 24 are insulative (or high resistance), the charge-removing rollers 23j and 24j connected to the ground potential are contacted and rotated on the downstream side of the fixing nip to remove electricity. Yes.

  The discharge roller 19 and the registration roller 22 are high resistance silicon rubber rollers.

The neutralizing brush 25 is connected to the ground potential and linearly contacts and rubs the recording material P to which the toner image has been transferred and separated from the intermediate transfer belt 1, thereby charging the recording material P. Remove static electricity. The neutralizing brush 25 uses conductive rayon fibers, which are conductive fibers, and the fiber thickness is 6 denier, the pile length is 5 mm, and the fiber density is 100,000 / inch ² . The distance L25 from the secondary transfer portion T2 to the static elimination brush 25 is set to 30 mm to 200 mm in order to reduce the size of the image forming apparatus 100. Here, the distance from the secondary transfer portion T2 to the static elimination brush 25 is set. Is 120 mm.

  The neutralizing brush 25 neutralizes the recording material P discharged to the paper discharge tray 18 by the discharge roller 19, thereby removing electrostatic sticking and curling of the recording material P stacked on the paper discharge tray 18. For this reason, it is directly connected to the ground potential so that the static elimination function is not hindered or the static elimination performance cannot be sufficiently exhibited.

  The belt cleaning device 18 slides the cleaning blade 18a on the intermediate transfer belt 1 to remove the transfer residual toner attached to the intermediate transfer belt 1 that has passed through the secondary transfer portion T2, and stores it in the waste toner box 18b.

<Constant current control>
In an image forming apparatus using a transfer member such as a transfer roller, the transfer voltage applied to the transfer member is controlled by constant voltage control or constant current control. However, the constant voltage control has the following drawbacks.

  As the transfer roller used as a transfer member, a roller having conductive particles dispersed in rubber and appropriately adjusting volume resistance is used, but this kind of substance has a resistance value of several orders of magnitude depending on the environmental temperature. Change across. For this reason, the current value that actually flows through the transfer portion and is involved in the movement of the toner image changes greatly according to the environmental temperature. That is, when a constant voltage is suitably set in an environment of normal temperature and normal humidity (N / N: temperature 23 ° C., relative humidity 60%), in a low temperature and low humidity environment (L / L: temperature 15 ° C., humidity 10%), the transfer roller Since the resistance value becomes large, transfer failure occurs.

  In order to eliminate such drawbacks, “ATVC (Active Transfer Voltage Control) type constant voltage control has been proposed. In the ATVC type, constant current control is performed during non-image formation where no recording material is present in the transfer portion. The output voltage value of the power supply is held, and at the time of image formation, the power supply is controlled at a constant voltage so that the held output voltage value is output, but the ATVC type constant voltage control has the following problems.

  Since the ATVC method uses an output voltage that is controlled at a constant current in the absence of the recording material P, the resistance value of the recording material is not reflected or is reflected as a constant value in the output voltage. For this reason, when the resistance value of the recording material fluctuates (changes), the current that actually flows to the transfer portion and is involved in the transfer of the toner image changes.

  When a constant voltage is set appropriately for a recording material having a high resistance value (for example, dry paper), when a recording material having a low resistance value (for example, paper with a high water content) is passed through the transfer portion, Excessive current flows through the transfer section, resulting in transfer failure. On the contrary, when a constant voltage is appropriately set for a recording material having a low resistance value, a transfer failure due to insufficient charge occurs in the recording material having a high resistance value.

  For this reason, in the image forming apparatus 100, a constant current-controlled voltage is output from the power source D <b> 2 to the secondary transfer roller 3, and the above-described defects due to changes in the resistance values of the secondary transfer roller 3 and the recording material P as described above. This eliminates the amount of charge necessary for transfer at all times.

  However, since the image forming apparatus 100 is designed to achieve a reduction in size, the recording material P has a trailing end at the fixing device 17, a discharge roller 19, and a static elimination device while the recording material P passes through the secondary transfer portion T <b> 2. It rushes into and comes into contact with the brush 25. Further, the rear end side of the recording material P is simultaneously held between the registration rollers 22 while the recording material P passes through the secondary transfer portion T2.

In recent years, recording materials used in electrophotographic image forming apparatuses are diversified, and recording materials having a wide range of resistance values such as 10 ⁸ Ω · cm to 10 ¹⁷ Ω · cm are used. The

  Further, the resistance value of the recording material is greatly affected by the influence of moisture in the air. Specifically, there is a resistance fluctuation of 5 digits or more in a transition from a low temperature and low humidity environment with an air temperature of 15 ° C. and a humidity of 10% to a high temperature and high humidity environment of an air temperature of 32 ° C. and a humidity of 80%. As a result, in a low resistance paper in a high humidity environment, a part of the current that should flow through the secondary transfer portion T2 is transmitted in the transport direction of the recording material P and leaks from the static elimination brush 25 to the ground potential. As a result, a current shortage occurs at the secondary transfer portion T2, and the toner image carried on the intermediate transfer belt 1 is not properly transferred to the recording material P.

  Such a phenomenon becomes more conspicuous by downsizing the image forming apparatus (shortening the distance L25) and by simultaneously contacting other rollers and a charge removal member. However, in order to solve these problems, it is not possible to adopt a method in which the members of the conveyance path that are simultaneously in contact with each other during transfer are grounded via a high resistance or varistor. As a result, each member is charged up, and it is easy to cause image defects, JAM due to adsorption, and the like.

  In the image forming apparatus 100, by controlling the power supply D2 so that the change in the output voltage before and after the recording material reaches the contact member is controlled, the image defect due to insufficient voltage at the transfer portion is solved without any problem.

<Example 1>
2 is an explanatory diagram of the configuration in the control of the first embodiment, FIG. 3 is a diagram of the output voltage of the power source through the control of the first embodiment, and FIG. 4 is a diagram of the output voltage when the constant current control is performed to the end. It is. In the first embodiment, the constant current control is switched to the constant voltage control using the voltage value output by the constant current control until the recording material reaches the contact member. Thereby, when transferring the toner image to the recording material P, the transfer control is changed before and after contacting the member in the apparatus that is simultaneously in contact, thereby preventing image defects and transfer omission due to leakage current.

As shown in FIG. 2, in the image forming apparatus 100, 300 sheets of A4-sized recording material P having low resistance in a high temperature and high humidity environment were continuously formed. The recording material P uses color laser copier paper manufactured by Canon Inc., and its volume resistivity is 1 × 10 ¹⁰ Ωcm and high temperature and high temperature in a normal temperature and normal humidity (temperature 23 ° C., humidity 60%) environment. The volume resistivity under humidity (H / H: temperature 32 ° C., humidity 80%) was 1 × 10 ¹⁰ Ωcm. The optimum current value involved in the transfer of the toner image through the secondary transfer portion T2 was 20 μA.

  Since the distance from the secondary transfer portion T2 to the neutralization brush 25 is 120 mm and the process speed is 100 mm / second, the recording material P contacts the neutralization brush 25 1200 msec after passing through the secondary transfer portion T2.

  Before the leading edge of the recording material P contacts the static elimination brush 25, a single current path (path A) flows through the resistance R3 of the secondary transfer roller 3 + the resistance Rp in the thickness direction of the recording material P + the resistance R1 of the intermediate transfer belt 1. Is formed.

  On the other hand, after the leading end of the recording material P comes into contact with the static elimination brush 25, another current path (path B) that flows through the resistance R1 of the secondary transfer roller 3 and the resistance Rh in the conveyance direction of the recording material P is formed. Is done. For this reason, the resistance value from the resistance R3 of the secondary transfer roller, which is the load of the power supply D2, to the ground potential is lowered.

  As shown in FIG. 3 with reference to FIG. 2, in the control of the first embodiment, the control unit 110 starts constant current control of the power supply D2 100 msec before the recording material P enters the secondary transfer unit T2. . When the recording material P enters the secondary transfer portion T2, a resistance Rp in the thickness direction of the recording material P is added, so that the output voltage of the power source D2 increases from 1.5 kV to 2.0 kV so as to keep the current value at 20 μA. To do.

  Thereafter, when the recording material P is conveyed 120 mm and the leading end contacts the static eliminating brush 25 (after 1200 msec from entering), the control unit 110 switches the constant current control so far to the constant voltage control. Specifically, the control unit 110 captures the output voltage value of the power source D2 after 1100 msec from the rush through the voltage detection circuit V2, sets the captured output voltage to the power source D2, and starts constant voltage control 1200 msec after the rush. I let you.

  In this case, since the output voltage value after 1100 msec from the rush was 2.0 kV, a constant voltage of 2.0 kV is applied from the power source D2 to the secondary transfer roller 3, and the current value flowing into the path A and the path B is It has risen from 20 μA to 40 μA.

  That is, the control unit 110 maintains the voltage value (2.0 kV) applied to the path A before the neutralization brush 25 is contacted, so that even if another parallel path B is formed after the neutralization brush 25 is contacted, the path A The current value flowing through is secured to 20 μA. As a result, a good image in which transfer omission due to insufficient transfer current did not occur was obtained.

  In the control of the first embodiment, since the constant current control up to that time is switched to the constant voltage control so as to compensate the current value leaking to the ground potential through the recording material P such as the static elimination brush 25, a good image with no transfer omission is obtained. It is possible to obtain. Further, even for various recording materials P whose resistance values vary over a wide range, it is possible to optimally secure a current value related to the transfer of the toner image through the secondary transfer portion.

  As shown in FIG. 4 with reference to FIG. 2, when the constant current control of 20 μA is continued to the end, the current value flowing from the power source D2 to the resistance R3 of the secondary transfer roller is determined by the recording material P as the secondary transfer portion T2. Is constant at 20 μA throughout.

  However, as can be read from the area B of the diagram, the output voltage of the power source D2 rapidly decreases when the leading edge of the recording material P comes into contact with the static elimination brush 25. This is because a current flows in the conveyance direction in the recording material P whose resistance has been reduced and leaks from the static elimination brush 25 to the ground potential.

  The output voltage of the power supply D2 decreases from 1/2 to 2.0 kV → 1.0 kV before and after coming into contact with the static elimination brush 25. Therefore, the leakage current through the static elimination brush 25 is constant that the power supply D2 outputs. It accounts for about 10 μA of half of the current.

  For this reason, only a current of 10 μA, which is about half of the appropriate current of 20 μA, flows through the path A involved in the transfer of the toner image through the recording material P and the intermediate transfer belt 1 from the secondary transfer roller 3 in the thickness direction. As a result, image omission due to insufficient transfer current occurred from a position 120 mm from the front end of the recording material P to the rear end.

<Example 2>
FIG. 5 is an explanatory diagram of a configuration in the control of Example 2, FIG. 6 is a diagram of a change in current value measured on the first recording material, and FIG. 7 is a diagram illustrating constant current control in the second and subsequent recording materials. FIG. In the second embodiment, a detection unit that detects a current value that flows through the transfer member is provided, and a current value for each position in the transport direction of the preceding recording material that is detected by applying a constant voltage to the transfer unit is used. Perform constant current control. Thereby, by determining the current value of the second and subsequent sheets based on the current history acquired by the constant voltage control of the first sheet, it is possible to prevent image failure and transfer omission due to leakage current.

  As shown in FIG. 5, similarly to Example 1, 300 sheets of A4-sized recording material P with reduced resistance are continuously formed in the image forming apparatus 100, and flow through the secondary transfer portion T <b> 2. The optimum current value involved in the transfer of the toner image is 20 μA. Since the distance from the secondary transfer portion T2 to the neutralization brush 25 is 120 mm and the process speed is 100 mm / second, the recording material P comes into contact with the neutralization brush 25 after 1200 msec from the secondary transfer portion T2.

  In the control of the second embodiment, the control unit 110 uses the current detection circuit A2 to measure a current profile along the conveyance direction of the recording material P through image formation of the preceding first recording material P. Then, in the image formation on the second and subsequent recording materials P, the control unit 110 sets constant constant current to the power source D2 along the measured current profile to perform constant current control.

  The control of the second embodiment is different from the control of the first embodiment in that the control method of the voltage applied to the secondary transfer roller 3 is switched between the first sheet and the second and subsequent sheets in a continuous image forming job. .

  Prior to the first image formation, ATVC control is performed to obtain a constant voltage to be set for the power source D2 in the first image formation.

Specifically, the control unit 110 sets a constant current of 20 μA to the power source D2 one second before the start of continuous image formation, controls the voltage applied to the secondary transfer roller 3, and outputs the output voltage at that time. The value V0 is taken in through the voltage detection circuit V2. The control unit 110 adds the recording material shared voltage Vp to the voltage value VO that can ensure a current of 20 μA in the secondary transfer unit T2 without the recording material P, and thereby obtains a constant voltage V1 that is used in image formation for the first sheet. Ask. The recording material sharing voltage Vp is prepared in advance in the storage device 109 as a data table for each combination of the type of the recording material P and the temperature and humidity.
V1 = V0 + Vp

The voltage value V0 captured in Example 2 was 1.5 kV, and the selected recording material sharing voltage Vp was 0.5 kV.
V1 = V0 + Vp = 1.5 kV + 0.5 kV = 2 kV

  As in the first embodiment, the control unit 110 controls the power source D2 at a constant voltage of 2.0 kV and controls the power supply D2 at a constant voltage of 100 msec before the recording material P enters the secondary transfer portion T2. An image is formed up to the rear end of P.

  As shown in FIG. 6 with reference to FIG. 5, the control unit 110 performs the secondary transfer after the leading end of the first recording material P has entered the secondary transfer unit T2 through the current detection circuit A2. The current history up to exiting the part T2 is acquired and recorded in the storage device 109.

  The control unit 110 sets the constant current of the power source D2 every 10 msec for the second and subsequent recording materials P so that the current history in the first recording material P held in the storage device 109 is reproduced. The set value is changed, and the voltage applied to the secondary transfer roller 3 is controlled with constant current.

  Here, the current value detected every 10 msec after the leading edge of the first recording material P enters the secondary transfer portion T2 is I0, and the current value detected every time 10 msec elapses thereafter. Are I1, I2, I3, I4,.

Then, the ratios r1, r2, r3, r4,... Between the current values I1, I2, I3, I4,... And the current value I0 are calculated and recorded in the storage device 109.
r1 = I1 / I0
r2 = I2 / I0
r1 = I3 / I0
・
・
・

As shown in FIG. 7 with reference to FIG. 5, in the second and subsequent recording materials P, the control unit 110 sets the optimum current value to Ip = 20 μA and sets the constant current control value Iq to a predetermined value as shown in the following equation. Switch every moment (10 msec).
Iq = Ip × rn n = 1, 2, 3,...

  By performing such control, the resistance change of the system in the recording material P to be passed through the first preceding sheet, specifically, the change in the resistance value of the path B shown in FIG. It is possible to secure the optimum current value according to the second and subsequent recording materials P.

  Therefore, it is possible to correct a minute leakage current through the path B, and it is possible to always keep the current value flowing through the path A which is a normal current path. According to the control of Example 2, there was no occurrence of image defect due to transfer omission, and a good image could be obtained.

<Example 3>
FIG. 8 is an explanatory diagram of a configuration in the control of the third embodiment, and FIG. 9 is an explanatory diagram of switching between constant current control and constant voltage control viewed on the recording material. In the third embodiment, there is provided a detecting means for detecting an instantaneous current value flowing to the ground potential through the contact member, and constant current control after reaching the contact member is performed using a current value obtained by increasing the detected current value. Thereby, an image forming apparatus capable of forming a high quality image can be provided.

  As shown in FIG. 9 with reference to FIG. 8, the control unit 110 sets the optimum transfer current value 20 μA in the power source D2, and performs constant current control from 100 msec before the recording material P enters the secondary transfer unit T2. Let it begin.

  The controller 110 acquires the output voltage value of the power source D2 through the voltage detection circuit V2 from 90 msec to 100 m after the recording material P enters the secondary transfer portion T2, and obtains an average value. The obtained average value is set as a constant voltage in the power source D2, and the power source D2 is controlled at a constant voltage from 110 msec to 150 msec after the recording material P enters the secondary transfer portion T2.

  Thereafter, the control unit 110 takes in the leakage current that leaks to the ground potential through the static elimination brush 25 through the leakage current detection circuit A25 from 130 msec to 140 msec after the recording material P enters the secondary transfer portion T2. Ask for. Then, a current value obtained by adding the obtained average value to the optimum transfer current 20 μA is set as a constant current leakage pressure in the power source D2, and from 150 msec to 220 msec after the recording material P enters the secondary transfer portion T2, The power source D2 is controlled at a constant current.

  As shown in FIG. 9, constant voltage control is applied in a section before and after the leading edge of the recording material P reaches the static elimination brush 25, so that leakage leaks to the ground potential through the static elimination brush 25 as in the first embodiment. Current is automatically taken from the power supply D2. For this reason, a density step is not formed at the position of the length L25 due to insufficient transfer current and lower transfer efficiency.

  In addition, since the actual leakage current is measured in a state where the recording material P is in contact with the static elimination brush 25 and the constant current value obtained by adding the leakage current is set, the transfer current is insufficient even after the constant voltage control is finished. A density step due to a decrease in transfer efficiency is not formed.

It is explanatory drawing of a structure of the image forming apparatus of 1st Embodiment. It is explanatory drawing of the structure in the control of Example 1. FIG. It is a diagram of the output voltage of the power supply through the control of Example 1. It is a diagram of the output voltage at the time of performing constant current control to the last. It is explanatory drawing of the structure in control of Example 2. FIG. It is a diagram of a change in current value measured with the first recording material. It is a diagram of constant current control in the second and subsequent recording materials. It is explanatory drawing of the structure in the control of Example 3. FIG. It is explanatory drawing of switching of the constant current control and constant voltage control which were seen on the recording material.

Explanation of symbols

1 Image carrier (intermediate transfer belt)
1a Drive roller 1b Transfer member (separation roller)
1c Tension roller 3 Transfer member (secondary transfer roller)
11a, 11b, 11c, 11d Photosensitive drums 12a, 12b, 12c, 12d Charging rollers 13a, 13b, 13c, 13d Exposure devices 14a, 14b, 14c, 14d Development device 17 Fixing device 19 Discharge roller 25 Static elimination brush

Claims (3)

Image forming means for forming a toner image and carrying the toner image on an image carrier;
A transfer member for forming a transfer portion between the image carrier and transferring a toner image from the image carrier to a recording material;
A contact member that is connected to a ground potential and contacts a recording material to which a toner image is transferred and separated from the image carrier;
A power supply capable of switching the output voltage controlled by constant current and the output voltage controlled by constant voltage to output to the transfer unit ;
Detecting means for detecting a current value flowing through the transfer member;
Using the current value for each position in the transport direction of the preceding recording material detected by applying a constant voltage to the transfer unit, the power source is controlled so that the constant current control is performed for each position in the transport direction of the recording material. an image forming apparatus comprising control means for controlling, that Ru comprising a. The image forming apparatus according to claim 1 , wherein the control unit controls the power source so as to output an output voltage subjected to constant current control to the transfer unit before the recording material reaches the contact member . A heating means for heating and pressurizing the recording material on which the toner image is transferred;
The contact member, an image forming apparatus according to claim 1 or 2, characterized in that it is arranged discharging brush to rub the recording material that has passed through the heating means.

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