JP4911146B2 - Image forming apparatus - Google Patents

Description

  The present invention includes an image carrier that is individually provided for a plurality of colors, and a forward current is applied to a developer that is adhered to an electrostatic latent image formed on the image carrier by a reversal development method. The present invention relates to an image forming apparatus that forms an image by transferring to a transfer medium.

  Conventionally, the surface of an image carrier such as a photosensitive drum is uniformly charged by charging means such as a scorotron charger, and an electrostatic latent image is formed on the surface of the image carrier to develop a developer such as toner. An image forming apparatus has been proposed in which the developer is transferred to a transfer medium after adhering. For example, in an image forming apparatus that forms an image by a reversal development method using a positively chargeable image carrier, a positively charged developer is attached to the electrostatic latent image of the image carrier, and the image carrier A forward current flowing toward the transfer means is energized to transfer the developer onto the recording medium.

  In such an image forming apparatus using a positively chargeable image carrier, when the forward current is controlled at the start-up of the apparatus, it is not preferable to give a negative charge to the image carrier. Thus, it is necessary to start the control after positively charging and the charged portion faces the transfer means. Conversely, when a negatively chargeable image carrier is used, the forward current control must be started after the surface is uniformly charged negatively so as not to give a positive charge to the image carrier. There is.

However, in this case, a large amount of forward current flows between the image carrier and the transfer means after the charging of the image carrier is started and when formation of the electrostatic latent image is not started. When the forward current control is performed by a self-excited oscillation circuit, the circuit may not be started. Therefore, in a monochrome image forming apparatus having only one image carrier, control is started by superimposing a reverse current on the forward current to be controlled, and the control of the forward current cannot be started. It has been proposed to suppress (see, for example, Patent Document 1).
JP 2003-295633 A

  However, if this configuration is applied to a multicolor image forming apparatus in which image carriers are individually provided for a plurality of colors and a belt or the like is circulated and driven at a portion opposed to each image carrier, the following problems occur: Will occur. That is, in this case, there is a possibility that the developer that has adhered to the belt or the like adheres to another image carrier due to the reverse current, and the developers having different colors are mixed.

  In view of this, the present invention provides a multicolor image forming apparatus provided with an image carrier for each of a plurality of colors, and controls the forward current supplied between the image carrier and the transfer unit. It was made for the purpose of enabling a good start while suppressing the mixing of different developers.

In order to achieve the above object, the present invention provides an image carrier individually provided for a plurality of colors and a surface of the corresponding image carrier provided for each of the image carriers. A plurality of charging means for uniformly charging to the same polarity as the charging polarity of the developer, and forming an electrostatic latent image on the surface of each image carrier uniformly charged by the charging means Means, a plurality of developing means provided for each of the image carriers, and a plurality of developing means for attaching a charged developer to the electrostatic latent image formed on the corresponding image carrier, and the image carriers The transfer medium is sequentially sandwiched between the image bearing member and a forward current is passed between the image bearing member and the developer on the electrostatic latent image of the image bearing member. An image forming apparatus comprising: a transfer unit that transfers the image to a transfer medium; A current detecting means for detecting a current value of the forward current passed between the body and the transfer means; after the start of the charging operation by the charging means; and by the electrostatic latent image forming means Current control that starts control before the start of the latent image forming operation, and controls the forward current that is passed between each image carrier and the transfer unit based on the current value detected by the current detection unit. and means, provided respectively to said each image carrier, after the start of the charging operation by the charging means, and a point earlier start of the electrostatic latent image forming operation by the electrostatic latent image forming unit When the absolute value of the current value detected by the current detection means is greater than or equal to a predetermined value before the control of the current control means, the surface charge of the image carrier uniformly charged by the charging means is reduced. A plurality of surface charge reducing means It is characterized by comprising a.

  In the present invention configured as described above, during normal image formation, the surface of each image bearing member is uniformly charged to the same polarity as the charging polarity of the developer by the charging means corresponding to each electrostatic latent image. An electrostatic latent image can be individually formed on the surface of each image carrier by the forming means. The electrostatic latent image formed on each image carrier has a developer corresponding to each of the electrostatic latent images, and a forward current is generated by sequentially sandwiching the transfer medium between each image carrier and the transfer unit. When energized, the developer is transferred to the transfer medium. The medium to be transferred may be a recording medium such as paper or an intermediate transfer member such as a so-called intermediate transfer belt.

Here, in the present invention, the current detection unit detects the current value of the forward current that is passed between each of the image carriers and the transfer unit, and based on the current value detected by the current detection unit, Control means controls the forward current. Then, after the start of the charging operation by the charging unit and before the start of the electrostatic latent image forming operation by the electrostatic latent image forming unit, the current detection is performed before the start of the control of the current control unit. When the current value of the forward current detected by the means is a predetermined value or more, the following control is performed in the present invention.

  That is, in this case, the surface charge reducing means reduces the surface charge (either positive charge or negative charge) of the image carrier uniformly charged by the charging means. For this reason, the forward current is reduced and the control of the current control means can be started. Further, in the present invention, it is not necessary to apply a reverse current to the transfer means, so that it is possible to suppress mixing of different color developing materials. The predetermined value may be, for example, a current value that makes it difficult to start control of the current control means, or may be a current value that is set in advance with a certain margin.

  In addition, the present invention is not limited to the following configuration, but the transfer means is provided for each of the image carriers, and sequentially with the corresponding image carrier with the transfer medium interposed therebetween. By passing a forward current between them, the developer attached to the electrostatic latent image of the image carrier is transferred to the transfer medium, and the current detection means includes the image carrier and the respective image carriers. A current value of the forward current passed between the transfer means and the current control means is individually detected, and the current control means detects the image carrier and the transfer means based on the current value detected by the current detection means. Each of the surface charge reducing means is controlled individually, and each surface charge reducing means is an absolute value of the current value at the time point detected by the current detecting means with respect to the image carrier on which the surface charge is provided. Is greater than the predetermined value, the charging means It may reduce the uniformly charged surface charge of the image bearing member.

  In this case, a transfer means is provided for each image carrier, and each set of image carriers and transfer means is individually controlled, so appropriate control according to the deterioration state of each image carrier is performed. Can be executed.

  The at least one surface charge reducing means may gradually reduce the surface charge while judging whether or not the absolute value of the current value detected by the current detecting means is equal to or greater than the predetermined value. If the surface charge of the image carrier is extremely reduced, the image carrier may be adversely affected. However, if the surface charge is gradually reduced until the current value falls below the predetermined value, it is more than necessary. It is possible to suppress the surface charge from being reduced.

  Further, the at least one surface charge reducing unit may be the developing unit configured such that a portion facing the corresponding image carrier can be controlled to a potential that reduces the surface charge of the image carrier. In this case, the surface charge can be reduced using the developing means without providing a new configuration.

  Further, at least one of the image carriers is a photoconductor whose surface charge is reduced by exposure, and the surface charge reducing means corresponding to the image carrier exposes the corresponding image carrier. The surface charge may be reduced. In this case, since the surface charge can be reduced in a state where the developing means such as the developing roller is separated from the image carrier, the deterioration of the image carrier can be further suppressed.

(Overall configuration of image forming apparatus)
Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram schematically illustrating the configuration of an image forming unit 1 of an image forming apparatus according to a first embodiment to which the present invention is applied. The image forming apparatus according to the present embodiment includes four image forming units 1 as shown in FIG. 1 for each color of black (K), yellow (Y), magenta (M), and cyan (C). (See FIG. 2). In the following description, when distinction is necessary, the image forming unit 1 for each color is referred to as an image forming unit 1K, 1Y, 1M, 1C.

  As shown in FIG. 1, the image forming apparatus according to the present embodiment is circulated and driven sequentially through the image forming units 1K, 1Y, 1M, and 1C, and is a sheet as an example of a recording medium and a transfer medium (illustrated). (Not shown) is provided on the upper surface. Each image forming unit 1 includes a photosensitive drum 5 as an example of an image carrier and a photosensitive member, and a transfer roller 7 as an example of a transfer unit, with the conveyance belt 3 interposed therebetween. The photosensitive drum 5 has a drum body grounded and a surface portion formed of a positively chargeable photosensitive layer. Further, a charger 11 as an example of a charging unit and an LED as an example of an electrostatic latent image forming unit are disposed around the photosensitive drum 5 above the conveying belt 3 from the upstream side in the rotation direction of the photosensitive drum 5. A unit 13 and a developing roller 15 as an example of a developing unit and a surface charge reducing unit are sequentially provided.

  The charger 11 is disposed above the photosensitive drum 5 so as to face the photosensitive drum 5 with a predetermined interval therebetween. The charger 11 is a positively charged scorotron charger that generates corona discharge from a charging wire, and is configured to uniformly charge the surface of the photosensitive drum 5 to a positive polarity. The LED unit 13 includes LEDs (not shown) arranged in the axial direction of the photosensitive drum 5 at the lower end, and exposes the photosensitive drum 5 according to image data to be described later. The developing roller 15 is provided at the lower end of a developing cartridge (not shown) filled with toner as a positively chargeable nonmagnetic one-component developer of a color corresponding to the image forming unit 1 and is triboelectrically charged to a positive polarity. The toner is supplied to the photosensitive drum 5 by rotating while carrying a thin layer of toner on the surface.

  The charger 11 generates the corona discharge by the positive voltage applied from the charging high-voltage power supply 21 to charge the photosensitive drum 5 to the positive polarity. The developing roller 15 is selectively applied with a positive voltage or a negative voltage from a developing high-voltage power supply 25. When the positive voltage is applied, the positively charged toner is supplied to the photosensitive drum 5 by the reverse developing method. The transfer roller 7 is configured by covering a metal roller shaft with a conductive rubber material. The transfer roller 7 is connected to a high-voltage power supply 27 for transfer, and is supplied with a forward current under constant current control in a direction from the photosensitive drum 5 toward the transfer roller 7.

  The surface of the photoconductive drum 5 is uniformly positively charged by the charger 11 as it rotates, and then exposed by the LED unit 13 to form an electrostatic latent image corresponding to the image data. Next, when the developing roller 15 rotates and the positively charged toner carried on the developing roller 15 comes into contact with the photosensitive drum 5 to face the static drum 5, An electrostatic latent image, that is, an exposed portion of the surface of the photosensitive drum 5 that is uniformly positively charged is exposed to the LED unit 13 and the potential is lowered. As a result, the electrostatic latent image on the photosensitive drum 5 is visualized, and a toner image by reversal development is carried on the surface of the photosensitive drum 5. Thereafter, the toner image carried on the surface of the photosensitive drum 5 is in the order in which the transfer roller 7 is energized while the paper carried on the transport belt 3 passes between the photosensitive drum 5 and the transfer roller 7. It is transferred to the paper by the direction current.

  While the paper is being transported by the transport belt 3, the toner images of the respective colors are sequentially transferred by the image forming units 1 </ b> K to 1 </ b> C in this manner, and the paper onto which the toner images of the respective colors are transferred is a known fixing device. After the toner is fixed by heat (not shown), the toner is discharged outside the image forming apparatus. Further, each developing roller 15 is provided with a developing separation motor 29 that presses / separates the developing roller 15 with respect to the photosensitive drum 5 via a developing separation shaft 28.

  FIG. 2 is a block diagram showing the configuration of the control system of the image forming apparatus. As shown in FIG. 2, each of the image forming units 1K to 1C including the developing separation motor 29 and the high-voltage power sources 21 to 27 includes a control unit together with an interface 37 to which image data is input from a personal computer (not shown). 30. The control unit 30 is configured as a known microcomputer including a CPU 31, a ROM 32, and a RAM 33.

(Configuration of high-voltage power supply for transfer)
Next, FIG. 3 is a circuit diagram showing in detail the configuration of the high-voltage power supply 27 for transfer. As shown in FIG. 3, the transfer high-voltage power supply 27 includes a self-excited transformer 40 in which energy stored in the primary winding 40A is transmitted to the secondary winding 40B by back electromotive force by energization from a 24V DC power supply. A transistor 41 that switches a current passed through the primary winding 40A and a drive current control unit 50 that controls a base current of the transistor 41 are provided.

  An auxiliary winding 40C of the transformer 40 is provided between the base of the transistor 41 and the drive current control unit 50, and the current supplied to the secondary winding 40B is as follows by the drive current control unit 50. Be controlled.

  That is, when a current is supplied from the drive current control unit 50 and a base current flows to the transistor 41 via the auxiliary winding 40C, the transistor 41 is turned on, and a collector current is supplied from the 24V DC power source via the primary winding 40A. Flows, and the magnetic flux of the transformer 40 increases. Since the collector current does not exceed the upper limit current value obtained by amplifying the current value of the base current by the amplification factor of the transistor 41, the collector current of the transistor 41 is saturated. Then, the magnetic flux supplied from the primary winding 40A is not increased, the potential across the auxiliary winding 40C is decreased, the base current of the transistor 41 is decreased, and the transistor 41 is turned off rapidly. At this time, the energy stored in the transformer 40 is transmitted to the secondary winding 40B by the back electromotive force of the transformer 40, the voltage is boosted, and a voltage is generated in the secondary winding 40B.

  Further, a rectifying diode 45 is connected in series to the secondary winding 40B, and a smoothing capacitor 46 and a discharging resistor are connected between both ends of the series circuit including the secondary winding 40B and the diode 45. The device 47 is connected in parallel. Then, a transfer output in which the transfer roller 7 is energized as the forward current is generated from the high voltage side of the secondary winding 40B. The low-voltage side of the secondary winding 40B is grounded via resistors 48 and 49, and the voltage divided by the resistors 48 and 49 is A as a transfer feedback (transfer FeedBack). / D port is input. That is, the forward current flows through resistors 48 and 49 as an example of current detection means, and the voltage is divided by the resistors 48 and 49, and the voltage drop between GND and the resistor 49 is It is detected as a value corresponding to the current value of the forward current.

  On the other hand, the drive current control unit 50 includes a series circuit that grounds the PWM signal output from the A / D port of the control unit 30 via the resistor 51 and the capacitor 52 in sequence, and between the resistor 51 and the capacitor 52. And a transistor 55 whose voltage is applied to the base via a resistor 53. The emitter of the transistor 55 is connected to a 3.3V DC power supply via a resistor 57, and the collector is connected to the auxiliary winding 40C via a resistor 58.

  Therefore, when a PWM signal is output from the A / D port of the control unit 30, the voltage of the PWM signal is smoothed by the resistor 51 and the capacitor 52 and applied to the base of the transistor 55. When the duty ratio of the PWM signal is 100%, the base voltage is 3.3 V, so that the transistor 55 is not turned on and the output of the drive current control unit 50 is also zero.

  When the duty ratio of the PWM signal output from the A / D port of the control unit 30 is reduced to some extent, the transistor 55 is turned on and a current corresponding to the collector current is supplied to the primary winding 40A. Therefore, the control unit 30 controls the forward current to a predetermined value by changing the duty ratio in accordance with the transfer feedback value. That is, the control unit 30 and the drive current control unit 50 correspond to current control means.

(Transfer output control)
Here, at the time of image formation, it is necessary to control the forward current to a constant current value prior to image formation (also referred to as printing) on a sheet. However, it is not preferable to apply a negative charge to the surface of the photosensitive drum 5 via the transfer roller 7 by the transfer output. Therefore, prior to the control of the transfer output, it is necessary to charge the photosensitive drum 5 positively by the charger 11 and to start the control after the charged portion faces the transfer roller 7.

  However, in this case, a large forward current flows between the surface of the photosensitive drum 5 that is uniformly positively charged and on which no toner image is formed, and the transfer roller 7, and the secondary winding. An excessive forward current may flow in 40B as illustrated in FIG. Here, the forward current that flows before the start of the electrostatic latent image forming operation by the electrostatic latent image forming unit not forming the toner image after the start of the positive charging operation by the charging unit as described above. Is the inflow current. Then, when an excessive inflow current flows, the control for adjusting the output of the self-excited transformer 40 by the drive current control unit 50 as described above may not be started.

  Therefore, when image data is input to the interface 37 and a print command is issued, the control unit 30 executes the following control. 4 to 5 are flowcharts showing processing executed by the control unit 30 when the print command is issued. At the start of this process, the developing rollers 15 of the image forming units 1K to 1C are separated from the photosensitive drum 5 via the developing separation motors 29 (see S52 described later).

  As shown in FIGS. 4 to 5, in this process, first, the charging high-voltage power supply 21 of the image forming units 1 </ b> K to 1 </ b> C is driven in S <b> 1 (S represents a step: the same applies hereinafter), and each charger 11. The charging output is turned on. In the subsequent S10, the inflow current flowing into the transformer 40 of the image forming unit 1K corresponding to K (black) color at a timing when the surface of the photosensitive drum 5 charged by the process of S1 faces the transfer roller 7 is predetermined. Whether the value is equal to or greater than the value is determined based on the transfer feedback. The predetermined value in this step may be a current value that makes it difficult to start the control by the drive current control unit 50, or a current value that is set in advance with a certain margin. May be.

  If the inflow current is less than the predetermined value (S10: N), in S11, the control of the transfer output by the drive current control unit 50 of the image forming unit 1K corresponding to K (black) color starts (transfer output). ON), and the process proceeds to S20 described later. Note that the target value of the current flowing into the transformer 40 at this time is set to, for example, +2 μA.

  On the other hand, when the inflow current is equal to or greater than the predetermined value (S10: Y), the process proceeds to S12, and the developing roller 15 corresponding to the K (black) color is pressed against the photosensitive drum 5 via the developing separation motor 29. Is done. In subsequent S13, voltage application to the developing roller 15 is started at a set value of −50 V (development output ON) via the high-voltage power supply 25 for development. However, it is not always necessary to apply a negative voltage to the developing roller 15 as long as the surface charge of the photosensitive drum 5 can be canceled by applying a developing bias lower than the reference potential of the photosensitive drum 5. In the subsequent S14, a predetermined time is waited until the influence of the development output being turned on is reflected, and when the predetermined time elapses, the inflow current of K (black) is equal to or greater than the predetermined value in S15 as in S10. It is determined whether or not. If the inflow current is equal to or greater than the predetermined value (S10: Y), the set value of the K (black) color development output is further reduced by 50 V in S16, and the process proceeds to S14 described above.

  In this way, when the K (black) inflow current becomes less than the predetermined value while the development output setting value is gradually reduced by repeatedly executing the processing of S14 to S16 (S15: N), the processing is performed. The process proceeds to S17, and the K (black) color transfer output is turned ON as in S11. In the subsequent S18, the K (black) color development output made in S13 or S16 is turned OFF. Further, in S19, after the development roller 15 corresponding to the K (black) color is separated from the photosensitive drum 5, The process proceeds to S20.

  In subsequent S20 to S29, processing similar to that in S10 to S19 is performed for the Y (yellow) color. That is, it is determined whether or not the Y (yellow) inflow current is greater than or equal to a predetermined value (S20). If it is less than the predetermined value (S20: N), the Y (yellow) color transfer output is turned on. (S21), the process proceeds to S30 described later. Also in this case, the target value of the inflow current to the transformer 40 by the transfer output is set to +2 μA, for example.

  On the other hand, when the inflow current is equal to or greater than the predetermined value (S20: Y), the developing roller 15 corresponding to the Y (yellow) color is pressed against the photosensitive drum 5 (S22). Then, the development output is turned ON at a set value of -50V (S23), and the set value of the development output is reduced by 50V each time until the inflow current becomes less than the predetermined value while waiting for a predetermined time (S24 to S26). When the inflow current becomes less than the predetermined value (S25: N), the Y (yellow) transfer output is turned on (S27). Subsequently, the Y (yellow) color development output is turned off (S28), the developing roller 15 corresponding to the Y (yellow) color is separated (S29), and the process proceeds to S30.

  In subsequent S30 to S39, the same processing as that in S10 to S19 is executed for the M (magenta) color. That is, it is determined whether or not the inflow current of M (magenta) is greater than or equal to a predetermined value (S30). If it is less than the predetermined value (S30: N), the transfer output of M (magenta) is turned on. (S31), the process proceeds to S40 described later. Also in this case, the target value of the inflow current to the transformer 40 by the transfer output is set to +2 μA, for example.

  On the other hand, when the inflow current is equal to or greater than the predetermined value (S30: Y), the developing roller 15 corresponding to M (magenta) color is pressed against the photosensitive drum 5 (S32). Then, the development output is turned ON at a set value of −50V (S33), and the set value of the development output is reduced by 50V each time until the inflow current becomes less than the predetermined value while waiting for a predetermined time (S34 to S36). When the inflow current becomes less than a predetermined value (S35: N), the transfer output of M (magenta) color is turned on (S37). Subsequently, the M (magenta) color development output is turned off (S38), the developing roller 15 corresponding to the M (magenta) color is separated (S39), and the process proceeds to S40.

  In subsequent S40 to S49, processing substantially similar to S10 to S19 is executed for the C (cyan) color. That is, it is determined whether or not the C (cyan) inflow current is greater than or equal to a predetermined value (S40). If it is less than the predetermined value (S40: N), the C (cyan) transfer output is turned on. (S41). Also in this case, the target value of the inflow current to the transformer 40 by the transfer output is set to +2 μA, for example. However, in this case, in S49 following S41, the developing roller 15 corresponding to the C (cyan) color is pressed against the photosensitive drum 5, and the process proceeds to S50 described later.

  On the other hand, if the inflow current is equal to or greater than the predetermined value (S40: Y), the developing roller 15 corresponding to C (cyan) is pressed against the photosensitive drum 5 (S42). Then, the development output is turned ON at a set value of -50V (S43), and the set value of the development output is reduced by 50V each time until the inflow current becomes less than the predetermined value while waiting for a predetermined time (S44 to S46). When the inflow current becomes less than the predetermined value (S45: N), the C (cyan) color transfer output is turned on (S47). Subsequently, the C (cyan) color development output is turned off (S48), and the process proceeds to S50.

  In S <b> 50, the developing rollers 15 corresponding to K (black), Y (yellow), and M (magenta) colors are pressed against the respective photosensitive drums 5. That is, the developing roller 15 corresponding to the C (cyan) color is already in pressure contact with the photosensitive drum 5 by the process of S42 or S49, and the other developing roller 15 is in pressure contact with the photosensitive drum 5. In subsequent S51, the development output of each color is turned ON with a set value of +400 V, and a known printing operation based on the image data is executed. After completion of the printing operation in S51, the developing roller 15 corresponding to each color of K (black), Y (yellow), M (magenta), and C (cyan) is separated from the photosensitive drum 5 in S52. The process is temporarily terminated.

(Effect of this embodiment)
As described above, in this embodiment, after the charging output is turned on (S1) and before the printing operation (S51) is started, the inflow current is a predetermined value or more (S10). : Y or S20: Y or S30: Y or S40: Y), the surface potential of the photosensitive drum 5 is reduced by making the set value of the development output relatively lower than the surface potential of the photosensitive drum 5. (S13 to S16, S23 to S26, S33 to S36, S43 to S46). For this reason, even when the inflow voltage is large, the constant current control by the drive current controller 50 can be favorably started as follows.

  As shown by the thick dotted line in FIG. 6, both the inflow current I and the surface potential V are kept constant, and if the inflow current I is large, control may not be possible. On the other hand, in the present embodiment, as shown by a thick solid line in FIG. 6, control is performed to gradually decrease the set value of the development output and gradually reduce the inflow current I and the surface potential V. For this reason, the constant current control is started by reducing the inflow current I and the surface potential V until the constant current control by the drive current control unit 50 can be started (transfer output ON), and further the printing operation is started. Can do.

  In addition, in the present embodiment, since the above control is executed for each image forming unit 1, it is possible to execute appropriate control according to the deterioration state of the individual photoconductive drums 5. In this embodiment, since the set value of the development output is gradually reduced, it is possible to suppress the surface potential V from being reduced more than necessary. Furthermore, in this embodiment, since the surface potential V is reduced using the developing roller 15, it is not necessary to provide a new configuration for the reduction. Furthermore, in this embodiment, the developing roller 15 that is not related to the start of constant current control at that time is separated from the photosensitive drum 5 (S19, S29, S39, S52). Deterioration can be suppressed even better.

(Another embodiment of the present invention)
Next, FIG. 7 is an explanatory diagram schematically showing the configuration of the image forming unit 101 of the image forming apparatus according to the second embodiment to which the present invention is applied. Also in this embodiment, four image forming units 101 are provided for each color of black (K), yellow (Y), magenta (M), and cyan (C), which are the same as those shown in FIG. It is connected to the control unit 30. The image forming unit 101 is different from the image forming unit 1 in that an erase lamp 117 as an example of a surface charge reducing unit is provided between the developing roller 15 and the transfer roller 7 around the photosensitive drum 5. The configuration is the same as that of the image forming unit 1.

  8 and 9 are flowcharts showing processing executed by the control unit connected to such an image forming unit 101. Since this process is almost the same as the process shown in FIGS. 4 and 5, only the differences will be described below.

  In the present embodiment, the surface potential of the photosensitive drum 5 can be reduced by turning on the erase lamp 117 while the developing roller 15 is kept away from the photosensitive drum 5. For this reason, the developing roller pressure contact / separation processing (S12, S19, S22, S29, S32, S39, S42, and S49) that has been performed to reduce the surface potential can be omitted. Instead, in S150 executed in place of S50 before the printing operation in S51, the developing rollers 15 for black (K), yellow (Y), magenta (M), and cyan (C) are replaced with the photosensitive drum 5. It is necessary to press contact with.

  In S113, S123, S133, and S143 instead of S13, S23, S33, and S43, the erase lamp 117 is turned on (EL lighting) in the corresponding color image forming unit 101 instead of the development output ON, and S16 and S26. In S116, S126, S136, and S146 instead of S36 and S46, the erase lamp 117 is brightened (EL brightening).

  In the present embodiment, after the charging output is turned on (S1) and before the printing operation (S51) is started, the inflow current is a predetermined value or more (S10: Y or S20). : Y or S30: Y or S40: Y), the erase lamp 117 is turned on, and the surface potential of the photosensitive drum 5 can be reduced by increasing the light as necessary (S113 to S116, S123 to S126, S133). -S136, S143-S146). For this reason, even when the inflow voltage is large, the constant current control by the drive current controller 50 can be favorably started as follows.

  That is, as illustrated in FIG. 10 where the inflow current I and the surface potential V of the photosensitive drum 5 are exemplified, when the surface potential V is high from the beginning and the inflow current I is large, there is no conventional way to reduce them. As shown by the thick dotted line in FIG. 10, the control by the drive current control unit 50 could not be started. On the other hand, in this embodiment, as shown by a thick solid line in FIG. 10, the erase lamp 117 is turned on (EL lighting), and the light is increased (EL brightening) as necessary, so that the inflow current I and the surface are increased. Control for gradually reducing the potential V is performed. For this reason, the constant current control is started by reducing the inflow current I and the surface potential V until the constant current control by the drive current control unit 50 can be started (transfer output ON), and further the printing operation is started. Can do.

  In addition, in the present embodiment, since the above-described control is executed for each image forming unit 1, it is possible to execute appropriate control according to the deterioration state of the individual photosensitive drums 5, and the erase lamp 117. Therefore, the surface potential V can be suppressed from being reduced more than necessary. Further, in the present embodiment, all the developing rollers 15 are kept away from the photosensitive drum 5 until the transfer output for all colors is turned on, so that the deterioration of the photosensitive drum 5 is further suppressed. can do.

  In addition, this invention is not limited to each said embodiment at all, It can implement with a various form in the range which does not deviate from the summary of this invention. For example, the present invention can be similarly applied to an image forming apparatus using a negatively chargeable developer. Further, as the electrostatic latent image forming means, a laser unit that forms an electrostatic latent image by exposing an image carrier with laser light, or an apparatus that forms an electrostatic latent image by a method other than exposure can be applied. it can. Further, as the surface charge reducing means, various configurations other than the above can be adopted, and the transfer means may be a means other than the roller. For example, a belt or the like that is driven to circulate through a portion facing the plurality of image carriers may be used. The transfer medium is not limited to a recording medium such as paper or an OHP sheet, and may be an intermediate transfer body such as an intermediate transfer belt or an intermediate transfer drum.

1 is an explanatory diagram schematically illustrating a configuration of an image forming unit of an image forming apparatus according to a first embodiment to which the present invention is applied. 2 is a block diagram illustrating a configuration of a control system of the image forming apparatus. FIG. 2 is a circuit diagram illustrating in detail a configuration of a high-voltage power supply for transfer of the image forming apparatus. FIG. It is a flowchart showing the process performed by the said control system. It is a flowchart showing the continuation of the process. It is explanatory drawing showing the effect by the process. It is explanatory drawing which represents typically the structure of the image formation part of the image forming apparatus of 2nd Embodiment to which this invention was applied. 3 is a flowchart illustrating processing executed by the image forming apparatus. It is a flowchart showing the continuation of the process. It is explanatory drawing showing the effect by the process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,101 ... Image forming part 3 ... Conveyor belt 5 ... Photoconductor drum 7 ... Transfer roller 11 ... Charger 13 ... LED unit 15 ... Developing roller 21 ... High voltage power source for charging 25 ... High voltage power source for development 27 ... High voltage power source for transfer DESCRIPTION OF SYMBOLS 30 ... Control part 28 ... Development separation shaft 29 ... Development separation motor 37 ... Interface 40 ... Transformer 41 ... Transistor 50 ... Drive current control part 117 ... Erase lamp

Claims (5)

An image carrier individually provided for a plurality of colors;
A plurality of charging means provided for each of the image carriers, and charging the corresponding surfaces of the image carriers uniformly with the same polarity as the charging polarity of the developer;
Electrostatic latent image forming means for forming an electrostatic latent image on the surface of each image carrier uniformly charged by the charging means;
A plurality of developing means provided for each of the image carriers, and for attaching a charged developer to the electrostatic latent image formed on the corresponding image carrier;
The transfer medium is sequentially sandwiched between each of the image carriers, and a forward current is applied to the corresponding image carrier, thereby being attached to the electrostatic latent image of the image carrier. Transfer means for transferring the developer to the transfer medium;
An image forming apparatus comprising:
Current detection means for detecting a current value of the forward current passed between each of the image carriers and the transfer means;
Control is started after the start of the charging operation by the charging unit and before the start of the electrostatic latent image forming operation by the electrostatic latent image forming unit. Current control means for controlling the forward current passed between the image carrier and the transfer means;
Provided for each of the image carriers, at a time after the start of the charging operation by the charging unit and before the start of the electrostatic latent image forming operation by the electrostatic latent image forming unit , If the absolute value of the current value detected by the current detection means is greater than or equal to a predetermined value before the control means starts control, a plurality of surface charges of the image carrier uniformly charged by the charging means are reduced. Surface charge reducing means;
An image forming apparatus comprising: The transfer means is provided for each of the image carriers, and by sequentially applying a forward current to the corresponding image carrier with the transfer medium interposed therebetween, Transfer the developer attached to the electrostatic latent image to the transfer medium,
The current detection means individually detects a current value of the forward current passed between each image carrier and each transfer means,
The current control means individually controls the forward currents passed between the image carriers and the transfer means based on the current value detected by the current detection means,
Each surface charge reducing means is uniformly applied by the charging means when the absolute value of the current value detected by the current detecting means with respect to the image carrier on which the surface charge is provided is equal to or greater than the predetermined value. 2. The image forming apparatus according to claim 1, wherein the surface charge of the charged image carrier is reduced.   The at least one surface charge reducing means gradually reduces the surface charge while determining whether or not the absolute value of the current value detected by the current detecting means is equal to or greater than the predetermined value. Item 3. The image forming apparatus according to Item 1 or 2.   The at least one surface charge reducing means is the developing means configured such that a portion facing the corresponding image carrier can be controlled to a potential that reduces the surface charge of the image carrier. Item 4. The image forming apparatus according to any one of Items 1 to 3. At least one of the image carriers is a photoreceptor whose surface charge is reduced by exposure,
The image forming apparatus according to claim 1, wherein the surface charge reducing unit corresponding to the image carrier reduces the surface charge by exposing the corresponding image carrier. .

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