Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent a rear face of a sheet from getting dirty with toner.SOLUTION: In transferring a toner image T formed on an image carrier (3) to a sheet P, while a first area β1 including at least a sheet area α1 that is an area corresponding to the sheet P on which the toner image is formed, on a surface 3a of the image carrier (3), is passing through a pre-transfer static eliminating device 16, an image forming apparatus 100 can execute sheet-to-sheet static elimination change control of driving the pre-transfer static eliminating device 16 in a predetermined first condition, and while a second area β2 that is at least a part of a sheet-to-sheet area α2 between a sheet area on which the toner image is formed on the surface (3a) of the image carrier (3) and a sheet area corresponding to the subsequent sheet P on which a toner image is to be formed next, is passing through the pre-transfer static eliminating device 16, driving the pre-transfer static eliminating device 16 in a predetermined second condition with a smaller static eliminating effect than the first condition.SELECTED DRAWING: Figure 13

Description

Technical Field

[0001] The present disclosure relates to an image forming apparatus such as a copying machine, a multifunction peripheral, a printer, a facsimile apparatus, etc.

Background Art

[0002] There has conventionally been known an image forming apparatus including a developing device that forms a toner image on a rotatable image carrier (for example, a photosensitive drum), a transfer member (for example, a transfer belt or a transfer roller) that abuts on the surface of the image carrier and transfers a sheet such as recording paper conveyed to the contact portion with the surface of the image carrier to the toner image, and a pre-transfer charge eliminating device that eliminates the potential of the surface of the image carrier on the downstream side of the developing device and on the upstream side of the transfer member in the rotation direction of the image carrier (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In such an image forming apparatus, when the transfer member transfers the toner image on the image carrier to the sheet, toner that should not originally adhere, so-called fogged toner, may adhere to the surface of the image carrier. Further, when the pre-transfer charge eliminating device eliminates the potential of the surface of the image carrier on the downstream side of the developing device and on the upstream side of the transfer member in the rotation direction of the image carrier, the fogged toner on the surface of the image carrier is likely to be transferred to the transfer member side. In this case, the fogged toner on the surface of the image carrier is likely to adhere to the transfer member to which the fogged toner has been easily transferred by the pre-transfer charge eliminating device. Then, the back surface of the sheet (the surface opposite to the image forming surface) is soiled with toner.

[0005] Therefore, an object of the present disclosure is to provide an image forming apparatus capable of suppressing soiling of the back surface of a sheet by toner.

Means for Solving the Problems

[0006] In order to solve the above problems, an image forming apparatus according to the present disclosure includes a developing device that forms a toner image on a rotatable image carrier, a transfer device having a transfer member that contacts the surface of the image carrier and transfers the toner image to a sheet conveyed to a contact portion with the surface of the image carrier, and a pre-transfer charge eliminating device that eliminates the potential of the surface of the image carrier on the downstream side of the developing device and on the upstream side of the transfer member in the rotation direction of the image carrier. The image forming apparatus is provided with a control unit that controls the pre-transfer charge eliminating device. When transferring the toner image formed on the image carrier to the sheet, the control unit drives the pre-transfer charge eliminating device under a predetermined first condition while at least a first region including a sheet region, which is a region corresponding to the sheet on the surface of the image carrier where the toner image is formed, passes through the pre-transfer charge eliminating device. The control unit is capable of executing inter-sheet charge elimination change control for driving the pre-transfer charge eliminating device under a predetermined second condition in which the charge elimination effect is smaller than the first condition while at least a part of a second region of an inter-sheet region between the sheet region on the surface of the image carrier where the toner image is formed and a subsequent sheet region corresponding to the subsequent sheet on which the toner image is to be formed next passes through the pre-transfer charge eliminating device.

Effects of the Invention

[0007] According to the present disclosure, it is possible to suppress soiling of the back surface of a sheet by toner.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

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Figure 8A

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Figure 14A

Figure 14B

Embodiments for Carrying Out the Invention

[0009] Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. In the following description, the same parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0010] [Image Forming Apparatus] FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 according to the present embodiment as viewed from the front. In the figure, reference symbol X represents the left-right direction as viewed from the front of the image forming apparatus 100, reference symbol Y represents the depth direction (front-rear direction), and reference symbol Z represents the vertical direction (vertical direction).

[0011] The image forming apparatus 100 according to the present embodiment is a monochrome image forming apparatus that forms a monochrome image on a sheet P such as recording paper. The image forming apparatus 100 performs an image forming process according to image data read by the image reading apparatus 90 or image data transmitted from the outside. Note that the image forming apparatus 100 may be another type of monochrome image forming apparatus. Further, the image forming apparatus 100 may be a color image forming apparatus that forms multi-color and single-color images.

[0012] The image forming apparatus 100 includes an image reading apparatus 90 and an image forming apparatus main body 101. The image forming apparatus main body 101 is provided with an image forming unit 102 and a sheet conveyance system 103.

[0013] The image reading apparatus 90 is provided above the image forming apparatus main body 101. The image reading apparatus 90 reads an image of a document (not shown), and includes a document conveyance unit 90a and a document reading unit 90b. The image reading apparatus 90 reads the image of the document with the document reading unit 90b while conveying the document with the document conveyance unit 90a, or scans and reads the document placed on the document table of the document reading unit 90b with the document reading unit 90b. The image of the document read by the image reading apparatus 90 is sent to the image forming apparatus main body 101 as image data.

[0014] The image forming unit 102 includes an exposure device 1 (exposure unit), a developing device 2 (developing unit), a photosensitive drum 3 acting as an image carrier, a cleaning device 4 (cleaning unit), a charging device 5, a transfer device 6 (transfer belt device), a pre-transfer charge elimination device 16 (pre-transfer charge elimination light source), a post-transfer charge elimination device 17, a fixing device 7 (fixing unit), and a toner cartridge 8. Further, the sheet conveyance system 103 includes a paper feed tray 9, a discharge roller 10, a discharge tray 11, and a sheet supply unit 12.

[0015] The photosensitive drum 3 has a grounded aluminum drum and a photosensitive layer formed on the outer side of the aluminum drum, which exhibits insulation in the dark and whose conductivity changes in the area irradiated with light. The charging device 5 charges the surface 3a (photosensitive layer) of the photosensitive drum 3 to a predetermined charging potential (for example, -650 V) of a predetermined polarity (minus polarity in this example). The exposure device 1 exposes (laser scans) the surface 3a of the photosensitive drum 3 charged to a predetermined charging potential by the charging device 5 to form an electrostatic latent image at the image forming portion potential (for example, -150 V). The developing device 2 develops the electrostatic latent image on the photosensitive drum 3 with toner charged to a predetermined polarity (minus polarity in this example) to form a toner image. The transfer device 6 transfers the toner image formed on the surface 3a of the photosensitive drum 3 by the developing device 2 onto the sheet P conveyed in the conveyance direction H. The cleaning device 4 removes the toner (waste toner) remaining on the photosensitive drum 3 without being transferred to the sheet P by the transfer device 6. The fixing device 7 fixes the toner image transferred onto the sheet P by the transfer device 6 to the sheet P. The post-transfer charge removal device 17 discharges and erases the electrostatic latent image remaining on the photosensitive drum 3, and returns the surface 3a of the photosensitive drum 3 to a value close to the ground potential (0 V). The pre-transfer charge removal device 16 will be described later.

[0016] The image forming apparatus main body 101 is provided with a sheet conveyance path W1. The sheet supply unit 12 supplies the sheets P accommodated in the paper feed tray 9 one by one to the sheet conveyance path W1. The sheet conveyance path W1 guides the sheet P through the transfer members (transfer belt 61 and transfer roller 62 in this example) of the transfer device 6, the fixing device 7, and the discharge roller 10 to the discharge tray 11. The fixing device 7 heat-fixes the toner image transferred onto the sheet P by the transfer device 6 to the sheet P. In the vicinity of the sheet conveyance path W1, the sheet supply unit 12, a plurality of conveyance rollers 13 to 13, the registration roller 14, the transfer device 6, the fixing roller 71 and the pressure roller 72 in the fixing device 7, and the discharge roller 10 are arranged.

[0017] In the image forming apparatus 100, the sheet P supplied from the paper feed tray 9 to the sheet conveyance path W1 by the sheet supply unit is conveyed to the registration roller 14 via the conveyance rollers 13 to 13. Next, the sheet P is conveyed to the transfer member of the transfer device 6 at a timing when the sheet P and the toner image on the photosensitive drum 3 are aligned by the registration roller 14, and the toner image formed on the photosensitive drum 3 by the transfer device 6 is transferred onto the sheet P. Thereafter, the sheet P passes through the fixing roller 71 and the pressure roller 72 in the fixing device 7, and is discharged onto the discharge tray 11 via the conveyance roller 13 and the discharge roller 10. When forming an image not only on the front surface but also on the back surface of the sheet P, the sheet P is conveyed in the reverse direction from the discharge roller 10 to the reverse sheet conveyance path W2. The sheet P passes through the reverse conveyance rollers 15 to 15, reverses the front and back surfaces of the sheet P, and is guided again to the registration roller 14. Then, in the same manner as the front surface, after a toner image is formed and fixed on the back surface, the sheet P is discharged toward the discharge tray 11.

[0018] FIG. 2 is a schematic diagram schematically showing a part of the image forming unit 102 in the image forming apparatus 100 shown in FIG. 1. FIG. 3 is a system block diagram showing the control configuration of the image forming apparatus 100 shown in FIG. 1.

[0019] As shown in FIG. 2, the photosensitive drum 3 is rotated in a predetermined rotation direction (first rotation direction R1) around the rotation axis V to form a toner image T.

[0020] The transfer device 6 transfers the toner image T formed on the photosensitive drum 3 to the sheet P sent from the paper feed tray 9 (see FIG. 1). The transfer device 6 includes a transfer member (in this example, a transfer belt 61 and a transfer roller 62) that transfers the toner image T to the sheet P conveyed to the contact portion (transfer nip portion TN) with the surface 3a of the photosensitive drum 3 in contact with the surface 3a of the photosensitive drum 3.

[0021] Specifically, the transfer device 6 includes an endless transfer belt 61, a transfer roller 62, a drive roller 63, a driven roller 64, a transfer cleaning member (transfer cleaning roller 65 in this example), and an opposing member (opposing roller 66 in this example). The transfer roller 62, drive roller 63, driven roller 64, and opposing roller 66 support the transfer belt 61. The transfer roller 62 contacts the inner side of the transfer belt 61 under the biasing force of a biasing member such as a coil spring (not shown) and contacts the outer surface of the transfer belt 61 against the photosensitive drum 3. The transfer belt 61 is driven to rotate in the second rotation direction R2 by the drive roller 63 being rotationally driven by a drive source (not shown), and is driven to rotate passively together with the transfer roller 62, driven roller 64, transfer cleaning roller 65, and opposing roller 66.

[0022] <Transfer belt> The transfer belt 61 is formed of a conductive resin material. The transfer belt 61 may have a release layer made of a fluororesin or the like formed on its surface. The transfer roller 62 is composed of a shaft member 621 and a roller portion 622 provided on the shaft member 621. The shaft member 621 is formed of a rigid material such as metal. The roller portion 622 is formed of a conductive resin material.

[0023] Here, examples of the conductive resin material constituting the transfer belt 61 include polyimide, polycarbonate, nitrile butadiene rubber (NBR), chloroprene rubber (CR), ethylene propylene rubber (EPDM), etc. in which fine powder-like conductive substances (such as carbon and metal powder) are blended. Examples of the conductive resin material constituting the transfer roller 62 and the roller portion 622 include nitrile butadiene rubber (NBR), chloroprene rubber (CR), ethylene propylene rubber (EPDM), etc. in which fine powder-like conductive substances (such as carbon black and metal powder) are blended.

[0024] <Pre-transfer charge elimination device and pre-transfer charge elimination power supply> The pre-transfer charge elimination device 16 reduces the potential of the surface 3a of the photosensitive drum 3 on the downstream side of the developing device 2 and on the upstream side of the transfer members (61, 62) in the rotation direction (first rotation direction R1) of the photosensitive drum 3. In this example, the pre-transfer charge elimination device 16 is a pre-transfer charge elimination lamp 162 that irradiates the surface 3a of the photosensitive drum 3 with light, and irradiates the surface 3a of the photosensitive drum 3 with light overall at a position upstream in the rotation direction (first rotation direction R1) from the transfer nip portion TN between the photosensitive drum 3 and the transfer members (61, 62).

[0025] <Post-transfer charge elimination device> The post-transfer charge elimination device 17 is disposed at a position on the downstream side of the transfer members (61, 62) and on the upstream side of the charging device 5 in the rotation direction (first rotation direction R1) of the photosensitive drum 3, and eliminates the potential of the surface 3a of the photosensitive drum 3 as described above. In this example, the post-transfer charge elimination device 17 is a post-transfer charge elimination lamp 172 that irradiates the surface 3a of the photosensitive drum 3 with light, and irradiates the surface 3a of the photosensitive drum 3 with light overall at a position downstream in the rotation direction (R1) from the transfer nip portion TN between the photosensitive drum 3 and the transfer members (61, 62), and erases the electrostatic latent image formed on the surface 3a of the photosensitive drum 3 formed by the exposure device 1.

[0026] The image forming apparatus 100 further includes a pre-transfer charge elimination power source 161, a post-transfer charge elimination power source 171, a transfer power source 623, a transfer cleaning power source 651, and a control unit 19 (see FIG. 3). The control unit 19 controls the entire image forming apparatus 100.

[0027] <Pre-transfer charge elimination power source> The pre-transfer charge elimination power source 161 supplies power (current in this example) to the pre-transfer charge elimination device 16 (pre-transfer charge elimination lamp 162) under the instruction of the control unit 19.

[0028] Here, the pre-transfer charge-removing power supply 161 is electrically connected to the output system of the control unit 19. The control unit 19 can select a current value (in this example, 5 mA, 10 mA, or 15 mA) from among current values within a predetermined range (in this example, 5 mA to 15 mA) and supply it to the pre-transfer charge-removing device 16 by transmitting an instruction signal to the pre-transfer charge-removing power supply 161. Therefore, in this example, the pre-transfer charge-removing power supply 161 includes a variable power supply unit 161a and a switch unit 161b. One terminal (positive-side terminal) of the variable power supply unit 161a is grounded via the pre-transfer charge-removing device 16, and the other terminal (negative-side terminal) is grounded via the switch unit 161b. The variable power supply unit 161a can selectively output currents of a plurality of current values (in this example, 5 mA, 10 mA, or 15 mA) under the instruction of the control unit 19. The switch unit 161b controls the energization (light emission) timing to the pre-transfer charge-removing device 16 under the instruction of the control unit 19. Since the amount of light emission of the pre-transfer charge-removing device 16 changes as the current value supplied from the pre-transfer charge-removing power supply 161 changes as described above, the charge-removing effect of removing the potential of the surface 3a of the photoreceptor drum 3 before transfer can be changed.

[0029] <Post-transfer charge-removing power supply> The post-transfer charge-removing power supply 171 supplies power to the post-transfer charge-removing device 17 under the instruction of the control unit 19.

[0030] Specifically, the post-transfer charge elimination power supply 171 supplies a current with a predetermined current value. The post-transfer charge elimination power supply 171 is electrically connected to the output system of the control unit 19. Thereby, the control unit 19 can supply a current with a predetermined current value from the post-transfer charge elimination power supply 171 to the post-transfer charge elimination device 17 (post-transfer charge elimination lamp 172) by transmitting an instruction signal to the post-transfer charge elimination power supply 171 to cause it to emit light. Due to the emission of the post-transfer charge elimination lamp 172, the potential of the surface 3a of the photosensitive drum 3 after transfer is discharged, and the electrostatic latent image formed on the photosensitive drum 3 is erased. For this purpose, the post-transfer charge elimination power supply 171 includes a power supply unit 171a and a switch unit 171b. One terminal (positive terminal) of the power supply unit 171a is electrically grounded via the post-transfer charge elimination device 17, and the other terminal (negative terminal) is electrically grounded via the switch unit 171b. The switch unit 171b controls the energization (light emission) timing to the post-transfer charge elimination device 17 under the instruction of the control unit 19.

[0031] <Transfer power supply> The transfer power supply 623 supplies a transfer bias, which is a predetermined voltage or current, to the transfer member (transfer roller 62 in this example) under the instruction of the control unit 19. When the transfer device 6 transfers the toner image T on the photosensitive drum 3 to the sheet P, the transfer power supply 623 applies a potential of the opposite polarity (positive polarity in this example) to the transfer member (62) to the charging polarity of the toner (negative polarity in this example).

[0032] Specifically, the transfer power supply 623 can supply different transfer biases to the transfer member (62) when transferring and when cleaning the toner on the transfer member (61). The transfer power supply 623 includes a first power supply unit 623a, a second power supply unit 623b, and a switching unit 623c.

[0033] The first power supply unit 623a supplies a transfer bias to the transfer member (62) to impart an adhesive force to the toner such that the toner on the photosensitive drum 3 moves toward the transfer member (61) side. In this example, the first power supply unit 623a supplies a current with a predetermined constant current value (specifically, 42 μA) of positive polarity by constant current control.

[0034] The second power supply unit 623b supplies a transfer bias to the transfer member (62) for imparting an adhesive force to the toner on the transfer member (61) such that the toner moves toward the photoreceptor drum 3 side. In this example, the second power supply unit 623b supplies a current in which an alternating current is superimposed on a direct current of a negative polarity.

[0035] The switching unit 623c is capable of selectively switching between the current supply from the first power supply unit 623a to the transfer member (62) and the current supply from the second power supply unit 623b to the transfer member (62). The first power supply unit 623a has its output side electrically connected to the first terminal 623d of the switching unit 623c. The second power supply unit 623b has its output side electrically connected to the second terminal 623e of the switching unit 623c. The output wiring 623f of the switching unit 623c is electrically connected to the transfer member (62).

[0036] The switching unit 623c is electrically connected to the output system of the control unit 19. Thereby, when transferring the toner image T on the photoreceptor drum 3 to the sheet P, the control unit 19 can connect the output wiring 623f to the first terminal 623d by transmitting an instruction signal to the switching unit 623c (see the solid line in FIG. 2). Thereby, the toner on the photoreceptor drum 3 can be made to move toward the transfer member (61) side. Further, when returning (cleaning) the toner on the transfer member (61) to the photoreceptor drum 3, the control unit 19 can connect the output wiring 623f to the second terminal 623e by transmitting an instruction signal to the switching unit 623c (see the broken line in FIG. 2). Thereby, the toner on the transfer member (61) can be made to move toward the photoreceptor drum 3 side.

[0037] Note that, in this example, the transfer device 6 uses the transfer belt 61. However, in the transfer device 6, the transfer belt 61, the driving roller 63, the driven roller 64, the transfer cleaning roller 65, and the opposing roller 66 may be removed, and the toner image T on the photoreceptor drum 3 may be transferred to the sheet P sandwiched between the transfer roller 62 and the photoreceptor drum 3.

[0038] <Transfer cleaning power supply> The transfer cleaning power supply 651 supplies a transfer bias to the transfer cleaning roller 65 under the instruction of the control unit 19.

[0039] Specifically, the transfer cleaning power supply 651 can selectively supply different transfer biases to the transfer member (61) when transferring and when cleaning the toner on the transfer member (61). The transfer cleaning power supply 651 includes a first power supply unit 651a, a second power supply unit 651b, and a switching unit 651c.

[0040] The first power supply unit 651a supplies a transfer bias to the transfer cleaning roller 65 to impart an adhesive force to the toner such that the toner on the photosensitive drum 3 moves toward the transfer member (61). The second power supply unit 651b supplies a transfer bias to the transfer cleaning roller 65 to impart an adhesive force to the toner such that the toner on the transfer member (61) moves toward the photosensitive drum 3.

[0041] The switching unit 651c can selectively switch between the current supply from the first power supply unit 651a to the transfer cleaning roller 65 and the current supply from the second power supply unit 651b to the transfer cleaning roller 65. The output side of the first power supply unit 651a is electrically connected to the first terminal 651d of the switching unit 651c. The output side of the second power supply unit 651b is electrically connected to the second terminal 651e of the switching unit 651c. The output wiring 651f of the switching unit 651c is electrically connected to the transfer cleaning roller 65.

[0042] The switching unit 651c is electrically connected to the output system of the control unit 19. Thereby, when the control unit 19 transfers the toner image T on the photosensitive drum 3 to the sheet P, it can connect the output wiring 651f to the first terminal 651d by sending an instruction signal to the switching unit 651c (see the solid line in FIG. 2). Thereby, the toner on the photosensitive drum 3 can be directed toward the transfer member (61). Further, when the control unit 19 returns (cleans) the toner on the transfer member (61) to the photosensitive drum 3, it can connect the output wiring 651f to the second terminal 651e by sending an instruction signal to the switching unit 651c (see the dashed line in FIG. 2). Thereby, the toner on the transfer member (61) can be directed toward the photosensitive drum 3 side.

[0043] <Control unit> As shown in FIG. 3, the control unit 19 includes a processing unit 191 and a storage unit 192. The processing unit 191 is composed of a microcomputer such as a CPU. The storage unit 192 includes a non-volatile memory such as a ROM and a volatile memory such as a RAM. The control unit 19 controls the operation of various components by loading and executing a control program stored in advance in the ROM of the storage unit 192 onto the RAM of the storage unit 192.

[0044] (First Embodiment) The control unit 19 controls the transfer device 6 and the pre-transfer charge removal device 16. The control unit 19 is capable of executing the inter-sheet charge removal change control described later and includes an inter-sheet charge removal change control unit Q1.

[0045] Before explaining the inter-sheet charge removal change control by the control unit 19, the charge removal effect of the pre-transfer charge removal device 16 will be explained.

[0046] FIG. 4 illustrates the influence of the charge removal by the pre-transfer charge removal device 16 on the transfer efficiency, and shows how the relationship between the transfer bias and the transfer efficiency Tr changes depending on the power supplied to the pre-transfer charge removal device 16. That is, it is a graph showing the relationship between the transfer bias and the transfer efficiency Tr with respect to the change in the power supply amount to the pre-transfer charge removal device 16.

[0047] Here, the transfer bias is the transfer current (current value It) supplied from the transfer power supply 623 to the transfer member (62), and the transfer efficiency Tr is the ratio of the amount of toner transferred to the sheet P (the amount of toner on the sheet P) when the toner image formed on the photosensitive drum 3 is transferred to the sheet P, divided by the amount of toner on the surface 3a of the photosensitive drum 3 before being transferred to the sheet P, under the condition of the transfer current (current value It). It is desirable that the value of the transfer efficiency Tr be higher. This is because it can reduce the amount of waste toner remaining on the photosensitive drum 3 without being transferred to the sheet P and removed by the cleaning device 4. Usually, in the image forming apparatus 100, it is set so as to achieve a predetermined (desired) transfer efficiency Trp. The amount of power supplied to the pre-transfer charge elimination device 16 is the current value I of the pre-transfer charge elimination current supplied to the pre-transfer charge elimination device 16 (pre-transfer charge elimination lamp 162). The larger the current value I, the greater (brighter) the amount of light emitted by the pre-transfer charge elimination lamp 162.

[0048] As shown in FIG. 4, the transfer efficiency Tr increases as the transfer current (current value It) increases from 0 mA. However, when the transfer efficiency exceeds 90%, there is a tendency that even if the transfer current (current value It) is increased, it hardly increases any further. This is because a re-transfer phenomenon occurs in which the toner transferred to the sheet P returns to the photosensitive drum 3 again as the current value It increases. Specifically explaining with reference to FIG. 4, when the current value I of the pre-transfer charge elimination current is I0 = 0 mA (the pre-transfer charge elimination device 16 is off), when the current value It of the transfer current supplied to the transfer member (62) is It0 = 50 μA, a predetermined (desired) transfer efficiency Tr (90% in this example) is reached. Even if the current value It of the transfer current exceeds 50 μA, the transfer efficiency Tr does not reach 100% and remains almost saturated at 90%. The reason for saturation is as described above. Note that the larger the current value It of the transfer current, the greater the damage (optical fatigue) to the photosensitive drum 3. Therefore, the current value It of the transfer current should be as small as possible as long as the transfer efficiency Tr exceeds 90%.

[0049] Next, the relationship between the power supplied to the pre-transfer charge elimination device 16 and the transfer efficiency Tr will be described. As shown in FIG. 4, the lower limit value of the current value It of the transfer current that achieves a predetermined transfer efficiency Trp (90% in this example) decreases sequentially as the current value I of the pre-transfer charge elimination current is I = 0 mA, I1 = 5 mA, I2 = 10 mA in the order of It = 50 μA, It1 = 40 μA, It2 = 30 μA with the increase of the current value I of the pre-transfer charge elimination current. This is because as the current value I of the pre-transfer charge elimination current increases, the amount of light of the pre-transfer charge elimination lamp 162 increases, and the potential of the electrostatic latent image formed on the surface 3a of the photoreceptor drum 3 decreases sequentially (is charge-eliminated), so that the electrostatic adhesion force between the toner image and the photoreceptor drum decreases. As a result, a predetermined transfer efficiency Trp (90% in this example) can be achieved with a lower transfer current. However, the effect also has a limit. Even if the current value I of the pre-transfer charge elimination current is increased to I3 = 15 mA, the lower limit value of the current value It of the transfer current that achieves the predetermined transfer efficiency does not fall below the lower limit value when the current value I of the pre-transfer charge elimination current is I2 = 10 mA and becomes the same value (It3 = 30 μA).

[0050] As described above, by changing the current value I of the current supplied to the pre-transfer charge elimination lamp 162, the magnitude of the charge elimination effect on the photoreceptor drum 3 can be changed, and thereby the transfer current that can achieve a predetermined transfer efficiency Trp can be lowered.

[0051] In this embodiment, when transferring the toner image formed on the photoreceptor drum 3 to the sheet P, the transfer pre - discharging device 16 is driven while power is supplied to it. More specifically, when transferring the toner image formed on the photoreceptor drum 3 to the sheet P, the control unit 19 drives the transfer pre - discharging device 16 with the current value I of the current supplied to the transfer pre - discharging lamp 162 being I2 = 10 mA. The control unit 19 stores this value of I2 = 10 mA in the storage unit 192 as the first condition. The value of the current It supplied to the transfer members (61, 62) is 42 μA, and the control unit 19 stores this value in the storage unit 192 as the reference transfer bias (reference transfer current value Is). The reference transfer bias (Is) is set with a margin of about +5% greater than It1 = 40 μA in consideration of variations in components, etc., but it may be set to a value greater than It1 = 40 μA and lower than It0 = 50 μA. These values are all obtained in advance through experiments.

[0052] By setting the conditions for transferring the toner image formed on the photoreceptor drum 3 to the sheet P as described above, the toner image formed on the photoreceptor drum 3 can be efficiently transferred to the sheet P. However, in some cases, the development may result in minute amounts of toner called fog toner being scattered outside the area where the toner image is formed on the photoreceptor drum 3. When continuously forming toner images in a state where such fog toner has occurred, between the sheets, that is, between the sheet P and the next sheet P, the fog toner may be transferred to the transfer member (61), soiling the surface of the transfer member (61), and the fog toner adhering to the transfer member (61) may adhere to the sheet P again, soiling the sheet P.

[0053] Therefore, in this embodiment, during the transfer between sheets when continuously transferring the toner image to the sheet P, the inter - sheet discharge change control is performed to change the driving conditions of the transfer pre - discharging device 16.

[0054] Next, this inter - sheet discharge change control will be described.

[0055] FIG. 5 schematically shows a region on the photoreceptor drum 3, namely, a sheet region α1 where a toner image to be transferred to the sheet P is formed, and an inter-sheet region α2 which is a region on the photoreceptor drum 3 where no toner image corresponding to the interval between the sequentially fed sheets P is formed. (FIG. 5 is a plan view schematically showing the case of transferring a toner image to two sheets P.) With reference to FIG. 5, the timing and conditions for driving the pre-transfer eraser 16 in the sheet region α1 and the inter-sheet region α2 by the control unit 19 will be described.

[0056] The control unit 19 has an inter-sheet erasure change control unit Q1 for performing inter-sheet erasure change control to change the driving conditions of the pre-transfer eraser 16 between the sheets P described above to conditions different from those at the time of toner image transfer.

[0057] When transferring the toner image T formed on the photoreceptor drum 3 to the sheet P, while at least a first region β1 including the sheet region α1 which is the region corresponding to the sheet P where the toner image T is formed on the surface 3a of the photoreceptor drum 3 is passing through the pre-transfer eraser 16, the pre-transfer eraser 16 is driven under a predetermined first condition (10 mA as described above). Then, while at least a part (all or part) of a second region β2 of the inter-sheet region α2 between the sheet region corresponding to the sheet P where the toner image T is formed on the surface 3a of the photoreceptor drum 3 and the sheet region corresponding to the subsequent sheet P where the toner image T is to be formed next is passing through the pre-transfer eraser 16, the pre-transfer eraser 16 is driven under a predetermined second condition (0 mA in this example) where the erasure effect is smaller than the first condition (the current value set thereby) by the inter-sheet erasure change control unit Q1. Here, the timing when the first region β1 passes through the pre-transfer eraser 16 and the timing when the second region β2 passes through the pre-transfer eraser 16 can be determined by the timing when various conveyance detection units (not shown) provided in the sheet conveyance path W1 and the reverse sheet conveyance path W2 detect the passage of the sheet P.

[0058] In this example, the first region β1 is the same as the sheet region α1, and the second region β2 is the same as the inter-sheet region α2. Therefore, the driving of the pre-transfer charge eliminator 16 under the first condition starts at the timing when the position corresponding to the leading end P1 of the preceding sheet P passes through the pre-transfer charge eliminator 16, and ends at the timing when the position corresponding to the trailing end P2 of the preceding sheet P passes through the pre-transfer charge eliminator 16. Also, the driving of the pre-transfer charge eliminator 16 under the second condition of the inter-sheet charge elimination change control starts at the timing when the position corresponding to the trailing end P2 of the preceding sheet P passes through the pre-transfer charge eliminator 16, and ends at the timing when the position corresponding to the leading end P1 of the subsequent sheet P passes through the pre-transfer charge eliminator 16.

[0059] According to the present embodiment, when transferring the toner image T onto the sheet P, while the first region β1 of the surface 3a of the photosensitive drum 3 is passing through the pre-transfer charge eliminator 16, the pre-transfer charge eliminator 16 is driven under the first condition, and while the second region β2 of the surface 3a of the photosensitive drum 3 is passing through the pre-transfer charge eliminator 16, the pre-transfer charge eliminator 16 is driven under the second condition where the charge elimination effect is smaller than the first condition. Therefore, even when there is residual toner on the surface 3a of the photosensitive drum 3 in the inter-sheet region α2 of the surface 3a of the photosensitive drum 3, it can be made difficult for it to be transferred to the transfer member (61, 62) side. As a result, it can be made difficult for the residual toner on the surface 3a of the photosensitive drum 3 to adhere to the transfer member (61). Therefore, it is possible to suppress the back surface of the sheet P (the surface opposite to the image forming surface) from being soiled with toner.

[0060] In addition, the charge removal by the pre-transfer charge removal device 16 has the advantage of being able to improve the transfer efficiency by reducing the electrostatic adhesion force of the toner image T to the photoreceptor drum 3. In this example, in the first region β1, by irradiating light on the surface 3a of the photoreceptor drum 3 after development under the first condition where the charge removal effect is greater than the second condition from the pre-transfer charge removal device 16, the negative charge on the surface 3a of the photoreceptor drum 3 can be weakened. Then, in the portion where the toner adheres in the first region β1, the electrical attractive force between the photoreceptor drum 3 and the toner can be weakened, thereby improving the transfer efficiency by the transfer operation in the first region β1. Note that the transfer bias supplied by the transfer power source 623 to the transfer member (62) is set to a value that can obtain a predetermined transfer efficiency (for example, 90% or more) under the first condition of the pre-transfer charge removal device 16.

[0061] In this embodiment, the current value I of the pre-transfer charge removal current under the first condition is set to 10 mA, and the current value I of the pre-transfer charge removal current under the second condition is set to 0 mA. However, the present invention is not limited to this value, as long as the current value I of the pre-transfer charge removal current under the second condition is set smaller than the current value I of the pre-transfer charge removal current under the first condition.

[0062] Thereby, while suppressing the damage (optical fatigue) given to the photoreceptor drum 3 by keeping the current value It of the transfer current low, a predetermined transfer efficiency can be achieved.

[0063] (Second Embodiment) FIG. 6 shows the amount of fogged toner adhering to the surface 3a of the photoreceptor drum 3, and is a graph showing the measured values of the fog density of the fogged toner on the surface 3a of the photoreceptor drum 3 with respect to the ambient humidity. In FIG. 6, the larger the measured value of the fog density (measurement value ID by the reflection densitometer), the larger the degree of fogging of the fogged toner (amount of fogged toner) is shown.

[0064] Incidentally, the fogged toner on the surface 3a of the photoreceptor drum 3 is easily affected by the ambient humidity of the image forming apparatus 100.

[0065] That is, as the ambient humidity around the image forming apparatus 100 increases, the charge amount of the toner decreases. Therefore, as shown in FIG. 6, it is easy for the fogged toner on the surface 3a of the photosensitive drum 3 to occur, and as a result, it is easy for the back surface of the sheet P to be soiled. Therefore, it is desirable to execute the inter-sheet static elimination change control when the ambient humidity around the image forming apparatus 100 is equal to or higher than the reference humidity.

[0066] In this regard, the image forming apparatus 100 further includes an environment detection unit 150 (see FIG. 3) that detects the ambient environment around the image forming apparatus 100. The environment detection unit 150 includes a humidity detection unit 151 (see FIG. 3) that detects the ambient humidity Ha around the image forming apparatus 100.

[0067] Based on the detection result of the humidity detection unit 151, when the ambient humidity Ha is equal to or higher than the reference humidity Hs, the control unit 19 executes the inter-sheet static elimination change control. The reference humidity Hs is the boundary for determining whether or not the fogged toner is acceptable. The reference humidity Hs is set in advance by experiments or the like and is stored in the storage unit 192 in advance.

[0068] In this example, the control unit 19 further includes a surrounding humidity detection control unit Q2 that detects the ambient humidity Ha from the detection signal of the humidity detection unit 151. Here, when the ambient humidity Ha is equal to or higher than the reference humidity Hs (for example, 50% or 75%), the measured value ID becomes 0.15 or more, which is the reference fog density value IDs, and the degree of fogging of the fogged toner exceeds the allowable degree. For this reason, when the ambient humidity Ha detected by the ambient humidity detection control unit Q2 is equal to or higher than the reference humidity Hs (for example, 50% or 75%), the inter-sheet static elimination change control unit Q1 executes the inter-sheet static elimination change control.

[0069] In the present embodiment, by executing the inter-sheet static elimination change control when the ambient humidity Ha is equal to or higher than the reference humidity Hs, in the reference humidity Hs or higher where the fogged toner is likely to occur on the surface 3a of the photosensitive drum 3, the pre-transfer static elimination device 16 can be driven under the first condition while the first region β1 passes through the pre-transfer static elimination device 16, and the pre-transfer static elimination device 16 can be driven under the second condition while the second region β2 passes through the pre-transfer static elimination device 16. As a result, the back surface of the sheet P can be prevented from being soiled at the reference humidity Hs or higher.

[0070] Here, the post-transfer charge eliminator 17 eliminates the potential remaining on the surface 3a of the photosensitive drum 3 after transferring the toner image T onto the sheet P, and ideally, it sets the potential on the surface 3a of the photosensitive drum 3 to 0V.

[0071] However, when the ambient humidity Ha of the image forming apparatus 100 is less than the reference humidity Hs, although fogging toner is less likely to occur, if the inter-sheet non-charge change control is executed, a phenomenon called transfer memory occurs, in which the potential remains on the photosensitive drum 3 due to the flow of the transfer bias (transfer current) to the non-image forming portion side where the toner image T is not formed on the surface 3a of the photosensitive drum 3.

[0072] In this regard, the charge elimination effect of the pre-transfer charge eliminator 16 has the advantage that it can effectively prevent transfer memory.

[0073] This will be described below with reference to FIGS. 7A to 7C and FIGS. 8A to 8C.

[0074] FIGS. 7A to 7C are graphs showing an example of the first to third processes before and after transfer of the charged potential state of the surface 3a of the photosensitive drum 3 in a state where it is not charged before transfer.

[0075] <When not charged before transfer> When the pre-transfer charge elimination device 16 fails to eliminate charge before transfer, for example, as shown in Fig. 7A, before transfer, the non-image forming section potential Vo (the potential of the non-image forming section where the toner image T is not formed) is about -650 V, and the image forming section potential VL (the potential of the image forming section where the toner image T is formed) is about -150 V. As shown in Fig. 7B, during transfer, a larger positive transfer current flows into the non-image forming section potential Vo compared to the image forming section potential VL. As shown in Fig. 7C, when charge is eliminated after transfer by the post-transfer charge elimination device 17, the state where there is a large positive transfer current in the non-image forming section potential Vo remains, and the potentials of the non-image forming section potential Vo and the image forming section potential VL are reversed. Then, instead of the potential on the surface 3a of the photoreceptor drum 3 being 0 V as it should be, a potential of about -50 V remains. Therefore, when the photoreceptor drum 3 is charged by the charging device 5 later, the potential on the surface 3a of the photoreceptor drum 3 becomes non-uniform.

[0076] In this regard, in the present embodiment, the transfer memory is effectively prevented by utilizing the charge elimination effect of the pre-transfer charge elimination device 16.

[0077] <When charged before transfer> Figs. 8A to 8C are graphs showing an example of the first to third processes before and after transfer of the charged potential state on the surface 3a of the photoreceptor drum 3 in a state where charge is eliminated before transfer.

[0078] In the state where pre-transfer charge elimination is performed by the pre-transfer charge eliminator 16, for example, as shown in FIG. 8A, before transfer, due to the charge elimination effect of the pre-transfer charge eliminator 16, the non-image forming portion potential Vo and the image forming portion potential VL (see the solid line in FIG. 8A) are lower than the non-image forming portion potential Vo and the image forming portion potential VL (see the broken line in FIG. 8A) in the state where they are not charge-eliminated before transfer. At this time, the potential (non-image forming portion potential Vo) at the location where there is no toner is lower than the potential (image forming portion potential VL) at the location where there is toner. As shown in FIG. 8B, during transfer, due to the charge elimination effect of the pre-transfer charge eliminator 16, the non-image forming portion potential Vo and the image forming portion potential VL have decreased, so the potential difference between the non-image forming portion potential Vo and the image forming portion potential VL becomes smaller, and the inflow of the positive transfer current to the image forming portion potential VL and the non-image forming portion potential Vo becomes almost equal. As shown in FIG. 8C, when charge elimination is performed after transfer by the post-transfer charge eliminator 17, when it is then charged by the charging device 5, the potential on the surface 3a of the photosensitive drum 3 becomes uniform. Thereby, the generation of transfer memory can be effectively prevented.

[0079] Therefore, the control unit 19 further includes an inter-sheet charge elimination non-change control unit Q3. When the ambient humidity Ha is less than the reference humidity Hs, the inter-sheet charge elimination non-change control unit Q3 executes inter-sheet charge elimination non-change control to drive the pre-transfer charge eliminator 16 under the first condition even when the second region β2 is passing through the pre-transfer charge eliminator 16 in addition to the first region β1. In this example, the inter-sheet charge elimination non-change control unit Q3 executes inter-sheet charge elimination non-change control when the ambient humidity Ha detected by the ambient humidity detection control unit Q2 is less than the reference humidity Hs (for example, 50% or 75%).

[0080] According to the present embodiment, by executing inter-sheet charge elimination non-change control when the ambient humidity Ha is less than the reference humidity Hs, in addition to the first region β1, even when the second region β2 is passing through the pre-transfer charge eliminator 16, the pre-transfer charge eliminator 16 can be driven under the first condition in the case where the transfer memory is likely to occur at a humidity less than the reference humidity Hs on the surface 3a of the photosensitive drum 3. Thereby, the generation of transfer memory can be effectively prevented.

[0081] (Third Embodiment) Incidentally, the fogged toner on the surface 3a of the photoreceptor drum 3 is liable to be affected by the ambient temperature of the image forming apparatus 100.

[0082] In this regard, in the present embodiment, the environment detection unit 150 further includes a temperature detection unit 152 (see FIG. 3) that detects the ambient temperature Ta of the image forming apparatus 100.

[0083] FIG. 9 is a first environment table TB1 showing an example of inter-sheet charge elimination change control or inter-sheet charge elimination non-change control for a combination of a plurality of predefined environment ranges. In FIG. 9, ON represents the execution of inter-sheet charge elimination change control, and OFF represents the execution of inter-sheet charge elimination non-change control. The first environment table TB1 is set in advance by experiments or the like and is stored in the storage unit 192 in advance.

[0084] Based on the detection results of the humidity detection unit 151 and the temperature detection unit 152, the control unit 19 determines that the ambient humidity Ha is greater than or equal to the reference humidity Hs in the first environment range [(in this example, Tr3, Hr4), (Tr3, Hr3 to Hr4)] within the temperature range (in this example, Tr3, Tr4) corresponding to the ambient temperature Ta among a plurality of predefined environment ranges (in this example, Tr1 to Tr4, Hr1 to Hr4) formed by combinations of a plurality of predetermined temperature ranges (in this example, Tr1 to Tr4) and a plurality of predetermined humidity ranges (in this example, Hr1 to Hr4). In this case, the inter-sheet charge elimination change control is executed. When it is determined that the ambient humidity Ha is less than the reference humidity Hs in the second environment range (in this example, Hr1 to Hr4, Hr1 to Hr4, Hr1 to Hr3, Hr1 to Hr2) within the temperature range (in this example, Tr1, Tr2, Tr3, Tr2) corresponding to the ambient temperature Ta, the inter-sheet charge elimination non-change control is executed.

[0085] In this example, the control unit 19 further includes an ambient temperature detection control unit Q4 that detects the ambient temperature Ta from the detection signal of the temperature detection unit 152. Here, when the ambient temperature Ta is 25°C or higher and less than 30°C (Tr3) and the ambient humidity Ha is 75% or higher (Hr4), or when the ambient temperature Ta is 30°C or higher (Tr4) and the ambient humidity Ha is 50% or higher (Hr3, Hr4), the measured value ID becomes 0.15 or higher, which is the reference overlapping density value IDs, and the overlapping degree of the overlapping toner exceeds the allowable degree. Therefore, the inter-sheet static elimination change control unit Q1 executes the inter-sheet static elimination change control when the ambient temperature Ta detected by the ambient temperature detection control unit Q4 is 25°C or higher and less than 30°C and the ambient humidity Ha detected by the ambient humidity detection control unit Q2 is 75% or higher, or when the ambient temperature Ta detected by the ambient temperature detection control unit Q4 is 30°C or higher and the ambient humidity Ha detected by the ambient humidity detection control unit Q2 is 50% or higher. Also, the inter-sheet static elimination non-change control unit Q3 executes the inter-sheet static elimination non-change control when the ambient temperature Ta is less than 25°C (Tr1, Tr2) regardless of the ambient humidity Ha, when the ambient temperature Ta is 25°C or higher and less than 30°C (Tr3) and the ambient humidity Ha is less than 75% (Hr1 to Hr3), or when the ambient temperature Ta is 30°C or higher (Tr4) and the ambient humidity Ha is less than 50% (Hr1 to Hr2).

[0086] Specifically, as shown in FIG. 9, the plurality of environmental ranges of the first environmental table TB1 are a matrix-like range in which a plurality of temperature ranges (Tr1 to Tr4) and a plurality of humidity ranges (Hr1 to Hr4) are combined.

[0087] In the first temperature range Tr1 below 20°C and the second temperature range Tr2 from 20°C or higher and less than 25°C, it is off in any humidity range (second environmental range), and the inter-sheet static elimination non-change control unit Q3 executes the inter-sheet static elimination non-change control.

[0088] In the third temperature range Tr3 where the temperature is above 25°C and below 30°C, in the first humidity range Hr1 of less than 25%, the second humidity range Hr2 of 25% or more and less than 50%, and the third humidity range Hr3 of 50% or more and less than 75%, it is off (second environmental range), and the inter-sheet static elimination non-change control unit Q3 executes inter-sheet static elimination non-change control. In the fourth humidity range Hr4 of 75% or more, it is on (first environmental range), and the inter-sheet static elimination change control unit Q1 executes inter-sheet static elimination change control.

[0089] Also, in the fourth temperature range Tr4 where the temperature is 30°C or more, in the first humidity range Hr1 and the second humidity range Hr2, it is off (second environmental range), and the inter-sheet static elimination non-change control unit Q3 executes inter-sheet static elimination non-change control. In the third humidity range Hr3 and the fourth humidity range Hr4, it is on (first environmental range), and the inter-sheet static elimination change control unit Q1 executes inter-sheet static elimination change control.

[0090] By doing so, in consideration of the change in the ambient temperature Ta of the image forming apparatus 100, inter-sheet static elimination change control can be executed at a reference humidity Hs or higher at which fogged toner is likely to occur on the surface 3a of the photoreceptor drum 3, and thereby, the back surface contamination of the sheet P can be suppressed at a reference humidity Hs or higher. On the other hand, in consideration of the change in the ambient temperature Ta of the image forming apparatus 100, even when the second region β2 in addition to the first region β1 is passing through the pre-transfer static eliminator 16 at less than the reference humidity Hs at which transfer memory is likely to occur on the surface 3a of the photoreceptor drum 3, the pre-transfer static eliminator 16 can be driven under the first condition, and thereby, the occurrence of transfer memory can be effectively prevented.

[0091] (Fourth Embodiment) Incidentally, when the ambient humidity Ha is less than the reference humidity Hs, the control unit 19 executes inter-sheet static elimination non-change control for driving the pre-transfer static eliminator 16 under the first condition even when the second region β2 in addition to the first region β1 is passing through the pre-transfer static eliminator 16. However, transfer memory is more likely to occur as the ambient humidity Ha of the image forming apparatus 100 decreases.

[0092] FIG. 10 is a graph showing the ease of occurrence of transfer memory with respect to the amount of power (pre-transfer charge removal current) supplied to the pre-transfer charge removal device 16 for each of a plurality of environmental ranges ("HH", "RR", "LL"). Here, "HH" represents the fourth humidity range Hr4 in the third temperature range Tr3, and the third and fourth humidity ranges Hr3 and Hr4 in the fourth temperature range Tr4.

[0093] "RR" represents the third and fourth humidity ranges Hr3 and Hr4 in the first temperature range Tr1, the second to fourth humidity ranges Hr2 to Hr4 in the second temperature range Tr2, the first to third humidity ranges Hr1 to Hr3 in the third temperature range Tr3, and the first and second humidity ranges Hr1 and Hr2 in the fourth temperature range Tr4.

[0094] "LL" represents the first and second humidity ranges Hr1 and Hr2 in the first temperature range Tr1 and the first humidity range Hr1 in the second temperature range Tr2.

[0095] As shown in FIG. 10, when the ambient humidity Ha is equal to or higher than the reference humidity Hs ( "HH" in FIG. 10), the occurrence of transfer memory is suppressed. On the other hand, when the ambient humidity Ha is less than the reference humidity Hs ( "RR" and "LL" in FIG. 10), in the case of "RR", unless the current value I of the pre-transfer charge removal current is set to I2 = 10 mA, the occurrence of transfer memory cannot be suppressed, and in the case of "LL", unless the current value I of the pre-transfer charge removal current is set to I3 = 15 mA, the occurrence of transfer memory cannot be suppressed.

[0096] In this regard, the first condition is preset to a value corresponding to a plurality of environmental ranges. Specifically, the first condition increases the amount of power (pre-transfer charge removal current) supplied to the pre-transfer charge removal device 16 as the ambient humidity Ha decreases.

[0097] FIG. 11 is a second environmental table TB2 showing an example of the current value I of the pre-transfer charge-removing current for a combination of a plurality of predefined environmental ranges. In this example, the current value I of the pre-transfer charge-removing current is set to I1 = 5 mA, I2 = 10 mA, and I3 = 15 mA. Also, in FIG. 11, the shaded portion represents the first environmental range. The second environmental table TB2 is set in advance by experiments or the like and is stored in the storage unit 192 in advance.

[0098] The control unit 19 sets, as a first condition, the current value I of the pre-transfer charge-removing current corresponding to the ambient temperature Ta and the ambient humidity Ha detected in the second environmental table TB2.

[0099] Specifically, as shown in FIG. 11, the plurality of environmental ranges in the second environmental table TB2 are a matrix-like range in which a plurality of temperature ranges (Tr1 to Tr4) and a plurality of humidity ranges (Hr1 to Hr4) are combined. In the first temperature range Tr1, the current value I is I3 = 15 mA in the first humidity range Hr1 and the second humidity range Hr2, and the current value I is I2 = 10 mA in the third humidity range Hr3 and the fourth humidity range Hr4.

[0100] In the second temperature range Tr2, the current value I is I3 = 15 mA in the first humidity range Hr1, and the current value I is I2 = 10 mA from the second humidity range Hr2 to the fourth humidity range Hr4.

[0101] In the third temperature range Tr3, the current value I is I2 = 10 mA from the first humidity range Hr1 to the third humidity range Hr3, and the current value I is I1 = 5 mA in the fourth humidity range Hr4.

[0102] Also, in the fourth temperature range Tr4, the current value I is I2 = 10 mA in the first humidity range Hr1 and the second humidity range Hr2, and the current value I is I1 = 5 mA in the third humidity range Hr3 and the fourth humidity range Hr4.

[0103] By doing so, in an ambient environment where a transfer memory with an ambient humidity Ha lower than the reference humidity Hs is likely to occur, as the ambient humidity Ha decreases, the amount of power supplied to the pre-transfer static eliminator 16 (pre-transfer static elimination current) can be increased, thereby suppressing the generation of transfer memory even in an ambient environment where a transfer memory with an ambient humidity Ha lower than the reference humidity Hs is likely to occur.

[0104] (Fifth Embodiment) Incidentally, when the control unit 19 is in the first environmental range [(Tr3, Hr4), (Tr4, Hr3 to Hr4) in this example], it executes the inter-sheet static elimination change control. However, in doing so, the burden due to the on / off operation of the pre-transfer static eliminator 16 is large. Also, in the first environmental range (Tr3, Hr4), (Tr4, Hr3 to Hr4) (see the shaded area in Fig. 11), the lower the electrical resistance value of the sheet P, the more likely the transferability to the back surface of the sheet P decreases, and thus, it is less likely that soiling occurs on the back surface of the sheet P. Therefore, when the electrical resistance value of the sheet P is smaller than a preset reference electrical resistance value, it is desirable to execute the inter-sheet static elimination non-change control without deliberately executing the inter-sheet static elimination change control.

[0105] In this regard, the control unit 19 detects a resistance equivalent value Ep corresponding to the electrical resistance value of the sheet P in the first environmental range (Tr3, Hr4), (Tr4, Hr3 to Hr4). When the resistance equivalent value Ep is equal to or greater than a preset reference resistance equivalent value Es, it determines that the electrical resistance value of the sheet P is equal to or greater than the preset reference electrical resistance value, and the inter-sheet static elimination change control unit Q1 executes the inter-sheet static elimination change control. Also, when the resistance equivalent value Ep is smaller than the reference resistance equivalent value Es, the control unit 19 determines that the electrical resistance value of the sheet P is smaller than the reference electrical resistance value, and the inter-sheet static elimination non-change control unit Q3 executes the inter-sheet static elimination non-change control. The reference resistance equivalent value Es is set in advance through experiments or the like and is stored in the storage unit 192 in advance.

[0106] According to this embodiment, in the case of the first environmental ranges (Tr3, Hr4) and (Tr4, Hr3 to Hr4), when the electrical resistance value of the sheet P is smaller than the reference electrical resistance value, it is possible to execute the inter-sheet static elimination non-change control without executing the inter-sheet static elimination non-change control, thereby reducing the burden due to the on / off operation of the pre-transfer static eliminator 16.

[0107] (Sixth Embodiment) FIG. 12 is a graph showing the voltage value Vp of the transfer voltage between the photoreceptor drum 3 and the transfer member (62) in the state of supplying a transfer current to the sheet P with respect to the electrical resistance value Rp of the sheet P in the first environmental ranges (Tr3, Hr4) and (Tr4, Hr3 to Hr4).

[0108] In the constant current control of the transfer power supply 623, as shown in FIG. 12, the smaller the electrical resistance value Rp of the sheet P, the smaller the voltage value Vp of the transfer voltage between the photoreceptor drum 3 and the transfer member (62) in the state of supplying a transfer current to the sheet P. Thereby, the electrical resistance value Rp of the sheet P can be estimated from the voltage value Vp of the transfer voltage.

[0109] That is, the control unit 19 supplies a transfer current to the transfer member (62) by constant current control as a transfer bias, and detects the voltage value Vp in the state of supplying a transfer current (42 μA in this example) to the sheet P as a resistance equivalent value.

[0110] In the example shown in FIG. 12, when the electrical resistance value Rp of the sheet P is about 10 10 Ω or more, the measured value ID becomes 0.15 or more which is the reference fog density value IDs, and the fog degree of the fogged toner exceeds the allowable degree. Therefore, 10 10 Ω is set as the reference electrical resistance value Rs, and the reference resistance equivalent value Es corresponding to 10 10 Ω can be set in advance. Specifically, when the electrical resistance value Rp of the sheet P is about 10 10 Ω or more and the voltage value Vp of the transfer voltage is the reference voltage value Vs (2.5 kV in this example), the fog degree of the fogged toner exceeds the allowable degree. Therefore, 10 10Let the reference electrical resistance value Rs be Ω, and the reference voltage value Vs corresponding to 10 10 Ω can be set in advance. Here, the resistance equivalent value Ep corresponds to the voltage value Vp, and the reference resistance equivalent value Es corresponds to the reference voltage value Vs.

[0111] The control unit 19 further includes a constant current control unit Q5, a voltage value detection control unit Q6, and a determination control unit Q7 (see FIG. 3). The constant current control unit Q5 performs constant current control so that the transfer current becomes a constant current value (42 μA in this example) in the transfer power supply 623 when image formation is performed.

[0112] The voltage value detection control unit Q6 detects (monitors) the voltage value between the photoreceptor drum 3 and the transfer member (62) in a state where the transfer current is supplied to the sheet P by the constant current control by the constant current control unit Q5. Here, since the transfer current is constant at the constant current value (42 μA), the voltage value Vp detected by the voltage value detection control unit Q6 in a state where the transfer current is supplied to the sheet P is proportional to the electrical resistance value Rp of the sheet P. That is, the larger the detected voltage value Vp, the larger the electrical resistance value Rp of the sheet P.

[0113] Therefore, when the voltage value Vp detected by the voltage value detection control unit Q6 is smaller than the preset reference voltage value Vs, the determination control unit Q7 can determine that the electrical resistance value Rp of the sheet P is smaller than the reference electrical resistance value Rs.

[0114] In this configuration, a configuration for determining whether the resistance equivalent value Ep is smaller than the reference resistance equivalent value Es can be realized with a simple configuration such as detecting the voltage value between the photoreceptor drum 3 and the transfer member (62).

[0115] (Embodiment 7) Incidentally, in the portion on the surface 3a of the photoreceptor drum 3 where the toner image T is not formed (non-image forming portion), the greater the electric attractive force between the surface 3a of the photoreceptor drum 3 after transfer and the sheet P, the more difficult it is for the sheet P after transfer to separate from the surface 3a of the photoreceptor drum 3, that is, the so-called sheet peelability deteriorates. This becomes more prominent as the ambient humidity Ha is greater than the reference humidity Hs and / or as the electric resistance value of the sheet P is greater.

[0116] In this regard, in the present embodiment, the sheet peelability is improved by utilizing the charge removal effect of the pre-transfer charge removal device 16.

[0117] FIG. 13 is a plan view schematically showing another example of the execution timing under the first condition and another example of the execution timing under the second condition in the inter-sheet charge removal change control for the pre-transfer charge removal device 16.

[0118] As shown in FIG. 13, the first region β1 on the surface 3a of the photoreceptor drum 3 is a region from the position corresponding to a predetermined distance d (for example, 5 mm) before the leading edge P1 of the preceding sheet P to the position corresponding to the trailing edge P2 of the preceding sheet P. The second region β2 on the surface 3a of the photoreceptor drum 3 is a region from the position corresponding to the trailing edge P2 of the preceding sheet P to the position corresponding to a predetermined distance d (for example, 5 mm) before the leading edge P1 of the subsequent sheet P.

[0119] That is, the execution under the first condition for the pre-transfer charge removal device 16 starts at the timing when the position corresponding to a predetermined distance before the leading edge P1 of the preceding sheet P passes through the pre-transfer charge removal device 16, and ends at the timing when the position corresponding to the trailing edge P2 of the preceding sheet P passes through the pre-transfer charge removal device 16. The execution under the second condition for the pre-transfer charge removal device 16 starts at the timing when the position corresponding to the trailing edge P2 of the preceding sheet P passes through the pre-transfer charge removal device 16, and ends at the timing when the position corresponding to a predetermined distance before the leading edge P1 of the subsequent sheet P passes through the pre-transfer charge removal device 16. Note that a margin is provided across the entire width direction orthogonal to the conveyance direction H within a predetermined distance (5 mm in this example) from the leading edge P1 of the sheet P.

[0120] By doing so, the electric attractive force between the surface 3a of the photoreceptor drum 3 after transfer and the sheet P can be weakened at positions corresponding to the position just before a predetermined distance from the leading edge P1 of the sheet P to the position corresponding to the leading edge P1 of the sheet P, thereby improving the sheet peelability. This is particularly effective when the ambient temperature Ta and the ambient humidity Ha are within the fourth humidity range Hr4 in the third temperature range Tr3, and / or the third humidity range Hr3 and the fourth humidity range Hr4 in the fourth temperature range Tr4, and / or when the control unit 19 determines that the electrical resistance value of the sheet P is smaller than a preset reference electrical resistance value.

[0121] <Processing example> Next, a processing example of the inter-sheet static elimination change control and the inter-sheet static elimination non-change control will be described below with reference to FIG. 14.

[0122] FIGS. 14A and 14B are flowcharts showing the first half and the second half of an example of the flow of the inter-sheet static elimination change control and the inter-sheet static elimination non-change control, respectively.

[0123] As shown in FIG. 14A, the control unit 19 first detects the ambient temperature Ta and the ambient humidity Ha of the image forming apparatus 100 (S1), and reads out the current value I of the pre-transfer static elimination current corresponding to the ambient temperature Ta and the ambient humidity Ha from the second environment table TB2 (see FIG. 11) (S2).

[0124] Specifically, when the ambient temperature Ta and the ambient humidity Ha are within the fourth humidity range Hr4 of the third temperature range Tr3, the third humidity range Hr3 and the fourth humidity range Hr4 of the fourth temperature range Tr4, I1 = 5 mA is read out.

[0125] When the ambient temperature Ta and the ambient humidity Ha are within the third humidity range Hr3 and the fourth humidity range Hr4 of the first temperature range Tr1, the second humidity range Hr2 to the fourth humidity range Hr4 of the second temperature range Tr2, the first humidity range Hr1 to the third humidity range Hr3 of the third temperature range Tr3, the first humidity range Hr1 and the second humidity range Hr2 of the fourth temperature range Tr4, I2 = 10 mA is read out.

[0126] Also, when the ambient temperature Ta and the ambient humidity Ha are within the first humidity range Hr1 and the second humidity range Hr2 of the first temperature range Tr1, and within the first humidity range Hr1 of the second temperature range Tr2, I3 = 15 mA is read out.

[0127] Next, the control unit 19 sets the read current value as the first condition, and sets a current value smaller than the first condition (specifically, 0 mA, that is, turning off the pre-transfer charge removal device 16) as the second condition (S3).

[0128] Next, the control unit 19 determines whether the ambient temperature Ta and the ambient humidity Ha correspond to the first environment range (Tr3, Hr4), (Tr4, Hr3 - Hr4) in the first environment table TB1 (see FIG. 9) (S4). If it is determined that the environment range is the first environment range (S4: Yes), the process proceeds to S5 shown in FIG. 14B.

[0129] As shown in FIG. 14B, the control unit 19 starts the image forming process (job) in the first environment range (S5).

[0130] Next, the control unit 19 determines whether the voltage value Vp of the transfer voltage is greater than the reference voltage value (specifically, 2.5 kV) (S6). If it is determined that the voltage value Vp of the transfer voltage is greater than the reference voltage value Vs (S6: Yes), the image forming process is performed with inter-sheet charge removal change control (S7), and the process proceeds to S9. Also, if the control unit 19 determines that the voltage value Vp of the transfer voltage is less than or equal to the reference voltage value Vs (S6: No), the image forming process is performed with inter-sheet charge removal non-change control (S8), and the process proceeds to S9.

[0131] Next, if the control unit 19 determines that the image forming process is continuing (S9: Yes), the process proceeds to S6. On the other hand, if it is determined that the image forming process has ended (S9: No), the process ends.

[0132] Also, as shown in FIG. 14A, if the control unit 19 determines that the environment range is the second environment range (S4: No), the process proceeds to S10 shown in FIG. 14B.

[0133] As shown in FIG. 14B, the control unit 19 starts the image forming process (job) in the second environmental range (S10).

[0134] Next, the control unit 19 performs the image forming process under the inter-sheet charge elimination non-change control (S11). When it is determined that the image forming process continues (S12: Yes), the process proceeds to S11. On the other hand, when it is determined that the image forming process has ended (S12: No), the process ends.

[0135] The present disclosure is not limited to the embodiments described above and can be implemented in various other forms. Therefore, such embodiments are merely illustrative in all respects and should not be construed in a limiting sense. The scope of the present disclosure is indicated by the claims and is not restricted by the main text of the specification. Further, all modifications and changes belonging to the equivalent scope of the claims are within the scope of the present disclosure.

Explanation of Reference Numerals

[0136] 100 Image forming apparatus 150 Environmental detection unit 151 Humidity detection unit 152 Temperature detection unit 16 Pre-transfer charge elimination device 161 Pre-transfer charge elimination power supply 17 Post-transfer charge elimination device 171 Post-transfer charge elimination power supply 19 Control unit 191 Processing unit 192 Storage unit 2 Developing device 3 Photoconductor drum (an example of an image carrier) 3a Surface 5 Charging device 6 Transfer device 61 Transfer belt 61a Surface 62 Transfer roller 623 Transfer power supply 65 Transfer cleaning roller 651 Transfer cleaning power supply Ep equivalent resistance value Es reference resistance equivalent value H conveying direction Ha ambient humidity Hr1 first humidity range Hr2 second humidity range Hr3 third humidity range Hr4 fourth humidity range Hs reference humidity P sheet P1 tip P2 rear end Q1 inter-sheet charge removal change control unit Q2 ambient humidity detection control unit Q3 inter-sheet charge removal non-change control unit Q4 ambient temperature detection control unit Q5 constant current control unit Q6 voltage value detection control unit Q7 determination control unit R1 first rotation direction (rotation direction) T toner image Ta ambient temperature TB1 first environment table TB2 second environment table TN transfer nip section Tr1 first temperature range Tr2 second temperature range Tr3 third temperature range Tr4 fourth temperature range d predetermined distance α1 sheet area α2 inter-sheet area β1 first area β2 second area

Claims

1. An image forming apparatus including a developing device that forms a toner image on a rotatable image carrier, a transfer device having a transfer member that contacts the surface of the image carrier and transfers the toner image to a sheet conveyed to a contact portion with the surface of the image carrier, a pre-transfer charge eliminating device that eliminates the charge of the surface of the image carrier on the downstream side of the developing device and on the upstream side of the transfer member in the rotation direction of the image carrier, wherein the image forming apparatus further includes a control unit that controls the pre-transfer charge eliminating device, and the control unit drives the pre-transfer charge eliminating device under a predetermined first condition when at least a first region including a sheet region corresponding to the sheet on which the toner image is formed on the surface of the image carrier passes through the pre-transfer charge eliminating device when transferring the toner image formed on the image carrier to the sheet, and drives the pre-transfer charge eliminating device under a predetermined second condition having a smaller charge eliminating effect than the first condition when at least a part of a second region of an inter-sheet region between the sheet region on the surface of the image carrier where the toner image is formed and a sheet region corresponding to a subsequent sheet on which the toner image is to be formed next passes through the pre-transfer charge eliminating device, and is capable of executing inter-sheet charge elimination change control. An image forming apparatus characterized by this.

2. The image forming apparatus according to claim 1, further including a humidity detection unit that detects the ambient humidity of the image forming apparatus, wherein the control unit executes the inter-sheet charge elimination change control when the ambient humidity is equal to or higher than a reference humidity, and is capable of executing inter-sheet charge elimination non-change control in which the pre-transfer charge eliminating device is driven under the first condition even when the second region passes through the pre-transfer charge eliminating device in addition to the first region when the ambient humidity is less than the reference humidity. An image forming apparatus characterized by this.

3. The image forming apparatus according to claim 2, further including a temperature detection unit that detects the ambient temperature of the image forming apparatus, wherein when it is determined that the ambient humidity is in a first environmental range equal to or higher than the reference humidity in a temperature range corresponding to the ambient temperature among a plurality of predetermined environmental ranges defined by combinations of a plurality of predetermined temperature ranges and a plurality of predetermined humidity ranges, the control unit executes the inter-sheet charge elimination change control, and when it is determined that the ambient humidity is in a second environmental range less than the reference humidity in the temperature range corresponding to the ambient temperature, the control unit executes the inter-sheet charge elimination non-change control. An image forming apparatus characterized by this. ​ ​ ​

4. The image forming apparatus according to claim 3, wherein the first condition is preset to a value corresponding to the plurality of environmental ranges, the image forming apparatus characterized by this.

5. The image forming apparatus according to claim 3 or claim 4, wherein the control unit detects a resistance equivalent value corresponding to the electric resistance value of the sheet in the first environmental range, and when the resistance equivalent value is equal to or greater than a preset reference resistance equivalent value, the control unit executes the inter-sheet charge elimination change control, and when the resistance equivalent value is less than the reference resistance equivalent value, the control unit executes the inter-sheet charge elimination non-change control, the image forming apparatus characterized by this.

6. The image forming apparatus according to claim 5, further comprising a transfer power source that supplies a transfer bias to the transfer member, wherein the control unit supplies a transfer current to the transfer member by constant current control as the transfer bias, detects a voltage value in a state where the transfer current is supplied to the sheet, and when the voltage value is less than a preset reference voltage value, determines that the electric resistance value of the sheet is less than the reference electric resistance value, the image forming apparatus characterized by this.

7. The image forming apparatus according to claim 1, wherein the first region on the surface of the image carrier is a region from a position corresponding to a predetermined distance before the leading end of the preceding sheet to a position corresponding to the trailing end of the preceding sheet, and the second region on the surface of the image carrier is a region from a position corresponding to the trailing end of the preceding sheet to a position corresponding to a predetermined distance before the leading end of the subsequent sheet, the image forming apparatus characterized by this.

Images