Image forming device

Abstract

To prevent a reduction in image quality due to a transfer failure in an image forming apparatus that outputs voltages having a positive polarity and a negative polarity in a superimposed manner from a common power supply to a transfer member, and outputs a voltage with any one of the polarities from the common power supply to a static eliminating member.SOLUTION: An image forming apparatus has: transfer means that transfers a developer image on an image carrier to a recording material; static eliminating means that eliminates static electricity on the recording material; a first circuit that applies a first voltage having a polarity opposite to the electrification polarity of developer to the transfer means; a second circuit that is connected in series with the first circuit and applies a second voltage having the same polarity as the electrification polarity of the developer to the transfer means and the static eliminating means; and a control unit. The control unit applies, to the transfer means, a transfer voltage in which the first and second voltages are superimposed with each other to transfer the developer image to the recording material, applies a partial voltage from the second voltage to the static eliminating means as a static eliminating voltage to eliminate static electricity on the recording material, and when taking the static eliminating voltage as a first value causes the transfer voltage to fall below a target value, decreases the static eliminating voltage.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] In an electrophotographic image forming apparatus, a transfer voltage is output from a power source to a transfer member disposed opposite an image carrier such as a drum-shaped photosensitive member or an intermediate transfer member, thereby electrostatically transferring a toner image carried by the image carrier to a recording material such as paper. The recording material onto which the toner image has been transferred in a transfer section formed by the image carrier and the transfer member is then transported to a fixing means, where the toner image is fixed to the recording material by applying heat and pressure.

[0003] In such image forming devices, toner may adhere to the transfer member during the image formation operation, which may then adhere to the recording material being transported to the transfer section. To prevent the recording material from being soiled by toner adhering to the transfer member, a known configuration has been proposed in which a power supply outputs a voltage of the opposite polarity to the transfer voltage to the transfer member. This configuration can prevent toner from adhering to the transfer member, or remove and recover toner adhering to the transfer member. In this case, the power supply that outputs voltage to the transfer member can output both "positive" and "negative" voltages.

[0004] Furthermore, with the advancement of miniaturization in image forming devices, Patent Document 1 discloses a configuration in which a common power supply is used in series to output a voltage to a discharge member disposed downstream of the transfer unit in the conveyance direction of the recording material, and to output a voltage to the transfer member. In the configuration of Patent Document 1, while a toner image is transferred from an image carrier to the recording material in the transfer unit, the discharge member may discharge the recording material that has passed through the transfer unit. At this time, a negative voltage is output from the power supply to the discharge member, and a voltage in which negative and positive voltages are superimposed is output from the power supply to the transfer member so as to maintain a transfer voltage sufficient to transfer the toner image in the transfer unit. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-184340 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when forming a transfer voltage by superimposing positive and negative voltages as in Patent Document 1, it is necessary to increase the output of the positive voltage by the amount of the negative voltage compared to when only a positive voltage is output from the power supply to the transfer member. Meanwhile, in the power supply, the circuit that outputs the positive voltage and the circuit that outputs the negative voltage each have an upper limit voltage that they are allowed to output.

[0007] In compact, inexpensive devices, voltages close to the upper limit voltage are often used to enable faster image formation, longer life, and image formation on various recording materials. However, if the transfer voltage output is controlled to increase in response to the increase in resistance associated with use of the transfer member, the positive voltage may reach its upper limit. As a result, the transfer voltage obtained by superimposing negative and positive voltages may not be sufficiently high (or obtainable). In such cases, transfer defects, such as insufficient toner transfer, may occur. The same problem also occurs when the positive and negative polarities are reversed.

[0008] The present invention has been made in view of the above-mentioned problems, and provides a positive and negative polarity current to a transfer member from a common power source. The present invention aims to suppress deterioration of image quality due to transfer defects, etc., in an image forming apparatus that outputs voltages of different polarities in a superimposed manner and further outputs a voltage of either polarity to a discharging member from a common power source. [Means for solving the problem]

[0009] The present invention employs the following configuration: an image carrier that carries a developer image; a transfer means for transferring the developer image carried on the image carrier onto a recording material; a charge eliminating means for eliminating charge from the recording material; a first circuit for applying a first voltage having a polarity opposite to the normal charging polarity of the developer to the transfer means; a second circuit connected in series with the first circuit and configured to apply a second voltage having the same polarity as the normal charging polarity of the developer to the transfer means and the charge removing means; a control unit that controls the first circuit and the second circuit; An image forming apparatus having: The control unit applying the first voltage and the second voltage superimposed on each other as a transfer voltage to the transfer means so that the developer image is transferred onto the recording material; applying a partial voltage from the second voltage to the charge-removing means as a charge-removing voltage so that the recording material is discharged; When the transfer voltage falls below a predetermined target value in a case where the neutralization voltage is set to a first value, the neutralization voltage is changed to a second value that is lower than the first value but has the same polarity. The image forming apparatus is characterized by the above. [Effects of the Invention]

[0010] According to the present invention, in an image forming apparatus in which a common power source outputs superimposed positive and negative voltages to a transfer member, and the common power source also outputs a voltage of either polarity to a discharge member, degradation of image quality due to transfer failures, etc. can be suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view illustrating a configuration of an image forming apparatus according to a first embodiment. [Figure 2] Block diagram showing the configuration of power supply control in the first embodiment. [Figure 3] Schematic diagram of the static elimination needle in Example 1 [Figure 4] Circuit diagram of secondary transfer power supply in embodiment 1 [Figure 5] 1 is a timing chart illustrating secondary transfer control and charge removal control in the first embodiment. [Figure 6] FIG. 10 is a diagram illustrating the change in impedance versus the number of sheets passed in the first embodiment. [Figure 7] FIG. 10 is a diagram illustrating the relationship between static elimination voltage and image quality in the first embodiment. [Figure 8] FIG. 10 is a diagram illustrating the relationship between the output of a voltage circuit and image quality in the first embodiment. [Figure 9] 10 is a timing chart illustrating secondary transfer control and charge removal control in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Preferred embodiments of the present invention will now be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following embodiments may be changed as appropriate depending on the configuration of the device to which the present invention is applied and various conditions. Therefore, unless otherwise specified, they are not intended to limit the scope of the present invention.

[0013] Example 1 [Configuration of image forming device] 1 is a schematic cross-sectional view illustrating the configuration of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 has four image forming units SY, SM, SC, and SK arranged in a line. This is a so-called tandem image forming apparatus. Each image forming unit SY, SM, SC, and SK forms an image of each color: yellow (Y), magenta (M), cyan (C), and black (K). In this embodiment, the configuration and operation of each image forming unit SY, SM, SC, and SK are essentially the same except for the color of toner (developer) used. Therefore, in the following description, unless a distinction is required, the suffixes Y, M, C, and K that indicate an element for one of the colors will be omitted.

[0014] The image forming unit S has a drum-type (cylindrical) photosensitive drum 1, which is a first image carrier. This photosensitive drum 1 is driven to rotate in the direction of arrow R1 in the figure. The following devices are arranged around the photosensitive drum 1 in this order along its rotation direction. First, a charging roller 2, which is a roller-shaped charging member, is arranged as charging means. Further arranged are an exposure device 3 as exposure means, a development device 4 as development means, and a drum cleaning device 6 as means for cleaning residual toner on the photosensitive drum 1.

[0015] The developing device 4 contains non-magnetic single-component toner as the developer, and includes a developing sleeve 41 as a developer carrier, a developer application blade 42 as developer regulating means, etc. In each image forming station S, the photosensitive drum 1, the charging roller 2 as process means acting on it, the developing device 4, and the drum cleaning device 6 are integrally configured as a process cartridge 7 that is detachable from the main body of the image forming apparatus 100. The exposure device 3 is configured as a scanner unit that scans laser light using a polygonal mirror, and irradiates the photosensitive drum 1 with a scanning beam modulated based on an image signal.

[0016] An intermediate transfer belt 8, which serves as a second image carrier and a movable intermediate transfer body, is disposed so as to contact all of the photosensitive drums 1Y, 1M, 1C, and 1K of the image forming units SY, SM, SC, and SK. The intermediate transfer belt 8 is an endless belt, stretched with a predetermined tension by three rollers: a drive roller 9, a tension roller 10, and a secondary transfer opposing roller 11. An intermediate transfer motor (not shown) is connected to the drive roller 9, and the drive roller 9 is driven to rotate by the intermediate transfer motor in response to instructions from the control unit 301. The intermediate transfer belt 8 moves (rotates) in the belt conveyance direction indicated by arrow R2 in the figure in accordance with the rotation of the drive roller 9.

[0017] A primary transfer roller 5 as a first transfer member is urged with a predetermined pressure against the photosensitive drum 1 via the intermediate transfer belt 8, forming a primary transfer portion (primary transfer nip) N1 where the intermediate transfer belt 8 and the photosensitive drum 1 come into contact. A secondary transfer roller 15 as a second transfer member is disposed on the outer circumferential surface of the intermediate transfer belt 8, facing the secondary transfer opposing roller 11. The secondary transfer roller 15 is urged with a predetermined pressure against the secondary transfer opposing roller 11 via the intermediate transfer belt 8, forming a secondary transfer portion (secondary transfer nip) N2 where the secondary transfer roller 15 comes into contact with the intermediate transfer belt 8, and is rotated by the rotation of the intermediate transfer belt 8 directly or via a recording material P such as recording paper. A belt cleaning device 12 as an intermediate transfer body cleaning means is disposed on the outer circumferential surface of the intermediate transfer belt 8, facing the tension roller 10.

[0018] FIG. 2 is a block diagram of the image forming apparatus 100. The controller circuit 301 is capable of communicating with the host computer 300. When the interface unit 310 of the controller circuit 301 receives (inputs) print data from the host computer 300, it develops the print data and converts it into image data for image formation. Then, based on the image data, it generates video signals for exposure of four colors for exposure. After that, when the generation of the video signals is completed, the interface unit 310 sends an instruction to start image formation to the power supply control unit 311, and the power supply control unit 311 starts various actuators to start preparations for image formation. The power supply control unit 311 controls the secondary transfer power supply E2 as described below. It can be done.

[0019] When the image forming operation starts, each photosensitive drum 1 and intermediate transfer belt 8 start to rotate in the directions of arrows R1 and R2 in the figure at a predetermined process speed. In this embodiment, the peripheral speed of the intermediate transfer belt 8 is 300 mm / sec, which is the image forming process speed, and the intermediate transfer belt 8 rotates at a speed substantially equal to the peripheral speed of the photosensitive drum 1. The surface of the rotating photosensitive drum 1 is substantially uniformly charged to a predetermined polarity (negative polarity in this embodiment) by the charging roller 2. At this time, a predetermined charging voltage is applied to the charging roller 2 from a charging voltage application means (not shown).

[0020] Next, the charged surface of the photosensitive drum 1 is exposed by the exposure device 3 in accordance with image information corresponding to each image forming station S, thereby forming an electrostatic latent image on the surface of the photosensitive drum 1 in accordance with the image information. Meanwhile, the toner in the developing device 4 is negatively charged by the developer application blade 42 and applied to the developing sleeve 41. Here, the normal charging polarity of the toner is negative, but this is not limited to this. A predetermined developing voltage is applied to the developing sleeve 41 by a developing voltage application means (not shown). Then, when the latent image formed on the photosensitive drum 1 reaches the opposing portion (developing portion) between the photosensitive drum 1 and the developing sleeve 41, the latent image on the photosensitive drum 1 is visualized by the negative toner, and a toner image (developer image) is formed on the photosensitive drum 1.

[0021] Next, the toner image formed on the photosensitive drum 1 is transferred (primary transfer) to the intermediate transfer belt 8, which is rotated, at the primary transfer portion N1 by the action of the primary transfer roller 5. The intermediate transfer belt 8 is made of PEN (polyethylene naphthalate) resin. The surface resistivity of the intermediate transfer belt 8 is 5.0×10 11 Ω / □, and the volume resistivity is 8.0×10 11The resistance is Ωcm. Alternatively, the intermediate transfer belt 8 may be an endless belt made of resin such as PVdF (vinylidene fluoride resin), polyimide, or PET (polyethylene terephthalate). Alternatively, the intermediate transfer belt 8 may be an endless belt made of a rubber base layer such as EPDM coated with urethane rubber in which a fluororesin such as PTFE is dispersed.

[0022] The primary transfer roller 5 is made of an elastic material such as sponge rubber. The primary transfer roller 5 in this embodiment is a Φ14 rubber roller made of a 6 mm diameter nickel-plated steel rod core covered with a 4 mm thick foamed NBR-hydrin rubber. The electrical resistance of the primary transfer roller 5 is 1.0×10 6 The electrical resistance of the primary transfer roller 5 is Both ends of the roller were pressed with 500 g against an aluminum cylinder with a diameter of 30 mm, and 1000 V was applied while rotating at 30 rpm for measurement. The ambient environment during measurement was a temperature of 23°C and a humidity of 50%. The primary transfer roller 5 rotates following the movement of the intermediate transfer belt 8.

[0023] A primary transfer voltage having a polarity opposite to the normal charging polarity of the toner during development (positive polarity in this embodiment) is applied to the core of the primary transfer roller 5 from a primary transfer power source E1 serving as a primary transfer voltage application means. For example, when forming a full-color image, a latent image is formed on the photosensitive drum 1 for each color station, and this is developed into a toner image. The toner images of each color formed on each photosensitive drum 1 of each image forming station S are then transferred to the intermediate transfer belt 8 at each primary transfer station N1Y, N1M, N1C, and N1K so as to be sequentially superimposed on the intermediate transfer belt 8, forming a full-color toner image of four colors on the intermediate transfer belt 8.

[0024] The recording material P loaded in a recording material storage cassette (not shown) is conveyed to a registration roller 14 as a conveying roller by a feeding roller (not shown) and an intermediate conveying roller (not shown). The registration roller 14, the feeding roller, and the intermediate conveying roller are driven to rotate by a driving force transmitted from a feeding motor (a second motor) via a drive train (not shown). After the skew of the recording material P is corrected by the registration roller 14, the recording material P is transferred to an intermediate transfer roller (an image carrier) as an image carrier in this embodiment. In synchronization with the toner image on the belt 8, the toner image is conveyed via the intermediate transfer belt 8 to a secondary transfer portion N2, which serves as a transfer portion formed by a secondary transfer opposing roller 11 and a secondary transfer roller 15. Then, the four-color multiple toner image carried on the intermediate transfer belt 8 is transferred all at once to the recording material P at the secondary transfer portion N2 by the action of the secondary transfer roller 15. At this time, a secondary transfer voltage is applied to the secondary transfer roller 15 from a secondary transfer power source E2 as a transfer voltage having a polarity opposite to the normal charging polarity of the toner during development (positive polarity in this embodiment).

[0025] The secondary transfer roller 15 is made of an elastic material such as sponge rubber. The secondary transfer roller 15 in this embodiment is a rubber roller with a diameter of 18 mm, in which a core metal on a nickel-plated steel rod with a diameter of 8 mm is covered with foamed NBR-hydrin rubber with a thickness of 5 mm. The electrical resistance of the secondary transfer roller 15 is 3.0×10 7 The measurement conditions for the electrical resistance of the secondary transfer roller 15 are as follows: The conditions are the same as those for the primary transfer roller 5. The secondary transfer roller 15 forms a secondary transfer portion N2 at the contact position with the intermediate transfer belt 8, and rotates following the movement of the intermediate transfer belt 8, or the intermediate transfer belt 8 and the recording material P. In this embodiment, the secondary transfer opposing roller 11, which is the opposing electrode (opposing member) of the secondary transfer roller 15, is a nickel-plated aluminum roller, has a diameter of 20 mm, and is grounded. An elastic layer such as low-resistance rubber may be provided on the surface of the secondary transfer opposing roller 11 to give it elasticity.

[0026] Discharge needles 18 are disposed downstream of the secondary transfer portion N2 in the conveyance direction of the recording material P, as discharge members for removing charge from the recording material P that has passed through the secondary transfer portion N2 (hereinafter referred to as "discharging the recording material P"). The discharge needles 18 are made of stainless steel and have a needle shape with a pitch of 2 mm, a height of 3 mm, and a thickness of 0.1 mm (Figure 3). The discharge needles 18 have the function of discharging the recording material P by generating an ion flow at the tip due to the concentration of the electric field. The shape of the discharge needles is not limited to the configuration described above, and any configuration is acceptable as long as it can discharge the recording material P evenly over the entire longitudinal length.

[0027] The static elimination needle 18 is connected to a secondary transfer power source E2, and a static elimination voltage generated by the secondary transfer power source E2 is output at a predetermined timing to eliminate static electricity from the recording material P. The static elimination voltage output to the static elimination needle 18 by the secondary transfer power source E2 has the opposite polarity to the secondary transfer voltage applied to the secondary transfer roller 15 during image formation, i.e., a negative voltage. By eliminating static electricity from the recording material, the amount of charge on the recording material is reduced, and image defects caused by discharge due to peeling of the recording material downstream of the secondary transfer portion N2 are suppressed.

[0028] Thereafter, the recording material P onto which the toner image has been transferred is transported to a fixing device 16 as a fixing means. The recording material P is pressed and heated while being sandwiched between a fixing roller and a pressure roller of the fixing device 16 and transported, and the toner image is fixed onto the recording material P. The recording material P onto which the toner image has been fixed is discharged to the outside of the main body of the image forming apparatus 100. Any toner remaining on the intermediate transfer belt 8 after the toner image has been transferred onto the recording material P is removed from the intermediate transfer belt 8 by a cleaning blade 21 of a belt cleaning device 12.

[0029] <Configuration of secondary transfer power supply E2> 4 is a circuit diagram illustrating the configuration of the secondary transfer power supply E2. The secondary transfer power supply E2 includes a positive voltage circuit 200a as a first circuit, a negative voltage circuit 200b as a second circuit, and a current detection circuit 200c as a detection unit. The positive voltage circuit 200a and the negative voltage circuit 200b are connected in series, and the secondary transfer power supply E2 functions as a common power supply for the secondary transfer roller 15 and the static elimination needle 18.

[0030] The positive voltage circuit 200a can generate a transfer voltage for secondary transfer of a toner image onto the recording material P, and outputs a DC voltage (first voltage) of positive polarity (first polarity) that is opposite to the normal charging polarity of the toner to the secondary transfer roller 15. The negative voltage circuit 200b can generate a transfer voltage for secondary transfer of a toner image onto the recording material P. The detection circuit 200c is a circuit for detecting the current flowing from the positive voltage circuit 200a toward the secondary transfer roller 15. Hereinafter, the voltage will be described as "high" or "low" in terms of the potential difference from 0V, regardless of whether it is a positive or negative voltage. For example, -2000V will be described as a higher voltage than -1000V.

[0031] The positive voltage circuit 200a can output up to +8000V, and the negative voltage circuit 200b can output up to -5000V. Resistors 211 and 216 are bleeder resistors for the positive voltage circuit 200a and the negative voltage circuit 200b, respectively, with resistances of 120 MΩ and 20 MΩ. The negative voltage circuit 200b, which outputs a negative voltage to the secondary transfer roller 15 and the static elimination needle 18, is shared. The positive voltage circuit 200a and the negative voltage circuit 200b are connected to each other, and the high DC voltage generated by each circuit is output to the secondary transfer roller 15 via each other's bleeder resistor. As a result, a voltage obtained by superimposing the first voltage and the second voltage is applied to the secondary transfer roller 15 as the transfer voltage. Therefore, when the positive voltage circuit 200a and the negative voltage circuit 200b are simultaneously turned on to superimpose the output voltages, the ground level of the positive voltage circuit 200a drops by the absolute value of the negative voltage output by the negative voltage circuit 200b. That is, when a transfer voltage is output from the secondary transfer power supply E2 to the secondary transfer roller 15 to perform secondary transfer of a toner image, the positive voltage circuit 200a outputs a positive voltage that is greater by the absolute value of the negative voltage output by the negative voltage circuit 200b.

[0032] In this embodiment, the bleeder resistor 216 of the negative voltage circuit is divided equally into 216a and 216b, and half of the negative voltage is output to the static elimination needle. As a result, a divided voltage from the second voltage is applied to the static elimination needle. Even when the negative voltage circuit 200b does not generate a negative voltage, a current flows through the bleeder resistor, generating a voltage in the static elimination needle according to the current flowing through the bleeder resistor and the bleeder resistance value. When paper absorbs moisture in a high-temperature, high-humidity environment, constant voltage control may be performed to maintain appropriate image quality. In this case, a large current flows through the secondary transfer roller 15 and the bleeder resistor. In this case, a high voltage is generated in the static elimination needle, which may result in discharge from the static elimination needle flowing through the paper to the secondary transfer unit N2, causing secondary transfer failure. Therefore, in this embodiment, the bleeder resistor is divided into two to prevent such unintended voltage generation.

[0033] On the other hand, another aspect of this configuration is that because only half the voltage of the negative voltage circuit 200b is output to the static elimination needle, if a higher static elimination needle voltage is required, the negative voltage circuit 200b must generate a higher voltage. As a result, to obtain the same secondary transfer voltage, the positive voltage circuit 200a must also generate a higher voltage. To cite a specific example, if a transfer voltage of +3000V and a static elimination voltage of -1000V are to be superimposed and output, the positive voltage circuit 200a must be controlled to output +5000V (=3000V + |-1000V| × 2). While the bleeder resistor 216 is divided equally here, the bleeder resistor may not need to be divided depending on the device configuration, and the resistance ratio of 216a to 216b may be other than 1:1.

[0034] When the image forming operation starts, the power supply control unit 311 drives the secondary transfer power supply E2 at a predetermined timing using the voltage control unit 312. The power supply control unit 311 performs constant current control or constant voltage control based on the voltage applied to the secondary transfer roller 15 by the secondary transfer power supply E2 and the current value detected by the current detection circuit 200c.

[0035] [Secondary transcription control] The secondary transfer control involves constant current control, which controls the output of the secondary transfer power supply E2 so that a constant current flows through the secondary transfer roller 15, and constant voltage control, which outputs a constant voltage from the secondary transfer power supply E2 to the secondary transfer roller 15. To implement the constant current control and constant voltage control, the storage unit 314 stores in advance an array (hereinafter referred to as a look-up table (LUT)) that associates voltage setting values ​​when the power supply control unit 311 controls the secondary transfer power supply E2 with voltage values ​​actually output from the secondary transfer power supply E2.

[0036] In constant current control, the detection circuit 200c monitors the current value flowing through the secondary transfer roller 15 at a predetermined cycle, and determines the voltage setting value to be output from the secondary transfer power supply E2 in the next cycle based on the difference between the target current value It and the detected current value Id. This controls the current value flowing through the secondary transfer roller 15 to be constant.

[0037] Here, we will explain the control (hereinafter referred to as secondary transfer control) of the secondary transfer power supply E2 when the toner image is secondarily transferred from the intermediate transfer belt 8 to the recording material P at the secondary transfer portion N2. Figure 5 is a timing chart (timings t0 to t10) when toner is transferred to one sheet of recording material P (P1 in the figure) at the secondary transfer portion N2.

[0038] Before the leading edge of the recording material P reaches the secondary transfer portion N2 in the conveyance direction of the recording material P, constant current control known as paper-free impedance detection is performed (timings t1 to t4). In this embodiment, impedance detection involves obtaining a voltage value Vpr to obtain a target current during constant current control. In paper-free impedance detection, the impedance of the secondary transfer portion N2 is detected when the recording material P is not sandwiched therein. Based on the result, a voltage value for constant voltage control (timings t4 to t5, hereinafter referred to as first constant voltage control) is determined. In the first constant voltage control, the voltage value Vtp output from the secondary transfer power source E2 to the secondary transfer roller 15 has a reference voltage setting value Vtp0 based on a lookup table (LUT) stored in advance in the memory unit 314, which is determined based on the installation environment of the apparatus and the type of recording material P. The paper-free impedance also has a reference value Vpr0.

[0039] As will be described later, the difference (Vpr-Vpr0) between the no-paper impedance detection result Vpr and the reference value Vpr0 is almost the same as the difference (Vav-Vav0) between the paper-present impedance detection result Vav and its reference value Vav0. Therefore, the target voltage when the recording material P is clamped in the secondary transfer section N2 can be corrected based on the no-paper impedance detection result.

[0040] The difference (Vpr-Vpr0) is added to the reference voltage Vtp0 in the first constant voltage control to obtain a voltage value Vtp=Vtp0+(Vpr-Vpr0) for the first constant voltage control.

[0041] By determining the impedance of the secondary transfer portion N2 and adjusting the constant voltage value at the leading edge of the recording material P, it is possible to prevent poor transfer performance at the leading edge of the recording material P, which would occur if the secondary transfer voltage were insufficient or excessive at the leading edge of the recording material P. With the first constant voltage control, even if the electrical resistance value at the secondary transfer portion N2 fluctuates due to the recording material P entering the secondary transfer portion N2, it is possible to suppress image defects that occur at the leading edge of the recording material P due to variations in the voltage output from the secondary transfer power supply E2. In this embodiment, the first constant voltage control starts 5 mm before the recording material P reaches the secondary transfer portion N2 and continues until a position 25 mm from the leading edge of the recording material P reaches the secondary transfer portion N2.

[0042] After the first constant voltage control is performed, the toner image is transferred from the intermediate transfer belt 8 to the recording material P while performing constant current control (timing t5 to t6). The constant current control allows the image forming apparatus 100 to be responsive to fluctuations in the environment (temperature, humidity) around the image forming apparatus 100 and fluctuations in the recording material P used for image formation. Even if the resistance value differs depending on the type, the toner image can be appropriately secondarily transferred from the intermediate transfer belt 8 to the recording material P. Note that the target current It used in the constant current control is a predetermined value stored in the memory unit 314 depending on the type of recording material P and the ambient environment of the image forming apparatus 100.

[0043] In addition, in parallel with the constant current control described above, a control called paper-present impedance detection is performed. Paper-present impedance detection is a control performed to measure the voltage setting value for flowing the optimal current relative to the electrical resistance value at the secondary transfer unit N2, including the recording material P actually sandwiched therein. Paper-present impedance detection begins when the detected current Id detected by the detection circuit 200c converges within a predetermined range relative to the target current It after constant current control begins. Once impedance detection begins, the secondary transfer voltage value is obtained 46 times at 4-msec intervals, i.e., for one rotation of the secondary transfer roller 15, and the average value, the optimal voltage Vav for the image formation, is calculated. The period and number of times the voltage setting value is obtained can be selected depending on the device.

[0044] After the constant current control is performed, constant voltage control (timings t6 to t7, hereinafter referred to as second constant voltage control) is performed before and after the trailing edge of the recording material P passes through the secondary transfer portion N2 in the conveyance direction of the recording material P. In the second constant voltage control, the voltage Vav is output from the secondary transfer power source E2 to the secondary transfer roller 15 for constant voltage control, thereby optimizing the secondary transfer of the toner image at the trailing edge of the recording material P. Furthermore, by performing the second constant voltage control, it is possible to suppress variations in the voltage output from the secondary transfer power source E2 even if the electrical resistance value at the secondary transfer portion N2 fluctuates due to the recording material P passing through the secondary transfer portion. Thereafter, processing is performed between sheets (timings t7 to t10), and the system waits for the next recording material P.

[0045] [Static elimination control] Next, we will explain the charge removal control that occurs when a negative voltage is output from the negative voltage circuit 200b of the secondary transfer power supply E2 to the charge removal needle 18 to remove static electricity from the recording material P. The recording material P becomes strongly charged during the secondary transfer of the toner image, and peeling discharge occurs on the recording material P downstream of the secondary transfer section, which can result in water droplet-like discharge marks appearing on the image. In charge removal control, the recording material P is removed by constant voltage control to suppress the aforementioned peeling discharge.

[0046] The voltage Vds as the neutralization voltage output from the secondary transfer power source E2 to the neutralization needle 18 in order to neutralize the recording material P is stored in advance in the storage unit 314. In this embodiment, the neutralization voltage to be output is determined depending on the type of recording material P and the surrounding environment of the image forming apparatus 100.

[0047] The secondary transfer control and charge removal control, which are features of this embodiment, will now be described. In the constant current control described above, a target current value during control is set so that transfer defects do not occur even when the image on the recording material P is an image with a large amount of toner per unit area, such as a so-called secondary color solid image. The target current value is determined taking into consideration the toner tribo (charge amount per unit volume of toner) and the toner amount per unit area, as well as the installation environment of the image forming apparatus and the type of recording material P.

[0048] The resistance values ​​of the recording material P and the secondary transfer roller 15 change depending on the conditions of use. In both cases, the resistance becomes lower when the ambient environment is high in temperature and humidity, and becomes higher as the ambient environment becomes low in temperature and humidity. In other words, to ensure secondary transfer performance in a low-temperature, low-humidity environment, a high voltage must be applied to the secondary transfer roller 15. Furthermore, even in the same environment, the resistance value tends to increase with the passage of paper. In other words, when using an image forming apparatus that has passed many papers (hereinafter referred to as an apparatus after paper passage durability) in a low-temperature, low-humidity environment, a higher voltage must be applied to the secondary transfer roller 15 to obtain the required secondary transfer current.

[0049] As an example, Figure 6 shows the results of Canon CS-060F paper (basis weight 60 g / m) that was conditioned in a low-temperature, low-humidity environment of 15°C / 10%RH for two days. 2 This figure shows how the secondary transfer voltage required to ensure transferability changes with use of the device when passing a sheet of paper through the device. Hereafter, paper that has been conditioned for two days is referred to as "2-day-left paper." At the beginning of device use, when no recording material P is sandwiched in the secondary transfer section N2 and constant current control is performed with a target current of 30 μA, the secondary transfer voltage (i.e., the reference no-paper impedance detection result Vpr0) is 1300 V, and the reference impedance detection result Vav0 with recording material P sandwiched is 2300 V. The reference voltage Vtp0 for the first constant voltage control is set to 2500 V. This reference voltage value Vtp0 is set slightly higher to prevent transfer defects such as insufficient toner transfer, even when the device is in use.

[0050] The secondary transfer voltage increased with use, and after 150,000 sheets (150,000 sheets) of paper feed, the no-paper impedance detection result Vpr rose to 4,200 V, and the paper-present impedance detection result Vav rose to 5,200 V. This indicates that the resistance of the secondary transfer roller 15 increased with paper feed. It also shows that the required secondary transfer voltages Vtp and Vav can be predicted from the no-paper impedance detection result Vpr depending on the recording material P (Vtp = Vpr + 1,200, Vav = Vpr + 1,000).

[0051] Furthermore, in the static elimination control, in order to prevent the aforementioned discharge marks that tend to occur on recording material P that has a low basis weight and high resistance, a voltage value for static elimination control is set so as to eliminate static from the recording material P downstream of the secondary transfer portion N2 in the conveyance direction of the recording material P. Paper that is prone to the aforementioned discharge marks requires a high voltage to perform sufficient static elimination.

[0052] The aforementioned CS-060F paper has a relatively high resistance, and it charges up during the toner image transfer process at the secondary transfer unit N2. This makes it one of the papers most susceptible to the occurrence of discharge marks, regardless of the device's operating environment. In any environment, applying a discharge voltage of -1000 V or higher can generally suppress the occurrence of discharge marks, but applying an even higher discharge voltage is effective in minimizing their frequency. Figure 7 shows the results of the discharge marks in a low-temperature, low-humidity environment. Regarding discharge marks, "○" indicates good (suppressed occurrence), "×" indicates problematic (significant occurrence), and "△" indicates moderate occurrence. "○△" indicates discharge marks occurring at a level below the user's tolerance level. At discharge voltages lower than -1000 V, discharge marks were significant and below the user's tolerance level. However, applying a high discharge voltage of -1000 V or higher suppressed discharge marks, and no discharge marks occurred at -2000 V. Therefore, a discharge voltage of -2000 V is typically applied to virtually eliminate the occurrence of discharge marks. This neutralization voltage of −2000 V is defined as a first value.

[0053] As mentioned above, the maximum voltage that can be output from the positive voltage circuit 200a is 8000V. Furthermore, when a neutralization voltage of -2000V is applied, -4000V is superimposed on the output from the positive voltage circuit 200a due to the circuit configuration. This means that the maximum voltage that can be applied to the secondary transfer roller 15 is +4000V. Figure 8 shows the secondary transfer target voltage, which is a predetermined target value for ensuring transferability, the outputs of the positive voltage circuit 200a and the negative voltage circuit 200b, the relationship between the actually obtained secondary transfer voltage and neutralization voltage, and the image quality indicators of transfer failure (incomplete toner transfer) and discharge mark occurrence levels. Regarding transfer failure and discharge mark occurrence, "○" indicates good (occurrence suppressed), "×" indicates problematic (occurrence significant), and "○△" indicates occurrence at a level acceptable to the user.

[0054] First, a conventional example of control will be explained using FIG. 8(a). The secondary transfer voltage and neutralization voltage operate to output their respective targets, but cannot output more than the upper limit of the high voltage output of each circuit. As mentioned above, in the device after paper feed durability, the secondary transfer voltage needs to be +5200V, but as shown in FIG. 8(a), it is -200V regardless of the impedance of the secondary transfer section N2. When a neutralization voltage of 0 V is applied, the secondary transfer voltage can only be output up to +4000 V. At +4000 V, the target current of 30 uA cannot be obtained, resulting in transfer failure in which the toner on the intermediate transfer belt 8 is not sufficiently transferred onto the recording material P ((4) and (5) in Figure 8(a)). However, no discharge marks are observed. From the perspective of image quality, it is more desirable to suppress transfer failures than discharge marks, which rarely occur.

[0055] 8(b) shows an example of the case where the control characteristic of this embodiment is implemented, and the predetermined target value of the secondary transfer voltage is the same as that of FIG. 8(a). In FIG. 8(b), when the output of the positive voltage 200a does not reach the maximum value of 8000 V (first mode, (1) and (2) in FIG. 8(b)), a voltage sufficient from the viewpoint of image quality is applied to both the secondary transfer voltage and the static elimination voltage.

[0056] On the other hand, in FIG. 8(b), when high voltages are desired for both the secondary transfer voltage and the neutralization voltage, the neutralization voltage is reduced to prioritize transferability at the secondary transfer portion N2. In this embodiment, when the output voltage from the positive voltage circuit 200a reaches the upper limit of 8000V (second mode), a secondary transfer voltage that provides the target secondary transfer current is maintained, and the output of the negative voltage circuit 200b is reduced. In this embodiment, when the no-paper impedance detection result Vpr is 2800V or less, the secondary transfer voltage, including the first constant voltage control voltage Vtp when the recording material P is sandwiched at the secondary transfer portion N2, is kept below 4000V, so it is possible to apply the neutralization voltage of −2000V, which is the first value ((1) to (3) in FIG. 8(b)).

[0057] On the other hand, if the neutralization voltage remains at -2000V and the secondary transfer voltage falls below the target value, i.e., if the no-paper impedance detection result Vpr exceeds 2800V, the output of the positive voltage circuit 200a is set to the upper limit of 8000V, and the output of the negative voltage circuit 200b is reduced to lower the neutralization voltage ((4) and (5) in FIG. 8(b)). In this case, the neutralization voltage has the same polarity as the first value but is lower than the first value, and this value is set to the second value. At this time, the negative voltage circuit 200b outputs -(8000-(Vpr+1200))V, and the neutralization voltage is -((8000-(Vpr+1200)) / 2)V. Comparing (4) and (5) in FIG. 8(a) of the prior art, when the neutralization voltage is set to -2000V, the difference between the secondary transfer voltage and the target value is greater in (5) than in (4). Therefore, in FIG. 1B of this embodiment, the value of the neutralization voltage is set lower in the case of (5) than in the case of (4) so ​​that the secondary transfer voltage reaches the target value.

[0058] As an effect of the control of this embodiment, it was possible to suppress transfer defects and to suppress discharge marks within an allowable range within the ranges of use of the positive voltage circuit 200a and the negative voltage circuit 200b.

[0059] Up to this point, in the color image forming apparatus of the intermediate transfer type, the intermediate transfer belt 8 has been described as an image carrier. However, the photosensitive member as the first image carrier and the transfer roller are pressed together to form a transfer section. The effect of this embodiment remains the same even in a monochrome printer, for example, in which a toner image is transferred onto the recording material P while the recording material P is nipped in the transfer section.

[0060] Example 2 Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0061] In Example 1, we have shown a case where the output of the negative voltage circuit 200b is suppressed based on the paper-free impedance detection result Vpr, but in Example 2, we will explain a case where the output of the negative voltage circuit 200b is controlled based on the paper-present impedance detection result Vav.

[0062] 9 shows a timing chart of this embodiment. Here, an example is shown in which two sheets of recording material P are printed consecutively. After voltage control (timings t1 to t7) for the first recording material P1, there is a paper interval (timings t7 to t10), and then voltage control (timings t10 to t13) for the second recording material P2.

[0063] As explained in the first embodiment, the paper-presence impedance detection makes it possible to calculate the optimum voltage Vav for image formation in the constant current control region of the recording material P. The advantage of the paper-presence impedance detection is that it is possible to optimize the voltage value of the first constant voltage control for the second and subsequent sheets of recording material P (P2 in the figure) to which images are transferred when sheets are continuously fed.

[0064] The feature of this embodiment is that it is possible to correct the deviation of the no-paper impedance detection result Vpr based on the paper-present impedance detection result Vav, and for the following reasons, it is possible to set a more optimal voltage than in the first embodiment.

[0065] In the paper-free impedance detection, the secondary transfer voltage required when recording material P is sandwiched between the secondary transfer portion N2 is predicted from the impedance Vpr of the secondary transfer portion N2 when recording material P is not sandwiched between the secondary transfer portion N2, as described in Example 1. However, variations in the paper-present and paper-absent impedances can occur depending on factors such as the recording material P's familiarity with the environment in which it is used and component variations, resulting in slight deviations in the secondary transfer voltage. As a result, transfer failures can occur in the 25-mm region at the leading edge of the recording material P where the first constant voltage control described in Example 1 is performed, or conversely, low density, known as "strong transfer dropout," can occur when the secondary transfer voltage is set too high. On the other hand, when paper-present impedance detection is used in this example, the optimal transfer voltage for the recording material P being fed can be determined, thereby minimizing deviations in the secondary transfer voltage.

[0066] As explained in Example 1, when using CS-060F in a low-temperature, low-humidity environment of 15°C / 10%RH and paper left for two days, the impedance detection result Vav with paper and the impedance without paper were compared. The difference in impedance detection results Vpr was 1000V. In contrast, when the aforementioned paper was left in a low-temperature, low-humidity environment immediately after being taken out of the package (hereinafter referred to as opened paper), Vav - Vpr was 500V. In another comparison, when the aforementioned paper was left in a low-temperature, low-humidity environment for about a month, the difference in impedance detection results increased to Vav - Vpr of 1500V. Here, we are explaining the difference in time required to acclimate the same CS-060F paper to a low-temperature, low-humidity environment, but the same explanation can be given for differences in the type of recording material P.

[0067] Based on the control of Example 1, the first constant voltage control value is 500 V too high for the open paper, resulting in a transfer failure with low density known as strong transfer loss, and is 500 V too low for the paper that has been left for one month, resulting in a transfer failure.

[0068] On the other hand, according to the control of this embodiment, the result of detecting the impedance in the paper Vav obtained by passing the recording material P can be applied as the first constant voltage Vtp to the recording material P on which the next image is formed, so that an optimal voltage value can be obtained even at the leading edge of the image. Furthermore, an optimal neutralization voltage can be set within a range that does not exceed the upper limit of the high voltage output.

[0069] In this embodiment, for the first sheet of a print job, control is performed based on the paper-free impedance detection described in embodiment 1, and for the second sheet and thereafter, the outputs of the positive voltage circuit 200a and the negative voltage circuit 200b are determined based on the voltage value obtained by paper-present impedance detection control during the passage of the first sheet, thereby using a voltage within the upper limit that can be output from the voltage circuit from the leading edge of the recording material P, making it possible to set the voltage to obtain optimal image quality. [Explanation of symbols]

[0070] 8: intermediate transfer belt, 15: secondary transfer roller, 18: charge removal needle, 200a: positive voltage circuit, 200b: negative voltage circuit, 311: power supply control unit

Claims

1. an image carrier that carries a developer image; a transfer means for transferring the developer image carried on the image carrier onto a recording material; a charge eliminating means for eliminating charge from the recording material; a first circuit for applying a first voltage having a polarity opposite to the normal charging polarity of the developer to the transfer means; a second circuit connected in series with the first circuit and configured to apply a second voltage to the transfer means and the discharging means, the second voltage having the same polarity as the normal charging polarity of the developer; a control unit that controls the first circuit and the second circuit; An image forming apparatus having: The control unit applying a superimposed voltage of the first voltage and the second voltage to the transfer means as a transfer voltage so that the developer image is transferred to the recording material; applying a partial voltage from the second voltage to the charge-removing means as a charge-removing voltage so that the recording material is discharged; When the neutralization voltage is set to a first value, if the transfer voltage falls below a predetermined target value, the neutralization voltage is changed to a second value that is lower than the first value but has the same polarity. An image forming apparatus characterized by:

2. The charge eliminating means is provided downstream of the transfer means in the conveying direction of the recording material.

2. The image forming apparatus according to claim 1, wherein:

3. The control unit determines the second value according to a difference between the transfer voltage and the predetermined target value when the neutralization voltage is set to the first value.

3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. The control unit reduces the second value as the difference between the transfer voltage and the predetermined target value increases when the static elimination voltage is set to the first value.

4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium.

5. Further, a detection means for detecting a current value flowing through the transfer means is provided, The control unit calculates the impedance between the transfer unit and the image carrier based on the current value detected by the detection unit, and determines a predetermined target value of the transfer voltage based on the impedance.

5. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

6. the detecting means detects the current value in a state where the recording material is not sandwiched between the transfer means and the image carrier, The control unit determines a predetermined target value of the transfer voltage based on the current value detected by the detection unit.

6. The image forming apparatus according to claim 5,

7. the detecting means detects the current value in a state in which the first recording material is sandwiched between the transfer means and the image carrier; The control unit determines a predetermined target value of the transfer voltage for a second recording material onto which the developer image is transferred after the first recording material, based on the current value detected by the detection unit.

6. The image forming apparatus according to claim 5,

8. The image carrier is an intermediate transfer member that carries the developer image transferred from the surface of the photosensitive member.

8. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

9. The image carrier is a photoreceptor on whose surface the developer image is formed.

8. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

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