Image forming apparatus

Abstract

To suppress occurrence of density unevenness of an image on a recording material after secondary transfer due to charging unevenness of toner on an intermediate transfer belt.SOLUTION: In the black monochrome image formation of an image forming device 100 having an intermediate transfer belt 7, the value of a primary transfer current supplied to the 5K of a primary transfer member is set higher than that in middle-humidity environments and lower than that in low-humidity environments in high-humidity environments, thereby suppressing the occurrence of "horizontal streak-like concentration unevenness".SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunction machine having a plurality of functions among these, which uses an electrophotographic system. [Background technology]

[0002] Conventionally, electrophotographic image forming apparatuses use toner as a developer for developing electrostatic latent images formed on a photoreceptor. The toner is charged by friction. The charge amount of the toner is easily affected by environmental conditions (temperature and humidity). For example, when humidity changes, the amount of moisture adhering to the toner surface changes, which in turn changes the charge amount of the toner, and this can cause fluctuations in the density of the output image.

[0003] To minimize fluctuations in image density, it is desirable to continuously monitor the environment of the image forming apparatus and adjust various image formation conditions appropriately. Patent Document 1 proposes changing the conditions for the primary transfer of a toner image from a photosensitive drum to an intermediate transfer belt and the conditions for the secondary transfer of the toner image from the intermediate transfer belt to a recording material such as paper in response to environmental changes. In the configuration described in Patent Document 1, the primary transfer current value and the secondary transfer current value are reduced proportionally with an increase in humidity. This is because the toner charge amount is high in low-humidity environments and low in high-humidity environments, and the transfer current is adjusted according to the toner charge amount. In other words, the primary transfer current value and the secondary transfer current value are increased in low-humidity environments and decreased in high-humidity environments according to the toner charge amount.

[0004] Furthermore, in an image forming apparatus using an intermediate transfer belt, if uneven charging of toner occurs on the intermediate transfer belt after primary transfer, differences in the amount of charge on the toner can cause differences in secondary transfer efficiency, resulting in uneven density of the image on the recording material after secondary transfer. Patent Document 2 calls this uneven density "folding noise" because it appears as striped density unevenness. Patent Document 2 also proposes, as a countermeasure to this uneven density, detecting unevenness in the amount of secondary transfer residual toner on the intermediate transfer belt, and, if it is determined that there is uneven charging of the toner, changing the value of the primary transfer output to a larger value.

[0005] Furthermore, Patent Document 3 proposes changing the amount of "offset" depending on environmental conditions. This moves the separation position where the intermediate transfer belt separates from the primary transfer roller, stabilizing the discharge between the intermediate transfer belt and the primary transfer roller. In Patent Document 3, "offset" refers to a state in which the contact point (nip) between the photosensitive member and the intermediate transfer member is misaligned with the contact point (nip) between the intermediate transfer member and the primary transfer roller. Furthermore, "offset amount" refers to the distance by which this misalignment occurs. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-139140 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-104978 [Patent Document 3] Japanese Patent Application Laid-Open No. 2014-85523 Summary of the Invention [Problem to be solved by the invention]

[0007] In the case of a configuration in which the primary transfer current value is reduced in proportion to an increase in humidity as described in Patent Document 1, when the toner charge amount is low in a high humidity environment and the primary transfer current value is low, density unevenness referred to as "folding noise" in Patent Document 2 may occur. Note that such density unevenness in the image on the recording material after secondary transfer due to uneven charging of the toner on the intermediate transfer belt is referred to here as "horizontal streak-like density unevenness."

[0008] Furthermore, in the configuration described in Patent Document 2, unevenness in the amount of secondary transfer residual toner on the intermediate transfer belt is detected, so density unevenness occurs in the images on the first one or two sheets of recording material, for example.

[0009] Furthermore, the configuration described in Patent Document 3 aims to stabilize the discharge between the intermediate transfer belt and the primary transfer roller to prevent a shortage of charge to hold the toner image on the intermediate transfer belt, but does not address the problem of "horizontal streak-like density unevenness."

[0010] SUMMARY OF THE INVENTION An object of the present invention is to prevent uneven density of an image on a recording material after secondary transfer due to uneven charging of toner on an intermediate transfer belt. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus comprising a rotatable image carrier carrying a toner image, an image forming means for forming the toner image on the image carrier, a rotatable intermediate transfer belt to which the toner image is transferred from the image carrier, a primary transfer member for transferring the toner image from the image carrier to the intermediate transfer belt, a primary transfer power supply for supplying a primary transfer current to the primary transfer member for transferring the toner image from the image carrier to the intermediate transfer belt, a secondary transfer member forming a secondary transfer section for transferring the toner image from the intermediate transfer belt to a recording material, a secondary transfer power supply for supplying a secondary transfer current to the secondary transfer section for transferring the toner image from the intermediate transfer belt to a recording material, an environment detection means for detecting humidity inside or outside the image forming apparatus, and a control unit for controlling the primary transfer power supply, and a black toner image formed on the recording material is transferred to the intermediate transfer belt, and then transferred to a recording material to form a black monochrome image on the recording material. When forming the black monochrome image, the control unit sets the primary transfer current supplied to the primary transfer member to a first current when the humidity detection result by the environment detection unit is a first humidity, sets the primary transfer current supplied to the primary transfer member to a second current having an absolute value smaller than the first current when the detection result is a second humidity higher than the first humidity, and sets the primary transfer current supplied to the primary transfer member to a third current having an absolute value smaller than the first current and larger than the second current when the detection result is a third humidity higher than the second humidity. [Effects of the Invention]

[0012] According to the present invention, it is possible to suppress the occurrence of uneven density of an image on a recording material after secondary transfer, which is caused by uneven charging of toner on an intermediate transfer belt. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus (multi-color image forming apparatus). [Figure 2] FIG. 1 is a schematic cross-sectional view of an image forming apparatus (black monochrome image forming apparatus). [Figure 3] FIG. 2 is a block diagram showing a control configuration of the image forming apparatus. [Figure 4] FIG. 10 is a schematic diagram of "horizontal stripe-like density unevenness." [Figure 5] 10A and 10B are schematic diagrams for explaining the mechanism of occurrence of "horizontal stripe-like density unevenness." [Figure 6] 10 is a graph showing the setting of the primary transfer current value with respect to the relative humidity [% RH] in the first embodiment. FIG. [Figure 7] FIG. 10 is a graph showing the setting of the primary transfer current value relative to the absolute moisture content [g / Kg] in the first embodiment. [Figure 8] FIG. 3 is a flowchart of an example of control in the first embodiment. [Figure 9] FIG. 10 is a flowchart of another example of control in the first embodiment. [Figure 10] FIG. 10 is a graph showing the setting of the secondary transfer current value with respect to the relative humidity [% RH] in the second embodiment. [Figure 11] FIG. 10 is a graph showing the setting of the secondary transfer current value relative to the absolute moisture content [g / Kg] in the second embodiment. [Figure 12] FIG. 10 is a flowchart of an example of control in the second embodiment. [Figure 13] FIG. 10 is a flowchart of another example of control in the second embodiment. [Figure 14] FIG. 10 is a flowchart of the control of the third embodiment. [Figure 15] 10A and 10B are schematic diagrams for explaining a countermeasure against density unevenness caused by offset of the primary transfer roller in the fourth embodiment. [Figure 16] FIG. 10 is a schematic diagram illustrating an example of a setting screen for density unevenness countermeasure control. [Figure 17] FIG. 10 is a schematic diagram illustrating an example of a setting screen for a density unevenness countermeasure mode. DETAILED DESCRIPTION OF THE INVENTION

[0014] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0015] [Example 1] <Overall Configuration and Operation of Image Forming Apparatus> 1 is a schematic cross-sectional view of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem-type full-color multifunction peripheral that uses an intermediate transfer system and is capable of forming a full-color image on a sheet-shaped recording material using an electrophotographic system.

[0016] The image forming apparatus 100 includes an apparatus main body 96, an output unit 97, a control unit 15, an image reading unit 98, and an operation unit 99 serving as a UI (user interface). The image forming apparatus 100 can form an image on a recording material P in accordance with image information (image signals) of an image read by the image reading unit 98, or image information from a host device such as a personal computer, a digital camera, a smartphone, or other external device. The recording material (recording medium, transfer material, sheet) P is a material on which a toner image is formed, and is typically paper, although a wide variety of materials can be used. Specific examples of the recording material P include plain paper, cardboard, synthetic resin sheets that are substitutes for plain paper, transparent resin overhead projector sheets, envelopes, postcards, coated paper, embossed paper, bond paper, label paper, and waterproof paper (resin-coated paper).

[0017] The image forming apparatus 100 has four image forming units (stations) SY, SM, SC, and SK that form images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. Elements having the same or corresponding functions or configurations provided for each color may be generally described by omitting the Y, M, C, or K suffix to the reference numeral indicating the element for a particular color. In this embodiment, the image forming unit S includes a photosensitive drum 1, a charging roller 2, a developing device 4, an exposure device 3, a primary transfer roller 5, and a drum cleaning device 6, which will be described later. In this embodiment, the exposure device 3 is configured as a single unit capable of exposing the four photosensitive drums 1Y, 1M, 1C, and 1K, but may also be provided independently for each of the photosensitive drums 1Y, 1M, 1C, and 1K.

[0018] In addition to full-color images, the image forming apparatus 100 of this embodiment is also capable of forming monochrome images of one of four colors, such as a black monochrome image, monochrome images using several of the four colors, and multicolor images.

[0019] Photosensitive drum 1, a drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier, is rotatable and carries an electrostatic latent image and a toner image. In this embodiment, photosensitive drum 1 is a negatively charged organic photosensitive member (OPC) with an outer diameter of approximately 80 mm. In this embodiment, photosensitive drum 1 has three layers: an undercoat layer, a photocharge generation layer, and a charge transport layer, which are coated and laminated in this order on the surface of a substrate formed by an aluminum cylinder. Photosensitive drum 1 is driven to rotate at a predetermined peripheral speed (process speed) in the direction of arrow R1 (clockwise direction) by a drive motor 153 (FIG. 3) that constitutes a driving means.

[0020] The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 2, a roller-type charging member serving as a charging means. The charging roller 2 is disposed so as to contact the surface of the photosensitive drum 1 and rotates in accordance with the rotation of the photosensitive drum 1. The charging roller 2 is made of a conductive rubber roller. During charging, a predetermined charging bias (charging voltage) is applied to the charging roller 2 by a charging power source E1 (FIG. 3). Depending on the bias applied to the charging roller 2, charging methods are classified into AC charging methods, which apply a superimposed voltage of AC voltage (alternating current voltage) and DC voltage (direct current voltage), and DC charging methods, which apply only DC voltage. In this embodiment, the AC charging method was adopted, which produces less charging unevenness.

[0021] The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device (laser scanner) 3 serving as exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. The exposure device 3 irradiates the surface of each uniformly charged photosensitive drum 1 with laser light (3Y, 3M, 3C, 3K) corresponding to image information of the separated colors corresponding to each image forming unit S output from the control unit 15. In this way, the exposure device 3 forms an electrostatic latent image (electrostatic image) of the color component corresponding to each image forming unit S on each photosensitive drum 1.

[0022] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4 as a developing means with toner, forming a toner image (toner image, developer image) on the photosensitive drum 1. In this embodiment, a two-component development method is adopted, using a secondary component developer containing toner (non-magnetic toner particles) and carrier (magnetic carrier particles). The developing device 4 has a developer container and a developing sleeve as a developer carrier (developing member). The developing sleeve carries the developer contained in the developer container and rotates, supplying the developer to the development area where the developing sleeve and the photosensitive drum 1 face each other. During development, a predetermined developing bias (developing voltage) is applied to the developing sleeve by a development power source E2 (Figure 3). In this embodiment, a superimposed voltage of a DC voltage and an AC voltage is applied to the developing sleeve as the developing bias. As a result, toner is transferred from the developing sleeve to the photosensitive drum 1 in accordance with the electrostatic latent image formed on the photosensitive drum 1. The developing device 4 is appropriately replenished with toner from a replenishment toner bottle 14. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 1 (negative in this embodiment) adheres to the exposed portion (image portion) on the photosensitive drum 1, where the absolute value of the potential has been reduced by exposure after being uniformly charged (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative.

[0023] An intermediate transfer belt 7, which is an endless belt serving as an intermediate transfer body, is disposed facing the four photosensitive drums 1Y, 1M, 1C, and 1K. The intermediate transfer belt 7 is wound around a drive roller 8 and a driven roller 9, which serve as multiple tension rollers, and is stretched under a predetermined tension. The intermediate transfer belt 7 rotates (circulates) in the direction of arrow R2 (counterclockwise) in the figure when the drive roller 8 is driven to rotate by a drive motor 153 (FIG. 3) that constitutes a driving means. The intermediate transfer belt 7 rotates at a peripheral speed (process speed) that is approximately the same as the peripheral speed of the photosensitive drums 1. The driven roller 9 is a tension roller that keeps the tension of the intermediate transfer belt 7 constant. A biasing force from a biasing spring (not shown) is applied to the driven roller 9, pushing the intermediate transfer belt 7 from its inner peripheral surface toward its outer peripheral surface. This force applies a tension of approximately 20 to 50 [N] to the intermediate transfer belt 7 in the conveyance direction. Note that "to" in relation to a numerical range means that the preceding and following numerical values ​​are included. Primary transfer rollers 5Y, 5M, 5C, and 5K, which are roller-type primary transfer members serving as primary transfer means, are arranged on the inner circumferential surface of the intermediate transfer belt 7, corresponding to the respective photosensitive drums 1Y, 1M, 1C, and 1K. The driven roller 9 and each primary transfer roller 5 are rotated in accordance with the rotation of the intermediate transfer belt 7. Note that, although the intermediate transfer belt 7 is supported by two shafts in this embodiment, it may also be supported by three shafts, with one of the shafts controlling the deviation of the intermediate transfer belt 7, for example.

[0024] In this embodiment, the intermediate transfer belt 7 is a three-layer belt having a base layer, an elastic layer, and a surface layer, from the inner peripheral surface (back surface) to the outer peripheral surface (front surface). That is, in this embodiment, the intermediate transfer belt 7 is an endless belt having an elastic layer. The base layer is preferably made of a resin such as polyimide (PI) or polycarbonate (PC), or various rubbers containing an appropriate amount of carbon black as an antistatic agent. The thickness of the base layer is, for example, 0.05 to 0.15 mm. The elastic layer is preferably made of various rubbers such as urethane rubber or silicone rubber containing an appropriate amount of ion conductive material. The thickness of the elastic layer is, for example, 0.1 to 0.5 mm. The surface layer is preferably made of a resin such as fluororesin. The surface layer reduces the adhesion of toner to the surface of the intermediate transfer belt 7, improving the transfer of the toner image from the intermediate transfer belt 7 to the recording material P at the secondary transfer section N2, which will be described later. In this embodiment, the intermediate transfer belt 7 used has a base layer made of polyimide resin (thickness 70 μm) with a conductive agent dispersed therein, an elastic layer made of conductive CR rubber (chloroprene rubber) (thickness 300 μm), and a surface layer made of PTFE (polytetrafluoroethylene) (thickness 5 μm), for a total thickness of 375 μm. In this embodiment, the intermediate transfer belt 7 has a volume resistivity of 1×10 7 ~1×10 9 The resistivity [Ω·cm] (measurement conditions: 23°C, 50% RH environment, resistivity meter, 100 V applied), and the surface hardness is 65° (measurement conditions: 23°C, 50% RH environment, Wallace hardness meter). In this embodiment, a belt having an elastic layer is used as the intermediate transfer belt 7, but it is also possible to use, for example, a PI single-layer belt without an elastic layer or a coated PI belt in which a PI base layer is coated with a fluororesin or the like.

[0025] The primary transfer roller 5 is disposed opposite the photosensitive drum 1. The primary transfer roller 5 is pressed against (contacts) the photosensitive drum 1 with the intermediate transfer belt 7 sandwiched between it and the photosensitive drum 1. This forms a primary transfer portion (primary transfer nip portion) N1, which is a pressure contact portion between the photosensitive drum 1 and the primary transfer roller 5 via the intermediate transfer belt 7. In this embodiment, the primary transfer roller 5 is an elastic roller having a core metal and an elastic layer formed of ion-conductive foam rubber (NBR rubber) around the core metal. In this embodiment, the outer diameter of the primary transfer roller 5 is 20 mm. In this embodiment, the primary transfer roller 5 has an electrical resistance of 1×10 5 ~1×10 7 [Ω] (measurement conditions: 23°C 50% RH environment, 30 mm outer diameter aluminum drum, approximately 7 mm nip width, 2 kV applied), medium resistance roller with hardness of 24 degrees (Asker C, 500 g load).

[0026] The toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto the rotating intermediate transfer belt 7 by the action of the primary transfer roller 5 at the primary transfer portion N1. During the primary transfer, a primary transfer bias (primary transfer voltage), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the primary transfer roller 5 by a primary transfer power supply E3 (FIG. 3). Although not shown in the figure, in this embodiment, a primary transfer power supply E3 is provided independently for each of the primary transfer rollers 5Y, 5M, 5C, and 5K, and the primary transfer bias applied to each of the primary transfer rollers 5Y, 5M, 5C, and 5K can be individually controlled by the control unit 15. For example, when forming a full-color image, the yellow, magenta, cyan, and black toner images formed on the photosensitive drums 1Y, 1M, 1C, and 1K are sequentially transferred onto the intermediate transfer belt 7 so as to be superimposed on top of each other.

[0027] A secondary transfer roller 10, a roller-type secondary transfer member serving as a secondary transfer means, is disposed on the outer peripheral surface of the intermediate transfer belt 7, facing the drive roller 8, which also serves as a secondary transfer opposing roller. The secondary transfer roller 10 is pressed against (contacts) the drive roller 8, with the intermediate transfer belt 7 sandwiched between them. This forms a secondary transfer nip N2, which is a pressure contact area between the drive roller 8 and the secondary transfer roller 10 via the intermediate transfer belt 7. At the secondary transfer nip N2, the toner image formed on the intermediate transfer belt 7 is transferred (secondarily transferred) onto the recording material P, which is being conveyed between the intermediate transfer belt 7 and the secondary transfer roller 10, by the action of the secondary transfer roller 10. During the secondary transfer, a secondary transfer bias (secondary transfer voltage), which is a DC voltage of a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment), is applied to the secondary transfer roller 10 by a secondary transfer power source E4 (FIG. 3). In this embodiment, the secondary transfer roller 10 is an elastic roller having a core and an elastic layer formed of ion-conductive foam rubber (NBR rubber) around the core. In this embodiment, the outer diameter of the secondary transfer roller 10 is 25 mm. In this embodiment, the secondary transfer roller 10 has an electrical resistance of 1×10 5 ~1×10 7 The secondary transfer roller 10 is a medium-resistance roller with a hardness of 26 degrees (Asker C, 500 g load) and a resistance of 0.01 [Ω] (measurement conditions: 23°C, 50% RH environment, to an aluminum drum with an outer diameter of 30 mm, nip width of approximately 8 mm, 2 kV applied). In this embodiment, the electrical resistance of the secondary transfer roller 10 is approximately equal to that of the primary transfer roller 5. The drive roller 8 is connected to a ground potential (electrically grounded). In this embodiment, for example, a secondary transfer bias of +1 to +5 kV is applied to the secondary transfer roller 10, and the toner image on the intermediate transfer belt 7 is secondarily transferred onto the recording material P. Note that an inner roller corresponding to the drive roller 8 in this embodiment may be used as a secondary transfer member, and a secondary transfer bias of the same polarity as the normal charging polarity of the toner may be applied to it. In this case, an outer roller corresponding to the secondary transfer roller 10 in this embodiment may be used as an opposing roller and connected to a ground potential.

[0028] The recording material P is stored in a cassette 12 serving as a recording material storage unit. The recording material P is sent out from the cassette 12 by a feeding roller 11 serving as a feeding member, conveyed in the direction indicated by the arrow in the figure, and supplied to the secondary transfer unit N2 at a predetermined timing.

[0029] The recording material P onto which the toner image has been transferred is transported to a fixing device 13 serving as a fixing means. The fixing device 13 has a fixing roller 13a and a pressure roller 13b that is in pressure contact with the fixing roller 13a. The fixing device 13 transports the recording material P carrying the unfixed toner image while sandwiching it between the heated fixing roller 13a and pressure roller 13b. In this way, the fixing device 13 applies heat and pressure to the toner image on the recording material P, fixing (melting and adhering) it to the recording material P. The recording material P onto which the toner image has been fixed is discharged (output) to a discharge section 97 provided outside the apparatus main body 96 of the image forming apparatus 100. This completes the image formation on the recording material P.

[0030] Furthermore, toner remaining on the photosensitive drum 1 after the primary transfer (primary transfer residual toner) is removed and collected from the photosensitive drum 1 by a drum cleaning device 6 serving as a photosensitive body cleaning means. In this embodiment, the drum cleaning device 6 uses a rubber cleaning blade that is brought into contact with the photosensitive drum 1 with a predetermined pressing force to scrape and remove the primary transfer residual toner from the surface of the rotating photosensitive drum 1. The surface of the photosensitive drum 1 is then neutralized by a pre-exposure device (not shown), and the photosensitive drum 1 is then used for the next image formation process.

[0031] Furthermore, toner remaining on the intermediate transfer belt 7 after the secondary transfer (secondary transfer residual toner) and paper dust and other adhering matter are removed and collected from the intermediate transfer belt 7 by a belt cleaning device 20 serving as an intermediate transfer body cleaning means. In this embodiment, the belt cleaning device 20 uses a fur brush to which a bias is applied to remove adhering matter from the surface of the intermediate transfer belt 7. Note that in the case where the intermediate transfer belt 7 does not have an elastic layer, for example, the intermediate transfer belt 7 is made of a single layer of PI or a coated PI, a rubber blade may be pressed against the intermediate transfer belt 7 to remove adhering matter from the intermediate transfer belt 7.

[0032] Furthermore, the voltage or current value of the primary transfer bias and the voltage or current value of the secondary transfer bias are determined depending on the material of the photosensitive drum 1, the charge amount of the toner, the image formation speed (i.e., the rotation speed of the photosensitive drum 1 and intermediate transfer belt 7), the electrical resistance value of each primary transfer roller 5, the electrical resistance value of the secondary transfer roller 10, the electrical resistance value of the intermediate transfer belt 7, the environmental temperature and humidity, etc. Furthermore, the secondary transfer bias is an arbitrary value determined depending on the material and electrical resistance value of the recording material P.

[0033] In this embodiment, the process speed of the image forming apparatus 100 is 350 mm / sec, and the image forming speed of the image forming apparatus 100 is 80 A4 landscape sheets per minute.

[0034] The primary transfer power supply E3 is connected to or incorporates a voltage detection circuit (voltage detection unit) that detects its output voltage and a current detection circuit (current detection unit) that detects its output current. In this embodiment, the image forming apparatus 100 acquires information (voltage-current characteristics) related to the electrical resistance of the primary transfer unit N1 during non-image formation, sets a target voltage so that a predetermined target current flows, and outputs a bias voltage under constant voltage control. Information related to the electrical resistance of the primary transfer unit N1 (voltage-current characteristics) can be acquired, for example, by supplying multiple test voltages or test currents to the primary transfer unit N1 during non-image formation and detecting the current or applied voltage. For example, during the pre-rotation process (when there is no toner in the primary transfer unit N1), the voltage V1 is detected to cause a current of, for example, 45 μA to flow from the primary transfer roller 5 to the photosensitive drum 1 via the intermediate transfer belt 7. During primary transfer, a primary transfer bias that is constant-voltage controlled with the voltage V1 as a target voltage is applied to the primary transfer roller 5.

[0035] Similarly, in this embodiment, the secondary transfer power supply E4 is connected to or incorporates a voltage detection circuit (voltage detection unit) that detects its output voltage and a current detection circuit (current detection unit) that detects its output current. Furthermore, in this embodiment, the image forming apparatus 100 is configured to acquire information (voltage-current characteristics) regarding the electrical resistance of the secondary transfer unit N2 during non-image formation, set a target voltage so that a predetermined target current flows, and output a bias voltage under constant voltage control. Information regarding the electrical resistance of the secondary transfer unit N2 (voltage-current characteristics) can be acquired, for example, by supplying multiple test voltages or test currents to the secondary transfer unit N2 during non-image formation and detecting the current or applied voltage. For example, during the pre-rotation process (when there is no toner or recording material P in the secondary transfer unit N2), a voltage (base voltage) V0 is detected to cause a current of, for example, 65 μA to flow from the secondary transfer roller 10 to the drive roller 8 via the intermediate transfer belt 7. Voltage V2 (=V0+VP) is calculated by adding voltage VP (recording material distribution voltage) to voltage V0. This voltage VP is determined in advance through experiments or the like depending on the environment (temperature and humidity), the type and basis weight of recording material P, the first side, and the second side in automatic double-sided printing. In the case of the above voltage V0, this voltage VP is a voltage that causes a current of 65 μA to flow when recording material P is present at secondary transfer section N2 (when paper is passing through). Then, during secondary transfer, a secondary transfer bias that is constant-voltage controlled with voltage V2 (=V0+VP) as the target voltage is applied to secondary transfer roller 10.

[0036] In this embodiment, the image forming apparatus 100 has a first environment sensor 30 as an environment detection means for detecting the temperature and humidity inside the apparatus main body 96. Here, the environment may be at least one of the temperature and humidity inside or outside the image forming apparatus 100. The humidity may be expressed as relative humidity [% RH], absolute moisture content [g / Kg], absolute humidity [g / m 3The first environmental sensor 30 is an environmental detection unit that mainly estimates fluctuations in the charge amount of toner due to the environment inside the apparatus main body 96. Therefore, it is preferable to place the first environmental sensor 30 near the image forming unit. In this embodiment, the first environmental sensor 30 is placed near the black developing device 4K, which is particularly convenient for estimating the charge amount of black toner.

[0037] In this embodiment, the image forming apparatus 100 also includes a second environmental sensor 31 as an environmental detection unit for detecting the temperature and humidity outside the apparatus main body 96. The second environmental sensor 31 is an environmental detection unit for mainly estimating the electrical resistance values ​​of the recording material P and transfer members (intermediate transfer belt 7, primary transfer roller 5, secondary transfer roller 10) that depend on the environment outside the apparatus main body 96. Therefore, it is preferable to place the second environmental sensor 31 at a convenient location for detecting the environment outside the apparatus main body 96, away from the fixing device 13, which is subject to large changes in temperature and humidity. In this embodiment, the second environmental sensor 31 is placed near the cassette 12.

[0038] In this embodiment, the image forming apparatus 100 is configured to be capable of forming images in at least a color mode and a black monochrome mode (monochrome mode). The color mode is an image forming mode in which toner images can be formed in all four image forming stations SY, SM, SC, and SK to form a full-color image. The black monochrome mode is an image forming mode in which toner images can be formed only in the black image forming station SK among the four image forming stations SY, SM, SC, and SK to form a black monochrome image. In this embodiment, the image forming apparatus 100 is configured such that, in the black monochrome mode, the intermediate transfer belt 7 and primary transfer rollers 5Y, 5M, and 5C are spaced apart from the photosensitive drums 1Y, 1M, and 1C in the yellow, magenta, and cyan image forming stations SY, SM, and SC. Furthermore, in the black monochrome mode, the image forming apparatus 100 is configured to stop the rotation of rotating members (photosensitive drums, developing sleeves, etc.) and the application of biases (charging bias, developing bias, etc.) in the yellow, magenta, and cyan image forming units SY, SM, and SC. Therefore, in the black monochrome mode, the life count of the elements in the yellow, magenta, and cyan image forming units SY, SM, and SC is stopped. In this way, in the black monochrome mode, the image forming units for the other colors (yellow, magenta, and cyan) are essentially turned off.

[0039] In this embodiment, for each image forming station S, the charging roller 2, the exposure device 3, the developing device 4, etc. constitute an image forming means for forming a toner image on the photosensitive drum 1.

[0040] In this embodiment, the photosensitive drum 1, charging roller 2, developing device 4, and drum cleaning device 6 are unitized for each color as cartridges for each part, and are configured to be detachable from the apparatus main body 96. As a result, by replacing the cartridges with new ones when the life of each part has expired, it is possible to form a toner image on the intermediate transfer belt 7 similar to that of a new image forming apparatus 100. However, the configuration of the cartridge is not limited to this. For example, in each image forming station S, the photosensitive drum 1, charging roller 2, developing device 4, and drum cleaning device 6 may be unitized as a cartridge, and configured to be detachable from the apparatus main body 96. Also, for example, in each image forming station S, a unit including the photosensitive drum 1, charging roller 2, and drum cleaning device 6, and the developing device 4 may each be configured to be detachable from the apparatus main body 96.

[0041] In this embodiment, the intermediate transfer belt 7, the multiple tension rollers, the primary transfer rollers 5, and the belt cleaning device 20 are configured as an intermediate transfer belt unit that can be detached from the device main body 96. This makes it possible to detach the intermediate transfer unit from the device main body 96 and replace it with a new one, for example, when a part reaches the end of its life or breaks down.

[0042] <Control configuration> FIG. 3 is a block diagram showing an outline of the electrical configuration of the image forming apparatus 100 in this embodiment.

[0043] The image forming apparatus 100 has a control unit 15 that controls the overall operation of the image forming apparatus 100. The control unit 15 is configured to include, for example, a CPU 150 as a calculation processing unit (calculation processing section) that executes predetermined calculation processes, a ROM 151 and a RAM 152 as storage units (storage units), and peripheral circuits therefor. The ROM 151 stores predetermined control programs and the like. The RAM 152 temporarily stores data such as various calculation results and detection results of various sensors. The control unit 15 executes, for example, the control programs stored in the ROM 151 using the RAM 152, thereby functioning as various functional blocks such as an image formation control unit, an image formation speed control unit, and a cleaning control unit.

[0044] The control unit 15 is connected to, for example, an operation unit 99, environmental sensors (first environmental sensor 30 and second environmental sensor 31), a communication I / F 156, an image reading unit 98, an image processing unit 157, the fixing device 13, image forming units SY, SM, SC, and SK, a drive motor 153, a recording material conveying mechanism 154, and a high-voltage power supply 155. Although the drive motor 153 is shown as a single block, it may be composed of a single motor or multiple motors. The high-voltage power supply 155 may be composed of multiple power supplies (high-voltage circuits), such as a charging power supply E1, a developing power supply E2, a primary transfer power supply E3, and a secondary transfer power supply E4. The operation unit 99 includes an input unit (input unit) for inputting information to the control unit 15 based on the operation of an operator such as a user or a service representative, and a display unit (display unit) for displaying information to the operator under the control of the control unit 15. The operation unit 99 may include a touch panel that functions as both an input unit and a display unit.

[0045] The drive motor 153 is controlled by the control unit 15 (image formation control unit) to, for example, drive and rotate the photosensitive drum 1. The drive motor 153 is also controlled by the control unit 15 (image formation control unit) to, for example, drive and rotate the drive roller 8, thereby circulating the intermediate transfer belt 7. The drive motor 153 is also controlled by the control unit 15 (image formation control unit) to, for example, drive and rotate the fixing roller 13a of the fixing device 13, thereby fixing the toner image on the recording material P. Individual drive motors may be provided to drive these drive targets, or a common drive motor may be used for multiple drive targets. Furthermore, if, for example, a malfunction or abnormality occurs in the fixing device 13, the control unit 15 (image formation control unit) stops the entire image forming apparatus 100 and displays information on the operation unit 99 to notify the user of the malfunction or abnormality. In this way, each device connected to the control unit 15 is controlled by bidirectional communication with the control unit 15.

[0046] The high-voltage power supply 155 is connected to, for example, the primary transfer roller 5 and the secondary transfer roller 10. The high-voltage power supply 155 is controlled by the control unit 15 (image formation control unit) and applies a bias that is individually controlled by constant current or constant voltage to the primary transfer roller 5 and the secondary transfer roller 10, thereby performing primary transfer and secondary transfer, respectively.

[0047] The image processing unit (controller) 157 processes, forms, processes, and separates colors of image information received from, for example, the communication I / F unit 156, the image reading unit 98, a network, etc., in order to form an image in the image forming apparatus 100.

[0048] The image forming apparatus 100 executes a job (printing operation), which is a series of operations that starts with a single start command and forms and outputs an image on one or more recording materials P. A job generally includes an image forming process, a pre-rotation process, a sheet-to-sheet process when forming images on multiple recording materials P, and a post-rotation process. The image forming process is a period during which electrostatic image formation, toner image formation, and primary and secondary transfer of the toner image are performed for the image that will actually be formed and output on the recording materials P. This period is referred to as the image formation time. More specifically, the timing of the image formation process differs depending on the positions where the electrostatic image formation, toner image formation, and primary and secondary transfer of the toner image are performed. The pre-rotation process is a period from when a start command is input until the actual start of image formation, during which preparatory operations are performed before the image forming process. The sheet-to-sheet process is a period corresponding to the interval between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is a period during which a tidying up operation (preparatory operation) is performed after the image forming process. Non-image formation refers to periods other than image formation, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multi-rotation process, which is a preparatory operation when the image forming device 100 is turned on or when it returns from a sleep state.

[0049] <Mechanism of "horizontal streaks"> Next, an image defect (density unevenness) called "horizontal stripe-like density unevenness" will be described.

[0050] Figure 4 is a schematic diagram of a solid black image in which "horizontal stripe-like density unevenness" has occurred. As shown in Figure 4, density unevenness occurs in the form of horizontal stripes that are approximately perpendicular to the direction of travel (conveyance direction) of the recording material P. The horizontal stripes are not all connected, but rather several fragments of density unevenness come together to appear as horizontal stripe-like density unevenness.

[0051] 5 is a schematic diagram of the vicinity of the primary transfer nip N1K of the black image forming unit SK. The primary transfer nip N1 is the pressure contact area between the photosensitive drum 1 and the primary transfer roller 5 via the intermediate transfer belt 7. Furthermore, the terms "upstream" and "downstream" in the configuration around the intermediate transfer belt 7 refer to the upstream and downstream in the transport direction of the intermediate transfer belt 7. For convenience, the magnitude (high / low) of potential, voltage, and current refers to the magnitude (high / low) when compared in absolute value.

[0052] The horizontal stripe-like density unevenness is thought to occur as follows. As shown in FIG. 5, a primary transfer bias of a polarity opposite to the normal charge polarity of the toner is applied to the primary transfer roller 5K at the primary transfer nip N1K, causing the toner T to be transferred from the photosensitive drum 1 to the intermediate transfer belt 7. Subsequently, a discharge occurs between the photosensitive drum 1 and the intermediate transfer belt 7 near the downstream side (exit) of the primary transfer nip N1K. The horizontal stripe-like density unevenness is thought to occur due to this discharge. In this embodiment, the primary transfer roller 5 is positioned downstream offset from the photosensitive drum 1 in the black image forming unit SK (all image forming units S in this embodiment). In this case, the horizontal stripe-like density unevenness tends to occur, but the principle described here also applies when the primary transfer roller 5 is positioned without being offset from the photosensitive drum 1. Offset will be described in more detail in Example 4.

[0053] In other words, when the potential of the intermediate transfer belt 7 is high near the downstream side of the primary transfer nip N1, the intermediate transfer belt 7 is susceptible to negative discharge from the photosensitive drum 1. In this case, negative creeping discharges that occur on the surface (outer periphery) of the intermediate transfer belt 7 form elliptical discharge shapes, causing uneven charge (charge unevenness) in the toner on the intermediate transfer belt 7. The uneven charge of the toner caused by these elliptical discharges accumulates to form horizontal stripes. The potential of the discharged portion of the intermediate transfer belt 7 drops and it is not subject to further discharge. However, when a high-potential portion of the intermediate transfer belt 7 that is not affected by the discharge appears at the discharge point, it is subject to a new discharge. This "intermittent discharge" causes periodic unevenness in the charge amount of the toner on the intermediate transfer belt 7. The charge amount of the toner increases in the discharged portion of the intermediate transfer belt 7 due to charging up. Meanwhile, the charge amount of the toner in the undischarged portion of the intermediate transfer belt 7 is not charged up. As a result, uneven charge (charge amount unevenness) occurs in the toner on the intermediate transfer belt 7 after the primary transfer.

[0054] This periodic unevenness in the charge amount of the toner causes differences in secondary transfer efficiency at the secondary transfer portion N2. As a result, the toner transferred onto the recording material P with different secondary transfer efficiencies becomes apparent as "horizontal stripe-like density unevenness." As such, because it is charge unevenness that occurs in the toner on the intermediate transfer belt 7 after the primary transfer, density unevenness does not occur in the toner image on the intermediate transfer belt 7 after the primary transfer. However, density unevenness occurs in the toner image on the recording material P after the secondary transfer.

[0055] In particular, in a solid black image, "horizontal streak-like density unevenness" is likely to become apparent on the recording material P after secondary transfer. This is because the black image forming station SK is typically located at the most downstream of the four image forming stations. The toner that has been primarily transferred onto the intermediate transfer belt 7 in the yellow, magenta, and cyan image forming stations SY, SM, and SC is then subjected to one or more additional primary transfer bias applications. The toner that has been primarily transferred onto the intermediate transfer belt 7 in the yellow image forming station SY is subjected to the primary transfer bias a total of four times.

[0056] It is known that the charge of the toner on the intermediate transfer belt 7 increases when it is subjected to the primary transfer bias. The charge of the yellow toner on the intermediate transfer belt 7, which receives the primary transfer bias four times, increases particularly. Thus, the yellow, magenta, and cyan toners receive the primary transfer bias four times, three times, and two times, respectively, increasing the charge of the toner on the intermediate transfer belt 7. The charge unevenness caused by the discharge in these toners as described above is reduced by the increased charge of the toners due to the multiple exposures to the primary transfer bias. Therefore, yellow, magenta, and cyan images are less likely to exhibit horizontal stripe-like density unevenness. On the other hand, after receiving the primary transfer bias and being primarily transferred onto the intermediate transfer belt 7, if charge unevenness occurs due to discharge near the downstream side of the primary transfer nip N1K, the black toner reaches the secondary transfer portion N2 as is. Therefore, black images are more likely to exhibit horizontal stripe-like density unevenness.

[0057] Furthermore, when the amount of charge on the toner is small, the effect of charge-up due to discharge is large, so "horizontal stripe-like density unevenness" is likely to occur in a high humidity environment where the amount of charge on the toner is reduced.

[0058] Furthermore, as for the recording material P, density unevenness is more easily visible on coated paper, which has a smooth surface, than on plain paper, so "horizontal streak-like density unevenness" is more likely to occur on coated paper.

[0059] Furthermore, in a high-humidity environment, the electrical resistance of the moisture-absorbed recording material P decreases. Therefore, when the secondary transfer bias is controlled to a constant voltage as in this embodiment, the secondary transfer current value increases in a high-humidity environment, the discharge amount increases, and the amount of toner charge-up increases. As a result, in a high-humidity environment, "horizontal streak-like density unevenness" is likely to occur.

[0060] Therefore, examples of stress conditions include the following: Horizontal stripe-like density unevenness is likely to occur in the most downstream image forming unit, in a high humidity environment, and in solid black images made on coated paper (paper left in a high humidity environment). Also, when the primary transfer current value is low, the pitch of intermittent discharge increases, making horizontal stripe-like density unevenness more likely to occur. Furthermore, when the secondary transfer current value is high, the transfer latitude becomes smaller in relation to uneven toner charging (only toner with the charge amount in some areas is transferred), making horizontal stripe-like density unevenness more likely to occur.

[0061] <Countermeasures against horizontal streaks in density> Considering the conditions under which the above-mentioned "horizontal streak-like density unevenness" is likely to occur, countermeasures that can be considered to suppress "horizontal streak-like density unevenness" include (1) increasing the primary transfer current value, (2) decreasing the secondary transfer current value, and (3) increasing the toner charge amount.

[0062] In this embodiment, (1) increasing the primary transfer current value will be described, while (2) decreasing the secondary transfer current value will be described in embodiment 2 below.

[0063] As a countermeasure against the "horizontal streak-like density unevenness," if the value of the primary transfer current supplied to the black primary transfer roller 5K is increased, the "intermittent discharge" near the downstream side of the black primary transfer nip N1K changes to "continuous discharge." This continuous discharge reduces or eliminates periodic unevenness, and also reduces or eliminates charging unevenness of the black toner on the intermediate transfer belt 7 after primary transfer. As a result, it was found that the "horizontal streak-like density unevenness" of the black toner on the recording material P after secondary transfer is also reduced or does not occur.

[0064] However, if the primary transfer current value in the black image forming station SK is increased, the polarity of the yellow, magenta, and cyan toner on the intermediate transfer belt 7 may be reversed from negative to positive, resulting in transfer (also referred to as "retransfer") to the black photosensitive drum 1K. In this case, the density of the yellow, magenta, and cyan toner on the intermediate transfer belt 7 may become low, resulting in white spots appearing in the image, and it may become difficult to obtain a desired color image on the recording material P after secondary transfer.

[0065] Therefore, the primary transfer control (setting of the primary transfer current value) according to the present invention is effective in the black monochrome mode of a multicolor image forming apparatus (color image forming apparatus) or in a black monochrome image forming apparatus (monochrome image forming apparatus) (FIG. 2) as described in Example 5. In these cases, in a high humidity environment, the occurrence of "horizontal streak-like density unevenness" can be suppressed by increasing the primary transfer current value in the black image forming unit SK.

[0066] Here, we consider the conditions for increasing the primary transfer current value. "Horizontal streak-like density unevenness" is likely to occur in high-humidity environments where the toner charge amount is reduced. Therefore, it is preferable to decrease the primary transfer current value as the humidity increases up to a certain humidity level, and then increase the primary transfer current value as the humidity increases above that level. In this case, typically, the primary transfer current value can be decreased in proportion to the increase in humidity up to the certain humidity level, and then increased in proportion to the increase in humidity above that level.

[0067] If the primary transfer current value is set too high in a high humidity environment, the polarity of the black toner may be reversed during the primary transfer, causing re-transfer, which may result in low image density or white spots. Therefore, an upper limit can be set for the primary transfer current value as appropriate.

[0068] FIG. 6 is a graph illustrating an example of primary transfer control (primary transfer current value setting) for the black image forming unit SK in this embodiment. In FIG. 6, the horizontal axis represents relative humidity (%RH) and the vertical axis represents primary transfer current (μA). The primary transfer current value described here is the target current value flowing from the primary transfer roller 5K to the photosensitive drum 1K during primary transfer. In this embodiment, the primary transfer bias is controlled at a constant voltage, so the current value may differ depending on whether toner is present during primary transfer. Therefore, the primary transfer current value described here is the target current value flowing to the photosensitive drum 1K when no toner is present. The control unit 15 can control the primary transfer power supply E3 based on information related to the primary transfer current value setting, as shown in FIG. 6, stored in ROM 151. Information indicating the relationship between humidity and primary transfer current value can be stored in ROM 151 as table data or a formula.

[0069] The solid line in Figure 6 represents "normal control," in which the primary transfer current value is controlled to decrease proportionally with an increase in relative humidity (%RH). For example, at 50%RH, the primary transfer current value is 45 μA.

[0070] Meanwhile, the dashed line in Figure 6 represents "density unevenness countermeasure control 1," which controls the primary transfer current value to increase as the relative humidity (%RH) increases at 50%RH or higher. However, at 85%RH or higher, the primary transfer current value is controlled to be constant at 53 μA to prevent excessive increase in the primary transfer current value. The upper limit of the primary transfer current value in this high-humidity environment is set to, for example, less than 57 μA, which is the upper limit for a low-humidity environment of 10%RH. Note that, while not limited to, examples of low humidity include 10%RH or lower, medium humidity includes 45%RH or higher and 55%RH or lower, and high humidity includes 70%RH or higher.

[0071] In this embodiment, primary transfer control (primary transfer current value setting) in color mode is "normal control" regardless of humidity. In this case, the primary transfer current value relationship with humidity is "(high current value) low humidity > medium humidity > high humidity (low current value)." On the other hand, primary transfer control (primary transfer current value setting) in black monochrome mode is "normal control" when the humidity is below a predetermined level and "density unevenness prevention control 1" when the humidity is above the predetermined level. In this embodiment, this predetermined humidity (control switching threshold) is 50% RH. In this case, the primary transfer current value relationship with humidity is "(high current value) low humidity > high humidity > medium humidity (low current value)." In other words, in this embodiment, primary transfer control (primary transfer current value setting) in color mode is "normal control" regardless of relative humidity. In this case, the primary transfer current value relationship with relative humidity is "(high current value) low relative humidity > medium relative humidity > high relative humidity (low current value)." Meanwhile, primary transfer control (primary transfer current value setting) in black monochrome mode is set to "normal control" when the relative humidity is below a predetermined level, and to "density unevenness countermeasure control 1" when the relative humidity is above the predetermined level. As described above, in this embodiment, this predetermined relative humidity (control switching threshold) is set to 50% RH. In this case, the relationship between the primary transfer current value and the relative humidity is "(high current value) low relative humidity > high relative humidity > medium relative humidity (low current value)." In this way, by switching the primary transfer control so that the primary transfer current value is increased in a high-humidity environment, the occurrence of "horizontal streak-like density unevenness" can be suppressed.

[0072] 8 is a flowchart showing an outline of an example of a control procedure in the black monochrome mode in this embodiment, in which an image is formed on one sheet of recording material P in the black monochrome mode.

[0073] When image formation begins, the control unit 15 acquires the temperature and humidity detected by the environmental detection unit (S101). In this embodiment, the control unit 15 uses the detection data from the first environmental sensor 30, which detects the temperature and humidity inside the image forming apparatus 100. Next, the control unit 15 determines whether the relative humidity detected by the first environmental sensor 30 is 50% RH or higher (S102). If the control unit 15 determines in S102 that the relative humidity is 50% RH or higher, it controls the primary transfer process by using "density unevenness countermeasure control 1" to control the primary transfer current value (S104). That is, the control unit 15 controls the primary transfer current value to be higher than in normal control and the primary transfer bias voltage value to be higher than in normal control. This is because it is determined that "horizontal stripe-like density unevenness" is likely to occur in this case. This "density unevenness countermeasure control 1" can suppress "horizontal stripe-like density unevenness." On the other hand, if the control unit 15 determines in S102 that the relative humidity is less than 50%, it controls the primary transfer process to be performed under "normal control" for primary transfer control (setting of the primary transfer current value) (S103). This is because it is determined that "horizontal stripe-like density unevenness" is unlikely to occur in this case. Next, after the primary transfer process is completed, the control unit 15 controls the secondary transfer process (S105) and the fixing process (S106) to be performed in sequence, thereby completing image formation.

[0074] Furthermore, the absolute moisture content [g / Kg] may be used as an index of humidity instead of the relative humidity [%RH]. The absolute moisture content is determined by the temperature and humidity of the environment. The absolute moisture content [g / Kg] can be calculated by the control unit 15 based on the temperature and humidity detection results. An example of the relationship between temperature [°C], relative humidity [%RH], and absolute moisture content [g / Kg] is as follows: Low humidity (low moisture content): 23℃10%RH=1.773[g / Kg] Medium humidity (medium moisture content): 23℃45%RH=7.979[g / Kg] 23℃50%RH=8.865[g / Kg] 23℃55%RH=9.752[g / Kg] High humidity (high moisture content): 30℃70%RH=19.026[g / Kg] 30℃80%RH=21.744[g / Kg] 30℃85%RH=23.103[g / kg]

[0075] 7 is a graph showing another example of primary transfer control (setting of primary transfer current value) in the black image forming unit SK in this embodiment. In FIG. 7, the horizontal axis represents absolute moisture content [g / Kg], and the vertical axis represents primary transfer current value [μA]. The control unit 15 can control the primary transfer power supply E3 based on information related to the setting of the primary transfer current value as shown in FIG. 7, which is stored in the ROM 151.

[0076] 7 represents "normal control," in which the primary transfer current value is controlled to decrease as the absolute moisture content [g / Kg] increases. For example, when the absolute moisture content is 8.865 [g / Kg], the primary transfer current value is 45 [μA].

[0077] On the other hand, the dashed line in Figure 7 represents "density unevenness countermeasure control 1," which controls the primary transfer current value to increase as the absolute moisture content (g / kg) increases above 8.865 [g / kg]. However, above 23.103 [g / kg], the primary transfer current value is controlled to be constant at 53 [μA] to avoid excessive increase in the primary transfer current value. The upper limit of the primary transfer current value in this high-humidity environment is set to, for example, less than 57 [μA] for a low-humidity environment of 1.773 [g / kg]. Note that, while not limited to these, for example, low moisture content (low humidity) is 1.75 [g / kg] or less, medium moisture content (medium humidity) is 8.0 [g / kg] or more to 9.75 [g / kg] or less, and high moisture content (high humidity) is 19.0 [g / kg] or more.

[0078] In this embodiment, primary transfer control (primary transfer current value setting) in color mode is "normal control" regardless of the absolute moisture content. In this case, the relationship between the primary transfer current value and the absolute moisture content is "(high current value) low moisture content > medium moisture content > high moisture content (low current value)." On the other hand, primary transfer control (primary transfer current value setting) in black monochrome mode is "normal control" when the absolute moisture content is less than a predetermined absolute moisture content and "density unevenness countermeasure control 1" when the absolute moisture content is equal to or greater than the predetermined absolute moisture content. In this embodiment, this predetermined absolute moisture content (control switching threshold) is 8.865 [g / kg]. In this case, the relationship between the primary transfer current value and the absolute moisture content is "(high current value) low moisture content > high moisture content > medium moisture content (low current value)." In this way, by switching primary transfer control to increase the primary transfer current value in a high-humidity environment, the occurrence of "horizontal streak-like density unevenness" can be suppressed.

[0079] 9 is a flowchart outlining another example of the control procedure in the black monochrome mode in this embodiment, where an image is formed on one sheet of recording material P in the black monochrome mode.

[0080] When image formation begins, the control unit 15 acquires the temperature and humidity detected by the environmental detection unit (S201). In this embodiment, the control unit 15 uses the detection data from the first environmental sensor 30, which detects the temperature and humidity inside the image forming apparatus 100. Next, the control unit 15 determines whether the absolute moisture content calculated from the temperature and humidity detected by the first environmental sensor 30 is equal to or greater than 8.865 [g / kg] (S202). If the control unit 15 determines in S202 that the absolute moisture content is equal to or greater than 8.865 [g / kg], it controls the primary transfer process by using "density unevenness countermeasure control 1" for primary transfer control (setting of the primary transfer current value) (S204). That is, the control unit 15 controls the primary transfer current value to be higher than in normal control, and the primary transfer bias voltage value to be higher than in normal control. This is because it is determined that "horizontal stripe-like density unevenness" is likely to occur in this case. This "density unevenness countermeasure control 1" can suppress "horizontal stripe-like density unevenness." On the other hand, if the control unit 15 determines in S202 that the moisture content is less than 8.865 [g / Kg], it controls the primary transfer process to be performed with the primary transfer control (setting of the primary transfer current value) set to "normal control" (S203). This is because it is determined that "horizontal stripe-like density unevenness" is unlikely to occur in this case. Next, after the primary transfer is completed, the control unit 15 controls the secondary transfer process (S205) and the fixing process (S206) to be performed in sequence, thereby completing image formation.

[0081] Here, the control unit 15 may automatically switch between "normal control" and "density unevenness countermeasure control 1," or an operator such as a user or service personnel may manually switch between them, for example, when "horizontal streak-like density unevenness" occurs.

[0082] For example, in this embodiment, the control unit 15 automatically switches between "normal control" and "density unevenness countermeasure control 1" for the primary transfer control at the black image forming unit SK depending on whether the image forming unit is in color mode or black monochrome mode. However, this is not limited to this. In black monochrome mode, an operator, such as a user or service technician, may manually switch between "normal control" and "density unevenness countermeasure control 1" for the primary transfer control. For example, when "horizontal stripe-like density unevenness" occurs, the operator can enable primary transfer control using "density unevenness countermeasure control 1." FIG. 16A is a schematic diagram of an example of a primary transfer control setting screen 301 displayed on the operation unit 99 in this case. The operator can use a switch 311 on this setting screen 301 to input an instruction to the control unit 15 to turn on "density unevenness countermeasure control 1" (or "density unevenness countermeasure control 2," described later). When "density unevenness countermeasure control 1" (or "density unevenness countermeasure control 2" described later) is turned ON, the control unit 15 controls the primary transfer control to be switched according to the humidity in the black monochrome mode according to the procedure described above. On the other hand, when it is turned OFF, the primary transfer control is set to "normal control" in the black monochrome mode regardless of the humidity.

[0083] Alternatively, instead of the controller 15 automatically switching between “normal control” and “density unevenness countermeasure control 1,” an operator, such as a user or service technician, may manually switch between these modes. For example, the operator may use “density unevenness countermeasure control 1” of the primary transfer control when “horizontal stripe-like density unevenness” occurs. In this case, the operator can turn on “density unevenness countermeasure control 1” (or density unevenness countermeasure control 2, described later) using a setting screen similar to the setting screen 301 described above. When this setting is turned on, the controller 15 can increase the primary transfer current setting by a predetermined amount in black monochrome mode, regardless of the environment. This predetermined amount of change may be fixed, selectable from multiple options, or arbitrarily increased or decreased in predetermined increments. FIG. 16B is a schematic diagram of an example of a setting screen 302 for primary transfer control displayed on the operation unit 99 in this case. The operator can set the predetermined amount of change using button 321 on the setting screen 302 shown in FIG. 16B. In this case, it is possible to make it so that the primary transfer current value can be adjusted only in the direction of increasing it (positive direction) compared to the case of normal control.

[0084] Furthermore, as shown in FIGS. 6 and 7, the "density unevenness countermeasure control" may have multiple settings with different primary transfer current values ​​for the same humidity. For example, a user or an operator such as a service representative may manually select from multiple controls, such as density unevenness countermeasure control 1, density unevenness countermeasure control 2, etc., based on the strength or weakness of the effect. In this case, the operator may select the control using a setting screen such as that shown in FIG. 16(a) on the operation unit 99, as described above. For example, "density unevenness countermeasure control 1" may eliminate "horizontal streak-like density unevenness," but the primary transfer current value may be too high, resulting in noticeable white spots due to re-transfer of black toner. In this case, the operator may be able to switch to "density unevenness countermeasure control 2," which has a slightly lower primary transfer current value (up to approximately 5 μA lower). This may allow, for example, adjustments to reduce white spots while still slightly increasing "horizontal streak-like density unevenness."

[0085] In this embodiment, the case where the relative humidity [% RH] or the absolute moisture content [g / Kg] is used as the humidity index has been described. 3 The humidity index can be appropriately selected depending on the configuration of the image forming apparatus, for example, and the same effect can be obtained.

[0086] In addition, relative humidity [%RH], absolute moisture content [g / Kg], absolute humidity [g / m 3 As an environment detection means for acquiring information about humidity, either one or both of a device that detects the environment inside the image forming apparatus 100 (first environment sensor 30) and a device that detects the environment outside the image forming apparatus 100 (second environment sensor 31) can be used. These can be appropriately selected depending on, for example, the configuration of the image forming apparatus, even in a configuration in which the primary transfer current value is switched, and similar effects can be obtained.

[0087] As described above, the image forming apparatus 100 of this embodiment includes a rotatable image carrier (photosensitive drum) 1K that carries a toner image, image forming means (charging roller 2K, exposure device 3, developing device 4K, etc.) that form a toner image on the image carrier 1K, a rotatable intermediate transfer belt 7 to which the toner image is transferred from the image carrier 1K, a primary transfer member (primary transfer roller) 5K that transfers the toner image from the image carrier 1K to the intermediate transfer belt 7, a primary transfer power supply E3 that supplies a primary transfer current to the primary transfer member 5K for transferring the toner image from the image carrier 1K to the intermediate transfer belt 7, and a developing device 4K that transfers the toner image from the intermediate transfer belt 7 to the primary transfer member 5K. The image forming apparatus 100 includes a secondary transfer member (secondary transfer roller) 10 that forms a secondary transfer section N2 that transfers a toner image to the recording material P, a secondary transfer power supply E4 that supplies a secondary transfer current to the secondary transfer section N2 to transfer the toner image from the intermediate transfer belt 7 to the recording material P, an environmental detection means (environmental sensor) 30 that can detect the humidity inside or outside the image forming apparatus 100, and a control unit 15 that can control the primary transfer power supply E3, and is capable of forming a monochrome black image on the recording material P by transferring a black toner image formed on the image carrier 1K with black toner to the intermediate transfer belt 7 and then transferring it to the recording material P. In this embodiment, when forming a black monochrome image, if the humidity detection result by the environment detection means 30 is a first humidity, the control unit 15 sets the primary transfer current supplied to the primary transfer member 5K to a first current; if the detection result is a second humidity higher than the first humidity, the control unit 15 sets the primary transfer current supplied to the primary transfer member 5K to a second current having an absolute value smaller than the first current; and if the detection result is a third humidity higher than the second humidity, the control unit 15 sets the primary transfer current supplied to the primary transfer member 5K to a third current having an absolute value smaller than the first current and larger than the absolute value of the second current.

[0088] In this embodiment, the image carrier 1K is a first image carrier 1K, the image forming means is a first image forming means (e.g., a charging roller 2K, an exposure device 3, a developing device 4K, etc.), the primary transfer member 5K is a first primary transfer portion 5K, and the image forming apparatus 100 includes a rotatable second image carrier (e.g., a photosensitive drum 1Y) that carries a toner image, and second image forming means (e.g., a charging roller 2Y, an exposure device 3, a developing device 4Y, etc.) that forms a toner image on the second image carrier 1Y, The image forming apparatus 100 has a second primary transfer member (e.g., primary transfer roller 5Y) that transfers a toner image from the second image carrier 1Y to the intermediate transfer belt 7, and is a multi-color image forming apparatus that can perform multi-color image formation by transferring a black toner image formed with black toner on the first image carrier 1K and a toner image of a color other than black formed with toner of a color other than black on the second image carrier 1Y to the intermediate transfer belt 7 and then transferring them to a recording material P, thereby forming a multi-color image on the recording material P. In this embodiment, when forming a multi-color image, if the detection result is the first humidity, the control unit 15 sets the primary transfer current supplied to the first primary transfer member 5K to the first current, if the detection result is the second humidity, the control unit 15 sets the primary transfer current supplied to the first primary transfer member 5K to the second current, and if the detection result is the third humidity, the control unit 15 sets the primary transfer current supplied to the first primary transfer member 5K to a fourth current whose absolute value is smaller than the second current. Note that, as will be described in a fifth embodiment, the image forming apparatus may be a monochrome image forming apparatus (FIG. 2) capable of forming only a black monochrome image as image formation in which a toner image is transferred from the intermediate transfer belt 7 to the recording material P to form an image on the recording material P.

[0089] In this embodiment, the primary transfer member 5K is pressed against the image carrier 1K via the intermediate transfer belt 7. In this embodiment, with such a configuration, when forming a black monochrome image, the control unit 15 sets the primary transfer current supplied to the primary transfer member 5K to a first current if the humidity detection result by the environment detection unit 30 is a first humidity, and sets the primary transfer current supplied to the primary transfer member 5K to a second current whose absolute value is greater than the first current if the detection result is a second humidity higher than the first humidity.

[0090] Furthermore, the control unit 15, when forming a black monochrome image, sets the primary transfer current supplied to the primary transfer member 5K to the first current when the detection result is the first humidity, sets the primary transfer current supplied to the primary transfer member 5K to the second current when the detection result is the second humidity, and sets the primary transfer current supplied to the primary transfer member 5K to a fourth current whose absolute value is smaller than the second current when the detection result is the third humidity; and and a second mode (control to counter density unevenness) in which, when forming a black monochrome image, if the detection result is the first humidity, the primary transfer current supplied to the primary transfer member 5K is set to the first current, if the detection result is the second humidity, the primary transfer current supplied to the primary transfer member 5K is set to the second current, and if the detection result is the third humidity, the primary transfer current supplied to the primary transfer member 5K is set to the third current. The image forming apparatus 100 may have an input unit (operation unit) 99 that inputs instructions to the control unit based on operation by an operator, and the control unit 15 can control to selectively execute the first mode and the second mode based on the instructions input by the input unit 99.

[0091] According to this embodiment, it is possible to suppress the occurrence of uneven density of the image on the recording material after secondary transfer, which is caused by uneven charging of the toner on the intermediate transfer belt.

[0092] [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.

[0093] In the first embodiment, it was explained that the occurrence of "horizontal streak-like density unevenness" is suppressed by "increasing the primary transfer current value" in a high humidity environment. In this embodiment, it is explained that the occurrence of "horizontal streak-like density unevenness" is suppressed by "reducing the secondary transfer current value" in a high humidity environment.

[0094] It was found that when the value of the secondary transfer current supplied to the secondary transfer roller 10 is reduced, even if there is uneven charging of the black toner on the intermediate transfer belt 7, "horizontal streak-like density unevenness" is less likely to occur in the image on the recording material P after secondary transfer. This is thought to be because reducing the value of the secondary transfer current widens the transfer latitude during secondary transfer. As a result, even if there is unevenness in the amount of charge of the toner, "horizontal streak-like density unevenness" is less likely to occur in the image on the recording material P after secondary transfer.

[0095] However, if the secondary transfer current value supplied to the secondary transfer roller 10 is reduced, and the transfer current is insufficient during secondary transfer, the image may be scattered or the image density may be low.

[0096] Therefore, the secondary transfer control (setting of the secondary transfer current value) according to the present invention is effective in the black monochrome mode of a multicolor image forming apparatus, or in a black monochrome image forming apparatus (FIG. 2) as described in Example 5. In these cases, the occurrence of "horizontal streak-like density unevenness" can be suppressed by lowering the secondary transfer current value during the secondary transfer of black toner.

[0097] In color mode, to generate a secondary color, for example, red using yellow toner and magenta toner, two colors of toner are transferred to the recording material P. In other words, in color mode, it is necessary to transfer approximately twice the amount of toner as in black monochrome mode to the recording material P in the secondary transfer. Therefore, in color mode, it is difficult to reduce the voltage value of the secondary transfer bias. However, in the black monochrome mode of a multicolor image forming apparatus, or in the case of a black monochrome image forming apparatus as described in Example 5, since it is black monochrome (one color), the amount of toner to be secondarily transferred can be half that when transferring a secondary color. Therefore, the secondary transfer current value can be reduced accordingly.

[0098] However, if the secondary transfer current value is set too low in a high-humidity environment, there is a possibility that transfer failure will occur, where even black toner cannot be transferred. Therefore, a lower limit for the secondary transfer current value can be set as appropriate.

[0099] FIG. 10 is a graph illustrating an example of secondary transfer control (setting of the secondary transfer current value) in this embodiment. In FIG. 10, the horizontal axis represents relative humidity (%RH) and the vertical axis represents secondary transfer current value (μA). The secondary transfer current value described here is the target current value flowing from the secondary transfer roller 10 to the drive roller 8 via the recording material P and the intermediate transfer belt 7 during secondary transfer. In this embodiment, the secondary transfer bias is controlled at a constant voltage, so the current value may differ depending on whether toner is present during secondary transfer. Therefore, the secondary transfer current value described here is the target current value flowing to the drive roller 8 when there is no toner. The control unit 15 can control the secondary transfer power supply E4 based on information related to the setting of the secondary transfer current value, as shown in FIG. 10, stored in the ROM 151. Information indicating the relationship between humidity and the secondary transfer current value can be stored in the ROM 151 as table data or a formula.

[0100] The solid line in Figure 10 indicates the "normal mode" in which normal control is performed, in which the secondary transfer current value is proportionally reduced as the relative humidity (%RH) increases. For example, at 50%RH, the secondary transfer current value is 65 μA.

[0101] Meanwhile, the dashed line in Figure 10 represents "density unevenness prevention mode 1," which controls the secondary transfer current value to be smaller than in the normal mode as the relative humidity (%RH) increases at 50%RH or higher. This reduces the occurrence of "horizontal streak-like density unevenness." However, at 85%RH or higher, the secondary transfer current value is controlled to be constant at 35μA to prevent it from being reduced too much.

[0102] 12 is a flowchart outlining an example of a control procedure for the black monochrome mode in this embodiment, where an image is formed on one sheet of recording material P in the black monochrome mode.

[0103] When image formation starts, the control unit 15 acquires the temperature and humidity detection results from the environment detection unit (S301). The moisture absorption of the recording material P, i.e., the electrical resistance value of the recording material P, significantly affects the secondary transfer bias voltage VP (paper distribution voltage). Therefore, in this embodiment, to estimate the electrical resistance value of the recording material P, the detection data of the second environment sensor 31, which detects the temperature and humidity outside the image forming apparatus 100, is used. Next, the control unit 15 controls the image forming apparatus 100 to perform a primary transfer process (S302). In this embodiment, the primary transfer control described in the first embodiment (FIGS. 6 and 8) is performed in this primary transfer process. Next, the control unit 15 determines whether the relative humidity detected by the second environment sensor 31 is 50% or higher (S303). If the control unit 15 determines in S303 that the relative humidity is 50% or higher, it controls the secondary transfer control (setting of the secondary transfer current value) to "density unevenness countermeasure mode 1" to perform the secondary transfer process (S304). That is, it controls the secondary transfer current value to be smaller than in "normal mode" and the secondary transfer bias voltage value to be lower than in normal mode. This is because it is determined that "horizontal stripe-like density unevenness" is likely to occur in this case. This "density unevenness countermeasure mode 1" can suppress "horizontal stripe-like density unevenness." On the other hand, if the control unit 15 determines in S303 that the relative humidity is less than 50%, it controls the secondary transfer control (setting of the secondary transfer current value) to be "normal mode" to perform the secondary transfer process (S305). This is because it is determined that "horizontal stripe-like density unevenness" is unlikely to occur in this case. Next, after the secondary transfer process is completed, the control unit 15 controls the fixing process (S306) to be performed, thereby completing image formation.

[0104] Furthermore, the absolute moisture content [g / Kg] may be used as an index of humidity instead of the relative humidity [%RH]. As described above, the absolute moisture content is determined by the temperature and humidity of the environment. Also, as described above, the absolute moisture content [g / Kg] can be calculated by the control unit 15 based on the detection results of the temperature and humidity. An example of the relationship between the temperature [°C], relative humidity [%RH], and the absolute moisture content [g / Kg] is as shown in the first embodiment.

[0105] 11 is a graph illustrating another example of secondary transfer control (setting of the secondary transfer current value) in this embodiment. In FIG. 11, the horizontal axis represents the absolute moisture content [g / Kg], and the vertical axis represents the secondary transfer current value [μA]. The control unit 15 can control the secondary transfer power supply E4 based on information related to the setting of the secondary transfer current value as shown in FIG. 11, which is stored in the ROM 151.

[0106] The solid line in Figure 11 indicates the "normal mode" where normal control is performed, and the secondary transfer current value is controlled to decrease as the absolute moisture content [g / kg] increases. For example, when the absolute moisture content is 8.865 [g / kg], the secondary transfer current value is 65 [μA].

[0107] On the other hand, the dashed line in Figure 11 represents "density unevenness prevention mode 1," in which the secondary transfer current value is controlled to be smaller than that in the normal mode as the absolute moisture content increases (g / kg) above 8.865 [g / kg]. This can suppress the occurrence of "horizontal streak-like density unevenness." However, above 23.103 [g / kg], the secondary transfer current value is controlled to be constant at 35 [μA] to prevent the secondary transfer current value from being reduced too much.

[0108] 13 is a flowchart outlining another example of the control procedure in the black monochrome mode in this embodiment, in which an image is formed on one sheet of recording material P in the black monochrome mode.

[0109] When image formation begins, the control unit 15 acquires the temperature and humidity detection results from the environmental detection unit (S401). The moisture absorption of the recording material P, i.e., the electrical resistance of the recording material P, significantly affects the secondary transfer bias voltage VP (paper distribution voltage). Therefore, in this embodiment, the detection data of the second environmental sensor 31, which detects the temperature and humidity outside the image forming apparatus 100, is used to estimate the electrical resistance of the recording material P. Next, the control unit 15 controls the image forming apparatus 100 to perform a primary transfer process (S402). In this embodiment, the primary transfer control described in the first embodiment (FIGS. 7 and 9) is performed in this primary transfer process. Next, the control unit 15 determines whether the absolute moisture content calculated from the temperature and humidity detected by the second environmental sensor 31 is equal to or greater than 8.865 g / kg (S403). If the control unit 15 determines in S403 that the absolute moisture content [g / Kg] is equal to or greater than 8.865 [g / Kg], it controls the secondary transfer control (setting of the secondary transfer current value) to "density unevenness countermeasure mode 1" to perform the secondary transfer process (S404). That is, it controls the secondary transfer current value to be smaller than in "normal mode" and the secondary transfer bias voltage value to be lower than in normal mode. This is because it is determined that "horizontal streak-like density unevenness" is likely to occur in this case. This "density unevenness countermeasure mode 1" can suppress "horizontal streak-like density unevenness." On the other hand, if the control unit 15 determines in S403 that the absolute moisture content is less than 8.865 [g / Kg], it controls the secondary transfer control (setting of the secondary transfer current value) to be "normal mode" to perform the secondary transfer process (S405). This is because it is determined that "horizontal streak-like density unevenness" is unlikely to occur in this case. Next, when the secondary transfer step is completed, the control unit 15 controls to perform the fixing step (S406), thereby completing the image formation.

[0110] Here, the "normal mode" and the "density unevenness countermeasure mode 1" may be switched automatically by the control unit 15, or may be switched manually by an operator such as a user or a service representative, for example, when "horizontal streak-like density unevenness" occurs.

[0111] For example, in this embodiment, the control unit 15 automatically switches between the "normal mode" and the "density unevenness countermeasure mode 1" of the secondary transfer control depending on whether the mode is color mode or black monochrome mode. However, this is not limited to this. The user, service personnel, or other operator may manually switch between the "normal mode" and the "density unevenness countermeasure mode 1" of the secondary transfer control. For example, when "horizontal stripe-like density unevenness" occurs, the operator can enable secondary transfer control using the "density unevenness countermeasure mode 1." FIG. 17A is a schematic diagram of an example of a secondary transfer control setting screen 401 displayed on the operation unit 99 in this case. The operator can use a switch 411 on this setting screen 401 to input an instruction to the control unit 15 to turn on the "density unevenness countermeasure mode 1" (or the "density unevenness countermeasure mode 2," described later). When "density unevenness countermeasure mode 1" (or "density unevenness countermeasure mode 2," described later) is turned on, the control unit 15 controls the secondary transfer control to be switched in accordance with the humidity in the black monochrome mode according to the procedure described above. On the other hand, when it is turned off, the secondary transfer control is set to "normal mode" in the black monochrome mode regardless of the humidity. Also, the density unevenness countermeasure control of the primary transfer control and the density unevenness countermeasure mode of the secondary transfer control may be linked and turned on / off with a single operation. That is, for example, when the density unevenness countermeasure control of the primary transfer control is turned on in the setting screen of FIG. 16(a) described in the first embodiment, the density unevenness countermeasure mode of the secondary transfer control may also be linked and turned on in accordance with the setting screen of FIG. 16(a). On the other hand, when the density unevenness countermeasure control of the primary transfer control is turned off in the setting screen of FIG. 16(a) described in the first embodiment, the density unevenness countermeasure mode of the secondary transfer control may also be linked and turned off in accordance with the setting screen of FIG.

[0112] Alternatively, instead of the controller 15 automatically switching between the "normal mode" and the "density unevenness countermeasure mode 1," an operator, such as a user or a service representative, may manually switch between them. For example, when "horizontal stripe-like density unevenness" occurs, the operator may use the "density unevenness countermeasure mode 1" of the secondary transfer control. In this case, the operator can turn on the "density unevenness countermeasure mode 1" (or the density unevenness countermeasure mode 2, described later) using a setting screen similar to the setting screen 401 described above. When this setting is turned on, the controller 15 can reduce the secondary transfer current setting by a predetermined amount in the black monochrome mode, regardless of the environment. This predetermined amount of change may be fixed, or may be selectable from multiple options, or may be increased or decreased arbitrarily in predetermined increments. FIG. 17B is a schematic diagram of an example of a setting screen 402 for secondary transfer control displayed on the operation unit 99 in this case. The operator can set the predetermined amount of change using a button 421 on the setting screen 402 shown in FIG. 17B. In this case, it is possible to make it so that the secondary transfer current value can only be adjusted in the direction of decreasing it (negative direction) compared to the normal mode.

[0113] Furthermore, in order to suppress "horizontal stripe-like density unevenness," it is also possible to use only the switching of secondary transfer control described in this embodiment, without using the switching of primary transfer control described in the first embodiment.

[0114] As shown in FIGS. 10 and 11 , the “density unevenness countermeasure mode” may have multiple settings with different secondary transfer current values ​​for the same humidity. For example, a user or an operator, such as a service representative, may manually select from multiple modes, such as density unevenness countermeasure mode 1, density unevenness countermeasure mode 2, etc., based on the strength of the effect. In this case, the operator may select the mode using a setting screen such as that shown in FIG. 17( a) on the operation unit 99, as described above. For example, while “density unevenness countermeasure mode 1” may eliminate “horizontal streak-like density unevenness,” the secondary transfer current value may be too small, resulting in poor transfer of black toner and blurred image edges. In this case, the operator may be able to switch to “density unevenness countermeasure mode 2,” which has a slightly higher secondary transfer current value (up to approximately 10 μA higher). This may allow, for example, adjustments to suppress blurred image edges while still slightly increasing “horizontal streak-like density unevenness.”

[0115] As described above, when forming a black monochrome image, the control unit 15 sets the primary transfer current supplied to the primary transfer member 5K to a first current when the humidity detection result is a first humidity, sets the primary transfer current supplied to the primary transfer member 5K to a second current when the humidity detection result is a second humidity, and sets the primary transfer current supplied to the primary transfer member 5K to a fourth current whose absolute value is smaller than the second current when the humidity detection result is a third humidity; and When forming a black monochrome image, the control unit 15 may be capable of executing a second mode (control to counter density unevenness) in which, if the detection result is the first humidity, the primary transfer current supplied to the primary transfer member 5K is set to the first current, if the detection result is the second humidity, the primary transfer current supplied to the primary transfer member 5K is set to the second current, if the detection result is the second humidity, and the primary transfer current supplied to the primary transfer member 5K is set to the third current, if the detection result is the third humidity. Typically, when executing the first mode, the control unit 15 sets the secondary transfer current supplied to the secondary transfer portion N2 to the first secondary transfer current if the detection result is the third humidity, and when executing the second mode, the control unit 15 sets the secondary transfer current supplied to the secondary transfer portion N2 to the second secondary transfer current whose absolute value is smaller than the first secondary transfer current if the detection result is the third humidity.

[0116] In this embodiment, a second environmental sensor 31 that detects the temperature and humidity outside the image forming apparatus 100 is disposed near the cassette 12 that stores the recording material P. This is to estimate the electrical resistance value of the recording material P, since the secondary transfer current value varies depending on the moisture absorption state (electrical resistivity) of the recording material P. In this embodiment, the detection result of the second environmental sensor 31 is used to switch the secondary transfer current value according to the relative humidity [% RH] or the absolute moisture content [g / Kg].

[0117] In this embodiment, the case where the relative humidity [% RH] or the absolute moisture content [g / Kg] is used as the humidity index has been described. 3The humidity index can be appropriately selected depending on the configuration of the image forming apparatus, for example, and the same effect can be obtained.

[0118] In addition, relative humidity [%RH], absolute moisture content [g / Kg], absolute humidity [g / m 3 As an environment detection means for acquiring information about humidity, either one or both of a device that detects the environment inside the image forming apparatus 100 (first environment sensor 30) and a device that detects the environment outside the image forming apparatus 100 (second environment sensor 31) can be used. These can be appropriately selected depending on, for example, the configuration of the image forming apparatus, even in a configuration in which the secondary transfer current value is switched, and similar effects can be obtained.

[0119] [Example 3] 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.

[0120] In the first and second embodiments, the suppression of the occurrence of "horizontal streak-like density unevenness" in a high humidity environment was described by adjusting the primary transfer current value or the secondary transfer current value. In this embodiment, the suppression of the occurrence of "horizontal streak-like density unevenness" in a high humidity environment is described by adjusting the primary transfer current value or the secondary transfer current value in accordance with the humidity and the attributes of the image within the page.

[0121] In the image forming device 100, the image processing unit 157, which generates image data for the image to be printed, detects the attributes of the image within a page and can detect for each page whether it is a text image (characters and lines) or a graphic image (photographs and halftones).

[0122] In black monochrome mode, if the image attribute within the page is "text image (characters and lines)," "horizontal streak-like density unevenness" is unlikely to occur. Therefore, the "normal control" described in Example 1 (solid lines in Figures 6 and 7) is used as the primary transfer control, and the "normal mode" described in Example 2 (solid lines in Figures 10 and 11) is used as the secondary transfer control. Thus, in the case of text images, "horizontal streak-like density unevenness" does not occur even when "normal control" is performed in a high humidity environment.

[0123] On the other hand, when the image attribute within a page is a "graphic image (photo or halftone)," "horizontal streak-like density unevenness" is likely to occur. Therefore, the "density unevenness countermeasure control 1" (dotted chain lines in FIGS. 6 and 7) described in Example 1 is used as the primary transfer control, and the "density unevenness countermeasure mode 1" (dotted chain lines in FIGS. 10 and 11) described in Example 2 is used as the secondary transfer control. In this way, by using "density unevenness countermeasure control 1" and "density unevenness countermeasure mode 1" in a high humidity environment, the occurrence of "horizontal streak-like density unevenness" can be suppressed.

[0124] It should be noted that "density unevenness countermeasure control 1" and "density unevenness countermeasure mode 1" can be used in any combination. That is, only one of "density unevenness countermeasure control 1" or "density unevenness countermeasure mode 1" may be used, or both "density unevenness countermeasure control 1" and "density unevenness countermeasure mode 1" may be used. In other words, three types of control can be selected.

[0125] 14 is a flowchart showing an outline of the control procedure for the black monochrome mode in this embodiment, where an image is formed in the black monochrome mode as an example.

[0126] When image formation starts, the control unit 15 acquires the temperature and humidity detection results from the environmental detection unit (S501). In this embodiment, for primary transfer control, the detection data from the first environmental sensor 30, which detects the temperature and humidity inside the image forming apparatus 100, is used, and for secondary transfer control, the detection data from the second environmental sensor 31, which detects the temperature and humidity outside the image forming apparatus 100, is used. Next, the control unit 15 acquires the image attribute detection results for each page from the image processing unit 157 and determines whether to switch control based on the humidity and image attribute (S502). That is, if the control unit 15 determines in S502 that the image is a text image or that the environment is low humidity, the control unit 15 controls the primary transfer control to "normal control" so that the primary transfer process is performed (S505). In this case, the control unit 15 also controls the secondary transfer control to "normal mode" so that the secondary transfer process is performed (S506). This is because it can be determined that "horizontal stripe-like density unevenness" is unlikely to occur in this case. Then, when the secondary transfer process is completed, the control unit 15 controls to perform the fixing process (S507). Thereafter, the control unit 15 determines whether or not there is a next page (S508). If the control unit 15 determines in S508 that there is a next page (Yes), it returns to S501 and repeats the processes from S501 onwards for the next page, but if it determines that there is not (No), it ends image formation.

[0127] On the other hand, if the control unit 15 determines in S502 that the image is a graphic image and that the environment is high humidity, it controls the primary transfer control to, for example, "density unevenness countermeasure control 1" to perform the primary transfer process (S503). In this case, the control unit 15 also controls the secondary transfer control to, for example, "density unevenness countermeasure mode 1" to perform the secondary transfer process (S504). This is because it is determined that "horizontal stripe-like density unevenness" is likely to occur in this case. However, as described above, there are three combinations (1) to (3) of the primary transfer control in S503 and the secondary transfer control in S504. These three combinations (1) to (3) may be selected arbitrarily depending on, for example, the configuration of the image forming apparatus 100. Alternatively, as described in the first and second embodiments, the user, a service representative, or other operator may be able to select the combination arbitrarily. Then, after the secondary transfer process is completed, the control unit 15 controls the fixing process to be performed (S507). Thereafter, the control unit 15 determines whether or not there is a next page (S508). If the control unit 15 determines in S508 that there is a next page (Yes), it returns to S501 and repeats the processes from S501 onwards for the next page, and if it determines that there is not a next page (No), it ends image formation.

[0128] By such control, it is possible to suppress the occurrence of "horizontal stripe-like density unevenness" in a high humidity environment.

[0129] In this embodiment, since the image attributes can be detected for each page, it is possible to switch between primary transfer control and secondary transfer control for each page.

[0130] In addition, relative humidity [%RH], absolute moisture content [g / Kg], absolute humidity [g / m 3 As an environment detection means for acquiring information about humidity, either one or both of a device that detects the environment inside the image forming apparatus 100 (first environment sensor 30) and a device that detects the environment outside the image forming apparatus 100 (second environment sensor 31) can be used. These can be selected appropriately depending on, for example, the configuration of the image forming apparatus, and similar effects can be obtained.

[0131] Alternatively, for example, image processing unit 157 may detect the area ratio of a graphic image within a page and select a combination of primary transfer control and secondary transfer control based on this area ratio. For example, when the graphic image area ratio is 80% or more (above the first threshold), the primary transfer control is set to "density unevenness countermeasure control 1" and the secondary transfer control is set to "density unevenness countermeasure mode 1," which is the strongest countermeasure combination. Also, when the graphic image area ratio is 10% or less (below the second threshold), the primary transfer control is set to "normal control" and the secondary transfer control is set to "density unevenness countermeasure mode 2," which is the weakest countermeasure combination. This type of control makes it easy to suppress "horizontal streak-like density unevenness" in a balanced manner according to the image attributes, without countermeasures being too strong or too weak.

[0132] Furthermore, similarly to the explanation of the primary transfer control and secondary transfer control in the first and second embodiments, it may be possible to turn on / off the primary transfer control and whether or not to switch between the primary transfer control and secondary transfer control based on the image attributes.

[0133] Typically, the control unit 15 can selectively execute the first mode (normal control) and the second mode (control to prevent density unevenness) based on the attribute of the image formed on the recording material in black monochrome image formation. In this case, the control unit 15 can execute the first mode when the attribute is a text image, and execute the second mode when the attribute is a graphic image.

[0134] [Example 4] 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.

[0135] In this embodiment, the position of the primary transfer roller 5K in the conveyance direction of the intermediate transfer belt 7 (the circumferential direction of the photosensitive drum 1) is changed depending on the humidity, and the relative position of the primary transfer roller 5K with respect to the photosensitive drum 1K is changed. This prevents the occurrence of "horizontal streak-like density unevenness."

[0136] FIG. 15 is a schematic diagram of the vicinity of the primary transfer nip N1K of the black image forming unit SK (showing a cross section approximately perpendicular to the rotational axis direction of the photosensitive drum 1). As described above, the primary transfer nip N1K is the pressure contact area between the photosensitive drum 1K and the primary transfer roller 5K via the intermediate transfer belt 7. In other words, the primary transfer nip N1K is the area where the contact area between the photosensitive drum 1K and the intermediate transfer belt 7 overlaps with the contact area between the primary transfer roller 5 and the intermediate transfer belt 7 in the transport direction of the intermediate transfer belt 7. Furthermore, "upstream" and "downstream" in the configuration around the intermediate transfer belt 7 refer to upstream and downstream in the transport direction of the intermediate transfer belt 7.

[0137] In the cross section shown in FIG. 15, a line passing through the rotation center of the photosensitive drum 1K and substantially perpendicular to the surface of the intermediate transfer belt 7 (a perpendicular line drawn to the intermediate transfer belt 7) is defined as a first line L1. In addition, in the cross section shown in FIG. 15, a line passing through the rotation center of the primary transfer roller 5K and substantially perpendicular to the surface of the intermediate transfer belt 7 (a perpendicular line drawn to the intermediate transfer belt 7) is defined as a second line L2. In this case, the distance (interval) between the first line L1 and the second line L2 is referred to as an "offset value [mm]." Note that, if the surface of the intermediate transfer belt 7 is inclined near the primary transfer nip N, the first and second lines L1 and L2 may be set at an angle with respect to the surface of the intermediate transfer belt 7 so that the first line L1 and the second line L2 are substantially parallel. In the multi-color image forming apparatus 100, the first and second straight lines L1 and L2 are substantially perpendicular to a common tangent line of the plurality of photosensitive drums 1 on the side that contacts the intermediate transfer belt 7.

[0138] 15(a), an arrangement in which the second line L2 is located downstream of the first line L1 in the transport direction of the intermediate transfer belt 7 is called a downstream offset (first position), and the offset value in this case is expressed as a positive value (+X [mm]). Also, as shown in FIG. 15(b), an arrangement in which the second line L2 is located upstream of the first line L1 in the transport direction of the intermediate transfer belt 7 is called an upstream offset (second position), and the offset value in this case is expressed as a negative value (-X [mm]).

[0139] In this embodiment, the primary transfer roller 5K of the black image forming unit SK is normally (in low- to medium-humidity environments) arranged with a downstream offset (+3.0 mm) as shown in FIG. 15(a). The downstream offset position ensures that the intermediate transfer belt 7 wraps around the photosensitive drum 1K on the upstream side of the primary transfer nip N1K for a long distance. This reduces the chance of toner T on the photosensitive drum 1K flying onto the intermediate transfer belt 7 in the gap upstream of the primary transfer nip N1K. This reduces image scattering, which has the advantage of reducing image scattering. In this embodiment, the yellow, magenta, and cyan image forming units SY, SM, and SC always use this arrangement during color mode image formation. However, this downstream offset position reduces the distance between the primary transfer nip N1K and the discharge position downstream of the primary transfer nip N1K, resulting in a large amount of discharge, which can lead to uneven charging of the toner on the intermediate transfer belt 7. Therefore, the "horizontal stripe-like density unevenness" of the image on the recording material P after the secondary transfer tends to become large.

[0140] On the other hand, in this embodiment, the primary transfer roller 5K of the black image forming unit SK is positioned with an upstream offset (-3.0 mm) as shown in FIG. 15(b) when addressing density unevenness (high humidity environment). With the upstream offset, the intermediate transfer belt 7 has a short wraparound around the photosensitive drum 1K upstream of the primary transfer nip N1K, which tends to cause toner T on the photosensitive drum 1K to fly and transfer onto the intermediate transfer belt 7 in the gap upstream of the primary transfer nip N1K. This tends to result in a lot of image scattering. However, this upstream offset positioning reduces the distance between the primary transfer nip N1K and the discharge position downstream of the primary transfer nip N1K, which reduces the amount of discharge, thereby reducing charging unevenness of the toner on the intermediate transfer belt 7. This has the advantage of reducing horizontal stripe-like density unevenness in the image on the recording material P after secondary transfer.

[0141] Therefore, in this embodiment, under normal circumstances, i.e., in low to medium humidity environments, the charge amount of black toner is high and "horizontal streak-like density unevenness" is unlikely to occur, so a "downstream offset" configuration is used that prioritizes reducing image scattering.

[0142] On the other hand, in this embodiment, in a high humidity environment, the charge amount of black toner is low and "horizontal stripe-like density unevenness" is likely to occur, so an "upstream offset" configuration is used, which prioritizes measures against density unevenness.

[0143] Specifically, the control unit 100 controls a movement mechanism (offset mechanism) 500 provided in the image forming apparatus 100 to move the primary transfer roller 5 along the conveyance direction of the intermediate transfer belt 7 (the circumferential direction of the photosensitive drum 1). This movement of the primary transfer roller 5 is performed, for example, during a pre-rotation process.

[0144] In this way, by switching the offset position of the primary transfer roller 5K depending on the humidity, it is possible to reduce image scattering under normal conditions while suppressing the occurrence of "horizontal stripe-like density unevenness" under high humidity conditions.

[0145] Note that the suppression of "horizontal streak-like density unevenness" by switching the offset position described in this embodiment can be used in combination with at least one of the switching of primary transfer control described in embodiment 1 or the switching of secondary transfer control described in embodiment 2. Therefore, the suppression of "horizontal streak-like density unevenness" by switching the offset position described in this embodiment can also be used in combination with the control described in embodiment 3. However, to suppress "horizontal streak-like density unevenness", it is also possible to use only the switching of the offset position described in this embodiment, without using the switching of primary transfer control described in embodiment 1 or the switching of secondary transfer control described in embodiment 2.

[0146] Also, similarly to the explanations of the primary transfer control and secondary transfer control in the first and second embodiments, whether or not to switch the offset position may be turned on / off.

[0147] Thus, in this embodiment, the image carrier 1K is cylindrical, the primary transfer member 5K is a roller, and the image forming apparatus 100 has a movement mechanism 500 that can move the primary transfer member 5K to a first position where the second line L2 is located downstream of the first line L1, and a second position where the second line L2 is located upstream of the first line L1. When forming a black monochrome image, the control unit 15 can control the movement mechanism 500 to position the primary transfer member 5K at the first position when the humidity detection result is a first humidity, and to position the primary transfer member 5K at the second position when the detection result is a third humidity higher than the first humidity. In addition, when forming a black monochrome image, if the detection result is a second humidity that is higher than the first humidity and lower than the third humidity, the control unit 15 can control the moving mechanism 500 to position the primary transfer member 5K at the first position.

[0148] In addition, relative humidity [%RH], absolute moisture content [g / Kg], absolute humidity [g / m 3As an environment detection means for acquiring information about humidity, either one or both of a device that detects the environment inside the image forming apparatus 100 (first environment sensor 30) and a device that detects the environment outside the image forming apparatus 100 (second environment sensor 31) can be used. These can be selected appropriately depending on, for example, the configuration of the image forming apparatus, and similar effects can be obtained.

[0149] [Example 5] Next, another embodiment of the present invention will be described. 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 the first embodiment will be assigned the same reference numerals as those of the first embodiment, and detailed descriptions thereof will be omitted.

[0150] FIG. 2 is a schematic cross-sectional view of an image forming apparatus 101 according to this embodiment. The image forming apparatus 101 according to this embodiment is a black monochrome image forming apparatus configured by retaining only the components and units necessary for black monochrome image formation from among the components and units of the multi-color image forming apparatus 100 shown in FIG. 1 . The image forming apparatus 101 according to this embodiment has an image forming unit (station) SK that forms black images. In this embodiment, the black image forming unit SK is configured with a photosensitive drum 1K, a charging roller 2K, an exposure device 3, a developing device 4K, a primary transfer roller 5K, a drum cleaning device 6K, and the like. The exposure device 3 may be the same as that of the image forming apparatus 100 shown in FIG. 1, or may be a device obtained by removing unnecessary elements from the image forming apparatus 101 according to this embodiment. In this embodiment, the charging roller 2K, the exposure device 3, the developing device 4K, and the like constitute an image forming unit that forms a toner image on the photosensitive drum 1K.

[0151] In the image forming apparatus 101 of this embodiment, for example, the elements of the yellow, magenta, and cyan image forming units SY, SM, and SC in the image forming apparatus 100 of Fig. 1 have been removed. However, the image forming apparatus 101 of this embodiment and the image forming apparatus 100 of Fig. 1 have the same black primary transfer position (primary transfer unit N1) and secondary transfer position (secondary transfer unit N2) in the apparatus body 96 of the image forming apparatuses 100 and 101. Such a black monochrome image forming apparatus 101 has the advantage that if a multi-color image forming apparatus 100 is developed, it can be brought to market with reduced development costs.

[0152] The image forming operation of the image forming apparatus 101 of this embodiment is the same as the operation in the black monochrome mode of the image forming apparatus 100 in Fig. 1. The electrical configuration, such as the control unit 15, is also substantially the same. Therefore, a description of these will be omitted.

[0153] The control applied to the black monochrome mode in the multi-color image forming apparatus 100 described in the above-mentioned Examples 1 to 4, and the configuration adapted to the black image forming section SK (offset mechanism, etc.) can also be applied to the black monochrome image forming apparatus 101 of this embodiment.

[0154] Therefore, the image forming apparatus 101 of this embodiment can also suppress the occurrence of "horizontal stripe-like density unevenness."

[0155] In the black monochrome image forming apparatus 101 of this embodiment, for example, when the relative humidity is 50% RH or higher in FIGS. 6 and 10, only "density unevenness countermeasure control 1" and "density unevenness countermeasure mode 1" can be used, respectively. Furthermore, for example, when the absolute moisture content is 8.865 g / kg or higher in FIGS. 7 and 11, only "density unevenness countermeasure control 1" and "density unevenness countermeasure mode 1" can be used, respectively. This is because "normal control" and "normal mode" used in the color mode are not required in these humidity ranges. However, in the black monochrome image forming apparatus 101, as described in the first and second embodiments, if it is possible to select whether to switch between primary transfer control and secondary transfer control, "normal control" and "normal mode" can be set in the humidity ranges.

[0156] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0157] The various numerical values ​​mentioned in the above examples are not limited to these.

[0158] Furthermore, the above-described embodiments (the respective controls of the first to fourth embodiments and the respective configurations of the first and fifth embodiments) can be implemented in any suitable combination.

[0159] Furthermore, the black monochrome image forming apparatus is not limited to the configuration obtained by removing unnecessary parts from the multi-color image forming apparatus as described in the fifth embodiment.

[0160] In addition, the operations and displays that are performed by the operation unit provided in the image forming apparatus in the above-described embodiment may be performed by an external device connected to the image forming apparatus, in which case the communication I / F or the like functions as an input unit that inputs instructions to the control unit.

[0161] 6, 7, 10, and 11, for convenience, the control (mode) is switched using a threshold as a boundary, but the present invention is not limited to such an embodiment. For example, in a case where each control (mode) can be turned on / off, humidity and current values ​​(primary transfer current value, primary transfer current value) in all humidity ranges may be set for each control (mode), and the control (mode) may be switched as a whole.

[0162] Furthermore, according to the inventors' investigations, "horizontal streak-like density unevenness" tends to occur more easily in systems that use a two-component developer containing non-magnetic toner and magnetic carrier. However, the present invention can be applied to both systems that use a one-component developer containing magnetic toner or non-magnetic toner, and a two-component developer in which non-magnetic toner and magnetic carrier are mixed in a predetermined ratio. [Explanation of symbols]

[0163] 1 Photosensitive drum 2 Charging roller 3 Exposure equipment 7 Intermediate transfer belt 10 Secondary transfer roller 15 Control Unit 30 First Environmental Sensor 31 Second Environmental Sensor 99 Control section 100 Image forming device (multicolor image forming device) 101 Image forming apparatus (black monochrome image forming apparatus)

Claims

1. In the image forming apparatus, a rotatable image carrier that carries a toner image; an image forming means for forming a toner image on the image carrier; a rotatable intermediate transfer belt onto which a toner image is transferred from the image carrier; a primary transfer member that transfers a toner image from the image carrier to the intermediate transfer belt; a primary transfer power supply that supplies a primary transfer current to the primary transfer member for transferring a toner image from the image carrier to the intermediate transfer belt; a secondary transfer member forming a secondary transfer portion that transfers a toner image from the intermediate transfer belt to a recording material; a secondary transfer power supply that supplies a secondary transfer current to the secondary transfer unit to transfer a toner image from the intermediate transfer belt to a recording material; an environment detection unit capable of detecting humidity inside or outside the image forming apparatus; a control unit capable of controlling the primary transfer power supply; and a black toner image formed on the image carrier with black toner is transferred to the intermediate transfer belt, and then transferred to a recording material, thereby forming a black monochrome image on the recording material; and when the humidity detection result by the environmental detection means is a first humidity, the control unit sets the primary transfer current supplied to the primary transfer member to a first current; when the humidity detection result is a second humidity higher than the first humidity, the control unit sets the primary transfer current supplied to the primary transfer member to a second current having an absolute value smaller than the first current; and when the detection result is a third humidity higher than the second humidity, the control unit sets the primary transfer current supplied to the primary transfer member to a third current having an absolute value smaller than the first current and greater than the absolute value of the second current.

2. the image carrier is a first image carrier, the image forming means is a first image forming means, the primary transfer member is a first primary transfer portion, a rotatable second image carrier that carries a toner image; a second image forming means for forming a toner image on the second image carrier; a second primary transfer member that transfers a toner image from the second image carrier to the intermediate transfer belt; and a multi-color image forming apparatus capable of forming a multi-color image on a recording material by transferring a black toner image formed on the first image carrier with black toner and a toner image of a color other than black formed on the second image carrier with toner of a color other than black onto the intermediate transfer belt, and then transferring the transferred image onto the recording material; 2. The image forming apparatus according to claim 1, wherein, when forming the multi-color image, if the detection result is the first humidity, the control unit sets the primary transfer current supplied to the first primary transfer member to the first current; if the detection result is the second humidity, the control unit sets the primary transfer current supplied to the first primary transfer member to the second current; and if the detection result is the third humidity, the control unit sets the primary transfer current supplied to the first primary transfer member to a fourth current whose absolute value is smaller than that of the second current.

3. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a monochrome image forming apparatus capable of forming only a black monochrome image as the image forming for forming an image on a recording material by transferring a toner image from the intermediate transfer belt to the recording material.

4. In the image forming apparatus, a rotatable image carrier that carries a toner image; an image forming means for forming a toner image on the image carrier; a rotatable intermediate transfer belt onto which a toner image is transferred from the image carrier; a primary transfer member that transfers a toner image from the image carrier to the intermediate transfer belt, the primary transfer member being in pressure contact with the image carrier via the intermediate transfer belt; a primary transfer power supply that supplies a primary transfer current to the primary transfer member for transferring a toner image from the image carrier to the intermediate transfer belt; a secondary transfer member forming a secondary transfer portion that transfers a toner image from the intermediate transfer belt to a recording material; a secondary transfer power supply that supplies a secondary transfer current to the secondary transfer unit to transfer a toner image from the intermediate transfer belt to a recording material; an environment detection unit capable of detecting humidity inside or outside the image forming apparatus; a control unit capable of controlling the primary transfer power supply; and a black toner image formed on the image carrier with black toner is transferred to the intermediate transfer belt, and then transferred to a recording material, thereby forming a black monochrome image on the recording material; and when the humidity detection result by the environmental detection means is a first humidity, the control unit sets the primary transfer current supplied to the primary transfer member to a first current, and when the humidity detection result is a second humidity higher than the first humidity, the control unit sets the primary transfer current supplied to the primary transfer member to a second current having an absolute value larger than that of the first current, when forming the black monochrome image.

5. the image carrier is a first image carrier, the image forming means is a first image forming means, the primary transfer member is a first primary transfer member, a rotatable second image carrier that carries a toner image; a second image forming means for forming a toner image on the second image carrier; a second primary transfer member that transfers a toner image from the second image carrier to the intermediate transfer belt; and a multi-color image forming apparatus capable of forming a multi-color image on a recording material by transferring a black toner image formed on the first image carrier with black toner and a toner image of a color other than black formed on the second image carrier with toner of a color other than black onto the intermediate transfer belt, and then transferring the transferred image onto the recording material; 5. The image forming apparatus according to claim 4, wherein, when forming the multi-color image, if the detection result is the first humidity, the control unit sets the primary transfer current supplied to the first primary transfer member to the first current, and if the detection result is the second humidity, the control unit sets the primary transfer current supplied to the first primary transfer member to a third current whose absolute value is smaller than that of the first current.

6. 5. The image forming apparatus according to claim 4, wherein the image forming apparatus is a monochrome image forming apparatus capable of forming only a black monochrome image as the image forming for transferring a toner image from the intermediate transfer belt to a recording material to form an image on the recording material.

7. The control unit a first mode in which, when forming the black monochrome image, if the detection result is the first humidity, the primary transfer current supplied to the primary transfer member is set to the first current, if the detection result is the second humidity, the primary transfer current supplied to the primary transfer member is set to the second current, and if the detection result is the third humidity, the primary transfer current supplied to the primary transfer member is set to a fourth current having an absolute value smaller than that of the second current; a second mode in which, when forming the black monochrome image, if the detection result is the first humidity, the primary transfer current supplied to the primary transfer member is set to the first current, if the detection result is the second humidity, the primary transfer current supplied to the primary transfer member is set to the second current, and if the detection result is the third humidity, the primary transfer current supplied to the primary transfer member is set to the third current; 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is controllable to execute the above.

8. The control unit When the first mode is executed, if the detection result is the third humidity, the secondary transfer current supplied to the secondary transfer unit is set to a first secondary transfer current; 8. The image forming apparatus according to claim 7, wherein when the second mode is executed, if the detection result is the third humidity, the secondary transfer current supplied to the secondary transfer unit is set to a second secondary transfer current whose absolute value is smaller than that of the first secondary transfer current.

9. an input unit for inputting instructions to the control unit based on an operation by an operator; 8. The image forming apparatus according to claim 7, wherein the control unit controls the image forming apparatus to selectively execute the first mode and the second mode based on an instruction inputted through the input unit.

10. 8. The image forming apparatus according to claim 7, wherein the control unit controls to selectively execute the first mode and the second mode based on attributes of an image formed on a recording material in the black monochrome image formation.

11. 11. The image forming apparatus according to claim 10, wherein the control unit controls the first mode to be executed when the attribute is a text image, and the second mode to be executed when the attribute is a graphic image.

12. the image carrier is cylindrical, the primary transfer member is a roller, a moving mechanism that can move the primary transfer member to a first position where the second line L2 is located downstream of the first line L1 in a conveying direction of the intermediate transfer belt, and a second position where the second line L2 is located upstream of the first line L1, when a first line L1 is a line that passes through the rotation center of the image carrier and is substantially perpendicular to the intermediate transfer belt in a cross section substantially perpendicular to the rotation axis direction of the image carrier, and a second line L2 is a line that passes through the rotation center of the primary transfer member and is substantially perpendicular to the intermediate transfer belt, 2. The image forming apparatus according to claim 1, wherein the control unit controls the moving mechanism to position the primary transfer member at the first position when the detection result is the first humidity, and controls the moving mechanism to position the primary transfer member at the second position when the detection result is the third humidity.

13. 13. The image forming apparatus according to claim 12, wherein when forming the black monochrome image, if the detection result is the second humidity, the control unit controls the moving mechanism to position the primary transfer member at the first position.

Images