Image forming apparatus

Abstract

To prevent a harmful effect on an image due to a deterioration of a contact member that is in contact with a recording material simultaneously with a transfer member, while maintaining good transferability.SOLUTION: An image forming apparatus 100 has: an image carrier 10; a transfer member 20; a power supply 21 that outputs voltage to the transfer member 20; a detection unit 241 that detects at least either one of a current value or a voltage value when the voltage is output from the power supply 21 to the transfer member 20; a control unit 210 that controls the power supply; an environment detection unit 300; and a contact member 31 that is provided to be contactable with a recording material P simultaneously with the transfer member 20 at a portion other than a transfer part N2. When an absolute moisture amount obtained based on environment information is equal to or more than a predetermined threshold, the control unit 210 sets a limit voltage Vlimit based on a result of detection performed by the detection unit 241 when the recording material P is not at the transfer part N2, and controls the power supply 21 so that the absolute value of the voltage applied from the power supply 21 to the transfer member 20 becomes equal to or less than the absolute value of the limit voltage Vlimit while the recording material P is simultaneously in contact with the transfer member 20 and the contact member 31.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This invention relates to image forming apparatus such as printers, copiers, and facsimile machines that use electrophotographic or electrostatic recording methods. [Background technology]

[0002] In image forming apparatuses using electrophotography or similar methods, a toner image formed on an image carrier is transferred to a recording material such as paper that passes through a transfer section formed between the image carrier and a transfer member. In an intermediate transfer type image forming apparatus, for example, a toner image formed on a photoreceptor (a first image carrier) is first transferred to an intermediate transfer body (a second image carrier). Subsequently, the toner image on the intermediate transfer body is secondarily transferred to a recording material that passes through a transfer section (secondary transfer section) formed between the intermediate transfer body and a transfer member (secondary transfer member). The transfer of the toner image from the image carrier to the recording material is performed by applying a transfer voltage to the transfer member. Transfer rollers are widely used as transfer members. To obtain high-quality output (printed materials), it is important to apply an appropriate transfer voltage to the transfer member.

[0003] Patent Document 1 discloses a configuration in which the transfer voltage is controlled by a constant current when the electrical resistance value of the transfer roller (hereinafter simply referred to as "resistance value") is high, and controlled by a constant voltage when the resistance value of the transfer roller is low. Patent Document 2 discloses a configuration in which a lower limit value for the transfer voltage to which constant current control is applied is set, and constant voltage control is applied when the transfer voltage falls below the lower limit value. The constant voltage value is set so as to ensure that a transfer current of at least a certain level flows to the toner image even when the resistance value of the transfer member continues to increase with the cumulative usage time, or when the resistance value of the recording material decreases due to moisture absorption by the recording material. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-48965 [Patent Document 2] Japanese Patent Publication No. 2010-191276 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, if the resistance of the recording material decreases due to moisture absorption, a portion of the transfer current may flow through the recording material to a component other than the transfer member at the same time the recording material comes into contact with that component. Hereafter, components located downstream or upstream of the transfer member that come into contact with the recording material at the same time as the transfer member, other than the transfer section, will also be simply referred to as "contact components."

[0006] Therefore, when setting a constant voltage value so that a certain transfer current flows through the toner image, as described in Patent Document 2, for example, the constant voltage value may be set unnecessarily high in order to allow the toner image to be transferred even if the resistance of the recording material decreases. In that case, the current flowing through the contact members also increases, which may lead to deterioration of the contact members. For example, a contact member is a fixing member that fixes the toner image to the recording material. For example, if a large current flows through this fixing member for a long time, it may lead to deterioration of the member, such as deterioration of the fixing member's electrical conductivity, and ultimately result in image defects.

[0007] Therefore, the present invention aims to suppress image defects caused by the deterioration of the contact member that comes into contact with the recording material simultaneously with the transfer member, while maintaining good transferability. [Means for solving the problem]

[0008] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention is an image forming apparatus comprising: an image carrier that carries a toner image; a transfer member that contacts the image carrier to form a transfer section and transfers the toner image from the image carrier to a recording material that passes through the transfer section; a power supply that outputs a voltage to the transfer member; a detection unit that detects at least one of the current value flowing through the transfer member or the voltage value applied to the transfer member when the power supply outputs a voltage to the transfer member; a control unit that controls the power supply; an environmental detection unit that detects environmental information relating to at least one of the temperature or humidity of the environment; and a contact member that is provided to be able to contact the recording material simultaneously with the transfer member and other than the transfer section, wherein the control unit sets a limit voltage based on the detection result of the detection unit when the recording material is absent from the transfer section, when the absolute amount of moisture obtained based on the environmental information is above a predetermined threshold, and controls the power supply so that the absolute value of the voltage applied from the power supply to the transfer member while the recording material is simultaneously in contact with the transfer member and the contact member is less than or equal to the absolute value of the limit voltage.

[0009] According to another aspect of the present invention, the present invention comprises an image carrier that carries a toner image, a transfer member that contacts the image carrier to form a transfer portion and transfers the toner image from the image carrier to a recording material that passes through the transfer portion, a power supply that outputs a voltage to the transfer member, a detection unit that detects at least one of the current value flowing through the transfer member or the voltage value applied to the transfer member when the power supply outputs a voltage to the transfer member, a control unit that controls the power supply, an input unit that inputs information about the recording material to the control unit, and a contact member provided to be able to contact the recording material simultaneously with the transfer member and other than the transfer portion, wherein the control unit sets a limit voltage based on the detection result of the detection unit when the recording material is absent from the transfer portion, when the information input by the input unit satisfies predetermined conditions set in advance, and controls the power supply so that the absolute value of the voltage applied from the power supply to the transfer member while the recording material is simultaneously in contact with the transfer member and the contact member is less than or equal to the absolute value of the limit voltage. Furthermore, if the information does not satisfy the conditions, the control to set the limit voltage is not performed, and the conditions are set such that the information satisfies the conditions when the electrical resistance value of the recording material is the first recording material resistance value, and the information does not satisfy the conditions when the electrical resistance value of the recording material is the second recording material resistance value, and the first recording material resistance value is lower than the second recording material resistance value. An image forming apparatus characterized by the above is provided. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress image defects caused by the deterioration of the contact member that comes into contact with the recording material at the same time as the transfer member, while maintaining good transferability. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view of an image forming apparatus. [Figure 2] This is a schematic block diagram showing the control mechanism of an image forming apparatus. [Figure 3] This is a flowchart illustrating the control of the secondary transfer voltage in Example 1. [Figure 4] This chart shows the current and voltage transitions during the control of the secondary transfer voltage. [Figure 5] This graph shows the relationship between the resistance value of the secondary transfer section and the limit voltage. [Figure 6] This is a cross-sectional view and equivalent circuit diagram of the secondary transfer section and fixing device. [Figure 7] This is a flowchart illustrating the control of the secondary transfer voltage in the comparative example. [Figure 8] This is a flowchart illustrating the control of the secondary transfer voltage in Example 2. [Figure 9] This is a schematic diagram illustrating the relationship between image patterns and image defects. [Figure 10] This is a schematic diagram of the configuration around the primary transfer section of the image forming apparatus in Example 3. [Figure 11] This is a schematic diagram showing the cross-sectional configuration of the intermediate transfer belt in Example 3. [Figure 12] This is an equivalent circuit diagram relating to the secondary transfer section in Example 3. [Modes for carrying out the invention]

[0012] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0013] [Example 1] <Overall configuration and operation of the image forming apparatus> Figure 1 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic full-color laser printer employing an in-line method and an intermediate transfer method. The image forming apparatus 100 can form a full-color image on a recording material P (e.g., recording paper, plastic sheet, etc.) according to image information. Image information is input to the image forming apparatus 100 from an image reading device (not shown) provided with or connected to the image forming apparatus 100, or from a host computer 199 (Figure 2), such as a personal computer, that is communicatively connected to the image forming apparatus 100.

[0014] The image forming apparatus 100 has a plurality of image forming stations, the first, second, third, and fourth image forming stations Sa, Sb, Sc, and Sd, each for forming images of yellow (Y), magenta (M), cyan (C), and black (K). In this embodiment, the first, second, third, and fourth image forming stations Sa, Sb, Sc, and Sd are arranged in a line in a direction intersecting the vertical direction. In this embodiment, the configuration and operation of the first, second, third, and fourth image forming stations Sa, Sb, Sc, and Sd are substantially the same except for the different colors of the images they form. Elements having the same or corresponding functions or configurations in each image forming station Sa, Sb, Sc, and Sd may be described collectively by omitting the suffixes a, b, c, and d of the symbols indicating that they are elements for one of the colors. The image forming unit S is composed of the following components: a photosensitive drum 1 (1a, 1b, 1c, 1d), charging rollers 2 (2a, 2b, 2c, 2d), exposure device 3 (3a, 3b, 3c, 3d), developing device 4 (4a, 4b, 4c, 4d), primary transfer rollers 14 (14a, 14b, 14c, 14d), drum cleaning device 5 (5a, 5b, 5c, 5d), and the like.

[0015] The photosensitive drum 1, which is a rotatable drum-shaped (cylindrical) photoreceptor (electrophotographic photoreceptor) as the first image carrier, is driven to rotate at a predetermined peripheral speed (process speed) in the direction of arrow R1 (counterclockwise direction) in Figure 1 by a drive motor as the driving means (driving source). The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charging roller 2, which is a roller-type charging member as the charging means. The charged surface of the photosensitive drum 1 is scanned and exposed according to image information by an exposure device (laser scanner unit) 3 as the exposure means, and an electrostatic latent image (electrostatic image) according to the image information is formed on the photosensitive drum 1. The exposure device 3 irradiates the photosensitive drum 1 with laser light L based on the output calculated by the CPU circuit unit 150 (Figure 2), which will be described later, from the image information input from, for example, a host computer 199 (Figure 2). The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by a developing device 4, which is a developing means, when toner is supplied as a developer, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the exposed area (image area) on the photosensitive drum 1, where the absolute value of the potential has decreased after uniform charging treatment and exposure (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the charge polarity of the toner during development, is negative polarity.

[0016] An intermediate transfer belt 10, which is an intermediate transfer body composed of an endless belt as a second image carrier, is positioned opposite the four photosensitive drums 1a to 1d. The intermediate transfer belt 10 is stretched over a plurality of support members (tension rollers), namely drive rollers 11, tension rollers 12, and secondary transfer opposing rollers 13, and is taut with a predetermined tension. The intermediate transfer belt 10 contacts the four photosensitive drums 1a to 1d at the transfer surface M formed between the secondary transfer opposing rollers 13 and the drive rollers 11. The drive rollers 11 are rotated by a drive motor, which is a driving means (driving source), in the direction of arrow R2 in Figure 1 (clockwise). As a result, the intermediate transfer belt 10 rotates (circular movement, reciprocal movement) in the direction of arrow R3 in Figure 1 (clockwise) at a peripheral speed (process speed) corresponding to the peripheral speed of the photosensitive drum 1. On the inner circumferential surface of the intermediate transfer belt 10, primary transfer rollers 14a, 14b, 14c, and 14d, which are roller-type primary transfer members serving as primary transfer means, are arranged corresponding to each photosensitive drum 1a, 1b, 1c, and 1d. The primary transfer rollers 14 press the intermediate transfer belt 10 toward the photosensitive drum 1, forming a primary transfer section (primary transfer nip section) N1, which is the contact area between the photosensitive drum 1 and the intermediate transfer belt 10. The toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto the rotating intermediate transfer belt 10 in the primary transfer section N1 by the action of the primary transfer rollers 14. During primary transfer, a primary transfer voltage (primary transfer bias) with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner is applied to the primary transfer rollers 14 by a primary transfer power supply (high voltage power supply) 15, which serves as a primary transfer voltage application means (primary transfer voltage application section). For example, when forming a full-color image, the toner images of yellow, magenta, cyan, and black, formed on each of the photosensitive drums 1a, 1b, 1c, and 1d, are sequentially transferred onto the intermediate transfer belt 10 in a primary transfer process.

[0017] On the outer circumferential surface of the intermediate transfer belt 10, a secondary transfer roller (secondary transfer outer roller) 20, which is a roller-type secondary transfer member serving as a secondary transfer means, is positioned opposite the secondary transfer opposing roller (secondary transfer inner roller) 13. The secondary transfer roller 20 is pressed toward the secondary transfer opposing roller 13 and contacts the secondary transfer opposing roller 13 via the intermediate transfer belt 10, forming a secondary transfer section (secondary transfer nip section) N2, which is the contact area between the intermediate transfer belt 10 and the secondary transfer roller 20. The toner image formed on the intermediate transfer belt 10 is transferred (secondary transfer) in the secondary transfer section N2 to the recording material P, which is being transported sandwiched between the intermediate transfer belt 10 and the secondary transfer roller 20, by the action of the secondary transfer roller 20. During secondary transfer, a secondary transfer voltage (secondary transfer bias) with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner is applied to the secondary transfer roller 20 by a secondary transfer power supply (high voltage power supply) 21, which serves as a secondary transfer voltage application means (secondary transfer voltage application section). In this embodiment, the secondary transfer opposing roller 13 is connected to ground (earth potential). For example, when forming a full-color image, the four toner images on the intermediate transfer belt 10 are transferred collectively onto the recording material P in the secondary transfer section N2. The recording material P is housed in a cassette 51, which serves as a recording material storage section. The recording material P is separated one sheet at a time from the cassette 51 by a feeding roller 50, which serves as a feeding means, and transported to the resist roller pair 60, 60. The recording material P is then transported to the secondary transfer section N2 by the resist roller pair 60, 60 in timing with the toner image on the intermediate transfer belt 10. The transport timing of the recording material P by the resist roller pair 60, 60 is controlled based on the detection result of a resist sensor (not shown) that detects the leading edge of the recording material P in the transport direction. In this embodiment, it is also possible to apply a voltage with the same polarity as the normal charging polarity of the toner to the inner roller corresponding to the secondary transfer opposing roller 13, and connect the outer roller corresponding to the secondary transfer roller 20 to ground.

[0018] The recording material P onto which the toner image has been transferred is transported to a fixing device 30, which serves as a fixing means. The fixing device 30 includes a fixing roller 31 equipped with a heat source and a pressure roller 32 that presses against the fixing roller 31. At the fixing section (fixing nip section) N3, which is the contact area between the fixing roller 31 and the pressure roller 32, the fixing device 30 applies heat and pressure to the recording material P carrying the unfixed toner image, thereby fixing (melting and solidifying) the toner image onto the recording material P. For example, when forming a full-color image, the four-color toner images on the recording material P are heated and pressurized in the fixing section N3, causing them to melt and mix, and then fixed onto the recording material P. The recording material P with the fixed toner image is discharged (output) from the main body of the image forming apparatus 100.

[0019] On the other hand, any toner or other deposits remaining on the photosensitive drum 1 after the primary transfer (primary transfer residue toner) are removed and recovered from the photosensitive drum 1 by the drum cleaning device 5, which serves as a photoreceptor cleaning means. In addition, any toner or other deposits remaining on the intermediate transfer belt 10 after the secondary transfer (secondary transfer residue toner) are removed and recovered from the intermediate transfer belt 10 by the belt cleaning device 16, which serves as an intermediate transfer body cleaning means.

[0020] Furthermore, the image forming apparatus 100 can also form monochrome or multicolor images using only one image forming unit S or some (but not all) of the image forming units S.

[0021] Furthermore, in each image forming unit S, the photosensitive drum 1, the charging roller 2 which acts on the photosensitive drum 1 as a process means, the developing device 4, and the drum cleaning device 5 together constitute a process cartridge 6 that can be attached to and detached from the main body of the image forming apparatus 100. The process cartridge 6 can be attached to and detached from the main body of the apparatus via mounting means such as mounting guides and positioning members provided on the main body of the apparatus.

[0022] Furthermore, the image forming apparatus 100 of this embodiment can form and output images on A5 size paper, A4 size paper, LTR size paper, etc., at a process speed of 148 mm / sec.

[0023] In this embodiment, the secondary transfer roller 20 contacts the intermediate transfer belt 10 with a pressure of 50 N, forming the secondary transfer section N2. The secondary transfer roller 20 rotates in conjunction with the rotation of the intermediate transfer belt 10. The recording material P, such as paper, is held and conveyed between the intermediate transfer belt 10 and the secondary transfer roller 20 in the secondary transfer section N2. The secondary transfer roller 20 has a nickel-plated steel rod with an outer diameter of 8 mm as a core, surrounded by an elastic layer with a volume resistivity of 10 8 This is a roller with an outer diameter of 18 mm, covered with a 5 mm thick foamed sponge body mainly composed of NBR and epichlorohydrin rubber adjusted to Ω·cm. In this embodiment, the secondary transfer power supply 21 is capable of outputting in the range of 100V to 5000V, and applies a voltage with the opposite polarity to the normal charging polarity of the toner (negative polarity in this embodiment) to the secondary transfer roller 20. In this specification, numerical ranges indicated using "~" mean the range including the numbers before and after "~".

[0024] Furthermore, the configuration of the fixing device 30 will be further explained with reference to Figure 6(a). Figure 6(a) is a schematic cross-sectional view of the secondary transfer section N2 and the fixing device 30 of the image forming apparatus 100. In this embodiment, the fixing roller 31, which serves as the fixing member, is a roller with an outer diameter of 18 mm, in which an elastic layer of insulating silicone rubber is formed around a metal core tube, and the outer circumference of the elastic layer is further coated with an insulating PFA tube. This fixing roller 31 incorporates a halogen heater (not shown) as a heating means. The halogen heater is not in contact with the fixing roller 31 and generates heat when voltage is supplied by a power supply (not shown). In this embodiment, the pressure roller 32, which serves as the pressurizing member, is a roller with an outer diameter of 18 mm, in which an elastic layer of conductive silicone rubber is formed around a metal core, and the outer circumference of the elastic layer is further coated with a conductive PFA tube. The fixing roller 31 and the pressure roller 32 are pressed together with a pressure of 10 kgf to form the fixing section N3. The pressure roller 32 is rotationally driven by a drive motor, which serves as the driving means (driving source). The fixing roller 31 rotates in conjunction with the rotation of the pressure roller 32. The recording material P is held between the fixing roller 31 and the pressure roller 32 and transported in the fixing section N3. The fixing roller 31 is connected to ground (electrically grounded) via a 470kΩ resistive element 33 from the metal tube. The pressure roller 32 is connected to ground (electrically grounded) via a 1000MΩ resistive element 34 from the core metal. By discharging the charge on the fixing roller 31 and the pressure roller 32 to ground via the fixing roller 31 and the resistive element 33, and the pressure roller 32 and the resistive element 34, it is possible to suppress the charging of the surfaces of the fixing roller 31 and the pressure roller 32.

[0025] Figure 2 is a block diagram illustrating the configuration of the engine control unit 210, which controls the entire image forming apparatus 100 in this embodiment. The engine control unit 210 incorporates a CPU circuit unit 150, a ROM 151, and a RAM 152. The CPU circuit unit 150 comprehensively controls the primary transfer control unit 201, secondary transfer control unit 202, development control unit 203, exposure control unit 204, charging control unit 205, etc., according to the control program stored in the ROM 151. Control tables related to the control of the secondary transfer voltage (environment table, recording material width / recording material thickness correspondence table, etc.), which will be described later, are stored in the ROM 151 and are called by the CPU circuit unit 150 to reflect in the control. The RAM 152 temporarily holds control data and is also used as a workspace for calculation processing associated with the control.

[0026] The primary transfer control unit 201 and the secondary transfer control unit 202 control the primary transfer power supply 15 and the secondary transfer power supply 21, respectively, under the control of the engine control unit 210. The primary transfer control unit 201 and the secondary transfer control unit 202 control the voltage output from the primary transfer power supply 15 and the secondary transfer power supply 21, respectively, based on the current value detected by their respective current detection units (current detection circuits). The control of the secondary transfer voltage will be explained in detail later.

[0027] The engine control unit 210 is connected to an environmental sensor 300, which serves as an environmental detection means (environmental detection unit) for detecting at least one of the temperature or humidity inside or outside the image forming apparatus 100. In this embodiment, the environmental sensor 300 incorporates a temperature sensor 301 as a temperature detection means (temperature detection unit) and a humidity sensor 302 as a humidity detection means (humidity detection unit), and detects the temperature and humidity around the image forming apparatus 100. The environmental sensor 300 inputs a signal (temperature information) indicating the temperature detection result from the temperature sensor 301 and a signal (humidity information) indicating the humidity (relative humidity) detection result from the humidity sensor 302 to the engine control unit 210.

[0028] Furthermore, a controller 200 is connected to the engine control unit 210. The controller 200 receives print information (image information, various setting information) and print commands (instructions to start a print job) from an external device, the host computer (host equipment) 199. The engine control unit 210 then controls each control unit (primary transfer control unit 201, secondary transfer control unit 202, development control unit 203, exposure control unit 204, charging control unit 205, etc.) to execute the print job. In this embodiment, the engine control unit 210 obtains environmental information from the detection results of the environmental sensor 300 and information regarding the recording material P from the print information from the host computer 199 in order to control the secondary transfer voltage, which will be described later. The print information is input from the host computer 199 to the controller 200 via a printer driver installed on the host computer 199.

[0029] Here, the image forming apparatus 100 executes a print job (printing job, image output operation), which is a series of operations that are started by a single start instruction and involve forming and outputting an image on one or more recording materials P. A print job generally includes an image forming process, a pre-rotation process, a paper-to-paper process when forming an image on multiple recording materials P, and a post-rotation process. The image forming process (printing process) is the period during which the electrostatic latent image, toner image, primary transfer, and secondary transfer of the toner image are performed for the image to be actually formed and output on the recording material P. The image forming time (image forming period) refers to this period. More specifically, the timing of the image forming time differs depending on the position where each of these processes—the formation of the electrostatic latent image, the formation of the toner image, and the primary and secondary transfer of the toner image—is performed. The pre-rotation process is the period during which preparatory operations are performed before the image forming process, from when a start instruction is input until the image is actually formed. The paper-to-paper process (inter-recording material process, inter-image process) is the period corresponding to the space between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is the period during which tidying operations (preparation operations) are performed after the image forming process. The non-image forming period (non-image forming period) is the period other than the image forming process, and includes the pre-rotation process, inter-paper process, post-rotation process, and pre-multi-rotation process, which is the preparation operation when the image forming apparatus 100 is powered on or when it returns from sleep mode.

[0030] <Overview of Secondary Transfer Voltage Control> Next, an overview of the control of the secondary transfer voltage in this embodiment will be described.

[0031] As shown in Figure 1, the secondary transfer power supply 21 is connected to the secondary transfer roller 20, and the secondary transfer voltage output from the secondary transfer power supply 21 is supplied to the secondary transfer roller 20. When the secondary transfer voltage is applied from the secondary transfer power supply 21 to the secondary transfer roller 20, an electric field is formed between the secondary transfer roller 20 and the secondary transfer counter roller 13 installed opposite it, and toner is transferred from the intermediate transfer belt 10 onto the recording material P.

[0032] As shown in Figure 2, the secondary transfer control unit 202 has a current detection unit (ammeter) 241 as a current detection means that detects the current flowing through the secondary transfer unit N2 (secondary transfer roller 20) when the secondary transfer power supply 21 applies voltage to the secondary transfer roller 20. The secondary transfer control unit 202 can control the voltage value output by the secondary transfer power supply 21 so that the current flowing through the secondary transfer unit N2 is approximately constant at (approaching) a target current value. During image formation (secondary transfer), the current flowing through the secondary transfer unit N2 is detected by the current detection unit 241 at a predetermined period (current detection period). The secondary transfer control unit 202 then determines the voltage value of the secondary transfer voltage to be applied to the secondary transfer roller 20 in the next current detection period. The secondary transfer control unit 202 determines the voltage value of the secondary transfer voltage in the next current detection period by feeding back the difference between a preset target current value and the detected current value detected by the current detection unit 241, which is the actual output value, to the secondary transfer power supply 21. In other words, the voltage value of the secondary transfer voltage applied to the secondary transfer roller 20 in the next current detection cycle is adjusted so that the detected current value approaches the target current value. As a result, the secondary transfer voltage applied from the secondary transfer power supply 21 to the secondary transfer roller 20 is controlled so that the current flowing through the secondary transfer section N2 remains approximately constant. Here, this control, in which a voltage is applied from the secondary transfer power supply 21 to the secondary transfer roller 20 so that the current value detected by the current detection section 241 remains approximately constant at a predetermined current value, is called "constant current control".

[0033] On the other hand, as shown in Figure 2, the secondary transfer control unit 202 has a voltage detection unit 242 as a voltage detection means for detecting the voltage value that the secondary transfer power supply 21 applies to the secondary transfer roller 20. The secondary transfer control unit 202 can control the voltage value output by the secondary transfer power supply 21 so that it becomes approximately constant at (approaching) the target voltage value. The voltage detection unit 242 may detect (recognize) the voltage value from the output voltage value instructed to the secondary transfer power supply 21. In high temperature and high humidity environments, the recording material P, secondary transfer roller 20, intermediate transfer belt 10, etc., have reduced resistance due to moisture absorption. If "constant current control" of the secondary transfer voltage is performed in such a state, the secondary transfer voltage required to output the target current value will be low, which may result in a failure to form the electric field necessary to transfer toner to the recording material P, causing a transfer defect. Therefore, secondary transfer is performed by "constant voltage control" to ensure the minimum voltage required to transfer toner to the recording material P. Here, the control method that applies a nearly constant voltage at a predetermined voltage value from the secondary transfer power supply 21 to the secondary transfer roller 20 (control that makes the applied voltage nearly constant regardless of the current value) is called "constant voltage control".

[0034] In this embodiment, the CPU circuit 150 of the engine control unit 210 calculates the absolute moisture content of the environment in which the image forming apparatus 100 is installed based on the detection results of the temperature sensor 301 and humidity sensor 302 of the environmental sensor 300. Then, the CPU circuit 150 decides whether to control the secondary transfer voltage by the secondary transfer control unit 202 using "constant current control" or "constant voltage control" according to the calculated absolute moisture content, and commands the secondary transfer control unit 202. In this embodiment, the absolute moisture content is 21.7 g / m². 3 In the above cases, "constant voltage control" of the secondary transfer voltage is implemented, and the absolute moisture content is 21.7 g / m². 3 If the value is less than the specified value, "constant current control" of the secondary transfer voltage is implemented.

[0035] In this example, the absolute moisture content was 21.7 g / m². 3Even if the voltage is less than the specified value, constant voltage control will be implemented if the minimum voltage required to transfer the toner cannot be secured, as described later. In other words, a lower limit is set for the setting of the secondary transfer voltage, and if the secondary transfer voltage falls below that lower limit when constant current control is performed, the secondary transfer voltage will be controlled so that the voltage value becomes approximately constant at a target voltage value corresponding to that lower limit.

[0036] <Details of controlling secondary transfer voltage> As mentioned above, if the constant voltage value of the secondary transfer voltage is set higher than necessary to allow secondary transfer of the toner image even when the resistance value of the recording material P decreases, the current flowing through the contact members will also increase, which may lead to deterioration of the electrical conductivity of the contact members. As mentioned above, the "contact members" are members that contact the recording material P simultaneously with the secondary transfer roller 20 and other than the secondary transfer section N2, and are provided downstream or upstream of the secondary transfer section N2. For example, if a large current flows through the fixing member, which acts as a contact member, for a long time, it may lead to deterioration of the member, such as deterioration of the electrical conductivity of the fixing member, and ultimately the resistance value of the fixing member may change significantly, potentially resulting in image defects.

[0037] A feature of this embodiment is that, in a high-temperature, high-humidity environment where the resistance of the recording material P and the resistance of the secondary transfer roller 20 decrease, the constant voltage value for constant voltage control of the secondary transfer voltage is determined as follows. First, a test current is applied to the secondary transfer section (secondary transfer roller 20 and intermediate transfer belt 10) before the recording material P reaches the secondary transfer section N2. Based on the test current value and the voltage value applied to the secondary transfer roller 20 when the test current is applied, the following voltage values ​​are determined. One is (i) the lower limit of the secondary transfer voltage required to pass current to the toner image (hereinafter referred to as "lower limit voltage V"). under ) is (ii) the upper limit of the secondary transfer voltage (hereinafter referred to as "limit voltage V") to prevent the resistance value of the contact members from changing significantly due to current degradation. limit " ) ) Finally, (iii) Lower voltage V under and limit voltage V limitCompare them and determine the lower one as the secondary transfer voltage (final secondary transfer voltage) V. Then, apply this determined secondary transfer voltage V to the secondary transfer roller 20 during the secondary transfer of the toner image to the recording material P.

[0038] Hereinafter, referring to the flowchart of FIG. 3, the flow of control of the secondary transfer voltage from the start to the end of the print job will be described. Also, regarding the steps in the flow of FIG. 3 such as the "lower limit voltage determination step", "limit voltage determination step", "inter-paper voltage determination step" described later, they will be described with reference to FIGS. 4 to 6.

[0039] FIG. 3 is a flowchart showing the flow of control of the secondary transfer voltage from the start to the end of the print job in this embodiment (here, taking the print job of forming an image on one recording material P as an example). The control of the secondary transfer voltage is roughly divided into three steps: the pre-rotation step before the recording material P reaches the secondary transfer unit N2, the image forming step (print step) of secondarily transferring the toner image to the recording material P, and the post-rotation step after the secondary transfer. The pre-rotation step in this embodiment further includes the rise of high voltage (secondary transfer high voltage), the determination of the inter-paper voltage V t0 the determination of the lower limit voltage V under the determination of the limit voltage V limit the determination of the secondary transfer voltage V, and the inter-paper control. For convenience, even the control of applying the inter-paper voltage described later before the image forming step for the first recording material P of the print job is also referred to as "inter-paper control". In particular, it is a feature of this embodiment to determine the secondary transfer voltage V based on the lower limit voltage V under and the limit voltage V limit

[0040] When the engine control unit (hereinafter, also simply referred to as the "control unit") 210 receives an instruction to start a print job, it starts the print job (S11). Then, first, the control unit 210 calculates the absolute moisture content of the environment from the environmental information acquired by the environmental sensor 300, and determines whether the calculated absolute moisture content is 21.7 g / m 3 or more (S12). The control unit 210 determines in S12 whether the absolute moisture content is 21.7 g / m 3 ​If it is determined that the absolute moisture content is less than 21.7 g / m³, the control unit 210 decides to control the secondary transfer voltage with constant current control and proceeds to the process in S40. Also, if the absolute moisture content is 21.7 g / m³ in S12, the control unit 210 3 If it is determined that the above conditions are met, the control unit 210 decides to control the secondary transfer voltage with constant voltage control and proceeds to the process in S13. In both cases, whether constant voltage control or constant current control is performed, the control unit 210 performs high voltage rise-up (S13, S40) and paper-to-paper voltage determination steps (S14, S41) (Figure 4).

[0041] High voltage startup is performed with a predetermined current I t0 This process involves constant current control at a constant current of 20 μA (in this embodiment) to stably drive the secondary transfer power supply 21. During high-voltage startup, a predetermined current I is applied from a starting voltage of 500 V. t0 After raising the high voltage to the vicinity using coarse control, fine control is performed. Coarse control is performed by changing the output of the secondary transfer power supply 21 at a predetermined control period of 20 msec and a predetermined voltage change amount (100 V in this embodiment) until the current change amount converges to a predetermined threshold (2 μA in this embodiment) or less. After the coarse control has converged, fine control is performed by changing the output of the secondary transfer power supply 21 at a predetermined control period of 20 msec and a predetermined voltage change amount (20 V in this embodiment) until the current change amount converges to a predetermined threshold (0.8 μA in this embodiment) or less. The control unit 210 then sets the predetermined current I t0 The high voltage is increased to (20 μA) (S13, S40).

[0042] After the high voltage is started up, the control unit 210 will determine the paper-to-paper voltage V to be applied in the subsequent paper-to-paper control (S30, S42). t0 The process is called the "paper-to-paper voltage determination step". In the paper-to-paper voltage determination step, the control unit 210 sets a predetermined current I for a predetermined time (1000 ms in this embodiment). t0 Constant current control is performed. The control unit 210 samples the voltage value at a predetermined sampling period (20ms in this embodiment) to set a predetermined current I t0 Average voltage value V during constant current control t0 The control unit 210 calculates the average voltage value V calculated by this control. t0The value is determined as the paper-to-paper voltage, and the paper-to-paper voltage determination step is completed (S14, S41).

[0043] After the above-described steps of high voltage startup and paper-to-paper voltage determination, the control unit 210 proceeds to the process of determining the secondary transfer voltage V to be applied during the secondary transfer of the toner image if constant voltage control of the secondary transfer voltage is to be performed (S15-S19). Below, the flow when constant voltage control of the secondary transfer voltage is performed will be explained first.

[0044] First, the control unit 210 sets the lower limit voltage V under The lower limit voltage V is determined (S15). This step is called the "lower limit voltage determination step". The lower limit voltage determination step is to suppress the lower limit of the secondary transfer voltage, which is the lower limit of the secondary transfer voltage, because the current value flowing through the toner image changes depending on the resistance value of the recording material P. under This is the process of determining the lower limit voltage. The lower limit voltage determination step aims to determine a voltage setting value that ensures a constant amount of partial voltage applied to the toner on the recording material P, regardless of the resistance values ​​of the recording material P and the secondary transfer roller 20. Therefore, in the lower limit voltage determination step, the lower limit voltage V is determined to ensure efficient transfer of the toner image even on recording material P with low resistance values, such as moisture-absorbing paper. under Set the range.

[0045] In the lower voltage determination step, the control unit 210 determines the target current value I of the lower voltage determination step, which is a predetermined test current. under (Hereafter, "Lower voltage current value I under "The voltage is raised to allow the current to flow (Figure 4). Lower limit voltage current value I under This is predetermined according to the print mode, paper type, paper size, etc. Also, the lower voltage limit V under The setting range is predetermined according to the print mode, paper type, paper size, etc. Lower voltage V under The lower limit of the setting is "V under_min ", lower limit voltage V under The upper limit of the setting is "V under_max The control unit 210 sets the lower limit voltage current value I under The secondary transfer voltage V' required to pass through the voltage is the lower limit voltage V mentioned above.under Within the setting range (V under_min ≦V'≦V under_max If this is the case, the secondary transfer voltage V' is the lower limit voltage V under The control unit 210 then determines the lower voltage current value I and terminates the lower voltage determination step. under The secondary transfer voltage V' after high-voltage startup is the target voltage V under Less than the setting range (V' <V under_min If it is V under_min Lower voltage limit V under The control unit 210 then determines the lower voltage current value I under The secondary transfer voltage V' after high-voltage startup is the target voltage V under Larger than the setting range (V'>V under_max ) In that case, V under_max Lower voltage limit V under With this determined, the lower voltage determination step is completed. under_min This is pre-set to a value such that the transferability of isolated patch patterns on the moisture-absorbing recording material P is within an acceptable range. Here, "isolated patch pattern" refers to an image pattern in which clusters of high-print toner images are scattered within the width of the recording material P (length in the width direction approximately perpendicular to the transport direction). Also, V under_max This value is pre-set to prevent strong gaps in full-surface halftones and full-surface solid images (full-surface solid black patterns) on the moisture-absorbing recording material P. Here, "full-surface halftone" refers to an image pattern in which a toner image of halftone density exists across the entire image-forming area in the width direction of the recording material P. Also, "full-surface solid image (full-surface solid pattern)" refers to an image pattern in which a toner image of the highest density level exists across the entire image-forming area in the width direction of the recording material P.

[0046] As an example, Table 1 shows the basis weight of 75 g / m² in this embodiment. 2 Lower limit voltage V for the paper under The settings are shown.

[0047] [Table 1]

[0048] The wider the recording material P (paper width), the larger the area of ​​white space on the recording material P where the secondary transfer current escapes (the apparent resistance of the recording material P decreases). Therefore, to ensure that sufficient transfer current is supplied to the toner, the lower limit voltage current value I should be set considering the escape current. under The setting is high.

[0049] The control unit 210 completes the lower limit voltage determination step as described above (S15).

[0050] Next, the control unit 210 sets the limit voltage V limit The limit voltage is determined (S16). This process is called the "limit voltage determination step". The limit voltage determination step is performed to suppress the degradation of current conduction of the contact members for reasons described later, by determining the limit voltage V, which is the upper limit of the secondary transfer voltage. limit The purpose is to set it up.

[0051] In high-temperature, high-humidity environments, the recording material P absorbs moisture and its resistance decreases. In this state, when the recording material P is held in the secondary transfer section N2 and comes into contact with the contact member, the secondary transfer current flows not only to the secondary transfer opposing roller 13, which is the opposing roller to the secondary transfer roller 20, but also to the contact member via the low-resistance recording material P. Basically, the transport path of the recording material P is provided so that only insulating material (or material with high resistance) comes into contact with the recording material P to prevent the secondary transfer current from flowing in. However, when the recording material P becomes less resistance due to moisture absorption, and the secondary transfer roller 20 also becomes less resistance in a high-temperature, high-humidity environment, a portion of the secondary transfer voltage may be divided (applied) to the contact member made of insulating material or the like. Experiments conducted by the inventors have shown that if the image forming apparatus 100 is used continuously with a high voltage value, the resistance value of the contact member may decrease due to the deterioration of the contact member's conductivity.

[0052] If the resistance of the contact material decreases, the secondary transfer current may not be able to supply sufficient transfer current to the toner as it flows through the recording material P, potentially causing image defects. Therefore, an upper limit (limit voltage V) should be set for the secondary transfer voltage. limit By providing this, the voltage divided across the contact members is prevented from exceeding a certain level, thereby suppressing the deterioration of the contact members' conductivity. limit From the standpoint of power supply protection, it is preferable that the output voltage is less than the output limit voltage determined by the high-voltage element of the image forming apparatus 100.

[0053] The voltage divided across the contact members is determined by the resistance values ​​of the secondary transfer roller 20 and the intermediate transfer belt 10. In other words, the current value that flows during constant voltage control fluctuates with the resistance values ​​of the secondary transfer roller 20 and the intermediate transfer belt 10, and the lower the resistance value, the more current flows, and the higher the voltage divided across the contact members. Therefore, in this embodiment, the limit voltage V limit This is not set to a fixed value, but rather according to the resistance value of the secondary transfer section N2. Therefore, in this embodiment, the limit voltage V is set as follows. limit To decide.

[0054] In this embodiment, the limit voltage determination step is performed without causing extra downtime (period during which an image cannot be formed) by utilizing the mechanism of the lower limit voltage determination step (see Figure 4). First, the control unit 210 determines the lower limit voltage current value I in the lower limit voltage determination step. under And the applied voltage value V under Based on this, the resistance value R of the secondary transfer section N2 is calculated. Specifically, the control unit 210 calculates the resistance value R of the secondary transfer section N2 based on equation (1). Resistance value R=V of secondary transfer section N2 under / I under ...Equation (1)

[0055] Note that the lower limit voltage current value I under The output voltage when the current is passed through is V under_min If less than V under_max If the value is greater than the given value, the resistance R of the secondary transfer section N2 is calculated based on equation (2) or equation (3), respectively. Resistance value R=V of secondary transfer section N2 under_min / I under_min ...Equation (2) Resistance value R=V of secondary transfer section N2 under_max / I under_max ...Equation (3)

[0056] In equations (2) and (3), I under_min , I under_max These are V under_min Output current value, V under_max This is the current value at output.

[0057] The control unit 210 calculates the resistance value R of the secondary transfer section N2 using equations (1) to (3), and then sets a limit voltage V corresponding to the resistance value R of the secondary transfer section N2. limit Determine the limit voltage V corresponding to the resistance value R of the secondary transfer section N2. limit The method for determining this will be explained using Figures 5 and 6.

[0058] First, the limit voltage V limit The method for determining this will be explained using Figure 5. The resistance value R of the secondary transfer section N2 and the limit voltage V are as shown in Table 2. limit Information showing the relationship is stored in ROM151 in advance. The control unit 210 refers to this information to determine the limit voltage V corresponding to the calculated resistance value R of the secondary transfer unit N2. limit Determine the resistance value R and limit voltage V of the secondary transfer section N2 as shown in Table 2. limit The information showing the relationship is determined in advance based on design data and experimental data, etc., so that the voltage divided across the contact member does not exceed a certain level depending on the resistance value R of the secondary transfer section N2. Figure 5 shows the resistance value R of the secondary transfer section N2 and the limit voltage V shown in Table 2. limit This graph shows the relationship between the two. In Figure 5, the horizontal axis represents the lower limit voltage current value I. under and lower limit voltage V under The resistance value R of the secondary transfer section N2 is calculated based on equation (1), and the vertical axis represents the limit voltage V corresponding to the resistance value R of the secondary transfer section N2. limitIn this embodiment, the limit voltage V at the resistance value R of the secondary transfer section N2, which is not listed in Table 2, is... limit This is determined by linear interpolation, as shown in Figure 5.

[0059] [Table 2]

[0060] The resistance value R and limit voltage V of the secondary transfer section N2. limit The relationship varies depending on the configuration of the image forming apparatus 100. In the configuration of this embodiment, the insulating PFA tube on the surface of the fixing roller 31 is a contact member made of an insulating member that comes into contact with the recording material P during secondary transfer, and a limit voltage V is set to prevent the insulating PFA tube from degrading due to current conduction. limit They have decided.

[0061] Next, using Figure 6, we will explain the mechanism by which the voltage divided across the contact member (in this embodiment, the insulating PFA tube of the fixing roller 31) changes depending on the resistance value R of the secondary transfer section N2. Figure 6(a) is a schematic cross-sectional view of the secondary transfer section N2 and the fixing device 30 of the image forming apparatus 100, and Figure 6(b) shows a simplified equivalent circuit.

[0062] Fig. 6(a) shows a state where the recording material P is being sandwiched and conveyed by the secondary transfer unit N2, and the recording material P is in contact with the fixing roller 31 of the fixing device 30. If the resistance value of the recording material P is sufficiently higher than the resistance values of the secondary transfer roller 20 and the intermediate transfer belt 10, the secondary transfer current flows through the path of the secondary transfer roller 20, the intermediate transfer belt 10, the secondary transfer opposing roller 13, and the ground. Note that the secondary transfer opposing roller 13 is electrically grounded. However, when the resistance value of the recording material P decreases to the same level as that of the secondary transfer roller 20, the intermediate transfer belt 10, etc. due to moisture absorption or the like, the secondary transfer current flows through the recording material P. Therefore, even when the resistance value of the recording material P decreases, the pressure roller 32 is electrically grounded via the resistance element 34 (1000 MΩ) so that a large amount of the secondary transfer current does not flow into the ground through the fixing roller 31 or the like. Similarly, the fixing roller 31, which is the other path, is electrically grounded via the resistance element 33 (470 kΩ). As a result, overcurrent is suppressed.

[0063] Fig. 6(b) shows a simplified equivalent circuit of the configuration described above. In Fig. 6(b), R represents the resistance values of the respective members as follows. ·R ローラ : Resistance value of the secondary transfer roller 20 (10 6 ~10 7 Ω) in a high-temperature and high-humidity environment ·R ベルト : Resistance value of the intermediate transfer belt 10 (10 6 ~10 7 Ω) in a high-temperature and high-humidity environment ·R 対向 : Resistance value of the secondary transfer opposing roller 13 (10 4 ~10 5 Ω) ·R 加圧 : Resistance value of the pressure roller 32 (10 4 ~10 5 Ω) ·R 絶縁PFA : Resistance value of the insulating PFA tube on the surface of the fixing roller 31 (10 12 ~10 13 Ω) ·R 弾性層 : Resistance value of the elastic layer on the inner surface of the fixing roller 31 (106 ~10 7 Ω) ·R 記録材 : Resistance value of recording material P (when sufficiently humidified, 10 6 ~10 7 (Assuming Omega)

[0064] Also, P1 to P3 and V in Figure 6(b) P1 ~V P3 These represent the voltage and potential at each of the following points. P1: Core metal voltage V of secondary transfer roller 20 P1 (Secondary transfer voltage) P2: Surface potential V of secondary transfer roller 20 P2 • P3: Surface potential V of the insulating PFA tube on the surface of the fixing roller 31 P3

[0065] Voltage V from secondary power supply 21 P1 When applied, the core metal P1 of the secondary transfer roller 20 has approximately the same potential V P1 Therefore, on the surface P2 of the secondary transfer roller 20, the potential V of the core metal is P1 The resistance value R of the secondary transfer roller 20 ローラ The potential V is the voltage drop by a certain amount of minutes. P2 This is the result. The secondary transfer voltage V is the same as in constant voltage control. P1 Assuming that is constant, the higher the resistance of the secondary transfer roller 20, the larger the voltage drop will be, resulting in a potential V P2 As the voltage drops, the voltage V decreases, and the lower the resistance of the secondary transfer roller 20, the smaller the voltage drop. P2 The value of the recording material P decreases due to moisture absorption, and assuming that the resistance value decreases to a level that is almost negligible, the surface potential V of the secondary transfer roller 20 increases. P2 This is almost exactly the surface potential V of the insulating PFA tube on the surface P3 of the fixing roller 31. P3 In reality, the recording material P also has a certain degree of resistance, so VP2 >V P3 However, the lower the resistance value of the recording material P, the lower V P2 and V P3 The difference will become smaller.

[0066] Therefore, when the resistance value of the secondary transfer roller 20 changes, the surface potential V of the secondary transfer roller 20 changes. P2 This also changes, and as a result, the voltage applied to the insulating PFA tube on the surface of the fixing roller 31, which acts as a contact member, also changes. When the secondary transfer voltage is constant, as in constant voltage control, the lower the resistance value of the secondary transfer roller 20, the lower the potential V on the surface P2 of the secondary transfer roller 20. P2 The potential V increases, and the lower the resistance of the recording material P, the higher the potential V on the surface P3 of the fixing roller 31. P3 The price will increase.

[0067] In a high-temperature, high-humidity environment, both the secondary transfer roller 20 and the recording material P have reduced resistance due to moisture absorption, so the surface potential V of the fixing roller 31 decreases. P3 The surface potential V of the fixing roller 31 may become high. If the image forming apparatus is used in this state, the fixing roller 31 may deteriorate in terms of electrical conductivity and its resistance may change. Therefore, in this embodiment, the surface potential V of the fixing roller 31 is considered to be high. P3 To control and suppress current degradation, the lower the resistance value of the secondary transfer roller 20, the lower the limit voltage V of the secondary transfer voltage. limit Determine it so that it becomes smaller.

[0068] In the actual image forming apparatus 100, the resistance value R of the secondary transfer roller 20 alone is... ローラ It is not possible to measure this. Therefore, in this embodiment, the resistance value R of the secondary transfer section N2 (mainly the sum of the resistance value of the secondary transfer roller 20 and the resistance value of the intermediate transfer belt 10), which is correlated with the resistance value of the secondary transfer roller 20 alone, is measured in the lower limit voltage determination step. Then, the limit voltage V is determined according to the resistance value R of the secondary transfer section N2. limit They have decided.

[0069] Furthermore, the lower the resistance R of the secondary transfer section N2, the easier it is for the secondary transfer current to flow through the recording material P (potential difference V between P2 and P3). P2-V P3 (The value tends to decrease). Therefore, the lower the resistance R of the secondary transfer section N2, the lower the limit voltage V limit This reduces the limit voltage V. Therefore, the greater the absolute moisture content of the environment, and the lower the resistance value R of the secondary transfer section N2, the lower the limit voltage V. limit and lower limit voltage V under The difference becomes smaller. Also, the current flowing through the recording material P follows Ohm's law (current is inversely proportional to resistance), and the lower the resistance R of the secondary transfer section N2, the greater the sensitivity to changes in resistance. Therefore, the lower the resistance R of the secondary transfer section N2, the lower the limit voltage V in response to changes in the resistance R of the secondary transfer section N2. limit The changes are also becoming more significant.

[0070] The control unit 210 then sets the limit voltage V corresponding to the resistance value R of the secondary transfer unit N2 as described above. limit Determine the limit voltage and complete the limit voltage determination step (S16).

[0071] The control unit 210 controls the lower limit voltage V under and limit voltage V limit Once that is determined, the secondary transfer voltage V is then determined (S17-S19).

[0072] The secondary transfer voltage V is equal to the lower limit voltage V. under and limit voltage V limit It is determined based on the following. First, the control unit 210 sets the lower limit voltage V under and limit voltage V limit The magnitude of the following is compared (S17). Specifically, the control unit 210 controls the limit voltage V limit Lower voltage V under Whether it is higher or lower (V under >V limit Determine whether the condition is met.

[0073] Here, the limit voltage V limit Lower voltage V under If the lower limit voltage V is higher, underIf this voltage is applied as the secondary transfer voltage, it may lead to a deterioration in the electrical conductivity of the contact members in the transport path of the recording material P (in this embodiment, the insulating PFA tube of the fixing roller 31). Therefore, the control unit 210 sets the limit voltage V in S17. limit Lower voltage V under If it is determined that the latter is higher, the secondary transfer voltage V is the limit voltage V. limit (S18)

[0074] On the other hand, the lower limit voltage V under The limit voltage V limit In the following cases, the lower limit voltage V under Even if the lower limit voltage V is applied as the secondary transfer voltage, there is little possibility that the contact members in the transport path of the recording material P (in this embodiment, the insulating PFA tube of the fixing roller 31) will deteriorate in terms of conductivity. Therefore, the control unit 210 sets the lower limit voltage V in S17. under The limit voltage V limit If it is determined that the following conditions apply, the secondary transfer voltage V remains the lower limit voltage V. under (S19) The control unit 210 determines the secondary transfer voltage V in the manner described above.

[0075] The control unit 210 performs inter-paper control after determining the secondary transfer voltage V (S30). The purpose of inter-paper control is to prepare to quickly apply the desired voltage value when the recording material P reaches the secondary transfer section N2. The control unit 210 determines the inter-paper voltage V determined in the inter-paper voltage determination step (S14). t0 Apply the solution to prepare for secondary transfer. These steps constitute the pre-rotation process before the recording material P reaches the secondary transfer section N2.

[0076] Subsequently, the control unit 210 performs a secondary transfer of the toner image to the recording material P by controlling the secondary transfer voltage to a constant voltage when the recording material P reaches the secondary transfer unit N2 (S31). In the secondary transfer process, the secondary transfer voltage V determined in S18 or S19 is applied to the secondary transfer roller 20 by constant voltage control.

[0077] Furthermore, the leading edge of the recording material P coincides with the timing of the rise of high voltage from the inter-paper voltage, and also the timing when the recording material P gradually enters the secondary transfer section N2 from a state where there is no recording material P, and the impedance becomes large. In other words, the inter-paper voltage V t0 Between the initial voltage and the secondary transfer voltage V, the voltage is in the process of rising, and the resistance value of the recording material P in the secondary transfer section N2 is indeterminate, so the voltage applied to the tip of the recording material P may be low. For this reason, it is preferable to apply a different voltage (tip voltage) with a voltage value (constant voltage value) equal to or greater than the secondary transfer voltage V in the paper to the tip (tip portion, tip region) of the recording material P (see Figure 4). Similarly, at the rear end of the recording material P, the impedance decreases sharply as the recording material P passes through the secondary transfer section N2. Due to this sharp decrease in impedance, the secondary transfer voltage in constant voltage control may not follow, and the secondary transfer voltage may be insufficient. For this reason, it is preferable to apply a different voltage (rear end voltage) with a voltage value (constant voltage value) equal to or greater than the secondary transfer voltage V in the paper to the rear end (rear end portion, rear end region), similar to the tip of the recording material P (see Figure 4). Furthermore, the timing at which the recording material P enters the fixing section N3 coincides with the timing at which the insulating PFA tube on the surface of the fixing roller 31 becomes charged, and a secondary transfer current may momentarily flow to the fixing section N3. Therefore, in order to compensate for the current that flows to a location other than the secondary transfer section N2, it is sometimes preferable to apply another voltage (fixing inrush voltage) with a voltage value equal to or greater than the secondary transfer voltage V (a constant voltage value) at the timing when the recording material P enters the fixing section N3. Note that the leading edge and trailing edge of the recording material P refer to the leading edge and trailing edge with respect to the transport direction of the recording material P, respectively. Also, "in the paper" refers to the period during which the recording material P (more specifically, the image-forming region with respect to the transport direction of the recording material P) exists in the secondary transfer section N2. "In the paper" corresponds to the period during image formation (secondary transfer) in the secondary transfer section N2. As will be described later, the limit voltage V is applied to the leading edge voltage, trailing edge voltage, and fixing inrush voltage. limit You can set this.

[0078] Next, in S12, the absolute moisture content is 21.7 g / m². 3The following describes the flow when it is determined that the value is less than the threshold and constant current control of the secondary transfer voltage is performed. As mentioned above, in this case as well, the same high voltage rise-up (S40), paper-to-paper voltage determination step (S41), and paper-to-paper control (S42) are performed as when constant voltage control of the secondary transfer voltage is performed. This ensures that the recording material P is quickly controlled to the desired current value when it reaches the secondary transfer section N2. Then, when the recording material P reaches the secondary transfer section N2, the control unit 210 performs constant current control of the secondary transfer voltage and performs secondary transfer of the toner image to the recording material P (S43-S45).

[0079] First, the control unit 210 refers to a target current value I1 from a table pre-stored in the ROM 151, based on the print mode, paper type, paper size, and environmental information (temperature, humidity, absolute moisture content, etc.) acquired by the environmental sensor 300. Then, the control unit 210 determines whether the secondary transfer voltage V when controlled with a constant current at the target current value I1 falls below a predetermined voltage V1 (S43). This is to prevent transfer failures from occurring due to the secondary transfer voltage V falling below the predetermined voltage V1, which is the minimum voltage required to transfer the toner, when controlled with a constant current at the target current value I1. If the control unit 210 determines in S43 that the secondary transfer voltage V falls below the predetermined voltage V1, it performs constant voltage control of the secondary transfer voltage at the predetermined voltage V1 (S44) and secondary transfers the toner image to the recording material P. On the other hand, if the control unit 210 determines in S43 that the secondary transfer voltage V does not fall below a predetermined voltage V1 (i.e., it is equal to or greater than the predetermined voltage V1), it continues to perform constant current control of the secondary transfer voltage at the target current value I1 and transfers the toner image to the recording material P (S45). In this case, the voltage value of the secondary transfer voltage is varied according to the resistance value of the recording material P and the pattern of the toner image.

[0080] As described above, the image formation process (printing process) is completed. Here, a print job in which an image is formed on one recording material P is used as an example, but if there are multiple print jobs, the paper-to-paper control and secondary transfer processes are performed alternately. When a print job is finished, the control unit 210 performs a post-rotation process (S32) after the secondary transfer to the last recording material P is completed, and terminates the print job.

[0081] <Effect Confirmation> To confirm the effectiveness of this embodiment, a high-temperature, high-humidity environment (temperature 30°C / relative humidity 80% / absolute moisture content 24.0 / m³) was used. 3 In this example, a durability test was conducted using 50,000 sheets, and the presence or absence of image defects was verified every 10,000 sheets. As the recording material P, Xerox Business 4200 LETTER size (Xerox, product name) (paper moisture content 9.5%) that had absorbed sufficient moisture was used. The test was performed on the configuration of this example and the configuration of the comparative example. In the configuration of this example, the settings of the secondary transfer current and secondary transfer voltage in the lower limit voltage determination step and the limit voltage determination step are set to be LTR or larger in the paper size shown in Table 1. The paper moisture content was measured using a Moistrex MX8000 microwave paper moisture meter manufactured by Shinmei General Co., Ltd.

[0082] Here, the configuration of the comparative example is the same as that described in Patent Document 2. The configuration of the comparative example is the same as that of the embodiment, except that the control of the secondary transfer voltage differs from that of the embodiment as follows. For the configuration of the comparative example, elements corresponding to the configuration of the embodiment will be described using the same reference numerals as those of the embodiment. Figure 7 is a flowchart showing the flow of control of the secondary transfer voltage in the comparative example. In the configuration of the comparative example, after starting the print job (S51), the control unit 210 performs the same high-voltage startup (S52), paper-to-paper voltage determination step (S53), and paper-to-paper control (S54) as in the embodiment, without determining the absolute moisture content of the environment. After that, the control unit 210 performs secondary transfer of the toner image to the recording material P (S55-S57). At this time, the control unit 210 determines whether the secondary transfer voltage V when controlled with a constant current at the target current value I2 is below a predetermined voltage V2 (S55). If the control unit 210 determines in S55 that the secondary transfer voltage V is below a predetermined voltage V2, it performs constant voltage control of the secondary transfer voltage at the predetermined voltage V2 (S56). On the other hand, if the control unit 210 determines in S55 that the secondary transfer voltage V is not below the predetermined voltage V2 (i.e., it is above the predetermined voltage V2), it performs constant current control of the secondary transfer voltage at the target current value I2 (S57). After that, the control unit 210 performs a post-rotation (S58) and finishes the print job.

[0083] Table 3 shows the occurrence of image defects at each durability count for the configurations of this embodiment and the comparative example. In the table, "○" indicates no image defects, and "×" indicates image defects occurred. Note that this durability test was terminated when an image defect occurred, and "―" in the table means that the test was not performed.

[0084] [Table 3]

[0085] In the comparative example configuration, no image defects occurred up to the point where the durability reached 30,000 sheets. However, in the comparative example configuration, at the point where the durability reached 40,000 sheets, the resistance value of the fixing roller 31 decreased due to the deterioration of the fixing roller's electrical conductivity, resulting in image defects (image defects due to transfer defects caused by leakage of secondary transfer current with each rotation cycle of the fixing roller 31).

[0086] On the other hand, in the configuration of this embodiment, no excessive voltage was applied to the surface of the fixing roller 31, and therefore no image defects occurred throughout the durability test.

[0087] As described above, according to this embodiment, it is possible to suppress the deterioration of the durability of the contact members that come into contact with the recording material P at the same time as the secondary transfer roller 20, while maintaining good secondary transfer properties. In other words, according to this embodiment, it is possible to suppress image defects caused by the deterioration of the contact members that come into contact with the recording material at the same time as the transfer members, while maintaining good transfer properties.

[0088] <Other configurations> In this embodiment, the limit voltage determination step (S15) was performed after the lower limit voltage determination step (S14), but the present invention is not limited to this embodiment. The steps may also be performed in the reverse order (limit voltage determination step (S15) followed by the lower limit voltage determination step (S14)), and the same effects as in this embodiment can be obtained.

[0089] In this example, the absolute moisture content was 21.7 g / m². 3 In cases where the absolute moisture content is less than 21.7 g / m³, the limit voltage determination step (S16) is not performed, but the present invention is not limited to this embodiment. 3 Even if it is less than the limit voltage, the limit voltage determination step is performed and the secondary transfer voltage V is determined in the same flow as in this embodiment (limit voltage V limit It may be preferable to set the limit voltage. For example, it may be known in advance that a low-resistance recording material P, whose resistance is sufficiently lower than that of the secondary transfer roller 20 or the intermediate transfer belt 10, will be used for image formation. For example, a user (operator) can input information to the control unit 210 indicating that a low-resistance recording material P will be used for image formation by operating the host computer 199 or the operation unit provided on the image forming apparatus 100. This information may include information that directly specifies the low-resistance recording material P, or information that specifies a specific operation setting associated with the low-resistance recording material P, such as "low-resistance paper mode". The control unit 210 can determine the secondary transfer voltage V in the same flow as in this embodiment by performing a limit voltage determination step according to this information. Note that the limit voltage determination step may be performed to control whether or not to set the limit voltage regardless of the environment, depending on whether or not the low-resistance recording material P is used. Here, even when the secondary transfer voltage is controlled by constant current, the limit voltage can be set as the upper limit of the secondary transfer voltage.

[0090] The input means (input unit) that inputs information indicating that the low-resistance recording material P is being used to the control unit 210 is not limited to inputting the information to the control unit 210 in response to user operation. The input means that inputs the information to the control unit 210 in response to user operation could be, for example, an input / output unit that receives signals from a host computer, or an operation unit of the image forming apparatus 100 operated by an operator. In other words, the image forming apparatus 100 may be provided with means to detect that the low-resistance recording material P is being used for image formation. For example, information regarding the resistance value of the recording material P, such as its thickness and resistance value, can be acquired by a media sensor installed in the transport path of the recording material P from the recording material P feeding unit to the secondary transfer unit N2. Media sensors using light, ultrasound, etc., are known that can be used to detect or estimate basis weight correlated with the thickness of the recording material P, surface properties of the recording material P, and even moisture content. The information regarding the thickness of the recording material P is just one example of information regarding the recording material P. Information regarding the recording material P includes any information that can distinguish the recording material P, such as attributes based on general characteristics like plain paper, cardboard, or thin paper (so-called paper type category), numerical values ​​or ranges of values ​​such as basis weight, thickness, size, and rigidity, or brand name (including manufacturer, product name, part number, etc.). Each recording material P distinguished by the information regarding the recording material P can be considered to constitute a type of recording material P. Furthermore, when the recording material P is present in the secondary transfer unit N2, a test current or test voltage can be applied to the secondary transfer unit N2 to obtain information on the resistance value of the secondary transfer unit N2 including the recording material P, and the resistance value of the recording material P can be detected by subtracting the resistance value of the secondary transfer unit N2 without the recording material P, obtained in the same manner as in this embodiment. In this case, for example, the resistance value of the first recording material P in a print job can be detected and reflected in the control of the secondary transfer voltage from the time of image formation on the subsequent recording material P. Furthermore, a mechanism (such as a pair of conductive rollers and a power supply) capable of detecting the resistance value of the recording material P in the same manner as the method for detecting the resistance value in the secondary transfer section N2 described above may be provided in the transport path of the recording material P from the recording material P feeding section to the secondary transfer section N2.

[0091] In this embodiment, the resistance value R of the secondary transfer section N2 was determined based on equation (1) (or equations (2) and (3)), but the present invention is not limited to this embodiment. For example, a voltage V can be applied to the secondary transfer opposing roller 13. t1 The following are applied: secondary transfer voltage V and secondary transfer opposing roller voltage V t1 A configuration can be used in which secondary transfer is performed by the potential difference between the two. In this case, the resistance value R of the secondary transfer section N2 can be determined based on equation (4). The resistance value R = (V under -V t1 ) / I under ...Equation (4)

[0092] In other words, in equation (1) of this embodiment, since the secondary transfer opposing roller 13 is connected to ground, V in equation (4) t1 This corresponds to the case where =0V, and it can be seen that the same effect as in this embodiment can be obtained when using equation (4). Similarly, the resistance value R of the secondary transfer section N2 can be determined using equation (5) instead of equation (2) in this embodiment, and equation (6) instead of equation (3) in this embodiment. The resistance value R = (V under_min -V t1 ) / I under_min ...Equation (5) The resistance value R = (V under_max -V t1 ) / I under_max ...Equation (6)

[0093] In this example, since a toner with a normal charge polarity of negative polarity is used, V under and V limit These are voltages of positive polarity. Therefore, although the magnitudes are simply compared in S17 of Figure 3, the present invention is not limited to this embodiment. When using toner with a normal charging polarity of positive polarity, V under and V limit These will each be negative voltages. In this case, we only need to compare the magnitude of their absolute values. The same applies to the descriptions of the high and low and magnitude of other voltages and potentials in this embodiment.

[0094] In this embodiment, the limit voltage V limit In determining the limit voltage V of the fixing roller 31, an insulating PFA tube on the surface of the fixing roller 31 was given as an example of a contact member composed of an insulating member on the transport path of the recording material P. However, the present invention is not limited to this embodiment. For example, if the resist roller 60 (at least one of the resist roller pair 60, 60) in contact with the recording material P is conductive and current flows through it, the wear of the surface of the resist roller 60 may be accelerated due to deterioration of current conduction, which may affect the transport speed of the recording material P. Therefore, the control of this embodiment can be applied from the viewpoint of suppressing wear of the resist roller 60. In addition, the same limit voltage V of this embodiment can be applied to the fixing device 30 and the resist roller 60, respectively. limit Apply a control to set the limit voltage V for each. limit This can be set. And in this case, the limit voltage V determined for each can be set. limit Among them, the lower voltage (smallest absolute value) is V under It is preferable to compare it with the above. In addition, it is conceivable that molded parts that come into contact with the recording material P on the transport path of the recording material P may also experience a decrease in resistance due to electrical degradation, causing secondary transfer current to flow. Therefore, the same limit voltage V as in this embodiment should be applied to these molded parts as well. limit You can apply controls to set this.

[0095] In this embodiment, V under and V limit Although we have compared this, the present invention is not limited to such embodiments. V limit In comparison, control similar to that of this embodiment may be performed. That is, in this embodiment, since the component that could deteriorate with current was the insulating PFA tube on the surface of the fixing roller 31, the timing at which the secondary transfer section N2 and the fixing section N3 simultaneously contacted the recording material P was during constant voltage control in the paper. However, as mentioned above, if there is deterioration with current in a component that may come into contact with the recording material P on the transport path, the same limit voltage V as in this embodiment may be applied to the tip voltage as well. limitControl may be implemented to set the following. For example, as the recording material P is held and conveyed between the resist roller pair 60, 60, its leading edge enters the secondary transfer section N2. Therefore, the tip voltage may also lead to a deterioration of the current supply to the resist roller 60. For this reason, the voltage applied to the secondary transfer section N2 at the timing when the recording material P is in contact with both the resist roller pair 60, 60 and the secondary transfer section N2 simultaneously may be limited by control similar to that in this embodiment (V limit It is preferable to set the following. Similarly, the voltage applied to the secondary transfer unit N2 at the timing when the recording material P is in contact with both the secondary transfer unit N and the fixing unit N3, such as the trailing end voltage mentioned above, should be limited by control in the same manner as in this embodiment (V limit It is preferable to set it to (

[0096] In this example, the absolute moisture content of the environment was 21.7 g / m². 3 In the above cases, the lower limit voltage V under Control parameter (I) for setting the control under , V under_min , V under_max The parameters were set as shown in Table 1. However, the present invention is not limited to this embodiment. For example, the control parameters may be changed according to the absolute moisture content, as shown in Table 4. Table 4 shows the lower limit voltage V for LTR size width. under This shows the control parameters for setting the control. The higher the absolute moisture content of the environment, the higher the moisture content of the paper of the recording material P. Therefore, the resistance value of the recording material P also decreases, making it easier for current to escape to areas other than the toner on the recording material P (white areas). For this reason, in order to ensure the transfer current to the toner area and suppress transfer defects, the transfer voltage (lower limit voltage V) must be set. under As shown in Table 4, it is preferable to increase this value as the absolute moisture content of the environment increases.

[0097] [Table 4]

[0098] Thus, in this embodiment, the image forming apparatus 100 includes an image carrier 10 that carries a toner image, a transfer member 20 that contacts the image carrier 10 to form a transfer section N2 and transfers the toner image from the image carrier 10 to the recording material P that passes through the transfer section N2, a power supply 21 that outputs a voltage to the transfer member 20, a detection unit 241 that detects at least one of the current value flowing through the transfer member 20 or the voltage value applied to the transfer member 20 when the power supply 21 outputs a voltage to the transfer member 20, a control unit 210 that controls the power supply 21, an environmental detection unit 300 that detects environmental information relating to at least one of the temperature or humidity of the environment, and a contact member 31 that is provided to be able to contact the recording material P simultaneously with the transfer member 20, other than the transfer section.Then, when the absolute moisture content obtained based on the above environmental information is above a predetermined threshold, the control unit 210 sets a limit voltage V based on the detection result of the detection unit 241 when there is no recording material P in the transfer section N2. limit The absolute value of the voltage applied from the power supply 21 to the transfer member 20 while the recording material P is in simultaneous contact with the transfer member 20 and the contact member 31 is set to the limit voltage V. limit The power supply 21 is controlled so that the absolute value is less than or equal to the limit voltage V. The image forming apparatus 100 has an input unit that inputs information about the recording material P to the control unit 210, and the control unit 210 controls the limit voltage V if the information input by the input unit satisfies predetermined conditions set in advance. limit The control may be configured to set the above. The input unit may input the above information to the control unit 210 in accordance with the operator's operation. The above information may also include the type of recording material P, the operation setting provided for a predetermined type of recording material P, or information indicating an index value correlated with the electrical resistance value of the recording material P. The input unit may also be a sensor that inputs the detection result of an index value correlated with the electrical resistance value of the recording material P to the control unit 210 as the above information. The above conditions are the limit voltage V limit When performing control to set the limit voltage V, the electrical resistance value of the recording material P is limit It may indicate that the limit voltage V is lower than the electrical resistance value of the recording material P when no control is performed to set it. limitThe absolute value of is less than the absolute value of the output limit voltage determined from the characteristics of the high-voltage element of the power supply 21. In this embodiment, the image forming apparatus 100 has a fixing member 31 as a contact member for fixing a toner image to the recording material P. The fixing member 31 is electrically grounded via a resistive element. The fixing member 31 also has an electrically insulating surface layer that can contact the recording material P. In this embodiment, the image carrier 10 is an intermediate transfer body that transports a toner image that has been primary transferred from another image carrier 1 to the recording material P in the transfer unit N2 for secondary transfer.

[0099] Furthermore, the control unit 210 sets a limit voltage V when the electrical resistance value of the transfer unit N2 indicated by the detection result of the detection unit 241 is the first resistance value. limit Let be the first limit voltage, and the limit voltage V when the electrical resistance value is a second resistance value that is lower than the first resistance value. limit The limit voltage V is set such that the second limit voltage has a smaller absolute value than the first limit voltage. limit The control unit 210 can set a limit voltage V when the electrical resistance value of the transfer unit N2 indicated by the detection result of the detection unit 241 is the first resistance value. limit Let be the first limit voltage, and the limit voltage V when the electrical resistance value is a second resistance value that is a predetermined value lower than the first resistance value. limit Let be a second limit voltage whose absolute value is smaller than the first limit voltage, and when the electrical resistance is a third resistance which is lower than the second resistance, the limit voltage V limit Let be a third limit voltage whose absolute value is smaller than the second limit voltage, and the limit voltage V when the electrical resistance is a fourth resistance which is lower than the third resistance by the predetermined value mentioned above. limit The fourth limit voltage is set to have an absolute value smaller than the third limit voltage, and the difference between the third limit voltage and the fourth limit voltage is set to be larger than the difference between the first limit voltage and the second limit voltage, thus setting the limit voltage V limit This can be set. In this embodiment, the limit voltage V limit This is set to limit the voltage that is divided and sent to the contact member 31 via the recording material P from the voltage applied from the power supply 21 to the transfer member 20.

[0100] Furthermore, in this embodiment, the control unit 210, based on the detection result of the detection unit 241 when there is no recording material P in the transfer unit N2, indicates the lower limit voltage V of the absolute value of the voltage applied from the power supply 21 to the transfer member 20 during transfer. under The voltage applied from the power supply 21 to the transfer member 20 while the recording material P is in simultaneous contact with the transfer member 20 and the contact member 31 is set to a lower limit voltage V under and limit voltage V limit The power supply 21 is controlled to use the one with the smaller absolute value. The control unit 210 controls the lower limit voltage V when the absolute moisture content obtained based on the above environmental information is the first moisture content. under Let be the first lower limit voltage, and let be the lower limit voltage V when the absolute amount of water is greater than the first amount of water. under The lower limit voltage V is set such that the second lower limit voltage has a larger absolute value than the first lower limit voltage. under The control unit 210 can set the lower limit voltage V when the width in the direction approximately perpendicular to the transport direction of the recording material P is the first width. under Let be the first lower limit voltage, and when the width is wider than the first width, the lower limit voltage V under The lower limit voltage V is set such that the second lower limit voltage has a larger absolute value than the first lower limit voltage. under The control unit 210 can set the lower limit voltage V when the absolute moisture content obtained based on the above environmental information is the first moisture content. under and limit voltage V limit The difference between these two values ​​becomes the first difference, and the lower limit voltage V is the case when the absolute amount of moisture is greater than the first amount of moisture for the second amount of moisture. under and limit voltage V limit The lower limit voltage V is set such that the difference between it and the second difference is smaller than the first difference. under and limit voltage V limit You can set this and that.

[0101] [Example 2] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatus in 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 in Embodiment 1 are denoted by the same reference numerals as in Embodiment 1, and detailed descriptions are omitted.

[0102] In Example 1, the lower limit voltage V under and limit voltage V limit The two values ​​were compared, and the lower value was applied to the secondary transfer roller 20 as the secondary transfer voltage V. A feature of this embodiment is the lower limit voltage V under This concerns the control method for applying corrections.

[0103] In this example, the lower limit voltage determined by the same lower limit voltage determination step as in Example 1 is V under And this lower limit voltage V under The corrected lower limit voltage V offset We will explain them separately.

[0104] Figure 8 is a flowchart showing the control flow of the secondary transfer voltage from the start to the end of a print job in this embodiment (here, a print job that forms an image on one recording material P is used as an example). In the flow of Figure 8, the same or corresponding steps as those in the flow of Figure 3 described in Example 1 are given the same step numbers, and detailed explanations are omitted as appropriate.

[0105] The control unit 210, similar to Example 1, calculates the absolute moisture content of the environment when the print job is started (S11), and the calculated absolute moisture content is 21.7 g / m². 3 It is determined whether or not the above is true (S12). Also, the control unit 210 determines, as in Example 1, that the absolute moisture content is 21.7 g / m². 3 If the value is less than 21.7 g / m³, the process proceeds to S40 to control the secondary transfer voltage with a constant current, and the absolute moisture content is 21.7 g / m³. 3In the above cases, the process proceeds to S13 in order to control the secondary transfer voltage at a constant voltage. Then, the control unit 210, as in Embodiment 1, performs high voltage rise-up (S13, S40) and paper-to-paper voltage determination steps (S14, S41) regardless of whether constant voltage control or constant current control is performed.

[0106] When performing constant voltage control of the secondary transfer voltage, the control unit 210 then performs a lower limit voltage determination step, similar to Example 1, to V under Determine (S15), and V by the limit voltage determination step limit Determine (S16). Note that the order of S15 and S16 may be reversed.

[0107] Next, unlike in Example 1, the control unit 210 has a lower limit voltage V under V with correction applied offset The following is calculated (S20~S22). As explained in Example 1, the lower limit voltage determination step is to determine a voltage (lower limit voltage V) to suppress the effect of the resistance value of the recording material P on the transferability, because the current value flowing through the toner image changes depending on the resistance value of the recording material P. under This is the step to determine the lower limit voltage. In other words, the lower limit voltage determination step aims to determine a voltage that improves secondary transferability regardless of the state of the recording material P. Therefore, by applying corrections to further optimize transferability according to the amount and arrangement of toner on the recording material P, in addition to the state of the recording material P, a more accurate voltage value can be obtained.

[0108] In this embodiment, as an example, a method for correcting the lower limit voltage according to the amount of toner will be described. In explaining the relationship between the amount of toner and secondary transferability, Figure 9 will be used to explain image defects (hereinafter referred to as "patch defects") that occur when an isolated patch pattern is transferred to a moisture-absorbing recording material P. Figure 9(a) is an example of an image containing an isolated patch pattern, and Figure 9(b) is a schematic diagram of the cross-section of the secondary transfer section N2 of the image containing the isolated patch pattern. Here, "isolated patch pattern" refers to an image pattern in which clumps of high-print toner are scattered within the width of the recording material P, as described above.

[0109] In a hot and humid environment, the secondary transfer roller 20 and recording material P absorb moisture, causing their resistance to decrease. Therefore, when transferring low-print images with a small amount of toner per page, such as isolated patch patterns as shown in Figure 9(a), the following occurs: In other words, as shown in Figure 9(b), the transfer current tends to flow selectively to areas where toner T is not present (also called "white areas"), rather than to areas where toner T has high resistance (also called "toner areas" or "patch areas"). Transfer current becomes less likely to flow to areas where toner T has high resistance (toner areas, patch areas). The arrows in Figure 9(b) represent the path of the secondary transfer current, and the thickness of the arrows schematically represents the magnitude of the current. In constant voltage control, insufficient transfer current can prevent adequate transfer of isolated patch patterns, leading to "patch blemishes" where toner is partially untransferred and missing. Therefore, to suppress patch blemishes, it is preferable to set the secondary transfer voltage higher, taking into account the escape of transfer current to the white areas.

[0110] Thus, by determining the transfer voltage according to the amount of toner in the toner image transferred to the recording material P, such as the image pattern, the amount of toner in one page, or the amount of toner in the secondary transfer section N2, it becomes possible to perform secondary transfer with greater accuracy. Therefore, in this embodiment, if it is known in advance that the amount of toner will be small (when the amount of toner is below a predetermined threshold), the lower limit voltage V under The lower limit voltage is corrected by adding a correction ΔV to it. That is, the control unit 210 adjusts the corrected voltage V offset This is calculated based on equation (7). V offset =V under +ΔV (where toner amount ≤ X%) V offset =V under (However, toner amount > X%) ...Equation (7)

[0111] In this embodiment, the control unit 210 predicts, for example, the amount of toner on one page from the number of laser drive signals emitted by the exposure device 3, which are generated by converting from the image data. In this embodiment, the amount of toner is determined with 100% being the case when a full-page monochrome image is printed on one page. In this embodiment, the threshold X in equation (7) is set to 20%.

[0112] The control unit 210 then adjusts the corrected lower limit voltage V offset The decision is made (S20-S22).

[0113] Next, the control unit 210 adjusts the corrected lower limit voltage V offset and limit voltage V limit Based on this, the secondary transfer voltage V is determined (S17-S19) in the same manner as in S17-S19 of Figure 3 in Example 1. That is, first the control unit 210 determines the corrected lower limit voltage V offset and limit voltage V limit The magnitude of the following is compared (S17). Specifically, the control unit 210 controls the limit voltage V limit Lower voltage limit V after correction offset Whether it is higher or lower (V offset >V limit The control unit 210 then determines whether the condition is met. offset The limit voltage V limit If it is higher than the limit voltage V, continuous use may cause the contact components to degrade in conductivity, so limit This is defined as the secondary transfer voltage V (S18). Meanwhile, the control unit 210 sets the corrected lower limit voltage V offset The limit voltage V limit In the following cases, the corrected lower limit voltage V offset This is defined as the secondary transfer voltage V (S19). The control unit 210 determines the secondary transfer voltage V in the manner described above.

[0114] The subsequent processes S30 to S32 when the secondary transfer voltage is controlled by a constant voltage, and the processes S42 to S45 and S32 when the secondary transfer voltage is controlled by a constant current, are the same as the processes with the same step numbers in Figure 3 in Example 1, so a detailed explanation is omitted.

[0115] Thus, in this embodiment, the control unit 210 determines the lower limit voltage V based on the detection result of the detection unit 241 when there is no recording material P in the transfer unit N2 and information regarding the amount of toner used for the toner image. under Set it.

[0116] As described above, this embodiment makes it possible to obtain even better secondary transfer properties than in Embodiment 1 while suppressing the deterioration of electrical conductivity of the contact members due to the continuous use of the image forming apparatus 100.

[0117] <Other configurations> In this embodiment, the amount of toner in the toner image transferred to the recording material P was predicted from the number of laser drive signals, but the present invention is not limited to this embodiment. For example, the controller 200 may transmit image information (toner amount information) along with the video signal to the control unit 210, and the control unit 210 may determine a correction value for the lower limit voltage based on that information.

[0118] Furthermore, although the toner amount was calculated page by page in this embodiment, the present invention is not limited to this embodiment. For example, correction may be performed at any period, such as after each rotation of the secondary transfer roller 20 or after each rotation of the photosensitive drum 1. Since it is preferable to change the voltage value according to the amount of toner in the secondary transfer section N2, the secondary transfer voltage may be changed according to the amount of toner in the secondary transfer section N2 when the response of the secondary transfer power supply is sufficiently fast.

[0119] Furthermore, in this embodiment, the amount of correction for the lower limit voltage applied to the secondary transfer roller 20 was determined based on the amount of toner, but the present invention is not limited to this embodiment. For example, in a configuration in which a secondary transfer opposing roller voltage is applied to the secondary transfer opposing roller 13 and secondary transfer is performed by the potential difference between the secondary transfer voltage and the secondary transfer opposing roller voltage, the amount of correction for the lower limit voltage may be changed by changing the secondary transfer opposing roller voltage without applying any correction to the voltage applied to the secondary transfer roller 20.

[0120] [Example 3] Next, other embodiments 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 Examples 1 and 2 are denoted by the same reference numerals as in Examples 1 and 2, and detailed descriptions are omitted.

[0121] The image forming apparatus of this embodiment is an image forming apparatus that does not have a primary transfer power supply. As an example of a configuration without a primary transfer power supply, a drum voltage configuration, which will be described later, in which the primary transfer member is connected to ground can be considered. In this embodiment, the drum voltage configuration in which the primary transfer member is connected to ground, the intermediate transfer belt used in the drum voltage configuration, and the effects of applying the present invention to the drum voltage configuration will be explained.

[0122] First, let's explain the drum voltage configuration. An image forming apparatus with a drum voltage configuration in which the primary transfer member is connected to ground is an image forming apparatus having a high-voltage power supply configuration as shown in Figure 10. Figure 10 is a schematic diagram showing the connection and grounding status of the high-voltage power supply for each part around the primary transfer section N1 in the image forming apparatus 100 of this embodiment. In this embodiment, the primary transfer roller 14, which is the primary transfer member, is connected to ground (0V) (electrically grounded). Also in this embodiment, during image formation, a voltage of -300V as the drum voltage (reference voltage) is applied from the high-voltage power supply 200 to the core metal (not shown) of the photosensitive drum 1. An image formation potential Vl (-400V) with an absolute value greater than the absolute value of the drum voltage is formed on the surface of the photosensitive drum 1. Then, the toner on the image section (the part with the image formation potential Vl) of the photosensitive drum 1 is primary transferred onto the intermediate transfer belt 10 by the difference (primary transfer contrast) between the potential of the primary transfer roller 14 (0V) and the image formation potential Vl (-400V) on the surface of the photosensitive drum 1.

[0123] Next, the intermediate transfer belt 10 used in the drum voltage configuration will be described. In a configuration without a primary transfer power supply, as in this embodiment, it is difficult to increase the primary transfer contrast. In order to increase the primary transfer contrast, it is necessary to increase the absolute value of the drum voltage, which may lead to an increase in the size and cost of the equipment. Therefore, in order to allow sufficient primary transfer current to flow even with a small primary transfer contrast, it is preferable that the electrical resistance value of the intermediate transfer belt 10 is low.

[0124] Figure 11 is a schematic diagram showing the cross-sectional structure of the intermediate transfer belt 10 in this embodiment. In this embodiment, an endless belt with a circumference of 700 mm and a thickness of 65 μm was used as the intermediate transfer belt 10. As shown in Figure 11, in this embodiment, the intermediate transfer belt 10 consists of two layers: a base layer 10e with a thickness of 64 μm and an inner layer 10f with a thickness of 1 μm. The base layer 10e side (outer surface side) contacts the photosensitive drum 1, and the inner layer 10f side (inner surface side) contacts the primary transfer roller 14. In this embodiment, polyethylene terephthalate (PET) resin mixed with an ionic conductive agent was used as the material for the base layer 10e. In this embodiment, polyester resin mixed with carbon, an electronically conductive agent, was used as the material for the inner layer 10f. The inner layer 10f is formed inside the base layer 10e and contacts the drive roller 11, tension roller 12, and secondary transfer opposing roller 13. In this embodiment, polyethylene terephthalate (PET) resin was used as the material for the base layer 10e, but other materials can also be used. For example, polyester, acrylonitrile-butadiene-styrene copolymer (ABS), and mixed resins thereof can be used as the material for the base layer 10e. Also, in this embodiment, polyester resin was used as the material for the inner layer 10f, but other materials can also be used, such as acrylic resin.

[0125] In this example, the electrical resistance of the inner layer 10f of the intermediate transfer belt 10 is lower than that of the base layer 10e. In this example, the volume resistivity of the intermediate transfer belt 10 is 1 × 10⁻⁶. 10It is Ω·cm. In this embodiment, the surface resistivity of the inner surface of the intermediate transfer belt 10 is 1.0 × 10⁻⁶. 6 The ratio is Ω / □. In this embodiment, the measurement environment for the electrical properties of the intermediate transfer belt 10 is an indoor temperature of 23°C and an indoor humidity of 50%. In this embodiment, the volume resistivity actually measured for the intermediate transfer belt 10 reflects the electrical resistance of the base layer 10e, based on the relationship between electrical resistance and thickness between the base layer 10e and the inner layer 10f. On the other hand, in this embodiment, the surface resistivity of the inner surface actually measured for the intermediate transfer belt 10 reflects the electrical resistance of the inner layer 10f.

[0126] Volume resistivity was measured using a Hiresta-UP (MCP-HT450) measuring instrument from Mitsubishi Chemical Corporation, with a ring probe of type UR (model MCP-HTP12). Surface resistivity was measured using the same measuring instrument as for volume resistivity, but with a ring probe of type UR100 (model MCP-HTP16). Volume resistivity was measured by applying the probe to the surface side (base layer 10e side) of the intermediate transfer belt 10, with an applied voltage of 100V and a measurement time of 10 seconds. Surface resistivity was measured by applying the probe to the inner side (inner layer 10f side) of the intermediate transfer belt 10, with an applied voltage of 10V and a measurement time of 10 seconds. In this example, the volume resistivity of the intermediate transfer belt 10 is 1 × 10⁻¹⁶. 9 Ω cm or more, 1×10 10 A range of Ω·cm or less is preferred, and the surface resistivity of the inner surface of the intermediate transfer belt 10 is 4.0 × 10 6 Ω / □ or less (typically 1.0 × 10) 5 Ω / □ or greater is preferable.

[0127] As described above, the intermediate transfer belt 10 in the electrical resistance range has low electrical resistance, allowing current to flow in the circumferential direction of the intermediate transfer belt 10. Therefore, even if the primary transfer contrast is small, sufficient primary transfer current can be supplied. For this reason, in a drum voltage configuration without a primary transfer power supply, as in this embodiment, it is preferable to use a low-resistance intermediate transfer belt 10 having the electrical resistance value described above.

[0128] Next, the effects of applying the present invention to the drum voltage configuration will be explained. In a high-temperature, high-humidity environment, the lower the electrical resistance of the intermediate transfer belt 10, as in this embodiment, the easier it is for the secondary transfer current to flow to the white area rather than the toner area (patch area). For example, when printing images as shown in Figures 9(a) and (b) above, an equivalent circuit as shown in Figure 12 can be considered for the secondary transfer section N2. Each symbol in Figure 12 represents the following: • Rr: Electrical resistance value of secondary transfer roller 20 • Rp: Electrical resistance value of recording material P • Rt: Electrical resistance of the toner in an isolated patch pattern • Ri: Electrical resistance value of the intermediate transfer belt 10 I1: Current passing through the white area I2: Current passing through the toner section (patch section)

[0129] The ratio of I1 to I2 is given by equation (8) below. I1 / I2 =(Ri+Rt+Rp) / (Ri+Rp) =1+Rt / (Ri+Rp) ...Equation (8)

[0130] As shown in equation (8), the smaller Ri becomes, the larger the ratio of I1 to I2 becomes. In other words, the smaller the electrical resistance value Ri of the intermediate transfer belt 10, the more easily the secondary transfer current flows to the white areas rather than the toner areas (patch areas). Therefore, in a configuration using a low-resistance intermediate transfer belt 10 as described above, patch blurring may occur in images with a small amount of toner. On the other hand, if the absolute value of the secondary transfer voltage is set to be unnecessarily large in order to suppress patch blurring, as explained in Example 1, the secondary transfer current will flow through the recording material P and into the contact members that come into contact with the recording material P on the transport path. If the flow of secondary transfer current is repeated, the contact members may deteriorate in terms of electrical conductivity.

[0131] Therefore, in this embodiment, the present invention is applied to a configuration using the low-resistance intermediate transfer belt 10 described above. In other words, the limit voltage V that suppresses the occurrence of current-carrying deterioration of the contact member.limit The limit voltage V is determined according to the electrical resistance R of the secondary transfer section. limit The secondary transfer voltage is controlled as follows. This suppresses the degradation of the contact members' conductivity while also suppressing patchy blurring. As a result, even with a configuration using a low-resistance intermediate transfer belt 10, it is possible to suppress image defects such as patchy blurring while also suppressing the degradation of the contact members' conductivity. Furthermore, as explained in the above embodiment, strong dropouts can also be suppressed. This enables a simple configuration without a primary transfer power supply, as in this embodiment. In this embodiment, the method of Embodiment 2 can be suitably applied as the method for controlling the secondary transfer voltage, but the method of Embodiment 1 may also be applied.

[0132] Thus, in this embodiment, the intermediate transfer body is composed of an endless belt, and the belt is capable of carrying current in the circumferential direction. Furthermore, in this embodiment, the volume resistivity of the belt is 1 × 10⁻⁶. 9 Ω cm or more, 1×10 10 It is less than or equal to Ω·cm.

[0133] As explained above, according to this embodiment, even when a low-resistance intermediate transfer belt 10 is used, the limit voltage V that suppresses the occurrence of current degradation of the contact member is maintained. limit By applying the method for controlling the secondary transfer voltage described below, it is possible to suppress current degradation of the contact members while also suppressing patchiness and strong disconnections. Therefore, according to this embodiment, in realizing a simple configuration without a primary transfer power supply, it is possible to suppress current degradation of the contact members while also suppressing patchiness and strong disconnections.

[0134] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the embodiments described above.

[0135] For example, in the above-described embodiment, the image forming apparatus was a color image forming apparatus having multiple image forming units, but the present invention is not limited thereto, and the image forming apparatus may be a monochrome image forming apparatus having only one image forming unit. In this case, the present invention may be applied to a transfer unit that directly transfers a toner image from a photoreceptor or the like, which serves as an image carrier, to a recording material. [Explanation of symbols]

[0136] 1 Photosensitive drum 3. Exposure apparatus 10 Intermediate transfer belt 20 Secondary transfer roller 21 Secondary Transfer Power Supply 100 Image forming apparatus P recording material

Claims

1. An image carrier that holds the toner image, A transfer member that contacts the image carrier to form a transfer portion and transfers the toner image from the image carrier to a recording material passing through the transfer portion, The transfer member is provided with a power supply that outputs a voltage, A detection unit that detects at least one of the current value flowing through the transfer member or the voltage value applied to the transfer member when a voltage is output from the power supply to the transfer member, A control unit that controls the power supply, An environmental detection unit that detects environmental information relating to at least one of the temperature or humidity of the environment, The transfer member has a contact member that is provided to be able to contact the recording material simultaneously with the transfer member, except for the transfer portion. The image forming apparatus is characterized in that the control unit sets a limit voltage based on the detection result of the detection unit when the absolute amount of moisture obtained based on the environmental information is greater than or equal to a predetermined threshold, when the recording material is absent from the transfer unit, and controls the power supply so that the absolute value of the voltage applied from the power supply to the transfer member while the recording material is in simultaneous contact with the transfer member and the contact member is less than or equal to the absolute value of the limit voltage.

2. An image carrier that holds the toner image, A transfer member that contacts the image carrier to form a transfer portion and transfers the toner image from the image carrier to a recording material passing through the transfer portion, The transfer member is provided with a power supply that outputs a voltage, A detection unit that detects at least one of the current value flowing through the transfer member or the voltage value applied to the transfer member when a voltage is output from the power supply to the transfer member, A control unit that controls the power supply, An input unit that inputs information regarding the recording material to the control unit, The transfer member has a contact member that is provided to be able to contact the recording material simultaneously with the transfer member, except for the transfer portion. The control unit, when the information input by the input unit satisfies predetermined conditions, sets a limit voltage based on the detection result of the detection unit when there is no recording material in the transfer unit, and controls the power supply so that the absolute value of the voltage applied from the power supply to the transfer member while the recording material is in simultaneous contact with the transfer member and the contact member is less than or equal to the absolute value of the limit voltage, and if the information does not satisfy the conditions, it does not perform the control to set the limit voltage. The above conditions are set such that the information satisfies the conditions when the electrical resistance value of the recording material is the first recording material resistance value, and the information does not satisfy the conditions when the electrical resistance value of the recording material is the second recording material resistance value. The resistance value of the first recording material is lower than the resistance value of the second recording material. An image forming apparatus characterized by the following features.

3. The image forming apparatus according to claim 2, characterized in that the input unit inputs the information to the control unit in response to the operator's operation.

4. The image forming apparatus according to claim 3, characterized in that the information includes the type of recording material, an operating setting provided in accordance with a predetermined type of recording material, or an index value that correlates with the electrical resistance value of the recording material.

5. The image forming apparatus according to claim 2, characterized in that the input unit is a sensor that inputs the detection result of an index value correlated with the electrical resistance value of the recording material to the control unit as the information.

6. The image forming apparatus according to any one of claims 1 to 5, characterized in that the absolute value of the limit voltage is less than the absolute value of the output limit voltage determined from the characteristics of the high-voltage element of the power supply.

7. The image forming apparatus according to any one of claims 1 to 6, characterized in that the control unit sets the limit voltage to a first limit voltage when the electrical resistance value of the transfer unit indicated by the detection result of the detection unit is a first resistance value, and sets the limit voltage to a second limit voltage with an absolute value smaller than the first limit voltage when the electrical resistance value is a second resistance value lower than the first resistance value.

8. The control unit sets the limit voltage to a first limit voltage when the electrical resistance value of the transfer unit indicated by the detection result of the detection unit is a first resistance value, to a second limit voltage with an absolute value smaller than the first limit voltage when the electrical resistance value is a second resistance value that is lower by a predetermined value than the first resistance value, to a third limit voltage with an absolute value smaller than the second limit voltage when the electrical resistance value is a third resistance value that is lower than the second resistance value, to a fourth limit voltage with an absolute value smaller than the third limit voltage when the electrical resistance value is a fourth resistance value that is lower by a predetermined value than the third resistance value, and sets the limit voltage to be greater than the difference between the third limit voltage and the fourth limit voltage than the difference between the first limit voltage and the second limit voltage, as described in any one of claims 1 to 6.

9. The image forming apparatus according to any one of claims 1 to 8, characterized in that the limit voltage is set to limit the voltage applied from the power supply to the transfer member that is divided and transmitted to the contact member via the recording material.

10. The image forming apparatus according to any one of claims 1 to 9, characterized in that the control unit sets a lower limit voltage indicating the lower limit of the absolute value of the voltage applied from the power supply to the transfer member during transfer, based on the detection result of the detection unit when there is no recording material in the transfer unit, and controls the power supply so that the voltage applied from the power supply to the transfer member while the recording material is in contact with the transfer member and the contact member simultaneously is the lower limit voltage and the limit voltage, whichever has the smaller absolute value.

11. It has an environmental detection unit that detects environmental information relating to at least one of the temperature or humidity of the environment. The image forming apparatus according to claim 10, characterized in that the control unit sets the lower limit voltage to a first lower limit voltage when the absolute amount of moisture obtained based on the environmental information is a first amount of moisture, and sets the lower limit voltage to a second lower limit voltage with an absolute value greater than the first lower limit voltage when the absolute amount of moisture is a second amount of moisture which is greater than the first amount of moisture.

12. The image forming apparatus according to claim 10, characterized in that the control unit sets the lower limit voltage to a first lower limit voltage when the width in a direction substantially perpendicular to the transport direction of the recording material is a first width, and sets the lower limit voltage to a second lower limit voltage with an absolute value greater than the first lower limit voltage when the width is a second width wider than the first width.

13. It has an environmental detection unit that detects environmental information relating to at least one of the temperature or humidity of the environment. The image forming apparatus according to claim 10, characterized in that the control unit sets the lower limit voltage and the limit voltage such that the difference between the lower limit voltage and the limit voltage when the absolute moisture content obtained based on the environmental information is a first moisture content becomes the first difference, and the difference between the lower limit voltage and the limit voltage when the absolute moisture content is a second moisture content greater than the first moisture content becomes the second difference which is smaller than the first difference.

14. The image forming apparatus according to any one of claims 10 to 13, characterized in that the control unit sets the lower limit voltage based on the detection result of the detection unit when there is no recording material in the transfer unit and information regarding the amount of toner used for the toner image.

15. The image forming apparatus according to any one of claims 1 to 14, characterized in that the contact member has a fixing member for fixing the toner image to the recording material.

16. The image forming apparatus according to claim 15, characterized in that the fixing member is electrically grounded via a resistive element.

17. The image forming apparatus according to claim 15 or 16, characterized in that the fixing member has an electrically insulating surface layer that can come into contact with the recording material.

18. The image forming apparatus according to any one of claims 1 to 17, characterized in that the image carrier is an intermediate transfer body that transports a toner image, which has been primary transferred from another image carrier, to the recording material in the transfer unit for secondary transfer.

19. The image forming apparatus according to claim 18, characterized in that the intermediate transfer body is composed of an endless belt, and the belt is capable of carrying an electric current in the circumferential direction.

20. The volume resistivity of the aforementioned belt is 1 × 10 ９ Ω・cm or more, 1×10 １０ The image forming apparatus according to claim 19, characterized in that it is Ω·cm or less.

Images