JP7512081B2 - Image forming device - Google Patents

Description

The present invention relates to image forming devices such as copiers, printers, and facsimile machines that use electrophotographic or electrostatic recording methods.

Conventionally, in image forming devices such as electrophotographic copying machines and page printers, intermediate transfer methods have been widely adopted to accommodate high functionality such as colorization and high speed. In this image forming device, a toner image formed on a photosensitive member is primarily transferred to an intermediate transfer member in a primary transfer section, and this toner image is secondarily transferred from the intermediate transfer member to a recording material such as recording paper in a secondary transfer section. Primary and secondary transfers are performed using electrostatic forces. An intermediate transfer belt, which is an endless belt stretched over multiple tension rollers, is widely used as the intermediate transfer member. Secondary transfer is often performed by applying a secondary transfer voltage to a secondary transfer member that contacts one of the multiple tension rollers via the intermediate transfer belt. Below, an electrophotographic image forming device equipped with an intermediate transfer belt as the intermediate transfer member will be mainly described as an example.

The secondary transfer voltage is generally controlled by constant voltage or constant current. When the secondary transfer voltage is controlled by constant voltage, a current corresponding to the constant voltage can be passed through the area on the recording material where the toner image is present, regardless of the current flowing through the area outside the recording material in the secondary transfer section or the area on the recording material where there is no toner image. However, the secondary transfer voltage that can supply an appropriate secondary transfer current changes depending on the variation in the electrical resistance (hereinafter also simply referred to as "resistance") of the secondary transfer member and the recording material. The resistance of the secondary transfer member varies depending on the variation between products, the environment (at least one of temperature and humidity), and the increase in the amount of use. The resistance of the recording material varies depending on the type of recording material and the environment. Therefore, when the secondary transfer voltage is controlled by constant voltage, it is necessary to detect the resistance of the secondary transfer member before or during continuous image formation, and to control the secondary transfer voltage by adding the recording material share voltage corresponding to the type of recording material to the voltage corresponding to the resistance of the secondary transfer member. Therefore, the productivity of continuous image formation may decrease due to such control, or the image density may change stepwise before and after such control.

On the other hand, when the secondary transfer voltage is controlled by constant current, for example, during continuous image formation, the secondary transfer current supplied to each of the multiple recording materials is adjusted in real time during continuous image formation according to the resistance of the secondary transfer member and the recording material. Therefore, it is possible to suppress a decrease in productivity of continuous image formation. In addition, it is possible to suppress a gradual change in image density between multiple recording materials. In addition, it is easy to deal with variations in parts, usage conditions of the image forming apparatus (such as the cumulative number of images formed), the type of recording material, and moisture absorption state. However, when the secondary transfer voltage is controlled by constant current, current flows selectively to areas with low resistance, such as the outer area of the recording material in the secondary transfer section and areas on the recording material where there is no toner image. Therefore, there may be a shortage of current flowing through the areas on the recording material where there is a toner image, causing transfer defects (hereinafter also referred to as "transfer noise") in which the toner image is not transferred sufficiently to the recording material and the image is missing.

Patent document 1 proposes a method in which a lower limit (lower limit voltage) is set for the secondary transfer voltage, and if the voltage detected when there is recording material in the secondary transfer section falls below the lower limit voltage, the secondary transfer voltage is controlled at a constant voltage of the lower limit voltage. In this method, the lower limit voltage is set based on the voltage detected when there is no recording material in the secondary transfer section (resistance of the secondary transfer member) and the type of recording material.

Patent Publication No. 5361435

However, even when the resistance of the secondary transfer member and the resistance of the recording material are the same, the secondary transfer voltage at which good image quality can be obtained may vary greatly depending on the resistance of the intermediate transfer belt. Specifically, when the resistance of the intermediate transfer belt is low due to a high temperature and high humidity environment or the usage conditions of the image forming apparatus (such as the cumulative number of images formed), the current may concentrate more in areas where there is no toner image, and a larger secondary transfer voltage may be required to transfer the toner image sufficiently. In other words, depending on the influence of the resistance of the intermediate transfer belt, the set secondary transfer voltage may be insufficient, resulting in poor transfer.

Therefore, the present invention aims to prevent transfer failures in the secondary transfer section caused by electrical resistance in the intermediate transfer body.

The above objective is achieved by the image forming device of the present invention. In summary, the present invention is an image forming apparatus having an image carrier that carries a toner image, an intermediate transfer member to which the toner image is primarily transferred from the image carrier, a secondary transfer member that forms a secondary transfer section that secondarily transfers the toner image from the intermediate transfer member to a recording material, a conductive member that contacts the intermediate transfer member, a first power source that applies a voltage to the conductive member, a first detection means that detects at least one of the current or voltage of the first power source, a second power source that applies a voltage to the secondary transfer member, a second detection means that detects at least one of the current or voltage of the second power source, and a control means that controls the second power source so that the current becomes a predetermined current when the absolute value of the voltage detected by the second detection means is equal to or greater than a predetermined voltage when a recording material is present in the secondary transfer section, and controls the second power source so that the voltage becomes the predetermined voltage when the absolute value of the voltage detected by the second detection means is smaller than the absolute value of the predetermined voltage, and the control means is capable of setting the predetermined voltage based on the detection result of the first detection means.

According to the present invention, it is possible to suppress the occurrence of transfer defects in the secondary transfer section due to the electrical resistance of the intermediate transfer body.

FIG. 1 is a schematic cross-sectional view of an image forming apparatus. FIG. 2 is a schematic cross-sectional view of an image forming unit and a primary transfer unit. 6 is a graph showing the relationship between the tension distance and resistance of the intermediate transfer belt in the primary transfer section. FIG. 4 is a schematic diagram for explaining a secondary transfer current. 6 is a graph showing the relationship between the resistance of the intermediate transfer belt and the lower limit voltage. FIG. 2 is a schematic block diagram showing a control mode of a main part of the image forming apparatus according to the first embodiment. FIG. 4 is a flow chart of control according to the first embodiment. FIG. 11 is a graph showing a comparison of secondary transfer voltage settings between the conventional example and the first embodiment. FIG. 4 is a graph illustrating an example of a change in the characteristics of a photosensitive drum. FIG. 11 is a schematic block diagram showing a control mode of a main part of an image forming apparatus according to a second embodiment. FIG. 11 is a flow chart of control according to the second embodiment.

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

[Example 1]
1 is a schematic cross-sectional view of an image forming apparatus 100 according to the present embodiment. The image forming apparatus 100 according to the present embodiment is a tandem printer that employs an intermediate transfer method and is capable of forming full-color images using an electrophotographic method.

The image forming apparatus 100 has a plurality of image forming units (stations), image forming units 1a, 1b, 1c, and 1d, which form toner images of the colors yellow (Y), magenta (M), cyan (C), and black (K), respectively. Elements having the same or corresponding functions or configurations in each image forming unit 1a, 1b, 1c, and 1d may be generally described by omitting the suffixes a, b, c, and d indicating that the element is for one of the colors. In this embodiment, the image forming unit 1 is configured with a photosensitive drum 2, a charging roller 3, an exposure device 4, a developing device 5, a primary transfer roller 14, a drum cleaning device 6, and the like, which will be described later. Note that here, the magnitude of the voltage is expressed by the magnitude of the potential difference from 0 (i.e., the magnitude of the absolute value), regardless of the polarity. The polarity of the voltage changes depending on the normal charging polarity of the toner. In this embodiment, the normal charging polarity of the toner is negative, and the toner during development is mainly charged to the negative polarity.

The photosensitive drum 2, which is a drum-shaped (cylindrical) photosensitive body (electrophotographic photosensitive body) serving as an image carrier, is rotated at a predetermined peripheral speed Vdr (FIG. 2(a)) in the direction of the arrow R1 in FIG. 1. The surface of the rotating photosensitive drum 2 is uniformly charged to a predetermined dark potential (charge potential) VD by a charging roller 3, which is a roller-shaped charging member serving as a charging means. During charging, a predetermined charging voltage is applied to the charging roller 3 by a charging power source D3. The surface of the charged photosensitive drum 2 is scanned and exposed according to image information by an exposure device (laser scanner unit) 4 serving as an exposure means, and an electrostatic latent image (electrostatic image) corresponding to the image information of the color component corresponding to each image forming unit 1 is formed on the photosensitive drum 2. Here, the dark potential VD refers to the potential of the surface of the photosensitive drum 2 before it is charged by the charging roller 3 and exposed by the exposure device 4. The electrostatic latent image formed on the photosensitive drum 2 is developed (visualized) by the developing device 5 as a developing means, which supplies toner as a developer, and a toner image is formed on the photosensitive drum 2. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 2 (negative polarity in this embodiment) adheres to the exposed portion (image portion) of the photosensitive drum 2, which has been uniformly charged and then exposed to light to reduce the potential (reverse development).

An intermediate transfer belt 31, which is an endless belt serving as an intermediate transfer body, is arranged to face the four photosensitive drums 2a, 2b, 2c, and 2d. The intermediate transfer belt 31 is stretched across a tension roller 32 and a secondary transfer opposing roller 34, which serve as multiple tension rollers (support members). The tension roller 32 applies a predetermined tension to the intermediate transfer belt 31. The secondary transfer opposing roller (inner roller) 34 functions as an opposing member (opposing electrode) of the secondary transfer roller 25 described later, and also functions as a drive roller that drives the intermediate transfer belt 31. Primary transfer rollers 14a, 14b, 14c, and 14d, which are roller-shaped primary transfer members serving as primary transfer means, are arranged on the inner peripheral surface side of the intermediate transfer belt 31 in correspondence with each of the photosensitive drums 2a, 2b, 2c, and 2d. The primary transfer roller 14 presses the intermediate transfer belt 31 toward the photosensitive drum 2, forming a primary transfer section T1 where the photosensitive drum 2 and the intermediate transfer belt 31 come into contact with each other. The intermediate transfer belt 31 rotates (circulates) at a predetermined peripheral speed Vb (FIG. 2(a)) in the direction of the arrow R2 in FIG. 1 by the rotation of the secondary transfer opposing roller 34, which also serves as a drive roller. The primary transfer roller 14 contacts the intermediate transfer belt 31 and rotates following the rotation of the intermediate transfer belt 31. The peripheral speed Vb of the intermediate transfer belt 31 corresponds to the process speed of the image forming apparatus 100. The toner image formed on the photosensitive drum 2 is primarily transferred onto the rotating intermediate transfer belt 31 at the primary transfer portion T1 by the action of the primary transfer roller 14. During the primary transfer, a primary transfer voltage, which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the primary transfer roller 14 by the primary transfer power source D1. For example, when a full-color image is formed, the toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 2 are sequentially primarily transferred onto the intermediate transfer belt 31 so as to be superimposed on each other.

In this embodiment, the charging power source D3 and the primary transfer power source D1 are provided independently for each image forming unit 1, but at least one of the charging power source and the primary transfer power source may be shared by multiple image forming units.

On the outer peripheral surface side of the intermediate transfer belt 31, a secondary transfer roller (outer roller) 25, which is a roller-shaped secondary transfer member serving as a secondary transfer means, is disposed at a position facing the secondary transfer opposing roller 34. The secondary transfer roller 25 is pressed toward the secondary transfer opposing roller 34 and abuts against the secondary transfer opposing roller 34 via the intermediate transfer belt 31 to form a secondary transfer section T2 where the intermediate transfer belt 31 and the secondary transfer roller 25 come into contact. The toner image formed on the intermediate transfer belt 31 is secondarily transferred onto a recording material (transfer material, sheet) S, such as a recording paper, which is sandwiched and conveyed between the intermediate transfer belt 31 and the secondary transfer roller 25, by the action of the secondary transfer roller 25 at the secondary transfer section T2. During secondary transfer, a secondary transfer voltage, which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the secondary transfer roller 25 by the secondary transfer power source D2. The recording material S is stored in a cassette 11 serving as a recording material storage section. The recording material S is sent from the cassette 11 by a feed roller 12 and conveyed to the registration roller 13. The registration roller 13 synchronizes the timing of the recording material S with the toner image on the intermediate transfer belt 31 and supplies it to the secondary transfer section T2.

The recording material S onto which the toner image has been transferred is transported to a fixing device 15 as a fixing means. The fixing device 15 heats and pressurizes the recording material S carrying the unfixed toner image, thereby fixing (melting and bonding) the toner image onto the recording material S. The recording material S onto which the toner image has been fixed is discharged (output) onto a tray 16 provided outside the device body 110 of the image forming device 100.

Toner that remains on the photosensitive drum 2 without being transferred onto the intermediate transfer belt 31 during the primary transfer (primary transfer residual toner) is removed from the photosensitive drum 2 and collected by a drum cleaning device 6 as a photosensitive body cleaning means. In addition, toner that remains on the intermediate transfer belt 31 without being transferred onto the recording material S during the secondary transfer (secondary transfer residual toner) is removed from the intermediate transfer belt 31 and collected by a belt cleaning device 33 as an intermediate transfer body cleaning means.

In this embodiment, in each image forming unit 1, the photosensitive drum 2 and the charging roller 3, developing device 5, and drum cleaning device 6 acting on the photosensitive drum 2 as process means are integrated into a cartridge to form a process cartridge 7. The process cartridge 7 is detachable from the device body 110 of the image forming device 100. The process cartridge 7 is replaced with a new one, for example, when the toner contained in the developing device 5 runs out or when the photosensitive drum 2 has been used up to a predetermined amount.

In addition, in this embodiment, the image forming apparatus 100 is provided with an environmental sensor (temperature and humidity sensor) 50 that detects the temperature and humidity inside the apparatus main body 110. The environmental sensor 50 is an example of an environmental detection means for detecting at least one of the temperature and humidity inside or outside the image forming apparatus 100. The environmental sensor 50 outputs a signal indicating the detection result of the temperature and humidity inside the apparatus main body 110 to the control unit 150 described later.

2. Control mode FIG. 6 is a schematic block diagram showing the control mode of the main part of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 is provided with a control unit 150 as a control means equipped with an electric circuit for controlling the image forming apparatus 100. The control unit 150 is configured with a CPU 151 as an arithmetic control means which is a central element for performing arithmetic processing, a memory (storage medium) 152 such as a RAM or ROM as a storage means, and an input/output circuit (not shown) for controlling input/output of signals between the control unit 150 and each part. The RAM, which is a rewritable memory, stores information input to the control unit 150, detected information, arithmetic results, etc., and the ROM stores a control program (algorithm), a data table obtained in advance, etc. The CPU 151 and the memory 152 such as a RAM or ROM can transfer and read data to each other.

The control unit 150 is connected to each part of the image forming apparatus 100 as exemplified below. The control unit 150 is connected to a drive control circuit (not shown) such as a drive source (not shown) for conveying the recording material S, a drive source (not shown) for the intermediate transfer belt 31, and a drive source (not shown) for each image forming unit 1. The control unit 150 is also connected to high-voltage power supply circuits such as a primary transfer power supply D1, a secondary transfer power supply D2, and a charging power supply D3. The control unit 150 is also connected to a current detection unit (current detection circuit) and a voltage detection unit (voltage detection circuit) which may be included in the high-voltage power supply circuit. The control unit 150 is also connected to various sensors provided in the device main body 110, such as the environment sensor 50. The control unit 150 is also connected to an operation unit 140 provided in the device main body 110. The operation unit 140 has a display unit for displaying information to an operator such as a user or a service person under the control of the control unit 150, and an input unit for the operator to input information to the control unit 150. In addition, an external device 200 such as a personal computer is connected to the control unit 150. In addition, an image reading device (not shown) that is connected to or provided in the device main body 110 may be connected to the control unit 150. The control unit 150 controls the operation of each part of the image forming apparatus 100 and executes the image forming operation based on job information input from the operation unit 140 or the external device 200 and signals from various sensors.

The job information (image forming signal) includes image data and control commands for executing the job. In this embodiment, the job information includes information about the type of recording material S. The information about the type of recording material S includes any information that can distinguish the recording material S, such as attributes based on general characteristics such as plain paper, high-quality paper, glossy paper, coated paper, embossed paper, thick paper, thin paper, etc. (so-called paper type categories), numerical values or numerical ranges such as basis weight, thickness, and size, or brand (including manufacturer, product number, etc.). The information about the type of recording material S may be the information about the type of recording material S itself, or may be information corresponding to the type of recording material S, such as information about a print mode selected corresponding to the type of recording material S, which is included in the job information. Although not shown here, the control unit 150 may be configured to input job information via a printer controller that exchanges information between the image forming apparatus 100 and the external device 200 and processes image information received from the external device 200.

In this embodiment, the control unit 150, which functions as a type acquisition unit, acquires information regarding the type of recording material S based on information (signals) input from the operation unit 140 or the external device 200, but the present invention is not limited to this aspect. For example, the image forming apparatus 100 may be provided with a recording material type detection unit for detecting the type of recording material S, and the control unit 150 may acquire information regarding the type of recording material S based on the detection result of this recording material type detection unit. As the recording material type detection unit, an optical recording material type sensor (media sensor) that detects the thickness and surface roughness of the recording material S on the transport path of the recording material S can be used.

Here, the image forming apparatus 100 executes a job (print job) which is a series of operations for forming and outputting an image on a single or multiple recording materials S, which is started by one start instruction. A job generally has an image forming process, a pre-rotation process, a paper-interval process in the case of forming an image on multiple recording materials S, and a post-rotation process. The image forming process is a period during which the electrostatic image of the image to be actually formed on the recording material S is formed, the toner image is formed, and the primary and secondary transfer of the toner image is performed, and the image forming time refers to this period. More specifically, the timing during the image formation differs depending on the positions at which the electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer are performed, and corresponds to the period during which the image area on the photosensitive drum 2 or the intermediate transfer belt 31 passes through each of the above positions. The pre-rotation process is a period during which a preparatory operation is performed before the image forming process, from when a start instruction is input until the image actually starts to be formed. The paper-interval process is a period corresponding to the period between recording materials S when image formation is performed continuously on multiple recording materials S (continuous image formation). The post-rotation process is a period in which a sorting operation (preparatory operation) is performed after the image formation process. Non-image formation time is a period other than image formation time, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and further the pre-multiple rotation process, which is a preparatory operation when the image forming apparatus 100 is turned on or when it returns from a sleep state. More specifically, the timing of non-image formation time corresponds to a period in which the non-image area on the photosensitive drum 2 or the intermediate transfer belt 31 passes through each position where the above-mentioned electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer are performed. Note that the image area on the photosensitive drum 2 or the intermediate transfer belt 31 is an area in which an image that is transferred to the recording material S and output from the image forming apparatus 100 can be formed, and the non-image area is an area other than the image area.

3. Configuration of the primary transfer unit Fig. 2(a) is a schematic cross-sectional view (a cross-section substantially perpendicular to the rotation axis of the photosensitive drum 2) showing some elements of the image forming unit 1 in the image forming apparatus 100 of this embodiment. Fig. 2(b) is a schematic cross-sectional view (a cross-section substantially perpendicular to the rotation axis of the photosensitive drum 2) showing a further enlarged view of the vicinity of the primary transfer unit T1 shown in Fig. 2(a).

In this embodiment, the photosensitive drum 2 has an outer diameter of 24 mm. In this embodiment, the photosensitive drum 2 is configured by providing a conductive layer, an undercoat layer, a charge generating layer, a charge transport layer, and a protective layer as a surface layer, in this order, on the outer periphery of an aluminum drum-shaped support. In this embodiment, the protective layer constituting the outermost layer of the photosensitive drum 2 is formed using an acrylic resin. The thickness (film thickness) of the charge transport layer is preferably 5 to 40 μm. The thickness (film thickness) of the protective layer is preferably 0.5 to 20 μm.

In this embodiment, the intermediate transfer belt 31 is an endless belt (film) made of polyethylene terephthalate (PET) resin with a circumference of 800 mm and a thickness of 70 μm. In this embodiment, PET resin is used as the material of the intermediate transfer belt 31, but the material of the intermediate transfer belt 31 is not limited to this. For example, other thermoplastic resins such as the following may be used. For example, materials such as polyimide, PEEK, polycarbonate, polyarylate, acrylonitrile-butadiene-styrene copolymer (ABS), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF), polyethylene naphthalate (PEN), and mixed resins thereof. In this embodiment, the material of the intermediate transfer belt 31 contains a conductive agent. In this embodiment, an ionically conductive conductive agent is used as the conductive agent. Examples of ionically conductive conductive agents include polyvalent metal salts and quaternary ammonium salts. Examples of the cation portion of the quaternary ammonium salt include tetraethylammonium ion, tetrapropylammonium ion, tetraisopropylammonium ion, tetrabutylammonium ion, tetrapentylammonium ion, and tetrahexylammonium ion, while examples of the anion portion include halogen ions, fluoroalkyl sulfate ions with 1 to 10 carbon atoms in the fluoroalkyl group, fluoroalkyl sulfite ions, and fluoroalkyl borate ions. The intermediate transfer belt 31 can also be made mainly of polyether ester amide resin, with potassium perfluorobutanesulfonate or the like added to it.

The intermediate transfer belt 31 may have other layers, such as a protective layer on the surface of the base material (base layer) formed by the above-mentioned belt. In other words, the intermediate transfer belt 31 may have a layer formed of an ionically conductive material.

Here, compared to members using electronic conductive agents, members using ion conductive agents have the advantage of being able to suppress resistance fluctuations when used for a long period of time. On the other hand, members using ion conductive agents tend to have greater resistance fluctuations due to the environment (at least one of temperature and humidity) compared to members using electronic conductive agents. In this embodiment, as described above, an ion conductive intermediate transfer belt 31 is used as the intermediate transfer belt 31, and the resistance of the intermediate transfer belt 31 tends to increase in a low-temperature, low-humidity environment and decrease in a high-temperature, high-humidity environment. Members using electronic conductive agents have smaller resistance fluctuations due to the environment compared to members using ion conductive agents. However, even with an electronically conductive intermediate transfer belt 31, for example, when carbon black or the like is used as the electronically conductive conductive agent, the number of leak sites through which current is likely to flow locally increases with deterioration due to long-term use, and the resistance may decrease. In this case, image quality may also decrease in a high-temperature, high-humidity environment. Thus, in this embodiment, an ionically conductive intermediate transfer belt 31 is used, but the present invention can also be applied when an electronically conductive intermediate transfer belt 31 is used as the intermediate transfer belt 31, and the same effects as those of this embodiment described below can be obtained.

2A, in this embodiment, the primary transfer roller 14 is arranged offset downstream from the photosensitive drum 2 with respect to the rotation direction (surface movement direction, transport direction) of the intermediate transfer belt 31. In other words, the primary transfer roller 14 is arranged to push up the intermediate transfer belt 31 from the inner peripheral surface side to the outer peripheral surface side (photosensitive drum 2 side) to bend the intermediate transfer belt 31, and is brought into contact with the intermediate transfer belt 31. With respect to the rotation direction of the intermediate transfer belt 31, the photosensitive drum 2 and the intermediate transfer belt 31 contact each other at a first contact portion ND from the contact start position 38 to the contact end position 39. With respect to the rotation direction of the intermediate transfer belt 31, the primary transfer roller 14 and the intermediate transfer belt 31 contact each other at a second contact portion NT from the contact start position 40 to the contact end position 41. And the first contact portion ND and the second contact portion NT are shifted with respect to the rotation direction of the intermediate transfer belt 31. In particular, in this embodiment, the first contact portion ND and the second contact portion NT do not overlap with each other in the rotation direction of the intermediate transfer belt 31. Therefore, in the rotation direction of the intermediate transfer belt 31, the intermediate transfer belt 31 is stretched between the photosensitive drum 2 and the primary transfer roller 14 from the contact end position 39 between the photosensitive drum 2 and the intermediate transfer belt 31 to the contact start position 40 between the primary transfer roller 14 and the intermediate transfer belt 31. The distance between the contact end position 39 and the contact start position 40 in the rotation direction of the intermediate transfer belt 31 is defined as the "stretching distance Dk". In this embodiment, the stretching distance Dk is set to 3.0 mm.

In this embodiment, the primary transfer roller 14 is made of a metal roller. More specifically, in this embodiment, the primary transfer roller 14 includes a roller portion that can contact the intermediate transfer belt 31 and a rotating shaft portion provided at both ends of the roller portion with respect to the rotation axis direction of the primary transfer roller 14, and is entirely made of metal. In particular, in this embodiment, the primary transfer roller 14 is a stainless steel metal roller with an outer diameter of the roller portion of 5.0 mm. The material of the metal roller constituting the primary transfer roller 14 is not limited to stainless steel, and aluminum or iron with a plated surface can also be used. The advantage of using a metal roller without an elastic layer on the surface as the primary transfer roller 14 is that there is almost no variation in the resistance of the primary transfer roller 14 between products or with an increase in the amount of use. On the other hand, when using a metal roller without an elastic layer on the surface as the primary transfer roller 14, it is desirable to sufficiently secure the clearance Dc shown in FIG. 2(b). The clearance Dc is defined as the sum of the distance between the photosensitive drum 2 and the intermediate transfer belt 31, and the distance between the primary transfer roller 14 and the intermediate transfer belt 31 on the straight line 36 connecting the center of rotation of the photosensitive drum 2 and the center of rotation of the primary transfer roller 14 in the cross section shown in FIG. 2B. If the clearance Dc is too small, foreign matter may become evident on the image if it is mixed in. Generally, foreign matter is no larger than 0.1 mm (maximum diameter), and it is desirable to ensure the clearance Dc so that the influence of foreign matter on the image can be sufficiently suppressed, taking this into consideration. Although not limited to this, the clearance Dc is typically set to about 0.1 mm or more and 0.5 mm or less.

In addition, when the tension distance Dk is large, the resistance of the primary transfer portion T1 is increased, and as a result, the required primary transfer voltage is increased. Therefore, if the tension distance Dk is too large, the risk of image defects due to abnormal discharge may increase, especially in a low temperature and low humidity environment. The surface resistivity of the intermediate transfer belt 31 used in this embodiment in a normal temperature and normal humidity environment is approximately 1.0×10 ¹⁰ Ω/□. The relationship between the tension distance Dk and the resistance of the primary transfer portion T1 in a normal temperature and normal humidity environment of the intermediate transfer belt 31 used in this embodiment is as shown in FIG. 3. In FIG. 3, the horizontal axis indicates the tension distance Dk, and the vertical axis indicates the resistance value of the primary transfer portion T1 (for example, 1.0E+08 in the figure means 1.0×10 ^8. The same applies to the others). In the configuration of this embodiment, if the tension distance Dk exceeds 10 mm, the primary transfer voltage exceeds 3.0 kV even in a normal temperature and normal humidity environment, and abnormal discharge is likely to occur. Therefore, in the configuration of this embodiment, it is desirable to set the tension distance Dk to 10 mm or less (greater than 0 mm). In this embodiment, the tension distance Dk is set to 3.0 mm and the clearance Dc is set to 0.36 mm so that the tension distance Dk is the shortest possible within a range in which the clearance Dc can be ensured even when component tolerances are taken into consideration.

The electrical characteristics of the intermediate transfer belt 31 are measured, for example, using a Hiresta-UP MCP-HT450 (manufactured by Mitsubishi Chemical Corporation) at an applied voltage of 250 V in an environment with a temperature of 23°C and a relative humidity of 50%.

In this embodiment, as shown in FIG. 6, the primary transfer power source D1 has a voltage output unit 121 and an output control unit 122 that controls the output of the voltage output 121. The output control unit 122 has a current detection unit (current detection circuit) 123 as a current detection means and a voltage detection unit (voltage detection circuit) 124 as a voltage detection means. The current detection unit 123 detects the value of the current flowing through the primary transfer roller 14 (primary transfer unit T1, primary transfer power source D1) when the primary transfer power source D1 applies a voltage to the primary transfer roller 14 (primary transfer unit T1). The voltage detection unit 124 detects the value of the voltage output by the primary transfer power source D1. The voltage detection unit may detect the voltage value based on the set value of the constant voltage output of the voltage output unit 121. In this embodiment, the primary transfer power source D1 can apply a constant current controlled voltage to the primary transfer roller 14 (primary transfer unit T1). That is, the output control unit 122 adjusts the output of the voltage output unit 121 so that the value of the current detected by the current detection unit 123 becomes substantially constant (approaching the target current value), thereby performing constant current control of the output of the primary transfer power source D1. In addition, in this embodiment, the primary transfer power source D1 can apply a constant voltage controlled voltage to the primary transfer roller 14 (primary transfer unit T1). That is, the output control unit 122 adjusts the output of the voltage output unit 121 so that the value of the voltage detected by the voltage detection unit 124 becomes substantially constant (approaching the target voltage value), thereby performing constant voltage control of the output of the primary transfer power source D1. In addition, the current detection unit 123 and the voltage detection unit 124 are capable of outputting signals indicating the detection results to the control unit 150.

4. Configuration of the Secondary Transfer Section In this embodiment, the secondary transfer roller 25 is pressed against the intermediate transfer belt 31, the inner surface of which is supported by a secondary transfer opposing roller 34 connected to a ground potential, to form a secondary transfer section T2 between the intermediate transfer belt 31 and the secondary transfer roller 25. A positive DC voltage is applied from a secondary transfer power source D2 to the secondary transfer roller 25, to form a transfer electric field at the secondary transfer section T2. As a result, the negative toner image carried by the intermediate transfer belt 31 is secondarily transferred to the recording material S passing through the secondary transfer section T2.

In this embodiment, the secondary transfer opposing roller 34 also serves as a driving roller that rotates and drives the intermediate transfer belt 31 by frictional force. In this embodiment, the secondary transfer opposing roller 34 is configured by forming a conductive rubber layer of 1 mm thickness as an elastic layer on the outer periphery of a stainless steel pipe with an outer diameter of 18 mm as a core metal (core material), and the outer diameter is 20 mm. As the conductive rubber constituting the elastic layer, nitrile butadiene rubber, ethylene propylene diene rubber, urethane, etc. containing an ionically conductive conductive agent can be used. The elastic layer may contain an electronically conductive conductive agent. In this embodiment, the resistance of the secondary transfer opposing roller 34 is adjusted to 1×10 ⁵ Ω or less. Here, the resistance of the secondary transfer opposing roller 34 was measured as follows. The secondary transfer opposing roller 34 was pressed against a conductive cylindrical member by applying a pressure of 10 N. The secondary transfer opposing roller 34 was rotated by rotating this conductive cylindrical member. Then, while rotating the secondary transfer opposing roller 34, the current flowing when a voltage of 50 V was applied to the rotation shaft of the secondary transfer opposing roller 34 was measured, and the resistance was calculated from the voltage value and the current value. In this embodiment, the surface hardness of the secondary transfer opposing roller 34 is 70 degrees in Asker-C hardness (load: 4.9 N).

In this embodiment, the secondary transfer roller 25 is configured by forming a conductive rubber sponge layer of 5 mm thickness as an elastic layer on the outer periphery of a stainless steel roller shaft having an outer diameter of 6 mm as a core metal (core material), and the outer diameter is 16 mm. As the conductive rubber sponge constituting the elastic layer, styrene butadiene rubber, nitrile butadiene rubber, ethylene propylene diene rubber, urethane, etc. containing an ionically conductive conductive agent can be used. The elastic layer may contain an electronically conductive conductive agent. In this embodiment, the resistance of the secondary transfer roller 25 is adjusted to 1×10 ⁷ to 1×10 ⁸ Ω. Here, the resistance of the secondary transfer roller 25 was measured as follows. The secondary transfer roller 25 was pressed against a conductive cylindrical member by applying a pressure of 10 N. The secondary transfer roller 25 was rotated by rotating this conductive cylindrical member. Then, while rotating the secondary transfer roller 25, the current flowing when a voltage of 1 kV was applied to the rotating shaft of the secondary transfer roller 25 was measured, and the resistance was calculated from the voltage value and the current value. In this embodiment, the surface hardness of the secondary transfer roller 25 is 35 degrees in Asker-C hardness (load: 4.9 N).

In this embodiment, as shown in FIG. 6, the secondary transfer power source D2 has a voltage output unit 131 and an output control unit 132 that controls the output of the voltage output 131. The output control unit 132 has a current detection unit (current detection circuit) 133 as a current detection means and a voltage detection unit (voltage detection circuit) 134 as a voltage detection means. The current detection unit 133 detects the value of the current flowing through the secondary transfer roller 25 (secondary transfer unit T2, secondary transfer power source D2) when the secondary transfer power source D2 applies a voltage to the secondary transfer roller 25 (secondary transfer unit T2). The voltage detection unit 134 detects the value of the voltage output by the secondary transfer power source D2. The voltage detection unit may detect the voltage value based on the set value of the constant voltage output of the voltage output unit 131. In this embodiment, the secondary transfer power source D2 can apply a constant current controlled voltage to the secondary transfer roller 25 (secondary transfer unit T2). That is, the output control unit 132 adjusts the output of the voltage output unit 131 so that the value of the current detected by the current detection unit 133 becomes substantially constant (approaching the target current value), thereby performing constant current control of the output of the secondary transfer power source D2. In addition, in this embodiment, the secondary transfer power source D2 can apply a constant voltage controlled voltage to the secondary transfer roller 25 (secondary transfer unit T2). That is, the output control unit 132 adjusts the output of the voltage output unit 131 so that the value of the voltage detected by the voltage detection unit 134 becomes substantially constant (approaching the target voltage value), thereby performing constant voltage control of the output of the secondary transfer power source D2. In addition, the current detection unit 133 and the voltage detection unit 134 are capable of outputting signals indicating the detection results to the control unit 150.

In addition, a roller (inner roller) corresponding to the secondary transfer opposing roller 34 in this embodiment may be used as a secondary transfer member, and a secondary transfer voltage of the same polarity as the normal charging polarity of the toner may be applied to this roller during secondary transfer. In this case, a roller (outer roller) corresponding to the secondary transfer roller 25 in this embodiment may be used as an opposing member (opposing electrode), and this roller may be electrically grounded.

5. Overview of Secondary Transfer Voltage Control The resistance (electrical characteristics) of the secondary transfer roller 25 changes significantly due to variations in each product during manufacturing, changes in the environment (at least one of temperature and humidity), and increased usage. The resistance (electrical characteristics) of the recording material S also changes significantly due to moisture absorption and drying of the recording material S. Therefore, in order to keep the secondary transfer current flowing through the recording material S constant during image formation, it is desirable to control the secondary transfer voltage applied to the secondary transfer roller 25 at a constant current. Therefore, in this embodiment, the secondary transfer voltage is controlled at a constant current so that the current detected when the recording material S is present at the secondary transfer portion T2 is approximately constant.

In this embodiment, the secondary transfer power supply D2 can output a variable constant voltage. Also, in this embodiment, the secondary transfer power supply D2 can detect the current flowing through the secondary transfer roller 25. Then, in this embodiment, the secondary transfer power supply D2 can adjust the voltage applied to the secondary transfer roller 25 so that the detected current becomes the target current required for transferring the toner image. With this configuration, a mixture of constant voltage control and constant current control is possible, and the set values of the constant voltage and constant current can be changed as desired.

Figures 4(a) and (b) are schematic diagrams showing the current at the secondary transfer section T2 as viewed from the upstream side in the transport direction of the recording material S. Figure 4(a) shows the current in a normal temperature and normal humidity environment, and Figure 4(b) shows the current in a high temperature and high humidity environment. The thickness of the white arrows in Figures 4(a) and (b) represents the magnitude of the current flowing through each part of the secondary transfer section T2 in a direction approximately perpendicular to the transport direction of the recording material S. Also, in Figures 4(a) and (b), the recording material S and intermediate transfer belt 31 are actually in contact in areas where there is no toner image Ti.

First, the current flowing through the secondary transfer portion T2 in the normal temperature and humidity environment shown in FIG. 4A will be described. When transferring the toner image Ti from the intermediate transfer belt 31 to the recording material S, it is necessary to ensure that the current flowing through the portion (image portion) on the recording material S where the toner image Ti is present is in a necessary and sufficient range. When transferring the toner image Ti to the recording material S, at the end of the secondary transfer portion T2 in the direction approximately perpendicular to the conveying direction of the recording material S, the recording material S is not present, and the intermediate transfer belt 31 and the secondary transfer roller 25 are in direct contact. The portion without the recording material S has a lower resistance than the portion where the recording material S is present. Therefore, the current density of the secondary transfer current is higher in the portion without the recording material S than in the portion where the recording material S is present. In addition, the portion without the toner image Ti on the recording material S (non-image portion) has a lower resistance than the portion where the toner image Ti is present on the recording material S. Therefore, the current density of the secondary transfer current is higher in the portion of the recording material S where there is no toner image Ti than in the portion of the recording material S where there is a toner image Ti. As a result, even in a normal temperature and humidity environment, the current density in the portion of the recording material S where there is a toner image Ti is lower than the average current density of the secondary transfer current flowing to the entire secondary transfer portion T2.

Next, referring to FIG. 4B, the current flowing through the secondary transfer portion T2 in a high temperature and high humidity environment will be described. In a high temperature and high humidity environment, the current density in the portion on the recording material S where the toner image Ti is present is even lower than in a normal temperature and humidity environment. This is because the recording material S absorbs moisture and has a smaller resistance, so the difference in resistance between the portion on the recording material S where the toner image Ti is present and the portion on the recording material S where the toner image Ti is not present is larger. Therefore, even if the difference in the secondary transfer current flowing between the portion on the recording material S where the toner image Ti is present and the portion on the recording material S where the toner image Ti is not present is small in a normal temperature and humidity environment and there is no problem with the image quality, in a high temperature and high humidity environment, the following occurs. In other words, the secondary transfer current is concentrated in the portion on the recording material S where the toner image Ti is not present and the portion on the recording material S where the toner image Ti is not present, and the proportion of the secondary transfer current flowing through the portion on the recording material S where the toner image Ti is present is reduced. As a result, in a high temperature and high humidity environment, the toner image Ti cannot be transferred sufficiently to the recording material S, and transfer failure due to a lack of secondary transfer current, i.e., "bumpy transfer" is likely to occur.

Therefore, in this embodiment, at least when the image forming apparatus 100 is used in a high-temperature, high-humidity environment, under conditions where the transferability of the toner image cannot be ensured by constant current control due to the difference in resistance depending on the presence or absence of a toner image, a lower limit value (lower limit voltage) Vlmt is set for the secondary transfer voltage. Then, when the secondary transfer voltage is under constant current control, if the secondary transfer voltage required to pass a predetermined secondary transfer current falls below the lower limit voltage Vlmt, the secondary transfer voltage is switched to constant voltage control in which the lower limit voltage Vlmt is applied as the secondary transfer voltage. In other words, the lower limit voltage Vlmt is set for the secondary transfer voltage, and if the voltage detected when the recording material S is present at the secondary transfer portion T2 falls below the lower limit voltage Vlmt, the secondary transfer voltage is constant voltage controlled at the lower limit voltage Vlmt. In this embodiment, the control unit 150 sets the lower limit voltage Vlmt, and the output control unit 132 of the secondary transfer power source D2 executes switching from constant current control to constant voltage control of the secondary transfer voltage based on the detection results of the current detection unit 133 and the voltage detection unit 134 under the control of the control unit 150.

Here, the following method is conventionally known for setting the lower limit voltage Vlmt. That is, the lower limit voltage Vlmt can be set based on information about the environment (at least one of temperature and humidity) and information about the type of recording material S. This is because the resistance of the recording material S when it absorbs moisture differs depending on the environment and the type of recording material S. In general, the higher the temperature and humidity of the environment (the higher the weight absolute humidity), the higher the set value of the lower limit voltage Vlmt. In general, the more likely the resistance of the recording material S used to decrease due to moisture absorption, the higher the set value of the lower limit voltage Vlmt.

The lower limit voltage Vlmt may also be set based on information about the resistance of the secondary transfer roller 25 or information about the usage of the secondary transfer roller 25. This is for the following reason. That is, the resistance of the secondary transfer roller 25 increases with an increase in the usage of the secondary transfer roller 25, and the shared voltage of the secondary transfer roller 25 at the secondary transfer portion T2 may increase with an increase in the usage of the secondary transfer roller 25. As a result, the shared voltage of the recording material S may decrease with an increase in the usage of the secondary transfer roller 25. Therefore, the lower limit voltage Vlmt may be set based on information about the resistance of the secondary transfer roller 25 or information about the usage of the secondary transfer roller 25 to maintain the shared voltage of the recording material S at the secondary transfer portion T2. In general, the higher the resistance of the secondary transfer roller 25, the higher the set value of the lower limit voltage Vlmt. In general, the more the usage of the secondary transfer roller 25, the higher the set value of the lower limit voltage Vlmt. The information on the resistance of the secondary transfer roller 25 can be information on any index that correlates with the resistance of the secondary transfer roller 25. Examples of this index include the value of the voltage generated when a predetermined current is supplied to the secondary transfer roller 25, the value of the current that flows when a predetermined voltage is applied to the secondary transfer roller 25, or the resistance value calculated from these current and voltage values. In addition, the information on the usage of the secondary transfer roller 25 can be information on any index that correlates with the usage of the secondary transfer roller 25 from the beginning of its use (when it is new), such as the number of rotations and rotation time of the secondary transfer roller 25.

The method for setting the lower limit voltage Vlmt according to the environment, the type of recording material S, or the resistance or usage of the secondary transfer roller 25 can be appropriately selected from available methods, such as the known method described in Patent Document 1.

6. Setting of the Lower Limit Voltage Vlmt in this Example Next, the setting of the lower limit value (lower limit voltage) Vlmt of the secondary transfer voltage in this example will be described.

<Challenges>
First, the problem will be described in more detail. As described above, it was found that when the lower limit voltage Vlmt is set by the conventional setting method, there are cases where the transfer noise cannot be sufficiently suppressed. This is because, even if the resistance of the secondary transfer roller 25 and the recording material S is the same, the occurrence of the transfer noise is also greatly affected by the resistance of the intermediate transfer belt 31.

Table 1 shows an example of the relationship between the occurrence of secondary color transfer roughness and single color transfer strong voids in a high temperature and high humidity environment and the secondary transfer voltage. The surface resistivity of the intermediate transfer belt 31 used in this embodiment is about 1.0×10 ¹⁰ Ω/□ in a normal temperature and normal humidity environment, but drops to a range of 6.0×10 ⁸ to 2.0×10 ⁹ Ω/□ in a high temperature and high humidity environment. Therefore, the occurrence of secondary color transfer roughness and single color transfer voids was investigated using intermediate transfer belts 31 having surface resistivities at the upper and lower limits of the resistance variation range in this high temperature and high humidity environment. Here, for each of the secondary color transfer roughness and single color strong voids, "◯" in Table 1 means that they do not occur, "△" means that they occur but at a relatively mild level and therefore are not problematic in practice, and "×" means that they occur at a level that can be recognized as an image defect. The secondary color transfer roughness is an image defect (transfer roughness) that occurs due to insufficient secondary transfer current when a solid image of two colors is secondarily transferred to the recording material S. The secondary color transfer blur is a phenomenon that is likely to occur due to the need to transfer two solid color images per unit area, and therefore insufficient secondary transfer current. On the other hand, the monochrome transfer strong drop is an image defect in which the polarity of a part of the toner in the toner image is reversed and not transferred due to an excessive secondary transfer voltage when a monochrome solid image is secondarily transferred to the recording material S, resulting in a drop in the density. The environment was a 30°C/80% RH environment, which is an example of a high temperature and high humidity environment. As the recording material S, Xerox Vitality Multipurpose Paper 75 g/ ^m2 paper that had been left in the above 30°C/80% RH environment for 96 hours was used.

From Table 1, it can be seen that within the range of variation in the resistance of the intermediate transfer belt 31 in a high temperature and high humidity environment, the relationship between the secondary transfer voltage and the occurrence of monochrome strong loss and secondary color transfer roughness is as follows. In other words, for an intermediate transfer belt 31 with high resistance, increasing the secondary transfer voltage to 800V will prevent both monochrome strong loss and secondary color transfer roughness from occurring. On the other hand, for an intermediate transfer belt 31 with low resistance, the secondary transfer voltage must be increased to at least 1400V in order for both monochrome strong loss and secondary color transfer roughness to reach an acceptable level. In addition, increasing the secondary transfer voltage to preferably 1600V will prevent both monochrome strong loss and secondary color transfer roughness from occurring.

In the configuration of this embodiment, the secondary transfer voltage applied by constant current control when performing secondary transfer of a toner image onto a recording material S that has absorbed moisture in a high temperature and high humidity environment is approximately 500 V. Therefore, in the configuration of this embodiment, it is desirable to transition the secondary transfer voltage to constant voltage control at the lower limit voltage Vlmt in any case within the range of variation in the resistance of the intermediate transfer belt 31 in a high temperature and high humidity environment. In this case, if the resistance of the intermediate transfer belt 31 in a high temperature and high humidity environment can be detected, the lower limit voltage Vlmt can be set (the setting of the lower limit voltage Vlmt can be changed) according to that resistance, thereby making it possible to more effectively suppress the occurrence of image defects (low transfer quality, strong dropout).

<Detection of resistance of intermediate transfer belt>
Next, detection of the resistance of the intermediate transfer belt 31 will be described. The resistance of the secondary transfer portion T2 is the combined resistance of the secondary transfer roller 25, the intermediate transfer belt 31, and the secondary transfer opposing roller 34. Of these members, the resistance of the secondary transfer opposing roller 34 is lower than the resistance of the other members. In addition, the intermediate transfer belt 31 is thinner than the rubber portion of the secondary transfer roller 25. For these reasons, the resistance of the secondary transfer portion T2 is greatly influenced by the resistance of the secondary transfer roller 25. Therefore, it is difficult to correctly estimate the resistance of the intermediate transfer belt 31 from the detection result of the resistance of the secondary transfer portion T2.

In this embodiment, information on the resistance of the primary transfer section T1 is obtained as information on the resistance of the intermediate transfer belt 31. That is, in this embodiment, a voltage is applied from the primary transfer power source D1 to the primary transfer roller 14, which functions as an electrical resistance detection member (conductive member). Then, a current generated by the potential difference between the dark potential VD of the photosensitive drum 2 and the potential of the primary transfer roller 14 is detected via the intermediate transfer belt 31. In this way, information on the resistance of the primary transfer section T1 is obtained as information on the resistance of the intermediate transfer belt 31.

In this embodiment, since a metal roller is used for the primary transfer roller 14, if the surface potential of the photosensitive drum 2 is controlled, it can be said that the resistance of the primary transfer portion T1 indicates the resistance of the intermediate transfer belt 31. In this embodiment, the outermost layer of the photosensitive drum 2 is a protective layer formed using acrylic resin, and its wear rate is 1 to 10 nm per 1000 rotations of the photosensitive drum 2. In this embodiment, the change in thickness (film thickness) of the outermost layer throughout the life range (usable period) of the photosensitive drum 2 is less than 1 μm. Also, in this embodiment, the film thickness dependency of the dark potential VD of the photosensitive drum 2 is about Δ8 to 10 V/μm. In other words, when compared under substantially the same charging processing conditions, the dark potential VD increases by 8 to 10 V for every 1 μm decrease in the thickness of the outermost layer of the photosensitive drum 2. Therefore, in this embodiment, if the photosensitive drum 2 is within its lifespan, the change in the dark potential VD of the photosensitive drum 2 is sufficiently small compared to the potential difference (approximately 1050 V) between the potential of the primary transfer roller 14 (approximately +500 V) and the dark potential VD (approximately -550 V). In this way, when the change in the surface potential of the photosensitive drum 2 is sufficiently small, it is relatively easy to accurately detect the resistance of the intermediate transfer belt 31 based on the detection result of the resistance of the primary transfer portion T1. In this embodiment, the detection result of the resistance of the primary transfer portion T1 is regarded as the detection result of the resistance of the intermediate transfer belt 31, but the present invention is not limited to such an embodiment. For example, a value obtained by performing a predetermined process, such as multiplying the detection result of the resistance of the primary transfer portion T1 by a predetermined coefficient, may be used as the detection result of the resistance of the intermediate transfer belt 31.

Here, the resistance of the intermediate transfer belt 31 is calculated from the current flowing through the primary transfer portion T1 and the voltage applied at that time. However, in terms of control, the voltage required to make the current flowing through the primary transfer portion T1 a constant value may be used as a substitute for the resistance of the intermediate transfer belt 31. In other words, the control unit 150 can perform control using information on any index that correlates with the resistance of the intermediate transfer belt 31. Examples of this index include the value of the voltage generated when a predetermined current is supplied, the value of the current flowing when a predetermined voltage is applied, or the resistance value calculated from these current values and voltage values. Here, these voltages, currents, and resistances may all be described as the resistance of the intermediate transfer belt 31. In particular, in this embodiment, the voltage required to make the current flowing through the primary transfer portion T1 a constant value is used as a substitute for the resistance of the intermediate transfer belt 31.

The timing for detecting the resistance of the intermediate transfer belt 31 may be any time as long as the rotation of each member and the voltage applied from each power source are stable and no toner image is present at the primary transfer portion T1. The above members include the photosensitive drum 2, the charging roller 3, the intermediate transfer belt 31, and the primary transfer roller 14. The above power sources include the charging power source D3 and the primary transfer power source D1. Typically, when setting the lower limit voltage Vlmt in the pre-rotation process or pre-multi-rotation process as a non-image formation time each time a job is executed, the resistance of the intermediate transfer belt 31 is detected as necessary, as described later. However, the present invention is not limited to such an embodiment, and for example, when setting the lower limit voltage Vlmt in the paper interval process or post-rotation process as a non-image formation time, the resistance of the intermediate transfer belt 31 may be detected as necessary, as described later. The process for setting the lower limit voltage Vlmt is not limited to being performed each time a job is executed. For example, it may be executed when the first job is executed after the image forming apparatus 100 is turned on, and thereafter, it may be executed in the pre-rotation process or the inter-sheet process of the job for every predetermined number of images formed.

The resistance of the intermediate transfer belt 31 can be detected based on the detection results of the resistance of at least one primary transfer unit T1 of the multiple image forming units 1. For example, the resistance of the intermediate transfer belt 31 may be detected based on the detection results of the resistance of all primary transfer units T1 of the multiple image forming units 1, or may be detected based on the detection results of the resistance of one primary transfer unit T1 of the multiple image forming units 1. When using the detection results of the resistance of multiple primary transfer units T1, a representative value such as the average value (which may be a maximum value, minimum value, median value, etc.) can be used.

In this embodiment, after job information is input, in the pre-rotation process immediately before image formation begins, the control unit 150 detects the resistance of the primary transfer unit T1d in the black image forming unit 1d as necessary when setting the lower limit voltage Vlmt, as described below. Then, before the first recording material S of the job reaches the secondary transfer unit T2, the control unit 150 can appropriately set the lower limit voltage Vlmt according to the resistance of the primary transfer unit T1 (intermediate transfer belt 31). This makes it possible to set the optimal lower limit voltage Vlmt for all recording materials S of the job.

<How to set the lower limit voltage Vlmt>
Next, a method for setting the lower limit voltage Vlmt in this embodiment will be described. In this embodiment, the control unit 150 substitutes the voltage required to make the current flowing through the primary transfer unit T1 constant as the resistance of the intermediate transfer belt 31, and can set the lower limit voltage Vlmt based on that voltage. In particular, in this embodiment, the control unit 150 can set the lower limit voltage Vlmt based on the detected voltage when a current of 30 μA flows through the primary transfer unit T1.

As described above, it is known that the lower limit voltage Vlmt is conventionally set based on information about the environment and information about the type of recording material S. In contrast, in this embodiment, the control unit 150 is able to change the setting of the lower limit voltage Vlmt in a high temperature and high humidity environment based on information about the resistance of the intermediate transfer belt 31 in addition to information about the environment and information about the type of recording material S. Specifically, as described below, in a high temperature and high humidity environment, the setting of the lower limit voltage Vlmt is made variable according to the resistance of the intermediate transfer belt 31 based on the relationship between the intermediate transfer belt 31 and the lower limit voltage Vlmt that is determined in advance according to the environment and the type of recording material S.

FIG. 5A is a graph for explaining the difference in the set value of the lower limit voltage Vlmt in a 30° C./80% RH environment as an example of a high temperature and high humidity environment between the conventional setting method and the setting method of this embodiment. The horizontal axis of FIG. 5A is the detection voltage (detection voltage when a current of 30 μA is applied to the primary transfer portion T1) used as a substitute for the resistance of the intermediate transfer belt 31, and the larger the value, the higher the resistance of the intermediate transfer belt 31. The vertical axis of FIG. 5A indicates the set value of the lower limit voltage Vlmt. The set value of the lower limit voltage Vlmt shown in FIG. 5A is an example in which Xerox Vitality Multipurpose Paper 75 g/m ² is used as the recording material S.

As shown in FIG. 5(a), in the conventional setting method, the lower limit voltage Vlmt was set to a constant value (950 V) regardless of the resistance of the intermediate transfer belt 31 in order to keep the transfer roughness and strong dropout within a certain range within the range of variation in the resistance of the intermediate transfer belt 31.

In contrast, in the setting method of this embodiment, as shown in FIG. 5(a), the higher the resistance of the intermediate transfer belt 31, the smaller the set value of the lower limit voltage Vlmt.

In FIG. 5A, the detected voltage value (resistance of intermediate transfer belt 31) where the line (dashed line) showing the set value of the lower limit voltage Vlmt by the conventional setting method intersects with the line (solid line) showing the set value of the lower limit voltage Vlmt by the setting method of this embodiment is defined as point P. With the set value of the lower limit voltage Vlmt by the conventional setting method, transfer noise is likely to occur in areas where the resistance of the intermediate transfer belt 31 is lower than point P. This is because the lower limit voltage Vlmt is too low, making the secondary transfer current insufficient. Also, with the set value of the lower limit voltage Vlmt by the conventional setting method, strong dropout is likely to occur in areas where the resistance of the intermediate transfer belt 31 is higher than point P. This is because the lower limit voltage Vlmt is too high, making the secondary transfer current excessive.

In contrast, the setting value of the lower limit voltage Vlmt according to the setting method of this embodiment is smaller the higher the resistance of the intermediate transfer belt 31 is (larger the lower the resistance of the intermediate transfer belt 31 is). Therefore, it is possible to suppress the occurrence of transfer defects in areas where the resistance of the intermediate transfer belt 31 is lower than the above-mentioned point P, and it is possible to suppress the occurrence of strong defects in areas where the resistance of the intermediate transfer belt 31 is higher than the above-mentioned point P.

In the configuration of this embodiment, in an environment where the weight absolute humidity is lower than the weight absolute humidity of 27°C/70% RH (normal temperature and humidity environment, low temperature and low humidity environment), it is not necessary to change (correct, adjust) the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31. As can be seen from the results of Table 1, in such an environment, the dependency of the occurrence of transfer roughness and strong dropout on the resistance of the intermediate transfer belt 31 is sufficiently low. Therefore, in such an environment, the lower limit voltage Vlmt can be set in the same way as the conventional setting method, for example, based on information about the environment and information about the type of recording material S (and also information about the resistance and usage amount of the secondary transfer roller 25). In other words, when these conditions are substantially the same, the lower limit voltage Vlmt can be set to a constant value regardless of the resistance of the intermediate transfer belt 31.

5B is a graph similar to FIG. 5A, showing the set values of the lower limit voltage Vlmt in the 27° C./70% RH environment, the 28° C./75% RH environment, and the 30° C./80% RH environment according to the setting method of this embodiment. The set values of the lower limit voltage Vlmt shown in FIG. 5B are an example of a case where Xerox Vitality Multipurpose Paper 75 g/m ² is used as the recording material S. In this embodiment, in an environment with a weight absolute humidity equal to or higher than 27° C./70% RH (high temperature and high humidity environment), the lower limit voltage Vlmt is set according to the resistance of the intermediate transfer belt 31, in accordance with the relationship shown in FIG. 5B. That is, in this embodiment, in the range of 27° C./70% RH (weight absolute humidity 15.9 g/kg (DA)) to 30° C./80% RH (weight absolute humidity 21.7 g/kg (DA)), the relationship between the resistance of the intermediate transfer belt 31 and the lower limit voltage Vlmt is as follows. In this range, in any environment, the lower the resistance of the intermediate transfer belt 31, the larger the set value of the lower limit voltage Vlmt is (the higher the resistance of the intermediate transfer belt 31, the smaller the set value of the lower limit voltage Vlmt is). Also, in this range, the higher the temperature and humidity environment (the higher the weight absolute humidity), the larger the change amount (slope) of the lower limit voltage Vlmt relative to the change amount of the resistance of the intermediate transfer belt 31 is. Here, the relationship between the resistance of the intermediate transfer belt 31 and the lower limit voltage Vlmt in the above-mentioned several exemplary environments as the high temperature and high humidity environment is shown, but similar relationships in many more environments can be obtained in advance. Further, although the relationship between the resistance of the intermediate transfer belt 31 and the lower limit voltage Vlmt when the above-described exemplary recording material S is used has been shown here, a similar relationship can be obtained in advance for each type of recording material S. Information indicating the relationship between the resistance of the intermediate transfer belt 31 and the lower limit voltage Vlmt according to the environment for each type of recording material S is stored in advance in the memory 152 as, for example, a data table.

The environmental boundary (threshold) for determining whether or not the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31 needs to be changed is not limited to the above example environment, and can be set appropriately depending on, for example, the configuration of the image forming device 100.

Depending on the type of recording material S, it may not be necessary to change (correct, adjust) the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31. For example, when a recording material S whose resistance is not easily reduced by moisture absorption is used, the dependency of the occurrence of transfer roughness and strong dropout on the resistance of the intermediate transfer belt 31 may be sufficiently low. When such a recording material S is used, it is not necessary to change the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31. Therefore, in this case, the lower limit voltage Vlmt can be set based on, for example, information about the environment and information about the type of recording material S (and also information about the resistance and usage amount of the secondary transfer roller 25) in the same manner as in the conventional setting method. In other words, when these conditions are substantially the same, the lower limit voltage Vlmt can be set to a constant value regardless of the resistance of the intermediate transfer belt 31.

The lower limit voltage can also be set differently between print modes with different numbers of image forming units 1 that form toner images, such as full-color mode and monochrome mode (e.g., black monochromatic mode). For example, the relationship between the intermediate transfer belt 31 and the lower limit voltage Vlmt according to the environment and type of recording material S as shown in FIG. 5 can be determined in advance for each of the full-color mode and monochrome mode. Typically, in the full-color mode, the amount of toner transferred to the recording material S at the secondary transfer unit N2 is greater than in the monochrome mode, so the lower limit voltage Vlmt in the full-color mode can be made greater than the lower limit voltage Vlmt in the monochrome mode.

In addition, in this embodiment, in order to increase the dynamic range of the detection voltage value, the current (detection current) flowing through the primary transfer portion T1 when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1) is set to 30 μA, which is larger than the primary transfer current during image formation. However, the present invention is not limited to such an embodiment, and the current flowing through the primary transfer portion T1 when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1) may be 10 to 20 μA, which is close to the primary transfer current during image formation. This current may also be changed depending on the print mode. For example, in the case of the plain paper mode, it is set to 30 μA, and in the case of the thick paper mode, which has a slower process speed than the plain paper mode, it can be set to 10 to 20 μA, which is smaller than the plain paper mode, in order to prevent the current flowing through the photosensitive drum 2 from becoming excessive. In addition, in this embodiment, the dark potential VD of the photosensitive drum 2 when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1) is set to be equal to (almost the same in this embodiment) the dark potential VD during image formation. However, the present invention is not limited to this embodiment, and the dark potential VD (dark potential for detection) of the photosensitive drum 2 when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1) may be made larger or smaller than when forming an image.

<Procedure for setting the lower limit voltage Vlmt>
Next, a procedure for setting the lower limit voltage Vlmt in this embodiment will be described. Fig. 7 is a flow chart showing an outline of the procedure. Here, a case where the control unit 150 executes a job based on job information input from the external device 200 will be described as an example.

When job information is input, the control unit 150 obtains information on the environmental temperature and humidity from the environmental sensor 50, and obtains information on the type of recording material S from the job information (S101). Next, the control unit 150 determines whether or not it is necessary to change the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31 (S102). As described above, for an environment with a weight absolute humidity lower than 27°C/70% RH, or for a recording material S (or a print mode corresponding to the recording material S) for which it is not necessary to change the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31, it is not necessary to change the setting of the lower limit voltage Vlmt. The control unit 150 determines whether or not it is necessary to change the setting of the lower limit voltage Vlmt based on such predetermined conditions.

When the control unit 150 determines in S102 that the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31 needs to be changed ("Yes"), it determines whether the photosensitive drum 2 is within its life span (S103). That is, in this embodiment, the control unit 150 stores information about the usage of the photosensitive drum 2 in the cartridge memory 71 (FIG. 6) as a storage means of the process cartridge 7 each time the photosensitive drum 2 is driven. Also, the memory 152 of the control unit 150 stores in advance threshold information for the information about the usage of the photosensitive drum 2 corresponding to the life span of the photosensitive drum 2. The control unit 150 acquires information about the current usage of the photosensitive drum 2 from the cartridge memory 71, and acquires threshold information from the memory 152. Then, the control unit 150 determines that the photosensitive drum 2 has reached its life span if the information about the current usage of the photosensitive drum 2 has reached the threshold, and determines that the photosensitive drum 2 is within its life span if the information about the current usage of the photosensitive drum 2 has not reached the threshold. The information regarding the usage of the photosensitive drum 2 can be any index information that correlates with the usage of the photosensitive drum 2 from the beginning of use (when it is new), such as the number of rotations of the photosensitive drum 2, the rotation time, and the amount of charging (the number of rotations and the rotation time of the photosensitive drum 2 that has been charged). Here, the information regarding the usage of the photosensitive drum 2 can be considered as information regarding the usage of the process cartridge 7, and the life range of the photosensitive drum 2 can be considered as the life range of the process cartridge 7.

If the control unit 150 determines in S103 that the photosensitive drum 2 is within its life span ("Yes"), it executes an operation of detecting the resistance of the intermediate transfer belt 31 (S104). That is, the control unit 150 applies a predetermined charging voltage to the charging roller 3 by the charging power source D3, and charges the surface of the photosensitive drum 2 to a predetermined dark potential VD. In addition, when the charged area on the photosensitive drum 2 passes through the primary transfer portion T1, the primary transfer power source D1 applies a voltage to the primary transfer roller 14 under constant current control. That is, the output of the primary transfer power source D1 is constant current controlled so that the current detected by the current detection unit 123 of the primary transfer power source D1 becomes a constant value (30 μA in this embodiment). Then, the control unit 150 obtains the voltage detected by the voltage detection unit 124 at that time as information on the resistance of the intermediate transfer belt 31 (primary transfer portion T1). As described above, in this embodiment, the detection of the resistance of the intermediate transfer belt 31 is performed in the black image forming unit 1d. Next, the control unit 150 determines the lower limit voltage Vlmt based on the resistance of the intermediate transfer belt 31 acquired in S104 (S105) using the relationship between the resistance of the intermediate transfer belt 31 and the lower limit voltage Vlmt corresponding to the environment and type of recording material S acquired in S102 (FIG. 5). As described above, information showing the relationship between the resistance of the intermediate transfer belt 31 and the lower limit voltage Vlmt according to the environment for each type of recording material S as shown in FIG. 5 is obtained in advance and stored in the memory 152 as a data table or the like.

On the other hand, if the control unit 150 determines in S102 that it is not necessary to change the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31 ("No"), it sets the lower limit voltage Vlmt in the same manner as the conventional setting method (S106). Also, if the control unit 150 determines in S103 that the photosensitive drum 2 is outside its life span ("No"), or if information regarding the usage amount of the photosensitive drum 2 cannot be obtained, it sets the lower limit voltage Vlmt in the same manner as the conventional setting method (S106). In this case, since the dark potential VD of the photosensitive drum 2 is not guaranteed, the detection accuracy of the resistance of the intermediate transfer belt 31 decreases, and there is a possibility that the setting of the lower limit voltage Vlmt will not be changed correctly. Note that in S106, the lower limit voltage Vlmt can be set to a preset constant value, or the lower limit voltage Vlmt can be not set.

An experiment was conducted to set the lower limit voltage Vlmt using the above-mentioned procedure, using two intermediate transfer belts 31 with different resistances and two secondary transfer rollers 25 with different resistances. As a result, it was confirmed that under conditions where the setting of the lower limit voltage Vlmt needs to be changed according to the resistance of the intermediate transfer belt 31, the lower limit voltage Vlmt is set according to the resistance of the intermediate transfer belt 31, not the resistance of the secondary transfer roller 25.

＜Effects＞
Next, the results of an experiment conducted to confirm the effect of this embodiment will be described. Here, five different levels of resistance of the intermediate transfer belt 31 were used, and the lower limit voltage Vlmt was set by the control of this embodiment in a 30° C./80% RH environment as an example of a high temperature and high humidity environment, and image formation was performed. As the recording material S, Xerox Vitality Multipurpose Paper 75 g/m ² paper that had been left in the above-mentioned 30° C./80% RH environment for 96 hours was used. Then, the occurrence of strong dropout on a black solid image (single color transfer strong dropout) and transfer roughness on a secondary color image (blue, red, or green) (secondary color transfer roughness) was examined. The results are shown in Table 2. The method of indicating the levels of strong dropout and transfer roughness in Table 2 is the same as that of the results shown in Table 1. The detection voltage in Table 2 is the detection voltage when a current of 30 μA is passed through the primary transfer portion T1. The lower limit voltage in Table 2 is the lower limit voltage Vlmt set based on the detection voltage.

As shown in Table 2, according to the control of this embodiment, neither transfer roughness nor strong dropout, which are in a trade-off relationship, occurred when any intermediate transfer belt 31 was used. Thus, according to the control of this embodiment, by setting the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31 in a high temperature and high humidity environment, it is possible to suppress the occurrence of both transfer roughness and strong dropout, even when using a recording material S whose resistance is easily reduced by moisture absorption.

Figure 8 is a graph showing a comparison of the secondary transfer voltage applied to the secondary transfer roller 25 during secondary transfer in the conventional example and this embodiment. Figure 8 shows an example of the relationship between the usage environment of the image forming apparatus 100 and the setting magnitude of the secondary transfer voltage, regardless of whether the secondary transfer voltage is constant voltage controlled or constant current controlled. Also, Figure 8 shows an example in which paper A and paper B are used as the recording material S. Paper A is a paper whose resistance is more likely to decrease due to moisture absorption than paper B. In Figure 8, the settings of the secondary transfer voltage in a high temperature and high humidity environment above 27°C/70% RH (weight absolute humidity 15.9 g/kg (DA)) for each of paper A and paper B are different between the conventional example and this embodiment (plots for low resistance belt and high resistance belt).

Because the resistance of the components and recording material S changes depending on the environment, the highest secondary transfer voltage is required in a low temperature, low humidity environment. As the weight absolute humidity increases toward 27°C/70% RH (weight absolute humidity 15.9 g/kg (DA)), the required secondary transfer voltage decreases. On the other hand, in a high temperature, high humidity environment of 30°C/80% RH (weight absolute humidity 21.7 g/kg (DA)), a secondary transfer voltage higher than that in the above 27°C/70% RH environment is often applied so that the toner image can be transferred even if the recording material S has absorbed moisture.

Here, in the conventional example, the secondary transfer voltage in a high-temperature, high-humidity environment is determined according to the environment and the recording material S. Therefore, depending on the recording material S, either rough transfer or strong dropout may occur. For example, in FIG. 8, when paper B is used, the difference in the optimal secondary transfer voltage in a high-temperature, high-humidity environment due to the difference in the resistance of the intermediate transfer belt 31 (the difference between the plot of the low-resistance belt of paper B and the plot of the high-resistance belt) is small. On the other hand, when paper A is used, the difference in the optimal secondary transfer voltage in a high-temperature, high-humidity environment due to the difference in the resistance of the intermediate transfer belt 31 (the difference between the plot of the resistance belt of paper A and the plot of the high-resistance belt) is large. However, in the conventional example, since the secondary transfer voltage is set according to the environment and the recording material S, it is difficult to set the optimal secondary transfer voltage that provides good image quality in a high-temperature, high-humidity environment when paper A is used. In contrast, in this embodiment, the control unit 150 can change the setting of the lower limit voltage Vlmt in a high-temperature, high-humidity environment based on information about the environment and information about the type of recording material S, as well as information about the resistance of the intermediate transfer belt 31. Specifically, in a high-temperature, high-humidity environment, the setting of the lower limit voltage Vlmt is varied according to the resistance of the intermediate transfer belt 31, based on the relationship between the intermediate transfer belt 31 and the lower limit voltage Vlmt that is determined in advance according to the environment and the type of recording material S. As a result, in this embodiment, when the resistance of the intermediate transfer belt 31 is high in a high-temperature, high-humidity environment, the secondary transfer voltage is set to be small, and when the resistance of the intermediate transfer belt 31 is low, the secondary transfer voltage is set to be large. Therefore, in this embodiment, it is possible to set an optimal secondary transfer voltage that provides good image quality in a high-temperature, high-humidity environment.

Thus, in this embodiment, the image forming apparatus 100 has a conductive member 14 in contact with the intermediate transfer body 31, a first power source D1 that applies a voltage to the conductive member 14, and a first detection means 124 that detects at least one of the current or voltage of the first power source D1. The image forming apparatus 100 also has a second power source D2 that applies a voltage to the secondary transfer member 25, and a second detection means 134 that detects at least one of the current or voltage of the second power source D2. The image forming apparatus 100 also has a control means 150 that controls the second power source D2 so that the current becomes a predetermined current when the absolute value of the voltage detected by the second detection means (secondary transfer voltage detection unit) 134 is equal to or greater than the absolute value of a predetermined voltage when the recording material S is present at the secondary transfer portion T2, and controls the second power source D2 so that the voltage becomes the predetermined voltage when the absolute value of the voltage detected by the second detection means 134 becomes smaller than the absolute value of the predetermined voltage. The control unit 150 can set the above-mentioned predetermined voltage based on the detection result of the first detection unit (primary transfer voltage detection unit) 124. In this embodiment, the control unit 150 sets the above-mentioned predetermined voltage so that the absolute value of the above-mentioned predetermined voltage when the electrical resistance of the intermediate transfer body 31 indicated by the detection result of the first detection unit 124 is a second electrical resistance smaller than the first electrical resistance is greater than the absolute value of the above-mentioned predetermined voltage when the electrical resistance is the first electrical resistance.

The image forming apparatus 100 may also have an environment detection means 50 that detects information about the environment. In this case, the control means 150 can set the above-mentioned predetermined voltage based on the detection result of the first detection means 124 when the weight absolute humidity indicated by the detection result of the environment detection means 50 is equal to or greater than a predetermined value. The image forming apparatus 100 may also have a type acquisition means (control unit 150 in this embodiment) that acquires information about the type of recording material S. In this case, the control means 150 can set the above-mentioned predetermined voltage based on the detection result of the first detection means 124 when the type of recording material S acquired by the type acquisition means 150 is a predetermined type. The image forming apparatus 100 may also have a storage means 152 that stores the relationship between the detection results of the first detection means 124 and the above-mentioned predetermined voltage, which is obtained for at least two different environments for each of at least two different types of recording material S. In this case, the control means 150 can set the above-mentioned predetermined voltage based on the detection result of the first detection means 124, using the above-mentioned relationship selected from the multiple above-mentioned relationships based on the detection result of the environment detection means 50 and the acquisition result of the type acquisition means 150.

In particular, in this embodiment, the conductive member 14 is a primary transfer member, and the control unit 150 sets the predetermined voltage based on the detection result of the first detection unit 124 detected when there is no toner image at the primary transfer portion T1. The image forming apparatus 100 also has a charging unit 3 that charges the surface of the image carrier 2, and a third power source D3 that applies a voltage to the charging unit 3. The control unit 150 sets the predetermined voltage based on the detection result of the first detection unit 124 detected when the surface of the image carrier 2 charged to a predetermined potential by the charging unit 3 passes through the primary transfer portion T1. In addition, in this embodiment, the primary transfer member 14 contacts the inner peripheral surface of the endless intermediate transfer body 31 that can move in a circular motion, and brings the intermediate transfer body 31 into contact with the image carrier 2, and the contact portion between the image carrier 2 and the intermediate transfer body 31 and the contact portion between the intermediate transfer body 31 and the primary transfer member 14 do not overlap with each other in the moving direction of the intermediate transfer body 31. In particular, in this embodiment, the primary transfer member 14 is made of a metal roller.

As described above, in this embodiment, in a high temperature and high humidity environment, the lower limit voltage Vlmt of the secondary transfer voltage is changed according to the detection result of the resistance of the intermediate transfer belt 31 in accordance with the installation environment of the image forming apparatus 100 and the recording material S used. As a result, the secondary transfer voltage in a high temperature and high humidity environment can be optimized to obtain good image quality. In other words, according to this embodiment, an appropriate secondary transfer voltage can be set in a situation where the current flowing through the part where the toner image is present is likely to be insufficient, regardless of the electrical resistance of the intermediate transfer belt 31. Therefore, according to this embodiment, the occurrence of transfer failure at the secondary transfer section N2 due to the electrical resistance of the intermediate transfer belt 31 can be suppressed. More specifically, according to this embodiment, in a high temperature and high humidity environment, an appropriate lower limit voltage Vlmt can be set for the recording material S whose resistance has decreased due to moisture absorption, according to the resistance of the intermediate transfer belt 31. As a result, according to this embodiment, good image quality can be obtained even in a high temperature and high humidity environment.

<Modification>
In the configuration of this embodiment, since the surface resistivity of the intermediate transfer belt 31 has a high correlation with the image quality, it is desirable to sufficiently secure the tension distance Dk of the primary transfer portion T1 and detect the resistance in the rotation direction of the intermediate transfer belt 31. When the variation in the resistance detection result is large, the tension distance Dk may be increased within a range in which the primary transfer voltage does not become too large, so that the resistance is detected as being large. On the other hand, when the resistance in the thickness direction of the intermediate transfer belt 31 (hereinafter also referred to as "volume resistance") is highly correlated with the image quality, the tension distance Dk may be reduced so that the influence of the volume resistance is increased in the detection of the resistance of the primary transfer portion T1. The first contact portion ND and the second contact portion NT may be overlapped in the rotation direction of the intermediate transfer belt 31. For example, if the primary transfer roller 14 is covered with a low-resistance rubber (a rubber layer constituting the elastic layer on the surface) having a thickness of about 1 to 2 mm, the tension distance Dk can be reduced because the influence of foreign matter on the image is not generated even if the clearance Dc is small. In this case, for example, by making the first contact portion ND and the second contact portion NT overlap in the rotation direction of the intermediate transfer belt 31, the influence of the volume resistance of the intermediate transfer belt 31 on the resistance of the primary transfer portion T1 can be increased. The volume resistivity of the intermediate transfer belt 31 in this embodiment is approximately 1.0×10 ¹⁰ Ω·cm. In this case, the volume resistivity of the rubber of the primary transfer roller 14 covered with rubber as described above is set to less than 1.0×10 ⁷ Ω·cm. This sufficiently reduces the influence of the resistance of the primary transfer roller 14, and the volume resistance of the intermediate transfer belt 31 can be accurately detected even if there is a difference in the thickness of the rubber as described above. As a result, the lower limit voltage Vlmt can be set according to the resistance of the intermediate transfer belt 31.

The image forming apparatus 100 may also have a means for changing the relative position (offset amount) of the primary transfer roller 14 with respect to the photosensitive drum 2 in the rotation direction of the intermediate transfer belt 31. For example, the image forming apparatus 100 may be provided with a moving mechanism for changing the relative position of the primary transfer roller 14 with respect to the photosensitive drum 2 in the rotation direction of the intermediate transfer belt 31 by moving the primary transfer roller 14. The control unit 150 can control the mechanism so that the relative position of the primary transfer roller 14 with respect to the photosensitive drum 2 in the rotation direction of the intermediate transfer belt 31 is different from that during image formation when detecting at least the resistance of the intermediate transfer belt 31 (primary transfer portion T1). For example, when it is desired to increase the tension distance Dk as described above, the primary transfer roller 14 can be moved so that the tension distance Dk is larger than that during image formation when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1). When it is desired to reduce the tension distance Dk as described above or to overlap the first contact portion ND and the second contact portion NT, the following can be done. In other words, when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1), the primary transfer roller 14 can be moved so that the tension distance Dk is smaller than during image formation, or so that the first contact portion ND and the second contact portion NT overlap.

As described above, even when an electronically conductive intermediate transfer belt 31 whose resistance has decreased due to deterioration caused by increased usage is used in a high humidity environment, the method of this embodiment makes it possible to set an optimal lower limit voltage Vlmt that provides good image quality according to the resistance of the intermediate transfer belt 31.

The configuration for acquiring information on the resistance of the intermediate transfer belt 31 is not limited to the configuration of this embodiment, and can be appropriately selected according to the configuration of the image forming apparatus 100, the characteristics of the members, and the image quality. In this embodiment, information on the resistance of the intermediate transfer belt 31 is acquired by detecting the resistance of the primary transfer portion T1, but the present invention is not limited to such an embodiment. A dedicated member may be brought into contact with the inner or outer peripheral surface of the intermediate transfer belt 31 as an electrical resistance detection member (conductive member). For example, an electrical resistance detection member that can be brought into contact with the inner or outer peripheral surface of the intermediate transfer belt 31 is disposed near an electrically grounded member such as the tension roller 32. Then, in the same manner as the detection at the primary transfer portion T1 in this embodiment, the resistance of the intermediate transfer belt 31 stretched between the electrical resistance detection member and the tension roller 32 can be detected. For example, a metal roller similar to the primary transfer roller 14 in this embodiment can be used as this electrical resistance detection member.

In the present embodiment, the secondary transfer voltage is controlled at a constant current, and a lower limit voltage is set in a high-temperature, high-humidity environment, and the control is switched to a constant voltage control as necessary. However, the present invention is not limited to such an embodiment. The technology described in this embodiment can be applied even when the secondary transfer voltage is controlled at a constant voltage. When the secondary transfer voltage is controlled at a constant voltage, the setting value of the secondary transfer voltage is made variable according to the resistance of the intermediate transfer belt 31 in a high-temperature, high-humidity environment in the same manner as the setting of the lower limit voltage Vlmt in this embodiment. As a result, the same effect as in this embodiment can be obtained even when the secondary transfer voltage is controlled at a constant voltage. In other words, the control unit 150 may control the second power source D2 so that the voltage becomes a predetermined voltage when the recording material S is present at the secondary transfer portion T2. In this case, the control unit 150 can also set the above-mentioned predetermined voltage based on the detection result of the first detection unit 124.

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

This embodiment differs from the first embodiment in that the process conditions for detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1) are adjusted so that the change in the characteristics of the photosensitive drum 2 can be detected and the resistance of the intermediate transfer belt 31 (primary transfer portion T1) can be detected with sufficient accuracy. In this embodiment, after the entire surface of the photosensitive drum 2 is exposed by the exposure device 4, the charging current Ic required for recharging to the dark potential VD is detected by the charging current detection means, and the change in the characteristics of the photosensitive drum 2 is detected based on the detection result. Note that exposing the entire surface of the photosensitive drum 2 means exposing at least the entire image forming area (area where a toner image can be formed) in the rotation axis direction of the photosensitive drum 2 (main scanning direction of the exposure device 4). Substantially the entire surface of the photosensitive drum 2 may be exposed.

In Example 1, a protective layer was provided on the outermost layer of the photosensitive drum 2 to improve the wear resistance of the photosensitive drum 2. In contrast, photosensitive drums 2 that do not have a protective layer on the outermost layer of the photosensitive drum 2 and have a charge transport layer formed from a material such as polycarbonate or arylate that is more easily worn than the protective layer of the photosensitive drum 2 used in Example 1 as the outermost layer are also commonly used.

Figure 9 is a graph showing an example of the change in characteristics between the initial use (when new) and after wear of a photosensitive drum 2, whose outermost layer is relatively susceptible to wear. In the example shown, when the outermost layer of the photosensitive drum 2 wears by 1 μm, the charging potential changes by about 10 V. In a photosensitive drum 2 whose outermost layer is made of a material that is susceptible to wear as described above, the outermost layer may wear by about 15 μm with use, and when the charging voltage is constant, the dark potential VD may change by about 150 V.

If the dark potential VD deviates from the expected value, the detection accuracy of the resistance of the primary transfer section T1 decreases, and the detection accuracy of the resistance of the intermediate transfer belt 31 decreases. Therefore, it is desirable to be able to accurately grasp the dark potential VD even if the photosensitive drum 2 is prone to wear.

In this embodiment, the outermost layer of the photosensitive drum 2 is a charge transport layer formed using arylate. The outermost layer of the photosensitive drum 2 may wear down by about 10 μm with use. Therefore, in this embodiment, the charging current Ic that recharges the exposed portion potential VL to the dark portion potential VD is detected by a charging current detection means, and the change in the characteristics of the photosensitive drum 2 due to the wear of the outermost layer is detected. In this embodiment, the exposure for obtaining the exposed portion potential VL is equivalent to printing a solid image, and the image width in the main scanning direction for detecting the charging current is the maximum image width of the image forming apparatus 100. In particular, in this embodiment, when detecting the charging current Ic, the charging voltage is −1050 V and the exposure amount is 0.30 μJ/cm ^2. In addition, in this embodiment, the dark portion potential VD at the beginning of use (when new) of the photosensitive drum 2 is −500 V.

In the photosensitive drum 2 used in this embodiment, the dark potential VD and the charging current Ic increase when the outermost layer is worn. According to an experiment, the change in the dark potential VD per 1 μm of wear was +10.1 V, and the change in the charging current was +2.2 μA. In addition, it was found that the relationship between the charging current Ic and the current dark potential VDn can be expressed by the following formula 1.
VDn=-4.7×Ic-332 (Equation 1)

In addition, when the dark potential of the photosensitive drum 2 at the beginning of use (when it is brand new) is VDi, the amount of change ΔVD in the dark potential VD is expressed by the following formula 2.
ΔVD=VDn−VDi (Equation 2)

In this embodiment, when detecting the resistance of at least the intermediate transfer belt 31 (primary transfer portion T1), a voltage obtained by subtracting ΔVD from a predetermined charging voltage during image formation (a constant value throughout the life of the photosensitive drum 2) is applied to the charging roller 3 as a charging voltage for resistance detection. This makes it possible to detect the resistance of the intermediate transfer belt 31 (primary transfer portion T1) with high accuracy. The information of the above formulas 1 and 2 is stored in memory 152.

Figure 10 is a schematic block diagram showing the control mode of the main parts of the image forming apparatus 100 of this embodiment. The control mode of the image forming apparatus 100 of this embodiment is similar to that of the image forming apparatus 100 of embodiment 1 shown in Figure 6, but in this embodiment, a charging current detection unit (charging current detection circuit) 160 is provided as a charging current detection means. The charging current detection unit 160 detects the current flowing through the charging roller 3 when the charging power source D3 applies a voltage to the charging roller 3, and can output a signal indicating the detection result to the control unit 150.

FIG. 11 is a flow chart showing an outline of the procedure for setting the lower limit voltage Vlmt in this embodiment. Here, an example is described in which the control unit 150 executes a job based on job information input from the external device 200. Also, a description of the same process as the procedure for setting the lower limit voltage Vlmt in the first embodiment shown in FIG. 7 will be omitted as appropriate.

The processing of S201 and S202 in FIG. 11 is similar to the processing of S101 and S102 in FIG. 7.

If the control unit 150 determines in S202 that the setting of the lower limit voltage Vlmt according to the resistance of the intermediate transfer belt 31 needs to be changed ("Yes"), it detects the charging current Ic (S203). That is, the control unit 150 exposes the entire surface of the photosensitive drum 2 by the exposure device 4, and charges the exposed area on the photosensitive drum 2 by applying a predetermined charging voltage (a constant value throughout the life of the photosensitive drum 2) from the charging power source D3 to the charging roller 3. Then, the control unit 150 acquires the charging current Ic detected by the charging current detection unit 160 while the charging process is being performed. That is, it acquires the charging current Ic when the fully exposed area on the photosensitive drum 2 passes through the charging position. Here, the charging position is the position where the charging process of the photosensitive drum 2 is performed by the charging roller 3. In this embodiment, the photosensitive drum 2 is charged by discharge occurring in at least one of the gaps between the charging roller 3 and the photosensitive drum 2 formed on the upstream side and downstream side of the contact portion between the charging roller 3 and the photosensitive drum 2 with respect to the rotation direction of the photosensitive drum 2. However, for simplicity, it may be considered that the charging process is performed at the contact portion between the charging roller 3 and the photosensitive drum 2. In this embodiment, the detection of this charging current is performed in the black image forming unit 1d that detects the resistance of the intermediate transfer belt 31. Next, the control unit 150 judges whether or not the difference between the charging current Ic detected this time and the charging current Ic detected at the photosensitive drum 2 at the beginning of use (when new) stored in the cartridge memory 71 exceeds a predetermined threshold value (5 μA in this embodiment) (S204). In this embodiment, when starting to use a new photosensitive drum 2 (process cartridge 7), the control unit 150 detects the charging current Ic in the same manner as above and stores it in the cartridge memory 71. The charging current Ic corresponding to the initial use of the photosensitive drum 2 (when it is new) is not limited to the one obtained immediately after the start of use of the photosensitive drum 2 (process cartridge 7), but may be the one obtained while the wear amount of the outermost layer of the photosensitive drum 2 is sufficiently small (for example, less than 1 μm).

If the control unit 150 determines in S204 that the difference exceeds the threshold value, it performs calculations based on the above-mentioned formulas 1 and 2, corrects (changes, adjusts) the setting of the charging voltage when detecting the resistance of the intermediate transfer belt 31 (primary transfer unit T1), and stores the result in the memory 152 (S205).

The processes of S206, S207, and S208 in FIG. 11 are the same as the processes of S104, S105, and S106 in FIG. 8, respectively. However, if the charging voltage is corrected in S205, the charging process of the photosensitive drum 2 is performed using the corrected charging voltage setting stored in memory 152 when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1) in S206.

An experiment was conducted to confirm the effect of this embodiment under the same conditions as the experiment described in Example 1. It was found that when the control of this embodiment was performed, the level of strong cut-outs and transfer defects was improved compared to when the control of this embodiment was not performed.

The effects of the control of Example 1 and the control of this Example were confirmed using an image forming apparatus 100 equipped with the photosensitive drum 2 of this Example, the outermost layer of which had worn down by about 10 μm due to use. When the control of Example 1 was used, the level of strong bleeding could be reduced to the aforementioned "△" level when using a photosensitive drum 2 whose outermost layer had worn down by about 10 μm due to use, compared to when a new photosensitive drum 2 was used. In contrast, when the control of this Example was used, the level of strong bleeding could be improved when using a photosensitive drum 2 whose outermost layer had worn down by about 10 μm due to use, compared to when the control of Example 1 was used.

In other words, in a photosensitive drum 2 whose outermost layer has worn down by about 10 μm, the dark potential VD changes by about -100 V compared to a new photosensitive drum 2. Therefore, in the control of Example 1, the detected voltage value as the detection result of the resistance of the intermediate transfer belt 31 (primary transfer portion T1) also shifts by about -100 V, and the resistance of the intermediate transfer belt 31 (primary transfer portion T1) is estimated lower than the actual resistance. For example, in an environment of 30°C/80% RH, the selected lower limit voltage Vlmt is about 160 V higher. As a result, as can be seen from the relationship between the lower limit voltage Vlmt and image defects described with reference to Table 1, this is disadvantageous for strong dropout.

In contrast, in this embodiment, by detecting the charging current Ic, it is possible to grasp and control the surface potential of the photosensitive drum 2 when detecting the resistance of the intermediate transfer belt 31 (primary transfer portion T1). As a result, in this embodiment, the resistance of the intermediate transfer belt 31 (primary transfer portion T1) can be detected with greater accuracy, and a more appropriate lower limit voltage Vlmt can be set. Therefore, according to this embodiment, even if the photosensitive drum 2 is worn, the occurrence of both transfer roughness and strong dropout can be suppressed.

In this way, in this embodiment, the image forming apparatus 100 has a charging means 3 that charges the surface of the image carrier 2, a third power source D3 that applies a voltage to the charging means 3, an exposure means 4 that exposes the surface of the image carrier 2, and a third detection means (charging current detection unit) 160 that detects the current of the third power source D3. The control means 150 sets the above-mentioned predetermined voltage based on the detection result of the first detection means 124 detected when the surface of the image carrier 2 charged to a predetermined potential by the charging means 3 passes through the primary transfer section T1. In this embodiment, the control means 150 can set the voltage that the third power source D3 applies to the charging means 3 to charge the surface of the image carrier 2 to the above-mentioned predetermined potential based on the detection result of the third detection means 160 when the surface of the image carrier 2 exposed by the exposure means 4 is being charged by the charging means 3.

In this embodiment, an example is described in which the resistance of the primary transfer portion T1 is detected after controlling the dark potential VD to a predetermined value, but the present invention is not limited to this aspect. It is not necessary to adjust the charging voltage to detect the resistance of the primary transfer portion T1. The resistance of the primary transfer portion T1 may be derived from the potential difference between the dark potential VD, which has changed in response to wear of the photosensitive drum 2, and the primary transfer portion T1 (primary transfer roller 14).

In addition, depending on the difference between the charging current Ic detected this time and the charging current Ic detected before this time (typically the previous time), it may be possible not to change the setting of the lower limit voltage Vlmt corresponding to the resistance of the intermediate transfer belt 31. This is because, depending on the difference, the state of the photosensitive drum 2 (process cartridge 7) may be outside the expected range, and the resistance of the intermediate transfer belt 31 may not be detected with sufficient accuracy. For example, if the difference exceeds a predetermined threshold, it is possible not to change the setting of the lower limit voltage Vlmt corresponding to the resistance of the intermediate transfer belt 31.

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

In the above embodiment, the intermediate transfer body is configured as an endless belt stretched over multiple tension rollers, but the present invention is not limited to this form. The intermediate transfer body may be, for example, a drum-shaped intermediate transfer body configured by stretching a sheet over a frame.

In addition, in the above embodiment, a roller-shaped member is exemplified as the electrical resistance detection member (conductive member), but the present invention is not limited to such an embodiment. The electrical resistance detection member (conductive member) may be, for example, a brush-shaped member or a sheet-shaped member.

Reference Signs List 1 Image forming unit 2 Photosensitive drum 3 Charging roller 4 Exposure device 5 Developing device 14 Primary transfer roller 25 Secondary transfer roller 31 Intermediate transfer belt 150 Control unit

Claims (10)

an image carrier that carries a toner image;
an intermediate transfer member onto which a toner image is primarily transferred from the image carrier;
a secondary transfer member forming a secondary transfer section for secondarily transferring a toner image from the intermediate transfer body to a recording material;
a conductive member in contact with the intermediate transfer body;
a first power source that applies a voltage to the conductive member;
a first detection means for detecting at least one of a current or a voltage of the first power supply;
a second power source that applies a voltage to the secondary transfer member;
a second detection means for detecting at least one of a current or a voltage of the second power source;
a control means for controlling the second power source so that a current becomes a predetermined current when an absolute value of a voltage detected by the second detection means is equal to or greater than an absolute value of a predetermined voltage when a recording material is present at the secondary transfer portion, and for controlling the second power source so that a voltage becomes the predetermined voltage when an absolute value of a voltage detected by the second detection means is smaller than an absolute value of the predetermined voltage;
having
The image forming apparatus according to claim 1, wherein the control means is capable of setting the predetermined voltage based on a detection result of the first detection means. 2. The image forming apparatus according to claim 1, wherein the control means sets the predetermined voltage so that the absolute value of the predetermined voltage when the electrical resistance of the intermediate transfer body indicated by the detection result of the first detection means is a second electrical resistance smaller than the first electrical resistance is greater than the absolute value of the predetermined voltage when the electrical resistance of the intermediate transfer body indicated by the detection result of the first detection means is a first electrical resistance. An environment detection means for detecting information related to the environment is provided,
3. The image forming apparatus according to claim 1, wherein the control means sets the predetermined voltage based on the detection result of the first detection means when the weight absolute humidity indicated by the detection result of the environment detection means is equal to or higher than a predetermined value. A type acquisition means for acquiring information regarding the type of recording material,
4. An image forming apparatus according to claim 1, wherein the control means sets the predetermined voltage based on the detection result of the first detection means when the type of recording material acquired by the type acquisition means is a predetermined type . An environment detection means for detecting information regarding an environment;
A type acquisition means for acquiring information regarding the type of recording material;
a storage means for storing a relationship between the detection results of the first detection means and the predetermined voltage, the relationship being obtained for each of at least two different environments for each of at least two different types of recording material;
having
The image forming apparatus according to any one of claims 1 to 4, characterized in that the control means sets the specified voltage based on the detection result of the first detection means, using a relationship selected from the multiple relationships based on the detection result of the environment detection means and the acquisition result of the type acquisition means. the conductive member is a primary transfer member that forms a primary transfer portion that primarily transfers a toner image from the image carrier to the intermediate transfer member,
6. The image forming apparatus according to claim 1, wherein the control unit sets the predetermined voltage based on a detection result of the first detection unit detected when there is no toner image at the primary transfer unit. A charging means for charging the surface of the image bearing member;
a third power source that applies a voltage to the charging means;
having
7. The image forming apparatus according to claim 6, wherein the control means sets the predetermined voltage based on a detection result of the first detection means detected when the surface of the image carrier, which has been charged to a predetermined potential by the charging means, passes through the primary transfer section. an exposure unit for exposing a surface of the image carrier;
a third detection means for detecting a current of the third power supply;
having
The image forming apparatus according to claim 7, characterized in that the control means is capable of setting a voltage to be applied by the third power source to the charging means in order to charge the surface of the image carrier to the predetermined potential based on the detection result of the third detection means when the surface of the image carrier exposed by the exposure means is being charged by the charging means . 9. The image forming apparatus according to claim 6, wherein the primary transfer member contacts an inner peripheral surface of the intermediate transfer body, which is endless and capable of circulating movement, to bring the intermediate transfer body into contact with the image carrier, and in the movement direction of the intermediate transfer body, a contact portion between the image carrier and the intermediate transfer body and a contact portion between the intermediate transfer body and the primary transfer member do not overlap. 10. The image forming apparatus according to claim 9 , wherein the primary transfer member is a metal roller.