JP2022190662A - Image forming apparatus - Google Patents

Abstract

To set an appropriate transfer voltage even in an environment in which prevention of patch void and prevention of strong void are difficult, such as a high temperature and high humidity environment.SOLUTION: An image forming apparatus 100 has an image carrier 10, a transfer member 20, an application unit 21, and a control unit 210. In performing constant voltage control on a transfer voltage so that a voltage that the application unit 21 applies to the transfer member 20 becomes substantially constant, the control unit 210 perform control to define the transfer voltage as a first voltage when the amount of toner used for a toner image is a first amount of toner, and defines the transfer voltage as a second voltage that has an absolute value smaller than the absolute value of the first voltage when the amount of toner is a second amount of toner larger than the first amount of toner.SELECTED DRAWING: Figure 1

Description

The present invention relates to an image forming apparatus such as a printer, a copier, a facsimile machine, etc. using an electrophotographic method or an electrostatic recording method.

In an image forming apparatus using an electrophotographic method or the like, a toner image formed on an image carrier is transferred onto a recording material such as paper passing through a transfer section formed between the image carrier and a transfer member. be done. In an intermediate transfer type image forming apparatus, a toner image formed on a photosensitive member or the like as a first image carrier is primarily transferred onto an intermediate transfer member as a second image carrier, and then transferred to an intermediate transfer member. The toner is secondarily transferred onto a recording material passing through a secondary transfer portion formed between the transfer member and the secondary transfer member.

A toner image is transferred from the image carrier to the recording material by applying a transfer voltage to the transfer member. In order to obtain a high-quality product (printed matter), it is important to set an appropriate transfer voltage.

Japanese Patent Application Laid-Open No. 2002-200000 discloses a configuration in which a transfer current is changed according to the printing rate and the number of pixels in order to obtain a uniform final image regardless of the image pattern or printing rate. In this configuration, the transfer current amount is increased as the printing ratio and the number of pixels are increased, that is, as the amount of toner transferred to the recording material increases, thereby suppressing defective transfer in high-quality printed images.

JP 2010-191088 A

However, if the transfer current is increased as the amount of toner transferred to the recording material increases, as in the configuration of Japanese Patent Application Laid-Open No. 2002-200014, for example, in a high-temperature and high-humidity environment, the following two types of image defects may occur. have a nature.

(1) When transferring a low-printing image including an isolated patch pattern, etc. Generally, in a high-temperature and high-humidity environment, the electrical resistance of a transfer member such as a transfer roller and a recording material decreases. Therefore, instead of a portion where high resistance (high electrical resistance) toner exists (hereinafter also referred to as a “toner portion (or patch portion)”), low resistance (low electrical resistance) toner is present. A transfer current is more likely to selectively flow through nonexistent portions (hereinafter also referred to as “white portions (or white background portions)”). Therefore, in order to transfer the isolated patch pattern satisfactorily, it is necessary to apply a large amount of transfer current. Here, the term "isolated patch pattern" refers to an image pattern in which clusters of high-performance toner images are interspersed within the width of the recording material (length in the width direction substantially perpendicular to the conveying direction). doing. However, in the configuration of Patent Document 1, the absolute value of the transfer voltage is reduced because it is determined that the amount of toner in the image including the isolated patch pattern is small. As a result, a sufficient transfer current cannot be supplied to the toner portion, which may cause an image defect (hereinafter also referred to as "patch blur") in which toner is not transferred onto the recording material.

(2) When transferring a high-quality print image including a solid image on the entire surface In this case, since the transfer current does not selectively flow to the white background portion, a sufficient transfer current is supplied to the toner portion. When the transfer voltage is set so as not to cause the above-mentioned patch blurring, that is, so that a sufficient transfer current can be supplied to the isolated patch pattern, the minimum current required for toner transfer or more is required for a solid image on the entire surface. of transfer current is supplied. Here, the “whole surface solid image (whole surface solid pattern)” means an image pattern in which a toner image of the highest density level exists in the entire image formable area in the width direction of the recording material. However, in the configuration of Patent Document 1, the absolute value of the transfer voltage is made larger than the transfer voltage required to transfer the "overall solid image (overall solid pattern)". As a result, an excessive transfer current is supplied to the toner portion, causing an image defect (hereinafter also referred to as "strong dropout") in which some toner is not transferred to the recording material due to, for example, reversal of charge polarity due to discharge. there's a possibility that.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to set an appropriate transfer voltage even in an environment such as a high-temperature and high-humidity environment in which it is difficult to simultaneously suppress patch blurring and suppressing strong dropout.

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention comprises an image carrier that carries a toner image, a transfer member that forms a transfer portion that transfers the toner image from the image carrier to a recording material, and a transfer voltage that is applied to the transfer member. and a control unit for controlling the application unit, wherein the control unit performs constant voltage control on the transfer voltage so that the voltage applied to the transfer member by the application unit is substantially constant. and setting the transfer voltage to the first voltage when the toner amount used for the toner image is the first toner amount, and when the toner amount is the second toner amount larger than the first toner amount. The image forming apparatus is characterized in that the transfer voltage is controlled to be a second voltage having an absolute value smaller than the absolute value of the first voltage.

According to the present invention, it is possible to set an appropriate transfer voltage even in an environment such as a high-temperature and high-humidity environment in which it is difficult to achieve both suppression of patch blurring and suppression of strong omission.

1 is a schematic cross-sectional view of an image forming apparatus; FIG. 3 is a block diagram showing a control mode of the image forming apparatus; FIG. 3 is a functional block diagram regarding calculation of toner amount; FIG. 5 is a graph diagram for explaining a method of determining a secondary transfer voltage in Example 1. FIG. FIG. 10 is a flow chart showing a control procedure of the secondary transfer voltage; It is a schematic diagram for explaining the effect of the embodiment. 5 is a timing chart showing control timing of the secondary transfer voltage; FIG. FIG. 3 is a schematic cross-sectional view of another example of an image forming apparatus; FIG. 11 is a graph diagram for explaining a method of determining a secondary transfer voltage in Example 2; FIG. 11 is a graph diagram for explaining a method of determining a secondary transfer voltage in another configuration of Example 2; FIG. 11 is a graph diagram for explaining a method of determining a secondary transfer voltage in Example 3; FIG. 10 is a schematic diagram of a configuration around a primary transfer section of an image forming apparatus of Example 4; FIG. 11 is a schematic diagram showing a cross-sectional configuration of an intermediate transfer belt in Example 4; FIG. 11 is an equivalent circuit diagram of a secondary transfer unit in Example 4;

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

[Example 1]
<Overall Configuration and Operation of Image Forming Apparatus>
FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic full-color laser printer that employs an in-line method and an intermediate transfer method. The image forming apparatus 100 can form a full-color image on a recording material P (for example, recording paper, plastic sheet, etc.) according to image information. Image information is sent to an image reading device (not shown) provided in or connected to the image forming apparatus 100, or a host device 199 such as a personal computer communicably connected to the image forming apparatus 100 (FIGS. 2 and 3). ) to the image forming apparatus 100 .

Image forming apparatus 100 includes first and second image forming units (stations) for forming yellow (Y), magenta (M), cyan (C), and black (K) images, respectively. , third and fourth image forming units Sa, Sb, Sc, and Sd. In this embodiment, the first, second, third, and fourth image forming units Sa, Sb, Sc, and Sd are arranged in a row in a direction crossing the vertical direction. In this embodiment, the configurations and operations of the first, second, third, and fourth image forming units Sa, Sb, Sc, and Sd are substantially the same except that the colors of the formed images are different. is. For elements having the same or corresponding functions or configurations in the image forming units Sa, Sb, Sc, and Sd, the suffixes a, b, c, and d are omitted to indicate that they are elements for one of the colors. may be described in a comprehensive manner. The image forming unit S includes photosensitive drums 1 (1a, 1b, 1c, 1d), charging rollers 2 (2a, 2b, 2c, 2d), exposure devices 3 (3a, 3b, 3c, 3d), developing devices 4, which will be described later. (4a, 4b, 4c, 4d), primary transfer rollers 14 (14a, 14b, 14c, 14d), and drum cleaning devices 5 (5a, 5b, 5c, 5d).

A photosensitive drum 1, which is a rotatable drum-shaped (cylindrical) photosensitive member (electrophotographic photosensitive member) as a first image bearing member, is driven by a driving motor as driving means (driving source). It is rotationally driven in the R1 direction (counterclockwise direction) at a predetermined peripheral speed (process speed). The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-type charging member as charging means. The charged surface of the photosensitive drum 1 is scanned and exposed according to image information by an exposure device (laser scanner unit) 3 as exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1 according to the image information. be done. The exposure device 3 irradiates the photosensitive drum 1 with laser light L based on an output calculated by a CPU 221 (FIG. 3), which will be described later, from image information input from, for example, the host device 199 (FIGS. 2 and 3). . The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by a developing device 4 as developing means, and a toner image (toner image, developed image) is formed on the photosensitive drum 1 . agent image) is formed. In this embodiment, the charging polarity of the photosensitive drum 1 (in this embodiment, negative (Reversal development). In this embodiment, the normal charge polarity of the toner, which is the charge polarity of the toner during development, is negative.

An intermediate transfer belt 10, which is an endless belt serving as a second image carrier, is arranged to face the four photosensitive drums 1a to 1d. The intermediate transfer belt 10 is stretched over a driving roller 11, a tension roller 12, and a secondary transfer counter roller 13 as a plurality of supporting members (stretching rollers) with a predetermined tension. The intermediate transfer belt 10 contacts the four photosensitive drums 1 a to 1 d on the transfer surface M formed between the secondary transfer opposing roller 13 and the driving roller 11 . The driving roller 11 is rotationally driven in the direction of arrow R2 (clockwise direction) in FIG. 1 by a driving motor as driving means (driving source). As a result, the intermediate transfer belt 10 rotates (circulates, circulates) in the direction of arrow R3 (clockwise) in FIG. 1 at a peripheral speed (process speed) corresponding to the peripheral speed of the photosensitive drum 1 . Primary transfer rollers 14a, 14b, 14c, and 14d, which are roller-type primary transfer members as primary transfer means, are provided on the inner peripheral surface side of the intermediate transfer belt 10 corresponding to the respective photosensitive drums 1a, 1b, 1c, and 1d. are placed. The primary transfer roller 14 presses the intermediate transfer belt 10 toward the photosensitive drum 1 to form a primary transfer portion (primary transfer nip portion) N1, which is a contact portion between the photosensitive drum 1 and the intermediate transfer belt 10 . The toner image formed on the photosensitive drum 1 is transferred (primarily transferred) onto the rotating intermediate transfer belt 10 by the action of the primary transfer roller 14 at the primary transfer portion N1. At the time of primary transfer, the primary transfer roller 14 is supplied with a primary transfer power source (high voltage power source) 15 as a primary transfer voltage applying means (primary transfer voltage applying section), which has a polarity opposite to the normal charging polarity of the toner (in this embodiment, A positive polarity primary transfer voltage (primary transfer bias) is applied. In this embodiment, a primary transfer voltage of +100 V is applied to the primary transfer roller 14, for example. For example, when forming a full-color image, yellow, magenta, cyan, and black toner images formed on the respective photosensitive drums 1a, 1b, 1c, and 1d are superimposed on the intermediate transfer belt 10, and are sequentially primary-transferred. be transcribed.

On the outer peripheral surface side of the intermediate transfer belt 10 , a secondary transfer roller ( A secondary transfer outer roller) 20 is arranged. The secondary transfer roller 20 is pressed toward the secondary transfer counter roller 13 and comes into contact with the secondary transfer counter roller 13 via the intermediate transfer belt 10, and the contact portion between the intermediate transfer belt 10 and the secondary transfer roller 13 A secondary transfer portion (secondary transfer nip portion) N2 is formed. The toner image formed on the intermediate transfer belt 10 is nipped and conveyed between the intermediate transfer belt 10 and the secondary transfer roller 20 by the action of the secondary transfer roller 20 at the secondary transfer portion N2. It is transcribed (secondary transcribed) onto P. During secondary transfer, a secondary transfer power supply (high voltage power supply) 21 serving as secondary transfer voltage applying means (secondary transfer voltage applying section) supplies the secondary transfer roller 20 with a polarity opposite to the normal charging polarity of the toner. A (positive polarity in this embodiment) secondary transfer voltage (secondary transfer bias) is applied. Incidentally, in this embodiment, the secondary transfer counter roller 13 is connected to the ground potential. For example, when forming a full-color image, the four-color toner images on the intermediate transfer belt 10 are collectively transferred onto the recording material P at the secondary transfer portion N2. The recording material P is housed in a cassette 51 as a recording material housing portion. The recording material P is separated one by one from a cassette 51 by a feeding roller 50 or the like as a feeding means and fed out, and conveyed to a registration roller 60 . Then, the recording material P is conveyed to the secondary transfer portion N2 in timing with the toner image on the intermediate transfer belt 10 by the registration rollers 60 . The conveying timing of the recording material P by the registration rollers 60 is controlled based on the detection result of a registration sensor 61 that detects the leading edge of the recording material P in the conveying direction.

A voltage having the same polarity as the normal charging polarity of the toner is applied to the inner roller corresponding to the secondary transfer opposing roller 13 in this embodiment, and the outer roller corresponding to the secondary transfer roller 20 in this embodiment is grounded. It can also be configured to connect to

The recording material P onto which the toner image has been transferred is conveyed to a fixing device 30 as fixing means. The fixing device 30 has a fixing roller 31 having a heat source and a pressure roller 32 that presses against the fixing roller 31 . The fixing device 30 applies heat and pressure to the recording material P bearing an unfixed toner image at a fixing nip portion, which is a contact portion between the fixing roller 31 and the pressure roller 32 , thereby fixing the toner onto the recording material P. Fix (melt, fix) the image. For example, when forming a full-color image, a four-color toner image on the recording material P is fixed on the recording material P by being heated and pressurized at the fixing nip portion so as to be melted and color-mixed. The recording material P on which the toner image is fixed is discharged (output) from the main body of the image forming apparatus 100 .

The image forming apparatus 100 of the present embodiment conveys the recording material P having the toner image transferred and fixed on the first side to the secondary transfer portion N2 again, and transfers the toner image to the second side of the recording material P and fixes it. It is possible to perform double-sided printing (automatic double-sided printing) in which the paper is ejected to the outside of the apparatus main body after printing. The image forming apparatus 100 has a double-sided conveying mechanism (not shown) for conveying the recording material P on which the toner image is fixed on the first side to the secondary transfer portion N2 again in order to execute double-sided printing. there is In the case of single-sided printing, the recording material P with the toner image fixed on the first side is discharged directly to the outside of the apparatus main body.

On the other hand, adherents such as toner remaining on the photosensitive drum 1 after the primary transfer (primary transfer residual toner) are removed from the photosensitive drum 1 by a drum cleaning device 5 as photoreceptor cleaning means and collected. Adhered matter such as toner remaining on the intermediate transfer belt 10 after the secondary transfer (secondary transfer residual toner) is removed from the intermediate transfer belt 10 by a belt cleaning device 16 as intermediate transfer body cleaning means and collected. be done.

Note that the image forming apparatus 100 can also form a single-color or multi-color image using only one image forming unit S or some (not all) image forming units S.

In each image forming section S, the photosensitive drum 1, the charging roller 2 as a process means acting on the photosensitive drum 1, the developing device 4, and the drum cleaning device 5 are integrated into the main body of the image forming apparatus 100. It constitutes a detachable process cartridge 6 . The process cartridge 6 can be attached to and detached from the apparatus main body through attachment means such as a mounting guide and a positioning member provided in the apparatus main body.

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

Here, in this embodiment, the primary transfer roller 14 is a cylindrical metal roller having an outer diameter of 6 mm, and is made of nickel-plated SUS. The primary transfer roller 14 is arranged at a position offset by 8 mm from the center position of the photosensitive drum 1 toward the downstream side in the moving direction of the intermediate transfer belt 10 so that the intermediate transfer belt 10 wraps around the photosensitive drum 1 . configuration. The primary transfer roller 14 moves the intermediate transfer belt 10 toward the photosensitive drum 1 with respect to a horizontal plane formed by the photosensitive drum 1 and the intermediate transfer belt 10 so that the intermediate transfer belt 10 can be wound around the photosensitive drum 1 by an appropriate amount. is placed at a position raised by 1 mm. The primary transfer roller 14 presses the intermediate transfer belt 10 toward the photosensitive drum 1 with a force of about 200 gf. Further, the primary transfer roller 14 rotates following the rotation of the intermediate transfer belt 10 .

In this embodiment, the secondary transfer roller 20 contacts the intermediate transfer belt 10 with a pressure of 50N to form the secondary transfer portion N2. The secondary transfer roller 20 rotates following the rotation of the intermediate transfer belt 10 . A recording material P such as paper is nipped and conveyed between the intermediate transfer belt 10 and the secondary transfer roller 20 at the secondary transfer portion N2. The secondary transfer roller 20 has a nickel-plated steel bar with an outer diameter of 8 mm as a core, and a thickness mainly composed of NBR adjusted to a volume resistivity of 10 ⁸ Ω·cm as an elastic layer and epichlorohydrin rubber. It is a roller with an outer diameter of 18 mm covered with a foamed sponge body of 5 mm. In this embodiment, the secondary transfer power supply 21 can output in the range of 100V to 5000V. In the present specification, a numerical range indicated using "-" means a range including the numerical values before and after "-".

In this embodiment, the fixing roller 31 as a fixing member is a roller having an outer diameter of 18 mm, which is formed by forming an elastic layer of insulating silicone rubber around a metal tube, and further coating the outer periphery of the elastic layer with an insulating PFA tube. is. The fixing roller 31 includes a halogen heater (not shown) as heating means. The halogen heater is not in contact with the fixing roller 31 and generates heat by being supplied with voltage from a power source (not shown). In this embodiment, the pressure roller 32 as a pressure member is formed by forming an elastic layer of conductive silicone rubber around the metal core, and further coating the outer periphery of the elastic layer with a conductive PFA tube. It is a roller with a diameter of 18 mm. The fixing roller 31 and the pressure roller 32 form a fixing nip portion by being pressed with a pressure of 10 kgf. The pressure roller 32 is rotationally driven by a driving motor as driving means (driving source). The fixing roller 31 rotates following the rotation of the pressure roller 32 . The recording material P is nipped and conveyed between the heating roller 31 and the pressure roller 32 at the fixing nip portion. The pressure roller 32 is connected to the ground (ground potential) via a resistance element (not shown) of 1000 MΩ from the metal core. By allowing the charge on the fixing roller 31 and the pressure roller 32 to escape to the ground via the pressure roller 32 and the resistance element, it is possible to suppress the surfaces of the fixing roller 31 and the pressure roller 32 from being charged.

FIG. 2 is a block diagram for explaining the configuration of the engine control unit 210 that controls the entire image forming apparatus 100 of this embodiment. The engine control unit 210 incorporates a CPU circuit unit 150, a ROM 151 and a RAM 152. The CPU circuit unit 150 comprehensively controls the primary transfer control unit 201, the secondary transfer control unit 202, the development control unit 203, the exposure control unit 204, the charging control unit 205, etc. according to the control program stored in the ROM 151. . A control table (environment table, recording material width/recording material thickness correspondence table, etc.) related to the control of the secondary transfer voltage, which will be described later, is stored in the ROM 151, and the CPU 221 (FIG. 3) mounted in the CPU circuit section 150 is stored in the ROM 151. Call it and reflect it in the control. A RAM 152 temporarily holds control data and is used as a work area for arithmetic processing associated with control.

A primary transfer control unit 201 and a secondary transfer control unit 202 control the primary transfer power source 15 and the secondary transfer power source 21, respectively. The primary transfer control unit 201 and the secondary transfer control unit 202 adjust the voltages output from the primary transfer power supply 15 and the secondary transfer power supply 21, respectively, based on the current values detected by the respective current detection units (current detection circuits). Control. Control of the secondary transfer voltage will be described later in detail.

The engine control unit 210 is connected to an environment sensor 300 as environment detection means (environment detection unit) for detecting at least one of temperature and humidity inside or outside the image forming apparatus 100 . In this embodiment, the environment sensor 300 incorporates a temperature sensor 301 as temperature detection means (temperature detection section) and a humidity sensor 302 as humidity detection means (humidity detection section). Detects ambient temperature and humidity. The environment sensor 300 inputs to the engine control unit 210 a signal (temperature information) indicating the result of temperature detection by the temperature sensor 301 and a signal (humidity information) indicating the result of humidity (relative humidity) detection by the humidity sensor 302 .

A controller 200 is also connected to the engine control unit 210 . The controller 200 receives print information (image information, various setting information) and a print command (instruction to start a print job) from the host device 199, which is an external device. Then, the engine control unit 210 controls each control unit (primary transfer control unit 201, secondary transfer control unit 202, development control unit 203, exposure control unit 204, charging control unit 205, etc.) to perform the print job operation. Run. In this embodiment, the engine control unit 210 acquires environment information from the detection result of the environment sensor 300 and information on the recording material P is obtained from the host device 199 for controlling the secondary transfer voltage, which will be described later. Get from information. The print information is input from the host device 199 to the controller 200 via a printer driver or the like installed in the host device 199 .

Here, the image forming apparatus 100 performs a print job (print job, image output operation), which is a series of operations for forming and outputting an image on a single or a plurality of recording materials P, which is started by one start instruction. Run. A print job generally includes an image forming process, a pre-rotation process, an inter-paper process when forming images on a plurality of recording materials P, and a post-rotation process. The image forming process is a period during which an electrostatic latent image of an image to be actually formed and output on the recording material P is formed, a toner image is formed, a primary transfer of the toner image, and a secondary transfer are performed. formation period) refers to this period. More specifically, the timing during image formation differs depending on the position where each step of forming the electrostatic latent image, forming the toner image, primary transfer, and secondary transfer of the toner image is performed. The pre-rotation process is a period from when the start instruction is input to when the image formation is actually started, during which preparatory operations are performed before the image forming process. The inter-paper process (inter-recording material process) is a period corresponding to the interval between recording materials P when image formation is continuously performed on a plurality of recording materials P (continuous image formation). The post-rotation process is a period during which an arrangement operation (preparation operation) is performed after the image forming process. The non-image forming period (non-image forming period) is a period other than the image forming period, and includes the pre-rotation process, the inter-paper process, the post-rotation process, and further, when the power of the image forming apparatus 100 is turned on or from the sleep state. It includes a pre-multi-rotation step, etc., which is a preparatory operation at the time of return.

<Overview of control of secondary transfer voltage>
Next, an outline of control of the secondary transfer voltage in this embodiment will be described.

As shown in FIG. 1 , the secondary transfer power supply 21 is connected to the secondary transfer roller 20 , and the secondary transfer voltage output from the secondary transfer power supply 21 is supplied to the secondary transfer roller 20 . A secondary transfer voltage is applied from the secondary transfer power supply 21 to the secondary transfer roller 20, so that an electric field is formed between the secondary transfer roller 20 and the secondary transfer counter roller 13 installed in the opposite portion. be. As a result, an induced polarization is generated between the intermediate transfer belt 10 and the recording material P, and an electrostatic adsorption force is generated between them.

As shown in FIG. 2, the secondary transfer control unit 202 detects current flowing through the secondary transfer unit N2 (secondary transfer roller 20) when the secondary transfer power supply 21 applies a voltage to the secondary transfer roller 20. It has a current detection unit (ammeter) 241 as current detection means for detecting current. The secondary transfer control unit 202 can control the voltage value output by the secondary transfer power supply 21 so that the current flowing through the secondary transfer unit N2 is substantially constant at the target current value (approaching the target value). During image formation (during secondary transfer), current flowing through the secondary transfer portion N2 is detected by the current detection portion 241 at a predetermined cycle (current detection cycle). Then, the secondary transfer control unit 202 determines the voltage value of the secondary transfer voltage to be applied to the secondary transfer roller 20 in the next current detection cycle. The secondary transfer control unit 202 feeds back the difference between the preset target current value and the detected current value detected by the current detection unit 241, which is the actual output value, to the secondary transfer power supply 21. A voltage value of the secondary transfer voltage in the next current detection cycle is determined. That is, the voltage value of the secondary transfer voltage applied to the secondary transfer roller 20 is adjusted in the next current detection cycle so that the detected current value approaches the target current value. As a result, the secondary transfer voltage applied from the secondary transfer power source 21 to the secondary transfer roller 20 is controlled so that the current flowing through the secondary transfer portion N2 is substantially constant. Here, the secondary transfer voltage is applied from the secondary transfer power supply 21 to the secondary transfer roller 20 so that the current value detected by the current detection unit 241 is substantially constant at a predetermined current value set in advance. This control is called “constant current control”.

On the other hand, as shown in FIG. 2, the secondary transfer control section 202 has a voltage detection section 242 as voltage detection means for detecting the voltage value applied to the secondary transfer roller 20 by the secondary transfer power supply 21 . The secondary transfer control unit 202 can perform control so that the voltage value output by the secondary transfer power source 21 is substantially constant at a target voltage value (approaching the target value). The voltage detection unit 242 may detect (recognize) the voltage value from the indicated value of the output voltage value for the secondary transfer power source 21 . In a high-temperature and high-humidity (high-temperature and high-humidity) environment, the electrical resistance values of the recording material P, the secondary transfer roller 20, the intermediate transfer belt 10, and the like decrease due to moisture absorption. If the “constant current control” of the secondary transfer voltage is performed in such a state, the absolute value of the secondary transfer voltage required to output the target current value becomes small. An electric field necessary for the transfer may not be formed, resulting in poor transfer. Therefore, a lower limit is set for the voltage value of the secondary transfer voltage. The secondary transfer voltage is controlled so that the value is substantially constant at the target voltage value corresponding to the lower limit. As a result, the minimum required voltage for transferring the toner onto the recording material P can be ensured and the secondary transfer can be performed. In this case, the secondary transfer voltage is applied from the secondary transfer power supply 21 to the secondary transfer roller 20 at a predetermined voltage value that is substantially constant (the applied voltage is substantially constant regardless of the current value). control) is called “constant voltage control”.

In this embodiment, the CPU 221 (FIG. 3) of the engine control unit 210 calculates the absolute moisture content of the environment in which the image forming apparatus 100 is installed based on the detection results of the temperature sensor 301 and the humidity sensor 302 of the environment sensor 300. . Then, the CPU 221 determines whether the control of the secondary transfer voltage by the secondary transfer control unit 202 is to be “constant current control” or “constant voltage control” according to the calculated absolute water content. The next transfer control unit 202 is instructed. In this embodiment, the “constant voltage control” of the secondary transfer voltage is performed when the absolute moisture content is 21.7 g/m ³ or more, and the second transfer voltage is performed when the absolute moisture content is less than 21.7 g/m ³ . "Constant current control" of the next transfer voltage is performed.

<Details of control of secondary transfer voltage in this embodiment>
One of the features of this embodiment is that, in the constant voltage control of the secondary transfer voltage, the amount of toner to be transferred to the recording material P is calculated based on the image information, and based on the result of the calculation, there are two preset standards. To correct the next transfer voltage value. In particular, at that time, decreasing the absolute value of the secondary transfer voltage as the toner amount increases (increasing the absolute value of the secondary transfer voltage as the toner amount decreases) is a feature of this embodiment. is one.

(Calculation of toner amount)
A method of calculating the amount of toner to be transferred onto the recording material P in this embodiment will be described with reference to FIG. FIG. 3 is a functional block diagram relating to calculation of the amount of toner to be transferred onto the recording material P in the image forming apparatus 100 of this embodiment. Here, a calculation method and physical meaning of the toner amount information regarding the toner amount to be transferred onto the recording material P used in the control of the secondary transfer voltage in this embodiment, that is, the toner amount X in one page will be described.

The controller section 200 can communicate with the host device 199 and the engine control section 210 . Upon receiving print information input from the host device 199, the controller unit 200 develops the print information and converts it into image data for forming an image. Based on the image data, the controller unit 200 generates video signals for exposure of four colors for the exposure devices 3 to expose the photosensitive drums 1 in the four image forming units S. FIG. When the generation of the video signal is completed, controller section 200 inputs a print job start instruction to video interface section 220 of engine control section 210 . Thereafter, upon receiving a print job start instruction from the video interface unit 220, the CPU 221 of the engine control unit 210 activates various actuators to start preparation for image formation. When preparation for image formation is completed, the CPU 221 notifies the controller 200 of completion of preparation for image formation via the video interface section 220 . The controller 200 transmits a video signal to the video interface unit 220 upon receiving the signal indicating the completion of preparation for image formation.

The video interface section 220 transmits the received video signal to the image processing GA 222 of the engine control section 210 . The image processing GA 222 receives a video signal from the video interface section 220 , converts it into a laser drive signal, and transmits the laser drive signal to the laser drive section 230 of the exposure device 3 . The laser drive unit 230 controls the current supplied to the laser diode 231 as the light source of the exposure device 3 according to the laser drive signal, thereby controlling the light emission of the laser diode 231 . In addition, the image data counting unit 223 of the engine control unit 210 samples the laser drive signal and counts the number of times the signal becomes High (emission) (hereinafter referred to as “H”). The image data counting unit 223 does not count when the signal is Low (turned off) (hereinafter referred to as “L”) when sampling the laser drive signal. In this embodiment, the video interface section 220 , the image processing GA, and the image data counting section 223 are implemented by an ASIC mounted on the CPU circuit section 150 of the engine control section 210 .

The CPU 221 calculates the pixel count values ny, nm, nc, and nk of each color, which are the number of times the laser drive signal becomes "H" for each of the four colors Y, M, C, and K, by the image data counting unit 223, Each counts one page. Subsequently, the CPU 221 calculates a pixel count value n (=ny+nm+nc+nk), which is the sum of the pixel count values of each color. Assuming that the total number of samples per color for one page is N, the toner amount X [%] in one page is calculated by the following formula (1). In this embodiment, the toner amount X in one page is calculated by the toner amount calculator 224 implemented by the CPU 211 mounted on the CPU circuit section 150 of the engine controller 210 .
Amount of toner in one page X [%]
= {(pixel count value n)/(total sampling number per color N)} x 100
... (1)

Sampling for the total number N of samplings per color is performed at different timings because the lasers are driven at different timings for each color. In this embodiment, the sampling period is set to a short period (100 MHz) so that all pixels in one page can be counted. Therefore, the total sampling numbers Ny, Nm, Nc, and Nk of the four colors Y, M, C, and K are Ny=Nm=Nc=Nk=N. In this embodiment, the maximum value of the pixel count values ny, nm, nc, and nk of each color is the total number N of samples per color, so X can take a value of 0 to 400 [%] (per color can take a value of 0 to 100 [%]). That is, in this embodiment, the toner amount information regarding the amount of toner to be transferred to the recording material P is the ratio of the amount of toner to be transferred to the recording material P to the total amount of toner of each color that can be transferred to the recording material P. Amount X [%] is shown.

As described above, the amount of toner X in one page is the total amount (more specifically, its predicted value) of toner transferred onto one sheet of recording material P (more specifically, its image formable area). be. The fact that the toner amount X in one page is less than 100% roughly represents how much the inside of one sheet of recording material P is covered with toner. At this time, it means that the smaller the value of the toner amount X in one page, the more the white portion. In particular, when the toner amount X in one page is less than 10%, it means a state in which several texts and isolated patch patterns, which will be described later, are present in white areas covering almost the entire surface of one sheet of recording material P. represents approximately. When the toner amount X in one page is 100% or more, it is a state in which almost the entire surface of one sheet of recording material P is covered with toner, and what is the amount of toner for four colors in the height direction. It roughly represents At this time, the larger the numerical value of the toner amount X in one page, the more toner exists in the height direction. A state in which the toner amount X in one page is 10 to 100% is a state in which a relatively large area in one sheet of recording material P is covered with toner for an isolated patch pattern or a halftone image, which will be described later. It is in a state that may be mixed.

In this embodiment, the purpose of calculating the toner amount X in one page is to predict how the inside of this one sheet of recording material P is covered with toner. In this embodiment, the correction amount (correction value) .DELTA.V of the secondary transfer voltage is determined based on the predicted state of toner coverage in one sheet of recording material P. FIG.

(Method for determining secondary transfer voltage)
A method of determining a reference secondary transfer voltage when performing constant voltage control of the secondary transfer voltage in this embodiment, and a method of correcting the secondary transfer voltage based on the toner amount X in one page (secondary transfer voltage determination method) will be explained.

First, the method of determining the voltage V1 (reference value), which is the reference secondary transfer voltage when performing the constant voltage control of the secondary transfer voltage, will be described. In this embodiment, the voltage V1 is determined by the CPU 221 based on the information of the recording material P obtained from the printing information input from the host device 199 and the environment information obtained from the detection result of the environment sensor 300. . In this embodiment, the information on the recording material P includes information on the length of the recording material P in the width direction substantially orthogonal to the conveying direction of the recording material P (here, also simply referred to as “width”) and the size of the recording material P (hereinafter referred to as “width”). , also referred to as “paper size”), information on indices (thickness, basis weight, etc.) related to the thickness of the recording material P, category of the recording material P (plain paper, thick paper, glossy paper, etc.). (also referred to as “paper quality”). Also, in this embodiment, the environment information includes information on the absolute moisture content of the environment calculated by the CPU 221 based on the detection results of the temperature sensor 301 and the humidity sensor 302 of the environment sensor 300 . Note that the information about the recording material P includes attributes (so-called categories (paper quality)) based on general characteristics such as plain paper, high-quality paper, glossy paper, glossy paper, coated paper, embossed paper, thick paper, and thin paper. It includes any information that can distinguish the recording material P, such as numerical values or numerical ranges of quantity, thickness, size, rigidity, etc., or brands (including manufacturers, product names, product numbers, etc.). . Each recording material P distinguished by information on the recording material P can be regarded as constituting the type of the recording material P. FIG. The information on the recording material P may be specified directly, for example, or may be included in print mode information specifying the operation settings of the image forming apparatus 100, such as "plain paper mode" and "thick paper mode." Alternatively, it may be replaced with print mode information. That is, in this embodiment, for each type of recording material P, information indicating the relationship between the paper size, basis weight, paper quality, absolute moisture content, and voltage V1 is set in advance and stored in the ROM 151 as a table. there is Then, the CPU 221 retrieves necessary information from the table based on the obtained information of the recording material P and the environment information, and determines the voltage V1 corresponding to the paper size, basis weight, paper quality, and absolute moisture content.

Next, a method for determining the correction amount ΔV of the secondary transfer voltage (method for determining the secondary transfer voltage) in this embodiment will be described. FIG. 4 is a graph for explaining a method of determining the correction amount ΔV of the secondary transfer voltage in this embodiment. In FIG. 4, the horizontal axis indicates the toner amount X in one page, and the vertical axis indicates the secondary transfer voltage V that is actually applied. The toner amount X in one page on the horizontal axis can take values from 0 to 400 [%]. The voltage value when the toner amount X=0 [%] in one page is set as the voltage V1, which is the reference value of the secondary transfer voltage in this embodiment. Also, the voltage value at the toner amount X=400[%] in one page is V2. It should be noted that in this embodiment, the voltage V2 is determined based on the information of the recording material P and the environmental information, like the voltage V1. That is, in this embodiment, for each type of recording material P, information indicating the relationship between the paper size, basis weight, paper quality, absolute moisture content, and voltage V2 is set in advance and stored in the ROM 151 as a table. there is Then, the CPU 221 retrieves necessary information from the table based on the obtained information of the recording material P and the environmental information, and determines the voltage V2 corresponding to the paper size, basis weight, paper quality, and absolute moisture content.

As shown in FIG. 4, in this embodiment, as the toner amount X in one page increases with respect to the voltage V1 when the toner amount X in one page is 0 [%], the secondary transfer voltage actually applied is increased. The absolute value of the voltage V is monotonously decreased. That is, the correction amount ΔV is the difference between the voltage V1 and the actually applied secondary transfer voltage V, and is determined (calculated) by the following formula (2). In this embodiment, the secondary transfer voltage correction amount ΔV is determined (calculated) by the secondary transfer voltage correction amount calculator 225 realized by the CPU 221 mounted on the CPU circuit section 150 of the engine control section 210. done.
ΔV={(V1−V2)/400}×X[V] (0≦X≦400) (2)

Then, the secondary transfer voltage V is determined (calculated) by the following formula (3) so that the absolute value of the secondary transfer voltage decreases as the toner amount X in one page increases. In this embodiment, determination (calculation) of the secondary transfer voltage V using the correction amount ΔV is performed by the CPU 221 mounted in the CPU circuit section 150 of the engine control section 210 .
V=V1-ΔV (3)

Table 1 shows, as an example, a table of voltages V1 and V2 for plain paper. In the example shown in Table 1, in the table, voltages V1 and V2 are set for the surface of the recording material P to which the toner image is transferred (the first or second surface), the absolute water content, and the paper size. . Note that the paper size indicates the width of the recording material P. As shown in FIG.

Note that the voltages V1 and V2 for paper sizes and absolute moisture amounts not listed in Table 1 are determined by linear interpolation between the paper sizes and absolute moisture amounts listed in Table 1. Regarding the paper size, the LTR setting is selected for the LTR width (215.9 mm) or more, and the A5 setting is selected for the A5 width (148.0 mm) or less. Also, the A4 width is 210.0 mm. Regarding the absolute moisture content, the setting of 27.1 g/m ³ is selected for 27.1 g/m ³ or more, and if it is less than 21.7 g/m ³ , the constant current control of the secondary transfer voltage is executed as described above. be done.

Next, the reason why the difference (absolute value) between the voltages V1 and V2 in Table 1 is changed according to the absolute water content, the first and second sides, and the width of the recording material P will be described.

First, it is preferable to increase the difference between the voltage V1 and the voltage V2 as the absolute water content increases. In other words, the resistance of the recording material P is lowered as the hygroscopicity thereof increases. Therefore, in the present embodiment, the difference between the voltage V1 and the voltage V2 is increased as the absolute water content increases, from the viewpoint of suppressing the patch blurring and strong omission.

Secondly, it is preferable that the difference between the voltages V1 and V2 is larger on the first surface than on the second surface. On the second side, heat is applied to the recording material P by going through the fixing process once, the water content of the recording material P evaporates, and the electric resistance increases. As a result, compared to the first side, the second side is less likely to have patchy patches and strong omissions. Therefore, in this embodiment, the difference between the voltage V1 and the voltage V2 is larger on the first surface than on the second surface.

Third, it is preferable that the difference between the voltage V1 and the voltage V2 is increased as the width of the recording material P is increased. This is because, when the size of the isolated patch pattern, which will be described later, is the same, the smaller the width of the recording material P, the larger the area ratio of the toner occupying the white portion, and the less patch blurring occurs. Therefore, in this embodiment, the larger the width of the recording material P, the larger the difference between the voltage V1 and the voltage V2.

Note that Table 1 is an example for plain paper, and depending on the type of recording material P, the degree of moisture absorption and the electric resistance value of the recording material P itself change. Therefore, the table can be appropriately set according to the type of recording material P. FIG. There is a tendency that the smaller the basis weight of the recording material P, the lower the electrical resistance. Therefore, from the viewpoint of suppressing patch blurring and strong omission, it is preferable to increase the difference between the voltage V1 and the voltage V2 as the basis weight of the recording material P decreases. In order to increase the difference (absolute value) between the voltages V1 and V2, the absolute value of the voltage V1 can be increased, the absolute value of the voltage V2 can be decreased, or both of these can be performed. That is, at least one of the voltage V1 and the voltage V2 can be changed so that the absolute value of the difference between the voltages V1 and V2 is increased.

A specific example of a method of determining (calculating) the secondary transfer voltage will be shown. For example, consider a case where an image with a toner amount X of 160% per page is printed on one side of plain paper (width 179 mm) in an environment with an absolute moisture content of 24.4 g/m ³ .

First, the voltage V1 and voltage V2 on the first surface of A5 size (width 148 mm) or less and A4 size (width 210 mm) in an environment with an absolute moisture content of 24.4 g / m ³ are set to an absolute moisture content of 27.1 g / m ³ Obtained by linear interpolation between the water content of 21.7 g/m ³ .
・A5 size V1 = 1000 + (1100-1000) x (24.4-21.7) / (27.1-21.7) = 1050V
・A4 size V1 = 1200 + (1300-1200) x (24.4-21.7) / (27.1-21.7) = 1250V
・A5 size V2 = 900 + (900-900) x (24.4-21.7) / (27.1-21.7) = 900V
・A4 size V2 = 800 + (850-800) x (24.4-21.7) / (27.1-21.7) = 825V

Next, the voltages V1 and V2 of the recording material P with a width of 179 mm are obtained by linear interpolation between the width of 148 mm and the width of 210 mm.
・V1 of width 179mm = 1050 + (1250-1050) x (179-148) / (210-148) = 1150V
・V2 of width 179mm=825+(900-825)×(179-148)/(210-148)=862.5V

Subsequently, the correction amount ΔV for the toner amount X=160[%] in one page is linearly adjusted between the toner amount X=0[%] in one page and the toner amount X=400[%] in one page. Obtained by interpolating.
ΔV={(1150−862.5)/400}×160=115[V]

Finally, the secondary transfer voltage V to be actually applied is obtained by correcting the voltage V1 using the correction amount ΔV obtained above.
V=V1-ΔV=1150-115=1035[V]

The CPU 221 instructs the secondary transfer control unit 202 to apply the secondary transfer voltage V determined as described above from the secondary transfer power source 21 to the secondary transfer roller 20, thereby executing the secondary transfer.

(Control procedure)
FIG. 5 is a flow chart diagram showing an outline of the procedure for controlling the secondary transfer voltage in this embodiment described so far.

When a print job start instruction is input, the CPU 221 starts preparation for image formation (S101). Control of the secondary transfer voltage in this embodiment starts from the start of a print job (print start (i=1)). The CPU 221 acquires the print information for the i-th page from the host device 199 (S102), and acquires information on the absolute moisture content from the detection result of the environment sensor 300 (S103). The CPU 221 determines whether the control of the secondary transfer voltage should be constant voltage control or constant current control based on the information on the absolute water content (S104). In this embodiment, the CPU 221 executes constant voltage control when the absolute moisture content is 21.7 g/m ³ or more (S105), and the absolute moisture content is less than 21.7 g/m ³ In the case of , constant current control is executed (S106). Since this embodiment is characterized by the control when the constant voltage control of the secondary transfer voltage is executed, the control when the constant voltage control of the secondary transfer voltage is executed will be described.

While the CPU 221 determines the control method of the secondary transfer voltage, the CPU 221 measures the toner amount X (0 to 400%) of the i-th page by the method described above based on the print information from the host device 199 ( S107). That is, a video signal from the controller 200 is sent to the image processing GA 222 via the video interface section 220, converted into a laser drive signal, and the toner amount X (0 to 400%) of the i-th page is measured. The CPU 221 causes the RAM 152 to store the measured toner amount X (0 to 400%) for the i-th page (S108). The CPU 221 determines the voltage V1 and A correction amount ΔV corresponding to the toner amount X is calculated (S110). In parallel with this, when the next page is to be printed, the CPU 221 measures the toner amount X of the (i+1) page, stores it in the RAM 152, and stores the voltage V1 and Determining the correction amount ΔV is repeated (S111, S112, S113).

After determining the correction amount ΔV corresponding to the voltage V1 and the toner amount X of the i-th page, the CPU 221 performs secondary transfer of V=V1−ΔV at the timing when the recording material P passes through the secondary transfer portion N2. A voltage is applied (S114) to finish printing page i. If there is a subsequent print, the CPU 221 repeats applying V=V1-ΔV for the i+1th page determined in advance as described above in the same manner as described above, and ends the print job (S115, S116, S117).

<Effect>
Next, in the constant voltage control of the secondary transfer voltage, the absolute value of the secondary transfer voltage is decreased as the toner amount X in one page increases (the absolute value of the secondary transfer voltage decreases as the toner amount X in one page decreases). increasing the value) will be described. FIG. 6A is a schematic diagram showing an example of an image including an isolated patch pattern, and FIG. 6B is a schematic diagram showing an example of a full solid image (for example, a full solid black image). FIG. 6C is a schematic cross-sectional view of the secondary transfer portion N2 during secondary transfer of an image including an isolated patch pattern, and FIG. 4 is a schematic cross-sectional view of a secondary transfer portion N2 during secondary transfer; FIG. In FIGS. 6(c) and 6(d), the arrows represent the paths of the transfer currents, and the thicknesses schematically represent the magnitudes of the currents. Note that, as described above, the “isolated patch pattern” means an image pattern in which clusters of high-printing toner images are scattered within the width of the recording material P. FIG. Further, the “whole surface solid image (whole surface solid pattern)” means an image pattern in which a toner image of the highest density level exists in the entire image formable area of the recording material P in the width direction.

In this embodiment, constant voltage control of the secondary transfer voltage is performed in a high temperature and high humidity environment. In a high-temperature and high-humidity environment, the recording material P, the secondary transfer roller 20, the intermediate transfer belt 10, and the like absorb moisture, thereby reducing their electrical resistance values.

Therefore, when transferring a poorly printed image including an isolated patch pattern, etc., in which the amount of toner X per page is small, as shown in FIG. 6A, the following is the case. That is, as shown in FIG. 6C, the transfer current tends to selectively flow in the white portion with low resistance rather than in the toner portion (patch portion) T with high resistance. , it becomes difficult for the transfer current to flow. At this time, according to the control of this embodiment, the absolute value of the secondary transfer voltage can be increased as the toner amount X in one page decreases, so the total current amount of the secondary transfer current can be increased. . That is, although the current to the white portion increases, the current to the toner portion (patch portion) T also increases at the same time, so that a sufficient current can be supplied to the toner portion (patch portion) T for secondary transfer. As a result, it is possible to suppress "patch blur".

Conversely, when transferring a high-quality print image including a full-surface solid image with a large amount of toner X per page as shown in FIG. That is, since there is no way out of the transfer current as shown in FIG. The current supply to the toner portion T becomes excessive. At this time, according to the control of this embodiment, the absolute value of the secondary transfer voltage can be reduced as the toner amount X in one page increases, so that the excessive current supply to the toner portion T can be prevented. can be suppressed, and as a result, "strong omission" can be suppressed.

In line with the above mechanism, ideally, it is preferable to change the correction amount ΔV of the secondary transfer voltage according to the amount of toner existing in the secondary transfer portion N2 and the area of the toner portion. That is, when the amount of toner present in the secondary transfer portion N2 or the area coverage of the toner portion is large, the absolute value of the secondary transfer voltage is decreased, and when the amount of toner or the area coverage of the toner portion is small, the secondary transfer voltage is reduced. It is preferable to increase the absolute value of the transfer voltage. However, if the voltage is frequently increased and decreased during one page during constant voltage control, the secondary transfer voltage does not follow the control due to the limit of the responsiveness of the secondary transfer power source 21, resulting in It may become impossible to apply the optimum secondary transfer voltage according to the image pattern. Therefore, in this embodiment, the secondary transfer voltage is corrected according to the toner amount X in one page in order to improve the average image quality.

Here, the application (change) timing of the secondary transfer voltage will be further described. FIG. 7 is a timing chart for explaining the application timing of the secondary transfer voltage. In FIG. 7, the horizontal axis indicates time and the vertical axis indicates voltage. Note that the period during which the recording material P (more specifically, its image formable area) exists in the secondary transfer portion N2 is also referred to as "inside the paper". The period on paper corresponds to the time of image formation (during secondary transfer) at the secondary transfer portion N2. Further, the period between the preceding recording material P and the recording material P following the recording material P is also referred to as "paper interval". "Paper interval" is a period corresponding to the above-described paper interval process in the secondary transfer portion N2. In addition, in FIG. 7, the time of non-image formation (corresponding to the above-mentioned pre-rotation process) before the first page "inside the paper" is also referred to as the "paper interval" for the sake of convenience.

FIG. 7A shows application timing of the secondary transfer voltage in this embodiment. In this embodiment, the secondary transfer voltage of V=V1-ΔV is applied only at the timing (inside the paper) when the recording material P passes through the secondary transfer portion N2. In this embodiment, at other timings (between sheets), a sheet-to-sheet voltage having a smaller absolute value than the secondary transfer voltage of V=V1-ΔV is applied. There are two reasons for this. First, the timing at which the secondary transfer voltage Vi for the i-th page described with reference to FIG. This is because it depends on the speed and the like. In other words, since it is not known at what timing the secondary transfer voltage Vi can be determined, the inter-paper voltage is applied until immediately before the application of the secondary transfer voltage for the i-th page, thereby determining the secondary transfer voltage Vi in the paper. be prepared for Second, if a secondary transfer voltage with a large absolute value is applied in a state where the recording material P does not exist in the secondary transfer portion N2, the load on the secondary transfer power supply 21 becomes heavy, and oscillation may occur in some cases. This is because it is possible.

For these reasons, in this embodiment, at the timing (paper interval) when the recording material P is not present in the secondary transfer portion N2, the paper interval voltage having a smaller absolute value than the secondary transfer voltage in the paper is applied. However, if the secondary transfer voltage determination timing is sufficiently early and the responsiveness of the secondary transfer power supply 21 to the control is sufficiently fast, for example, as shown in FIG. It may be provided in the paper by applying the next transfer voltage.

Also, the timing of applying the secondary transfer voltage according to the toner amount will be described in more detail. In this embodiment, the secondary transfer voltage to be applied during a period when toner may exist (timing when the image formable area in the conveying direction of the recording material P passes through the secondary transfer portion N2) is set to From the viewpoint of suppressing strong dropout, control is performed to change the amount of toner. In the present embodiment, the voltage to be applied at the timing when the leading edge or trailing margin of the recording material P where no toner is present passes through the secondary transfer portion N2 is controlled to change according to the toner amount. did not go Note that the leading edge and the trailing edge of the recording material P are the leading edge and the trailing edge of the recording material P in the conveying direction, respectively. In this embodiment, at the timing when the leading edge and the trailing edge of the recording material P pass through the secondary transfer portion N2, the leading edge and trailing edge voltages different from the secondary transfer voltage of V=V1-ΔV are applied. there is This is for the following reasons. That is, at the leading edge of the recording material P, the recording material P abruptly enters the secondary transfer portion N2 from a low impedance state where the recording material P does not exist at the secondary transfer portion N2, and the impedance becomes high. Therefore, at the leading end of the recording material P, a leading end voltage is applied so that the transfer voltage does not become insufficient. At the rear end of the recording material P, on the contrary to the above, the recording material P gradually passes through the secondary transfer portion N2 from a high impedance state where the recording material P exists, and the impedance decreases. Therefore, a trailing edge voltage is applied to the trailing edge of the recording material P so that the transfer current does not run short. However, when there is no margin or the margin is narrow and there is a possibility that the toner may be present up to near the leading edge or the trailing edge of the recording material P, secondary As for the transfer voltage, the secondary transfer voltage may be corrected according to the toner amount according to this embodiment. Note that the leading end voltage and the trailing end voltage are predetermined voltages set in advance from the above viewpoint. This voltage is determined, for example, based on information about the recording material P and environmental information, like the voltages V1 and V2 described above. That is, for example, for each type of recording material P, information indicating the relationship between the paper size, basis weight, paper quality, absolute moisture content, leading edge voltage, trailing edge voltage is set in advance and stored in the ROM 151 as a table. ing. Then, the CPU 221 retrieves the necessary information from the table based on the acquired information about the recording material P and the environmental information, and sets the leading edge voltage and the trailing edge voltage corresponding to the paper size, basis weight, paper quality, and absolute moisture content. decide. Typically, the leading edge voltage and the trailing edge voltage are larger in absolute value than the secondary transfer voltage of V=V1-ΔV. However, at least one of the leading edge voltage and the trailing edge voltage set as described above may be substantially the same as the secondary transfer voltage of V=V1-ΔV.

In this embodiment, in an environment where the absolute moisture content is less than 21.7 g/m ³ , the electrical resistance of the recording material P is high enough to prevent the leakage current to the white portion. "Constant current control" is being executed. In this environment, even if the image patterns are as shown in FIGS. 6A and 6B, the "constant current control" is applied so that the secondary transfer voltage is substantially uniform in the width direction of the recording material P so as to obtain an appropriate secondary transfer voltage. A transfer current can be supplied.

Thus, in this embodiment, the image forming apparatus 100 includes the image carrier 10 that carries a toner image, and the transfer member 20 that forms the transfer portion N2 that transfers the toner image from the image carrier 10 to the recording material P. , an application unit 21 for applying a transfer voltage to the transfer member 20 and a control unit (engine control unit) 210 for controlling the application unit 21 . In this embodiment, when the control unit 210 performs constant voltage control on the transfer voltage so that the voltage applied to the transfer member 20 by the application unit 21 is substantially constant, the amount of toner used for the toner image is set to When the toner amount is 1, the transfer voltage is set to the first voltage, and when the toner amount is the second toner amount larger than the first toner amount, the transfer voltage is set to have an absolute value higher than the absolute value of the first voltage. Control is performed so that the second voltage is small. In this embodiment, the image forming apparatus 100 uses an acquisition unit (CPU) 221 for acquiring toner amount information related to the toner amount, and another image carrier 1 that carries toner to be transferred to the image carrier 10 as image information. The acquisition unit 221 acquires toner amount information based on a drive signal for causing the exposure unit 3 to emit light according to image information. Further, in this embodiment, toner amount information regarding the toner amount is obtained for each toner image transferred onto one sheet of recording material P. FIG.

Here, the controller 210 controls the recording material P onto which the toner image is to be transferred when the basis weight is the second basis weight smaller than the first basis weight, rather than when the basis weight is the first basis weight. , at least one of the first voltage and the second voltage can be changed such that the absolute value of the difference between the first voltage and the second voltage is increased. Further, the control unit 210 controls the width in the direction substantially perpendicular to the conveying direction of the recording material P on which the toner image is transferred to be the second width, which is larger than the first width, rather than the first width. In this case, at least one of the first voltage and the second voltage can be changed so that the absolute value of the difference between the first voltage and the second voltage is increased. Further, the image forming apparatus 100 can execute an operation of transferring the toner image to the second side of the recording material P by conveying the recording material P having the toner image transferred and fixed on the first side to the transfer portion N2. In this case, control unit 210 sets the absolute value of the difference between the first voltage and the second voltage when the toner image is transferred to the first side more than when the toner image is transferred to the second side. At least one of the first voltage or the second voltage can be changed to be greater. Further, the image forming apparatus 100 may include an environment detection unit 300 that detects environmental information regarding at least one of environmental temperature and humidity. A first voltage and a second voltage when the absolute water content of the environment is a second absolute water content larger than the first absolute water content than when the absolute water content is the first absolute water content At least one of the first voltage and the second voltage can be changed so that the absolute value of the difference between the voltage and the voltage is increased.

In this embodiment, the controller 210 increases the transfer voltage between the transfer voltage when the toner amount is the minimum and the transfer voltage when the toner amount is the maximum, as the toner amount increases. Control is performed so that the absolute value is gradually reduced. In this embodiment, the control unit 210 can perform constant current control of the transfer voltage so that the current supplied to the transfer member 20 by the application unit 21 is substantially constant. constant voltage control of the transfer voltage is performed when the absolute moisture content indicated by is equal to or greater than a predetermined value, and constant current control of the transfer voltage is performed when the absolute moisture content indicated by the detection result of the environment detection unit 300 is less than the predetermined value.

As described above, according to the present embodiment, even when performing constant voltage control of the secondary transfer voltage in an environment in which it is difficult to simultaneously suppress patch blurring and suppressing strong dropout, such as a high-temperature and high-humidity environment, an appropriate secondary transfer voltage can be obtained. A transfer voltage can be set. Further, as described above, in this embodiment, even in an environment other than the high-temperature and high-humidity environment, it is possible to supply an appropriate secondary transfer voltage by constant current control of the secondary transfer voltage. Therefore, according to this embodiment, it is possible to perform good secondary transfer regardless of the environment in which the image forming apparatus 100 is used.

<Effect confirmation>
In order to confirm the effects of this embodiment, a test was conducted to verify the presence or absence of image defects in a high-temperature and high-humidity environment (temperature: 30° C./relative humidity: 80%/absolute moisture content: 21.7/m ³ ). As the recording material P, XEROX Business 4200 LETTER size (Xerox, trade name) was used. The test was conducted on the configuration of this example and the configurations of Comparative Examples 1 and 2. In the configuration of this embodiment, the voltage V1 was 1400V, and the voltage V2 was 1000V when the toner amount X=400[%] in one page.

In the configurations of Comparative Examples 1 and 2, a substantially constant secondary transfer voltage was applied regardless of the toner amount X in one page. In the configuration of Comparative Example 1, 1400 V, which is the same as the voltage V1 in this embodiment, was applied as the secondary transfer voltage. In the configuration of Comparative Example 2, 1000 V, which is the same as the voltage V2 in this example, was applied as the secondary transfer voltage. The configurations of Comparative Examples 1 and 2 are substantially the same as the configuration of the present embodiment except for the above points.

Table 2 shows the evaluation results. Table 2 shows the applied voltage V in the low print image (X = 5 [%]) and the high print image (X = 300 [%]), and the level of the image in each case (patch blur in the low print image, high (strong omission in the printed image). The image level is divided into three levels, from the top, Good (good), Fair (slightly bad), and Poor (bad). Also, Fair (slightly bad) and Poor (bad) are judged to be the occurrence of image defects.

In the configuration of Comparative Example 1, in a poorly printed image including an isolated patch pattern, since a sufficient current can be supplied to the toner portion (patch portion), "patch blur" does not occur. However, in the configuration of Comparative Example 1, the absolute value of the secondary transfer voltage became large and the current was excessively supplied in high-printing images including solid images, and "strong omission" occurred.

In the configuration of Comparative Example 2, the setting of the secondary transfer voltage was such that the current was not excessively supplied to the high-printing image including the solid image, so the "strong dropout" did not occur. However, in the configuration of Comparative Example 2, in a poorly printed image including an isolated patch pattern, sufficient current could not be supplied to the toner portion (patch portion), resulting in "patch vomit".

On the other hand, in the configuration of this embodiment, the absolute value of the secondary transfer voltage can be increased in low-print images including isolated patch patterns, and a sufficient current can be supplied to the toner portion (patch portion). Good transferability was obtained without any occurrence of splattering. In addition, in the configuration of this embodiment, the absolute value of the secondary transfer voltage can be reduced in high-printing images including solid images, and the flow of current more than necessary can be suppressed, so that "strong dropout" occurs. good transferability was obtained.

In both the present example and Comparative Examples 1 and 2, in an environment where the absolute water content is less than 21.7 g/m ³ , the secondary transfer voltage is under “constant current control”. was transferable.

<Other configurations>
In this embodiment, the calculation of the toner amount and the correction of the secondary transfer voltage are performed for each page, but the present invention is not limited to such an aspect. For example, the amount of toner is calculated and the secondary transfer voltage is adjusted at any given cycle such as every predetermined amount of rotation (for example, one rotation) of the secondary transfer roller 20 or every predetermined amount of rotation (for example, one rotation) of the photosensitive drum 1 . Corrections may be made. Alternatively, as described above, ideally, it is preferable to change the secondary transfer voltage according to the toner amount at the secondary transfer portion N2. Therefore, for example, in a configuration in which the response of the secondary transfer power supply is sufficiently fast, the secondary transfer voltage may be changed according to the toner amount at the secondary transfer portion N2.

In this embodiment, the laser drive signal is sampled in the engine control unit 210, the pixel count value n (the number of times of "H") is measured, and the toner amount is calculated. It is not limited. For example, image information (toner amount information) may be transmitted from the controller 200 to the engine control section 210 together with a video signal, and the engine control section 210 may determine the correction amount of the secondary transfer voltage based on the information. Further, in this embodiment, the toner amount is calculated using the pixel count values for four colors with respect to the total number of samplings per color, but the present invention is not limited to such an aspect. For example, a pixel count value corresponding to a solid image (for example, a solid black image) for each paper size is stored as a constant in the RAM 152 in advance, and the toner amount is calculated from the ratio between the number and the actual pixel count value. good too.

In this embodiment, it is determined whether to execute the "constant voltage control" of the secondary transfer voltage or the "constant current control" of the secondary transfer voltage based on the detection result of the absolute moisture content by the environment sensor 300. However, the invention is not limited to such an embodiment. For example, by measuring the impedance of the secondary transfer portion N2 (the secondary transfer roller 20 and the intermediate transfer belt 10) before image formation, it is possible to determine whether the secondary transfer voltage is controlled by "constant current control" or "constant voltage control". may decide whether to execute Specifically, during non-image formation (pre-rotation process or pre-multi-rotation process) before the image formation process of a print job, a predetermined current is passed through the secondary transfer portion N and the voltage value applied at that time is The impedance is measured by measuring Alternatively, the impedance may be measured by applying a predetermined voltage and measuring the value of the current flowing at that time. When the measured impedance is small, the electrical resistance of the secondary transfer roller 20 and the intermediate transfer belt 10 is low, so it is considered that the electrical resistance of the recording material P is also low. Therefore, when the impedance is lower than a predetermined threshold value, the "constant voltage control" of the secondary transfer voltage may be executed to correct the secondary transfer voltage according to this embodiment. In addition to the impedance itself, the voltage value when the predetermined current is applied or the current value when the predetermined voltage is applied may be used as an index value correlated with the impedance for control. Alternatively, for example, for each type of recording material P, the relationship between the paper size, basis weight, paper quality, absolute moisture content, and the lower limit voltage value of the secondary transfer voltage, and the relationship between the paper size, basis weight, paper quality, and absolute moisture content. Information indicating both the relationship with the target current value for the next transfer is set in advance and stored in the ROM 151 as a table. Then, when the transfer current of the target current value is passed during the secondary transfer and the voltage value at that time is lower than the lower limit voltage value, "constant voltage control" may be executed at the lower limit voltage value. That is, the image forming apparatus 100 may include resistance detection units (current detection unit, voltage detection unit) 241 and 242 that detect an index value correlated with the electrical resistance of the transfer unit N2. performs constant voltage control of the transfer voltage when the electric resistance of the transfer portion N2 indicated by the detection results of the resistance detection units 241 and 242 is lower than a predetermined value, and controls the transfer portion N2 indicated by the detection results of the resistance detection units 241 and 242. When the electric resistance is equal to or higher than the predetermined value, constant current control of the transfer voltage can be performed. Alternatively, the control section 210 may perform constant voltage control of the transfer voltage when the voltage applied by the application section 21 becomes smaller than a predetermined value by performing constant current control of the transfer voltage.

In this embodiment, the secondary transfer voltage V is calculated from the voltage V1 and the correction amount ΔV as V=V1−ΔV, but the present invention is not limited to this aspect. For example, there is a configuration in which a primary transfer current can be supplied from a secondary transfer power source 21, like an image forming apparatus 100 in FIG. In such a configuration, determination of the secondary transfer voltage V requires further correction. In the image forming apparatus 100 of FIG. 8, elements having the same or corresponding functions or configurations as those of the image forming apparatus 100 of FIG. 1 are denoted by the same reference numerals as in FIG. In the image forming apparatus 100 of FIG. 8, the secondary transfer opposing roller 13 (furthermore, the driving roller 11 and the tension roller 12) and the primary transfer rollers 14 each include a Zener diode 17 as a voltage maintaining element (voltage stabilizing element). It is connected to ground potential through Thus, by supplying a voltage of a predetermined value or more from the secondary transfer power source 21, the potential of each primary transfer roller 14 (and the secondary transfer opposing roller 13) is maintained at a predetermined potential, and each primary transfer portion N1 is can be supplied with the primary transfer current. Note that the voltage stabilizing element (voltage sustaining element) for stabilizing the primary transfer voltage Vt1 is not limited to the Zener diode 17, and other voltage stabilizing elements such as varistors may be used as long as the same effect can be obtained. A conversion element may also be used. In the configuration shown in FIG. 8, the primary transfer voltage Vt1 is applied to the secondary transfer opposing roller 13. In the configuration shown in FIG. Therefore, the effective voltage for secondary transfer is the difference between the voltage applied to the secondary transfer roller 20 and the voltage applied to the secondary transfer opposing roller 13 . Therefore, the secondary transfer voltage to be actually applied must be the value calculated by the following formula (4) in consideration of the primary transfer voltage Vt1.
V=(V1-ΔV)+Vt1 (4)

In this embodiment, the correction amount of the secondary transfer voltage is changed linearly with respect to the change in the toner amount X, but the present invention is not limited to such an embodiment, and the correction amount of the secondary transfer voltage can be set arbitrarily. can be changed by the curve of

In this embodiment, the normal charging polarity of the toner was negative, but the present invention is not limited to such an embodiment, and the normal charging polarity of the toner may be positive. In that case, the polarity of the bias such as the secondary transfer voltage is opposite to that of this embodiment. In that case, the increase/decrease relationship of the secondary transfer voltage value including the sign in the control of the secondary transfer voltage is opposite to that of the present embodiment, but the increase/decrease relationship of the absolute value of the secondary transfer voltage is the same as that of the present embodiment. be the same.

Further, in this embodiment, the image forming apparatus 100 is configured to use toner of four colors of Y, M, C, and K, but the present invention is not limited to such an aspect. The image forming apparatus 100 may be configured to use transparent toner, metallic color toner, or the like in addition to Y, M, C, and K or in place of any one of these colors. In that case, the maximum value of the toner amount X is not limited to 400[%] in this embodiment, and may be changed in accordance with the total amount of the type of toner used.

[Example 2]
Another embodiment of the present invention will now 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 the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In Example 1, correction was performed to monotonically decrease the absolute value of the secondary transfer voltage as the toner amount X in one page increased with respect to the reference secondary transfer voltage V1. In this embodiment, there is a section of the toner amount X in which the absolute value of the secondary transfer voltage is substantially constant with respect to the change in the toner amount X in one page.

A method of determining the correction amount ΔV of the secondary transfer voltage in this embodiment will be described. FIG. 9 is a graph for explaining a method of determining the correction amount ΔV of the secondary transfer voltage in this embodiment. In FIG. 9, the horizontal axis indicates the toner amount X in one page, and the vertical axis indicates the secondary transfer voltage V that is actually applied. The toner amount X in one page on the horizontal axis can take values from 0 to 400 [%]. The voltage value in the section (section a) where the toner amount X in one page is 0≦X≦A [%] is V1. Also, the voltage value in the section (section c) where the toner amount X in one page satisfies B≦X≦C is V2. Also, the voltage value in the section (section e) where the toner amount X in one page is D≦X (D≦X≦400) [%] is V3. Here, the relationship among voltages V1, V2, and V3 is V1>V2>V3. Also, the voltage in the section of A<X<B (section b) is determined by linearly interpolating the voltage V1 and the voltage V2. Also, the voltage value in the interval C<X<D (interval d) is determined by linearly interpolating the voltage V2 and the voltage V3.

That is, the correction amount ΔV of the secondary transfer voltage is given by the following formula (5).
ΔV=0 (0≦X≦A)
ΔV=V1-{(V2-V1)/(B-A)×(X-B)+V2} (A<X<B)
ΔV=V1-V2 (B≦X≦C)
ΔV=V1-{(V3-V2)/(D-C)×(X-D)+V3} (C<X<D)
ΔV=V1-V3 (D≦X≦400)
... (5)

As described above, in this embodiment, when the toner amount X is larger than the section of the toner amount X corresponding to the voltage V1, the correction is performed to reduce the absolute value of the secondary transfer voltage. In other words, in this embodiment, the absolute value of the secondary transfer voltage is increased when the toner amount X is less than the section of the toner amount X of the voltage V2, and When the toner amount X is large, correction is performed to reduce the absolute value of the secondary transfer voltage. In this embodiment, the voltage V1 and the voltage V2 can be considered as reference values for the secondary transfer voltage.

In this embodiment, the voltages V1, V2 and V3 and the toner amounts A, B, C and D are determined based on the information on the recording material P and the environmental information. That is, in this embodiment, the relationship between the paper size, basis weight, paper quality, absolute moisture content, voltages V1, V2, V3, and toner amounts A, B, C, and D is shown for each type of recording material P. Information is preset and stored in the ROM 151 as a table. Then, the CPU 221 reads the necessary information from the table based on the acquired information of the recording material P and the environmental information, and sets the voltages V1, V2, V3 and V3 corresponding to the paper size, basis weight, paper quality, and absolute moisture content. Toner amounts A, B, C, and D are determined.

Next, the effects of this embodiment will be described. In general, "patch blurring" tends to occur in very low print, while "strong dropout" occurs not only in full solid images as shown in Fig. 6(b), but also in full halftone images. there's a possibility that. Here, the full-surface halftone image is an image in which the amount of toner is on average smaller than that of a full-surface solid image as shown in FIG. 6B. A halftone image is typically an image having a toner amount X of 20 to 100%. A full-surface halftone image is likely to cause "strong omission" due to the same mechanism as described in the first embodiment. In other words, while "patch blurring" tends to occur in a narrow print amount section with very low toner amount, "strong dropout" occurs in a relatively wide print amount section with a medium or higher toner amount. Likely to happen.

As a result of diligent studies by the inventors, it has been found that "patch blurring" tends to occur when the toner amount X is about 0 to 20%, and "strong omission" tends to occur when the toner amount X is about 100% or more. In the case of a full-surface halftone image, even if the toner amount X is about 20% or more, "strong dropout" is likely to occur. It has been found that even with a toner amount X of about 20% or more, "patch blurring" may occur in a normal image. In other words, it has been found that, depending on the image pattern, both "strong missing" and "poor patch" may occur in the interval of the toner amount X of about 20% to 100%. Therefore, the appropriate transfer voltage differs for each section of the toner amount X of low print, medium print amount, and high print, and it may not be preferable to change the transfer voltage too much in each section. Also, with regard to very low printing, it may not be preferable to change the transfer voltage from the viewpoint of the load on the secondary transfer power source 21, which will be described later.

Based on the above, the voltage V1 for suppressing patch blurring in ultra-low printing is applied between 0% and A of the toner amount X, and the voltage V2 for suppressing both strong missing and patch blurring in medium printing amount is applied. A, B, and C for determining the section B to C of the toner amount X to be applied, and the section D or more (D to 400%) of the toner amount X for applying the voltage V3 for suppressing strong dropout in high-speed printing, It has been found that the value of D is preferably in the following range. In other words, A is 3-10% to suppress patch sagging, B is 15-25%, C is 75-90% to suppress strong sagging and patch sagging, and D is 95% or more to suppress strong sagging. (typically 150% or less) was found to be preferable.

Therefore, in this embodiment, in order to suppress patch blurring in extremely low printing, voltage V1 (voltage with a large absolute value at which patch blurring does not occur) is set in the interval where the toner amount X is 0≦X≦A=5 [%]. did. In addition, in order to suppress strong missing and patch blurring in images such as full-page halftone images in which a large area of the page is covered with toner, or in the case where an isolated patch pattern partially exists, the toner amount X is set to B=20.ltoreq. A voltage V2 in the section of X≦C=80[%] (a voltage at which both patch distortion and strong omission are unlikely to occur) was set. Also, in order to suppress strong dropout in high-speed printing, a voltage V3 (a voltage at which strong dropout of a solid image of a single color or more does not occur) is set in a section where the toner amount X is X≧D=100[%]. As a result, compared with the control of Example 1, it becomes possible to set a more suitable voltage value for each image defect, and it is possible to obtain better transferability compared with Example 1. rice field.

As an example, the set values of the voltages V1, V2, and V3 when the absolute moisture content for the first side of plain paper in this embodiment is 21.7 g/m ³ are shown in the following equation (6).
V=V1=1400V (0≤X≤5%)
V=(V2-V1)/(B-A)×(X-B)+V2 (5%<X<20%)
V=V2=1000V (20%≤X≤80%)
V=(V3-V2)/(D-C)×(X-D)+V3 (80%<X<100%)
V=V3=900V (100%≤X≤400%)
... (6)

Patch blur is a phenomenon that occurs in low-print images such as isolated patch patterns, and that transfer current escapes to white areas. Therefore, as shown in equation (6), in order to supply an appropriate transfer current to the toner portion, the difference (absolute value) between the voltage V2 and the voltage V3 is compared with the voltage V1 and the voltage V2 (absolute value) is increased. The reason why the difference between the voltages V2 and V3 is smaller than the difference between the voltages V1 and V2 is as follows. In other words, strong dropout is a phenomenon that occurs when current is supplied to the entire surface of the toner area, and there is no leakage current to the white area, unlike the case of patch blurring. This is because the current increases and decreases. The reason why the secondary transfer voltage is gradually changed in the interval from A to B and in the interval from C to D is that if the secondary transfer voltage changes discontinuously with respect to the amount of toner, the discontinuity may occur. This is because there is a possibility that an appropriate secondary transfer voltage cannot be applied with an appropriate toner amount.

As described above, in this embodiment, the controller 210 sets the toner amount between the transfer voltage when the toner amount used for the toner image is the minimum and the transfer voltage when the toner amount is the maximum. Control is performed so as to have a toner amount section in which the transfer voltage is substantially constant with respect to changes in . In this embodiment, in the interval 0≦X≦A defined by the toner amount value A (0<A), the transfer voltage is substantially constant with respect to the change in the toner amount. In particular, in this embodiment, the toner amount indicates the toner amount X [%], which is the ratio of the toner amount transferred to the recording material P to the total amount of toner of each color that can be transferred to the recording material P. The interval 0≤X≤A defined by the values A, B, C, and D (0<A<B<C<D) of the quantity X is defined as interval a, the interval A<X<B as interval b, and B≤ The section of X≦C is section c, the section of C<X<D is section d, the section of D≦X is section e, the average value of the absolute values of the transfer voltage in section a is Vave1, and the transfer voltage in section c. Vave2 is the average value of the absolute values of , and Vave3 is the average value of the absolute values of the transfer voltages in the interval e, then Vave1>Vave2>Vave3 is satisfied. Further, in this embodiment, the value A is 3 to 10 [%], the value B is 15 to 25 [%], the value C is 75 to 90 [%], and the value D is 95 [%] or more. In this embodiment, (Vave1-Vave2)>(Vave2-Vave3) is satisfied. Further, in this embodiment, the transfer voltage is substantially constant with respect to the change in the toner amount X in the sections a, c, and e. In addition, in this embodiment, the absolute value of the transfer voltage gradually decreases as the toner amount X increases in the sections b and d. The average values Vave1, Vave2, and Vave3 are used because, as will be described later, the absolute values of the transfer voltages can be changed in the intervals a, c, and e.

As described above, according to the present embodiment, the secondary transfer voltage is set more appropriately than in the first embodiment in an environment such as a high-temperature and high-humidity environment in which it is difficult to simultaneously suppress the patch blurring and the strong omission. As a result, it is possible to obtain better transferability. Further, as in the first embodiment, in the present embodiment, it is possible to supply an appropriate secondary transfer voltage by constant current control of the secondary transfer voltage even in an environment other than the high-temperature and high-humidity environment. Therefore, according to this embodiment, it is possible to perform good secondary transfer regardless of the environment in which the image forming apparatus 100 is used.

<Other configurations>
In this embodiment, four thresholds A, B, C, and D are provided for the toner amount X, and the toner amount X is 0≦X≦A, B≦X≦C, and D≦X (D≦X≦400). Although the voltages V1, V2, and V3 are set to be substantially constant in the respective sections, the present invention is not limited to such an aspect. For example, as shown in FIG. 10, a section (section a) where the toner amount X is 0≦X≦A, a section (section c) where B≦X≦C, and a section (section) where D≦X (D≦X≦400). In section e), the correction amount of the secondary transfer voltage with respect to the change in the toner amount X may be changed. It is possible to change the correction amount of the secondary transfer voltage with respect to the change in the toner amount X in at least one of these sections a, c and e. In this configuration, in addition to the voltage V1, the voltage V4 at the toner amount C, the voltage V5 at the toner amount D, and the voltage in the interval C≦X≦D of the toner amount X can be considered as reference values of the secondary transfer voltage. can.

First, with regard to low printing, since patch blurring is more likely to occur as the printing becomes extremely low, patch blurring can be easily suppressed by increasing the absolute value of the secondary transfer voltage as the toner amount X decreases. However, when the toner amount X is small (the electric resistance value is low), the transfer current also increases and the load on the secondary transfer power source 21 becomes heavy. Therefore, in this embodiment, considering the load on the secondary transfer power supply 21, the secondary transfer voltage is substantially constant at voltage V1 in the interval where the toner amount X is 0≦X≦A as shown in FIG. However, if the capacity of the secondary transfer power supply 21 has sufficient capacity, control may be performed such that the absolute value of the secondary transfer voltage is gradually increased from the toner amount X from A to 0% of the entire white image.

In the section B≦X≦C, although isolated patch patterns and patterns with large areas covered by toner may coexist, as the amount of toner increases, the possibility of toner covering the page increases. Therefore, control may be performed such that the absolute value of the secondary transfer voltage is gradually decreased as the toner amount X changes from B to C. FIG. Similarly, in the section of D≦X (D≦X≦400), the greater the amount of toner, the higher the possibility of the presence of a solid image covering the entire page surface, and the more likely the occurrence of strong blanking. Therefore, in the section where the toner amount X is D≦X (D≦X≦400), the absolute value of the secondary transfer voltage may be gradually decreased as the toner amount X increases.

Note that the voltages V1, V2, V3, V4, V5, and V6 and the toner amounts A, B, C, and D in FIG. It may be decided.

In this embodiment, the secondary transfer voltage is linearly and monotonously decreased in the section where the secondary transfer voltage is changed with respect to the change in the toner amount X, but the present invention is not limited to such an aspect. Arbitrary change rate correction may be performed, such as by changing in a curved line.

Also, other configurations of the first embodiment may be applied to the above-described present embodiment, other configurations of the above-described present embodiment, or a combination thereof.

[Example 3]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatuses of the first and second embodiments. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatuses of Embodiments 1 and 2 are denoted by the same reference numerals as those of Embodiments 1 and 2. A detailed explanation is omitted.

This embodiment is a modification of the second embodiment. In this embodiment, there is a section of the toner amount X in which the absolute value of the secondary transfer voltage is increased with respect to the change in the toner amount X in one page.

FIG. 11 is a graph for explaining a method of determining the correction amount ΔV of the secondary transfer voltage in this embodiment. In FIG. 11, the horizontal axis indicates the toner amount X in one page, and the vertical axis indicates the secondary transfer voltage V that is actually applied. The toner amount X in one page on the horizontal axis can take values from 0 to 400 [%]. The voltage value in the section (section a) where the toner amount X in one page is 0≦X≦A [%] is V1. Also, the voltage value in the section (section c) where the toner amount X in one page satisfies B≦X≦C is V2. Also, the voltage value in the section (section e) where the toner amount X satisfies D≦X≦E is V3. Also, the voltage value in the section (section g) where the toner amount X is F≦X (F≦X≦400) [%] is V4. Here, the relationships among the voltages V1, V2, V3 and V4 are V1>V2>V3 and V1>V4>V3. This embodiment differs from the second embodiment in that the voltage V4 having such a relationship is provided.

That is, in this embodiment, the absolute value of the secondary transfer voltage is increased in some sections of the toner amount X while maintaining the relationship of V<V1. In this embodiment, the voltage V1 can be considered as a reference value for the secondary transfer voltage.

The voltages V1, V2 and V3 are set for the same purpose as explained in the second embodiment. That is, the voltage V1 is set for the purpose of suppressing the patch blurring of the isolated patch pattern in ultra-low printing. Also, the voltage V2 is set for the purpose of suppressing both strong missing and patch blurring in a medium print amount. Further, the voltage V3 is set for the purpose of suppressing strong dropout of a secondary color solid image from a single color solid image covering the entire page.

The voltage V4 applied in the section of F≤X (F≤X≤400) is set for the purpose of suppressing defective transfer when the page is covered with a multi-color solid image of secondary colors or more. is. This transfer failure is caused by insufficient transfer current with respect to the weight of the toner to be transferred, and thus the transfer failure becomes more likely to occur as the weight of the toner increases. Therefore, when the amount of toner such as multi-color toner increases, the absolute value of the secondary transfer voltage is made larger than the secondary transfer voltage V3 for suppressing strong dropout in order to suppress transfer failure.

Also, the voltage in the section of A<X<B (section b) is determined by linearly interpolating the voltage V1 and the voltage V2. Also, the voltage in the interval C<X<D (interval d) is determined by linearly interpolating the voltage V2 and the voltage V3. Also, the voltage in the section of E<X<F (section f) is determined by linearly interpolating the voltage V3 and the voltage V4.

Voltages V1, V2, V3, V4 and toner amounts A, B, C, D, E, and F in FIG. It's okay to be like this.

The values of A, B, C, D, E, and F preferably fall within the following ranges. As in Example 2, A, B, and C are 3 to 10% for suppressing patch swelling, 15 to 25% for B, and 75 to 90% for C for suppressing strong bleeding and patch swelling. It has been found that D is preferably set to 95% to 140% in order to suppress strong dropout. It was also found that E and F for coping with poor transfer are preferably 210 to 240% for E and 260% or more (less than 400%) for F.

Also, as described in the other configurations of the second embodiment, it is possible to change the absolute value of the secondary transfer voltage in the section a, section c, section e, and section g. In section g, as the toner amount X increases, the absolute value of the secondary transfer voltage can be gradually increased. This is because the absolute value of the secondary transfer voltage is increased as the amount of toner such as multi-color toner increases, thereby suppressing defective transfer.

As described above, in this embodiment, the control unit 210 controls the transfer voltage so that the voltage applied to the transfer member 20 by the application unit 21 is substantially constant. is the first toner amount, the transfer voltage is the first voltage, and when the toner amount is the second toner amount larger than the first toner amount, the transfer voltage is set to the absolute value of the first voltage. A second voltage having a smaller value is set, and when the toner amount is a third toner amount larger than the second toner amount, the transfer voltage is set to a value smaller in absolute value than the first voltage and higher than the second voltage. Control is performed so that the third voltage has a larger absolute value than the absolute value. More specifically, in this embodiment, the toner amount is defined as the toner amount X [%], which is the ratio of the toner amount transferred onto the recording material P to the total amount of toner of each color that can be transferred onto the recording material P. , and the interval a, A< The section where X<B is section b, the section where B≦X≦C is section c, the section where C<X<D is section d, the section where D≦X≦E is section e, and the section where E<X<F is section Section f, the section of F≦X is section g, the average value of the transfer voltage absolute values in section a is Vave1, the average value of the transfer voltage absolute values in section c is Vave2, the absolute value of the transfer voltage in section e When the average value of the values is Vave3 and the average absolute value of the transfer voltage in the section g is Vave4, Vave1>Vave2>Vave3 and Vave1>Vave4>Vave3 are satisfied. Further, in this embodiment, the value A is 3 to 10 [%], the value B is 15 to 25 [%], the value C is 75 to 90 [%], the value D is 95 to 140 [%], and the value E is 210 to 240 [%], value F is 260 "%" or more. Further, in this embodiment, the transfer voltage is substantially constant with respect to the change in the toner amount X in the sections a, c, e, and g. In addition, in this embodiment, the absolute value of the transfer voltage gradually decreases as the toner amount X increases in the sections b and d. In addition, in this embodiment, in the section f, as the toner amount X increases, the absolute value of the transfer voltage gradually increases. The reason why the average values Vave1, Vave2, Vave3, and Vave4 are set is that the absolute values of the transfer voltages can be changed in the intervals a, c, e, and g as described above. .

As described above, the voltage V1 is applied to suppress patch blurring in the case of extremely low printing. Also, in the case of an image that covers the entire page, a voltage V3 is applied to suppress strong blanking. Also, for a print amount of about 20% to 100%, which is neither low print nor high print, a voltage V2 is applied that balances both strong omission and patch blur. Thereby, each image defect can be suppressed. Furthermore, by applying the voltage V4 in the case of multi-color high-printing images in which the entire page is covered and the amount of toner increases in the height direction, transfer defects due to insufficient transfer current in high-speed printing are suppressed. be able to. In this way, image defects can be suppressed by selecting an appropriate voltage value for each image defect that may occur depending on the amount of printing.

In this embodiment, the secondary transfer voltage is linearly and monotonously increased in the section in which the secondary transfer voltage is increased with respect to the change in the toner amount X, but the present invention is limited to such an aspect. Instead, any rate of change may be corrected, such as by changing in a curved line.

Also, other configurations of the first embodiment may be applied to the configuration of the present embodiment.

[Example 4]
Another embodiment of the present invention will now be described. In the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatuses of Embodiments 1, 2 and 3 are assigned the same reference numerals as those of Embodiments 1, 2 and 3. Therefore, a detailed explanation is omitted.

The image forming apparatus of this embodiment is an image forming apparatus having no primary transfer power source. As a configuration without a primary transfer power supply, a drum voltage configuration described later in which the primary transfer member is grounded can be considered as an example. In this embodiment, a drum voltage configuration in which the primary transfer member is grounded, an intermediate transfer belt used in the drum voltage configuration, and effects when the present invention is applied to the drum voltage configuration will be described.

First, the drum voltage configuration will be described. An image forming apparatus having a drum voltage configuration in which the primary transfer member is grounded is an image forming apparatus having a high voltage power supply configuration as shown in FIG. FIG. 12 is a schematic diagram showing the connection state and grounding state of the high-voltage power supply of each portion around the primary transfer portion N1 in the image forming apparatus 100 of this embodiment. In this embodiment, the primary transfer roller 14 as the primary transfer member is connected (electrically grounded) to the ground (0 V). In this embodiment, during image formation, a voltage of −300 V is applied from the high-voltage power supply 200 to the metal core (not shown) of the photosensitive drum 1 as a drum voltage (reference voltage). On the surface of the photosensitive drum 1, an image forming potential Vl (-400 V) having an absolute value larger than that of the drum voltage is formed. Then, the difference (primary transfer contrast) between the potential (0 V) of the primary transfer roller 14 and the image forming potential Vl (-400 V) of the surface of the photosensitive drum 1 causes the image portion of the photosensitive drum 1 (image forming potential Vl portion) is primarily transferred onto the intermediate transfer belt 10 .

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

FIG. 13 is a schematic diagram showing the cross-sectional structure of the intermediate transfer belt 10 in this embodiment. In this embodiment, an endless belt having a circumference of 700 mm and a thickness of 65 μm is used as the intermediate transfer belt 10 . Further, as shown in FIG. 13, in this embodiment, the intermediate transfer belt 10 is composed of two layers, a base layer 10e having a thickness of 64 μm and an inner surface layer 10f having a thickness of 1 μm. The base layer 10 e side (outer peripheral surface side) contacts the photosensitive drum 1 , and the inner layer 10 f side (inner peripheral surface side) contacts the primary transfer roller 14 . In this embodiment, polyethylene terephthalate (PET) resin mixed with an ionic conductive agent is used as the material of the base layer 10e. Further, in this embodiment, a polyester resin mixed with carbon, which is an electronic conductive agent, is used as a conductive agent for the material of the inner surface layer 10f. The inner surface layer 10f is formed inside the base layer 10e and contacts the drive roller 11, the tension roller 12, and the secondary transfer counter roller 13. As shown in FIG. In addition, although polyethylene terephthalate (PET) resin is used as the material of the base layer 10e in this embodiment, it is also possible to use other materials. Materials such as polyester, acrylonitrile-butadiene-styrene copolymer (ABS), and mixed resins thereof can be used as the material of the base layer 10e. Moreover, in this embodiment, the polyester resin is used as the material of the inner surface layer 10f, but it is also possible to use other materials such as acrylic resin.

In this embodiment, the electrical resistance of the inner surface layer 10f is made lower than that of the base layer 10e of the intermediate transfer belt 10. FIG. In this embodiment, the volume resistivity of the intermediate transfer belt 10 is 1×10 ¹⁰ Ω·cm. Further, in this embodiment, the surface resistivity of the inner surface of the intermediate transfer belt 10 is 1.0×10 ⁶ Ω/□. In this embodiment, the environment for measuring the electrical characteristics of the intermediate transfer belt 10 is the room temperature of 23° C. and the room humidity of 50%. In this embodiment, the electrical resistance value of the base layer 10e is reflected in the volume resistivity actually measured for the intermediate transfer belt 10 because of the relationship between the electrical resistance and the thickness between the base layer 10e and the inner layer 10f. there is On the other hand, in this embodiment, the surface resistivity of the inner surface actually measured for the intermediate transfer belt 10 reflects the electric resistance value of the inner surface layer 10f.

The volume resistivity was measured using a Hiresta-UP (MCP-HT450) manufactured by Mitsubishi Chemical Corporation as a measuring instrument using a ring probe type UR (model MCP-HTP12). The surface resistivity was also measured using a ring probe type UR100 (model MCP-HTP16) in the same measuring instrument as the volume resistivity measurement. The volume resistivity was measured by applying a probe from the surface side (base layer 10e side) of the intermediate transfer belt 10 under conditions of an applied voltage of 100 V and a measurement time of 10 seconds. The surface resistivity was measured by applying a probe from the inner surface side (inner layer 10f side) of the intermediate transfer belt 10 under conditions of an applied voltage of 10 V and a measurement time of 10 seconds. In this embodiment, the volume resistivity of the intermediate transfer belt 10 is preferably in the range of 1×10 ⁹ Ω·cm or more and 1×10 ¹⁰ Ω·cm or less, and the surface resistivity of the inner surface of the intermediate transfer belt 10 is 4. 0×10 ⁶ Ω/□ or less (typically 1.0×10 ⁵ Ω/□ or more) is preferable.

Since the intermediate transfer belt 10 in the electrical resistance range as described above has a low electrical resistance to the extent that current can flow in the circumferential direction of the intermediate transfer belt 10, a sufficient primary transfer current can flow even if the primary transfer contrast is low. . Therefore, in a drum voltage configuration that does not have a primary transfer power source as in this embodiment, it is preferable to use a low resistance intermediate transfer belt 10 having the electrical resistance value as described above.

Next, the effect of applying the present invention to the drum voltage configuration will be described. In a high-temperature and high-humidity environment, the lower the electrical resistance of the intermediate transfer belt 10 as in the present embodiment, the easier it is for the secondary transfer current to flow not to the toner portion (patch portion) but to the white background portion. For example, when printing images as shown in FIGS. 6A and 6C, an equivalent circuit as shown in FIG. 14 can be considered for the secondary transfer portion N2. Each symbol in FIG. 14 represents the following.
Rr: electrical resistance value of secondary transfer roller 20 Rp: electrical resistance value of recording material P Rt: electrical resistance value of isolated patch pattern toner Ri: electrical resistance value of intermediate transfer belt 10 I1: white background portion Current passing through ・I2: Current passing through the toner portion (patch portion)

The ratio between I1 and I2 is given by the following formula (7).
I1/I2
= (Ri + Rt + Rp) / (Ri + Rp)
=1+Rt/(Ri+Rp) Expression (7)

As shown in equation (7), the smaller Ri is, the larger the ratio of I1 to I2 is. That is, the smaller the electrical resistance value Ri of the intermediate transfer belt 10 is, the easier it is for the secondary transfer current to flow not to the toner portion (patch portion) but to the white background portion. Therefore, in the configuration using the intermediate transfer belt 10 having a low resistance as described above, there are cases where patch blurring is likely to occur in an image with a small amount of toner.

Therefore, in this embodiment, the present invention is applied to a configuration using the intermediate transfer belt 10 with low resistance as described above. As a result, as in the above-described embodiment, for an image with a small amount of toner that causes patch blurring, it is possible to apply control that increases the absolute value of the secondary transfer voltage as the amount of toner decreases. As a result, even with a configuration using the intermediate transfer belt 10 of low resistance, it is possible to suppress image defects such as patch blur. Moreover, as described in the above embodiment, it is also possible to suppress the strong omission. As a result, a simple configuration without a primary transfer power supply can be realized as in this embodiment. Any of the methods of Embodiments 1, 2, and 3 may be applied as the method of controlling the secondary transfer voltage in this embodiment.

As described above, in this embodiment, the image carrier 10 is an endless belt that conveys a toner image that has been primarily transferred from another image carrier 1 to the recording material P at the transfer portion N2. configured, the belt is capable of conducting current in the circumferential direction. Further, in this embodiment, the belt has a volume resistivity of 1×10 ⁹ Ω·cm or more and 1×10 ¹⁰ Ω·cm or less.

As described above, according to this embodiment, even when the intermediate transfer belt 10 with a low resistance is used, by applying control to increase the absolute value of the secondary transfer voltage as the amount of toner decreases, It is possible to suppress patch vomit and strong omission. Therefore, according to this embodiment, it is possible to realize a simple configuration that does not have a primary transfer power source, while suppressing patch blurring and strong omission.

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

For example, in the above embodiments, the image forming apparatus is a color image forming apparatus having a plurality of image forming units, but the present invention is not limited to this, and the image forming apparatus has a single image forming unit. It may be a monochrome image forming apparatus having only In this case, the present invention may be applied to a transfer section that directly transfers a toner image from a photoreceptor as an image carrier onto a recording material.

1 photosensitive drum (photoconductor)
3 Exposure Device 10 Intermediate Transfer Belt 20 Secondary Transfer Roller 21 Secondary Transfer Power Supply 100 Image Forming Device P Recording Material

Claims (24)

an image carrier that carries a toner image;
a transfer member forming a transfer portion for transferring the toner image from the image carrier to a recording material;
an applying unit that applies a transfer voltage to the transfer member;
and a control unit that controls the application unit,
The control section controls the transfer voltage so that the voltage applied to the transfer member by the application section is substantially constant, and the toner amount used for the toner image is the first toner amount. When the toner amount is a second toner amount larger than the first toner amount, the transfer voltage is set to a smaller absolute value than the first voltage. An image forming apparatus characterized by performing control to set the voltage to the second voltage. an acquisition unit that acquires toner amount information related to the toner amount;
an exposure unit that exposes the image carrier or another image carrier carrying toner to be transferred to the image carrier according to image information;
2. The image forming apparatus according to claim 1, wherein the acquisition section acquires the toner amount information based on a drive signal for causing the exposure section to emit light according to the image information. 3. The image forming apparatus according to claim 1, wherein toner amount information regarding the toner amount is obtained for each toner image transferred onto one sheet of recording material. an operation of conveying the recording material having the toner image transferred and fixed on the first surface to the transfer unit and transferring the toner image onto the second surface of the recording material;
The controller controls the absolute value of the difference between the first voltage and the second voltage to be greater when the toner image is transferred to the first surface than when the toner image is transferred to the second surface. 4. The image forming apparatus according to claim 1, wherein at least one of said first voltage and said second voltage is changed. having an environment detection unit that detects environmental information about at least one of environmental temperature and humidity;
The controller controls a second absolute moisture content in which the absolute moisture content of the environment indicated by the detection result of the environment detection unit is greater than the first absolute moisture content. At least one of the first voltage and the second voltage is changed so as to increase the absolute value of the difference between the first voltage and the second voltage in the case of the moisture content. The image forming apparatus according to any one of claims 1 to 4. The controller gradually adjusts the absolute value of the transfer voltage between the transfer voltage when the toner amount is minimum and the transfer voltage when the toner amount is maximum, as the toner amount increases. 6. The image forming apparatus according to any one of claims 1 to 5, wherein control is performed so as to reduce the . Between the transfer voltage when the toner amount is minimum and the transfer voltage when the toner amount is maximum, the control unit keeps the transfer voltage substantially constant with respect to changes in the toner amount. 6. The image forming apparatus according to claim 1, wherein control is performed so as to have a certain toner amount section. 8. The method according to claim 7, wherein in a section of 0≦X≦A defined by the toner amount value A (0<A), the transfer voltage is substantially constant with respect to changes in the toner amount. image forming device. The toner amount indicates the toner amount X [%], which is the ratio of the toner amount transferred to the recording material to the total amount of toner of each color that can be transferred to the recording material. C, D (0<A<B<C<D) defined by 0≦X≦A as interval a, A<X<B as interval b, and B≦X≦C as interval c , the interval C<X<D is interval d, the interval D≦X is interval e, the average value of the absolute values of the transfer voltage in the interval a is Vave1, the absolute value of the transfer voltage in the interval c 6. The apparatus according to any one of claims 1 to 5, wherein Vave1>Vave2>Vave3 is satisfied, where Vave2 is an average value and Vave3 is an average value of the absolute values of the transfer voltages in the interval e. Image forming device. The value A is 3 to 10 [%], the value B is 15 to 25 [%], the value C is 75 to 90 [%], and the value D is 95 [%] or more. 10. The image forming apparatus according to Item 9. 11. The image forming apparatus according to claim 9, wherein (Vave1-Vave2)>(Vave2-Vave3) is satisfied. 12. The image forming method according to any one of claims 9 to 11, wherein the transfer voltage is substantially constant with respect to the change in the toner amount X in the section a, the section c, and the section e. Device. 13. The image forming apparatus according to claim 9, wherein in the interval b and the interval d, the absolute value of the transfer voltage gradually decreases as the toner amount X increases. When the control section performs constant voltage control on the transfer voltage so that the voltage applied to the transfer member by the application section is substantially constant, the control section adjusts the transfer voltage when the toner amount is a first toner amount. setting a first voltage, setting the transfer voltage to a second voltage whose absolute value is smaller than the absolute value of the first voltage when the toner amount is a second toner amount larger than the first toner amount, and When the toner amount is a third toner amount larger than the second toner amount, the transfer voltage has an absolute value smaller than the absolute value of the first voltage and an absolute value larger than the absolute value of the second voltage. 6. The image forming apparatus according to any one of claims 1 to 5, wherein control is performed so as to set the third voltage having a large value. The toner amount indicates the toner amount X [%], which is the ratio of the toner amount transferred to the recording material to the total amount of toner of each color that can be transferred to the recording material. C, D, E, F (0<A<B<C<D<E<F) defined by 0≦X≦A as interval a, A<X<B as interval b, and B≦B The section of X≦C is section c, the section of C<X<D is section d, the section of D≦X≦E is section e, the section of E<X<F is section f, and the section of F≦X is section g , Vave1 is the average value of the absolute values of the transfer voltage in the interval a, Vave2 is the average value of the absolute values of the transfer voltage in the interval c, and Vave2 is the average value of the absolute values of the transfer voltage in the interval e. 15. The image forming apparatus according to claim 14, wherein Vave1>Vave2>Vave3 and Vave1>Vave4>Vave3 are satisfied, where Vave3 is an average value of the absolute values of the transfer voltages in the interval g and Vave4 is an average value of the absolute values of the transfer voltages. . The value A is 3 to 10 [%], the value B is 15 to 25 [%], the value C is 75 to 90 [%], the value D is 95 to 140 [%], and the value E is 210 to 16. The image forming apparatus according to claim 15, wherein the value F is 240[%] or greater than 260%. 17. The image forming apparatus according to claim 15, wherein the transfer voltage is substantially constant with respect to changes in the toner amount X in the section a, the section c, the section e, and the section g. . 18. The image forming apparatus according to claim 15, wherein in the interval b and the interval d, the absolute value of the transfer voltage gradually decreases as the toner amount X increases. 19. The image forming apparatus according to any one of claims 15 to 18, wherein in the section f, the absolute value of the transfer voltage gradually increases as the toner amount X increases. having an environment detection unit that detects environmental information about at least one of environmental temperature and humidity;
The control section is capable of constant current control of the transfer voltage so that the current supplied to the transfer member by the application section is substantially constant. The constant voltage control of the transfer voltage is performed when the transfer voltage is equal to or greater than a predetermined value, and the constant current control of the transfer voltage is performed when the absolute moisture content indicated by the detection result of the environment detection unit is less than the predetermined value. The image forming apparatus according to any one of claims 1 to 19. a resistance detection unit that detects an index value correlated with the electrical resistance of the transfer unit;
The control section is capable of constant current control of the transfer voltage so that the current supplied to the transfer member by the application section is substantially constant. The constant voltage control of the transfer voltage is performed when the electrical resistance is lower than a predetermined value, and the constant current of the transfer voltage is performed when the electrical resistance of the transfer portion indicated by the detection result of the resistance detection portion is equal to or greater than the predetermined value. 20. The image forming apparatus according to any one of claims 1 to 19, wherein control is performed. The control section is capable of constant current control of the transfer voltage so that the current supplied to the transfer member by the application section is substantially constant, and the constant current control of the transfer voltage is performed to apply the transfer voltage. 20. The image forming apparatus according to claim 1, wherein the constant voltage control of the transfer voltage is performed when the voltage applied to the portion becomes smaller than a predetermined value. The image carrier is composed of an endless belt that conveys a toner image that has been primarily transferred from another image carrier so that the toner image is secondarily transferred onto the recording material at the transfer unit. 23. The image forming apparatus according to any one of claims 1 to 22, wherein the current can flow in one direction. 24. The image forming apparatus according to claim 23, wherein the belt has a volume resistivity of 1*10< ⁹ > [Omega].cm or more and 1* ¹⁰ <10 > [Omega].cm or less.

Images