JP5591024B2 - Image forming apparatus - Google Patents

Description

  The present invention is an image forming apparatus that forms a latent image on an image carrier and visualizes the latent image using toner (developer), and has low rigidity as a recording material (for example, recording material, OHP sheet). The present invention relates to an image forming apparatus capable of stably providing a high-quality image even with a recording material such as a thin recording material. In particular, the present invention is suitable for application to an image forming apparatus such as a copying machine or a printer (for example, a laser printer or an LED printer) that employs an electrophotographic system and forms an image on a recording material.

  In an electrophotographic image forming apparatus, each color toner image formed on an image carrier provided for each color is sequentially primary-transferred onto an intermediate transfer body such as an intermediate conveyance belt to superimpose a multicolor toner image. A color image forming apparatus that has been formed and secondary-transferred collectively onto a recording material has been put to practical use. By using the intermediate transfer member, it is possible to perform stable image formation since the transfer process to the recording material whose state is easily changed is only required once. The recording material on which the multicolor toner image is secondarily transferred is then separated from the intermediate transfer member.

  Here, with respect to the recording material, in recent years, as the application of the image forming apparatus is expanded, it is required to cope with a recording material with low rigidity such as a thin recording material. A thin recording material having low rigidity may not be stably transferred by the secondary transfer process because the leading edge of the recording material is deformed or delayed due to the curling condition of the leading edge of the recording material or the friction with the guide. Therefore, the recording material is electrostatically adsorbed on an endless conveying belt that conveys the recording material, and passes through the secondary transfer portion.

  The recording material separated from the intermediate transfer member passes through the secondary transfer portion and is separated from the conveying belt which is a recording material conveying means. When the stiffness of the recording material becomes extremely low, it becomes difficult to separate the recording material from the conveying belt. Therefore, it is conceivable to reduce the diameter of the roller located at the separation portion that separates the recording material from the conveying belt. In this case, although it is advantageous for the separation property, the increase in the curvature causes the peeling discharge at the separation portion to increase, thereby disturbing the arrangement of the toner image on the recording material and causing an image defect. Will occur.

  The peeling discharge occurs when the recording material is in a state of being largely biased to either positive or negative. Therefore, in order to suppress this peeling charge, it is known to arrange a corona charger in the separation part to remove the charge causing the peeling discharge (Patent Document 1).

  Apart from this, considering that the toner layer potential varies depending on the charge amount per unit mass of the toner as the charge amount of the toner, the applied voltage to the discharge electrode of the pre-transfer charging means is set according to the detected toner charge amount. It is known to control (Patent Document 2).

JP-A-2005-3731 JP 2006-251288 A

  The present invention relates to a further improvement of the device according to Patent Document 1. The purpose is to provide an image forming apparatus that can be separated from the conveying belt in a direction that reduces peeling discharge without causing image defects due to elimination of the charge of the toner even for a recording material such as a thin recording material with low rigidity. It is to provide.

In order to achieve the above object, a typical configuration of an image forming apparatus according to the present invention includes an image carrier, an image forming unit that forms an image using toner holding electric charges on the image carrier, and a plurality of image forming units. A flexible endless belt that is stretched and circulated by a tension member, a belt that carries and transports a recording material, and the image carrier and transfer nip portion via the belt A transfer member that forms a toner image , an adsorbing unit that supplies electric charge to the recording material upstream of the transfer nip portion in the belt conveyance direction, and adsorbs the recording material to the belt, and a charge amount per unit mass held by the toner And a mode switching means for switching a plurality of image forming modes having different maximum toner image loading amounts, and the recording material from the belt on the downstream side of the transfer nip portion in the belt conveyance direction. The An image forming apparatus for separated and supplies at least the detection result of said detecting means, and the length in the width direction of the recording material the recording material intersecting the direction to be conveyed, and the image forming mode, the suction means in response to And a control means for controlling the amount of current .

In addition, a typical configuration of another image forming apparatus according to the present invention includes an image carrier, an image forming unit that forms an image using a toner having a charge held on the image carrier, and a plurality of stretching members. A flexible endless belt that is stretched around and circulated, and that carries and conveys a recording material, and a transfer that forms a transfer nip portion with the image carrier via the belt A member, suction means for supplying charge to the recording material upstream of the transfer nip portion in the belt conveyance direction to attract the recording material to the belt, and detection for detecting the amount of charge per unit mass held by the toner And an image for separating the recording material from the belt on the downstream side in the belt conveyance direction from the transfer nip portion, and a mode switching means for switching a plurality of image forming modes having different maximum loading amounts of toner images. form In the device, a second means provided separately from said suction means be in the transfer nip belt conveyance direction upstream side than the portion, and the detection result of at least the detecting means, the width crossing the direction in which the recording material is conveyed Control means for controlling the amount of current supplied to the second means according to the length of the recording material in the direction and the image forming mode .

  According to the present invention, even a recording material such as a thin recording material having low rigidity can be separated from the conveying belt in a direction that reduces peeling discharge without causing image defects due to elimination of the charge of the toner.

It is a figure which shows the 1st Embodiment of this invention. (A) is a diagram showing that separation from the belt is possible when the recording material is a thick recording material, and (b) is a diagram showing that separation from the belt is not possible when the recording material is a thin recording material. . It is a figure which shows the behavior of the toner at the time of image development. FIG. 8A is an explanatory diagram of charge distribution in a comparative example, and FIG. 7B is an explanatory diagram of charge distribution in the first embodiment, in a case where the toner holding amount is large. In this case, the toner retention amount is small, (a) is an explanatory diagram of the charge distribution in the comparative example, and (b) is an explanatory diagram of the charge distribution of the first embodiment. It is a block diagram of a 1st embodiment. It is a figure which shows the flowchart of 1st Embodiment. FIG. 4 is a diagram illustrating a state of occurrence of peeling discharge due to an amount of charge per unit mass of toner and an adsorption bias, according to the first exemplary embodiment. It is a figure which shows 2nd Embodiment. FIG. 6A is an explanatory diagram of charge distribution in a comparative example, and FIG. 6B is an explanatory diagram of charge distribution in the second embodiment in a case where the toner holding amount is large. FIG. 8A is an explanatory diagram of charge distribution in a comparative example, and FIG. 9B is an explanatory diagram of charge distribution of the second embodiment in a case where the toner holding amount is small. It is a figure which shows the flowchart of 2nd Embodiment. FIG. 10 is a diagram illustrating a state of occurrence of peeling discharge due to an amount of charge per unit mass of toner and an adsorption bias in the second embodiment. It is a figure which shows 3rd Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings of the following embodiments, the same or corresponding parts are denoted by the same reference numerals.

<< First Embodiment >>
(Image forming device)
(Image formation on photosensitive drum)
FIG. 1 is an explanatory diagram of an image forming apparatus according to the first embodiment. Corresponding to yellow (Y), magenta (M), cyan (C) and black (k) toners of chromatic toner, 1Y, 1M, 1C and 1k are provided as photosensitive drums (latent image carriers).

  The photosensitive drum 1 is rotationally driven in the direction of arrow A at a predetermined peripheral speed (process speed). The peripheral surface of the photosensitive drum 1 is charged to a predetermined polarity and potential by the charging means 2 (primary charging). The laser beam scanner as the image exposure unit 3 outputs a laser beam modulated on / off in accordance with image information input from an external device such as an image scanner (not shown) or a computer, and performs charging processing on the drum 1. Scan exposure of the surface. By this scanning exposure, an electrostatic latent image corresponding to target image information is formed on the surface of the photosensitive drum 1.

Developers 4Y, 4M, 4C, and 4k containing toners of color components of Y, M, C, and Bk are arranged in the vicinity of the drum 1, and the developer 4 to be used is selected based on the image information, and the surface of the photosensitive drum 1 The developer (toner) is developed on top, and the electrostatic latent image is visualized as a toner image. A reversal development method is used in which toner is attached to the exposed portion of the electrostatic latent image for development. The charge amount per unit mass in each developing device 4 is set to −30 uC / g, and the stirring condition and the replenishment amount in the developer device are controlled. However, depending on the environment, toner replenishment timing, etc., −10 Vary between -40 uC / g. Further, the amount of applied toner per unit area of each color is controlled between 0.50 and 0.65 mg / cm ² .

(Primary transfer from photosensitive drum to intermediate transfer belt)
An endless belt-shaped intermediate transfer belt 6 is disposed opposite to the drum 1, and an unfixed toner image of each color on the drum 1 is formed for each rotation of the intermediate transfer belt 6. The primary transfer means 5 sequentially and electrostatically transfers sequentially onto the intermediate transfer belt 6 to obtain a full-color image in which four color unfixed toner images are superimposed on the intermediate transfer belt. On the other hand, every time one rotation of the photosensitive drum 1 after the primary transfer, the surface of the photosensitive drum 1 is subjected to cleaning of the transfer residual toner by the cleaning device 11 and repeatedly enters an image forming process.

  The intermediate transfer belt 6 is stretched around a plurality of stretching rollers 20, 21, and 22 and is rotated in the direction of arrow G at 250 to 300 mm / sec. In this embodiment, the tension roller 20 is a tension roller that controls the tension of the intermediate transfer belt 6 to be constant, the tension roller 21 is a counter roller for the upstream secondary transfer roller, and the tension roller 22 is an intermediate transfer belt. 6 drive rollers.

  As the intermediate transfer belt 6, an appropriate amount of carbon black as an antistatic agent is contained in a resin such as polyimide and polycarbonate, or various rubbers, the volume resistivity is 1E + 9 to 1E + 14 [Ω · cm], and the thickness is 0.07 to The one set to 0.1 [mm] is used.

(Secondary transfer from intermediate transfer belt to recording material)
A constant voltage bias (transfer bias) controlled to a predetermined value is applied from the power supply unit 13 to the secondary transfer roller 9 as a transfer member in the transfer nip N. A transfer bias having a polarity opposite to that of the toner is applied to the secondary transfer roller 9 to collectively transfer the four full-color images superimposed on the intermediate transfer belt 6 to the recording material P at the secondary transfer portion. A full-color unfixed toner image is formed on the recording material P. The secondary transfer current is +30 to 60 uA, and the secondary transfer current needs to change depending on factors such as the dry state of the recording material, which is the recording material, the environment, the charge amount and the applied amount of toner to be transferred. This is because the amount of current changes.

  The secondary transfer roller 9 is disposed opposite to the stretching roller 21 and can contact and separate from the intermediate transfer belt 6. While the unfixed toner image is superimposed on the intermediate transfer belt 6, it is not brought into contact, but is contacted when secondary transfer is performed on the recording material P at the end of stacking to form a contact nip to form a secondary transfer portion. .

  The secondary transfer roller 9 to which a secondary transfer high-voltage power supply 13 with a variable supply bias is attached is composed of an elastic layer of ion conductive foamed rubber (NBR rubber) and a cored bar. The secondary transfer roller 9 has an outer diameter of 16 mm, a roller surface roughness Rz = 6.0 to 12.0 (μm), a longitudinal width of 340 mm, and a resistance value of N / N (23 ° C., 50% RH). ) In the measurement, it is 1E + 5 to 1E + 7Ω under the condition of 2 kV application.

  After being transferred by the secondary transfer roller 9, the intermediate transfer belt 6 separated from the recording material P is cleaned by the cleaning device 12 with residual transfer toner, recording material powder, and the like, and repeatedly enters an image forming process.

(Adsorption of recording material to the conveyor belt)
A recording paper (not shown) stores recording paper as a recording material P, and a paper supply roller (not shown) is driven based on a paper supply start signal so that the recording materials P in the paper supply cassette are one by one. The paper is fed and conveyed in the direction of arrow B by the registration roller 8. The recording material P conveyed by the registration roller 8 is first used as an adsorption unit to which a bias is applied (current is supplied) by a tension roller 25 of the conveyance belt 24 and an adsorption bias application unit 30 (FIG. 1) as will be described later. By being pinched by the suction roller 28, it is attracted to the transport belt 24.

  After the suction, the recording paper P is fed by controlling the conveyance of the recording material P by the registration roller 8 so as to synchronize with the timing when the leading end of the toner image on the intermediate transfer belt 6 reaches the secondary transfer site.

With respect to the suction, a suction current of −15 to −30 μA flows through the suction bias controlled by the constant current control by the suction bias applying means 30 (FIG. 1) . The suction bias applied by the suction bias applying means 30 is controlled by the suction bias control means 50, and the relationship is shown in the block diagram of FIG. That is, based on the recording material size information designated by the user operation unit 40, the printing mode, the development current detected by the ammeter 4A of the developing device 4, and the density information detected by the patch sensor 17 for patch detection, The suction bias applied to the bias applying means 30 is determined.

  The suction roller 28 is constituted by a pair of a roller 25 disposed inside the transport belt 24 and a roller disposed outside. The roller disposed inside the conveyor belt 24 is made of an elastic layer of ion conductive solid rubber (NBR rubber) and a cored bar. The rubber roller has an outer diameter of 18 mm and a resistance value of N / N (23 ° C., 50% RH), and becomes 1E + 5 to 1E + 6Ω under the condition of 50V application. The roller disposed outside the conveyor belt 24 is a fur brush roller, and enters the conveyor belt 24 by 1.5 to 2 mm. Then, a hair having a hair length of 5 mm, a cored bar diameter of 8 mm, an outer diameter of 18 mm, and a resistance value of 1E + 5 to 1E + 6Ω by applying 100 V of N / N (23 ° C., 50% RH) measurement was used.

  The conveying belt 24 is a flexible endless belt that is stretched around a plurality of stretching members and carries the recording material P. That is, the conveying belt 24 is stretched around a plurality of stretching rollers 25, 26, and 27 and is rotated in the direction of arrow B at 250 to 300 mm / sec. In the present embodiment, the tension roller 26 is a separation tension roller positioned at a separation portion that separates the recording material P from the conveyance belt 24. In addition, an elastic body having a Young's modulus measured by a tensile test method (JIS K 6301) of 10 MPa or less is used as the transport belt 24.

(Separation of recording material from conveyor belt)
The recording material P that has passed through the transfer nip N is conveyed to the stretching roller 26 downstream of the transfer nip N in the belt conveyance direction. Further, the recording material P is separated from the conveyance belt 24 by the separation claw 29 on the downstream side, conveyed to a fixing device (not shown), subjected to a heating and pressure fixing process of a toner image, and then discharged outside the apparatus. The

  When the recording material is a thick recording material, the recording material P is easily separated from the transport belt 24 by the separation claw 29 as shown in FIG. However, when the recording material P is a thin recording material, as shown in FIG. 2B, it is difficult for the recording material P to be separated from the conveying belt 24 by the separation claw 29. In this case, separation is facilitated by reducing the diameter of the separation tension roller 26 and increasing the curvature. The separation tension roller 26 is a SUS roller having an outer diameter of 16 mm.

  Here, as means for assisting the separation, in addition to the separation claw 29 provided downstream of the separation stretching roller 26, a separation assist roller is provided on the back surface of the conveying belt 24 upstream of the separation stretching roller 26. Also good.

(Detection of charge held by toner)
As a detection method, which is known from Patent Document 2, FIG. 3 shows the behavior of toner during development. In developing the electrostatic latent image on the image carrier ZT (photosensitive drum 1) to form a toner image on the image carrier ZT, a developing bias voltage is applied to the developing sleeve GS of the developing device 4 by the power source E. Is done. In the developing process, the toner T carrying a charge (negative charge in FIG. 3) moves from the developing sleeve GS to the image carrier ZT, whereby a developing current flows through the bias voltage applying circuit.

  That is, each of the color developing devices 4Y to 4K is provided with a developing ammeter for detecting a developing current when developing the toner onto the photosensitive drums 1Y to 1K, and the toner T is transferred from the developing sleeve GS to the image carrier ZT. Since current flows only by movement, the development current is measured with an ammeter.

  Further, by measuring the amount (mass) of the toner adhering to the image carrier ZT, the charge amount q which is the amount of charge (so-called tribo) of the unit mass of toner is detected.

  That is, the charge amount q of the toner of the unit mass is obtained by the following equation by detecting the development current I when the toner image having the predetermined area S and the toner adhesion amount M per unit area is formed by time integration. Calculated.

I = dQ / dt, Q = q × M × S
Specifically, the image exposure means 3 is driven to form an electrostatic latent image of a patch image consisting of a solid image having a predetermined area on the photosensitive drum 1. Here, the toner charge amount per unit area (q × M) can be calculated by detecting the developing current value when the electrostatic latent image is developed by the developing device 4.

  Here, the detection result of the density sensor is fed back to the development bias current of the developing device 4, and the development bias current is controlled so that the output of the density sensor becomes constant, and a toner image of a predetermined density is formed. It is assumed that the development current I is measured with an ammeter.

  Further, the density of the toner image on the intermediate transfer body 6 when the developed toner image is transferred to the intermediate transfer body 6 is detected by a density sensor, and the amount of applied toner per unit area is calculated.

If the toner charge amount per unit area is divided by the toner applied amount per unit area, a tribo which is the charge amount per unit mass can be obtained as the retained charge amount of the toner.
Table 1 shows the relationship between the average developing current I and the toner charge amount q which is the charge amount of the toner of unit mass.

The toner charge amount q varies depending on the environment such as temperature and humidity, the usage history of the developer and toner, the image formation conditions, and the like. The toner charge amount q, which is the charge amount of the unit mass toner shown in Table 1, is This is detected by forming a toner image under the following development conditions.
Toner: 6.5 μm, cyan toner (specific gravity 1.1)
Toner image: 50 mm wide x 10 mm long (formation time 0.2 sec)
Toner adhesion amount 6g / square m
Photosensitive drum speed: 220mm / sec
(Charge amount held by toner and adsorption bias)
When the amount of charge held by the toner changes, the charge balance on the recording material changes, and sometimes the peeling discharge occurs at the separation portion of the conveying belt 24, and the toner image on the recording material is disturbed. Therefore, when the charge amount of the toner increases, the charge balance goes in the negative direction. Therefore, it is conceivable to apply the adsorption bias in the direction in which the bias having the polarity opposite to that of the toner increases to maintain the charge balance on the recording material. Here, the larger the toner tribo (the amount of charge per unit mass of toner), the larger the maximum applied amount of toner in the print mode designated by the user, the larger the value of the suction bias.

  The adsorption current table is shown in Table 2. As shown in FIG. 4A, the larger the retained charge amount of the toner, the more easily the charge amount on the recording material including the toner becomes negative after the secondary transfer. As a result, when the conveying belt is separated, a peeling discharge that relaxes the polarity of the negatively charged recording material is generated, resulting in an image defect. Therefore, when the amount of charge held by the toner increases with respect to the predetermined reference value, the charge supplied to the recording material is previously controlled by the suction bias applying means 30 in a direction in which a bias having a polarity opposite to that of the toner increases. As a result, when the recording material is separated from the conveying belt 24, the recording material is in a balanced state as shown in FIG. 4B even when the toner is placed. Therefore, the peeling discharge at the time of separation is alleviated, and the image defect caused by the peeling discharge is suppressed.

  Conversely, when the amount of charge held by the toner is small, as shown in FIG. 5A, the charge amount on the recording material after the secondary transfer tends to be positive even if the toner is included, and the recording is positively charged. An exfoliation discharge that relaxes the polarity of the material occurs to cause image defects. Therefore, when the amount of charge held by the toner is smaller than the predetermined reference value, as shown in FIG. 5B, the recording material is applied by the suction bias applying means 30 in the direction in which the bias having a polarity opposite to that of the toner decreases in advance. The electric charge supplied to is controlled. As a result, when the recording material is separated from the conveyance belt 24, the recording material is in a state of being balanced in terms of charge even when the toner is placed. Image defects are suppressed.

FIG. 8 shows the state of occurrence of peeling discharge due to the charge amount per unit mass and the adsorption bias as described above. That is, as long as the recording material P is separated from the transport belt 24, the discharge when the recording material P is separated from the conveyance belt 24 can be suppressed as long as it falls within the white background portion of FIG. Is applied. The relationship shown in FIG. 8 is a relationship when the recording material width of the recording material P is 236 to 267 mm. If the width of the recording material is increased, the current that leaks to the portion with a small impedance in the suction roller pair can be reduced. Therefore, even if the charge amount per unit mass is the same, the suction current is reduced. Therefore, the value of the suction bias determined by the charge amount per unit mass of the toner and the recording material width is a matrix shown in Table 1.

Here, FIG. 7 shows details of a flow in which the suction bias control circuit 50 determines the suction bias. First, the user selects the paper size (the length of the recording material in the width direction intersecting the transport direction) with the user operation unit 40. Next, the developing current is measured by the developing ammeter of the developing device 4 to calculate the charge amount of the toner. Then, the applied toner amount is calculated from the toner patch density measurement using the patch sensor 17. Here, the toner tribo (the charge amount per unit mass) is calculated from the charge amount of the toner and the applied toner amount. Next, when the user selects a print mode (image formation mode) of full-color image formation or single-color image formation by the user operation unit 40, the toner image is adsorbed when the maximum applied amount is 200% (full-color image formation). The adsorption current is derived from the current table. In cases other than 200% (for example, monochromatic image formation), the charge amount information per unit mass is converted according to the toner image loading amount, and the adsorption current is derived from the adsorption current table. That is, in modes other than full-color image formation, the amount of charge per unit area of the toner image is changed during full-color image formation according to the amount of toner applied on the recording material, and the adsorbed charge is determined accordingly. The amount is controlled. In the present exemplary embodiment, at least the current supplied to the suction unit according to the detection result of the detection unit that detects the amount of charge per unit mass held by the toner, the length of the recording material in the width direction, and the image forming mode. The amount can be controlled.

  In this embodiment, the tribo (the charge amount per unit mass of toner) is obtained as the charge amount information of the toner for the following reason. The density gradation of the toner image is performed by changing the density of dots of a certain density. For this reason, the charge per unit area of the individual dot regions is almost the same whether the image is a dense toner image or a halftone image in which toner images are sparsely arranged.

  Here, assuming that the toner has a certain amount of charge, a method of calculating the amount of charge in the entire image area from the image ratio and controlling the amount of adsorbed charge accordingly is considered. In this method, the charge amount of the entire image area is small like a halftone image, but when the individual dot image has sufficient charge, the balance of the charge amount is lost in the area where the image is placed and peeling is performed. Discharge may occur. Therefore, the amount of charge per unit area of the area where the dot image is placed is obtained, and the amount of charge that can be balanced is supplied to the entire surface regardless of the image ratio.

In this embodiment, the charge amount per unit mass is obtained as the retained charge amount of the toner. This is because the maximum amount of toner on the recording material changes depending on the image creation mode, and the amount of charge per unit area of the toner image on the recording material changes accordingly. . In other words, in the present embodiment, the maximum toner image amount transferred to the recording material when forming a full-color image is 1.2 mg / cm ² corresponding to 200% toner amount for the maximum secondary color image. Is assumed to be transcribed.

  However, when a monochromatic image is formed or there is a low toner consumption mode, the maximum applied amount of the toner image is smaller than that, and the charge amount per unit area is also changed. For this reason, it is necessary to change the amount of charge supplied by the suction bias supply means 30 in order to balance the charge.

  The matrix shown in Table 2 assumes that the maximum applied toner amount is 200% during full-color image formation. In other image forming modes, the amount of charge per unit area of the toner image is determined by the amount of toner loaded on the recording material, and the amount of charge absorbed is controlled accordingly. doing. For example, when forming a monochromatic image, the amount of charge per unit area of the toner image is halved compared to when forming a full color image. Therefore, a value obtained by halving the amount of charge detected per unit mass is applied to the matrix shown in Table 1. Determine the adsorption bias.

<< Second Embodiment >>
In the first embodiment, the amount of charge supplied to the recording material by the suction bias applying unit 30 is switched according to only the amount of charge held by the toner. In the present embodiment, the amount of charge on the recording material after the secondary transfer varies depending on the secondary transfer bias applied at the time of secondary transfer. Accordingly, the amount of adsorption current is controlled.

FIG. 9 is an explanatory diagram of a configuration of the image forming apparatus according to the second embodiment. Also in this embodiment, the charge amount per unit mass is detected as the retained charge amount of the toner. Then, the suction bias charge amount control circuit 50 controls the suction bias application means 30 according to the flow shown in FIG. The following points differ from the flow shown in FIG. That is, the user first selects the paper basis weight in addition to the paper size with the user operation unit 40, and then acquires humidity information with the environment sensor 41, and then derives the secondary transfer current value from the secondary transfer current table. The subsequent flow is the same as that shown in FIG. In this embodiment, the detection result of the detection unit that detects the amount of charge per unit mass held by the toner, the length of the recording material in the width direction, the image forming mode, and the transfer current amount applied by the transfer bias Accordingly, the amount of current supplied to the adsorption means can be controlled.

  That is, the suction applied to the suction bias applying means 30 from the detected charge amount per unit mass of the toner and the secondary transfer current value applied with the secondary transfer bias with reference to the suction current table shown in Table 3. The bias is determined. The larger the secondary transfer current value of the polarity opposite to that of the toner, the smaller the amount of current applied by the attracting bias applying means, and the larger the detected charge amount per unit mass of the toner, the greater the attracting bias value. In this embodiment, the value of the suction bias is generally smaller than that in the first embodiment.

  Here, as shown in Table 4, the secondary transfer current value is determined based on the environmental information such as humidity detected by the environmental sensor 41 and the recording material basis weight information designated by the user on the user operation unit 40. The current value is applied according to the transfer current table.

Here, as in the adsorption current table shown in Table 3, the meaning of controlling the adsorption bias according to the amount of charge per unit mass of toner will be described in detail. As the amount of negative charge held by the toner becomes larger than the positive charge supplied to the recording material by the secondary transfer current, the recording including the toner is performed after the secondary transfer as shown in FIG. The amount of charge on the material tends to be negative. Then, at the time of separation from the conveying belt, a peeling discharge that relaxes the polarity of the negatively charged recording material occurs, and an image defect occurs. Therefore, when the amount of charge held by the toner increases with respect to the predetermined reference value in relation to the secondary transfer current amount, the suction bias applying means 30 applies the recording material to the recording material in a direction in which a bias having a polarity opposite to that of the toner increases in advance. The supplied charge is controlled. Thus, when the recording material is separated from the conveying belt 24, the recording material is in a balanced state as shown in FIG. 10B even when the toner is placed. Peeling discharge is mitigated and image defects caused by peeling discharge are suppressed. Conversely, when the amount of charge held by the toner is smaller than the positive charge supplied to the recording material by the secondary transfer current, as shown in FIG. 11A, the recording material after the secondary transfer is included even if the toner is included. The amount of charge above tends to be positive. Then, a peeling discharge that relaxes the polarity of the positively charged recording material is generated, causing an image defect. Therefore, when the amount of charge held by the toner becomes smaller than the predetermined reference value, as shown in FIG. 11B, the suction bias applying means 30 applies the recording material to the recording material in the direction in which the bias having a polarity opposite to that of the toner decreases in advance. The supplied charge is controlled. As a result, when the recording material is separated from the conveyance belt 24, the recording material is in a state of being balanced in terms of charge even when the toner is placed. Image defects are suppressed.

  FIG. 13 shows the state of occurrence of peeling discharge due to the toner tribo (charge amount per unit mass) and the adsorption bias as described above. That is, if the recording material P is separated from the conveying belt 24 as long as it falls within the region indicated by the white background portion in FIG. Therefore, a necessary suction bias is applied according to the amount of charge per unit mass of toner. The relationship shown in FIG. 13 is a relationship when the secondary transfer current is 40 to 50 μA. If the secondary transfer current increases, the amount of positive charge supplied to the recording material P increases. Therefore, even if the charge amount per unit mass is the same, the adsorption current may be small. For this reason, the charge amount per unit mass of toner and the value of the adsorption bias determined by the secondary transfer current amount are in the matrix shown in Table 3.

<< Third Embodiment >>
This embodiment is different from the first embodiment or the second embodiment in that the amount of electric charge supplied to the recording material is controlled by a second means provided separately from the suction roller. That is, as shown in FIG. 14, in addition to the suction roller 28, the conveying roller 100 is provided upstream of the transfer nip portion in the belt conveying direction. The recording material is adsorbed to the conveyance belt by the adsorption roller 28, but the charge amount control on the recording material is not performed by the adsorption roller but by the conveyance roller 100. In this embodiment, at least the detection result of the detection unit that detects the amount of charge per unit mass held by the toner, the length of the recording material in the width direction intersecting the direction in which the recording material is conveyed, and the image forming mode , The amount of current supplied to the second means can be controlled. Furthermore, the amount of current supplied to the second means can be controlled in accordance with the amount of transfer current applied by the transfer bias.

(Modification)
In the above embodiment, the toner image formed on the photosensitive drum 1 is primarily transferred to the intermediate transfer belt 6 and the toner image formed on the intermediate transfer belt is secondarily transferred to the recording material P. The toner image formed on the drum 1 may be directly transferred to the recording material P.

  Further, the charge amount held by the toner is detected in the developing device 4 related to the photosensitive drum 1 as the first image carrier, but it is related to the intermediate transfer belt 6 as the second image carrier. You may make it detect.

  In addition, regarding the detection of the amount of charge held by the toner, in the case of monochrome image formation only with a monochrome image forming apparatus, in addition to detecting the amount of charge per unit mass of toner, the amount of charge per unit area is also determined. It is also possible to detect.

1 .... photosensitive drum, 2 .... primary charger, 3 .... exposure device, 4 .... developer, 5 .... primary transfer roller, 6 .... intermediate transport belt, 8 .... registration roller, ... Secondary transfer roller,
11 .. Drum cleaning device, 12 .... Belt cleaning device, 13 .... Secondary transfer high voltage power supply, 17 .... Patch sensor, 20 .... Tension roller, 21 ... Counter roller of secondary transfer roller, 22 .... Drive Roller, 24 ... Conveying belt, 28 ... Suction roller, 29 ... Separation claw, 30 ... Suction bias application means, 40 ... User operation unit,
50 ·· Adsorption bias control circuit, P ·· Recording material, N ·· Transfer nip

Claims (6)

An image carrier;
An image forming means for forming an image using a toner having a charge held on the image carrier;
A flexible endless belt that is stretched around a plurality of stretching members and carries the recording material;
A transfer member that forms a transfer nip with the image carrier via the belt;
An adsorbing means for supplying an electric charge to the recording material on the upstream side in the belt conveyance direction from the transfer nip portion and adsorbing the recording material to the belt;
Detecting means for detecting the amount of charge per unit mass held by the toner;
Mode switching means for switching a plurality of image forming modes having different maximum loading amounts of toner images;
In the image forming apparatus for separating the recording material from the belt on the downstream side in the belt conveyance direction from the transfer nip portion,
Control means for controlling the amount of current supplied to the suction means in accordance with at least the detection result of the detection means, the length of the recording material in the width direction intersecting the direction in which the recording material is conveyed, and the image forming mode And an image forming apparatus. A power supply unit configured to apply a transfer bias to the transfer member when the recording material carried and conveyed by the belt passes through the transfer nip, and the control unit includes a detection result of the detection unit; The amount of current supplied to the suction unit is controlled according to the length of the recording material in the width direction, the image forming mode, and the amount of transfer current applied by the transfer bias. The image forming apparatus described in 1. The image forming apparatus according to claim 1, wherein the image carrier is an intermediate transfer belt. An image carrier;
An image forming means for forming an image using a toner having a charge held on the image carrier;
A flexible endless belt that is stretched around a plurality of stretching members and carries the recording material;
A transfer member that forms a transfer nip with the image carrier via the belt;
An adsorbing means for supplying an electric charge to the recording material on the upstream side in the belt conveyance direction from the transfer nip portion and adsorbing the recording material to the belt;
Detecting means for detecting the amount of charge per unit mass held by the toner;
Mode switching means for switching a plurality of image forming modes having different maximum loading amounts of toner images;
In the image forming apparatus for separating the recording material from the belt on the downstream side in the belt conveyance direction from the transfer nip portion,
A second means provided on the upstream side of the transfer nip portion in the belt conveyance direction and provided separately from the suction means;
Control for controlling the amount of current supplied to the second means in accordance with at least the detection result of the detection means, the length of the recording material in the width direction intersecting the direction in which the recording material is conveyed, and the image forming mode. And an image forming apparatus. A power supply unit configured to apply a transfer bias to the transfer member when the recording material carried and conveyed by the belt passes through the transfer nip, and the control unit includes a detection result of the detection unit; The amount of current supplied to the second means is controlled according to the length of the recording material in the width direction, the image forming mode, and the amount of transfer current applied by the transfer bias. 5. The image forming apparatus according to 4 . The image forming apparatus according to claim 4 , wherein the image carrier is an intermediate transfer belt.