JP2011248128A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of effectively performing a two-stage electrostatic fur brush cleaning and reducing the amount of toner carried on an intermediate transfer belt even in the case of image forming of a monochrome low-density halftone image.SOLUTION: A control part 110 extracts a halftone image from the received job image data to determine a total tone value of the halftone image of each color. A ratio of areas in which a total tone value is less than 128/255 when converted into monochrome is determined. If the area ratio is 80% or more, a voltage applied to a secondary transfer part T2 is reduced to suppress the occurrence of reversed polarity toner. In contrast, if the area ratio is less than 80%, the absolute value of a voltage applied to fur brushes 51a and 51b is increased to improve the electrostatic fur brush cleaning performance.

Description

  The present invention relates to an image forming apparatus that electrostatically cleans transfer residual toner adhering to an intermediate transfer belt, and more particularly to transfer voltage control that suppresses deterioration in electrostatic cleaning performance in image formation of a halftone image.

  2. Description of the Related Art Image forming apparatuses that form a full-color toner image by transferring a toner image of a halftone image of each color to an intermediate transfer member and transferring the toner image onto an intermediate transfer member are widely used in a transfer unit. (Patent Document 1). An image forming apparatus has also been put to practical use in which a two-stage electrostatic member is disposed on an intermediate transfer body on the downstream side of a transfer section, and residual transfer toner is electrostatically removed from the intermediate transfer body in two stages (Patent Document). 2, 3).

  Patent Document 1 discloses a tandem type image forming apparatus in which image forming units of yellow, magenta, cyan, and black are arranged along an intermediate transfer belt. Here, on the downstream side of the transfer portion, a two-stage fur brush is brought into contact with the intermediate transfer belt to electrostatically collect the transfer residual toner.

  Patent Document 2 discloses an image forming apparatus that electrostatically collects untransferred toner by bringing a two-stage charging roller into contact with a photosensitive drum. Here, a voltage having the same polarity as the charging polarity of the toner is applied to the first charging roller that is in contact with the photosensitive drum on the downstream side of the transfer portion, and the charging polarity is made uniform while dispersing the transfer residual toner adhering to the photosensitive drum. ing. A voltage opposite in polarity to the charging polarity of the toner is applied to the second charging roller that is in contact with the photosensitive drum on the downstream side of the first charging roller so that the transfer residual toner is efficiently transferred from the photosensitive drum to the second charging roller. Collected.

  Patent Document 3 discloses an image forming apparatus in which a web cleaning device is arranged downstream of an electrostatic fur brush cleaning device that electrostatically collects untransferred toner from an intermediate transfer belt. The web cleaning device contacts and removes the transfer residual toner from the intermediate transfer belt by bringing a web-like fiber material into contact therewith.

JP 2002-207403 A Japanese Patent Application Laid-Open No. 07-306617 Japanese Patent Laid-Open No. 10-149033

  In the electrostatic fur brush cleaning device disclosed in Patent Document 1, similarly to Patent Document 2, a voltage having the same polarity as the charging polarity of the toner is applied to the upstream fur brush, and the toner fur is applied to the downstream fur brush. A voltage having a polarity opposite to the charging polarity is applied. The upstream fur brush electrostatically collects the toner charged to the opposite polarity while dispersing the toner charged to the normal polarity on the intermediate transfer belt. Further, in addition to the normal polarity toner that has passed through the upstream fur brush, the downstream fur brush electrostatically collects the toner that is in contact with the upstream fur brush and charged to the normal polarity.

  However, when the intermediate transfer belt is cleaned with a two-stage electrostatic fur brush cleaning device, it has been found that if an image is formed with a high ratio of low-density halftone images, the cleaning efficiency is significantly reduced. did. Rather than secondary and tertiary full-color images with a large amount of applied toner per image, a single color low density halftone image with much less toner applied passes through the electrostatic fur brush cleaning device. Turned out to increase.

  As a result, the transfer residual toner that adheres to the intermediate transfer belt increases, and “fogging image defect” in which unnecessary toner is transferred to the white background portion of the image easily occurs. In addition, when the web cleaning device is disposed downstream of the electrostatic fur brush cleaning device, the web of the web cleaning device is easily contaminated, resulting in an increase in web consumption per hour and an increase in cleaning cost. It became.

  The present invention provides an image forming apparatus capable of efficiently performing two-stage electrostatic cleaning and reducing the amount of toner accompanying the intermediate transfer body even in image formation in which the area ratio of a single color low density halftone image is high. It is an object.

  The image forming apparatus of the present invention applies a voltage to the intermediate transfer member, a toner image forming unit capable of transferring a halftone image of each color on the intermediate transfer member, and a transfer unit sandwiching the recording material. A voltage having the same polarity as the charging polarity of the toner is applied to the transfer means for transferring the toner image from the intermediate transfer member to the recording material and the first electrostatic member in contact with the intermediate transfer member on the downstream side of the transfer portion. A first electrostatic processing means capable of dispersing the transfer residual toner on the intermediate transfer body and aligning the charging polarity of the transfer residual toner passing therethrough, and the downstream side of the first electrostatic member; A second electrostatic processing unit that applies a voltage having a polarity opposite to the charging polarity of the toner to the second electrostatic member that contacts the intermediate transfer member, and collects the transfer residual toner on the intermediate transfer member; It is. In the image formation in which the total amount of applied toner for each color is less than a predetermined level, the transfer voltage is applied so that the absolute value of the voltage applied to the transfer unit is lower than in the image formation in which the total amount of applied toner for each color is greater than or equal to the predetermined level. Control means for controlling the means is provided.

  In the image forming apparatus of the present invention, in the case of a halftone image in which the area density of the transferred toner is small, the voltage applied to the transfer unit is reduced to reduce the transfer current. For this reason, the generation of reversal polarity toner generated by injecting excessive charge into the toner at the time of transfer is suppressed, and the reversal polarity toner generated when the reversal polarity toner is pulled back from the recording material to the intermediate transfer member is reduced. . When the reversal polarity toner is reduced, the transfer residual toner that follows the intermediate transfer member without being captured by the first electrostatic member or charged to the normal polarity is reduced.

  Therefore, even in the image formation in which the area ratio of the single-color, low-density halftone image is high, the two-stage electrostatic cleaning can be efficiently executed to reduce the toner accompanying the intermediate transfer belt.

1 is an explanatory diagram of an overall configuration of an image forming apparatus. FIG. 6 is an explanatory diagram of a cleaning configuration of an intermediate transfer belt. It is explanatory drawing of operation | movement of a 2nd cleaning apparatus. 2 is a block diagram of a secondary transfer voltage control system of the image forming apparatus. FIG. 3 is a flowchart of cleaning control according to the first exemplary embodiment. 6 is a time chart of cleaning control in normal image formation. It is a time chart of the cleaning control when the halftone image is 80% or more. It is a time chart of the cleaning control when the halftone image is less than 80%. FIG. 6 is an explanatory diagram of a transfer residual toner reduction effect in Embodiment 1. FIG. 10 is an explanatory diagram of a cleaning configuration of an intermediate transfer belt in Embodiment 2. 10 is a flowchart of cleaning control according to the second embodiment. It is a time chart of the cleaning control when the halftone image is 80% or more. FIG. 10 is an explanatory diagram of a transfer residual toner reduction effect in Embodiment 2.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention provides another embodiment in which part or all of the configuration of the embodiment is replaced with the alternative configuration as long as the amount of reverse polarity toner generated in the transfer unit is controlled to maintain the electrostatic cleaning performance. But it can be done.

  Accordingly, in the case of an image forming apparatus that electrostatically cleans an intermediate transfer member, there is no distinction between an image carrier charging method, an electrostatic image forming method, a developer and developing method, a primary transfer method, and a tandem type / 1 drum type. Can be implemented.

  In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. The image forming apparatus can be used for various purposes such as a printer.

  In addition, about the general matter of the image forming apparatus shown by patent document 1, 2, 3, and a cleaning apparatus, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<Image forming apparatus>
FIG. 1 is an explanatory diagram of the overall configuration of the image forming apparatus. As shown in FIG. 1, the image forming apparatus 1 is a tandem intermediate transfer type full-color printer in which yellow, magenta, cyan, and black image forming units 10 a, 10 b, 10 c, and 10 d are arranged along an intermediate transfer belt 16. is there.

  In the image forming unit 10a, a yellow toner image is formed on the photosensitive drum 11a and transferred to the intermediate transfer belt 16. In the image forming unit 10b, a magenta toner image is formed on the photosensitive drum 11b and transferred to the intermediate transfer belt 16. In the image forming units 10c and 10d, a cyan toner image and a black toner image are formed on the photosensitive drums 11c and 11d, respectively, and transferred to the intermediate transfer belt 16. The four-color toner images transferred to the intermediate transfer belt 16 are conveyed to the secondary transfer portion T2 and secondarily transferred to the recording material P.

  The recording material P stored in the feeding unit 30 is held at a predetermined feeding position by the lifter plate 31 and fed one by one by a feeding separation unit 32 adopting an air conveyance system. The recording material P is conveyed by the feeding / conveying unit 33, adjusted in registration by the registration unit 34, and then conveyed to the secondary transfer portion T 2 in synchronization with the toner image on the intermediate transfer belt 16.

  In the process in which the intermediate transfer belt 16 and the recording material P are overlapped and sandwiched and conveyed by the secondary transfer portion T2, a positive voltage is applied to the secondary transfer outer roller 35, whereby the recording material is transferred from the intermediate transfer belt 16 to the recording material. The toner image is secondarily transferred to P. The recording material P onto which the toner image has been secondarily transferred is transported to the fixing unit 37 by the pre-fixing transport unit 36 and is heated and pressurized by the fixing unit 37 to fix the toner image to the recording material P.

  In the case of the single-sided printing process, the recording material P is conveyed by the discharge unit 38 and discharged to the discharge tray 39. On the other hand, in the case of the double-sided printing process, the recording material P is conveyed to the reversing unit 40, the front and back are reversed, and then conveyed again to the feeding / conveying unit 33 and the registration unit 34 via the double-sided conveying unit 41. Then, after the back side image is transferred and fixed, it is discharged to the discharge tray 39.

  The image forming units 10a, 10b, 10c, and 10d, which are examples of toner image forming units, are configured substantially the same except that the colors of the toners used in the developing devices 13a, 13b, 13c, and 13d are different. Hereinafter, the image forming unit 10a will be described, and the image forming units 10b, 10c, and 10d will be described by replacing “a” at the end of the reference numerals attached to the components of the image forming unit 10a with “b”, “c”, and “d”. To do.

  In the image forming unit 10a, a corona charger 22a, an exposure device 12a, a developing device 13a, a primary transfer roller 15a, and a drum cleaning device 14a are arranged around the photosensitive drum 11a.

  A photosensitive drum 11a, which is an example of an image carrier, is formed with a photosensitive layer having a negative polarity on the outer peripheral surface of an aluminum cylinder, and rotates in the direction of arrow R1 at a predetermined process speed. The corona charger 22a charges the surface of the photosensitive drum 11a to a uniform negative dark portion potential VD. The exposure device 12a scans the laser beam with a rotating mirror and writes an electrostatic image of the image on the surface of the charged photosensitive drum 11a. The developing device 13a develops the electrostatic image using a developer containing toner and carrier to form a toner image on the surface of the photosensitive drum 11a.

  The primary transfer roller 15 a presses the inner surface of the intermediate transfer belt 16 to form a primary transfer portion between the photosensitive drum 11 a and the intermediate transfer belt 16. By applying a positive DC voltage to the primary transfer roller 15a, the negative toner image carried on the photosensitive drum 11a is primarily transferred to the intermediate transfer belt 16. The drum cleaning device 14a collects the transfer residual toner remaining on the photosensitive drum 11a by avoiding the transfer to the recording material P.

  The intermediate transfer belt 16, which is an example of an intermediate transfer body, is a so-called elastic belt in which a resin endless belt is used as a base material and the surface thereof is covered with an elastic rubber layer. Resin materials such as polyimide, PET, and PVDF are used as the material of the base material, and silicon rubber, urethane rubber, and the like are used as the material of the elastic layer. The elastic layer forms a stable transfer nip in the secondary transfer portion T2, and enables transfer of high-quality toner images to various recording materials P.

  The intermediate transfer belt 16 is stretched around a tension roller 17, a driving roller 18, and a secondary transfer inner roller 19, and given tension is applied by the tension roller 17. The drive roller 18 is rotationally driven by a drive motor (not shown) to rotate the intermediate transfer belt 16 in the arrow R2 direction. A toner patch reading sensor 20 for reading a belt position detection patch is disposed in the vicinity of the intermediate transfer belt 16.

<Elastic intermediate transfer belt>
In recent years, in an image forming apparatus using an electrostatic process, an intermediate transfer belt has been used because of the need for high-quality images on a wide variety of papers, and toner particle size reduction and particle shape non-sphericalization have been promoted. Yes. As the intermediate transfer belt, a resin belt typified by polyimide or the like is widely used because of its high image quality, long life, and highly stable characteristics. As the intermediate transfer belt cleaning device, a cleaning blade system having a high cleaning ability is widely used. It is used.

  However, the resin belt has a problem of a hollowing out phenomenon that occurs at the time of transfer due to a change in toner. The void phenomenon is a phenomenon in which a large pressure acts locally when a toner image is transferred to cause toner particles to aggregate and adhere to the intermediate transfer belt, and is prominent at the center of a character image or line image. Appears in In the case of a resin belt, the pressure acting on the toner image at the time of transfer tends to concentrate locally, so that a hollowing out phenomenon is likely to occur.

  Therefore, in order to eliminate the hollow-out phenomenon, an elastic intermediate transfer belt in which a rubber elastic layer is disposed on the surface layer is employed instead of the resin intermediate transfer belt. In the elastic intermediate transfer belt, the surface layer exhibits rubber elasticity and is soft, and the pressure is dispersed throughout the toner image so that local pressure concentration does not occur. The elastic intermediate transfer belt has better adhesion to the surface of the recording material in the secondary transfer area, so the transfer efficiency is higher than that of the resin belt, and the transferability to thick paper and the transferability to recording materials with large surface irregularities are also achieved. Are better.

  However, the elastic intermediate transfer belt has a problem that a cleaning blade type belt cleaning device cannot be adopted. In the elastic intermediate transfer belt, the pressed surface layer is depressed, so that the edge of the cleaning blade bites into the surface layer and the contact resistance increases, the behavior of the edge of the cleaning blade becomes unstable, and cleaning failure tends to occur. Become. The cleaning blade is likely to be turned over, chattered, squealed, and the like, and the surface of the photosensitive drum is easily damaged and the toner is fused.

  For this reason, in an elastic intermediate transfer belt, an electrostatic fur brush cleaning device or a web cleaning device with a small contact load is generally employed as a belt cleaning device.

  In the electrostatic fur brush cleaning device, a fur brush in which conductive fibers are wound around a metal core is brought into contact with an elastic intermediate transfer belt on the downstream side of the secondary transfer portion. Then, by applying a DC voltage having a polarity opposite to the charging polarity of the toner to the fur brush, the transfer residual toner is electrostatically adsorbed to the fur brush and removed from the intermediate transfer member.

  However, the electrostatic fur brush cleaning method has restrictions on the amount of toner that can be cleaned and the charging polarity of the toner, compared with the cleaning blade method that mechanically removes toner. In the electrostatic fur brush cleaning method, after the toner is electrostatically adsorbed to the fur brush, the original effect of the fur brush cannot be obtained unless the toner is further transferred from the fur brush by a metal cylindrical member to which a voltage is applied. . As the amount of toner adsorbed by the fur brush increases, the cleaning performance deteriorates, so that the general cleanable amount is inferior to the cleaning blade method.

  Therefore, a web cleaning device that removes the adhering matter adhering to the intermediate transfer belt so as to be wiped off by the cleaning web is disposed on the downstream side of the fur brush cleaning device. A cleaning web is brought into contact with the intermediate transfer belt to remove substances such as toner and external additives that cannot be collected by the fur brush alone, thereby improving the total cleaning performance.

<Belt cleaning device>
FIG. 2 is an explanatory diagram of a cleaning configuration of the intermediate transfer belt. FIG. 3 is an explanatory diagram of the operation of the second cleaning device.

  As shown in FIG. 2, the image forming apparatus 1 includes a first cleaning device 50 using a two-stage electrostatic fur brush and a second cleaning device 60 using a cleaning web. Toner, external additives, and the like remaining on the intermediate transfer belt 16 after the secondary transfer are removed by the electrostatic cleaning type first cleaning device 50 and the cleaning web type second cleaning device 60.

  The secondary transfer outer roller 35 as an example of a secondary transfer unit abuts on the intermediate transfer belt 16 whose inner surface is supported by the secondary transfer inner roller 19 to form a secondary transfer portion T2 as an example of a transfer portion. To do. The power source 56 applies a voltage to the secondary transfer portion T2 that sandwiches the recording material to transfer the toner image from the intermediate transfer belt 16 to the recording material P. The control unit 110 executes constant current control that varies the voltage applied to the secondary transfer unit T2 so that the transfer current detected by the current measurement unit 57 becomes a predetermined value.

  The first cleaning device 50 electrostatically cleans the intermediate transfer belt 16 using an upstream first electrostatic processing unit and a downstream second electrostatic processing unit. The first electrostatic processing means 50a includes a fur brush 51a, a counter roller 21a, a bias roller 52a, and a cleaning blade 53a. The second electrostatic processing means 50b includes a fur brush 51b, a counter roller 21b, a bias roller 52b, and a cleaning blade 53b.

  The fur brush 51a, which is an example of a first electrostatic member, contacts the intermediate transfer belt 16, which is an example of an intermediate transfer member, on the downstream side of the secondary transfer portion T2. The first electrostatic processing unit 50a applies a voltage having the same polarity as the charging polarity of the toner to the fur brush 51a to disperse the transfer residual toner on the intermediate transfer body, and aligns the charging polarity of the transfer residual toner passing therethrough. It is possible. A fur brush 51b, which is an example of a second electrostatic member, contacts the intermediate transfer belt 16 on the downstream side of the fur brush 51a. The second electrostatic processing unit 50b applies a voltage having a polarity opposite to the charging polarity of the toner to the fur brush 51b, and collects the transfer residual toner on the intermediate transfer belt 16.

  The opposing rollers 21a and 21b are disposed in contact with the inner surface side of the intermediate transfer belt 16, and are connected to the ground potential. Fur brushes 51a and 51b are arranged on the outer surface side of the intermediate transfer belt 16 facing the opposing rollers 21a and 21b. The fur brushes 51a and 51b are formed by implanting conductive fibers on the surface of the metal shaft, and the tips of the conductive fibers are in contact with the surface of the intermediate transfer belt 16 at a predetermined penetration depth. Since the fur brushes 51a and 51b are driven to rotate in the counter direction with respect to the traveling direction of the intermediate transfer belt 16, the fur brushes 51a and 51b rotate to push the intermediate transfer belt 16 back and rub against the surface.

  A metal bias roller 52a, which is an example of a metal cylindrical member, is in contact with the conductive fibers of the upstream fur brush 51a, and is driven to rotate in the forward direction with respect to the fur brush 51a. The bias application power supply 55a, which is an example of a power supply, applies a negative cleaning bias to the bias roller 52a, and transfers the transfer residual toner charged to the positive polarity from the intermediate transfer belt 16 to the bias roller 52a via the fur brush 51a. Electrostatically move sequentially. The toner transferred to the surface of the bias roller 52a is scraped off by the cleaning blade 53a.

  A metal bias roller 52b, which is an example of a metal column member, is in contact with the conductive fiber of the fur brush 51b on the downstream side, and is rotationally driven in the forward direction with respect to the fur brush 51b. A bias application power supply 55b, which is an example of a power supply, applies a positive cleaning bias to the bias roller 52b, and transfers the negatively charged transfer residual toner from the intermediate transfer belt 16 to the bias roller 52b via the fur brush 51b. Electrostatically move sequentially. The toner adhering to the surface of the bias roller 52b is scraped off by the cleaning blade 53b.

  DC voltages having opposite polarities are applied to the bias rollers 52a and 52b. For this reason, the transfer residual toner is electrostatically attracted to either the fur brushes 51a and 51b and cleaned from the intermediate transfer belt 16 regardless of the charging polarity.

  As shown in FIG. 3, the second cleaning device 60, which is an example of a web cleaning unit, brings the web-like fiber material into contact with the intermediate transfer belt 16 and wipes the toner from the intermediate transfer belt 16.

  In the second cleaning device 60, a non-woven cleaning web 61 is wound around a web roller 63 from an attachment roller 62 and stretched around a take-up roller 64. The cleaning web 61 contacts the outer surface of the intermediate transfer belt 16 whose inner surface is supported by the driving roller 18 and cleans the transfer residual toner by rubbing the surface of the intermediate transfer belt 16. The take-up roller 64 is rotationally driven by a drive motor 68 and the cleaning web 61 is taken up, whereby new cleaning webs 61 are sequentially supplied to the contact portion with the intermediate transfer belt 16.

  As the material of the cleaning web 61, one or more kinds selected from polyester, acrylic, vinylon, water-soluble vinylon, rayon, nylon, polypropylene, cotton and the like can be used. However, it is not limited to the above materials.

  The second cleaning device 60 is supported so as to be able to contact and separate from the intermediate transfer belt 16 by rotating about a rotation center shaft 65. The second cleaning device 60 is driven by a motor actuator 66 and is configured to be rotatable about a rotation center shaft 65. When the entire second cleaning device 60 is rotated by the motor actuator 66, the cleaning web 61 is set to the contact state shown in FIG. 2 and the separated state shown in FIG. 3 with respect to the intermediate transfer belt 16. This is to prevent a large amount of toner from being taken into the cleaning web 61 when a non-transferred high density toner is supplied on the intermediate transfer belt 16 when a jam occurs in the recording material P.

  The motor actuator 66 may be replaced with a solenoid, a cam, or the like, and the configuration of separating and contacting the intermediate transfer belt 16 may be configured of separating and contacting by parallel movement instead of rotation.

  By the way, when the developer charging performance decreases according to the deterioration state of the developer or the temperature and humidity of the installation environment, the reverse polarity toner increases in the transfer residual toner on the intermediate transfer belt. When a halftone image of a single color and low density is formed in a state where the charging performance of the developer is deteriorated, the reverse polarity toner further increases in the transfer residual toner on the intermediate transfer belt.

  If the reversal polarity toner on the intermediate transfer belt exceeds the limit amount, it passes through the two-stage electrostatic fur brush cleaning device and is taken to the intermediate transfer belt to be mixed with the next full-color image and cause color unevenness. End up.

  When the toner image is secondarily transferred from the intermediate transfer belt 16 to the recording material P, the transfer current is set assuming that the toner image of each color is sufficiently loaded with toner. In the secondary transfer portion, the secondary transfer current is set to be large so that the toner images of a plurality of colors can be transferred to the recording material with high transfer efficiency in a state where the toner is sufficiently loaded thereon.

  However, among full-color images, there are also monochrome and low-density halftone images. In this case, the transfer current set in accordance with the toner images of a plurality of colors of medium to high density gradation is considerably excessive as compared with the amount of toner actually transferred in the secondary transfer portion, and excessive charge injection into the toner. Will be done. In the case of image formation in which there is an image signal for performing exposure of a single-tone halftone image, a toner amount corresponding to the transfer current set assuming a sufficient toner amount is not transferred.

Table 1 shows the toner charge amount at each part when the toner images having the same transfer current setting and the toner application amount M / S of 1.63 [mg / cm ² ] and 0.45 [mg / cm ² ] are secondarily transferred. It is a measurement result of Q / M [μC / g].

  As shown in Table 1, when the applied toner amount is 1.63 and 0.45, the toner charge amount Q / M [μC / g] of the toner collected by the developing sleeve Slv and the photosensitive drum Dr is equal, but the secondary amount The toner charge amount Q / M [μC / g] after transfer is greatly different. When the applied toner amount is 0.45, the secondary transfer current assuming that the applied toner amount is 1.63 becomes excessive, and when the toner passes through the secondary transfer portion T2, the toner charge amount Q / M is a plus having a reverse polarity. Get higher on the side.

  For this reason, the secondary transfer current is excessive as compared with the toner density on the intermediate transfer member. If the secondary transfer current is set as usual, the secondary transfer current becomes excessive for a single-tone halftone image, so-called strong omission occurs in the secondary transfer portion T2, and the image is reversed on the intermediate transfer belt 16. Polar toner increases. A large amount of reversal polarity toner is generated at the time of secondary transfer, and is pulled back from the recording material P to the intermediate transfer belt 16 and is carried around.

  Here, since the DC voltages having different polarities are applied to the fur brushes 51a and 51b, the transfer residual toner should be adsorbed and removed regardless of whether the transfer residual toner on the intermediate transfer belt 16 is positive or negative. It is.

  However, since the reverse polarity toner is not captured by the fur brush 51b to which a positive voltage of the same polarity is applied, the intermediate transfer belt 16 must be recovered by the upstream fur brush 51a or charged to the normal polarity. I will take you around. When the reverse polarity toner is generated in the secondary transfer portion T2 in an overlapping state, the voltage at the tip of the fur brush 51a is lowered, and there is no room for attracting the reverse polarity toner, and the toner is leaked to the fur brush 51b on the downstream side. The upstream fur brush 51a cannot be sufficiently cleaned, causing reverse polarity toner to slip through.

  Then, since the fur brush 51b on the downstream side cannot collect the reversal polarity toner, the reversal polarity toner passes through the fur brush 51b and is rotated around the intermediate transfer belt 16. The reverse polarity toner that has passed through the fur brush 51a due to insufficient dispersion passes through 100% of the fur brush 51b on the downstream side to which a plus voltage is applied.

  As a result, a large amount of untransferred toner flows and stays in the cleaning web 61 of the second cleaning device 60, the cleaning web 61 becomes clogged with toner, and finally the toner is discharged downstream. When the cleaning web 61 exceeds the limit of the toner holding capacity, the transfer residual toner passes through the cleaning web 61 and reaches the primary transfer portion of the photosensitive drum 11a. The transfer residual toner is mixed into the yellow toner image which is primarily transferred from the photosensitive drum 11a to the intermediate transfer belt 16, and color unevenness occurs in the toner image on the intermediate transfer belt 16 to greatly reduce the quality of the output image.

  Therefore, it has been proposed to increase the absolute value of the negative voltage applied to the fur brush 51a to improve the cleaning performance for the reverse polarity toner. However, if the absolute value of the voltage to be applied is increased, the current that always flows through the fur brush 51a increases, the resistance increase speed of the fur brush increases, and the cleaning performance of the fur brush 51a deteriorates at an early stage. When the life of the fur brush 51a is shortened, there is a concern that cleaning failure starts frequently before reaching the expected life. For this reason, it is desirable to reduce the time for increasing the high pressure applied to the fur brush 51a as much as possible to prevent the resistance from increasing and to achieve a long life.

  Therefore, in the following embodiments, the toner density parameter on the recording material is calculated from the image signal that is exposed in the image forming unit of each color, and the secondary transfer current is set to be smaller than usual in image formation with a low toner density. ing. The reverse polarity toner on the intermediate transfer belt 16 is reduced by avoiding the occurrence of strong omission transfer failure at the secondary transfer portion T2.

<Example 1>
FIG. 4 is a block diagram of a secondary transfer voltage control system of the image forming apparatus. FIG. 5 is a flowchart of cleaning control according to the first embodiment. FIG. 6 is a time chart of cleaning control in normal image formation. FIG. 7 is a time chart of the cleaning control when the halftone image is 80% or more. FIG. 8 is a time chart of the cleaning control when the halftone image is less than 80%. FIG. 9 is an explanatory diagram of the transfer residual toner reduction effect in the first embodiment.

  As shown in FIG. 1, image forming units 10 a, 10 b, 10 c, and 10 d, which are examples of toner image forming means, can transfer toner images of halftone images of respective colors while being superimposed on the intermediate transfer belt 16.

  As illustrated in FIG. 2, the control unit 110, which is an example of a control unit, performs image formation in which the total amount of applied toner for each color is less than a predetermined level, compared to image formation in which the amount of applied toner for each color is greater than or equal to a predetermined level. The absolute value of the voltage applied to the secondary transfer portion T2 is lowered. The control unit 110 lowers the voltage applied to the secondary transfer unit T2 in image formation with a single color and less than a predetermined gradation, compared to image formation in which a plurality of colors are superimposed. The controller 110 determines the voltage applied to the fur brush 51a and the fur brush 51b in image formation in which the area where the total amount of applied toner of each color is less than a predetermined level is less than a predetermined ratio than in image formation in which the area is greater than or equal to a predetermined ratio. Increase the absolute value.

  As shown in FIG. 4, when the toner application amount M / S of the halftone image is less than a predetermined level, the control unit 110 performs the secondary transfer unit more than when the toner application amount of the halftone image is equal to or higher than the predetermined level. Reduce the voltage applied to T2. Specifically, when the total gradation of the image signals of a plurality of colors each composed of a halftone image with halftone dots is less than a predetermined level, it is more secondary than when the total gradation is above a predetermined level. The voltage applied to the transfer portion T2 is lowered.

  The control unit 110 increases the absolute value of the voltage applied to the fur brush 51a and the fur brush 51b when the ratio of the image area where the total gradation of each color is less than a predetermined level occupies the entire image is less than the predetermined level. Specifically, based on image data transmitted as an image forming job, image data for exposure of each color to be supplied to the image forming units 10a, 10b, 10c, and 10d is formed. Then, a halftone image is extracted from the image data, and a total gradation of overlap of each color is obtained. Then, an area ratio of the image area in which the total gradation of each color is less than a halftone level described later to the entire image is obtained.

  In the first embodiment, the area ratio threshold for switching the cleaning control is set to 80% of the entire image forming area. The control unit 110 determines that the area ratio of a halftone image having a predetermined gradation (128/255) or less in the entire image is 80% as a cleaning control threshold.

  In image formation in which a halftone image having a predetermined gradation or less is included more than that, the voltage applied to the secondary transfer portion T2 is lowered to give priority to the prevention of the generation of reverse polarity toner. However, if the halftone image having a predetermined gradation or less is less than 80%, the voltage applied to the secondary transfer portion T2 is set as normal, and the absolute value of the voltage applied to the fur brushes 51a and 51b is increased to perform cleaning. Increase performance. On the other hand, when there is no halftone image of a predetermined gradation or less, the control of normal setting is applied without suppressing the generation of polarity reversal toner and improving the cleaning performance.

  As shown in FIG. 5 with reference to FIG. 4, when receiving image forming data from the outside, the control unit 110 activates the image forming units 10a, 10b, 10c, and 10d to start preparation for image formation ( S10). The control unit 110 detects the image signal of each color and calculates the ratio of the halftone image having a predetermined gradation or less to the entire image (S11).

  The control unit 110 determines whether or not there is an image having a single color and a later-described halftone level (halftone image is a predetermined density) (S13). When there is no halftone image (N in S13), the controller 110 executes normal cleaning control.

  As shown in FIG. 6, during normal image formation, the constant current C applied to the secondary transfer portion T2 and the constant voltages A and B applied to the fur brushes 51a and 51b are controlled by normal settings. After the transfer of each color toner image to the intermediate transfer belt 16 is completed and the recording material P reaches the secondary transfer portion T2, the constant current C is applied. Subsequently, the constant voltage A is applied to the upstream fur brush 51a, and the constant voltage B is further applied to the downstream fur brush 51b. Then, after the recording material P passes through the secondary transfer portion T2, the constant current C and the constant voltages A and B are turned off in the same order.

  When there is a halftone image (Y in S13), the control unit 110 determines whether the halftone area is 80% or more of the image area (S14). When the halftone area is 80% or more of the image area (Y in S14), the controller 110 reduces the constant current applied to the secondary transfer portion T2 and improves the cleaning performance (S15).

  As shown in FIG. 7, when a low-density halftone image occupies most of the entire image, the control timing of the constant current C and the constant voltages A and B is the same as in the normal image formation of FIG. However, the constant current C is set lower than that during normal image formation to prevent the toner charge amount Q / M from changing to the reverse polarity side during the secondary transfer, and the strong transfer failure at the secondary transfer portion T2 is prevented. Suppressed. As a result, the reverse polarity toner on the intermediate transfer belt 16 is reduced to make it difficult for the transfer residual toner to slip through the fur brushes 51a and 51b.

  When the halftone area is less than 80% of the image area (N in S14), the controller 110 increases the constant voltage applied to the fur brushes 51a and 51b to improve the cleaning performance (S16).

  As shown in FIG. 8, when the proportion of the low-density halftone image in the entire image is not so high, the control timing of the constant current C and the constant voltages A and B is the same as in the normal image formation of FIG. . However, the constant current C is set equal to that during normal image formation, and instead, the absolute value of the voltage applied to the fur brushes 51a and 51b is set large.

  That is, when there are very few low-tone gradation halftone images in the image formed by each color, if the constant current C is decreased, the so-called weak omission transfer defect becomes serious in the portion where the density gradation of the image is high. Become. As a result of a shortage of transfer current compared to the amount of toner to be transferred, a large amount of normal polarity toner remains on the intermediate transfer belt 16 and the image density of the image transferred to the recording material is reduced.

  Therefore, the constant current C applied to the secondary transfer portion T2 is made equal to that during normal image formation, and priority is given to the transfer efficiency of the portion where the density gradation of the image is high. Instead, in the low-density halftone image portion, so-called strong omission transfer failure occurs, and the reversal polarity toner on the intermediate transfer belt 16 increases. Therefore, the cleaning performance of the fur brushes 51a and 51b is improved. . Although increasing the constant voltage applied to the fur brushes 51a and 51b is negative for the life of the fur brushes 51a and 51b, priority is given to reliably cleaning the reverse polarity toner on the intermediate transfer belt 16. This avoids a situation in which the transfer residual toner is mixed into the toner image on the intermediate transfer belt 16 and the image quality is deteriorated.

  In other words, when a toner image of a plurality of colors is secondarily transferred to the recording material P, the amount of toner transferred is large, so that the current flowing through the toner tends to be insufficient at the time of transfer and reversal polarity toner hardly occurs. For this reason, the amount of reverse polarity toner that is pulled back from the recording material P to the intermediate transfer belt 16 is small, and almost no reverse polarity toner passes through the upstream fur brush 51a.

  At this time, there is a large amount of normal polarity residual transfer toner (so-called weak missing toner) generated due to insufficient current flowing through the toner during secondary transfer. However, since the transfer residual toner of normal polarity is scattered on the surface of the intermediate transfer belt 16 when passing through the upstream fur brush 51a, it can be reliably collected by the downstream fur brush 51b. For this reason, when a plurality of color toner images are secondarily transferred to the recording material P, the amount of reversal polarity toner is increased by increasing the current flowing to the secondary transfer portion as compared with the case of a single color halftone toner image. Therefore, the residual toner with normal polarity can be reduced.

  If there is a next image formation (Y in S19), the image signal of the next image is detected (S11), and if there is no next image formation (N in S19), the image formation is terminated (S19).

  Table 2 shows set values of the constant current C and the constant voltages A and B in the first embodiment. A specific set value example of the control method of the first embodiment described so far will be described. Here, an example in which the usage environment is 25 ° C./50% RH is presented, but the set value can be freely changed depending on the usage environment.

  Next, the halftone level of the image used in the first embodiment will be described. The halftone level is defined by the density gradation of the black image of the most downstream image forming unit 10d where an image defect occurs in the yellow image formed by the most upstream image forming unit 10a.

  As shown in Table 3, the image signal of the black image was varied in the range of 0 to 255. Then, a normal constant current was set in the secondary transfer portion T2, and the presence or absence of an image defect in the yellow image was confirmed in a state where a normal constant voltage was applied to the fur brushes 51a and 51b.

  As a result, it was found that when the reflection density of the black image was less than 1.0, an image defect of the yellow image occurred due to the mixing of the black toner. When the reflection density 1.0 is made to correspond to the image signal level, it becomes 128/255, and the image signal level below this value is defined as the halftone level in the first embodiment.

  If there is a single-tone halftone image for magenta and cyan other than black, and the image signal level of the image is 128/255 or lower, an image defect similarly occurs in the next yellow image.

  Even when a halftone image with a combination of a plurality of colors exists, if the total image signal level of the images is 128/255 or less, an image defect similarly occurs in the next yellow image.

  In any case, when the total image signal level of each color is 128/255 or less, the toner density on the intermediate transfer belt 16 is too low as compared with the transfer current, so that the generation of reverse polarity toner becomes excessive. Because. This is because when the reverse polarity toner becomes excessive, the cleaning performance of the fur brushes 51a and 51b deteriorates, the fog toner around the intermediate transfer belt 16 increases, and color mixing occurs in the most upstream image forming unit 10a.

  Therefore, in the first embodiment, the case where the total image signal level of the halftone image is 128/255 or less is defined as the halftone level, and the control for improving the cleaning performance is performed. As described above, if a halftone image having an image signal level of 128/255 or less is 80% or more of the image, the constant current C applied to the secondary transfer portion T2 is lowered to suppress the generation of reverse polarity toner. If a halftone image having an image signal level of 128/255 or less is less than 80% of the image, the absolute value of the constant voltage applied to the fur brushes 51a and 51b is increased to improve the electrostatic cleaning capability.

Next, an experimental result confirming the effect of the cleaning control of Example 1 will be described. In the experiment method, the toner is collected by attaching and peeling the adhesive tape to the intermediate transfer belt 16 after passing through the first cleaning device 50, and the reflection density of the adhesive tape to which the toner is attached is measured to measure the intermediate transfer belt 16. The amount of toner fog was quantified.
(1) Using the image forming apparatus 1 (actual product), the intermediate transfer belt is applied when the normally set constant current C is applied to the secondary transfer portion T2 and when the cleaning control of the first embodiment is performed. 16 toner fog amounts were compared.
(2) By varying the image signal of the image forming unit 10d and measuring the toner fog amount of the intermediate transfer belt 16, the relationship between the gradation of the image signal of the image forming unit 10d and the cleaning performance of the first cleaning device 50 is obtained. It was measured.
(3) The gradation of the image signal of the image forming unit 10d and the first cleaning device in a state where the constant current C applied to the secondary transfer unit T2 or the constant voltages A and B applied to the fur brushes 51a and 51b are varied. A relationship of 50 cleaning performances was measured.

  As shown in FIG. 9A, when an image signal having a halftone level is 80% or more of the entire image forming area, when the normal constant current C is set, the image signal is displayed as shown by a solid line. When it becomes 128/255 or less, the secondary transfer residual toner amount rapidly increases. Therefore, when the image signal becomes 128/255 or less, the constant current C applied to the secondary transfer portion T2 is varied to suppress an increase in the secondary transfer residual toner amount as indicated by a broken line. This is because the toner charge amount Q / M after the secondary transfer is prevented from changing to the reverse polarity side by reducing the transfer current flowing through the secondary transfer portion T2, and the strong transfer failure is suppressed and the reverse polarity toner This is because the generation could be suppressed.

  As shown in FIG. 9B, when the image signal of the halftone level is less than 80% of the entire image forming area, when the normal constant voltages A and B are set, the image is shown as indicated by the solid line. When the signal becomes 128/255 or less, the secondary transfer residual toner amount suddenly increases. Therefore, when the image signal becomes 128/255 or less, an increase in the absolute values of the constant voltages A and B applied to the fur brushes 51a and 51b causes an increase in the amount of reverse polarity toner and an image defect occurs. It is now possible to remove it to a level that does not.

  From these results, it became possible to control the polarity of the transfer residual toner and the amount of adhesion on the intermediate transfer belt 16 by using the cleaning control of Example 1. The effect of preventing image defects of the yellow image formed by the most upstream image forming unit 10a by suppressing the toner passing through the fur brushes 51a and 51b was obtained.

<Example 2>
FIG. 10 is an explanatory diagram of a cleaning configuration of the intermediate transfer belt in the second embodiment. FIG. 11 is a flowchart of cleaning control according to the second embodiment. FIG. 12 is a time chart of the cleaning control when the halftone image is 80% or more. FIG. 13 is an explanatory diagram of the transfer residual toner reduction effect in the second embodiment.

  As shown in FIG. 10, in the second embodiment, the second cleaning device 60 is removed from the cleaning configuration of FIG. 2 and the cleaning web 61 is eliminated, but the rest is the same as the first embodiment.

  In the configuration without the cleaning web 61, the transfer residual toner is easily slipped through to the most upstream image forming unit 10a. Therefore, it is necessary to improve the cleaning capability of the first cleaning device 50. Therefore, in the second embodiment, the first cleaning device 50 is more than in the first embodiment when “there is 80% or more of a halftone level image signal” that the most reverse polarity toner is generated and is accompanied by the intermediate transfer belt 16. Has improved cleaning ability.

  As shown in FIG. 11 with reference to FIG. 4, the control from S10 to S16, S19, and S20 is the same as that in the first embodiment. When there is a halftone image (Y in S13), the control unit 110 determines whether the halftone area is 80% or more of the image area (S14). When the area of the halftone level is less than 80% of the image area (N in S14), the controller 110 increases the constant voltage applied to the fur brushes 51a and 51b to improve the cleaning performance (S14). S16).

  On the other hand, when the area of the halftone level is 80% or more of the image area (Y in S14), the control unit 110 reduces the constant current C applied to the secondary transfer unit T2 to generate the reverse polarity toner. Is suppressed (S15). In addition, the constant voltage applied to the fur brushes 51a and 51b is increased to improve the cleaning performance of the intermediate transfer belt 16 (S17).

  As shown in FIG. 12, the generation amount of reverse polarity toner in the secondary transfer portion T2 is reduced, and at the same time, the cleaning performance of the intermediate transfer belt 16 by the fur brushes 51a and 51b is enhanced. That is, when a single-tone halftone image occupies most of the entire image, the constant current C is set lower than the normal setting to prevent the toner charge amount Q / M from changing to the reverse polarity side during secondary transfer. As a result, so-called strong omission transfer failure at the secondary transfer portion T2 is suppressed, and the toner with reverse polarity on the intermediate transfer belt 16 can be reduced. At the same time, since the absolute value of the constant voltage applied to the fur brushes 51a and 51b is increased to improve the cleaning performance of the reverse polarity toner by the first cleaning device 50, the transfer residual toner is difficult to slip through the fur brushes 51a and 51b.

  Table 4 shows set values of the constant current C and the constant voltages A and B in the second embodiment. A specific set value example of the control method of the second embodiment described so far will be described. Here, an example in which the usage environment is 25 ° C./50% RH is presented, but the set value can be freely changed depending on the usage environment.

  Using the cleaning control of Example 2, an experiment for confirming the effect was performed in the same manner as in Example 1. As shown in FIG. 13, when a halftone image signal is 80% or more of the entire image forming area, a constant current C applied to the secondary transfer portion T2 and a constant voltage applied to the fur brushes 51a and 51b. A and B were changed. As a result, the fog toner on the intermediate transfer belt 16 can be reduced as compared with the cleaning control of the first embodiment, and even when the cleaning web 61 is not provided, an effect of sufficiently preventing image defects can be obtained.

  As shown in FIG. 13, in the configuration of the second embodiment without the cleaning web 61, the range in which the transfer residual toner that passes through the fur brushes 51a and 51b does not cause a defective image of the yellow image is narrower than that of the first embodiment. The effect of preventing image defects can be obtained.

  As described above, when the image signal formed on the photosensitive drum of each color includes a single color and halftone level signal, the voltage applied to the secondary transfer portion T2 is lowered so that the transfer residual toner in the next image forming process. Can prevent image defects from moving and achieve high image quality.

  In the image forming apparatus 1 capable of multicolor formation, when a single color and halftone level signal is present in the image signal formed in each color, the setting of the cleaning bias and the secondary transfer bias is varied. As a result, it is possible to prevent the transfer residual toner from moving to the next image forming process and to prevent image defects caused by the transfer residual toner.

<Example 3>
In the above embodiment, the total image signal level of the halftone image is calculated, and the case where the image signal level is 128/255 or less is defined as the halftone level that requires cleaning control. However, in the case of a single-tone halftone image such as a gray scale, it is assumed that the image formation is uniformly low in toner density, and as a control to lower the voltage applied to the secondary transfer portion T2 than during full-color image formation. Also good.

1 Image forming apparatuses 10a, 10b, 10c, 10d Image forming units 11a, 11b, 11c, 11d Photosensitive drums 12a, 12b, 12c, 12d Exposure apparatuses 13a, 13b, 13c, 13d Developing apparatuses 14a, 14b, 14c, 14d Drum cleaning Devices 15a, 15b, 15c, 15d Primary transfer roller 16 Intermediate transfer belt, 17 Tension roller, 18 Drive roller 19 Secondary transfer inner roller, 35 Secondary transfer outer roller 50 First cleaning device, 51a, 51b Fur brushes 52a, 52b Bias roller, 53a, 53b Cleaning blade 55a, 55b Bias applying power source 60 Second cleaning device, 61 Cleaning web 62 Mounting roller, 63 Web roller, 64 Take-up roller P Recording material

Claims (5)

An intermediate transfer member;
A toner image forming means capable of transferring a toner image of a halftone image of each color while being superimposed on the intermediate transfer member;
Transfer means for applying a voltage to a transfer portion that sandwiches the recording material to transfer a toner image from the intermediate transfer member to the recording material;
A voltage having the same polarity as the charging polarity of the toner is applied to the first electrostatic member that contacts the intermediate transfer member on the downstream side of the transfer portion, and the transfer residual toner on the intermediate transfer member is dispersed and passed. First electrostatic processing means capable of aligning the charging polarity of the residual toner to be transferred;
A voltage having a polarity opposite to the toner charging polarity is applied to the second electrostatic member that is in contact with the intermediate transfer member on the downstream side of the first electrostatic member to collect the transfer residual toner on the intermediate transfer member. An image forming apparatus comprising: a second electrostatic processing unit;
In the image formation in which the total amount of applied toner for each color is less than a predetermined level, the transfer unit is configured to lower the absolute value of the voltage applied to the transfer unit compared to the image formation in which the total amount of applied toner for each color is greater than or equal to the predetermined level. An image forming apparatus comprising control means for controlling.   2. The control unit according to claim 1, wherein the control unit controls the transfer unit so as to lower a voltage applied to the transfer unit in image formation with a single color and less than a predetermined gradation, rather than image formation in which a plurality of colors are superimposed. Image forming apparatus.   In the image formation in which the area where the total amount of applied toner of each color is less than a predetermined level is less than a predetermined ratio, the control unit is more than the first electrostatic member and the second electrostatic member than in image formation in which the area is equal to or greater than the predetermined ratio. 3. The image forming apparatus according to claim 2, wherein the absolute value of the voltage applied to the member is increased.   4. A web cleaning unit that wipes toner from the intermediate transfer member by bringing a web-like fiber material into contact with the intermediate transfer member on the downstream side of the first electrostatic processing unit. The image forming apparatus according to any one of the above.   The first electrostatic processing means is a rotationally driven fur brush that contacts the intermediate transfer member, a metal cylindrical member that is rotationally driven and contacts the fur brush, and contacts the fur brush to scrape off toner. The image forming apparatus according to claim 1, further comprising: a cleaning blade; and a power source that applies a voltage to the metal cylindrical member.

Images