JP2007298768A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of minimizing disturbance of a toner image resulting from a peeling discharge applied to separate a transfer material from a transfer material conveying means, and capable of restraining a bias applied to the separating part and a transfer bias from interfering with each other. <P>SOLUTION: The image forming apparatus includes: image carriers 1; the transfer material conveying means 7; a transfer means 5d for transferring a toner image on the image carrier 1 to the surface of a transfer material P conveyed while held on the transfer material conveying means 7; a transfer bias output means 51d for outputting a transfer bias to the transfer means 5d; a bias application member 71 for applying a bias to the separating part D where the transfer material P is separated from the transfer material conveying means 7; and a separating part bias output means 73 for outputting a bias to the bias application member 71. The image forming apparatus further includes a bias control means 110 by which the bias output to the bias application member 71 by the separating part bias output means 73 is controlled based on the bias output to the transfer means 51d by the transfer bias output means 51d. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms an image by transferring a toner image formed on an image carrier onto a transfer material using an electrophotographic system or an electrostatic recording system.

  2. Description of the Related Art Conventionally, there is an image forming apparatus including an image carrier on which a toner image is formed on a surface, a transfer unit that transfers a toner image on the image carrier to a transfer material, and a transfer material conveyance unit that conveys the transfer material. .

  FIG. 11 shows a schematic configuration of an example of a conventional image forming apparatus. An image forming apparatus 300 illustrated in FIG. 11 includes a photosensitive drum 301 as an image carrier. Around the photosensitive drum 301, devices for performing electrophotographic image formation are arranged. In the illustrated example, these devices include a charging device 302, a latent image forming unit 303 including an optical writing device, a developing device 304, a transfer material conveyance belt 305, a cleaning device 306, a static elimination device 307, and a pre-transfer static elimination device 309. Etc. are included. The latent image forming unit 303 uses a document image exposure device, a laser, an LED array, or the like.

  The transfer material conveyance belt 305 is stretched around a plurality of rollers such as a driving roller 311, a driven roller 312, and a transfer roller 313. A transfer bias is applied to the transfer material conveyance belt 305 by a transfer bias application power source 315 via a transfer roller 313, and the transfer material P is electrostatically attracted and conveyed. Then, by using the bending of the belt at one roller (the driving roller 311 in the illustrated example) and the rigidity of the transfer material P among the plurality of rollers that stretch the transfer material transport belt 305, the transfer material P is used. P is peeled off from the transfer material conveying belt 305 to separate them. However, when the transfer material P is separated from the transfer material transport belt 305, there is a problem that peeling discharge occurs between the two.

  Next, the problem of peeling discharge when the transfer material P is separated from the transfer material conveyance belt 305 will be further described. It is assumed that the photosensitive drum 301 is negatively charged and the transfer bias is positive. In this case, when the photosensitive drum 301 and the transfer material P are separated, a positive charge moves toward the photosensitive drum 301 and a negative charge moves toward the transfer material P due to the peeling discharge. At this time, the peel discharge amount is greatly influenced by the application condition of the transfer bias. For example, a transfer bias is applied using the transfer roller 313 so that a transfer current of 50 μA flows under constant current control.

  On the other hand, the transfer material P is negatively charged by the charge injection such as peeling discharge generated at the time of separation from the photosensitive drum 301 in addition to the toner. Further, the transfer material conveyance belt 305 is positively charged. For this reason, when the transfer material P is separated from the transfer material transport belt 305 in the separation portion D, peeling discharge occurs.

  With respect to this problem, in Patent Document 1, a bias applying member is provided inside a transfer material transport unit that moves to face the image carrier. That is, in Patent Document 1, a positive charge is applied to the roller 311 positioned at the separation portion D of the transfer material P from the transfer material conveyance belt 305 so that the peeling discharge is performed uniformly and as close to the separation point as possible. It is disclosed to provide a power source 316 that provides the same polarity as the transfer bias.

In this way, a charge having the same polarity as the transfer bias is applied to the roller 311 which is a belt support member positioned at the separation portion D of the transfer material P from the transfer material conveyance belt 305. As a result, electric discharge is generated even in a small gap between the transfer material P and the transfer material conveyance belt 305 that normally does not generate electric discharge. In this way, it is possible to reduce the impact of the peeling discharge when the transfer material P is separated from the transfer material conveying belt 305, and to reduce the degree of disturbance of the toner image.
JP-A-9-134082

  However, as described above, the image forming apparatus that applies a bias to the roller 311 that stretches the transfer material conveyance belt 305 in the separation portion D of the transfer material P from the transfer material conveyance belt 305 has the following problems. There was a case.

  That is, it is an interference between the bias applied to the roller 311 located in the separation portion D and the transfer bias. This phenomenon is a phenomenon in which the bias interferes through the transfer material P, and particularly occurs when the resistance of the recording material P is low. In the portion where these biases interfere, the degree of transfer of the toner image onto the transfer material P becomes stronger or weaker. Thereby, for example, in a halftone image, light and shade unevenness may occur.

  For example, when the bias applied to the roller 311 located in the separation part D is lower than the transfer bias, the charge due to the transfer bias flows through the transfer material P and flows into the roller 311 located in the separation part D. As a result, the portion may be defective in transfer and the image density may be reduced.

  On the other hand, when the charge due to the bias applied to the roller 311 located in the separation part D is higher than the transfer bias, the bias applied to the roller 311 located in the separation part D flows into the transfer part N. Therefore, a so-called drum memory phenomenon may occur. That is, an excessive bias may be applied from the transfer portion N to the photosensitive drum 301. When an excessive transfer bias is applied to the photosensitive drum 301, a transfer bias memory is generated on the photosensitive drum 301. Therefore, it becomes difficult to charge the photosensitive drum 301 to a desired potential by the charging unit by the charging device 302. In addition, a portion of the photosensitive drum 301 that has received an excessive transfer bias may be charged only to a value lower than a desired charging potential. For this reason, unevenness in the exposure potential when exposed by the latent image forming unit 303 may also occur, and as a result, unevenness in development may occur.

  Accordingly, an object of the present invention is to reduce the degree of toner image disturbance due to the peeling discharge during separation of the transfer material from the transfer material conveying means, while reducing the bias applied to the separation portion and the transfer bias. It is an object of the present invention to provide an image forming apparatus capable of suppressing the interference.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to an image carrier on which a toner image is formed on a surface, a transfer material conveying unit that carries a transfer material and moves to face the image carrier, and a toner on the image carrier. Transfer means for transferring an image onto a transfer material carried by the transfer material carrying means, transfer bias output means for outputting a bias for the transfer to the transfer means, and the transfer material carrying means A bias applying member that is disposed on the opposite side of the transfer material carrying surface of the transfer material and applies a bias to a separation portion of the transfer material from the transfer material conveying means, and outputs a bias to the bias applying member A bias output from the separation bias output unit to the bias applying member is applied to the transfer unit from the transfer bias output unit. Having a bias control means for controlling on the basis of the bias output Te is an image forming apparatus according to claim.

  According to the present invention, the interference between the bias applied to the separation portion and the transfer bias can be reduced while making it possible to reduce the degree of disturbance of the toner image due to the peeling discharge at the time of separation of the transfer material from the transfer material conveying means. Can be suppressed.

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

Example 1
[Entire configuration of image forming apparatus]
FIG. 1 shows a schematic sectional configuration of an embodiment of an image forming apparatus according to the present invention. The image forming apparatus 100 according to this embodiment includes four image forming units (process stations) Sa, Sb, Sc, and Sd that form toner images of different colors, so-called in-line four-color full-color image formation. Device. The image forming apparatus 100 according to the present exemplary embodiment includes a conveyance belt (transfer belt) 7 serving as a transfer material conveyance unit, and forms an image on a transfer material P such as a recording sheet or an OHP sheet by a direct transfer method. To do. This will be described in detail below.

  The image forming apparatus 100 includes first, second, third, and fourth image forming units Sa, Sb, Sc, and Sd as a plurality of image forming units. The image forming portions Sa, Sb, Sc, and Sd are for forming magenta (M), cyan (C), yellow (Y), and black (K) images, respectively.

  In the following description, elements provided in common for each color are given to the reference numerals in the drawings to indicate that they are elements provided for any color unless it is particularly necessary to distinguish them. The subscripts a, b, c, and d will be omitted and will be described generally.

  The image forming unit S includes a cylindrical electrophotographic photosensitive member as an image carrier, that is, a photosensitive drum 1. In this embodiment, the photosensitive drum 1 is rotationally driven so that the outer periphery moves at a speed of about 95 (mm / sec). Around the photosensitive drum 1, a charging roller 2 as a primary charging unit, a developing device 4 as a developing unit, a transfer roller 5 as a transfer unit, a cleaning unit 6 as a cleaning unit, and the like are arranged. Further, an exposure device 3 as an exposure means (latent image forming means) is arranged so that the surface of the photosensitive drum 1 charged by the charging roller 2 can be exposed in each of the image forming portions Sa to Sd.

  A transport belt 7 is disposed as a transfer material transport unit that moves to face the photosensitive drums 1a to 1d of the image forming units Sa to Sd. The conveying belt 7 is stretched around two rollers, a driving roller 71 and a driven roller 72, as a plurality of supporting members, and moves endlessly in the direction of the arrow in the figure. The four image forming units Sa to Sd are arranged in a line along the conveyance direction of the transfer material P by the conveyance belt 7. That is, the first, second, third, and fourth image forming portions Sa, Sb, Sc, and Sd are arranged in series in order from the upstream in the conveyance direction of the transfer material P by the conveyance belt 7. The conveying belt 7 is pressed by the transfer rollers 8a to 8d provided on the inner side thereof, thereby forming transfer portions (transfer nips) Na to Nd that are in contact with the photosensitive drums 1a to 1d, respectively.

  It should be noted that all or some of the components of the image forming units Sa to Sd may be detachable from the image forming apparatus main body as a process cartridge. In this embodiment, the photosensitive drum 1, the charging roller 2, the developing device 4, and the cleaning device 6 are integrated together by a frame to constitute a process cartridge.

  At the time of image formation, the surface of the photosensitive drum 1 that is rotationally driven is charged substantially uniformly by the charging roller 2 to a predetermined potential having a predetermined polarity (negative polarity in this embodiment). Thereafter, the charged surface of the photosensitive drum 1 is subjected to exposure based on image information by an exposure device 3 having a light source such as an LED or a laser. As a result, an electrostatic latent image is formed on the photosensitive drum 1. Next, the electrostatic latent image is developed as a toner image by attaching toner charged to a predetermined polarity (negative polarity in this embodiment) by the developing device 4. The toner image formed on the photosensitive drum 1 is electrostatically transferred onto the transfer material P carried and transported on the transport belt 7 by the action of the transfer roller 5 in the transfer portion N. At this time, a transfer bias having a polarity (positive in this embodiment) opposite to the normal charging polarity of the toner output from a transfer bias power source 51 serving as a transfer bias output unit is applied to the transfer roller 5.

  For example, when a full color image is formed, the above operation is repeated in each of the image forming portions Sa to Sd, and magenta, cyan, yellow, and black toners are sequentially superimposed and transferred onto the transfer material P on the transport belt 7. A multiple toner image is formed.

  The transfer material P is supplied from the transfer material supply device 10 to the conveyor belt 7 in synchronization with the toner image forming operation on the photosensitive drums 1a to 1d in the image forming portions Sa to Sd. That is, the transfer material P is fed into the main body of the image forming apparatus by the supply roller 12 from the cassette 11 serving as a transfer material container. Next, the recording material P is conveyed by the registration roller 13 and the registration counter roller 14 and is carried on the conveyance belt 7. The transport belt 7 carries the transfer material P by electrostatically adsorbing it. The transfer material P adsorbed on the transport belt 7 sequentially passes through the transfer portions Na to Nd of the image forming portions Sa to Sd. As described above, for example, when a full-color image is formed, the toner images of the respective colors on the respective photosensitive drums 1a to 1d are electrostatically sequentially transferred at the respective transfer portions Na to Nd.

  Thereafter, the transfer material P onto which the toner image has been transferred is separated from the conveyance belt 7 in the separation unit D located on the downstream side in the conveyance direction of the transfer material P with respect to the transfer unit Nd of the fourth image forming unit Sd. Next, the transfer material P is conveyed to a heating / pressure fixing device 8 as a fixing unit. The toner image on the transfer material P is fixed on the transfer material P by being heated and pressed by the fixing device 8. The transfer material P on which the toner image is fixed and the permanent image is formed is then discharged out of the image forming apparatus main body.

  Further, the toner (transfer residual toner) remaining on the photosensitive drum 1 after the transfer process is removed from the surface of the photosensitive drum 1 by the cleaning device 6. That is, the cleaning device 6 scrapes off toner from the photosensitive drum 1 by a cleaning blade as a cleaning member that contacts the photosensitive drum 1, and collects the toner in a waste toner container.

  Note that the image forming apparatus 100 can also form a single-color or multi-color image using only a desired single or plural (not all) image forming units S. Typically, when a black single color image is formed, a toner image is formed on the transfer material P only in the fourth image forming unit Sd by the above-described image forming process. In an image forming unit that is not used for image formation, the photosensitive drum 1 and the conveyance belt 7 may be separated from each other.

  As will be described in detail later, in this embodiment, a bias is applied to a bias application member provided inside the conveyor belt 7 so as to reduce the impact of the peeling discharge at the separation portion D of the transfer material P from the conveyor belt 7. (Separator bias) is applied. In particular, in the present embodiment, as a bias application member, a driving roller 71 that is one of the support members of the conveyance belt 7 disposed inside the conveyance belt 7 in the separation portion D of the transfer material P from the conveyance belt 7 is provided. Use. The bias applied to the driving roller 71 as the bias applying member will be described in detail later.

  Further, as shown in FIG. 1, a separation static eliminator 9 may be provided as a static elimination unit that removes charges from the transfer material P on the downstream side of the separation part D in the transport direction of the transfer material P. For example, the separation static eliminator 9 includes a static elimination needle 91, a static elimination bias power source 92 that applies a bias to the static elimination needle, and a shield case 93 that is electrically grounded. The neutralization bias power source 92 applies a bias (in this embodiment, a positive bias because the transfer material P is charged negatively) to the static elimination needle 91 so as to cancel the charged charge of the transfer material P. Alternatively, the static elimination needle 91 may be electrically grounded.

[Transfer bias]
Next, a method for determining a transfer bias for transferring a toner image to the transfer material P will be described.

  Conventionally, before the transfer material P reaches the transfer portion N, the impedance of the entire transfer portion is detected, and based on the detection result, the transfer material P enters the transfer portion N and transfers toner to the transfer material P. There is a method for determining a transfer bias applied to the transfer roller 5 sometimes. Such a transfer bias control system is called an ATVC (Active Transfer Voltage Control) system (see, for example, Japanese Patent Laid-Open No. 2-123385).

  More specifically, in this transfer bias control method, the transfer bias is determined by the control shown in FIG. That is, as shown in FIG. 2, when there is no transfer material P in the transfer portion N, a constant current is applied to the transfer portion N for a predetermined time T1 by constant current control. Then, a voltage value calculated and determined from a voltage generated at that time by a preset control equation is applied when the transfer material P exists in the transfer portion N.

  In this embodiment, when the transfer material P is not in the transfer portion N, a bias controlled by constant current is output from the transfer bias power source 51 to the transfer roller 5. Further, the output voltage value of the transfer bias power source 51 at that time is detected by a voltage detection means provided in or connected to the transfer bias power source 51. Based on the detected voltage value, a constant voltage-controlled bias value output from the transfer bias power source 51 to the transfer roller 5 when the transfer material P is in the transfer portion N is determined. In this embodiment, the transfer bias value is determined by the controller 110 based on the detection result input from the voltage detection means. The controller 110 controls the transfer bias power source 51 so that the transfer bias determined as described above is output to the transfer roller 5 when the toner image is transferred onto the transfer material P. The controller 110 may also serve as a controller of a control unit that comprehensively controls the operation of the image forming apparatus 100.

  Since it is only necessary to obtain information relating to the impedance of the entire transfer portion, a bias controlled by constant voltage is applied to the transfer roller 5 when the transfer material P is not in the transfer portion N, and transfer is performed from the output current value at that time. The bias may be determined. In this case, a current detection unit is provided in or connected to the transfer bias power source 51. Then, the controller 110 controls the transfer bias output from the transfer bias power source 51 to the transfer roller 5 when the toner image is transferred, based on the detected current value. Alternatively, the information on the impedance itself is obtained from the result of detecting the output voltage value or the output current value when the bias controlled by the constant current control or the constant voltage is applied to the transfer roller 5, and the toner image is correspondingly detected. The transfer bias at the time of transfer may be controlled.

  Note that, in order to determine the transfer bias in each of the image forming units Sa to Sd, the transfer bias determination operation as described above can be performed in each transfer unit N. Alternatively, the transfer bias for an image forming unit different from the image forming unit may be controlled by performing the transfer bias determining operation as described above in any of the image forming units.

[Separator bias]
Next, the control of the separation unit bias in the present embodiment will be described.

  As described above, in this embodiment, a bias (separator bias) is applied to the drive roller 71 located inside the conveyor belt 7 in the separation portion D of the transfer material P from the conveyor belt 7. More specifically, in this embodiment, a constant-voltage controlled bias output from a separation unit bias power source 73 serving as a separation unit bias output unit is applied to the drive roller 71. Thereby, the impact of the peeling discharge at the separation portion D of the transfer material P from the transport belt 7 is reduced.

  In this embodiment, the separation unit bias is determined with reference to the voltage value of the transfer bias applied to the transfer roller 5d in the transfer unit (fourth transfer unit) Nd of the fourth image forming unit Sd. Is done. In other words, in this embodiment, the value of the separation unit bias applied to the driving roller 71 disposed in the separation unit D is based on the transfer bias value of the transfer unit Nd adjacent to the separation unit D in the transfer direction of the transfer material P. Determined.

More specifically, in this embodiment, assuming that the transfer bias of the fourth transfer portion Nd is Vt and the separation portion bias applied to the drive roller 71 is Vdr, Vt and Vdr have the relationship of the following formula (1). .
Vt = aVdr + b (1)
(A and b are constants)

  In this way, the separation unit bias to be applied to the driving roller 71 can be determined from the transfer bias value of the fourth transfer unit N4. Therefore, the above equation (1) is set so that the separation portion bias and the transfer bias do not leak, and an arbitrary separation portion bias can be applied to the drive roller 71 according to the equation (1). .

  By determining the separation unit bias value in this way, interference between the transfer bias Vt of the fourth transfer unit Nd and the separation unit bias Vdr applied to the driving roller 71 can be prevented, and density unevenness can be prevented. I was able to.

  Next, the results of evaluating the effects of this example will be described.

  The graph of FIG. 3 shows the results of evaluating density unevenness due to bias interference and toner image disturbance due to peeling discharge in the separation portion D when the separation portion bias is controlled according to this embodiment. FIG. 4 shows, as Comparative Example 1, density unevenness due to bias interference and toner image due to peeling discharge in the separation portion D when the separation portion bias is constant regardless of the transfer bias of the fourth transfer portion Nd. The result of evaluating the disturbance is shown.

  Here, for each of this example and Comparative Example 1, a black halftone image was formed on a letter size of XEROX brand 4024 and a basis weight of 75 g in a normal temperature and humidity environment of 23 ° C. and 50% RH. The evaluation results are shown. 3 and 4 is the transfer bias Vt applied to the transfer roller 5d in the transfer unit Nd of the fourth image forming unit Sd that forms a black image. Further, the vertical axis of the graphs of FIGS. 3 and 4 is a value obtained by rank evaluation of density unevenness. Further, for each of the present example and the comparative example 1, this evaluation is performed so that the electric resistance of the transfer roller is different in order to obtain an optimum transfer current according to the transfer bias of the voltage value shown on the horizontal axis. I prepared something and went.

  FIG. 5 shows the relationship between the separation portion bias Vdr and the transfer bias Vt of the fourth transfer portion Nd for each of the present embodiment and the comparative example 1.

  Density unevenness is caused by a leak of bias that occurs when the transfer material P is located in a portion that extends over the separation portion D and the fourth transfer portion Nd, and a phenomenon in which the image of the portion becomes darker or thinner. It is. In the case of the halftone image used in this evaluation, when the charge flows from the driving roller 71 to the fourth transfer portion Nd, the transfer bias is strong and the toner scatters on the transfer material P by penetrating the transfer material P. It looks darker than other parts. In addition, when the charge due to the transfer bias of the fourth transfer portion Nd flows into the drive roller 71, the transfer current becomes insufficient, so that it appears thinner than other portions.

Here, the density unevenness was ranked in five stages. Ranks 1 to 5 indicate the following cases, respectively.
Rank 1: No density unevenness can be seen at all Level Rank 2: Density unevenness is slight but almost unknown Level Rank 3: Density unevenness is slightly permissible Level Rank 4: Density unevenness is present in the halftone pattern used in this evaluation I understand that
Acceptable level because it is difficult to understand with practical images (level where improvement is desired)
Rank 5: Immediately found density unevenness, unacceptable level

  In addition, the toner image was evaluated for rank using a black halftone image based on the same standard as the density unevenness.

  As shown in the evaluation results of FIG. 4, in Comparative Example 1, the toner image is not disturbed by all transfer biases, but density unevenness may occur. That is, density unevenness in which the image becomes darker occurs on the side where the transfer bias is strong, and density unevenness in which the image becomes lighter on the side where the transfer bias is weak.

  More specifically, FIG. 6 is a graph showing the relationship between the transfer bias Vt of the fourth transfer portion Nd and the leakage current to the drive roller 71 in Comparative Example 1. As can be seen from FIGS. 4 and 6, when the current flowing through the driving roller 71 is large, density unevenness that causes an image to darken occurs. Conversely, when the current flowing through the driving roller 71 is small, density unevenness that causes the image to become thin. Has occurred. Thus, it can be seen that the density unevenness is caused by the interference between the transfer bias and the separation unit bias. Further, density unevenness occurs when the transfer material P exists between the transfer portion Nd and the separation portion D. From this, it can be seen that the transfer bias and the separation unit bias interfere through the transfer material P.

  On the other hand, as shown in the evaluation results of FIG. 3, in this embodiment, although the toner image may be slightly disturbed on the high transfer bias side, it is within the allowable level. Moreover, it was a good level with no density unevenness.

  As described above, the relationship between the transfer bias Vt of the fourth transfer portion Nd and the separation portion bias Vdr in this embodiment is as shown in FIG. In this embodiment, the separation unit bias is determined based on the above equation (1). As shown in the figure, in this embodiment, the constants a and b in the formula (1) are set to a = 1 and b = 0. That is, in this embodiment, Vt = Vdr (that is, the same polarity and the same potential), and both biases are difficult to interfere with each other.

  In the present embodiment, the information related to the equation (1) is stored in advance in a storage unit built in or connected to the controller 110 as a bias control unit. Then, the controller 110 obtains the separation unit bias from the transfer bias of the fourth transfer unit Nd determined as described above using the information of the equation (1). The controller 110 controls the separation unit bias power source 73 so as to output a bias that has been subjected to constant current control with the voltage value of the obtained value from the separation unit bias power source 73 to the drive roller 71.

  The constants a and b are not limited to those of the present embodiment, and are appropriately set according to conditions such as the environment as will be described later. Further, the relationship between Vt and Vdr is not limited to a linear relationship, and may be set to have a higher order relationship. Further, the information indicating the relationship between the separation bias Vdr and the transfer bias Vt may be, for example, a combination of a plurality of relational expressions set for each transfer bias range.

  As described above, according to the present embodiment, the bias applying member that applies a bias to the separation portion D of the transfer material P from the conveyance belt 7 is provided from the side opposite to the carrying surface of the transfer material P of the conveyance belt 7. Then, bias control means is provided for controlling the separation unit bias to be applied to the bias applying member based on the transfer bias. That is, when the basic transfer bias is different, the separation bias is different. Thereby, it is possible to control so that the separation unit bias and the transfer bias do not interfere with each other. In this embodiment, a driving roller 71 that is a member that stretches the conveyor belt 7 is used as the bias applying member. Thereby, the discharge between the transfer material P and the transport belt 7 can be stabilized at the separation portion D of the transfer material P from the transport belt 7 without adding a special member. In this embodiment, the separation unit bias applied to the driving roller 71 as the bias applying member is applied to the transfer roller 5d provided in the fourth transfer unit Nd which is a member adjacent to the driving roller 71. Control is based on bias. Thereby, it is possible to efficiently prevent bias interference between the adjacent transfer portion and the separation portion.

  Thus, according to the present embodiment, the impact of the peeling discharge at the time of separation of the transfer material P from the conveyance belt 7 can be reduced, and the degree of disturbance of the toner image can be reduced. Interference between the applied bias and the transfer bias can be suppressed. Thereby, density unevenness and drum memory can be prevented.

Example 2
Next, another embodiment according to the present invention will be described. The basic configuration of the image forming apparatus of the present embodiment and the basic operation from when the transfer material P is supplied until the toner image is fixed are the same as those of the first embodiment. Accordingly, elements having the same or corresponding functions and configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals, detailed description thereof is omitted, and characteristic points of the present invention will be described.

  FIG. 5 shows the relationship between the separation unit bias Vdr and the transfer bias Vt of the fourth transfer unit Nd in this embodiment. As shown in FIG. 5, in this embodiment, compared to the first embodiment, on the side where the transfer bias Vt of the fourth transfer portion Nd is higher, the separation portion bias Vdr higher than the transfer bias Vt on the same polarity side. Is used.

  FIG. 7 shows the results of evaluating density unevenness due to bias interference and toner image disturbance due to peeling discharge in the separation portion D in this example. The evaluation method and evaluation criteria are the same as in Example 1.

  As shown in FIG. 7, according to the present embodiment, it was possible to improve the disturbance of the toner image that occurred slightly when the transfer bias Vt was high in the first embodiment (FIG. 3).

  Next, the mechanism by which the above results were obtained in this example will be described.

  When the transfer bias Vt is high, the conveying belt 7 and the transfer material P tend to be more charged up at the transfer portion. Then, a peeling discharge is easily generated when the transfer material P is separated from the transport belt 7 by the separation portion D. The transfer material P after being separated from the conveyance belt 7 is strongly charged on the negative polarity side, and discharge occurs between the transfer belt P and the conveyance belt 7 charged to the positive polarity in the drive roller 71 portion. This discharge is generated by a potential difference between the driving roller 71 and the transfer material P. At this time, by applying a bias having a stronger positive polarity (that is, a polarity opposite to the charged charge of the transfer material P and the same polarity as the transfer bias) to the driving roller 71, peeling discharge is generated at a position closer to the separation point. Therefore, the toner image can be prevented from being disturbed.

  As described above, when the transfer material P is easily charged up, the peeling discharge is positively performed, and the stable peeling discharge is performed near the point where the transfer material P is separated from the conveyance belt 7. It is valid.

  In this embodiment, as a condition for the transfer material P to be easily charged up, when a strong transfer bias is applied, the separation portion bias is made stronger. For example, the constant a in the formula (1) is set to a> 1. Similarly to the first embodiment, the controller 110 serving as the bias control unit outputs an output from the separation unit bias power source 73 to the drive roller 71 according to the transfer bias of the fourth transfer unit Nd according to the equation (1). The separation unit bias to be controlled is controlled. In other words, in this embodiment, the controller 110 outputs the separation unit bias output from the separation unit bias power source 73 to the drive roller 71 and the transfer bias power source 51d of the fourth transfer unit Nd to the transfer roller 5d. The difference from the transfer bias can be controlled. Then, the controller 110 increases the separation bias to the same polarity side as the transfer bias (increases the absolute value) in a range where the transfer bias of the fourth transfer portion Nd is stronger than the predetermined value (the absolute value is large). The difference between the separation unit bias and the transfer bias can be increased.

  In addition, as an application example of the present embodiment, other conditions that the transfer material P is likely to be charged up include conditions such as the atmosphere (environment) of the image forming apparatus 100 and the type of the transfer material.

  For example, the image forming apparatus 100 can be provided with environment detecting means (such as a temperature / humidity sensor) 120 that detects the atmosphere in which the image forming apparatus 100 is installed. When the atmosphere of the image forming apparatus 100 is a low humidity environment, it is effective to increase the separation unit bias to the same polarity side as the transfer bias (increase the absolute value). That is, the environment detection unit 120 that detects the atmosphere in which the image forming apparatus 100 is installed is provided, and the controller 110 determines the separation unit bias and the transfer bias of the fourth transfer unit Nd based on the detection result of the environment detection unit 120. Can be controlled. In other words, the constants a and b in Expression (1) can be made variable according to the atmosphere in which the image forming apparatus 100 is installed. That is, when the atmosphere in which the image forming apparatus 100 is installed is different, the relationship between the separation unit bias and the transfer bias of the fourth transfer unit Nd is different. Typically, when the atmosphere detected by the environment detection unit 120 is lower than a predetermined value, the separation bias is increased to the same polarity side as the transfer bias (the absolute value is increased), and the separation unit bias and the fourth bias are increased. The difference from the transfer bias of the transfer portion Nd can be increased.

  The controller 110 can be compared with a preset reference value by, for example, obtaining an absolute water content from information related to the atmosphere detection result input from the temperature / humidity sensor 120. When the controller 110 determines that the atmosphere is a low humidity environment lower than a predetermined value, for example, the controller 110 selects and uses the constants a and b of the formula (1) set in advance for the low humidity environment, and uses the separation unit bias. Can be controlled to be stronger on the same polarity side as the transfer bias.

  It is also effective for the image forming apparatus 100 to make the control of the separation unit bias variable according to the type of the transfer material P used for image formation. When the transfer material P that is easily charged up is used for image formation, the separation bias is increased to the same polarity side as the transfer bias (the absolute value is increased). That is, when the image forming apparatus 100 has a plurality of image forming modes for forming an image on different types of transfer materials, the controller 110 determines whether the separation unit bias and the first bias are set according to the image forming mode. The difference from the transfer bias of the fourth transfer portion Nd can be controlled. In other words, the constants a and b in Expression (1) can be made variable according to the type of transfer material P used for image formation. That is, when the transfer material P used for image formation is different, the relationship between the separation portion bias and the transfer bias of the fourth transfer portion Nd is different. Typically, in an image forming mode in which an image is formed on a preset electrically high-resistance transfer material P, the separation bias is increased to the same polarity side as the transfer bias (increase the absolute value), and the separation unit bias. And the transfer bias of the fourth transfer portion Nd can be increased.

For example, FIG. 8 shows the relationship between the transfer bias Vt of the fourth transfer portion Nd and the separation portion bias Vdr when each of plain paper and high resistance paper is used as the transfer material P. As shown in FIG. 8, when an electrically high resistance transfer material P is used, control is performed so that a higher separation unit bias is used, and when a low resistance transfer material P is used, a lower bias is used. be able to. The controller 110 instructs the transfer material P used for image formation by an operation unit 130 provided in the image forming apparatus 100 or an operation unit of an external device such as a personal computer connected to be communicable with the image forming apparatus 100. Is done. For example, the controller 110 can select and use the preset constants a and b of the formula (1) according to the instructed transfer material P to control the separation unit bias. For example, the types of the transfer material P set as the optimum values shown in FIG. 8 are as follows.
Plain paper: Letter size of XEROX brand 4024, basis weight 75g
High resistance paper: Sanichi monument brand 80g Neutral paper A4 size, basis weight 80g

  As described above, according to this embodiment, as a condition for the ease of charge-up of the transfer material P, when the transfer bias of the transfer portion adjacent to the separation portion is larger than a predetermined value, the separation portion bias and the separation portion are adjacent to each other. Control can be performed so that the difference from the transfer bias of the transfer portion becomes large. Accordingly, it is possible to prevent the toner image from being disturbed easily when the transfer bias of the transfer portion adjacent to the separation portion is high. Further, as a condition for the ease of charge-up of the transfer material P, the separation unit bias can be controlled according to the atmosphere in which the image forming apparatus 100 is installed. Thereby, in an atmosphere in which peeling discharge is likely to occur, a high separation portion bias can be applied, so that discharge at the peeling portion can be stabilized. In addition, as a condition for the ease of charge-up of the transfer material P, the image forming apparatus 100 having an image formation mode corresponding to the type of the transfer material P controls the separation unit bias according to the image formation mode. Can do. As a result, when the transfer material P, on which peeling discharge is likely to occur, is used for image formation, a high separation portion bias can be applied, so that discharge at the peeling portion can be stabilized.

  Thus, according to the present embodiment, the impact of the peeling discharge at the time of separation of the transfer material P from the conveyance belt 7 can be reduced, and the degree of disturbance of the toner image can be reduced. Interference between the applied bias and the transfer bias can be suppressed. Thereby, density unevenness and drum memory can be prevented.

  As mentioned above, although this invention was demonstrated according to the specific Example, it should be understood that this invention is not limited to the above-mentioned embodiment.

  For example, in each of the above embodiments, a power source for supplying a bias to the driving roller 71 as a bias applying member is individually provided. However, the present invention is not limited to this, and as another embodiment, for example, the same effect can be obtained by supplying from the transfer bias power source 51d of the fourth image forming unit Sd or the bias power source 92 of the separation static eliminator 9. can get.

  The image forming apparatus according to each of the embodiments described above relates to the separation portion of the transfer material P from the transfer material conveying means for conveying the transfer material P onto which the toner image is transferred from the photosensitive drum 1 as the image carrier. Applied. As an application example, as shown in FIG. 10, the present invention can be applied to an image forming apparatus 200 using an intermediate transfer method. In the image forming apparatus 200 of FIG. 10, elements having the same or corresponding functions as those of the image forming apparatus 100 shown in FIG.

  The intermediate transfer type image forming apparatus 200 includes an intermediate transfer belt 201 that is a belt body as an intermediate transfer body that moves to face the photosensitive drums 1 a to 1 d of the image forming units Sa to Sd. The intermediate transfer belt 201 is stretched around two rollers, a driving roller 202 and a driven roller 203 as a plurality of support members, and moves endlessly in the direction of the arrow in the figure.

  The toner images formed on the respective photosensitive drums 1a to 1d are sequentially superimposed on the intermediate transfer belt 201 by the action of the primary transfer rollers 5a to 5d as primary transfer units in the primary transfer portions N1a to N1d. Transfer (primary transfer) is performed. At this time, a primary transfer bias having a polarity opposite to the normal charging polarity of the toner is applied to the primary transfer rollers 5a to 5d from primary transfer bias power sources 51a to 51d as primary transfer bias output means. . Then, the toner image primarily transferred onto the intermediate transfer belt 201 is collectively transferred onto the transfer material P (2 by the action of the secondary transfer roller 204 as a secondary transfer unit in the secondary transfer portion N2. Next transfer). At this time, a secondary transfer bias having a polarity opposite to the normal charging polarity of the toner is applied to the secondary transfer roller from a secondary transfer bias power source 207 as a secondary transfer bias output unit.

  The transfer material P is supplied from, for example, the transfer material supply device 10 and is transported to the secondary transfer unit N2 by a transport belt 7 as a transfer material transport unit. The secondary transfer roller 204 comes into contact with the intermediate transfer belt 201 at a position facing the support roller (driven roller) 203 of the intermediate transfer belt 201 via the conveyance belt 7, and is a secondary transfer portion (secondary transfer nip) N2. Form. The conveying belt 7 is stretched around two rollers, a driving roller 71 and a driven roller 72, as a plurality of supporting members, and moves endlessly in the direction of the arrow in the figure. The transfer material P onto which the toner image has been secondarily transferred is separated from the conveyance belt 7 and then conveyed to the fixing device 8.

  At this time, the present invention can be applied to the separation portion D of the transfer material P from the transport belt 7 and the secondary transfer portion N2. In this case, the separation bias is output from the separation bias applying means 73 to the drive roller 71, and this separation bias is controlled based on the secondary transfer bias. In the same manner as described in the second embodiment, the conditions for ease of charge-up of the transfer material P depend on the strength of the secondary transfer bias, the atmosphere of the image forming apparatus 200, and the type of the transfer material P. Of course, it is also possible to control the separation bias.

  That is, the separation portion D of the transfer material P from the conveyance belt 7 that moves opposite to the intermediate transfer belt 201 as an image carrier that carries the toner image, and the transfer of the toner image from the intermediate transfer belt 201 to the transfer material P The present invention can be applied to the portion (secondary transfer portion) N2. In this case, the same effects as those in the above embodiments can be obtained.

  Further, as a matter of course, even in a single color image forming apparatus having only one image forming unit, the present invention can be applied to the transfer material separating unit and the transfer unit from the transfer material conveying unit.

1 is a schematic cross-sectional configuration diagram of an image forming apparatus according to an embodiment of the present invention. It is a chart for demonstrating the control method of a transfer bias. It is a graph for demonstrating the effect of one Example of this invention. It is a graph regarding the comparative example for demonstrating the effect of one Example of this invention. It is a graph for demonstrating the setting of the isolation | separation part bias in the Example according to this invention, and a comparative example. It is a graph regarding the comparative example for demonstrating the effect of one Example of this invention. It is a graph for demonstrating the effect of the other Example of this invention. It is a graph for demonstrating the setting of the isolation | separation part bias in the further another Example of this invention. FIG. 6 is a schematic cross-sectional configuration diagram of an image forming apparatus according to another embodiment of the present invention. FIG. 6 is a schematic cross-sectional configuration diagram of an image forming apparatus according to still another embodiment of the present invention. It is a schematic cross-sectional block diagram of an example of the image forming apparatus of a prior art example.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (primary charging means)
3 Exposure equipment (exposure means)
4 Developing device (Developing means)
5 Transfer roller (transfer means)
6 Cleaning device (cleaning means)
7 Conveying belt (transfer material conveying means)
51 Transfer bias power supply (transfer bias output means)
71 Drive roller (bias application member, belt support member)
73 Separator Bias Power Supply (Separator Bias Output Unit)
D Separation part N Transfer part P Transfer material

Claims (6)

An image carrier on which a toner image is formed, a transfer material conveying unit that carries a transfer material and moves to face the image carrier, and a toner image on the image carrier to the transfer material conveying unit A transfer means for transferring onto a transfer material carried and conveyed; a transfer bias output means for outputting a bias for the transfer to the transfer means; and a transfer material carrying surface of the transfer material conveyance means; Is disposed on the opposite surface side, and a bias applying member that applies a bias to the separating portion of the transfer material from the transfer material conveying means, and a separating portion bias output means that outputs a bias to the bias applying member, In an image forming apparatus having
An image having bias control means for controlling a bias output from the separation unit bias output means to the bias applying member based on a bias output from the transfer bias output means to the transfer means. Forming equipment.   The image forming apparatus according to claim 1, wherein the bias applying member is a member that stretches the transfer material conveying unit.   The image forming apparatus according to claim 1, wherein the bias applying member is a member disposed adjacent to the transfer unit.   4. The bias control unit controls a bias output from the separation unit bias output unit to the bias applying member in accordance with an atmosphere in which the image forming apparatus is installed. The image forming apparatus according to any one of the items.   The bias control unit includes a detection unit configured to detect an atmosphere in which the image forming apparatus is installed. The image forming apparatus according to claim 1, wherein a difference between an output bias and a bias output from the transfer bias output unit to the transfer unit is controlled. A plurality of image forming modes for forming images on different types of transfer materials, and the bias control unit is configured to switch the bias applying member from the separation unit bias output unit according to the image forming mode. The image forming apparatus according to claim 1, wherein a difference between a bias output to the transfer unit and a bias output from the transfer bias output unit to the transfer unit is controlled. .

Images