JP2006259571A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form high quality recording images without generating image omission even at the time of a recording sheet with ruggedness existing on an image receiving surface like embossed paper for instance by improving the transfer efficiency of a toner image from an intermediate transfer body to the recording sheet at the time of transferring the toner image from a plurality of imaging engines through the intermediate transfer body to the recording sheet altogether. <P>SOLUTION: In this tandem type image forming apparatus of an intermediate transfer system, a distribution adjusting charger 5 which is provided on the downstream side of the imaging engine 2-2 of at least the second or succeeding stage around the intermediate transfer body 1, to which the offset voltage of the same polarity as the normal charge polarity of toner is applied, and which is capable of adjustment to the state that the entire toner charge distribution of the toner image on the intermediate transfer body 1 is biased to the normal charge polarity side at a primary transfer part by the respective imaging engines 2 and a secondary transfer part by a batch transfer device 4, is added. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer that forms a recorded image using an electrophotographic method, and in particular, a plurality of image forming engines for forming a toner image around an intermediate transfer member are arranged, The present invention relates to an image forming apparatus configured to secondarily transfer a toner image primarily transferred from an image forming engine onto an intermediate transfer member onto a recording sheet.

  In a copying machine or a laser beam printer that forms a recorded image using an electrophotographic system, a toner corresponding to image information is formed on the image carrier using an image forming engine having an image carrier such as a photosensitive drum. After the image is formed, the toner image is transferred to a recording sheet such as paper, and the recording sheet onto which the toner image has been transferred is inserted into a fixing device to fix the toner image to the recording sheet. As a color image forming apparatus that forms a full-color image, a so-called tandem type image forming apparatus having four image forming engines corresponding to yellow, magenta, cyan, and black colors is also known. In this tandem type image forming apparatus, four image forming engines are arranged in series along the circumferential direction of an intermediate transfer member formed in a belt shape or a drum shape, and yellow, formed according to image information, The magenta, cyan, and black color toner images are primarily transferred from the image forming engines onto the intermediate transfer member, and a composite toner image in which these toner images overlap is formed on the intermediate transfer member. Then, the composite toner image is secondarily transferred from the intermediate transfer member to the recording sheet, and finally the recording sheet is inserted into a fixing device, thereby forming a full-color recording image on the recording sheet (for example, Patent Documents). 1).

  As a method for transferring a toner image to a recording sheet or an intermediate transfer member, so-called electrostatic transfer is used. In the image forming engine, the toner held in the developing device develops the electrostatic latent image written on the image carrier to form a toner image, but the toner is image-bearing by electrostatic attraction. Therefore, each toner forming a toner image has a certain polarity and a predetermined charge or more. For this reason, in electrostatic transfer, a transfer member such as a transfer roll is disposed on the back side of the image receiving surface of the recording sheet or on the back side of the image receiving surface of the intermediate transfer member, and the recording sheet or intermediate transfer member is brought into close contact with the image carrier. A transfer bias voltage having a polarity opposite to the charging polarity of the toner is applied to the transfer member, and an electric field is formed between the image carrier and the transfer member, whereby the toner is transferred from the image carrier to the recording sheet. Yes.

JP 2002-148895 A (Embodiment of the Invention, FIG. 2) JP 2002-258576 A (Embodiment of the Invention, FIG. 1)

Here, the individual toners transferred from the image bearing member to the image receiving member such as a recording sheet or an intermediate transfer member do not have a uniform charge amount, but have a certain amount of charge amount. .
FIG. 10 is a graph showing a model distribution of the charge amount of each toner in a toner image developed on the image carrier, with the horizontal axis indicating the charge amount and the vertical axis indicating the charge amount indicated on the horizontal axis. It shows the amount of toner. The toner in this model distribution is charged with a negative polarity as shown by the solid line in FIG. In electrostatic transfer, a transfer electric field is formed between the image receiver and the image carrier, and the toner image is transferred by electrostatic attraction based on the transfer electric field, so that the transfer efficiency of the toner image is increased. In other words, all toners must be charged to either positive or negative polarity, and it is preferable that the toner has a charge amount of a certain level or more. In particular, in a recording sheet having an uneven surface such as embossed paper, when the toner image is transferred from the photosensitive drum to the recording sheet or from the intermediate transfer body to the recording sheet, the physical adhesion between the two is weak, When the charge amount of the toner is reduced, the transfer efficiency of the toner image is extremely deteriorated.

  FIG. 11 is a model diagram showing a change in the charge amount distribution of the toner primarily transferred to the intermediate transfer member 104 in the tandem type image forming apparatus including the four image forming engines 100 to 103. The primary transfer of the toner image is performed from the image forming engine 100 located at the uppermost stream in the rotation direction of the intermediate transfer member 104, and the charge amount of the toner image immediately passes through each of the image forming engines 101, 102, 103. Shows how it changes. As is clear from this figure, when the toner image is transferred from the first-stage image forming engine 100 located at the uppermost stream of the intermediate transfer member to the intermediate transfer member 104, the toner in the toner image is charged. The quantity distribution is wider than the distribution on the image carrier shown by the solid line in FIG. This is considered to be because when the toner image is primarily transferred from the image forming engine 100 to the intermediate transfer member 104, a transfer bias voltage having a polarity opposite to the charging polarity of the toner is applied from the back side of the intermediate transfer member 100. It is done. However, the toner charge distribution immediately after being transferred from the image forming engine 100 to the intermediate transfer member 104 still maintains the same polarity before and after the transfer, and if this distribution spreads to this extent, secondary transfer of the toner image is performed. Is considered to have almost no effect.

  However, when the toner image primarily transferred from the first-stage image forming engine 100 passes through the primary transfer portion of the second-stage image forming engine 101, as shown by a dotted line in FIG. The distribution of the charge amount is further expanded, and the charge polarity of some toners is reversed. This is because when the toner image formed by the second-stage image forming engine 101 is transferred to the intermediate transfer member 104, a transfer bias voltage having a polarity opposite to the charging polarity of the toner is applied. This is because the toner image primarily transferred from the image engine 100 is affected when it passes through the second-stage image forming engine 101. In addition, according to an actual experiment, it has been confirmed that the toner image primarily transferred from the first-stage image forming engine 100 passes through the primary transfer portion of the second-stage image forming engine 101 and further has three stages. Even if it passes through the primary transfer portion of the eye image forming engine 102 or further passes through the primary transfer portion of the fourth stage image forming engine 103, the toner charge amount distribution is the second stage image forming engine. Almost the same as after passing 101.

  Thus, when the toner charge distribution is expanded and toner with extremely low charge amount or toner with reversed charge polarity is generated, the electrostatic attraction force is generated when the toner image is electrostatically transferred from the intermediate transfer member to the recording sheet. The transfer efficiency of the toner image is reduced. In the case of a flat recording sheet with almost no unevenness on the image receiving surface, the image receiving surface can be uniformly adhered to the intermediate transfer member, and there is also assistance in physical adhesion between the toner and the recording sheet, Although the actual transfer efficiency of the toner image does not decrease so much, for example, in the case of a paper having a large unevenness on the image receiving surface such as embossed paper, it becomes difficult to transfer the toner image in the recessed portion on the image receiving surface, and the transfer to the recorded image is lost ( White spots) will occur.

  The present invention has been made in order to solve the above technical problems. In transferring a toner image from a plurality of image forming engines to a recording sheet via an intermediate transfer member, the transfer from the intermediate transfer member to the recording sheet is performed. A tandem type image that can increase the transfer efficiency of toner images and can form high-quality recorded images without image loss even on recording sheets with unevenness on the receiving surface such as embossed paper. It is to provide a forming apparatus.

  That is, according to the present invention, as shown in FIG. 1, an intermediate transfer body 1 on which toner images are multiplex-transferred and the periphery of the intermediate transfer body 1 are arranged in series to form a toner image corresponding to image information. In addition, a plurality of stages (n stages: a natural number of n = 2 or more) of image forming engines 2 (specifically 2-1, 2-2,..., 2-) that primarily transfer the toner image onto the intermediate transfer body 1. n), a batch transfer unit 4 for secondary transfer of the toner images sequentially transferred onto the intermediate transfer body 1 by the respective image forming engines 2, and at least two stages around the intermediate transfer body 1. Provided on the downstream side of the image forming engine 2-2 after the first, an offset voltage having the same polarity as the normal charging polarity of the toner is applied, and the primary transfer site by each image forming engine 2 and the secondary transfer by the batch transfer unit 4 are applied. The entire toner charge distribution of the toner image on the intermediate transfer body 1 is positive at the site. The state of being biased to the charging polarity side are those having an adjustable distribution control charger 5.

In such technical means, the present case is based on a tandem type image forming apparatus adopting an intermediate transfer method.
In the present invention, the intermediate transfer member 1 may have a drum shape as well as a belt shape.
The image forming engine 2 includes an electrophotographic method and an electrostatic recording method widely as long as an image forming method using toner is employed. In addition, the number of image forming engines 2 may be plural, and may be five or more.
Furthermore, the distribution adjustment charger 5 is located at the downstream side of at least the second and subsequent image forming engines 2-2. The toner charge distribution is downstream of the first image forming engine 2-1. This is because there is almost no change in the reverse charging polarity.
Furthermore, the distribution adjusting charger 5 needs to be applied with an offset voltage having the same polarity as the normal charging polarity of the toner. However, the degree of the offset voltage can be appropriately selected depending on the number and location of the distribution adjusting charger 5 installed, but the intermediate transfer is performed at the primary transfer site by each image forming engine 2 and the secondary transfer site by the batch transfer unit 4. The entire toner charge distribution of the toner image on the body 1 is limited to a range that can be adjusted so as to be biased toward the normal charge polarity.

Here, as the distribution adjusting charger 5, at least one charger is provided around the intermediate transfer body 1 from the primary transfer portion of the toner image in the second stage image forming engine 2-1. 4 may be provided between the paths leading to the secondary transfer site 4, and the specific arrangement position thereof is any one of the second and subsequent image forming engines 2 and the image forming engine of the next step. Or between the n-th image forming engine 2-n and the secondary transfer site.
When a normal charging polarity of the toner, that is, a voltage having the same polarity as the normal charging polarity of the toner is applied to the distribution adjusting charger 5 provided at such a position, the charging of the toner primarily transferred onto the intermediate transfer body 1 is performed. The distribution can be corrected, and the toner whose charge polarity has been reversed by repeatedly passing through the primary transfer site is restored to the normal charge polarity, and the charge amount is lowered even if the polarity is not reversed. The charge amount of the toner can be increased. Accordingly, when the toner image is secondarily transferred from the intermediate transfer body 1 to the recording sheet 3, the electrostatic attraction force toward the recording sheet 3 is surely applied to the individual toners forming the toner image. It is possible to prevent a decrease in image transfer efficiency. Therefore, even when the toner image is secondarily transferred to the recording sheet 3 such as an embossed paper having an uneven surface on the image receiving surface, it is possible to prevent the transfer efficiency from being extremely lowered, and due to transfer omission. It is possible to prevent a decrease in image quality.
In addition, the distribution adjustment charger 5 of the present case adjusts the entire toner charge distribution of the toner image on the intermediate transfer body 1 to be biased to the normal charging polarity side at each primary transfer site by the image forming engine 2. The toner having the opposite polarity to the toner image on the intermediate transfer body 1 is not retransferred to the image carrier (for example, a photosensitive drum) on the image forming engine 2 side at each primary transfer portion.

Further, it is sufficient that at least one distribution adjusting charger 5 is provided. However, as a preferable aspect, there is an aspect in which the distribution adjusting charger 5 is provided in all or a plurality of locations on the downstream side of each image forming engine 2 in the second stage and thereafter. Can be mentioned. According to this aspect, it is possible to reliably keep the entire toner charge distribution of the toner image in a state of being biased toward the normal charging polarity.
More specifically, if one charger is provided as the distribution adjustment charger 5, the arrangement position is between the n-th image forming engine 2-n and the secondary transfer site. preferable. By disposing at this position, the toner image primarily transferred to the intermediate transfer body 1 by the (n-1) th stage image forming engine 2-n-1 becomes the primary transfer site of the nth stage image forming engine 2-n. Even when the toner has passed and the charge distribution of the toner in the toner image has spread, it can be corrected. However, when attention is paid to the toner image primarily transferred to the intermediate transfer body 1 by the first-stage image forming engine 2-1, the toner generated by the toner image passing through the second-stage image forming engine 2-2. Even when the distribution adjustment charger 5 is provided between the second-stage image forming engine 2-2 and the third-stage image forming engine 2-3, This is effective, and the secondary transfer efficiency of the toner image on the intermediate transfer body 1 to the recording sheet 3 can be increased. In other words, in the present invention, even if the position of the distribution adjusting charger 5 is selected between any one of the second and subsequent image forming engines and the next image forming engine, the transfer of the toner image is performed. This is effective to some extent.

On the other hand, when the distribution adjusting charger 5 is provided between the n-th image forming engine 2-n and the secondary transfer portion, the distribution adjusting charger 5 is formed on the intermediate transfer body 1 that has passed through the n-th image forming engine 2-n. Since the toner images are transferred in multiple layers and the total amount of toner adhering to the intermediate transfer body 1 is large, the charge distribution of the toner must be applied unless a large voltage is applied to the distribution adjusting charger 5. Cannot be corrected as described above. However, it has been confirmed that when the absolute value of the voltage applied to the distribution adjusting charger 5 increases, toner scattering occurs in the toner image that has passed through the distribution adjusting charger 5 and the image quality is deteriorated. .
Therefore, from the viewpoint of avoiding image quality degradation due to toner scattering, it is preferable that the absolute value of the voltage applied to the distribution adjusting charger 5 is low, and the toner image is transferred while the absolute value of the applied voltage is reduced. From the viewpoint of preventing omission, the distribution adjustment charging is performed between the path from the primary transfer portion of the toner image to the secondary transfer portion of the toner image with respect to the recording sheet 3 in the second stage image forming engine 2-2. It is preferable to provide a plurality of devices 5 (all or a plurality of locations downstream of each image forming engine 2 in the second stage and thereafter). With this configuration, it is possible to reliably correct the charge distribution of the toner forming the toner image while keeping the voltage applied to each distribution adjustment charger 5 low, so that the image quality is deteriorated due to toner scattering. It is possible to prevent the toner image from being transferred to the recording sheet.
Furthermore, as an installation mode of the distribution adjustment charger 5, the distribution adjustment charger 5 may be installed on the downstream side immediately after the first stage image forming engine 2-1. In this case, it is preferable in that the charge distribution of the toner can be adjusted in advance to suppress the deviation toward the reverse charge polarity.

Further, although the prior art approximated to the distribution adjustment charger 5 of the present case is presented (see, for example, Patent Document 2), it will be supplemented for the sake of caution that it is unrelated to the present case.
That is, the prior art described in Patent Document 2 has two printing stations for double-sided printing (intermediate transfer type tandem image forming apparatus mode), and toner of the same polarity is used in each printing station. In addition, before the batch transfer, the charging polarity of the toner image formed at the second printing station is reversed by a corona charger to enable batch transfer processing.
However, the charging process by the corona charger in this prior art is a technique for reversing the normal charging polarity to the reverse polarity, and requires a sufficient applied voltage to reverse, so that the scatter phenomenon of multiple transferred toner images In addition, there is a concern that it may lead to image quality defects, and the effect of the distribution adjustment charger 5 of this case (the entire toner charge distribution can be adjusted to a state deviated to the normal charge polarity side, and the toner charge distribution is reversed polarity) By adjusting the charge so that it does not reach the target, not only the batch transfer property can be kept good, but also the re-transfer during the primary transfer by the image forming engine can be prevented.) It is.

Further, as a typical mode of the distribution adjustment charger 5, a non-contact charger arranged on the front surface side of the intermediate transfer member 1 may be used, or alternatively, it may be arranged in contact with the back surface of the intermediate transfer member 1. It may be an electrode member.
Here, when a non-contact charger is used as the distribution adjusting charger 5, the back side of the intermediate transfer body 1 may be used, but the front side is preferable in terms of charging efficiency. Further, in the case of using a contact-type charger, from the viewpoint of preventing the toner image on the intermediate transfer body 1 from being disturbed, it is limited to a mode in which the electrode member is brought into contact with the back surface of the intermediate transfer body 1.

Furthermore, the following are mentioned as a preferable control aspect using the distribution adjustment charger 5.
As one aspect, a control device (not shown) capable of controlling the voltage applied to the distribution adjusting charger 5 is provided. The control device is provided with an environmental sensor, and the distribution is determined according to the detection value of the environmental sensor. The voltage applied to the adjustment charger 5 may be controlled. Since the toner charge distribution is strongly dependent on the environment, the voltage applied to the distribution adjusting charger 5 is controlled based on information from the environment sensor.
As another aspect, a control device (not shown) capable of controlling the voltage applied to the distribution adjusting charger 5 is provided, and this control device is provided with a recording sheet type determining unit, and the determined recording sheet type Accordingly, the presence / absence of voltage application to the distribution adjusting charger may be controlled. Whether or not charging by the distribution adjusting charger 5 is necessary depends on the recording sheet type (plain paper, embossed paper, etc.), and therefore the voltage application to the distribution adjusting charger is controlled according to the recording sheet type. is there.

Further, a preferred embodiment of the present invention includes an embodiment in which the intermediate transfer member 1 is configured as an endless belt, and the volume resistivity of the intermediate transfer member 1 is set to 10 ¹² Ω · cm or more. By making the intermediate transfer body 1 have a high resistance, it becomes possible to secure a charge retention force, so that it is possible to effectively prevent toner scattering. Along with this, it is possible to set a large voltage applied to the distribution adjusting charger 5, which is preferable in terms of further improving transfer omission with respect to the recording sheet 3 such as embossed paper.

According to the present invention, in the intermediate transfer type tandem image forming apparatus, an offset having the same polarity as the normal charging polarity of the toner is provided on the downstream side of at least the second and subsequent image forming engines around the intermediate transfer member. Distribution that can be adjusted so that the toner charge distribution of the toner image on the intermediate transfer member is biased to the normal charge polarity at the primary transfer site by each image forming engine and the secondary transfer site by the batch transfer device when voltage is applied Since the adjustment charger is added, the following basic effects can be obtained.
Since the entire toner charge distribution at the secondary transfer site is adjusted to a state deviated to the normal charge polarity side, it is possible to secure a sufficient amount of toner transfer to the recording sheet, and to prevent unevenness such as embossed paper. Transfer defects such as white spots can be effectively prevented even with a certain recording sheet.
Also, since the entire toner charge distribution at the primary transfer site is adjusted to a state deviated to the normal charge polarity side, it is possible to suppress the retransfer of the toner image primarily transferred to the intermediate transfer member, and each color component toner image. The image disturbance can be effectively prevented.

Hereinafter, the image forming apparatus of the present invention will be described in detail with reference to the accompanying drawings.
Embodiment 1
First, FIG. 2 is an explanatory diagram showing the overall configuration of the first embodiment of the image forming apparatus to which the present invention is applied.
In the figure, the image forming apparatus shows an example of a color laser beam printer. This laser beam printer has four image forming engines 10Y, 10M for forming toner images for each color of yellow, magenta, cyan and black. 10C, 10Bk, and an intermediate transfer belt 20 to which toner images are primarily transferred from the respective image forming engines. A full-color image is obtained by secondary transfer of the toner images transferred onto the intermediate transfer belt 20 to a recording sheet P. Is formed.

  The intermediate transfer belt 20 is formed in an endless shape and is wound around three belt transporting rolls 21 to 23 including a driving roll 21, and each color image forming engine 10Y, 10M, while rotating in the direction of the arrow. It is configured to receive a primary transfer of a toner image formed with 10C and 10Bk. Further, a secondary transfer roll 30 is disposed at a position facing the belt conveying roll 23 with the intermediate transfer belt 20 in between, and the recording sheet P is formed between the secondary transfer roll 30 and the intermediate transfer belt 20 that are pressed against each other. The toner image is inserted between them to receive secondary transfer of the toner image from the intermediate transfer belt 20. That is, the belt transport roll 23 functions as a backup roll for the secondary transfer roll 30, and the secondary transfer roll 30 and the belt transport roll 23 constitute a secondary transfer unit.

  The intermediate transfer belt 20 is stretched between three belt conveyance rolls 21 to 23, and a substantially horizontal image receiving span is formed between the belt conveyance rolls 21 and 22, while the two are directly below the image receiving span. A third belt conveyance roll 23 facing the next transfer roll 30 is disposed and stretched in an inverted triangular shape as a whole.

  The four image forming engines 10Y, 10M, 10C, and 10Bk described above are arranged in parallel on the image receiving span of the intermediate transfer belt 20, and a toner image formed according to image information of each color is intermediate transferred. Primary transfer is performed on the belt 20. These four image forming engines are arranged in the order of yellow 10Y, magenta 10M, cyan 10C, and black 10Bk along the rotational direction of the intermediate transfer belt 20, and the most frequently used black image forming engines are arranged. The image engine 10Bk is disposed closest to the secondary transfer portion. Each of the image forming engines 10Y, 10M, 10C, and 10Bk includes a raster scanning unit (ROS) 12 that exposes the photosensitive drums 11 included in the image forming engines according to image information. The laser beam Bm modulated according to the image information exposes the photosensitive drums 11 of the image forming engines 10Y, 10M, 10C, and 10Bk.

  Each of the image forming engines 10Y, 10M, 10C, and 10Bk is sensitized by exposure of the photosensitive drum 11, the charger 13 that charges the photosensitive drum 11 to a uniform background portion potential, and the laser beam Bm. A developing device 14 that develops the electrostatic latent image formed on the photosensitive drum 11 to form a toner image, and residual toner and paper dust from the surface of the photosensitive drum 11 after the toner image is transferred to the intermediate transfer belt 20. And a drum cleaner 15 that removes the toner image, and is configured so that a toner image corresponding to the image information of each color can be formed on the photosensitive drum 11.

  Primary transfer rolls 17Y, 17M, 17C, and 17Bk are disposed at positions facing the photoconductive drums 11 of the respective image forming engines 10Y, 10M, 10C, and 10Bk so as to sandwich the intermediate transfer belt 20 therebetween. By applying a predetermined transfer bias voltage to the rolls 17Y, 17M, 17C, and 17Bk, an electric field is formed between the photosensitive drum 11 and the primary transfer rolls 17Y, 17M, 17C, and 17Bk. The charged toner image is transferred onto the intermediate transfer belt 20 by Coulomb force. In this printer, since the charging polarity of the toner when developing the electrostatic latent image is set to a negative polarity, the transfer bias voltage applied to the primary transfer rolls 17Y, 17M, 17C, and 17Bk is a positive polarity. Is applied.

  Accordingly, in forming a full color image by this color laser beam printer, first, the raster scanning unit 12 applies the photosensitive drums 11 of the image forming engines 10Y, 10M, 10C, and 10Bk at a predetermined timing according to the image information of each color. After the exposure to form an electrostatic latent image, the electrostatic latent image is developed by the developing unit 14, so that the image forming engines 10 </ b> Y, 10 </ b> M, 10 </ b> C, and 10 </ b> Bk on the photosensitive drums 11 correspond to image information. A toner image is formed. The toner images formed by the image forming engines 10Y, 10M, 10C, and 10Bk are sequentially transferred to the rotating intermediate transfer belt 20, and a multiple toner image in which the respective color toner images are overlapped on the intermediate transfer belt 20. It is formed.

  On the other hand, after the recording sheet P is pulled out from the sheet cassette 40 one by one by the sheet feed roll 41, the intermediate transfer belt 20 and the secondary transfer roll 30 are passed through a predetermined sheet feeding path 46 indicated by a one-dot chain line in the drawing. Is fed to the secondary transfer section where the A registration roll 42 is provided on the front side of the secondary transfer portion, and the recording sheet P is primary-transferred onto the intermediate transfer belt 20 by controlling the entry timing with respect to the secondary transfer portion by the registration roll 42. Registration with the toner image is performed.

  The backup roll 23 facing the secondary transfer roll 30 with the intermediate transfer belt 20 interposed therebetween is formed by covering the surface of the insulating roll with a semiconductive sheet, and a predetermined transfer bias voltage is applied to the surface. The conductive contact roll 50 is in contact. Accordingly, when a transfer bias voltage having the same polarity as that of the toner image is applied to the contact roll 50 and a charge is applied to the surface of the backup roll 23, the secondary transfer roll 30 positioned on the back side of the recording sheet P A transfer electric field is formed between the intermediate transfer belt 20 and the backup roll 23 located on the back side, and the toner image held on the intermediate transfer belt 20 is electrostatically transferred to the recording sheet P.

  The recording sheet P on which the toner image has been secondarily transferred is peeled off from the intermediate transfer belt 20 and then conveyed to the fixing device 44 by a series of two sheet conveying belts 43 arranged in series. The recording sheet P is fixed on the toner image by the fixing device 44 and then discharged to the paper discharge tray 45, whereby the formation of the full color image on the recording sheet P is completed. Further, in this printer, an inverter passage 47 for inverting the front and back of the recording sheet P between the fixing device 44 and the discharge tray 45 in order to enable so-called face-down discharge with the image surface of the recording sheet P facing downward. Is formed.

  On the other hand, the transfer residual toner is removed from the surface of the intermediate transfer belt 20 after the secondary transfer of the toner image is completed by the belt cleaner 24. The belt cleaner 24 has a cleaning blade that scrapes off toner, paper dust, and the like from the surface of the intermediate transfer belt 20, and the cleaning blade is provided at a position facing the belt conveying roll 21. As a result, the surface of the intermediate transfer belt 20 after the secondary transfer of the toner image is cleaned and used for the formation of the next toner image.

FIG. 3 is an explanatory diagram showing a main part of the image forming apparatus according to the present embodiment.
As already described with reference to FIG. 11, in the so-called tandem type image forming apparatus in which a plurality of image forming engines 10Y, 10M, 10C, and 10Bk are arranged in series, the image forming engines 10Y, 10M, and 10C except for the final stage are used. When the toner image primarily transferred from the photosensitive drum 11 to the intermediate transfer belt 20 passes through the primary transfer portion of the next-stage image forming engine located on the downstream side, the toner is transferred to the primary transfer rolls 17M, 17C, and 17Bk. Since a polar transfer bias voltage is applied, the charge distribution of the toner image is slightly shifted to a positive polarity while spreading. For this reason, during the secondary transfer in which the multiple toner images on the intermediate transfer belt 20 are collectively transferred to the recording sheet P, toner that is difficult to be transferred from the intermediate transfer belt 20 to the recording sheet P is generated and transferred to the recording sheet P in a batch. There is a technical problem that white spots are likely to occur in the toner image.

Therefore, in the first embodiment, a plurality of distribution adjustment chargers 60 are provided downstream of the second stage image forming engine 10M and upstream of the secondary transfer roll 30 in the periphery of the intermediate transfer belt 20. The configuration to install is adopted.
In this example, a corotron 61 which is a non-contact type charger is used as the distribution adjustment charger 60, and the second-stage image forming engine 10M and the third-stage image forming engine around the intermediate transfer belt 20 are used. 10C, and at two locations downstream of the black image forming engine 10Bk located at the final stage and upstream of the secondary transfer roll 30, so as not to face the surface of the intermediate transfer belt 20 Has been placed. The corotron 61 is connected to a variable power supply 65 that can adjust the voltage, and a voltage having the same polarity as the normal charging polarity (negative polarity in this embodiment) of the toner is applied. For example, the toner image primarily transferred from the yellow image forming engine 10Y to the intermediate transfer belt 20 passes through the magenta, cyan, and black image forming engines 10M, 10C, and 10Bk, and the charging of the toner constituting the yellow toner image therebetween. Although the distribution fluctuates, a corotron 61 as a distribution adjustment charger 60 is provided on the downstream side of the second-stage image forming engine 10M and the final-stage image forming engine 10Bk, so that the magenta image forming engine 10M is passed through. The yellow toner image and the yellow toner image that has passed through the black image forming engine 10Bk can be recharged with the same charge as the normal charge polarity of the toner, and the charge distribution of the yellow toner can be corrected. it can.
The same applies to the toner images primarily transferred onto the intermediate transfer belt 20 from the magenta image forming engine 10M, the cyan image forming engine 10C, and the black image forming engine 10Bk.

In this way, for each color toner image that has passed through the primary transfer portion of the second-stage image forming engine 10M and the final-stage image forming engine 10Bk, the corotron 60 applies a charge having the same polarity as the normal charging polarity of the toner. As a result, the charge amount is increased for the toner whose charge amount has decreased, and the charge polarity for the toner whose charge polarity is reversed to the reverse polarity (in this case, the positive polarity) Reversed to polarity. As a result, when these toner images reach the secondary transfer portion, a sufficient electrostatic attraction can be exerted on the multiple toner images on the intermediate transfer belt 20, and the transfer efficiency of the toner images to the recording sheet P can be increased. Can be increased. In particular, when a recording sheet having a large unevenness on the receiving surface of the toner image, such as embossed paper, is used, the toner image can be reliably transferred to the concave portion of the receiving surface, and white spots occur in the recorded image. It is possible to prevent this.
Further, even in the primary transfer portion of each image forming engine 10, the entire toner charge distribution of the toner image on the intermediate transfer belt 20 is biased to the normal charging polarity side, so that the toner on the intermediate transfer belt 20 is subjected to primary transfer. The concern that the image is retransferred onto the photoreceptor drum 11 is effectively avoided.

  In particular, in the present embodiment, a plurality of distribution adjusting chargers 60 (in this example, the corotron 61 is used) are arranged around the intermediate transfer belt 20, and each corotron 61 has the same polarity as the normal charging polarity of the toner. If a voltage is applied, charges are repeatedly injected from the plurality of corotrons 61 to the toner image on the intermediate transfer belt. Therefore, even if the absolute value of the voltage applied to each corotron 61 is kept low, a toner image is formed. It becomes possible to accurately correct the charge distribution of the toner. That is, the charge amount is increased by injecting a charge to the toner whose charge amount is reduced, and the polarity is reversed to the original normal polarity for the toner whose charge polarity is reversed. It becomes possible to increase the transfer efficiency of the secondary transfer of the toner image to P, and it is possible to prevent the toner image from being transferred to the embossed paper. Further, since the voltage applied to each corotron 61 can be kept low, it is possible to effectively avoid toner scattering that occurs during correction of the charge distribution, and it is possible to prevent deterioration in the image quality of the recorded image.

In the present embodiment, the distribution adjustment charger 60 (corotron 61 in this example) is installed in the layout shown in FIG. 3, but the present invention is not limited to this. For example, as shown in FIG. As shown in the drawing, the distribution adjustment charger 60 (on the downstream side of the black image forming engine 10Bk of the final stage black image forming engine 10Bk and between the cyan image forming engine 10C of the third stage and the black image forming engine 10Bk of the final stage, respectively. For example, a corotron 61) may be installed, or, as shown in FIG. 4B, between the first and second imaging engines 10Y and 10M, the second and third stages. The distribution adjusting charger 60 (for example, the corotron 61) may be installed between the image forming engines 10M and 10C and between the third and final image forming engines 10C and 10Bk, respectively. 2nd and later A distribution adjustment charger 60 (for example, corotron) is installed immediately after each image forming engine, that is, immediately after each image forming engine 10M, 10C, 10Bk for magenta, cyan, and black. The distribution adjusting charger 60 is installed immediately after the downstream side of each of the image forming engines 10Y to 10Bk, or one distribution adjusting charger 60 is installed after the second stage image forming engine 10M. There is no problem.
In each of these deformation modes, it is only necessary to adjust so that the entire charge distribution of the toner on the intermediate transfer belt 20 when passing through at least the primary transfer portion and the secondary transfer portion is biased to the normal charging polarity side.

  Further, as one of the deformation modes, in the mode in which the distribution adjustment charger 60 is installed between the first-stage and second-stage image forming engines 10Y and 10M, the yellow toner image having a wide charge distribution is applied. By applying a charge of the same polarity in advance, it becomes possible to further shift the toner charge distribution in a direction away from the reverse charge polarity side, and by that amount, it passed through the primary transfer portion of the subsequent image forming engine. However, there is an advantage that the shift of the charge distribution of the toner to the reverse charge polarity side is suppressed.

Furthermore, in the embodiment using one distribution adjusting charger 60, the applied voltage needs to be increased because the degree of correction of the toner charge distribution increases. Under such circumstances, it is preferable to employ a technique for effectively preventing the toner image on the intermediate transfer belt 20 from scattering.
Here, as an effective means for suppressing toner scattering, it is conceivable to set the volume resistivity of the intermediate transfer belt 20 high. According to the present inventors, if the volume resistivity of the intermediate transfer belt 20 is 10 ¹² Ω · cm or more, for example, in a mode in which one distribution adjusting charger 60 is installed, for example, the distribution adjusting charger Even when a voltage of 1 to 4 kV is applied to the corotron 61 as 60, the scattering of toner on the intermediate transfer belt 20 is reduced, and the occurrence of white spots in the secondary transfer of the toner image is prevented, and the recorded image It was possible to avoid image quality degradation.

Embodiment 2
5 (a) and 5 (b) show the main part of Embodiment 2 of the image forming apparatus according to the present invention.
In the drawing, the basic configuration of the image forming apparatus includes a plurality of corotrons 61 as distribution adjustment chargers 60, as in the first embodiment. However, unlike the first embodiment, the image forming apparatus responds to environmental changes. Thus, the voltage applied to the corotron 61 is controlled. Components similar to those in the first embodiment are denoted by the same reference numerals as those in the first embodiment, and detailed description thereof is omitted here. The same applies to the following embodiments.

In the present embodiment, the change in the toner charge distribution that occurs when the toner image primarily transferred onto the intermediate transfer belt 20 passes through the next-stage image forming engine differs depending on changes in temperature and humidity in the printer. Therefore, in a high temperature and high humidity environment, the toner charge distribution changes remarkably, and there is a tendency that more toner whose polarity is reversed is generated. This is presumably because the resistance value of the semi-conductive intermediate transfer belt 20 slightly changes under the influence of temperature and humidity, and the resistance value decreases in a high temperature and high humidity environment.
Therefore, in order to more accurately correct the toner charge distribution by the corotron 61 as the distribution adjusting charger 60, as shown in FIG. 5B, the voltage applied to the corotron 61 as the distribution adjusting charger 60. The control device 66 is provided with an environmental sensor 67 for measuring temperature and humidity in the printer, and the control device 66 supplies the variable power source 65 to the variable power supply 65 in accordance with the detected value of the environmental sensor 67. Thus, a voltage control signal is sent to control the voltage applied to the corotron 61.
With this configuration, the voltage applied to the corotron 61 can be changed in accordance with the amount of change in the charge distribution of the toner. For example, the absolute value of the negative polarity voltage applied to the corotron 61 in a high temperature / high humidity environment. By setting a large value, the toner whose polarity has been reversed can be reliably returned to the normal charging polarity, and transfer white spots in the secondary transfer can be reliably prevented.

Embodiment 3
6 (a) and 6 (b) show the main part of Embodiment 2 of the image forming apparatus according to the present invention.
In the figure, the basic configuration of the image forming apparatus is provided with corotrons 61 as a plurality of distribution adjustment chargers 60 in substantially the same manner as in the first embodiment. Accordingly, the presence or absence of a voltage applied to the corotron 61 is controlled.
In the present embodiment, white spots when the toner image is secondarily transferred to the recording sheet P are likely to occur in the recording sheet P such as embossed paper having unevenness on the image receiving surface. As described above, there is almost no problem with the recording sheet P having a smooth image receiving surface. Accordingly, it is not always necessary to operate the distribution adjustment charger 60 for all types of recording sheets P, and the distribution adjustment charger 60 is operated on high-quality paper and glossy paper that do not cause a problem of toner image transfer whiteout. If so, there is a concern that toner scattering caused by the charging process becomes obvious and the image quality of the recorded image is deteriorated.
Therefore, depending on the type of the recording sheet P on which the toner image is transferred, the presence or absence of the operation of the distribution adjusting charger 60 is properly used. It is preferable to correct the toner charge distribution on the intermediate transfer belt 20.

In the present embodiment, for example, as shown in FIG. 6B, a control device 66 capable of controlling the voltage applied to the corotron 61 as the distribution adjusting charger 60 is provided. The control device 66 determines the recording sheet type. And the presence or absence of voltage application to the distribution adjusting charger 60 may be controlled according to the determined recording sheet type.
Here, as a method of identifying the type of recording sheet P, the user himself / herself inputs the type of recording sheet loaded in the paper feed cassette 40 (see FIG. 2) from the sheet type selection switch 68 of the control panel which is a user interface. The input information may be used for discrimination, or the recording sheet P in the sheet feeding cassette 40 or the image receiving surface of the recording sheet P sent through the sheet feeding path 46 may be a semiconductor. The type of the recording sheet P may be discriminated from the amount of reflected light by illuminating with illumination means such as a laser.

Embodiment 4
7 (a) and 7 (b) show the essential parts of Embodiment 4 of the image forming apparatus to which the present invention is applied.
In this figure, the basic configuration of the image forming apparatus is substantially the same as that of the first embodiment. However, unlike the first embodiment, a contact-type charger is used as the distribution adjustment charger 60, and at least the second stage operation is performed. For example, a plurality of the contact chargers are installed on the downstream side of the image engine 10M, in this example, between the second and third imaging engines 10M and 10C, and on the downstream side of the final imaging engine 10Bk. It is what you do.
Here, as the contact-type charger, for example, a plate-like electrode member 62 that is in contact with the back surface of the intermediate transfer belt 20 is disposed in contact, and a variable power source 65 is connected to the electrode member 62, whereby A voltage having the same polarity as the normal charging polarity of the toner is applied. The electrode member 62 disposed in contact with the back surface side of the intermediate transfer belt 20 is not limited to a plate shape, and may be a roll-shaped electrode member that rotates around the intermediate transfer belt 20.

Therefore, when the electrode member 62 serving as the distribution adjusting charger 60 is arranged in contact with the back surface side of the intermediate transfer belt 20 in this way, charge is injected from the back surface side, and the toner on the surface of the intermediate transfer belt 20 is injected. The charge distribution can be corrected. Accordingly, even in this case, the transfer efficiency of the toner image to the recording sheet P can be increased, and the toner image can be reliably transferred to the embossed paper.
Even in the present embodiment, the number and positions of the electrode members 62 as the distribution adjusting charger 60 may be appropriately selected as in the first embodiment.
Further, with respect to the control mode of the distribution adjusting charger 60, as shown by the phantom line in FIG. 7B, the electrode member according to the environmental change using the control device 66 and the environmental sensor 67 similar to those of the second embodiment. The voltage applied to 62 may be controlled, or the presence or absence of the voltage applied to the electrode member according to the recording sheet P type using the control device 66, the sheet type selection switch 68, etc., similar to the third embodiment. May be controlled.

Example 1
This example uses a model in which only the corotron 61 as the distribution adjustment charger 60 installed on the downstream side of the black image forming engine 10Bk in the final stage is used in the image forming apparatus according to the first embodiment. The charge distribution of the toner image primarily transferred to the intermediate transfer belt 20 is corrected using the corotron 61.
In this model, depending on the voltage applied to the corotron 61, it is possible to prevent the whitening of the toner image in the secondary transfer, but the toner is scattered on the intermediate transfer belt 20 and the image quality of the recorded image is deteriorated. It was observed.
Here, in this embodiment, the absolute value of the DC voltage applied to the corotron 61 is changed so that the toner image transfer white spot occurs on the recording sheet P and the toner scattering occurs on the intermediate transfer belt 20. And the results shown in FIG. 8 were obtained. In FIG. 8, the abscissa indicates the voltage value applied to the corotron 61, and the ordinate indicates the grade in which the occurrence of transfer white spots and the occurrence of toner scattering are evaluated in seven stages. The details of the grade are as shown in Table 1 below. Further, as a recording sheet P on which the toner image is secondarily transferred, Rezac 66 (250 gsm) which is embossed paper is used. Transfer white spots were determined by transferring a 20% square blue patch image onto the recording sheet P with a toner amount of 200% (each toner image of yellow and magenta components is 100% solid image). Further, toner scattering was determined by transferring a toner amount of 400% (a solid image with 100% of each color component toner image) and a 20 mm square patch image onto a recording sheet.

  In the experimental results shown in FIG. 8, when the voltage applied to the corotron 61 has the same polarity (negative polarity) as that of the toner and the absolute value exceeds 4.0 kV, the grade relating to the transfer white drop is lowered, and the whiteness of the transferred image on the embossed paper is reduced. It is observed that omissions are decreasing. On the other hand, if the voltage applied to the corotron 61 has the same polarity (negative polarity) as that of the toner and the absolute value exceeds 4.0 kV, toner scattering is recognized on the intermediate transfer belt 20 after passing through the corotron 61. As the voltage increased, the image quality deterioration due to toner scattering became more significant. Therefore, from the viewpoint of preventing the transfer white spot, it is preferable that the voltage applied to the corotron 61 has the same polarity (negative polarity) as that of the toner and the absolute value exceeds 4.0 kV. However, the image quality deterioration due to the scattering of the toner is avoided. In this case, it is understood that the absolute value of the voltage applied to the corotron 61 that is the distribution adjusting charger 60 needs to be suppressed to a certain level.

Example 2
In this example, the performance evaluation of the image forming apparatus according to the first embodiment in which a plurality of corotrons 61 as the distribution adjustment charger 60 are installed is examined.
Here, in this embodiment, the applied voltage to the corotron 61 is set to −4.5 kV or −5 kV, and the occurrence of white spots in the toner image on the recording sheet P is generated for each model in which the number of corotrons 61 is changed. When the occurrence of toner scattering on the intermediate transfer belt 20 was examined, the result shown in FIG. 9 was obtained. In FIG. 9, among the horizontal axes, “None” indicates a model that does not use the corotron 61, and “1/2 color” indicates that the corotron 61 is installed immediately downstream of the first and second image forming engines. The model “1/2/3 color” is the model in which the corotron 61 is installed immediately downstream of the first, second, and third stage imaging engines, “1/2/3/4 color”. Indicates a model in which the corotron 61 is installed immediately after the downstream side of each imaging engine from the first stage to the last stage, and the vertical axis indicates the occurrence of transfer white spots and the occurrence of toner scattering in seven stages. Indicates the grade. The details of the grade are as shown in Table 1 above.

According to FIG. 9, it was found that if the corotron 61 as a plurality of distribution adjusting chargers 60 is installed, toner scattering is reduced. For example, when the corotron applied voltage is -4.5 kV, in the "1/2 color" model, the toner scattering grade is G0.5 and the whiteout grade is G5. On the other hand, in the model of the first embodiment (a mode in which the corotron 61 is installed only on the downstream side of the final stage image forming engine), when the toner scattering grade is G0.5, the whiteout grade is G6. It is understood that the whiteout grade is improved by one level from G6 to G5 by installing the distribution adjusting charger 60.
In addition, it is understood that the “1/2/3 color” model or the “1/2/3/4 color” model further improves the blank grade compared to the “1/2 color” model. However, on the contrary, it is understood that the scattering grade is deteriorated as the number of passes by the corotron 61 increases.
However, the scattering grade is substantially in the range up to G3.

1 is a diagram illustrating a schematic configuration of an image forming apparatus of the present invention. 1 is an explanatory diagram illustrating an overall configuration of an image forming apparatus according to a first embodiment to which the present invention is applicable. (a) is an explanatory view showing a main part of the image forming apparatus according to the first embodiment, and (b) is an explanatory view showing an example of a method of applying a voltage to the distribution adjusting charger. (a) (b) is explanatory drawing which shows the deformation | transformation aspect of the example of installation of the distribution adjustment charging device which concerns on Embodiment 1. FIG. (a) is explanatory drawing which shows the principal part of the image forming apparatus which concerns on Embodiment 2, (b) is schematic which shows the example which controls the applied voltage of a distribution adjustment charger according to an environmental change. (a) is explanatory drawing which shows the principal part of the image forming apparatus which concerns on Embodiment 3, (b) is the schematic which shows the example which controls the presence or absence of the applied voltage of a distribution adjustment charger according to a sheet | seat type. (a) is explanatory drawing which shows the principal part of the image forming apparatus which concerns on Embodiment 4, (b) is explanatory drawing which shows an example of the voltage application system to a distribution adjustment charger. 6 is a graph showing a relationship between an applied voltage to a distribution adjusting charger, toner image transfer omission (white omission), and toner scattering. 6 is a graph showing a relationship between an applied voltage, toner image transfer omission (white omission), and toner scattering for each installation example of a distribution adjusting charger. 6 is a graph showing an example of toner charge distribution in a toner image developed on an image carrier. In the conventional intermediate transfer type tandem type image forming apparatus, the toner image on the intermediate transfer body changes in the toner charge distribution due to passing through the image forming engine.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Intermediate transfer body, 2 ... Image forming engine, 2-1 ... First stage image forming engine, 2-2 ... Second stage image forming engine, 2-n ... N stage image forming engine, 3 ... Recording sheet, 4 ... Batch transfer device, 5 ... Distribution adjustment charger

Claims (8)

An intermediate transfer member on which a toner image is transferred in multiple layers;
A plurality of image forming engines which are arranged in series around the intermediate transfer body, form a toner image according to image information, and primarily transfer the toner image onto the intermediate transfer body;
A batch transfer device that secondarily transfers the toner images sequentially transferred onto the intermediate transfer member by each image forming engine to a recording sheet;
Provided on the downstream side of at least the second and subsequent image forming engines around the intermediate transfer body, an offset voltage having the same polarity as the normal charging polarity of the toner is applied, and the primary transfer site and the batch of each image forming engine A distribution adjustment charger capable of adjusting the entire toner charge distribution of the toner image on the intermediate transfer member to a normal charge polarity side at a secondary transfer site by the transfer device; and
An image forming apparatus comprising:   The image forming apparatus according to claim 1, wherein the distribution adjusting chargers are provided at all or a plurality of locations on the downstream side of at least the second and subsequent image forming engines.   The image forming apparatus according to claim 1, wherein the distribution adjusting charger is also provided between the first-stage image forming engine and the second-stage image forming engine.   The image forming apparatus according to claim 1, wherein the distribution adjusting charger is a non-contact charger arranged on the surface side of the intermediate transfer member.   2. The image forming apparatus according to claim 1, wherein the distribution adjusting charger is an electrode member disposed in contact with the back surface of the intermediate transfer member.   A control device capable of controlling the voltage applied to the distribution adjustment charger is provided. The control device is provided with an environment sensor, and the voltage applied to the distribution adjustment charger is controlled according to a detection value of the environment sensor. The image forming apparatus according to claim 1.   A control device capable of controlling the voltage applied to the distribution adjustment charger is provided. The control device is provided with a recording sheet type determination unit, and whether or not voltage is applied to the distribution adjustment charger according to the determined recording sheet type. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled. The image forming apparatus according to claim 1, wherein the intermediate transfer member is configured as an endless belt, and the volume resistivity of the intermediate transfer member is 10 ¹² Ω · cm or more.

Images