JP3201030B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser printer, and more particularly, to an image forming apparatus formed on a latent image carrier such as a photosensitive drum via an intermediate transfer member. The present invention relates to an improvement in an image forming apparatus that transfers an unfixed toner image to a recording medium such as paper.

[0002]

2. Description of the Related Art Conventionally, as this type of image forming apparatus,
JP-A-49-209 and JP-A-62-206567 are known. Specifically, as shown in FIG. 5, the unfixed toner image T formed on the latent image carrier 100 such as a photosensitive drum is primarily transferred to the intermediate transfer body 101, and the unfixed toner image T is newly transferred to the intermediate transfer body 101. It is configured to form a desired image on the recording medium 102 by secondary transfer from the body 101 to the recording medium 102, particularly black (Bk), yellow as shown in the figure.
(Y), magenta (M) and cyan (C) in the color image forming apparatus that includes developing units 103 to 106 corresponding to each color, and forms a full-color image by superimposing unfixed toner images of each color, An unfixed toner image of each color formed for each rotation of the latent image carrier 100 is superimposed on the intermediate transfer body 101, and the combined image is transferred to the recording medium 102 at a time. In this figure, the transfer of the unfixed toner image T is performed by an electrostatic transfer method, and a corona discharger 107 or a bias roll 108 disposed on the back side of the transfer target at each transfer position has a polarity opposite to that of the toner. When a voltage is applied, an unfixed toner image is transferred from the latent image carrier 100 to the intermediate transfer member 101 or from the intermediate transfer member 101 to the recording medium 102.

In a conventional color image forming apparatus, since unfixed toner images of each color are directly multiplex-transferred onto a recording medium, the recording medium thickness and surface characteristics, the recording medium conveyance characteristics with respect to the latent image carrier, and the like. Many factors affect the image quality of a color image formed on a recording medium. However, in the color image forming apparatus using the intermediate transfer body 101 as described above, the composite image already subjected to the multiple transfer is recorded on the recording medium 102.
Since the image is transferred to the non-transferred image, it is possible to eliminate the above-mentioned unstable factors and to effectively prevent the occurrence of image disturbance and color shift during multiple transfer.

In the above-described image forming apparatus, in order to continuously form an image on a plurality of recording media such as a large number of copies, before starting an image forming cycle for each of the recording media, it is necessary to execute the preceding cycle. It is necessary to uniformly remove the charge remaining on the intermediate transfer member. This is because if the transfer of the unfixed toner image in the next image forming cycle is performed in a state where the charge remains, uneven transfer of the unfixed toner image occurs due to uneven potential of the intermediate transfer member. Conventionally, the following two methods have been performed as a method of the static elimination work. That is, the first method is a method of using a corotron to which an AC voltage is applied to neutralize residual charges to a certain potential level. In the second method, the volume resistivity of the intermediate transfer body is set to be small so that the time constant of the charged charge is smaller than the rotation cycle of the intermediate transfer body. Is discharged from the intermediate transfer member (see JP-A-64-74571).

[0005]

However, these methods have the following problems. First, the first method has a problem that the static elimination efficiency by the AC corotron is poor and the intermediate transfer member cannot be neutralized to a uniform potential. Further, since the AC corotron generates about four times as much ozone as the DC corotron, the life of a general photoconductor as a latent image carrier is adversely affected, and the human body may be adversely affected. Further, in the second method, since the volume resistivity of the intermediate transfer member is set small, the unfixed toner image is electrostatically primary-transferred or secondary-transferred using a transfer corotron or a bias roll as shown in FIG. When the next transfer is attempted, the electric charge given from the transfer corotron is conducted along the surface of the intermediate transfer body, and the transfer of the unfixed toner image is performed before the intermediate transfer body and the latent image carrier are brought into close contact with the recording medium. Has been started. For this reason,
In the transferred unfixed toner image, transfer unevenness and image disorder occurred, and a high-quality image could not be formed.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems. It is an object of the present invention to uniformly adjust the potential level of an intermediate transfer member before an image forming operation. It is an object of the present invention to provide an image forming apparatus capable of preventing occurrence of image disturbance and transfer unevenness during transfer of a toner image and forming a good transfer image.

[0007]

In order to achieve the above object, an image forming apparatus according to the present invention primarily transfers an unfixed toner image formed on a latent image carrier onto an intermediate transfer member, and re-transfers the image. In an image forming apparatus in which an unfixed toner image is secondarily transferred from the intermediate transfer body to a recording medium to form an image on the recording medium, the image forming apparatus exhibits a small volume resistivity under a high electric field and a large volume resistance under a low electric field. And a pre-transfer charger for charging the intermediate transfer body to a predetermined potential or more before the start of the primary transfer.

In such a technical means, the intermediate transfer body has at least the following electrical characteristics, that is, a small volume resistivity so that electric charges can easily move under a high electric field, while the intermediate transfer body exhibits a low volume resistivity under a low electric field. It is formed of a semiconductive material having a large volume resistivity so that electric charges are retained. In other words, according to such electric characteristics, when electric charges flow into the intermediate transfer member and the surface potential of the intermediate transfer member increases, the electric field applied to the intermediate transfer member increases. Has a small volume resistivity, and the electric charge gradually flows out of the intermediate transfer member. For this reason, as the surface potential of the intermediate transfer member gradually increases, the ratio of the outflow charge to the inflow charge increases, and finally, the inflow charge and the outflow charge balance as shown in FIG. The surface potential of the intermediate transfer member becomes saturated (hereinafter, this surface potential is referred to as saturation potential).

FIG. 3 shows an example of the electrical characteristics of the intermediate transfer member used in the present invention, and shows the characteristic that the volume resistivity decreases under a high electric field. This characteristic diagram differs depending on the material of the intermediate transfer member, and as a result, the electric field strength at which the inflow electric field and the outflow electric field are balanced also differs depending on the material of the intermediate transfer member. Therefore, the saturation potential can be arbitrarily set by arbitrarily selecting the material of the intermediate transfer member.

Next, the pre-transfer charger includes a device which applies a charge to the intermediate transfer member before an unfixed toner image is primarily transferred and charges the intermediate transfer member to a predetermined surface potential or more. May be changed as appropriate. Here, the term “predetermined surface potential or higher” means higher than the saturation potential of the intermediate transfer member. That is, when the pre-transfer charger charges the intermediate transfer body to a surface potential equal to or higher than the saturation potential, electric charge flows out of the intermediate transfer body, and the surface potential of the intermediate transfer body becomes saturated potential before the primary transfer of the unfixed toner image. Will be stable.

The charging of the intermediate transfer member by the pre-transfer charger may be performed only once before the start of the image forming operation on each recording medium. Therefore, it is not always necessary to provide a dedicated charger. For example, when an unfixed toner image is transferred using a corona discharger or a bias roll, the corona discharger or the bias roll for transfer may be diverted to the pre-transfer charger. In this case, it is necessary to switch the applied voltage and the applied polarity to the corona discharger and the bias roll in two stages between the time of transfer and the time of charge before transfer. Further, when the intermediate transfer body is to be neutralized before the intermediate transfer body is cleaned or before the secondary transfer of the unfixed toner image, the corona discharger provided for neutralization is diverted as a pre-transfer charger. You can also.

Further, when a color image is formed by this image forming apparatus, the unfixed toner images of the respective colors are superimposed on the intermediate transfer body, so that the intermediate transfer body has the number of colors to be superimposed on each recording medium. However, even in such a case, the charging of the intermediate transfer body by the pre-transfer charger only needs to be performed once each time an image forming operation on each recording medium is started. Further, if the process of removing the charge of the intermediate transfer body is not included in the middle of the image forming operation, even if the image forming operation is repeatedly performed on a large number of recording media, the intermediate transfer by the pre-transfer charger may be performed. It is sufficient that the transfer member is charged only before the start of the image forming operation on the first recording medium.

[0013]

According to the above technical means, the intermediate transfer member is formed of a material having a small volume resistivity under a high electric field and a large volume resistivity under a low electric field, and before the start of the primary transfer. Since the intermediate transfer body is charged to a predetermined surface potential (saturated potential of the intermediate transfer body) or more by the pre-transfer charger, the surface potential converges to the saturated potential and stabilizes due to the electrical characteristics of the intermediate transfer body. The surface potential of the intermediate transfer member does not become unstable due to the residual charges in the image forming cycle. Therefore, at the time of the primary transfer or the secondary transfer of the unfixed toner image, the transferred image is not disturbed or the transfer unevenness does not occur.

According to the above technical means, a high electric field is not applied in the creeping direction of the intermediate transfer body, and the insulating property of the intermediate transfer body is maintained. Premature transfer of the unfixed toner image before the transfer position does not occur.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows a schematic configuration of a color electrophotographic copying machine to which the present invention is applied. Reference numeral 1 denotes a photosensitive drum (latent image carrier), on the surface of which an electrostatic latent image corresponding to image information is formed by a well-known electrophotographic process (not shown) with rotation in the direction of arrow A. Is done. Further, black (Bk), yellow (Y),
Developing devices 5 to 8 corresponding to each color of magenta (M) and cyan (C)
The electrostatic latent image formed on the photosensitive drum 1 is developed by any one of the developing devices to form a toner image T. Therefore, if the electrostatic latent image written on the photosensitive drum 1 corresponds to the yellow image information, this electrostatic latent image is developed by the developing device 6 containing yellow (Y) toner, and On the body drum 1, a yellow toner image is formed.

Reference numeral 2 denotes a belt-shaped intermediate transfer member disposed so as to be in contact with the surface of the photosensitive drum 1;
An electric charge is applied to a predetermined surface potential by a charging corotron (pre-transfer charger) 4, and is stretched over a plurality of rolls and rotated in the direction of arrow B. The unfixed toner image T formed on the photosensitive drum 1 is transferred from the photosensitive drum 1 to the surface of the intermediate transfer member 2 at a primary transfer position where the photosensitive drum 1 and the intermediate transfer member 2 are in contact. At this primary transfer position, a corona discharger 9 is provided on the back side of the intermediate transfer body 2. By applying a voltage having a polarity opposite to the charging polarity of the toner to the corona discharger 9, the photosensitive drum The unfixed toner image T on 1 is electrostatically attracted to the intermediate transfer body 2.

When a single-color image is to be formed, the unfixed toner image T primarily transferred to the intermediate transfer member 2 is immediately secondarily transferred to the recording medium 3. However, a color image in which a plurality of color toner images are superimposed is used. When the toner image is formed, the process of forming the toner image on the photosensitive drum 1 and the primary transfer process of the toner image are repeated by the number of colors. For example, when a full-color image is formed by superimposing four color toner images, unfixed toner images T of black, yellow, magenta, and cyan are formed on the photosensitive drum 1 every one rotation thereof, and these unfixed toner images are formed. The image T is primarily transferred to the intermediate transfer body 2 sequentially. On the other hand, the intermediate transfer body 2 rotates at the same cycle as the photosensitive drum 1 while holding the black primary untransferred toner image T that has been primarily transferred, and the intermediate transfer body 2 has yellow, The magenta and cyan unfixed toner images T are transferred to be superimposed on the black unfixed toner image T.

The unfixed toner image T primarily transferred to the intermediate transfer body 2 in this manner is conveyed to a secondary transfer position facing the conveyance path of the recording medium 3 with the rotation of the intermediate transfer body 2. You. At the secondary transfer position, a semiconductive bias roll 10
Is in contact with the intermediate transfer member 2 and is discharged from the tray 12 by the feed roller 11 at a predetermined timing.
3 is sandwiched between the bias roller 10 and the intermediate transfer member 2. A voltage having a polarity opposite to the charge polarity of the toner is also applied to the bias roll 10, and the unfixed toner image T carried on the intermediate transfer member 2 is electrostatically transferred to the recording medium 3 at the secondary transfer position. .

The recording medium onto which the unfixed toner image has been transferred is peeled off from the intermediate transfer member by the peeling claw 13 and fed to a fixing device (not shown) by the transport belt 14 so that the fixing process of the unfixed toner image is performed. Done. On the other hand, the residual toner is removed by the cleaner 15 from the intermediate transfer body 2 after the secondary transfer of the unfixed toner image is completed.
To give a predetermined surface potential.

The bias roll 10, the peeling claw 13 and the cleaner 15 are disposed so as to be able to freely contact and separate from the intermediate transfer body 2. When a color image is formed, the final unfixed toner image is formed as an intermediate image. These members are separated from the intermediate transfer member 2 until the primary transfer to the transfer member 2 is performed.

In this embodiment, the intermediate transfer member 2 is formed by adding a suitable amount of an antistatic agent such as carbon black to a resin such as acryl vinyl chloride, polyester, polycarbonate, or various rubbers, for example, to a thickness of 0.1 mm. And its volume resistivity is adjusted to 10 ⁶ to 10 ¹⁴ Ω · cm.

Further, as shown in FIG.
Is formed so as to exhibit a small volume resistivity under a high electric field and a large volume resistivity under a low electric field. Such electrical characteristics are provided by controlling the resistance, particle diameter, and interparticle distance of the antistatic agent contained in the intermediate transfer body 2 when forming the intermediate transfer body 2. Considering the withstand voltage of the material forming the intermediate transfer body 2, the voltage applied to the intermediate transfer body 2 is in the range of 100 to 1000 V, that is, the electric field strength is 10 ⁴ to 10 ⁵ as shown in FIG. It is preferable that the volume resistivity is set to fall in the range of V / cm.If the electrical characteristics of the intermediate transfer body 2 are set in this way, a voltage higher than the withstand voltage is applied to the intermediate transfer body 2 due to accumulation of electric charge. Can be prevented from acting, and dielectric breakdown can be effectively avoided. However, as long as the intermediate transfer member 2 is formed of a material having a sufficient withstand voltage, there is no problem even if the volume resistivity is set to decrease at an electric field intensity higher than this.

Therefore, as shown in FIG. 3, when a surface potential of 500 V or more is applied to the intermediate transfer member 2, the intensity of the electric field applied to the intermediate transfer member becomes 5 × 10 ⁴ V / cm or more. The volume resistivity becomes 10 ¹² Ω · cm or less, and the charge flowing into the intermediate transfer body 2 is rapidly eliminated, and the surface potential of the intermediate transfer body 2 decreases. On the other hand, when the surface potential of the intermediate transfer member 2 becomes 300 V or less, the intensity of the electric field applied to the intermediate transfer member 2 is 1.7 ×
Since it is 10 ⁴ V / cm or less, its volume resistivity is 10 ¹³ Ω
m, the electric charge flowing into the intermediate transfer member 2 is accumulated, so that the intermediate transfer member 2 is stably charged at a constant surface potential. That is, as shown in FIG. 2, the surface potential of the intermediate transfer body 2 converges to a constant value regardless of the amount of the inflowing charge.

Here, it is assumed that continuous copying is performed by the copying machine of this embodiment without using the charging corotron 4. FIG. 4 shows the surface potential of the intermediate transfer body 2 at that time. Since the intermediate transfer body 2 is charged by a corona discharger 9 for primary transfer and a bias roll 10 for secondary transfer for each copy, its surface potential increases as the number of copies increases. However, the electric field applied to the intermediate transfer member 2 increases as the surface potential increases, so that the volume resistivity of the intermediate transfer member 2 gradually decreases. As a result, the charge that has flowed into the intermediate transfer body 2 is easily discharged, so that the amount of change in the surface potential for each copy gradually decreases,
Eventually, the surface potential of the intermediate transfer member 2 becomes constant at the saturation potential regardless of the increase in the number of copies. However, if the surface potential of the intermediate transfer body 2 changes at the beginning of the continuous copying, sufficient transfer efficiency cannot be obtained unless the conditions of the primary transfer and the secondary transfer of the unfixed toner image are sequentially changed.

Therefore, in the present embodiment, before the start of the copying operation, the charging corotron 4 applies a surface potential higher than the saturation potential to the intermediate transfer body 2, and from the first copy, the surface potential of the intermediate transfer body 2 is set to the saturation potential. Stabilized. At this time,
The surface potential applied to the intermediate transfer body 2 by the charging corotron 4 may be a saturation potential or higher, and if it is higher than the saturation potential, excess charge is removed, and the surface potential of the intermediate transfer body 2 naturally decreases to the saturation potential. Stabilize. Therefore, even if the amount of charge injected into the intermediate transfer body 2 due to the discharge of the charged corotron 4 is unstable, the surface potential of the intermediate transfer body 2 can be stabilized, and The unfixed toner image T can be primarily transferred from the photosensitive drum 1 to the intermediate transfer body 2 while being kept at the potential, and further, the unfixed toner image T can be secondarily transferred from the intermediate transfer body 2 to the recording medium 3. .

The electric charge flowing from the corona discharger 9 for the primary transfer and the bias roll 10 for the secondary transfer into the intermediate transfer body 2 flows toward the earth roll 16 on which the intermediate transfer body 2 is stretched. However, since the ground roll 16 is disposed at a certain distance from the corona discharger and the bias roll, the electric field applied to the intermediate transfer body 2 along the creeping direction is low. That is, the volume resistivity of the intermediate transfer body 2 in the creeping direction becomes high, and the creeping conduction of the charge flowing into the intermediate transfer body 2 can be effectively limited. Therefore, before the primary transfer position or the secondary transfer position, the photosensitive drum 1 and the intermediate transfer member 2 or the intermediate transfer member 2
It is possible to prevent an unfixed toner image from being transferred before the recording medium 3 and the recording medium 3 come into close contact with each other.

In this embodiment, since no step of removing the charge of the intermediate transfer member 2 is included, the charging of the intermediate transfer member 2 by the charging corotron 4 need only be performed before the image forming operation for the first copy. In this way, the intermediate transfer body
The surface potential of No. 2 is always kept at the saturation potential until the end of the continuous copying. However, if the process of removing the charge of the intermediate transfer body 2 for each copy (for example, removing the charge as a pre-cleaning process) is included, the charging corotron 4 is used for each copy.
To raise the surface potential of the intermediate transfer member 2 to the saturation potential.

[0027]

As described above, according to the image forming apparatus of the present invention, before the primary transfer of the unfixed toner image from the latent image carrier to the intermediate transfer member, the intermediate transfer is performed by the pre-transfer charger. When the body is charged to a predetermined surface potential or more, the surface potential of the intermediate transfer body converges to the saturation potential and stabilizes, so that during the primary transfer or the secondary transfer of the unfixed toner image, disturbance of the transferred image and uneven transfer are caused. Generation can be prevented, and a high-quality recorded image can be formed on a recording medium.

Further, since the insulating properties of the intermediate transfer member are maintained in the creeping direction, the unfixed toner image is not transferred early before the primary transfer position or the secondary transfer position. In this regard, it is also possible to prevent the transfer image from being disturbed and transfer unevenness from occurring.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a graph showing a relationship between a surface potential of an intermediate transfer member and an amount of inflow charge according to an example.

FIG. 3 is a graph showing the relationship between the volume resistivity of an intermediate transfer member according to an example and the intensity of an applied electric field.

FIG. 4 is a graph showing a change in surface potential for each copy number of an intermediate transfer member according to an example.

FIG. 5 is a schematic view showing a conventional image forming apparatus.

[Explanation of symbols]

1 ... Photoreceptor drum (latent image carrier), 2 ... Intermediate transfer body, 3 ... Recording medium, 4 ... Charging corotron (charger before transfer), T ... Unfixed toner image

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G03G 15/16 G03G 15/01 114

Claims (4)

(57) [Claims] 1. An unfixed toner image formed on a latent image carrier is primarily transferred onto an intermediate transfer member, and the unfixed toner image is secondarily transferred from the intermediate transfer member to a recording medium again to perform recording. In an image forming apparatus for forming an image on a medium, the intermediate transfer body is formed of a material having a small volume resistivity under a high electric field and a large volume resistivity under a low electric field, and before starting the primary transfer. Saturates the above intermediate transfer member
An image forming apparatus comprising a pre-transfer charger for charging to a potential or higher . 2. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein the pre-transfer charger is also used as another charger disposed around the intermediate transfer member. 3. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein the charging of the intermediate transfer member by the pre-transfer charger is performed for each image forming operation corresponding to each recording medium. 4. The image forming apparatus according to claim 1, wherein
An image forming apparatus, wherein the charging of the intermediate transfer member by the pre-transfer charger is performed only for an image forming operation corresponding to a first recording medium.