JPH09292780A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a transfer dust from being generated by electric discharge on a gap area adjacent to a transferring nip part from the upstream side in the traveling direction of an intermediate transfer belt. SOLUTION: This device is provided with toner image forming means for forming the toner image on a photoreceptor 1, the intermediate transfer belt 5 making a transfer surface opposite to the photoreceptor 1 possible to travel, a conductive blush 9 as intermediate transfer means for transferring the toner image of on the photoreceptor 1 to the intermediate transfer belt 5, and a transfer roller 11 for transferring the toner image on the intermediate transfer belt 5 to a recording paper 12. In this case, a surface potential of a non-image part on the photoreceptor 1 and a transfer bias voltage applied to the transfer rear side on the intermediate transfer belt 5 is severally set, so as not to generate the electric discharge in between the surface of the non-image part on the photoreceptor 1 and the transfer surface of the intermediate transfer belt 5, in the gap area in between the intermediate transfer belt 5 adjacent to the transferring nip part from the upstream side in the traveling direction of the intermediate transfer belt and the photoreceptor 1.

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 a copying machine, a facsimile machine, a printer, a printing machine, and more specifically, a toner image formed on an image carrier is transferred onto an intermediate transfer body to form an intermediate image. The present invention relates to an image forming apparatus that transfers a toner image on a transfer body onto a transfer material.

[0002]

2. Description of the Related Art In recent years, an electrophotographic image forming apparatus capable of copying and printing a full-color image has been put to practical use. As a method of transferring a full-color image onto a transfer material in this type of image forming apparatus, (A) Transfer drum method: Each image of yellow (Y), magenta (M), cyan (C), and black (BK) formed for each color on an image carrier such as a photoconductor is transferred onto a transfer drum. (B) an intermediate transfer body double transfer method (also simply referred to as an intermediate transfer method): Y formed on an image carrier such as a photoreceptor for each color; M, C, and BK images are sequentially superimposed and transferred on an intermediate transfer body, and a full-color toner image on the intermediate transfer body is collectively transferred and transferred to a transfer material. Paper that can be transferred to thick paper etc. Points with Lee property, and the transfer drum system in terms that can be entirely copied without inability image formed on the clamp holding portion of the tip as an intermediate transfer type (b) above are advantageous.

[0003]

Conventionally, in the above-described image forming apparatus of the intermediate transfer system, a phenomenon of toner dust (hereinafter referred to as "transfer dust") occurs when a toner image is transferred from an image carrier to an intermediate transfer member. It is known that may occur. Here, the transfer dust refers to the toner image (visible image) formed on the image carrier when the toner image is transferred from the image carrier to the intermediate transfer member (primary transfer). It is a phenomenon that the image is not transferred to the position but is diffused and transferred to the surrounding area, resulting in blurred image.
In particular, the sharpness of an image in a thin line portion is impaired.

As a result of intensive studies to solve the above-mentioned problem of transfer dust, the present inventors have found that the intermediate transfer body is in contact with the image carrier from the upstream side in the moving direction of the surface of the intermediate transfer body. It has been found that discharge between the intermediate transfer member and the non-image portion of the image carrier in the adjacent gap region is one of the causes of the transfer dust. For example, when discharge occurs in the gap area, the charge amount of the toner image (q /
There is a possibility that the absolute value of m) becomes small and the adhesive force of the toner to the image carrier is lowered, and the above-mentioned transfer dust occurs. Particularly, in the case of the image forming apparatus adopting the negative / positive developing method, when discharge occurs in the gap area as shown in FIG. 2A, the absolute value of the potential of the non-image portion of the image carrier becomes small. As a result, the potential distribution of the well shape for holding the toner changes, and the toner of the toner image on the image carrier is non-imaged by electrostatic repulsion (Coulomb repulsion) between the toners as shown in FIG. 2B. It becomes easy to move to the copy side, and there is a possibility that the above transfer dust occurs.

The present invention has been made under the above background, and an object of the present invention is to provide a gap region adjacent to the contact facing portion of the intermediate transfer member and the image bearing member from the upstream side in the moving direction of the surface of the intermediate transfer member. It is an object of the present invention to provide an image forming apparatus capable of preventing the occurrence of transfer dust due to electric discharge.

[0006]

In order to achieve the above object, the invention of claim 1 is directed to a toner image forming means for forming a toner image on an image carrier, and a transfer surface in contact with and facing the image carrier. A movable intermediate transfer member, an intermediate transfer unit that transfers the toner image on the image carrier to the intermediate transfer member, and a transfer material transfer unit that transfers the toner image on the intermediate transfer member to a transfer material. In an image forming apparatus provided with the intermediate transfer body and the image carrier, in a gap area between the intermediate transfer body and the image carrier, which is adjacent to the contact facing portion of the intermediate transfer body from the upstream side in the moving direction of the surface of the intermediate transfer body. , The surface potential of the non-image area of the image carrier and the transfer surface of the intermediate transfer body so that no discharge occurs between the surface of the non-image area of the image carrier and the transfer surface of the intermediate transfer body. It is characterized in that the surface potential is set.

According to a second aspect of the present invention, a belt member driven so that the transfer surface is moved is used as the intermediate transfer member, and a transfer charge is applied from a transfer back surface opposite to the transfer surface. In the image forming apparatus of item 1, the intermediate transfer member has a volume resistivity of 1 × 10 ¹² Ωcm or more. Here, the volume resistivity is measured by a measuring instrument manufactured by Mitsubishi Yuka (trade name: Hiresta, probe: HR).
It is the volume resistivity measured using S). The same applies to the volume resistivity in claim 4.

According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the surface potential of the transfer back surface of the intermediate transfer member and the surface potential of the non-image portion of the image carrier are Vp (V) and Vs (V, respectively). ), And the thickness and relative permittivity of the intermediate transfer member are d (μm) and ε ′, respectively, and b = 12.9 (d /
ε ') + 777, 1.07 Vp-b <Vs <
It is characterized by satisfying the condition represented by 1.07 Vp + b.

According to a fourth aspect of the present invention, a belt member driven so that the transfer surface is moved is used as the intermediate transfer member, and a transfer charge is applied from a transfer back surface opposite to the transfer surface. In the image forming apparatus of item 1, the intermediate transfer member has a volume resistivity of 1 × 10 ⁹ Ωcm or less.

According to a fifth aspect of the present invention, in the image forming apparatus according to the fourth aspect, the surface potential of the transfer back surface of the intermediate transfer member and the surface potential of the non-image portion of the image carrier are Vp (V) and Vs (V, respectively). ), 1.06Vp-656 <Vs
It is characterized in that the condition represented by <1.06Vp + 656 is satisfied.

According to the present invention, the surface potential of the non-image portion of the image carrier and the surface potential of the intermediate transfer member are set to a predetermined potential, so that the intermediate transfer member is contacted to the intermediate transfer member. In the gap area between the intermediate transfer body and the image carrier that are adjacent to each other from the upstream side in the body surface moving direction, discharge should not occur between the surface of the non-image part of the image carrier and the transfer surface of the intermediate transfer body. I have to. By suppressing the occurrence of discharge in the gap area in this way, a large change in the surface potential of the non-image area due to the discharge is suppressed, and the toner on the image area of the image carrier is electrostatically charged to each other. So that it does not move to the non-image area side due to repulsive force.

Further, in general, the potential difference between the surface potential of the transfer surface of the intermediate transfer member and the surface potential of the non-image portion of the image carrier is determined by the surface potential of the transfer surface and the surface potential of the image portion of the image carrier. Is larger than the potential difference between the transfer surface and the non-image portion of the image carrier, and discharge is more likely to occur than between the image portion. In such a situation, if the occurrence of discharge between the surface of the non-image portion of the image carrier and the transfer surface of the intermediate transfer member is suppressed, even between the image portion of the image carrier and the transfer surface of the intermediate transfer member. Since the discharge does not occur, it is possible to suppress the decrease in the toner charge amount on the image portion.

Particularly, in the invention of claim 2, in the image forming apparatus of claim 1, a belt member driven so that the transfer surface is moved is used as an intermediate transfer member, and the intermediate transfer member is opposite to the transfer surface. A transfer charge is applied from the side of the transfer back surface to make the transfer surface a potential necessary for transferring the toner from the image carrier. The volume resistivity of the intermediate transfer member is 1 × 10.
^When it is ¹² Ωcm or more, the electric resistance in the thickness direction of the intermediate transfer member is sufficiently large and it can be regarded as an insulator when the surface moving speed of the intermediate transfer member is within the range of 100 to 300 mm / sec which is usually used. Yes, the potential of the transfer surface of the intermediate transfer body is determined by the electrostatic capacity of the intermediate transfer body. Therefore, when the volume resistivity of the intermediate transfer body changes within the range of 1 × 10 12 Ωcm or more due to environmental conditions, especially temperature and humidity. However, the potential of the transfer surface of the intermediate transfer body hardly changes.

Particularly, in the invention of claim 3, the invention of claim 2
In this image forming apparatus, the surface potential of the transfer back surface of the intermediate transfer member and the surface potential of the non-image portion of the image carrier are set to Vp (V) and Vs (V), respectively, and the thickness and relative dielectric constant of the intermediate transfer member are set. The rates are d (μm) and ε ′, respectively, and b = 1
When set to 2.9 (d / ε ') + 777, 1.07 Vp
By setting Vp and Vs so as to satisfy the condition expressed by −b <Vs <1.07Vp + b, it is possible to ensure the occurrence of the discharge when an intermediate transfer member having a volume resistivity of 1 × 10 12 Ωcm or more is used. suppress.

Further, in particular, in the invention of claim 4,
In this image forming apparatus, a belt member driven so that the transfer surface is moved is used as an intermediate transfer member, and a transfer charge is applied from the transfer back surface on the side opposite to the transfer surface of the intermediate transfer member. To a potential necessary for the transfer of toner from the image carrier. The volume resistivity of the intermediate transfer member is 1 ×
When it is 10 ⁹ Ωcm or less, the surface moving speed of the intermediate transfer member is usually 100 to 300 mm / sec.
Within the range, the electric resistance in the thickness direction of the intermediate transfer member is sufficiently small and can be regarded as a conductor, and the potential of the transfer surface of the intermediate transfer member is Even if the volume resistivity of the intermediate transfer member changes within the range of 1 × 10 9 Ωcm or less due to environmental conditions, especially temperature / humidity, etc., it depends on the applied transfer charge.
The potential of the transfer surface of the intermediate transfer body hardly changes.

Further, in particular, in the invention of claim 5, claim 4
In the image forming apparatus of No. 1, when the surface potential of the transfer back surface of the intermediate transfer member and the surface potential of the non-image portion of the image carrier are Vp (V) and Vs (V), respectively, 1.06 Vp-
By setting Vp and Vs so as to satisfy the condition represented by 656 <Vs <1.06Vp + 656, it is possible to ensure that the above discharge occurs when an intermediate transfer member having a volume resistivity of 1 × 10 ⁹ Ωcm or less is used. Hold down.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. The intermediate transfer member may be configured as an intermediate transfer drum in addition to the intermediate transfer belt. However, in the following description, an example in which the intermediate transfer belt is configured as an intermediate transfer belt will be described.

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to this embodiment. A toner charged to a predetermined polarity (negative in this embodiment) by a negative-positive developing toner image forming unit using a known electrophotographic technique is applied to the photoconductor 1 as an image carrier that is driven to rotate in the direction of the arrow. An image has been formed. The toner image forming means includes a charging device 2 for negatively charging the surface of the photosensitive member 1 to a desired potential, and a light image 3 corresponding to image information is exposed on the photosensitive member 1 to form an electrostatic latent image (not shown). Exposure device, predetermined polarity (negative polarity in this embodiment)
And a developing device 4 that develops an electrostatic latent image on the photoconductor 1 using a charged toner to form a visible image. In the case of a full-color image forming apparatus, for example, four developing devices configured to use four color toners of yellow, magenta, cyan, and black are provided. An endlessly movable intermediate transfer belt 5 serving as an intermediate transfer member is stretched around a plurality of rollers 6, 7, and 8, and a photosensitive member is provided at a portion facing the photosensitive member 1 (hereinafter referred to as “transfer nip portion”). It is driven so as to move in the arrow direction (forward direction) at a linear velocity almost equal to that of one surface. The rollers 6 and 7 are
The intermediate transfer belt 5 is arranged so as to contact and face the photoconductor 1.

The intermediate transfer belt has, for example, a thickness of about 100 μm to 1 mm and a volume resistivity of 1 × 10 ⁹ Ω.
It is possible to use a material formed of a medium resistance material having a cm to 1 × 10 ¹² Ωcm.

Transfer charges are applied to the intermediate transfer belt 5 on the transfer back surface opposite to the transfer surface of the intermediate transfer belt 5 which is in contact with the photosensitive member 1 to transfer the toner image on the photosensitive member 1 to the intermediate transfer. A conductive brush 9 as an intermediate transfer unit that transfers the image onto the belt 5 is brought into contact with the surface of the intermediate transfer belt 5. To the conductive brush 9, a transfer bias voltage having a polarity (positive in this embodiment) opposite to the charging polarity of the toner is applied by a DC power supply 10. This conductive brush 9
The negative toner image on the photoconductor 1 is transferred onto the intermediate transfer belt 5 by the transfer electric field formed between the photoconductor 1 and the intermediate transfer belt 5 to which the electric charge has been applied. In this embodiment, the conductive brush 9 to which the transfer bias voltage is applied is used as the intermediate transfer means, but the conductive brush 9 is not necessarily a conductive brush, and a roller formed of conductive elastic rubber or Alternatively, a blade-shaped member made of a conductive material may be used. Further, a corona charger that applies electric charges to the transfer back surface of the intermediate transfer belt 5 by discharging may be used.

Further, a pre-transfer charge eliminating device is provided between the developing device 4 and the transfer nip portion for eliminating the surface potential of the photoconductor 1 before the transfer when the absolute value of the surface potential is larger than a predetermined potential. May be.

The toner image on the intermediate transfer belt 5 is transferred onto a recording paper 12 as a transfer material by a transfer roller 11 to which a transfer bias voltage is applied from a power source (not shown), and a fixing device including a fixing roller pair 13. Is fixed on the recording paper 12 to form a recorded image.

Next, the conditions for suppressing the occurrence of discharge, which is one of the causes of transfer dust, will be described. In order to suppress the occurrence of the discharge that causes the transfer dust, the potential difference between the intermediate transfer belt 5 and the photoconductor 1 facing each other in the gap region on the upstream side (entrance side) of the transfer nip portion is set. It is effective to make it small. Therefore, in the present embodiment, the photoconductor 1 is provided in a gap region (hereinafter referred to as “upstream gap region”) adjacent to the transfer nip portion between the intermediate transfer belt 5 and the photoconductor 1 from the upstream side in the moving direction of the intermediate transfer belt. The non-image area on the surface of the photoconductor 1 in the upstream gap area is prevented so that electric discharge that changes the potential of the non-image area to the extent that the toner image on the surface is moved to the non-image area side by the Coulomb repulsion between the toners. And the potential of the surface of the intermediate transfer belt 5 are set.

Here, the results of considering the conditions under which the above-mentioned discharge does not occur by performing a simulation by numerical calculation for the photoconductor 1 and the intermediate transfer belt 5 in the transfer nip portion will be shown. In this simulation, an intermediate transfer belt (relative permittivity = 8, thickness = 150 μm) to which a transfer bias voltage was applied to the transfer back surface was charged with a photoconductor (relative permittivity = 3, relative permittivity = 3, which was charged to a predetermined potential through a gap. The surface potential of the transfer surface of the photoconductor and the transfer surface of the intermediate transfer belt, which face each other, is obtained by applying the difference method to a two-dimensional model facing each other (thickness = 25 μm), and a modified Paschen curve (RM Schaffert, “Electron "Photo", Kyoritsu Shuppan, pp.290-292), and the transfer bias voltage (V) and the photoconductor surface potential (V), which are the discharge generation limit, are changed by changing the size of the gap between them. I asked. Here, the resistivity and the linear velocity (moving velocity) of the intermediate transfer belt were changed as parameters for obtaining the above conditions.

3A to 3C, the linear velocities of the intermediate transfer belt are 100 mm / sec and 200 mm / s, respectively.
In the case of ec and 300 mm / sec, the results of the above simulation were obtained for the relationship between the transfer bias voltage applied to the transfer back surface of the transfer nip portion of the intermediate transfer belt and the surface potential of the photoconductor at the limit at which the above-mentioned discharge occurs. Shows. In each figure, the volume resistivity is 1 × 10 ⁹ Ωcm or less (○), 1 × 10 ¹⁰ Ωcm (△), 1 × 10 ¹¹ Ωcm
The results at (●) and 1 × 10 ¹² Ωcm (▲) are plotted together. Further, in each case, since the relationship between the transfer bias voltage and the surface potential of the photoconductor is almost linear, the transfer bias voltage is from 200V to 1000V.
The data up to this point are not shown.

In FIGS. 3 (a) to 3 (c), the region above each straight line is the region showing the condition that no discharge occurs. For example, in FIG. 3A, the volume resistivity of the intermediate transfer belt is 1 × 10 ¹¹ Ωcm, and the transfer bias voltage is 200.
In the case of V, if the surface potential before entering the transfer nip portion of the photoconductor is −530 V or less in absolute value, it means that no discharge occurs between the photoconductor and the intermediate transfer belt.

From the results of the simulations of FIGS. 3A to 3C, the volume resistivity of the intermediate transfer belt is 1 × 10.
^It can be seen that when the line width is ⁹ Ωcm or less or 1 × 10 ¹² Ωcm or more, the straight line indicating the limit of discharge generation does not change even if the linear velocity of the intermediate transfer belt changes. On the other hand, the volume resistivity of the intermediate transfer belt is 1 × 10 ⁹ Ωcm to 1 × 10 ¹²
When the value is up to Ωcm, it can be seen that if the linear velocity of the intermediate transfer belt changes, the straight line indicating the limit of discharge generation also shifts significantly. This change in the discharge occurrence limit is considered to be due to the time when the intermediate transfer belt passes through the transfer nip portion and the electrical time constant of the intermediate transfer belt being in the same order. Focusing on whether the charge applied to the transfer back surface of a part of the intermediate transfer belt while passing through the transfer nip part can be sufficiently moved to the transfer surface side facing the surface of the photoconductor, the volume resistivity is Is 1
If the intermediate transfer belt is 10 ⁹ Ωcm or less, the belt resistance can be regarded as almost 0 within the above linear velocity range.
If the intermediate transfer belt has a volume resistivity of 1 × 10 ¹² Ωcm or more, the belt resistance can be regarded as infinite (insulator) within the above linear velocity range.

Further, the volume resistivity of the intermediate transfer belt is 1 ×.
In the high resistance region of 10 ¹² Ωcm or more, it is considered that the electrostatic capacity of the intermediate transfer belt, that is, the relative dielectric constant and thickness of the belt greatly affects the straight line indicating the discharge limit.
Therefore, the volume resistivity of the intermediate transfer belt is 1 × 10 ¹² Ωcm.
In the above case, the above simulation was performed using the relative dielectric constant of the belt as a parameter. In the case of this high resistance region, since there is no influence of the linear velocity of the intermediate transfer belt, calculation was performed only for linear velocity = 100 mm / sec. (Hereinafter, margin)

FIGS. 4A and 4B show the transfer bias voltage applied to the transfer back surface of the transfer nip portion of the intermediate transfer belt and the above discharge when the thickness of the intermediate transfer belt is 150 μm and 300 μm, respectively. The results obtained by the above simulation for the relationship between the limit of occurrence and the surface potential of the photoconductor are shown. In each figure, the relative permittivity of the intermediate transfer belt is 11 (○), 8 (△), 5 (●) and 3
The results in the case of (▲, only FIG. 4B) are also shown. From FIGS. 4A and 4B, when the volume resistivity of the intermediate transfer belt is 1 × 10 ¹² Ωcm or more, the straight line indicating the discharge limit changes greatly depending on the thickness of the intermediate transfer belt and the relative dielectric constant. You can see that When the volume resistivity of the intermediate transfer belt was in the low resistance region of 1 × 10 ⁹ Ωcm or less, there was no influence of the relative permittivity or thickness of the intermediate transfer belt.

From the results of the simulations shown in FIGS. 3 and 4, a conditional expression for preventing the above-mentioned discharge from occurring at each belt resistance can be obtained in consideration of the result when the transfer bias voltage (not shown) has a negative polarity. , For example, the following equations 1 and 2
It becomes like the formula shown in. Here, the volume resistivity of the intermediate transfer belt is defined as the volume resistivity measured by a measuring instrument (trade name: Hiresta, probe: HRS) manufactured by Mitsubishi Yuka, and Vs (V) in the formula is the transfer nip portion. Vp (V) is a transfer bias voltage applied to the transfer back surface of the intermediate transfer belt in the transfer nip portion, dp
(Μm) and ε ′ are the thickness and the relative permittivity of the intermediate transfer belt, respectively.

[0031]

## EQU1 ## The volume resistivity ρv of the intermediate transfer belt is 1 × 10 ⁹
Ωcm or less: 1.06 · Vp−656 <Vs <1.06 · Vp + 65
6

[0032]

## EQU00002 ## The volume resistivity ρv of the intermediate transfer belt is 1 × 10 ¹²
In the case of Ωcm or more: 1.07Vp-b <Vs <1.07Vp + bb b = 12.9 (d / ε ′) + 777

3 and 4 by the above simulation
As can be seen from the result and the conditional expressions of Formulas 1 and 2, the volume resistivity of the intermediate transfer belt is 1 × 10 ⁹ Ωcm to 1 × 10 ¹²
If it is in the range of Ωcm, the setting conditions of the transfer bias voltage and the surface potential of the photoconductor are complicated, and if the belt resistance fluctuates due to changes in the environment or the linear velocity of the intermediate transfer belt is changed, It may be necessary to change the settings of the transfer bias voltage and the surface potential of the photoconductor in order to prevent transfer dust due to discharge.

On the other hand, the volume resistivity of the intermediate transfer belt is 1 ×.
When it is 10 ⁹ Ωcm or less or 1 × 10 ¹² Ωcm or more, the above discharge generation limit condition is relatively stable, and when the belt resistance fluctuates due to changes in the environment or the setting of the linear velocity of the intermediate transfer belt. The volume resistivity is 1 × 1 even when
As long as it is within the range of 0 ⁹ Ωcm or less or 1 × 10 ¹² Ωcm or more, it is not necessary to change the settings of the transfer bias voltage and the surface potential of the photoconductor. Therefore, from the viewpoint of preventing transfer dust due to the above-mentioned discharge, it can be said that the volume resistivity of the intermediate transfer belt is preferably 1 × 10 ⁹ Ωcm or less or 1 × 10 ¹² Ωcm or more. In particular, when the volume resistivity of the intermediate transfer belt is 1 × 10 ⁹ Ωcm or less, it is not affected by the thickness of the belt or the relative dielectric constant, so that the transfer bias voltage and the surface potential of the photosensitive member can be easily set. .

Next, the result of evaluation of the transfer rate and the transfer dust in the more specific embodiment of the image forming apparatus having the above-mentioned structure will be described. FIG. 5 is a schematic configuration diagram of the image forming apparatus according to this embodiment. The basic configuration is the same as that of the apparatus of FIG. 1 described in the above embodiment, and the photoconductor 1 before transfer
The apparatus is different from the apparatus shown in FIG. 1 in that a pre-transfer charge eliminating lamp (PTL) 14 is provided so that the surface potential can be eliminated as necessary. An organic photoconductor (OPC) was used as the photoconductor 1.
The intermediate transfer belt 5 has a relative permittivity ε ′ = 8 ± 3 and a thickness = 1.
Made of 50 μm fluororesin and has an average volume resistivity of 1
Three types of belts, x10 ¹⁰ Ωcm (belt 1), 2x10 ¹¹ Ωcm (belt 2) and 5x10 ¹² Ωcm (belt 3), were used. The linear velocities of the photoconductor 9 and the intermediate transfer belt 19 are both 200 mm / sec, and they are driven so that the surfaces of both transfer nip portions move in the same direction. In this copying apparatus, the photoconductor 1 is charged to −400 V by the charging device 2 (corona charger), and a line image (line width: 100 μm, line width: 100 μm;
A line interval: 100 μm, toner: magenta toner) was formed on the photoconductor 1. Then, the transfer bias voltage Vp is applied to the transfer back surface of the intermediate transfer belt 5 in the transfer nip portion by the conductive brush 9 to which the transfer bias voltage Vp is applied from the DC power supply 10, so that the surface of the photoconductor 1 and the intermediate transfer belt 5 are separated from each other. The line image on the photoconductor 1 is transferred onto the intermediate transfer belt 5 by the transfer electric field formed on. The device was stopped during this transfer, and the mass per unit area of the transferred image on the intermediate transfer belt 5 and the image of the untransferred portion on the photoconductor 1 was measured to determine the transfer rate. Also, the toner image transferred onto the intermediate transfer belt 5 is observed with a microscope, and in a predetermined observation area (500 μm × 500 μm), the toner image is moved from a position to be originally transferred to a non-image portion and apparently scattered. The number of dust toners observed was counted.

Table 1 shows the results of evaluating the transfer rate and the transfer dust when the pre-transfer charge eliminating lamp (PTL) 14 is not operated in the image forming apparatus of this embodiment, and Table 2 shows the pre-transfer charge eliminating lamp (PTL). 14) is operated to decrease the surface potential of the photoconductor to near −100 V, the results of evaluation of the transfer rate and transfer dust are shown. From the results of Tables 1 and 2, as the transfer bias voltage of each of the belts 1, 2 and 3 is reduced to make it difficult for discharge to occur in the gap region on the upstream side of the transfer nip portion, the transfer dust decreases. Recognize. It is also understood that a good transfer rate can be obtained by setting the transfer bias voltage large within the range in which the above-mentioned discharge does not occur.

The numerical values of the transfer dust rank in Tables 1 and 2 above are predetermined observation areas (500 μm × 500 μm).
It was determined from a correspondence table (Table 3) showing the relationship between the number of dust toners per m) and the transfer dust rank. FIG.
(E) is an enlarged view of an evaluation reference image (line image in the sub-scanning direction with a line width of 100 μm and a line interval of 100 μm) for each numerical value of the transfer chile rank. The number of dust toners per predetermined observation area (500 μm × 500 μm) is counted for each transfer chile rank evaluation reference image, and the average value of count values for successive transfer chile ranks (for example, rank 1 and rank 2) is calculated between the transfer chile ranks. The range of the number of dust particles in each transfer dust rank in the correspondence table of Table 3 is set as the boundary value of. Here, an image whose transfer dust rank is 4 or more (see FIG.
(A) and (b)) were good images with less transfer dust as a result of observation with the naked eye. In addition, FIG.
The white portion in (e) is toner particles, and the line width and line interval dimensions of the image in FIG. 6 (a) corresponding to transfer dust rank 5 are 100 μm.

[0038]

[Table 1] (Hereinafter, margin)

[0039]

[Table 2]

[0040]

[Table 3]

Next, the apparatus of the above-mentioned embodiment was installed in a dark room, and light (hereinafter referred to as "discharge light") due to discharge was observed in the gap region on the upstream side of the transfer nip portion in the belt moving direction. An image intensifier (V1366P, manufactured by Hamamatsu Photonics KK) composed of a photomultiplier tube capable of observing weak light was used for the observation of the discharge light.
Table 4 shows the results of observation of discharge light when a transfer bias voltage was applied as in Table 1 above. In the figure, “x” indicates that discharge light was not observed, and “◯” indicates that discharge light was observed. From the comparison results of Table 1 and Table 4, it is clear that the presence or absence of the discharge light in the transfer nip portion is related to the transfer dust and the transfer rate. It was confirmed that by suppressing the occurrence of the occurrence of the toner, it is possible to obtain a toner image having a transfer dust rank of an allowable level of 4 or more and obtain a sufficient transfer rate. Also, from this result,
It can be said that it was confirmed that an image with less transfer dust and transfer defects can be obtained by setting the transfer bias voltage and the surface potential of the photoconductor 1 so that the discharge in the gap region does not occur. As shown in the results of Tables 1 and 4, the belt 3 (volume resistivity = 5 × 10 ¹² Ω
cm), the transfer bias voltage is 800 V, and the surface potential of the photoconductor is −400 V. The data shown in FIG. 4A of the above simulation shows that the transfer dust is small even though discharge occurs. Although the discharge light is not observed, the difference in the results is considered to be due to the measurement error of the dust toner and the observation accuracy of the discharge light. In addition, the pre-transfer charge eliminating lamp (PTL) 1
4 is operated, the pre-transfer charge eliminating lamp (PT
Since the light from L) 14 was strong, the discharge light could not be observed sufficiently. (Hereinafter, margin)

[0042]

[Table 4]

Next, the results of another embodiment of the image forming apparatus will be described. FIG. 7 is a schematic configuration diagram of the image forming apparatus according to this embodiment. The basic configuration is the same as that of the embodiment shown in FIG. 5, the scorotron charger 15 is used as an intermediate transfer means instead of the conductive brush 9, and the linear velocity of the intermediate transfer belt 5 is 200 mm. / Se
FIG. 5 shows that 300 mm / sec is set instead of c.
Device. When the scorotron charger 15 is used as in the present embodiment, the transfer back surface of the intermediate transfer belt 5 is charged to substantially the same potential as the potential of the grid electrode to which the grid bias voltage is applied. It can be regarded as the potential of the transfer back surface of the intermediate transfer belt 5 in the area.

Table 5 shows the results of evaluation of the transfer rate and the transfer dust when the pre-transfer charge eliminating lamp (PTL) 14 is not operated in the image forming apparatus of this embodiment. From the results of Table 5, as in the case of Table 1, the transfer bias voltage is decreased for each of the belts 1, 2 and 3 so that discharge is less likely to occur in the gap region on the upstream side of the transfer nip portion. It can be seen that a good transfer rate can be obtained by setting the transfer bias voltage large within a range in which the amount of dust is reduced and the above-mentioned discharge does not occur. In this embodiment, the scorotron charger is used as the intermediate transfer means. However, a corotron charger without a grid electrode may be used instead of the scorotron charger, and transfer dust and transfer failure can be similarly prevented.

[0045]

[Table 5]

As described above, according to the present embodiment, by setting the surface potentials of the non-image portion of the photoconductor 1 and the intermediate transfer belt 5 to the predetermined potentials, the transfer nip portion is moved from the upstream side in the moving direction of the intermediate transfer belt. In the adjacent upstream gap region, no electric discharge occurs between the surface of the non-image portion of the photoconductor 1 and the transfer surface of the intermediate transfer belt 5. By suppressing the occurrence of the discharge in this way, a large change in the surface potential of the non-image portion due to the discharge is suppressed, and the static electricity between the toner on the image portion of the photoconductor 1 and the surface of the non-image portion is suppressed. Since the electric repulsive force does not decrease, the transfer dust is less likely to occur due to the toner in the image area on the photoconductor 1 moving to the non-image area side.

Further, according to the present embodiment, by preventing the discharge between the intermediate transfer belt 5 and the non-image portion of the photoconductor 1 in the upstream gap region, the discharge is generated more than the non-image portion. The electric discharge in the image portion, which is unlikely to occur, will not occur. In this way, since no discharge occurs in the image area,
The charge amount (q / m) of the toner image on the photoconductor 1 does not decrease, and thus, it is possible to prevent transfer dust from being generated by the toner with the decreased charge amount, and the toner image on the photoconductor 1 can be prevented. When transferring to the intermediate transfer belt 5, it is possible to prevent the occurrence of transfer failure due to a decrease in the electrostatic amount acting on the toner for transfer due to a decrease in the charge amount.

In the above-described embodiment, the N / P developing system in which the photosensitive member 9 is uniformly charged in the negative polarity and the electrostatic latent image formed on the photosensitive member 9 is developed by using the negatively charged toner. However, the present invention can be applied without being limited to the charging polarity and the developing method. For example, the toner having the positive polarity uniformly charged on the photoconductor 9 and the positive polarity is used. The same effect can be obtained even when it is used or when the P / P developing method is adopted.

In the above embodiment, the case of the image forming apparatus provided with only one developing device 4 has been described.
The present invention is not limited to the number of developing devices, and is also applicable to a color image forming apparatus including four developing devices configured to use toners of four colors of yellow, magenta, cyan, and black, for example. The same effect can be obtained.

[0050]

According to the first to sixth aspects of the present invention, it is possible to suppress the occurrence of discharge in the gap region adjacent to the contact facing portion of the intermediate transfer member and the image carrier from the upstream side in the moving direction of the surface of the intermediate transfer member. As a result, a large change in the surface potential of the non-image portion due to the discharge is suppressed, and the toner on the image portion of the image carrier does not move to the non-image portion side due to electrostatic repulsion between the toners. Further, there is an effect that it is possible to prevent transfer dust from being generated in the gap region due to discharge.

Further, since no discharge is generated between the image portion of the image carrier and the transfer surface of the intermediate transfer member, the toner charge amount on the image portion is prevented from decreasing, and the toner having the reduced charge amount is suppressed. It is possible to prevent the occurrence of transfer dust, and when the toner image on the image carrier is transferred to the intermediate transfer member, the electrostatic force for transfer that acts on the toner is reduced due to the decrease in the charge amount, and the transfer failure occurs. There is an effect that it can be prevented from occurring.

Particularly, according to the invention of claim 2 or 4, by using an intermediate transfer member having a volume resistivity of 1 × 10 ¹² Ωcm or more or 1 × 10 ⁹ Ωcm or less, environmental conditions, particularly temperature / humidity, etc. Therefore, the volume resistivity of the intermediate transfer member is 1 × 10 ¹²
Even if it changes within the range of Ωcm or more or 1 × 10 ⁹ Ωcm or less, the potential of the transfer surface of the intermediate transfer body hardly changes, so that the above discharge is suppressed and transfer dust and transfer failure are prevented. The effect is that you can.
Further, the volume resistivity is 1 × 10 ¹² Ωcm or more or 1 × 10 ⁹
When an intermediate transfer member having an Ωcm or less is used, the surface moving speed of the intermediate transfer member is usually 100 to 300 mm /
Even when the value is changed within the range of sec, the transfer bias voltage applied to the surface potential of the image carrier or the transfer back surface of the intermediate transfer member is set in order to prevent the occurrence of discharge that causes the transfer dust and transfer failure. The effect is that there is no need to change.

Further, according to the invention of claim 3 or 5, the surface potential Vp (V) of the transfer back surface of the intermediate transfer member and the surface potential Vs (V) of the non-image portion of the image carrier are predetermined conditions. By satisfying the formula, the volume resistivity of the intermediate transfer member is 1
Under the condition of not less than × 10 ¹² Ωcm or not more than 1 × 10 ⁹ Ωcm, it is possible to suppress the occurrence of the above discharge and more reliably prevent the occurrence of transfer dust and transfer failure.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the invention.

2A and 2B are explanatory views showing the occurrence of discharge between a photoconductor and an intermediate transfer belt and the movement of toner after the discharge.

3A to 3C are graphs of simulation results showing the relationship between the transfer bias voltage and the surface potential of the photosensitive member with the volume resistivity and linear velocity of the intermediate transfer belt as parameters.

FIGS. 4A and 4B are graphs of simulation results showing the relationship between the transfer bias voltage and the surface potential of the photosensitive member with the relative permittivity and thickness of the intermediate transfer belt as parameters.

FIG. 5 is a schematic configuration diagram of an image forming apparatus according to an example of the exemplary embodiment.

6A to 6E are enlarged views of an evaluation reference image corresponding to each numerical value of a transfer dust rank.

FIG. 7 is a schematic configuration diagram of an image forming apparatus according to another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Charging device 4 Developing device 5 Intermediate transfer belt 9 Conductive brush 10 DC power supply 11 Transfer roller 12 Recording paper 13 Fixing roller pair 14 Pre-transfer static elimination lamp 15 Scorotron charger

Claims (5)

[Claims] 1. A toner image forming means for forming a toner image on an image carrier, an intermediate transfer member having a transfer surface movable in contact with the image carrier, and an intermediate transfer member on the image carrier. In the image forming apparatus including an intermediate transfer unit that transfers the toner image of the intermediate transfer member and a transfer material transfer unit that transfers the toner image on the intermediate transfer member to a transfer material, contact between the intermediate transfer member and the image carrier. The surface of the non-image portion of the image carrier and the transfer surface of the intermediate transfer member in the gap region between the intermediate transfer member and the image carrier that are adjacent to the facing portion from the upstream side in the moving direction of the surface of the intermediate transfer member. The image forming apparatus is characterized in that the surface potential of the non-image portion of the image carrier and the surface potential of the transfer surface of the intermediate transfer member are set so that no discharge is generated between the image carrier and the intermediate transfer member. 2. The image forming method according to claim 1, wherein a belt member driven so that the transfer surface is moved is used as the intermediate transfer member, and transfer charges are applied from a transfer back surface opposite to the transfer surface. An image forming apparatus, wherein the intermediate transfer member has a volume resistivity of 1 × 10 ¹² Ωcm or more. 3. The image forming apparatus according to claim 2, wherein the surface potential of the transfer back surface of the intermediate transfer member and the surface potential of the non-image portion of the image carrier are Vp (V) and Vs (V), respectively.
And the thickness and relative permittivity of the intermediate transfer member are respectively d
(Μm) and ε ′, b = 12.9 (d / ε ′) + 7
The image forming apparatus satisfies the condition represented by 1.07Vp-b <Vs <1.07Vp + b when 77 is set. 4. The image forming method according to claim 1, wherein a belt member driven so that the transfer surface is moved is used as the intermediate transfer member, and transfer charges are applied from a transfer back surface opposite to the transfer surface. In the image forming apparatus, the intermediate transfer member has a volume resistivity of 1 × 10 ⁹ Ωcm or less. 5. The image forming apparatus according to claim 4, wherein the surface potential of the transfer back surface of the intermediate transfer member and the surface potential of the non-image portion of the image carrier are Vp (V) and Vs (V), respectively.
In this case, the image forming apparatus satisfies the condition represented by 1.06Vp−656 <Vs <1.06Vp + 656.