JPH1010887A - Image forming device - Google Patents

Abstract

(57) [Problem] To provide an image forming apparatus of an intermediate transfer type capable of preventing transfer dust without providing any additional mechanism. A non-image portion of a photoreceptor and an intermediate transfer belt are provided.
Due to the occurrence of discharge between the non-image portions, the relative permittivity and thickness of the photoconductor 1 and the relative permittivity and thickness of the intermediate transfer belt are controlled so that the polarity of the potential of the non-image portion does not reverse after transfer. A transfer bias voltage applied to the back surface of the intermediate transfer belt 5 is set.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer, a printing machine, and the like. More specifically, the present invention relates to a method for transferring a toner image formed on an image bearing member onto an intermediate transfer member. The present invention also relates to an image forming apparatus for transferring a superimposed toner image on an intermediate transfer member 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 technique for preventing the transfer dust, a technique of non-electrostatically transferring a high-resistance toner to an intermediate transfer medium and then pressing and fixing the toner with a heating roller via a recording sheet (for example, Japanese Patent Application Laid-Open No. JP-A-63-34570), a technique in which a conductive toner is non-electrostatically transferred to an intermediate transfer medium, and then pressed and transferred and fixed by a heating roller with a recording sheet interposed (see JP-A-63-34571). A technology for removing charge from a toner image transferred by a paper peeling charger every time a toner image is transferred to an intermediate transfer medium (Japanese Patent Laid-Open No. 1-282)
No. 571), a technique in which the transfer potential in the final transfer stage is made higher than the immediately preceding transfer potential, and a predetermined voltage is applied to the intermediate transfer medium during each transfer stage (Japanese Patent Laid-Open No. 2-18).
Japanese Patent Application Laid-Open No. 4-147170), and a technique for providing a means for removing charges on the intermediate transfer member before the means for transferring a visible image from the intermediate transfer member to a sheet (see Japanese Patent Application Laid-Open No. 4-147170). . However, among these technologies, Japanese Patent Application Laid-Open Nos. 63-34570 and 63-34
In the technique disclosed in Japanese Patent No. 571, a recording sheet capable of being pressed and transferred and fixed by a heating roller is required, and the paper-free property which is the advantage of the intermediate transfer method (b) cannot be utilized. Further, in the techniques disclosed in JP-A-1-282571, JP-A-2-183276, and JP-A-4-147170, it is necessary to provide a means for static elimination and voltage application and a control means for controlling these means. There are problems that the control mechanism becomes complicated and that it also hinders downsizing of the device.

The inventors of the present invention have conducted intensive studies to solve the problem of the occurrence of transfer dust, and as a result, in the transfer portion, a portion of the intermediate transfer member and the image bearing member where the toner is not attached (hereinafter referred to as a non-conductive portion). It is found that one of the causes of the transfer dust is that a discharge occurs between the non-image portion and the polarity of the potential of the non-image portion is reversed after the transfer. Hereinafter, this will be described. The force F acting on the toner moving from the photoreceptor surface to the intermediate transfer member during transfer to the intermediate transfer member is as follows: (1) The toner receives the electric field from the electric charge on the photoreceptor surface and the transfer bias voltage applied to the intermediate transfer member. Force, (2) Coulomb repulsion between toners (3)
It is determined by the sum of the forces such as the image force of each toner other than (1) and (2). Here, the mirror image force of each toner of the above (3) contributes when the toner adheres to the photoreceptor, but the distance between the toner and the photoreceptor increases after the toner is separated from the photoreceptor during transfer. , Contributes little and can be ignored.

Next, the forces (1) and (2) acting on the toner will be described with reference to FIG. Figure 4 shows the negative
FIG. 4 is an explanatory diagram of electric lines of force showing electric fields due to charges on the surface of a photoconductor and a transfer bias voltage applied to an intermediate transfer body during image formation using a positive developing method. Here, it is assumed that the potential of the non-image portion of the photoconductor before transfer is negative, the polarity of toner is negative, and the transfer bias applied to the intermediate transfer belt during transfer is positive. FIG. 4A shows the photosensitive member 1 and the intermediate transfer belt 5.
The photosensitive member 1 and the intermediate transfer belt 5 in the case where no discharge occurs between the photosensitive members 1 or when the potential of the non-image portion does not reverse even if the discharge occurs.
It shows the lines of electric force between them. FIG. 4 (b)
Means that a discharge occurs between the photoreceptor 1 and the intermediate transfer belt 5,
FIG. 4 shows lines of electric force between the photoconductor 1 and the intermediate transfer belt 5 when the potential of the non-image portion is reversed. In the drawing, reference symbol A indicates a non-image portion, and B indicates an image portion to which toner adheres. If no discharge occurs, or if the potential of the non-image part A does not reverse even if it occurs, the charge on the non-image part A is negative. As shown, the image is transferred onto the intermediate transfer belt 5 along a line of electric force that is sparse on the surface of the photoconductor and becomes denser as the distance from the intermediate transfer belt 5 increases. Therefore, the force received by the toner from the electric field caused by the charge on the photoreceptor surface and the transfer bias voltage applied to the intermediate transfer member in the above (1) suppresses the spread of the toner in the above (2) due to the Coulomb repulsion. Work on. Here, for example, assuming that the charge amount per unit mass of the toner is -20 μC / g, the distance between the toners is 10 μm, and the transfer electric field is 10 MV / m, the charge on the photoconductor surface of (1) and the transfer given to the intermediate transfer body The force that the toner receives from the electric field due to the bias voltage is 4.24 × 10 ⁻⁸ N, and the Coulomb repulsion between the toners in (2) is 1.69.
× 10 ⁻⁹ N. Therefore, the force received by the toner from the electric field caused by the charge on the photoreceptor surface in (1) and the transfer bias voltage applied to the intermediate transfer member is much larger than the Coulomb repulsion between the toners in (2). The spread of toner due to the Coulomb repulsion is sufficiently suppressed, and transfer dust is less likely to occur. On the other hand, when a discharge occurs and the electric charge of the non-image portion A becomes positive and the potential of the non-image portion is reversed, the toner on the image portion B becomes dense on the surface of the photoconductor as shown in FIG. That is, the image is transferred onto the intermediate transfer belt 5 along lines of electric force that are sparse on the surface of the intermediate transfer belt 5. Therefore, the force received by the toner from the electric field caused by the electric charge on the surface of the photoreceptor and the transfer bias voltage applied to the intermediate transfer member in the above (1) acts in a direction in which the toners are separated from each other, and transfer dust tends to occur. . Further, when the discharge further proceeds, a discharge also occurs between the toner on the image portion B and the intermediate transfer belt 5, and the amount of charge per unit mass of the toner becomes small, so that the Coulomb repulsion becomes small. The force received by the toner from the electric field due to the charge on the photoreceptor surface and the transfer bias voltage applied to the intermediate transfer member in the above (1) is strong in the direction of separating the toners, so that transfer dust is more likely to occur. Become. Here, only the case of the negative-positive developing method has been described. However, even in the case of an image forming apparatus employing the positive-positive developing method, when the polarity of the potential of the non-image portion is reversed by discharge, the photosensitive member The force received by the toner from the electric field due to the charge on the surface and the transfer bias voltage applied to the intermediate transfer member acts in a direction to separate the toner particles, causing a problem that transfer dust tends to occur.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of preventing transfer dust without providing any additional mechanism.

[0008]

In order to achieve the above-mentioned object, according to the present invention, there is provided a toner image forming means for forming a toner image on an image carrier, and a toner image forming means for contacting and facing the image carrier. An intermediate transfer body having a movable surface, an intermediate transfer means for sequentially superimposing and transferring the toner image on the image carrier to the intermediate transfer body, and a superimposed toner image on the intermediate transfer body as a transfer material. In the image forming apparatus provided with a transfer material transferring means for transferring, the polarity of the potential of the portion where the toner is not adhered on the image carrier after transferring the toner image on the image carrier to the intermediate transfer member is changed. The relative permittivity of the image carrier, the thickness, the relative permittivity of the intermediate transfer member so as to be equal to the polarity of the potential of the portion before the toner image on the image carrier is transferred to the intermediate transfer member, That the thickness and the potential of the lower surface of the intermediate transfer body are set. It is an butterfly.

In this image forming apparatus, after the toner image on the image carrier is transferred to the intermediate transfer member, the polarity of the potential of the portion of the image carrier on which the toner is not adhered is the same as that of the image carrier. Since the polarity of the potential of the above-mentioned portion before the upper toner image is transferred to the intermediate transfer member is equal to that of the portion, the force received by the toner from the electric field due to the charge on the photosensitive member surface and the transfer bias voltage applied to the intermediate transfer member is It works in a direction to suppress the spread of each other.

Here, even when a discharge occurs between the non-image portion and the intermediate transfer member where the toner is not adhered on the image carrier, the true charge of the non-image portion becomes zero. If the image forming apparatus is configured such that no further discharge occurs when the toner image is transferred, the polarity of the potential of the non-image portion is changed to the value of the non-image portion before the toner image on the image carrier is transferred to the intermediate transfer member. There is no polarity opposite to the polarity of the potential.

Therefore, the invention of claim 2 provides a toner image forming means for forming a toner image on an image carrier, a surface-movable intermediate transfer member arranged so as to contact and face the image carrier, and An image forming apparatus comprising: an intermediate transfer unit that sequentially superimposes and transfers the toner image on the image carrier to the intermediate transfer body; and a transfer material transfer unit that transfers the superimposed toner image on the intermediate transfer body to a transfer material. In the apparatus, the relative permittivity of the image carrier,
The thicknesses are ε ₁ and d ₁ [m], the gap width between the image carrier and the intermediate transfer member is d ₂ [m], and the relative permittivity and thickness of the intermediate transfer member are ε ₃ , respectively. , D ₃ [m], and the absolute value of the potential on the lower surface of the intermediate transfer member is V _p ,

(Equation 1) as well as

(Equation 2) Wherein the relative permittivity and thickness of the image carrier, the relative permittivity and thickness of the intermediate transfer body, and the potential of the lower surface of the intermediate transfer body are set. is there.

In this image forming apparatus, when the true charge of the non-image portion is set to 0, the voltage V ₂ applied to the gap is

V ₂ = C · V _p / C ₂ where C = (C ₁ ^-1 + C ₂ ^-1 + C ₃ ^-1 ) ^-1 C 1 = ε ₁ ε ₀ / d ₁ C 2 = ε ₀ / d ₂ C3 = ε ₃ ε ₀ / d ₃ ε ₀ is the dielectric constant of vacuum. Further, according to Paschen's discharge law, the voltage V _{pa at} which the discharge starts in the above-mentioned gap is expressed as follows when the unit of d ₂ is [m].

(Equation 4)_pa= 312 + 6.2 × 10⁶・ D_Two  It is. In this image forming apparatus, Equations 1 and 2 are
The relative permittivity and the thickness of the image carrier are
Relative dielectric constant and thickness of the intermediary transfer member, and
Since the potential of the surface is set, it is always
V_Two≤V_paHolds. That is, the image carrier and the
The voltage across the gap with the intermediate transfer member is
May be higher than the voltage at which discharge starts
Absent. Thereby, the non-image portion and the intermediate transfer body are
Even if a discharge occurs between the non-image portions, the true charge of the non-image portion becomes zero.
Then, no further discharge occurs. Therefore,
The polarity of the potential of the image portion is the same as that of the toner image on the image carrier.
The polarity of the potential of the non-image part before transfer to the intermediary transfer member and the opposite polarity
Never be. Therefore, assuming no toner particles
The force received by the toner from the electric field when
It works in the direction that suppresses the burrs.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a full-color copying machine (hereinafter, referred to as a "color copying machine") as an image forming apparatus will be described. 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 a color copying apparatus according to the present 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. An endlessly movable intermediate transfer belt 5 as an intermediate transfer member is stretched around a plurality of rollers 6, 7, and 8, and the photosensitive member 1
(Hereinafter referred to as “nip portion”) at a linear velocity substantially equal to the surface of the photoreceptor 1 in the direction of the arrow (forward direction).
It is driven to move. The rollers 6 and 7 are arranged such that the intermediate transfer belt 5 comes into contact with and faces the photoconductor 1.

The intermediate transfer belt has a thickness of 10
0 μm to 1 mm, volume resistivity is 1 × 10 ⁹ Ωcm
A medium resistance material of up to 1 × 10 ¹³ Ωcm or an insulator can be used.

A transfer charge is applied to the intermediate transfer belt 5 on the surface of the intermediate transfer belt 5 opposite to the surface facing the photoreceptor 1 (hereinafter, referred to as a “back surface”). A conductive brush 9 as an intermediate transfer unit for transferring the toner image onto the intermediate transfer 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. A negative toner image on the photoconductor 1 is transferred onto the intermediate transfer belt 5 by a transfer electric field formed between the intermediate transfer belt 5 to which electric charge is applied by the conductive brush 9 and the photoconductor 1. . In this embodiment, the conductive brush 9 to which the transfer bias voltage is applied is used as the intermediate transfer unit.
It is not necessary to use a conductive brush, and a roller made of conductive elastic rubber or a blade made of a conductive material may be used. Further, a corona charger that applies electric charges to the back surface of the intermediate transfer belt 5 by discharging may be used. Further, an indirect bias application method in which a predetermined bias is applied to the rollers 6 and 7 to form a transfer electric field in the nip portion may be used. When a corona discharger or an indirect bias application method is used, the potential of the back surface of the intermediate transfer belt at the nip is transferred to the back surface of the intermediate transfer belt by the conductive brush 9 in the present embodiment. It can be considered as a bias voltage.

A full-color toner image is formed on the intermediate transfer belt 5 by repeating the transfer process from the photoreceptor 1 to the intermediate transfer belt 5 four times for each of the four types of toner, yellow, magenta, cyan, and black. can do.
The toner image superimposed 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 supply (not shown), and is provided with a fixing roller pair 13. The image is fixed on the recording paper 12 by the apparatus and becomes a full-color recorded image.

In the above-described series of steps, if the toner image transferred onto the intermediate transfer belt 5 is disturbed, only a disturbed recorded image can be obtained. Here, the discharge between the photoconductor 1 and the intermediate transfer belt 5 which causes the toner image transferred on the intermediate transfer belt 5 to be disturbed will be described. FIG. 2 shows the result of a simulation calculation of how the toner image formed on the photoconductor 1 moves on the intermediate transfer belt 5. FIG.
2A is a result of calculation under the condition that no discharge occurs between the photoconductor 1 and the intermediate transfer belt 5, and FIG. 2B shows that a discharge occurs between the photoconductor 1 and the intermediate transfer belt 5, and It is a result calculated under the condition that the potential of the non-image portion is reversed. In this simulation, the adhesion between one toner and the photoconductor 1 was 5 × 10 ⁻⁸ N, the charge per unit mass of the toner was −20 μC / g, and the toner diameter was 7 μm. In FIG. 2A, the ten toners T arrive at the surface of the intermediate transfer belt 5 as a unit while keeping their shape on the photoconductor, but in FIG. 2B, the ten toners T The toner image spreads and reaches the surface of the intermediate transfer belt 5, and it can be seen that the toner image on the belt is disturbed.

Table 1 shows a transfer bias voltage (hereinafter, referred to as a transfer bias voltage) applied to the back surface of the intermediate transfer belt 5 by the conductive brush 9 under the condition that a discharge occurs between the photosensitive member 1 and the intermediate transfer belt 5. The results obtained by calculating the line width of the transferred image when a line image having a width of 90 μm formed on the photoreceptor is transferred onto the belt by simulation are shown. Further, the potential after the transfer of the non-image portion on the photoconductor 1 is also shown. The potential of the photoconductor 1 before the transfer is-
500 V, but the photoconductor and the intermediate transfer belt 5
Since the discharge occurs between the non-image portions, the non-image portion potential changes at the end of the transfer. In Table 1, the presence or absence of expected dust is determined when the line width of the transferred image is larger than 90 μm, and is not determined when the line width of the image remains 90 μm.

[0020]

[Table 1]

From Table 1, it is considered that no transfer dust occurs if the potential of the non-image portion of the photosensitive member after the transfer is the same polarity as that before the transfer. On the other hand, when a large discharge occurs and the polarity of the potential of the non-image portion of the photoconductor after the transfer becomes opposite to the polarity before the transfer, transfer dust is considered to occur. This is because, as described above, when the polarity of the non-image area of the photoreceptor after the transfer becomes opposite to the polarity before the transfer, the toner is discharged from the electric field by the charge on the photoreceptor surface and the transfer bias voltage applied to the intermediate transfer body. This is because the received force acts in a direction to separate the toners. Further, when the discharge further proceeds, a discharge also occurs between the toner on the image portion B and the intermediate transfer belt 5, and the amount of charge per unit mass of the toner becomes small, so that the Coulomb repulsion becomes small. The force received by the toner from the electric field due to the electric charge on the body surface and the transfer bias voltage applied to the intermediate transfer member acts strongly in the direction of separating the toners, and transfer dust is more likely to occur. Here, if the image forming apparatus is configured such that the polarity of the non-image portion of the photoconductor after the transfer is the same as that before the transfer, the electric lines of force between the photoconductor 1 and the intermediate transfer belt 5 will be as described above. FIG.
The surface of the photoconductor described with reference to FIG. 5A is sparse, and becomes denser as it comes closer to the intermediate transfer belt 5. Therefore, it depends on the charge on the surface of the photoconductor and the transfer bias voltage applied to the intermediate transfer member. The force that the toner receives from the electric field is
This works in the direction opposite to the direction in which the toners are separated from each other. Therefore, generation of transfer dust can be prevented.

Next, the condition under which the potential polarity of the non-image portion of the photosensitive member is not reversed will be described with reference to the model shown in FIG.
In the model shown in FIG. 3, the photoconductor 1 in the nip portion, a gap between the photoconductor 1 and the intermediate transfer belt 5 (hereinafter, referred to as a gap D), and the intermediate transfer belt 5 are respectively connected to capacitors C ₁ , C ₂ , approximated by C _3. Here, strictly speaking, the resistance of the intermediate transfer belt 5 should also be taken into consideration. However, the following condition is a necessary condition that must be satisfied even under the influence of the resistance. . FIG.
In the image forming apparatus described above, the non-image portion of the photoreceptor 1 enters the nip portion in a state where negative charge is given by the charging device 2. Here, in order to determine a condition under which the potential polarity of the non-image portion of the photoreceptor does not reverse even when a discharge occurs, the polarity of the true charge held in the non-image portion after passing through the nip portion of the non-image portion is determined before transfer. A condition for preventing the polarity from becoming opposite to the above, that is, a condition under which the discharge does not occur any more when the true charge becomes 0 may be obtained. Therefore, when the true charge on the photoreceptor is set to 0, a condition is determined such that no discharge occurs even if the width of the gap D changes. Here, the thickness of the photoconductor 1, the width of the gap D, and the thickness of the intermediate transfer belt 5 are d ₁ [m], d ₂ [m], and d ₃ [m], respectively. D, the relative dielectric constant of the intermediate transfer belt 5 is ε ₁ ,
Assuming that ε ₂ and ε ₃ and the dielectric constant of vacuum is ε ₀ ,

## EQU5 ## Photoconductor capacity per unit area C ₁ = ε ₁ ε ₀ / d ₁

[Equation 6] Void volume per unit area C ₂ = ε ₂ ε ₀ / d ₂

## EQU7 ## The belt capacity per unit area is C ₃ = ε ₃ ε ₀ / d ₃ . Since the relative dielectric constant of air can be regarded as substantially 1, ε ₂ = 1. At this time, the combined capacity C of the photoconductor 1, the gap D, and the intermediate transfer belt 5 is

C = (C ₁ ^-1 + C ₂ ^-1 + C ₃ ^-1 ) ^-1 Therefore, when the absolute value of the transfer bias voltage is V _p, the gap voltage V ₂ applied to the gap D is the above-mentioned

## EQU3 ## V ₂ = CV _p / C ₂ If the air gap voltage V ₂ is equal to or lower than the discharge limit obtained from Paschen's discharge law, no discharge in which the potential of the non-image portion of the photoconductor is reversed occurs. Discharge starting voltage V _pa in the air gap of the gap width d ₂ by the discharge law of the Paschen, the aforementioned

## EQU4 ## Since V _pa = 312 + 6.2 × 10 ⁶ · d ₂ , V ₂ ≤V _pa

## EQU9 ## The condition CV _p / C ₂ ≦ 312 + 6.2 × 10 ⁶ · d ₂ is derived. That is, even if changes in gap width d ₂ by the photosensitive member 1 and the intermediate transfer belt 5 approaches the time of transfer, so as to always satisfy the above conditions, constituting the photosensitive member 1 and the intermediate transfer belt in an image forming apparatus Then, the transfer bias voltage may be set.

Next, the gap width d ₂ is also vary, always relative dielectric constant epsilon ₁ and the thickness of the photosensitive member 1 that satisfies the above conditions by the photosensitive member 1 and the intermediate transfer belt 5 is close to the time of the transfer It is d ₁ and the dielectric constant epsilon ₃ and the thickness d ₃ of the intermediate transfer belt 5
When, described setting conditions of the transfer bias voltage V _p. The above setting condition is V ₂ = V _pa

Equation 10] One or no d ₂ disintegrated in _{CV p / C 2 = 312 +} 6.2 × 10 6 · d 2 only obtained by determining the conditions such that no solution. This condition

B ² -4ac ≦ 0 where a = 6.2 × 10 ⁶ b = 6.2 × 10 ⁶ (d ₁ / ε ₁ + d ₃ / ε ₃ ) +312
Obtained by _{V p c = 312 (d 1} / ε 1 + d 3 / ε 3), than the number 11,

(Equation 12) Becomes Therefore,

(Equation 1) as well as

(Equation 2) The dielectric constant epsilon ₁ of the photosensitive member 1 so as to satisfy the thickness d _1, the dielectric constant epsilon ₃ of the intermediate transfer belt 5, the thickness d _3, and, by setting the transfer bias voltage V _p, even if discharge Even if it occurs, the potential of the non-image portion of the photoconductor does not reverse. Therefore, the force received by the toner from the electric field caused by the charge on the photoconductor surface and the transfer bias voltage applied to the intermediate transfer member acts in a direction to suppress the spread of the toner. Therefore, the occurrence of transfer dust can be easily prevented.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a color copying apparatus as schematically shown in FIG. 1, an organic photoconductor OPC having a thickness of 25 .mu.m and a relative dielectric constant of 3 is applied to an organic photoconductor OPC of -500 by a corona charger as a charging device.
V is uniformly charged, and then a laser beam is irradiated in an exposure process to obtain a line image (line width: 100 μm) in the sub-scanning direction.
m, line interval: 100 μm, toner: magenta toner) was formed on the photoreceptor 1. Here, the location irradiated with the laser beam had a potential of -150V. Next, negative latent toner was adhered to the photoconductor 1 by negative-positive development to visualize the latent image, and it was confirmed that the line image was formed clearly on the photoconductor 1. The intermediate transfer belt 5 has a thickness of 150 μm and a volume resistivity of 10 ^13.
Using carbon dispersed ETFE of Ωcm, the conductive brush 9 was pressed against the back side of the nip portion of the intermediate transfer belt 5 to apply a transfer bias voltage of 1000 V to perform primary transfer. During the primary transfer, the processing by the color copying machine is stopped, and the line image transferred on the intermediate transfer belt is photographed. At a predetermined observation area (500 μm × 500 μm), the line image is taken from the two line images on the photograph. The number of dusty toners that were clearly off was counted.

In the above experiment, the relative dielectric constant was in the range of 3 to 12,
The measurement was performed using the intermediate transfer belts 5 having different dielectric constants. The above-mentioned intermediate transfer belts 5 having different dielectric constants were prepared by subjecting carbon to kraft treatment, covering the carbon with an aromatic group, and forming dielectric particles. In this embodiment, Formula 1 is satisfied when d ₃ / ε ₃ ≧ 2.31 × 10 ⁻⁵ . Here, the thickness of the intermediate transfer belt is 150 μm.
Therefore, when the relative dielectric constant of the belt satisfies ε ₃ ≦ 6.5, Expression 1 is satisfied. In addition, it is d that satisfies Equation 2
_{Since 3} / ε ₃ ≦ 3.83 × 10 ⁻⁴ , Expression 2 is satisfied when the belt dielectric constant satisfies ε ₃ ≧ 0.391. Therefore, in the case of this embodiment, Equation 2 is always satisfied. Table 2 summarizes the relationship between the relative dielectric constant ε ₃ of the intermediate transfer belt 5 and the number of transfer dust. Table 2 also shows the results obtained by measuring the potential of the non-image portion of the photoconductor after the completion of the transfer using a surface voltmeter (Model 344, manufactured by TREK).

[0026]

[Table 2]

As shown in Table 2, when the relative dielectric constant ε _{3 of the} belt is smaller than 6.5, the polarity of the non-image portion potential of the photosensitive member after the transfer is the same as that before the transfer, and the number of dust toners is small. Image was obtained. On the other hand, if the relative dielectric constant ε _{3 of the} belt is larger than 6.5, the polarity of the non-image portion potential of the photoreceptor after transfer becomes positive, which is the opposite polarity to that before transfer, and the number of dust toners increases. Was confirmed.

In the above embodiment, only the image forming apparatus for negative / positive development has been described. However, the same effects can be obtained by applying the present invention to an image forming apparatus for positive / positive development. it can.

[0029]

According to the first aspect of the present invention, after the toner image on the image carrier is transferred to the intermediate transfer member, the polarity of the potential of the portion where the toner is not adhered on the image carrier is: Since the polarity becomes equal to the polarity before the transfer, the force received by the toner from the electric field due to the charge on the photoreceptor surface and the transfer bias voltage applied to the intermediate transfer member acts to suppress the spread between the toners. Therefore, there is an excellent effect that transfer dust can be prevented.

According to the second aspect of the present invention, when the true charge of the portion of the image carrier on which the toner is not adhered, that is, the non-image portion becomes 0, no further discharge occurs. Therefore, the polarity of the potential of the non-image portion does not become opposite to the polarity of the potential of the non-image portion before the toner image on the image carrier is transferred to the intermediate transfer member. Therefore, the force received by the toner from the electric field caused by the charge on the photosensitive member surface and the transfer bias voltage applied to the intermediate transfer member acts in a direction to suppress the spread of the toner particles. Therefore, there is an excellent effect that transfer dust can be easily prevented.

[Brief description of the drawings]

FIG. 1 is a front view showing a schematic configuration of a color copying apparatus according to an embodiment.

FIGS. 2A and 2B are simulation results showing how a toner image formed on a photoreceptor 1 of the apparatus moves onto an intermediate transfer belt 5. FIGS.

FIG. 3 is an explanatory diagram in which a photoconductor, an intermediate transfer belt, and a gap between the two are modeled.

FIGS. 4A and 4B are explanatory diagrams of electric lines of force showing an electric field due to a charge on a photosensitive member surface and a transfer bias voltage applied to an intermediate transfer member.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Charging device 3 Optical image 4 Developing device 5 Intermediate transfer belt 6, 7, 8 Roller 9 Conductive brush 10 DC power supply 11 Transfer roller 12 Recording paper 13 Fixing device

Claims (2)

[Claims] 1. A toner image forming means for forming a toner image on an image carrier, a surface-movable intermediate transfer member arranged in contact with and facing the image carrier, and an image formed on the intermediate transfer member. An image forming apparatus comprising: an intermediate transfer unit configured to sequentially transfer toner images on a carrier in a superimposed manner; and a transfer material transfer unit configured to transfer the superimposed toner image on the intermediate transfer member to a transfer material. The polarity of the potential of the portion where the toner is not adhered on the image carrier after transferring the upper toner image to the intermediate transfer member is the same as that before transferring the toner image on the image carrier to the intermediate transfer member. The relative permittivity and thickness of the image carrier, the relative permittivity and thickness of the intermediate transfer member, and the potential of the lower surface of the intermediate transfer member are set to be equal to the polarity of the potential of the portion. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the relative permittivity and the thickness of the image carrier are ε ₁ and d, respectively.
₁ [m], the gap width between the image carrier and the intermediate transfer member is d ₂ [m], the relative permittivity and the thickness of the intermediate transfer member are ε ₃ and d ₃ [m], respectively. When the absolute value of the potential on the lower surface of the intermediate transfer member is V _p , And Wherein the relative permittivity and thickness of the image carrier, the relative permittivity and thickness of the intermediate transfer member, and the potential of the lower surface of the intermediate transfer member are set so that apparatus.