JPH08234580A - Image forming device - Google Patents

Abstract

PURPOSE: To obviate the need for a complex transfer-belt cleaning mechanism, to improve the reliability and durability of an image forming device, to eliminate unsatisfactory transfer such as transfer smudge and transfer repellence and, therefore, to improve sharpness in image quality. CONSTITUTION: In the image forming device equipped with a belt transfer-device 30 which transfers a toner image, formed on an image carrier 10, to recording material P in a transfer part and carries it, a first electrode 32 and a second electrode 34 are arranged on the back of a transfer belt 31 opposed to the image carrier 10 and upstream and downstream, respectively, in such a manner that they cross the transfer belt 31 in the direction of its movement. To the second electrode 34, the potential which makes the polarity of the potential of the image carrier 10 opposite to that of toner is impressed only when the recording material P passes through the transfer part. To the first electrode 37, the voltage that makes the polarity of the potential of the image carrier 10 the same as that of the toner is impressed when the recording material P has passed and/or when it is not passed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic recording type image forming apparatus such as an electrophotographic copying machine or an electrostatic printer, which rotates a toner image formed by developing a latent image on an image carrier. The present invention relates to an image forming apparatus that transfers a recording material on which a toner image is adhered to a recording material by a belt transfer device having a transfer belt, and then conveys the recording material to which a toner image is attached, and fixes the recording material by a fixing device.

[0002]

2. Description of the Related Art In an image forming apparatus such as an electrophotographic copying machine and a printer, after a peripheral surface of a photosensitive drum, which is an image carrier, is uniformly charged by a charger, an image is formed on the peripheral surface of the photosensitive drum. Exposure is performed to form an electrostatic latent image. This latent image is developed and visualized by developing means to become a toner image.
The toner image formed on the peripheral surface of the photosensitive drum is transferred by a physical means onto a recording material (recording paper) that is fed at the same timing, and the recording material on which the toner image is transferred is transferred from the photosensitive drum. The separated toner image is conveyed to the fixing unit, and the toner image is fixed on the recording material and then discharged to the outside of the apparatus.

The toner image adhering to the peripheral surface of the photoconductor drum is transferred to the recording material by applying a charge having a polarity opposite to the charging polarity of the toner from the back surface to the recording material by a transfer discharger. After that, the recording material to which the high voltage of alternating current is applied is discharged by the separator and separated from the photosensitive drum. However, the multicolor toner image has a diameter of 100 mm.
It is difficult to reliably transfer the color toner image formed by laminating on the photoconductor drum having a considerable outer diameter to the recording material and to stably separate it from the photoconductor drum. It has a drawback that the finished recording material easily winds around the photosensitive drum without being separated and causes paper jams. In particular, when a relatively thin recording material such as 16 to 20 pound paper is used in a high humidity environment of 75% RH or more, the tendency is remarkable, and the condition for transferring a vivid color image and the white paper It has a drawback that the conditions for separation are not compatible. A belt transfer device is used as a technique for improving this drawback.

In the belt transfer device, a transfer conveyance belt (hereinafter also referred to as a transfer belt) stretched by a plurality of holding rollers (driving roller, driven roller) is rotated at the same speed as the photosensitive drum. The recording material is conveyed while being in close contact with the transfer belt, passes through the transfer portion, and contacts the photosensitive drum integrally with the transfer belt at the transfer portion. At the transfer portion, a transfer electrode (corona discharge electrode, roller-shaped electrode or brush-shaped electrode) provided inside the transfer belt applies a charge having a polarity opposite to the toner polarity to the transfer belt, and the toner image on the photosensitive drum is Is configured to be transferred to a recording material. In such a belt transfer device, excellent transfer efficiency and separation effect can be obtained in the transfer section.

The above-mentioned belt transfer device is used as a particularly preferable device in a color image recording device in which toner images are superposed on a photosensitive drum and transferred onto a recording material at once. In such a color image recording apparatus, since a plurality of developing devices are provided on the peripheral portion of the photosensitive drum in order to superpose toner images on the photosensitive drum, the drum diameter becomes large, and sufficient separation performance can be obtained by the conventional electrostatic transfer separation method. Since it requires no more reliable separation performance, and the toner adhesion amount increases by 2-3 times in the method of overlapping toner images, the transfer charge amount increases almost in proportion to the adhesion amount, compared to a single color. Then, it is necessary to increase the size by 2-3 times. With respect to the above two requirements, the belt transfer device is excellent for the following reasons. In the belt transfer device, the transfer charge is applied to the recording material while the recording material is in close contact with the photosensitive member via the transfer belt, so that the transfer belt can hold sufficient transfer charge and ensure reliable transfer. To be In addition, the recording material can obtain a reliable separation performance while being electrostatically attracted by the transfer charge in the transfer belt and holding a transfer force sufficient to separate from the photosensitive drum.

[0006]

However, in an image recording apparatus equipped with a belt transfer device, the image carrier is transferred from the image carrier to the transfer belt due to a jam of the recording material at the transfer portion, fog outside the area of the recording material, or the like. It is necessary to clean the transferred toner. Many cleaning methods for this have been proposed. For example, the blade cleaning method disclosed in JP-A-3-116174, JP-A-6-59581,
There is an electrostatic recovery roller cleaning method and the like in JP-A-6-59586. Since the blade and the roller are brought into pressure contact with the transfer belt, there remain problems such as deviation of the transfer belt in the axial direction, transfer deviation due to uneven transfer belt speed, and deterioration of parts such as the blade and transfer belt. Further, there is a problem that the transfer device itself becomes complicated and the reliability is managed or the manufacturing cost is increased.

Furthermore, there are the following problems in terms of image quality.

In a high-humidity environment, an image in which white grains are transferred is generated, and as the humidity rises, a severe frost-like transfer failure may occur over the entire toner image. The generation conditions and generation levels also differ depending on the type of paper. This is commonly called "transcription repellency". This flows into the transfer entrance through the belt and recording paper provided by the corona transfer pole, the potential of the entrance approaches the transfer pole, and the gap potential between the image carrier and the recording paper exceeds the Paschen critical. The cause is the air discharge that occurs.

Even if the transfer repelling does not occur even at room temperature and normal humidity, "character dust" and "image blurring" in which toner is scattered around the characters occur. This is because the toner on the image carrier flies to the recording paper side by electrostatic force before the recording paper comes into close contact with the image carrier due to the increase in the gap potential.

The present invention solves the above-mentioned cleaning problem by improving the cleaning mechanism of the belt transfer device, and has no structural problems such as transfer deviation, pitch unevenness, and belt deviation, and is structurally simple and inexpensive as an apparatus. We provide a belt transfer device with a long service life, prevent the occurrence of transfer defects such as image blurring and transfer repellency, and even when transferring toner images formed by superposition, high quality on recording material without impairing color tone. It is an object of the present invention to provide a belt transfer device capable of transferring as a color image.

[0011]

An image forming apparatus of the present invention that achieves the above object is equipped with a belt transfer device for transferring a toner image formed on an image carrier to a recording material at a transfer portion and conveying the recording material. In the image forming apparatus, a first electrode on the upstream side and a second electrode on the downstream side are arranged to intersect with the moving direction of the transfer belt on the back surface side of the transfer belt facing the image carrier, Only when the recording material passes through the transfer portion, a potential having a characteristic opposite to that of the toner with respect to the potential of the image carrier is applied to the second electrode, and the recording material is passed to the first electrode. It is characterized in that a voltage having the same polarity as that of the toner is applied to the electric potential of the image carrier during paper feeding and paper non-passing, and a voltage having the same polarity as the toner is applied to the first electrode, No paper bleeding or image blurring occurs during paper passing No so that the toner of the transfer belt is reversely transferred to the drum in it is obtained by an entirely unnecessary cleaning mechanism of the belt transfer device.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of the embodiments, the structure and operation of an image forming apparatus having a belt transfer device will be described.

FIG. 1 is an overall configuration diagram of a color image forming apparatus (color printer) as an embodiment of the image forming apparatus according to the present invention.

(Charging / Electrification) In FIG. 1, reference numeral 10 denotes a photosensitive drum which is an image bearing member, which is formed by coating an OPC photosensitive layer on the drum, and is grounded and rotated clockwise. Reference numeral 12 denotes a scorotron charger, which applies a uniform charge of V _{H to} the peripheral surface of the photosensitive drum 10 by corona discharge by a grid and a corona discharge wire whose potential is held at V _G. Prior to the charging by the scorotron charger 12, the precharging pre-exposure means (PCL) 11 using a light emitting diode or the like is used to remove the charge on the peripheral surface of the photoconductor in order to eliminate the history of the photoconductor until the previous printing. deep.

(Exposure) After uniformly charging the photosensitive drum, the image exposing means 13 performs image exposure based on the image signal. The image exposure means 13 passes through a polygon mirror 131, a collimator lens 132, etc., which rotate using a laser diode (not shown) as a light emitting source, has its optical path bent by reflection mirrors 133A and 133B, and scans via an fθ lens 134. A latent image is formed by the rotation (sub-scanning) of the photoconductor drum 10. In this embodiment, the character portion is exposed to form an inverted latent image in which the character portion has a lower potential V _L.

(Development) At the peripheral edge of the photosensitive drum 10, a developing device 14Y, which contains a developer composed of toner such as yellow (Y), magenta (M), cyan (C), black (K) and a carrier, is provided. 14M, 14C and 14K are provided, and first, the development of the first color is performed by the developing sleeve 141 which has a magnet built therein and holds the developer and rotates. The developer consists of a carrier in which ferrite is used as a core and is coated with an insulating resin around it, and a toner whose main component is polyester and pigments according to the color and charge control agents, silica, titanium oxide, etc. are added. Agent is layer forming rod 142
Then, the layer is regulated to a layer thickness of 300 to 600 μm on the developing sleeve 141 and conveyed to the developing area.

The gap between the developing sleeve 141 and the photosensitive drum 10 in the developing area is 0.4 to larger than the layer thickness (developer).
As 1.0mm, V _AC AC ice and V _DC D
The C bias is superimposed and applied. Since V _DC and V _H and the toner are charged with the same polarity, the toner given the opportunity to be separated from the carrier by V _AC does not adhere to the V _H portion, which has a higher potential than V _{DC, and} has a potential higher than V _DC. attached to a lower V _L portion of visualized (reversal development) is carried out.

After the visualization of the first color is completed, the process proceeds to the image forming process of the second color, the uniform charging is performed again by the scorotron charger 12, and a latent image based on the image data of the second color is formed by the image exposing means 13. To be done. At this time, the charge elimination by the PCL 11 performed in the image forming process of the first color is not performed because the toner attached to the image portion of the first color scatters due to the rapid decrease in the potential around the image.

Again, V is applied over the entire surface of the photosensitive drum 10.
A latent image similar to that of the first color is formed on the portion of the photoconductor having the _H potential and the image of the first color is not developed, but development is performed again on the portion having the image of the first color. In the portion to be performed, the latent image of V _M ′ is formed by the light shielding of the toner of the first color and the charge of the toner itself, and V _{D C}
And development is performed according to the potential difference between V _M ′ and V _M ′. In the overlapping portion of the images of the first color and the second color, the development of the first color is V _L
When the latent image of is formed, the balance between the first color and the second color is lost, so the exposure amount of the first color is reduced and V _H > V _M > V
_It may be an intermediate potential that becomes _L.

An image forming process similar to that for the second color is performed for the third and fourth colors, and a visible image of four colors is formed on the peripheral surface of the photosensitive drum 10.

(Paper feeding) On the other hand, the recording material P fed from the paper feeding cassette 21 and fed by the paper feeding rollers 221 and 222 is
The multi-color image on the peripheral surface of the photoconductor drum 10 is collectively transferred onto the recording material P by being fed to the transfer area by the belt transfer device 30 in which the transfer belt 31 is stretched. 223 is a resist shutter, 2
Reference numeral 24 is a recording material leading edge detection sensor.

(Transfer) FIG. 2 is a sectional view of the belt transfer device 30 showing a transfer state in which the transfer belt 31 is in contact with the photosensitive drum 10 via the recording material P. FIG. 3 is a sectional view showing a state in which the transfer belt 31 is separated from the peripheral surface of the photosensitive drum 10. Transfer belt 31 used in the transfer belt device 30
Has a FLC layer formed on the outside of a urethane rubber substrate
It is an endless rubber belt with a volume resistivity of 10 ¹⁰ to 10 ¹¹ Ωcm (20 ° C, 60% RH), a thickness of 0.61 mm, and a perimeter (diameter) of 52 cm. It is a driving roller 33 and a driven roller (transferring). Front roller)
It is stretched between 32 and 32 and is used at an extension rate of 3% by the bias of the tension roller 35.

The volume resistivity is 300 mse when the transfer belt 31 is sandwiched by electrodes and 2 kV is applied between the electrodes.
It is a value calculated from the current value after c has elapsed. However, this volume resistance value is highly dependent on the environment and varies by about two orders of magnitude depending on the environment, especially the temperature. Therefore, the charge injected into the transfer belt 31 is applied to the electrodes by the in-machine humidity monitor in order to set it in a desired range. It is more desirable to switch and control the voltage.

The pre-transfer roller 32, the drive roller 33 and the tension roller 35 are the outer frame 30 of the belt transfer device 30.
The compression spring 30 is supported by the inner frame 30b housed in a.
It is biased counterclockwise by the shaft 33a of the drive roller 33 as a fulcrum by c.

The pre-transfer roller 32 is a pre-transfer roller that is wound by abutting rollers (not shown) provided on the shafts at both ends being brought into contact with a non-image area on the side edge of the photosensitive drum 10.
The photosensitive drum 10 is supported so that the distance between the photosensitive drum 10 and the peripheral surface of the photosensitive drum 10 is regulated to a predetermined value, and is rotated by the conveyance of the transfer belt 31. A bias voltage can be applied to the pre-transfer roller 32.

On the other hand, the drive roller 33 is connected to the power system of the main body of the apparatus and is driven to rotate in the counterclockwise direction.
A roller of 5.3 mm, which has a surface roughness of Rmax. 55 to 85 μm, reliably circulates in the counterclockwise direction without slipping with the transfer belt 31, and synchronizes with the peripheral speed of the photoconductor drum 10. Transport.

In the transfer belt 31, the belt surface between the pre-transfer roller 32 corresponding to the first electrode and the driving roller 33 is pressed against the peripheral surface of the photosensitive drum 10 to form the nip portion N, and A corona discharger 34 corresponding to the second electrode is disposed inside the transfer belt 31 facing each other to form a transfer area of the toner image.

The corona discharger 34 is composed of a transfer wire 34a made of WO ₃ and having a wire diameter of 0.08 mm, and an electrode plate made of a stainless steel plate SUS304 in which the distance between the side plate and the transfer wire 34a is 7.5 mm. 34b, and the transfer wire 34a is arranged to face the surface of the photoconductor drum 10 at a distance of 7 mm.

An insulating member 37 is provided between the corona discharger 34 and the pre-transfer roller 32, and the corona discharger 34 is separated.
It is prevented that the discharge charge due to the flow directly into the pre-transfer roller 32. The insulating member 37 is made of polyethylene terephthalate (PET) thin plate, and the electrode plate
It is fixed to 34b.

Further, the pre-transfer roller 32 is configured such that the transfer belt 31 is wound by setting the outer diameter of the abutting roller.
The transfer belt 31 is appropriately pressed by the step portion so that the outer periphery of the transfer belt 31 is in a contact state over its downstream side so as not to generate excessive pressure on the drum surface.

A variable voltage in the range of +3.5 to +7.5 kV is applied to the transfer wire 34a of the corona discharger 34 by the transfer electrode power supply HV1, and the bias power supply HV2 is applied to the shaft 32a of the pre-transfer roller 32. Thus, the variable voltage VPR is applied in the range of -1000 to -2500V.

At the time of transfer, the transfer electrode power supply HV1 and
The bias power supply HV2 is activated, but the charge (+) injected from the transfer corona discharger to the transfer belt is applied to the pre-transfer roller 32.
By making it larger than the electric charge (-) injected from the transfer belt 31 to the transfer belt 31, a total of + charges is injected into the transfer belt 31, and the toner is transferred to the recording material P. Transfer electrode power supply H
The output voltage from V1 can be appropriately adjusted according to the thickness and material of the recording material P or environmental conditions. When not passing the paper,
The transfer electrode power supply HV1 is deactivated and the bias power supply HV2 is set.
Is operated to inject a negative charge as a total into the transfer belt 31 to prevent unnecessary toner on the image carrier 10 from adhering to the transfer belt 31, or the residual toner adhering to the transfer belt 31. The toner is reversely transferred to the image carrier 10 and the transfer belt is constantly cleaned. Immediately after the jam or immediately after the power is turned on, it is assumed that unnecessary toner has adhered to the transfer belt 31, and the transfer belt 31 is brought into the non-paper passing state.
The transfer electrode power supply H
The transfer belt 31 is cleaned by deactivating V1 and activating the bias power supply HV2 to rotate the transfer belt 31. At this time, it is more preferable in terms of cleaning stability that the surface potential of the photoconductor drum 10 is maintained at the potential during image formation by the charger 12 and the static eliminator 15.

Next, a belt transfer device according to claims 1 to 4 of the present invention will be described with reference to the following first and second embodiments.

(Embodiment 1) In the belt transfer device according to the first to fourth aspects of the present invention, the upstream first electrode (pre-transfer roller) is provided on the back surface side of the transfer belt 31 facing the image carrier 10.
32 and a second electrode (corona discharger) 36 on the downstream side are arranged so as to intersect with the moving direction of the transfer belt 31, and the recording material P passes through the transfer portion to the second electrode 36. Only sometimes
A potential having a characteristic opposite to that of the toner with respect to the potential of the image carrier 10 is applied, and the potential of the image carrier 10 is applied to the first electrode 32 when the recording material P is fed and when it is not fed. Is applied with a voltage of the same polarity as that of the toner.

Further, the first with respect to the potential of the image carrier 10
The voltage applied to the electrode 32 is set to a different voltage value when the recording material P is fed and when it is not fed.

Further, the voltage applied to the first electrode 32 with respect to the potential of the image carrier 10 is set to have a larger voltage value with the same polarity as the toner when the recording material P is not passed than when it is passed.

The belt transfer device of the present invention is a pre-transfer roller which is configured to bring the transfer belt into direct contact with the image carrier (photosensitive drum) when the recording material does not pass through the transfer portion, and which presses the inside of the transfer belt. A potential is applied to the (first electrode) to inject into the transfer belt a charge (having the same polarity as the toner) enough to prevent the toner on the image carrier from adhering to the transfer belt. In addition, it is possible to prevent transfer of toner from the image carrier when the paper is not passed, and at the same time, to transfer the toner on the transfer belt to the image carrier side almost completely in reverse. By applying, without providing a transfer belt cleaning means,
Good cleaning properties can be realized.

The corona discharger (second electrode) is
Only when the recording material passes, a potential is applied to inject into the transfer belt a charge (charge having a polarity opposite to that of the toner) enough to transfer the toner on the image carrier to the recording material.

The pre-transfer roller (first electrode) is
Regardless of whether the recording material passes or does not pass, the transfer belt is cleaned by applying the electric potential of the same polarity of the toner to the electric potential of the image carrier, and at the same time when the paper is passed, the photoconductor Toner on the drum flies toward the recording material, and a gap potential difference or polarity is formed so that air discharge does not occur, thereby making it possible to eliminate image blurring and transfer cissing.

Further, according to the present invention, the total amount of charges injected from the first and second electrodes to the transfer belt is set to a predetermined value so that the original image transfer is not affected when the paper is passed. In addition, the output conditions of both electrodes were made appropriate.

Furthermore, the present invention does not have a structure in which the bias voltage is lowered to prevent transfer blurring and transfer repelling when the paper is passed, and when the paper is not passed, defective cleaning such as a tab-like cleaning and excessive rise of the potential of the photoreceptor are caused. The bias voltage was raised to a level that would not occur.

By applying a bias voltage to the transfer roller 32 to inject a negative charge having the same polarity as the toner into the transfer belt 31 and interrupting or significantly weakening the application of the positive charge from the transfer pole, the transfer belt A negative charge is applied to 31 as a total, and the toner remaining on the transfer belt 31 is reversely transferred to the image carrier 10 to clean the transfer belt 31. Further, the toner on the image carrier 10 is prevented from adhering to the transfer belt 31. The toner transferred onto the image carrier 10 is cleaned by the cleaning device 16 on the downstream side.

The conditions for realizing the above functions will be described below.

First, a model was constructed to find out how much charge should be injected into the transfer belt 31.

Two layers of toner adhere to the image carrier 10, and the amount of electric charge to the transfer belt 31 required to prevent the toner from adhering to the transfer belt 31 was determined.

FIG. 4 is a diagram showing the charge distribution of the image carrier (photosensitive drum) 10, the toner layer, and the transfer belt 31. The surface of the photosensitive layer 10a on the drum substrate 10b of the image carrier 10 is charged with a charge density σ _p . A charge density σ _b is injected into the transfer belt 31. σ _t is the amount of charge per unit area of the toner layer. ε _p represents the electric flux density of the photosensitive layer 10 a, and ε _t represents the electric flux density of the toner layer T.

FIG. 5 is a characteristic diagram showing the electric flux density εE in the layer thickness direction x. D _p is the layer thickness of the photosensitive layer 10a, and D _t is the layer thickness of the toner layer. x = 0 is the position of the surface of the drum substrate 10b, x
= D _p indicates the position of the photosensitive layer 10 a, and x = D _p + D _t indicates the position of the contact interface between the toner and the transfer belt 31.

Next, the force F _t acting on the surface of the transfer belt 31.
Consider.

F _t = q _t · E _t (D _t + D _p ) + F _ad (1) ε _t · E _t (D _t + D _p ) = σ _b (2) q _t : charge amount E _{t of} one toner (D _t + D _p ): electric field of x = D _t + D _p ε _t : dielectric constant of the toner layer (here, 3 ε _o ,
ε _o is the vacuum dielectric constant 8.85 × 10 ⁻¹² F / m) In the above formula, F _ad is the adhesive force between the toner and the belt, and it is generally said that the van den Waals force is dominant. I tried to find this experimentally. It can be obtained by applying a centrifugal force to the toner and the sample attached to the transfer belt 31 to obtain the force required for the toner to separate. The toner used in this experiment was found to be able to separate 95% with a force of 20 mdyn. After that, F _ad ≈ 20 mdyn
Treat as.

The condition of not adhering to the transfer belt 31 is F _ad >
0, σ _b <F _ad · ε _t / q _t (3) The toner used this time is a two-component developer, the amount of charge per weight is 20 μC / gr, and the average particle size is 15 μm. q
_{The t} is −3.5 × 10 ⁻¹⁴ clones. By substituting the above numerical values into the equation (3), σ _b <−51 μC / m ² (4) was obtained.

The current I _b injected from the pre-transfer roller 32 to the transfer belt 31 can be expressed by the following equation.

[0052] _{I b = σ b · W ·} V L (5) W: the effective image width (drum axis) 300 mm V _L in Example: 74 mm / sec above embodiment W is a process speed embodiment, the V _L ( Substituting into equation (5), the injection current required to prevent adhesion is obtained as follows.

I _b <−1.12 μA (6) That is, in order to prevent the toner on the image carrier 10 from adhering, it is necessary to inject a negative current of 1.12 μA or more into the transfer belt 31. The result was obtained.

FIG. 6 shows an experimental result showing the relationship between the current injected into the transfer belt 31 and the toner transferred to the belt.

(Conditions) In the example using the image forming apparatus shown in FIG. 1, a _Red image of 5 cm × 2 cm was formed. First, a Y-color toner image was formed, and an M-color toner image was formed thereon in an overlapping manner. The amount deposited is controlled within the range of 2.0 mg / cm ² ± 0.3 mg.

The process speed and the image width are as described above. The pressing load on the pre-transfer roller 32 and the image carrier 10 is 500 to 700 gf / (per full width). It is recharged before transfer, and image carrier 10
Is uniformly charged to -650V. Transfer belt 3
The injection current into 1 was measured by connecting an ammeter between the pre-transfer roller 32 and the bias power supply.

The parameters represent the relationship when the transfer belt 31 made of semiconductive urethane rubber having different surface states shown in the attached sheet is used. From FIG. 6, it was found that the amount of adhesion on the transfer belt 31 can be reduced by increasing the transfer belt current, and a current of about 1 to 2 μA is required. This substantially matches the equation (6). The urethane rubber single layer has a surface roughness (R _z ) of about 8 μm, requires a larger current, and tends to have a large amount of residual toner that cannot be solved even if the current is increased. The transfer belt 31 for which the adhesive force F _ad with the toner is experimentally obtained is a transfer belt having a fluororesin coat layer.

The relationship between the bias voltage applied to the pre-transfer roller 32 and the volume resistivity ρ _b of the transfer belt 31 will be described. Here, first, with respect to the charge density σ _b injected into the transfer belt 31, a model was constructed and a required applied voltage was obtained. Furthermore, the relationship between the volume resistivity ρ _b and σ _b was considered.

Pre-transfer roller 32 made of conductive rubber roller
A model of electric charge / current injected from the transfer belt 31 into the transfer belt 31 is shown in FIG. FIG. 8 shows an equivalent circuit in the process of being injected into the transfer belt 31 while passing through the nip width W.

On the surface of the pre-transfer roller 32, 10 ⁵ to 10 ⁷ Ω.
Since the transfer belt 31 is covered with rubber having a volume resistivity of 10 cm and the volume resistivity of the transfer belt 31 is in the order of 10 ⁹ to 10 ¹¹ Ωcm, the rubber layer of the pre-transfer roller 32 is ignored. There is.

When the charge density injected into the transfer belt 31 is obtained, the charge density σ _b2 which is injected into the transfer belt 31 and is accumulated on the surface of the transfer belt on the image carrier 10 side and the transfer belt 31.
_Is the sum of the charge density σ _b1 charged on the transfer belt on the side of the pre-transfer roller 32 when separated from.

Between the pre-transfer roller 32 and the transfer belt 31, there is a potential threshold V _{z at} which charging starts.

Σ _b = σ _b1 + σ _b2 (7) Next, the equivalent circuit of FIG. 8 will be described.

C _p is the capacity of the photosensitive layer 10a, C _t is the toner layer T
, C _b is the capacity of the transfer belt 31 layer, V _z is the charging start threshold potential between the pre-transfer roller 32 and the transfer belt 31, and R _b is the resistance value of the transfer belt 31. E _o is the bias voltage applied to the roller surface, V _p is the potential of the image carrier 10 before transfer, and V _t
Is the potential of the toner layer T, and V _p + V _t is the surface potential of the image carrier 10 having the toner layer T. The time required to pass the nip width W is t _n .

In FIG. 8, there are two switches SW. Before entering the nip of the transfer part, the switch SW is in the state of a solid line,
When the transfer belt 31 is in the nip of the transfer portion, the switch SW is in a broken line state. By calculating the charge densities σ _b1 and σ _b2 after the time t _n after the switch SW is in the solid line state, the injected charge amount σ _b to the transfer belt 31 can be calculated.

[0066]

[Equation 1]

Considering per unit area, C _p = ε _p / D _p , C _t = ε _t / D _t , R _b = ρ _b D _b , and (9) and (10) are given by the following equations. Become.

[0068]

[Equation 2]

By substituting the following implementation constants from the equation (12), the transfer belt charge density shows the dependency on the volume resistivity as shown in FIG.

V _p + V _t = −650 V V _o = −1650 V V _z = −400 V (obtained from literature) D _p = 21 μm D _t = 30 μm (assuming two toner layers) D _b = 0.6 × 10 ⁻⁴ m ε _p = 3 ε _o ε _t = 2 ε _o ε _b = 6 ε _o (obtained from literature) ε _o = 8.85 × 10 ⁻¹² F / m t _n = 1.6 × 10 ⁻² sec Semi-conductivity in which carbon is dispersed in urethane rubber The belt (used recently in an electrophotographic apparatus) changes in volume resistivity as shown in FIG. 10 depending on the environment of temperature and humidity.

The bias voltage of the pre-transfer roller 32 necessary for reversely transferring the toner adhered on the transfer belt 31 to about 2.0 mg / cm ² to the image carrier 10 has a volume resistivity ρ _{p of the} transfer belt 31.
On the other hand, the results obtained experimentally are shown in FIG.

The characteristic curve is the boundary line of good or bad cleaning, the upper right side is a bad (No-good) area, and the lower left side is a good cleaning (Good) area. FIG. 11 shows, for example, 10
It is shown that a bias voltage of −1800 V or more should be applied to the transfer belt 31 of ⁹ to 10 ¹² Ω · cm. It is also shown that the applied voltage can be lowered by the belt resistivity. For example, with a transfer belt 31 of 10 ⁹ Ω · cm, −135
If it is 0 V or more, it means that it is good.

FIG. 12 shows an image carrier on which the transfer belt 31 comes into direct contact when a fixed power source of a constant voltage of -2000 V is applied.
10 This is the result of examining the potential increase on the surface. The horizontal axis represents the volume resistivity ρ _b (Ω · cm) of the transfer belt 31, and the vertical axis represents the potential of the image carrier 10 after transfer.

The image carrier 10 is charged to −650 V before transfer. The surface potential of the image carrier 10 increases due to an increase in injected charges when the volume resistivity of the transfer belt 31 decreases,
It has been shown that there is a sharp rise at ρ _b <10 ¹⁰ Ω · cm. Raising the surface potential of the image carrier 10 to about -900 V is not preferable in terms of durability and repeated memory. The transfer belt 31 used in the embodiment has a volume resistivity that varies by two digits depending on environmental conditions (humidity). For this reason, even with a belt that normally has suitable characteristics on the order of 10 ¹⁰ to 10 ¹¹ Ω ・ cm, it is 10 ⁹ Ω at high temperature and high humidity.
There is a problem that the volume resistivity ρ _b decreases to the cm order, and the surface potential of the image carrier 10 rises with a fixed voltage power supply. Further, when a bias of −2000 V is applied in the vicinity of 10 ⁹ Ω · cm, gap discharge occurs in the pre-transfer roller 32 at the entrance portion of the transfer belt 31, and a small spot-like reverse transfer defect causes the transfer belt 31.
It was confirmed that it was happening above. From these facts, it was found that it is preferable to variably control the bias voltage according to the change in the volume resistivity of the transfer belt.

Next, another feature will be described.

First, the principle of preventing transfer cissing will be described. In conventional corona transfer and roller transfer, under high humidity conditions, the resistance of the transfer paper (recording material) decreases to the order of the 9th power, so the potential of the transfer paper causes the transfer paper contact potential and the contact area of the transfer electrode. It is almost the same at a certain transfer part. as a result,
When the transfer paper potential at the transfer portion is increased, the potential at the transfer paper entrance portion also becomes high, which is a factor that makes it difficult to achieve both the transfer performance and the prevention of transfer blurring and transfer cissing. Also, regulate the potential of the transfer paper entry guide plate,
Floating was also an important point. When a multi-color image is formed in the color image forming apparatus as shown in FIG. 1, two or more color toners are layered on the photosensitive drum, and the transfer charge density is two times or more as compared with the single color transfer. That is, the transfer gap electric field is required, which makes the compatibility more difficult.

The image forming apparatus shown in Japanese Patent Laid-Open No. 6-59586 uses belt transfer, but this problem has not been solved. The reason is the same as above, but the situation will be described in detail below.

First, the transfer paper is conveyed by being electrostatically attracted to the transfer belt. The gap between the pre-transfer roller unit and the photoconductor drum is regulated to 0.2 mm, and the pre-transfer roller unit is configured to come into contact with the photoconductor drum while advancing to the transfer unit of about 20 mm downstream. Moreover, the pre-transfer roller is grounded. The volume resistivity of the transfer belt used is 10 ⁸ to 10 ¹⁰ Ω ・ c
Since m and the relative permittivity are about 6, it is estimated that the transfer charge injected from the corona discharge to the transfer belt is at the interface of the transfer paper. On the other hand, since the transfer paper has the above-mentioned resistance, the transfer paper develops at substantially the same potential from the transfer portion to the upstream side thereof. Therefore, when the transfer paper is separated from the photosensitive drum on the upstream side, the gap potential determined by the following equation increases,
You will cross the Paschen's curve.

Vg = (Vm−Vh) × G / (Dp + Dt + G) Vg; Gap potential Vm; Transfer paper potential Vh; Photoconductor surface potential G having toner layer G; Gap length Dp; Photosensitive layer / photosensitive layer dielectric constant Dt; Toner layer thickness / toner layer dielectric constant This countermeasure is to make the gap between the photoconductor drum and the pre-transfer roller equal to or less than the thickness of the transfer belt at the entrance, and to apply a negative potential to the pre-transfer roller to maintain a steady state. The negative charge is injected into the transfer belt, and even into the transfer paper in some cases, while the transfer side attempts to give the positive charge to the transfer belt to compensate the injected negative charge. As a method of injecting a negative charge into the transfer paper from the transfer belt side, a method of giving a high potential difference in a strong contact state between the transfer belt and the transfer paper (image of conductive paper) and a method of transferring the transfer paper in contact with the photoconductor It is considered to be a method of forming a gap between the back surface and the belt to cause a steady Paschen discharge. In any case, the potential difference between the transfer paper potential of the transfer part and the transfer paper potential of the entrance part is obtained by moving the charge of the opposite polarity to the transfer pole side from the entrance part into the transfer paper in the transfer area. It was made possible to obtain stable transfer performance without transfer cissing even under high humidity conditions.

Next, the situation concerning the transferability under the actual high humidity condition in the belt transfer device using the transfer belt as shown in FIGS. 5 to 10 was examined.

In the transfer section, the electric field E in the gap between the transfer belt and the transfer paper is 56 when assuming the transfer of the two-layer toner.
It is estimated that the transfer paper potential is about Mv / m, and the transfer paper potential is calculated from the following equation.

Vm = E × (Dp + Dt + g + Dm + g) + Vh where Dm = 0 (Considering that the volume resistivity of the transfer paper is of the order of 10 ⁹ the transfer paper thickness is small enough to be ignored) Dt is a vacuum gap of 8 to 15 Micron and Vm are 300-800v.

Further, when the condition that discharge does not occur in the gap between the transfer paper and the photosensitive drum at the entrance portion (strictly, the condition that the polarity of the toner is not reversed by the discharge) is determined, it depends on the toner layer thickness. Was estimated to be -100V to + 200V.

Assuming that the surface resistance of the transfer paper is 3000 MΩ.
The image width of 30 cm is 100 MΩ, and the current injected into the transfer paper at the entrance portion and moving to the transfer portion requires 4 to 6 μA.
The charge density of transfer paper is 180 to 270 μC / sm, which is a considerable amount. Next, considering the amount of current that moves laterally in the layer of the transfer belt, it is obtained by the following equation.

Ib = (Vbt−Vr−Vz) × W × Db / ρb × L = (800 + 1100−400) × 30 × 0.06 / 3 × 10 ⁹ = 0.9 μA Vbt; Belt potential of transfer portion Vr; Roller before transfer (Here, it is assumed that -1100V is applied.) Vz; Contact potential difference between roller and transfer belt [from literature 400
V] Db; transfer belt thickness 0.06 cm ρb; transfer belt volume resistivity [Ωcm] assumed to be 3 × 10 ⁹ Further, charge density accumulated at the transfer belt interface of the transfer belt −
It is estimated to be about 70 μC / sm, and when converted to current, 1.5 μ
A. This means that from the pre-transfer roller to the transfer belt side
This means that it is necessary to flow a current of about 6.5 μA to 8.5 μA.

The volume resistivity of the transfer belt is 3 × when the humidity is low.
Since it is as high as 10 ¹¹ Ωcm, the transfer belt inflow current and the transfer paper injection current in the pre-transfer roller portion shown above are extremely small, 1 μA or less, which can be ignored, and when the charge required for transfer is converted to a current, it is about 11 μA. Considering that, the current flowing from the corona discharge electrode to the transfer belt side is estimated to be 17.4 to 19 μA at 30 ° C. and 80% high humidity, which is about twice the current required at low humidity. Means that.

However, since the volume resistivity of the transfer belt is reduced by two digits in high humidity as compared with low humidity, the potential of the transfer belt layer itself becomes negligibly small. The conditions (total current or discharge wire voltage) of the corona electrode for passing the double current do not need to be changed so much for low humidity, and can be made equal by adjusting the electrode shape or dimensions.

(Embodiment 2) FIG. 13 is an overall configuration diagram of a color image forming apparatus (color printer) as a second embodiment of the present invention.

In this embodiment, the humidity sensor 39 is fixedly installed in the main body of the image forming apparatus, the internal humidity of the main body is monitored by the humidity sensor 39, and the humidity detection signal is controlled by the CPU. As described above, the bias application voltage of the pre-transfer roller 32 is switched and controlled. The humidity sensor 39 is installed near the belt transfer device 30. The structure of the belt transfer device 30 is the same as that of the first embodiment described above. In this way, the bias voltage applied to the pre-transfer roller 32 is switched in a plurality of stages in accordance with the change in humidity and is variably controlled, whereby the granularity due to the change in volume resistivity of the transfer belt 31 due to the excessive bias applied to the pre-transfer roller 32. It has become possible to prevent the cleaning failure and the rise of the surface potential of the image carrier 10. The humidity sensor also controls the output of the second electrode, that is, the corona discharger 34, at the same time, so that stable transfer performance can be obtained simultaneously under a wide range of environmental conditions from low humidity to high humidity. May be.

For example, the humidity sensor 39 detects a change in humidity inside the apparatus and controls the voltage applied to the pre-transfer roller 32 as shown in the table below.

[0091]

[Table 1]

The transfer belt 31 used varies with humidity in the range of 20 to 80% within the range of 5 × 10 ^{9 to} 5 × 10 ¹⁰ Ω · cm.

As the humidity sensor 39, an impedance change type humidity sensor element using a moisture sensitive polymer material was used. The change in impedance of the sensor element of the humidity sensor 39 is linear, and is linearized by a linearization circuit, and after the humidity is corrected, output is made to be a full scale.

FIG. 14 is a characteristic diagram showing the relationship between the humidity condition and the transferability in the image forming apparatus according to the conventional method. That is, it is an experimental result showing the humidity condition in the machine and the transfer current allowable region. The experimental conditions are shown below.

Bias voltage of pre-transfer roller: 0v Recharge: Yes _VH = -650V Two-color toner adhesion pattern Transfer paper: Xerox 4024 (20 lb paper) [transfer paper exposed to each environment for 8 hours or more] Fig. 14 The upper part of the polygonal line A shown in FIG. 3 shows the area where transfer cissing occurs, and the shaded area at the upper left of the polygonal line A is the high voltage abnormal discharge occurrence area. Also,
The lower part of the broken line B is a defective transfer area. Therefore, the narrow intermediate area sandwiched between the folding line A and the folding line B is the transferable region.

FIG. 15 is a characteristic diagram showing the relationship between humidity conditions and transferability in the image forming apparatus according to the present invention. That is, it is an experimental result showing the humidity condition in the machine and the transfer current allowable region.
The experimental conditions are almost the same as the case in FIG. 12, but
A feature is that a bias voltage of -900V is applied to the pre-transfer roller.

The upper part of the broken line C shown in FIG. 15 shows the area where transfer repelling occurs, and the upper left diagonally shaded area of the broken line C is the high voltage abnormal discharge occurrence area. The lower part of the broken line D is a transfer failure area. Therefore, a wide intermediate area sandwiched between the folding lines C and D is a transferable area. This transferable area is wider than the transferable area shown in FIG. 12, and a stable transfer current can be obtained in the humidity range of 20 to 80% RH. However, under high humidity of 70 to 80% RH, the range of transfer current value becomes narrow.

As shown here, the transfer belt is brought into contact with the photosensitive drum by the pre-transfer roller, and the potential of the same polarity as the toner is applied to the pre-transfer roller in comparison with the conventional method. It shows that it has stable transfer performance in a wide environment.

The upper part of the broken line E shown in FIG. 14 is the high-voltage abnormal discharge occurrence region, the lower part of the broken line F is the transfer failure occurrence region, and the wide intermediate region between the broken line E and the broken line F is the transferable region.

In the above-mentioned high voltage abnormal discharge occurrence region, a high voltage is applied to the surface of the transfer belt, the discharge wire voltage of the corona discharger becomes excessive and leaks to the side plate or the like of the corona discharger. However, the above-mentioned high-voltage abnormal discharge generation region changes depending on the shape of the corona discharger, the volume resistivity of the transfer belt, the thickness of the transfer belt, and the like.

FIG. 16 is a characteristic diagram obtained by an experimental result showing the relationship between the bias voltage applied to the pre-transfer roller and the permissible transfer condition by the corona discharger when the inside of the image forming apparatus is under low temperature and low humidity. is there. The experimental conditions are shown below.

V _H = -650 V Recharge: Yes Two-color toner adhesion pattern Transfer paper: Xerox 4024 (20 lb paper) [transfer paper exposed to each environment for 8 hours or more] Environmental conditions: low temperature (15 ° C), low humidity (20% RH) As shown in Fig. 16, the transferable area is wide under low temperature and low humidity, and the bias voltage of the pre-transfer roller applied to the pre-transfer roller and the transfer current value applied to the corona discharger are set in a wide range. It is possible.

FIG. 17 is a characteristic diagram obtained by an experimental result showing the relationship between the bias voltage applied to the pre-transfer roller and the transfer permission condition by the corona discharger when the inside of the image forming apparatus is under high temperature and high humidity. is there. The experimental conditions are almost the same as the case in FIG.

The upper part of the broken line G shown in FIG. 17 is a region where transfer repelling occurs, and the lower part of the broken line H is a region where transfer failure occurs. The narrow intermediate region between the broken line G and the broken line H is a transferable region. is there. The transferable area is extremely narrow under high temperature and high humidity, and therefore the setting of the bias voltage and the corona discharge current value is extremely limited.

As shown here, if the voltage applied to the pre-transfer roller has the same polarity as the toner, the transferable area is gradually sensitized. Moreover, there is an appropriate value for the voltage applied to the pre-transfer roller under high humidity, which indicates that the range of -800V to -1500V is appropriate under the conditions implemented this time.

Under the above environmental conditions of low temperature and low humidity to high temperature and high humidity, the bias voltage to the pre-transfer roller and the current value of the corona discharger when the transfer sheet is passed through the transfer section and when it is not passed. Table 2 shows the specifications.

[0107]

[Table 2]

As shown in Table 2, the bias voltage applied to the pre-transfer roller was controlled to different voltage values when the transfer sheet was passed and when it was not passed (-900 V when the sheet was passed and when the sheet was not passed). -1
500V, -2000V).

The current value applied to the corona discharger is
When the transfer paper was passed, the current value was controlled to be larger than that when it was not passed (350μA, 400μA when passed, 0μ when not passed).
A).

Further, the corona discharger at the time of passing the paper is made to pass a smaller current value at high temperature and high humidity (HH) than at room temperature and normal humidity (NN) and low temperature and low humidity (LL). 400 μA under NN and LL conditions, 350 μA under HH conditions). Also, it is -2000V under NN and LL conditions and -1500V under HH conditions with respect to the pre-transfer roller.
The bias voltage value was controlled so that FIG. 18 is a timing chart showing an image forming process operation of the image forming apparatus in the monochrome mode. Note that this timing chart shows a mode in which five black sheets are continuously printed on the recording material P of A4 size. In this mode, only the black development is performed by the developing device 14K having black toner, and the transfer belt 31 and the cleaning blade 16 are always operating. In addition, the pre-transfer roller (front roller) 32
The bias voltage application may be constantly turned on when the transfer belt 31 is in pressure contact with the image carrier 10, but here it is sufficient to clean the transfer belt 31 when the recording material P does not pass. An electric potential for injecting an appropriate electric charge (having the same polarity as the toner) into the transfer belt 31 is applied. The bias voltage applied to the pre-transfer roller 32 at the on-timing during non-sheet passing is controlled according to environmental conditions, and is -2000 V at normal temperature and normal humidity (NN) and low temperature and low humidity (LL), and high temperature and high humidity (H).
H) At time, switch to -1500V. A bias voltage of -900V is applied during paper passing regardless of the environment inside the machine.

The corona discharger (transfer pole) 34 is turned on only when the recording material P passes through the transfer portion, and the charge on the image carrier 10 is transferred to the recording material P (polarity different from that of the toner). The electric potential for injecting () is applied to the transfer belt 31 is applied. The corona discharger (transferring electrode) 34 at this on timing has 200 μA in the NN and LL environments and 2 in the HH environment.
It is controlled to supply 50 μA. A pre-transfer roller 32 that presses the inside of the transfer belt 31 so that the transfer belt 31 directly contacts the image carrier 10 when the recording material P does not pass through.
A voltage for injecting into the transfer belt 31 a charge (having the same polarity as the toner) enough to prevent the toner on the image carrier 10 from adhering to the transfer belt 31 is applied to the (first electrode).
Further, not only is the transfer of the toner from the image carrier 10 prevented when the recording material P is not passed, but the toner on the transfer belt 31 is simultaneously transferred onto the image carrier 10 (almost completely reverse transfer). Then, the toner of the image carrier 10 is cleaned by a cleaning device.
It is also possible to clean and remove by 16. Further, the voltage applied to the pre-transfer roller 32 is selected in such a range that the gap discharge does not occur at the entrance portion immediately before the transfer belt 31 comes into contact with the image carrier 10 to prevent the occurrence of a pot-like cleaning failure. There is.

Further, in order to prevent an increase in the surface potential of the image carrier 10 due to the direct contact with the image carrier 10 or to prevent a gap discharge, the pre-transfer roller is changed according to the change in the resistivity.
When the voltage applied to 32 is variably controlled, the cleaning system can be further stabilized in all environments even when using a transfer belt whose volume resistivity is highly dependent on humidity, and a wide range of environmental conditions from low humidity to high humidity. Based on this, it is possible to obtain stable transfer performance without transfer cissing and transfer defects.

In the case where the bias voltage is always applied to the pre-transfer roller 32 regardless of whether the paper is passed or not, the second electrode, that is, the corona discharger, is compared with the case where the bias voltage is applied only when the paper is not passed. It is necessary to increase the charge injected from the corona discharger 34 in order to increase the voltage of 34 to cancel the negative charge injected from the pre-transfer roller 32, but there is an advantage that the control is simple. .

FIG. 19 is a timing chart showing the image forming process operation of the image forming apparatus in the full color mode with four color toners. It should be noted that this timing chart shows a mode in which color printing of four colors is continuously performed on the recording material P of A4 size. In this mode, the bias voltage application to the pre-transfer roller 31 is turned on when the passage of the recording material is completed, and turned off when the transfer belt 31 is continuously operated for one rotation or more. Also transfer belt 31
The pressure is released from the front end of the image by pressing the peripheral surface of the image carrier 10 and the bias voltage of the pre-transfer roller 32 is turned on, and after the transfer belt 31 has moved more than one revolution, the bias voltage is It is separated from the image carrier 10 almost at the same time as the turning off. The timing of releasing the pressure of the blade 16a of the cleaning device 16 to the image carrier 10 is started by pressing the upstream side of the image carrier point released by the blade 16a in the previous printing, and the transfer belt 31 is separated from the image carrier point. It is separated at the image carrier point on the further downstream side. In this way, the transfer belt 31
Each print is reliably cleaned. Transfer belt
The toner on the image carrier 10 reversely transferred from 31 is also reliably removed from the image carrier 10 by the blade 16a for each print.

The recording material P separated from the photosensitive drum 10 is
The shaft 33a of the drive roller 33 on the downstream side which stretches the transfer belt 31
Transfer belt 31 after receiving static electricity by AC corona discharge with the counter electrode as the counter electrode, or while receiving AC corona discharge.
Separate from. The transfer belt 31 of the belt transfer device 30 is separated from the photoconductor drum 10 around the shaft 33a of the drive roller 33 on the downstream side as a rotation center during the formation of the multicolor image.

At the bias-on timing at the time of cleaning, it is set to -2000 V at room temperature and normal humidity (NN) and low temperature and low humidity (LL), and switched to -1500 V at high temperature and high humidity (HH). In addition, the corona discharger 34 has 400μ in NN and LL environments.
A, HH environment is controlled so that 350 μA is supplied during paper feeding.

The recording material P holding the multicolor image separated from the belt transfer device 30 is conveyed to a fixing device 23 composed of two pressure bonding rollers having a heater inside at least one of the rollers, and is transferred between the pressure bonding rollers. The applied toner is melted by applying heat and pressure, fixed on the recording paper P, and then discharged to the outside of the apparatus.

After the transfer, the residual toner remaining on the peripheral surface of the photosensitive drum 10 is subjected to static elimination by the static eliminator 15 using an AC corona discharger, and then is moved to the cleaning device 16 or from the rubber material contacting the photosensitive body. It is scraped off into the cleaning device 16 by the cleaning blade 16a, discharged by a screw or the like, and then stored in the recovery box. Depending on the arrangement of the static eliminator 15, the static eliminator 15 can also serve as static eliminator for the recording paper for separating the recording paper closely attached to the transfer belt.

The photosensitive drum 10 from which the residual toner has been removed by the cleaning device 16 is exposed by the PCL 11 and then uniformly charged by the scorotron charger 12.
Enter the next image forming cycle. The cleaning blade 16a is separated from the surface of the photoconductor during the formation of the multicolor image,
AC neutralization by 15 is kept in OFF state.

[0120]

As described above, according to the present invention, it is possible to provide an image forming apparatus in which the cleaning mechanism can be eliminated and the manufacturing cost can be reduced with a simple structure. Further, by doing so, the problem of deterioration of the transfer belt and the cleaning parts is solved, and it is not necessary to process the collected toner, so that the life of the transfer unit can be dramatically improved.

Further, the flying of the toner from the surface of the image bearing member is prevented at the initial stage of the image transfer, so that the occurrence of so-called transfer bleeding is prevented, and as a result, the image repelling occurs on the recording material even in a high humidity environment where the conditions are bad. I was able to transfer a clear color image with no color tone. It should be noted that the effect of the present invention can be obtained similarly even if the second electrode is a conductive brush-shaped electrode or a conductive roller electrode that contacts the transfer belt to apply an electric charge, although the applied voltage value is different. It goes without saying that it will be done.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a color image forming apparatus including a belt transfer device of the present invention.

FIG. 2 is a sectional view of a belt transfer device.

FIG. 3 is a cross-sectional view showing a state where the transfer belt is separated from the surface of the image carrier.

FIG. 4 is a diagram showing a charge distribution of an image carrier, a toner layer, and a transfer belt.

FIG. 5 is a characteristic diagram showing the electric flux density in the layer thickness direction.

FIG. 6 is a characteristic diagram of an injection current to a transfer belt and a toner adhesion amount.

FIG. 7 is a model diagram of electric charges injected from a conductive rubber roller to a transfer belt.

FIG. 8 is an equivalent circuit of charge injection to the transfer belt.

FIG. 9 is a characteristic diagram of volume resistivity and charge density of the transfer belt.

FIG. 10 is a temperature / humidity dependence characteristic diagram of the volume resistivity of the transfer belt.

FIG. 11 is a characteristic diagram of the volume resistivity of the transfer belt and the bias voltage of the pre-transfer roller.

FIG. 12 is a characteristic diagram of the volume resistivity of the transfer belt and the surface potential of the image carrier after transfer.

FIG. 13 is an overall configuration diagram showing a second embodiment of a color image forming apparatus including a belt transfer device of the present invention.

FIG. 14 is a characteristic diagram showing a relationship between humidity conditions and transferability in an image forming apparatus according to a conventional method.

FIG. 15 is a characteristic diagram showing humidity conditions and transferability in the image forming apparatus according to the present invention.

FIG. 16 is a characteristic diagram showing a relationship between a pre-transfer roller voltage and a transfer permission condition under low temperature and low humidity.

FIG. 18 is a timing chart of first and second transfer electrode control in monochrome mode.

FIG. 19 is a timing chart of first and second transfer electrode control in the full color mode.

[Explanation of symbols]

 10 Photoconductor drum (image carrier) 12 Scorotron charger (belt electrode) 122 Band electrode wire 123 Grid 126 Charging power source 127 Grid power source 13 Image exposure means 14Y, 14M, 14C, 14K Developing device 16 Cleaning device 224 Passing through the leading edge of recording material Detection sensor (switch SW) 30 Belt transfer device 31 Transfer belt 32 First electrode (pre-transfer roller, driven roller) 33 Drive roller 34 Second electrode (corona discharger, transfer electrode) 35 Tension roller 37 Insulating member ( Discharge prevention plate) 39 Humidity sensor

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] July 5, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a color image forming apparatus including a belt transfer device of the present invention.

FIG. 2 is a sectional view of a belt transfer device.

FIG. 3 is a cross-sectional view showing a state where the transfer belt is separated from the surface of the image carrier.

FIG. 4 is a diagram showing a charge distribution of an image carrier, a toner layer, and a transfer belt.

FIG. 5 is a characteristic diagram showing the electric flux density in the layer thickness direction.

FIG. 6 is a characteristic diagram of an injection current to a transfer belt and a toner adhesion amount.

FIG. 7 is a model diagram of electric charges injected from a conductive rubber roller to a transfer belt.

FIG. 8 is an equivalent circuit of charge injection to the transfer belt.

FIG. 9 is a characteristic diagram of volume resistivity and charge density of the transfer belt.

FIG. 10 is a temperature / humidity dependence characteristic diagram of the volume resistivity of the transfer belt.

FIG. 11 is a characteristic diagram of the volume resistivity of the transfer belt and the bias voltage of the pre-transfer roller.

FIG. 12 is a characteristic diagram of the volume resistivity of the transfer belt and the surface potential of the image carrier after transfer.

FIG. 13 is an overall configuration diagram showing a second embodiment of a color image forming apparatus including a belt transfer device of the present invention.

FIG. 14 is a characteristic diagram showing a relationship between humidity conditions and transferability in an image forming apparatus according to a conventional method.

FIG. 15 is a characteristic diagram showing humidity conditions and transferability in the image forming apparatus according to the present invention.

FIG. 16 is a characteristic diagram showing a relationship between a pre-transfer roller voltage and a transfer permission condition under low temperature and low humidity.

FIG. 17 shows a voltage applied to the pre-transfer roller under high temperature and high humidity.
FIG. 6 is a characteristic diagram showing a relationship with a transfer permission condition.

FIG. 18 is a timing chart of first and second transfer electrode control in monochrome mode.

FIG. 19 is a timing chart of first and second transfer electrode control in the full color mode.

[Explanation of symbols] 10 photoconductor drum (image carrier) 12 scorotron charger (belt electrode) 122 band electrode wire 123 grid 126 charging power source 127 grid power source 13 image exposure means 14Y, 14M, 14C, 14K developing device 16 cleaning device 224 Recording material front end detection sensor (switch SW) 30 Belt transfer device 31 Transfer belt 32 First electrode (pre-transfer roller, driven roller) 33 Drive roller 34 Second electrode (corona discharger, transfer electrode) 35 Tension roller 37 Insulating member (discharge prevention plate) 39 Humidity sensor

Claims (4)

[Claims] 1. An image forming apparatus including a belt transfer device that transfers a toner image formed on an image carrier to a recording material at a transfer portion and conveys the toner image, the back side of a transfer belt facing the image carrier. A first electrode on the upstream side and a second electrode on the downstream side are arranged so as to intersect in the moving direction of the transfer belt, and when the recording material passes through the transfer portion, the second electrode is arranged. Only, an electric potential having a characteristic opposite to that of the toner is applied to the electric potential of the image carrier, and the first electrode is applied to the electric potential of the image carrier when the recording material is passed or not. An image forming apparatus characterized in that a voltage having the same polarity as that of toner is applied. 2. The voltage applied to the first electrode with respect to the potential of the image carrier is set to different voltage values when the recording material is fed and when the recording material is not fed. Image forming apparatus. 3. The voltage applied to the first electrode with respect to the potential of the image carrier is set to have a larger voltage value with the same polarity as the toner when the recording material is not passed than when the recording material is passed. The image forming apparatus according to claim 1. 4. The non-image area on the image carrier is set to a low electric potential state so that the recording material passes through the transfer portion, according to any one of claims 1 to 3. Image forming apparatus.