JPH08160784A - Color image forming device - Google Patents

Abstract

PURPOSE: To provide a transfer means with the stable transfer holding force by controlling the transfer bias voltage applied in-between the image carrier and the transfer electrode based on the surface potential detection result. CONSTITUTION: With use of the surface potential sensor 5 for detecting the surface potential of the transfer material, the surface potential of the transfer material 3 right before the transfer nip part is read. The surface potential detected by the potential sensor 5 is inputted to the CPU 10 through the measuring circuit 12, and outputted to the transfer bias power source 8, the attraction bias power source 7 or the like as the signal after applying the specified process. Then, by obtaining the prior transfer attraction potential difference, the strength of the attraction force by the attraction bias Va is detected. In the same time, whether the potential difference of the transfer material on the transfer drum 1 to the photosensitive body drum 4 is appropriate or not is judged. That is done by directly comparing the potential measurement value and the transfer drum potential target value. Then, the transfer bias value can be made most preferably by feeding back the difference with the target value to the transfer bias VT.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic color image forming apparatus such as a color printer or a color copying machine, which is characterized by a transfer means.

[0002]

2. Description of the Related Art A color image forming apparatus that obtains a color image on a transfer material by superposing toner images of a plurality of colors forms a toner image on an image carrier by charging, exposing and developing, and the toner image is formed. There is a method of obtaining a color image by repeating a step of transferring onto a transfer material for each color for each color so that toner images of a plurality of colors are superposed on the transfer material.

FIG. 11 shows an example of the color image forming apparatus. As shown in the figure, an electrophotographic photosensitive drum 101 is provided in the apparatus as an image bearing member, a charging roller 102 is provided around the photosensitive drum 101, a rotary developing unit 104 having a plurality of developing devices, and a transfer drum 108 serving as a transfer unit. , Cleaner 103, etc. are installed. Above the photosensitive drum 1, a laser diode 105 forming an exposure device,
Polygon mirror 106, imaging optical system 115, mirror 10
7 etc. are arranged.

The developing means 104 includes four color developing devices 104Y, 104M, and 1 for yellow, magenta, cyan, and black.
04C and 104K, each developing device accommodates toner of each color, and each developing sleeve 116.
A toner layer is formed on top.

Then, according to the latent image pattern formed on the photosensitive drum 101 by the above-mentioned exposure device, the toner of each color is sequentially developed in the exposed portion in the case of the reversal development method and in the non-exposed portion in the case of the regular development method.

On the other hand, the transfer drum 108 has a transfer material 1 on its surface.
Each of the developed color toner images carrying 09 is sequentially transferred from the surface of the photosensitive drum 101 to the surface of the transfer material 109.

The structure of the transfer drum 108 will be described. As the transfer drum 108, conventionally, a hollow dielectric sheet is held in a cylindrical shape, and transfer is performed from the inner surface by using a transfer corona charger, or the inner and outer surfaces are simultaneously corona charged for adsorbing a transfer material and discharging electricity. The so-called hollow transfer drum system was generally used. This method has a large degree of freedom in setting and is stable because each area of transfer, adsorption, and charge removal is independent, but on the other hand, it has a complicated structure and poor durability. There was a flaw.

On the other hand, in the solid transfer drum having the structure shown in FIG. 11 (hereinafter referred to as a transfer drum), an elastic layer 108b is provided on a cylinder 108a made of a metal such as a substrate, and a dielectric layer 108c is formed thereon. With the structure having the above-mentioned structure, the above-mentioned drawbacks of the hollow transfer drum are improved.

Here, it is necessary to provide a transfer electrode made of a conductive material on the back surface of the dielectric layer 108c. This is because a conductive layer is separately provided on the back surface of the dielectric layer 108c, and an appropriate electrode is further provided. The elastic layer 108b itself may be made conductive. Alternatively, a conductive layer may be further provided between the dielectric layer 108c and the conductive elastic layer 108b to prevent the roughness of the sponge surface of the elastic layer from affecting the transfer. It can have a layered structure.

The transfer material 109 is attracted to the transfer drum 108 having the above-mentioned structure, transfer is performed between the transfer drum 108 and the photosensitive drum 101, and finally the surface of the transfer drum 108 is destaticized to electrically transfer the transfer drum 108. A series of steps for initialization is performed by the suction roller 110, the photosensitive drum 101,
The electric field is formed by the bias voltage applied to the charge eliminating roller 111 and the bias voltage applied to the back surface of the dielectric layer 108c of the transfer drum, and the transfer drum 108 is different from the hollow transfer drum system described above. It has a significantly simple structure.

What is important here is that, when electric charges exist on the surfaces of the dielectric layer 108c and the transfer material 109 in advance during the respective steps, the potential difference caused by the electric charges is directly formed by the aforementioned bias. As a result of being added to the electric field, it becomes 2, 3, 4 compared to the conventional first color.
A method of optimizing the transfer by temporarily increasing the transfer bias value by a predetermined amount when advancing the color and transfer has been performed.

As shown in FIG. 11, the transfer drum 10
8 is a grip member 11 for holding the tip of the transfer material.
3, a cleaner 114 for cleaning the surface of the transfer drum 108, and a separation claw 112 for separating the transfer material from the transfer drum 108. Further, in the figure, members marked with a double-headed arrow mark repeat the reciprocating motion in the direction of the arrow.

As described above, the four-color image transferred onto the transfer material 109 carried on the transfer drum 108 is fixed in a fixing device (not shown) to be a permanent image.

[0014]

However, in the above-mentioned conventional color image forming apparatus using the above-mentioned transfer drum, regarding the adsorbing means for pre-adsorbing the transfer material and the optimization of the transfer bias at the time of transfer, the following is described. There was a problem.

(1) When the resistance value or the electrostatic capacity of the dielectric layer 108c of the transfer drum and the resistance value or the electrostatic capacity of the transfer material 109 change depending on the environmental conditions, the value of the attracting current to the transfer material 109 changes. Or, once the supplied charge is greatly attenuated via the transfer electrode, the holding force of the transfer material at the time of pre-adsorption becomes insufficient. This is a phenomenon that also occurs when the type and thickness of the transfer material, the material (paper and overtransparency sheet, abbreviated as OHT sheet, etc.) change. Furthermore, (2) the transfer performance changes significantly depending on the environmental conditions and the type of transfer material, and the optimum transfer bias value also changes.

In particular, regarding the transfer condition (2), the residual charge on the transfer material 109 and the dielectric layer 108C before the transfer is performed, and the adsorption condition (1), that is, the actual charge due to the adsorption. Is also affected by how much the applied charge remains during transfer, so not only the state of the transfer material and the initial state of the dielectric layer,
There is also a situation in which it is difficult to discuss independently of the adsorption condition of (1), and it is very difficult to optimize the transfer bias value.

(3) Static elimination means 111 depending on environmental conditions
The residual charge of the dielectric layer 108c of the transfer drum is not sufficiently removed by the above, and the surface of the dielectric layer is not initialized prior to the pre-adsorption of the transfer material for the next printing, resulting in adsorption failure or transfer failure. On the contrary, there are cases where the static elimination bias value becomes too large and the dielectric is damaged.

Therefore, a first object of the present invention is to provide a color image forming apparatus having a transfer means having a stable transfer material holding force.

A second object of the present invention is to provide a color image forming apparatus having a transfer means to which an optimum transfer bias is always applied.

A third object of the present invention is to provide a color image forming apparatus having a transfer means capable of favorably initializing the residual charge on the surface of the dielectric layer of the transfer means.

[0021]

The above object can be achieved by a color image forming apparatus according to the present invention. In summary, the present invention relates to a visual image forming means for forming an electrostatic latent image on an image bearing member and visualizing the latent image with a toner, and a transfer material is adsorbed to a dielectric layer on the surface of the image forming means. In a color image forming apparatus having a transfer means for sequentially transferring a visible image on the image carrier onto a transfer material by a transfer electrode provided on the back surface of the dielectric layer in contact with the image carrier, the color image forming apparatus faces the transfer means. A color image forming apparatus is characterized in that a surface potential detecting means is provided, and a transfer bias voltage applied between the image carrier and the transfer electrode is controlled based on the surface potential detection result.

It is preferable that the surface potential detecting means is provided on the upstream side in the moving direction of the transfer means with respect to the transfer position where the image carrier and the transfer means face each other.

It is preferable that the surface potential detecting means is provided on the downstream side in the moving direction of the transfer means with respect to the transfer position where the image carrier and the transfer means face each other.

According to another aspect of the present invention, a visible image forming means for forming an electrostatic latent image on the image bearing member and visualizing the latent image with toner, and a transfer material is adsorbed to the surface dielectric layer. And a transfer means for contacting the image bearing body with the transfer electrode provided on the back surface of the dielectric layer to sequentially transfer the visible images on the image bearing body onto a transfer material. Prior to the transfer of a visible image, in a color image forming apparatus having a suction means for pre-sucking a transfer material on the dielectric layer of the transfer means, a surface potential measuring means is provided facing the transfer means, and the surface potential measuring means is provided. A color image forming apparatus is provided, which controls an attraction bias voltage applied between the attraction means and the transfer electrode or a transfer bias voltage applied between the image carrier and the transfer electrode based on a detection result. It

It is preferable that one or a plurality of the surface potential detecting means are provided between the preliminary adsorption position and the transfer position.

It is preferable that the surface potential detecting means is provided downstream of the transfer position in the moving direction of the transfer means.

It is preferable that the surface potential detecting means is provided between the preliminary adsorption position and the transfer position, and at least at two positions downstream of the transfer position in the moving direction of the transfer means.

Discharge means for eliminating residual charge on the surface of the dielectric layer of the transfer means is provided on the downstream side of the transfer position in the moving direction of the transfer means, and potential measuring means for measuring the surface potential of the transfer means is provided. A transfer bias voltage applied between the image carrier and the transfer electrode based on the surface potential measurement result, which is provided on the downstream side of the charge removing unit in the moving direction of the transfer unit, or between the charge removing unit and the transfer electrode. It is preferable to control the static elimination bias voltage applied.

It is preferable to provide an environment sensor for detecting the atmosphere in the vicinity of the main body of the apparatus, and to change each control parameter at the time of controlling the adsorption bias, the transfer bias, or the static elimination bias according to the detection result.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A color image forming apparatus according to the present invention will be described below in more detail with reference to the drawings. In the embodiment described below, the present invention is embodied in the color image forming apparatus shown in FIG. Therefore, detailed description of the overall configuration and functions of the color image forming apparatus will be omitted, and the characteristic part of the present invention will be described.

Example 1 The solid transfer drum 1 shown in FIG.
A foamed conductive EPDM rubber layer 1b having a thickness of about 5 mm and a hardness of about 60 degrees as measured by Asker F is provided on the top, and further covered with a PVdF sheet 1c having a thickness of 75 μm. Here, the volume resistance value of the conductive rubber 1b is about 10
^The PVdF sheet 1c has a volume resistance value of about 10 ¹⁴ Ω · cm to 10 ¹⁵ Ω · cm by dispersing a high resistance substance (for example, PMMA) in the range of several% to several tens%. A material having a dielectric constant of about 8 to 12 was used.

As the photosensitive drum 4, a negatively-charged OP in which a charge transport layer having a thickness of about 25 μm is provided on the charge generation layer.
Using a C drum, the contact pressure at the transfer section is about 500g in total pressure.
To about 1500 g.

Next, a suction roller 2 is used as a suction means, and the suction roller 2 is kept in contact with and separated from the transfer drum 1 in the direction of the arrow. Same polarity as T (ie minus)
The bias voltage is applied. As the suction roller 2, a conductive EPDM rubber having a Asker C hardness of 20 to 40 degrees (volume resistance value of 10 ⁵ Ω · cm or less) was used.

Further, the charge eliminating roller 6 has the same structure as that of the attraction roller 2, and the structure is such that the AC bias power source 9 is used between the charge removing roller 6 and the transfer drum 1 as shown in FIG.

First, the mechanism for pre-sucking the transfer material 3 onto the transfer drum 1 by the suction roller 2 will be described. 2 and 3 show an equivalent circuit at this time,
First, when the transfer paper 3 comes to the abutting portion of the suction roller 2 while the front end of the transfer paper 3 is held by the gripper 11, a closed circuit as shown in FIG. 2 is formed.

Here, V _a is the applied bias of the attraction roller 2, C _P is the electrostatic capacity of the transfer material 3, R _P is the same resistance, C _T is the electrostatic capacity of the dielectric sheet 1C, and R _T is the same resistance. (Both at the nip portion).

Next, FIG. 3 shows an equivalent circuit after the transfer material 3 has passed through the nip portion of the suction roller 2. Here, V _S1 indicates the surface potential of the transfer material 3 immediately after passing through the nip, and normally, there is a discharge threshold V _{TH in the} contact charging, so | V _S1 | = | V _a | −V _TH ... (1) The discharge threshold value V _TH is determined by the combined capacity of C _P and C _T , but in this embodiment, it was about 500 V to 1000 V (in absolute value).

On the other hand, while the transfer material rotates from the attracting position to the surface facing the photosensitive drum, the charge applied to the surface of the transfer material is attenuated at the attracting portion via the resistances R _P and R _T. This attenuation occurs in both the transfer material and the dielectric sheet, but generally R
_{Since P} <R _T and C _P R _P <C _T R _T , the attenuation at the transfer material portion is faster.

Thus, the surface potential of the transfer material 3 is V _S1 →
Attenuate to V _S2 (| V _S1 |> | V _S2 |). However, in the above description, the bias voltage of the transfer drum 1 is set to 0 for simplification.
It was set to V. Actually, the surface potential V _S2 rises by the transfer bias during transfer.

Here, the transfer material 3 faces the photosensitive drum 4, and when the toner T is transferred, the transfer material 1 is more firmly fixed.
Since it is electrostatically adsorbed to the dielectric sheet 1c, the above-mentioned pre-adsorption operation is only for holding the transfer material in the course of conveyance to the transfer portion and for preventing the displacement at the transfer nip portion. Therefore, it suffices to set the adsorption bias V _a so that the above-mentioned surface potential V _S2 becomes a predetermined value or more.

However, since the surface potential V _S2 changes depending on the threshold value V _TH of the charging at the adsorption portion and the attenuation of the charge to the transfer portion, the change of V _TH due to the capacitance change of the conventional transfer material and the resistance change (similarly). In this case, the dielectric sheet 1c also has a resistance reduction of about one digit under high humidity), which may cause adsorption failure, or conversely, V _S2 may become too high and transfer may be adversely affected. It was

Next, the transfer potential V _{S2 at the} transfer nip portion
The relationship between the value of and the transfer efficiency will be described. In order to transfer the toner T from the photosensitive drum 4 onto the transfer material of the transfer drum 1, it is necessary to shift the surface potential of the transfer material 3 with respect to the photosensitive drum 4 to the opposite polarity side to the toner. By the way, when a transfer bias V _T having a polarity opposite to that of the toner T, that is, a positive transfer bias V _T is applied to the transfer drum, the surface potential of the transfer material 3 is changed from V _S2 to V _T + V.
Change to _S2 . (Where V _S2 is a negative value).

On the other hand, since the photosensitive drum 4 is a negative polarity OPC drum, in the case of the reversal development system in which the toner T is negatively charged by the charging roller 13 and the toner T is placed on the exposed portion irradiated with the laser exposure 14, When the potential of the riding portion and V _L, as shown in FIG. 4, between the surface and the V _L portion of the transfer _{material, V = V T + V S2} -V L becomes a potential difference is generated. Here, since V _L is a negative value, when a capacitor model such as the circuit diagram of FIG. 5 is assumed, the apparent applied voltage V is V = V _T − | V _S2 | + | V _L | (2) (In FIG. 5, C _d is the photosensitive drum 4, C _p is the transfer material,
C _T is the electrostatic capacitance at each transfer nip portion of the dielectric sheet 1C. Here, in the same model, the transfer efficiency when the toner T is transferred onto the transfer material 3 is the magnitude of the amount of electric charge induced when this voltage V is applied to the model of the series capacitor as shown in FIG. If it is too small, the transfer efficiency is reduced due to abnormal discharge or retransfer. (To be precise, the abnormal discharge is caused by an electric discharge in the air due to an electric field near the transfer nip). Thus, as is clear from the equation (2), the V _T value for obtaining the proper potential difference V changes depending on the adsorption bias voltage V _a during adsorption.

Next, the present invention will be described through the operation of the transfer drum which is a characteristic part of the present invention. First, as shown in FIG. 1, a surface potential sensor 5 for detecting the surface potential of the transfer material.
Is used to read the surface potential V _ST of the transfer material 3 immediately before the transfer nip portion. The surface potential V _ST detected by the potential sensor 5 is input to the CPU 10 via the measurement circuit 12, and after being subjected to predetermined processing, is output to the transfer bias power source 8, the suction bias power source 7, etc. as a signal.

At this time, the transfer bias V _T is applied with the initial value V _T0 as shown in the sequence of FIG. next,
If the transfer bias is 0 V, and the surface potential of the transfer material 3 generated by the adsorption charge is V _S2 (described above), then | V _S2 | = | V _ST −V _T0 | . By _obtaining this | V _S2 | (hereinafter referred to as the pre-transfer attraction potential difference), the strength of the attraction force by the attraction bias V _a can be detected. At the same time, it is determined whether V _ST + | _VL |, that is, the potential difference between the photosensitive drum 4 and the transfer material on the transfer drum 1 is appropriate. (Assuming that the potential V _L of the photosensitive drum 4 is a known value, the potential measurement value V _ST and the transfer drum potential target value may be directly compared.) Then, the difference from the target value is fed back to the transfer bias V _T. Therefore, the transfer bias value can be optimized.

In this embodiment, the transfer drum 1 has a diameter of 16
The experiment was conducted with 0 mm, the diameter of the photosensitive drum 4 set to 40 mm, and the process speed set to about 100 mm / sec. Further, the potential sensor 5 was placed about 20 mm before the transfer nip portion, and this was set as a substantially transfer upstream nip potential. First, when the pre-transfer adsorption potential difference | V _S2 | for optimum adsorption was determined, it was found that | V _S2 | = about 250 V is preferable. When | V _S2 | becomes smaller than this, the adsorption becomes weaker, and when it becomes larger, the adsorption bias V _a itself becomes too large and _a discharge pattern is formed on the surface of the transfer material 3, which disturbs the image at the time of transfer. Occurs.

Next, when the optimum target value V _ST0 of the surface potential V _ST of the transfer material 3 including the transfer bias is obtained, V _ST0
Good transfer could be performed at about +800 V. If V _ST is smaller than this, transfer defects are likely to occur, and if V _ST is too large, abnormal discharge patterns occur. Therefore, when using a standard transfer material 3 weighing 80 g / m ² at room temperature and normal humidity, | V _S2 | = 25
When the adsorption bias value V _a that becomes 0 V is obtained, it is approximately −9.
Since it was found to be 00V, as an initial value, V _a0
= -900V, V _T0 = + 1050V (according to the equation (3).
However, V _S2 is a negative value), and the suction bias and the transfer bias are controlled at the timings shown in FIG.

As a result, even if the environmental conditions and the type of transfer material are changed, the surface potential (ie, transfer potential) V _ST of the transfer material 3 is immediately V which is the optimum target value by controlling the transfer bias V _{T. ST0} is controlled to +800 V, and the attraction bias V _a at the next printing can be controlled so that the attraction potential difference before transfer │V _S2 │ is about 250 V. You can now get images.

The sequence of FIG. 6 shows a method of gradually increasing the transfer biases of the second to fourth colors by applying a bias value of about 200 V to the preceding color. Of course, for the fourth color, _VST may be measured in the same manner as for the first color, and the transfer bias value of each color may be controlled so that this becomes a predetermined amount.

According to a study made by the present inventors, after the second color, the surface potential V _ST (+8 in this example) similar to that of the first color is obtained.
It has been found that it is better to set it to 00V). In addition, V _ST
Dielectric sheet 1 but there needs to be gradually increased transfer bias V _T in the second color to fourth color spite of identical
This is because electric charges are accumulated in c and the transfer material 3 to form a reverse electric field.

Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIG. The present embodiment is characterized in that two surface potential sensors 15 and 16 are arranged between adsorption and transfer, and the potential of the transfer material surface is measured at two points immediately after adsorption and immediately before transfer.

In this way, by measuring the potentials at a plurality of points and finding the difference, it is possible to know the charge decay state in the transfer material 3 and the insulating sheet 1c. In the first embodiment, the potential in the vicinity of the transfer nip is treated as the potential of the transfer nip portion. However, since the potential decay state of the charge can be known by the potential sensors 15 and 16, the potential of the transfer nip portion is more accurate. In addition, when the attenuation amount exceeds a predetermined upper limit value, it is possible to give a warning that the transfer material 3 or the insulating sheet 1c is abnormal.

Third Embodiment Next, a third embodiment of the present invention will be described with reference to FIG.

In the above-described first embodiment, the transfer efficiency is changed to the photosensitive drum 4, the dielectric sheet 1c, and the transfer material 3 with reference to FIG.
It was explained that it depends on the capacitance of the. Among these constituent members, the photosensitive drum 4 is rubbed with a cleaning member (not shown) or the like for cleaning the surface of the photosensitive drum 4 for a long period of time,
The thickness of the CTL layer, which is the surface layer portion, gradually decreases. As a result, the electrostatic capacitance Cd in FIG. 5 may increase and electric charges may be excessively supplied to disturb the image due to abnormal discharge or reduce transfer efficiency due to retransfer.

Therefore, as shown in FIG. 8, the potential on the transfer drum 1 downstream of the transfer nip portion is detected by the potential sensor 19 and input to the CPU 10 via the measuring circuit 12, and the detection result becomes an appropriate value. By controlling the transfer bias V _T in this manner, the above-mentioned problem can be prevented.

Regarding the potential measurement timing, the potential on the dielectric layer 1c is measured by applying the transfer bias V _T before the transfer process and without the transfer material 3 (for example, during the so-called pre-rotation process). Alternatively, the same measurement may be performed on the transfer material 3.

According to the former method, the control of the transfer voltage can be completed before the printing of the first sheet. On the other hand, if the latter method is used, the transfer voltage can be adjusted to 2 depending on the printing result of the first sheet.
Although it is a follow-up method of controlling the transfer voltage of the first sheet (or the second color of the first sheet), there is an advantage that the transfer bias V _T can be controlled in consideration of the change of the electrostatic capacity of the transfer material 3. .

Further, instead of the configuration of FIG. 8, potential sensors 20 and 21 are respectively arranged before and after the contact nip portion between the photosensitive drum 4 and the transfer drum 1 as shown in FIG. By inputting to the CPU 10 via 22 and 23, the control of the suction bias power supply 7 described in the first embodiment and the control considering the photosensitive drum film thickness change described in the third embodiment can be used together. it can.

In this case, regarding the pre-transfer attraction potential difference | V _S2 | described in the first embodiment, the attraction bias V _a based on the measurement result of the potential sensor 20 is completely the same as in the first embodiment.
Can be determined.

On the other hand, regarding the optimization of the transfer bias V _T , for example, a predetermined transfer bias is applied during the pre-rotation step, the potential V _PT on the dielectric layer 1c is measured by the potential sensor 21, and it is carried out according to the value. Control target value V _ST0 of the surface potential V _ST (measured by the potential sensor 20) of the transfer material described in Example 1
(+ 800V in the first embodiment) may be changed.
That is, for example, when the film thickness of the photosensitive drum 4 becomes thin, the potential V _PT on the dielectric layer 1c tends to increase accordingly, and the transfer current tends to flow easily. Therefore, if V _{ST 0} is changed to a small value. Good. And the potential sensor 20
The transfer bias V _T may be changed so that the surface potential measurement value V _{ST in the above step} becomes the control target value V _ST0 , as in the first embodiment.

Furthermore, in FIG. 9, the environment sensor 2
4, the control parameters such as the target value of | V _S2 | and the target value V _ST0 of the surface potential V _ST may be changed according to the environment. (Such a configuration can be similarly implemented in the first and second embodiments). In this case, for example, when the absolute humidity (= absolute water content) is small, each target value is increased, and when the absolute humidity is large, the target value is decreased to obtain a good result. Environmental sensor 24
For example, there is a method of using both a temperature sensor and a relative humidity sensor to calculate the absolute water content.

Fourth Embodiment Next, a fourth embodiment of the present invention will be described with reference to FIG. The present embodiment is characterized in that the potential sensor 24 is arranged downstream of the charge eliminating roller 6.

Even in such a configuration, if the amount of potential attenuation from the transfer nip portion to the position of the potential sensor 24 is taken into consideration in advance, the same method as described with reference to FIG. 7 of the third embodiment is used. It is possible to properly control the transfer bias V _T.

Furthermore, in this configuration, it is possible to measure the surface potential of the transfer drum 1 after the charge removal when the charge removal roller 6 is applied to the transfer drum 1. As a result, the following effects can be obtained. That is, as an output of the high-voltage power supply 9 for static elimination, for example, an AC bias voltage of 3 KV _PP (peak-to-peak voltage) and about 500 Hz is required. However, the voltage V _PP of a magnitude required for removing this voltage value depends on the environmental condition and the durability of the dielectric layer 1c.
The amount of charge remaining on the dielectric layer 1c before static elimination greatly differs.

However, for example, in terms of environmental conditions, the voltage V _PP is small at high humidity and needs to be large at low humidity, but if applied excessively at high humidity, the dielectric layer 1c will be damaged. The resistance value lowers, which causes an adverse effect such as a suction failure of the transfer paper 3. Therefore, the potential sensor 24 measures the potential after static elimination, and this is ± 1.
The above problem can be prevented by controlling the output of the AC bias power supply 9 to a required minimum amount so that the value is within 00V.

The potential after the charge removal is the transfer bias V _T.
When 0V is set to 0V, it means that static electricity is eliminated because it becomes approximately 0V as described above. However, when the transfer bias V _T (V) is applied, the surface potential also becomes approximately V _T (V). It will represent the state of being made.

In this embodiment, the control of the transfer bias, which is performed in the same manner as in the third embodiment, is performed during the pre-rotation, for example. On the other hand, the control of the static elimination bias described above is performed during the post-rotation, for example. It is possible to run together.

Although the present invention has been described above according to the mode of embodiment, the present invention is not necessarily limited to the solid transfer drum having the structure shown in the embodiment. For example, as a transfer electrode, a conductive elastic material is used. Instead of using a layer, a conductive thin film may be formed on the back surface of the dielectric layer and used as a transfer electrode. Further, instead of the attraction roller or the static elimination roller, a corona charger may be used to perform the attraction charging and the static elimination charging, respectively. In this case, it goes without saying that the DC or AC corona high voltage may be changed based on the surface potential of the solid transfer drum.

[0069]

As described above, according to the color image forming apparatus of the present invention, the surface potential detecting means is provided so as to face the transfer means, and based on the result of the surface potential detection, the distance between the image carrier and the transfer electrode is increased. By controlling the transfer bias voltage applied to, the transfer unit can have a stable transfer material holding force. Further, the optimum transfer bias can be always applied. Furthermore, the residual charge on the surface of the dielectric layer of the transfer means can be initialized well. Therefore, according to the color image forming apparatus of the present invention, a high quality image can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of a transfer unit according to the present invention.

FIG. 2 is a circuit diagram for explaining a mechanism in the case where the transfer material is pre-sucked on the transfer drum by the suction roller in FIG.

FIG. 3 is an equivalent circuit diagram after the transfer material has passed through the nip portion of the suction roller in FIG.

FIG. 4 is an explanatory diagram showing a relationship between a photosensitive drum, a transfer material, and a transfer drum during transfer.

FIG. 5 is a circuit diagram showing a model of a capacitor.

FIG. 6 is a time chart showing a transfer sequence.

FIG. 7 is an explanatory diagram showing a second embodiment of a transfer unit.

FIG. 8 is an explanatory diagram showing a third embodiment of a transfer unit.

FIG. 9 is an explanatory diagram showing a modified example of the third embodiment.

FIG. 10 is an explanatory diagram showing a fourth embodiment of the transfer means.

FIG. 11 is a schematic configuration diagram showing a conventional color image forming apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Solid transfer drum (transfer means) 1a Cylinder (transfer electrode) 1b Conductive elastic layer 1c Dielectric layer 2 Adsorption roller 3 Transfer material 4 Photosensitive drum (image carrier) 5, 15, 16, 19, Potential sensor (surface potential detection means) ) 6 charge eliminating rollers 20, 21, 24 potential sensor (surface potential detecting means) 24 environment sensor 104 rotary developing device (visual image forming means)

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takao Kume 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Toshiaki Miyashiro 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Incorporated (72) Inventor Takehiko Suzuki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (9)

[Claims] 1. A visible image forming means for forming an electrostatic latent image on an image carrier and developing the latent image with toner, and a transfer material is adsorbed on a dielectric layer on the surface of the image carrier. In a color image forming apparatus having a transfer means for sequentially transferring the developed image on the image carrier onto a transfer material by a transfer electrode provided on the rear surface of the dielectric layer in contact with the A color image forming apparatus, characterized in that detection means is provided, and a transfer bias voltage applied between the image carrier and the transfer electrode is controlled based on the surface potential detection result. 2. The color image forming method according to claim 1, wherein the surface potential detecting means is provided on an upstream side in a moving direction of the transfer means with respect to a transfer position where the image carrier and the transfer means face each other. apparatus. 3. The color image forming apparatus according to claim 1, wherein the surface potential detecting means is provided on a downstream side in a moving direction of the transfer means with respect to a transfer position where the image carrier and the transfer means face each other. . 4. An image forming means for forming an electrostatic latent image on an image carrier and developing the latent image with a toner, and a transfer material is adsorbed on a surface dielectric layer, and the transfer material is adsorbed on the image carrier. A transfer means for sequentially transferring the developed image on the image carrier onto a transfer material by a transfer electrode provided on the back surface of the dielectric layer in contact with the transfer layer, and before transferring the developed image by the transfer means, In a color image forming apparatus having a suction means for pre-sucking a transfer material on a dielectric layer of the transfer means, a surface potential measuring means is provided facing the transfer means, and the suction means is based on the surface potential detection result. And a transfer bias voltage applied between the image carrier and the transfer electrode, or a suction bias voltage applied between the transfer electrode and the transfer electrode. 5. The color image forming apparatus according to claim 4, wherein one or a plurality of the surface potential detecting means are provided between the preliminary suction position and the transfer position. 6. The color image forming apparatus according to claim 4, wherein the surface potential detecting means is provided downstream of the transfer position in the moving direction of the transfer means. 7. The surface potential detecting means is provided between the pre-adsorption position and the transfer position and at least at two locations downstream of the transfer position in the moving direction of the transfer means. Color image forming apparatus. 8. A charge eliminating means for eliminating residual charges on the surface of the dielectric layer of the transfer means is provided on the downstream side of the transfer position in the moving direction of the transfer means, and a potential measurement for measuring the surface potential of the transfer means. Means is provided downstream of the discharging means in the moving direction of the transfer means, and a transfer bias voltage applied between the image carrier and the transfer electrode based on the surface potential measurement result, or the discharging means and the transfer electrode. The color image forming apparatus according to claim 1 or 4, wherein a charge eliminating bias voltage applied between them is controlled. 9. An environment sensor for detecting an atmosphere in the vicinity of the main body of the apparatus is provided, and each control parameter at the time of controlling the adsorption bias, the transfer bias, or the static elimination bias is changed according to the detection result. 1, 4, or 8 color image forming apparatus.