JP2003295587A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control fogging by making the mirror image-force of toner not act on an independent electrode, in an image forming apparatus which forms an electrostatic latent image on an image carrier by a writing electrode using the image carrier having an independent floating electrode layer on the surface. <P>SOLUTION: The image forming apparatus is provided with at least the image carrier 2 which has the independent floating electrode layer 2d on the surface, a writing head 3 which writes the electrostatic latent image on the latent image carrier, and a developing device 4 which develops the electrostatic latent image on the image carrier. When electric resistance of the independent electrode of the independent floating electrode layer 2d is set to R1, and electric resistance of a developer carrier 4a of the developing device 4 is set to R2, and a relation R1>R2 is satisfied, development is carried out with a non-contact type development method. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming an image by forming an electrostatic latent image on an image carrier by a write electrode of a write head.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrostatic copying machine or a printer, the surface of a photosensitive member is generally charged uniformly by a charging device, and a laser beam is applied to the surface of the uniformly charged photosensitive member. Alternatively, an electrostatic latent image is formed on the surface of the photoconductor by exposing light from an exposure device such as LED lamp light. Then, the electrostatic latent image on the surface of the photoconductor is developed by a developing device to form a toner image on the surface of the photoconductor, and the toner image is transferred to a transfer material such as paper by a transfer device to form an image. There is.

In such a conventional general image forming apparatus, the image forming apparatus is large in size because the exposure device, which is a device for writing an electrostatic latent image, is composed of a laser light or LED lamp light generator. It has a complicated structure.

Therefore, as an electrostatic latent image writing device, an image forming apparatus for writing an electrostatic latent image on the surface of an image carrier by a writing electrode without using laser light or LED lamp light is disclosed.
01-287396, and the present applicant has filed a patent application in Japanese Patent Application No. 2001-227630.

FIG. 1 shows the Japanese Patent Application No. 2001-227630.
It is a figure which shows typically the basic composition. This image forming apparatus 1 is an image carrier having a charging body layer 2b which is provided on the outer periphery of a base material 2a which is made of a conductive material and which is grounded, and which has an insulating property, and on which an electrostatic latent image is formed. 2 and a flexible base material 3a having a high insulating property and being relatively soft and elastic
Written for writing an electrostatic latent image on the charged body layer 2b, which is supported on the charged body layer 2b and is brought into surface contact with the charged body layer 2b of the image carrier 2 by being lightly pressed by the weak elastic restoring force due to the bending of the base material 3a. At least a writing head 3 having an electrode 3b, a developing device 4 having a developing roller 4a which is a developer carrier, and a transfer device 6 having a transfer roller 6a which is a transfer member are provided.

In the image forming apparatus 1 thus constructed, after the charged layer 2b of the image carrier 2 is brought into a uniform charge state, the write voltage is applied to the write electrode 3b by the IC of the write electrode 3b.
An electrostatic latent image is generated by the charge transfer (for example, charge injection) between the image carrier 2 and the write electrode 3b of the write head 3 which are applied via the driver 11 and are in surface contact with each other. Writing is performed on the image carrier 2 in a uniform charge state. Then, the electrostatic latent image on the image carrier 2 becomes the charged body layer 2 of the image carrier 2.
written on b. Then, the electrostatic latent image on the charged body layer 2b is developed by the developer conveyed by the developing roller 4a of the developing device 4, and the developer image is transferred onto paper or the like by the transfer roller 6a to which a transfer voltage is applied. It is transferred to the material 5.

By the way, from the write electrode 3b to the charged body layer 2b.
There is a problem that the electric charges injected into the battery easily leak inside the charging layer 2b. Therefore, as shown in FIG. 2, the charging layer 2b is exposed to the dielectric layer and the surface layer of the dielectric layer. It can be considered to be composed of a large number of independent electrodes. In this case, at the time of image writing, for example, a positive write voltage V ₁ is applied from the write electrode 3b to the independent electrode on the surface layer of the dielectric layer, and the negative charge of the dielectric layer is dielectrically polarized around the independent electrode. Image writing is performed, and a predetermined charge can be held between immediately after image writing with the writing voltage to the independent electrode and until development, and the electrostatic latent image is developed by the developing device. To be done.

[0008]

However, when the independent electrode is provided on the surface layer of the dielectric layer as described above, both the independent electrode and the developing roller are conductors, and the toner is pressed against the independent electrode and the developing roller. Then, the image force of the toner may act on the independent electrode. As a result, fogging is likely to occur, and the toner on the image bearing member bound by such strong energy causes problems such as transfer residual and cleaning residual.

The present invention has been made to solve the above-mentioned problems and is an image forming apparatus for forming an electrostatic latent image on a write electrode by using an image carrier having an independent floating electrode layer on its surface. In the above, it is an object of the present invention to provide an image forming apparatus capable of suppressing fog by preventing the mirror image force of toner from acting on the independent electrode.

[0010]

In order to achieve the above object, an image forming apparatus according to claim 1 of the present invention comprises an image carrier having an independent floating electrode layer on the surface thereof, and the latent image carrier having the image carrier. In an image forming apparatus including at least a writing head for writing an electrostatic latent image and a developing device for developing the electrostatic latent image on the image carrier, an electric resistance of an independent electrode of the independent floating electrode layer is set to R1. And R2 is the electric resistance of the developer carrier of the developing device, R1
When there is a relation of> R2, the development is carried out by a contact development system. According to the invention of claim 2, in the writing head and the developing device, the writing head and the developing device are of black, yellow, magenta and cyan. 4. The image forming apparatus is provided for each color, and the writing head for each color and the developing device for each color are formed to be superimposed on each other on the image bearing member. According to the invention described in claim 1, the image carrier, the writing head, and the developing device are provided for each color of black, yellow, magenta, and cyan, so that the image forming apparatus for each of these colors is tandem. According to a fourth aspect of the present invention, there is provided an intermediate transfer device for intermediately transferring a developer image for each color by the image forming device for each color. It is characterized in that is.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. 3A and 3B show an embodiment of the image forming apparatus of the present invention, FIG. 3A is a diagram schematically showing a basic configuration, FIG. 3B is a perspective view showing a specific configuration of FIG. 4 is an enlarged view partially and schematically showing the image carrier of FIG.

As shown in FIG. 3, the image forming apparatus 1 is provided with an insulating property on the outer circumference of a base material 2a which is made of a conductive material such as aluminum and is grounded. Image carrier 2 having a charged body layer 2b formed
And FPC (Flexible Print Circuit) or PE
The charged body layer 2b of the image carrier 2 is supported by a flexible base material 3a having a high insulating property such as T (polyethylene terephthalate) and is relatively soft and elastic, and is weakly elastically restored by the bending of the base material 3a. A writing head 3 having a writing electrode 3b for writing an electrostatic latent image on the charged body layer 2b, which is lightly pressed upward to make surface contact, and a developing roller (developer carrying body) 4a which is a developer carrying body. And a transfer device 6 having a transfer roller 6a as a transfer member.

The charging body layer 2b is a dielectric layer 2 which is an insulating layer.
c and an independent floating electrode layer 2d which is an image writing portion provided on the surface of the dielectric layer 2c. As shown in FIG. 4, the independent floating electrode layer 2d is composed of a large number of independent electrodes 2d ₁ arranged so as to be exposed on the surface of the dielectric layer 2c. These independent electrodes 2d ₁ are electrically independent from each other, and the dielectric layer 2c
It is formed in a sea-island structure exposed from the surface. In addition,
FIG. 3 shows the dielectric layer 2c and the independent floating electrode layer 2d.
Are shown as being partitioned, but this is only partitioned for convenience of explanation, and as clearly shown in FIG. 4, the dielectric layer 2c and the independent floating electrode layer 2d are clearly partitioned. The floating electrode layer 2d is not a part of the surface of the dielectric layer 2c where many independent electrodes 2d ₁ are present.

Then, at the time of image writing, for example, a positive voltage applied via the IC driver 11 is applied from the write electrode 3b to the independent floating electrode layer 2d as the write voltage V ₁ , and this independent floating electrode layer is applied. Image writing is performed on the independent floating electrode layer 2d by charging positive charges to the image writing portion 2d.

Materials for the dielectric layer 2c include polycarbonate, polyethylene, polypropylene, polystyrene, poly-p-xylylene, polyvinyl acetate, polyacrylate, polymethacrylate (methacrylic resin), polyvinyl chloride, polyvinylidene chloride, polyvinyl ether. , Polyvinylketone, polyether, thermoplastic polyester, polyamide, silicone or the like is used alone, or two or more materials are used, but not limited thereto as long as the materials have similar electric characteristics.

The independent floating electrode layer 2d is obtained by dispersing the resin of the above-mentioned dielectric layer 2c and a large number of conductive fine particles in a solvent by adjusting the mixing ratio (concentration) of the resin and diluting. After applying to the surface of the body layer 2c by a general appropriate method such as a spray coating method or a dipping method,
A large number of independent electrodes 2d ₁ are exposed on the surface layer by puff polishing, sandblast polishing, solvent etching or the like. In that case, the surface smoothness is improved, the contact resistance with the write electrode 3b is reduced, and the abrasion between the write head 3 and the charging body layer 2b is reduced. Alternatively, conductive fine particles may be regularly arranged in a matrix on the surface of the dielectric layer 2c by a screen printing method, an inkjet method, or the like.

Materials for the conductive fine particles include Cu, Al, Ni, Ag, C, Mo, W, Fe and Ta.
Etc., fine particles of metal such as ZnO (zinc oxide), tin oxide, antimony oxide, titanium oxide, etc. (for example, those obtained by conducting conductivity by doping with antimony, indium, etc.), polyacetylene, polythiophene, polypyrrole Conductive fine particles obtained by doping iodine into a polymer complex, etc. are used, but not limited thereto as long as they are conductive fine particles having a resistance component of.

In the image forming apparatus 1 thus constructed, the charge voltage is applied to the write electrode 3b after the write voltage is applied to the write electrode 3b after the charge layer 2b of the image carrier 2 is brought into a uniform charge state.
An electrostatic latent image is generated by the charge transfer (for example, charge injection) between the image carrier 2 and the write electrode 3b of the write head 3 which are applied via the driver 11 and are in surface contact with each other. Writing is performed on the independent floating electrode layer 2d having a uniform charge state. Then, the independent floating electrode layer 2d
The upper electrostatic latent image is developed by the developer conveyed by the developing roller 4a of the developing device 4, and the developer image is transferred to the transfer material 5 such as paper by the transfer roller 6a to which the transfer voltage is applied. .

FIG. 5 is a diagram showing a basic process of image formation in the image forming apparatus 1 of FIG. As the basic process of image formation in the image forming apparatus 1 of the present invention, (1)
Uniform charge removal-contact charge writing-regular development, (2) Uniform charge removal-contact charging write-reverse development, (3) Uniform charge-contact charge writing-
There are four image forming processes of normal development and (4) uniform charge removal-contact charge removal writing-reversal development. Hereinafter, these image forming processes will be described. (1) Uniform charge removal-contact charging writing-normal development As an example of this image forming process, as shown in FIG.
Is used, and a charge eliminating roller 7b is used as the image carrier uniform charge control device 7. The charge of the independent floating electrode layer 2d is removed by the charge removing roller 7b, and the charge on the dielectric 2b is brought to a uniform charge state close to 0V. Writing electrode 3 of writing head 3
b is configured to write an electrostatic latent image on the independent floating electrode layer 2d by contacting the independent floating electrode layer 2d and (+) charging the image portion of the independent floating electrode layer 2d. A (-) DC bias voltage, for example, is applied to the developing roller 4a of the contact developing type developing device 4 as in the conventional case. It is also possible to apply a bias voltage in which AC is superimposed on (-) DC to the developing roller 4a. Further, an AC bias voltage is applied to the charge eliminating roller 7b. (2) Uniform charge removal-contact charging writing-reversal development As an example of this image forming process, there is a process shown in FIG. In this image forming process, the independent floating electrode layer 2d is used as the image carrier 2 as in the example shown in FIG. 7A, and the charge eliminating roller 7b is used as the image carrier uniform charge control device 7. There is. Further, the write electrode 3b of the write head 3 comes into contact with the dielectric 2b to charge the non-image portion of the dielectric 2b (-). The other configuration of the image forming process of this example is the same as the example shown in FIG.

In the image forming process of this example, the charge eliminating roller 7b is in contact with the independent floating electrode layer 2d, and the charge of the independent floating electrode layer 2d is eliminated by this charge eliminating roller 7b so that the upper surface of the independent floating electrode layer 2d is removed. , A uniform charge state close to 0 V with the charge removed. The subsequent image forming operation is the same as the example shown in FIG. (3) Uniform charging-contact charge elimination writing-normal development As an example of this image forming process, there is a process shown in FIG. In this image forming process, the independent floating electrode layer 2d is used as the image carrier 2 and the charging corona discharger 7d is used as the image carrier uniform charge control device 7. Although not shown, the charging corona discharger 7d is applied with a (−) DC bias voltage or a (−) DC direct current with an alternating current superimposed thereon as in the conventional case. In addition, the write electrode 3b of the write head 3 comes into contact with the independent floating electrode layer 2d to eliminate the (−) charge in the non-image portion of the independent floating electrode layer 2d. Further, a (+) DC bias voltage is applied to the developing roller 4a, so that the developing roller 4a conveys the developer 8 charged to (+) to the independent floating electrode layer 2d. There is.

In the image forming process of this example, the independent floating electrode layer 2d is negatively charged by the charging corona discharger 7d to a uniform charge state of a predetermined voltage, and then the writing head 3 writes. The negative electrode 3b removes the (-) charges in the non-image portion on the independent floating electrode layer 2d, and an electrostatic latent image is written on the independent floating electrode layer 2d. Then, the (+) charged developer 8 conveyed by the developing roller 4a of the developing device 4 adheres to the (-) charged image portion of the independent floating electrode layer 2d to normally develop the electrostatic latent image. . (4) Uniform charging-contact charge elimination writing-reverse development As an example of this image forming process, there is a process shown in FIG. In this image forming process, the independent floating electrode layer 2d is used as the image carrier 2, and the charging corona discharger 7d is used as the image carrier uniform charge control device 7. Although not shown, a bias voltage of (+) DC or a bias voltage in which AC is superimposed on (+) DC is applied to the charging corona discharger 7d as in the conventional case.

In the image forming process of this example, the independent floating electrode layer 2d is (+) charged by the charging corona discharger 7d to a uniform charge state of a predetermined voltage, and then the writing head 3 writes. The (+) charges in the image portion on the independent floating electrode layer 2d are removed by the built-in electrode 3b, and an electrostatic latent image is written on the independent floating electrode layer 2d. Then, the (+) charged developer 8 conveyed by the developing roller 4a of the developing device 4 becomes the dielectric 2b.
The electrostatic latent image is reversely developed by adhering to the (+) non-charged image area.

FIG. 6 illustrates the principle of writing an electrostatic latent image by charging or discharging the writing electrode 3b of the writing device 3, and FIG. 6A shows the writing electrode 3b and the image carrier 2. An enlarged view of the contact portion, FIG. 6B is an electrical equivalent circuit diagram of the contact portion, and FIGS. 6C to 6F are diagrams showing the relationship between each parameter and the surface potential of the image carrier 2. FIG. 7 illustrates charging or discharging of the image carrier, and FIG. 7A is an explanatory view of charging or discharging of the image carrier by charge injection, and FIG.
Is an explanatory diagram of charging or discharging of an image carrier by electric discharge, and FIG. 7C is a diagram illustrating Paschen's law.

As shown in FIG. 6A, the image carrier 2 is made of a conductive material such as aluminum and has a base 2a that is grounded and an insulating material formed on the outer periphery of the base 2a. It is composed of a charged body layer 2b. FP of writing device 3
The writing electrode 3b supported by the base material 3a made of C or the like is in contact with the charging body layer 2b with a predetermined small pressing force as described above, and the image carrier 2 moves at a predetermined speed V ( It is rotating). This small pressing force has a width of 300 mm
A pressing force of 10 N or less, that is, a linear pressure of 0.03 N / mm or less is preferable for stabilizing the contact between the writing electrode 3b and the image carrier 2 and stabilizing the charge injection or the discharge. It is desirable to reduce the linear pressure as much as possible while maintaining a stable state.

A predetermined high voltage V ₀ or a predetermined low voltage V ₁ is selectively switched and applied to the write electrode 3b via the base material 3a (± as described above).
High voltage means a voltage with a high absolute value,
In addition, the low voltage refers to a voltage having a low absolute value or 0 V with the same polarity as the high voltage. In the description of the present invention in this specification, all of the low voltage is the ground voltage. Therefore, in the following description, the high voltage V _{0 is referred} to as the predetermined voltage V ₀ ,
The low voltage V _{1 is referred} to as the ground voltage V ₁ . It goes without saying that the ground voltage V ₁ is 0V).

That is, at the contact portion (nip portion) between the write electrode 3b and the image carrier 2, the electrical equivalent circuit shown in FIG. 6B is constructed. In FIG. 6B, R
Indicates the resistance of the writing electrode 3b, and C indicates the capacitance of the image carrier 2. The resistance R of the write electrode 3b is selectively switched to the (−) predetermined voltage V ₀ on the A side or the ground voltage V ₁ on the B side.

In the equivalent circuit, the resistance R of the write electrode 3b and the resistance of the image carrier 2 when the write electrode 3b is connected to the A side and a predetermined voltage V ₀ of (-) is applied to the write electrode 3b. As for the relationship with the surface potential, as shown by the solid line in FIG. 6C, in the region where the resistance R of the writing electrode 3b is small, the surface potential of the image carrier 2 becomes a constant predetermined voltage V ₀ , and the writing electrode 3b If the resistance R is in a region larger than a predetermined value, the absolute value of the surface potential of the image carrier 2 decreases. On the other hand, the relationship between the resistance R of the write electrode 3b and the surface potential of the image carrier 2 when the write electrode 3b is connected to the B side and the write electrode 3b is grounded is shown in FIG.
In the region where the resistance R of the write electrode 3b is small as indicated by the dotted line, the surface potential of the image carrier 2 becomes a constant ground voltage V ₁ , and the resistance R of the write electrode 3b is larger than a predetermined value. The absolute value of the surface potential of the image carrier 2 increases.

Then, in a region where the resistance R of the write electrode 3b is small and the surface potential of the image carrier 2 is a constant voltage V ₀ or a constant ground voltage V ₁ , as shown in FIG. Between the write electrode 3b in contact with the carrier 2 and the charged body layer 2b of the image carrier 2, direct (-) charge injection from low voltage to high voltage is performed. That is, the image carrier 2 is charged or discharged by the charge injection. Further, in a region where the resistance R of the writing electrode 3b is large and the surface potential of the image carrier 2 starts to change, the charging or discharging of the image carrier 2 due to the charge injection gradually decreases, and the resistance of the writing electrode 3b decreases. As R becomes large, as shown in FIG. 7B, electric discharge is generated between the later-described conductive pattern of the base material 3a and the image carrier 2.

The conductive pattern of the substrate 3a and the image carrier 2
The discharge generated between the base material 2a and the base material 2a of the image carrier 2
This occurs when the absolute value of the voltage (predetermined voltage V ₀ ) between the discharge start voltage V _{th and the} discharge start voltage V _th becomes larger than the discharge start voltage V _th. Is as shown in FIG. 7C according to Paschen's law. That is, the discharge start voltage V _th is smallest when the gap is about 30 μm, and the discharge start voltage V _th is large when the gap is smaller or larger than about 30 μm.
Discharge is less likely to occur. The image carrier 2 is also generated by this discharge.
The surface of will be charged or discharged. However, when the resistance R of the writing electrode 3 is in this region, the charging or discharging of the image carrier 2 due to the charge injection is large, and the charging or discharging of the image carrier 2 due to the discharge is small, and the charging or discharging of the image carrier 2 occurs. The charge elimination or charge elimination by charge injection is dominant. In the charging or discharging by the charge injection, the surface potential of the image carrier 2 becomes the predetermined voltage V ₀ or the ground voltage V ₁ applied to the writing electrode 3b. In the case of charging by charge injection, the predetermined voltage V ₀ supplied to the write electrode 3b is set to be equal to or lower than the discharge start voltage V _th at which discharge is generated between the write electrode 3b and the substrate 2a of the image carrier 2. Is desirable.

When the resistance R of the writing electrode 3b is further large, the charge or charge elimination of the image carrier 2 due to the charge injection is small, and the charge or charge elimination of the image carrier 2 due to the discharge is more than the charge or charge elimination due to the charge injection. Image carrier 2
As for the charging or discharging, the charging or discharging due to discharge gradually becomes dominant. That is, the resistance R of the write electrode 3b
Becomes larger, the surface of the image carrier 2 is mainly charged or discharged by discharge, and the image carrier 2 is hardly charged or discharged by charge injection. In the charging or discharging by this discharge, the surface potential of the image carrier 2 is changed to the writing electrode 3
It is a voltage obtained by subtracting the discharge start voltage V _th from the predetermined voltage V ₀ or the ground voltage V ₁ applied to b. The same applies when the predetermined voltage V ₀ is a (+) voltage.

Therefore, the resistance R of the electrode 3b is set to a predetermined voltage │V ₀ │ (which is an absolute value because there is a voltage of ± because the surface potential of the image carrier 2 is constant) or a constant ground voltage V.
_The write electrode 3b is set to a small area of ₁
By controlling the voltage applied to the image carrier between the predetermined voltage V ₀ and the ground voltage V ₁ , the image carrier 2 can be charged or discharged by injecting charges.

The capacitance C of the image carrier 2 and the surface potential of the image carrier 2 when the write electrode 3b is connected to the side A and a predetermined voltage V ₀ of (-) is applied to the write electrode 3b. As shown by the solid line in FIG. 6D, the capacitance C of the dielectric 2b is
The surface potential of the image carrier 2 becomes a constant predetermined voltage V _{0 in a} small area, and the absolute value of the surface potential of the image carrier 2 decreases in a area where the capacitance C of the dielectric 2b is larger than a predetermined value.
On the other hand, by connecting the write electrode 3b to the B side,
The relationship between the capacitance C of the image bearing member 2 and the surface potential of the image bearing member 2 when the image bearing member 2 is grounded is that in the region where the capacitance C of the image bearing member 2 is small as shown by the dotted line in FIG. The surface potential of 2 becomes a constant ground voltage V _{1 and} the capacitance C of the image carrier 2
In a region where is larger than a predetermined value, the absolute value of the surface potential of the image carrier 2 increases.

Then, in a region where the capacitance C of the image carrier 2 is small and the surface potential of the image carrier 2 is a constant voltage V ₀ or a constant ground voltage V ₁ , the write electrode contacting the image carrier 2 is formed. Direct (-) charge injection is performed between 3b and the charged body layer 2b of the image carrier 2. That is, the image carrier 2 is charged or discharged by the charge injection. Further, in a region where the capacity C of the image carrier 2 is large and the surface potential of the image carrier 2 starts to change, the charge or charge elimination of the image carrier 2 due to the charge injection becomes gradually smaller, and the capacity of the image carrier 2 becomes smaller. When C becomes large, electric discharge occurs between the base material 3a and the image carrier 2 as shown in FIG. 7 (b). The surface of the image carrier 2 is also charged or discharged by this discharge. However, when the image carrier capacity C is in this region, the charge or charge removal of the image carrier 2 due to the charge injection is large and the charge or charge removal of the image carrier 2 due to the discharge is small, so that the charge or charge removal of the image carrier 2 is prevented. Charging or charge elimination by charge injection is dominant. In the charging or discharging by the charge injection, the surface potential of the image carrier 2 is the predetermined voltage V applied to the writing electrode 3b.
It becomes ₀ or the ground voltage V ₁ .

In a region where the capacitance C of the image carrier 2 is further large, this charge injection is hardly performed between the write electrode 3b and the charged body layer 2b of the image carrier 2. That is, the image carrier 2 is not charged or discharged by the charge injection. The same applies when the predetermined voltage V ₀ is a (+) voltage.

Therefore, the capacitance C of the image carrier 2 is represented by a predetermined voltage │V ₀ │ (there is an absolute value because there is a voltage of ± because the surface potential of the image carrier 2 is constant) or a constant ground voltage V.
_The write electrode 3b is set to a small area of ₁
By controlling the voltage applied to the image carrier between the predetermined voltage V ₀ and the ground voltage V ₁ , the image carrier 2 can be charged or discharged by injecting charges.

Further, the speed (peripheral speed) v of the image carrier 2 and the image carrier when the write electrode 3b is connected to the A side and a predetermined voltage V ₀ of (-) is applied to the write electrode 3b. 2 has a relationship with the surface potential as shown by the solid line in FIG.
In a region in which the speed v is relatively small, the surface potential of the image carrier 2 increases as the speed v increases, and when the speed v of the image carrier 2 exceeds a predetermined value, the surface potential of the image carrier 2 increases. The absolute value is a constant voltage. The surface potential of the image carrier 2 increases as the speed v of the image carrier 2 increases. It is easy to inject charges into the image carrier 2 due to friction between the writing electrode 3b and the image carrier 2. It is thought that this is due to This facilitation of charge injection due to friction does not change when the speed v of the image carrier 2 increases to some extent, and remains almost constant. On the other hand, when the write electrode 3b is connected to the B side and the write electrode 3b is grounded, the speed v of the dielectric 2b and the dielectric 2
The relationship with the surface potential of b is a constant ground voltage V ₁ irrespective of the speed v of the dielectric 2b as shown by the dotted line in FIG. 6 (e). The same applies when the predetermined voltage V ₀ is a (+) voltage.

Further, when the write electrode 3b is connected to the A side and a predetermined voltage V ₀ of (-) is applied to the write electrode 3b, the pressing force of the write electrode 3b on the image carrier 2 (hereinafter The relationship between the write electrode 3b pressure and the surface potential of the image carrier 2 is as follows. In the region where the pressure of the write electrode 3b is extremely small as shown by the solid line in FIG. The surface potential of (1) rises relatively rapidly as the pressure of the write electrode 3b increases, and when the pressure of the write electrode 3b exceeds a predetermined value, the absolute value of the surface potential of the image carrier 2 becomes a constant voltage. The surface potential of the image carrier 2 suddenly rises in accordance with the increase in the pressure of the write electrode 3b, so that the contact between the write electrode 3b and the image carrier 2 is more surely performed as the pressure of the write electrode 3b increases. It is believed that this is due to The certainty of the contact between the writing electrode 3b and the image carrier 2 is determined by the writing electrode 3
When the pressure of b is increased to some extent, it does not change and remains almost constant. On the other hand, the relationship between the pressure of the write electrode 3b and the surface potential of the image carrier 2 when the write electrode 3b is connected to the B side and the write electrode 3b is grounded is indicated by the dotted line in FIG. 6 (f). As shown, a constant ground voltage V ₁ irrespective of the pressure of the write electrode 3b.
Becomes The same applies when the predetermined voltage V ₀ is a (+) voltage.

In this way, the resistance R of the writing electrode 3b and the capacitance C of the image carrier 2 are set so that the surface potential of the image carrier 2 becomes a constant predetermined voltage, and at the same time, the image carrier 2
Of the write electrode 3b and the pressure of the write electrode 3b are controlled so that the surface potential of the image carrier 2 becomes a constant predetermined voltage.
By switching control of the voltage applied to b between the predetermined voltage V ₀ and the ground voltage V ₁ , it becomes possible to reliably and easily charge or remove the image carrier 2 by charge injection.

In the above example, the predetermined voltage V ₀ applied to the write electrode 3b is a DC voltage, but an AC voltage may be superimposed on the DC voltage. When superimposing an AC voltage, a DC component is used as a voltage applied to the image carrier 2, and
It is preferable that the amplitude of the AC component is set to twice or more the discharge start voltage V _th , and the frequency of the AC component is about 500 to 1000 times the frequency of the rotation of the image carrier 2 (for example, the diameter of the image carrier 2 is If the peripheral speed of the image carrier 2 is 30 mm and the peripheral speed of the image carrier 2 is 180 mm / sec, the frequency of the rotation of the image carrier 2 is 2 Hz, and the frequency of the AC component is 1000 to 2000 Hz.

By superposing the AC voltage on the DC voltage in this way, the charging or discharging by the discharge of the write electrode 3b becomes more stable, and the write electrode 3 is made by the AC voltage.
By vibrating b, the foreign matter adhering to the write electrode 3b can be removed, and the write electrode 3b can be prevented from being contaminated.

FIG. 8 is a diagram showing a switching circuit for switching and connecting the predetermined voltage V ₀ and the ground voltage V ₁ to the write electrode 3b. For example, the write electrodes 3b arranged in four columns
Are the corresponding high voltage switches (High Voltage
Switch; HVSW) 15, and these high voltage switches 15 switch and connect the corresponding electrodes 3b to a predetermined voltage V ₀ and a ground voltage V ₁ , respectively. An image writing control signal from a shift register (SR) 16 is input to each high voltage switch 15, and an image signal stored in a buffer 17 and a clock 18 are input to the shift register 16. Clock signals are input respectively. Then, the image writing control signal from the shift register 16 is input to each high voltage switch 15 by the AND circuit 19 based on the writing timing signal from the encoder 20. Each high voltage switch 15 and AND circuit 19 causes each write electrode 3
The above-mentioned driver 11 that controls switching of the supply voltage to b is configured.

FIG. 9 shows each high voltage switch 1 of each electrode 3b.
5 shows a state in which 5 is selectively switched to a predetermined voltage V ₀ or a ground voltage V ₁ , respectively, (a) is a diagram showing a voltage state of each electrode, and (b) is a normal state in the voltage state of (a). FIG. 6 is a diagram showing a developer image when developed, and FIG. 6C is a diagram showing a developer image when reverse development is performed in the voltage state of FIG.

In FIG. 9, for example, n-2, n-1
The 3rd, nth, n + 1th, and n + 2th electrodes 3b
However, it is assumed that the respective high voltage switches 15 are switching-controlled to be in the voltage state shown in (a). Therefore, when the electrostatic latent image is written on the image carrier 2 by each electrode in such a voltage state and the electrostatic latent image is developed by the regular development, the developer 8 causes the developer 8 to have a predetermined voltage. A developer image is obtained which adheres to the V ₀ portion and is hatched in (b). Further, when the electrostatic latent image is written in the same manner and the electrostatic latent image is developed by the reversal development, the developer 8 adheres to the ground voltage V ₁ portion of the image carrier 2 and is shown in (c). A developer image as shown by hatching is obtained.

According to the image forming apparatus 1 using the writing head 3 thus constructed, the writing electrode 3b is brought into contact with the image carrier 2 with a light pressing force by the small elastic restoring force of the base material 3a. Therefore, the writing electrode 3b can be brought into stable contact with the image carrier 2. Therefore, it is possible to more stably and highly accurately charge the image carrier 2 with the write electrode 3b. Thereby, the electrostatic latent image can be written more stably, so that a good image can be reliably obtained with high accuracy.

Further, since the write electrode 3b is only brought into contact with the image carrier 2 with a light pressing force, the image carrier 2 can be prevented from being damaged by the write electrode 3b and the durability of the image carrier 2 can be prevented. Can be improved. Furthermore, since only the writing electrode 3b is used as the writing device 3, a large-sized laser light generator, LED lamp light generator, and the like as in the related art are not provided, so that the device can be further downsized. In addition, the number of parts can be further reduced, and a simpler and cheaper image forming apparatus can be obtained. Further, the write electrode 3b can further suppress the generation of ozone.

FIG. 10 is a plan view schematically showing a specific example of the write head of FIG. Each driver 11 is electrically connected to each other by a conductive pattern 9 formed on the base material 3a and having a rectangular flat cross section and made of, for example, a copper foil. Similarly, each driver 11 and a plurality of write electrodes 3b are connected. And are electrically connected by a conductive pattern 9 formed on the base material 3a. These conductive patterns 9 can be formed by a conventional thin film pattern forming method such as etching. Then, line data, a write timing signal, and a high-voltage power supply are supplied to each driver 11 from the upper conductive pattern 9 in the drawing.

Next, the features of the present invention will be described. FIG. 11 is a schematic diagram for explaining the problem of the present invention. When the independent floating electrode layer 2d is provided on the surface of the dielectric layer 2b as described above, the independent electrode 2d ₁ and the developing roller 4 are
Both a are conductors, and the independent electrode 2d ₁ and the developing roller 4
When the toner T is pressed into contact with the a, the image force Q of the toner is generated on the independent electrode 2d ₁ and the developing roller 4a, and the independent electrode 2d _{1
If the} image force Q on the ₁ side is larger than the image force on the developing roller 4a side, fog will occur. The toner on the image carrier bound with such strong energy causes problems such as transfer residual and cleaning residual. This image force Q is empirically expressed by the following equation.

[0048]

[Equation 1] Here, a is a proportional constant, q is the charge amount of the external additive on the toner surface layer, R is the electric resistance of the independent electrode or the developing roller, r is the distance between the toner and the independent electrode or the developing roller, and when the distance r becomes small. The image force Q becomes large. This means that in the contact developing method, the distance r becomes small, so that the mirror image force Q becomes large, and the smaller the resistance R, the larger the mirror image force Q becomes.
It should be noted that the proportional constant a includes complicated parameters such as relative permittivity and measurement system, but it is considered that there is no problem in comparison using the same value for comparative hairs in the same experiment.

FIG. 11 shows a case where the layer thickness is _{one and} the electric resistance R1 of the independent electrode 2d _{1 is} the electric resistance R2 of the developing roller 4a.
When it becomes larger, the toner T acting on the independent electrode 2d ₁
Is smaller than the mirror image force acting on the developing roller 4a, and therefore fogging is suppressed.

From the above, in the present invention, when the electric resistance R1 of the independent electrode is larger than the electric resistance R2 of the developing roller (R1> R2), the development is carried out by the contact developing system shown in FIG. Examples of the material of the developing roller having a low electric resistance include metallic materials such as Ni and Al. Although no fog is observed even in the non-contact developing method, the amount of toner in the portion to be adhered is reduced, so that the image density is reduced.

[0051]

[Embodiment 1] Conductive titanium oxide having an electric resistance value of 10 ⁸ Ω is used for an independent electrode, and an electric resistance value of 1 is used for a developing roller.
An aluminum roller of 0 ^-6 Ω was used. The write electrode is
The FPC with bump plating was brought into contact with the image bearing member, and the writing voltage was set to V _on at 400 V and V _0ff at 0 V. A rectangular wave with a duty ratio of 50% was input as the write pulse, and the frequency was 200 to 500 Hz. The peripheral speed of the image carrier (drum) was set to 50, 100, and 150 mm / sec, and the peripheral speed ratio of the developing roller was set to be equal to or twice the peripheral speed of the drum. The developing bias is DC component (100V) and AC component (0 to 800V).
Was applied and the frequency was set to 0 to 1500 Hz. The development gap was changed from 0 to 300 μm. The evaluation result was judged by the presence or absence of fog in the transferred image. Figure 12 shows the experimental results,
It was found that when the electric resistance of the independent electrode is larger than that of the developing roller, fog does not occur when the development is carried out by the contact developing method.

[0052]

Example 2 An experiment was conducted in the same manner as in Example 1 except that the developing roller used was an iron roller having an electric resistance value of 10 ⁻⁶ Ω and having a surface layer plated with Ni. FIG. 13 shows the result of the experiment, and it was found that when the electric resistance of the independent electrode is larger than the electric resistance of the developing roller, fog does not occur if the development is carried out by the contact developing method.

Next, the application of the developing bias will be described. In the case of the image carrier 2 having the independent floating electrode layer 2d on its surface, the relative permittivity of the independent electrode 2d ₁ is large (for example, infinite in the case of metal), and the relative permittivity of the dielectric part is small (about 3.0). ). Therefore, when an attempt is made to develop the toner particles by applying only the DC voltage to the developing roller 4a, the instability of the image portion and the non-image portion becomes conspicuous. Therefore, by superimposing the AC component on the DC component, the unevenness of the potential contrast near the surface layer of the image bearing member is alleviated. Further, the frequency of the AC component is f (Hz), and the peripheral speed of the image carrier 2 is V (mm
/ sec) divided by the average distance L (mm) between the conductive fine particles satisfies a relation of f> V / L, it becomes possible to form a more uniform image.

A method of calculating the distance L will be described. The conductive fine particles to be dispersed have a fixed outer diameter. For example, Mo fine particles having an average particle diameter of 3 μm are used. Then, the volume ratio or mass ratio of the conductive fine particles to be dispersed and the dielectric resin, or the ratio of the solvent is obtained, and thus the approximate distance can be calculated. Next, a method of measuring the distance L when it becomes a product will be described. Microscopic observation may be performed in the axial direction of the image bearing member, but it is difficult to distinguish between the exposed portion and the buried portion, and therefore the measurement is performed by the following method. A contact charging member such as a conductive brush or a conductive sheet is brought into contact with the image carrier, a voltage not higher than the discharge start voltage is applied, uniform charging is performed, and the needle electrode is even scanned in the axial direction of the image carrier. , The waveform is recorded by monitoring whether or not current flows, and the current flows at the moment when the needle comes into contact with the exposed part of the conductive fine particles, and the next current flows at the next adjacent fine particles. The point of contact is repeatedly recorded, and the average particle spacing is measured from the waveform.

FIG. 14 is a diagram schematically showing another example of the image forming apparatus of the present invention. In the following example, the developing device uses a non-contact method. Image forming apparatus 1 of this example
Is an image forming apparatus that performs full-color development by superimposing four colors of black K, yellow Y, magenta M, and cyan C on the image carrier 2 and includes the image carrier 2 in the form of an endless belt. . The image carrier 2 in the form of an endless belt is stretched between two rollers 22 and 23, and among the rollers 22 and 23,
The roller on the driving side rotates clockwise in the figure.

Along the straight portion of the endless belt of the image carrier 2.
Writing head 3 for each color_K, 3_Y, 3_M, 3_CAnd
And developing device 4_K, 4_Y, 4_M, 4_CIs on the rotation direction of the image carrier 2.
The colors K, Y, M, and C are arranged in this order from the flow side. Rice cake
Of course, these developing devices 4_K, 4_Y, 4_M, 4_CIs not shown
They can be arranged in any order. Writing to each color
Dead 3_K, 3_Y, 3_M, 3_CEach writing electrode 3b_K, 3b_Y, 3b_M,
3b_CIs the flexible base material 3a as described above._K, 3
a_Y, 3a_M, 3a_CIt is provided in. Not shown
However, even in the image forming apparatus 1 of this example, the image carrier 2
Endless belt writing device 3 _K, 3_Y, 3_M, 3_CAnd the straight line on the opposite side
The above-mentioned image carrier uniform charge control device is provided in this section.
It

In the image forming apparatus 1 thus constructed, the electrostatic latent image of black K is first written on the surface of the image carrier by the electrode 3b _K of the writing head 3 _K of black K, and at the same time, this black black is written. The K electrostatic latent image is developed by the developing device 4 _K to form a black K developer image on the surface of the image carrier 2. Next, an electrostatic latent image of cyan C is written on the surface of the image carrier 2 _C and on the developer image of black K by the electrode 3b _C of the writing head 3 _C of cyan C, and the cyan C Is developed by the developing device 4 _C to form a cyan C developer image on the surface of the image carrier 2. Similarly, the electrode 3 of the write head 3 _M of magenta M
After the electrostatic latent image of magenta M is written on the image carrier 2 and the developer images of black K and cyan C by b _M, the electrostatic latent image of magenta M is developed by the developing device 4 _M. As a result, a developer image of magenta M is formed on the surface of the image carrier 2 and on each developer image of black K and cyan C. Further, the image carrier by the electrode 3b _C of the write head 3 _C of cyan C is formed. 2 and black K, cyan C, and magenta M, respectively, after the electrostatic latent image of cyan C is written on each of the developer images, the electrostatic latent image of cyan C is developed by the developing device 4 _C to form an image. A cyan C developer image is formed and color-matched on the surface of the carrier 2 and on each of the black K, cyan C, and magenta M developer images, after which these developer images are transferred by the transfer device 6 to the transfer material 5 A full-color developer image in which each color developer image is color-matched It is formed. The order of forming the developers of the respective colors can be set to any order other than the order described above.

FIG. 15 shows still another example of the image forming apparatus of the present invention.
It is a figure which shows the example of FIG. Image forming apparatus 1 of this example
Is an image forming apparatus 1 for each color_K, 1_C, 1_M, 1_YIs transfer material 5
Are arranged in tandem from the upstream side in the moving direction of
Has been. Image forming apparatus 1 for each color_K, 1_C, 1_M, 1_YArrangement of
The order can be set to any order. Each picture
Image forming apparatus 1_K, 1_C, 1_M, 1_YAre the image carriers
Two_K, 2_C, 2_M, 2_Y, Writing head 3_K, 3_C, 3_M, 3_Y, Development equipment
Set 4_K, 4_C, 4_M, 4_Y, And transfer device 6_K, 6_C, 6_M, 6_YTo
I have it. Although not shown, each image forming apparatus 1_K, 1
_C, 1_M, 1_YAre the same image carrier as above.
The load control device 7 uses the writing device 3 for each color._K, 3_C, 3 _M, 3_YThan
Each image carrier 2_K, 2_C, 2_M, 2_YIs installed on the upstream side of the
It may be.

In the image forming apparatus 1 of this example configured as described above, first, in the black K image forming apparatus 1 _K , the surface of the image carrier 2 _K is controlled by the black K image carrier uniform charge control device 7. Are uniformly charged, the electrostatic latent image of black K is written on the surface of the image carrier 2 _K by the electrode 3b _K of the writing head 3 _K , and the electrostatic latent image of black K is developed by the developing device. After being developed at 4 _K , a black K developer image is formed on the surface of the image carrier 2 _K. This image carrier 2
In _K on the black K developer image transfer apparatus 6 _K, is transferred to the transfer material 5 that has been supplied, the developer image of black K is formed on the transfer material 5. Next, in the cyan C image forming apparatus 1 _C , the cyan C image carrier uniform charge control device 7
After the surface of the image carrier 2 _C is uniformly charged by the above, an electrostatic latent image of cyan C is written on the surface of the image carrier 2 _C by the electrode 3b _C of the writing head 3 _C , and the cyan C Is developed by the developing device 4 _C to form a cyan C developer image on the surface of the image carrier 2 _C. This image carrier 2 _C
The above cyan C developer image is transferred by the transfer device 6 _C onto the transfer material 5 on which the supplied black K developer image has been formed, and a cyan C developer image is formed on the transfer material 5. To be done. Similarly, writing of an electrostatic latent image of magenta M to the image carrier 2 _M by electrodes 3b _M of the writing head 3 _M in the image forming apparatus 1 _M of magenta M, electrostatic latent image developing device 4 _M Of the developer image of magenta M onto the transfer material 5 of the transfer device 6 _M , and the image is carried by the electrode 3b _Y of the writing head 3 _Y in the image forming device 1 _Y of yellow Y after the development. Writing an electrostatic latent image of yellow _Y on the body 2 _Y , developing the electrostatic latent image of the developing device 4 _Y , and transferring material 5 of the transfer device 6 _Y.
The yellow Y developer image is transferred to the transfer material 5
A full-color developer image is formed by color-developing the developer images of the respective colors.

FIG. 16 is a diagram schematically showing still another example of the image forming apparatus of the present invention. Image forming apparatus 1 of this example
In this case, the developer images of the respective colors formed on the image carriers 2 _K , 2 _C , 2 _M and 2 _Y of the respective colors are temporarily transferred to the transfer material 5 after the intermediate transfer. That is, in the image forming apparatus 1 of this example, the intermediate transfer device 24 is provided. The intermediate transfer device 24 is provided with an endless belt-shaped intermediate transfer member 25. The intermediate transfer member 25 has two rollers 26, 2.
It is stretched between 7 and is rotated counterclockwise in the figure by driving one roller. The image forming apparatuses 1 _K , 1 _C , 1 _M and 1 _Y are arranged along the linear portion of the intermediate transfer member 25, and the roller 2 is provided.
One transfer device 6 is provided at the position 7.

The image forming apparatus of this example configured as described above
In the apparatus 1, the image carrier 2 of each color _K, 2_C, 2_M, 2_YTo each
A developer image of each color is formed, and development of each of these colors is performed.
The developer image of each color is color-matched on the intermediate transfer member 25.
Will be overlaid and transferred. Intermediate transfer is performed on this intermediate transfer body 25.
The developed developer image of each color is transferred to the transfer material 5 by the transfer device 6.
As a result, a full-color developer image is formed on the transfer material 5. This
As described above, the image forming apparatus for each color is provided with the intermediate transfer device 24.
1_K, 1_C, 1_M, 1_YOf the full color arranged in tandem
The writing head 3 of the present invention is also used in an image forming apparatus.
To make it smaller and simpler.
You can

[0062]

As is apparent from the above description, according to the present invention, an image carrier having an independent floating electrode layer on its surface, and a write head for writing the electrostatic latent image on the latent image carrier. An image forming apparatus comprising at least a developing device for developing the electrostatic latent image on the image carrier,
The electric resistance of the independent electrode of the independent floating electrode layer is R1, and the electric resistance of the developer carrying member of the developing device is R1.
When it is set to 2, when the relationship of R1> R2 is satisfied, the development is performed by the contact development method. Therefore, by preventing the image force of the toner from acting on the independent electrode, fogging can be suppressed.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a basic configuration of Japanese Patent Application No. 2001-227630.

FIG. 2 is a diagram for explaining a problem of the present invention.

3A and 3B show an example of an image forming apparatus according to the present invention, FIG. 3A is an overall configuration diagram, and FIG. 3B is a partial perspective view of the image carrier and the charging / writing device of FIG. 3A. .

FIG. 4 is an enlarged view partially and schematically showing the image carrier of FIG.

FIG. 5 is a diagram showing a basic process of image formation in the image forming apparatus of the present invention.

FIG. 6 illustrates the principle of writing an electrostatic latent image by charging or discharging a writing electrode of a writing device, FIG. 6A is an enlarged view of a contact portion between the writing electrode and an image carrier, and FIG. 8C is an electrical equivalent circuit diagram of this contact portion, and FIGS. 9C to 9F are diagrams showing the relationship between each parameter and the surface potential of the image carrier.

FIG. 7 illustrates charging or discharging of an image carrier,
(A) is an explanatory view of charging or discharging of an image carrier by electric charge injection, (b) is an explanatory view of charging or discharging of an image carrier by discharging, and (c) is a diagram explaining Paschen's law.

FIG. 8 is a diagram showing a switching circuit for switching and connecting a predetermined voltage V ₀ and a ground voltage V ₁ to a write electrode.

FIG. 9 shows a state in which each high voltage switch of each electrode is selectively switched to a predetermined voltage V0 or a ground voltage V1, and FIG. 9A is a diagram showing a voltage state of each electrode,
(B) is a figure which shows the developer image at the time of normal development in the voltage state of (a), (c) is a figure which shows the developer image at the time of reversal development in the voltage state of (a).

FIG. 10 is a plan view schematically showing a specific example of the write head of FIG.

FIG. 11 is a schematic diagram for explaining a problem of the present invention.

FIG. 12 is a diagram for explaining an experimental result according to the present invention.

FIG. 13 is a diagram for explaining an experimental result according to the present invention.

FIG. 14 is a diagram schematically showing another example of the image forming apparatus of the present invention.

FIG. 15 is a diagram schematically showing another example of the image forming apparatus of the present invention.

FIG. 16 is a diagram schematically showing another example of the image forming apparatus of the present invention.

[Explanation of symbols]

2 ... image bearing member 2a ... substrate 2b ... charging layer 2c ... dielectric layer 2d ... independent floating electrode layer 2d ₁ ... independent electrode 3 ... write head 3a ... substrate 3b ... writing electrode 4 ... developing device 4a ... Developing roller (developer bearing member) Q ... Image force

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kenjiro Yoshioka             Seiko, 3-3-3 Yamato, Suwa City, Nagano Prefecture             -In Epson Corporation F-term (reference) 2C162 AE21 AE31 AE47 AE62 AE84                       AF51 AH03 AJ09 EA19                 2H029 AA06 AB03 AB04 AB16 AB18                       AC05 AD01 AD07 AE01                 2H300 EC02 EC05 EF03 EF08 EF10                       EG03 EH08 EH38 EJ09 EJ25                       EJ29 EJ32 FF05 GG01 GG02                       GG29 GG35 KK03 KK05 KK13                       PP06 PP07 PP10

Claims (4)

[Claims] 1. An image carrier having an independent floating electrode layer on its surface, a write head for writing the electrostatic latent image on the latent image carrier, and developing the electrostatic latent image on the image carrier. In an image forming apparatus including at least a developing device, when the electric resistance of the independent electrode of the independent floating electrode layer is R1 and the electric resistance of the developer carrier of the developing device is R2, R1> R2. An image forming apparatus that develops by a contact development method at a certain time. 2. The writing head and the developing device,
It is provided for each color of black, yellow, magenta, and cyan, and the developer image of each color is formed on the image carrier by the writing head of each color and the developing device in a superimposed manner. The image forming apparatus according to claim 1, wherein: 3. The image carrier, the writing head, and the developing device are provided for each color of black, yellow, magenta, and cyan, and the image forming device for each of these colors is arranged in tandem. The image forming apparatus according to claim 1, wherein: 4. The image forming apparatus according to claim 3, further comprising an intermediate transfer device for intermediately transferring the developer image for each color by the image forming device for each color.