JP2003107834A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device which prevents an implantation electrostatical charging member from decreasing in performance and prolongs the life of a photoreceptor. SOLUTION: The image forming device has a plurality of image formation units which each make an electrostatic charging member applied with a voltage abut against an electrostatically charged member to electrostatically charge the electrostatically charged member, expose the electrostatically charged body to image information to form an electrostatic latent image, and visualize the electrostatically latent image formed on the electrostatically charged body into a toner image by a developing device, and transfers toner images formed by the respective image forming units onto a transferred material one after another to form an image; while image formation processing is carried out by at least one image forming unit, electrostatically changed bodies of image forming units which are not performing image formation processing are electrostatically charged.

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 using a magnetic brush type injection charging technology and a cleanerless technology.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic or electrostatic recording type image forming apparatus, an image carrier (charged body) such as an electrophotographic photosensitive member or an electrostatic recording dielectric is subjected to a charging process (including a destaticizing process). A corona charger was often used as a means for doing this.

This is a device in which a corona charger is disposed so as not to contact a member to be charged, and the surface of the member to be charged is charged to a predetermined polarity and potential by exposing the surface of the member to be charged to a discharge corona generated by the corona charger. Is. In recent years, a contact charging device (direct charging device) has been put into practical use because it has advantages such as low ozone and low power consumption over a corona charger.

The contact charging device is a device in which a charging member to which a voltage is applied is brought into contact with a member to be charged to charge the surface of the member to a predetermined polarity and potential. A contact charging device using a magnetic brush as a charging member is preferably used from the viewpoint of stability of charging and contact. In this magnetic brush type contact charging device, electrically conductive magnetic particles are magnetically directly held on a magnet or a sleeve containing the magnet as a magnetic brush, and the magnetic brush is stopped or rotated on the surface of the body to be charged. By bringing them into contact with each other and applying a voltage to them, the charging during the non-charged period is started.

In the magnetic brush charging, a charge injection layer is provided on the surface of the body to be charged, and a charge member to which a voltage is applied is brought into contact with the body to be charged to inject the charge into the charge injection layer so that the surface of the body to be charged is predetermined. With the injection charging method of charging to the polarity and potential, it is possible to obtain the surface potential of the body to be charged that is substantially the same as the applied DC bias (direct current voltage) regardless of whether or not the AC bias (alternating voltage) is superimposed on the charging member. it can.

Further, since this magnetic brush charger also has a function as a cleaning device for collecting the transfer residual toner on the charged body after the transfer, it is possible to omit a dedicated cleaning device. . As described above, it is necessary to separately provide a cleaning device for collecting the transfer residual toner on the surface of the body to be charged without using the discharge phenomenon in which the body to be charged is charged by using the corona charger. Since it is not present, the deterioration of the charged body due to discharge and the abrasion of the surface layer by the cleaning device are small, and as a result, the service life of the charged body (photoconductor) can be extended.

Next, the multicolor image forming apparatus using the electrophotographic system or the electrostatic recording system can be classified into two types according to the image forming method. One is that the photoconductors used for image formation are prepared in parallel for each color, and the images are obtained by sequentially transferring the photoconductors to a transfer target (intermediate transfer member or a transfer target material such as paper or OHT). This is a so-called tandem drum type image forming apparatus that performs formation, and the other is that a developing device for each color is prepared in parallel on one photoconductor,
This is a so-called one-drum type image forming apparatus that forms an image by developing the image on the photoconductor for the number of colors and sequentially transferring the image on the transfer target.

Further, the tandem drum type image forming apparatus is
When forming an image of a single color (mainly black), there are two main types due to the difference in the control method of the process unit other than the process unit (including the photoconductor, charging device, exposure device, developing device, etc.) of the target color. Divide. One is a type that does not rotate the photoconductor of a process unit that is not used during monochromatic image formation. When this method is used, there is a difference in peripheral speed between the photoconductor and the transfer device in contact with the photoconductor. In order to prevent the occurrence of scratches on the photoconductor and the like, it is necessary to have a function of separating the photoconductor from the transfer device.

Further, since the time required for the separating operation is generated when switching between the multicolor and the single color, it is conceivable that the actual printing speed is lowered. In the other method, when a monochromatic image is formed, the photosensitive member of the unused process unit is also rotated, so that the above-mentioned problem does not occur. In any of the methods, it is possible to prevent meaningless wear of the charged body (photoreceptor) by performing the same charging process as in image formation on a process unit that is not used during monochromatic image formation. Is intended.

[0010]

However, in the multi-color image forming apparatus in which the above-mentioned contact injection charging method and the method of rotating the photoconductor of the process unit which is not used during the image formation of a single color are combined, The following problems have occurred.

That is, the most commonly used transfer material is neutral paper for copying in copying machines or neutral paper for printers, which contains fillers made of various materials. Calcium carbonate is the most used of these fillers, but when paper comes into contact with the photoconductor during image formation, a small amount of calcium carbonate may adhere and accumulate on the photoconductor. Is commonly known. In a photoconductor having a filler such as calcium carbonate accumulated on the surface, the surface free energy is increased and the adhesion force of the photoconductor is increased, so that dust, magnetic fine powder, or the like contacting the photoconductor surface is easily attached. As a result, in the contact type injection charging method in which the magnetic fine powder is used for charging, the amount of the magnetic particles used for charging adheres to the photosensitive member and is easily separated from the charging device, and as a result, There is a possibility that problems will occur as the charging property decreases.

Further, since calcium carbonate accumulated on the surface of the photoconductor binds to water molecules in the air, the surface resistance of the photoconductor is lowered and a problem called so-called image deletion occurs in which the electrostatic latent image on the photoconductor is disturbed. Easier to do. Here, in the usual image forming process, the adhering fillers and the like are removed by scraping the surface of the photoconductor in the charging process and the cleaning process of the photoconductor, so that there is substantially no problem. .

However, in the method of rotating the photoconductor of the process unit which is not used during the formation of a monochromatic image, the photoconductor is not subjected to the charging process in order to prevent unnecessary wear of the photoconductor. Since it comes into contact with the transfer material (paper or the like), the ability to remove the filler or the like adhering to the surface of the photoconductor cannot be kept up, and the above-mentioned problems occur.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of achieving both prevention of performance deterioration of an injection charging member and extension of life of a photosensitive member. Especially.

[0015]

In order to achieve the above object, the present invention charges a charged member by bringing a charging member, to which a voltage is applied, into contact with the charged member, and image information is transferred to the charged member. Each image forming unit has a plurality of image forming units that form an electrostatic latent image by exposure and visualize the electrostatic latent image formed on the charged body as a toner image by a developing device. In an image forming apparatus that forms an image by sequentially transferring a toner image formed by a transfer target material onto a transfer material, in a state where an image forming process is performed in at least one image forming unit among a plurality of image forming units. It is characterized in that the charged body of the image forming unit which has not been subjected to the image forming processing is subjected to the charging processing.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

[Printer Main Body] FIG. 1 is a sectional view of a main portion of an image forming apparatus according to the present invention, and FIG. 2 is a sectional view of the image forming apparatus.

The image forming apparatus shown in FIG. 2 is an LED printer using a transfer type electrophotographic process, and is composed of Y, M, C, and
The four process units K are arranged in parallel.
In addition, Y, M, C, which is a part of the image forming apparatus shown in FIG.
Each of the K process units has the same configuration.

Reference numeral 1 denotes a rotary photosensitive drum type electrophotographic photosensitive member (hereinafter referred to as photosensitive drum or drum) which is an image bearing member as a member to be charged. The photosensitive drum 1 has a diameter in the present embodiment. Using an OPC photosensitive member of 30 mm, it is rotationally driven in the direction of the arrow (clockwise direction) at a process speed (peripheral speed) of 100 mm / sec.

Reference numeral 2 denotes a sleeve-type magnetic brush charging member for uniformly charging the peripheral surface of the photosensitive drum 1 to a predetermined polarity and potential, and temporarily collecting the transfer residual toner on the surface of the body to be charged and then discharging it. Is. A predetermined charging bias is applied to the sleeve of the magnetic brush charging member 2 from a charging bias applying power source (not shown), and the outer peripheral surface of the rotary photosensitive drum 1 is almost uniformly charged by charge injection charging. Image exposure corresponding to the time-series electric digital pixel signal of the target image information output from the LED 3 is performed on the charged surface of the rotary photosensitive drum 1, and the target image information is formed on the peripheral surface of the rotary photosensitive drum 1. A corresponding electrostatic latent image is formed.

The electrostatic latent image uses a developer in which toner particles and magnetic particles (development carrier) are mixed at a fixed ratio.
It is developed as a toner image by the component contact developing device 4. Reference numeral 4a designates a non-magnetic developing sleeve having a diameter of 16 mm and containing a magnet 4b. The developing sleeve is coated with toner particles and a developing carrier, and is kept at a constant speed of 500 .mu.m with the surface of the photosensitive drum 1 at a constant speed. And the developing bias voltage is applied to the developing sleeve 4a from a developing bias applying power source (not shown).

In this embodiment, a DC voltage of -500 V and a rectangular AC voltage having a frequency of 2 kHz and a peak-to-peak voltage of 1.5 kV are superposed, and development is performed between the developing sleeve 4a and the photosensitive drum 1. Let That is, the negatively charged toner carried by the developing sleeve 4a is attached to the image portion of the latent image by an electric field to develop the latent image. The two-component developer used in the present embodiment is a toner in which 1% by weight of titanium oxide having an average particle diameter of 20 nm is externally added to a negative band toner having an average particle diameter of 6 μm manufactured by a pulverizing method. Used as a developing carrier and having a saturation magnetization of 205 emu.
A magnetic carrier having an average particle size of 35 μm / cm ³ was used.
A mixture of this toner and carrier in a weight ratio of 6:94 was used as a developer.

On the other hand, the transfer belt 5a is fed from a paper feeding unit (not shown).
Then, the transfer material 6 as a recording material is supplied and is introduced at a predetermined timing to the photosensitive drum 1 and the transfer roller 5b which is brought into contact with the photosensitive drum 1 through the transfer belt 5a with a predetermined pressing force. A predetermined transfer bias voltage is applied to the transfer roller 5b from a transfer bias applying power source (not shown). In this embodiment, the transfer roller 5b has a resistance value of 5 × 10 ⁸ Ω measured by a roller resistance meter when 100 V is applied, and a DC voltage of +2 kV is applied to transfer.

The transfer material 6 introduced into the transfer section is nipped and conveyed at the transfer section, and the toner images formed and carried on the surface of the rotary photosensitive drum 1 are sequentially transferred to the surface side by electrostatic force and pressing force. Will be done. The transfer material 6 that has received the transfer of the toner image is separated from the surface of the photosensitive drum 1 and is conveyed to the transfer portion of the next process unit. After the image formation of all process units is completed, the transfer material 6 is transferred to the upper heating roller 7a. It is conveyed to the fixing device 7 composed of the lower heating roller 7b,
The fixing process is performed and the image is output as a print image.

The surface of the photosensitive drum 1 after the transfer of the toner image to the transfer material 6 is cleaned by the magnetic brush charging member 2 which also serves as a cleaning device to remove adhered contaminants such as residual toner, and is repeatedly imaged. Be served.

[Photoreceptor] As the photosensitive drum 1 as a member to be charged or an image carrier, a commonly used organic photoreceptor or the like can be used, but it is desirable that the resistance on the organic photoreceptor be 10 ⁹ to 10 −. When a material having a surface layer having a material of 10 ¹⁴ Ω · cm is used, charge injection charging can be realized, which is effective in preventing ozone generation and reducing power consumption. In addition, it becomes possible to improve the charging property.

In this embodiment, a 100-mm photosensitive drum 1 is a negatively charged organic photosensitive member, in which the following first to fifth layers are provided in order from the bottom on an aluminum drum substrate having a diameter of 30 mm. It was used by rotating at a rotation speed of / sec.

First layer: an undercoat layer having a thickness of 20 μm provided to smooth out defects and the like of the aluminum substrate.
Of the conductive layer.

Second layer: a positive charge injection preventing layer, which plays a role of preventing the positive charges injected from the aluminum substrate from canceling out the negative charges charged on the surface of the photoreceptor, and the amylan resin and methoxymethylated nylon. By 10 ⁶ Ω
-It is a medium resistance layer with a thickness of 1 μm whose resistance is adjusted to about cm.

Third layer: a charge generation layer, which is exposed to light with a layer having a thickness of about 0.3 μm in which a disazo pigment is dispersed in a resin to generate positive and negative charge pairs.

Fourth layer: a charge transport layer, which is a polycarbonate resin in which hydrazone is dispersed, and is a P-type semiconductor. Therefore, the negative charges charged on the surface of the photoconductor cannot move in this layer, and only the positive charges generated in the charge generation layer can be transported to the surface of the photoconductor.

Fifth layer: a charge injection layer, which is a coating layer of a material in which SnO ₂ ultrafine particles as conductive fine particles are dispersed in a binder of an insulating resin. Specifically, SnO having a particle size of 0.03 μm, which is obtained by doping an insulating resin with antimony which is a light-transmitting conductive filler to reduce resistance (conductivity).
It is a coating layer of a material in which 70% by weight of ₂ particles are dispersed in a resin. The coating solution prepared in this manner is applied to an appropriate coating method such as a dipping coating method, a spray coating method, a roll coat coating method or a beam coat coating method to have a thickness of about 3
It was applied to a thickness of μm to form a charge injection layer.

[Magnetic Brush Charging Member] The magnetic brush charging member 2 is a fixed magnet roller 2a having a diameter of 16 mm.
And a non-magnetic SUS sleeve 2b rotatably fitted on the magnet roller 2a, and a magnetic brush layer 3c of magnetic particles (charge carrier) attached and held by the magnetic force of the magnet roller 2a on the outer peripheral surface of the sleeve 2b. It is of the sleeve rotation type.

The charge carrier constituting the magnetic brush layer 2c has an average particle size of 10 to 100 μm and a saturation magnetization of 2.
0-250 emu / cm ³ , resistance 1 × 10 ² -1 × 1
It is preferably 0 ¹⁰ Ω · cm, and it is 1 × in consideration of the existence of insulation defects such as pinholes on the photosensitive drum 1.
It is preferable to use one having a resistance of 10 ⁶ Ω · cm or more. Resistance value of the charging carrier, after which the bottom area is placed 2g of the magnetic particles in a metal cell of 228 mm ^2, weighted by 6.6 kg / cm ^2, are measured by applying a voltage of 100 V.

In order to improve the charging performance, it is preferable to use the one having the smallest possible resistance. Therefore, in the present embodiment, the average particle size is 25 μm and the saturation magnetization is 200 emu / cm.
³ , the resistance of 5 × 10 ⁶ Ω · cm was used, and 40 g of this was magnetically adhered to the outer peripheral surface of the sleeve 2b to form the magnetic brush layer 2c. The structure of the charging carrier is a resin carrier formed by dispersing a magnet as a magnetic material in a resin to disperse carbon black for conductivity and resistance adjustment, or a magnetite simple substance surface such as ferrite is oxidized and reduced. The resistance is adjusted, or the resistance of the magnetite simple substance such as ferrite is coated with resin to adjust the resistance.

The magnetic brush layer 2c of the magnetic brush charging member 2 is disposed in contact with the surface of the photosensitive drum 1. The width of the contact nip portion (charging nip portion) between the magnetic brush layer 2c and the photosensitive drum 1 was 6 mm. Then, a predetermined charging bias is applied from the charging bias applying power source E1 to the developing sleeve 2b, and an arrow which is in a counter direction (reverse direction) to the rotation direction of the photosensitive drum 1 at the contact nip portion where the developing sleeve 2b contacts the photosensitive drum 1. Photosensitive drum 1 in the direction (clockwise)
Rotation speed of 100 mm / sec, peripheral speed of 150 m
The magnetic brush layer 2c to which the charging bias is applied is applied to the surface of the rotating photosensitive drum 1 by being driven to rotate at m / sec.
The surface of the photosensitive layer of the photosensitive drum 1 is uniformly rubbed by a primary charging process by an injection charging method to a desired potential.

In this embodiment, the DC voltage is -70.
Frequency 0Hz, frequency 500Hz, peak voltage 800V
By applying to the magnetic brush charging member 2 a charging bias in which the rectangular electric current AC electric field is superimposed, good charging properties could be obtained. Here, in a state in which at least one image forming unit among the plurality of image forming units is performing the image forming process, the magnetic force that is rotationally driven while being in contact with the photosensitive drum 1 of the image forming unit that is not performing the image forming process. By applying only a rectangular-wave AC electric field 800V having a frequency of 500 Hz and a peak-to-peak voltage of 800 V to the brush-type injection charging device 2, it is possible to remove deposits such as calcium carbonate accumulated on the photosensitive drum 1.

FIG. 3 is a graph in which the horizontal axis represents the number of sheets passed in the process unit Y, and the vertical axis represents the value obtained by the contact angle meter when pure water on the surface of the photosensitive member 1 of the process unit Y is used. is there. A contact angle meter is a device that drops a water drop of a predetermined reagent on the surface of an object and measures the angle calculated from the angle between the top of the water drop and the ends in contact with the object surface. It is a value that generally indicates the surface free energy of the object, and the surface free energy increases as the contact angle decreases. That is, it can be considered that the adhesive force on the surface is increased.

FIG. 3 shows the transition of the contact angle of the photoconductor 1 by dropping a water droplet of 10 nl of pure water on the surface of the photoconductor 1 of the process unit Y for each predetermined number of sheets passed. A in the graph of FIG. 3 indicates a case where a charging bias in which a rectangular wave AC electric field having a frequency of 500 Hz and a peak-to-peak voltage of 800 V is superimposed on a DC voltage of −700 V for each sheet is applied to the magnetic brush charging member 2 which is rotationally driven. , B, when the magnetic brush type injection charger is driven and rotated without charging, C shows a frequency of 50.
FIG. 6 is a graph showing changes in contact angle on the surface of each photoconductor when a charging bias in which a rectangular-wave AC electric field of 0 Hz and a peak-to-peak voltage of 800 V is superimposed is applied to the magnetic brush charging member 2 that is rotationally driven.

As can be seen from the graph of FIG. 3, the photoconductor of C in which a predetermined AC electric field is applied to the rotating magnetic brush type charging charger is higher than the photoconductor of B without charging,
It can be seen that the surface contact angle is maintained at about the same level as that of the photoconductor of A to which the bias in which the direct current and the alternating current are superimposed is applied.
The surface resistance of each of the photoconductors A, B, and C when the number of sheets passed was 6000 under a high temperature and high humidity environment of room temperature 30 ° C. and humidity 80%. While the resistance was less than 1 × 10 ¹¹ Ω · cm ² ,
The photoconductors of A and C maintained the surface resistance of 1 × 10 ¹² Ω · cm ² or more.

When an image was actually formed using the A, B, and C photoconductors, with respect to the A and C photoconductors, a character size of 7 points could be output without any problem.
With respect to the photoconductor of B, characters with a character size of 11 points or less were blurred and blurred characters were output.

Next, in FIG. 4, the horizontal axis represents the number of sheets passed in the process unit Y, and the vertical axis represents the magnetic brush in the image forming apparatus according to the present embodiment using A, B, and C photoconductors. This is the amount of magnetic particles separated from the expression injection charger 2. Here, the collected amount of the magnetic particles is the amount of magnetic particles adhered to the surface of the developing sleeve 4a by magnetic force after a predetermined charging number of sheets by performing the charging process similar to the image formation with the developer in the developing device 4 removed. It was investigated by measuring.

As can be seen from the graph of FIG. 4, no charging bias was applied to the magnetic brush type injection charger B.
It was found that when the photoconductors of A and C were used, only 1/10 or less of the amount of magnetic particles was released, compared with the case of using the photoconductor of 1.

Next, in FIG. 5, the horizontal axis represents the number of sheets passed in the process unit Y, and the vertical axis represents the amount of abrasion of the resin layer of the photoconductor 1 of A, B, and C. As can be seen from the graph of FIG. 5, the abrasion amount of the resin layer of the photoconductor of A applied to the magnetic brush type injection charger rotatably driving the charging bias in which the AC electric field is superimposed on the DC electric field is 1 in 10,000 sheets on average.
On the other hand, the abrasion amount of the resin layer of the C photoconductor applied to the magnetic brush type injection charger rotatively driving only the AC electric field is about 0.01 to 0.015 μm per 10,000 sheets, which is about 1 μm. It is suppressed to wear of less than / 100.

In this embodiment, the example in which the present invention is applied to the process unit Y has been described.
The same effect can be obtained with any of the process units arranged in parallel.

[0046]

As is apparent from the above description, according to the present invention, a charging member to which a voltage is applied is brought into contact with the charged member to charge the charged member, and image information is transferred to the charged member. Each image forming unit includes a plurality of image forming units that form an electrostatic latent image by exposure and visualize the electrostatic latent image formed on the charged body as a toner image by a developing device. In an image forming apparatus that forms an image by sequentially transferring a toner image formed by a transfer material onto a transfer material, in a state where image forming processing is performed in at least one image forming unit among a plurality of image forming units. Since the charged body of the image forming unit that has not been subjected to the image forming process is subjected to the charging process, it is possible to achieve both prevention of performance deterioration of the injection charging member and extension of the life of the photoconductor. Obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a main part of an image forming apparatus according to the present invention.

FIG. 2 is a sectional view of the image forming apparatus according to the present invention.

FIG. 3 is a transition correlation diagram of the number of passed sheets and the contact angle of the surface of the photoconductor.

FIG. 4 is a transition correlation diagram of the number of passed sheets and the amount of magnetic particles leaked from the magnetic brush type charger in the developing device.

FIG. 5 is a transition correlation diagram of the number of passed sheets and the scraped amount of the photoconductor.

[Explanation of symbols]

1 Photosensitive drum (image carrier) 2 Magnetic brush type charging charger 2a Magnet (injection charger) 2b Sleeve (injection charger) 2c Magnetic particles (injection charger) 3 LED (image exposure device) 4 Developing device 4a Sleeve (injection charger) 4b Magnet (injection charger) 5a transfer belt 5b Transfer roller 6 Transfer material 7 Fixing device

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 15/02 101 G03G 15/02 101 102 102 15/24 15/24 21/10 21/00 316 F term (Reference) 2H030 AB02 AD02 AD03 AD17 BB46 BB71 2H068 AA03 AA05 AA08 BB01 CA37 2H078 AA13 BB01 BB12 CC08 DD01 DD15 DD40 DD42 DD75 2H134 GA01 GB02 HC02 HC12 HC14 GA14 GB53 GA57 GA14 GA23 GA45 GA14 GA23 GA52 GA14 GA23 GA02 GAHGA FAGA GA12 HA21 HA28 HA29 HB12 HB17 HB22 HB45 HB46 HB47 HB48 JA02 JA26 JA28 JB06 JB10 MA01 MA02 MA14 MA20 MB06 MC13 MC15 NA02 NA03 NA06 NA09 NA10 PA02

Claims (6)

[Claims] 1. A charging member, to which a voltage is applied, is brought into contact with a charged body to charge the charged body, and image information is exposed on the charged body to form an electrostatic latent image on the charged body. A plurality of image forming units that visualize the electrostatic latent image formed on the toner image as a toner image by a developing device are provided, and the toner image formed by each image forming unit is sequentially transferred onto a transfer material. In the image forming apparatus for forming an image by the above, in the plurality of image forming units, at least one of the image forming units is performing the image forming process, and the charged body of the image forming unit not performing the image forming process. An image forming apparatus characterized in that a charging process is performed on the image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the charging member of the image forming unit is a magnetic brush type charging member made of conductive magnetic particles. 3. The image forming apparatus according to claim 1, wherein the magnetic brush charging member, which is a charging member of the image forming unit, also functions as a cleaning device that collects transfer residual toner on the charged body. 4. A charging process performed on an object to be charged of an image forming unit that is not performing the image forming process while at least one of the plurality of image forming units is performing the image forming process. 3. The image forming apparatus according to claim 1, wherein is an AC electric field only. 5. The member to be charged is a photosensitive layer and resistors 10 ⁹ to 10 ¹⁴
The image forming apparatus according to claim 1, further comprising a surface layer formed of a resin and conductive fine particles of Ω · cm. 6. The image forming apparatus according to claim 4, wherein the conductive particles are SnO ₂ .