JP2005215626A - Developing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing device which when performing two-component contact development under an alternative electric field to an electrostatic latent image using an image carrier having a surface layer containing amorphous silicon with 1×10<SP>9</SP>to 1×10<SP>14</SP>Ωcm of volume resistance, performs development to a steady state at an image part, exhibits wide latitude to an SD gap and amount of developer coating, etc and performs successful image formation by preventing the occurrence of a defective image due to fogging and image density deterioration, sweep and edge enhancement such as void. <P>SOLUTION: In the developing device 4 in which a developer carrier 11 performs contact development of the electrostatic latent image formed on the surface of the image carrier to form a toner image by conveying developer containing nonmagnetic toner and magnetic carrier to a developing part 40 and the magnetic carrier has the volume resistance value of ≥1×10<SP>10</SP>Ωcm ≤1×10<SP>14</SP>Ωcm, a plurality of developer carriers 11 are provided and a plurality of developing parts 40 are formed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a developing device provided in an image forming apparatus that develops an electrostatic latent image formed on an image carrier with a two-component developer to obtain an image.

  Conventionally, as an image forming apparatus, various methods are described in Patent Document 1, Patent Document 2, Patent Document 3, and the like, particularly as electrophotography. In any of these methods, an electrostatic latent image is formed by irradiating the photoconductive layer of the image carrier with a light image corresponding to the original, and then the opposite polarity is formed on the electrostatic latent image. The electrostatic latent image is developed by adhering colored fine powder called toner contained in the developer, and the toner image is transferred onto a transfer material such as paper as necessary. A copy or printout is obtained through an image forming process such as fixing by, for example.

  In the image forming process, the developing process for developing the electrostatic latent image moves the charged toner particles onto the electrostatic latent image on the image carrier using the electrostatic interaction of the electrostatic latent image. In this step, a developer image (toner image) is formed. In general, among the methods for developing such an electrostatic latent image, a two-component developing method employing a two-component developer in which toner is dispersed in a medium called a carrier is particularly required to have high image quality. It is preferably used for full-color copying machines and full-color printers. The developing device is provided with a developer carrier that carries toner particles and conveys them to the surface of the image carrier.

  In recent years, with the development of multimedia, computer image processing, etc., means for outputting a higher-definition full-color image has been demanded. For this purpose, efforts are being made to make full-color copy images and print-out images even higher in image quality and higher in image quality than silver halide photography. In response to these requirements, studies have been made from the viewpoint of processes and materials.

  The two-component development method has been often adopted due to the tendency to improve the image quality. However, the two-component development method described above is a contact two-component method in which development is performed while the developer magnetic brush rubs the surface of the image carrier. Development methods and non-contact two-component development methods in which the developer magnetic brush does not contact the image carrier are classified. The non-contact two-component development method has an advantage that the carrier adhesion phenomenon that the carrier adheres to the image carrier is less likely to occur. However, in order to obtain a high-definition full-color image as described above, excellent fine line reproducibility and sufficient A contact two-component development capable of obtaining a high image density is preferably used.

  Further, the developing device is provided with a developer carrier formed of a rotatable conductive cylinder or the like having a built-in magnetic field generating unit that carries toner particles and conveys them to the surface of the image carrier. In order to convey the image on the electrostatic latent image, a developing bias is applied from the developer carrying member to the electrostatic latent image side, but in order to achieve the above-described high image quality, an AC bias is superimposed on the DC bias. The method is preferably used.

  By the way, in recent years, with the progress of full color, systemization, and digitalization, the demand for higher image quality, higher speed, and higher stability of output images is increasing, and it is expected that copiers and various printers will enter the light printing market. Is done. In order to enter the printing market with the electrophotographic method generally used in copying machines and various printers, high image quality and high stability are the minimum issues.

  Conventionally, selenium-based photoconductors, amorphous silicon photoconductors, organic photoconductors, and the like have been put to practical use as photoconductors that are image carriers used in electrophotographic processes (steps). Amorphous silicon photoreceptors containing crystalline silicon are known to have excellent image quality and durability, and are suitably used when high image quality, high speed, and high stability are required.

  Here, the amorphous photosensitive drum will be described. Amorphous silicon has a conductive support such as a cylindrical aluminum cylinder, a charge injection blocking layer, a photoconductive layer, and a surface layer sequentially deposited on the conductive support. Here, the charge injection blocking layer is for blocking the injection of charges from the conductive support to the photoconductive layer, and the photoconductive layer is made of an amorphous material mainly composed of silicon atoms, Shows photoconductivity. Furthermore, the surface layer often takes charge of holding an electron latent image formed on the surface containing silicon atoms and carbon atoms and improving the durability of the film.

  However, these amorphous photosensitive drums have the following problems.

  As a characteristic of the amorphous silicon photoconductor drum, compared to other organic photoconductors, the photogenerated carriers generated in the photoconductive layer by exposure in the latent image forming process performed in the image forming process are less likely to recombine, In other words, it is difficult to disappear.

  When an electrophotographic photosensitive member is used, a latent image is formed based on external information by exposing the surface of the photosensitive drum with an exposure unit such as a laser driver. In the latent image forming step, the photosensitive drum is formed. As a result of electron transition accompanying light absorption by exposure, photogenerated carriers that can move in the material by an electric field are generated in the photoconductive layer of the.

  However, amorphous silicon photoconductor drums have many tangling bonds (unbonded hands), which become localized levels and trap some of the photogenerated carriers to reduce their runnability. Or reduce the recombination probability of photogenerated carriers. Therefore, a part of the photogenerated carrier generated by exposure continues to exist in the subsequent process. That is, the property that photogenerated carriers are difficult to recombine and are difficult to disappear is derived from the amorphous nature of amorphous silicon.

  As will be described in detail below, this partially remaining photogenerated carrier may combine with the charge of the developed toner during development, causing problems.

  Usually, in the image forming process, after the photosensitive drum is charged to the charging potential Vd in the charging process, the potential Vl of the image portion is formed by exposure and is used for the developing process.

  The electrostatic latent image is determined by the property of light irradiated on the uniformly charged surface of the photosensitive drum. Here, after the electrostatic latent image is uniformly charged on the photosensitive drum, the exposure process is performed. The portion that has not been exposed in step 1 is defined as a non-image portion, and the photosensitive drum is uniformly charged and the portion that has been exposed is defined as an image portion.

  In the developing step, a developing bias is applied to the developing sleeve of the developer carrying member, which is a conductive cylindrical member provided in the developing device so as to face the photosensitive drum, so that the electrostatic latent image becomes a developer image ( If the developing bias at this time includes a DC component having a potential of Vdc, the latent image potential Vl on the surface of the photosensitive drum becomes the developing bias potential Vdc in the image portion. The state in which the latent image portion on the surface of the photosensitive drum is filled with toner is a steady state in which the potential difference between the drum surface layer and the developing sleeve disappears, and the development ends there. When the potential of the toner layer on the surface of the photosensitive drum after the development is measured, a value almost the same as the development bias potential Vdc is measured.

  If development is performed to a steady state in this way, the development process is performed stably, so that even if there is some variation in development capability, for example, the distance between the development sleeve and the photosensitive drum varies, The variation in the toner development amount is small.

  On the other hand, if the developing capability is low, such as when the AC component is not superimposed on the developing bias, toner is not filled in the image area until the developing bias potential Vdc is reached in the developing step, and the toner after the development is completed. There is a case where the layer potential is in an unfilled state where the value is between the latent image potentials Vl and Vdc.

  When the image portion is not filled, that is, not in a steady state, for example, the distance (SD gap) between the developing sleeve and the photosensitive drum reacts sensitively to fluctuations, and the toner development amount tends to vary.

  For this reason, it is desirable that the potential of the toner layer in the image portion after the development is filled up to the development bias potential Vdc, that is, in a steady state.

  However, in the case where an amorphous silicon photosensitive drum is used, as described above, when the amorphous silicon photosensitive drum is charged and then exposed to form a latent image potential Vl, it is generated during exposure. Then, the process proceeds to the next development step while a part of the photogenerated carrier remains.

  In the development process, development is performed by applying a development bias. According to the study by the present inventors, when an amorphous silicon photosensitive drum is used, the toner layer potential of the image portion after development is up to Vdc. It has been found that there are many cases where the filled state is not reached.

  This is considered to be due to the following reason.

  At the time of development, development is performed in the image portion so that the toner fills the potential difference between the development bias potential Vdc and the latent image potential Vl. Usually, the sum of the latent image potential Vl and the potential of the toner formed on the drum is summed. In the state in which Vdc becomes Vdc, there is no potential difference between the sleeve and the drum, the steady state is reached, and development is completed.

  However, in the case of an amorphous silicon photoconductor, the photogenerated carrier generated during the exposure for forming the electrostatic latent image remains, so that the toner adheres to the drum even if the toner is transported to the image area. Then, the charge of the toner and the photogenerated carrier are combined, and the charge of the toner disappears. That is, in the case of an amorphous silicon-based photoreceptor, most of the charge of the developed toner disappears due to the combination with the remaining photogenerated carrier, so that the toner charge is the potential difference between the latent image potential Vl and the development bias potential Vdc. As a result, the development is completed in an unfilled state where a potential difference exists.

  The unfilled state in which the potential of the toner layer in the image area after the development is not filled up to the developing bias potential Vdc is not a steady state, that is, as described above, for example, the SD coat or the developer coating amount on the developing sleeve Because of this, it reacts sensitively to changes in developing ability, such as fluctuations in density, and therefore, unevenness of density or the like is likely to occur on an image.

  As described above, when developing a latent image formed on the surface of an amorphous silicon photoconductor, there is a tendency that the development ends in an unfilled state. To solve this problem However, it is necessary to improve the developing ability of the developing device. As one of the methods, for example, the resistance of the developer can be lowered. The low resistance of the developer means that the potential of the magnetic brush near the photosensitive drum is close to the potential of the developing sleeve. Then, a large potential difference is generated in a narrow gap near the photosensitive drum, and the developability is enhanced due to the large electric field. Note that the resistance of the developer is lowered by selecting a magnetic carrier contained in the developer having a relatively low volume resistance.

  However, according to the study by the inventors, when an amorphous silicon photosensitive drum is used as an image carrier and a developer having a relatively low resistance is used, fogging occurs in the image, and the output image is output. There was a problem that only a low concentration was obtained.

As a result of various investigations on the factors that cause the fogging and the image density reduction in the development process, the above problem is that the volume resistance value of the surface layer is 10 ⁹ to 10 as in the case of an amorphous silicon photosensitive drum. It has been found that this phenomenon occurs when a charge is injected from a magnetic carrier contained in a two-component developer to a photosensitive drum of about ¹⁴ Ω · cm during development.

By the way, it is possible to use a charging method called a charge injection charging method in a charging process for a photosensitive drum having a surface layer volume resistance of about 10 ⁹ to 10 ¹⁴ Ω · cm, not limited to an amorphous silicon drum. is there.

The charge injection charging method is a charging method in which a magnetic drum is used to frictionally charge a photosensitive drum and a bias is applied. Specifically, a ferrite having a volume resistance value of less than 10 ¹⁰ Ω · cm is used. A magnetic particle of approximately 100 μm or less, preferably 15 to 50 μm, is carried on a charging sleeve containing a magnet as a magnetic field generating means, and a bias is applied while rubbing against the photosensitive drum adjusted to the above-mentioned volume resistivity. This is a charging method for charging the photosensitive drum.

The same phenomenon as the charge injection charging in the charging process occurs in the developing process when a relatively low resistance magnetic carrier having a volume resistance of about 10 ⁶ to 10 ⁹ Ω · cm is used. The carrier behaves like a magnetic particle in the charge injection charging system, and the developing sleeve behaves like a charging sleeve, so that the two-component development in which the magnetic carrier is in contact with the photosensitive drum is performed. In such a case, it is considered that charge injection occurs in the developing portion, causing the above-described fogging and a decrease in image density.

  In charge injection charging, it has also been confirmed that charging efficiency is improved when an AC electric field is superimposed.

That is, the charge injection charging with the AC electric field superimposed thereon is replaced with a development step, and in the conventional development method, the volume resistance of the image carrier is 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm. Using an amorphous silicon photoconductor to improve development efficiency and image quality, adopt two-component development using a low-resistance magnetic carrier, and in addition, apply a development bias with an AC electric field superimposed on the charging process. It is considered that the same phenomenon as in charge injection charging has occurred.

That is, a photosensitive drum whose surface layer has a volume resistance adjusted to about 10 ⁹ to 10 ¹⁴ Ω · cm, such as an amorphous silicon photoconductor, has a low volume resistance, for example, 10 ⁶ to 10 ⁹ Ω · cm. Considering a case where reversal development is performed by superimposing an AC electric field using a two-component developer containing about a magnetic carrier, charge injection from the magnetic carrier contained in the developer to the photosensitive drum is performed in the developing unit. Therefore, the potential of both the non-image area and the image area converges on the DC component of the voltage applied to the developing sleeve. For this reason, the potential difference between the non-image portion and the developing sleeve is reduced, fogging occurs, and the potential difference between the image portion and the developing sleeve is reduced, so that the image density is lowered. This can occur in the same manner not only in the reverse development but also in the normal development.

Such a problem is essentially caused by a low volume resistance value of the magnetic carrier in the two-component development, and the volume resistance value of the magnetic carrier is increased, that is, a volume resistance value of 10 ¹⁰ Ω · cm or more is increased. By using the magnetic carrier, no charge is injected into the photoreceptor during development, and the above-described problem such as fog does not occur.

Therefore, when an amorphous silicon photoconductor having a volume resistance of 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm is used, if a relatively high resistance magnetic carrier having a volume resistance of 10 ¹⁰ Ω · cm or more is used, Although the above-mentioned problems such as fogging and density reduction can be avoided, as described above, there arises a problem that the development is completed while the potential difference between the sleeve and the drum is not filled.

  Furthermore, when development is completed without filling, as another problem, edge enhancement occurs in the copied image.

  Edge enhancement occurs in the form as in the examples described in (A) and (B) below.

  (A) When a solid black (image portion) area exists in an image area of a solid white portion (non-image portion) or a relatively low density portion, the density of the edge portion of the solid black density region is high and the solid portion The density is lighter than the edge. Among these, the edge emphasis level is highest at the trailing edge in the transport direction when the movement direction of the developing sleeve and the photosensitive drum is the same direction (forward direction) at the closest part. This edge enhancement at the solid black rear end is usually referred to as “crushing”.

  (B) When there is a solid black image (for example, solid black characters) in the halftone density area, the density of the halftone area around the solid black area becomes light. In this edge emphasis, when the moving direction of the developing sleeve and the photosensitive drum is the same direction (forward direction) at the closest part, the halftone at the tip of the solid black region is whitened out in the transport direction. This phenomenon in which the halftone density region is white is usually referred to as “white blank”.

  Edge enhancement such as (A) sweeping and (B) whiteout occurs more strongly when the potential difference between the solid black portion and the low density portion around it is large. As described above, edge enhancement is also affected by the moving direction of the developing sleeve and the photosensitive drum. However, the movement direction dependence of the developing sleeve and the photosensitive drum is not described in detail in this specification.

  As can be seen from the situation in which edge enhancement occurs as described above, the phenomenon of electric field wrapping around the border between adjacent images with a large potential difference, such as the halftone area and solid black area, is an essential factor for edge enhancement. is there.

  At the time of development, in the development region near the closest position of the photosensitive drum and the developing sleeve, the potential of the electrostatic latent image formed on the photosensitive drum and the potential of the developing sleeve (DC component of the developing bias) The electric field in between is attempted to be canceled by the toner contained in the developer moving from the developer toward the photosensitive drum. Normally, the toner moves so as to cancel out the electric field between the potential of the electrostatic latent image formed on the photosensitive drum and the potential of the magnetic brush near the photosensitive drum, and is developed in a steady state. Ends. Therefore, the potential difference between the halftone area and the solid area is canceled by the developed toner by the end of the development, and the electric field does not wrap around and edge enhancement does not occur.

  However, in the case of an amorphous silicon photoconductor, the potential difference between the potential of the electrostatic latent image and the potential of the developing sleeve is not canceled, and the development is completed without being filled. Is likely to occur.

  Further, when the volume resistance value of the developer is high, the edge portion of the image where the difference in electric field occurs is particularly emphasized, coupled with the fact that the counter electrode effect does not work in the vicinity of the electrostatic latent image.

As described above, in the developing device and the image forming apparatus that perform development using the conventional two-component development method for the amorphous silicon photoconductor as the image carrier, the following (1) and (2) Two problems arise as described.
(1) If a carrier having a relatively high volume resistance is used as the carrier contained in the two-component developer, the photogenerated carrier tends to remain on the surface of the amorphous silicon photosensitive member by exposure in the latent image process. Since the potential difference between the developing bias potential and the latent image potential Vl on the surface of the photosensitive member is not sufficiently small in the developing process, the steady state is not easily affected. Is likely to occur.
(2) As a countermeasure, if the volume resistance of the carrier contained in the two-component developer is relatively low, the volume resistance value of the surface layer is 10 ⁹ to 10 ¹⁴ Ω like an amorphous silicon photosensitive drum. In a photoconductor of about cm, charges are injected from the magnetic carrier into the photoconductor, so that the potential difference between the non-image area and the developing sleeve potential is reduced, and the formed image is likely to be fogged or the density is lowered.
US Patent No. 2,297,691 Japanese Patent Publication No.42-23910 Japanese Patent Publication No.43-24748

An object of the present invention is to use an amorphous silicon photosensitive drum having amorphous silicon as a main component and having a volume resistance of 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm as an image carrier. When two-component contact development is performed on an electrostatic latent image formed on the surface under an alternating electric field, the toner charge is transferred from the developing sleeve and the photosensitive member in the image area without causing fogging or a decrease in image density. Development is performed to a steady state that almost cancels the potential difference of the drum, and the latitude for the SD gap, developer coating amount, etc. is wide, and image formation is not caused by edge enhancement such as close-up or whiteout, and good image formation Is to provide a developing device capable of performing the above.

The above object is achieved by the developing device according to the present invention. In summary, according to the present invention, an electrostatic image formed on an image carrier having a surface layer having a volume resistance of 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm is contacted with a developer containing toner and carrier. In the developing device for developing,
A plurality of developer carriers that carry and transport the developer to a portion facing the image carrier are provided, and the volume resistance value of the carrier is set to 1 × 10 ¹⁰ Ω · cm to 1 × 10 ¹⁴ Ω · cm. A developing device is provided.

In the developing device of the present invention, an electrostatic image formed on an image carrier having a surface layer with a volume resistance of 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm is contact-developed with a developer containing toner and carrier. In the developing device, a plurality of developer carriers that carry and transport the developer to a portion facing the image carrier are provided, and the volume resistance value of the carrier is 1 × 10 ¹⁰ Ω · cm to 1 × 10 ¹⁴ Ω · cm. Therefore, in the developing device that develops the electrostatic latent image formed on the surface of the amorphous silicon photosensitive drum, the developing sleeves as a plurality of developer carrying members are moved upstream and downstream in the moving direction along the photosensitive drum surface. Since it can be arranged side by side and a plurality of development opportunities can be provided, it is possible to prevent charge injection in the developing portion, and to solve the problem that the development ends in a state where the image portion is not filled with toner. Gad A wide latitude for the developer coating amount, etc., fog, density decrease, and no image failure occurs in the edge enhancement and the like, it is possible to form a high quality image.

  Hereinafter, the developing device according to the present invention will be described in more detail with reference to the drawings. Note that the dimensions, materials, shapes, relative positions, and the like of the developing device and the components of the image forming apparatus provided with the developing device limit the scope of the present invention only to those unless otherwise specified. It is not intended.

Example 1
FIG. 1 is a schematic configuration diagram showing an example of an image forming apparatus embodying the present invention. Although the present embodiment is a single color image forming apparatus, the present invention is not limited thereto, and the present invention can be preferably applied to, for example, a full color image forming apparatus.

  The image forming apparatus (copier) includes a reader unit 110 having a document table 10 and an optical scanning unit 9, a photosensitive drum 1 as an image carrier, a developing device 4, a laser scanning unit 100, and the like disposed below the reader unit 110. The image forming means including the printer unit 120, and the image forming process is performed. When the copy G is placed on the document table 10 with the surface on which the document G is to be copied facing down and the copy button is pressed, Copying of the original G, that is, image formation is started.

  In the reader unit 110, external information that is the document G is converted into an image signal and transmitted to the printer unit 120.

  The optical scanning unit 9 provided in the reader unit 110 is configured by integrally integrating a document irradiation lamp (not shown), a short focus lens array (not shown), and a CCD sensor (not shown). When a copy button (not shown) provided on the outside of the apparatus is pressed, the unit 9 scans while irradiating the original G with the irradiation lamp, and the reflected light of the irradiated light from the original G surface has a short focal point. An image is formed by the lens array and incident on the CCD sensor. An image signal (analog signal) obtained by the CCD sensor is converted into a digital signal by well-known image processing, and then sent to the printer unit 120.

  In the printer unit 120, an image forming process is performed. First, an electrostatic latent image is formed on the surface of the photosensitive drum 1 serving as an image carrier based on the image signal of the document G transmitted from the reader unit 110. Here, the photosensitive drum 1 is driven to rotate at a predetermined peripheral speed around the central support shaft, and each image forming process is performed in the rotating process.

  For this purpose, in the charging step, which is the first step, the surface of the photosensitive drum 1 is subjected to a uniform charging process so that the surface becomes, for example, 650 V by the charger 3 which is a contact charging member as a charging means.

  Next, the surface of the uniformly charged photoconductive drum 1 is subjected to a latent image forming step, which is a second step, in a latent image forming unit, here, a solid state laser element 102 (see FIG. 6) receives the image signal (digital signal) transmitted from the reader unit 110, generates laser light L by ON / OFF light emission, and uses the rotating polygon mirror to expose the laser light L that is the exposure. Thus, the surface of the photosensitive drum 1 is scanned, and electrostatic latent images corresponding to the original image are sequentially formed on the surface of the photosensitive drum 1.

  The electrostatic latent image thus formed on the photosensitive drum 1 is developed by the two-component developing device 4 installed around the photosensitive drum 1 in a third development process, and a developer image (toner image). Is visualized as

  The toner image formed on the photosensitive drum 1 is transferred onto the transfer material P conveyed from the paper feed cassette 80 in the transfer process which is the fourth process.

  Here, on the lower side of the photosensitive drum 1, a transfer belt 71 that is wound around a drive roller 72 and a driven roller 73 and rotates in the direction of arrow D is installed as a transfer unit. The transfer material P is taken out from the paper feed cassette 80, is fed onto the transfer belt 71 at an appropriate timing in synchronization with the rotation of the photosensitive drum 1, and the photosensitive drum 1 and the transfer belt 71 are moved at a predetermined timing. It is conveyed to the transfer unit 70 that has come into contact. A transfer charging blade 74 is installed inside the transfer portion 70 of the transfer belt 71. The transfer charging blade 74 presses the transfer belt 71 in the direction of the photosensitive drum 1, and the transfer charging blade 74 is supplied with a high voltage power supply (not shown). By supplying power, the transfer material P is charged from the back side with a polarity opposite to that of the toner, and the toner image formed on the photosensitive drum 1 is electrostatically transferred onto the transfer material P.

  In this embodiment, a polyimide resin sheet having a thickness of 75 μm was used as the transfer belt 71. Other examples of the transfer belt 71 include a polycarbonate resin, a polyethylene terephthalate resin, a polyvinylidene fluoride resin, a polyethylene naphthalate resin, a polyether ether ketone resin, a polyethersulfone resin, a polyurethane resin, or a fluorine-based or silicone resin. A rubber sheet of the type can be preferably used. The thickness of the transfer belt 71 is not limited to 75 μm, and a transfer belt having a thickness of about 25 to 2000 μm, preferably 50 to 150 μm can be suitably used.

A transfer charging blade 74 having a resistance of 10 ⁵ to 10 ⁷ Ω, a thickness of 2 mm, and a length of 306 mm was used. At the time of transfer, the current applied to the transfer charging blade 74 was +15 μA, and this was supplied with power controlled under constant current control.

  As described above, the transfer material P onto which the toner image has been transferred is separated from the transfer belt 71 and then conveyed to the fixing device 6 as fixing means, and the transfer material P is heated in the fixing step as the fifth step. Then, the image is fixed by pressurization and output as a print image as an image formed product to the outside of the image forming apparatus.

  The photosensitive drum 1 after transferring the toner image is used for repeated image formation by removing contaminants such as transfer residual toner adhering to the surface by the cleaner 5.

  The image formation is performed through the image forming process described above. However, the present invention is not limited to the example in which the image formation is performed by one photosensitive drum as described above. Also applicable to the tandem system, in which a plurality of photosensitive drums and a plurality of developing devices (for example, yellow, magenta, cyan, black, etc.) provided for each of the photosensitive drums are arranged side by side as a set. It is. Further, the present invention can also be applied to an intermediate transfer system configuration in which a toner image is primarily transferred from a photosensitive drum to an intermediate transfer member and then secondarily transferred from the intermediate transfer member to a transfer material.

By the way, as the photosensitive drum 1, an amorphous silicon photosensitive member is used which is configured by providing an amorphous silicon photosensitive layer containing amorphous silicon on a metal drum base made of aluminum having a diameter of 60 mm. Yes. The volume resistivity is 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm. In this embodiment, the photosensitive drum 1 is negatively charged.

As described above, in the present invention, since the photosensitive drum 1 having a surface layer with a relatively low volume resistivity of 10 ⁹ to 10 ¹⁴ Ω · cm is used, the magnetic carrier contained in the developer has a low resistivity. In order to prevent charge injection in the developing part, which is caused by the above, a relatively high volume resistance of 10 ¹⁰ Ω · cm to 10 ¹⁴ Ω · cm as a magnetic carrier in the two-component developer used in the development process Use one with a value.

  Here, the charger 3 that is a charging unit, the laser exposure operation unit 100 that is a latent image forming unit, and the developing device 4 that is a developing unit that perform an image forming process that acts on the photosensitive drum 1 will be described.

First, the charger 3 as charging means will be described. In the charging step in the first step of the image forming method, not only an amorphous silicon drum but also a photosensitive drum having a surface layer with a volume resistance of about 10 ⁹ to 10 ¹⁴ Ω · cm is referred to as a charge injection charging method. It is possible to use a method.

  In this embodiment, charging of the photosensitive drum 1 provided with the amorphous silicon photosensitive layer was performed by using a charge injection charging type contact charging member, particularly a magnetic brush charging type charger, as the charger 3.

  The magnetic brush charger 3 is configured by fixing a non-rotating magnet 3b, which is a magnetic field generating means, inside a rotatable nonmagnetic sleeve 3a having an outer diameter of 16 mm. The magnetic particles 3c held on the surface are formed in a magnetic brush so that the magnetic brush 3c comes into contact with the surface of the photosensitive drum 1.

  The magnetic particles 3c, which are magnetic brushes, are conveyed by the rotation of the nonmagnetic sleeve 3a, and by applying a charging voltage to the nonmagnetic sleeve 3a, electric charges are applied to the photosensitive drum 1 through the magnetic particles 3c, and the photosensitive member The surface of the drum 1 is charged to a potential corresponding to the charging voltage.

  The charging of the photosensitive drum 1 with the magnetic brush 3c of magnetic particles is performed by adjusting the nip width of the contact portion of the magnetic brush 3c with the photosensitive drum 1 to about 6 mm, and applying a charging bias applied to the nonmagnetic sleeve 3a to -700V. It was successfully implemented by setting a bias in which a rectangular wave alternating voltage having a frequency of 1000 Hz and an amplitude of 800 V was superimposed on the direct current voltage.

  As shown in the figure, the rotation direction of the non-magnetic sleeve 3 (the rotation direction of the magnetic brush 3c) is preferably the counter direction in the B direction with respect to the photosensitive drum 1 rotating in the A direction. The charging of becomes better. Also, the higher the rotational speed of the nonmagnetic sleeve 3a, the more uniform the charging of the photosensitive drum 1 tends to be. In this embodiment, the rotational speed of the nonmagnetic sleeve 3a is 1.5 times the peripheral speed of the photosensitive drum 1 at a rotational speed of 300 mm / second.

The magnetic particles 3c used in the magnetic brush charger 3 are preferably those having an average particle diameter of 10 to 100 μm, a saturation magnetization of 20 to 250 emu / cm ³ , and a volume resistance value of 10 ² to 10 ¹⁰ Ωcm. In view of the case where there is an insulation defect such as a pinhole, a more preferable volume resistance value is about 10 ⁶ to 10 ¹⁰ Ωcm. In this example, magnetic particles having an average particle diameter of 25 μm, a saturation magnetization of 200 emu / cm ³ , and a volume resistance of 5 × 10 ⁶ Ωcm were used. The method of measuring the resistance value of the magnetic particles is the same as that of the magnetic carrier for development, and will be described later.

  For magnetic particles, a resin carrier formed by dispersing maglite as a magnetic material in a resin, and carbon black dispersed for conductivity and resistance adjustment, or the surface of a magnetite simple substance such as ferrite is oxidized and reduced for resistance. What adjusted, or what adjusted resistance by coating the surface of a magnetite simple substance with resin, etc. are used.

  Note that the charging process is not limited to the above-described system, and for example, a corona charging system may be used. However, the charge injection charging system directly injects charges instead of discharging and exposes the surface of the photoreceptor. In the case of an image forming apparatus using a photosensitive drum having a relatively low surface resistance as used in the present invention, the charge injection charging method is easier. In some cases, the charge injection charging method is suitable.

  After the charging step, which is the first step by the charger 3, is completed, a second step is performed in which an electrostatic latent image is formed on the photosensitive drum 1 by the method described above.

  A latent image forming unit that performs the latent image forming step, which is the second step, here, the laser operation unit 100 as an exposure unit will be described. FIG. 6 shows a schematic configuration of a laser scanning unit 100 that is an exposure unit including the solid-state laser element 102 described above. In the laser scanning unit 100, first, the solid-state laser element 102 is blinked at a predetermined timing by the light emission signal generator 101 based on the input image signal. The laser light L0 emitted from the solid-state laser element 102 in this way is converted into a substantially parallel light beam L1 by the collimator lens system 103, and further scanned by the rotating polygon mirror 104 that rotates in the direction of arrow b, and the fθ lens group Images are formed in spots on the scanned surface 106 of the photosensitive drum 1 by 105a, 105b, and 105c. By scanning with such laser light L, an exposure distribution for one image scan is formed on the scanned surface 106, and the scanned surface 106 is scrolled by a predetermined amount perpendicular to the scanning direction for each scan. As a result, an exposure distribution corresponding to the image signal is obtained on the surface 106 to be scanned. That is, an electrostatic latent image is formed.

  In the third step, the electrostatic latent image is developed by the two-component developing device 4 installed around the photosensitive drum 1 and visualized as a developer image (toner image).

In the development process, as described above, in order to prevent charge injection in the developing unit 40 with respect to the photosensitive drum 1 having a surface layer with a relatively low volume resistance value of 10 ⁹ to 10 ¹⁴ Ω · cm, As the magnetic carrier in the two-component developer used in the development process, one having a volume resistance value of 10 ¹⁰ Ω · cm to 10 ¹⁴ Ω · cm is used.

  The physical property values of the magnetic carrier used in this example will be described below.

When the volume resistance value (specific resistance) of the magnetic carrier is smaller than 10 ⁹ Ω · cm, charge injection at the developing unit 40 is likely to occur. When a carrier larger than 10 ¹⁴ Ω · cm is used, the developer becomes electrically insulative as a developer, and the developing ability is lowered and the edge emphasis is noticeable. It becomes difficult.

Therefore, in the present invention, it is necessary that the resistance of the carrier be 10 ¹⁰ Ω · cm or more and 10 ¹⁴ Ω · cm or less. Also in this embodiment, the one having a volume resistance value in this region is used.

The specific resistance, which is the volume resistance value of the magnetic carrier, is obtained by filling the cell with the magnetic carrier, placing one of the pair of electrodes in contact with the filled carrier, and applying a voltage between these electrodes. Measured by measuring the current flowing at that time. The specific resistance was measured under the condition that the contact area between the filled carrier and the electrode was about 2.3 cm ² , the carrier filling thickness was about 2 mm, the load on the upper electrode was 180 g, and the applied voltage was 100V. In this case, since the magnetic carrier is powder, the filling rate may change, and the specific resistance changes accordingly. Therefore, care must be taken when filling the carrier so as not to happen. The volume resistance value of the magnetic powder used in the charging process was also measured by the same measurement method.

By the way, as described in the conventional example, when the amorphous silicon photosensitive drum is used, the photogenerated carrier generated at the time of exposure is combined with the toner at the time of development and takes away a lot of toner charge. It is difficult to cancel the latent image potential formed thereon due to the charge of the toner, and as a result, development ends in an unfilled state. As a result, it is weak against fluctuations in the distance between the sleeve and the drum, and unevenness tends to occur, and edge emphasis tends to occur. Such a problem is more conspicuous because when the developer containing a carrier having a volume resistance value of 10 ¹⁰ Ω · cm or more and 10 ¹⁴ Ω · cm or less is used, the developing ability is lowered. is there.

  However, according to the study by the present inventors, even if an amorphous silicon photosensitive drum is used, if there is an opportunity for development of one electrostatic latent image at least twice, that is, a portion of the electrostatic latent image that has been developed once. It was found that the development can be completed in a filled state with almost no potential difference between the drum and the sleeve by developing again. Therefore, in the present invention, in the developing means, a plurality of developing sleeves are arranged side by side along the surface of the photosensitive drum, upstream and downstream in the movement direction, and a plurality of development opportunities are given to one electrostatic latent image. The purpose was achieved by providing it.

  Therefore, in this embodiment, as a process (hardware) countermeasure against such edge enhancement, a plurality of developing devices are used, and a developing sleeve 11 which is a developer carrier provided in each of them is used.

  The two-component developing device 4 as developing means will be described. In the image forming apparatus of the present invention, as in the conventional case, the surface of the developer carrying member (developing sleeve) in which the magnetic field generating means is disposed includes the toner and the carrier having the volume resistance value described above. The component developer is carried and transported to the developing unit 40 that is opposite to the photosensitive drum 1, and a developer magnetic brush 19a (see FIG. 2) is formed on the developing unit 40 by the developing magnetic pole of the magnetic field generating means. Thus, a two-component contact developing method for developing the electrostatic latent image on the photosensitive drum (image carrier) 1 is employed. As shown in FIG. 1, in the developing device 4 of the present embodiment, the developing sleeve 11 is a developing device provided upstream and downstream along the moving direction (rotating direction) of the surface of the photosensitive drum 1. 4a and 4b are installed as one component.

  The configuration of the developing devices 4a and 4b constituting the developing device 4 is shown in detail in FIG. Since the developing unit 4a and the developing unit 4b have the same configuration and are arranged upstream and downstream of the outer periphery of the photosensitive drum 1, there is no difference in the magnet pattern of the mag roller 12 that is a magnetic field generating means in the developing sleeve 11. However, since it is made according to the same principle, the description will be made with the developing device 41 as a representative of the upstream developing device 4a in the rotation direction of the photosensitive drum 1. The developing device 41 can be replaced with 4a or 4b.

  The developing device 41 is a two-component magnetic brush developing device. The developing device 41 includes a developing container 16 containing the two-component developer 19 containing the toner and the magnetic carrier described above. A developing sleeve 11, which is a developer carrying member that carries the developer 19 and conveys it to the developing unit 40 that faces the photosensitive drum 1, is installed in an opening portion of the developing container 16 that faces the photosensitive drum 1. The sleeve 11 rotates in a direction in which a portion facing the photosensitive drum 1 moves in the same direction.

Here, the diameter of the developing sleeve 11 is 30 mm, the diameter of the photosensitive drum 1 is 80 mm, and the closest region between the developing sleeve 11 and the photosensitive drum 1 is set to a distance of about 400 μm, so The developer 19 is set so that development can be performed with the conveyed developer 19 in contact with the photosensitive drum 1. In this embodiment, at this time, the developer coating amount per unit area on the developing sleeve 11 by the regulating blade 15 which is the regulating member facing the developing sleeve 11 upstream of the developing portion in the rotation direction of the developing sleeve 11. Is regulated to 30 mg / cm ² . The developing sleeve 11 is made of a conductive material such as Al or SUS.

  A magnet roller 12 which is a magnetic field generating means is fixedly disposed in the developing sleeve 11, and in the present embodiment, the conductive magnetic regulating blade 15 described above is disposed perpendicular to the substantially top portion of the developing sleeve 11. ing. The gap between the regulating blade 15 and the developing sleeve 11 is set to 200 to 1000 μm, preferably 300 to 700 μm. In this embodiment, it is set to 600 μm.

  Further, in the developing container 16, two developer agitating and conveying screws 13 and 14 for agitating and conveying the developer 19 are disposed at the bottom. A partition wall 16a that is open at both axial ends of the screw is provided between the screws 13 and 14, and the screw 13 closer to the developing sleeve 11 is provided in the developing sleeve 11. The developing chamber 13a for supplying the developer 19, and the one where the distant screw 14 is disposed is the stirring chamber 14a for stirring the developer 19 to be uniform. The developer 19 is circulated between the developing chamber 13a and the stirring chamber 14a by the rotation of the screws 13 and 14. The stirring chamber 14a is often replenished with new toner.

  The electrostatic latent image formed on the photosensitive drum 1 is developed by the two-component magnetic brush developing method using the developing device 41 as follows. First, the developing sleeve 11 is rotated, and the developer 19 in the developing container 16 is pumped onto the developing sleeve 11 by the magnetic pole N3 located on the inner side of the developing container 16 of the magnet roller 12 along with the rotation. The developer 19 is conveyed from the magnetic pole S2 to N1 by the rotational movement of the developing sleeve 11. During this conveyance process, the developer is formed into a thin developer layer on the developing sleeve 11 by magnetically regulating the thickness of the developer layer by the regulating blade 15. When the developer thin layer is conveyed to the developing main pole S1 of the magnet roller 12, the developer thin layer rises toward the photosensitive drum 1 by the magnetic force of the developing main pole S1. The developer spike (magnetic brush) 19a comes into contact with the surface of the photosensitive drum 1 to develop the electrostatic latent image on the photosensitive drum 1, and the electrostatic latent image is visualized as a toner image.

  At the time of this development, a developing bias in which a DC voltage and an AC voltage are superimposed is applied to the developing sleeve 11 from the bias power source 17 between the photosensitive drum 1. In this embodiment, a DC voltage of −500 V, a peak-to-peak voltage Vpp of 2 kV, and a frequency f of 12 kHz are AC voltages. The DC voltage value and the AC voltage waveform are not limited to this. However, in general, in the two-component magnetic brush development method, when an AC voltage is applied, the development efficiency increases and the image becomes high quality. For example, the peak-to-peak voltage of the AC voltage is increased or the AC voltage is increased. As the frequency increases, the image becomes higher quality. Therefore, a high-quality image can be obtained by setting the peak-to-peak voltage Vpp to 1.0 kV or more, more preferably 1.5 kV or more and the frequency f to 8 kHz or more.

However, on the other hand, when a drum with a low surface volume resistance is used as in the present invention, an AC voltage is applied, the peak-to-peak voltage is further increased, and the frequency is increased. In such a case, charge injection into the photosensitive drum 1 in the developing unit 40 tends to occur. When high-resistance carriers of about 10 ¹⁰ to 10 ¹⁴ Ω · cm are used, charge injection generally does not occur, but peak-to-peak voltage Vpp is 2.5 kV or less, preferably 2.1 kV or less, and frequency f is By setting the frequency to 20 kHz or less, for example, when there is a low-resistance portion due to unevenness in manufacturing the photosensitive drum 1, or when the resistance of the developer is effectively reduced in an extremely high humidity environment, etc. However, charge injection is unlikely to occur.

  By the way, when an AC voltage is applied, the development efficiency increases, but conversely, fogging easily occurs. Therefore, in this embodiment, fogging is prevented by providing a potential difference between the DC voltage applied to the developing sleeve 11 and the charged potential (non-image portion potential) of the photosensitive drum 1.

  In the developing unit 40, both the developing sleeves 11 of the developing devices 4a and 4b are moved in the forward direction and the moving direction of the photosensitive drum 1, and the peripheral speed ratio is 1.75 times the photosensitive drum. ing. This peripheral speed ratio is set between 0 and 3.0 times, and preferably any number as long as it is set between 1.2 and 2.5 times. The larger the moving speed ratio, the higher the development efficiency. However, if the moving speed ratio is too large, problems such as toner scattering and agent deterioration occur. Therefore, the moving speed ratio is preferably set within the above range.

  In this embodiment, when development is performed using the two developing devices 4a and 4b having the above-described configuration and the two-component developer 19 having low magnetization and high volume resistance, the rotation of the photosensitive drum 1 is performed. In the developing portion 11 on the opposite side of the photosensitive drum 1 and the developing sleeve 11 of the developing device 4 a located upstream in the direction, the first development is performed on the electrostatic latent image on the photosensitive drum 1.

  At the first development, the development is completed in an unfilled state that does not fill the potential difference between the developing sleeve 11 and the photosensitive drum 1. That is, as a result of the toner flying from the developer 19 coated on the developing sleeve 11 according to the electric field formed between the developing sleeve 11 and the electrostatic latent image formed on the photosensitive drum 1. In this state, the development end stage has not been reached.

  The development end stage described here is a steady state in which the potential difference between the potential of the electrostatic latent image and the potential of the developing sleeve 11 is eliminated after the development is completed, that is, the toner adheres after the development. When the potential of the state is measured, the potential is almost converged to the DC potential (Vdc) applied to the developing sleeve 11. In this case it is defined as such.

  In the case of an amorphous silicon photoconductor, the photogenerated carrier at the time of exposure remains, and when the toner is developed, the toner is deprived of the charge by the photogenerated carrier, so it is difficult to bring it to a so-called development end stage. Further, in the case of the configuration of the present embodiment, a high-resistance carrier is used, which is particularly difficult to carry until the development end stage.

  Therefore, in the second development, the combination of the toner charge and the photogenerated carrier has been completed to some extent in the first development, and the remaining photogenerated carrier has almost disappeared. Further, by performing the development for the second time, the loss of charge due to the combination of the toner charge and the photogenerated carrier hardly occurs, and the development end stage can be reached.

  As a result, unevenness and edge enhancement due to fluctuations in the distance (SD gap) between the developing sleeve 11 and the photosensitive drum 1 due to the fact that the development end stage has not been reached do not occur.

  Here, the developer used in the present invention will be described.

In the present invention, a magnetic carrier having the above-described volume resistance and a relatively high volume resistance value is used. As a metal oxide constituting the magnetic carrier, MO · Fe ₂ O ₃ or MFe ₂ O ₄ exhibiting magnetism is used. Magnetite, ferrite and the like represented by the general formula can be preferably used. Here, M corresponds to a divalent or monovalent metal ion Mn, Fe, Ni, Co, Cu, Mg, Zn, Cd, Li or the like, and M can be used alone or as a plurality of metals. For example, iron oxides such as magnetite, γ iron oxide, Mn—Zn ferrite, Ni—Zn ferrite, Mn—Mg ferrite, Ca—Mg ferrite, Li ferrite and Cu—Zn ferrite can be exemplified. .

The above-mentioned metal oxide can be used alone as a carrier core, but in that case, it is necessary to use a core specific resistance of 1 × 10 ¹⁰ Ωcm or more by performing a treatment such as intense oxidation of the core surface. It is. Alternatively, it may be 1 × 10 ¹⁰ Ω · cm or more by adjusting the carrier surface coat type, thickness, and the like.

  When a high-resistance carrier core is used as the development magnetic carrier, a particularly preferable carrier form is to use the above metal oxide dispersed in a resin as the carrier core. In this case, one kind of metal oxide can be dispersed in the resin and used, but it is particularly preferred that at least two kinds of metal oxides are mixed.

In this case, in addition to the above magnetic metal oxide, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Nb, Mo , Cd, Sn, Ba, Pb, or other nonmagnetic metal oxides using one or more metals, and metal oxides exhibiting the above-mentioned magnetism can be used. For example, nonmagnetic metal oxides such as Al ₂ O ₃ , SiO ₂ , CaO, TiO ₂ , V ₂ O ₅ , CrO ₂ , MnO ₂ , Fe ₂ O ₃ , CoO, NiO, CuO, ZnO, SrO, Y ₂ O ₃ , ZrO ₂ system, etc. can be used.

In this case, it is more preferable to use particles having similar specific gravity and shape in order to improve adhesion to the binder and carrier strength. For example, magnetite and hematite, magnetite and SiO ₂ , magnetite and Al ₂ O ₃ , magnetite and TiO ₂ , magnetite and Ca—Mn ferrite, magnetite and Ca—Mg ferrite can be preferably used. Among these, a combination of magnetite and hematite can be preferably used from the viewpoint of price and carrier strength.

The average particle size of the developing magnetic carrier of the present invention is preferably 5 to 100 μm in number average particle size, and the saturation magnetization at 100 mT is preferably 20 to 250 emu / cm ³ . When the number average particle size is less than 5 μm, the development magnetic carrier having a substantially high resistance as described above is used particularly in the case of a contact two-component development process such as the image forming method of the present invention and a development method using an AC development bias. In some cases, carrier adhesion could not be avoided. On the other hand, if the number average particle diameter exceeds 100 μm, a high quality toner image excellent in fine line reproducibility which is an object of the present invention may not be obtained. More preferably, it is 20 to 45 emu / cm ³ . Regard the saturation magnetization, when the 20 emu / cm ³ or less, carrier adhesion is a problem, whereas A large problem is not even 250 emu / cm ³ or more, so conspicuous sweep Me unevenness by rubbing from the viewpoint of high image quality . More preferably, it is good to set it as 100-220emu / cm < ³ >.

  A method for measuring the carrier particle size used in the present invention will be described. As for the particle size of the carrier of the present invention, 300 or more carrier particles having a particle size of 0.1 μm or more are randomly extracted by a scanning electron microscope (100 to 5000 times), and the image processing analyzer Luzex3 manufactured by Nireco Corporation is used. The carrier diameter is measured with the horizontal ferret diameter, and the number average particle diameter is calculated.

On the other hand, for saturation magnetization, the magnetization (emu / g) of a carrier packed in an external magnetic field of 100 mT is obtained by using an oscillating magnetic field type automatic magnetic recording device manufactured by Riken Denshi Co., Ltd. The amount of magnetization (emu / cm ³ ) was calculated by applying the true specific gravity (g / cm ³ ).

  As the toner used for the developer together with the above magnetic carrier, a conventionally known toner, for example, a toner prepared by a so-called pulverization method or a polymerization method can be used. The volume average particle size of the toner is preferably 4 to 15 μm. From the viewpoint of high image quality, a toner having a smaller particle size is preferable, and 4 to 7 μm is more preferable. However, as the fine line reproducibility is increased by reducing the particle size, the image reproducibility such as edge enhancement becomes more prominent. Therefore, the present invention is suitable for making edge enhancement inconspicuous even with such a small particle size toner.

  The volume average particle diameter of the toner can be measured by, for example, the following measurement method.

  A call counter TA-II type (manufactured by Coulter Inc.) is used as a measuring device, and an interface (manufactured by Nikka Ki) and CX-i personal computer (manufactured by Canon) for outputting the number average distribution and volume average distribution are connected thereto. As the electrolytic solution, 1% NaCl aqueous solution is prepared using sodium chloride (reagent grade 1).

  In 100 to 150 ml of the above electrolytic solution, 0.1 to 5 ml of a surfactant (preferably alkyl benzene sulfonate) is added as a dispersant, and 0.5 to 50 mg of a measurement sample toner is added and suspended. The electrolytic solution in which the sample is suspended is dispersed for about 1 to 3 minutes with an ultrasonic disperser, and then the particle size distribution of toner particles of 2 to 40 μm is obtained using the 100 μm aperture by the above-mentioned call counter TA-II type. Measure and determine the toner volume distribution. From the volume distribution of the toner thus obtained, the volume average particle diameter of the toner is obtained.

  Further, the external additive used in the present invention preferably has a particle size of 1/10 or less of the weight average particle size of the toner particles from the viewpoint of durability when added to the toner. The particle size of the external additive means the average particle size obtained by observing the surface of the toner particles with a microscope. The external additive is used in an amount of 0.01 to 15 parts by weight, preferably 0.05 to 12 parts by weight, based on 100 parts by weight of the toner.

  Examples of the external additive include the following. Metal oxides such as aluminum oxide, titanium oxide, strontium titanate, cerium oxide, magnesium oxide, chromium oxide, tin oxide, and zinc oxide; nitrides such as silicon nitride; carbides such as silicon carbide; calcium sulfate, barium sulfate, carbonic acid Metal salts such as calcium; fatty acid metal salts such as zinc stearate and calcium stearate; carbon black; silica and the like. These external additives may be used alone or in combination. Those subjected to hydrophobic treatment are preferred.

The charging polarity of the toner composed of the above components can be either negative polarity or positive polarity, but in this embodiment, toner of negative charging polarity is used and the average charging amount (unit) is charged by friction with the carrier. charge amount per weight; hereinafter Q / M) was used as the ^{-1.0 × 10- 2 C / kg~-} 6.0 × 10 -2 C / kg.

As described above, the present invention uses a magnetic carrier having a volume resistance of 10 ¹⁰ Ω · cm or more and 10 ¹⁴ Ω · cm or less in a developing device using an amorphous silicon drum, and a plurality of developments. The present invention relates to an image forming apparatus that is used for development by arranging sleeves along the photosensitive drum surface upstream and downstream in the moving direction, that is, by increasing the resistance value of the carrier, charge injection in the development region is performed. In addition to preventing the problem of development in an unfilled state, it was solved by providing multiple development opportunities, wide latitude for the SD gap, developer coating amount, etc., and further preventing edge enhancement from occurring. In summary.

  In this embodiment, the number of development opportunities is increased twice by using two developing units each having one developing sleeve. Incidentally, the development opportunity may be more than twice, and if it is increased to three times or more, the toner is sufficiently moved to the electrostatic latent image, and good development can be achieved. Therefore, three or more developing devices may be provided.

  Further, the developing device may have another configuration as long as it has a developing sleeve.

  In the developing device according to the present invention, a so-called contact two-component AC development is performed in which a two-component developer is used as a developer, and a developer magnetic brush contacts and rubs the latent image carrier and develops while applying an alternating electric field. Is the best. This is for obtaining a high-quality toner image having excellent fine line reproducibility as described above.

  Furthermore, by substantially increasing the resistance of the magnetic carrier constituting the two-component developer, an image with good fine line reproducibility can be obtained while preventing toner fog and carrier adhesion.

  Further, by developing the electrostatic latent image on the image carrier with a plurality of developer carriers, development is performed to a filling state where the charge of the toner almost cancels the potential difference between the developer carrier and the image carrier. A latitude with respect to the gap, developer coating amount, etc. is wide, and furthermore, a high-quality image free from image defects due to edge enhancement such as close-up or whiteout is obtained.

If this structure is taken, a high resistance carrier is contained with respect to the photosensitive drum which can move the surface which has the surface layer containing at least amorphous silicon whose volume resistance is 1 * 10 < ⁹ > -1 * 10 < ¹⁴ > ohm * cm. Depending on the developer to be developed, there are two or more development opportunities for the latent image formed on the photosensitive drum, so that the toner charge is developed to a filling state that almost cancels the potential difference between the developing sleeve and the photosensitive drum. High image quality can be realized for a long time.

Example 2
The image forming apparatus of the present embodiment has the same configuration as that of the first embodiment except for the configuration of the developing device 4.

  As shown in FIG. 3, in the first embodiment, two independent developing devices 4a and 4b are used as the developing device 41 constituting the developing device 4, whereas in the present embodiment, one developing device 4c is used. Two developing sleeves 11c and 11d are contained therein.

  Thus, if one developing device having a plurality of developing sleeves is used, there are two development opportunities, and the object of the present invention can be achieved. As an example of such a developing device, the developing device used in this embodiment will be described with reference to FIGS.

  The developing device 4c is configured as a two-component magnetic brush developing device like the developing devices 4a and 4b of the first embodiment, and the two-component developer 19 containing the toner and the magnetic carrier described in the first embodiment. And a developing container 16 containing the. As shown in FIG. 4, the developer is a developer carrying member that carries the developer 19 in the opening facing the photosensitive drum 1 of the developing container 16 and transports it to the developing unit 40 facing the photosensitive drum 1. Two developing portions are provided with two sleeves 11c and 11d adjacent to each other so as to face the photosensitive drum 1, and are provided on the upstream side and the downstream side along the moving direction thereof. Here, the developing sleeves 11c and 11d rotate in the directions of arrows C and D, respectively, in the direction in which the portion facing the photosensitive drum 1 moves in the same direction.

  As shown in FIG. 4, the S1 pole and developer layer thickness regulating blade 15 of the mag roller 12c in the developing sleeve 11c on the upstream side in the moving direction of the photosensitive drum 1 are positioned upstream of the developing unit 40 in the rotating direction of the developing sleeve 11c. The developer layer thickness is regulated. Similarly to the first embodiment, a developing bias is applied in common to both developing sleeves 11c and 11d from a power source (not shown). The same developing bias as that in Example 1 is applied.

  The configuration of each of the two developing sleeves 11c and 11d is the same as that of the developing sleeve 11 provided in the developing devices 4a and 4b of the first embodiment, and a magnet roller 12c that is a magnetic field generating unit fixed inside, respectively. It is composed of rotatable nonmagnetic cylinders 11c and 11d on which 12d is disposed.

  The flow of the agent on the two developing sleeves 11c and 11d is that the developer that has completed the first development by the developing sleeve 11c on the upstream side in the moving direction of the photosensitive drum 1 is received by the developing sleeve 11d on the downstream side. The second development is performed. The developer is transferred between the developing sleeves 11c and 11d at the N3 pole and the S3 pole facing each other provided in each of the mag rollers 12c and 12d in the developing sleeves 11c and 11d.

  In this configuration, since the T / D ratio at the time of the second development is surely lowered, the developer 19 after the first development is not reliably stirred and is induced by the carrier in the developer 19. If a reverse charge (reverse charge (+ side) induced on the carrier (ie, counter charge) due to the toner being peeled off from the carrier) remains, uneven peeling or the like is likely to occur during the second development. However, according to this configuration, the developed agent in which the counter charge has occurred moves on the upstream sleeve 11c and is further transferred to the downstream sleeve 11d until the second development is performed. Counter charge disappears due to the movement of toner and carrier. As a result, although the T / D ratio is decreased, no unevenness occurs.

  In the case of this system, two developing devices are not arranged around the photosensitive drum 1 and can be handled by one developing device, and the configuration can be simplified.

  In the developing device 41 of the first embodiment described with reference to FIG. 2 and the developing device 4c of the present embodiment shown in FIG. 4, the stirring and conveying screws 13 and 14 provided in the developing container 16 are stirred and conveyed in the longitudinal direction. The screws 13 and 14 are horizontally arranged on the left and right, but may be arranged vertically as in the developing device 4d shown in FIG. Even if the screws 13 and 14 are arranged horizontally or vertically, the basic function does not change. Here, the upper accommodating portion in which the screw 13 is disposed is defined as the developing chamber 13a. The developing unit 4d is characterized in that the developing unit 4c shown in FIG. 4 in which the screws 13 and 14 are arranged horizontally feeds the developer 19 to the developing sleeve 11c while the developing chamber 13a in which the screw 13 is arranged, Whereas the developer is collected from the developing sleeve 11d, the developing device 4d shown in FIG. 5 is supplied with the developer 19 from the screw 13 to the developing sleeve 11c according to the arrow E in the developing chamber 13a. From the developing sleeve 11d, the developer 19 is collected in the stirring chamber 14a in which the screw 14 below is disposed in accordance with the arrow F. When the developing chamber 13a for supplying the developer 19 to the developing sleeve 11c also serves as a collection chamber as in the developing device 4c shown in FIG. 4, the developer once used for development is immediately used again. When the developing chamber 13a for supplying the developer 19 to the developing sleeve 11c and the agitating chamber 14a for recovery are separately provided as in the developing device 4d, this is not the case, and toner density unevenness in the developer 19 is not caused. It is possible to always supply a fresh developer 19 having no equality to the developing sleeve 11c.

  The configuration in which the developing chamber and the stirring chamber are provided above and below can be applied to a configuration in which one developing sleeve 11 is provided in one developing device. In this case, the developer is supplied to the developing sleeve at the upper side. It is a developing chamber, and a lower stirring chamber is collected. Furthermore, regardless of the number of developing sleeves, the structure of the developing chamber for supplying the developer at the upper portion and the stirring chamber for collecting at the lower portion can be applied.

  The amorphous silicon drum used in the image forming apparatus of the present invention is generally said to have a non-uniform electric potential on the drum. However, by adopting a configuration like the developing device of FIG. Thus, development can be performed, and the potential unevenness peculiar to the amorphous silicon drum can be made inconspicuous.

As described above, in an image forming apparatus equipped with such a developing device, the surface layer has a volume resistance of about 10 ⁹ to 10 ¹⁴ Ω · cm, and two-component contact development is performed under an alternating electric field. In this case, by using a magnetic carrier having a relatively high volume resistance value, the toner charge can be separated from the developing sleeve even if an amorphous silicon photoreceptor is used without causing fogging and a decrease in image density. Development was performed to a filling state that almost canceled the potential difference of the photosensitive drum, and high image quality could be achieved in a small space / low cost.

  Although the rotation direction of the developing sleeve and the flow of the developer are not limited to the present embodiment, when the rotation direction of the developing sleeve changes, the position of the developer regulation on the sleeve is changed.

  The number of developing sleeves may be three or more. A plurality of developing sleeves are arranged side by side along the moving direction of the image carrier, and development is performed three times or more.

Example 3
In the present invention, only the development bias waveform is different from that of the first embodiment, and all other configurations are the same as those of the first embodiment. The developing bias used in this embodiment will be described.

  In the developing bias used in the present invention, the developing bias is applied by applying a voltage for applying a force in a direction from the image carrier (photosensitive drum) 1 to the developer carrier (developing sleeve) 11 for a certain period of time. And a step of applying a voltage that applies a force from the developing sleeve 11 to the photosensitive drum 1 to the developer for a certain period of time alternately, and an AC voltage that repeats a plurality of times alternately, and from the developing sleeve of the AC voltage to the photosensitive member. After applying a voltage that gives a force toward the drum, a DC voltage, which is a voltage value between the potential of the image portion of the electrostatic latent image on the photosensitive drum 1 and the potential of the non-image portion, is fixed to the developing sleeve 11. It is performed by applying time and repeating a combination cycle of these AC voltage and DC bias. This developing bias is generally referred to as “blank pulse bias” (see FIG. 7).

  By applying a developing bias as described above, a DC bias that causes toner to fly only to the image area (hereinafter referred to as “blank bias”) is applied, and then an AC voltage that vibrates the toner in the vicinity of the photosensitive drum is applied. As a result, the T / D ratio of the developer is increased in the image area, and as a result, the toner can be supplied sufficiently evenly to the halftone area. An inconspicuous smooth image can be obtained.

  Further, the behavior of the toner due to the developing bias improves the developability even in the entire density region. For example, even when the T / D ratio is lowered in the latter half of the durability, the configuration of the present invention can be applied. It maintains high developability and works effectively not only for edge emphasis but also for sharpness. This also increases the latitude for toner density control, SD gap, developer coating amount, and the like.

  In this embodiment, a developing bias is used in which two pulses of a rectangular wave of 2 kVpp / 12 kHz per pulse are turned on for two pulses and turned off for six pulses.

  Also, such a blank pulse bias can be applied to each developing sleeve even in a developing device having a plurality of developing sleeves for one developing device described in the second embodiment. The effect is improved.

  As described above, if the development is performed with a plurality of development sleeves, and if a blank pulse bias is used as the development bias, only high image quality / long life can be achieved and problems such as edge enhancement can be suppressed. Thus, it was possible to achieve a developer structure with a high latitude.

Example 4
In the present invention, only the development bias waveform is different from that of the first embodiment, and all other configurations are the same as those of the first embodiment. The developing bias used in this embodiment will be described.

  The developing bias used in the present invention is opposite to the step of applying a voltage that applies a force in the direction from the image carrier (photosensitive drum) 1 to the developer carrier (developing sleeve) 11 to the two-component developer 19. The step of applying a voltage for applying a force from the developing sleeve 11 toward the photosensitive drum 1 to the two-component developer 19 is alternately applied with an AC voltage that is repeatedly applied a plurality of times. The characteristic is that the ratio of time to be applied, that is, the duty ratio is varied.

  At this time, the voltage (value from the DC component Vdc of the developing bias) that applies a force in the direction of the developing sleeve 11 at the AC voltage is V1, the application time in one cycle of the above steps is T1, and the direction of the photosensitive drum The voltage that applies force to the voltage (also the value from the DC component of the developing bias) is V2, and similarly, the application time in one cycle is T2, and the duty ratio D is V2 / (V1 + V2) = T1 / (T1 + T2), 90 ≧ D ≧ 60 (see FIG. 8).

  In general, as the voltage applied in the direction of the photosensitive drum 1 increases, the developability improves. On the other hand, when the photosensitive drum 1 having a low resistance surface layer is used, charge injection in the developing unit 40 is likely to occur. Conceivable.

  Therefore, in this embodiment, the duty ratio is increased. As a result, the voltage applied to the photosensitive drum 1 increases, but on the other hand, since the application time is shortened, it is possible to prevent charge injection from being particularly likely to occur. Furthermore, on the contrary, the peak-to-peak voltage value Vpp = (V1 + V2) can be reduced by the amount of improved developability. As a result, charge injection is less likely to occur without impairing developability. It is possible to make Vpp as relatively small as 1.5 kV ≧ Vpp ≧ 0.5 kV. However, when Vpp is smaller than 0.5 kV, developability may be extremely lowered particularly when a high-resistance carrier is used.

  If the duty ratio is larger than 90, the application time for applying a voltage in the direction of the photosensitive drum 1 is shortened, and the developability may be lowered.

  As described above, even when an AC bias with a duty ratio changed so as to increase the voltage applied to the photosensitive drum 1 is applied to the configuration in which development is performed by the plurality of developing sleeves 11, high image quality / long life is achieved. The development of a developing device having a high latitude with respect to charge injection in the development region can be achieved.

  This configuration can also be applied to the developing device having the configuration of Example 2 in which a plurality of developing sleeves are provided in one developing unit.

1 is a schematic configuration diagram illustrating an example of an image forming apparatus according to the present invention. It is sectional drawing which shows an example of the developing device which concerns on this invention. It is a schematic block diagram which shows the other example of the image forming apparatus which concerns on this invention. It is sectional drawing which shows the other example of the developing device which concerns on this invention. It is sectional drawing which shows the other example of the developing device which concerns on this invention. It is a schematic block diagram which shows the structure of an exposure means. It is explanatory drawing which shows an example of the developing bias which concerns on this invention. It is explanatory drawing which shows the other example of the developing bias which concerns on this invention.

Explanation of symbols

1 Photosensitive drum (image carrier)
3 Charging device 4 Developing device 4a, 4b, 4c, 4d Developing device 11, 11c, 11d Developing sleeve (developer carrier)
12, 12c, 12d Magnet (magnetic field generating means)
13a developing chamber 14a stirring chamber 16 developing container 19 two-component developer 40 developing section

Claims (4)

In a developing device for contact-developing an electrostatic image formed on an image carrier having a surface layer having a volume resistance of 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm with a developer containing toner and carrier,
A plurality of developer carriers that carry and transport the developer to a portion facing the image carrier are provided, and the volume resistance value of the carrier is set to 1 × 10 ¹⁰ Ω · cm to 1 × 10 ¹⁴ Ω · cm. A developing device.   2. The developing device according to claim 1, further comprising bias applying means for applying a developing bias in which a DC voltage and an AC voltage are superimposed on the plurality of developer carriers.   The developing device according to claim 2, wherein the bias applying unit intermittently applies an AC voltage.   The bias applying means is configured to apply a first voltage for urging toner to the image carrier and a second voltage for urging toner to the developer carrier during one cycle. 4. The developing device according to claim 3, wherein the time for applying the second voltage is made longer than the time for applying the voltage.

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