JP2007240995A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which retains an advantage of using deep-color toner and light-color toner and, at the same time, can suppress the carrier deposition to an image carrier apt to be caused on an image formation part using the light-color toner in particular. <P>SOLUTION: The image forming apparatus comprises: first and second image carriers 1a, 1c on which electrostatic images are formed; and first and second developing means 4a, 4c which respectively develop the electrostatic images formed on the first and second image carriers 1a, 1c by using a developer provided with the toners and a carrier, wherein the first toner used by the first developing means 4a and the second toner used by the second developing means 4c have the same hue each other, and the first toner has relatively thin concentration as compared to the second toner. Further, the image forming apparatus which forms an image of the hue by using the first toner and the second toner has such a constitution that capacitance per unit area of the first image carrier 1a is larger than capacitance per unit area of the second image carrier 1c. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer that forms an image by developing an electrostatic image formed on an image carrier with a developer. More specifically, the present invention relates to an image forming apparatus that forms an image using dark toner and light color toner having the same hue and different densities as a developer.

  2. Description of the Related Art Conventionally, for example, there are image forming apparatuses such as a copying machine and a laser beam printer that form an image by developing an electrostatic image formed on an image carrier using an electrophotographic method with a developer.

  An electrophotographic image forming apparatus forms an electrostatic image (latent image) on an image carrier, which is generally a cylindrical electrophotographic photosensitive member (photosensitive member), and the electrostatic image is formed with a developer toner. Develop as a toner image. The toner image is transferred onto a transfer material such as paper using an electrostatic force. Thereafter, the toner image on the transfer material is melted and fixed (fixed) on the transfer material by heat and pressure applied by the fixing device, and becomes stable.

  In general, there is a full-color image forming apparatus that forms images by superimposing color component images of Y (yellow), M (magenta), C (cyan), and Bk (black). In a full-color image forming apparatus, a two-component developer in which mainly nonmagnetic toner particles (toner) and magnetic carrier particles (carrier) are mixed is widely used.

  In such a full-color image forming apparatus, development of an image forming apparatus using both dark color toner and light color toner is in progress in order to further improve image quality. For example, not only dark toners such as Y, M, C, and Bk but also light color toners having the same spectral components as dark toners such as LM (light magenta), LC (light cyan), and LBk (light black) are used as developing means. Fill and use dark and light toners together.

  By using dark toner and light toner together, light toner is actively used in the low density area, light toner and dark toner are mixed in the intermediate density area, and dark toner is mixed in the high density area. Can be used actively. As a result, the roughness of the dots in the low density portion can be made inconspicuous, and the amount of toner used in the high density portion can be suppressed. Accordingly, it is possible to reduce the graininess particularly in the low density portion, to widen the reproduction color range, and to achieve improvement in image quality and quality.

  However, the method using both the dark color toner and the light color toner has the above-mentioned advantages, but the problem that the carrier of the developer adheres to the photoreceptor easily occurs in the image forming unit using the light color toner. .

  This is because a general color image has a high image ratio in the intermediate density portion, and an image forming portion using light color toner often develops a solid latent image (a latent image of a solid image that is an image of the highest density level). This is considered to be the cause. In this regard, development is performed by a reversal development method using a negatively charged photoconductor (a toner charged to the same polarity as the charged polarity of the photoconductor is attached to an image portion whose potential is attenuated by exposure on the photoconductor). In the following, further explanation will be given by taking the case of performing the above as an example.

  In other words, during the formation of the solid image, the negatively charged toner is developed on the photosensitive member (reversal development) by the potential difference Vcont (development contrast) between the potential of the developer carrying member and the exposed portion potential on the photosensitive member. . At this time, inside the carrier ear exposed to the potential difference, a negative charge is injected into the leading end side of the carrier ear due to the potential difference Vcont.

  When the negative charge is accumulated in a predetermined amount or more on the carrier, the negatively charged carrier adheres to the photoconductor by the force of the electric field, as in the case of developing with normal toner.

  Accordingly, carrier adhesion to the image area tends to occur remarkably during high density development with a large potential difference Vcont. This is a factor of increasing the frequency of carrier adhesion to the image area during development with light toner. When such carrier adhesion occurs, the image on the photoconductor may be disturbed and the image quality may deteriorate.

  Patent Document 1 proposes a method for recovering the carrier attached to the photosensitive member by providing a carrier recovery roller on the downstream side of the developing unit with respect to the problem of carrier attachment. Further, in Patent Document 2, a magnet having the same polarity as the developing magnetic pole is disposed at a position facing the developing magnetic pole of the magnet in the developing sleeve inside the photosensitive member so that the carrier adheres to the photosensitive member. Methods for preventing it have been proposed.

  However, these methods have problems such as a complicated apparatus configuration, an increase in size of the apparatus, and complicated control.

  On the other hand, as another method for preventing carrier adhesion in the image forming unit using light color toner without taking the complicated configuration as described above, a method of reducing the charge amount of the toner or increasing the covering power is also considered. It is done.

However, if the charge amount of the toner is lowered, the scattering of the toner is deteriorated, and there is a possibility that the whole apparatus is soiled. Also, increasing the covering power will darken the hue of the light color toner and will be close to that of the dark color toner, thus offsetting the merit of the light color toner.
Japanese Patent Application Laid-Open No. 07-098545 Japanese Patent Laid-Open No. 07-121022

  Accordingly, an object of the present invention is to maintain an advantage of using dark toner and light toner, and in particular, an image capable of suppressing carrier adhesion to an image carrier that is likely to occur in an image forming unit using light toner. A forming apparatus is provided.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention includes first and second image carriers on which electrostatic images are formed, and toners and carriers for the electrostatic images formed on the first and second image carriers. First and second developing means for developing each using the developer, and the first toner used by the first developing means and the second toner used by the second developing means An image forming apparatus that has the same hue as each other and the first toner has a relatively lower density than the second toner, and forms an image using the first toner and the second toner. The electrostatic capacity per unit area of the first image carrier is larger than the electrostatic capacity per unit area of the second image carrier, and the maximum density image on the first image carrier. The potential difference between the electrostatic image potential and the DC component potential of the voltage applied to the first developing means The potential difference between the potential of the electrostatic image of the maximum density image formed on the second image carrier and the potential of the DC component of the voltage applied to the second developing means is smaller. An image forming apparatus.

  According to the present invention, it is possible to maintain the advantage of using dark toner and light toner, and to suppress carrier adhesion to an image carrier that is likely to occur particularly in an image forming unit that uses light toner.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
[Overall Configuration and Operation of Image Forming Apparatus]
FIG. 1 shows a schematic cross section of an embodiment of an image forming apparatus according to the present invention. The image forming apparatus 100 according to the present exemplary embodiment is a full color image forming apparatus capable of forming a full color image corresponding to an image information signal on a transfer material 17 such as a recording sheet or an OHP sheet using an electrophotographic method. . The image forming apparatus 100 according to the present embodiment employs a tandem method, a reversal development method, an intermediate transfer method, a heating / pressure fixing method, and the like well known to those skilled in the art.

  The image forming apparatus 100 includes a printer unit 10 and a reader unit 20. The reader unit 20 optically reads a document image on the document placement table, converts it into an electric signal subjected to color separation, and sends it to the printer unit 10. The printer unit 10 forms, for example, a full color image according to the image information signal.

  The printer unit 10 includes first, second, third, fourth, fifth, and sixth image forming units (stations) Pa, Pb, Pc, Pd, Pe, and Pf as a plurality of image forming units. ing. Each of the image forming units Pa to Pf includes image carriers 1a to 1f, respectively. Each of the six image carriers 1a to 1f is provided with one developing means 4a to 4f each loaded with a developer having a different type of toner. The image forming units Pa to Pf each including a combination of the image carriers 1a to 1f and the developing units 4a to 4f are juxtaposed in series in the image feeding direction.

  In this embodiment, the first image forming unit Pa uses light density magenta (LM) toner, and the second image forming unit Pb uses light density cyan (LC) toner. The third, fourth, fifth, and sixth image forming portions Pc, Pd, Pe, and Pf are respectively yellow (Y), dark magenta (DM), dark cyan (DC), and black ( An image is formed using each color toner of Bk). In this embodiment, the LM toner and the LC toner are referred to as “light toner”. In this embodiment, DM toner, DC toner, Y toner, and Bk toner are referred to as “dark toner”.

  In the following description, elements provided in common for each color are given symbols to indicate that they are elements provided for any color unless it is necessary to distinguish between them. Subscripts a, b, c, d, e, and f will be omitted for general explanation.

  A drum-shaped photoconductor (photoconductor drum) 1 as an image carrier is supported so as to be rotatable in the direction of an arrow in the figure. Around the photosensitive drum 1, there are a charging roller 2 as a charging means, an exposure device 3 as an exposure means, a developing device 4 as a developing means, a primary transfer roller 5 as a primary transfer means, a cleaner 6 as a cleaning means, and the like. Is arranged.

  The intermediate transfer belt 12 as an intermediate transfer member is disposed so as to face all of the photosensitive drums 1a to 1f of the first to sixth image forming portions Pa to Pf. The intermediate transfer belt 12 is stretched around a driving roller 13, a driven roller 14, and a secondary transfer counter roller 15. When the rotational driving force is transmitted to the driving roller 13, the intermediate transfer belt 12 moves endlessly in the direction of the arrow (circular movement). ) On the inner peripheral surface side of the intermediate transfer belt 12, primary transfer rollers 5 a to 5 f are arranged facing the photosensitive drums 1 a to 1 f of the image forming units Pa to Pf. The primary transfer rollers 5a to 5f are in contact with the intermediate transfer belt 12 and press the intermediate transfer belt 12 toward the photosensitive drums 1a to 1f. As a result, primary transfer portions (primary transfer nips) N1a to N1f where the intermediate transfer belt 12 contacts the photosensitive drums 1a to 1f are formed. Further, a secondary transfer roller 11 is provided as a secondary transfer unit that abuts against the secondary transfer counter roller 15 via the intermediate transfer belt 12. As a result, a secondary transfer nip (secondary transfer nip) N2 in which the secondary transfer roller 11 contacts the intermediate transfer belt 12 is formed.

  At the time of image formation, first, an original placed on the original table glass of the reader unit 20 is scanned. The reader unit 20 converts the document information into an electrical signal by the CCD, and converts it into a digital signal by the A / D converter. In addition, the reader unit 20 processes digitalized data with an image processing block, performs color conversion of RGB signals into CMYK signals, and then performs gamma correction and lookup table (hereinafter referred to as “LUT”) conversion processing. Finally, binarization is performed. Then, the binarized image data is D / A converted as an image memory and transferred to the LED driver. With the transferred data, the LED of the exposure device 3 is driven to form an image as will be described later (FIG. 2).

  In the printer unit 10, the photosensitive drum 1 is rotated in the direction of the arrow in the drawing, and the surface of the rotating photosensitive drum 1 is uniformly charged by the charger 2. Then, the exposure apparatus 3 irradiates a light image according to the separated color image information corresponding to each image forming unit P to form an electrostatic image (latent image) on the photosensitive drum 1.

  Next, the electrostatic image on the photosensitive drum 1 is reversed and developed by the developing device 4 to form a toner image on the photosensitive drum 1 using a resin and a pigment as a base. At this time, a developing bias is applied to the developing device 4.

  The toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto an intermediate transfer belt 12 as a transfer medium by a primary transfer roller 5. At this time, a primary transfer bias is applied to the primary transfer roller 5.

  For example, when a full color image is formed, the above-described operation is performed in the first to sixth image forming units Pa to Pf. As a result, the toner images are sequentially superimposed and primarily transferred onto the intermediate transfer belt 12 at the primary transfer portions N1a to N1f, and a full-color toner image is formed on the intermediate transfer belt 12.

  Thereafter, the full color toner image on the intermediate transfer belt 12 is collectively transferred (secondary transfer) to the transfer material 17. At this time, a secondary transfer bias is applied to the secondary transfer roller 11.

  The transfer material 17 is conveyed one by one from the storage unit, and is conveyed to the secondary transfer unit N2 at a desired timing. That is, in the recording material supply unit 30, the transfer material 17 stored in the recording material storage unit (cassette) 31 is sent out one by one by a pickup roller 32 or the like as a recording material supply unit. Next, the recording material 17 is conveyed to the secondary transfer portion N2 by the registration roller 33 at a desired timing.

  After the toner image is transferred at the secondary transfer portion N2, the transfer material 17 passes through the transport portion and is transported to a heat roller fixing device 9 as a fixing unit. After the unfixed toner image is fixed by the fixing device 9, the transfer material 17 is discharged to a discharge tray or a post-processing device (not shown).

  Further, the toner remaining on the photosensitive drum 1 after the primary transfer process is collected by the cleaner 6. Further, the toner remaining on the intermediate transfer belt 12 after the secondary transfer process is collected by a belt cleaner 16 as an intermediate transfer member cleaning unit.

Here, for example, the principle of image formation using a dark color toner and a light color toner will be further described by taking cyan toner as an example. FIG. 8 shows the covering power of LC toner (broken line) and DC toner (solid line). For example, LC toner has an optical density of 0.7 when the applied amount on the transfer material is 0.5 mg / cm ² , and DC toner has an applied amount of 0.5 mg / cm ² on the transfer material. Has an optical density of 1.4. In image formation using LC toner and DC toner, image formation is performed based on the LC lookup table (LUT) and the DC lookup table (LUT) shown in FIGS. Is called. 9A shows an LC toner lookup table (LUT), and FIG. 9B shows a DC toner lookup table (LUT). Then, by superimposing the LC toner and the DC toner, as shown in FIG. 10, cyan density gradation reproduction faithful to the image signal is performed.

  In FIGS. 9A and 9B, the horizontal axis represents the gradation value (0 to 255 level) of the image before separation into DC toner (dark color toner) and LC toner (light color toner). In addition, the vertical axis in FIGS. 9A and 9B represents the respective gradation values (0 to 255 levels) when the color toner is separated into DC toner and LC toner. Separation refers to dividing image data of a certain color (also referred to as a plate or a channel) into two image data for dark color toner and light color toner.

  In the above description, the principle of image formation using cyan toner and dark toner is described. However, not only cyan hue but also other hue toners are used. When forming an image, a substantially similar method is used. In this embodiment, the same method as the image forming method using the LC toner and the DC toner is used for the image formation using the LM toner and the DM toner.

  Unlike image formation with dark toner alone, by using light color toner actively in the low to medium density areas of the image signal, the density per unit dot can be reduced and the graininess can be reduced. is there.

  In image formation using only dark toners such as Y and Bk, image formation is performed for the corresponding colors according to a look-up table relating output image density and input signal as shown in FIG.

[Photoconductor]
Next, the photosensitive drum 1 will be further described.

  Referring to FIG. 4, the photosensitive drum generally has a cylindrical conductive substrate (photoconductor support) 51 that is rotationally driven. A photosensitive layer (photosensitive film) 57 composed of a plurality of coating layers is formed on the surface of the conductive substrate 51. The photosensitive layer 57 may be formed by laminating a charge generation layer 54 that generates charge carriers and a charge transport layer 55 that has the ability to move the generated charge carriers, and the surface protective layer 56 is the outermost layer. May be laminated. Thereby, the characteristics can be improved. In addition, the thickness of the surface protective layer 56 is preferably about 1 to 30 μm, and more preferably about 1 to 10 μm. The photosensitive layer may be a single layer.

  Further, an intermediate layer 58 may be provided between the conductive substrate 51 and the charge generation layer 54. Thereby, the adhesiveness of the electroconductive base | substrate 51 and the photosensitive layer 57 can be improved, or the coating property of a photosensitive layer can be improved. In addition, the conductive substrate 51 is protected, defects on the surface of the conductive substrate 51 are covered, the photosensitive layer 57 is protected from electrical breakdown, and the charge from the conductive substrate 51 to the photosensitive layer 57 is increased. Injectability can be improved.

  The layers that contribute to the electrostatic capacity of the photoconductor are the charge generation layer 54, the charge transport layer 55, and the surface protective layer 56, and the electrostatic capacity of the photoconductor changes depending on the thickness, as will be described later. The thickness of the intermediate layer provided between the conductive substrate 51 and the charge generation layer 54 does not contribute to the capacitance. Therefore, when the capacitance is changed, the total thickness of the charge generation layer 54, the charge transport layer 55, and the surface protective layer 56 is changed.

  The conductive substrate 51 is made of a metal such as aluminum or copper, cardboard, plastic, or the like.

  The photosensitive layer 57 may be formed by vacuum-depositing a chalcogenide compound such as selenium, arsenic selenide, selenium-tellurium-arsenic alloy, silicon, germanium, phthalocyanine pigment, cadmium sulfide, or the like. Silicon, germanium, or the like may be formed by a CVD method. Furthermore, it may be formed by applying a dye-sensitized zinc oxide, selenium powder, amorphous silicon powder, polyvinyl carbazole, phthalocyanine pigment, oxadiazole pigment or the like together with an adhesive resin as necessary.

  Here, when the photosensitive layer 57 has a laminated structure of the charge generation layer 54 and the charge transport layer 55 and an organic photoconductive layer is used, the charge generation layer 54 disperses the charge generation material in the binder resin. And then formed. Examples of the charge generating substance include azo pigments such as Sudan Red and Diane Blue, disazo pigments, quinone pigments such as Argol Yellow or pyrenequinone, quinocyanine pigments, perylene pigments, indigo pigments such as indigo or thioindigo, and Indian first orange. Bisbenzoimidar pigments, quinacridone pigments, pyrylium salts, azulenium salts and the like. Examples of the binder resin include polyester, polyvinyl acetate, acrylic, polycarbonate, polyarylate, polystyrene, polyvinyl butyral, polyvinyl pyrrolidone, methyl cellulose, hydroxypropyl methyl cellulose, and cellulose esters. It can also be formed by vapor deposition. The thickness of the charge generation layer 54 is preferably about 0.05 to 0.2 μm.

  When the photosensitive layer 57 has a laminated structure of the charge generation layer 54 and the charge transport layer 55 and an inorganic photoconductive layer is used, the charge generation layer 54 may be formed as follows. That is, a chalcogenide compound such as selenium or arsenic selenide, silicon, germanium, cadmium sulfide, or the like may be deposited or applied, or deposited by CVD or the like. In this case, the thickness of the charge generation layer 54 is preferably about 0.1 to 10 μm.

  The charge transport layer 55 can be formed by dissolving a hole transporting substance in a film-forming resin. As the hole transport material, for example, in the main chain or the side chain, a polycyclic aromatic structure or indole, carbazole, oxadiazole, isoxadiazole, thiazole, imidazole, pyrazole, oxadiazole, pyrazoline, thiadiazole, Examples thereof include compounds having a nitrogen-containing cyclic structure such as triazole, hydrazone compounds, and the like. Examples of the resin having a film forming property include polycarbonate, polyarylate, polystyrene, polymethacrylic acid esters, styrene-methyl methacrylate copolymer, polyester, styrene-acrylonitrile copolymer, and polysulfone. The reason why the resin having a film forming property is used is that the charge transporting substance generally has a low molecular weight and itself has a poor film forming property. The thickness of the charge transport layer 55 is preferably about 5 to 30 μm. The thickness of the charge transport layer 55 is more preferably about 5 to 20 μm.

  On the other hand, the intermediate layer 58 described above may have a single-layer configuration, or may have a stacked configuration of the conductive layer 52 and the undercoat layer 53.

  When the intermediate layer 58 has a single layer structure, it may be formed of polyvinyl alcohol, polyvinyl methyl ether, poly-N-vinylimidazole, ethyl cellulose, methyl cellulose, ethylene-acrylic acid copolymer, casein, gelatin, polyamide, or the like.

  When the intermediate layer 58 has a laminated structure, the conductive layer 52 on the side in contact with the conductive substrate 51 is formed to be relatively thick for the purpose of covering defects on the surface of the conductive substrate. An undercoat layer 53 is formed. Among these, the conductive layer 52 may be formed by containing a conductive substance instead of the resin alone, and the resistance value may be lowered to prevent the generation of a residual potential. Examples of the conductive substance include fine powders of metals such as aluminum, copper, silver, gold, and nickel, and powders such as carbon, titanium oxide, and tin oxide. The undercoat layer 53 may be formed of polyvinyl alcohol, polyvinyl methyl ether poly-N-vinylimidazole, ethyl cellulose, methyl cellulose, ethylene-acrylic acid copolymer, casein, gelatin, polyamide, or the like.

  Further, with reference to FIG. 5, a photoconductor generally called amorphous silicon (a-Si) photoconductor having amorphous silicon as a main component will be further described. The a-Si photosensitive member has a photoconductive layer mainly composed of amorphous silicon.

  In the a-Si photosensitive member shown in FIG. 5A, a photosensitive film (photosensitive layer) 62 is provided on a photosensitive member support 61. The photosensitive film 62 is composed of a photoconductive layer 63 made of a-Si: H, X (H is a hydrogen atom, X is a halogen atom) and having photoconductivity. In the a-Si photosensitive member shown in FIG. 5B, a photosensitive film 62 is provided on a photosensitive member support 61. The photosensitive film 62 includes a photoconductive layer 63 made of a-Si: X, X and having photoconductivity, and an amorphous silicon-based surface layer 64. In the a-Si photosensitive member shown in FIG. 5C, a photosensitive film 62 is provided on a photosensitive member support 61. The photosensitive film 62 includes a photoconductive layer 63 made of a-Si: H, X and having photoconductivity, an amorphous silicon-based surface layer 64, and an amorphous silicon-based charge injection blocking layer 65. In the a-Si photosensitive member shown in FIG. 5D, a photosensitive film 62 is provided on a photosensitive member support 61. The photosensitive film 62 is composed of a charge generation layer 66 and charge transport layer 67 made of a-Si: H, X constituting the photoconductive layer 63, and an amorphous silicon surface layer 64.

  These photoconductive layer, surface layer, charge injection blocking layer, charge generation layer, charge transport layer and the like may constitute a normal a-Si photoreceptor. The support used for the a-Si photosensitive member may be conductive or electrically insulating. Examples of the conductive support include metals such as Al, Cr, Mo, Au, In, Nb, Te, V, Ti, Pt, Pd, and Fe, and alloys thereof such as stainless steel. In addition, at least the surface of the electrically insulating support such as polyester, polyethylene, polycarbonate, cellulose acetate, polypropylene, polyvinyl chloride, polystyrene, polyamide, or other synthetic resin film or sheet, glass, ceramic, etc., is formed on the surface. A conductively treated support can also be used.

  In this embodiment, as the photosensitive drum 1, an organic photosensitive drum is used in all the image forming portions Pa to Pf.

[Developer]
Next, the developer used in this embodiment will be described.

  In this embodiment, the developing device 4 is loaded with a two-component developer in which mainly non-magnetic toner particles (toner) and magnetic carrier particles (carrier) are mixed.

  The toner has colored resin particles containing a binder resin, a colorant, and, if necessary, other additives, and colored particles to which external additives such as colloidal silica fine powder are externally added. It's okay. As the toner, a negatively chargeable polyester resin can be suitably used. The volume average particle diameter of the toner is preferably 5 μm or more and 8 μm or less. In this example, a toner having a volume average particle diameter of 7.0 μm was used.

In this embodiment, the light color toner is designed so that the optical density after fixing becomes 0.7 when the toner amount on the transfer material 17 is 0.5 mg / cm ² . The dark toner is designed so that the optical density after fixing becomes 1.4 when the toner amount on the transfer material 17 is 0.5 mg / cm ² . The light color toner may be a toner whose pigment is adjusted so that the optical density is less than 1.0 per 0.5 mg / cm ² of the toner amount on the transfer material 17. The dark toner may be a toner in which the pigment is adjusted so that the optical density is 1.0 or more per 0.5 mg / cm ² of toner on the transfer material 17. Here, the same hue means that the spectral characteristics of the coloring components (pigments) are the same. However, even if they are not exactly the same, generally, such as magenta, cyan, yellow, black, etc. The range of colors that can be called the same color.

  In this embodiment, the amount of external additive is adjusted so that the charge amount of the toner is 25 μC / g in an environment of 23 ° C./50% RH.

As the carrier, for example, metal such as surface oxidized or unoxidized iron, nickel, cobalt, manganese, chromium, rare earth, and alloys thereof, or oxide ferrite can be preferably used. The method for producing these magnetic particles is not particularly limited. And the volume average particle diameter of a carrier becomes like this. Preferably it is 20-50 micrometers, More preferably, it is 30-40 micrometers. Further, the resistivity of the carrier is preferably 10 ⁷ Ωcm or more and 10 ¹² Ωcm or less, more preferably 10 ⁸ Ωcm or more and 10 ¹² Ωcm or less. In this example, a carrier having a volume average particle diameter of 35 μm, a resistivity of 5 × 10 ⁸ Ωcm, and a magnetization of 200 emu / cc was used.

[Developer]
Next, the developing device 4 will be described with reference to FIG. In this embodiment, the developing devices 4a to 4f of the image forming units Pa to Pf have the same configuration.

The developing device 4 has a developing container (developing device main body) 43 as a developer accommodating portion. The interior of the developing container 43 is divided into a developing chamber (first chamber) R ₁ and a stirring chamber (second chamber) R ₂ by a partition wall 44, and a toner storage chamber R ₃ is formed above the stirring chamber R _2. , the replenishing toner (non-magnetic toner particles) 49 is housed in the said toner storage chamber R _3. The toner storage chamber R ₃ is provided with a replenishing port 48, and an amount of replenishing toner 49 corresponding to the toner consumed by development is dropped and replenished into the stirring chamber R ₂ through the replenishing port 48. On the other hand, a developer (two-component developer) in which nonmagnetic toner particles (toner) and magnetic carrier particles (carrier) are mainly mixed is used as a developer in the developing chamber R ₁ and the stirring chamber R ₂ . Contained.

A first conveying screw 45 as a developer agitating / conveying means is accommodated in the developing chamber R ₁ . When the first transport screw 45 is driven to rotate, the developer in the developing chamber R ₁ is transported along the longitudinal direction of a developing sleeve 41 serving as a developer carrier to be described later.

The stirring chamber R in ₂ second conveying screw 46 as the developer stirring and conveying means are accommodated. The second conveying screw 46 conveys the developer along the longitudinal direction of the developing sleeve 41 by its rotation. The developer conveying direction by the second conveying screw 46 is the opposite direction to that by the first conveying screw 45.

  The partition wall 44 has openings at both ends in the longitudinal direction (front side and back side in the figure). The developer conveyed by the first conveying screw 45 is transferred from one of the openings to the second conveying screw 46, and the developer conveyed by the second conveying screw 46 is transferred to the opening. It is delivered to the first conveying screw 45 from the other one. Thereby, the developer is circulated and conveyed in the developing container 43. The toner is charged to a predetermined polarity (negative polarity in this embodiment) for developing the latent image by friction with the carrier.

As the image is formed, the replenishment toner 49 is agitated from the toner storage chamber R ₃ at a desired timing so as to keep the toner concentration in the developing device 4 (weight ratio of toner with respect to the total weight of the developer) constant. The room R ₂ is replenished at any time. Replenishment toner 49 is appropriately supplied to the toner storage chamber R _{3 from} a replenishment toner storage container (not shown) for each color.

  An opening is provided in a portion of the developing container 43 close to the photosensitive drum 1, and a cylindrical body formed of a nonmagnetic material such as aluminum or nonmagnetic stainless steel as a developer carrier in this opening, that is, A developing sleeve 41 is provided.

  The developing sleeve 41 rotates in the direction of arrow β in the figure, and carries the developer in which the toner and the carrier are mixed to the developing portion (developing region) D where the photosensitive drum 1 and the developing sleeve 41 face each other. Transport. The photosensitive drum 1 rotates in the direction of arrow α in the figure. That is, in the present embodiment, the developing sleeve 41 and the photosensitive drum 1 rotate so that the surface movement directions thereof are the same in the developing portion D.

  The riser (magnetic brush) of the developer carried on the developing sleeve 41 comes into contact with the photosensitive drum 1 in the developing portion D, and the electrostatic image on the photosensitive drum 1 is developed in the developing portion D.

  Note that a vibration bias voltage obtained by superimposing a DC voltage on an AC voltage is applied to the developing sleeve 41 by a developing bias power source 18 as a developing bias output means. The dark part potential (non-exposed part potential) VD and the bright part potential (exposed part potential) VL of the electrostatic image are located between the maximum value and the minimum value of the vibration bias voltage. As a result, an alternating electric field whose direction changes alternately in the developing portion D is formed. In this alternating electric field, the toner and the carrier vibrate vigorously, and the toner adheres to the photosensitive drum 1 corresponding to the electrostatic image while shaking off the electrostatic binding force to the developing sleeve 41 and the carrier.

  The difference (peak-to-peak voltage) between the maximum value and the minimum value of the vibration bias voltage is preferably 1 kV to 5 kV, and the frequency is preferably 1 KHz to 10 KHz. A rectangular wave, a sine wave, a triangular wave, or the like can be used as the vibration bias voltage waveform. The direct current voltage component (Vdc) of the vibration bias voltage is a value between the dark part potential VD and the bright part potential VL of the electrostatic image, but the dark part potential VD is smaller than the minimum bright part potential VL in absolute value. A value closer to this is preferable in terms of preventing fog toner from adhering to the dark potential region.

  The minimum gap between the developing sleeve 41 and the photosensitive drum 1 (the minimum gap position is in the developing portion D) is preferably 0.2 to 1 mm.

  In the present embodiment, a regulating blade 47 as a developer layer thickness regulating member is disposed on the upstream side of the developing portion D in the rotation direction of the developing sleeve 41 so as to face the developing sleeve 41 above the developing sleeve 41. ing. The regulating blade 47 regulates the layer thickness of the two-component developer that the developing sleeve 41 carries and conveys to the developing section D. The developer amount (developer coating amount) regulated by the developing blade 41 and conveyed to the developing portion D is preferably set as follows. That is, the free height of the developer on the surface of the developing sleeve 41 of the magnetic brush at the minimum gap between the developing sleeve 41 and the photosensitive drum 1 is between the developing sleeve 41 and the photosensitive drum 1. The amount is preferably 1.2 to 2 times the minimum gap value. The magnetic brush of the developer is formed by a magnetic field in the developing portion D by the developing magnetic pole S1 described later.

  Inside the developing sleeve 41, a roller-shaped magnet (magnet) 42 is fixedly disposed as a magnetic field generating means. The magnet 42 has a developing magnetic pole S1 that faces the developing portion D. A magnetic brush before development is formed by the development magnetic field formed by the development magnetic pole S1 in the development portion D, and the magnetic brush contacts the photosensitive drum 1 to develop the dot distribution electrostatic latent image. At this time, the toner adhering to the magnetic carrier ears (brushes) and the toner adhering to the surface of the developing sleeve 41 instead of the ears are transferred to the exposed portion of the electrostatic image on the photosensitive drum 1. Develop this. In the present embodiment, the strength of the developing magnetic field on the surface of the developing sleeve 41 by the developing magnetic pole S1 (the magnetic flux density in the direction perpendicular to the surface of the developing sleeve 41 (normal direction)) is set to 100 mT. . In this embodiment, the magnet 42 has N1, N2, N3, and S2 poles in addition to the developing magnetic pole S1. The N1, N2, and N3 poles are the “N” poles of the magnet, and the S1 and S2 poles are the “S” poles of the magnet.

With such a configuration, the developer pumped up at the N2 pole by the rotation of the developing sleeve 41 is then conveyed from the S2 pole to the N1 pole, and is regulated by the regulating blade 47 in the middle thereof to form a developer thin layer. . Next, the developer that stands up in the magnetic field of the developing magnetic pole S1 develops the electrostatic image on the photosensitive drum 1. Thereafter, the developer on the developing sleeve 41 falls into the stirring chamber R ₁ due to the repulsive magnetic field between the N3 pole and the N2 pole. The developer dropped into the stirring chamber R ₁ is transported while being stirred by the first transport screw 45 and the second transport screw 46.

[Carrier adhesion]
Next, a carrier adhesion preventing method that is most characteristic in the present embodiment and that is likely to occur in the image forming portion P using particularly light color toner will be described.

  First, the potential difference Vcont (development contrast) between the potential of the developing sleeve 41 (DC voltage component of the developing bias) and the exposed portion potential of the solid latent image (solid image latent image that is the image with the highest density level) and the transfer material 17. The relationship with the toner applied amount will be described.

FIG. 6 shows the relationship between the potential difference Vcont for the light color toner and the dark color toner and the amount of applied toner on the transfer material 17 in this embodiment. From FIG. 6, it can be seen that the potential difference Vcont needs to be 250 V in order to secure a toner applied amount of 0.5 mg / cm ² (solid image) on the transfer material 17 in the image forming portion P that uses dark toner. . In the figure, the light color toner will be described later.

  Next, the relationship between the potential difference Vcont and the carrier adhesion amount will be described. FIG. 7 shows the relationship between the potential difference Vcont and the carrier adhesion amount.

Here, the carrier adhesion amount was measured by the following method. That is, the carrier adhesion amount was measured by a method in which the toner image visualized / developed on the photosensitive drum 1 was peeled off with a transparent tape and adhered to paper, and the number of carriers in a specific range was counted with a loupe. Specifically, the number of carriers in 5 cm ² was counted, and the number of carriers per 1 cm ² was observed. When the carrier adhesion amount was 2 pieces / cm ² or less, it was determined that the carrier adhesion was at a level at which there was no problem.

  From FIG. 7, it can be seen that when the potential difference Vcont is 250 V, carrier adhesion is noticeable. That is, normally, the carrier is restrained by the developing sleeve 41 by the magnetic pole of the magnet 42 inserted into the developing sleeve 41 and does not leave the developing sleeve 41. However, as described above, when the potential difference Vcont is large, negative charges are injected into the tip of the carrier ear. Therefore, similarly to the development with normal toner, the negatively charged carrier is more greatly affected by the electric field applied between the developing sleeve 41 and the photosensitive drum 1 than the magnetic binding force of the magnet 42, and the photosensitive It adheres on the body drum 1.

  Note that when the potential difference Vcont is 250 V, carrier adhesion occurs even with dark toner, but dark toner develops a solid latent image far less than light toner. For this reason, the carrier adhesion of the dark toner does not greatly contribute to the deterioration of the image quality. On the other hand, since light color toner often develops a solid latent image, it cannot be used with this potential difference Vcont.

  More specifically, FIG. 11 shows the latent image potential when a solid image (image of the highest density level) is formed in the case of reversal development. Here, a case where the electrostatic image formed on the negatively charged photosensitive drum 1 is reversely developed using negatively charged toner will be described as an example.

  During solid image formation, the latent image potential shown in FIG. 11 is formed on the photosensitive drum 1. In this case, the negatively charged toner is supplied to the photosensitive drum 1 by the potential difference Vcont between the potential (Vdc) of the developing sleeve 41 and the exposure portion potential (VL).

  At this time, in the inside of the carrier ear exposed to the potential difference, the negative charge is injected into the leading end side of the carrier ear by the potential difference Vcont. When this negative charge is accumulated in a predetermined amount or more on the carrier, the negatively charged carrier adheres to the photosensitive drum 1 by the force of the electric field, as in normal development with toner.

  Accordingly, carrier adhesion to the image area tends to occur remarkably when developing a high-density toner image having a large potential difference Vcont. This leads to a factor that the frequency of carrier adhesion to the image area is increased when developing with the light color toner as compared with when developing with the dark color toner. That is, as described above, the image formation using the light color toner and the dark color toner is performed using the look-up table (LUT) shown in FIGS. In the case of a commonly used average use density (for example, an image signal level of 0 to 255 and an average density of 100 to 140), the image output signal for light color toner is 200 to 255, that is, high density development equivalent to a solid image is performed. Will do. That is, the potential difference Vcont at this time is relatively large. On the other hand, the image output signal for dark toner is an area where low density development is performed in the vicinity of the average density. That is, the potential difference Vcont at this time is relatively small. Therefore, when an image having an average density is formed, the chance of carrier adhesion in the image area at the time of development with the light color toner is increased several times as compared with the case of the development with the dark color toner.

  One of the objects of the present invention is to provide a photosensitive drum that is likely to be generated particularly in the image forming portion P that uses light color toner while securing the amount of light color toner in the image forming apparatus 100 using both dark color toner and light color toner. 1 to prevent carrier adhesion to 1. Thus, it is also an object of the present invention to make it possible to obtain a high-quality and high-quality color image by enabling reduction of graininess in the low-density portion and expansion of the color reproduction range. .

Therefore, in this embodiment, the electrostatic capacity of the photoconductor (photosensitive layer) carrying the light color toner image is made larger than the electrostatic capacity of the photoconductor (photosensitive layer) carrying the dark color toner image. Then, the potential difference Vcont in the image forming unit P using the light color toner is lowered. That is, in this embodiment, the development contrast is lowered only in the image forming portion P using light color toner. As a result, in the image forming portion P that uses light toner, carrier adhesion does not occur while securing a light toner loading amount of 0.5 mg / cm ² on the transfer material 17.

That is, when the electrostatic capacity of the photosensitive member (photosensitive layer) is C and the potential difference Vcont is V, the total charge amount Q on the photosensitive member is expressed by the following equation:
Q = CV
Saved in relation to Therefore, the potential difference Vcont can be lowered by increasing the capacitance C. The electrostatic capacity of the photoreceptor is represented by the following formula (1). The electrostatic capacity of the photoreceptor can be adjusted by adjusting each parameter shown in Expression (1).
C = ε × S / d
C: Capacitance of the photoreceptor ε: Dielectric constant d: Film thickness of the photoreceptor S: Surface area of the photoreceptor

  In this embodiment, since the diameter of the photosensitive drum 1 is the same in all the image forming portions P, S is a constant value. In this embodiment, the thickness d of the charge transport layer 55 and the surface protective layer 56 is reduced in order to reduce the electrostatic capacitance component of the photosensitive drum 1.

  In this embodiment, the photosensitive drum 1 carrying a dark toner image, that is, in this embodiment, the charge transport layer 55 of the photosensitive drums 1c to 1f of the third to sixth image forming portions Pc to Pf. The thickness was 10 μm, and the thickness of the surface protective layer 56 was 10 μm. On the other hand, the thickness of the charge transport layer 55 of the photosensitive drum 1 carrying the light toner image, that is, the photosensitive drums 1a and 1b of the first and second image forming portions Pa and Pb in this embodiment is 5 μm, The thickness of the surface protective layer 56 was 5 μm. In this embodiment, the relative dielectric constant of the photoconductor was 3 in all the image forming portions P.

Here, it is assumed that the relative permittivity of the photoreceptor carrying the light color toner image is ε and the film thickness is d. Further, the relative permittivity of the photosensitive member carrying the dark toner is ε ′ and the film thickness is d ′. At this time, in this embodiment, d = 10 μm and d ′ = 20 μm. Further, since the photosensitive drum 1 made of the same material is used in all the image forming portions P, the relative dielectric constant is the same value ε = ε ′ = 3. Therefore, as shown in FIG. 6, in the image forming portion P that uses light color toner, it is possible to secure a toner applied amount of 0.5 mg / cm ² (solid image) on the transfer material 17 with a potential difference Vcont of 125V. became. Further, as shown in FIG. 7, since the potential difference Vcont was suppressed to 125V, the carrier adhesion amount could be reduced.

  The electrostatic capacity of the photosensitive member of the image forming unit P using the light color toner is less sensitive to the photosensitivity of the image forming unit P using the dark color toner so that the carrier adhesion in the image forming unit P can be sufficiently reduced. Make it sufficiently larger than the body's capacitance. It is not critical how much the electrostatic capacity of the photoreceptor of the image forming unit P using the light color toner is larger than the electrostatic capacity of the photoreceptor of the image forming unit P using the dark color toner. Those skilled in the art will appropriately set through experiments and the like in relation to the covering power of each of the light color toner and the dark color toner and the usage mode of the combination so as to obtain the desired effect of reducing the carrier adhesion as described above. can do. However, according to the study of the present inventor, it is preferable that the electrostatic capacity of the light-colored toner photoconductor is 1.5 to 3 times that of the dark-colored toner photoconductor. That is, it is possible to reduce the development contrast in the image forming unit P using light color toner to 1/3 to 2/3 of the development contrast in the image forming unit P using dark color toner. When the electrostatic capacity of the light-colored toner photoreceptor is less than 1.5 times the electrostatic capacity of the dark-colored toner photoreceptor, generally, the image forming portion P using the light-colored toner has a lower development contrast and the image formation. In part P, the effect of reducing carrier adhesion is not significant. On the other hand, if it exceeds three times, there is a possibility that an image defect is caused by excessively reducing the development contrast.

  On the other hand, in the image forming portion P using dark toner, if the photosensitive drum 1 having the above-described thin film (that is, having a large capacitance) is used, the graininess in the low density portion is deteriorated. Further, there arises a problem that a desired density cannot be obtained unless gamma is applied. For this reason, reducing the potential difference Vcont by reducing the thickness of the photoconductor is not suitable for the image forming portion P using dark toner.

  As described above, in this embodiment, the image forming portion P using the dark toner, that is, the third to sixth image forming portions Pc to Pf in this embodiment, is arranged between the photosensitive drum 1 and the developing sleeve 41. The applied potential difference Vcont is set to 250V. On the other hand, the potential difference Vcont applied between the photosensitive drum 1 and the developing sleeve 41 is set to 125 V in the image forming portion P using light toner, that is, in the present embodiment, the first and second image forming portions Pa and Pb. To do. Thereby, in particular, it is possible to reduce carrier adhesion to the photosensitive drum 1 that is likely to occur in the image forming portion P using light color toner.

In the present embodiment, the photosensitive drum 1 made of the same material is used in all the image forming portions P, but the material may be different. Also in this case, if the electrostatic capacity C of the photoconductor carrying the light color toner image is larger than the electrostatic capacity C of the photoconductor carrying the dark color toner image, the image forming unit using the light color toner is used. Carrier adhesion to the photosensitive drum 1 that is likely to occur at P can be reduced. That is, due to the configuration of the image forming apparatus 100, the surface area S of the photosensitive drum 1 often has to be the same. Therefore, always
ε / d> ε ′ / d ′
If this relationship is established, it will suffice. If this relationship is satisfied, the potential difference Vcont in the image forming portion P using the light color toner is always smaller than the potential difference Vcont in the image forming portion P using the dark color toner, so that the carrier adhesion amount can be reduced.

  As described above, according to the present exemplary embodiment, in the image forming apparatus 100 using both the dark color toner and the light color toner, only in the image forming unit P using the light color toner, the film thickness of the photoconductor is reduced and the capacitance is increased. Reduce development contrast. Accordingly, it is possible to prevent carrier adhesion to the photosensitive drum 1 that is likely to occur particularly in the image forming portion P using light color toner, while securing the amount of light color toner applied. In addition, it is possible to reduce the graininess in the low density portion and expand the color reproduction range, and obtain a high-quality and high-quality color image.

Example 2
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Accordingly, elements having the same or corresponding functions and configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted, and description will be focused on characteristic points of the present embodiment. To do.

  In this embodiment, an amorphous silicon (a-Si) photosensitive member is used as a method of reducing the development contrast of the image forming portion P using light color toner, that is, in this embodiment, the first and second image forming portions Pa and Pb. Was used.

That is, in this embodiment, an OPC (organic photoconductor) photosensitive drum is used as the photosensitive drum 1 of the image forming unit using dark toner, and the photosensitive drum 1 of the image forming unit P using light color toner is used. An amorphous silicon (a-Si) photoreceptor drum was used. The relative dielectric constant of the a-Si photosensitive drum is 10, which is about three times the relative dielectric constant 3 of the OPC photosensitive drum (organic photosensitive drum). Therefore, the electrostatic capacity of the photosensitive member is increased according to the following formula (1).
C = ε × S / d
C: Capacitance of the photoreceptor ε: Dielectric constant d: Film thickness of the photoreceptor S: Surface area of the photoreceptor

In this embodiment, the thickness of the charge transport layer 55 is 10 μm, both of the OPC photosensitive drum of the image forming portion P using dark color toner and the a-Si photosensitive drum of the image forming portion P using light color toner. The thickness of the protective layer 56 was 10 μm. Therefore, the capacitance of the a-Si photosensitive drum increases as the relative dielectric constant increases. Further, the total charge amount Q on the photosensitive member is expressed by the following equation:
Q = CV
Saved in the relationship.

  Therefore, also in the present embodiment, the potential difference Vcont can be lowered as in the first embodiment. That is, also in the present embodiment, as in the first embodiment, it is possible to reduce the potential difference Vcont in the image forming portion P that uses light color toner while securing the amount of light color toner applied.

  As described above, according to the present exemplary embodiment, in the image forming apparatus 100 using both the dark color toner and the light color toner, the image forming unit P using the dark color toner uses the OPC photoreceptor, and the image forming unit P using the light color toner uses a -Si photoconductor is used. With such a configuration, it is possible to increase the capacitance of the photoreceptor and to reduce the development contrast. Accordingly, it is possible to prevent carrier adhesion to the photosensitive drum 1 that is likely to occur particularly in the image forming portion P using light color toner, while securing the amount of light color toner applied. In addition, it is possible to reduce the graininess in the low density portion and expand the color reproduction range, and obtain a high-quality and high-quality color image.

[Measurement method]
Measuring methods for various parameters mentioned in the above examples are shown below.

<Specific resistance of carrier>
The specific resistance of the magnetic carrier was measured as follows. First, the cell is filled with magnetic carriers or core particles. Next, specific resistance was obtained by arranging electrodes at both ends so as to be in contact with the filled magnetic carrier or core particles, applying a voltage between these electrodes, and measuring the current flowing at that time. The specific resistance was measured under the condition that the contact area S between the filled magnetic carrier or core particle and the electrode was about 2.3 cm ² , the thickness d was about 2 mm, the load on the upper electrode was 180 g, and the measured electric field strength was 5 × 10 ⁴ V. / M.

<Average particle diameter of carrier>
The average particle diameter of the carrier was measured as follows. The carrier particle size is indicated by the maximum horizontal chord length, and the measurement method is an arithmetic operation by randomly selecting 300 or more carriers having a particle size of 0.1 μm or more with a scanning electron microscope (100 to 5000 times) and measuring the diameter. The carrier particle diameter of the present invention was obtained by taking the average.

<Carrier magnetic properties>
The magnetic characteristics of the magnetic carrier were measured using an oscillating magnetic field automatic magnetic recording apparatus BHV-30 manufactured by Riken Denshi Co., Ltd. The magnetic properties of the carrier powder were 795.7 kA / m and 79.58 kA / m external magnetic fields, respectively, and the magnetization strength of the magnetic carrier was determined. The sample for measuring the magnetic carrier is prepared in a state packed in a cylindrical plastic container so as to be sufficiently dense. In this state, the magnetization moment is measured, and the actual weight of the sample filled as described above is measured to determine the magnetization strength (emu / g). Further, the true specific gravity of the magnetic carrier particles is obtained by, for example, a dry automatic densitometer Accupic 1330 (manufactured by Shimadzu Corporation), and the true specific gravity is multiplied by the magnetization intensity obtained as described above. The intensity of magnetization per unit volume can be obtained.

<Average particle diameter of toner>
The volume average particle diameter of the toner can be measured, for example, by the following measurement method. A Coulter Counter TA-II type (manufactured by Coulter Inc.) is used as a measuring device, and an interface (manufactured by Nikkiki) that outputs a number average distribution and volume average distribution and a CX-i personal computer (manufactured by Canon) are connected. As the electrolytic solution, a 1% NaCl aqueous solution is prepared using primary sodium chloride. As a measurement method, 0.1 to 5 ml of a surfactant (preferably alkylbenzene sulfonate) is added as a dispersant to 100 to 150 ml of the above electrolytic aqueous solution, and 0.5 to 50 mg of a measurement sample is further added. The electrolytic solution in which the sample is suspended is subjected to a dispersion treatment with an ultrasonic disperser for about 1 to 3 minutes, and a particle size distribution of 2 to 40 μm particles is measured using a 100 μm aperture as an aperture by a Coulter counter TA-II type. Find the volume distribution. From the obtained volume distribution, the volume average particle diameter of the sample can be obtained.

<Capacitance>
The capacitance can be measured by the following procedure. The capacitance is obtained by converting per unit area. The value of the capacitance depends on the dielectric constant and the layer thickness of the layer in which various materials are mixed. FIG. 12 shows a schematic diagram of a capacitance measuring device. The measuring method is shown below.
1) A sample (photoconductor) 204 whose electrostatic capacity (C _X ) is to be measured and a capacitor 206 having a known electrostatic capacity (C ₀ ) are connected as shown in FIG. 12, and a predetermined DC voltage is applied. The sample 204 is charged by the corona charger 201.
2) With the switch SW turned OFF, the surface potential of the sample 204 is measured by the surface potential meter 202. The measured value at this time is V1.
3) Next, the switch SW is turned ON, and the surface potential of the sample 204 is again measured by the surface potential meter 202. The measured value at this time is V2.

The calculation method of the capacitance C _X is as follows.
V ₁ = V ₀ + V _X = q / C ₀ + q / C _X (1)
V ₂ = V _X = q / C _X (2)
If q is eliminated from the above equations (1) and (2),
C _X = [(V ₁ −V ₂ ) / V ₂ ] · C ₀
It becomes.

Then, by dividing the measured capacitance C _X by the surface area of the sample 204, the capacitance per unit area is obtained. In FIG. 12, 203 is an electric charge, and 205 is an electrode.

<Relative permittivity>
The relative dielectric constant can be measured by the following method.

Measuring instrument LCR meter: HP4284A Precision LCR meter (manufactured by Hewlett-Packard Company)
Electrode: Electrode for dielectric measurement HP16451B (manufactured by Hewlett-Packard Company)
Electrode type: C

Sample A photoconductor is prepared, and a part of the photoconductor is cut into a circle having a diameter of 56 mm. After cutting, a main electrode having a diameter of 50 mm and a guard electrode having an inner diameter of 51 mm are provided by a Pt—Pd vapor deposition film. The Pt—Pd vapor deposition film can be obtained by performing a mild sputtering E1030 (manufactured by Hitachi) vapor deposition operation for 2 minutes. The sample after the vapor deposition operation is used as a measurement sample.

Measurement conditions Measurement atmosphere: temperature 22-23 ° C., humidity 50-60%. The measurement sample is previously left in an atmosphere at a temperature of 22 to 23 ° C. and a humidity of 50 to 60% for 24 hours or more.
Applied voltage: 1V _PP (HP4284A auto level control ON)
Frequency: 100Hz
Measurement mode: CP-RP or CP-D

・ Calculation formula of relative permittivity Relative permittivity ε = t × CP / (1.738 × 10 ⁻¹⁴ )
Here, t: thickness of the sample (unit: m, except for the thickness of the aluminum sheet). The unit of CP is F (farad).

  As mentioned above, although this invention was demonstrated according to the specific Example, it should be understood that this invention is not limited to the aspect of the said Example. The material of the photoconductor used in the image forming apparatus, the developer, the configuration of the image forming apparatus, and the like are not limited to these, and the present invention can be applied to various developers and image forming apparatuses. For example, the order of developing with the toner of each color, the maximum amount of applied toner, and the like are not limited to those in the above embodiments.

  In each of the above embodiments, one photosensitive drum is provided for one developing device, but the present invention is not limited to this. For example, as shown in FIG. 13, one photosensitive drum 1A is provided for developing units for dark toner (four in the illustrated example) 4A1 to 4A4, and two developing units for light toner (two in the illustrated example). ) One photosensitive drum 1B may be provided for 4B1 and 4B2. In FIG. 13, elements having the same or equivalent functions and configurations as those of the image forming apparatus 100 shown in FIG.

  In each of the above embodiments, the image forming apparatus 100 has been described as adopting the intermediate transfer method. However, the present invention can be equally applied to a direct transfer type image forming apparatus. FIG. 14 shows an example of an image forming apparatus that employs the direct transfer method. In FIG. 14, elements having the same functions or configurations as those of the image forming apparatus 100 shown in FIG. The image forming apparatus shown in FIG. 14 includes a transfer material carrier 92 that is, for example, an endlessly moving belt (conveyance belt) instead of the intermediate transfer body 12 in the image forming apparatus 100 of FIG. Further, a transfer means (transfer roller or the like) 5 having the same function as the primary transfer means in the image forming apparatus 100 of FIG. 1 is provided with the photosensitive drums 1a to 1 of the image forming portions Pa to Pf via the transfer material carrier 92. It is arranged to face 1f. The toner images formed in the image forming portions Pa to Pf are sequentially superimposed and transferred onto the transfer material 17 on the transfer material carrier 92 by the action of the transfer unit 5.

1 is a schematic cross-sectional configuration diagram of an example of an image forming apparatus to which the present invention can be applied. It is a flowchart for demonstrating the flow of the image signal in an image formation operation | movement. FIG. 2 is a schematic cross-sectional configuration diagram of a developing device provided in the image forming apparatus of FIG. 1. FIG. 3 is a schematic cross-sectional view for explaining a layer structure of a photoreceptor. FIG. 3 is a schematic cross-sectional view for explaining a layer structure of a photoreceptor. FIG. 6 is a graph showing the relationship between development contrast and applied toner amount in an image forming apparatus configured according to the present invention. It is a graph which shows the relationship between development contrast and carrier adhesion amount. It is a graph for demonstrating the covering power of LC toner and DC toner. 4A is a graph showing an LC toner lookup table, and FIG. 4B is a graph showing a DC toner lookup table. It is a graph which shows the relationship with the cyan density | concentration with respect to an image input signal. It is an explanatory diagram for explaining a latent image potential at the time of image formation by reversal development. It is the schematic of an electrostatic capacitance measuring apparatus. It is a schematic sectional block diagram of the other example of the image forming apparatus which can apply this invention. It is a schematic sectional block diagram of the other example of the image forming apparatus which can apply this invention.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (charging means)
3 Exposure equipment (exposure means)
4 Developer (Developer)
5 Primary transfer roller (primary transfer means)
6 Cleaner (cleaning means)
9 Fixing device (fixing means)
11 Secondary transfer roller (secondary transfer means)
12 Intermediate transfer belt (intermediate transfer member)

Claims (4)

The first and second image carriers on which electrostatic images are formed, and the electrostatic images formed on the first and second image carriers are developed using a developer including toner and carrier, respectively. The first toner used by the first developing unit and the second toner used by the second developing unit have the same hue, and In the image forming apparatus, the first toner has a relatively lower density than the second toner, and forms an image using the first toner and the second toner.
The electrostatic capacity per unit area of the first image carrier is larger than the electrostatic capacity per unit area of the second image carrier, and the static image of the maximum density image on the first image carrier. The potential difference between the potential of the electric image and the potential of the DC component of the voltage applied to the first developing means is the potential of the electrostatic image of the maximum density image formed on the second image carrier. An image forming apparatus, wherein the potential difference is smaller than a potential of a direct current component of a voltage applied to the second developing unit. The first and second image carriers are electrophotographic photoreceptors having a photosensitive film, and the dielectric constant of the first image carrier is ε, and the film thickness of the photosensitive film of the first image carrier is d, where the dielectric constant of the second image carrier is ε ′, and the film thickness of the photosensitive film of the second image carrier is d ′,
ε / d> ε ′ / d ′
The image forming apparatus according to claim 1, wherein the relationship is satisfied.   The first and second image carriers are electrophotographic photosensitive members having a photosensitive film, and the film thickness of the photosensitive film of the first image carrier is the same as that of the photosensitive film of the second image carrier. The image forming apparatus according to claim 1, wherein the image forming apparatus is thinner than the film thickness.   2. The image forming apparatus according to claim 1, wherein the first image carrier includes an a-Si photoconductor, and the second image carrier includes an OPC photoconductor.

Images