JP7790144B2 - Developing device and image forming apparatus - Google Patents

Description

This disclosure relates to a developing device that develops an electrostatic latent image, and an image forming apparatus equipped with the developing device.

In image forming devices that use electronic processes, a latent image formed on the surface of a photosensitive drum (image carrier) is developed with developer attached to a developing roller (developer carrier). For example, Patent Document 1 proposes a technology for adjusting the pressure between the photosensitive drum and the developing roller to improve image quality.

JP 2009-15111 A (see abstract)

Depending on the pressure between the photosensitive drum and the developing roller, toner that should be held on the developing roller may adhere to the surface of the photosensitive drum, resulting in a printing defect (fog) in which toner is scattered across a white background.

This disclosure has been made to solve the above problems and aims to prevent printing defects such as fogging.

The developing device disclosed herein includes an image carrier that carries an electrostatic latent image, and a developer carrier that contacts the image carrier and develops the electrostatic latent image with a developer. The developer carrier has a shaft, an elastic layer that covers the shaft, and a surface layer that covers the elastic layer and contacts the image carrier . The surface layer covers the elastic layer over its entire circumference, preventing contact between the elastic layer and the image carrier. The developer has a Si content of 0.98 to 1.28 [wt %] as measured by elemental analysis using energy dispersive X-ray spectroscopy. The overall resistance value R _ALL [Log Ω] of the surface layer and the elastic layer combined is 6.15 [Log Ω] or more and 8.45 [Log Ω] or less. The difference R _ALL −R _IN between the overall resistance value R _ALL [Log Ω] and the resistance value R _IN [Log Ω] of the elastic layer is 0.56 [Log Ω] or more and 1.74 [Log Ω] or less.

This disclosure makes it possible to suppress a decrease in the surface potential of the developer carrier, thereby reducing printing defects such as fogging.

1 is a diagram illustrating a configuration of an image forming apparatus according to an embodiment; FIG. 2 is a block diagram showing a control system of the image forming apparatus according to the embodiment; FIG. 2 is a diagram illustrating a cross-sectional structure of a photosensitive drum according to an embodiment. FIG. 2 is a diagram showing a cross-sectional structure of a developing roller according to an embodiment. 5A and 5B are schematic diagrams showing a method for measuring the resistance of a developing roller and a supply roller. FIG. 2 is a diagram illustrating a cross-sectional structure of a supply roller according to an embodiment. FIG. 2 is a perspective view showing a support structure for a photosensitive drum and a developing roller according to an embodiment. 5A and 5B are schematic diagrams showing a method for measuring the conveying force between a photosensitive drum and a developing roller. 10 is a flowchart showing a method for measuring the overall resistance and inner layer resistance of a developing roller. 10 is a schematic diagram showing a resistance measurement method performed in steps S12 and S15 of FIG. 9. 5A and 5B are schematic diagrams showing a method for cutting the surface layer of a developing roller. FIG. 4 is a diagram showing a method for measuring the residual potential of a developing roller. FIG. 2 is a schematic diagram showing a printing pattern used in continuous printing. FIG. 2 is a schematic diagram showing a print pattern used for test printing. 5A and 5B are schematic diagrams showing print patterns used in test printing. 1A, 1B, and 1C are diagrams showing a method for evaluating the influence of cutting of the surface layer of a developing roller. 10 is a graph showing a comparison of R _ALL −R _IN values when the surface layer of the developing roller is cut and when it is not cut. 10 is a graph showing print results for R _ALL −R _IN and R _ALL in the embodiment and the comparative example. 5A and 5B are schematic diagrams illustrating the state of toner movement from a developing roller to a photosensitive drum. Graph (A) shows the relationship between the hue difference ΔE of a printed image and the initial Q/M of the toner on the developing roller, and graph (B) shows the relationship between the initial Q/M of the toner on the developing roller and the residual potential of the developing roller. 10 is a graph showing the relationship between the residual potential and the resistance value of the developing roller.

<Configuration of Image Forming Apparatus>
1 is a diagram showing an image forming apparatus 1 according to an embodiment. The image forming apparatus 1 is a printer that forms images using an electrophotographic process. The image forming apparatus 1 includes a medium supply unit 40, a developing device 10, a fixing device 50, a medium discharge unit 60, and a housing 1A that houses these components.

The media supply unit 40 has a media tray 41 that stores media P such as printing paper, a paper feed roller 42 that sends the media P from the media tray 41 one sheet at a time onto the transport path, a transport roller 43 that transports the media P sent onto the transport path to the developing device 10, and a media guide 44 that guides the transported media P.

An exposure head 30 serving as an exposure device is positioned opposite the photosensitive drum 11 (described below) of the developing device 10. The exposure head 30 has an LED array in which LEDs (light-emitting diodes) are arranged as light-emitting elements, and a lens array, and irradiates the surface of the photosensitive drum 11 with light. The exposure head 30 is suspended and supported by the top cover 1B that covers the top of the housing 1A.

The developing device 10 has a photosensitive drum 11 as an image carrier, a charging roller 15 as a charging member, a developing roller 12 as a developer carrier, a supply roller 13 as a supply member, a regulating blade 14 as a layer regulating member, a toner cartridge 16 as a developer container, a cleaning member 17, a waste toner transport unit 18, and a unit housing 20 that houses these components.

The developing device 10 does not need to have all of the above components; it only needs to have at least a photosensitive drum 11 as an image carrier and a developing roller 12 as a developer carrier. In addition, it may have a charging roller 15 as a charging member, a supply roller 13 as a supply member, and a regulating blade 14 as a layer regulating member. Furthermore, it may have a toner cartridge 16 as a developer container, a cleaning member 17, a waste toner transport unit 18, and a unit housing 20 that houses these.

The photoreceptor drum 11 is a cylindrical member with a photosensitive layer formed on the surface of a conductive support, and rotates clockwise in the figure. The photoreceptor drum 11 carries an electrostatic latent image on its surface. Details of the configuration of the photoreceptor drum 11 will be described later.

The charging roller 15 is positioned so that it abuts against the photosensitive drum 11 and rotates in conjunction with the photosensitive drum 11. A charging voltage is applied to the charging roller 15 from the charging voltage power supply 105 (Figure 2), uniformly charging the surface of the photosensitive drum 11.

The developing roller 12 is positioned so that it contacts the surface of the photosensitive drum 11 and rotates in the opposite direction to the photosensitive drum 11 (the direction in which the surface moves at the contact point is the forward direction). A developing voltage is applied to the developing roller 12 from the developing voltage power supply 106 (Figure 2), and the electrostatic latent image on the surface of the photosensitive drum 11 is developed with toner (developer).

The supply roller 13 is positioned so that it abuts against the surface of the developing roller 12 and rotates in the same direction as the developing roller 12 (the direction in which the surface moves at the abutment point is opposite). A supply voltage is applied to the supply roller 13 from the supply voltage power source 107 (Figure 2), and it supplies toner to the developing roller 12.

The regulating blade 14 is positioned so that it abuts against the surface of the developing roller 12. A blade voltage is applied to the regulating blade 14 from the blade voltage power supply 108 (Figure 2), and it regulates the toner layer on the surface of the developing roller 12 to a constant thickness.

The toner cartridge 16 is a container that contains toner (indicated by the reference symbol 9) as a developer. The toner 9 is, for example, black toner, but is not limited to this. The toner cartridge 16 is detachably attached to the top of the unit housing 20 and supplies the toner 9 to the developing roller 12 and the supply roller 13.

Above the developing roller 12 and supply roller 13 within the unit housing 20, there is a toner storage section, which is a space for storing toner 9 supplied from the toner cartridge 16. Crank-shaped stirring bars 25, 26, and 27 are disposed in the toner storage section. The stirring bars 25, 26, and 27 rotate in the directions indicated by the arrows to stir and transport the toner 9. A seal member 28 is provided below the developing roller 12 to prevent toner leakage.

The cleaning member 17 is a blade or roller that is positioned to contact the surface of the photosensitive drum 11 and scrapes off the toner 9 remaining on the surface of the photosensitive drum 11. The waste toner transport unit 18 has a screw (not shown) and transports the waste toner scraped off by the cleaning member 17 to a waste toner collection unit.

The developing device 10 is also referred to as an image forming unit, process unit, or image drum unit (ID unit).

A transfer roller 19 serving as a transfer member is positioned so as to contact the surface of the photosensitive drum 11. A transfer voltage is applied to the transfer roller 19 from a transfer voltage power supply 109 (Figure 2). This transfer voltage causes the toner image on the surface of the photosensitive drum 11 to be transferred to a medium P passing between the photosensitive drum 11 and the transfer roller 19.

The image forming device 1 forms a monochrome image using the developing device 10, but is not limited to this example. It may also be configured to form a color image by arranging multiple image forming units for yellow, magenta, cyan, black, etc. in the transport direction of the medium P.

The fixing device (also referred to as the fixing unit) 50 is located downstream of the developing device 10 in the transport direction of the medium P. The fixing device 50 has a fixing roller 51 and a pressure roller 52. The fixing roller 51 has a built-in heater such as a halogen lamp. The pressure roller 52 is pressed against the fixing roller 51 to form a fixing nip. The fixing roller 51 and the pressure roller 52 apply heat and pressure to the medium P passing through the fixing nip, fixing the toner image to the medium P.

The medium discharge unit 60 is located downstream of the fixing device 50 in the transport direction of the medium P. The medium discharge unit 60 has a discharge roller 61 that discharges the medium P that has passed through the fixing device 50 from the discharge outlet, and a medium guide 62 that guides the medium P from the fixing device 50 to the discharge outlet. A stacker 63 on which the discharged medium P is placed is formed in the top cover 1B.

In Figure 1, the axial direction of the photosensitive drum 11 is the X direction. The X direction is the axial direction of each roller within the image forming device 1, and is also the width direction of the medium P being transported. The direction of movement of the medium P as it passes through the developing device 10 is the Y direction. The direction perpendicular to the X and Y directions is the Z direction. Here, the Z direction is the up-and-down direction.

With regard to the Y direction, the transport direction of the medium P as it passes through the developing device 10 is the +Y direction, and the opposite direction is the -Y direction. With regard to the X direction, the right-hand direction when facing the +Y direction is the +X direction, and the left-hand direction is the -X direction. With regard to the Z direction, the upward direction in Figure 1 is the +Z direction, and the downward direction is the -Z direction.

<Control system of image forming apparatus>
2 is a block diagram showing a control system of the image forming apparatus 1. The image forming apparatus 1 has a main control unit 100, an I/F (interface) control unit 101, a receiving memory 102, an image data editing memory 103, a voltage control unit 104, a head control unit 110, a drive control unit 111, a fixing control unit 112, a fixing drive control unit 113, and a paper feed/conveyance control unit 114. These control units and memories constitute a control device.

The main control unit 100 has a microprocessor, ROM (Read Only Memory), RAM (Random Access Memory), input/output ports, a timer, etc. The main control unit 100 receives print data and control commands from a higher-level device via the I/F control unit 101 and executes the printing operation of the image forming device 1.

The main control unit 100 receives operation signals from an operation panel (operation unit) 121 and detection signals from a group of sensors 122 that detect the state of the image forming device 1. The group of sensors 122 includes, for example, a media sensor that detects the position of the medium P on the transport path, and a temperature and humidity sensor that detects temperature and humidity.

Receiving memory 102 temporarily stores print data input from a higher-level device via I/F control unit 101. Image data editing memory 103 receives the print data stored in receiving memory 102 and records the image data, i.e., image data, formed by editing the print data.

The voltage control unit 104 controls the charging voltage applied to the charging roller 15 from the charging voltage power supply 105, the development voltage applied to the development roller 12 from the development voltage power supply 106, the supply voltage applied to the supply roller 13 from the supply voltage power supply 107, the blade voltage applied to the regulating blade 14 from the blade voltage power supply 108, and the transfer voltage applied to the transfer roller 19 from the transfer voltage power supply 109.

The head control unit 110 controls the emission of each LED in the exposure head 30 based on the image data recorded in the image data editing memory 103.

The drive control unit 111 controls the rotation of the drive motor (drum motor) 115, which drives the photosensitive drum 11 to rotate. The rotation of the photosensitive drum 11 is also transmitted to the developing roller 12 and the supply roller 13.

The fixing control unit 112 has a temperature adjustment circuit and supplies current to the heater 53 of the fixing roller 51 based on the output signal of a temperature sensor such as a thermistor provided in the fixing device 50. The fixing drive control unit 113 controls the rotation of the fixing motor 116, which drives the fixing roller 51 to rotate. The discharge roller 61 rotates due to rotation transmitted from the fixing motor 116.

The paper feed and transport control unit 114 controls the rotation of the paper feed motor 117, which drives the paper feed roller 42, and the transport motor 118, which drives the transport roller 43.

<Basic Operation of Image Forming Apparatus>
1 and 2, the printing operation of the image forming apparatus 1 will be described. When the main control unit 100 receives a print command and print data from a host device via the I/F control unit 101, the main control unit 100 starts the printing operation.

The main control unit 100 temporarily records print data received from a higher-level device in the reception memory 102, edits the recorded print data to generate image data, and records it in the image data editing memory 103.

Furthermore, the fixing drive control unit 113 drives the fixing motor 116, causing the fixing roller 51 and pressure roller 52 to start rotating. Furthermore, the fixing control unit 112 energizes the heater 53, causing the fixing roller 51 to heat up to a predetermined fixing temperature.

The paper feed transport control unit 114 also drives the paper feed motor 117, causing the paper feed roller 42 to feed the medium P in the media tray 41 to the transport path as indicated by arrow A1. The transport motor 118 also rotates the transport roller 43, transporting the medium P to the developing device 10 as indicated by arrow A2.

In addition, the voltage control unit 104 applies a charging voltage, a developing voltage, a supply voltage, and a blade voltage from each power supply 105-108 to the charging roller 15, the developing roller 12, the supply roller 13, and the regulating blade 14.

The drive control unit 111 also drives the drive motor 115, causing the photosensitive drum 11 to rotate. As the photosensitive drum 11 rotates, the charging roller 15, developing roller 12, and supply roller 13 also rotate. The charging roller 15 uniformly charges the surface of the photosensitive drum 11.

The head control unit 110 also drives the exposure head 30 to irradiate the surface of the photosensitive drum 11 with light. This forms an electrostatic latent image on the surface of the photosensitive drum 11.

The electrostatic latent image formed on the surface of the photosensitive drum 11 is developed by toner attached to the developing roller 12, forming a toner image on the surface of the photosensitive drum 11. Furthermore, the voltage control unit 104 applies a transfer voltage from the transfer voltage power supply 109 to the transfer roller 19.

This transfer voltage transfers the toner image on the surface of the photosensitive drum 11 to the medium P passing between the photosensitive drum 11 and the transfer roller 19. Toner that is not transferred to the medium P is scraped off by the cleaning member 17.

In the fixing device 50, heat and pressure are applied to the medium P as it passes through the fixing nip between the fixing roller 51 and the pressure roller 52, fixing the toner image to the medium P. The medium P with the fixed toner image is sent to the medium discharge section 60, as shown by arrow A3.

In the medium discharge unit 60, discharge rollers 61 discharge the medium P from the discharge port as indicated by arrow A4. The discharged medium P is then stacked on the stacker 63. This completes the formation of an image on the medium P.

<Configuration of each component of developing device>
Next, the configuration of each component of the developing device 10 will be described in detail.

<Toner>
First, the toner 9 will be described. The toner 9 is a non-magnetic, single-component, negatively charged toner, which is obtained by adding an external additive (external additive) such as inorganic fine powder or organic fine powder to toner base particles containing at least a binder resin. Note that, although a single-component development method is used in this embodiment, a two-component development method may also be used. Furthermore, the toner is not limited to a negatively charged toner, and a positively charged toner may also be used.

Preferred binder resins are polyester resins, styrene-acrylic resins, epoxy resins, and styrene-butadiene resins. A release agent, colorant, etc. may be added to the binder resin, and additives such as a charge control agent, conductivity adjuster, flowability improver, or cleaning improver may also be added. Multiple types of binder resins may also be mixed. In this example, a mixture of two or more types of amorphous polyester resins and a crystalline polyester resin with a crystalline structure is used.

Toner 9 has an average particle size of approximately 7.0 μm and a circularity of approximately 0.93. The average particle size is measured using a Multisizer 3 manufactured by Coulter Corporation. The circularity is measured using a Flow Particle Image Analyzer FPIA-3000 manufactured by Sysmex Corporation.

Silica (SiO ₂ ) is used as the external additive. In this embodiment, in order to evaluate the amount of the external additive, the Si (silicon) content (detected amount) [wt %] in the toner 9 is measured by elemental analysis using energy dispersive X-ray spectroscopy (EDX). For the elemental analysis, an energy dispersive X-ray fluorescence analyzer "EDX-800HS" manufactured by Shimadzu Corporation is used. The measurement environment is a helium (He) gas atmosphere, and the X-ray tube voltage is 15 [kV] or 50 [kV]. The Si content in the toner 9 of this embodiment is 0.98 to 1.28 [wt %].

Generally, when a sample is irradiated with X-rays, fluorescent X-rays, which are X-rays specific to the atoms contained in the sample, are generated and emitted from the sample. Because fluorescent X-rays have wavelengths (energy) specific to each element, qualitative analysis can be performed by examining the wavelengths of the fluorescent X-rays. Furthermore, the intensity of fluorescent X-rays is a function of concentration. For this reason, quantitative analysis can be performed by measuring the amount of X-rays at each element-specific wavelength.

In this embodiment, toner is irradiated with X-rays emitted from the X-ray tube of an energy dispersive X-ray fluorescence analyzer, and the Si content is measured based on the fluorescent X-rays emitted from Si atoms contained in the toner's external additives. In the energy dispersive X-ray fluorescence analyzer, the atmosphere in the sample chamber is replaced with helium (He) gas, and X-rays are irradiated at a voltage of 15 kV and a current of 100 μA.

Furthermore, the blow-off charge of toner 9 is -94.6 μC/g or less. In other words, in this embodiment, toner with a lower chargeability than general toner (blow-off charge: -104.6 μC/g) is used. The method for measuring the blow-off charge of toner is as follows.

That is, a mixture of 0.5 g of toner and 9.5 g of carrier (EF96-35, manufactured by Powder Tech Co., Ltd.) was placed in a container, and the container was shaken using a shaker "YS-LD" manufactured by Yayoi Co., Ltd. The number of shakes was 200 times per minute, and the shaking time was 600 seconds.

After shaking, a powder charge amount measuring device (Kyocera Chemical Corporation's "TB-203") was used to perform suction for 10 seconds at a blow pressure of 7.0 kPa and a suction pressure of -4.5 kPa, and the charge amount and suction amount for 0.1 seconds were output to a PC (personal computer). The charge amount per unit weight of the toner particles, Q/M (unit: μC/g), was calculated from the average values of the charge amount and suction amount output during the final 2 seconds of the suction time (10 seconds). The measurement environment was a temperature of 25°C and a relative humidity of 50%.

<Photosensitive drum>
Next, the photoreceptor drum 11 will be described. Fig. 3 is a diagram showing the cross-sectional structure of the photoreceptor drum 11. The photoreceptor drum 11 has a cylindrical conductive support 11b and a photosensitive layer 11c formed on the surface of the conductive support 11b. An undercoat layer may be formed between the conductive support 11b and the photosensitive layer 11c.

The photoreceptor constituting the photosensitive layer 11c can be a photoreceptor applicable to general electrophotographic photoreceptors. Specific examples include single-layer photoreceptors and multi-layer photoreceptors. A single-layer photoreceptor has a single photosensitive layer (i.e., a single-layer photoreceptor) in which a photoconductive material is dissolved or dispersed in a binder resin. A multi-layer photoreceptor is formed by laminating a charge generation layer containing a charge generation substance and a charge transport layer containing a charge transport substance. It is generally known that photoreceptors exhibit equivalent performance whether they are single-layer or multi-layer.

In this embodiment, the photosensitive layer 11c is preferably a laminated photosensitive layer, taking into consideration mechanical properties, electrical characteristics, manufacturing stability, and other factors. Among these, a sequentially laminated photosensitive layer, in which a charge generation layer and a charge transport layer are laminated in this order on the conductive support 11b, is particularly preferred. Note that laminated photosensitive layers are also referred to as function-separated photosensitive layers, as the charge generation layer and charge transport layer share the responsibility of generating and transporting charges.

When forming the charge transport layer of a function-separated photoreceptor, or the photosensitive layer of a single-layer photoreceptor, it is common to disperse a compound (such as a charge transport material) in a binder resin to ensure film strength. The charge transport layer of a function-separated photoreceptor is formed by applying and drying a coating liquid obtained by dissolving or dispersing a charge transport material and a binder resin in a solvent. Similarly, a single-layer photoreceptor is formed by applying and drying a coating liquid obtained by dissolving or dispersing a charge generation material, a charge transport material, and various binder resins in a solvent.

Binder resins commonly used in the charge generation layer of function-separated photoreceptors include, for example, polyvinyl butyral resin, polyvinyl formal resin, polyvinyl acetal resins such as partially acetalized polyvinyl butyral resins in which part of the butyral is modified with formal or acetal, polyarylate resin, polycarbonate resin, polyester resin, modified ether-based polyester resin, phenoxy resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyvinyl acetate resin, polystyrene resin, acrylic resin, methacrylic resin, polyacrylamide resin, polyamide resin, polyvinylpyridine resin, cellulose-based resin, polyurethane resin, epoxy resin, and silica. The binder resin can be selected from among silicone resins, polyvinyl alcohol resins, polyvinylpyrrolidone resins, casein, vinyl chloride-vinyl acetate copolymers such as vinyl chloride-vinyl acetate copolymers, hydroxy-modified vinyl chloride-vinyl acetate copolymers, carboxyl-modified vinyl chloride-vinyl acetate copolymers, and vinyl chloride-vinyl acetate-maleic anhydride copolymers; insulating resins such as styrene-butadiene copolymers, vinylidene chloride-acrylonitrile copolymers, styrene-alkyd resins, silicone-alkyd resins, and phenol-formaldehyde resins; and organic photoconductive resins such as poly-N-vinylcarbazole, polyvinyl anthracene, and polyvinylperylene. However, the binder resin is not limited to these resins. These binder resins may be used alone or in any combination and ratio.

Binder resins used in the charge transport layer include, for example, polyvinyl acetal resins, polyarylate resins, polycarbonate resins, polyester resins, modified ether-based polyester resins, phenoxy resins, polyvinyl chloride resins, polyvinylidene chloride resins, polyvinyl acetate resins, polystyrene resins, acrylic resins, methacrylic resins, polyacrylamide resins, polyamide resins, polyvinylpyridine resins, cellulose-based resins, polyurethane resins, epoxy resins, silicone resins, polyvinyl alcohol resins, polyvinylpyrrolidone resins, casein, vinyl chloride-vinyl acetate copolymers, styrene-butadiene copolymers, vinylidene chloride-acrylonitrile copolymers, styrene-alkyd resins, silicone-alkyd resins, phenol-formaldehyde resins, and organic photoconductive resins. Examples of vinyl chloride-vinyl acetate copolymers include vinyl chloride-vinyl acetate copolymers, hydroxy-modified vinyl chloride-vinyl acetate copolymers, carboxyl-modified vinyl chloride-vinyl acetate copolymers, and vinyl chloride-vinyl acetate-maleic anhydride copolymers. Examples of organic photoconductive resins include poly-N-vinylcarbazole, polyvinylanthracene, and polyvinylperylene.

The compound contained in the charge transport layer includes, for example, one or more charge transport materials. The type of charge transport material is not particularly limited, but examples include aromatic amine derivatives, stilbene derivatives, butadiene derivatives, hydrazone derivatives, carbazole derivatives, aniline derivatives, and enamine derivatives. The charge transport material may be, for example, a compound to which one or more of the above-mentioned aromatic amine derivatives are bonded. The charge transport material may also be, for example, a polymer (electron donating material) having a group consisting of the above-mentioned aromatic amine derivative or the like in its main chain or side chain. In particular, the charge transport material is preferably an aromatic amine derivative, stilbene derivative, hydrazone derivative, enamine derivative, or a compound to which one or more of these are bonded, and more preferably a compound to which an aromatic amine derivative and an enamine derivative are bonded.

The photosensitive layer 11c of the photoreceptor drum 11 is generally formed by applying a coating liquid containing the materials for each layer onto the conductive support 11b using a known coating method, followed by drying, and repeating this process for each layer. Examples of solvents and dispersion media that dissolve the binder resin when preparing the coating liquid include saturated aliphatic solvents such as pentane, hexane, octane, and nonane; aromatic solvents such as toluene, xylene, and anisole; halogenated aromatic solvents such as chlorobenzene, dichlorobenzene, and chloronaphthalene; amide solvents such as dimethylformamide and N-methyl-2-pyrrolidone; alcohol solvents such as methanol, ethanol, isopropanol, n-butanol, and benzyl alcohol; aliphatic polyhydric alcohols such as glycerin and polyethylene glycol; linear, branched, and cyclic ketone solvents such as acetone, cyclohexanone, methyl ethyl ketone, and 4-methoxy-4-methyl-2-pentanone; and methyl formate. Examples of solvents that do not dissolve the undercoat layer include ester solvents such as ethanol, ethyl acetate, and n-butyl acetate; halogenated hydrocarbon solvents such as methylene chloride, chloroform, and 1,2-dichloroethane; chain and cyclic ether solvents such as diethyl ether, dimethoxyethane, tetrahydrofuran, 1,4-dioxane, methyl cellosolve, and ethyl cellosolve; aprotic polar solvents such as acetonitrile, dimethyl sulfoxide, sulfolane, and hexamethylphosphoric triamide; nitrogen-containing compounds such as n-butylamine, isopropanolamine, diethylamine, triethanolamine, ethylenediamine, triethylenediamine, and triethylamine; mineral oils such as ligroin; and water. These solvents may be used alone or in any combination and ratio.

Examples of methods for applying the coating liquid include dip coating, spray coating, spinner coating, bead coating, wire bar coating, blade coating, roller coating, air knife coating, and curtain coating, but other coating methods may also be used. These methods may be used alone or in any combination of two or more. The coating liquid is preferably dried to the touch at room temperature (usually 25°C), and then heated and dried at a temperature in the range of 30 to 190°C for a drying time of 1 minute to 2 hours, either without air or with a fan. The heating temperature may be constant, or the heating temperature may be varied while drying.

The thickness of the charge transport layer of a normal layered photoreceptor is generally in the range of 5 to 50 μm, but from the perspective of extending life and image stability, it is preferably 10 to 45 μm. Furthermore, from the perspective of improving resolution, it is even more preferably 10 to 30 μm. The outer diameter of the photoreceptor drum 11 in this embodiment is 30.0 mm.

<Developing roller>
Next, the developing roller 12 will be described. Fig. 4 is a diagram showing the cross-sectional structure of the developing roller 12. The developing roller 12 includes a conductive shaft 12a, an inner layer (elastic layer) 12b that covers the surface of the shaft 12a, and a surface layer (coating layer) 12c that covers the surface of the inner layer 12b.

The inner layer 12b can be formed from common rubber materials such as silicone rubber and urethane. When using polyurethane as the rubber material, polyurethanes based on polyether-based polyols (ether-based polyurethanes) are preferred. Ether-based polyurethanes are cast-type polyurethanes obtained by reacting polyols based on polyether-based polyols with polyisocyanates. This is done to reduce compression set. On the other hand, ester-based polyurethanes have poor hydrolysis properties, making them difficult to use stably over long periods of time.

The inner layer 12b is formed by adding carbon black to a rubber base material made from the above-mentioned rubber material, and then heat-curing the rubber while maintaining the carbon black's dispersed state.

The surface layer 12c is formed by impregnating the surface of the inner layer 12b with a surface treatment liquid. The surface treatment liquid is made by dissolving isocyanate in an organic solvent, and then dissolving acrylic resin and urethane resin in a specific ratio. If the total of the acrylic resin and urethane resin in the surface treatment liquid is taken as 100%, it is desirable for the acrylic resin to comprise 40-60% (more desirably 50%). Carbon black, such as acetylene black, is added to the surface treatment liquid as a conductivity-imparting agent.

By immersing the inner layer 12b in the surface treatment solution, the surface treatment solution is impregnated into the surface portion of the inner layer 12b. The inner layer 12b is then dried and hardened, and the surface portion of the inner layer 12b becomes the surface layer 12c. The thickness of the surface layer 12c is 10 μm to 20 μm (i.e., 0.01 mm to 0.02 mm).

The roller portion of the developing roller 12 (excluding the shaft end portion 12e, described below) may be straight and have a constant outer diameter, or may have a shape where the outer diameter varies between the axial center and the ends (e.g., crowned). A shape that ensures uniform contact pressure between the developing roller 12 and the photosensitive drum 11, supply roller 13, and regulating blade 14 is desirable. Here, the roller portion of the developing roller 12 has a straight shape and an outer diameter of 12.0 mm.

The rubber hardness (Asker C hardness) of the inner layer 12b is preferably 55 to 85 degrees. If the Asker C hardness of the inner layer 12b is lower than 55 degrees, when the developing device 10 is stopped for an extended period of time, depressions may occur at the contact points between the developing roller 12 and the photosensitive drum 11 or regulating blade 14, potentially resulting in horizontal streaks in the image. If the Asker C hardness of the inner layer 12b is higher than 85 degrees, the mechanical load on the developing roller 12 increases, making it more likely that toner will adhere to the surface of the developing roller 12 (known as filming). Here, the Asker C hardness of the inner layer 12b is 82 degrees.

The surface roughness Rz (ten-point average roughness: JIS B0601-1994) of the developing roller 12 is preferably 2.0 to 7.0 μm. If the surface roughness Rz is less than 2.0 μm, the toner layer on the surface of the developing roller 12 will be thin, and the stress applied to each toner particle will be greater. As a result, more external additives will be released from the toner, and this external additive may become stuck between the developing roller 12 and the regulating blade 14, causing filming on the surface of the regulating blade 14. Furthermore, if the surface roughness Rz exceeds 7.0 μm, the toner layer on the surface of the developing roller 12 will be thick, preventing the supply roller 13 from sufficiently scraping off the toner, causing filming on the surface of the developing roller 12. Furthermore, more toner than necessary may migrate to the photosensitive drum 11, resulting in image contamination.

The surface roughness Rz of the developing roller 12 is measured using a surface roughness measuring instrument, "Surfcorder SEF3500," manufactured by Kosaka Laboratory Co., Ltd. The stylus radius of the surface roughness measuring instrument is 2 μm, the stylus pressure is 0.7 mN, and the measurement is performed by moving the stylus in the axial direction of the developing roller 12 a distance of 4.0 mm at a speed of 0.1 mm/sec. The cutoff value is 0.8 mm.

The resistance value of the developing roller 12 (total resistance, described below) is preferably 6.15 to 8.45 [log Ω]. The resistance of the developing roller 12 is measured as follows:

Figures 5(A) and (B) are schematic diagrams showing a method for measuring the resistance of the developing roller 12. A Hewlett-Packard High Resistance Meter (model number: 4339B) was used as the measuring device 71. As shown in Figure 5(A), stainless steel bearings 72 with a diameter of 10.0 mm were pressed against the surface of the developing roller 12 at six locations along the length of the developing roller 12 with a load W1 of 20±0.5 gf.

As shown in Figure 5 (B), a measuring device 71 is connected to the shaft end 12e of the developing roller 12 and the bearing 72, the developing roller 12 is rotated at a speed of 50 rpm, and a DC voltage of -40 V is applied to the shaft end 12e of the developing roller 12 relative to the bearing 72. In this state, the resistance value of the developing roller 12 is determined from the current flowing between the shaft end 12e of the developing roller 12 and the bearing 72. The measurement environment is a temperature of 20°C and a relative humidity of 50%.

<Supply roller>
Next, the supply roller 13 will be described. Fig. 6 is a cross-sectional view showing the supply roller 13. The supply roller 13 has a conductive shaft 13a and a foamed elastic layer 13b formed on the surface of the shaft 13a. The shaft 13a may be made of any material as long as it has good conductivity, and iron, aluminum, stainless steel, etc. are generally used.

The rubber composition forming the foamed elastic layer 13b contains rubber, a foaming agent, and a conductivity imparting agent, and may further contain additives as needed. The rubber is preferably silicone rubber or silicone-modified rubber, which has excellent heat resistance and electrostatic charge characteristics. The foaming agent may be any foaming agent used in foamed rubber. Examples of inorganic foaming agents include sodium bicarbonate and ammonium carbonate. Examples of organic foaming agents include organic azo compounds such as diazoamino derivatives, azonitrile derivatives, and azodicarboxylic acid derivatives. Inorganic foaming agents are used to form open cells in the foamed elastic layer 13b, while organic foaming agents are used to form closed cells. Additives include, for example, fillers, colorants, and release agents.

The roller portion of the supply roller 13 may be straight and have a constant outer diameter, or may have a shape where the outer diameter varies between the axial center and the ends (e.g., crowned). A shape that ensures uniform contact pressure between the supply roller 13 and the developing roller 12 in the axial direction is desirable. Here, the roller portion of the supply roller 13 has a crowned shape with an outer diameter of 16.2 mm at the axial center and 15.8 mm at the ends.

The outer diameter of the shaft 13a of the supply roller 13 is, for example, 6.0 mm. The average cell diameter of the elastic foam layer is preferably 200 to 500 μm, but is not limited to this range. The hardness of the elastic foam layer is preferably approximately 53 to 63 degrees on the Asker F hardness scale; here, an Asker F hardness of 58 degrees is used.

The resistance value of the supply roller 13 is preferably 6.0 to 8.0 [log Ω]. Here, a supply roller 13 with a resistance value of 7.0 [log Ω] is used. The method for measuring the resistance of the supply roller 13 is the same as the resistance measurement method described with reference to Figures 5 (A) and (B).

As shown in Figure 5(A), stainless steel bearings 72 with a diameter of 10.0 mm are pressed against the surface of the supply roller 13 at six locations along the length of the supply roller 13 with a load W1 of 10.5 gf.

As shown in Figure 5 (B), a measuring device 71 is connected to the shaft end 13e of the supply roller 13 and the bearing 72. The supply roller 13 is rotated at a speed of 63 rpm, and a DC voltage of -300 V is applied to the shaft end 13e of the supply roller 13 relative to the bearing 72. In this state, the resistance value of the supply roller 13 is determined from the current flowing between the shaft end 13e of the supply roller 13 and the bearing 72. The measurement environment is a temperature of 20°C and a relative humidity of 50%.

The supply roller 13 is made by integrally molding the shaft 13a, which has been washed with an organic solvent or similar to remove oil, with conductive silicone rubber foam using an extrusion molding machine. The foam is then expanded and hardened in an infrared oven or similar. A secondary vulcanization process is then performed at a temperature of approximately 180-225°C for approximately 5-10 hours, and the surface of the foamed elastic material is polished with a polishing machine to obtain the desired outer diameter.

<Regulatory blade>
1 is a plate-shaped member made of stainless steel, with a thickness of, for example, 0.08 mm. The regulating blade 14 is bent at the contact point with the developing roller 12, with the radius of curvature of the bent portion being approximately 0.15 to 0.35 mm. The pressure (linear pressure) between the regulating blade 14 and the developing roller 12 is approximately 25 to 50 gf/cm.

<Support structure for photosensitive drum and developing roller>
7 is a schematic diagram showing a support structure for the photosensitive drum 11 and the developing roller 12 in the developing device 10. The axial direction of the photosensitive drum 11 and the developing roller 12 is the X direction.

The unit housing 20 of the developing device 10 has side frames 21 and 22 on both sides of the photosensitive drum 11 and developing roller 12 in the X direction. The photosensitive drum 11 and developing roller 12 are supported by the side frames 21 and 22. The supply roller 13 and regulating blade 14 (Figure 1) are also supported by the side frames 21 and 22, but are omitted from Figure 7.

The photosensitive drum 11 has a shaft 11a that axially penetrates a conductive support 11b (Figure 3). Both ends 11e of the shaft 11a fit into holes 21a and 22a formed in the side frames 21 and 22. This fixes the shaft 11a to the side frames 21 and 22. The shaft 11a does not rotate by itself.

The conductive support 11b (Figure 3) of the photosensitive drum 11 is rotatably supported on the shaft 11a by a bearing (not shown) provided on its inner periphery. A drum flange 11f is provided at the end of the photosensitive drum 11 in the -X direction (toward the side frame 21), and a drum gear 11g is provided at the end in the +X direction (toward the side frame 22).

The drum flange 11f and drum gear 11g are located on either side of the photosensitive layer 11c in the X direction. The drum gear 11g meshes with a gear (not shown) to which rotation from the drive motor 115 (Figure 2) is transmitted, causing the photosensitive drum 11 to rotate.

The developing roller 12 has shaft end portions 12e at both ends in the X direction of the shaft body 12a (Figure 4) supported by bearings arranged in holes 21b and 22b in the side frames 21 and 22. This allows the developing roller 12 to be rotatably supported by the side frames 21 and 22.

A roller gear 12g that meshes with the drum gear 11g is fixed to the shaft end 12e of the developing roller 12 in the +X direction. When the photosensitive drum 11 rotates, the developing roller 12 also rotates due to the meshing of the drum gear 11g and roller gear 12g.

The photosensitive layer 11c of the photosensitive drum 11 and the surface layer 12c of the developing roller 12 abut against each other. The distance L1 between the centers of the photosensitive drum 11 and the developing roller 12 is determined by the center-to-center distance between the holes 21a and 21b of the side frame 21 (i.e., the center-to-center distance between the holes 22a and 22b of the side frame 22). This distance L1 determines the contact pressure between the photosensitive drum 11 and the developing roller 12.

Next, we will explain the transport force exerted by the photosensitive drum 11 and the developing roller 12. The transport force is the force with which an object (in this case, a film) inserted between the photosensitive drum 11 and the developing roller 12 is pulled by the rotating photosensitive drum 11 and the developing roller 12. The transport force is also called the pulling force or the feed force.

Figure 8(A) is a schematic diagram showing a method for measuring the conveying force between the photosensitive drum 11 and the developing roller 12. Figure 8(B) is a schematic diagram showing a measuring jig 80 used to measure the conveying force.

As shown in Figure 8 (A), a film 82 is inserted between the photosensitive drum 11 and the developing roller 12, and the photosensitive drum 11 and the developing roller 12 are rotated. The force with which the film 82 is pulled is measured with a force gauge 81, thereby measuring the transport force.

The longitudinal direction of the film 82 is the tangent direction to the surface of the photosensitive drum 11 at the contact point (indicated by the symbol N) between the photosensitive drum 11 and the developing roller 12. The width direction of the film 82 is the X direction.

As shown in Figure 8 (B), a support plate 83 is fixed to the upper end of the film 82 (the end away from the contact portion N). One end of a hook 85 is attached to the support plate 83, and the other end of the hook 85 is attached to the force gauge 81.

The film 82 is made of, for example, PET (polyethylene terephthalate). The length of the film 82 is 95 mm, the width is 5 mm, and the thickness is 1 mm. The width direction of the film 82 is the X direction.

The support plate 83 is, for example, an acrylic plate, and is fixed to one longitudinal end of the film 82. The support plate 83 is 20 mm long and 12 mm wide. The support plate 83 has a hole 84 formed therein into which one end of the hook 85 can be hooked.

As shown in Figure 8 (A), the photosensitive drum 11 and the developing roller 12 are rotated with the film 82 inserted between them. The rotational speed (circumferential speed) of the photosensitive drum 11 is 234.0 mm/sec, which corresponds to a printing speed of 49 ppm. The peripheral speed of the developing roller 12 is 1.26 times the peripheral speed of the photosensitive drum 11.

The peripheral speed ratio between the photosensitive drum 11 and the developing roller 12 (here, 1.26) is determined by the outer diameter ratio between the photosensitive drum 11 and the developing roller 12 and the gear ratio between the drum gear 11g and the roller gear 12g shown in Figure 7.

With the photosensitive drum 11 and developing roller 12 rotating as described above, the value [N] of the force gauge 81 is read. In this case, the force gauge 81 is a digital force gauge, and the measured value is output to a personal computer.

The transport force is measured at three locations: the center of the developing roller 12 in the X direction and both sides of that center in the X direction. At each location, the output value of the force gauge 81 is recorded at a sampling interval of 10 milliseconds, and the average value over a 5-second period is calculated. This gives the transport force [N] at each location. The maximum transport force value at the three locations is then taken as the transport force [N].

Here, the maximum value of the transport force at three locations is used, but this is not limited to this; for example, the average value of the transport force at three locations may be used. Alternatively, since the transport force on both sides in the X direction tends to be higher than at the center of the developing roller 12, the average value of the transport force on both sides (two locations) in the X direction may be used.

<Total Resistance and Inner Layer Resistance of Developing Roller>
Next, the overall resistance and inner layer resistance of the developing roller 12 will be described. Fig. 9 is a flowchart showing a method for measuring the overall resistance and inner layer resistance of the developing roller 12. First, the outer diameter of the developing roller 12 is measured (step S11). For the outer diameter measurement, an automatic roller measuring machine manufactured by Apollo Seiko Co., Ltd. is used.

Next, the overall resistance of the developing roller 12 is measured (step S12). Figure 10 is a schematic diagram showing the resistance measurement method. A "High Resistance Meter" (model number: 4339B) manufactured by Hewlett-Packard Company is used as the measuring device 71.

The resistance measurement method is generally the same as that described with reference to Figures 5(A) and (B), but uses only one bearing 72. As shown in Figure 10, a stainless steel bearing 72 with a diameter of 10.0 mm is pressed against the axial center portion M of the developing roller 12, acting as a contact, with a load W1 of 20±0.5 gf.

A measuring device 71 is connected to the shaft end 12e of the developing roller 12 and the bearing 72, the developing roller 12 is rotated at a speed of 50 rpm, and a DC voltage of -40 V is applied to the shaft end 12e of the developing roller 12 relative to the bearing 72. The resistance of the developing roller 12 is determined from the current flowing between the bearing 72 and the shaft end 12e in this state. The average value of the resistance over two revolutions of the developing roller 12 is taken as the overall resistance value R _ALL [Log Ω].

Next, the surface of the developing roller 12 is cut (step S13). Figures 11(A) and (B) are schematic diagrams showing the cutting process of the developing roller 12. As shown in Figure 11(A), a flat blade 90 (made of cutlery tool steel, 20 mm wide, 25 mm long) manufactured by Toho Co., Ltd. is used for cutting. The shaft end 12e of the developing roller 12 is rotatably supported by a shaft holder 92, and the flat blade 90 is pressed against the surface of the center portion M of the developing roller 12.

As shown in Figure 11 (B), the developing roller 12 is rotated in one direction (counterclockwise in the figure) at a rotational speed of 150 rpm or more. The cutting edge 91 of the flat blade 90 is pressed against the surface of the developing roller 12 below the center of rotation, in a counter-direction to the direction of movement of the surface of the developing roller 12. The cutting edge 91 of the flat blade 90 is oriented at 45 degrees to the vertical.

The cutting depth should be deeper than the thickness of the surface layer 12c of the developing roller 12 (here, 0.01 to 0.02 mm), and is set to 0.03 mm here. By cutting, the surface layer 12c is removed from the center portion M (20 mm wide) of the developing roller 12, exposing the inner layer 12b.

After cutting, any shavings remaining on the surface of the developing roller 12 are blown away with an air blower and then removed by pressing a nonwoven fabric wiper "BEMCOT" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd. against the surface.

Next, the outer diameter of the center portion M of the developing roller 12 after cutting is measured (step S14). An automatic roller measuring machine manufactured by Apollo Seiko Co., Ltd. is used to measure the outer diameter. At this time, it is confirmed that the difference between the outer diameter measured in step S11 and the outer diameter measured in step S14 (i.e., the cutting depth of the center portion M of the developing roller 12) is 0.03 mm or more, and if the outer diameter difference is less than 0.03 mm, cutting in step S13 is repeated. Note that in this embodiment, the center portion M of the developing roller 12 is cut, but other locations may also be cut as long as the surface layer 12c is formed.

Next, the resistance of the center portion M of the developing roller 12 after cutting is measured (step S15). The resistance measurement method is the same as in step S12. That is, as shown in FIG. 10, the bearing 72 is pressed against the center portion M of the developing roller 12, the developing roller 12 is rotated at a speed of 50 rpm, and a DC voltage of -40 V is applied to the shaft end 12e of the developing roller 12 relative to the bearing 72.

In this state, the resistance of the developing roller 12 is calculated from the current flowing between the bearing 72 and the shaft end 12e. The average resistance over two revolutions of the developing roller 12 is defined as the resistance value R _IN [Log Ω] of the inner layer 12b. Because the surface layer 12c has been removed from the center portion M of the developing roller 12, the resistance of the inner layer 12b (i.e., the resistance value R _IN of the inner layer 12b) can be calculated.

Next, the resistance value R _IN [Log Ω] of the inner layer 12b obtained in step S15 is subtracted from the overall resistance value R _ALL [Log Ω] obtained in step S12 to obtain the value R _ALL -R _IN [Log Ω].

The resistance values R _ALL and R _IN are expressed in logarithmic notation. For example, if R _ALL = 7.27 [Log Ω], converting the unit to [Ω] gives R _ALL = 10 ^7.27 [Ω]. Because logarithmic notation is used in this way, the value R _ALL - R _IN [Log Ω] is also referred to as the resistance ratio.

In this embodiment, a plurality of developing rollers 12 (Examples 1 to 10 and Comparative Examples 1 to 10) having different overall resistance values R _ALL and different resistance values R _IN of the inner layer 12 b are prepared, and test printing is performed using each of them to evaluate the relationship with the occurrence of printing defects such as uneven density, fogging, and smearing.

<Residual Potential of Developing Roller>
Next, a description will be given of the measurement of the residual potential of the developing roller 12. Fig. 12 is a schematic diagram showing a method for measuring the residual potential of the developing roller 12. The measuring device 300 used to measure the residual potential of the developing roller 12 is a dielectric relaxation measuring device "DRA2000" manufactured by Quality Engineering Associates.

The measuring device 300 has a carrier 301, a corona discharge electrode 302, a probe 303, and a surface electrometer 304. The corona discharge electrode 302 and the probe 303 are supported by the carrier 301. The carrier 301 is movable along the surface of the developing roller 12 in the axial direction of the developing roller 12. The probe 303 is connected to the surface electrometer 304. The distance between the corona discharge electrode 302 and the probe 303 is 23 mm.

When measuring the residual potential, the shaft end 12e of the developing roller 12 is grounded via resistor 305. In this state, the carrier 301 is moved to a predetermined position (for example, the center position in the axial direction of the developing roller 12), and corona discharge is performed using the corona discharge electrode 302. The corona discharge voltage is set to 6.0 [KV].

Then, the carrier 301 is moved 23 mm, and the probe 303 is positioned at the position of the corona discharge electrode 302 during corona discharge. The residual potential [-V] is then measured 2.15 seconds after corona discharge. The reason for measuring the residual potential after 2.15 seconds is that there is large variation in the residual potential of the developing roller 12 immediately after corona discharge (especially before 0.2 seconds have passed). The residual potential can be measured after the variation in the residual potential has become small; for example, it can be measured after 2.10 seconds or 2.30 seconds have passed.

As described below, the residual potential is measured for multiple types of developing rollers 12 (Examples 1-10 and Comparative Examples 1-10) using measuring device 300. Note that because a negative developing voltage is applied to the developing roller 12, the unit of residual potential is [-V]. Hereinafter, a large absolute value of the potential (residual potential, surface potential) is referred to as a "high potential," and a small absolute value of the potential is referred to as a "low potential."

<Test print>
Next, test printing will be described. In recent years, image forming apparatuses using electrophotographic processes have been considered for printing using toner with fewer external additives. When toner with fewer external additives is used, the toner tends to have a lower chargeability. Therefore, when the surface potential of the developing roller 12 decreases, the toner is not retained on the developing roller 12 and tends to adhere to the unexposed portions of the photosensitive drum 11 that are not exposed by the exposure head 30. As a result, a phenomenon occurs in which toner is printed in a scattered state on a blank sheet of paper (fog).

Furthermore, if the toner transfer from the developing roller 12 to the photosensitive drum 11 is insufficient, the image density will be partially reduced, resulting in a printing defect known as blurring. Furthermore, if the voltage responsiveness of the developing roller 12 is low, the surface potential of the developing roller 12 will fluctuate, resulting in uneven density.

Here, continuous printing and subsequent test printing were performed using the image forming device 1 to determine the presence or absence of fog, blurring, and density unevenness. The image forming device 1 used was an LED printer "B820" manufactured by Oki Electric Industry Co., Ltd., and the developing device 10 was pre-filled with 15 g of toner.

Black toner was used for continuous printing and test printing. The desirable range for the Si content in the toner is 0.98 to 1.28 wt. %, but here it was set to 0.98 wt. The Si content was measured by elemental analysis using energy dispersive X-ray spectroscopy (EDX), as described above.

Figure 13 shows the medium P1 used for continuous printing. Medium P1 is A4-sized plain paper. The transport direction was horizontal, and the transport speed (circumferential speed) was 202.5 mm/sec, which corresponds to a printing speed of 45 ppm. The printing environment was a temperature of 28°C and a relative humidity of 80%.

The charging voltage applied to the charging roller 15 was -1050 V, the developing voltage applied to the developing roller 12 was -200 V, the supply voltage applied to the supply roller 13 was -350 V, the blade voltage applied to the regulating blade 14 was -350 V, and the transfer voltage applied to the transfer roller 19 was +1500 V. The potential of the portion of the surface of the photosensitive drum 11 exposed by the exposure head 30 dropped to between -30 V and -50 V.

In continuous printing, an image LP with a duty ratio of 1.25% was printed on medium P1. An image LP with a duty ratio of 1.25% (i.e., a print image density of 1.25%) is a solid line image as shown in Figure 13.

The duty ratio is also called print image density and is defined as follows:
Print image density (duty ratio) = [Cm(i) / (Cd x C0)] x 100
Cm(i) is the number of dots emitted by the exposure head 30 while the photosensitive drum 11 rotates Cd. C0 is the number of dots that the exposure head 30 can emit while the photosensitive drum 11 rotates once. Cd x C0 is the number of dots that the exposure head 30 can emit while the photosensitive drum 11 rotates Cd.

In other words, if a solid image is printed across the entire printable area of medium P1, the print image density will be 100%. If an image with an area of 1% of this 100% print image density is printed, the print image density will be 1%.

Continuous printing was performed on 10,000 sheets of media P1, and three types of test patterns described below were printed every 2,000 sheets.

Figure 14 shows the medium P2 and test pattern used for test printing. Medium P2 was A3-sized plain paper. The transport direction was vertical, and the transport speed (circumferential speed) was 202.5 mm/sec, which corresponds to a printing speed of 45 ppm. A halftone image HT was printed on medium P2 as the test pattern. The halftone image HT is a 2x2 pattern. The 2x2 pattern forms 4 dots in 2 vertical dots and 2 horizontal dots, out of 16 squares formed by 4 dots in the vertical direction and 4 dots in the horizontal direction, with 4 squares of dots formed by 2 dots D in the vertical direction and 2 dots D in the horizontal direction.

Figure 15 (A) shows the medium P3 and test pattern used for test printing. Medium P3 is the same as medium P2 shown in Figure 14, and the transport direction and transport speed are also the same. A solid image SD with a duty ratio of 100% was printed on medium P3 as the test pattern.

Figure 15 (B) shows the medium P4 and test pattern used for test printing. Medium P4 was A3-sized glossy paper. The transport direction and transport speed were the same as for medium P2 shown in Figure 14. On medium P3, an image WH with a duty ratio of 0% was printed as the test pattern.

As described below, a plurality of developing rollers 12 with different resistance values R _ALL and R _IN were created, and then each developing roller 12 was incorporated into the developing device 10 of the image forming apparatus 1, and the above-mentioned continuous printing was performed. After completing the continuous printing of 2,000 sheets, test patterns (images HT, SD, WH) were printed on media P1, P2, and P3, respectively, and the surfaces of the printed media P1, P2, and P3 were visually observed to evaluate the presence or absence of fog, blurring, and uneven density.

A total of five test prints were performed after each 2,000-sheet continuous print run. Visual fogging assessment was conducted. If even one sheet was judged to be defective, the fogging rating was rated "X." If all five sheets were judged to be good, the fogging rating was rated "O."

Similarly, in a total of five test prints, if even one sheet was judged to be defective through visual inspection, the smearing was rated as "X", and if all five sheets were judged to be good, the smearing was rated as "O".

Similarly, in a total of five test prints, if even one sheet was judged to be defective through visual inspection of density unevenness, the density unevenness was rated as "X", and if all five sheets were judged to be good, the density unevenness was rated as "O".

This makes it possible to evaluate the correlation between the resistance characteristics including the overall resistance R _ALL of the developing roller 12 and the resistance R _IN of the inner layer 12b, and print defects such as uneven density, fogging, and blurring.

<Correction of Resistance Value R _IN of Inner Layer 12 b>
Before describing the relationship between the resistance characteristics of the developing roller 12 and print quality, we will explain how to correct the resistance value R _IN of the inner layer 12 b. When measuring the resistance value R _IN of the inner layer 12 b (steps S13 to S15 shown in FIG. 9 ), the surface layer 12 c of the developing roller 12 is cut to expose the inner layer 12 b, so it is necessary to consider the effect of cutting on the resistance value R _IN of the inner layer 12 b.

Here, the influence of cutting was eliminated from the resistance value R _IN of the inner layer 12 b as follows: First, as shown in Fig. 16(A), the inner layer 12 b was formed on the surface of the shaft body 12 a (not shown), and before the surface layer 12 c was formed, the resistance value R IN of the inner layer 12 b was measured by bringing a bearing 72 into contact with the inner layer 12 b (point A) using the resistance measurement method described with reference to _Fig . 10 .

Next, as shown in FIG. 16(B), a surface layer 12c was formed on the surface of the inner layer 12b of FIG. 16(A), and the overall resistance value R _ALL of the developing roller 12 was measured by pressing a bearing 72 against the surface layer 12c (point A) using the resistance measurement method described with reference to FIG. 10.

Furthermore, as shown in FIG. 16(C), the surface layer 12c of FIG. 16(B) was cut with a flat blade 90 to expose the inner layer 12b, and the resistance value R _IN of the inner layer 12b was measured by pressing the bearing 72 against the inner layer 12b (point A) using the resistance measurement method described with reference to FIG. 10.

R _ALL −R _IN when the surface layer 12 c is not ground can be obtained by subtracting the resistance value R _IN of the inner layer 12 b measured in Fig. 16(A) from the overall resistance value R _ALL measured in Fig. 16(B). On the other hand, R _ALL −R IN when the surface layer 12 c is ground can be obtained by subtracting the resistance value R _IN of the inner layer 12 b measured in Fig. 16(C) from the overall resistance value R _ALL measured in Fig. 16(B ₎ .

Here, 12 types of developing rollers 12 were created by varying the contents of acrylic resin and urethane resin, and the amount of carbon black added, in the surface treatment solution used to form the surface layer 12c of the developing roller 12. For each developing roller 12, R _ALL −R _IN was determined when the surface layer 12c was not scraped, and R _ALL −R _IN was determined when the surface layer 12c was scraped.

17 is a graph showing the relationship between R _ALL −R _IN when the surface layer 12 c is not ground and R _ALL −R _IN when the surface layer 12 c is ground, where the vertical axis represents R _ALL −R _IN (y) when the surface layer 12 c is not ground, and the horizontal axis represents R _ALL −R _IN (x) when the surface layer 12 c is ground.

The 12 plots shown in Figure 17 are on the approximate line of y = 0.998x + 0.417, and the coefficient of determination ^R2 is 0.954. The slope of the line, 0.998, can be considered to be roughly 1, indicating a high correlation.

On the other hand, the y-intercept of the above line, 0.417, is due to the effect of cutting the surface layer 12c, more specifically, the increase in surface roughness of the inner layer 12b due to cutting the surface layer 12c.

The surface roughness Rz of the inner layer 12b in Figure 16(A) was measured using the above-mentioned surface roughness measuring instrument (Kosaka Manufacturing's "Surfcorder SEF3500"), and was found to be Rz = 6.3 [μm]. In contrast, the surface roughness Rz of the inner layer 12b in Figure 16(C) was measured, and was found to be Rz = 20.3 [μm].

From this, it can be seen that the y-intercept of the line shown in Figure 17, 0.417, is due to the fact that the surface roughness Rz of the inner layer 12b increases as a result of cutting the surface layer 12c, resulting in a decrease in adhesion between the inner layer 12b and the bearing 72 (Figure 10).

From the above, in this embodiment, the resistance value R _IN of the inner layer 12 b is corrected by subtracting 0.417 from the resistance value R _IN of the inner layer 12 b measured in step S15 of FIG.

<Example>
Next, we will explain Examples 1 to 10 and Comparative Examples 1 to 10. In the developing rollers 12 of Examples 1 to 10 and Comparative Examples 1 to 10, the amount of carbon black added to the rubber material of the inner layer 12b and the amount of carbon black added to the surface treatment solution used to form the surface layer 12c were adjusted to make the overall resistance value R _ALL and the resistance value R _IN of the inner layer 12b different.

In the developing roller 12 of Example 1, the amount of carbon black added to the rubber material of the inner layer 12b and the amount of carbon black added to the surface treatment solution used to form the surface layer 12c were adjusted to set the overall resistance R _ALL to 7.27 [Log Ω] and the resistance R _IN of the inner layer 12b to 6.45 [Log Ω]. The value of _{R ALL} -R _IN was 0.82 [Log Ω] and the residual potential was 3.66 [-V].

The developing roller 12 of Example 2 had an overall resistance R _ALL of 7.29 [Log Ω] and an inner layer 12b resistance R _IN of 6.07 [Log Ω]. The value of _{R ALL} -R _IN was 1.22 [Log Ω], and the residual potential was 2.98 [-V].

The developing roller 12 of Example 3 had an overall resistance value R _ALL of 7.84 [Log Ω] and an inner layer 12b resistance value R _IN of 6.34 [Log Ω]. The value of _{R ALL} -R _IN was 1.50 [Log Ω] and the residual potential was 9.33 [-V].

The developing roller 12 of Example 4 had an overall resistance R _ALL of 8.00 [Log Ω] and an inner layer 12b resistance R _IN of 6.26 [Log Ω]. The value of _{R ALL} -R _IN was 1.74 [Log Ω], and the residual potential was 21.3 [-V].

The developing roller 12 of Example 5 had an overall resistance value R _ALL of 6.69 [Log Ω] and an inner layer 12b resistance value R _IN of 6.13 [Log Ω]. The value of _{R ALL} -R _IN was 0.56 [Log Ω] and the residual potential was 3.83 [-V].

The developing roller 12 of Example 6 had an overall resistance value R _ALL of 7.19 [Log Ω] and an inner layer 12b resistance value R _IN of 6.39 [Log Ω]. The value of _{R ALL} -R _IN was 0.80 [Log Ω] and the residual potential was 5.08 [-V].

The developing roller 12 of Example 7 had an overall resistance value R _ALL of 6.15 [Log Ω] and an inner layer 12b resistance value R _IN of 4.89 [Log Ω]. The value of _{R ALL} -R _IN was 1.26 [Log Ω] and the residual potential was 3.02 [-V].

The developing roller 12 of Example 8 had an overall resistance value R _ALL of 6.15 [Log Ω] and an inner layer 12b resistance value R _IN of 5.45 [Log Ω]. The value of _{R ALL} -R _IN was 0.70 [Log Ω] and the residual potential was 3.06 [-V].

The developing roller 12 of Example 9 had an overall resistance value R _ALL of 8.45 [Log Ω] and an inner layer 12b resistance value R _IN of 7.34 [Log Ω]. The value of _{R ALL} -R _IN was 1.11 [Log Ω] and the residual potential was 8.01 [-V].

The developing roller 12 of Example 10 had an overall resistance value R _ALL of 8.45 [Log Ω] and an inner layer 12b resistance value R _IN of 6.90 [Log Ω]. The value of _{R ALL} -R _IN was 1.55 [Log Ω] and the residual potential was 13.20 [-V].

In the developing rollers 12 of Examples 1 to 10, the weight ratio of the acrylic resin to the urethane resin in the surface layer 12c (also referred to as the A/U ratio) was 50:50. This is the weight ratio of the acrylic resin to the urethane resin contained in the surface treatment liquid used when surface treating the inner layer 12b to form the surface layer 12c.

The developing roller 12 of Comparative Example 1 had an overall resistance value R _ALL of 6.26 [Log Ω] and an inner layer 12b resistance value R _IN of 7.05 [Log Ω]. The value of _{R ALL} -R _IN was -0.79 [Log Ω] and the residual potential was 1.36 [-V].

The developing roller 12 of Comparative Example 2 had an overall resistance value R _ALL of 6.52 [Log Ω] and an inner layer 12b resistance value R _IN of 6.61 [Log Ω]. The value of _{R ALL} -R _IN was -0.09 [Log Ω] and the residual potential was 1.81 [-V].

The developing roller 12 of Comparative Example 3 had an overall resistance value R _ALL of 6.49 [Log Ω] and an inner layer 12b resistance value R _IN of 6.99 [Log Ω]. The value of _{R ALL} -R _IN was -0.50 [Log Ω] and the residual potential was 4.83 [-V].

The developing roller 12 of Comparative Example 4 had an overall resistance value R _ALL of 7.69 [Log Ω] and an inner layer 12b resistance value R _IN of 8.63 [Log Ω]. The value of _{R ALL} -R _IN was -0.94 [Log Ω] and the residual potential was 5.84 [-V].

The developing roller 12 of Comparative Example 5 had an overall resistance value R _ALL of 7.52 [Log Ω] and an inner layer 12b resistance value R _IN of 8.89 [Log Ω]. The value of _{R ALL} -R _IN was -1.37 [Log Ω] and the residual potential was 5.64 [-V].

The developing roller 12 of Comparative Example 6 had an overall resistance value R _ALL of 5.70 [Log Ω] and an inner layer 12b resistance value R _IN of 5.45 [Log Ω]. The value of _{R ALL} -R _IN was 0.25 [Log Ω] and the residual potential was 3.05 [-V].

The developing roller 12 of Comparative Example 7 had an overall resistance value R _ALL of 8.50 [Log Ω] and an inner layer 12b resistance value R _IN of 8.39 [Log Ω]. The value of _{R ALL} -R _IN was 0.11 [Log Ω] and the residual potential was 8.02 [-V].

The developing roller 12 of Comparative Example 8 had an overall resistance value R _ALL of 7.20 [Log Ω] and an inner layer 12b resistance value R _IN of 4.73 [Log Ω]. The value of _{R ALL} -R _IN was 2.47 [Log Ω] and the residual potential was 5.43 [-V].

The developing roller 12 of Comparative Example 9 had an overall resistance value R _ALL of 6.05 [Log Ω] and an inner layer 12b resistance value R _IN of 4.29 [Log Ω]. The value of _{R ALL} -R _IN was 1.76 [Log Ω] and the residual potential was 5.02 [-V].

The developing roller 12 of Comparative Example 10 had an overall resistance value R _ALL of 9.08 [Log Ω] and an inner layer 12b resistance value R _IN of 7.78 [Log Ω]. The value of _{R ALL} -R _IN was 1.30 [Log Ω] and the residual potential was 16.28 [-V].

Of these, the developing roller 12 of Comparative Example 1 had a weight ratio of acrylic resin to urethane resin in the surface layer 12c of 20:80. The developing rollers 12 of Comparative Examples 2 to 10 had a weight ratio of acrylic resin to urethane resin in the surface layer 12c of 50:50.

The developing rollers 12 of Examples 1 to 10 and Comparative Examples 1 to 10 were each installed in the developing device 10 of the image forming apparatus 1, and continuous printing and test printing were performed under the conditions described above, and uneven density, blurring, and fogging were evaluated. The results are shown in Tables 1 and 2.

Tables 1 and 2 show the ratio (A/U ratio) of acrylic resin to urethane resin in the surface layer 12c of the developing roller 12, the overall resistance value R _ALL , the resistance value R _IN of the inner layer 12b, the difference between these (resistance ratio) R _ALL -R _IN , the residual potential of the developing roller 12, and the image evaluation results (uneven shading, blurring, fogging).

In Comparative Example 1, the value of R _ALL - R _IN was -0.79 [Log Ω], and fogging was observed. This was due to the following reason. That is, because the resistance value R _IN of the inner layer 12 b was higher than the overall resistance value R _ALL , the charge applied to the surface layer 12 c of the developing roller 12 did not easily move to the inner layer 12 b, which had a higher resistance. Therefore, the charge applied to the surface layer 12 c was easily dissipated into the atmosphere, and the surface potential of the developing roller 12 decreased.

Here, when the overall resistance value R _ALL is equal to or greater than the resistance value R _IN of the inner layer 12 b as in Examples 1 to 10, the charge applied to the surface layer 12 c is likely to move to the inner layer 12 b and is therefore less likely to dissipate into the atmosphere, and therefore the charge is likely to be retained throughout the entire developing roller 12.

On the other hand, when the overall resistance value R _ALL is lower than the resistance value R _IN of the inner layer 12 b as in Comparative Examples 1 to 10, the charge applied to the surface layer 12 c is likely to dissipate into the atmosphere, and therefore less charge is retained in the entire developing roller 12. As a result, fogging is more likely to occur.

When the surface potential of the developing roller 12 decreases, toner that would normally be retained on the developing roller 12 (e.g., positively charged toner) is not retained on the developing roller 12 and instead adheres to the photosensitive drum 11. This toner also adheres to the unexposed parts of the photosensitive drum 11, causing fogging in the printed image. The fact that the charge on the developing roller 12 is easily dissipated is also consistent with the low residual potential in Comparative Example 1.

In Comparative Example 2, the value of R _ALL - R _IN was -0.09 [Log Ω], and fogging was observed. This was for the same reason as in Comparative Example 1. That is, because the resistance value R _IN of the inner layer 12b was higher than the overall resistance value R _ALL , the charge applied to the surface layer 12c of the developing roller 12 did not move easily to the highly resistive inner layer 12b and was easily dissipated into the atmosphere, resulting in a decrease in the surface potential of the developing roller 12. The fact that the charge on the developing roller 12 was easily dissipated is also consistent with the low residual potential in Comparative Example 2.

In Comparative Examples 3 to 5, the values of R _ALL - R _IN were -0.50 [Log Ω], -0.94 [Log Ω], and -1.37 [Log Ω], respectively, and fogging was observed in all cases. This was for the same reason as in Comparative Examples 1 and 2. That is, because the resistance value R _IN of the inner layer 12b was higher than the overall resistance value R _ALL , the charge applied to the surface layer 12c of the developing roller 12 did not easily move to the highly resistive inner layer 12b and was easily dissipated into the atmosphere, resulting in a decrease in the surface potential of the developing roller 12.

In Comparative Example 6, the overall resistance value R _ALL was 5.70 [Log Ω], and fogging was observed. This was because both the overall resistance value R _ALL and the resistance value R _IN of the inner layer 12b were low, and therefore the amount of conductivity imparting agent (carbon black) added was large. When the amount of conductivity imparting agent added is large, the attenuation rate of the development voltage applied to the developing roller 12 increases, and as a result, charge is more likely to dissipate throughout the entire developing roller 12 via the shaft 12a, and the surface potential of the developing roller 12 decreases.

In Comparative Example 7, the overall resistance value R _ALL was 8.50 [Log Ω], and blurring was observed. This was because the overall resistance value R _ALL and the resistance value R _IN of the inner layer 12b were both high, resulting in a decrease in the voltage responsiveness of the developing roller 12. When the voltage responsiveness decreases, the surface potential of the developing roller 12 does not increase sufficiently even when a developing voltage is applied, resulting in an insufficient amount of toner on the developing roller 12 and an insufficient amount of toner adhering to the photosensitive drum 11, causing blurring.

In Comparative Example 8, the R _ALL −R _IN value was 2.47 [Log Ω], and density unevenness was observed. This is because the R _ALL −R _IN value was large, and therefore the content of the conductivity imparting agent (carbon black) in the surface layer 12c was low. If the content of the conductivity imparting agent is low, the conductivity imparting agent is dispersed sparsely within the surface layer 12c, causing variations in the charge on the surface of the developing roller 12 and resulting in density unevenness.

In Comparative Example 9, the overall resistance value R _ALL was 6.05 [Log Ω], and fogging was observed. This was for the same reason as in Comparative Example 6. That is, both the overall resistance value R _ALL and the resistance value R _IN of the inner layer 12b were low, and therefore the amount of conductivity imparting agent (carbon black) added was large, so the attenuation rate of the development voltage applied to the developing roller 12 increased, and as a result, charge was more likely to dissipate throughout the entire developing roller 12 via the shaft 12a, and the surface potential of the developing roller 12 decreased.

In Comparative Example 10, the overall resistance value R _ALL was 9.08 [Log Ω], and blurring was observed. This was because, as in Comparative Example 7, both the overall resistance value R _ALL and the resistance value R _IN of the inner layer 12 b were high, resulting in a decrease in the voltage responsiveness of the developing roller 12.

In contrast, the evaluation results for fog, blur, and unevenness in shading were all good in Examples 1 to 10. In Examples 1 to 10, the R _ALL −R _IN value was 0.56 to 1.74 [Log Ω], so the charge on the surface of developing roller 12 was less likely to dissipate (hence the surface potential was less likely to decrease), and charge variation due to an excessively small amount of the conductivity imparting agent in surface layer 12 c was also less likely to occur, which is thought to have suppressed the occurrence of fog and unevenness in shading.

Furthermore, in Examples 1 to 10, the overall resistance value R _ALL was 6.15 to 8.45 [Log Ω], so the charge was less likely to dissipate across the entire developing roller 12, and the voltage responsiveness of the developing roller 12 was also sufficient, which is thought to have suppressed the occurrence of blurring.

Fig. 18 is a graph showing the evaluation results shown in Tables 1 and 2. The vertical axis represents R _ALL -R _IN [Log Ω], and the horizontal axis represents the overall resistance value R _ALL [Log Ω]. In Fig. 18, the range in which R _ALL -R _IN is 0.56 to 1.74 [Log Ω] and the overall resistance value R _ALL is 6.15 to 8.45 [Log Ω] is the range in which blurring, fogging, and density unevenness do not occur.

On the other hand, in FIG. 18, a range R1 where the value of R _ALL −R _IN exceeds 1.74 [Log Ω] is a range where density unevenness occurs because the content of the conductivity imparting agent (carbon black) in the surface layer 12c is small.

Furthermore, a range R2 in which the overall resistance value R _ALL exceeds 8.45 [Log Ω] is a range in which blurring occurs due to a decrease in the voltage responsiveness of the developing roller 12.

In addition, in the range R3 where the value of R _ALL −R _IN is below 0.56 [Log Ω], the charge on the surface of the developing roller 12 is likely to dissipate, and the surface potential of the developing roller 12 drops, causing fogging.

In addition, in a range R4 where the overall resistance value R _ALL is below 6.15 [Log Ω], the charge of the entire developing roller 12 is likely to dissipate, the surface potential of the developing roller 12 drops, and fogging occurs.

<Consideration>
Here, the relationship between the surface potential of the developing roller 12 and fogging will be further examined. Fig. 19A is a schematic diagram showing the state of toner movement from the developing roller 12 to the photosensitive drum 11. Fig. 19B is a schematic diagram showing the state of toner retention on the surface of the developing roller 12. In this embodiment, toner with a small amount of external additives is used, and the blow-off charge amount is 94.6 [μC/g] or less.

When the amount of external additive in the toner is small, the toner tends to have low chargeability. Even in this case, as shown in Figure 19 (B), if the surface potential of the developing roller 12 is high, there will be a large amount of charge on the surface layer 12c of the developing roller 12, and the toner can be held on the developing roller 12 by electrostatic force. On the other hand, if the surface potential of the developing roller 12 decreases, the ability to hold toner on the developing roller 12 decreases.

As shown in Figure 19 (A), the surface potential of the developing roller 12 is highest at the contact point with the regulating blade 14 and decreases as it approaches the contact point N with the photosensitive drum 11. If the circumferential distance from the regulating blade 14 to the photosensitive drum 11 on the developing roller 12 is L1, the time it takes for the developing roller 12 to rotate the distance L1 is approximately 0.12 seconds.

If the surface potential of the developing roller 12 drops while the developing roller 12 rotates from the regulating blade 14 to the photosensitive drum 11, positively charged toner that should be held on the surface of the developing roller 12 by electrostatic force (and not move to the photosensitive drum 11) may leave the developing roller 12 and move to the photosensitive drum 11. This toner also adheres to the unexposed areas of the photosensitive drum 11, causing toner to scatter over white characters and result in printing fog.

In particular, if the transport force exerted by the photosensitive drum 11 and the developing roller 12 is increased (for example, to 1.0 N or greater), the pressure between the photosensitive drum 11 and the developing roller 12 will be high, increasing the amount of toner adhering to the photosensitive drum 11 and making fogging even more likely to occur.

Here, we will explain the relationship between the residual potential of the developing roller 12, the residual charge of the toner on the developing roller 12, and fogging. First, in addition to Examples 1 to 10 and Comparative Examples 1 to 10, we created several types of developing rollers 12 with different compositions of the surface layer 12c (for example, the amount of carbon black added).

The developing roller 12 was mounted in the developing device 10 of the image forming apparatus 1 (LED printer "B820" manufactured by OKI Electric Industry Co., Ltd.), and the pattern (image LP) shown in Figure 13 was printed on A4-sized plain paper. The transport direction was horizontal, and the transport speed (circumferential speed) was 202.5 mm/sec, which corresponds to a printing speed of 45 ppm. The charging voltage was -1050 V, the developing voltage was -200 V, the supply voltage was -300 V, and the transfer voltage was +1500 V. The printing environment was a temperature of 28°C and a relative humidity of 80%.

During this printing operation, the image forming apparatus 1 was stopped, the developing roller 12 was removed from the image forming apparatus 1, and the charge Q/M [μC/g] of the toner on the developing roller 12 was measured. The charge Q/M measured in this manner is referred to as the "initial Q/M."

The toner charge Q/M was measured using the measuring device 300 shown in Figure 12. Corona discharge was performed while moving the carrier 301 at a speed of 230 mm/s, and the charge was measured 0.1 seconds after the corona discharge.

In addition, the hue difference ΔE, which is an index of fog, was measured for the image printed during the above printing operation (before the image forming apparatus 1 was stopped). The hue difference ΔE was measured as follows.

First, an adhesive tape ("Scotch Mending Tape" manufactured by Sumitomo 3M Limited) was attached to the surface of the developing roller 12 and then peeled off to collect the fog toner. This adhesive tape is referred to as the collection adhesive tape.

This collection adhesive tape was attached to a white recording paper ("Excellent White A4" manufactured by Oki Electric Industry Co., Ltd.: basis weight 80 [g/m ² ]). In addition, a standard adhesive tape for comparison (hereinafter referred to as the standard adhesive tape) was attached to another part of the same recording paper.

Then, using a spectrophotometer (Konica Minolta, Inc., CM-2600d), the hue difference ΔE (L*a*b color system chromaticity) between the sample adhesive tape and the reference adhesive tape was calculated using the following formula (1):

Generally, if the hue difference ΔE is 1.5 or less, the fog assessment result can be considered good.

Figure 20(A) is a graph showing the relationship between the hue difference ΔE of the printed image measured as described above and the initial Q/M of the toner on the developing roller 12. The vertical axis represents the hue difference ΔE, and the horizontal axis represents the initial Q/M [μC/g] of the toner on the developing roller 12.

20A, the hue difference ΔE(y) of the printed image and the initial Q/M(x) of the toner on the developing roller 12 are in a relationship that can be approximated by the curve y = -1.386Ln(x) + 5.9571, with a coefficient of determination ^R2 of 0.88. The hue difference ΔE of the printed image can be reduced to 1.5 or less (i.e., fog can be suppressed) when the initial Q/M of the toner on the developing roller 12 is 30 μC/g or more.

Figure 20 (B) is a graph showing the relationship between the initial Q/M of the toner on the developing roller 12 and the residual potential of the developing roller 12. The vertical axis represents the initial Q/M [μC/g] of the toner on the developing roller 12, and the horizontal axis represents the residual potential [V] of the developing roller 12.

The residual potential of the developing roller 12 is the potential measured using the method described with reference to Figure 12 after the developing roller 12 is manufactured and before it is installed in the developing device 10.

20B, the initial Q/M(y) of the toner on the developing roller 12 and the residual potential (x) of the developing roller 12 are in a relationship that can be approximated by a curve of y = 7.9155Ln(x) + 17.697, with a coefficient of determination ^R2 of 0.834. The initial Q/M of the toner on the developing roller 12 can be made equal to or greater than 30 μC/g when the residual potential of the developing roller 12 is 4.4 -V or greater.

Figure 21 is a graph showing the relationship between the residual potential of the developing roller 12 and the resistance value of the developing roller 12. The vertical axis represents the residual potential [-V] of the developing roller 12, and the horizontal axis represents the resistance value [Log Ω] of the developing roller 12.

The residual potential of the developing roller 12 was measured using the method described with reference to Figure 12. The resistance value of the developing roller 12 was measured using the method described with reference to Figure 10. In Figure 21, the circular plots show the measurement results for nine types of developing rollers 12 that have a surface layer 12c. The square plots show the measurement results for eight types of developing rollers 12 that do not have a surface layer 12c.

From Figure 21, it can be seen that by providing a surface layer 12c covering the inner layer 12b on the developing roller 12 and making the overall resistance value R _ALL [Log Ω] higher than the resistance value R _IN [Log Ω] of the inner layer 12b, the residual potential [-V] of the developing roller 12 increases, the residual charge of the toner on the developing roller 12 increases, and this leads to the suppression of fogging.

In this embodiment, the developing roller 12 has an inner layer 12b and a surface layer 12c, and the difference R _{ALL -R} in between the overall resistance value R _ALL [Log Ω] of the inner layer 12b and the surface layer 12c combined and the resistance value R _IN [Log Ω _] of the inner layer 12b is 0.56 to 1.7 4 [Log Ω], so that the decrease in the surface potential of the developing roller 12 is suppressed, thereby reducing printing defects such as fogging.

Note that the Si content in the toner is set to a range of 0.98 to 1.28% by weight. If the amount of external additive in the toner is greater than 1.28% by weight, the pressure applied to the toner between the photosensitive drum 11 and the developing roller 12 will cause the external additive to fall off the toner base particles, resulting in poor toner charging.

Furthermore, if the amount of external additive in the toner is less than 0.98% by weight, most of the surface of the toner base particles is exposed and not covered by the external additive, making it easier for the toner base particles to adhere to each other. As a result, the toner becomes less likely to separate from the surface of the developing roller 12, and less toner moves to the photosensitive drum 11, resulting in smearing.

In test printing using the developing rollers 12 of Examples 1 to 10 described above, smearing was evaluated with a toner Si content of 0.98% by weight, and the smearing evaluation results were good in all cases. The lower the Si content in the toner, the more likely smearing occurs, and the higher the Si content, the less likely smearing occurs. Therefore, it can be seen that smearing does not occur at least when the Si content is in the range of 0.98 to 1.28% by weight.

Furthermore, the developing rollers 12 of Examples 1 to 10 described above had a weight ratio of acrylic resin to urethane resin in the surface layer 12c of 50:50. In other words, the weight ratio of acrylic resin to the total of acrylic resin and urethane resin was 50%. However, it has been confirmed that similar results to those described above can be obtained if the weight ratio of acrylic resin to the total of acrylic resin and urethane resin is in the range of 40% to 60%.

Furthermore, when the developing roller 12 of Examples 1 to 10 described above was used, the conveying force between the photosensitive drum 11 and developing roller 12 measured using the method shown in Figures 8(A) and (B) was 1.0 [N] or more. By setting the conveying force to 1.0 [N] or more, the pressure between the photosensitive drum 11 and developing roller 12 can be sufficiently increased, making it possible to suppress white voids when printing solid images.

However, if the conveying force is too large, friction between the photosensitive drum 11 and the developing roller 12 can cause the temperature to rise during continuous printing, which can lead to print defects such as blurring. Therefore, it is desirable for the conveying force to be 1.4 N or less.

<Effects of the embodiment>
As described above, in the developing device 10 of this embodiment, the developing roller 12 has an inner layer 12b and a surface layer 12c, and the difference R _{ALL -R} in between the overall resistance value R _ALL [Log Ω] of the inner layer 12b and the surface layer 12c combined and the resistance value R _IN [Log Ω] of the inner layer 12b is 0.56 [Log Ω] or more, so that the decrease in the surface potential of the developing roller 12 is suppressed, thereby reducing the occurrence of fogging.

Furthermore, since the difference R _{ALL -R} in between the overall resistance value R _ALL [Log Ω] of the developing roller 12 and the resistance value R _IN [Log Ω] of the inner layer 12 b is 1.74 [Log Ω] or less, it is possible to suppress uneven density that occurs when the amount of conductivity imparting agent is too small (i.e., occurs due to uneven distribution of the conductivity imparting agent).

Furthermore, since the overall resistance value R _ALL of the developing roller 12 is 6.15 [Log Ω] or more, the decrease in surface potential due to dissipation of charge across the entire developing roller 12 is suppressed, thereby reducing the occurrence of fogging.

Furthermore, since the overall resistance value R _ALL of the developing roller 12 is 8.45 [Log Ω] or less, the occurrence of blurring due to a decrease in the voltage response of the developing roller 12 can be suppressed.

Furthermore, since the surface layer 12c contains an acrylic resin and a urethane resin, with the weight ratio of the acrylic resin being 40% or more and 60% or less of the total, a decrease in the surface potential of the developing roller 12 can be suppressed.

Furthermore, by setting the toner's Si content, as measured by elemental analysis using energy dispersive X-ray analysis, to 0.98 to 1.28 weight percent, it becomes less likely for external additives containing Si to separate from the toner base particles, reducing printing defects associated with poor toner charging.

This disclosure can be used in image forming devices such as printers, copiers, facsimile machines, and MFPs (Multi Function Peripherals) that form images on media.

1 Image forming apparatus, 10 Developing device, 11 Photosensitive drum (image carrier), 12 Developing roller (developer carrier), 12a Axial body (shaft), 12b Inner layer, 12c Surface layer, 13 Supply roller (supply member), 14 Regulating blade (layer regulating member), 15 Charging roller (charging member), 16 Toner cartridge (developer container), 19 Transfer roller (transfer member), 30 Exposure head (exposure device), 40 Medium supply unit, 50 Fixing device, 60 Medium discharge unit, 80 Measuring jig, 90 Flat blade, 100 Main control unit, 300 Measuring device, R _ALL resistance value (total resistance value), R _ALL - R _IN Difference between total resistance value and inner layer resistance value (resistance ratio), R _IN resistance value (inner layer resistance value).

Claims (5)

an image carrier that carries an electrostatic latent image;
a developer carrier that contacts the image carrier and develops the electrostatic latent image with a developer,
The developer carrier is
A shaft body,
an elastic layer covering the shaft body;
a surface layer that covers the elastic layer and is in contact with the image carrier ;
The surface layer covers the entire periphery of the elastic layer, so that the elastic layer does not come into contact with the image carrier,
the developer has a Si content of 0.98 to 1.28 [wt %] as measured by elemental analysis using energy dispersive X-ray analysis,
an overall resistance value R _ALL [Log Ω] of the surface layer and the elastic layer combined is 6.15 [Log Ω] or more and 8.45 [Log Ω] or less;
a difference R _ALL −R _IN between the overall resistance value R _ALL [Log Ω] and the resistance value R _IN [Log Ω] of the elastic layer is 0.56 [Log Ω] or more and 1.74 [Log Ω] or less. 2. The developing device according to claim 1, wherein a conveying force exerted by the image carrier and the developer carrier is 1.0 [N] or more. the surface layer contains an acrylic resin and a urethane resin,
3. The developing device according to claim 1, wherein a weight ratio of the acrylic resin to the total weight of the acrylic resin and the urethane resin is 40% or more and 60% or less. 4. The developing device according to claim 1, wherein the surface roughness Rz of the developer carrier is 2.0 [μm] or more and 7.0 [μm] or less. A developing device according to any one of claims 1 to 4;
a fixing device that fixes the developer image developed with the developer onto a medium.