Drum unit and image forming apparatus

Abstract

To maintain high image quality across various types of media.SOLUTION: A drum unit (20) comprises: a photosensitive drum (11) which is provided with a photosensitive layer (114) having a first charge transport material of a first polarity and a second charge transport material of a second polarity; and a frame (17). When the surface potential of the photosensitive drum (11) is defined as V0 [V] in a case where the photosensitive drum (11) is rotated at a linear speed of 194 [mm / s] and the potential of the first polarity having an absolute value of 1000 [V] is applied to the surface of the photosensitive drum (11) for a predetermined time, and the surface potential is defined as VL [V] in a case where the surface with the surface potential V0 [V] is exposed with an amount of 1.69 [μJ / cm2], a reverse charging potential attenuation rate α obtained through a test operation is defined as α=(1-(VL / V0))×100[%] and the reverse charging potential attenuation rate α satisfies 11.5%≤α≤17.2%.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present disclosure relates to a drum unit having a photosensitive drum, and an image forming apparatus including the drum unit. [Background technology]

[0002] Generally, in an image forming apparatus using an electrophotographic system, an exposure device exposes the surface of a photosensitive drum that has been uniformly charged by a charging roller to form an electrostatic latent image, a toner image based on the electrostatic latent image is formed using toner (developer) supplied by a developing roller, the toner image is transferred to a recording medium (paper) by a transfer roller, and the toner image is fixed to the recording medium by a fixing device. In addition, any toner remaining on the surface of the photosensitive drum after the toner image has been transferred is removed by a cleaning member. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-288672 Summary of the Invention [Problem to be solved by the invention]

[0004] However, with the above-mentioned conventional techniques, there is a risk that the image quality may not be satisfactory depending on the medium, and it may be difficult to maintain high image quality across a variety of media.

[0005] An object of the present disclosure is to provide a drum unit that can maintain high image quality on a variety of media, and an image forming apparatus having the same. [Means for solving the problem]

[0006] The drum unit of the present disclosure includes a photosensitive drum provided with a photosensitive layer having a first charge transport material that transports charges of a first polarity and a second charge transport material that transports charges of a second polarity different from the first polarity, and to which toner of the second polarity can be attached, and a frame that holds the photosensitive drum; The surface potential of the photosensitive drum when a potential of the first polarity and an absolute value of 1000 [V] is applied to the surface of the photosensitive drum for a predetermined time while the photosensitive drum is rotated at a linear velocity of 194 [mm / s] is defined as V0 [V], and the surface at the surface potential of V0 [V] is defined as 1.69 [μJ / cm 2 ], the surface potential of the photosensitive drum when exposed to V L In the case of [V], α=(1-(V L The reverse charge potential decay rate α defined as / V0)) × 100[%] is It is characterized by satisfying 11.5%≦α≦17.2%.

[0007] The image forming apparatus of the present disclosure is characterized by including the above-described drum unit and an exposure device that exposes the surface of the photosensitive drum. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to prevent printing defects from occurring and also to prevent a decrease in density in halftone printing. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view showing a configuration of an image forming apparatus (including an image forming unit) according to an embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view showing the configuration of an image forming unit (including a drum unit) according to the embodiment. [Figure 3] FIG. 2A is a partially cutaway perspective view showing a photosensitive drum according to an embodiment, and FIG. 2B is an enlarged cross-sectional view showing a cross section of a portion of the photosensitive drum shown in FIG. 2A. [Figure 4]10A and 10B are diagrams showing a method for measuring the resistance value of a developing roller of an image forming unit according to an embodiment. [Figure 5] FIG. 2 is a block diagram showing a configuration of a control system of the image forming apparatus according to the embodiment. [Figure 6] FIG. 2 is a schematic diagram showing a state in which a recording medium passes between a photosensitive drum and a transfer roller. [Figure 7] 1A to 1E are schematic diagrams showing the state of charges in the conductive support, undercoat layer, charge generating layer, and charge transport layer of a photosensitive drum. [Figure 8] (A) to (C) are schematic diagrams showing examples of printing defects (occurrence of density steps) that occur in printed images, and (D) is a schematic diagram showing an example of printing defects (occurrence of low halftone print density) that occur in printed images. [Figure 9] (A) is a schematic diagram showing the charge transport layer of the photosensitive layer of a conventional photosensitive drum, and (B) is a diagram showing a state in which positive charges, which are reversely charged charges present on the surface of a conventional photosensitive drum, are difficult to remove by exposure to light. [Figure 10] (A) is a schematic diagram showing the charge transport layer of the photosensitive layer of the photosensitive drum in the embodiment, and (B) is a diagram showing the state in which positive charges, which are reversely charged charges present on the surface of the photosensitive drum in the embodiment, are removed by exposure to light. [Figure 11] 10A and 10B are diagrams illustrating a method for measuring the reverse charging potential decay rate by a test operation. [Figure 12] FIG. 12 is a table showing the surface potential measured using the device of FIG. 11, the calculated reverse charge potential attenuation rate, whether or not printing defects (density step) occurred, and whether or not low halftone print density occurred due to exposure for Comparative Examples #1 to #3 and Examples #1 and #2. DETAILED DESCRIPTION OF THE INVENTION

[0010] A drum unit and an image forming apparatus according to an embodiment of the present disclosure will be described below with reference to the drawings. The following embodiment is merely an example, and various modifications are possible within the scope of the present disclosure.

[0011] <Configuration of Image Forming Apparatus> FIG. 1 is a schematic cross-sectional view showing the configuration of an image forming apparatus 1 (including an image forming unit) according to an embodiment. The image forming apparatus 1 forms a color image on a recording medium M using an electrophotographic system, and is, for example, a color printer. The image forming apparatus 1 is not limited to a color printer, and may be a monochrome printer, a facsimile machine, a copier, an MFP (Multifunction Peripheral), or the like. The image forming apparatus 1 may be any type of device that forms an image using an electrophotographic system.

[0012] The image forming apparatus 1 includes a medium supply section 40 that supplies a recording medium M such as paper, image forming units 10K, 10Y, 10M, and 10C that form toner images of black (K), yellow (Y), magenta (M), and cyan (C), a transfer unit 30 that transfers the toner image to the recording medium M, a fixing device 50 that fixes the toner image to the recording medium M, and a medium discharge section 60 that discharges the recording medium M.

[0013] The medium supply unit 40 has a medium tray 41, a hopping roller 42, a pair of registration rollers 43, and a pair of transport rollers 44. The medium tray 41 accommodates recording media M in a stacked state. The recording media M include, for example, printing paper, overhead projector (OHP) sheets, envelopes, copy paper, and special paper. The printing paper includes plain paper and coated paper (for example, waterproof paper).

[0014] The hopping roller 42 is rotated by the driving force of the transport motor 45 (FIG. 5) and sends the media M from the media tray 41 one by one onto the transport path. The registration roller pair 43 is rotated by the driving force of the transport motor 45 (FIG. 5) and transports the recording medium M along the transport path. The registration roller pair 43 starts rotating a predetermined time after the leading edge of the recording medium M contacts the nip portion between the two rollers, thereby correcting any skew of the recording medium M. The transport roller pair 44 is rotated by the driving force of the transport motor 45 (FIG. 5) and transports the recording medium M to the transfer unit 30.

[0015] Image forming units 10K, 10Y, 10M, and 10C as image forming sections are arranged from the upstream side to the downstream side, i.e., from the right side to the left side in Fig. 1, along the conveyance path of recording medium M. When there is no particular need to distinguish between image forming units 10K, 10Y, 10M, and 10C, they will be referred to as "image forming units 10."

[0016] An exposure head 21 serving as an exposure device is disposed opposite a photosensitive drum 11 (described later) of the image forming unit 10. The exposure head 21 is an LED head that has an LED array consisting of a plurality of LEDs (light-emitting diodes) as a plurality of light-emitting elements arranged in the axial direction of the photosensitive drum 11, and irradiates light onto the surface of the photosensitive drum 11. The exposure head 21 is suspended and supported by a top cover 1B that covers the top of a housing 1A of the image forming apparatus 1. Alternatively, an exposure device having a laser scanning device may be used as the exposure head 21.

[0017] <Configuration of Image Forming Unit> 2 is a schematic cross-sectional view showing the configuration of an image forming unit 10 (including a drum unit 20) according to an embodiment. The image forming unit 10 includes a photosensitive drum 11 as an image carrier, a charging roller 12 as a charging member that uniformly charges the surface of the photosensitive drum 11, a developing roller 13 as a developer carrier that develops an electrostatic latent image formed by exposure of an exposure device to form a toner image as a developer image, a supply roller 14 as a supply member, a developing blade 15 as a layer regulating member, a cleaning member (cleaning blade) 16, and a frame 17 as a housing that houses these components. A toner cartridge 18 as a developer container is attached to the frame 17.

[0018] In the above configuration, the photosensitive drum 11 and the frame 17 that holds it constitute the drum unit 20. In other words, the drum unit 20 has at least the photosensitive drum 11 and the frame 17 that holds it. The drum unit 20 may also have the photosensitive drum 11, the charging roller 12, and the frame 17 that holds them. The drum unit 20 may also have other components that are held by the frame 17, and the image forming unit 10 may also be called the drum unit 20.

[0019] The photosensitive drum 11 is a cylindrical member with a photosensitive layer formed on the surface of a conductive support, and rotates clockwise around a rotation axis in FIGS. 1 and 2. The photosensitive drum 11 carries an electrostatic latent image on its surface. The configuration of the photosensitive drum 11 will be described in detail later.

[0020] The charging roller 12 is disposed so as to be in contact with the surface of the photosensitive drum 11, and rotates following the rotation of the photosensitive drum 11. A charging voltage is applied to the charging roller 12 from a charging roller power supply 131 (FIG. 5), and the charging roller 12 uniformly charges the surface of the photosensitive drum 11.

[0021] Developing roller 13 is disposed so as to be in contact with the surface of photosensitive drum 11, and rotates in the opposite direction to photosensitive drum 11. Therefore, at the contact portion between photosensitive drum 11 and developing roller 13, the moving direction of the surface of photosensitive drum 11 and the moving direction of the surface of developing roller 13 are the forward direction. A developing voltage is applied to developing roller 13 from developing roller power supply 132 (FIG. 5), and the electrostatic latent image on the surface of photosensitive drum 11 is developed with toner (developer).

[0022] Supply roller 14 is disposed so as to contact the surface of developing roller 13, and rotates in the same direction as developing roller 13. Therefore, at the contact portion between developing roller 13 and supply roller 14, the movement direction of the surface of developing roller 13 is opposite to the movement direction of the surface of supply roller 14. A supply voltage is applied to supply roller 14 from supply roller power supply 133 (FIG. 5), and supply toner to developing roller 13.

[0023] The developing blade 15 is a blade that is arranged so as to come into contact with the surface of the developing roller 13. A blade voltage is applied to the developing blade 15 from a developing blade power supply 134 (FIG. 5), and the toner layer on the surface of the developing roller 13 is regulated to a constant thickness.

[0024] Cleaning member 16 has a plate-shaped elastic body and a conductive plate-shaped holder for holding it. Cleaning member 16 is arranged so as to contact the surface of photoreceptor drum 11, and scrapes off (i.e., removes) toner remaining on the surface of photoreceptor drum 11. The waste toner scraped off by cleaning member 16 is transported by a transport screw (not shown) serving as a transport member to a collection container (not shown) serving as a waste toner collection unit.

[0025] The material for forming the plate-like elastic body is not particularly limited, but an elastic body composition is generally used so as not to damage the surface of the photosensitive drum when it is in sliding contact with the surface of the photosensitive drum to scrape off residual toner. Examples of such materials include compositions containing polyurethane, silicone resin, fluororesin, fluororubber, etc., blended with appropriate additives. Among these, polyurethane compositions are preferred because of their excellent mechanical strength and elastic pressure-contact properties.

[0026] The polyurethane composition can usually be obtained using a polyisocyanate, a polyol, a curing agent, and a catalyst.

[0027] The polyisocyanate is not particularly limited, and examples thereof include 4,4'-diphenylmethane diisocyanate (MDI), 2,4-tolylene diisocyanate (2,4-TDI), 2,6-tolylene diisocyanate (2,6-TDI), 3,3'-tolylene-4,4'-diisocyanate, 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, 2,4-tolylene diisocyanate uretidinedione (a dimer of 2,4-TDI), 1,5-naphthylene diisocyanate, metaphenylene diisocyanate, hexamethylene diisocyanate, ... Examples include diisocyanates such as isocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate (water-added MDI), carbodiimide-modified MDI, orthotoluidine diisocyanate, xylene diisocyanate, paraphenylene diisocyanate, and lysine diisocyanate methyl ester, triisocyanates such as triphenylmethane-4,4',4"-triisocyanate, and polymeric MDI. These may be used alone or in combination of two or more. Of these, MDI is preferred from the viewpoint of abrasion resistance.

[0028] The polyol used together with the polyisocyanate is not particularly limited, and examples thereof include polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexylene adipate, and polyether polyols such as polycaprolactone, polyoxytetramethylene glycol, and polyoxypropylene glycol. These may be used alone or in combination of two or more. Among these, PBA is preferred because of its excellent abrasion resistance.

[0029] The curing agent used together with the polyisocyanate and polyol is not particularly limited, and examples thereof include polyols having a molecular weight of 300 or less, such as 1,4-butanediol, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, hexanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, xylene glycol, triethylene glycol, trimethylolpropane, glycerin, pentaerythritol, sorbitol, and 1,2,6-hexanetriol. These may be used alone or in combination of two or more.

[0030] The linear pressure of the cleaning member 16 against the photosensitive drum 11 is preferably 15 gf / cm or more and 30 gf / cm or less, and in this embodiment, it is set to 20 gf / cm. The cleaning angle is set to 10° to 15°.

[0031] Frame 17 is a housing that forms the outer casing of image forming unit 10. Within frame 17, the area where developing roller 13, supply roller 14, and developing blade 15 are arranged is toner storage section 22 that stores toner. In addition to developing roller 13, supply roller 14, and developing blade 15, toner storage section 22 also contains crank-shaped stirring bars 19a, 19b, and 19c that stir and transport toner T. Image forming unit 10 may also be provided with a static elimination light source at a position facing photosensitive drum 11.

[0032] A toner cartridge 18 serving as a developer container is detachably attached to the upper portion of the frame 17 of the image forming unit 10. The toner cartridge 18 is a container that contains toner (denoted by the symbol T) as the developer, and supplies the toner to the toner storage section 22. In this embodiment, an example in which the developer is a one-component developer will be described, but the developer may also be a two-component developer containing toner and a carrier.

[0033] As shown in FIG. 1, the transfer unit 30 has four transfer rollers 31 arranged to face the photosensitive drums 11 of each image forming unit 10, a transfer belt 32 passing between the photosensitive drums 11 and the transfer rollers 31, and a drive roller 33 and a driven roller 34 around which the transfer belt 32 is stretched.

[0034] The transfer unit 30 also has a belt cleaning member 35 that removes residual toner from the transfer belt 32 , and a waste toner storage unit 36 that stores the residual toner removed by the belt cleaning member 35 .

[0035] The transfer belt 32 is an endless belt made of a semi-conductive plastic film with high resistance. The transfer belt 32 has a glossy surface, and conveys the recording medium M by adsorbing and holding it on the surface.

[0036] The drive roller 33 is rotated by the drive force of a drive motor 108 (FIG. 5), and causes the transfer belt 32 to travel in the direction indicated by arrow B. The driven roller 34 applies a predetermined tension to the transfer belt 32.

[0037] The transfer roller 31, which serves as a transfer member, is made by forming a semiconductive elastic layer on the surface of a metal shaft. A transfer voltage is applied to the transfer roller 31 by a transfer roller power supply 135 (FIG. 5), and the toner image on the surface of the photosensitive drum 11 is transferred to the recording medium M on the transfer belt 32.

[0038] 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 fixing roller 51 is rotated by the driving force of a fixing motor 53 (FIG. 5). The fixing device 50 is, for example, a fixing unit that is detachable from the housing 1A.

[0039] The pressure roller 52 is pressed against the fixing roller 51, forming a fixing nip between the fixing roller 51 and the pressure roller 52. The fixing roller 51 and the pressure roller 52 apply pressure and heat to the toner image transferred onto the recording medium M, thereby fixing the toner image to the recording medium M.

[0040] The medium discharge unit 60 is disposed downstream of the fixing unit 50 in the transport direction of the recording medium M. The medium discharge unit 60 has a pair of discharge rollers 61 that discharge the recording medium M that has passed through the fixing unit 50 from a paper discharge port 62. The pair of discharge rollers 61 rotates by rotation transmitted from the fixing motor 53 (FIG. 5), and discharges the recording medium M from the paper discharge port 62. A stacker 63 that stacks the discharged recording medium M is provided above the top cover 1B.

[0041] 1 and 2, the axial direction of the photosensitive drum 11 (i.e., the direction of the rotation axis) is defined as the X direction. The X direction is the axial direction of each roller in the image forming apparatus 1 and is also the width direction of the recording medium M being transported. The direction of movement of the recording medium M as it passes through the image forming unit 10 is defined as the Y direction. The direction perpendicular to the X direction and the Y direction is defined as the Z direction. Here, the Z direction is the up-down direction.

[0042] Next, each component of the image forming unit 10 will be described in more detail.

[0043] "toner" In this embodiment, the toner used in the image forming unit 10 will be described. The toner is a non-magnetic, single-component, negatively charged toner. The average particle size of the toner is approximately 6.0 μm, and the circularity is approximately 0.96. The average particle size is measured using a Multisizer III manufactured by Coulter Corporation. The circularity is measured using a Flow Particle Image Analyzer FPIA-3000 manufactured by Sysmex Corporation.

[0044] The toner 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.

[0045] The binder resin is preferably a polyester resin, a styrene-acrylic resin, an epoxy resin, or a styrene-butadiene resin. Alternatively, multiple types of binder resins may be mixed. Here, a mixture of two or more types of amorphous polyester resins and a crystalline polyester resin having a crystalline structure is used.

[0046] A release agent and a colorant are added to the binder resin, and in addition, additives such as a charge control agent, a conductivity adjuster, a flowability improver, or a cleaning improver may also be added.

[0047] The release agent is not particularly limited, but may include low-molecular-weight polyethylene, low-molecular-weight polypropylene, olefin copolymers, aliphatic hydrocarbon waxes such as microcrystalline wax, paraffin wax, and Fischer-Tropsch wax, oxides of aliphatic hydrocarbon waxes such as oxidized polyethylene wax, or block copolymers thereof, waxes primarily composed of fatty acid esters such as carnauba wax and Montan acid ester wax, and partially or completely deoxidized fatty acid esters such as deoxidized carnauba wax. The content of the release agent is preferably in the range of 0.1 to 20 parts by weight, more preferably 0.5 to 12 parts by weight, per 100 parts by weight of binder resin. Multiple waxes may also be used in combination.

[0048] The colorant is not particularly limited, but dyes and pigments commonly used as colorants for black, yellow, magenta, and cyan toners can be used alone or in combination. Examples of colorants that can be used include carbon black, iron oxide, phthalocyanine blue, permanent brown FG, brilliant fast scarlet, pigment green B, rhodamine B base, solvent red 49, solvent red 146, pigment blue 15:3, solvent blue 35, quinacridone, carmine 6B, and disazo yellow. The content of the colorant is preferably in the range of 2 to 25 parts by weight, more preferably 2 to 15 parts by weight, per 100 parts by weight of binder resin.

[0049] Known charge control agents can be used. For example, in the case of negatively charged toner, azo complex charge control agents, salicylic acid complex charge control agents, calixarene charge control agents, etc. are available. The content of the charge control agent is preferably in the range of 0.05 to 15 parts by weight, and more preferably in the range of 0.1 to 10 parts by weight, per 100 parts by weight of binder resin.

[0050] The external additives are added to improve environmental stability, charge stability, developability, fluidity, and storage stability. Known additives can be used. The amount of external additive added is within the range of 0.01 to 10 parts by weight per 100 parts by weight of binder resin, but preferably within the range of 0.05 to 8 parts by weight. In this embodiment, several types of silica (one with a positive charge polarity and one with a negative charge polarity) with an average particle size of 14 μm, colloidal silica (negatively charged) with an average particle size of 110 μm, and melamine (positively charged) with an average particle size of 200 μm are added to 100 parts by weight of the base particles, with the total amount falling within the above range.

[0051] The toner charge (blow-off charge) was measured by shaking and stirring the toner and carrier. Powder Tech Co., Ltd.'s ferrite carrier "EF96-35" was used as the carrier, and 0.5 g of toner and 9.5 g of carrier were mixed. The toner and carrier mixture (150 mg) was placed in a container and shaken using a Yayoi Co., Ltd.'s shaker "YS-LD." The shaking rate was 200 times per minute, and the shaking time was 300 seconds.

[0052] After shaking, the powder was suctioned for 10 seconds using a Kyocera Chemical Corporation powder charge analyzer "TB-203" with a blow pressure of 7.0 kPa and a suction pressure of -4.5 kPa. The charge and suction volume were output every 0.1 seconds to a personal computer (PC). The charge per unit weight of the toner particles, Q / M, calculated from the average charge and suction volume output during the final 2 seconds of the suction time (10 seconds), was approximately -35 μC / g. The charge measurement was performed at a temperature of 25°C and a relative humidity of 50%.

[0053] Photosensitive drum Figure 3(A) is a partially cutaway perspective view showing a photosensitive drum 11 according to an embodiment, and Figure 3(B) is an enlarged cross-sectional view showing a cross-section of a portion (part 3a) of the photosensitive drum 11 shown in Figure 3(A).

[0054] The photosensitive drum 11 is a member that carries an electrostatic latent image on its surface (surface layer portion). As shown in FIG. 3(A), the photosensitive drum 11 is a cylindrical member that has a drum gear 11a at one axial end and a drum flange 11b at the other end. The drum gear 11a is a portion that receives driving force from a drive motor 108 (FIG. 5). The outer diameter of the cylindrical portion of the photosensitive drum 11 (excluding the drum gear 11a and drum flange 11b) is, for example, 30 mm, which is within the range of 30.0±0.2 mm considering manufacturing tolerances. The axial length of the cylindrical portion of the photosensitive drum 11 is, for example, 327 mm.

[0055] 3(B), the photosensitive drum 11 has a conductive support 110 and a photosensitive layer 114 covering the surface of the conductive support 110. The conductive support 110 is a pipe made of a metal such as aluminum or stainless steel.

[0056] The photosensitive layer 114 has a structure in which a charge generating layer 112 and a charge transport layer 113 are laminated in this order. An undercoat layer 111 may be formed between the conductive support 110 and the photosensitive layer 114. The undercoat layer 111 is also called a blocking layer.

[0057] For example, a binder resin in which particles of metal oxide or the like are dispersed is used as the undercoat layer 111. The undercoat layer 111 may be configured as a single layer or multiple layers.

[0058] Examples of metal oxide particles used in the undercoat layer 111 include metal oxide particles containing one type of metal element such as titanium oxide, aluminum oxide, silicon oxide, zirconium oxide, zinc oxide, and iron oxide, and metal oxide particles containing multiple metal elements such as calcium titanate, strontium titanate, and barium titanate. These may be used alone or in any combination and ratio.

[0059] Among these metal oxide particles, titanium oxide and aluminum oxide, and particularly titanium oxide, are preferred as materials for the undercoat layer 111. The surfaces of the titanium oxide particles may be treated with an inorganic substance such as tin oxide, aluminum oxide, antimony oxide, zirconium oxide, or silicon oxide, or with an organic substance such as stearic acid, polyol, or silicone.

[0060] These treatments may be performed by any one type, or by two or more types. The crystalline form of the titanium oxide particles may be, for example, rutile, anatase, brookite, or amorphous. The titanium oxide particles may have only one crystalline form, or may contain two or more crystalline forms in any ratio and combination.

[0061] The particle size of the metal oxide particles is arbitrary as long as it does not significantly impair the effects of the present disclosure, but from the viewpoint of the properties of the binder resin and the like that are the raw materials of the undercoat layer 111 and the stability of the solution, the average primary particle size is preferably 10 nm or more and 100 nm or less, and more preferably 10 nm or more and 50 nm or less. This average primary particle size can be measured, for example, by a transmission electron microscope (TEM).

[0062] The undercoat layer 111 is preferably formed, for example, by dispersing metal oxide particles in a solution in which a binder resin is dissolved, and applying this solution (hereinafter also referred to as "coating liquid for forming an undercoat layer") onto the conductive support 110. Examples of binder resins used in the undercoat layer 111 include epoxy resins, polyethylene resins, polypropylene resins, acrylic resins, methacrylic resins, polyamide resins, vinyl chloride resins, vinyl acetate resins, phenolic resins, polycarbonate resins, polyurethane resins, polyimide resins, vinylidene chloride resins, polyvinyl acetal resins, vinyl chloride-vinyl acetate copolymers, polyvinyl alcohol resins, polyurethane resins, polyacrylic acid resins, polyacrylamide resins, polyvinylpyrrolidone resins, polyvinylpyridine resins, water-soluble polyester resins, cellulose ester resins such as nitrocellulose, cellulose ether resins, casein, gelatin, polyglutamic acid, starch, starch acetate, amino starch, organic zirconium compounds such as zirconium chelate compounds and zirconium alkoxide compounds, organic titanyl compounds such as titanyl chelate compounds and titanyl alkoxide compounds, and silane coupling agents. These may be used alone or in any combination and ratio. They may also be used in a cured form with a curing agent. In particular, alcohol-soluble copolymerized polyamides and modified polyamides are preferred because they exhibit good dispersibility and coatability.

[0063] The photosensitive layer 114 may have any structure applicable to known electrophotographic photoreceptors. Specific examples include a single-layer photoreceptor having a single-layer photosensitive layer (i.e., a single-layer photosensitive layer) in which a photoconductive material is dissolved or dispersed in a binder resin, and a multi-layer photoreceptor having a photosensitive layer (i.e., a multi-layer photosensitive layer) consisting of multiple layers stacked together, each layer comprising a charge generating layer containing a charge generating material and a charge transport layer containing a charge transport material (first charge transport material). It is generally known that photoconductive materials, whether single-layer or multi-layer, exhibit similar functional performance.

[0064] The photosensitive layer 114 of the photosensitive drum 11 of the present embodiment may be in any known form, but a laminated type electrophotographic photosensitive member is preferred in consideration of the mechanical properties, electrical characteristics, manufacturing stability, etc. of the electrophotographic photosensitive member. In particular, a normal laminated type photosensitive member in which a charge generating layer 112 and a charge transport layer 113 are laminated in this order on a conductive support 110 is more preferred.

[0065] When forming the charge transport layer 113 of a multilayer photoreceptor (functionally separated photoreceptor) and the photosensitive layer of a single-layer photoreceptor, a binder resin is usually used to disperse the compound in order to ensure film strength. The charge transport layer of a functionally separated photoreceptor can be obtained by applying and drying a coating liquid obtained by dissolving or dispersing a charge transport material (first charge transport material) and various binder resins in a solvent. Also, a single-layer photoreceptor can be obtained by applying and drying a coating liquid obtained by dissolving or dispersing a charge generation material, a charge transport material (first charge transport material), and various binder resins in a solvent.

[0066] Examples of binder resins used in the charge generation layer 112 of the function separated type photoreceptor include polyvinyl acetal resins such as polyvinyl butyral resin, polyvinyl formal resin, and partially acetalized polyvinyl butyral resin in which a part of butyral is modified with formal or acetal, polyarylate resin, polycarbonate resin, polyester resin, modified ether 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 resin, polyurethane resin, epoxy resin, silicone resin, polyvinyl a Examples of binder resins that can be used include, but are not limited to, 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 polymers such as poly-N-vinylcarbazole, polyvinylanthracene, and polyvinylperylene. These binder resins may be used alone or in any combination and ratio.

[0067] Examples of binder resins used in the charge transport layer 113 include 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.

[0068] The charge transport material may contain, for example, one or more of the above-mentioned 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, enamine derivatives, nitro-substituted polycyclic fused aromatic compounds, and aromatic quinone compounds. Alternatively, the charge transport material may be, for example, a compound to which one or more of the above-mentioned aromatic amine derivatives are bonded. Furthermore, the charge transport material may be, for example, a polymer (electron-donating material) having a group consisting of the above-mentioned aromatic amine derivative or the like as a main chain or side chain. Among these, the charge transport material is preferably an aromatic amine derivative, a stilbene derivative, a hydrazone derivative, an 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. For example, when a nitro-substituted polycyclic condensed aromatic compound or an aromatic quinone compound is used as an additive, if the amount of the additive exceeds the amount of the charge transport substance, the charge transport capacity decreases, so the amount of the charge transport substance added is set to an amount that does not impair the image quality of the toner image.

[0069] In this embodiment, the charge transport layer 113 of the photosensitive layer 114 includes (i.e., contains) a first charge transport material that transports charges of a first polarity and a second charge transport material that transports charges of a second polarity different from the first polarity, in order to reduce printing defects. Toner of the second polarity can be attached to the photosensitive layer 114. In this embodiment, the first polarity is positive, and the first charge transport material is a hole transport material that transports holes. The second polarity is negative, and the second charge transport material is an electron transport material that transports electrons. Known examples of electron transport materials (second charge transport materials) include aromatic quinones. In the charge transport layer 113, if the amount of electron transport material added is large, the charge transport capacity (here, hole transport capacity) decreases. Therefore, the amount of electron transport material added is set to a level that does not impair the image quality of the toner image.

[0070] Each layer constituting the photoreceptor drum 11 is usually formed by repeatedly applying a coating liquid containing the materials constituting each layer onto the conductive support 110 using a known coating method and drying the coating liquid for each layer. Examples of solvents and dispersion media used to dissolve the binder resin and prepare 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, methyl formate, ethyl acetate, and the like. Examples of suitable solvents include ester solvents such as ethanol 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 (also referred to as THF), 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.

[0071] The coating solution for forming the charge transport layer of the single-layer photoreceptor and the multi-layer photoreceptor typically has a solids concentration of 5% by weight or more, preferably 10% by weight or more, with an upper limit of 40% by weight or less, preferably 35% by weight or less. The viscosity of the coating solution is 10 mPa·s or more, preferably 50 mPa·s or more, with an upper limit of 500 mPa·s or less, preferably 400 mPa·s or less.

[0072] In the case of the charge generating layer of a multilayer photoreceptor, the solids concentration is usually 0.1 wt % or more, preferably 1 wt % or more, and the upper limit is usually 15 wt % or less, preferably 10 wt % or less. The viscosity of the coating liquid is usually 0.01 mPa·s or more, preferably 0.1 mPa·s or more, and the upper limit is 20 mPa·s or less, preferably 10 mPa·s or less.

[0073] 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 known coating methods can 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, typically in the absence of wind or with a fan, at a temperature ranging from 30°C to 200°C for a period of from 1 minute to 2 hours. The heating temperature during heating and drying may be constant, or the heating temperature may be varied.

[0074] The thickness of the photosensitive layer of a single-layer photoreceptor is generally 5 μm or more, preferably 10 μm or more, and generally 100 μm or less, preferably 50 μm or less. The thickness of the charge transport layer of a normal laminated photoreceptor is generally 5 μm or more and 50 μm or less, but from the viewpoint of long life and image stability, it is preferably 10 μm or more and 45 μm or less, and from the viewpoint of high resolution, it is preferably 10 μm or more and 30 μm or less.

[0075] Developing roller The developing roller 13 includes a conductive core (shaft), an elastic layer formed on the surface of the core, and a surface layer covering the surface of the elastic layer. The core may be made of any material having good conductivity, such as iron, aluminum, or stainless steel.

[0076] The elastic layer of the developing roller 13 can be made of common rubber materials such as silicone rubber and urethane. When polyurethane is used for the elastic layer, it is preferable that the polyurethane be made primarily of polyether-based polyol. Ether-based polyurethane is a so-called cast-type polyurethane obtained by reacting a polyol, primarily polyether-based polyol, with polyisocyanate. This is done to reduce compression set. On the other hand, when ester-based polyurethane is used, it has poor hydrolysis properties and cannot be used stably for long periods of time.

[0077] When polyurethane is used as the elastic layer of the developing roller 13, the isocyanate to be reacted with the polyol may be, for example, a trifunctional isocyanate alone such as triphenylmethane triisocyanate, tris(isocyanatophenyl)thiophosphate, or bicycloheptane triisocyanate, or a mixture such as nerate-modified polyisocyanate of hexamethylene diisocyanate or polymeric MDI.

[0078] The elastic layer of the developing roller 13 may be a mixture of these trifunctional or higher polyisocyanates and a general bifunctional isocyanate compound. Examples of bifunctional isocyanate compounds include 2,4-tolylene diisocyanate (TDI), 4,4-diphenylmethane diisocyanate (MDI), paraphenylene diisocyanate (PPDI), 1,5-naphthalene diisocyanate (NDI), 3,3-dimethyldiphenyl-4,4-diisocyanate (TODI), and modified products and polymers of prepolymers having these isocyanates at both ends.

[0079] The elastic layer of the developing roller 13 is formed by adding carbon black to the rubber base material as described above, and then heating and curing the carbon while maintaining its dispersed state. This allows the carbon black, which has a specific resistance in the range of about 0.1 [Ω·cm] to 10 [Ω·cm], to be mixed with an elastomer (which can be considered an insulator) with a specific resistance of 10 12 [Ω·cm] to 10 16 [Ω·cm]) for 10 4 [Ω·cm] to 10 8 It is possible to form a stable medium resistance region of [Ω·cm].

[0080] In this embodiment, the surface layer of the developing roller 13 is formed, for example, by impregnating the surface layer portion of the elastic layer with a surface treatment liquid. The surface treatment liquid is obtained by dissolving at least an isocyanate component in an organic solvent. Examples of the organic solvent include methyl acetate, butyl acetate, and pentyl acetate. When such an organic solvent is used, the isocyanate component contained in the surface treatment liquid may be, for example, an isocyanate compound such as 2,4-tolylene diisocyanate (TDI) or 4,4-diphenylmethane diisocyanate (MDI), or a polymer or modified product thereof.

[0081] The surface treatment solution may contain a polyether polymer. The polyether polymer is preferably soluble in an organic solvent and has active hydrogen, which can react with an isocyanate compound to form a chemical bond. Suitable polyether polymers having active hydrogen include polymers having a hydroxyl group or an allyl group, such as polyols and glycols used in isocyanate-terminated prepolymers.

[0082] The surface treatment liquid may also contain a polymer selected from an acrylic fluorine-based polymer and an acrylic silicone-based polymer. The acrylic fluorine-based polymer and the acrylic silicone-based polymer are soluble in a predetermined solvent and can react with an isocyanate compound to form a chemical bond. The acrylic fluorine-based polymer is, for example, a solvent-soluble fluorine-based polymer having a hydroxyl group, an alkyl group, or a carboxyl group, such as a block copolymer of an acrylic ester and an alkyl fluoride acrylate, or a derivative thereof. The acrylic silicone-based polymer is, for example, a solvent-soluble silicone-based polymer, such as a block copolymer of an acrylic ester and an acrylic acid siloxane ester, or a derivative thereof.

[0083] Furthermore, carbon black such as acetylene black may be added to the surface treatment solution as a conductivity imparting agent.

[0084] The polyether polymer, acrylic fluorine polymer, and acrylic silicone polymer in the surface treatment solution are preferably adjusted so that the total amount of the polyether polymer, acrylic fluorine polymer, and acrylic silicone polymer relative to the isocyanate component is 10 to 70% by mass. If the amount is less than 10% by mass, the effect of retaining carbon black and the like in the surface treatment solution is reduced. On the other hand, if the amount is more than 70% by mass, there are problems such as an increase in electrical resistance or a relative decrease in the isocyanate component, making it impossible to form an effective surface treatment layer.

[0085] The elastic layer of the developing roller 13 is immersed in the surface treatment liquid, which is then applied and dried and cured, so that the surface treatment liquid is impregnated into the surface layer portion of the elastic layer of the developing roller 13 and becomes the surface layer of the developing roller 13. The outer diameter of the developing roller 13 is, for example, 16.0 mm.

[0086] 4A and 4B are diagrams showing a method for measuring the resistance value of the developing roller 13 of the image forming unit 10 according to the embodiment. A high resistance meter (model number: 4339B) manufactured by Hewlett-Packard was used as the measuring device 25. As shown in FIG. 4A, a load of W=300 g was applied to the shaft portions 13e at both ends of the core metal of the developing roller 13, and the developing roller 13 was brought into contact with a stainless steel metal roller 26 having a diameter of 30 mm.

[0087] A potential difference of -100 V was applied to the shaft 27 of the metal roller 26 and the shaft 13e of the developing roller 13, and the metal roller 26 was rotated at a speed of 50 rpm. The resistance was measured at 100 points per revolution of the developing roller 13, and the average value was taken as the resistance value of the developing roller 13. The resistance value of the developing roller 13 was 1×10 4 [Ω] to 1×10 7 [Ω], and here the resistance value is preferably in the range of 1×10 5 A developing roller 13 of [Ω] was used.

[0088] <<Supply Roller>> Next, the supply roller 14 will be described. The supply roller 14 has a conductive core (shaft) and a sponge-like foam elastic layer formed on the surface of the core. The core of the supply roller 14 may be made of any material as long as it has good conductivity, such as iron, aluminum, or stainless steel.

[0089] The rubber composition forming the foamed elastic layer of the supply roller 14 contains rubber, a foaming agent, and a conductivity imparting agent, and further contains 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 of the supply roller 14, while organic foaming agents are used to form closed cells. Examples of additives include fillers, colorants, and release agents.

[0090] The outer diameter of the supply roller 14 is, for example, 15.5 mm. The average cell diameter of the foamed elastic layer of the supply roller 14 is preferably within a range of 200 μm to 500 μm. The hardness of the foamed elastic layer is preferably within a range of approximately 50 to 65 degrees on the Asker F hardness scale, and is 58 degrees here.

[0091] The resistance value of the supply roller 14 is preferably within the range of 3.5 [log Ω] to 7.5 [log Ω]. Here, a supply roller 14 with a resistance value of 5.5 [log Ω] is used. The method for measuring the resistance value of the supply roller 14 is the same as the method for measuring the resistance value of the development roller 13, as described with reference to Figures 4(A) and (B).

[0092] 《Developing Blade》 2 is a metal plate-shaped elastic body. More specifically, the developing blade 15 is made of stainless steel and has a plate thickness of, for example, 0.08 mm. The developing blade 15 is formed in an elongated shape that is long in the X direction.

[0093] One widthwise end (fixed end) of the developing blade 15 is fixed to a blade holder 23 provided on the frame 17. The other widthwise end (free end) of the developing blade 15 is bent, with a curvature radius of approximately 0.18 mm. The bent portion of the developing blade 15 abuts against the surface of the photosensitive drum 11. The pressure (linear pressure) between the developing blade 15 and the developing roller 13 is approximately 40 gf / cm.

[0094] Considering the setting conditions of the developing blade 15, the surface roughness and resistance value of the developing roller 13 are set to obtain the desired toner layer thickness and toner charge amount on the developing roller 13. The surface roughness of the developing roller 13 is preferably such that the ten-point average roughness Rz (JIS B0601-1994) in the circumferential direction is within the range of 2 μm to 10 μm.

[0095] 《Charging roller》 The charging roller 12 has a core (shaft), an elastic layer formed on the surface of the core, and a surface layer covering the surface of the elastic layer.

[0096] The core of the charging roller 12 is made of a conductive material, such as electroless nickel-plated free-cutting steel (SUM) or stainless steel (SUS). The elastic layer of the charging roller 12 is made of rubber, thermoplastic elastomer, resin, or the like, so as to form a nip between the charging roller 12 and the photosensitive drum 11 that allows appropriate discharge to occur. The elastic layer of the charging roller 12 may be a single layer, or may have a multi-layer structure of two or more layers.

[0097] The elastic layer of the charging roller 12 is formed of a rubber composition whose main component is one or a mixture of two or more of the following: epichlorohydrin rubber (CO, ECO, GECO), ethylene propylene rubber (EPM, EPDM), acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), chloroprene rubber (CR), urethane rubber, and silicone rubber.

[0098] It is particularly desirable to use rubber containing epichlorohydrin rubber (ECO) as the main component or rubber containing a mixture of epichlorohydrin rubber (ECO) and acrylonitrile-butadiene rubber (NBR) as the main component for the elastic layer of the charging roller 12. In this embodiment, rubber containing epichlorohydrin rubber (ECO) as the main component is used.

[0099] If the electrical resistance of the elastic layer of the charging roller 12 is too high, printing defects occur due to uneven charging or overcharging of the surface of the photosensitive drum 11. Conversely, if the electrical resistance of the elastic layer of the charging roller 12 is too low, printing defects occur due to current leakage caused by scratches on the surface of the photosensitive drum 11. Therefore, there is an optimum range for the electrical resistance of the elastic layer of the charging roller 12. To keep the electrical resistance of the elastic layer of the charging roller 12 within the optimum range, an ion-conductive material, ion-conductive agent, carbon black, metallic oxide, or the like is added to the elastic layer to impart a predetermined conductivity.

[0100] The elastic layer of the charging roller 12 may have electronic conductivity or ionic conductivity. Since partial resistance unevenness in the elastic layer of the charging roller 12 is likely to lead to charging unevenness of the photosensitive drum 11, an elastic layer having ionic conductivity is often used from the viewpoint of suppressing resistance unevenness, but an elastic layer having electronic conductivity may also be used.

[0101] The volume resistance of the elastic layer of the charging roller 12 is 10 6 Omega to 10 9 It is desirable that the resistance be in the range of Ω. If the charging roller 12 has ionic conductivity, the volume resistance value will change depending on the temperature and humidity, but here the value shown is measured in an environment with a temperature of 20°C and a relative humidity of 50%.

[0102] The hardness of the elastic layer of the charge roller 12 is adjusted so that a small gap is formed between the surface of the charge roller 12 and the surface of the photosensitive drum 11, allowing for proper discharge based on Paschen's law. The hardness of the elastic layer of the charge roller 12 is measured using a micro rubber hardness tester "MD-1capa" (Type_A) manufactured by Kobunshi Keiki Co., Ltd., and peak measurements are performed. When measured using this method, the hardness of the elastic layer of the charge roller 12 is preferably within the range of 35 to 80 degrees. If the hardness of the elastic layer of the charge roller 12 falls within this range, it can absorb eccentricity or shape variations between the charge roller 12 and the photosensitive drum 11. However, the hardness range is not limited to this range as long as a proper nip is formed between the charge roller 12 and the photosensitive drum 11.

[0103] The surface (i.e., the outer peripheral surface) of the elastic layer of the charging roller 12 is given a predetermined surface roughness by cutting, polishing, molding, or the like. The ten-point mean roughness Rz of the charging roller 12, depending on the applied voltage and the usage environment, is preferably within a range of, for example, approximately 1 μm to 30 μm according to Paschen's law. The surface of the elastic layer of the charging roller 12 may be subjected to surface treatment, coating, ultraviolet irradiation, or electron beam irradiation. These treatments can prevent contamination of the photosensitive drum 11 or adjust the resistance of the elastic layer. Furthermore, toner and its external additives adhering to the photosensitive drum 11 are less likely to adhere to the surface of the charging roller 12.

[0104] The surface layer of the charging roller 12 is formed by applying a solution of ethyl acetate (solvent) mixed with a urethane polymer and porous particles to the surface of the elastic layer of the charging roller 12 (see Examples 1-1 to 1-5 described later). The application is performed by dipping, spraying, or using a coater. The urethane polymer is a homopolymer or copolymer having a urethane bond formed by condensation of an isocyanate group and an alcohol group.

[0105] For example, toluene diisocyanate (TDI), methylene diisocyanate (MDI), xylylene diisocyanate (XDI), naphthalene diisocyanate (NDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), polyester polyol, polycarbonate polyol, silicone diol, acrylic fluorine polymer, acrylic silicone polymer, fluorine polymer, and their multimers and modified products can be used to form the surface layer of the charging roller 12. Furthermore, conductive agents such as carbon black, ionic conductive agents, and electronic conductive agents can be added as needed.

[0106] The porous particles contained in the surface layer of the charging roller 12 are, for example, urethane resin, acrylic resin, nylon resin, fluororesin, polyamide resin, polycarbonate resin, polyester resin, isocyanate resin, etc., and these can be used alone or in combination of two or more.

[0107] Here, as an example, the outer diameter of the core metal of the charging roller 12 is set to 8 [mm], and the outer diameter of the elastic layer 12b is set to 12 [mm].

[0108] <Cleaning material> 2 is made of a plate-shaped elastic body. The cleaning member 16 is supported by a plate-shaped jig 24 provided on the frame 17.

[0109] The material for cleaning member 16 is not particularly limited, but it is preferable to use an elastic composition so as not to damage the surface of photoreceptor drum 11 when it comes into sliding contact with the surface of photoreceptor drum 11 to scrape off residual toner. Examples of this elastic composition include compositions containing additives blended with polyurethane, silicone resin, fluororesin, fluororubber, etc. A particularly preferable example of this elastic composition is a polyurethane composition, which has excellent mechanical strength and elastic pressure-contact properties.

[0110] The polyurethane composition can be obtained using a polyisocyanate, a polyol, a curing agent, and a catalyst.

[0111] The polyisocyanate is not particularly limited, but examples thereof include 4,4'-diphenylmethane diisocyanate (MDI), 2,4-tolylene diisocyanate (2,4-TDI), 2,6-tolylene diisocyanate (2,6-TDI), 3,3'-tolylene-4,4'-diisocyanate, 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, 2,4-tolylene diisocyanate uretidinedione (a dimer of 2,4-TDI), 1,5-naphthylene diisocyanate, metaphenylene diisocyanate, and hexamethylene diisocyanate. Examples of usable diisocyanates include isocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate (water-added MDI), carbodiimide-modified MDI, orthotoluidine diisocyanate, xylene diisocyanate, paraphenylene diisocyanate, lysine diisocyanate methyl ester, and other diisocyanates; triisocyanates such as triphenylmethane-4,4',4"-triisocyanate; and polymeric MDI. These may be used alone or in combination of two or more. MDI is particularly preferred from the viewpoint of abrasion resistance.

[0112] The polyol used together with the polyisocyanate is not particularly limited, but examples thereof include polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexylene adipate, and polyether polyols such as polycaprolactone, polyoxytetramethylene glycol, and polyoxypropylene glycol. These may be used alone or in combination of two or more. PBA is particularly preferred because of its excellent abrasion resistance.

[0113] The curing agent used together with the polyisocyanate and polyol is not particularly limited, but examples of polyols that can be used include 1,4-butanediol, ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, hexanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, xylene glycol, triethylene glycol, trimethylolpropane, glycerin, pentaerythritol, sorbitol, and 1,2,6-hexanetriol, each having a molecular weight of 300 or less. These may be used alone or in combination of two or more.

[0114] The linear pressure of the cleaning member 16 against the photosensitive drum 11 is preferably 15 gf / cm or more and 30 gf / cm or less, and is set to 20 gf / cm here. The cleaning angle is set to 10° or more and 15° or less.

[0115] Transfer roller The transfer roller 31 has a conductive core (shaft) and an elastic layer of an elastic body formed on the surface of the core. The core of the transfer roller 31 is made of a conductive material such as iron, aluminum, or stainless steel. The elastic layer of the transfer roller 31 is a foam rubber layer made of, for example, acrylonitrile butadiene rubber (NBR).

[0116] Transfer belt 1 is an endless belt made of a high-resistance semiconductive plastic film. The transfer belt 32 is made of, for example, a resin such as polyamide, polyimide, or polyether ether ketone, or a mixture of these materials. The transfer belt 32 may also have a base layer and a surface layer.

[0117] <<Control system of image forming apparatus>> 5 is a block diagram showing the configuration of a control system of the image forming apparatus 1. The image forming apparatus 1 has a control unit 100, a receiving memory 121, an image data editing memory 122, an operation unit 123, a sensor group 124, and a power supply circuit 130.

[0118] The control unit 100 includes a print control unit (main control unit) 101, an I / F (interface) control unit 102, a head control unit 103, a fixing control unit 104, a fixing drive control unit 105, a conveyance control unit 106, and a drive control unit 107.

[0119] The print control unit 101 has a processor as a processing circuit, a ROM (Read Only Memory), a RAM (Random Access Memory), an input / output port, a timer, etc. The print control unit 101 executes a predetermined program to control the overall operation of the image forming apparatus 1. Specifically, the print control unit 101 receives print data and control commands from an external device via the I / F control unit 102, and performs overall control of the head control unit 103, the fixing control unit 104, the fixing drive control unit 105, the conveyance control unit 106, the drive control unit 107, and the power supply circuit 130 to perform the printing operation.

[0120] The I / F control unit 102 receives print data and control commands from an external device such as a personal computer, and also transmits information about the state of the image forming apparatus 1 to the external device.

[0121] The receiving memory 121 temporarily stores print data input from an external device via the I / F control unit 102 .

[0122] The image data editing memory 122 receives the print data stored in the receiving memory 121 and records image data obtained by editing the print data.

[0123] The operation unit 123 has a display unit that displays information about the state of the image forming apparatus 1, and an input unit that accepts operation inputs from the user. The display unit is configured, for example, by an LED lamp, and the input unit is configured, for example, by a button or a touch panel.

[0124] The sensor group 124 includes various sensors that monitor the operating state of the image forming apparatus 1. Specifically, these sensors include a position detection sensor that detects the position of the recording medium M on the conveyance path, a temperature and humidity sensor, a print density sensor, and a toner remaining amount sensor.

[0125] The power supply circuit 130 includes a charging roller power supply 131 that applies a charging voltage to the charging roller 12, a developing roller power supply 132 that applies a developing voltage to the developing roller 13, a supply roller power supply 133 that applies a supply voltage to the supply roller 14, a developing blade power supply 134 that applies a blade voltage to the developing blade 15, and a transfer roller power supply 135 that applies a transfer voltage to the transfer roller 31.

[0126] The head control unit 103 controls the light emission of each LED of the exposure head 21 based on the image data recorded in the image data editing memory 122 .

[0127] The fixing control unit 104 has a temperature adjustment circuit, and supplies current to the heater in the fixing roller 51 based on the output signal of a temperature sensor such as a thermistor provided in the fixing unit 50 .

[0128] The fixing drive control unit 105 controls the rotation of the fixing motor 53 that drives the fixing roller 51 to rotate. The pair of discharge rollers 61 is rotated by the rotation transmitted from the fixing motor 53.

[0129] The conveyance control unit 106 controls the rotation of a conveyance motor 45 that drives the hopping roller 42, the pair of registration rollers 43, and the pair of conveyance rollers 44. The rotation of the conveyance motor 45 is transmitted to the hopping roller 42, the pair of registration rollers 43, and the pair of conveyance rollers 44 via an electromagnetic clutch or the like (not shown).

[0130] The drive control unit 107 controls the rotation of a drive motor 108 that rotates and drives the photosensitive drum 11. The rotation of the photosensitive drum 11 is also transmitted to the developing roller 13, the supply roller 14, and the drive roller 33 via a gear train (not shown).

[0131] <<Printing Operation of Image Forming Apparatus>> Next, we will explain the printing operation of the image forming apparatus 1. When the print control unit 101 receives a print command and print data from a host device via the I / F control unit 102, the print operation starts.

[0132] The print control unit 101 temporarily records print data received from a host device in a reception memory 121 , edits the recorded print data to generate image data, and records the image data in an image data editing memory 122 .

[0133] Furthermore, the fixing drive control unit 105 drives the fixing motor 53, and the fixing roller 51 and the pressure roller 52 start to rotate. Furthermore, the fixing control unit 104 energizes the heater in the fixing roller 51, and the fixing roller 51 is heated to a predetermined fixing temperature.

[0134] Furthermore, the conveyance control unit 106 drives the conveyance motor 45, and the hopping roller 42 sends the recording medium M in the medium tray 41 to the conveyance path. Furthermore, the pair of registration rollers 43 and the pair of conveyance rollers 44 each rotate, and convey the recording medium M to the transfer unit 30.

[0135] Further, power sources 131 to 135 of a power supply circuit 130 apply a charging voltage, a developing voltage, a supply voltage and a blade voltage to the charging roller 12, the developing roller 13, the supply roller 14 and the developing blade 15 of each image forming unit 10, respectively.

[0136] In addition, the drive control unit 107 drives the drive motor 108, causing the photosensitive drum 11 to rotate in each image forming unit 10. As the photosensitive drum 11 rotates, the charging roller 12, developing roller 13, and supply roller 14 also rotate. The charging roller 12 uniformly charges the surface of the photosensitive drum 11.

[0137] Furthermore, the head control unit 103 drives the exposure head 21 to irradiate the surface of the photosensitive drum 11 with light. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 11.

[0138] The electrostatic latent image formed on the surface of the photosensitive drum 11 is developed by the toner attached to the developing roller 13, and a toner image is formed on the surface of the photosensitive drum 11. Furthermore, a transfer voltage is applied to the transfer roller 31 from the transfer roller power supply 135.

[0139] This transfer voltage causes the toner image on the surface of the photosensitive drum 11 to be transferred to the recording medium M passing between the photosensitive drum 11 and the transfer roller 31. Toner that has not been transferred to the recording medium M is scraped off by the cleaning member 16. The toner images are transferred from the photosensitive drum 11 of each image forming unit 10 to the recording medium M and are superimposed on each other.

[0140] In the fixing unit 5, heat and pressure are applied to the recording medium M passing through the fixing nip between the fixing roller 51 and the pressure roller 52, and the toner image is fixed to the recording medium M. The recording medium M with the fixed toner image is sent to the medium discharge unit 6.

[0141] In the medium discharge unit 6, a pair of discharge rollers 61 discharges the recording medium M from a discharge port. The discharged recording medium M is stacked on a stacker 63. This completes the formation of an image on the recording medium M.

[0142] <<Operation of the Image Forming Unit>> The operation of the image forming unit 10 during the above printing operation is as follows: The photosensitive drum 11, charging roller 12, developing roller 13, and supply roller 14 of the image forming unit 10 rotate in the directions indicated by the arrows in FIG.

[0143] In this embodiment, the printing speed of the image forming apparatus 1 is set to 45 ppm (sheets per minute) when printing on A4 paper in landscape orientation as the recording medium M, and the peripheral speeds of the developing roller 13 with an outer diameter of 16.0 mm and the supply roller 14 with an outer diameter of 15.5 mm are set to 0.3 m / s and 0.2 m / s, respectively. The photosensitive drum 11 has an outer diameter of 30.0 mm and a peripheral speed of 0.194 m / s.

[0144] In the image forming unit 10, the supply roller 14, which has a foamed elastic layer that is a sponge-like elastic body, rotates while carrying toner on its outer circumferential surface and in its cells, and reaches the contact point with the development roller 13. A DC voltage of −330 [V] is applied to the supply roller 14 by a supply roller power supply 133. A DC voltage of −200 [V] is applied to the development roller 13 by a development roller power supply 132. Then, due to the potential difference generated between the development roller 13 and the supply roller 14, negatively charged toner is supplied to the development roller 13.

[0145] A DC voltage of −330 V is applied to the developing blade 15 from a developing blade power supply 134. The developing blade 15 thins the toner on the surface of the developing roller 13.

[0146] A DC voltage of −1000 [V] is applied to the charging roller 12 from a charging roller power supply 131. The charging roller 12 uniformly charges the surface of the photosensitive drum 11 to about −420±30 [V].

[0147] The uniformly charged surface of the photosensitive drum 11 is exposed to light by the exposure head 21, forming an electrostatic latent image. Then, toner carried by the developing roller 13 is supplied to the electrostatic latent image formed on the photosensitive drum 11 by the exposure of the exposure head 21, and the electrostatic latent image is developed.

[0148] When printing a solid image with a print image density of 100%, the surface potential of the photosensitive drum 11 decays from -450 [V] before exposure to a surface potential within a range of approximately -50 [V] to -50 [V] due to exposure of all LEDs of the exposure head 21.

[0149] The toner on the developing roller 13 that has not been supplied to the photosensitive drum 11 is scraped off by the supply roller 14 at the portion facing the supply roller 14.

[0150] The toner image on the surface of the photosensitive drum 11 is transferred onto the recording medium M by a transfer roller 31 (FIG. 1) to which a predetermined transfer voltage is applied.

[0151] On the other hand, the toner remaining on the surface of the photosensitive drum 11 without being transferred to the recording medium M and the external additives that have been released from the toner particles and adhered to the surface of the photosensitive drum 11 are scraped off by the cleaning member 16.

[0152] After the toner and external additives are scraped off the surface of the photosensitive drum 11 by the cleaning member 16, the surface is neutralized by irradiating it with neutralization light from a neutralization light unit (not shown). The neutralized surface of the photosensitive drum 11 is then recharged by the charging roller 12.

[0153] "Strong transcription history" Next, we will explain strong transfer history, which is one type of printing defect. When a high-resistivity medium is used as the recording medium M, applying the same transfer voltage as for plain paper will not provide the current necessary to transfer the toner image, resulting in insufficient transfer and transfer blur. Therefore, when a high-resistivity medium is used as the recording medium M, it is common to set a higher transfer voltage (i.e., a strong transfer voltage) than the transfer voltage when plain paper is used as the recording medium M (i.e., a normal transfer voltage).

[0154] High-resistivity media are recording media with high volume resistivity, such as coated paper (waterproof paper, etc.) with a resin coating, film media made of polyester, etc. In contrast, plain paper is a recording medium with a relatively low volume resistivity (i.e., low-resistivity media).

[0155] 6 is a schematic diagram showing a state in which a recording medium M passes between the photosensitive drum 11 and the transfer roller 31. The area where the photosensitive drum 11 contacts the recording medium M is referred to as the medium area R1. Meanwhile, in areas on both sides of the recording medium M in the width direction, the photosensitive drum 11 and the transfer roller 31 face each other via the transfer belt 32. This area is referred to as the non-medium area R2.

[0156] Here, we will explain the case where a transfer belt 32 is used, but it is also possible to have a configuration in which the photosensitive drum 11 and the transfer roller 31 are in contact with each other without using a transfer belt 32, as in an image forming device that forms a monochromatic image (such as a monochrome printer).

[0157] When a high resistance medium is used as the recording medium M as described above, a high transfer voltage (i.e., a strong transfer voltage) is applied to the transfer roller 31, and therefore a relatively large current C flows from the transfer roller 31 to the photosensitive drum 11 in the outside-medium region R2 where the photosensitive drum 11 and the transfer roller 31 face each other (i.e., directly) without the recording medium M in between. In the area where the large current flows in, the photosensitive drum 11 is charged to a polarity (first polarity, positive polarity in this embodiment) opposite to the original polarity (second polarity, negative polarity in this embodiment) of the charging potential during printing operation.

[0158] When printing operations are repeated, positive potential accumulates in the outside-of-medium area R2 on the surface of the photosensitive drum 11, causing charging defects. The accumulation of positive potential, which is charge that is charged to the polarity opposite to the original polarity (second polarity, negative in this embodiment) during printing operations (first polarity, positive in this embodiment), does not disappear even if left for several hours, and is therefore called strong transfer history. When a solid image is printed in area R1 in a state where strong transfer history has occurred, the density becomes high in the areas in the outside-of-medium area R2 where positive potential has accumulated, causing printing defects.

[0159] 7(A) to 7(E) are schematic diagrams showing the state of charges in the conductive support 110, undercoat layer 111, charge generation layer 112, and charge transport layer 113 of the photosensitive drum 11. In FIGS. 7(A) to 7(E), the transfer roller 31 and transfer belt 32 are positioned above the photosensitive drum 11 in the drawings.

[0160] When a toner image is transferred to a high-resistance medium M1, as shown in Figures 7(A) and (B), a strong current C flows from the transfer roller 31 to the photosensitive drum 11 in the outer-medium region R2, and positive charges accumulate in the charge transport layer 113.

[0161] When the transfer operation is repeated, the positive charges in the charge transport layer 113 move to the charge generation layer 112, and the positive charges are accumulated in the charge generation layer 112, as shown in FIG. 7(C).

[0162] When positive charges are accumulated in the charge generation layer 112 in this way, as shown in Figure 7(D), part of the negative charges imparted from the charging roller 12 during the next printing operation is canceled out by the positive charges that have moved from the charge transport layer 113 to the charge generation layer 112. As a result, a step (potential step) occurs in the charge distribution state on the surface of the charge transport layer 113, as shown in Figure 7(E).

[0163] In this state, when a solid image (or an image with half-tone density or higher) is printed on a recording medium M with a wide width (dimension in the direction perpendicular to the conveying direction F), the difference in the distribution of charge on the surface of the photosensitive drum 11 appears as a difference in the amount of toner adhesion, resulting in density differences in the printed image.

[0164] 8A to 8C are schematic diagrams showing examples of printing defects (the occurrence of density differences due to the influence of strong transfer history) that occur in printed images, and FIG. 8D is a schematic diagram showing an example of printing defects (the occurrence of low halftone print density) that occur in printed images. For example, if an image 70 is continuously printed on a high-resistance medium M1 having a width (dimension perpendicular to the conveying direction F) W1 as shown in FIG. 8A, and then a solid image (area 71) is printed on a recording medium M2 having a width W2 (>W1) as shown in FIG. 8B, as shown in FIG. 8C, high-density areas (areas 72) occur on both ends of the solid image on the recording medium M2 in the width direction, i.e., outside the portion (area 71) corresponding to the width W1 of the high-resistance medium M1 shown in FIG. 8A, and a density difference occurs between areas 71 and 72. In FIG. 8C, 71a indicates an example of a measurement position of the density of the region 71, and 72a indicates an example of a measurement position of the density of the region 72.

[0165] Furthermore, if the property of photosensitive layer 114 that makes it difficult for positive charge to accumulate is strengthened too much, when a halftone density image (area 73) is printed on recording medium M3 (for example, various recording media), the density of the halftone density image (for example, position 73a) does not increase to the predetermined density, resulting in low density, as shown in Figure 8(D). Note that 73a is an example of the measurement position for the density of area 73.

[0166] Features of the present disclosure In order to reduce the influence of strong transfer history, the image forming unit 10 and drum unit 20 according to this embodiment are equipped with a photosensitive drum 11 that has the property of making it difficult for positive charge to accumulate on its surface even when a high transfer voltage is applied.

[0167] The property of making it difficult for positive charges to accumulate on the surface of the photosensitive drum 11 can be achieved, for example, by adding an electron transport material (second charge transport material) to the charge transport layer 113. Although adding an electron transport material (second charge transport material) to the charge transport layer 113 reduces the inherent capacity (charge transport capacity) of the charge transport layer 113, it is an effective measure to counteract the positive charges (positive charges in FIG. 7C) generated by the above-mentioned strong transfer voltage.

[0168] FIG. 9(A) is a schematic diagram showing the charge transport layer 113 of the photosensitive layer 114 of a conventional photosensitive drum 11, and FIG. 9(B) illustrates a state in which the positive charges, which are oppositely charged charges, present on the surface of the conventional photosensitive drum 11 are difficult to remove by exposure to light. As shown in FIG. 9(A), the charge transport layer 113 of the photosensitive layer 114 of the conventional photosensitive drum 11 can be obtained by applying and drying a coating liquid obtained by dissolving or dispersing a charge transport material, i.e., a hole transport material (first charge transport material), and various binder resins in a solvent. In this example, the first charge transport material is a hole transport material. As shown in FIG. 9(B), a positive charge, which is oppositely charged charges, may exist on the surface of this photosensitive drum 11 due to a strong transfer voltage. This state is also illustrated in FIGS. 7(A) and 7(B). When static elimination exposure is performed, charges are generated in the charge generation layer 112. However, because only a hole transport material (first charge transport material) is added to the charge transport layer 113 of the photosensitive layer 114 of the conventional photosensitive drum 11, the negative charges generated in the charge generation layer 112 cannot move toward the surface of the charge transport layer 113, as shown in FIG. 9(B). Therefore, in an image forming apparatus using the conventional photosensitive drum 11, it is not possible to reduce the degradation of image quality due to the occurrence of strong transfer history (for example, the density step in FIG. 7(E)). As a result, the strong transfer history shown in FIG. 7(C) is likely to occur.

[0169] FIG. 10(A) is a schematic diagram showing the charge transport layer 113 of the photosensitive layer 114 of the photosensitive drum 11 according to the present embodiment, and FIG. 10(B) is a diagram showing the removal of positive charges, which are oppositely charged charges, present on the surface of the photosensitive drum 11 according to the present embodiment by exposure to light. As shown in FIG. 10(A), the charge transport layer 113 of the photosensitive layer 114 of the photosensitive drum 11 according to the present embodiment can be obtained by applying and drying a coating liquid obtained by dissolving or dispersing a charge transport material, which is a hole transport material (first charge transport material), an electron transport material (second charge transport material), and various binder resins in a solvent. In this example, the second charge transport material is an electron transport material. As shown in FIG. 10(B), positive charges, which are oppositely charged charges, may be present on the surface of the photosensitive drum 11 due to a strong transfer voltage. When the charge-removing exposure is performed, charges are generated in the charge generation layer 112. However, because the charge transport layer 113 contains an electron transport material (second charge transport material), the negative charges generated in the charge generation layer 112 can move toward the surface of the charge transport layer 113 (in the direction of the arrow in the figure) as shown in FIG. 10(B). Therefore, in the image forming apparatus 1 using the photoreceptor drum 11 according to this embodiment, the positive charges present on the surface of the photoreceptor drum 11 can be canceled out by the negative charges generated by the exposure. As a result, the occurrence of strong transfer history can be suppressed, and degradation of image quality (for example, the density step in FIG. 7(E)) can be reduced.

[0170] Test Method FIG. 11 is a diagram showing a method for measuring the reverse charge potential attenuation rate of the photosensitive layer 114 of the photosensitive drum 11 by a test operation.

[0171] The surface potential was measured using a test machine simulating the image forming unit 10. The environment was a temperature of 25°C and a humidity of 50%. As shown in Figure 11, probes 84 and 85 of a Trek surface potential meter (Model 344) were installed at positions after the surface of the photosensitive drum 11 passed the charging roller 12 and at positions after it passed the static elimination exposure unit 82, respectively.

[0172] First, a voltage of 1000 [V] in absolute value and of the opposite polarity to that normally used in a printer is applied to the charging roller 12, and the exposure energy is 1.69 μJ / cm , which is the same as that of the charge removal exposure unit (not shown) of the image forming unit 10. 2 The photosensitive drum 11, which rotates at a linear speed of 194 mm / s, the same as the interior of the image forming apparatus 1, is charged to a polarity opposite to the polarity during printing for a predetermined time (for example, 10 seconds or longer). Note that this predetermined time is not particularly limited as long as it is a time that allows the photosensitive drum 11 to be sufficiently charged.

[0173] The surface potential measured by probe 84 after applying the charging voltage for a predetermined time (for example, 10 [s] or longer) is defined as V0 [V], and the surface potential measured by probe 85 is defined as V L The measurement was performed at a distance of 34 mm from the charging roller 12 to the probe 84, and at a distance of 10 mm from the charge removal exposure unit 82 to the probe 85. Here, the reverse charge potential attenuation rate α is calculated by the following formula: α=(1-(V L / V0))×100[%] is defined by

[0174] Next, we will explain the evaluation test for solid density step and halftone density thinness. This evaluation test was performed in an environment of 25°C and 50% humidity using a printer C844dnw manufactured by Oki Electric Industry Co., Ltd. The solid pattern in Figure 8(B) and the halftone pattern in Figure 8(D) were printed. After that, 100 sheets of the high resistance medium shown in FIG. 8(A) were printed. Thereafter, the solid pattern shown in FIG. 8(C) was printed. The density of the resulting pattern was measured using an X-Rite eXact (registered trademark) densitometer. To measure the print density difference ΔD of the solid image, the density of the center of region 71 of the solid image in FIG. 8(C) (for example, solid density D1 [OD] at measurement position 71a of central region 71 in FIG. 8(C)) and the density of the center of regions 72 provided at both ends of region 71 (for example, solid density D2 [OD] at measurement position 72a of end region 72 in FIG. 8(C)) were measured, and the print density difference ΔD [OD] was obtained from ΔD = D2 - D1. When measuring the density loss of a halftone density image, the halftone pattern shown in Figure 8(D) was printed. The density of the resulting pattern was measured using an X-Rite eXact (registered trademark) densitometer. The print density (e.g., halftone print density Dm[OD] at measurement position 73a of region 73) in the center of region 73 of the halftone image in Figure 8(D) was measured. Because the density loss of halftones is a phenomenon that can occur throughout region 73 in Figure 8(D), the measurement position is not limited to the vicinity of the center of region 73, and measurement may be performed at other positions.

[0175] Test Results FIG. 12 shows the surface potentials V0 [V] and V1 [V] measured using the device shown in FIG. 11 for Comparative Examples #1, #2, and #3 and Examples #1 and #2. LThis figure shows in table format the [V], the calculated reverse charge potential attenuation rate α, the presence or absence of strong transfer history (density step), and the presence or absence of low halftone print density due to exposure. In Figure 12, when a solid print density OD value of 1.50 is considered 100%, the halftone print density is the density when printing at 40% or 70% density across the entire surface. Solid print density step (density difference) (density difference ΔD > density difference evaluation standard value ΔDr = 0.05) is marked as "x" indicating a poor result (there is a density step), and solid print density step (density difference ΔD ≦ density difference evaluation standard value ΔDr = 0.05) is marked as "o" indicating a good result (no density step). Furthermore, cases where low halftone print density occurred (halftone print density Dm<density evaluation reference value Dmr=0.05) were marked with an "X" indicating poor (low halftone print density present), and cases where there was almost no low halftone print density (halftone print density Dm≧density evaluation reference value Dmr=0.05) were marked with an "O" indicating good (no low halftone print density present). Note that the value of the density difference evaluation reference value ΔDr used in evaluating the density difference ΔD is not limited to 0.05 and can be other values. Also, the value of the density evaluation reference value Dmr used in evaluating the halftone print density Dm is not limited to 0.25 and can be other values.

[0176] It was confirmed that the photosensitive drums 11 of Examples #1 and #2 were less likely to produce density steps and low halftone print density.

[0177] Regarding the relationship between density step in solid printing and reverse charge potential attenuation rate α, it was confirmed that the larger the value of α, the better the density step in solid printing (lower density step). This is because when printing on a narrow, high-resistance medium with an image forming device, a strong transfer voltage is applied to the photosensitive drum 11 in the non-printing area, causing the photosensitive drum 11 to become charged with a polarity opposite to the polarity applied by the charging member.

[0178] If the opposite charge potential decay rate α is small, it is not possible to cancel out this opposite polarity charge (for example, the positive charge on the surface of the charge transport layer 113 in FIG. 10(B)). As a result of repeating this process, when wide printing is performed, charges of the opposite polarity accumulate in areas that were previously non-printed regions of narrow media, and the effect of canceling out the polarity charge applied by the charging member becomes stronger, resulting in a higher density, which is clearly visible as a density step in solid printing.

[0179] On the other hand, when the reverse charge potential decay rate α is large, charges of the opposite polarity (for example, positive charges on the surface of the charge transport layer 113 in FIG. 10B) can be canceled out. Therefore, even when repeatedly printing on a narrow, high-resistance medium (FIG. 8A), the charges of the opposite polarity applied from the transfer roller can be canceled out, and no charges accumulate. As a result, when performing wide-width printing (FIG. 8B), the non-printed area of the narrow medium (area 72 in FIG. 8C) can achieve the same density as the printed area of the narrow recording medium (area 71 in FIG. 8C), resulting in a good density gradient.

[0180] We also confirmed that the relationship between low density in halftone prints and the reverse charge potential attenuation rate α is such that the larger the value of α, the worse the low density in halftone prints. This is because a large reverse charge potential attenuation rate α reduces the ability to cancel out the charge of the polarity applied from the charging member to the surface of the photosensitive drum 11 during exposure to generate a latent image. As a result, the surface potential cannot be reduced to the level originally required for halftone prints, and this is clearly manifested as low density in halftone prints.

[0181] From the test results shown in FIG. 12, in drum unit 20 having photosensitive layer 114 having a first charge transport material that transports charges of a first polarity and a second charge transport material that transports charges of a second polarity different from the first polarity, photosensitive drum 11 to which toner of a second polarity can adhere, and frame 17 that holds photosensitive drum 11, when photosensitive drum 11 is rotated at a linear velocity of 194 mm / s and a potential of the first polarity and an absolute value of 1000 V is applied to the surface of photosensitive drum 11 for a predetermined time (for example, 10 seconds or more), the surface potential of photosensitive drum 11 when the surface potential of V0 V is set to V0 [V], and the surface potential of V0 V is set to 1.69 μJ / cm 2 ], the surface potential of the photosensitive drum 11 when exposed to V L [V], then α=(1-(V L The reverse charge potential decay rate α defined as / V0)) × 100[%] is It is desirable to configure the drum unit 20 so that 11.5%≦α≦17.2% is satisfied.

[0182] Furthermore, a linear speed of 194 mm / s corresponds to about 45 ppm in an actual printer, but if the speed is faster than this, density step problems are likely to occur even in an actual printer. Therefore, a printing speed of 8 ppm to 45 ppm is preferable, and 8 ppm to 35 ppm is even more preferable.

[0183] Effects of the embodiment From the above, by optimizing the reverse charge potential attenuation rate α of the photosensitive drum 11, even when an image is printed on a medium that is prone to high transfer history (for example, a high resistance medium or a perforated medium), density differences do not occur and low halftone print density does not occur.

[0184] Specifically, the drum unit 20 applies a voltage of 1000 [V] in absolute value to the charging roller 12, which has a polarity opposite to that used in a normal printer, and the exposure energy is 1.69 [μJ / cm 2By providing a photosensitive drum 11 characterized in that the reverse charge potential attenuation rate α, which represents the attenuation rate of the drum surface potential after the neutralization exposure unit 82 is turned on and the photosensitive drum 11, which is rotating at a linear speed of 194 mm / s, is charged to the opposite polarity for 10 seconds, is within the range of 11.5% to 17.2%, it is possible to suppress the occurrence of density steps and the occurrence of low density in halftone prints.

[0185] <<Variation>> In the diagrams used in the above explanation, an example is shown in which the first polarity is positive, the second polarity is negative, the first charge transport material is a hole transport material, and the second charge transport material is an electron transport material. A positive charge exists on the surface of the photosensitive drum 11 when a strong transfer history occurs. The surface of the photosensitive drum 11 is uniformly charged negatively by the charging roller 12 during normal printing operations, and negatively charged toner is used. However, these polarities may be reversed. For example, the same effect can be achieved by configuring the drum unit and image forming apparatus so that the first polarity is negative, the second polarity is positive, the first charge transport material is an electron transport material, and the second charge transport material is a hole transport material. A negative charge exists on the surface of the photosensitive drum 11 when a strong transfer history occurs. The surface of the photosensitive drum 11 is uniformly charged positively by the charging roller 12 during normal printing operations, and positively charged toner is used.

[0186] Aspects of the present disclosure Various aspects of the present disclosure are summarized below as appendices. (Appendix 1) a photosensitive drum provided with a photosensitive layer having a first charge transport material that transports charges of a first polarity and a second charge transport material that transports charges of a second polarity different from the first polarity, and to which toner of the second polarity can be attached; a frame for holding the photosensitive drum; and The surface potential of the photosensitive drum when a potential of the first polarity and an absolute value of 1000 [V] is applied to the surface of the photosensitive drum for a predetermined time while the photosensitive drum is rotated at a linear velocity of 194 [mm / s] is defined as V0 [V], and the surface at the surface potential of V0 [V] is defined as 1.69 [μJ / cm 2], the surface potential of the photosensitive drum when exposed to V L In the case of [V], α=(1-(V L The reverse charge potential decay rate α defined as / V0)) × 100[%] is Satisfy 11.5%≦α≦17.2% A drum unit characterized by: (Appendix 2) The photosensitive drum further includes a charging member for charging the surface of the photosensitive drum. 2. The drum unit according to claim 1, (Appendix 3) When the photosensitive drum is used in a printing operation, the charging member charges the surface of the photosensitive drum to the second polarity. 3. The drum unit according to claim 2, (Appendix 4) the first polarity is positive and the second polarity is negative; When the photosensitive drum is used for printing, the charging member charges the surface of the photosensitive drum to a negative voltage. 4. The drum unit according to claim 2 or 3. (Appendix 5) the first polarity is positive and the second polarity is negative; When the photosensitive drum is used for printing, the charging member charges the surface of the photosensitive drum to a voltage within a range of −1200 [V] to −700 [V]. 5. The drum unit according to claim 2, wherein: (Appendix 6) the first polarity is positive and the second polarity is negative; When the photosensitive drum is used for printing, the photosensitive drum rotates at a linear speed within a range of 70 mm / s to 220 mm / s. 6. The drum unit according to claim 2, wherein: (Appendix 7) the first polarity is negative and the second polarity is positive; When the photosensitive drum is used for printing, the charging member charges the surface of the photosensitive drum to a positive voltage. 3. The drum unit according to claim 2, (Appendix 8) The photosensitive layer is a charge generating layer containing a charge generating material; a charge transport layer covering the outside of the charge generation layer and containing the first charge transport material and the second charge transport material; 8. The drum unit according to claim 1, further comprising: (Appendix 9) a drum unit according to any one of claims 1 to 8; a charging member for uniformly charging the surface of the photosensitive drum; an exposure device for forming an electrostatic latent image on the uniformly charged surface; a developer carrier for developing the electrostatic latent image to form a toner image on the surface thereof; a transfer member that transfers the toner image formed on the surface; An image forming apparatus comprising: [Explanation of symbols]

[0187] 1 image forming apparatus, 10 image forming unit (image forming section), 11 photosensitive drum (image carrier), 12 charging roller (charging member), 13 developing roller (developer carrier), 14 supply roller (supply member), 15 developing blade (layer regulating member), 16 cleaning member, 17 frame (housing), 18 toner cartridge (developer container), 20 drum unit, 21 exposure head (exposure device), 22 toner storage section (developer storage section), 30 transfer unit, 31 transfer roller (transfer member), 32 transfer belt, 40 medium supply section, 50 fixing unit, 60 medium discharge section, 80 electrometer, 110 conductive support, 111 undercoat layer (blocking layer), 112 charge generation layer, 113 charge transport layer, 114 Photosensitive layer, M recording medium, M1 high resistance medium, M2 normal medium, R1 medium area, R2 outside medium area, Q exposure amount, V0, VL Surface potential of the photosensitive drum, α is the reverse charge potential decay rate.

Claims

1. a photosensitive drum provided with a photosensitive layer having a first charge transport material that transports charges of a first polarity and a second charge transport material that transports charges of a second polarity different from the first polarity, and to which toner of the second polarity can be attached; a frame for holding the photosensitive drum; and The surface potential of the photosensitive drum when a potential of the first polarity and an absolute value of 1000 [V] is applied to the surface of the photosensitive drum for a predetermined time while the photosensitive drum is rotated at a linear velocity of 194 [mm / s] is V ０ [V], and the surface potential is V ０ [V], the surface of 1.69 [μJ / cm ２ ], the surface potential of the photosensitive drum when exposed to V Ｌ When [V] is used, α = (1-(V Ｌ / V ０ )) × 100 [%], the reverse charge potential attenuation rate α is Satisfy 11.5%≦α≦17.2% A drum unit characterized by:

2. The photosensitive drum further includes a charging member for charging the surface of the photosensitive drum.

2. The drum unit according to claim 1.

3. When the photosensitive drum is used for a printing operation, the charging member charges the surface of the photosensitive drum to the second polarity.

3. The drum unit according to claim 2.

4. the first polarity is positive and the second polarity is negative; When the photosensitive drum is used for printing, the charging member charges the surface of the photosensitive drum to a negative voltage.

3. The drum unit according to claim 2.

5. the first polarity is positive and the second polarity is negative; When the photosensitive drum is used for printing, the charging member charges the surface of the photosensitive drum to a voltage within a range of −1200 [V] to −700 [V].

3. The drum unit according to claim 2.

6. the first polarity is positive and the second polarity is negative; When the photosensitive drum is used for printing, the photosensitive drum rotates at a linear velocity within a range of 70 mm / s to 220 mm / s.

3. The drum unit according to claim 2.

7. the first polarity is negative and the second polarity is positive; When the photosensitive drum is used for printing, the charging member charges the surface of the photosensitive drum to a positive voltage.

3. The drum unit according to claim 2.

8. The photosensitive layer is a charge generating layer containing a charge generating material; a charge transport layer covering the outside of the charge generation layer and containing the first charge transport material and the second charge transport material; 2. The drum unit according to claim 1, further comprising:

9. A drum unit according to any one of claims 1 to 8; a charging member for uniformly charging the surface of the photosensitive drum; an exposure device for forming an electrostatic latent image on the uniformly charged surface; a developer carrier for developing the electrostatic latent image to form a toner image on the surface thereof; a transfer member that transfers the toner image formed on the surface; An image forming apparatus comprising:

Images