JP7825992B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus that forms an image on a recording material.

A configuration known as a simultaneous development and cleaning system or cleanerless system is known in electrophotographic image forming devices. In this configuration, residual toner remaining on the surface of an image carrier, such as a photosensitive drum, after a toner image has been transferred from the image carrier to a receiving material (recording material or intermediate transfer material) is not collected by a cleaning device, but is instead collected by a developing member, such as a developing roller, simultaneously with the next development process.

Patent Document 1 describes a cleanerless image forming apparatus in which a brush member is positioned downstream of the transfer roller and upstream of the charging roller in the rotation direction of the photosensitive drum to scatter residual toner adhering to the photosensitive drum. According to this document, by using the brush member to scatter residual toner distributed in the same pattern as the toner image transferred to the recording material, the behavior of residual toner in the charging device and developing device is made uniform, making it less likely that residual images caused by residual toner will appear on the recording material.

JP 2010-14982 A

Generally, the residual toner remaining on the image carrier surface after the transfer process includes toner particles charged to the toner's normal polarity (normal charging polarity), toner particles charged to an irregular polarity opposite to the normal polarity, and toner particles with a charge amount close to zero. If the residual toner reaches the charging member or developing member while still in a wide charge distribution state, various problems can occur, mainly due to toner with an irregular polarity, such as poor charging due to contamination of the charging roller and the generation of residual images (fog images) due to poor collection of residual toner by the developing member.

For this reason, it is conceivable to stabilize the charge distribution of the residual toner at the normal polarity by placing a brush member downstream of the transfer member and upstream of the charging member in the rotational direction of the image carrier, and interposing the residual toner between the transfer section and charging section of the image carrier on the brush member. However, depending on the contact conditions between the brush member and the image carrier, the residual toner may not be sufficiently normalized.

The present invention therefore aims to provide an image forming device that can stabilize the charge distribution of residual toner at a normal polarity.

One aspect of the present invention is an image forming apparatus comprising: a rotating image carrier; a developing member that develops an electrostatic latent image formed on the surface of the image carrier using toner in a developing unit; a transfer member that transfers the toner image developed by the developing member from the image carrier to a transfer recipient in a transfer unit; and a brush member that contacts the surface of the image carrier at a contact unit that is downstream of the transfer unit and upstream of the developing unit in the rotation direction of the image carrier, wherein toner that has not been transferred to the transfer recipient is collected by the developing member, the image forming apparatus is characterized in that the toner is located on the same side of the brush member as its normal charge polarity in a triboelectric series, the maximum value of the contact pressure at the contact unit is 0.7 gf/ ^mm2 or more and 3.5 gf/ ^mm2 or less, the maximum contact area ratio at the contact unit is 18% or more and 74% or less, and the Clark-Evans index of the brush member at the contact unit is 1 or more.

Another aspect of the present invention is an image forming apparatus comprising: a rotating image carrier; a developing member that develops an electrostatic latent image formed on the surface of the image carrier using toner in a developing unit; a transfer member that transfers the toner image developed by the developing member from the image carrier to a transferee in a transfer unit; a brush member that contacts the surface of the image carrier at a contact portion downstream of the transfer member and upstream of the developing member in the rotation direction of the image carrier; and voltage application means that applies a voltage to the brush member, wherein the toner that has not been transferred to the transferee is collected by the developing member, the voltage applied to the brush member by the voltage application means being on the same side as the normal charging polarity of the toner with respect to the surface potential of the image carrier that reaches the contact portion; a maximum value of the contact pressure at the contact portion is 0.7 gf/ ^mm2 or more and 3.5 gf/ ^mm2 or less; a maximum contact area ratio at the contact portion is 18% or more and 74% or less; and a Clark-Evans index of the brush member at the contact portion is 1 or more.

According to the present invention, the charge distribution of residual toner can be stabilized at a normal polarity.

4 is a diagram showing the peak pressure and the maximum contact area ratio of the brush member; 1 is a schematic diagram of an image forming apparatus according to a first embodiment. 1A is a front view of a brush member according to a first embodiment, and FIG. 1B is a cross-sectional view of the brush member according to the first embodiment. 4A and 4B are diagrams showing a method for measuring a normal force acting on a brush member. 10A and 10B are diagrams showing a method for observing a brush member using a glass plate. FIG. 4 is a diagram for explaining a method for calculating a peak pressure. FIG. 1A shows an example of a brush contact area observed using a glass plate; FIG. 1B explains a method for calculating the contact area ratio; and FIG. 1C explains a method for calculating the Clark-Evans index. FIG. 4 is a schematic diagram of an image forming unit according to Reference Example 1. FIG. 9 is an enlarged view of a part of FIG. 8 . 10A is a schematic diagram of a brush contact portion according to Reference Example 2, and FIG. 10B is a schematic diagram of a photosensitive drum surface beyond the brush contact portion. FIG. 2 is a schematic diagram of an image forming unit according to the first embodiment. FIG. 12 is an enlarged view of a part of FIG. 11 . FIG. 10 is a schematic view of a brush member according to a second embodiment. FIG. 10 is a schematic view of a brush member according to a second embodiment. FIG. 10 is a diagram showing a state in which the penetration amount of the brush member is increased in the second embodiment. FIG. 10 is a schematic diagram of an image forming unit according to a third embodiment. FIG. 17 is an enlarged view of a part of FIG. 16 .

Embodiments of the present disclosure will be described below with reference to the drawings.

First Embodiment
An overview of the image forming apparatus 100 according to the first embodiment will be described using Figure 2. The image forming apparatus 100 is a monochrome printer that forms a monochrome image on a sheet S based on image information received from an external device. The sheet S, which is the recording material, can be a variety of sheets of different sizes and materials, including paper such as plain paper and cardboard, surface-treated sheet materials such as plastic film, cloth, and coated paper, and sheet materials of special shapes such as envelopes and index paper.

As shown in FIG. 2, the image forming apparatus 100 includes an electrophotographic image forming unit 101 that forms an image on a sheet S, and a sheet transport mechanism (6, 8, 12) that feeds and transports the sheet S. The image forming unit 101 includes a photosensitive drum as an image carrier, a charging roller 2 as a charging means, an exposure device 3 as an exposure means, a developing device 4 as a developing means, a transfer roller 5 as a transfer means, a brush member 11, and a fixing device 9 as a fixing means.

The photosensitive drum 1 is an electrophotographic photosensitive member molded into a drum shape. The charging roller 2 is a contact charging member that contacts the photosensitive drum 1. The contact point between the charging roller 2 and the photosensitive drum 1 is the charging point P2 (charging position) where the surface of the photosensitive drum 1 is charged.

The developing device 4 includes a developing roller 41, a supply roller 42, an agitating member 43, a developing blade 44, and a toner storage unit 45. The developing roller 41 is a developing member or developer carrier that carries toner T and rotates to supply toner T to a developing unit P4 (developing position) where the developing roller 41 faces the photosensitive drum 1. In this embodiment, a so-called contact development method is used, in which a toner layer carried by the developing roller 41 contacts the surface of the photosensitive drum 1 at the developing unit P4. The developing roller 41 is disposed in the opening of the toner storage unit 45, which is located opposite the photosensitive drum 1. The supply roller 42 supplies (applies) toner T from the toner storage unit 45 to the developing roller 41. The agitating member 43 is disposed in the toner storage unit 45 and agitates the toner T in the toner storage unit 45 by rotating. The toner storage unit 45 is a container that stores toner T as a developer. The developing blade 44 is brought into contact with the surface of the developing roller 41, which rotates from inside the toner storage section 45 toward the developing section P4, with a predetermined pressure. The developing blade 44 is mainly composed of a material (for example, iron or copper) that is on the positive polarity (non-regular polarity) side of the toner T's main component (binder resin) in the electrification series. As a result, the developing blade 44 rubs against the toner T, causing the toner T to be tribo-charged to the regular polarity (regular charging polarity).

The transfer roller 5 is positioned in contact with the surface of the photosensitive drum 1. The nip where the transfer roller 5 and the photosensitive drum 1 face each other is the transfer section P5 where the toner image is transferred from the photosensitive drum 1 to the sheet S.

The brush member 11 is positioned downstream of the transfer portion P5 and upstream of the charging portion P2 in the rotation direction R1 of the photosensitive drum 1. The brush member 11 is positioned so that it comes into contact with the surface of the photosensitive drum 1 under predetermined contact conditions. Details of the brush member 11 will be described later.

The fixing device 9 has a fixing roller or flexible fixing film as a first rotating body, a pressure roller as a second rotating body that contacts the first rotating body with a predetermined pressure, and heating means that heats the image on the sheet S via the first rotating body. The heating means can be a halogen lamp that generates radiant heat or a heater substrate with a pattern of heating resistors formed on a ceramic substrate.

The sheet transport mechanism includes a cassette 6, a pair of transport rollers 8, and a pair of discharge rollers 12. The cassette 6 is a stacking unit on which sheets S are stacked. The pair of transport rollers 8 is a transport member that transports sheets S fed from the cassette 6 to the transfer unit P5. The pair of discharge rollers 12 is a discharge member that discharges sheets S on which an image has been formed by the image forming unit 101.

The image forming operation performed by the image forming apparatus 100 is outlined below. When an instruction to perform an image forming operation is input to the image forming apparatus 100, the photosensitive drum 1 is rotated clockwise, and the charging roller 2 uniformly charges the surface of the photosensitive drum 1. The exposure device 3 irradiates the photosensitive drum 1 with laser light L based on image information received from an external device, exposing the surface of the photosensitive drum 1. This writes an electrostatic latent image onto the surface of the photosensitive drum 1.

This embodiment uses a reversal development method. Therefore, the charging roller 2 is applied with a negative voltage (charging voltage) that is the same as the normal polarity of the toner T, thereby charging the surface of the photosensitive drum 1 to a negative dark potential Vd. After charging, the light potential Vl of the area (image area) exposed by the exposure device 3 becomes lower than the dark potential Vd. In the configuration example of this embodiment, Vd = -700 (V) and Vl = -100 (V).

In the developing device 4, the toner T contained in the toner storage compartment 35 is homogenized by the agitator 43 and supplied to the developing roller 41 by the supply roller 42. The toner T carried by the developing roller 41 is frictionally charged to the normal polarity by friction with the developing blade 44 and is restricted to a predetermined layer thickness as the developing blade 44 passes. As the developing roller 41 rotates, the toner T charged to the normal polarity is supplied to the developing section P4. In addition, a voltage (developing voltage) of the same negative polarity as the normal polarity of the toner T is applied to the developing roller 41, causing the toner T to transfer to the photosensitive drum 1 in accordance with the potential distribution on the surface of the photosensitive drum 1. This develops the electrostatic latent image on the surface of the photosensitive drum 1 and makes it visible as a toner image. The toner image formed on the surface of the photosensitive drum 1 is transported to the transfer section P5 while still carried by the photosensitive drum 1.

In parallel with the above process, sheets S are fed one by one from cassette 6 by a feeding unit (not shown) and transported to transfer section P5 by a pair of transport rollers 8. A voltage of positive polarity (transfer voltage), opposite to the normal polarity of toner T, is applied to transfer roller 5, causing the toner image to be transferred from photosensitive drum 1 to sheet S at transfer section P5.

After passing through transfer section P5, sheet S is transported to fixing device 9. In fixing device 9, sheet S is sandwiched between the nip portion of the first and second rotating bodies and transported, while the image on sheet S is heated by the first rotating body heated by the heating means, thereby fixing the image. After passing through fixing device 9, sheet S is discharged outside image forming apparatus 100 by a pair of discharge rollers 12.

(Cleanerless brush system)
Next, we will explain the operation unique to the cleanerless brush system using the brush member 11. This embodiment employs a simultaneous development and cleaning system in which residual toner that has not been transferred to the transfer target (sheet S) at the transfer unit P5 is collected by the developing roller 41 when the residual toner next reaches the developing unit P4. In the simultaneous development and cleaning system, the residual toner collected by the developing roller 41 is agitated with other toner T in the toner storage unit 45 and is reused for development.

In the simultaneous development and cleaning method, the developing roller 41 collects the residual toner that was not transferred to the transfer medium at the transfer section P5, so the brush member 11 basically allows the residual toner to pass through. Therefore, the "brush member" in this embodiment differs from the brush member that serves as a cleaning device (drum cleaner) intended to remove residual toner from the photosensitive drum 1. Note that the simultaneous development and cleaning method is sometimes called a cleanerless method because it does not employ a cleaning device to collect residual toner.

In the cleanerless brush system, a brush member 11 is placed downstream of the transfer station P5 and upstream of the charging station P2 in the rotation direction R1 of the photosensitive drum 1 to scatter residual toner adhering to the surface of the photosensitive drum 1 that has passed through the transfer station P5. By placing the brush member 11, it is possible to alleviate the condition in which residual toner is locally concentrated on the photosensitive drum 1. If residual toner is locally concentrated on the photosensitive drum 1, this can lead to poor charging due to contamination of the charging roller 2 or poor collection at the developing station P4, resulting in poor image quality. In contrast, the cleanerless brush system scatters residual toner using the brush member 11, which uniforms the behavior of residual toner at the charging station P2 and the developing station P4, thereby minimizing the above-mentioned problems.

(Operation specific to the cleanerless brush method)
In the cleanerless brush system, residual toner that passes through the contact point between the brush member 11 and the photosensitive drum 1 reaches the charging section P2. Because a charging voltage of the same polarity as the normal polarity is applied to the charging roller 2, toner particles among the residual toner that are charged to the normal polarity are pressed against the photosensitive drum 1 and pass through the charging section P2. On the other hand, toner particles among the residual toner that are charged to the non-normal polarity or toner particles with a charge amount close to zero partially adhere to the charging roller 2 at the charging section P2. When residual toner accumulates on the charging roller 2, uniform charging of the photosensitive drum 1 is hindered, and image defects caused by poor charging become apparent.

In this embodiment, a peripheral speed difference is set between the charging roller 2 and the photosensitive drum 1 to reduce adhesion of residual toner to the charging roller 2. Specifically, the peripheral speed of the charging roller 2 is set to a value 5% or more faster than the peripheral speed of the photosensitive drum 1. Furthermore, the materials of the surface layers of the charging roller 2 and the photosensitive drum 1 are selected so that the residual toner is charged to the normal polarity through friction with the charging roller 2 or the photosensitive drum 1. In other words, in the charging series, the materials of the surface layers of the charging roller 2 and the photosensitive drum 1 are positioned higher (on the positive polarity side, the non-normal polarity side) than the toner. By configuring the peripheral speed difference and materials as described above, the residual toner is charged to the normal polarity through friction with the charging roller 2 or the photosensitive drum 1 at the charging section P2, thereby reducing adhesion of residual toner to the charging roller 2.

The remaining toner that has passed through the charging section P2 reaches the developing section P4 as the photosensitive drum 1 rotates. The toner particles charged to the normal polarity among the remaining toner carried in the non-image area (non-exposed area) on the photosensitive drum 1 are transferred to the developing roller 41 due to the potential difference between the dark area potential Vd and the developing voltage, and are then collected in the toner storage section 45. Meanwhile, the toner particles charged to the normal polarity among the remaining toner carried in the image area (exposed area) on the photosensitive drum 1 are retained on the photosensitive drum 1 without transferring to the developing roller 41 due to the potential difference between the light area potential Vl and the developing voltage. In this case, the toner particles are sent to the transfer section P5 as part of the developed toner image. The voltage value of the developing voltage is the same polarity as the normal polarity of the toner, and is higher than the light area potential Vl and lower than the dark area potential Vd.

Ideally, residual toner particles charged to an irregular polarity and toner particles with a charge close to zero are converted to a regular polarity at charging unit P2, do not adhere to charging roller 2, and are collected by developing roller 41 at developing unit P4. However, when a large amount of residual toner charged to an irregular polarity enters charging unit P2, the residual toner that was not converted to a regular polarity at charging unit P2 is more likely to adhere to charging roller 2. Furthermore, when residual toner that was not converted to a regular polarity at charging unit P2 reaches developing unit P4, it passes through developing unit P4 without being collected by developing roller 41. This can result in contamination of transfer roller 5 or image defects (white background fog) in which a faint toner image is formed on the white background (non-image area) on the sheet.

The components of the image forming device 100 are described in detail below.

(Brush member)
First, the brush member 11 in this embodiment will be described. As shown in Fig. 2, the brush member 11 contacts the surface of the photosensitive drum 1 downstream of the transfer portion P5 and upstream of the charging portion P2 in the rotation direction R1 of the photosensitive drum 1. In other words, the image forming apparatus 100 includes the brush member 11, which is disposed downstream of the transfer member and upstream of the developing member in the rotation direction of the image carrier, and which contacts the surface of the image carrier. Hereinafter, the area where the brush member 11 contacts the photosensitive drum 1 will be referred to as the "brush contact portion."

Figure 3(a) is a front view (viewed from one side in the short direction) of the brush member 11 in a standalone state. The standalone state means that the brush member 11 is not attached to the image forming device 100, i.e., no external force is acting on the brush member 11. Figure 3(b) is a cross-sectional view of the standalone brush member 11 cut along a plane perpendicular to the longitudinal direction. Figure 3(c) is a cross-sectional view of the brush member 11 in contact with the photosensitive drum 1.

As shown in Figures 3 (a-c), the brush member 11 has a base fabric 11b as a base and conductive threads 11a as bristles (fibers) supported by the base fabric 11b. The base fabric 11b is made of synthetic resin fibers containing carbon as a conductive agent. The conductive threads 11a are made of, for example, nylon fibers mixed with a conductive agent, and are woven into the base fabric 11b for implantation. The material of the conductive threads 11a is not limited to nylon; rayon or other synthetic resin fibers may also be used.

The brush member 11 is a member in which the base fabric 11b extends in an elongated manner in a predetermined direction. Hereinafter, the extension direction of the base fabric 11b will be referred to as the longitudinal direction LD of the brush member 11, and the direction along the base fabric 11b that is perpendicular to the longitudinal direction will be referred to as the lateral direction SD of the brush member 11. When no external force is acting on the brush member 11 (Figure 3(b)), the conductive threads 11a protrude in a direction (normal to the base fabric 11b) that is approximately perpendicular to both the longitudinal direction LD and the lateral direction SD.

As shown in Figure 3(c), the brush member 11 is positioned so that its longitudinal direction LD is approximately parallel to the rotational axis of the photosensitive drum 1.

As shown in Figure 3(b), the distance from the base fabric 11b to the tip of the conductive thread 11a in the brush member 11 in its standalone state is defined as bristle length L1. In this embodiment, the bristle length L1 of the brush member 11 is 5.75 mm. The base fabric 11b of the brush member 11 is fixed to a support member 11c installed in a predetermined position on the image forming apparatus 100 using a fixing means such as double-sided tape. The position of the support member 11c is set so that the tip of the conductive thread 11a penetrates the photosensitive drum 1. Therefore, the brush member 11 is bent as the tip of the conductive thread 11a is pressed against the surface of the photosensitive drum 1.

In this embodiment, the fixing surface of the base fabric 11b on the support member 11c is arranged approximately parallel to the surface of the photosensitive drum 1, and the distance (clearance) between the support member 11c and the photosensitive drum 1 is approximately constant. In other words, when viewed in the longitudinal direction LD, a straight line passing from the rotation axis of the photosensitive drum 1 through the center position of the base fabric 11b in the transverse direction SD is perpendicular to the fixing surface of the support member 11c.

In this embodiment, the shortest distance from the base fabric 11b of the brush member 11 fixed to the support member 11c to the photosensitive drum 1 is defined as L2. In this embodiment, the difference between L2 and L1 is defined as the maximum penetration amount of the brush member 11 into the photosensitive drum 1, where L2 < L1.

In this embodiment, the maximum penetration amount of the brush member 11 into the photosensitive drum 1 is, for example, 1.2 mm. Also, in this embodiment, as shown in Figure 3(b), when the brush member 11 is in a standalone state, the lateral width L3, which is the length of the brush member 11 in the lateral direction SD, is, for example, 4 mm. When pressed against the photosensitive drum 1 as shown in Figure 3(c), the occupancy width of the conductive threads 11a in the lateral direction SD is approximately 5 mm to 6 mm.

In this embodiment, the length L4 of the brush member 11 in the longitudinal direction LD is 216 mm. The length L4 is set so that the brush member 11 can contact the entire image formation area (the area where a toner image can be formed, the maximum area of the latent image formed by the exposure device 3) on the photosensitive drum 1 in the longitudinal direction (the direction of the rotation axis of the photosensitive drum 1). In this embodiment, the conductive threads 11a have a thickness of, for example, 2 denier and a density of, for example, 240 kF/ ^inch² . The thickness and density of the conductive threads 11a can be changed as long as they fulfill the functions required of the brush member 11. For example, the conductive threads 11a preferably have a thickness of 1 denier to 6 denier and a density of 150 kF/ ^inch² to 350 kF/ ^inch² . Note that 1 kF/ ^inch² represents a bristle density of 1,000 threads per square inch.

In the case of the nylon conductive thread 11a used in this embodiment, the constant length unit of 1 to 6 denier is converted to a fiber diameter of approximately 10 μm to approximately 30 μm. Therefore, when using bristle materials other than nylon for the brush member, bristle materials with a constant length unit of 1 to 6 denier or a fiber diameter of 10 to 30 μm can be used.

"1 denier or more and 6 denier or less" can be rephrased as "1.1 dtex or more and 6.7 dtex or less." Also, since 1 ^inch2 is approximately 6.45 ^cm2 , "150 kF/ ^inch2 or more and 350 kF/ ^inch2 or less" can be rephrased as "23 kF/ ^mm2 or more and 54 kF/ ^mm2 or less."

In this embodiment, the brush member 11 is configured to allow the remaining toner adhering to the surface of the photosensitive drum 1 that has passed through the transfer portion P5 to pass through while scattering the remaining toner. Therefore, if the conductive threads 11a are too thick, the remaining toner cannot be uniformly scattered, and the remaining toner may pass through the brush contact area in a streak-like pattern, potentially resulting in streak-like contamination of the charging roller 2. Furthermore, if the conductive threads 11a are too dense, the remaining toner may be blocked at the brush contact area, not only hindering the development roller 41 from collecting the remaining toner, but also potentially contaminating the inside of the image forming apparatus by falling or scattering from the photosensitive drum 1. Furthermore, the brush member 11 in this embodiment is configured to triboelectrically charge the remaining toner at the brush contact area. Therefore, if the conductive threads 11a are too thin, the conductive threads 11a may easily bend when the remaining toner comes into contact with the conductive threads 11a, causing the toner particles to escape, preventing the toner particles from rolling, and potentially preventing the remaining toner from being sufficiently triboelectrically charged. Furthermore, if the density of the conductive threads 11a is too low, the frequency with which the residual toner collides with the conductive threads 11a will decrease, which may result in the residual toner not being sufficiently triboelectrically charged.

The above explanation describes the preferred ranges for the thickness and density of the conductive threads 11a from the perspective of the function of dispersing residual toner and the function of frictionally charging the residual toner, but the details of the conductive threads 11a, such as the thickness, density, material, and bristle length, can be changed as appropriate depending on the function required of the brush member 11. The brush member 11 of this embodiment may also have the function of blocking foreign matter other than residual toner (e.g., paper dust) at the brush contact portion.

(Developer)
In this embodiment, toner T, which is a one-component developer whose normal polarity is negative, is used as the developer. Therefore, in the following description of this embodiment, unless otherwise specified, "negative polarity" is synonymous with the normal polarity of toner T, and "positive polarity" is synonymous with the irregular polarity of toner T.

Toner T contains a binder resin and a colorant, and may also contain a release agent, charge control agent, and external additives as needed. Styrene-acrylic resin or polyester resin, which are lower (negative polarity) in the triboelectric series than nylon and rayon, can be suitably used as the binder resin. In other words, it is desirable for the main component (binder resin) of toner T to be on the normal polarity side (lower) of the triboelectric series relative to the material of the bristles of the brush member 11. In this embodiment, styrene-acrylic resin is used as the binder resin for toner T.

Known colorants can be used, such as dyes and pigments. Known release agents can be used, such as hydrocarbon waxes and ester waxes. Known charge control agents can be used, such as charge control agents that have an acid value and a hydroxyl value, and preferably have a negative polarity equal to or greater than that of the binder resin. Known external additives can be used, such as silica, alumina, titania, and titanium composite oxides. It is preferable that the colorants and release agents be encapsulated in the binder resin so as not to affect the charge polarity of the toner particle surface.

The toner T can be a polymerized toner produced by polymerization. A particle size (volume average particle size) of 4 to 10 μm, more preferably 6 to 8 μm, is preferably used. In this embodiment, spherical toner with a particle size of 7 μm prepared by polymerization is used. The toner T of this embodiment is a non-magnetic single-component developer that does not contain a magnetic component and is supported on the developing roller 41 primarily by intermolecular forces and electrostatic forces (image forces). However, a single-component developer made of toner containing a magnetic component may also be used. Alternatively, a two-component developer made of non-magnetic toner and magnetic carrier may also be used. When a magnetic developer is used, a cylindrical developing sleeve with a magnet disposed inside is used as the developer support. In addition to the toner or carrier, the developer may contain additives (e.g., wax or silica particles) to adjust the fluidity and charging performance of the toner.

(Photosensitive drum)
The photosensitive drum 1 has a cylindrical or columnar conductive support (metal core) as the bottom layer, on which an undercoat layer, a charge generation layer, and a charge transport layer are sequentially laminated. The charge transport layer is formed by applying and drying a coating material in which a charge transport material and a binder resin are mixed in a solvent. Known charge transport materials can be used as the main charge transport material. Examples include various triarylamine compounds and hydrazone compounds. Examples of binder resins include polycarbonate resins and polyarylate resins.

When it comes to frictional charging with toner, it is the charge transport layer, which is the surface layer (outermost layer), that is dominant, and furthermore, the binder resin that makes up the majority of that layer. The polycarbonate resins and polyarylate resins listed here are on the non-regular polarity side (higher) of the triboelectric series relative to the styrene acrylic, which is the binder resin of toner T. In other words, it is preferable that the outermost layer of the image carrier be made of a material that can triboelectrically charge the toner to the regular polarity when rubbed against the resin that is the main component of the toner. In this embodiment, polycarbonate was selected as the binder resin for the outermost layer.

In this embodiment, the photosensitive drum 1 is cylindrical and has an outer diameter of 24 mm. The way in which the brush member 11 is applied (for example, the penetration amount described above and the angle described in the third embodiment) is changed appropriately depending on the outer diameter of the photosensitive drum 1.

(Charging roller)
The charging roller 2 according to this embodiment will now be described. The charging roller 2 has a core metal as a conductive support, a 2 mm elastic layer provided on the outer periphery of the core metal, and a 25 μm resin layer as a surface layer provided on the outer periphery of the elastic layer. The surface of the surface layer comes into contact with the photosensitive drum 1 and serves as a surface that discharges electricity to the photosensitive drum 1.

The elastic layer is made of an electronically conductive rubber material. Electronically conductive rubber materials are, for example, binder polymers that are not themselves electrically conductive, with carbon black dispersed as conductive particles (electronically conductive agents) to adjust the electrical resistance. Known binder polymers used in the conductive elastic layers of charging rollers for electrophotographic devices can be used. Examples include hydrin rubber and butadiene rubber. In this embodiment, hydrin rubber was selected.

The type of carbon black to be compounded in the elastic layer is not particularly limited, as long as it is a conductive carbon black that can impart conductivity to the elastic layer. Furthermore, if necessary, fillers, processing aids, crosslinking aids, crosslinking accelerators, crosslinking accelerator aids, crosslinking retarders, softeners, dispersants, colorants, and other additives commonly used as compounding agents for rubber may also be added to the elastic layer.

The resin for the surface layer is a resin material that is on the non-regular polarity side (higher) of the main component (binder resin) of the toner T in the triboelectric series. For example, the surface layer is formed by coating the outer periphery of the elastic layer with a conductive resin such as polycarbonate urethane. By forming the surface layers of the charging roller 2 and photosensitive drum 1 from the above materials and setting a peripheral speed difference between the charging roller 2 and photosensitive drum 1 as described above, the remaining toner can be frictionally charged to the regular polarity at the charging section P2.

Furthermore, roughening particles of a polarity that does not impair frictional charging can be added to the surface layer of the charging roller 2. For example, one method is to disperse the same polycarbonate urethane material as the surface layer in a particulate form. In other words, the charging roller 2 does not need to be in close contact with the surface of the photosensitive drum 1 at the charging section P2, but can be configured so that the peaks of the roughening particles form contact with the surface of the photosensitive drum 1.

(Transfer roller)
The transfer roller 5 is a roller-type transfer member disposed opposite the photosensitive drum 1. The transfer roller 5 is pressed against the photosensitive drum 1 at a predetermined pressure. The transfer roller 5 in this embodiment is an elastic roller with an outer diameter of 12 mm, in which an elastic layer made of conductive nitrile butadiene rubber-hydrin sponge rubber is formed around a core metal.

(Contact conditions of brush members)
In this embodiment, the brush member 11 applies an electric charge to the remaining toner by frictional charging while scattering the remaining toner on the photosensitive drum 1. At this time, the material of the bristles (conductive threads 11a) of the brush member 11 is one that is on the positive side (higher) of the triboelectric series relative to the main component of the toner T so that a negative charge is applied to the remaining toner by frictional charging. In addition, the contact pressure between the brush member 11 and the photosensitive drum 1 at the brush contact portion is ensured so that the conductive threads 11a can rub the remaining toner with sufficient force.

In terms of the triboelectric series, the main component of the toner T in this embodiment is styrene-acrylic resin. The bristle material of the brush member 11 is preferably a material such as nylon or rayon, in which styrene-acrylic is on the negative (lower) side of the triboelectric series and the triboelectric series difference is large. In this embodiment, as described above, nylon resin was selected as the main component of the conductive thread 11a. Polyester and acrylic fibers are not desirable materials for the conductive thread 11a, as styrene-acrylic is on the positive side of the triboelectric series and the triboelectric series difference is small. However, if the main component of the toner T is different, polyester or acrylic fibers may be used for the conductive thread 11a.

The surface layer of the photosensitive drum 1 can also affect the frictional charging of the toner T at the brush contact area. Therefore, it is preferable that the main component of the surface layer of the photosensitive drum 1 be a material that is on the positive side of the triboelectric series relative to the main component of the toner T. In this embodiment, as described above, the main component of the surface layer of the photosensitive drum 1 is polycarbonate.

The contact conditions of the brush member 11 at the brush contact area will be further explained. To examine the physical properties (parameters) that indicate the contact conditions of the brush member 11, four samples with different bristle thicknesses and densities were prepared. Sample 1 is a brush member 11 with thick bristles and low density. Sample 2 is a brush member 11 with thin bristles and medium density. Sample 3 is a brush member 11 with thin bristles and high density. Sample 4 is a brush member 11 with medium bristle thickness and low density. The brush member 11 of each sample was then brought into contact with the photosensitive drum 1, and the peak pressure and maximum contact area ratio at the brush contact area were calculated using the following method. Note that the peak pressure is the maximum value of the average contact pressure within a 1 mm width in the short direction of the brush contact area, and the maximum contact area ratio is the contact area ratio between the brush member 11 and the photosensitive drum 1 in a 1 mm wide region where the peak pressure is obtained.

The peak pressure was calculated as follows. As shown in Figure 4, a compression test fixture for Shimadzu Corporation's small benchtop testing machine EZTest was used to measure the normal force when the pressure plate was pressed against the brush member 11 while aligning the bristles of the brush member 11, with the brush member placed horizontally. This resulted in the relationship between the penetration amount and the normal force. Meanwhile, a glass plate, as shown in Figure 5, was moved horizontally while being pressed against the brush member 11 so that the bristles of the brush member 11 were aligned. The contact width in the short direction SD was measured by observing under a microscope.

When the density and thickness of the bristles of the brush member 11 are uniform, the peak pressure can be calculated using the following formulas (1) to (3). First, when an object is pressed against the brush member 11 with a given penetration, the average value of the contact pressure (average pressure) at the brush contact point can be expressed by the following formula: Note that the normal force and contact width in the formula are values measured when a pressure plate (FIG. 4) or a glass plate (FIG. 5) is pressed against the brush member 11 with a given penetration.
(Formula 1) Average pressure = normal force / (contact width x longitudinal width) (gf/mm ² )

In an actual brush contact portion between the brush member 11 and the photosensitive drum 1, the contact pressure is greatest at the portion where the brush member 11 penetrates the photosensitive drum 1 to the greatest extent. This maximum value of contact pressure is called the peak pressure. The peak pressure is calculated by (Equation 3) using the average penetration amount (Equation 2) obtained from the maximum and minimum penetration amounts of the brush member 11.
(Formula 2) Average penetration amount = (Maximum penetration amount + Minimum penetration amount)/2 (mm)
(Equation 3) Peak pressure = average pressure x maximum penetration amount / average penetration amount (gf/mm ² )

The above calculation method actually means linearly approximating the contact pressure acting on the surface of the photosensitive drum 1, which describes an arc as shown in Figure 6. Specifically, let's say a brush member 11 with a lateral width L3 of 4 mm is brought into contact with a photosensitive drum 1 with an outer diameter of 24 mm, from directly above the photosensitive drum 1, so that the penetration depth is 1.2 mm at the center in the lateral direction SD. In this case, the maximum penetration depth is 1.2 mm, the minimum penetration depth is 1.03 mm, and the average penetration depth is 1.115 mm, and the peak pressure can be calculated using (Equation 3).

The contact area ratio was measured by bringing the brush member 11 into contact with a glass plate as shown in Figure 5, and distinguishing the areas where the bristles of the brush member 11 were in contact (contact areas) and the areas where they were not in contact (non-contact areas) based on color. Figure 7(a) is an actual photograph observed under a microscope, and Figure 7(b) is an image of Figure 7(a) that was binarized so that the contact areas were white and the non-contact areas were black. The contact area ratio is the ratio of the area of the contact areas to the area of the object being observed (the number of white pixels divided by the total number of pixels). The maximum area contact ratio is the position where peak pressure is obtained, i.e., the contact area ratio at the center of the short side direction SD.

To summarize the above explanation, for an image forming apparatus having the brush member 11 as in this embodiment, the peak pressure and the maximum contact area ratio can be confirmed by the following procedure.
1) The outer diameter of the photosensitive drum 1, the bristle length L1 and width L3 of the brush member 11, and the shortest distance L2 from the base fabric 11b of the brush member 11 to the surface of the photosensitive drum 1 are measured, and the maximum penetration amount (L1-L2) is calculated (see Figures 3(b) and 3(c)).
2) The minimum intrusion amount is calculated based on the outer diameter of the photosensitive drum 1, the width L3 of the brush member 11, and the maximum intrusion amount measured in 1) above, based on the geometric relationship shown in FIG.
3) The average intrusion amount is calculated based on the maximum intrusion amount and the minimum intrusion amount obtained in 1) and 2) above, using (Equation 2).
4) Using the average intrusion calculated in 3) above, a compression test of the brush member 11 is carried out using the method shown in FIGS. 4 and 5, and the average pressure is obtained based on (Equation 1).
5) Using the maximum intrusion amount, average intrusion amount, and average pressure obtained in 1), 3), and 4), the peak pressure is calculated based on (Equation 3).
6) Using the maximum penetration amount obtained in 1) above, the brush member 11 is pressed against the glass plate using the method shown in FIG. 5, the contact surface is observed, and the maximum contact area ratio is calculated.

Furthermore, to achieve the desired peak pressure and maximum contact area ratio of the brush member 11, parameters such as the outer diameter of the photosensitive drum 1, the bristle length L1 and width L3 of the brush member 11, and the above-mentioned shortest distance L2 can be appropriately changed, and the peak pressure and maximum contact area ratio can be confirmed using the above procedure.

Using the above method, the peak pressure and maximum contact area ratio were calculated for each sample under multiple contact conditions with different maximum penetration amounts. Figure 1 shows a map plotting peak pressure on the vertical axis and maximum contact area ratio on the horizontal axis. In Figure 1, black dots indicate that no image defects occurred, while white dots indicate that image defects occurred.

As shown in Figure 1, image defects occurred when the peak pressure was less than 0.7 gf/ ^mm2 and the maximum contact area ratio was less than 18%. This is thought to be because when the peak pressure was too low, the contact between the bristles of the brush member 11 and the toner particles was weak, resulting in insufficient triboelectric charging of the residual toner to the normal polarity. Also, when the maximum contact area ratio was too low, the frequency of contact between the toner particles and the bristles of the brush member 11 was low in the peak pressure region, where triboelectric charging is most likely to occur, resulting in insufficient triboelectric charging of the residual toner to the normal polarity. In either case, if the residual toner reaches the charging section P2 without being sufficiently charged to the normal polarity at the brush contact point, residual toner charged to the non-normal polarity or residual toner with a charge amount close to zero will adhere to the charging roller 2, causing contamination of the charging roller 2.

In addition, image defects also occurred when the peak pressure was greater than 3.5 gf/ ^mm2 and the maximum contact area ratio was greater than 74%. This is thought to be because, in both cases where the peak pressure was too high and the maximum contact area ratio was too high, the remaining toner could not pass through some of the brush contact areas, concentrating in areas where the remaining toner could pass through (areas with relatively low contact pressure or bristle density). In this case, the remaining toner adhered to the surface of the photosensitive drum 1 that had passed through the brush contact area in stripes (lines extending in the direction of rotation), causing striped contamination of the charging roller 2. Note that in areas where the peak pressure or maximum contact area ratio was particularly high, the remaining toner was blocked by the brush member 11, which not only hindered the development roller 41 from collecting the remaining toner, but also potentially caused the blocked toner to scatter and contaminate the interior of the image forming apparatus.

Therefore, it is desirable to configure the brush member 11 so that the peak pressure and maximum contact area ratio at the brush contact portion fall within the following region enclosed by the dotted line in FIG.
Peak pressure: 0.7 gf/mm2 or ^more and 3.5 gf/mm2 or ^less Maximum contact area ratio: 18% or more and 74% or less

This allows the charge distribution of the residual toner to be stabilized at a normal polarity in a cleanerless brush system in which the brush member 11 scatters the residual toner adhering to the surface of the photosensitive drum 1 that has passed through the transfer portion P5. In other words, the charge distribution of the residual toner after it leaves the brush contact portion has a peak on the normal polarity side (negative polarity side) of the toner T, and is sharper than the charge distribution of the residual toner before it enters the brush contact portion.

The image defects described above are less likely to occur near the center of the area surrounded by the dotted line in Fig. 1 than near the periphery. Therefore, it is preferable to configure the brush member 11 so that the peak pressure and/or the maximum contact area ratio falls within the following ranges.
Peak pressure: 1.4 gf/mm2 or ^more and 2.8 gf/mm2 or ^less Maximum contact area ratio: 32% or more and 60% or less

Since 1 gf is approximately equal to 9.8 x mN (millinewtons), "0.7 gf/ ^mm2 or more and 3.5 gf/ ^mm2 or less" can be rephrased as "6.9 mN/ ^mm2 or more and 34 mN/ ^mm2 or less." Similarly, "1.4 gf/ ^mm2 or more and 2.8 gf/mm2 or ^less " can be rephrased as "14 mN/ ^mm2 or more and 28 mN/ ^mm2 or less."

Furthermore, in this embodiment, the Clark-Evans Index is introduced as an index that indicates whether the brush member 11 is contacting the photosensitive drum 1 evenly. When multiple points are distributed in a certain planar area, the Clark-Evans Index indicates the tendency of the points to be distributed in a locally concentrated manner or to be distributed at a distance from each other.

The method for calculating the Clark-Evans index will be described below. First, if the distance from point i to the nearest neighbor point is d_i and the number of points is n, the average value of the distance from each point to the nearest point (average nearest neighbor distance) W is expressed by the following equation (Equation 1).

Here, as an evaluation criterion, consider a case where points are randomly distributed (following a uniform Poisson distribution) on a plane of area S. In this case, the expected value E(W) of the average nearest neighbor distance W is expressed by the following equation (Equation 2):

In order to compare cases where the number of points or density is different, the average nearest neighbor distance W is normalized by the above expected value E(W) as expressed in the following equation (Equation 3), and the resulting w is called the Clark-Evans index.

To find the Clark-Evans index of the brush member 11, the brush member 11 is pressed against a glass surface as shown in Figure 5, and the brush contact area is observed from the glass surface opposite to where the brush member 11 is pressed. If the tips of the bristles in a certain area (100 mm ² ) at the brush contact area are represented by points, a distribution of points like that shown in Figure 7(c) is obtained. The Clark-Evans index is calculated from this distribution of points using the above equations (1) to (3).

Note that some bristles are in contact with the glass surface at the base rather than the tip. It is believed that the effect of such bristles in frictionally charging the toner T is largely due to the contribution of the point where the bristles are in strongest contact with the glass surface (the most pressured point). However, the distribution of the most pressured points of the bristles is largely the same as the distribution of the bristles' tips, and the properties of the distribution are almost the same. Therefore, in this embodiment, the Clark-Evans index is calculated from the distribution of the tips of each bristle in contact with the glass surface.

The Clark-Evans index is w=1 for a random distribution, w<1 for a clustered distribution, and w>1 for a regular distribution. An extreme example of a regular distribution is a grid-like distribution that covers the entire area of the object being observed. An extreme example of a clustered distribution is a distribution in which points are densely concentrated in one or a few places within the area of the object being observed.

The Clark-Evans index was calculated for an actual sample of the brush member 11 and the results are as follows:
Sample 1 w = 1.01
Sample 2 w = 1.13
Sample 3 w = 1.15
Sample 4 w = 1.07
Sample 2 was intentionally twisted into a bundle w=0.7

The properties of the Clark-Evans index indicate that if w>1, the tips of the bristles of the brush member 11 are loose and not bundled; conversely, if w<1, it suggests that the bristles of the brush member 11 are bundled (clusters, clumps) for some reason.

For the brush member 11 to function properly, the bristles of the brush member 11 must be loose and not bunched together when they contact the photosensitive drum 1. In this embodiment, the brush member 11 is configured so that the Clark-Evans index is 1 or greater (w≧1). This condition can also be said to be a condition that ensures that the bristles are not bunched together for some reason.

Note that depending on the configuration of the brush member 11, the Clark-Evans index value may differ depending on the location of the brush contact area. In such cases, the Clark-Evans index at the point where the brush member 11 penetrates the most (the point where the contact pressure reaches its peak) is assumed to be 1 or greater. This is because at the point where the penetration is greatest, the force applied to the bristles makes it easier for the bristles to clump together.

(Qualitative description of the phenomenon)
We will now explain how the phenomenon changes depending on the peak pressure and maximum contact area ratio calculated as above. Figures 8 and 9 show a case where the peak pressure or maximum contact area ratio is insufficient (Reference Example 1). Figure 8 is a schematic diagram showing the main components of the image forming unit 101. Figure 9 is an enlarged view of the same, showing three types of toner charge: positive (+), negative (-), and weak (0).

As shown in Figure 9, residual toner with a wide charge distribution enters the brush member 11. In this example, the peak pressure or maximum contact area ratio of the brush member 11 is insufficient, resulting in low frictional charging performance, so the residual toner passes through the brush contact area while maintaining a wide charge distribution. When the residual toner then reaches the charging section P2, as described above, toner that is positively charged in particular may adhere to the charging roller 2, causing poor charging, or may not be collected by the developing roller 41, resulting in white background fogging. Furthermore, if there is a large amount of weakly charged toner, it may not be possible to impart a sufficient negative charge at the charging section P2, in which case the weakly charged toner is difficult to collect by the developing roller 41.

Next, we will explain what happens when the peak pressure or maximum contact area ratio is too high (Reference Example 2). Figure 10(a) is a schematic diagram of a model test in which a glass plate, serving as a model of the photosensitive drum 1 in Figure 5, is pressed firmly against the brush member 11 (maximum penetration amount: 3 mm) and toner T is supplied. It can be seen that toner is supplied along with the movement of the glass plate, creating a flow of toner T.

When the peak pressure or maximum contact area ratio is high, the brush member 11 is pressed firmly against the photosensitive drum 1. As shown in Figure 10(a) at the location indicated by X, in the brush contact area, where the bristles (conductive threads 11a) are bundled (collected), the brush is pressed more firmly against the photosensitive drum 1, and the passage of toner T is strongly restricted. Conversely, at the location indicated by Y, the conductive threads are not bundled, and the flow of toner T converges, resulting in a large amount of toner T slipping through.

In Figure 10(b), x schematically represents the amount and charge of toner adhering to the portion of the surface of the photosensitive drum 1 that has passed through the brush contact area, corresponding to the location indicated by X in Figure 10(a). In Figure 10(b), y schematically represents the amount and charge of toner adhering to the portion of the surface of the photosensitive drum 1 that has passed through the brush contact area, corresponding to the location indicated by Y in Figure 10(a). In locations (X, x) where toner T is difficult to remove, the toner T is rubbed strongly against the brush member 11 as it is removed, and most of the toner T is given a negative charge. On the other hand, in locations (Y, y) where toner T is concentrated and removed, some of the toner T is likely to pass through without sufficient rubbing against the brush member 11, and the amount of toner is also large. As a result, the residual toner adhering in a streak-like manner to the location (Y, y) where toner T is concentrated and removed adheres to the charging roller 2, making it more likely that the charging roller 2 will be stained in a striped pattern.

Figures 11 and 12 show diagrams illustrating this embodiment. Figure 11 is a schematic diagram showing the main components of the image forming unit 101. Figure 12 is an enlarged view of that diagram, and, like Figure 9, shows three types of toner charge. Compared to Figures 8 and 9, the brush member 11 is pressed strongly against the photosensitive drum 1, and the contact conditions are set so that the peak pressure and maximum contact area ratio do not become excessive (Figure 10).

As shown in Figure 12, the brush member 11 of this embodiment has an appropriate peak pressure and maximum contact area ratio, so that residual toner with a wide charge distribution rubs against the bristles (conductive threads 11a) of the brush member 11 as it passes through the brush contact area, causing frictional charging. Furthermore, because the peak pressure and maximum contact area ratio of the brush member 11 are not excessively large and are configured to satisfy the Clark-Evans index w≧1, residual toner that has passed through the brush contact area does not concentrate in streaks. This reduces the likelihood of contamination of the charging roller 2 or poor collection of residual toner by the developing roller 41, ensuring high image quality over a long period of time.

Second Embodiment
A second embodiment of the present disclosure will be described. This embodiment differs from the first embodiment in that a voltage is applied to the brush member 11. Hereinafter, elements with the same reference numerals as those in the first embodiment will be considered to have substantially the same configurations and functions as those described in the first embodiment, and differences from the first embodiment will be mainly described.

As shown in the schematic diagram of Figure 13, in the cleanerless brush system, toner particles can become entangled and trapped near the base of the bristles (conductive threads 11a) of the brush member 11. The toner T trapped in this area is generally pushed downstream in the direction of rotation of the photosensitive drum 1 as new residual toner successively reaches the brush contact area. However, compared to the toner T that passes through the brush contact area while rubbing against the tips of the bristles of the brush member 11 and rolling, the toner T that is trapped at the base of the bristles and then pushed out tends to have an insufficient amount of charge of the normal polarity (negative polarity).

In this embodiment, therefore, a voltage is applied to the brush member 11 to urge the toner T toward the area where the bristles of the brush member 11 come into contact with the surface of the photosensitive drum 1 (on the photosensitive drum 1 side).

In this embodiment, during image formation, the surface of the photosensitive drum 1 is charged to a dark potential Vd of -700V at the charging section P2. The image area on the photosensitive drum 1 is exposed by the exposure device 3 and becomes a light potential Vl of -100V. Then, by passing through the transfer section P5, where a transfer voltage of +1000V is applied to the transfer roller 5, the dark potential becomes approximately -200V and the light potential becomes approximately -50V. Therefore, the surface potential of the photosensitive drum 1 that reaches the brush contact point during image formation is approximately -50V to approximately -200V.

As shown in Figure 14, the brush member 11 is electrically connected to a brush power supply E11 as a voltage application means. During image formation, a predetermined brush voltage E is applied to the brush member 11 by the brush power supply. The predetermined brush voltage E is a potential that has the same polarity as the normal polarity of the toner T with respect to the surface potential of the photosensitive drum 1 that reaches the brush contact point during image formation (particularly the potential after passing through the transfer section where the dark area potential is higher than the light area potential). In this embodiment, a voltage of -400V is applied to the brush member 11.

Negatively charged (normally charged) toner T entering the brush contact area is electrostatically biased toward the photosensitive drum 1 due to the potential difference between the brush voltage E (-400V) and the surface potential of the photosensitive drum 1 (-50V to -200V). This forces the negatively charged toner T against the photosensitive drum 1, where it rolls while in contact with the bristles of the brush member 11 and the surface of the photosensitive drum 1, thereby providing sufficient frictional charging. By sufficiently frictionally charging the toner T at the brush contact area, the charge distribution of the residual toner can be stabilized at normal polarity, preventing problems such as contamination of the charging roller 2 and poor collection of residual toner by the developing roller 41.

Here, the brush voltage E is set to a value that does not cause discharge between the photosensitive drum 1 and the brush. This is because unnecessary discharge can contaminate the photosensitive drum 1 with discharge products and accelerate deterioration.

In this embodiment, as in the first embodiment, the brush member 11 is arranged approximately parallel to the surface of the photosensitive drum 1. Therefore, as shown in Figure 14, the contact pressure between the brush member 11 and the photosensitive drum 1 is maximum (peak value) at the center of the brush contact area in the width direction.

Here, among the remaining toner particles that enter the brush contact area, toner T that is positively charged (irregular polarity) is more likely to be attracted to the base of the bristles when it enters the brush contact area because brush voltage E is applied to brush member 11. Toner T attracted to the base of the bristles is pushed downstream by newly supplied toner T from upstream, and as it moves downstream, it becomes frictionally charged to negative polarity as it rubs against the bristles. As this negatively charged toner T moves inside brush member 11, it is pressed against photosensitive drum 1 by brush voltage E, and rolls while in contact with the bristles of brush member 11 and the surface of photosensitive drum 1, becoming frictionally charged. However, if the negative polarity of toner T attracted to the base of the bristles is delayed, toner T with an insufficient charge may slip through brush member 11.

Therefore, in this embodiment, in order to quickly negatively polarize the toner attracted to the base of the bristles, it is desirable to set lower limits on the contact pressure and contact area ratio of the brush member 11 at the most upstream position of the brush contact area in the rotation direction R1 of the photosensitive drum 1. As an example, the penetration amount of the brush member 11 at the most upstream position of the brush contact area is set to 1.2 mm, and the penetration amount of the brush member 11 at the center position of the brush contact area in the short direction (rotation direction R1) is set to 1.34 mm. As a result, the following relationship is satisfied at the most upstream position of the brush contact area.
Contact pressure ≧0.7 gf/ ^mm2
Contact area rate ≧18%

The conditions for peak pressure, maximum contact area ratio, and Clark-Evans index are the same as in the first embodiment.

By setting the contact pressure and contact area ratio at the most upstream position of the brush contact area as described above, the toner T can be quickly converted to negative polarity at the upstream portion of the brush contact area, as shown in Figure 15, making it difficult for the toner T to accumulate at the base of the bristles of the brush member 11. Furthermore, by sufficiently frictionally charging the toner T at the brush contact area, the charge distribution of the remaining toner can be more stabilized at a normal polarity.

In this embodiment, the toner T is pressed toward the photosensitive drum 1 by the action of the brush voltage E, so even if the brush member 11 is not necessarily located on the positive polarity side (non-normal polarity side) relative to the toner T in the charging series, the toner T can be made negatively polarized at the brush contact point. However, a configuration in which the brush member 11 is located on the positive polarity side relative to the toner T in the charging series is more advantageous for making the toner T negatively polarized.

In addition, in this embodiment, the main function of the brush voltage E has been described from the perspective of pressing the toner T that has entered the inside of the brush member 11 toward the photosensitive drum 1. However, this is not limiting, and the brush voltage E may be applied to the brush member 11 to inject (supply) a charge of normal polarity into the toner T via the brush member 11, thereby promoting the negative polarity of the toner T. Furthermore, the application of the brush voltage E may simultaneously perform the function of pressing the toner T toward the photosensitive drum 1 and the function of injecting charge.

Third Embodiment
A third embodiment of the present disclosure will be described. This embodiment differs from the second embodiment in that the brush member 11 is disposed at an angle with respect to the photosensitive drum 1. Hereinafter, elements having the same reference numerals as those in the first or second embodiment will be considered to have substantially the same configurations and functions as those described in the first or second embodiment, and differences from the first or second embodiment will be mainly described.

Figures 16 and 17 show diagrams illustrating this embodiment. Figure 16 is a schematic diagram showing the main components of the image forming unit 101. Figure 17 is an enlarged view of that diagram, and, like Figure 9, shows three types of toner charge.

In this embodiment, the brush member 11 is positioned at an angle with respect to the tangent direction of the photosensitive drum 1 so that the contact pressure and contact area ratio at the most upstream portion of the brush contact area in the rotation direction R1 of the photosensitive drum 1 become the peak pressure and maximum contact area ratio. The tangent line TL of the photosensitive drum 1 is defined as the tangent line of the photosensitive drum 1 at the intersection 1a of the photosensitive drum 1 with a straight line drawn perpendicular to the lateral direction SD (the direction in which the base fabric 11b extends when viewed longitudinally) from the center position of the brush member 11 in the lateral direction SD. In this embodiment, the brush member 11 is tilted in such a way that the base fabric 11b of the brush member 11 moves farther away from the tangent line TL as it moves downstream in the rotation direction R1.

The angle between the short-side direction SD of the brush member 11 and the tangent line TL is the inclination angle of the brush member 11. In this embodiment, it is preferable to set the inclination angle to, for example, 12 degrees and to position the brush member 11 so that the penetration amount (maximum penetration amount) at the most upstream part of the brush contact area is 1.2 mm. In this embodiment as well, the following relationship is satisfied at the most upstream position of the brush contact area.
Contact pressure ≧0.7 gf/ ^mm2
Contact area rate ≧18%
The inclination angle and the penetration amount of the brush member 11 can be changed as appropriate depending on the outer diameter of the photosensitive drum 1, the required peak pressure, and the contact area ratio.

The conditions for peak pressure, maximum contact area ratio, and Clark-Evans index are the same as in the first embodiment.

In this embodiment, the toner T that enters the brush contact area is pressed toward the photosensitive drum 1 by the action of the brush voltage E, as in the second embodiment. This allows the toner T to be sufficiently frictionally charged at the brush contact area, stabilizing the charge distribution of the remaining toner at a normal polarity and preventing problems such as contamination of the charging roller 2 and poor collection of the remaining toner by the developing roller 41.

In addition, in this embodiment, as shown in Figure 17, the contact pressure and contact area ratio are maximized at the most upstream part of the brush contact area. Therefore, among the residual toner that enters the brush contact area, toner T that is positively charged (irregular polarity) can be quickly converted to negative polarity by friction with the bristles of the brush member 11, and the charge distribution of the residual toner can be more stabilized at regular polarity.

In this embodiment, the brush member 11 is arranged at an angle so that the contact pressure and contact area ratio are maximized at the most upstream portion of the brush contact area. However, this is not limiting, and for example, the bristle length of the brush member 11 may be configured to decrease from one side in the short direction SD (the upstream side in the rotation direction R1) to the other side (the downstream side), thereby maximizing the contact pressure and contact area ratio at the most upstream portion of the brush contact area.

(Other embodiments)
In the above-described embodiments, the charging roller 2 is a contact charging type charging member, but a charging member other than a contact charging type (for example, a corona discharge type) may be used. Even in this case, by applying the configuration described in each embodiment, it is possible to at least suppress the failure of the remaining toner collection on the developing roller 41.

While the above-described embodiments have been described as employing a direct transfer system in which a toner image is transferred directly from the photosensitive drum 1 (image carrier) to a sheet (recording material) as a receiver, the present technology may also be applied to an intermediate transfer image forming apparatus. In the case of an intermediate transfer system, the transfer member refers to, for example, a transfer roller (primary transfer roller) that performs primary transfer of the toner image from the photosensitive drum 1 (image carrier) to an intermediate transfer material (transfer material). The intermediate transfer material may be an endless belt stretched over multiple rollers. The toner image primarily transferred to the intermediate transfer material is then secondarily transferred from the intermediate transfer material to the sheet (recording material) by a secondary transfer means, such as a secondary transfer roller that forms a secondary transfer nip between the intermediate transfer material and the belt. Even in such an intermediate transfer system, the same effects as those of the above-described embodiments can be achieved by replacing the transfer roller in the above-described embodiments with a primary transfer roller.

1... Image carrier (photosensitive drum) / 5... Transfer member (transfer roller) / 11... Brush member / 41... Development member (development roller)

Claims (13)

a rotating image carrier;
a developing member that develops the electrostatic latent image formed on the surface of the image carrier using toner in a developing section;
a transfer member that transfers the toner image developed by the developing member from the image carrier to a transfer-receiving member in a transfer section;
a brush member that contacts the surface of the image carrier at a contact portion that is downstream of the transfer portion and upstream of the development portion in the rotation direction of the image carrier;
an image forming apparatus comprising: a developing member for recovering toner that has not been transferred to the transfer object;
In the triboelectric series, the toner is located on the same side as the normal charge polarity of the toner with respect to the brush member,
the maximum value of the contact pressure at the contact portion is 0.7 gf/ ^mm² or more and 3.5 gf/ ^mm² or less,
a maximum contact area ratio in the contact portion is 18% or more and 74% or less;
The Clark-Evans index of the brush member at the contact portion is 1 or more.
An image forming apparatus characterized by: a rotating image carrier;
a developing member that develops the electrostatic latent image formed on the surface of the image carrier using toner in a developing section;
a transfer member that transfers the toner image developed by the developing member from the image carrier to a transfer-receiving member in a transfer section;
a brush member that contacts the surface of the image carrier at a contact portion downstream of the transfer member and upstream of the developing member in the rotation direction of the image carrier;
a voltage applying means for applying a voltage to the brush member;
an image forming apparatus comprising: a developing member for recovering toner that has not been transferred to the transfer object;
the voltage applied to the brush member by the voltage application means is on the same side as the normal charging polarity of the toner with respect to the surface potential of the image carrier that reaches the contact portion;
the maximum value of the contact pressure at the contact portion is 0.7 gf/ ^mm² or more and 3.5 gf/ ^mm² or less,
a maximum contact area ratio in the contact portion is 18% or more and 74% or less;
The Clark-Evans index of the brush member at the contact portion is 1 or more.
An image forming apparatus characterized by: In the triboelectric series, the toner is located on the same side as the normal charge polarity of the toner with respect to the brush member.
3. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium. At the most upstream portion of the contact portion in the rotation direction of the image carrier,
The contact pressure at the contact portion is 0.7 gf/mm ² or more,
a contact area ratio at the contact portion is 18% or more;
The Clark-Evans index is greater than or equal to 1.
4. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium. the contact pressure and the contact area ratio at the contact portion are maximum at the most upstream portion of the contact portion in the rotation direction of the image carrier;
5. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium. the brush member has a base portion extending in a longitudinal direction parallel to the rotational axis direction of the image carrier and in a lateral direction perpendicular to the longitudinal direction, and bristles supported by the base portion;
When viewed in the direction of the rotation axis, the brush member is disposed at an incline with respect to the image carrier so that the base portion becomes farther away from the tangent line of the image carrier toward the downstream side in the rotation direction.
6. The image forming apparatus according to claim 5, the maximum value of the contact pressure at the contact portion is 1.4 gf/mm ² or more and 2.8 gf/mm ² or less,
The maximum contact area ratio at the contact portion is 32% or more and 60% or less.
7. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium. a charging member that charges the surface of the image carrier at a charging section;
the brush member is disposed downstream of the transfer unit and upstream of the charging unit in the rotation direction of the image carrier;
8. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium. the charging member is a charging roller that is arranged in contact with the image carrier and rotates in the charging section;
9. The image forming apparatus according to claim 8, In the triboelectric series, the toner is located on the same side as the normal charging polarity of the toner with respect to the material of the surface layer of the charging member and the material of the surface layer of the image bearing member.
10. The image forming apparatus according to claim 8, wherein the image forming apparatus is a recording medium. The brush member has a base and bristles supported by the base,
The hair material is a synthetic resin fiber having a thickness of 1 denier or more and 6 denier or less,
The density of the bristle material is 150 kF/inch ² or more and 350 kF/inch ² or less,
11. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium. the transfer medium is a recording material,
12. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium. the transfer medium is an intermediate transfer medium,
a secondary transfer unit that transfers the toner image transferred to the intermediate transfer body onto a recording material;
12. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.