JP4262191B2 - Image forming apparatus having contact charging means and process cartridge included in image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a contact charging unit and a process cartridge included in the image forming apparatus.

  2. Description of the Related Art Conventionally, for example, in an image forming apparatus employing an electrophotographic method, as a means for charging a surface of a charged body such as a photoreceptor, a charging member is directly contacted with the surface of the charged body to charge the surface of the charged body. So-called contact charging means is widely known.

  Here, the electrophotographic image forming apparatus includes, for example, a copying machine, a printer (for example, an LED printer, a laser beam printer, etc.), a facsimile apparatus, and a word processor.

  The process cartridge is a cartridge in which a charging unit, a developing unit, a cleaning unit, and an electrophotographic photosensitive member (a member to be charged) are integrally formed, and the cartridge can be attached to and detached from the main body of the image forming apparatus. Or at least the electrophotographic photosensitive member and the charging means are integrated into a cartridge so as to be detachable from the main body of the image forming apparatus.

  In Patent Document 1, in order to ensure contact of the charging roller of the contact charging means with the surface of the photosensitive drum as the member to be charged, the charging roller is continuously removed from the longitudinal center to both ends of the charging roller. A so-called crown shape with a small diameter has been proposed.

  This will be described with reference to FIGS. In FIG. 5, reference numeral 101 denotes a photosensitive drum as an object to be charged which has an organic photosensitive layer and is driven to rotate at a predetermined process speed. Reference numeral 102 denotes a charging roller as a contact charging member. The conductive cored bar 102a and an elastic layer 102b coated on the outer peripheral surface of the conductive cored bar 102a. The both ends of the conductive core metal 102a are held by bearings 103, respectively. Further, the bearings 103 are urged with a predetermined pressure F in the direction of the photosensitive drum by the pressurizing springs 104 that are contracted between the fixed spring receiving members 107. As a result, the charging roller 102 is brought into pressure contact with the photosensitive drum 101.

  The charging roller 102 is driven to rotate as the photosensitive drum 101 rotates. Then, for example, an oscillating voltage in which an alternating current component is superimposed on a direct current component is applied as a charging bias from a high voltage power source 105 through a member such as a contact 106, a pressure spring 104, a bearing 103, and a conductive metal core 102a. As a result, the surface of the photosensitive drum 101 is charged to a predetermined potential.

The bearing 103 is made of a plastic material. Of these, at least the bearing on the side that supplies power to the charging roller 102 has a resistance value of 10 ³ to 10 ⁴ [Ω · cm] by dispersing carbon fiber or the like in the plastic material at the time of molding. Is made conductive.

In order to ensure contact with the surface of the photosensitive drum, the charging roller 102 continuously decreases in outer diameter from the central portion in the longitudinal direction of the charging roller toward both ends as shown in FIG. The so-called crown shape is adopted. As described above, the charging roller 102 is urged by the pressure spring 104 at both ends in the longitudinal direction and is in contact with the surface of the photosensitive drum, so that the charging roller 102 is bent in a bow shape. For example, in the case of a so-called straight-shaped charging roller having a substantially cylindrical shape, inconveniences such as local charging failure due to the formation of a minute gap between the photosensitive drum and the charging roller in the central portion in the longitudinal direction. The purpose is to prevent. That is, by making the charging roller a crown shape, the central portion in the longitudinal direction is prevented from floating from the surface of the photosensitive drum.
JP-A-4-230776 (Fig. 2)

  The above-described prior art sufficiently satisfies the performance desired at that time. However, in recent years, further improvement in performance has been demanded.

  The present invention is a further development of the above prior art. The purpose is to keep the pressing force of the charging member against the surface of the object to be charged as low as possible, but to make sure that the charging member is in contact with the surface of the object to be charged, thereby further preventing the occurrence of charging failure and permanent deformation of the charging member. It is to suppress.

In order to achieve the above object, a typical configuration of the image forming apparatus according to the present invention includes at least a rotatable charged body and a surface of the charged body that is in contact with the charged body surface to charge the charged body surface to a predetermined potential. In the image forming apparatus having a rotatable charging member having an elastic portion, the outer diameter of the elastic portion of the charging member continuously increases from the central portion in the longitudinal direction toward both ends. It has an inverted crown shape, and the rotational axis of the member to be charged and the rotational axis of the charging member are in a positional relationship where they intersect at a predetermined angle θ at the center of the member to be charged and the charging member. And

In addition, a typical configuration of the process cartridge according to the present invention for achieving the above object includes at least a rotatable charged body and a surface of the charged body that is in contact with the surface of the charged body and has a predetermined potential. In a process cartridge that is charged and has a rotatable charging member having an elastic part and is detachable from the main body of the image forming apparatus, the elastic part of the charging member extends from the longitudinal center to both ends. It has a so-called inverted crown shape whose outer diameter is continuously increased, and the rotating shaft of the charged body and the rotating shaft of the charging member are a predetermined portion at the center of the charged body and the charging member . It is characterized by being in a positional relationship that intersects with an angle θ.

  According to the present invention, a positive biasing force such as a pressure spring is used to make the charging member have an inverted crown shape and to be placed in a positional relationship that intersects the charged body at an angle. The charging member can be brought into contact with the surface of the body to be charged substantially while keeping the pressure applied to the surface of the body to be charged as low as possible without requiring any means, and charging failure or permanent deformation of the charging member can be achieved. Etc. can be further suppressed.

  Examples according to the present invention are shown below. FIG. 1 is a schematic configuration diagram of an image forming apparatus provided with contact charging means according to the present invention.

(1) Overall Description This image forming apparatus of an image forming apparatus using an electrophotographic process, a contact charging system, a process cartridge detachable, a laser over beam printer.

Reference numeral 1 denotes a photosensitive drum as an image bearing member (charged member). A photosensitive material such as OPC is formed on the outer peripheral surface of a cylindrical substrate such as aluminum, and the outer diameter thereof is 30 [mm]. When the image forming apparatus receives a print command from a host device (not shown) such as a personal computer, the photosensitive drum 1 is rotationally driven at a peripheral speed of about 100 mm / sec in the direction of the arrow. By the charging roller 2 serving as the rotation driving photosensitive drum surface is a contact charging member is uniformly charged to about -600 [v] as a dark portion potential V _D. The charging roller 2 is a vibration in which an AC voltage having a frequency of 1000 Hz and a peak-to-peak voltage of 1800 V is superimposed on a DC voltage of −610 V, for example, as a charging bias from a charging bias application power source (not shown). A voltage is applied.

Next, the uniformly charged surface of the photosensitive drum is subjected to scanning exposure with the laser beam 3a which is ON / OFF controlled according to the image information by the laser scanner 3, and the bright portion potential _VL is about −150 [ v] electrostatic latent image is formed.

  The electrostatic latent image formed in this way is developed and visualized by the developing device 4 disposed opposite to the photosensitive drum 1. The developing device 4 includes a developing roller 4a that is rotationally driven, and includes a negatively charged toner 4b. The developing roller 4a is supplied with a developing bias from a charging bias application power source (not shown). For example, an oscillating voltage obtained by superimposing an AC voltage having a frequency of 2000 [Hz] and a peak-to-peak voltage of 1800 [v] on a DC voltage of −350 [v] is applied. As a developing method, for example, a jumping developing method is used, and image exposure and reversal development are used in combination.

  On the other hand, the sheet feeding roller 11 is driven at a predetermined control timing, the transfer material (recording material) P in the sheet feeding cassette 12 is separated and fed, and is conveyed through the sheet path 13 including the registration roller 14. The sheet is fed at a predetermined timing to the transfer portion 6 that is pressed against the photosensitive drum 1 and faces the transfer roller 5 that is rotationally driven. As a result, the toner image on the photosensitive drum 1 is electrostatically transferred onto the surface of the transfer material P at the transfer portion 6. A voltage (transfer bias) having a polarity opposite to the normal charging polarity of the toner is applied to the transfer roller 5 from a transfer bias applying power source (not shown) while the transfer material P is nipped and conveyed through the transfer portion 6. The

  The transfer roller 5 is obtained by coating the outer peripheral surface of a core metal such as SUS with foamed rubber in which a conductive material is dispersed in, for example, epichlorohydrin rubber, and molding the outer diameter to 16 [mm].

The transfer material P is separated from the photosensitive drum surface at the transfer portion 6 and is conveyed to the fixing device 8 through the sheet path 15, and the transferred toner image is fixed as a fixed image. Then, it passes through the sheet path 16 and is discharged to the paper discharge tray unit 17 as an image formed product (print, copy).

  In addition, the surface of the photosensitive drum (the surface of the photosensitive drum after the transfer material is separated) on which the toner image is transferred to the transfer material P at the transfer portion 6 is cleaned by removing a residual amount of transfer residual toner by the cleaning device 7. Repeatedly used for image formation. The cleaning device 7 includes a cleaning blade 7 a having an elastic member made of urethane rubber or the like at the tip of a support member made of sheet metal or the like, and the tip of the elastic member is placed on the surface of the photosensitive drum 1. The transfer residual toner is removed from the surface of the photosensitive drum by contacting with a predetermined pressing force in a so-called counter direction.

  In this embodiment, a process cartridge 10 is formed by integrating the photosensitive drum 1, the charging roller 2, the developing device 4, and the cleaning device 7. The process cartridge 10 is detachable from the main body of the image forming apparatus. It has become. Reference numeral 18 denotes a mounting support member for the process cartridge 10.

(2) Charging Member FIG. 2 is a schematic diagram showing the shape of the charging roller 2 as a contact charging member. The charging roller 2 was produced by the following method. That is, EPDM compound having a Mooney viscosity of 20 at 100 [° C.]: 100 [parts by weight], conductive carbon: 10 [parts by weight], EPDM compound: 50 [parts by weight], and silicone rubber: 100 [parts by weight] , Dicumyl peroxide: 3 [parts by weight] were mixed to prepare a silicone rubber, and then the silicone series was concentrically with a 6 mm diameter SUS round bar as the conductive core 2a. The rubber is vulcanized by transfer molding at 160 [° C.] and 10 [min], the length is 230 [mm], the outer diameter is 11.90 [mm] in the center in the longitudinal direction, and the outer diameter is 12.02 [both ends]. mm] reverse-crown conductive rubber roller portion (elastic portion) 2b.

The resistance of each part along the longitudinal direction of the conductive rubber roller is wound with an aluminum foil having a width of 10 [mm] closely attached to the outer periphery of the roller, and a voltage of 250 [v] is applied between the conductive core metal and the aluminum foil. When applied and measured with a tester, the resistance value was almost stable in the range of 8 × 10 ^{6 to} 5 × 10 ⁷ [Ω · cm].

Furthermore, the conductive rubber roller is coated with a coating material in which carbon 3% dispersed in methylolated nylon is coated to a film thickness of 80 [μm] to form a resistance layer. Finally, as shown in FIG. A so-called 60 [μm] reverse crown-shaped charging roller 2 having an outer diameter R _{C of} 12.06 [mm] at the central portion in the longitudinal direction and an outer diameter R _{S of} 12.18 [mm] at both ends was produced.

Next, a method for bringing the charging roller 2 into contact with the surface of the photosensitive drum 1 will be described. In the present embodiment, as shown in the schematic diagram of FIG. 3, the charging roller 2 is relative to the photosensitive drum 1 with respect to the axis DD ′ of the rotating shaft 1 a of the photosensitive drum 1. Are in a positional relationship in which the axis CC ′ intersects at a predetermined angle θ in the central portion of the photosensitive drum 1 and the charging roller 2. In this embodiment, the crossing angle θ is arranged with an inclination of 0.3 °. Also, as shown in FIG. 4, the distance between the opposite ends of the conductive core 2a of the charging roller 2 is such that the distance from the center of the rotating shaft of the opposing photosensitive drum 1 becomes a predetermined value. It is fixedly supported by the fixing member 11 and does not have a biasing member such as a pressurizing spring that can be displaced and changed.

  That is, the charging roller 2 that is a contact charging member is supported by the fixing member 11 that does not have an urging member that can be displaced and displaced so as to substantially press the charging roller 2 toward the surface of the photosensitive drum at both ends in the longitudinal direction. As a result, the elastic portion 2b is brought into contact with the surface of the photosensitive drum. In FIG. 4, the rotation shaft 1a of the photosensitive drum 1 and the rotation shaft 2a of the charging roller 2 have an inter-shaft distance fixing member 11 so that the distance L becomes a predetermined value at both ends in the longitudinal direction. Is supported by

  As described above, the charging roller 2 has an inverted crown shape and has a positional relationship that intersects with the photosensitive drum 1 at an angle as described above. Even if the biasing means is not required, it can be brought into contact with the surface of the photosensitive drum so as to substantially follow.

According to the study by the present inventors, the distance L [mm] between the charging roller 2 and the photosensitive drum 1 is the radius R _C / 2 at the center in the longitudinal direction of the charging roller 2 and the radius of the photosensitive drum 1. A value obtained by subtracting the half value of the reverse crown amount of the charging roller 2 from the sum of R _D / 2, ie,
L = (R _C + R _D ) / 2-((R _S -R _C ) / 2) / 2 = (R _C + R _D ) / 2- (R _S -R _C ) / 4
It was found preferable to set to.

Further, as the reverse crown amount of the charging roller 2, if the value is too small, it becomes close to a straight shape, so that an area where the charging roller 2 is not sufficiently in contact with the surface of the photosensitive drum is likely to be generated. for the contact pressure against the photosensitive drum surface at both longitudinal ends becomes stronger if too large, the outer diameter R _S of the longitudinal ends is 0.5 than the outer diameter R _C of the central portion [ %] Or more and 10 [%] or less, and more preferably 1 [%] or more and 5 [%] or less.

  Further, if the crossing angle θ of the charging roller 2 with respect to the photosensitive drum 1 is too small, an area where the charging roller 2 is not sufficiently in contact with the surface of the photosensitive drum 1 is likely to be generated. In this case, the space required for arranging the photosensitive drum 1 in the short side direction becomes large, leading to an increase in the size of the apparatus. Further, it is preferable that the angle be 0.2 [°] or more and 0.4 [°] or less.

  The contact charging means is disposed in the main body of the image forming apparatus, and the photosensitive drum surface is charged by applying the vibration voltage as a charging bias to the conductive core 2a of the charging roller 2 to output an image. No image failure due to charging failure or the like occurred, and a good image could be output stably.

  In addition, after the process cartridge 10 having this configuration is stored stationary for one month in a harsh environment of 40 [° C.] and 95 [%], an image is output using the process cartridge 10 to confirm the presence or absence of defects. The acceleration test on permanent deformation of the charging roller 2 was performed.

  As a comparative example, a charging roller having a so-called crown shape of 60 [μm] is urged by a pressure spring (one side 700 [gf]) at both ends in the longitudinal direction of the conductive mandrel against the surface of the photosensitive drum. The process cartridge which was brought into pressure contact was also tested at the same time.

  As a result, all of the process cartridges according to the present invention did not cause any problem even if an image of any image pattern was output. However, in the process cartridge as a comparative example, in the part in the longitudinal direction, However, streaks corresponding to the rotation period due to permanent deformation of the charging roller were observed particularly in the halftone image.

1 is a schematic configuration diagram of an image forming apparatus including a contact charging unit according to Embodiment 1. FIG. It is a figure which shows the shape of a charging roller. It is a figure which shows arrangement | positioning of a charging roller and a photosensitive drum. It is a figure which shows the positioning method of a charging roller and a photosensitive drum. It is a figure which shows the prior art example of a contact charging means. It is a figure which shows the shape of the conventional charging roller.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum, 2 ... Charging roller, 11 ... Interaxial distance fixing member

Claims (10)

In an image forming apparatus comprising at least a rotatable member to be charged and a rotatable charging member having an elastic portion that contacts the surface of the member to be charged and charges the surface of the member to be charged to a predetermined potential.
The elastic portion of the charging member has a so-called reverse crown shape in which the outer diameter continuously increases from the longitudinal central portion toward both ends, and the rotating shaft of the charged body and the charging member An image forming apparatus, characterized in that a rotational axis is in a positional relationship that intersects with a predetermined angle θ at a central portion of the charged body and the charging member .   2. The image according to claim 1, wherein an outer diameter of both ends in the longitudinal direction of the charging member is larger than an outer diameter of a central portion in a range of 0.5% to 10%. Forming equipment.   The angle θ formed by the rotation axis of the member to be charged and the rotation axis of the charging member is 0.1 [°] or more and 0.5 [°] or less. Image forming apparatus.   The charging member is supported at its both ends in the longitudinal direction by a fixing member that does not have a biasing member that can be displaced and displaced so as to substantially press the charging member toward the surface of the member to be charged. The image forming apparatus according to claim 1, wherein the surface of the image forming apparatus is brought into contact with the surface of the member to be charged.   The rotating shaft of the member to be charged and the rotating shaft of the charging member are supported by an inter-shaft distance fixing member so that the distance between them is a predetermined value at both ends in the longitudinal direction. The image forming apparatus according to claim 1. The image forming apparatus main body includes at least a rotatable member to be charged and a rotatable charging member having an elastic portion that contacts the surface of the member to be charged and charges the surface of the member to be charged to a predetermined potential. On the other hand, in the process cartridge that can be attached and detached,
The elastic portion of the charging member has a so-called reverse crown shape in which the outer diameter continuously increases from the longitudinal central portion toward both ends, and the rotating shaft of the charged body and the charging member A process cartridge provided in an image forming apparatus, characterized in that a rotational axis is in a positional relationship where the object to be charged and a central portion of the charging member intersect at a predetermined angle θ.   7. The image according to claim 6, wherein an outer diameter of both ends in the longitudinal direction of the charging member is larger than an outer diameter of the central portion in a range of 0.5% to 10%. Process cartridge provided in the forming apparatus.   The angle θ formed by the rotation axis of the member to be charged and the rotation axis of the charging member is 0.1 [°] or more and 0.5 [°] or less, 8 or 7, A process cartridge included in the image forming apparatus.   The charging member is supported at its both ends in the longitudinal direction by a fixing member that does not have a biasing member that can be displaced and displaced so as to substantially press the charging member toward the surface of the member to be charged. The process cartridge provided in the image forming apparatus according to claim 6, wherein the toner is brought into contact with the surface of the member to be charged.   The rotating shaft of the member to be charged and the rotating shaft of the charging member are supported by an inter-shaft distance fixing member so that the distance between them is a predetermined value at both ends in the longitudinal direction. A process cartridge provided in the image forming apparatus according to claim 6.

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