JP3305180B2 - Charging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device used in an image forming apparatus such as a laser printer or a copying machine using an electrophotographic system.

[0002]

2. Description of the Related Art Generally, an image forming apparatus includes a photosensitive member as a member to be charged, a charging device for charging the photosensitive member to a predetermined potential, an exposure device for forming an electrostatic latent image on the photosensitive member, and a developing device for developing the electrostatic latent image. Developing device for visualizing with an agent (hereinafter referred to as “toner”);
A transfer device for transferring the visualized toner image on the photoconductor onto transfer paper (recording medium), a fixing device for fixing the toner on the transfer paper, a cleaning device for removing the toner remaining on the photoconductor after transfer, etc. The toner image is formed with the rotation of the photoconductor, the toner image is transferred to a transfer sheet, and then the toner on the transfer sheet is fixed.

[0003] A corona charger is widely used in the conventional charging device. The corona charger has an advantage that the surface of the photoreceptor can be uniformly and uniformly charged to a predetermined potential. In addition, a high voltage is required because a high voltage is required, and a large amount of ozone is generated during corona discharge.

For this reason, instead of corona chargers, contact type charging devices having the advantages of lowering the voltage of the power supply and generating less ozone have been researched and developed for practical use. Has been put to practical use.

The above-mentioned contact charging device performs a charging process by bringing a conductive charging member to which a voltage is applied into contact with the surface of a photoreceptor, and a charging roller is generally used as the charging member.

[0006] The charging roller is constituted by surrounding two or more layers on the outer periphery of a metal core to which a voltage is supplied, and the base layer is mainly constituted by solid rubber and the sponge. It is divided into two types.

The charging method using the charging roller includes a method of applying only a DC voltage component to the charging roller, a method of applying a superimposed voltage in which an AC voltage component is superimposed on a DC voltage component to the charging roller, There is. In the former case, since the resistance unevenness in the charging roller and the change in resistance due to toner attached on the surface of the charging roller appear directly as the potential unevenness on the surface of the photoconductor, the resistance control in the charging roller and The demands on uniformity of resistance and accuracy of roller shape are very severe. On the other hand, in the case of the latter superimposed voltage, the charging roller and the photosensitive member vibrate due to the AC voltage component, and a so-called charging noise is generated. It can have a wide range of performance tolerances.

Therefore, the vibration and the charging noise due to the AC voltage component increase in the case of the charging roller using the solid rubber for the base layer, but are reduced in the case of the charging roller using the sponge. It is desired to use.

[0009]

However, in an image forming apparatus using a method of charging by applying a superimposed voltage, if a charging roller having a sponge as a base layer is used, the toner which cannot be removed by the cleaning device is removed. In the contact nip portion (hereinafter, simply referred to as “nip portion”) between the surface of the charging roller and the surface of the photoreceptor, the silica contained contains the photoreceptor due to vibration of the charging roller due to an AC voltage component and friction between the two. May be attached to the surface of the substrate and be embedded. As a result, the resistance of the portion of the photoreceptor in which the silica is embedded increases, so that the potential is lost and charging failure occurs, and the toner adheres to the photoreceptor, causing toner filming and fusing.

Further, in a charging device in which the base layer of the charging roller is formed of solid rubber and both ends of the core are pressed against the surface of the photoreceptor, the charging roller has a high hardness. In contrast, the nip width of the charging roller can be formed almost uniformly small along the longitudinal direction, whereas the charging device in which the base layer of the charging roller is formed by a sponge and pressed against the surface of the photoconductor has a small hardness of the charging roller. If an attempt is made to set the nip width as in the case of solid rubber, both ends where pressure is applied are widened, and it is difficult to form the nip width uniformly along the longitudinal direction.

That is, in a charging roller having a sponge base layer, the nip width between the charging roller and the photosensitive member is increased by pressing both ends of the cored bar, and the frictional resistance accompanying rotation of the photosensitive member is increased. As a result, silica is more strongly embedded in the surface of the photoreceptor, toner filming occurs on the surface of the photoreceptor, and the level of fusion is deteriorated. Therefore, there is a problem that the life of the photoconductor is shortened.

The present invention has been made in order to solve the above-mentioned problems, and uses a charging member having a small surface layer hardness, eliminates vibration and charging noise due to an AC component, and furthermore, a charging member. It is an object of the present invention to provide a charging device in which the embedding force of silica in the toner is reduced to prevent toner filming and fusing.

[0013]

The invention according to claim 1 is
In a charging device that performs charging by bringing a conductive charging member to which a voltage is applied into pressure contact with the surface of a member to be charged, the charging member is formed by coating at least a soft base layer and a surface layer on the outer periphery of a cored bar, The surface layer is formed without changing the thickness at the end and the center in the direction along the axis of the cored bar.
Make the mixing ratio of the material different between the end and the center.
Thus, the surface hardness at the end portion is smaller than that at the center portion .

[0014]

[0015]

[0016]

[Operation] Based on the above configuration, the charging member is brought into pressure contact with the member to be charged. In this state, in order to reduce the surface hardness of the end surface layer portion with respect to the central surface layer portion of the charging member, for example, the thickness is reduced or the mixing ratio of the materials is changed. As a result, the force of embedding the silica in the toner into the charged body at the end of the charging member is reduced, and the filming and fusion of the toner generated at both ends of the charged body are prevented.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, with reference to the drawings, first reference
An example will be described, followed by a detailed description of an embodiment of the present invention. < Reference Example > FIG. 1 is a schematic sectional view showing an image forming apparatus according to the present invention.

In FIG. 1, the image forming apparatus rotates a photosensitive member 1 as a member to be charged in a direction indicated by an arrow A, and a charging roller as a charging member around the photosensitive member 1 (details will be described later). 2, a developing unit 7, a transfer roller 13 as a transfer member, and a cleaner 14 are provided, and a fixing unit 15 is provided downstream of the transfer paper P as a recording medium in the transport direction. The process devices of the developing unit 7 and the cleaner 14 are integrated into the process cartridge 43. In addition, the image forming apparatus is provided with a laser scanner 4, a mirror 6, and the like as an optical system.

The photosensitive member 1 is formed by a drum having a diameter of 24 mm and is rotated at a process speed of 24 mm / sec.

Next, an image forming process of the image forming apparatus will be described.

The photosensitive member 1 is uniformly charged by a charging roller 2 during the rotation process, and then a laser beam 5 corresponding to a time-series electric digital image signal of image information sent from a video controller (not shown) is applied to the laser. Output from the scanner 4, scanning exposure is performed at a resolution of 300 dpi, and an electrostatic latent image is formed on the surface via a mirror 6 installed in the main body of the image forming apparatus.

The electrostatic latent image on the photoreceptor 1 is reversal-developed by the toner 8 carried on the developing sleeve 10 in the developing section 7 and visualized to obtain a toner image. And
The toner image is transferred onto the transfer paper P by the action of the transfer roller 13. The transfer paper P on which the toner image has been transferred is separated from the photoreceptor 1 and conveyed to the fixing unit 15, where the unfixed toner image is fixed, and then discharged from the image forming apparatus main body.

The residual toner remaining on the surface of the photoreceptor 1 after the transfer of the toner image is removed by the cleaner 14 in preparation for the next image forming process.

The core metal of the charging roller 2 has a DC bias and A
The charging roller 2 is electrically connected to a charging bias application power supply 17 capable of superimposing the C bias, and applies a bias to the charging roller 2 via the metal core to charge the surface of the photoconductor 1 to a predetermined potential. In this reference example , the DC voltage component Vd
The setting was such that a sine wave bias of -700 V, an AC voltage component Vpp of 1600 V, and a frequency f of 300 Hz was applied. With this setting, the surface of the photoconductor 1 is charged to about -680V.

The developing section 7 employs a non-contact developing method, and has a developing sleeve 10 as a toner carrier for carrying the toner 8 and transporting the toner 8 to the photoreceptor 1, and the toner accommodating chamber 3. The developing sleeve 10 is formed by coating a base tube with a coating material in which carbon is dispersed. Developing sleeve 1
Numeral 0 is non-magnetic, and the base tube is made of aluminum, stainless steel or the like. Further, the roughness on the surface of the developing sleeve 10 due to the coating of the coating is usually R on average.
a is set in a range of 0.4 μm to 3.5 μm, and Rz is set in a range of 3 μm to 30 μm. In this reference example , the average Ra is 1.8 μm.
m, Rz 15 μm were used.

Furthermore, the developing sleeve 10 is rotatably supported by bearings (not shown), the present reference example in diameter 12 mm, rotating in the arrow H direction at a peripheral speed 36 mm / sec. The developing sleeve 10 is connected to a DC bias by AC.
It is connected to a power supply 12 which bias can superposition, the ginseng
In the example , a rectangular wave bias voltage having a DC voltage component Vdc of −400 V, an AC voltage component Vpp of 1300 V, and a frequency f of 1800 Hz is applied.

The developing sleeve 10 is supported opposite the photosensitive member 1 at a predetermined developing interval, and the developing interval is 300
It is set to μm. Toner 8 on developing sleeve 10
The toner layer thickness regulating member 9 for regulating the layer thickness of
Urethane rubber having a hardness of 67 mm and a thickness of 67 mm is used, and an appropriate tribo is given to the toner 8 by frictional charging.

The toner 8 is supplied to the power supply 12
When a developing bias is supplied from the photoconductor 1, it flies at a position closest to the photoconductor 1 and develops an electrostatic latent image formed on the photoconductor 1. Further, the toner 8 is a magnetic one-component negative toner, and has an average particle diameter of 6.4 μm.

A magnet roller 11 is fixedly provided inside the developing sleeve 10. Developing sleeve 1
The magnet roller 11 in 0 has, for example, four magnetic poles, and the S1 pole of the four magnetic poles faces the photoconductor 1. When the toner 8 flies to develop the electrostatic latent image formed on the photoreceptor 1, the magnet roller 11 applies the toner 8 which is a cause of fogging to the developing sleeve 10.
Needed to keep on top. The S2 pole is disposed on the opposite side of the S1 pole, and the toner 8 in the toner storage chamber 3 is attracted to the developing sleeve 10 so that the developing sleeve 10
Has a function of circulating the toner 8 in the vicinity of the S2 pole (in the direction of the arrow E in the figure) with the rotation of. This circulation contributes to the tribo application of the toner 8. The N1 pole and the N2 pole both contribute to the conveyance of the toner 8 coated on the developing sleeve 10 and the provision of a tribo. In this reference example , the S1 pole is 780
The Gauss and S2 poles used had a peak value of 700 Gauss, and the N1 and N2 poles each had a peak value of 720 Gauss.

Next, the charging roller will be described in detail.

FIG. 2 is a sectional view showing a reference example of the charging roller according to the present invention. In FIG. 2, the charging roller 2 is
Since it is symmetrical with respect to the center line B, only one half is shown.

The charging roller 2 has a sponge 22 as a soft base layer adhered to the outer periphery of the cored bar 21.
Has a two-layer structure in which a surface layer 23 is provided on the outer periphery of the two layers. Sponge 2
No. 2 is obtained by foaming urethane in a mold, and has an average foam diameter of 120 μm. The surface layer 23 is formed by dipping the sponge 22 in a solution in which carbon is mixed with a base solvent of urethane and acrylic, and then drying. The film thickness is 110 μm at the center of the charging roller 2 in the longitudinal direction.
It is. Further, the surface layer 23 is a central surface layer portion (a central portion of the surface layer).
24 and an end surface portion (end portion of the surface layer) 25. In addition,
The material of the surface layer 23 is not limited to urethane, but may be acrylic or nylon resin. The hardness is about 41 degrees when the sponge is provided on the cored bar and about 48 degrees when the surface layer 23 is provided. Also, the charging roller 2
Has a total length of 240 mm.

Both ends of the core metal 21 of the charging roller 2 are rotatable by bearings (not shown), and both ends of the core metal 21 indicated by arrows C in the figure so that the charging roller 2 is always pressed against the photosensitive member 1. The part is pressurized. At both ends in this case, 3.4
It is appropriate to apply the same pressing force in the range of 3 to 6.86 N. That is, if the pressing force is 3.43 N or less, a sufficient nip width between the charging roller 2 and the photoconductor 1 cannot be obtained, and a favorable charging process cannot be performed.
If the pressing force is 6.86 N or more, the charging roller 2
Since the nip width between the photoconductor 1 and the photoconductor 1 is large, filming or fusion occurs over the entire surface of the photoconductor 1.

Therefore, it is generally set so that a pressing force of 4.90 N is applied to both ends of the charging roller 2. In this case, the nip width between the charging roller 2 and the photoconductor 1 is larger at both ends than at the center. Further, the charging roller 2 is driven to rotate with respect to the photoconductor 1.

The surface hardness of the charging roller 2 is determined by the material used for the surface layer 23 and the thickness of the surface layer. The surface layer thickness and the surface layer hardness have a linear linear correlation. The appropriate thickness of the surface layer is about 50 to 300 μm. The surface hardness of the charging roller 2 is desirably 80 degrees or less in the entire region (the surface hardness is measured by Kobunshi Keiki Co., Ltd.).
(Measured with a micro rubber hardness tester MD-1). In the relationship between the surface hardness and the image (excluding images at positions corresponding to both ends of the charging roller 2), if the surface hardness is larger than 80 degrees, silica is strongly embedded in the surface of the photoconductor 1.
Filming and fusion occur on the image. In addition, 70
When the surface hardness is lower than the degree, filming and fusion can be prevented from occurring on the photoconductor 1 (excluding the photoconductors at positions corresponding to both ends of the charging roller 2). However, when the surface hardness is reduced, the plastic set, which is generated on the surface layer 23 of the charging roller 2 in the nip portion, is deteriorated due to long-term storage.

In consideration of these points, the surface hardness of the reference example was set to 67 degrees at the central surface portion 24 of the surface layer 23.

However, if the surface layer hardness is made uniform over the entire length of the charging roller 2 in the longitudinal direction, even if the hardness is 70 degrees or less, the nip between the photosensitive member 1 and both ends A of the charged charging roller 2 is maintained. Since the width is large, filming and fusion occur. Since the area where the filming or the fusion has occurred corresponds to both ends A (position of about 30 mm) of the charging roller 2 shown in FIG. Table 1 shows the results when the filming and the fusion level were confirmed by changing.

[0039]

[Table 1] From Table 1, the thickness of the end surface layer portion 25 corresponding to both end portions A is 9
If the thickness is set to 0 μm or less, filming and fusion will not be a problem. However, in consideration of the reduction in breakdown voltage due to small film thickness, in the present reference example , the thickness of the end surface layer portion 25 is 86 μm, and the surface layer hardness is 53 degrees.

In the end surface layer portion 25 of this reference example, the set is deteriorated by long-term storage, but the deteriorated portion is the end surface layer portion 25 of the charging roller 2. Does not cause a decrease in the surface potential of the photoreceptor 1 due to excess current. Therefore, an abnormal image due to set does not occur.

In this embodiment , the same material is used to change the surface hardness of the charging roller 2 in the longitudinal direction, so that the same type of surface material can be used.

[0042] When the charging device of the present Example was subjected to a durability test of 5000 sheets of a lifetime of the process cartridge 43 is attached to the image forming apparatus, filming or fusion by the toner 8 on the photosensitive member 1 occurs Did not.

Therefore, the end surface layer portion 25 of the charging roller 2
Can be reduced by making the film thickness smaller than that of the central surface portion 24, so that filming and fusion can be prevented. <First Embodiment> Next, a first embodiment of the present invention will be described with reference to FIG.

FIG. 3 is a sectional view showing the charging roller 30 of the present embodiment. 3 is similar to FIG.
Are symmetrical with respect to the center line B of the cored bar 31, and only half are shown.

The charging roller 30 of the present embodiment is
Like the charging roller 2 of the embodiment of the reference example , the core roller 3 has a two-layer structure including a sponge 32 as a soft base layer and a surface layer 33.
1. Sponge 32 and central surface layer (central part of surface layer) 34
The same material as the surface layer 23 of the charging roller 2 was used.

Edge surface layer of charging roller 30 ( edge of surface layer )
Reference numeral 35 denotes a material formed by dipping the sponge 32 in a solution in which carbon is mixed in a base solvent of urethane and acrylic and increasing the ratio of urethane in the material of the central surface layer portion 34, followed by drying. The surface hardness is smaller. The surface layer thickness of the end surface layer portion 35 is 110 μm, and the surface layer hardness is 55 degrees.

Therefore, the surface hardness of the central surface portion 34 of the charging roller 30 of the first embodiment is almost the same as the surface hardness of the central surface portion 24 of the charging roller 2. Also,
The entire length of the charging roller 30 and the region of the end surface layer portion 35 having a small surface hardness are the same as those of the charging roller 2.

Further, the charging roller 30 of the first embodiment
The method of pressing the photosensitive member 1 against the photosensitive member 1 and the method of charging the same are the same as those in the case where the charging roller 2 of the reference example is used.

In this embodiment, as in the reference example , at both ends A of the charging roller 30, the setting is deteriorated by long-term storage, but the deteriorated portion is the end surface layer 35 of the charging roller 30. During image formation, a decrease in the surface potential of the photoconductor is not caused by an excessive amount of current due to a change in the capacitance of the sag portion. Therefore, an abnormal image due to set does not occur.

In this embodiment, since the hardness of the surface layer is changed without changing the thickness of the surface layer, there is no need to worry about a decrease in the breakdown voltage due to the small thickness of the surface layer.

When the charging device of this embodiment is mounted on the image forming apparatus, the life of the process cartridge 43 is 500.
When the endurance test was performed on 0 sheets, the toner 8
No filming or fusion occurred.

Therefore, the end surface layer 35 of the charging roller 2
Can be made smaller than that of the central surface layer portion 34 by changing the material, so that filming and fusion can be prevented.

[0053]

As described above, according to the present invention,
Since the surface hardness of the end surface layer portion is smaller than that of the central surface layer portion of the charging member that is in contact with the charged member in a pressurized state, the force for embedding silica in the toner into the charged member at both ends of the charging member. And filming and fusing of the toner generated at both ends of the charging member can be prevented. Also,
The surface hardness can be changed without changing the surface thickness
It is possible to prevent a decrease in breakdown voltage due to a small surface layer thickness.
You.

As a result, a high-quality image can be obtained consistently.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an image forming apparatus equipped with a charging device according to the present invention.

FIG. 2 is a sectional view showing a reference example of the charging device according to the present invention.

FIG. 3 is a sectional view showing a first embodiment of the charging device according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 charged member (photosensitive member) 2 charging member (charging roller) 21 core metal 22 soft base layer (sponge) 23 surface layer 24 central surface layer portion 25 end surface layer portion 30 charging member (charging roller) 31 core metal 33 surface layer 34 central surface portion 35 Edge surface

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-7-295365 (JP, A) JP-A-5-297681 (JP, A) JP-A-6-295116 (JP, A) JP-A-5-295116 257362 (JP, A) JP-A-3-246566 (JP, A) JP-A-6-102783 (JP, A) JP-A-5-318497 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/02 G03G 15/16 103

Claims (1)

(57) [Claims] 1. A charging device for performing charging by bringing a conductive charging member to which a voltage is applied into contact with a surface of a member to be charged under pressure, wherein the charging member covers at least a soft base layer and a surface layer on an outer periphery of a cored bar. It is to form the surface layer, an end portion on the direction along the axis of the core metal
The end portion and the center portion without changing the thickness at the center portion
By making the mixing ratio of the materials different between
A charging device , wherein the surface layer hardness at the end is smaller than that at the center .