JP7187332B2 - image forming device - Google Patents

Description

The present invention relates to an image forming apparatus having a charging roller that charges the surface of a photoreceptor by contacting the surface of the photoreceptor.

2. Description of the Related Art In order to form an electrostatic latent image on a surface of a photoreceptor, the image forming apparatus needs to charge the surface of the photoreceptor in advance.
Such an image forming apparatus conventionally employs a corona charging system in which a corona discharger incorporated in the apparatus charges the surface of the photosensitive member.
Such a corona charging type image forming apparatus has many problems such as the need for a high-voltage power supply for power supply to the corona discharger and the generation of harmful ozone by discharge.

Therefore, in order to solve the problems of the corona charging method, a contact charging type image forming apparatus has been used in which the photosensitive member is charged by bringing the charging roller into contact with the photosensitive member with a uniform contact pressure.

A long charging roller is required for such a contact charging type large-sized image forming apparatus.
However, it is difficult to bring the long charging roller into contact with the photoreceptor with a uniform contact pressure over the entire length in the width direction due to problems such as bending and difficulty in processing.
For this reason, it is difficult for a contact charging type image forming apparatus using a long charging roller to contact the photoreceptor with a uniform contact pressure and charge the photoreceptor uniformly. was not available.

Therefore, for example, Patent Document 1 describes a charging device provided with a plurality of charging rollers facing the photoreceptor in an axial direction perpendicular to the rotation direction of the photoreceptor.
The charging device described in Patent Document 1 includes a plurality of charging rollers in a direction perpendicular to the rotation direction of the photoreceptor. The entire width direction can be covered.
Therefore, the charging device described in Patent Document 1 can solve the problems of bending of the charging roller and workability.

However, in the charging device described in Patent Document 1, the charging potential of the photosensitive member in the overlapping portion of the adjacent charging rollers is higher than the charging potential in the non-overlapping portion, resulting in a halftone image density in the overlapping portion. There was a problem that was expressed thinly.

Therefore, in Japanese Patent Laid-Open No. 2002-100000, by setting the outer diameter of the end portion where the charged regions of the adjacent contact charging members (charging rollers) overlap to be smaller than the outer diameter of the central portion in the axial direction, An image forming apparatus is described which can obtain good image quality even in the overlapping portions of adjacent contact charging members (charging rollers).

JP-A-8-305128 Japanese Patent Application Laid-Open No. 2007-178460

However, in the image forming apparatus described in Patent Document 2, the outer diameter of the end portion where the charging regions of the adjacent contact charging members (charging rollers) overlap is set to the outer diameter of the central portion in the axial direction. There is a problem that the shape is limited to a relatively small shape, a so-called crown shape, and a problem that individual differences cannot be accommodated.
Considering that charging rollers of various shapes have been proposed in order to solve the problem of the contact charging system, the fact that the charging roller is limited to a crown shape poses a serious problem.

The present invention has been made in view of the above, and can uniformly charge a long photoreceptor, and can apply charging rollers having different shapes in consideration of individual differences. An object of the present invention is to provide an image forming apparatus.

In order to solve the above-described problems and achieve the object, an image forming apparatus according to claim 1 of the present invention includes a photoreceptor, and a surface of the photoreceptor that is moved by a rotational movement around a rotation axis of the photoreceptor. and a charging section having a charging roller that charges the surface of the photoreceptor by contacting the charging section. A plurality of charging rollers are provided along a direction intersecting the moving direction of the surface of the photoreceptor so as to overlap with the moving direction of the body surface, and one or more of the charging rollers among the plurality of charging rollers rotates in the rotation axis direction of the photosensitive member. It is characterized in that it is provided at an angle of inclination with respect to a direction perpendicular to the moving direction of the body surface.

According to a second aspect of the present invention, in the image forming apparatus described above, the inclination angle is 0.2 degrees or more and 2 degrees or less.

According to a third aspect of the present invention, there is provided an image forming apparatus according to the above-described invention, wherein the plurality of charging rollers are provided so that the inclination angles thereof can be adjusted within a predetermined range.

In the image forming apparatus according to claim 4 of the present invention, in the above invention, the discharge start voltage from the plurality of charging rollers to the photoreceptor is timely detected, and the detected discharge start voltage is determined in advance. and an applied voltage automatic control section for controlling the application of the DC voltage to the plurality of charging rollers.

Further, in the image forming apparatus according to claim 5 of the present invention, in the above invention, the discharge start voltage from the plurality of charging rollers to the photoreceptor is timely detected, and the detected discharge start voltage is not less than twice the detected discharge start voltage. (1), an application voltage automatic control section for performing control to apply to the plurality of charging rollers a voltage obtained by superimposing an AC voltage having a peak-to-peak voltage of a predetermined multiple on a DC voltage.

In the image forming apparatus according to the present invention, the plurality of charging rollers provided in the charging unit move the surface of the photoreceptor so that the ends of adjacent charging areas overlap each other in the moving direction of the surface of the photoreceptor. One or more of the plurality of charging rollers are provided along a direction intersecting with the direction of rotation, and one or more of the charging rollers have a rotation axis direction inclined with respect to a direction perpendicular to the moving direction of the surface of the photoreceptor. is provided.
Therefore, in the image forming apparatus according to the present invention, the inclination angle corresponding to charging rollers having different shapes is set in consideration of individual differences. The photoreceptor can be uniformly charged.
Therefore, the image forming apparatus according to the present invention can uniformly charge a long photoreceptor and can use charging rollers having different shapes.

FIG. 1 is a diagram schematically showing the overall structure including a control unit of an image forming apparatus according to an embodiment of the invention. FIG. 2 is an enlarged view of the periphery of the photoreceptor of the image forming apparatus shown in FIG. FIG. 3 is a diagram for explaining the positional relationship between the photosensitive member and the three charging rollers, and also shows the cleaning roller. 4(a) is a perspective view of an assembly in which the bearings of three charging rollers are attached to a fixed frame, and FIG. is arranged. FIG. 5 is a diagram showing a charging roller and a cleaning roller. 6, (a) is a view of the assembly shown in FIG. 4 viewed from the back side with respect to the bearing fixing surface of the fixing frame, and (b) and (c) are the bearing portions fixed by screws. It is the figure which expanded the periphery of the part which carries out. FIG. 7 is an axial view of the vicinity of the bearing attached to the fixed frame, and is a partial cross-sectional view. FIG. 8 is a view of the vicinity of the bearing portion attached to the fixed frame as seen from a direction orthogonal to the axial direction, and is a view partially showing a cross section. FIG. 9 is a diagram for explaining the cross amount and the inclination angle of the charging roller. FIG. 10(a) is a diagram for explaining the photoreceptor charging characteristics when a DC voltage is applied to the charging roller according to the experimental results of the applicants of the present patent application, and FIG. 3A and 3B are diagrams for explaining a method of detecting a discharge starting voltage; FIG. FIG. 11 is a diagram for explaining crown-shaped and reverse-crown-shaped charging rollers. FIG. 12 is a diagram schematically showing the overall structure including the control section of the image forming apparatus of the modification. FIG. 13 is a diagram for explaining the photoreceptor charging characteristics when a voltage obtained by superimposing an AC voltage on a DC voltage is applied to the charging roller, based on experimental results obtained by the applicants of the present patent application. FIG. 14 is a diagram showing charging characteristics when a DC voltage superimposed with an AC voltage of 1600 VPP/1 KHz is applied to the charging roller. FIG. 15 is a diagram for explaining a modification of the positional relationship between the photoreceptor and the three charging rollers, and also shows the cleaning roller.

Preferred embodiments of the image forming apparatus according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram schematically showing the overall structure including a control unit 50 of an image forming apparatus 1 according to an embodiment of the invention. FIG. 2 is an enlarged view of the periphery of the photoreceptor 11 of the image forming apparatus 1 shown in FIG. FIG. 3 is a diagram for explaining the positional relationship between the photoreceptor 11 and the three charging rollers 40, and also shows the cleaning roller 60. As shown in FIG. 4A is a perspective view of the assembly in which the bearings 21 of the three charging rollers 40 are attached to the fixed frame 30, and FIG. 4B is a perspective view of the assembly shown in FIG. 1 is a diagram in which a photoreceptor 11 is arranged at the same position. FIG. 5 is a diagram showing the charging roller 40 and the cleaning roller 60. As shown in FIG. 6, (a) is a view of the assembly shown in FIG. It is the figure which expanded the circumference|surroundings of the part fixed by screwing. FIG. 7 is an axial view of the periphery of the bearing portion 21 attached to the fixed frame 30, and is a partially sectional view. FIG. 8 is a view of the periphery of the bearing portion 21 attached to the fixed frame 30 as seen from a direction orthogonal to the axial direction, and is a view partially showing a cross section. FIG. 9 is a diagram for explaining the cross amount T and the inclination angle A of the charging roller 40. As shown in FIG. FIG. 10(a) is a diagram for explaining the photoreceptor charging characteristics when a DC voltage is applied to the charging roller 40 according to the experimental results of the applicants of the present patent application, and FIG. 10(b). is a diagram for explaining a method of detecting a discharge starting voltage. FIG. 11 is a diagram for explaining the charging roller 40 having a crown shape and an inverted crown shape.
An image forming apparatus 1 according to an embodiment of the present invention is of a tandem type in which a plurality of photoreceptors are arranged corresponding to toner colors, and is moved by a photoreceptor 11 and a rotational movement around a rotation axis. a contact charging type charging unit 20 including a charging roller 40 that uniformly charges the photoreceptor surface 11a by contacting the photoreceptor surface 11a with a uniform contact pressure.

The overall configuration of the image forming apparatus 1 will be described below.
This image forming apparatus 1 includes four color-specific units 10 corresponding to the four primary toner colors of cyan C, magenta M, yellow Y, and black K, and a transfer unit for transferring a toner image onto a transfer material P. 15, a conveying unit 16 that conveys the transfer material P such as paper, a fixing unit 17 that fixes the toner image transferred to the transfer material P, and a control unit 50 that controls the operation of each unit of the image forming apparatus 1. have.

First, the color correspondence unit 10 will be described.
Note that the four color-specific correspondence units 10 have substantially the same configuration except that the toner colors are different, and thus individual descriptions will be omitted.

The color correspondence unit 10 includes a photoreceptor 11 , a charging unit 20 that uniformly charges the surface of the photoreceptor 11 , an exposure unit 12 that forms an electrostatic latent image on the photoreceptor 11 , and an electrostatic latent image of the photoreceptor 11 . It has a developing unit 13 that develops toner into an image to make it visible, and a photoreceptor cleaner 14 that cleans unnecessary residual toner.

The photosensitive member 11 is drum-shaped and has an outer diameter of 80 mm in this embodiment.
Further, the primary band potential of the photoreceptor 11 is controlled to about 550V by the controller 50 .

The exposure unit 12 is a combination of a known LED array and imaging means. Note that the exposure unit 12 may use a laser scanning type exposure device using a laser light source, a polygon mirror, or the like.

The developing unit 13 is supported by a frame with a predetermined distance between the toner storage unit 13a containing toner of a predetermined color and the photoreceptor 11, and is a developing roller that contacts the photoreceptor 11 during actual operation. 13b and.
The electrostatic latent image formed on the surface of the photoreceptor 11 by the exposure unit 12 is developed with toner by the development unit 13 to become a visible image.

The photoreceptor cleaner 14 is composed of a rubber blade or the like, and removes unnecessary toner or foreign matter remaining on the photoreceptor 11 .

The charging unit 20 is a characteristic part of the present invention, and includes three charging rollers 40 that charge the surface of the photoreceptor 11a by contacting the surface of the photoreceptor 11a that is moved by the rotational movement of the photoreceptor 11 about the rotation axis. , and three cleaning rollers 60 provided corresponding to each charging roller 40 so as to remove toner adhering to each charging roller 40 or foreign matter.

As shown in FIG. 3, the charging roller 40 intersects the moving direction of the photoreceptor surface 11a so that the ends of the adjacent charging regions (elastic contact portions 42) overlap in the moving direction of the photoreceptor surface 11a. A plurality of them are provided along the direction.
In this embodiment, in the charging unit 20, two charging rollers 40 out of the three charging rollers 40 are rotated at an angle A with respect to the direction perpendicular to the moving direction of the photoreceptor surface 11a. is provided.
More specifically, one central charging roller 40 is provided with its rotational axis direction perpendicular to the moving direction of the photoreceptor surface 11a, and the two charging rollers 40 on both sides are provided with their rotational axis directions perpendicular to the direction of movement of the photoreceptor surface 11a. It is provided with an inclination angle A with respect to the direction orthogonal to the movement direction of 11a.

Each charging roller 40 has a straight shape in which an elastic contact portion 42, which will be described later, has a constant outer diameter in the axial direction.
The charging roller 40 may have a so-called crown shape in which the central portion of the elastic contact portion 42 is convex, or an inverted crown shape in which the central portion of the elastic contact portion 42 is concave. It is preferable that the outer diameter deviation of the portion 42 is 0.2 mm or less.

The charging roller 40 has a metal shaft portion 41 and an elastic contact portion 42 which is a portion that is elastically contacted with the photoreceptor 11 and is also a charging area. A function-separated roller covered with an oil seepage prevention layer, a resistance adjustment layer, and a surface protection layer in this order is applied (see, for example, Japanese Patent Application Laid-Open No. 2-311868).
If the charging roller 40 is a single-layer roller in which the metal shaft portion 41 is simply covered with a semi-conductive rubber, or a double-layer roller in which a surface protective layer is added to the semi-conductive rubber layer, the charging roller 40 will not be electrically charged. The voltage drop borne by the resistance component cannot be ignored, and a step is likely to occur in the charging potential between the first rotation of the photoreceptor 11 and subsequent rotations. It is preferable to use a function-separated roller because a step is likely to occur.
In order to minimize the voltage drop carried by the electrical resistance component of the charging roller 40, it is not enough to simply reduce the electrical resistance. In order to avoid the occurrence of pinholes due to insufficient withstand voltage of the photoreceptor 11, the volume resistivity of the resistance adjustment layer, which substantially controls the electrical resistance component of the charging roller 40, is set to about 1×10 7 [Ω·cm]. It is preferable to make the layer as thin as possible within a range in which the thickness does not cause defective withstand voltage.

Here, a more specific configuration example of the charging roller 40 will be described.
In this embodiment, the elastic contact portion 42 has an outer diameter of 12 mm and a length of 322 mm.
Metal shaft: 8 mm in diameter,
Conductive elastic layer: about 2 mm thick, ethylene-propylene-diene rubber (EPDM), SBR, NR, etc., mixed with an electronic conductive agent such as carbon black, volume resistivity is 1 x 10 3 [Ω·cm] or less and low hardness of 30 degrees (Hs: JIS A).
Layer for preventing oil seepage from the conductive elastic layer: thickness 10 μm
Resistance adjustment layer: An ion-conducting layer that enhances voltage resistance (leakage resistance), and has a minimum thickness of 50 μm or more that satisfies the dielectric breakdown voltage.
The electronic conductive agent such as carbon black contained in the resistance adjusting layer is 10% by volume or less with respect to the rubber.
As a specific example, perchlorate (quaternary ammonium salt), an antistatic agent, etc., to adjust the volume resistivity to 1×10 7 [Ω·cm].
Surface protective layer: thickness 10 μm, electronically conductive particles (magnetite or titanium black) mixed with fluororesin (vinylidene fluoride-tetrafluoroethylene copolymer resin), volume resistivity 10 7 ~ 10 to the 10th power and Shore hardness (D type) of 65 degrees or less.
In addition, the surface protective layer forms a distribution of convexes caused by the rough surface-forming particles, the outer diameter of the rough surface-forming particles is 13 μm, and the thickness other than the convex portions is 10 μm (Japanese Unexamined Patent Application Publication No. 2007-178559). , see JP-A-2007-264491).
By applying such a surface protective layer, the contact area between the photoreceptor 11 and the charging roller 40 can be reduced, and a constant discharge area can be secured in the gap, thereby improving the charging performance. Therefore, it is possible to consider a DC voltage system that is advantageous in cost reduction, and it is also possible to reduce vibration noise that tends to occur in an AC voltage system.

Since the charging roller 40 is in contact with the photoreceptor 11, when it is used for a long period of time, problems such as a decrease in charging performance and uneven charging occur due to adhesion of toner and foreign matter adhering to the surface 11a of the photoreceptor.
In order to solve such a problem, in this embodiment, the cleaning roller 60 disclosed in Japanese Patent Application Laid-Open No. 8-137208, for example, is applied.
The cleaning roller 60 can solve the problems such as the deterioration of the charging performance and the uneven charging.

As shown in FIG. 5, the cleaning roller 60 has a metal shaft portion 61 and a cleaning material 62 spirally wound around the metal shaft portion 61, and follows the charging roller 40 while being in contact therewith. It is designed to be rotated by

The cleaning material 62 is a belt-like member made of foamed polyurethane or the like, and is spirally wound around the metal shaft 61 to cause clogging between the protrusions with the cleaning material 62 and the recesses where the shaft is exposed therebetween. can be prevented.
As disclosed in Japanese Patent Application Laid-Open No. 2012-103641, the cleaning material 62 should preferably have a width of 5 mm to 24 mm and a thickness of 1.9 mm to 3.6 mm along the metal shaft portion 61 .

The charging section 20 also has bearing portions 21 for the metal shaft portions 41 of the three charging rollers 40 and a fixed frame 30 to which the bearing portions 21 of the three charging rollers 40 are fixed.

The bearing portions 21 are provided in pairs so as to axially support both end portions of each charging roller 40. As shown in FIGS. , and a movable bearing portion 23 in which the charging roller 40 is attached to the fixed portion 22 so as to be able to press in the direction of the photosensitive member 11 .

The fixed portion 22 is made of an insulating material, and includes a spring accommodating portion 22 a that accommodates a spring S for pressing the elastic contact portion 42 of the charging roller 40 against the photosensitive member 1 by urging the movable bearing portion 23 . and a guide wall portion 22b that guides the movable bearing portion 23 in the biasing direction.

The movable bearing portion 23 includes an electrode holding portion 24 that holds an electrode plate E for applying a voltage to the charging roller 40, and an axial end rotation support that supports the metal shaft portion 41 of the charging roller 40 so as to be rotatable. a portion 26;

The electrode holding portion 24 is made of an insulating material, and includes an axial biasing member holding portion 25 that holds the electrode plate E and an axial biasing member 25a that biases the end face of the metal shaft portion 41 in the axial direction. is formed so as to be connected to the shaft end rotation support portion 26 along the axial direction of the metal shaft portion 41 .

The axial biasing member holding portion 25 biases the end face of the metal shaft portion 41 of the charging roller 40 in the axial direction with a metal spring S to the conductive axial biasing member 25a connected to the electrode plate E. I hold it so that I can.

The shaft end rotation support portion 26 is made of a conductive resin material, and a material with good slidability is applied, in particular, in consideration of the slidability in the rotational direction and the axial direction of the metal shaft portion 41. there is
The shaft end rotation support portion 26 is formed with a metal shaft portion holding recessed portion 27 that is formed so as to be connected to the electrode holding portion 24 along the axial direction of the metal shaft portion 41 .
The inner diameter of the metal shaft portion holding recess 27 is set slightly larger than the outer diameter of the metal shaft portion 41 .
The axial depth of the metal shaft portion holding recess 27 is set so that the metal shaft portion 41 can slide in the axial direction while receiving the biasing force of the axial biasing member 25a.

Appropriate urging force acts on the charging roller 40 whose both ends of the metal shaft portion 41 are supported by the bearing portions 21 as described above, not only in the direction of contact with the photoreceptor 11 but also in the axial direction of the metal shaft portion 41 . It is designed to be
Therefore, even if the charging roller is provided with the rotation axis direction inclined at an angle A with respect to the direction orthogonal to the moving direction of the photoreceptor surface 11a, the charging roller can rotate stably while being pressed against the photoreceptor 11a. is now possible.

The electrode plate E may be provided on both of the pair of bearing portions 21 or may be provided on only one of the bearing portions 21 .

In this embodiment, the cleaning roller 60 is also rotatably supported by the bearing portion 21 in the same manner as the charging roller 40 .
That is, the movable bearing portion 23 also holds the electrode plate E at the position where the electrode holding portion 24 corresponds to the cleaning roller 60 .
Further, the shaft end rotation support portion 26 is formed with a metal shaft portion holding recess portion 28 corresponding to the cleaning roller 60 so as to be aligned with the metal shaft portion holding recess portion 27 corresponding to the charging roller 40 .
The axial depth of the metal shaft holding recess 28 is set so that the metal shaft 61 of the cleaning roller 60 can slide in the axial direction while receiving the biasing force of the axial biasing member 25b.

The fixed frame 30 is fixed to a housing (not shown) or a frame (not shown) of the image forming apparatus 1 so as to extend along the width direction of the photoreceptor 11 .
As shown in FIG. 6, the fixed frame 30 is arranged so as to extend along the width direction of the photoreceptor 11 and a fixing portion 31 fixed to the housing or the frame of the image forming apparatus 1. and a multiple charging roller arrangement portion 32 in which a bearing fixing surface 32a to which each bearing portion 21 of the three charging rollers 40 is fixed is formed.

The multiple charging roller arrangement portion 32 has a plurality of screw fixing portions 33 of each bearing portion 21 corresponding to the three charging rollers 40 on the bearing fixing surface 32a, and when fixing each bearing portion 21 with the screw fixing portions 33, , there are formed a plurality of angle adjustment guide slits 34 which serve as guide portions when the rotation axis direction of the charging roller 40 is fixed at an inclination angle A with respect to the direction orthogonal to the moving direction of the photoreceptor surface 11a. .

Two screw fixing portions 33 are provided corresponding to each bearing portion 21 . More specifically, the screw fixing portion 33 is a screw insertion hole provided corresponding to two screw holes (not shown) formed in each of the fixed portions 22 of each bearing portion 21 .

The angle adjustment guide slit 34 is an elongated hole formed in the bearing fixing surface 32 a , and is provided in one position corresponding to each bearing portion 21 .
More specifically, the angle adjustment guide slits 34 serve as slide guides for the guide protrusions 22c formed one by one on the fixed portion 22 of each bearing portion 21. As shown in FIG.

In this embodiment, the three charging rollers 40 are set so that the cross amount T with respect to the photoreceptor 11 is 0.1 mm at the central portion of the charging roller 40 in the specific configuration described above. It is designed to be rotated following the body 11 .
Here, the cross amount T is a value calculated by [cross amount T=radius R1 of the photoreceptor+radius R2 of the charging roller−distance D between the shaft centers of the photoreceptor 11 and the charging roller 40] (see FIG. 9). ).

In addition, the charging roller 40 is moved three times along the direction intersecting the moving direction of the photoreceptor surface 11a so that the ends of the elastic contact portions 42 as adjacent charging regions overlap with each other in the moving direction of the photoreceptor surface 11a. Each charging roller 40 is set so that the inclination angle A with respect to the direction orthogonal to the moving direction of the photosensitive member surface 11a can be adjusted within a range of 0.2 degrees or more and 2 degrees or less.
In such an adjustment range of the inclination angle A, when each charging roller 40 is set to the above-described dimensions, the difference B in the axial center position between the both end positions of the elastic contact portion 42 of the charging roller 40 (see FIG. 9) is 1.1 mm or more and 11.2 mm or less.

The reason why the inclination angle A is in the range of 0.2 degrees or more and 2 degrees or less is based on the results of experiments conducted by the applicants of the present patent.
That is, when the inclination angle A is less than 0.2 degrees, the charging potential of the photoreceptor 11 in the overlapping portion of the two adjacent charging rollers 40 is higher than the charging potential of the photoreceptor 11 in the non-overlapping portion. Due to the increase in height, the halftone image density of the photoreceptor 11 in the portion where the charging roller 40 overlaps is expressed lightly.
On the other hand, if the inclination angle A exceeds 2 degrees, the deviation of the pressure distribution on the photoreceptor 11 becomes too large, and a correct charge distribution cannot be obtained, resulting in image unevenness.

In the charging section 20 having such a configuration, the inclination angle A can be easily adjusted within a predetermined range (0.2 degrees or more and 2 degrees or less in this embodiment).
That is, the charging unit 20 inserts the guide projections 22c formed in the fixed portions 22 of the pair of bearings 21 into the corresponding angle adjustment guide slits 34 of the fixed frame 30, and then inserts the guide projections 22c into the corresponding angle adjustment guide slits 34 of the fixed frame 30. After moving each guide protrusion 22c within the slit 34 to adjust the inclination angle A within a predetermined range, the fixed portion 22 is fixed to the fixed frame 30 by screwing.

By the way, the applicants of the present patent have found that the image forming apparatus using the crown-shaped charging roller described in the above-mentioned Patent Document 2 (Japanese Patent Application Laid-Open No. 2007-178460) initially causes overlapping of the adjacent charging rollers. It was confirmed by experiment that the density difference in the part can be reduced, but the density difference is generated as the durability is continued.
The applicants of the present patent speculate that this is due to factors such as the fact that the film thickness of the photoreceptor has become thinner due to durability and that the space charge in the photoreceptor layer has increased.
In addition, the applicants of the present patent have found that the degree of the density difference in the overlapping portion of the adjacent charging rollers 40 is not necessarily the same because of individual differences between the photoreceptor 11 and the charging roller 40, contact pressure differences, dimensional errors, environmental differences, and the like. Experiments have also confirmed that this is not the case.

In the image forming apparatus 1 of this embodiment, since the charging unit 20 can adjust the inclination angle A, individual differences, contact pressure differences, dimensional errors, and environmental differences of the photoreceptor 11 and charging roller 40 are possible. Alternatively, by adjusting the inclination angle A according to the durability difference, the density step in the overlapping portion W of the charging roller 40 can be stably reduced.

Next, the transfer section 15 will be described.
The transfer section 15 has a transfer belt 15a, a belt drive roller 15b, a belt tension roller 15c, a primary transfer roller 15d, and a belt cleaner 15e.

The transfer belt 15a is an endless belt, and is stretched around a belt driving roller 15b, a belt tension roller 15c, and a primary transfer roller 15d.

The belt drive roller 15b is driven by a drive unit (not shown) to rotate the transfer belt 15a that is stretched by the belt tension roller 15c and the primary transfer roller 15d.

The belt tension roller 15c adjusts the tension of the transfer belt 15a and prevents the transfer belt 15a from meandering.

The primary transfer roller 15d is arranged to face the photosensitive member 11 of each color corresponding unit 10 with the transfer belt 15a interposed therebetween. The resulting toner image is electrostatically transferred to the transfer belt 15a.

The belt cleaner 15e removes unnecessary toner or foreign matter remaining on the transfer belt 15a.

Next, the transport section 16 will be described.
The conveying unit 16 conveys the transfer material P such as paper from the supply unit to the fixing device with the toner image transferred thereon.
The conveying unit 16 has a plurality of conveying rollers 16a arranged in the conveying direction and a roller driving unit (not shown) that drives the conveying rollers 16a. As one of the conveying rollers 16a, a secondary transfer roller 16b is provided.
The secondary transfer roller 16b is arranged to face the transfer belt 15a with the transfer material P such as paper interposed therebetween, and is transferred onto the transfer belt 15a by applying a bias having a polarity different from the charging polarity of the toner. The toner image is transferred onto a transfer material P such as paper.

The fixing unit 17 uses heat to fix the toner image transferred onto the transfer material P such as paper.

The control unit 50 is implemented by a CPU or the like, and is electrically connected to each unit of the image forming apparatus 1 .
The control unit 50 has an applied voltage automatic control unit 50 a that automatically controls the DC voltage applied to each charging roller 40 .

Here, the DC voltage applied to each charging roller 40 will be described in detail.
Applicants of the present patent have used the charging roller 40 in which the elastic contact portion 42 includes a semi-conductive elastic layer and a surface protective layer as in Examples, and found that when a DC voltage is applied to the charging roller 40, the photoreceptor 11 charging potentials were confirmed.
At the entrance side of the nip between the photoreceptor 11 and the charging roller 40, the gap between the charging roller 40 and the photoreceptor 11 gradually narrows, and when the potential difference ΔV between the charging roller 40 and the photoreceptor 11 becomes larger than the discharge start voltage, discharge occurs. The photoreceptor 11 is charged until the potential difference ΔV between the charging roller 40 and the photoreceptor 11 becomes the same as the discharge start voltage. (See FIG. 10(a)).
On the other hand, it was also confirmed by experiments that the discharge start voltage fluctuates greatly depending on the environment (temperature and air pressure) and durability, so that the charging potential also fluctuates greatly.

Therefore, the DC voltage applied to each charging roller 40 is automatically applied by the applied voltage automatic control section 50a.
More specifically, the applied voltage automatic control unit 50a timely detects the discharge start voltage from each charging roller 40 to the photosensitive member 11, and applies a predetermined voltage of, for example, 580 V to the detected discharge start voltage. The added DC voltage is applied to each charging roller 40 .

To detect the firing voltage, the DC current is measured when the DC voltage is applied.
For example, as shown in the graph of FIG. 10(b), DC current is measured when two DC voltages of -800 VDC and -1100 VDC are applied.
Since the relationship between DC current and DC voltage is represented by a linear function (y=12.931x-536.21) as shown in the graph of FIG. 10(b), the firing voltage is about 536V.

In this manner, the applied voltage automatic control unit 50a provides an optimum voltage for each charging roller 40 in response to the environment and durability, that is, contamination due to durability of the charging roller 40, and changes in film thickness due to durability of the photoreceptor 11. The photosensitive member surface 11a can be stably charged at all times by detecting and applying a constant DC voltage each time.
In addition, since AC voltage is not superimposed, filming of the photoreceptor surface 11a at the overlapping portion W of the charging roller 40 can be prevented, and a high quality image can be obtained for a long time.

Although the charging roller 40 having a straight shape is exemplified in the embodiment, the shape of the charging roller 40 is not limited to this, and other shapes can be applied by adjusting the inclination angle A. For example, as shown in FIG. 11, a crown shape (see FIG. 11(a)) or an inverted crown shape (see FIG. 11(b)) can also be applied.

Below, charging rollers 40 having a crown shape, a straight shape, and an inverted crown shape are applied to the image forming apparatus 1 of the embodiment. Table 1 shows the results of a 50 km durability test.
It should be noted that "⊚" in the evaluation result of image quality in Table 1 indicates that both filming and density unevenness are good.
In addition, the crown-shaped and reverse-crown-shaped charging rollers 40 have an outer diameter deviation of the elastic contact portion 42 of 0.1 mm.
From the results of this durability test, it can be seen that by adjusting the inclination angle A, the image forming apparatus 1 can apply charging rollers 40 having a crown shape, a straight shape, and an inverted crown shape.

In the image forming apparatus 1 according to the embodiment of the present invention, the three charging rollers 40 provided in the charging unit 20 are adjacent to each other, and the ends of the elastic contact portions 42 are adjacent to each other in the moving direction of the photosensitive member surface 11a. Two charging rollers 40 among the three charging rollers 40 are provided along a direction intersecting the moving direction of the photoreceptor surface 11a so as to overlap each other, and two charging rollers 40 rotate in a direction orthogonal to the moving direction of the photoreceptor surface 11a. is provided at an inclination angle A with respect to
For this reason, in the image forming apparatus 1 according to the embodiment of the present invention, the inclination angle A is set according to the charging rollers 40 having different shapes, including individual differences. The portion W of the photosensitive member 11 can also be uniformly charged.
Therefore, the image forming apparatus 1 according to the embodiment of the present invention can uniformly charge the elongated photoreceptor 11 and can apply charging rollers 40 having different shapes.

Further, in the image forming apparatus 1 according to the embodiment of the present invention, the inclination angle A is 0.2 degrees or more and 2 degrees or less. can be applied.

Further, in the image forming apparatus 1 according to the embodiment of the present invention, the three charging rollers are provided so that the inclination angle A can be adjusted within a predetermined range. It is possible to adjust including differences and individual differences. Readjusting the tilt angle A can easily remedy the problem. As a result, the image forming apparatus 1 according to the embodiment of the present invention can stably and uniformly charge the long photoreceptor 11 even in long-term use.

Further, in the image forming apparatus 1 according to the embodiment of the present invention, the applied voltage automatic control unit 50a timely detects the discharge start voltage from the three charging rollers 40 to the photoreceptor 11, and the detected discharge start voltage is Since a DC voltage to which a predetermined voltage is added is applied to the three charging rollers 40, the optimum DC voltage is applied to each charging roller 40 in a timely manner, so that the photoreceptor surface 11a is always stabilized. In addition, filming of the photoreceptor surface 11a at the overlapping portion W of the charging roller 40 can be prevented, and high-quality images can be obtained for a long period of time.
(Modification)

Next, an image forming apparatus 2 that is a modification of the image forming apparatus 1 according to the embodiment of the present invention will be described with reference to FIGS. 12 to 14. FIG.
FIG. 12 is a diagram schematically showing the overall structure including the control section 50 of the image forming apparatus 2 of the modification. FIG. 13 is a diagram for explaining the experimental results of the applicants of the present patent application. FIG. 14 is a diagram showing charging characteristics when a DC voltage superimposed with an AC voltage of 1600 VPP/1 KHz is applied to the charging roller 40. As shown in FIG.

In the image forming apparatus 2 of this modified example, the voltage applied to each charging roller 40 is a DC voltage superimposed with an AC voltage having a peak-to-peak voltage VPP that is at least twice the discharge start voltage. is different from the image forming apparatus 1 of .
The rest of the configuration is the same as that of the embodiment, and the same components as those of the embodiment are denoted by the same reference numerals, and the description thereof will be omitted.

By the way, according to the technique disclosed in Japanese Patent Application Laid-Open No. 149668/1988, "an AC voltage having a peak-to-peak voltage VPP which is at least twice as high as the discharge start voltage in a low temperature and low humidity environment" is superimposed on the charging roller 40 on a DC voltage. By applying a voltage, a charging potential equivalent to the DC component (strictly speaking, the average value of the positive peak value and the negative peak value) can be stably obtained regardless of the environment.
Further, when the voltage applied to the charging roller 40 is only a DC voltage, it is difficult to avoid fogging in reversal development.
Further, when the voltage applied to the charging roller 40 is only the DC voltage, the charging potential microscopically looks jagged. become.

Applicants of the present patent have used the charging roller 40 in which the elastic contact portion 42 includes a semi-conductive elastic layer and a surface protective layer as in the examples, and applied a voltage obtained by superimposing an AC voltage on a DC voltage to the charging roller 40. was confirmed. Applicants of the present patent have fixed the positive peak value at a constant value of 0 V and changed only the negative peak value, until the negative peak value reaches the discharge start voltage (about 550 V), only the DC voltage is applied. It was confirmed by experiments that no discharge (no charging) occurred (see FIG. 13).
When the negative peak value reaches the firing voltage (about 550 V) and then doubles the firing voltage (about 1100 V), the potential difference ΔV between the charging potential of the photosensitive member 11 and the positive peak value (=0 V). becomes equal to or higher than the firing voltage, reverse discharge begins to occur at the microscopic timing of the positive peak value.
On the other hand, since a normal discharge occurs at the microscopic timing of the negative peak value, the total charged potential is the average value of the charged potentials at the positive and negative microscopic timings (see FIG. 14).
The method of applying a voltage in which an AC voltage is superimposed on a DC voltage to the charging roller 40 solves the problems in the case of applying only a DC voltage, and also has the effect of suppressing the generation of ozone, which is the greatest advantage of the contact charging method. Also, although the amount of ozone generated increases compared to the case where only a DC voltage is applied, the amount of ozone generated can be reduced by an order of magnitude compared to the corona charging method.
Ideally, if a voltage obtained by superimposing an AC voltage having a peak-to-peak voltage VPP that is at least twice the discharge start voltage in a low temperature and low humidity environment on a DC voltage is applied to the charging roller, the charging roller passes through the origin regardless of the environment. The charging characteristic has a slope of 1.
It is ideal whether only a DC voltage is applied to the charging roller 40 or a voltage obtained by superimposing an AC voltage having a peak-to-peak voltage VPP that is at least twice the discharge start voltage on the DC voltage is applied to the charging roller 40. , the charging characteristic should have a slope of 1, but in reality, there is an effect of the space charge in the photoreceptor 11 layer due to exposure and the voltage drop carried by the electrical resistance component of the charging roller 40, so the theoretical value and there is a slight difference between the experimental values.
On the other hand, when a voltage obtained by superimposing an AC voltage on a DC voltage is applied to the charging roller 40, filming on the photosensitive member 11 tends to occur more easily than when only a DC voltage is applied. A method of reducing filming, which is an adverse effect of AC voltage, is to avoid passing AC current more than necessary. (See, for example, JP-A-9-218560)
For this reason, when a voltage obtained by superimposing an AC voltage on a DC voltage is applied to the charging roller 40, instead of applying a constant peak-to-peak voltage VPP, the environment and durability, that is, contamination due to durability of the charging roller 40, is applied. Furthermore, it is important to detect the minimum necessary peak-to-peak voltage VPP that can cope with changes in film thickness due to durability of the photosensitive member 11 each time, and to apply the minimum necessary peak-to-peak voltage VPP each time. is.

In consideration of the circumstances described above, the image forming apparatus 2 of this modified example has an applied voltage automatic control section 50 b that automatically controls the voltage applied to each charging roller 40 .
The applied voltage automatic control unit 50b timely detects the discharge start voltage from each charging roller 40 to the photoreceptor 11, and converts the DC voltage to an AC voltage having a peak-to-peak voltage VPP that is 2.2 times the detected discharge start voltage. is applied to each charging roller 40 .
In this embodiment, a DC voltage superimposed with an AC voltage having a peak-to-peak voltage VPP that is 2.2 times the detected discharge start voltage is applied to each charging roller 40. Even if a voltage obtained by superimposing an AC voltage having a peak-to-peak voltage VPP (a predetermined multiple that is equal to or greater than twice the discharge start voltage) is applied to each charging roller 40 . good.

More specifically, the application voltage automatic control unit 50b applies a voltage obtained by superimposing an automatically controlled AC voltage on a DC voltage of 580V to each charging roller 40, thereby constantly maintaining the photoreceptor surface 11a. It can be stably charged to about 550V.

In addition, in order to detect the firing voltage, the current of the DC component may be measured in the same manner as in the case of applying the DC voltage in the embodiment.

In this manner, the applied voltage automatic control unit 50b applies to each charging roller 40 a voltage obtained by superimposing a DC voltage and an AC voltage having a peak-to-peak voltage VPP that is at least twice the discharge start voltage, thereby only DC voltage is applied. Compared to the case where the voltage is applied to each charging roller 40, the filming of the photoreceptor surface 11a at the overlapping portion W of the adjacent charging rollers 40 is a little more likely to occur, and the amount of ozone generated is slightly increased, but the voltage is applied over a long period of time. A higher quality image can be obtained.

The charging rollers 40 having a crown shape, a straight shape, and an inverted crown shape are applied to the image forming apparatus 2 of the modified example. Table 2 shows the results of a 50 km durability test.
It should be noted that "○" in the image quality determination results in Table 2 indicates that very slight filming occurs but does not lead to image steps.
In addition, the crown-shaped and reverse-crown-shaped charging rollers 40 have an outer diameter deviation of the elastic contact portion 42 of 0.1 mm.
From the result of this durability test, it can be seen that by adjusting the inclination angle A, the image forming apparatus 2 can use the charging roller 40 having a crown shape, a straight shape, or an inverted crown shape.

In the image forming apparatus 2 according to this modified example, the applied voltage automatic control unit 50b timely detects the discharge start voltage from the three charging rollers 40 to the photoreceptor 11, and the detected discharge start voltage Since a voltage obtained by superimposing an AC voltage having a peak-to-peak voltage VPP of a predetermined multiple on a DC voltage is applied to the three charging rollers, high-quality images can be obtained for a long period of time.

In the image forming apparatuses 1 and 2 according to the embodiments and modifications of the present invention, one charging roller 40 in the center is provided with the rotation axis direction perpendicular to the moving direction of the photoreceptor surface 11a. The two charging rollers 40 are provided with the rotation axis direction inclined at an angle A with respect to the direction perpendicular to the moving direction of the photoreceptor surface 11a. 15A, all three charging rollers 40 may be provided with an inclination angle A (see FIG. 15A), and one charging roller 40 in the center may rotate in the rotation axis direction of the photoreceptor surface 11a. , and the two charging rollers 40 on both sides are provided so that the rotation axis direction is oriented in the direction perpendicular to the moving direction of the photoreceptor surface 11a. (See FIG. 15(b)).

Further, in the image forming apparatuses 1 and 2 according to the embodiments and modifications of the present invention, the charging unit 20 has three charging rollers 40, but the number of charging rollers 40 is not limited to three, and a plurality of charging rollers 40 may be provided. Just do it. For example, the number of charging rollers 40 may be two, or four or more.

Further, although the image forming apparatuses 1 and 2 according to the embodiment and the modified example of the present invention have been exemplified as those in which the inclination angle A of the charging roller 40 can be adjusted within a predetermined range, the inclination angle A is fixed at one value. may be
For example, by determining an optimum tilt angle A corresponding to the shape of the charging roller 40, such as a crown shape, a straight shape, or an inverted crown shape, the apparatus specification may be fixed at the determined tilt angle A. .

Finally, as a reference material, Table 3 below shows the relationship between the difference B in the axial center position at both end positions of the elastic contact portion 42 of the charging roller 40 and the inclination angle A.
The crown-shaped and reverse-crown-shaped charging rollers 40 have an outer diameter deviation of the elastic contact portion 42 of 0.1 mm.

As described above, the invention made by the present inventor has been specifically described based on the above-described embodiments of the invention, but the present invention is not limited to the above-described embodiments of the invention, and is within the scope of the invention. can be changed in various ways.

REFERENCE SIGNS LIST 1, 2 image forming apparatus 10 color unit 11 photoreceptor 11a photoreceptor surface 12 exposure unit 13 developing unit 13a toner storage unit 13b developing roller 14 photoreceptor cleaner 15 transfer unit 15a transfer belt 15b belt drive roller 15c belt tension roller 15d Primary transfer roller 15e Belt cleaner 16 Conveying section 16a Conveying roller 16b Secondary transfer roller 17 Fixing section 20 Charging section 21 Bearing section 22 Fixed section 22a Spring accommodating section 22b Guide wall section 22c Guide protrusion 23 Movable bearing section 24 Electrode holding section 25 Axial biasing member holding portions 25a, 25b Axial biasing member 26 Shaft end rotation support portions 27, 28 Metal shaft portion holding recess 30 Fixed frame 31 Fixed portion 32 Multiple charging roller arrangement portion 32a Bearing fixing surface 33 Screw fixation Portion 34 Angle adjustment guide slit 40 Charging roller 41 Metal shaft portion 42 Elastic contact portion (charging area)
50 control section 50a, 50b applied voltage automatic control section 60 cleaning roller 61 metal shaft section 62 cleaning material A inclination angle B difference in axial center position between both ends of charging roller D distance between axial centers of photoreceptor and charging roller E electrode Plate P Transfer material such as paper R1 Photoreceptor radius R2 Charging roller radius S Spring T Cross amount W Overlapping portion

Claims (5)

An image forming apparatus comprising: a photoreceptor; and a charging unit having a charging roller that charges the surface of the photoreceptor by contact with the surface of the photoreceptor that is moved by a rotational movement about a rotation axis of the photoreceptor. ,
The charging unit is
a plurality of the charging rollers are provided along a direction intersecting the moving direction of the photoreceptor surface such that the ends of adjacent charging regions overlap in the moving direction of the photoreceptor surface;
An image forming apparatus, wherein one or more of the charging rollers among the plurality of charging rollers are provided with a rotation axis direction inclined with respect to a direction perpendicular to a movement direction of the surface of the photoreceptor. The tilt angle is
The image forming apparatus according to claim 1, wherein the angle is 0.2 degrees or more and 2 degrees or less. The plurality of charging rollers are
3. The image forming apparatus according to claim 1, wherein the inclination angle is adjustable within a predetermined range. A discharge start voltage from the plurality of charging rollers to the photoreceptor is timely detected, and control is performed to apply a DC voltage obtained by adding a predetermined voltage to the detected discharge start voltage to the plurality of charging rollers. 4. The image forming apparatus according to claim 1, further comprising an applied voltage automatic control section. The discharge start voltages from the plurality of charging rollers to the photoreceptor are timely detected, and an AC voltage having a peak-to-peak voltage of a predetermined multiple that is twice or more the detected discharge start voltage is superimposed on the DC voltage. 4. The image forming apparatus according to claim 1, further comprising an automatic application voltage control section that performs control to apply the applied voltage to the plurality of charging rollers.

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