JP2010191340A - Charging device, image forming cartridge and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide: a charging device capable of achieving high durability by simple voltage control operation; an image forming cartridge using the same; and an image forming apparatus. <P>SOLUTION: The charging device includes: a charging roller 2a opposed to a turning photoreceptor 1; charging power sources 30 and 33 applying charging voltage obtained by superposing AC voltage on DC voltage to the charging roller 2a; and charging voltage control means 40 and 41 making the AC voltage of the charging voltage to be applied to the charging roller by the charging power sources equal to the voltage in a charging section in one or several units assuming that a charging area of image-forming of one sheet is minimum unit, but switching it to a high value 2.3 kV and a low value 2.0 kV alternately in one or several units. The charging voltage control means measures charging saturation current IS and charging saturation voltage VPs, and hold them in memory 43 so as to use them. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a charging device for charging a photosensitive member, and an image forming cartridge and an image forming apparatus using the charging device. In particular, although not intended to be limited thereto, a charging roller facing a photosensitive member that is a photosensitive member in a non-contact manner. The present invention also relates to a charging device that is a combination with a charging power source that applies a charging voltage in which an AC voltage is superimposed on a DC voltage.

  Conventionally, as a charging device of an image forming apparatus, a charger-type device such as Scorotron has been mainly used, but there is a problem that a large amount of discharge products such as ozone is generated. The charging device of the contact type has come to be widely used.

  Patent Document 1 uses an elastic roller portion made of a material containing an ionic conductive electrical resistance adjusting material, a power source capable of outputting a DC voltage and an AC voltage, and a power source control means. The output voltage from the power source is made of a material in which carbon powder is dispersed by performing control to switch at a predetermined timing from a superimposed voltage obtained by superimposing an AC voltage on a DC voltage to an AC voltage consisting of only an AC component. There has been described a charging device that uses a discharge member that is less expensive than the discharge member and charges the photosensitive member at a stable potential over a long period of time.

  Patent Document 2 describes an image forming apparatus in which a charging bias is applied before the position on the drum with which the cleaning blade is in contact with the charging roller contact portion when the apparatus is stopped. The charging bias is an AC bias that is larger than the bias at the time of image formation, or a bias in which AC and long currents are superimposed so that the DC component changes alternately between positive and negative, thereby removing foreign matter and the coating agent of the cleaning means from the photosensitive drum surface. Even when the cleaning means is slipped through, the adhesion of foreign matter to the charging roller is suppressed.

  In Patent Document 3, in an image forming apparatus capable of applying a charging bias in which a DC voltage is superimposed on at least three or more AC peak-to-peak voltages, a charging AC bias at the time of image formation selected by AC current detection can be applied. When a peak-to-peak voltage that is two or more steps lower than the maximum peak-to-peak voltage is selected, a bias that is two or more steps higher than the charging AC bias at the time of image formation is applied at least during a pre-rotation before image formation By doing so, it is described that image defects caused by foreign matters remaining on the photosensitive drum during the pre-rotation are removed.

  In the contact-type charging device, since dust such as toner on the photosensitive member (photosensitive member) is electrostatically attracted to the charging member, uneven charging due to contamination of the toner and the like over time occurs, and the life of the charging device It was a big factor to decide.

  In order to reduce the contamination of the charging roller (charging member), a method has been proposed in which a sheet-like member is attached to both ends of the charging roller to form a minute charging gap between the photoreceptor and the charging roller ( For example, Patent Document 4). As a method for forming a charging gap without using a sheet-like member, there is a method in which a step or groove is provided at the end of the charging roller and a spacer member is attached thereto (for example, Patent Document 5). As a material of the charging roller, an elastic member such as rubber or sponge is generally used, but a method using a resin material is also known (for example, Patent Document 6 and Patent Document 7). There has also been proposed a method in which a roller is attached to the end of the charging member to form a charging gap while reducing friction with the photoreceptor (for example, Patent Document 8 and Patent Document 9). In this case, the gap holding member (roller) is disposed so as to contact the non-coated portion (non-photosensitive layer) outside the image area of the photoconductor. Also, inorganic fine particles are dispersed to improve the wear resistance and mechanical strength of the organic photoreceptor (for example, Patent Document 10), or fluororesin fine particles are dispersed to improve the lubricity (for example, Patent Document). 11) etc., a method of forming a protective layer on the surface of the photoreceptor in relation to the charging member is also known. In the method of providing the charging roller in a non-contact state with respect to the photoconductor, the contamination of the charging roller can be reduced as compared with the contact method.

  However, when an AC (alternating current) voltage is applied to the charging roller, the amount of ozone generated is smaller than that of the corona charger, but there is an adverse effect due to the discharge product. That is, the discharge product, toner, and toner additive are fused and adhered to the photoconductor surface (commonly known as photoconductor filming), thereby reducing the resistance of the photoconductor surface and reducing the resolution of the latent image. However, blurring, image flow, etc. are likely to occur. This impairs the smoothness of the surface of the photosensitive member, induces poor cleaning of the photosensitive member, and makes the quality with time unstable. Therefore, a means for suppressing photoconductor filming by applying a lubricant to the photoconductor has been proposed and commercialized, but by applying the lubricant, the charging roller can be brought into a non-contact state. Even in such a case, the charging roller is easily soiled, and the life of the charging device is not extended more than expected.

  SUMMARY An advantage of some aspects of the invention is that it provides a charging device capable of realizing high durability by a relatively simple voltage control operation, an image forming cartridge using the charging device, and an image forming apparatus. To do. Specifically, an object is to suppress both contamination of the charging roller and toner filming on the surface of the photosensitive member, and to increase the durability of the charging roller and the photosensitive member.

(1) A charging roller (2a) facing the rotating photoreceptor (1);
A charging power source (30, 33) for applying a charging voltage obtained by superimposing an AC voltage on a DC voltage to the charging roller; and
The AC voltage of the charging voltage applied to the charging roller by the charging power source is set to the same voltage in one or several units of charging sections with the charging unit for image formation of one sheet as the minimum unit. A charging device comprising: a high value (2.3 kV) and a low value (2.0 kV) alternately (FIG. 7) and charging voltage control means (40, 41);

  In addition, in order to make an understanding easy, the code | symbol or corresponding matter of the corresponding element of the actual command which is shown in a drawing and mentions later is added to the parenthesis as reference for reference. The same applies to the following (2) to (12).

  When an alternating voltage having a low value is applied consistently, filming of the photoconductor is suppressed, but vertical streaks due to charging roller contamination tend to occur. However, when a high AC voltage is applied consistently, the occurrence of vertical streaks due to contamination of the charging roller is suppressed, but filming of the photoreceptor is likely to occur. According to the present invention, the charging voltage control means (40, 41) are alternately switched between a high value (2.3 kV) and a low value (2.0 kV) (FIG. 7), so by applying a low value (2.0 kV) The charging roller contamination is suppressed by applying a high value (2.3 kV), and the filming of the photoconductor by applying a high value is suppressed by applying a low value. Overall, it is possible to obtain the effect of suppressing the charging roller contamination and the filming of the photosensitive member.

1 is a longitudinal sectional view showing an outline of a mechanism of a color printer according to a first embodiment of the present invention. FIG. 2 is an enlarged longitudinal sectional view showing one of the image forming cartridges shown in FIG. 1. FIG. 4 is a block diagram showing a combination of power sources 30 and 33 for applying a charging voltage to the charging roller 2a shown in FIG. 2 and control means 40 and 41 for controlling the charging voltage. FIG. 4 is a flowchart showing an outline of charging current control in which a CPU shown in FIG. 3 measures a saturation current value IS (FIG. 8) of the charging roller 2a. 4 is a flowchart showing an outline of charging voltage control in which a CPU shown in FIG. 3 measures a saturation voltage value VPs (FIG. 7) of the charging roller 2a. 4 is a flowchart showing an outline of charge voltage control during image formation in which a CPU shown in FIG. 3 switches a charge voltage applied to the charging roller 2a for image formation. 4 is a time chart showing several examples of change patterns of charging voltage applied to the charging roller 2a by image forming charging voltage control of a CPU 42 shown in FIG. 3 is a graph showing a charging potential saturation characteristic of the photosensitive member 1 shown in FIG. 2 according to a charging current of a charging roller 2a. 3 is a graph showing a charging potential saturation characteristic of the photosensitive member 1 shown in FIG. 2 according to a charging voltage of a charging roller 2a. It is an enlarged plan view showing the shape of the toner, and also shows an index value calculation formula tail representing the shape. FIG. 3 is an enlarged plan view showing a toner shape and parameters defining the shape.

  (2) The charging section in which the high AC voltage is applied is equal to or less than the charging section in which the low AC voltage is applied; the charging device according to (1) above.

  (3) The charging voltage control means (40, 41) obtains the AC voltage (VPs) that becomes the charging current value (IS) when the charging potential of the photoconductor (1) is saturated, and stores the memory (43). (FIG. 5), one of the high and low values of the AC voltage is the AC voltage (VPs) held in the memory (43), and the other is higher than the set value (0.3 kV); The charging device according to 1) or (2).

  (4) The charging voltage control means (40, 41) obtains a charging current value (IS) when the charging potential of the photoconductor (1) is saturated and holds it in the memory (43) (FIG. 4); The charging device according to (3) above.

  (5) The charging device according to any one of (1) to (4), further including a lubricant application member that applies a lubricant to the photoconductor.

(6) the photoreceptor (1);
The charging device according to any one of (1) to (5) above;
A developing device for applying toner to the electrostatic latent image formed on the photoreceptor (1) to form a toner image; and
Cleaning means for cleaning the surface of the photosensitive member (1) after the toner image is transferred to a sheet or an intermediate transfer member;
An image forming cartridge (FIG. 2).

  (7) The toner of the developing device has a volume average particle diameter of 3 to 8 μm and a ratio (Dv / Dn) of the volume average particle diameter (Dv) to the number average particle diameter (Dn) of 1.00 to 1.n. 40. The image forming cartridge according to (6) above.

  (8) The toner of the developing device has a shape factor SF-1 in the range of 100 to 180 and a shape factor SF-2 in the range of 100 to 180; the image forming cartridge according to (6) above.

  (9) The toner of the developing device is obtained by crosslinking, in an aqueous medium, a toner material liquid in which at least a polyester prepolymer having a functional group containing a nitrogen atom, polyester, a colorant, and a release agent are dispersed in an organic solvent. And / or a toner obtained by an extension reaction; The image forming cartridge according to (6) above.

  (10) The toner of the developing device has a substantially spherical shape; the image forming cartridge according to (6).

(11) The image forming cartridge according to any one of (6) to (10) above;
An exposure device (9) for forming an electrostatic latent image by projecting image light onto the charging surface of the photoconductor of the image forming cartridge;
Transfer means (51, 61) for transferring the toner image directly or indirectly to the sheet via an intermediate transfer member (56); and
A fixing device (71 to 74) for fixing the toner image on the paper to the paper;
An image forming apparatus comprising:

(12) The image forming apparatus includes an intermediate transfer belt (56) and a plurality of image forming cartridges arranged along the intermediate transfer belt;
The transfer means (51, 61) includes a primary transfer means (51) that superimposes and transfers each toner image formed by each cartridge to the same position on the intermediate transfer belt (56), and a superposition transfer on the transfer belt. And a secondary transfer means (61) for transferring the toner image to the paper. The image forming apparatus according to any one of (6) to (11).

  Other objects and features of the present invention will become apparent from the following description of embodiments with reference to the drawings.

  FIG. 1 shows a printer PRT which is an image forming apparatus according to a first embodiment of the present invention. The printer PRT includes an intermediate transfer belt 56 in the approximate center of the inside thereof. The intermediate transfer belt 56 is an endless belt made of a heat-resistant material such as polyimide or polyamide and made of a base adjusted to a medium resistance. The intermediate transfer belt 56 is supported around four rollers 52 to 55 in the direction of arrow A in the figure. Driven by rotation. Below the intermediate transfer belt 56, four image forming cartridges corresponding to yellow (Y), magenta (M), cyan (C), and black (K) toners are arranged along the belt surface of the intermediate transfer belt 56. It is out.

  FIG. 2 shows an enlarged view of one PRC of the four image forming cartridges. All of the image forming cartridges have the same configuration. Each of the image forming cartridges shown in FIG. 1 has a photoconductor 1 (Y, M, C, and K). Around each photoconductor 1, a charging device 2 that applies a charge to the surface of the photoconductor 1, a photoconductor A developing device 4 that develops a latent image formed on one surface with each color toner to form a toner image, a lubricant application device 3 that applies a lubricant to the surface of the photoreceptor 1, and the surface of the photoreceptor 1 after the toner image is transferred Each of the cleaning devices 8 is arranged.

Referring to FIG. 1 again, below the four image forming cartridges, there are provided exposure devices 9 for exposing the surfaces of the charged photoconductors based on the image data of the respective colors to form latent images.
A primary transfer roller 51 that primarily transfers a toner image formed on the photoconductor 1 onto the intermediate transfer belt 56 is disposed at a position facing each photoconductor 1 with the intermediate transfer belt 56 interposed therebetween. The primary transfer roller 51 is connected to a power source (not shown) and is applied with a predetermined voltage.

  A secondary transfer roller 61 is pressed against the outside of the portion of the intermediate transfer belt 56 supported by the roller 52. The secondary transfer roller 61 is connected to a power source (not shown) and is applied with a predetermined voltage. A contact portion between the secondary transfer roller 61 and the intermediate transfer belt 56 is a secondary transfer portion, and the toner image on the intermediate transfer belt 56 is transferred to a sheet. An intermediate transfer belt cleaning device 57 for cleaning the surface of the intermediate transfer belt 56 after the secondary transfer is provided outside the portion of the intermediate transfer belt 56 supported by the roller 55.

  A fixing device 70 is provided above the secondary transfer unit to fix the toner image on the paper semi-permanently on the paper. The fixing device 70 includes a heating roller 72 having a halogen heater therein and an endless fixing belt 71 wound around the fixing roller 73, and a pressurization disposed so as to be opposed to and in pressure contact with the fixing roller 73 via the fixing belt 71. And a roller 74. Below the image forming apparatus, there is provided a paper feeding device 20 for placing paper and feeding the paper toward the secondary transfer unit.

  Referring to FIG. 2 again, one image forming cartridge can be attached to and detached from the main body of the printer PTR in which the photoconductor 1, the charging device 2, the lubricant applying device 3, the developing device 4 and the cleaning device 8 are integrally assembled. An image process cartridge. The photoreceptor 1 is an organic photoreceptor, and a surface protective layer is formed of a polycarbonate-based resin. The charging device 2 includes a charging roller 2a configured by covering a conductive cored bar with a medium resistance elastic layer as a charging member.

  The charging roller 2a is connected to charging power sources 30 and 33 (FIG. 3), and a predetermined voltage is applied thereto. The charging roller 2 a is disposed with a small gap with respect to the photoreceptor 1. The minute gap is set by, for example, winding a spacer member having a certain thickness around the non-image forming regions at both ends of the charging roller 2a to bring the surface of the spacer member into contact with the surface of the photoreceptor 1. can do. Further, the charging roller 2a is provided with a charging cleaning member 2b for cleaning in contact with the surface of the charging roller 2a.

  The lubricant application device 3 includes a solid lubricant 3 b housed in a fixed case, and a brush roller 3 a that scrapes the lubricant in contact with the solid lubricant 3 b and applies it to the photoreceptor 1. The solid lubricant 3b is formed in a rectangular parallelepiped shape and is urged toward the brush roller 3a by the pressure member 3c. The pressurizing member 3c is preferably a spring such as a leaf spring or a compression spring, and a compression spring can be preferably used as shown in the drawing. The solid lubricant 3b is scraped off and consumed by the brush roller 3a, and the thickness of the solid lubricant 3b decreases with time. However, since the solid lubricant 3b is pressurized by the pressure member 3c, it is always in contact with the brush roller 3a. The brush roller 3 a applies the lubricant scraped off while rotating to the surface of the photoreceptor 1.

Brush thickness of the fibers of the brush roller 3a is preferably 3 to 8 deniers, the density of the brush fibers 20000-100000 present / inch ² is preferable. If the thickness of the brush fiber is too thin, the brush roller 3a is liable to fall down when the brush roller 3a comes into contact with the surface of the photoconductor 1. Conversely, if the brush fiber is too thick, the density of the fiber cannot be increased. In addition, if the density of the brush fibers is low, the number of brush fibers in contact with the surface of the photoreceptor 1 is small, so that the lubricant cannot be uniformly applied. The gap becomes small, and the amount of the lubricant powder that has been scraped off decreases, resulting in an insufficient amount of coating. Therefore, the brush roller 3a having the above-described setting range has the thickness of the brush fiber for making the hair fall difficult to occur and the density of the brush fiber that can efficiently apply the lubricant uniformly.

  As the solid lubricant 3b, a dry solid hydrophobic lubricant can be used. Besides zinc stearate, barium stearate, lead stearate, iron stearate, nickel stearate, cobalt stearate Those having a stearic acid group such as copper stearate, strontium stearate, calcium stearate, cadmium stearate and magnesium stearate can be used. In addition, the same fatty acid groups zinc oleate, manganese oleate, iron oleate, cobalt oleate, lead oleate, magnesium oleate, copper oleate, and palmitic acid, zinc cobalt palmitate, copper palmitate, palmitate Magnesium acid, aluminum palmitate, and calcium palmitate may be used. In addition, fatty acids such as lead caprylate, lead caproate, zinc linolenate, cobalt linolenate, calcium linolenate and cadmium ricolinolenate, metal salts of fatty acids, and the like can also be used. Further, waxes such as candelilla wax, carnauba wax, rice wax, wax, ooba oil, beeswax, and lanolin can be used.

  In the present embodiment, a cleaning blade 8a as a cleaning unit is brought into contact with the surface of the photoreceptor downstream in the moving direction with respect to a lubricant application position by the brush roller 3a, and lubrication as a lubricant leveling unit is performed. It has a function as a leveling agent. The cleaning blade 8a is made of rubber which is an elastic body.

  In the developing device 4, a developing sleeve 4 a provided with a magnetic field generating unit is disposed at a position facing the photoconductor 1. Below the developing sleeve 4a, there are provided two screws 4b for mixing toner introduced from a toner bottle (not shown) with the developer and pumping it up to the developing sleeve 4a while stirring. The developer composed of toner and magnetic carrier pumped up by the developing sleeve 4a is regulated to a predetermined developer layer thickness by the doctor blade 4c and is carried on the developing sleeve 4a. The developing sleeve 4a carries and conveys the developer while moving in the same direction at a position facing the photoconductor 1, and supplies toner to the latent image surface of the photoconductor 1.

  The toner of the developing device 4 is obtained by crosslinking and / or crosslinking a toner material liquid in which at least a polyester prepolymer having a functional group containing a nitrogen atom, polyester, a colorant and a release agent are dispersed in an organic solvent in an aqueous solvent. A toner obtained by an extension reaction. In order to reproduce minute dots of 600 dpi or more, the volume average particle diameter of the toner of the developing device 4 is preferably 3 to 8 μm. The ratio Dv / Dn between the volume average particle diameter Dv and the number average particle diameter Dn is preferably in the range of 1.00 to 1.40. The closer the Dv / Dn is to 1.00, the sharper the particle size distribution. With such a toner having a small particle size and a narrow particle size distribution, the toner charge amount distribution is uniform, a high-quality image with little background fogging can be obtained, and the electrostatic transfer method has a high transfer rate. can do.

  Further, the shape factor SF-1 of the toner is preferably in the range of 100 to 180, and the shape factor SF-2 is preferably in the range of 100 to 180. (A) and (b) of FIG. 10 are diagrams schematically showing the shape of the toner in order to explain the shape factor SF-1 and the shape factor SF-2. The shape factor SF-1 indicates the ratio of the roundness of the toner shape and is expressed by the following equation (1). This is a value obtained by dividing the square of the maximum length MXLNG of the shape formed by projecting toner on a two-dimensional plane by the graphic area AREA and multiplying by 100π / 4.

SF-1 = {(MXLNG) ² / AREA} × (100π / 4) (1)
When the value of SF-1 is 100, the shape of the toner is a true sphere, and becomes larger as the value of SF-1 increases. The shape factor SF-2 indicates the ratio of the unevenness of the toner shape, and is represented by the following equation (2). This is a value obtained by dividing the square of the peripheral length PERI of the figure formed by projecting the toner on the two-dimensional plane by the figure area AREA and multiplying by 100π / 4.

SF-2 = {(PERI) ² / AREA} × (100π / 4) (2)
When the value of SF-2 is 100, unevenness does not exist on the toner surface, and as the value of SF-2 increases, the unevenness of the toner surface becomes more prominent. Specifically, the shape factor is measured by taking a photograph of the toner with a scanning electron microscope (S-800: manufactured by Hitachi, Ltd.), introducing it into an image analyzer (LUSEX 3: manufactured by Nireco) and analyzing it. Calculated.

  When the shape of the toner is close to a spherical shape, the contact state between the toner and the toner or the toner and the photoconductor becomes a point contact, so that the adsorbing force between the toners becomes weak and the fluidity increases, and the toner and the photoconductor The attraction force becomes weaker and the transfer rate becomes higher. If either of the shape factors SF-1 and SF-2 exceeds 180, the transfer rate is lowered, which is not preferable.

  The toner of the developing device 4 has a substantially spherical shape and can be expressed by the following shape rule. FIG. 11 is a diagram schematically showing the shape of the toner of the present invention. In FIG. 11, when a substantially spherical toner is defined by a major axis r1, a minor axis r2, and a thickness r3 (where r1 ≧ r2 ≧ r3), the toner of the present invention has a major axis and a minor axis. The ratio (r2 / r1) (see FIG. 11B) is 0.5 to 1.0, and the ratio of thickness to minor axis (r3 / r2) (see FIG. 11C) is 0. It is preferable that it exists in the range of 7-1.0. When the ratio of the major axis to the minor axis (r2 / r1) is less than 0.5, the dot reproducibility and transfer efficiency are inferior because of being away from the true spherical shape, and high-quality image quality cannot be obtained. On the other hand, if the ratio of thickness to minor axis (r3 / r2) is less than 0.7, the shape is close to a flat shape, and a high transfer rate like a spherical toner cannot be obtained. In particular, when the ratio of the thickness to the minor axis (r3 / r2) is 1.0, the rotating body has a major axis as a rotation axis, and the fluidity of the toner can be improved. Note that r1, r2, and r3 were measured with a scanning electron microscope (SEM) while changing the angle of field of view and taking pictures.

  FIG. 3 shows a configuration of a charging power source that applies a charging voltage to the charging roller 2a. The DC constant voltage circuit 31 of the DC power supply 30 is controlled by constant voltage control (feedback voltage control) so that the output voltage detected by the voltage sensor 31 matches the target voltage Vdco provided by the CPU 42 of the process controller 41 that controls the image forming process. ) To control the DC voltage. In this embodiment, the DC voltage for image formation is -750V.

  Although not shown, a system controller (also referred to as a printer controller) that instructs the process controller 41 to perform printing in response to a print command from a user terminal (for example, a personal computer), and gives a user instruction to the system controller. There are external communication devices that perform communication between an operation board and system controller and a user terminal. The system controller sets the initial state or operation (printing conditions) of the printer PRT according to a user instruction from the operation board and the user terminal, and instructs the process controller 41 to form an image when there is a printing instruction. In addition to the charging power interface 40 shown in FIG. 3, the process controller 41 is connected to an interface with drivers and sensors of other image forming related devices. The system controller 41 responds to instructions from the system controller. Performs initial settings, image formation process control, etc.

  When the CPU 42 of the process controller 41 instructs the AC power supply 33 to output an AC constant voltage, the constant voltage control 36 of the AC power supply 33 matches the output voltage detected by the voltage sensor 37 with the target voltage Vaco given by the CPU 42. As described above, the output AC voltage Vpp (peak-to-peak voltage) of the AC power supply circuit 34 is controlled by constant voltage control (feedback voltage control). When the CPU 42 instructs the constant current output, the constant current control 38 of the AC power supply 33 causes the target current Iaco provided by the CPU 42 to match the output current (charging current) detected by the current sensor 39. The circuit 34 is subjected to constant current control (feedback current control) to control the current (charging current) flowing between the charging roller 2a / photosensitive member 1.

  The target voltages Vdco and Vaco and the target current Iaco output from the CPU 42 are digital data. However, the charging power interface (electric circuit) 40 converts the digital data into an analog signal, and the DC constant voltage circuit 31 and constant voltage control. 36 and constant current control 38.

  Next, the control of the charging voltage applied to the charging roller 2a will be specifically described. FIG. 9 shows a state in which the surface of the photoreceptor 1 is charged by applying a charging voltage obtained by superimposing a constant voltage AC voltage of a peak-to-peak voltage Vpp to a DC constant voltage of −750 V to the core of the charging roller 2a. The relationship between the peak-to-peak voltage Vpp and the surface potential of the photoreceptor 1 is shown. Each of the lines G1, G2, G3, and G4 shows a charging potential saturation characteristic in which the gap G between the photoconductor 1 / charging roller 2a is 80 μm, 60 μm, 40 μm, and 0 μm, respectively. The frequency of the AC voltage is the same. As can be seen from FIG. 9, whatever the gap G is, when the AC peak-to-peak voltage Vpp exceeds a certain value, the surface potential of the photosensitive member 1 exhibits a substantially constant value, and this value is applied to the charging roller 2a. The DC constant voltage (-750 V in the illustrated example) is substantially the same. That is, when the gap is G1 (80 μm), when the peak-to-peak voltage is equal to or higher than Vp1, the surface potential of the photoreceptor 1 becomes a constant value of approximately −750 V, and the gaps are similarly G2 (60 μm) and G3 (40 μm), respectively. And G4 (0 μm), when the peak-to-peak voltages are Vp2, Vp3, and Vp4, respectively, the surface potential of the photosensitive member 1 is almost −750 V (constant value).

  On the other hand, since the thickness of the spacer that separates the charging roller 2a from the surface of the photoreceptor is known in advance, even if the gap G varies, the size of the maximum gap can be grasped in advance by experiments or the like. Therefore, in the case of a printer in which the maximum gap G when the photosensitive member 1 and the charging roller 2a are rotated is, for example, 80 μm, the DC constant voltage is an alternating current with a peak-to-peak voltage value controlled at a constant voltage of Vp1 or higher. A voltage on which the voltage is superimposed is applied to the charging roller 2a. In the case of a printer with a maximum gap G of 60 μm, an AC voltage having a peak-to-peak voltage controlled at a constant voltage of Vp2 or higher is controlled, and in the case of a printer with a maximum gap G of 40 μm, a constant voltage of Vp3 or higher is controlled. The AC voltage of the peak-to-peak voltage is applied to the charging roller 2a together with the DC constant voltage.

  In this way, the surface of the photoreceptor can be charged to a substantially constant potential (approximately −750 V in the example of FIG. 9) when each printer is operated. In addition, since the AC voltage applied to the charging roller 2a is not controlled at constant current but is controlled at constant voltage, the problem of uneven density of the halftone image, that is, the occurrence of a horizontal stripe pattern observed in the case of constant current control is prevented. it can. Thus, by setting the value of the peak-to-peak voltage controlled at a constant voltage to a value at which the surface potential of the member to be charged is substantially constant regardless of the size of the gap G, All of the drawbacks that have occurred in the case of the AC application method can be eliminated.

  By the way, in the case of the AC application method of this example, the peak-to-peak voltage capable of charging the surface potential of the photosensitive member 1 to a substantially constant value may be applied to the charging roller 2a. The large voltage shown may be applied. In this way, the above-described effects can be achieved when the maximum gap is 40 μm, when it is 60 μm larger than this, and when the maximum gap is 80 μm. In addition, the peak-to-peak voltage value applied to the charging roller 2a can be easily set.

  However, when the peak-to-peak voltage value becomes too large, the photoreceptor 1 is easily fatigued. For example, when the maximum gap is 80 μm and the peak-to-peak voltage of Vp5 is applied to the charging roller 2a, especially when the gap G between the rotating photoreceptor 1 and the charging roller 2a is minimized. The voltage applied to the charging roller 2a becomes excessive, the electric field formed between the charging roller 2a and the photoconductor 1 becomes too strong, and the fatigue of the photoconductor 1 is promoted to shorten its life. It is done. In addition, toner filming is likely to be formed on the surface of the photoconductor, which may cause abnormal images.

  Therefore, the value of the peak-to-peak voltage under constant voltage control is set to the minimum voltage value among the peak-to-peak voltage values at which the surface potential of the photoreceptor 1 as a charged body is almost constant regardless of the size of the gap G. It is preferable to set to. For example, in FIG. 9, when the maximum gap G is 80 μm, the peak-to-peak voltage value is set to Vp1, when it is 60 μm, it is set to Vp2, and when it is 40 μm, it is set to Vp3. In this way, even when the gap G between the rotating photosensitive member 1 and the charging roller 2a is minimized, an excessive voltage is not applied to the charging roller 2a, thereby preventing the above-described problems. can do.

  The peak-to-peak voltage value Vpp applied to the charging roller 2a can also be set as follows. That is, as shown in FIG. 8, when the method of superimposing AC on DC is adopted, if the charging current value supplied to the charging roller 2a is equal to or greater than IS, the surface of the photoreceptor is almost constant at any gap G. (-750 V in the example of FIG. 8). If the current value is greater than or equal to IS, the potential on the surface of the photoreceptor is saturated. Therefore, an AC constant current is supplied from the charging roller 2a to the photosensitive member 1 by constant current control, and the photosensitive member 1 is supplied from the charging roller 2a when the surface potential of the photosensitive member 1 is saturated to a substantially constant value. If the current value IS is referred to as a saturation current value IS, this saturation current value IS is detected (FIG. 4), and the value is stored in the nonvolatile memory 43.

  FIG. 4 shows an outline of the “detection of saturation current value IS” CSD. In response to an instruction to set the “saturation current value IS” in the initial settings from the operation board (not shown), the system controller (not shown) responds to the process controller 41 shown in FIG. When “detection of value IS” is instructed, the process controller 41 (CPU 42 thereof) executes “detection of saturation current value IS” CSD. Here, first, rotation of the photosensitive member 1 is started (s1). As a result, the photoreceptor 1 and the charging roller 2a start to rotate. Next, the output of the DC constant voltage circuit 31 is set to −750V. That is, the target voltage Vdco specifying -750V is applied to the DC constant voltage circuit 31, and the DC constant voltage circuit 31 is turned on (target voltage output) (s2).

  Next, a constant current output (38 operation, 36 stop) is instructed to the AC power source 33 (s3), and the target current Iaco representing the initial current value (the minimum value of the target current range determined for IS detection) is set to the constant current control 38. The AC power supply circuit 34 and the constant current control 38 are turned on (charging current output) (s4).

  Then, the CPU 42 repeatedly reads the detection potential of the potential sensor 5, that is, the surface potential of the photosensitive member 1 at a predetermined period while the charging roller 2a rotates a set number of times, and the charging potential (surface potential) becomes a set value (−750V). If it has not been reached, the target current Iaco given to the constant current control 38 is changed to a high value by one step (s6), and the detection potential of the potential sensor 5 is read to charge the potential. It is searched whether the value has reached the set value (−750V) (s5). When the set value is reached, the target current Iaco given to the constant current control 38 at that time is stored in the nonvolatile memory 43 as the saturation current value IS (s7). That is, the saturation current value IS held in the nonvolatile memory 43 is updated to the value detected this time. Then, the power supplies 30 and 33 are turned off (output stop) (s8), and the rotation drive of the photosensitive member 1 is stopped (s9). Since the charging roller 2a is driven in conjunction with the photoreceptor 1, the charging roller 2a also stops rotating.

  On the other hand, at the time of warming up the printer PTR or at the time of non-image formation such as after the end of printing, AC voltages having several peak-to-peak voltage values are respectively applied to the charging roller 2a by constant voltage control. If the voltage application at this time is referred to as preliminary application, the peak-to-peak voltage value VPs when the value of the current supplied to the charging roller 2a at the preliminary application becomes the saturation current value IS is detected (FIG. 5). At the time of printing (image formation), a charging voltage obtained by superimposing the alternating current of the peak-to-peak voltage value VPs detected as described above on the above-described DC constant voltage (in this example, −750 V) is applied to the charging roller 2a, Charge the surface of the photoreceptor. The peak-to-peak voltage value VPs as the saturation current value IS is set to the peak-to-peak voltage value Vpp of the AC voltage applied to the charging roller 2a.

  FIG. 5 shows an outline of the “detection of peak-to-peak voltage value VPs” VSD. When the printer PTR warms up, printing is terminated, and when the integrated value of the number of prints after the previous “detection of peak-to-peak voltage value VPs” VSD has increased by more than the set value, or “ When “detection of peak-to-peak voltage value VPs” “VSD” is instructed, the CPU 42 starts “detection of peak-to-peak voltage value VPs” VSD. Here, first, rotation of the photosensitive member 1 is started (s11). As a result, the photoreceptor 1 and the charging roller 2a start to rotate. Next, the output of the DC constant voltage circuit 31 is set to −750V. That is, the target voltage Vdco specifying -750 V is applied to the DC constant voltage circuit 31, and the DC constant voltage circuit 31 is turned on (target voltage output) (s12).

  Next, a constant current output (36 operation, 38 stop) is instructed to the AC power source 33 (s13), and the target voltage Vaco representing the initial voltage value (the minimum value of the target voltage range determined for VPs detection) is controlled by the constant voltage control 36. The AC power supply circuit 34 and the constant voltage control 36 are turned on (charging AC voltage output) (s14).

  The CPU 42 repeatedly reads the current detected by the current sensor 39, that is, the charging current at a predetermined cycle while the charging roller 2a rotates a set number of times, and the charging current reaches the saturation current value IS stored in the nonvolatile memory 43. If the target voltage Vaco given to the constant voltage control 36 is changed to a high value by one step (s16), the detection voltage of the current sensor 39 is read, and the charging current is It is searched whether the saturation current value IS has been reached (s15). When the saturation current value IS is reached, the target voltage Vaco applied to the constant voltage control 36 at that time is stored in the nonvolatile memory 43 as the saturation voltage value VPs (s17). That is, the saturation voltage value VPs held in the nonvolatile memory 43 is updated to the value detected this time. Then, the power sources 30 and 33 are turned off (output stop) (s18), and the rotation of the photosensitive member 1 is stopped (s19). Since the charging roller 2a is driven in conjunction with the photoreceptor 1, the charging roller 2a also stops rotating.

  At the time of image formation, the surface of the photoreceptor is charged by applying to the charging roller 2a a voltage obtained by superimposing the AC voltage of the saturation voltage value VPs (peak-to-peak voltage) on the aforementioned DC constant voltage (-750 V in this example). Even when the environment changes or the wear of the spacer progresses, the current of the saturation current value IS is always applied to the charging roller 2a, in other words, the surface of the photosensitive member is set to a predetermined potential (-750 V in this example). A peak-to-peak voltage that can be charged is applied to the charging roller 2a. When this applied voltage setting method is adopted, the user needs to set the peak-to-peak voltage value to be applied to the charging roller 2a based on the detection result of the environment detection means and the gap detection means or the detection result of the usage time of the charging member 2. The maintenance work can be simplified.

  By the way, as described above, the saturation voltage value VPs through which the saturation current value IS flows is set to the peak-to-peak AC voltage because the AC voltage applied to the charging roller 2a is excessive and the surface of the photoconductor is fatigued. This is to suppress problems such as toner filming as much as possible. However, the lubricant application device 3 that protects the surface of the photosensitive member by toner filming causes the finely divided lubricant to adhere to the roller surface even when the charging roller 2a is kept in contact with the photosensitive member 1 and charged. Prone to defects. Next, experimental examples are shown.

-Items common to Comparative Example 1 and Example 1-
Conditions related to charging are:
Linear velocity: 205 mm / sec.
Charging AC frequency: 1500Hz
AC saturation current IS: 1.6 mA
Evaluation conditions are
Original / transfer material size: A4
Original feed direction: Horizontal (210mm)
Color mode: Full color (4 colors) mode Number of prints per time (number of transfer material output per time): Two sheets were used.

-Comparative Example 1-
The application condition of the AC voltage (peak-to-peak voltage) Vpp for charging of each machine is as follows:
Machine A: VPs where the AC saturation current IS is 1.6 mA
Machine B: VPs + 100V
Machine C: VPs + 150V
Machine D: VPs + 200V
Machine E: VPs + 300V
It was. With the above five units, vertical streaks relating to charging roller contamination, density unevenness relating to photoconductor filming, poor cleaning, and photoconductor surface filming were evaluated. The results are shown in Table 1.

  The above VPs + 150V (Machine C) was the most balanced in image quality. However, paying attention to the vertical streak that is a measure of the contamination of the charging roller, the filming of the photoconductor slightly occurred, and although there is density unevenness due to this, it is better to use the machine D, that is, the above VPs + 200V. Therefore, the following Example 1 was carried out and evaluated.

Example 1
In response to the result of Comparative Example 1, Vpp was switched as follows for each predetermined image formation;
Normal application: VPs corresponding to the above IS × 1.00
Image formation period of a predetermined sheet: VPs + 300V (for example, 2300V for normal 2000V)
Predetermined sheet: 2 sheets (A4 paper feed is counted as 1 sheet)
In summary, the pattern A shown in FIG. 7 switches between the normal application of 2000 V and the application of 2300 V for every predetermined sheet each time two A4 landscape feed images are formed. The upper peak value of Vaco = 2.0 kV shown in FIG. 7 is + 250V, and the lower peak value is −1750V. The upper peak value of Vaco = 2.3 kV is +400 V, and the lower peak value is −1900 V. Evaluation was performed in the same manner as in Comparative Example 1 under the above conditions. The results are shown in Table 2.

  FIG. 6 shows an outline of “charging voltage control” CVC at the time of image formation executed by the CPU 42 of the process controller 41 shown in FIG. 3 in response to a print instruction from a system controller (not shown). Here, first, rotation of the photosensitive member 1 is started (s21). As a result, the photoreceptor 1 and the charging roller 2a start to rotate. Next, when the charging start timing for the first image formation is reached (s22), the CPU 42 sets the output of the DC constant voltage circuit 31 to -750V. That is, the target voltage Vdco specifying -750V is applied to the DC constant voltage circuit 31, and the DC constant voltage circuit 31 is turned on (target voltage output) (s23). Further, the AC power supply 33 is instructed to output a constant voltage (36 operation, 38 stop), and the target voltage Vaco representing the saturation voltage VPs of the nonvolatile memory 43 is given to the constant voltage control 36, so that the AC power supply circuit 34 and the constant voltage control 36 are provided. Is turned on (charging AC voltage output) (s24).

  After waiting for the charging end timing of the first sheet (s25), if printing is not completed, the data “0” (first sheet) in the flag register Ffp (one area of the internal memory of the CPU 42) is displayed to indicate the second sheet. ) Is rewritten to "1" (s27, s28), and the charging voltage output is continued (s25). When the charging end timing of the second sheet is reached, if the printing is not completed, the data in the flag register Ffp is returned to “0” (same as the third sheet: the first sheet) (s26, s27, s29), and the charging power source 30 and 33 are turned off (charging output is stopped) (s30), and the charging start timing for the next third image formation is waited (s31).

  Next, when the charging start timing for image formation on the third sheet comes (s31), the CPU 42 sets the output of the DC constant voltage circuit 31 to -750 V (s32). Further, the AC power supply 33 is instructed to output a constant voltage (36 operation, 38 stop), and a target voltage Vaco representing a voltage value obtained by adding 300 V to the saturation voltage VPs of the nonvolatile memory 43 is given to the constant voltage control 36, and the AC power supply The circuit 34 and the constant voltage control 36 are turned on (charging AC voltage output) (s33). Then, after waiting for the charging end timing of the third sheet (s34), if the printing is not completed, the data “0” (third sheet) in the flag register Ffp is displayed to indicate the fourth sheet (synonymous with the second sheet). Is rewritten to "1" (s36, s37), and the charging voltage output is continued (s34). When the charging end timing of the fourth sheet is reached, if the printing is not completed, the data in the flag register Ffp is returned to “0” (same as the fifth sheet: the first sheet) (s35, s36, s38), and the charging power source 30 and 33 are turned off (s39), and the process waits for the charging start timing for image formation of the next fifth sheet (s22).

  Until the printing is completed, the same charging control as the charging control for the first to fourth sheets is repeated. As a result, an AC voltage is applied to the charging roller 2a in the pattern A shown in FIG. When the printing is finished, the CPU 42 turns off the charging power sources 30 and 33 (s40), and stops the rotation of the photosensitive member 1 (s41). As a result, the charging roller 2a also stops rotating.

  As described above, by applying a voltage VPs + 300 V, which is different from the AC voltage value VPs to which the AC voltage Vpp normally applied to the charging roller 2a every time a predetermined number of images are formed, during the predetermined time of image formation, Photoreceptor filming is reduced. That is, the contamination of the charging roller 2a and the toner filming of the photosensitive member 1 resulted in a trade-off relationship in Comparative Example 1, but the saturation current value at which the occurrence of vertical stripes due to the contamination of the charging roller 2a is improved. “VPs + 300V” is consistently applied by alternately combining Vpp (VPs + 300V), which is greater than the saturation voltage value VPs, which is IS, and VPs, which is the saturation current value IS where the hazard is minimized, to the photosensitive member 1. Filming of the photosensitive member 1 when applied (Comparative Example 1) was suppressed, and vertical streaks due to charging roller contamination when VPs were applied consistently were also suppressed.

  Similar effects were obtained with the patterns B to D in FIG. 7 in which the application time of the voltage value different from the normal voltage value was 1: 1. Therefore, when the number of times of image formation (number) per time is arbitrarily different, it is effective to switch Vpp for each predetermined number of times of image formation (number of sheets) as in patterns E to H in FIG. It is. Furthermore, in this embodiment, the switching of the output Vpp is managed by the number of image formations (number of images), but the same effect can be obtained even if it is an arbitrary predetermined time.

  The printer PRT detects the saturation current value IS and holds it in the nonvolatile memory 43. It is preferable that the user adjust the increase / decrease in the initial setting. Further, instead of detecting the saturation current value IS in each printer, each printer of the same model has a saturation current value obtained by experiment or obtained by a prototype equipped with a charging device having the configuration shown in FIG. The IS can also be written in the nonvolatile memory. That is, the detection of the saturation current value IS can be omitted in each printer.

1: Photoconductor 2: Charging device 2a: Charging roller 2b: Charging cleaning member 3: Coating device 3a: Brush roller 3b: Solid lubricant 3c: Pressurizing member 4: Developing device 5: Potential sensor 51: Primary transfer roller 56 : Intermediate transfer belt 61: Secondary transfer roller 8: Cleaning device 8a: Blade

JP 2007-072030 A JP 2005-173273 A JP 2004-258539 A JP 2001-194868 A JP 2002-055508 A JP 2001-337515 A JP 2003-066693 A JP 2001-312121 A JP 2000-206805 A Japanese Patent Laid-Open No. 08-339092 JP 11-218945 A

Claims (12)

A charging roller facing the rotating photoreceptor;
A charging power source that applies a charging voltage in which an AC voltage is superimposed on a DC voltage to the charging roller; and
The AC voltage of the charging voltage applied to the charging roller by the charging power source is set to the same voltage in one or several units of charging sections with the charging unit for image formation of one sheet as the minimum unit. Charging voltage control means that switches between a high value and a low value alternately every time;
A charging device comprising:   The charging device according to claim 1, wherein the charging interval in which the high-value AC voltage is applied is equal to or less than the charging interval in which the low-value AC voltage is applied.   The charging voltage control means obtains the AC voltage that is a charging current value when the charging potential of the photoconductor is saturated and holds it in a memory, and holds one of the high value and the low value of the AC voltage in the memory. The charging device according to claim 1 or 2, wherein an AC voltage is used and the other is set higher than the AC voltage.   The charging device according to claim 3, wherein the charging voltage control unit obtains a charging current value when a charging potential of the photoconductor is saturated and holds the value in a memory.   The charging device according to claim 1, further comprising: a lubricant application member that applies a lubricant to the photoconductor. The photoreceptor;
The charging device according to any one of claims 1 to 5;
A developing device for applying toner to the electrostatic latent image formed on the photoreceptor to form a toner image; and
Cleaning means for cleaning the surface of the photoreceptor after the toner image is transferred to a sheet or an intermediate transfer member;
An image forming cartridge comprising:   The toner of the developing device has a volume average particle diameter of 3 to 8 μm and a ratio Dv / Dn of the volume average particle diameter Dv to the number average particle diameter Dn is in a range of 1.00 to 1.40; The imaging cartridge described in 1.   The image forming cartridge according to claim 6, wherein the toner of the developing device has a shape factor SF-1 in the range of 100 to 180 and a shape factor SF-2 in the range of 100 to 180.   The toner of the developing device is obtained by crosslinking and / or crosslinking a toner material liquid in which at least a polyester prepolymer having a functional group containing a nitrogen atom, a polyester, a colorant, and a release agent in an organic solvent is dispersed in an aqueous medium. The image forming cartridge according to claim 6, wherein the toner is obtained by an extension reaction.   The image forming cartridge according to claim 6, wherein the toner of the developing device has a substantially spherical shape. An image forming cartridge according to any one of claims 6 to 10;
An exposure apparatus for projecting image light onto a charged surface of the photosensitive member of the image forming cartridge to form an electrostatic latent image;
Transfer means for transferring the toner image to a sheet directly or indirectly via an intermediate transfer member; and
A fixing device for fixing the toner image on the paper to the paper;
An image forming apparatus comprising: The image forming apparatus includes an intermediate transfer belt and a plurality of image forming cartridges arranged along the intermediate transfer belt;
The transfer unit includes a primary transfer unit that superimposes and transfers the toner images formed by the cartridges at the same position on the intermediate transfer belt, and a secondary that transfers the toner image superimposed and transferred on the transfer belt to a sheet. Transfer means;
The image forming apparatus according to claim 6.

Images