Imaging system with non-contact charging device and controller thereof

An imaging system includes a photoreceptor including a surface to form a static latent image, a non-contact charging device being spaced apart from the photoreceptor, a power source to apply a voltage to the charging device, and a controller. The charging device charges an image-forming portion of the surface of the photoreceptor during an image-forming period and charges a non-image-forming portion of the surface of the photoreceptor during a non-image-forming period. The controller changes a signal parameter of the voltage to be applied by the power source during the non-image-forming period, in order to adjust a current flowing from the charging device to the photoreceptor.

BACKGROUND

An imaging apparatus may include a charging device that is disposed in a non-contact manner with respect to a photoreceptor, in order to reduce the wear of the photoreceptor. In such an imaging apparatus, a voltage in which an AC voltage component is superimposed on a DC voltage component is applied to the charging device, and thus, the surface of the photoreceptor is homogeneously charged.

DETAILED DESCRIPTION

Hereinafter, an example imaging system will be described with reference to the drawings. The imaging system may be an imaging apparatus such as a printer or the like, according to some examples, or may be a device that is used in an imaging apparatus, such as a developing device or the like, according to other examples.

In the following description, with reference to the drawings, the same reference numbers are assigned to the same components or to similar components having the same function, and overlapping description is omitted.

With reference toFIG.1, an example imaging apparatus1is a device that forms a color image by using the colors of magenta, yellow, cyan, and black. The imaging apparatus1includes a conveying device10that conveys a recording medium such as paper (or paper sheet) P, a developing device20that develops a static latent image, a transfer device30that secondarily transfers a toner image to the paper P, a photoreceptor40that includes a surface (a circumferential surface) on which the static latent image is formed, a fixing device50that fixes the toner image onto the paper P, and an ejection device60that ejects the paper P.

The conveying device10conveys the paper P as the recording medium on which an image is to be formed, along a conveyance route R1. The paper P is stacked and contained in a cassette K, and is conveyed by being picked up with a paper feeding roller11. The conveying device10allows the paper P to reach a transfer nip region R2through the conveyance route R1at a timing when the toner image to be transferred to the paper P reaches the transfer nip region R2.

Four developing devices20are provided, with one developing device20for each color. Each of the developing devices20includes a developer carrying body24that carries the toner on the photoreceptor40. In the developing device20, a two-component developer containing a toner (e.g., in the form of toner particles) and a carrier (e.g., in the form of carrier particles) is used as a developer. A mixing ratio of the toner and the carrier is adjusted in the developing device20, to a predetermined or targeted mixing ratio, and the toner is homogeneously dispersed by being mixed and stirred, to obtain a developer with an optimal charge amount. The developer is carried on the developer carrying body24. When the developer is transferred, for example by a rotational movement of the developer carrying body, to a developing region facing the photoreceptor40, the toner in the developer that is carried on the developer carrying body24is moved or transferred to the static latent image that is formed on the circumferential surface of the photoreceptor40, so as to develop the static latent image.

The transfer device30conveys the toner image that is formed by the developing device20to the transfer nip region R2where the toner image is secondarily transferred to the paper P. The transfer device30includes a transfer belt31to which the toner image is primarily transferred from the photoreceptor40, suspension rollers34,35,36, and37suspending (or supporting) the transfer belt31, a primary transfer roller32adjacent the photoreceptor40to interpose the transfer belt31between the primary transfer roller32and the photoreceptor40, and a secondary transfer roller33adjacent the suspension roller37to interpose the transfer belt31between the secondary transfer roller33and the suspension roller37. The transfer belt31is an endless belt that engages the suspension rollers34,35,36, and37to move circularly. The suspension rollers34,35,36, and37are rollers that are rotatable around respective axis lines. The suspension roller37may be a driving roller that is rotationally driven around the axis line, and the suspension rollers34,35, and36may be driven rollers that are driven to be rotated in accordance with the rotational driving of the suspension roller37. The primary transfer roller32is positioned to press against the photoreceptor40from an inner circumferential side of the transfer belt31. The secondary transfer roller33extends in parallel with and adjacent to the suspension roller37to interpose the transfer belt31between the secondary transfer roller33and the suspension roller37. The secondary transfer roller33is positioned to press against the suspension roller37from an outer circumferential side of the transfer belt31. Accordingly, the secondary transfer roller33forms the transfer nip region R2between the secondary transfer roller33and the transfer belt31.

The photoreceptor40may also be referred to as a static latent image carrying body, a photoreceptor drum, and/or the like. Four photoreceptors40are provided for the respective four colors. The photoreceptors40are spaced apart along a movement direction of the transfer belt31. The developing device20, a charging device41, an exposure unit (or exposure device)42, and a cleaning unit (or cleaning device)43are positioned circumferentially about the photoreceptor40.

The charging device41includes a charging roller that is provided in a non-contact manner with respect to the photoreceptor40, to homogeneously (or uniformly) charge the surface of the photoreceptor40to a predetermined potential. The exposure unit (or device)42exposes the surface of the photoreceptor40that has been previously charged by the charging device41, in accordance with the image to be formed on the paper P. Accordingly, the potential of a portion of the surface of the photoreceptor40that has been exposed by the exposure unit42, is changed to form the static latent image. The four developing devices20develop the static latent image formed on the respective four photoreceptors40with the toner that is supplied from respective toner tanks N that face the respective developing devices20, to generate the toner image. The toner tanks N are filled with toner of the respective colors of magenta, yellow, cyan, and black. The cleaning unit (or device)43collects the toner that remains on the adjacent photoreceptor40after the toner image that is formed on the photoreceptor40is primarily transferred to the transfer belt31.

The fixing device50directs the paper P to pass through a fixing nip region in which heating and pressuring are performed, to attach and fix the toner image that has been secondarily transferred to the paper P from the transfer belt31to the paper P. The fixing device50includes a heating roller52that heats the paper P, and a pressure roller54that presses against the heating roller52and that is rotationally driven. The heating roller52and the pressure roller54have a cylindrical shape. The heating roller52may include a heat source such as a halogen lamp. The fixing nip region provides a contact region between the heating roller52and the pressure roller54. The paper P is conveyed to pass through the fixing nip region, to melt the toner image and fix the toner image to the paper P.

The ejection device60includes ejection rollers62and64for ejecting the paper P to which the toner image has been fixed, to the outside of the imaging apparatus.

The imaging apparatus1further includes a power source70and a controller72. The power source70applies a voltage to the charging device41for charging the photoreceptor40. The controller72controls the overall operation of the imaging apparatus1. The example power source70and the example controller72will be described further below.

A printing process of the imaging apparatus1will be described. When an image signal of an image to be recorded on a recording medium, is input to the imaging apparatus1, the controller72(cf.FIG.2) of the imaging apparatus1controls the paper feeding roller11to rotate, to pick-up (or lift) a sheet of the paper P that is stacked in the cassette K and to convey the sheet of paper P. The surface of the photoreceptor40is homogeneously charged to a predetermined potential by the charging device41(a charging operation). The surface of the photoreceptor40having been charged is subsequently irradiated with a laser light by the exposure unit (or device)42, based on the received image signal, to form the static latent image (an exposure operation).

In the developing device20, the static latent image is developed, and the toner image is formed (a developing operation). The formed toner image is primarily transferred to the transfer belt31from the photoreceptor40, from a region of the photoreceptor40that faces the transfer belt31(a transfer operation). Four toner images are formed by the four photoreceptors40and are sequentially layered on the transfer belt31, to form a single composite toner image. Then, the composite toner image is secondarily transferred to the paper (or paper sheet) P that is conveyed from the conveying device10, at the transfer nip region R2where the suspension roller37and the secondary transfer roller33face each other.

The paper P to which the composite toner image has been secondarily transferred, is then conveyed to the fixing device50. When the paper P passes through the fixing nip region, the paper P is heated and pressured by the fixing device50between the heating roller52and the pressure roller54, to melt the composite toner image and to fix the toner image to the paper P (a fixing operation). After that, the paper P is ejected to the outside of the imaging apparatus1by the ejection rollers62and64.

The example photoreceptor40, the example charging device41, the example power source70, and the example controller72will be described, with reference toFIG.2.

As illustrated inFIG.2, the photoreceptor40may include a surface40sfor forming the static latent image. The photoreceptor40, for example, may be an organic photoconductor (OPC), and may include a substrate (or substrate member)40a, and a photosensitive layer40bthat is provided over the substrate40a.

The substrate40amay have a substantially cylindrical or columnar shape, and supports the photosensitive layer40b. The substrate40amay include a conductive material such as aluminum, copper, chromium, nickel, zinc, and stainless steel. An outer circumferential surface of the substrate40amay be covered with the photosensitive layer40b.

The photosensitive layer40bis a layer on which the static latent image is formed, and for example, in the photosensitive layer40b, a charge generating layer and a charge transport layer may be layered in this order from the substrate40alayer (e.g. in a radially outward direction). Accordingly, the charge transport layer forms a surface layer of the photoreceptor40. The charge generating layer has a charge generating function, and for example, may include a binder resin in which charge generating substance(s) are dispersed. The charge generating substance may be of one or more among: a monoazo pigment, a disazo pigment, an asymmetric disazo pigment, a trisazo pigment, an azo pigment having a carbazole skeleton, an azo pigment having a distyryl benzene skeleton, an azo pigment having a triphenyl amine skeleton, an azo pigment having a diphenyl amine skeleton, a perylene pigment, a phthalocyanine pigment, and/or the like, for example. The charge generating substance(s) may include a single one of the above-listed types according to examples, or two or more types thereof may be mixed together according to other examples. Furthermore, an intermediate layer, such as an undercoat layer, may be further formed between the substrate40aand the charge generating layer of the photosensitive layer40b.

The charge transport layer (a layer configured of an organic compound) is formed of an organic compound, and includes the outermost layer of the photoreceptor40. The charge transport layer has a charge transport structure, and for example, may be formed of a binder resin in which charge transport substances are dispersed. The charge transport substance contained in the charge transport layer, for example, may include a hole transport substance. The hole transport substance may include one or more among: Poly(N-vinyl carbazole) and/or a derivative thereof, poly(γ-carbazolyl ethyl glutamate) and/or a derivative thereof, pyrene-formaldehyde condensation and/or a derivative thereof, polyvinyl pyrene, polyvinyl phenanthrene, polysilane, an oxazole derivative, an oxadiazole derivative, an imidazole derivative, a monoaryl amine derivative, a diaryl amine derivative, a triaryl amine derivative, a stilbene derivative, an α-phenyl stilbene derivative, an aminobiphenyl derivative, a benzidine derivative, a diaryl methane derivative, a triaryl methane derivative, a 9-styryl anthracene derivative, a pyrazoline derivative, a divinyl benzene derivative, a hydrazone derivative, an indene derivative, a butadiene derivative, a pyrene derivative, a bisstilbene derivative, a distyryl benzene derivative, an enamine derivative, and/or the like. The hole transport substances may include a single one of the above-listed types according to examples, or two or more types thereof may be mixed together in other examples.

Furthermore, the charge transport substance contained in the charge transport layer may include an electron transport substance. Examples of electron transport substances include: a benzoquinone-based compound, a cyan ethylene-based compound, a cyanoquinodimethane-based compound, a fluorenone-based compound, a phenantraquinone-based compound, a phthalic anhydride-based compound, a thiopyran-based compound, a naphthalene-based compound, a diphenoquinone-based compound, and a stilbene quinone-based compound. Some additional examples of electron transport substances include: chloranil, bromanil, tetracyanoethylene, tetracyanoquinodimethane, 7-trinitro-9-fluorenone, and the like. The charge transport layer may include one among such electron transport substances according to some examples, or may include two or more types thereof mixed together.

The charge transport layer may further contain filler particles. Organic filler particles and/or inorganic filler particles may be used as the filler particles contained in the charge transport layer. Examples of organic filler particles include a urethane resin, a polyamide resin, a fluorine resin, a nylon resin, an acrylic resin, a urea resin, and the like. Examples of organic filler particles include silica, alumina, and the like. The charge transport layer may include one of such filler particles according to some examples, or may include two or more types thereof mixed together according to other examples.

The charge transport layer includes filler particles formed of a material that is less likely to be affected by discharge than an organic compound, to suppress wear of the photoreceptor40due to discharge that occurs between the photoreceptor40and the charging device41. In order to more effectively suppress the wearing of the photoreceptor40, the filler particles may have an average particle diameter of approximately 50 to 500 nm. In addition, the charge transport layer contains a ratio of approximately 1 to 30 mass % of the filler particles.

An outer circumferential surface of the photosensitive layer40bforms the surface40sof the photoreceptor40. The surface40sof the photoreceptor40is homogeneously or uniformly charged by the charging device41, to form a charged surface on the surface40sof the photoreceptor40. The charged surface is irradiated with light L from the exposure unit (or device)42, to form the static latent image. As described above, the charge transport layer (or surface layer) of the photosensitive layer40bmay form the uppermost (or outermost) layer of the photoreceptor40, and a protective layer formed of an acrylic resin or the like may be further formed on the charge transport layer to increase the hardness of the surface40sof the photoreceptor40, and improve wear resistance.

The photoreceptor40extends longitudinally, and the substrate member40ahas opposite ends that are rotatably supported on a support member, and are rotationally driven by power from a driving source such as a motor. The photoreceptor40is rotated at a rotational velocity according to a process speed of the imaging apparatus1. The process speed of the imaging apparatus1is coincident with a tangential velocity (a circumferential velocity) Vt of the surface40sof the photoreceptor40. For example, the tangential velocity Vt of the photoreceptor40may be approximately 50 [mm/sec].

The charging device41is a non-contact charging device, and is provided by being spaced apart from the photoreceptor40.

The charging device41is in the shape of a column, and includes a conductive support body41a, a conductive elastic body layer41bthat is layered on an outer circumferential surface of the conductive support body41a, and a conductive resin layer41cthat is layered on an outer circumferential surface of the conductive elastic body layer41b.

The conductive support body41amay have a columnar or cylindrical shape, and extends in parallel with the substrate40aof the photoreceptor40. The conductive support body41ais formed of a conductive metal such as iron, copper, aluminum, nickel, and stainless steel. In an example, the outer circumferential surface of the conductive support body41amay be subjected to a plating treatment in order to provide antirust performance and scratch resistance performance. In addition, the outer circumferential surface of the conductive support body41amay be coated with a conductive adhesive agent or a conductive primer in order to protect the conductive elastic body layer41b. The conductive support body41ahas opposite ends that may be rotatably supported on a support member.

The conductive elastic body layer41bcovers at least a part of the outer circumferential surface of the conductive support body41a. The conductive elastic body layer41b, for example, may be formed of a resin containing a conductive material. The conductive elastic body layer41b, for example, may be formed of a material in which a conductive material is added to natural rubber, synthetic rubber, a synthetic resin (a polyamide resin, a polyurethane resin, a silicone resin, and the like), and the like. Examples of the conductive agent contained in the conductive elastic body layer41b, include: carbon black, graphite, potassium titanate, ferric oxide, conductive titanium oxide (c-TiO2), conductive zinc oxide (c-ZnO), conductive tin oxide (c-SnO2), a quaternary ammonium salt, and the like.

The conductive resin layer41ccovers the outer circumferential surface of the conductive elastic body layer41b, and configures the outermost layer of the charging device41. The conductive resin layer41cmay be formed of a material having an electrical resistance that is greater than an electrical resistance of the conductive elastic body layer41b. The conductive resin layer41cmay be formed of a material in which a conductive material is added to a base polymer such as a fluorine resin, a polyamide resin, an acrylic resin, a nylon resin, a polyurethane resin, a silicone resin, a butyral resin, a styrene-ethylene⋅butylene-olefin copolymer (SEBC), and an olefin-ethylene⋅butylene-olefin copolymer (CEBC).

The charging device41is spaced apart from the photoreceptor40. A gap G is formed between the photoreceptor40and the charging device41. The charging device41extends in parallel with the photoreceptor40. A spaced distance (e.g., a closest distance) between the surface40sof the photoreceptor40and the outer circumferential surface of the charging device41(which corresponds to the width or distance of the gap G in a facing direction of the photoreceptor40and the charging device41) is set to be substantially constant along an axis line direction (or the longitudinal direction) of the photoreceptor40. In an example, in order to ensure the homogeneity of the charge of the photoreceptor40, the gap G in the facing direction of the photoreceptor40and the charging device41may have a constant width (or distance) of approximately 10 to 100 μm. As described above, the charging device41may be positioned in a non-contact manner with respect to the photoreceptor40, as described above, to prevent wear of the photoreceptor40due to friction.

The power source70is electrically connected to the charging device41. The power source70applies the voltage to the conductive support body41aof the charging device41, in order to charge the photoreceptor40. The voltage that is applied to the conductive support body41afrom the power source70is a voltage in which a DC voltage and an AC voltage are superimposed. When the voltage is applied to the charging device41, discharge occurs between the charging device41and the photoreceptor40. By such discharge, an AC current flows from the charging device41to the photoreceptor40, to charge a portion facing the charging device41on the surface40sof the photoreceptor40. The entire circumference of the surface (circumferential surface)40sof the photoreceptor40is homogeneously charged with the rotation of the photoreceptor40.

In addition, a DC component of the voltage that is applied to the conductive support body41afrom the power source70is a negative bias voltage, and for example, includes a voltage of approximately −900 to −300 [V]. An AC component of the voltage that is applied to the conductive support body41a, for example, includes a peak-to-peak voltage of approximately 1500 to 3000 [V]. Such a voltage is applied to the charging device41, and thus, discharge occurs between the charging device41and the photoreceptor40, and the portion facing the charging device41on the surface40sof the photoreceptor40is charged.

The surface40sof the photoreceptor40that is charged by the charging device41is exposed by the light L that is output from the exposure unit (or device)42. The exposure unit42, for example, includes a laser light scanning device to scan a part of the surface40swith laser light, in accordance with the image to be formed on the paper P. Accordingly, the potential of the portion on the surface of the photoreceptor40that is exposed by the exposure unit42is changed, to form the static latent image.

FIG.3is a diagram schematically illustrating the static latent image that is formed on the surface40sof the photoreceptor40. With reference toFIG.3, in an imaging process that forms an image on a plurality of paper sheets (or sheets of paper) P, the surface40sof the photoreceptor40is formed with a plurality of image-forming portions80in which the static latent image is generated, and with a plurality of non-image-forming portions82in which no static latent image is generated.

The image-forming portion80is a portion on the surface40sof the photoreceptor40, that is exposed by the exposure unit42while the photoreceptor40is rotated (e.g., by one rotation), such that the static latent image is formed. The image-forming portion80receives light corresponding to the image to be formed on the paper (or paper sheet) P, so as to form the static latent image. The non-image-forming portion82is a portion on the surface40sof the photoreceptor40, that is charged to remain free of exposure from the light from the exposure unit42while the photoreceptor40is rotated (e.g., by one rotation) such that no static latent image is not formed in the non-image-forming portion82. The non-image-forming portion82and the image-forming portion80are mutually exclusive regions. Accordingly, the non-image-forming portion82is a region that excludes the image-forming portion80on the surface40sof the photoreceptor40. In some examples, the non-image-forming portion82may include a portion corresponding to a timing of a pretreatment period in which a charge potential of the photoreceptor40is stabilized, a portion corresponding to a white space, a portion corresponding to an interval between the plurality of paper sheets P to be printed, and a portion corresponding to a timing of an post-treatment period in which the photoreceptor40is neutralized.

The controller72is a computer including a processor, a storage (or storage device), an input device, a display device, and the like, and has a function of controlling the overall operation of the imaging apparatus1. The storage device of the controller may store processor-readable data and instructions. For example, the processor-readable data and instructions may be executed by the processor as a control program for controlling various processes to be carried out by the imaging apparatus1. The controller72is connected to or in communication with the power source70such that communication can be performed, and the signal parameter of the AC voltage that is applied to the conductive support body41amay be adjusted in order to suppress the wear of the photoreceptor40.

A wear volume of the photoreceptor40depends on the amount of AC current flowing to the photoreceptor40from the charging device41.FIG.4is an example graph showing a relationship between the amount of the AC current flowing to the photoreceptor40from the charging device41, and the wear volume of the photoreceptor40. As shown inFIG.4, the wear volume of the photoreceptor40increases, as the AC current that flows to the photoreceptor40increases. Therefore, in order to suppress the wear of the photoreceptor40, it is necessary that the AC current flowing to the photoreceptor40is suppressed.

In order to suppress the wear of the photoreceptor40, the controller72changes the signal parameter of the AC voltage that is applied to the charging device41at the time of charging the non-image-forming portion82of the photoreceptor40so as to decrease the AC current flowing to the photoreceptor40from the charging device41.

FIG.5is an example graph showing an example relationship between the frequency of the AC voltage to be applied to the charging device41and the amount of the AC current flowing to the photoreceptor40from the charging device41. As shown inFIG.5, the AC current that flows to the photoreceptor40from the charging device41increases substantially linearly as the frequency of the AC voltage that is applied to the charging device41increases. Accordingly, the frequency of the AC voltage that is applied to the charging device41may be decreased to decrease the wear volume of the photoreceptor40.

In addition,FIG.6is an example graph showing an example relationship between the frequency of the AC voltage to be applied to the charging device41and a surface potential of the photoreceptor40. In this example, for a frequency of the AC voltage that is equal to or greater than 300 [Hz], the surface potential of the photoreceptor40is substantially constant, and for a frequency of the AC voltage that is less than 300 [Hz], the surface potential of the photoreceptor40decreases exponentially. Accordingly, the frequency of the AC voltage may be set to be equal to or greater than a certain value in order to achieve an improved printing quality.

In view of the above-described, in some examples, the controller72controls the power source70such that a frequency of the AC voltage that is applied to the charging device41during a non-image-forming period (in which the non-image-forming portion82of the photoreceptor40is charged) is lower than a frequency of the AC voltage that is applied to the charging device41during an image-forming period (in which the image-forming portion80of the photoreceptor40is charged).

FIG.7is a flowchart illustrating an example of a control flow of the controller72. At operation ST1, the controller72determines whether or not a portion to be charged by the charging device41, on the surface40sof the photoreceptor40(hereinafter, referred to as a “target portion”) is an image-forming portion80. In a case where the static latent image is formed in the target portion by the exposure unit (or device)42, the controller72determines that the target portion is the image-forming portion80.

In a case where it is determined that the target portion is the image-forming portion80, at operation ST2, the controller72controls the power source70, and sets the frequency of the AC voltage to be applied to the charging device41during the image-forming period (in which the target portion is charged) to a first frequency f1. The first frequency f1 is a high frequency for achieving a stable printing quality. In addition, the controller72causes the charging device41to charge the image-forming portion80by using the AC voltage having the first frequency f1. The controller72synchronously controls the charging device41and the exposure unit42such that the static latent image is formed in the image-forming portion80by the exposure unit42.

Where it is determined that the target portion is not the image-forming portion80but instead the non-image-forming portion82, at operation ST3, the controller72controls the power source70such that the frequency of the AC voltage that is applied to the charging device41during the non-image-forming period (in which the target portion is charged) is set to a second frequency f2. The second frequency f2 is a frequency for suppressing the AC current flowing to the photoreceptor40from the charging device41, and is a frequency that is lower than the first frequency f1.

In some examples, the first frequency f1 and the second frequency f2 satisfy the following relationships.
f1 [Hz]/Vt[mm/sec]≥8
1.7<f2 [Hz]/Vt[mm/sec]<8

As described above, a ratio of the first frequency f1 [Hz] to the tangential velocity Vt [mm/sec] of the surface40sof the photoreceptor40may be equal to or greater than 8. In addition, a ratio of the second frequency f2 [Hz] to the tangential velocity Vt [mm/sec] of the surface40sof the photoreceptor40is of approximately 1.7 to 8. The AC voltage having the second frequency f2 that is a relatively low frequency is applied to the charging device41during the non-image-forming period, to decrease the AC current flowing to the photoreceptor40during the non-image-forming period, as compared with the AC current flowing to the photoreceptor40during the image-forming period, in order to suppress wear of the photoreceptor40.

In some examples, the controller72may control the power source70such that the waveform of the AC voltage to be applied to the charging device41during the non-image-forming period (in which the non-image-forming portion82of the photoreceptor40is charged) is a waveform that is different from the waveform of the AC voltage to be applied to the charging device41during the image-forming period (in which the image-forming portion80of the photoreceptor40is charged).

FIG.8is a flowchart illustrating a control flow of the controller72according to another example. At operation ST11, the controller72determines whether or not the target portion of the surface40sof the photoreceptor40is an image-forming portion80.

Where it is determined that the target portion is an image-forming portion80, at operation ST12, the controller72controls the power source70, and sets the waveform of the AC voltage to be applied to the charging device41during the image-forming period (in which the target portion is charged) to a sine wave. In addition, the controller72causes the charging device41to charge the image-forming portion80by using the AC voltage of the sine wave, and then, synchronously controls the charging device41and the exposure unit42such that the static latent image is formed in the image-forming portion80by the exposure unit42.

Where it is determined that the target portion is not the image-forming portion80but the non-image-forming portion82, at operation ST13, the controller72controls the power source70, and sets the waveform of the AC voltage that is applied to the charging device41during the non-image-forming period (in which the target portion is charged) to a triangle wave (operation ST13). An average voltage of the triangle wave is lower than an average voltage of the sine wave, and accordingly, the waveform of the AC voltage that is applied to the charging device41during the non-image-forming period is changed to the triangle wave, to decrease the AC current flowing to the photoreceptor40while maintaining the peak-to-peak voltage, in order to suppress or inhibit wear of the photoreceptor40.

Furthermore, the controller72may change both of the frequency and the waveform of the AC voltage that is applied to the charging device41during the non-image-forming period. For example, the controller72may set the waveform of the AC voltage that is applied to the charging device41to the sine wave, and may set the frequency of the AC voltage to the first frequency f1, during the image-forming period. The controller72may set the waveform of the AC voltage that is applied to the charging device41to the triangle wave, and may set the frequency to the second frequency f2, during the non-image-forming period. The controller72may control both the frequency and the waveform, to suppress or inhibit wear of the photoreceptor40.

FIG.9is a timing diagram illustrating an example of a voltage that is supplied to the charging device41from the power source70. Such a timing diagram illustrates an example in which an image is formed on two paper sheets P.

The timing diagram ofFIG.9includes a period T1in which the charge potential of the photoreceptor40is stabilized, a period T2in which the image-forming portion80for forming the static latent image of the image to be formed on the first sheet of paper (or first paper sheet) P, is charged, a period T3in which the non-image-forming portion82corresponding to a white space and a paper interval, is charged, a period T4in which the image-forming portion80for forming the static latent image of the image to be formed on the second sheet of paper (or second paper sheet) P is charged, and a period T5in which the photoreceptor40is neutralized. The periods T1, T3, and T5are the non-image-forming period for charging the non-image-forming portions82in which no static latent image is formed. The periods T2and T4are the image-forming periods for charging the image-forming portions80in which a static latent image is formed.

As illustrated inFIG.9, the controller72controls the power source70, sets the waveform of the AC voltage that is applied to the charging device41to the triangle wave, and sets the frequency of the AC voltage to the second frequency f2, in the periods T1, T3, and T5. As a result, the AC current flowing to the photoreceptor40during the non-image-forming period decreases, and the wear of the photoreceptor40is suppressed.

During the periods T2and T4, the controller72controls the power source70, sets the frequency of the AC voltage that is applied to the charging device41to the sine wave, and sets the frequency of the AC voltage to the first frequency f1. Accordingly, the photoreceptor40is homogeneously charged during the image-forming period, and the printing quality of the image is maintained. Furthermore, in all periods of the periods T1to T5, the DC voltage may be kept constant.

An influence on the wear volume of the photoreceptor40at the time of changing the frequency of the AC voltage that was applied to the charging device41was evaluated by using the graphs shown inFIG.4toFIG.6. In the following example, the controller72applied the AC voltage of the sine wave of 2500 [Hz] to the charging device41during the image-forming period, and applied the AC voltage of the sine wave of 300 [Hz] to the charging device41during the non-image-forming period. A sequence of printing four paper sheets P was performed, and the wear volume of the photoreceptor40was examined by a simulation. The number of rotations of the photoreceptor40in one sequence was set to 25 rotations. In addition, a time ratio between the image-forming period and the non-image-forming period was set to 10:15.

In a case where the frequency that is applied to the charging device41is 2500 [Hz], a AC current of 2.0 [mA] flows to the photoreceptor40(refer toFIG.5), and accordingly, the wear volume of the photoreceptor40per one rotation is 0.018 [nm] (refer toFIG.4). In a case where the frequency that is applied to the charging device41is 300 [Hz], the amount of AC current flowing to the photoreceptor40is 0.3 [mA] (refer toFIG.5), and accordingly, the wear volume of the photoreceptor40per one rotation is 0.010 [nm] (refer toFIG.4).

The wear volume of the photoreceptor40at the time of executing one sequence by applying an AC voltage having a frequency of 2500 [Hz] during both of the image-forming period and the non-image-forming period to the charging device41is 0.45 [nm] (=0.018 [nm/cycle]×25 [cycle]). The wear volume of the photoreceptor40at the time of applying an AC voltage of the sine wave of 2500 [Hz] to the charging device41during the image-forming period, and of applying an AC voltage of the sine wave of 300 [Hz] to the charging device41during the non-image-forming period is 0.32 [nm] (=0.018 [nm/cycle]×10 [cycle]+0.010 [nm/cycle]×15 [cycle]). Based on such results, when an AC voltage of 2500 [Hz] is applied to the charging device41during the image-forming period, and an AC voltage of 300 [Hz] is applied to the charging device41during the non-image-forming period, the wear volume of the photoreceptor40can be reduced by approximately 30%, as compared with a case where an AC voltage of 2500 [Hz] is applied to the charging device41during both of the image-forming period and the non-image-forming period.

For example, in the example illustrated inFIG.9, both the frequency and the waveform of the AC voltage that is applied to the charging device41during the non-image-forming period are changed. In other examples, either one of the frequency or the waveform of the AC voltage may be changed. In addition, in some of the examples, in order to decrease the AC current flowing to the photoreceptor40, the waveform of the AC voltage during the AC non-image-forming period is changed to the triangle wave from the sine wave. In other examples, the waveform of the AC voltage during the non-image-forming period may be changed to another waveform, for example, a saw waveform, to decrease the AC current flowing to the photoreceptor40.