Image forming apparatus

An image forming apparatus includes an image carrier, a charging device to be applied with a charging voltage, an exposure device, a developer carrier to be applied with a development voltage, a developer supplying member to be applied with a supply voltage, a developer regulation member to be applied with a regulation voltage, and a voltage controller. The developer carrier has thereon a developer having a charge amount Q/M. The charge amount Q/M satisfies 25 [μC/g]≦|Q/M|≦40 [μC/g]. The image carrier has a surface potential Vopc, the development voltage is expressed as Vdb, and the developer carrier has a developer layer potential Vtnr thereon. The developer layer potential Vtnr relative to a difference between the surface potential Vopc and the development voltage Vdb is expressed by a ratio γ. The ratio γ satisfies 0.1≦γ≦0.2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

2. Description of Related Art

A related art image forming apparatus employing an electrophotographic method has served as, for example, a printer, a photocopier, a facsimile device, and a multi-functional peripheral. In the related art printer, for example, a surface of a photosensitive drum is uniformly charged by a charging roller, and is irradiated with the light by a light emitting diode (LED) head to form an electrostatic latent image thereon. After a development device disposed in the related art printer develops the electrostatic latent image to form a toner image on the photosensitive drum, the toner image is transferred to a sheet by a transfer roller and is fixed onto the sheet by a fixing device.

The development device includes a development roller, a toner supplying roller, and a development blade, and allows toner supplied from a toner cartridge to be charged, so that a thin layer of the toner is uniformly formed on the development roller. Accordingly, a development voltage, a supply voltage, and a regulation voltage are adjusted and respectively applied to the development roller, the toner supplying roller, and development blade. The toner undergoes friction between the development roller and the toner supplying roller, and between the development roller and the development blade (e.g., Patent Document 1).

Such a related art printer, however, may cause phenomena such as blur by transfer, fog, smudge, and filming which cause deterioration of image quality in a case where a surface potential of the photosensitive drum and a setting value of the development voltage are not appropriate. The blur by transfer is a phenomenon in which the toner is not sufficiently adhered across an image portion on a sheet in the course of transfer of the toner image to the sheet. The fog is another phenomenon in which a small amount of the toner is adhered across a non-image portion (blank portion), while the smudge is a phenomenon in which the toner is locally adhered to a non-image portion, a half-tone portion and the like in a granular manner, a streak manner, or a band-like manner. The filming is another phenomenon in which a white portion is locally formed in a granular shape, a streak shape, or a band-like shape when a solid image is printed across the sheet.

The present invention is proposed in consideration of the aforementioned situations, and provides an image forming apparatus capable of reducing occurrences of blur by transfer, fog, smudge, filming, and the like, thereby improving image quality.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, an image forming apparatus includes: an image carrier; a charging device, applied with a charging voltage, charging a surface of the image carrier; an exposure device forming a latent image by allowing the charged surface of the image carrier to be exposed to light thereof; a developer carrier, applied with a development voltage, forming a developer image by development of the latent image by allowing a developer to be adhered to the image carrier; a developer supplying member, applied with a supply voltage, supplying the developer to the developer carrier; a developer regulation member, applied with a regulation voltage, regulating an amount of the developer on the developer carrier; and a voltage controller controlling the charging voltage, the development voltage, the supply voltage, and the regulation voltage. The developer carrier has thereon the developer having a charge amount Q/M, and the charge amount Q/M satisfies 25 [μC/g]≦|Q/M|≦40 [μC/g]. The image carrier has a surface potential Vopc; the development voltage is expressed as Vdb; and the developer carrier has a developer layer potential Vtnr thereon. The developer layer potential Vtnr relative to a difference between the surface potential Vopc and the development voltage Vdb is expressed by a ratio γ satisfying 0.1≦γ≦0.2.

Additional features and advantages of the present invention will be more fully apparent from the following detailed description of embodiments, the accompanying drawings and the associated claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A printer serving as an image forming apparatus according to preferred embodiments of the present invention is now described more fully hereinafter with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. The embodiments, therefore, may be modified or varied without departing from the scope of the present invention.

In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of the patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Preferred embodiments of the present invention are described in detail referring to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.

Referring toFIGS. 2 and 3, the printer serving as the image forming apparatus and a drive system of a photosensitive drum in the printer according to the first embodiment of the present invention are respectively illustrated.

The printer serving as the image forming apparatus includes thereinside: image forming portions Bk, Y, M, and C disposed with respect to respective toners14serving as developers of black, yellow, magenta, and cyan, respectively; and a transfer unit21disposed along the image forming portions Bk, Y, M, and C, as illustrated inFIG. 2. Each of the image forming portions Bk, Y, M, and C includes: an image forming unit15; a light emitting diode (LED) head23serving as an exposure device disposed corresponding to the image forming unit15; and a transfer roller22serving as a transfer member.

Since the image forming portion Bk, Y, M, and C are substantially similar to one another except for the color of the toner, the image forming unit15in the image forming unit Bk will be described.

The image forming unit15includes a toner cartridge13, serving as a developer storage unit, detachably disposed thereto. The toner cartridge13stores the toner14thereinside.

The image forming unit15includes: a photosensitive drum11serving as an image carrier; a charging roller12serving as a charging device; a development roller16serving as a developer carrier; and a cleaning blade19serving as a first cleaning member. The photosensitive drum11is formed in a drum shape, and carries an electrostatic latent image as a latent image on a surface thereof. The charging roller12is disposed in the vicinity of the photosensitive drum11in such a manner as to be in contact with the photosensitive drum11, and uniformly charges the surface of the photosensitive drum11. The development roller16is disposed in contact with the photosensitive drum11, and allows the toner14to be adhered to the surface of the photosensitive drum11, so that the electrostatic latent image on the surface of the photosensitive drum11is developed to form a toner image serving as a developer image.

Moreover, the image forming unit15includes: a toner supplying roller18not only serving as a developer supplying member but also serving as a developer supplying roller; and a development blade17serving as a developer regulation member. The toner supplying roller18is disposed in contact with the development roller16. The toner supplying roller18allows the toner14supplied from the toner cartridge13to be charged, and supplies the toner14to the development roller16. The development roller16, the development blade17, and the toner supplying roller18, for example, form a development unit.

The photosensitive drum11is formed of a charge generation layer having a film thickness of 0.5 μm and a charge transport layer having a film thickness of 18 μm provided on an aluminum tube having a thickness of 0.75 mm and an outside diameter of 30 mm. The charge generation layer can be formed of a charge generation material such as organic pigment and dye made of, for example, selenium, selenium alloy, a selenium arsenic compound, cadmium sulfide, zinc oxide, other inorganic photoconductive material, phthalocyanine, azo color, quinacridon, polycyclic quinone, pyrylium salt, thiapyrylium salt, indigo, thioindigo, anthanthrone, pyranthrone, and cyanine. The charge transport layer can be formed of a charge transport material made of, for example, aniline derivative, a hydrazone compound, aromatic amino-derivative, stilbene derivative, and a heterocyclic compound such as carbazol, indole, imidazole, oxazole, pyrazole, oxadiazole, pyrazoline, and thiadiazole, or a electron-donating material such as a polymer including a group made by combination of such substances or materials in a main chain or a side chain.

The charging roller12uniformly charges the surface of the photosensitive drum11. For example, the charging roller12is formed of a conductive member serving as a shaft made of stainless steel and the like and a conductive elastic layer made of epichlorohydrin and the like, and is disposed in contact with the photosensitive drum11.

The LED head23allows the uniformly charged surface of the photosensitive drum11to be selectively exposed to the light thereof, thereby forming the electrostatic latent image (latent pattern) on the surface of the photosensitive drum11. The LED head23includes an LED element, an LED drive element, and a lens array, and allows image formation on the surface of the photosensitive drum11with the irradiation light from the LED element.

The development roller16is formed of a conductive shaft (e.g., metal core) made of a SUS material (SUS represents stainless used steel standardized by Japanese Industrial Standards), an elastic layer provided on the conductive shaft in such a manner as to be in a roll shape, and a surface layer covering the elastic layer. For the elastic layer, urethane rubber or silicone rubber is used. The surface layer is formed by processing a surface of the elastic layer with urethane solution, or by applying acrylic resin, acrylic-fluoro copolymer resin and the like on the surface of the elastic layer. In a case where the surface layer is formed of the acrylic resin, acrylic-fluoro copolymer resin and the like, carbon black is blended in the acrylic resin to impart the conductivity to the surface layer. Herein, one (1) part by weight of the carbon black is blended to one hundred (100) parts by weight of the acrylic resin.

The toner supplying roller18is formed of a conductive shaft (metal core) made of a SUS material, and an elastic layer. For the elastic layer, conductive silicone rubber foam or conductive urethane rubber foam is used. In a case where an elastic layer having a semi-conductive property is used, for example, acetylene black and carbon black are added.

The development blade17is formed of a SUS material having a thickness of 0.08 mm. The development blade17includes a contact portion contacting the development roller16, and the contact portion undergoes a bending process to form a bending portion having a curvature radius R of 0.2 mm. The development blade17has a linear pressure of 30 gf/cm with respect to the development roller16. The curvature radius R and the linear pressure of the development blade17can be adjusted according to the charge amount of the toner14.

The cleaning blade19is, for example, formed of a rubber blade, and is disposed in such a manner that an edge of the rubber blade contacts the surface of the photosensitive drum11.

The photosensitive drum11is rotated at a constant linear speed (i.e., a circumferential speed which is a speed of an outer circumference) by gear transmission provided with a drum drive gear41as illustrated inFIG. 3. The drum drive gear41is rotated with the drive of a drum motor53serving as a first drive unit. The charging roller12is rotated by friction transmission provided with the surface of the photosensitive drum11, and the development roller16is rotated by the gear transmission provided with the photosensitive drum11. The toner supplying roller18is rotated by the gear transmission provided with the development roller16through an idler gear42.

The development roller16is rotated in a direction opposite to the rotation direction of the photosensitive drum11at a linear speed ratio of 1.6 times relative to the photosensitive drum11. The toner supplying roller18is rotated in the same direction as the development roller16at a linear speed ratio of 0.7 time. According to the first embodiment, the linear speed ratio represents a circumferential speed ratio, and the linear speed ratio of the development roller16relative to the photosensitive drum11represents a ratio of the circumferential speed of the development roller16relative to that of the photosensitive drum11.

The transfer unit21includes: a drive roller25aserving as a first roller; an idler roller25bserving as a second roller; a transfer belt24, serving as belt member, tightly stretched by the drive roller25aand the idler roller25and travelably disposed in contact with each of the photosensitive drums11; and the transfer rollers22disposed opposite to the respective photosensitive drums11through the transfer belt24. Each of the transfer rollers22is, for example, formed of a foam elastic member having a conductive property.

In a lower portion of the transfer belt24, a cleaning blade26, serving as a second cleaning member, is disposed in the vicinity of the idler roller25bdisposed on the most downstream side relative to a traveling direction of the transfer belt24, thereby cleaning the transfer belt24.

A sheet feed cassette31, serving as a medium storage unit, is disposed below the transfer unit21to store a sheet P serving as a medium or a plurality of sheets Ps. A hopping roller32is disposed in one end of the sheet feed cassette31. Each of the sheets Ps is separately fed from the sheet feed cassette31by the hopping roller32, and is fed to the image forming portions Bk, Y, M, and C through a conveyance roller34.

A fixing unit35, serving as a fixing device, is disposed on a downstream side of the image forming portions Bk, Y, M, and C relative to a conveyance direction of the sheet P. The fixing unit35includes a fixing roller R1serving as a first roller and a pressure roller R2serving as a second roller. The fixing unit35allows a color toner image on the sheet P conveyed from the image forming portions Bk, Y, M, and C to be fixed by the fixing roller R1and the pressure roller R2, thereby forming the color image. The color image formed on the sheet P is ejected outside the printer by an ejection roller36.

The photosensitive drum11, the charging roller12, the LED head23, the development roller16, the toner supplying roller18, the transfer roller22, the drive roller25a, the idler roller25band the fixing unit35are controlled by a control unit (not shown). The control unit applies the direct voltage to the charging roller12, the transfer roller22, the development roller16, and the toner supplying roller18at a prescribed timing arranged beforehand, and drives the drum motor, so that the photosensitive drum11, the charging roller12, the development roller16, the toner supplying roller18, and the fixing roller R1are rotated in directions indicated by respective arrows shown inFIG. 2. Accordingly, the control unit allows a belt motor (not shown) serving as a drive unit for traveling to be driven, thereby rotating the drive roller25ain a direction indicated by an arrow shown inFIG. 2.

A description is now given of the toner14.

According to the first embodiment of the present invention, the toner14is formed as follows: toner particles are formed by an emulsion polymerization method; and silica, titanium oxide fine particles, and the like are added to the toner particles and mixed by a mixer. The toner14is formed to have an average particle size smaller than or equal to 6 μm.

The emulsion polymerization method represents a method for producing a toner particle as follows: a primary particle of a styrene-acrylic copolymer resin serving as a blinding resin for the toner14is produced in solvent; a colorant emulsified by an emulsifying agent (a surface active agent) is mixed with the solvent used for primary particle production; a wax, a charge control agent, and the like are mixed as may be needed; and such a mixture is aggregated to produce the toner particles. The toner particles removed from the solvent are washed and dried, so that an unnecessary solvent component and a by-product component are removed from the toner particle.

According to the first embodiment, the styrene-acrylic copolymer resin is generated from styrene, acrylic acid, and methylmethacrylate. The colorant includes the carbon black, pigment yellow 74, pigment red 238, and pigment blue 15:3 used as black, yellow, magenta, and cyan, respectively. Moreover, the wax includes stearyl stearate used as high-class fatty acid ester wax.

A description is now given of a control device of the printer.

Referring toFIG. 1, the printer according to the first embodiment of the present invention is illustrated in a control block diagram.

As illustrated inFIG. 1, a controller50is connected to a motor controller51and a print controller52. The motor controller51is connected to the drum motor53and a belt motor54. The motor controller51, functioning as a drive controller, drives the drum motor53to rotate the photosensitive drum11and the like, and drives the belt motor54to allow the transfer belt24to travel. Upon receipt of a print instruction from the controller50, the print controller52functions as a voltage controller and a light emission controller, so that print operation is performed. Herein, the printer controller52, functioning as the voltage controller, controls each of a charging voltage, a development voltage, a supply voltage, a regulation voltage, and a transfer voltage to be applied to the charging roller12, the development roller16, the toner supplying roller18, the development blade17, and the transfer roller22, respectively. The print controller52, functioning as the light emission controller, controls a light emission amount of the LED head23.

A description is now given of operation of the printer.

First, the printer controller52allows the charging voltage to be applied to the charging roller12to uniformly charge the surface of the photosensitive drum11, and allows the LED head23to emit the light based on image data from the controller50, thereby forming the electrostatic latent image on the surface of the photosensitive drum11. Second, the print controller52allows the development voltage to be applied to the development roller16including a thin toner layer formed on a surface thereof to develop the electrostatic latent image on the photosensitive drum11, thereby forming the toner image. Herein, the print controller52adjusts each of the supply voltage and the regulation voltage so that the thin toner layer having a uniform thickness, that is, the toner layer serving as a developer layer, is formed on the development roller16, and an average charge amount serving as a charge amount of the toner in the toner layer is adjusted to a prescribed value. The printer controller52also changes one of the charging voltage and the development voltage to adjust the average charge amount to the prescribed value.

Third, the print controller52allows the transfer voltage to be applied to the transfer roller22, and allows the transfer belt24to travel by the belt motor54, so that the toner images of the respective colors on the respective photosensitive drums11are sequentially superimposed and transferred to the sheet P, thereby forming a color toner image. The color toner image formed on the sheet P is fixed by the fixing unit35, thereby forming a color image. The sheet P with the color image formed thereon is ejected outside the printer by the ejection roller36, and the print operation is completed.

After the toner image is transferred from the photosensitive drum11, the surface of the photosensitive drum11may have a small amount of the toner14remained thereon. The cleaning blade19scrapes and removes the remaining residual toner14from the surface of the photosensitive drum11.

In a case where the printer is operated in the normal temperature and humidity environment using the toner to be charged with a negative polarity, that is, a toner having a negative charge property, for example, the voltages are set as follows: the charging voltage is set to −100 V, the development voltage is set to −200 V, the supply voltage is set to −300 V, and the regulation voltage is set to −300 V. When the charging roller12is applied with the charging voltage higher than or equal to the prescribed value, the surface of the photosensitive drum11is charged, and a surface potential being proportional to the charging voltage is generated. For example, where the charging voltage is −1100 V, the surface potential of −600 V is generated on the surface of the photosensitive drum11. Accordingly, a potential on a portion of the electrostatic latent image formed with the light emitted from the LED head23, that is, a latent image potential, becomes −50 V, and the toner14supplied from the development roller16is adhered to the electrostatic latent image, thereby performing reverse development. In a case where a toner to be charged with a positive polarity, that is, a toner having a positive charge property, is used, each of the voltages has a reverse property in terms of positive and negative.

According to the first embodiment of the present invention, the charging voltage is set to be in a range between −1000 V and 1200 V (i.e., higher than or equal to −1000 V and lower than or equal to 1200 V), the surface potential of the photosensitive drum11is set to be in a range between −500 V and 700 V (i.e., higher than or equal to −500 V and lower than or equal to 700 V), and the development voltage is set to be in a range between −100 V and 300 V (i.e., higher than or equal to −100 V and lower than or equal to 300 V).

In a case where the print speed increases, the toner image formed on the surface of the photosensitive drum11is not surely transferred to the sheet P, causing blur by transfer. The blur by transfer is a phenomenon in which the toner14is not sufficiently adhered across an image portion on the sheet P in the course of transfer of the toner image to the sheet P.

A description is now given of a relationship between a charge amount Q/M of the toner14on the development roller16, the print speed, and an evaluation level of the blur by transfer with reference toFIG. 4. A horizontal axis indicates the charge amount Q/M of the toner14, and a vertical axis indicates the evaluation level of the blur by transfer. Since the toner charge amount Q/M has a plus or minus sign depending upon the positive charge property or negative charge property of the toner, the toner charge amount Q/M indicates an absolute value or is expressed using the absolute value |Q/M| as may be necessary.

As illustrated inFIG. 4, the print speed is set to 20 PPM, 40 PPM, and 60 PPM, and the charge amount Q/M of the toner14is changed to examine occurrences of the blur by transfer. The higher the print speed, the greater the possibility of the blur by transfer to occur.

Accordingly, the print speed was set to 60 PPM, and an experiment was conducted to obtain a condition under which the blur by transfer does not occur even in a case where the print speed is fast. Where the print speed was set to 60 PPM, the photosensitive drum11had a linear speed of 265 mm/s.

In such a case, the charge amount Q/M of the toner14was calculated as follows: an electrical charge Q of the toner14was measured by a portable draw-off type measurement instrument (Model 210HS-3 available from TREK, Inc.); a mass M of the toner14was measured by an electronic balance instrument (CP225D available from Sartorius Corp.) at the time of measuring the electrical charge Q; and the electrical charge Q was divided by the mass M. The surface potential of the photosensitive drum11and a potential of the toner layer on the development roller16, that is, a toner layer potential, were measured by a surface potential measurement instrument (Model 344 available from TREK, Inc.).

The charge amount Q/M of the toner14on the development roller16and the toner layer potential on the development16were measured in a measurement point between a contact point with the development blade17on the circumference of the development roller16and a contact point with the photosensitive drum11. The surface potential on the photosensitive drum11was measured in a measurement point between a contact point with the charging roller12on the circumference of the photosensitive drum11and a contact point with the development roller16.

Moreover, a color difference ΔE was used as an index to indicate a level of fog. The color difference ΔE was calculated by comparing a Lab value of the sheet P prior to the print operation, that is, the Lab value of a not-yet-print sheet, and the Lab value of a non-image portion (i.e., blank portion) on the sheet P subsequent to the print operation. The Lab value was measured using a spectral colorimeter (CM-2600d available from Konica Minolta Holdings, Inc.). The lower the value of the color difference ΔE, the lower the possibility of the fog occurrence. Therefore, the image quality is high.

The evaluation level of the blur by transfer was evaluated on a ten-level scale according to density of a solid image printed on the sheet P. Herein, the solid image was printed across the sheet P having a size of A4, and the density of the solid image on the sheet P was measured. The density was measured using a spectral densitometer (X-Rite 528 available from X-Rite, Incorporated). The higher the level of evaluation scale, the lower the possibility of the blur occurrence. Therefore, the image quality is high.

Referring toFIG. 5, a description is given of a relationship between the charge amount of the toner14, the evaluation level of the blur by transfer, and the color difference ΔE according to the first embodiment of the present invention. A horizontal axis indicates the charge amount of the toner14, a left vertical axis indicates the evaluation level of the blur by transfer, and a right vertical axis indicates the color difference ΔE.

As illustrated inFIG. 5, the greater the toner charge amount Q/M, the lower the evaluation level of the blur by transfer and the smaller the color difference ΔE. Consequently, the blur by transfer tends to occur, and the fog tends not to occur. According to the first embodiment, following ranges are applied:A suppression range of the fog occurrence: The color difference ΔE is smaller than or equal to 0.5A suppression range of the blue by transfer: Level 8 or higherA preferable range for the charge amount Q/M of the toner14:
25 [μC/g]≦|Q/M|≦40 [μC/g]

Although the toner has the positive charge property and negative charge property, a similar trend is provided except for a reverse sign. Therefore, a range for the charge amount Q/M of the toner14is expressed using the absolute value as above.

Now, a description is given of a relationship between a ratio “r” and smudge and filming. The ratio γ is determined as follows:
γ=Vtnr/(Vopc−Vdb)
where “Vtner” indicates the potential of the toner layer on the development roller16, that is, the toner layer potential serving as the developer layer potential, “Vopc” indicates the surface potential of the photosensitive drum11, and “Vdb” indicates the development voltage. Accordingly, the ratio γ presents the toner layer potential “Vtnr” relative to a difference between the surface potential Vopc and the development voltage Vdb. The relationship between the ratio γ and the smudge and the filming is shown in TABLE 1 below.

As shown in TABLE 1, where the ratio γ is below 0.1, the filming occurs. Where the ratio γ is greater than or equal to 0.2, on the other hand, the smudge occurs. Accordingly, the print controller52adjusts the ratio γ to be in a range stated below by changing the surface potential Vopc and the development voltage Vdb so as to enhance the image quality.
0.1≦γ≦0.2

The print controller52can change one of the surface potential Vopc and the development voltage Vdb to adjust the ratio γ.

The first embodiment, therefore, can suppress the occurrences of the blur by transfer, the fog, the smudge, the filming, and the like by changing the surface potential Vopc and the development voltage Vdb in such a manner that the ratio γ is arranged in the range of 0.1≦γ≦0.2 in a case where the preferable range for the charge amount Q/M of the toner14is expressed using the absolute value as 25 [μC/g]≦|Q/M|≦40 [μC/g].

A description is now given of a printer serving as an image forming apparatus according to a second embodiment of the present invention. Components and configuration of the printer similar to the first embodiment are given the same reference numerals as above, and an advantage derived from the same configuration as above can be provided.

Referring toFIG. 6, a drive system of a photosensitive drum according to the second embodiment of the present invention is illustrated.

According to the second embodiment, a roller motor (not shown) serving as a second drive unit is coupled with a development roller16in such a manner that a linear speed ratio of the development roller16serving as a developer carrier relative to a photosensitive drum11serving as an image carrier can be changed according to density of an image formed on a sheet P serving as a medium.

The photosensitive drum11is rotated at a constant linear speed by gear transmission provided with a drum drive gear41as illustrated inFIG. 6. The drum drive gear41is rotated by the drive of a drum motor53(i.e., substantially similar to the drum motor53ofFIG. 1described in the above embodiment) serving as a first drive unit. A charging roller12serving as a charging device is rotated by friction transmission provided with a surface of the photosensitive drum11, and the development roller16is rotated by the gear transmission provided with a roller drive gear43. The roller drive gear43is rotated by the drive of the roller motor. A toner supplying roller18, serving as a developer supplying member and a developer supplying roller, is rotated by the gear transmission provided with the development roller16through an idler gear42.

The development roller16is rotated at a prescribed linear speed ratio in a direction opposite to the rotation direction of the photosensitive drum11. The toner supplying roller18is rotated in the same direction as the development roller16at a linear speed ratio of 0.7 time that is set beforehand. A motor controller51, functioning as a drive controller, can change one of the rotation speeds of the drum motor53and the roller motor, so that the linear speed ratio of the development roller16relative to the photosensitive drum11can be changed. According to the second embodiment, the motor controller51changes the rotation speed of the roller motor, so that the linear speed ratio of the development roller16relative to the photosensitive drum11is changed.

Referring toFIG. 7, the printer according to the second embodiment of the present invention is illustrated in a control block diagram.

A density detector56detects the density of the toner image serving as a developer image formed on a transfer belt24(i.e., substantially similar to the transfer belt24ofFIG. 2described in the above embodiment) serving as a belt member. The density detector56is disposed opposite to the transfer belt24, and transmits the detected density to a controller50. Accordingly, the toner image is formed in a prescribed pattern on the transfer belt24. After the density is detected, the toner image is scraped and removed from the transfer belt24by a cleaning blade26(i.e., substantially similar to the cleaning blade26ofFIG. 2described in the above embodiment) serving as a second cleaning member.

The motor controller51is connected to the drum motor53, a belt motor54, and a roller motor55. The motor controller51drives the drum motor53to rotate the photosensitive drum11and the like, drives the belt motor54to allow the transfer belt24to travel, and drives the roller motor55to rotate the development roller16.

According to the second embodiment, when the density detected by the density detector56is transmitted to the controller50, the motor controller51changes the linear speed ratio of the development roller16relative to the photosensitive drum11by changing the rotation speed of the roller motor55based on an instruction from the controller50.

Referring toFIG. 8, a description is given of a relationship between the linear speed ratio and the density according to the second embodiment of the present invention. A horizontal axis indicates the linear speed ratio, and a vertical axis indicates the density.

As illustrated inFIG. 8, the higher the linear speed ratio of development roller16relative to the photosensitive drum11, the higher the density.

A print controller52functions as a rotation controller. Where the density detected by the density detector56is high, the print controller52decreases the linear speed ratio by reducing the rotation speed of the roller motor55. Where the density is low, on the other hand, the print controller52increases the linear speed ratio by increasing the rotation speed of the roller motor55.

According to the first embodiment described above, the surface potential Vopc and the development voltage Vdb are adjusted, so that the likelihood of the smudge and the filming occurrences are suppressed. However, the adjustment of the development voltage Vdb may cause deterioration of the image density. According to the second embodiment, on the other hand, the motor controller51increases the linear speed ratio of the development roller16relative to the photosensitive drum11by changing the rotation speed of the roller motor55based on the density detected by the density detector56, thereby reducing the likelihood of the image density deterioration.

Therefore, the image quality can be enhanced according to the second embodiment of the present invention.

Each of the above first and second embodiments of the present invention is described using the printer as the example of the image forming apparatus. However, the present invention can be applied to a photocopier, a facsimile device, a multi-functional peripheral, and the like.

As can be appreciated by those skilled in the art, numerous additional modifications and variation of the present invention are possible in light of the above-described teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.