Image forming apparatus

An image forming apparatus includes a photoconductor; an intermediate transfer belt to which a toner image formed on the photoconductor is transferred, the intermediate transfer belt holding the toner image transferred thereto; a first transfer roller that nips the intermediate transfer belt between the first transfer roller and the photoconductor to transfer the toner image formed on the photoconductor to the intermediate transfer belt; a second transfer roller that transfers the toner image held by the intermediate transfer belt to a recording medium; a detection unit that detects an environmental condition; and a separation position changing unit that changes a first separation position in accordance with the environmental condition, the first separation position being a position at which the intermediate transfer belt becomes separated from the first transfer roller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-234553 filed Oct. 24, 2012.

BACKGROUND

(i) Technical Field

The present invention relates to image forming apparatuses.

(ii) Related Art

An image forming apparatus, such as a photocopier or a printer, includes a transfer device that transfers a toner image formed on a photoconductor to an intermediate transfer belt, which is stretched around components such as a backup roller and transportation rollers, and then transfers the transferred toner image to a recording medium. An image forming apparatus including such a transfer device is capable of being made compact if, for example, the intermediate transfer belt is allowed to be wound around freely.

SUMMARY

According to an aspect of the invention, an image forming apparatus includes a photoconductor; an intermediate transfer belt to which a toner image formed on the photoconductor is transferred, the intermediate transfer belt holding the toner image transferred thereto; a first transfer roller that nips the intermediate transfer belt between the first transfer roller and the photoconductor to transfer the toner image formed on the photoconductor to the intermediate transfer belt; a second transfer roller that transfers the toner image held by the intermediate transfer belt to a recording medium; a detection unit that detects an environmental condition; and a separation position changing unit that changes a first separation position in accordance with the environmental condition, the first separation position being a position at which the intermediate transfer belt becomes separated from the first transfer roller.

DETAILED DESCRIPTION

Exemplary Embodiment

Referring to the drawings, an exemplary embodiment of the present invention is described below.FIG. 1schematically illustrates an image forming apparatus1according to the exemplary embodiment of the present invention. The image forming apparatus1according to the exemplary embodiment includes a controller2, image forming units3K,3Y,3M, and3C, a detection unit4, a transfer unit50, a fixing unit60, and a sheet storage80. The detection unit4detects environmental conditions. The image forming apparatus1forms an image on a sheet P, which serves as a recording medium, or on other types of media on the basis of input image data supplied thereto.

The image forming units3K to3C form toner images of black (K), yellow (Y), magenta (M), and cyan (C). As illustrated inFIG. 1, each of the image forming units3K to3C includes a corresponding one of photoconductors10K to100, a corresponding one of charging units20K to20C, a corresponding one of exposure units30K to30C, a corresponding one of developing units40K to40C, a corresponding one of electric-charge eliminators70K to70C, and a corresponding one of cleaning units71K to71C. In the case where these components do not have to be distinguished from one another by their colors, for example, when the image forming units3K to3C do not particularly have to be distinguished from one another, each of the image forming units3K to3C is described simply as an image forming unit3.

The controller2includes an arithmetic unit such as a central processing unit (CPU) and a memory to control operations of components of the image forming apparatus1.

Each of the photoconductors10(10K to10C) is a cylindrical rotating body that rotates in a direction of the arrow ofFIG. 1and that has a photosensitive layer made of an organic photosensitive material to hold an image.

Each of the charging units20(20K to20C) applies a predetermined charging voltage to the surface of the corresponding photoconductor10using, for example, a charging roller that rotates while coming into contact with the surface of the photoconductor10. Each charging unit20may be a contact-type charging unit that charges the photoconductor10while coming into contact with the photoconductor10using a brush or may be a non-contact-type charging unit that charges the photoconductor10using a corona discharge.

Each of the exposure units30(30K to30C) emits light based on image data to the surface of the corresponding photoconductor10charged by the corresponding charging unit20and forms an electrostatic latent image having a latent image potential by using a potential difference. As the photoconductor10rotates, the electrostatic latent image moves to a position at which the corresponding developing unit40is disposed.

Each of the developing units40(40K to40C) has a rotatable developing roller41(a corresponding one of the developing rollers41K to41C) and a toner adhering to the developing roller41transfers to the corresponding photoconductor10. Specifically, the toner transfers to the surface of the photoconductor10due to there being a potential difference between the charged toner and the electrostatic latent image formed on the photoconductor10. Consequently, a toner image is formed on the photoconductor10. The toner image moves to a position at which the transfer unit50is disposed as the photoconductor10rotates.

The transfer unit50includes an intermediate transfer belt51, a back-up roller52, transportation rollers53aand53b, first transfer rollers54(54K to54C), a second transfer roller55, a belt-lifting roller56, and a roller shifter57. The transfer unit50transfers the toner images formed on the intermediate transfer belt51to a sheet P that has been transported thereto by transportation rollers91. The sheet P to which the toner images have been transferred is transported to the fixing unit60.

The intermediate transfer belt51is stretched around the transportation rollers53aand53band the back-up roller52. The intermediate transfer belt51is driven to rotate by driving, for example, the transportation roller53a. The first transfer rollers54are disposed so as to face the photoconductors10with the intermediate transfer belt51interposed therebetween. Specifically, the first transfer roller54K faces the photoconductor10K and the other first transfer rollers54face the corresponding photoconductors10.

Each of the first transfer rollers54K to54C is driven to rotate as the intermediate transfer belt51rotates. While rotating, the first transfer rollers54K to54C press the intermediate transfer belt51to transfer toner images formed on the photoconductors10K to10C to the intermediate transfer belt51so that the toner images are held on the intermediate transfer belt51in a stacked manner.

The second transfer roller55is disposed so as to face the back-up roller52. The intermediate transfer belt51and a sheet P are nipped between the second transfer roller55and the back-up roller52and thus the toner images that have been transferred to the intermediate transfer belt51are transferred to the sheet P.

The belt-lifting roller56is a transportation roller that lifts the surface of the intermediate transfer belt51so that a nip portion in which the intermediate transfer belt51and the photoconductor10are in contact with each other is flattened. The roller shifter57moves the belt-lifting roller56in accordance with control of the controller2in a direction perpendicular to the direction in which the intermediate transfer belt51moves. The belt-lifting roller56and the roller shifter57controlled by the controller2are included in a separation position changing unit according to the exemplary embodiment, which will be described below.

The fixing unit60includes a heating roller61, which includes a heat source, and a pressing roller62, which is disposed opposite the heating roller61. The heating roller61is pressed by the pressing roller62. The fixing unit60fixes unfixed toner images formed on a sheet P to the sheet P by heating and pressing the toner images. The sheet P to which the toner images have been fixed by the fixing unit60is transported by transportation rollers91and then ejected to the outside.

Each electric-charge eliminator70(70K to70C) eliminates, by, for example, irradiating the surface of the corresponding photoconductor10with light, electric charge remaining on the surface of the photoconductor10after the toner image formed on the photoconductor10has been transferred to the intermediate transfer belt51by the corresponding first transfer roller54. The electric-charge eliminator70increases an amount of electric charge to be eliminated as the separation position changing unit starts operating.

Each of the cleaning units71(71K to71G) removes remnants such as a toner remaining on the surface of the corresponding photoconductor10after the toner image has been transferred to the sheet P. Multiple sheets P are stored in the sheet storage80. The sheets P are picked up by a pick-up roller90from the sheet storage80and transported to the transfer unit50by the transportation rollers91.

Referring now toFIG. 2, a separation position changing unit according to the exemplary embodiment will be described. As mentioned above, the separation position changing unit includes the belt-lifting roller56and the roller shifter57controlled by the controller2.

The detection unit4detects, for example, at least one of temperature or humidity as an environmental condition.

When the temperature and/or the humidity detected by the detection unit4exceeds a predetermined temperature and/or a predetermined humidity, the controller2determines the temperature and/or the humidity is high and transmits a signal to the roller shifter57. Upon receipt of the signal, the roller shifter57moves the belt-lifting roller56in a direction perpendicular to the direction in which the intermediate transfer belt51moves.

Specifically, as illustrated inFIG. 2, upon receipt of the signal from the controller2, the roller shifter57moves the belt-lifting roller56from, for example, a first roller position R1to a second roller position R2. According to the movement of the belt-lifting roller56, the intermediate transfer belt51is changed from a first belt position B1to a second belt position B2.

According to the movement of the intermediate transfer belt51, a position at which the intermediate transfer belt51and the first transfer roller54K, which have been in contact with each other, become separated from each other is changed from a first separation position H1to a second separation position H2.

Specifically, as illustrated inFIG. 2, the position at which the intermediate transfer belt51becomes separated from the first transfer roller54K and then moves toward the second transfer roller55is changed from the first separation position H1to the second separation position H2, which is positioned upstream from the first separation position H1in the direction in which the intermediate transfer belt51moves. Here, an example of the roller shifter57is a driving mechanism that moves the belt-lifting roller56by using a cam. The temperature and/or the humidity detected by the detection unit4is input into the controller2, which determines whether or not the temperature/humidity exceeds the predetermined temperature/humidity.

Now, a process in which the intermediate transfer belt51and the first transfer roller54K become separated from each other will be described. The intermediate transfer belt51is typically controlled on the basis of the surface resistance, which is a resistance in the surface direction, and the volume resistance, which is a resistance in the thickness direction. If the intermediate transfer belt51has a low surface resistance, the electric field is more likely to expand, whereas if the intermediate transfer belt51has a high surface resistance, the electric field is more likely to be localized without expanding.

Typically, the surface resistance of the intermediate transfer belt51is affected to a lesser extent by environmental changes including temperature and humidity. On the other hand, the resistance of the first transfer roller54K changes to a large extent due to such environmental changes. Thus, the first transfer roller54K is often made of a material whose resistance decreases in hot and humid conditions and increases in cold and dry conditions.

In hot and humid conditions, the first transfer roller54K has a low resistance and thus a small electric field occurs at the separation position at which the back side of the intermediate transfer belt51and the first transfer roller54K become separated from each other.

If the electric field that occurs at the separation position is small, the amount of electric charge discharged on the back side of the intermediate transfer belt51may be too small to attract the toner image that has been transferred to the intermediate transfer belt51to it. If so, the toner image may be defectively transferred to a sheet P by the second transfer roller55. Such defective transfer is prevented by, for example, the separation position changing unit appropriately regulating the amount of electric charge discharged during separation between the intermediate transfer belt51and the first transfer roller54K.

The image forming apparatus1according to the exemplary embodiment achieves stable electric discharge between the intermediate transfer belt51and the first transfer roller54K by having a simple configuration in which the roller shifter57causes the belt-lifting roller56, disposed downstream from the image forming unit3K, to lift the intermediate transfer belt51up and down.

In this configuration, the electric field that occurs at the separation position at which the intermediate transfer belt51and the first transfer roller54K become separated from each other becomes larger and thus an allocation of a gap between the photoconductor10K and the intermediate transfer belt51becomes smaller. Thus, the amount of electric charge discharged on the back side of the intermediate transfer belt51increases and a larger amount of electric charge transfer from the first transfer roller54K to the intermediate transfer belt51.

In this manner, the electrically charged states on the front and back sides of the intermediate transfer belt51are made stable, thereby preventing defective image formation. Secondary troubles due to an effect of electric discharge that occurs during separation between the photoconductor10K and the intermediate transfer belt51are prevented from occurring by increasing the amount of electric charge to be eliminated from the photoconductor10by the electric-charge eliminator70to an optimum level.

Another Exemplary Embodiment

The image forming apparatus1according to the exemplary embodiment has been described thus far. The present invention, however, is not limited to the above-described exemplary embodiment and other exemplary embodiments are also conceivable. Now, other exemplary embodiments will be described.

In the separation position changing unit according to the exemplary embodiment, the roller shifter57moves the belt-lifting roller56in response to a signal from the controller2, but the present invention is not limited to this configuration. For example, as illustrated inFIG. 3, the separation position changing unit may have a configuration in which an offset amount changing unit58, which changes an amount of offset, is controlled by a signal from the controller2.

In this separation position changing unit, upon receipt of a signal from the controller2, the offset amount changing unit58moves the first transfer roller54K in the same direction as the direction in which the intermediate transfer belt51moves. In other words, the offset amount changing unit58is a driving mechanism that moves the first transfer roller54K by using a cam or other devices.

Here, offset is used to describe the state where a point at which the photoconductor10and the intermediate transfer belt51are in contact with each other (nip point) and a point at which the intermediate transfer belt51and the first transfer roller54K are in contact with each other (nip point) do not coincide with each other. The amount of offset is a distance by which these points are separated from each other. In this manner, a sufficiently large transfer area between the photoconductor10and the intermediate transfer belt51is secured.

The offset amount changing unit58, which changes the amount of offset, moves the first transfer roller54K to change the amount of offset from a first offset amount OF1to a second offset amount OF2, as illustrated inFIG. 3, so as to reduce the distance between the nip points. In this manner, the separation position at which the intermediate transfer belt51and the first transfer roller54K become separated from each other is changed.

Specifically, when a position at which the intermediate transfer belt51becomes separated from the first transfer roller54K is denoted by a roller separation position HR and a position that is upstream from the roller separation position HR and at which the intermediate transfer belt51becomes separated from the photoconductor10and then moves toward the second transfer roller55is denoted by a photoconductor separation position HK, the roller separation position HR is changed to a upstream side in accordance with the change in amount of offset from the first offset amount OF1to the second offset amount OF2. This change in amount of offset reduces a gap between the roller separation position HR and the photoconductor separation position HK, thereby changing the position at which the intermediate transfer belt51and the first transfer roller54K become separated from each other.

In the exemplary embodiment, the amount of electric charge to be eliminated by the electric-charge eliminator70is changed in accordance with the change in the position at which the intermediate transfer belt51and the first transfer roller54K become separated from each other. However, the present invention is not limited to this. For example, an image forming apparatus that does not include an electric-charge eliminator70is also conceivable.