Image forming apparatus with suction unit controller

Provided is an image forming apparatus including: an image carrying unit on which a toner image is carried; a transfer unit that transfers a toner image carried by the image carrying unit onto a sheet at a transferring position; a sheet conveyor unit that conveys the sheet having the toner image transferred thereto by the transfer unit; a suction device that sucks the sheet to the sheet conveyor unit; and a controller that stops or reduces a sucking operation of the suction device where the toner image carried by the image carrying unit passes the transferring position without being transferred onto the sheet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus for forming an image on a sheet.

2. Description of the Related Art

As a conventional image forming apparatus such as a copying machine, a printer, or a facsimile, there is one having a structure in which a toner image formed on a photosensitive drum is primarily transferred onto an endless intermediate transfer belt and is then secondarily transferred onto the sheet. In addition, a sheet transport apparatus for transporting the sheet having the toner image secondarily transferred thereto to a fixing portion is provided between a secondary transfer portion for secondarily transferring the toner image onto the sheet, and the fixing portion for fixing the toner image onto the sheet.

As the sheet transport apparatus, there is one which includes a conveyor belt, a roller for driving the conveyor belt, and a fan for attaching the sheet to the conveyor belt, and transports the sheet having the toner image transferred thereto to the fixing portion while adsorbing the sheet (for example, see Japanese Patent Application Laid-Open No. 2005-316184).

FIG. 8is a diagram illustrating a structure of a related sheet transport apparatus. A sheet transport apparatus200includes an endless sheet conveyor belt201which is suspended over multiple rollers203and has flexibility, and a fan202provided inside the sheet conveyor belt, for attracting a sheet.

InFIG. 8, a toner image formed on a photosensitive drum (not shown) is primarily transferred onto an intermediate transfer belt211, and a second transfer portion210transfers a toner image T formed on the intermediate transfer belt211onto the sheet.

In a case where the sheet passing through the secondary transfer portion210and having the toner image secondarily transferred thereto is transported to a fixing portion (not shown), the sheet conveyor belt201is rotated in a direction of the fixing portion while the sheet is adsorbed to the sheet conveyor belt201by the fan202. When a full-color image is formed, the intermediate transfer belt211is first rotated multiple times, and then the toner images T for each color are sequentially superposed on the intermediate transfer belt, thereby forming a full-color toner image T. After that, the full-color toner image T is secondarily transferred onto the sheet from the intermediate transfer belt211.

In order to reliably adsorb and transport the sheet in the sheet transport apparatus and the image forming apparatus, it is necessary to increase an adsorbability (suction force) of the fan202. However, in a case where the adsorbability of the fan202is increased, an air flow from the sheet conveyor belt201to the fan202is generated.

Particularly in a case of forming a full-color image, when the toner images T sequentially superposed on the intermediate transfer belt211reach a position at which the toner images T face the fan202before the toner images T are transferred onto the sheet as illustrated inFIG. 8, the toner images T may be scattered from the intermediate transfer belt211by the air flow. When the toner images T are scattered from the intermediate transfer belt211, a sheet transport path may be contaminated, and the sheet is also contaminated when the sheet is transported. When the toner images T are scattered from the intermediate transfer belt211, an image quality is degraded.

Japanese Patent Application Laid-Open No. H08-310686 discloses a structure in which a fan for adsorbing a sheet to a conveyor belt is stopped/driven according to a position of the sheet to be transported. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-310686, noise due to a change of the attraction, which is caused when a trailing edge of the sheet passes through an adsorbing portion, is prevented from generating by stopping a sheet adsorbing operation before the trailing edge of the sheet passes through an adsorbing unit. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-310686, the above-mentioned problem in that the toner image is scattered when the toner image formed on an image bearing member faces the conveyor belt is not taken into consideration. In the structure disclosed in Japanese Patent Application Laid-Open No. 08-310686, the toner image developed on the photosensitive drum serving as the image bearing member is immediately transferred onto the sheet by the transfer portion. Accordingly, the toner image formed on the photosensitive drum is not in a state of facing the conveyor belt.

Japanese Patent Application Laid-Open No. H08-110738 discloses a structure in which a fan for ventilation of the apparatus main body is turned up during a time when formation of an image on the sheet is not performed, and is turned down during image formation. In a case of operating the fan for ventilation during image formation, the fan can be driven with only a small power which does not affect the image formation. Accordingly, in the structure disclosed in Japanese Patent Application Laid-Open No. H08-110738, in consideration of the fact that the ventilation by the fan for ventilation when the fan is insufficient with such a small power that does not affect the image formation, the exhaust fan is strongly driven during a time when the image formation is not performed. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-110738, the above-mentioned problem in that the toner image is scattered when the toner image formed on the image bearing member faces the conveyor belt is not taken into consideration. In the structure disclosed in Japanese Patent Application Laid-Open No. H08-110738, because the toner image developed on the photosensitive drum serving as the image bearing member is immediately transferred onto the sheet by the transfer portion, the toner image formed on the photosensitive drum is not in a state of facing the conveyor belt.

Japanese Patent Application Laid-Open No. 2002-258722 discloses a structure in which disturbance of toner particles on a sheet is prevented by turning down or stopping a cooling fan which is used for preventing temperature rise within the apparatus due to heat generated from the fixing device when the leading edge of the sheet having an image formed thereon is present between the transfer portion and the fixing device. In the structure disclosed in Japanese Patent Application Laid-Open No. 2002-258722, an effect on the toner on the sheet due to the fan for preventing temperature rise within the apparatus is taken into consideration. However, the problem in that the toner image is scattered when the toner image formed on the image bearing member faces the conveyor belt is not taken into consideration.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentioned circumstances, and the present invention provides a sheet transport apparatus and an image forming apparatus capable of transporting a sheet in a state where a toner image is hardly scattered.

According to the preset invention, there is provided an image forming apparatus including: an image carrying unit on which a toner image is carried; a transfer unit that transfers a toner image carried by the image carrying unit onto a sheet at a transferring position; a sheet conveyor unit that conveys the sheet having the toner image transferred thereto by the transfer unit; a suction device that sucks the sheet to the sheet conveyor unit; and a controller that stops or reduces a sucking operation of the suction device where the toner image carried by the image carrying unit passes the transferring position without being transferred onto the sheet.

According to the present invention, it is possible to reduce scattering of the toner image formed on the image bearing member.

Further features of the present invention will become apparent from the following description of an exemplary embodiment (with reference to the attached drawings).

DESCRIPTION OF THE EMBODIMENT

Hereinafter, an exemplary embodiment for carrying out the present invention will be described in detail with reference to the drawings.

FIG. 1is a diagram schematically illustrating a color laser printer which is an example of an image forming apparatus including a sheet transport apparatus according to an embodiment of the present invention.

InFIG. 1, a color laser printer main body (hereinafter, referred to as “apparatus main body”)101of a color laser printer (image forming apparatus)100includes an image forming portion102, a sheet feeding portion103for feeding a sheet P to the image forming portion102, a fixing portion6, and a conveyor belt unit16which is an example of a sheet transport apparatus for transporting a sheet having an unfixed full-color toner image transferred thereto, to the fixing portion6. The conveyor belt unit16transports a sheet through rotation of a sheet conveyor belt31in a state where the sheet is adsorbed to the sheet conveyor belt by a conveying fan30(seeFIG. 2). A structure of the conveyor belt unit16will be described later.

The image forming portion102includes a cylindrical photosensitive drum1, a primary charger2, a rotary developing device3having multiple developing devices3Y,3M,3C, and3Bk each being integrated with a toner cartridge. The image forming portion102includes an endless intermediate transfer belt (image carrying unit)8onto which four color toner images sequentially formed on the photosensitive drum1are sequentially primarily transferred, and a secondary transfer roller5for secondarily transferring the toner images primarily transferred onto the intermediate transfer belt8onto the sheet P.

An exposure system4includes a laser unit4afor emitting a laser beam L and on/off modulating by an image signal (VDO signal) to be described later, and a polygon mirror4bused for horizontal scanning by the laser beam L. The exposure system4further includes a scanner motor4cfor rotating the polygon mirror4bat low speed, an fθ lens4dfor imaging the laser beam L, a photodetector4efor detecting a BD signal indicating a start point of the horizontal scanning, and a reflection mirror4ffor guiding the laser beam L to the photosensitive drum1.

The sheet feeding portion103includes cassettes12that contains sheets P and is detachably mounted onto the apparatus main body101, and a manual feed cassette19. The sheets P are supplied to the image forming portion102from the cassettes12and the manual feed cassette19. On an upstream side of the image forming portion102, there is provided a registration roller14for enhancing a posture position accuracy of the sheet P and for feeding the sheet P at an appropriate timing for the toner image formed on the transfer belt.

A detector9detects a detection target provided at a predetermined position on the intermediate transfer belt8. The detector9outputs a reference signal which is determined in consideration of a transfer position when the toner image formed on the photosensitive drum1is transferred onto the intermediate transfer belt8, and which is a trigger of a start of image formation by the laser unit4a.

InFIG. 10which is a block diagram of the control of the apparatus main body, a control unit (control means)62controls the entire image forming operation of the apparatus main body101. The control unit62is connected to the detector9. A detection result obtained by the detector9is input to the control unit62. The control unit62is connected to an image forming control portion for controlling each of the image forming portion102and the sheet feeding portion103, and controls each of the image forming portion102and the sheet feeding portion103through the image forming control portion. The control unit62is connected to a fan drive motor30A for driving the conveying fan30of the conveyor belt unit16. The control unit62controls drive of the fan drive motor30A.

The control unit62outputs the image signal (VDO signal) to the exposure system4in synchronization with a reference signal (TOP signal) from the detector9. In other words, the detector9detects the detection target provided at the predetermined position on the intermediate transfer belt8, and transmits the TOP signal to the control unit62. The control unit62having received the TOP signal transmits the image signal (VDO signal) to the exposure system4. The photosensitive drum1is subjected to scanning exposure by the laser beam L emitted from the laser unit4aand on/off modulated by the image signal (VDO signal), thereby forming an electrostatic latent image corresponding to the image signal on the photosensitive drum1.

Next, an operation of the image forming apparatus100will be described.

When an image forming signal is output from the control unit62provided to the apparatus main body101, an image signal (VDO signal) from, for example, an image reading portion or a personal computer (not shown) is input to the exposure system4in synchronization with the reference signal (TOP signal) from the detector9.

In the exposure system4, the laser beam L is radiated from the laser unit4ain response to the image signal (VDO signal), and the horizontal scanning by the laser beam L is performed using the polygon mirror4b. After that, the laser beam L passes through the fθ lens4dand the reflection mirror4f, and the photosensitive drum1is radiated with the laser beam L. At this time, the detector4edetects the signal (BD signal) indicating the start point of the horizontal scanning by exposure, thereby performing exposure in synchronization with the BD signal.

At this time, the photosensitive drum1is charged with a predetermined polarity and a predetermined voltage in advance by the primary charger2, and is radiated with the laser beam L, thereby forming the electrostatic latent image on a surface of the photosensitive drum1. Then, a toner image for a selected color is formed by the multiple developing devices3Y,3M,3C, and3Bk provided in the rotary developing device3. After that, the toner image formed on the photosensitive drum is primarily transferred onto the intermediate transfer belt8by a primary transfer roller1aat the prescribed transfer position.

In a case of a full-color mode, first, in synchronization with the reference signal (TOP signal) from the detector9, the control unit62inputs an image signal (VDO signal) related to yellow to the exposure system4so as to form an electrostatic latent image related to yellow on the photosensitive drum1. In order to develop the electrostatic latent image related to yellow which is formed on the photosensitive drum1, the yellow developing device3Y is placed at a position opposing the photosensitive drum1. By the yellow developing device3Y, the yellow toner image, which is a first color, is formed on the photosensitive drum1. The yellow toner image is primarily transferred onto the intermediate transfer belt8by application of a transfer bias voltage with a polarity opposite to that of the toner, to the primary transfer roller1a.

After that, the intermediate transfer belt8having the yellow toner image transferred thereto further rotates so that a subsequent toner image is primarily transferred thereto. During that time, the rotary developing device3is caused to rotate in a counterclockwise direction so that the developing device3M for magenta, which is a subsequent color, faces the photosensitive drum1.

Subsequently, an image signal (VDO signal) related to magenta is input to the exposure system4so as to form an electrostatic latent image related to magenta on the photosensitive drum1. The control unit62inputs the image signal to the exposure system4at a timing after the elapse of a predetermined time after receiving the reference signal (TOP signal) from the detector9.

The predetermined time is set as a time for superimposing the magenta toner image, which is to be primarily transferred, on the yellow toner image formed on the intermediate transfer belt8, based on a circumferential speed of the photosensitive drum1, a peripheral length of the photosensitive drum1from a point at which the laser beam is radiated from the laser unit4a, to the primary transfer portion, and based on a circumferential speed and a peripheral length of the intermediate transfer belt8. In other words, it is possible to obtain a position of a toner image formed on the intermediate transfer belt8according to the elapse of time from a time when the reference signal (TOP signal) from the detector9is transmitted, based on the circumferential speed of the photosensitive drum1, a rotational speed of the intermediate transfer belt8, and the like. After the elapse of the time which is set as the time for superimposing the magenta toner image to be primarily transferred, on the yellow toner image formed on the intermediate transfer belt8, after the reference signal (TOP signal) from the detector9is received, the control unit62inputs the image signal to the exposure system4to thereby cause the laser unit4ato start latent image formation.

The magenta electrostatic latent image formed on the photosensitive drum1is developed on the photosensitive drum1by the magenta developing device3M. The toner image for magenta, which is the second color, developed on the photosensitive drum1is primarily transferred onto the transfer belt8by the primary transfer roller1aso that the magenta toner image is superimposed on the yellow toner image formed on the intermediate transfer belt8.

The intermediate transfer belt8to which the yellow toner image and the magenta toner image are transferred further rotates so that the subsequent toner image is primarily transferred thereto. During that time, the rotary developing device3causes the developing device3C for cyan, which is a subsequent color, to rotate in a counterclockwise direction so that the developing device3C faces the photosensitive drum1.

The control unit62inputs an image signal (VDO signal) related to cyan to the exposure system4so as to form an electrostatic latent image related to cyan on the photosensitive drum1. The control unit62inputs the image signal to the exposure system4at a timing after the elapse of a predetermined time which is set as a time for superimposing the cyan toner image to be primarily transferred, on the yellow toner image and the magenta toner image that are formed on the intermediate transfer belt8after the reference signal (TOP signal) from the detector9is received.

The cyan electrostatic latent image formed on the photosensitive drum1is developed on the photosensitive drum1by the cyan developing device3C. The toner image for cyan, which is the third color, developed on the photosensitive drum1is primarily transferred onto the intermediate transfer belt8by the primary transfer roller1aso that the cyan toner image is superimposed on the yellow toner image and the magenta toner image that are formed on the intermediate transfer belt8.

The intermediate transfer belt8having the yellow toner image, the magenta toner image, and the cyan toner image transferred thereto further rotates so that the subsequent toner image is primarily transferred thereto. During that time, the rotary developing device3causes the developing device3Bk for black, which is the subsequent color, to rotate in a counterclockwise direction so that the developing device3Bk faces the photosensitive drum1.

The control unit62inputs an image signal (VDO signal) related to black to the exposure system4so as to form an electrostatic latent image related to black on the photosensitive drum1. The control unit62inputs the image signal to the exposure system4at a timing after the elapse of a predetermined time which is set as a time for superimposing the black toner image to be primarily transferred, on the yellow toner image, the magenta toner image, and the cyan toner image that are formed on the intermediate transfer belt8after the reference signal (TOP signal) from the detector9is received.

The black electrostatic latent image formed on the photosensitive drum1is developed on the photosensitive drum1by the black developing device3Bk. The toner image for black, which is the fourth color, developed on the photosensitive drum1is primarily transferred onto the intermediate transfer belt8by the primary transfer roller1aso that the black toner image is superimposed on the yellow toner image, the magenta toner image, and the cyan toner image that are formed on the intermediate transfer belt8.

As described above, in the full-color mode, formation of an electrostatic latent image, development, and primary transfer are repeated until the four color toner images are transferred onto the intermediate transfer belt8, thereby forming a full-color toner image, which is obtained by superimposing four colors, on the intermediate transfer belt8.

On the other hand, the control unit62causes a sheet feed roller13and a transport roller15, which are provided to the sheet feeding portion103, to rotate in response to the reference signal (TOP signal) from the detector9serving as a detecting unit, thereby feeding the sheet P from the cassettes12or the manual feed cassette19. After that, a skew of the sheet P fed from the sheet feeding portion103is corrected by the registration roller14, and the sheet P is transported to a secondary transfer portion which includes the intermediate transfer belt8and the secondary transfer roller5at an appropriate timing for the toner image formed on the intermediate transfer belt8.

Then, the full-color toner image formed on the intermediate transfer belt8is secondarily transferred onto the sheet P, which is fed to the secondary transfer portion, by the secondary transfer roller5, and then the sheet P is transported to the fixing portion6by the conveyor belt unit16. After that, an unfixed transfer image is permanently fixed onto the sheet P through heating and pressurization by the fixing portion6. The sheet P having the image thus fixed thereon is delivered onto a delivery tray64from the apparatus main body101by a transport roller17and a delivery roller18.

In a case where the image forming apparatus100has a duplex image forming function and images are formed on both surfaces of the sheet, after completion of a fixing process on a first surface of the sheet by the fixing portion6, the sheet P is transported to the image forming portion102again through a reversing and conveying path105. Then, with respect to the sheet P which is transported again to the image forming portion102, the toner image is formed on a second surface of the sheet P by the above-mentioned imaging process.

Residual toner on the surface of the photosensitive drum1which has finished transfer of the toner image onto the intermediate transfer belt8, and on the surface of the intermediate transfer belt8which has finished transfer of the toner image onto the sheet P, is removed by cleaning devices7and11each having a blade-like cleaning member.

FIG. 2is a diagram illustrating a structure of the color laser printer in the vicinity of the conveyor belt unit16. InFIG. 2, a carrying space S is provided between a lower surface11aof the cleaning device11, and a conveying guide G provided at a downstream of the secondary transfer portion. The sheet P obtained after the secondary transfer is transported to the conveyor belt unit16through the carrying space S.

A part of the peripheral surface of the intermediate transfer belt8faces the carrying space S. For example, in a case where the full-color toner image is formed on the intermediate transfer belt8, when the intermediate transfer belt8having the first to third color toner images transferred thereto rotates, the toner images formed on the intermediate transfer belt8are carried while facing the carrying space S.

As illustrated inFIG. 3, the conveyor belt unit16includes four suspension rollers32and32a, and four sheet conveyor belts31which are suspended by the four rollers32and32a, and conveying fans30provided inside the sheet conveyor belt31(seeFIG. 4). The conveying fans30sucks air for adsorbing the sheet to the conveyor belt31.FIG. 4is a perspective view of the conveyor belt unit16in a state where the sheet conveyor belts31are removed.

One suspension roller32aamong the four suspension rollers32and32ais a drive roller, and has a knurled shape so as to reliably rotate the sheet conveyor belt31serving as the sheet transport unit. The sheet conveyor belt31has multiple holes31aformed therein so that attraction is enabled by the conveying fan30serving as the adsorbing unit provided inside the sheet conveyor belt31. By providing the plurality of holes31a, it is possible to stably adsorb and transport the sheet.

When the sheet is adsorbed and transported by the conveyor belt unit16, in a case where the adsorbability (suction force) of the conveying fan30is large, the air flow from the carrying space side to the conveying fan30is generated as illustrated inFIG. 5. When the air flow is generated, for example, when the full-color toner image is formed, in a case where the toner image formed on the intermediate transfer belt8is present at the position where the toner image faces the carrying space S, there is a fear that the toner on the intermediate transfer belt8scatters in the carrying space S, which may contaminate the conveying guide G (sheet transport path).

In this embodiment, the adsorbability of the conveying fan30is set to be variable. In the case where the toner image formed on the intermediate transfer belt8is present at the position where the toner image faces the carrying space S, the conveying fan30is stopped, thereby removing (reducing) the adsorbability. In a case where the toner image is present in the carrying space S, the adsorbability of the conveying fan30is removed, thereby preventing scattering of the toner and contamination of the sheet transport path.

In this embodiment, the position of the toner image formed on the intermediate transfer belt8can be recognized based on the elapse of time from the time when the reference signal (TOP signal) from the detector9is transmitted. Based on the elapse of time from the time when the reference signal from the detector9is transmitted, in a case where the toner image formed on the intermediate transfer belt8is present in the carrying space S, the fan drive motor30A is stopped by the control unit62so as to remove the adsorbability of the conveying fan30.

Next, referring toFIGS. 6A to 6C, description will be made of the control of the adsorbability by the control unit62according to this embodiment in the case where the full-color toner image is formed.

FIG. 6Aillustrates a state where a toner image T transferred onto the intermediate transfer belt8is present in the carrying space S in the case where the full-color toner image is formed. In this case, the control unit62stops the fan drive motor30A so as to stop the conveying fan30.

FIGS. 6B and 6Cillustrate a state where the toner image T formed on the intermediate transfer belt8has passed through the carrying space S. Also in this case, the drive of the fan drive motor30A is continuously stopped by the control unit62so as to remove the adsorbability of the conveying fan30. It should be noted that when the toner image is transferred onto the intermediate transfer belt8, the secondary transfer roller5is moved to a position spaced apart from the intermediate transfer belt8.

In this case, when the full-color toner image is formed, the operations illustrated inFIGS. 6A to 6Care repeated. Specifically, in the case of forming the full-color toner image, when the toner image for the first color is transferred onto the intermediate transfer belt8and then the toner image for the second color is transferred onto the intermediate transfer belt8, a trailing end portion of the toner image T is present in the carrying space S as illustrated inFIG. 6A. After that, in association with the rotation of the intermediate transfer belt8, the trailing end portion of the toner image T formed on the first surface passes through the carrying space S as illustrated inFIGS. 6B and 6C.

Then, when the toner image for the third color is transferred onto the intermediate transfer belt8, each trailing end portion of the toner images T for the first color and the second color is present in the carrying space S as illustrated inFIG. 6A. After that, in association with the rotation of the intermediate transfer belt8, each trailing end portion of the toner images T for the first color and the second color passes through the carrying spaces S as illustrated inFIGS. 6B and 6C.

When the toner image for the fourth color is transferred onto the intermediate transfer belt8, the trailing end portions of the toner images T for the first color to the third color are present in the carrying space S as illustrated inFIG. 6A. After that, in association with the rotation of the intermediate transfer belt8, the trailing end portions of the toner images T for the first color to the third color pass through the carrying space S as illustrated inFIGS. 6B and 6C.

In this embodiment, the control unit62stops the conveying fan30immediately before the leading end of the toner image for the first color transferred onto the intermediate transfer belt8reaches the carrying space S. Then, as illustrated inFIG. 6B, after each trailing end portion of the toner images T for the first to third colors has passed through the carrying space S, the toner images T are not present in the carrying space S, so the control unit62controls the fan drive motor30A so that the adsorbability of the conveying fan30is restored to an adsorbability necessary for adsorbing the sheet.

By restoration of the adsorbability of the conveying fan30at the above-mentioned timing, when the sheet reaches the conveyor belt unit16after the four color toner images are transferred onto the sheet by the secondary transfer portion, the intermediate transfer belt8exerts a sufficient adsorbability.

The control unit62performs control of the adsorbability of the conveying fan30in response to the reference signal (TOP signal) from the detector9.

FIG. 7is a diagram for illustrating a relationship between the operation of controlling the conveying fan30by the control unit62and the position of the toner image on the intermediate transfer belt8.

As illustrated inFIG. 7, the control unit62controls the exposure system4so as to start image formation after T1seconds in response to the reference signal (TOP signal) from the detector9. After receiving the reference signal (TOP signal) from the detector9, the control unit62controls the fan drive motor30A so as to stop the conveying fan30after time T2(>time T1) seconds. The time T2is shorter than the time T4which is immediately before the time when the toner image T for the first color reaches the carrying space S (T4>T2).

A time between the time when the control unit62receives the reference signal (TOP signal) from the detector9and the time when the toner images T for the first to third colors pass through the carrying space S as illustrated inFIG. 6B, is represented as T5. The control unit62controls the fan drive motor30A to drive the conveying fan30after the toner images have passed through the carrying space S, that is, after the elapse of a time T3which is larger than a time T5.

FIG. 9is a flowchart illustrating the operation of controlling the conveying fan30by the control unit62. InFIG. 9, the control unit62determines whether or not the reference signal (TOP signal) of the detector9is received in Step1(hereinafter, a step is represented as “S”).

After receiving the reference signal (TOP signal) from the detector9, the control unit62drives the conveying fan30after the elapse of the time T3from the time when the reference signal (TOP signal) is received from the detector9(S2and S3). The time T3is set as a time after the toner images T for the first to third colors has passed through the carrying space S after the reference signal is received as illustrated inFIG. 6B.

Then, the control unit62determines whether or not there is a subsequent page (S4). In a case where there is the subsequent page, the control unit62determines whether or not the reference signal related to the subsequent page has been received (S5).

Upon reception of the reference signal (TOP signal) related to the subsequent page from the detector9, the control unit62stops the conveying fan30after the elapse of the time T2after the reference signal (TOP signal) related to the subsequent page is received. The time T2is set as a time before the toner image for the first color related to the subsequent page reaches the carrying space S after the reference signal (TOP signal) related to the subsequent page is received, that is, a time before the toner image for the first color faces the conveyor belt.

Then, after the elapse of the time T3after receiving the reference signal (TOP signal) related to the subsequent page, the control unit62drives the conveying fan30(S2).

In a case where it is determined that there is not the subsequent page in S4, the control unit62stops the conveying fan30after the elapse of time sufficient for the conveyor belt unit16to transport the sheet to the fixing portion6(S8).

The conveying fan30is stopped before the toner image primarily transferred onto the intermediate transfer belt reaches the position at which the toner image faces the sheet conveyor belt31, thereby enabling reduction in scattering of the toner image. As a result, it is possible to provide a sheet having a high-quality image formed thereon without adhesion of stain to the sheet. It is also possible to reduce contamination due to toner within the apparatus main body.

In this embodiment, description has been made of the mode in which the conveying fan30is stopped in the case where the toner image is present in the carrying space S. Alternatively, in the case where the toner image is present in the carrying space S, it is also possible to control the conveying fan30to operate in a state where the adsorbability is reduced to an adsorbability necessary for adsorbing the sheet. In this case, by application of a voltage of 12 V to, for example, the conveying fan30having a rating of 24 V, by the control unit62, the adsorbability of the conveying fan30may be reduced.

In the embodiment, illustrated is the mode in which the timing for changing the drive of the conveying fan30is determined based on the time after the control unit62receives the reference signal (TOP signal) from the detector9. Alternatively, a detector for detecting a position of a toner image transferred onto the intermediate transfer belt8may be provided, and the control unit62may control the drive of the conveying fan30in response to a signal from the detector.

Depending on types of sheets (materials), for example, in a case where a thick sheet which requires a large amount of heat is used, there is adopted a method of increasing the amount of heat to be given per unit time by reducing a fixing speed of the fixing portion when the image is fixed onto the sheet. In other words, the imaging operation (primary transfer) onto the intermediate transfer belt is performed at a normal speed. After the four-color toner image is primarily transferred onto the intermediate transfer belt8, the circumferential speed of the intermediate transfer belt8is reduced immediately before the toner image is transferred onto the sheet. In this case, the circumferential speed of the intermediate transfer belt8is low, so the toner image is more liable to be scattered in the carrying space. Also in this case (condition), when the toner image is present in the carrying space S, the adsorbability of the conveying fan30is reduced, thereby preventing adhesion of stain to the transport path.

This application claims the benefit of Japanese Patent Application No. 2006-212613, filed Aug. 3, 2006, which is hereby incorporated by reference herein in its entirety.