Fixing device, image forming apparatus, and non-transitory computer readable medium

A fixing device includes a fixing member, a first roller, a second roller, a detector, and an adjustment unit. The fixing member fixes, on a recording medium, a toner image formed on the recording medium. The first roller performs cleaning of toner sticking to a surface of the fixing member. The second roller collects the toner sticking to a surface of the first roller. The detector detects a temperature of the second roller. The adjustment unit adjusts the temperature of the second roller on a basis of a result of detection performed by the detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-034532 filed Feb. 25, 2016.

BACKGROUND

Technical Field

The present invention relates to a fixing device, an image forming apparatus, and a non-transitory computer readable medium.

SUMMARY

According to an aspect of the invention, there is provided a fixing device including a fixing member, a first roller, a second roller, a detector, and an adjustment unit. The fixing member fixes, on a recording medium, a toner image formed on the recording medium. The first roller performs cleaning of toner sticking to a surface of the fixing member. The second roller collects the toner sticking to a surface of the first roller. The detector detects a temperature of the second roller. The adjustment unit adjusts the temperature of the second roller on a basis of a result of detection performed by the detector.

DETAILED DESCRIPTION

Hereinafter, examples of exemplary embodiments will be described in detail with reference to the drawings.

First Exemplary Embodiment

The configuration of an imaging device10according to an exemplary embodiment will be described with reference toFIG. 1. In the following description, Y denotes yellow, M denotes magenta, C denotes cyan, and K denotes black. If components and toner images (images) need to be discriminated based on color, one of references (C, M, Y, and K) for the colors is suffixed to the corresponding reference numeral. If the components and the toner images are collectively referred to without being discriminated based on color, the references for the colors to be suffixed to the respective reference numerals are omitted.

The imaging device10includes a sheet container20that contains sheets P that are recording media, a toner-image forming section22, a transportation section24, a discharging section28, a refilling mechanism30, a controller32, and a fixing device70.

The toner-image forming section22includes imaging units40and a first transfer unit50. Each imaging unit40includes an image holder42, a charger43, an exposure device44, a developing device45, and a remover49. Each of the imaging units40C,40M,40Y, and40K forms a toner image in a corresponding one of the colors C, M, Y, and K on the circumferential surface of the corresponding image holder42.

The charger43charges the circumferential surface of the image holder42. The exposure device44radiates exposure light to the circumferential surface of the image holder42charged by the charger43and thereby forms a latent image. The developing device45develops the latent image formed on the image holder42by using a developer (for example, negative charged toner) and makes the latent image visible as a toner image. The developing device45includes a toner supplier46that supplies the toner to the circumferential surface of the image holder42and two transportation members47and48that transport the toner and the developer containing carriers to the toner supplier46. The developing devices45for the respective colors are connected to toner cartridges34through refilling pipes (not illustrated), respectively. The four toner cartridges34(34C,34M,34Y, and34K) are filled with the toner to be refilled in the respective developing devices45and are arranged for the respective colors along the width of the imaging device10in such a manner as to be detachable from the imaging device10. The toner cartridges34for the respective colors are each shaped into a column extending along the depth of the imaging device10. Further, the remover49removes, from the circumferential surface of the image holder42, the toner, an additive, and the like that remain on the circumferential surface of the image holder42without undergoing the first transfer after the toner image formed on the circumferential surface of the image holder42undergoes the first transfer onto an intermediate transfer belt52that is an example of an image holder.

The first transfer unit50includes the endless intermediate transfer belt52and a driving roller54around which the intermediate transfer belt52is wound. The driving roller54is rotated by a motor (not illustrated) and causes the intermediate transfer belt52to move in an arrow A direction. The first transfer unit50also includes a tension roller56around which the intermediate transfer belt52is wound and an auxiliary roller58. The tension roller56tensions the intermediate transfer belt52, and the auxiliary roller58is rotated in accompany with the intermediate transfer belt52. The first transfer unit50also includes first transfer rollers51that are each disposed to face the corresponding color image holder42across the intermediate transfer belt52.

With the configuration described above, the C, M, Y, and K toner images serially formed on the respective image holders42of the imaging units40for the respective colors are transferred in an overlapping manner onto the intermediate transfer belt52by the first transfer rollers51for the respective colors.

A cleaning device59removes, from the circumferential surface of the intermediate transfer belt52, the toner, the additive, and the like that are not fixed in the second transfer and thus remain on the circumferential surface of the intermediate transfer belt52after the toner image on the circumferential surface of the intermediate transfer belt52undergoes the second transfer onto one of the sheets P.

In addition, a second transfer roller60is disposed to face the auxiliary roller58across the intermediate transfer belt52. The second transfer roller60transfers the toner image transferred on the intermediate transfer belt52onto the transported sheet P. The second transfer roller60is grounded, and the auxiliary roller58forms a counter electrode for the second transfer roller60. When a second transfer voltage is applied, the toner image is transferred onto the sheet P by using the auxiliary roller58.

The transportation section24includes a feed roller63that feeds each sheet P stacked in the sheet container20to a transport path62, separation rollers64that separate, one by one, the sheet P fed by the feed roller63, and registration rollers65that times the transportation of the sheet P. The feed roller63, the separation rollers64, and the registration rollers65are arranged on the transport path62in this order in a transportation direction of the sheet P.

The rotating registration rollers65feed each sheet P supplied from the sheet container20to a contact portion between the intermediate transfer belt52and the second transfer roller60(a second transfer position) at scheduled timing.

Further, the sheet P is transported to the fixing device70, and the fixing device70fixes the toner image on one of the surfaces (image-formed surface) of the sheet P.

The fixing device70of the exemplary embodiment will be described with reference toFIG. 2. The fixing device70of the exemplary embodiment includes a fixing roller72, a pressure roller74, a cleaning roller76, and a collection roller78.

The fixing roller72, the pressure roller74, the cleaning roller76, and the collection roller78are columnar rollers that extend from the front of the imaging device10toward the rear (the depth end) and are rotated about the rotary axis that extends in a direction orthogonal to the transportation direction of the sheet P.

The fixing roller72is rotated by a motor110(seeFIG. 3) in an arrow B direction. The fixing roller72has a heater84incorporated in the fixing roller72and heats the sheet P transported on the transport path62. The fixing roller72of the exemplary embodiment is an example of a fixing member.

The pressure roller74is disposed to face the fixing roller72across the transport path62and is rotated in an arrow C direction by a motor112(seeFIG. 3). The pressure roller74presses the sheet P transported on the transport path62.

The cleaning roller76is in contact with the fixing roller72and rotated in an arrow D direction by a motor114(seeFIG. 3). The cleaning roller76removes (performs cleaning of) the toner, a foreign substance, and the like sticking to the surface of the fixing roller72. The size (a diameter φ) of the cleaning roller76is not particularly limited but is set smaller than that of the fixing roller72in the exemplary embodiment. The material of the cleaning roller76is not particularly limited as long as the material does not damage the surface of the fixing roller72in the cleaning. The cleaning roller76of the exemplary embodiment is an example of a first roller.

The collection roller78is in contact with the cleaning roller76and rotated in an arrow E direction by a motor116(seeFIG. 3). The collection roller78collects and holds the toner, a foreign substance, and the like sticking to the surface of the cleaning roller76. The size (a diameter φ) of the collection roller78is not particularly limited, but in the exemplary embodiment, the size is set smaller than that of the fixing roller72and also is set equal to or slightly smaller than that of the cleaning roller76. The material of the collection roller78is not particularly limited as long as the collection roller78collects and holds the toner, the foreign substance, and the like. The collection roller78of the exemplary embodiment is an example of a second roller.

The fixing device70also includes temperature sensors80and82. The temperature sensor80detects the temperature of the surface of the fixing roller72. In the exemplary embodiment, as illustrated as a detection position80A inFIG. 2that is viewed in the transportation direction, the temperature sensor80detects the temperature of a portion outside an area (the widest paper-passing area) of the surface of the fixing roller72through which the sheet P passes. More specifically, the detection position80A is located upstream of a nip (contact portion) with the cleaning roller76in a direction of the rotation of the fixing roller72and in a rear portion of the imaging device10. The temperature sensor82detects the temperature of the surface of the collection roller78. In the exemplary embodiment, as illustrated as a detection position82A inFIG. 2that is viewed in the transportation direction, the temperature sensor82detects the temperature of a portion outside an area (the widest paper-passing area) of the surface of the collection roller78through which the sheet P passes. More specifically, the detection position82A is located upstream of a nip (contact portion) with the cleaning roller76in a direction of the rotation of the collection roller78and in the rear portion of the imaging device10.

The temperature sensors80and82are not particularly limited as long as the temperature sensors80and82respectively detect the temperatures of the surfaces of the fixing roller72and the collection roller78. However, the temperature sensors80and82preferably detect the temperatures in such a manner as not to be in contact with the fixing roller72and the collection roller78, respectively. For example, the temperature sensors80and82are each preferably an infrared radiation thermometer or the like.

As illustrated inFIG. 1, the transportation section24of the imaging device10of the exemplary embodiment is provided with a reverse transportation section66that transports the sheet P turned over to further form an image on each side of the sheet P. The reverse transportation section66reverses the transportation direction of the sheet P on which the fixing device70fixes the toner image. Thereafter, the reverse transportation section66transports the sheet P to the second transfer position to cause the back side of the sheet P to face the circumferential surface of the intermediate transfer belt52.

The discharging section28is disposed downstream of the fixing device70in the transportation direction of the sheet P and on the outer and upper surface of the imaging device10. The sheet P on which the fixing device70fixes the toner image is discharged by discharge rollers67to the discharging section28.

A process for forming an image by the imaging device10illustrated inFIGS. 1 and 2(image formation process) is performed in the following manner.

First, the controller32serially outputs pieces of tone data for the respective colors to the respective exposure devices44. Exposure light emitted from each exposure device44in accordance with the corresponding piece of the tone data is radiated onto the surface of the corresponding image holder42charged by the charger43. An electrostatic latent image is thereby formed on the surface of the image holder42. The electrostatic latent image formed on the image holder42is developed by the developing device45in the corresponding color and made visible as a toner image in a corresponding one of the colors of C, M, Y, and K.

Further, each color toner image formed on the image holder42is transferred onto the rotating intermediate transfer belt52in the overlapping manner by the first transfer roller51of the first transfer unit50.

Each color toner image transferred onto the intermediate transfer belt52in the overlapping manner undergoes the second transfer onto the sheet P transferred along the transport path62from the sheet container20by using the feed roller63, the separation rollers64, and the registration rollers65. The second transfer is performed by the second transfer roller60at the second transfer position.

Further, the sheet P having the toner image transferred thereon is transported to the fixing device70. The fixing device70then fixes the toner image on the sheet P. The sheet P having the toner image fixed thereon is discharged to the discharging section28by the discharge rollers67.

If images are to be formed on both surfaces of the sheet P, the discharge rollers67do not discharge, to the discharging section28at this stage, the sheet P one of the surfaces (a front surface) on which the fixing device70fixes the toner image. The discharge rollers67are reversely rotated to thereby switch the transportation direction of the sheet P. The sheet P is transported by the reverse transportation section66and is again transported to the registration rollers65with the surfaces being upside down. The sheet P undergoes transfer and fixing of a toner image on the other surface (a back surface) and is thereafter discharged to the discharging section28by the discharge rollers67.

Subsequently, the configuration of a principal part of an electrical system of the imaging device10of the exemplary embodiment will be described with reference toFIG. 3.

As illustrated inFIG. 3, the controller32according to the exemplary embodiment includes a central processing unit (CPU)100, a read only memory (ROM)102, a random access memory (RAM)104, a nonvolatile memory106such as a flash memory, and a communication network interface (I/F)108. The CPU100controls the overall operation of the imaging device10. The ROM102in advance stores various programs including a temperature control program103(described later in detail), various parameters, and the like. The RAM104is used as a work area or the like for the CPU100to run the various programs. The communication network I/F108exchanges communication data with an external device.

The CPU100, the ROM102, the RAM104, the memory106, the communication network I/F108, the motors110,112,114, and116, the temperature sensors80and82, and the heater84are connected to each other through a bus117including an address bus, a data bus, a control bus, and other buses.

With the configuration illustrated inFIG. 3of the imaging device10of the exemplary embodiment, the CPU100accesses to the ROM102, the RAM104, and the memory106and exchanges communication data with the external device through the communication network I/F108.

The CPU100controls the motors110,112,114, and116to thereby control the rotation of the fixing roller72, the pressure roller74, the cleaning roller76, and the collection roller78, respectively. The CPU100also controls the heater84to thereby heat the fixing roller72. The CPU100(controller32) and the heater84of the exemplary embodiment are each an example of an adjustment unit of the exemplary embodiment.

Further, the CPU100acquires a detection signal output from the temperature sensor80to thereby detect a temperature t(H/R) of the fixing roller72. The CPU100acquires a detection signal output from the temperature sensor82to thereby detect a temperature t(W/R) of the collection roller78.

Subsequently, actions of the imaging device10of the exemplary embodiment will be described. In the imaging device10of the exemplary embodiment, the controller32controls the temperature of the collection roller78of the fixing device70and the temperature of the fixing device70so as to restrain the toner collected by the collection roller78from causing a defect in the image quality of an image (toner image) fixed on the sheet P. Hereinafter, an operation for controlling the temperature of the fixing device70performed by the controller32will be described.

FIG. 4is a flowchart illustrating the flow of a process for controlling the temperature of the fixing device70performed by the controller32according to the exemplary embodiment. When the CPU100runs the temperature control program103installed in advance in the ROM102, the temperature control process illustrated inFIG. 4is thereby performed. In addition, when the imaging device10receives an instruction for forming an image on the sheet P, the temperature control process illustrated inFIG. 4is executed. Note that the instruction causes the entire imaging device10(including the toner-image forming section22, the first transfer unit50, and other components) to start operations, but the image formation is not started at this stage.

In step S100, the controller32starts the motors110,112,114, and116of the fixing device70. The fixing roller72, the pressure roller74, the cleaning roller76, and the collection roller78thereby start rotation.

In subsequent step S102, the controller32sets, to0(No=0), a variable No for counting the number of times the process is performed (described later in detail).

In subsequent step S104, the controller32acquires the temperature t(H/R) of the fixing roller72from the temperature sensor80and judges whether the temperature t(H/R) exceeds a lower limit Tlow of the temperature of the fixing roller72(t(H/R)>Tlow). In the imaging device10of the exemplary embodiment, the lower limit Tlow and an upper limit Thi of the temperature t(H/R) of the fixing roller72are in advance acquired based on experiments. The lower limit Tlow and the upper limit Thi are needed to fix a toner image on the sheet P and are determined based on, for example, the types of the toner and the sheet P, and the configuration of the toner-image forming section22. The controller32controls the temperature t(H/R) of the fixing roller72to be higher than the lower limit Tlow and lower than the upper limit Thi (Tlow<t(H/R)<Thi) when the toner image is formed.

If the temperature t(H/R) does not exceed the lower limit Tlow, the judgment has a negative result, and the process moves to step S106. In step S106, the controller32turns on the heater84to heat the fixing roller72, and the process thereafter returns to step S104. If the heater84has been turned on, the controller32maintains the on state in step S106. If the temperature t(H/R) exceeds the lower limit Tlow, the judgment has an affirmative result, and the process moves to step S108.

In step S108, the controller32acquires the temperature t(H/R) of the fixing roller72from the temperature sensor80, and judges whether the temperature t(H/R) is lower than the upper limit Thi (t(H/R)<Thi). If the temperature t(H/R) is not lower than the upper limit Thi, the judgment has a negative result, and the process moves to step S110. In step S110, the controller32turns off the heater84and stops heating the fixing roller72, and the process thereafter returns to step S104. If the heater84has been turned off, the controller32maintains the off state in step S110. If the temperature t(H/R) is lower than the upper limit Thi, the judgment has an affirmative result, and the process moves to step S112.

In step S112, the controller32acquires the temperature t(W/R) of the collection roller78from the temperature sensor82and judges whether the temperature t(W/R) is equal to or higher than a softening temperature Ts of the toner (t(W/R)≧Ts).

After the imaging device10forms the image, the collection roller78collects and holds the toner. If the temperature of the collection roller78drops in this state, the toner thus held adheres to the surface of the collection roller78. As illustrated inFIG. 5, the toner adheres to a portion between the collection roller78and the cleaning roller76(nip) in a shape (a shape79) corresponding to the surface of the cleaning roller76.

Consider a case where the collection roller78is rotated with the toner being adhered in this way and with the temperature t(W/R) of the collection roller78being lower than the softening temperature Ts of the toner. In this case, as illustrated in the case of t(W/R)<Ts inFIG. 5, the collection roller78is rotated with the toner still being in the adhered state in the shape79. The shape79leads to an even surface of the collection roller78. Accordingly, when the collection roller78is brought into contact with the cleaning roller76, the shape79prevents some area of the surface of the collection roller78from being brought into contact with the surface of the cleaning roller76. In the area of the surface of the cleaning roller76with which the collection roller78is not brought into contact, the toner is not collected and remains. If the cleaning roller76attempts to perform cleaning on the fixing roller72in this state, the toner remaining on the cleaning roller76contrarily sticks to the fixing roller72. The toner sticking to the fixing roller72sticks to the sheet P, and this causes an image defect in some cases. In particular, when borderless printing is performed, cleaning of the fixing roller72matters. A larger amount of toner is collected and held in portions outside the widest paper-passing area of the collection roller78(seeFIG. 2) than in the widest paper-passing area. Accordingly, as described above, this increases the probability that the adhesion causes an image defect.

In contrast, consider a case where the collection roller78is rotated with the temperature t(W/R) of the collection roller78being equal to or higher than the softening temperature Ts of the toner. As illustrated in the case of t(W/R)≧Ts inFIG. 5, the collection roller78is rotated with the softening toner being spread over the entire surface of the collection roller78. The collection roller78does not have the uneven surface that otherwise would be caused by the shape79, is thus brought into contact with the entire surface of the cleaning roller76, and is restrained from having the non-contact area. This causes the toner to be appropriately collected from the surface of the cleaning roller76, and the toner is thus restrained from remaining on the surface of the cleaning roller76. Accordingly, the image defect as described above is restrained.

In the imaging device10of the exemplary embodiment, after the temperature of the collection roller78becomes equal to or higher than the softening temperature Ts of the toner, an image (toner image) is formed or at least fixed by the fixing device70.

If the temperature t(W/R) is lower than the softening temperature Ts in step S112, the judgment has a negative result, and the process moves to step S114. In step S114, the controller32turns on the heater84and heats the fixing roller72. In subsequent step S116, the controller32idles the fixing roller72for a predetermined period of time, and the process thereafter returns to step S112. If the fixing roller72has been rotated, the fixing roller72is maintained in a standby state in step S116until the predetermined period of time elapses. Note that the predetermined period of time is a time taken for the temperature t(W/R) of the collection roller78to increase after transmission of the heat of the fixing roller72to the collection roller78and is in advance obtained based on experiments or the like.

If the temperature t(W/R) of the collection roller78is equal to or higher than the softening temperature Ts of the toner, the judgment has an affirmative result, and the process moves to step S118. In step S118, the controller32judges whether the temperature t(W/R) is lower than a melt temperature Tm of the toner. If the temperature of the collection roller78is equal to or higher than the melt temperature Tm of the toner, the storage modulus of the collection roller78for the toner is lowered. The toner is not held on the collection roller78and may flow backwards to the cleaning roller76. That is, if the temperature t(W/R) of the collection roller78is equal to or higher than the melt temperature Tm of the toner, the toner collected by the collection roller78may be melted and flow backwards to the cleaning roller76. The backwards flowing toner sticks to the fixing roller72, and the toner sticking to the fixing roller72sticks to the sheet P. This may cause an image defect.

In the imaging device10of the exemplary embodiment, a state where the storage modulus of the collection roller78takes on a value sufficient to hold the toner is taken into consideration. If the temperature t(W/R) of the collection roller78is lower than the melt temperature Tm of the toner, an image (toner image) is formed or at least fixed by the fixing device70.

If the temperature t(W/R) is not lower than the melt temperature Tm in step S118, the judgment has a negative result, and the process moves to step S120. In step S120, the controller32increments the variable No by one (No=No+1). In subsequent step S122, the controller32judges whether the variable No is larger than a predetermined constant K1so as to control the iteration of the steps (loop). If the variable No is equal to or smaller than the constant K1, the judgment has a negative result, and the process moves to step S124.

In step S124, the controller32turns off the heater84and stops heating the fixing roller72. If the heater84has been turned off, the controller32maintains the off state in step S124. In subsequent step S126, the controller32idles the fixing roller72for a predetermined period of time, and the process thereafter returns to step S104. If the fixing roller72has been rotated, the fixing roller72is maintained in a standby state in step S126until the predetermined period of time elapses. Note that the predetermined period of time is a time taken to lower the temperature of the collection roller78and is in advance obtained based on experiments or the like. The period of time for the idling in step S126may be equal to or different from that for the idling of the fixing roller72in step S116.

If the variable No is larger than the constant K1in step S122, the judgment has an affirmative result, and the process moves to step S128. In step S128, the controller32decreases a process speed for forming the image (toner image).

In steps S104to S118described above, the heater84is turned on or off to adjust the temperature t(H/R) of the fixing roller72and the temperature t(W/R) of the collection roller78. However, in some cases, the adjustment is not performed sufficiently, and the iteration of the steps (loop) is continuously performed. In such cases (where the variable No exceeds the predetermined constant K1), the imaging device10of the exemplary embodiment performs the temperature adjustment by decreasing the process speed for forming the image (toner image), not by using the heater84. In the imaging device10of the exemplary embodiment, the temperature at which the fixing device70fixes the toner is lowered by decreasing the process speed, and the iteration of the steps described above (loop) is avoided by lowering the temperature of the fixing roller72.

In subsequent step S130, the controller32changes the lower limit Tlow of the temperature of the fixing roller72to a lower limit for a case where the process speed is decreased. Specifically, a temperature Tp by which the process speed is to be decreased in accordance with a decreased process speed is in advance obtained based on experiments or the like. The controller32subtracts the temperature Tp from the current lower limit Tlow (Tlow=Tlow−Tp). The controller32also changes the upper limit Thi of the temperature of the fixing roller72to an upper limit for a case where the process speed is decreased. Specifically, the controller32subtracts the temperature Tp from the current upper limit Thi (Thi=Thi−Tp). After step S130is complete, the process returns to step S104.

If the temperature t(W/R) is lower than the melt temperature Tm in step S118, the judgment has an affirmative result, and the process moves to step S132. In step S132, the controller32starts image formation. In the imaging device10, an image is formed on the sheet P in the image formation process described above, and the sheet P is discharged to the outside (the discharging section28) of the imaging device10.

In subsequent step S134, the controller32judges whether to terminate the image formation. If the formation of all of images for which image forming instructions are received to start the temperature control process is complete, or if an instruction for terminating the image formation (forced termination) is received from a user or the like, the controller32judges that the image formation is to be terminated. If the image formation is not to be terminated, the judgment has a negative result, the process returns to step S102, and steps S104to S130are repeated.

If the image formation is to be terminated, the judgment has an affirmative result, and the process moves to step S136. In step S136, the controller32turns off the heater84and stops heating the fixing roller72. If the heater84has been turned off, the controller32maintains the off state in step S136.

In subsequent step S138, the controller32stops the motors110,112,114, and116of the fixing device70. The rotation of the fixing roller72, the pressure roller74, the cleaning roller76, and the collection roller78is thereby stopped.

In subsequent step S140, the controller32judges whether the temperature t(W/R) of the collection roller78is lower than a temperature that is 5 degrees higher than the softening temperature Ts of the toner (t(W/R)<Ts+5). As long as the temperature t(W/R) is equal to or higher than the temperature of the softening temperature Ts plus 5 degrees, the judgement has a negative result. If the temperature t(W/R) is lower than the temperature of the softening temperature Ts plus 5 degrees, the judgment has an affirmative result, and the process moves to step S142.

In step S142, the controller32starts the motors110,112,114, and116of the fixing device70. The fixing roller72, the pressure roller74, the cleaning roller76, and the collection roller78are thereby rotated again.

As described above, when the imaging device10forms an image, the collection roller78collects and holds the toner. If the collection roller78stops rotating at a high temperature with the toner being held thereon, the toner adheres to the portion (nip) between the collection roller78and the cleaning roller76in the shape (shape79) corresponding to the surface of the cleaning roller76as described above. The adhesion of the toner in the shape79, in some cases, prevents the cleaning roller76and the collection roller78from rotating when image formation is started next time.

In the imaging device10of the exemplary embodiment, when the image formation is to be terminated, the collection roller78and the cleaning roller76are rotated after the temperature of the collection roller78becomes around the softening temperature and until the temperature is lowered to a predetermined temperature. That is, the collection roller78and the cleaning roller76are rotated while the toner is kept transformable and not easy to flow backwards. As illustrated in the case of t(W/R)≧Ts inFIG. 5, unevenness of the shape79of the toner held on the surface of the collection roller78is reduced, and the toner spreads over the entire surface. Accordingly, the rotation of the cleaning roller76and the collection roller78is restrained from being prevented when image formation is started next time.

In the exemplary embodiment, in steps S138to S142, the motors of the fixing device70are turned off, and if the temperature t(W/R) of the collection roller78becomes lower than the temperature of the softening temperature Ts of the toner plus 5 degrees, the motors are turned on again. However, steps S138to S142may be omitted. The motors of the fixing device70may be kept on. However, for example, if the imaging device10is turned off while the control is being performed, the rotation of the collection roller78may be stopped with the temperature of the collection roller78still being high. Accordingly, it is preferable that the temperature of the collection roller78be lowered in a short time. It is thus preferable that the motors of the fixing device70be turned off as in the temperature control process according to the exemplary embodiment because the temperature of the collection roller78is lowered in a shorter time.

In subsequent step S144, the controller32judges whether the temperature t(W/R) of the collection roller78is higher than a temperature that is 5 degrees lower than the softening temperature Ts of the toner (t(W/R)>Ts−5). As long as the temperature t(W/R) is higher than the temperature of the softening temperature Ts minus 5 degrees, the judgment has an affirmative result. If the temperature t(W/R) is equal to or lower than the temperature of the softening temperature Ts minus 5 degrees, the judgment has a negative result, and the process moves to step S146.

In subsequent step S146, the controller32turns off the motors of the fixing device70and terminates the temperature control process. In the temperature control process according to the exemplary embodiment as described above, if the temperature t(W/R) of the collection roller78is within a range of plus or minus 5 degrees of the softening temperature Ts of the toner after the image formation is complete, the motors of the fixing device70(at least the motor116of the collection roller78) are kept on. However, the range of plus or minus 5 degrees of the softening temperature Ts of the toner is an example. The temperature range, a time, or the like for keeping the motors of the fixing device70on after the end of the image processing is not particularly limited and may be experimentally obtained in advance.

FIG. 6illustrates a graph of evaluation of image defects of images (the sheets P having the images formed thereon) formed by the imaging device10of the exemplary embodiment.FIG. 7illustrates a graph, in a comparative example, of evaluation of image defects in images formed by an image forming apparatus without performing the temperature control process according to the exemplary embodiment.FIG. 8illustrates a graph of the worst-grade values (the highest values) in the exemplary embodiment and the comparative example, respectively. In each graph illustrated inFIGS. 6 and 7, the horizontal axis represents the number of sheets P having images formed thereon, and the vertical axis represents the level of a smear (defect). Note that whether each sheet P having an image thereon has a smear is checked by visual observation and is assigned a level. The higher the level is, the dirtier the smear is. Generally, if the smear level is equal to or higher than Level3assigned to occurrence of clusters of spot smears in a diameter φ of 0.2 mm, the image defect causes a problem. The smear level is thus preferably lower than Level3, and Level3is set as a target value.

As illustrated inFIGS. 6 to 8, it is understood that if the temperature control process according to the exemplary embodiment is not executed, as the number of sheets P having images formed thereon is increased, the level of image defects in the images becomes high, that is, smears are increased. In contrast, it is understood that if the imaging device10of the exemplary embodiment executes the temperature control process, there is only a slight change in the level of image defects in images despite an increase in the number of sheets P having the images formed thereon, that is, smears are not increased, and the level is kept lower than Level3that is the target value.

Second Exemplary Embodiment

A second exemplary embodiment will be described. The same components as those of the imaging device10according to the first exemplary embodiment are denoted by the same reference numerals, and detailed description is omitted.

As illustrated inFIG. 9, an imaging device10of the second exemplary embodiment is different from the imaging device10of the first exemplary embodiment in that the fixing device70includes a temperature sensor86that detects the temperature of the cleaning roller76and a cooling fan88that cools the collection roller78.

The temperature sensor86detects the temperature of the surface of the cleaning roller76. In the exemplary embodiment, the temperature sensor86detects the temperature of a portion outside an area (widest paper-passing area) of the surface of the cleaning roller76through which the sheet P passes. More specifically, the detection position is located upstream of the nip (contact portion) with the collection roller78in a direction of the rotation of the cleaning roller76and in the rear portion of the imaging device10. The temperature sensor86is not particularly limited, either, as long as the temperature sensor86detects the temperature of the surface of the cleaning roller76. However, like the temperature sensors80and82, the temperature sensor86preferably detects a temperature in such a manner as not to be in contact with the cleaning roller76. For example, the temperature sensor86is preferably an infrared radiation thermometer or the like.

The cooling fan88is disposed to face the surface of the collection roller78and rotated by a motor118(seeFIG. 10). The cooling fan88is rotated and thereby sends air to cool the collection roller78. Note that the cooling fan88and the motor118of the exemplary embodiment are each an example of a cooling device for cooling the collection roller78. The cooling device is not limited to that in the exemplary embodiment as long as the cooling device cools the collection roller78.

As in the configuration of a principal part of an electrical system of the imaging device10of the exemplary embodiment illustrated inFIG. 10, the CPU100, the ROM102, the RAM104, the memory106, the communication network I/F108, the motors110,112,114,116, and118, the temperature sensors80,82, and86, and the heater84of the imaging device10of the exemplary embodiment are connected to each other through the bus117including the address bus, the data bus, and the control bus, and other buses.

The CPU100controls the rotation of the cooling fan88by controlling the motor118and thereby controls cooling of the collection roller78. Further, the CPU100acquires a detection signal output from the temperature sensor86and thereby detects a temperature t(C/R) of the cleaning roller76.

Subsequently, the actions of the imaging device10of the exemplary embodiment will be described.FIG. 11is a flowchart illustrating the flow of a temperature control process for the fixing device70executed by the controller32according to the exemplary embodiment. Since the temperature control process executed by the imaging device10of the exemplary embodiment includes the same steps as those in the temperature control process (seeFIG. 4) executed in the imaging device10of the first exemplary embodiment, only different steps will be described.

As illustrated inFIG. 11, the temperature control process according to the exemplary embodiment does not have step S102(seeFIG. 4) in the temperature control process in the first exemplary embodiment. Accordingly, after the controller32starts the motors of the fixing device70in step S100, the process moves to step S104.

The temperature control process according to the exemplary embodiment does not have steps S120, S122, S128, and S130(seeFIG. 4) in the temperature control process in the first exemplary embodiment. Accordingly, if the judgment has a negative result in step S118, the process moves to step S124.

The temperature control process according to the exemplary embodiment has steps S127A, S127B, and S127C before step S132. Accordingly, if the judgment has an affirmative result in step S118, the process moves to step S127A.

In step S127A, the controller32detects the temperature t(C/R) of the cleaning roller76and the temperature t(W/R) of the collection roller78and judges whether a difference between the temperature t(C/R) of the cleaning roller76and the temperature t(W/R) of the collection roller78is larger than a constant K2predetermined to control the backflow of the toner.

The toner is easier to melt at a high surface temperature than at a low surface temperature and thus sticks to an area that has a lower surface temperature than the other area in the nip between the cleaning roller76and the collection roller78. When heating or the like of the fixing roller72increases the temperature t(W/R) of the collection roller78and when a difference from the temperature t(C/R) of the cleaning roller76becomes lower than the constant K2, the storage modulus of the collection roller78for the toner may be decreased, and the toner may flow backwards from the collection roller78to the cleaning roller76. The backwards flowing toner sticks to the fixing roller72, and the toner sticking to the fixing roller72sticks to the sheet P. This may cause an image defect.

In the imaging device10of the exemplary embodiment, the constant K2for the difference between the temperature t(W/R) of the collection roller78and the temperature of the temperature t(C/R) of the cleaning roller76is in advance experimentally obtained to maintain the storage modulus sufficient to hold the toner. The temperature of the collection roller78is controlled such that the difference between the temperature t(C/R) of the cleaning roller76and the temperature t(W/R) of the collection roller78is larger than the constant K2.

If the judgment has a negative result in step S127A, the process moves to step S127B. In step S127B, the controller turns on the cooling fan88. Specifically, the controller32turns on the motor118to rotate the cooling fan88. If the cooling fan88has been turned on, the controller32maintains the on state in step S127B. The temperature of the collection roller78is thereby lowered.

In subsequent step S127C, the controller32idles the fixing roller72for a predetermined period of time, and the process thereafter returns to step S104. If the fixing roller72has been rotated, the fixing roller72is maintained in a standby state in step S127C until the predetermined period of time elapses. Note that the predetermined period of time is a time taken to lower the temperature of the collection roller78and is in advance obtained based on experiments or the like. The period of time for the idling in step S127C may be equal to or different from any of the periods of time for the idling of the fixing roller72in steps S116and S126.

If the judgment has an affirmative result in step S127A, the process moves to step S132. If the cooling fan88has been turned on before the controller32starts the image formation in step S132, the cooling fan88may be turned off.

Since the temperature control process according to the exemplary embodiment does not have steps S138to S142(seeFIG. 4) in the temperature control process in the first exemplary embodiment, the process moves to step S144after step S136.

In the temperature control process according to the exemplary embodiment, step S145A or step S145B is performed after step S144.

If the judgment has an affirmative result in step S144in the temperature control process according to the exemplary embodiment, the process moves to step S145A. In step S145A, the controller32turns on the cooling fan88as in step S127B, the process thereafter returns to step S144. The controller32thereby lowers the temperature of the collection roller78. As described above in the exemplary embodiment, the cooling fan88is turned on to lower the temperature of the collection roller78, and the rotation of the collection roller78is restrained from being stopped with the temperature of the collection roller78being still high. The imaging device10of the exemplary embodiment thereby restrains the rotation of the cleaning roller76and the collection roller78from being prevented when image formation is started next time.

If the judgment has an affirmative result in step S144, the process moves to step S145B. In step S145B, the controller32turns off the cooling fan88, and the process thereafter moves to step S146. If the cooling fan88has been turned off, the controller32maintains the off state.

FIG. 12illustrates a graph of the worst values (the highest values) in level of image defects in the exemplary embodiment and the comparative example, respectively. The image defects have respectively occurred in an image (on the sheet P having the image formed thereon) formed by the imaging device10of the exemplary embodiment and in an image formed by an image forming apparatus in the comparative example without performing the temperature control process according to the exemplary embodiment. As in the first exemplary embodiment inFIGS. 6 to 8, the higher the level is, the dirtier a smear is. The smear level is thus preferably lower than Level3, and Level3is set as the target value.

As illustrated inFIG. 12, it is understood that if the temperature control process according to the exemplary embodiment is not executed, the level of the image defect in the image is high and exceeds the target value. In contrast, it is understood that if the imaging device10of the exemplary embodiment executes the temperature control process, the level of the image defect in the image is lower than Level3that is the target value.

It goes without saying that each exemplary embodiment is an example and may be changed without departing from the spirit of the exemplary embodiment. For example, the exemplary embodiments described above may be combined.