IMAGE FORMING APPARATUS

There is provided an image forming apparatus that heat-fixes a toner image by passing a recording sheet through a fixing nip, and the image forming apparatus includes: a heater that raises a temperature of a fixing member forming the fixing nip; and a determiner that determines, by using at least one of a heat storage state of a peripheral member that affects a temperature rise rate of the fixing member and a power supply voltage of the image forming apparatus, a print wait period until the recording sheet is caused to enter the fixing nip after a target temperature for temperature control of the fixing member is switched to a heat fixing temperature such that the recording sheet enters the fixing nip before the temperature of the fixing member is raised to exceed an allowable temperature range.

The entire disclosure of Japanese patent Application No. 2020-067364, filed on Apr. 3, 2020, is incorporated herein by reference in its entirety.

BACKGROUND

Technological Field

The present disclosure relates to an image forming apparatus, and more particularly to a technique for preventing overheating of a fixing member that heat-fixes a toner image on a recording sheet.

Description of the Related Art

In the electrophotographic image forming apparatus, when a pressure member is used to press-contact a recording sheet carrying a toner image to a high-temperature fixing member, the toner image is melted and pressed onto the recording sheet. In recent years, innovations have been added in which in order to speed up the fixing process, a large amount of electric power is applied to shorten the time required for raising the temperature of the fixing member, and in order to save this electric power, the heat capacity of the fixing member is reduced. In this way, a first copy out time (FCOT) can be shortened, so that a printed matter can be provided quickly without making the user of the image forming apparatus wait.

However, when the heat capacity of the fixing member is reduced, the temperature of the fixing member is easily fluctuated. As a result, when the temperature of the fixing member deviates from an appropriate temperature range and becomes low, toner particles cannot be sufficiently softened, and fixing failure may occur.

The pressure member has a larger heat capacity than the fixing member. For example, in a case where the pressure member is a pressure roller having an elastic layer provided on the outer peripheral surface of a core metal, and the fixing member is a metal fixing belt, the heat capacity of the pressure roller is considerably larger than that of the fixing belt.

Therefore, even when the temperature of the fixing member is within the appropriate temperature range, in a case where the temperature of the pressure member is not sufficiently raised, and a temperature difference between the pressure member and the fixing member is large, the amount of heat absorbed from the fixing member by the pressure member increases. Thus, when the recording sheet is passed through a fixing nip, the heat absorption by the recording sheet and the heat absorption by the pressurizing member combine so that the temperature of the fixing member is significantly lowered beyond the lower limit of the appropriate temperature range, and fixing failure may occur.

In response to such a problem, for example, a technique is proposed which monitors the temperature of a fixing member and passes the recording sheet through the fixing nip after the temperature of the fixing member rises above a predetermined temperature from the lower limit of the appropriate temperature range (see, for example, JP 10-333485 A).

In this way, as compared with a case where the recording sheet is passed through the fixing nip at the timing when the temperature of the fixing member reaches the lower limit of the appropriate temperature range, the temperature of the pressure member can be raised in the time until starting passing the recording sheet. Thus, it is possible to suppress the temperature drop of the fixing member due to the endothermic effect of the pressure member.

The temperature of the fixing member is higher than the lower limit of the appropriate temperature range. Thus, the temperature of the fixing member can be maintained within the appropriate temperature range even when the temperature of the fixing member is lowered by passing the recording sheet. Therefore, it is possible to prevent the occurrence of fixing failure due to the temperature drop of the fixing member.

When the temperature of the fixing member is easily fluctuated by reducing the heat capacity of the fixing member, an overshoot in which the temperature of the fixing member deviates from the appropriate temperature range and becomes high easily occurs. When the overshoot occurs, the toner particles are thermally melted and, spot-off occurs, which may deteriorate an image quality. Further, when the overshoot occurs repeatedly, the fixing member and the pressure member may deteriorate acceleratively over time or be damaged.

In response to such a problem, the above-described related art allows the temperature of the fixing member to be higher than the upper limit of the appropriate temperature range. Thus, for example, in FIG. 2 of JP 10-333485 A, the fixing member is warmed up until the temperature of the fixing member reaches a temperature TFS higher than an upper limit TFH of the appropriate temperature range, so that the overshoot cannot be avoided.

SUMMARY

The present disclosure has been made in view of the above-described problems, and an object thereof is to provide an image forming apparatus which is capable of preventing an occurrence of overshoot in a case where a temperature of a fixing member having a small heat capacity is raised at a high speed.

To achieve the abovementioned object, according to an aspect of the present invention, there is provided an image forming apparatus that heat-fixes a toner image by passing a recording sheet through a fixing nip, and the image forming apparatus reflecting one aspect of the present invention comprises: a heater that raises a temperature of a fixing member forming the fixing nip; and a determiner that determines, by using at least one of a heat storage state of a peripheral member that affects a temperature rise rate of the fixing member and a power supply voltage of the image forming apparatus, a print wait period until the recording sheet is caused to enter the fixing nip after a target temperature for temperature control of the fixing member is switched to a heat fixing temperature such that the recording sheet enters the fixing nip before the temperature of the fixing member is raised to exceed an allowable temperature range.

DETAILED DESCRIPTION OF EMBODIMENTS

[1] Configuration of Image Forming Apparatus

First, the configuration of the image forming apparatus according to this embodiment will be described.

As illustrated inFIG. 1, the image forming apparatus1is a so-called tandem type color multi-function peripheral (MFP), and includes an image reading part110, an image forming part120, and a sheet feeding part130.

The image reading part110includes an automatic document feeder (ADF)111and a scanner device112. In a case where originals are read by a sheet-through method, the automatic document feeder111feeds out and conveys the originals one by one from an original bundle placed on an original tray113and causes the scanner device112to read the originals to generate image data. Thereafter, the originals are discharged onto an output tray114. In a case where the originals are read by a platen set method, the scanner device112reads the original placed on a platen glass (not illustrated).

The image forming part120forms an image by using the image data generated by the image reading part110and the image data received from another device by the controller151. In this embodiment, imaging parts121Y,121M,121C and121K form toner images of colors of yellow (Y), magenta (M), cyan (C) and black (K), respectively.

The toner images of YMCK colors formed by the imaging parts121Y,121M,121C and121K are sequentially electrostatically transferred (primary transfer) so as to be aligned and overlapped with each other in an intermediate transfer belt122, thereby forming the color toner image. The intermediate transfer belt122is an endless belt and travels around to convey a color toner image to a secondary transfer nip125formed by a secondary transfer roller pair123.

As illustrated inFIG. 2, in this embodiment, a so-called system speed is set to 300 mm/sec. Therefore, the speed at which the intermediate transfer belt122travels around is 300 mm/sec, and the speed at which a recording sheet S is conveyed is also 300 mm/sec. Further, the distance by which the intermediate transfer belt122travels around from the start of image formation of the Y color toner image in the imaging part121Y to the primary transfer of the corresponding Y color toner image to the intermediate transfer belt122is 90 mm, and the required time therefor is 0.3 seconds (=90 mm/300 mm per second).

The traveling distance of the intermediate transfer belt122from the primary transfer of the Y color toner image to the secondary transfer of the corresponding Y color toner image is 300 mm, and the required time therefor is 1.0 second (=300 mm/300 mm per second). The total of 1.3 seconds is called an image formation time. Since the toner image of each MCK color forms a color toner image together with the Y color toner image and is 30 secondarily transferred, the primary transfer and the secondary transfer are performed during this image formation time.

In parallel with the image formation process as described above, a sheet feed roller132feeds out a sheet S from a sheet feed cassette131containing the sheet S of the type specified by a user. The sheets S contained in the second to fourth-stage sheet feed cassettes131are fed out by the sheet feed rollers132and then conveyed toward a timing roller pair124by a vertical conveying roller133.

The sheet S fed in this way is conveyed to the secondary transfer nip125in accordance with the secondary transfer timing after skew is corrected by bringing the tip of the sheet S into contact with the timing roller pair124and forming a loop.

The secondary transfer nip125is formed by pressing two rollers of the secondary transfer roller pair123to each other with the intermediate transfer belt122sandwiched therebetween. A secondary transfer bias is applied between the two rollers in the secondary transfer roller pair123, and in at the secondary transfer nip125, the color toner image carried by the intermediate transfer belt122is electrostatically transferred to the sheet S (secondary transfer).

The sheet S to which the color toner image is secondarily transferred is conveyed to the fixing device100, and the color toner image is heat-fixed. Then, the sheet is discharged to the output tray127by an output roller pair126.

As illustrated inFIG. 2, the conveying distance by which the recording sheet S is fed out from the sheet feed roller132until the tip thereof reaches the timing roller pair124is 90 mm, and the required time is 0.3 seconds (=90 mm/300 mm per second). The conveying distance by which the recording sheet S is fed out from the timing roller pair124until the tip thereof reaches the secondary transfer nip125of the secondary transfer roller pair123is also 90 mm, and the required time is 0.3 seconds (=90 mm/300 mm per second).

The conveying distance by which the recording sheet S is fed out from the secondary transfer roller pair123until the tip thereof reaches the fixing nip306(seeFIG. 3) of the fixing device100is also 90 mm, and the required time is 0.3 seconds (=90 mm/300 mm per second).

The controller151monitors and controls the operation of each part of the image forming apparatus1.

[2] Configuration of Fixing Device100

Next, the configuration of the fixing device100will be described.

The fixing device100is a so-called upper two-axis belt fixing system, and as illustrated inFIG. 3, has a configuration in which a fixing belt300, a fixing pad301, a pressure roller302, a heating roller303, and the like are provided in a housing304. The fixing belt300is hung around the fixing pad301, the heating roller303, and the belt support member305. The fixing belt300is a belt having a low heat capacity and a small diameter of φ40 mm.

The heating roller303has a hollow cylindrical shape, is made of a metal material such as aluminum (Al), steel use stainless (SUS), and iron (Fe), and has high thermal conductivity. A halogen heater307is arranged inside the heating roller303. When the halogen heater307is turned on, the heating roller303is heated, so that the portion of the fixing belt300in contact with the heating roller303is heated.

When the pressure roller302is rotationally driven in the direction of arrow C, the fixing belt300is driven by the pressure roller302to rotate and travel in the direction of arrow B. The range of the fixing belt300coming into contact with the heating roller303fluctuates in accordance with this rotation traveling, and thus the entire fixing belt300is heated evenly. A temperature sensor311detects the surface temperature of the fixing belt300.

The fixing pad301is provided with a fluorine layer on the surface on which the fixing belt300is in sliding contact. The fluorine layer reduces the frictional resistance when the fixing belt300slides on the fixing pad301. As the fluorine layer, a fluorine sheet, a sheet obtained by impregnating a woven glass fiber cloth with fluorine, or the like can be used. Considering wear resistance, it is desirable to use an insulating fluororesin.

The pressure roller302is pressure-contacted to the fixing pad301with the fixing belt300sandwiched therebetween, whereby the fixing nip306is formed. Further, the heating roller303rotates in the direction of arrow Aby following the rotation traveling of the fixing belt300in the direction of arrow B. The temperature sensor317detects the temperature of the outer peripheral surface of the pressure roller302. The temperature of the outer peripheral surface of the pressure roller302is used to estimate the temperature rise rate R [° C. per second] of the fixing belt300as will be described later.

The recording sheet S entering the fixing device100along a conveying path308is guided to the fixing nip306by conveying guides309and310. A peeling claw312is arranged on the downstream side of the fixing nip306in a sheet conveying direction to prevent the recording sheet S having passed through the fixing nip306from winding around the pressure roller302. After passing through the fixing nip306, the recording sheet S which separates from the fixing device100along a conveying path313is guided to the outside of the fixing device100by conveying guides314and315.

Incidentally, in order to prevent electrostatic offset, the inner peripheral surface of the fixing belt300is conductive, and the conductive heating roller303coming into contact with the inner peripheral surface of the fixing belt300is electrically grounded. Specifically, both the cylindrical portion and the rotation shaft of the heating roller303are made of a conductive material and are electrically connected to each other. Since the rotation shaft of the heating roller303is electrically connected to a grounding circuit316, the inner peripheral surface of the fixing belt300is grounded via the cylindrical portion and the rotation shaft of the heating roller303and the grounding circuit316.

Among the members arranged around the fixing belt300, the fixing pad301and the pressure roller302are indirect contact with the fixing belt300. Further, the support member of the fixing pad301and the rotation shaft of the pressure roller302are supported by the housing304. Further, the housing304is directed to the main body of the image forming apparatus1.

That is, the fixing pad301, the pressure roller302, and the housing304form a heat conduction path for releasing the heat of the fixing belt300to the main body of the image forming apparatus1. In this sense, the fixing pad301, the pressure roller302, and the housing304are peripheral members which affect the temperature rise rate of the fixing belt300.

[3] Configuration of Controller151

Next, the configuration of the controller151will be described.

As illustrated inFIG. 4, in the controller151, a central processing unit (CPU)401, a read only memory (ROM)402, a random access memory (RAM)403, and the like connected in an internal bus408to communicate with each other. When a reset signal is input due to a power supply or the like to the image forming apparatus1, the CPU401reads a boot program from the ROM402and starts up and executes an operating system (OS) and a control program read from a hard disk drive (HDD)404with the RAM403used as a work storage area.

A network interface card (NIC)405executes processing for communicating with other devices via a communication network such as a local area network (LAN) or the Internet. Accordingly, the image forming apparatus can accept an image forming job or the like from another apparatus.

A timer406is used by the controller151for timing, and in this embodiment, the timer is particularly used for timing a print wait time Δt of the fixing device. The temperature sensor407is used to acquire an environmental temperature, and in this embodiment, the in-apparatus temperature of the image forming apparatus1is measured as the environmental temperature. However, needless to say, the present disclosure is not limited to this, and an outside temperature may be measured as the environmental temperature.

The controller151uses the operation panel410to provide information to the user of the image forming apparatus1and to receive an instruction input from the user.

As illustrated inFIG. 5, the operation panel410includes a touch panel500, a speaker501, a power key511, a hard key512, a startkey513, a stop key514, a reset key515, a menu key516, and an ID key517. The touch panel500includes a liquid crystal display (LCD) and a touch pad, and displays the screen to the user or accepts touch input by the user. The speaker501is used to output audio to the user.

The power key511is a key for turning on the power of the image forming apparatus1, and the hard key512is customized by the user to set a function to be executed when pressed. The start key513is a key for starting the execution of jobs after the user completes the setting of the execution conditions of jobs such as copying. The stop key514is a key for stopping a running job. The reset key515is a key for resetting setting on a displayed screen.

The menu key516is a key for displaying a top menu. The ID key517is a key for starting an authentication process when the operation panel410is in a log-out state and for logging out when the operation panel is in a login state. Further, the operation panel410includes a short-range wireless communication interface518for linking with a mobile terminal or the like.

With reference to the output signal of the temperature sensor311of the fixing device100, the controller151acquires the surface temperature of the fixing belt300and turns on or off the halogen heater307to control the surface temperature of the fixing belt. In addition to these, the controller151starts the state of other parts of the image forming part120such as the imaging parts121Y,121M,121C and121K, the image reading part110, and the sheet feeding part130, and controls the operations thereof.

[4] Operation of Controller151

Next, the operation of the controller151will be described with particular attention to the print wait time Δt.

FIG. 6is a flowchart for describing the operation of the controller151regarding the first page in a case where the user of the image forming apparatus1operates the operation panel410to form an image of a plurality of pages. Incidentally, in a case where the number of pages to be image-formed is only one, the same applies to the operation of the controller151regarding the corresponding one page. Further, in a case where an image forming job is received from another apparatus via the communication network, the operation of the controller151starts from step S604inFIG. 6.

As illustrated inFIG. 6, when the controller151detects a user operation on the operation panel410(S601: YES), the controller151starts a preliminary rotation operation of the fixing device100(S602). The preliminary rotation operation of the fixing device100is an operation to raise the temperature of the fixing belt300with a temperature (referred to as a “preliminary rotation temperature Ts”) lower than the temperature (referred to as a “print set temperature Ti”) of the fixing belt at the time of fixing defined as a control target value.

Thereafter, when the user of the image forming apparatus1presses the start key513of the operation panel410to instruct the start of job execution (S603: YES), a process of estimating the temperature rise rate R of the fixing belt300is executed as described later (S604), and the print wait time Δt is calculated from the temperature rise rate R obtained by estimation (S605). The print wait time Δt is a set value of the time since the temperature rise is started with the target temperature of the fixing belt300set as the print set temperature Ti until the tip of the recording sheet S enters the fixing nip306.

In this embodiment, the print wait time Δt is calculated using the following equation (1).

For example, in a case where the environmental temperature is 25° C., the estimation value of the temperature rise rate R of the fixing belt300is 4° C./sec, the print set temperature Ti is 176° C., and the preliminary rotation temperature Ts is 160° C., the print wait time Δt is

Next, the timing to start imaging is calculated (S606). In this embodiment, as illustrated inFIG. 7, the time required for the recording sheet S carrying the toner image to enter the fixing nip306after the imaging part121Y starts the imaging of the Y-color toner image is 1.6 seconds which is obtained by adding 0.3 seconds which is the time taken for the recording sheet S to be conveyed from the secondary transfer roller pair123to the fixing nip306to the above-described image formation time of 1.3 seconds. Time T1 obtained by subtracting this time from the print wait time Δt is

Therefore, the image formation start timing is 2.4 seconds after the start of the temperature rise of the fixing belt300.

Next, the sheet feed timing of the recording sheet S is calculated (S607). As illustrated inFIG. 2, the time required for the recording sheet S to enter the fixing nip306via the timing roller pair124and the secondary transfer roller pair123after the start of sheet feeding 0.9 seconds (=0.3 seconds+0.3 seconds+0.3 seconds) or more in consideration of performing skew-correction or waiting to adjust the print position on the recording sheet S. Therefore, in comparison with the value obtained by subtracting this time from the print wait time Δt, time T2 is set such that

For example, T2 may be set to 2.8 seconds with a margin of 0.3 seconds.

Further, the timing at which the timing roller pair124starts passing a sheet is time T3 which is obtained by rewinding the time of the recording sheet S entering the fixing nip306by the time taken for the recording sheet S to enter the fixing nip306after the timing roller pair124starts passing the sheet. Thus,

After the calculation of times T1, T2 and T3, these times are set to timer406(S608) and the temperature rise of the fixing belt300is started (S609).

When time T1 (=2.4 seconds) elapses after the start of the temperature rise of the fixing belt300(S610: YES), the imaging parts121Y,121M,121C and121K are controlled to start image formation in order from the Y color toner image (S611).

When time T2 (2.8 seconds in the above example) elapses after the start of the temperature rise of the fixing belt300(S612: YES), the sheet feeding part130is controlled to start the sheet feeding of the recording sheet (S613).

When time T3 (=3.4 seconds) elapses after the start of the temperature rise of the fixing belt300(S614: YES), the timing roller pair124are rotationally driven to start passing the recording sheet S (S615).

Thereafter, in the recording sheet S, the toner image carried on the outer peripheral surface of the intermediate transfer belt122is secondarily transferred at the secondary transfer nip125(S616), and the toner image is heat-fixed at the fixing nip306(S617) and then discharged to the outside of the apparatus (S618).

When the members configuring the fixing device100are warm as in a case where the next image forming job is executed immediately after the previous image forming job is completed or a case where the preliminary rotation time is sufficiently long, the temperature rise rate R is increased. For example, in a case where the environmental temperature is 25° C., the estimation value of the temperature rise rate R of the fixing belt300is 8° C./sec, the print set temperature Ti is 176° C., and the preliminary rotation temperature Ts is 160° C., the print wait time Δt is

As illustrated inFIG. 8, in the timing of starting image formation, the time required for the recording sheet carrying the toner image to enter the fixing nip306after the imaging part121Y starts the imaging of the Y-color toner image is 1.6 seconds which is obtained by adding 0.3 seconds which is the time taken for the recording sheet S to be conveyed from the secondary transfer roller pair123to the fixing nip306to the above-described image formation time of 1.3 seconds. Time T1 obtained by subtracting this time from the print wait time Δt is

Therefore, the image formation start timing is 0.4 seconds after the start of the temperature rise of the fixing belt300.

In the timing of feeding the recording sheet S, the time required for the recording sheet S to enter the fixing nip306via the timing roller pair124and the secondary transfer roller pair123after the start of sheet feeding 0.9 seconds (=0.3 seconds+0.3 seconds+0.3 seconds) or more in consideration of performing skew-correction or waiting to adjust the print position on the recording sheet S. Therefore, in comparison with the value obtained by subtracting this time from the print wait time Δt, time T2 is set such that

For example, T2 may be set to 2.8 seconds with a margin of 0.3 seconds.

Further, the timing at which the timing roller pair124starts passing a sheet is time T3 which is obtained by rewinding the time of the recording sheet S entering the fixing nip306by the time taken for the recording sheet S to enter the fixing nip306after the timing roller pair124starts passing the sheet. Thus,

The processing after determining times T1, T2 and T3 is the same as above.

In this way, the recording sheet S enters the fixing nip306at the timing when the print wait time Δt elapses after the start of the temperature rise of the fixing belt300, and thus it is possible to prevent the overshoot of the temperature of the fixing belt300.

[5] Estimating Process (S504) of Temperature Rise Rate R

Next, the estimation process of the temperature rise rate R will be described.

FIG. 9is a graph illustrating the temperature change of the fixing belt300when the control target value of the temperature of the fixing belt300is switched from the preliminary rotation temperature Ts to the print set temperature Ti. A vertical axis represents the temperature of the fixing belt300, and a horizontal axis represents an elapsed time. Further, line graph901shows the control target value of the temperature of the fixing belt300.

As illustrated inFIG. 9, in a case where the amount of heat stored in the members configuring the fixing device100such as the pressure roller302is large, and the temperature is considerably high, the time until the temperature of the fixing belt300reaches the print set temperature Ti after the temperature rise start timing when the control target value of the temperature of the fixing belt300is switched from the preliminary rotation temperature Ts to the print set temperature Ti becomes shorter as shown in graph902. That is, the temperature rise rate R of the fixing belt300is high.

As illustrated inFIG. 10, the temperature rise rate R of the fixing belt300is obtained by dividing the temperature rise amount ΔT by the print wait time Δt. Before the temperature rise start timing, the temperature of the fixing belt300is adjusted to be the preliminary rotation temperature Ts. Further, the temperature of the fixing belt300immediately after the print wait time Δt elapses is the print set temperature Ti. Therefore, the temperature rise amount ΔT is

Therefore, the temperature rise rate R is

Incidentally, even after the temperature of the fixing belt300reaches the print set temperature Ti, the temperature of the fixing belt300continues to rise at the temperature rise rate R until the recording sheet S reaches the fixing nip306. Thus, particularly in a case where the temperature rise rate R is high, the possibility of overshoot increases.

On the other hand, in a case where the amount of heat stored in the members configuring the fixing device100is not large, and the temperature is not so high, the time until the temperature of the fixing belt300reaches the print set temperature Ti becomes longer as shown in graph903ofFIG. 9. Therefore, the temperature rise rate R of the fixing belt300is low. As described above, the temperature rise rate R of the fixing belt300depends on the amount of heat stored in the members configuring the fixing device100.

Therefore, when the recording sheet S is conveyed such that the time from the temperature rise start timing to the entry of the recording sheet S into the fixing nip306is constant regardless of the heat storage state of the members configuring the fixing device100, the overshoot occurs easily particularly in a case where the temperature rise rate R is high. Therefore, in order to prevent the occurrence of overshoot, it is necessary to adjust the entry timing of the recording sheet S according to the heat storage state of the members configuring the fixing device100.

In this embodiment, the temperature rise rate R of the fixing belt300is estimated by referring to the surface temperature of the pressure roller302, which is considered to have a large influence particularly on the temperature rise rate R due to the direct contact with the fixing belt300, among the members configuring the fixing device100as an index value for indexing the heat storage state of the members configuring the fixing device100.

That is, as illustrated inFIG. 11, in the process of estimating the temperature rise rate, first, the surface temperature of the pressure roller302is acquired by referring to the output signal of the temperature sensor317(S1101), and then with reference to the temperature rise rate table (S1102), the temperature rise rate corresponding to the surface temperature of the pressure roller302is estimated (S1103). As illustrated inFIG. 12, the temperature rise rate table is a table in which the range of the surface temperature of the pressure roller302and the temperature rise rate R of the fixing belt300are stored in association with each other.

The numerical value in each column of the temperature rise rate table can be determined, for example, by measurement with experiments. For example, the temperature rise rate table may be stored in the HDD404of the controller151or may be stored in the ROM402. Further, the temperature rise rate table may be stored in another non-volatile memory.

For example, in a case where the surface temperature of the pressure roller302is higher than 70° C. and lower than 80° C., the temperature rise rate R of the fixing belt300from the temperature rise start timing to the entry of the recording sheet S to the fixing nip306can be estimated to be 7.0° C. per second.

Needless to say, the stored contents of the temperature rise rate table are not limited toFIG. 12, and it is desirable to specify the numerical values stored in the temperature rise rate table by experiments for each image forming apparatus to which the present disclosure is applied. Further, as a matter of course, instead of the temperature rise rate table, a function that outputs the surface temperature R of the pressure roller302as an independent variable and the temperature rise rate R of the fixing belt300as a dependent variable may be used.

[6] Suppression of Overshoot

For example, in the fixing device100according to this embodiment, in a fixing pad type of fixing device which is a high-speed machine with a system speed of 300 mm per second and uses a fixing belt having a low heat capacity and a small diameter (D40 mm), when the amount of heat applied to the fixing belt is increased due to the need to control the temperature of the belt at high speed, the temperature fluctuation (ripple) of the fixing belt tends to increase. In particular, in the temperature control before printing the first sheet of the job, the temperature rise width of the fixing belt is large, so that the temperature fluctuation becomes large, and there is a risk that an overshoot occurs in which the temperature of the fixing belt exceeds the upper limit of an allowable range. In particular, in a state where the fixing device is warmed after continuous printing (=a state where the heat storage amount is large), the temperature rise rate of the fixing belt is faster than that in a state where the fixing device is not warmed. Thus, the overshoot occurs further easily, and for example, as illustrated inFIG. 13A, the width of the overshoot easily becomes large.

In response to such a problem, a countermeasure can be considered which prevents an overshoot by causing the recording sheet S to enter the fixing nip before the temperature of the fixing belt exceeds the upper limit of the allowable range so that the overshoot occurs and absorbing heat from the fixing belt to the recording sheet S. In a case where the heat capacity of the fixing belt is low, the temperature lowering width of the fixing belt due to the heat absorption of the recording sheet S can be secured sufficiently largely. Thus, this method is particularly effective in that case.

Therefore, even in a case where the amount of heat stored in the fixing device is large, and the temperature rise rate is high, as illustrated inFIG. 13B, when the first recording sheet S of the job is caused to enter the fixing nip in accordance with the timing when the temperature of the fixing belt reaches the print set temperature Ti, the overshoot can be suppressed.

However, the timing for causing the first recording sheet S of the job to enter the fixing nip is uniformly advanced regardless of the amount of heat stored in the fixing device, which has an adverse effect. For example, immediately after the power is turned on to the image forming apparatus, especially in a case where the environmental temperature is low such as in winter, the amount of heat stored in the fixing device is small, and thus it takes time to raise the temperature of the fixing belt. In such a case, when the timing of causing the recording sheet S to enter the fixing nip is advanced, for example, as illustrated inFIG. 13C, the fixing belt is not sufficiently heated, so that fixing failure may occur.

In response to such a problem, in this embodiment, the temperature rise rate of the fixing belt300is estimated from the measured value of the surface temperature of the pressure roller302at the temperature rise start timing of the fixing belt300, and the entry timing of the recording sheet S is determined such that the timing of causing the recording sheet S to enter the fixing nip306is advanced when the temperature rise rate of the fixing belt300increases. Therefore, it is possible to suppress both the occurrence of overshoot in which the temperature rise of the fixing belt300is excessive and the fixing failure caused by insufficient temperature rise of the fixing belt300.

Incidentally, in this embodiment, it is assumed that the environmental temperature is room temperature (25° C.) since the image forming apparatus1is often installed in an air-conditioned room such as an office. In a case where the environmental temperature is room temperature, the temperature of the fixing belt300can be set to be the preliminary rotation temperature Ts at the temperature rise start timing by performing preliminary rotation.

In the example ofFIG. 14, a vertical axis represents a temperature, and a horizontal axis represents the passage of time. The temperature of the fixing belt300represented by graph1401is the same temperature as the room temperature (for example, 25° C.) until the halogen heater307is turned on. After the halogen heater307is turned on, the temperature of the fixing belt300is controlled to be the preliminary rotation temperature Ts, and the temperature is close to the preliminary rotation temperature Ts at the temperature rise start timing.

Herein, the preliminary rotation is a control operation for controlling the temperature while rotating the fixing belt300so that fixing can be performed at any time when a printing instruction comes. Incidentally, in consideration of energy saving, the target temperature (preliminary rotation temperature Ts) for temperature control during the preliminary rotation is set to be lower than the print set temperature Ti. In this embodiment, the print set temperature Ti is 176° C. while the preliminary rotation temperature Ts is 160° C.

As described above, at the temperature rise start timing, the fixing belt300reaches approximately the preliminary rotation temperature Ts, and in order to cause the recording sheet S to enter the fixing nip306at the timing when the temperature of the fixing belt300rises to the print set temperature Ti, the time obtained by dividing the temperature difference obtained by subtracting the preliminary rotation temperature Ts from the print set temperature Ti by the temperature rise rate may be defined as the print wait time Δt.

In this embodiment, the temperature rise rate is estimated by using the surface temperature of the pressure roller302, but the temperature rise rate may be estimated by using another method as described later. Further, since the temperature difference obtained by subtracting the preliminary rotation temperature Ts from the print set temperature Ti is constant, the print wait time Δt may be estimated directly from the surface temperature of the pressure roller302or the like without estimating the temperature rise rate.

Incidentally, as well as in a case where the temperature rise of the fixing belt300is started by the reception or the like of a printing instruction during the preliminary rotation, even in a case where the temperature rise of the fixing belt300is started by the reception or the like of a printing instruction during the time of the rotation standby in which the rotation and temperature control of the fixing belt300continue during a predetermined time in preparation for the next job after the completion of the image forming job, the occurrence of overshoot can be suppressed by causing the recording sheet S to enter the fixing nip306at an appropriate timing in a similar way to this embodiment.

Incidentally, due to the low environmental temperature or the like, the fixing belt300may not reach the preliminary rotation temperature Ts at the time (temperature rise start timing) of receiving the printing instruction. Further, even in a case where the halogen heater307is turned off, the fixing belt300is returned from a sleep mode in which the temperature is not heated or raised, and printing is performed, as illustrated inFIG. 15, the fixing belt300may not reach the preliminary rotation temperature Ts at the temperature rise start timing.

In such a case, the time taken for the fixing belt300to reach the print set temperature Ti is longer compared to a case where the fixing belt300reaches the preliminary rotation temperature Ts at the temperature rise start timing. Thus, in order to accurately suppress the occurrence of overshoot, the print wait time Δt is desirably estimated from the temperature difference between the temperature rise start temperature obtained by measuring the temperature of the fixing belt300at the temperature rise start timing and the print set temperature Ti by using the temperature rise rate.

Hereinbefore, the present disclosure has been described above based on the embodiment. However, it goes without saying that the present disclosure is not limited to the above-described embodiment, and the following modifications can be implemented.

(7-1) In the above-described embodiment, a case where the temperature rise rate of the fixing belt300is estimated by using the surface temperature of the pressure roller302has been described as an example. However, needless to say, the present disclosure is not limited thereto, and the temperature rise rate of the fixing belt300may be estimated by using the surface temperature of the fixing pad301and the temperature of a member which can affect the temperature rise rate of the fixing belt300instead of the pressure roller302.

The temperature rise rate of the fixing belt300may be estimated by estimating the heat storage state of the fixing device100from the heating rotation information indicating that the fixing belt300is rotationally driven while being heated, the sheet passing information indicating that the recording sheet S is passed through the fixing nip306, the non-operating information indicating that the heating of the fixing belt300is stopped, and the like.

For example, a case is considered in which a plain sheet of A4 size with a basis weight of 65 g/m2is fed horizontally (LED: Long Edge Feeding) at the system speed of 300 mm/s, the image formation speed of 60 ppm (Pages Per Minute), and the print setting temperature of 170° C., and one-sided continuous printing is performed.

As illustrated inFIG. 16A, the index indicating the amount of heat generated during the printing (fixing) operation of passing a sheet while the fixing belt300is rotationally driven after the halogen heater307is turned on is set as heat generation index A, and the index indicating the amount of heat generated during the preliminary rotation operation in which the halogen heater307is turned on, and the fixing belt300is rotationally driven, but a sheet is not passed is defined as heat generation index B. Further, the index indicating that the amount of heat dissipation in a sleep-off state in which the halogen heater307is turned off, and the fixing belt300is not rotationally driven is defined as heat dissipation index C.

As illustrated inFIG. 16B, heat generation index A is calculated by using linear equation (12) including coefficients a and b, and

heat generation index B is calculated by using linear equation (13) including coefficients c and d.

Further, heat dissipation index C is calculated by using logarithmic equation (14) including coefficients f and g.

Here, coefficients a, b, c, d, f, and g are constants determined experimentally. The time is the elapsed time from the start of the printing operation in heat generation index A and is the elapsed time from the start of the preliminary rotation operation in heat generation index B. Further, in heat dissipation index C, the time is the elapsed time from the transition of the operation mode of the image forming apparatus1to the sleep-off state.

In logarithmic equation (14), ln is a natural logarithm. However, even when the base of a logarithm such as a common logarithm is a logarithm function different from that of the natural logarithm, the same value can be calculated as heat dissipation index C by adjusting coefficient f according to the difference in the base.

Heat storage index x is calculated by using the following equation (15), which includes heat generation indexes A and B and heat dissipation index C,

FIG. 16Cis a graph illustrating a change of heat storage index x over time. InFIG. 16C, an aspect in which heat storage index x increases by executing a printing operation for only 16 minutes is shown in graph1601, then transition is made to the sleep-off state, and an aspect in which heat storage index x decreases when heat dissipation progresses is shown in graph1602. Further, the preliminary rotation is executed for only four minutes, and the heat storage index x increases again as shown in graph1603. Then, when transition is made to the sleep-off state, heat storage index x decreases as shown in graph1604.

Incidentally, as illustrated inFIG. 16B, heat dissipation index C is defined only within 120 minutes after the operation mode of the image forming apparatus1transitions to the sleep-off state. In a case where 120 minutes or more elapses after the transition to the sleep-off state, the value of heat storage index x is reset to zero.

After heat storage index x is specified as described above, the temperature rise rate is estimated with reference to the temperature rise rate table as illustrated inFIG. 17. The temperature rise rate table is a table in which the numerical range of the heat storage coefficient and the temperature rise rate are stored in association with each other. The numerical value in each column of the temperature rise rate table can be determined, for example, by experiment.

When the temperature rise rate is estimated, the numerical range to which heat storage coefficient x specified as described above belongs is specified, and further the temperature rise rate associated with the numerical range in the temperature rise rate table is specified. The process after the temperature rise rate is specified is the same as in step S605and subsequent steps inFIG. 6.

In this way, the temperature rise rate can be estimated according to the operation history of the image forming apparatus1, and thus it is possible to accurately suppress the overshoot of the temperature of the fixing belt300.

(7-2) In the above-described embodiment, a case where the timing of causing the recording sheet S to enter the fixing nip306is determined by estimating the temperature rise rate has been described as an example. However, needless to say, the present disclosure is not limited thereto, and the following may be used instead.

That is, the heat storage state of the fixing device100mainly depends on the length of the preliminary rotation time until the temperature rise start timing. By paying attention to this characteristic and determining the timing of causing the recording sheet S to enter the fixing nip306according to the preliminary rotation time, it is possible to avoid the occurrence of extreme overshoot which cannot be put to practical use.

In a case where the preliminary rotation time is short, the print wait time of about three seconds is sufficient, but when the preliminary rotation time increases, the amount of heat stored in the fixing device100increases. For example, in a case where the preliminary rotation time is 20 seconds or more, when the timing of causing the recording sheet S to enter the fixing nip306is fixed without advancing, the overshoot occurs.

For example, inFIG. 18A, the print wait time is fixed, and thus by the time the recording sheet S enters the fixing nip306, the overshoot occurs in which the temperature (graph1801) of the fixing belt300is significantly higher than the print set temperature Ti (graph1802).

When the overshoot occurs, a problem such as uneven gloss of the toner image and separation failure of the recording sheet sticking to the fixing belt300easily occurs. In order to suppress the occurrence of overshoot, it is necessary to shorten the print wait time and advance the timing of causing the recording sheet S to enter the fixing nip306.

For example, inFIG. 18B, the print wait time is set in accordance with the time from when the target value (graph1812) for temperature control of the fixing belt300is switched to the print set temperature Ti until the temperature (graph1811) of the fixing belt300reaches the print set temperature Ti. In accordance with this printing wait time, the recording sheet S enters the fixing nip306to absorb heat from the fixing belt300, and thus the occurrence of overshoot is suppressed.

Therefore, as illustrated inFIG. 19, a print wait time table in which the print wait time is associated with the numerical range of the preliminary rotation time until the temperature rise start timing is stored in advance. With reference to the print wait time table, the numerical range to which the preliminary rotation time until the temperature rise start timing belongs may be specified, and the print wait time associated with the numerical range may be specified.

The numerical value in each column of the print wait time table can be determined, for example, by experiment. Further, the print wait time table may be stored in a non-volatile memory such as the ROM402or the HDD404and read out as needed. The process after the print wait time is specified is the same as in step S606and subsequent steps inFIG. 6. In this way, the overshoot can be suppressed with high accuracy.

(7-3) In the above-described embodiment, a case where the print wait time is changed according to the heat storage state of the fixing device100to suppress the overshoot has been described as an example. However, needless to say, the present disclosure is not limited thereto, and the following may be used instead of or in addition to this.

That is, when the external power supply voltage for supplying power to the image forming apparatus1fluctuates, the temperature rise rate of the fixing belt300also fluctuates. Thus, when the print wait time is adjusted according to the power supply voltage of the image forming apparatus1, the overshoot can be suppressed even more accurately.

For example, in the period until the temperature (graph2001) of the fixing belt300reaches the print set temperature Ti after the target value (graph2002) for temperature control of the fixing belt300is switched to the print set temperature Ti in a case where the power supply voltage D of the image forming apparatus1is 100V as illustrated inFIG. 20A, in a case where the power supply voltage D of the image forming apparatus1is 90V with respect to the temperature rise rate of the fixing belt300as illustrated inFIG. 20B, the amount of heat generated by the halogen heater307is lower compared to the case where the power supply voltage D is 100V. Thus, the temperature rise rate of the fixing belt300becomes low in the period until the temperature (graph2011) of the fixing belt300reaches the print set temperature Ti after the target value (graph2012) for temperature control of the fixing belt300is switched to the print set temperature Ti.

Therefore, when the print wait time is fixed (3,100 milliseconds in the example ofFIGS. 20A and 20B), in a case where the power supply voltage is 90V, the recording sheet S enters the fixing nip306before the fixing belt300is sufficiently heated, and thus fixing failure may occur.

In response to such a problem, a print wait time table as illustrated inFIG. 21is used to store the print wait time in a non-volatile memory such as the ROM402in association with the combination of the numerical range of the preliminary rotation time until the temperature rise start timing and the numerical range of the power supply voltage D, a combination of the numerical range to which the preliminary rotation time and the power supply voltage D belong is specified before the temperature rise start, and the print wait time corresponding to the combination is read, whereby an appropriate print wait time can be specified. The process after the print wait time is specified is the same as in step S606and subsequent steps inFIG. 6.

In this way, the overshoot and fixing failure can be accurately suppressed even when the preliminary rotation time and the power supply voltage D fluctuate.

(7-4) Although not particularly described in the above-described embodiment, in a case where the temperature difference between the temperature of the fixing belt300at the temperature rise start timing and the print set temperature Ti is less than 10° C., the time required for the temperature of the fixing belt300to reach the print set temperature Ti is excessively short when considering the time required for upstream processes such as image formation and sheet feeding. Thus, it is difficult to adjust the timing of causing the recording sheet S to enter the fixing nip306. Therefore, the image formation may be started when the printing instruction is received without adjusting the print wait time.

(7-5) In the above-described embodiment, a case where the heat capacity of the fixing belt300is small has been described as an example. However, in a case where the fixing belt300having a large heat capacity is used, the temperature rise rate of the fixing belt300becomes low, and thus the temperature of the fixing belt300can be repeatedly measured by using the temperature sensor311to actually measure the temperature rise rate of the fixing belt300.

Therefore, unlike the above-described embodiment, it is not necessary to estimate the temperature rise rate, and the timing of causing the recording sheet S to enter the fixing nip306may be determined by using the actual temperature rise rate.

(7-6) The following can be further considered as a method for estimating the temperature rise rate. In the preliminary rotation period, the target temperature for temperature control of the fixing belt300may be changed to the print set temperature Ti for a predetermined short period, the temperature of the fixing belt300at the start of the short period and the temperature of the fixing belt300at the end of the short period may be acquired by using the temperature sensor331, and the temperature rise rate may be estimated by dividing the temperature difference by the length of the short period.

The length of the preliminary rotation period is not always constant. Thus, the above-described estimation of the temperature rise rate may be repeated during a certain period, and the print wait time may be determined by using the temperature rise rate which is estimated finally until the temperature rise start timing is reached.

The temperature rise rate obtained by the estimation in this way is a value which reflects the heat storage state of the fixing device100, the power supply voltage of the image forming apparatus1, or the environmental conditions. Further, since it is not necessary to carry out an experiment in advance and store the result in the table, the storage capacity in the controller151can be saved.

(7-7) In the above-described embodiment, a case where the toner image is heat-fixed to the recording sheet S by using the fixing belt300has been described as an example. However, needless to say, the present disclosure is not limited thereto, and a member other than the fixing belt300, such as a fixing roller, may be used instead of300.

(7-8) Although not described in detail in the above-described embodiment, the temperature control of the fixing belt300at the time of fixing is performed such that the temperature of the fixing belt300is within a predetermined allowable temperature range including the print set temperature Ti. Therefore, the overshoot of the temperature of the fixing belt300indicates that the temperature of the fixing belt300exceeds the upper limit of the allowable temperature range.

(7-9) Although not described in detail in the above-described embodiment, the surface temperature of the pressure roller302is warmed to a temperature close to the temperature of the fixing belt300by bringing the pressure roller302into pressure contact with the fixing belt300in the fixing nip306. On the other hand, the outer peripheral surface of the pressure roller302is cooled by coming into contact with the surrounding air after being separated from the fixing nip306. As a matter of course, when the cooling time increases, the temperature of the outer peripheral surface of the pressure roller302becomes low. Thus, the surface temperature of the pressure roller302immediately before entering the fixing nip306is the lowest.

It is this lowest surface temperature that has the largest effect on the temperature rise rate of the fixing belt300. Thus, in order to accurately estimate the temperature rise rate of the fixing belt300, the temperature sensor317is desirably arranged on the downstream side in the rotation direction of the pressure roller302in the outer peripheral surface of the pressure roller302other than the fixing nip, for example, at a position close to the conveying guide310such that the temperature sensor317can detect the lowest possible surface temperature. The position close to the conveying guide310means a portion, which belongs to at least the downstream half in the rotation direction of the pressure roller302, of the outer peripheral surface of the pressure roller302other than the fixing nip.

(7-10) In the above-described embodiment, a case where the image forming apparatus1is a tandem type color multi-function peripheral has been described as an example. However, needless to say, the present disclosure is not limited thereto, a color multi-function peripheral other than the tandem type may be used, and a monochrome multi-function peripheral may be used. Further, the same effect can be obtained when the present disclosure is applied to a single-function device such as a printer device, a copying device equipped with a scanner, or a facsimile device having a facsimile communication function.

The image forming apparatus according to the present disclosure is useful as an apparatus capable of accurately suppressing the overshoot of a fixing member and achieving high image quality.