JP2020095170A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that can prevent the occurrence of overshoot and undershoot during warm-up.SOLUTION: An image forming apparatus comprises a fuser 8 that has a heating unit 81, a pressing unit 82, and a heater 83. The control unit 100 can execute first heating control of energizing the heater 83 such that the temperature of the heating unit 81 becomes a fixing temperature for fixing a developer image to a sheet S based on a temperature detected by a temperature detection unit 9, second heating control of energizing the heater 83 such that the temperature of the heating unit 81 becomes the fixing temperature based on the detection temperature, the second control of reducing an upper limit of the amount of energization of the heater 83 per unit time compared with that in the first heating control, and conveyance control of conveying the sheet S toward a developer image forming unit 5 when the detection temperature becomes equal to or higher than a first temperature that is lower than the fixing temperature. The control unit 100 executes the second heating control from a predetermined timing after the first heating control is started at least until when the sheet S reaches the fuser 8.SELECTED DRAWING: Figure 1

Description

The present invention relates to an image forming apparatus including a fixing device that fixes a developer image on a sheet.

Conventionally, there is known an image forming apparatus including a heating roller, a pressure roller, a heater for heating the heating roller, a fixing device for fixing a developer image on a sheet, and a temperature detection unit for detecting the temperature of the heating roller. (For example, Patent Document 1). In this technique, the electric power supplied to the heater is controlled so that the temperature of the heating roller becomes the fixing temperature based on the temperature detected by the temperature detection unit.

JP, 2008-063395, A

By the way, during warm-up in which the temperature of a heating unit such as a heating roller is raised to the fixing temperature, the temperature of the heating unit may rapidly rise and an overshoot that greatly exceeds the fixing temperature may occur. In a situation where such an overshoot occurs, the heating of the heating unit is stopped before the sheet enters the fixing device, so that the heat is taken away by the sheet when the sheet enters the fixing device. There is a possibility that the temperature of the heating portion drops sharply and an undershoot that is much lower than the fixing temperature occurs.

Therefore, an object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of overshoot and undershoot during warm-up.

In order to achieve the above-mentioned object, the image forming apparatus of the present invention includes a developer image forming unit that forms a developer image on a sheet, a conveying unit that conveys the sheet toward the developer image forming unit, and a heating unit. A heater for heating the heating unit, a pressure unit for sandwiching the sheet between the heating unit and the fixing unit for fixing the developer image on the sheet, a temperature detection unit for detecting the temperature of the heating unit, and a control unit. And a section.
The controller controls the first heating control to energize the heater so that the temperature of the heating unit becomes the fixing temperature for fixing the developer image on the sheet based on the detected temperature of the temperature detecting unit, and the heating unit of the heating unit based on the detected temperature. Second heating control for energizing the heater so that the temperature becomes the fixing temperature, in which the upper limit value of the energization amount to the heater per unit time is smaller than the first heating control, and the detected temperature It is possible to execute the conveyance control for conveying the sheet toward the developer image forming unit when the temperature becomes equal to or higher than the first temperature lower than the fixing temperature.
The control unit executes the second heating control from a predetermined timing after starting the first heating control until at least the sheet reaches the fixing device.

With such a configuration, the temperature of the heating unit can be gently increased from the predetermined timing after the first heating control is started until at least the sheet reaches the fixing device, and therefore, the temperature during the warm-up may be exceeded. It is possible to suppress the occurrence of shoots. Also, heating of the heating unit can be continued until the sheet enters the fixing unit, and it is possible to prevent the temperature of the heating unit from dropping sharply even if heat is taken by the sheet when the sheet enters the fixing unit. Therefore, the occurrence of undershoot can be suppressed.

In the above-described image forming apparatus, the control unit controls the temperature of the heating unit with a duty ratio according to the difference between the fixing temperature and the detected temperature in the first heating control and the second heating control, and the duty ratio has a large difference. The upper limit value of the duty ratio may be set to 100% in the first heating control, and the upper limit value of the duty ratio may be set to be smaller than 100% in the second heating control.

In the above-described image forming apparatus, when the image is continuously formed on a plurality of sheets, the control unit executes the second heating control until the fixing of the predetermined number of sheets is completed, and the predetermined number of sheets is performed. The first heating control may be performed after the fixing of No. 1 is completed.

According to this, when an image is continuously formed on a plurality of sheets, a predetermined number of sheets can be fixed by limiting the energization amount to the heater per unit time. Can be suppressed. On the other hand, after fixing a predetermined number of sheets, by releasing the upper limit value of the amount of electricity supplied to the heater per unit time, insufficient heating of the heating unit can be suppressed, so that fixing failure occurs. Can be suppressed.

In the above-described image forming apparatus, the control unit may be configured to execute the second heating control on condition that the detected temperature when the first heating control is started is equal to or lower than a predetermined temperature threshold.

In the case of cold start, in which the detected temperature is low when the first heating control is started, the temperature of the heating portion is likely to rise rapidly when the heating portion is heated, and thus overshooting is likely to occur. Therefore, in the case of cold start, by executing the second heating control, it is possible to effectively suppress the occurrence of overshoot and undershoot during warm-up.

In the above-described image forming apparatus, the control unit may be configured to execute the second heating control based on the detected temperature being equal to or higher than the second temperature lower than the fixing temperature after starting the first heating control. You can

In the above-described image forming apparatus, the second temperature can be set to be equal to or higher than the first temperature.

In the above-described image forming apparatus, the controller does not rotate the fixing device when the detected temperature is lower than the third temperature which is lower than the first temperature, and does not rotate when the detected temperature is equal to or higher than the third temperature. It is possible to adopt a configuration in which the rotation control for driving the rotation can be executed.

According to the present invention, it is possible to suppress the occurrence of overshoot and undershoot during warm-up.

FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus according to an embodiment. It is a figure which shows the structure for controlling a heater. A table (a) for setting a duty ratio in the first heating control and a table (b) for setting a duty ratio in the second heating control. It is a flow chart which shows operation of a control part. 6 is a time chart showing the operation of the control unit.

Next, an embodiment of the invention will be described in detail with reference to the drawings as appropriate.
As shown in FIG. 1, a laser printer 1 as an example of an image forming apparatus includes a main body housing 2, a sheet supply unit 3, an exposure device 4, a developer image forming unit 5, a fixing unit 8, and a temperature. The detection unit 9, the sheet detection unit 12, and the control unit 100 are provided.

The sheet supply unit 3 is provided in the lower portion of the main body housing 2, and has a sheet tray 31 in which a sheet S such as paper is stored, a pressure plate 32, and a conveyance that conveys the sheet S toward the developer image forming unit 5. And part 33. The transport unit 33 includes a pickup roller 33A, a separation pad 33B, a first transport roller 33C, a registration roller 33R, and the like. The sheet supply unit 3 draws the sheets S in the sheet tray 31 to the pickup roller 33A by the pressure plate 32, separates them one by one by the pickup roller 33A and the separation pad 33B, and sends them out, and by the first conveyance roller 33C and the registration roller 33R. The image is conveyed toward the developer image forming unit 5.

The exposure device 4 is arranged in the upper portion of the main body housing 2 and includes a light source device (not shown), a polygon mirror, a lens, a reflecting mirror, etc., which are not shown. The exposure device 4 exposes the surface of the photoconductor 61 by scanning the surface of the photoconductor 61 at high speed with a light beam (see the alternate long and short dash line) based on the image data emitted from the light source device.

The developer image forming unit 5 is a device that forms a developer image on the sheet S, and is arranged below the exposure device 4. The developer image forming unit 5 is detachably attached as a process cartridge to the main body housing 2 through an opening formed when the front cover 21 provided at the front portion of the main body housing 2 is opened. The developer image forming section 5 includes a photoconductor cartridge 6 and a developing cartridge 7.

The photoconductor cartridge 6 includes a photoconductor 61 that is a cylindrical photoconductor drum, a charger 62 that is a corona charger, and a transfer roller 63. The developing cartridge 7 is attachable to and detachable from the photoconductor cartridge 6, and includes a developing roller 71, a supply roller 72, a layer thickness regulating blade 73, an accommodating portion 74 for accommodating a developer made of dry toner, and an agitator 75. It has and.

The developer image forming section 5 uniformly charges the surface of the photoconductor 61 by the charger 62. Then, the surface of the photoconductor 61 is exposed by the light beam emitted from the exposure device 4, so that an electrostatic latent image based on image data is formed on the photoconductor 61. Further, the developer in the container 74 is supplied to the supply roller 72 while being stirred by the agitator 75, and then supplied from the supply roller 72 to the developing roller 71. Then, as the developing roller 71 rotates, the developer enters between the developing roller 71 and the layer thickness regulating blade 73 and is carried on the developing roller 71 as a thin layer having a constant thickness.

The developer image forming unit 5 supplies the developer carried on the developing roller 71 to the electrostatic latent image formed on the photoconductor 61 from the developing roller 71. As a result, the electrostatic latent image is made visible and a developer image is formed on the photoconductor 61. After that, the sheet S passes between the photoconductor 61 and the transfer roller 63, so that the developer image on the photoconductor 61 is transferred onto the sheet S.

The fixing device 8 is a device that fixes the developer image on the sheet S, and is arranged behind the developer image forming unit 5. The fixing device 8 includes a heating unit 81, a pressure unit 82, and a heater 83.
The heating unit 81 is a cylindrical heating roller made of metal. The heating unit 81 is configured to contact the sheet S conveyed between the heating unit 81 and the pressure unit 82 to heat the sheet S.

The pressure unit 82 is a pressure roller having an elastic layer around the cored bar. The pressurizing unit 82 is arranged in a state of being pressed by being in contact with the heating unit 81, and sandwiches the sheet S with the heating unit 81.
The heater 83 is a heat source that heats the heating unit 81. The heater 83 is configured as a halogen heater having a filament 83A (see FIG. 2), and heats the heating unit 81 with radiant heat. The heater 83 is arranged inside the heating unit 81.

The fixing device 8 is rotationally driven by driving a motor M provided inside the main body housing 2. As an example, in the fixing device 8, when the driving force is input from the motor M, the heating unit 81 is rotationally driven, and the pressurizing unit 82 is driven to rotate in accordance with the rotational driving of the heating unit 81.

The fixing device 8 fixes the developer image transferred on the sheet S to the sheet S by passing the sheet S between the heating unit 81 and the pressure unit 82. The sheet S having the developer image fixed thereon is discharged onto the discharge tray 22 by the second transport roller 23 and the discharge roller 24.

The control unit 100 is a device that controls each unit of the laser printer 1, such as the fixing device 8 and the motor M, and is configured by a single or a plurality of electric circuits. The control unit 100 outputs a control signal or a drive voltage to each unit of the laser printer 1 to control each unit. As shown in FIG. 2, the control unit 100 includes a CPU 110, a ROM 120, a RAM 130, a heater controller 140, a switching circuit 150, and the like.

The CPU 110 performs a process of instructing the operation timing of each part of the laser printer 1 and sending a command value as the target temperature of the fixing device 8 to the heater controller 140.
The ROM 120 stores data such as programs for controlling each unit of the laser printer 1 and various setting information.
The RAM 130 is used as a work area when the CPU 110 executes various programs and a temporary storage area for data.

The heater controller 140 sets the duty ratio of the switching circuit 150 based on the target temperature of the fixing device 8 and the detected temperature T detected by the temperature detection unit 9. In this embodiment, the CPU 110 and the heater controller 140 are integrated as a single semiconductor element.
The switching circuit 150 energizes the heater 83 by switching the AC voltage at the set duty ratio.

The temperature detection unit 9 is a sensor that detects the temperature of the heating unit 81. In the present embodiment, the temperature detection unit 9 is arranged so as to face the surface of the heating unit 81 with a predetermined distance from the surface of the heating unit 81. The temperature detection unit 9 outputs a signal according to the temperature of the heating unit 81 to the control unit 100. The control unit 100 acquires the detected temperature T as the temperature of the heating unit 81 from the signal output from the temperature detection unit 9. As the temperature detection unit 9, for example, a thermistor whose electric resistance changes according to temperature can be used.

The detected temperature T acquired by the control unit 100 based on the output signal from the temperature detection unit 9 does not linearly correspond to the output signal from the temperature detection unit 9, and does not correspond to the actual surface temperature of the heating unit 81. The temperature may be corrected to correspond. For example, the detected temperature T may be a temperature obtained by correcting the output of the temperature detection unit 9 according to the elapsed time from the start of heating of the heating unit 81.

As an example, the control unit 100 can acquire the detected temperature T (correction value) by the following equation.
T=a(t)×TS+b(t)
Here, TS is the temperature detected by the temperature detection unit 9, specifically, the output signal of the temperature detection unit 9. Further, a(t) is a correction coefficient, and b(t) is a correction term. The correction coefficient a(t) and the correction term b(t) are functions of the time t from the start of heating of the heating unit 81, respectively.

The sheet detection unit 12 is a sensor that detects the sheet S conveyed between the heating unit 81 and the pressure unit 82 toward the second conveyance roller 23. The sheet detection unit 12 is arranged downstream of the heating unit 81 and upstream of the second transport roller 23 in the transport direction of the sheet S. The sheet detection unit 12 is arranged closer to the heating unit 81 than the second transport roller 23.

The sheet detection unit 12 includes, for example, a swing lever that is pushed by the conveyed sheet S and swings, and an optical sensor that detects swing of the swing lever. In the present embodiment, the sheet detection unit 12 outputs an ON signal to the control unit 100 when the swing lever is tilted by the seat S, and outputs an OFF signal to the control unit 100 when the swing lever is not tilted. (Specifically, the ON signal is not output).

The control unit 100 can execute first heating control, second heating control, rotation control, and conveyance control.
The first heating control and the second heating control are controls for energizing the heater 83 so that the temperature of the heating unit 81 becomes the fixing temperature Tf based on the detection temperature T of the temperature detection unit 9.

The fixing temperature Tf is a target temperature for temperature control of the heating unit 81 for fixing the developer image on the sheet S. The fixing device 8 can fix the developer image on the sheet S if the temperature of the heating unit 81 is within a predetermined temperature range including the fixing temperature Tf. As an example, the fixing temperature Tf is 200°C. Note that the specific numerical values shown in this specification are examples.

In the first heating control and the second heating control, the control unit 100 uses a heater having a duty ratio according to a difference between the fixing temperature Tf and the detected temperature T, specifically, a difference ΔT obtained by subtracting the detected temperature T from the fixing temperature Tf. The temperature of the heating unit 81 is controlled by energizing 83. In the first heating control and the second heating control, the control unit 100 increases the duty ratio as the difference ΔT increases.

In the second heating control, the control unit 100 sets the upper limit value of the energization amount to the heater 83 per unit time to be smaller than that in the first heating control. Specifically, in the second heating control, the control unit 100 makes the upper limit value of the duty ratio smaller than that in the first heating control.

As an example, when the first heating control is executed, the control unit 100 sets the duty ratio based on the table as shown in FIG. 3A, and when the second heating control is executed, the duty ratio is set. The duty ratio is set based on the table shown in FIG.

For example, based on the tables shown in FIGS. 3A and 3B, the control unit 100 sets the duty ratio to 0% when the difference ΔT is less than 0° C., and the difference ΔT is 0° C. or more and 5° C. When it is less than or equal to less than 30%, the duty ratio is set to 30%. Further, the control unit 100 sets the duty ratio to 40% when the difference ΔT is 5° C. or more and less than 10° C. or more, and sets the duty ratio to 50% when the difference ΔT is 10° C. or more and less than 15° C. Set. Further, the control unit 100 sets the duty ratio to 60% when the difference ΔT is 15° C. or more and less than 20° C. or more, and sets the duty ratio to 70% when the difference ΔT is 20° C. or more and less than 25° C. Set.

As shown in FIG. 3A, the control unit 100 sets the upper limit value of the duty ratio to 100% in the first heating control. Specifically, in the first heating control, the control unit 100 sets the duty ratio to 100% of the upper limit value when the difference ΔT is 25° C. or more based on the table shown in FIG.

On the other hand, as shown in FIG. 3B, the controller 100 sets the upper limit value of the duty ratio to be smaller than 100 in the second heating control. Specifically, the control unit 100 sets the upper limit value of the duty ratio to 70% in the second heating control. Specifically, in the second heating control, the control unit 100 sets the duty ratio to 70% of the upper limit value when the difference ΔT is 20° C. or more based on the table shown in FIG.

In the rotation control, after the first heating control is started, the fixing device 8 is not rotationally driven when the detected temperature T is lower than the third temperature Tth3, and the fixing is performed when the detected temperature T is equal to or higher than the third temperature Tth3. This is the control for driving the container 8 to rotate. Specifically, after starting the first heating control, the control unit 100 does not rotate and drive the heating unit 81 and the pressurizing unit 82 until the detected temperature T reaches the third temperature Tth3. After heating, and after the detected temperature T reaches the third temperature Tth3, the motor M is driven to rotate the heating unit 81 and the pressurizing unit 82 to heat the heating unit 81. The third temperature Tth3 is a temperature lower than a first temperature Tth1 described later and is set in advance. As an example, the third temperature Tth3 is 125°C.

The transport control is control to transport the sheet S toward the developer image forming unit 5 when the detected temperature T becomes equal to or higher than the first temperature Tth1 after starting the first heating control. Specifically, when the detected temperature T becomes equal to or higher than the first temperature Tth1 after starting the first heating control, the control unit 100 transmits the driving force from the motor M to the pickup roller 33A by the clutch mechanism (not shown). Then, the pickup roller 33A is rotationally driven to feed the sheet S toward the developer image forming section 5, and is conveyed toward the developer image forming section 5 by the registration roller 33R or the like. The first temperature Tth1 is lower than the fixing temperature Tf and is set in advance. As an example, the first temperature Tth1 is 170°C.

When a print job including a print instruction or image data to be printed is input to the laser printer 1, the control unit 100 starts the first heating control. Then, the control unit 100 executes the second heating control from a predetermined timing after the first heating control is started until at least the sheet S reaches the fixing device 8.

Specifically, after starting the first heating control, the controller 100 executes the second heating control based on the fact that the detected temperature T becomes equal to or higher than the second temperature Tth2. In the present embodiment, the control unit 100 switches from the first heating control to the second heating control at a timing when a predetermined time tp elapses after the detection temperature T reaches the second temperature Tth2 after starting the first heating control. Switch.

The second temperature Tth2 is a temperature equal to or higher than the first temperature Tth1 and lower than the fixing temperature Tf, and is preset. In the present embodiment, the second temperature Tth2 is equal to the first temperature Tth1. That is, in the present embodiment, the second temperature Tth2 is the first temperature Tth1. The predetermined time tp is shorter than the time from when the detected temperature T reaches the second temperature Tth2 until the sheet S reaches the fixing device 8.

When forming images on a plurality of sheets S continuously, the control unit 100 executes the second heating control until the fixing of the predetermined number of sheets S is completed, and the fixing of the predetermined number of sheets S is performed. After the completion, the first heating control is executed. Specifically, when performing continuous printing on a plurality of sheets S, the control unit 100, after switching from the first heating control to the second heating control, until the fixing of the Nth sheet S is completed. The second heating control is executed, and after the fixing of the Nth sheet S is completed, the second heating control is switched to the first heating control.

More specifically, the control unit 100 causes the sheet detection unit 12 to move when the N-th sheet S discharged from between the heating unit 81 and the pressure unit 82 tilts the swing lever of the sheet detection unit 12. An ON signal is output to the control unit 100, and then the Nth sheet S is conveyed toward the discharge tray 22, whereby the swing of the swing lever of the sheet detection unit 12 is released and the sheet detection unit 12 is released. When the OFF signal is output to the control unit 100, the second heating control is switched to the first heating control. N can be set to any natural number.

In the present embodiment, the control unit 100 executes the second heating control on condition that the detected temperature T0 when the first heating control is started is equal to or lower than the predetermined temperature threshold value Tth0. That is, the control unit 100 does not execute the second heating control when the detected temperature T0 when the first heating control is started is higher than the temperature threshold value Tth0. The temperature threshold Tth0 is a temperature lower than the third temperature Tth3 and is set in advance. As an example, the temperature threshold Tth0 is 50°C.

Next, the operation of the control unit 100 will be described with reference to the flowchart.
As shown in FIG. 4, when a print job is input (START), the control unit 100 acquires the detected temperature T0 at the start of printing (S101) and starts the first heating control (S102).

Next, the control unit 100 determines whether the detected temperature T has become equal to or higher than the third temperature Tth3 (S103). When the detected temperature T is equal to or higher than the third temperature Tth3 (S103, Yes), the control unit 100 drives the motor M to rotate the fixing device 8 (S104).

Next, the control unit 100 determines whether the detected temperature T has become equal to or higher than the first temperature Tth1 (S105). Then, when the detected temperature T becomes equal to or higher than the first temperature Tth1 (S105, Yes), the control unit 100 drives the pressure plate 32 and the transport unit 33 to transport the sheet S toward the developer image forming unit 5. It starts (S106).

Next, the control unit 100 determines whether the detected temperature T0 at the start of printing is equal to or lower than the temperature threshold Tth0 (S107).

When the detected temperature T0 at the start of printing is equal to or lower than the temperature threshold value Tth0 (S107, Yes), the control unit 100 sets the predetermined time tp from the time when the detected temperature T becomes the first temperature Tth1 (=the second temperature Tth2). It is determined whether the time has passed (S108). Then, when the predetermined time tp has elapsed (S108, Yes), the control unit 100 starts the second heating control (S109).

If the detected temperature T0 at the start of printing is higher than the temperature threshold Tth0 in step S107 (No), the controller 100 fixes the sheet S by the first heating control, and proceeds to step S113.

After starting the second heating control in step S109, the control unit 100 determines whether to perform continuous printing (S110). Then, when performing continuous printing (S110, Yes), the control unit 100 determines whether the fixing of the Nth sheet S is completed (S111). Then, when the fixing of the Nth sheet S is completed (S111, Yes), the control unit 100 starts the first heating control (S112), and the sheet S after the Nth sheet is subjected to the first heating control. Fix it.

When the continuous printing is not performed in step S110 (No), the control unit 100 fixes the sheet S by the second heating control.

Then, when printing is completed in step S113 (Yes), the control unit 100 ends the process (END).

Next, the operation of the control unit 100 and the operational effects of the laser printer 1 according to this embodiment will be described with reference to a time chart.
As shown in FIG. 5, when a print job is input at time t0, the control unit 100 starts the first heating control. As a result, the temperature of the heating section 81 (detection temperature T) rises.

After that, at time t1, when the detected temperature T reaches the third temperature Tth3, the control unit 100 rotationally drives the fixing device 8. By rotating the fixing device 8, the heat of the heating part 81 is easily transferred to the pressurizing part 82, so that the gradient of the temperature rise of the heating part 81 is slightly reduced.

After that, when the detected temperature T reaches the first temperature Tth1 at time t2, the control unit 100 starts the conveyance of the sheet S toward the developer image forming unit 5. Since the difference ΔT obtained by subtracting the detection temperature T from the fixing temperature Tf is sufficiently large from the start of the first heating control to the start of the conveyance of the sheet S (time t0 to t2), the duty ratio Is set to 100% which is the upper limit value in the first heating control.

After that, at time t3, the predetermined time tp elapses from the time (time t2) when the detected temperature T reaches the first temperature Tth1 (=second temperature Tth2). At this time, the control unit 100 starts the second heating control because the detected temperature T0 at the start of printing (time t0) is equal to or lower than the temperature threshold Tth0. Since the upper limit value of the duty ratio in the second heating control is 70%, when the second heating control is started at time t3, the duty ratio is changed from 100% to 70%.

Here, if the first heating control continues after time t3, the duty ratio remains 100% for a while until time t31 as indicated by the chain double-dashed line, so that the temperature of the heating unit 81 increases. As shown by, there is a case where the temperature continues to rise rapidly and an overshoot that greatly exceeds the fixing temperature Tf occurs (see around time t41).

Further, in such a situation where overshooting occurs, heating of the heating unit 81 is stopped (duty ratio becomes 0%) for a while after time t32 before the sheet S enters the fixing device 8. When the sheet S enters the fixing device 8 at time t4, heat may be taken by the sheet S and the temperature of the heating unit 81 may drop sharply, causing an undershoot that is much lower than the fixing temperature Tf (time t42). See near).

On the other hand, according to the present embodiment, from the time t3, by performing the second heating control in which the upper limit value of the duty ratio is 70% which is smaller than 100%, the temperature of the heating unit 81 is moderated after the time t3. Can be raised. As a result, it is possible to suppress the occurrence of overshoot during warm-up in which the temperature of the heating unit 81 is heated toward the fixing temperature Tf.

Further, until the sheet S enters the fixing device 8, specifically, before and after the time t4, the duty ratio becomes larger than 0%, and the heating of the heating unit 81 can be continued. As a result, at time t4, it is possible to prevent the temperature of the heating unit 81 from rapidly decreasing even if heat is taken by the sheet S when the sheet S enters the fixing device 8, so that the occurrence of undershoot occurs. Can be suppressed.

Further, by suppressing the overshoot and the undershoot, the fluctuation range of the temperature of the heating unit 81 becomes smaller after the time t4, so that the ratio of the time when the duty ratio becomes 0% becomes smaller.

Further, when the detected temperature T0 when the first heating control is started (time t0) is as low as the temperature threshold value Tth0 or less, that is, when the temperature is low, the temperature of the heating unit 81 rapidly increases when the heating unit 81 is heated. Since it is easy, overshoot is likely to occur, but in such a case, by performing the second heating control, it is possible to effectively suppress the occurrence of overshoot and undershoot during warm-up.

Further, in the present embodiment, when performing continuous printing on a plurality of sheets S, up to the Nth sheet S, fixing is performed by the second heating control by limiting the energization amount per unit time to the heater 83. Since it can be performed, the power consumption of the heater 83 can be suppressed. On the other hand, the first heating control is started after fixing the Nth sheet S, and the restriction on the upper limit value of the energization amount to the heater 83 per unit time is released, so that the insufficient heating of the heating unit 81 is suppressed. Therefore, it is possible to prevent the fixing failure from occurring in the sheet S after the Nth sheet.

Although the embodiment has been described above, the present invention is not limited to the embodiment. The specific configuration can be changed as appropriate without departing from the spirit of the invention.
For example, in the above-described embodiment, the duty ratio according to the difference ΔT is set based on the table as shown in FIG. 3, but a method such as PI control or PID control is used from the deviation between the fixing temperature and the detected temperature. The duty ratio (heater operation amount) may be set.

Further, in the above embodiment, in the case of performing continuous printing, the second heating control is executed until the fixing of the Nth sheet S is completed, and the first heating control is performed after the fixing of the Nth sheet S is completed. Although the heating control is executed, in the case where the continuous printing is performed, for example, after the second heating control is started, the second heating control may be continuously executed until the printing is completed.

Further, in the above-described embodiment, after the first heating control is started, the second heating control is executed at the timing when the predetermined time tp elapses from the time when the detected temperature T becomes the second temperature Tth2. The second heating control may be executed at the timing when the temperature reaches the second temperature. Further, in the above-described embodiment, the second temperature Tth2 is equal to the first temperature Tth1, but the second temperature may be higher than the first temperature.

Further, in the above embodiment, in the rotation control, the fixing device 8 is rotationally driven by driving the motor M. However, in the rotation control, for example, the driving force from the motor is transmitted to the fixing device 8 by the clutch mechanism. The fixing device 8 may be driven to rotate.

Further, in the above-described embodiment, the temperature detection unit 9 is provided so as to detect the temperature of the heating unit 81, but the temperature detection unit is provided so as to detect the temperature of the pressurizing unit, the heater, or the like. It may be. Further, the temperature detection unit may be a temperature sensor other than the thermistor. Further, the temperature sensor may be a non-contact type temperature sensor or a contact type temperature sensor.

Further, although the heating roller is exemplified as the heating unit 81 in the above-described embodiment, the heating unit may be, for example, a heating unit having an endless heating belt. Further, in the above-described embodiment, the pressure roller is illustrated as the pressure unit 82, but the pressure unit may be, for example, a pressure unit including an endless pressure belt.

Further, in the above-described embodiment, the halogen heater that uses radiant heat is illustrated as the heater 83, but the heater may be, for example, a ceramic heater or a carbon heater that uses heat generated by a resistor. Further, the heater may be arranged outside the heating unit instead of inside the heating unit.

Further, in the above-described embodiment, the laser printer 1 that forms a monochrome image on the sheet S is exemplified as the image forming apparatus, but the image forming apparatus is, for example, a printer configured to form a color image on the sheet. It may be. Further, the image forming apparatus is not limited to the printer, and may be, for example, a copying machine or a multi-function machine including a document reading device such as a flatbed scanner.

Further, in the above-described embodiment, the process cartridge is illustrated as the developer image forming unit 5, but the developer image forming unit 5 is not limited to this. For example, an image forming apparatus is capable of forming a color image on a sheet and has a process unit having a plurality of photoconductors arranged, and a transfer unit for transferring the developer image formed on the photoconductors to the sheet. In the case of including a transfer unit having a belt or the like, the developer image forming unit can be configured to include a process unit and a transfer unit.

In addition, the respective elements described in the above-described embodiment and modification can be implemented in an appropriate combination.

1 Laser Printer 5 Developer Image Forming Section 8 Fixing Unit 9 Temperature Detection Section 33 Conveying Section 81 Heating Section 82 Pressurizing Section 83 Heater 100 Control Section S Sheet

Claims (7)

A developer image forming portion for forming a developer image on the sheet;
A transport unit that transports the sheet toward the developer image forming unit;
A heating unit, a heater that heats the heating unit, and a fixing unit that has a pressing unit that sandwiches the sheet between the heating unit and the fixing unit that fixes the developer image on the sheet,
A temperature detection unit for detecting the temperature of the heating unit,
And a control unit,
The control unit is
First heating control for energizing the heater so that the temperature of the heating unit becomes a fixing temperature for fixing the developer image on the sheet based on the temperature detected by the temperature detecting unit;
It is the second heating control for energizing the heater so that the temperature of the heating unit becomes the fixing temperature based on the detected temperature, wherein the upper limit value of the energization amount per unit time to the heater is the first value. A second heating control that is smaller than the heating control,
Transport control for transporting the sheet toward the developer image forming unit when the detected temperature is equal to or higher than a first temperature lower than the fixing temperature, and
An image forming apparatus, wherein the second heating control is executed at least until a sheet reaches the fixing device from a predetermined timing after the first heating control is started. The control unit is
In the first heating control and the second heating control, the temperature of the heating unit is controlled with a duty ratio according to the difference between the fixing temperature and the detected temperature, and the duty ratio is increased as the difference is increased,
In the first heating control, the upper limit of the duty ratio is 100%,
The image forming apparatus according to claim 1, wherein an upper limit value of the duty ratio is set to be smaller than 100% in the second heating control. When continuously forming images on a plurality of sheets, the control unit executes the second heating control until the fixing of the predetermined number of sheets is completed, and the fixing of the predetermined number of sheets is completed. The image forming apparatus according to claim 1, wherein the first heating control is executed after the heating. The control unit executes the second heating control on condition that the detected temperature when the first heating control is started is equal to or lower than a predetermined temperature threshold. The image forming apparatus according to any one of 3 above. The control unit executes the second heating control based on that the detected temperature is equal to or higher than a second temperature lower than the fixing temperature after starting the first heating control. The image forming apparatus according to any one of claims 1 to 4. The image forming apparatus according to claim 5, wherein the second temperature is equal to or higher than the first temperature. The controller does not rotate the fixing device when the detected temperature is lower than the third temperature which is lower than the first temperature, and the fixing device when the detected temperature is equal to or higher than the third temperature. The image forming apparatus according to any one of claims 1 to 6, characterized in that it is capable of executing a rotation control for rotating and driving.

Images