JP4332397B2 - Fixing device - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine, and more particularly to temperature control of a fixing device used in the image forming apparatus.

  Generally, in an image forming apparatus, in order to fix an unfixed toner image transferred on a recording sheet onto the recording sheet, a fixing unit using a heat fixing method is used. In this type of fixing unit, a fixing roller is used. The outer surface temperature of the (heating roller) is detected, and a halogen lamp, which is a heating element, is turned on and off to control the outer surface temperature of the fixing roller to the fixing temperature (predetermined temperature).

  The temperature detection element (temperature sensor) is arranged in contact with the fixing roller outside the sheet passing area of the fixing roller (outside the maximum sheet passing size) in consideration of the sheet passing width of the recording paper. As described above, when the temperature detecting element is disposed outside the paper region, when the fixing roller is heated to the fixing temperature, a temperature difference occurs between the paper passing region (particularly the central portion) and the temperature of the fixing roller ( It is difficult to stabilize the temperature in the paper passing area.

For this reason, when the temperature of the fixing roller is raised to a predetermined fixing temperature, the temperature (T0) of the fixing roller at the start of temperature increase detected by the temperature detecting element is compared with a predetermined threshold (T ^* ) set in advance. In some cases, when the temperature T0 is lower than the threshold value T ^* , a heating element such as a halogen lamp is driven with a smaller amount of heat generation than when the temperature T0 is higher than the threshold value T ^* (see Patent Document 1).

In the fixing device described in Patent Document 1, when the detected temperature T0 is compared with the threshold value T ^* and the detected temperature T0 is higher than the threshold value T ^* , the heating element is fully turned on to raise the temperature of the fixing roller. . In this case, since the temperature difference between the detection temperature T0 and the fixing temperature is small and the heat generation time until the fixing temperature is short, the temperature difference between the temperature detection element and the sheet passing area is within a predetermined range even if the heat generation amount is increased. Will be paid.

On the other hand, when the detected temperature T0 is lower than the threshold value T ^* , the heat generation amount of the heating element is decreased. In this case, although the temperature difference between the detection temperature T0 at the start of temperature rise and the fixing temperature is large and the heat generation time to the fixing temperature is long, the heat generation amount of the heating element is small, so the temperature detection element when the fixing temperature is reached And the temperature difference between the sheet passing area and the sheet passing area are maintained within a predetermined range.

  That is, in Patent Document 1, when the temperature raising operation of the fixing roller is performed, the temperature difference between the sheet passing area and the portion where the temperature detecting element is arranged is always maintained within a predetermined range, and overshoot occurs in the sheet passing area. I try not to.

Japanese Patent No. 3347375 (paragraph [0017] to paragraph [0026], FIGS. 1 to 5)

  By the way, in order to shorten the time required to raise the temperature of the fixing device to the fixing temperature (warm-up time), it is necessary to increase the heat generation amount of the heating element as the temperature rise start temperature is lower. As described above, in the fixing device described in Patent Document 1, if the detected temperature at the start of temperature rise is lower than a predetermined threshold, the amount of heat generated by the heating element is reduced, so that the temperature rise start temperature is low. There is a problem that the warm-up time becomes extremely long.

  On the other hand, as described above, if the amount of heat generated by the heating element is increased in order to shorten the warm-up time, overshoot will occur in the sheet passing area, and it becomes difficult to stabilize the temperature of the fixing roller. End up.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a fixing device in which the warm-up time is shortened regardless of the temperature rise start temperature and overshoot does not occur in the sheet passing area. To do.

Accordingly, in order to solve such a problem, the present invention provides a fixing unit that heat-fixes an unfixed image on a recording medium in which a recording medium passage area and a recording medium non-passage area are defined and passes through the recording medium passage area, In the fixing device, comprising: a temperature detection unit that detects the temperature of the fixing unit as a detection temperature in a recording medium non-passing region; and a heating control unit that performs heating control of the fixing unit according to the detected temperature. Is set with a first fixing start temperature and a second fixing start temperature higher than the first fixing start temperature, and the fixing means is provided with a heating element having a non-uniform temperature distribution in the longitudinal direction. , when performing the heating control of the fixing means hitting the forming an image wherein the heating control means, the detection temperature at the start of temperature increase by comparing whether a predefined threshold temperature, the Inspection When the temperature is equal to or lower than the threshold temperature, heating control of the fixing unit is performed so that the detected temperature becomes the first fixing start temperature, and when the detected temperature exceeds the threshold temperature, the detected temperature is Heating control of the fixing unit is performed so as to reach the second fixing start temperature.

Furthermore, in the present invention, a recording medium passing area and a recording medium non-passing area are defined, a fixing unit that heat-fixes an unfixed image on the recording medium passing through the recording medium passing area, and the recording medium non-passing area In the fixing device, which includes a temperature detection unit that detects the temperature of the fixing unit as a detection temperature, and a heating control unit that performs heating control of the fixing unit according to the detected temperature, the heating control unit includes a first standby temperature. And a second standby temperature higher than the first standby temperature is set, and the fixing unit is provided with a heating element having a non-uniform temperature distribution in the longitudinal direction so that the fixing unit is in a standby state. The heating control means compares whether or not the detected temperature at the start of temperature rise is a predetermined threshold temperature, and determines whether the detected temperature is equal to or lower than the threshold temperature. Characterized in that the detected temperature has to perform the heating control of the fixing means such that the first standby temperature.

  In this case, when the detected temperature exceeds the threshold temperature, the heating control unit performs heating control of the fixing unit so that the detected temperature becomes the second standby temperature.

  Further, in the present invention, the heating control unit is set with a first fixing start temperature and a second fixing start temperature higher than the first fixing start temperature, and the detected temperature is the first standby temperature. When the image formation is started before the temperature is reached, the heating control unit controls the heating of the fixing unit so that the detected temperature becomes the first fixing start temperature.

  Further, if the image formation does not start after the detected temperature reaches the first standby temperature, the heating control unit controls the heating of the fixing unit so that the detected temperature becomes the second standby temperature. When the image formation is started before the detected temperature reaches the second standby temperature, the heating control unit causes the second fixing to be performed when the detected temperature is equal to or higher than the first fixing start temperature. Heating control of the fixing unit is performed so that the third fixing start temperature is equal to or lower than the start temperature. When the image formation is started after reaching the second standby temperature, the heating control unit controls the heating of the fixing unit so that the detected temperature becomes the second fixing start temperature. .

  In addition, in the present invention, when the image formation is started during the heating control to the second standby temperature, the heating control unit causes the detected temperature to be the second fixing start temperature. Heating control of the fixing unit is performed.

  As described above, the fixing device according to the present invention compares whether or not the detected temperature at the start of the temperature rise is a predetermined threshold temperature when performing heating control of the fixing unit during image formation. When the detected temperature is equal to or lower than the threshold temperature, heating control of the fixing unit is performed so that the detected temperature becomes the first fixing start temperature. If the detected temperature exceeds the threshold temperature, the detected temperature becomes the second temperature. Since the heating of the fixing unit is controlled so as to reach the fixing start temperature, the temperature of the recording medium passage area can be made substantially the same regardless of the temperature at the start of the temperature rise, and the temperature of the recording medium passage area is overshooted. And it can prevent rising more than necessary.

Further, in the present invention, when the fixing unit is set to the standby state, it is compared whether or not the detected temperature at the start of temperature rise is a predetermined threshold temperature, and when the detected temperature is equal to or lower than the threshold temperature, the detected temperature Is controlled so as to reach the first standby temperature, and when the detected temperature exceeds the threshold temperature, the fixing means is controlled so that the detected temperature becomes the second standby temperature. since the, regardless of the temperature at the start of heating, the temperature of the recording medium passing area (standby temperature) there is an effect that can be made substantially the same. Furthermore, the warm-up time can be shortened when the detected temperature is equal to or lower than the threshold temperature.

  In the present invention, when the image formation is started before the detected temperature reaches the first standby temperature, the fixing unit is heated so that the detected temperature becomes the first fixing start temperature. If the image formation does not start after the temperature reaches the first standby temperature, the heating control means controls the fixing means so that the detected temperature becomes the second standby temperature. When the image formation is started before the standby temperature of 2 is reached, the heating control means causes the detected temperature to become a third fixing start temperature not lower than the first fixing start temperature but lower than the second fixing start temperature. When the heating control of the fixing unit is performed and image formation is started after reaching the second standby temperature, the heating control unit controls the heating of the fixing unit so that the detected temperature becomes the second fixing start temperature. Because I'm going Regardless of the timing of the formation start, it is possible to stabilize the temperature of the recording medium passing area substantially the same, there is an effect that it prevents the occurrence of high-temperature offset due to low temperature offset or overshoot.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  FIG. 1 is a diagram schematically showing a first embodiment of a fixing device according to the present invention. In this fixing device, an unfixed toner image transferred to a recording paper is recorded on the recording paper in an image forming apparatus (not shown). Used when fixing. In the image forming apparatus, for example, an electrostatic latent image is formed on a photosensitive drum (not shown) in accordance with image data, and the electrostatic latent image is developed by a developing device (not shown) to form a toner image. Is done. The toner image is transferred to a recording sheet at the transfer position, and the recording sheet is conveyed to a fixing device. In the fixing device, after the toner image on the recording paper is heat-fixed, the recording paper is discharged to a paper discharge tray, for example.

  Referring to FIG. 1, the fixing device 10 includes a fixing unit (fixing unit) 11, and the fixing unit 11 includes a fixing roller (heating roller) 11a and a pressure roller (pressing roller) 11b. The fixing roller 11a and the pressure roller 11b are in contact with each other at their peripheral surfaces, and the pressure roller 11b presses the fixing roller 11a with a predetermined pressure. The fixing roller 11a and the pressure roller 11b are rotationally driven in the direction indicated by the broken line arrow, whereby the recording paper P is conveyed in the direction indicated by the solid line arrow and passes through the nip portion between the fixing roller 11a and the pressure roller 11b. In this case, the unfixed toner K on the recording paper P is fixed and becomes a fixed toner K ′.

  The fixing roller 11a and the pressure roller 11b extend in a predetermined direction (in FIG. 1, a direction extending from the front side to the back side of the paper surface: a longitudinal direction), and the peripheral surface of the fixing roller 11a passes along the longitudinal direction. An area (recording medium passing area) and a non-sheet passing area (non-recording medium passing area) are defined, and the non-sheet passing area is positioned on both ends in the longitudinal direction of the sheet passing area. During fixing, the recording paper P passes through the nip portion between the fixing roller 11a and the pressure roller 11b on the paper passing area. The sheet passing area is set to the maximum sheet size that can be used by the image forming apparatus.

  As shown in FIG. 1, a heating element (for example, a halogen heater, a ceramic heater, a nichrome wire resistor, or an electromagnetic induction coil) 12 extending in the longitudinal direction along the central axis is disposed in the fixing roller 11a. The heating element 12 is connected to an AC power supply 13 via a power supply line 13a. The heating element 12 generates heat by the power supplied from the AC power supply 13, and the fixing roller 11a is heated. A switching element (for example, triac) 14 is disposed on the power line 13a, and the switching element 14 is controlled to be turned on and off as described later, so that power is selectively supplied to the heating element 12.

  As shown in FIGS. 1 and 2, a temperature detection element (temperature sensor: for example, a thermistor element) 15 is disposed in the non-sheet passing region in contact with the outer peripheral surface of the fixing roller 11a. A control circuit (CPU) 16 is connected. A switching element control circuit (hereinafter simply referred to as a control unit) 17 is connected to the CPU 16. As will be described later, the CPU 16 issues a heating element heat generation command to the control unit 17 in accordance with the detected temperature detected by the temperature sensor 15. The controller 17 controls the switching element 14 on and off based on the heating element heat generation command.

  When the fixing roller 11a is heated by the heating element 12, the temperature distribution shown in FIG. 3 is generated in the longitudinal direction of the fixing roller 11a, and the temperature detection element 15 is disposed in the paper passing region, although the temperature is substantially constant. In the portion (non-sheet passing area), the temperature is lower than that in the sheet passing area, and a temperature difference is generated between the sheet passing area and the non-sheet passing area. That is, the detected temperature detected by the temperature detecting element is different from the temperature of the sheet passing area.

Here, referring to FIG. 4, when the image forming apparatus has an input signal (for example, power-on of the image forming apparatus or image formation start) (step S1), the CPU 16 inputs the input signal as a print start signal. If it is a print start signal (image formation start), the CPU 16 starts temperature increase (step S3). When the temperature raising start is executed, the CPU 16 takes in the detected temperature T from the temperature sensor 15 (step S4), and refers to a preset threshold temperature T ^* (for example, about 80 ° C.) (step S5). It is determined whether or not the temperature T is equal to or lower than a threshold temperature T ^* (step S6).

If T ≦ T ^* , the CPU 16 sets the fixing start temperature to Tpl (first fixing start temperature preset in the CPU 16) (step S7). On the other hand, if T> T ^* , the CPU 16 sets the fixing start temperature to Tpu (second fixing start temperature preset in the CPU 16) (step S8). Here, Tpu> Tpl, and for example, Tpl is a temperature 20 ° C. lower than Tpu (the fixing start temperature Tpu is about 180 ° C.).

In this way, the CPU 16 selectively sets the fixing start temperature Tpl or Tpu according to the comparison result between the detected temperature T and the threshold temperature T ^*, and controls the switching element 14 on / off via the control unit 17. Then, the fixing roller 11a is heated by the heating element 12, and the fixing is started when the detected temperature T reaches the fixing start temperature Tpl or Tpu (in this case, the fixing start means that the leading end of the recording paper is the fixing nip portion). Is reached when the heating and pressurizing process is started).

Referring to FIG. 5, when the threshold temperature T ^* is 80 ° C., the fixing temperature Tpu is 180 ° C., and the detected temperature at the start of temperature rise is T0 (T0 <T ^* ), as described above, The fixing start temperature is set to Tpl, and the fixing roller 11a is heated. As a result, the detected temperature (fixing roller temperature) detected by the temperature sensor 15 increases from the temperature T0 as indicated by the solid line M1, and reaches the point B (fixing start temperature Tpl). Then, when the detected temperature T reached point B, fixing was started. At this time, when the temperature in the paper passing region was measured, the temperature increased as indicated by the alternate long and short dash line N1 and reached point B ′.

On the other hand, when the detected temperature at the start of temperature rise is T0 ′ (T0 ′> T ^* ), as described above, the fixing start temperature is set to Tpu, and the fixing roller 11a is heated. As a result, the detected temperature (fixing roller temperature) detected by the temperature sensor 15 increases from the temperature T0 ′ as indicated by the solid line M2, and reaches the point A (fixing start temperature Tpu). Then, when the detected temperature T reached point A, fixing was started. At this time, when the temperature in the paper passing region was measured, the temperature increased as indicated by a one-dot chain line N2 and reached point A ′. When the temperatures at points B ′ and A ′ were measured, they were substantially the same, the temperature difference ΔT ′ between point A and point A ′ was 30 ° C., and the temperature difference ΔT between point B and point B ′ was It was 50 ° C.

As described above, the temperatures of the points A ′ and B ′ (that is, the temperature of the sheet passing area) are substantially the same, and as a result, no overshoot or the like occurs even if the temperature rise start temperature is different. In other words, if the detection temperature T ≦ the threshold temperature T ^* , the temperature difference between the detection temperature T and the sheet passing area becomes large because the heat generation time of the heating element becomes long. Therefore, the fixing start temperature is set to Tpl (Tpl <Tpu), the fixing start temperature is lowered, and the temperature of the sheet passing area is prevented from overshooting and increasing more than necessary. Furthermore, if the fixing start temperature is lowered, it is possible to shorten the time from the input of the print start signal to the end of fixing.

On the other hand, if the detected temperature T> the threshold temperature T ^* , the temperature difference between the detected temperature T and the temperature of the sheet passing area becomes smaller because the heat generation time of the heat generating element 12 can be shortened. Accordingly, the temperature of the sheet passing area does not rise too much due to overshoot, and the fixing start temperature is set to Tpu, and the temperature of the sheet passing area at the start of fixing is substantially the same regardless of the detected temperature T at the start of raising the temperature. Can be the same.

Here, for comparison, referring to FIG. 6, FIG. 6 is an example of heating the fixing roller 11a and controls the detected temperature T with the target temperature (fixation starting temperature) T _STB, 6, one point chain line Represents the temperature rise in the paper passing area, and the solid line represents the temperature rise of the detected temperature T. In the example shown in FIG. 6, if the temperature at the time of starting temperature rise is T0, the detected temperature T (point Q) when the detected temperature T reaches the target temperature _TSTB and the temperature of the sheet passing area (point Q ′). ΔT is large, and if the temperature at the start of temperature rise is T0 ′, the detected temperature T when the detected temperature T reaches the target temperature _TSTB (point R) and the temperature of the sheet passing area (point R ′) ΔT ′ is small.

In other words, the temperature of the sheet passing area differs depending on the detected temperature at the start of the temperature rise (that is, the temperature difference increases as the time to reach the target temperature T _STB increases), and fixing is caused by this temperature difference. In some cases, the amount of heat required for the heat treatment becomes insufficient and the temperature is offset, or conversely, the temperature is too high and a high temperature offset occurs.

Referring to FIG. 4 again, when it is determined in step S2 that the input signal is not a print start signal (for example, power on), the CPU 16 executes the first subroutine (* 2). Referring to FIG. 7, first, the CPU 16 starts to raise the temperature (step S9), the CPU 16 takes in the detected temperature T from the temperature sensor 15 (step S10), and refers to a preset threshold temperature T ^* ( Step S11), it is determined whether or not the detected temperature T is equal to or lower than the threshold temperature T ^* (Step S12).

If T ≦ T ^* , the CPU 16 sets the standby temperature to Trl (stable temperature: a first standby temperature preset in the CPU 16) (step S13). On the other hand, if T> T ^* , the CPU 16 sets the stable temperature to Tru (second standby temperature preset in the CPU 16) (step S14). Here, Tru> Trl, and for example, Trl is a temperature 20 ° C. lower than Tru (the stable temperature Tru is 120 ° C. to 130 ° C.).

In this way, the CPU 16 selectively sets the stable temperature Trl or Tru according to the result of comparing the detected temperature T and the threshold temperature T ^*, and controls the switching element 14 on and off via the control unit 17. The fixing roller 11a is heated by the heating element 12. That is, heating control is performed. When the stable temperature is set to Trl and the heating control is performed, the CPU 16 monitors whether or not a print start signal is input (step S15), and the detected temperature T reaches the stable temperature Trl. If a print start signal is input before, the CPU 16 sets the fixing start temperature to Tpl (step S16), and controls the heating of the fixing roller 11a as described above.

  On the other hand, if the print start signal is not input before the detected temperature T reaches the stable temperature Trl, the CPU 16 executes a second subroutine to be described later.

  Similarly, when the stable temperature is set to Tru and heating control is performed, the CPU 16 monitors whether or not a print start signal has been input (step S17), and the detected temperature T is the stable temperature Tru. If the print start signal is input before reaching the value, the CPU 16 sets the fixing start temperature to Tpu (step S18), and controls the heating of the fixing roller 11a.

On the other hand, the detection when the temperature T is no input of the print start signal before reaching a stable temperature Tr u, CPU 16 has the detected temperature T off controls the switching elements 14 to maintain a stable temperature Tru (stability control: step S19 ), Whether or not a print start signal has been input is monitored (step S20). When the print start signal is input, the CPU 16 sets the fixing start temperature to Tpu (step S21), and controls the heating of the fixing roller 11a.

Here, referring to FIG. 5, when the detected temperature at the start of temperature rise is T0 (T0 <T ^* ), as described above, the stable temperature is set to Trl, and the fixing roller 11a is heated. Is called. As a result, the detected temperature (fixing roller temperature) detected by the temperature sensor 15 increased from the temperature T0 as indicated by the solid line M1, and reached a point D (stable temperature Trl). At this time, when the temperature in the paper passing region was measured, the temperature increased as indicated by the alternate long and short dash line N1 and reached point D ′.

On the other hand, when the detected temperature at the start of temperature rise is T0 ′ (T0 ′> T ^* ), as described above, the stable temperature is set to Tru, and the fixing roller 11a is heated. As a result, the detected temperature detected by the temperature sensor 15 rises from the temperature T0 ′ as indicated by the solid line M2, and reaches a point C (stable temperature Tru). At this time, when the temperature in the paper passing region was measured, the temperature increased as indicated by a one-dot chain line N2 and reached point C ′. When the temperatures at points D ′ and C ′ were measured, they were substantially the same, the temperature difference ΔTr ′ between points C and C ′ was 15 ° C., and the temperature difference ΔTr between points D and D ′ was It was 35 ° C.

As described above, the temperatures of the points C ′ and D ′ (that is, the stable temperature of the sheet passing region) are substantially the same, and even when the temperature at the start of the temperature rise is low, the warm-up time does not become long. In practice, the warm-up time can be shortened. When the detected temperature is T ≦ threshold temperature T ^*, on the relationship between heating time of the heating element increases, the temperature difference between the detected temperature T and the paper feeding area is increased. For this reason, the stable temperature is set to Trl (Trl <Tru), and the stable temperature is lowered.

On the other hand, if the detection temperature T> the threshold temperature T ^* , the temperature difference between the detection temperature T and the temperature of the sheet passing area is small because the heat generation time of the heating element can be short. Therefore, the stable temperature can be set to Tru, and the stable temperature of the sheet passing area can be made substantially the same regardless of the detected temperature T at the start of the temperature rise.

Here, for comparison, referring to FIG. 8, FIG. 8 is an example of heating the fixing roller 11a and controls the detected temperature T to a stable temperature (set stable temperature), 8, one point chain line passing The temperature rise in the paper region is represented, and the solid line represents the temperature rise of the detected temperature T. In the example shown in FIG. 8, if the temperature at the time of starting temperature rise is T0, the difference ΔTr between the detected temperature T and the temperature of the paper passing area is large when the detected temperature T reaches the stable temperature. As the temperature of the paper region decreases, the temperature difference ΔTr 2 becomes smaller and becomes a temperature difference corresponding to the stable temperature. Thus, in the example shown in FIG. 8, it takes time to warm up.

  Referring to FIG. 7 again, in step S15, if the print start signal is not input before reaching the stable temperature Tr1, the CPU 16 executes the second subroutine as described above (* 3). Referring to FIG. 9, when the detected temperature T reaches the stable temperature Trl (step S22), the CPU 16 operates the built-in timer 16a (FIG. 1) to start timing, and controls the heating of the fixing roller 11a. To increase the detection temperature T. For example, the CPU 16 increases the detected temperature T according to the stable temperature Tr = at + Trl (a is a predetermined constant, t is a timer count) (Trl ≦ Tr ≦ Tru: Step S23).

  The CPU 16 monitors whether or not a print start signal has been input during the above-described heating control (step S24). If the print start signal is input before reaching the stable temperature Tru, the heating control of the fixing roller 11a is performed. The detected temperature T is raised. For example, the CPU 16 sets the fixing start temperature Tp (third fixing start temperature) = b + Tpl (where b is a constant defined according to the stable temperature Tr or the timer count when the print start signal is input). Then, the fixing roller 11a is heated to the fixing start temperature Tp (Tpl ≦ Tp ≦ Tpu: step S25). Then, when the detected temperature T reaches the fixing start temperature Tp, fixing is started.

  On the other hand, if no print start signal is input before reaching the stable temperature Tru in step S24, the CPU 16 performs on / off control of the switching element 14 so as to maintain the detected temperature T at the stable temperature Tru (stability control: step S26). It is monitored whether a start signal is input (step S27). When a print start signal is input, the CPU 16 sets the fixing start temperature to Tpu (step S28), and controls the heating of the fixing roller 11a.

  Here, referring to FIG. 5, when the heating control described in step S23 is performed after the detected temperature T reaches the point D (stable temperature Trl), the detected temperature T reaches the point D1, and printing is performed. If there is no input of the start signal, the heating control is performed so as to stabilize at the point D2. At this time, the temperature of the fixing roller 11a (FIG. 1) hardly increased in the central portion (sheet passing region), and stabilized at the points D1 'and D2'. The temperature difference ΔTr1 between the point D1 and the point D1 ′ was 25 ° C., and the temperature difference ΔTr2 between the point D2 and the point D2 ′ was 15 ° C.

  On the other hand, when a print start signal is input in step S23, the detected temperature T rises from the point D1 (stable temperature Tr) to the point B1 (fixing start temperature Tp). At this time, the temperature of the paper passing area is measured. The temperature increased as indicated by the alternate long and short dash line N3 and reached point B1 ′. The temperature difference ΔT1 between the point B1 and the point B1 ′ was 40 ° C.

  When a print start signal is input in step S27, the detected temperature T rises from point D2 (stable temperature Tru) to point B2 (fixing start temperature Tpu). At this time, the temperature of the sheet passing area is measured. The temperature rose as indicated by the alternate long and short dash line N4 and reached point B2 ′. The temperature difference ΔT2 between the point B2 and the point B2 ′ was 30 ° C. Further, when the temperatures of the points A ', B', B1 'and B2' were compared, they were substantially the same.

  Thus, after reaching the stable temperature Trl, the temperature of the sheet passing area can be stabilized substantially the same regardless of the input timing (timing) of the print start signal, and the high temperature due to the low temperature offset or overshoot. Occurrence of offset or the like can be prevented.

  As is clear from the above description, the heating element 12, the AC power supply 13, the switching element 14, the temperature detecting element 15, the CPU 16, and the switching element control circuit 17 collectively function as a heating control means.

  When performing heating control of the fixing roller 11a when performing image formation, the detected temperature is compared with whether or not the detected temperature of the fixing roller 11a at the start of temperature rise is a predetermined threshold temperature. When the temperature is equal to or lower than the temperature, the heating control of the fixing roller 11a is performed so that the detected temperature becomes the first fixing start temperature. If the detected temperature exceeds the threshold temperature, the detected temperature is higher than the first fixing start temperature. Since the heating control of the fixing roller 11a is performed so as to reach a high second fixing start temperature, the temperature of the sheet passing area can be made substantially the same regardless of the temperature at the start of the temperature increase. As a result of preventing over-shooting and unnecessarily rising, the present invention can be applied to a fixing device in an image forming apparatus such as a copying machine, a printer, or a facsimile machine.

1 is a block diagram schematically illustrating a first embodiment of a fixing device according to the present invention. FIG. FIG. 2 is a diagram illustrating a position of a temperature detection element in the fixing device illustrated in FIG. 1. FIG. 2 is a view showing a temperature distribution in a longitudinal direction of a fixing roller by heating of a heating element in the fixing device shown in FIG. 4 is a flowchart for explaining heating control in the fixing device shown in FIG. 1. FIG. 2 is a diagram for explaining an increase in a detected temperature and a temperature increase in a sheet passing area in temperature control in the fixing device shown in FIG. 1. FIG. 5 is a diagram for explaining a detected temperature increase and a temperature increase in a sheet passing area during temperature control in comparison with Example 1 (first comparative example). 4 is a flowchart for explaining heating control in the fixing device shown in FIG. 1. It is a figure for demonstrating the detection temperature rise in the case of temperature control, and the temperature rise of a paper passing area | region compared with Example 1 (2nd comparative example). 4 is a flowchart for explaining heating control in the fixing device shown in FIG. 1.

10 Fixing Device 11 Fixing Unit 11a Fixing Roller (Heating Roller)
11b Pressure roller (pressure roller)
12 Heating element 13 AC power supply 14 Switching element 15 Temperature detection element (temperature sensor)
16 Main control circuit (CPU)
17 Switching element control circuit

Claims (8)

A recording medium passing area and a recording medium non-passing area are defined, a fixing means for heating and fixing an unfixed image on the recording medium passing through the recording medium passing area, and a temperature of the fixing means is detected in the recording medium non-passing area In a fixing device having temperature detecting means for detecting as temperature, and heating control means for controlling heating of the fixing means according to the detected temperature,
The heating control means is set with a first fixing start temperature and a second fixing start temperature higher than the first fixing start temperature, and the heating means has a non-uniform temperature distribution in the longitudinal direction. Is provided,
When performing heating control of the fixing unit when performing image formation, the heating control unit compares the detected temperature at the start of temperature rise with a predetermined threshold temperature to determine whether the detection is performed. When the temperature is equal to or lower than the threshold temperature, heating control of the fixing unit is performed so that the detected temperature becomes the first fixing start temperature, and when the detected temperature exceeds the threshold temperature, the detected temperature is the second to the the perform heating control of the fixing unit so that the fixing initiation temperature you said constant Chakusochi. A recording medium passing area and a recording medium non-passing area are defined, a fixing means for heating and fixing an unfixed image on the recording medium passing through the recording medium passing area, and a temperature of the fixing means is detected in the recording medium non-passing area In a fixing device having temperature detecting means for detecting as temperature, and heating control means for controlling heating of the fixing means according to the detected temperature,
The heating control means is set with a first standby temperature and a second standby temperature higher than the first standby temperature, and the fixing means is provided with a heating element having a non-uniform temperature distribution in the longitudinal direction. And
When the fixing unit is in a standby state, the heating control unit compares the detected temperature at the start of temperature rise with a predetermined threshold temperature, and the detected temperature is equal to or lower than the threshold temperature. In this case, the fixing device is controlled to be heated so that the detected temperature becomes the first standby temperature.   The heating control unit controls the heating of the fixing unit so that the detected temperature becomes the second standby temperature when the detected temperature exceeds the threshold temperature. Item 3. The fixing device according to Item 2. The heating control unit is set with a first fixing start temperature and a second fixing start temperature higher than the first fixing start temperature,
When the fixing unit is in a standby state, the heating control unit compares the detected temperature at the start of temperature rise with a predetermined threshold temperature, and the detected temperature is equal to or lower than the threshold temperature. When the image formation is started before the detected temperature reaches the first standby temperature, the heating control unit heats the fixing unit so that the detected temperature becomes the first fixing start temperature. The fixing device according to claim 2, wherein control is performed. If the image formation does not start after the detected temperature reaches the first standby temperature,
The fixing device according to claim 4, wherein the heating control unit performs heating control of the fixing unit so that the detected temperature becomes the second standby temperature. When the image formation is started before the detected temperature reaches the second standby temperature,
The heating control means controls the heating of the fixing means so that the detected temperature becomes a third fixing start temperature not less than the first fixing start temperature and not more than the second fixing start temperature. The fixing device according to claim 5, wherein: When the image formation is started after reaching the second standby temperature,
The fixing device according to claim 5, wherein the heating control unit controls the heating of the fixing unit so that the detected temperature becomes the second fixing start temperature. The heating control unit is set with a first fixing start temperature and a second fixing start temperature higher than the first fixing start temperature,
When image formation is started before or after reaching the second standby temperature, the heating control unit performs heating control of the fixing unit so that the detected temperature becomes the second fixing start temperature. The fixing device according to claim 3, which is configured as described above.

Images