JP2017161739A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the occurrence of image defects while suppressing the generation of UFPs by reducing a fixing temperature at the end of printing in an image forming apparatus of an electrophotographic system.SOLUTION: There is provided an image forming apparatus comprising: fixing means that thermally fixes a toner image transferred from an image carrier onto a sheet by passing the toner image through a fixing nip; and control means that controls a printing operation including the operation of the fixing means. The control means performs control of reducing a fixing temperature immediately before the end of printing and compensating for a time for the sheet to pass through the fixing nip.SELECTED DRAWING: Figure 12

Description

  The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus such as an electrophotographic copying machine, a printer, a facsimile machine, etc., in which a toner image is transferred onto a sheet and then heated and fixed.

  2. Description of the Related Art An image forming apparatus that forms a toner image on a sheet by using an electrophotographic method includes a fixing device that heats and fixes the toner image transferred on the sheet. It is known that volatile substances are generated from a roller member constituting a fixing nip by heat generated when the fixing device is driven. Furthermore, recently, it has been recognized that ultrafine particles (hereinafter referred to as UFP (Ultrafine Particle), mainly including siloxane) are generated from this volatile substance and discharged outside the apparatus.

  Although UFP does not affect the human body or the environment, even if its emissions are extremely small, in recent years, with increasing environmental awareness, attention has been paid to emission standards for substances discharged outside the aircraft. In the future, it will be expected to become stricter (for example, BA standard).

  The amount of UFP generated increases as the fixing temperature increases. Regarding the control of the fixing temperature, Patent Document 1 describes that the fixing temperature is lowered at the end of printing in accordance with the number of printed sheets in order to prevent the occurrence of overshoot of the fixing roller or the pressure roller after the printing is completed. Yes. However, if the fixing temperature is simply lowered, there is a risk of image quality failure due to the lowering of the fixing temperature.

  That is, as shown in FIG. 21, the amount of heat given to the paper at the time of fixing the toner image needs to be within a fixed image guarantee area for maintaining a constant image quality, and when the fixing temperature is lowered, the amount of heat given to the paper can be secured. Therefore, an image quality defect occurs.

JP-A-8-166742

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of reducing the fixing temperature at the end of printing and suppressing the occurrence of image quality while suppressing the generation of UFP.

An image forming apparatus according to an aspect of the present invention is
Fixing means for heating and fixing the toner image transferred from the image carrier onto the paper by passing through the fixing nip;
Control means for controlling the printing operation including the fixing means;
In an image forming apparatus comprising:
The control means lowers the fixing temperature immediately before the end of printing and performs control to compensate the time for the paper to pass through the fixing nip;
It is characterized by.

  In the image forming apparatus, before the paper leaves the fixing nip at the end of printing, control is performed to reduce the fixing temperature and compensate for the time for the paper to pass through the fixing nip in order to ensure the amount of heat. Here, the compensation control is control that substantially increases the time for the sheet to pass through the fixing nip in order to compensate for the amount of heat corresponding to the decrease in the fixing temperature. For example, it means reducing the system speed including the paper conveyance speed or increasing the fixing nip pressure. Decreasing the system speed increases the amount of time the paper passes through the fixing nip to compensate for the amount of heat. When the fixing nip pressure is increased, the nip portion is crushed and the nip width is increased to compensate for the amount of heat.

  According to the present invention, since the fixing temperature is lowered at the end of printing, the occurrence of UFP can be suppressed, and the occurrence of image quality defects can be suppressed because temperature compensation is performed.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. It is a block diagram which shows a fixing heater part and its control mechanism. It is a block diagram which shows a control mechanism. It is a graph which shows the amount of UFP discharge | emission at the time of overshoot occurrence. It is a graph which shows the amount of UFP discharge | emission when overshoot does not generate | occur | produce. It is a graph which shows the relationship between fixing temperature and UFP generation amount. 6 is a graph showing the relationship between the roller temperature and the amount of UFP generated when the fixing roller overshoots at a high temperature. 6 is a graph showing the relationship between the roller temperature and the amount of UFP generated when the fixing roller overshoots at a low temperature. 6 is a graph showing a relationship between a fixing temperature and a sheet conveyance speed. 6 is a graph showing a relationship between a fixing temperature and a fixing nip pressure. It is a graph which shows the amount of UFP generation | occurrence | production at the time of ending printing by normal control. 6 is a graph showing the amount of UFP generated when the fixing temperature is lowered and the conveyance speed is lowered at the end of printing. It is a graph which shows the change of the actual fixing temperature with respect to a target value. 6 is a graph showing a change in actual fixing temperature with respect to a target value when a conveyance speed is lowered. 6 is a graph showing the amount of UFP generated when the conveyance speed is lowered while the fixing temperature is lowered just before the end of printing. 6 is a graph showing a change in actual fixing temperature with respect to a target value when the fixing temperature and the conveyance speed are lowered stepwise. 6 is a graph showing the amount of UFP generated when the fixing temperature is lowered and the fixing nip pressure is increased at the end of printing. 6 is a graph showing a change in actual fixing temperature with respect to a target value when the fixing nip pressure is increased. 6 is a graph showing the amount of UFP generated when the fixing nip pressure is increased while lowering the fixing temperature just before the end of printing. 5 is a graph showing a change in actual fixing temperature with respect to a target value when the fixing temperature is lowered stepwise and the fixing nip pressure is raised stepwise. 6 is a graph showing the relationship between the fixing temperature and the amount of heat applied to a sheet.

  Embodiments of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the same member and part in each figure, and the overlapping description is abbreviate | omitted.

  An image forming apparatus 1 according to an embodiment forms a color image by a tandem method as shown in FIG. That is, four process units 10 (a unit for yellow image 10y, a unit for magenta image 10m, a unit for cyan image 10c, and a unit for black image 10k) are juxtaposed and formed on each photoconductor 11. The toner images of the respective colors are transferred / combined (primary transfer) onto the intermediate transfer belt 31 that is rotationally driven in the direction of arrow A by an electric field applied from each transfer roller 32. Thereafter, the composite toner image is secondarily transferred onto a recording medium (paper) by an electric field applied from the transfer roller 35.

  Around each photoconductor 11, a charging roller 12, a developing device 13, a residual toner cleaning blade 14, and the like are arranged, and a laser scanning optical unit 20 that forms an electrostatic latent image on the photoconductor 11. Has been placed. A process in which a toner image is formed on the photoconductor 11 by an electrophotographic method by this type of process unit 10, the toner image is primarily transferred onto the intermediate transfer belt 31, and then further transferred onto a sheet is well known. .

  The sheets are stacked and accommodated in a sheet feeding cassette 50 and are fed one by one by a sheet feeding roller 51. The fed paper is conveyed to the secondary transfer section via the registration roller pair 52, and the toner image is secondarily transferred from the intermediate transfer belt 31. Thereafter, the sheet is heated and fixed with toner by the fixing unit 60, and is discharged from the discharge roller pair 56 to the discharge unit 57.

  The fixing unit 60 includes a heating roller 61 and a pressure roller 62. The pressure-contact portions of the rollers 61 and 62 form a fixing nip, and the pressure roller 62 is variable in pressure with respect to the heating roller 61 or with a predetermined pressure. A mechanism for applying pressure to the heating roller 61 by the pressure roller 62 and a mechanism for changing the pressure are well known. As shown in FIG. 2, the heating roller 61 includes a large size heater 65 and a small size heater 66 in the axial direction. The heaters 65 and 66 are driven by drivers 67 and 68 controlled by the CPU 101. The heating roller 61 is provided with a sensor SE 1 that detects the temperature of the outer peripheral portion, and the detected value is input to the CPU 101 via the input circuit 69.

  The control mechanism of the image forming apparatus 1 is configured around a CPU 101 as shown in FIG. The CPU 101 includes a main body-attached nonvolatile memory 102 and a unit-attached nonvolatile memory 103. Further, the CPU 101 controls the process unit 10 and controls a load 121 including a paper transport system, a toner replenishment motor, or a motor such as a cooling fan. The control mechanism is connected to the controller unit 130 and exchanges necessary information such as image data.

  FIG. 4 shows the amount of UFP generated (see point B) in a state where overshoot has occurred after the end of printing. While the target value is PER10 = 5.00E + 10 (pieces / 10 min), the calculated value at point B is PER10 = 7.25E + 10 (pieces / 10 min), which does not satisfy the BA standard.

  FIG. 5 shows the amount of UFP generated (see point B) when no overshoot occurs after printing is completed. The calculated value at the point B here with respect to the target value is PER10 = 3.92E + 10 (pieces / 10 min), which satisfies the BA standard.

  As described above, when overshoot occurs after the printing is completed, the calculated value of the UFP discharge amount may be doubled. Here, the end of printing means when the trailing edge of the paper (the last paper in the case of continuous printing) has passed through the fixing nip. If overshoot of the UFP discharge does not occur after such printing, the standard value can be cleared. However, there is a possibility that the standard value cannot be cleared due to overshoot.

  The cause of the overshoot of the UFP discharge amount is that when the fixing roller 61 and the exhaust fan continue to rotate after the printing is finished (the same state as during printing), the paper is removed from the fixing nip, It can be estimated that the amount of heat that has been received cannot be released from the fixing roller 61, the temperature of the fixing roller 61 increases, and the amount of UFP generated increases from this temperature increase portion.

  Here, FIG. 6 shows the relationship between the fixing (during printing) temperature and the amount of UFP generated (dissipated). FIG. 6 shows measured values for three models, where black circles indicate measured values for the first model, black squares indicate the second model, and black triangles indicate measured values for the third model. Here, the fixing temperature refers to the temperature of the central portion in the width direction orthogonal to the sheet passing direction. The amount of UFP generated varies greatly depending on the fixing temperature, and the amount generated with respect to the temperature rise from around 150 ° C. tends to increase exponentially. The vertical axis | shaft of FIG. 6 is an index display, According to this, when temperature rises 10 degreeC, generation | occurrence | production amount may become about 5 to 10 times.

  The state of UFP generation when the fixing roller 61 overshoots at a high temperature is as shown in FIG. 7, and the amount of UFP generated increases as the roller (actual) temperature slightly increases immediately after the end of printing. On the other hand, FIG. 8 shows a UFP generation state when the fixing roller 61 overshoots at a low temperature. When the sheet comes out of the fixing nip at the end of printing, the temperature of the fixing roller 61 rises. If the temperature of the fixing roller 61 (150 ° C. or more) differs by several degrees, the amount of UFP generated is about 2 to 3 times. This means that even if the temperature rise of the fixing roller 61 is the same several degrees Celsius, if the original temperature of the fixing roller 61 is low, the amount of UFP generated can be reduced to 1/2 to 1/3.

  Specifically, when the temperature of the fixing roller 61 is 190 ° C. and increases by 2 ° C. at the end of printing and reaches 192 ° C., the amount of UFP generated increases by 2 ° C. to 187 ° C. at the end of printing at 185 ° C. The amount of UFP generated in this case may be 2 to 5 times. If the fixing temperature after the paper at the end of printing is discharged from the fixing nip can be 185 ° C., the amount of UFP generated is reduced to 1/2 to 1/5 compared to 190 ° C. There is a possibility to let you.

  Therefore, the amount of UFP generated at the end of printing can be suppressed by reducing the temperature of the fixing roller 61 immediately before the sheet is discharged from the fixing nip at the end of printing. On the other hand, as shown in FIG. 21, when the fixing temperature is lowered to prevent overshoot, the sheet (toner) cannot secure a sufficient amount of heat, and image quality may be deteriorated.

  Therefore, the fixing temperature is lowered immediately before the end of printing, and the control for compensating the time for the sheet to pass through the fixing nip is performed, thereby suppressing the occurrence of image quality defects and the amount of UFP discharged. Specifically, control is performed to lower the temperature of the fixing roller 61 while maintaining the heat quantity in the fixed image guarantee area shown in FIG.

  FIG. 9 shows the relationship between the fixing temperature and the sheet conveyance speed (the rotation speed of the fixing roller 61) at the fixing nip. By reducing the system speed (from paper feeding to paper discharge and the weekly rotational speed of the photoreceptor 11 and the intermediate transfer belt 31) including the conveyance speed at the fixing nip, the time for the sheet to pass through the fixing nip is lengthened. The amount of heat applied to the paper can secure the fixed image guarantee area.

  The control for compensating the time for which the sheet passes through the fixing nip may be to increase the fixing nip pressure. FIG. 10 shows the relationship between the fixing temperature and the fixing nip pressure. When the fixing nip pressure is increased, the nip width is increased, and as a result, the amount of heat applied to the sheet is increased, and a fixed image guarantee area can be secured.

  FIG. 11 shows the amount of UFP generated when printing is terminated by normal control, and FIG. 12 shows the amount of UFP generated when the fixing temperature is lowered and the conveyance speed is lowered just before printing is finished. As shown in FIG. 11, if the printing operation is terminated without taking countermeasures for lowering the temperature of the fixing roller 61, the amount of UFP generated after the printing is very large. On the other hand, as shown in FIG. 12, if the countermeasure for lowering the temperature of the fixing roller 61 is executed immediately before the end of printing to end the printing, the amount of UFP generated after the end of printing can be suppressed.

  In consideration of a decrease in the temperature of the fixing roller 61 from the viewpoint of securing the amount of heat to the sheet, FIG. 13 shows a change in the actual fixing temperature with respect to the target value of the fixing temperature. In FIG. 13, when one sheet is fixed, the temperature of the fixing roller 61 decreases by about 4 to 5 ° C., and the temperature of the fixing roller 61 has a fixed image guarantee area of about ± 5 ° C. with respect to the target value. Show. Therefore, if the fixing temperature is simply lowered by 5 ° C., the temperature will be outside the image guarantee area, which may cause image quality defects.

  FIG. 14 shows the target value when the conveyance speed is lowered and the actual change in the fixing temperature as a countermeasure for preventing image quality defects. By lowering the fixing temperature and lowering the conveyance speed, it is possible to prevent the occurrence of image quality defects by securing the amount of heat given to the paper.

  The system control speed including the rotation speed of the fixing roller 61 may be decreased after the fixing control temperature is decreased immediately before the end of printing and the actual temperature at the fixing nip is decreased. This control is shown in FIG. In the case of a fixing unit having a large heat capacity, there is a possibility that the actual fixing temperature does not decrease to the target value even if the sheet is passed with the fixing temperature lowered. In this case, the sheet is allowed to pass without changing the conveyance speed (system speed), and the conveyance speed is lowered after the fixing temperature is lowered to the target value, thereby securing a necessary amount of heat. Further, in this case, if the fixing temperature is lowered and the conveyance speed is lowered at the same time, there is a possibility that the amount of heat given to the paper is outside the image guarantee area on the side where the amount of heat is too large.

  FIG. 16 shows control when the fixing temperature and the conveyance speed are lowered stepwise. When the difference G between the normal fixing temperature and the fixing temperature for suppressing UFP generation is large and the image security areas do not overlap, the fixing temperature and the conveyance speed are decreased stepwise for each temperature at which the image security areas overlap. Finally, the target temperature is reached when the paper at the end of printing runs out (when the trailing edge of the paper passes through the fixing nip).

  In this case, if the temperature is lowered to the target fixing temperature at the same time and simultaneously reduced to the target speed, the temperature of the fixing roller 61 does not decrease immediately. There is. Further, if the fixing temperature is lowered without changing the conveyance speed, there is a possibility that the amount of heat given to the paper is too small and the image is out of the image guarantee area. Therefore, it is necessary to adjust the fixing temperature and the conveyance speed so that the amount of heat given to the paper is always in the image guarantee area. In the case of a fixing unit having a large heat capacity, the fixing temperature and the conveyance speed are gradually reduced at the timing shown in FIG. 15 as shown in FIG.

  In order to execute the above control accurately and according to the situation, it is preferable to always detect the surface temperature of the fixing roller 61. In this case, the upper and lower temperature limits of the image guarantee area are set, the temperature of the fixing roller 61 (fixing nip) is monitored, and the conveyance speed is switched when the limit temperature of the image guarantee area is reached. Accordingly, it is possible to keep the fixing temperature just before the end of printing low while always applying an appropriate amount of heat to the paper.

  Further, as shown by the dotted lines in FIGS. 12 and 15, the conveyance speed at the fixing nip may be returned to the normal speed after the printing is completed. According to such control, the fixing roller 61 can be efficiently cooled after the printing is completed.

  FIG. 17 shows the amount of UFP generated when the fixing temperature is lowered and the nip pressure of the fixing nip is increased immediately before the end of printing. As shown in FIG. 11, if the printing operation is terminated without taking countermeasures for lowering the temperature of the fixing roller 61, the amount of UFP generated after the printing is very large. On the other hand, as shown in FIG. 17, if the countermeasure for decreasing the temperature of the fixing roller 61 is executed immediately before the end of printing and the printing is ended, it is possible to suppress the amount of UFP generated after the end of printing.

  In consideration of a decrease in the temperature of the fixing roller 61 from the viewpoint of securing a heat quantity to the sheet, it is preferable to consider a change in the actual fixing temperature with respect to the target value of the fixing temperature shown in FIG. In FIG. 13, when one sheet is fixed, the temperature of the fixing roller 61 decreases by about 4 to 5 ° C., and the temperature of the fixing roller 61 has a fixed image guarantee area of about ± 5 ° C. with respect to the target value. Show. Therefore, if the fixing temperature is simply lowered by 5 ° C., the temperature will be outside the image guarantee area, which may cause image quality defects.

  FIG. 18 shows the target value when the fixing nip pressure is increased and the actual fixing temperature change as a countermeasure for preventing image quality defects. By lowering the fixing temperature and increasing the nip pressure, it is possible to prevent the occurrence of image quality defects by securing the amount of heat applied to the paper.

  The fixing control temperature may be decreased immediately before the end of printing, and the fixing nip pressure may be increased after the actual temperature at the fixing nip has decreased. This control is shown in FIG. In the case of a fixing unit having a large heat capacity, there is a possibility that the actual fixing temperature does not decrease to the target value even if the sheet is passed with the fixing temperature lowered. In this case, the sheet is passed without changing the nip pressure, and the nip pressure is increased after the fixing temperature is lowered to the target value to secure a necessary amount of heat. In this case, if the nip pressure is increased at the same time as the fixing temperature is lowered, there is a possibility that the amount of heat applied to the paper is outside the image guarantee area on the side where the amount of heat is too large.

  FIG. 20 shows control when the fixing temperature is lowered stepwise and the nip pressure is raised stepwise. When the difference G between the normal fixing temperature and the fixing temperature for suppressing UFP generation is large and the image security areas do not overlap, the fixing temperature is lowered step by step for each temperature at which the image security areas overlap, and the nip The pressure is increased step by step, and finally the target temperature is reached when the paper at the end of printing runs out (when the trailing edge of the paper passes through the fixing nip).

  In this case, if the temperature is lowered to the target fixing temperature at the same time and simultaneously increased to the target nip pressure, the temperature of the fixing roller 61 does not decrease immediately. There is sex. Further, if the fixing temperature is lowered without changing the nip pressure, there is a possibility that the amount of heat given to the sheet is too small and the image is out of the image guarantee area. Therefore, it is necessary to adjust the fixing temperature and the nip pressure so that the amount of heat given to the paper is always within the image guarantee area. In the case of a fixing unit having a large heat capacity, the fixing temperature is lowered stepwise and the nip pressure is increased at the timing shown in FIG. 19 as shown in FIG.

  In order to execute the above control accurately and according to the situation, it is preferable to always detect the surface temperature of the fixing roller 61. In this case, the upper and lower temperature limits of the image guarantee area are set, the temperature of the fixing roller 61 (fixing nip) is monitored, and the nip pressure is switched when the limit temperature of the image guarantee area is reached. Accordingly, it is possible to keep the fixing temperature just before the end of printing low while always applying an appropriate amount of heat to the paper.

  In addition to the two types described above (conveying speed control and nip pressure control), in order to perform more precise and detailed control, as a control for compensating the time for the sheet to pass through the fixing nip, the nip is used. You may combine the said 2 types of form of raising a pressure and reducing a paper conveyance speed.

  The control according to the present invention is an image stabilization control during a printing operation (a conventionally well-known control, a predetermined toner image is formed on a photoconductor or a transfer belt, and a developing bias is adjusted by detecting the density thereof. It may be executed when performing control. When performing image stabilization control for about several minutes during printing, the above-described UFP generation overshoot phenomenon occurs. Therefore, it is effective to execute the control according to the present invention during image stabilization control. It is.

  Even when the print target paper is switched from a large size (for example, A3 size) to a small size (for example, B5 size), the fixing temperature is lowered and the control for compensating the time for the paper to pass through the fixing nip is executed. It is preferable to do. When the large size paper is switched to the small size paper, both end portions of the fixing roller 61 are in a state where the above-described overshoot phenomenon occurs. Therefore, even when the paper size is switched from the large size to the small size, the present invention is applied. It is effective to execute such control.

  As shown in FIG. 2, when the large-size heater 65 and the small-size heater 66 are independently arranged in the fixing roller 61, before the paper to be printed is switched from the large size to the small size, the large size heater 65 and the small size heater 66 are arranged. Even when the temperature of the size heater 65 is lowered and the temperature rise at both ends of the fixing roller 61 is suppressed, if the paper is not passed through the both ends, the both ends lose heat escape. End up. As a result, UFP generation overshoots at both ends of the fixing roller 61, but overshooting can be suppressed by executing the control according to the present invention when switching to a small size sheet.

  In addition, when a plurality of images are continuously printed, it is preferable that a monochrome image be the final image. Since the monochrome image has a low fixing temperature, the fixing temperature immediately before the end of printing can be set low. By bringing the monochrome image to the end of printing, it is easy to suppress the occurrence of UFP. For example, in the case of a color image, the temperature can be decreased by 5 ° C. just before the end of printing, but in the case of a monochrome image, it can be decreased by 10 ° C. That is, the fact that the target temperature at the end of printing can be (largely) changed between a color image and a monochrome image is used.

  Furthermore, when a plurality of images are continuously printed, an image with little influence of image quality deterioration may be used as the final printed image. Basically, the fixing temperature changes within the upper and lower limit temperatures of the image guarantee area, but in the case where the image quality is affected more or less, by setting an image with little influence of image quality deterioration as the final print image, The effect can be minimized. Further, when it is desired to suppress the generation amount of UFP even if the image quality is somewhat sacrificed, the fixing temperature can be set to be equal to or lower than the temperature of the image guarantee area.

  As shown in FIG. 1, the image forming apparatus 1 includes an exhaust unit that sucks the air in the sheet conveyance path 70 after passing through the fixing nip by the rotation of the fan 71 and exhausts the air out of the apparatus through a filter. The fan 71 may be stopped or its rotational speed may be reduced immediately before the end of printing. Usually, as a countermeasure against UFP overshoot, the fan 71 needs to be rotated at a high speed even after printing is completed. However, when the apparatus enters a standby state, if the fan 71 is rotated at full speed, the noise becomes annoying. Noise can be suppressed by stopping the fan 71 or reducing the rotation speed.

  The image forming apparatus according to the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the gist thereof.

  In particular, the basic configuration and detailed configuration of the image forming apparatus are arbitrary, and the image forming apparatus may be a multi-function machine having a communication function, a facsimile function, and the like in addition to a print function.

  As described above, the present invention is useful for an image forming apparatus, and is particularly excellent in that the generation of UFP can be suppressed and the occurrence of poor image quality can be suppressed.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 10 ... Process unit 11 ... Photoconductor 31 ... Intermediate transfer belt 60 ... Fixing unit 61 ... Fixing roller 62 ... Pressure roller 65, 66 ... Heater 101 ... CPU

Claims (18)

Fixing means for heating and fixing the toner image transferred from the image carrier onto the paper by passing through the fixing nip;
Control means for controlling the printing operation including the fixing means;
In an image forming apparatus comprising:
The control means lowers the fixing temperature immediately before the end of printing and performs control to compensate the time for the paper to pass through the fixing nip;
An image forming apparatus.   The image forming apparatus according to claim 1, wherein the control for compensating the time for which the sheet passes through the fixing nip is to reduce a system speed including a sheet conveyance speed.   The image forming apparatus according to claim 2, wherein the control unit reduces the fixing control temperature and the system speed.   The image forming apparatus according to claim 3, wherein the control unit lowers the system speed after the fixing control temperature is lowered and the actual temperature at the fixing nip is lowered.   4. The control unit according to claim 3, wherein the control unit decreases the fixing control temperature and the system speed in a stepwise manner, and always gives a heat amount of the image guarantee area to the sheet passing through the fixing nip. Item 5. The image forming apparatus according to Item 4.   6. The control unit according to claim 5, wherein the upper and lower temperature limits of the image guarantee area are set to monitor the temperature of the fixing nip, and the system speed is switched when the limit temperature of the image guarantee area is reached. The image forming apparatus described in 1.   The image forming apparatus according to claim 3, wherein the control unit returns the sheet conveyance speed at the fixing nip to a normal speed after printing is completed.   2. The image forming apparatus according to claim 1, wherein the control for compensating the time for the sheet to pass through the fixing nip is to increase the nip pressure of the fixing nip.   The image forming apparatus according to claim 8, wherein the control unit lowers the fixing control temperature and increases the nip pressure.   The image forming apparatus according to claim 9, wherein the control unit increases the nip pressure after the fixing control temperature is decreased to decrease the actual temperature at the fixing nip.   The control means reduces the fixing control temperature stepwise and raises the nip pressure stepwise, and always gives the heat amount of the image guarantee area to the paper passing through the fixing nip. The image forming apparatus according to claim 9 or 10.   12. The control unit according to claim 11, wherein the controller sets the upper and lower temperature limits of the image guarantee area, monitors the temperature of the fixing nip, and switches the nip pressure when the limit temperature of the image guarantee area is reached. The image forming apparatus described.   2. The image forming apparatus according to claim 1, wherein the control for compensating the time for which the sheet passes through the fixing nip includes increasing a nip pressure of the fixing nip and decreasing a system speed including a sheet conveying speed. .   2. The control unit according to claim 1, wherein when the image stabilization control is executed during printing, the control unit lowers the fixing temperature and compensates for the time for the sheet to pass through the fixing nip. The image forming apparatus described.   The control means performs control for reducing the fixing temperature and compensating for the time for the paper to pass through the fixing nip when the printing target paper is switched from a large size to a small size. Item 2. The image forming apparatus according to Item 1.   The image forming apparatus according to claim 1, wherein when the plurality of images are continuously printed, the control unit sets a monochrome image as a final print image.   The image forming apparatus according to claim 1, wherein when the plurality of images are continuously printed, the control unit sets an image that is less affected by image quality deterioration as a final print image. An exhaust unit that sucks air on the sheet conveyance path after passing through the fixing nip by rotation of a fan and exhausts the air out of the apparatus through a filter;
The control means stops the fan immediately after printing or reduces the rotational speed,
The image forming apparatus according to claim 1.

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