JPH08241011A - Image forming device - Google Patents

Abstract

PURPOSE: To provide an image forming device constituted so that printing quality is prevented from being lowered caused by the variation of fixing temperature at a first printing time, an error caused by an abnormally high temperature does not occur and a fixing device is not damaged. CONSTITUTION: This device is provided with a means counting the number of printed sheets after a power source is turned on. Then, a heater 101 is controlled to be forcibly turned on according to the counted number before a recording paper 22 arrives at the fixing device at the first printing time. Since the temperature of a heating roller 29 is extremely lowered because a pressing roller 30 is not sufficiently warmed when the counted number is small, the forcible turning-on time of the heater 101 is prolonged. Since the roller 30 is sufficiently warmed when the counted number is large, the heater 101 is forcibly turned on but the time thereof is controlled to be short. Besides, when time elapses after a printing action is finished, the counted number is reset a fixed time after the printing action was finished has elapsed, because the temperature of the roller 30 is gradually lowered and it becomes nearly normal at the part other than the part where it is brought into contact with the roller 29.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for controlling a fixing temperature of a fixing device used in a laser beam printer, a copying machine or the like.

[0002]

2. Description of the Related Art Conventionally, printers of various principles have been proposed as output terminals for personal computers, workstations and the like. In particular, laser beam printers using electrophotographic processes and laser technology (hereinafter referred to as "LB
"P") is highly advantageous in terms of recording speed and printing quality, and is rapidly spreading.

On the other hand, there is an increasing demand for full-color LBP in the market, but in the case of full-color LBP, it is necessary to stack toners of three primary colors of printing: cyan, magenta, yellow, and black. Each color image must be formed on top of it in the correct position. Generally, for this alignment, a method is used in which a reference mark is provided on a latent image carrier on which images are superimposed, and the image forming process for each color is started when the mark is detected.

The LBP which forms a color image by applying the electrophotographic process technique selectively irradiates a light beam corresponding to each color on a photosensitive member having a photosensitive layer to form an image, and forms an image among a plurality of predetermined color components. A method of forming a color image on one transfer material by developing a plurality of electrostatic latent images corresponding to the specific components of the above with respective predetermined toners and superposing the toner images of the single color ing.

Hereinafter, the conventional image forming apparatus will be described with reference to FIGS. 8, 9, 10, 11, 12, 13 and 1.
4 will be described. 8 is a configuration diagram of a conventional image forming apparatus, FIG. 9 is a configuration diagram of a fixing device of the conventional image forming device, and FIG. 10 is an enlarged view of a fixing portion of the fixing device of the conventional image forming device. FIG. 11 is a control block diagram of a fixing device of a conventional image forming apparatus, FIG. 12 is a flow chart of a fixing device temperature control of a conventional image forming apparatus, and FIG. 13 is printing after powering on the conventional image forming apparatus. FIG. 14 is a diagram showing the fixing temperature control result when the number of sheets is small, and FIG. 14 is a diagram showing the fixing temperature control result when the number of printed sheets is large after power-on of the conventional image forming apparatus.

In FIG. 8, reference numeral 1 denotes a photosensitive member in which a photosensitive layer such as an organic photoconductor (OPC) is applied in a thin film on the outer peripheral surface of a Ni (nickel) belt base material and has a joint structure. Has become. Reference numerals 2 and 4 denote photoconductor transport rollers, and 3 denotes a photoconductor position detection sensor that detects the reference position of the photoconductor 1. The reference position is detected by a photoconductor reference mark such as a slit arranged at one end of the photoconductor 1. To do. The photoconductor 1 is supported by two photoconductor transport rollers 2 and 4 so as to form a vertical plane between the photoconductor transport rollers 2 and 4, and is supported by a drive motor (not shown) on the photoconductor transport rollers 2 and 4. Along arrow A
Rotate in the direction. On the peripheral surface of the belt-shaped photoconductor 1, the charger 5, the exposure optical system 6, the black (K), the yellow (Y), the magenta (M), and the cyan (C) are arranged in this order in the photoconductor rotation direction indicated by an arrow A. Developing devices 7K, 7Y, 7 for each color
M, 7C, an intermediate transfer member unit 8, a photoconductor cleaning device 9, and a static eliminator 10 are provided. Charger 5
Is composed of a charging wire 11 made of a tungsten wire or the like, a shield plate 12 made of a metal plate, and a grid plate 13. By applying a high voltage of about 4 to 5 kV to the charging line 11, the charging line 11 causes corona discharge to uniformly charge the photoconductor 1 via the grid plate 13.
An exposure light beam 14 is emitted from the exposure optical system 6. The exposure light beam 14 is obtained by subjecting an image signal from a gradation conversion device (not shown) to light intensity modulation or pulse width modulation by a laser drive circuit (not shown), and a static image corresponding to a specific color on the photoconductor 1. Form a latent image. The developing devices 7K, 7Y, 7M and 7C store toners corresponding to the respective colors. In the development of each color, the separating / contacting cams 15K, 15Y, 15M and 15C, both ends of which are rotatably supported by the main body of the machine corresponding to each color, rotate corresponding to the color selection signal, and the selected developing device, for example, 7K.
Is brought into contact with the photoconductor 1 to carry out. At this time, the unselected developing devices 7Y, 7M, and 7C are separated from the photoconductor 1. The intermediate transfer body unit 8 includes a seamless loop belt-shaped intermediate transfer body 16 made of a conductive resin or the like, and two intermediate transfer body transport rollers 17 and 18 supporting the intermediate transfer body 16.
And an intermediate transfer roller 19 arranged to face the photosensitive member 1 with the intermediate transfer member 16 interposed therebetween for transferring the toner image on the photosensitive member 1 to the intermediate transfer member 16.

Reference numeral 20 denotes an intermediate transfer member cleaning device for scraping off the residual toner on the intermediate transfer member 16, and while the composite image is being formed on the intermediate transfer member 16, the intermediate transfer member 1 is provided.
It is separated from 6, and contacts only when it is used for cleaning. Reference numeral 21 is a recording paper cassette that houses recording paper 22. The recording papers 22 are sent from the recording paper cassette 21 one by one to a paper transport path 24 by a half-moon shaped paper feed roller 23. Reference numeral 25 denotes a registration roller for temporarily stopping and waiting the recording paper 22 to match the positions of the recording paper 22 and the composite image on the intermediate transfer body 16, and is in pressure contact with the driven roller 26. Reference numeral 27 denotes a paper transfer roller for transferring the composite image on the intermediate transfer member 16 to the recording paper 22.
Only during transfer, the intermediate transfer member 16 is contacted and rotated. Reference numeral 28 denotes a fixing device including a heating roller 29 having a heat source inside and a pressure roller 30, and a combined image transferred onto the recording paper 22 is subjected to pressure as the heating roller 29 and the pressure roller 30 are nipped and rotated. It is fixed on the recording paper 22 by heat to form a color image.

The operation of the image forming apparatus configured as described above will be described below. The image is developed in the order of black, cyan, magenta, and yellow. The photosensitive member 1 and the intermediate transfer member 16 are driven by respective drive sources (not shown), and are controlled so that their peripheral speeds are the same constant speed.

In this state, first, a high voltage of about -4000 V to -5000 V is applied to the charging wire 11 in the charger 5 connected to the high-voltage power source to cause corona discharge, and the surface of the photoconductor 1 is made uniform. It is charged to about 700V. Next, the photoconductor 1 is rotated in the direction of arrow A, and the uniformly charged surface of the photoconductor 1 is irradiated with an exposure light beam 14 of a laser beam corresponding to a predetermined black (K) of a plurality of color components. Then, the charge disappears in the irradiated portion on the photoconductor 1 to form an electrostatic latent image. On the other hand, the black developing device 7K is pushed in the direction of arrow B by the rotation of the separation cam 15K and comes into contact with the photoconductor 1 after a lapse of a predetermined time. On the other hand, immediately before contact, a negative potential of about -300 V is applied to the developing sleeve 31K having toner on its surface. If a negative charge is given to the toner in advance, the toner adheres only to a portion (electrostatic latent image portion) on the photoconductor 1 where the exposure light beam is irradiated and the charge disappears, so that development is performed by a so-called negative-positive process. The developing device 7K that has completed the development is moved from the contact position with the photoconductor 1 to the separation position by the 180 ° rotation of the separation cam 15K. Developing device 7K
The toner image formed on the photoconductor 1 by the
6 is transferred for each color by applying a high voltage of +800 V to the intermediate transfer roller 19 arranged in contact with the photoconductor 1. The residual toner that has not been transferred from the photoconductor 1 to the intermediate transfer body 16 is removed by the photoconductor cleaning device 9, and the charge on the photoconductor 1 from which the residual toner has been scraped off is removed by the static eliminator 10.

Next, when the color of cyan (C) is selected, the separation cam 15C is rotated, and this time the developing device 7C is pushed toward the photoconductor 1 and brought into contact with the photoconductor 1 to develop the cyan (C). To start. In the case of a copying machine or printer that uses four colors, the above-described developing operation is sequentially repeated four times, and the intermediate transfer member 16
Toner images of four colors B, C, M, and Y are superposed on each other to form a composite image. In the composite image formed in this way, the paper transfer roller 27, which has been separated so far, comes into contact with the intermediate transfer body 16, applies a high voltage of about +1000 V to the paper transfer roller 27, and pressurizes the recording paper cassette 21. It is collectively transferred to the recording paper 22 sent along the paper carrying path 24. Then, the recording paper 22 on which the toner image is transferred is fixed to the fixing device 2.
8 where the heat of the heating roller 29 and the pressure roller 3
It is fixed by a nip pressure of 0 and is output as a color image. The residual toner on the intermediate transfer body 16 which is not completely transferred onto the recording paper 22 by the paper transfer roller 27 is removed by the intermediate transfer body cleaning device 20. The intermediate transfer body cleaning device 20 is at a position apart from the intermediate transfer body 16 until one synthetic image is obtained, and a synthetic image is obtained.
After being transferred to, the contact state is established and the residual toner is removed.

Recording of one image is completed by the above operation. Next, the configuration of the fixing device 28 of the conventional image forming apparatus will be described with reference to FIGS. 9 and 10. In FIG. 9, 29
The heating roller is connected to a drive source (not shown) and is rotatably supported by a side wall (not shown). A heater 101 is housed in the heating roller 29. 54
Is a temperature detecting section using a thermistor or the like, and detects the surface temperature of the heating roller. The pressure roller 30 is supported by the pressure roller support member 102 so as to be freely contactable to and detachable from the heating roller 29. The pressure roller support member 102 is supported on a side wall (not shown) by a pin 106, and an end portion of the pressure roller support member 102 is biased by a biasing member (not shown) in the direction of arrow A by a biasing force f. Is pressed against the heating roller 29. At that time, the pressure contact force F between the pressure roller 30 and the heating roller 29
Becomes like Equation 1 by the lever principle.

[0012]

[Equation 1]

The heating roller 29 is a metal roller whose surface is thinly coated with silicon rubber or the like.
Reference numeral 0 is a rubber roller having a thin fluororesin film on its surface. By pressing the rollers 29 and 30 into contact with each other, as shown in FIG.
The pressure roller 30 is deformed as shown in 0 to increase the contact area, and the heating time of the recording paper 22 (time to move from point a to point b) is lengthened.

Next, the operation of the fixing device 28 will be described with reference to FIG. 9 again. During the fixing operation, the heating roller 29 rotates in the C direction and the pressure roller 30 rotates in the D direction at the same peripheral speed, and the recording paper 22 on which the toner image 108 is formed is conveyed in the B direction. The recording paper 22 guided to the fixing device by the recording paper guide 107 passes between the heating roller 29 heated by the heater 101 and the pressure roller 30.
At that time, the unfixed toner image is brought into pressure contact with the surface of the heating roller 29, and heat is instantaneously applied to melt the toner, and the toner is fused to the recording paper 22, and the toner image is fixed on the recording paper 22.
After that, the recording paper 22 is discharged onto the discharge tray 105 by the discharge rollers 103 and 104.

Next, the temperature control of the fixing device 28 will be described in detail with reference to FIG. The CPU 51 outputs digital data corresponding to the actual temperature to the data latch unit 52. The latched data is output to the data comparison unit 53. On the other hand, the fixing temperature is detected by the temperature detecting unit 54 installed in contact with the surface of the heating roller 29. The temperature detecting section 54 uses a thermistor and utilizes the fact that the resistance value of the thermistor changes with temperature, and the A / D converting section 55 converts the potential across the thermistor into digital data.
This is output to the data comparison unit 53. Data comparison unit 53
Then, the data in the data latch section 52 is compared with the data in the A / D conversion section 55, and if the value of the data latch section 52 indicates a higher temperature, a high level is set to the gate section 57, and otherwise a low level is set to the gate section 57. Output.

The error detection unit 56, which will be described later, normally outputs a high level (when no error is detected) to the gate unit 57.
The gate unit 57 is composed of an AND circuit, and when no error is detected, the output level of the data comparison unit 53 is directly input to the heater driver unit 58. Heater driver section 5
Reference numeral 8 controls the heater 101 to be turned on when the input is at the high level and to turn off the heater 101 when the input is at the low level.

On the other hand, the output of the temperature detector 54 is also input to the error detector 56. Although the error detector 56 normally outputs a high level as described above, the temperature detector 54
If the output potential of the above is, for example, a potential corresponding to −10 ° C. or less, the thermistor is opened, and if it is a potential corresponding to 230 ° C. or more, it is determined that the control has failed and a low level is output to the gate unit 57. At this time, the gate section 57, which is an AND circuit, always outputs a low level, so that the input of the heater driver section 58 becomes a low level and the heater 101 is turned off.

Further, since the fixing device generally handles high temperature, a thermostat or the like is arranged on the heating roller 29 for safety (not shown), and the surface temperature of the heating roller 29 is independently monitored, and a temperature higher than the set value is detected. When it is detected, the heater 101 is forcibly turned off. The output of the A / D converter 55 and the signal of the error detector 56 are the CPU
Since the CPU 51 is connected to the CPU 51, the CPU 51 can monitor the current fixing device temperature and can know the occurrence of an error.

Next, the temperature control flow of the fixing device 28 of the conventional image forming apparatus will be described in detail with reference to FIG. First, the power of the entire device is turned on (step (ST
EP) 3-1). The heater control is started by the heater driver unit 58 at a preset fixing temperature (for example, 170 ° C.) (step 3-2). At this time, the heating roller 29 and the pressure roller 30 in the fixing device 28 also start to rotate by a drive source (not shown) (step 3-3) and at the same time, the initialization of the electrophotographic process also starts (step 3-).
4) Wait until the initialization of the electrophotographic process is completed (step 3-5). When the initialization of the electrophotographic process is completed, it waits until the heating roller surface temperature detected by the temperature detection unit 54 reaches the fixing set temperature. During this time, the surface temperature of the pressure roller also approaches the fixing temperature (step 3-
6). When the surface temperature of the heating roller reaches the fixing set temperature, the heater 101 is turned off, the rotations of the heating roller 29 and the pressure roller 30 are stopped, and the warm-up is completed (step 3-7). At this point, the image forming apparatus is in a standby state and is in a standby state for a print start command (step 3-8). At this time, the CPU 51 checks for an error (step 3-9). If the error detector 56 detects an error such as an abnormally high temperature or thermistor open, error processing is performed (step 3-10). The error processing includes forcibly turning off the heater 101 in the heating roller 29. If no error is detected, the process returns to step 3-8.

If there is a print start command in step 3-8, each print process is sequentially started to form an image (step 3-11). CPU 51 even during printing
Checking for errors is performed (Step 3-1)
2). If the error detection unit 56 detects an error such as an abnormally high temperature or thermistor open, error processing is performed (step 3-13). Error handling stops the entire electrophotographic process. If no error occurs and normal printing is performed, it is checked whether the entire printing process is completed (step 3-14). If it is in the middle of the printing process, the process returns to step 3-11. If the entire printing process is completed, the process returns to step 3-8.

Next, the actual control result based on the above operation flow will be described in detail with reference to FIGS. 13 and 14.
13 and 14, the solid line shows the set value of the heating roller surface temperature set by the CPU 51, and the solid line shows the actual temperature detected by the thermistor (temperature detection unit 54) on the heating roller surface. FIG. 13 is a diagram showing a fixing device temperature control result when the number of printed sheets is small after the power is turned on.

At the time of the time axis X0, the image forming apparatus is powered on. After the power is turned on, both the heating roller 29 and the pressure roller 30 start to rotate, the heater 101 in the heating roller 29 is turned on, and the surface temperature of the heating roller is the temperature T.
It starts rising from on. At this point, the initialization of the electrophotographic process is also started. In the initialization of the electrophotographic process, the presence / absence of a unit necessary to execute the electrophotographic process is checked and the operation of various motors is checked. If an error is detected in these checks, error processing is performed and all operations are stopped. The heater 101 continues to be turned on until the surface temperature of the heating roller reaches the fixing set temperature Tst. If the time point when the temperature Tst is reached is X1, the period during which the heater 101 is on is from X0 to X1.

At the time of X1, the heater 101 is turned off and the warm-up period ends. After that, the process is in a standby state, and if a print start command is input, the image forming operation is started. X2
It is assumed that the print start command is given at the time of. During standby, the heating roller 29 and the pressure roller 30 are always in contact with each other at the same portion, so the surface temperature on the pressure roller 30 side gradually decreases except for the vicinity of the portion in contact with the heating roller 29. After the printing start command is given, the heating roller 29 and the pressure roller 30 start to rotate, but the surface temperature of the heating roller 29 rapidly decreases due to heat conduction from the heating roller 29 to the pressure roller 30. Therefore, when the recording paper 22 reaches the entrance of the fixing device 28, the heating roller surface temperature does not reach the fixing set temperature Tst. To solve this problem, at the time of first printing, the heater 101 is forcibly turned on before the recording paper 22 enters the fixing device 28 in consideration of the temperature drop on the surface of the heating roller due to heat conduction from the heating roller 29 to the pressure roller 30. There is a way to do it. However, since the time for which the heater is forcibly turned on at the time of first printing is always constant, a temperature difference occurs in the fixing temperature depending on the state of the image forming apparatus when the print command is issued.

FIG. 14 is a view showing the fixing device temperature control result when the number of printed sheets is large after the power is turned on. At the time of the time axis X0, the image forming apparatus is powered on. After the power is turned on, both the heating roller 29 and the pressure roller 30 start rotating, the heater 101 in the heating roller 29 is turned on, and the surface temperature of the heating roller starts to rise from the temperature Ton. At this point, the initialization of the electrophotographic process is also started. In the initialization of the electrophotographic process, the presence / absence of a unit necessary to execute the electrophotographic process is checked and the operation of various motors is checked. If an error is detected in these checks, error processing is performed and all operations are stopped. The heater 101 continues to be turned on until the surface temperature of the heating roller reaches the fixing set temperature Tst. If the time point when the temperature Tst is reached is X1, the period during which the heater 101 is on is from X0 to X1.

At the time of X1, the heater 101 is turned off and the warm-up period ends. After that, the process is in a standby state, and if a print start command is input, the image forming operation is started. X2
It is assumed that the print start command is given at the time of. During standby, the heating roller 29 and the pressure roller 30 are always in contact with each other at the same portion, so the surface temperature on the pressure roller 30 side gradually decreases except for the vicinity of the portion in contact with the heating roller 29. After the print start command is given, the heating roller 29 and the pressure roller 30 start rotating, but the forced lighting control of the heater 101 in consideration of the temperature decrease due to heat conduction from the heating roller 29 to the pressure roller 30 is applied. In this case, if a large number of printing operations are performed after the power is turned on, the entire circumference of the pressure roller 30 is also warmed, so a large temperature drop does not occur, and this time an abnormal high temperature error occurs and the fixing device 2
8 will be the cause of damage.

[0026]

SUMMARY OF THE INVENTION In the above conventional configuration,
At the time of first printing, the heating roller 29 and the pressure roller 30 start rotating from the heating roller 29 to the pressure roller 30.
In consideration of the temperature decrease of the heating roller 29 due to the heat conduction to the heater 10, before the recording paper 22 enters the fixing device 28, the heater 10
When the forced lighting control of 1 is performed, the image forming apparatus is left for a long time and the power is turned on from the state where the heating roller 29 is at the normal temperature, and the printing is performed immediately after the warm-up is completed and the power is turned on. When a printing operation is performed to some extent after the printing operation is performed again, the fixing temperature is different, the printing quality is deteriorated such as cold offset or hot offset, an abnormal high temperature error is generated, and the fixing device 2 is used.
Problems such as breakage 8 occur.

An object of the present invention is to prevent deterioration of print quality due to variations in fixing temperature during first printing,
An object of the present invention is to provide an image forming apparatus in which an abnormal high temperature error does not occur and a fixing device is not damaged.

[0028]

According to another aspect of the present invention, there is provided an image forming apparatus, wherein one of a first roller and a second roller for nipping and conveying a recording medium and one of the first and second rollers are set to a preset temperature. A fixing device having heating means for heating, driving means for rotating the first and second rollers, and temperature detecting means for detecting the temperature of at least one of the first and second rollers, and the first and second fixing means. The recording medium detection means disposed upstream of the roller for detecting the recording medium and the counting means for counting the number of image formations are provided, and after the heating means once heats the temperature to the set temperature, the recording medium reaches the fixing device. Prior to this, one of the first and second rollers is forcibly heated by the heating means, and the forcible heating time is made longer as the count value of the counting means becomes smaller.

An image forming apparatus according to a second aspect of the present invention includes first and second rollers that sandwich and convey the recording medium, and heating means that heats one of the first and second rollers to a preset temperature. A fixing device having a driving means for rotating the first and second rollers and a temperature detecting means for detecting the temperature of at least one of the first and second rollers, and a fixing device at a position upstream of the first and second rollers. A heating unit is provided which is provided with a recording medium detecting unit for detecting the recording medium and a counting unit for counting the number of times of image formation, and which is heated to a preset temperature by the heating unit and before the recording medium reaches the fixing device. The set temperature is increased as the count value of the counting means decreases.

The image forming apparatus according to claim 3 is the image forming apparatus according to claim 1.
Alternatively, in the image forming apparatus described in 2, the timer means for counting the time from the end of the image forming operation to the start of the next image forming operation is provided, and the counting means is reset when the count value of the timer means exceeds a certain value. It is characterized by doing so.

[0031]

According to the structure of claim 1, the heating means forcibly heats one of the first and second rollers before the recording medium reaches the fixing device after being heated to the set temperature by the heating means. The heating is performed so that the forced heating time becomes longer as the count value (the number of times of image formation) of the counting means becomes smaller. That is, the number of times of image formation (the number of prints) after the power is turned on is counted, and when the count value is small in the first print, the roller that is not heated (pressurizing roller) is not sufficiently warmed. Since the temperature of the heating roller (heating roller) at the start of rotation of the second roller is remarkably lowered, the forced heating time before the recording medium reaches the fixing device is lengthened. When the count value is large, it is considered that the roller not heated is sufficiently warmed, and therefore the heating time is controlled to be short.

According to the second aspect of the present invention, after the heating means once heats the temperature to the set temperature and before the recording medium reaches the fixing device, the set temperature of the heating means is counted by the counting means (the number of times of image formation). ) Becomes smaller, the higher it rises. That is, the number of times of image formation (the number of prints) after the power is turned on is counted, and when the count value is small in the first print, the roller that is not heated (pressurizing roller) is not sufficiently warmed. Since the temperature of the roller to be heated (heating roller) at the start of rotation of the second roller is remarkably lowered, the set temperature of the heating means is raised to a high temperature before the recording medium reaches the fixing device, whereby forced heating is performed for a long time. If the count value is large, it is conceivable that the roller that is not heated is also sufficiently warmed, so the set temperature of the heating means is set low, and the forced heating time is shortened or not heated.

According to the third aspect of the invention, the timer means for counting the time from the end of the image forming operation to the start of the next image forming operation is provided, and when the count value of the timer means exceeds a certain value. I am trying to reset the counting means. This is because after the end of the image forming operation, that is, the time from the end of printing, the temperature of the pressure roller gradually decreases to near room temperature except in the vicinity where it is in contact with the heating roller. After a lapse of time, the count value of the counting means is reset so that the recording medium is forcibly heated before reaching the fixing device.

[0034]

Embodiments of the present invention will be described below with reference to FIGS. 1, 2, 3, 4, 5, 5, 6, 7, 8 and 9. 1 is a control block diagram of a fixing device of an image forming apparatus according to an embodiment of the present invention, and FIG. 2 is a diagram showing the number of printed sheets after the power is turned on and the heater compulsory lighting time of the image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a diagram showing the relationship between the number of printed sheets after the power is turned on and the fixing set temperature during the heater forced lighting period of the image forming apparatus of the second embodiment of the present invention. 4 is the first of the present invention
5 is a flow chart of fixing device temperature control of the image forming apparatus of the embodiment of the present invention, FIG. 5 is a flow chart of fixing device temperature control of the image forming apparatus of the second embodiment of the present invention, and FIG. 6 is an embodiment of the present invention. FIG. 7 is a diagram showing a fixing device temperature control result when the number of printed sheets of the image forming apparatus is small after the power is turned on, and FIG. 7 is a fixing process when the number of printed sheets of the image forming apparatus of the embodiment of the present invention is large after the power is turned on. It is a figure which shows a container temperature control result. The schematic diagrams of the image forming apparatus and the fixing device thereof according to the embodiment of the present invention are the same as those of the conventional example shown in FIGS. 8, 9 and 10, and detailed description thereof will be omitted.

FIG. 1 is a control block diagram of the fixing device showing the first and second embodiments of the present invention together. First
In addition to the structure of the conventional example shown in FIG. 11, the embodiment of FIG.
1 is provided. In the second embodiment, in addition to the configuration of the conventional example shown in FIG. 11, a fixing set temperature calculating means 60 and a fixing set temperature table 62 during the heater forced lighting period are provided. .

Further, a recording medium detecting means arranged upstream of the fixing device 28 for detecting the conveyed recording paper 22, a counting means for counting the number of times of image formation after the power is turned on, and a timer means. And are provided. The other parts of the first and second embodiments are the same as those of the conventional example, and detailed description thereof will be omitted. The operation of the image forming apparatus of this embodiment will be described below with reference to FIG. The image is developed in the order of black, cyan, magenta, and yellow.

The photosensitive member 1 and the intermediate transfer member 16 are driven by respective driving sources (not shown), and are controlled so that their peripheral speeds are the same constant speed. A photoconductor reference mark for determining the reference position is provided at one end of the photoconductor 1 so that the reference position can be detected. When a print command is input, first, a high voltage of about −4000 V to −5000 V is applied to the charging wire 11 in the charger 5 connected to the high voltage power source to cause corona discharge, and the surface of the photoconductor 1 is made uniform. To -700V. Next, photoreceptor 1
Is rotated in the direction of arrow A, and the exposure beam 14 of a laser beam corresponding to a predetermined black (K) of a plurality of color components is irradiated on the surface of the photoreceptor 1 which is uniformly charged. In the upper irradiated portion, the electric charge disappears and an electrostatic latent image is formed. After the elapse of a predetermined time, the black developing device 7K is pushed in the direction of arrow B by the rotation of the separation cam 15K and comes into contact with the photoconductor 1. On the other hand, immediately before contact, a negative potential of about -300 V is applied to the developing sleeve 31K having toner on its surface. If a negative charge is given to the toner in advance, the toner adheres only to a portion (electrostatic latent image portion) on the photoconductor 1 where the exposure light beam is irradiated and the charge disappears, so that development is performed by a so-called negative-positive process. Developing device 7 after development
K moves from the contact position with the photoconductor 1 to the separation position by the 180 ° rotation of the separation cam 15K. The toner image formed on the photoconductor 1 by the developing device 7K is transferred to the intermediate transfer body 16 by applying a high voltage of about +800 V to the intermediate transfer roller 19 arranged in contact with the photoconductor 1 for each color. Residual toner not transferred from the photoconductor 1 to the intermediate transfer body 16 is removed by the photoconductor cleaning device 9,
Further, the charge on the photoconductor 1 from which the residual toner has been scraped off by the static eliminator 10 is removed.

Next, when cyan (C) color is selected, the separation cam 15C rotates, and this time the developing unit 7C is pushed toward the photoconductor 1 and brought into contact with the photoconductor 1 to develop cyan (C). To start. In the case of a copying machine or printer that uses four colors, the above-described developing operation is sequentially repeated four times, and the intermediate transfer member 16
Toner images of four colors B, C, M, and Y are superposed on each other to form a composite image. Also in the cyan printing process, the reference of the image forming timing is the intermediate transfer member reference position. In the composite image formed in this way, the paper transfer roller 27, which has been separated so far, comes into contact with the intermediate transfer body 16, and the paper transfer roller 27
A high voltage of about +1000 V is applied to the recording paper 22 and the recording paper 22 is collectively transferred to the recording paper 22 sent from the recording paper cassette 21 along the paper transport path 24 by pressure. Then, the recording paper 22 on which the toner image is transferred is sent to a fixing device 28, where it is fixed by the heat of the heating roller 29 and the nip pressure of the pressure roller 30 and is output as a color image. The residual toner on the intermediate transfer body 16 which is not completely transferred onto the recording paper 22 by the paper transfer roller 27 is removed by the intermediate transfer body cleaning device 20. The intermediate transfer member cleaning device 20 keeps the intermediate transfer member 1 until one composite image is obtained.
6, a composite image is obtained, and after the composite image is transferred to the recording paper 22 by the paper transfer roller 27,
Contact is made and residual toner is removed.

With the above operation, recording of one image is completed. FIG. 2 is a diagram showing the relationship between the number of printed sheets after the power is turned on and the heater forced lighting time in the first embodiment. In FIG. 2, the horizontal axis represents the heater forced lighting time before the recording paper 22 enters the fixing device 28, and the vertical axis represents the number of prints after the power is turned on. The heater lighting time table 61 of FIG. 1 stores the relationship of FIG. Specifically, the number of printed sheets after the power is turned on is set as an address, and the recording paper 2 is set at each address.
The data of the heater compulsory lighting time before 2 enters the fixing device 28 is stored. The heater lighting time calculation means 59 in FIG. 1 calculates the heater forced lighting time according to the number of prints after the power is turned on, and creates the table shown in FIG.

FIG. 3 is a diagram showing the relationship between the number of printed sheets after the power is turned on and the set fixing temperature during the heater forced lighting period in the second embodiment. In FIG. 3, the horizontal axis shows the fixing set temperature during the heater forced lighting period before the recording paper 22 enters the fixing device 28, and the vertical axis shows the number of prints after the power is turned on. The fixing set temperature table 62 shown in FIG.
Is remembered. Specifically, the number of printed sheets after the power is turned on is used as an address, and each address stores the fixing set temperature during the heater forced lighting period before the recording paper 22 enters the fixing device 28. The fixing set temperature calculating means 60 in FIG. 1 calculates the fixing set temperature during the heater forced lighting period according to the number of prints after the power is turned on, and creates the table shown in FIG.

Next, the temperature control flow of the fixing device of the image forming apparatus according to the first embodiment of the present invention will be described in detail with reference to FIG. First, the power of the entire apparatus is turned on (step (STEP) 1-1). At the same time, the fixing device temperature control is started (step 1-2), and the heating roller 2
The surface temperature of 9 begins to rise. Then, rotation of the heating roller 29 and the pressure roller 30 is also started by the driving means (not shown) (step 1-3). Further, the initialization of the electrophotographic process is started (step 1-4) and waits until the initialization of the electrophotographic process is completed (step 1-
5). The surface temperature of the heating roller continues to rise even during the initialization of the electrophotographic process. Note that the surface temperature of the heating roller may reach the fixing set temperature during the initialization of the electrophotographic process. Although not shown here, the heater 101 is independently managed, so that the heater 101 is not always on and the fixing device 28 does not reach an abnormally high temperature. When the initialization of the electrophotographic process is completed, the process proceeds to step 1-6. Step 1
At -6, the heating roller surface temperature is waited until it reaches the fixing set temperature. During this time, the surface temperature of the pressure roller also approaches the set fixing temperature. When the heating roller surface temperature reaches the fixing set temperature, the rotations of the heating roller 29 and the pressure roller 30 are stopped (step 1-7). At this point in time, the warm-up ends, the image forming apparatus enters the standby state, and the standby time starts counting (step 1-8).

In this standby state, it is judged whether or not there is a print start command (step 1-9). If there is no print start command, the CPU 51 checks for an error (step 1-10). If the error detection unit 56 detects an error such as abnormally high temperature or thermistor open, error processing is performed and the entire electrophotographic process is stopped (step 1-11). If no error is detected, the process returns to step 1-9. If there is a print start command in step 1-9, each print process is sequentially started to form an image (step 1-12), and at the same time, rotation of the heating roller 29 and the pressure roller 30 is also started. At this time, it is determined whether or not the standby time of the image forming apparatus exceeds the specified value (step 1-13), and if it exceeds the specified value, the number of printed sheets after power-on is reset to 0 (step 1). -14). After that, the heater forced lighting time is set according to the count value of the number of printed sheets after the power is turned on (step 1-15). This heater forced lighting time is obtained by accessing the heater lighting time table 61.

Thereafter, after waiting for a prescribed time (step 1-16), the CPU 51 also checks for errors during printing (step 1-17). If the error detection unit 56 detects an error such as abnormal high temperature or thermistor open, error processing is performed and the entire electrophotographic process is stopped (step 1-18). If no error is detected, heater forced lighting control according to the number of counts is started (step 1-19), and the end of the heater forced lighting control period is awaited (step 1-20). During this time, the heater 101 remains on.

Next, when the heater forced lighting control period ends, it is checked whether or not the recording paper 22 is ejected outside the machine (step 1-21), and if the recording paper 22 is ejected outside the machine, the printed sheet count value is obtained. Is incremented (step 1-22). The ejection of the recording paper 22 to the outside of the machine is checked by a recording paper detection sensor (not shown) provided downstream of the fixing device 28. Then, it is determined whether or not the entire printing process is completed (step 1-23), and if the entire printing process is completed, the rotation of the heating roller 29 and the pressure roller 30 is stopped, and the process returns to step 1-8. .

Next, the temperature control flow of the fixing device of the image forming apparatus according to the second embodiment of the present invention will be described in detail with reference to FIG. First, the power of the entire device is turned on (step (2-1)). At the same time, the fixing device temperature control is started (step 2-2), and the heating roller 2
The surface temperature of 9 begins to rise. Then, rotation of the heating roller 29 and the pressure roller 30 is also started by the driving means (not shown) (step 2-3). Further, the initialization of the electrophotographic process is started (step 2-4) and waits until the initialization of the electrophotographic process is completed (step 2-
5). The surface temperature of the heating roller continues to rise even during the initialization of the electrophotographic process. Note that the surface temperature of the heating roller may reach the fixing set temperature during the initialization of the electrophotographic process. Although not shown here, the heater 101 is independently managed, so that the heater 101 is not always on and the fixing device 28 does not reach an abnormally high temperature. When the initialization of the electrophotographic process is completed, the process proceeds to step 2-6. Step 2
At -6, the heating roller surface temperature is waited until it reaches the fixing set temperature. During this time, the surface temperature of the pressure roller also approaches the set fixing temperature. When the surface temperature of the heating roller reaches the fixing set temperature, the rotations of the heating roller 29 and the pressure roller 30 are stopped (step 2-7). At this point, the warm-up is completed, the image forming apparatus enters the standby state, and the standby time counting is started (step 2-8).

In this standby state, it is judged whether or not there is a print start command (step 2-9). If there is no print start command, the CPU 51 checks for an error (step 2-10). If the error detection unit 56 detects an error such as an abnormally high temperature or thermistor open, error processing is performed and the entire electrophotographic process is stopped (step 2-11). If no error is detected, the process returns to step 2-9. If there is a print start command in step 2-9, each print process is sequentially started to form an image (step 2-12), and at the same time, rotation of the heating roller 29 and the pressure roller 30 is also started. At this time, it is determined whether or not the standby time of the image forming apparatus exceeds the specified value (step 2-13), and if it exceeds the specified value, the number of printed sheets after power-on is reset to 0 (step 2). -14). After that, the fixing set temperature is set according to the count value of the number of printed sheets after the power is turned on (step 2-15). This is obtained by accessing the fixing set temperature table 62.

Thereafter, after waiting for a prescribed time (step 2-16), the CPU 51 also checks for errors during printing (step 2-17). If the error detection unit 56 detects an error such as an abnormally high temperature or thermistor open, error processing is performed and the entire electrophotographic process is stopped (step 2-18). If no error is detected, heater forced lighting control according to the number of counts is started (step 2-19), and the end of the heater forced lighting control period is awaited (step 2-20). During this time, the heater 101 remains on. Next, when the heater forced lighting control period ends, it is checked whether or not the recording paper 22 is ejected outside the machine (step 2-21). When the recording paper 22 is ejected outside the machine, the print sheet count value is incremented. (Step 2-22). The ejection of the recording paper 22 to the outside of the machine is checked by a recording paper detection sensor (not shown) provided downstream of the fixing device 28. Then, it is determined whether or not the entire printing process is completed (step 2
-23), if the entire printing process is completed, the rotation of the heating roller 29 and the pressure roller 30 is stopped, and the process returns to step 2-8.

Next, the actual control results based on the operation flows of the first and second embodiments will be described in detail with reference to FIGS. 6 and 7. In FIG. 6 and FIG. 7, the solid line shows the fixing set temperature (Tst), and the solid line shows the actual temperature detected by the thermistor (temperature detection unit 54) on the surface of the heating roller. First, the fixing device temperature control result when the number of printed sheets is small after the power is turned on will be described with reference to FIG.

At the time of the time axis X0, the image forming apparatus is powered on. After the power is turned on, both the heating roller 29 and the pressure roller 30 start to rotate, the heater 101 in the heating roller 29 is turned on, and the surface temperature of the heating roller is the temperature T.
It starts rising from on. At this point, the initialization of the electrophotographic process is also started. In the initialization of the electrophotographic process, the presence / absence of a unit necessary to execute the electrophotographic process is checked and the operation of various motors is checked. If an error is detected in these checks, error processing is performed and all operations are stopped. The heater 101 continues to be turned on until the surface temperature of the heating roller reaches the fixing set temperature Tst. If the time point when the temperature Tst is reached is X1, the period during which the heater 101 is on is from X0 to X1.

At the time of X1, the heater 101 is turned off and the warm-up period ends. After that, the process is in a standby state, and if a print start command is input, the image forming operation is started. X2
It is assumed that the print start command is given at the time of. During standby, the heating roller 29 and the pressure roller 30 are always in contact with each other at the same portion, so the surface temperature on the pressure roller 30 side gradually decreases except for the vicinity of the portion in contact with the heating roller 29. After the print start command is given, the heating roller 29 and the pressure roller 30 start rotating, and heat conduction from the heating roller 29 to the pressure roller 30 causes the heating roller 2 to rotate.
The surface temperature of No. 9 should drop sharply, but since the heater forced lighting control or the fixing set temperature is increased at the time of X3, even if the heating roller surface temperature exceeds the predetermined fixing set temperature, the heater Is turned on, and a sudden decrease in the surface temperature of the heating roller can be prevented. Therefore, when the recording paper 22 reaches the entrance of the fixing device 28, the surface temperature of the heating roller is around the fixing set temperature Tst, and the fixing process is performed without delay. The termination of the forced heater lighting at X4 or the normal restoration of the fixing set temperature is performed according to the output result from the heater lighting time calculating means 59 or the fixing set temperature calculating means 60.

Next, the fixing device temperature control result when the number of printed sheets is large after the power is turned on will be described with reference to FIG. At the time of the time axis X0, the image forming apparatus is powered on. After the power is turned on, both the heating roller 29 and the pressure roller 30 start rotating, the heater 101 in the heating roller 29 is turned on, and the surface temperature of the heating roller starts to rise from the temperature Ton. At this point, the initialization of the electrophotographic process is also started. In the initialization of the electrophotographic process, the presence or absence of the unit necessary for executing the electrophotographic process is checked,
Check the operation of various motors. If an error is detected in these checks, error processing is performed and all operations are stopped. The heater 101 continues to be turned on until the surface temperature of the heating roller reaches the fixing set temperature Tst. Temperature Ts
If the time point when t is reached is X1, the period during which the heater 101 is on is from X0 to X1.

At the time of X1, the heater 101 is turned off and the warm-up period ends. After that, the process is in a standby state, and if a print start command is input, the image forming operation is started. X2
It is assumed that the print start command is given at the time of. During standby, the heating roller 29 and the pressure roller 30 are always in contact with each other at the same portion, so the surface temperature on the pressure roller 30 side gradually decreases except for the vicinity of the portion in contact with the heating roller 29. After the print start command is given, the heating roller 29 and the pressure roller 30 start rotating, and heat conduction from the heating roller 29 to the pressure roller 30 causes the heating roller 2 to rotate.
The surface temperature of No. 9 should drop sharply, but since the heater forced lighting control or the fixing set temperature is increased at the time of X3, even if the heating roller surface temperature exceeds the predetermined fixing set temperature, the heater Is turned on, and a sudden decrease in the surface temperature of the heating roller can be prevented. Therefore, when the recording paper 22 reaches the entrance of the fixing device 28, the surface temperature of the heating roller is around the fixing set temperature Tst, and the fixing process is performed without delay. The termination of the forced heater lighting at X4 or the normal restoration of the fixing set temperature is performed according to the output result from the heater lighting time calculating means 59 or the fixing set temperature calculating means 60.

As described above, according to the above embodiment, the number of printed sheets after the power is turned on is counted, and the heater 101 corresponding to the counted number before the recording paper 22 reaches the fixing device 28 during the first printing. Performs forced lighting control of. That is, when the count number is small, the pressure roller 30 is not sufficiently warmed, and the temperature of the heating roller 29 and the temperature of the heating roller 29 at the start of rotation of the pressure roller 30 is significantly lowered, so that the heater forced lighting time is lengthened. Alternatively, the heater 101 is turned on by increasing the fixing set temperature without introducing the heater forced lighting sequence. When the number of counts is large, it is considered that the pressure roller 30 is also sufficiently warm, so the heater 101 is forcibly turned on, but the time is controlled to be short. Alternatively, the heater 101 is turned off by lowering the set fixing temperature. When the time from the end of printing further elapses, the temperature of the pressure roller 30 gradually decreases to near room temperature except in the vicinity of contact with the heating roller 29. Try to reset the number. As a result, even if the print start command is issued in any case, the fixing process is performed at the set fixing temperature, and the fixing temperature varies from print to print, resulting in poor print quality such as cold offset and hot offset, and high temperature error. However, there is no problem such as the occurrence of the problem or the breakage of the fixing device, and it is possible to prevent the deterioration of the print quality due to the variation of the fixing temperature in the first print with an extremely simple configuration.

[0054]

According to the image forming apparatus of the present invention, one of the first and second rollers is heated by the heating means after the heating means once heats the temperature to the set temperature and before the recording medium reaches the fixing device. Is forcibly heated, and the forcible heating time is made longer as the count value (the number of times of image formation) of the counting means becomes smaller. That is, the number of times of image formation (the number of prints) after the power is turned on is counted, and when the count value is small in the first print, the roller that is not heated (pressurizing roller) is not sufficiently warmed. Since the temperature of the heating roller (heating roller) at the start of rotation of the second roller is remarkably lowered, the forced heating time before the recording medium reaches the fixing device is lengthened. When the count value is large, it is considered that the roller not heated is sufficiently warmed, and therefore the heating time is controlled to be short.

In the image forming apparatus according to the second aspect of the present invention, after the heating means once heats the temperature to the set temperature and before the recording medium reaches the fixing device, the set temperature of the heating means is counted by the counting means (the number of times of image formation). ) Becomes smaller, the higher it rises. That is, the number of times of image formation (the number of prints) after the power is turned on is counted, and when the count value is small in the first print, the roller that is not heated (pressurizing roller) is not sufficiently warmed. Since the temperature of the roller to be heated (heating roller) at the start of rotation of the second roller is remarkably lowered, the set temperature of the heating means is raised to a high temperature before the recording medium reaches the fixing device, whereby forced heating is performed for a long time. If the count value is large, it is conceivable that the roller that is not heated is also sufficiently warmed, so the set temperature of the heating means is set low, and the forced heating time is shortened or not heated.

Further, the image forming apparatus according to claim 3 is provided with timer means for counting the time from the end of the image forming operation to the start of the next image forming operation, and when the count value of the timer means exceeds a certain value. I am trying to reset the counting means. This is because after the end of the image forming operation, that is, the time from the end of printing, the temperature of the pressure roller gradually decreases to near room temperature except in the vicinity where it is in contact with the heating roller. After a lapse of time, the count value of the counting means is reset so that the recording medium is forcibly heated before reaching the fixing device.

As a result, even if the print start command is issued in any case, the fixing step is carried out at the set fixing temperature, and a difference in the fixing temperature occurs for each printing, and the print quality is deteriorated such as cold offset and hot offset. A problem such as occurrence of a high-temperature abnormal error or breakage of the fixing device does not occur, and it is possible to prevent deterioration of printing quality due to variations in fixing temperature in first printing with a very simple configuration.

[Brief description of drawings]

FIG. 1 is a control block diagram of a fixing device of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the number of printed sheets after the power is turned on and the heater forced lighting time of the image forming apparatus according to the first embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the number of printed sheets after the power is turned on and the set fixing temperature during the heater forced lighting period of the image forming apparatus according to the second embodiment of the present invention.

FIG. 4 is a flow chart of a fuser temperature control of the image forming apparatus according to the first embodiment of the present invention.

FIG. 5 is a flow chart of a fuser temperature control of the image forming apparatus according to the second embodiment of the present invention.

FIG. 6 is a diagram showing a fixing device temperature control result when the number of printed sheets is small after the power is turned on in the image forming apparatus according to the embodiment of the present invention.

FIG. 7 is a diagram showing a fixing device temperature control result when the number of printed sheets is large after the power is turned on in the image forming apparatus according to the embodiment of the present invention.

FIG. 8 is a configuration diagram of a conventional image forming apparatus.

FIG. 9 is a configuration diagram of a fixing device of a conventional image forming apparatus.

FIG. 10 is an enlarged view of a fixing portion of a fixing device of a conventional image forming apparatus.

FIG. 11 is a control block diagram of a fixing device of a conventional image forming apparatus.

FIG. 12 is a flowchart of a fixing device temperature control of a conventional image forming apparatus.

FIG. 13 is a diagram showing a fixing temperature control result when the number of printed sheets is small after the power of the conventional image forming apparatus is turned on.

FIG. 14 is a diagram showing a fixing temperature control result when the conventional image forming apparatus has a large number of printed sheets after the power is turned on.

[Explanation of symbols]

 1 Photoconductor 2 Photoconductor Conveying Roller 3 Photoconductor Position Detection Sensor 4 Photoconductor Conveying Roller 5 Charging Device 6 Exposure Optical System 7K Black Developer 7Y Yellow Developer 7M Magenta Developer 7C Cyan Developer 8 Intermediate Transfer Unit 9 Photoconductor Cleaning device 10 Static eliminator 11 Charged wire 12 Shield plate 13 Grid plate 14 Exposure beam 15K Separation cam (black) 15Y Separation cam (yellow) 15M Separation cam (magenta) 15C Separation cam (cyan) 16 Intermediate transfer body 17 Intermediate transfer body transport roller 18 Intermediate transfer body transport roller 19 Intermediate transfer roller 20 Intermediate transfer body cleaning device 21 Recording paper cassette 22 Recording paper 23 Paper feed roller 24 Paper transport path 25 Registration roller 26 Follower roller 27 Transfer roller 28 Fixer 29 Heating Roller 30 Pressure roller 31K Black developing sleeve 31C Cyan developing sleeve 31M Magenta developing sleeve 31Y Yellow developing sleeve 51 CPU 52 Data latch section 53 Data comparing section 54 Temperature detecting section 55 A / D converting section 56 Error detecting section 57 Gate section 58 Heater driver section 59 Heater lighting time Calculating means 60 Fixing setting temperature calculating means 61 Heater lighting time table 62 Fixing setting temperature table 101 Heater 102 Pressure roller support member 103 Discharging roller 104 Discharging roller 105 Discharging tray 106 Pin 107 Recording paper guide 108 Unfixed toner image

Claims (3)

[Claims] 1. A first and a second roller for nipping and conveying a recording medium, a heating means for heating one of the first and the second roller to a set temperature, and the first and the second roller. A fixing unit having a driving unit for rotating the roller and a temperature detecting unit for detecting the temperature of at least one of the first and second rollers; and a fixing unit disposed at a position upstream of the first and second rollers. A recording medium detection unit that detects a recording medium and a counting unit that counts the number of times of image formation are provided, and the heating unit temporarily heats the recording medium to a set temperature, and then the heating unit before the recording medium reaches the fixing device. An image forming apparatus in which one of the first and second rollers is forcibly heated by means, and the forcible heating time becomes longer as the count value of the counting means becomes smaller. 2. A first and a second roller for nipping and conveying a recording medium, a heating means for heating one of the first and the second roller to a preset temperature, and the first and the second roller. A fixing unit having a driving unit for rotating the roller and a temperature detecting unit for detecting the temperature of at least one of the first and second rollers; and a fixing unit disposed at a position upstream of the first and second rollers. A recording medium detection unit that detects a recording medium and a counting unit that counts the number of times of image formation are provided, and the heating unit temporarily heats the recording medium to a set temperature, and then the heating unit before the recording medium reaches the fixing device. An image forming apparatus in which the set temperature of the means is increased as the count value of the counting means decreases. 3. A timer means for counting the time from the end of the image forming operation to the start of the next image forming operation is provided, and the counting means is reset when the count value of the timer means exceeds a certain value. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.