JP2009103986A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which does not require a cooling device, and can prevent tacking while achieving miniaturization of the apparatus. <P>SOLUTION: The image forming apparatus equipped with a fixing part having a heater temporarily turns off a heater to stop heating of the fixing part (S32) before starting print of a first OHP sheet when continuously performing a job to perform print of the OHP sheet after a job to perform print of two or more sheets of plain paper. Time measurement is started (S33). When determining that a predetermined time T1, assumed to be required to lower temperature of a housing tray to temperature at which tacking is not caused even though print of the OHP sheet is started, elapses ("YES" in S34), 200[°C] is set as fixing target temperature for the OHP sheet (S35), and fixing temperature control to raise the temperature of the fixing part so as to attain the target temperature is performed (S36), then print of the first OHP sheet is started. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method for thermally fixing an image on a sheet.

In an image forming apparatus such as a printer, a sheet is conveyed one by one, a toner image is formed on the conveyed sheet, and the formed toner image is melted by heat of a fixing unit and thermally fixed on the sheet. After that, there are many structures that are accommodated in an accommodation tray.
In such a heat fixing method, if a sheet having a larger heat capacity than plain paper such as an OHP sheet or coated paper is used, the temperature of the sheet is difficult to decrease even after fixing, so that the sheet is stored in the storage tray with the temperature kept high. . In addition, since the OHP sheet and the coated paper have high flatness, the sheets are likely to adhere to each other after being accommodated in the accommodation tray. Therefore, for example, when a plurality of OHP sheets having a high temperature are continuously stacked on the storage tray, the toner images are softened and attracted by the heat of the OHP sheets themselves while the OHP sheets are in close contact with each other. Tacking is likely to occur. If the OHP sheets stuck by the tacking are forcibly separated, the toner image is peeled off and it becomes necessary to restart printing.

Since the above-described tacking is likely to occur when the sheet immediately after fixing is stored in the storage tray while the temperature of the sheet remains high, a configuration for cooling the sheet after fixing has been proposed. Patent Document 1 discloses a configuration in which a cooling device is disposed at a position downstream of the fixing device in the sheet conveying direction to cool the sheet after fixing. Patent Document 2 discloses a configuration in which the conveyance speed from the fixing device to the discharge roller is lowered to cool the sheet during the conveyance.
JP 2004-252045 A JP 2003-287972 A

However, the configuration of Patent Document 1 has a problem that it is necessary to provide a dedicated cooling device, which increases costs. Further, in the configuration of Patent Document 2, unless the transport distance from the fixing device to the discharge roller is increased to some extent, the cooling effect by lowering the transport speed cannot be obtained, and the apparatus cannot be downsized.
SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus and an image forming method capable of preventing tacking while reducing the size of the apparatus without requiring a cooling device. .

  In order to achieve the above object, the present invention provides an image forming apparatus for forming an image on a conveyed sheet, the image forming apparatus being disposed in the vicinity of a discharge port of a sheet conveying path, wherein the first rotating body is a second rotating body. Fixing means for heating at least one rotating body with a heating means while securing a fixing nip between the two while being in pressure contact with each other, and thermally fixing an image formed on the sheet through the fixing nip to the conveyed sheet; When receiving an instruction to carry out image formation by conveying a specific type of sheet, the heating unit is controlled so that the rotating body is rotated and the temperature of the rotating body is lower than the target temperature before starting image formation. And control means for switching the control of the heating means so as to raise the temperature to the target temperature when a predetermined condition for raising the temperature to the target temperature is satisfied.

  According to another aspect of the present invention, there is provided an image forming method in an image forming apparatus for forming an image on a conveyed sheet, wherein the first rotating body is a second rotating member in a fixing unit disposed near the discharge port of the sheet conveying path. Fixing is performed in such a manner that at least one of the rotating bodies is heated by a heating unit while the fixing nip is secured between the two rotating bodies and the image formed on the sheet is heat-fixed through the fixing nip. When receiving an instruction to perform image formation by transporting a specific type of sheet, the rotating body is rotated and heated so that the temperature of the rotating body is lower than a target temperature before starting image formation. A control step of switching the control of the heating means so as to raise the temperature to the target temperature when the predetermined condition for controlling the means and raising the temperature to the target temperature is satisfied. And wherein the executing step.

  Here, “in the vicinity of the discharge port of the sheet conveyance path” is used within the range in which the influence of heat by the heating unit extends to the tray for storing the discharged sheet.

  As a result, for example, when an image is formed on a specific type of sheet, even if the temperature of the sheet already accommodated in the tray reaches the tacking temperature of the specific type of sheet, the sheet rotates before the start of image formation. Once the body temperature is lowered, image formation can be started after waiting for the temperature of the tray and the accommodated sheets to drop as the temperature of the rotating body drops, preventing the occurrence of tacking and dedicated cooling. Since no apparatus is required, costs can be reduced, and further downsizing of the apparatus can be realized because there is no need to lengthen the sheet conveyance path to the discharge port.

  In addition, the control unit includes a first timer that measures an elapsed time from the completion of the previous image formation, and the instruction is performed after the time measured by the first timer reaches a first predetermined time. Is received, the control of the heating means before the image formation is prohibited, and the image forming is executed by controlling the heating means so that the temperature of the rotating body becomes the target temperature.

And a temperature detecting means for detecting the temperature of the rotating body, wherein the control means is when the temperature of the rotating body when receiving the instruction is equal to or lower than a first predetermined temperature lower than the target temperature. Is characterized in that the control of the heating means before the image formation is prohibited and the image forming is executed by controlling the heating means so that the temperature of the rotating body becomes a target temperature.
In this way, it is possible to start the image formation earlier and to reduce the waiting time of the user because it is not necessary to control the heating means before the image formation as well as preventing the tacking.

Further, the control means has a second timer for measuring an elapsed time since the start of the control so as to reach the low temperature, and when the measurement time by the second timer reaches a second predetermined time, The condition is satisfied.
In this way, it can be determined by measuring time that the predetermined condition is satisfied.

  Further, the control unit includes a tray that is disposed immediately below the discharge port and accommodates a sheet discharged from the discharge port, and a sheet detection unit that detects the presence or absence of the discharged sheet on the tray. When it is detected that there is no discharged sheet on the tray, the second predetermined time value is set to a smaller value than when the presence of the sheet is detected. Features.

In this way, when the discharged sheet does not exist on the tray, image formation can be started earlier, and the waiting time of the user can be further shortened.
Here, the second predetermined time is the time required for the temperature of the rotating body to decrease to the second predetermined temperature after starting the control of the heating means so as to be the low temperature, or The time required for the temperature of the tray, which is disposed immediately below the discharge port and that accommodates the sheet discharged from the discharge port, to be lowered to a third predetermined temperature.

In addition, a temperature detection unit that detects the temperature of the rotating body is provided, and the control unit starts controlling the heating unit so that the temperature becomes the low temperature, and then the temperature of the rotating body reaches a second predetermined temperature. When lowered, the condition is satisfied.
In this way, it can be determined by detecting the temperature of the rotating body that the predetermined condition is satisfied.

  Further, the control unit includes a tray that is disposed immediately below the discharge port and accommodates a sheet discharged from the discharge port, and a sheet detection unit that detects the presence or absence of the discharged sheet on the tray. When it is detected that there is no discharged sheet on the tray, the second predetermined temperature value is set to a higher value than when the presence of the sheet is detected. Features.

In this way, when the discharged sheet does not exist on the tray, image formation can be started earlier, and the waiting time of the user can be further shortened.
And a tray that is disposed immediately below the discharge port and that accommodates a sheet discharged from the discharge port; and a temperature detection unit that detects a temperature of the tray, and the control unit controls the temperature of the tray. And determining means for determining whether or not the condition is satisfied.

In this way, it can be determined by detecting the temperature of the tray that the predetermined condition is satisfied.
Further, controlling the heating means so that the temperature of the rotating body is lower than a target temperature is to stop supplying power to the heating means.
In this way, the temperature of the rotating body and thus the tray that accommodates the discharged sheet can be lowered more quickly, and at least power can be saved until the temperature rises, and the control of power supply is simplified. Can be planned.

The specific type of sheet is an OHP sheet or coated paper.
In this way, it is possible to prevent tacking of the OHP sheet and the coated paper.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of an image forming apparatus and an image forming method according to the present invention will be described below by taking a tandem color digital printer (hereinafter simply referred to as “printer”) as an example.
<First Embodiment>
(1) Overall Configuration of Printer FIG. 1 is a diagram showing the overall configuration of the printer 10.

  As shown in the figure, the printer 10 forms an image by a well-known electrophotographic system, and includes an image processing unit 11, a feeding unit 12, a fixing unit 13, and a control unit 14, and includes a network ( For example, when an instruction to execute a print (print) job is received from an external terminal device (not shown) connected to a LAN, an image of a color composed of yellow, magenta, cyan, and black is formed based on the instruction. Execute. Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, and the Y, M, C, and K may be added as subscripts to the numbers of the components related to the reproduction colors. is there.

The image processing unit 11 includes image forming units 20Y, 20M, 20C, and 20K corresponding to Y to K colors, an intermediate transfer belt 21, and the like.
The image forming unit 20Y includes a photosensitive drum 1, a charger 2, an exposure unit 3, a developing unit 4, a primary transfer roller 5, a cleaner 6 for cleaning the photosensitive drum 1, and the like disposed around the photosensitive drum 1. A Y-color toner image is formed on the photosensitive drum 1. The other image forming units 20M to 20K have the same configuration as that of the image forming unit 20Y, and create corresponding color toner images. In the figure, the reference numerals are omitted.

The intermediate transfer belt 21 is an endless belt, is stretched around a driving roller 22 and a driven roller 23, and is circulated and driven in the direction of an arrow.
The feeding unit 12 includes paper feeding cassettes 31 and 32, feeding rollers 33 and 34, a conveyance roller pair 35, a timing roller pair 36, and a driving roller 22 across the intermediate transfer belt 21 at the secondary transfer position 371. An opposing secondary transfer roller 37 is provided.

The paper feed cassette 31 accommodates plain paper S as a recording sheet. A specific type of special paper P having a larger heat capacity than the plain paper S, such as an OHP sheet or coated paper, is set in the paper feed cassette 32. Here, it is assumed that an OHP sheet is set.
The feed roller 33 feeds the plain paper in the paper feed cassette 31 to the sheet transport path 38 one by one, and the feed roller 34 feeds the OHP sheet in the paper feed cassette 32 to the sheet transport path 38 one by one. Hereinafter, when the plain paper and the OHP sheet are not particularly distinguished, they are collectively referred to as a sheet.

The conveyance roller pair 35 conveys the sheet fed on the sheet conveyance path 38 to the timing roller pair 36. The timing roller pair 36 takes a timing to send the conveyed sheet to the secondary transfer position 371.
The fixing unit 13 includes a fixing roller 51, a pressure roller 52, a fixing heater 53, a temperature detection sensor 54, a roller driving motor 55, and the like.

The fixing roller 51 includes a metallic cylindrical cored bar and a release layer such as a fluororesin provided on the surface thereof. The pressure roller 52 is composed of a metal cylindrical cored bar and an elastic layer such as silicone rubber provided on the surface thereof. The pressure roller 52 is pressed against the opposing fixing roller 51, and the fixing nip 50 is formed between the two by this pressure contact. Secured.
The fixing roller 51 is rotationally driven in the direction of the arrow in the drawing by the driving force of the roller driving motor 55, and the pressure roller 52 rotates following the fixing roller 51.

The fixing heater 53 is a halogen lamp heater, is inserted into the fixing roller 51, is supplied with electric power from a power supply source (not shown), generates heat, and heats the fixing roller 51. The temperature detection sensor 54 is a temperature detection means including a known thermistor and detects the temperature of the surface of the fixing roller 51.
The control unit 14 converts an image signal from an external terminal device into a digital signal for Y to K colors, and generates a drive signal for driving the laser diode of the exposure unit 3 for each image forming unit. With the generated drive signal, the laser diode of the exposure unit 3 for each color is driven to emit a laser beam L, and the photosensitive drum 1 is exposed and scanned.

Before the exposure scan, the photosensitive drum 1 is uniformly charged by the charger 2 for each image forming unit, and an image to be formed on the photosensitive drum 1 is exposed by the laser beam L exposure. An electrostatic latent image is formed, and the formed electrostatic latent image is developed with toner by the developing device 4.
The toner images of the respective colors are primarily transferred onto the intermediate transfer belt 21 under the action of electrostatic force due to the electric field generated between the primary transfer roller 5 and the photosensitive drum 1. At this time, the image forming operations for the respective colors are executed at different timings so that the toner images are superimposed and transferred at the same position on the intermediate transfer belt 21. Each color toner image superimposed on the intermediate transfer belt 21 is moved to the secondary transfer position 371 by the rotation of the intermediate transfer belt 21.

  In accordance with the timing of the image forming operation, a sheet is fed from the feeding unit 12 via a pair of timing rollers 36. The sheet is fed to the intermediate transfer belt 21 that is driven to circulate, and to this. The toner images on the intermediate transfer belt 21 are batched by being sandwiched and conveyed between the secondary transfer rollers 37 that are in pressure contact and subjected to the action of electrostatic force generated by the electric field generated between the secondary transfer roller 37 and the drive roller 22. Secondarily transferred to the sheet.

The sheet that has passed the secondary transfer position 371 is conveyed to the fixing unit 13, and when passing through the fixing nip 50, the toner image is heated and pressurized to be fixed on the sheet, and then discharged from the discharge port 39. It is stored in a storage tray 40 provided at a position directly below the discharge port 39.
(2) Configuration of Control Unit 14 FIG. 2 is a block diagram showing the configuration of the control unit 14.

As shown in the figure, the control unit 14 includes a CPU 71, a communication interface (I / F) unit 72, a ROM 73, a RAM 74, a fixing target temperature storage unit 75, a timer 76, and the like as main components. Prepare.
The communication I / F unit 72 is an interface for connecting to a LAN, such as a LAN card or a LAN board, and receives print job data from the outside.

  The CPU 71 reads a necessary program from the ROM 73, controls the operations of the image processing unit 11, the fixing unit 13, etc. in a unified manner with timing, and prints based on the print job data received by the communication I / F unit 72. Make the operation run smoothly. At this time, the sheet information added to the job data is read. The sheet information is information indicating which type of sheet is used in the job. This sheet information is designated by input or the like from the screen of the terminal device when the user instructs the execution of the job at the terminal device, and is added to the job data.

The CPU 71 selects whether to use plain paper or an OHP sheet in the job from the read sheet information, and feeds the selected sheet from the feeding unit 12 to perform printing.
The CPU 71 receives a detection signal from the temperature detection sensor 54, detects the surface temperature of the fixing roller 51, and controls the fixing heater 53 so that the surface temperature of the fixing roller 51 is maintained at a target temperature. For example, energization control such as alternately turning on and off is performed. Hereinafter, the surface temperature of the fixing roller 51 is referred to as “fixing portion temperature”, and the target temperature is referred to as “fixing target temperature”.

  Further, in the job for printing on the OHP sheet, the CPU 71 sets the fixing heater 53 for the first OHP sheet to be printed first among the OHP sheets specified in the job before image formation. OHP fixing temperature control is performed in which the image forming is started by switching the control so that the temperature of the fixing unit is lowered for a predetermined time, the temperature of the fixing unit is turned on again, and the temperature is increased to the target temperature. Such control is performed to prevent tacking that tends to occur in the OHP sheet. Details of the control will be described later.

  The fixing target temperature storage unit 75 is a non-volatile storage unit, and data indicating the fixing target temperature, for example, 180 [° C.] as a target temperature during standby (while waiting for an instruction to execute the next job), plain paper Data indicating 190 [° C.] as a target temperature for 200 and 200 [° C.] as a target temperature for the OHP sheet are stored. The value of the fixing target temperature is set in advance through experiments or the like and stored in the fixing target temperature storage unit 75. Needless to say, the fixing target temperature is not limited to the above value. An appropriate value is set according to the device configuration.

(3) Processing Contents of Print Job by CPU 71 FIGS. 3 to 5 are flowcharts showing processing contents executed by the CPU 71 when an execution instruction is received for each print job. FIG. 6 shows a fixing unit during job execution. 6 is a graph showing an example of an experimental result in which the temperature of the storage tray and the temperature of the surface of the storage tray (hereinafter referred to as “tray temperature”) change over time. Here, an example will be described in which a job β for printing on a plurality of sheets, for example, two OHP sheets, is executed continuously after a job α for printing on a plurality of sheets, for example, eight plain papers.

  In FIG. 6, time A corresponds to the time when the power is turned on, and the time up to time B is warming up. In the warm-up, the target temperature is set to the standby temperature (180 [° C.]), and the warm-up is completed at the point B when the temperature of the fixing unit is raised to 180 [° C.]. Between the time points A and B, the tray temperature gradually increases so as to follow the temperature increase of the fixing unit. This is because the storage tray 40 is disposed in the vicinity of the fixing unit 13 due to the downsizing of the apparatus, and is affected by the heat generated from the fixing unit 13. Further, since the temperature of the sheets discharged continuously during the job execution cannot be directly measured, the tray temperature is measured, and this is set as the temperature of the sheet bundle stored in the storage tray 40. The figure shows an example in which an execution instruction for job α is given at time point B and an execution instruction for job β is given at time point C (immediately after the end of plain paper printing).

The relationship between the temperature of the fixing unit and the temperature of the tray during execution of two jobs will be described below with reference to FIG.
As shown in FIG. 3, when there is an instruction to execute job α, the value of variable n is set to “1” (step S1). Then, the fixing roller 51 is driven to rotate (step S2). If the fixing roller 51 is actually being driven to rotate, the rotation is continued.

Next, it is determined whether or not the designated sheet is an OHP sheet (step S3). Here, it is determined that the paper is plain paper (“NO” in step S3), and the plain paper fixing temperature control is executed (step S4).
FIG. 4 is a flowchart showing the contents of a subroutine for controlling the plain paper fixing temperature.
As shown in the drawing, the fixing target temperature data for plain paper stored in the fixing target temperature storage unit 75 is read, the fixing target temperature is set to 190 [° C.] (step S21), and the fixing unit The fixing temperature control for controlling the fixing heater 53 is started so as to maintain the temperature at the fixing target temperature (step S22), and the process returns to the main routine. Immediately before the job α execution instruction is received, the temperature of the fixing unit is about 180 [° C.], so the fixing heater 53 is controlled so that the temperature is raised to 190 [° C.].

Returning to FIG. 3, in step S <b> 5, an image forming operation for the nth sheet, here, the first plain sheet is started. Specifically, an operation of feeding plain paper from the paper feed cassette 31 is executed along with operations such as charging, exposure, and development in the image processing unit 11.
When the image formation on the first plain paper is completed (step S6), it is determined whether or not the current n, here the first sheet, is the last (step S7). If it is determined that it is not the last ("NO" in step S7), "1" is incremented to the current value of n, here n = 2 is set (step S8), and the process returns to step S3.

  In step S3, it is determined whether or not the second sheet is an OHP sheet. In the example of job α, since it is plain paper, the process proceeds to step S4. The processing after step S4 is the same as the first sheet. Until n = 8, the processes in steps S3 to S8 are repeatedly executed. When n = 8 is determined (“YES” in step S7), the process ends. As a result, eight plain papers after printing are stored in the storage tray 40 so as to be stacked (time points B to C in FIG. 6).

Between time points B to C in FIG. 6, the plain paper immediately after fixing is continuously stored in the storage tray 40, so that the tray temperature gradually rises. Enters into the fiber of the paper, has high fixability, and has a small heat capacity, so that it is easy to cool, and the adhesion between plain papers is also low, so that tacking hardly occurs.
On the other hand, since the OHP sheet is a resin film, the toner particles cannot enter the fiber of the paper, and the fixing property is lower than that of plain paper. In addition, since the temperature at the time of fixing is higher than that of plain paper and the heat capacity is large, it is easy to maintain a high temperature state even after storage, and the flatness of the surface is high, so the sheets are easy to adhere to each other. For this reason, when the OHP sheet is stored in the storage tray 40, the toner image is in close contact with the surface of another sheet while being softened by the heat of the OHP sheet itself, and is likely to cause tacking. The tray temperature (tacking limit temperature) that becomes the boundary of whether or not tacking occurs is, for example, 50 [° C.].

At time point C, the tray temperature has risen to about 60 [° C.]. Therefore, if printing is performed on the OHP sheet by job β, the OHP after fixing is performed in a situation where the tray temperature exceeds the tacking limit temperature. The sheet is accommodated, and the probability of occurrence of tacking becomes extremely high.
Therefore, in order to prevent the occurrence of tacking, a process of lowering the temperature of the fixing unit is once performed before the start of the printing (time points C to D). That is, when an execution instruction for job β is received, n = 1 is set in step S1 in FIG. 3, and the fixing roller is driven to rotate in step S2, and it is determined that the designated sheet is an OHP sheet (step S1). “YES” in S3), OHP fixing temperature control is executed (step S9).

FIG. 5 is a flowchart showing the contents of the OHP fixing temperature control subroutine.
As shown in the figure, it is first determined whether or not the nth OHP sheet is the first (first) sheet in the job β (step S31). Here, the first sheet is determined (“YES” in step S31), heating of the fixing unit 13 is stopped (step S32), and time measurement by the timer 76 is started (step S33). Here, the stop of heating specifically turns off the energization to the fixing heater 53. The rotation driving of the fixing roller 51 is continued as it is. By continuing the rotation drive, the heat of the fixing roller 51 is transferred to the pressure roller 52 and is easily radiated.

As a result, the temperature of the fixing unit gradually decreases after time point C in FIG. When the temperature of the fixing unit decreases, the heat radiation by the storage tray 40 that has been suppressed by the influence of the heat of the fixing unit proceeds, and the tray temperature decreases following the temperature decrease of the fixing unit.
If the above heating is not stopped, the tray temperature will remain flat as shown by the broken line in the figure. In order to slow down the sheet conveyance speed, it is necessary to make the conveyance path from the fixing unit 13 to the discharge port 39 longer. If the conveying path is made longer, the OHP sheet can be cooled before discharging, but the position of the discharge port 39 is shifted to the downstream side in the sheet discharging direction by the length of the conveying path, and the storage tray 40 is also The entire apparatus is shifted to the downstream side in the sheet discharge direction by the same length, so that the apparatus becomes wider and consequently the apparatus becomes larger.

  Returning to FIG. 5, it is determined whether or not the measurement time by the timer 76 has passed the predetermined time T1 (step S34). Here, the predetermined time T1 corresponds to a time that is assumed to be required for the tray temperature to be lowered to a temperature at which tacking does not occur from the stop of heating of the fixing unit to 45 [° C.] in this example. From the example of FIG. 6, in the present embodiment, T1 = 35 [seconds].

As the value of the predetermined time T1, an appropriate value is obtained in advance from experiments or the like according to the apparatus configuration, and the data is stored in the ROM 73 or the like.
If it is determined that the predetermined time T1 has elapsed (“YES” in step S34), the timer 76 is stopped and the fixing target temperature data for the OHP sheet stored in the fixing target temperature storage unit 75 is read. The fixing target temperature is set to 200 ° C. (step S35), and fixing temperature control for controlling the fixing heater 53 is started so that the temperature of the fixing unit is maintained at the fixing target temperature (step S36). Return to the main routine.

  As a result, the temperature of the fixing unit starts to rise (after time D in FIG. 6). However, since the tray temperature follows the temperature change of the fixing unit with a certain time delay, the tray temperature decreases to about 40 ° C. Since it rises slowly and changes within the range of about 40-45 [° C], there is a margin of about 5-10 [° C] between the tacking limit temperature of 50 [° C] and the next OHP sheet If printing is performed on the head, the occurrence of tacking can be prevented.

  Returning to FIG. 3, in step S5, an image forming operation for the first OHP sheet is started (time D in FIG. 6). Immediately before the OHP sheet reaches the fixing nip 50, the temperature of the fixing portion has increased to nearly 200 ° C. (time E in FIG. 6). After the end of image formation (step S6), the discharged OHP sheet is stored on the storage tray 40 so as to be stacked on the already stored plain paper bundle (time point F in FIG. 6). At this time, since the tray temperature is about 45 [° C.], even if an OHP sheet with a high temperature is accommodated, heat is absorbed by the plain paper bundle and the accommodation tray 40, thereby preventing the occurrence of tacking.

If n = 2 is set in step S8 in FIG. 3, the process returns to step S3, and the OHP fixing temperature control in step S9 is executed again.
In step S31 of the OHP fixing temperature control, the first sheet is not determined (“NO” in step S31), the sheet interval extension setting is performed (step S37), and the process proceeds to step S35. This paper interval expansion setting facilitates heat dissipation of the sheet bundle on the storage tray 40 by increasing the paper supply interval of the second and subsequent OHP sheets slightly larger than the paper supply interval in the case of plain paper, thereby increasing the tray temperature. This is intended to prevent tacking of a plurality of OHP sheets. When the paper interval extension setting is performed, the start timing is controlled to be delayed by a time corresponding to the setting when the image forming operation in step S5 is executed next.

Returning to FIG. 3, the image forming operation for the second OHP sheet is executed in a state where the paper interval expansion setting is performed (step S5), and is stored in the storage tray 40 after the image is formed (step S6) (FIG. 3). 6 time point G).
When it is determined that the nth sheet is the last ("YES" in step S7), the process ends. With the paper interval expansion setting, the timing at which the second OHP sheet is fed, that is, the timing at which the second OHP sheet is accommodated in the accommodating tray 40 is delayed as compared with the case of plain paper. In the example 6 as well, at time point G, the tray temperature falls below the tacking limit temperature of 50 [° C.] by about 5 [° C.], and the occurrence of tacking is prevented.

  As described above, when the job for printing on the OHP sheet is continuously executed after the job for printing on the plain paper, the tray temperature (the temperature of the sheet bundle) is set by the plain paper after fixing stored in the storage tray 40. Even if the tacking limit temperature of the OHP sheet is temporarily exceeded, the fixing heater 53 is turned off and the fixing roller 51 is rotated to temporarily lower the temperature of the fixing unit before starting the printing of the first OHP sheet. Since the printing is started after the temperature of the sheet bundle is lowered to a temperature at which tacking does not occur, the occurrence of tacking can be prevented, and the cost can be reduced because a dedicated cooling device is not required, and the sheet up to the discharge port Since it is not necessary to lengthen the conveyance path, the apparatus can be downsized.

  In the above description, an example in which an instruction to execute job β (an instruction to carry out image formation by conveying a specific type of sheet) is received immediately after job α has been described. If an instruction is given when the job β cannot be executed, the instruction may be temporarily held and accepted when the job β can be executed. This can also be applied, for example, when jamming.

The tacking limit temperature is determined by the toner melting (softening) property, the adhesion property of the toner particles to the sheet surface, the flatness of the surface of the sheet, and the like. Absent.
<Second Embodiment>
In the above embodiment, when a job for printing on plain paper and a job for printing on an OHP sheet are successively executed in this order, the fixing heater 53 is turned off before the start of image formation on the OHP sheet, and the temperature of the fixing unit is set. In the present embodiment, the necessity of executing the temperature drop control is determined according to the elapsed time from the completion of the previous job. This point is different from the first embodiment. Hereinafter, in order to avoid duplication of description, the description of the same contents as those of the first embodiment will be omitted, and the same components will be denoted by the same reference numerals.

FIG. 7 is a flowchart showing only the parts different from the OHP fixing temperature control according to the first embodiment regarding the processing contents of the OHP fixing temperature control according to the present embodiment.
As shown in the figure, the OHP fixing temperature control according to the present embodiment is different in that the process of step S51 is executed between steps S31 and S32 of the OHP fixing temperature control according to the first embodiment. Yes.

  That is, when it is determined that the sheet to be fed is the first OHP sheet (“YES” in step S31), it is determined whether or not the elapsed time from the previous job completion is equal to or longer than the predetermined time T2. (Step S51). Here, the elapsed time from the completion of the previous job is measured by starting the timer 76 when the job is completed for each job (when image formation on the last sheet of the job is completed). The predetermined time T2 corresponds to a time that is assumed to be required for the tray temperature to fall to a temperature at which no tacking occurs in a state where the job has shifted to the standby state from the completion of the previous job. In the present embodiment, the predetermined time T2 is set to 400 [seconds].

FIG. 8 is a graph showing how the tray temperature changes with the elapsed time from the completion of the previous job.
As shown in the figure, the tray temperature gradually decreases from the completion of the previous job (time point H), and reaches about 45 ° C. at time point J after about 400 seconds. This 45 [° C.] corresponds to the temperature at the time point D in the first embodiment, that is, the temperature at the start of image formation on the first OHP sheet, and even when printing on the OHP sheet is started. It means the temperature at which tacking does not occur. Therefore, if the predetermined time T2 is set to 400 [seconds], even when printing on the OHP sheet is started after the lapse of T2, the tray temperature has sufficiently decreased at that time, and the occurrence of tacking can be prevented. As the value of the predetermined time T2, an appropriate value is obtained in advance by experiments or the like according to the apparatus configuration, and the data is stored in the ROM 73 or the like.

Returning to FIG. 7, when it is determined that the elapsed time from the completion of the previous job is equal to or longer than the predetermined time T2 (“YES” in step S51), the process proceeds to step S35. In this case, since printing on the OHP sheet can be started immediately, the temperature drop control in steps S32 to S34 is not executed (prohibited).
On the other hand, if it is determined that the elapsed time from the previous job completion has not reached the predetermined time T2 ("NO" in step S51), the process proceeds to step S32. In this case, temperature drop control is executed as in the first embodiment.

  In this way, whether or not the temperature drop control is necessary is determined based on the length of time elapsed since the completion of the job. Therefore, if unnecessary, the temperature drop control can be omitted and printing of the OHP sheet can be started earlier. Will be able to get the output faster. The previous print job may be an OHP sheet job or a plain paper job.

<Third Embodiment>
In the second embodiment, the determination as to whether or not the temperature drop control is to be executed, that is, the determination as to whether or not the tray temperature has been lowered to a level at which tacking does not occur, is the length of time that has elapsed since the completion of the job. However, in the present embodiment, the determination is made based on the temperature of the fixing unit immediately after the power is turned on.

FIG. 9 is a flowchart showing the processing contents of the OHP fixing temperature control according to this embodiment. This processing is performed when a job execution request for printing on an OHP sheet is received during warm-up after power-on. Is called. Further, it is assumed that the temperature of the fixing unit immediately after the power is turned on is detected and the temperature is temporarily held.
As shown in the figure, the OHP fixing temperature control according to the present embodiment is basically the same as the OHP fixing temperature control according to the first embodiment, but step S61 is performed between steps S31 and S32. The difference is that the process is executed.

  That is, when it is determined that the sheet to be fed is the first OHP sheet (“YES” in step S31), whether or not the temperature of the fixing unit immediately after the power supply is a predetermined temperature, for example, 100 [° C.] or less. Is determined (step S61). Here, when the temperature is 100 [° C.] or less immediately after the power is turned on, the power off state (power supply to the fixing heater 53 is cut off) lasts for a long time, and the temperature of the fixing unit is the place where the printer 10 is installed (office or the like ), When the power is turned on this time. In such a state, it is assumed that the tray temperature is also lowered to about room temperature, and it can be said that the probability of occurrence of tacking is extremely low even when printing on the OHP sheet is started.

Therefore, if it is determined that the temperature of the fixing unit is 100 [° C.] or less (“YES” in step S61), the process proceeds to step S35, the temperature drop control is prohibited, and printing on the OHP sheet is performed after the warm-up is completed. To start.
On the other hand, when it is determined that the temperature of the fixing unit has exceeded 100 [° C.] (“NO” in step S61), the process proceeds to step S32 to execute temperature drop control. The fact that the temperature exceeded 100 ° C immediately after the power supply can be assumed that the power supply has been on for a long time and has been turned off once, but it has been turned on immediately. Since it is expected that the temperature is close to the tacking limit temperature, temperature drop control is executed.

By determining whether or not the temperature drop control needs to be executed based on the temperature of the fixing unit in this manner, the temperature drop control can be omitted and the start of printing of the OHP sheet can be accelerated when unnecessary.
Note that the above 100 [° C.] is an example, and an appropriate value is obtained from an experiment or the like according to the apparatus configuration, and the data is stored in the ROM 73 or the like.
In the above description, an example in which an OHP sheet print job is received during warm-up immediately after power-on has been described. However, if a job is received in a state where the tray temperature is assumed to be lowered, Not limited. For example, when a job is received during the power saving mode in which the temperature of the fixing unit is maintained at a temperature lower than the standby target temperature to reduce the power consumption of the fixing heater 53, the power saving mode is canceled and the warm-up is started. Applicable to configuration. When the low temperature is set to 80 ° C., for example, when the power saving mode is continued for a long period of time, the temperature of the fixing unit is maintained at 80 ° C., and the tray temperature is also maintained in this state. The temperature drop control is not performed because it is sufficiently lowered.

On the other hand, when the state is maintained at the standby temperature (180 [° C.]) and the power saving mode is canceled immediately after that, the temperature of the fixing unit exceeds 100 [° C.], Since there is a high probability that the tray temperature is still high, temperature drop control is performed.
<Fourth embodiment>
In the above embodiment, the temperature drop control execution time (predetermined time T1) is uniform, but in this embodiment, whether or not there is a sheet in the storage tray 40 (whether or not it remains due to the user forgetting it). )), This time is different. Here, the presence / absence of a sheet is determined by a discharge paper detection sensor 61 (FIG. 1), which is disposed on the storage tray 40 and is formed of a known micro switch or the like. A signal indicating the detection result of the presence or absence of a sheet detected by the discharged paper detection sensor 61 is sent to the control unit 14.

  FIG. 10 is a flowchart showing the processing contents of the OHP fixing temperature control according to the present embodiment, and shows the parts different from the OHP fixing temperature control according to the third embodiment. Specifically, steps S71 to S73 are executed between steps S32 and S33 of the OHP fixing temperature control process according to the third embodiment, and step S74 is executed instead of step S34. Is different.

  That is, heating of the fixing unit 13 is stopped (step S32), and it is determined whether or not a sheet exists on the storage tray 40 (step S71). The existing sheet indicates a previous job, for example, a bundle of plain paper stored in the storage tray 40 by the job α in the first embodiment. In continuous jobs, the user may not be able to take out the sheet on the storage tray 40 in time, and may exist (is placed) as it is. However, if there is a certain time interval between jobs, the user is already present. It may be removed by and not placed. If the stored sheet remains, the tray temperature is unlikely to decrease due to the heat storage of the sheet as described above, but conversely, if the sheet is immediately removed by the user, the heated sheet disappears, so the tray temperature Is likely to fall.

  Therefore, in this embodiment, if it is determined that a sheet is present (“YES” in step S71), the temperature drop control execution time T3 is set to 30 [seconds], which is the same as in the first embodiment ( Step S72). On the other hand, if it is determined that there is no sheet (“NO” in step S71), it is assumed that the time required for the tray temperature to fall to a temperature at which no tacking occurs is reduced as compared with the case where there is a sheet. Is set to a value smaller than the temperature when the sheet exists, for example, 10 [seconds] (step S73). For this time, an appropriate value is obtained in advance from an experiment or the like according to the apparatus configuration as in the above 30 [seconds], and the data is stored in the ROM 73 or the like.

In step S33, time measurement is started. In step S74, when it is determined that the elapsed time from the start of time measurement has reached the time T3 set in step S72 or S73, the process proceeds to step S35, and the processing after S35, that is, OHP. Print the sheet.
When the sheet does not exist, the printing start of the OHP sheet can be advanced by the amount corresponding to the time T3 being shortened, and the waiting time of the user can be shortened.

Note that the configuration in which the value of the time T3 is made different depending on the presence or absence of the sheet may be applied to the OHP fixing temperature control according to other embodiments, for example, the first and second embodiments. The same effect can be obtained.
<Fifth embodiment>
In the above embodiment, it is determined whether or not the tray temperature has been lowered to a temperature at which tacking does not occur based on the elapsed time from the heating stop of the fixing unit 13 (steps S32 to S34). In this embodiment, it is performed according to the temperature of the fixing portion, and this point is different.

FIG. 11 is a flowchart showing the processing contents of the OHP fixing temperature control according to the present embodiment. As shown in the figure, the OHP fixing temperature control is basically the same as the OHP fixing temperature control according to the first embodiment, but steps S81 and S82 are executed instead of steps S33 and S34. Is different.
That is, when heating of the fixing unit is stopped (step S32), the temperature of the fixing unit is detected (step S81), and it is determined whether or not the detected temperature is lower than the threshold value Tf (step S82). Here, the threshold value Tf is a temperature value that is assumed that when the temperature of the fixing unit is lowered to the value, the tray temperature is also lowered to a temperature at which no tacking occurs due to the heat radiation effect of the storage tray 40. The threshold value Tf is a value lower than the fixing target temperature of the OHP sheet, and can be set to 160 [° C.] at the point D in FIG. 6, for example. As this threshold value (predetermined temperature) Tf, an appropriate value is obtained in advance by experiments or the like according to the apparatus configuration, and the data is stored in the ROM 73 or the like.

If it is determined that the detected temperature is lower than Tf (“YES” in step S82), it is determined that the tray temperature has dropped to a temperature at which tacking does not occur, and the processing from step S35 is executed.
Thus, not only the elapsed time from the heating stop of the fixing unit 13 but also the temperature of the fixing unit can be used to control the print start timing of the first OHP sheet. Since the detection signal of the temperature detection sensor 54 for fixing temperature control can be used, a timer for measuring the elapsed time and its measurement processing are not required, and the configuration can be simplified correspondingly.

<Sixth Embodiment>
In the present embodiment, a process of setting the threshold value Tf in the fifth embodiment to a different value depending on whether or not a sheet exists in the storage tray 40 is performed.
FIG. 12 is a flowchart showing the processing contents of the OHP fixing temperature control according to the present embodiment, and shows mainly the parts different from the OHP fixing temperature control according to the fifth embodiment. Specifically, steps S91 to S93 are executed between steps S32 and S81 of the OHP fixing temperature control process according to the fifth embodiment, and step S94 is executed instead of step S82. Is different.

  That is, heating of the fixing unit 13 is stopped (step S32), and it is determined whether or not a sheet exists on the storage tray 40 (step S91). This determination is performed by the same method as the determination in step S71 in the fourth embodiment. As in the fourth embodiment, the tray temperature is likely to decrease when the sheet is removed by the user and is not present in the storage tray 40.

  If it is determined that the sheet exists ("YES" in step S91), the threshold value Tf is set to, for example, 160 [° C] as in the fifth embodiment (step S92), and the sheet does not exist. Is determined (“NO” in step S91), the temperature is set to a higher temperature than when the sheet exists, for example, 170 [° C.] (step S93). If the sheet is not present, the value set higher than the case where the sheet is present is that heat dissipation is facilitated if the sheet is not present, so that the temperature when the sheet is present, for example, 160 [° C.] must be lowered. Even if it is not waited, it is assumed that if the temperature is lowered to 160 [° C.], for example, 170 [° C.], the tray temperature is also lowered to a temperature at which tacking does not occur. As this temperature value, an appropriate value is obtained in advance from experiments or the like according to the apparatus configuration and stored in the ROM 73 or the like.

In step S81, the temperature of the fixing unit is detected. In step S94, if it is determined that the temperature of the fixing unit is lower than the threshold value Tf set in step S92 or S93, the process proceeds to step S35, and the processing in and after S35. That is, printing of the OHP sheet is executed.
When there is no sheet, it is possible to spend less time waiting for the temperature of the fixing unit to be lower than when there is a sheet, and the start of printing on the OHP sheet can be accelerated by that time.

<Seventh embodiment>
In the above embodiment, by detecting the elapsed time since the heating of the fixing unit 13 is stopped and the temperature of the fixing unit, it is determined that the tray temperature will be lowered to a temperature at which tacking does not occur. However, in this embodiment, the tray temperature detection sensor 62 (FIG. 1) for detecting the tray temperature is provided in the storage tray 40, and the tray temperature detection sensor 62 directly detects the tray temperature. Yes. The tray temperature detection sensor 62 is a known sensor such as a thermistor, and sends a detection signal to the control unit 14.

FIG. 13 is a flowchart showing the processing content of the OHP fixing temperature control according to the present embodiment. As shown in the figure, the OHP fixing temperature control is basically the same as the OHP fixing temperature control according to the third embodiment, but steps S98 and S99 are executed instead of steps S33 and S34. Is different.
That is, when heating of the fixing unit is stopped (step S32), the tray temperature is detected (step S98), and it is determined whether or not the detected temperature is lower than a predetermined temperature as a threshold, for example, 45 [° C.] (step S98). S99). This threshold is a value set as a temperature at which tacking does not occur, and 45 [° C.] is a value corresponding to the tray temperature at time D in FIG. As this threshold value (predetermined temperature), an appropriate value is obtained in advance from experiments or the like according to the apparatus configuration, and the data is stored in the ROM 73 or the like.

If it is determined that the detected temperature is lower than 45 ° C. (“YES” in step S99), it is assumed that the tray temperature has dropped to a temperature at which tacking does not occur, and the processing after step S35 is executed.
If this configuration is taken, the actual tray temperature can be accurately known. For example, if it is detected that the tray temperature has dropped to a temperature at which tacking does not occur before the predetermined time T1 elapses, The idle rotation time of the fixing roller 51 can be shortened, for example, printing can be started immediately at the time of detection without waiting for the predetermined time to elapse, and finer print control can be performed.

  The present invention is not limited to the fixing device and the image forming apparatus, and may be an image forming method for a specific sheet such as an OHP sheet. Furthermore, the method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It is possible to record on various computer-readable recording media.

<Modification>
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the above embodiment, when the heating of the fixing unit 13 is stopped (step S32), the fixing heater 53 is turned off for a predetermined time to reduce the temperature of the fixing unit. However, if the temperature of the fixing unit can be decreased, The method is not limited to the method of completely turning off the power supply and turning it off.

For example, the energization of the fixing heater 53 may be controlled so that the temperature of the fixing unit becomes a predetermined temperature lower than the fixing target temperature, for example, 50 [° C.]. In the fifth and sixth embodiments, The threshold value Tf (160, 170 [° C.], etc.) may be controlled.
(2) In the OHP fixing temperature control, the extended setting is made to make the sheet interval wider than that of plain paper for the second and subsequent sheets. However, the configuration is not limited to this setting. In consideration of job execution efficiency and the like, for example, the same sheet interval as that of plain paper may be set.

  (3) In the above embodiment, the fixing roller 51 and the pressure roller 52 are also used as a pair of discharge rollers, and the length of the conveyance path from the fixing unit 13 to the discharge port 39 is made as short as possible to reduce the size of the apparatus. However, the present invention is not limited to this. As long as the apparatus can be reduced in size, for example, the conveyance path to the discharge port 39 may be slightly extended and a discharge roller pair may be separately disposed at the discharge port 39.

  (4) In the above embodiment, the temperature drop control is performed on the OHP sheet. However, the tacking has a larger heat capacity than the plain paper and / or has a higher surface flatness, specifically a coating. It tends to occur on paper and cardboard. Therefore, when an image is formed on a specific type of OHP sheet or coated paper that is susceptible to tacking, temperature drop control can be executed. It should be noted that a method for determining whether or not a specific type of sheet is set is a known method, for example, a method of registering in advance which sheet is set for each paper feed tray, A method is conceivable in which the reflectance and transmittance are obtained in advance, and the sheet type is specified by measuring the reflectance of the set sheet.

  (5) In the above embodiment, an example in which the image forming apparatus according to the present invention is applied to a tandem color digital printer has been described. However, the present invention is not limited to this. The present invention can be applied to general image forming apparatuses including a fixing device that thermally fixes an unfixed image such as a toner image, for example, a copying machine, a FAX, an MFP (Multiple Function Peripheral), and the like. Further, the present invention is not limited to color image formation, and can be applied to an image forming apparatus that performs monochrome image formation. For example, in the case of a copying machine, it is possible to accept an instruction to convey a specific type of sheet and form an image when a key for instructing the start of copying is pressed. Furthermore, the configuration example in which the fixing roller 51 and the pressure roller 52 are pressed against each other as the first and second rotating bodies to secure the fixing nip between them has been described, but the present invention is not limited to this. For example, the fixing nip may be secured by a combination of a roller and a belt, or a belt and a belt. Furthermore, although the halogen lamp is used as the heating means, a general heater can be used as long as it can heat the rotating body.

Furthermore, although the configuration example in which the paper feed cassette 32 is arranged to set the OHP sheet has been described, the present invention is not limited to this. For example, a known manual feed tray can be arranged on the side surface of the apparatus as long as the apparatus can be downsized.
The contents of the above embodiment and the above modification may be combined.

  The present invention can be widely applied to image forming apparatuses that thermally fix an image formed on a conveyed sheet.

1 is a diagram illustrating an overall configuration of a printer according to a first embodiment. It is a block diagram which shows the structure of the control part of the said printer. 6 is a flowchart illustrating processing contents executed by a CPU of a control unit when an execution instruction is received for each print job. It is a flowchart which shows the content of the subroutine of plain paper fixing temperature control. It is a flowchart which shows the content of the subroutine of OHP fixing temperature control. 10 is a graph showing an example of an experimental result showing that the temperature of the fixing unit and the temperature of the storage tray change with time during job execution. It is a flowchart which shows a part of processing content of the OHP fixing temperature control which concerns on 2nd Embodiment. It is a graph which shows a mode that tray temperature changes with the elapsed time from the completion of the last job. It is a flowchart which shows the processing content of the OHP fixing temperature control which concerns on 3rd Embodiment. It is a flowchart which shows a part of processing content of the OHP fixing temperature control which concerns on 4th Embodiment. It is a flowchart which shows the processing content of the OHP fixing temperature control which concerns on 5th Embodiment. It is a flowchart which shows a part of processing content of the OHP fixing temperature control which concerns on 6th Embodiment. It is a flowchart which shows the processing content of the OHP fixing temperature control which concerns on 7th Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Printer 11 Image process part 12 Feeding part 13 Fixing part 14 Control part 38 Sheet conveyance path 39 Discharge port 40 Storage tray 50 Fixing nip 51 Fixing roller 52 Pressure roller 53 Fixing heater 55 Roller drive motor 61 Discharged paper detection sensor 62 Tray Temperature detection sensor 71 CPU
75 Fixing target temperature storage section 76 Timer P Plain paper S OHP sheet

Claims (12)

An image forming apparatus for forming an image on a conveyed sheet,
Located near the discharge port of the sheet conveying path, the first rotating body is brought into pressure contact with the second rotating body to secure a fixing nip therebetween, and at least one of the rotating bodies is heated by a heating unit and conveyed. Fixing means for thermally fixing an image formed on the sheet through the fixing nip;
When receiving an instruction to carry out image formation by conveying a specific type of sheet, the heating unit is controlled so that the rotating body is rotated and the temperature of the rotating body is lower than the target temperature before starting image formation. And when the predetermined condition for raising the temperature to the target temperature is satisfied, the control means for switching the control of the heating means so that the temperature is raised to the target temperature, and executing the image formation;
An image forming apparatus comprising: The control means includes
A first timer for measuring an elapsed time since the previous image formation was completed;
When the instruction is received after the time measured by the first timer reaches the first predetermined time, control of the heating unit before the image formation is prohibited, and the temperature of the rotating body is set to the target temperature. The image forming apparatus according to claim 1, wherein the image forming apparatus executes the image formation by controlling the heating unit so as to become. Comprising temperature detecting means for detecting the temperature of the rotating body;
The control means includes
When the temperature of the rotating body when the instruction is received is equal to or lower than a first predetermined temperature lower than the target temperature, the control of the heating unit before the image formation is prohibited, and the temperature of the rotating body is determined. The image forming apparatus according to claim 1, wherein the image forming apparatus executes the image formation by controlling the heating unit so that the temperature becomes a target temperature. The control means includes
A second timer for measuring an elapsed time since the start of control so as to reach the low temperature;
4. The image forming apparatus according to claim 1, wherein the condition is satisfied when a time measured by the second timer reaches a second predetermined time. 5. A tray disposed at a position directly below the discharge port and containing a sheet discharged from the discharge port;
Sheet detecting means for detecting the presence or absence of a discharged sheet on the tray,
The control means includes
When it is detected that there is no discharged sheet on the tray, the value of the second predetermined time is set to a smaller value than when the presence of the sheet is detected. The image forming apparatus according to claim 4. The second predetermined time is:
The time required for the temperature of the rotating body to drop to the second predetermined temperature after starting the control of the heating means so as to become the low temperature, or the exhaust disposed just below the exhaust port. The image forming apparatus according to claim 4, wherein the time required for the temperature of the tray that accommodates the sheet discharged from the outlet to drop to the third predetermined temperature is the time. Comprising temperature detecting means for detecting the temperature of the rotating body;
The control means includes
4. The method according to claim 1, wherein the condition is satisfied when the temperature of the rotating body is lowered to a second predetermined temperature after the control of the heating unit is started so as to be the low temperature. The image forming apparatus according to claim 1. A tray disposed at a position directly below the discharge port and containing a sheet discharged from the discharge port;
Sheet detecting means for detecting the presence or absence of a discharged sheet on the tray,
The control means includes
When it is detected that there is no discharged sheet on the tray, the second predetermined temperature is set to a higher value than when the presence of the sheet is detected. The image forming apparatus according to claim 7. A tray disposed at a position directly below the discharge port and containing a sheet discharged from the discharge port;
Temperature detecting means for detecting the temperature of the tray,
The control means includes
The image forming apparatus according to claim 1, further comprising a determination unit that determines whether the condition is satisfied based on a temperature of the tray.   The control of the heating means so that the temperature of the rotating body is lower than a target temperature is to stop power supply to the heating means. The image forming apparatus described in the item. The specific type of sheet is
The image forming apparatus according to claim 1, wherein the image forming apparatus is an OHP sheet or a coated paper. An image forming method in an image forming apparatus for forming an image on a conveyed sheet,
In the fixing unit disposed near the discharge port of the sheet conveying path, the first rotating body is pressed against the second rotating body to secure a fixing nip therebetween, and at least one rotating body is heated by the heating unit. A fixing step of thermally fixing an image formed on the sheet to be conveyed through the fixing nip;
When receiving an instruction to carry out image formation by conveying a specific type of sheet, the heating unit is controlled so that the rotating body is rotated and the temperature of the rotating body is lower than the target temperature before starting image formation. When the predetermined condition for raising the temperature to the target temperature is satisfied, a control step of switching the control of the heating unit so that the temperature is raised to the target temperature and executing the image formation;
An image forming method comprising: executing a step including:

Images