JPH1063132A - Fixing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To smoothly execute a fixing operation without causing hot-offset when an image is formed on an OHP sheet and the like, by preventing a heating roll and a pressure roll from being rotated in a state where they are brought into contact with each other in idle time in fixing. SOLUTION: When the fixing operation on the OHP sheet is finished, the OHP sheet is ejected to a downstream side from a fixing nip area further. At this time, it is detected by a sensor for detecting a paper after fixing 8 disposed on the downstream side of the fixing nip area that the trailing edge part of the OHP sheet is passed through the detection part of the sensor 8 and the signal thereof is transmitted to a CPU. By the CPU, it is recognized that the OHP sheet is not caught in the fixing nip area any more and it is detected that the idle time in the fixing comes. Then, the CPU being as a stop means is actuated and rotation driving force from a driving motor is not transmitted to the heating roll 1 in order to turn off the power source of a fixing driving clutch functioning also as the stop means. Besides, the rotation of the heating roll 1 is stopped and the rotation of the pressure roll 4 is also simultaneously stopped.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device incorporated in an image forming apparatus such as a copying machine or a printer, and more particularly, to a fixing device having a heating type fixing device.

[0002]

2. Description of the Related Art Conventionally, as a fixing device for fixing an unfixed toner image transferred onto a recording medium such as paper or film, the recording medium is heated under pressure to melt the toner on the surface and penetrate into the recording medium. Alternatively, a heating type fixing device for fixing by welding is known. In general, a full-color fixing device that has become popular recently has a heating source such as a heating lamp disposed in a pressure roll as well as a heating roll in order to improve the color development of each color toner. Has become.

As shown in FIG. 3, the fixing portion of such a full-color image forming apparatus comprises a heating roll 1 and a pressure roll 2 pressed against the heating roll 1. Heating lamps 3 and 3 'are disposed in the sections, respectively, and the heat generated by the heating lamps 3 and 3' heats the heating roll 1 and the pressure roll 2 from inside thereof to a predetermined temperature. At this time, generally, the surface temperature of the pressure roll 2 is controlled to maintain a control temperature lower than the surface temperature of the heating roll 1.

The reason is to cope with double-sided copying which has become commonplace in recent years. That is, when making a double-sided copy, first, an unfixed toner image is fixed on the surface of a recording medium, then the recording medium is inverted in the image forming apparatus, and then another unfixed toner image is transferred to the back side. Then, the unfixed toner image on the back side is fixed by passing through the fixing nip area again.When the unfixed toner image on the back side is fixed, the surface contacts the surface of the pressure roll and heat is applied. Exposed to

Therefore, if the surface temperature of the pressure roll 2 is too high at this time, the toner image once fixed on the surface of the recording medium is melted by being heated again, and a trouble called "hot offset" occurs. When the same recording medium is heated and pressed twice consecutively, the recording medium itself may be deformed and image quality may be degraded. Therefore, in order to prevent this, the surface temperature of the pressure roll is controlled to be lower than the surface temperature of the heating roll as described above.

There are cases where copying is performed on a specific recording medium such as an OHP sheet or cardboard in addition to plain paper. However, as described above, an OHP sheet or cardboard is fixed by a fixing device provided with a pressure roll having a heat source as described above. In such a case, troubles such as deterioration of image quality and paper jam often occur.

[0007] Then, when various investigations were made to elucidate the cause, it was found that the surface temperature of the pressure roll was increased to be higher than an intended target temperature. It has been found that the heat of the surface of the heating roll is supplied to the pressure roll because the roll continues to rotate in the contact state during the fixing operation.

That is, since the amount of heat absorbed in the fixing operation of the OHP sheet or the thick paper is larger than that of the plain paper, it is necessary to fix the OHP sheet or the like when the recording medium such as the OHP sheet passes through the fixing nip area. It is necessary to increase the amount of heat supplied to the recording medium. As a specific method, for example, a method of heating the surface of a heating roll to a high temperature when forming an image on an OHP sheet or the like, and a method of lowering a fixing speed itself to a recording medium passing through a fixing nip area It is conceivable to increase the amount of heat supplied to the fuel cell.

However, the method of heating the surface of the heating roll to a high temperature requires a waiting time for heating, and when performing a fixing operation on plain paper after fixing on an OHP sheet or the like. There is a problem that a waiting time for cooling is required, and fixing efficiency may be reduced by providing the waiting time. Therefore, a method of slowing the fixing speed and increasing the amount of supplied heat is generally performed. ing.

However, in such a fixing device of this type, OHP
To make a copy on a sheet or the like, the fixing speed must be considerably reduced, and a dummy cycle or the like is required to decelerate after the transfer is completed. As a result, as shown in FIG. 4, the pressure roll receives heat from the surface of the heating roll, and as a result, the surface temperature of the pressure roll rises so as to greatly exceed the intended target temperature. This causes problems such as deterioration of image quality and paper jam.

Japanese Patent Application Laid-Open No. 4-3088881 discloses that when making a double-sided copy, the amount of heat supplied when fixing the front side is set to be low, and when the back side is fixed, the front side is heated again. There is disclosed a fixing device having a mechanism capable of performing perfect fixing only after passing through a fixing nip region twice for convenience of a front surface and a back surface. However, when the temperature is increased and the amount of heat is increased during the fixing of the back surface in the double-sided mode in such an apparatus, there is a problem that a waiting time occurs and continuous operation cannot be performed.

Therefore, as a result of intensive studies by the present inventors,
Between the time when the trailing edge of the preceding recording medium passes through the fixing nip area and the time when the leading edge of the subsequent recording medium enters the fixing nip area (hereinafter referred to as “fixing free time”), the heating roll and the pressure roll By preventing the rotation of the heating roll in a contact state, it is possible to prevent the heat from being excessively supplied from the surface of the heating roll to the pressure roll, thereby preventing the surface temperature of the pressure roll from rising. Thus, the present invention has been completed by finding that the conventional problems described above can be solved.

Accordingly, an object of the present invention is to provide an OHP sheet in a fixing device in which a heating source is incorporated in both a heating roll and a pressure roll and the surface temperature of the pressure roll is controlled to a temperature lower than the surface temperature of the heating roll. It is an object of the present invention to provide a fixing device that can smoothly perform fixing without causing a problem such as hot offset even when an image forming mode is selected.

[0014]

According to the present invention, there is provided a heating roll having a built-in heating source, a recording medium having a built-in heating source and being pressed against the heating roll to form a fixing nip region and transferring an unfixed toner image. A pressure roll for fixing an unfixed toner image in cooperation with the heating roll while the medium passes through the fixing nip area; and a heating source in each roll based on the surface temperature of the heating roll and the pressure roll. And controlling the heating source of the heating roll so that the surface temperature of the heating roll becomes the target temperature ( _TH ) in the standby state, and the target temperature at which the surface temperature of the pressure roll is lower than the surface temperature of the heating roll. temperature control means for controlling so as to (T _P), a rotary drive means for rotating the heating roller and the pressure roll during fixing, after the trailing end of the preceding recording medium after passing through the fixing nip area A free time detecting means for detecting a free time until the leading edge of the recording medium enters the fixing nip area, and when a specific image forming mode is designated, or when the free time becomes equal to or more than a set value. If
And a stopping means for stopping the rotation driving means during the fixing free time.

In the present invention, the heating roll is not particularly limited as long as it can surely heat the unfixed toner image on the transfer recording medium sandwiched between the fixing nip areas and fix it on the recording medium. Although not limited, a typical example is a structure in which an outer surface of a core member formed of a metal cylindrical member is covered with a tube-shaped member made of an elastic body such as silicon rubber. Further, a heating source such as a heating lamp is provided inside the heating roll, and the heating roll is heated from the inner surface by the heat of the heating source.

In the present invention, the pressure roll can form a fixing nip area with the heating roll, and the transfer recording medium sandwiched between the fixing nip area is surely pressed toward the heating roll so as to form the recording medium. Any structure can be used as long as it can be fixed thereon, and the structure and material are not particularly limited.
A typical example is a structure in which an outer surface of a core made of a metal cylindrical member is covered with a tube-shaped member made of an elastic material such as silicon rubber. In addition, the coating of the elastic body is generally thinner than the coating of the heating roll, but is not limited thereto.

A heating source such as a heating lamp is provided inside the pressure roll similarly to the heating roll, and the heating is generally performed from the inside by the heat generated by the heating source. . In general, the surface temperature of the pressure roll is generally controlled to a temperature lower than the surface temperature of the heating roll in a two-sided copying machine, so that the heating source disposed in the pressure roll is disposed in the heating roll. A heat source having a lower calorific value than the provided heating source can be used.

The temperature control means of the present invention is not particularly limited as long as it can accurately and reliably control the surface temperatures of the heating roll and the pressure roll, and is not particularly limited. A temperature sensor such as a thermistor that detects the surface temperature of each, and based on signals from these temperature sensors, recognizes the surface temperature of each of the heating roll and pressure roll, and is installed inside each of the heating roll and pressure roll Turn on / off the heating source such as the heating lamp
And a temperature control device for controlling turning off. The temperature control device is, for example, a central processing unit (hereinafter, referred to as a “CPU”).

As a temperature control method, for example, a target temperature (T _H ) of a heating roll is set in advance as a temperature control device C
The read-only recording device incorporated in the PU (hereinafter, this read-only recording device is referred to as “ROM”) is stored, and the surface temperature of the heating roll detected by the temperature sensor is lower than the target temperature (T _H ). In this case, the heating source is turned on to perform heating, while the heating source is turned off to stop heating while the surface temperature of the heating roll is equal to or higher than the target temperature ( _TH ). And a method of controlling the surface temperature of the heating roll to be substantially equal to the target temperature (T _H ).

In the same manner, for the pressure roll, the on / off of a power source of a heating source disposed in the pressure roll is controlled so as to maintain the surface temperature of the pressure roll at a control temperature. A control temperature (T _P ) lower than the target temperature (T _H ).

The rotation driving means of the present invention is a means for rotating the heating roll and the pressure roll during the fixing operation. The rotation is performed by using one of the heating roll and the pressure roll as a driving roll and the other as a driven roll, and transmitting a rotational driving force to the driving roll. During fixing, the pressure roll is pressed against the heating roll, so that whichever drive roll is used, the other roll rotates as a driven roll.

The method of transmitting the rotational driving force is not particularly limited as long as it can surely and accurately rotate the heating roll and the pressure roll, and is not particularly limited. There are those that transmit the rotational driving force supplied from the provided main motor via a gear train, a belt, and the like, and those that are driven by a dedicated driving motor provided separately from the main motor.

In the case where a dedicated drive motor is used, when a paper jam occurs during the fixing operation, each area of the other copying apparatus is driven independently of the movement of the peripheral members of the heating roll and the pressure roll. When the copier is opened and the module inside is pulled out to remove the jammed paper, the jammed paper cannot be torn off in other areas even if this module is pulled out. After the paper is moved, the driving can be stopped. Therefore, when such control is performed, it is advantageous in that a trouble called “guillotine” in which paper is torn off during paper jam processing can be prevented.

Further, the rotation driving means of the present invention may have a function of displacing the pressure roll in a direction of coming and coming from the heating roll side. In that case, in addition to a main motor and a dedicated drive motor for supplying the rotation drive force to the heating roll and the pressure roll, a fixing drive system for displacing the pressure roll, for example, a pressure system is provided. Is required. Examples of the pressure system include a pressure system using a combination of a spring clutch and a latch solenoid provided for the pressure roll, and a pressure system using a linear drive motor or the like.

The vacant time detecting means of the present invention is not particularly limited as long as it can surely detect the fixing vacant time. Typically, the vacant time detecting means includes a sheet sensor provided on a sheet conveying path. , CP connected to this paper sensor
U and the like. Then, as a method of detecting the fixing free time, before and after the fixing nip region, that is, a paper detector such as a paper sensor is disposed on each of the upstream and downstream sides in the moving direction of the recording medium with respect to the fixing nip region, The time when the downstream end (hereinafter, referred to as “rear end”) of the preceding recording medium in the moving direction of the recording medium passes through the detection portion of the downstream paper sensor is defined as the start of the fixing free time, while the upstream paper is detected. There is a method in which a time when a downstream end (hereinafter, referred to as “front end”) of the subsequent recording medium in the movement direction of the recording medium passes through the detection portion of the sensor is regarded as an end of the fixing free time.

However, in practice, the time that is grasped as the fixing free time is generally longer than the time that the recording medium is actually sandwiched in the fixing nip area.
This is because the heating roll and the pressurizing roll have a margin of time to be rotationally driven, and the heating roll and the pressurizing roll are rotationally driven longer than the time when the recording medium is sandwiched in the fixing area. This is to ensure fixing.

For example, immediately after the trailing edge of the preceding recording medium has passed the detection unit of the downstream paper sensor, the start of the fixing idle time is defined as the start of the fixing free time. If you want after entering the straight and the end of the fixing of free time, and the like.

The stopping means of the present invention is a means for stopping the rotation driving means during the fixing free time when a specific image forming mode is designated or when the fixing free time exceeds a set value. is there. Here, the specific image forming mode is an image forming mode in which the free fixing time becomes equal to or longer than a set value.

The value of the free fixing time is determined by the cycle of the image forming mode. For example, the fixing free time in the OHP mode is the time from when the leading edge of the preceding OHP sheet enters the fixing area to when the leading edge of the following OHP sheet enters the fixing area (hereinafter, “one cycle time”). )), That is, the time required for fixing, that is, the remaining time obtained by subtracting the time in which the OHP sheet is sandwiched in the fixing nip area and a value obtained by adding a margin time to the time. Here, the time during which the OHP sheet is sandwiched in the fixing nip area is determined as a time that can supply a necessary amount of heat when fixing the OHP sheet.
Further, a margin time for safety is further added to this time, and this is grasped as a time for fixing (hereinafter, referred to as “fixing time”). On the other hand, the remaining time of the one cycle time is a time determined by the image forming cycle.

For example, in the case of the OHP mode, during the fixing free time, the image of the next original is written on the electrostatic latent image carrier, developed, and transferred to the transfer drum. In order to perform a dummy cycle in which the speed is reduced to the same speed as the fixing speed, a longer fixing time is required than the time required for performing these steps.

Similarly, in the case of the double-sided copy mode, after the image is fixed on the front side of the first sheet, the recording medium is temporarily discharged from the fixing nip area, turned over, transferred to the back side, and transferred again to the fixing nip. Since a process of sending the toner to the area and fixing is performed, time for performing each of the fixing, discharging, reversing, and transferring processes is required. Therefore, the free fixing time needs to be longer than the minimum time required for performing these steps. As described above, the fixing free time is determined by the cycle of each mode.

The value of the free fixing time is determined by the contents of the steps performed during one cycle of each image forming mode,
Since it varies depending on the processing speed of each process, the structure of the image forming apparatus, the material of each part, and the like, it is actually determined for each fixing device based on actual measurement data obtained in an actual machine test.

The set value of the fixing free time means that when the pressure roll and the heating roll are kept rotated in contact with each other during the fixing operation, the pressure roll supplies heat from the heating roll. The surface temperature rises in response to this, and it is a value that is a boundary between causing or not causing troubles such as image quality deterioration and paper jam.If the free fixing time is less than this set value, the above trouble does not occur, but This value is a boundary at which the above-described trouble occurs when the value exceeds the set value.

As for the specific values of the set values, the contents of the steps performed during one cycle of each image forming mode, the processing speed of each step, the image forming apparatus Since it differs depending on the structure, the material of each part, etc., ultimately, actual measurement data is obtained for each fixing device, and it is determined based on this data, and this data is combined with the fixing time with a margin to ensure the fixing. Although it is determined, a typical example is 15 seconds. Here, setting the value to the 15 seconds, when the fixing free time than this becomes longer, because it too high surface temperature of the pressure roll can no longer be controlled to the desired target temperature (T _P) There, shorter than this Conversely, the amount of heat supplied from the heating roll is small, can control the surface temperature of the pressure roll to the desired target temperature (T _P).

Examples of the specific image forming mode include an OHP mode and a cardboard mode. As other image forming modes, there are a double-side mode corresponding to double-sided copying, and other modes such as postcard, illuminated paper, and special paper such as tack film.

The stop means according to the present invention operates the rotation driving means accurately and reliably during the fixing free time when a specific image forming mode is designated or when the fixing free time exceeds a set value. Any means may be used as long as it can be stopped, and is not particularly limited. Typically, a fixing unit that controls transmission of rotational driving force from a main motor to a heating roll or a pressing roll as a driving roll during a fixing free time. An example includes a drive clutch and a CPU that controls the drive clutch.

When the heating roll and the pressurizing roll are driven to rotate by a dedicated drive motor instead of the main motor, the dedicated drive motor and a CPU for controlling ON / OFF of the power supply of the dedicated drive motor are provided. Things. Further, in the case where the rotation driving means is provided with a function of moving the pressure roll in a direction of moving toward and away from the heating roll, in addition to applying the rotation driving force to the heating roll and the pressure roll as described above. A fixing drive system that separates the pressure roll from the heating roll, such as a device that includes a latch solenoid, a direct drive motor, and a CPU that controls on / off of the power of these components.

According to the present invention, when a specific image forming mode is designated, or when a fixing free time becomes equal to or longer than a set value, the stopping means operates. That is, even if the plain paper mode or the like is designated as the image forming mode, the fixing free time when fixing plain paper is short, and during this time, the pressure roll and the heating roll can be rotationally driven in contact with each other. ,
Since the amount of heat supplied from the heating roll to the pressure roll is small, and the surface temperature of the pressure roll is in a temperature range that can be sufficiently controlled by the temperature control means, it is not necessary to particularly operate the stop means. Therefore, when an image forming mode such as a plain paper mode in which the free time for fixing is short is designated, the stopping means does not operate and the pressure roll continues to rotate in contact with the heating roll during the fixing operation.

On the other hand, a specific image forming mode, for example, O
The stop unit operates when the HP mode, the cardboard mode, the duplex mode, or the like is designated, or when the idle time detecting unit detects that the fixing idle time is equal to or more than the set value. That is, when a fixing operation is performed using a recording medium having a large heat absorption amount such as an OHP sheet, a fixing free time is set to be long. When allowed to rotate in contact with the state, the amount of heat supplied is large, the surface temperature of the pressure roll is heated during this time exceeds greatly the desired target temperature (T _P) from the heating roll to the pressure roll side It will rise as much. Therefore,
When the fixing operation is performed in this state, a large amount of heat is supplied to the OHP sheet when the OHP sheet passes through the fixing nip area, and the toner transferred onto the OHP sheet is excessively melted or heated. Peeling to the roll side causes problems such as deterioration of image quality.

However, in the fixing device of the present invention,
The stop means operates to stop the rotation of the heating roll and the pressure roll during the fixing free time, and to separate the pressure roll from the heating roll during the fixing free time. As a result, the surface temperature of the pressure roll does not become too high. Therefore, even when a large number of OHP sheets or the like are continuously fixed, troubles such as deterioration of image quality and paper jams caused by supply of an excessive amount of heat when passing through the fixing nip area may occur. Is prevented.

[0041]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) The present invention will be described below based on an embodiment of the present invention. FIG. 1 is a schematic diagram showing a fixing device of the present embodiment, and FIG. 2 is a diagram showing a temporal change of the surface temperature of a heating roll and the surface temperature of a pressure roll, a paper detection state of a paper sensor, and a fixing drive clutch. 6 is a timing chart showing an operation.

The fixing device of this embodiment has a heating roll 1 containing a heating lamp (heating source) 2 and a heating lamp (heating source) 5 and is pressed against the heating roll 1 to form a fixing nip area. A pressure roll 4 for fixing the unfixed toner image 9 together with the heating roll 1 while the sheet (recording medium) 10 onto which the unfixed toner image 9 is transferred passes through the fixing nip area; Heating roll 1 and pressure roll 4
Heating lamps in each roll on the basis of the surface temperature of the controls (heat source) 2,5, heating roll 1 so that the surface temperature of the heating roll 1 in the standby state is the target temperature (T _H)
And a temperature control means for controlling the surface temperature of the pressure roll 4 to be a target temperature ( _TP ) lower than the surface temperature of the heating roll 1 while controlling the heating lamp (heating source) 2. A fuser drive motor and a gear train (rotation drive means, both not shown) for rotating and driving the heating roll 1 and the pressure roll 4, and the trailing edge of the preceding paper (recording medium) after passing through the fixing nip area An idle time detecting unit for detecting an idle time until the leading edge of the sheet enters the fixing nip area, and stopping the rotation driving unit during the idle time when a specific image forming mode is designated. And a CPU (stop means) for controlling ON / OFF of a power supply for the fixing drive clutch.

The heating roll 1 has a structure in which an iron roll core having a thickness of 2 mm is covered with a silicone rubber layer having a thickness of 2 mm, and a surface thereof is further covered with a fluorine rubber layer having a thickness of 30 μm. The outer diameter is 40 mm. The pressure roll 4 is a 0.5 mm thick iron roll core having a thickness of 2 mm.
And has an outer diameter of 40 mm.

The contact pressure and contact width between the heating roll 1 and the pressure roll 4 are 80 kg / cm ² and 4.5 mm, respectively. Further, the temperature control means specifically includes temperature sensors 3 and 6 for measuring the respective surface temperatures of the heating roll 1 and the pressure roll 4, and the heating roll 1 based on signals from the temperature sensors 3 and 6. And a CPU for recognizing the surface temperature of each of the pressure rolls 4.
The CPU incorporates a ROM in which a program for controlling each of the heating lamps 2 and 5 is written.

In the fixing device of this embodiment, a ROM incorporated in a CPU is used as a temperature control means.
Target temperature in the standby state of the heating roll 1 in OM (T _H) and the standby state of the pressure roll 4 target temperature (T _P) is set.

In the fixing device of this embodiment, a CPU is used as a temperature control means, and a ROM incorporated in the CPU is connected to the heating roll 1 based on the target temperatures (T _H ) and (T _P ). A program is written to control the power on / off of each of the heating lamps 2 and 5 so that the respective surface temperatures of the pressure rolls 4 reach these target temperatures.

Here, in the fixing device of this embodiment, the target
Temperature (T_H), Target temperature (T_P) Is the value of both
Image defects on the pressure roll side during side copy, especially back side copy
Has been decided not to
Each, T_H= 145 ° C, T _P= 115 ° C
is there.

In the fixing device of the present embodiment, as the idle time detecting means, a pre-fixing paper sensor disposed before the fixing nip area, a post-fixing paper sensor disposed behind the fixing nip area, and these sensors Recognizes when the trailing edge of the preceding sheet has passed through the fixing nip area and when the leading edge of the succeeding sheet has begun to enter the fixing nip area based on signals from these sensors. Then, the fixing free time is detected from these points.

The fixing device of this embodiment has a structure in which the stop means is activated when a specific image forming mode is designated. Specifically, when an operator designates on a switching panel, the signal is sent to the CPU. The CPU determines whether to activate the stopping means.
And as a stopping means, F as a main motor
A gear train transmitting a rotational driving force from a user drive motor, a fixing drive clutch interposed between gears driving a heating roll, and a CPU for controlling on / off of a power supply of the fixing drive clutch were used.

In the fixing device of this embodiment, a ROM incorporated in a CPU is used as a storage device for preliminarily storing the fixing speed for each image forming mode, and the fixing speed stored in the ROM is a plain paper. a fixing speed of copying only one side (V _1), and fixing speed of copying to OHP sheets (V _2), the fixing speed of copying cardboard (V ₃₎ is stored.

In the apparatus of the present embodiment, a CPU is used as a device for controlling the fixing speed, and the ROM incorporated in the CPU includes fixing speeds (V ₁ ), (V ₁ ), (V ₁ ) stored in the ROM as the storage device. from among _2), a program for controlling the rotational speed of fuser drive motor providing a selected fixing speed rotational driving force to the heating roll 1 is written by the specified image formation mode.

Here, in the fixing device of the present embodiment, the values of the first fixing speed (V ₁ ), the second fixing speed (V ₂ ), and the third fixing speed (V ₃ ) are respectively set for plain paper. The speed for appropriately setting the gloss of the fixing toner, the speed for obtaining the transparency of the toner with the OHP, the gloss for the thick paper, and the speed for satisfying the fixing were determined based on the test data.
Specifically, V ₁ = 106 mm / sec. ,
V ₂ = 70 mm / sec. , V ₃ = 40 mm / sec.
Is set to Further, the value of the free fixing time when copying in plain paper one-sided mode using A3 paper is 1
4.8 seconds. On the other hand, the value of the fixing free time in the OHP mode is 37 seconds.

Hereinafter, a case where an image is formed using a copying machine incorporating the fixing device of the present embodiment and a fixing operation is performed will be described. Now, an example in which three OHP sheets are used and a full-color image is copied thereon will be described. First, when the copier is turned on and enters a standby state, the heating roll 1 is controlled to maintain its target temperature (T _H ), and the pressure roll 4 also maintains its target temperature (T _P ). Controlled.

Next, when the OHP mode is designated on the switching panel of the copying machine, the designated signal is sent to the CPU as the speed control device, and the CPU stores the signal in the ROM as the speed storage device based on the signal. A fixing speed (V ₂ ) corresponding to the OHP mode is selected from a plurality of stored fixing speeds, and a rotational driving force is applied to the heating roll 1 so that the fixing is performed at the selected fixing speed (V ₂ ). The number of rotations of the supplied Fuser drive motor is controlled.

Next, when the start button of the copying machine is pressed, image information is read from the original by an image reading device (not shown), and this is temporarily converted into a digital signal. Written in
An electrostatic latent image is formed. The electrostatic latent image is black, yellow,
This is performed for each color of red and blue. First, black will be described. When an electrostatic latent image corresponding to red is formed on the electrostatic latent image carrier, black toner is then supplied from a developing device that supplies black toner, and the electrostatic latent image on the electrostatic latent image carrier is Is developed to form a red unfixed toner image. This black unfixed toner image is then transferred to a transfer drum,
A black unfixed toner image is carried on the transfer drum. On the other hand, the surface of the electrostatic latent image carrier that had carried the black unfixed toner image was completely removed of the red toner with a cleaner,
After the static elimination, an electrostatic latent image corresponding to the next color, for example, yellow, is written, developed with yellow development yellow, and then a black unfixed toner image is transferred to the transfer drum, where Is transferred so as to overlap the unfixed toner image. Hereinafter, similarly, the red and blue toner images are superimposed on the transfer drum, and a full-color unfixed toner image is formed on the transfer drum.

Next, this full-color unfixed toner image is transferred to the OHP sheet conveyed from the paper tray, but before that, a kind of idling operation called "dummy cycle" is performed. The reason is that the transfer step is performed at a relatively high speed, while the speed of the fixing step after the transfer step is low. The OHP that has passed through the transfer drum must be
This is because the sheet causes a paper loop just before the fixing roll.

During the dummy cycle of about 5 seconds, the rotation speed of the transfer drum is reduced, and when the transfer speed of the transfer drum matches the fixing speed, the full-color unfixed image carried on the transfer drum is removed from the paper tray. OH transported
The image is transferred onto a P sheet and sent to a subsequent fixing portion.

In the fixing section, a pre-fixing paper sensor is attached just before the fixing nip area where the heating roll and the pressure roll are arranged, that is, at a position upstream in the moving direction of the recording medium. In a state before the sensor detects the passage of the leading end of the OHP sheet, the stopping means operates, and thus the CPU as the rotation driving means does not operate, and therefore, the fixing driving clutch does not operate, so that the fuser driving is performed on the heating roll. The rotation driving force from the motor is not transmitted, and the heating roll does not rotate. Also, the pressure roll pressed against the heating roll does not rotate.

Next, when an OHP sheet onto which an unfixed toner image has been transferred is fed from the transfer area provided with the transfer drum toward the fixing nip area, the pre-fixing paper sensor detects the OHP sheet. The passage of the tip is detected. Then, the operation of the stopping means is completed, the power of the fixing drive clutch is turned on, the rotational driving force from the fuser driving motor is transmitted from the gear train to the heating roll via the fixing driving clutch, and the heating roll starts rotating. .
Then, the pressure roll pressed against the heating roll also starts rotating. When the OHP sheet passes through the fixing nip area, the toner of the unfixed toner image is fixed on the OHP sheet.

Next, when the fixing to the OHP sheet is completed, the sheet is discharged further downstream from the fixing nip area.
At this time, when the rear end portion of the OHP sheet passes through the detection portion of the post-fixing paper sensor disposed downstream of the fixing nip area, the post-fixing paper sensor detects and transmits a signal to the CPU. . Then, the CPU recognizes that the OHP sheet is no longer sandwiched in the fixing nip area, and detects that the fixing nip area has entered the free fixing time. Then, the CPU as the stopping means is operated, and the power of the fixing drive clutch as the stopping means is turned off, so that the rotational driving force from the fuser driving motor is not transmitted to the heating roll, and the rotation of the heating roll is stopped. Stops, and the pressure roll also stops rotating at the same time.

On the other hand, while the previous OHP sheet is being fixed, the steps of reading, writing, developing and transferring the image information for the next original are performed, and the transfer drum is set to a full-color image for the next original. An unfixed image is formed, and the system is ready to transfer to a subsequent OHP sheet at any time. Then, when a predetermined time elapses after the preceding OHP sheet has passed through the fixing nip area, a full-color unfixed image is transferred to the subsequent OHP sheet and sent to the fixing portion.

Then, the pre-fixing paper sensor disposed in front of the fixing area detects the passage of the leading edge of the succeeding OHP sheet, and sends a signal to the CPU. Then, the CPU detects that the fixing free time has ended. Then, the fixing drive clutch is operated to start the rotation of the heating roll.

By repeating the above operations, the rotation of the heating roll and the pressure roll is stopped during the fixing free time, so that heat is transferred from the heating roll to the pressure roll during the fixing free time. Thus, the surface of the pressure roll is prevented from being heated.

On the other hand, when the plain paper mode is designated as the image forming mode, the stopping means of this embodiment does not operate. However, when the fixing time is short as in the plain paper mode, the fixing operation is performed. Even when the pressure roll and the heating roll are rotated while being pressed against each other, the degree of increase in the surface temperature of the pressure roll is small, so that the rotation drive between the heating roll and the pressure roll is stopped. There is no trouble even if you do not perform the operation.

(Second Embodiment) In the fixing device of the present embodiment, a pressure roll is disposed so as to be displaceable in a direction of coming and going with respect to a heating roll, and a rotation driving force is applied to the heating roll as rotation driving means. In addition to a supplied fuser drive motor and gear train, a latch solenoid for turning on / off a spring clutch and a CPU for driving the same are used to provide a function of pressing and separating a pressure roll against a heating roll, In addition to using the latch solenoid and a CPU for controlling the power on / off of the latch solenoid as a stopping means, the pressure roll is separated from the heating roll during the idle time when the OHP mode is designated. The same fixing device as in the first embodiment was used. In the fixing device of the present embodiment, since the pressing roll is separated from the heating roll during the fixing free time when the OHP mode is designated, heat is supplied from the heating roll to the pressing roll during the fixing free time. Characteristic effect that almost completely can be prevented.

[0066]

According to the present invention, there is provided a full-color fixing device in which a heating member is incorporated in both a heating roll and a pressure roll, wherein the surface temperature of the pressure roll is controlled to a temperature lower than the surface temperature of the heating roll. It is possible to provide a fixing device capable of performing a fixing operation on an OHP sheet or the like in a fixing device of a model corresponding to the duplex copying.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a fixing device according to a first embodiment of the present invention.

FIG. 2 is a timing chart of the fixing device according to the first embodiment of the present invention.

FIG. 3 is a schematic sectional view of a conventional fixing device.

FIG. 4 is a timing chart of a conventional fixing device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Heating roll, 2 ... Heating lamp (heating source), 3 ... Temperature sensor, 4 ... Pressure roll, 5 ... Heating lamp (heating source), 6 ... Temperature sensor, 9 ... Unfixed toner image, 10 ...
Paper (recording medium).

Claims (5)

[Claims] 1. A heating roll having a built-in heating source, and a heating medium having a built-in heating source and being pressed against the heating roll to form a fixing nip area, and a recording medium onto which an unfixed toner image has been transferred passes through the fixing nip area. A pressure roll for fixing an unfixed toner image in conjunction with the heating roll, and controlling a heating source in each roll based on the surface temperature of the heating roll and the pressure roll to heat in a standby state. The heating source of the heating roll is controlled so that the surface temperature of the roll becomes the target temperature (T _H ), and the surface temperature of the pressure roll becomes the target temperature (T _P ) lower than the surface temperature of the heating roll. A temperature control unit for controlling, a rotation driving unit for rotating the heating roll and the pressure roll at the time of fixing, and a leading end of a succeeding recording medium is fixed after a rear end of the preceding recording medium passes through the fixing nip area. A free time detecting means for detecting a fixing free time until entering the nip area; and a stopping means for stopping the rotary driving means during the fixing free time when a specific image forming mode is designated. A fixing device, comprising: 2. The fixing device according to claim 1, wherein the specific image forming mode is an OHP mode or a duplex copy mode. 3. A heating roll having a built-in heating source, and a heating medium having a built-in heating source and being pressed against the heating roll to form a fixing nip area, and a recording medium onto which an unfixed toner image has been transferred passes through the fixing nip area. A pressure roll for fixing an unfixed toner image in conjunction with the heating roll, and controlling a heating source in each roll based on the surface temperature of the heating roll and the pressure roll to heat in a standby state. The heating source of the heating roll is controlled so that the surface temperature of the roll becomes the target temperature (T _H ), and the surface temperature of the pressure roll becomes the target temperature (T _P ) lower than the surface temperature of the heating roll. A temperature control unit for controlling, a rotation driving unit for rotating the heating roll and the pressure roll at the time of fixing, and a leading end of a succeeding recording medium is fixed after a rear end of the preceding recording medium passes through the fixing nip area. A vacant time detecting unit for detecting a vacant fixing time before entering the nip area; and a stopping unit for stopping the rotary driving unit during the vacant fixing time when the vacant fixing time is equal to or more than a set value. A fixing device. 4. The fixing device according to claim 3, wherein the set value of the free fixing time is 5 to 15 seconds. 5. A pressure roll is disposed so as to be able to freely contact and separate from the heating roll, and a rotation driving means applies a rotation driving force to the heating roll or the pressure roll, and also connects the pressure roll to the heating roll. The device according to any one of claims 1 to 4, wherein the device is a unit that displaces the fixing roller in a direction in which the pressing roller is separated from the heating roller. The fixing device as described in the above.