JPH08262918A - Fixing device for image forming device - Google Patents

Abstract

PURPOSE: To prevent the temp. of a pressure roll from excessively rising, even if a fixing device is consecutively operated during the stand by of a paper sheet, in an image forming device for making the paper stand by with the exact timing of the formation of an image. CONSTITUTION: The image forming device transfers the image on a photoreceptor 1 to the paper sheet in such a manner that the papers are consecutively fed to an image forming part 7 provided with the photoreceptor 1 and temporarily stopped in the halfway of a paper carrying path, to be in a standby state and the heating roll 16 and pressure roll 17 of a fixing device 15 are idly rotated in a press-contact state to release the standby state with the exact timing of the formation of the image in the photoreceptor 1 and feed the paper to the image forming part 7 and fixes the image on the paper to another paper sheet in such a manner that the paper after the transfer passes through the nip part between the heating roll 16 and the pressure roll 17. Moreover, a temperature control part 24 for setting the set temp. of the heating roll 16 lower than that during the passage of the paper, while the paper is in the standby state is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device used in an image forming apparatus such as a copying machine.

[0002]

2. Description of the Related Art For example, in an image forming apparatus such as a copying machine, a toner image is formed on the surface of a moving image carrier such as a photosensitive drum, and the toner image is transferred from a paper feed tray in synchronization with the moving toner image. The paper is fed, and the toner image on the image carrier is transferred onto the paper at the transfer portion. And
The paper on which the toner image has been transferred is supplied to a fixing device including a heating roll and a pressure roll, and heat and pressure are applied to the paper to fix the toner image on the paper.

Further, the paper is also conveyed from the paper feed tray to a stopping means provided in advance on the upstream side of the transfer section, temporarily stopped there, and then the paper is sent out on the image carrier at the timing of image formation. ing.

Further, as described in Japanese Patent Application Laid-Open No. 2-231342, paper is continuously sent out from a paper feed tray, and when the processing is delayed in the image forming unit, the paper is temporarily stopped. Then, the sheet is put on standby and re-conveyed at the timing of image formation.

[0005]

In such an image forming apparatus in which the sheet is temporarily made to stand by at the timing of image formation, the fixing device is in the operating state even when the sheet is in the standby state. That is, the heating roll and the pressure roll of the fixing device are rotating in a state of being pressed against each other,
Further, temperature control is performed so that the temperature of the heating roll is maintained constant. This is to ensure that the fixing operation is performed by applying sufficient heat and pressure to the sheet carrying the toner image when the sheet whose standby state has been released is supplied to the fixing device.

However, when the fixing device is operated under the same conditions as in the normal operating state, the temperature of the pressure roll rises excessively, and a high temperature is again applied to the fixed image on the back surface during double-sided copying. It was found that the problem of image loss occurs.

FIG. 2A is a graph showing a temperature change of the heating roll and a temperature change of the pressure roll in the conventional image forming apparatus for making a sheet stand by. In the figure, period A
Indicates a continuous sheet running state in which the sheet conveying device and the fixing device are in operation, and period B indicates a period in which the sheet is in a standby state.

For example, if the set temperature for the temperature control of the heating roll is 195 ° C., the temperature of the pressure roll will be as shown in FIG.
As shown by curve b in (a), about 160 ° C
However, in the standby state, the fixing device rotates idly, that is, the fixing device rotates without pinching the sheet.
Therefore, the heat from the heating roll is directly applied to the surface of the pressure roll, and the heat is not taken from the surface of the heating roll and the surface of the pressure roll to the paper. 2A, the temperature rises to 180 ° C. or higher as shown by the curve b. When the temperature of the pressure roll becomes 180 ° C. or higher, if the next sheet is double-sided sheet, the image already fixed on the back surface (viewed from the heating roll) is again heated to a high temperature and the toner peels off. ,
Toner loss occurs in the image on the back side.

Therefore, according to the present invention, in the image forming apparatus for making the paper stand by at the timing of image formation, even if the fixing device is continuously operated during the paper stand-by, the temperature of the pressure roll excessively rises. The purpose is not to.

[0010]

In order to achieve the above-mentioned object, the present invention continuously feeds a sheet to an image forming section provided with an image carrier and temporarily stops the sheet in the middle of a sheet conveying path. To a standby state, the heating roll and the pressure roll of the fixing device are idled in a pressure contact state, the standby state is released at the timing of image formation on the image carrier, and the sheet is fed to the image forming unit. An image forming apparatus for fixing an image on paper by passing the image on the image carrier onto the paper and passing the transferred paper through a nip portion between the heating roll and the pressure roll. In the above, a temperature control unit is provided for setting the set temperature of the heating roll to be lower than the set temperature during the passage of the sheet while the sheet is in the standby state.

Further, the present invention is characterized in that a temperature control unit is provided for setting the set temperature of the heating roll to a temperature equal to or lower than the toner melting temperature after the standby state of the paper is released.

Further, the temperature control unit is for returning the set temperature of the heating roll to the normal temperature before the sheet reaches the nip portion between the heating roll and the pressure roll. .

[0013]

When the paper is in the standby state, the heating roll and the pressure roll run idle, but the temperature of the heating roll becomes lower than the temperature during the passage of the paper while the paper is in the standby state. When the paper reaches the nip portion between the heating roll and the pressure roll, the temperature of the pressure roll does not become higher than the toner melting temperature, and the toner image is not damaged.

Further, after the standby state of the paper is released, the temperature of the heating roll is set to the toner melting temperature or lower, and the temperature of the heating roll is returned to the normal temperature before the paper reaches the nip portion between the heating roll and the pressure roll. As a result, overheating of the pressure roll is prevented and the fixing process for the next sheet is surely performed.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be specifically described below based on embodiments with reference to the drawings.

FIG. 1 is a schematic view showing an embodiment of an image forming apparatus to which the fixing device of the present invention is applied. In the present exemplary embodiment, an image forming apparatus that forms images on both sides of a sheet will be described as an example.

A charging device 2, an exposure device 3, a developing device 4, a charging device 5, a cleaning device 6 and the like are sequentially arranged around a drum-shaped photosensitive member 1 which is an image carrier to form an image forming section 7. are doing. Below the image forming unit 7, a paper feed tray 9 including a paper feed mechanism 8 and a standby tray 11 including a paper feed mechanism 10 are arranged. Each paper feed mechanism 8, 10
For example, it is equipped with a paper feed roll and a paper feed motor. The paper fed from the paper feed tray 9 or the standby tray 11 is
It is supplied to the transfer portion 13 between the photoconductor 1 and the charging device 5 by the conveyance roller 12. The sheet that has passed through the transfer unit 13 is conveyed to the fixing device 15 by the belt conveyor 14. The fixing device 15 includes a heating roll 16 and a pressure roll 17, which are in pressure contact with each other. A heater 18, which is a heat source, is arranged on the shaft portion of the heating roll 16, and the heating roll 1
A temperature sensor 19 is arranged on the surface of 6 in a pressure contact state.

Next, the control system and the signal system of the image forming apparatus will be described.

The image signal S1 corresponding to the image to be output is supplied to the image forming processing section 21 and, after undergoing predetermined image processing, is supplied to the exposure device 3 as a drive signal S2. The exposure device 3 includes, for example, a laser (not shown) modulated by the drive signal S2 and a polygon mirror (not shown) that deflects the laser beam L from the laser to scan the surface of the photoconductor 1. It is configured.

The paper feed mechanisms 8 and 10 are connected to a paper transport control section 23 via a drive circuit 22. Also,
A sheet standby signal S3 is supplied from the image forming processing section 21 to the sheet conveyance control section 23, and the sheet standby signal S3 is supplied.
The operation and stop of the paper feed mechanisms 8 and 10 are controlled based on the above.

Further, the output of the temperature sensor 19 is supplied to the fixing device temperature control section 24, and the current flowing through the heater 18 is controlled so that the temperature of the heating roll 16 matches the set temperature.

Next, an outline of processing for forming images on both sides of the paper in the above-mentioned image forming apparatus will be described.

First, the surface of the photoconductor 1 rotating in the direction of the arrow is uniformly charged by the charging device 2. Next, the image signal S1 corresponding to the image on the first surface is supplied to the image forming processing unit 21, the exposure device 3 is driven, and the surface of the photoconductor 1 is irradiated with the laser beam L, so that the surface of the photoconductor 1 is exposed. An electrostatic latent image is formed on. This electrostatic latent image is developed by the developing device 4, and a toner image is formed on the surface of the photoconductor 1. The paper P is fed from the paper feed tray 9 in time with the formation of this toner image, passes through the conveyance paths A and B, and is conveyed by the conveyance roller 12 to the transfer unit 13 between the photoconductor 1 and the charging device 5. Supplied. In addition, the first side of the paper in the paper feed tray 9 facing downward
Surface and the upward surface is referred to as the second surface.
In the transfer unit 13, the toner image on the surface of the photoconductor 1 is transferred to the first surface of the paper. The transferred sheet is conveyed to the fixing device 15 by the belt conveyor 14 and heated by the heating roll 1
6 passes through the nip portion between the pressure roller 17 and the pressure roller 17 and undergoes fixing processing. At this time, the heating roll 16 contacts the first surface of the sheet on which the unfixed image is formed, and the pressure roll 17 contacts the second surface on which the image is not yet formed. The paper after the fixing process is indicated by an arrow C by a switching gate (not shown).
The course is switched in the direction indicated by and reaches the reversing roll 20. The reversing roll 20 is initially rotated in the direction of arrow a,
The sheet is drawn in near the rear end of the sheet, then rotated in the direction of arrow b, the sheet is sent out in the direction of arrow b, and the sheet is stored in the standby tray 11 with the first surface facing upward, and is in a standby state.

Next, the sheets in the standby tray 11 are released from the standby state at the timing of image formation in the image forming section 7, pass through the paths E and B, and are transferred again to the transfer section 13.
And the toner image is transferred to the second surface of the paper this time. The transferred sheet passes through the fixing device 15 again. At this time, the heating roll 16 contacts the second surface of the sheet on which the unfixed image is formed, and the second image on which the fixed image is formed is formed.
The pressure roll 17 comes into contact with the surface. The sheet after fixing is switched in the direction indicated by the arrow F this time by a switching gate (not shown), and is discharged to the outside of the machine.

As described above, when images are formed on both sides of the paper, it is necessary to pass the paper through the fixing device 15 twice, but at the time of the second fixing process, that is, the second paper of the paper.
When the image applied to the first surface of the sheet formed earlier is applied with a high temperature during the fixing process of the image on the first surface, the toner of the image on the first surface may be lost. .

Therefore, in the present embodiment, the image forming processing section 21.
The paper standby signal S3 from the image forming apparatus 21 is also supplied to the fixing device temperature control unit 24, and the paper standby signal S3 from the image forming processing unit 21 is input to the paper transportation control unit 23, so that the image forming apparatus enters the paper transportation interruption standby state. When it enters, it is controlled to lower the set temperature in the fixing device temperature control section 24. For example, if the normal set temperature is 195 ° C, the set temperature is lowered to about 175 to 180 ° C. This allows
As shown in FIG. 2B, the temperature of the heating roll 16 (see the curve a) decreases, and accordingly, the temperature of the pressure roll 17 (see the curve b) does not cause back surface toner loss, that is, 170 ° C below 180 ° C which is the toner melting temperature
Maintained below. Therefore, even when the images are formed on both sides of the paper, the toner of the already formed image on the first side is not lost when the image on the second side is fixed.

In the above embodiment, the set temperature is lowered during the standby state and the normal temperature is used after the standby is released, but the present invention is not limited to this. For example,
After the standby state is released, the leading edge of the paper is heated by the heating roll 16
The set temperature may be lowered during the period until it reaches the nip portion of the pressure roll 17. In addition, the set temperature is lowered during the standby state, and after the standby is released, the leading edge of the paper is heated by the heating roll 16
The set temperature may be set to the normal temperature during the period until it reaches the nip portion of the pressure roll 17.

[0028]

As described above, according to the present invention, the temperature of the pressure roll becomes equal to or lower than the toner melting temperature even when the sheet is supplied to the fixing device after the standby state, and the backside toner loss occurs. There is no temperature. Therefore, even when the images are formed on both sides of the paper, the toner of the already formed image on the first side is not lost when the image on the second side is fixed.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of an image forming apparatus to which a fixing device of the present invention is applied.

FIG. 2 is a graph showing changes in temperature of a heating roll and a pressure roll in an operating state and a standby state of the image forming apparatus.

[Explanation of symbols]

1 ... Photosensitive member, 2 ... Charging device, 3 ... Exposure device, 4 ... Developing device, 5 ... Charging device, 6 ... Cleaning device, 7 ... Image forming device, 8, 10 ... Paper feeding mechanism, 9 ... Paper feeding tray, 11
... standby tray, 12 ... transport tray, 13 ... transfer section, 14
... belt conveyor, 15 ... fixing device, 16 ... heating roll, 17 ... pressure roll, 18 ... heater, 19 ... temperature sensor, 20 ... reversing roll, 21 ... image forming processing section, 22 ...
Drive circuit, 23 ... Paper transport controller

Claims (3)

[Claims] 1. A heating roll and a pressure roll of a fixing device, which continuously feeds a sheet to an image forming unit provided with an image carrier, temporarily stops the sheet in the middle of a sheet conveying path to put it in a standby state. Is rotated in a pressure contact state, the standby state is released at the timing of image formation on the image carrier, the sheet is fed to the image forming unit, and the image on the image carrier is transferred onto the sheet. Then, in the image forming apparatus that fixes the image on the sheet by passing the sheet after transfer through the nip portion between the heating roll and the pressure roll, the set temperature of the heating roll is set in the standby state of the sheet. A fixing device for an image forming apparatus, comprising: a temperature control unit for setting a temperature lower than a set temperature during paper passage. 2. A sheet is continuously fed to an image forming unit provided with an image carrier, the sheet is temporarily stopped in the middle of a sheet conveying path to be in a standby state, and a heating roll and a pressure roll of a fixing device are provided. Is rotated in a pressure contact state, the standby state is released at the timing of image formation on the image carrier, the sheet is fed to the image forming unit, and the image on the image carrier is transferred onto the sheet. In the image forming apparatus that fixes the image on the sheet by passing the sheet after transfer through the nip portion between the heating roll and the pressure roll, the setting of the heating roll is performed after the standby state of the sheet is released. A fixing device for an image forming apparatus, which is provided with a temperature control unit for setting a temperature to be equal to or lower than a toner melting temperature. 3. The temperature control unit returns the set temperature of the heating roll to a normal temperature before the sheet reaches a nip portion between the heating roll and the pressure roll. Item 2. A fixing device for an image forming apparatus according to item 2.