JPH11258941A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To save power consumption and to satisfactorily fix a toner image by shortening the warm-up time prior to double-side copy or single surface copy and waiting time at the time of switching from back surface copy mode to both-side or single surface copy mode, in an image forming device which fixes all at once a transfer material having the toner images held on both its sides. SOLUTION: In this image forming device by which the toner image is transferred to single side of double sides of recording paper P being a transfer material and fixed, an upper roller 310 being a fixing means for the surface of the recording paper P has an internal fixing means and also an external heating roller 331 being an external heating means. At the time of the warm-up prior to double-side or single surface copy, or at the time of switching the modes, the upper roller 310 is heated by actuating the external heater H3 of the external heating roller 331.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming an image by an electrophotographic method, and more particularly, to forming a toner image on both sides of a transfer material and heating and fixing the toner images collectively to form a double-sided image. The present invention relates to an image forming apparatus that performs the following.

[0002]

2. Description of the Related Art Conventionally, in double-sided image formation, an image on one side formed on an image forming body is transferred and fixed on a transfer material, and is temporarily stored in a double-sided reversal feeding device, and is then re-imaged. A method has been adopted in which a transfer material is fed from a two-sided reversing feeder in time with an image formed on a formed body, and an image on the other surface is transferred and fixed on the transfer material.

In this double-sided image forming apparatus, as described above, the transfer of the transfer material such as feeding to the two-sided reversing feeder and passing through the fixing device twice is performed, so that the transfer material transfer reliability is low.
This causes the transfer material to jam or wrinkle. Further, as is well known, there is another problem that a long processing distance is required for image formation because the transfer distance of the transfer material is long.

On the other hand, JP-B-49-37538, JP-B-54-28740, and JP-A-1-444.
No. 57, Japanese Patent Application Laid-Open No. 4-214576, and the like have proposed a device in which a toner image is formed on both surfaces of a transfer material and then fixed once.

Further, the present inventor provides an apparatus for transferring a toner image to both surfaces of a transfer material and simultaneously fixing a transfer material having toner images on both surfaces by providing two image carrying means and two transfer means. The authors conducted a study on a double-sided image forming apparatus that has high reliability in transfer material transfer, does not cause jams and wrinkles, and can significantly increase the processing speed of double-sided image formation.

In the above-described double-sided image forming apparatus, the transfer material on which the toner images have been transferred on the front and back surfaces only needs to be passed through the fixing device once, so that the transfer material transfer reliability is high, and the transfer material transfer path is increased. The processing speed of image formation can be increased by shortening.

[0007]

However, in the above-described double-sided image forming apparatus, this is not easy because a transfer material having toner images on both sides is fixed collectively. It is necessary to have heating means on both sides of the transfer material in order to collectively fix the transfer material having the toner images on both sides, and when using rollers which contact the front and back of the transfer material as the heating means, In order to lengthen the nip portion of the roller, at least one of the fixing units in contact with the front and back surfaces of the transfer material needs to be a soft roller. In order to efficiently and completely fix a transfer material having toner images on both surfaces without causing offset, it is necessary to maintain the surface temperatures of the upper and lower rollers at or above the offset limit temperature Tc at which offset occurs. However, since the heat conduction of the soft roller is poor, it takes a long time to reach the offset limit temperature Tc or more using only the heating means inside the roller, and if image formation is continued, the heat supply is insufficient and the surface temperature of the soft roller decreases. There is a problem that the temperature becomes lower than the offset limit temperature Tc.
Another problem is that the control is unstable because the temperature rise of the soft roller is slow.

For example, if the upper roller is a hard roller and the lower roller is a soft roller, the temperature rise of the lower roller for fixing the backside image is slow, so that the front side image forming mode is changed to the double side image forming mode (or the backside image forming mode). There is a problem that the waiting time when switching to the (forming mode) becomes long.

On the other hand, when the upper roller is a soft roller and the lower roller is a hard roller, the temperature rises quickly because the lower roller is a hard roller. There is an advantage that the switching loss when switching to is small. However, in this case, since the temperature rise of the upper roller for fixing the front surface image is slow, there is a problem that the warm-up time prior to the formation of the double-sided image or the front surface image is long. Further, although the switching frequency is very low, there is a problem that the waiting time when switching from the back side image forming mode to the double side image forming mode or the front side image forming mode is long.

Further, in the case of forming images continuously using a large-size transfer material or a thick transfer material in the double-sided image forming mode, the temperature of the upper and lower rollers greatly decreases.
There is a problem that the surface temperature of the upper roller, which is a soft roller, becomes lower than the offset limit temperature Tc.

The present invention solves the above-mentioned problems, and provides a warm-up time in a double-sided or surface image forming mode,
The waiting time at the time of switching the image forming mode is shortened, and the surface temperature of the upper roller falls below the offset limit temperature Tc even when a large-sized transfer material or a thick transfer material is used in the double-sided image forming mode. It is an object of the present invention to provide an image forming apparatus which can prevent the image formation and perform satisfactory fixing.

[0012]

According to an aspect of the present invention, there is provided an image forming apparatus in which a toner image is transferred onto one or both surfaces of a transfer material and then fixed. On the other hand, having an external heating means together with the internal heating means, warming up prior to double-sided image formation or front side image formation, or when changing from the back side image formation mode to the two sided or front side image formation mode, heating by the external heating means. This is achieved by a characteristic image forming apparatus (first invention).

According to another aspect of the present invention, there is provided an image forming apparatus in which a toner image is transferred onto one or both surfaces of a transfer material and then fixed. Having external heating means,
This is achieved by an image forming apparatus (second invention) characterized in that heating is performed by the external heating means during double-sided image formation.

In the above invention, the image forming apparatus according to the present invention is characterized in that the external heating means is controlled according to the type (thickness, size) of the transfer material.

In the above two inventions, the fixing means facing the surface of the transfer material is a soft roller having an elastic layer provided on the roller surface, and the fixing means facing the back surface of the transfer material is an elastic layer provided on the roller surface. It is a preferred embodiment that the hard roller is not provided with.

[0016]

Embodiments of the present invention will be described below. The description in this section does not limit the technical scope of the claims and the meaning of the terms. The assertive description below shows the best mode, and limits the meaning of the terms and the technical scope of the present invention. It does not do. In the following description of the embodiment, the surface of the transfer material facing the image forming body in the transfer area is referred to as the front surface, and the other surface of the transfer material is referred to as the back surface, and the image to be transferred to the surface of the transfer material is referred to as the front surface. The image to be transferred onto the other side of the image or the transfer material is called a back side image.

(Embodiment 1) An image forming process and each mechanism of an image forming apparatus according to Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a cross-sectional configuration diagram of an image forming apparatus showing one embodiment of the present invention, and FIG.
FIG. 3 is a diagram showing a state of forming a toner image and supply of the transfer material when images are formed on both surfaces of the transfer material.

The image forming apparatus 100 includes an image forming body,
An intermediate transfer member, a transfer material charging unit for charging the transfer material and adsorbing the intermediate transfer member, a first transfer unit for transferring the toner image on the image forming member to the surface of the intermediate transfer member or the transfer material, A second transfer unit for transferring the toner image on the transfer material to the back surface of the transfer material; and a fixing unit for fixing the toner image on the transfer material. The toner includes a charging unit, an image exposure unit, and a developing unit. The toner image formed on the image forming body by the image forming means is once transferred onto the intermediate transfer body by the first transferring means and then transferred to the back surface of the transfer material by the second transferring means, and / or on the image forming body. The toner image formed on the transfer material is transferred to the surface of the transfer material by the first transfer means, and the toner image transferred to both sides or one side of the transfer material is fixed by the fixing means to form an image on both sides or one side of the transfer material. Image forming apparatus.

The photosensitive drum 10, which is an image forming body, has, for example, a transparent conductive layer, a-, on the outer periphery of a cylindrical substrate formed of a transparent member of optical glass or transparent acrylic resin.
A photosensitive layer such as an Si layer or an organic photosensitive layer (OPC) is formed, and is rotated clockwise as indicated by an arrow in FIG. 1 with the conductive layer grounded.

As shown in FIG. 2, the photosensitive drum 10 has flange members 10a and 10b on both end surfaces for engaging and fixing the photosensitive drum.
Is rotatably supported by bearings 110a and 110b fitted into flange members 10a and 10b on both end surfaces of a drum shaft 110 which is erected and fixed to the apparatus main body. The apparatus is driven at a constant speed in a predetermined direction by being driven by meshing with a drive gear on the apparatus body side.

Scorotron charger 1 as charging means
1. An exposure unit 12 as an image exposure unit and a developing unit 13 as a developing unit are used in an image forming process of each color of yellow (Y), magenta (M), cyan (C) and black (K). Are arranged in the order of Y, M, C, and K with respect to the rotation direction of the photosensitive drum 10 indicated by the arrow in FIG.

The scorotron charger 11 serving as a charging means for each color has a corona discharge electrode 11a formed of a saw-toothed electrode or a wire electrode and a control for maintaining the organic photosensitive layer of the photosensitive drum 10 at a predetermined potential. The photoconductive drum 10 is attached to the photoconductive drum 10 in a direction orthogonal to the moving direction of the photoconductive drum 10. The scorotron charger 11 performs corona discharge of the same polarity as the toner (in this embodiment, negative charge), and applies a uniform potential to the photosensitive drum 10.

The exposure unit 12 as an image exposure means for each color is arranged such that the exposure position on the photosensitive drum 10 is on the downstream side in the rotation direction of the photosensitive drum 10 with respect to the scorotron charger 11 for each color. , Are arranged inside the photosensitive drum 10.

The exposure units 12 for each color are respectively
A linear exposure element 12a in which a plurality of LEDs (light emitting diodes) as image exposure light emitting elements arranged in the main scanning direction in parallel with the axis 110 of the photosensitive drum 10 are arranged in an array.
And a SELFOC lens 12b as an equal-magnification imaging element are configured as an exposure unit attached to a holder (not shown). The exposure unit 12 for each color is attached to the holding member 20 together with the uniform exposure unit 12c and the simultaneous transfer exposure unit 12d, and the holding unit 20 is fixed to the apparatus main body. The unit 12c and the simultaneous transfer exposure unit 12d are integrally housed inside the base of the photosensitive drum 10. As the exposure element, a linear element in which a plurality of light emitting elements such as FL (phosphor emission), EL (electroluminescence), and PL (plasma discharge) are arranged in an array is used. The exposure unit 12 performs image exposure on the photosensitive drum 10 based on image data of each color read by a separate image reading device and stored in a memory, and forms a latent image on the photosensitive drum 10. The light-emitting wavelength of the light-emitting element used in this embodiment is usually 6 which has high transmittance for Y, M, and C toners.
The wavelength in the range of 80 to 900 nm is good, but the wavelength may be shorter than this, since the color toner does not have sufficient transparency because the image is exposed from the back surface.

The developing device 13 as a developing means for each color includes
For example, each has a thickness of 0.5 to 1 mm and an outer diameter of 15 to 25, for example.
mm developing sleeve 131 made of a non-magnetic stainless steel or aluminum material, and a developing casing 1
And contains therein a one-component or two-component developer of Y, M, C and K. Developing sleeve 13
Reference numeral 1 denotes a non-illustrated abutting roller that keeps the photosensitive drum 10 in a non-contact state with a gap of a predetermined value, for example, 100 to 1000 μm, and rotates in a forward direction with respect to the rotation direction of the photosensitive drum 10. By applying to the developing sleeve 131 a DC voltage of the same polarity as the toner (in the present embodiment, a negative polarity) or a developing bias to which an AC is further applied, the developing sleeve 131 is in a non-contact state with the exposed portion of the photosensitive drum 10. Reversal development is performed. At this time, the precision of the development interval needs to be about 20 μm or less in order to prevent image unevenness.

A toner image receiver 14a as an intermediate transfer member
Is an endless belt stretched between the driving roller 14d and the driven roller 14e and provided in contact with the photosensitive drum 10, and has a thickness of, for example, 0.5 to 2.0 mm and a volume resistivity of 10 ⁸ to 10 ⁸ . It has a two-layer structure in which a semiconductive fluorine coating having a thickness of 5 μm to 50 μm is applied as a toner filming preventing layer to the outside of a semiconductive rubber belt base made of 10 ¹⁴ Ω · cm silicon rubber or urethane rubber. . Instead of rubber belt base, thickness 0.1-0.5m
m, semiconducting polyester, polystyrene, polyethylene, polyethylene terephthalate, polyimide, modified polyimide or ET having a volume resistivity of 10 ⁸ to 10 ¹⁴ Ω · cm.
FE (ethylene-tetrafluoroethylene copolymer) or the like can also be used.

The paper charger 14f as a transfer material charging means is a charging brush capable of coming into contact with and releasing the toner image receiver 14a, and is opposed to a grounded driven roller 14e which stretches the toner image receiver 14a. Placed. Paper charger 1
4f shows the recording paper P as the recording paper P as a transfer material passes.
, And the recording paper P has the same polarity as the toner (negative polarity in the present embodiment) by the paper charger 14f.
And is attracted to the toner image receiver 14a and fed to the transfer area 14b. By charging the toner to the same polarity as the toner, the recording paper P is transferred to the toner image receiving member 14a at a portion other than the transfer portion.
The toner image is prevented from being attracted to the upper toner image or the toner image on the photosensitive drum 10, thereby preventing the toner image from being disturbed. As the transfer material charging means, the toner image receiving member 14
It is also possible to use a conductive roller or conductive film capable of contacting and releasing contact with a, or a corona discharger that does not contact the toner image receiver 14a.

The transfer device 14c as the first transfer means includes:
Polarity opposite to toner (positive polarity in this embodiment)
Is applied to the photosensitive drum 10 via the toner image receiver 14a. The transfer device 14c charges the back surface of the toner image receiving member 14a to the opposite polarity to the toner, thereby causing the photosensitive drum 1 to charge.
0, a transfer area 14b is formed, the toner image of the back side image on the photosensitive drum 10 is transferred to the toner image receiver 14a, and the toner image of the front side image on the photosensitive drum 10 is Transfer to the surface.

Back transfer device 14g as second transfer means
Is a corotron charger to which a voltage having a polarity opposite to that of the toner (positive polarity in the present embodiment) is applied, and is opposed to a grounded roller member provided in contact with or close to the back surface of the toner image receiver 14a. Toned image receiver 1
4a is arranged on the front side of the recording paper P conveyed on the upper side. The back transfer unit 14g charges the surface of the recording paper P to the opposite polarity to the toner, and transfers the toner image of the back image on the toner image receiver 14a to the back of the recording paper P.

As will be described in detail later, the fixing device 30 as a fixing means is a heat roller type fixing device composed of two upper rollers 310 and lower rollers 320 each having a heater therein. Heat and pressure are applied to the recording paper P, which is separated from the body 14a and has toner images on both sides, by nipping and transporting between the upper roller 310 and the lower roller 320, thereby fixing the toner image on the recording paper P.

The photosensitive drum 10 is rotated clockwise as indicated by an arrow in FIG. 1 by the start of a photoconductor drive motor (not shown) by the start of copying, and the photosensitive drum 10 is rotated by the charging action of the yellow (Y) scorotron charger 11. Is started.

As a document image, an image read by an image pickup device of an image reading apparatus separate from the present apparatus or an image edited by a computer is converted into image data for each of Y, M, C and K colors. Once stored and stored in memory.

After the photosensitive drum 10 is applied with a potential,
The first color signal, that is, Y
Exposure scanning by an electric signal corresponding to the image data is started, and an electrostatic latent image corresponding to the Y image of the original image is formed on the surface of the photosensitive drum 10.

The Y electrostatic latent image formed on the photosensitive drum 10 is reversely developed in a non-contact state by the Y developing unit 13 to form a yellow (Y) toner image on the photosensitive drum 10. Is done.

Next, the photosensitive drum 10 is placed on the magenta (M) scorotron charger 11 from above the Y toner image.
A potential is given by the charging action of the M exposure unit 1
Exposure using an electrical signal corresponding to the second color signal, that is, the M image data, is performed by 2, and an M electrostatic latent image is formed. The M electrostatic latent image is reversely developed in a non-contact state by the M developing unit 13, and a magenta (M) toner image is formed on the yellow (Y) toner image.

By the same process, the scorotron charger 11 for cyan (C), the exposure unit 12 for C, and the
The cyan (C) toner image corresponding to the third color signal is further produced by the developing device 13 of the third color, and the scorotron charger 11 of black (K), the exposure unit 12 of the K and the developing device 13 of the fourth color produce the fourth color. A black (K) toner image corresponding to the signal is sequentially superimposed and formed, and a superposed color toner image of Y, M, C and K is formed on the peripheral surface within one rotation of the photosensitive drum 10. .

Since the exposure of the organic photosensitive layer of the photosensitive drum 10 by the Y, M, C, and K exposure units 12 is performed from the inside of the drum through the above-described transparent substrate, the second, third, and fourth exposure units are used. The exposure of the image corresponding to the color signal is also performed without being affected by the previously formed toner image at all, and an electrostatic latent image equivalent to the image corresponding to the first color signal can be formed. .

By the above-described image forming process, a superimposed color toner image serving as a back surface image is formed on the photosensitive drum 10 (image forming body), and in the transfer area 14b, a transfer unit 14c (first transfer means). Are collectively transferred onto the toner image receiver 14a (intermediate transfer member). At this time, for example, LE, which is provided inside the photosensitive drum 10 so as to
Uniform exposure may be performed by the transfer simultaneous exposure device 12d using D (light emitting diode).

The toner remaining on the peripheral surface of the photoconductive drum 10 after the transfer is subjected to static elimination by the photoconductive drum AC static eliminator 16 and then reaches a cleaning device 19 as a cleaning means for the photoconductive drum 10. The toner is cleaned by a cleaning blade 19a made of a rubber material in contact with the drum 10, and collected by a screw 19b in a not-shown waste toner container. Also, the photosensitive drum 10
Is removed by exposure by a uniform exposure device 12c before charging using, for example, an LED (light emitting diode),
The history of the photosensitive drum 10 in the previous image formation is eliminated.

After the superimposed color toner image serving as the back surface image is formed on the toner image receiving member 14a as described above, the superposed color toner image serving as the front surface image is continuously formed on the photosensitive drum 10. It is formed in the same manner as the above color image forming process. It is necessary to change the image data so that the front surface image formed on the photosensitive drum 10 is a mirror image of the back surface image formed earlier.

The recording paper P is sent out by a feed roller 15a from a paper feed cassette 15 serving as a transfer material storage means, fed by a feed roller 15b, and conveyed to a timing roller 15c serving as a transfer material supply means.

The recording paper P is driven by the timing roller 15c, as shown in FIG. 3, to form a color toner image of the front surface image carried on the photosensitive drum 10 and a back surface carried by the toner image receiver 14a. The image is fed to the transfer area 14b in synchronization with the color toner image. At this time, the recording paper P is charged to the same polarity as the toner by the paper charger 14f, and is transferred via the color toner image of the back surface image carried on the toner image receiver 14a.
a and is fed with the movement of the toner image receiver 14a.

In the transfer area 14b, the color toner image of the surface image on the photosensitive drum 10 is collectively transferred to the surface of the recording paper P by the transfer device 14c (first transfer means). At this time, the color toner image of the back side image carried on the toner image receiver 14a is not transferred to the recording paper P but exists on the toner image receiver 14a. At the time of transfer by the transfer unit 14c, uniform exposure may be performed by the simultaneous transfer exposure unit 12d so that good transfer is performed.

The recording paper P on which the color toner image has been transferred to the front side is then conveyed to a backside transfer unit 14g (second transfer means), where the toner image receiver 14a is transferred by the backside transfer unit 14g.
The color toner of the back side image carried on the recording paper P
Is collectively transferred to the back surface of.

Since the toner images of the respective colors overlap with each other, it is preferable that the toner in the upper layer and the toner in the lower layer of the toner layer are charged to the same polarity with the same charge amount in order to enable batch transfer. In a double-sided image formation in which the polarity of a color toner image formed on the toner image receiving member 14a is inverted by corona charging or the polarity of a color toner image formed on the photosensitive drum 10 is inverted by corona charging, the lower layer toner is It is not preferable because it is difficult to be sufficiently charged to the same polarity, resulting in poor transfer.

That is, reversal development is repeated on the photoreceptor drum 10, and the color toner images of the same polarity formed by superimposition are collectively transferred to the toner image receiving member 14a without changing the polarity, and then, without changing the polarity. Batch transfer to the recording paper P is preferable because it contributes to improvement of the transferability of the back side image formation.
Also for the surface image formation, reversal development is repeatedly performed on the photoreceptor drum 10, and the color toner images of the same polarity formed by superposition are collectively transferred to the recording paper P without changing the polarity. It is preferable because it contributes to the improvement of transferability.

As described above, in the color image formation, the first transfer means is operated to form a color toner image on the surface of the transfer material by using the above-described image forming method for the front surface and the back surface. A two-sided image forming method of forming a color toner image on the back surface of the transfer material by operating the second transfer means is preferably employed.

The recording paper P on which the color toner images have been transferred on both sides as described above is neutralized by a paper separation AC neutralizer 14h for separating the transfer material disposed opposite to the grounded drive roller 14d. After being separated from the toner image receiving member 14a and conveyed to a fixing device 30 as fixing means, the toner adhered to the front surface and the back surface is fixed by heat and pressure, and then discharged rollers 18a, 18b and 18c.
Is discharged to a tray outside the apparatus.

The toner remaining on the toner image receiving member 14a after transferring the toner image of the back side image onto the recording paper P is provided to face the driven roller 14e, and the toner image receiving member 14a
a having a blade member that can abut and abut on the a.
The toner image receiver 14a is cleaned by a toner image receiver cleaning device 14i as cleaning means. Further, the toner remaining on the photosensitive drum 10 after the toner image of the front surface image is transferred onto the recording paper P is subjected to static elimination by the photosensitive drum AC static eliminator 16 in the same manner as when the back surface image is formed. The residual toner is removed by the body drum cleaning device 19. The peripheral surface of the photoreceptor drum 10 from which the residual toner has been removed is a uniform exposure unit 12 before charging.
The charge is removed by the exposure in step (c), and the process proceeds to the next image forming cycle.

In the above-described image forming apparatus 100, since the color toner images are superimposed and transferred at once to form a color image on both sides of the recording paper P, the color image on the toner image receiver 14a is formed. It is possible to form a favorable double-sided color image with less color shift, toner scattering and rubbing, and less image deterioration.

In the image forming apparatus 100 described above, in addition to the double-sided image forming mode for forming an image on both sides of the recording paper P as described above, a surface image forming mode for forming an image only on the surface of the recording paper P Alternatively, it is possible to select a backside image forming mode in which an image is formed only on the backside of the recording paper P.

Next, a state of the toner image formed on the photosensitive drum 10 until it is transferred to both sides or one side of the recording paper P and enters the fixing device 30 will be described with reference to FIG.
This will be described with reference to FIG. Here, the photosensitive drum 10 serving as an image forming body is negatively charged, and the latent image on the photosensitive drum 10 is subjected to reversal development by a toner having a negative charge, so that a toner image having a negative charge is exposed. The case in which it is formed on the body drum 10 will be described.

FIG. 4 shows a state in which the toner image has adhered only to the surface of the recording paper P by the first image forming step of directly transferring the toner image of the surface image formed on the photosensitive drum 10 onto the recording paper P. FIG. 4 is an explanatory diagram when the vehicle enters the fixing device 30. The toner image of the negative polarity surface image formed on the photosensitive drum 10 is transferred to a transfer unit 14c as a first transfer unit via a recording sheet P and a toner image receiver 14a as an intermediate transfer member behind the recording sheet P. As a result, a voltage having a positive polarity is applied, and transfer is performed on the recording paper P (FIG. 4A).
The recording paper P holding the toner image of the negative polarity surface image on the upper surface is subjected to static elimination by the paper separation AC static eliminator 14h and separated from the toner image receiving body 14a.
And enters the fixing device 30 having the lower roller 320 (FIG. 4B).

FIG. 5 shows a first example in which the toner image of the surface image formed on the photosensitive drum 10 is directly transferred onto the recording paper P.
And a second image forming step in which the toner image of the back side image formed on the photosensitive drum 10 is transferred onto the recording paper P via the intermediate transfer member, toner images are formed on both sides of the recording paper P. FIG. 4 is an explanatory diagram when the toner enters the fixing device 30 in a state of being attached. The toner image of the negative polarity rear surface image formed on the photoconductor drum 10 is a toner image receiver 1
A positive voltage is applied by the transfer device 14c from behind the transfer member 4a, and the transfer is performed on the toner image receiver 14a (FIG. 5A). Subsequently, a toner image having a negative polarity surface image is formed on the photosensitive drum 10. The toner image of the front surface image is transferred to the upper surface of the recording paper P by applying a positive polarity voltage by the transfer device 14c via the recording paper P and the toner image receiving body 14a holding the toner image of the rear surface image behind the recording paper P. (FIG. 5B). Next, the toner image of the back surface image adhered on the toner image receiver 14a is
From the upper side of the recording paper P, a back transfer unit 1 as a second transfer unit
When a positive polarity voltage is applied by 4 g, transfer is performed on the lower surface of the recording paper P. At this time, the recording paper P
The potential of the toner image of the front surface image on the upper surface is changed from a negative polarity to a positive polarity by applying a voltage of a positive polarity from directly above by the rear transfer unit 14g (FIG. 5).
(C)). The recording paper P is separated from the recording paper P in a state where the toner image of the positive polarity front surface image is held on the upper surface of the recording paper P and the toner image of the negative polarity rear surface image is held on the lower surface.
At 4h, the charge is removed, separated from the toner image receiver 14a, and enters the fixing device 30 (FIG. 5D).

Next, a fixing device as a fixing means of the present invention will be described. FIG. 6 is a configuration diagram showing an embodiment of the fixing device 30 of the present invention. The fixing device 30 includes an upper roller 310 that is located on the upper side and is in charge of fixing the surface toner image on the recording paper P.
And a lower roller 320, which is located below and is in charge of fixing the back surface toner image of the recording sheet P, faces and presses and heats to fix the toner image held by the recording sheet P passing therethrough.

The upper roller 310 has a first heater H1 (300W halogen lamp in this embodiment) as an internal heating means composed of a halogen lamp or the like at an inner core portion. The upper roller 310 has a thickness of 0.1 mm on the outer periphery of a cored bar 310a formed of a thin metal pipe having the first heater H1 built therein.
An elastic layer 310b made of an elastic material such as silicon rubber having a rubber hardness Asker C of about 55 degrees and a rubber hardness of 8 to 5 mm is provided.
FA (perfluoroalkyl vinyl ether) and PTF
It is a soft roller coated with a heat-resistant, high-release layer such as a fluororesin such as E (polytetrafluoroethylene) or a silicone resin. It is preferable to attach or mix a substance that enhances thermal conductivity to the elastic body of the elastic layer 310b to increase the thermal conductivity.

The upper roller 310 has a halogen lamp (5 in this embodiment) as an external heater H3 on the inner core.
An external heating roller 331, which is an external heating means having a built-in 00W halogen lamp), is pressed against the upper roller 310 and is driven to rotate. In addition, as the external heating means, a heating means composed of a rod-shaped halogen lamp and a gutter-shaped reflecting mirror can be used.

The lower roller 320 has a second heater H2 (500W halogen lamp in this embodiment) as a heating means composed of a halogen lamp or the like at the inner core. The lower roller 320 extends around the outer periphery of a metal core 320a made of a thin metal pipe having the second heater H2 built therein, for example, 30 to
This is a hard roller having a surface layer 320c coated with a heat-resistant, high-release layer such as a 200-μm fluororesin such as PFA or PTFE, or a silicone resin.

On the peripheral surfaces of the upper and lower rollers 310 and 320,
Cleaning rollers 312 and 322 for adsorbing and removing toner, paper powder and the like adhering to the peripheral surface, and oil applying rollers 313 and 323 for applying silicone oil as a release agent for preventing offset are provided. I have.
For the surfaces of the cleaning rollers 312 and 322, a material that does not absorb oil, such as foamed silicone rubber, is used. If an absorbent material is used, the collected absorbed oil will be discharged, resulting in uneven oil. Therefore, a non-absorbable material is used for the cleaning rollers 312 and 322, and the variation in the amount of applied oil is suppressed. In order to improve the performance of removing foreign matter such as toner, a material having a low releasability and a material having a small heat capacity so as not to absorb heat from the upper and lower rollers 310 and 320 are selectively used.
The cleaning rollers 312 and 322 and the oil application rollers 313 and 323 are upper and lower rollers 310 and 3 respectively.
20 and is driven to rotate.

Further, a first temperature sensor S1 and a second temperature sensor S2, which are first and second temperature detecting means including a thermistor or the like, are in contact with or in close proximity to each of the upper roller 310 and the lower roller 320. The surface temperature of the roller is detected, and based on this detection signal, energization control of the first heater H1, the second heater H2, and the external heater H3 is performed as described later, and the temperature is maintained within a predetermined temperature range. Is done.

Between the upper roller 310 and the lower roller 320, a linear pressure of 0.8 to 1..
The nip portion is pressed at 8 kg / cm. At this time, the length of the nip portion varies depending on the linear pressure and the hardness of the roller, but is approximately 2 to 7 mm. The upper roller 310 and the lower roller 320 are driven by the same drive source so as not to cause a slip at the nip, rotate at the same linear speed (160 mm / sec in this embodiment), and perform double-sided fixing at the nip.

In the image forming apparatus 100, the fixing device 30 as a fixing means forms an electric field between each of the upper and lower rollers 310 and 320 and the recording paper P sandwiched and conveyed therebetween. The roller surface is charged by providing a corona charger or a roller charger capable of conducting sponge-like driven rotation, or the upper and lower rollers 31
The surface layers 310c and 320c and the elastic layer 310b of the first and second layers 0 and 320 are made of a conductive material in which carbon black or titanium oxide is included to impart conductivity. 320a is connected to each of the upper rollers 310 via a safety resistor, and a bias voltage (for example, + 200V or -200V to the upper roller 310, and -200V to the lower roller 320) is applied. By charging the same polarity as the toner charge of the upper toner image and charging the lower roller 320 with the same polarity as the toner charge of the lower toner image to increase the electric repulsion between the toner and the roller, The electric electrostatic offset when the double-sided toner images are fixed collectively is completely eliminated.

The image forming apparatus 100 forms a toner image on both surfaces of a transfer material and performs fixing at once, and a two-sided image forming mode (double-sided copy mode), and forms a toner image only on the surface of the transfer material. A function of switching an image forming process between a front side image forming mode for fixing (front side copy mode) and a back side image forming mode for forming and fixing a toner image only on the back side of the transfer material (back side copy mode); When a mode is set by an image forming mode setting button (copy mode setting button), excellent control of image formation and fixing according to the mode is performed, and good image recording is performed. Hereinafter, this control method will be described.

FIG. 7 is a block diagram showing an example of the control system according to the first embodiment. In the figure, reference numeral 80 denotes a control unit; 81, a copy mode setting button on an operation panel of the image forming apparatus main body;
Reference numeral 30 denotes a D / A converter, and reference numerals 831, 832, and 833 denote heater power supplies for the first heater H1, the second heater H2, and the external heater H3. The control signal sent from the control unit 80 in accordance with the copy mode set by the copy mode setting button 81 is D / A converted through the D / A converter 830, and then input to the heater power supplies 831, 833, 833. ON / OFF of the first heater H1, the second heater H2, and the external heater H3 according to the copy mode selected by the user.
While performing the control, the control unit 80 performs the process control corresponding to the mode of the image forming apparatus 100. in this case,
The surface temperature of the upper roller 310 is detected by the first temperature sensor S1, and the surface temperature of the lower roller 320 is detected by the second temperature sensor S2. The control is performed to control the temperature.

In addition to the above, an automatic copy mode for automatic selection is provided, and when an image of a document is read by a document reading device (not shown), the presence or absence of an image on the back of the document is detected. Image formation and fixing are also performed by automatically selecting the copy mode, the duplex copy mode when the front and back images are detected, and the back copy mode when only the back image is detected.

When the front side copy mode is selected, the control unit 80 calls up the image information on the front side from a memory or from a document reading device (not shown) to form a toner image of the front side image on the photosensitive drum 10, and The toner image of the surface image on the body drum 10 is transferred onto the upper surface (front surface) of the recording paper P fed synchronously by the transfer device 14c (first image forming step). The fixing device 30 uses the electric field forming means (not shown) so as to form a negative electric field with respect to the upper roller 310 for the recording paper P which holds the toner image on the upper surface and is conveyed to the fixing device 30. Activate. On the other hand, lower roller 3
For 20, the electric field forming means (not shown) is not operated.

In the present embodiment in which the upper roller 310 is a soft roller, when the double-sided or front-side copy mode is selected, the heat of the upper roller 310 is taken off by the recording paper P, and the surface temperature becomes the offset limit temperature Tc ( 150 ° C.) or less, and the heat recovery is slow because of a soft roller using an elastic member having low thermal conductivity. Therefore, when the first heater H1 inside the upper roller 310 is a high output heater, the temperature inside the roller rises abnormally due to the heat inside the roller, and the material of the elastic member is deteriorated. For this reason, in the present embodiment, the upper roller 310 is provided with an external heating roller 331 as an external heating means, and is operated in the two-sided copy mode and the front side copy mode. Since the lower roller 320 is a hard roller, the heat conductivity at the roller portion is good, and it is possible to return the surface temperature of the roller lowered by conveyance and fixing to the original temperature within a short copy interval. . With such a configuration and control, the surface image on the recording paper P conveyed at a short copy interval is fixed, and the recording paper P after the fixing is sent by the paper discharge rollers 18a, 18b, 18c, and is fixed to the apparatus. It is discharged to an external tray.

When the double-sided copy mode is selected, the control unit 80 first retrieves the image information of the back side from the memory or from the document reading device, forms a toner image of the back side image on the photosensitive drum 10, and then transfers the image. The backside toner image is transferred onto the toner image receiving body 14a as an intermediate transfer body by the device 14c. Next, a toner image of a surface image is formed on the photosensitive drum 10 after the transfer and cleaning. Then, a toner image of a front surface image is transferred to the upper surface (front surface) of the recording paper P fed in synchronization by the transfer device 14c (first image forming step), and the back surface is transferred to the lower surface (back surface) of the recording paper P. The toner image of the back side image on the toner image receiver 14a is transferred by the container 14g (second image forming step). The toner image is held on both sides, and the recording paper P conveyed to the fixing device 30 is
In the fixing device 30, the electric field forming means (not shown) is operated so as to form a positive electric field with respect to the upper roller 310 which has operated the external heater H <b> 3 of the external heating roller 331. The electric field forming means (not shown) is operated so as to form a negative electric field.

In the image forming apparatus 100 of the present invention, when the two-sided copy mode is set, the upper roller 310
The first heater H1 and the external heater H3 of the external heating roller 331 are in an operating state, and ON / OFF control is performed by comparing the detected temperature detected by the first temperature sensor S1 with a preset fixing temperature Ts. You. Here, the set fixing temperature Ts is a temperature exceeding the offset limit temperature Tc,
In the present embodiment, the temperature is set at 180 ° C to 200 ° C. Further, the second heater H2 of the lower roller 320 is also in the operating state, and ON / OFF control is performed by comparing the detected temperature detected by the second temperature sensor S2 with the set fixing temperature Ts. FIG. 8 shows the roller surface temperature in the duplex copying mode.
As shown in FIG.

When the front copy mode is set,
The first heater H1 of the upper roller 310 and the external heating roller 33
The first external heater H3 is in the operating state, and the first external heater H3
1 and the external heater H3 are turned on / off by comparing the detected temperature detected by the first temperature sensor S1 with the set fixing temperature Ts.
OFF control is performed. The second heater H2 of the lower roller 320 is also in the operating state and ON / OFF control is performed. However, since it is not necessary to fix the toner image by the lower roller 320 in the front copy mode, the second heater H2 of the lower roller 320 is not required. As for H2, ON / OFF control is performed by comparing the detected temperature detected by the second temperature sensor S2 with a set warming temperature Th lower than the set fixing temperature Ts. Here, the set warming temperature Th is a temperature for preheating so as to reach the set fixing temperature Ts in a short time when switching to perform fixing by the lower roller 320, and is a temperature lower than the offset limit temperature Tc. Th is set to 110 ° C. FIG. 8B shows the roller surface temperature in the front copy mode.
It becomes as shown in.

When the back side copy mode is set,
Since the toner image formed on the photosensitive drum 10 is once transferred onto the toner image receiving member 14a and then transferred onto the recording paper P, the same image forming process as in the duplex copy mode is employed. In the backside copy mode, since it is not necessary to fix the toner image with the upper roller 310, the external heater H3 of the external heating roller 331 is disabled and the first heater H3 is not operated.
Only the heater H1 is in the operating state, and the first heater H1 is ON / OFF controlled by comparing the detected temperature detected by the first temperature sensor S1 with the set warming temperature Th. The second heater H2 of the lower roller 320 for fixing the toner image is in the operating state, and the detected temperature detected by the second temperature sensor S2 is compared with the set fixing temperature Ts to determine the temperature.
N / OFF control is performed. The roller surface temperature in the back side copy mode is as shown in FIG.

Table 1 shows the control state of the heater in each copy mode described above.

[0073]

[Table 1]

By such control, the upper roller 310
Of course, also for the lower roller 320, the surface temperature of the roller for fixing the toner image is the offset limit temperature T during copying.
c or more, and good fixing is performed while increasing the processing speed.

Next, warming up in the present embodiment will be described.

FIG. 9A shows the upper roller 310 during warm-up prior to image formation in the duplex copy mode.
And the surface temperature of the lower roller 320. In the two-sided copy mode, the roller heating during the warm-up following the main power ON is performed by the operation of the three halogen lamps of the first heater H1, the second heater H2, and the external heater H3. When each of the temperature sensors S2 detects that the temperature has reached the set fixing temperature Ts, the warm-up is completed, and the display of "copy operation is possible" is displayed on the operation panel. Note that FIG.
The graph shown by the dotted line in (a) is the surface temperature of the upper roller 310 when the external heater H3 is not operated, and the time required for warming up is determined by the external heater H3.
It can be seen that the operation can be greatly reduced by providing and operating.

FIG. 9B shows the roller surface temperature during warm-up in the front copy mode. Also in the front side copy mode, the roller heating during the warm-up following the main power ON is performed by the first heater H1 and the second heater H2.
This is performed by operating three halogen lamps of the heater H2 and the external heater H3. However, the second heater H2 is set at the set warming temperature Th based on the temperature detected by the second temperature sensor S2.
Is controlled. The upper roller 3 is controlled by the first temperature sensor S1.
When it is detected that the surface temperature of No. 10 has reached the set fixing temperature Ts, the warm-up is finished, and the display of "copying is possible" is displayed on the operation panel. As in the double-sided copy mode, the time required for the warm-up in the front-side copy mode can be greatly reduced by providing the external heater H3 for operation.

FIG. 9C shows the roller surface temperature during warm-up in the back side copy mode. In the backside copy mode, the roller heating during the warm-up following the main power ON is performed by the first heater H1 and the second heater.
This is performed by operating two halogen lamps of the heater H2. However, the first heater H1 is controlled to the set warming temperature Th based on the temperature detected by the first temperature sensor S1. When it is detected by the second temperature sensor S2 that the surface temperature of the lower roller 320 has reached the set fixing temperature Ts, the warm-up is completed, and the display of "copy operation enabled" is displayed on the operation panel. In the present embodiment, since the lower roller 320 is a hard roller, the time required for warm-up in the backside copy mode is shorter than in the duplex copy mode or the frontside copy mode.

Next, switching of the copy mode will be described.

In the case of switching from the double-sided copy mode to the front-side copy mode or the back-side copy mode, the surface temperatures of the upper and lower rollers 310 and 320 have reached the set fixing temperature Ts in the both-sided copy mode. It is possible to shift to the next front side or back side copy mode without time.

On the other hand, in the case of switching from the front side copy mode to the double side or back side copy mode, or the mode of switching from the back side copy mode to the both side or front side copy mode, the surface temperature of the lower roller 320 or the upper roller 310 is set to the set fixing temperature. Since Ts has not been reached, a waiting time when switching the copy mode is required.

FIG. 10A shows the upper and lower rollers 310, 3 when switching from the front copy mode to the duplex copy mode.
20 is a graph showing the surface temperature of No. 20. When the mode is switched from the front copy mode to the duplex copy mode, the control unit 80 immediately switches the control temperature of the second heater H2 from the set warming temperature Th to the set fixing temperature Ts, and the surface temperature of the upper and lower rollers 310 and 320. Is controlled to reach the set fixing temperature Ts. Until the surface temperature of the lower roller 320 reaches the set fixing temperature Ts from the set keeping temperature Th, double-sided copying is prohibited, and this is a waiting time. Lower roller 32
When the surface temperature of 0 reaches the set fixing temperature Ts, double-sided copying is enabled. In the present embodiment, since the lower roller 320 is a hard roller, the waiting time when switching from the front side copy mode to the double side copy mode can be shortened.

FIG. 10B shows the upper and lower rollers 310, 3 when switching from the front side copy mode to the back side copy mode.
20 is a graph showing the surface temperature of No. 20. When the mode is switched from the front copy mode to the back copy mode, the control unit 80 immediately switches the control temperature of the second heater H2 from the set warming temperature Th to the set fixing temperature Ts, turns off the external heater H3, The control temperature of the heater H1 is switched from the set fixing temperature Ts to the set warming temperature Th,
The surface temperature of the lower roller 320 is controlled to the set fixing temperature Ts, and the surface temperature of the upper roller 310 is controlled to the set heat retaining temperature Th. Until the surface temperature of the lower roller 320 reaches the set fixing temperature Ts, the back side copying is prohibited,
This is the waiting time. When the surface temperature of the lower roller 320 reaches the set fixing temperature Ts, the back side copy can be performed. In the present embodiment, since the lower roller 320 is a hard roller, the waiting time when switching from the front side copy mode to the back side copy mode can be shortened.

FIG. 11A shows the upper and lower rollers 310, 3 when switching from the back side copy mode to the both side copy mode.
20 is a graph showing the surface temperature of No. 20. When the mode is switched from the back side copy mode to the both side copy mode, the control unit 80 immediately turns on the external heater H3,
The control temperatures of the first heater H1 and the external heater H3 are switched to the set fixing temperature Ts, and the upper and lower rollers 310 and 320 are switched.
Is controlled so as to reach the set fixing temperature Ts.
Until the surface temperature of the upper roller 310 reaches the set fixing temperature Ts from the set warming temperature Th, double-sided copying is prohibited, and this is a waiting time. When the surface temperature of the upper roller 310 reaches the set fixing temperature Ts, double-sided copying becomes possible. In the present embodiment, the upper roller 310 is a soft roller. However, since the upper roller 310 has an external heater H3 in addition to the first heater H1, the waiting time when switching from the backside copy mode to the duplex copy mode can be reduced. Can be.

FIG. 11B shows the upper and lower rollers 310, 3 when switching from the back side copy mode to the front side copy mode.
20 is a graph showing the surface temperature of No. 20. When the mode is switched from the backside copy mode to the frontside copy mode, the control unit 80 immediately turns on the external heater H3 and sets the control temperatures of the first heater H1 and the external heater H3 to the set fixing temperature T.
s, the control temperature of the second heater H2 is switched from the set fixing temperature Ts to the set warming temperature Th, the surface temperature of the upper roller 310 is set to the set fixing temperature Ts, and the lower roller 32
The surface temperature of 0 is controlled so as to become the set heat retaining temperature Th. The surface temperature of the upper roller 310 is equal to the set insulation temperature T
From the time h until the set fixing temperature Ts is reached, front side copying is prohibited, and this time is a waiting time. Upper roller 310
When the surface temperature reaches the set fixing temperature Ts, the surface copy can be performed. In this embodiment, the upper roller 310 is a soft roller. However, since the upper roller 310 has the external heater H3 in addition to the first heater H1, the waiting time when switching from the back side copy mode to the front side copy mode can be reduced. Can be.

(Embodiment 2) FIG. 12 is a block diagram showing a control system according to Embodiment 2. The image forming apparatus main body is the same as the above-described image forming apparatus 100 shown in FIG. 1, and the control system has the same parts as those in FIG. In the figure, reference numeral 82 denotes a designation button for designating the recording paper size and thickness.

The control of the heating means in the double-sided copy mode in the image forming apparatus 100 of this embodiment is exactly the same as that of the first embodiment, and the first heater H of the upper roller 310 is controlled.
1, the external heater H3 of the external heating roller 331, and the second heater H2 of the lower roller 320 are in an operating state, and the first heater H1 and the external heater H3 set the detection temperature detected by the first temperature sensor S1 to the fixing temperature. ON / OFF control is performed by comparing with Ts, and ON / OFF control is performed on the second heater H2 by comparing the detected temperature detected by the second temperature sensor S2 with the set fixing temperature Ts.

When an image is formed on both sides of the recording paper P by the image forming apparatus 100 of the present embodiment, first, the first rotation of the toner image receiver 14a exposes a back toner image corresponding to one page of the recording paper P to light. Formed on the body drum 10 and transferred onto the toner image receiver 14a, and then transferred to the toner image receiver 14a.
In the second rotation of a, a front surface toner image corresponding to one page of the recording paper P is formed on the photosensitive drum 10 and transferred to the front surface of the recording paper P, and the back toner image on the toner image receiver 14a is formed. After the image is transferred to the back side of the recording sheet P, the toner images transferred to the front side and the back side of one sheet of the recording sheet P are fixed on both sides simultaneously (hereinafter, referred to as one-page copy mode). Further, the image forming apparatus 100 of the present embodiment has a two-sheet copy mode in order to increase the copy processing speed. For example, when forming an image on both sides of a small-sized recording paper P of A4 or B5 size, 1 of the toner image receiver 14a
During the rotation, two pages of the back surface toner images to be transferred onto the two recording papers P are formed and transferred onto the toner image receiver 14a, and the two recordings are performed during the second rotation of the toner image receiver 14a. While continuously supplying the paper P, the toner images are transferred to the front and back surfaces of the recording paper P, respectively.
By rotation, a two-sided image is formed on the two recording sheets P. FIG.
3 is an explanatory diagram showing this situation, and shows a state in which two pages of the back surface toner image of the small-sized recording paper P are formed on the toner image receiver 14a.

The image forming apparatus 100 of this embodiment can record an image up to A3 size recording paper, and the toner image receiving body 14a corresponds to the length of A3 size and the copy interval required for continuous copying. It has a circumference that is the sum of the length α and the circumference. FIG. 14 is a view for explaining the formation of toner images on the toner image receiver 14a when images are formed on both sides of the recording paper P of each size. FIG.
FIG. 14B is a cross-sectional view illustrating a state in which a toner image is formed on the toner image receiver 14a in the sheet copy mode.
FIG. 7G is a developed view showing a toner image forming state on the toner image receiving body 14a in the copy mode for one sheet of the recording paper P of each size and the copy mode for two sheets. In the figure, T is a toner image, α is a length corresponding to a copy interval required for continuous copying using A3 size recording paper P, that is, a toner image interval in the case of one A3 size sheet, and β is This figure shows the toner image interval in the case of one sheet of B4 size. The roller surface temperature of the fixing device 30 is set within a predetermined time.
, The larger the amount of recording paper P passing through, that is, the larger the size of the toner image T formed on the toner image receiver 14a and the smaller the interval between the formation of the toner images T, the lower the tendency. Therefore, the order in which the temperature of the roller tends to decrease is as shown in FIG. 14 (c), (b), (e), (d),
(F) and (g).

In the present embodiment, in order to prevent the lowered roller surface temperature from falling below the offset limit temperature Tc, the fixing is set according to the size of the recording paper P and the copy mode for one sheet or the two sheets copy mode. The temperature Ts is changed, and as shown in Table 2, the respective copy conditions shown in FIG. 14 are classified into three types, and a plurality of set fixing temperatures Ts are set in accordance with the order in which the temperature of the roller tends to decrease. I have.
That is, in the case of (c) and (b) of FIG. 14, Ts ₁ = 20
0 ° C., Ts ₂ = 190 ° C. for (e) and (d),
In the cases of (f) and (g), Ts ₃ is set to 180 ° C., and the fixing temperature Ts is set higher for the copy condition in which the temperature drop is larger, so that the roller surface temperature becomes the offset limit temperature Tc in any copy condition. It is prevented from dropping to.

[0091]

[Table 2]

In this embodiment, in addition to changing the set fixing temperature Ts according to the size of the recording paper P and the copy mode for one sheet or the two-sheet copy mode, the set fixing temperature Ts is also changed according to the thickness of the recording paper P. When the thickness of the recording paper P is large, control is performed to increase the set fixing temperature Ts by one rank by inputting the thick paper information with the designation button 82.

The inventions described in the above two embodiments are also applied to an image forming apparatus having a belt-type fixing unit using a ceramic heater instead of the lower roller 320, and the same effects are obtained. be able to.

[0094]

According to the first aspect of the present invention, when the upper roller as the fixing means for the surface toner image is a soft roller, the upper roller is provided with external heating means together with internal heating means. The warm-up time during double-sided image formation and surface image formation and the waiting time during copy mode switching are significantly reduced, the deterioration of the elastic layer of the soft roller is prevented, and the power of the heating means inside the soft roller is reduced. As compared with the case where the surface temperature is prevented from lowering by increasing the power consumption, an image forming apparatus is provided in which power consumption is greatly reduced and favorable fixing is performed.

According to the present invention, when the upper roller, which is a fixing means for the surface toner image, is a soft roller, the upper roller is provided with an external heating means together with an internal heating means, so that a double-sided image formation can be achieved. During copying, it is possible to reliably prevent the surface temperature of the upper roller from falling below the offset limit temperature Tc.
An image forming apparatus is provided in which the power consumption of the heating means inside the soft roller is increased and power consumption is remarkably reduced as compared with the case where the surface temperature is prevented from lowering, and good fixing is stably performed. .

Further, according to the third aspect, even when a large-size transfer material or a thick-size transfer material is used as the transfer material, the surface temperature of the upper roller falls below the offset limit temperature Tc during copying. An image forming apparatus has been provided which can surely prevent the occurrence.

[Brief description of the drawings]

FIG. 1 is a cross-sectional configuration diagram illustrating an example of an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a sectional view of the image forming body of FIG. 1;

FIG. 3 is an explanatory diagram illustrating a state where toner images are formed on both sides.

FIG. 4 is an explanatory diagram illustrating a state of a toner image when an image is formed on the surface of a transfer material.

FIG. 5 is an explanatory diagram illustrating a state of a toner image when images are formed on both surfaces of a transfer material.

FIG. 6 is a cross-sectional view illustrating a configuration of an embodiment of a fixing device.

FIG. 7 is a block diagram illustrating a control system according to the first embodiment.

FIG. 8 is a graph showing surface temperatures of upper and lower rollers in a copy mode of both sides, front side, and back side in the present invention.

FIG. 9 is a graph showing surface temperatures of upper and lower rollers during warm-up prior to image formation in each copy mode in the present invention.

FIG. 10 is a graph showing surface temperatures of upper and lower rollers when switching from a front side copy mode to a double side or back side copy mode.

FIG. 11 is a graph showing surface temperatures of upper and lower rollers when switching from a back side copy mode to a both side or front side copy mode.

FIG. 12 is a block diagram illustrating a control system according to a second embodiment.

FIG. 13 is a cross-sectional view illustrating a case where two images are formed on a toner image receiver.

FIG. 14 is a diagram illustrating a state of forming a toner image on a toner image receiver when images are formed on both sides of recording paper of each size.

[Explanation of symbols]

 REFERENCE SIGNS LIST 10 photoconductor drum (image forming body) 11 scorotron charger 12 exposure unit 13 developing unit 14a toner image receiving body (intermediate transfer body) 14c transfer unit (first transfer unit) 14g back surface transfer unit (second transfer unit) 14h Paper separation AC neutralizer 30 Fixing device (fixing device) 100 Image forming device 310 Upper roller 320 Lower roller 331 External heating roller (External heating device) 310a, 320a Core 320b Elastic layer 310c, 320c Surface layer H1 First heater H2 Second heater H3 External heater P Recording paper (transfer material) S1 First temperature sensor (first temperature detecting means) S2 Second temperature sensor (second temperature detecting means)

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kuki Nagase 2970 Ishikawacho, Hachioji-shi, Tokyo Konica Corporation

Claims (4)

[Claims] 1. An image forming apparatus for fixing a toner image after transferring a toner image to one or both sides of a transfer material, comprising: an external heating means together with an internal heating means for a fixing means facing the surface of the transfer material; An image forming apparatus wherein the external heating unit heats the image forming apparatus during warm-up before forming a double-sided image or a front-side image, or when changing from a back-side image forming mode to a double-sided or a front-side image forming mode. 2. An image forming apparatus for transferring a toner image onto one or both sides of a transfer material and then fixing the toner image, comprising: an external heating means together with an internal heating means for a fixing means facing the surface of the transfer material; An image forming apparatus comprising: heating at the time of double-sided image formation by the external heating means. 3. The image forming apparatus according to claim 2, wherein the external heating unit is controlled according to the type (thickness, size) of the transfer material. 4. The fixing device according to claim 1, wherein the fixing unit facing the front surface of the transfer material is a soft roller, and the fixing unit facing the back surface of the transfer material is a hard roller. The image forming apparatus as described in the above.