JP2002049264A - Image-forming device with fixing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image-forming device provided with a fixing device which prevents a fixing irregularity on a recording medium, without causing displacing of images, and therefore, obtains a satisfactory image. SOLUTION: The fixing device 45, which applies heat and pressure to a recording medium P, in contact therewith so that a toner image is fixed to the recording medium P, is provided with a heat roller 45a whose peripheral face contacts the recording medium in the direction of carrying the recording medium. The circumference of the heat roller is set greater than or equal to short sides of a normal size (A-4 size) for the recording medium. When an overhead projector sheet is used as a recording medium and a sheet size sensor detects an overhead projector sheet, whose length in the direction of carrying on a manual insertion tray is greater than the short sides of the normal size of recording medium, image formation on the overhead projector is inhibited.

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 provided with a fixing device for fixing a toner image by applying heat to an object to be fixed by contact with a heat fixing roller or the like, and more particularly, to an image forming apparatus generally used most commonly. The present invention relates to a measure for obtaining a good image by eliminating unevenness in fixing to a frequently-fixed standard-size object.

[0002]

2. Description of the Related Art Generally, in an image forming apparatus of an electrophotographic type, a heat roller type fixing device in which the surface is controlled to a predetermined temperature by a heater provided inside in order to fix a formed image on a recording medium. An apparatus is used, and the fixing object is conveyed to the heat roller, and heat and pressure are applied to the fixing object to fix the fixing object. In such a fixing device, the surface of the heat heat roller is deprived of heat by contact with toner or a recording medium as a fixing object, and the surface temperature of the heat heat roller decreases as the fixing object passes. Then, the surface temperature of the heat heat roller is detected, and electric power is supplied to the heater inside the heat heat roller, so that the temperature of the heat heat roller is controlled to be constant.

[0003]

However, in the above-described fixing apparatus of the heat heat roller type, good fixability can be obtained in the first rotation of a fixing member such as a heat heat roller, but in the second rotation, the first rotation of the fixing member is not sufficient. Due to the large amount of heat taken away by the fixing object in the circumference, the temperature has not completely recovered to the temperature before fixing (the temperature in the first cycle), and the fixing property (glossiness, transparency) of the fixing member in the second cycle ) Will be reduced. For this reason, when the fixing member is fixed to one recording medium over the second rotation, fixing unevenness occurs after the second rotation of the fixing member in one recording medium, and a good image is obtained. Can not get.

Therefore, as disclosed in Japanese Utility Model Publication No. Hei 4-1564 and Japanese Patent Laid-Open Publication No. Hei 8-95422,
The fixing speed in the first rotation of the fixing member is reduced so that the amount of heat taken by the object to be fixed in the second rotation is recovered by the second rotation, and the fixing is performed in the second and subsequent rotations of the fixing member in one recording medium. There has been proposed one that prevents the occurrence of unevenness.

However, also in the above-mentioned proposal, since the fixing speed of the fixing member, that is, the rotation speed, is changed during the fixing, the image may be misaligned when the conveying speed of the recording medium by the fixing member changes. As a result, a good image cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing, and an object of the present invention is to prevent the occurrence of fixing unevenness on one recording medium without causing image displacement, and to provide a good An object of the present invention is to provide an image forming apparatus including a fixing device capable of obtaining a stable image.

[0007]

In order to achieve the above-mentioned object, the present invention is based on an image forming apparatus provided with a fixing device for hot-pressing an object to be fixed so as to fix a toner image on the object. The fixing device is provided with a fixing member whose peripheral surface is in contact with the conveyance direction of the object to be fixed. The peripheral length of the fixing member is set to be equal to or longer than the length of the standard size recording medium most frequently used for the image forming apparatus in the transport direction.

According to this specific matter, the fixing is performed before the fixing member makes one rotation on the standard size recording medium most frequently used for the image forming apparatus. The fixing member is not fixed over the second rotation in the sheet, and the fixing unevenness that occurs after the second rotation of the fixing member in one recording medium is reliably prevented. Further, unlike the case where the fixing member is fixed on one recording medium over the second rotation, there is no need to reduce the fixing speed (circular speed) of the first rotation of the fixing member, and the recording by the fixing member is performed. Image displacement occurring when the medium transport speed changes can be reliably prevented. Thereby, a good image can be obtained.

In particular, the following configuration is proposed to prevent an increase in the size of the fixing device.

That is, the peripheral length of the fixing member is set to be equal to or larger than the width of the short side of the standard size recording medium.

According to this specific matter, the size of the fixing member is minimized, the size (perimeter) of the fixing member is reduced, and the size of the fixing device can be increased by enlarging the circumference of the fixing member. And the size of the image forming apparatus can be prevented from increasing.

[0012] In particular, the following arrangements are provided to prevent poor image formation on a special fixing object.

That is, there is provided paper size detecting means for detecting that the length of the recording medium in the transport direction is longer than the short size of the standard size recording medium, and the overhead projector paper used in the overhead projector is fixed. In the case of application as an object, receiving an output from the paper size detecting means, it is prohibited to form an image on an overhead projector paper whose length in the transport direction is longer than the short size of the standard size recording paper.

According to this specific matter, the standard size of the overhead projector paper is basically the mainstream. Therefore, the overhead projector paper whose length in the transport direction is longer than the short size of the standard size recording medium is specified. The conveyance is determined to be from a direction other than the conveyance direction (longitudinal direction), and image formation is prohibited. For this reason, fixing over the second round of the fixing member to the overhead projector paper from an erroneous transport direction (longitudinal direction) is prohibited, so that a poor image with uneven fixing is formed on the overhead projector paper. This can be prevented beforehand.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

In this embodiment, an electrophotographic digital color copying machine equipped with a fixing device according to the present invention will be described.

FIG. 1 is a longitudinal sectional front view schematically showing the internal structure of a digital color copying machine 1 according to the present embodiment from the front. As shown in FIG. 1, the digital color copying machine 1 includes a double-sided automatic document feeder 2, an image reading unit 3 for reading an image, and an image forming unit 4 for forming an image on a recording medium P. . Hereinafter, each unit will be described.

<Description of Double-sided Automatic Document Feeding Section 2> The double-sided automatic document feeding section 2 is supported on a document table 21 made of transparent glass or the like so as to be openable and closable with respect to the document table 21.
1 are mounted with a predetermined positional relationship. The two-sided automatic document feeder 2 transfers the document to the image reading unit 3 on one side (for example, the front surface) of the document table 21 at a predetermined position.
After the reading of the image on one side is completed, the document is temporarily retracted and turned over, and the document is placed on the other side (for example, the back side) of the document reading unit 21 at a predetermined position on the document reading unit 21. 3 and is conveyed toward the document table 21. Then, when the reading of the two-sided image for one document is completed, the two-sided automatic document feeder 2 discharges this document and then executes the two-sided conveyance operation for the next document. The operation of transporting and reversing the document described above is controlled by the control unit 7 (see FIG. 2) in relation to the operation of the entire digital color copying machine 1.

<Description of Image Reading Unit 3> The image reading unit 3
A portion for reading an image of a document placed on a document table 21 or a document fed one by one by a two-sided automatic document feeder 2 to create image data; That is, it is provided below the document table 21. The image reading unit 3 includes an exposure light source 31, first to third
The optical system includes reflecting mirrors 32, 33, and 34, an imaging lens 35, and a photoelectric conversion element 36. In this case, the exposure light source 31 and the first
The first scanning unit 37 is constituted by the reflecting mirror 32, while the second scanning unit 38 is constituted by the second and third reflecting mirrors 33 and 34.

The above-mentioned exposure light source 31 irradiates light to the image surface of a document placed on the document table 21 of the double-sided automatic document feeder 2 or a document conveyed through the double-sided automatic document feeder 2. is there. Each of the reflecting mirrors 32, 33, and 34 deflects (reflects) the reflected light image from the document once in a predetermined direction (left direction in FIG. 1) and then deflects the reflected light image downward, as indicated by the broken line in FIG. After that, the light is deflected rightward in the figure so as to be directed to the imaging lens 35.

The first scanning unit 37 (the exposure light source 31 and the first reflecting mirror 32) reciprocates in parallel to the lower surface of the document table 21 at a predetermined scanning speed while maintaining a constant speed (FIG. 1).
Then, it moves left and right). Second scanning unit 38
The (second and third reflecting mirrors 33, 34) reciprocate (left and right in FIG. 1) in parallel to the lower surface of the document table 21 while maintaining a constant speed relationship with the first scanning unit 37. .

The imaging lens 35 includes the second scanning unit 3
The reflected light image from the document deflected by the third reflecting mirror 34 is reduced, and the reduced light image is formed at a predetermined position on the photoelectric conversion element 36.

The photoelectric conversion element 36 is for photoelectrically converting the formed light image sequentially and outputting it as an electric signal. The photoelectric conversion element 36 is a three-line color CCD that can read a black-and-white image or a color image, and output line data separated into R (red), G (green), and B (blue) color components. . The document image information converted into an electric signal by the photoelectric conversion element 36 (CCD) is transferred to an image processing unit (not shown) and subjected to predetermined image data processing.

<Description of Image Forming Unit 4>
An image forming system 41 and a transfer paper transport system 42 are provided.

The image forming system 41 is provided on the upper part of the main body 10 of the digital color copying machine 1, and the paper feeding mechanism 4 is provided below it.
3 is provided. The paper feed mechanism 43 separates the recording media P stored in the paper feed cassette 12 mounted on the lower end of the main body 10 one by one and supplies the separated recording media P to the image forming system 41. The recording medium P supplied to the image forming system 41 is a cut sheet-shaped normal sheet or an overhead projector sheet, and a pair of upper and lower registration rollers 4 provided in front of the image forming system 41 (right side in FIG. 1).
0 and 40 control the supply timing to the image forming system 41 by the control unit 7. The recording medium P on which an image is formed on one side is also supplied (conveyed) to the image forming system 41 again by the registration rollers 40 in synchronization with the image formation in the image forming system 41.

Below the image forming system 41, a transfer / conveying belt mechanism 44 is provided. The transfer / conveyance belt mechanism 44 is provided substantially at the center of the main body 10 of the digital color copying machine 1. The transfer conveyance belt mechanism 44 includes a drive roller 44a provided on one side (the left side in FIG. 1),
A driven roller 44b provided on the other side (the right side in FIG. 1)
The endless transfer / conveying belt 44 stretched between the rollers 44a and 44b and driven in the direction of arrow Z shown in FIG.
and the recording medium P supplied from the registration rollers 40, 40 is transferred to the other side (upstream side) by electrostatically adsorbing the recording medium P on the surface of the transfer conveyance belt 44c.
From one side (downstream side). A fixing device 45 is provided on the downstream side of the transfer paper transport belt mechanism 44 in the recording medium P transport direction (the left side in FIG. 1), and the fixing device 45 transfers the toner image transferred and formed on the recording medium P onto the recording medium P. Is fixed. The fixing device 45 is provided with a heat roller 45a as a fixing member and a pressure roller 45b on the upper and lower sides.
4 (transfer conveyance belt 44c), the recording medium P conveyed
Is passed through a nip between the heat heat roller 45a and the pressure roller 45b. Fixing device 45
A switching gate 46 is provided on one side. The switching gate 46 is connected to the heat roller 45a and the pressure roller 45.
b, a discharge path for discharging the fixed recording medium P having passed through the nip between the discharge tray 11 and a pair of upper and lower discharge rollers 11a, 11a. It is configured to selectively switch to a resupply path that resupplies the image forming system 41 below the mechanism 44. The resupply path includes a switchback transport mechanism 47, and the front and back surfaces of the recording medium P conveyed to the resupply path side by the switching gate 46 are turned upside down by the switchback transport mechanism 47 and then toward the image forming system 41. To be resupplied.

A manual tray 6 is provided downstream of the switchback transport mechanism 47 in the transport path of the recording medium P. The recording medium P supplied one by one from the manual feed tray 6 is supplied toward the image forming system 41. The manual feed tray 6 includes a paper presence / absence sensor 61 that detects the presence or absence of the recording medium P on the manual feed tray 6 by contact, and a pair of upper and lower light emitting units 62a and light receiving units 62b. A light transmitting sensor 62 for detecting whether the recording medium P is overhead projector paper or other paper (normal paper) based on the amount of light transmission; Paper size sensor 6 as paper size detecting means for detecting paper size by contact
3 are provided. As shown in FIG. 2, these sensors 61 to 63 are connected so that detection signals can be output to the control unit 7.

A first image forming station S1, a second image forming station S2, a third image forming station S3, and a fourth image forming station S4 are transferred and conveyed above the transfer and convey belt mechanism 44, respectively. In the vicinity of the belt 44c, the upstream side of the recording medium conveyance path (FIG. 1)
(The left side). In this case, the recording medium P on the transfer conveyance belt 44c is
, The second image forming station S2, the third image forming station S3, and the fourth image forming station S4.

Each of the image forming stations S1 to S4 has substantially the same configuration, and includes a photosensitive drum 5 which rotates in the direction of arrow F shown in FIG. Around the photosensitive drums 5, a charger 51 that charges each photosensitive drum 5 and forms an electrostatic latent image on the outer peripheral surface of each photosensitive drum 5, and a charger 51 on the outer peripheral surface of the photosensitive drum 5 A developing device 52 for developing the electrostatic latent image formed on the photosensitive drum 5 into a visible image using toner as a powder, and a toner image (visible image) developed on the outer peripheral surface of the photosensitive drum 5 on a recording medium P. Transfer discharger 5 for transfer
3 and a cleaning device 54 for removing the toner remaining on the outer peripheral surface of the photosensitive drum 5 are sequentially provided along the rotation direction (the direction of arrow F) of the photosensitive drum 5.

Above each photosensitive drum 5, a laser beam scanner unit 55 (hereinafter referred to as LSU) is provided. Each LSU 55 includes a semiconductor laser device (not shown) that emits dot light modulated in accordance with image data, and a polygon mirror 55a (deflecting device) for deflecting a laser beam from the semiconductor laser device in the main scanning direction. ) And an fθ lens 55b and a mirror 55 for forming an image of the laser beam deflected by the polygon mirror 55a on the outer peripheral surface of the photosensitive drum 5.
c, 55d.

The LSU of the first image forming station S1
The pixel signal corresponding to the black component image of the color original image is input to 55, and the LS of the second image forming station S2 is
A pixel signal corresponding to the cyan color component image of the color original image is input to U55, and a third image forming station S3
The pixel signal corresponding to the magenta color component image of the color original image is input to the LSU 55, and the pixel signal corresponding to the yellow color component image of the color original image is input to the LSU 55 of the fourth image forming station S4. It has been made. As a result, an electrostatic latent image corresponding to the color-converted document image information is formed on the outer peripheral surface of each photosensitive drum 5.

The developing device 52 of the first image forming station S1 contains black toner, and the developing device 52 of the second image forming station S2 contains cyan toner. The magenta toner is stored in the developing device 52 of S3.
The yellow toner is stored in the developing device 52 of the image forming station S4. The electrostatic latent image on the outer peripheral surface of each photoconductor drum 5 is developed into a visible image by the toner of each color, whereby the original image information color-converted in the image forming system 41 is converted into a toner image by the toner of each color. It is to be reproduced as.

A recording medium suction charger 56 is provided between the first image forming station S1 and the paper feed mechanism 43. The recording medium attracting charger 56 charges the surface of the transfer / transport belt 44c.
The recording medium P supplied from the third transfer belt 44c is transferred and conveyed.
The recording medium P is conveyed without shifting between the first image forming station S1 and the fourth image forming station S4 by securely adsorbing the recording medium P on the upper side.

On the other hand, between the fourth image forming station S4 and the fixing device 45, a discharger 57 for static elimination is provided almost directly above the drive roller 41a. An AC current is applied to the discharger 57 for separating the recording medium P electrostatically attracted to the transfer / transport belt 44c from the transfer / transport belt 44c.

When the recording medium P used in the digital color copying machine 1 is sent out from the paper supply cassette 12 or the manual feed tray 6 and supplied to the guide of the paper supply mechanism 43 in the recording medium transport path, The leading end of the recording medium P is detected by a sensor (not shown), and is temporarily stopped by a pair of registration rollers 40, 40 based on a detection signal output from the sensor. And the recording medium P
Is sent onto the transfer / conveying belt 44c rotating in the direction of arrow Z in FIG. 1 at the timing of each of the image forming stations S1 to S4. At this time, the transfer conveyance belt 44
The recording medium P is stably conveyed and supplied while passing through each of the image forming stations S1 to S4, since the recording medium P is given a predetermined charge by the recording medium suction charger 56.

In each of the image forming stations S1 to S4, a toner image of each color is formed, and the recording medium P conveyed by being electrostatically attracted and conveyed by the transfer / conveyance belt 44c.
The toner images of the respective image forming stations S1 to S4 are superimposed on the supporting surface of, and the images are transferred. And the fourth
When the transfer of the image by the image forming station S4 is completed, the recording medium P is sequentially separated from the transfer conveyance belt 44c from the leading end portion by the negative discharger 57 and guided to the fixing device 45. The recording medium P on which the toner image has been fixed by the fixing device 45 is supplied to a discharge roller 11a.
Is discharged onto the discharge tray 11 through a discharge port (not shown).

As a characteristic part of the present invention, as shown in FIG. 3, the fixing device 45 removes the recording medium P which has been heated and pressed at the nip between the upper and lower heat heat rollers 45a and the pressure rollers 45b. A pair of upper and lower peeling claws 45c and 45d for peeling are provided. Peeling nails 45c, 45d
Is formed of a resin such as PPS (polyphenylene sulphite) having a high toner release property.

The upper heat heat roller 45a is formed by coating a hollow aluminum tube with an elastic body such as heat-resistant silicon rubber, while the lower heat roller 45b is used.
Is applied to a shaft in which an elastic body such as heat-resistant silicone rubber is coated. The circumferential length of the heat heat roller 45a is set to be equal to or longer than the short size of the A4-size (standard size) recording medium P most frequently used for the image forming apparatus in the transport direction. More specifically, the heat heat roller 45a has a diameter D of 7 so as to be longer than the length (210 mm) of the A4 size recording medium P in the short direction.
It is set to 0 mm and the circumference is 220 mm.

Heaters 45e and 45f such as halogen lamps are provided in the aluminum tubes of the heat heat roller 45a and the pressure roller 45b, respectively. The upper and lower heaters 45e and 45f are connected to upper and lower heater power supplies 45m and 45n.
It is connected to the. Temperature detection sensors 45g and 45h such as thermistors are in contact with the surfaces of the heat heat roller 45a and the pressure roller 45b, respectively, and the control unit 7 is controlled based on the temperatures detected by the upper and lower temperature detection sensors 45g and 45h. Are the upper and lower heater power supply 45m, 45m
The surface temperatures of the heat roller 45a and the pressure roller 45b are kept constant by individually turning on or off n. In this case, the upper heater 45e of the heat heat roller 45a outputs 650W during standby.
Is controlled by ON / OFF switching by the upper heater power supply 45m at the time of image formation, and is controlled by ON / OFF switching by the upper heater power supply 45m at 850 to 950W at the time of image formation. The temperature is kept at 180 ° C. On the other hand, the pressure roller 45b
The upper heater 45f is controlled by ON / OFF switching by the lower heater power supply 45n at 450W during standby, and is turned off without performing ON / OFF switching by the lower heater power supply 45n during image formation.

A heat heat roller 45a and a pressure roller 45b formed by the elasticity of heat-resistant silicone rubber.
Heat is supplied to the toner image on the recording medium P in the nip portion between them, and the toner is melted and pressed to fix the toner image on the recording medium P. Further, the recording medium P discharged from the nip between the heat roller 45a and the pressure roller 45b reaches the peeling claw 45c while being stuck to the surface of the heat heat roller 45a by the melted toner. 45c, heat heat roller 45
a and is delivered to the discharge rollers 11a, 11a on the downstream side (left side in the figure) in the transport direction. Further, a cleaning roller 45i is provided around the heat roller 45a.
And an oil application roller 45j are provided.
A cleaning roller 45k is provided around the pressing roller 45b.
Are arranged. Each cleaning roller 45i, 45
k is a roller whose surface is a felt, which individually comes into sliding contact with the surfaces of the heat heat roller 45a and the pressure roller 45b to collect the offset toner adhered to the surface of the heat heat roller 45a and the pressure roller 45b. It has been made like that. The oil application roller 45j is a roller whose surface is made of rubber.
The surface of the heat heat roller 45a is contacted with the surface of the heat heat roller 45a so as to be in contact with the surface of the heat heat roller 45a. Then, the control unit 7 controls the temperature detection sensor 45
g and 45h are connected so as to be able to input detection signals, and are connected to the heater power supplies 45m and 45n so as to be able to output ON operation signals or OFF operation signals. The control unit 7 includes a memory (not shown), and is connected to a display unit 10a formed of a liquid crystal screen provided on the upper surface of the main body 10 of the digital color copying machine 1 so as to display a warning sentence in the memory. . In this case, as the warning text displayed on the display unit 10a, the content such as "Please check the paper size in the recording medium conveyance direction" is displayed based on the detection signal detected by the paper size sensor 63. I have to.

Here, the flow of control by the control unit 7 based on the detection signals from the sensors 61 to 63 when the recording medium P is supplied from the manual feed tray 6 will be described with reference to the flowchart shown in FIG.

First, step S in the flowchart of FIG.
In 1, the paper presence sensor 61 determines whether or not the recording medium P (cut sheet ordinary paper or overhead projector paper) is present on the manual feed tray 6, and this determination is YES. In step S2, whether or not the recording medium P on the manual feed tray 6 is overhead projector paper (hereinafter referred to as OHP paper) is determined by the transmission sensor 62 based on the light transmission amount of the recording medium P in step S2. To detect and determine.

If the determination in step S2 is YES that the recording medium P is an OHP sheet, the process proceeds to step S3.
, The paper size sensor 63 detects whether or not the paper size of the OHP paper supplied from the manual feed tray 6 in the transport direction is the short size.

If the determination in step S3 is NO, that is, if the paper size in the transport direction of the OHP paper is the longitudinal size, in step S4, the display unit 10a of the main body 10 of the digital color copying machine 1 is stored in the memory. After displaying the warning text "Please check the paper size in the transport direction of the recording medium", the process returns to step S1. On the other hand, step S
If the determination of No. 3 is YES in which the paper size in the transport direction of the OHP paper is the short size, the display unit 10a of the main body 10 of the digital color copying machine 1 is determined in step S5.
If the warning message "Please check the paper size in the conveyance direction of the recording medium" is displayed, the warning message is deleted.

Thereafter, in step S6, the start button for starting the image formation by the image forming section 4 is turned on.
After waiting until the operation, the image forming unit 4 forms an image in step S7.

Then, in step S8, the recording medium P (O
HP paper (HP paper) is discharged onto the discharge tray 11 and the process is terminated.

On the other hand, if the determination in step S2 is NO, that is, if the recording medium P is normal paper, in step S9 the paper size of the normal paper supplied from the manual feed tray 6 in the transport direction is short. The paper size sensor 63 determines whether or not there is a sheet.

If the determination in step S9 is NO, ie, if the paper size in the normal paper transport direction is the longitudinal size, a warning is issued to the display unit 10a of the main body 10 of the digital color copying machine 1 in step S10. After displaying the sentence "Please check the paper size in the transport direction of the recording medium", the process proceeds to step S6. On the other hand, step S9
Is YES, the paper size in the transport direction of the normal paper is the short size, the process proceeds to step S5.

Next, a case where the recording medium P is fixed by heat heat rollers having different diameters D1 and D2 will be described. In this case, as the recording medium P, a standard size (A4 size) set on the manual feed tray 6 so that the paper size in the transport direction is the short size is applied.

First, as shown in FIG.
When fixing the recording medium P using the heat heat roller 45a of the present invention set to
Has a circumferential length of 220 mm, which is longer than the short-side length 210 mm of the A4-size recording medium P,
As shown in FIG. 6, the recording medium P is fixed by the end of the first rotation, and it can be seen that the recording medium P is not fixed over the second rotation in one sheet of the recording medium P.

On the other hand, as shown in FIG. 6, when the recording medium P is fixed using a conventional heat heat roller a in which the diameter D2 is set to 40 mm similarly to the pressure roller 45b, the heat heat roller a Its circumference is about 126 mm (125.
66 mm), which is shorter than 210 mm in the width direction of the A4-size recording medium P, and is about 60% of the conveying direction of the recording medium P at the end of the first round (the oblique line shown in FIG. 6). It can be seen that only one portion of the recording medium P is fixed, and is fixed over the second rotation in one sheet of the recording medium P.

Therefore, in this embodiment, the diameter D1 is set to 70
Since the heat heat roller 45a having a long circumference set to mm is used, it takes a long time for the heat heat roller 45a to make one rotation with respect to the recording medium P (OHP paper and normal paper) of the most frequently used standard size. Is fixed on the recording medium P, and the heat roller 45a
It is possible to reliably prevent the fixing unevenness occurring after the second round of the above. Moreover, the fixing speed of the heat heat roller a in the first rotation (circular rotation speed) is the same as that of the heat heat roller a, which has a short circumferential length in which the heat heat roller a is fixed over the second rotation on one recording medium P.
It is not necessary to decelerate, and it is possible to reliably prevent an image deviation occurring when the transport speed of the recording medium P by the heat roller a changes. As a result, a good image can be obtained.

The circumferential length of the heat heat roller 45a is set to 220 mm, which is equal to or greater than the short side length 210 mm of the standard size (A4 size) recording medium P. (Circumferential length) is set to be short without significantly increasing, and the heat heat roller 45a
The increase in the size of the fixing device 45 due to the increase in the circumference D1 is suppressed as much as possible, and the increase in the size of the digital color copying machine 1 can be prevented.

Further, the image forming section 4 includes a manual feed tray 6
When the overhead size of the overhead projector paper is detected by the paper size sensor 63 when the length of the overhead projector paper in the transport direction is longer than the short size of the standard size recording medium P, control is performed to prohibit image formation on the overhead projector paper. Since the control is performed by the unit 7, the overhead projector paper, which is basically the mainstream of the standard size, is determined to be transported from a direction (longitudinal direction) other than the specified transport direction, and image formation is prohibited. Will be. For this reason, the heat heat roller 45a cannot be used for the overhead projector paper from the wrong transport direction (longitudinal direction).
Therefore, the formation of a poor image with uneven fixing on the overhead projector paper can be prevented beforehand.

In the above embodiment, the outer diameter D1 of the heat heat roller 45a is enlarged, but the outer diameter of the pressure roller may be changed to 70 mm as in the case of the heat heat roller.

In the above embodiment, the LSU 55 of each of the image forming stations S1 to S4 scans and exposes a laser beam to perform light writing on the photosensitive drum 5, but instead of the LSU 55, Alternatively, a writing optical system (LED head) including a light emitting diode array and an imaging lens array may be used. In this case, L
Since the ED is smaller in size than the LSU and has no moving parts and is silent, it is preferably used in an image forming apparatus such as a tandem digital color copying machine that requires a plurality of optical writing units. Will be done.

[0057]

As described above, according to the present invention, the following effects are exhibited. First, by setting the peripheral length of the fixing member to be equal to or longer than the length of the standard size recording medium in the conveying direction, the fixing over the second circumference of the fixing member in one standard size recording medium is eliminated, and the fixing unevenness is reduced. Can be reliably prevented. In addition, it is possible to reliably prevent the image from shifting due to a change in the fixing speed of the fixing member during the first rotation. As a result, a good image can be obtained.

In particular, by setting the peripheral length of the fixing member to be equal to or longer than the width of the short side of the standard size recording medium, it is possible to suppress the enlargement of the fixing device due to the increase in the peripheral length of the fixing member as much as possible. In addition, the size of the image forming apparatus can be prevented from increasing.

In particular, by prohibiting the image formation of the overhead projector paper longer in the transport direction than the short size of the standard-size recording medium, the overhead projector paper from the wrong transport direction is fixed in the second rotation of the fixing member. It is possible to prohibit straddling fixing, thereby preventing a poor image formation on the overhead projector paper.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a color laser copying machine provided with a fixing device according to an embodiment of the present invention as viewed from the front.

FIG. 2 is a block diagram showing components of a control system for inputting and outputting signals to and from a control unit.

FIG. 3 is a cross-sectional view illustrating a configuration near a fixing device.

FIG. 4 is a flowchart illustrating a flow of control by a control unit based on a detection signal from each sensor when a recording medium is supplied from a manual feed tray.

FIG. 5 is an explanatory diagram in a case where a recording medium is fixed by a heat heat roller having an enlarged diameter.

FIG. 6 is an explanatory diagram of a case where a recording medium is fixed by a conventional heat roller.

[Explanation of symbols]

Reference Signs List 1 color laser copying machine (image forming apparatus) 45 fixing device 45a heat roller (fixing member) 63 paper size sensor (paper size detecting means) P recording medium

Claims (3)

[Claims] An image forming apparatus including a fixing device that heat-presses a fixing object so as to fix a toner image to the fixing object, wherein the fixing device has a peripheral surface in a conveying direction of the fixing object. A fixing member is provided in contact with the fixing member, and a peripheral length of the fixing member is set to be equal to or longer than a length in a conveying direction of a standard size recording medium most frequently used for an image forming apparatus. An image forming apparatus provided with the device. 2. An image forming apparatus provided with the fixing device according to claim 1, wherein a peripheral length of the fixing member is set to be equal to or larger than a width of a short side of a standard size recording medium. An image forming apparatus provided with a fixing device. 3. An image forming apparatus comprising the fixing device according to claim 2, wherein a sheet for detecting that the length of the recording medium in the transport direction is longer than the short size of the standard size recording medium. In the case where the overhead projector paper used for the overhead projector is provided as a fixing object, the output from the paper size detection means is provided, and the length in the transport direction is a short size of the recording paper having the standard size. An image forming apparatus provided with a fixing device, wherein image formation on a longer overhead projector sheet is prohibited.