JP3492089B2 - Image forming device - Google Patents

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 such as a copying machine or a laser beam printer capable of forming images on a single recording medium a plurality of times.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus such as a copying machine or a laser beam printer is provided with front and back reversing means capable of automatically forming images on both front and back sides of a sheet-like recording medium such as paper. Are known.

In this apparatus, first, a toner image is formed on a first surface (front surface) of a recording medium such as paper in a toner image forming section, and an unfixed toner image on the recording medium is heated in the fixing section. And fix it on the recording medium. Next, the recording medium after fixing is turned upside down by the front side upside down means, and is supplied again to the toner image forming section. Then, the same image formation (toner image formation and fixing) as described above is performed on the second side (back side) of the recording medium this time. As described above, images are automatically formed on both sides of the recording medium.

By the way, recently, there is known a fixing device of a so-called quick system, which reaches a fixable temperature in a short time.

In this fixing device, a heat-resistant fixing film is formed in an annular shape, and is sandwiched between a heating member having a low heat capacity and a pressure roller from inside and outside, respectively. When fixing
The pressure roller and the fixing film rotate, and between them,
While a recording medium carrying an unfixed toner image is nipped and conveyed, heat and pressure are applied to fix the toner image on the recording medium. At this time, the front surface side of the recording medium carrying the unfixed toner image contacts the surface of the fixing film, while the back surface side of the recording medium contacts the surface of the pressure roller. According to this fixing device, unlike the conventional roller type fixing device, the wait time at the start of the image forming apparatus is short.

[0006]

However, according to the above-mentioned quick-type fixing device, the fixing film which the first surface (front surface) of the recording medium contacts when fixing the toner image on the first surface is the second surface (rear surface). Since the temperature is higher than that of the pressure roller in contact with each other and the temperature difference between the two is large, a large temperature difference occurs between the front surface side and the back surface side of the recording medium. For example,
When the recording medium is paper, the fibers of the paper have a larger expansion on the front surface side than the expansion on the rear surface side. Therefore, the paper after fixing has a downward curl wound on the rear surface side (pressure roller side). Will occur.

In order to form an image on the second surface (rear surface) of this recording medium, the paper is turned upside down by the switchback method, and then the paper is supplied again to the toner image forming section. At this time, the paper is turned upward. Supplied in curled state. Then, the toner image on the second side is transferred and fixed on the paper curled upward.

By the way, the toner image forming section has, for example, as shown in FIG. 12, a photosensitive drum 51 and a transfer roller 54 in pressure contact with the photosensitive drum 51. The toner image formed on the photosensitive drum 51 is transferred to the transfer roller 54. To the photosensitive drum 5 by utilizing the curvature of the photosensitive drum 51.
In the case of the structure in which the paper is separated from the first surface, if the paper is curled upward, the curvature separation effect is reduced, and the separation may not be performed well, which may cause a separation failure.

In the figure, the tangent line of the photosensitive drum 51 and the transfer roller 54 at the transfer nip N ₁ is S, the shortest distance between the tangent line S and the bottom of the cleaner container 25 is B, and the cleaner container 25 parallel to the tangent line S and the transfer nip are shown. The shortest distance of N ₁ is A
And More specifically, A = 25 mm, B = 10 mm
In the case of the above configuration, in the paper on which the image formation on the first side is completed, if there is curl of 10 mm or more from the position 25 mm inward from the rear end to the rear end, the image formation on the second side curled upward At this time, the leading edge of the paper gets caught in the bottom of the cleaner container, causing paper jam.

The above example shows an example in which the paper is jammed due to the separation failure when the image is formed again on the paper with the leading edge curled upward. Not only when the tip is curled upward, but also when the tip is curled downward.

Therefore, according to the present invention, in an image forming apparatus that forms an image a plurality of times on one recording medium, the occurrence of curl at the leading end portion, which is the leading end side at the time of the next image formation, is prevented at the time of fixing this time. An object of the present invention is to provide an image forming apparatus that is suppressed.

[0012]

According to a first aspect of the present invention, there is provided a toner image forming portion for forming a toner image on a recording medium conveyed by a conveying means, and the toner image is formed on the recording medium by a heating member. To the recording medium after the first fixing so that the image formation including the toner image formation and the fixing is performed a plurality of times on one recording medium. And a re-feeding unit that re-feeds the toner to the toner image forming unit, wherein the fixing unit is a heat-resistant fixing film.
A heating member having a low heat capacity and the heating member
And a pressure roller that holds and conveys the fixing film.
A recording medium carrying a toner image on the fixing film
Fixes the toner image while conveying it with the pressure roller
Of a roller type having a fixing device or a cylindrical fixing roller
The temperature control means is provided with a fixing device, and the temperature control means controls the temperature of the recording medium at the time of fixing on one recording medium by measuring the temperature of the recording medium with respect to the front end portion which is the front side in the transport direction at the time of the next image formation. It is characterized in that the temperature is controlled to be lower than the temperature for other parts except the part.

According to a second aspect of the present invention, the re-feeding means sets the recording medium to the front and back sides so that the second image is formed on the back side of the recording medium on which the first image is formed on the front side. The temperature control means controls the temperature of the rear end portion of the recording medium at the time of the first fixing to be lower than the temperature of other portions excluding the rear end portion. It is characterized by:

According to a third aspect of the present invention, there is provided first designating means for designating the first image formation on the front side when image formation is performed on both front and back surfaces of the recording medium, and the control means comprises: The first fixing is performed based on the designation by the first designating unit.

According to a fourth aspect of the present invention, the re-feeding means is a loop system for performing the second and subsequent image formation on the same front side of the recording medium on which the first side image formation is performed on the front side. Re-conveying means, and the temperature control means controls the temperature of the leading end portion of the recording medium at each fixing time except the last time to be lower than the temperature of the other portion except the leading end portion. Is characterized by.

According to a fifth aspect of the present invention, there is provided second designating means for designating the first image formation on the front surface side in the case of performing multiple image formation on one surface of the recording medium. The control means is characterized in that the fixing is performed each time except the last time, based on the designation by the second designating means.

In the present invention according to claim 6, the toner image forming section conveys the image carrier on the surface of which the toner image is formed and the toner image formed on the image carrier by the conveying means. And a transfer means for transferring to the surface of the recording medium.

According to a seventh aspect of the present invention, the recording medium after the toner image is transferred from the image carrier is separated from the surface of the image carrier by utilizing the curvature of the surface of the image carrier. Is characterized by.

The present invention according to claim 8 relates to the toner image.
The charge of the recording medium on which the toner image is formed by the forming unit is removed.
A static elimination unit having a static elimination needle is provided .

[Operation] Regarding temperature control, the temperature of the leading end portion of the recording medium, which is the leading end side in the transport direction at the time of the next image formation, is controlled to be lower than the temperature of the other portions excluding the leading end portion. As a result, it is possible to reduce the temperature difference between the front and back surfaces of the leading end portion of the recording medium and reduce the curl at the leading end portion.

Since this leading end portion is on the leading end side in the carrying direction at the next image forming, it is possible to prevent the carrying defect at the next image forming.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. <Embodiment 1> FIG. 1 shows a schematic configuration of an image forming apparatus equipped with an automatic double-sided image forming mechanism (switchback inverting means) as a re-feeding device. The configuration of the entire image forming apparatus and the operation of forming images on both front and back surfaces of a recording medium will be briefly described with reference to FIG.

First, a recording medium (hereinafter appropriately referred to as "paper")
The first image is formed on the first surface (front surface) of 19.

That is, the recording medium 19 such as paper in the paper cassette 36 arranged in the lower portion of the image forming apparatus main body 47 is fed one by one by the rotation of the paper feed roller 25 in the direction of the arrow (counterclockwise in the figure). The paper is conveyed and conveyed to the conveyance roller pair 33 along the guide 34. The recording medium 19 includes a pair of conveying rollers 33.
Is further conveyed by the rotation in the direction of the arrow, and is conveyed to the registration roller pair 31 along the guide 32. The registration roller pair 31 rotates in accordance with the toner image formed on the drum type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 51 as an image bearing member, and moves to the transfer nip N ₁ along the guide 14. The recording medium 19 is supplied.

The toner image is formed on the photosensitive drum 51 by
Do the following: The photosensitive drum 51 is rotationally driven in the arrow direction (clockwise in the figure) by a driving unit (not shown), and the surface of the photosensitive drum 51 is uniformly charged by the charger 29.
The surface of the photosensitive drum 51 after charging is exposed by the exposure device 28 based on image information, whereby an electrostatic latent image is formed. Toner is attached to the electrostatic latent image by the developing device 30 and is developed as a toner image.

The toner image on the photosensitive drum 51 is nipped and conveyed by the photosensitive drum 51 and the transfer roller 54 which rotate in the directions of the arrows. At this time, a transfer bias is applied to the transfer roller 54, so that the transfer nip N ₁ is transferred. It is transferred onto the first surface of the recording medium 19 via the recording medium.

After the transfer of the toner image, the transfer drum remaining on the surface of the photosensitive drum 51, which has not been transferred to the recording medium 19, is removed by the cleaner 12, and is used for the next image formation.

On the other hand, the recording medium 19 after the toner image transfer is
The charge removing needle unit 58 and the photosensitive drum 51 are separated from the surface of the photosensitive drum 51 by the action of the curvature and the like, and are conveyed to the fixing device (fixing device) 1 along the guide 60 while the first surface carries an unfixed toner image. Here, heat and pressure are applied to fix the toner image. The fixing device 1 will be described in detail later.

As described above, the first image formation on the first surface (front surface) of the recording medium 19 is completed.

Next, the second image formation is performed on the second surface (back surface) of the recording medium 19. The recording medium 19 on which the toner image is fixed on the first surface is conveyed downward by a swingable flapper 37 arranged on the downstream side of the fixing device 1 (switched by a dotted line in the figure) to be guided to a guide 39. It is conveyed to the reversing roller (reversing means) 40 along. The reversing roller 40 conveys the leading end side of the recording medium 19 downward along the guide 41, stops immediately after the trailing end of the recording medium 19 passes through the branch portion 39a, and performs reverse rotation this time. This allows
The recording medium 19 is conveyed along the guide 42 to the conveying roller pair 43 in a state where the portion which has been the rear end until then becomes the front end side, is guided to the guide 42 side, and is turned upside down. The recording medium 19 is transported along the guide 44 by the rotation of the pair of transport rollers 43 in the direction of the arrow, and is transported to the pair of transport rollers 33, and is supplied to the transfer nip N ₁ in the same manner as the first surface. By this time the photosensitive drum 5
A toner image to be transferred to the second surface of the recording medium 19 is formed on one surface, and this toner image is transferred to the second surface of the recording medium 19 via the transfer nip N ₁ . After the transfer of the toner image, the transfer residual toner on the surface of the photosensitive drum 51 is removed by the cleaner 12, and the photosensitive drum 51 is used for the next image formation. On the other hand, in the recording medium 19 after the toner image transfer, after the toner image on the second surface is fixed by the fixing device 1, the flapper 37 is switched to the lower side (solid line in the figure), so that the upper guide 3
8 and is discharged onto the paper discharge tray 46 by the discharge roller pair 45.

The above is a brief description of the configuration of the entire image forming apparatus and the operation of forming images on both front and back surfaces (first surface and second surface) of the recording medium 19.

Next, the structure of the transfer portion will be described with reference to FIG.

The photosensitive drum 51 shown in the figure has a diameter of 30 m.
m OPC (organic optical semiconductor) on aluminum cylinder 51a
The transfer roller 54 has a volume resistivity of about 10 ⁹ Ω · cm and a core metal 53.
And the elastic layer 52 of foamed EPDM. Then, the surface of the transfer roller 54 is fixed to the photosensitive drum 51.
By pressing the surface with a predetermined pressing force, a transfer nip N ₁ is formed between the two. The charger 29 shown in FIG.
To transfer the toner image formed on the surface of the photosensitive drum 51 by the exposure device 28, the developing device 30 and the like onto the recording medium 19, a power source 55 applies to the core metal 53 a bias having a polarity opposite to the charging polarity of the toner. Therefore, the transfer is performed via the transfer nip N ₁ .

The recording medium 19 that has passed through the transfer nip N ₁ is
It is separated from the surface of the photosensitive drum 51 by its own rigidity (bending stiffness) and the effect of the curvature of the photosensitive drum 51, and the effect of the electric field and the electric charge by the static elimination needle unit 58. The static elimination needle unit 58 is composed of a static elimination needle 56 and an insulating member 57. The static elimination needle 56 is electrically grounded or a bias having a polarity opposite to the transfer bias is applied. In the figure, the grounding is illustrated. The static elimination needle 56 has a saw shape as shown in FIG. 3, and the height b of the saw peak is 3 mm and the pitch a is 2 mm.

The cleaner 12 includes a cleaner container 25 and a cleaner blade 24, and the transfer residual toner on the surface of the photosensitive drum 51 removed by the cleaner blade 24 is collected in the cleaner container 25 as waste toner.

Next, the fixing device 1 will be described with reference to FIG. The fixing device 1 shown in the figure is a film heating type fixing device using an endless fixing film 5.

The fixing device 1 is a laterally long heating element (hereinafter referred to as "the following direction") in the left-right direction (the direction which is the same plane as the carrying direction and orthogonal to the carrying direction; hereinafter the same) in the carrying direction (arrow K1 direction) of the recording medium 19. 2). The heaters 2 are stays 3 and 4 that also serve as film guides having an arcuate longitudinal section and are arranged substantially symmetrically on the right side and the left side (upstream side and downstream side in the transport direction) around the heater 2 in the figure. Supported by. An endless (endless) heat resistant film (fixing film) 5 is fitted onto the heater 2 and the stays 3 and 4. The inner peripheral length of the fixing film 5 and the outer peripheral length of the stays 3 and 4 including the heater 2 are, for example, 3 in the inner peripheral length of the former fixing film 5.
It is set to a value as large as mm, which allows the fixing film 5
Is loosely fitted to the stays 3 and 4 including the heater 2 with a sufficient circumferential length. Stay 3,
4 is fixedly supported by a stationary member (not shown), and the heater 2 is fixedly supported by stays 3 and 4.

Below the heater 2, a pressure roller 6 as a fixing film driving roller is arranged. The pressure roller 6 is biased upward by a biasing member (not shown), sandwiches the fixing film 5 with the upper heater 2, and has a predetermined width in the transport direction with the fixing film 5. At the same time, a belt-shaped fixing nip N ₂ that is long in the left-right direction is formed. Since the pressure roller 6 conveys the recording medium 19,
It is made of solid rubber that has a small change in outer diameter due to heat and durable use. As the material, for example, silicone rubber can be used.

When the pressure roller 6 is rotationally driven in the direction of arrow R6, the frictional force thereof causes the endless fixing film 5 to slide its inner peripheral surface in close contact with the lower surface of the heater 2 in the direction of arrow R5. Is orbitally driven at a predetermined peripheral speed (process speed).

Next, the heater 2 described above will be described in detail. As shown in FIG. 4, the heater 2 having a long insulating property, high heat resistance, and low heat capacity in the left-right direction, and a lower surface of the heater substrate 2a in the longitudinal direction thereof. An electric resistance heating electric resistance layer (hereinafter simply referred to as "electric resistance layer") 2b formed along the surface, and a surface protection layer 2c formed thereon such as glass or fluororesin.
And a thermistor 2d as a heating element temperature detecting means arranged in contact with the upper surface of the heater substrate 2a (the surface opposite to the surface on which the surface protective layer 2c is formed), which is a low heat capacity linear heating element.

The heater 2 having the above-described structure is arranged between the stays 3 and 4 in a state where the heater 2 is in contact with the inner peripheral surface of the fixing film 5 with the surface protective layer 2c side facing downward. More specifically, the heater substrate 2a is, for example, an alumina substrate having a thickness of 1 mm, a width in the transport direction of 5 to 10 mm, and a length in the left-right direction of 340 mm. The electric resistance layer 2b is, for example, Ag / Pd.
It is a coating layer made of an electric resistance material such as (silver palladium) or Ta ₂ N by screen printing or the like with a thickness of 10 μm and a width in the transport direction of 1 to 3 mm.

In the heater 2 having the above-described structure, a voltage is applied (power supply) between both ends of the electric resistance layer 2b in the longitudinal direction (between both left and right ends), so that the electric resistance layer 2b generates heat,
The heater substrate 2a is heated, and the temperature of the entire heater having a low heat capacity rises rapidly with a good temperature rise.

To control the temperature of the heater 2, the output of the thermistor 2d on the upper surface of the heater substrate 2a is A / D converted and taken into a CPU (temperature control means) 7, and based on the information, the triac 8 electrically lowers the heater 2. AC for energizing the anti-layer 2b
The voltage is pulse-width modulated such as phase / wave number control to control the heater power supply. In addition, triac 8
The upper 9 is an AC power supply. In the present embodiment,
The temperature controlled temperature by the temperature control of the heater 2 is 190 ° C.

At the time of fixing, electric power is supplied to the heater 2 and the fixing film 5 is orbitally driven in the direction of arrow R5 by the rotation of the pressure roller 6 in the direction of arrow R6. The recording medium 19 as the material to be heated guided and conveyed from the drum 51 or the like) to the paper feed guide 10 is a fixing film with the first surface (front surface) carrying the unfixed toner image Ta facing upward. 5
Between the pressure roller 6 and the pressure roller 6. As a result, in the recording medium 19, the first surface (front surface) of the fixing nip (heating nip) N ₂ formed by pressure contact between the heater 2 and the pressure roller 6 is brought into close contact with the outer peripheral surface of the fixing film 5. It passes with the fixing film 5. In the recording medium 19, the unfixed toner image Ta on the recording medium 19 is heated by the heat of the heater 2 via the fixing film 5 in the process of passing through the fixing nip N ₂ , and becomes the fixed toner image Tb.

When the recording medium 19 passes through the fixing nip N ₂ , the recording medium 19 is sequentially separated from the outer peripheral surface of the fixing film 5 by curvature separation, and guided by the paper discharge guide 11 to output the recording medium 19 after the fixing process. To be done.

As shown in FIG. 5, the temperature control of the fixing device 1 is performed by using the fixing device 1 as a recording medium 19 (indicated as "paper" in the figure).
It is designed to control the temperature when passing by. In the above-described film heating type fixing device 1 using the fixing film 5, since the heat capacity of the heating portion (the heater 2 and the fixing film 5) is small, the temperature change in the fixing nip N is instantaneous with respect to the temperature control. Respond to. In the present invention, the temperature control of the fixing device 1 shown in FIG. 5 is performed at the time of fixing when an image is formed on only one surface of the recording material 19, or at the final fixing at the time of double-sided or multiple image formation. It indicates time. Temperature control other than these will be described later.

The above-mentioned image forming apparatus main body 47 is provided with an operation panel (not shown), and on this operation panel,
Switches for selecting a single-sided image forming mode, a double-sided image forming mode, a multiple image forming mode (described later), and the like are arranged. In the above description, the double-sided image forming mode is selected by this switch, and the image formation is performed twice on the one recording medium 19, the first time and the second time. That is, a toner image was formed on the first surface (front surface) of the recording medium 19 in the first image formation, and a toner image was formed on the second surface (back surface) of the recording medium 19 in the second image formation.

Next, the temperature control according to the present invention will be described with reference to FIG. In the following, "Recording medium 1
Instead of "9", simply "paper" is used.

In the temperature control shown in the figure, first, the first side (front side) of the first sheet, then the second side (back side) of the first sheet,
Next, the first side (front side) of the second sheet of paper, then the second side of the second sheet of paper 2.
This is temperature control when images are sequentially formed in the order of the first surface (back surface).

According to the temperature control shown in the figure, the temperature control of the second side is the same as the conventional one over the entire length of the length of the paper in the conveyance direction (hereinafter referred to as "whole area"). The controlled temperature at the rear edge of the paper is lowered.

This effect will be described. It has been confirmed that the curl amount of the trailing edge of the paper during fixing depends on the temperature difference between the temperature of the heater 2 (see FIG. 4) and the temperature of the drive roller 6. This is because the difference in the degree of expansion of the fibers of the paper due to heat when passing through the fixing nip N ₂ tends to increase when the temperature difference between the front and back of the paper is large.

In the structure of the fixing device 1 described above, the temperature of the pressure roller at the time of fixing is about 90.degree. Temperature control temperature is 1
Since it is 90 ° C, the temperature difference between the front and back of the paper is 100 ° C. As a result of examining the relationship between the temperature difference between the front and back of the paper at the time of fixing and the curl amount, it was found that there is a tendency as shown in FIG.

When the controlled temperature is 190 ° C., the temperature difference is 1
Since it is 00 ° C., the curl amount is 20 mm as a minus curl wound on the pressure roller 6 side (lower side).

In order to prevent the paper jam, the minus curl amount at the trailing edge of the first side of the paper must be less than 10 mm, which is less than 80 ° C. when the temperature difference is 170 ° C. when the temperature is controlled. Must be less than.

On the contrary, in order to reliably fix the unfixed toner image Ta on the paper, a temperature of 180 ° C. or higher is required. However, if it is assumed that the toner image does not fall off even if the toner image on the paper after fixing is lightly rubbed,
Even about 60 ° C is sufficient.

FIG. 6 shows the temperature adjustment control when images are automatically formed on both the front and back sides of the paper based on the above.

In the case of performing automatic double-sided image formation, the temperature control temperature is set to 160 ° C. at least at the trailing edge of the paper of 25 mm or more at the time of image formation on the first side. Although it is possible to control the temperature of 160 ° C. over the entire area of the paper, 190 ° C. is preferable in order to improve the margin of fixability.
The temperature was adjusted to ℃, and thereafter it was adjusted to 160 ℃.

As a result, the minus curl at the trailing edge of the first side is 5 mm, and the paper jam at the transfer portion is 10 mm.
Since it can be made smaller than that, it is possible to stabilize the paper conveyance on the second surface.

At the time of fixing the second surface, the temperature control temperature is set to 190 ° C. over the entire area of the paper to secure the fixability. At the time of fixing the second side, the assumed attachment part of the first side is also heated at the same time.
It is possible to obtain sufficient fixability on both sides.

With the above-mentioned structure, it is possible to improve the paper transportability during automatic double-sided image formation.

In the case of single-sided printing, the entire temperature from the leading edge to the trailing edge of the paper is controlled at 190 ° C. as usual.

The present sequence is not limited to the film heating type fixing device, but is also applied to a roller type fixing device, particularly when a cylindrical fixing roller having a thickness of 2 mm or less is used. The same effect can be achieved. <Second Embodiment> In the second embodiment, unlike the first embodiment, a case will be described where an image is formed on both sides of a paper by an image forming apparatus that does not have a paper reversing unit.

When double-sided image formation is performed by a single-sided image forming apparatus, the paper on which the first-side image formation has been completed is
Set it in the paper feed section and start printing. In this case, unlike the case of automatic double-sided printing, the leading edge of the first side printing is also the leading edge of the second side printing (because the top and bottom of the image are aligned on the first and second sides).

First, in the case where the image forming apparatus main body 47 is continuously used in the operation section of the image forming apparatus 47 or a computer such as a printer apparatus, the first side of double-sided image formation can be designated from the outside.

In the case of single-sided image formation, the conventional temperature control is performed, but in the case of double-sided image formation, the temperature control as shown in FIG. 8 is performed.

As for the control contents, first, when the image formation for the first side is performed, the first side designation for double-sided image formation is performed by the above-mentioned designation means. In this case, the temperature adjustment is performed at the assumed contact temperature of 160 ° C. at the front end of the paper, and the normal temperature adjustment temperature 1
90 ° C.

As a result, the curl at the leading edge of the paper on which the image formation on the first side is completed can be made small, so that the second side can stably convey the paper without causing separation failure at the transfer section. It can be performed.

When the image formation for the second side is performed, the normal temperature control temperature of 190 ° C. is set over the entire area from the front end to the rear end of the paper as in the case of the normal single-sided image formation.

With the above-mentioned structure, paper jam can be prevented and stable image formation can be performed even when double-sided image formation is performed by the image forming apparatus for single-sided image formation. <Third Embodiment> In the third embodiment, the same surface of the paper 2
A case of performing image formation of more than one color, that is, a case of performing multiple image formation will be described.

FIG. 9 shows the configuration of an image forming apparatus equipped with an automatic multiplex image forming mechanism (loop type re-conveying means) capable of automatically forming two color images on the same side of paper 19. An outline is shown.

Only parts different from the image forming apparatus having the automatic double-sided image forming mechanism shown in FIG. 1 will be described here.

First, when the first color image is formed,
The developing device 62 is separated from the photosensitive drum 51 and turned off, and the electrostatic latent image on the photosensitive drum 51 becomes a toner image with toner attached by the developing device 61. The toner image of the first color is transferred onto the paper 19 and then heated and pressed by the fixing device 1 to be fixed. The paper 19 after fixing is the flapper 37.
Is switched to the position above the dotted line, through the guide 39, the conveyance roller pair 63, the guide 42, the conveyance roller pair 43, the guide 44, the conveyance roller pair 33, the guide 32, the registration roller 31, and the guide 14. It is supplied to the transfer unit again.

When the image of the second color is formed, the developing device 61 is separated from the photosensitive drum 51 and turned off, and the developing device 62 causes toner to adhere to the electrostatic latent image on the photosensitive drum 51 to form a toner image.

The transfer of the second color onto the paper 19 is performed on the same surface as the first color. After the transfer of the second color is completed, the fixing device 1 fixes the second color, and the flapper 37 is switched to the lower position of the solid line, so that the upper guide 38 and the discharge roller pair 45 are used. , Output tray 46
Discharged on top.

Here, the problem is that, when the toner image of the second color is transferred, if the leading edge of the paper has a downward curl, the leading edge of the paper is caught by the static elimination needle unit 58 and jams the paper. .

In the third embodiment, as shown in FIG. 10, the distance C in the tangential line S direction from the transfer nip N ₁ to the static elimination needle unit 58 is 12 mm, and the distance C between the tangent line S and the static elimination needle unit 58 is the same. The distance D is 5 mm.

In this case, when the curl from the position 12 mm from the front end of the paper to the front end of the paper becomes 5 mm or more, paper jam occurs.

Therefore, in the third embodiment, as shown in FIG.
As shown in FIG. 1, when fixing the first color, the paper leading edge 1
By lowering the temperature of 2 mm and then adjusting the temperature normally, the curl amount of the leading end of the paper 19 after the fixing process of the first color is reduced.

The controlled temperature at the leading edge of the first color will be described.
In the case of the first embodiment, the length is 25 by adjusting the temperature at 160 ° C.
The curl amount was set to 5 mm. This is length 12
When it corresponds to mm, the curl amount is 2.5 mm, which is also applicable to the third embodiment. Therefore, also in the present embodiment, the temperature control temperature of the leading end of the first color is set to 160 ° C., and thereafter the temperature control is controlled to 190 ° C. Further, the temperature control temperature at the time of fixing the second color is set to 190 ° C. to secure the fixability of the second color.

With the above configuration, even in an image forming apparatus capable of performing automatic multiple image formation, paper jam at the transfer portion is eliminated, and stable paper conveyance becomes possible.

In the above description, the case of forming an image of two colors has been described, but the same applies to the case of forming an image of three colors or four colors. That is, in the image formation other than the final image formation (for example, the third color when forming a three-color image or the fourth color when forming a four-color image), the temperature control temperature of the leading end is always 160 ° C.
By setting to 1, it is possible to effectively prevent conveyance failure during image formation each time from the first time to the final time.

Needless to say, the same applies to the case where double-sided image formation and multiple image formation are performed on one recording medium. That is, when image formation is performed a plurality of times on one recording medium,
At the time of fixing this time to the portion on the leading end side in the next image formation, the temperature control temperature may be lowered to reduce the curl of that portion.

In all of the above-mentioned embodiments, the temperature control temperature is explained as 190 ° C. and 160 ° C. However, this is merely an example, and these temperature control temperatures are based on the properties of the toner to be used and the like. , Can be set appropriately.

[0084]

As described above, according to the present invention,
In an image forming apparatus that forms an image a plurality of times on one recording medium, the temperature of the leading end portion of the recording medium, which is the leading end side in the transport direction at the time of the next image formation, is
By controlling the temperature to be lower than the temperature of the other portion excluding the leading end portion, it is possible to reduce the temperature difference between the front and back surfaces of the leading end portion of the recording medium, and to reduce the curl at the leading end portion.

Since this leading end portion is on the leading end side in the carrying direction at the next image forming, it is possible to prevent the carrying defect at the next image forming, and to effectively make the jam such as the paper jam due to the carrying defect. Can be prevented. It

Note that image formation on one recording medium a plurality of times means, for example, double-sided image formation in which images are formed on both front and back surfaces of one recording medium, or a plurality of images is formed on one surface of one recording medium. There is multiple image formation that forms the image of.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus according to a first embodiment.

FIG. 2 is a diagram showing a configuration of a transfer unit according to the first embodiment.

FIG. 3 is a front view showing the configuration of a charge elimination needle.

FIG. 4 is a diagram showing a configuration of a fixing device.

FIG. 5 is a diagram showing a fixing temperature control sequence of a conventional image forming apparatus.

FIG. 6 is a diagram showing a fixing temperature control sequence of the image forming apparatus according to the first embodiment.

FIG. 7 is a diagram showing a relationship between a heater / pressure roller temperature difference and a curl amount.

FIG. 8 is a diagram showing a fixing temperature control sequence of the image forming apparatus according to the second embodiment.

FIG. 9 is a diagram showing a schematic configuration of an image forming apparatus according to a third embodiment.

FIG. 10 is a diagram showing a configuration in the vicinity of a transfer portion of the image forming apparatus according to the third embodiment.

FIG. 11 is a diagram showing a fixing temperature control sequence of the image forming apparatus according to the third embodiment.

FIG. 12 is a diagram showing a configuration near a transfer portion of a conventional image forming apparatus.

[Explanation of symbols]

1 fixing unit (fixing device) 2 Heating member (heater) 5 fixing film 6 pressure roller 7 Temperature control means (CPU) 8 triacs 9 AC power supply 19 Recording medium (paper) 40 Reversing means (reversing roller) 51 image carrier 54 transfer roller 58 Static elimination unit

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Claims (8)

(57) [Claims] 1. A toner image forming portion for forming a toner image on a recording medium conveyed by a conveying means, a fixing portion for fixing the toner image on the recording medium by a heating member, and a temperature of the heating member. Re-feeding to re-supply the recording medium after the first fixing to the temperature control means for controlling and the toner image forming and fixing to form a plurality of image formations on one recording medium a plurality of times. In the image forming apparatus, the fixing unit includes a heat-resistant fixing film and a low heat capacity.
The fixing film is sandwiched between a heating member and the heating member.
And a pressure roller for carrying and carrying the toner image.
A recording medium is placed between the fixing film and the pressure roller.
Fixing device or cylinder that fixes the toner image while being transported by
The temperature control means includes a roller type fixing device having a fixing roller, and the temperature control means controls the temperature at the time of fixing on one recording medium to be the front end side in the conveying direction at the time of the next image formation of the recording medium. The image forming apparatus is characterized in that the temperature of the leading end portion is controlled to be lower than the temperature of other portions excluding the leading end portion. 2. A reversal of a switchback system in which the re-feeding means reverses the recording medium so as to perform the second image formation on the back side of the recording medium on which the first image formation is performed on the front side. The temperature control means controls the temperature of the rear end portion of the recording medium at the time of the first fixing to be lower than the temperature of other portions excluding the rear end portion. The image forming apparatus according to claim 1. 3. When image formation is performed on both the front and back sides of a recording medium, the image forming apparatus has a first designating means for designating a first image formation on the front surface side, and the control means comprises the first designating means. The image forming apparatus according to claim 2, wherein the first fixing is performed based on the designation. 4. The re-feeding unit has a loop-type re-conveying unit for performing the second and subsequent image formations on the same front side of the recording medium on which the first image formation has been performed on the front side. The temperature control unit controls the temperature of the leading end portion of the recording medium at the time of fixing each time except the final time to be lower than the temperature of the other portion excluding the leading end portion. The image forming apparatus described. 5. When a plurality of images are formed on one surface of a recording medium, the image forming apparatus further comprises a second specifying unit for specifying a first image formation for the front side, and the control unit is the second unit. The image forming apparatus according to claim 4, wherein fixing is performed each time except the final time based on the designation by the designation unit. 6. The toner image forming section transfers an image carrier on which a toner image is formed, and a toner image formed on the image carrier to the surface of a recording medium carried by the carrying means. And a transfer means for controlling the transfer means.
The image forming apparatus according to the item. 7. The recording medium after the toner image is transferred from the image carrier is separated from the surface of the image carrier by utilizing the curvature of the surface of the image carrier. To claim 1
The image forming apparatus according to the item. 8. A toner image is formed by the toner image forming unit.
Equipped with a charge elimination needle for eliminating charge on the formed recording medium
A static elimination unit is provided, and any one of claims 1 to 7 is provided.
The image forming apparatus according to the item.