JPH0934180A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the difference of glossiness on images printed on both surfaces of a transfer paper. SOLUTION: In this image forming device capable of performing both-side printing, in which a toner image developed on image carriers 9B, 9Y, 9M, and 9C is transferred on the first surface of the transfer paper 3, then the toner image is fixed by a fixing part 13, and transfer paper 3 where the toner image is fixed is once housed in an intermediate tray 16, and the toner image formed on the image carriers 9B, 9Y, 9M and 9C again is transferred and fixed on the second surface of the paper 3 which is fed again, the surface of the paper 3 which is fed again and on which the image is not transferred is preheated by a pre-heating means 18, so that the difference between temperature which is given to the toner image on the first surface passing through the part 13 twice and the temperature which is given to the toner image on the second surface can be reduced.

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 printing a toner image formed on an image carrier by an electrophotographic method on both sides of a transfer paper.

[0002]

2. Description of the Related Art Conventionally, a toner image formed on an image carrier is transferred onto a first surface of a transfer paper, the transferred image is fixed by a fixing device, and the transfer paper after fixing is reversed and re-fed. Thus, there is an image forming apparatus that transfers and fixes the next toner image on the second surface of the transfer paper.

[0003]

In the case of double-sided printing, the toner image transferred to the first surface of the transfer paper passes through the fixing device once when fixing the toner image, and then the second transfer image is transferred to the transfer paper. When the toner image transferred to the surface is fixed, the toner image also passes through the fixing device. That is, the toner image transferred later on the second surface of the transfer paper is heated only once by the fixing device, whereas the toner image transferred first on the first surface is heated twice by the fixing device. . On the other hand, toner (particularly color toner) increases in gloss level as the heat becomes higher. Therefore, as described above, if the temperature applied varies depending on the number of times the image passes through the fixing device, the glossiness of the images on both surfaces of the transfer paper varies, and a uniform printed image cannot be obtained.

[0004]

According to the first aspect of the present invention,
The toner image developed on the image carrier by the developing unit is transferred to the first surface of the transfer paper by the transfer unit and then fixed by the fixing unit, and the transfer paper after fixing is temporarily stored in the intermediate tray and then by the developing unit. In the image forming apparatus capable of double-sided printing, in which the re-formed toner image is transferred and fixed on the second surface of the transfer paper re-fed by the re-feed path, the image non-transfer side of the transfer paper to be re-fed By preheating the surface of No. 2 by the preheating unit, the difference between the temperature given to the toner image of the first surface and the temperature given to the toner image of the second surface passing twice through the fixing portion is reduced.

According to a second aspect of the invention, in the first aspect of the invention, the preheating means is arranged in the re-feeding path so that the toner image on the image carrier is transferred immediately before the step of transferring the toner image onto the transfer sheet. The non-transferred side of the paper was preheated.

According to a third aspect of the invention, in the invention according to the first or second aspect, between the fixing section and the conveyance direction switching section,
By providing a curl correcting unit having a cooling unit for cooling the image transfer surface of the transfer paper to correct the curl of the transfer paper due to the fixing, the curl of the transfer paper caused by the fixing is corrected by the curl correcting unit. Further, although the toner image on the second surface passes through the fixing section twice, the amount of heat applied from the fixing section for the second time is suppressed to a lower level by the cooling action by the cooling section before that.

[0007]

An embodiment of the present invention will be described with reference to the drawings. First, FIG. 1 shows the overall structure of the image forming apparatus. 1 is a housing. A paper feed section 5 for feeding the transfer papers 3 stacked in the paper feed tray 2 by a paper feed roller 4 is provided on one side of the housing 1, and a paper discharge section is provided on the other side of the housing 1. A certain paper discharge tray 6 is provided. Between the paper feed unit 5 and the paper discharge receiver 6, a separation roller 7 for separating the transfer papers 3 fed by the paper feed roller 4 one by one, and a tip position of the transfer paper 3 is determined by a nip portion. Pair of registration rollers 8
And a developing unit 10 for developing toner images of different colors on the photoconductors 9B, 9Y, 9M and 9C which are image carriers.
B, 10Y, 10M, 10C and the photoconductors 9B, 9Y, 9
Transfer belt 11 rotating along the arrangement direction of M and 9C
Then, transfer portions 12B, 12Y, 12M, 12C for transferring the toner images on the photoconductors 9B, 9Y, 9M, 9C are arranged on the transfer paper 3 conveyed by the transfer belt 11. The photoconductor, the developing unit, and the transfer unit are provided for each color. Typically, the photoconductor is 9, the developing unit is 10, and the transfer unit is 1.
A description will be given using the reference numeral 2.

The developing unit 10, although only schematically shown, charges the outer periphery of the photoreceptor 9 and irradiates the charged portion with laser light to form an electrostatic latent image, and the electrostatic latent image is formed. It is a known one that is visualized with toner.

On the downstream side of the transfer belt 11,
The fixing unit 13, the curl correcting unit 14, and the transport direction switching unit 15 that switches the transport direction of the transfer paper 3 by the rotational movement are arranged. Further, below the belt conveyor 11, there is provided an intermediate tray 16 for reversing the transfer paper 3 guided by the conveyance direction switching unit 15 along an S-shaped path and temporarily storing it. Re-feeding path 17 for guiding 3 to the upstream side of the registration roller 8.
Is provided. The re-feeding path 17 is provided with pre-heating means 18 for pre-heating the untransferred surface of the transfer paper 3 re-fed from the intermediate tray 16 (the second surface which is the lower surface in FIG. 1).

The fixing section 13 is a fixing roller 1 in which the outer circumference of a cylindrical cored bar containing a heater is covered with silicone rubber.
9 and a pressure roller 20 in which the outer circumference of a cylindrical cored bar having a built-in heater is covered with silicone rubber or fluororubber is elastically brought into contact with each other.

As shown in FIG. 2, the curl correcting means 14 has a roller 22 having a surface layer 21 that elastically bends.
And a roller 23 having a high rigidity with which the roller 22 is elastically pressed, and a heat pipe 24 which is a cooling unit pressed against the outer circumference of the roller 22. As shown in FIG. 3, a plurality of fins 26 that are cooled by the air blown by the fan 25 are formed at one end of the heat pipe 24. Either one of the rollers 22 and 23 is driven by a motor (not shown).

Next, the structure of the preheating means 18 will be described with reference to FIGS. 4 and 5. This preheating means 18
In both cases, a heating roller 28 having a built-in heater 27 and a pressure roller 29 are elastically brought into contact with each other, and at least one of the heating roller 28 and the pressure roller 29 is driven by a motor (not shown). . In the example shown in FIG. 4, the heating roller 28 has increased rigidity, and the pressure roller 29 has a surface layer 30 that elastically bends. Further, in the example shown in FIG. 5, instead of using the pressure roller 29 having high rigidity,
This is an example of using the heating roller 27 having the surface layer 31 that elastically bends.

In such a structure, in the double-sided printing mode, the transfer sheet 3 initially fed from the sheet feeding section 5 is conveyed by the registration roller 8 and the transfer belt 11 which rotate in synchronization with the operation of the developing section 10. , Transfer paper 3 during the transportation process
The toner image on the photoconductor 9 is transferred onto the first surface of the sheet by the transfer section 12, and then fixed by the fixing section 13. In this fixing process, the transfer paper 3 is heated and contracts, but due to the difference in contraction rate between the transfer paper 3 and the toner, the transfer paper 3 curls such that the first surface (image surface) is recessed. In particular, in the case of a color image, different toners are overlaid, so that the toner layer becomes thicker, and the influence of shrinkage increases, so that curling easily occurs.

However, the transfer paper 3 that has passed through the fixing unit 13
Is curved in a direction opposite to the curl direction due to the fixing by the rollers 22 and 23 of the curl correcting unit 14, and the heat received by the fixing is applied to the heat pipe 2 via the roller 22.
Since it is robbed by 4, the curl due to fixing is corrected.

The transfer paper 3 that has passed through the curl correcting means 14 is changed in direction by the transfer direction switching portion 15, is transferred through the S-shaped path, and is temporarily stored in the intermediate tray 16. In this state, the transfer side surface transferred to the first surface is as shown in FIG.
In the above, the surface (second surface) on the untransferred side is directed downward. The next toner image is developed on the outer peripheral surface of the photoconductor 9 until the transfer paper 3 on the intermediate tray 16 is preheated by the preheating means 18 and reaches the registration roller 8 via the re-feeding path 17. . The transfer sheet 3 re-fed by the registration roller 8 and the transfer belt 11 has the transfer surface (first surface) on which the image was previously transferred facing downward and the non-transfer side surface (second surface) upward. The toner image on the photoconductor 9 is transferred to the second surface, is fixed by the fixing unit 13 as before, and the curl is corrected by the curl correcting unit 14. By this time, the transport direction switching unit 15
Is rotated in the clockwise direction, the transfer paper 3 after the second printing is discharged to the paper discharge tray 6.

In this case, the glossiness of the printed image is determined by the temperature when it is finally heated by the fixing unit 13. The temperature of the first surface when the first image is transferred to the first surface of the transfer paper 3 and once passes through the fixing unit 13 is t.
1. The glossiness of the image at that time is G1, and the preheating means 1
8. If the temperature of the first surface when passing through the fixing unit 13 twice without the cooling unit 24 is t4 and the glossiness of the image at that time is G4, the first passing through the fixing unit 13 twice While the glossiness of the image on the surface is as high as G4, the fixing unit 13
The glossiness of the image on the second surface that passes only once is as low as G1, and as shown in FIG. 7, the difference ΔG is extremely large.

On the other hand, in the present invention, the toner image transferred to the second surface of the transfer paper 3 passes through the fixing section 13 only once, but before that, the surface on the untransferred side (corresponding to the second surface). ) Is preheated by the preheating means 18, the temperature applied to the toner image on the second surface during fixing can be increased from t1 to t2. The black circle plot shown in FIG. 6 indicates the temperature at t2. The glossiness of the image at this time is G corresponding to t1.
It should be G2 higher than 1. Therefore, the glossiness of the image on the second surface can be made closer to the glossiness of the image on the first surface. This is an effect corresponding to the first aspect of the present invention.

Next, the toner image transferred as the first image on the first surface of the transfer paper 3 is given a temperature of t1 when passing through the fixing section 13 once, and at this time, the glossiness is G.
However, as shown by the dotted line in FIG. 6, the place where the temperature is increased to t4 when the fixing portion 13 is passed for the second time is cooled by the heat pipe 24 of the curl straightening means 14 before that. The temperature at the second passage through the fixing unit 13 is t3, which is lower than t4. As a result, the glossiness of the image finally determined when passing through the fixing unit 13 is G3, which is lower than G4. Therefore, the difference ΔG ′ between the glossiness G3 of the image on the first surface and the glossiness G2 of the image on the second surface can be made much smaller than the conventional difference ΔG. This is an effect corresponding to the third aspect of the present invention.

Further, by disposing the preheating means 18 in the re-feeding path 17, just before the step of transferring the toner image on the image carrier 9 to the transfer paper 3, the surface of the transfer paper 3 on the untransferred side ( The second surface) can be preheated to raise the temperature in the fixing unit 13. This is the effect corresponding to claim 2.

[0020]

According to the first aspect of the invention, the toner image developed on the image bearing member by the developing unit is transferred to the first surface of the transfer paper by the transferring unit and then fixed by the fixing unit. An image forming apparatus capable of double-sided printing, in which the transfer paper is temporarily stored in the intermediate tray, and the toner image re-formed by the developing unit is transferred and fixed on the second surface of the transfer paper re-fed by the re-feeding path. In the above, by preheating the surface on the image non-transfer side of the transfer paper to be re-fed by the preheating means, the temperature given to the toner image on the first surface passing twice through the fixing section, and
The difference from the temperature applied to the toner image on the second surface can be reduced. As a result, the difference in glossiness between the images printed on both sides of the transfer paper can be reduced, and a good-looking print result can be obtained.

According to a second aspect of the present invention, in the first aspect of the invention, by disposing the preheating means in the re-feeding path, immediately before the step of transferring the toner image on the image carrier to the transfer sheet, It is possible to preheat the surface (second surface) on the untransferred side of the transfer paper to raise the temperature in the fixing portion.

According to a third aspect of the invention, in the invention according to the first or second aspect, between the fixing unit and the conveyance direction switching unit,
By providing a curl straightening unit having a cooling unit for cooling the image transfer surface of the transfer paper to correct the curl of the transfer paper due to the fixing, the curl of the transfer paper caused by the fixing can be straightened by the curl straightening unit. it can. Further, the toner image on the second surface passes through the fixing unit twice, but the amount of heat applied from the fixing unit the second time can be suppressed to a lower level by the cooling action by the cooling unit before that. This makes it possible to further reduce the difference in glossiness between the images printed on both sides of the transfer paper.

[Brief description of drawings]

FIG. 1 is a vertical sectional side view showing an overall structure of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a vertical cross-sectional side view showing the structure of the curl correcting means of FIG.

FIG. 3 is a perspective view showing the structure of the curl correcting means of FIG.

4 is a vertical sectional side view showing an example of the preheating means in FIG.

5 is a vertical sectional side view showing another example of the preheating means in FIG. 1. FIG.

FIG. 6 is a graph showing a relationship between a change in temperature applied to a toner image when passing through a fixing unit and a gloss level of the image at that time.

FIG. 7 is a graph showing a relationship between a change in temperature applied to a toner image and a glossiness of the image at that time when the toner image passes through the fixing unit in the conventional configuration.

[Explanation of symbols]

 5 Paper Feeding Section 6 Paper Discharging Section 9B, 9Y, 9M, 9C Image Carriers 10B, 10Y, 10M, 10C Developing Section 12B, 12Y, 12M, 12C Transfer Section 13 Fixing Section 14 Curling Correction Section 15 Conveying Direction Switching Section 16 Intermediate Tray 17 Refeeding path 18 Preheating means 24 Cooling unit

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Yasuhiro Kodaira 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd.

Claims (3)

[Claims] 1. A developing unit for developing a toner image on an image carrier, a transfer unit for transferring the toner image on the image carrier to a transfer sheet fed from a sheet feeder, and a transfer unit for transferring the toner image on the transfer sheet. A heating type fixing unit for fixing the toner image to be transferred, a transfer direction switching unit for selectively guiding the fixed transfer paper to the discharge unit or the intermediate tray, and the transfer paper in the intermediate tray to the transfer unit. And a preheating unit that preheats the surface of the transfer sheet on the image non-transfer side in the process of passing from the transport direction switching unit to the intermediate tray and the refeed path. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the preheating means is arranged in the re-feeding path. 3. A curl correcting means for correcting a curl of the transfer paper due to fixing is provided between the fixing unit and the conveyance direction switching unit, the cooling unit cooling the image transfer surface of the transfer paper. The image forming apparatus according to claim 1 or 2, characterized in that the image forming apparatus comprises: