JP2000315031A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of both-side simultaneous fixing where generation of a toner blister, etc., is surely prevented even when a both-side simultaneous fixing is carried out by a both-side fixing device of a roll nip method utilizing coated paper and this prevention effect is obtained without enlargement of the device with a simple constitution. SOLUTION: In this device, a sheet heating device 5 is provided. In the sheet heating device 5, when coated paper Pc is utilized as recorded sheet P, a pair of heating rolls 50 is provided for heating coated paper Pc once at a heating temperature not lower than that near a lower side of both-side fixing temperature at a both-side fixing device 4 before transfer forming of an unfixed toner image by toner image forming device 2 and 3.

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, a printer or a multifunction machine using an electrophotographic method, and more particularly to an unfixed toner image transferred and formed on both sides of a recording sheet. The present invention relates to an image forming apparatus equipped with a double-side fixing device capable of heating the sheet from both sides of the sheet and fixing the sheet at the same time.

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus employing a new double-sided image forming method, two toner image forming means for forming an unfixed toner image are arranged in a state of being opposed to a recording sheet conveyance path. After each toner image forming means forms an unfixed toner image on both the front and back surfaces of the recording sheet, the recording sheet is passed through one heat fixing device (a pair of heating rolls), and the toner images on both surfaces of the sheet are simultaneously formed. There has been proposed an image forming apparatus capable of performing heat fixing (Japanese Patent Laid-Open No. 4-371969).

An image forming apparatus employing such a double-sided image forming system includes one toner image forming means and one heat fixing device, and separates formation and fixing of a toner image for each side of a recording sheet. This makes it possible to form images on both sides of the recording sheet faster than a general known double-sided image forming method that is performed twice in total, and to provide a compact image forming apparatus without a complicated structure. It becomes possible.

That is, since a series of steps of forming an unfixed toner image and fixing the same is not repeated twice as in the above-described general double-sided image forming system, double-sided image formation can be performed faster. . Further, since it is not necessary to provide a sheet reversing mechanism for reversing the recording sheet on which image formation on one side is completed and then re-sending the recording sheet and re-sending the sheet to the toner image forming means, and a transport path therefor, the structure becomes simpler.
Further, since the fixing process of the unfixed toner image formed on both sides of the sheet only needs to be performed once, the image on one side of the sheet passing through the fixing device twice and the fixing device as in the general double-sided image forming method described above are used. It is possible to avoid that an image on the other side of the sheet that has passed only once may differ in image quality (particularly gloss).

[0005]

However, on the other hand,
In the image forming apparatus of the new double-sided image forming method, the recording sheet is simultaneously heated from both sides by a heating roll pair (roll nip method) in the fixing device for simultaneous double-side fixing. The amount of heat applied to the recording sheet (including the toner image) in the double-sided simultaneous fixing step is greatly increased as compared with the case where the sheet on which the toner image is formed is heated from one side as in the fixing step. As a result, for example, when a color image is formed on both sides of a recording sheet with high image quality, especially as the recording sheet, coated paper (coated paper, art paper, etc.) in which a coating layer is coated on a fiber layer surface of a sheet substrate. Etc.) to form a double-sided image, there are the following problems.

That is, when the coated paper having the toner image formed on both sides is passed through the nip portion of the pair of heating rolls of the fixing device to perform simultaneous double-sided fixing, a portion of the coated paper having a high image density can be obtained. An image defect (toner blister) that appears to be dotted with white particles occurs. This is because the air permeability of the coated paper is lower than that of plain paper (recording paper in which the fiber layer is exposed on the surface) by the presence of the coating layer. Moisture inside the paper (fibrous layer) is less likely to be released to the outside as steam, which increases the pressure inside the coated paper. As a result, the pressure causes water vapor to blow out from the gaps in the coated layer. It is presumed that this is caused by the fact that some of them remain as bubbles in the toner layer constituting the image.

By the way, the applicant of the present invention has provided a pre-heating means for pre-heating the temperature of the coated paper to 60 ° C. or higher when using the coated paper as a fixing device for performing single-side fixing. A fixing device has already been proposed (Japanese Unexamined Patent Application Publication No.
-3255631).

However, the fixing device provided with the preheating means is intended to prevent the occurrence of toner blisters when performing so-called single-sided fixing, in which the unfixed toner image formed on one side of the coated paper is thermally fixed. It is not intended to prevent the occurrence of toner blisters when performing so-called simultaneous double-sided fixing, in which the toner images formed on both sides of the coated paper are thermally fixed at the same time.

For this reason, as shown in Example 5 and the like described in the above-mentioned publication, a non-contact type preheating device
After preheating in the temperature range of 0 to 90 ° C.,
When single-side fixing is performed by a single-side fixing device, which is a heating roll heated to 0 ° C. (fixing temperature for coated paper), air and moisture in the coated paper are released in the preheating step. Although it is possible to prevent the occurrence of toner blisters and the like on the coated paper after fixing, such preheated coated paper is passed, for example, through the nip portion of a heated roll pair heated to about 140 to 160 ° C. When the double-sided simultaneous fixing is performed, a larger amount of heat is applied to the coated paper in the double-sided fixing process than in the preheating described above, so that new steam is generated from inside the coated paper in the double-sided fixing process. Toner blisters and the like. In this case, measures to raise the temperature at the time of the preheating may be considered, but another problem called a so-called paper blister occurs in which the surface of the coated paper is deformed into a blister depending on a set level such as the heating temperature. May be done.

In addition, before the transfer of the paper web (roll paper) used in the electrostatographic printer to the electrophotographic printer, the paper web (roll paper) touches the surface of a rotating heating drum or a cylinder. (Patent Document 1) Japanese Patent Application Laid-Open No. 7-72769 proposes a heat treatment apparatus in which the water content in the paper is reduced so that the heat treatment is performed, and the heat treatment is performed. ). According to this refining device, a high yield of toner transfer can be obtained in the electrophotographic printing machine after refining and cooling.

However, this refining apparatus is merely intended for refining a paper web to a state suitable for transfer, and focuses on preventing occurrence of toner blisters and the like in simultaneous double-side fixing. Since it is not a countermeasure, there are the following problems. First, since it is mainly intended for paper webs, it is inferior in compatibility with recording sheets (so-called cut sheets) cut to a predetermined size from the beginning.
In addition, when the present invention is applied to an image forming apparatus having a high process speed, it may be difficult to accurately refin the cut recording sheet. Further, since the heating means and the cooling means are provided side by side, it is inevitable that the apparatus becomes large, and the cut recording sheet may be used,
It is not suitable for an image forming apparatus that places importance on miniaturization of the apparatus.

Accordingly, the present invention has been made in view of the above circumstances, and in an image forming apparatus capable of simultaneous double-sided fixing, in particular, a double-sided simultaneous fixing using a coated paper as a recording sheet. It is an object of the present invention to provide an image forming apparatus capable of reliably preventing the occurrence of toner blisters and the like when the image forming is performed, and obtaining the effect of the prevention with a simple configuration without increasing the size of the apparatus. And

[0013]

According to a first aspect of the present invention, there is provided a toner image forming apparatus capable of transferring and forming an unfixed toner image on both sides of a recording sheet, and a toner image forming apparatus. The recording sheet on which the unfixed toner image is transferred is passed through the nip portion of a pair of heating rolls that rotate under pressure and heated from both sides of the sheet at a predetermined fixing temperature and time to form each unfixed toner image on both sides of the sheet. Assuming that the image forming apparatus includes a double-sided fixing device capable of fixing simultaneously, when using a coated paper as the recording sheet, the coated paper is unfixed by the toner image forming apparatus. An image forming apparatus provided with a sheet heating device for heating once at a heating temperature equal to or higher than a temperature near the lower side of the double-sided fixing temperature in the double-sided fixing device before the transfer formation is performed.

The temperature near the lower side of the double-side fixing temperature, which is the heating temperature of the sheet heating device, is preferably a temperature lower by about 10 ° C. than the double-side fixing temperature. The heating temperature is more preferably a temperature equal to or higher than the double-sided fixing temperature, more preferably about 30 ° C. higher than the double-sided fixing temperature, from the viewpoint that toner blisters and the like can be more reliably prevented. On the other hand, the upper limit of the heating temperature is appropriately set, but can be set based on, for example, a temperature level within a range where paper blister does not occur on the coated paper due to heating of the sheet heating device. Also,
The heating time of the sheet heating device can be set to be shorter than the fixing time at the time of double-sided fixing, and in particular, can be set to be shorter as the heating temperature is set higher.

The sheet heating device of the image forming apparatus according to the first aspect of the present invention is not particularly limited as long as it can heat coated paper satisfying at least the above-mentioned heating conditions (particularly temperature). Although it is possible to use a non-contact type heating device capable of heating without directly touching the coated paper, it is preferable to provide a pair of press roll rotating heating rolls, An apparatus is used in which the coated paper is passed through the nip portion once and heated and pressed at the heating temperature.

According to a second aspect of the present invention, which can achieve the above-mentioned object, there is provided an image forming apparatus based on the first aspect.
When a coated paper is used as the recording sheet, the coated paper is subjected to the lower limit of the double-sided fixing temperature and time in the double-sided fixing device before the transfer formation of the unfixed toner image by the toner image forming apparatus is performed. This is an image forming apparatus provided with a sheet heating device that performs heating a plurality of times at a heating temperature and time that are higher than the temperature and time that are close to the sheet heating device.

The temperature near the lower side of the both-side fixing temperature, which is the heating temperature of the sheet heating device, is 10 degrees lower than the both-side fixing temperature.
It is preferable that the temperature is lower by about ° C., and that the time near the lower side of the double-sided fixing time, which is the heating time, is about 20 msec shorter than the double-sided fixing time. On the other hand, the upper limit of the heating temperature is the same as the case of the heating temperature in the first invention described above, and
The upper limit of the heating time is also set as appropriate,
For example, it can be set based on a temperature level within a range where paper blisters do not occur on the coated paper due to heating of the sheet heating device.

The heating by the sheet heating device is performed in a plurality of times, and the heating temperature and time in each heating step are set to the same conditions within the above-mentioned heating conditions. Or different conditions may be set for each heating step, but from the viewpoint of more reliably preventing paper blisters and the like, the heating temperature is determined by the recording sheet (actually, coated paper). It is preferable to set the temperature so that the temperature gradually increases as the recording sheet advances, and the heating time increases gradually as the recording sheet advances in the traveling direction.

The sheet heating apparatus of the image forming apparatus according to the second aspect of the present invention is not particularly limited as long as it can heat coated paper satisfying at least the above-mentioned heating conditions (temperature and time). For example, it is possible to use a non-contact type heating device capable of heating without directly touching the coated paper, but it is preferable to provide a plurality of pairs of press roll rotating heating rolls, and each heating roll thereof. An apparatus for applying pressure and heating by sequentially passing coated paper through a pair of nip portions is used.

In each of the sheet heating apparatuses according to the first and second aspects of the present invention, the discriminating means for discriminating whether or not the recording sheet is a coated paper is used to determine whether or not the recording sheet is a coated paper. If there is, it is controlled by a control means appropriately set to operate under the above-described heating conditions for a predetermined time.

Further, the double-sided fixing temperature and time in each of the above sheet heating devices are determined based on the heating temperature and time which are within a range capable of satisfying the fixing strength required for double-sided fixing. It is. The fixing strength in this case is determined based on a predetermined test or evaluation method that can be adopted in a technical field related to electrophotographic image formation. Examples of the test or evaluation method for determining the fixing strength include a bending (Crease) test method, a tape peeling evaluation method, and a rubbing test method.

The bending test method is such that a recording sheet on which a fixed toner image of a fixed density and area is formed is folded under a constant load, and the area of the image portion where the toner has peeled off at the fold portion at that time. Is measured, and the fixing strength is determined based on the ratio of the area. The tape peeling evaluation was performed by attaching the tape to the fixed toner image on the recording sheet and pressing it with a certain force, peeling off the tape, measuring the area of the portion where the toner was removed at that time, and calculating the ratio of the area. This is to determine the fixing strength. In the rubbing test method, a blank sheet is rubbed with a fixed force on a fixed toner image on a recording sheet, and the area of the portion where the toner is removed at that time is measured, and the fixing strength is determined based on the ratio of the area. is there. And each of these methods
The larger the measured area, the lower the fixing strength is.It is an index that indicates that the sample with different peeling conditions of the toner is numerically defined in advance as a limit sample and compared with the limit sample. Thus, the actual degree of peeling of the evaluation sample can be quantified or confirmed. Incidentally, any other method for evaluating the fixing strength may be used, but it is preferable to use a method having a strong correlation between the toner and the adhesive force on the adhesive surface of the recording sheet.

According to the first and second image forming apparatuses, when coated paper is used as a recording sheet,
Prior to the duplex fixing, the coated paper is heated in advance by each sheet heating device at a level substantially equivalent to the amount of heat applied at the time of the duplex fixing (or a level higher than that), whereby moisture contained in the paper is reduced. It evaporates and decreases under almost the same conditions as during double-sided fixing. The coated paper thus heated by the sheet heating device is heated under the same heating conditions as the heating conditions of the sheet heating device during the subsequent double-side fixing, but the coated paper is heated during the double-side fixing. The water does not evaporate again from the inside. As a result, toner blisters (including paper blisters) are not generated after both-side fixing.

[0024]

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

[First Embodiment] FIG. 1 is a schematic diagram showing an embodiment of a color image forming apparatus capable of forming a double-sided image to which the present invention is applied. This image forming apparatus has a main body 1
0, a recording sheet storage unit 1 capable of storing various recording sheets P in a plurality of trays 12 in a distributed manner, and one side (first surface) of the recording sheet P fed from the recording sheet storage unit 1. First color image forming system 2 for forming first toner image
And a second color image forming system 3 for forming a second toner image on the other surface (second surface) of the sheet, and a first surface formed on both surfaces (first and second surfaces) of the recording sheet P And a double-sided fixing device 4 capable of simultaneously thermally fixing the unfixed toner image on the second side, and furthermore, a recording sheet is provided between the recording sheet accommodating section 1 and the first color image forming system 2. A sheet heating device 5 for heating P under predetermined conditions is provided. A dashed line in the drawing is a conveyance path of the recording sheet P in the main body 1, and this conveyance path is configured by a conveyance roll pair, a conveyance guide, and the like so that the recording sheet can be conveyed at a predetermined conveyance speed. .

The color image forming systems 2, 3
Are arranged on different sides of the conveyance path of the recording sheet P, and all of them are full-color images (yellow: Y, magenta; M, cyan: C, and black: B, four color toners). (A color toner image formed by superimposing images) on the recording sheet P. That is, the photosensitive drums 20 and 30 rotating in the direction of the arrow are uniformly charged by the chargers 21 and 31, and the charged drum surfaces are irradiated with light images 22 and 32 corresponding to image information from an image exposure device (not shown). After the electrostatic latent image is formed, the electrostatic latent image is
The first and third color toner images are formed by the developing device that contains the toner of the first color among the above-described four colors by 3 and 33, and the image exposure and development process is performed by the remaining color toner image components. By repeating in the same manner, a multi-toner image formed so as to be superimposed on the photosensitive drums 10 and 30 is formed. Then, the unfixed multi-toner image is electrostatically transferred to the recording sheet P being conveyed at each transfer position where the photosensitive drums 10 and 30 and the transfer devices 24 and 34 face each other. I have. After this transfer is completed, the photosensitive drums 10, 30
Is cleaned by the cleaning members 25 and 35 so that the next image forming operation can be started.

Further, the double-sided fixing device 4 includes a heating roll pair 40 provided in a device main body (not shown) so as to be rotationally driven while two heating rolls 41 and 42 are pressed against each other. The recording sheet P is passed through a nip portion (pressure contact portion) 40 for heat fixing. Heating rolls 41 and 42 constituting the heating roll pair 40
Is formed by forming a heat-resistant rubber layer on the surface of a metal cylindrical roll, a heater 43 is built in the cylindrical roll, and a load is applied to both ends of the cylindrical roll by a pressing mechanism (not shown). . And both heating rolls 4
Reference numerals 1 and 42 control the supply of electric power to the heater 43 based on detection information from a temperature detection sensor 44 for detecting the temperature of the roll surface by a temperature controller (not shown) so that the heater 43 is uniformly heated to a predetermined surface temperature. It has become. Further, the nip portion is set to have a desired nip width by adjusting the load. Further, the heating rolls 41, 4
Reference numeral 2 denotes an oil supply device 45 in which oil for imparting toner releasability is applied, and a cleaning pad 46 removes residual toner on the roll surface.

Further, the sheet heating device 5 includes a pair of heating rolls 50 provided in an apparatus main body (not shown) so as to be rotationally driven while two heating rolls 51 and 52 are pressed against each other.
The recording sheet P is simply conveyed or heated by passing the recording sheet P through the nip portion of the heating roll pair 50. Therefore, the heating roll pair 50 also functions as a transport roll pair for the recording sheet P. The heating rolls 51 and 52 constituting the heating roll pair 50 are the heating roll pair 40 of the double-sided fixing device 4.
And a heat-resistant rubber layer formed on the surface of a metal cylindrical roll, and a heater 5 inside the cylindrical roll.
3, and a load is applied to both ends of the cylindrical roll by a pressing mechanism (not shown). In addition, both the heating rolls 51 and 52 are connected to the heater 53 by the controller 55 in the same manner as the heating roll pair 40 of the double-sided fixing device 4 at the time of image formation in which the recording sheet P is recognized as “coated paper”. The supply of electric power is controlled so as to be heated and maintained at a predetermined surface temperature. Similarly, the nip portion is set to have a desired nip width by adjusting the load. The controller 55 is set so as to control the rotation operation of the heating roll pair 50 as well.

The sheet heating device 5 has a heating temperature and a heating time (this time is equivalent to a value obtained by dividing a nip width of a nip portion by a conveying speed of a recording sheet). It is set appropriately within a range that satisfies. The heating temperature is set by incorporating it into the control program for controlling the heating operation of the controller 55 as heating temperature setting information (supply voltage and the like). One heating time is set by adjusting the load to adjust the nip width. (In this case, the heating time may be set by adjusting the conveyance speed of the recording sheet P together).

In addition, the image forming apparatus is provided with an input switch for inputting an operation when the recording sheet P is coated paper, for example, on its operation panel. When used, the user operates the input switch. The information of the input switch is also transmitted to the controller 55 of the sheet heating device 5.

When a coated paper is used as the recording sheet P, double-sided image formation by this image forming apparatus is performed as follows.

First, information that the recording sheet P to be used is "coated paper" is transmitted to the controller 55 of the sheet heating device 5, and the heating device 5 is brought into a heated state. That is, the heating roll pair 50 is heated to a predetermined heating temperature and starts rotating at a predetermined speed. Then, the coated paper Pc stored in the sheet storage unit 2 is
After being sent out of the storage section 2 at a predetermined timing, the sheet is conveyed so as to pass through the heating roll pair 50 of the sheet heating device 5 in a heated state. Thus, the coated paper Pc is heated and pressed from both sides of the sheet by the heating roll pair 50.

Subsequently, the coated paper Pc that has been heated by passing through the sheet heating device 5 is supplied to the first color image forming system 2.
And the second color image forming system 3 are sequentially conveyed, so that each of the image forming systems 2,
3, the first and second color toner images are transferred and formed on both the front and back surfaces. Next, the coated paper Pc on which the color toner image is formed on both sides is conveyed so as to pass through the heating roll pair 40 of the double-sided fixing device 4. As a result, the heating roll pair 40 heats and presses from both sides of the sheet, so that the unfixed color toner images on both sides of the coated paper Pc are simultaneously thermally fixed. Thus, an image is efficiently formed on both sides of the coated paper Pc. When such double-sided image formation is performed on coated paper, toner blisters and the like as in the case of the related art do not occur as described later.

On the other hand, this image forming apparatus uses a recording sheet P
When performing double-sided image formation using a material other than coated paper (plain paper), the sheet heating device 5 does not enter the heating state as described above (the heating is performed only by rotating the heating roll pair 50). However, it is different in that it functions as a mere transport roll, but otherwise performs the same double-sided image forming operation as in the case of using the above-described coated paper. Therefore, in this case, the recording sheet P is conveyed so as to sequentially pass through the image forming systems 2 and 3 without being heated by the sheet heating device 5, and an unfixed toner image is formed on both surfaces of the sheet. Thereafter, the sheet is conveyed so as to pass through the duplex fixing device 4. Further, the image forming / forming apparatus can perform general single-sided image forming. In this case, one of the image forming systems 2 and 3 (usually the first image forming system 2) operates, and the sheet of the heating roll pair 50 of the duplex fixing device 4 on which the toner image is formed is formed. Heating roll 51 on the side facing the surface
Or 52 is in a heated state. Even in the case of performing the one-sided image formation, when using the coated paper as the recording sheet P, the sheet heating device 5 is set to the heating state, and the coated paper is heated before the transfer of the toner image.

Next, the setting contents of the heating temperature and time of the sheet heating device 5 in the color image forming apparatus and the effect of preventing the toner blister and the like by the sheet heating device 5 will be specifically described.

First, FIG. 2 shows the fixing strength required for double-sided fixing of the double-sided fixing device 4. The relationship between the fixing strength and the setting conditions of the fixing temperature and time in the fixing device 4 is determined by a bending test method. This is shown based on the evaluation results. The test conditions at this time are as follows. The outer diameter of the heating rolls 41 and 42 of the fixing device 4 is 50 m
mφ, the thickness of the heat-resistant rubber layer is 3 mm (rubber hardness according to JIS-A 30 °), and the nip width is 8 mm (total load:
120 kg · f). Plain paper was used as the recording sheet P, and a full-color image was formed as a sample image on the plain paper.

Regarding the bending test method, a test was performed in the same manner as described above for each sample when the conditions of the fixing temperature and time were variously changed, and the area of the toner peeling portion at that time was set to “0, 10, 20, ..., 5
0, 60, 70, 80,..., 160 ". The evaluation result at this time is an evaluation level for each set condition of the fixing temperature and time (this corresponds to “fixing strength”, and a smaller value indicates that the fixing strength is stronger and better). The values are plotted in the figure. Note that “160” in FIG.
<> Indicates that the level is greater than 160. In FIG. 2, the fixing (temperature and time) conditions when the evaluation level is "10" or "80" are shown by connecting them with dotted lines. Among them, the evaluation level “80” is the maximum allowable value.

Therefore, as is apparent from the results shown in FIG. 2, in this image forming apparatus, the temperature and time are conditionally set so as to be a combination of the fixing temperature and time above the dotted line indicating the evaluation level 80. In this case, sufficient fixing strength can be obtained. Incidentally, the same fixing strength as shown by the dotted line in the figure is not always obtained constantly even if other conditions such as the recording sheet, toner, environment, etc. are changed, but other conditions may not be obtained. In the case of the same, if the fixing temperature and the time are the same, it is always possible to obtain a constant.

FIG. 3 shows the test results of the heating temperature and time of the sheet heating device 5 and the effect of preventing the occurrence of toner blisters after double-sided fixing.

In this test, the sheet heating device 5
Heating temperature and time for black circles (including white circles)
Are set to various conditions consisting of a combination of temperature and time, and the heating temperature and time of the double-sided fixing device 4 are set to the respective conditions (140 ° C. and 120 m
sec, 150 ° C and 100msec, 160 ° C and 80m
sec). Then, a coated paper Pc (coated paper: Colotech, trade name: 135 gsm, air permeability: 7000 s) was used as a recording sheet, and the coated paper Pc was heated by a sheet heating device 5 set to each condition, and then any one of them was used. The same test color toner image is formed on both sides of each coated paper Pc by each of the image forming systems 2 and 3. The double-side fixing was performed by the set double-side fixing device 4. The occurrence of toner blisters in each coated paper Pc thus obtained was examined, and the results were evaluated according to the following criteria (G0 to G5). The allowable level of this evaluation is G2 or lower. Further, a test in the case where heating by the sheet heating device 4 is not performed is also performed, and the evaluation result at that time is shown in a square frame at the tip of a dotted arrow drawn from each fixing condition indicated by a white circle. G0: No blister is generated. G1: There are one or two blisters confirmed by observation under magnification (× 50). G2: There are several blisters confirmed by magnification observation, and a few blisters confirmed by visual observation. G3: Blisters of the size confirmed by visual observation are present in some places. G4: Blisters of the size confirmed by visual observation are present on the entire surface. G5: Large blisters confirmed by visual observation are present on the entire surface.

FIG. 3 shows the fixing strength required at the time of double-side fixing of the coated paper Pc used in this test by the bending test method as described above, and shows the number of evaluation levels obtained at that time. Examples are shown by dotted lines AD. Among these, the dotted line A is obtained by connecting the combined conditions of the heating temperature and the time at the time of double-sided fixing indicating “evaluation level = 80” which is the minimum necessary fixing strength. Therefore, it is shown that the fixing temperature and the time in the double-sided fixing device 4 need to be set at least based on the combination of the heating time and the time in the upper right area of the dotted line A. Dotted lines B to D indicate the heating temperature and time during double-sided fixing at 140 ° C. and 120 msec, respectively.
50 ° C and 100msec, or 160 ° C and 100msec
This is based on the assumption of a combination condition of the heating temperature and the time which has an evaluation level equivalent to the fixing strength realized in the case of c.

According to FIG. 3, for example, 14
When the coated paper Pc is heated under the heating conditions of 0 ° C. and 80 msec, and the duplex fixing is performed under the fixing conditions of 150 ° C. and 100 msec, the toner blister level is “G3”, and the heating conditions of 160 ° C. and 120 msec. When the double-sided fixing is performed under the fixing conditions of 150 ° C. and 100 msec, the toner blister level is “G2”. After the heating under the heating conditions of 150 ° C. and 180 msec, the fixing condition of 140 ° C. and 120 msec is performed. It can be seen that the toner blister level is "G1" when both-side fixing is performed. Further, when heating is performed under heating conditions consisting of the same heating time and time as the fixing temperature and time under each fixing condition (both heating and fixing are performed under the conditions of white circles, and an arrow in a state of making a full circle in FIG. 3). It is also understood that each of them is “G2”. Further, without heating the coated paper Pc by the sheet heating device 5,
Thereafter, when double-sided fixing is performed under the fixing conditions of 150 ° C. and 100 msec, the toner blister level is found to be “G4”.

Therefore, as is apparent from the results shown in FIG. 3, in this image forming apparatus, it is possible to obtain the effect of suppressing the generation of toner blisters by heating the coated paper Pc by the sheet heating device 5. . It can be seen that the effect of improving the toner blister level is obtained when the heating temperature of the sheet heating device 5 is at least 10 ° C. lower than the fixing temperature, and the allowable level “ It can be seen that the effect of "G2 or less" is reliably obtained when the fixing temperature and time are equal to or higher than the fixing temperature and time. For example, 140 ° C.
When the heating is performed under the condition of 120 msec, the heating condition is set to 140
° C and 120 msec, the toner blister level is “G2”, whereas the heating condition is 150 ° C.
When the temperature is increased to 180 msec, the toner blister level becomes "G1" and the effect is improved.

For example, the heating conditions are 140 ° C. and 120 ms.
Specifically, the sheet heating device 5 having an outer diameter of 50 mmφ and a heat-resistant rubber layer having a thickness of 3 mm (JIS-
A heating roll pair 50 composed of heating rolls 51 and 52 having a rubber hardness of 30 ° according to A) is used.
Heat to 0 ° C and total load on both ends of the roll: 130k
A nip width of 9.6 mm with a load of g · f was used. In this case, the conveyance speed of the recording sheet (coated paper) in the sheet heating device 5 and the duplex fixing device 4 was set to 80 mm / sec. In this case, the fixing device 4 having the configuration used in the above-described bending test was used.

[Second Embodiment] FIG. 4 is a schematic diagram showing another embodiment of a color image forming apparatus to which the present invention is applied. The color image forming apparatus according to the present embodiment is the same as that of the first embodiment except that a sheet heating apparatus 500 including two pairs of heating rolls 60 and 70 is provided instead of the sheet 5 in the first embodiment. Has the same configuration as the image forming apparatus according to the first embodiment.

That is, in the sheet heating apparatus 500 according to the present embodiment, the heating roll pairs 60 and 70 are arranged so as to be pressed and rotated before and after the conveyance path (dashed line) of the recording sheet P. , 70 are constituted by heating rolls 61, 62 and heating rolls 71, 72 having substantially the same structure as the heating rolls 51, 52 in the first embodiment, and other than the heating conditions (temperature and time). The configurations are almost the same except for the setting conditions. Then, similarly to the sheet heating device 5 in the first embodiment, the sheet heating device 500 is in a heated state when the coated paper Pc is used as the recording sheet P, and the coated paper is heated to each of the heating roll pairs 60 and 70. , The recording sheet is heated twice, and when it is other recording sheets, it simply functions as a sheet conveying roll.

Next, the setting contents of the heating temperature and time of the sheet heating device 500 in this color image forming apparatus,
The effect of preventing the toner blister and the like by the sheet heating device 500 will be specifically described.

First, FIGS. 5 and 6 show the relationship between the heating by the sheet heating device (5, 500) and the occurrence of paper blisters on the coated paper. Among them, FIG.
As coated paper Pc, coated paper A (1
35 gsm) and a coated paper B having an air permeability of 14,000, and both coated papers A and B are heated by a sheet heating device (one set of heating rolls) in which the heating temperature and time are variously set. Of paper blisters (P / B) on coated papers A and B
It is a thing which investigated. On the other hand, FIG. 6 shows the results of examining the occurrence of paper blisters on the coated paper A when the coated paper A was heated once or twice at the heating temperature and time shown in the table.

As is clear from the results shown in FIGS. 5 and 6, in any case of the coated papers A and B, the paper blister is increased by increasing the heating temperature and time by a sheet heating device by a certain value or more. It can be seen that the occurrence of cracks easily occurs (FIG. 5). Here, when the boundary of paper blister generation for coated paper A in FIG. 5 is described as a solid line E in FIG. 3, the respective heating conditions and time combination conditions in the sheet heating device 5 are all It can be seen that the condition is such that paper blister does not occur in the area inside the boundary line E (lower left area).

In addition, even if the heating temperature and time are combined such that paper blisters are generated only by one heating, the heating temperature is not changed for each one and only the heating time is slightly shortened. It can be seen that paper heating does not generate paper blisters when heating is performed twice (see FIG. 6). Moreover, at this time, it can be seen that paper blisters are not generated even when heating is performed twice while changing both the heating temperature and time for each batch (FIG. 6). Incidentally, when the paper blister is generated, a transfer failure of the toner image in the image forming systems 2 and 3 occurs before the generation of the toner blister, and the image quality is deteriorated. Also, 2
If the second heating temperature is set higher than the first heating temperature, or if the second heating time is set longer than the first heating time, both the first and second heating temperatures are the same. It can be seen that the occurrence of paper blisters can be more reliably prevented as compared with the case where the time is set to the time and the time.

FIG. 7 shows the test results of the heating temperature and time of the sheet heating device 500 and the effect of preventing the occurrence of toner blisters after double-sided fixing.

In this test, the sheet heating device 5
The heating temperature and time and the number of times of heating are set to various conditions shown in the table of FIG. 7, and the heating temperature and time of the double-sided fixing device 4 are set to 160 ° C. and 80 msec.
Set to. The coated paper A is used as the coated paper Pc.
And the same test as the toner blister generation state test in Embodiment 1 was performed, and the results were evaluated based on the same criteria (G0 to G5). In FIG. 7, “○” indicates that toner blister has not occurred, and “X” indicates that toner blister has occurred.

According to FIG. 7, for example, 19
When the coated paper Pc (coated paper A) is heated once at 0 ° C. and 80 msec, and then double-sided fixed at 160 ° C. and 80 msec, paper blisters are generated at the heating stage. (See FIGS. 5 and 6)
The toner blisters were slightly generated (however, the generation level was close to G2). On the other hand, the heating temperature was not changed and the heating was performed twice at a heating condition of 190 ° C. and 40 msec. It can be seen that when both-side fixing is performed under the fixing condition, no paper blister is generated (see FIG. 5), and no toner blister is generated, and the generation level is "G2". 180 ° C,
When both sides are fixed under the above-mentioned fixing condition after heating once under the heating condition of 60 msec, neither the paper blister nor the toner blister level occurs (see FIGS. 3 and 7), but the heating condition is 180 ° C. and 30 msec. When heated twice, the same toner blister level is also "G2"
Is obtained. Further, when performing the heating twice, if the second heating temperature or time is set to be higher or longer than that of the first heating, the generation of toner blister is reliably prevented (G1). ).

In the case where the heating condition at the same temperature and time shown in FIG. 7 is repeated twice, when the same heating condition as the heating condition is repeated three or more times, compared with the case of heating twice, It was confirmed that a good toner blister generation preventing effect was obtained. Further, in the case of the heating conditions marked with “*” in FIG. 7, when additional heating was performed as the third heating under the heating conditions of 150 ° C. and 40 msec, a good toner blister generation preventing effect (G2) was obtained. It was confirmed that it could be obtained.

Further, as illustrated in FIG. 6, compared with the case where heating is performed twice continuously under the same heating conditions, the heating is performed two times more than the first time.
It was also confirmed that when the first heating temperature was set higher or when the second heating time was set longer than the first heating temperature, it was possible to more reliably avoid the occurrence of paper blisters. .

Accordingly, as is apparent from the results shown in FIG. 7, in this image forming apparatus, the generation of toner blisters can be suppressed by heating the coated paper Pc twice or more by the sheet heating device 500. The effect is obtained,
In particular, in this case, it can be seen that the effect of reliably preventing the generation of paper blisters is also obtained. When the heating is performed in a plurality of times, the heating conditions (conditions for each heating step) are more than those in the case of heating once.
It is also possible to secure a wider settable range.
The effect of improving the toner blister level is obtained because the heating temperature of the sheet heating device 500 is at least 10 ° C. lower than the fixing temperature, and the heating time is at least shorter than 20 msec than the fixing time. It can be seen that the time is longer than the time. In addition, it can be seen that the effect that the allowable level of “G2 or less” is reliably obtained when the fixing temperature and the time are equal to or more than the fixing temperature.

For example, as the sheet heating apparatus 500 that performs heating twice at 140 ° C. and 120 msec, specifically, the outer diameter is 40 mmφ, and the thickness of the heat-resistant rubber layer is 2 mm.
A heating roll 61 (rubber hardness 30 ° according to JIS-A),
A pair of heating rolls 62, 71, 72 was used, and the roll surface was heated to 160 ° C. and a total load of 100 kg · f was applied to both ends of the roll to set the nip width to 4.8 mm. One used. In this case, the conveying speed of the recording sheet (coated paper) in the sheet heating device 5 and the duplex fixing device 4 is 120 mm / sec.
(This is 1.5 times the transport speed in the first embodiment.
Double the speed). In this case, the fixing device 4 having the configuration used in the above-described bending test was used.

[Other Embodiments] FIG. 8 shows the effect of preventing the occurrence of toner blisters according to the distance from the coated paper when heated by the sheet heating device. The test at this time uses the image forming apparatus according to the first embodiment,
When the heating rolls 51 and 52 of the sheet heating device 5 are brought into pressure contact with each other (by forming a nip portion) and brought into close contact with the coated paper that passes during fixing (when the separation distance is zero), Are separated from each other by a distance as shown in FIG. 8 from the coated paper passing during fixing, and a test similar to the toner blister generation status test in the first embodiment is performed. The results were evaluated according to the same criteria (G0 to G5). When fixing is performed in a state of being separated, the coated paper is heated rolls 51, 5
The coated paper is conveyed between the rolls in such a manner that the coated paper is guided using a conveyance guide member or the like so as not to touch any of the two.

As is apparent from the results shown in FIG.
It was found that when heating was performed in a state of being separated from the coated paper, the effect of preventing the generation of toner blisters could not be obtained irrespective of the heating temperature. Therefore,
In order to surely and quickly obtain the effect of preventing the occurrence of toner blisters and the like, it can be seen that contact-type heating in which heating is performed in contact with coated paper is extremely advantageous. In addition, the sheet heating apparatus does not only perform heating in a state of being in contact with coated paper, but also employs a roll nip type double-sided fixing apparatus 4.
From the point of approaching the state at the time of double-sided fixing in
It is more desirable to heat under pressure. This makes it possible to more reliably prevent toner blisters and the like.

[0060]

As described above, according to the image forming apparatus of the present invention, at the time of forming a double-sided image using coated paper, the coated paper is subjected to a specific heating by the sheet heating device before the transfer of the toner image. Since the heating is performed under the conditions, even if the coated paper is subjected to simultaneous double-sided fixing by a roll nip type double-sided fixing device after forming the toner image, generation of toner blisters and the like is reliably prevented. As a result, it is possible to form a favorable double-sided image with little deterioration in image quality. Further, since the sheet heating device can be constituted by, for example, a pair of heating rolls, the above-described prevention effect can be obtained with a simple configuration without increasing the size of the device.

[Brief description of the drawings]

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

FIG. 2 is a graph showing the relationship (test results) between the fixing temperature and time and the fixing strength during double-sided fixing.

FIG. 3 is a graph showing test results of a heating temperature and time of a sheet heating device and an effect of preventing a toner blister.

FIG. 4 is a schematic diagram illustrating an image forming apparatus according to a second embodiment.

FIG. 5 is a graph showing test results of the heating temperature and time of the sheet heating device and the effect of preventing paper blister.

FIG. 6 is a table showing test results of the heating temperature and time of the sheet heating device, the number of heating times thereof, and the paper blister prevention effect.

FIG. 7 is a table showing test results of the heating temperature and time of the sheet heating device, the number of times of heating, and the toner blister prevention effect.

FIG. 8 is a graph showing a test result of a heating distance of a sheet heating device to coated paper and an effect of preventing toner blister.

[Description of Symbols] 2 ... First toner image forming system (toner image forming apparatus), 3 ... Second toner image forming system (toner image forming apparatus), 4 ... Double-sided fixing apparatus, 5, 500 ... Sheet heating apparatus , 40 ... heating roll pair, 50 ... one heating roll pair, 60, 70 ... two heating roll pairs, P ... recording sheet, Pc ... coated paper.

Claims (4)

[Claims] A toner image forming apparatus capable of transferring and forming an unfixed toner image on both sides of a recording sheet;
The recording sheet on which the unfixed toner image is transferred and formed by the toner image forming apparatus is passed through a nip portion of a pair of heating rolls which rotate in a pressure-contact manner, and is heated from both sides of the sheet at a predetermined fixing temperature and a predetermined time so that each of the two sides is heated. An image forming apparatus including a double-sided fixing device capable of simultaneously fixing an unfixed toner image, wherein when using a coated paper as the recording sheet, the coated paper is undetermined by the toner image forming apparatus. An image forming apparatus comprising: a sheet heating device for heating once at a heating temperature not lower than a temperature near a lower side of a double-sided fixing temperature in the double-sided fixing device before transfer formation of a toner image is performed. . 2. The sheet heating apparatus according to claim 1, wherein the sheet heating apparatus includes a pair of rotating heating rolls, and the coated paper is passed once through a nip portion of the pair of heating rolls to heat and press at the heating temperature. Item 2. The image forming apparatus according to Item 1. 3. A toner image forming apparatus capable of transferring and forming an unfixed toner image on both sides of a recording sheet, respectively.
The recording sheet on which the unfixed toner image is transferred and formed by the toner image forming apparatus is passed through a nip portion of a pair of heating rolls which rotate in a pressure-contact manner, and is heated from both sides of the sheet at a predetermined fixing temperature and a predetermined time, thereby forming each of the two sides of the sheet. An image forming apparatus including a double-sided fixing device capable of simultaneously fixing an unfixed toner image, wherein when using a coated paper as the recording sheet, the coated paper is undetermined by the toner image forming apparatus. Before the transfer formation of the toner image is performed, a sheet heating device is provided which is heated a plurality of times at a heating temperature and time that are near the lower side of the double-sided fixing temperature and time in the double-sided fixing device. An image forming apparatus characterized in that: 4. A sheet heating apparatus comprising a plurality of pairs of heating roll pairs which rotate under pressure, wherein a coating paper is sequentially passed through a nip portion of each pair of heating rolls to heat and press at the heating temperature and time. The image forming apparatus according to claim 3.