JP2002244466A - Image forming device having double-sided printing function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate a transfer failure, such as a transfer irregularity which occurs during printing on the second side, and also solve the problem of such as fixing strength in a fixing part, in an image forming device having a double- sided printing function. SOLUTION: A fixing means for fixing an unfixed toner image, formed on a transfer material, to the transfer material by heating and pressure contact is composed of a fixing roller having a heating means, and a pressure roller 1 pressed against the fixing roller. The pressure roller 1 has a metal core 2 and an elastic body 3 covering the periphery of the metal core 2. Parallel through-holes 4 are made in the elastic body 3 in the longitudinal and peripheral directions of the metal core 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a copying machine,
Among the electrophotographic image forming apparatuses such as a laser beam printer, the image forming apparatus has an automatic double-sided printing function, which has a plurality of process speeds and a plurality of resolutions, and can form an image on both sides of a transfer material. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus with a double-sided printing function, which includes a fixing device for heating and fusing a toner image by hot-pressing a transfer material on which a transferred toner image is formed to form a permanent image. .

[0002]

2. Description of the Related Art Hitherto, various image forming apparatuses with an automatic double-sided printing function capable of forming images on both sides of a transfer material such as transfer paper have been developed. FIG. 4 shows a schematic configuration of a laser beam printer as an example of an image forming apparatus for forming images on both sides of a transfer sheet. As shown in FIG. 4, a conventional laser beam printer includes, as main components, a photosensitive drum 51, a charging roller 52, an exposing device (including a semiconductor laser) 53, a developing device 54, a transfer roller 55, and a fixing device 5.
6, a static eliminator 57, a cleaning device 58, and a transfer paper re-transporting means 59 for back side printing.

An image signal input to the laser beam printer is sent to a laser driver (not shown), where ON / OFF of a semiconductor laser (not shown) is controlled. The laser light emitted from the semiconductor laser is deflected by a polygon mirror (not shown) to become scanning light in the longitudinal direction of the photosensitive drum 51, and the mirror (not shown)
Is projected on the photosensitive drum 51 via the. The photoreceptor drum 51 rotates in the direction of the arrow in the drawing, and is uniformly charged by the charging roller 52, and then receives an exposure corresponding to ON / OFF of the laser beam to form an electrostatic latent image on the surface thereof. .

Then, toner is applied by a developing device 54 to form a visible image (toner image).
As a result, the toner image is transferred to the transfer paper taken out one by one from the paper supply cassette 60 by the paper supply roller 61. The untransferred toner is wiped off from the surface of the photoconductor drum 51 by the cleaning device 58, and the photoconductor drum 51 prepares for the next image forming process. On the other hand, the transfer paper carrying the unfixed toner image is conveyed to the fixing device 56, and after a permanent fixed image is obtained, is conveyed in the direction of the arrow in FIG. The arrows in the drawing indicate the feed trajectory of the transfer paper taken out of the paper feed cassette 60 and conveyed.

The fixing device 56 has a heater (fixing heater) 63 inside a hollow fixing roller 62. When the heater 63 is energized, the fixing roller 62 is heated, and the surface of the fixing roller 62 is heated. The output of the temperature sensor that detects the temperature is input to a control unit (not shown). Then, under the control of a fuser control circuit (not shown), the heater 63 is turned on / off, and a predetermined surface temperature is maintained. The pressure roller 64 is pressed against the fixing roller 62 by an urging means (not shown), and the unfixed toner on the transfer paper is transferred to the transfer paper in a nip formed by the fixing roller 62 and the pressure roller 64. At the same time, it is heated and pressed to be permanently fixed.

The transfer paper on which the toner image is permanently fixed on the first surface (front surface) in this way is normally discharged as it is, but when printing is also performed on the second surface (back surface). Is guided to the reverse conveyance path by switching the flapper 65 by a flapper solenoid (not shown),
The sheet is guided to the re-feeding conveyance path 67 by the forward / reverse rotation roller 66.

The transfer paper guided to the re-feeding conveyance path 67 is conveyed by the conveyance rollers 68, re-supplied to the photosensitive drum 51 by the re-feeding rollers 69, and similarly to the first surface.
After printing on the second side, the sheet is discharged. In addition to such a function, it is also possible to continuously perform automatic double-sided printing such as performing first-side printing and then performing second-side printing.

[0008]

In recent years, with the growing need for higher image quality, higher resolution has been promoted even in low-cost monochrome machines, and printers having a high resolution mode of 1200 dpi (dot / inch) have been developed. Are being manufactured. However, if the resolution is increased to, for example, 600 dpi or 1200 dpi, the number of rotations of the polygon mirror in the scanner unit of the laser beam printer must be doubled, which is not only technically not easy but also increases the manufacturing cost. The problem arises.

In order to deal with such a problem, for example,
26 dpi / min at 00 dpi (process speed about 122m
m / sec), the process speed is switched to half in the 1200 dpi mode,
13 sheets / minute at 200 dpi (process speed about 61m
m / sec), printers of the resolution and process speed switching type are manufactured. With such a process speed switching method, the number of rotations of the polygon mirror can be kept constant irrespective of the resolution, so that there is no problem in terms of technical and manufacturing costs in the scanner unit.

[0010] However, in the above-described conventional technology, the process speed changes at the time of resolution switching, so that the optimum value of the process condition of electrophotography may be different. That is, in the electrophotographic image forming apparatus, when the process speed is switched and printing is performed on both sides of the transfer paper, the following problems may occur.

<Defective point 1> After fixing the toner image, the moisture contained in the transfer paper evaporates and decreases due to the heating action of the fixing device, and the resistance value of the transfer paper increases. In addition, when the process speed is slow, the heating time of the transfer paper is long, and the evaporation amount of the moisture content of the transfer paper is also large, so that the resistance is high (depending on the type of paper and the heating conditions for fixing,
Resistance of about ¹⁰ 10 Ω · cm is about 10 ¹⁴ Ω · cm). For this reason, a higher transfer voltage is required at the time of printing on the back surface (second surface) than at the time of printing on the front surface (first surface). However, if the transfer voltage is increased in order to obtain the necessary transfer density in the solid portion, This causes image quality deterioration such as reverse transfer in the halftone portion. In addition, it may cause deterioration of the photoconductor or the like due to abnormal discharge or the like. In addition, the type of paper,
Depending on environmental conditions, a transfer bias exceeding the withstand voltage (4 kV or more) of the photoreceptor may be required to perform good transfer.

<Problem 2> The amount of evaporation of the water content of the transfer paper due to the fixing of the toner image differs depending on the type of transfer paper, the atmospheric environment, and the fixing temperature. In particular, the fixing temperature varies depending on the temperature ripple of the fixing roller (normally about 5 ° C. to 10 ° C.) and the surface temperature of the pressure roller (about 20 ° C. to 150 ° C.). The surface temperature of the pressure roller has a large temperature gap between when the machine is started and when continuous multi-sheet feeding is performed. Therefore, the degree of change in resistance after fixing is also different. Therefore, the back surface (second
(Surface) For printing, process conditions such as process speed and transfer voltage must be controlled, but it is difficult to set optimal process conditions for all types of transfer paper and each environment.

<Defect 3> The transfer paper is heated by the fixing device, and the heated transfer paper is re-fed to the transfer unit again. Is done. In particular, the temperature rise of the transfer roller during continuous printing becomes remarkable. In addition, when printing on the back side, the transfer roller is heated by the heat of the transfer paper re-fed to the transfer unit, and heat is also radiated from the transfer paper, so that the ambient temperature is lower than the internal temperature during normal single-sided printing. To rise. This phenomenon becomes remarkable especially in multi-printing in continuous paper feeding. Due to such changes in the surrounding environment,
In the case of the transfer roller having the environment dependency, the volume resistance value largely fluctuates, and when the initial stage of continuous printing is compared with the image after passing a plurality of sheets, the image quality is significantly deteriorated. Therefore, in order to avoid image quality deterioration, electric control is performed such that the bias applied to the transfer roller is changed between the initial stage and the continuous paper feed, or the constant voltage control and the constant current control are switched at a predetermined timing. In addition, various complicated process controls are required, such as detecting the resistance value of the transfer roller at the time of paper transfer at the time of transfer, and applying a transfer bias corresponding to the detected resistance value.

Next, the disadvantages of the prior art will be discussed in more detail. In an image forming apparatus provided with a double-sided printing function, as a material property required for a transfer roller, it is important to reduce a change in a volume resistance value due to an environmental change (particularly, a temperature change). A factor that largely affects the transfer image quality is a change in the resistance value of the transfer paper between single-sided printing and double-sided printing. However, when the change in the volume resistance value of the transfer roller is large, the adverse effect on the transfer image quality is further affected. Factors are added, and image quality deterioration such as insufficient density and transfer dust occurs.

In the prior art, in order to avoid such image quality deterioration and to obtain a good transfer image, the above-described complicated process control must be performed, and the transfer bias must be changed for various conditions. Did not. For example, the first
For the transfer paper that has once passed through the fixing unit during surface printing, the volume resistivity before heating is about 10 ¹⁰ Ω · cm, but after heating is about 10 ¹⁴ Ω · cm. This is due to evaporation of water due to heating, and depends on the temperature of the fixing roller, the temperature of the pressure roller, and the heating time (nip time). Therefore, in order to reduce the fluctuation range of the volume resistivity of the transfer paper, a method of keeping the temperature of the fixing roller low can be considered.

However, if a method of keeping the temperature of the fixing roller low is adopted, it may cause a decrease in fixing strength in the fixing section and eventually cause a cold offset. Such a phenomenon of a decrease in fixing strength and a phenomenon of cold offset are particularly caused by a first copy when the initial power is turned on, a copy when the machine is placed in a low-temperature environment, and a thick paper (specifically, a basis weight of 128 / gcm ² or more). This is likely to occur in a situation where the toner is insufficiently melted, such as copying on paper.

A method of keeping the temperature of the pressure roller low is also conceivable. However, in consideration of cost and the like, the pressure roller is often made of a cored bar and a silicone solid rubber coated with the cored bar, which is excellent in releasability, heat resistance and rubber strength. Furthermore, it is often the case that, in addition to having no heating means inside, there is no detection means for detecting the temperature of the pressure roller, and the temperature of the pressure roller is substantially controlled. Can not.

In continuous multi-printing, the fixing roller and the pressure roller rotate continuously. Therefore, assuming that the temperature of the fixing roller is Tu (deg), the temperature T _L (deg) of the pressure roller is about T _L1 = (Tu−30) (deg) in the case of silicone solid rubber, and the silicone sponge is in the case of rubber, and heated up to _{T L2 = (Tu-40)} (deg) extent which is less than the temperature is stabilized approximately at that temperature. In this case, Table 1 below shows the volume resistivity of the transfer paper when the pressure roller passed the transfer paper at a temperature close to room temperature, the volume resistivity of the transfer paper when the paper passed at the stable temperature, and the image quality at the time of the second surface transfer. The experimental results shown in FIG.

[0019]

[Table 1]

In optimizing the heating time, the required process speed is determined on the machine side. Therefore, the heating time is optimized by adjusting the nip width. However, since the nip width is considerably restricted by the roller outer diameter, the cost, and the like, the nip width is appropriately determined by taking into consideration the fixing temperature and the non-offset region of the toner (the region where the fixing is excellent except for the hot offset and the cold offset). You have to decide.

The resolution desired by the user (for example, 120
When an image forming apparatus having different process speeds according to 0 dpi and 600 dpi) is used, 1200 dpi is used.
At the time of printing, the process speed is half speed (for example, the process speed at a resolution of 1200 dpi is 61 mm / sec while the process speed at a resolution of 600 dpi is 122 mm / sec). For this reason, at the time of high resolution, the nip time is twice as long as at the time of low resolution, so that the volume resistivity of the transfer paper further increases, and increases up to about 2 × 10 ¹⁴ Ω · cm. As a result,
In the second-side printing, image defects such as transfer unevenness in a transfer portion occur.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art. In an image forming apparatus with a double-sided printing function, transfer defects such as transfer unevenness during second-side printing are eliminated. It is an object of the present invention to solve problems in the fixing section such as fixing strength.

[0023]

An image forming apparatus with a double-sided printing function according to the present invention comprises: an electrostatic latent image forming means for forming an electrostatic latent image on an image carrier; and the electrostatic latent image forming means. Developing means for adhering toner to the electrostatic latent image formed by the developing means to make the toner visible, and applying a voltage to the toner image developed by the developing means to transfer the toner image onto a transfer material. Transfer means, fixing means for fixing the unfixed toner image formed on the transfer material onto the transfer material by heating and pressing, and forming an image on the back surface of the transfer material after fixing the toner image. To perform the transfer again, in the image forming apparatus with a double-sided printing function provided with a re-conveying unit for supplying the transfer material to the transfer unit, the fixing unit includes a fixing roller having a heating unit, and a pressing roller pressed against the fixing roller. Pressure roller, The pressure roller includes a cored bar and an elastic body coated on the outer periphery of the cored bar. Inside the elastic body, a plurality of through holes parallel to the longitudinal direction of the cored bar are provided in the circumferential direction. It is characterized by the following.

Further, in the image forming apparatus with the double-sided printing function, the transfer paper transport speed may be switchable in a plurality of stages.

Further, in the image forming apparatus with a double-sided printing function, it is preferable that the elastic body is made of a foam. In the image forming apparatus with a double-sided printing function, the elastic body is preferably made of silicone rubber. Further, in the image forming apparatus with a double-sided printing function, it is preferable that a fluororesin tube is coated on an outer periphery of the elastic body. In this case, the fluororesin tube is made of a tetrafluoroethylene-perfluoroisopropyl vinyl ether copolymer (PF
A) It can be a tube.

In the image forming apparatus with a double-sided printing function, the elastic body has a thermal conductivity of 5 × 10 ⁻⁴ cal.
/ Cm · sec · ° C. or less. Also,
In the image forming apparatus with a double-sided printing function, the elastic body preferably has a heat capacity of 2 × 10 ⁻³ J / mm ³ · ° C. or less. Further, in the image forming apparatus with a double-sided printing function, the core of the elastic body preferably has a hollow shape.

Specifically, the surface layer of the pressure roller is coated with a flexible synthetic resin having excellent heat insulation properties, and the base layer is formed of silicone sponge rubber having a low thermal conductivity. Also,
By providing a plurality of through holes parallel to the longitudinal direction of the core metal in the circumferential direction of the base metal layer, the thermal conductivity can be further reduced. Furthermore, due to the compression effect provided by the through holes, a large nip is obtained even when compared with a roller having the same diameter and the same pressure as before, so that the toner heating efficiency is increased, and the temperature of the fixing roller can be reduced, The temperature of the pressure roller can also be reduced, and an increase in the volume resistance of the transfer paper due to excessive heating can be prevented without lowering the toner fixing strength.

As described above, the heat insulating property of the pressure roller is increased.
By suppressing the temperature rise due to the fixing roller
In both cases, the pre-heating time of the fixing roller can be shortened
You. In addition, by setting the wide nip, the set temperature of the fixing roller
The degree itself can also be reduced. As a result of this,
When the temperature of the pressure roller rises stably during continuous printing,
In this case, the fixing roller temperature is set to Tu (deg).
And the temperature T of the pressure roller _L3= (Tu-70) (deg)
Before and after. Therefore, the volume of transfer paper due to overheating
There is no increase in resistivity. Table 2 below shows the measurement results.
Shown in

[0029]

[Table 2]

As is apparent from Table 2, when the image forming apparatus with a double-sided printing function of the present invention is used, the most severe condition under which the volume resistivity of the transfer paper increases, that is, in the case of continuous multi-printing, Even during two-sided printing, good transfer can be performed without causing transfer failure such as transfer unevenness.

As for the fixing strength, a wide nip can be realized by using a pressure roller having a through hole. For example, while the nip width of the conventional φ30 mm pressure roller is 3.2 mm, the pressure roller used in the image forming apparatus with a double-sided printing function according to the present invention includes:
The nip width is 4.3 mm, the set temperature of the fixing roller can be reduced from 165 ° C. to 158 ° C., and the fixing strength is almost the same as the conventional one (toner remaining ratio is 80%).
Above) or more.

Further, a pressure roller (core φ10 mm, outer diameter φ30 m) used in the image forming apparatus with a double-sided printing function of the present invention.
As the basic physical properties of m), the results of comparison with a conventional pressure roller are shown. Conventionally, when the pressure roller is formed of a silicone solid body, the thermal conductivity is about 5 × 10 ^-4 ca1 /
cm.sec..degree. C. and a heat capacity of about 2.times.10.sup.- ^3.
J / mm ³ · ° C. On the other hand, in the pressure roller used in the image forming apparatus with a double-sided printing function of the present invention,
The thermal conductivity is about 2 × 10 ⁵ cal / cm · sec · ° C., and the heat capacity is about 6 × 10 ² J / mm ³ · ° C. Table 3 shows the temperature of each pressure roller when the fixing roller was set at 158 ° C. and rotated for 10 minutes.

[0033]

[Table 3]

As is clear from Table 3, by making the core portion of the pressure roller from solid to hollow, the temperature of the pressure roller after idling for 10 minutes becomes 89 ° C., which is lower than that of the conventional pressure roller. Can be reduced by about 10 ° C. Further, with respect to the fixing strength, the image quality at the time of the second surface transfer was further improved in a state where the toner remaining ratio of 80% or more was kept.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an image forming apparatus with a double-sided printing function according to the present invention will be described below with reference to the drawings.

First, the pressure roller used in the image forming apparatus with a double-sided printing function according to the present invention will be described. FIG.
1 is a cross-sectional view of a pressure roller used in an image forming apparatus with a double-sided printing function according to the present invention.

As shown in FIG. 2, this pressure roller 1
A metal core 2 made of an aluminum material or an iron material is provided, and the metal core 2 is covered with a foamable elastic body 3 mainly made of silicone rubber. further,
A large number of through holes 4 are formed in the elastic body 3 in the circumferential direction of the core 2 in parallel with the longitudinal direction of the core 2.

To manufacture this pressure roller, first, 100 parts by weight of silicone rubber, 4 parts by weight of a vulcanizing agent, and 2 parts by weight of a foaming agent
The silicone rubber mixture (hereinafter referred to as S
i mixture). Next, a raw rubber tube having holes is continuously extruded and formed by an extruder using the Si mixture as a raw material. In the die inside the extruder, a pin is disposed in advance in a part to be the through hole 4 at a distance from the core, extruded by a screw, and vulcanized and foamed by heating. Continue at 250 ° C
For 20 minutes to form a silicone rubber sponge tube having through-holes 4. Subsequently, a φ10 mm core metal having an adhesive applied to the outer peripheral surface is inserted into the sponge tube. Next, after the adhesive is cured, the sponge tube is polished until the sponge tube has a desired outer diameter to obtain the elastic body 3.

The elastic body 3 has a large number of through holes 4 formed in the circumferential direction parallel to the longitudinal direction of the metal core 2.
The through-holes 4 may be formed in two stages in the circumferential direction, and each of the outer through-holes 4 may be arranged between adjacent inner through-holes 4.

Further, silicone rubber or the like is cast on the outer peripheral surface of the elastic body 3 integrated with the cored bar 2 by a casting method and cured, and then polished to a desired outer diameter to form the surface coating layer 5. Form. In the present embodiment, as the material of the surface coating layer 5, a fluororesin having excellent toner releasability, strength, abrasion resistance, and heat insulation is used. In the pressure roller shown in FIG. 2, the cross-sectional shape of the through-hole 4 is a teardrop shape, but the cross-sectional shape of the through-hole is not limited to this. For example, as shown in FIG. May have a circular cross section.

FIG. 1 is a schematic configuration diagram of a laser beam printer which is an embodiment of an image forming apparatus with a double-sided printing function according to the present invention. A laser beam printer according to an embodiment of the present invention uses the above-described pressure roller 1, and as shown in FIG. 1, transfers a transfer roller 6 via a transfer sheet 7 as a transfer material. The transfer roller 6 is brought into contact with the photosensitive drum 8, and a voltage is applied between the transfer roller 6 and the photosensitive drum 8 using a high-voltage power supply (not shown). By generating an electric field therebetween, the toner on the photosensitive drum 8 is transferred to the transfer paper 7. In the figure, the arrow indicates the moving direction of the transfer paper.
This laser beam printer can perform both single-sided printing and double-sided printing. When performing double-sided printing, the transfer material once fixed is re-sent to the image forming unit and transferred to the transfer paper 7.
Print on the back side of.

Hereinafter, the structure of the laser beam printer will be described in more detail. This laser beam printer is shown in FIG.
As shown in FIG. 1, a photosensitive drum 8, a charging member 9, an exposure device (including a semiconductor laser) 10, a developing device 11, a transfer roller 6, a fixing device 12, a pressure roller 1, a cleaning device 13,
Transfer paper re-conveying means 14 for back side printing is a main component.

The image signal input to the laser beam printer is sent to a laser driver (not shown), where ON / OFF of a semiconductor laser (not shown) is controlled. The laser light emitted from the semiconductor laser (not shown) is deflected by a polygon mirror (not shown) to become scanning light in the longitudinal direction of the photosensitive drum 8, and passes through the mirror (not shown) It is projected on the drum 8.
The photoreceptor drum 8 rotates in the direction of the arrow in the drawing, and is charged by the charging member 9, and then receives exposure corresponding to ON / OFF of the laser beam, and forms an electrostatic latent image on the surface thereof.

Then, after the toner is applied by the developing device 11 and a visible image (toner image) is obtained, the transfer paper 7 taken out one by one from the paper supply cassette 15 by the paper supply roller 16 by the transfer roller 6. Is transferred to the toner image. The untransferred toner is wiped off from the surface of the photosensitive drum 8 by the cleaning device 13, and the photosensitive drum 8 is prepared for the next image forming step. On the other hand, the transfer paper 7 on which the unfixed toner image is placed is conveyed to the fixing device 12, and after a permanent fixed image is obtained, is conveyed in the direction of the arrow in FIG. The arrows in the drawing indicate the feed trajectory of the transfer paper 7 taken out of the paper feed cassette 15 and conveyed.

The fixing device 12 is provided with a heater (fixing heater) inside a hollow fixing roller. When the heater is energized, the fixing roller is heated, and the temperature for detecting the surface temperature of the fixing roller is detected. The output of the sensor is input to a control unit (not shown). Then, under the control of the fuser control circuit (not shown), the heater is turned on / off and a predetermined surface temperature is maintained. The pressure roller 1 is pressed against the fixing roller by urging means (not shown), and the unfixed toner on the transfer paper 7 is heated together with the transfer paper 7 in a nip formed by the fixing roller and the pressure roller 1. It is pressed and permanently fixed.

The transfer paper 7 on which the toner image is permanently fixed on the first surface (front surface) is normally discharged as it is, but is printed on the second surface (back surface). The flapper 17 has a flapper solenoid (not shown)
To switch to the paper path route at the time of back side printing. Then, the transfer paper 7 guided to the paper path route at the time of back side printing is transported by the transport roller 18 and
The sheet is re-fed to the photosensitive drum 8 and printed on the second surface in the same manner as the first surface, and then discharged.

Next, another function of the laser beam printer according to this embodiment will be described. The laser beam printer according to the present embodiment includes a speed switching unit (not shown) for the transport speed and a resolution switching unit (not shown), and can switch to a plurality of transport speeds. A latent image can be formed on the transfer paper 7 depending on the resolution. For example, the transfer speed of the transfer paper 7 is 122 mm.
/ Sec and 61 mm / sec, and switching is performed by the speed switching unit according to the resolution.

The resolution is 1200 dpi and 6
The resolution is switched by the resolution switching means in accordance with the image signal. And 600 dpi
26 sheets / min during printing (process speed about 122
Each engine unit is controlled to operate at 13 sheets / min (process speed of about 61 mm / sec) during 1200 dpi printing.

In the laser beam printer according to the present embodiment, by mounting the pressure roller 1 having the above-described structure, even under severe conditions such as continuous double-sided paper feeding,
In the printing on the second surface (back surface), a high-quality image having a high image quality of 1200 dpi was formed and transfer was performed.

[0050]

The image forming apparatus with a double-sided printing function according to the present invention has the above-described configuration, and can provide the following effects. That is, according to the image forming apparatus with the double-sided printing function of the present invention, the pressure roller is constituted by the core metal and the elastic body coated on the outer periphery of the core metal, and the elastic body is provided in the longitudinal direction of the core metal. By providing a plurality of through holes parallel to each other in the circumferential direction, the temperature of the pressure roller does not rise more than necessary even during continuous multi-sided printing. For this reason, in the printing on the second surface after fixing due to excessive heating, a good transferred image can be formed without increasing the resistance of the transfer material.

When the transfer speed of the transfer material can be switched in a plurality of stages, when the transfer speed changes, the influence of heat from the fixing roller on the pressure roller also changes. Is much smaller than when a conventional pressure roller is used, so that the influence on the increase in the resistance value of the transfer material can be minimized.

Further, the heat insulating effect can be improved by forming the elastic body by a foam. Further, when the elastic body is made of silicone rubber, the heat conductivity can be made relatively small and the elastic body can be made excellent in heat resistance. In addition, by covering the outer periphery of the elastic body with a fluororesin tube, a skin layer having relatively low thermal conductivity and excellent toner releasability can be obtained.

Further, by forming the fluororesin tube as a tetrafluoroethylene-perfluoroisopropylvinyl ether copolymer (PFA) tube, a skin layer having relatively small heat conductivity, excellent toner releasability and excellent mechanical strength can be obtained. It can be.

The thermal conductivity of the elastic body is 5 × 10 ⁻⁴ c
By setting the temperature to al / cm · sec · ° C. or less, the pressure roller is not heated more than necessary even in the case of double-sided multi-printing, and the volume resistance value of the transfer material causing the transfer failure is increased. Can be prevented. Further, by setting the heat capacity of the elastic body to 2 × 10 ⁻³ J / mm ³ · ° C. or less, the pressure roller is not heated more than necessary even in the case of double-sided multi-printing, which causes transfer failure. Can be prevented from increasing in the volume resistance of the transfer material.

By making the core of the elastic body hollow, the heat insulating effect can be further improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a laser beam printer according to an embodiment of the present invention.

FIG. 2 is a sectional view of a pressure roller.

FIG. 3 is a cross-sectional view of a pressure roller according to another embodiment.

FIG. 4 is a schematic configuration diagram of a conventional laser beam printer.

DESCRIPTION OF SYMBOLS 1 Pressure roller 2 Core metal 3 Elastic body 4 Through hole 5 Coating layer 6 Transfer roller 7 Transfer paper 8 Photoconductor drum 9 Charging member 10 Exposure device 11 Developing device 12 Fixing device 13 Cleaning device 14 Transfer paper Transporting means 15 Paper feeding cassette 16 Paper feeding roller 17 Flapper 18 Transporting roller 51 Photoreceptor drum 52 Charging roller 53 Exposure device 54 Developing device 55 Transfer roller 56 Fixing device 57 Static eliminator 58 Cleaning device 59 Transfer paper re-transporting device 60 Paper feeding cassette 61 Paper Feeding Roller 62 Fixing Roller 63 Heater 64 Pressure Roller 65 Flapper 66 Forward / Reverse Roller 67 Refeeding Transport Path 68 Transporting Roller 69 Refeeding Roller

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F16C 13/00 F16C 13/00 E G03G 15/00 106 G03G 15/00 106 15/16 15/16 (72 ) Inventor Shiro Wakahara 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Japan, inside Sharp Corporation (72) Inventor Hideki Onishi 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka inside Sharp Corporation (72) Inventor Road Hiroshi Masada 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Japan F-term (reference) 2H028 BA06 BA09 BA16 2H033 AA02 AA46 BA08 BA10 BA11 BA25 BB18 BB29 BB30 BB34 CA36 2H200 FA20 GA23 GA44 GB22 GB23 GB25 JA10 JA18 JA18 JB12 JB17 JB18 JB49 PA11 3J103 AA02 AA24 BA41 FA01 GA02 GA57 GA58 HA03 HA04 HA12 HA43 HA53

Claims (9)

[Claims] 1. An electrostatic latent image forming means for forming an electrostatic latent image on an image carrier, and a toner attached to the electrostatic latent image formed by the electrostatic latent image forming means to make the electrostatic latent image visible. Developing means for applying the voltage to the toner image developed by the developing means to transfer the toner image onto a transfer material, and an unfixed toner image formed on the transfer material. A fixing unit for fixing the transfer material to the transfer material by heating and pressing, and supplying the transfer material to the transfer unit again to form an image on the back surface of the transfer material after fixing the toner image. In the image forming apparatus with a double-sided printing function provided with a re-conveying unit, the fixing unit includes a fixing roller having a heating unit, and a pressing roller pressed against the fixing roller. Gold and an elastic body coated on the outer periphery of the core metal Provided, inside the elastic body, double-sided printing function image forming apparatus is characterized in that a plurality of parallel through-holes in the circumferential direction along the longitudinal direction of the metal core. 2. The image forming apparatus with a double-sided printing function according to claim 1, wherein the transfer material transport speed can be switched in a plurality of stages. 3. The image forming apparatus according to claim 1, wherein the elastic body is a foam. 4. The image forming apparatus according to claim 1, wherein the elastic body is a silicone rubber. 5. The image forming apparatus with double-sided printing function according to claim 1, wherein a fluororesin tube is coated on an outer periphery of the elastic body. 6. The image forming apparatus according to claim 5, wherein the fluororesin tube is a tetrafluoroethylene-perfluoroisopropylvinyl ether copolymer (PFA) tube. 7. The thermal conductivity of the elastic body is 5 × 10 ⁻⁴ c.
3. The image forming apparatus with a double-sided printing function according to claim 1, wherein the temperature is al / cm · sec · ° C. or less. 8. The heat capacity of the elastic body is 2 × 10 ⁻³ J /
^{3. The} image forming apparatus with a double-sided printing function according to claim 1, wherein the temperature is equal to or less than mm3.degree. 9. The image forming apparatus with double-sided printing function according to claim 1, wherein the core of the elastic body has a hollow shape.