Fixing apparatus and fixing method using same

Abstract

 A fixing apparatus for forming fixed images has a heat roller (8) having a conductive elastomer layer (23) formed in a uniform thickness; a pressure roller (9); and a current supplying means. The heat roller (8) and the pressure roller (9) rotate in contact with each other to thermally fix a visible image onto the recording medium (7) at the contacted surfaces, and the electric current is passed into the conductive elastomer layer (23) by the current supplying means. Since the fixing is carried out at a fixing temperature of not more than 130°C, the fixing apparatus can be simplified, thereby making it possible to miniaturize and lower its overall cost.

Description

[0001] The present invention relates to a fixing apparatus for forming fixed images used for plain paper copying machines, laser printers, plain paper facsimiles, etc. and to a fixing method using such an apparatus.

[0002] Conventionally, when images are formed with copying machines, laser beam printers, etc., the Carlson Method has been generally used (U.S. Patent 2,221,776, 2,297,691 and 2,357,809, "Electrophotography," p22-p41, R.M. Shaffert, 1965, The Focal Press).

[0003] In a conventional method of forming fixed images, after the electrostatic latent image formed on a photoconductor by optical means is developed in a developing process, it is transferred to a recording medium such as a recording paper in a transfer process and then fixed into the final image generally with heat and pressure in a fixing process. Since the photoconductor is repeatedly used, a cleaning device is provided for cleaning the residual toner after the transfer process with its rotation.

[0004] In the conventional method of forming fixed images as described above, however, through the processes from the formation of the electrostatic latent image up to the transfer and then the fixing thereof onto the recording medium, the energy consumed in the fixing process is extremely large as compared to that of each of other processes. Therefore, the temperature of the heating element of the fixing apparatus has to remain at a very high level (usually around 200°C) and further a high pressure is required (usually between 2.0 and 6.0 kg/cm).

[0005] On the other hand, since both the photoconductor and the developer device have to be maintained at around room temperature, a considerable distance has to be maintained between the fixing apparatus and the developer device, which necessitates making the machine larger. In addition, it is necessary to force the removal of the generated heat from the system, but the noise produced by the forced radiation device is not negligible.

[0006] Further, in the conventional method of forming fixed images, since the fixing process works independently and fixing is carried out at such a high temperature of around 200°C, as mentioned above, expensive heat-resistant materials such as heat-resistant resins, heat-resistant rubbers, etc. have to be provided in the periphery of the fixing apparatus.

[0007] Since the fixing is carried out at a high temperature, problems as curling and jamming of the paper, etc. are likely to take place. In addition, when the fixing requires a high temperature, it takes more time to reach the set temperature so that a quick printing becomes impossible. In such a case, therefore, this method is unsuitable for devices such as a facsimile which requires quick printings.

[0008] As for solving these problems, a device for low temperature fixing using a cold pressing method carried out at a temperature of not more than 100°C is known (Japanese Patent Laid-Open No. 159174/1984). In this reference, however, although the fixing temperature is low, the nip pressure has to be elevated normally to not less than 4 kg/cm in this method, making the machine heavier. Moreover, it poses problems in the gloss of the images, deformation of the paper copy sheets and an insufficient fixing strength. Also, some attempts for enabling quick printings and fixing with conserved energy by using heating sheets in place of the conventionally used quartz heaters, nichrome wires, etc. as heat sources for the heat rollers in thermally fixing apparatuses have been known (Japanese Patent Laid-Open Nos. 196562/1983, 150183/1989 and 260475/1989).

[0009] However, in these methods, since the fixing temperature is set at such a high temperature of about 200°C, the heating element is inserted in a cylinder in order to avoid temperature unevenness, thereby delaying the rate of the temperature rise and making the energy efficiency of the fixing apparatus poor.

[0010] In order to solve such problems, it has been demanded to improve conducting effects by winding a heating layer in a helical structure to form a slit; or by providing a heat-resistant, protective layer made of expensive imide resins, fluororesins, etc. in the periphery of the heat roller when the releasing properties of the toner at a high temperature are insufficient. These may undesirably make the apparatuses complicated and also expensive.

[0011] In addition, in the conventional fixing apparatus, the relation between a radius of the elastic material roller and a nip width are such that the radius of the roller is from about 15 mm to at most 50 mm, and that the nip width thereof is from about 3 mm to at most 10 mm. Therefore, since the ratio of the nip width to the radius of the elastic material roller is usually from 0.05 to at most 0.20, those expensive elastic materials whose compression set at a temperature of 180°C to 200°C are extremely small have been used. The reasons for using elastic materials having such properties are as follows: The toner is usually fixed at a surface temperature of the heat roller of close to 200°C, and a high nip pressure is applied thereto, so that the toner can be melted and adhered onto a recording paper, etc. Specifically, in order to thermally melt the toner using a heat roller and to adhere it onto the recording paper at a high nip pressure, it is preferred that the nip width is kept wide so that the time for heat conduction from the heat roller to the toner can be kept long. At the same time, however, it is more necessary to increase the nip pressure for the purpose of increasing the adhesion strength of the toner with the paper by pressing the thermally softened toners into the pulp fibers of the surface of the paper.

[0012] In the conventional methods, since the surface temperature of the heat roller is needed to be at a high temperature of close to 200°C as described above, those having small compression set at a high temperature are chosen among various heat-resistant elastic materials. However, when a large plastic deformation takes place by applying such a high temperature and high nip pressure as described above, the compression set becomes large, and the nip pressure becomes uneven, resulting not only in poor fixing ability but also in the problems incurred due to large loads on the driving systems.

[0013] From these standpoints, the development of a novel compact fixing apparatus is in demand.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide a novel fixing apparatus which is compact in size and highly reliable with low consumed energy.

[0015] Another object of the present invention is to provide a fixing method using an encapsulated toner which is highly reliable with low consumed energy.

[0016] The present invention essentially relates to a heat roller-type fixing apparatus for forming fixed images at a temperature of not more than 130°C, comprising a heat roller, a pressure roller and a current supplying means, the rollers rotating in contact with each other to thermally fix a visible image onto the recording medium at the contacted surfaces, wherein a conductive elastomer layer is formed in a uniform thickness on a cylinder of the heat roller, and the electric current is passed into the layer by the current supplying means. In the present invention, excellent properties are fully exhibited under the conditions of the specific resistivity of the conductive elastomer layer of not more than 100 Ω·cm; the maximum heating temperature of not more than 150°C; the nip width of a pair of rollers in the fixing apparatus of not less than 0.2 times the radius of the elastic material roller; and the rubber hardness of at least one of the rollers of not more than 90 degrees according to JIS A-type hardness.

[0017] The present invention further essentially relates to a fixing method using the above fixing apparatus of the present invention. Specifically, a method of fixing a visible image formed by an encapsulated toner using a heat roller-type fixing apparatus comprises the steps of passing a current into a conductive elastomer layer formed in a uniform thickness on a cylinder of the heat roller; and thermally fixing the toner by heat generated thereby at a temperature of not more than 130°C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein:

Figure 1 is a schematic view showing an electrophotographic process used in the present invention;

Figure 2 is a schematic view showing one example of a fixing apparatus as defined by the present invention;

Figure 3 is a schematic view showing one example of a fixing apparatus as defined by the present invention; and

Figure 4 is a schematic view showing one example of a fixing apparatus as defined by the present invention.

[0019] The reference numerals in Figures 1 through 4 denote the following elements:
   Element 1 is a photoconductive drum, element 2 a charger, element 3 an exposure device, element 4 a developer device, element 5 a transfer device, element 6 a toner, element 7 a recording medium (a recording paper), element 8 a heat roller, element 9 a pressure roller, element 10 a radiator device, element 11 a cleaner device, element 12 an insulating layer, element 13 a heating layer, element 14 a developer sleeve, element 21 an elastic heating element, element 22 a conductive layer, element 23 a releasing layer, element 31 an elastic heating element, element 32 a releasing layer, element 41 an elastic heating element, element A a rotating shaft and element B a rotating shaft.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The fixing apparatuses of the present invention are detailed below, referring to the drawings.

[0021] Figure 1 is a schematic view showing an electrophotographic process used in the present invention;
   Element 1 is a photoconductive drum such as of amorphous selenium, amorphous silicon or organic photoconductor, etc., in which a photoconductive layer is provided on a conductive supporter.

[0022] Element 2 is a charger arranged opposite to the photoconductive drum 1. The charging means is not particularly restricted, and any of the ordinarily used chargers, for example, a charger by corotron, a roller charger using a conductive roller, a brush charger using a conductive brush, etc. can be used.

[0023] Element 3 is an exposure device arranged opposite to the photoconductive drum 1 for forming electrostatic latent images on the surface of the photoconductor. For an exposure device 3, light sources such as semiconductor laser beams, LED or EL arrays, etc. are used in combination with an image-forming optical system. Alternatively, a device based on optical systems projecting a reflected light of a document usually provided in the copying machine can be used.

[0024] Element 4 is a developer device for making visible the electrostatic latent image formed on the surface of the photoconductor with the toner. For a developer device, any of the commonly used two-component magnetic brush developer devices, the one-component magnetic brush developer devices, and the one-component non-magnetic developer devices, etc. can be used. The toner 6 charged inside the developer device passes through the developer sleeve 14 to visualize the electrostatic latent images formed on the surface of the photoconductor.

[0025] Element 5 is a transfer device which has the same function as that of the charger 2 with essentially the same mechanism. In the transfer process, the visualized images formed on the surface of the photoconductive drum 1 by using a developer is transferred onto the surface of the recording medium 7. Element 11 is a cleaner device, by which about 5% to 20% of the toners which remain untransferred in the transfer process are removed.

[0026] Element 8 is a heat roller, and element 9 is a pressure roller. The visible image is formed by transferring the toner onto the recording medium 7 and fixing thereonto by passing through a fixing apparatus comprising a pair of these rollers. Element 10 is a radiator device. Conventionally, driving systems or fans have been necessary in order to discharge forcefully high heat generated from the fixing apparatus from the printing machine. By contrast, when the fixing apparatus has a low heating temperature with low heat radiation as in the case of the present invention, a simple radiator device of a slit type or honeycomb type as shown in the figures serves its purposes.

[0027] The fixing apparatus of the present invention is constituted by a heat roller 8, which has a conductive elastomer layer, and a pressure roller 9 made of an elastic material or a rigid material. Specifically, for example, as illustrated in Figure 1, the fixing apparatus of the present invention is constituted by a heat roller 8 comprising an insulating layer 12 formed in the periphery of the rotating shaft A and a conductive elastomer layer, which is a heating layer 13, formed thereon; and a pressure roller 9 made of an elastic material. In another embodiment, as illustrated in Figure 2, the fixing apparatus of the present invention is constituted by a heat roller 8 comprising a conductive elastomer layer, which is an elastic heating element 21, formed in the periphery of the rotating shaft A, a conductive layer 22 formed thereon and a releasing layer 23; and a pressure roller 9 made of an elastic material. In a further embodiment, as illustrated in Figure 3, the fixing apparatus of the present invention is constituted by a heat roller 8 comprising a conductive elastomer layer, which is an elastic heating element 31, formed in the periphery of the rotating shaft A and a releasing layer 32 formed thereon; and a pressure roller 9 made of an elastic material. In a still further embodiment, as illustrated in Figure 4, the fixing apparatus of the present invention is constituted by a heat roller 8 comprising a conductive elastomer layer, which is an elastic heating element 41 formed in the periphery of the rotating shaft A; and a pressure roller 9 made of a rigid material. As described above, although the heat roller is provided with a conductive elastomer layer having a thickness without being subject to limitation as long as it has a uniform thickness, the thickness is preferably about 0.5 to 3.0 mm.

[0028] In addition to conductive materials such as SUS steel and aluminum, insulating materials such as heat-resistant resins and insulating ceramics can be used as a roller base material for the heat roller 8.

[0029] Examples of the heat-resistant resins include polyamides, polyamide-imides, polyacetals, polycarbonates, denatured PPOs, polyethylene terephthalates, polybutylene terephthalates, polyarylates, polysulfones, polyether sulfones, polyether ether ketones, polyetherimides, aromatic polyesters, polyphenylene sulfides, fluorine polymers, ABS resins, AS resins, AAS resins, AES resins, ACS resins, methylpentene polymers, ultrahigh molecular polyethylenes, polypropylene resins, phenol resins, diallyl phthalate resins, unsaturated polyester resins, epoxy resins, polyimides, polyurethanes, cyclic polyolefins and liquid crystal polymers.

[0030] These resins may be used singly or in combination of two or more kinds. In addition, they may be supplemented with fillers such as glass fibers.

[0031] Examples of the insulating ceramic materials include metallic oxides such as alumina, magnesia, beryllia, zirconia, silica, forsterite, wollastonite, zircon, mullite, cordierite, spodumene, aluminum titanate, spinel and barium titanate, and non-oxide metallic compounds such as silicon nitride, sialon, aluminum nitride, titanium nitride, silicon carbide, boron carbide, titanium carbide, tungsten carbide, lanthanum borate, titanium borate and zirconium borate.

[0032] When these insulating ceramic materials are used as roller base materials, one or more kinds thereof are used in any structural form selected from the group consisting of sintered bodies, glass and crystallized glass.

[0033] In such a case of using an insulating material as the roller base material, the insulating layer 12 becomes unnecessary.

[0034] The specific resistivity of the conductive elastomer layer is not more than 100 Ω·cm, preferably 1 to 100 Ω·cm, and more preferably 2 to 50 Ω·cm. When the resistivity exceeds 100 Ω·cm, the heat roller may fail to reach the set fixing temperature because the elastomer layer is not sufficiently heated unless a high charge is applied, and, in many cases, it takes too much time to reach the set temperature. Also, when the resistivity is less than 1 Ω·cm, the temperature control of the heat roller is difficult due to excess heating speed, and the resulting high temperature may undesirably damage the fixing apparatus including the fixing roller. The maximum heating temperature of the conductive elastomer layer used in the present invention is not more than 150°C.

[0035] Examples of the conductive elastomers for the fixing apparatus of the present invention include tetrafluoroethylenepropylene, vinylidene fluoride, silicone, fluorosilicone and other fluorine-based or silicone-based heat-resistant elastomers; and acrylic elastomers, nitrile elastomers, epichlorohydrin elastomers, ethylene-propylene-non-conjugated diene terpolymer (EPDM) elastomers, and other general-purpose elastomers. Under certain conditions, neoprene, butadiene and isoprene elastomers are also usable. In the present invention, a preference is given to the fluorine-based and silicone-based heat-resistant elastomers and the acrylic elastomer.

[0036] The conductive elastomer used for the fixing apparatus of the present invention is prepared by dispersing a conductive material such as conductive carbon, conductive inorganic powder or conductive potassium titanate whisker in the above elastic material and shaping the dispersed mixture. Further, a conductive layer may be provided which comprises a dispersion of an organic polymer such as polypyrrole, polythiophene, polyparaphenylene or polyaniline or a charge transfer complex such as that of anthracene and tetracyanoquinoline or that of pyrene and tetracyanoethylene in the above elastic material. Moreover, in some cases, to prevent the toner adhesion to the heat roller, the heat roller may be provided with a releasing layer by coating silicone oil or by forming a film such as that of polyethylene terephthalate, polybutylene terephthalate, polycarbonate, nylon, Teflon or PFA. The thickness of the releasing layer is preferably not more than 200 µm, since thicknesses exceeding 200 µm result in decreased thermal conductivity. More preferably, the thickness of the releasing layer is not more than 100 µm, but it needs to be not less than 10 µm, since thicknesses under this level can result in pinholes, which in turn lead to leakages of electricity.

[0037] In the conventional fixing apparatuses, as described above, the ratio of the nip width to the roller radius is normally from 0.05 to at most 0.20. By contrast, in the fixing apparatus of the present invention, this ratio is remarkably larger than those of the conventional fixing apparatuses, and it is adjusted to not less than 0.2, preferably 0.25 to 0.8, relative to the radius of the roller made of the elastic material. By adjusting the ratio of the nip width to the roller radius to not less than 0.2, the amount of heat supplied to toner upon fixing is increased so that fixing can be carried out at a lower fixing temperature. This effect is highly remarkable when the roller radius is not more than 15 mm, particularly not more than 10 mm. In the present invention, a roller made of an elastic material means a roller comprising a roller base material in the form of a rod or cylinder made of SUS steel, aluminum or another material having an outer diameter of not less than 8 mm, and an elastic material coated thereon having a hardness of not more than 90 degrees as determined by using a JIS A-type rubber hardness tester.

[0038] In the conventional fixing apparatuses, the rubber hardness of pressure roller is 40 to 90 degrees, as determined by a JIS A-type rubber hardness tester, for the pressure rollers having a slow peripheral speed and a small roller radius, and it is 40 to 90 degrees as determined by a JIS C-type rubber hardness tester, for the pressure rollers having a high peripheral speed and a large roller radius. However, in the present invention, at least one of the pair of rollers has a rubber hardness of not more than 90 degrees, preferably 20 to 90 degrees as determined by the JIS A-type rubber hardness tester. When the rubber hardness of the roller made of an elastic material exceeds 90 degrees as determined by the JIS A-type rubber hardness tester, the nip pressure becomes too high in the case where the roller-to-roller nip width is set in a specified range, which in turn causes a nip pressure difference between the center and both ends of the roller due to rotating shaft torsion of the roller, resulting in uneven fixing in the case of a small roller radius. When the rubber hardness is not more than 20 degrees, a sufficiently high nip pressure cannot be achieved when the nip width is set at a specified range, thereby resulting in an unsatisfactory fixing of the toner onto the recording medium. When the nip pressure is raised to a specified level, the pressure deformation of the roller becomes too large, resulting in an increased load on the driving system during operation, which can interfere with the rotation of the roller of the fixing apparatus. Hardness is tested in accordance with JIS K6301-1975, which may also be tested using a Durometer in accordance with ASTM 2240-75.

[0039] As described above, in the present invention, at least one of the pair of rollers is made of a material having a low rubber hardness. However, as illustrated in Figure 2, both the heat roller and the pressure roller made of materials having a low rubber hardness may be used. Also, as illustrated in Figure 3, the heat roller may be made of an elastic material having a high rubber hardness from the aspect of deterioration due to thermal load, etc. In another embodiment, as illustrated in Figure 4, in order to widen the nip width by using a heat roller made of a material having a low rubber hardness and to lower the cost by using a pressure roller made of an inexpensive tough material, the heat roller is made of a material having a low rubber hardness, and the pressure roller is made of SUS steel or another steel material insulated with a PET film or the like. In this case, when the surface of the heat roller is conductive, it is necessary to form a protective film with an insulating sheet on the surface of the heat roller.

[0040] In the fixing apparatus of the present invention, the toner can be heated at a fixing temperature of not more than 130°C, a sufficiently wide nip width can be obtained even when the roller radius is smaller, and the fixing can be carried out at a remarkably low nip pressure so as not to cause permanent set even when the strain is large. Specifically, the nip pressure in the fixing apparatus of the present invention is 0.01 to 4.0 kg/cm, preferably 0.1 to 3.0 kg/cm.

[0041] The heat roller of the fixing apparatus of the present invention is heated by applying a voltage to a conductive brush in contact with both ends of the heat roller as current supplying means, when an insulating layer is provided in the periphery of the rotating shaft and a heating layer of conductive elastomer is formed thereon as illustrated in Figure 1. Examples of the conductive brushes include those formed in the periphery of a metal shaft in a brush-like manner with conductive resin fibers, such as nylon or rayon having conductive carbon dispersed therein, or with fibers having conductive carbon, conductive paint or the like adhered thereto. To enhance the effects of applying a voltage, both end surfaces of the roller may be coated with a conductive paint or the like having conductive carbon, tin oxide or copper powder dispersed therein. Also, in the case illustrated in Figure 2, the voltage may be applied between the rotating shaft and the conductive elastomer layer.

[0042] The fixing apparatus of the present invention is thus capable of thermally fixing a visible image onto a recording medium conveyed, by the pair of the heat roller and the pressure roller as described above, which rotate in contact with each other. The fixing is carried out at the contacted surfaces of the rollers at a fixing temperature of not more than 130°C by heat generated by passing the electric current into the conductive elastomer layer.

[0043] The heat roller and the pressure roller rotate in contact with each other by specified driving means not illustrated in the figures in the direction shown in the respective drawings at a constant peripheral speed. The peripheral speed is not particularly limitative, and it may be properly chosen.

[0044] In the conventional apparatuses, the fixing is usually carried out at a high temperature of about 200°C because the softening point of the resin contained as the main component in the toner for forming the visible image is high. In order to eliminate this problem, various references proposing toners which can be fixed at low temperatures have been known.

[0045] The toner used in the present invention is an encapsulated toner wherein the function separation for the storage stability and the fixing ability, and the offset resistance can be achieved. Examples of these toners are not particularly limitative, but those comprising a shell having improved storage stability and chargeabiity and a core having improved coloring, fixing ability and offset resistance are suitably used (see Japanese Patent Laid-Open Nos. 176642/1983, 176643/1983, 56352/1986, 128357/1988, 128358/1988, 267660/1989 and 51175/1990).

[0046] The construction of the encapsulated toner are described in detail below.

[0047] For shell materials, styrene resins (Japanese Patent Laid-Open No.205162/1983), polyamide resins (Japanese Patent Laid-Open No.66948/1983), epoxy resins (Japanese Patent Laid-Open No.148066/1984), polyurethane resins (Japanese Patent Laid-Open No.179860/1982), polyurea resins (Japanese Patent Laid-Open No.150262/1987) and many others have been proposed. And as substances fixable under heat and pressure contained in the core material, those thermoplastic resins whose glass transition points (Tg) are not less than 10°C and not more than 50°C such as polyester resins, polyamide resins, polyester-polyamide resins and polyvinyl resins can be used.

[0048] Such encapsulated toners can be obtained usually by the following production method.

(1) A spray-drying method, wherein after the core material is dispersed in a non-aqueous solution of polymer or polymer-emulsion, the dispersed liquid is spray-dried.

(2) A phase separation method (coacervation method), wherein phase separation is conducted around the core material in a solution of ionic polymer colloids and the core material, so that a simple emulsion is first prepared, which in turn is converted to a complex emulsion, in which the core materials are micro-encapsulated.

(3) An interfacial polymerization method, wherein a core material solution or dispersion is dispersed in a water in oil or oil in water type emulsion system, while at the same time collecting the shell material monomers (A) around the surfaces, which in turn is followed by reacting monomers (A) with monomers (B) around the surfaces in the subsequent step.

(4) Other methods include an in situ polymerization method, a submerged cure coating method, an air suspension coating method, an electrostatic coalescing method, a vacuum vapor deposition coating method, etc.

[0049] According to the fixing method using the fixing apparatus of the present invention, since the toner adhered onto the recording medium while conveying is fixed at a fixing temperature of not more than 130°C in the fixing process, it is no longer necessary to use expensive heat-resistant materials for the fixing apparatus and the periphery thereof, and inexpensive materials can be used. In addition, since the durability of the parts used become long, the service life of the overall apparatus becomes long, thereby making it possible to be low-cost printing machines. Further, since the fixing is carried out at a fixing temperature of not more than 130°C and the nip pressure is also kept low, paper sheets are not likely to curl or jam, thereby conserving in its maintenance.

[0050] Since the fixing is carried out at a low temperature, the temperature of the heating element in the fixing device can be set low with only a small rise of the temperature in the printing machine. Accordingly, a forced radiation device such as an electric fan can be made smaller or is not required, thereby reducing the noise. Also, since the waiting time for the temperature rise in the fixing apparatus can be shortened, quick printing becomes possible, making it suitable in the field requiring quick printings such as facsimile. Further, since the fixing is carried out at a fixing temperature of not more than 130°C, the fixing apparatus can be simplified, thereby making it possible to miniaturize and lower its overall cost.

EXAMPLES

[0051] The present invention is hereinafter described in more detail by means of the following production examples, test examples and comparative test example, but the present invention is not limited by them.

Production Example 1 of Encapsulated Toner:

[0052] 100 parts by weight of a polyester resin (Tg: 50°C; softening point: 120°C; and acid value: 25) obtained by a condensation polymerization of bisphenol A propylene oxide adduct, bisphenol A ethylene oxide adduct, terephthalic acid, dodecenylsuccinic anhydride and trimellitic acid; 10 parts by weight of carbon black "#44" (manufactured by Mitsubishi Kasei Corporation); and 4 parts by weight of low-molecular weight polypropylene "Biscol 660p" (manufactured by Sanyo Chemical Industries, Ltd.) are together premixed, and the resulting mixture is kneaded using a roller. After cooling this kneaded mixture, it is roughly pulverized to a particle size of 1 to 2 mm and then finely pulverized using a jet mill. The pulverized mixture is classified with an air classifier to give core material particles having an average particle size of 10 µm. To 20 parts by weight of a separately prepared styrene·butyl methacrylate·butyl acrylate·acrylate copolymer emulsion (the resin having Tg: 70°C; Mw: 230,000; Mw/Mn: 6.3; softening point: 140°C; and solid component: 40%), 58 parts by weight of the above core material particles, 0.4 parts by weight of nigrosine dye "Bontron N-01" (manufactured by Orient Chemical) and 240 parts by weight of water are added. The resulting mixture is spray-dried, while mixing and dispersing the mixture well, to give an encapsulated toner. The softening point of the obtained encapsulated toner is 118°C.

Production Example 2 of Encapsulated Toner:

[0053] 60 parts by weight of paraffin wax "SP-0145" (manufactured by Nippon Seiro), 8 parts by weight of carnauba wax (manufactured by Noda Wax), 32 parts by weight of a urethane compound (an equimolar reaction product of hexamethylene diisocyanate and ethyl alcohol; molecular weight: 450; and softening point: 100°C) and 10 parts by weight of carbon black "#44" (manufactured by Mitsubishi Kasei Corporation) are together mixed, and the resulting mixture is kneaded using a heat roller. After cooling this kneaded mixture, it is roughly pulverized to a particle size of 1 to 2 mm and then finely pulverized using a jet mill. The pulverized mixture is classified with an air classifier to give core material particles having an average particle size of 10 µm. Thereafter, the fine particles are spray-dried to give the encapsulated toner. The softening point of the resulting encapsulated toner is 98°C.

Test Example 1:

[0054] The encapsulated toner obtained in Production Example 1 of Encapsulated Toner is surface-treated with 0.5% of silica "R972" (manufactured by Nippon Aerosil Co., Ltd.), and then 50 g of the toner is blended together with 1 kg of a commercially available ferrite carrier to obtain a developer.

[0055] Also, a commercially available copying machine is modified to produce an apparatus according to Figure 1, in which a fixing portion comprising a heat roller and a pressure roller is produced according to Figure 2. Specifically, each of the heat roller and the pressure roller has a diameter of 20 mm, the rubber hardness measured according to an A-type hardness tester is 60 degrees for the heat roller and 40 degrees for the pressure roller. The heat roller is obtained by coating the rotating shaft of the heat roller with a silicone resin as an insulating layer, and then forming a conductive elastomer layer thereon with a silicone resin dispersing a carbon black to provide the specific resistivity of 3 Ω·cm. The pressure roller is made of an elastic material using silicone foam rubber.

[0056] The nip pressure of the fixing apparatus is 0.3 kg/cm and the nip width is 3 mm, and voltage is supplied to both ends of conductive elastomer layer using a conductive brush. Here, the nip width is measured by a roll nip tester manufactured by Toshiba Silicone Co., Ltd. The ratio of the nip width to the roller radius is 0.30.

[0057] The obtained developer is used to carry out copying by using the modified copying machine to develop images. The fixing temperature is measured, and it has been found that a sufficient fixing strength is exhibited even at a temperature of 100°C (lowest fixing temperature) at a peripheral speed of 25 mm/sec, and that it does not show any hot offsetting to the heat roller at a temperature of 150°C. Further, a continuous copying test for 10,000 sheets is carried out at a fixing temperature of 100°C, and substantially no failures caused on the formed images by the fixing apparatus are observed. Also, although no separation claw normally used in heat rollers and pressure rollers is used, substantially no deformation of paper sheets such as curling, jamming, wrinkling of the recording paper is observed, at the time of discharging of the recording medium from the fixing apparatus after fixing.

[0058] The lowest fixing temperature for the toner is the temperature of the paper surface at which the fixing rate of the toner exceeds 70%. This fixing rate of the toner is determined by placing a load of 500 g on a sand-containing rubber eraser having a bottom area of 15 mm x 7.5 mm which contacts the fixed toner image, placing the loaded eraser on a fixed toner image obtained in the fixing device, moving the loaded eraser on the image backward and forward five times, measuring the optical reflective density of the eraser-treated image with a reflective densitometer manufactured by Macbeth Co., and then calculating the fixing rate from this density value and a density value before the eraser treatment using the following equation.

Test Example 2:

[0059] The encapsulated toner obtained in Production Example 2 of Encapsulated Toner is surface-treated with 0.5% of silica "R972" (manufactured by Nippon Aerosil Co., Ltd.), and then 50 g of the toner is blended together with 1 kg of a commercially available ferrite carrier to obtain a developer.

[0060] The obtained developer is used to carry out copying by using the same apparatus and the same method as in Test Example 1 above, and the fixing temperature is measured. As a result, it has been found that a sufficient fixing strength is exhibited even at a temperature of 100°C (lowest fixing temperature) at a peripheral speed of 25 mm/sec, and that it does not show any hot offsetting to the heat roller at a temperature of 150°C. Further, a continuous copying test for 10,000 sheets is carried out at a fixing temperature of 100°C, and substantially no failures caused on the formed images by the fixing apparatus are observed. Also, although no separation claw normally used in heat rollers and pressure rollers is used, substantially no deformation of paper sheets such as curling, jamming, wrinkling of the recording paper is observed, at the time of discharging of the recording medium from the fixing apparatus after fixing.

Comparative Test Example:

[0061] The encapsulated toner obtained in Production Example 1 of Encapsulated Toner is surface-treated with 0.5% of silica "R972" (manufactured by Nippon Aerosil Co., Ltd.), and then 50 g of the toner is blended together with 1 kg of a commercially available ferrite carrier to obtain a developer.

[0062] The fixing apparatus used in the commercially available copying machine is modified so as to make it variable in fixing temperature and roller rotating speed. The heat roller comprises an aluminum cylinder coated with a fluorine resin, which has a roller radius of 30 mm and a quartz heater disposed inside the cylinder. The pressure roller has a rubber hardness measured by the A-type tester of 80 degrees, a roller radius of 20 mm, and a nip pressure is 0.5 kg/cm and a nip width is 3 mm. The ratio of the nip width to the roller radius is 0.15.

[0063] The obtained developer is used to carry out copying using the copying machine to develop images, and the fixing temperature is measured. As a result, it has been found that although a sufficient fixing strength is exhibited even at a temperature of 100°C (lowest fixing temperature) at a peripheral speed of 25 mm/sec, a hot offsetting to the heat roller takes place at a temperature of 120°C.

[0064] The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. A fixing apparatus for forming fixed images at a temperature of not more than 130°C, comprising:
   a heat roller having a conductive elastomer layer formed in a uniform thickness;
   a pressure roller; and
   a current supplying means,
wherein said heat roller and said pressure roller rotate in contact with each other to thermally fix a visible image onto the recording medium at the contacted surfaces, and the electric current is passed into said conductive elastomer layer by said current supplying means.

2. The fixing apparatus according to claim 1, wherein the specific resistivity of the conductive elastomer layer is not more than 100 Ω·cm, and the maximum heating temperature thereof is not more than 150°C.

3. The fixing apparatus according to claim 1 or 2, wherein the nip width of a pair of rollers in said fixing apparatus is not less than 0.2 times the radius of the elastic material roller.

4. The fixing apparatus according to claim 1, 2, or 3, wherein the rubber hardness of at least one of the rollers is not more than 90 degrees according to JIS A-type hardness.

5. A method of fixing a visible image formed by an encapsulated toner using a heat roller-type fixing apparatus, said heat roller-type fixing apparatus comprising a heat roller having a conductive elastomer layer formed in a uniform thickness, a pressure roller and a current supplying means, comprising the steps of:
   passing an electric current into the conductive elastomer layer; and
   thermally fixing the visible image at a temperature of not more than 130°C by heat generated in the conductive elastomer layer.

6. The method according to claim 5, wherein the specific resistivity of the conductive elastomer layer is not more than 100 Ω·cm, and the maximum heating temperature thereof is not more than 150°C.

7. The method according to claim 5 or 6, wherein the nip width of a pair of rollers in said fixing apparatus is not less than 0.2 times the radius of the elastic material roller.

8. The method according to claim 5, 6, or 7, wherein the rubber hardness of at least one of the rollers is not more than 90 degrees according to JIS A-type hardness.

Drawing