1. Technical Field
The present invention relates to a fixing device employing an electromagnetic induction heating method, and to an image forming apparatus incorporating such a fixing device.
2. Related Art
Conventionally, an image forming apparatus such as a copier, a printer, and the like, includes a fixing device employing electromagnetic induction, which is both fast and energy-efficient.
For example, JP-2006-350054-A discloses a fixing device employing the electromagnetic induction heating method, which includes a support roller as a heat roller to generate heat, a fixing support roller as a fixing roller, a fixing belt stretching around the support roller and the fixing support roller, an induction heater disposed opposite the support roller via the fixing belt, and a pressure roller pressing against the fixing support roller via the fixing belt.
The induction heater is formed of a coil such as an excitation coil wound in the longitudinal direction and a core disposed opposite the coil. The fixing belt is configured to be heated at a portion opposite the induction heater. The thus-heated fixing belt heats to fix a toner image formed on a recording medium conveyed to a position opposite the fixing support roller and the pressure roller. More specifically, when a high frequency alternating current is supplied to the coil, an alternate magnetic field is formed around the coil, and an eddy current is generated near the surface of the support roller. When the eddy current is generated to the support roller as a heat roller, joule heat is generated by the electrical resistance of the support roller itself and the fixing belt wound around the support roller is heated.
The fixing device employing the electromagnetic induction heating method as described above has better thermal conversion efficiency and thus consumes less energy than a conventional halogen heater, and is capable of increasing a surface temperature of the fixing belt up to a prescribed level in a short time because a heat generator used in the electromagnetic induction fixing device is directly heated.
A coil used for the induction heating includes an excitation coil and a core to introduce alternate magnetic field generated by the excitation coil to the heat generator. The fixing device disclosed by JP-2008-032944-A includes a flux path by using cores 28, 29 from the excitation coil 25 to the fixing roller 20 including the heat generator.
As illustrated in
In addition, JP-2003-215957-A (or JP3452920) discloses a fixing device in which the excitation coil 5 is surrounded by the cores 32, 33, and 38 (see
However, dividing the core as described above causes the magnetic flux to leak from joint portions between adjacent cores, thereby reducing heat generation efficiency. In addition, segmentation of the core increases the number of parts, resulting in a cost rise.
As an approach to the above disadvantage, provision of a gap between all arch-shaped cores and side cores is conceived to afford a unified contact status to reduce temperature fluctuation in the longitudinal direction due to dimensional variations of the opening. In this case, decrease in the heat generation efficiency cannot be prevented.
In addition, JP-2009-216751-A discloses a structure in which both side ends of the arch-shaped cores are bent in the direction to the heat generator. Specifically,
In such a structure, heat rises at opposed surfaces of the heat generator, i.e., a front end of the bent portions of the arch-shaped cores, and therefore it is difficult to maintain a uniform temperature distribution along the axial direction of the roller or the coil longitudinal direction.
In addition, the excitation coil needs to be held in the arch-shaped core. However, the disclosed structure with both ends bent is unsuitable for mounting the arch-shaped cores from above the coil because bent portions at both ends interfere with the coil.
The present invention solves the above problem in the fixing device employing the induction heating method, and provides a fixing device that includes a rotary fixing member; a pressure roller pressed against the fixing member to form a nip in association with the fixing member; and an induction heater, as a heat source, to heat the fixing member. The induction heater includes an excitation coil to induction-heat the fixing member; a side core disposed along an outer circumference in a longitudinal direction of the excitation coil; and a plurality of arch-shaped cores disposed to cover the excitation coil in the longitudinal direction thereof. The arch-shaped cores include center portions corresponding to an inner side of the excitation coil and bent to the fixing member; and outer end portions extending in the direction leading to the side core without interfering with the excitation coil.
These and other objects, features, and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.
Hereinafter, preferred embodiments of the present invention will be described with reference to accompanying drawings.
The image forming apparatus is a printer that employs an electrophotographic method and includes four sets of image forming units 10Y, 10M, 10C, and 10Bk, each mainly including photoreceptor drums 1Y, 1M, 1C, and 1Bk as an image carrier, so that a full-color image using four colors of toner, yellow (Y), magenta (M), cyan (C), and black (Bk) can be formed. However, the structure of the image forming apparatus is not limited to the illustrated example alone. For example, the illustrated printer herein employs a direct transfer method, in which a toner image is directly transferred onto a recording medium such as a sheet; however, the printer may employ an indirect transfer method, in which the toner image is transferred to the sheet via an intermediate transfer member. In addition, the number or order of colors can be varied. Further, the present invention is not limited to a printer but is applicable to, a copier, a facsimile machine, or a multi-function apparatus having one or more capabilities of the above devices.
As illustrated in
The four sets of image forming units 10Y, 10M, 10C, and 10Bk each are similar in structure. Therefore, the image forming unit 10Y disposed most upstream is taken as a representative and is described in detail below. To avoid complication, reference numerals for the image forming units 10M, 10C, and 10Bk other than the yellow image forming unit 10Y are omitted. In addition, suffixes representing different colors Y, M, C, and Bk will be omitted in the explanation below.
Each image forming unit 10 includes a photoreceptor drum 1 disposed in the center and rotatably contacting the conveyance belt 20. Around a circumference of the photoreceptor drum 1 are disposed a charger 2, an exposure device 3, a developing device 4, a transfer roller 5, a cleaner 6, and a discharge lamp, not shown, in this order along a rotation direction of the photoreceptor drum 1. The charger 2 charges a surface of the photoreceptor drum 1 so that the photoreceptor drum 1 has a predetermined electric potential. The exposure device 3 exposes the charged drum surface based on color-decomposed image signals and forms an electrostatic latent image on the surface of the drum. The developing device 4 supplies toner to develop the electrostatic latent image formed on the drum surface and renders the latent image visible. The transfer roller 5 transfers the developed toner image on the sheet conveyed via the conveyance belt 20. The cleaner 6 removes residual toner remaining, without being used in the transfer, on the surface of the drum. The discharge lamp, not shown, removes any electrical charge remaining on the surface of the drum.
Next, the fixing device according to an embodiment of the present invention will be described with reference to
The heat roller 41 includes a metal core formed of non-magnetic stainless steel, having a thickness of from 0.2 to 1.0 mm. The heat roller 41 includes a heat generation layer formed of Cu on the surface thereof, to thus improve the heat generation effect. In this case, Nickel coating is preferably applied on the surface of the Cu layer for preventing corrosion. In addition, in order to further improve the heat generation effect, a ferrite core can be disposed inside the heat roller.
Alternatively, any magnetic shunt alloy with a Curie point of approximately 160 to 220 degrees C. may be used. An aluminum material is disposed inside the magnetic shunt alloy, so that a temperature increase stops around the Curie point. Even when the magnetic shunt alloy is used for the heat roller, a Cu coating layer is formed on the surface of the heat roller, so that the heat generation effect can be improved.
The fixing roller 42 includes a metal core 42a formed of, for example, stainless steel, carbon steel, or the like, and an elastic material 42b covering the metal core with solid or foamed silicon rubber having heat resistivity. Then, the pressure roller 44 presses against the fixing roller 42, so that a contact portion, that is, a fixing nip N, with a predetermined width is formed between the pressure roller 44 and the fixing roller 42. An external diameter of the fixing roller 42 is from 30 to 40 mm, the thickness of the elastic material 42b is from 3 to 10 mm, and the roller hardness is from 10 to 50 degrees according to Japanese Industrial Standards Class A (JIS-A).
The fixing belt 43 serving as a fixing member is stretched around the heat roller 41 and the fixing roller 42. The fixing belt 43 according to the present embodiment includes a base 43a, an elastic layer 43b, and an outer release layer 43c. The elastic layer 43b and the release layer 43c are laminated on the base 43a.
Properties required for the base 43a include mechanical strength required when stretched around the rollers, flexibility, and heat resistivity capable of withstanding the fixing temperature. In the present invention, the base 43a to induction-heat the heat roller 41 is preferably formed of insulating heat-resisting resins, such as, polyimide, polyamideimide, polyetheretherketone (PEEK), polyethersulfone (PES), polyphenylene sulfide (PPS), fluorine resins, and the like. The thickness thereof is from 30 to 200 μm considering the thermal capacity and the strength.
The elastic layer 43b is provided to give flexibility to the surface of the belt so as to obtain a uniform image without uneven glossiness, and preferably has a rubber stiffness of 5 to 50 degrees (according to JIS-A), and a thickness ranging from 50 to 500 μm. In addition, preferable materials include silicon rubbers, fluorosilicon rubbers, and the like, for obtaining heat resistivity for the fixing temperature.
Materials used for the release layer 43c include fluorine resins such as: polytetrafluoroethylene (PTFE); tetrafluoroethylene-perfluoroalkyl vinylether copolymer (PFA); and tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or mixture of these resins, or heat resistant resins dispersed with above resins.
When the elastic layer 43b is coated with the release layer 43c, toner can be released easily and paper dust can be prevented from sticking without using silicon oil and the like, and an oil-less structure is enabled. However, the resins having releaseability usually have no elasticity like a rubber material, so that if the thick release layer is formed on the elastic layer, elasticity of the belt surface forming the release layer is lost, thereby generating uneven glossiness in the output image. To balance the releaseability and the elasticity, the thickness of the release layer 43c is preferably ranging from 5 to 50 μm and is more preferably 10 to 30 μm.
In addition, if necessary, a primer or undercoat layer is disposed between adjacent layers. Further, a layer to improve durability against slidable movement can be disposed on an interior surface of the base 43a.
The base 43a may include a heat generation layer. For example, the one in which a Cu layer having a thickness of 3 to 15 μm is formed on the base layer formed of polyimide can be used as a heat generation layer.
The pressure roller 44 is formed of a release layer 44c, an elastic layer 44b having a high heat resistance, and a metal core 44a including a metallic cylinder portion. The pressure roller 44 presses against the fixing roller 42 via the fixing belt 43, so that a fixing nip N is formed at the pressed portion. An outer diameter of the pressure roller is set to some 30 to 40 mm and the elastic layer 44b has a layer thickness of 0.3 to 5 mm and has an Asker stiffness of 20 to 50 degrees. A favorable material for the pressure roller 44 is silicon rubber because of necessity of heat resistance. Further, in order to improve releaseability when duplex printing is performed, the release layer 44c formed of fluorine resins and having a layer thickness of 10 to 100 μm is disposed on the elastic layer 44b.
Because the stiffness of the pressure roller 44 is greater than that of the fixing roller 42, the pressure roller 44 bites into the fixing roller 42 and the fixing belt 43. As a result, the recording medium that is conveyed along the fixing belt 43 is distorted on the way out of the fixing nip and has a curvature relative to the surface of the fixing belt 43, and thus, the releaseability of the recording medium is increased.
As illustrated in
The excitation coil 52 is formed such that 50 to 500 electrical leads or wire strands each having a diameter of approximately 0.05 to 0.2 mm are wound together to form a litz wire, which is wound around 5 to 15 times. The litz wire includes a fusion layer on its surface thereof. The fusion layer is solidified by being heated electrically or heated in a constant temperature reservoir, and thus, the shape of the wound coil can be maintained. Alternatively, the litz wire without the fusion layer can be shaped by press molding. Because the litz wire requires a heat resistance in excess of the predetermined fixing temperature, preferable materials for an insulation coated layer of base wires include resins such as polyamideimide, polyimide, and the like having heat resistance and insulation properties.
If finished winding the coil 52, the coil 52 is attached to the case using a silicon adhesive or the like. The case 51 should be heat-resistant up to a temperature exceeding the fixing temperature and is preferably formed with highly heat-resistant resins such as PET or crystal liquid polymers.
Preferred materials for the cores 53, 54, 55 are ferrite ones such as Mn—Zn ferrite and Ni-Zn ferrite. The ferrite core is obtained by compressing and molding ferrite powder and by sintering the obtained ferrite mold. During sintering, the core shrinks. In particular, the opening of the arch-shaped core 53 is due to a difference in the shrinkage ratio between the opening portion and the connection portion thereof. As the arch-shaped core is large in size at the connection portion, the shrinkage ratio is greater, so that variations in shape are remarkable due to the shrinkage. As a result, production yield decreases, thereby increasing the cost for production. Considering the above, the arch-shaped cores 53 according to the present embodiment are formed in a compact size so as to cover one side of the wound coil 52.
Coil center portions 53b of the arch-shaped cores 53 are bent to the heat generating side, i.e., in the direction of the heat roller 41. With such a configuration, the magnetic flux generated from the coil 52 can be more efficiently led to the heat roller 41 serving as the heat generating member.
In addition, outer end portions 53c of the arch-shaped cores 53 are not bent to a side of the heat generating member, do not interfere with the coil 52 disposed inside the arch-shaped cores 53, and extend in the direction leading to the side cores 54. In the present embodiment, the outer end portions 53c extend substantially parallel to the coil center portions 53b; however, the portions 53c can retain the arch shape. With such a configuration, because the outer end portions 53c of the arch-shaped cores 53 extend in the direction toward the side cores 54 without interfering with the excitation coil, in assembling the induction heating unit 50, there is no interference of the end portions of the arch-shaped cores with the coil even when mounting the arch-shaped cores 53 to the coil 52 from above the coil 52 (i.e., from the left in
Referring to
As illustrated in
In the present embodiment, the side cores 54 include a planar surface and plural side cores 54 are disposed along the axial direction of the heat roller. Because the ferrite core shrinks through sintering process, the longer one tends to be warped. Therefore, plural cores are used to avoid warping. In addition, the side cores 54 are disposed up to a bent portion of the excitation coil 52, that is, a portion of the coil at a longitudinal end where the straight coil starts to be curved.
End cores 55 are disposed at both ends of the coil 52 to prevent reduction of heat at the end of the recording material passing through the nip and to increase temperature at the end. When the temperature at the nip is sufficiently uniform, provision of the end cores 55 can be omitted.
Next, the induction heating unit 50 will be described.
In the structure as illustrated in
In particular, if the heat roller 41 that serves as a heat generator in the present embodiment is produced using magnetic alloys, if each line of the arch-shaped cores 53 is disposed to oppose to each other (that is, not the staggered structure), heat is concentrated at the portion where the cores are disposed due to good magnetic coupling, resulting in uneven temperature distribution in the longitudinal direction. However, in the structure according to the third embodiment, because the arch-shaped cores 53 are displaced in the staggered manner, uneven distribution of the temperature in the longitudinal direction is suppressed, thereby obtaining the smoothed temperature distribution.
In the third embodiment, the shape of the end of the coil center portions 53b of the arch-shaped cores 53 may be either the one as illustrated in
Temperature tends to be decreased at an axial end portion of the heat roller 41 because heat dissipates outside. According to the present embodiment, by increasing the density of the arch-shaped cores 53 at both ends in the longitudinal direction, the temperature at the end of the heat roller 41 can be prevented from reducing.
In the fourth embodiment, the shape of the end of the coil center portions 53b of the arch-shaped cores 53 may be either the one according to the first embodiment and the other according to the second embodiment, that is, the shape substantially parallel to the tangent line of the heat roller 41. In the example as illustrated in
Specifically, as illustrated in the cross-sectional view of
With this structure according to the fifth embodiment, the temperature distribution in the axial direction of the heat generator can be smoothed.
In the fifth embodiment, the shape at the end of the coil center portions 53b of the arch-shaped cores 53 may be either the one according to the first embodiment and the other according to the second embodiment, that is, the shape substantially parallel to the tangent line of the heat roller 41. In the example as illustrated in
In addition, in an experiment for comparison which will be described later, an induction heater mounted in the copier ‘imagio C5000’ (registered trademark; manufactured by Ricoh Company, Ltd.) is used as a comparative example 2.
The above comparative example 1 is appropriate to compare an effect of the arch-shaped core according to the present embodiment of the invention, in which the arch-shaped portion 53a and the center portion 53b are continuously provided. In addition, the comparative example 2 represents uniform temperature at the nip of an actual commercial product level and is effectively used for comparing the temperature distribution.
Hereinafter, a comparison experiment will be described.
In the comparison experiment, the above described actual printer (imagio C5000) is used, and the heating experiments have been done by sequentially changing the induction heater from the ones described in the first to fifth embodiments, the comparative example 1, and the comparative example 2. A temperature sensor is mounted upstream of the nip of the fixing unit and the temperature is obtained.
First, as illustrated in
1) Elevated Temperature Experiment
Experiments are done using the induction heaters according to the comparative example 1 and the embodiments 1 and 2, so as to verify temperature increase when starting the temperature increase test.
If comparing the comparative example and the first embodiment, it can be seen that the temperature increase is faster in the first embodiment. By using the arch-shaped core according to the present invention, the heat generation efficiency is improved and the temperature rise becomes faster.
If comparing the first embodiment and the second embodiment, it can be seen that the temperature increase is much faster in the second embodiment. From this result, it can be seen that the heat generation efficiency is improved when the leading end of the coil center portions 53b is shaped parallel to the heat generator.
From the above experiment of the temperature increase, it can be seen that the heat generation efficiency is improved by using the arch-shaped core according to the present invention.
2) Temperature Distribution in the Nip
Temperature distribution in the nip along the axial direction of the heat roller is measured, and it is verified whether or not the temperature distribution applicable to the fixing device may be actually obtained.
As described above, the temperature distribution that can secure the fixabilty is obtained in the above embodiments and the uniformity of the temperature distribution can be improved.
As a result, by applying the present invention, leaked magnetic flux from the coil can be reduced and the heat generation property can be improved without degrading the uniformity of the temperature required for the nip portion of the fixing device, whereby the present invention can provide an optimal induction heating means excellent in the faster temperature rising when starting printing and an optimal image forming apparatus with an excellent energy saving property.
Finally, a description will be given of the sixth embodiment of the present invention in which the present invention is applied to the fixing device employing a heat roll method.
The structure and operation of the induction heating unit 50 which is used in the sixth embodiment are the same as those explained in the first embodiment, and therefore, the redundant description thereof will be omitted.
Specifically, the fixing roller 45 has an outside diameter from 30 to 40 mm and includes an elastic layer 45b, a heat generation layer 45c, and a release layer (not shown) are laminated on a metal core 45a. The fixing roller 45 rotates in a direction as indicated by an arrow in the figure, i.e., in a counterclockwise direction, is heated by induction heating, and fuses the toner image carried on a recording sheet, to be conveyed to the fixing nip portion.
As described above, the fixing device according to the present invention includes arch-shaped cores 53 having ends 53b disposed at inner sides of the excitation coil 52 bent toward the side of the fixing member or the heat generator; and opposite side ends 53c each extending to the side cores 54 without interfering with the excitation coil 52. As a result, without increasing the number of parts for producing the core, heat generating efficiency can be improved. In addition, the end portion opposite the bent portion is not bent toward the fixing member, so that centralization of heat is eliminated and uneven temperature along the longitudinal direction of the heat generator can be suppressed. Further, because the end of the arch-shaped cores do not interfere with the excitation coil in assembling operation, thereby not degrading workability in assembling.
Further, because the leading end of the arch-shaped core is substantially parallel to the tangent line of the fixing member or the heat generator, magnetic fluxes from the arch-shaped core leading to the fixing member (heat generator) can be increased and the heating efficiency can be improved.
Furthermore, the plurality of arch-shaped cores are arranged in two lines along each longitudinal side of the excitation coil so that the arch-shaped cores in one line are disposed in the staggered manner at different positions relative to the arch-shaped cores in the opposite line. With this structure, the temperature distribution in the longitudinal direction of the excitation coil can be smoothed.
Furthermore, the plurality of arch-shaped cores is disposed denser in the end portions in the longitudinal direction of the excitation coil than in the center portion. Thus, the temperature at the end of the fixing member (heat generator) can be prevented from reducing.
Further, the gap between each arch-shaped core and the side core is adjusted so that the elevated heat distribution along the axial direction of the rotary fixing member is smoothed, whereby occurrence of uneven temperature in the fixing member axial direction can be prevented.
In addition, the present invention can be applied to both the fixing device employing the belt fixing method and that employing the heat roll method.
The present invention may also be applied to, not limited to the copier, a printer, a facsimile machine, or a multi-function apparatus having one or more capabilities of the above devices.
Additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.
Number | Date | Country | Kind |
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2013-008252 | Jan 2013 | JP | national |
The present application claims priority pursuant to 35 U.S.C. §119 from Japanese patent application number 2013-008252, filed on Jan. 21, 2013, the entire disclosure of which is incorporated by reference herein.